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TABLE I9.|

Sectio

General Characteristics of Forming and Shaping Processes for Plastics andComposite Materials

Process CharacteristicsExtrusion

Injection molding

Structural foam molding

Blow molding

Rotational molding

Thermoforming

Compression molding

Transfer molding

Casting

Processing of composite materials

Continuous, uniformly solid or hollow, and complexcross sections; high production rates; relatively lowtooling costs; wide tolerancesComplex shapes of various sizes; thin walls; very highproduction rates; costly tooling; good dimensionalaccuracyLarge parts with high stiffness-to-weight ratio; lessexpensive tooling than in injection molding; lowproduction ratesHollow, thin-walled parts and bottles of various sizes;high production rates; relatively low tooling costsLarge, hollow items of relatively simple shape; relativelylow tooling costs; relatively low production ratesShallow or relatively deep cavities; low tooling costs;medium production ratesParts similar to impression-die forging; expensivetooling; medium production ratesMore complex parts than compression molding;higher production rates; high tooling costs; somescrap lossSimple or intricate shapes made with rigid or flexiblelow-cost molds; low production ratesLong cycle times; expensive operation; tooling costsdepend on process

require much less force and energy to process. Plastics in general can be molded, cast,formed, and machined into complex shapes in few operations, with relative ease, andat high production rates (Table 19.1). They also can be joined by various means(Section 32.6) and coated (generally for improved appearance) by various techniques(described in Chapter 34). Plastics are shaped into discrete products or as sheets,plates, rods, and tubing that may then be formed by secondary processes into a vari-ety of discrete products. The types and properties of polymers and the shape andcomplexity of components that can be produced are influenced greatly by theirmethod of manufacture and processing parameters.

Plastics usually are shipped to manufacturing plants as pellets, granules, orpowders and are melted (for thermoplastics) just before the shaping process. Liquidplastics that cure into solid form are used especially in the making of thermosetsand reinforced-plastic parts. With increasing awareness of our environment, rawmaterials also may consist of reground or chopped plastics obtained from recyclingcenters. As expected, however, product quality is not as high for such materials.

In this chapter, we follow the outline shown in Fig. 19.1, which describes thebasic processes and economics of forming and shaping plastics and reinforced plas-tics. We also describe processing techniques for metal-matrix and ceramic-matrixcomposites, which have become increasingly important in various applications withcritical requirements. We begin with melt-processing techniques (starting with ex-trusion) and continue on to molding processes-both categories involving the appli-cation of external pressure during processing.

n 19.1 Introduction 48

86 Chapter 19 Plastics and Composite Materials: Forming and Shaping

Extruded products y _ Y _ Y , _ , , _

Sheet Thermoformings,_, ,_t,s,t _ tg ~ ` ' ~` " ` ~ " -TPE Extrusion i i i i' ` i j - Blow molding Tube f ~ s

TP, TS, E injection ,s_ _ g gy i t f-c.tmQlcd"7Q, Blown Film ' " ' "` ' a' " '

Granu|eS, TP, Rotational i_ c o d f 1 .TF?'1'09TP, TS isirtiihrai foam 'TQld?'79. TS TP, iiio6&ibki

Section 19.2 Extrusion

Barrel Barrel Wire filter Meltliner heater/cooler screen thermocouple

Thermocouples

Throat Barrel Bleak fiff:;;m:m:: Ame'

Channel Feed sectior1'(l\/lelt section lVlelt-pumping section ScrewGear reducer

box (3)

Pitchlj Barrelif llr'r rrrrl Fright

D =ee;;f;22fEs;er_rL22~;;;;;;;;r.;;2;;;,; `,'~ .";>fs;e2;&r,r if Barrellb)

FIGURE I9.2 (a) Schematic illustration of a typical screw extruder. (b) Geometry of anextruder screw. Complex shapes can be extruded with relatively simple and inexpensive dies.

The lengths of these individual sections can be changed to accommodate themelting characteristics of different types of plastics. The molten plastic is forcedthrough a die in a process similar to that of extruding metals. A metal-wire filterscreen (Fig. 19.2a) usually is placed just before the die to filter out unmelted or con-gealed resin. This screen also helps build up back pressure in the barrel and is re-placed periodically. Between the screen and the die is a breaker plate, which hasseveral small holes in it and helps improve mixing of the polymer prior to its enter-ing the die. The extruded product is then cooled, generally by exposing it to blowingair or by passing it through a water-filled channel (trough).

Controlling the rate and uniformity of cooling is important to minimize prod-uct shrinkage and distortion. In addition to single-screw extruders, other designs in-clude twin (two parallel screws side by side) and multiple screws for polymers thatare difficult to extrude (see also reciprocating screw, Section 193).

A typical helical screw is shown in Fig. 19.2b and indicates the important pa-rameters that affect the mechanics of polymer extrusion. At any point in time, themolten plastic is in the shape of a helical ribbon with thickness H and width W, andis conveyed towards the extruder outlet by the rotating screw flights. The shape,pitch, and flight angle of the helical screw are important parameters, as they affectthe flow of the polymer through the extruder. The ratio of the barrel length, L, to itsdiameter, D, is also important. The L/D ratio in typical commercial extruders rangesfrom 5 to 30, and barrel diameters are generally in the range from 25 to 200 mm.

487

Melt distribution


Melt inlet

End latep Die b0ClY Melt-distribution

manifoldmanifold Die -|-hermocou, ple wellPreland bo" Die land Fixed lipEnd Seal Adjustable lip

(2)

0 Eii ljDie Extruded

Shape product Die shape Extruded Die shape Extrudedproduct product

(bl (C)

FIGURE l9.3 Common extrusion die geometries: (a) coat-hanger die for extruding sheet;(b) round die for producing rods; and (c) and (d) nonuniform recovery of the part after it exitsthe die. Source: (a) Encyclopedia of Polymer Science and Engineering, 2nd ed., Copyright 1985. Reprinted by permission of John Wiley Sc Sons, Inc.

