rapport final.docx

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PLASTIC INJECTION MOLD DESIGN 2012/2013

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Table of contents

Introduction ............................................................................................................................................. 2

Objectives ................................................................................................................................................ 2

State of the Art ........................................................................................................................................ 4

Chapter I: Injection Molding Materials ............................................................................................... 5

1. Introduction: ............................................................................................................................ 5

2. Thermoplastic polymers and injection molding: ..................................................................... 5

3. Thermoplastic Characteristics: ................................................................................................ 6

4. Polypropylene (PP): ................................................................................................................. 8

4.1 Introduction: .................................................................................................................... 8

4.2 Chemical and physical properties: ................................................................................... 9

5. Conclusion: ............................................................................................................................ 10

Chapter II: Injection molding process ............................................................................................... 11

1. Introduction: .......................................................................................................................... 11

2. The principle of injection molding: ........................................................................................ 11

3. Advantages of injection molding: .......................................................................................... 11

4. The injection molding machine: ............................................................................................ 12

4.1 Injection System: ................................................................................................................. 13

4.2 Hydraulic system: ................................................................................................................ 13

4.3 Clamping System: ............................................................................................................... 134.4 Control system: .................................................................................................................... 14

4.5 Molded system: ................................................................................................................... 14

5. Mold components: ................................................................................................................ 16

6. Injection molding cycle: ......................................................................................................... 16

7. Cavity pressure variation: ...................................................................................................... 18

8. Important Parameters in injection molding: ......................................................................... 19

9. Conclusion: ............................................................................................................................ 19

Chapter3: Shrinkage behavior of molded parts ................................................................................. 20


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1. Introduction: .......................................................................................................................... 20

2. Definition of shrinkage: ......................................................................................................... 20

3. Different forms of shrinkage: ................................................................................................ 21

3.1 In-mold and post-mold shrinkage: ................................................................................. 21

3.2 Isotropic and anisotropic shrinkage: .............................................................................. 22

4. Factors that influence shrinkage: .......................................................................................... 24

4.1 Material properties: ....................................................................................................... 25

4.2 Injection molding processing conditions: ...................................................................... 27

4.3 Molded part geometry: .................................................................................................. 28

5. Basic shrinkage formulas: ...................................................................................................... 30

6. Thermodynamic approach with pvT diagrams to predict shrinkage: ................................... 33

6.1 Definition: ........................................................................................................................... 33

6.2 Description of injection molding process with PVT diagram: ............................................ 34

6.3 Shrinkage measurement with pvT diagrams: ...................................................................... 35

6.4 Conventional measurements of polymer PVT properties: .................................................. 36

6.4.1 The piston-die technique: ............................................................................................. 36

6.4.2 Confining-fluid technique: ........................................................................................... 38

6. 4.3 Application of polymer PVT data: .............................................................................. 39

7. Conclusion: ............................................................................................................................ 42

Experimental work ................................................................................................................................ 43

Chapter I: Design and Modeling of Injection Molded Parts ............................................................. 44

1. Introduction: .......................................................................................................................... 44

2. Part design: ............................................................................................................................ 44

3. Mold design: .......................................................................................................................... 45

3.1 Parting line position: ...................................................................................................... 45

3.2 Sprue position: ............................................................................................................... 463.3 Air vents dimensions: .................................................................................................... 47

4. Conclusion: ............................................................................................................................ 47

Chapter II: Injection molding machine calculations .......................................................................... 48

1. Introduction: .......................................................................................................................... 48

2. Clamping force calculation: ................................................................................................... 48

3. Injection capacity and weight calculations: .......................................................................... 50

4. Injection molding machine choosing:.................................................................................... 51

5. Conclusion: ............................................................................................................................ 51


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Conclusion: ............................................................................................................................................ 52

Introduction

Injection molding technology is a method of processing predominantly used for

thermoplastic polymers. It consist of heating thermoplastic material until it melts, then forcing

this melted plastic into a steel mold, where it cools and solidifies. The increasingly

sophisticated use of injection molding is one of the principal tools in the battle to produce

elegant product structures with reduced parts counts.

In order to exploit the versatility of injection molding technology for economical

manufacture, it is necessary to understand the basic mechanisms of the process and the related

aspects of the molding equipments and materials used.

This report will be divided into two parts. The first part is concerned with a state-of-

the-art, and it includes three chapters: injection molding materials, injection molding process

and finally shrinkage behavior. The second part is mainly about the conception of a plastic

grating panels mold.

Objectives

For this project, there are two main objectives to achieve:

(i) Design of the plastic grating panels mold.

(ii) Search about shrinkage.


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State of the Art


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Chapter I: Injection Molding Materials

1. Introduction:

There are many types of materials that may be used in the injection molding process.

Most polymers may be used, including all thermoplastics, some thermosets, and some

elastomers. In this chapter we will focus on the thermoplastic polymers and especially

polypropylene which is used subsequently in the fabrication of the plastic grating panel.

2. Thermoplastic polymers and injection molding:

In general, polymers that are capable of being brought to a state of fluidity can be

injection-molded. The vast majority of injection molding is applied to thermoplastic polymers.

This class of materials consists of polymers that always remain capable of being softened by

heat and of hardening on cooling, even after repeated cycling. This is because the long-chain

molecules always remain separate entities and do not form chemical bonds to one another.

