19511.2SourcesandoriginBakelite was thefirst entirelysyntheticpolymer andwassdevelopedintheearly twentiethcentury. It waslightweightanddurableandcouldbemouldedintoalmost anyshape, soits use quickly expandedas manufacturersrealisedits potential. Bakelitewas usedtomakejewelleryboxes, kitchenware, lamps, clocks, radios, telephonesandmanymore products. ExtractionandconversionMost modernpolymers aretotallysynthetic. Thechemicalsfromwhichthey aremadearesynthesisedfromcarboncompounds that arelargelyobtainedfromcrudeoil. Thecrude oil is first fractionallydistilledandthensomeof theproducts areprocessedfurther toproducethechemical. Only a small amount (about 4per cent) of total crudeoil productiongoes intomakingpolymers. One of the chemicals obtainedfromcrudeoil isethene. Thisis usedtomaketheplasticpolythene, andif youlookat howpolythene is madeyoucanlearnabout thegeneral structureof plastics andbegintounderstandtheir properties. Figure 11.2 Awoodenbox finishedwithshellacFigure11.3ABakeliteradioHHHHCCHHHHCCHHHHCCHHHHCCFigure 11.4 An ethenemoleculeFigure11.5EthenemoleculesfloatingasagasKEYTERMSSynthesised:Madebycombiningpartsof,e.g.combiningdifferentcarboncompounds.
196Chapter 11Polymers Anethenemolecule(seeFigure11.6)ismadeoffourhydrogenatomsandtwocarbonatoms- t helinesrepresent thebondsbetweenthem. Ethenegasismadeupofmillionsofthesemoleculesfloatingaround. Polytheneismadebygettingtheseethenemoleculestojointogethertoformlongchainmolecules. Thisisdoneusingotherchemicalscalledcatalysts.Duringtheprocess,thousandsof ethenemoleculesjointogethertoformpolythenemolecules.=Hydrogen=CarbonEthene(monomer) Polythene(polymer) Figure11.6EthenemoleculesjoinedtogethertoformpolythenePolythenemoleculesdonotneedtobemadetojointogether;theyattracteachotherandbecometangledandtwistedtogether, whichformsthesolid,high-densitypolythene.Small moleculeslikeethenethatcandothisarecalledmonomers(mono=single).Oncejoinedtogether theybecomeapolymer(poly=many). Theprocessofjoiningthemtogetheris calledpolymerisation. AdditivesThemechanical propertiesof polymerscanbeimprovedbytheuseofadditives:lPlasticisersaresubstancesaddedtopolymerstoimprovetheirflowpropertiesformoulding. PVCisusedfortoughdrainpipes, yetbyaddingasuitableplasticiseritcanbemadeflexibleenoughtomakeagardenhose. lPigmentsaddcolour. lStabilisershelppreventdamagefromultravioletlight, whichcancauseseveralpolymerstobecomebrittle. lFillersarepowderedsolidsthatbulkuppolymersandreducecosts,buttheycanalsoimprovestrengthbyreducingbrittlenessandincreasingresistancetoimpact.lCatalystscanbeusedtospeedupthesynthesisofpolymers.Theyincludeepoxyresin,whichhardensinonehourcomparedtotheusual12. lAntioxidantspreventoxidation. Ecological,socialandethicalissuesassociatedwithprocessingOnlyasmall amount (4per cent) of crudeoil productiongoesintomakingpolymers.Thevast majoritygoestowardsprovidingenergyforheating, transportandelectricity(86percent). Worldproductionof polymershasbeenrisingsteadily, however,andisnowintheregionof 300milliontonnesayear, anditwill continuetoriseapproximately4percenteachyear. Duringthemanufactureofpolymerssignificantquantitiesoftoxicchemicalsareproduced. Producinga500ml PETbottlegeneratesmorethanahundredtimesthetoxicemissionstoair andwater thanmakingthesamebottleoutofglass.SafetyconcernsProducingplasticscanbehazardoustoworkers. Seriousaccidentshaveincludedexplosions,chemical fires, chemical spills, andcloudsoftoxicvapour. Thesekindsofoccurrenceshavecauseddeaths, injuries, evacuationsandmajorpropertydamage.KEYTERMSAdditives: Substances addedtopolymers toimprove their mechanical properties. Catalysts: Chemicals that cause a reactiontohappen. Polymerisation: Theprocess of joiningsmall molecules toformpolymers (longchains).
19711.2SourcesandoriginsEnvironmental andhealtheffectsMany chemical additives that giveplasticproductsdesirableperformancepropertiesalsohave negativeenvironmental andhumanhealtheffects. Phthalatesareaddedtoplasticstomake themsofter (plasticisers) andseveral of themhavebeenclassedascarcinogens-substances capableof causingcancer. The most obvious formof pollutionassociatedwithpolymers is thewasteproducts that areeither dumpedor sent tolandfill. Between10 and20 milliontonnes of plasticends upinthe oceanevery year. This plasticdebrisdamages marineecosystems. Animals suchas seabirds, whales anddolphins caningest or become entangledinplasticmatter, andfloatingplastic items - s uchas discardednets, docks andboats - c antransport microbes, algae, invertebrates andfishintonon-nativeregions, affectingthelocal ecosystems. It is estimatedthat everyadult maleintheUKgenerates aroundonetonneof wasteeachyear. Around9per cent of this is polymer- basedpackagingwastesuchas milkbottlesandfoodpackaging. This wastecouldactuallybe recycledandmadeintonewpolymer- basedproducts. LifecyclePolymers area very stablematerial andtendtostayintheenvironmentforalongtimeafter they arediscarded, especiallyif theyareshieldedfromdirectsunlightbybeingburiedinlandfills. They decomposeveryslowly, especiallyasmostpolymerscontainantioxidantstoresist attack by chemicals. Different kindsof polymer degradeatdifferenttimes, buttheaverage timefor a simplePETbottletodegradeisat least 450years. Somemighteventakea thousandyears. Whiletheyslowlydegradetheycanreleasesignificantquantitiesoftoxicchemicals suchas trichloroethaneandmethylenechloride. Recycling, reuseanddisposalRecyclingRecyclingis animportant factor inconservingnatural resourcesandgreatlycontributestowards improvingtheenvironment. Recyclingrecoversmaterialsusedinthehomeorindustry for further uses. It reduces theamount of energyandnatural resources(suchaswater, petroleumandnatural) neededtocreatenewplastic. Italsoreducestheamountofmaterial dumpedinlandfill sites. Many polymers, suchas PET, canberecycledwithout anylossof quality. Infact, theplasticfromPET drinks bottles canberecycledintoclothingproductssuchasfleeces. Severalproducts, especially thosethat comeintocontact withfood, mustuseneworvirginplasticfor healthandsafety reasons. Other products areallowedtouseacertainpercentageof recycledpolymermixedinwithvirginmaterial. Recyclingaproduct intoalower qualityproductiscalleddowncycling. TheHDPE usedinmilk bottles is anexcellent polymer for downcyclingandhasbeenusedtocreate products suchas playgroundequipment andpicnicbenches. Figure11.7ThemajorityofpolymerproductsendupinlandfillKEYTERMSDowncycling:Recyclingintoalower-qualityproduct.
198Chapter 11Polymers PolymerrecyclingcodesIf youlookcarefullyat aplasticproductyouwill seeasymbolandanumber.Theseidentifythetypeof polymer usedtomaketheproduct. Forrecyclingpurposesitisessentialtoknowwhichplasticiswhich. Figure 11.8 Polymer recyclingcodes ReuseDespiteitbeingbetterthancreatingnewplastic,recyclingstillusesalotof energy. Thewastepolymerneedstobesorted,cleanedandchippedbeforebeingmelteddownintouseablegranules.Reusingacontainerorpackagingisthereforeevenbetterthanrecycling.Forexample, onceyouhaveconsumedabottleofwater,refillitfromatapandreuseit. Reuseplasticjarsbywashingthemoutandusingthemforstorage. Useareuseablewrapperforpackedlunchesratherthancontinual useofclingfilm. Sellorgiveawayoldtoysinsteadofthrowingthemaway. Consideradaptinganexistingplasticproductintosomethingmoreuseful. ReduceAsaconsumer, thinkabouttheproductsyoubuyandtrytochooseproductswithminimal packaging. In2014over7.6billionsingle-useplasticbagsweregiventocustomersbymajorsupermarketsinEngland. That'ssomethinglike140bagsperperson,ortheequivalentofabout61,000tonnesintotal. FromOctober2015largeshopsintheUKhavehadtocharge5pforeverysingle-useplasticcarrierbag,whichhasencouragedshopperstobuythicker, morereuseable,'bagsforlife'.Asadesigner, thinkabouttheproductsyoudesignandhowyoucanreducetheamountofpolymermaterialbeingused.Forexample,thenewNestleEco-Shapebottleuses30percentlessplasticandathinnerlabel thanthepreviousversion.Figure 11.9 Pencil cases madefromreusedPETdrinks bottles
19911.2SourcesandoriginsThe slogan'Reduce, reuseandrecycle' appearsonthewastehierarchy, whichisasetofprinciples for theefficient useof resources. Thelargest partof thepyramid(thebase)istheworst place tobe, as that is what wedonow- wedisposeof mostof ourwaste. Thebestplace tobe is at thetop, wherewecompletelyeliminatetheneedforthewaste. Inpractice, this is the differencebetweenbuyingaregular bananafromalocal greengrocerorashrink-wrappedversiononatrayfromamajor supermarket. Disposal EnvironmentallyfriendlypolymersEnvironmentally friendlypolymers containpolymersthat are biobased(fromarenewableresource) or biodegradable. Polylactic acid(PLA) is abiobasedpolymer derivedfromcornstarchthat breaks downintoharmlesschemicalswhencomposted. It has beenusedtomakedisposableitems suchas cups, cutleryandfoodcontainers. Biodegradablebags aremadefrombiodegradableplastics that containadditives that causethemtodecay morerapidly inthepresenceof light andoxygen(moistureandheat alsohelp). Unlikebioplastics, biodegradableplastics aremadeof normal (petrochemical) plastics anddon't always breakdownintoharmless substances: sometimes theyleavebehinda toxic residueandthat makes themgenerally(but not always) unsuitablefor composting. Figure11.11Anover-packagedbananaFigure11.10ThisPETbottlehasbeendesignedtouse30percentlesspolymer. PreventionReductionReuseRecyclingEnergyrecoveryDisposalFigure11.12Thethree'Rs' inthewastehierarchyKEYTERMSBiobased: Aproduct madefromarenewableresource. Biodegradable: Theability of asubstanceor object tobreakdownnaturallyintheenvironment throughtheactionof micro-organisms, therebyavoidingpollution.