Because it has a direct bearing on the quality of the product extruded and onthe design of the extruder and the die, the mechanics of this operation has been stud-ied extensively. Several relationships have been established between the dimensionsshown in Fig. 19.2b, the screw rotational speed, and the viscosity of the polymer todescribe what are known as extruder characteristics and die characteristics. Thesecharacteristics then determine such quantities as the pressure and flow rate at anylocation during the operation of the extruder. (Further details are beyond the scopeof this book and are available in various references cited in the bibliography at theend of this chapter.)

Because there is a continuous supply of raw material from the hopper, longproducts (such as solid rods, sections, channels, sheet, tubing, pipe, and architecturalcomponents) can be extruded continuously. Complex shapes with constant cross sec-tion can be extruded with relatively inexpensive tooling. Some common die profilesare shown in Fig. 19.3b. Note that some of the profiles are not intuitive, but this isattributable to the polymer usually undergoing much greater and uneven shaperecovery than is encountered in metal extrusion. Furthermore, since the polymer willswell at the exit of the die, the openings shown in Fig.19.3b are smaller than theextruded cross sections. After it has cooled, the extruded product may subsequentlybe drawn (sized) by a puller and coiled or cut into desired lengths.

The control of processing parameters such as extruder-screw rotational speed,barrel-wall temperatures, die design, and rate of cooling and drawing speeds areimportant in order to ensure product integrity and uniform dimensional accuracy.Defects observed in extruding plastics are similar to those observed in metal extru-sion (described in Section 15 .5 ). Die shape is important, as it can induce high stress-es in the product, causing it to develop surface fractures (as also occur in metals).Other surface defects are bambooing and slvarleskin effects-due to a combinationof friction at the die-polymer interfaces, elastic recovery, and nonuniform deforma-tion of the outer layers of the product with respect to its bull< during extrusion.

Sec

Extruders generally are rated by the diameter, D, of the barrel and the length-to~diameter (L/D) ratio of the barrel. Machinery costs can be on the order of$300,000, including the cost for the equipment for downstream cooling and wind-ing of the extruded product.

I9.2.l Miscellaneous Extrusion ProcessesThere are several variations of the basic extrusion process for producing a numberof different products.

Plastic Tubes and Pipes. These are produced in an extruder with a spider die, asshown in Fig. 19.421 (see also Fig. 15.8 for details). Woven fiber or wire reinforce-ments also may be fed through specially designed dies in this operation for the pro-duction of reinforced hoses that need to withstand higher pressures. The extrusionof tubes is also a necessary first step for related processes, such as extrusion blowmolding and blown film.

Breaker plate

Polymer meltExtruder barrel

Section A-AB Section f

Screen pack %Melt flow _, 5/ Q :'f Spider |995 (3)direction V

Blegs (3)

|\/|and,e| Air channel

Air in

(8)

Extruder 1

.EE "

.zjgi

fi Plastic melt:X two or more layers Parison

'~ ~ Mandrel V \;

Extruder 2

(D)

FIGURE l9.4 Extrusion of tubes. (a) Extrusion using a spider die (see also Fig. 15.8) andpressurized air. (b) Coextrusion for producing a bottle.

tion 19.2 Extrusion 489


Rigid Plastic Tubing. Extruded by a process in which the die is rotated, rigid plas-tic tubing causes the polymer to be sheared and biaxially oriented during extrusion.As a result, the tube has a higher crushing strength and a higher strength-to-weightratio than conventionally extruded tubes.

Coextrusion. Shown in Fig. 19.4b, coextrusion involves simultaneous extrusion oftwo or more polymers through a single die. The product cross section thus containsdifferent polymers-each with its own characteristics and function. Coextrusioncommonly is performed in shapes such as flat sheets, films, and tubes, and is usedespecially in food packaging where different layers of polymers have different func-tions, such as (a) providing inertness for food, (b) serving as barriers to fluids such aswater or oil, and (c) labeling of the product.

Plastic-coated Electrical Wire. Electrical wire, cable, and strips also are extrudedand coated with plastic by this process. The wire is fed into the die opening at a con-trolled rate with the extruded plastic in order to produce a uniform coating. Plastic-coated wire paper clips also are made by this process. To ensure proper insulation,extruded electrical wires are checked continuously for their resistance as they exitthe die; they also are marked automatically with a roller to identify the specific typeof wire.

Polymer Sheets and Films. These can be produced by using a specially designedflat-extrusion die, such as that shown in Fig. 19.3a. Also known as the coat-/rangerdie, it is designed to distribute the polymer melt evenly throughout the width. Thepolymer is extruded by forcing it through the die, after which the extruded sheet istaken up--first on water-cooled rolls and then by a pair of rubber-covered pull-offrolls. Generally, polymer slieet is considered to be thicker than 0.5 mm, and #lm isthinner than 0.5 mm.

Thin Polymer Films. Common plastic bags and other thin polymer film productsare made from blown film, which in turn is made from a thin-walled tube producedby an extruder (Fig. 19.5 ). In this process, a tube is extruded continuously verticallyupward and then expanded into a balloon shape by blowing air through the centerof the extrusion die until the desired film thickness is reached. Because of the molec-ular orientation of thermoplastics (Section 7.3), a frost line develops on the balloonand its transparency is reduced.

The balloon usually is cooled by air from a cooling ring around it, which canalso act as a barrier to further expansion of the balloon, thus controlling its dimen-sions. The cooled bubble is then slit lengthwise, becoming wrapping 191141, or it ispinched and cut off, becoming a plastic bag. The width of the film produced afterslitting can be on the order of 6 m or more.

