模具设计外文翻译塑料成型过程.docx

模具设计外文翻译塑料成型过程.docx

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模具设计外文翻译塑料成型过程

外文翻译原文

Plasticsformingprocesses

Thereisawiderangeofprocessingmethodsthatmaybeusedforplastics.Nevertheless,theyallinvolvethreeorfourbasicstages:

softening,shaping,solidifyingandcoolingofthemoulds（forthermoplasticsonly）.Commonmaterialsformouldingprocessesarethermoplasticsandthermosetpolymers.Principalmethodsofprocessingthermoplasticsincludeextolsion,blowmoulding,rotationalmoulding,thermoformingandinjectionmoulding;butasforthermosets,compression,transferandreactioninjectionmouldingarefrequentlyused.

1Extrusion

Extrusionisoneofthemostimportantformingprocessesforthereasonthatpellets,whichareusedformanyothermouldingprocesses,arenormallyproducedbythisprocess.Infact,somemouldingprocessesarepost-extrusionoperations,suchasblowmouldingandthermoformmoulding.Extnlsionisbasicallyaprocessofcontinuouslyshapingafluidpolymerthroughtheorificeofadie,andsubsequentlysolidifyingitintoaproductofauniformcross-section.Anextrudingmachinemayhaveoneortwoscrews,ornoscrew（screwless）.Single-screwextruders,asseeninFigure1,arethemostcommonlyusedmachines.Screwless（ram）extrudersallowaprecisecontrolofthemeltflowrateandaregainingpopularity.Theyareparticularlysuitedforhighviscosity.Inrecentyears,therehasbeenasteadyincreaseintheuseoftwinscrewextruders.Thesemachinespermitawiderrangerofpossibilitiesintermsofoutputrates,mixingefficiencyandheatgeneration.Theyare,however,considerablymoreexpensive.Commonextrusionproductsincludefilamentsofcircularcross-section,profilesofirregularcross-section,axisymmetrictubesandpipes,andflatproductssuchasfilmsorsheets.Almostalltypesofintricatecross-sectionalshapeswithlargelengthsaremadebyextrusionmoulding,whichmanyotherdiscreteformingprocesses,suchascompression,transferandinjectionmoulding,areincapableofproducing.

FIGURE1.Single-screwextruder.

2Blowmoulding

Thisprocessbeginswiththepreparationofasoft,extrudedandpreformedthermoplastictubeoveracorepin.Asthemouldhalvesclose,airpressureinflatesthethinwalledpreformandforcesitoutwardsagainstthemouldsides.Figure2showstheprocessattwostages.Thepreformcanbemadebyeitherextrasionorinjection.Blowmouldsaresubjectedtomoderatepressuresandclampingforces,comparedtoinjectionmoulds.Thus,theycanbemadeofalightmaterialsuchasaluminmm,whichhasadvantagesoflightweightandhighheatconductivity.

Blown-warecontainersarecommonlyusedforpackagingbeverageandotherfluidfood,e.g.narrowneckplasticbottlesformineralwater,milk,alcoholicbeverageandcarbonatedbeverages.Othernon-foodproductspackedintheblown-warecontainersincludecosmetics,pharmaceuticals,paintandpowderproducts.Blowmouldingisalsousedtoproducesomehugeproductsinsize,suchasshippingdrumsandstationarystoragetankswhosevolumesmayreachashighas10000litres[5].Thesetanksareusedforundergroundfuelstorageandseptictanks.

Stage1:

PreformextrusionStage2:

Blowing

FIGURE2.Extrusionblowmoulding

3Rotationalmoulding

Likeblowmoulding,rotationalmouldingisalsousedtoproducehollowplasticarticles,thoughtheprinciplesineachmethoddifferalot.Duringtheprocess,acarefullyweighedchargeofplasticpowderisplacedinonehalfofametalmould.Themouldhalvesarethenclampedtogetherandheatedonanoven.Whenheated,themouldrotatesabouttwoaxesatrightanglestoeachother.Afteratimetheplasticswillbesufficientlysoftenedtoformahomogeneouslayeronthesurfaceofthemould.Theprocessisattractiveforanumberofreasons.Firstly,asitisalow-pressureprocess,themouldsarerelativelysimpleandinexpensive.Secondly,theproductisvirtuallystrata-free.Thirdly,auniformthicknesscanbeeasilyachieved.Finally,itispossibletointroducereinforcementintotheproducts,andtheirsurfacecanbetexturedasdesired.However,thecycletimesarelongercomparedtobloworinjectionmoulding.Themould-handlingdevice,capableofimpartingdoublerotations,isthecentralelementofrotationalmouldingequipment.Therearetwomajortypesofequipment:

shuttlecartsystem,asshownFigure3,andswing/rotaryarmsystem.Rotationalmouldingisgoodatproducingverylarge,thick-walledarticleswhichcouldnotbeproducedeconomicallybyanyotherprocesses.Thelargestcapacityofarotational-mouldingmadetankisrecordedatabout75000litres[4].

FIGURE3.Shuttlecartrotationalmoulding.

