Injection Molding

INJECTION MOLDING refers to a variety of processes that generally involve forcing or inject- ing a fluid plastic material into a closed mold (Ref 1, 2). It is differentiated from compression mold- ing, in which plastic materials in a soft but not fluid condition are formed by transferring them into an open mold, which is then forcibly closed. This latter process is fully discussed in the article "Compression Molding and Stamping" in this Volume. The injection molding process generally has the advantages, first, of being more readily automated and, second, of permitting finer part detail, in contrast to compression molding. The part and mold often can be designed so that no subsequent trimming or machining operations are required. However, not all plastic materials can be injection molded successfully; for exam- ple, there is a limit to the amount and types of fibrous reinforcement that can be incorporated in an injection molded part.

There are two basic categories of plastic injec- tion molding: thermoplastic and thermoset. In the former, a thermoplastic material is melted and forced through an orifice, or "gate," into a rela- tively cool mold in which the material solidifies and from which it can then be removed. In ther- moset injection molding, a reacting material is forced into a generally warm mold in which the material further polymerizes or cross-links into a solid part.

and bosses, often without secondary operations or direct labor.

While the flow process has the potential to physically change the molding compound, these changes are not always negative. Improved ho- mogeneity, better reinforcement wetting, and higher physical properties may be achieved if the response of the molding compound to the injec- tion process conditions is well understood. Where this understanding is absent, reinforce- ments may be broken, and polymers may be either degraded or prematurely cured. Table 1 itemizes the more common thermoplastic and thermosetting molding compounds and indicates where, within the four subprocesses of injection molding, specific formulations demand special care or attention.

Thermoplastic Injection Molding

Injection molding came into significant com- mercial use following the development of plastic screws. This development started in the 1930s, and culminated in 1952 with the invention of the reciprocating screw plasticizing injection unit by W.H. Willert (Ref 3). The reciprocating screw type of injection machine has largely supplanted the early plunger type of injection machine.

Reciprocating-Screw Injection Molding Machine

Although there are many variations in thermo- plastic injection molding machines, the recipro- cating-screw machine has become the most com- mon type, and the following description of its operation illustrates the basic principles of all types. The functions performed by the machine include heating the plastic until it is able to flow readily under pressure, pressurizing this melt to inject it into a closed mold, holding the mold closed both during the injection and while the material is solidifying in the mold, and opening the mold to allow removal of the solid part.

As depicted in Fig. 1, the main components of the machine are the hopper, heated extruder bar- rel, reciprocating screw, nozzle, and mold clamp. The hopper feeds the unmelted thermoplastic, usually in pellet form, into the barrel. The hopper is often equipped with a desiccant-type drying system to remove moisture degradation. A mag- net is placed in the hopper throat to remove any iron that has accidentally entered the feedstock.

In the heated extruder barrel, which contains the reciprocating screw, the thermoplastic is gradually melted by a combination of shear heat- ing (caused by the mechanical working of the material as it is conveyed down the barrel) and heat conduction from the barrel. The thermoplas-

Molding Compounds Injection-molding compounds are thermoplas-

tic or thermosetting materials and their compos- ites that are specifically formulated for the injec- tion-molding process. This process requires materials capable of being fed into a molding machine, transported to accumulate pressure, in- jected through channels, and made to flow into a small opening in the mold. The process may cause major changes in both the physical and chemical properties of the molding compound. Because of their resistance to flow, neither high- molecular-weight resins nor long reinforcing fi- bers, or flakes, can be effectively manipulated through the molding process. Consequently, parts produced from molding compounds represent a compromise between optimal physical properties and the essential ability to flow under pressure. This compromise is offset by the ability to pro- duce three-dimensional products with holes, ribs,

Fixed platen mold clamp Extruder barrel I Mate~ d

/ / J hopl: r Tachometer Hydrauhc motor / Standard / Reciprocating \ / drive / Screw drive , - - ~ extruder head [ extruder screw\ / ~ / /

\ ~ / /~J l l_ _~ I ~ / ~ systeT~rust Nozxz'e ~ b e a r i n

~ ect ion "Ai r lift for s c r e w r e m o v a l c h a m b e r cy l inders

I Pullqn cylinder Heating bands

Fig. 1 The injection end of a reciprocating-screw injection molding machine. Source: Ref 2

Engineered Materials Handbook Desk Edition, 1995Michelle M. Gauthier, Editor, p 299-307

Copyright © 1995 ASM International® All rights reserved.

www.asminternational.org

300 / Processing and Fabrication of Engineering Plastics

Table 1 Common thermoplastic and thermoset molding compounds

Base polymer Principal applications Feeding Potential subprocess problem areas(a)

Transporting lnjectiag

Thermoplastics

Acrylonitrile-butadiene-styrene Aeetal Acrylic Cellulose Polycarbonate Polyester Polyethylene Fluoroplastics Polyimide Ionomer Nylon Polyphenylene oxide and alloys Polypropylene Polystyrene Polysulfone Polyvinylchloride

Thermosets

Alkyd Allyl Epoxy Polyester Polyimide Melamine Phenolic Urethane Vinyl ester

(a) Key to problem areas: Feeding

Furniture, cabinets, containers, trim B Clock gears, miniature engineered parts B Automobile light lenses, plastic glazing B Esters trim, moldings, screwdrivers B Auto bumpers, traffic lights, lenses B C Appliance parts, pump and electrical housings B C Housewure, food storage, dunnage B Corrosion/solvent-resistant parts B C Aerospace items, electrical insulators B C Bumper rub strips, golf ball covers B Auto parts, bearing retainers, appliances B C Auto instrument panels B Battery cases, auto parts, containers B Toys, advertising displays, picture frames B Camera cases, aircraft parts, connectors B C Soft steering wheels, Irim items B

