Engineering Material LEC#2

7/30/2019 Engineering Material LEC#2

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METAL ALLOYS

There are two types of metal alloys

Ferrous Alloys

Non-Ferrous Alloys

Compared to other engineering materials, the carbon steels offer high strength and high

stiffness, coupled with reasonable toughness. Unfortunately, they also rust easily and

generally require some form of surface protection, such as paint, galvanizing, or other

coating. The plain-carbon steels are generally the lowest-cost steel material and shouldbe given first consideration for many applications.

The differentiation between plain-carbon and alloy steel is often somewhat arbitrary.

Both contain carbon, manganese, and usually silicon. Copper and boron are possible

additions to both classes. Steels containing more than 1.65% manganese, 0.60%

silicon, or 0.60% copper are usually designated as alloy steels. Also, a steel isconsidered to be an alloy steel if a definite or minimum amount of other alloying elementis specified. The most common alloy elements are chromium, nickel, molybdenum,

vanadium, tungsten, cobalt, boron, and copper, as well as manganese, silicon,

phosphorus, and sulphur in amounts greater than are normally present. If the steelcontains less than 8% of total alloy addition, it is considered to be a low-alloy

steel. Steels with more than 8% alloying elements are high-alloy steels.


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IMPORTANT ORES OF COPPERCopper pyrite or chalcopyrite (CuFeS2).

Chalocite (Cu2S) or copper glance.

Malachite green [CuCO3.Cu(OH)2].Azurite blue [2CuCO3.Cu(OH)2].

Bornite (3Cu2S.Fe2S3) or peacock ore.

Melaconite (CuO) etc.EXTRACTION OF COPPER FROM SULPHIDE ORE

Large amount of copper are obtained from copper pyrite (CuFeS2) bysmelting. Ores containing 4% or more copper are treated by smeltingprocess. Very poor ores are treated by hydro-metallurgical process(Leaching, Solution concentration and purification, Metal recovery).

EXTRACTION OF COPPER BY SMELTING PROCESSFollowing steps are involved in the extraction of copper.

1. CONCENTRATIONThe finely crushed ore is concentrated by Froth-Floatation process. Froth

flotation is a process for separating minerals from gangue by taking

advantage of differences in their hydrophobicity. The finely crushed ore is

suspended in water containing a little amount of pine oil. A blast of air is

passed through the suspension. The particles get wetted by the oil and floatas a froth which is skimmed. The gangue sinks to the bottom.

EXTRACTION OF COPPER


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2. ROASTINGThe concentrated ore is then roasted in a furnace in the presence of a

current of air. Sulphur is oxidized to SO2 and impurities of arsenic (As) and

antimony (Sb) are removed as volatile oxides. The following reaction takes

place during the roasting process.2CuFeS2 + O2 Cu2S + 2FeS + SO2

S + O2 SO2

4As + 3O2 As2O3

4Sb + 3O2 2Sb2O3Cuprous sulphide and ferrous sulphide are further oxidized into their oxides.

2Cu2S + 3O2 2Cu2O + 2SO2

2FeS + 3O2 2FeO + 2SO23. SMELTINGThe roasted ore is mixed with coke and silica (sand) SiO2 and is introduced

in to a blast furnace. The hot air is blasted and FeO is converted in toferrous silicate (FeSiO3).

FeO + SiO2 FeSiO3

Cu2O + FeS Cu2S + FeOFeSiO3 (slag) floats over the molten matte of copper.


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4. BESSEMERIZATIONCopper metal is extracted from molten matte through bessemerization .

The matte is introduced into Bessemer converter which uphold by tuyers.

The air is blown through the molten matte. Blast of air converts Cu2S partly

into Cu2O which reacts with remaining Cu2S to give molten copper.2Cu2S + 3O2 2Cu2O + 2SO2

2Cu2O + Cu2S 6Cu + SO2The copper so obtained is called "Blister copper" because, as it solidifies,

SO2 hidden in it escapes out producing blister on its surface.IMPURITIES IN BLISTER COPPER

AND THEIR EFFECTSBlister copper is 99% pure. It contains

impurities mainly iron but little amount

of As, Zn, Pb, Ag and Au may also be

present. These impurities adversely

affect the electrical as well as

mechanical properties of copper.

Therefore, they must be removed.


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ZINC Ores are found in the forms of

1) Zinc sulphide2) Smithsonite (ZnCO3) 67% Zn

3) Hemimorphite or (Zn4Si2O7(OH)2.H2O).

54.2% Zn4) Zincite (ZnO)5) Willemite (Zn2SiO4) 58.5%.

