Molecular weight and molecular weight distribution

Average molecular weight :- A simple compound has a fixed molecular weight . The molecular weightwill not change regardless of how the compound is made. A simple compound cannot have more thanone molecular weight . So the molecular weight of simple compounds is definite. In contract, a polymercomprises molecular of different molecular weights and hence it’s molecular weight is expressed interms of an ‘average’ value . For example ethylene gas is a low molecular weight simple chemicalcompound. Each of it’s molecular has the same chemical structure (CH2 = CH2) and hence a fixedmolecular weight of 28. Ethylene is polymerized to make polyethylene. (CH2 – CH2)n , Where in canchange its value from one polyethylene molecule to another present in the same polymer sample .Because different molecular have different sizes, their molecular weights are different. A polymersample can therefore be considered as a mixture of molecular of the same chemical type but ofdifferent molecular weights. In this situation the molecular weight of the polymer sample can only beexpressed as some average of molecular weights contributed by the individual molecular that make thesame.

There are mainly four types of molecular weight averages used in polymer science for characterizing thepolymers .

1) Number average molecular weight (Mn)2) Weight average molecular weight (Mw)3) Sedimentation average molecular weight (Mz)4) Viscosity average molecular weight (Mv)

Determination of Average molecular weight :-

The average molecular weights of a polymer can be computed by using either the number fraction orthe weight fraction of the molecular present in the polymer . Suppose that there are n number ofmolecular in a polymer sample and n1 of them have M1 molecular weight , n2 have M2 molecular weightand so and ni molecular have Mi molecular weight .

So , we have a total number of molecular (n) , given by , n = n1 + n2 + n3 +-----------+ n1 = Eni

Number of molecular in fraction 1 = n1

Number fraction of fraction 1 = n = n1

n Eni

molecular weight contribution by fraction 1 = n1M1

Eni

Similarly , molecular weight contribution by other fractions will be

Number average molecular weight of the whole polymer will then be given by

Similarly ,Total weight of the polymer W = Eni Mi

Weight of fraction 1 = n1 M1 = W1

Weight fraction of fraction 1 = n1 M1 = n1M1

W Eni Mi

Molecular weight contribution by fraction 1 is given by ,

Similarly , the molecular weight contribution by the other fractions will be

The weight average molecular weight of the whole polymer will then be _

For all synthetic polymers , Mw is greater than Mn .

Similarly the sedimentation or Z-average molecular weight is given by

And viscosity average molecular weight is expressed as _

Where ni is the number of molecules having molecular weight of Mi present in the sample and a is avariable in the mark – Aouwink equation , which relates intrinsic viscosity (n) with the viscosity averagemolecular weight . value of a , ranges from 0.5 to 1.0 .

Degree of Polymerization :-The size of a polymer molecule depends on the number of repeat units

it contains and this number is known as the degree of polymerization . For example , if there are 1000repeat units in a polymer molecule , the degree of polymerization (DP) is 1000 .Molecular weight of a polymer can also be expressed in terms of degree of polymerization as follows .

M = Dp.M.Where M is the molecular weight of the polymer , Dp, it’s degree of polymerization and m is themolecular weight of the monomer or the repeat unit .Degree of polymerization can also be average degree of polymerization

And the weight average degree of polymerization

The average molecular weights can be related to the corresponding degree of polymerization as .

Poly disparity :- A simple chemical compound contains molecules , each of which has the samemolecular weight . This is called a monoclisperse system . On the other hand a polymer contains

molecules , each of which can have different molecular weights . Polymer are mixtures of molecules ofdifferent molecular weights and are known as poly dispersed systems and this property of a polymersample having molecules of different molecular weights is termed ‘poly disparity ‘ . The reason for thepoly disparity of polymers his in the statistical variations present in the polymerization processes .Poly disparity is a very important far meter . It gives an idea of the lowest and the highest molecularspecies as well as the distribution pattern of the intermediate molecular species . It is calculated as theratio of Mw and Mn . So .

For synthetic polymers , the value of poly disparity is always greater than one . Moreover polymersobtained from different polymerization techniques may have different values for poly disparity .Polymers obtained from cationic / anionic chain polymerization using homogeneous catalyst systemgenerally have poly disparity values less than 1.5, where as polymerization systems leading to chainbranching may field polymers with poly disparity index of greater than 20 .Molecular Weight distribution in polymers :- The poly disparity nature of the polymer is the basic ofthe concept of average molecular weight . This average by itself conveys nothing on the disparitypattern in a given polymer sample . Two polymer samples of same average molecular weight can displaysimilar properties in some respects but not in some others . To know a polymer properly , we must havea knowledge of both the average molecular weight as well as it’s dispersion pattern . This disparity withrespect to the lowest to the highest molecular weight homologues is expressed by a simple molecularweight distribution curve . Such a curve for a polymer sample is computed by plotting the numberfraction (ni) of molecules having a particular molecular weight (Mi) against the corresponding molecularweight as given below .

Such a curve reveals the pattern of the different molecular species present in a polymer sample . It canbe noted from the above curve that Mw is greater than Mn and that Mv is closer to Mw than to Mn .The following figure shows molecular weight distribution curves of three hypothetical samples withsame number average molecular weights but with different poly disparities .

In the above figure

Some properties of the three samples will be different .

The practical significance of polymer molecular weight :-1) Many commercially useful polymers are selected on the basic of their properties such as melt

viscosity , impact strength or tensile strength . These properties are directly dependent on themolecular weight on the polymer . For example tensile and impact strength increase upto apoint and then level off . At high molecular weights , the melt viscosity rises more steeply thanat low molecular weights . A commercially useful polymer should have a low melt viscosity forease of processing , but at the same time , it should have good strength .Every polymer has a threshold value (TV) for it’s molecular weight (DP) , below which thepolymer does not possers any strength and exists as a powder or as liquid resin . After reachinga certain value of molecular weight , they attain optimum properties . The useful range ofdegree of polymerization (DP) for most of the polymers is from 200 to 2000, which correspondsto molecular weights of roughly 20000 to 200000 .Measurement of molecular weight of polymer :- The type of average obtained depends on theexperimental technique and the property of the polymer molecules being measured . Anyexperiment , in which , the measured property is dependent on the total number of moleculespresent , regardless of their masses , will field the number average molecular weight . Methodsbased on cryoscopy , ebulliometry and osometry give Mn chemical methods such as the endgroup analysis are also used to evaluate Mn . Experiments based on viscosity measurementsyield the viscosity average molecular weight Mv . Techniques such as light scattering andultracentrifugation are used to find out the weight average molecular weight Mw .It is important to note that all these measurements are made on polymer molecules in solutionsand that they are concentration dependent . Results are good at very low concentration of lessthan 10gms/liter .Measurement of molecular weight of difnte solution viscometry :- viscometry is usefultechnique for determining the polymer molecular weight . The molecular weight obtained bythis technique is the viscosity average molecular weight (Mv) . Viscosity of a polymer solution isconsiderably high compared to that of a pure solvent . The increase in viscosity is a directfunction of the molecular weight of the macromolecule . Different relationship have beendefined between the viscosity of a polymer solution and molecular weight . However , the mark– hoodwink equation is most appropriate –

Where [ ] is the intrinsic viscosity, M, the molecular weight and a and k are constants for aparticular polymer / solvent/temperature system .

The intrinsic viscosity also known as Staudinger index or Limiting Viscosity number can becalculated from the flow times of a constant volume of solvent and the solutions through aparticular capillary tube . This principle is used in the viscometeric technique of molecularweight determination.In actual experiment, solutions of known concentrations of the polymer sample are made . Thesolvent flow time (to) and the solution flow times (t) for different concentrations are measuredusing the same viscometer. For each concentration, the corresponding reduced viscosity and theinherent viscosity are calculated. Then double date exploration plots of reduced viscosity againstconcentration and inherent viscosity against concentration are prepared and extrapolated tozero concentration. The common ordinate intercept of these graphs gives the intrinsic viscosity.Molecular weight determination by viscosity measurement is a simple and convenient method .The solution viscosities are measured by viscometers . The Ostwald viscometer and theubbelohde suspended level viscometer (USLV) are commonly used for this purpose . Theseviscometers are simple glass capillary devices . While using the Ostwald viscometer , care mustbe taken to see that all the measurements are made using a constant volume of the solution .The USLV design is such that the measurement is unaffected by the volume of solution taken .The main advantage of USLV is that only a single solution of a known concentration is requiredto be made to start with . A known volume of this solution is taken in the viscometer and theflow time measured . Subsequent concentrations can be achieved by adding known volumes ofpure solvent and mixing inside the viscometer itself . In Ostwald viscometer each time , we haveto empty , clean and refill the viscometer with a fresh solution before measuring the flow time .

End Group Analysis :- The end group analysis is a chemical method used for calculating thenumber average molecular weight of polymer samples whose molecules contain reactinefunctional group at one end or both ends of the molecule . such as hydroxyl , carboxyl , aldehyde, amino ,esler or methyl groups .Usual chemical methods are employed to find out the total number of functional groupspresent in a given weight of the sample and this is expressed as a functional groupequivalent/100gms . From a knowledge of the functionality, the molecular weight can becalculated using the equation –

The end group analysis method can be used reliably only for samples consisting of linearmolecules with determinable end groups and for those obtained by a known polymerizationmechanism without side reactions , because side reactions during polymerization may introduceerrors in the assumed functionality and hence may lead to errors in the result . This methodcould however be used to find out the average functionality of any polymer sample.Membrane Osmometery :- Membrane Osmometery is a widely used technique to determinethe number average molecular weight of polymers . It is based on the phenomenon of osmosis.If a pure solvent is repeated from the solution by a semi permeable membrane, the solventmolecules diffuse into the solution through the membrane till the solution becomes infinitelydilute. This flow of solvent into solution may be practically stopped by applying an appropriatepressure on the solution side . Osmotic pressure of a solution can thus be defined as thepressure that must be applied to the solution so as to totally prevent the follow of solventthrough the semi permeable membrane in to the solution .The Osmotic pressure ( ) of a polymer solution is related to the number average molecularweight of a polymer by the relation .

Where – R is universal gas constantT – Absolute temp. 0kC – Concentration of solution andB – is a constant called second visual constantA plot of /RTC vessus C, therefore gives a straight line , the ordinate intercept of which will giveOsmometers are basically of two types .1) Static equilibrium Osmometers and2) Dynamic equilibrium Osmometers

The static equilibrium Osmometers are simple but take very long time and are therefore notwidely used now a days . The Osmometers which operate on the dynamic equilibriumprinciple e.f. the high speed membrane Osmometer (HSMO), can provide results within fewminutes and are in common use . Membrane Osmometery is useful in the molecular weightrange of 30000 to 1000000 .Vapour phase Osmometery :- This technique is based on the principle that at a giventemperature the vapour pressure of a solution is less than that of a pure solvent . If we keepa drop each of a pure solvent and the solution in an atmosphere saturated with the solventvapour , condensation of the solvent takes place from the saturated vapour phase on thesolution droplet , because vapour pressure of the solution is lower than that of the puresolvent . The solution droplet , therefore starts getting diluted as well as heated up by thelatent heat of condensation of the solvent condensing on it. Because of the rise in

temperature and increased concentration of solvent , the vapour pressure of the solutiondroplet increases steadily . The process of condensation and the resultant temperature risecontinues till the vapour pressure of the solution droplet at the new elevated temperaturebecomes equal to that of the pure solvent at the original temperature . The total rise in thetemperature T, will be proportional to the mole fraction of the solute , n, in the solution asper the following equation –

Where Hv, is the Heat of vaporization of the solvent .In an actual vapour phase Osmometer (or vapour pressure Osmometer) , the solution andthe solvent droplets are placed directly on two thermostats arranged in a Wheatstonebridge circuit in such way that the temperature rise can Measured very accurately as afunction of bridge in balance output voltage v . The factor v is related to the molecularweight of the solute by the equation –

Where K is the calibration constant . This method is useful for measurement molecularweights upto 30000 .Cryoscopy :- The freezing point of a solvent is lowered when a non volatile solute isdissolved in it . In general , solutions freeze at a lower temperature than that at which apure solvent will do so . This phenomenon of depressing the freezing point of a liquid by theaddition of a solute is made use of in the technique known as cryoscopy . The extent of thefreezing point depression depends on the number of solute molecules dissolved per unitvolume of solution and is independent of the size and nature of the solute molecule . Thismeans that cryoscopy is used to determine the number average molecular weight (Mn) of agiven polymer sample . If the depression of the freezing point is denoted by Tf, it related tonumber average molecular weight and concentration through the following equation –

Where , C is the concentration of the solution ,P , the density of the solventR , the universal gas constantTf , the freezing point of the solventB , is a constant called second virial constant and Hf – the Heat of fusion of solvent .The freezing points the pure solvent and of very dilute solutions of the polymer sample indifferent concentrations are determined using standard freezing point techniques . Once thefreezing points of the solvent and solutions of different concentrations are known . Tf canbe calculated for all these concentrations . Using these data , a graph of Tf versus C isplotted and extrapolated to infinite dilution i.e zero concentration . From the value of theordinate intercept of the plot at zero concentration , Mn can be computed from theequation –

Molecular weight upto 30,000 can be determined accurately with this method .

Ebulliometry :- Ebulliometry is a useful technique for the determination of molecularweights of polymers . It is based on the principle that the boiling point of a solution is higherthan that of the pure solvent . for example , a salt solution in water boils at a temperaturehigher than 1000c .The basic equations , which relate the boiling points elevation, Tb to the solute molecularweight and concentration, are as following –

Where Hv and Tb are the heat of vaporization and boiling point of the solvent , respectively. For calculating the molecular weight by this method , boiling points of pure solvent andpolymer solutions of different concentrations are determined and compared to find out Tb

. Number average molecular weight of polymer sample can be calculated using equation 1& 2 above . This method can be used to determine number average molecular weights upto30,000 .Light scattering :- The light scattering phenomenon is used to measure the weight averagemolecular weight Mw of polymers .It has been observed that in solutions, scattering of light occurs due to changes in density orrefractive index within the system arising from compositional variations . In light scatting ,the amplitude of scattering is found to be proportional to the mass M of the particle , whichscatters the beam of light . Experimentlly , the intensity of scattering light is equal to thesquare of the amplitude .Debye in 1944, derived an expression , as given below , to relate the molecular weight of thesolute particle to the intensity of scattered light .

Where R90 is the Rayleigh ratio , defined as

Where I q is the density of the scattered light per unit volume V of scattering material whichis observed at a distance and at an angle of Q, with reference to the incident beam and Io isthe intensity of the incident beam .T is the turbidity of the medium which results from the scattering of light .K and H are light scattering calibration constants , defined as –

Where = 22/7n is the refrective index of the solutiondn/dc is the specific refractive index increment- Wavelength of incident light and

NA is Avogadro’s number .Similarly,H =

In case of polymer solutions , it may be noted that some molecular are larger in sizecompared to the wavelength of light and are in the form of randomly coiled swollenchains in which the different regions of the coil simultaneously scatter the incident lightbeam . Thus different regions of the same molecular get exposed to incident lightwaves interfere with each other and it’s intensity is reduced . So equation1) Is modified as given below .

Where P (Q) is the particle scattering factorAt Q = O , P(Q) =1 . At infinitely low concentration (c-0.), BC = 0, So the equationcan be rewritten as –

Two methods , namely the Debye method and the Zimm method are used formeasurement of molecular weight by this technique . Zimm method is some whatbetter as we donot need to know the shape of the molecule in the solution .The light scattering technique is a convenient method for measuring the molecularweight of polymers in the range of 10,000 to 10,000,000 .

Ultra Centrifugation :- In ultra centrifugation, centrifugal force is used to spatesuspended particles from liquids . Polymer molecules , in solution , when subjectedto a very high centrifugal force settle down . The sedimentation rate of polymermolecules settling down under the influence of a constant centrifugal force isrelated to their molecular weight . There are two different methods adopted todetermine the molecular weight of a polymer .1) Sedimentation velocity method :- In this method the solution is subjected to

very high gravitational fields . this process is done by an ultracentrifuge whichcan provide spin as high as 65000 rpm. Due to the high centrifugal force thepolymer modules will start regimenting by applying stake’s law, the molecularweight can be correlated to the sedimentation coefficients, by the followingexpression.

Where so and do are the sedimentation and diffusion constants respectively ,obtained by extrapolation the sedimentation coefficient (S) and diffusioncoefficient (D) at different concentrations to zero concentration , V is thespecific volume of the polymer in solution and P the density of solvent . Theabove equation is known as the Svedberg equation .In actual experiment , a homogeneous solution of known concentration istaken and subjected to ultra centrifugation . As the sedimentation starts , aboundary between the solvent phase and the solution phase is formed . As thecentrifugation proceeds, the boundary between the solvent and solutionphases slowly recedes away from the axis of rotation . The rate at which theboundary moves away is the sedimentation velocity . It can be calculated usingthe equation.

Where x1 & x2 are the distances of the boundary from the axis of rotation attimes t1 & t2 respectively. The sedimentation coefficient S is given as –

Where W is the angular velocity .

The sedimentation constant so can be computed by extrapolating S values fordifferent concentrations to zero concentration according to following equation–

The diffusion coefficient D can be similarly calculated by measuring theconcentration gradients at different times . Diffusion coefficients for differentconcentrations are measured and extrapolated to zero concentration to obtaindo as per the equation –

D = Dot kd c _ _ (5)The terms Ks and Kd are constants for a given polymer solvent system .

2) Sedimentation equilibrium method :- In this method , a thermodynamicequilibrium is achieved by practically equalizing the sedimentation anddiffusion velocities .The centrifuge spin is kept at around 15000 rpm . Underthis centrifugal force an equilibrium concentration gradient is established alongthe cell and the macromolecules get distributed according to their molecularweight . At equilibrium , the sedimentation and diffusion balance ont and themolecular weight is given by the equation –

Where ca and cb represent the concentration values in the cell at distance x1

and x2 from the axis of rotation . This method is used to find out the weightaverage molecular weight of polymer sample .

Simple small molecules like those of ethyl alcohol , water and sodium chloride can exist in any one ofthe three physical states i.e. the solid state , the liquid state and the gaseous state, the liquid state andthe gaseous state . These three phases have sharply defined boundaries. At fixed ambient pressure , themelting and boiling points of such materials occur at definite temperatures .Polymer are large molecules with strong intermolecular force and tan fled chains , and donot have avapour phase, they decompose before the temperature gets high enough to form a vapour . The lengthof the polymer molecules also makes it difficult to form the large crystals found in solid phases of mostsmall molecules .Polymers, on the basic of the arrangement of their molecules the sample , may be classified into twobroad categories .

1) Amorphous polymers :- polymer chains with branches or irregular pendant groups cannot packtogether regularly enough to form crystals . These polymers are said to be amorphous . Theamorphous phase is characterized by the absence of any long range order and there is norepetitive pattern or orderliness of the positions occupied by the molecules over long distances .The amorphous nature of polymers is analogous to a plateful of spaghetti long and randomlycoiled . These polymers do not show a definite arrangement or order of molecules . Polymermolecules in an amorphous polymer are randomly oriented and entangled with each other andresult in a disordered molecular configuration . Following are the main characteristics ofamorphous polymers .

Degree of crystalline :- The crystallinity of a polymer sample is expressed in terms of that fraction of thesample , which is crystalline and it effects the properties of the polymer . The overall property (Q) of apartially crystalline polymer can be expressed as a sum total of it’s two components as follows .

Q = Qc + Qa -------------(1)Where Qc & Qa are the contridntions of the crystalline and amorphous components of the sample,respectively. For example as we know that the density of a material increases when it changes its phasefrom liquid to crystal.Amorphous components in a polymer are in liquid phase and the crystalline components in thecrystalline phase . The density of the crystalline component , is therefore , higher than that of theamorphous component . For a given polymer, a 100% crystalline sample will have the highest densityand a 100% amorphous sample will have the lowest . Many of the actual polymers are only partiallycrystalline and their densities will naturally be in between those of their crystalline and amorphouscomponents . From the knowledge of the densities of the crystalline and amorphous components of apolymer , and that of the aggregate sample , the degree of crystallinity of the sample can be computedas follows –

Where xv is the degree of crystallinity by volume and d, da and dc are densities of sample , the fullyamorphous and fully crystalline components respectively .Similarly, the specific volume (V) of the sample could also be used to measure the degree of crystallinityas follows –

Where xm is the degree of crystallinity by mass and v, va and vc are the specific volumes of the sample,the fully amorphous and fully crystalline components , respectively .Enthalpy (H) of a sample can also be used to find out the degree of crystallinity by using the followingequation –

Where Ha, Hc and H are fully amorphous, fully crystalline and sample enthalpies respectively .Crystallisability :- It is the maximum crystallinity that a polymer can achieve at a particular temperature,regardless of the other conditions of crystallization . Crystallisability at the particular temperaturedepends on the chemical nature of the polymer chain , it’s geometrical structure , molecular weight andmolecular weight distribution on the other hand , the extent of crystallinity depend on such conditions

as the rate of cooling , residence fine , temperature of the molten polymer and heat dissipation, underwhich crystallization takes place .Polymer Crystallization :- Any polymer at a temperature higher than it’s melting temperature is in theform of a visions melt . The melt is in a liquid phase where polymer chains exist in a completelydisordered manner . They have conformations of random coils interpenetrating each other and areconstantly undergoing regimental and molecular motions due to thermal energy. The degree of chaincoiling and chain entanglement in the melt , depends on the chemical nature and geometrical structureof the polymer composed of non-polar flexible chains assume maximum chain coiling and hence aremore entangled . Polar polymers with bulky side groups are less coiled and less entangled. Also linearpolymers are less entangled than branched polymers.When a polymer melt is cooled, it will solidify. The type of solid obtained will depend on the nature ofthe polymer. At the point of solidification, the molecular mobility of the chains is arrested and they arefrozen to fixed conformations. Depending on the conformations taken by individual chains and also onthe nature of aggregation of neigh bouring chain at the time of solidification, the polymer may eithercrystallize or it will become a glassy (amorphous) material.The crystalline state is characterized by the existence of a long range order with respect to themolecular arrangement. It is therefore essential that a polymer melt has to crystallize, the chains whichwere completely disordered in molten state, take up conformations leading to a highly ordered andoriented arrangement of chain aggregates at the time of solidification.

At the point of solidification , first formation of several nuclei takes place for crystal growth . Thesenuclei are formed either by impurities present or by elements of chain regiments aggregating togetherin a satirically ordered manner . Once the nuclei are formed, the crystal growth around them takes placedue to chain regiments from the melt regions diffusing on the nuclei and taking orderly orientationaround them . This process is accompanied by a decrease in free energy . The process however does notgo on endlessly . The diffusion of chain regiments continues smoothly in regions where there is no chainentanglement . At regions of chain entanglement , the chain regiments lying between the alreadycrystallized portion of the chain and the portion caught within the entanglement get strained .Crystallization is therefore favored in polymer melts where chain entanglement is minimum and onlysuch polymers to a highly ordered molecular arrangement can crystallize . Other polymers, whose chainstructure and geometry does not help their chains to an orderly arrangement cannot crystallize and canonly form glassy solids .

In the case of crystallisable polymers too, it is very difficult to get a fully crystalline solid . When the meltis cooled , the viscosity increases rapidly and at the point of crystallization, it becomes extremely high .This high viscosity considerably slows down the process of chain diffusion from the disorderly state tothe highly ordered state . Therefore some chains settle in the orderly state , where as the others remainin entangled and disorderly state . Also process of crystal growth does not go to completion becausedifferent portions of the same chain , spaced quite for a part , may simultaneously diffuse into morethan one crystalline region and get involved in the crystal growth around more than one nucleus . As theprocess continues , at some stage , the chain regiments lying between those different regions of crystalgrowth get strained and prevent further regimental diffusion . Within the same chain , therefore , onlysome regiments get settled in orderly state , while the remaining are still in a disorderly state . Theprocess of crystallization , therefore , never goes to completion . The resultant solid contains bothcrystalline as well as amorphous regions . The ratio of crystalline region to amorphous region dependsupon the chemical nature of the polymer and physical parameters of crystallization such as temperatureand rate of cooling . Maximum crystallization becomes possible if the polymer is annealed just at itsmelting point for a sufficiently long time so as to allow the maximum number of chains or chainregiments to diffuse into an orderly arrangement .Crystallisation can, sometimes , be completely avoided by rapidly cooling down (quenching ) thepolymer melt much below its melting point without allowing any time for the process of chainorientation to take place and the polymer is obtained as a completely glassy (amorphous) solid .Crystallites :- The crystalline regions in a polymer do not have a regular shape nor do they have a perfectlattice structure . The chain regiments are so arranged as to form small orderly bundles or aggregatesresembling the limited portion of a three dimensional crystal lattice of low molecular weight crystals .These orderly regions in a polymeric substance are called crystallites instead of crystals . They areregions composed of imperfect crystal-like chain regiments . A partially crystalline polymeric materialconsists of several such crystallites co-existing with amorphous regions of disorderly placed chainregiments . No sharp boundaries however exist between the crystallites and the amorphous regions .Both are connected by polymer chains running through then .Spherulites :- Some polymers when crystallize from a melt develop complex polycrystalline structures .They show circular birefringent regions when viewed under a polarizing microscope . These birefringentregions are called Spherulites . If a molten polymer such as polypropylene is made into a thin filmbetween two hot glass plates and cooled showly , it is seen that from different nucleating centers,Spherulites start developing . The Spherulites grow to attain a spherical structure with a very highgeometrical symmetry . Transparency of crystalline polymers is dependent on the size of Spherulites .Transparency of a given material decreases as the size of the Spherulites increases . Experiments haveshown that the Spherulites consist of fibrils of crystalline nature having a thickness of 100A0 . thus abunch of fibrils of constitutes a Spherulites .Polymer Single Crystals :- Single crystals of polymers may be produced by crystallization from a verydilute solution of a concentration of about 0.01% by weight . Single crystals of various polymers appearsimilar when viewed under electron microscope .A polymer single crystal looks as though it consists of many platelets (lamella) kept one over the other ina decreasing order of size . The use all lamellar thickness of the single crystal is around 100A0 and theyhave a very high internal order .

Factors affecting Crystallisability :- There are a number of factors that determine whether a polymerwill crystallize or not . Most important among than are structural regularly, polarity, presence of bulkyside groups etc.The most important factor which determines whether a polymer can crystallize or not is it’s geometricalregularity i.e. the configuration of chain. Stereo –regular polymers i.e. is tactic and syndiotactio arefound to crystallize where as the atactic polymers do not crystallize. Similarly linear polymers crystallizeeasily but the branched polymers do not . Branching imparts irregularity to the molecular structure andreduces the ability of molecules to get themselves packed closely and hence they do not crystallize.The effect of macromolecular geometry on Crystallisability can be illustrated with the examples ofnatural rubber and gutta-percha . Both are polyisprenes . Natural rubber is a cis-isomer , there is abending back of successive isoprene units , giving the molecule a coiled structure . Gutta-percha, whichis the trans isomer , exhibits a straightening out of the successive isoprene units , giving a rod likestructure to the molecules and therefore gutta-percha is more crystalline .

Homopolymers are more crystalline than copolymers . Strictly alternating copolymers can show atendency to crystallize because of their structural regularity .Polarity in a molecule is also an important factor for crystallinity . Polarity results in formation ofhydrogen bonds and the hydrogen bonds between adjacent chains increases the inter chain forces ofattraction and facilitates tighter packing and perfect bonding of the chain elements with each other andresult in a crystalline structure .One more factor , i.e. presence of bulky side groups affects the ability of polymers to crystallize in anadvise manners as they come in the way of closer molecular packing .Effect of crystallinity on the properties of polymers :- The properties of polymer such as density ,modulus, hardness, permeability and heat capacity are affected by it’s polymer, it’s crystalline andamorphous regions the density of the crystalline regions will be many properties of the polymer dependon the percentage of crystalline material present in the bulk . Following graph shows the values ofyoung’s modulus of natural rubber with changes in crystallinity .

Initially when the value of crystallinity is low , value of young’s modulus is also low, which is acharacteristic of young’s modulus increases steeply with the amount of crystalline component in thesamples A & B of polyethylene are compared below . ( B is more crystalline)Property Sample A (less crystalline) Sample B (more crystalline)DensityHardness Shore DTensile StrengthFlexural modulus

0.917-0.93244-5083-314*105Pa241-331 MPa

0.952-0.96566-73221-314*105Pa1000-1550 MPa.

Molecular weight also has a strong effect on a polymer’s physical properties . A combined effect of themolecular weight and crystallinity on the physical properties of a polymer is shown below –

Permeability is also affected by crystallinity . Permeability of a polymer decreases with increasingcrystallinity . Permeability depends upon the extent and rate of penetration of liquid or vapourmolecules through the polymer matrix and is a factor of great importance in the packaging industry .Chemical degradation of polymers is also dependent upon the crystallinity . The amorphous regionsdegrade more easily than the crystalline regions .

Polymer Dissolution

When a low molecular weight solute such as salt or sugar is added to a solvent , the solute moleculesstart diffusing into the solvent phase . There is a progressive reduction in the volume of solute phase ,which ultimately merges with the solvent phase and forms the solution of low molecular weight solutesis factor when stirred or agitated .

When a solid polymer is added to a solvent , nothing seems to happen for some time . Slowly howeverthe fragments swell and increase in size and the polymer fragments take a lot of time before undergoingany visible change .

This is because the inorganic low molecular weight solids have discrete individual molecules , that areseparate from one another and are held together within the solid matrix only by van der wall’s forcesor electrostatic forces of attar action . As soon as the solvent and the solute are brought together , thesolvent molecules are round the solute molecules at the surfaces , establish solvent-solute interactionand break the solute-solute attar action . As this happens, the solute molecules , are isolated from thesolid phase and as their size is comparable to that of the solvent molecules , they diffuse fast into thesolvent phase and therefore the dissolution in this case is quite fast .

Polymer molecules on the other hand are very large molecules as compared to the solvent moleculesand are made up of hundreds of chain regiments and are in the form of tightly folded random coils .Moreover these molecular coils are interpenetrating and entangled with one another .

There are also varying degrees of cohesive and attar active forces between different regiments of thesame molecular coil as well as neigh bouring coils . Strong forces such as dispersion , induction dipole-dipole interaction and hydrogen bonding hold the molecular coils and their regiments together tightly .Because of the large size and coiled nature of the polymer molecules and also because of strong forcesof attar action between then , solvent molecules take five to establish interaction with the polymermolecules, to overcome the forces of attar action , to release individual molecules out of chainentanglement and get them out of the polymer phase. That is why there is difference in the dissolutionbehavior of low molecular weight substances and polymers .

The Process of Polymer Dissolution :- When a polymer fragment is added to a solvent , forces of attaraction or dispersion start acting between them, depending on their chemical nature , polarity andsolubility parameter . When the solvent solute attar action , the forces holding the polymer regimentstogether become weak and the solvent molecules force their way between the regiments, break theregiment-regiment contacts, surround individual regiments and establish contact with them . In thisprocess, the regiments unfold or loosen out from their tightly coiled conformation and the regimentsare said to get solvated .

The first stage in polymer dissolution is therefore characterized by a slow penetration of the solventmolecules into the interstices of the polymer coil and forcing them to swell . During this stage, thevolume of the polymer matrix increases and the solvent molecules leave the solvent phase and diffuseinto the polymer matrix . The polymer molecules , however, remain with the matrix itself and donot

diffuse out . The phenomenon of swelling depends on the forces of interaction between solventmolecules and polymer regiments and is not influenced by stirring or agitation. As swelling continues,more and more regiments of the polymer molecule are solvated and loosened out. When all theregiments are solvated, the molecule as a whole, in the form of a loose coil, separates out from theswollen polymer. The loosened polymer molecule than diffuses slowly out of the polymer phase anddisperses in the solvent phase, forming the solution. Because of the entanglement, the polymermolecules during dissolution do not diffuse out of the swollen polymer, one after another but a streamof entangled molecules simultaneously diffuse and disperse in the solvent. The rate of dissolution can bepolymer is fully swollen.

Even though all the chain regiments of a polymer molecule in solution are unfolded and fully solvated,the molecule does not assume the shape of an extended straight chain . The coiled nature of themolecule is still retained but with a very much expanded coil volume and with the empty space betweenthe unfolded regiments of the coil being occupied by solvent molecules . The polymer coil, along withthe solvent molecules filled within moves as a whole in the solvent phase forming what ellipsoid . Thesolvent held within the polymer coil can be called the ‘bound’ solvent and that surrounding the coil asthe ‘free’ solvent . The apparent volume occupied by the expanded coil is reffered to as the‘hydrodynamic volume’ of the polymer molecule in the solution under flow .

In case of low molecular weight solutions, the molecular size of the solute and that of the solvent arecomparable and the solute does not swell in volume in the process of dissolution, there is a freemolecular mobility with respect to the solute without much increase in intermolecular friction . Intensethe viscosity of the solution is not very much different from that of solvent . In case of polymersolutions, however , the solute molecules are much bigger than the solvent molecules, swellconsiderably in volume during the process of dissolution . The extremely large size and the increasedvolume of the polymer molecule restrict it’s molecular mobility in the solution and increase theintermolecular friction . The polymer solutions are therefore highly viscous .

Thermodynamics of Polymer dissolution :- Dissolution of a solute in a solvent is dependent on thechanges in the entropy and the enthalpy of the system . Any given system tries to shift in the directionof a state in which the enthalpy or the heat content (H) is minimum and/or the entropy or degree of

disorderliness or randomness (S) is maximum . Any system can spontaneously shift from a state A to astate B, if in the process these is a loss of enthalpy (- H) and /or a gain in the entropy (+ S) . In theprocess, if there is a decrease in the enthalpy with a simultaneous increase in the entropy ( - H and +S), both of them support the change of state and the process occurs simultaneously . On the other hand,if there is an increase in the enthalpy along with a decrease in the entropy (+ H and - S ) , both theparameters oppose the change of state and the process does not occur . If a situation arises where oneof them supports the process, where as the other opposes it , the feasibility of the process depends onthe magnitude of these counter factors . The resultant free energy change ( F) as per the equation –

Will then decide whether the process is feasible or not when there is a resultant decrease in the freeenergy ( F), the process will occur .

Dissolution of a low molecular weight solute always results in a positive entropy change due to thefreedom of mobility acquired by the solute molecules in the dissolved state as against the bound naturein the undisclosed state . Hence the only other parameter which will decide if the process can occur ornot is the change in enthalpy . If there is a decrease in the enthalpy (- H), the process becomesspontaneous . The process there is no heat change ( H = 0), since the entropy gain alone results in adecrease process , where there is an increase in the enthalpy (+ H), the process will be opposed by the+ H factor, while the same is favored by the + S . Te process can occur as long as + H is less than T.S.

When + H becomes equal to or greater than T S, the dissolution process is exothermic, the solute isfreely soluble in the solvent in all proportions . In endothermic dissolution, however, at the beginning ofthe dissolution process, the + H value is less than the T S value and the dissolution occurs in thestarting . As the dissolution process proceeds, T S value progressively decreases . When the T S valuedecreases and becomes equal to + H, F becomes zero and hence no further dissolution is possible .At this stage , an equilibrium is established between the entropy gain and enthalpy gain and the numberof solute molecules going into the solute equals those precipitating out . The solution at this point ofequilibrium is said to be saturated .

The same basic principle applies to polymer dissolution also . Whenever strong attar action forces actbetween polymer regiments and solvent molecules, H becomes negative and the process ofdissolution becomes favorable. The polymer is the completely soluble in the solvent in all proportions . Ifattar action forces are not strong, only weak dispersion forces act between the polymer and the solventand H becomes the dissolution can however occur even when H is the as long as it’s value is lessthan T S .

When a polymer changes it’s state from solid to solution, not only the molecular mobility but theregiments, rotating freely around fixed angles, they assume innumerable ‘conformations’ in thedissolved state . The freedom of molecular mobility and regimental mobility give an entropy increase tothe dissolved polymer . To start with, the process of dissolution is highly favorable . As the concentration

of the solution increases, the T S value becomes progressively lower . An equilibrium is finally attained, when + H becomes equal to T S .

At equilibrium, some regiments of ach molecule are solvated, while others are in solvated. There is aconstant diffusion of the solvent molecule between the solvated and in solvated regiments and adynamic equilibrium is set up between the solvated and the in solvated regiments within the swollenmass . The polymer molecules, therefore never move out of the polymer phase and no separate solutionphase is formed . When some fresh quantity of the polymer is added to this swollen mass, the solventmolecules penetrate into the newly added polymer and make it swell . The attack on the freshly addedpolymer sample by the solvent molecules and the resultant polymer swelling continue till a freshequilibrium is reached between the solvated and the in solvated chain regiments . The newly addedpolymer losses it’s identity and comes one with the already existing swollen mass .

In a polymer-solvent system, where + H is very high and exceeds the T S value, the dissolution cannotoccur at all and such a solvent is known as a ‘now solvent’ or precipitant for that polymer .

Solubility of crystalline and amorphous polymers :- Amorphous and crystalline polymer differ in theirphase state and hence behave differently as for as the physical process of their dissolution is conserved .Dissolution of amorphous polymers can be compared to the mixing of two liquids, since it exists in theliquid phase state and can easily undergo dissolution in a solvent, which is also in the liquid phase stateso long as F is negative . On the other hand dissolution of a crystalline polymer in values mixing of twosubstances existing in two different phase states . The crystalline polymer is in the crystalline phasestate while the solvent is in the liquid phase state . The two different phases can not mix together andform a single homogeneous system, mixing or dissolution is impossible unless both of them are broughtinto the same phase state . The first step involved in dissolving a crystalline polymer, therefore , is tobring the same into the liquid phase state . This can be done in two ways –

1) By heating the crystalline polymer to it’s melting point , or2) By bringing in conditions so as to destroy the long range order .