Switches, motor housings, pot/pan handles A B Electrical connectors, circuit boards A B Electrical insulators, electronic cases B Automotive structural parts Aircraft components, aerospace parts A Dinnerware, microwave cookware B Distributor caps, plastic ash trays A B Automotive body panels, bumpers A Composite car/truck springs, wheels B

A, Moisture may chemically react to degrade the polymer base B, Drying is recommended to avoid splay in molded product C, Drying is essential to prevent molecular-weight attrition D, Drying may volatilize monomers essential to the curing reaction E, Fiber reinforcement breakage may occur during force feeding

Transporting F, Overheating may cause explosive depolymerization G, Overheating may cause premature curing of (thermose0 compound H. Venting is recommended to remove volatiles and reduce splay I, Fiber reinforcement lengths may be severely reduced J, Overheating may produce chemical changes in the base polymer

H I K L N F H I J

H I K L N H I K L N

F H I J K L N H I J

I F I J K L N

H I J K L N H I K L N H I J H I O ) K L N H I H I K L N H I J K L N

F H I J K L N

o Q O P o Q O Q R O Q O P O P O Q O Q O P O P O Q R O P O Q O Q O Q R

G H I J K L M N O G H I J K L M N O

E G H I J K L M N O D E G I J K L M N O D E G H I J K L M N O

G H I J K L M N O G H I J K L M N O

D E G I L M O D E G I J K L M N O

Injecting K, Fast injection of the molding compound can lead to serious overheating L, Filled or reinforced compounds will exhibit much higher viscosity M, Open mnners required as material can cure in closed channels N, Melt fracture may occur with high injection speeds O, Fiber-filler orientation will occur if molding compound is reinforced

Flowing P, Major sink marks may develop if part sections are thick, not uniform Q. Weak knitlines may develop if compound packing pressure is low R, Large mold vents recommended to allow volatiles to escape S, Hot molds required to promote cure, crystalline growth T, Curing reaction may produce peak exothenn leading to degradation

Q R S T Q R S T Q R S T Q R S T Q R S T Q R S T Q R S T Q R T Q R S T

tic is gradually conveyed by the rotating screw from the rear of the barrel to the front. The barrel is usually heated by band-type electrical resis- tance heaters fitted around its periphery. Because the amount of heat required varies with position along the barrel, the band heaters are controlled in several zones along the barrel. Some barrels are equipped about halfway down with a vent to remove gases from the melting feedstock. A vac- uum is usually applied to the vent, and a specially designed screw is required to depressurize the melt in the region of the vent to keep it from extruding through the vent.

The reciprocating screw usually has three suc- cessive zones, each with specific functions: The feed zone of relatively deep screw flights con- veys the unmelted plastic from the hopper throat into the heated barrel, where it begins to melt; the compression zone of decreasing flight depth, and thus volume, provides gas removal and melt den- sification, along with a material mixing action for better material and temperature uniformity; and the metering zone, generally of constant, shal- low-flight depth, provides the final shear heating and mixing of the melt. After the melt passes

through the metering section and a check valve at the end of the screw, it joins the melt pool in front of the screw. As the volume of melt in front of the screw increases, it forces the screw to the rear of the barrel against an adjustable "back pressure." This back pressure is applied hydraulically to the back end of the screw. Increasing the pressure increases the amount of mechanical working of the feedstock. Screw rotation and feedstock melt- ing continue until a sufficient amount of melt is available in front of the screw to fill the mold. At this stage, the screw rotation is stopped, and the machine is ready for injection. The stages in the operation of a reciprocating screw are shown in Fig. 2.

The melted thermoplastic is injected into the mold through the nozzle, under high pressure (typically 70 to 205 MPa, or 10 to 30 ksi, depend- ing on the mold-filling resistance). Injection oc- curs as the screw is hydraulically forced forward in the barrel. The hydraulic cylinder is located at the rear of the screw and barrel. A check valve at the tip of the screw keeps the melt from flowing back along the screw as the screw is pushed forward. In contemporary machines, the injection

rate (determined by the forward velocity of the screw) and injection pressure are closely control- led throughout the mold-filling stage.

The mold clamp, which holds the halves of the mold closed against the injection pressure of the melt, opens the mold to allow part removal after the thermoplastic has solidified. During the cool- ing and solidification period, the screw begins to rotate and melt new material for the next shot.

The clamping system consists of a fixed platen and a movable platen, each of which has half of the mold attached. The fixed platen (shown in Fig. 1) has a hole in its center to allow the injec- tion cylinder nozzle to be placed in contact with the sprue bushing of the mold. A movable platen is moved along tie bars by either hydraulic or mechanical means, or a combination of both. The amount of clamping force the machine can apply, rated in tons, generally determines the size of the part that the machine can process. It is this clamp- ing force that overcomes the injection pressure of the melt in the mold and keeps the mold halves together. The clamping force, or "tonnage," re- quired is determined by the amount of injection pressure required to fill the mold and the pro-

Injection Molding / 301

Frozen molding in clamped mold

/ / / / / / / / / / / / / / / / / / k ~ x ~

Sh;ekvmlO vesYt; k, r . . . . . . ~ Stage4

41Fd open

Stage 1 Reser//voir ~ - / ~ full Ejected molding

" / / / / / / / / ~ ' / / / / / / / / / Mold clamped, ~ ~ cavities filling

with melt

Stage 2 Screw moves forward, check valve closed

/ / / / / / . ' / / , . ' ~ / / / / . ' , " / e " Mold clamped, ~ ~ cavities full,

melt freezing Stage 3 Screw (almost)

stationary

Fig. 2 Stages in the operation of a reciprocating-screw in- jection molding machine

jected area of the part (that is, the area perpen- dicular to the axis of the machine). If the machine has inadequate clamping force, the two halves of the tool will begin to separate during injection, causing the melt to squirt out, or "flash," at the mold parting line, potentially causing an incom- plete mold-fill, or "short shot."

