Overview Polymer Processing

OverviewPolymer processingJ Vlachopoulos and D Strutt

After a brief introduction to the various types of polymers in use and the size and impact of the polymer processingindustry this overview focuses on the most important processes for thermoplastic materials namely extrusion andinjection moulding Single and twin screw extruders are used for melting and pumping of polymers and for dieextrusion for the production of lm sheet pipe tubing pro les and bres Injection moulding is the process used forthe production of numerous parts small and large by injecting a molten polymer into mould cavities Variousproblems associated with these processes are discussed Other processes described include calendering compressionmoulding rotational moulding powder injection moulding and thixomoulding The overview concludes with a briefdiscussion of current trends and future challenges faced by the polymer industry MST5866

Dr Vlachopoulos (vlachopjmcmasterca) is at the Centre for Advanced Polymer Processing and Design (CAPPA-D)Department of Chemical Engineering McMaster University Hamilton ON L8S 4L7 Canada and Dr Strutt is withPolydynamics Inc 1685 Main St W Suite 305 Hamilton ON L8S 1G5 Canada Manuscript received 8 November 2002accepted 10 January 2003 2003 IoM Communications Ltd Published by Maney for the Institute of Materials Minerals and Mining

Introduction

Mankind has used natural polymeric materials such as woodleather and wool since the beginning of history but syntheticpolymers became possible only after the development ofrubber technology in the 1800s The rst synthetic polymermaterial celluloid was invented by John Wesley Hyatt in1869 from cellulose nitrate and camphor A major break-through in synthetic polymers was the invention of Bakeliteby Leo Hendrik Baekeland in 1907 Hermann Staudingerrsquoswork in the 1920s clearly demonstrated the macromolecularnature of long chains of repeating units1 The wordlsquopolymerrsquo comes from Greek and it means lsquomany partsrsquoThe rapid growth of the polymer industry started shortlybefore the Second World War with the development ofacrylic polymers polystyrene nylon polyurethanes and thesubsequent introduction of polyethylene polyethylene tere-phthalate polypropylene and other polymers in the 1940sand 1950s While only about 1 million tons were producedin 1945 the production of plastics in volume surpassed thatof steel in 1981 and the gap has been continuously growingever since

Pure polymers are seldom processed on their own They arecompounded with other materials typically by mechanicalblending or melt state mixing to produce pellets powdersor akes to be used in subsequent processing operations2

Such compoundedproductsare referred to as lsquoplasticsrsquo whichmeans lsquopliablesrsquo in Greek The compounds may involve llers(to reduce cost) reinforcements other polymers colourants ame retardants stabilisers (to prevent deterioration fromlight heat or other environmental factors) and various pro-cessing aids

Synthetic polymers can be classi ed in two categoriesThermoplastics (by far the largest volume) can be meltedby heating solidi ed by cooling and remelted repeatedlyMajor types are polyethylene(PE) polypropylene(PP) poly-styrene (PS) polyvinyl chloride (PVC) polycarbonate (PC)polymethyl methacrylate (PMMA) polyethylene tereph-thalate (PET) and polyamide (PA nylon) Thermosets arehardened by the application of heat and pressure owingto crosslinking ie the creation of permanent three-dimensional networks They cannot be softened by heatingfor reprocessing Bakelite epoxies and most polyurethanesare thermosets

The present overview is exclusively devoted to the pro-cessing of thermoplastics Commercial thermoplastics areclassi ed according to their performance as lsquocommodityrsquo(low performance such as PE PP PS and PVC) lsquoEngineer-ingrsquo (such as PC nylon and PET) or lsquoadvancedrsquo (highestperformance such as liquid crystal polymers (LCPs)polyphenylene sulphide (PPS) and polyetheretherketone(PEEK)) The anticipated explosive growth in engineeringand advanced polymers did not materialise The use ofplastics has been continuously growing throughout thepast three decades but mainly in the commodity categoryCurrently commodity polymers amount to ~88 of thevolume produced3 engineering plastics ~12 and advancedless than 1 Although the prices of advanced polymers perkilogram are much higher than those for commodity poly-mers their global value to the economy is still very small

Commodity plastics have low strengths and stiffnesseswhen compared with metals or ceramics and they tend toexhibit creep under an applied force They also have tem-perature limitations in their use as solids (most melt in therange 100ndash 250degC) The tensile moduli of commodity plas-tics are ~1 GPa (compared with 210 GPa for steel) Signi- cant improvement can be achieved by alignment of thepolymer chains Actually the carbonndash carbon bonds arevery strong and single lament polyethylenes have beenproduced with modulus values exceeding that of steel Highorientation can be achieved by special processing techni-ques for example extrusion and subsequent drawing at lowtemperatures At low temperatures the polymer chains havelimited mobility and the orientation remains after stretch-ing Recent discoveries and developments of single sitemetallocene based catalysts have resulted in new grades ofcommodity polymers having controlled molecular archi-tecture with improved properties

The world production of polymers increased3 from27 million tons in 1975 to ~200 million tons per year in2000 and is still growing According to a recent report4

shipments of plastics products in the USA in 2000 amountedto $330 billion and upstream supplying industries had salesof $90 billion bringing the annual total to $420 billionTotal employment was estimated to be 24 million ndash about2 of the US workforce The growth of the polymer indus-try is a result of the unique combination of properties ofplastic products which include easy shaping and fabrica-tion low densities resistance to corrosion electrical and

DOI 101179026708303225004738 Materials Science and Technology September 2003 Vol 19 1161

thermal insulation and often favourable rigidity and tough-ness per unit weight2

Thermoplastics are usually processed in the molten stateMolten polymers have very high viscosity values and exhibitshear thinning behaviour As the rate of shearing increasesthe viscosity decreases owing to alignments and disentang-lements of the long molecular chains The viscosity alsodecreases with increasing temperature In addition to theviscous behaviour molten polymers exhibit elasticity Elasti-city is responsible for a number of unusual rheological phen-omena1 5 ndash 7 These include stress relaxation and normal stressdifferences Slow stress relaxation is responsible for frozenin stresses in injection moulded and extruded products Thenormal stress differences are responsible for some owinstabilities during processing and also extrudate swellingie the signi cant increase in cross-sectional area when amolten material is extruded out of a die

