Kutz M. Handbook of materials selection

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12 TRANSFER MOLDING 985

Fig. 26 Plunger transfer molding: , loading; , molding; , unloading; W, loading well;LM U

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D, distributing channel; G, gate; C, cavity; R, plunger; M, material charge; T⬘, stationary mold

half; S, cull (scrap); P, product; T, moving mold half. (From Charrier, J.-M., Polymeric Materials

and Processing: Plastics, Elastomers and Composites, Hanser Publishers, Munich, 1990.)

Fig. 27 Pot transfer molding: , loading; , molding; , unloading; C, cavity; L, pot;LM U

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M, material charge; R, transfer piston; S, sprue (scrap); G, gate; T, T⬘, mold halves; P, product.

(From Charrier, J.-M., Polymeric Materials and Processing: Plastics, Elastomers and

Composites, Hanser Publishers, Munich, 1990.)

Tooling

There are two basic methods for transfer molding. These are plunger transfer

molding and pot transfer molding. In plunger transfer molding (Fig. 26), the

material charge is ﬁrst placed in a loading well, which extends to the mold

parting line . The plunger then transfers the material to the cavities, through

䡺

distributing channels and gates . The material left at the bottom of the loadingM

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well, known as the ‘‘transfer pad’’ (cushion, cull), is essential for pressure control

but should be minimal. Products and a certain amount of material, to be

scrapped, are ejected after mold opening .

䡺

In pot transfer molding (Fig. 27), the material charge is loaded into the pot

(chamber), and subsequently transferred through the sprue and gate into the

cavity. The press stroke can normally assure both old clamping and material

transfer functions, without a need for an auxiliary transfer ram.

Materials and Products

Transfer molding is normally used with materials that have fairly high precuring

ﬂuidity, facilitating the ﬂow from the loading areas to mold cavities.

986 PLASTIC PARTS PROCESSING I

Fig. 28 Injection-molding principle: , feed unit; , mold unit; , plastication stage; ,FM P I

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injection;; , product ejection (demolding); D, axial screw drive; D⬘, rotation screw drive; F, feedE

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hopper; S, reciprocating screw; R, material ‘‘reservoir’’; V, valve; T, T⬘, mold halves; C, mold

clamping device. (From Charrier, J.-M., Polymeric Materials and Processing: Plastics, Elastomers

and Composites, Hanser Publishers, Munich, 1990.)

13 INJECTION MOLDING

The injection-molding process involves the rapid pressure ﬁlling of a shape-

speciﬁc mold cavity with a ﬂuid material, followed by the solidiﬁcation of the

material into a product. The process is used for thermoplastics, thermosetting

resins, and rubbers.

Principle

The injection molding of thermoplastics can be subdivided into several stages,

as illustrated in Fig. 28. At the plastication stage , the feed unit operates

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in much the same way as an extruder, melting and homogenizing the material

in the screw/barrel system. The screw, however, is allowed to retract, to make

room for the molten material in a space at the cylinder head, referred to here as

the material ‘‘reservoir,’’ between the screw tip and a closed valve or an obstruc-

tion of solidiﬁed material from the previous shot. At the injection stage , the

䡺

screw is used as a ram (piston) for the rapid transfer of the molten material from

the reservoir to the cavity between the two halves (T and T

⬘) of the closed mold.

Since the mold is kept at a temperature below the solidiﬁcation temperature of

the material, it is essential to inject the molten material rapidly to ensure com-

plete ﬁlling of the cavity. A high holding or packing pressure (10,000–30,000

psi or 600–2000 atm) is normally exerted, to partially compensate for the ther-

mal contraction (shrinkage) of the material upon cooling. The cooling of the

material in the mold is often the limiting time factor in injection molding because

of the low thermal conductivity of polymers. After the cooling stage, the mold

can be opened and the solid product removed (ejection stage ).

䡺

Equipment

Injection molding machines are now most commonly of the reciprocating screw

type, as illustrated in Fig. 29. Two distinct units referred to as the feed unit

䡺

13 INJECTION MOLDING 987

Fig. 29 Injection-molding machine: , feed unit; , mold unit; D, axial screw drive;FM

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D⬘, rotation screw drive; F, feed hopper; C, plastication / injection cylinder; P, stationary platen;

T, mold; P⬘, moving platen; R, tie bars; B, back platen; C⬘, mold clamping mechanism;

F⬘, maching grame; H⬘, hydraulic accumulator; H, hydraulic pump; M, electric motor;

In, injection unit. (From Charrier, J.-M., Polymeric Materials and Processing: Plastics, Elastomers

and Composites, Hanser Publishers, Munich, 1990.)