Contemporary molding machines have two or more closing speeds: a high-speed closing, re- quiting only a low force, followed by a slow- speed closing stage, which generates the high clamping force to close the mold fnrnly (stretch- ing the tie bars) prior to injection. The clamp usually opens slowly at first, followed by a rapid traverse. These high-speed motions reduce the time of the overall process cycle.

Injection Mold Design Injection molds, in their simplest form, consist

of two halves, often called the core and cavity (Fig. 3) (Ref 4, 5). A hole or spree bushing con- ducts the melt from the injection nozzle through the sprue and through a gate into the mold cavity.

Gates and Runners. Commonly, more than one gate is used to deliver material into the mold, and each gate is fed by a "runner" channel leading from the spree to the gate. After the mold is filled and the melt has solidified, the material in the sprue and runners also solidifies and must be removed with the part before the next shot. The spree and runner material are usually reground in a granulator machine and fed back, along with virgin material, into the injection cylinder for reuse. To reduce the amount of material that must be recycled, some molds are equipped with a hot

Locating ring Spruebushing/ /Runner Cla?pingplate

~/'~1 It/,~/[~-'l u " u / .:Plate: Parting line IV/A4 ' [, / / / / ~ .~'N~ Core ,~X'~

t , , ,~l ( ~ 0 'Plate Temperature ~_ '~ "~-~---- ~ control

channels s u00o

, ~ ~ Sprue / tKnockout F / plate ::!~:~!~;:: puller pin • plates

__ _ 1 _ ~ _ ' 1 " / Ejector ~ - ~ ~ :'housing

Support Knockout bar pillar

Fig. 3 Two-plate injection mold. Source: Ref 2

runner manifold (Fig. 4), which keeps the mate- rial in the sprue and runners molten to become a part of the next shot.

With larger and more complex parts, it is com- mon to use more than one gate for injecting ma- terial into the mold. The configuration of the runners and gates determines the way the mold is filled. This can change the amount of injection pressure required, the location and condition of "knit" lines, the orientation of material flow (which can affect mechanical properties), and the ability to vent air from the mold. Knit lines are surfaces along which the flow fronts meet and are generally weaker (Ref 6). Nonuniform mold fill- ing, caused by poor gate locations, can result in overpacking the mold in the regions which fill first, causing residual stresses and possible warp- age in the part. Computer programs are now available to assist in mold design and the layout of runners and gates for effective mold filling at minimum pressure.

Mold Cooling. With thermoplastic injection molding, a method of cooling is usually incorpo- rated into the mold to speed the solidification of the plastic. This usually consists of holes bored in each half of the mold through which a heat-ex- change fluid, usually water, can circulate. The mold temperature is usually controlled above room temperature. The optimal mold temperature depends on the type of plastic being molded, but typical mold temperatures for the more common thermoplastics vary from 40 to 120 °C (100 to 250 °F).

Material Shrinkage. All thermoplastic solidi- fication is accompanied by a volumetric shrink- age. For crystalline plastics, the shrinkage is as- sociated with crystallization. For amorphous plastics, the shrinkage is generally less and is associated with the glass transition. In both cases, the amount of shrinkage depends on various processing parameters, including the mold tem- perature and rate of cooling of the melt. This shrinkage can continue for a period of time after

Sprue Support bushin ̂ ~-Locating pins ~ S " ' ~ r....i ~ ring /

Air W / / / / / , ~ / K / Electric

gaps--~'~l / / / / / / " I Kd~l ch;t;i;rg t

plastic- , ~ " - ~ . ~ " ~ " q l ' - . \ \ '~\~llk~] runner ~'N~'q~,~.'~"~ ]~L~-'~'-NL--~ manifold

Band heater ~ . ~ . . ~ . . . . . . ~ . . . . . . . . . . . " .................... ::;:::;:::;:::::;:;:: ~A r gaps : :::::::::::::::: : " Nozz e:.~::::':::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: I ~:.......:.:.......:==============================

Plastic ~ Plate

~ Core/ ~ " ~ Plate

Auxiliary Knockout bushing pin Cooling

channels

Fig. 4 Hot runner injection mold. Source: Ref 2

the part is removed from the mold. It is important that the shrinkage be repeatable, so the mold can be appropriately sized. For parts requiring tight dimensional control or optimal mechanical prop- erties, uniform cooling is essential. If the part does not solidify uniformly in the mold, residual stresses will occur as a result of differential shrinkage. Computer programs are available to assist in the optimal layout of cooling channels for uniform part cooling.

Part Removal. The mold is usually designed such that the part remains on the moving half of the mold when it is opened. Ejector pins are then actuated to separate the part from the mold. These ejector, or knockout, pins are activated either as a direct result of the moving platen (mechanical knockouts), or hydraulically.

The geometry of the mold must be such that the part can be readily removed after the material has solidified, sometimes by means of a mechanical part remover. This requires careful part design and selection of the "parting surface" across which the two halves of the mold separate. Nei- ther half of the mold can have undercuts or "die locks" that would trap the part or keep it from being ejected. If the part cannot be designed with- out undercuts, then moving cores, slides, or lifters must be incorporated to eliminate the trapped condition when the mold is opened.

Mold Venting. Air must be removed as the mold fills with plastic. This is usually accom- plished by grinding channels, or vents, into the parting line of the mold. These vents must be narrow enough to keep molten plastic from es- caping, while allowing air to escape. They should be located adjacent to the last region of the mold to fill. Sometimes undersized ejector pins are also used to assist in venting the mold. Insufficient venting can result in an incompletely filled mold, or in cosmetic defects called diesel bums, which are caused by the heating of the air as it is com- pressed.