200Chapter 11Polymers RecyclingandthelawTherearemanylawsandregulationsthatrequireconsumersandmanufacturerstoconsidertheenvironment whenaproductcomestotheendofitslife.Someexamplesof waysinwhichtheselawsencouragemanufacturersandconsumerstoconsider theenvironmentwhenbuyingordisposingofproductsinclude:lElectrical itemssuchasrefrigeratorsandwashingmachinesnowhavealabelonthemthat describestheir energyefficiencyrating. Thishelpsconsumersmakeaninformedchoiceabout whether theproducttheychooseisbetterfortheenvironment.lConsumersareencouragedtotaketheirolddevicestocollectionpoints,fromwhichmanufacturersarrangecollection. Asaresultdesignershavehadtomakeproductseasiertodismantle, reuseandrecycle. lNewvehiclesdonot usetoxicmaterialsandhaveallpolymerpartslabelledtohelpwithrecycling. Vehiclemanufacturerspublishinformationabouthowtodismantletheir vehicles. KEYPOINTPolymers havebeenmanufacturedsincetheearlytwentiethcentury.Theyareversatilematerials whosepropertiescanbealteredbyvariouschemicaladditives.Despitetheiramazingpropertiestheycancauseserioushealthissuesandecologicalproblemsfortheplanet. It's important that youalwaysdesignwithsustainabilityandsafetyinmind.ACTIVITYGather arangeof polymerproductsandidentifythepolymerusedbyfindingthepolymerrecyclingcode. ACTIVITYDesignandmakeaproductthatreusesapolymerproductthatwouldnormallybedisposedof. For example, ascoopfromanoldmilkbottle,aphonecasefromwovencrisppacketsorastationaryorganiser fromadrinksbottle.Figure 11.13 PLAfoodpackagingFigure11.14Abiodegradablecarrierbag
20111.3Commonlyavailableforms11.3CommonlyavailableformsLEARNINGOUTCOMESBy theendof this sectionyoushouldknowandunderstand: ➜that polymers areavailableinarangeof stockforms➜that thereis awiderangeof standardisedcomponentsmadefrompolymers.StockformsStockforms of polymer arebasicallytheformsthat it canbeboughtinandthatcanbestored'instock', ready for usefor moulding, cutting, bending, etc. lSheets areoftenusedfor signage, roofingandpanels. Theycanalsobeusedforlinebending. lGranules areusedfor injectionmouldingastheymeltquickerwhenfedintothemouldingmachines. lRods aregenerally usedfor CNCmachining. lExtrudedsections aresimilar torods, whereacontinuousprofileismaintainedthroughout thesection. Awiderangeof shapesisavailable, suchasT, HandCsections. Several companies specialiseincustomplasticextrusions. lTubes canbemadeinanyshape- r ound, squareandtriangularareexamplesyoumaycome across inschool. lFoamedplastics areavailableinsheet or roll form, suchasPlastazote®, orblockformforexpandedpolystyrene. lPowderedpolymers areoftenusedtoprovideacoatingonmetal surfaces. lReels of plastic wireareusedincertain3Dprinters. KEYTERMSStockform:Commonlyavailableformsofpolymerthatcanbebought.Figure 11.15 Some commonforms of polymer
202Chapter 11Polymers StandardcomponentsThereareseveral standardisedcomponentscommonlymanufacturedfrompolymers:lNutsandbolts- usuallymadefromnylon. lWashers- usedtosecurefittings. lWall plugs- usedtoprovideanchorsforscrewsinwalls.lEndcaps- usedtohidetheheadsofscrewsortoclosepipes/tubing.lPlasticgearwheels- usedinvarioustoysandmechanicaldevices.Figure11.16AnassortmentofpolymerfixingsSTRETCHANDCHALLENGEPrepareastocklist of thepolymerformsavailableinyourschoolworkshop.Presentthis as atableandgivedetailsofthefollowing:nameofthepolymer,type(thermoorthermosetting), form(sheet, rod, square,etc.),size,totallength.STRETCHANDCHALLENGEProduceaninformation/exampleboardwithsamplesofthedifferentpolymerformsavailabletostudentsinyourschool workshop.KEYPOINTAkey strengthof polymers is their ability tobe mouldedandformedintoany shape. Anawareness of the stock forms, however, will helpyour iterativemodelling. KEYTERMSStandardisedcomponent: Anindividual part or component, manufacturedinthousands or millions, tothe samespecification.
20311.4Manipulatingandjoining11.4ManipulatingandjoiningLEARNINGOUTCOMESBy theendof this sectionyoushouldknowaboutandunderstand:➜a rangeof specialist techniquesusedtoshape, fabricate, constructandassemblehighquality prototypes usingpolymersinaworkshop. WastageWastageis theprocess of cuttingawaymaterial toleaveadesiredshape. Itiscalledthisbecause thematerial removedis oftenthrownaway. MarkingoutBefore a designcanbecut it must bemarkedout. Thereareanumberofwaystomarkoutona piece of polymer. Achinagraphpencil isoftenusedasitcanbewipedoffeasily, buttheline cansometimes betoothicktofolloweffectively. Sometimesanon-permanentmarkercanbe used, but thelines canbesmudgedeasily. Apermanentmarkercanbeusedonquickearly models whereprogress is moreimportant thanahigh-qualityfinish. Figure 11.17 Cuttingacrylic onascroll sawFigure11.18CuttingacryliconalasercutterCuttingScissors canbeusedonthinfoamsheets and0.5mmHIPS. Craftknivescanbeusedtocutthicker sheets of HIPSbyfirst scoringalongthelinethenflexingthematerial until itsnapsalongthe line. Copingsaws canbeusedtocut curves inthinplasticsheet, andjuniorhacksawscanbeusedtocut sections. Scroll saws canalsobeusedfor curvedshapes, butcaremustbetakentoensure that theplastic'dust' does not heat upandre-seal thecut. Leavingtheprotectivefilmonor applyingstickytapetotheareathat istobecut canhelppreventthishappening. Your teacher may useabandsawfor cuttinglarger, moreprecisestraightlinesandshallowcurves. KEYTERMSWastage:Cuttingawaymaterialtoleaveadesiredshape.
204Chapter 11Polymers Alaser cutter canbeusedtocutany2Dshapeoutofacrylicsheet.Itsonlylimitationisthesizeof acrylicsheet themachinecantake. Itcanalsoengravedesignsintoacrylicfordecoration.Several laser cuttershavea'deepengrave' settingthatcanbeusedtocreateshallowchannels.MillingandturningMillingmachinescanbeusedtocutslotsandgroovesinblocksofsuitablepolymermaterials. Theworkpieceisfixedtoatablethatcanmovebackwards,forwards,upanddownunder afixedcutter. Avarietyof differentcutterscanbeused,dependingonthetypeofjob.CNCmillingmachinescanbeusedtocutmorecomplexshapes.Centrelathescanbeusedtomakeroundcomponents. Theworkpieceisheldsecurelyandrotateswhileacuttingtool removesmaterial. CNClathescanbeusedtocutmorecomplexshapes. Vertical millingmachineCuttingtool heldsecurelyinchuckWorktableholdsmaterial tobemilledTraversehandletomovetablesidewaysTiltingheadMillingcuttertocutmaterialCrosstraversehandlemovestablebackandforthKneeelevatinghandleraisesandlowerstableMotor inside Traverse handle moves table sideways Millingcutter tocut material Horizontal millingmachineWorktableholdsmaterial tobemilledHandleraisesandlowerstableFigure 11.20 Millingmachines Figure 11.19 ACNCmillingmachine KEYTERMSCNC: Acomputer- controlledmachineusedinindustry. Figure11.22ACNClathefortraininguseHeadstock spindle Headstock SaddleTool post CompoundslideCross slide TailstockBedLeadscrewApronApronhandwheel ONOFF Figure 11.21Centrelathe
206Chapter 11Polymers KEYTERMSWelding:Afusionofmetalscausedbyintenseheat.HeatweldingHeatweldingcanbecarriedoutonspecificpolymersusingahotairweldingtool.Differentaccessorynozzlescanbeusedfordifferentpolymersandapplications.Atackingnozzleisfirstusedtoholdthematerialsinplacebeforemovingontoanextrusiontoolthatheatsandfeedsathinwireofpolymerintothejoinforapermanentjoin.WeldingrodWelder Welding directionWeldingtipWelding beadFigure 11.26 Heat weldingplastic Ultrasonicweldingissimilar toheatweldingandusesahighfrequencyelectromagneticwavetosoftenthepolymersforjoining. Itismostcommonlyusedonclamshellpackagingandpolymer filmsealing. Polymerscanalsobejoinedusingawiderangeofmechanicalfixings.Theseincludemachinescrewsandbolts, self-tappingscrews, rivetsandvariousspringfastenersandclips.PneumaticPistonConverterSonotrodeHornPlasticmaterialsAnvil Transducer Booster Power supplyFigure11.27UltrasonicweldingKEYTERMSLinebending: Bendinga polymer sheet after softeninganarrowstrip. DeformingandreformingDependingonthescopeof yourworkshop, youwillhaveaccesstoarangeofmachinerythat canbeusedtodeformandreformpolymers. LinebendingLinebendingisaprocessusedtocreatesimplebendsinpolymersheetsuchasacrylicandHIPS. Thesheet material isheatedalongalineusingamachinecalledastripheater.Oncethematerial hassoftenedtheheatercanberemovedandthesheetbenttothedesiredangleusingaformer or bendingjig. Itmustthenbeheldinplaceuntilthepolymerhascooledandbecomehardagain.
