一种改善微孔注射液型注塑件表面质量的新方法毕业课程设计外文文献翻译Word文档下载推荐.docx
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一种改善微孔注射液型注塑件表面质量的新方法毕业课程设计外文文献翻译Word文档下载推荐.docx
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Received25October2010
Receivedinrevisedform
12January2011
Accepted14January2011
Availableonline21January2011
Keywords:
Microcellularinjectionmolding
Plasticfoaming
Swirl-freesurface
abstract
Microcellularinjectionmoldingisthemanufacturingmethodusedforproducingfoamedplasticparts.Microcellularinjectionmoldinghasmanyadvantagesincludingmaterial,energy,andcostsavingsaswellasenhanceddimensionalstability.Inspiteoftheseadvantages,thistechniquehasbeenlimitedbyitspropensitytocreatepartswithsurfacedefectssuchasaroughsurfaceorgasflowmarks.Methodsforimprovingthesurfacequalityofmicrocellularplasticpartshavebeeninvestigatedbyseveralresearchers.Thispaperdescribesanovelmethodforachievingswirl-freefoamedplasticpartsusingthemicrocellularinjectionmoldingprocess.Bycontrollingthecellnucleationrateofthepolymer/gassolutionthroughmaterialformulationandgasconcentration,microcellularinjectionmoldedpartsfreeofsurfacedefectswereachieved.Thispaperpresentsthetheoreticalbackgroundofthisapproachaswellastheexperimentalresultsintermsofsurfaceroughnessandprofile,microstructures,mechanicalproperties,anddimensionalstability.
●Introduction
Thecommerciallyavailablemicrocellularinjectionmoldingprocess(a.k.a.theMuCellProcess)consistsoffourdistinctivesteps,namely,gasdissolution,nucleation,cellgrowth,andshaping[1].Inthegasdissolutionstage,polymerintheinjectionbarrelismixedwithsupercriticalfluid(SCF)nitrogen,carbondioxide,oranothertypeofgasusingaspecialscrewwhichisdesignedtomaximizethemixinganddissolvingofthegasinthepolymermelt.Duringinjection,alargenumberofnucleationsites(ordersofmagnitudehigherthanconventionalfoamingprocesses)areformedbyarapidandsubstantialpressuredropasthepolymer/gassolutionisinjectedintothemoldcavity,thuscausingtheformationofcells(bubbles).Duringtherestoftheinjectionmoldingcycle,cellscontinuetogrowtofillandpackoutthemoldandsubsequentlycompensateforthepolymershrinkageasthematerialcoolsinsidethemold.Thecellgrowthisdrivenbytheamountandspatialdistributionofthedissolvedgas.Thecellgrowthisalsocontrolledbyprocessingconditionssuchasmeltpressureandtemperatureaswellasmaterialpropertiessuchasmeltstrengthandgassolubility.Finally,theshapingoftheparttakesplaceinsidethemolduntilthemoldopensallowingtheparttobeejected.
Sincethemicrocellularinjectionmoldingprocesswasinvented,therehavebeennumerousstudiesonprocess,material,andtechnicaldevelopmentsaimedatmaterializingthefullprocesspotential.Accordingtopreviousstudies[1-5],microcellularinjectionmoldingoffersanumberofadvantagessuchascostsavings,weightreduction,easeinprocessingduetolowviscosity,andoutstandingdimensionalaccuracy.Duetotheseadvantages,themicrocellularinjectionmoldingprocesshasbeenusedinmanyindustriessuchasautomotive,electricalgoods,andhomeappliancesusingabroadrangeofthermoplastics.Despitetheseadvantages,however,thesurfaceimperfectionsassociatedwithmicrocellularinjectionmoldedpartsdsuchasuniquegasflowmarks,referredtoasswirlmarksthroughoutthispaper,andalackofsmoothnessdstillremainoneofthemaindrawbackssurroundingmicrocellularinjectionmolding.Inordertoeliminateorreducethesesurfaceimperfectionstherehavebeenseveralstudiesattempted,asreportedinRefs.[6-14].Someresearchershavefocusedontemperaturemodificationofthemoldsurfacetoimprovethesurfacequalityofmicrocellularinjectionmoldedparts[6-8].Withpolymericfoam,itwasfoundthatbubblesformingattheadvancingmeltfrontarefirststretchedbythefountainflowbehaviortowardthemoldsurfaceandsubsequentlydraggedagainstthemoldwallcausingswirlmarks[9].Duringthefillingstageofpolymermelts,keepingthemoldwalltemperaturehighenoughforbubblesatthemoldsurfacetobeeliminatedimprovesthesurfacequalityofmicrocellularinjectionmoldedparts.Bycontrollingthemoldtemperaturerapidlyandpreciselyusingmoldtemperaturecontrolunitsorotherkindsofthermalorsurfaceheatingdevices,microcellularfoamedplasticswithglossyandswirl-freesurfacescanbeproduced.
Therehavealsobeeneffortstoeliminatetheswirlmarksonmicrocellularinjectionmoldedpartswithoutanymoldtemperaturecontroller.Inparticular,itwasproposedthatinsertinganinsulatorontothemoldwallmighthelpkeepingtheinterfacetemperaturebetweenthemoldandthepolymermelthigh.Thistechniquebasicallyyieldsthesameresultastemperaturemodificationofthemold[10].Thermalanalysisandexperimentalresultsprovethattheadditionofaninsulatorlayeronthemoldcanimprovethesurfacequalityofmicrocellularinjectionparts[11].