Fig. 30 Mold-clamping mechanisms: , mechanical (double toggle); , hydraulic (dual cylin-MH

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der); , mold open conﬁguration; , mold closed conﬁguration; P, stationary platen; T, mold;OC

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P⬘, moving platen; L, link, D, toggle drive; C⬘, lock cylinder; C, traverse cylinder. (From Charrier,

J.-M., Polymeric Materials and Processing: Plastics, Elastomers and

Composites, Hanser Publishers, Munich, 1990.)

and the mold unit , are mounted on a frame (F⬘). The feed unit consistsMF

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of the plastication/injection cylinder (screw, barrel, and feed hopper), the axial

screw drive, and the rotation screw drive.

While injection-molding machines may occasionally be dedicated to the

molding of a single product, a machine is normally used with a variety of tools

(molds), which may imply frequent mold changes and the associated costly setup

period. Injection-molding machines are available in a broad range of sizes. They

are normally rated by their maximum clamping force, with normal ranges from

about 25–150 tons for ‘‘small’’ machines, 150–750 tons for ‘‘medium-size’’

machines, and 750–5000 tons for ‘‘large’’ machines; the current maximum is

10,000 tons. Figure 30 shows several common mold-clamping mechanisms em-

ployed in the injection-molding process.

Tooling

The interchangeable injection-molding tool, the mold, must (a) provide a cavity

corresponding to the geometry of the product and (b) allow the ejection of the

988 PLASTIC PARTS PROCESSING I

Fig. 31 Pin/ busing: , centering pin; , angled pin; , mold closed conﬁguration; , moldLAC O

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open conﬁguration; P, pin; B, busing; T, stationary mold half; S, secondary mold section;

T⬘, moving mold half. (From Charrier, J.-M., Polymeric Materials and Processing: Plastics,

Elastomers and Composites, Hanser Publishers, Munich, 1990.)

Fig. 32 Multicavity molding: , , linear arrangements; , , radial arrangements;LL⬘ RR⬘

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C, cavity; R, main runner; S, sprue; G, gate; R⬘, secondary runner; W, cold slug well.

(From Charrier, J.-M., Polymeric Materials and Processing: Plastics, Elastomers and

Composites, Hanser Publishers, Munich, 1990.)

product after its solidiﬁcation. Primary mold opening is achieved by fastening

one-half of the mold to the stationary platen (T), as shown in Fig. 31, and the

other half to the moving platen (T

⬘). The stationary mold half is sometimes

referred to as the ‘‘front,’’ ‘‘cavity,’’ or ‘‘negative block,’’ and the moving mold

half as the ‘‘rear,’’ ‘‘force,’’ or ‘‘positive block.’’ The removal of a product from

a cavity surface requires, in addition to an ejection system, a suitable surface

ﬁnish and an appropriate taper or draft. It need not require a mold release agent.

During injection, the material ﬂows from the nozzle at the tip of the injection

unit to the single cavity, or to each of several cavities, through what is referred

to here as the ‘‘feed system,’’ generally comprising sprues, runners, and gates,

as illustrated in Fig. 32. In most cases, injection-molded products need to be

removed from one mold half by an ejection (knockout, stripping) device. This

device is normally incorporated in the moving mold half. Retractable secondary

mold sections may be required when products feature undercuts, reentrant

shapes, internal or external threads, etc.

Runners are machined in mold halves, next to the parting surface. One so-

lution, applicable to chemically stable thermoplastics, consists of having large

runners cooled in such a way that a sleeve of insulating solid plastic forms

around a molten core, where the intermittent injection ﬂow takes place; this

method is referred to as ‘‘insulated’’ or ‘‘Canadian’’ runner molding. Another

solution referred to as ‘‘hot runner molding’’ involves a heated runner, or man-

13 INJECTION MOLDING 989

ifold block, and is often used in conjunction with valve gating. Gates serve

several purposes in injection molding. Their easily altered, smaller cross section

permits a convenient control of the ﬂow of the molten material, the rapid freezing

of the material to shut off the cavity after injection, and the easy separation of

the products from the feed appendage (degating). Important savings can be made

by using hot runners.