20711.4ManipulatingandjoiningClamp Clamp Heldinvice Simple fomer Guide Fixed stop Adjustable stop AdjustableboardFixedboardHingeFigure 11.28 Line bendingHeatedacrylicPlasticpipeClothBlockforviceFigure11.29DrapeformingDrapeformingDrape formingis usedif alargecurveor bendisrequired. Thepolymersheetisusuallyheatedinanoventoensurethewholepieceisheatedevenly. Oncesoftened, itisdrapedover a former anda pieceof clothis pulledtightlyacrossit toholditinshapeuntil thepolymer has cooled. PressmouldingPress mouldingis usedtoproducemorecomplexshapessuchas trays anddishes. It canbeusedonthinpolymer sheet materials suchas acrylicandHIPSandalsoonfoamedpolymers suchas Plastazote®. Press mouldingusestwo-partformers calleda yoke(theupper piece) andaplug(thebottompiece). Thepolymer sheet needs tobeheatedinanoventoensureit is heatedevenly. It is thenpositionedover the plugandpusheddownover it usingtheyoke. Theseare thenclampedtogether until thematerial hascooled. Itis important that theplughas angledsidesandroundededges sothat thefinishedpiececanbeeasilyremovedfromit. Oncethepiecehas cooledyoucanremovetheexcess plastic. VacuumformingVacuumformingis quitesimilar topressmouldingbut it uses a special machine. Theplasticsheet isheatedevenly until soft, andthenair pressureis usedtoshapeit over a mould. Theheatingelement is similar tothegrill ona cooker andis usuallymoveabletoallowaccesstothemachine. Asheet of suitablepolymer (usuallyHIPSbut sometimes acrylic) is fixedacross thetopof themachinebyclamping. This must formanairtight seal. Belowtheplasticsheet intheair chamber is themould. Whenthesheet ishotandsoft theheater is movedout of theway, themouldisraisedandtheair betweenthemouldandtheplasticsheetis evacuatedby anair pump. KEYTERMSMould: Madetotheshaperequiredtobeusedmanytimes. Pressmoulding:Formingahollowshapefromasoftenedpolymersheet.Vacuumforming:Producingthinhollowitemsoverashapedmould.YokePlugGuidepegsFinishedmouldingFigure11.30Pressmoulding
208Chapter 11Polymers Theair pressureontheoutsideof thesheetthenpressestheplasticintoclosecontactwiththemould. Theshapeof themouldmustbecarefullydesignedtoallowtheplasticsheettobeeasilyremovedandthemouldreused. Thesidesmustbeslightlytaperedtoproduceadraft angleandcornersshouldberadiused(roundedoff). Sometimesventholesarerequiredtoallowtheplastictoformintosmall recesses. Vacuum-formedcomponentsalwayshaveashell ofthesamethicknessthroughout.Thismethodof constructionisextensivelyusedinthepackagingindustrywhereitisusedtoformclear plasticintoshapesthatallowaproducttobeviewed.Muchlargermachinesareusedtoformacrylicbaths. STRETCHANDCHALLENGEThefigurebelowshowsdetails of thebaseof alamp. Stateasuitablepolymer for thebaseof theadjustablelampandgivetwopropertiesor characteristicsthat makethematerial suitablefor this use. Describeindetail howthebasewouldbemanufacturedinabatchof 250. Includedetailsof anyjigs and/or formersused. Useannotateddiagrams tosupport your answer. Figure 11.31 Vacuum-formingmachineRoundedcornersDraftangleFlatbaseCounterboredventholesFigure11.33Vacuum-formingmouldClampHeatHeaterRaiseplatenVacuumMouldPlatenPlasticsheetFigure11.32Thevacuum-formingprocessKEYPOINTDespite lack of access toindustrial polymer productionmethods suchas injectionandblowmoulding, polymers canbeusedsuccessfully ina school workshopusinga broadrangeof techniques. Always think about what youcandowitha polymer as youdesignanddevelopyour ideas.
20911.5Structuralintegrity11.5Structural integrityLEARNINGOUTCOMESBy theendof this sectionyoushouldknowaboutandunderstand:➜howpolymers canbereinforcedtowithstandexternal forcesandstresses➜theprocesses that canbeusedtoensurethestructural integrityofaproduct.ThermosettingpolymersThermosettingpolymers produceahardbut brittlematerial. Thiscanbemadestrongerandtougher by usingother materials toreinforceit. Glass-reinforcedplasticGlass-reinforcedplastic(GRP) is thereinforcement of polyester resinusingstrandsofglassfibre. This gives thematerial amuchhigher tensileandcompressivestrength. Itprovidesalight, hard-wearingsurfacewithathinsection, whichhasexcellentresistancetocorrosion. The glass fibreusedis availableas awovenmat or loosestrands. Figure 11.34 Afibreglass product ThermopolymersThermopolymers tendtobemouldedas thinhollowshapes. Theyareusedforawiderangeof products that areexposedtovarious forcesthat test their flexibilityandtoughness. If youlook closely at asimpleproduct suchasadisposablecupyouwill seethatithasbeendesignedtoinclude'lips' and'ridges', whichmaketheshapemorerigidandlesslikelytotwist, flex andultimatelycrack. Injection-mouldedproducts canbestrengthenedbyincreasingtheirwall thickness. Thiswouldleadtohigher productioncosts, however, sotheyarecarefullydesignedtoincludefeatures suchas stiffeningribs that helpreinforceandstrengthentheproductandincrease rigidity. KEYPOINTJustbecausepolymerscanbemouldedintoanyshapedoesn'tmeanthattheyaresuitableinallsituations.Anawarenessofthestressesandstrainsthatapartwillcomeunderwillinfluencethedesignofapartandtheadditionofextramaterialtohelpstrengthenandreinforceit.KEYTERMSReinforce:Tostrengthenorsupportanobjectorsubstancewithadditionalmaterial.
210Chapter 11Polymers Several thermopolymers, suchaslow-densitypolyethylene(LDPE)andpolypropylene(PP),areattackedbyultravioletlightandwill becomebrittleovertime-thisprocessisknownasUVdegradation.Pigmentsanddyescanalsobeaffected, andyouwillhaveseenmanyoutdoorpolymerproductsthathavefadedovertime.TopreventthismanufacturersaddUVstabilisersthatabsorbtheUVradiationanddissipateitaslow-levelheat.Thesechemicalsaresimilartothoseusedinsuncream.Figure11.36Aninjection-mouldedpiecewithstiffeningribsFigure 11.35 AdisposablecupwithridgesandarolledlipKEYTERMSStabiliser: Anadditiveaddedtopolymers tohelpthemwithstandUVdegradation. UVdegradation: The weakeningof polymers whenexposedtothe ultraviolet light insunlight. ACTIVITYVacuum-formabasicshell design, orfindadisposableplasticcup.Useoffcutsofhighimpact polystyrene(HIPS) toaddstiffeningribstoreinforcetheshapeandstopitflexing.ACTIVITYTakeadisposableplasticcupandusescissorstoremovethelipatthetop-howdoesitaffect thestructural integrityofthecup?
21111.6Makingiterativemodels11.6MakingiterativemodelsLEARNINGOUTCOMESBy theendof this sectionyoushouldknowaboutandunderstand:➜theprocesses andtechniques usedtoproduceearlymodelstosupportiterativedesigning. Polymers canbeusedinavarietyof useful andinterestingwaystohelpyouexploreyourearly designideas andtosupport iterativedesigninginschool. Sheet materials areexcellent for developinginitial ideas. Theyarequickandeasytoworkandare not tooexpensive. AcrylicsheetsThese are probably themost commonformof polymer tobefoundinschools. Acrylicsheetcanbe cut by handor machinebut this canrequireagreatdeal of skill andtheedgeswilltake considerabletimetocleanandtidyup. Thisprocessisnotparticularlyuseful forquickiterativemodelling. Usinga suitableCADprogramsuchas 2DDesignandalaser cutterwill resultinmorepreciseanduseful models that areasteponfromandmoreresilientthancard. Thesemodelscanbe usedtoexplorelinkages andother movingparts. Layersof acryliccanalsobebuiltuptoforma morecomplex model for testing. Offcuts of acrylic fromthelaser cutter canoftenbeusedtoquicklyrepresentspecificaspectsona variety of models. Figure 11.37 An iterativemodel madefromlayersof acrylicsheetKEYTERMSIterativemodelling:Repeatedmodellingtodevelopanidea.
212Chapter 11Polymers Abendingstripheatercouldbeusedtocreatesimpleangledpieces, orifmorecomplexshapesarerequiredtheacrylicshapecanbeheatedinanovenandformedaroundasimplewoodenmould. Forevenmorerapidmodelling,small piecesofacryliccanbemanipulatedquicklybyhandwhilewearingwelder'sglovesforprotection.Acrylicsheetcanbequicklygluedtogetherwithdichloromethane(DCM)oranothersuitablepolymerglue, butyoumustensurethatsurfacesarecompletelyflushforthegluetohold. Ahotmeltglueguncanalsobeusedbuttheresultsdonotlookascleanandprecise.Drillingcanbeusedtocreateholesbutyoumustbecareful toensurethatsuitable'plasticbits'areused(seeSection11.4). StyrenesheetsHIPSsheetsarecommonlyusedforvacuumforming.Thesheetscanbemanipulatedquickly,however,whichalsomakesitanidealmaterialforcertaintypesofiterativemodelling. Styrenecanbecut byscoringandthenbendinguntilitsnapsalongtheline.Thismakesitideal for rapidboxconstruction-typemodels. Simplecurvedshapescanalsobeachievedwiththis'scoreandsnap' method. Notethatstyrenemustnotbecutonalasercutter.As withacrylicsheet, styrenecanbebentonalinebender. Dependingonitsthicknessthiswill befaster thanfor acrylicsoextracaremustbetaken. DCMis againthebest methodforgluingstyrenepartstogether.Useasyringeapplicatororthinbrushtoruntheliquidalongthejoint, thenholdthepiecesinforafewseconds.Figure11.39GluingpiecesofHIPStogetherwithDCMDrills canbeusedfor small holesinstyrene; ifthesheetisthinenoughaholecanalsobeused. For larger holesareamingtool canbeusedtoenlargeaholethathasalreadybeendrilled. Figure 11.38Amodel made usinglaser-cut acrylic anda line bender KEYTERMSDichloromethane: Asolvent usedtojoinpolymer pieces together.