The most important polymer processing operations areextrusion and injection moulding Extrusion is materialintensive and injection moulding is labour intensive Boththese processes involve the following sequence of steps (a)heating and melting the polymer (b) pumping the polymerto the shaping unit (c) forming the melt into the requiredshape and dimensions and (d) cooling and solidi cationOther processing methods include calenderingblow mould-ing thermoforming compression moulding and rotationalmoulding There are more than 30 000 grades of polymersprocessed by these methods The suitability of a material fora particular process is usually decided on the basis of the

melt ow index (MFI also called melt ow rate or MFR)This is an inverse measure of viscosity based on a rathercrude test involving the extrusion of a polymer through adie of standard dimensions under the action of a prescribedweight8 The MFI is the number of grams of polymer col-lected from the test apparatus in 10 min Low MFI valuesmean high viscosity and high molecular weight and highMFI values indicate the opposite The following is the usualMFI range for some processes extrusion 001ndash 10 injec-tion moulding 1 ndash 100 blow moulding 001 ndash 1 rotationalmoulding 15 ndash 20

Screw extrusion

Most polymeric materials are extruded at least twice in theirlifetime rst through a pelletising die after the reactor andthen for nal shaping8 In simplest terms screw extrudersare polymer pumps with the capacity to melt the materialwhich they are fed Screw extruders comprise one or twoArchimedean screws rotating in a heated barrel

The single screw extruder (SSE) is the workhorse of theplastics industry Figure 1a shows a schematic diagram ofan SSE9 and Fig 1b shows the geometry dimensions ofa screw channel8 Polymer resins in the form of pelletspowders or akes ow from a hopper to the gap between arotating screw and a heated barrel The depth of the con-veying channel in the screw is contoured from large to smallin the ow direction to account for the density change fromthe particulate solid feed to the molten polymer extrudateand for pressure development The SSEs normally havediameters between 25 and 250 mm and lengthdiameterratios between 20 and 36 Usual rotation speeds range from20 to 150 rev min21 A 60 mm diameter machine maydeliver up to 200 kg h2 1 while a 150 mm diameter machinecan exceed 1000 kg h21

In the rst or solids conveying zone of the extruder thesolid polymer particles are compacted together in the screwchannel by the rotating action of the screw to form a solidbed of material At the start of the next extruder section theplastication (melting) zone barrel heaters cause a thin lmof molten polymer to form in the gap between the solid bedand the barrel wall The melt lm is subjected to intenseshearing in the thin gap and because of the extremely highviscosities of molten polymers high rates of viscous dissi-pation result The generated heat melts the solid bed withina short distance of the start of melting Figure 2 illustratesthis process1 0 In the last zone of the extruder the meteringsection the polymer melt ow is stabilised in the shallowscrew channels and nally the material passes out throughthe die at the end of the machine

Pressurebuildupowing to frictional forces in the solidscon-veying section of the screw has been modelled by Darnelland Mol1 1 1 2 (along with some more recent research1 3) onthe basis of force and torque balances which result in anexponential expression For forward motion of the solidbed the friction coef cient must be larger on the barrel thanon the screw Screws have polished surfaces The barrelsurfaces are sometimes intentionally roughened (as ingrooved barrels)

Industrial experience shows that for good function ofsingle screw extruders the throughput should not be morethan 15m

D or less than 075mD where m

D is the drag ow

a schematic diagram9 b screw channel geometry8

1 Single screw plasticating extruder

2 Melting in screw channel10

1162 Vlachopoulos and Strutt Polymer processing

Materials Science and Technology September 2003 Vol 19

capacity of the metering section given by8

_mmD~rmQD

~12

rmp2D2HmN sin w cos w (1)

where QD is the volumetric ow rate rm is the melt densityD is the screw outer diameter Hm is the channel depth in themetering section N is the screw rotation rate and w is thescrew channel helix angle (frequently w~1766deg)

The power provided by the rotating action of the screwgoes to heating the polymer from room temperature to theextrusion temperature melting the polymer and pumping itthrough the die8 The barrel heaters usually provide lessthan 25 of the power required to raise the temperatureandmelt the polymer

Po~rmQCp(ToutTin)zrmQHfzDPQ (2)

where Po is the power Cp is heat capacity T is the tem-perature Q is the volume rate of ow DP is the pressurechange and Hf is the heat of fusion (zero for amorphouspolymers such as PS and PMMA and up to 300 kJ kg2 1 forsome high density PE grades) Extruders have poor pump-ing ef ciency with the pumping term DPQ accountingusually for 10 of the total motor power supplied by therotating screw With the help of equation (2) it is easy tocalculate that for extrusion of 400 kg h21 a motor of~120 kW would be required In contrast with metalsenergy costs for polymer processing are relatively low Therule of thumb is that the plastics manufacturing cost isroughly equal to the raw materials cost Power requirementscontribute ~5 to the manufacturing cost of extrusion

To improve the mixing capabilities of single screw extru-ders various types of mixing devices have been developedMost prominent among these is the Maddock (or UnionCarbide) mixer in both straight and spiral variations Thesedevices are normally located at the downstream end of ascrew and improve melt quality by reducing temperaturenon-homogeneities in the polymer stream and increasingthe dispersion and distribution of additives Used increas-ingly in the extrusion industry are barrier screws andgrooved barrels Barrier screws have a secondary screw ight in the melting section of the screw which serves tosegregate the solid bed from the molten polymer Byindependently controlling the dimensions of the solids andmelt channels the melting process can be accelerated andmade more stable thereby increasing the extruder outputand melt quality Grooved barrel extruders feature axialgrooves or slots in the part of the barrel immediately follow-ing the feed throat The grooved barrel can signi cantlyenhance feeding of resin pellets and akes and therebysharply increase the output of the extruder above the drag ow output of equation (1)

