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外文翻译--气辅注射成型.doc

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外文翻译--气辅注射成型.doc

附件2外文原文GasAssistedInjectionMoldingInjectionmoldingisaverypopularoperationforproductionofcommercialplasticpartswithitssophisticatedcontrolandsuperiorsurfacedetails.However,ithaslimitations,suchaslongcycletimeforpartswiththicksectionsduetoslowcooling.Alsopackingofthicksectionscanproducesinkmarksonthepartsurface.Largethinpartscanhavewarpagebecausetheresidualstressandstraininducedduringfillingandpacking.Thustraditionalinjectionmoldingcanbemodifiedtosolvethesekindsofproblems,alsotoimprovethequalityofthepartandlowerthecostofproduction.Currently,gasassistedinjectionmoldingisinuseandbeingdevelopedworldwide.IntheUS,theprocessisknownasGasAssistedInjectionMoldingGAIMitisalsocalledGasInjectionTechniqueGITinEuropeseeFig.4.3.1.Thisprocessisdevelopedfortheproductionofhollowplasticpartswithseparateinternalchannels.Itisuniquebecauseitcombinestheadvantagesofconventionalinjectionmoldingandblowmoldingwhiledifferingfromboth.GAIMoffersacosteffectivemeansofproducinglarge,smoothsurfacedandrigidpartsusinglowerclampingpressurewithlittleornofinishing.Byintroducingthegasbeforecompletefilling,numerousproblemssuchaswarpage,sinkmarks,andhighfillingpressurearemostlyovercome.Moreover,theprocessgivesgreatbenefitsintermsofhigherstiffnesstoweightratiothanthesolidpartswiththesameoveralldimensionsduetotheeliminationofmaterialplacedinefficientlyneartheneutralaxisofthecrosssection,thusincreasingthefreedomofpartdesign.Incomparisonwithconventionalinjectionmolding,thegasassistedprocessismorecriticalintermsofprocesscontrol,especiallyformulticavityapplications.Thequalityofthepartisdeterminedbybothtoolandprocessvariablessuchasdegreeofunderfill,gasinjectionconditions,andmoldtemperature,thusindicatingtheimportanceofprocesscontrol.Theprocessisattractingmanymoldersduetothedemandforhighlyautomatedproductionofgasassistedinjectionmoldedparts.Thegasassistedinjectionmoldingprocessisthemostrapidlygrowingfieldwithconsiderableworkgoingoninthefieldofcontrolsandtheprocessdevelopment.Researchinterestisdrawntowardsthedevelopmentofnewgasinjectionunits,thestudyoftheprocessvariable,theefficiencyoftheproductionprocess,andadvantagesofferedbythenewprocess.Manydifferentcompaniesareofferinggasinjectionmoldingunitswiththevariousoptions,whicharemainlypressurecontrolledorvolumecontrolledprocesses.Ingasinjectionmolding,themoldispartiallyfilledwithmoltenthermoplastic,andaninertgas,usuallynitrogen,isinjectedintotheplastic.Gasisinjectedintothemoltenthermoplasticmaterialusingeitheroftwoprocedures.Inonemethod,ameasuredvolumeofgasispressurizedinacontainer.Avalveisopenedtoallowthegastoflowintothepolymer,andapistonisactivatedtoforceallgasfromthecontainerintothemold.Asthegasexpandsinthemold,itspressuredrops.Asecondmethodholdsgaspressure,ratherthangasvolume,constant.Thegasrapidlytravelsdownthethickestandthereforethehottestsectionofthepart,advancingthemeltfrontandfillingandpackingthemold.Additionalplasticvolumemaybedisplacedbythepressurizedgasasthematerialshrinks.Aftertheplasticcools,thegasisallowedtoescape,leavingamoldedplasticpartcontaininginternalvoids.ThestandardGAIMprocesscanbedividedintofourpartialsteps.ThefirststepisastageofmeltsinjectionFig.4.3.2a.Thecavityispartiallyfilledwithadefinedamountofmelt.Therequiredvolumeisempiricallydeterminedbyperformingfillingstudiesinordertoavoidblowingthegasthroughattheflowfrontandtoensureanidealblowholevolume.Typicallythepolymerfillsthecavitybetween7595beforethemeltandgastransition.Thegasinletphaseisthesecondstage,whichisshowninFig.4.3.2b.Gasmaybeaddedatanypointintimeeitherduringorshortlyaftermeltsinjection.Thegascanenteronlyifthegaspressureexceedsthemeltpressured.Intheinteriorofthemoldedpart,thegasexpelsthemeltfromtheplasticnucleusuntiltheremainderofthecavityiscompletelyfilled.Gasinjectionpressuresrangefrom0.530Mpa704500psi.Atthegasholdingpressurephase,Fig.4.3.2cthegascontinuestopushthepolymermeltintotheextremitiesofthecavityofthemoldedarticleactsasaholdingpressuretocompensateforpathofleastresistanceasitpushesthroughthepolymer.ThefinalstageisagasreturnforrecyclingoragasreleasetoatmosphereFig.4.3.2d.Afterthegasholdingphase,thegaspressureinthemoldedarticleisreleasedtotheoutsidebysuitablegasreturnand/orbypressurerelease.A.AdvantagesoftheGAIMprocessGasinjectionprovidesasolutiontoanumberofproblemsthatoccursinconventionalinjectionmolding.1ReducingstressandwarpageWithgas,thepressureisequaleverywherethroughoutthecontinuousnetworkofhollowchannels.Whendesignedproperly,theseprovideaninternalrunnersystemwithinthepart,enablingtheappliedpressure,andthereforetheinternalstressgradients,tobereducedmarkedly.Thisreducesapartstendencytowarp.2EliminationofsinkmarksSinkmarksresultingfromribsorbossesonthebacksideofaparthavelongbeenaproblem.Thesesurfacemarksresultfromthevolumecontractionofthemeltduringcooling.Sinkmarkscanbeminimizedoreliminatedifahollowgaschannelcanbedirectedbetweenthefrontsurfaceofthepartandthebacksidedetail.Withgasinjection,thebaseoftheribmadesomewhatthickertohelpdirectthegaschannel.Withagaschannelatthebaseofarib,materialshrinksareawayfromtheinsidesurfaceofthechannelasthemoldedpartcoolsbecausethematerialisthehottestatthecenter.Therefore,nosinkmarkoccursontheoutsidesurfaceasthepartshrinksduringcooling.3SmoothsurfaceUnlikestructuralfoam,gasinjectionpermitslighterweightandsavesmaterialinastructurallyrigidpart.Withgasholding,agoodsurfacequalitycanbeachieved.4ReducedclamptonnageInconventionalinjection,thehighestpressureoccursduringthepackingphase.ThemaximuminjectionpressureissignificantlylowerinGAIMandacontrolledgaspressurethroughanetworkofhollowchannelsisusedtofilloutthemold.Thismeansthatclamptonnagerequirementscanbereducedbyasmuchas90.5EliminationofexternalrunnersOneofthebestfeaturesofgasinjectionisthatflowrunnerscanbebuiltrightintothepart.Frequently,allexternalrunnersbothhotandcoldcanbeeliminated,evenonalargerandcomplexpart.Thesebenefitsincludethereducedtoolingcosts,thelowerquantitiesofregrindfromrunners,andtheimprovementoftemperaturecontrolovertheplasticmelt.Oftentheinternalrunnerscanimprovetheflowpatterninthemoldandeliminateorcontrolknitlinelocationresultingfrommultipleinjectionsfrommultipleinjectiongates.Inadditiontoservingasflowchannels,theribsandthicksectionscanprovidestructuralrigiditywhenrequired.6PermittingdifferentwallthicknessAconstantwallthicknessismaintainedintheplasticparts.Withgasinjection,thisdesignruleisflexible.Differentwallthicknessesarepossibleifgaschannelsaredesignedintothepartatthetransitionpoints.Thispermitsuniformmaterial

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