21311.6MakingiterativemodelsFoamsheetsFoamsheets areavailableinavarietyof sizesandthicknesses, from2mm-thickA4craftsheets tothicker Plastazote® sheets likethetypeusedincampingmats. Thinfoamcanbeusedtorepresent moreflexiblematerials onamodel, suchastextileelements, paddingorgripdetailing. These sheets canbecut quicklyandeasilyusingscissorsor acraftknifeandcanbegluedusingcraft foam. Acool melt guncanbeusedoncertainpiecesdependingonthesizeandshape. Somecraft foamsheets comewithaself-adhesivebackingandthiscanbeanexcellent way of addinggripdetails tocertainmodels. Youcanmakeyourownself-adhesivepieces by applyingdouble-sidedtapetothebackof sheetsbeforecuttingthem. Figure 11.40 Asimple model madewithfoamsheet Sticky-backedvinyl Sticky-backedvinyl canbecut byhandusingscissorsor acraftknifetocreateavarietyofinterestingshapes that couldbeusedfor decorativeor informativedetailsonyourmodels. Abroadrangeof colours is availableas well asmirrored, metallicandprintabletypesthatcouldbe usedtocreateyour ownsticker designs. Morepreciseandintricateshapescanbedesignedona suitableCADprogramandthencut onaCAMknifecuttingmachine. FoamboardFoamboardis a thinsheet of expandedfoamsandwichedbetweentwolayersofcardboard. It is usually approximately5mmthick, availableinwhiteor blackandinsheetsizesfromA4uptoA1. It is best cut witha sharpcraft knifeandsafetyruler instraightlines, butcurvescanbeachievedwitha littlemorecareandpractice. It canbegluedwithanall-purposeadhesivesuchas UHU, or withagluegunif timeis anissue. Foamboardis oftenusedfor quickbox-likeconstructionssuchasinitial architecturalmodels, andinterestingeffects canbeachievedif youlaminateit withprintoutsoftexturesbeforecuttingit. Figure11.41Afoamboardmodel
21111.6Makingiterativemodels11.6MakingiterativemodelsLEARNINGOUTCOMESBy theendof this sectionyoushouldknowaboutandunderstand:➜theprocesses andtechniques usedtoproduceearlymodelstosupportiterativedesigning. Polymers canbeusedinavarietyof useful andinterestingwaystohelpyouexploreyourearly designideas andtosupport iterativedesigninginschool. Sheet materials areexcellent for developinginitial ideas. Theyarequickandeasytoworkandare not tooexpensive. AcrylicsheetsThese are probably themost commonformof polymer tobefoundinschools. Acrylicsheetcanbe cut by handor machinebut this canrequireagreatdeal of skill andtheedgeswilltake considerabletimetocleanandtidyup. Thisprocessisnotparticularlyuseful forquickiterativemodelling. Usinga suitableCADprogramsuchas 2DDesignandalaser cutterwill resultinmorepreciseanduseful models that areasteponfromandmoreresilientthancard. Thesemodelscanbe usedtoexplorelinkages andother movingparts. Layersof acryliccanalsobebuiltuptoforma morecomplex model for testing. Offcuts of acrylic fromthelaser cutter canoftenbeusedtoquicklyrepresentspecificaspectsona variety of models. Figure 11.37 An iterativemodel madefromlayersof acrylicsheetKEYTERMSIterativemodelling:Repeatedmodellingtodevelopanidea.
21511.6Makingiterativemodels3DprintingThe use of suitable3Dmodellingsoftwareanda3Dprinter canalsobebeneficial wheniterative modelling. Designs canbechangedquicklyandeasilyandthenoutputtoasuitableprinter for testing. Productiontimecanvary, though, dependingonthesizeandcomplexityofthe design. ACTIVITYCreate a model skills boardby makingaseries of small modelsusingeachofthemethodsin this section. STRETCHANDCHALLENGEDesignandmake aninstructionsheet for creatingmodelsusingeachmethodforuselowerdowntheschool.Colouredcard• Colouredcardis suppliedinarangeof colours, shadesandthicknesses. • Card canevenbe bought withdifferent coloursonthefrontandbacksurfaces. • It is suitable for colourful models, especiallytheearlystagesofmakinga series of models. • Agoodexample of cardas amodellingmaterial isinthemanufactureofa prototype boardgame. Theequipment needed: Whatyouneedtodo: Roll thecardintoatubeFixwithtapeAdddouble-sidedtapetotheblackcardTrimofftheexcesswithscissorsPeeloffthedouble-sidedtapeFix theblackcardtothebaseof thetube- dothesamewiththeredstripandstickaroundtheoppositeendCut acircleusingthecirclecutterCutfromtheedgetothecentretomakeashallowcone-fixwith4spotsofhotglueAddsomedetailswiththepen-andyouhaveapostbox!Finishedwithtimetospare?Tryaddingsomedetails(likeahandle)usingthepermanentpen.2 x red card Sellotape Circle cutter Pen Thin strip of red card Thin strip of black cardDouble-sidedtape Figure 11.45 Exampleinstructionsheet for creatingamodel KEYPOINTPolymerscanbemanipulatedquicklyandeasilyandcanbeusedforawidevarietyofiterativemodelling.Becarefulnottoletpre-formedpiecesinfluenceyourdesignchoices,though.
216Chapter 11Polymers 11.7FinishesLEARNINGOUTCOMESBytheendof thissectionyoushouldknowaboutandunderstand:➜theprocessesusedforfinishingplasticsforspecificpurposes.IndustryfinishesPolymersareusuallydescribedasself-finishing, whichmeanstheyrequirenofurtherfinishing. Injection-mouldedparts, forexample, areformedinamouldthatiseithertexturedor highlypolished- t hisfinishisimpartedtoeachproductproducedbythemould.Mostplastics aremanufacturedwithaverygoodsurfacefinish. Certainproducts(suchastoothbrushes)areco-injectionmoulded,whichproducesatwo-colour product combiningahardpolymerwithasofterelastomerforimprovedgrip.Onceaproduct hasbeeninjectionmoulded, thepartmaybesubjectedtooneormorefinishingprocesses: lDegating- i njectionmouldinginvolvesinjectingapressurisedflowofmoltenpolymerthroughachannel systemof runnersandgatesintoamouldcavity.Thegateandtherunner typicallyremainattachedtothepartwhenit'sejectedfromthemould.Degatingis theprocessof removingthisexcessmaterial. lDeflashing- duringmoulding, excessmaterialcalledflashmayleakoutbetweenthemouldcavityhalves. Deflashingistheremoval ofthisexcessmaterial,oftenusingaknifeor other cuttingutensil. lCleaning- polymer partsoftenrequiresomeformofcleaningaftermoulding.Mouldreleaseagentsmayleavearesidue, andgreaseanddirtcanbepickedupfromthemachine. Cleaningisaccomplishedbysprayingordippingpartsinamilddetergentsolutionthenrinsinganddrying. KEYTERMSCo-injectionmoulding: Aninjection-mouldingprocess that uses twodifferent polymers. Flash: Excess material formedbetweenthejoint of a mould. Gate: The entry point for moltenplastic toflowintoa mould. Self-finishing: Amaterial that requires nofurther coatings or finishingprocesses. ColdslugSubrunner SprueMainrunner Gate Product Figure 11.47 The parts of aninjection-mouldedproductFigure11.48FlashonaplasticboxFigure 11.46 Acoinjection-mouldedtoothbrush
21711.7FinisheslDecorating- s omepolymer parts mayrequireapplieddecorationaftermoulding. Thismay includeplating, vacuummetallising, printingandpainting. Figure 11.49 ApaintedactionfigureSchool-basedfinishesInthe school workshopyoumayberequiredtocleanupandpolishtheedgesofacrylic. Thiscanbe doneby first draw-filingtheedges, thenmovingupthroughprogressivelyhighergrades of wet anddry paper. Afinal polishwithabuffer or byhandwithasuitablepolishingcompoundwill bethefinal stage. Whenvacuumforming, onceyouhavetrimmedthepart youmayneedtocleanuptheedges andthis canbedonebyscrapingasteel ruler alongtheedges. Youmight alsohaveaccess toasublimationprinter at school. Thisisaprinterthatusesheattotransfer dyeontosheets of plasticandcouldprint aspecificdesignortexture. ACTIVITYTake a permanent marker andaddadesigntoasheet of HIPS. Tryvacuumformingwithitto see howthe designdeforms aroundvarious moulds. ACTIVITYMake a keyringfromacrylic. Cut ashapeout of acrylicsheet andtrytogettheedgestoamirror finish. KEYPOINTDuetothefactthattheyaremoulded,polymerscanbegivenanytextureforfunctionaloraestheticreasons.Modernprintingtechnologycanalsobeusedtofurtherenhancetheiraestheticappeal.
218Chapter 11Polymers 11.8UsingdigitaldesigntoolsLEARNINGOUTCOMESBytheendof thissectionyoushouldknowaboutandunderstand:➜theuseof 2Dand3Ddigital technologyandtoolstopresent,model,designandmanufacturesolutions. Industryprofessionalsusearangeof digital toolswhenexploringanddevelopingdesignideas that will bemanufacturedusingpolymers. RapidprototypingRapidprototypingisaprocessof makingthree-dimensionalsolidobjectsfromadigitalfile. Thecreationof a3Dprintedobjectisachievedusingadditiveprocesses.Inanadditiveprocess anobject iscreatedbylayingdownsuccessivelayersofmaterialuntiltheobjectiscreated. Eachof theselayerscanbeseenasathinlyslicedhorizontalcross-sectionoftheeventual object. Original part (CAD) PrintedpartFigure11.50RapidprototypingbuildsobjectsupinlayersDigitalmanufactureComputer-aidedmanufacture(CAM)canbeusedinvariousformswithpolymers. StereolithographyThecomponentisbuiltupinlayersontheplatform.Theliquidisaphotopolymer. Whenitisexposedtoultravioletlightfromthelaserbeam, itcuresorsolidifies.Theplatformmovesdownwards, andthesweeperpassesoverthenewlyformedlayer, breakingthesurfacetensionandensuringthataflatsurfaceisproducedforthenextlayer.Subsequentlayersarelaiddown, andbindtogether. Thepartisthenremovedfromthevat. LasersinteringThisisatotallydifferentmanufacturingprocess,inwhichpowderisspreadoveraplatformbyaroller.Thelaserthensintersselectedareas, whichmakesthepowderedpolymermeltandthenharden. KEYTERMSCAM: Computer- aidedmanufacture. Rapidprototyping: The process of makinga 3Dshape fromadigital file. Computer guided laser Liquid resinMovable platformPrototypeTankFigure11.51Stereolithography
21911.8UsingdigitaldesigntoolsFuseddepositionmodellingThe buildmaterial for fuseddepositionmodelling(FDM) isapolymerfilament; thisispassedthrougha heatingelement, meltedandextruded. Eachsliceof themodel isdrawnfromacontinuous lengthof themoltenfilament. Typical buildmaterialsareacrylonitrile-butadiene-styrene (ABS) andpolylacticacid(PLA). Thesearenowcommonlyusedinmostschools. The filament is normally pushed in the heating block. This can present problems with less ridged filaments. Filament spool. Aheater block melts the filament to a usable temperature. Then the heated filament is extruded through the head of the printer. Theextrudedmaterial islaiddownonthemodel whereit is needed. The bed often will move inthex- andy-axes, andthe extruder will move inthe z-axis. Figure 11.52 Fuseddepositionmodelling(FDM) 3Dprinting3Dprintingis thetermusedfor all theaboveprocesses, butitactuallydescribesmachinesthat use a powder-basedprintingsystem. Athinlayer of buildmaterial, typicallyplaster-orstarch-based, is graduallygluedtogether byaprint headthatcanalsocolourthematerialusingregular printer ink. Thefinal product isremovedfromthepowder, cleanedandtheouter surfacestrengthenedbydippinginSuperglue. Figure 11.53 A3Dprinter Figure11.543Dprintedmodels
220Chapter 11Polymers Digitaltechnologies:CAD,CAEandFAEComputer-aideddesignComputer-aideddesign(CAD)softwareapplicationsareusedthroughoutdesignandtechnologytocreateandexploreideas, fromsimple2Dsketchingprogramsthathelpyouquicklyvisualiseideas,toadvanced3Dmodellingsoftwarethatallowsyoutocreatefullyrealisedproducts. Usingcomputerstohelpyoudesignallowsyoutomakequickalterationstocomponents.Itallowsyoutoincorporatecommonlyusedcomponentsfromabuilt-inoronlinelibrary. Youcancollaborateondesignswithotherprofessionalsfromaroundtheworldandrenderproductdesignsinavarietyofdifferentcoloursandmaterials.Computer-aidedengineeringComputer-aidedengineering(CAE)isthebroaduseofcomputersoftwaretoaiddesignengineersinanalysistasks.FiniteelementanalysisFiniteelementanalysis(FEA)allowsyoutomodelproductsinavirtual environmentandtestforweaknesses.Thiscouldincludestressingyourdesigntofindweaknesses.Withpolymerproductsthisisparticularlyusefulforfindingareasthatwouldneedreinforcingwithstiffeningribsorforestablishingareaswherewallthicknesswouldhavetobeincreasedtoinsulateagainstheatfromaninternalmotor.VirtualmouldingSimulationsoftwarecanbeusedtoseehowaliquidpolymerwouldflowintoandaroundamouldcavity,quicklyidentifyingpotentialdesignfaultswithinaproductthatmaycausethemouldtofail.Polymerinjectionpressurecanbemeasuredtodeterminewhethermachinerycouldactuallycope.Coolingtimecanalsobecalculated. Partscanbevirtuallyarrangedtoensureonlyasingle'shot' ofinjectionmouldingisrequired.Changescanbemadetothemodelandthesimulationsquicklyre-runtoestablishtheireffect. Figure 11.55 On-screenmodellingFigure 11.56Injection-mouldingsimulationKEYTERMSCAD: Computer-aideddesign. CAE: Computer-aidedengineering. FEA: Finite element analysis. DraftanalysisFor polymer productstosuccessfullyreleasefromamouldcavity,adraftangleisrequired.Computer simulationcanalsocalculatetheoptimumdraftanglefortheparttoensurerapidproductiontime. KEYPOINTNoother material worksaswell withCADaspolymers.Theabilitytogeneratea3Dmodelandoutput it toa3Dprinterallowsforrapidproductdevelopmentthatmirrorsindustrialpracticelikenoother technique.