EXTRACTION OF ZINC FROM SULPHIDE ORE

There are two methods of zinc extraction1) Pyrometallurgy

2) HydrometallurgyEXTRACTION OF ZINC & ALUMINIUM BY SMELTING PROCESS

Following steps are involved in the extraction of zinc .1. CONCENTRATION

The finely crushed ore is concentrated by Froth-Floatation process. Froth

flotation is a process for separating minerals from gangue by takingadvantage of differences in their hydrophobicity. The finely crushed ore is

suspended in water containing a little amount of oil. A blast of air is passed

through the suspension. The particles get wetted by the oil and float as a

froth which is skimmed. The gangue sinks to the bottom.

EXTRACTION OF ZINC


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2. ROASTINGThe concentrated ore is then roasted in a furnace in the presence of a

current of air.

3. SMELTINGThe roasted ore is mixed with coke and silica (sand) SiO2 and is introduced

in to a blast furnace, where the hot air is blasted.


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CORROSION

Definition: Corrosion is the deterioration of materials by chemical interaction with theirenvironment. Deteriorative mechanisms are different for metals, ceramics, and

polymers.

In metals, there is actual material loss either by dissolution (chemical corrosion)or by the formation of non-metallic scale or film (oxidation/electrochemical corrosion).

Ceramic materials are relatively resistant to deterioration, which usually occurs at

elevated temperatures or in rather extreme environments; the process is frequently also

called corrosion.

For polymers, the term degradation is most frequently used. Polymers maydissolve when exposed to a liquid solvent, or they may absorb the solvent and swell.

Also, electromagnetic radiation (primarily ultraviolet) and heat may cause alterations in

their molecular structures.

Corrosion processes are occasionally used to advantage. For example, etching

procedures, make use of the selective chemical reactivity of grain boundaries or variousmicrostructural constituents.


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Buried gas or water supply pipes can suffer

severe corrosion which is not detected untilan actual leakage occurs, by which time

considerable damage may be done.


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EFFECTS OF CORROSION

The consequences of corrosion are many and varied and

the effects of these on the safe, reliable and efficientoperation of equipment or structures are often more seriousthan the simple loss of a mass of metal. Failures of variouskinds and the need for expensive replacements may occureven though the amount of metal destroyed is quite small.

Losses are economic and safety:

Reduced Strength

Downtime of equipment

Escape of fluids

Lost surface properties

Reduced value of goods


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In chemical corrosion, or direct dissolution, a material dissolves in a corrosive liquid

medium. The material continues to dissolve until either it is consumed or the liquid is

saturated.

CHEMICAL CORROSION

Zn + 2HCl ZnCl2 + H2

Chlorine only peripherally involved

Zn + 2H+ Zn 2+ + H2


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Electrochemical corrosion, the most common form of attack of metals, occurs when

metal atoms lose electrons and become ions. As the metal is gradually consumed by

this process, a by-product of the corrosion process is typically formed. Metal atoms

characteristically lose or give up electrons in what is called an oxidation reaction. For

example, a hypothetical metal M that has a valence of n (or n valence electrons) may

experience oxidation according to the reaction:

ELECTROCHEMICAL CORROSION

releasing electrons

Electrons absorbed by oxygen

Reactions

Oxidation:(Anodic reaction) Zn Zn2+ + 2e-

Reduction:

(Cathodic reaction) 2H+ + 2e- H2

Electrochemical corrosionoccurs most frequently in an

aqueous medium, in which

ions are present in water, soil,

or moist air.


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Galvanization is the process of applying a protective zinc coating to steel or iron, inorder to prevent rusting.

The galvanization can be done with electrochemical and electrodeposition processes,

the most common method in current use is hot-dip galvanization, in which steel parts aresubmerged in a bath of molten zinc.

Galvanization


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CONCEPT OFSTRESS AND STRAIN

W k h d i i h d i If l t S1 th t i t th th


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flow at stress

level S1

the new flow

stress is S2

Each time we apply a higher

stress, the flow stress and

tensile strength increase,and the ductilit decreases

Strain Hardening Mechanism

Work hardening, orstrain hardening, If we apply a stress S1 that is greater than theyield strength Sy, it causes a permanent deformation or strain. When the stress is

removed, a strain of e1 remains. Our new test specimen would begin to deform

plastically or flow at stress level S1. We define the flow stress as the stress that is

needed to initiate plastic flow in previously deformed material. Strain hardening results in

an increase in the strength of a material due to plastic deformation. Plastic deformation =adding dislocations

HEAT TREATMENT


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Heat treatment is the term used to describe the controlled heating and cooling of metals

and alloys for the purpose of altering their structures and properties.