Owing to the long chain link nature of the polymer molecule and the presence of strong inter chaininteractions, crystalline polymers are not soluble in most of the solvents below their melting points .They are soluble below their melting points only in a few solvents where strong interactions betweenthe polymer and the solvent over comes the inter chain cohesive forces within the polymer and destroysthe crystalline lattice structure . For example polyethylene can be dissolved in hydrocarbon solventssuch as toluene only when heated . Atactic polystyrene is soluble in many solvents at ambienttemperature, but is tactic polystyrene can be dissolved only at elevated temperatures . Nylon 6, in whichthe crystallinity is due to intermolecular hydrogen bonding, can be dissolved at ambient temperatureonly in solvents such as formic acid or phenol, which are capable of forming strong hydrogen bandings .The fully aromatic polyamides can be dissolved only in concentrated sulphuric acid . The free energyequation governing the solubility of crystalline polymers near their melting point can be expressed as –

Where the subscript M and F denote mixing and fusion respectively .

Heat of dissolution and solubility parameter :- The heat of mixing ( HM) of two liquids, whosedissolution involves only dispersion forces, is expressed by the following equation

Where, HM – Heat of Mixing

VM - Total volume of two liquids

E1 and E2 - Energy of vaporization of liquids 1 and 2 .

V1 and V2 – The Molar volumes of two liquids, and

1 and2 – Volume fractions of liquids 1 and 2 .

The term ( E/V) indicates the amount of energy required to vaporize a unit volume of the liquid and isa measure of the cohesive force that holds molecules in the liquid together and has been designated asthe ‘cohesive energy density’ (CED) . The square root of CED is known as the solubility parameter .

HM and solubility parameters of two liquids are related by the following equation –

(S1-S2)2 decides the magnitude of heat of mixing . When S1 = S2, HM (i.e. Heat of mixing) becomes zeroand mixing will occur readily .

For polymer dissolution , equation (3) can be rewritten as –

Where the subscripts p and s denote the polymer and the solvent respectively .

According to this equation a polymer will dissolve only in solvents having S values very close to that ofthe polymer . If sp and ss values are very much different dissolution will not take place .

The theory however fails when applied to crystalline polymers as well as to solvents with strong polargroups and hydrogen bonding .

The solubility theory is based on the dispersion forces and hence does not apply to polymer-solventsystems with positive interaction where H is negative dissolution can occur in such cases even when Sp

and Ss are quite apart .

Polymerization :- Polymerization is a process which allows simple low molecular weight compounds tocombine and form a complex high molecular weight compound . For this , each molecule of thecompound should have the capability to react at least with two other molecules of the same or someother compound . In other words, they should have a functionality of at least two . The functionality of acompound depends on the number of reactive sites it has . A compound assumes functionality becauseof the presence of reactive functional groups like –

-OH, -COOH, -SH, -NCO etc.

The number of such functional groups per molecule of the compound defines it’s functionality .

Some compounds do not contain any reactive functional groups but the presence of double or triplebond in the molecules results in poly functionality in them . For example, ethylene, propylene,butylenes, vinyl chloride, styrene have a functionality of two . Acetylene has a functionality of four .There are some other compounds in which presence of easily replaceable hydrogen (e.g. phenol) atomsimparts functionality .

Low molecular weight compounds having a functionality of two or more are called monomers .

These monomer molecules combine to form higher molecular weight molecules (polymers) . Dependingupon the functionality of the monomers used, we get linear, branched or three dimensional cross-linkedpolymers . The molecular weight of the polymer formed depends on the conditions of polymerization.

There are two pathways for polymerization –

1) Chain polymerization or addition polymerization2) Condensation polymerization .1) Chain polymerization :- Chain polymerization is characterized by a self addition of the

monomer molecules, to each other, very rapidly through a chain reaction . No byproduct isformed . The product/polymer has the same elemental composition as that of the monomer .The bifunctionality is provided by the double bonds present in the monomer . Typical examplesare vinyl compounds (CH2 = CHX), ally Compounds (CH2 = CH.CH2X), olefins (CH2 = CHR) anddieses (CH2 = CR-CH = CH2) . Chain polymerization is also termed ‘vinyl polymerization’ chainpolymerization consists of three major steps, namely, initiation, propagation, and termination .Depending on the mechanism , there are three types of chain polymerization -1) Free radical polymerization :- In free radical polymerization , the initiation of polymer chain

growth is brought about by free radicals produced by the decomposition of compoundscalled initiators . Chain growth implies a process involving a continuous and very rapidaddition of the monomer units to form polymer molecule .INITIATORS :- Initiators are thermally unstable compounds and decompose into productscalled free radicals . If R-R is an initiator and the pair of electrous forming the bond betweenthe two R’S, can be represented by dots, the initiator can be written as –

R:R

When energy is supplied to this compound, the molecule is split into two symmetricalcomponents . Each component carries with it one of the electrons from the electron pair .This type of decomposition, where the molecule is split into two identical fragments , iscalled hemolytic decomposition . The two fragments, each carrying one unpaired electronwith it, are called free – radicals –

R%R – R0

A large number of low molecular weight compounds such as azo compounds, peroxides, peracids and per esters are useful as initiators .Some commonly used compounds for initiators are –Sr. No. commercial name chemical structure free radical formed1) Azo bisdiphenyl Methane

2) Azo bisisobutyro nitrite

3) Benzyl peroxide

4) Cumyl peroxide

5) Nitrogen peroxide

6) Parasitic acid

7) Per benzoic acid

8) Potassium persulphate

Decomposition of Benzyl peroxide takes place as given below –

The rate of decomposition of initiators depends on their chemical nature, reaction temperature and thesolvents used.

When initiators are decomposed by heat, it is known as thermal decomposition.

Decomposition of initiators by ultraviolet light is termed photodecomposition. Here the rate ofdecomposition depends on the intensity and wavelength of radiation and the solvent used. Initiators canalso be induced to decompose into free radicals by using suitable catalysts.

Initiation :- A free radical contains a lone (unpaired, single) electron and is therefore highly reactiveand can attack any molecule which either has a lone electron or is ready to part with one of it’selectrons.

This is what happens in the process of initiation. The free radical R0 attunes the double bond in themonomer molecule, resulting in the following chemical change.

Now the monomer unit is linked to the free radical unit through a sigma bond forming a single molecule.The other electron of the -bond becomes unpaired and the free radical site is now shifted from theinitiator fragment to the monomer unit. It is an exothermic process.

This whole sequence, in which one free radical attunes a monomer molecule, adds the monomermolecule ti itself and simultaneously transfers the free radical site from itself to the monomer unit, istermed the initiation step.

Propagation :- After initiation comes propagation In this step, the radical site at the first monomer unitattacks the double bond of a fresh monomer molecule. This results in the linking up of the secondmonomer unit to the first and the transfer of the radical site from the first monomer unit to the secondby the unpaired electron transfer process.

This chain still contains a radical site at it’s end carbon atom and can therefore attack yet anothermonomer molecule with a simultaneous transfer of the radical site to the new monomer unit added.

This process involving a continuing attack on fresh monomer molecules, which in turn, keep successivelyadding to the growing chain one after another is termed propagation.

The propagation lasts till the chain growth is stopped by the free radical site being killed by someimpurities or till there is no further monomer left for attack. The structure of the growing chain can berepresented by.

Where n denotes the number of monomer units added up in the chain growth.

The mode of addition of the incoming monomer to the growing chain can be of any type as given below-

Termination :- After propagation comes termination. Any further addition of the monomer units to thegrowing chain is here after stopped and the growth of the polymer chain is arrested. During propagationthere may be quite a few growing chains present in the system. Depending on factors such astemperature, time and monomer and initiator concentrations, the two growing chains may come closerand collide with each other. When such a collision takes place, the following two reactions occur.

In the first case, the two growing chains unite by coupling of the lone electron present in each chain toform an electron pair and thus nullity their reactiveness. Since the process in values the coupling of thetwo lone electrons, this kind of termination is known as termination by coupling.

In the second case, one hydrogen from one growing chain is taken by the other growing chain andutilized by the lone electron for getting stabilized, while the other chain gets stabilized by the formationof a double bond. This type of termination is called termination by disproportionate. In the formation oftwo polymer molecules of shorter chain length.

After termination, the product molecules formed do not contain any free radical site and hence cannotgrow further. The process of termination results in the deactivation of the growing chain and thepolymer molecule formed can be reffered to as a ‘dead’ polymer chain.

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UNIT- 1

MELT PROCESSES AND THERMOPLASTIC BEHAVIOR:

Most of the polymer processing technology can be recalled as: ‘get the shapethen set the shape’. Processing of plastic materials concerned with theproduction of objects of definite shape and form. Such objects may be shapedby the following general techniques:

1. Deformation of a polymer melt – either a thermoplastic or athermosetting melt. Processes using this approach include blowmolding, calendaring, extrusion and injection molding.

2. Deformation of a polymer in the rubbery state - This approach isused in sheet shaping techniques such as thermoforming and the shapingof acrylic sheet.

3. Deformation of a polymer solution either by spreading or by extrusionto make films and fibers.

4. Deformation of a polymer suspension. This approach is used in rubberlatex technology and in PVC plastisol technology.

5. Deformation of a low molecular weight polymer. This approach isused in the manufacture of acrylic sheet and in the preparation of glass-reinforced product.

6. Machining operations.

Note: Our concerned in this subject is only on the melt deformationprocess.

MELT PROCESSING OF THERMOPLASTICS:

The most important conversion methods used by the thermoplastics processingindustry are extrusion and injection molding. Whether extrusion or injectionmolding is being used, there are certain factors that should be consideredbefore a thermoplastics material is processed.

1.2

These factors include:

1. Hygroscopic Behavior

2. Granule Characteristics

3. Thermal Properties and Heat Input

4. Thermal Stability

5. Flow Properties

6. Thermal Properties and Cooling

7. Crystallization and Shrinkage

8. Molecular Orientation

1. Hygroscopic Behavior:

If a polymer compound contains water or any other material with a low boilingpoint, then visible bubbles and rough surface will form within thethermoplastic material, when it emerges out from the mold or die. Higher theprocessing temperatures, the lower is the amount of water that can betolerated. This is because higher temperatures will generate a larger amount ofsteam from the same quantity of water.

Commodity thermoplastics do not suffer from water-related problems ascomparison to the engineering thermoplastics. For example, PET and Nylonabsorb water i.e. they are hygroscopic and must be carefully dried beforeprocessing. Some additives may also contain excessive water.

2. Granule Characteristics:

Processes such as extrusion, blow molding and injection molding use plasticmaterial as feed. The material must be in the granular form. If the material isavailable in more than one feed form (different granule size), then feedingproblems will be encountered, if a mixture of feed forms is used.

In terms of feeding efficiency, spherical granules (of approximately 3 mm or0.125 inch diameter) are the most suitable and fine powder is usually the

1.3

worst. Re-granulated material, because it may contain a range of particle sizes,can be almost as bad. Cube cut granules are better and lace cut granules arebetter.

3. Thermal Properties and Heat Input:

Thermoplastic materials require large amount of heat to increase theirtemperatures required for melt processing. They also require different amountof heat energy that is needed to bring them up to processing temperatures.These differences are not simply due to the different processing temperaturesrequired, but also to the fact that different plastics materials have differentspecific heats. (The amount of heat required to raise the temperature of aspecific weight of a material by 1oC.)

Different materials require different amounts of heat to raise their temperatureto a fixed number of degrees. For example, when melt processing a semi-crystalline, thermoplastic material heat must be supplied to melt the crystalstructures. This extra heat input is not needed in the case of an amorphousresin. Both types of material will, however, require a large amount of heat tobe put into the material quickly.

4. Thermal Stability:

Thermoplastic materials differ widely in their thermal stability. For example,UPVC is very unstable even when stabilized and can only be held atprocessing temperatures (175ºC/347ºF) for a few minutes. (Un-stabilized PVCwill show some degradation in boiling water).

On the other hand, polysulfones require melt temperatures in the region of400ºC/752ºF, where they are stable. The thermal stability of a material is notonly depend upon the temperature, but the residence time at that temperature,the atmosphere surrounding the material (oxygen or inert) and the materials incontact with the plastics material. For example, copper causes degradation ofpolypropylene (PP). In general, the compounded plastics should be properlyvented during processing, as they should contain potentially harmful gases andvapors.

5. Flow Properties:

Because of thermal stability problems, the processing temperatures employedfor thermoplastics are frequently limited to relatively low values. This means

1.4

that melt viscosities are generally high. Process melt viscosities are notuniform and differ from one material to another and from one grade of thesame material to another grade.

These differences may be due to intrinsic differences in the nature of thepolymers, they may also be affected by temperature and by molecular weight.In general, viscosity decreases with an increase in temperature and as themolecular weight is reduced. Small variations of temperature, molecularweight, and molecular weight distribution can cause large differences to arisein melt viscosity. This, in turn, affects output and quality in both extrusion andinjection molding. For this reason, strict control over both the processingconditions, and the material fed to the machine, must be employed. This iswhy there is such an interest in flow testing of thermoplastics materials. (It isalso worth noting that since the viscosity goes down with an increase in outputrate, through a given die, the energy used per unit output, tends to go down asthe extrusion rate is increased)

6. Thermal Properties and Cooling:

Thermoplastic materials require large heat inputs to raise their temperatures tothose required for melt processing. As these materials are good thermalinsulators, the removal of the large amounts of heat poses severe problems forhigh-speed production. Table 3 shows some heat removal data for differentthermoplastic materials. Variations in the cooling rate may have a pronouncedeffect on the crystalline morphology of the product and on factors such asmolecular orientation and shrinkage.

7. Crystallization and Shrinkage:

Generally, all polymeric products shrink on being cooled from processingtemperatures. The shrinkage of an amorphous thermoplastic material, such aspolystyrene (PS), is much less than that for a semi-crystalline thermoplasticsuch as high-density polyethylene (HDPE). (For PS the amount of shrinkagemay be 0.6%, whereas for HDPE it may reach 4 %.)

This is because when polymer molecules crystallize they tend to pack moreefficiently than they do in the disorganized amorphous state. With thick-sectioned moldings and extrusions, the cooling rates will differ from theoutside edges to the center. This results in differences in the degree ofcrystallinity throughout the part. This, in turn, explains why a shrinkage rangeis always quoted for each material. This range is always higher for a semi-

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crystalline thermoplastic material than for an amorphous one. For example, theshrinkage of PS is listed as 0.02 to 0.08%, while that of HDPE is 1.5 to 4%.

8. Molecular Orientation:

Polymer melts are extensively deformed during melt processing. The hotmaterial is then cooled extremely fast to achieve the high output ratesdemanded. The shearing processes result in the molecules taking up adeformed, or oriented, shape and the rapid cooling results in this deformedshape being frozen-in the product. This orientation, which results in theproduct having different properties in different directions, is known asanisotropy. In many cases, such orientation is undesirable, however, in somecases, orientation in introduced or enhanced, to improve the properties ofproduct. The process of deliberately orienting extrudate is used in themanufacture of fibrillated tape, many types of fibers, and in the extrusion blowmolding of bottles.

Effect of Polymer properties on process technique:When processing thermoplastic material, the following factors should be takeninto consideration in order to both process efficiently and obtain good qualityproducts.Thermal stability of polymers PVC thermally sensitive material - Little higher melt temperature may

lead to degradation - HCL is released - This can leads to corrosion andharmful to human being. PMMA, POM upon degradation liberatesMMA & formaldehyde respectively - MMA volatilize and causebubbles - Formaldehyde gas causes “eye-irritation”.

PVC & POM (acetal) should never be processed one after the other.This may lead to explosion

Adhesion of melt to metal: Wetting of the polymer melt against the metal wall of processing

equipment can lead to strong adhesion of polymer to metal. ForExample: difficulty in removing PVC - Mix from two-roll mill.

PC has a strong adhesion to metal. It can take away the skin of thebarrel if not properly purged

Thermal properties affecting heating and cooling: In the case of polymer melts, the specific heat varies with temperature.

For crystalline polymers such as POM, NYLON etc. latent heat offusion and specific heat should be taken in to account. I.e. Total heatcontent (Enthalpy) =LH of fusion + specific heat.

1.6

POLYMER PROCESS TEMP C ENTHALPY/KJ/ KG

PS 200 310

LDPE 200 500

HDPE 260 810

PP 260 670

Because of higher enthalpy PP requires more cooling time than LDPE and PS.

Cooling and Compressibility:When polymers are in molten stage the vibrations of the molecules results inthe polymer chain being pushed apart so that the volume occupied by a givenpolymer mass is higher than when the material is solid.

POLYMERDENSITY AT20C(G/CC)

DENSITY AT PROCESS TEMP(G/CC)

LDPE 0.923 0.746(210c)

PP 0.905 0.765(210c)

PMMA 1.180 1.105(210c)

SPVC 1.48 1.390(190c)

Because polymer melts are compressible, molding shrinkage is much less, thanthe above figure.

Basic Process Factors in Injection Moulding Material Parameters Amorphous, Semicrystalline, Blends and Filled Systems Pressure-Volume-Temperature (PVT) Behaviour Viscosity Geometry Parameters Wall Thickness of Part Number of Gates Gate Location Gate Thickness and Area Type of Gates: Manually or Automatically Trimmed Constraints from Ribs, Bosses and Inserts

1.7

Manufacturing Parameters Fill Time Packing Pressure Level Mold Temperature Melt Temperature

2.1

Unit -2

Preparing plastic raw materials before processing:The plastic raw material, we are obtained from refinery by-product as apetrochemical product. The materials are prepared and nomenclatureaccording to their grades. The raw material is then distributed in packingof bags of various size packing. For the distribution of raw material, wehave to store it at suitable locations. During the storage, plastic rawmaterial may be stay from one week to several months.During storage and distribution some plastic materials absorbs moisturefrom environment. Before processing, the plastic material must be freefrom moisture and at suitable temperature (preheat).

Therefore, we need following pretreatment to raw material beforeprocessing. These are explained as below:

1. Drying2. Preheat

1. Drying:Some polymers are hygroscopic in nature, means that they absorbmoisture from the air. Water or Moisture is the greatest enemy forprocessing of plastics. The polymer molecular structure determines themoisture level that can be absorbed, effect moisture has on the polymerduring extrusion and the permissible moisture content allowed prior toextrusion

In general, polar polymer molecules containing oxygen and/ornitrogen atoms in the polymer backbone tend to absorb moisture throughhydrogen bonding. While hydrogen bonds are weaker than the polymermolecular bonds, they are strong enough for the polymer to absorb andhold moisture at an equilibrium level based on the polymer structure.Once the moisture is absorbed, it can affect the extrusion and polymerproperties if it is not removed. Moisture can turn into steam in theextruder, having deleterious effects on the polymer structure and/or theextruded product.Drying Definitions and Factors Affecting Drying:Drying: Drying occurs when a vapour pressure differential existsbetween the pellet moisture and the surrounding air. Moisture migrates tothe medium with the lowest vapour pressure. If the air is drier than thepellets, moisture migrates from the pellets to the air.

On the other hand, if the air moisture is greater than in the pellets,water will migrate to the pellets. Removing moisture from pellets is adiffusion process and requires time. As moisture migrates from the pelletsurface to the air (assume the air has lower moisture content), moisturefrom the pellet centre diffuses to the pellet surface, where the moisture

2.2

content is lower. Heating the pellets increases moisture diffusion throughthe pellets. This also increases the moisture migration from the pelletsurface to the air.Terms used to describe drying factors are

• Relative humidity• Dew point• Moisture weight percent in the plastic

Relative humidity is the actual air moisture compared to air saturatedwith water at that temperature. The higher the air temperature, the moremoisture the air can hold. Hot summer air can hold significantly moremoisture than cold winter air.

Similarly, hot air in a drying oven can hold more moisture at highertemperature. If the hot air has a high humidity, plastics can easily absorbthe moisture from the air, increasing their moisture content.Dew point: the dew point determines the air moisture. The dew point isthe temperature where moisture condenses out of the air. Lower moisturein the air correlates with a lower dew point. A dew point of –4˚F (–20˚C)means that the air must be cooled to –4˚F (–20˚C) before moisture willcondense out of the air. If the dew point is –4˚F (–20˚C), the relativehumidity in 250˚F (121˚C) air is very low, and this assists moisturemigration from pellets to air.

The weight percent of moisture in plastic is given by EquationWeight % = [(Sample Weight – Sample Weight Dry) /Sample Weight]✕100

Five percent moisture means each 100 grams of plastic resinactually weighs 95 grams, as it contains 5 grams of water.

Factors affecting the drying rate are:1. Air temperature surrounding the pellets—at highertemperatures, air can hold more water and the diffusion rate in the pelletsis faster.2. Weight percent moisture in the plastic—higher moisture contentrequires more time to remove or reduce the moisture to acceptableconcentration.3. Air flow around the pellets—Airflow around each pellet iscritical to remove wet air (air where moisture has already migrated fromthe pellet to the air) and replace it with dry air.

Necessary Actions taken for using plastic material:1. Use granules as soon as the seal bag is opened.2. Pre-drying ovens, Hopper drier, Dehumidifying drier are to be

used.3. For PC - Dehumidifying drier preferred.

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4. Use of Slab form material in Calendaring, Compression Molding.5. Use of uniform Granular pellet size material in general to ensures

even and faster feeding.6. Use of Powder form –where difficulty in feeding.

Types of Drying Equipment:Drying equipment ranges from general-purpose ovens with trays tocomplex dehumidifying dryers, drying thousands of pounds per hour withdry air conveying systems to transport resin from dryers to feed hopperwithout exposing the resin to the environment.Drying equipment can be divided into three categories:

1. Ovens2. Hopper dryers3. Central drying systems

Further, there are three general drying systems used for hopper dryers andcentral drying systems:

a. Hot airb. Refrigerant dehumidifyingc. Desiccant dehumidifying

Dryers are used mainly for the following function:1. To remove surface moisture from hygroscopic and non-

hygroscopic material.2. To Preheat pellets before processing to increase the throughput rate3. To dry hygroscopic material to acceptable moisture content for

processing.

1. Ovens:Oven drying is the least used system and is generally used for small lotsin the product development or laboratory-type applications, where smallsamples have to be dried. Ovens are available in different sizes anddesigns that satisfy different applications. Ovens available includes hotair, vacuum, and dehumidifying with regenerative beds.

A hot air oven, containing samples for drying having blower tocirculates air through a heater, where the air is heated to the desired setpoint and then flows through the oven from bottom to top. In some ovens,the air flows from side to side. In most designs, the air is recycled backthrough the blower and heater.

If the air relative humidity entering the oven is high, the ovendrying capacity is limited. At a given temperature, the air can absorbmoisture only as long as the moisture in the pellet is greater than the airmoisture. At some point, the water no longer migrates from the resin toair. Unless the dew point or air moisture content is lowered, hot aircontinues to circulate around the pellets, but no additional moisture isremoved. Heating the air hotter allows the air to absorb more moisture as

2.4

the relative humidity drops; however, the higher temperature may causethe resin being dried to become tacky, melt, or degrade as the thermalstabilizers are consumed. To improve the drying efficiency, a bed is usedto lower the air dew point or a vacuum is used to increase the pressuredifference.

Another type is desiccant oven with the exception that a desiccantbed is added between the blower and the heater. With lower moisturecontent in the hot air, the water partial pressure differential between thepellets and air is greater, improving the drying efficiency. Moisturetransfers from the pellets to the air, and the moisture in the recycled air isremoved in the desiccant bed before reheating to the set point. Normallythere are at least two desiccant beds, one drying the air while the other isregenerating. In the regeneration cycle, the desiccant is heated to driveout the moisture. After regeneration, the dry bed is rotated into the airstream to remove the moisture while the other desiccant bed isregenerating.

A second approach to lower the air moisture content is to apply avacuum, lowering the oven pressure. As the water, partial pressuredifferential is increased between the pellets and the surrounding air, watertransfers to the lower pressure. The vacuum pump removes the moistureleaving the pellets.

To improve oven drying efficiency, the pellet bed thickness in thedrying pans needs to be as thin as possible. It is more efficient to usemore trays with less resin in each tray, as the circulating air does not flowbetween pellets. Oven shelving is perforated to provide good airflowthrough the oven. Solid shelving restricts airflow and reduces oven

2.5

efficiency. Another factor that may lead to poor drying is overloading theoven. This restricts the airflow and may generate excessive moisture inthe oven as it diffuses from the pellets.

Once all the material in an oven is dried, addition of new materialwith higher moisture content may affect the dried material in the oven. Ifthe air moisture content becomes higher than the moisture content of thedried pellets, moisture migrates from the air back into the previouslydried material. Without adding a desiccant bed to circulating dry air or avacuum to remove moisture, ovens do not provide acceptable means todry hygroscopic resins. If dry air is not used, hygroscopic resins like PETand PBT can actually act as desiccants, with moisture migrating from theair to the resin. Properly dried hygroscopic resins placed in a standard hotair oven without a desiccant dryer or vacuum will increase in moisturecontent over time. Standard hot air ovens do not lower the dew point ofthe air, which is required to dry hygroscopic resins.

2. Hopper Dryers and Central Drying Systems:

Hopper Dryer: An off-line hopper dryer is used in a central dryingsystem where dried materials are pneumatically conveyed to the extrusionequipment. An in-line dryer is used on a single extrusion line. In singlescrew extrusion, the dryer may sit directly above the feed throat, feedinghot pellets to the extruder. An in-line dryer may be coupled with an off-line dryer if the off-line dryer is supplying many extruders.

Large off-line dryers can dry resin for multiple extruders; driedresin is conveyed from the off-line dryer to the in-line hopper dryer withdry air. The in-line dryer either maintains or lowers the resin moisturecontent as the resin awaits extrusion. The third hopper dryer is a portabledryer that can be moved around the plant to the extrusion line requiringdry resin.

2.6

Hopper dryers are designed for plug flow, meaning the firstmaterial added to the dryer is the first material out. In normal operation,as material is removed from the bottom, additional resin is added to thetop through an automatic conveying system. It is important to scale thedryer to the equipment throughput requirements. A very significantproduction problem occurs when extruder capacity exceeds dryingcapacity.The dryer hopper has to be properly insulated to provide uniformtemperature around and across the dryer. The resin and air temperature atthe walls need to be the same as the centre temperature in the dryer topromote uniform drying. Insulation is important both for energyefficiency and for safety. Hot hoses can cause burns if touched. Supplyand return lines have to be properly designed for the drying temperaturesbeing used. Above 200˚F (93˚C), high-temperature silicone hoses arerecommended for both air supply and resin discharge. Above 250˚F(121˚C), high-temperature silicone hoses are required on the airlines.Coolers on the return air reduce the air temperature, improving thedesiccant efficiency to lower the moisture content. After-coolers are usedwhen drying temperature is greater than 160˚F (71˚C).

Calendering ProcessCalendering is a finishing process applied to textiles and plastic. During calenderingrolls of the material are passed between several pairs of heated rollers, to give a shinysurface. Extruded PVC sheeting is produced in this manner as well other plastics.Calendering is a final process in which heat and pressure are applied to a fabric bypassing it between heated rollers, imparting a flat, glossy, smooth surface. Lustreincreases when the degree of heat and pressure is increased. Calendering is applied tofabrics in which a smooth, flat surface is desirable, such as most cotton, many linensand silks, and various man-made fabrics.

The molten material is fed to the calendar roll from a Banbury mixer and two-roll millsystem, or from a large extruder. The major plastic material that is calendered is PVC.Products range from wall covering and upholstery fabrics to reservoir linings andagricultural mulching materials .Owing to the large separating forces developed in thecalendar gap, the rolls tend to bend. This may result in undesirable thickness variationsin the finished product. Compensations for roll deflections are provided by usingcrowned rolls having a larger diameter in the middle than at the ends or by roll bendingor roll skewing.Calender installations require large initial capital investment. Film and sheet extrusionare competitive processes because the capital investment for an extruder is only afraction of the cost of a calender. However, the high quality and volume capabilities ofcalendering lines make them far superior for many products. Calendering in principle issimilar to the hot rolling of steel into sheets. It is interesting to note that strip casting ofsemi-solid alloys can be modeled with the help of the hydrodynamic lubricationapproximation for a power-law viscosity model, just like plastics calendering. Theprocess of calendering is also used extensively in the paper industry.

Introduction

A calender is a device used to process a polymer melt into a sheet or film. It hasbeen in use for over a hundred years and when first developed it was mainly usedfor processing rubber, but nowadays is commonly used forproducing thermoplasticW sheets, coatings and films [1]. The calender never didbecome very popular when it was first invented mainly because it was difficult toadjust the desired gap between rollers; consequently, it was difficult to get anaccurate sheet thickness. The process did not start to become popular until the

1930's when the machines became easier to adjust[2]. Nowadays calenders canachieve tolerances around 0.005mm[2]

How it works

The calender concept is fairly easy to understand. The basic idea of the machine is thatsquishes a heat softened polymer between two or more rollers (this area is called a nip)to form a continuous sheet. To begin the process the polymer must go through blendingand fluxing before it goes through the calender. Blending is a process that creates thedesired polymer and fluxing heats and works this blended polymer to make it aconsistency easier for the calender to handle.[3]. The polymer is then ready to gothrough the calender and will leave it at a thickness dependent mainly on the gapbetween the last two rollers. The last set of rollers also dictate the surface finish; forexample, they can influence the glossiness and texture of the surface. One thing aboutpolymers being calendered is that the sheet going through the rollers tends to follow thefaster moving roller of the two that it's in contact with and it also sticks more to the hotterrolls. That is why calenders typically end with a smaller roller at a higher speed to peelthe sheet off. It is also why the middle roller is normally kept cooler so that the sheetwon't stick to the other rollers nor will it split by sticking to both rollers which canhappen. This splitting phenomenon has forced calender operators to desire a highfriction ratio between two rollers, which ranges from 5/1 to 20/1.Uses:

floor tile continuous flooring rainwear shower curtains table covers pressure-sensitive tape automotive and furniture upholstery wall coverings luminous ceilings signs and displays etc.

Material Specifications

The best polymers for calendering are thermoplastics. One reason for this is becausethey soften at a temperature much lower than their melting temperature, giving a widerange of working temperatures. They also adhere well to the rollers, allowing them tocontinue through the chain well, but they don't adhere too well and get stuck on theroller. The last reason is that thermoplastic melts have a fairly low viscosity, but they arestill strong enough to hold together and not run all over the place. Heat sensitivematerials are also great for calenders because calenders put immense pressures on the

1930's when the machines became easier to adjust[2]. Nowadays calenders canachieve tolerances around 0.005mm[2]

How it works

The calender concept is fairly easy to understand. The basic idea of the machine is thatsquishes a heat softened polymer between two or more rollers (this area is called a nip)to form a continuous sheet. To begin the process the polymer must go through blendingand fluxing before it goes through the calender. Blending is a process that creates thedesired polymer and fluxing heats and works this blended polymer to make it aconsistency easier for the calender to handle.[3]. The polymer is then ready to gothrough the calender and will leave it at a thickness dependent mainly on the gapbetween the last two rollers. The last set of rollers also dictate the surface finish; forexample, they can influence the glossiness and texture of the surface. One thing aboutpolymers being calendered is that the sheet going through the rollers tends to follow thefaster moving roller of the two that it's in contact with and it also sticks more to the hotterrolls. That is why calenders typically end with a smaller roller at a higher speed to peelthe sheet off. It is also why the middle roller is normally kept cooler so that the sheetwon't stick to the other rollers nor will it split by sticking to both rollers which canhappen. This splitting phenomenon has forced calender operators to desire a highfriction ratio between two rollers, which ranges from 5/1 to 20/1.Uses:

floor tile continuous flooring rainwear shower curtains table covers pressure-sensitive tape automotive and furniture upholstery wall coverings luminous ceilings signs and displays etc.

Material Specifications

The best polymers for calendering are thermoplastics. One reason for this is becausethey soften at a temperature much lower than their melting temperature, giving a widerange of working temperatures. They also adhere well to the rollers, allowing them tocontinue through the chain well, but they don't adhere too well and get stuck on theroller. The last reason is that thermoplastic melts have a fairly low viscosity, but they arestill strong enough to hold together and not run all over the place. Heat sensitivematerials are also great for calenders because calenders put immense pressures on the

1930's when the machines became easier to adjust[2]. Nowadays calenders canachieve tolerances around 0.005mm[2]

How it works

The calender concept is fairly easy to understand. The basic idea of the machine is thatsquishes a heat softened polymer between two or more rollers (this area is called a nip)to form a continuous sheet. To begin the process the polymer must go through blendingand fluxing before it goes through the calender. Blending is a process that creates thedesired polymer and fluxing heats and works this blended polymer to make it aconsistency easier for the calender to handle.[3]. The polymer is then ready to gothrough the calender and will leave it at a thickness dependent mainly on the gapbetween the last two rollers. The last set of rollers also dictate the surface finish; forexample, they can influence the glossiness and texture of the surface. One thing aboutpolymers being calendered is that the sheet going through the rollers tends to follow thefaster moving roller of the two that it's in contact with and it also sticks more to the hotterrolls. That is why calenders typically end with a smaller roller at a higher speed to peelthe sheet off. It is also why the middle roller is normally kept cooler so that the sheetwon't stick to the other rollers nor will it split by sticking to both rollers which canhappen. This splitting phenomenon has forced calender operators to desire a highfriction ratio between two rollers, which ranges from 5/1 to 20/1.Uses:

floor tile continuous flooring rainwear shower curtains table covers pressure-sensitive tape automotive and furniture upholstery wall coverings luminous ceilings signs and displays etc.

Material Specifications

The best polymers for calendering are thermoplastics. One reason for this is becausethey soften at a temperature much lower than their melting temperature, giving a widerange of working temperatures. They also adhere well to the rollers, allowing them tocontinue through the chain well, but they don't adhere too well and get stuck on theroller. The last reason is that thermoplastic melts have a fairly low viscosity, but they arestill strong enough to hold together and not run all over the place. Heat sensitivematerials are also great for calenders because calenders put immense pressures on the

materials to work them and therefore do not need as high of temperatures to processthem limiting the chances of thermal degradation. This is why calendering is often themethod of choice for processing PVC. Due to the nature of the process the polymersmust have a shear and thermal history that is consistent across the width of the sheet.

Advantages

The best quality sheets of plastic today are produced by calenders; in fact, the onlyprocess that competes with the calender in sheet forming is extruding. The calenderalso is very good at handling polymers that are heat sensitive as it causes verylittle thermal degradation. Another advantage to calendering is that it is good at mixingpolymers that contain high amounts of solid additives that don't get blended or fluxed invery well. This is true because compared to extrusion the calender produces a largerate of melt for the amount of mechanical energy that is put in. Due to this companiesare able to add more filler product to their plastics and save money on raw materials.Calenders are very versatile machines meaning that it is very easy to change settingslike the size of the roller gap.

Disadvantages

Although the calendering process produces a better product than the extruding processthere are a couple of disadvantages. One disadvantage is that the process is moreexpensive to perform which is a major deterrent for many companies. The calenderingprocess also is not as good at too high of gauges or too low of gauges. If the thicknessis below 0.006 inches then there is a tendency for pinholes and voids to appear in thesheets. If the thickness is greater than about 0.06 inches though there is a risk of airentrapment in the sheet. Any desired thickness within that range though would turn outmuch better using a calender process.

Types

There are 3 main types of calender: the I type, L type and Z type

I Type

Fig 1: Roller setup in a typical 'I' type calender

The I type, as seen in Figure 1, was for many years the standard calender used. It canalso be built with one more roller in the stack. This design was not ideal though becauseat each nip there is an outward force that pushes the rollers away from the nip.

L TypeFig 2: Roller setup in a typical inverted 'L' type calender

The L type is the same as seen in Figure 2 but mirrored vertically. Both these setupshave become popular and because some rollers are at 90o to others their roll separatingforces have less effect on subsequent rollers. L type calenders are often used forprocessing rigid vinyls and inverted L type calenders are normally used for flexiblevinyls.

Z Type

Fig 3: Roller setup in a typical 'Z' type calender

The z type calender places each pair of rollers at right angles to the next pair in thechain. This means that the roll separating forces that are on each roller individually willnot effect any other rollers. Another feature of the Z type calender is that is that theylose less heat in the sheet because as can be seen in Figure 3 the sheet travels only aquarter of the roller circumference to get between rollers. Most other types this is abouthalf the circumference of the roller.

Z Type

Fig 3: Roller setup in a typical 'Z' type calender

The z type calender places each pair of rollers at right angles to the next pair in thechain. This means that the roll separating forces that are on each roller individually willnot effect any other rollers. Another feature of the Z type calender is that is that theylose less heat in the sheet because as can be seen in Figure 3 the sheet travels only aquarter of the roller circumference to get between rollers. Most other types this is abouthalf the circumference of the roller.

Z Type

Fig 3: Roller setup in a typical 'Z' type calender

The z type calender places each pair of rollers at right angles to the next pair in thechain. This means that the roll separating forces that are on each roller individually willnot effect any other rollers. Another feature of the Z type calender is that is that theylose less heat in the sheet because as can be seen in Figure 3 the sheet travels only aquarter of the roller circumference to get between rollers. Most other types this is abouthalf the circumference of the roller.

Rotational molding

Rotational Molding involves a heated hollow mold which is filled with a charge or shotweight of material. It is then slowly rotated (usually around two perpendicular axes),causing the softened material to disperse and stick to the walls of the mold. In order tomaintain even thickness throughout the part, the mold continues to rotate at all timesduring the heating phase and to avoid sagging or deformation also during the coolingphase. The process was applied to plastics in the 1940s but in the early years was littleused because it was a slow process restricted to a small number of plastics. Over thepast two decades, improvements in process control and developments with plasticpowders have resulted in a significant increase in usage.

Rotocasting (also known as rotacasting), by comparison, uses self-curing resins in anunheated mould, but shares slow rotational speeds in common with rotationalmolding. Spin casting should not be confused with either, utilizing self-curing resins orwhite metal in a high-speed centrifugal casting machine.

A three-motor powered (tri-power) rotational-molding or spin-casting machine

History

In 1855, R. Peters of Britain documented the first use of biaxial rotation and heat. Thisrotational molding process was used to create metal artillery shells and other hollowvessels. The main purpose of using rotational molding was to create consistency in wallthickness and density. In 1905 in the United States F.A. Voelke used this method for thehollowing of wax objects. This led to G.S. Baker's and G.W. Perks's process of makinghollow chocolate eggs in 1910. Rotational molding developed further and R.J. Powellused this process for molding plaster of Paris in the 1920s. These early methods usingdifferent materials directed the advancements in the way rotational molding is usedtoday with plastics.