Twin screw extruders (TSEs) are extruders with two screwsof the same diameter which turn side by side within theextruder barrel at the same speed In recent years twin screwextruders have come into increasingly wider use in applica-tions such as mixing blending compounding of thermo-plastic polymers with additives devolatilisation and reactiveextrusion The TSEs offer greater control over residence timedistribution (RTD) and mixing than that of single screwextruders and have superior heat and mass transfer capa-bilities The disadvantage of TSEs compared with SSEs istheir signi cantly higher capital cost

There are three major classes of twin screw extruderscorotating intermeshing counter rotating intermeshing andcounter rotating non-intermeshing Figure 3 shows thethree TSE classes1 4 The screws are made up of conveyingkneading block and mixing sections The screw design isfrequently modular which allows for a nearly unlimited

number of possible screw con gurations Kneading blockscomprise several discs staggered at an angle to one anotherthe motion of which causes intense shearing and has achopping effect on the material stream Most of the dis-persive mixing (melt homogenisation and solids breakup)and melting that occurs within the extruder takes place inkneading blocks

Corotating intermeshing twin screw extruders are themost common type of TSE These machines have the fea-ture that their screws are self-cleaningowing to their specialdesign This leads to narrower RTDs in these extruders thanin SSEs making them particularly attractive for use withthermal and shear sensitive materials as there are no deadspots in which material can collect and degrade Large twinscrew extruders can rotate at more than 1200 rev min2 1

RPM with outputs of more than 10 tonsh

Die extrusion

Once a polymer has been melted mixed and pressurised inan extruder it is pumped through an extrusion die for

a corotating fully intermeshing b counter rotating intermesh-ing c counter rotating non-intermeshing

3 Twin screw con gurations14

Vlachopoulos and Strutt Polymer processing 1163


continuous forming (after cooling and solidi cation) into a nal product The most common die types are at annularand pro le Products made by extrusion include pipe tub-ing coating of wire plastic bottles plastic lms and sheetsplastic bags coating for paper and foil bres lamentsyarns tapes and a wide array of pro les (eg windowframes and sealing systems)8

Polymer extrusion through dies has certain similarities tothe hot extrusion of metals1 5 However there are also signi- cant differences In metal extrusion the material is pushedforward by a ram while in polymer extrusion the material iscontinuously supplied by a rotating screw In hot metalextrusion the temperatures range from 340degC for magne-sium to 1325degC for steel and the corresponding pressuresrange from 35 to over 700 MPa1 5 In polymer extrusion thetemperatures seldom exceed 350degC and pressures usuallydo not go much above 50 MPa at the screw tip Solid phaseextrusion of polymers has been developed for the produc-tion of certain high strength products1 6 At low tempera-tures the molecular orientation imparted by the forcing ofthe material through the shaping die remains in the extru-date Solid state polymer extrusion has certain similarities tothe cold extrusion of metals

Polymer melts in extrusion dies undergo creeping ow(characterised by very low Reynolds numbers typically lessthan 102 4) owing to their extremely large viscosities Whilethe viscosity of molten metals is the same order of magni-tude as that of water commercial polymers in the moltenstate exhibit viscosities usually more than 106 times largerThe simpli ed form of the Navier ndash Stokes momentum con-servation equation used in modelling these ows re ects thebalance of viscous stress and pressure forces as follows5

=Pz= t~0 (3)

where P is the pressure and t is the stress tensor A largenumber of constitutive relationships have been formulatedto relate the stress and strain rate elds in polymer ows the

simplest being the power law model for the shear viscosity5

g~m_ccn1

txy~g _cc

) (4)

where g is the viscosity tx y is the shear stress cis the shear

rate and m and n are model parameters (m decreasesexponentially with increasing temperature) The model pre-dicts shear thinning of the uid with exponent n valuesranging from 02 to 08 for most polymers To represent thedual viscous and elastic behaviour of polymer melts theMaxwell viscoelastic model has been used1 7

l _ttzt~m _cc (5)

where l is a relaxation time constant and m is the viscosityTo use this equation for ow problems it must be expressedin a tensorially invariant frame of reference Althoughnumerous viscoelastic constitutive equations have beendeveloped1 7 they have had little impact on polymer processtechnology Equipment design and process analysis of indu-strial installations is virtually always based on the solutionof purely viscous shear thinning models

Blown lm extrusion is the most important process forthe production of thin plastic lms from polyethylenes Themolten polymer is extruded through an annular die (nor-mally of spiral mandrel construction) to form a thin walledtube which is simultaneously axially drawn and radiallyexpanded In most cases the blown lm bubble is formedvertically upwards The maximum bubble diameter is usually12 ndash 4 times larger than the die diameter The hot melt iscooled by annular streams of high speed air from externalair rings and occasionally also from internal air distribu-tors The solidi ed lm passes through a frame which pin-ches the top of the bubble and is taken up by rollersCoextruded lms with 3 ndash 8 layers (sometimes up to 11) arealso produced by this process for use in food packaging8

Figure 4 shows a schematic diagram of the blown lmprocess1 8

Cast lm and sheet extrusion involves extruding a poly-mer through a at die with a wide opening (up to 10 m)onto a chilled steel roller or rollers which quench and soli-dify the molten material Film is generally de ned as a pro-duct thinner than 025 mm while sheet is thicker than thisThe cast lm process is used for very tight tolerances of thin lm or for low viscosity resins Most at dies are of T slotor coathanger designs which contain a manifold to spreadthe owing polymer across the width of the die followeddownstream by alternating narrow and open slits to createthe desired ow distribution and pressure drop Most cast lm lines manufactured today are coextrusion lines com-bining layers from as many as seven extruders into theproduct through multimanifolddies or single manifold dieswith the aid of feedblocks Figure 5 shows a schematicdiagram of the cast lm process8