Heat treatment is the process of combination of heating and cooling operations, timed

and applied to metals and alloys in their solid state to obtain desired properties.

HEAT TREATMENT

AIM OF HEAT TREATMENT

The aim is to obtain a desired microstructure to achieve certain predetermined

properties (physical, mechanical, magnetic or electrical).

The same metal or alloy can bemade weak and ductile for ease

in manufacture, and then

retreated to provide high strength

and good fracture resistance for

use and application

SOFTENING HEAT TREATMENT


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SOFTENING HEAT TREATMENT

Annealing Full Annealing

Process Annealing

Stress-relief Annealing

Spheroidization Annealing

Normalizing

Tempering


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ANNEALING

Annealing is done to improve ductility (the ability to be drawn and extruded) and

reduce brittleness.

Annealing consists of softening the metal by heating it between 30 & 50 degrees C

above it upper critical point and allowing it to cool slowly. This can be done in either hot sand, ashes of a fire or by leaving the metal in an oven

or furnace until cooled.

TEMPERING

Tempering is done to remove some of the brittleness and hardness of

steel after hardening.

Suitable temperatures for tempering vary considerably

Tempering by colour still provides an accurate and reliable method ofdealing with plain carbon steels

NORMALISING The main purpose of normalizing is to obtain a structure that is uniform throughout

the work piece and is free from any locked up stresses. Similar to annealing, but the cooling rate is accelerated by taking the work piece

from the furnace and allowing it to cool in free air.

This more rapid cooling results in a finer grain structure which in turn leads to

improved physical properties and improved finishes when machining.

ANNEALING


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The term annealing refers to a heat treatment in which a material is exposed to an

elevated temperature for an extended time period and then slowly cooled. Annealing is

carried out to (1) relieve stresses; (2) increase softness, ductility, and toughness; and/or

(3) produce a specific microstructure.

Annealing is a heat treatment used to eliminate some or all of the effects of cold working.

Annealing at a low temperature may be used to eliminate the residual stresses produced

during cold working without affecting the mechanical properties of the finished part.Any

annealing process consists of three stages: (1) heating to the desired temperature, (2)

holding or soaking at that temperature, and (3) cooling, usually to room temperature.

Time is an important parameter in these procedures. During heating and cooling,temperature gradients exist between the outside and interior portions of the piece; their

magnitudes depend on the size and geometry of the piece. If the rate of temperature

change is too great, temperature gradients and internal stresses may be induced that

may lead to warping or even cracking.

PROCESS ANNEALING is a heat treatment that is used to negate the effects of coldwork that is, to soften and increase the ductility of a previously strain-hardened metal.Process annealing is applied to low carbon steels (


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STRESS-RELIEF ANNEAL also known as sub-critical annealing is employed toreduce the residual stresses in large steel castings, welded assemblies, and cold-formed

products. Parts are heated to temperatures below the A1(between 550 and 650C or

1000 and 1200F), held for a period of time, and then slow cooled to prevent the

creation of additional stresses. Times and temperatures vary with the condition of the

component, but the basic microstructure and associated mechanical properties generallyremain unchanged.

SPHEROIDIZATION ANNEALING: When high-carbon steels are to undergo extensivemachining or cold forming, a process known as spheroidization is often employed. Here

the objective is to produce a structure in which all of the cementite is in the form of small

spheroids or globules dispersed throughout a ferrite matrix. This can be accomplished

by a variety of techniques, including (1) prolonged heating at a temperature just below

the A1followed by relatively slow cooling, (2) prolonged cycling between temperatures

slightly above and slightly below the or (3) in the case of tool or high-alloy steels, heating

to 750 to 800C (1400 to 1500F) or higher and holding at this temperature for several

hours, followed by slow cooling.

TEMPERING is a process of heat treating, which is used to increase the toughnessof iron-based alloys. It is also a technique used to increase the toughness of glass. For

metals, tempering is usually performed after hardening, to reduce some of the excess

hardness, and is done by heating the metal to a temperature below its "lower critical

temperature. Tempering is usually performed after quenching, which is rapid cooling of

the metal to put it in its hardest state.


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NORMALIZING: In plastically deformed metals (for example, by rolling operation),the grains are irregularly shaped and relatively large, but vary substantially in size. An

annealing heat treatment called normalizing is used to refine the grains (i.e., todecrease the average grain size) and produce a more uniform and desirable size

distribution; fine-grained pearlitic steels are tougher than coarse-grained ones.