22111.9ManufacturingmethodsandscalesofproductionSTRETCHANDCHALLENGECreate a series of designs for anorganicshapedstool usinglayersofcorrugatedcardasyour modellingmaterial. 11.9ManufacturingmethodsandscalesofproductionLEARNINGOUTCOMESBy theendof this sectionyoushouldknowaboutandunderstand:➜themethods usedfor manufacturingat differentscalesofproduction➜manufacturingprocesses usedfor larger scalesofproduction➜methods of ensuringaccuracyandefficiencywhenmanufacturingatlargerscales.ScalesofproductionDecidingona suitablescaleof productionandmanufacturingmethodforyourpolymerparts depends ona number of factors: 1Form: What shapeareyour parts? Thiswill belimitedbytherestrictionsofthemanufacturingprocesses available. 2Budget: Part cost +toolingcost. Somemethodshavehighset-upcosts. Somearequitecheapbut there's usuallyatrade-off. Most high-volumemanufacturingprocessesareexpensivetotool but offer cheapparts- t heoppositeisalsotrue: low-volumeprocessesare cheaptoset upbut parts areexpensive. 3Time: Toolingtakes time, manufacturingtakestime. Moreexpensivetoolingusuallymeans longer set-uptime. Injection-mouldingtoolscantake10-16weekstomanufacture. 4Material: Material choiceis determinedmostlybyform, functionandcost; it'll alsodependonwhat manufacturingtechniqueyouchoose, forexampleifyouwantamelamineformaldehyde(MF) bowl, youwon't beabletoinjectionmouldit.
222Chapter 11Polymers One-off/low-volumepolymerproductionmethodsOne-off productioninvolvesmakingonlyoneorasmallnumberofproducts.GRPlayupThelay-uptechniquefor glassreinforcedplastics(GRPs)involvesacomparativelysimpleprofilemouldof metal, woodorplasterandthefollowingprocesses:1Liquidpolyester resin, mixedwithacatalyst(orhardener),isappliedtothemouldtoformapre-gelledcoat. 2Glassfibreinmat or wovenfabricformislaidonthefirstgelcoatandaliquidpolyesterresin/catalyst mixissprayedonuntil thefibrelayerissaturated.3Whentheresinmixhashardened, themouldingisremovedfromthemould.Curing(setting) cantakeplaceinthecoldorcanbespeededupbyheating.KEYTERMSFabricating: Makingparts by gluing, turning, carvingor welding. GRP: Glass-reinforcedplastic. One-off production: Makingonly oneor a small number of products. Usually usedfor specialist products. ResinMixingheadCatalystVentFibreglassmatMould(closedbeforeinjection)Figure11.58Matched-diemouldingofGRP1. Polish mould 2. Brushonreleaseagent 3. Brushongel coat 5mmthick4. Brush on lay-up r esin 5. “Stipple”glass fibremat intor esin6. Trimmouldingtofinal shapeFigure 11.57GRPlayupFabricatingInfabricating, partsaremadefromeithergluing,turning,carvingorweldingexistingmaterials.CNCmachiningCNCmachinesareanexampleofsubtractivemanufacturing, whichmeansmaterialiscutawayfromablock. Theshapeofthepartspossibleislimitedasinternalstructuresaredifficulttocreate.Figure 11.59TurningnylonrodonacentrelatheGRPmouldingTwoother techniques usedwithGRParetherubber- bagandmatched-diemouldingmethods, inwhichpressureis appliedtothetopsurfaceof themouldingduringprocessing. Various compositionsof polyester resin/catalyst/glass fibreareusedtoproducemouldings inboththesepressurisedprocesses. Byheating, comparativelyfast hardeningof theresinispossible.
22311.9Manufacturingmethodsandscalesofproduction3Dprinting3Dprintingis actually cheaper thanCNCbut cantakelonger. Printingisfromthebottomsosupport structures may needtobeincorporated. Thesewill alsoneedremovingoncethemachine has finishedprinting. SeeSection8.8for moreinformationon3Dprinting. Batch/mediumvolumepolymerproductionmethodsBatchproductioninvolves makingaset number of identical products. VacuumformingSheets of plastic arewarmedandsuckedontoamould. Thereisnoneedforpressureorhightemperatures somoulds canbemadefromcheapmaterials. Theshape, however, isdeterminedby a single-sidedmouldandcanbequitelimited. SeeSection11.4formoredetails onthevacuum-formingprocess. CastingLiquidplastic intheformof aresin(andahardener) ispouredintoamouldandthensolidifies. Moulds arecheapandcanbereusedanumber of times. Partstendtobequitesmall but canbesolid/thick. Rotational mouldingThe rotational mouldingprocess consists of rotatingaheatedmouldcontainingplasticpasteor powder. As theplasticmelts andthemouldrotates, theplasticcoatsthesurfaceofthemouldcavity withanevenlayer of plastic. Themouldisthencooledbeforeopening. Typicalobjects madethis way includefarmtanks, barrels, septictanksandlargehollowtoys. Figure 11.60 Rotational mouldingKEYTERMSBatchproduction:Makingasetnumberofidenticalproducts.Powder Mould Mould charging Heat Two-axis rotationCool Product demouldingFigure11.61Outdoortoysmadeusingrotationalmoulding
224Chapter 11Polymers Mass/highvolumepolymerproductionmethodsMass/highvolumeproductioninvolvesproducingverylargenumbersofproducts.CompressionmouldingCompressionmouldingistheprocessmostoftenusedforshapingthermosettingplastics.Theplastic'mouldingpowder' isheatedandcompressedintoshape.Typicalobjectsincludechildren'stableware, electricplugs, socketsandlightswitches.Upper moveablemouldhalf ChargeLower fixedmouldhalf Ejector pinFigure11.62CompressionmouldingInjectionmouldingInjectionmouldingistheprocesswherebythemoltenplasticisinjectedintoamouldviaaninjectionscrewor ram, cooled, andtheobjectisthenejected.Typicalobjectsproducedthis wayincludemodel kits, audio/videocassettes, bottlecrates,buckets,carbumpers,dashboards, gear wheels, phonecases, plasticchairsandwashing-upbowls.Figure11.63Industrial injection-mouldingmachineKEYTERMSMass/highvolumeproduction: Producingvery largenumbers of products.
22511.9ManufacturingmethodsandscalesofproductionExtrusionblowmouldingExtrusionblowmouldingis theprocess wherebyashort tube of meltedplastic is extrudedandtrappedinamould, thenair is blowninsothat theplastictakestheshape of themouldcavity. Typical objectsincludebottlesmade frompoly(chloro-ethene), PVC, poly(propene), PPor highdensity poly(ethane), HDPE, petrol tanks, drums. MouldRamHeatingchamberHopperShotchamberFeedscrewFigure11.64Aninjection-mouldingmachineThermoformingThermoforming, or sheet moulding, is aprocessusedfor vacuumformingformassproduction, whereby anextrudedsheet of plasticisheatedandthenshapedbypressureand/or by vacuum. Typical objects includechocolateboxtrays, refrigeratorlinings, packagingtrays, vendingcups, groups of yoghurt pots, bathsandacrylicsinks. Air escapevPositivemouldHeatedplastic sheet Negative mould Figure 11.66 The principleof thermoformingExtrusionContinuous extrusionis theprocess wherebymoltenplasticispushedcontinuouslythrougha shapedhole(profileor die) beforebeingcooled. Typical objectsincludecurtaintrack, drainpipes, gardenhose, guttering, rods, rulers, sheets, tubesandpipes, unplasticisedpoly(chloro-ethene), uPVCwindowframes. Theshapeof thediedetermineswhetherasolidrod, hollowpipe or plastic sheet is made. HopperDieHeatingchamberExtrudedtubeMould Air inFeedscrewFigure 11.65 Diagramof acombinedextrusion/blow-mouldingmachineFigure11.67Thermoformingmouldusedforplasticcups
226Chapter 11Polymers Sheathextrusionisusedfor makingcables. Electricwiresarefedthroughanextrusionlinetogether withtheplastictheyaregoingtobecoatedwith. Typicalobjectsincludeelectricandoptical cables. CalenderingSuitablethermoplasticcompositionsarepassedthroughheatedmetalrollerswithprogressivelysmaller gapstoproducecontinuousfilmandprecisionthinsheet.Themethodis usedtoproducepoly(chloro-ethene), PVCflexiblefilminwidthsofupto4m,thinPVCandpoly(phenylethene), andPSrigidfoilsforuseinthermoformingprocessesormakingsheetmaterial. Embossingtechniquescanalsobeincorporatedintotherollingprocess.Feed hopper Plastic pellets Heaters ThermocouplesShapingdieTurningscrewBarrel MoltenplasticExtrudateMotor StructuralpartsSheetandfilmTubingandpipesFigure 11.68Diagramof theextrusionprocessRollsaresubsequentlycooledastheypassthroughthecoolingcylindersFinishing rolls determine the appearance of the rolls Figure 11.69 Diagramof thecalenderingprocess
22711.10CostandavailabilityBlownfilmextrusionTubular sheet or sheathextrusionis theprocesswherebyextrudedplasticisexpandedintoa sheathandthenwoundontoreels. Typical objectsincludepackagingfilms, 'plastic'bags, greenhousecovers. Niprolls CollapsingframeBlownfilmlineBubbleAir ringDieIdlerroll Idlerroll Roll offilmFigure 11.70 Diagramof blownfilmextrusionKEYPOINTThe choice of productionmethodmust bebasedonthetypeofproductandthequantityto be made. STRETCHANDCHALLENGECreate a reference tableof theprocesses inthissection. Usethefollowingheadings:lMaterial category lProcess lScale of productionlExample products. 11.10CostandavailabilityLEARNINGOUTCOMESBy theendof this sectionyoushouldknowaboutandunderstand:➜howthecost andavailabilityof specificpolymersandcomponentscanaffectdecisions whendesigning. ThesignificanceofcostDuringthe designanddevelopment of anewproduct therearemanyfactorsthatwillinfluence your choiceof materials. This sectionlooksat howthecostandavailabilityofpolymers canaffect your material choices.
