PlasticProcessing.docx

PlasticProcessing.docx

《PlasticProcessing.docx》由会员分享，可在线阅读，更多相关《PlasticProcessing.docx（13页珍藏版）》请在冰豆网上搜索。

PlasticProcessing

PlasticProcessing

Polymersarenewmaterialsthathavebeenextensivelyusedinengineering,startingfromthenineteenthcentury.Amonomeristhesinglebuildingblockusedincreatingthepolymerbytheprocessofpolymerization.Apolymerisbasicallyalinkedmonomerwithanumberofthembeinglinkedbasedonthetypeofthepolymer.Mostofthepolymersarebasedoncarbonandtothatextentthearecalledorganicmaterials.However,therearecertaininorganicpolymersalsoinexistence.

Thoughengineeringuseofpolymersstartedinthe1800's,theareinexistencefromalongtime.Mostofthemareofbiologicalnatureandareusedasasourceoffood.However,weshallstudyonlyengineeringpolymersinthischapter.Thereweresignificantdevelopmentsintheinventionofrubbervulcanizationandphenolicplasticsduringthenineteenthcentury.TherealuseofengineeringplasticswentuptremendouslyaftertheSecondWorldWarinthetwentiethcentury.

Plasticareformedwhensmallorganicmoleculesarestitchedtogethertoformalongchain.Thisprocessiscalledpolymerization.Thoseorganicmoleculessuitableforpolymerizationarecalledmonomers.Itisnecessaryforamonomertobeatleastbifunctional（capableofformingtwocovalentbonds）topolymerize.Thebifunctionalmonomerwillattachothermonomersatthefrontandbackendtoformachain.Somemonomersarepolyfunctional,capableofformingthreeormorebonds.Thesecanformthree-dimensionalarrangementsofchainscalledanetwork.

Polymerizationisgenerallyoftwotypes:

additionandcondensation.Additionpolymerizationconsistsofbreakingthedoublecarbonbond（C=C）inbifunctionalpolymerssothatthechaincanbeformed.SomeofthemonomersthatarecommonlyusedinadditionpolymerizationtoproducethermoplasticmaterialsareshowninTable12.1.Forexample,ethylenewhenpolymerizedwillbecalledpolyethylene.

Incondensationpolymerization,twodifferentorganicmoleculesreacttoformaplasticmolecule.ThereactiongenerallyresultsintheseparationofasmallmoleculesuchasH2Oasaby-product.SometypicalplasticmaterialsformedbycondensationpolymerizationareshowninTable12.2.

Resinisthetermgenerallyusedforuncompoundedingredientsormonomersthataremixedbutnotyetpolymerized.Anexampleisthermosettingresin.Sometimesitisalsousedasasynonymoustermforplastics,forexample,thermoplasticresininsteadofthermoplasticmaterial.

Aspolymerizationproceeds,thenumberofcarbonchainswillintermingletoformlargevolumes.Dependingontheorderlinessofthesechains,thepolymerswillbetermedamorphous（disorderly）orcrystalline（orderly）.Crystallinityororderlinessofthechainscontrolthepropertiesoftheplastics.Amorphouspolymersarelessdensecomparedtocrystallineplastics.Crystallinityinplasticsincreasesthestrengthandtoughness.

12.1.1Additives

Anumberofadditivesarenormallyusedwithetheplasticmaterialstomodifytheirbehaviour,improvepropertiesorreducetheoverallcosttherebyincreasingtherangeofapplicationofplastics.

Plasticizersaremixedwithplasticstoimprovetheirflowcharacteristicsanddecreasetheirbrittleness.Thereareliquidorganiccompoundswithfairlylargemoleculeweight.Theglasstransitiontemperatureisthetemperatureatwhichanamorphousbrittleoncooling,orsoftonheating.Plasticizerslowertheglasstransitiontemperaturethusmakingtheplasticsofterandmoreflexibleatroomtemperature.Forexample,polyvinylchloride（PVC）isbrittleattheroomtemperature.However,whenaplasticizersuchasdi-iso-octylismixed,PVCbecomesflexibleandisusedaswrappingtapeforelectricalinsulation.

Fillersareinexpensivematerialsthatareaddedtoplasticstoreducetheircost.Theyaregenerallyinertandwillnotreactwithplastics.However,manyofthefillermaterialsincreasethehardnessandimpactstrengthofplastics.Commonfillersarewoodflour,quartz,glassspheres,talc,calciumcarbonateandaluminatrihydrate.

Flameretardantsareaddedtoplasticstoreducetheflammabilityofplasticsbypreventingoxygenreactionandimprovingcharring.Elementssuchasboron,nitrogen,chlorine,antimonyandphosphorousareaddedforthispurpose.Thesecansometimescauseproblemsbyreducingflexibility,tearresistance,tensilestrengthandheatdeflection.

Reinforcingagentsarethematerialsthatarespeciallyaddedtoplasticmaterialstoraisetheirmechanicalproperties.Reinforcingcanaffectpracticallyalloftheproperties.SomeexampleofthechangeinmechanicalpropertiesbytheadditionofglassfibretothecommonengineeringplasticsaregiveninTable12.3.Someoftheagentsbondwiththeplasticstructureimpartingregularity.

Stabilizersareusedtothepropertiesoftheplasticthroughoutitsusefullife.Theyresistheat,strengthdeteriorationandreducetheeffectofradiationonbondswithinthechains.