One should note a key difference between full annealing and normalizing. In the fullanneal, the furnace imposes identical cooling conditions at all locations within the metal,

which results in identical structures and properties. With normalizing, the cooling will be

different at different locations. Properties will vary between surface and interior, and

different thickness regions will also have different properties. When subsequent

processing involves a substantial amount of machining that may be automated, the

added cost of a full anneal may be justified, since it produces a product with uniformmachining characteristics at all locations.

HARDENING HEAT TREATMENT


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To harden by quenching, a metal (usually steel or cast iron) must be heated above the

upper critical temperature and then quickly cooled. Depending on the alloy and other

considerations (such as concern for maximum hardness vs. cracking and distortion),cooling may be done with forced air or other gases, (such as nitrogen). Liquids may be

used, due to their better thermal conductivity, such as water, oil, a polymer dissolved in

water, or a brine. Upon being rapidly cooled, a portion of austenite (dependent on alloy

composition) will transform to martensite, a hard, brittle crystalline structure. The

quenched hardness of a metal depends on its chemical composition and quenching

method.

QUENCHING

HARDENING HEAT TREATMENT

Quenching

Surface hardening by changing the surface chemistry


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HARDENING

Hardening of steels is done to increase

the strength and wear properties.

Carbon steel is heated 30 & 50degrees C above the upper critical point

and then quenched quickly

The quicker the steel is cooled theharder it will be.


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ROLLING PROCESS


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Rolling is a deformation process in which the thickness of the work is reduced by

compressive forces exerted by two opposing rolls. The rolls rotate to pull and

simultaneously squeeze the work between them.

Hot Rolling is generally free of residual stresses, and its properties are isotropic.

Disadvantages of hot rolling are that the product cannot be held to close tolerances, andthe surface has a characteristic oxide scale. High Reduction ratio.

Cold Rolling strengthens the metal and permits a tighter tolerance on thickness. Butcauses residual stresses in the work.

TYPES OF ROLLING PROCESSESFlat Rolling reduce the thickness of a rectangular cross section.

Shape Rolling a square cross section is formed into a shape such as an I-beamThread Rolling is used to form threads on cylindrical parts by rolling them between twodies.

Gear Rolling is a cold working process to produce certain gears. The automotiveindustry is an important user of these products. The setup in gear rolling is similar to

thread rolling, except that the deformed features of the cylindrical blank or disk are

oriented parallel to its axis (or at an angle in the case of helical gears) ratherthan spiralled as in thread rolling.

ROLLING PROCESS

Ring Rolling is a deformation process in which a thick walled ring of smaller diameter is


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Ring Rolling is a deformation process in which a thick-walled ring of smaller diameter isrolled into a thin-walled ring of larger diameter.

Roll Piercing is a specialized hot working process for making seamless thick-walledtubes. It utilizes two opposing rolls, and hence it is grouped with the rolling processes.

Roll Forging In this operation (also called

cross rolling), the cross section of a roundBar is shaped by passing it through a pair

of rolls with profiled grooves .

Skew Rolling is typically used for makingball bearings. Round wire or rod is fed into

the roll gap, and roughly spherical blanks

are formed continuously by the action ofthe rotating rolls.

Roll piercing

Ring Rolling

Roll ForgingSkew Rolling

POLYMERS (Poly=many+ Mers=unit )


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POLYMERS (Poly many+ Mers unit )

Polymers consist of chains of molecules.

Polmolecules consisting of one unit (known as a monomer) or a few units (known as

oligomers) that are chemically joined (by covalent bonding) to create these giant

molecules.

Most polymers are organic, meaning that they are carbon-based; however, polymers

can be inorganic (e.g., silicones based on a Si-O network).

What is Polymerization?

Polymerization is the process by which small molecules consisting of one unit (known as

a monomer) or a few units (known as oligomers) are chemically joined to create these

giant molecules. Polymerization normally begins with the production of long chains in

which the atoms are strongly joined by covalent bonding.

Classification of Polymers


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Classification of PolymersThere are several ways of classification of polymers based on some special

considerations. The following are some of the common classifications of polymers:

Classification Based on SourceUnder this type of classification, there are three sub categories.

1. Natural polymersThese polymers are found in plants and animals. Examples are proteins, cellulose,

starch, resins and rubber.

Naturally occurring polymers are derived from plants and animals, these materials

include wood, rubber, cotton, wool, leather, and silk.

2. Semi-synthetic polymersCellulose derivatives as cellulose acetate (rayon) and cellulose nitrate, etc. are the usualexamples of this sub category.