Mold Materials. For high-volume production applications, the mold cavity and core are usually

302 / Processing and Fabrication of Engineering Plastics

machined from special mold-making steels. Steel is chosen for its wear resistance and durability. For low-volume production or prototype applica- tions, it is common to cast molds from low melt- ing point metals, such as aluminum and zinc. These cast molds are usually less expensive and faster to build, but are not as durable and often do not have as good a surface finish as do machined steel molds.

Process Control Molding process controls can be particularly

important for parts with tight dimensional or per- formance requirements. Some thermoplastic molding materials require tighter control over these parameters than others.

Predrylng. In a molten state, some ther- moplastics, such as polyesters and polycarbon- ates, are very sensitive to the presence of water. Under such conditions, these polymers will de- grade or depolymerize, reducing the molecular weight, which in turn reduces impact strength and may cause cosmetic defects on the surface of the part that usually radiate outward from the gates. Known as splay, these marks are caused by mi- croscopic water vapor bubbles at the surface of the part. Predrying is usually achieved by ade- quate residence time (3 to 4 h) in a hopper drier at a temperature just below the softening point of the material. Too low a temperature will result in inadequate water removal, but too high a tem- perature may result in fusion of the pellets.

Controlling Shear. Excessive shear can be caused by restrictive check valve assemblies (Ref 7), shut-off nozzles, gates, and even by the part geometry itself. Excessive shear stress during the molding process can cause molecular breakdown or chain scission in thermoplastic materials. Some materials are more susceptible than others and need to be processed with great care. Control of shear stress is particularly critical if mechani- cal properties, such as impact strength, are impor- tant. Shear can be minimized by carefully select- ing the injection screw profile (Ref 7), by minimizing the back pressure on the material in the barrel, or by reducing flow restrictions.

Overheating. Thermoplastics are subject to degradation when they are overheated. Some, such as polyvinylchloride (PVC) and acetal, are far more sensitive than others. Degradation can result from chain scission, oxidation, hydrolytic degradation if moisture is present, or chemical interactions, such as transesterification. Material suppliers commonly add stabilizers to those ma- terials that are particularly susceptible to degra- dation. It is desirable to mold the material at the lowest temperature and shear possible and to check for hot spots in the machine.

Other Types of. Injection-Molding Machines

Instead of having a reciprocating screw, a ram injection machine has a simple plunger (ram) than pushes the unmelted plastic down the heated

barrel. A spreader or torpedo at the end of the barrel can increase the shear and compression on the material to assist in heating and melting. Be- cause the plunger does not mix and homogenize the melt as well as a reciprocating screw does, the ram injection machine is not often used.

In a two-stage injection or accumulator-type machine, the melting function is separated from the melt accumulation and injection functions. Either a stationary screw in a heated barrel or a plunger in a heated barrel performs the melting function. The melt is then fed into an accumulator cylinder before injection. Once sufficient melt has been accumulated for the next part, the ram forces the melt through the nozzle into the mold. A check valve keeps the melt from being forced back through the melting cylinder. This type of unit is generally not used because of the amount of degradation of the engineering materials.

Thermoplastic Structural Foam Injection Moldinsl

Structural-foam injection molding was intro- duced on a commercial basis in the early 1960s. It became a desirable extension of injection mold- ing, based on the concept that large parts could be produced on molding equipment having a rela- tively low clamping force if pressure on the mold surfaces could be kept low.

The mold cavity pressure developed during the structural-foam process is indeed low, repre- senting a fraction of that developed in solid injec- tion molding. Because the clamping force needed to mold a given part is a function of cavity pres- sure and the projected surface area of the part, low cavity pressure allows a low clamping force. Clamping capacity relates directly to molding equipment size and economics, as well as stress levels in the molded part.

The structural-foam injection-molding concept was once the only practical method to produce most large parts. For example, a projected part

2 2 area of 1600 cm (250 in. ) would require a clamp tonnage of 530 to 1070 kN (60 to 120 tonf) for a foam machine versus 4.5 to 10.7 MN (500 to 1200 tonf) for an injection machine. However, there is now molding equipment with the clamp- ing capacity to produce many of these large parts in solid form, although the associated penalties are that these large injection-molding machines are much larger than foam machines for the same clamp capacity, are more expensive, cost more per hour to run, take up more space, and weigh more. However, the structural-foam concept con- tinues to be practiced commercially, using a vari- ety of thermoplastic materials, because it offers economic value when producing large parts. An- other important reason is that low pressure results in low-stress parts that are more dimensionally stable and flatter. The structural foam process offers the lowest stress of any thermoplastic molding method.

This technology, like any other, has both ad- vantages and disadvantages. For example, one disadvantage is that the mechanical property val-

ues for structural-foam materials are significantly lower than those obtained with solid material, although foam is more rigid than an injection product on a weight-to-weight basis because of increased wall thickness. In addition, the struc- tural-foam process limits the quality of the part surface. It was previously impossible to obtain a class A part finish. In fact, some structural-foam parts still require extensive finishing, such as sanding and painting, to be commercially accept- able. However, at least one plastics manufacturer has been working with automotive manufacturers to improve class A surface fmishes by using a gas counterpressure technique, which is discussed at the end of this article, and reports favorable re- suits. Two other technologies being used are the co-injection structural foam and expanding mold, high-pressure methods.

Part Performance The level of performance of any structural-

foam part is determined by the material selected and the molding technique used. In fact, the molding technique can have a major effect on part properties, although poor molding using any process will result in poor properties.

The high level of interaction between struc- tural-foam properties and process is not surpris- ing, because a portion of the polymer is replaced by gas. This factor alone results in a reduction of physical properties. The reduction of mechanical properties can vary considerably, depending on the specific property. This is generally referred to as density reduction. Less polymer per cubic vol- ume results in lower properties. This can be over- come by mechanical design, such as thicker walls.