This property differentiates thermoplastic materials from thermosetting ones. In the latter type

of polymer, chemical bonds are formed between the separate molecule chains during

processing. Thermosetting polymers are generally more expensive to mold than thermoplastic

and represent only about 5% of plastic processing.

Most thermoplastic materials offer high impact strength, good corrosion resistance,

and easy processing with good flow characteristics for molding complex design.

The mechanical properties of thermoplastic, while substantially lower than those of

metals, can be enhanced for some applications by adding glass fiber reinforcement. This takesthe form of short-chopped fibers, a few millimeters in length, which randomly mixed with the

thermoplastic resin. The fiber can occupy up to one-third of the material volume to

considerable improve the material strength and stiffness. The negative effect of this

reinforcement is usually a decrease in impact strength and increase abrasiveness. The latter

also has an effect on processing.

The main weakness of injection-molded parts is the relatively low service

temperatures to which they can be subjected. Thermoplastic components can only rarely be


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operated continuously above 250C, with an absolutely upper service temperature of about

400C. The temperature at which a thermoplastic can be operated under load can be defined

qualitatively by the heat deflection temperature. This is the temperature at which a simply

supported beam specimen of the material, with a centrally applied load, reaches a predefined

deflection. The temperature value obviously depend upon condition of the test and the

allowed deflection, and for this reason, the test value are only really useful for comparing

different polymers.

3. Thermoplastic Characteristics:

Thermoplastic materials generally fall within two classes of molecular arrangement,

amorphous and semi-crystalline.

Amorphous polymers have random orientation of their polymer chains, whereas

crystalline polymers form highly ordered crystal structures within an amorphous matrix

(Figure 1). The term semi-crystalline polymers is used for polymers containing both

crystalline and amorphous regions.

As a general rule, amorphous polymers have advantages of transparency and

toughness. Semi-crystalline polymers have advantages in chemical resistance and temperature

performance.

Because of the ordered arrangement of molecules, the crystalline polymers reflect

most incidents light and generally appear opaque. They also undergo a high shrinkage or

reduction in volume during solidifications. Crystalline polymers also generally are denser and

have better mechanical properties than amorphous polymers.


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These are general statements however, and the designer must consult product-specific

literature and test data for specific properties.

Figure1: Schematic of structure in the solid state for amorphous and semi-crystalline polymers

Figure2: Classification of amorphous and semi-crystalline polymers by performance

Figure 2 shows various amorphous and crystalline plastics segmented by performance.

Generally, the higher in the triangle, the higher is the temperature.

Polymers are often used in combination with other ingredients to make a useful

product. This combination of polymer and additives (Table 1) is often referred to as a plastic,

or a composite. Typical ingredients used to produce composites are fiberglass, mineral, heat

stabilizers, flame retardants and other processing aids. Fiberglass reinforcement provides

strength and stiffness particularly as the temperature is increased beyond the polymers glass

transition temperature (Tg), where the amorphous region becomes mobile.


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Table1: Common additives for plastics:

4. Polypropylene (PP):

4.1 Introduction:

For this research, polypropylene is selected as the material for plastic grating panel

product because of the criteria and characteristic that was stated below. Polypropylene or (PP)

is a thermoplastic polymer, used in a wide variety of applications, including food packaging,

textiles, plastic parts and reusable containers of various types, laboratory equipment,

loudspeakers, automotive components, and polymer bauknotes.

An addition polymer made from the monomer propylene, it is rugged and unusually

resistant to many chemical solvents, bases and acids. Its resin identification code is:

Polypropylene is an economical material that offers a combination of outstanding

physical, chemical, mechanical, thermal and electrical properties not found in any other

thermoplastic. Compared to low or high density polyethylene, it has a lower impact strength, but superior working temperature and tensile strength.


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Polypropylene possesses excellent resistance to organic solvents, degreasing agents

and electrolytic attack. It has lower impact strength, but its working temperature and tensile

strength are superior to low or high density polyethylene. It is light in weight, resistant to

staining, and has a low moisture absorption rate. This is a tough, heat-resistant, semi- rigid

material, ideal for the transfer of hot liquids or gases. It is recommended for vacuum systems

and where higher heats and pressures are encountered. It has excellent resistance to acids and

alkalies, but poor aromatic, aliphatic and chlorinated solvent resistance.

4.2 Chemical and physical properties:

Most commercial polypropylene has a level of crystallinity intermediate between that

of low density polyethylene (LDPE) and high density polyethylene (HDPE); its Young's

modulus is also intermediate. Although it is less tough and flexible than LDPE, it is much less

brittle than HDPE. This allows polypropylene to be used as a replacement for engineering

plastics. Polypropylene is rugged, often somewhat stiffer than some other plastics, reasonably

economical, and can be made translucent when uncolored but not completely transparent as

polystyrene, acyrylic or certain other plasics can be made. It can also be made opaque and/or

have many kinds of colors.

Polypropylene has very good resistance to fatigue, so that most plastic living hinges,

such as those on flip-top bottles, are made from this material.

When liquid, powdered, or similar consumer products come in disposable plastic

bottles which do not need the improved properties of polypropylene, the containers are often

made of slightly more economical polyethylene, although transparent plastics such as

polyethylene are also used for appearance. Plastic pails, car batteries, wastebaskets, coolercontainers, dishes and pitchers are often made of polypropylene or HDPE, both of which

commonly have rather similar appearance, feel, and properties at ambient temperature.