228Chapter 11Polymers RawmaterialsManyof thepolymersdescribedinthischapteraretheonesyouaremostlikelytocomeacross asadesigner, andtheyall costroughlythesame. Manufacturingthemrequirescrudeoil sotherisingpriceof oil affectsthepriceofneworvirginpolymerpellets.Costs canbecontrolledslightlybyusingrecycledplastic, whichisaround25percentcheaper. Manymanufacturersaretryingtoincreasetheamountofrecycledpolymerintheproductstheyproduce. Four-fifthsof all polymer productsmanufacturedendupinlandfill,sotheeconomicsuccessor failureof usingpolymersreliesonthecostoftherawmaterials(petroleumandnaturalgas) usedtomakevirginpolymersandthecostofrecyclingversusdisposal.Thecostofrecyclingcanvaryfromcitytocityandcountrytocountry. Newandexcitingpolymersarebeingdevelopedallthetime,andhigh-performancepolymerscost morethanregularpolymers. Theseinclude: lPolyacetals- usedinmedical devicesandmechanicalgears.lKevlar - af ibreusedinbulletproofvests. lPolyetheretherketone(PEEK) -averyhard-wearingandlong-lastingpolymerusedintheaerospaceindustry; oneof themostexpensivecommercialpolymers.Your choiceof polymer will affectyourdesigndevelopmentandtheviabilityofyourdesign.RespondingtostakeholderneedsAkeystrengthof usingapolymerinyourproductistheabilitytofinetuneitspropertiesindirect responsetowhatyourstakeholdersrequire. Youhaveseenthatadditivescanbeintroducedtopolymerstoproduceaspecificresult. Withregardtostakeholders,thefollowingmayoccur: lPlasticisersmayberequestedbythedevelopers/manufacturerstoensurethepolymerflowsquicklyandefficientlyintoamouldtokeepcycletimestoaminimum.lIncreasingtheamountof recycledpolymermayberequiredbymanagerswhowanttoreducecosts. lPigmentsmayberequiredbyusers, togivethemmorechoiceofcoloursinthefinalproduct. ManufacturingThemost commonmethodsof polymerproductionareinjectionmoulding,blowmouldingandthermoforming. Theprocessyouchoosedependsonanumberoffactors,includingthepart sizeandcomplexity, thematerial selected, howmanyyouwant,howmuchyouwanteachpart tocost, andhowsoonyouwantit. lInjectionmouldingiscomplexandrequiresaspecificmouldtobemade,andforthatreasonit cancost more. Thevolumeofpartsproducedishigh,though,socostscanbespreadover thetotal numberof partsmade. lBlowmouldingissimplerthaninjectionmouldingandisgenerallylessexpensive.lThermoformingisasingle-sidedprocessandsimplerthanblowmoulding.Itisusedforlower-volumeproductionandofferslowertoolingcosts. ExampleIf youwantedtoproduce1000washersperyear,youwoulduseasinglecavitymould, meaningitwouldmakeonewasherpermachinecycle.Inthat case, themouldwouldprobablycost£1000-£2000.Ontheotherhand,if youaregoingtoneed100,000Xboxcontrollerseverymonth,thenyouwouldneeda12-cavityhardened'family'mould,whichwouldmakefourfronts, four backsandfoursetsofbuttonseverycycle.Thiswouldcostyouaround£70,000to£80,000. KEYTERMSStakeholder: Apersonwithaninterest inthe success of a product. Viability: The ability towork successfully. Figure 11.71 Industrial injection moulds cost thousands of pounds
22911.10CostandavailabilityCalculatingquantities,costsandsizesof materialsWorkingwiththecommonlyavailableformsof polymersseeninsection8.3 will requireyoutomakeavarietyof calculations. Sheet material suchas acryliccanbeusedfor linebendinganditmaybenecessary tocalculatetheoverall sizes neededfor apiece. Theexamplebelowis a menuholder for acafé. What size pieceof clear acrylicwouldbeneeded?Theanswer is200mmx750mm. ExplanationTocalculate theoverall lengthof thesinglepieceof acrylicneededyouwouldaddeachmeasurement: 100+50+300+300=750mm(theextra300mmis needtocreatethebendat thebacktoholdthemenuinplace)Therefore, theoverall dimensions of thesinglepieceof acrylicneededwouldbe 200mmx750mm. Usinga lasercutter oftenrequires youto'nest' your itemstogether inthesmallest spacepossibletominimisewaste. For example, imagineyouhaveadesignfor apenmadefromlayeringtwopieces of lasercut acrylictogether liketheexample below. The overall sizeof eachpieceis 50mmx100mm. The laser cutter cantakesheets of material upto600mmx300mm. Howmany pens couldyoumakefromasinglesheet?The answer wouldbe18pens. ExplanationThe widthof eachpieceis 50mmsoyoucancalculatethatyouwouldfitarowoftwelvebydividingthelengthof material inthelaser cutter (600mm) bythewidthofeachpenpiece(50mm) 600dividedby50is12. The lengthof eachpieceis 100mmsoyoucancalculatethatyouwouldfitcolumnsofthreeby dividingthewidthof thematerial (300mm) bythelengthof eachpenpiece(100mm)300dividedby100is 3. Multiplyingtherows by thecolumns will giveyou: 3x12=36pieces. Andfinally eachpenis madeupof twohalvesso36dividedby2is18.If a sheet of acrylic (600mmx300mm) costs£4.95, eachpenwouldcost27.5ptomake. (£4.95 dividedby 18) ACTIVITYBased onthe measurements of themenuholder abovehowmanypieceswouldyougetfroma sheet of acrylic measuring1000mmx500mm?Howmany sheets wouldyouneedtomake50menuholders?If a sheet of 3mmclear acrylic 1000mmx500mmcosts£9.85. Howmuchwouldeachmenu holder cost tomake?Could youcalculate howmuchwastageyouhaveleft at theend?30020010050
230Chapter 11Polymers CalculatingthecostofplasticproductsThereareseveral onlinecalculatorsthatcanhelpcustomersestablisharoughestimateforseveral polymer productionprocesses, andespeciallyforinjectionmoulding.Thecost typicallydependsonthecostofthemouldrequired,whichdependsonthesizeof theproduct, thewall thicknessandhowmanyproductsyouwanttocomeoutofeachinjection. For example, asingleproductroughly101cmwithaweightof10gwouldcostapproximately£2000justforthemould. Itwouldcostanother£100forthepolypropylenepellets androughly£400for thelabour/energytogiveyouapproximately2500pieces.Eachpiecewouldcost £1tomake, anditwouldtakearoundsixdaystoproduce2500pieces(ifthemachinewasproducingoneitemaminutethrougharegularworkingday).Table11.4Howthecostof makinggoesdownasthenumberofpartsmadeincreasesNumber of partsmade10002000500010,000Fixedcost (£) 2000200020002000Variablecost (£) @50pperpart500100025005000Total cost (£) 2500300045007000Cost per part (£) 2.501.500.900.70Thefixedcost isthecost of makingthetoolsandfittingthemintothemachine.Thevariablecost is thecost of actuallymakingtheparts, includingtherawmaterial,labourandenergycosts. ControllingthecostatthedesignstageInjection-mouldedpartscanbecarefullydesignedusingCADandchangescanbemadetofeatures, suchaswall thicknessandthenumberofcomponentsformedinasingleinjection.Thesecanall contributetoestablishingtheviabilityofapolymerpartbyloweringcostswherever possible. Figure11.72Makingchangesatthedesignstage
21511.6Makingiterativemodels3DprintingThe use of suitable3Dmodellingsoftwareanda3Dprinter canalsobebeneficial wheniterative modelling. Designs canbechangedquicklyandeasilyandthenoutputtoasuitableprinter for testing. Productiontimecanvary, though, dependingonthesizeandcomplexityofthe design. ACTIVITYCreate a model skills boardby makingaseries of small modelsusingeachofthemethodsin this section. STRETCHANDCHALLENGEDesignandmake aninstructionsheet for creatingmodelsusingeachmethodforuselowerdowntheschool.Colouredcard• Colouredcardis suppliedinarangeof colours, shadesandthicknesses. • Card canevenbe bought withdifferent coloursonthefrontandbacksurfaces. • It is suitable for colourful models, especiallytheearlystagesofmakinga series of models. • Agoodexample of cardas amodellingmaterial isinthemanufactureofa prototype boardgame. Theequipment needed: Whatyouneedtodo: Roll thecardintoatubeFixwithtapeAdddouble-sidedtapetotheblackcardTrimofftheexcesswithscissorsPeeloffthedouble-sidedtapeFix theblackcardtothebaseof thetube- dothesamewiththeredstripandstickaroundtheoppositeendCut acircleusingthecirclecutterCutfromtheedgetothecentretomakeashallowcone-fixwith4spotsofhotglueAddsomedetailswiththepen-andyouhaveapostbox!Finishedwithtimetospare?Tryaddingsomedetails(likeahandle)usingthepermanentpen.2 x red card Sellotape Circle cutter Pen Thin strip of red card Thin strip of black cardDouble-sidedtape Figure 11.45 Exampleinstructionsheet for creatingamodel KEYPOINTPolymerscanbemanipulatedquicklyandeasilyandcanbeusedforawidevarietyofiterativemodelling.Becarefulnottoletpre-formedpiecesinfluenceyourdesignchoices,though.