4 Blown lm process18

5 Cast lm process CR chill roll8



In lm extrusion the shear rates at the die lips are usually~103 s21 When the wall shear stress exceeds a certainvalue (usually 014 MPa in research papers higher in indus-try with the help of additives) the extrudate surface loses itsgloss owing to the sharkskin melt fracture phenomenon1 9

Sharkskin can be described as a sequence of ridges visible tothe naked eye perpendicular to the ow direction

Pipe and tubing extrusion involves pumping a moltenpolymerthroughanannulardie following which the extrudedproduct while being pulled passes through a vacuum sizerwhere it attains its nal dimensions This is followed byspray or immersion cooling and cutting to xed lengthsPipe of diameter up to 2 m or greater is made by thisprocess and tubing with diameters from 10 mm down tobelow 1 mm The annular dies are normally of spider orspiral mandrel design8 2 0

In wire and cable coating processes individual wires orwire assemblies are pulled at very high speed through acrosshead die at right angles to the extruder axis In highpressure extrusion the polymer melt meets the wire or cablebefore the die exit for example insulating of individualwires In low pressure extrusion the melt meets the cableafter the die exit for example jacketing of assemblies ofinsulated cables1 4 Very high shear rates are frequentlyencountered in this process (up to 106 s2 1) and low viscosityresins are used Figure 6 shows a wire coating die2 1

Pro le extrusion is a manufacturing process used forproducts of constant cross-section These can range fromsimple shapes to very complex pro les with multiple cham-bers and ngers Examples range from picture frame mould-ings to automotive trim to edging for tabletops to windowlineals The extruded materials are classi ed (roughly) asrigid or exible The typical pro le extrusion line consistsof an extruder pumping a polymer through a pro le diefollowed by a sizing tank or calibrator additional coolingtroughs a puller and a cutoff device The design of pro ledies requires considerable experience and patience8 Outputlimitations in pro le extrusion are encountered owing toeither sharkskin (for thin products produced from high vis-cosity polymers) or the ability to cool thick walled productsPolymer pipe and pro le extrusion is similar to the hotextrusion of metals for the production of continuous hollowshapes of barlike objects However the mathematicalmodel-ling of these processes for metals is based mainly on elastic ndashplastic ow hypotheses2 2

In melt spinning the molten polymer ows throughnumerous capillaries in a spinneret (up to 1000) The poly-mer is delivered under pressure by a gear pump for accuratemetering after passing through a lter which follows theextruder On exiting the capillaries the laments are atten-uated to the desired diameter2 3 2 4 For the production ofvery thin bres the melt blowing process2 5 is used In thisprocess the bres are attenuated by the drag force exertedby a high velocity air jet

Calendering

The calendering of thermoplastics is an operation used forthe production of continuous sheet or lm of uniform thick-ness by squeezing the molten material between a pair ofheated driven rolls2 6 Figure 7 illustrates the calenderingprocess The molten material is fed to the calender rollsfrom a Banbury mixer and two roll mill system or from alarge extruder The major plastic material to be calenderedis PVC Products range from wall covering and uphols-tery fabrics to reservoir linings and agricultural mulchingmaterials

Owing to the large separating forces developed in thecalender gap the rolls tend to bend This may result in unde-sirable thickness variations in the nished product Com-pensations for roll de ections are provided by crowned rollshaving a larger diameter in the middle than at the ends or byroll bending or roll skewing2 7

Calender installations require large initial capital invest-ment Film and sheet extrusion are competitive processesbecause the capital investment for an extruder is only afraction of the cost of a calender However the high qualityand volume capabilities of calendering lines make them farsuperior for many products

Calendering in principle is similar to the hot rolling ofsteel into sheets It is interesting to note that strip castingof semisolid alloys can be modeled with the help of thehydrodynamic lubrication approximation for a power lawviscosity model as in plastics calendering2 8 The process ofcalendering is also used extensively in the paper industry

Injection moulding

Injection moulding is a two step cyclical process (a) meltgeneration by a rotating screw and (b) lling of the mouldwith molten polymer by the forward ramming of the screw(called a reciprocating screw) followed by a very short pack-ing stage necessary to pack more polymer in the mould tooffset the shrinkage after cooling and solidi cation Thematerial is held in the mould under high pressure until it hassolidi ed suf ciently to allow ejection

Polymer injection molding has certain similarities to thedie casting process of metals1 5 in which molten metal isforced under high pressure into a steel mould or die Themetal as the polymer is pressed into all the crevices of themould and the pressure is held while the metal freezes

In polymer injection moulding the melt path into themould starts with a sprue and splits off into individualrunners each feeding one of the multiplicity of mould cavi-ties through a gate Figure 8 shows schematically how thesemould components are connected Moulds can contain over100 cavities each producing a part per injection cycle Cycletimes range from a few seconds to over a minute Numerousproducts ranging from boat hulls lawn chairs and appliance

6 Wire coating die21

7 Calendering rolls producing plastic sheet26



housings to radio knobs and bottle caps are injectionmoulded Very high shear rates arise in injection mouldingoperations (usually up to 104 s21) and to limit temperatureincreases from viscous heating and also to facilitate easy lling low viscosity thermoplastic polymer grades are used

Injection moulding machines are rated by the size of theirclamping systems which are hydraulic electrical or mech-anical toggle systems used to hold the mould closed withsuf cient force to resist the injection pressure Machinesavailable usually range from 5 tons for a shot of ~10 g to5000 tons for shots of more than 50 kg (shot is the totalamount of material pumped into the mould in a cycleincluding that in the sprue runners and cavities) Somebigger machines have also been built for the production ofvery large moulded products Micromoulding in which theshot size is below 1 g is recently nding increased use inbiomedical and nanotechnological applications

