注塑模具设计注射模具毕业课程设计外文文献翻译.docx

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注塑模具设计注射模具毕业课程设计外文文献翻译

TheInjectionMolding

TheIntroductionofMolds

Themoldisatthecoreofaplasticmanufacturingprocessbecauseitscavitygivesapartitsshape.Thismakesthemoldatleastascritical-andmanycasesmoreso-forthequalityoftheendproductas,forexample,theplasticitingunitorothercomponentsoftheprocessingequipment.

MoldMaterial

Dependingontheprocessingparametersforthevariousprocessingmethodsaswellasthelengthoftheproductionrun,thenumberoffinishedproductstobeproduced,moldsforplasticsprocessingmustsatisfyagreatvarietyofrequirements.Itisthereforenotsurprisingthatmoldscanbemadefromaverybroadspectrumofmaterials,including-fromatechnicalstandpoint-suchexoticmaterialsaspapermatchedandplaster.However,becausemostprocessesrequirehighpressures,oftencombinedwithhightemperatures,metalsstillrepresentbyfarthemostimportantmaterialgroup,withsteelbeingthepredominantmetal.Itisinterestinginthisregardthat,inmanycases,theselectionofthemoldmaterialisnotonlyaquestionofmaterialpropertiesandanoptimumprice-to-performanceratiobutalsothatthemethodsusedtoproducethemold,andthustheentiredesign,canbeinfluenced.

Atypicalexamplecanbeseeninthechoicebetweencastmetalmolds,withtheirverydifferentcoolingsystems,comparedtomachinedmolds.Inaddition,theproductiontechniquecanalsohaveaneffect;forinstance,itisoftenreportedthat,forthesakeofsimplicity,aprototypemoldisfrequentlymachinedfromsolidstockwiththeaidofthelatesttechnologysuchascomputer-aided（CAD）andcomputer-integratedmanufacturing（CIMS）.Incontrasttothepreviouslyusedmethodsbasedontheuseofpatterns,theuseofCADandCAMoftenrepresentsthemoreeconomicalsolutiontoday,notonlybecausethisproductioncapabilityisavailablepin-housebutalsobecausewithanyothertechniqueanorderwouldhavetobeplacedwithanoutsidesupplier.

Overall,althoughhigh-gradematerialsareoftenused,asarulestandardmaterialsareusedinmoldmaking.New,state-of-theart（high-performance）materials,suchasceramics,forinstance,arealmostcompletelyabsent.Thismayberelatedtothefactthattheirdesirablecharacteristics,suchasconstantpropertiesuptoveryhightemperatures,arenotrequiredonmolds,whereastheirnegativecharacteristics,e.g.lowtensilestrengthandpoorthermalconductivity,haveaclearlyrelatedtoceramics,suchassinteredmaterial,isfoundinmildmakingonlytoalimiteddegree.Thisreferslesstothemodernmaterialsandcomponentsproducedbypowdermetallurgy,andpossiblybyhotisocraticpressing,thantosinteredmetalsinthesenseofporous,air-permeablematerials.

Removalofairfromthecavityofamoldisnecessarywithmanydifferentprocessingmethods,andithasbeenproposedmanytimesthatthiscanbeaccomplishedusingporousmetallicmaterials.Theadvantagesoverspeciallyfabricatedventingdevices,particularlyinareaswheremeltflowfrontsmeet,I,e,atweldlines,areasobviousasthepotentialproblemareas:

ononehand,preventingthetextureofsuchsurfacesfrombecomingvisibleonthefinishedproduct,andontheotherhand,preventingthemicrosporesfromquicklybecomingcloggedwithresidues（brokenoffflash,depositsfromthemoldingmaterial,so-calledplateout,etc.）.Itisalsointerestinginthiscasethatcompletelynewpossibilitieswithregardtomolddesignandprocessingtechniqueresultfromtheuseofsuchmaterials.

A.Designrules

Therearemanyrulesfordesigningmolds.Theserulesandstandardpracticesarebasedonlogic,pastexperience,convenience,andeconomy.Fordesigning,moldmaking,andmolding,itisusuallyofadvantagetofollowtherules.Butoccasionally,itmayworkoutbetterifaruleisignoredandanalternativewayisselected.Inthistext,themostcommonrulesarenoted,butthedesignerwilllearnonlyfromexperiencewhichwaytogo.Thedesignermusteverbeopentonewideasandmethods,tonewmoldingandmoldmaterialsthatmayaffecttheserules.

B.Thebasicmold

1.Moldcavityspace

Themoldcavityspaceisashapeinsidethemold,“excavated”insuchamannerthatwhenthemoldingmaterialisforcedintothisspaceitwilltakeontheshapeofthecavityspaceand,therefore,thedesiredproduct.Theprincipleofamoldisalmostasoldashumancivilization.Moldshavemetalsintosandforms.Suchmolds,whicharestillusedtodayinfoundries,canbeusedonlyoncebecausethemoldisdestroyedtoreleasetheproductafterithassolidified.Today,wearelookingforpermanentmoldsthatcanbeusedoverandover.Nowmoldsaremadefromstrong,durablematerials,suchassteel,orfromsofteraluminumormetalalloysandevenfromcertainplasticswherealongmoldlifeisnotrequiredbecausetheplannedproductionissmall.Ininjectionmoldingtheplasticisinjectedintothecavityspacewithhighpressure,sothemoldmustbestrongenoughtoresisttheinjectionpressurewithoutdeforming.