MFI (Melt Flow Index) identifies the flow speed of the raw material in the process. It

helps to fill the plastic mold during the production process. The higher MFI increases, the

weaker the raw material gets.


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It also has Copolymer and Random Copolymer. Copolymer helps stiffness of the PP

(Polypropylene). Random Copolymer helps transparent look. Colpolymer is more expensive

than Homopolypropylene. Random Copolymer is even higher than copolymer.

Table2: The specification for Polypropylene:

5. Conclusion:

In this chapter we have got a general idea about the different categories of thermoplastics and

the different specification of polypropylene. In the following chapter will focus on the

injection molding process.


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Chapter II: Injection molding process

1. Introduction:

Polymer processing is an important segment of the manufacturing industries in our

modern society, to which polymer products have greatly contributed. Injection molding is the

most widely used polymer processing operation.

Injection molded products range from simple drinking cups to large automobile

components through to intricate electronic parts.

2. The principle of injection molding:

Injection Molding is the technique of injecting molten plastic into a cold mold and

forming a part. A schematic of the main elements of a molding machine are shown below. It

consists of a hopper that holds the raw plastic pellets. The hopper feeds the barrel with the

plastic. The plastic is melted in the barrel and with the help of the screw (piston) is injected

into a mold. The mold being colder than the plastic rapidly cools and solidifies the plastic.

The mold is then opened and the part is ejected out of the mold.

Figure3: Injection molding machine

3. Advantages of injection molding:

- Accuracy in weight of articles ;

- Choice of desired surface finish and colours;

- Choice of ultimate strength of articles;

Hopper


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- Faster production and lower rejection rates;

- Faster start-up and shut down procedures;

- Minimum wastage;

- Stability of processing parameters;

- Versatality in processing different raw materials;

- Option in article sizes by changing the mould;

- Minimum post moulding operations.

4. The injection molding machine:

In order to get injected parts, a great number of injection molding machine types were

developed. In general, a typical injection molding machine consists of the following major

components, as illustrated in Figure 4 below.

- Injection system;

- Hydraulic system;

- Mold system;

- Clamping system;

- Control system.

Figure 4: A single screw injection molding machine for thermoplastics


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4.1 Injection System:

The injection system consists of a hopper, a reciprocating screw and barrel assembly,

and an injection nozzle, as shown in Figure 5 This system confines and transports the plasticas it progresses through the feeding, compressing, degassing, melting, injection, and packing

stages.

Figure 5: A single screw injection molding machine for thermoplastics, showing theplasticizing screw,

a barrel, band heaters to heat the barrel, a stationary platen, and amovable platen .

4.2 Hydraulic system:

The hydraulic system on the injection molding machine provides the power to open and close

the mold, build and hold the clamping tonnage, turn the reciprocating screw, drive the

reciprocating screw, and energize ejector pins and moving mold cores.

A number of hydraulic components are required to provide this power, which includes pumps,

valves, hydraulic motors, hydraulic fittings, hydraulic tubing, and hydraulic reservoirs.

4.3 Clamping System:

The clamping system opens and closes the mold, supports and carries the constituent parts of

the mold, and generates sufficient force to prevent the mold from opening. Clamping force

can be generated by a mechanical (toggle) lock, hydraulic lock, or a combination of the two

basic types.


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- The sprue must not freeze before any other cross section in order to permit sufficient

transmission of holding pressure;

- The sprue must de-mold easily and reliably.

Runners:- The runners deliver the plastic into the part cavities (usually multiple);

- Should exhibit low flow resistance;

- Should minimize cooling;

- Should be easily machined .

Gates:

- The gates determine the flow field into the part cavity;

- The cross section of the gate is smaller than that of the runner and the part so that the

part can be separated from the runner;

- The gate freezes off first because it is thinner than the cavity;

- When gates freeze packing of the cavity ends. A larger gate reduces viscous heating,

permit lower velocities, and allow the application of high packing pressure;

- The gate location should be selected in such a way that rapid and uniform mold filling

is ensured and the weld/meld lines (if any) and air vents are positioned properly. The

gate should be positioned away from load-bearing areas.

The delivery system design has a great influence on the filling pattern and thus the quality of

the molded part.

Part of the plastic in the mold has to be recycled (sprue, runners). Special designs (hot runners)

are used to minimize recycling.


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5. Mold components:

Figure 7: Mold components

6. Injection molding cycle:

The injection molding process can be divided into five separate steps: plastification, injection,

holding, cooling, and finally ejection.

Figure 8: The injection molding cycle and the steps included in the cycle time


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Cooling time generally dominates cycle time :

A typical sequence of operations from startup is as follows:

1. Starting with an empty cylinder, raw material from the feed hoper falls onto the rear flights

of the screw which conveys material to the front of the cylinder. During its passage along the

cylinder it is plasticised to a fluid state with the help of external heaters on the barrel.

2. The mould closes and the cylinder moves forward units carriage until the nozzle is in

contact with the entrance of the mould.

3. The screw is moved forward by the hydraulic cylinder at the rear of the machine and the

injection takes place.

4. When the cavity is completely filled, a hold pressure is applied. Plastics shrink from the

melt temperature as they cool. The hold pressure applied by the screw compensates this loss

of volume by pushing in more melt. The smallest passage into the cavity is called the gate,

and when the gate has solidified, the entrance to the cavity is stopped (Figure9), and no more

melt can be pushed into the cavity. The cooling phase starts.