Mould cavity lling is characterised by the fountain effectin which elements of the molten polymeric uid undergocomplex shear and stretching motions as they catch up thefree ow front and then move outwards to the cold wallsThis phenomenon can impart considerable orientation tothe resulting injection moulded part While molecular orien-tation is used in extrusion to improve the mechanical pro-perties in injection moulding orientation is generally anuisance2 The orientation is further exacerbated during thepacking stage The consequent frozen in stresses can causeparts to become distorted especially at elevated tempera-tures Figure 9 shows streamlines and uid element defor-mation in fountain ow2 9 Simulation of cavity lling isgenerally carried out on the basis of the Hele ndash Shaw owapproximation3 0 3 1 which applies to viscous ow betweentwo closely spaced plates With this approximation the twocomponents of the equation of conservation of momentumare reduced into a single partial differential equation withpressure P as the unknown

ccedilccedil x

Sccedil Pccedil x

sup3 acutez

ccedilccedil y

Sccedil Pccedil y

sup3 acute~0 (6)

where S~bdquo b0 z2=g dz and b is the gap between the plates

and g is the viscosity which is a function of local shear rateSimulation software is used extensively in the design of

injection moulds because of the ability of the Hele ndash Shaw ow approximation to describe reasonably well the mould lling process More recently however three-dimensionalmodels of ow simulation have come into use Economicsalso provide an incentive for the use of software as mouldscan be extremely expensive to make so minimising trialand error procedures on the factory oor is desirable The

software is used to design the part cavities to balancerunners to visualise the lling process and to predictorientation shrinkage warpage and weld lines in theproduct3 2 3 3

Among the challenging problems faced in computersimulation is the prediction of shrinkage and warpageShrinkage is the difference in dimensionsbetween the mouldand the cooled moulded part The main cause is the densityincrease which occurs as the melt freezes Crystalline poly-mers such as polyamide (PA nylon) high density PE PETand PP give the worst problems2 with shrinkages of 1 ndash 4Amorphous polymers such as PS PMMA and PC havefewer problems shrinking only 03 ndash 07 Warpage iscaused by the density changes mentioned above and orien-tation imparted to the part during cavity lling and packingdecidedly in a non-uniform manner

Gas assist injection moulding involves the injection ofnitrogen with the plastic which creates a hollow void in themoulded part3 4 This allows large parts to be moulded withlower clamp tonnage and signi cant material savings Thisinnovative process started in the 1980s and its use is rapidlygrowing

The process of reaction injection moulding (RIM) involvesthe injection of low viscosity liquids which become reactivewhen mixed and polymerise within the mould3 5 The advan-tage of this process is that the pressures are low owing to thelow viscosities A disadvantage is the requirement to handlehighly toxic substances Although initial projections calledfor signi cant growth in RIM this did not materialise Infact some manufacturers of automotive parts by RIM haveswitched to conventional injection moulding

Blow moulding

Blow moulding is the process by which articles are formedby in ation of a molten resin to ll a mould cavity havingthe desired shape and dimensions Bottles for soft drinksand other liquids are blow moulded The two most impor-tant process variants are extrusion blow moulding andinjection blow moulding3 6 In extrusion blow moulding anextruder pumps the melt through an annular die to form amolten tube or parison with well de ned and controlleddimensions The parison is clamped between the two mouldhalves and is in ated by internal air pressure to take theshape of the mould cavity which is usually cooled Finallythe formed article solidi es as a result of cooling and themould is opened to eject the article without damageDepending on the thermal stability of the material theextrusion process can be continuous or intermittent Inter-mittent systems can use reciprocating screws which operateas rams in the same manner as in injection moulding

Injection blow moulding is a two stage process In the rst stage the plastic is injected into a cavity where thepreform is moulded The preform is then transferred tothe blow mould for in ation Injection blow moulding

8 Injection mould components IM injection moulded

9 Fountain ow streamlines and uid element deforma-tion illustration29



offers the advantages of accurate dimensional controlelimination of scrap and the moulding in of threads beforeblowing Extrusion blow moulding is preferred for contain-ers with high lengthdiameter ratios and for products withhandles3 7

Polyethylene terephthalate or PET is the polymer mostwidely used in injection blow moulding for carbonateddrinks and water bottles Stretch blow moulding is used toproduce PET bottles of enhanced physical properties Inthis process stretching induces the formation of small lamel-lar crystals These crystals result in more transparent andtougher products than those produced without stretch blow-ing which have spherulitic crystals

Compression moulding

Compression moulding is the oldest technique for the pro-duction of polymer products3 0 and is mainly used for ther-mosets In this process the compound is pressed in themould by the heated platens of a hydraulic press This pro-cess is to some extent analogous to sheet metal stampingInjection moulding of polymers has replaced compressionmoulding for some polymers because of the advantagesin materials handling and automation However compres-sion moulding has an advantage in the processing ofreinforced polymers3 8 Owing to modest levels of deforma-tion and stress involved in compression moulding thereinforcing bres are not damaged Very high bre concen-trations and longer bres can be included in compressionmoulded products

Thermoforming

Thermoforming techniques involve the softening of thermo-plastic sheets by heat followed by forming by the appli-cation of vacuum pressure or a moving plug3 9 The sheetmay be stretched over a male mould (positive forming) orinto a female mould (negative forming) On contact with themould heat is lost and the material regains stiffness as itcools Geometries of thermoformed products are usuallysimple (boxes food trays various containers refrigeratorliners computer cases) Thermoforming competes withblow moulding and injection moulding The main advan-tages of this process are the relatively low cost of thermo-forming machines and the very low cost of the moulds andthe ease of forming large area thin section parts Disa-dvantages are the limited product shapes possible dif cultiesin obtaining the required thickness distribution dif culty incontrolling molecular orientation and limitations in service

temperature which may induce strain recovery or shrinkageThermoforming of thermoplastics has certain similaritiesto sheet metal forming4 0 However the strain rates involvedin polymer sheet forming are much larger than for metalsFigure 10 shows a vacuum thermoforming process