The ratio of the blown diameter to the extruded tube diameter is known as theblow ratio, which is about 3:1 in Fig. 19.5. Note that, as described in Section 2.2.7,the polymer has to have a high strain-rate sensitivity exponent, rn, to successfully beblown by this process without tearing.

Plastic Films. Plastic films, especially polytetrafluoroethylene (PTFE, trade name:Teflon), can be produced by shaving the circumference of a solid, round plastic bil-let with specially designed knives in a manner similar to producing veneer from alarge piece of round wood. The process is called skiving (see also Section 24.4).

Pellets. Used as raw material for other plastic-processing methods described inthis chapter, pellets also are made by extrusion. A small-diameter, solid rod isextruded continuously and then chopped into short lengths (pellets). With some

Pinch rolls\@@l .U

WinC|'UD 0 0 Guide rollsO Q

I 0

Blowntube

Mandrel `Extruder

"

DieII' I

iAir

(H) (0)

Section 19.2 Extrusion 49|

FIGURE l9.5 (a) Schematic illustration of the production of thin film and plastic bags fromtube-first produced by an extruder and then blown by air. (b) A blown-film operation. Thisprocess is well developed, producing inexpensive and very large quantities of plastic film andshopping bags. Source: (b) Courtesy of Wind Moeller 86 Hoelscher Corp.

modifications, extruders also can be used as simple melters for other shapingprocesses, such as injection molding and blow molding.

EXAMPLE |9.l Blown Film

Assume that a typical plastic shopping bag made byblown film has a lateral dimension (width) of 400 mm.(a) What should be the extrusion-die diameter?(b) These bags are relatively strong in use. How is thisstrength achieved?

Solution

a. The perimeter of the flat bag is (2)(400) =800 mm. Since the original cross section of thefilm is round, the blown diameter should be11D = 800, thus D = 255 mm. Recall that inthis process a tube is expanded from 1.5 to

2.5 times the extrusion-die diameter. Taking themaximum value of 2.5, we calculate the diediameter as 255/2.5 = 100 mm.Note in Fig. 19.5a that, after extrusion, the bal-loon is being pulled upward by the pinch rolls.Thus, in addition to diametral stretching andthe attendant molecular orientation, the film isstretched and oriented in the longitudinal direc-tion. The resulting biaxial orientation of thepolymer molecules significantly improves thestrength and toughness of the plastic bag.

l9.2.2 Production of Polymer Reinforcing Fibers

Polymer fibers have numerous important applications. In addition to their use asreinforcement in composite materials, these fibers are used in a wide variety ofconsumer and industrial products, including clothing, carpeting, fabrics, rope, andpackaging.


V.; powmer Most synthetic fibers used in reinforced plastics are~ _~ Chi 5 ol mers that are extruded throu h the tin holes of a device

p "t P iy bl if 11 Zi f ,M cal ed a spinneret (resem ing a s ower ea ) to orm contin-`~ Feed M uous filaments of semisolid polymer. The extruder forces the\ hopper spinneret polymer through the spinneret, which may have from onei to several hundred holes. If the polymers are thermoplas-

ies, iii tics, they first are melted in the extruder, as described in&Hh?;'*~~* s 19 2 Th 1 1 b f d,`,,_`*m_m,;g ~ ection _ _ ermosetting po ymers a so can e orme Cold air into fibers by first dissolving or chemically treating them so\ _ yy" that they can be extruded. These operations are performed at

Melter/extruder high production rates and with very high reliability.|\/lelt As the filaments emerge from the holes in the spinneret,

fy, SP'V"'"VQ M the liquid polymer is first converted to a rubbery state andV//wwf, W//f/Q _

i ~i then solidified. This process of extrusion and solidification of Bobbin continuous filaments is called spinning. The term spinning also is used for the production of natural textiles (such as

~i-~~~ cotton or wool), where short pieces of fiber are twisted intoSt t h_ I yarn. There are four methods of spinning fibers: melt, wet,

re C mg dry, and gel spinning.5 Twisting and Winding I. In melt spinning (shown in Fig. 19.6), the polymer is

:V melted for extrusion through the spinneret and then

FIGURE l9.6 The melt-spinning process for producingpolymer fibers. The fibers are used in a variety of appli-cations, including fabrics and as reinforcements forcomposite materials. In the stretching box the right rollrotates faster than the left roll.

2.

3.

4.

solidified directly by cooling. A typical spinneret forthis operation has about 50 holes around 0.25 mm indiameter and is about 5 mm thick. The fibers thatemerge from the spinneret are cooled by forced-air con-vection and are simultaneously pulled, so that theirfinal diameter becomes much smaller than the spin-neret opening. Polymers (such as nylon, olefin, poly-ester, and PVC) are produced in this matter. Because ofthe important applications of nylon and polyesterfibers, melt spinning is the most important fiber-manu-facturing process.

Melt-spun fibers also can be extruded from the spinneret in various othercross sections, such as trilobal (a triangle with curved sides), pentagonal,octagonai, and hollow shapes. Hollow fibers trap air and thus provide addi-tional thermal insulation, while other cross sections have specific applications.

Wet spinning is the oldest process for fiber production and is used for polymersthat have been dissolved in a solvent. The spinnerets are submerged in a chemi-cal bath. As the filaments emerge, they precipitate in the bath, producing a fiberthat is then wound onto a bobbin. The term wet spinning refers to the use ofa precipitating liquid bath, resulting in wet fibers that require drying before theycan be used. Acrylic, rayon, and aramid fibers can be produced by this process.