232Chapter 12Fibres andfabrics CHAPTER12FibresandfabricsYoulearnt about thedifferenttypesoffibresandfabricsinSection5.1.Thischapterincludesmorein-depthinformationonfibresandfabrics. 12.1PhysicalandworkingpropertiesLEARNINGOUTCOMESBytheendof thissectionyoushouldknowaboutandunderstandthephysicalandworkingpropertiesofarangeofnatural andsyntheticfibres,including:➜howeasytheyaretoworkwith➜howwell theyfulfil therequiredfunctionsofproductsindifferentcontexts.PhysicalandworkingpropertiesoffibresEachnatural andsyntheticfibrecomeswitharangeofproperties-suchasabsorbency,stain-resistance, crease-resistanceandinsulation-thatcanbeputtogooduseinthedesignof newproducts. Afibre'spropertieswill directlyaffecttheenduseofaproduct.Adesignerwillworktoafabricspecificationandconsiderthephysical andworkingpropertiesoffibresandfabricscarefully, beforeselectinganappropriatematerial touse. Thisisimportantinmakingsurethefinishedproduct will functioncorrectly. Thechart belowshowswhatabathtowel needstodo, andwhatfabricpropertiesarethereforerequired. Table12.1ThefabricpropertiesrequiredforbathtowelsWhat doesit needtodo?PropertiesrequiredComfortabletohandleGoodhandleSoakupmoisturetoaiddryingAbsorbentKeepus warmInsulatingBe easy tocarefor Crease-resistantWithstandregular useDurableFigure 12.1 The fibres in bath towels areselectedfor their workingproperties. KEYTERMSAbsorbency: Theability of a fibretosuck upmoisture. This canbe beneficial inproducts like towels. Anabsorbent fibre, however, is alsomoreprone tostaining. Crease-resistance: Afibre's ability torecover after beingwrinkled. Ingeneral, natural cellulosefibres, like cotton, havelowcrease-resistanceandnatural proteinandsynthetic fibres havehighcrease-resistance. Fabric specification: Alist of guidelines usedby designers toensure the correct fabric is usedfor aproduct. The list will include the key fabric properties required, as well as guidanceoncolour, textureandpattern. Function: What aproduct will doandhowit will work. Handle: The level of comfort against the skin. Silk has anexcellent handle, as it has a soft andsmoothtexture. Coarse wool does not have agoodhandle, as its texturecanbe roughandirritating.
23312.1PhysicalandworkingpropertiesNatural fibresNatural fibres areproducedfor useinanextensiverangeof textileapplications. Theyarechosenspecifically for their physical andworkingproperties. Ingeneral, all naturalfibresare absorbant. Table 12.2 The specific physical andworkingpropertiesof arangeofnatural fibresFibre Properties/workingcharacteristicsUsesCotton Strong Cool to wear Creases easily Easy to handle andsewWill not stretchBurns easily Medical dressingsT-shirtsSocksDenimjeansCosmeticpadsNappiesBedsheetsUpholsteryCanvasCar tyrecordsFishingnetsLinen (flax) Strong Cool to wear Natural lustre (shine) Creases easily Easy to handle andsewWill not stretchBurns easily BedsheetsTablecoveringsTentsSkirtsSuitsUpholsteryCanvasWallpaper BanknotesWool Strong WarmCrease-resistant Shrinks easily Difficult tohandle andsewStretches easily Some flame-resistanceCoatsJumpersSportswear BlanketsSocksInsulationSound-proofingSnooker tablesCarpetingSilk Good handle Good insulator (cool insummer andwarmin winter) High natural lustre (shine) Crease-resistant Difficult tohandle andsewLowstretch(canbestretchedout of shape) Burns slowly Eveningwear TiesHandkerchiefsBedsheetsMedical dressingsParachutesSutures(stitches) Wall coveringsKEYPOINTAllnaturalfibressharealotofphysicalandworkingproperties.Forexample,theyareallabsorbent.Makesureyouknowthefewdifferencesbetweenthefibreswhenyourevisethistopic.KEYTERMSPhysicalandworkingproperties:Thepropertiesofafibre,suchasabsorbency,strengthorresistancetochemicalsthataffectthewaythefibreisused.
234Chapter 12Fibres andfabrics SyntheticfibresSyntheticfibreshaveadiverserangeofproperties. Thesepolymer-basedfibrescanalsobeengineeredtomimicthepropertiesofnaturallyoccurringfibres,makingthemextremelyversatile. Table12.3Thephysical andworkingpropertiesofarangeofsyntheticfibresFibreProperties/workingcharacteristicsUsesPolyester Non-absorbent StrongGoodhandlePoor insulator DurableCrease-resistant Will not stretchMeltseasilyClothingPillowfillingUpholsterypaddingBeddingCarpetingThreadRopesBoatsailsNylon(polyamide) Non-absorbent Abrasion-resistant VerystrongSomeelasticityDurableResistant tochemicalsandperspirationSeatbeltsTentsParachutesRucksacksShoelacesToothbrushbristlesUmbrellasLifejacketsTightsUnderwearCarpetingAcrylicWater-resistantQuick-dryingStrongGoodinsulator Resistant tochemicalsandperspirationFleeceSkijacketsBlanketsRugsOutdoorfurnitureKnitwearCleaningclothsViscose(rayon) Absorbent Goodinsulator CreaseseasilyWill not stretchWeakfibre, particularlywhenwetBlousesSportswearShirtsBlanketsCurtainsTableclothsUpholsteryElastaneNon-absorbent Excellent elasticityResistant tochemicalsandperspirationQuick-dryingSwimwearSportswearDenimjeansLeggingsTightsKEYTERMSWater-resistance: The ability of a fabric toresist water droplets for a periodof time, thoughthey will eventually soak intothe fibre. Awaterproof fibre resists water droplets indefinitely. Inrecent years, super- hydrophobic coatings have beendevelopedthat actually repel water.
23512.1PhysicalandworkingpropertiesMixed/blendedfibresAs we learnt inChapter 5sometimes fibresaremixedor blendedtogether, togiveafibrethat benefits fromtheproperties of morethanonefibre. Thispracticeisverycommonanditcanimprovethefunction, aestheticvalueandcost of thefinal product. Figure 12.2 Bales of rawfibre, prior toprocessingandblendingTable 12.4 Some commonfibreblends andtheir physical andworkingpropertiesMix/blend Benefits fromfibre1Benefitsfromfibre21 Polyester / 2 CottonStrength, durability, crease-resistance, stain-resistance, lowcost Handle, absorbency, cool towear,strength, drape1 Cotton / 2 ElastaneHandle, absorbency, cool towear, strength, drape Crease-resistance, stretch, snap-back,flexibility1 Acrylic / 2 Wool Quick-drying, strength, resistancetochemicals, lowcost Insulation, crease-resistance,absorbency, drapeSTRETCHANDCHALLENGEComplete the followingactivities for eachof theproductsinthelistbelow.1 List the key fibreproperties neededtomaketheproduct suitableforuse.2 Basedontheir properties, suggest asuitableblendof fibresfortheproduct.lSki jacket lRucksack lDeckchair lChild's sleepsuit lGardenparasol KEYTERMSBlending:Mixingfibresofdifferentoriginstogetherinordertoimprovethepropertiesofthefinishedyarn.Drape:Thewayafabrichangsunderitsownweight.Chiffonisanexampleofamaterialwithexcellentdrape;feltisamaterialthatdoesnotdrapewell,duetothestiffnessofitsstructure.
236Chapter 12Fibres andfabrics 12.2SourcesandoriginsLEARNINGOUTCOMESBytheendof thissectionyoushouldknowaboutandunderstand:➜thesourcesandoriginsofarangeoffibres➜theprocessesusedtoproducefibres,yarnsandfabrics➜theecological, social andethical issuesassociatedwiththeproductionoffibresandfabrics➜thelifecycleofnatural andsyntheticfibres➜recycling, reuseanddisposal ofnatural andsyntheticfibresandfabrics.OriginsofnaturalfibresNatural fibrescanbeclassifiedintotwocategories: lnatural proteinlnatural cellulose. Figure12.4Natural plantfibrescontaincellulose.Figure 12.3 Keratinproteinchains fromhair or fur fibres. Natural proteinfibrescomefromtheanimal kingdomintheformofhair,furorexcretions.Hair andfur fibresconsistof keratinproteins, muchlikehumanhair.Silkfibreisexcretedbysilkwormsandconsistsmostlyof fibroinproteins, whichgivesilkitsuniqueproperties.Natural cellulosefibrescomefromplant-basedsourcesandaremadeupofanaturalpolymer calledcellulose. Naturalpolymersarechainsofsinglemolecules,andconsistofspecificsugars, suchasglucosemolecules. Cellulosefibresarefoundintheseeds,stemsandleaves of arangeof plants. Celluloseisalsotheprimarysubstanceinwood.KEYTERMSNatural cellulosefibres: Fibres that come fromplant-basedsources, for examplecottonandlinen. Natural proteinfibres: Fibres that come fromanimal- basedsources. Theseinclude hair, fur or silkfibres. Natural polymers: Chains of proteinor cellulose molecules (monomers), suchas keratin, glucoseor fibroin. These chains are the basis of all natural fibres.