Plastics were introduced to the rotational molding process in the early 1950s. One of thefirst applications was to manufacture doll heads. The machinery was made of an E Bluebox-oven machine, inspired by a General Motors rear axle, powered by an externalelectric motor and heated by floor-mounted gas burners. The mold was made out ofelectroformed nickel-copper, and the plastic was a liquid PVC plastisol. The coolingmethod consisted of placing the mold into cold water. This process of rotational moldingled to the creation of other plastic toys. As demand for and popularity of this processincreased, it was used to create other products such as road cones, marine buoys, andcar armrests. This popularity led to the development of larger machinery. A new systemof heating was also created, going from the original direct gas jets to the current indirecthigh velocity air system. In Europe during the 1960s the Engel process was developed.This allowed large hollow containers to be manufactured in low-density polyethylene.The cooling method consisted of turning off the burners and allowing the plastic toharden while still rocking in the mold.

In 1976, the Association of Rotational Moulders (ARM) was started in Chicago as aworldwide trade association. The main objective of this association is to increaseawareness of the rotational molding technology and process

In the 1980s, new plastics, such as polycarbonate, polyester, and nylon, wereintroduced to rotational molding. This has led to new uses for this process, such as thecreation of fuel tanks and industrial moldings. The research that has been done sincethe late 1980s at Queen's University Belfast has led to the development of more precisemonitoring and control of the cooling processes based on their development of the“Rotolog system”

Equipment and tooling

Rotational molding machines are made in a wide range of sizes. They normally consistof molds, an oven, a cooling chamber, and mold spindles. The spindles are mounted ona rotating axis, which provides a uniform coating of the plastic inside each mold.

Molds (or tooling) are either fabricated from welded sheet steel or cast. The fabricationmethod is often driven by part size and complexity; most intricate parts are likely madeout of cast tooling. Molds are typically manufactured from stainless steel or aluminum.Aluminum molds are usually much thicker than an equivalent steel mold, as it is a softermetal. This thickness does not affect cycle times significantly since aluminum's thermalconductivity is many times greater than steel. Due to the need to develop a model priorto casting, cast molds tend to have additional costs associated with the manufacturingof the tooling, whereas fabricated steel or aluminum molds, particularly when used forless complex parts, are less expensive. However, some molds contain both aluminumand steel. This allows for variable thicknesses in the walls of the product. While this

process is not as precise as injection molding, it does provide the designer with moreoptions. The aluminum addition to the steel provides more heat capacity, causing themelt-flow to stay in a fluid state for a longer period.

Standard setup and equipment for rotational molding

Normally all rotation molding systems have a number of parts including molds, oven,cooling chamber and mold spindles. The molds are used to create the part, and aretypically made of aluminium. The quality and finish of the product is directly related tothe quality of the mold being used. The oven is used to heat the part while also rotatingthe part to form the part desired. The cooling chamber is where the part is placed until itcools, and the spindles are mounted to rotate and provide a uniform coat of plasticinside each mold.

Rotational molding Machines

Rock and roll machine

This is a specialized machine designed mainly to produce long narrow parts. Some areof the clamshell type, thus one arm, but there are also shuttle-type Rock & Rollmachines, with two arms. Each arm rotates or rolls the mold 360 degrees in onedirection and at the same time tips and rocks the mold 45 degrees above or belowhorizontal in the other direction. Newer machines use forced hot air to heat the mold.These machines are best for large parts that have a large length-to-width ratio. Becauseof the smaller heating chambers, there is a saving in heating costs compared to bi-axialmachines.

Clamshell machine

This is a single arm rotational molding machine. The arm is usually supported by otherarms on both ends. The clamshell machine heats and cools the mold in the samechamber. It takes up less space than equivalent shuttle and swing arm rotationalmolders. It is low in cost compared to the size of products made. It is available insmaller scales for schools interested in prototyping and for high quality models. Morethan one mold can be attached to the single arm.

Vertical or up & over rotational machine

The loading and unloading area is at the front of the machine between the heating andcooling areas. These machines vary in size between small to medium compared toother rotational machines. Vertical rotational molding machines are energy efficient dueto their compact heating and cooling chambers. These machines have the same (orsimilar) capabilities as the horizontal carousel multi-arm machines, but take up muchless space.

Shuttle machine

Most shuttle machines have two arms that move the molds back and forth between theheating chamber and cooling station. The arms are independent of each other and theyturn the molds bi-axially. In some cases, the shuttle machine has only one arm. Thismachine moves the mold in a linear direction in and out of heating and coolingchambers. It is low in cost for the size of product produced and the footprint is kept to aminimum compared to other types of machines. It is also available in smaller scale forschools and prototyping.

Swing arm machine

The swing-arm machine can have up to four arms, with a bi-axial movement. Each armis independent from each other as it is not necessary to operate all arms at the sametime. Each arm is mounted on a corner of the oven and it swings in and out of the oven.On some swing-arm machines, a pair of arms is mounted on the same corner, thus afour-arm machine has two pivot points. These machines are very useful for companiesthat have long cooling cycles or require a lot of time to demold parts, compared to thecook time. It is a lot easier to schedule maintenance work or try to run a new moldwithout interrupting production on the other arms of the machine.

Carousel machine

A Carousel machine with four independent arms

This is one of the most common bi-axial machines in the industry. It can have up to 4arms and six stations and it comes in a wide range of sizes. The machine comes in twodifferent models, fixed and independent. A fixed-arm carousel consists of 3 fixed armsthat must move together. One arm will be in the heating chamber while the other is inthe cooling chamber and the other in the loading/reloading area. The fixed-arm carouselworks well when working with identical cycle times on each arm. The independent-armcarousel machine is available with 3 or 4 arms that can move separately from the other.This allows for different size molds, with different cycle times and thickness needs.

Production process

The rotational molding process is a high-temperature, low-pressure plastic-formingprocess that uses heat and biaxial rotation (i.e., angular rotation on two axes) toproduce hollow, one-piece parts.[7] Critics of the process point to its long cycle times—only one or two cycles an hour can typically occur, as opposed to other processes suchas injection molding, where parts can be made in a few seconds. The process doeshave distinct advantages. Manufacturing large, hollow parts such as oil tanks is mucheasier by rotational molding than any other method. Rotational molds are significantlycheaper than other types of mold. Very little material is wasted using this process, andexcess material can often be re-used, making it a very economically andenvironmentally viable manufacturing process.

Production process

The rotational molding process is a high-temperature, low-pressure plastic-formingprocess that uses heat and biaxial rotation (i.e., angular rotation on two axes) toproduce hollow, one-piece parts.[7] Critics of the process point to its long cycle times—only one or two cycles an hour can typically occur, as opposed to other processes suchas injection molding, where parts can be made in a few seconds. The process doeshave distinct advantages. Manufacturing large, hollow parts such as oil tanks is mucheasier by rotational molding than any other method. Rotational molds are significantlycheaper than other types of mold. Very little material is wasted using this process, andexcess material can often be re-used, making it a very economically andenvironmentally viable manufacturing process.

Production process

The rotational molding process is a high-temperature, low-pressure plastic-formingprocess that uses heat and biaxial rotation (i.e., angular rotation on two axes) toproduce hollow, one-piece parts.[7] Critics of the process point to its long cycle times—only one or two cycles an hour can typically occur, as opposed to other processes suchas injection molding, where parts can be made in a few seconds. The process doeshave distinct advantages. Manufacturing large, hollow parts such as oil tanks is mucheasier by rotational molding than any other method. Rotational molds are significantlycheaper than other types of mold. Very little material is wasted using this process, andexcess material can often be re-used, making it a very economically andenvironmentally viable manufacturing process.

Unloading a molded polyethylene tank in a Shuttle machineUnloading a molded polyethylene tank in a Shuttle machineUnloading a molded polyethylene tank in a Shuttle machine

Rotational Molding Process

The rotational molding process consists of four distinct phases:

Loading a measured quantity of polymer (usually in powder form) into the mold.

Heating the mold in an oven while it rotates, until all the polymer has melted andadhered to the mold wall. The hollow part should be rotated through two or more axes,rotating at different speeds, in order to avoid the accumulation of polymer powder. Thelength of time the mold spends in the oven is critical: too long and the polymer willdegrade, reducing impact strength. If the mold spends too little time in the oven, thepolymer melt may be incomplete. The polymer grains will not have time to fully melt andcoalesce on the mold wall, resulting in large bubbles in the polymer. This has anadverse effect on the mechanical properties of the finished product.

Cooling the mold, usually by fan. This stage of the cycle can be quite lengthy. Thepolymer must be cooled so that it solidifies and can be handled safely by the operator.This typically takes tens of minutes. The part will shrink on cooling, coming away fromthe mold, and facilitating easy removal of the part. The cooling rate must be kept withina certain range. Very rapid cooling (for example, water spray) would result in coolingand shrinking at an uncontrolled rate, producing a warped part.

Removal of the part.

Recent improvements

Until recently, the process largely relied on both trial and error and the experience of theoperator to determine when the part should be removed from the oven and when it wascool enough to be removed from the mold. Technology has improved in recent years,allowing the air temperature in the mold to be monitored, removing much of theguesswork from the process.

Much of the current research is into reducing the cycle time, as well as improving partquality. The most promising area is in mold pressurization. It is well known that applyinga small amount of pressure internally to the mold at the correct point in the heatingphase accelerates coalescence of the polymer particles during the melting, producing apart with fewer bubbles in less time than at atmospheric pressure. This pressure delaysthe separation of the part from the mold wall due to shrinkage during the cooling phase,aiding cooling of the part. The main drawback to this is the danger to the operator ofexplosion of a pressurized part. This has prevented adoption of mold pressurization ona large scale by rotomolding manufacturers.

Mold release agents

A good mold release agent (MRA) will allow the material to be removed quickly andeffectively. Mold releases can reduce cycle times, defects, and browning of finishedproduct. There are a number of mold release types available; they can be categorizedas follows:

Sacrificial coatings: the coating of MRA has to be applied each time because most ofthe MRA comes off on the molded part when it releases from the tool. Silicones aretypical MRA compounds in this category.

Semi-permanent coatings: the coating, if applied correctly, will last for a number ofreleases before requiring to be re-applied or touched up. This type of coating is mostprevalent in today's rotational molding industry. The active chemistry involved in thesecoatings is typically a polysiloxane.

Permanent coatings: most often some form of PTFE coating, which is applied to themold. Permanent coatings avoid the need for operator application, but may becomedamaged by misuse.

Materials

More than 80% of all the material used is from the polyethylene family: cross-linkedpolyethylene (PEX), low-density polyethylene (LDPE), linear low-density polyethylene(LLDPE), high-density polyethylene (HDPE), and regrind. Other compounds are PVCplastisols, nylons, and polypropylene.

Order of materials most commonly used by industry:[8]

Polyethylene

Polypropylene

Polyvinyl chloride

Nylon

Polycarbonate

These materials are also occasionally used (not in order of most used):[8]

Aluminum

Acrylonitrile butadiene styrene (ABS)

Acetal

Acrylic

Epoxy

Fluorocarbons

Ionomer

Polybutylene

Polyester

Polystyrene

Polyurethane

Silicone

Various foods (especially chocolate)

Natural materials

Recently it has become possible to use natural materials in the molding process.Through the use of real sands and stone chip, sandstone composite can be createdwhich is 80% natural non-processed material.

Rotational molding of plaster is used to produce hollow statuettes.

Chocolate is rotationally molded to form hollow treats.

Products

Designers can select the best material for their application, including materials that meetU.S. Food and Drug Administration (FDA) requirements. Additives for weatherresistance, flame retardation, or static elimination can be incorporated. Inserts,graphics, threads, handles, minor undercuts, flat surfaces without draft angles, or finesurface detail can be part of the design. Designs can also be multi-wall, either hollow orfoam filled.

Products that can be manufactured using rotational molding include storage tanks,furniture, road signs and bollards, planters, pet houses, toys, bins and refusecontainers, doll parts, road cones, footballs, helmets, canoes, rowing boats, tornadoshelters[9], kayak hulls and playground slides. The process is also used to make highlyspecialised products, including UN-approved containers for the transportation of nuclearfissile materials,[10] anti-piracy ship protectors,[11] seals for inflatable oxygenmasks[12] and lightweight components for the aerospace industry.[13]

Mold ingraphic

A blind brass threadedhex insert molded into a liquidstorage tank.

Rotational MoldedFlamingo

Edon roto mouldedrowing boat

Mold ingraphic

A blind brass threadedhex insert molded into a liquidstorage tank.

Rotational MoldedFlamingo

Edon roto mouldedrowing boat

Mold ingraphic

A blind brass threadedhex insert molded into a liquidstorage tank.

Rotational MoldedFlamingo

Edon roto mouldedrowing boat

What is Casting?

Casting involves introducing a liquefied plastic into a mold and allowing it to solidify. Incontrast to molding and extrusion, casting relies on atmospheric pressure to fill the moldrather than using significant force to push the plastic into the mold cavity. Somepolymers have a viscosity similar to bread dough even when they are at elevatedtemperature so they are not candidates for the casting process. Examples of this arepolymers like POM, PC, PP and many others. Casting includes a number of processesthat take a monomer, powder or solvent solution and pur them into a mold. Theytransition from liquid to solid by either evaporation, chemical action, cooling or externalheat. The final product can be removed from the mold once it solidifies.

Casting has several advantages:-Cost of equipment, tooling and molds are low.

-The process is not complex.

-Products have little or no internal stress.

Casting can have some disadvantages:-The output rate is slow and has long cycle times.

-Dimensional tolerances are not very good.

-Moisture and air bubbles can be difficult to manage and may cause problems.

Resin castingResin casting is a method of plastic casting where a mould is filled with aliquid synthetic resin, which then hardens. It is primarily used for small-scale productionlike industrial prototypes and dentistry. It can be done by amateur hobbyists with littleinitial investment, and is used in the production of collectible toys, models and figures,as well as small-scale jewellery production.The synthetic resin for such processes is a monomer for making a plastic thermosettingpolymer. During the setting process, the liquid monomer polymerizes into the polymer,thereby hardening into a solid.Single-monomer resins may be used in the process, which form homopolymers(polymers containing only one type of polymer). In such uses, the "curing agent" mixedwith the resin contains what is loosely referred to as a "catalyst," but which is moretechnically an initial source of free radicals (such as MEKP) to act as an initiator in afree-radical chemical chain reactionpolymerization. Alternately, resin casting may beaccomplished with a resin plus a nearly equal amount of a "hardener" liquid (as inmany epoxy resin or polyester resin systems), which functionally contains a second

polymer, for use in forming a final product plastic which is a copolymer. Copolymerscontain two different alternating chemical entities in the final polymer molecule.

Process

Most commonly a thermosetting resin is used that polymerizes by mixing witha curing agent (polymerization catalyst) at room temperature and normal pressure. Theresins are named by analogy with plant resins, but are synthetic monomers for makingpolymer plastics. The so-called synthetic resins used include polystyrene resin,polyurethane resin, epoxy resin, unsaturated polyester resin, acrylic resin and

silicone resin.Epoxy resin has a lower viscosity than polyurethane resin[citation needed]; polyester resinalso shrinks markedly while curing.[1]Acrylic resin, in particular the methylmethacrylate type of synthetic resin, produces acrylic glass (also called PMMA, Lucite,Plexiglass), which is not a glass but a plastic polymer that is transparent, and very hard.It is suitable for embedding objects (such as, for example, acrylic trophies), for displaypurposes. Styrene is a similar liquid monomer at room temperature, which will alsopolymerize into clear glass-like polystyrene plastic, with addition of a suitable catalyst.A flexible mold can be made of latex rubber, room temperature vulcanized siliconerubber or other similar materials at relatively low cost, but can only be used for a

limited number of castings.The simplest method is gravity casting where the resin is poured into the mold andpulled down into all the parts by gravity. When the two part resin is mixed air bubblestend to be introduced into the liquid which can be removed in a vacuum chamber. Thecasting can also be done in a vacuum chamber (when using open molds) to eitherextract these bubbles, or in a pressure pot, to reduce their size to the point where theyaren't visible. Pressure and/or centrifugal force can be used to help push the liquid resininto all details of the mold. The mold can also be vibrated to expel bubbles.Each unit requires some amount of hands-on labor, making the final cost per unitproduced fairly high. This is in contrast to injection molding where the initial cost ofcreating the metal mold is higher, but the mold can be used to produce a much highernumber of units, resulting in a lower cost per unit.

Collectibles and models

A custom resin cast Pinky:St part and two-part silicone mold

Resin casting is used to produce collectible and customized toys and figureslike designer toys, garage kits and ball-jointed dolls, as well as scale models, eitherindividual parts or entire models of objects like trains, aircraft or ships. They aregenerally produced in small quantities, from the tens to a few hundred copies,compared to injection-molded plastic figures which are produced in many thousands.Resin casting is more labor intensive than injection molding, and the soft molds usedare worn down by each cast. The low initial investment cost of resin casting means thatindividual hobbyists can produce small runs for their own use, such as customization,while companies can use it to produce small runs for public sale.The creation of a toy or figure start with the traditional sculpting process where the artistdesigns a clay sculpture. Where appropriate, for example when making a garage kit, thesculpture is dissected into several parts like head, torso, arms and legs. A flexible moldmade from room temperature vulcanized (RTV) silicone rubber is made for each part.After the mold has been made, a synthetic resin - such as polyurethane or epoxy -mixed with a curing agent, is poured into each mold cavity. Mixing the two liquid partscauses an exothermic reaction which generates heat and within minutes causes thematerial to harden, yielding castings or copies in the shape of the mold into which it hasbeen poured. The molds are commonly half-divided (like the hollowed chocolate Eastereggs with candy inside) and a release agent may be used to make removal of thehardened/set resin from the mold easier. The hardened resin casting is removed fromthe flexible mold and allowed to cool.

A Baldwin 6-axle locomotive kit cast in resin in HO Scale

Due to aggressive nature of most compounds used for casting and the high temperatureof the reaction the mold gradually degrades and loses small details. Typically, a flexiblemold will yield between 25 and 100 castings depending upon the size of the part, theintensity of the heat generated.Depending on the type of product it may then be cut or sanded to remove any castingartefacts like sprues and seams. Some products are also assembled and painted, whilesome models and kits, which are intended for the consumer to assemble, are leftunfinished.The ability of RTV silicone molds to reproduce even the tiniest detail means that manyof these low volume castings are of very high quality. Quality of both original mastersand resin castings varies due to differences in creator's skill, as well as castingtechniques.

Why Structural Foam?Structural foam molding is a low pressure injection molding process where an inert gasis introduced into melted polymer for the purpose of reducing density and weight of thefinished product while increasing the strength. The lower pressure and forces involvedallow more economical molding equipment and tooling to be utilized resulting the massproduction of very large or parts or multiple parts being produced on a single machine ina single cycle at a lower cost than conventional injection molding.The structural foam molding process utilizes a molten resin that has been injected withnitrogen gas or a chemical blowing agent. This mixture is injected into the mold, wherethe gas expands and fills the mold with foam. As the foam flows through the mold, thesurface cells collapse. Solid skins are formed against the walls of the mold, while thecore of the part remains structurally foamed. Because the outer skin is solid and thecenter of the wall is foam, the part weight is reduced up to 20%.Structural foam yields parts that are larger and sturdier than injection molding. Theprocess pressures are much lower than in injection molding, thereby producing partsthat are structurally sound, nearly stress-free and have minimal warpage. Parts madeare thicker and sturdier than with other processes (doubling the parts' thickness yieldsparts up to eight times stiffer).

KEY BENEFITS OF THE STRUCTURAL FOAM INJECTION MOLDING PROCESSDesign Advantages

Part weight reduced 10% to 30% Increased strength and stiffness due to honeycomb structure Capable of molding large, complex parts without sink marks Capable of molding parts .500 inches thick or greater Parts may weigh up to several hundred pounds

Product Advantages HIGHEST STRENGTH-TO-WEIGHT RATIO compared to alternative manufacturing

methods and materials. Can replace concrete, sheet metal, metal castings, wood, fiberglass, rotational molding

and blow molding in a variety of applications Superior impact resistance More rigid than a solid part Low part stress and warpage Consistent surface finish Cleanability Chemical and corrosion resistance does not rust Parts that are impervious to the elements Sound deadening characteristics Electrical /thermal insulating properties Parts can be sawn, screwed, nailed or stapled like wood

Production Advantages Faster cycles due to better heat transfer of aluminum High dimensional stability over the entire production run Multiple molds can be run at the same time Functions as excellent substrate for high quality painted finish applications Two different materials and/or colors can be molded simultaneously

Cost Advantages Low multi-nozzle injection pressure allows lower cost aluminum molds Can use recycled post consumer plastics Parts are recyclable and returnable for supply chain cost effectiveness Significant reduction in cost and increases in productivity Lower raw material cost due to use of commodity resins Lifetime tooling

Processes & CapabilitiesAt Polycel we have over 40 years of experience in structural foam molding. We canprocess a wide range of plastic materials from commodity grades to custom blends toconductive and engineering thermoplastic resins. Whether your product requires asingle cavity, open and shut mold, or a complex, multi-cavity, multiple-gate, multiple-action tool, Polycel can handle all your requirements.

Molding capabilities include: Low pressure structural foam molding Gas assist foam molding Gas counter-pressure molding High pressure structural foam molding Over-molding Plastic insert molding Underwriters Laboratories (UL) Certified EMI/RFI Shielding

Markets / ApplicationsPolycel's structural foam and/or injection molded products are found in a wide range ofmarkets and industries.Structural foam and injection molding allow virtually unlimited freedom of design foraesthetic value. Complex, precise shapes often combining two or more parts orfunctions, or eliminating the need for additional hardware, can be produced. Productscan be made from materials which provide ruggedness for both indoor and outdoorapplications. They include corrosion resistance, moisture resistance, and long outdoorlife expectancy.

Markets served include: Automotive Business and Office Equipment Construction Electronics Furniture General Industrial Housewares HVAC Lawn and Garden Equipment Leisure and Recreation Marine Material Handling Medical Renewable Energy Retail Telecommunications Utilities

Examples of products manufactured include: Cases and Tote Boxes Electronic Housings Highway Crash Barriers Junction Boxes Material Handling Containers and Pallets Medical Housings Outdoor Playground Equipment Pallets Patio Tables Pipe Conduit Point-of-Purchase Displays Shelving Units Shutters Telecomm Enclosures Underground Electrical Enclosures Utility Boxes

Part design features of Plastic Product Design.

Holes

Holes can be possibly made on slides but can result in generation of weld lines. Minimum spacing between 2 holes or a hole and a sidewall should be equal to the diameter of

the hole. The hole should be located at a minimum distance of 3 times the diameter from the edge of a

part, to minimize stresses. A through hole is preferred over a blind hole because core pin that produces a hole can be

supported at both ends and is less likely to bend. Holes in the bottom of a part are better than holes in side, which require retractable core pins. Depth of blind holes should not be more than 2 times the diameter. Steps should be used to increase the depth of a deep blind hole. For through holes, cutout sections in the part can shorten the length of a small-diameter pin. Use overlapping and offset mold cavity projections instead of core pins to produce holes parallel

to the die parting line (perpendicular to the mold movement direction).

FilletSharp corners increase concentrations, which are prone to air entrapments, air voids, and sinkmarks hence weakening the structural integrity of the plastic part. It must be eliminated using radiiwhenever is possible. It is recommended that an inside radius be a minimum of one times thethickness. At corners, the suggested inside radius is 0.5 times the material thickness and the outsideradius is 1.5 times the material thickness. A bigger radius should be used if part design allows

RibsRib features help in strengthening the molded part without adding to wall thickness. In some cases,they can also act as decorative features. Ribs also provide alignment in mating parts or providestopping surfaces for assemblies. However, projections like ribs can create cavity filling, venting, andejection problems. These problems become more troublesome for taller ribs. Ribs need to bedesigned in correct proportion to avoid defects such as short shots and provide the requiredstrength. Thick and deep ribs can cause sink marks and filling problems respectively. Deep ribs canalso lead to ejection problems. If ribs are too long or too wide, supporting ribs may be required. It isbetter to use a number of smaller ribs instead of one large rib.

Recommended values for parameters: Generally, the rib height is recommended to be notmore than 2.5 to 3 times the nominal wall thickness. Similarly, rib thickness at its base should bearound 0.4 to 0.6 times the nominal wall thickness.

Minimum base radius for ribs: A fillet of a certain minimum radius value should be provided atthe base of a rib to reduce stress. However, the radius should not be so large that it results inthick sections. The radius eliminates a sharp corner and stress concentration. Flow and coolingare also improved. Fillet radius at the base of ribs should be between 0.25 and 0.4 times thenominal wall thicknesses of the part.

Draft angle for ribs: Draft angle design is an important factor when designing plastic parts.Such parts may have a greater tendency to shrink onto a core. This creates higher contactpressure on the core surface and increases friction between the core and the part, thus makingejection of the part from the mold difficult. Hence, draft angles should be designed properly toassist in part ejection. This also reduces cycle time and improves productivity. Draft anglesshould be used on interior or exterior walls of the part along the pulling direction. It is

Part design features of Plastic Product Design.

Holes

Holes can be possibly made on slides but can result in generation of weld lines. Minimum spacing between 2 holes or a hole and a sidewall should be equal to the diameter of

the hole. The hole should be located at a minimum distance of 3 times the diameter from the edge of a

part, to minimize stresses. A through hole is preferred over a blind hole because core pin that produces a hole can be

supported at both ends and is less likely to bend. Holes in the bottom of a part are better than holes in side, which require retractable core pins. Depth of blind holes should not be more than 2 times the diameter. Steps should be used to increase the depth of a deep blind hole. For through holes, cutout sections in the part can shorten the length of a small-diameter pin. Use overlapping and offset mold cavity projections instead of core pins to produce holes parallel

to the die parting line (perpendicular to the mold movement direction).

FilletSharp corners increase concentrations, which are prone to air entrapments, air voids, and sinkmarks hence weakening the structural integrity of the plastic part. It must be eliminated using radiiwhenever is possible. It is recommended that an inside radius be a minimum of one times thethickness. At corners, the suggested inside radius is 0.5 times the material thickness and the outsideradius is 1.5 times the material thickness. A bigger radius should be used if part design allows

RibsRib features help in strengthening the molded part without adding to wall thickness. In some cases,they can also act as decorative features. Ribs also provide alignment in mating parts or providestopping surfaces for assemblies. However, projections like ribs can create cavity filling, venting, andejection problems. These problems become more troublesome for taller ribs. Ribs need to bedesigned in correct proportion to avoid defects such as short shots and provide the requiredstrength. Thick and deep ribs can cause sink marks and filling problems respectively. Deep ribs canalso lead to ejection problems. If ribs are too long or too wide, supporting ribs may be required. It isbetter to use a number of smaller ribs instead of one large rib.

Recommended values for parameters: Generally, the rib height is recommended to be notmore than 2.5 to 3 times the nominal wall thickness. Similarly, rib thickness at its base should bearound 0.4 to 0.6 times the nominal wall thickness.

Minimum base radius for ribs: A fillet of a certain minimum radius value should be provided atthe base of a rib to reduce stress. However, the radius should not be so large that it results inthick sections. The radius eliminates a sharp corner and stress concentration. Flow and coolingare also improved. Fillet radius at the base of ribs should be between 0.25 and 0.4 times thenominal wall thicknesses of the part.

Draft angle for ribs: Draft angle design is an important factor when designing plastic parts.Such parts may have a greater tendency to shrink onto a core. This creates higher contactpressure on the core surface and increases friction between the core and the part, thus makingejection of the part from the mold difficult. Hence, draft angles should be designed properly toassist in part ejection. This also reduces cycle time and improves productivity. Draft anglesshould be used on interior or exterior walls of the part along the pulling direction. It is

Part design features of Plastic Product Design.

Holes

Holes can be possibly made on slides but can result in generation of weld lines. Minimum spacing between 2 holes or a hole and a sidewall should be equal to the diameter of

the hole. The hole should be located at a minimum distance of 3 times the diameter from the edge of a

part, to minimize stresses. A through hole is preferred over a blind hole because core pin that produces a hole can be

supported at both ends and is less likely to bend. Holes in the bottom of a part are better than holes in side, which require retractable core pins. Depth of blind holes should not be more than 2 times the diameter. Steps should be used to increase the depth of a deep blind hole. For through holes, cutout sections in the part can shorten the length of a small-diameter pin. Use overlapping and offset mold cavity projections instead of core pins to produce holes parallel

to the die parting line (perpendicular to the mold movement direction).

FilletSharp corners increase concentrations, which are prone to air entrapments, air voids, and sinkmarks hence weakening the structural integrity of the plastic part. It must be eliminated using radiiwhenever is possible. It is recommended that an inside radius be a minimum of one times thethickness. At corners, the suggested inside radius is 0.5 times the material thickness and the outsideradius is 1.5 times the material thickness. A bigger radius should be used if part design allows

RibsRib features help in strengthening the molded part without adding to wall thickness. In some cases,they can also act as decorative features. Ribs also provide alignment in mating parts or providestopping surfaces for assemblies. However, projections like ribs can create cavity filling, venting, andejection problems. These problems become more troublesome for taller ribs. Ribs need to bedesigned in correct proportion to avoid defects such as short shots and provide the requiredstrength. Thick and deep ribs can cause sink marks and filling problems respectively. Deep ribs canalso lead to ejection problems. If ribs are too long or too wide, supporting ribs may be required. It isbetter to use a number of smaller ribs instead of one large rib.

Recommended values for parameters: Generally, the rib height is recommended to be notmore than 2.5 to 3 times the nominal wall thickness. Similarly, rib thickness at its base should bearound 0.4 to 0.6 times the nominal wall thickness.

Minimum base radius for ribs: A fillet of a certain minimum radius value should be provided atthe base of a rib to reduce stress. However, the radius should not be so large that it results inthick sections. The radius eliminates a sharp corner and stress concentration. Flow and coolingare also improved. Fillet radius at the base of ribs should be between 0.25 and 0.4 times thenominal wall thicknesses of the part.

Draft angle for ribs: Draft angle design is an important factor when designing plastic parts.Such parts may have a greater tendency to shrink onto a core. This creates higher contactpressure on the core surface and increases friction between the core and the part, thus makingejection of the part from the mold difficult. Hence, draft angles should be designed properly toassist in part ejection. This also reduces cycle time and improves productivity. Draft anglesshould be used on interior or exterior walls of the part along the pulling direction. It is

recommended that draft angle for rib should be around 1 to 1.5 deg. Minimum draft should be0.5 per side.

Spacing between two parallel ribs: Mold wall thickness gets affected due to spacing betweenvarious features in the plastic model. If features like ribs are placed close to each other or thewalls of the parts, thin areas are created which can be hard to cool and can affect quality. If themold wall is too thin, it is also difficult to manufacture and can also result in a lower life for themold due to problems like hot blade creation and differential cooling. It is recommended thatspacing between ribs should be at least 2 times the nominal wall. Ribs in plastic part improve stiffness (relationship between load and part

deflection) of the part and increases rigidity. It also enhances mouldability asthey hasten melt flow in the direction of the rib.

Ribs are placed along the direction of maximum stress and deflection on non-appearance surfaces of the part. Mould filling, shrinkage and ejection shouldalso influence rib placement decisions.

Ribs that do not join with vertical wall should not end abruptly. Gradualtransition to nominal wall should reduce the risk for stress concentration.

Ribs should have following dimensions.

Rib thickness should be between 0.5 to 0.6 times nominal wall thickness toavoid sink mark.

Rib height should be 2.5 to 3 times nominal wall thickness. Rib should have 0.5 to 1.5-degree draft angle to facilitate ejection. Rib base should have radius 0.25 to 0.4 times nominal wall thickness. Distance between two ribs should be 2 to 3 times (or more) nominal wall

thickness.

BossBoss, a basic design element in plastics, is typically cylindrical and used as a mounting fixture,location point, reinforcement feature or spacer. Under service conditions, bosses are often subjectedto loadings not encountered in other sections of a component.

Minimum radius at base of boss: Provide a generous radius at the base of the boss forstrength and ample draft for easy part removal from the mold. A fillet of a certain minimumradius value should be provided at the base of boss to reduce stress. The intersection of thebase of the boss with the nominal wall is typically stressed and stress concentrationincreases if no radii are provided. Also, the radius at the base of the boss should not exceeda maximum value to avoid thick sections. The radius at base of boss provides strength andample draft for easy removal from the mold. It is recommended that the radius at the base ofboss should be 0.25 to 0.5 times the nominal wall thickness.

Boss height to outer diameter ratio: A tall boss with the included draft will generate amaterial mass and thick section at the base. In addition, the core pin will be difficult to cool,can extend the cycle time and affect the cored hole dimensionally. It is recommended thatheight of boss should be less than 3 times of outer diameter.

Minimum radius at tip of boss: Bosses are features added to the nominal wall thickness ofthe component and are usually used to facilitate mechanical assembly. Under serviceconditions, bosses are often subjected to loadings not encountered in other sections of acomponent. A fillet of a certain minimum radius value should be provided at the tip of boss toreduce stress.

Wall thickness of boss: Wall thicknesses for bosses should be less than 60 percent of thenominal wall to minimize sinking. However, if the boss is not in a visible area, then the wallthickness can be increased to allow for increased stresses imposed by self-tapping screws.It is recommended that wall thickness of boss should be around 0.6 times of nominal wallthickness depending on the material.

Radius at base of hole in boss: Bosses find use in many part designs as points forattachment and assembly. The most common variety consists of cylindrical projections withholes designed to receive screws, threaded inserts, or other types of fastening hardware.Providing a radius on the core pin helps in avoiding a sharp corner. This not only helpsmolding but also reduces stress concentration. It is recommended that the radius at base ofhole in boss should be 0.25 to 0.5 times the nominal wall thickness.

Minimum draft for boss inner and outer diameter: An appropriate draft on the outerdiameter of a boss helps easy ejection from the mold. Draft is required on the walls of bossto permit easy withdrawal from the mold. Similarly, designs may require a minimum taper onthe ID of a boss for proper engagement with a fastener. Draft is required on the walls of bossto permit easy withdrawal from the mold. It is recommended that minimum draft on outersurface of the boss should be greater than or equal to 0.5 degree and on inner surface itshould be greater than 0.25 degrees.

Spacing between bosses: When bosses are placed very close to each other, it results increating thin areas which are hard to cool and can affect the quality and productivity. Also, ifthe mold wall is too thin, it is very difficult to manufacture and often results in a lower life forthe mold, due to problems like hot blade creation and differential cooling. It is recommendedthat spacing between bosses should be at least 2 times the nominal wall thickness.

Standalone boss: Bosses and other thick sections should be cored. It is good practice toattach the boss to the sidewall. In this case the material flow is uniform and providesadditional load distribution for the part. For better rigidity and material flow, the generalguideline suggests that boss should be connected to nearest side wall.

Radius at cornersGenerously rounded corners provide a number of advantages. There is less stress concentration onthe part and on the tool. Because of sharp corners, material flow is not smooth and tends to bedifficult to fill, reduces tooling strength and causes stress concentration. Parts with radii and filletsare more economical and easier to produce, reduce chipping, simplify mold construction and addstrength to molded part with good appearance.

Sharp Corners general design guidelines in injection molding suggest that corner radii should be atleast one-half the wall thickness. It is recommended to avoid sharp corners and use generous filletsand radii whenever required. During injection molding, the molten plastic has to navigate turns orcorners. Rounded corners will ease plastic flow, so engineers should generously radius the cornersof all parts. In contrast, sharp inside corners result in molded-in stress particularly during the coolingprocess when the top of the part tries to shrink and the material pulls against the corners. Moreover,the first rule of plastic design i.e. uniform wall thickness will be obeyed. As the plastic goes around a

well-proportioned corner, it will not be subjected to area increases and abrupt changes in direction.Cavity packing pressure stays consistent. This leads to a strong, dimensionally stable corner that willresist post-mold warpage.

CORNERS

When two surfaces meet, it forms a corner. At corner, wall thickness increases to 1.4times the nominal wall thickness. This results in differential shrinkage and moulded-in stress and longer cooling time. Therefore, risk of failure in service increases atsharp corners.

Draft anglesDraft angle design is an important factor when designing plastic parts. Because of shrinkage ofplastic material, injection molded parts have a tendency to shrink onto a core. This creates highercontact pressure on the core surface and increases friction between the core and the part, thusmaking ejection of the part from the mold difficult. Hence, draft angles should be designed properlyto assist in part ejection. This also reduces cycle time and improves productivity. Draft angles shouldbe used on interior and exterior walls of the part along the pulling direction.

The minimum allowable draft angle is harder to quantify. Plastic material suppliers and molders arethe authority on what is the lowest acceptable draft. In most instances, 1degree per side will besufficient, but between 2 degree and 5 degree per side would be preferable. If the design is notcompatible with 1 degree, then allow for 0.5 degree on each side. Even a small draft angle, such as0.25 degree, is preferable to none at all.

Mold Wall ThicknessNon-uniform wall sections can contribute to warpage and stresses in molded parts. Sections whichare too thin have a higher chance of breakage in handling, may restrict the flow of material and maytrap air causing a defective part. Too heavy a wall thickness, on the other hand, will slow the curingcycle and add to material cost and increase cycle time.

Generally, thinner walls are more feasible with small parts rather than with large ones. The limitingfactor in wall thinness is the tendency for the plastic material in thin walls to cool and solidify beforethe mold is filled. The shorter the material flow, the thinner the wall can be. Walls also should be asuniform in thickness as possible to avoid warpage from uneven shrinkage. When changes in wallthickness are unavoidable, the transition should be gradual and not abrupt.

Some plastics are more sensitive to wall thickness than others, where acetal and ABS plastics maxout at around 0.12 in. thick (3 mm), acrylic can go to 0.5 in. (12 mm), polyurethane to 0.75 in.

(18 mm), and certain fiber-reinforced plastics to 1 in. (25 mm) or more. Even so, designers shouldrecognize that very thick cross sections can increase the likelihood of cosmetic defects like sink.