The maximum pressure in injection molds is normally in the range of 4000–

12,000 psi corresponding to a clamping force per unit projected area of cavity

and feed system in the range of 2–6 tons/in.

. The construction of injection

molds requires materials with a combination of good thermal conductivity and

resistance to mechanical wear and abrasion. Prototype molds can be cast from

low melting alloys. Short-run molds (about 1000 moldings) can be machined in

aluminum or mild steel. For medium runs (about 10,000–100,000 moldings),

tool steel is normally used. Long runs involving millions of moldings require

special hardened and chrome-plated steels.

A variety of techniques are used to form mold cavities: cutting (machining)

of a solid block, computer-aided machining (CAM) centers, hobbing (cold form-

ing), electrochemical machining (ECM), electrical discharge machining (EDM),

or spark erosion, electroforming, plating, and etching.

Auxiliaries

Many thermoplastic resins require thorough drying prior to molding, to avoid

the formation of voids or a degradation of the material at molding temperatures.

Mold temperature control is often achieved by the circulation of a ﬂuid through

a separate heater/chiller device. With increased interest in automation, robots

have been introduced for the removal of products and feed appendages from

open molds, and for separation (degating) and sorting. Feed appendages, startup

scrap, and occasional production scrap are normally reground in granulators and

recycled as a fraction of the feed material.

Materials

All thermoplastics are, in principle, suitable for injection molding, but since fast

ﬂowrates are needed, grades with good ﬂuidity (high melt index) are normally

preferable.

Products

A major advantage of injection-molded products is the incorporation of ﬁne

details such as bosses, locating pins, mounting holes, bushings, ribs, ﬂanges,

etc., which normally eliminates assembly and ﬁnishing operations. Thermoset-

ting resin systems, such as phenolics (PF) or unsaturated polyester (UP), often

used with ﬁllers or reinforcements, are increasingly injection molded at relatively

high speeds. Curing, which involves chemical reactions, takes generally much

longer than the injection, and multimold machines are thus often used with

shuttle or rotary systems. Injection molding is increasingly used for producing

relatively small rubber products signiﬁcantly faster than by compression molding

and, normally, with a smaller amount of scrap and a better dimensional accuracy.

As in the case of thermosetting resins, a heated mold is needed for vulcanization

(curing).

990 PLASTIC PARTS PROCESSING I

Fig. 33 RIM equipment: T, mold; H, mixing head; E, heat exchanger; R, recirculating pump;

R, material tank; A, stirrer; C, metering cylinder. (From Charrier, J.-M., Polymeric Materials and

Processing: Plastics, Elastomers and Composites, Hanser Publishers, Munich, 1990.)

14 REACTION INJECTION MOLDING

Principle

Reaction (or reactive) injection molding (RIM) is a relatively new process that

involves the rapid mixing, in precise proportions, of two or more highly reactive

low-molecular-weight liquid components, and the immediate injection of the

mixture into a closed mold. Polymerization and/or molecular network formation

take place in the mold in a very short time. The RIM process was developed

with urethane resins and is still used primarily with these materials.

Equipment

RIM process equipment involves two, and sometimes more, components, each

one corresponding to a stream up to the mixing head, as illustrated in Fig. 33.

A suitable batch of each component is held in a material tank (day tank, work

tank), which is pressure-rated (about 7.5 atm or

⬃110 psi). Tanks generally

feature a nitrogen gas overlay (blanket) to protect the chemicals from oxygen

or moisture, and a vortex-free stirrer (agitator), particularly for materials con-

taining ﬁller particles. A heat exchanger permits the control of the temperature

of each component. Some resin systems (polyamide systems in particular) re-

quire much higher processing temperatures; corresponding equipment featuring

jacketed tanks and lines is often referred to as ‘‘heat-traced’’ (HT) equipment.

Tooling

RIM molds do not have to withstand high pressures (4–8 atm or ⬃50–100 psi),

and for most polyurethane systems their temperature remains moderate. De-

pending on the number of parts required, mold materials can range from epoxy,

metal-coated epoxy (nickel is common), kirksite, aluminum, and steel.