Dry spinning is used for thermosets carried by a solvent. However, instead ofprecipitating the polymer by dilution as in wet spinning, solidification isachieved by evaporating the solvent in a stream of air or inert gas. The fila-ments do not come in contact with a precipitating liquid, thus eliminating theneed for drying. Dry spinning may be used for the production of acetate, triac-etate, polyether-based elastane, and acrylic fibers.

Gel spinning is a special process used to obtain high strength or special fiberproperties. The polymer is not melted completely or dissolved in liquid, butthe molecules are bound together at various points in liquid-crystal form. Thisoperation produces strong interchained forces in the resulting filaments

Section 19.

that can significantly increase the tensile strength of the fibers. In addition, theliquid crystals are aligned along the fiber axis by the strain encountered duringextrusion. The filaments emerge from the spinneret with an unusually high de-gree of orientation relative to each other-further enhancing their strength.This process also is called dry-ir/et spinning, because the filaments first passthrough air and then are cooled further in a liquid bath. Some high-strengthpolyethylene and aramid fibers are produced by gel spinning.

A necessary step in the production of most fibers is the application of signifi-cant stretching to induce orientation of the polymer molecules in the fiber direction.This orientation is the main reason for the high strength of the fibers, comparedwith the polymer in bulk form. The stretching can be done While the polymer is stillpliable-just after extrusion from the spinneret-or it can be performed as a cold-drawing operation. The strain induced can be as high as 800%.

Graphite fibers are produced from different polymer fibers by pyrolysis. In thisoperation, controlled heat in the range from 1500 to 3000C is applied to the poly-mer fiber (typically polyacrylonitrile, PAN) to drive off all elements except the car-bon. The fiber is under tension in order to develop a high degree of orientation in theresulting fiber structure. (See also Section 9.2.1 on the properties of graphite fibersand other details.)

l9.3 Injection Molding

Injection molding is similar to hot-chamber die casting (Fig. 19.7, see alsoSection 11.3.5 ). The pellets or granules are fed into the heated cylinder, and the melt

Powder Hopperpenets Heatingzones Nozzle Mold

\ /1Vent

Piston(ram) \P

ressCooiing ping (C|amp) a"e'>g,;1;;;,;Q Torpedo /force

(spreader) Sprue

Moldedpart Vent

(H)

Rotating and reciprocatingscrew

FIGURE I9.1 Schematic illustration of injection molding with (a) a plunger and (b) a recipro-cating rotating screw.

3 Injection Molding 493

Chapter 19 Plastics and Composite Materials: Forming and Shaping

1 1 Em '. iiss iffffssa ssa~s 'i' 4 a a };;V W_W_l,E ,ii,, ,,,,,, _,,;; ,i,, _ i

Rotating and if if ifreciprocating screw

1. Build up polymer in front of sprue bushing; 2. When the mold is ready, the screw is pushedpressure pushes the screw backwards. forward by a hydraulic cylinder, filling the sprueWhen sufficient polymer has built up, bushing, sprue, and mold cavity with polymer.rotation stops. The screw begins rotating again to build up

more polymer.

1 _'__g,._ _ .g. ._ ,_za VV;_V7VVW` ___~_ ____ff.....ff f f M __f.t

Section 19.3 Injection Molding

gi

(H) CT (D)

FIGURE l9.9 Typical products made by injection molding, including examples of insertmolding. Source: (a) Courtesy of Plainfield Molding, Inc. (b) Courtesy of Rayco Mold andMfg. LLC.

Gate Cavity

Sprue

MainQ runner\ _ 1 Part Gate

" lii \ .rf z _ ;;;. if

,~, LM

Cold slug well lc 5223?\ Cavity Main Sprue Guide Branch Guide pin

runner pin runner

(H) (D)

FIGURE l9.I0 Illustration of mold features for injection molding. (a) Two-plate mold withimportant features identified. (b) Schematic illustration of the features in a mold. Source:Courtesy of Tooling Molds West, Inc.

dies), cores, cavities, cooling channels, inserts, knockout pins, and ejectors. Thereare three basic types of molds:

I. Cold-runner, two-plate mold: This design is the simplest and most common, asshown in Fig. 19.11a.

2. Cold-runner, three-plate mold (Fig. 19.11b): The runner system is separatedfrom the part When the mold is opened.


Spruebushing._+

StripperPlate Gate Plate Plate Plate Plate

(patS Sprue

prue Ejector bushing Ejector Pins 3 pins'r= Paris

Runner

(3) (U)

Hot plate;Runner stays molten Plate

Plate

bushing Eiff_+ T pinsParts

(C)

FIGURE l9.| I Types of molds used in injection molding: (a) two-plate mold; (b) three-platemold; and (c) hot-runner mold.

3. Hot-runner mold (Fig. 19.11c), also called runnerless mold: The molten plasticis kept hot in a heated runner plate.

In cold-runner molds, the solidified plastic remaining in the channels connect-ing the mold cavity to the end of the barrel must be removed, usually by trimming.Later, this scrap can be chopped and recycled. In hot-runner molds (which are moreexpensive), there are no gates, runners, or sprues attached to the molded part. Cycletimes are shorter, because only the molded part must be cooled and ejected.

Multicomponent injection molding (also called coinjection or sandwichmolding) allows the forming of parts with a combination of various colors andshapes. An example is the molding of automobile rear-light covers made of differ-ent materials and colors, such as red, amber, and white. Also, for some parts,printed film can be placed in the mold cavity, so they need not be decorated orlabeled after molding.

Insert molding involves metallic components (such as screws, pins, and strips)that are placed in the mold cavity prior to injection and then become an integral partof the molded product (Fig. 19.9). The most common examples of such combina-tions are hand tools, Where the handle is insert molded onto a metal component.Other examples include electrical and automotive components and faucet parts.