Solid shape moulding is not desired in injection moulding due to following reasons. Cooling time is proportional to square of wall thickness. Large cooling time for

solid will defeat the economy of mass production. (poor conductor of heat) Thicker section shrink more than thinner section, thereby introduce differential

shrinkage resulting in warpage or sink mark etc. (shrinkage characteristics ofplastics and pvT characteristics)

Therefore we have basic rule for plastic part design; as far as possible wall thicknessshould be uniform or constant through out the part. This wall thickness is callednominal wall thickness.If there is any solid section in the part, it should be made hollow by introducing core.This should ensure uniform wall thickness around the core.What are the considerations for deciding wall thickness?

It must be thick and stiff enough for the job. Wall thickness could be 0.5 to5mm.

It must also be thin enough to cool faster, resulting lower part weight andhigher productivity.

Any variation in wall thickness should be kept as minimum as possible.A plastic part with varying wall thickness will experience differing cooling rates anddifferent shrinkage. In such case achieving close tolerance becomes very difficult andmany times impossible. Where wall thickness variation is essential, the transitionbetween the two should be gradual.

WELD LINE IN MOULDINGWeld line occurs when two melt streams join. Meltstream gets divided at cut-out (core) in the part and they join at the other end of thecut out. Normally weld line region is filled at the end of injection stroke or duringpressure phase. Strength of the weld line is weak when partially frozen melt frontmeet. The orientation at the joint remains perpendicular to direction of flow -a sign ofweakness. Weld line can form by melt stream flowing in same direction or in oppositedirection. It is not possible to eliminate weld line, but it can be made sufficientlystronger or its position can be altered.

Reasons To Texture A Plastic Part

Visual Effect

• To give a part the appearance of leather, wood, stipple, sand, or whatever effect you are simulating.

• To give parts a more even, planned effect, or to get rid of a glossy appearance and change to a mattefinish. This can add richness to a part’s appearance, therefore making the part more marketable, and givingit a perception of higher value and quality.

• To build a company’s logo or a pattern into the appearance of the part that immediately identifies the partas belonging to that particular corporation.

• To diffuse light on clear parts, such as serrations or frosting on a lens. Texture can also be used to make aclear part translucent.

• For visual contrast - through the use of two different textures on one part or by frosting the backgroundor foreground of a logo, for example.

• Texture can provide visual improvement on a difficult part to mold. Certain textures can hide splay lines,flow lines, knit lines, blush marks, and other molding flaws. Even sink marks can sometimes be disguisedby the application of texture.

• Accugrave engraving to add logos, part numbers, designs, instructions for consumers, or partidentification, thus eliminating secondary processes such as hot stamping or applying of labels.

Improving Molding

• Adding texture to a core can help to hold the part onto the tool without manual undercuts which couldcreate sink marks. The texture disperses the pressure over a larger even area, lessening the likelihood ofsink marks and yet still holding the part, allowing the mold to eject properly, thus lessening the potentialfor drag or scuff marks on the Class "A" surface of the molded part.

• Texture applied on the core side and across lifters and/or slides, for some materials, can hide the shadowmarks which sometimes will show through from the front of the part when the Class "A" side is polished.

• Texture applied to some molds will allow trapped gasses to escape more quickly, by venting to theparting lines from within the cavity.

• Texture can be applied to hold paint better during a secondary molding operation.

• Texture applied in the correct design and location can help to minimize turbulence created by plasticflow.

• Texture can provide a functional rough surface finish, on a roll, for example to help the roll stock throughthe rolling process.

Some of the factors that can affect texture pricing are:

1) The complexity of the mold: • The areas to be textured. • The areas that will NOT be textured. • Size ofthe tool. • Molding Process: Blow mold, Vacuum mold, Injection mold, Rubber mold, etc. • ToolComponents: Cams, Cores, Lifters, Pins, Inserts, etc. • Additional disassembly of the mold.

2) The complexity of the surface: • Contours. • Accessibility of textured areas. • Tool Characteristics:Ribs, Pockets, Vents, Standing Bosses, etc. • Polished surfaces.

3) The complexity of the texture pattern: • Random or geometric pattern. • Pattern depth. • Even orgradient application for low draft sidewall. • Number of pattern levels or applications. • Toleranceallowed in the pattern. • Pattern fit to the tool. • Number of different patterns in the tool. • Patternintricacy or boldness.

4) 4) Material of which the mold is constructed.

PLASTICS PART DESIGN and MOULDABILITY

Injection moulding is popular manufacturing method because of its high-speed productioncapability. Performance of plastics part is limited by its properties which is not as strong (asgood) as metal. There are applications where the available properties of the plastics can beuseful. The strength of plastics can be improved with reinforcement of glass fiber, mica, talketc.

Plastics generally have following characteristics,

Light weight - low density, Low conductivity of heat and electricity - insulating properties, Low hardness, Lower strength than metals, Ductile, Dimensional stability- not as good as metal,

WALL THICKNESS

Solid shape moulding is not desired in injection moulding due to following reasons.

Cooling time is proportional to square of wall thickness. Large cooling time for solidwill defeat the economy of mass production. (poor conductor of heat)

Thicker section shrink more than thinner section, thereby introduce differentialshrinkage resulting in warpage or sink mark etc. (shrinkage characteristics of plasticsand pvT characteristics)

Therefore we have basic rule for plastic part design; as far as possible wall thickness shouldbe uniform or constant through out the part. This wall thickness is called nominal wallthickness.

If there is any solid section in the part, it should be made hollow by introducing core. Thisshould ensure uniform wall thickness around the core.

What are the considerations for deciding wall thickness?

It must be thick and stiff enough for the job. Wall thickness could be 0.5 to 5mm.

It must also be thin enough to cool faster, resulting lower part weight and higherproductivity.

Any variation in wall thickness should be kept as minimum as possible.

A plastic part with varying wall thickness will experience differing cooling rates and differentshrinkage. In such case achieving close tolerance becomes very difficult and many timesimpossible. Where wall thickness variation is essential, the transition between the two shouldbe gradual.

CORNERS

When two surfaces meet, it forms a corner. At corner, wall thickness increases to 1.4 times thenominal wall thickness. This results in differential shrinkage and moulded-in stress and longercooling time. Therefore, risk of failure in service increases at sharp corners.

SINK MARK IS INEVITABLE.

Temperature dependent change in volume - 29% in crystalline and 8% in amorphous-.

Compressibility of melt under pressure is 10-15%.

On falling temperature of melt in the mould, decrease in volume is more than the increase involume on relaxation of pressure.

Therefore void can not be perfectly filled in. Hence sink mark is inevitable.

CHANGE IN VOLUME and DENSITY OF MATERIAL

Materials Specific volumeAT 20 degree C

Specificvolume AT

200 degree C

% agechange

cubic-cm / g cubic-cm / g

HDPE(crystalline)

1.03 1.33 29 %

PS (amorphous) 0.97 1.05 8%

Density Density

HDPE(crystalline)

0.97 0.75 22.7%

PS (amorphous) 1.03 0.952 7.8%

To solve this problem, the corners should be smoothened with radius. Radius should beprovided externally as well as internally. Never have internal sharp corner as it promotes crack.Radius should be such that they confirm to constant wall thickness rule. It is preferable to haveradius of 0.6 to 0.75 times wall thickness at the corners. Never have internal sharp corner as itpromotes crack.

RIBS for stifness consideration

Ribs in plastic part improve stiffness (relationship between load and part deflection) of the partand increases rigidity. It also enhances mouldability as they hasten melt flow in the direction ofthe rib.

Ribs are placed along the direction of maximum stress and deflection on non-appearancesurfaces of the part. Mould filling, shrinkage and ejection should also influence rib placementdecisions.

Ribs that do not join with vertical wall should not end abruptly. Gradual transition to nominalwall should reduce the risk for stress concentration.

Ribs should have following dimensions.

Rib thickness should be between 0.5 to 0.6 times nominal wall thickness to avoid sinkmark.

Rib height should be 2.5 to 3 times nominal wall thickness.

Rib should have 0.5 to 1.5-degree draft angle to facilitate ejection.

Rib base should have radius 0.25 to 0.4 times nominal wall thickness.

Distance between two ribs should be 2 to 3 times (or more) nominal wall thickness.

MOULDABILITY consideration

While designing plastic part, pitfalls in achieving quality, consistency and productivity must beconsidered. It is wrong to assume that shapes can be moulded successfully with out anydefects. All shapes may not be 100% mouldable. To improve the mouldability injectionmoulding process has to be understood in depth.

Part design obviously has to be influenced by the intricacies of the process.

Filling phase of the process is influenced by type of gate, location of gate, number of gates, sizeof gate (also dependent on material viscosity). Gate should be located at such a position fromwhere flow path to thickness ratio (flow ratio)is constant in all direction. The difference in flowratio could be as small as possible. In some cases where thickness variation is unavoidable,

melt must flow from thin section to thick section for better mouldability. Melt flow from thin tothick results in poor moulding. The size of gate should not result in excessive pressure dropacross it. It should be adequate to handle flow rate required.

Resistance to flow and viscosity determines the filling pressure. Filling pressure variationshould be gradual and not abrupt. It should be remembered that flow thinner section introducesshearing of melt, resulting in lowering of melt viscosity. This is the shear thinning nature ofthermoplastics melt.

Filling phase is influenced by wall thickness variation as it introduces variation in resistance toflow in all directions from the gate. Melt is held in cylindrical shape in plasticating cylinderbefore injection. When the melt is injected through gate and runner system, melt streams moveequally in all directions only when resistance to flow is equal in all direction.

It should be realised that variation in wall thickness, hole / slot, variation of mould surfacetemperature introduces variation in resistance to flow. Therefore melt moves in number ofstreams with different velocity in different direction and mould does not fill in balancedmanner.

When melt streams reach boundary at the same time it can be called balanced filling. Whensome stream reaches the boundary early and some other streams reach late - this time lag tocomplete the filling of part results in induction of moulded-in stresses in the part.

WELD LINE IN MOULDING

Weld line ocures when two melt streams join. Melt stream gets divided at cutout (core) in thepart and they join at the other end of the cut out.

Normally weld line region is filled at the end of injection stroke or during pressure phase.

Strength of the weld line is weak when partially frozen melt front meet. The orientation at thejoint remains perpendicular to direction of flow -a sign of weakness.

Weld line can form by melt stream flowing in same direction or in opposite direction.

It is not possible to eliminate weld line, but it can be made sufficiently stronger or its positioncan be altered.

MELT STREAM FROM OPPOSITE DIRECTION

CHANGING WELD LINE POSITION

Over cooled region can also freeze faster than lesser cooled region. When freezing is notuniform, melt moves through narrowing cross section of slow freezing stream and overpacksthe slow slow freezing stream region. Hence uniform mould surface temperature distribution isvery important. This has to be achieved through proper design of cooling channels for turbulentwater flow.

Melt temperature is highest near the gate. Hence freezing likely to be slower near the gate. Thishappens near the gate during pressure phase of the process. Here over packing can becontrolled through proper profiling of pressure - reducing with time.

COOLING consideration

Volumetric changes associated with changes in temperature and pressure should be understoodwell.

Dimensional variation of mould cavity and core during moulding, moulded part before ejectionand after ejection and thereafter sever hours later is described in this figure.

Balance in heat exchange during a cycle time ensures the consistency moulding.

EJECTION consideration.

Adequate draft angle, good surface finish, mechanism to handle undercut, stregic location ofejector pins etc should be the consideration of part designer.

Post-Mold Installed with Heat or Ultrasonic Installation Knurls are used to increase resistance to torque.Straight knurls, as opposed to diamond knurls, are the preferred design. Coarser knurls increase resistanceto torque but they also induce greater stress on the plastic. In addition, the circumference of the Insertdetermines the knurl pitch so there are practical limitations on knurl design. Helical knurls, in comparisonto straight knurls, lower torque resistance but increase axial pull-out resistance. In practice, knurl anglesbetween 30 and 45 degrees have a positive impact on pull-out resistance with a minimal loss of torquevalue. In some Inserts, different helical knurl bands as well as straight and helical knurls can be combinedon the same Insert to achieve an optimum combination of torque and pull-out resistance. The objective is todesign an Insert with sufficient torque resistance to accommodate the tightening torque necessary toachieve sufficient axial tension load on the threaded joint to keep it together and prevent loosening, whilealso achieving pull-out values necessary for the load conditions that the Insert will be exposed to while inservice. In general, resistance to torque is a function of diameter and resistance to pull-out is a function oflength. These functions, however, are interactive and the challenge for the designer is to achieve theoptimum combination of both. Some Inserts are designed with a slightly larger diameter knurl bandbetween two slightly smaller diameter knurl bands on either side separated from the larger knurl band bygrooves. With a properly designed Insert and in a hole manufactured as recommended, the plastic will flowover the larger knurl band into the groove and knurls behind the larger knurl band in the opposite directionof installation, significantly increasing pull-out resistance. All the plastic above the larger knurl band ineffect becomes a shear plane. A head facilitates plastic flow into the upper grooves of the Insert. Finally forbest performance, it is essential that the Insert is installed axially square to the hole. This can be facilitatedwith tapering the Insert or by providing a pilot. Pilots need to be of sufficient length and have a plain,unknurled diameter the same size or slightly smaller than the hole.

Drilling is a cutting process that uses a drill bit to cut a hole of circular cross-section in solid materials. The drill bitis usually a rotary cutting tool, often multipoint. The bit is pressed against the workpiece and rotated at rates fromhundreds to thousands of revolutions per minute. This forces the cutting edge against the workpiece, cuttingoff chips (swarf) from the hole as it is drilled.

A specific procedure must be chosen for drilling into plastic parts, in order to eliminate component defects as far aspossible.When drilling, particular attention must be paid to the insulating characteristics of plastic. These can cause plastics(especially semi-crystalline ones) to quickly build up heat during the drilling process, particularly if the drilling depthis more than twice the diameter. This can result in drill "smearing" and inner elongation occurring in the materialand can bring about compression stress in the component (particularly where drill holes are made in the cores ofrod sections). The stress levels can be high enough to cause a high level of warping, dimensional inaccuracy, oreven to or even cracks, fractures and bursting open of the finished component or blank. These effects can beavoided by choosing a machining method to suit the material.

Stress produced with a blunt drill

Stress produced with a sharp drill

For the above reasons, when drilling it is advisable to use coolants and also to frequently withdraw the drill bit toguarantee adequate cooling and chip removal. Drill holes can generally be produced using a well sharpenedstandard commercially available HSS drill bit. It is also advisable to use a drill bit with a reduced land width(synchronous drill) in order to reduce friction and consequently also heat accumulation. Manual feed should also beavoided to prevent the possibility of the drill becoming entangled and causing cracks.

Compared to unreinforced plastics, reinforced plastics have a higher residual tension level. The use of reinforcingmaterials and fillers also makes the products harder and more brittle. Impact strength also diminishes.Consequently, these products are particularly susceptible to cracks, particularly during the drilling process. Duringthe machining process, the residual tensions can be released, culminating in effects ranging from high levels ofwarping to crack formation or complete rupture.Consequently, the following notes should be taken into account during the machining process:

The semi-finished products should be heated where possible prior to drilling up to 120°C (heating time ~1 hour per10 mm of cross section)

Carbide tipped or even better diamond tipped tools should be used for machining When tensioning and fixing the workpieces, pay attention to freedom from distortion, i.e. subject the material to the

smallest possible bending, tensile or compressive forces.

Small diameters (0.5-25mm)High-speed steel (HSS) drill bits are usually sufficient here. Twist drills with an angle of twist of between 12 and 16°and very smooth helical flutes are very suitable for this purpose and also help to ensure good chip removal. Asmentioned above, ensure that the drill bit is withdrawn frequently (brief drilling periods) in order to improve theremoval of chips and prevent the build-up of heat.For drill holes in thin-walled workpieces, it is advisable to select a high cutting speed and where applicable aneutral (0°) rake angle. This will prevent the drill bit becoming entangled in the workpiece and tearing open the holeor lifting the workpiece.Large diameters (25mm and bigger)It is advisable when working with holes of this size to produce a pilot hole and then finish machine using an internalcutting tool / circle cutter. The pilot hole should not have a diameter greater than 25mm.Holes drilled into rod sections should be produced from one side only, in order to avoid the unfavourable stressconditions which occur when holes drilled from two sides meet, which can result in the fractures in the rod wall.In extreme cases, it can even be advisable for instance to heat the blank to around 120°C (heating time appr. 1hour per 10 mm of cross section) and to carry out pilot drilling in this condition. To ensure dimensional accuracy,finish machining then takes place after the blank has cooled down completely.

Inside sharp corners same as fillet and radius:

Generously rounded corners provide a number of advantages. There is less stress concentration on thepart and on the tool. Because of sharp corners, material flow is not smooth and tends to be difficult to fill,reduces tooling strength and causes stress concentration. Parts with radii and fillets are more economicaland easier to produce, reduce chipping, simplify mold construction and add strength to molded part withgood appearance.

Sharp Corners general design guidelines in injection molding suggest that corner radii should be at leastone-half the wall thickness. It is recommended to avoid sharp corners and use generous fillets and radiiwhenever required. During injection molding, the molten plastic has to navigate turns or corners.Rounded corners will ease plastic flow, so engineers should generously radius the corners of all parts. Incontrast, sharp inside corners result in molded-in stress particularly during the cooling process when thetop of the part tries to shrink and the material pulls against the corners. Moreover, the first rule of plasticdesign i.e. uniform wall thickness will be obeyed. As the plastic goes around a well-proportioned corner,it will not be subjected to area increases and abrupt changes in direction. Cavity packing pressure staysconsistent. This leads to a strong, dimensionally stable corner that will resist post-mold warpage.

Sharp corners increase concentrations, which are prone to air entrapments, air voids, and sink marks henceweakening the structural integrity of the plastic part. It must be eliminated using radii whenever is possible. Itis recommended that an inside radius be a minimum of one times the thickness. At corners, the suggestedinside radius is 0.5 times the material thickness and the outside radius is 1.5 times the material thickness. Abigger radius should be used if part design allows

Weld lines:

In manufacturing, the Weld line or Knit line or Meld line is the line where two flow fronts meet when there isthe inability of two or more flow fronts to "knit" together, or "weld", during the molding process. These linesusually occur around holes or obstructions and cause locally weak areas in the molded part. Knit lines areconsidered molding defects, and occur when the mold or/and material temperatures are set too low: thus thematerials will be cold when they meet, so that they do not bond perfectly. This can cause a weak area in thepart which can cause breakage when the part is under stress. There are many Computer Aided Engineeringtools that are available that can predict where these areas could occur.

Knit lines could be caused by different causes:

Low temperature of molding machine barrel Inadequate back pressure Injection pressure or injection speed is too low Low mold temperature Small injection gates and/or runners Improper location of injection gate Excessive gate land length Improper flow rate of injected materials Inconsistent process cycle

Draft angles or Taper:

Draft angle design is an important factor when designing plastic parts. Because of shrinkage of plasticmaterial, injection molded parts have a tendency to shrink onto a core. This creates higher contact pressureon the core surface and increases friction between the core and the part, thus making ejection of the part fromthe mold difficult. Hence, draft angles should be designed properly to assist in part ejection. This also reducescycle time and improves productivity. Draft angles should be used on interior and exterior walls of the partalong the pulling direction.

The minimum allowable draft angle is harder to quantify. Plastic material suppliers and molders are theauthority on what is the lowest acceptable draft. In most instances, 1degree per side will be sufficient, butbetween 2 degree and 5 degree per side would be preferable. If the design is not compatible with 1 degree,then allow for 0.5 degree on each side. Even a small draft angle, such as 0.25 degree, is preferable to noneat all.

Draft angles or Taper:

Draft angle design is an important factor when designing plastic parts. Because of shrinkage of plasticmaterial, injection molded parts have a tendency to shrink onto a core. This creates higher contact pressureon the core surface and increases friction between the core and the part, thus making ejection of the part fromthe mold difficult. Hence, draft angles should be designed properly to assist in part ejection. This also reducescycle time and improves productivity. Draft angles should be used on interior and exterior walls of the partalong the pulling direction.

The minimum allowable draft angle is harder to quantify. Plastic material suppliers and molders are theauthority on what is the lowest acceptable draft. In most instances, 1degree per side will be sufficient, butbetween 2 degree and 5 degree per side would be preferable. If the design is not compatible with 1 degree,then allow for 0.5 degree on each side. Even a small draft angle, such as 0.25 degree, is preferable to noneat all.

Draft angles or Taper:

Draft angle design is an important factor when designing plastic parts. Because of shrinkage of plasticmaterial, injection molded parts have a tendency to shrink onto a core. This creates higher contact pressureon the core surface and increases friction between the core and the part, thus making ejection of the part fromthe mold difficult. Hence, draft angles should be designed properly to assist in part ejection. This also reducescycle time and improves productivity. Draft angles should be used on interior and exterior walls of the partalong the pulling direction.

The minimum allowable draft angle is harder to quantify. Plastic material suppliers and molders are theauthority on what is the lowest acceptable draft. In most instances, 1degree per side will be sufficient, butbetween 2 degree and 5 degree per side would be preferable. If the design is not compatible with 1 degree,then allow for 0.5 degree on each side. Even a small draft angle, such as 0.25 degree, is preferable to noneat all.

Gate Location:

Each injection mold design must have a gate, or an opening that allows the molten plastic to beinjected into the cavity of the mold. Gate type, design and location can have effects on the partsuch as part packing, gate removal or vestige, cosmetic appearance of the part, and partdimensions & warping.

To avoid problems from your gate location, below are some guidelines for choosing the propergate location(s):

Place gates at the heaviest cross section to allow for part packing and minimize voids &sink.

Minimize obstructions in the flow path by placing gates away from cores & pins. Be sure that stress from the gate is in an area that will not affect part function or

aesthetics.o If you are using a plastic with a high shrink grade, the part may shrink near the gate

causing "gate pucker" if there is high molded-in stress at the gate Be sure to allow for easy manual or automatic degating. Gate should minimize flow path length to avoid cosmetic flow marks. In some cases, it may be necessary to add a second gate to properly fill the parts. If filling problems occur with thin walled parts, add flow channels or make wall thickness

adjustments to correct the flow.

Gates vary in size and shape depending upon the type of plastic being molded and the size ofthe part. Large parts will require larger gates to provide a bigger flow of resin to shorten themold time. Small gates have a better appearance but take longer time to mold or may need tohave higher pressure to fill correctly.

Gate TypesThere are two types of gates available for injection molding; manually trimmed andautomatically trimmed gates.

Manually Trimmed Gates:

These type of gates require an operator to separate the aprts from the runners manually aftereach cycle. Manually trimmed gates are chosen for several reasons:

The gate is too bulky to be automatically sheared by the machine Shear-sensitive materials such as PVC cannot be exposed to high shear rates Flow distribution for certain designs that require simultaneous flow distribution across a

wide front

Automatically Trimmed Gates

These type of gates incorporate features in the tool to break or shear the gates when the toolopens to eject the part. Automatically trimmed gates are used for several reasons:

Avoiding gate removal as a secondary operation, reducing cost Maintaining consistent cycle times for all parts Minimizing gate scars on parts

Undercuts:

In manufacturing, an undercut is a special type of recessed surface. In turning, it refers to a recess in adiameter. In machining, it refers to a recess in a corner. In molding, it refers to a feature that cannot bemolded using only a single pull mold. In printed circuit board, construction it refers to the portion of thecopper that is etched away under the photoresist. In welding, it refers to undesired melting and removalof metal near the weld bead.

Undercut - Any indentation or protrusion in a shape that will prevent its withdrawal from a one-piecemold.

Undercuts on molded parts are features that prevent the part from being directly ejected froman injection molding machine. They are categorized into internal and external undercuts, where externalundercuts are on the exterior of the part and interior undercuts are on the inside of the part. Undercutscan still be molded, but require a side action or side pull. This is an extra part of the mold that movesseparately from the two halves. These can increase the cost of the molded part due to an added 15 to30% cost of the mold itself and added complexity of the molding machine.

If the size of the undercut is small enough and the material is flexible enough a side action is not alwaysrequired. In these cases the undercut is stripped or snapped out of the mold. When this is done usually astripping plate or ring is used instead of stripper pins so that the part is not damaged. This technique canbe used on internal and external undercuts.

CONDUCTING POLYMERS

Conductive polymers or, more precisely, intrinsically conducting polymers (ICPs) are organicpolymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors.The biggest advantage of conductive polymers is their processability, mainly by dispersion. Conductivepolymers are generally not thermoplastics, i.e., they are not thermoformable. But, like insulating polymers,they are organic materials. They can offer high electrical conductivity but do not show similar mechanicalproperties to other commercially available polymers. The electrical properties can be fine-tuned using themethods of organic synthesis and by advanced dispersion techniques.

The linear-backbone "polymer blacks" (polyacetylene, polypyrrole, and polyaniline) and their copolymers arethe main class of conductive polymers. Poly(p-phenylene vinylene) (PPV) and its soluble derivatives haveemerged as the prototypical electroluminescent semiconducting polymers. Today, poly(3-alkylthiophenes)are the archetypical materials for solar cells and transistors.

Poly(fluorene)s

polyphenylenes

polypyrenes

polyazulenes

polynaphthalenes

The N is in the aromatic

cycle:

poly(pyrrole)s(PPY)

polycarbazoles

polyindoles

polyazepines

The N is outside the aromatic

cycle:

polyanilines (PANI)

The S is in the aromatic cycle:

poly(thiophene)s (PT)

poly(3,4-ethylenedioxythiophene)(PEDOT)

The S is outside the aromatic cycle:

poly(p-phenylene sulfide)(PPS)

Double bonds Poly(acetylene)s(PAC)

The conductivity of such polymers is the result of several processes. For example, in traditional polymers

such aspolyethylenes, the valence electrons are bound in sp3 hybridized covalent bonds. Such "sigma-

bonding electrons" have low mobility and do not contribute to the electrical conductivity of the material.

However, in conjugatedmaterials, the situation is completely different. Conducting polymers have backbones

of contiguous sp2 hybridized carbon centers. One valence electron on each center resides in a pz orbital,

which is orthogonal to the other three sigma-bonds. All the pz orbitals combine with each other to a molecule

wide delocalized set of orbitals. The electrons in these delocalized orbitals have high mobility when the

material is "doped" by oxidation, which removes some of these delocalized electrons. Thus, the conjugated

p-orbitals form a one-dimensional electronic band, and the electrons within this band become mobile when it

is partially emptied. The band structures of conductive polymers can easily be calculated with a tight binding

model. In principle, these same materials can be doped by reduction, which adds electrons to an otherwise

unfilled band. In practice, most organic conductors are doped oxidatively to give p-type materials. The redox

doping of organic conductors is analogous to the doping of silicon semiconductors, whereby a small fraction

silicon atoms are replaced by electron-rich, e.g., phosphorus, or electron-poor, e.g., boron, atoms to create n-

type and p-type semiconductors, respectively.

Although typically "doping" conductive polymers involves oxidizing or reducing the material, conductive

organic polymers associated with a protic solvent may also be "self-doped."

Undoped conjugated polymers state are semiconductors or insulators. In such compounds, the energy gap

can be > 2 eV, which is too great for thermally activated conduction. Therefore, undoped conjugated

polymers, such as polythiophenes, polyacetylenes only have a low electrical conductivity of around 10−10 to

10−8 S/cm. Even at a very low level of doping (< 1%), electrical conductivity increases several orders of

magnitude up to values of around 0.1 S/cm. Subsequent doping of the conducting polymers will result in a

saturation of the conductivity at values around 0.1–10 kS/cm for different polymers. Highest values reported

up to now are for the conductivity of stretch oriented polyacetylene with confirmed values of about 80

kS/cm. Although the pi-electrons in polyactetylene are delocalized along the chain, pristine polyacetylene is

not a metal. Polyacetylene has alternating single and double bonds which have lengths of 1.44 and 1.36 Å,

respectively. Upon doping, the bond alteration is diminished in conductivity increases. Non-doping increases

in conductivity can also be accomplished in a field effect transistor (organic FET or OFET) and by irradiation.

Some materials also exhibit negative differential resistance and voltage-controlled "switching" analogous to

that seen in inorganic amorphous semiconductors.

Despite intensive research, the relationship between morphology, chain structure and conductivity is still

poorly understood. Generally, it is assumed that conductivity should be higher for the higher degree of

crystallinity and better alignment of the chains, however this could not be confirmed for polyaniline and was

only recently confirmed for PEDOT, which are largely amorphous.

Properties and applications

Due to their poor processability, conductive polymers have few large-scale applications. They have promise

in antistatic materials and they have been incorporated into commercial displays and batteries, but there

have had limitations due to the manufacturing costs, material inconsistencies, toxicity, poor solubility in

solvents, and inability to directly melt process. Literature suggests they are also promising in organic solar

cells, printing electronic circuits, organic light-emitting diodes, actuators, electrochromism,

supercapacitors, chemical sensors and biosensors, flexible transparent displays, electromagnetic

shielding and possibly replacement for the popular transparent conductor indium tin oxide. Another use is

for microwave-absorbent coatings, particularly radar-absorptive coatings on stealth aircraft. Conducting

polymers are rapidly gaining attraction in new applications with increasingly processable materials with better

electrical and physical properties and lower costs. The new nanostructured forms of conducting polymers

particularly, augment this field with their higher surface area and better dispersability.

With the availability of stable and reproducible dispersions, PEDOT and polyaniline have gained some large

scale applications. While PEDOT (poly(3,4-ethylenedioxythiophene)) is mainly used in antistatic applications

and as a transparent conductive layer in form of PEDOT:PSS dispersions (PSS=polystyrene sulfonic acid),

polyaniline is widely used for printed circuit board manufacturing – in the final finish, for protecting copper

from corrosion and preventing its solderability.

Electroluminescence

light emission is observed when a voltage is applied to a thin layer of a conductive organic polymer film.

While electroluminescence was originally mostly of academic interest, the increased conductivity of modern

conductive polymers means enough power can be put through the device at low voltages to generate

practical amounts of light. This property has led to the development of flat panel displays using organic

LEDs, solar panels, and optical amplifiers.

Barriers to applications

Since most conductive polymers require oxidative doping, the properties of the resulting state are crucial.

Such materials are salt-like (polymer salt), which diminishes their solubility in organic solvents and water and

hence their processability. Furthermore, the charged organic backbone is often unstable towards

atmospheric moisture. The poor processability for many polymers requires the introduction of solubilizing or

substituents, which can further complicate the synthesis.

Experimental and theoretical thermodynamical evidence suggests that conductive polymers may even be

completely and principally insoluble so that they can only be processed by dispersion.

Polymeric membranesA membrane is an interphase between two adjacent phases acting as a selective barrier, regulating thetransport of substances between the two compartments. The main advantages of membrane technology ascompared with other unit operations in (bio)chemical engineering are related to this unique separationprinciple, i.e. the transport selectivity of the membrane. Separations with membranes do not requireadditives, and they can be performed isothermally at low temperatues and—compared to other thermalseparation processes—at low energy consumption. Also, upscaling and downscaling of membraneprocesses as well as their integration into other separation or reaction processes are easy.

Polymeric membranes lead the membrane separation industry market because they are very competitive in

performance and economics. Many polymers are available, but the choice of membrane polymer is not a

trivial task. A polymer has to have appropriate characteristics for the intended application. The polymer

sometimes has to offer a low binding affinity for separated molecules (as in the case of biotechnology

applications), and has to withstand the harsh cleaning conditions. It has to be compatible with chosen

membrane fabrication technology. The polymer has to be a suitable membrane former in terms of its chains

rigidity, chain interactions, stereoregularity, and polarity of its functional

groups. The polymers can form amorphous and semicrystalline structures (can also have different glass

transition temperatures), affecting the membrane performance characteristics. The polymer has to be

obtainable and reasonably priced to comply with the low cost criteria of membrane separation process. Many

membrane polymers are grafted, custom-modified, or produced ascopolymers to improve their

properties.[4] The most common polymers in membrane synthesis are cellulose acetate, Nitrocellulose,

and cellulose esters (CA, CN, and CE), polysulfone (PS), polyether sulfone(PES) ,polyacrilonitrile (PAN)

, polyamide , polyimide, polyethylene and polypropylene (PE and

PP), polytetrafluoroethylene(PTFE), polyvinylidene fluoride (PVDF), polyvinylchloride (PVC).

Some commonly used polymers in membrane technology are polysulfone (PSF), polyethersulfone (PES),polyacrilonitrile (PAN), polyamide (PA), polypolyethylene and polypropylene (PE and PP).

The choice for a particular polymer is not trivial. The selection of the right polymeric membrane depends on

the application and the chemical resistance level required in your separation process.

NanocompositeNanocomposite is a multiphase solid material where one of the phases has one, two or three dimensions of less than100 nanometers (nm), or structures having nano-scale repeat distances between the different phases that make up thematerial. In the broadest sense this definition can include porous media, colloids, gels andcopolymers, but is more usuallytaken to mean the solid combination of a bulk matrix and nano-dimensional phase(s) differing in properties due todissimilarities in structure and chemistry. The mechanical, electrical, thermal, optical, electrochemical, catalytic propertiesof the nanocomposite will differ markedly from that of the component materials. Size limits for these effects have beenproposed,[1] <5 nm for catalytic activity, <20 nm for making a hard magnetic material soft, <50 nm for refractiveindex changes, and <100 nm for achieving superparamagnetism, mechanical strengthening or restrictingmatrix dislocation movement.

Nanocomposites are found in nature, for example in the structure of the abalone shell and bone. The use of nanoparticle-rich materials long predates the understanding of the physical and chemical nature of these materials. Jose-Yacaman etal. [2] investigated the origin of the depth of colour and the resistance to acids and bio-corrosion of Maya blue paint,attributing it to a nanoparticle mechanism. From the mid-1950s nanoscale organo-clays have been used to control flow ofpolymer solutions (e.g. as paint viscosifiers) or the constitution of gels (e.g. as a thickening substance in cosmetics,keeping the preparations in homogeneous form). By the 1970s polymer/clay composites were the topic oftextbooks,[3] although the term "nanocomposites" was not in common use.

In mechanical terms, nanocomposites differ from conventional composite materials due to the exceptionally high surface tovolume ratio of the reinforcing phase and/or its exceptionally high aspect ratio. The reinforcing material can be made up ofparticles (e.g. minerals), sheets (e.g. exfoliated clay stacks) or fibres (e.g. carbon nanotubes or electrospun fibres). Thearea of the interface between the matrix and reinforcement phase(s) is typically an order of magnitude greater than forconventional composite materials. The matrix material properties are significantly affected in the vicinity of thereinforcement. Ajayan et al. [4] note that with polymer nanocomposites, properties related to local chemistry, degree ofthermoset cure, polymer chain mobility, polymer chain conformation, degree of polymer chain ordering or crystallinity canall vary significantly and continuously from the interface with the reinforcement into the bulk of the matrix.

This large amount of reinforcement surface area means that a relatively small amount of nanoscale reinforcement canhave an observable effect on the macroscale properties of the composite. For example, adding carbonnanotubes improves the electrical and thermal conductivity. Other kinds of nanoparticulates may result in enhanced opticalproperties, dielectric properties, heat resistance or mechanical properties such as stiffness,strength and resistance to wearand damage. In general, the nano reinforcement is dispersed into the matrix during processing. The percentage by weight(called mass fraction) of the nanoparticulates introduced can remain very low (on the order of 0.5% to 5%) due to the lowfiller percolation threshold, especially for the most commonly used non-spherical, high aspect ratio fillers (e.g. nanometer-thin platelets, such as clays, or nanometer-diameter cylinders, such as carbon nanotubes). The orientation andarrangement of asymmetric nanoparticles, thermal property mismatch at the interface, interface density per unit volume ofnanocomposite, and polydispersity of nanoparticles significanlty affect the effective thermal conductivity ofnanocomposites.

Ceramic-matrix nanocomposites

In this group of composites the main part of the volume is occupied by a ceramic, i.e. a chemical compound from the groupof oxides, nitrides, borides, silicides etc.. In most cases, ceramic-matrix nanocomposites encompass ametal as the secondcomponent. Ideally both components, the metallic one and the ceramic one, are finely dispersed in each other in order toelicit the particular nanoscopic properties. Nanocomposite from these combinations were demonstrated in improving theiroptical, electrical and magnetic properties [6] as well as tribological, corrosion-resistance and other protective properties.

The binary phase diagram of the mixture should be considered in designing ceramic-metal nanocomposites and measureshave to be taken to avoid a chemical reaction between both components. The last point mainly is of importance for themetallic component that may easily react with the ceramic and thereby lose its metallic character. This is not an easilyobeyed constraint, because the preparation of the ceramic component generally requires high process temperatures. Themost safe measure thus is to carefully choose immiscible metal and ceramic phases. A good example for such acombination is represented by the ceramic-metal composite of TiO2and Cu, the mixtures of which were found immiscibleover large areas in the Gibbs’ triangle of Cu-O-Ti.

The concept of ceramic-matrix nanocomposites was also applied to thin films that are solid layers of a few nm to some tensof µm thickness deposited upon an underlying substrate and that play an important role in the functionalization of technicalsurfaces. Gas flow sputtering by the hollow cathode technique turned out as a rather effective technique for the preparationof nanocomposite layers. The process operates as a vacuum-baseddeposition technique and is associated with highdeposition rates up to some µm/s and the growth of nanoparticles in the gas phase. Nanocomposite layers in the ceramicsrange of composition were prepared from TiO2 and Cu by the hollow cathode technique [9] that showed a high mechanicalhardness, small coefficients of friction and a highresistance to corrosion.

Metal-matrix nanocompositesMetal matrix nanocomposites can also be defined as reinforced metal matrix composites. This type of composites can beclassified as continuous and non-continuous reinforced materials. One of the more important nanocomposites is Carbonnanotube metal matrix composites, which is an emerging new material that is being developed to take advantage of thehigh tensile strength and electrical conductivity of carbon nanotube materials. Critical to the realization of CNT-MMCpossessing optimal properties in these areas are the development of synthetic techniques that are (a) economicallyproducible, (b) provide for a homogeneous dispersion of nanotubes in the metallic matrix, and (c) lead to strong interfacialadhesion between the metallic matrix and the carbon nanotubes. In addition to carbon nanotube metal matrix composites,boron nitride reinforced metal matrix composites and carbon nitride metal matrix composites are the new research areason metal matrix nanocomposites.