Materials

Urethane resin systems (RIM-PUR) are most widely used. Urea resin systems

(RIM-PUA) have been developed with a superior temperature resistance that

makes them suitable, for example, for automotive body components subjected

to high-temperature paint ovens.

BIBLIOGRAPHY 991

Products

Because of the low-viscosity ﬂuid involved and the low mold pressure, the RIM

process is particularly well suited to the fabrication of large thin parts, which

include exterior automotive body panels such as front-end fascias and fenders,

as well as furniture, electronic cabinets, and window frames.

15 SUMMARY AND CONCLUSIONS

This chapter has highlighted the most useful of the plastics parts processing

techniques. It has attempted to elucidate the fundamental processing principle

underlying each technique, describe the equipment and tooling employed to

carry out this processing technique, discuss the materials most commonly used

with each processing technique, and point out the most important products man-

ufactured using that technique.

To gain a more complete understanding of plastics parts processing, one

should refer to the core literature on the subject as listed in the following section.

BIBLIOGRAPHY

Belofsky, H., Plastics: Product Design and Process Engineering, Hanser Publishers, Munich, 1995.

Charrier, J.-M., Polymeric Materials and Processing: Plastics, Elastomers and Composites, Hanser

Publishers, Munich, 1990.

Malloy, R. A., Plastic Part Design for Injection Molding, Hanser Publications, Munich, 1994.

McCrum, N. G., C. P. Buckley, and C. B. Bucknall, Principles of Polymer Engineering, Oxford

Science Publications, Oxford, 1997.

993

CHAPTER 33

PLASTIC PARTS PROCESSING II

Dean O. Harper

Department of Chemical Engineering

University of Louisville

Louisville, Kentucky

1 CLASSIFICATION OF PLASTIC

PARTS PROCESSES 993

1.1 Continuous and Cyclic

Processes 994

1.2 Dominating Kinematics

of Flow 994

1.3 Sources of Momentum 996

2 INTRODUCTION 997

2.1 Design Problem 997

2.2 Design Solution Algorithm 998

2.3 Effects of Temperature 1007

2.4 Effects of Pressure 1008

2.5 Extrusion of a Rod as an

Example 1009

PART I. Continuous Processes:

Shear Dominated

3 EXTRUSION: SINGLE SCREW 1011

4 EXTRUSION: TWIN SCREW 1015

5 WIRE COATING 1016

5.1 Drag Flow Only 1017

5.2 Drag and Pressure Flow 1017

6 CALENDERING 1018

PART II. Continuous Processes:

Extension Dominated

7 FIBER SPINNING: MELT 1019

8 FILM BLOWING 1020

PART III. Cyclic Processes:

Shear Dominated

9 MELT INJECTION MOLDING 1022

10 REACTION INJECTION

MOLDING 1025

11 TRANSFER MOLDING 1025

PART IV. Cyclic Processes:

Extension Dominated

12 COMPRESSION MOLDING 1026

13 BLOW MOLDING 1026

14 THERMOFORMING 1027

15 ROTATIONAL MOLDING 1032

PART V. Cyclic Process:

Quiescent

16 CASTING 1032

17 CONCLUSION 1033

REFERENCES 1033

BIBLIOGRAPHY 1035

1 CLASSIFICATION OF PLASTIC PARTS PROCESSES

Plastic parts processes may be classiﬁed by the time mode of product formation

and subclassiﬁed by the dominating kinematics of the associated ﬂows. Some-

times dimensionality is used, referring to a product as ‘‘one,’’ ‘‘two,’’ or ‘‘three

dimensional.’’

HandbookofMaterialsSelection.EditedbyMyerKutz

994 PLASTIC PARTS PROCESSING II

1.1 Continuous and Cyclic Processes

The two time mode categories of processing are continuous and cyclic. Products

made continuously are characterized by having one dimension that is ‘‘unend-

ing.’’ Consequently, the other two dimensions constitute an area referred to as

the ‘‘proﬁle.’’ The proﬁle of a rod is a circle; the proﬁle of a pipe is the area

between two concentric circles. Of course, many useful proﬁles, such as door

closures, are geometrically complicated; however, they share the characteristic

of having the same cross section throughout their length.

