附件2：外文原文Gas-AssistedInjectionMoldingInjectionmoldingisaverypopularoperationforproductionofcommercialplasticpartswithitssophisticatedcontrolandsuperiorsurfacedetails.However,ithaslimitations,suchaslongcycletimeforpartswiththicksectionsduetoslowcooling.Alsopackingofthicksectionscanproducesinkmarksonthepartsurface.Largethinpartscanhavewarpagebecausetheresidualstressandstraininducedduringfillingandpacking.Thustraditionalinjectionmoldingcanbemodifiedtosolvethesekindsofproblems,alsotoimprovethequalityofthepartandlowerthecostofproduction.Currently,gas-assistedinjectionmoldingisinuseandbeingdevelopedworldwide.IntheUS,theprocessisknownasGas-AssistedInjectionMolding(GAIM)；itisalsocalledGasInjectionTechnique(GIT)inEurope(seeFig.4.3.1).Thisprocessisdevelopedfortheproductionofhollowplasticpartswithseparateinternalchannels.Itisuniquebecauseitcombinestheadvantagesofconventionalinjectionmoldingandblowmoldingwhiledifferingfromboth.GAIMoffersacosteffectivemeansofproducinglarge,smoothsurfacedandrigidpartsusinglowerclampingpressurewithlittleornofinishing.Byintroducingthegasbeforecompletefilling,numerousproblemssuchaswarpage,sinkmarks,andhighfillingpressurearemostlyovercome.Moreover,theprocessgivesgreatbenefitsintermsofhigherstiffness-to-weightratiothanthesolidpartswiththesameoveralldimensionsduetotheeliminationofmaterialplacedinefficientlyneartheneutralaxisofthecrosssection,thusincreasingthefreedomofpartdesign.Incomparisonwithconventionalinjectionmolding,thegas-assistedprocessismorecriticalintermsofprocesscontrol,especiallyformulti-cavityapplications.Thequalityofthepartisdeterminedbybothtoolandprocessvariablessuchasdegreeofunder-fill,gasinjectionconditions,andmoldtemperature,thusindicatingtheimportanceofprocesscontrol.Theprocessisattractingmanymoldersduetothedemandforhighlyautomatedproductionofgas-assistedinjectionmoldedparts.Thegas-assistedinjectionmoldingprocessisthemostrapidlygrowingfieldwithconsiderableworkgoingoninthefieldofcontrolsandtheprocessdevelopment.Researchinterestisdrawntowardsthedevelopmentofnewgasinjectionunits,thestudyoftheprocessvariable,theefficiencyoftheproductionprocess,andadvantagesofferedbythenewprocess.Manydifferentcompaniesareofferinggasinjection-moldingunitswiththevariousoptions,whicharemainlypressurecontrolledorvolumecontrolledprocesses.Ingasinjectionmolding,themoldispartiallyfilledwithmoltenthermoplastic,andaninertgas,usuallynitrogen,isinjectedintotheplastic.Gasisinjectedintothemoltenthermoplasticmaterialusingeitheroftwoprocedures.Inonemethod,ameasuredvolumeofgasispressurizedinacontainer.Avalveisopenedtoallowthegastoflowintothepolymer,andapistonisactivatedtoforceallgasfromthecontainerintothemold.Asthegasexpandsinthemold,itspressuredrops.Asecondmethodholdsgaspressure,ratherthangasvolume,constant.Thegasrapidlytravelsdownthethickest-andthereforethehottest-sectionofthepart,advancingthemeltfrontandfillingandpackingthemold.Additionalplasticvolumemaybedisplacedbythepressurizedgasasthematerialshrinks.Aftertheplasticcools,thegasisallowedtoescape,leavingamoldedplasticpartcontaininginternalvoids.ThestandardGAIMprocesscanbedividedintofourpartialsteps.ThefirststepisastageofmeltsinjectionFig.4.3.2(a).Thecavityispartiallyfilledwithadefinedamountofmelt.Therequiredvolumeisempiricallydeterminedbyperformingfillingstudiesinordertoavoidblowingthegasthroughattheflowfrontandtoensureanidealblowholevolume.Typicallythepolymerfillsthecavitybetween75%95%beforethemeltandgastransition.Thegasinletphaseisthesecondstage,whichisshowninFig.4.3.2(b).Gasmaybeaddedatanypointintimeeitherduringorshortlyaftermeltsinjection.Thegascanenteronlyifthegaspressureexceedsthemeltpressured.Intheinteriorofthemoldedpart,thegasexpelsthemeltfromtheplasticnucleusuntiltheremainderofthecavityiscompletelyfilled.Gasinjectionpressuresrangefrom0.530Mpa(704500psi).Atthegasholdingpressurephase,Fig.4.3.2(c)thegascontinuestopushthepolymermeltintotheextremitiesofthecavityofthemoldedarticleactsasaholdingpressuretocompensateforpathofleastresistanceasitpushesthroughthepolymer.ThefinalstageisagasreturnforrecyclingoragasreleasetoatmosphereFig.4.3.2(d).Afterthegasholdingphase,thegaspressureinthemoldedarticleisreleasedtotheoutsidebysuitablegasreturnand/orbypressurerelease.A.AdvantagesoftheGAIMprocessGasinjectionprovidesasolutiontoanumberofproblemsthatoccursinconventionalinjectionmolding.(1)ReducingstressandwarpageWithgas,thepressureisequaleverywherethroughoutthecontinuousnetworkofhollowchannels.Whendesignedproperly,theseprovideaninternalrunnersystemwithinthepart,enablingtheappliedpressure,andthereforetheinternalstressgradients,tobereducedmarkedly.Thisreducesapartstendencytowarp.(2)EliminationofsinkmarksSinkmarksresultingfromribsorbossesonthebacksideofaparthavelongbeenaproblem.Thesesurfacemarksresultfromthevolumecontractionofthemeltduringcooling.Sinkmarkscanbeminimizedoreliminatedifahollowgaschannelcanbedirectedbetweenthefrontsurfaceofthepartandthebacksidedetail.Withgasinjection,thebaseoftheribmadesomewhatthickertohelpdirectthegaschannel.Withagaschannelatthebaseofarib,materialshrinksareawayfromtheinsidesurfaceofthechannelasthemoldedpartcoolsbecausethematerialisthehottestatthecenter.Therefore,nosinkmarkoccursontheoutsidesurfaceasthepartshrinksduringcooling.(3)SmoothsurfaceUnlikestructuralfoam,gasinjectionpermitslighterweightandsavesmaterialinastructurallyrigidpart.Withgasholding,agoodsurfacequalitycanbeachieved.(4)ReducedclamptonnageInconventionalinjection,thehighestpressureoccursduringthepackingphase.ThemaximuminjectionpressureissignificantlylowerinGAIMandacontrolledgaspressurethroughanetworkofhollowchannelsisusedtofilloutthemold.Thismeansthatclamptonnagerequirementscanbereducedbyasmuchas90%.(5)EliminationofexternalrunnersOneofthebestfeaturesofgasinjectionisthatflowrunnerscanbebuiltrightintothepart.Frequently,allexternalrunners(bothhotandcold)canbeeliminated,evenonalargerandc