A recent study, comparing the mechanical properties (Young's modulus, compressive yield strength, flexural modulus andflexural yield strength) of single- and multi-walled reinforced polymeric (polypropylene fumarate—PPF) nanocomposites totungsten disulfide nanotubes reinforced PPF nanocomposites suggest that tungsten disulfide nanotubes reinforced PPFnanocomposites possess significantly higher mechanical properties and tungsten disulfide nanotubes are better reinforcingagents than carbon nanotubes.[11] Increases in the mechanical properties can be attributed to a uniform dispersion ofinorganic nanotubes in the polymer matrix (compared to carbon nanotubes that exist as micron sized aggregates) andincreased crosslinking density of the polymer in the presence of tungsten disulfide nanotubes (increase in crosslinkingdensity leads to an increase in the mechanical properties). These results suggest that inorganic nanomaterials, in general,may be better reinforcing agents compared to carbon nanotubes.

Another kind of nanocomposite is the energetic nanocomposite, generally as a hybrid sol–gel with a silica base, which,when combined with metal oxides and nano-scale aluminum powder, can form superthermitematerials.

Polymer-matrix nanocomposites

In the simplest case, appropriately adding nanoparticulates to a polymer matrix can enhance its performance, oftendramatically, by simply capitalizing on the nature and properties of the nanoscale filler (these materials are betterdescribed by the term nanofilled polymer composites ). This strategy is particularly effective in yielding high performancecomposites, when good dispersion of the filler is achieved and the properties of the nanoscale filler are substantiallydifferent or better than those of the matrix.

Nanoparticles such as graphene, carbon nanotubes, molybdenum disulfide and tungsten disulfide are being used asreinforcing agents to fabricate mechanically strong biodegradable polymeric nanocomposites for bone tissue engineeringapplications. The addition of these nanoparticles in the polymer matrix at low concentrations (~0.2 weight %) causesignificant improvements in the compressive and flexural mechanical properties of polymeric nanocomposites. Potentially,these nanocomposites may be used as a novel, mechanically strong, light weight composite as bone implants. The resultssuggest that mechanical reinforcement is dependent on the nanostructure morphology, defects, dispersion ofnanomaterials in the polymer matrix, and the cross-linking density of the polymer. In general, two-dimensionalnanostructures can reinforce the polymer better than one-dimensional nanostructures, and inorganic nanomaterials arebetter reinforcing agents than carbon based nanomaterials. In addition to mechanical properties, multi-walled carbonnanotubes based polymer nanocomposites have also been used for the enhancement of the electrical conductivity.

Nanoscale dispersion of filler or controlled nanostructures in the composite can introduce new physical properties andnovel behaviors that are absent in the unfilled matrices. This effectively changes the nature of the original matrix (suchcomposite materials can be better described by the term genuine nanocomposites or hybrids. Some examples of suchnew properties are fire resistance or flame retardancy, and accelerated biodegradability.

A range of polymeric nanocomposites are used for biomedical applications such as tissue engineering, drug delivery,cellular therapies. Due to unique interactions between polymer and nanoparticles, a range of property combinations can beengineered to mimic native tissue structure and properties. A range of natural and synthetic polymers are used to design

polymeric nanocomposites for biomedical applications including starch, cellulose, alginate, chitosan, collagen, gelatin, andfibrin, poly(vinyl alcohol) (PVA), poly(ethylene glycol) (PEG), poly(caprolactone) (PCL), poly(lactic-co-glycolic acid) (PLGA),and poly(glycerol sebacate) (PGS). A range of nanoparticles including ceramic, polymeric, metal oxide and carbon-basednanomaterials are incorporated within polymeric network to obtain desired property combinations.

Polymer nanocomposites (PNC) consist of a polymer or copolymer having nanoparticles or nanofillers dispersed in thepolymer matrix. These may be of different shape (e.g., platelets, fibers, spheroids), but at least one dimension must be inthe range of 1–50 nm. These PNC's belong to the category of multi-phase systems (MPS, viz. blends, composites, andfoams) that consume nearly 95% of plastics production. These systems require controlled mixing/compounding,stabilization of the achieved dispersion, orientation of the dispersed phase, and the compounding strategies for all MPS,including PNC, are similar.

Polymer nanoscience is the study and application of nanoscience to polymer-nanoparticle matrices, where nanoparticlesare those with at least one dimension of less than 100 nm.

The transition from micro- to nano-particles lead to change in its physical as well as chemical properties. Two of the majorfactors in this are the increase in the ratio of the surface area to volume, and the size of the particle. The increase insurface area-to-volume ratio, which increases as the particles get smaller, leads to an increasing dominance of thebehavior of atoms on the surface area of particle over that of those interior of the particle. This affects the properties of theparticles when they are reacting with other particles. Because of the higher surface area of the nano-particles, theinteraction with the other particles within the mixture is more and this increases the strength, heat resistance, etc. andmany factors do change for the mixture.

An example of a nanopolymer is silicon nanospheres which show quite different characteristics; their size is 40–100 nmand they are much harder than silicon, their hardness being between that of sapphire and diamond.

Bio-hybrid polymer nanofibersMany technical applications of biological objects like proteins, viruses or bacteria such as chromatography, opticalinformation technology, sensorics, catalysis and drug delivery require their immobilization. Carbon nanotubes, goldparticles and synthetic polymers are used for this purpose. This immobilization has been achieved predominantly byadsorption or by chemical binding and to a lesser extent by incorporating these objects as guests in host matrices. In theguest host systems, an ideal method for the immobilization of biological objects and their integration into hierarchicalarchitectures should be structured on a nanoscale to facilitate the interactions of biological nano-objects with theirenvironment. Due to the large number of natural or synthetic polymers available and the advanced techniques developedto process such systems to nanofibres, rods, tubes etc. make polymers a good platform for the immobilization of biologicalobjects.

Bio-hybrid nanofibres by electrospinningPolymer fibers are, in general, produced on a technical scale by extrusion, i.e., a polymer melt or a polymer solution ispumped through cylindrical dies and spun/drawn by a take-up device. The resulting fibers have diameters typically on the10-µm scale or above. To come down in diameter into the range of several hundreds of nanometers or even down to a fewnanometers, Electrospinning is today still the leading polymer processing technique available. A strong electric field of theorder of 103 V/cm is applied to the polymer solution droplets emerging from a cylindrical die. The electric charges, whichare accumulated on the surface of the droplet, cause droplet deformation along the field direction, even though the surfacetension counteracts droplet evolution. In supercritical electric fields, the field strength overbears the surface tension and afluid jet emanates from the droplet tip. The jet is accelerated towards the counter electrode. During this transport phase,the jet is subjected to strong electrically driven circular bending motions that cause a strong elongation and thinning of thejet, a solvent evaporation until, finally, the solid nanofibre is deposited on the counter electrode.

Bio-hybrid polymer nanotubes by wettingElectro spinning, co-electrospinning, and the template methods based on nanofibres yield nano-objects which are, inprinciple, infinitively long. For a broad range of applications including catalysis, tissue engineering, and surface modificationof implants this infinite length is an advantage. But in some applications like inhalation therapy or systemic drug delivery, awell-defined length is required. The template method to be described in the following has the advantage such that it allowsthe preparation of nanotubes and nanorods with very high precision. The method is based on the use of well definedporous templates, such as porous aluminum or silicon.

The basic concept of this method is to exploit wetting processes. A polymer melt or solution is brought into contact withthe pores located in materials characterized by high energy surfaces such as aluminum or silicon. Wetting sets in andcovers the walls of the pores with a thin film with a thickness of the order of a few tens of nanometers.

Gravity does not play a role, as it is obvious from the fact that wetting takes place independent of the orientation of thepores relative to the direction of gravity. The exact process is still not understood theoretically in detail but its known fromexperiments that low molar mass systems tend to fill the pores completely, whereas polymers of sufficient chain length justcover the walls. This process happens typically within a minute for temperatures about 50 K above the melting temperatureor glass transition temperature, even for highly viscous polymers, such as, for instance, polytetrafluoroethylene, and thisholds even for pores with an aspect ratio as large as 10,000. The complete filling, on the other hand, takes days. To obtainnanotubes, the polymer/template system is cooled down to room temperature or the solvent is evaporated, yielding porescovered with solid layers. The resulting tubes can be removed by mechanical forces for tubes up to 10 µm in length, i.e., byjust drawing them out from the pores or by selectively dissolving the template. The diameter of the nanotubes, thedistribution of the diameter, the homogeneity along the tubes, and the lengths can be controlled.

ApplicationsThe nanofibres, hollow nanofibres, core–shell nanofibres, and nanorods or nanotubes produced have a great potential fora broad range of applications including homogeneous and heterogeneous catalysis, sensorics, filter applications, andoptoelectronics. Here we will just consider a limited set of applications related to life science.

Tissue engineering

This is mainly concerned with the replacement of tissues which have been destroyed by sickness or accidents or otherartificial means. The examples are skin, bone, cartilage, blood vessels and may be even organs. This technique involvesproviding a scaffold on which cells are added and the scaffold should provide favorable conditions for the growth of thesame. Nanofibres have been found to provide very good conditions for the growth of such cells, one of the reasons beingthat fibrillar structures can be found on many tissues which allow the cells to attach strongly to the fibers and grow alongthem as shown.

Nanoparticles such as graphene, carbon nanotubes, molybdenum disulfide and tungsten disulfide are being used asreinforcing agents to fabricate mechanically strong biodegradable polymeric nanocomposites for bone tissue engineeringapplications. The addition of these nanoparticles in the polymer matrix at low concentrations (~0.2 weight %) leads tosignificant improvements in the compressive and flexural mechanical properties of polymericnanocomposites.[2][3] Potentially, these nanocomposites may be used as a novel, mechanically strong, light weightcomposite as bone implants. The results suggest that mechanical reinforcement is dependent on the nanostructuremorphology, defects, dispersion of nanomaterials in the polymer matrix, and the cross-linking density of the polymer. Ingeneral, two-dimensional nanostructures can reinforce the polymer better than one-dimensional nanostructures, andinorganic nanomaterials are better reinforcing agents than carbon based nanomaterials.

Delivery from compartmented nanotubes

Nano tubes are also used for carrying drugs in general therapy and in tumor therapy in particular. The role of them is toprotect the drugs from destruction in blood stream, to control the delivery with a well-defined release kinetics, and in idealcases, to provide vector-targeting properties or release mechanism by external or internal stimuli.

Rod or tube-like, rather than nearly spherical, nanocarriers may offer additional advantages in terms of drug deliverysystems. Such drug carrier particles possess additional choice of the axial ratio, the curvature, and the“all-sweeping”hydrodynamic-related rotation, and they can be modified chemically at the inner surface, the outer surface, and at theend planes in a very selective way. Nanotubes prepared with a responsive polymer attached to the tube opening allow thecontrol of access to and release from the tube. Furthermore, nanotubes can also be prepared showing a gradient in itschemical composition along the length of the tube.

Compartmented drug release systems were prepared based on nanotubes or nanofibres. Nanotubes and nanofibres, forinstance, which contained fluorescent albumin with dog-fluorescein isothiocyanate were prepared as a model drug, as wellas super paramagnetic nanoparticles composed of iron oxide or nickel ferrite. The presence of the magnetic nanoparticlesallowed, first of all, the guiding of the nanotubes to specific locations in the body by external magneticfields. Super paramagnetic particles are known to display strong interactions with external magnetic fields leading tolarge saturation magnetizations. In addition, by using periodically varying magnetic fields, the nanoparticles were heated up

to provide, thus, a trigger for drug release. The presence of the model drug was established by fluorescence spectroscopyand the same holds for the analysis of the model drug released from the nanotubes.

Immobilization of proteins

Core shell fibers of nano particles with fluid cores and solid shells can be used to entrap biological objects such asproteins, viruses or bacteria in conditions which do not affect their functions. This effect can be used among others forbiosensor applications. For example, Green Fluorescent Protein is immobilized in nanostructured fibres providing largesurface areas and short distances for the analyte to approach the sensor protein.

With respect to using such fibers for sensor applications fluorescence of the core shell fibers was found to decay rapidly asthe fibers were immersed into a solution containing urea: urea permeates through the wall into the core where i t causesdenaturation of the GFP. This simple experiment reveals that core–shell fibers are promising objects for preparingbiosensors based on biological objects.

Polymer nanostructured fibers, core–shell fibers, hollow fibers, and nanorods and nanotubes provide a platform for a broadrange of applications both in material science as well as in life science. Biological objects of different complexity andsynthetic objects carrying specific functions can be incorporated into such nanostructured polymer systems while keepingtheir specific functions vital. Biosensors, tissue engineering, drug delivery, or enzymatic catalysis is just a few of thepossible examples. The incorporation of viruses and bacteria all the way up to microorganism should not really pose aproblem and the applications coming from such biohybrid systems should be tremendous.

Bio polymer:

Biopolymers are polymers produced by living organisms; in other words, they are polymeric biomolecules. Since theyare polymers, biopolymers containmonomeric units that are covalently bonded to form larger structures. There are threemain classes of biopolymers, classified according to the monomeric units used and the structure of the biopolymerformed: polynucleotides (RNA andDNA), which are long polymers composed of 13 ormore nucleotide monomers;polypeptides, which are short polymers of amino acids; and polysaccharides, which are oftenlinear bonded polymeric carbohydrate structures.

Cellulose is the most common organic compound and biopolymer on Earth. About 33 percent of all plant matter iscellulose. The cellulose content of cotton is 90 percent, while wood's is 50 percent.

A major defining difference betweenbiopolymers and other polymers can be found in their structures. All polymers aremade of repetitive units calledmonomers. Biopolymers often have a well-defined structure, though this is not a definingcharacteristic (example:lignocellulose): The exact chemical composition and the sequence in which these units arearranged is called theprimary structure, in the case of proteins. Many biopolymers spontaneously fold into characteristiccompact shapes (see also "protein folding" as well as secondary structure and tertiary structure), which determine theirbiological functions and depend in a complicated way on their primary structures. Structural biology is the study of thestructural properties of the biopolymers. In contrast, most synthetic polymers have much simpler and more random (orstochastic) structures. This fact leads to a molecular mass distribution that is missing in biopolymers. In fact, as theirsynthesis is controlled by a template-directed process in most in vivo systems, all biopolymers of a type (say one specificprotein) are all alike: they all contain the similar sequences and numbers of monomers and thus all have the same mass.This phenomenon is called monodispersity in contrast to the polydispersityencountered in synthetic polymers. As a result,biopolymers have a polydispersity index of 1.

PolypeptidesThe convention for a polypeptide is to list its constituent amino acid residues as they occur from the amino terminus to thecarboxylic acid terminus. The amino acid residues are always joined by peptide bonds. Protein, though used colloquially torefer to any polypeptide, refers to larger or fully functional forms and can consist of several polypeptide chains as well assingle chains. Proteins can also be modified to include non-peptide components, such as saccharide chains and lipids.

Nucleic acidsThe convention for a nucleic acid sequence is to list the nucleotides as they occur from the 5' end to the 3' end ofthe polymer chain, where 5' and 3' refer to the numbering of carbons around the ribose ring which participate in forming thephosphate diester linkages of the chain. Such a sequence is called the primary structure of the biopolymer.

SugarsSugar-based biopolymers are often difficult with regards to convention. Sugar polymers can be linear or branched and aretypically joined with glycosidic bonds. The exact placement of the linkage can vary, and the orientation of the linkingfunctional groups is also important, resulting in α- and β-glycosidic bonds with numbering definitive of the linking carbons'location in the ring. In addition, many saccharide units can undergo various chemical modification, such as amination, andcan even form parts of other molecules, such as glycoproteins.

Biopolymers as materials

Some biopolymers- such as (PLA), naturally occurring zein, and poly-3-hydroxybutyrate can be used as plastics, replacingthe need for polystyrene or polyethylene based plastics.

Some plastics are now referred to as being 'degradable', 'oxy-degradable' or 'UV-degradable'. This means that they breakdown when exposed to light or air, but these plastics are still primarily (as much as 98 per cent) oil-based and are notcurrently certified as 'biodegradable' under the European Union directive on Packaging and Packaging Waste (94/62/EC).Biopolymers will break down, and some are suitable for domestic composting.

Biopolymers (also called renewable polymers) are produced from biomass for use in the packaging industry. Biomasscomes from crops such as sugar beet, potatoes or wheat: when used to produce biopolymers, these are classified as nonfood crops. These can be converted in the following pathways:

Sugar beet > Glyconic acid > Polyglyconic acid

Starch > (fermentation) > Lactic acid > Polylactic acid (PLA)

Biomass > (fermentation) > Bioethanol > Ethene > Polyethylene

Many types of packaging can be made from biopolymers: food trays, blown starch pellets for shipping fragile goods, thinfilms for wrapping.

Environmental impactsBiopolymers can be sustainable, carbon neutral and are always renewable, because they are made from plant materialswhich can be grown indefinitely. These plant materials come from agricultural non food crops. Therefore, the use ofbiopolymers would create a sustainable industry. In contrast, the feedstock’s for polymers derived from petrochemicals willeventually deplete. In addition, biopolymers have the potential to cut carbon emissions and reduce CO2 quantities in theatmosphere: this is because the CO2 released when they degrade can be reabsorbed by crops grown to replace them: thismakes them close to carbon neutral.

Biopolymers are biodegradable, and some are also compostable. Some biopolymers are biodegradable: they are brokendown into CO2 and water by microorganisms. Some of these biodegradable biopolymers are compostable: they can be putinto an industrial composting process and will break down by 90% within six months. Biopolymers that do this can bemarked with a 'compostable' symbol, under European Standard EN 13432 (2000). Packaging marked with this symbol canbe put into industrial composting processes and will break down within six months or less. An example of a compostablepolymer is PLA film under 20μm thick: films which are thicker than that do not qualify as compostable, even though theyare biodegradable. In Europe there is a home composting standard and associated logo that enables consumers to identifyand dispose of packaging in their compost heap.

A polymer blend or polymer mixture is a member of a class of materials analogous to metal alloys,in which at least two polymers are blended together to create a new material with different physicalproperties.

During the 1940s, '50s and '60s, the commercial development of the new monomers for productionof the new polymers seemed endless. In this period, it was discovered that the development of thenew techniques for the modification of the already existing polymers, would be economicallyviable.The first technique of modification developed was the polymerization, in other words, the jointpolymerization of more than one kind of polymer.A new polymers modification process, based on a simple mechanical mixture of two polymers firstappeared when Thomas Hancock got one mixture of natural rubber with Gutta-percha. Thisprocess generated a new polymer class called Polymer Blends.

Basic concepts

Polymer blends can be broadly divided into three categories:

Immiscible polymer blends (heterogeneous polymer blends): This is by far the most populousgroup. If the blend is made of two polymers, two glass transition temperatures will be observed.

Compatible polymer blends: Immiscible polymer blend that exhibits macroscopically uniformphysical properties. The macroscopically uniform properties are usually caused by sufficientlystrong interactions between the component polymers.

Miscible polymer blends (homogeneous polymer blend): Polymer blend that is a single-phasestructure. In this case, one glass transition temperature will be observed.

The use of the term polymer alloy for a polymer blend is discouraged, as the former term includesmultiphase copolymers but excludes incompatible polymer blends.Examples of miscible polymer blends:

Homopolymer - Homopolymer: Polyphenylene oxide (PPO) - polystyrene (PS): Noryl developed by General

Electric Plastics in 1966. The miscibility of the two polymers in Noryl is caused by thepresence of an aromatic ring in the repeat units of both chains;

Polyethylene terephthalate (PET) - Polybutylene terephthalate (PBT); Poly(methyl methacrylate) (PMMA) - Polyvinylidene fluoride (PVDF);

Homopolymer - Copolymer: Polypropylene (PP) - EPDM; Polycarbonate (PC) - Acrylonitrile butadiene styrene (ABS):

Polymer blends can be used as thermoplastic elastomer.

What Is an Immiscible Blend?Long ago someone came up with the idea of taking two polymers and mixing them together inorder to get a material with properties somewhere between those of the two polymers mixed.Materials made from two polymers mixed together are called blends. But if you read the blendspage, you'll know that it's not very often that two polymers will mix with each other. Most of thetime, if you try to mix two kinds of polymers, you'll end up with something that looks like chickensoup. Take a look at a bowl of good chicken soup, and you'll see that it has two phases: a waterphase and a chicken fat phase. The chicken fat is insoluble in water, so it forms little blobs in thesoup separate form the water phase. So we say mixtures like chicken soup are phase-separated.

Phase-separated mixtures are just what you get when you try to mix most polymers. But strangelyenough, the phase-separated materials often turn out to be rather nifty and useful. We even havea name for them. We call them immiscible blends. Ok, I have to admit, that name is an oxymoron.These materials aren't really blends; they can't be if they're immiscible, but that's the name peopleuse.

Anyway...Immiscible blends turn out to be useful, as I was saying. Would you like an example?Alright, then consider polystyrene and polybutadiene. These two polymers are immiscible. Whenyou mix polystyrene with a small amount of polybutadiene, the two polymers won't blend, ofcourse. Instead, the polybutadiene will separate from the polystyrene into little spherical blobs, justlike the chicken fat in your soup separates from the water into little blobs. If you looked at themixture under an electron microscope you'd see something that looks like the picture below.

The phase morphology of HIPS.

The little spheres of polybutadiene do a lot for the material. You see, polystyrene is a rather brittlematerial. It's stiff, but you can break it easily if you try to bend it. But those little polybutadienespheres are rubbery, remember, and they can absorb energy under stress. This keeps thepolystyrene from breaking. This immiscible blend has more ability to bend instead of breaking thanregular polystyrene. That is, it's tougher and more ductile. Immiscible blends of polystyrene andpolybutadiene are sold commercially under the name high-impact polystyrene, or HIPS for short.

Another immiscible blend you may be familiar with is one made froma polyester called poly(ethylene terephthalate) and poly(vinylalcohol). We call them PET and PVA, respectively, for short. If youput just the right amounts of the two polymers together under the rightconditions you'll get something that looks like the picture on the left,when you look at it under an electron microscope.

In this material, PET and PVA separate into sheetlike layers called lamellae. We call the resultingarrangement lamellar morphology. This particular immiscible blend is used to make plastic bottlesfor carbonated beverages. The PET makes the bottle strong, while the layers of PVA do somethingvery important if you want your sodas to stay fizzy. Carbon dioxide can't pass through PVA. If thecarbon dioxide in your soda leaked out (it can pass easily through plain PET), your soda would goflat.

MorphologyI hope you folks are asking a question right now. Did you notice a difference between the twoimmiscible blends we just talked about? Did you notice that in HIPS, one polymer forms into littlespheres dispersed in the polystyrene? Did you also notice that in the PET-PVA system, the twopolymers separated into layers? Why are the two different? Why do they separate in differentfashions?

We call the shape made by the two phases, and the arrangement of the two phases morphology.The biggest thing one can do to affect the morphology of an immiscible blend is to control therelative amounts of the two polymers one is using. Let's say you're trying to make an immiscibleblend from two polymers, polymer A and polymer B. If you have a lot more of polymer A thanpolymer B, polymer B separate into little spherical globs. The spheres of polymer B will beseparated from each other by a sea of polymer A, like you see in the picture below. In such a casewe call polymer A the major component and polymer B the minor component.

relative amount of polymer B in the immiscible blend

But if you put more polymer B into the immiscible blend, the spheres will get bigger and bigger,until they get so big that they become joined together. Now they aren't isolated spheres anymore,but a continuous phase. The immiscible blend now looks like the middle picture above. It mighthelp to think of a block of colby-jack cheese. The domains of polymer B are now joined together,but so are the domains of polymer A. When this happens we say that the polymer A phase and thepolymer B phase are co-continuous.

But if we keep adding more polymer B, eventually there will be so much more polymer B in theimmiscible blend that polymer A will become nothing but isolated spheres surrounded by acontinuous phase of polymer just like you see in the picture above on the right. Polymer B is nowthe major component and polymer A is the minor component, and the situation is reversed fromwhat we had at first.

Spheres, co-continuity, then more spheres...so how do we get theneat layers that you get in the PET-PVA immiscible blend?Sometimes the way in which a product is processed affects themorphology of the material. soft drink bottles are made by atechnique called blow molding. To make a bottle we take a smallpiece of plastic that looks like a test tube, about 1 inch (2.5 cm) indiameter and maybe 6 inches (15 cm) long. We heat the tube up,then inflate it like a balloon until it is the size we want it. Thiswhole procedure puts the material under stress. Think about asection of the skin of the bottle. When it is being inflated, it is putunder stress in two directions, like you see in this picture on the right.

This is called biaxial stress, and it causes the domains of PET and the domains of PVA to flattenout, just like pizza dough does when you roll it. This is how we get flat layers instead of spheres inour immiscible blend.

Processing under flow in one direction turns the spheres into rods.

Another interesting morphology you can get is one of rod-like domains of one polymer surroundedby a continuous phase of the other. This happens when the immiscible blend is put under stress inonly one direction, such as during extrusion.

Ok, we have to talk about one last thing when we're talking about morphology, and that is size.Let's go back to that simple case we talked about earlier, where we had spheres of polymer Bsurrounded by continuous phase of polymer A. How big are the spheres? How far apart are they?Could you see them if you looked at a sample of an immiscible blend?

I hate to disappoint everyone, but in most cases you're not going to be able to see the twoseparate phases with your own eyes. In fact, it usually takes an electron microscope. So the phasedomains, spherical or otherwise, are very small.

But the domains do try to be as big as they can. Take our spheres for example. The bigger thespheres are, the less surface area they will have. A few bigger spheres have less surface areathan a bunch of little ones. The less surface area, the better. Remember, the two polymers in animmiscible blend don't like each other, and the smaller the surface area of the spheres, the lessthe two polymers have to touch each other.

I figure you're going to want some numbers, so I'll tell you some specifics on an 80:20 immiscibleblend of high density polyethylene and polystyrene. Polystyrene is the minor component here, so itwill form the separated spherical domains, and they tend to be in the range of 5-10 mm indiameter.1

Properties of Immiscible BlendsHow do these immiscible blends behave? They have to behave in some interesting manner, orelse nobody would make them, and people like me wouldn't write about them.

One unusual property of immiscible blends is that one made from two amorphous polymers hastwo glass transition temperatures or Tgs for short. Since the two components are phaseseparated, the retain their separate Tgs. In fact, scientists often measure the Tg of a blend to findout if it is miscible or immiscible. If two Tgs are found, then the blend is immiscible. If only one Tgis observed, then the blend is likely to be miscible.

But what about mechanical properties? Let's consider an immiscible blend of a major componentpolymer A and a minor component polymer B, whose morphology is that of spheres of polymer Bdispersed in a matrix of polymer A. The mechanical properties of this immiscible blend are going todepend on those of polymer A, because the polymer A phase is absorbing all the stress andenergy when the material is under load. In addition, the immiscible blend is going to be weakerthan a sample of pure polymer A.

So why make immiscible blends then, if separate materials arestronger? It turn out there are some tricks one can do to makeimmiscible blends strong. One is to process them under flow. Ifwe process them under flow in one direction, the minorcomponent will form rods instead of spheres, Like you see inthe picture on your right. These rods act like the fibers of areinforced composite material. They make the material stronger in the direction of the rods.

Another way to make a strong immiscible blend is to use more equal amounts of the two polymers.Remember, when the relative amounts of the two polymers are equal, we get a differentmorphology than when one is in large excess. When polymer A and polymer B are present inroughly equal amounts, they form two co-continuous phases. This means both phases will bebearing the load of any stress on the material, so it will be stronger.

But one of the most interesting ways to make immiscible blends stronger is to use a compatibilizer.So what is a compatibilizer? A compatibilizer is anything that helps bond the two phases to eachother more tightly. You see, in an immiscible blend, the two phases are not bonded very strongly toeach other. Remember, they don't like each other, and that is why they are immiscible in the firstplace. But if stress and energy are going to be transferred between the components, they have to

be bound to each other in some fashion.

That's where compatibilizers come in. Often times acompatibilizer is a block copolymerof the twocomponents of the immiscible blend. Let's take ourexample of an immiscible blend of polymer A andpolymer B again. Let's make polymer A the major

component and polymer B the minor component, and then let's throw in a block copolymer of Aand B. For those of you who may not no, a block copolymer of A and B is a polymer with one longsegment of polymer A joined to another long segmentof polymer B, like you can see on your right.

Of course, the A block is going to want to be in thepolymer A phase, and the B block is going to want tobe in the polymer B phase. So the copolymer moleculehas to sit right on the phase boundary between the

polymer A and the polymer B phases. The A block can then be happy sitting in the polymer Aphase, and the B block is happy because it can stay in the polymer B phase, as you can see in thepicture on your left.

The block copolymers tie the two phases together, and allow energy to be transferred from onephase to the other. This means that the minor component can improve the mechanical propertiesof the major component rather than worsen them.

Graft copolymers are also used as compatibilizers. HIPS contains graft copolymers of polystyrenegrafted onto a polybutadiene backbone chain. These graft copolymers allow stress to betransferred from the polystyrene phase to the polybutadiene phase. Since polybutadiene isrubbery, it dissipates the energy which would otherwise cause the brittle polystyrene phase tobreak. This is why HIPS is tougher than regular polystyrene.

Compatibilizers also have another effect on immiscible blends. Remember, we talked earlier aboutthe size of spheres of the minor component in an immiscible blend. The bigger the spheres, themore stable they are, because a few larger spheres will have less surface area than lots and lotsof small ones. As the two polymers don't like each other, they try to minimize contact. The lesssurface area the spheres have, the less contact the two phases have. This means the spheres willtend to be relatively large.

But a compatibilizer lowers the energy of the phase boundary, as we say. What we mean is thatthe two phases can stand each other a little more when there is a compatibilizer present. So theneed to minimize contact between the two phases isn't as great. So when a compatibilizer is used,our spheres don't need to be as big. Remember our 80:20 immiscible blend of high densitypolyethylene and polystyrene? The polystyrene spheres were about 5-10 mm in diameter. Whenenough of a polystyrene-polyethylene block copolymer (enough being 9%) is added to theimmiscible blend, the size of the polystyrene spheres drops to about 1 mm.1

This is good for the mechanical properties of the immiscible blend. The smaller the spheres, thegreater the area of the phase boundary between the two phases, of course. The greater the areaof the phase boundary, the more efficiently energy can be transferred from one phase to the other,meaning better mechanical properties.

Keywordsamorphous, copolymer, entropy, hydrogen bonding

Sometimes we want a material that has the some of the properties of one polymer, and some of theproperties of another. Instead of going back into the lab and trying to synthesize a brand new polymer withall the properties we want, we try to mix two polymers together to form a blend that will hopefully have someproperties of both in the right combination.

Polymers Usually Don't MixSounds easy enough, but it turns out that blending two different kinds of polymers can be really trickybusiness. You see, very seldom is it that two different kinds of polymers will mix together. This doesn'tseem to make sense. Take a look at polyethylene and polypropylene below. Click on the model images ifyou want to play with the 3D model of each polymer.

Would you believe that these two polymers don't mix? Why is that? What happened to the old "likedissolves like" rule that you learned in high school chemistry? These are both very non-polar hydrocarbonpolymers. They should mix beautifully.

But they don't. And yes, there is a reason why. It has to do with that old culprit entropy. Entropy isthe name we scientists call disorder. This dog is named Entropy. Just say the word "Frisbee"around her and you'll get a good demonstration of what entropy is.

This brings us to a little rule we call the second law of thermodynamics. The second law ofthermodynamics says that when things change, they will change from a state of order to a state ofdisorder. Getting things to change in the other direction is very difficult. It's easy to mess up yourroom, but difficult to clean it up. It's easy to crash a car, but fixing it is much trickier. A change, inyour room, in life, in polymers, is more likely to happen if that which is changing changes from astate of more order to a state of less order; that is, if it changes from a state of less entropy tomore entropy.

So what does entropy have to do with polymer blends? This will take some explaining. Considerone type of polymer, in the amorphous state. When it's alone, by itself, all its chains are tangled upin each other randomly and chaotically. Entropy runs high in an amorphous polymer.

This presents a problem if you're trying to make polymer blends. You see, one of the biggestreasons two compounds will ever mix together is that they are more disordered mixed togetherthan they are separate. So, mixing is favored by the second law of thermodynamics. But anamorphous polymer is so disordered as it is, that it really doesn't gain that much entropy when it'sblended with another polymer. So, mixing is disfavored.

Making Polymers Mix This presents a challenge to would-be polymer blenders. Without entropy tomake polymers blend, how can we ever get two polymers to mix? To make that happen, we haveto go back to go back to the first law of thermodynamics. Aha! Just like lawyers, we can use onelaw to get around another. The first law of thermodynamics says that when things change, theychange from a state of more energy to a state of less energy. Think of it this way: it's easier to goto sleep than it is to get out of bed in the morning. Or if you'd like a physics example, a rock on topof a mountain will roll down to the bottom of the mountain more easily than a rock on the bottomwill roll to the top. (I learned this the hard way when I was mountain climbing one summer andalmost got killed by a falling boulder.)

What, then, does this first law of thermodynamics have to do with blending polymers? This: inorder to make two polymers mix, we have to make them have less energy when mixed than theywould be separate. Let me use an example to illustrate. Two polymers that do actually mixare polystyrene and poly(phenylene oxide). Again, you can view the 3D models by clicking on themodel images of the two polymers, right and left.

As you can see, both of these polymers have aromatic rings. As you may know, aromatic rings like to stackup like little hexagonal poker chips. For this reason, these two polymers like to associate with each other.So they blend very nicely.

There are a few other examples of polymer pairs which will blend. Here is a list of a few:

poly(ethyleneterephthalate) with poly(butyleneterephthalate)poly(methyl methacrylate) with poly(vinylidene fluoride)

Copolymers

But most of the time, the two polymers you want to blend won't be miscible. So you have to playsome tricks on them to make them mix. One is to use copolymers. Polystyrene doesn't blend withmany polymers, but if we use a copolymer made from styrene and p-(hexafluoro-2-hydroxyisopropyl)styrene, blending is a lot easier.

You see, those fluorine atoms are very electronegative, and they're going to draw electrons away from allthe nearby atoms. This leaves the alcohol hydrogen very lacking in electrons, which means it is left with apartial positive charge. So that hydrogen will form strong hydrogen bonds with any group with a partialnegative charge. Because of this, it's easy to form blends of this copolymerwith polycarbonates, poly(methyl methacrylate), and poly(vinyl acetate).2,3

There's another way copolymers can be used to help polymers blend. Let's consider a randomcopolymer of styrene and acrylonitrile. This copolymer will blend with poly(methylmethacrylate) (PMMA). This is where it gets weird. PMMA won't blend witheither polystyrene or polyacrylonitrile.

So why does the random copolymer blend with PMMA? The explanation is something like this: the styrenesegments and the acrylonitrile segments of the random copolymer may not like PMMA, but they like eachother even less. The styrene segments are non-polar, while the acrylonitrile segments are very polar. So,the styrene segments and the acrylonitrile segments blend into the PMMA to avoid coming into contact witheach other.

Making Your Own BlendsBlends are usually made in two ways. The first way is to dissolve two polymers in the same solvent, andthen wait for the solvent to evaporate. When the solvent has all gone away, you'll be left with a blend at thebottom of your beaker, presuming your two polymers are miscible.

While this method works fine in the laboratory, it could get expensive if you tried to do thisindustrially. Solvents aren't cheap, and if you're going to evaporate hundreds or thousands of

gallons of them, you'll be paying a lot of money. Not to mention the effects on the environment ofputting so much of your toxic solvents into the air, or the extra cost of recapturing all that solventso it could be reused.

So for making blends in large amounts, you heat the two polymers together until you're abovethe glass transition temperatures of both polymers. At this point they will be nice and gooey, andyou can mix them together like a cake mix. This is often done in machines such as extruders.When your material cools, you'll have a nice blend, again, presuming your two polymers aremiscible.

Properties of BlendsSo what are these blends like? How do they behave? In general, a miscible blend of two polymers is goingto have properties somewhere between those of the two unblended polymers. Take for example the glasstransition temperature, or Tg for short. If we take polymer A and blend it with polymer B, the Tg will dependon the ratio of polymer A to polymer B in the blend. You can see this in the graph below.

If polymer B has a higher Tg than polymer A, the Tg of the blend is going to increase as the relative amountof polymer B in the blend increases. The increase is generally linear, like you see in the graph. But the plotisn't perfectly linear. Sometimes if the two polymers bind more strongly to each other than to themselves,the Tg will be higher than expected, because the stronger binding lowers chain mobility. The plot will looklike you see in the graph on the right below.

Of course, in most cases, the two polymers bind less strongly with each other than with themselves, sothe Tgs of the blends are usually a little lower than expected. The Tg plot will look like the one you see aboveon the left.

We've been talking about Tgs up until now, but what holds for Tgs generally holds for otherproperties. Mechanical properties, resistance to chemicals, radiation, or heat; they all generallyplot the same way as the Tg does with respect to the relative amounts of each polymer in theblend.

This makes altering the properties of your blend fairly simple. When you vary the amount of thetwo polymers, you vary the properties. This can be very useful. I'll use the example ofpoly(phenylene oxide), a.k.a. PPO, to illustrate. PPO is a very heat resistant polymer. This is

wonderful. People need heat resistant materials. But it has some drawbacks. It's very hard toprocess. You see, it's too heat resistant. Amorphous polymers are usually processed by heatingthem above their Tgs so they get soft and gooey. But with a Tg of 210 oC, heating PPO enough tomake it soft and gooey is not only difficult, but expensive.

Enter polystyrene. Remember, polystyrene and PPO blend nicely with each other. Sincepolystyrene has a Tg of only about 100 oC, blending polystyrene with PPO drops the Tg of the blenddown to temperatures which make the blend much more processable than straight PPO.