Overmolding. This is a process for making products (such as hinge joints and ball-and-socket joints) in one operation and without the need for postmolding assembly.Two different plastics usually are used to ensure that no bonds will form betweenthe molded halves of the joint, as otherwise motion would be impeded.

Section 19 3 Injection Moldlng 7

In ice-cold molding, the same type of plastic is used to form both componentsof the joint. The operation is carried out in a standard injection-molding machineand in one cycle. A tvvo-cavity mold is used with cooling inserts positioned in thearea of contact between the first and the second molded component of the joint. Inthis way, no bonds develop between the two pieces, and thus the two componentshave free movements, as in a hinge or a sliding mechanism.

Process Capabilities. Injection molding is a high-rate production process and per-mits good dimensional control. Although most parts generally weigh from 100 to600 g, they can be much heavier, such as automotive-body panels and exterior com-ponents. Typical cycle times range from 5 to 60 seconds, although they can be sever-al minutes for thermosetting materials.

Injection molding is a versatile process capable of producing complex shapeswith good dimensional accuracy. As in other forming processes, mold design and thecontrol of material flow in the die cavities are important factors in the quality of theproduct and thus in avoiding defects. Because of the basic similarities to metal cast-ing regarding material flow and heat transfer, defects observed in injection moldingare somewhat of the same nature, as outlined next.

For example, in Fig. 10.13g, the molten metal flows in from two opposite run-ners and then meets in the middle of the mold cavity. Thus, a cold shut in cast-ing is equivalent to weld lines in injection molding.

If the runner cross sections are too small, the polymer may solidify prematurely,thus preventing full filling of the mold cavity. Solidification of the outer layers inthick sections can cause porosity or 1/oids due to shrinkage, as in the metal partsshown in Fig. 12.2.

If for some reason the dies do not close completely or due to die wear, a flashwill form in a manner similar to flash formation in impression-die forging (seeFigs. 14.5 and 19.17c).

A defect known as sink marks (or pull-in) similar to that shown in Fig. 19.31calso is observed in injection-molded parts.

Methods of avoiding defects consist of the proper control of temperatures, pres-sures, and mold design modifications using simulation software.

Much progress has been made in the analysis and design of molds and materialflow in injection molding. Modeling techniques and simulation software have beendeveloped for studying optimum gating systems, mold filling, mold cooling, and partdistortion. Software programs now are available to expedite the design process formolding parts with good dimensions and characteristics. The programs take intoaccount such factors as injection pressure, temperature, heat transfer, and the condi-tion of the resin.

Machines. Injection-molding machines are usually horizontal (Fig. 19.12).Vertical machines are used for making small, close-tolerance parts and for insertmolding. The clamping force on the dies generally is supplied by hydraulic means,although electrical means (which weigh less and are quieter than hydraulic machines)also are used. Modern machines are equipped with microprocessors in a controlpanel and monitor all aspects of the operation.

Injection-molding machines are rated according to the capacity of the moldand the clamping force. In most machines, this force ranges from 0.9 to 2.2 MN.The largest machine in operation has a capacity of 45 MN, and it can produce partsweighing 25 kg. The cost of a 1-MN machine ranges from about $60,000 to about$90,000 and of a 2.7-MN machine from about $85,000 to about $140,000. Die


Mold Ejector Moving StationaryClamp pins die die Barrel Hopper Motor

I I I I I

FIGURE I9.I2 A 2.2-MN injection-molding machine. The tonnage is the force applied tokeep the dies closed during the injection of molten plastic into the mold cavities and hold itthere until the parts are cool and stiff enough to be removed from the die.Source: Courtesy of Cincinnati Milacron, Plastics Machinery Division.

costs typically range from $20,000 to $200,000. Consequently, high-volume pro-duction is essential to justify such high expenditure.

The molds generally are made of tool steels, beryllium-copper, or aluminum.They may have multiple cavities, so that more than one part can be made in onecycle (see also Fig. 11.21). Mold costs can be on the order of $100,000 for largeones. Mold life may be on the order of 2 million cycles for steel molds, but it can beabout only 10,000 cycles for aluminum molds.

EXAMPLE l9.2 Force Required in Injection Molding

A 2.2-MNn injection-molding machine is to be usedto make spur gears 110 mm in diameter and 2.5 mmthick. The gears have a fine-tooth profile. How manygears can be injection molded in one set of molds?Does the thickness of the gears influence your an-swer?

Solution Because of the fine detail involved (finegear teeth), lets assume that the pressures requiredin the mold cavity will be on the order of 100 MPa.

7r(110)2/4 = 950Omm2. If we assume that the part-ing plane of the two halves of the mold is inthe middle of the gear, the force required is(9S00)(100) = 0.95 MN.

Since the capacity of the machine is 2.2 MN, wehave 22 MN of clamping force available. Hence, themold can accommodate two cavities and produce twogears per cycle. Because it does not influence the cross-sectional area of the gear, the thickness of the geardoes not directly influence the pressures involved and

The cross-sectional (projected) area of the gear is thus does not change the answer. pql9.3.l Reaction-injection Molding

In the reaction-injection molding (RIM) process, a monomer and two or more reac-tive fluids are forced at high speed into a mixing chamber at a pressure of 10 to20 MPa and then into the mold cavity (Fig. 19.13). Chemical reactions take placerapidly in the mold, and the polymer solidifies. Typical polymers are polyurethane,

Section 19 4 Blow Molding

Heat Wexchanger S tirrer

Displacement Heat cy||nders\.

exchanger ff l\/lOl`lOlT1l'2 - *fe fa Recirculation

Stirrer loopMonomer 1 MixingPump Gad

Reoi rculation iloop

FIGURE l9.l3 Schematic illustration of the reaction-injection molding process. Typicalparts made are automotive-body panels, water skis, and thermal insulation for refrigeratorsand freezers.

nylon, and epoxy. Cycle times may range up to about 10 minutes, depending on thematerials, part size, and shape.