Rotational moulding

In rotationalmouldinga chargeof plastic powder is placed inone half of a metal mould The mouldhalves are thenclampedtogether and heated while the mould rotates biaxially Thepowder particles melt and coalesce to form a homogeneouslayer on the surface of the mould The mould is then cooledand the polymer layer densi es and solidi es4 142 Rotationalmoulding competes with blow moulding thermoformingandinjection moulding for the production of hollow parts Themain advantages of this process are the simplicity of themoulds and the low capital investment requirement Cycletimes are large ~15ndash 30 min Rotationalmoulding is mainlyused for large tanks (up to 85 m3 reported) toys and hollowparts that require frequent design modi cations

The process of coalescence and subsequent densi ca-tion4 3 4 4 is usually referred to as sintering by rotationalmoulding practitioners In powder metallurgy however theterm sintering usually refers to compaction of powdersbelow their melting point

Powder injection moulding andthixomoulding

Powder injection moulding (PIM) and thixomoulding aretwo processes that use traditional polymer processing prin-ciples for the production of metal parts

In PIM ne powders are blended into a lsquobinderrsquo to holdthe particles together and are mixed with additives to formthe lsquofeedstockrsquo The feedstock is then used in suitably modi- ed but otherwise conventional polymer injection mould-ing machines for injection into moulds After debinding andheating the metal welds together by a sintering process toform strong metal or ceramic parts Various types of ferrousalloys and oxide ceramics are suited to this process4 5

In thixomoulding metal alloy granules are fed from ahopper into the gap between a barrel and a rotating screwand are heated by shearing to form a semisolid slurry Thematerial is then injected into a heated mould By coolingthe material is solidi ed and then it is removed from themould and trimmed Magnesium aluminium and zinc aresuited to thixomoulding Currently magnesium is mostlyused in this process4 6

a before application of vacuum b after application of vacuum

10 Thermoforming by application of vacuum40



Current trends and future challenges

The polymer industry experiencedexponentialgrowth duringthe latter half of the 20th century There are about 50 resinproducers around the world (the properties of many materialgrades are available from a single database4 7) the well knownbig chemical companies contributing most of the productionvolume of 200 million tons per year The processing industryon the other hand is fragmentedto tens of thousandsof smalland medium sized enterprisesaround the world For exampleGermanyhas 2500 plasticsprocessing companiesMost of themanufacturers of extruders injection moulding machinesand other types of equipment are also small or medium sizedenterprises having fewer than 500 employees The growth ofthe plastics industry is likely to continue especially in deve-loping countries Plastics consumption is likely to increase asmore people around the world try to satisfy their needs intransportation food packaging housing and electrical app-liancesHowever this industry is consideredto have reachedastage of maturity Research and development efforts by themajor resin producers have been severely curtailed in recentyears The plastics processors and original equipment manu-facturers are not big enough to sustain major RampD pro-grammes that could lead to lsquoquantum jumpsrsquo in technology

At a recent workshop of university and industry experts4 8

it was concluded that future efforts should go beyondmachinery design and process analysis and optimisationThe focus should be on predicting and improving the productproperties of polymer based products The term lsquomacro-molecular engineeringrsquo was introduced as being more des-criptive of future developments in the transformation ofmonomers into long chain molecules and their subsequentshaping or moulding into numerous useful products

The prediction of end use properties of polymeric pro-ducts is faced with some huge challenges The currentprocess simulation approach which is based on the con-tinuum mechanics of non-Newtonian uids4 9 ndash 5 4 must becombined with models describing macromolecular confor-mations relaxation and polycrystalline morphologies Thevarious types of constitutive models whether continuum1 7

reptation6 or pom pom5 5 have had very limited successes inpredicting the unusual rheological phenomena exhibited bypolymeric liquids even under isothermal conditions Deter-mination of heat transfer coef cients5 6 and modelling of ow induced crystallisation5 7 5 8 are necessary for the even-tual prediction of properties of lms and other extrudedproducts Numerous problems remain unresolved in otherpolymer processes such as the prediction of shrinkage war-page and stress cracking in injection moulding The goal ofprecise property prediction is likely to remain a challengefor a considerable length of time However new technol-ogies5 9 even without detailed scienti c understanding arelikely to play a signi cant role in the eld of polymersThese include nanocomposites with exceptional proper-ties conductive plastics for electronics self-assembly pro-cesses for the creation of special polymeric structures andfabrication of biomaterials and polymer based tissueengineering

Plastics are perceived as long lasting pollutants in the envi-ronment because of their dominant role in disposable itemsSocietal and legislative pressures for reuse and recycling arelikely to increase in the years to come Plastics waste col-lection reprocessing and burning for energy recovery aresome technologies of current and future development6 0 6 1

References

1 j l white lsquoPrinciples of polymer engineering rheologyrsquo 1990New York Wiley

2 d h morton-jones lsquoPolymer processingrsquo 1989 LondonChapman and Hall

3 h u schenck AIM Mag (Special issue lsquoPolymers in EuropeQuo vadisrsquo) 2001 15 ndash 18

4 Society of the Plastics Industry lsquoThe Size and impact of theplastics industryrsquo 2001 Washington DC SPI

5 r b bird r c armstrong and o hassager lsquoDynamics ofpolymeric liquidsrsquo Vol 1 lsquoFluid Mechanicsrsquo 2nd edn 1977New York Wiley

6 c w macosko lsquoRheology principles measurements andapplicationsrsquo 1994 New York VCH Publishers

7 j m dealy and k f wissbrun lsquoMelt rheology and its rolein plastics processingrsquo 1999 Dordrecht The NetherlandsKluwer

8 j vlachopoulos and j wagner jr (ed) lsquoThe SPE guide onextrusion technology and troubleshootingrsquo 2001 Brook eldCT Society of Plastics Engineers

9 e e agur and j vlachopoulos Polym Eng Sci 1982 221084ndash 1093

10 p n colby b p smith and m f durina lsquoPlasticatingcomponents technologyrsquo 1992 Youngstown OH Spirex Corp