Here's a nifty piece of information: NorylTM, the PPO/polystyrene blend that GE sells, uses a specialkind of polystyrene, called high-impact polystyrene, or HIPS for short. HIPS is really a mixture ofpolystyrene and polybutadiene. These two polymers don't blend. The rubbery polybutadieneseparates from the polystyrene. But the little blobs of rubbery polybutadiene make HIPS, andNorylTM, a lot tougher. We call a mixture of two polymers like polystyrene and polybutadiene thatphase separates an immiscible blend. Immiscible blends aren't really blends at all, because theyphase-separate like water and chicken fat in a bowl of homemade chicken soup. But such phase-separated mixtures are also useful, mind you. If you want to read more about them go visitthe Immiscible Polymer Blends Page.

To Blend or Not to BlendA few polymer pairs mix. Most don't. But there are also polymer pairs that sometimes mix and sometimesdon't. The variables that one can control to make them mix or not mix are usually temperature andcomposition. A lot of polymer pairs are only miscible when there is a lot more of one polymer than of theother. There will be a range of compositions for which the two polymers won't mix. For example, let's saywe have two polymers, polymer A and polymer B. Let's also say they are miscible when we have less than30% polymer B, that they are miscible when there is more than 70% polymer B. But between 30 and 70%polymer B, the blend phase-separates into two phases. Here's a graph for those who of you who like thatsort of thing:

Interestingly, one phase will have 30% polymer B and the other will have 70% polymer B. There's a reasonfor this. If we look at a plot of free energy versus composition, we'll see that these two compositions arelower in energy than any other compositions.4One note first: we chemists usually use the Greek letter torepresent the relative amount of one or the other component in a mixture of any kind, so we're going touse B instead of "% B" from here on.

So these are the moststable

compositionspossible, and any mixture between 30 and 70 % polymer B will phase separate into a 30% polymer Bphase and a 70 % polymer B phase.

But the composition range over which the two polymers phase-separate isn't constant. It canchange with temperature. For some polymer pairs that rangegets smaller as temperature increases. Eventually, if you heatsuch a pair high enough, that range of immiscibility will becomeso small that it will disappear. The temperature at which thishappens is called the upper critical solution temperature orUCST. The graph on the right shows this. The upside-downparabola is the boundary between those temperatures andcompositions at which there is one phase, and those at whichthere is phase separation.

But sometimes the opposite happens. For some polymer pairsthe range of immiscibility decreaseswith decreasing temperature. If one cools such a pair enough,eventually we'll reach a temperature at which the range gets sosmall that it disappears. This temperature is calledthe lower critical solution temperature or LCST. If one plots therange of immiscibility versus temperature, the plot looks like aninversion of the UCST plot, as you can see on the left.

Now for you thermodynamicists out there who are wonderingwhat happens to our plot of free energy versus composition oncewe've crossed either a UCST or LCST and out polymer pair hasbecome miscible in all compositions, we have a graph showing

just that right here:

This plot takes some explaining. Imagine, if you will, a blend of polymers A and B, of composition Z. Now,imagine it phase-separating into two phases, one of composition X and the other of composition Y. As youcan see, the two separate phases are both higher in free energy than the single phase at composition Z, sothey are less stable than that single phase at composition Z. So, if the two separate phases were somehowgenerated they would spontaneously merge into one phase, whose composition is, you guessed it, Z.

Interpenetrating polymer networkAn Interpenetrating polymer network (IPN) is a polymer comprising two or more networks which are at leastpartially interlaced on a polymer scale but not covalently bonded to each other. The network cannot beseparated unless chemical bonds are broken.[1] The two or more networks can be envisioned to be entangled insuch a way that they are concatenated and cannot be pulled apart, but not bonded to each other by anychemical bond. Simply mixing two or more polymers does not create an interpenetrating polymer network(polymer blend), nor does creating a polymer network out of more than one kind of monomers which are bondedto each other to form one network (heteropolymer or copolymer).

Compatibilizers:

Compatibilization in polymer chemistry is the addition of a substance to an immiscible blend of polymers thatwill increase their stability. Polymer blends are typically described by coarse, unstable phase morphologies. Thisresults in poor mechanical properties. Compatibilizing the system will make a more stable and better blendedphase morphology by creating interactions between the two previously immiscible polymers. Not only does thisenhance the mechanical properties of the blend, but it often yields properties that are generally not attainable ineither single pure component.

Block or graft copolymers as compatibilizing agentsBlock or graft copolymers are commonly used as compatibilizing agents. The copolymer used is made of the twocomponents in the immiscible blend. The respective portions of the copolymer are able to interact with the twophases of the blend to make the phase morphology more stable. The increased stability is caused by reducingthe size of the phase-separated particles in the blend. The size reduction comes from the lower interfacialtension, due to accumulating block copolymers at the many interfaces between the two copolymers. This helpsthe immiscible blends break up into smaller particles in the melt phase. In turn, these phase separated particleswill not be as inclined to consolidate and grow because the interfacial tension is now much lower. This stabilizesthe polymer blend to a usable product. An example of this are Ethylene/propylene copolymers. They are able toact as good compatibilizing agents for blends of polypropylene and low density polyethylene. In this specificapplication, longer ethylene sequences are preferred in the copolymer. This is because co crystallization alsofactors into this case, and the longer ethylene sequences will retain some residual crystallinity.

Reactive compatibilizationReactive compatibilization is a procedure in which immiscible polymer blends are compatibilized by creatingcopolymers in the solution or melt state. Copolymers are formed when the proper functional groups in eachcomponent of the immiscible blend interact in the compatibilization process. These interactions includehydrogen, ionic or covalent bonding. The functional groups that cause these interactions can be the end groupsthat are already present in the blend polymers (e.g., carboxylic acids or alcohols on polyesters, or amine groupson nylons). Another approach is to add functional groups to the component chains by grafting. The manypossible functional groups allow for many types of commercial polymer blends, including polyamide/polyalkeneblend systems. There are a number of advantages reactive compatibilization has over using the traditional blockor graft copolymer as the compatibilizing agent. Unlike the latter approach, reactive compatibilization does notrely on diffusing pre-formed copolymers. Copolymers form at the interfaces of the two immiscible blends and donot need to be dispersed. In the traditional approach the system needs to be well mixed when adding thecopolymers. Reactive compatibilization is also much more efficient than traditional compatibilization. This isbecause in reactive compatibilization, functional groups are either already present, or easily grafted on the blendcomponents. In the traditional compatibilization, copolymers must be synthesized on a case by case basis for thecomponents to blend.

PVC –NBR Blend

Blending of polymers for property improvement or for economic advantage has gained considerable importance inthe field of polymer science in the last decade. Miscibility of the constituent polymers is often a necessity for formingsuccessful blends. One of the commercially important and miscible polymer blends is that of NBR and PVC. 1-1 NBRacts as a permanent plasticizer for PVC in applications like wire and cable insulation, food containers, pond linersused for oil containment, etc. On the other hand, PVC improves ozone, thermal aging, and chemical resistance ofNBR in applications like feed hose covers, gaskets, conveyor belt covers, printing roll covers, etc. PVC also vastlyimproves abrasion resistance, tear resistance, and tensile properties. It also adds gloss and improves finish of theextruded stock and imparts flame-retardant character. NBR/PVC blends can be conveniently milled, extruded, and

compression-molded using traditional processing equipments for natural and synthetic rubbers., One difficulty informing successful blends of NBR and PVC is the lack of suitable stabilizers for PVC which do not affect NBR. Althoughthe barium stearate/cadmium stearate combination, the commonly used PVC stabilizer in NBR/PVC blends,, does notseem to produce any obvious deterioration in the physical properties, it does produce a yellowish tint which limitsthe color flexibility of the blends. Recently it was shown that magnesium oxide and zinc oxide combination alongwith stearic acid could efficiently stabilize plasticized PVC. These conventional ingredients in rubber compoundingdo not produce any color. Hence magnesium oxide and zinc oxide along with stearic acid were tried as the stabilizerfor PVC in this study on NBR/ PVC blends.

PVC – ABS Blend:

Poly(vinyl chloride) and acrylonitrile‐butadiene‐styrene terpolymer blends made via melt blending. Blendswere characterized by various thermal, morphological methods of analysis. Two distinct glass transitionswere recorded by differential scanning calorimetry (DSC).This suggested the need for a compatibilizer.Incorporation of ABS had marginal effect on rate, chemistry, and overall pattern of decomposition ofpoly(vinyl chloride).

Polymer blending is one of the fastest growing areas of polymer technology. Blending of polymer hasbecome an increasingly important technique because it is an economical, viable and versatile way inwhich new material can be produced with a wide range of properties by merely using conventionalprocessing equipment such as extruder or internal mixer.

Polyvinyl chloride is most versatile material in plastic family and second largest consumption materialin plastic industries compared to polyolefin's. It is characterized by rigidity, hardness, excellent tensilemodulus and low cost. However, it has low impact strength and poor thermal stability. Which limit's it'sused.

Similarly, Acrylonitrile-Butadiene-Styrene (ABS) is one of most largely used engineering plastics. It hasexcellent mechanical, thermal, electrical &chemical properties. Followed by inferior properties such aspoor weather resistance, highly flammable and merely high cost.

Polyvinyl chloride is largely blended with number of polymers and rubbers. In most cases, to improveproperties of PVC and rarely to improve properties of other materials.

One of most inferior properties of PVC is low impact strength. To overcome this problem, it is blendedwith many rubbery materials. It has been shown that impact strength of PVC increases by blendingwith rubbery material such as NBR, SBR etc. But, it follows the decrease in tensile strength, rigidityand in most cases thermal stability.

Hence to achieve high impact strength, better thermal properties along with rigidity, PVC is blendedwith ABS. The blend of PVC and ABS posses their advantage of impact strength, rigidity, chemicalresistance, electrical properties and overall low cost.

In ABS, generally the rubbery phase is made of emulsion polymerized polybutadiene, which constitutesthe main polymer chain .The glassy phase is made of styrene and acrylonitrile grafted onPolybutadiene. Thus, it combines the impact strength of rubber and tensile strength, heat stability ofstyrene Acrylonitrile (SAN) Matrix. Thus properties of styrene acrylonitrile (SAN) and polybutadiene areimparted in PVC/ABS blend.

PP-EPDM Blend:Polypropylene (PP) is one of the most widely used polyolefin polymers. The toughening has been one of themost active and significant theme in the field of the modification for PP resin. Ethylene-propylenecopolymer (EPR) or ethylene-propylene-diene copolymer (EPDM) copolymer has been often used totoughen PP. At present, the impact strength of the modified resin can be enhanced by four times relative to

the matrix by adding elastomers. In recent years, blends of PP containing soft elastomer and rigid fillers areof ever-increasing interest because both their stiffness and toughness can be partly controlled and materialswith balanced mechanical properties can be formulated.

If a material can be generated at a lower cost with properties meeting the specifications, the manufacturermust exploit it to remain competitive. Blending of PP/EPDM are the most common combination for impactstrength improvement of PP.

This blend has been used as material in the manufacture of car bumpers, fender extensions and rubber strips.Many recent works reported the structure property relationship of impact modified PP through its meltblending with EPDM . However, EPDM is a synthetic rubber and generally must be imported. This meansthat the cost is expensive. Therefore this research was carried out to investigate the possibility ofreplacement or partial replacement of EPDM by NR or ENR.

Ethylene-propylene diene terpolymer (EPDM) is obtained by polymerizing ethylene and propylene withsmall amount of a non-conjugated diene (3 to 9%). Due to the unsaturated positions in the terpolymers,which lie outside the main chain, the good ageing characteristics, the ozone and cold resistance and theexcellent electrical resistance of a saturated olefin remain. EPDM has a broad resistance to chemicals but notto oil and other hydrocarbons .Meanwhile, an isotactic polypropylene is a stiff material, highly crystallinewith high melting point. Typical uses of polypropylene include sterilizable hospital items, dishes, applianceparts, dishwasher components, containers, automotive ducts, trim, etc.

In the past decade, polymer blend technology has achieved an important position in the field of polymer science.With increased academic and industrial research interest, the application of polymer blend technology tocommercial utility has grown significantly. This review on the applications of polymer blends will cover the majorcommercial blends in the categories of styrene-based polymer blends, poly(vinyl chloride) blends, polyacrylateblends, polyester and polycarbonate blends, polyolefin blends, elastomer blends, polyelectrolyte complexes, andinterpenetrating polymer networks. New developments in polymer blend applications will be discussed in moredetail. These systems include linear low-density polyethylene blends with either low- or high-density polyethylene,styrenemaleic anhydride terpolymer/ABS (acrylonitrile-butadiene-styrene) blends, polycarbonate/poly(butylenetetephthalate) blends, new PPO/polystyrene blends, and tetramethyl bisphenol A polycarbonate/impact polystyreneblends. Areas for future research to enhance the potential for polymer blend applications will be presented. Theneed for improved methods for predicting miscibility in polymer blends is discussed. Weldline strength is a majorproperty deficiency of two-phase systems (even those with mechanical compatibility), and future research effortappears warranted to resolve this deficiency. The use of polymeric compatibilization additives to polymer blends hasshown promise as a method to improve mechanical compatibility in phase-separated blends, and will be expected tobe the subject of future research programs. Finally, the reuse of polymer scrap is discussed as a future applicationarea for polymer blends. Unique applications recently proposed for polymer blends include immobilization ofenzymes, permselective membranes, reverse osmosis membranes, selective ion-exchange systems, and medicalapplications using polyelectrolyte complexes.

Polymer concrete:

Polymer concrete is a risk-free material for long-term use under continuous stress with almostunbeatable physical and chemical features. The production is done by a simple casting and curingprocess and this product can have highly attractive results produced as part of the mouldingprocess (e.g. high gloss, coloured and sparkling, luminescent etc.) thus the need and cost of extradecoration is avoided.

Cures to full hardness and structural integrity within hours

Completely free to manufacture in any shape with non-porous surface structures

Able to manufacture to exceptional size and shape tolerances with lower weight than

conventional concrete

Does not absorb water, therefore completely frost proof - no need for a damp-proof course

3 to 5 times higher compressive and bending tensile strength than conventional concrete

Absolutely corrosion-resistant

Resistant to virtually all corrosive elements

No damage from gasoline, mineral oils, acids, bases, vapours and gases

High abrasion resistance, high UV-resistance

Unaffected by large variations in temperature, very low expansion

Different types of polymer concrete:

1) Polymer impregnated concrete

2) Polymer concrete

3) Polymer modified concrete

1) Polymer impregnated concrete (PIC) :

Polymer impregnated concrete was first known after wide and comprehensive investigations ofresearchers at Brookhaven National Laboratory and the Department of improvement in the UnitedStates of America (Fowler,1999). Polymer impregnated concrete is usually produced by injecting amonomer with low-density to hydrated portland cement which is followed by radiation or thermalcatalytic polymerization techniques. Modulus elasticity of this type of concrete is 50% to 100%higher than normal concrete; however, modulus of polymer is 10% more than normal concretemodules. With these excellent features, many of the applications for polymer impregnated concreteinclude the production of bridge deck, tubes, carriers of corrosive liquids, floor tiles, constructionlaminate, and concrete injection in place. These studies were also successful in detailed-deepinjection processes and can be used to inject the bridge deck and other concrete surfaces.Implementation process involves drying concrete to remove moisture from the concrete surface, theuse of monomers in a thin layer of sand, and then polymerization of monomers through using heatflow. This process is capable of producing deep injection of 20 to 50 mm. The concrete surfaces have

lower permeability against water absorption, high resistant to abrasion and generally are moredurable (Fowler, 1999) Full injection process includes:

Drying concrete Air discharge from hardened concrete Concrete injection with a monomer with low-density Polymerization of monomers with heat or radiation Advantages: Higher compressive strength Higher tensile strength and flexural strength Resistance to freezing , thawing and acid attacks Higher durability Disadvantages: Higher costs Slow run Complex run

2) Polymer concrete (PC)

Polymer concrete was first used in America in 1958 to produce construction laminate (Fowler, 1999).Polymer concrete is formed through combination of stones with a polymer binder material whichcontains no water or PC. Polystyrene, acrylic and epoxy are resins and monomers which are widelyused in the manufacture of this type of concrete. Sulphur is also considered as a polymer andsulphur concrete is used for applications that require high resistance to acid. Polymer concrete isprimarily used in manufacturing prefabricated laminates, but today is used in manufacturingnecessary laminates in health care industry as well. Polymer coatings and concrete coatings areoften used in decoration and architecture due to the ability to create thin layers, processing speed,and very low permeability. Other applications of polymer concrete are to cover bridge surfaces, floorcoverings in sports arenas, stadiums, laboratories, hospitals, factories and stores (San-Jose, Vegasand Ferreira, 2005). One of the innovations is the epoxy polymer concrete which has the ability tostore anti-ice fluids and then release it during snowfall and severe weather conditions (frost). Theresult is an appropriate and stable covering which is generally humid-proof as well as havingexcellent resistance to the slide. Its thickness is at least 9.5 mm and weighs 2 kilograms per squaremeters. These coverings are suitable for asphalt surfaces, bridge floor like concrete pavement.Surface preparation is usually used like other polymer coatings like epoxy coating layer. Antifreezematerial is applied in liquid form therefore, it can absorb aggregate. When snow falls or frost occurs,the antifreeze prevents sticking as well as lowers the freezing temperature. Antifreeze materialsshould be applied periodically depending on the frequency of storms, snow and ice (Fowler, 1999).

Prefabricated concrete polymer can be used to produce a range of products including the drainage,underground boxes, building cladding, acid tanks, hazardous waste containment, tile, etc.Prefabricated components represent an excellent use of materials for rapid processing, the ability toproduce complex shapes and very good vibration amortization (Kienow and Allen, 1993: Czarnecki,Garbacz, and Krystosiak, 2006). The advantages:

Fast processing Excellent adhesion to concrete and steel High resistance Durability Disadvantages Costs, expenses Contractors' unfamiliarity Competition with other repair or restoration materials

The use of additives in polymer concrete has also been considered. For example, Lokuge andcolleagues (2013) as well as Gorninskia and colleagues (2004) investigated the effect of fly ash onpolymer concrete made with epoxy, ester and polyester. The results showed that the use of fly ashreduces the amount of polymer and increases the compressive strength. Modulus of elasticity isincreased by increasing the amount of fly ash. The tensile strength and flexural strength is decreasedwith increasing amount of fly ash as well. Golestaneh et al. (2010) investigated the effect of usingsilica powder as filler in polymer concrete made with epoxy. It was shown that the use of 200% silicapowder and 20% polymer had the best mechanical properties. It has been also reported in this studythat the values of compressive strength were 128/9, tensile strength 22/5 and flexural strength 16/2mega Pascal respectively. Polymer concrete has very good durability against thermal cycles (Jung,Roh, and Chang, 2014: Mirza, Durand, Bhutta, and Tahir, 2014). Shokrieh and colleagues (2011) haveconsidered three temperature cycles including 25 to 30, 25 to 70 and 30 to 70 and put polymerconcrete made with epoxy in it for 7 days and 24-hour cycle which indicated the lack of a significantchange in tensile and flexural strength and a slight decline in the second cycle.

3) Polymer modified concrete (PMC):Polymer-modified concrete through using latex has been used since 1950s. Polymer modified

concrete is made of Portland cement concrete with a modified polymer such as acrylic, SBR,polyvinyl acetate and ethylene vinyl acetate. SBR is widely used mortar in repair and floors andbridge laminate (G Barluenga and Herna´ndez, 2004: Diab, Elyamany, 2014). Its minimum thicknessis about 30 mm. The advantages are good adhesion to concrete, high flexural strength and lowpermeability (MANSON, 1976: Beushausen, Gillmer, and Alexander, 2014). Acrylic latex and meta-crylic acid are used to produce mortar that can be sprayed on final architecture surfaces (Son andYeon, 2012). Acrylic polymer modified concrete has a constant color and that’s why it is consideredas an attractive material in the architecture. The polymer modified concrete is very similar toconventional cement concrete. The amount of polymer is usually between 10 to 20% of Portlandcement. Only a small number of polymers are suitable to be added to the concrete and most otherpolymers of polymer modified concrete are produced with low quality. Polymer modified concretecan be reinforced with fibers as well as increasing its tensile strength and reducing crackingpossibility. Polymer modified concrete costs are less than polymer concrete because less polymer is

Limitation of using polymers in concrete:

The first and the biggest limitation of using polymers in concrete is their cost. Polymers' cost can be10 to 100 times more than the cost of conventional Portland cement (Fowler, 1999). For this reason,the use of polymer concrete in polymer concrete is non- applicable in cases where the great amountof concrete is needed, such as sidewalks, foundations, hydraulic structures. Except in certain caseswhere high durability is required, the use of conventional concrete is inefficient. Other instability ofthese materials is in high temperatures, particularly in fire that makes it difficult to use in residentialbuildings. Other problems regarding the use of polymers refer to their dangerous gas and theirflammability that can be resolved, but safety should be considered during construction phase.

Flexural Test

Flexure tests are generally used to determine the flexural modulus or flexural strength ofa material. A flexure test is more affordable than a tensile test and test results areslightly different. The material is laid horizontally over two points of contact (lowersupport span) and then a force is applied to the top of the material through either one ortwo points of contact (upper loading span) until the sample fails. The maximumrecorded force is the flexural strength of that particular sample.

Why perform a flexure test?

Unlike a compression test or tensile test, a flexure test does not measure fundamentalmaterial properties. When a specimen is placed under flexural loading all threefundamental stresses are present: tensile, compressive and shear and so the flexuralproperties of a specimen are the result of the combined effect of all three stresses aswell as (though to a lesser extent) the geometry of the specimen and the rate the load isapplied.

The most common purpose of a flexure test is to measure flexural strength and flexuralmodulus. Flexural strength is defined as the maximum stress at the outermost fiber oneither the compression or tension side of the specimen. Flexural modulus is calculatedfrom the slope of the stress vs. strain deflection curve. These two values can be used toevaluate the sample materials ability to withstand flexure or bending forces.

Flexure Test Types:

The two most common types of flexure test are three point and four point flexurebending tests. A three point bend test consists of the sample placed horizontally upontwo points and the force applied to the top of the sample through a single point so thatthe sample is bent in the shape of a “V”. A four point bend test is roughly the sameexcept that instead of the force applied through a single point on top it is appliedthrough two points so that the sample experiences contact at four different points and isbent more in the shape of a “U”. The three point flexure test is ideal for the testing of aspecific location of the sample, whereas, the four point flexure test is more suitedtowards the testing of a large section of the sample, which highlights the defects of thesample better than a 3-point bending test.

A bend test is similar to a flexure test in the type of hardware and test procedureinvolved. Bend tests are used with ductile materials whereas flexural tests are used withbrittle materials.

Types of Materials:

Generally a flexure test is run until the sample experiences failure and is therefore idealfor the testing of brittle materials. The most common materials tested in flexure areplastic materials, composites, concrete, and ceramics. Because these materials have avery low ductility they will break before any permanent deformation of the sampleoccurs allowing for the accurate measurement of the flexural modulus and strength.

Flexural strengthFlexural strength, also known as modulus of rupture, or bend strength,or transverse rupture strength is a material property, defined as the stress in amaterial just before it yields in a flexure test.[1] The transverse bending test is mostfrequently employed, in which a specimen having either a circular or rectangular cross-section is bent until fracture or yielding using a three point flexural testtechnique. Theflexural strength represents the highest stress experienced within the material at itsmoment of yield. It is measured in terms of stress, here given the symbol

.

Introduction

Fig. 1 - Beam of material under bending. Extreme fibers at B (compression) and A(tension)

Flexural strengthFlexural strength, also known as modulus of rupture, or bend strength,or transverse rupture strength is a material property, defined as the stress in amaterial just before it yields in a flexure test.[1] The transverse bending test is mostfrequently employed, in which a specimen having either a circular or rectangular cross-section is bent until fracture or yielding using a three point flexural testtechnique. Theflexural strength represents the highest stress experienced within the material at itsmoment of yield. It is measured in terms of stress, here given the symbol

.

Introduction

Fig. 1 - Beam of material under bending. Extreme fibers at B (compression) and A(tension)

Flexural strengthFlexural strength, also known as modulus of rupture, or bend strength,or transverse rupture strength is a material property, defined as the stress in amaterial just before it yields in a flexure test.[1] The transverse bending test is mostfrequently employed, in which a specimen having either a circular or rectangular cross-section is bent until fracture or yielding using a three point flexural testtechnique. Theflexural strength represents the highest stress experienced within the material at itsmoment of yield. It is measured in terms of stress, here given the symbol

.

Introduction

Fig. 1 - Beam of material under bending. Extreme fibers at B (compression) and A(tension)

Fig. 2 - Stress distribution across beam

When an object formed of a single material, like a wooden beam or a steel rod, is bent(Fig. 1), it experiences a range of stresses across its depth (Fig. 2). At the edge of theobject on the inside of the bend (concave face) the stress will be at its maximumcompressive stress value. At the outside of the bend (convex face) the stress will be atits maximum tensile value. These inner and outer edges of the beam or rod are knownas the 'extreme fibers'. Most materials fail under tensile stress before they fail undercompressive stress, so the maximum tensile stress value that can be sustained beforethe beam or rod fails is its flexural strength.

Flexural versus tensile strength

The flexural strength would be the same as the tensile strength if the materialwere homogeneous. In fact, most materials have small or large defects in them whichact to concentrate the stresses locally, effectively causing a localized weakness. Whena material is bent only the extreme fibers are at the largest stress so, if those fibers arefree from defects, the flexural strength will be controlled by the strength of those intact'fibers'. However, if the same material was subjected to only tensile forces then all thefibers in the material are at the same stress and failure will initiate when the weakestfiber reaches its limiting tensile stress. Therefore, it is common for flexural strengths tobe higher than tensile strengths for the same material. Conversely, a homogeneousmaterial with defects only on its surfaces (e.g., due to scratches) might have a highertensile strength than flexural strength.If we don't take into account defects of any kind, it is clear that the material will fail undera bending force which is smaller than the corresponding tensile force. Both of theseforces will induce the same failure stress, whose value depends on the strength of thematerial.

Fig. 2 - Stress distribution across beam

When an object formed of a single material, like a wooden beam or a steel rod, is bent(Fig. 1), it experiences a range of stresses across its depth (Fig. 2). At the edge of theobject on the inside of the bend (concave face) the stress will be at its maximumcompressive stress value. At the outside of the bend (convex face) the stress will be atits maximum tensile value. These inner and outer edges of the beam or rod are knownas the 'extreme fibers'. Most materials fail under tensile stress before they fail undercompressive stress, so the maximum tensile stress value that can be sustained beforethe beam or rod fails is its flexural strength.

Flexural versus tensile strength

The flexural strength would be the same as the tensile strength if the materialwere homogeneous. In fact, most materials have small or large defects in them whichact to concentrate the stresses locally, effectively causing a localized weakness. Whena material is bent only the extreme fibers are at the largest stress so, if those fibers arefree from defects, the flexural strength will be controlled by the strength of those intact'fibers'. However, if the same material was subjected to only tensile forces then all thefibers in the material are at the same stress and failure will initiate when the weakestfiber reaches its limiting tensile stress. Therefore, it is common for flexural strengths tobe higher than tensile strengths for the same material. Conversely, a homogeneousmaterial with defects only on its surfaces (e.g., due to scratches) might have a highertensile strength than flexural strength.If we don't take into account defects of any kind, it is clear that the material will fail undera bending force which is smaller than the corresponding tensile force. Both of theseforces will induce the same failure stress, whose value depends on the strength of thematerial.

Fig. 2 - Stress distribution across beam

When an object formed of a single material, like a wooden beam or a steel rod, is bent(Fig. 1), it experiences a range of stresses across its depth (Fig. 2). At the edge of theobject on the inside of the bend (concave face) the stress will be at its maximumcompressive stress value. At the outside of the bend (convex face) the stress will be atits maximum tensile value. These inner and outer edges of the beam or rod are knownas the 'extreme fibers'. Most materials fail under tensile stress before they fail undercompressive stress, so the maximum tensile stress value that can be sustained beforethe beam or rod fails is its flexural strength.

Flexural versus tensile strength

The flexural strength would be the same as the tensile strength if the materialwere homogeneous. In fact, most materials have small or large defects in them whichact to concentrate the stresses locally, effectively causing a localized weakness. Whena material is bent only the extreme fibers are at the largest stress so, if those fibers arefree from defects, the flexural strength will be controlled by the strength of those intact'fibers'. However, if the same material was subjected to only tensile forces then all thefibers in the material are at the same stress and failure will initiate when the weakestfiber reaches its limiting tensile stress. Therefore, it is common for flexural strengths tobe higher than tensile strengths for the same material. Conversely, a homogeneousmaterial with defects only on its surfaces (e.g., due to scratches) might have a highertensile strength than flexural strength.If we don't take into account defects of any kind, it is clear that the material will fail undera bending force which is smaller than the corresponding tensile force. Both of theseforces will induce the same failure stress, whose value depends on the strength of thematerial.

For a rectangular sample, the resulting stress under an axial force is given by thefollowing formula:

This stress is not the true stress, since the cross section of the sample is consideredto be invariable (engineering stress).

is the axial load (force) at the fracture point b is width d is the depth or thickness of the materialThe resulting stress for a rectangular sample under a load in a three-point bendingsetup (Fig. 3) is given by the formula below (see "Measuring flexural strength").

The equation of these two stresses (failure) yields:Usually, L (length of the support span) is much bigger than d, so the fraction isbigger than one.

Measuring flexural strength

Fig. 3 - Beam under 3 point bending

For a rectangular sample under a load in a three-point bending setup (Fig. 3):

F is the load (force) at the fracture point (N) L is the length of the support span b is width d is thicknessFor a rectangular sample under a load in a four-point bending setup wherethe loading span is one-third of the support span:

For a rectangular sample, the resulting stress under an axial force is given by thefollowing formula:

This stress is not the true stress, since the cross section of the sample is consideredto be invariable (engineering stress).

is the axial load (force) at the fracture point b is width d is the depth or thickness of the materialThe resulting stress for a rectangular sample under a load in a three-point bendingsetup (Fig. 3) is given by the formula below (see "Measuring flexural strength").

The equation of these two stresses (failure) yields:Usually, L (length of the support span) is much bigger than d, so the fraction isbigger than one.

Measuring flexural strength

Fig. 3 - Beam under 3 point bending

For a rectangular sample under a load in a three-point bending setup (Fig. 3):

F is the load (force) at the fracture point (N) L is the length of the support span b is width d is thicknessFor a rectangular sample under a load in a four-point bending setup wherethe loading span is one-third of the support span:

For a rectangular sample, the resulting stress under an axial force is given by thefollowing formula:

This stress is not the true stress, since the cross section of the sample is consideredto be invariable (engineering stress).

is the axial load (force) at the fracture point b is width d is the depth or thickness of the materialThe resulting stress for a rectangular sample under a load in a three-point bendingsetup (Fig. 3) is given by the formula below (see "Measuring flexural strength").

The equation of these two stresses (failure) yields:Usually, L (length of the support span) is much bigger than d, so the fraction isbigger than one.

Measuring flexural strength

Fig. 3 - Beam under 3 point bending

For a rectangular sample under a load in a three-point bending setup (Fig. 3):

F is the load (force) at the fracture point (N) L is the length of the support span b is width d is thicknessFor a rectangular sample under a load in a four-point bending setup wherethe loading span is one-third of the support span:

F is the load (force) at the fracture point L is the length of the support (outer) span b is width d is thicknessFor the 4 pt bend setup, if the loading span is 1/2 of the support span (i.e.Li = 1/2 L in Fig. 4):

If the loading span is neither 1/3 nor 1/2 the support span for the 4 ptbend setup (Fig. 4):

Fig. 4 - Beam under 4 point bending

Li is the length of the loading (inner) span.

F is the load (force) at the fracture point L is the length of the support (outer) span b is width d is thicknessFor the 4 pt bend setup, if the loading span is 1/2 of the support span (i.e.Li = 1/2 L in Fig. 4):

If the loading span is neither 1/3 nor 1/2 the support span for the 4 ptbend setup (Fig. 4):

Fig. 4 - Beam under 4 point bending

Li is the length of the loading (inner) span.

F is the load (force) at the fracture point L is the length of the support (outer) span b is width d is thicknessFor the 4 pt bend setup, if the loading span is 1/2 of the support span (i.e.Li = 1/2 L in Fig. 4):

If the loading span is neither 1/3 nor 1/2 the support span for the 4 ptbend setup (Fig. 4):

Fig. 4 - Beam under 4 point bending

Li is the length of the loading (inner) span.

Melt flow indexThe melt flow index (MFI) is a measure of the ease of flow of the melt ofa thermoplastic polymer. It is defined as the mass of polymer, in grams, flowing in tenminutes through a capillary of a specific diameter and length by a pressure applied viaprescribed alternative gravimetric weights for alternative prescribed temperatures.Polymer processors usually correlate the value of MFI with the polymer grade that theyhave to choose for different processes, and most often this value is not accompanied bythe units, because it is taken for granted to be g/10min. Similarly, the test loadconditions of MFI measurement is normally expressed in kilograms rather than anyother units. The method is described in the similar standards ASTM D1238 and ISO1133.Melt flow rate is an indirect measure of molecular weight, with high melt flow ratecorresponding to low molecular weight. At the same time, melt flow rate is a measure ofthe ability of the material's melt to flow under pressure. Melt flow rate is inverselyproportional to viscosity of the melt at the conditions of the test, though it should beborne in mind that the viscosity for any such material depends on the applied force.Ratios between two melt flow rate values for one material at different gravimetricweights are often used as a measure for the broadness of the molecular weightdistribution.Melt flow rate is very commonly used for polyolefins, polyethylene being measured at190 °C and polypropylene at 230 °C. The plastics engineer should choose a materialwith a melt index high enough that the molten polymer can be easily formed into thearticle intended, but low enough that the mechanical strength of the final article will besufficient for its use.

Measurement

Overview of the measurement of melt flow index (MFI)

ISO standard 1133-1 governs the procedure for measurement of the melt flow rate.Theprocedure for determining MFI is as follows:

1. A small amount of the polymer sample (around 4 to 5 grams) is taken in thespecially designed MFI apparatus. A die with an opening of typically around2 mm diameter is inserted into the apparatus.

2. The material is packed properly inside the barrel to avoid formation of airpockets.3. A piston is introduced which acts as the medium that causes extrusion of the

molten polymer.4. The sample is preheated for a specified amount of time: 5 min at 190 °C

for polyethylene and 6 min at 230 °C for polypropylene.5. After the preheating a specified weight is introduced onto the piston. Examples of

standard weights are 2.16 kg, 5 kg, etc.6. The weight exerts a force on the molten polymer and it immediately starts flowing

through the die.7. A sample of the melt is taken after the desired period of time and is weighed

accurately.

8. MFI is expressed in grams of polymer per 10 minutes of duration of the test.

Synonyms of Melt Flow Index are Melt Flow Rate and Melt Index. More commonlyused are their abbreviations: MFI, MFR and MI.Confusingly, MFR may also indicate "melt flow ratio", the ratio between two melt flowrates at different gravimetric weights. More accurately, this should be reported as FRR(flow rate ratio), or simply flow ratio. FRR is commonly used as an indication of the wayin which rheological behavior is influenced by the molecular mass distribution of thematerial.The flow parameter that is readily accessible to most processors is the MFI. MFI is oftenused to determine how a polymer will process. However MFI takes no account of theshear, shear rate or shear history and as such is not a good measure of the processingwindow of a polymer. It is a single-point viscosity measurement at a relatively low shearrate and temperature. Earlier, it was often said that MFI give a ‘dot’ when actually whatis needed is a ‘plot’ for the polymer processors. However, this is not true now becauseof a unique approach developed for estimating the rheogram merely from theknowledge of the MFI.The MFI device is not an extruder in the conventional polymer processing sense in thatthere is no screw to compress heat and shear the polymer. MFI additionally does nottake account of long chain branching nor the differences between shear andelongational rheology. Therefore, two polymers with the same MFI will not behave thesame under any given processing conditions.The relationship between MFI and temperature can be used to obtain the activationenergies for polymers.The activation energies developed from MFI values has theadvantage of simplicity and easy availability. The concept of obtaining activation energyfrom MFI can be extended to copolymers as well wherein there exists an anomaloustemperature dependence of melt viscosity leading to the existence of two distinct valuesof activation energies for each copolymer.

Melt Flow Index Formula

MFI (In Grams) = Weight of Melted samples in 10 minutes

Melt Flow Rate ASTM D 1238, ISO 1133

Melt Flow Index, Melt Flow Rate, ASTM D1238, ISO 1133

Scope:Melt Flow Rate measures the rate of extrusion of thermoplastics through an orifice at aprescribed temperature and load. It provides a means of measuring flow of a meltedmaterial which can be used to differentiate grades as with polyethylene, or determinethe extent of degradation of the plastic as a result of molding. Degraded materials wouldgenerally flow more as a result of reduced molecular weight, and could exhibit reducedphysical properties. Typically, flow rates for a part and the resin it is molded from aredetermined, and then a percentage difference is calculated. Alternatively, comparisonsbetween "good" parts and "bad" parts may be of value.

Test Procedure:Approximately 7 grams of the material is loaded into the barrel of the melt flowapparatus, which has been heated to a temperature specified for the material. A weightspecified for the material is applied to a plunger and the molten material is forcedthrough the die. A timed extrudate is collected and weighed. Melt flow rate values arecalculated in g/10 min.

Specimen size:At least 14 grams of material

Data:Flow rate = ( 600/t x weight of extrudate )t = time of extrudate in secondsmelt flow rate = g/10 min.

Tensile test

ASTM D638 is one of the most common plastic strength specifications and covers thetensile properties of unreinforced and reinforced plastics. This test method uses standard“dumbell” or “dogbone” shaped specimens under 14mm of thickness. A universal testingmachine (tensile testing machine) is needed to perform this test. If you are going to performthis test, you should read the entire specification from ASTM. This is a quick summary todecide if this test is right for you, and to point out what equipment you need to perform thetest.

First off, do not perform this test if you have films or elastomers. If you have film under1mm in thickness use ASTM D882. If you have an elastomer use ASTM D412.

Test Procedure:

1. Cut or injection mold your material into one ofthe five “dumbbell” shapes. The exact shapeyou use is dependent upon your material’srigidity and thickness.

2. Load the specimen into tensile grips.3. Attach the extensometer to the sample4. Begin the test by separating the tensile grips at a constant rate of speed. Speed

depends on specimen shape and can range from 0.05 – 20 inches per minute. Thetarget time from start of test to break should be from 30 seconds to 5 minutes.