Major applications of this process include automotive parts (such as bumpersand fenders, steering wheels, and instrument panels), thermal insulation for refriger-ators and freezers, water skis, and stiffeners for structural components. Parts mademay range up to about 50 kg. Reinforcing fibers (such as glass or graphite) also maybe used to improve the products strength and stiffness. Depending on the number ofparts to be made and the part quality required, molds can be made of common ma-terials, such as steel or aluminum.

|9.4 Blow Molding

Blow molding is a modified extrusion- and injection-molding process. In extrusionblow molding, a tube or preform (usually oriented so that it is vertical) is first ex-truded. lt is then clamped into a mold with a cavity much larger than the tube diam-eter and blown outward to fill the mold cavity (Fig. l9.14a). Depending on thematerial, the blow ratio may be as high as 7:1. Blowing usually is done with a hot-air blast at a pressure ranging from 350 to 700 kPa. Drums with a volume as largeas 2000 liters can be made by this process. Typical die materials are steel, aluminum,and beryllium copper.

In some operations, the extrusion is continuous and the molds move with thetubing. The molds close around the tubing, sealing off one end, breaking the longtube into individual sections, and moving away as air is injected into the tubularpiece. The part is then cooled and ejected from the mold. Corrugated-plastic pipeand tubing are made by continuous blow molding in which the pipe or tubing isextruded horizontally and blown into moving molds.

In injection blow molding, a short tubular piece (parison) is injection molded(Fig. 19.14b) into cool dies. (Parisons may be made and stored for later use.) The

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Section 19.5 Rotational Molding 50|

dies then open, and the parison is transferred to a blow-molding die by an indexingmechanism (Fig. 19.14c). Hot air is injected into the parison, expanding it to thewalls of the mold cavity. Typical products made are plastic beverage bottles (typicallymade of polyethylene or polyetheretherketone, PEEK) and small, hollow containers.A related process is stretch blow molding, in which the parison is expanded and elon-gated simultaneously, subjecting the polymer to biaxial stretching and thus enhancingits properties.

Multilayer blow molding involves the use of coextruded tubes or parisons andthus permits the production of a multilayer structure (see Fig. 19.4b). A typical ex~ample of such a product is plastic packaging for food and beverages, having suchcharacteristics as odor and permeation barrier, taste and aroma protection, scuffresistance, the capability of being printed, and the ability to be filled with hot fluids.Other applications of this process are for containers in the cosmetics and the phar-maceutical industries.

l9.5 Rotational Molding

Most thermoplastics and some thermosets can be formed into large, hollow partsby rotational molding. In this process, a thin-walled metal mold is made in twopieces (split-female mold) and is designed to be rotated about two perpendicularaxes (Fig. 19.15 ). For each part cycle, a premeasured quantity of powdered plasticmaterial is placed inside the warm mold. (The powder is obtained from a polymer-ization process that precipitates a powder from a liquid.) Then the mold is heated(usually in a large oven) and is rotated continuously about the two principal axes.

This action tumbles the powder against the mold, where the heat fuses thepowder without melting it. For thermosetting parts, a chemical agent is added to thepowder; cross-linking occurs after the part is formed in the mold. The machines arehighly automated, with parts moved by an indexing mechanism similar to thatshown in Fig. 19.14c.

A large variety of parts are made byrotational molding, such as storage tanks ofvarious sizes, trash cans, boat hulls, buckets,housings, large hollow toys, carrying cases,and footballs. Various metallic or plastic in-serts or components also may be molded inte-grally into the parts made by this process.

In addition to powders, liquid polymers(plastisols) can be used in rotational molding-PVC plastisols being the most common mate-rial. In this operation (called slush molding orslush casting), the mold is heated and rotatedsimultaneously. Due to the tumbling action,the polymer is forced against the inside walls ofthe mold, where it melts and coats the moldwalls. The part is cooled while it is still rotatingand removed by opening the mold. Parts madeare typically thin-walled products, such asboots and toys.

Process Capabilities. Rotational moldingcan produce parts with complex, hollowshapes with wall thicknesses as small as

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FIGURE l9.I5 The rotational molding (rotomolding or rotocasting)process. Trash cans, buckets, and plastic footballs can be made by thisprocess.


0.4 mm. Parts as large as 1.8 m >< 1.8 m >< 3.6 m with a volume as large as80,000 liters have been produced. The outer surface finish of the part is a replica ofthe surface finish of the inside mold walls. Cycle times are longer than in othermolding processes. Quality-control considerations usually involve accurate weightof the powder, proper rotational speed of the mold, and temperature-time relation-ships during the oven cycle.

l9.6 Thermoforming

Thermoforming is a process for forming thermoplastic sheets or films over a moldthrough the application of heat and pressure (Fig. 19.16). In this process, a sheet is (a)clamped and heated to the sag point (above the glass-transition temperature, Tg, of thepolymer; Table 7.2), usually by radiant heating, and (b) forced against the mold surfacesthrough the application of a vacuum or air pressure. The sheets used in thermoformingare available as a coiled strip or as lengths and widths of various sizes. They also areavailable filled with various materials for making parts with specific applications.

The mold is generally at room temperature; thus, the shape produced becomesset upon contact with the mold. Because of the low strength of the materials formed,the pressure difference caused by a vacuum usually is sufficient for forming.However, thicker and more complex parts require air pressure, which may rangefrom about 100 to 2000 kPa, depending on the type of material and thickness ofthe sheet. Mechanical means, such as the use of plugs, also may be employed tohelp form the parts. Variations of the basic thermoforming process are shown inFig. 19.16.