11 w h darnell and e a j mol SPE J 1956 12 2012 z tadmor and c g gogos lsquoPrinciples of polymer processingrsquo

1979 New York Wiley13 k s hyun and m a spalding Proc SPE ANTEC Toronto

Canada May 1997 Society of Plastics Engineers14 c rauwendaal lsquoPolymer extrusionrsquo 4th edn 2001 Munich

Hanser15 l e doyle lsquoManufacturing processes and materials for

engineersrsquo 3rd edn 1985 Englewood Cliffs NJ Prentice-Hall16 i m ward p d coates and m m dumoulin (ed) lsquoSolid phase

processing of polymersrsquo 2000 Cincinnati OH Hanser Gardner17 r i tanner lsquoEngineering rheologyrsquo 2000 Oxford Oxford

University Press18 j vlachopoulos and v sidiropoulos in lsquoEncyclopedia of

materials science and technologyrsquo (ed K H J Buschow et al)7296ndash 7301 2001 Amsterdam Elsevier Science

19 m denn Ann Rev Fluid Mech 2001 33 265ndash 28720 w michaeli lsquoExtrusion dies for plastics and rubberrsquo 2nd edn

1992 Munich Hanser21 j-f agassant p avenas j-p sergent and p j carreau

lsquoPolymer processing principles and modelingrsquo 1991 MunichHanser

22 b avitzur lsquoMetal Forming processes and analysisrsquo 1968New York McGraw-Hill

23 a ziabicki lsquoFundamentals of bre formationrsquo 1976 LondonWiley

24 a ziabicki and h kawai (ed) lsquoHigh-speed bre spinningrsquo1988 New York Wiley

25 m a j uyttendaele and r l shambaugh AIChE J 1990 36175ndash 186

26 j vlachopoulos in lsquoConcise encyclopedia of polymerprocessing and applicationsrsquo (ed P J Corish) 105ndash 1071992 New York Pergamon Press

27 a w coaker in lsquoPlastic materials and technologyrsquo (ed I IRubin) 1099ndash 1118 1990 New York Wiley

28 tmatsumiya and c flemings Metall Trans B 198112 17ndash 3129 h mavridis a n hrymak and j vlachopoulos J Rheol

1988 32 639ndash 66330 a i isayev (ed) lsquoInjection and compression molding funda-

mentalsrsquo 1987 New York Marcel Dekker31 j a dantzig and c l tucker lsquoModelling in materials

processingrsquo 2001 Cambridge Cambridge University Press32 p kennedy lsquoFlow analysis reference manualrsquo 1993 Kilsyth

Australia Mold ow33 j beaumont r nagel and r sherman lsquoSuccessful injection

moldingrsquo 2002 Cincinnati OH Hanser Gardner34 j avery lsquoGas-assist injection moldingrsquo 2001 Cincinnati OH

Hanser Gardner35 c w macosko lsquoFundamentals of reaction injection moldingrsquo

1988 Munich Hanser36 d v rosato and d v rosato lsquoBlow molding handbookrsquo 1988

Munich Hanser37 m r kamal in lsquoConcise encyclopedia of polymer processing

and applicationsrsquo (ed P J Corish) 71 ndash 76 1992 New YorkPergamon Press

38 p k mallick and s newman (ed) lsquoComposite materialstechnologyrsquo 1990 Munich Hanser

39 j l throne lsquoTechnology of thermoformingrsquo 1996 MunichHanser



40 d bhattacharyya (ed) lsquoComposite Sheet formingrsquo 1997Amsterdam Elsevier Science

41 r j crawford lsquoRotational molding of plasticsrsquo 1992 NewYork Wiley

42 r j crawford and j l throne lsquoRotational moldingTechnologyrsquo 2002 New York William Andrew Publishing

43 m narkis and n rosenzweig lsquoPolymer powder technologyrsquo1995 New York Wiley

44 m kontopoulou and j vlachopoulos Polym Eng Sci 200141 155ndash 169

45 r m german and a bose lsquoInjection molding of metals andceramicsrsquo 1997 Princeton NJ MPIF Publications

46 r d carnahan in lsquoMagnesium technology 2000rsquo (ed H IKaplan et al) 90 ndash 97 2000Warrendale PA TMS

47 Internet httpwwwcampusplasticscom48 lsquoFinal workshop report touchstones of modern polymer

processingrsquo Polymer Processing Institute NJIT NewarkNJ USA June 2002

49 d g baird and d i collias lsquoPolymer processingrsquo 1998 NewYork Wiley

50 c l tucker lsquoFundamentals of computer modeling forpolymer processingrsquo 1989 Munich Hanser

51 a i isayev (ed) lsquoModeling of polymer processingrsquo 1991Munich Hanser

52 k t orsquobrien (ed) lsquoApplications of computer modeling forextrusion and other continuous polymer processesrsquo 1992Munich Hanser

53 j a covas j f agassant a c diogo j vlachopoulos and kwalters (ed) lsquoRheological fundamentals of polymer process-ingrsquo 1995 Dordrecht The Netherlands Kluwer

54 j vlachopoulos Can Chem News Sept 2000 29 ndash 3055 t c b mcleish and r g larson J Rheol 1998 42 8156 v sidiropoulos and j vlachopoulos Intern Polym Proc

2000 15 40 ndash 4557 t kanai and g a campbell lsquoFilm Processingrsquo 1999 Munich