TheInstitutionofProfessionalEngineersNewZealand

4Compressionmoulding

Compressionmouldingisoftenusedtoproducearticlesfromthermosetmaterials,thoughitcanalsobeusedforthermoplastics.ThemouldingoperationusedforthermosetsisillustratedinFigure4.Alargenumberofcompressionmouldedthermosetproductscanbefoundinelectricalandelectronicapplications.Glass-fibrereinforcementcanbeeasilyaddedtomeettheheatresistancerequirement.However,thelimitationwiththisprocessisthattheproducthastobesimpleinshapeandwithoutthinwallsorfragileinserts.Numerousrubberproductsarecompressionmoulded.Ausefulfeatureofitisitsabilitytohavemetalinsertsthatformstrongbondswiththeproductandareoftenusedtoattachtheproducttostructures.Tyresarethemostcommonproductsmadebycompressionmoulding.

FIGURE4.Compressionmoulding.

5Transfermoulding

Transfermouldingissimilartocompressionmouldingexceptthat,insteadofthemouldingmaterialbeingpressurisedinthecavity,itispressurisedinaseparatechamberandthenforcedthroughanopeningandintoaclosedmouldcavity.Theadvantageoftransfermouldingisthatthepreheatingofthematerialinjectedthroughanarroworificeimprovesthetemperaturedistributioninthematerialandhenceacceleratesthecross-linkingreactioninthermosets.Asaresultthecycletimeisreducedandthereislessdistortionintheproduct.Theimprovedflowofmaterialalsomeansthatmoreintricateshapescanbeproduced.Partswithfragileinsertslikeelectricapplianceparts,electroniccomponentsandconnectorsthatmayenclosecoils,integratedcircuits,andplugscanalsobeeasilymade.

6Thermoforming

Sheetthermoformingwasdevelopedinthe1950s.Thelimitationssuchaspoorwallthicknessdistributionandlargeperipheralwasterestricteditsusetosimplepackagingapplications.Inrecentyears,however,therehavebeenmajoradvancesinmachinedesignandmaterials,whichhaveresultedinawiderangeofproductsbeingmadebythermoforming.Therearethreetypesofthermoformingprocesses（Figure5）:

vacuummoulding,airpressuremoulding,andmechanicalmoulding.

Themoulds,whicharenotsubjectedtohighpressure,areoftenmadefromcastormachinedalumininmforsmallandmediumsizes,andtheydonotrequireagoodsurfacefinish.Theproductsurfacequalityislargelydependentuponthatofthesheetmaterial.

Productsmadebythermoformingcanbesmallaswellaslarge.Smallerproductsaremadeinhighoutputmachines,usingmulti-cavitymoulds.Suchproductsareoftenfoundinthefoodindustryandmedicalapplications,forexample,jellyorcreamcontainers,cups,robsandtrays.Thesesmallitemscanhaverelativelycomplexshapeswithreasonablyeventhickness.Largeproductsaregenerallymadefromcutsheetsatalowerthough-putrate,andtheyareusuallyofsimpleshapes.Fisher&Paykel'svacuumformmouldingmachinesproducethemajorityofpre-formsforrefrigeratorsandfreezers.Manyotherinteriorpartsarealsomadebythesameorsimilarprocesses.

FIGURE5.Threebasicmethodsofthermoforming.

7Injectionmoulding

Injectionmouldinghasalwaysbeenoneofthemostcommonprocessingmethodsforplastics.Nowadayscountlesspartsinmanyelectricalappliances,automobilesandofficeequipmentareinjectionmoulded.Themostcommoninjectionmouldingmachineryisthereciprocatingscrewmachine,whoseprocesscanbedividedintoseveralstagesasseeninFigure6.Attheplasticationstage,thefeedunitoperatesasanextntder,meltingandhomogenisingthematerialinthescrew/barrelsystem.Thescrew,however,isallowedtoretractinordertomakeroomforthemoltenmaterialinaspaceatthecylinderhead,calledmaterialreservoir,betweenthescrewtipandaclosedvalveoranobstntctionofsolidifiedmaterialfromthepreviousshot.Attheinjectionstage,thescrewisusedasaram（piston）forrapidtransferofthemoltenmaterialfromthereservoirtothecavitybetweenthetwohalvesoftheclosedmould.Sincethemouldiskeptatatemperaturebelowthesolidificationtemperatureofthematerial,itisessentialtoinjectthemoltenmaterialrapidlyenoughtoensurecompletefillingofthecavity.Ahighholdingorpackingpressureisnormallyexerted,topartiallycompensateforthethermalcontractionofthematerialuponcooling.Thecoolingofthematerialinthemouldoftenlimitstheproductiontimebecauseofthelowthermalconductivityofpolymers.Themould,afterbeingcooled,canbeopenedandthesolidproductejected.

Althoughthescrewmachineisbyfarthemostpopular,plungerinjectionmachinesarealsousedtogiveproductssomeuniquefeatures.Thereisnoshearingormixingaction,asaplungerdoesnotrotate.Theresultingmouldedpartcantakeonamarbledappearancewithswirlsoftwoormorecolours.Thismaybethedesiredfinishforcertainproducts.Regardlessofdifferentmachines,injectionmouldingyieldsahighproductivityandallowstheproductstohavemanyfinedetailssuchasbosses,loca