Process Capabilities. Typical parts made by thermoforming are packaging, traysfor cookies and candy, advertising signs, refrigerator liners, appliance housings, andpanels for shower stalls. Parts with openings or holes cannot be formed by thisprocess because the pressure difference cannot be maintained during forming.Because thermoforming is a combination of drawing and stretching operations(much like in some sheet-metal forming), the material must exhibit high, uniformelongation; otherwise, it will neck and tear. Thermoplastics have high capacities foruniform elongation by virtue of their high strain-rate sensitivity exponent, m, asdescribed in Section 2.2.7.

Molds for thermoforming usually are made of aluminum because highstrength is not required; hence, tooling is relatively inexpensive. Thermoforming

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FIGURE |9.l6 Various thermoforming processes for a thermoplastic sheet. These processescommonly are used in making advertising signs, cookie and candy trays, panels for showerstalls, and packaging.

Section 19.7

molds have small through-holes in order to aid vacuum forming. These holes typi-cally are less than 0.5 mm in diameter; otherwise, they would leave marks on theparts formed. Defects encountered in thermoforming include (a) tearing of the sheetduring forming, (bl nonuniform wall thickness, (c) improperly filled molds, (d) poorpart definition, and (e) lack of surface details.

l9.7 Compression Molding

In compression molding, a preshaped charge of material, premeasured volume of pow-der, or viscous mixture of liquid-resin and filler material is placed directly into a heatedmold cavity that typically is around 200C but can be much higher. Forming is doneunder pressure from a plug or from the upper half of the die (Fig. 19.17); thus, theprocess is somewhat similar to closed-die forging of metals.

Pressures range from about 10 to 150 MPa. As seen in Fig. 19.17, there is aflash formed, which subsequently is removed by trimming or by some other means.Typical parts made are dishes, handles, container caps, fittings, electrical and elec-tronic components, washing-machine agitators, and housings. Fiber-reinforced partswith chopped fibers also are formed exclusively by this process.

Compression molding is used mainly with thermosetting plastics, with theoriginal material being in a partially polymerized state. However, thermoplasticsand elastomers are also processed by compression molding. Curing times rangefrom about 0.5 to 5 minutes, depending on the material and on part thickness andshape. The thicker the material, the longer it will take to cure.

Compression Molding 503

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Process Capabilities. Three types of compression molds are available:

Flash type: for shallow or flat parts Positive type: for high-density parts Semipositive type: for quality production.

Undercuts in parts are not recommended; however, dies can be designed toopen sideways (Fig. 19.17d) to allow removal of the molded part. In general, thecomplexity of parts produced is less than that from injection molding, but the dimen-sional control is better. Surface areas of compression-molded parts may range up toabout 2.5 ml. Because of their relative simplicity, dies for compression molding gen-erally are less costly than those used in injection molding. They typically are made oftool steels and may be chrome plated or polished for an improved surface finish ofthe molded product.

l9.8 Transfer Molding

Transfer molding represents a further development of compression molding. The un-cured thermosetting resin is placed in a heated transfer pot or chamber (Fig. 1918),and after the material is heated, it is injected into heated closed molds. Depending onthe type of machine used, a ram, plunger, or rotating-screw feeder forces the materialto flow through the narrow channels into the mold cavity at pressures up to 300MPa. This viscous flow generates considerable heat, which raises the temperature ofthe material and homogenizes it. Curing takes place by cross-linking. Because theresin is in a molten state as it enters the molds, the complexity of the parts and the di-mensional control approach those of injection molding.

Process Capabilities. Typical parts made by transfer molding are electrical con-nectors and electronic components, rubber and silicone parts, and the encapsulationof microelectronic devices. The process is suitable particularly for intricate shapeswith varying wall thicknesses. The molds tend to be more expensive than those for

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Section 19 9 Castlng 0

compression molding, and some excess material is left in the channels of the moldduring filling, which is later removed.

I9.9 Casting

Some thermoplastics (such as nylons and acrylics) and thermosetting plastics (epox-ies, phenolics, polyurethanes, and polyester) can be cast into a variety of shapesusing either rigid or flexible molds (Fig. 1919). Compared with other methods ofprocessing plastics, casting is a slovv, but simple and inexpensive, process. However,the polymer must have sufficiently low viscosity in order to flow easily into themold. Typical parts cast are gears (especially nylon), bearings, wheels, thick sheets,lenses, and components requiring resistance to abrasive vvear.

In the basic conventional casting of thermoplastics, a mixture of monomer,catalyst, and various additives (activators) is heated to above its melting point, Tm,and poured into the mold. The part is formed after polymerization takes place atambient pressure. Degassing may be necessary for product integrity. lntricate shapescan be produced using flexible inolcls, Which are then peeled off (in a manner simi-lar to using rubber gloves) and reused. As with metals, thermoplastics may be castcontinuously, With the polymer carried over continuous stainless-steel belts andpolymerized by external heat.

Centrifugal Casting. This process, similar to centrifugal metal casting (Sec-tion 1l.3.6), is used with thermoplastics, thermosets, and reinforced plastics withshort fibers.

Potting and Encapsulation. As a variation of casting that is important, particularlyto the electrical and electronics industry, potting and encapsulation involve castingthe plastic material (typically a liquid resin, such as expoxy) around an electricalcomponent (such as a transformer) to embed it in the plastic. Potting (Fig. 19.19b) iscarried out in a housing or case, which becomes an integral part of the componentand fixes it in position. In encapsulation (Fig. 19.19c), the component is coated witha layer of the plastic, surrounding it completely and then solidifying.

In both of these processes, the plastic material can serve as a dielectric (non-conductor); consequently, it must be free of moisture and porosity, which wouldrequire processing in a vacuum. Mold materials may be metal, glass, or variouspolymers. Small structural members (such as hooks, studs, and similar parts) may beencapsulated partially by dipping them in a hot thermoplastic using polymers of var-ious colors.