5. End the test after sample break (rupture)

Analysis obtained:

1. Tensile Strength2. Elongation at Yield3. Elongation at Break4. Nominal Strain at Break (Grip Separation)5. Modulus of Elasticity6. Secant Modulus7. Poisson’s Ratio (Requires Transverse Extensometer)

Equipment required:

1. Universal testing machine (tensile testing machine)1. Needs to be servo controlled to keep a constant rate of speed.2. Capacity needs to be enough for your materials. A 1,000 lbf single column

system is usually sufficient for most non-reinforced plastics. A 2,000 lbf dualcolumn system is also very common. A high capacity 10,000 lbf model issometimes needed for larger samples and/or stronger materials such asreinforced plastics or composites.

2. Extensometer1. Required when measuring modulus, yield, and modulus. Why? For two

reasons: 1) The linear region of plastics is very small and happens suddenlyso grip separation is just not accurate enough. 2) Dumbbell specimens donot have uniform widths so there will be errors when both the wide andnarrow sections of the dumbbell shaped specimen elongate at different rates.

3. Data Acquisition1. Software or suitable electronics are required to operate the machine and to

take the measurements. Basic systems will provide the raw data, and stress-strain charts. Using these sources of data, you can determine and calculateall of the analysis listed above. However, fully PC based systems have thecapability to calculate all of these automatically. For example,our MTESTQuattro testing software has built in support for ASTM D638 andall of these calculations are provided immediately after performing the test.

4. Tensile Grips

1. Any grip with serrated faces is usually adequate for thistest. You can use wedge, pneumatic, vise, or other self tightening grips suchas eccentric roller or scissor grips.

HEAT TRANSFER

The concept of unit operations was introduced in the year 1905 by A.D litle. Theunit operations are carried out in the chemical industries involved physicalchanges. The unit operations are physical in nature. There are different types ofunit operations. They are classified as into the five categories.

1) Fluid flow operation2) Heat transfer operation3) Mass transfer operation4) Mechanic operation5) Thermodynamics operation

1) Fluid flow operation: - These operations are related to the fluid flow and theirphysical properties.

For eg. – Transportation of fluid from one point to another, fluid relatedproperties such as viscosity, pressure, temperature, density, specific gravity.The cause of fluid flow is pressure difference.

2) Heat transfer operation: - The heat transfer operation involved the transfer ofheat from one body to another due to the temperature difference. The causeof heat transfer is temperature difference. The heat transfer operations areheating, cooling, drying and evaporation.

3) Mass transfer operation: - The mass transfer operation involved the transferof mass from one point to another. The cause of mass transfer isconcentration difference. The e.g of mass transfers are drying,crystallization, distillation, gas absorption e.t.c.

Unit ProcessUnit processes are the process in the chemical engineering, In which there isa chemical reaction is involved. In the unit process the properties of thematerials is change due to the chemical process. The e.g of unit processesare polymerisation, alkylation, hydrogenation, oxidation and reduction.

HEAT TRANSFER OPERATION:Heat transfer deals with the study of the rate of change of heat takes placebetween a hot and a cold source. There are different types of heat transferoperations such as evaporation, drying, heating, cooling etc.

There are three mode of heat transfer:-1) Conduction2) Convection3) Radiation

1) Conduction: - It is a mode of transfer of heat from one part of the body tothe another from one particle to another particle in direction of fall oftemperature. The conduction takes place generally in the solid for e.g whenone end of a metal rod is heated after some time the other end also becomesnot due to the transfer of conductive heat.

2) Convection: - It is a mode of transfer of heat from one part of the fluidsmedium to another part by the actual motion of the heated particles. Theconvection heat transfer takes place generally in the fluid (liquid and gases)For e.g :- The boiling of water in a beaker blowing air .

Types of convection are:-

1) Free convection2) Forced convection

1) Free convection: - In the natural convection the force of gravity play animportant role in the formation of convection current. The e.g of naturalconvection is the cooling of boiled water, breezing.

2) Forced convection: - In the force convection the material particles of a fluidare forced to move by a pump or by a physical or a mechanical movement.

For e.g :- The air heating in the home cooling system in the automobile.

3) Radiation: - Radiation is a mode of heat transfer from the source to thereceiver without any movement on the source and also without any heatingmedium. The radiation requires no medium for the transfer of heat. The heatis transferred of heat. The heat is transferred by the electro-magnetic waves.

Fourier’s law of heat conduction:-Fourier’s law states that the rate of heat flow by the conduction through a uniformmaterial is directly proportional to the area normal to the direction of heat flow andthe temperature gradient in the direction of heat flow.

Principle of Fourier’s law:According to this law under the steady state, the temperature of the body isconstant. Let us consider a rectangular bar of solid to explain this law.

Heat conduction through composite wall

When a wall is formed by a series of layer of different materials, it is called as acomposite wall.

Let us consider a flat wall made of three diff. materials having thermalconductivity K1 k2 k3 there cross sectional area are A1, A2, A3 and theirthickness be x1 , x2 , x3

Heat conduction through the hollow cylinder

Let us consider a hollow cylinder made up of a material haring constant thermalconduturity (K).

Let r1 and r2 are the inner and outer radius of hollow cylinder.

Let T1 and T2 are the temperature of the inner surface and outer surface of cylinder

Overall coefficient of heat transfer:-

Let us consider to fluid separated by a wall. The heat will be transfer from not fluidto the cold fluid with the help of solid wall. The heat in not flowing by convectionhaving temp.

Heat Exchanger: - It may be defined as an equipment which transfers the energyfrom a not fluid to a cold fluid with maximum rate and min investment andrunning cost.

Type of heat exchanger1) Double pipe heat exchanger: - It is the simplest pipe of the heat exchanger

used in industries. It consist of concentric pipes, connecting tee, return head andreturn bend in this heat exchanger one of the fluid flow through the inner pipeand the other fluid flow through the outer pipe . Generally the not fluid ispassed in the inner pipe. According to the fluid through the pipe the heatexchanger are further classified into two types.

1) Co-rotating flow heat exchanger2) Counter rotating heat exchanger.

2) Shell and tube heat exchangerShell and tube heat exchanger is most commonly used in chemical industrysuch as refinery, fertilizer and heat transfer operation. It is suitable for highpressure application.

1) U-tube heat exchangerU-tube heat exchanger is used in nuclear power plant. They are used to boil waterin this exchange there are difference pass through which the fluid is following.

2) Straight tube heat exchangerIn the straight tube heat exchanger flow through the shell side controlled bybaffles there are two type.

Log mean temperature Difference

The temperature difference between the two fluids in the heat exchanger may varyfrom one point to another point. The mean temperature difference can becalculated from the terminal temperature of the two fluid steams.

The thermal temperature difference has no wide difference the mean temp.Difference is calculated by averaging temp. However when the terminaltemperature difference has wide difference the log mean temperature difference(LMTD) gives the correct value. The temperature profile of the Hot and cold fluidalong the length of heat exchanger is shown in the fig.

Thermodynamics1) Thermodynamics: - It is the branch of science which deals with the study of

the transformation of the heat into another form of energy and vice- versa.Thermodynamics is a macroscopic science. In the thermodynamics

macroscopic variable such as pressure volume, temperature, mass andcomposition are considered.

2) Thermodynamics system: - a thermodynamics system is defined as quantityof matter or a specific region. In the space upon which we focus our study.

3) Surrounding: - Everything external to the system is called as surrounding orthe environment.

4) Boundary: - It is a kind of envelope which separates the system fromsurrounding. It may be fixed or removal able.

5) Universe: - A system and its surrounding together from a universe.

Types of thermodynamic system:-

1) Open system: - In an open system there an exchange of mass and energyacross the system and surrounding. For e.g :- The boiling water placed in anopen blacker.

2) Closed system: - In a closed system energy may be transferred into or out tothe system. No transfer of mass takes place across the system boundary. Fore.g :- a) boiling water in a closed vessel. b) Piston cylinder arrangementcontaining a gas.

3) Isolated system:- In a isolated system there is no transfer of mass and energyinto end from the system

1) Homogenous system: - A system is said to be homogenous system if thequantity of matter is unformed throughout in the chemical compositionand physical structure (Phase).

2) Heterogeneous system: - A system is said to be heterogeneous system, ifthere are more than one phase are present.

CommunityCapacity Building

Program

Leadership and Motivation

CommunityCapacity Building

Program

Leadership and Motivation

Learning Objectives

• Understanding the relationshipbetween leadership and motivation

• Motivation through a psychologicalframework

• Leadership theories/styles• Participative leadership• Qualities of successful leaders

• Understanding the relationshipbetween leadership and motivation

• Motivation through a psychologicalframework

• Leadership theories/styles• Participative leadership• Qualities of successful leaders

Introduction

2 major influences affect howindividuals perform:

1. The type of leadership that exists2. Personal motivation

• Important to look at these twoconcepts as interconnected anddependent on individual situations

2 major influences affect howindividuals perform:

1. The type of leadership that exists2. Personal motivation

• Important to look at these twoconcepts as interconnected anddependent on individual situations

What is motivation?

• Motivation is defined as “the extent towhich persistent effort is directedtoward a goal”

1. Effort - must be defined in relation to its appropriateness tothe objectives being pursued.

2. Persistence - relates to the willingness of the individual tostay with a task until it is complete

3. Direction - measured in terms of how persistent effort isapplied in relation to the goals being pursued

4. Goals - individual goals and organizational goals (must becompatible)

• Motivation is defined as “the extent towhich persistent effort is directedtoward a goal”

1. Effort - must be defined in relation to its appropriateness tothe objectives being pursued.

2. Persistence - relates to the willingness of the individual tostay with a task until it is complete

3. Direction - measured in terms of how persistent effort isapplied in relation to the goals being pursued

4. Goals - individual goals and organizational goals (must becompatible)

Types of motivation

• Extrinsic Motivation- Factors in the external environment

such as pay, supervision, benefits,and job perks

• Intrinsic Motivation- Relationship between the worker and

the task

• Extrinsic Motivation- Factors in the external environment

such as pay, supervision, benefits,and job perks

• Intrinsic Motivation- Relationship between the worker and

the task

5 Ways to Motivate a Team

1. Figure out what makes them tick(individual needs)

2. Give clear expectations3. Consistent reinforcement and

consequences4. Healthy competition5. Change out team members

1. Figure out what makes them tick(individual needs)

2. Give clear expectations3. Consistent reinforcement and

consequences4. Healthy competition5. Change out team members

Activity 1 – Identifying Goals

• Create a thought web linking current andfuture goals of the organization to internalor external motivators

• Example:

Goal: Increase sales by 10%Motivators: Top seller receives financial

bonus if goal is reached (extrinsic)Top seller gets to choose their next project

(allowing seller to pursue work-basedinterest would increase intrinsic motivation)

• Create a thought web linking current andfuture goals of the organization to internalor external motivators

• Example:

Goal: Increase sales by 10%Motivators: Top seller receives financial

bonus if goal is reached (extrinsic)Top seller gets to choose their next project

(allowing seller to pursue work-basedinterest would increase intrinsic motivation)

Needs-based Motivation

• Maslow’s Hierarchy of Needs: Physiological Safety Relationship Esteem Self-actualization

• The more you move from basic to higher level needs, themore motivation depends on internal factors

• Important that organizations present opportunities to satisfysuch needs

• Maslow’s Hierarchy of Needs: Physiological Safety Relationship Esteem Self-actualization

• The more you move from basic to higher level needs, themore motivation depends on internal factors

• Important that organizations present opportunities to satisfysuch needs

Needs-based Motivation

• Alderfer’s ERG Theory: Existence Relatedness Growth

• Similar to Maslow’s Hierarchy in that it focuses on a differinglevels of needs, which are usually satisfied in order ofimportance

• Unlike Maslow’s theory in that it allows for higher needs tobe met before lower-level needs under certain circumstances

• Alderfer’s ERG Theory: Existence Relatedness Growth

• Similar to Maslow’s Hierarchy in that it focuses on a differinglevels of needs, which are usually satisfied in order ofimportance

• Unlike Maslow’s theory in that it allows for higher needs tobe met before lower-level needs under certain circumstances

Needs-based Motivation

• McClelland’s Theory of Needs: Achievement Affiliation Power

• Concerned with the behavioral consequences of need• Non-hierarchical

• McClelland’s Theory of Needs: Achievement Affiliation Power

• Concerned with the behavioral consequences of need• Non-hierarchical

Needs-based Motivation

• These three theories present a usefulapproach for thinking aboutorganizational behavior

• One is not inherently better than theother; The point is to apply theconcepts of internal/externalmotivation to individual situations

• These three theories present a usefulapproach for thinking aboutorganizational behavior

• One is not inherently better than theother; The point is to apply theconcepts of internal/externalmotivation to individual situations

Motivational Goals

Most goals fall within two categories:• Performance goal - individual is

concerned with acquiring favorablejudgment from his or her peers,supervisors, or authority figures(extrinsic)

• Learning goal - individual usesfeedback to increase his or hercompetence (intrinsic)

Most goals fall within two categories:• Performance goal - individual is

concerned with acquiring favorablejudgment from his or her peers,supervisors, or authority figures(extrinsic)

• Learning goal - individual usesfeedback to increase his or hercompetence (intrinsic)

Activity 2 – Needs and Motivation

• In small groups, answer thefollowing questions:

1. Whose needs are satisfied throughmy organization? (Owners,employees, clients, etc.)

2. What are some needs of eachgroup? (Relatedness?Achievement?)

3. What is the best way to addressthose needs to reach goals?

• In small groups, answer thefollowing questions:

1. Whose needs are satisfied throughmy organization? (Owners,employees, clients, etc.)

2. What are some needs of eachgroup? (Relatedness?Achievement?)

3. What is the best way to addressthose needs to reach goals?

Leadership

• Once it is known what motivatespeople, leadership can be thought ofin relation to individual situations

• Two main types of leaders:EmergentAssigned

• Once it is known what motivatespeople, leadership can be thought ofin relation to individual situations

• Two main types of leaders:EmergentAssigned

Leadership – Shaping Behavior

• Rewards – the most effective type ofreinforcement E.g. Compliments, tangible benefits, etc.

• Punishments – have minimal impacton behaviour E.g. Reprimands, withholding of raises,

unfavorable task assignments, etc.

• Rewards – the most effective type ofreinforcement E.g. Compliments, tangible benefits, etc.

• Punishments – have minimal impacton behaviour E.g. Reprimands, withholding of raises,

unfavorable task assignments, etc.

Leadership Styles

• Directive - includes scheduling work, maintainingperformance standards, and letting subordinatesknow what is expected from them

• Supportive - friendly, approachable, and concernedwith pleasant interpersonal relationships.

• Participative – leaders will consult with theirsubordinates, and consider their opinions.

• Achievement-oriented - encourages subordinatesto exert higher efforts and strive for a higher levelof goal accomplishment.

• Directive - includes scheduling work, maintainingperformance standards, and letting subordinatesknow what is expected from them

• Supportive - friendly, approachable, and concernedwith pleasant interpersonal relationships.

• Participative – leaders will consult with theirsubordinates, and consider their opinions.

• Achievement-oriented - encourages subordinatesto exert higher efforts and strive for a higher levelof goal accomplishment.

Situational Factors

• 2 major situational factors affectingleader success:Subordinate Characteristics –

aptitude, individual needsEnvironmental factors – task urgency,

clarity, appropriateness of leader’sstyle to the situation, timing

• 2 major situational factors affectingleader success:Subordinate Characteristics –

aptitude, individual needsEnvironmental factors – task urgency,

clarity, appropriateness of leader’sstyle to the situation, timing

Participative Leadership

• Has a wide range of applications – Can have totalinvolvement of subordinates in implementation,planning, etc.

• Possible benefits of participation: Motivation Quality Acceptance• Possible Pitfalls Requires a lot of time and energy Resentment• Best to use this style when employees are part of a

team for an extended period and areknowledgeable/proficient

• Has a wide range of applications – Can have totalinvolvement of subordinates in implementation,planning, etc.

• Possible benefits of participation: Motivation Quality Acceptance• Possible Pitfalls Requires a lot of time and energy Resentment• Best to use this style when employees are part of a

team for an extended period and areknowledgeable/proficient

Leadership Styles

• Vroom and Jago’s styles:AutocraticConsultativeGroup

• Vroom and Jago’s styles:AutocraticConsultativeGroup

Qualities of Successful Leaders

• Intellectual Stimulation• Energy• Self-confidence• Assertiveness• Dominance• Motivation• Honesty and Integrity• Charisma

• Intellectual Stimulation• Energy• Self-confidence• Assertiveness• Dominance• Motivation• Honesty and Integrity• Charisma

Final Activity – Leadership Considerations

• Through other activities, goals, needs, andpossible motivators were identified. Now, taketime in groups to discuss the best leadership stylefor one’s own situation.

• Questions to consider: How involved do others need or want to be? Is participative leadership an option? Why or why

not? What environmental considerations are there? Are

there external influences? Is it possible to adapt your leadership style to

different situations? Give examples

• Through other activities, goals, needs, andpossible motivators were identified. Now, taketime in groups to discuss the best leadership stylefor one’s own situation.

• Questions to consider: How involved do others need or want to be? Is participative leadership an option? Why or why

not? What environmental considerations are there? Are

there external influences? Is it possible to adapt your leadership style to

different situations? Give examples

STRATEGY & PLANNING RELATIONSHIPBUILDING

ORGANIZATIONAL SKILLS &MANAGEMENT

CO-OPERATIVEDEVELOPMENT

Strategic Planning CommunityDevelopment

Organizational Governance Basics of aCo-operative

Proposal Writing Public Participation Board Orientation Co-operatives andthe CommunityDevelopment

Process I

Project Management Alternative DisputeResolution

Meeting Management Co-operative andthe CommunityDevelopment

Process II

Alternative DisputeResolution

Co-operative andthe CommunityDevelopment

Process II

OpportunityIdentification

Group Dynamics Leadership and Motivation

OpportunityManagement

InterpersonalCommunications

CommunicationsPlanning

Legal Issues

• Review objectives• Review any additional expectations• Review Parking Lot• Point out Certificates of Participation• Complete evaluation• Thank you!

Conclusion and Evaluation

• Review objectives• Review any additional expectations• Review Parking Lot• Point out Certificates of Participation• Complete evaluation• Thank you!

Community Capacity Building Programhttp://www.ibrd.gov.nl.ca/regionaldev/capacitybuilding.html

Materials used for Dies Moulds

Every mold designer must have a basic understanding of the various types of materials used inmold making . The mold designer has many aspects to consider . He must know processingconditions and production requirements . The mold to be fabricated must be correct to produceparts economically and tough to withstand the hard use to which it will be subjected . The cost ofthe material which goes into the mold is the least important consideration but the houses of laborand expensive equipment used in the construction of mould parts represent an importantinvestment that will be lost of the design is poor or the materials unsuitable . In view of the abovethe greatest care and consideration must be given to the selection of materials used in moldbuilding .

Steel is the most common material used in making of the molds other materials not becommonly used are Beryllium copper , Aluminums , bronze , Kristie ( zinc alloy ) , brass , Epoxiesand phenol and wood .

1) Steel : In reference to the mold building steel may be defined as an alloy which has propertiesthat move it useful as a math from which to form the main body of tools . Steel is made from ironand elements such as carbon , manganese , chromium , nickel , cobalt , unfasten , vanadium andmolybdenum etc. are added singly or in combination in order to impart desired qualities to themetal .Effect of alloy of elements in steels used for mold making :

element EffectCarbon Increases strength and brittleness , lowers corrosion resistance and conductivity .Manganese Deoxidizes , increases strength , ductility , wear resistance .Silicon Di oxidizes , increases hardness .Nickel Increase toughness , strength and through hardness but lowers thermal and

electrical conductivity .Chromium Carbide former , increase hardness ,resistance to wear and corrosion , lowers

conductivity .Molybdenum Increase heat resistance , expands heat treatment range , raises creep strength .Vanadium De oxidizer , forms hard carbides , raises fatigue strength .Tungsten Forms extremely hard carbides , increases hardness and resistance to wear and

temperature , helps to maintain shop edges .Sulphur Increase mach inability but lowers corrosion resistance and weld ability and

interferes with texturing and plating .There are four general classes of steels used by the mold maker .

1) Plate steels - Plate steel is a low carbon steel such as AVSI 1020 , produced in relatively increasiveprocess , where in cleanliness is loss important than high volume and low cost . Use of thismaterial is restricted to non – critical components e.g. the frames of molds . After car busing orcase hardening it can also be used to make cavities and plumages for cheap qualities of molds .Plate steels have low core strength because they are prone to structural faults such as pipe ,seams , pits and other defects .

Mold plates are usually made from plate steel , while knockout bars or cold rolled steals of themold has to be used in corrosive atmosphere , an all stainless construction must be consideredmachinery steel is of the same general class as the AISI 1020 plate steel . The difference is thatmachinery steel is hot rolled into flat rolled product , square bars or round rods . For manyapplications such bars can be used without any special other than surface grinding on both sidesto ensure flatness .Typical composition of steel for moulds :

Elements in %

AISI Type C Mn P S1020 0.20 0.45 0.04 0.051030 0.30 0.75 0.04 0.051040 0.40 0.75 0.04 0.051095 0.95 0.40 0.04 0.05

2) Tool Steel - Tool steel was the first material widely used in making the molds of good qulity .There are three types of tools in use –

1) Water hardening2) Oil hardening3) Air hardening

Tool steels have first fair mach inability and are not suitable for hobbing after hardening a block oftool steel will have almost the same hardness strength out its body but may lack toughness . As aresult the mold may be brittle and tend to break rather than yield when excess pressure is applied. The initial cost of tool steel is high and it is sometimes the preffered material for injection moldsbecause it does not or deform as easily as some other steels water hardening tool steel may beused when maximum hardness is desired . Water hardening tool steels are not recommended foruse with plastic moulds . Oil hardening tool steel and even better air hardening tool steels arerecommended because they perform more predictably in applications where distortion must beheld to a minimum .Typical composition of tool steels for moulds :

Elements in percentAISI Type C Mn Si Cr Ni Mo V OthersS1 0.50 0.70 0.75 3.25 - - 0.2 w 2.5A2 1.00 2.00 0.35 5.00 - - - -D2 1.50 - - 12.00 - 1.0 1.0 -H13 0.35 0.40 1.00 5.00 0.30 1.4 1.0 -L6 0.75 0.75 - 0.90 1.75 0.35 - -P2 0.07 - - 2.00 - 0.20 - -P20 0.35 0.80 0.50 1.70 0.30 0.45 - -P21 0.20 0.30 0.30 0.30 4.25 0.45 0.2 Al. 1.2

Alloy Steels - The alloy steels differ. From tool steels in several ways . Their carbon content isreduced and various other elements are added to modify their properties . Typical composition ofalloy steels used in mold making is given below .AISI Type C Mn Si Cr Mo Others1330 0.3 1.75 0.25 - - -4130 0.3 0.50 0.25 0.9 0.2 -4340 0.4 0.70 0.25 0.8 0.25 2.0 Ni8630 0.3 0.80 0.25 0.5 0.20 0.7 Ni

The addition of phosphorus and sulphur is restricted to 0.035 and 0.04% respectively .Stainless Steel - While there are many alloys classed as stainless steel , only a few are to beconsidered for use in high pressure molds . For a desirable balance of hardenbility , corrosionresistance and thermal conductively , the AISI type in the 400 refries are generally proffered ofthese , type 420 is the most popular . It contains 12 to 14% cr. And can be satisfactorily heattreated to develop a through hardness of 48-50 RC . It is used where corrosion and resisting areproblems such as processing as vinyl’s particularly in lurid conditions . Type 414 is a preharderedsteel having a hardness of HRC 30-35 . It does not require further heat treatment after the mold isfinished . It is suitable for injection mold where pressures are relatively low . It is therecommended material for molding corrosine materials where rusting is a problem . It does notconductivity of ss is lower than non ss .Typical composition of stainless mold steels :Type C Mn Si Ni Cr.T 410 0.15 1.00 1.00 - 12.0T 414 0.05 1.00 1.00 2.00 12.0T 420 0.25 1.00 1.00 - 13.0T 440 0.60/120 1.00 1.00 - 17.5

Precipitation hardening & mar aging steels : These are relatively new types of steels being usedas mold steels . They are hardened by procedures completely different from theheating/quenching tempering sequence associated with the conventional mold steels .Precipitation hardening alloys are delivered in the soft , solution treated conditional in which theymachine easily , they are hardenal by slow and gentle heating to a temp. as low as 4800 c .Hardness is controlled by the length of time at which the alloy is held at the hardening temp. Themain advantages of precipitation hardening alloys are –

1) Good Mach inability2) Superior polish ability and ease of texturing .3) Controlled heat treatment .4) Avoidance of thermal shock .5) Good weld ability and reaper ability .

Composition of precipitation hardening steels -Universal No. C Mn Si Cr Ni Cu Others

UNS S 15500 0.07 1.0 1.0 15.0 5.0 3.5 0.30 NbUNS S 17400 0.07 1.0 1.0 17.0 4.5 4.0 0.30 NbUNS S 17700 0.09 1.0 1.0 17.0 7.0 - 1.00 Al.

The material cost of precipitation hardening alloys is 2-5 times the cost of a good quality p 20 alloyHowever since the material cost of the mold , this is not a very important factor .

Maraging steels are an advanced and expensive class of precipition hardening steel . Inaddition to exceptional fracture toughness and other advantages of precipitation hardening steels, maraging steels can be safely hardened to HRC 70 at which hardness they devlop a strength of3,00,000 PSI .Typical composition of maraging steels :

ElementsC Ni C Mo Ti All Cr

350 type 18% Nickel Maraging Steel .02 17.5 12.0 4.8 1.5 .1 25300 ,, 18% ,, ,, ,, .03 18.5 9.0 4.8 .6 - -250 ,, 18% ,, ,, ,, 0.03 18.25 7.75 4.8 .4 .1 -200 ,, 18% ,, ,, ,, 0.03 18.25 7.50 4.25 .2 .1 -220 ,, 12% Chromium ,, ,, 0.02 10.00 - - .35 1.3 12%

Mold steel requirements - There are certain qualities which are essential for a steel to be used inbuilding of molds – These include

1) Cleanliness - A good mold steel must be clean . It should not contain non-metal ling inclusionswhich will cause pitting during polishing .

2) Soundness - The steel must be dense an free from voids and porosity .3) Structure uniformity - It must be uniform structure and free from segregations analysis . Its

properties should be substantially the same both along and across the direction of rolling .4) Mach inability - Steels which can be machined easily and uniformly are needed for economical

mold construction . Extreme softness is as undesirable as extreme hardness .5) Hob ability - Hibbing steels must be very soft when annealed and be clean and ductile as well . In

got iron and low alloy steels are easiest to hob . The higher alloy content steels offer somedifficulty but give the best results in service .

6) Harden ability - During the heat treating process , good mold steels must acquire surface and atough strong core .

7) Strength , toughness & Fracture Toughness - Molds require a hard surface and a very tough core .The larger the projected area of the mold , the greater will be the core strength needed to resistcollapse , distortion or cracking due to brittleness .

8) Heat Treating safety - An important characteristic of a good mold steel is its ability to behardened satisfactorily in a wide range of section thickness and by a variety of hardening methodswhile still producing uniform results .

9) Polish ability or finish - All mold steels must reading take a mirror like finish , althengh a dullsurface is sometimes selected as a final finish .

10) Wear Resistance - Wear resistance is a fundamental requirement of a good general purpose moldsteel . Glass and mineral filled composites cause maximum tool wear and therefore steels usedfor such composites should offer max. resistance to wear or abrasion .

Steels for injection molds :Steel AISI ApplicationsP 20 suitable for all types and sizes of machine cut molds .

Usually used in the prehardened condition RC 30-35 . Thisshould be carburized and hardened for low viscosity &glass filled plastics and for usage in excess of 1,00,000 Pcsper cavity .

H 13. Used for large & small molds when toughness strengthare required . Good dimensional stability duringhardening . Hardness up to Rc 52.

A 2 for small and medium size molds when higher hardness isrequired as for molding abasing materials .

D 2 for small molds when abrasion becomes a problem . Alsofor molds operating at temps. Up to 4000 c .

420 stainless for molding corrosive resins . Hardens up to Rc-48-52 .SAE 4140 usually used for holds and shoes can be used for molds

where a high finish is not necessary usually used theprehardened condition HRC – 28-32 .

M2 HSS used if operating temps are above 6000 c , but not higherthan 7000 c and mold hardness must be higher than 60 RC.

Precipitation hardening :Mar aging steels : For large molds or molds containing deep cuts and heavy sections , to avoidstresses and brittleness associated with quenching and tempering , also for mold componentswhich require exceptional hardness and fracture toughness .

Steels for compression & Transfer molds :Steel ( AISI ) ApplicationP 20 Must be carburized of 0.030-0.065% deep , depending

upon size . Surface hardness RC – 60 . Core hardness RC45-50 .

H 13 Good for large compression molds if nitride after heattreating to RC 48 .

SAE 4145 Often used for holdness and chases can be carburized toRC 60 .

S 1 For small & medium sized molds . Tougher than A2 butnot as stable , dimensionally in hardening . It may be oilhardened use at RC 53-56 .

S 7 For medium and large molds has good combination oftoughness and stability sections thinner than 2” will airharden otherwise oil harden to RC 53-56 .

Precipitation hardening :Mar aging steels : For large molds and molds containing deep cuts and heavy sections to avoidstresses and brittleness associated with quenching & tempering . Also for mold components whichrequire exceptional hardness and for fracture toughness .

NON – FERROUS METAL MOLDS : Several types of non ferrous metals are used for molds ,especially for short runs or where the pressures or wear conditions are not so severe , or wherethe metal has some special characteristic such as high thermal conductivity , ease of casting etc .The most commonly used metal non-ferrous alloys are –

1) Beryllium Copper Alloys : Many different alloys containing beryllium and copper are in use .Selection depends upon the desired balance between hardness , strength , cast ability ,conductivity and resistance to wear and corrosion as well as on the particular technique used formold making . Certain alloys are used for making cores and mandrels rather than molds .

In general , the alloys with 1.7% or more beryllium provide better fluidity and thereforebetter reproducibility . They have less tendency to form dross thus making possible to cast themat a lower temp. Higher beryllium concentration results in better reproduction , but the costincreases .The c 82400 alloys are most common in use . C 82600 alloys can be used where lower patternprecision is required .

Alloys with less than 1.7% be content are generally used only for mold cores and mandrels, Where high fidelity of reproduction and very high strength are not required . Such use of Be Cualloys results in lower mold material cost and

There may be some difficulty in their disposal in an environmentally acceptable manner .Be – Cu Alloys : Properties & ApplicationsType composition others Thermal HRC. Characteristic &applications

( Balance cu ConductivityBe Co ( BTU/ft/w/

Of at f )C 172000 2.0 0.5 - Co 40 Good strength & wear

Resistance with goodelec.&

Thermal conductivity .

C 17510 0.6 2.5 - 145 22 High thermal & elect.Conductivity but lowerhardness used wheremax. heating coolingrates are report .

C 82400 1.7 0.3 - 58 37 Good strength , hardnessCorrosion resistance &conductivity .

C 82510 2.0 0.5 0.3 Si 56 41 Similar to 82400 but withBetter cast ability .

C 82600 2.3 0.5 0.3 Si 54 44 As 82400 but withimproved

Resistance to wear usedin pressure & ceramiccashings .

C 96700 1.2 - 30.0 Ni 21 50 Highest corrosionresistance

and strength & castability , but lower thermalconductivity , Resistsflame retardants ,blowing Agents and othercorrosive chemicalscontained in moldingresins .

2) Aluminum : All alloys injection mold are mainly used for short run production or for proto typefabrication . They are however used extensively for low pressure applications such as rotational

offers the advantages of lightweight , ease of machining , high thermal conductivity and moderate costs . Most commonly usedalloys for making of molds are 7075 T6, 6061 or 245 alloys . Alloys 7075 T6 is representative of thegroup . It contains 5.6% Zn , 2.5% mg , 1.6% Cu and 0.25% Cr . Al alloy molds have given goodservice for production runs up to 2.50,000 cycles . Anodizing increases the alloys wear resistance .

3) Zinc Alloys : Several alloys of zinc have been used for casting molds especially for pre productionproto types for injection molding . They are easier to handle , can be cost at 4750 c and yet theydevelop surface hardness and compressive strength equal to some of Al. alloys . The use of a steelframe is recommended if the mold is used in any high pressure application . Typical compositionof zinc alloy is zn – 92% , cu – 3.5% , Al- 4.0% , mf 0.04% .

4) Nickel : In general electroformed Nickel is used to produce molds such as pen barrel cavities andsimilar parts which might be very difficult to fabricate by normal machining . In some cases , thecavity is built up to a thickness of approve 1/6” using a hard nickel deposit and then the mold isdeposit faster . In some cases the hard nickel is rain forced by a copper deposit which builds up

even faster . Nickel molds are also produced by a vapor deposition technique which results inbetter tolerances , more uniformity of shell thickness , lither strength and better heat transfer .

5) Al-Bronze and Nickel Aluminum Bronze : This is a family of copper based alloys . The grades usedin the mold making typically contain about 80% cu , 12% Al , and 5% iron . To increase the strengthof the alloy about 5% Ni is added by reducing Cu & Al contents . This also raises the corrosionresistance while maintaining good ductility and fracture toughness . The Al bronze alloys havebeen used widely in applications such as wear plates , slides , moveable bushings and / or plungesin contact with steel .

6) Powdered Metal Components : Small cavities can be produced by compressing finely powderedmetals or mixture of such powders held together by the binder materials . The powder mix ismolded around a master from which the cavity is formed .

In this process different metals and binds are mixed together at room temperature andthen compacted to form hard and strong shapes such as injection moulding cavities . This processis known as mechanical alloying . The powdered mix may contain super hard particles such astungsten or titanium carbides for better wear resistance or nickel if toughness and resistance tocorrosion are rigid . It may also contain soft particle such as alloys of copper to impart lubricity .Some powders require heat treatment and HIP treatment ( HOP isocratic pressing ) Where as insome cases a chemical action is employed followed by sintering to remove any residue binder .

Nonmetallic mold materials : There are a number of non-metallic materials which are also usedto produce molds . In general these materials are used where the pressures are not high such ascasting , rotational molding , non-structural foam , thermo forming etc. The non-metallic moldmaterials are of two types –

1) The elastomeric or flexible materials : These include room temperature mechanizing silicons inmany different hardness , use them resins , vinyl plastically , gelatin , polysulfide and variousrubber latches . The mold is made by casting or brushing the material around a master and thenhardening to form a mold . The main advantage of using the elastomeric materials is that theparts can be made with under cuts and still se removed from the mold . the disadvantage is thatonly plastic casting can be made using such molds since any pressure will cause deformation . Aspecial characteristic of the silicon RTV molds is that usually mold releasing agent is not rigid evenwhen casting of epoxies or Pus is done .

2) The Rigid Materials : The rigid non-metallic’s include such materials as plaster of pass , wood ,glass fiber reinforced plastics such as epoxies and polyesters . A mold release agents is required inthese types of molds . Cast epoxies reinforced with metal powder can be made strong enough tobe used as an injection mold for small quotation of parts , if very high pressure are not employedand temps. Are kept below 200-2300 c . Backing up the mold with a metal frame is recommended .The main advantages of using non-metallic molds are -

1) In most cases the cost is considerably less then a similar metal mold .2) Prod time is usually much less than making a metal mold by machining , EDM or electroforming .3) The wt. is usually much less than a metal mold .4) Repair work is simple and can be done quickly .5) These molds are much more corrosive resistant then metal molds .

6) Transparent molds can be made and for some applications such molds allow better control of theprocess .The disadvantages are –Lower temp. resistance , Lower thermal conductivity and Lower pressure resistance as comparedto metal molds .

General Design Consideration for Various

Types of Molds & diesThe designing of mold and dies should be given a lot of careful attention . It is important torecognize that the forces involved resulting from high injection pressures in case of injectionmolds are very large and their effects on cavity strength must be carefully analyzed . The temp. ofmolding and its influence on cavity configuration must also be taken into account . Once thegeneral design concept has been established , a detailed analysis of strength requirements shouldbe done . After completion of this process , one should proceed with dimension and proportionsof vital components . Mold design should include calculations for runner size , position of gatesstrength that will assure safe and sates factory performance of the costly tools and so on .A mold should be designed to be changed with min. down time of the m/c . If should change timeis small lesser inventories may be kept leading to profitability .Designing a mold requires that a logical sequence be followed and the basic information beobtained about the part , the material to be molded and the equipment to be utilized .

1) Part Design : The part design should be analyzed to determine the location of parting line , theacceptable gate location and the type of ejection to be used to remove the part . There may besome part design features that require special design consideration in the mold design . Suchfeatures are molded threads , side cores , undercuts etc. Problem areas such as critical dimensions, minimal draft specification etc. should be identified .

2) Mold Configuration : The anticipated volume of parts to be produced by the mold will determineno. of cavities to be included in the mold and the type of the mold to be built .

3) Machine Selection : The design of the mold will also depend upon the type of the M/C. for aninjection mold , following data about the M/C should be available –a) Clamp force ( Tons )b) Injection capacity ( gms/ oj )c) Injection Pressure ( PSI/Newton/M2 )d) Platen size hxv ( in , cm )e) Distance between tie rods hxv ( in , cm )f) Clamp stroke max. ( in , cm )g) Min. Mold height ( in , cm ) .

The weight of the shot and the required clamp tonnage based on the projected area of thepart must be calculated .

4) Material : Physical properties of the plastic malt to be molded must be known . The flowcharacteristics , mold shrinkage , corrosiveness , and wear characteristics of the material must beconsidered .

5) Mold Layout : When the above information has been collected , a rough design of the mold canbe made . The size of the cavity and core components can be estimated and the size of the moldbase to be utilized can be selected .

6) Gating : A choice must be made for the type of material distribution system from the barred tothe cavity to be used . The conventional multicavity mold utilize a sprue and a parting line runner

system . Other choices which are dictated by part configuration , economics etc. are three plate ,back gated , insulated runner , hot manifold or lot runner .