Colourantsareusedtotheplastics.Bothorganicdyesandinorganicareusedtoaddcolourtoplasticmaterials.Pigmentsdisperseratherthandissolveinplasticstherebyreducingtransparencyofthematerial.Theyhideflawssuchasairbubbles,makingitdifficulttojudgequality.

12.2.PLASTICMATERIALS

PolymericmaterialsofinteresttoengineerscanbebroadlyclassifiedintoplasticsandelastomersasshowninFig.12.1.Plasticmaterialsareclassifiedintothermoplasticandthermosettingmaterials,basedontheprocessingmethods.

ThermoplasticsTherearetheplasticsthatcanbesoftenedandmeltedbyheatandcanthenbeformedintotherequiredshapewhentheyarehot.Thesematerialscanbemeltedanumberoftimes.Thatmeansitispossibletorecyclethermoplastics.However,frequentremeltingisavoidedinindustrysincesomechemicaldegradationoccursduringremelting.Thermoplasticmaterialstendtoconsistoflongpolymerchainswithlittlebreath,akintoatwo-dimensionalstructure.Thefabricationprocessesusedforthermoplasticssuchasblowmoulding,andinjectionmouldingarelessexpensivecomparedtothosemethodsusedforthermosettingmaterials.SomeofthecommonthermoplasticmaterialsaregiveninTable12.4withtheirmechanicalcharacteristics.

ThermosettingMaterialsThesearetheplasticthatcannotbemeltedoncetheyaresolidified.Therawmaterialsforthermosettingmaterialsareusuallycalledresins.Theyaremixedandplacedinthemould,andheatedandcompressedduringwhichprocessthematerialsachievestrengthandhardness.Polymerizationoccursbystrongnetworkbonds（cross-linking）withtheapplicationofheat,pressureand/odtime.Themanufacturingprocessesusedaremoreexpensivecomparedtothoseofthermoplasticmaterials.Thesematerialsarecharacterizedbyathree-dimensionalnetworkofmolecules.Thesematerialscannotberecycled.Whenheated,thesematerialsburnandchar.SomeofthecommonthermosettingplasticmaterialsaregiveninTable12.5withtheirmechanicalcharacteristics.

12.3EXTRUSIONOFPLASICS

ASexplainedinchapter,extrusionistheprocessofconfininginasclosedcavityandthenallowingittoflowfromonlyoneopeningsothatthemetalwilltaketheopening.Theoperationisidenticaltothesqueezingoftoothpasteoutofatoothpastetubes.

Extrusioncanbeprocessmostthermoplasticssuchaspolyurethane,polystyrene,polyamide,polyester,andflexiblepolyvinylchloride.Acharacteristicthatoftendifferentiatesextrudedfrominjection-mouldedplasticisviscosityoftheplasticatnormalprocessingtemperatures.Extrudedplasticsoftenhaveahighermeltviscositythatallowstheextrudatetoretaintheshapeimpartedtoitbyhediewhiletheextrudateisinthecoolingstages.

Itispossibletocombineavarietyofresinstogainspecialphysical,biologicalproperties.Also,asexplainedearlier,additivesthatincludelubricants,thermalstabilizers,antioxidantsandcolorants,canbeusedduringtheextrusionprocesstoenhanceprocessingcharacteristicsofthepolymerortoalterproductproperties.

TheplasticthatisgenerallyintheformofgranulesisfedintotheextrudingmachinetroughahopperasshowninFig.12.2.Thismachineisknownasanextruder.Pelletsmoveintothebarrelbygravity.Thebarrelisheatedbyelectricheaters,whichsoftengeneratesmoreheattosoftentheplastic.Theheatingofthebarrelmaybereducedoncesufficientheattosoftentheplastic.Theheatingofthebarrelmaybereducedoncesufficientlysothatitflowssmoothlylikealiquid.Finally,inthemeteringisforcedthroughthedie.Thefeedingcapacitycapacityofthescrewisdeterminedbyitsgeometryandspeedofrotation.

Thereisabreakerplate（aplatewithseveralsmallholes）betweenthedieandthebarrelwhichimprovesthemixingoftheplasticbeforeenteringthedie.Theextrudedproductcalledextrudateiscooledbyblowingairorbypassingthroughawater-filledchannel.

Thedieisametalplateplacedattheendfotheextruderwithacut-outoftheshapeoftheproductbeingmanufactured.Anycomplexexternalcontourcanbeextrudedwithrelativelysimpleandinexpensivetooling.AnexampleofthediecrosssectionandtheproductproducedbyitareshowninFig.12.3.Adie,whichisasimpleplatewiththeshapeofthepartcutthroughthediewithnotransitionguidingthematerial,isalow-costdesignusedforlow-volumeproduction.However,astreamlineddieprovidesabetterflowoftheplasticwithminimumnumberofdefects.Inthistypeofdie,channelsaremadetodirecttheflowofthematerialevenlythroughoutthedie.Thedesignismostexpensiveandidealforhigh-volumejobs.

Thepartsforextrusionshouldhaveconsistentwallthicknessasitallowsforanevenflowofmaterialthroughthediethatproducesmorecontrolledpartswithalowertoolingcost.Asfaraspossible,designingprofileswithhollowsectionsareavoided