These processes are also referred to as steady state. The design equations

used to relate the dimensions of the proﬁle to the plastic’s properties and op-

erating parameters do not include ‘‘time’’ as an independent variable.

Products that are produced cyclically are inherently three dimensional; that

is, no one dimension is unending—each is ﬁnite. Every part, though replicated

cycle after cycle, has its own identity.

Such processes are also referred to as unsteady state. There is a start and end

to each cycle, the cycle time. And, the design equations always include time as

an independent variable; however, in terms of the process economics, cycle time

is an important dependent variable.

1.2 Dominating Kinematics of Flow*

Kinematics is the study of motion, per se, without regard to the forces necessary

to produce that motion. In plastics parts processing, there are two such

categories—shear ﬂow and extensional ﬂow. Although these ﬂows are usually

characterized in rectangular, cylindrical, bipolar, or some other appropriate three-

dimensional coordinate system, it is convenient to use a generic ‘‘numerical’’

representation to describe them. Let the ﬂuid’s velocity v, be decomposed into

its three components, (

), in the three coordinate directions, N (x

where N is used to designate the numerical coordinate system.

Shear-Flow-Dominated Processes

Shear ﬂows are those motions for which (i, j ⫽ 1, 2, 3)

v ⫽ ƒ(x ) i ⫽ j (1.1)

The shear rate

is given by˙

␥

⭸v

␥

⬅ (1.2)

⭸x

Simple shear has the universally accepted description

⫽ (v ,0,0) ⫽ N(0,x ,0) (1.3)

There is only one nonzero component of motion, and it is a function of only

*Note: Scalars are italic, vectors are boldface, and tensors are boldface italic.

1 CLASSIFICATION OF PLASTIC PARTS PROCESSES 995

one other, and different, direction. More complicated ﬂows may be characterized

as complex shear, when there are two nonzero components of the velocity, or

total shear, when all three components are nonzero, provided in each case that

Eq. 1.1 is satisﬁed for each i and j.

Extrusion is a shear-dominated continuous process and injection molding is

a shear-dominated cyclic process.

Extensional-Flow-Dominated Processes

Extensional ﬂows are those motions for which

v ⫽ ƒ(x ) (1.4)

i.e., Eq. 1.1 with i ⫽ j. The extensional rate is

⭸v

˙␧ ⬅ (1.5)

⭸x

There are three such extensional ﬂows: uniaxial, planar, and uniform biaxial.

Uniaxial extensional ﬂow has the characteristics

⫽ (v ,v ,v ) ⫽ N(x ,x ,x ) (1.6)

123 123

where ⭸v

/⭸x

⬎ 0 and ⭸v

/⭸x

⫽ ⭸v

/⭸x

⫽ (⭸v

/⭸x

). It is uniaxial because

–

⫺

only one component of velocity is increasing with position; the other two are

decreasing at rates such that the volume remains constant. Fiber spinning is a

continuous uniaxial-dominated extensional process.

Planar extensional ﬂow has the characteristics

⫽ (v ,v ,0) ⫽ N(x ,x ,x ) (1.7)

12 12 3

where ⭸v

/⭸x

⬎ 0 and ⭸v

/⭸x

⫽⫺(⭸v

/⭸x

) and ⭸v

/⭸x

⫽ 0 since v

⫽ 0. It

is planar because only two components of velocity change; one is increasing

with position and another is decreasing at a rate such that the volume remains

constant. The third ‘‘does not change’’ with position because that component is

zero. Sheet extrusion, with pulling to reduce thickness, is a continuous planar-

dominated extensional process.

Uniform biaxial extensional ﬂow has the characteristics

⫽ (v ,v ,v ) ⫽ N(x ,x ,x ) (1.8)

123 123

where ⭸v

/⭸x

⫽ ⭸v

/⭸x

⬎ 0 and ⭸v

/⭸x

⫽⫺2(⭸v

/⭸x

). It is uniform biaxial

because two components of velocity increase with position at the same rate; the

third decreases at a rate such that the volume remains constant. Film blowing is

a continuous process dominated by uniform biaxial extension.

Quiescent Process

A plastic parts process that has no ﬂow is casting; it is a quiescent process and

is inherently cyclic.