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l9.l0 Foam Molding

Products such as styrofoam cups, food containers, insulating blocks, and shapedpackaging materials (such as for shipping appliances, computers, and electronics)are made by foam molding, using expandable polystyrene beads as the rawmaterial. As is readily seen upon close inspection, these products have a cellularstructure. The structure may have open and interconnected porosity (for polymerswith low viscosity) or have closed cells (for polymers with high viscosity).

There are several techniques that can be used in foam molding. In the basicoperation, polystyrene beads obtained by polymerization of styrene monomer areplaced in a mold with a blowing agent-typically pentane (a volatile hydrocarbon)or inert gas (nitrogen)-and exposed to heat, usually by steam. As a result, thebeads expand to as much as 50 times their original size and take the shape of themold cavity. The amount of expansion can be controlled by varying the tempera-ture and time. Various other particles, including hollow glass beads or plasticspheres, may be added to impart specific structural characteristics to the foamproduced.

Poiystyrene beads are available in three sizes: (a) small, for cups with a fin-ished part density of about 50 kg/m3, (b) medium, for molded shapes; and (c)large, for molding insulating blocks with a finished part density of about 15 to30 kg/m3 (which can then be cut to size). The bead size selected also depends on theminimum wall thickness of the product: The smaller the size, the thinner the part.The beads can be colored prior to expansion; thus, the part becomes integrally col-ored. Both thermoplastics and thermosets can be used for foam molding, but ther-mosets are in a liquid-processing form and hence are in a condition similar to that ofpolymers in reaction-injection molding.

A common method of foam molding is to use preexpanded polystyrene beads,in which the beads are expanded partially by steam (hot air, hot water, or an ovenalso can be used) in an open-top chamber. The beads then are placed in a storage binand allowed to stabilize for a period of 3 to 12 hours. They then can be molded intodesired shapes in the manner described previously.

Structural Foam Molding. This is a molding process used to make plastic productswith a solid outer slain and a cellular core structure. Typical products made are fur-niture components, computer and business-machine housings, and moldings (re-placing more expensive wood moldings). In this process, thermoplastics are mixedwith a blowing agent (usually an inert gas such as nitrogen) and injection moldedinto cold molds of desired shapes. The rapid cooling against the cold-mold surfacesproduces a skin that is rigid [which can be as much as 2 mm thick] and a core of thepart that is cellular in structure. The overall part density can be as low as 40% of thedensity of the solid plastic. Thus, with a rigid skin and a less dense bulk, moldedparts have a high stiffness-to-weight ratio (see also Fig. 3.2).

Polyurethane Foam Processing. Products such as furniture cushions and insulat-ing blocks are made by this process. Basically, the operation starts with the mixingof two or more components; chemical reactions then take place after the mixture is(a) poured into molds of various shapes or (b) sprayed over surfaces with a spraygun to provide sound and thermal insulation. Various low-pressure and high-pressure machines are available, having computer controls for proper mixing. Themixture solidifies with a cellular structure, the characteristics of which depend onthe type and proportion of the components used.

Section 19.12

l9.l I Cold Forming and Solid-phase Forming

Processes that have been used in the cold working of metals (such as rolling, closed-die forging, coining, deep drawing, and rubber forming-all described in Part III)also can be used to form thermoplastics at room temperature (cold forming). Typicalmaterials formed are polypropylene, polycarbonate, ABS, and rigid PVC. Importantconsiderations regarding this process are that (a) the polymer must be sufficientlyductile at room temperature (thus, polystyrenes, acrylics, and thermosets cannot beformed) and (b) its deformation must be nonrecoverable (in order to minimizespringback and creep of the formed part).

The advantages of the cold forming of plastics over other methods of shapingare as follows:

Strength, toughness, and uniform elongation are increased. Plastics with high molecular weights can be used to make parts with superior

properties. Forming speeds are not affected by part thickness because (unlike other plastic-

processing methods) there is no heating or cooling involved. Cycle times gener-ally are shorter than in molding processes.

Solid-phase Forming. Also called solid-state forming, this process is carried out ata temperature 10 to 20C below the melting temperature of the plastic (for a crys-talline polymer). Thus, the forming operation takes place while the polymer is stillin a solid state. The main advantages of solid-phase forming over cold forming arethat forming forces and springback are lower. These processes are not used as wide-ly as hot-processing methods and generally are restricted to special applications.

l9.l2 Processing Elastomers

We have described the properties, characteristics, and applications of elastomersand rubbers in Section 7.9. Recall that, in terms of its processing characteristics, athermoplastic elastomer is a polymer. In terms of its function and performance, it isa rubber. The raw material to be processed into various shapes is basically a com-pound of rubber and various additives and fillers. The additives include carbonblack-an important element that enhances properties such as tensile and fatiguestrength, abrasion and tear resistance, ultraviolet protection, and resistance tochemicals.

These materials are then mixed to break them down and lower their viscosity;the mixture subsequently is vulcanized, using sulfur as the vulcanizing agent. Thiscompound is then ready for further processing (such as calendering, extrusion, andvarious molding processes), which may also include reinforcements in such forms asfibers and fabric. During processing, the part becomes cross-linked, imparting thedesirable properties that we all associate with rubber products ranging from rubberboots to pneumatic tires.

Elastomers can be shaped by a variety of processes that also are used for shap-ing thermoplastics. Thermoplastic elastomers commonly are shaped by extrusion orinjection molding-extrusion being the more economical and the faster process.They also can be formed by blow molding or thermoforming. Thermoplasticpolyurethane, for example, can be shaped by all conventional methods. It also can be

Processing Elastomers 50

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