Figure 9: Gates and Freeze-off

polymersforsec10

thicknesshalf

33

2

cm

t cool


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5. The metal mold cools the plastic. The cooling rate is highly dependent on the thickness of

the part, but it can also to some extent be controlled by a mold temperature control unit where

the flow rate and the temperature of the fluid in the cooling channels are controlled.

6. The mold opens, the article is ejected and the mold closes again ready for the next cycle.

7. Stages (2) to (5) repeat.

7. Cavity pressure variation:

Cavity pressure has been found to be a reliable process indicator in injection molding

for both part quality and process monitoring.

Figure 10: Cavity pressure profile variations

Figure 10 shows that cavity pressure curve starts to rise during filling and packing, and

decays during cooling as the polymer shrinks.

The purpose of the holding pressure phase is to replace the volume lost during the

cooling down and solidifying of the melt. The holding pressure phase can last as long as the

gate remains open and molten material can still enter the cavity. The gate freezes off first

because it is thinner than the cavity.


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Excessive holding results in a highly stressed part and may cause ejection problems

whereas insufficient holding causes poor surface, sink marks, welds and non-uniform

shrinkage .

8. Important Parameters in injection molding:

Material Parameters

Amorphous, Semicrystalline, Blends and Filled Systems ;

Pressure-Volume-Temperature (PVT) Behavior

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

Manufacturing Parameters

Fill Time Packing Pressure Level

Mold Temperature

Melt Temperature

9. Conclusion:

In this chapter we got an idea about injection molding machine, process and parameters. In

the following chapter will focus on the shrinkage behavior of injected parts.


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Chapter III: Shrinkage behavior of molded parts

1. Introduction:

In manufacturing processes, materials that undergo a phase change (from liquid to

solid) involve a decrease in specific volume and a resulting shrinkage. A certain amount of

shrinkage is inevitable in any process that involves cooling of plastic from elevated

temperature and this is certainly the case with injection molded polymers. Therefore,

shrinkage predictions in injection molding represent quite a challenging problem.

2. Definition of shrinkage:

Shrinkage occurs due to the thermal contraction and the compressibility resulting in a

volume change.

In fact, molecules are expanded when exposed to heat. This is similar to the effect of

heating the air in a balloon. The balloon will expand. Then, as the air in the balloon cools

down the balloon will contract. The situation is the same with plastic molecules. They expandwhen heated and contract when cooled. We call that amount of contraction shrinkage. It is

expressed in inches per inches, mm per mm or in percentage.

Shrinkage is usually classified in three orders: low, medium, and high. Amorphous

materials have low shrinkage, semi-crystalline materials have medium shrinkage, and

crystalline materials have high shrinkage.

Low shrinkage has a range of from 0.000 to 0.005 inch per inch. That means

that every inch of the part will shrink that amount. So shrinkage is rated as so much

''inch per inch'' and is usually written as ''in/in.''

Medium shrinkage has a range of from 0.005 in/in to 0.010 in/in.

High shrinkage has a range that is anything above 0.010 in/in.


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3. Different forms of shrinkage:

3.1 In-mold and post-mold shrinkage:

Two different forms of shrinkage must be considered when designing to meettolerances: in-mold or die shrinkage, and post-mold shrinkage.

- In-mold shrinkage:

In-mold shrinkage occurs as the polymer cools and can vary in different directions. It

corresponds to the reduction in the dimensions of a plastic part, compared with the mold

dimensions.

To compensate in-mold shrinkage, the dimension of mold cavity is made larger than

the desired dimensions of the processed plastic part.

- Post-mold shrinkage:

Cold molds and rapid cycles tend to freeze stresses in a molded part while reducing its

apparent shrinkage.

Later, with exposure to time and/or temperature and moisture, additional shrinkage

can occur. Shrinkage that occurs more than twenty four hours after molding is considered to

be post-mold shrinkage.


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Figure 11: The different states of shrinkage of a molded part

3.2 Isotropic and anisotropic shrinkage:

Shrinkage is classified as isotropic and anisotropic.

- Isotropic shrinkage:

When the shrinkage in the flow direction is about equal to the shrinkage across the

flow direction shrinkage is called isotropic. However, practically shrinkage does not occur

uniformly and isotropically.

- Anisotropic shrinkage (differential shrinkage):

When shrinkage in the flow direction is substantially different to the shrinkage across

the flow direction shrinkage is called anisotropic.

I --> Shrinkage at demoldingII --> Shrinkage after coolingIII --> Long time shrinkage


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Figure 12: The anisotropic shrinkage phenomenon

Generating cavity dimensions for a plastic that have anisotropic mould shrinkage

behavior is not easy.

Anisotropic mould shrinkage in injection moulding leads to difficulties in maintaining

the desired dimensions, internal stress levels and warpage.

- Common Causes of anisotropic shrinkage:

Differential shrinkage may be due to any of the following conditions:

Differential Orientation : In general, oriented material with molecules or fibers

aligned or parallel shrinks in a more anisotropic manner than unoriented material.

The degree of orientation imparted to the melt during the mold filling process has a

large influence on the shrinkage exhibited by the plastic material. During mold filling, the

polymer molecules undergo a stretching that results in molecular orientation and anisotropic

shrinkage behavior.