2.Numberofcavities

Manymolds,particularlymoldsforlargerproducts,arebuiltforonlycavityspace,butmanymolds,especiallylargeproductionmolds,arebuiltwith2ormorecavities.Thereasonforthisispurelyeconomical.Ittakesonlylittlemoretimetoinjectseveralcavitiesthantoinjectone.Forexample,a4-cavitymoldrequiresonlyone-fourthofthemachinetimeofasingle-cavitymold.Conversely,theproductionincreasesinproportiontothenumberofcavities.Amoldwithmorecavitiesismoreexpensivetobuildthanasingle-cavitymold,butnotnecessarily4timesasmuchasasingle-cavitymold.Butitmayalsorequirealargermachinewithlargerplatenareaandmoreclampingcapacity,andbecauseitwilluse4timestheamountofplastic,itmayneedalargeinjectionunit,sothemachinehourcostwillbehigherthanforamachinelargeenoughforthesmallermold.

3.Cavityshapeandshrinkage

Theshapeofthecavityisessentiallythe“negative”oftheshapeofthedesiredproduct,withdimensionalallowanceaddedtoallowforshrinkingoftheplastic.Theshapeofthecavityisusuallycreatedwithchip-removingmachinetools,orwithelectricdischargemachining,withchemicaletching,orbyanynewmethodthatmaybeavailabletoremovemetalorbuilditup,suchasgalvanicprocesses.Itmayalsobecreatedbycastingcertainmetalsinplastermoldscreatedfrommodelsoftheproducttobemade,orbycastingsomesuitablehardplastics.Thecavityshapecanbeeithercutdirectlyintothemoldplatesorformedbyputtinginsertsintotheplates.

C.Cavityandcore

Byconvention,thehollowportionofthecavityspaceiscalledthecavity.Thematching,oftenraisedportionofthecavityspaceiscalledthecore.Mostplasticproductsarecup-shaped.Thisdoesnotmeanthattheylooklikeacup,buttheydohaveaninsideandanoutside.Theoutsideoftheproductisformedbythecavity,theinsidebythecore.Thealternativetothecupshapeistheflatshape.Inthiscase,thereisnospecificconvexportion,andsometimes,thecorelookslikeamirrorimageofthecavity.Typicalexamplesforthisareplasticknives,gamechips,orrounddiskssuchasrecords.Whiletheseitemsaresimpleinappearance,theyoftenpresentseriousmoldingproblemsforejectionoftheproduct.Thereasonforthisisthatallinjectionmoldingmachinesprovideanejectionmechanismonthemovingplatenandtheproductstendtoshrinkontoandclingtothecore,fromwheretheyarethenejected.Mostinjectionmoldingmachinesdonotprovideejectionmechanismsontheinjectionside.

PolymerProcessing

Polymerprocessing,initsmostgeneralcontext,involvesthetransformationofasolid（sometimesliquid）polymericresin,whichisinarandomform（e.g.,powder,pellets,beads）,toasolidplasticsproductofspecifiedshape,dimensions,andproperties.Thisisachievedbymeansofatransformationprocess:

extrusion,molding,calendaring,coating,thermoforming,etc.Theprocess,inordertoachievetheaboveobjective,usuallyinvolvesthefollowingoperations:

solidtransport,compression,heating,melting,mixing,shaping,cooling,solidification,andfinishing.Obviously,theseoperationsdonotnecessarilyoccurinsequence,andmanyofthemtakeplacesimultaneously.

Shapingisrequiredinordertoimparttothematerialthedesiredgeometryanddimensions.Itinvolvescombinationsofviscoelasticdeformationsandheattransfer,whicharegenerallyassociatedwithsolidificationoftheproductfromthemelt.

Shapingincludes:

two-dimensionaloperations,e.g.dieforming,calendaringandcoating;three-dimensionalmoldingandformingoperations.Two-dimensionalprocessesareeitherofthecontinuous,steadystatetype（e.g.filmandsheetextrusion,wirecoating,paperandsheetcoating,calendaring,fiberspinning,pipeandprofileextrusion,etc.）orintermittentasinthecaseofextrusionsassociatedwithintermittentextrusionblowmolding.Generally,moldingoperationsareintermittent,and,thus,theytendtoinvolveunsteadystateconditions.Thermoforming,vacuumforming,andsimilarprocessesmaybeconsideredassecondaryshapingoperations,sincetheyusuallyinvolvethereshapingofanalreadyshapedform.Insomecases,likeblowmolding,theprocessinvolvesprimaryshaping（pair-sonformation）andsecondaryshaping（pairsoninflation）.

Shapingoperationsinvolvesimultaneousorstaggeredfluidflowandheattransfer.Intwo-dimensionalprocesses,solidificationusuallyfollowstheshapingprocess,whereassolidificationandshapingtendtotakeplacesimultaneouslyinsidethemoldinthreedimensionalprocesses.Flowregimes,dependingonthenat