PlasticProcessing.docx
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PlasticProcessing.docx
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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
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