In general, mold shrinkage will tend to be more isotropic when the degree of

orientation imparted to the melt during mold filling is minimized, and when favorable

conditions for molecular relaxation exist.

Differential Crystallinity: For semicrystalline materials, if some part of the mold

cools at a slower rate, that area will have higher crystalline content and, hence, higher

Gate


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shrinkage. This is the case for parts with different thicknesses, and for hot spots such as where

material is in contact with outside corners of a core or with core pins.

Differential Cooling: This can occur when the mold surfaces are at different

temperatures, as they frequently are around core pins, inside and outside mold corners, near

gates, and where there are section thickness variations. Hot spots cause problems in two ways:

with added crystallinity, and with a longer/later cooling time. (The last area to cool acts as if it

were shrinking more).

Material Characteristics: Copolymers are better than homopolymers at resisting

warpage. Certain types of fillers reduce overall shrinkage and increase stiffness.

Differential Thermal Strain: This may be due to geometric effects, that is, where

there are section thickness changes, sharp inside corners, or other geometric conditions that

cause variable cooling or unusual orientation.

The more abrupt the change, or the greater the differential cooling rate, the more

severe the thermal strain.

Molding Conditions: These can lead to excessive stresses caused by unusually high

or low melt temperature or pressure, or unusually long injection time or short cycles.

Mold Constraints: Mold constraints can contribute to nonuniform shrinkage. Usually

the part is free to shrink in thickness. It is usually less free to shrink in length and width due to

the geometry of the part. There may be cores, ribs, or edges that are firmly anchored so that

the part cannot move until it is out of the mold.

4. Factors that influence shrinkage:

Shrinkage is influenced by a large number of parameters, as material properties,

processing conditions and both mold and molded part geometries.


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4.1 Material properties:- Amorphous vs semi crystalline materials:

Amorphous materials have a randomly ordered molecular structure which does not

have a sharp melt point but instead softens gradually as the temperature rises and change into

low viscosity liquid.

Amorphous polymer are isotropic in flow, shrinking uniformly in the direction of flow

and transverse to flow. As a result, amorphous materials typically exhibit lower mould

shrinkage and fewer tendencies to warp than the semi-crystalline materials.

Although semi-crystalline materials have a highly ordered molecular structure, withsharp melt points. They do not gradually soften with a temperature increase but, rather,

remain hard until a given quantity of heat is absorbed and then rapidly change into a low

viscosity liquid.

These materials are anisotropic in flow, shrinking less in the direction of flow vs.

transverse to flow. The crystallization occurs during cooling and is time and temperature

dependent. The more slowly cooling takes place, the higher the degree of crystallization and

the greater the level of shrinkage.


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Figure 13: Polymer molecular structure: Amorphous and semi-crystalline materials

- Reinforced materials:

Natural, unfilled plastic materials tend to shrink more along the direction of flow (in-

flow shrinkage) compared to the direction perpendicular to flow (cross-flow shrinkage), while

the shrinkage behavior of reinforced materials is restricted along the direction of fiber

orientation.

Figure 14: Differences in filled and unfilled materials


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4.2 Injection molding processing conditions:

Reducing shrinkage and warpage is one of the objectives to improve the quality of

injection-moulded parts. In addition to part design and material properties, process conditions

are the most important factor in determining the part quality. It is well known that process

conditions affect many properties of plastic parts including shrinkage (Figure 15):

- Holding pressure: Controls the compensating flow of material as it is cooling and

shrinking. The higher the holding pressure, the lower the mould shrinkage.

- Pressure holding time: Controls how long compensation flow is provided. If hold

time is too short, part will not be properly packed and will shrink more.

- Mould temperature: Can affect how much internal stress there is and amount of

crystallization. The moulding shrinkage increases with the mould temperature.

- Injection velocity: The injection velocity has almost no influence on overall

shrinkage. This parameter affects the amount of orientation of the polymer molecules.

- Melt temperature: Melt temperature affects the viscosity of the material, therefore,

affecting how well it can be packed. An elevated melt temperature increases the potential for

thermal contraction in the resin (increased shrinkage) and, secondly, it leads to a reduction in

the melt viscosity and hence to better packing and, ultimately, to a reduction in shrinkage.

- Demoulding time: Controls how long the part stays in its shape in the mould

before ejection. A longer time can allow the part to become more rigid and resist warpage or

linear shrinkage. It can also contribute to the crystallization of the material.

Several studies were carry out to analyse the effect of processing variables on the as-

moulded shrinkage. These studies concluded that the Holding pressure is the most significant

injection moulding parameter.


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Figure 15: Influence of processing parameters on shrinkage behaviour

4.3 Molded part geometry:

Geometry may affect shrinkage in two ways. First geometry may affect flow and

hence cause orientation effects (of amorphous phase, crystalline phase or filled particles)

resulting in shrinkage anisotropy. Second geometrical constraints affect the shrinkage

boundary conditions.

- Thickness:

Polymers have a very low thermal conductivity, compared with metals, cooling from

the melt proceeds unevenly, the surface cools more rapidly than the interior. This leads to

variations in the structure and crystallinity through the section thickness and can result in the

formation of voids or holes due excessive internal shrinkage.

As a general rule the shrinkage increase with the increasing part thickness. Thicker

plaques cool more slowly, and slower cooling rates allow the molecules to adopt a regular pattern, forming larger crystalline areas and a higher degree of crystallinity. The higher degree

of crystallinity results in higher shrinkage.