Foam Cell Effects. One of the reasons for the differences between foam and solid material property values is cell distribution (Fig. 5). The greatest density reduction with foam occurs at the center of the cross section. One would expect the compressive strength and modulus to be dispro- portionately low because the low-density core (shown in Fig. 6) is a load-beating area. Instead, flexural strength and modulus are only slightly lower than the values for solid material because these properties are very surface dependent and the surface of a structural-foam test specimen is nearly solid. Another way cell distribution affects test values or part performance is illustrated in Fig. 7. Even in a 200 mm (8 in.) tensile bar, there is a density gradient from one end to the other, except when counterpressure has been used.

Weight reduction is a major factor when con- sidering structural-foam parts. Because density reduction calculations are based on the weight ratio of the part as a solid shot to that same part in structural foam, the latter is referred to as having a 10 or 15% weight reduction. When appraising the required performance level, the actual density of material in critical loading areas of the part should be considered.

ImFmct Strength. In regard to the charac- teristic surface roughness, there are special roughness reduction techniques, which are de-

Injection Molding / 303

Neutral axis

OoO~OoO°O_O_Ooo o OoO_Ooo~Oo I o ~o~o o ° o ° ~ ° o oo°oo~°O ~o o

oG_o bOoo 6 o oOoo"o o 02 o 0 O00£OOC~_OuO 0 u O U OI 0%°0°o0°O 0°0%0 °0° °0°% /

0 0 0 O 0 0 O 0 0 0 U O u O O v O I

Fig. 5 Foam part cross section

Part cross section J

i 05 1.0 Specific gravity

Fig. 6 Low-density core of foam port

scribed in the Section "Aesthetic Concerns" in this article. The depth and frequency of surface flaws are a function of both material and process- ing conditions. Surface roughness influences any strength measurement, including impact strength. Surface defects can affect impact strength values, although these values are more affected by proc- essing, weight reduction, and types of material and filler.

Properties. Some of the more important physi- cal, mechanical, and thermal properties for com- mercially significant thermoplastic structural- foam materials are given in Table 2.

Blowing Agents The design of some structural-foam molding

equipment allows the direct addition of nitrogen gas to the polymer melt. The gas is forced into a solution and remains there until the external pres- sure on the melt is relieved as it enters the mold cavity. When the pressure is removed from the melt, the gas immediately expands, driving the melt to the extremities of the mold. Although nitrogen is low in cost and clean in terms of chemical residue, many commercial operations are phasing it out because of poor cell structure, as well as the fact that most modern equipment is not modified to accept direct nitrogen gas addi- tion.

On the other hand, the use of chemical blowing agents has increased. These agents generate the gas necessary for structural foam by either ther- mal decomposition or a chemical reaction in the melt.

Factors to consider when selecting a blowing agent for a particular thermoplastic are chemical compatibility, cost, convenience, safety, the exo- thermic or endothermic nature of the reaction, the nature and quantity of gas released, and cell struc- ture. Several important features of chemical blowing agents can be illustrated using polyphenylene ether (PPE) alloy structural foams. These alloys are particularly well suited

o o O 1 Io°o%1

0 o c

O 0 0 0 0 0 o %°g '22

. ;

O@ °

Gate J q

Fig. 7 Foam cell distribution

for blowing-agent studies because they can toler- ate exposure to a wide variety of chemicals at processing temperatures. In addition, they are not damaged by either water-containing or water- generating blowing agents. Consequently, these alloys are used commercially with several differ- ent generic types of blowing agents.

The following information is based on chemi- cal blowing agents that were tumble blended with pellets to form pelletized concentrates (Ref 8). The active chemicals, the gas they generated, the

Table 2 Thermoplastic structural foam properties

Flame- retardant

acrylonitrile- Polyphenylene ether alloy Poly- butadiene- Standard 10% glass

Properties ethylene styrene V-O grade fiber

~ame-remraam imlyester Polycarbonate 6.3 mm 9.5 mm

7% glass 30% glass Polyphenylene Thermoplastic (0,25im) (0.375in.) fiber fiber(a) ether/polyamide polyester thick sample thick sample

Physical Specific gravity 0.6 Density reduction, % 15 i5 i5 15 Mold shrinkage, % 1.8 0.5-0.8 0.5-0.7 0.4-0.6 Mechanical Tensile strength, MPa (ksi) 12.4 (1.8) 20.0 (2.9) 28.3 (4.1) 41.4 (6.0) Tensile modulus, GPa (106 psi) ... 1.41(0.205) 1.86(0.270) 2.62(0.380) Elongation, % Flexural strength, MPa (ksi) 18.6"i2.7) 41.4'('6.0) 60.;('8.7) 55.8"i8.1) Flexural modulus, GPa (106 psi) 0.827 (0.120) 1.69 (0.245) 2.28 (0.330) 3,03 (0.440) Impact strength, falling dart test, J (in.. lbf) ... 4 (36) 8 (72) 3 (28) Thermal

Deflection temperature At 1.8 MPa (0.264 ksi), °C (°F) 35 (94) 70 (162) 80 (180) 90 (190) At 455 kPa (66 psi), °C (°F) 80 (177) 90 (194)

~ammab~, ~ 94 ~ V-0/5V V - 0 / 5 V V-OM

(a) Test values determined on 6.4 mm (0.250 in.) thick samples

1.05 1.05 i5 1.23___ ]5 iO 15 15 15

0.4-0.7 0.2-0.4 1.2-1.6 0.2-0.4 0.5-0.7 0.5-0.7 0.5-0.7

44.8 (6.5) 66.9 (9.7) 51.2 (7.42) 84.8 (12.3) 72.4 (10.5) 15.9 (2.3) 17.2 (2.5) 2.28 (0.330) 1.52 (0.220) 3.38 (0.490) 1.52 (0.220) 1.52 (0.220)