Hanser58 a k doufas and a j mchugh J Rheol 2001 45 1085ndash

110459 p mukhopadhyay Plast Eng Sept 2002 28 ndash 3760 j brandrup m bittner w michaeli and g menges (ed)

lsquoRecycling and recovery of plasticsrsquo 1996 Munich Hanser61 g akovali c a bernardo j leidner l a utracki and

m xanthos lsquoFrontiers in the science and technology ofpolymer recyclingrsquo 1998 Dordrecht The Netherlands Kluwer



thermal insulation and often favourable rigidity and tough-ness per unit weight2

Thermoplastics are usually processed in the molten stateMolten polymers have very high viscosity values and exhibitshear thinning behaviour As the rate of shearing increasesthe viscosity decreases owing to alignments and disentang-lements of the long molecular chains The viscosity alsodecreases with increasing temperature In addition to theviscous behaviour molten polymers exhibit elasticity Elasti-city is responsible for a number of unusual rheological phen-omena1 5 ndash 7 These include stress relaxation and normal stressdifferences Slow stress relaxation is responsible for frozenin stresses in injection moulded and extruded products Thenormal stress differences are responsible for some owinstabilities during processing and also extrudate swellingie the signi cant increase in cross-sectional area when amolten material is extruded out of a die

The most important polymer processing operations areextrusion and injection moulding Extrusion is materialintensive and injection moulding is labour intensive Boththese processes involve the following sequence of steps (a)heating and melting the polymer (b) pumping the polymerto the shaping unit (c) forming the melt into the requiredshape and dimensions and (d) cooling and solidi cationOther processing methods include calenderingblow mould-ing thermoforming compression moulding and rotationalmoulding There are more than 30 000 grades of polymersprocessed by these methods The suitability of a material fora particular process is usually decided on the basis of the

melt ow index (MFI also called melt ow rate or MFR)This is an inverse measure of viscosity based on a rathercrude test involving the extrusion of a polymer through adie of standard dimensions under the action of a prescribedweight8 The MFI is the number of grams of polymer col-lected from the test apparatus in 10 min Low MFI valuesmean high viscosity and high molecular weight and highMFI values indicate the opposite The following is the usualMFI range for some processes extrusion 001ndash 10 injec-tion moulding 1 ndash 100 blow moulding 001 ndash 1 rotationalmoulding 15 ndash 20

Screw extrusion

Most polymeric materials are extruded at least twice in theirlifetime rst through a pelletising die after the reactor andthen for nal shaping8 In simplest terms screw extrudersare polymer pumps with the capacity to melt the materialwhich they are fed Screw extruders comprise one or twoArchimedean screws rotating in a heated barrel

The single screw extruder (SSE) is the workhorse of theplastics industry Figure 1a shows a schematic diagram ofan SSE9 and Fig 1b shows the geometry dimensions ofa screw channel8 Polymer resins in the form of pelletspowders or akes ow from a hopper to the gap between arotating screw and a heated barrel The depth of the con-veying channel in the screw is contoured from large to smallin the ow direction to account for the density change fromthe particulate solid feed to the molten polymer extrudateand for pressure development The SSEs normally havediameters between 25 and 250 mm and lengthdiameterratios between 20 and 36 Usual rotation speeds range from20 to 150 rev min21 A 60 mm diameter machine maydeliver up to 200 kg h2 1 while a 150 mm diameter machinecan exceed 1000 kg h21

In the rst or solids conveying zone of the extruder thesolid polymer particles are compacted together in the screwchannel by the rotating action of the screw to form a solidbed of material At the start of the next extruder section theplastication (melting) zone barrel heaters cause a thin lmof molten polymer to form in the gap between the solid bedand the barrel wall The melt lm is subjected to intenseshearing in the thin gap and because of the extremely highviscosities of molten polymers high rates of viscous dissi-pation result The generated heat melts the solid bed withina short distance of the start of melting Figure 2 illustratesthis process1 0 In the last zone of the extruder the meteringsection the polymer melt ow is stabilised in the shallowscrew channels and nally the material passes out throughthe die at the end of the machine

Pressurebuildupowing to frictional forces in the solidscon-veying section of the screw has been modelled by Darnelland Mol1 1 1 2 (along with some more recent research1 3) onthe basis of force and torque balances which result in anexponential expression For forward motion of the solidbed the friction coef cient must be larger on the barrel thanon the screw Screws have polished surfaces The barrelsurfaces are sometimes intentionally roughened (as ingrooved barrels)

Industrial experience shows that for good function ofsingle screw extruders the throughput should not be morethan 15m

D or less than 075mD where m

D is the drag ow

a schematic diagram9 b screw channel geometry8

1 Single screw plasticating extruder

2 Melting in screw channel10




_mmD~rmQD

~12












Die extrusion









=Pz= t~0 (3)



g~m_ccn1

txy~g _cc

) (4)



l _ttzt~m _cc (5)















Calendering





Injection moulding











ccedilccedil x

Sccedil Pccedil x

sup3 acutez

ccedilccedil y

Sccedil Pccedil y

sup3 acute~0 (6)








Blow moulding











Thermoforming



Rotational moulding


















References

































































_mmD~rmQD

~12












Die extrusion









=Pz= t~0 (3)



g~m_ccn1

txy~g _cc

) (4)



l _ttzt~m _cc (5)















Calendering





Injection moulding











ccedilccedil x

Sccedil Pccedil x

sup3 acutez

ccedilccedil y

Sccedil Pccedil y

sup3 acute~0 (6)








Blow moulding











Thermoforming



Rotational moulding


















References



































































=Pz= t~0 (3)



g~m_ccn1

txy~g _cc

) (4)



l _ttzt~m _cc (5)















Calendering





Injection moulding











ccedilccedil x

Sccedil Pccedil x

sup3 acutez

ccedilccedil y

Sccedil Pccedil y

sup3 acute~0 (6)








Blow moulding











Thermoforming



Rotational moulding


















References






































































Calendering





Injection moulding











ccedilccedil x

Sccedil Pccedil x

sup3 acutez

ccedilccedil y

Sccedil Pccedil y

sup3 acute~0 (6)








Blow moulding











Thermoforming



Rotational moulding


















References



































































ccedilccedil x

Sccedil Pccedil x

sup3 acutez

ccedilccedil y

Sccedil Pccedil y

sup3 acute~0 (6)








Blow moulding











Thermoforming



Rotational moulding


















References




































































Thermoforming



Rotational moulding


















References







































































References






















































































Documents

Overview Polymer Processing