Parts with thick wall sections are most difficult to cool (longer to cool) and require

additional packing.

When parts have both thick and thin sections (Figure 16), the location of the gate into

the thick section is preferred because it enables packing of the thick section, even if the


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- Corner parts:

The phenomenon of corner warpage is similarly attributable to shrinkage. The uneven

cooling behaviour in the corners causes the inside of the corner to shrink to a greater extent.

This leads to stresses and forces which produce corner warpage (Figure 18).

Figure 18: Corner warpage due to differential cooling

5. Basic shrinkage formulas:

The mold cavities are cut to dimensions larger than the desired part dimensions to

compensate for the plastic shrinkage which occurs during cooling. The cavity dimensions are

equal to the part dimensions plus some shrink factor supplied by the material manufacturer.

Dc = Dp+(D p*S)+(Dp*s) = Dp(1+s+s)

For practical purposes,because S is usually very small, S can be ignored,and the

formula simplified to:

Dc=Dp(1+S)

Dc : cavity dimensionDp : part dimension

S: shrinkage factor (mm/mm, inch/inch, %)


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Thus, the shrinkage formula is:

Linear mold shrinkage SL (what is ordinarily referred to as mold shrinkage) and

volume mold shrinkage SV are defined by the following equations:

Volumetric shrinkage:

Linear shrinkage:

There are usually two shrink factors given, one for dimensions in the direction of the

flow and the other for dimensions perpendicular to the direction of the flow.

Figure 19: The three shrinkage directions


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Mold shrinkage in the flow direction is calculated by:

SFlow= 100*(L M-Ls)/L M

Where L M is the length of the test section of the mold and L S is the corresponding

length of the specimen after it has cooled.

Mold flow in the transverse directions is calculated by:

STransverse = 100*(W M-WS)/W M

Where W M is the width of the test section of the mold cavity and W S is thecorresponding width of the test specimen after it has cooled.

Estimating shrinkage, however, is not straight forward. It is often difficult to predict

the melt flow path in parts with complex geometries and therefore, it is not clear which shrink

factor to apply.

Finally a coarse estimation of the shrinkage based on the thermal expansion can be

given by the difference of the thermal expansion coefficients:

l=l o [ polymer (T, p)- mold]. T

Where l is the characteristic length and is the thermal expansion coefficient.

This equation does not consider only process specific influence factors and illustrates only the

difference in thermal expansion between mold and polymer.

The shrinkage is influenced by injection mold process parameters in accordance with next

relation:


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Where:

Cv: volumetric shrinkage;

v: coefficient of volume dilatation, indicate the influence of temperature over the melt of

thermoplastics materials;

k pi: coefficient that indicate the influence of injection and holding pressure over the

coefficient of volume dilatation;

Tm: melt temperature;

T0: ambient temperature.

Coefficient of volume dilatation has a value that characterizes each thermo-plastic material,

and is influenced by the degree of purity of the material.

6. Thermodynamic approach with pvT diagrams to predict shrinkage:

6.1 Definition:

The PVT diagram is a condensed presentation of the interrelations of three variables

that affect the processing of a polymer: Pressure, Volume and Temperature.

The two classes of thermoplastics, amorphous and semi-crystalline, show a linear dependency

of the specific volume on the temperature in the melted state. In the solid state, the specific

volume of the semi-crystalline polymers decreases exponentially whereas the amorphous

polymer keeps a linear dependency, although with a different slope. This can be visualized by

the pvT diagram (Figure 20), where the specific volume is plotted as a function of the

temperature for different pressures.

Therefore a pvT diagram is simply the presentation of the series of curves obtained when the

measurement of specific volume versus temperature is repeated at different pressures.


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Figure 20: Exemple of pvT for semi crystalline polymer (left) and amorphous polymer (right)

6.2 Description of injection molding process with PVT diagram:

Injection molding is a cyclic process consisting of four phases: filling, melt

compressing (or packing), holding and cooling, as shown by the typical PVT diagram in

Figure21(a), cavity pressure profile in Figure21(b), and cavity temperature profile in

Figure21(c).

Figure 21: PVT diagram (a) , cavity pressure profile (b) and cavity temperature profile (c)

The filling process starts at Point A. The cavity pressure signal begins at Point B

where the melt plastics touch the pressure sensor for the first time and then the pressure

increases steadily as the filling proceeds. The filling phase is complete at Point C, where the

cavity is only volumetrically filled by the melt without being compressed.


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The packing process then embarks and the pressure rises rapidly to the peak value at

Point D. At point D, the injection pressure switches over to the holding pressure and the

holding pressure control sets in. Thereafter, the melt within the cavity is maintained at an

assigned pressure during the holding phase, when additional plastic melt can be packed into

the cavity to compensate for the plastic shrinkage caused by cooling, so as to have the mold

completely filled. This process continues until the gate is frozen, as marked at Point E. Point

E is the end-point of the holding phase. Beginning at Point E, a phase of constant volume is

maintained (isochoric phase). This isochoric phase is especially important because one strives

for a minimum of orientation, residual and distortion. This phase is decisive for the

dimensional accuracy of the molding. Reaching Point F in a uniform way is decisive for the

constancy of weight and dimensions of the molding. After Point F, the molding cannot be

influenced anymore. It shrinks unaffected, usually down to ambient temperature.