4 "3' "4" 86.2(12.5) 120(17.5) 64.8"('9.4) 84.8'i]2.3) 114(16.5) 39.3"(5.7) 42.0'('6.1)

2.79 (0.400) 6.72 (0.975) 1.62 (0.235) 4.31 (0.625) 5.17 (0.750) 1.90 (0.275) 1.62 (0.235) 20 (170) 11 (100) 33 (288) 27 (240) 4 (36) 3 (29) 9 (85)

135 (273) 140 (280) 135 (275) 205 (398) 150 (300) 75 (168) 76 (169) 145(290) 185(365) >205(>400) ... 84(183) 86(186) v-o/5v v - o ' / s v . . . . . . . . . v-o/5v v-o/5v

304 / Processing and Fabrication of Engineering Plastics

Table 3 Blowing agents

Clamp time Gas released Chemical necessary, s

Gas Quantity of permeability gas generated,

rate(a) cm3/g

Nitrogen Azodicarbonamide 160 Hydrogen Sodium borohydride 140 Carbon dioxide Sodium bicarbonate 110

(a) In cm3/100 in.2/24 h, atmospheric

70 220 1200 2700 550 120

quantity of gas generated per gram of chemical, and the nature of the reaction are given in Table 3. The clamp time values given are specific to one part. The relationship between blowing agent and clamp time may vary widely, depending on part thickness and other factors. The time that must elapse before a molded part can be painted is called the degas time, and the type of gas gener- ated by the chemical blowing agent plays an important role in determining the degas time. If a part is painted before it is completely degassed, the paint will blister.

Blisters result from the blowing agent gas, which is under greater than atmospheric pressure in the core of the part, and migrates through the plastic material to reach equilibrium with the sur- rounding atmosphere. As the data in Table 3 indi- cate, nitrogen is very sluggish in its permeability characteristics. All other factors being equal (level of internal gas pressure, part thickness, and type of paint system), both carbon dioxide and hydrogen-generating blowing agents can more quickly reach equilibrium with the surrounding atmosphere than can nitrogen-generating blow- ing agents.

The endothermic or exothermic nature of a blowing agent affects processing economics. One of the factors that frequently controls the cycle time of structural-foam parts, and therefore the number of parts that can be produced in an hour, is known as post blow. Post blow results when a part is removed from a mold before the core is sufficiently cooled, causing swelling in areas that have the greatest thickness. This swelling in thick cross sections is due to pressurized gas in the core of the part. These core areas cool slowly not only because plastics have low thermal conductivity but also because the cell structure produced by the blowing agent reduces thermal conductivity. When a structural-foam part is prematurely re- moved, the intemal pressure of the gas is suffi- cient to bend exterior skins outward, causing a visible defect in the part.

If a blowing agent is exothermic, it increases the temperature in the core of the part, which softens the plastic material and increases the gas pressure from the blowing agent. Endothermic blowing agents, on the other hand, draw heat from the core of the material during gas genera- tion, helping to solidify the cell wall and decrease internal gas pressure. The combination of lower core pressure and a higher-modulus structure al- lows the parts to be removed from the mold after a shorter time period.

With respect to chemical compatibility, it is essential to select a blowing agent and polymer

combination that will not produce undesirable side reactions affecting properties or appearance. Binder compatibility must also be considered when concentrates are used. The binder, usually a polymer material, must carry the active blowing agent but must not physically separate from the host material in an aesthetically or mechanically detrimental way. Fortunately, blowing-agent sup- pliers provide valuable information on appropri- ate combinations of blowing agents and foam materials.

Aesthetic Concerns

At this time, no well-known method produces a smlctural-foam surface whose appearance is equivalent to that achieved with a solid part. The lower-quality appearance is due to the swirl pat- tern that develops on the surface of a structural- foam part. The swirl results when the gas from the blowing agent breaks through the melt front and is trapped between the mold surface and the poly- mer. For most commercial applications, this sur- face is not acceptable.

To achieve a part finish suitable for exterior housings, extensive secondary operations are necessary. These operations could include use of

a filler coat, sanding, priming, and application of a top coat. With all of these finishing steps and the level of manual labor involved with each, part finishing costs can be high. Therefore, there has been an almost continuous effort to simplify part finishing and improve the initial part finish. As a result, improvements in the smoothness of the part surface, as molded, have been achieved through nucleating the molding compound and applying external molding techniques, such as gas counterpressure, as well as through a better understanding of the injection-molding process.

The use of nucleating agents reduces the depth but not the number of surface imperfections in a structural-foam surface. These smaller but more numerous flaws are easier to make aesthetically acceptable. In fact, both the filling and sanding operations can sometimes be eliminated.

A better understanding of the relationship be- tween molding parameters and surface quality has resulted in an increasing number of equip- ment modifications, such as hydraulic boosters, which are used to inject molten polymer at a faster rate to attain higher weight reduction in large parts.

Some structural-foam molders with either foam or standard injection-molding machines have fully adopted this technique. At least one plastics manufacturer plans to issue new material specifications that use this technique in the pro- duction stage. Molded-in color parts can be made that do not need painting. For applications that do require paint, only one coat needs to be applied, regardless of the type of surface, without sanding or priming or using filler paints.