23712.2SourcesandoriginsTable 12.5 Other natural fibres Other natural proteinfibres Other natural cellulosefibresAlpaca Camel Mohair Cashmere Angora RamieCoir HempJuteSisal Natural protein Natural celluloseNatural fibres Wool fleece is shearedby highly skilledworkersCottonfibreshownintheseedpod(boll)Pure silk fibre surrounds thecocoonof thebombyx mori mothLinencomesfromthestemoftheflaxplantFigure 12.5 The types of natural fibres withsomeexamples
238Chapter 12Fibres andfabrics OriginsofsyntheticfibresSyntheticfibresarealsomadeupof polymerchains. Thesechainsareformedbypolymerisinghydrocarbonmonomers(singlemoleculesthatbondtogether)thathavebeenextractedfrompetrochemicalssuchascoal andoil. Becausetheyaresourcedfromfiniteresources, syntheticfibresarenotsustainableandcausesignificantecologicaldamage.Natural polymers(cellulose) canbechemicallyextractedfromplant-basedsources,andprocessedtoformadifferenttypeof syntheticfibre, knownasaregeneratedfibre.Thesefibres aremoresustainableandsharesomeofthesamepropertiesasnaturalfibres.Theprocessof polymerisationcanbeadaptedtoproducefibreswithspecificproperties- t hesearecalledtechnical textiles. AnexampleofatechnicaltextileisKevlar®.Intheproductionof thisabrasion-resistantfibre, thelongpolymerchainsarejoinedtogetherinparallel rows, formingextremelystrongbonds. Thetablebelowlistssomecommonsyntheticandregeneratedfibres.Table12.6CommonsyntheticandregeneratedfibresSyntheticfibresfrompetrochemicalsRegeneratedfibresfromcellulosePolyester ViscoseNylonAcetateAcrylic ElastaneFigure 12.6 Synthetic polymer pellets areheatedandextrudedtoformfilament fibres. Figure12.7Celluloseextractedfromwoodpulpisusedtocreateregeneratedfibres.KEYTERMSFiniteresources: Non-renewablesources that cannot bereplacedina sufficient timeframetoallowfurther humanconsumption. Examples includecrudeoil andnatural gasandcoal, as theseresources havetakenmillionsof yearstoform. Regeneratedfibres: Fibres derivedfromnatural cellulose sources, usingsyntheticchemical processes. These include viscoseandbamboofibre. Sustainable: Renewablesources that canbereplacedina sufficient timeframetoallowcontinuedhumanconsumption.Theseresourcesincludecotton,linenandwool.Theprocessesusedtocultivateandextractthesefibresmustbeconsideredwhendecidinghowsustainablearesourceis.Technicaltextiles:Textilesmanufacturedspecificallyfortheirperformancepropertiesinsteadoftheiraestheticvalue.ExamplesoftechnicaltextilesincludeKevlarandStomatex®.
23912.2SourcesandoriginsExtractionandconversion-naturalfibresCottonBrazil, Pakistan, Turkey andtheUSAaretheworld'sleadingproducersofcottoncrops. Cottonis intensively farmedacross hugeareas of land, usingfertilisers, pesticidesandlargeamountsof water. Figure 12.8 Thousands of hectares of landareusedfor cotton crops. Figure12.9Machineryharvestingcottonbolls.The fibre is harvestedfromtheseedpod(boll) of thecottonplant. Asinglematurecottonplant canproducearoundahundredcottonbollsannually. Thisroughlyequatestoonebale(225kg). Toput this intoperspective, it takesaround1kgof cottontomakeasinglepairofjeans. Over 1billionpairs of jeans areproducedworldwide, eachyear. The cottonharvest begins inJulyandcontinuesuntil November. Cottonpickingcanbecompletedby handor machine, dependingonthescaleof productiononthefarm. Once picked, thecottongoes throughtheginningprocess, whichseparatesthefibrefromthe seed. Rawcottonbales aredriedtoremoveanymoisturecontentthatcouldcausefibrestoclump. Thedry fibreis passedthroughaginstand, wherecircularsawsareusedtoremovethe fibre fromtheseed. Thecottonis baledupandshippedtoadifferentfactoryforfurtherprocessing. Most cottonis usedtoproduceyarns for themanufactureof fabric. Forthis, itmustbecleanedandbleachedtoachieveconsistencyinthefinishedproduct. Thecottonfibregoesthroughseveral stages of cleaningbeforeit canbespunintoyarn. LinenCanada, Russia andFrancearetheworld'sleadingproducersof flax, whichiswherethelinenfibre is found. Theseplants flourishincooler climates, wheretheyareharvestedannually. Thefibre is foundinthestemof theflaxplant, andthisisalsoknownasabastfibre.
240Chapter 12Fibres andfabrics Toharvest thelinen, thestemsareuprootedusingmachinerythatpullsupthestem,thenlies theplant onitsside. Itisimportantthattheplantsarenotcut,asthisremovesthenaturalsapinthestemandaffectsthequalityofthelinen. Onceuprooted,thestemsareretted.Therettingprocessinvolvesleavingthestemsoutintheelementstorotawaythetough,outer layer of thestem, andtodrythemout. Oncerettingiscompletethestemscanbebrokenandthefibrescanberemovedfrominside. Thesefibresthengothroughaseriesofprocessestocleanandpreparethemformanufactureintoyarn.WoolWoolproductioniscenturiesoldandcanbetracedbackto4000bc.Australia,NewZealandandtheUSAaretheworld'slargestproducersofwool. Theprocessingofwoolfibrehasremainedunchangedthroughhistory,buttheuseofmoderntechnologymeanswecannowprocessover2milliontonnesofwoolannually.Theprocessbeginsinspring,whenthemajorityofsheeparesheared.Thishighlyskilledprocessinvolvestheuseofclipperstoremovethewholefleecefromthesheep.Workersattempttodothisquickly,tocauseaslittledistressaspossibletotheanimal.Fleecesareseparatedandgraded,asasinglefleecewillyielddifferenttypesofwool.Theouterwoolonthefleeceisthickerandcoarser,whichmakesitmoresuitableforcarpetorinsulation. Meanwhilethewoolontheinside,nexttotheskinofthesheep,isfinerandsofter.Thiswoolismoresuitableforclothingsuchassuitsorjumpers.Woolisnaturallygreasy.Itcontainsanaturaloilcalledlanolin, whichmustberemovedbeforethewoolfibrecanbeused.Aprocesscalledscouringisusedtoremovethewoolgrease.Thisinvolvesaseriesofbathsthatusewarmwateranddetergents.Thewastelanolinisoftenusedinskincareproducts.Figure 12.10 Flax plants areuprootedrather thancut toFigure12.11Rawlinenfibremustbetreatedbeforeuse.preserve their quality. Figure 12.12 Wool comes fromawidevarietyof animals, includingllamas andalpacas.
24112.2SourcesandoriginsThe wool is cleaned, driedandstored, readyfor further processing. Figure 12.13 Rawwool fibremust becleanedthoroughlybeforefurtherprocessing.SilkChina is the world's leadingproducer of silk, andsilkproductiontheredates backtotheNeolithicperiod. Silkisproducedby thesilkworm, whichis actuallythecaterpillar of the Bombyx mori moth. Figure 12.14 The Bombyx mori mothFigure12.15Boiledcocoonsareunravelledtoremovethesilkfibre. Silkworms arebredincaptivityandarefedadiet of mulberryleavesthroughouttheirshortlives. As it grows, thesilkwormmoults several times. After thefourthmoult, thewormencases itself ina cocoonof silkfibreandbeginstopupate. Itisatthispointthatthecocoonsare boiled, inorder tokill thepupainside. Theprocessof boilingalsohelpstoremovetheseracin, a sticky gumthat helps thesilkfibressticktogether whenproducingthecocoon. The cocoonmust not becut, inorder toretainthelengthof thesilkfibre.
242Chapter 12Fibres andfabrics Workerspull thesilkfibre, whichunravelsoffthecocoon, andwinditontobobbins.Thesilkfibreis thencleanedanddriedinpreparationforfurtherprocessing.Themanufactureofsilkis alabour-intensiveprocess, makingitoneofthemostexpensivefibresavailable.Recent innovationsintheproductionofmoresustainablenaturalfibreshaveledtothedevelopment of moreunusual fibresources. Theseincludesouredmilk,fermentedwine,spidersilk, cornhusksandlab-grownbacteria. Extractionandconversion-syntheticfibresSyntheticfibresareall producedusingextrusionprocesses. Manufacturersbeginwithpolymersolutionor meltedpelletsof polyesterorasimilarsyntheticfibre.Thesolutionisplacedinatankaboveaspinneret. Thespinneretisametal extrusionplatethatcontainsmanytinyholes,andactslikethespinneretthataspiderusestoproduceitsweb.Thesolutionisforcedthroughtheholesinthespinneretusingairpressure,orasyringe-likesystem, formingthesolutionintolongindividual fibres. Meltedpolymer fibresrequirecool airtosetthemastheyemergefromthespinneret.Regeneratedfibresrequireachemical solutiontosetthefibresastheyexitthespinneret.Thespinneretsarethenplacedinachemical bathandthefinishedfibreisthoroughlycleanedoff thembeforetheyareusedagaintoensurenotraceofchemicalsremains.Oncetheyareset anddried, thesyntheticfibresarewoundontobobbinsreadyforfurtherprocessing. Polymer pellets Hopper Extruder (heats thepellets, meltingthem) Closeupof spinneret headSpinneret Cooling air Finishes applied here Godets Glides WindupontospoolsFigure 12.16 The melt spinningprocess CellulosepolymersolutiontankPumpSpinneretGuidesAcidbathStretchingWashingWaterbathWarmairWindupontospoolsFigure12.17Thewetspinningprocess
244Chapter 12Fibres andfabrics ACTIVITYYouwill need: aball of cottonwool. Aball of cottonwool isessentiallyamassofcottonfibres.1Lookcloselyat thesurfaceofthecottonwool -youshouldseetheendsofhundredsoftinyfibres. Usetweezerstograbandpull outasinglefibre.Examineanddescribethefibre.2Pinchthecottonwool andgentlypull ittostretchitoutintoalongthinstrip.Atthispoint, thefibresshouldcomeaparteasily.3Start totwist thestripoffibres. Dotheystill comeaparteasily?Whathappenswhenyouaddmoretwist?Extension: Usetweezerstoremovesinglefibresfromarangeofdifferentyarns.Howdotheydiffer?Woven,non-wovenandknittedtextilesFabrics areconstructedusingarangeofdifferentmethods, dependingontheirintendeduse. Theoriginal methodsof construction, suchasweavingonwoodenlooms,havebeendevelopedandarenowmostlycompletedonindustrialmachinery,althoughtraditionalmethodsarestill favouredbydesignersforcreatingsmallcollectionsorone-offpieces.WovenfabricconstructionWovenfabricsareproducedusingwarpandweftyarns. Thewarpyarnisusuallythestronger of thetwo. Warpyarnsrunverticallydownthelengthofthefabric,whichisalsoknownasthestraight grain. Thestraightgrainiseasilyfoundbyusingtheselvedge,asthis alwaysrunsparallel tothestraightgrain. Theselvedgeisthefactory-finishededgeofthefabricroll. Theselvedgepreventsthefabricfromfrayingorunravelling.Weftyarnsrunhorizontallyacrossthefabric, alsoknownasthecrossgrain. FinishedyarniswoundontospoolsFinishedyarniswoundontospoolsRawfibre enters machine Cardingmachinecombs fibres Webof parallel fibresCut intostripscalledsliverSliverisstretchoutintorovingsRovingsarestretchedeventhinnerRovingsaretwistedonaspinningframetogivethemstrengthFigure 12.20Process for manufacturingspunyarns KEYTERMSWarp: The warpyarnis the stronger yarnina fabric constructionandruns vertically downthe lengthof the fabric. This is alsoknownas the straight grain. Weft: The weft yarnis wovenunder andover the warpyarnandruns horizontally, across the lengthof the fabric. This is alsoknownas the cross grain.