Therefore, Point D and E are the important transfer points to be controlled in order to obtain

optimized holding phase control.

6.3 Shrinkage measurement with pvT diagrams:

The pvT behavior is an important key to produce parts with low shrinkage.

Figure 22: Shrinkage prediction from pvT diagram

A

B


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Point A establishes the volumetric shrinkage and the volume of the plastic part is the

same as the volume of the cavity. At point B, the part reaches a thermal equilibrium.

Volumetric shrinkage is defined by the following equation:

6.4 Conventional measurements of polymer PVT properties:

Using a dilatometer is the most common technique to measure the bulk specific volume as a

function of temperature and pressure of polymers. There are two principally differentconventional techniques performing PVT measurements: the piston-die technique and the

confining-fluid technique.

6.4.1 The piston-die technique:

Figure 23: Piston-die technique

The material is enclosed and pressurized in a rigid die using a piston which is tightly fitted

into the die. During the measuring cycle the volume of the material is recorded by measuring

the displacement of the piston. Both temperature and pressure can be varied. The advantage of

this technique is the simplicity of the design that can be achieved. The disadvantage is that the

pressure applied is not hydrostatic because the material sticks to the wall. Other problems are


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the possible leakage between the piston and the die and the formation of voids in the sample

when solidifying.

The piston-die technique was applied by Chang et al. (1996) who used a PVT-100 apparatus

(figure 24).

Figure 24: Principle schematic diagram of PVT100

The specific volume is calculated from the displacement of the piston l by the following

formula:

mr l v pT

2

),(

Where :

v : difference in specific volume; l: displacement upper piston;

r: radius upper piston;

m: sample mass.


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6.4.2 Confining-fluid technique:

Figure 25: Confining-fluid technique

The sample is enclosed in a rigid sample chamber, and it is submerged into a fluid (mercury

or silicon oil). The cell is closed using a flexible wall or bellows. The bellows is used to apply

hydrostatic pressure to the fluid and polymer by reducing the sample chamber volume, and

sensing the cumulative volume change of fluid and polymer. The absolute specific volume of

the polymer can be obtained by correcting the relative volume difference with the specific

volume of the confining fluid. Both pressure and temperature can be varied. The advantages

of this technique are:

- Pressure is purely hydrostatic as the sample is surrounded by the confining fluid in

both a melted and solid state;- There is an absence of leakage and friction.

The disadvantages are:

- The volumetric changes measured are not that of the polymeric sample only;

- Sealing of the pressurized fluid and reactions may occur between polymers and the

confining fluid.


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at temperatures above the melting point for polymers. Nonetheless further developed

modified 2-domain Tait Equation has been used.

The PVT relationships can be represented by the 2-domain Tait EOS:

Where V (T, P) is the specific volume at temperature T and pressure P, V0 is the specific

volume on the zero gauge pressure, C is 0.0894 (universal constant), and B represents the

pressure sensitivity of the material. Two temperature domains are required to model the PVT

relationship, because the thermodynamic properties of polymers change at the transition to the

solid state. The volumetric transition temperature at zero gauge pressure is denoted by b 5, and

the linear increase in the transition with pressure is denoted by b 6.

The specific volume obtained by extrapolating the zero-isobar curve to the transition

temperature is denoted by b 1. This value is the same for both domains when crossing the glass

transition. However, when the material is semi-crystalline, the transition due to crystallization

is accompanied by an abrupt change in specific volume, such that b 1m (the melt specific

volume at b 5 and zero pressure) is greater than b 1s. The temperature dependence of thespecific volume is measured by b 2, while b 3 and b 4 characterize B(T) in the solid and melt

state. The specific volume becomes more pressure sensitive with increasing temperature when

b4 is positive. The constants, b 7, b8 and b 9 characterize V 1 in the solid state.


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Figure 27: Schematic pvT diagram for amorphous and crystalline polymers

b5, b 6, b 1m , b 2m, b 3m , b 4m , b 1s, b 2s, b 3s, b 4s, b 7, b 8 and b 9 were determined by fitting the

experimental PVT data using a nonlinear regression. Software SPSS (SPSS Inc., Chicago,

Illinois) could be used for the nonlinear regression. Before the nonlinear regression, the

experimental data should be divided into two phases with the transition temperature. With thetransition temperature at different pressure, b5 and b6 should be calculated firstly; then b 1m ,


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b2m , b3m , b4m in melt state and b 1s, b2s, b 3s, b 4s, b7, b 8, b 9 in solid state could be calculated

separately.

- Mold shrinkage estimation:

The volumetric shrinkage of the molded part can be predicted by imposing the known

processing conditions over the P-v-T behavior of the plastics.

The isotropic linear shrinkage and anisotropic linear shrinkage can be predicted and

controlled by the equations below if the melts pressure, temperature, and specific volume

history during a molding cycle are known.

where, s is the molded shrinkage, v(Tno_flow, Ppack) is the specific volume of the

plastic at the end of packing stage, v(Tend_flow, Pend_use) is the specific volume of the

plastic during end use of the molded part and a is the fraction of anisotropy in the flow

direction related to other two directions.

7. Conclusion:

In this chapter we have got an idea about in mold and post mold shrinkage, determined

shrinkage factor and finally introduced the equation of state widely used to describe the

polymer PVT data.