Gas Counterpressure. In the gas counterpres- sure method, inert gas charged in molten polymer is contained until the entire shot is injected into

Regulator on tank

Solenoid valve-inlet Check valve < o - - - -

)

N2

Mold cavity

690-3450 kPa

supply

Fig. 8 Counterpressure foam schematic

[ ~ Solenoi~ Pressure valve-outlet relief valve

)

j O-ring seal

To ambient

Injection Molding / 305

l Ho, ii coo, Manifold

Mold section

° - - L ~

Electric cartridge heaters

Fig. 9 Cold runner mold

the cavity. This prevents simultaneous expansion and produces a molded part with a smooth skin.

This method requires a sealed mold in order to control the gas pressure during injection. Sealing the mold usually involves machining an O-ring groove around the parting line to accept sealing material, such as neoprene. Recommended

• Insulation

©

(3

(3

©

Water cooling

channels

Clamping plate

groove dimensions are 4.3 mm deep by 4.6 mm wide (0.170 by 0.182 in.) for a 4.8 mm (0.188 in.) O-ring. It may also be necessary to seal ejector pins, the spme bushing, and moving slides or cores to maintain a seal adequate to hold 340 to 3450 kPa (50 to 500 psi) counterpressure. Thus, the process should be considered as a tooling

modification rather than one that requires special- ized molding equipment.

The steps in the process are as follows: clamp- ing the mold, pressurizing the cavity, injecting the resin, depressurizing, and ejecting the part. This sequence has no significant effect on either cycle or cure time.

Pressurization is accomplished by connecting the mold to a supply of filtered, dry compressed air or, more frequently, nitrogen. Solenoid-con- trolled valves regulate both gas intake and vent- ing to depressurize the cavity. The amount of gas is controlled by pressure transducers or regulators (Fig. 8).

With a tightly sealed mold, gas counterpressure processing produces a part with a cellular core and a smooth surface. Although a class A surface is difficult to achieve, slight imperfections are easily masked with a mist coat of paint. Density reduction is normally less than that of a similar part molded with the low-pressure foam process. The exact amount of density will be determined by wall thickness, flow length, part configura- tion, and processing conditions.

The gas counterpressure method does offer a number of advantages. Because expansion of the polymer does not occur until the entire shot is injected, cell structure is evenly distributed throughout the part, providing a predictable, uni- form mechanical property profile and reducing the possibility of over-packed and overstressed sections that can result in poor dimensional con- trol and warping. Counterpressure produces a much thicker and nonporous skin, sealing in the gases that usually permeate the porous skin of conventional foam parts. This means that parts usually can be painted directly from the mold without blistering. In addition, volatiles that sometimes rise to the surface of parts are kept in solution, greatly reducing liquid buildup in vent areas and on the parting line.

Thermoset Injection Molding

In thermoset injection molding, a reacting fluid is forced into a generally warm mold in which the material further polymerizes or cross-links into a solid part. The injected material can be either a one-component material or it can be two reacting liquids that are mixed just before injection into the mold, as in polyurethane reaction injection molding.

Selecting Injection-Moldable Thermosets

Reasons for specifying injection-moldable thermoset resins vary with the particular applica- tion. In general, the use of injection-moldable diaUyl phthalate (DAP), urea- and melamine-for- maldehyde, epoxy, phenolic, and alkyd is con- centrated in smaller parts with nigher precision requirements. Like the thermoplastics, these m a -

306 ] Processing and Fabrication of Engineering Plastics

Polyol component

Im Isocyanate component

11

Hydraulic oil

1

Fig. 11 Compression injection mold

(a)

Control piston

11 Hydraulic oil

Polyol 1~ ~1 Isocyanate component component

/ Mixing chamber

(b)

Fin 11~ RIM impingement mixing heed. (a) Recirculation "~" " - - position just before and just offer injection. (b}

Shot position during mixing and injection

terials benefit from the virtually infinite variety of reinforcing media and compounding procedures that allow material suppliers to devise optimal formulations for specific applications. Often, the material specifications for an application are de- fined by its dielectric, thermal, and environ- mental requirements. Underwriters' Laboratories UL temperature, flammability, and continuous- use ratings also narrow the list of candidate ma- terials. In general, if requirements are stringent, such as heat-deftection temperatures approaching 315 °C (600 °F), dimensional stability in severe environments, or ultralow mold shrinkage, it is productive to consider one of the thermosets named above. The inherent heat resistance of thermosets makes them good candidates for any high-heat application. Their only obvious per- formance drawback as a group is brittleness, but even this characteristic can often be made accept- able by means of special compounding.

\N?

The use of polyester and polyurethane com- pounds tends to be concentrated in larger part sizes, typically in automotive and business-ma- chine applications. Whether produced of bulk molding compound in a reciprocating screw in- jection molding machine, or of polyurethane in a reaction injection molding machine, large ther- moset parts may require secondary painting. The inherent heat resistance of parts molded of these materials permits them to endure the elevated temperatures of paint-baking ovens. Although these materials can be compounded for molded- in colors, end-use surface finish requirements often necessitate painting.

Both polyurethane and silicone compounds of- fer outstanding properties as elastomers. Polyure- thanes are typically specified for automotive ap- plications that require energy absorption, such as bumpers, bumper mountings, and suspension units. Silicones are typically specified for auto- motive under-the-hood applications requiring heat and chemical resistance. The machine and molds are very similar to those used for thermo- plastic injection molding, with the following ex- ceptions discussed below.

First, shear on the material in the barrel is kept to a minimum to prevent overheating and thus precuring. The screw compression ratio is kept very low, with screw flights at a constant depth for most of the length of the screw. Where pistons are used for polyester, no shear is created and no precuring takes place.

Second, barrel temperature is carefully control- led to keep the material from prematurely react- ing. This may actually require removing heat from the barrel and nozzle area, because some shear heating will occur as the material is con- veyed down the barrel.

Finally, mold temperature is generally higher than with thermoplastics. Hot oil heaters are often used.