7) Part ejection : The method by which a part will be ejected is lactated by the part design , the sizeof the mold the M/C. and the degree of automation to be utilized in the molding operation . Thechoice of ejector pins , ejector sleeves , stripper plate or air assisted ejection is determined by thepart design .

Some Considerations for Compression Molds :-1) Mold which are made with cavities or plumages in a section require especially good

backing and hardened back up plates are recommended .2) Loose pins used in molds may stick , therefore provision should be made for the addition

of an ejector pin behind the loose pin .3) When ejector pins but against the opposite half of the mold , use hardened strips under

the pins to prevent sinking .4) A solid part will shrink more than a part having thin walls , therefore additional shrinkage

allowance must be made .5) Removable plate molds must be designed with stop blocks between the top and bottom

so the mold can not be fully closed when the plate is out .6) All feather edges must be eliminated in mold designs . Molded threads must be designed

without feather edges if low cost operation is desired . Feather edges will require frequentmold replacements because of breakages .

7) The determination of bulk factor and shrinkage are of prime importance in the design of acompression mold .

8) The product design must be studied carefully to make sure that the selected design ismoldable .

General concepts of die design : Designing dies requires an accurate knowledge of flowcharacteristics of the particular plastic material to be processed . The designer mustdetermine the precise shape of flow channels in the die and the exact shape of the exitorifice . The designer must know how plastics flow under pressure . One important factorto be considered is die swell . The amount of die swell will vary from one material to theanother and will also vary with changes in the operating conditions for the extruder .Depending upon the material and the melt conditions , die swell will range from less than5% to over 100% of the orifice dimensions . The basic relationship used for die design isthe fellow will vary as the cube of the channel depth ( D ) and inversely as the channellength ( L ) with any given upstream pressure .

BASIC CONCEPTS OF DESIGNBefore the design of any part is begun , the usual procedure is to first determine the function ofthe part and the environment in which it is to operate . This environmental survey should includethe thermal conditions high and low temps. and the mechanical stresses on the part tensile load ,impact , compression etc. In some types of parts the Major factors may be electrical stresses highvoltages or frequencies , dielectric losses etc. Some parts may combine thermal stresses withchemical stresses such as strong acids , solvents , oils , high lucidity etc. Operational time mustalso be considered .

The next step is usually to prepare a list of the requirements of the part from the standpoint of user acceptance . Such a list may include items such as colour , transparency , specificgravity and cost . The list should also include hard to define items such as feel of the part etc. Afterall the requirements have been determined , the next step is to decide on the material . Inselecting the material , the most important factor should be used as the first screening guide . Forexample if a transparent article is to be made , the list of probable materials is considerablyshortened . The final choice is usually a material which will give the optimum performance at thelowest cost . In practice usually a compromise is made in the selection of material .

The next step for the designer is to make the selection of fabrication process . In manycases the choice is dictated by the no. of parts or the material chosen . In other cases thefabrication process is dictated by the type of part or sometimes the type of machine available .

Regardless of the type of part or the overall design , there are certain fundamental designrules which are applicable to almost all types of parts . There include avoidance of stressconcentrators , proper use of ribs and fillets , hole spacing etc.

SHRIN KAGE :- Shrinkage is the amount by which a molded product is smaller than the size of thecavity space , where in it was produced by injecting plastic under high pressure injection and athigh temperatures . It is expressed as MM/MM Dr in % . There is a definite relationship betweenpressure ( P ) , volume ( V ) and temp. ( T ) . This relationship is different for various plastics . Anyand all conditions that affect temp. pressure and timing will affect shrinkage .

- When a volume of plastic is heated , it expands . When it cools to the original temp. , it willcontract to the original volume .

- When a plastic is compressed , its volume is reduced . When the pressure is reduced to originalpressure , it will turn to its original volume .

Shrinkage depends on the following variables –1) The plastic material :- different materials have different heat expansion values . But even

materials with the same chemical and physical specifications may have significant differences inheat expansion and therefore in shrinkage .

2) Product geometry :- This applies mainly to variations in wall thickness and the shape of surfaces ,ribs etc .

3) Mold design :- The designer must take shrinkage into account , particularly in the waling layoutwithin the mold , the geometry of runners and gates and the uniformity of heating in hot runnersetc.

4) Type of molding machine :- Injection speed , available injection pressure , accuracy of time ,temp. and pressure controls etc. , all affect shrinkage .

5) Condition of molding M/C . & Mold :- The condition of molding M/C . & Mold may also affect theshrinkage . An old or neglected M/C . may have unreliable controls or a worn check valve etc. Amold which has not been properly maintained may have corroded or plugged waling lines .

6) Molding conditions :- This includes the M/C . set up , mold cooling temperatures , cycle timeelements , injection and hold pressures etc. When hot plastic is injected into the cavity , thepressure in the cavity is relatively low until the cavity is filled . After the molten plastic has filled upthe cavity , pressure builds up rapidly . This compresses the plastic in the cavity space . Thepressure in the cavity is , therefore , a major factor affecting mold shrinkage , Normally thispressure is maintained until the gate freezes or is closed sealing off the material in the cavity .From this time on wards , the pressure within the cavity will drop as the plastic cools and shrinks .Therefore at constant mold and melt temps. Variations in injection pressure and timing of sealingof the gate are the most important factors affecting variations in shrinkage . If the component isejected in fairly hot condition it will shrink more than the component ejected at lower temp.Thickness of the product also affect shrinkage . as a rule a thick walled product will shrink morethan one with a smaller wall thickness . Therefore confiscation of heavy and thin walls adjacent toeach other should be avoided , provided deformation is not required in the end product .

Shrinkage Formula :- The diagram and variables defined below are used in calculating shrinkagefactors .Dc = Dpt ( Dp*s ) + ( Dp*s2 )Or Dc = Dp ( I+S+S2 )Because S is usually very small , S2 can be ignored and the formula is simplified toDc = Dp ( I+S )Where Dc = cavity and core steel dimensionDp = Product dimension &S = shrinkage factor ( MM/MM or % )

Shrinkage may also be defined as –(A) Axial shrinkage - It is the shrinkage , Which occurs in the direction of flow of plastic .(B) Radial shrinkage – It is the shrinkage which is in the direction perpendicular to the flow .

This may be important when product tolerances are very tight and the plastic shows considerabledifference in the shrinkage in these two directions .

APPROXIMATE SHRINKAGE VALUES FOR VARIOUS MOLDING MATERIALS :-MATL SHRINKAGEABS 0.005 – 0.007

Acetal Axially 0.021 – 0.026Acetal radically 0.018 – 0.020Acrylic 0.004 – 0.007EVA 0.007 – 0.020Nylon 6 0.006 – 0.014Nylon 66 Axially 0.012 – 0.033

Radically 0.020 – 0.028Poly carbonate 0.006 – 0.008PE 0.015 – 0.050PE , 30% glass filled 0.014 – 0.045PET bottle grade 0.005 – 0.012PP 0.012 – 0.022PS 0.002 – 0.006PS 30% glass filled 0.0005 – 0.0010PVC 0.003 – 0.008PVC 30% glass filled 0.001 – 0.002

FLASH LINE OR PARTING LINE :- Parting line or flash line considerations depend on the functionthat the part is to perform with most products the P/L is in an obvious location . However in otherproducts the P/L is not so simple and requires considerable thought . Following points should beconsidered –

1) Ejection :- An important consideration in selecting the parting line is to ensure that the productwill stay on the side which will have the ejection mechanism .

2) Shut-offs, offset P/L :- If there are openings in the sides of a product , which do allow the productto pull off but which will require shut off areas between core and cavity , then to prevent slidingmotion of a shut off area between cavity and core , this area must be at an angle . Such an angle(a) Prevents flashing and (b) reduces wear .

3) Strength of P/L :- After establishing the P/L , it is necessary to check the strength of the areawhere cavity and core will touch . If strength is inefficient either the area should be increased orsupports should be added to take some of the clamping force .

4) Matching of P/L :- P/L should be produced to provide flash free matching of the mating surfaces .Usually the P/L is ground on both cavity and core side .

UNDER CUTS :- Under cuts are indentations or projections on the wall of a plastic part whichinterfere with simple ejection from a two part mould . Under cuts should be avoided wheneverpossible . The unnecessarily complicate the mould design and considerably increase the cost .Special split moulds are required for ejection of components having under cuts and retractingmechanisms etc may be needed .Designs with internal undercuts may be machined after moulding of the components at less costthan the cost which may be incurred if the component is moulded with under cuts . This isparticularly true of external undercuts .

WALL THICKNESS :- The first rule of good design is to use a uniform wall thickness whenever andwherever possible . A thick section , next to thin sections will cool last and shrink more away fromthe mold , resulting in a sink mark . The differential cooling will also result in internal stresses ,warpage or distortion and may sometime cause cracking . If a non uniform wall thickness can notbe avoided then the different wall sizes should be gradually blended as shown in the figure below .

The wall thickness should however not vary by more than a ratio of 3:1 if possible . In addition touniform thickness it is desirable to design parts with wall sections that are well within normalmolding capability , which in turn depends upon the machine , mold and material to be molded . Itshould also be kept in mind that higher wall thickness will result in longer cycle times because ofslow rates of cooling .

IMPRESSION :- The injection mould is an assembly of parts containing within it an impression intowhich plastic material is injected and cooled . It is the impression which gives the moulding itsform . The impression may therefore be defined as that part of the mold which imparts shape tothe molding . The impression is generally formed by two mould members –

1) The cavity – it is the female portion of the mould and it gives the moulding its external form .2) The core – It is the male portion of the mold and it forms the internal shape of the moulding .

Cavity & core Plates :- A basic mould consists of two plates . In to one plate , the cavity is sunkwhich shapes the outside form of the moulding and is therefore known as cavity plate . Similarly

the core which projects from the core plate forms the inside shape of the moulding . When themould is closed , the two plates come together forming a space between the cavity & core , whichis called the impression .Types of core & cavity :- There are two alternative ways by which the cavity and core can beincorporated into the mould , namely the integer method and the insert method . Anothermethod by which the cavity can be incorporated is by means of split inserts or splits .

1) Integer Cavity & Core Plates :- When the cavity or core is machined from a large plate or block ofsteel or is cost in one piece and used without bolstering as one of the mould plates . It is termedan integer cavity plate or integer core plate . This design is preferred for single impression mouldsbecause of the strength smaller size and lower cost characteristics . It is not used as much formulti impression moulds .

Normally two types of process are used for preparing integer type cavity and core . These are

a) A direct machining operation on a rough steel forming or blank using the conventionalmachine tools orb) The precision investment casting technique in which a master pattern is made of thecavity and core . the pattern is then used to prepare a costing of the cavity or core by a specialprocess . A 4.25% Ni – Cr – Mo steel ( BS- 970-835 M 30 ) is normally specified for integer mouldplates which are to made by the direct machining method . The precision investment castingmethod utilizes a high chrome steel .

2) INSERTS :- Core & Cavity :- For moulds containing intricate impressions and for multi impressionmoulds , It may not be satisfactory to machine the cavity and core plates from single blocks ofsteel . The machining sequences and operation would be very complicated and costly . The insertbolster assembly method is used for this type of moulds . The method consists of machining theimpression out of small blocks of steel . these machined components are known as core inserts orcavity inserts depending upon their use either in core or in cavity . These are then inserted andsecurely fitted into holds in a plate of steel called a bolster . The holes are either sunk part way orare machined right though the bolster plate . The inserts are mostly circular or rectangular inshape .Advantages & Disadvantages of integer and insert bolster method :-

Both the integer and the insert bolster methods have their advantages depending uponthe size , the shape of the moulding , the complexity of the mould , no. of impressions in themould etc. The comparison of the advantages and disadvantages is discussed below –

1) Cost :- The total cost of a mold includes – (a) the cost of the mold material and (b) thecost of matching & fitting . The integer method requires the whole mould plate to bemade of expansive mould steel , where as the insert – bolster method needs only parts ,which form the impression to be made of mould steel and the bolster is made fromconsiderably cheaper mild steel . However the machining & fitting of a single impressioninteger type mould is less costly in time and in no. of operations as compared with theinsert – bolster combination .

2) No. of Impressions :- The difficulty in machining and aligning the cavities and cores in aninteger type mould increases with the no. of impressions in the mould . Therefore formulti impression moulds it is usually preferable to use insert – bolster system .

3) Multi impression Mould alignment :- The non uniformity and non- conformity of the cores& cavities in a multi impression mould can be corrected more easily in an insert – bolstersystem as compared to integer type mould .

4) Mould size :- The fabrication of an integer type mould requires that very heavy steelblocks are to be handled during the manufacturing stage , where as in the case of an insert– bolster system , the smaller blocks will have to handled , for individual impressionswhich is much easier .

5) Heat Treatment :- It is often desirable to heat treat that part of the mould which containsthe impression to given a hard , wear resisting surface . During this heat treatment , theseis a possibility that the steel may distort . This possibility reduces for smaller blocks ofsteel . From this point of view , insert type moulds are preferable .

6) Replacement of damaged parts :- with the insert bolster type system , it is possible torepair a damaged impression while continuing to operate the mould with the remainingimpressions , resulting in minimum interruption to prod4 .

7) Cooling system :- It is easier for an integer mould .8) Conclusion :- For single impression moulds the integer design is to be preferred

irrespective of whether the component form is a simple or a complex one . The resultingmould will be stronger , smaller & less costly . For multi impression moulds , the insert –bolster system is most commonly used for the ease of manufacture , mould alignment andresulting lower mould costs .

BOLSTERSWhen the cores and the cavities are incorporated into a mould design as inserts , theymust be securely retained in the mould . This is achieved by fitting the inserts into abolster , which when fitted with suitable guiding arrangements , ensures that alignment ofthe cavities and cores is maintained . The fundamental requirements of a bolster are –1) It must provide a suitable pocket into which an insert can be fitted .2) It must provide some means for securing the insert after it is fitted in position .3) It must have sufficient strength to withstand the applied moulding forces .

The bolster is normally made from mild steel plate to the BS 970-040 A 15specification . In certain cases , a medium carbon steel ( BS – 970-080-M 40 ) is alsoused .Types of bolster :- Following are the main types of bolsters :-1) Solid bolster :- Solid bolster is suitable for use with both rectangular and circular

inserts . This is made by squaring up a block of steel . Then by a direct machiningoperation , a pocket is sunk in to the top surface to a predetermined depth . Theshape of the pocket is either rectangular or circular to suit the shape of the mouldinserts .

2) Strip type bolster :- in this type of bolsters , the poet is made by machining slotcompletely through the bolster block . Steel strips are then fitted at either end ofthe slot to complete a frame for the inserts . To prevent the strips from movingunder possible side thrust , a projection extends from the underside of the stripand this fits into a mating recess in the bolster with socket – headed screws .These types of bolsters can be used only for rectangular inserts . The advantage ofthis type of bolsters is that all the important surfaces are ground and thesubsequent fitting of the insert is simplified .

3) Frame type bolster :- This bolster consist of two parts , namely a frame andbacking plate . The frame is made by machining an aperture of the required shapecompletely through the bolster plate . The bottom of the insert is supported by abacking plate secure to the frame with a no. of socket headed screw . The insertsthem selves may be secured either in the same manner by screws through thebacking plate or altematively by the use of flanged inserts . This type of inserts areparticularly useful for small inserts .

4) Chase bolster :- This type of bolsters are used with split inserts . When splitsinserts are to be incorporated in the mould design it is necessary for one of thebolsters to lock the splits in their closed position . chase bolsters are of two types:-

a) The open channel :- This is used for shallow rectangular splits and is mad bymachining a channel across the width of the bolster plate . The sides of thechannel are stopped or angles . The stopping sides or faces are usually facedwith hardened area strip made from a carburized low carbon steel .

b) Enclosed chase bolster :- This is used for deep splits It is machined from asolid block and the pocket which is to accommodate the splits may be atapered circular or a tapered rectangular form .

5) Bolster plate :- Inserts can be mounted directly into a plane bolster plate . Thissystem provides no side support and the walls of the inserts must be of sufficientthickness to withstand the applied moulding pressure without undue defection .the inserts must also be securely screwed and dowelled in position to preventmisalignment .

Mould alignment :- mould alignment is one of the most important area infabrication and operation of a mold . If the mould haves are not perfectly aligned ,it will not be possible to produce quality components . Moreover the leader pinsand bushes will be subjected to excessive wear , reducing their life .During the fabrication , the aligning is done after the core and cavity have beensemi finished . This is achieved by clamping the two mould halves together incorrect position and boring guide halves together both the plates . Guide pillersand bushes are then fitted into the plates . The two mould plates are againbrought together to check the perfect alignment of core and cavity . A dummymoulding is often made at this stage , using wax , so that the wall sections of themold can be checked .During the injection moulding operation the mold halves are normally hold inplace by using mold clamps .

Dowel or leader pins :- The dowel or leader pins or the guide pillers are used toensure the alignment of mould halves during operation . This is necessary formouldling an even called product . The guide pillers pins are incorporated on onemould plate . Which then enter corresponding guide bushes in other mould plateas the mould closes . The size of guide pillers should be such that they maintainalignment inspective of the applied moulding force .A guide piller is designed so that the working diameter is smaller then the fittingdiameter D by a min. of 7mm. This introduces a shoulder into the piller , where itemerges from the mould plate so that the fitting diameter of the guide piller canbe made the same as the guide bust . Thus the holes of same dia can be bored andground thought both mould plates , when clamped together . Thus allows perfectalignment .The piller is normally machined from a low carbon steel ( BS- 970 – 050-M 15 )which is then case hardened . Save times carburized Nickel-chrome steel (BS-970-835 M 15 ) is used for better bending resistance . The normal size range of guide

pillers is between 10mm to 38mm working diameter . Some very large mouldsmay require guide pillers outside this range .

Size range of guide pillers :-Working

diameter(d)Size of mould

MM In (MM) (In)1013161922253238

3/5½5/8¾7/811 ¼1½

100*100100*150150*200200*250250*300300*400400*600600*700

4*44*66*88*1010*1212*1616*2424*28

Register Ring :- The registering or the locating ring , is a flat circular member normally fitted onthe front face of the mould . It is purpose is to locate the mould in its correct position on theinjection moulding machine plastic .

When the mould is mounted on the machine . the front mounted register ring fits into acircular hole which is accurately machined in the injection platen on the cylinder nozzle axis . Thisensures that the small aperture in the nozzle is in direct alignment with the sprue bush hole .Since the sprue bush is the connecting member between the machine nozzle and the mould face ,this alignment of nozzle aperture and sprue bush hole permits an uninterrupted flow of materialfrom the cylinder , through the nozzle and sprue hole into the mould runner system . The registerring infect forms a direct connection between the sprue bush and the hole in the injection platenof the machine .

As the register ring is permanently attached to the mould , correct alignment followsautomatically whenever the mould is set up on the machine and no adjustment by the setter istherefore necessary .

The register rings may be of various designs as shown in the fingers below :-

Attachment of mould to platen :- There are two ways by which the mould halves may beattached to the platen of the injection moulding machine .

1) Direct Bolting Method :- In principle , holes are provided in each mould half to correspond withthe holes tapped in the machines platen . Bolts are then used to directly secure the mould to theplaten . Now a days machine manufactures adopt a standard hole layout and therefore a molddesign for one machine can normally be fitted to another of similar size . Various alternative directbolting design can be chosen as shown in the figures below .

2) Indirect Bolting Method :- With this design the attachment of the mould to the machineis by means of a clamp plate . It is used when it is not possible to use the direct boltingmethod . The indirect bolting assembly consists of three parts , namely the clamp plate ,the bolt and the packing piece . There are two alternative designs –a) The front plate incorporates a projection (or flange) and a clamping force is applied to

this by the bolt via the clamp plate .b) A slot can be machined through the mould plate and the clamping force applied in

identical manner .

Since large clamping forces are provided by M/C Mfrs, and it is likely that if small land widths areadopted , the effective land area will be insufficient to withstand the applied forces and therelatwely narrow steel projection will deform . To overcome this possible hazard , the land area isincreased by insuring that other area of the mould face are left proud in places unlikely to beaffected by flash such as the corns of the mould .

Venting :- When the plastic material elves an impression , all the air inside should be displacesotherwise moulding defects such as discoloration sink marks , incomplete filling etc. will develop .Therefore it is recommended that vents be provided in the mould to allow air (and other gaseswhen present ) to escape freely . The vent is normally a shallow slot , not more than 0.05MM deepby 3MM wide , machined in the land . Positions where the vent is likely to be required are –

1) At the point where flow paths are likely to meet .2) At the point further most from the gate on symmetrical mouldings and3) At the bottom of projections .

The vent is normally machined into the mould plate once the mould has been tried out todetermine its best location .

FEED SYSTEMIt is necessary to provide a flow way in the injection mould to connect the nozzle to eachimpression . This flow way is termed as the feed system . Normally the feed system comprises asprue , runner and gate . The material passes through the sprue , main runner , branch runnersand gate before entering the impression . It is desirable to keep the distance that the material hasto travel down to a minimum to reduce pressure and heat losses .SPRUE :- During the injection process , plastic matl. is delivered to the nozzle of the machine as amelt , It is then transferred to the impression through a passage . In the simplest case this passageis a tapered hole within a bush . The material in this passage is termed the sprue , and the bush iscalled a sprue bush . Therefore sprue bush is the connecting member between the machine nozzleand the mould face and provides a suitable aperture through which the matl. can travel on its wayto the impressions or to start of the runner system in multi impression moulds . Sprue bush isnormally made from a 1 ½ % nickel chrome steel ( BS 970-817 – M 40 ) and should always behardened .RUNNER :- The runner is a channel machined into the mould plate to connect the sprue with theentrance (gate) to the impression . In the basic two plate mould the runner is positioned on theparting surface while in more complex designs the runner may be positioned below the partingsurface . The wall of the runner channel must be smooth to prevent any restriction to flow . Alsoas the runner has to be removed with the moulding , there must be no machine marks left whichwould land to retain the runner in the mould plate . There are some considerations for thedesigner to keep in mind during designing of a runner for a particular mould .

1) The shape of the cross section of the runner –The cross sectional shape of the runner used in a

mould is usually one of the four forms – (a) fully round (b) trapezoidal (c) modified trapezoidal and(d) hexagonal (fig.) The criteria of efficient runner design is that the runner should provide amaximum cross sectional area from the stand point of pressure transfer and a minimum contacton the periphery from the stand point of heat transfer . The runner efficiency is therefore definedas .

Runner efficiency = Cross sectional areaPeriphery of runner

Efficiencies of various types of runners are finer below –

It can be observed from the table that the round and square type of runners are the two mostsatisfactory designs from the stand point of efficiency . The square runner however becomesundesirable because it cannot be ejected easily . In practice , an angle of 100 is incorporated onthe runner wall , thus modifying the square to the trapezoidal section . The volume of thetrapezoidal runner is approximately 25% greater then that of a round runner with correspondingdimensions ( W=D) .

The hexagonal runner is basically a double trapezoidal runner , where he two halves of thetrapezium meet at the parting surface . The cross sectional area of this type of runner is about82% of that of corresponding round runner .The choice of runner is also influenced by the question whether positive ejection of the runnersystem is possible . This can be used in two plate moulds only and is not practicable for multiplatemoulds . Here the basic trapezoidal type runner is specified . Hence we can say that for simpletwo plate moulds which have flat parting surface the fully round runner or hexagonal runner is tobe preferred for moulds which have complex parting surface , the semicircular , trapezoidal ormodified trapezoidal runner should be used .

2) Runner size :- Following factors should be considered for deciding the size of the runner –a) The wall section and volume of moulding .b) The distance of the impression from the sprue .c) Runner cooling considerations .d) The range of mould maker’s cutter available , ande) The plastics materials to be used .The cross sectional area of the runner must be sufficient to permit the melt to pass through and fill

the impression before the runner freezes and for packing pressure to be applied for shrinkagecompensation of required . Runners below 2MM dia are not normally used . The longer the melthas to travel along the runner , the greater is the resistance to flow . Hence longer the runnerdistance , bigger should be the cross sectional area . If cooling of the runner system is very fast ,larger cross sectional area will have to be provided . If size of the cross sectional area of the runneris such that it is in between two sizes of the cutters available with the mould maker, then thedesigner should opt for the longer cutter size .

Type of the plastic material to be processed also affects the size of the runner . A less viscous andsmooth plastic melt will need smaller cross section than a more viscous and more abrasivematerial such as FR Ps. The following formula is used to find the size of the runner or branchrunner mouldings weighing up to 200gms. and with wall sections less then 3MM.

D =

Where, D = Runner dia (MM)W = Weight of moulding (gm)L = Length of runner (MM)

For rigid PVCs and acrylics , the calculated dia of the runner is increased by 25% . The runnershould not be below 2MM dia nor above 10 or 13MM dia wherever applicable and the calculatedsize . The cross sectional area of the main runner should be equal to or in excess of the area of thebranch runners . This relationship is ignored when the max. dia is reached . Intersection ofsecondary runners with the main runner should be filleted with a 3MM radius on the sprue side ofthe intersection . All main runners in a given mould should be equal in diameter , while thediameter of all secondary runners should be at least 0.7MM less than the diameter of the mainrunner .

RUNNER DIAMETERS FOR UNIFILLED MATERIALSMATL. RUNNER DIA.

Indult. MM.ABS, SAN 3/16 - 3/8ACETAL 1/8 - 3/8ACRYLIC 5/16 - 3/8NYCON 1/16 - 3/8P.C 3/16 - 3/8POLYESTER(TP)(UNFILLED) 1/8 - 5/16

Reinforced 3/16 - 3/8Poly ethylene LD – HD 1/16 - 3/8PP 3/16 - 3/8PS (All Types) 1/8 - 3/8PVC (Plastic used) 1/8 - 3/8

Rigid ¼ - 5/16

Weights of runner systems for matl. density 1gm/cm3 .RUNNER DIA Wf. (gm) per cm length of runner .

ROUND. TRAPEZOIDAL2 0.0314 0.03663 0.0707 0.08234 0.1256 0.14635 0.1963 0.22866 0.2826 0.329212 1.1304 1.316715 1.7663 2.057418 2.5434 2.9626

RUNNER LAYOUT :- The layout of the runner system depends upon the following factors –a) The no. of impressions .b) The shape of the mouldings .c) The type of the mould . ( Two plate or multi plate mould )d) The type of gate .

There are two main considerations for designing a runner layout –1) The runner length should always be kept to a min. and2) The runner system should be balanced .

Various types of balanced runner systems are shown in the fig. below –

BALANCING OF RUNNERS :- Runner balancing means that the distance the plastic materialtravels from the sprue to the gate should be the same for each moulding . This system ensuresthat all the impressions will fill uniformly and without interception provided the gate lands and thegate areas are identical .The melt should arrive at all gates with the same pressure & temp. so that all products within onemould have uniform characteristics .

To achieve balancing it is preferred to arrange the runners symmetrically from the pointof entry of plastic all the way to each gate . This is fairly easy with some layouts but becomesdifficult with a large no. of cavities even through the geometry of the product is symmetricalbecause of the following reasons .

- The flow of plastics through the channels changes at every point of division into branchchannels and depends on the amount of redirection of the melt at the point of change .

- The flow depends upon the accuracy of machining and finish inside the channel .- There may be temperature differences within the runner due to uneven heating and

cooling of the steel runner ding the runner .- There are machining tolerances of the gate .- Uneven venting can affect the filling of cavity .- Some resins are flow sensitive to changes in direction and generally will not flow evenly

around several bends in a runner system .- The phenomenon of the “plastic memory” can also affect the flow within the runners .

GATES

The gate is a channel or orifice connecting the runner with the impression . It has a small crosssectional area when compared with the rest of the feed system . The cross sectional area is smallbecause of following reasons .

1) The gate freezes soon after the impression is filled so that the injection plunger can be with drawnwithout the probability of void being created in the moulding by sunk back .

2) It allows for simple degating .3) After degating only a small witness mark remains .4) Better control of the filling of multi-impressions can be achieved .5) Packing the material impression with material in excess of that required to compensate for

shrinkage is minimized . The optimum size of a gate will depend on the following factors .a) The follow characteristics of the matl. to be moulded .b) The wall section of the moulding .c) The volume of the material to be injected into the impression .d) The temp. of the melt ande) The temp. of the mould .

Posting of the Gate :- Ideally the position of the gate should be such that there these is an evenflow of melt in the impression so that it is filled uniform ally and the advancing melt front spreadsout and reaches the various impression extremities at the same time . In this way two or moreadvancing fronts would really meet to form a weld line . Such an ideal position for gate is possiblein certain shopped moulding such as those with circular cross sections . Another reason for centralgating for slender cone like components such that pen caps is that side gating may cause defectionof the core . This results in a thinner wall section one side thus adding another weakness to that ofthe weld line .

For thin walled rectangular mouldings off centre multipoint gating or film gating is used it shouldbe positioned so that the melt flow immediately meet a restriction .

GATE BALANCING :- It is often necessary to balance the gates of a multimpression mould toensure that the impressions fill simultaneous . This method is adopted when the preferredbalanced runner system can not be used to achieve balanced filling it is necessary to cause thegreater restriction to the flow of the melt to those impressions closet to the sprue to progressivelyreduce the restriction as the distance from the sprue increases . These are two ways of varying therestriction –

a) By varying the land length andb) By varying the cross sectional area of the gate . Following formula can be used to calculate

the gate size for impressions of different shot weights for round gates –D2=d1 (w2 )1/4

W1

For rectangular gates –T2 = t1 (w2)1/3

W1

Where –D1 = The gate dia of first cavity (cm)D2 = “ “ “ second “ (cm)

T1 = depth of the gate in first cavityT2 = “ “ “ “ “ second “W1 = wt. of first cavity component (gm)W2 = “ “ second “ “ “

Types of Gate :- The type of gate must be chosen carefully to obtain optimum fillingconditions . The choice normally depends upon the plastic material , types of mould ,types of enjection of gate and permissible visibility of gate marks on the products . Thetypes of gate commonly used are – sprue gate , edge gate , overlap gate , fan gate , diapram gate , ring gate , film gate , pin gate and subsurface gate .1) Sprue gate :- when the moulding is directly fed from a sprue , the feed section is

termed a sprue gate . The main disadvantage with this type of gate is that it leaves alarge gate mark on the moulding . the size of this mark depends on – (a) The dia at thesmall end of the sprue (b) The sprue ingle and (c) The sprue length . The gate mark canbe minimized by keeping the dimensions of the above factors to a min .

Sprue sizes for PS.Wt. of moulding (gm) Min. sprue dia (mm)10 3.510-20 4.520-40 5.540-150 6.5150-300 7.5

For other materials the sprue dia should be multihued by following factors –Acrylic 2.0Plasticized PVC 0.8Nylon 0.8Cellulose acetate 1.0Polyethylene 0.5

2) Rectangular gate :- This is a general purpose gate and in it’s simplest form it is arectangular channel machined in one mould plate to connect the runner to theimpression . Following are the main advantages of this type of gate –1) The cross sectional form is simple and cheap to machine .2) Close accuracy in the gate dimensions can be achieved .3) The gate dimensions can be quickly and easily modified .4) The filling rate of the impression can be controlled relatively indepently of the

gate real time .5) All commonly used materials can be moulded through the type of gate .

The main disadvantage of this type of gate is however that after gate removal a witness mark isleft on the visible surface of the moulding .

Gate size :-

SPLITSThe mould designer is frequently can fronted with a component design that incorporates a recess orprojection which presents the simple removal of the moulding from the mould . A moulding which has arecess or projection is termed an undercut moulding . The mould design for this type of component ismore complex than for the in line of draw components , as it necessitates the removal of that part of theimpression which form the undercut prior to rejection . External undercut components

Any recess or projection on the outside surface of the component which pennants it’s removalfrom the cavity is termed an external undercut .

There are two forms to be considered –

1) The undercut may be local , in that the recess or projection occurs in on position only .2) The undercut may be a continuous recess or a projection on the periphery of the component . The water

connector has a number of such undercuts .In such cases it is necessary to split the cavity into parts and open these , generally at right

angles to the line of draw , to relieve the undercut , before the moulding is removed .Since the cavity is in two pieces , a joint line will be visible on the finished product . This joint line

on an undercut component , is compos able to the pasting line on a normal component . It is desirableto keep the splits movement to a minimum . The joint line for regular components should be positionedon the long studional centre line for unsymmetrical components , the choice of joint line is more critical. For very complex components , it is advantageous to have a model made to simplify the reelection ofthe joint line . The joint line should be as little visible or the component as possible .SPLITS - when the cavity form for a component is machined into separate blocks of steel , thesesplit cavity blocks are called splits . One half of the components form is sunk into each split and ,providing the splits can be opened the moulding can be extracted .

The splits can be incorporated in the mould design in several ways . The simplest is by fitting thesplits into a chase bolster . This method has a major disadvantage in that after each moulding operationthe splits must be removed and opened prior to the moulding being extracted . This manual operationlengthens the moulding cycle and therefore this design should be avoided .

In more complex systems , the splits are retained on the mould plate and actuated automatically. There are two basic designs –

1) Sliding splits and ,2) Angled lift splits .

In both the designs it is necessary to arrange fori) Guiding the splits in the desired direction .ii) Actuating the splits andiii) Securely locking the splits in position prior to the material being injected into the mould .

Sliding splits –In this design , the splits are mounted in guides on a flat mould plate and they are

actuated in one mould plane by mechanical or hydraulic means . The splits are positively locked in their

closed position by heals which projection from the other mould half . Sliding splits are generallymounted on the moving mould plate . The principle of sliding action is illustrated in the figure below .

Guiding and retention of splits - There are three main factors in design of guiding and retention systemfor a sliding splits type mould .

1) Side movement must be prevented to insure that the split halves always come together in the sameplace .

2) All parts of the guiding system must be of adequate strength .3) The two split halves must have a smooth , unimpeded movement .

In most designs, the guiding function is accomplished by providing an accurately machine of slot in themould plate , in which the splits can slide . The splits retaining system usually adopted is based upon a T-design . Each split in corporate shoulders , which are caused to slide in the T-shaped slot , that extendsacross the mould plate . The side walls of the T- slot can also be utilized for the guiding function also .Types of splits –

The split moulds are normally classified on the basic of various methods that are used to actnote the splits in relation to the mould plate . The most frequently used designs are based on varioustypes of cam e. g. finger cam , dog-leg cam and various methods of can track act nation .The basic operation with can act nation is as follows –As the mould is opened , the comes attached to the fixed mould half cause the splits to slide across themoving mould plate . When the mould halnes are brought together , the splits are progressively closed .Another method of actuating the splits is by the use of compression springs . This system , while simpleand cheap , has limited application as only relatively small split movement can be obtained . Mostmachine manufactures incorporate facilities in the hydraulic circuit for operation of additional actuates, if required . The main advantage of this system is that large split movements are practicable .FINGER CAM ACTUATION : In this system , hardened circular steel pins , termed finger cams , aremounted at an angle in the fixed mould plate . The splits , mounted in guides on the moving mould plate, have corresponding circular holes to accommodate these finger cams .

In a typical design in the figures given below , the split are shown in the closed position(a) as the mould opens , the finger cams forces the split to move outwards , sliding on the mould plate (b) Once contact with the finger cam is lost , the splits movement ceases immediately .

Continued movement of the moving half causes the ejector system to operate and the moulding to beejected (c) Du closing the servers action occurs . The finger cam re-enters the hole in the split andforces the split to move inwards . The final closing nip on the splits is achieved by the locking heels .The designer must aim to keep the splits movement down to a minimum and also to ensure that themould part can be easily and quickly removed from the mould . The clearance c , servers two purposes :

1) It ensures that the force , which is applied to the splits , during the injection phase , is not transferred tothe cam .It permits the mould to open a predetermined amount before the splits are accented. The value of angleof various between 100 and 250 . The diameter of splits ranges from 13mm to 76mm .

2) Dog-leg cam act nation -This method of act nation is used where a greater splits delay is required . The

dog-leg cam , is of a general rectangular section and is mounted in the fixed mould plate . Each splitincorporates a rectangular hole , the operating face of which has a corresponding angle to that of thecam .The sequence of operation is shown in the fingers below .

When the mould is closed , The splits are locked together by the locking heads of the fixedmould plate . When the mould halves are parted for ejection , the splits do not immediately start toopen because of the straight portion of the dog-leg cams . When the moving half of mould movesfurther , it causes the act nation of the splits by the dog-leg cams , there by releasing the moulding . Thesenses action occurs , when the mould is closed .

Typical cross-section dimensions of a dog-leg cam for a small mould are 13mm*18mm . Theangle is ideally 100 but may be increased to 250 . A lead in at the front – end of the cam should beprovided to facilitate the re-entry of the cam into the split as the mould closes .

Cam Track Actuation - This method utilizes a cam track machined into a steel plate attached to thefixed mould half . A boss fitted to both sides of split , runs in this track . The movement of splits cam thisbe accurately controlled by specific cam track design .

The splits are mounted on a mould plate . The posses screwed into the side faces of the split ,protrude into the cam track plates . The plates are securely attached to the side faces of the fixed mouldplate . A small clearance is provided between the cam track plate and the moving mould half .

As the mould opens , the bosses follow the cam track and thereby cause the splits to open .When the mould is closes , the boss reenters the cam track and the splits are progressing angle with thedesign various between 100 and 400 .Spring Actuation - This design incorporates compression springs to force the splits a part and utilizes theangled faces of the chose bolster to close them . The outward movement of the splits in the design islimited . This design is limited to mouldings , Which incorporate relatively shallow undercuts .

The splits are mounted on the mould plate and retained by guide strips : studs project from thebase of splits into a slot machined in the mould plate . The outward movement of each split is controlledby the length of this slot . A compression spring is fitted between the studs in a link shaped pocketsituated in the lower mould plate . The splits are held closed by the chase bolster during the injectionphase . When the mould begins to open the compression springs lowest a force to part the split halves .The split halves movement is stopped by the studs reaching the end of the slot in the mould plate .During the closing stroke the splits reenter the chase bolster and are progressively closed .A locking heel angle of 200-250 is recommended for splits and 76mm is width , one stud per split issuitable . Over this width , two studs should be used .