Anothermethodofproducingpartswithanimprovedsurfacequalityleadstoamicrocellularco-injectionmoldingprocess[12].Inthistechnique,aproperamountofsolidskinmaterialisinjectedpriortotheinjectionofafoamingcorematerial.Thiscanyieldasandwiched(solidskinefoamedcoreesolidskin)structurewithasurfacefinishsimilartoaconventionallymoldedcomponentwhilepartiallymaintainingtheadvantagesofmicrocellularinjectionmolding.
Anotherapproachforimprovingthesurfacequalityofmicrocellular
injectionmoldedpartsisthegascounterpressureprocess[13,14].Inthisprocess,ahigh-pressuregasisinjectedintothemoldpriortothepolymer/gassolutiontosuppresscellnucleationandbubblegrowthwhilethepolymer/gassolutionisbeinginjectedintothemoldcavity.Towardtheendofinjection,countergaspressureisreleasedandbubblesbegintoformwithinthecavity.Sinceamajorityofthepartsurfaceisalreadysolidified,gasflowmarksareeliminated.
Inspiteoftheseeffortstoimprovethesurfacequality,therehavebeendifficultiesinapplyingthemicrocellularinjectionmoldingprocessinindustriesrequiringpartswithhighsurfacequalitiesbecausethesetechniquesentailadditionalequipmentwhichresultsinhighcostsormaintenance.Therehavebeennoreportedstudiesonimprovingthesurfacequalityofmicrocellularinjectionmoldedpartswithoutanyadditionalequipmentormodificationtoexistingequipment.
Thispaperproposesanovelapproachtoimprovethesurfacequalityofmicrocellularinjectionmoldedpartsbycontrollingthecellnucleationrate.Inthisstudy,thecellnucleationratewasdramaticallyloweredordelayedbycontrollingthedegreeofsupersaturationsothatcellnucleationwasdelayedduringthefillingstage.Afterthepolymer/gassolutionvolumetricallyfilledthemoldcavity,intentionallydelayednucleationoccurredandbubblesformedinthepolymermatrix,exceptonthesurfacewherethematerialhadalreadysolidifiedupontouchingthemoldsurface.Theoreticalbackgroundandexperimentalresultsaredescribedinthispaper.Microstructure,surfaceprofile,surfaceroughness,mechanicalproperties,anddimensionalstabilityarealsoinvestigatedinthisstudy.
2.Theoretical
2.1.Nucleationtheoryforpolymericfoams
Inpolymericfoams,nucleationreferstotheinitialstageoftheformationofgasbubblesinthepolymeregassolution.Fornucleation,
gasbubblesmustovercomethefreeenergybarrierbeforetheycansurviveandgrowtomacroscopicsize[15].Accordingtoclassicalnucleationtheories[16-18],thenucleationrateiscontrolledbythemacroscopicpropertiesandstatesofthepolymerandgassuchassolubility,diffusivity,surfacetension,gasconcentration,temperature,andthedegreeofsupersaturation.
OnerepresentativeequationforthenucleationrateofpolymericfoamswasreportedbyColtonandSuh[19,20].Inadditiontothemathematicalrepresentation,theyalsoverifiedtheirnucleationtheoryexperimentallyforabatchfoamingprocessusingahighpressurevessel.Thenucleationequationformicrocellularfoamsdominatedbytheclassicalnucleationtheory[16e18]canbeexpressedas
whereNisthenucleationrate,fisthefrequencyofatomicmolecularlatticevibration,Cistheconcentrationofgasmolecules,kistheBoltzmann’sconstant,Tistheabsolutetemperature,and
istheactivationenergybarrierfornucleation.
Accordingtopreviousstudies[19,20],thenucleationrateofpolymericfoamsiscomposedoftwocomponents:
ahomogeneoustermandaheterogeneousterm.Theactivationenergyforhomogeneousnucleationisgivenby
wheregisthesurfaceenergyofthebubbleinterfaceand
is
assumedtobethegassaturationpressure.Moreprecisely,
wherePr`isthepressurethatisexertedinahigh
pressurevesselandPristhepressureofthesupersaturatedvaporin
thesample[16].Thatis,DPisthepressuredifferencebetweenthe
pressurethatisappliedtothesampleandthepressureofthesupersaturatedvaporinthesample.Whenthepressurethatsaturates
thegasinahighpressurevesselissuddenlyreleasedtotriggertheso-calledthermodynamicinstabilitybyrenderingthesampleintothesupersaturatedstate,Pr`becomes1bardsolowcomparedtoPrthatDPcanbeapproximatedasPr.
Ontheotherhand,theactivationenergyforheterogeneousnucleationisaffectedbyageometricfactorthatdependsonthecontact(wetting)anglebetweenthepolymerandtheparticleandcanbeexpressedas
(3a)
(3b)
wheref(q)isageometricfactorthatisdependentuponthecontact
angle,
oftheinterfacebetweenthepolymerandasecondphase,
andhasvaluesoflessthanorequalto1.Foratypicalwetting
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