In the next chapter we will focus on the conception of the plastic grating panel s mold.

3

_ _

_

),(

),(1

useend flowend

pack flowno

P T v

P T v s

)2(

)2(1),(

),( 32

_ _

_

a

as sa P T v

P T v

suseend flowend

pack flowno


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Experimental work


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Chapter I: Design and Modeling of Injection Molded Parts

1. Introduction:

In this chapter we will focus on modeling the plastic part and its characteristic mold

using SOLIDWORKS network.

2. Part design:

The first task was to measure the dimensions of the grating panel (Figure 28) using a

digital caliper and a gauge radius (Figure 29).

After measurement, the part was modeled with SOLIDWORKS (Figure 30). In order

to allow part ejection from the mold, plastic part was designed with a draft angle in thedirection of mold movement of about 0.3.

Figure 29: Measuring instruments Figure 28: Plastic grating panel in PP


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Figure 30: The plastic grating panels model

3. Mold design:

3.1 Parting line position:

The position was chosen by following the trace of the parting line existing on the

sample.

Figure31: Front view of the parting line

Parting line

Front cavity side

Rear cavity side

Demoldingdirection


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Figure 32: Trimetric view of the parting line

3.2 Sprue position:

The sprue was chosen in the middle of the part. In this case there will be neither runner norgate. The sprue will be ejected with the part and then will be cut.

Figure 33: Front cavity showing the sprue position

Sprueposition


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3.3 Air vents dimensions:

The concept of venting is simple: provide many pathways to allow trapped air and

volatile gases to escape from the mold quickly and cleanly. The pathways should lead directly

to the outside atmosphere surrounding the mold. These pathways need to be deep enough to

let air and gases out easily, but not deep enough to allow the molten plastic to escape through

them.

Figure 34: Air vents dimensions

4. Conclusion:

At the end of this part, the plastic grating panel s mold was designed. In the following chapter

we will focus on the required calculations while choosing injection molding machines.

Parting line

Extremity ofthe mold

0.02 mm


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- Total flow path length = 222.3 mm

- The wall thickness of the cavity: 2.75mm

- The flow path ratio = 222.3/ 2.75

= 80.83 [75:1; 100:1]

According to Figure 35 or to Table 4, the cavity pressure is about 180bars (18MPa) when the

flow path ratio is between 75:1 and 100:1 and the wall thickness is 2.75mm.

Figure 35: Cavity pressure as function of wall thickness and flow path length

Table 3: Cavity pressure for thermoplastic melt with various flow characteristics:


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3. Viscosity factor and safety factor:

- According to the table below the viscosity factor of PP is1.

Table 4: Viscosity factor:

- Usually a safety factor of 10 or more is common in an industrial design.

So we have:

Cavity pressure 18 MPa

Viscosity factor for PP 1Projected area 238 679.44 mmSafety factor 10%

Therefore, the clamping force can be calculated:

Clamping force = cavity pressure x viscosity factor x project area x (1+safety factor)

= 18 x 1 x 238 679.44 x (1+10%)

= 4.7259.10 6 N

= 4.7259.10 3 kN= 4.7259.10 2 tonnes = 472.59 tonnes

Clamping force = 472.6 tonnes

3. Injection capacity and weight calculations:

Injection capacity is given by the following formula:

Injection capacity = 1.25* total volume of component (part + sprue)


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- (Part + sprue) volume = (198 785.75+1744.12) +109.23= 200 639.1 mm 3;

- Injection capacity = 1.25* 200 639.1 = 250 798.875 mm 3

Injection capacity = 250 798.875 mm 3

Injection weight is given by the following formula:

Injection weight = injection capacity x Melt Density of PP

- Melt Density of PP= 0.9 g/cm 3 = 0.9.10-3 g/mm 3;

- Injection weight of component = 250 798.875 *0.9.10-3= 225.7190 g

4. Injection molding machine choosing:

Number of cavities 1

Clamping force 472.59 tonnes

Injection pressure 180 bars

Injection capacity 250 798.875 mm 3

Injection weight 225.7190 g

5. Conclusion:

In this chapter, we have got a general idea about the required calculations while choosing

injection molding machines.


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Conclusion:

In conclusion, working with Teem Engineering for four weeks was a very nice

experience that has enhanced my major understanding. In fact, Ive learnt a lot about

designing plastic injection molding and Ive ameliorated my knowledge about polymers.

In addition, I gained a good experience in term of self-confidence, real life working

situation and interactions among people in the same field.

Also, the training was an opportunity for me to increase my human relation bothsocially and professionally.


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References:

- Process Optimization for Plastic Injection Mould. Case Study: Container Mould (PDF)

- Handbook of Molded Part Shrinkage and Warpage

- Hot Embossing: Theory and Technology of Microreplication

- Injection moulding process (PDF)

- Injection_Mould (PDF)

- Shrinkage and Warpage (PDF)

- IN-situ shrinkage sensor for injection molding (pdf)

- Studies and experimental research over the influence of plastics ground material percentage

over the shrinkage of injection molded pieces (PDF)

- PVT Properties of Polymers for Injection Molding (PDF)

- Polymer engineering and science-2008 (PDF)

- Small Moulding Guide Index (PDF)

- How to select a plastic injection moulding machin e by Tat Ming Technology Co. Ltd,

October 1999

- Technical Papers by Prabodh C. Bolur

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