When standard injection molding of any of these thermoset materials using a conventional reciprocating-screw machine is required, eco- nomic drawbacks sometimes arise, especially in high-volume projects. The economic drawbacks result from the generally longer cycle times re- quired for thermosets, in comparison with ther- moplastics, except when wall thicknesses are less than 3.2 nun (0.125 in.), and the lack of use for

thermoset runners, sprues, and other scrap. These drawbacks have been more pronounced as high- performance thermoplastics have been developed with heat-resistance properties that can compete with thermosets. The generally higher material cost for these thermoplastics over thermosets is offset by fast cycle times and the regrind recovery potential. Several thermoset injection molding processes for reciprocating-screw molding ma- chines have been developed to improve overall economics by eliminating regrind loss and mini- mizing molding cycle times.

Warm-runner molding (sometimes referred to as cold-runner molding, runnerless thermoset molding, or cold-manifold molding) is so called because the molten thermoset resin is kept below its curing temperature in the runner system of the mold, allowing the runner and sprue to act as an extension of the nozzle of the molding machine. The objective is to maintain the runner and sprue system in a precured state during the next mold- ing cycle so that this material may become the next part rather than allowing it to cure and then throwing it away. As shown in Fig. 9, insulation board and water cooling channels are used in the injection side of the mold, much as the water jacket around the injection barrel is used in the injection-molding machine to keep temperatures below curing levels. The basic requirement for use of this technique is sufficient volume of parts to warrant operating the molding equipment con- tinuously. When a machine can be dedicated to this process, material savings by elimination of sprues and runners can exceed 15% of material costs. Cycle times are generally faster as well. Although this process has achieved wide accep- tance in Europe, it is not commonly used in the United States.

Reaction Injection Molding (RIM). In this process, two reacting liquids (such as a polyol and an isocyanate, the precursors for polyure- thanes) are delivered separately to the machine. The machine mixes the liquids just before injec- tion into a closed mold. The liquids react in the mold to form a cross-linked solid part. The two liquid streams, under high pressure, are inti- mately and instantaneously mixed by impinging on each other in an impingement mixing head, shown in Fig. 10, just before the combined stream flows into the mold. The mixing head plunger

Injection Molding / 307

eliminates all the mixed material from the head after injection, which precludes the need to flush out the head with solvent after each shot to keep the mixer from clogging, as is required with me- chanical-type mixers.

Just after mixing, the liquids have a low enough viscosity to allow large, complex molds to be filled using low injection pressure. Thus, a relatively low clamping force is required, which makes this process particularly attractive for large parts such as automotive bumper covers and body panels.

Fillers, such as short glass fibers or flakes, can be incorporated in one or both of the liquid streams to provide a part that is somewhat stiffer, and more dimensionally stable. This process is known as reinforced reaction injection molding (RR1M). Special mixing tanks are required to keep the filler in suspension in the liquid. Also, abrasion-resistant equipment is required.

Another modification of the RIM process con- sists of placing glass reinforcement in the mold before injecting the reacting liquid. The liquid wets the glass and reacts to form a glass-rein- forced solid part. This process, which is either called structural RIM, resin transfer molding, or resin injection molding, is discussed in the article "Resin Transfer Molding and Structural Reaction Injection Molding" in this Volume.

Compression injection molding (CIM) is an- other thermoset molding technique used to mini- mize material scrap and to enhance performance of the molded part. A reciprocating-screw ma-

chine injects the molten resin (Fig. 11) into a mold that is held slightly open. After the complete charge is injected, the mold is closed completely, which displaces the material into the cavities, almost completely eliminating any scrap from sprue or runner system. Generally, faster cycle times are obtained and improved performance in properties such as impact strength is gained through higher-density parts with minimized weld lines. The benefits of this process may be achieved by the use of either special hydraulic systems installed on the molding machine or two- stage clamping mechanisms built into the mold itself.

ACKNOWLEDGMENTS

The information in this article is largely taken from the following articles in Composites, Vol- ume 1, Engineered Materials Handbook, ASM International, 1987:

• EJ. Meyer, Injection Molding Compounds, p 164-167

• A.D. Murray, Injection Molding, p 555-558

and in Engineering Plastics, Volume 2, Engineered Materials Handbook, ASM International, 1988:

• M. Blaetterlein and S. Bornemeier, Thermoset Injection Molding, p 319-323

• G.W. Brewer, Properties of Thermoplastic Structural Foams, p 508-513.

REFERENCES

1. I.I. Rubin, Injection Molding--Theory and Practice, John W'dey & Sons, 1972

2. I.I. Rubin, Injection Molding, chapter 10, Intro- duction to Polymer Science and Technology: An SPE Textbook, H.S. Kaufman and J.J. Falcetta, Ed., Society of Plastics Engineers

3. J.H. DuBois, Plastics History U.S.A., Catmers Books, 1972

4. A.B. Glanvill and E.N. Denton, Injection MouM Design Fundamentals, Industrial Press, 1965

5. J.H. DuBois and W.I. Pribble, Ed., Plastics MoM Engineering Handbook, 3rd ed., Van Nostrand Reinhold, 1978

6. R.M. Criens and H.G. Mosle, "The Influence of Kafit-Lines on the Tensile Properties of Injection Molded Parts," Polym. Eng. Sci., Vol 23, July 1983, p 591

7. A.J. Keeney, 'Tree Flow Check Ring and Zero Metering Screw," paper presented at the 37th Annual Conference, Reinforced Plastics/Com- posites Institute, 1982

8. N. I.~nardi, C. Rosis, S. Driscoll, and S. Leitch, Chemical Blowing Agents, Their Effect on De- gassing Time and Paint Adhesion of Engineering Resins, Proc. 15th Annual Structural Foam Conf. and Parts Competition, Society of the Plastics Industry, 1987, p 56-60