3. Synthetic polymersA variety of synthetic polymers as plastic (polythene), synthetic fibres (nylon) and

synthetic rubbers (Buna S) are examples of man-made polymers extensively used in

daily life as well as in industry. The synthetics can be produced inexpensively, and their properties may be managed

to the degree that many are superior to their natural counterparts.

Plastics are materials that are composed principally of naturally occurring and

modified or artificially made polymers often containing additives such as fibers,

fillers, pigments, and the like that further enhance their properties.

Classification Based on Structure


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Classification Based on Structure

1. Linear polymersThese polymers consist of long and straight chains. The examples of Polymers are high

density polythene, polyvinyl chloride, etc. These are represented as:

2. Branched chain polymersThese polymers contain linear chains having some branches, e.g., low density

polythene. These are depicted as above:

3. Cross linked or Network polymers

These are usually formed from bi-functional and tri-functional monomers and containstrong covalent bonds between various linear polymer chains, e.g. bakelite, melamine,

etc. These polymers are depicted as above:

Classification Based on Molecular Forces


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Under this category, the polymers are classified into the following four sub groups on the

basis of magnitude of intermolecular forces present in them.

1. Elastomers: These are rubber like solids with elastic properties. In theseelastomeric polymers, the polymer chains are held together by the weakest

intermolecular forces. These weak binding forces permit the polymer to be stretched.

A few crosslinks are introduced in between the chains, which help the polymer to

retract to its original position after the force is released as in vulcanised rubber.

2. Fibres: Fibres are the thread forming solids which possess high tensile strength andhigh modulus. These characteristics can be attributed to the strong intermolecular

forces like hydrogen bonding. These strong forces also lead to close packing ofchains and thus impart crystalline nature. The examples are polyamides (nylon 6, 6),

polyesters (terylene), etc.

3. Thermoplastic polymersThese are the linear or slightly branched long chain molecules capable of repeatedly

softening on heating and hardening on cooling. These polymers possess

intermolecular forces of attraction intermediate between elastomers and fibres.

Some common thermoplastics are polythene, polystyrene, polyvinyls, etc

4 Thermosetting polymersThese polymers are cross linked or heavily branched molecules, which on heating

undergo extensive cross linking in moulds and again become infusible. These cannot

be reused. Some common examples are bakelite, urea-formaldehyde resins, etc.


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Polymers of Commercial Importance

Stress-Strain Behaviour of Polymers


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The fracture strengths of polymeric materials are low relative to those of metals and

ceramics. As a general rule, the mode of fracture in thermosetting polymers (heavily

cross linked networks) is brittle (curve B). The behaviour for a plastic material, curve B, is

similar to that for many metallic materials; the initial deformation is elastic, which is

followed by yielding and a region of plastic deformation. Finally, the deformation displayedby curve C is totally elastic; this elasticity is displayed by a class of polymers termed the

elastomers.

The influence of temperature on the

stressstrain characteristics ofpoly(methyl methacrylate)

The stressstrain behaviour for brittle,

plastic, and highly elastic polymers.

Stress Strain Behaviour of Polymers

FORMING TECHNIQUES FOR POLYMERS


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Moulding is the most common method for forming plastic

polymers. The several moulding techniques used include

compression, transfer, blow, injection, and extrusion moulding.

FORMING TECHNIQUES FOR POLYMERS

Injection Molding.

Compression Molding.

Extrusion Molding.

Blow Molding.

Rotational Molding.


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Injection MouldingCompression Moulding

Extrusion Moulding


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Blow Moulding

Blow Moulding


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The mould is held between the heated platens of the hydraulic press;

A prepared quality of moulding compound is placed in the mould, usually by

hand, and the mould placed in the press;

The press closes with sufficient pressure to prevent or minimize flash at the

mould part line;

The compound softens and flows to shape; the chemical cure then occurs

as the internal mould temperature becomes high enough;

If necessary, cooling takes place, although for the vast majority of

thermosets this is not needed;The press is opened and the moulding removed.

Process description

Compression Moulding


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ROTATIONAL MOLDING

A predetermined amount ofplastic, powder or liquid form, isdeposited in one half of a mold.

The mold is closed. The mold is rotated biaxially

inside an oven.

The plastics melts and forms acoating over the inside surface of

the mold. The mold is removed from the

oven and cooled.

The part is removed from the

mold.

Rotational molding process consists of six steps


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ROTATIONAL MOLDING

Advantages

Molds are relatively inexpensive.

Rotational molding machines are much less

expensive than other type of plastic

processing equipment.

Different parts can be molded at the same

time.

Very large hollow parts can be made.

Parts are stress free.

Very little scrap is produced


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