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1Copyright©1999byASMEProceedingsofMaterialsProcessingSymposium1999ASMEInternationalMechanicalEngineeringCongressExpositionNovember1419,1999,Nashville,TennesseeTOWARDSCONTROLLABILITYOFINJECTIONMOLDINGDavidKazmerDepartmentofMech.Ind.EngineeringUniversityofMassachusettsAmherstDavidHatchDepartmentofMech.Ind.EngineeringUniversityofMassachusettsAmherstABSTRACTProcesscontrolhasbeenrecognizedasanimportantmeansofimprovingtheperformanceandconsistencyofthermoplasticparts.However,nosinglecontrolstrategyorsystemdesignhasbeenuniversallyaccepted,andmoldingsystemscontinuetoproducedefectivecomponentsduringproduction.Thecapabilityoftheinjectionmoldingprocessislimitedbythethermalandflowdynamicsoftheheatedpolymermelt.Thispaperdiscussessomeofthedifficultiesposedbycomplexanddistributednatureoftheinjectionmoldingprocess.Theflowandthermaldynamicsoftheprocessareanalyzedwithrespecttotransportandrheology.Then,twonovelprocessingmethodsaredescribedtoenableincycleflow,pressure,andthermalcontrol.Simulationandexperimentalresultsdemonstrateeffectivenessoftheseinnovationstoincreasetheconsistencyandflexibilityinpolymerprocessing.Suchsystemdesignchangessimplifytherequisitecontrolstructureswhileimprovingtheprocessrobustnessandproductivity.INTRODUCTIONInjectionmoldingiscapableofproducingverycomplexcomponentstotightspecifications.Theprocessconsistsofseveralstagesplastication,injection,packing,cooling,andejection.Ininjectionmoldinganditsvariantscoinjection,injectioncompression,gasassistmolding,etc.,thermoplasticpelletsarefedintoarotatingscrewandmelted.Withahomogeneousmeltcollectedinfrontofthescrew,thescrewismovedforwardaxiallyatacontrolled,timevaryingvelocitytodrivethemeltintoanevacuatedcavity.Oncethemeltissolidifiedandthemoldedcomponentissufficientlyrigidtoberemoved,themoldisopenedandthepartisejectedwhilethenextcyclesthermoplasticmeltisplasticizedbythescrew.Cycletimesrangefromlessthanfoursecondsforcompactdiscstomorethanthreeminutesforautomotivecomponents.Controlofinjectionmoldingissignificantlychallengedbythenonlinearbehaviorofthepolymericmaterials,dynamicandcoupledprocessphysics,andconvolutedinteractionsbetweenthemoldgeometryandfinalproductqualityattributes.Arevisedsystemsviewofthemodernconventionalinjectionmoldingprocess1ispresentedinFig.1.Themachineparametersareindicatedontheleftsideofthefigure,andsomecommonmoldedpartmeasuresofqualityarelistedontheright.Inthisfigure,theprocessisdecomposedintofivedistinctbutcoupledstages.Theoutputofeachstagenotonlydirectlydeterminestheinitialconditionsofthenextstage,butalsoinfluencessomeofthefinalqualitiesofthemoldedpart.BarrelTemp1000PLASTICATIONINJECTIONPACKINGCOOLINGEJECTIONPROCESS/PARTQUALITYMeltPressureThermoplasticPelletsScrewPres0.02ScrewRPM0.5DistortionDimensionsClarityEconomicsResid.StressIntegrityEjectedPartRelaxationSolidifiedLayerDevelopmentStrengthAppearanceResidenceTimeMeltVolumeMeltTempMeltQualityInjectionVelocityProfile0.02MaximumInjectionPressure0.1PackingPressureProfile0.2PackingTime0.01MeltViscosityInletPressureFlowRateMoldCoolantTemperature200CoolingTime0.01MeltFrontVelocityMeltPresMeltDensityMeltTempSolidifiedLayerDevelopmentClampTonnageSolidifiedLayerDevelopmentCycleTimePartTempPartStrainPartStressEjectionStroke0.02EjectionVelocity0.01FlashMoldFailureShotSize0.02MACHINEINPUTSQUALITYATTRIBUTESSTATEVARIABLESFigure1SystemsviewoftheinjectionmoldingprocessThincavityfillingofpolymermeltcorrespondstocreepingflowRe1surroundingahotcoreregion2.Asanexample,considerareferencevelocityof10cm/sec,referencethicknessof3mm,andaviscosityof100PaSeconds.TheReynoldsnumberbasedonthiscaseisverysmall,ϑ103,indicatingthevalidityofthehighlyviscouscreepingflowassumption.Furthermore,theflowregionsareconsideredfullydeveloped,andboththeunsteadyandthegravitationalforceeffectscanbeignoredduetonegligiblelocalacceleration.Ontheotherhand,thethermaldiffusivity,2Copyright©1999byASMEk/Cp,oftypicalpolymermeltsisϑ103cm3/sec,andthekinematicviscosity,η/103cm2/sechence,thePrandtlnumberisaboutϑ106andPecletnumber,PeRePr,isϑ103.Usingtheseassumptions,themass,momentum,andenergyequationsreducetothefollowingformsintheCartesiancoordinatesystem0∂∂∂∂∂∂wzvxtrrr1xPzvz∂∂∂∂∂∂h2222ghr∂∂∂∂∂∂zTkxTvtTCp3wherezandxarethethicknessandstreamwisedirectionsvisthevelocitycomponentPisthepressurehistheshearviscosityr,Cp,andkarethethermalpropertiesgistheshearrate,and2ghistheviscousheatingterm.Thesolutionofthepressurefieldininjectionmoldingisobtainedbycouplingthemassandmomentumequations.Generally,themassequationprovidesaconvergencecriterionforflowrateaboutwhichthemomentumequationisiterativelysolvedtoproduceanaccuratepressurefield.Foreachinstantoftime,allthenodalpressuresonthemesharesolvedsimultaneously.Iterationisrequiredtoupdatetheshearrate,viscosity,andflowrateestimatesuntilfullconvergenceisachieved.Foracompressibleflow,thenetmassfluxmustequalanymassgainsorlosseswithintheelement3.Thenecessarysystemofequationscanbedeveloped,assembled,andsolvedusingaconventionalGalerkinformulationforafixedmeshandtransientmeltfront.Suchasimulationhasbeendeveloped,andwillbeutilizedinassessingstrategiesforprocessdevelopmentalongwithexperimentalvalidation.PROCESSDEVELOPMENTAnoverviewofinjectionmoldingcontrolisshowninFig.2.Attheinnermostlevel,onlythemachineactuatorsareregulated.ThislevelofcontrolwillensureproperexecutionoftheprogrammedmachineinputsFig.1.Atthesecondlevel,statevariablessuchasmelttemperatureandmeltpressurearecontrolledtotrackprespecifiedprofiles.Thiswillprovidemoreprecisecontrolofthestateofthemelt.Attheoutermostlevel,themachineinputsareadjustedtoimprovethequalityofthepartthroughbettersetpointsgivenqualityfeedback.MachineActuatorsProcessMachineControlSetPointControlStateVariableControlMachineFeedbackQualityFeedbackStateVariableFeedbackPartAttributesMachineInputsFigure2SystemdiagramofinjectionmoldingcontrolWhilemachinecontrolisimportant,itisthepolymerstatepressure,temperature,andmorphologywhichdirectlydeterminesthemoldedpartquality4,5.Assuch,thispaperfocusesonclosingtheloopbetweenthemachineparametersandthepolymerstate.Ifachieved,theseadvancedcontrolstrategieswouldprovideincreasedmoldedpartqualityandconsistency.CavityPressureControlAfundamentalstatevariablethatcanberegulatedduringthemoldingcycleiscavitypressure.Closedloopcontrolofcavitypressurecouldautomaticallycompensateforvariationsinmeltviscosityandinjectionpressuretoachieveaconsistentprocessanduniformsetofproductattributes6.Mannintroducedoneofthefirstpressurecontrolschemesbyusingmodulatedpressurereliefvalves7,andAbuFaradevelopedaprocesscontrolmodelbyrelatingthecavitypressureresponsetoopenloopperturbations8.Srinivasanlaterusedthesemodelstoproposealearningcontrollerforclosedloopcavitypressurecontrol9.Adaptivecontrolmethodshavealsobeenproposedtotrackcavitypressureprofileatusuallyonelocationinthemold1012.Unfortunately,cavitypressurecontrolsuffersfromthelackofasystematicmethodofdeterminingthepressureprofile.Inaddition,itishandicappedbytheabsenceofappropriateactuatorsfordistributedpressurecontrol,asconventionalmoldingmachinesareequippedwithonlyoneactuatorthescrewwhichdoesnotallowsimultaneouscavitypressurecontrolatmultiplepointsinthemold.ConsiderthemelttransportsysteminaconventionalcoldrunnermoldasshowninFig.3.Itisevidentthatthegeometryishardwiredintothemold.Therunnerlocationsarefixedandthegatedimensionsarealsofixed.Theresultingpressuredistributioncannotbecontrolledwithoutretoolingmoldsteel.Figure3TypicalPackingPressureDistributionToinvestigatethecontrollabilityoftheinjectionmoldingprocess,ahalffactorialdesignofexperiments13wasperformedtodeterminethemaineffectsbetweenthecriticalprocessparametersandthepartdimensions3Copyright©1999byASME−−−ScrewSpeedeTemperaturVelocityPressureLLL10.018.005.023.000.029.018.051.002.043.010.057.03214Inthisequation,themachineparametershavebeenscaledtotherangeof0to1,indicativeofthemaximumfeasibleprocessingrangeforthisapplication.Theresultingcoefficientsofthelinearmodelareactualchangeinpartdimensionsmeasuredinmm.Itshouldbenotedthatoncetoolingiscompleted,thedimensionalchangesavailablethroughprocessingarequitelimitedthoughfunctionallysignificant.Theprimaryconclusionthatshouldbedrawnfromeq.4,however,isthatallthedimensionsreactsimilarlytochangesintheprocesssettings.Thus,themoldingprocessbehavesasaonedegreeoffreedomprocessinwhichonlyonequalityattributeiscontrollable.OneofNamSuhsaxioms14ofdesignstatesthatindependenceoffunctionalrequirementsshouldbemaintained.Thisaxiomwasappliedtodevelopmultipledegreesoffreedomforcontrolofmeltflowandpressureinthemoldcavity.AsshowninFig.4,thevalvesmetertheflowofmeltfromtherunnersintothemoldcavity.Thepressuredropandflowrateofthemeltisdynamicallyvariedbytheaxialmovementofeachvalvestemwhichcontrolsthegapbetweenthevalvestemandthemoldwall.Bydecouplingthecontrolofthemeltatdifferentvalvestempositions,meltcontrolateachgatecanoverridetheeffectsofthemoldingmachineandprovidebettertimeresponseanddifferentialcontrolofthemelt.Eachvalveactsasanindividualinjectionunit,lesseningdependencyonmachinedynamics.Forclosedloopcontrol,manifoldpressuretransducerswereusedintherunnerdropsinsteadofinthecavity.Thisimplementationnotonlyprovideslowercostandgreaterreliability,butalsorendersaconventionalappearanceforthesystem.MeltInletValve1Valve2Cavity1Cavity2P1P2Figure4DynamicFlowRegulationDesignTheresultingcontrollabilityoftheinjectionmoldingprocessisdemonstratedinFig.5wheremultiplepressureprofilescanbemaintainedinthemoldcavityofasinglepart.Inthesamecycle,threedifferentmagnitudesofmeltpressurewereexertedatdifferentgatesinthesamemoldcavity.ThecontrolpressurefortheholdingstageatGate1is41.4MPa6000psi.,Gate2is41.4MPa6000psi.,Gate3is20.7MPa3000psi.,andGate4is62.1MPa9000psi..Inconventionalinjectionmolding,themeltpressurewouldbethesameatallgates.Thislevelofprocesscontrolhasnotpreviouslybeenachievedbyanymoldingtechnologythusfar.Eachgatecanexertaspecificholdingpressure.010203040506070024681012TimesecFigure5DynamicFlowRegulationDesignThematerialshrinkageanddimensionschangeatdifferinglocationsinthepartbasedonthepressurecontoursandhistoriesaroundthegates.Theabilitytochangeindividualdimensionsorotherqualityattributeswithoutretoolingmoldsteelprovidessignificantprocessflexibility.Itispossibletoaugmenteq.4withtheadditionaldegreesoffreedomandreexaminethecontrollabilityofthethreepartdimensions−−−−−−−P4P3P2P1ScrewSpeedeTemperaturVelocityPressureL3L2L121.000.002.000.016.000.017.010.000.060.031.000.001.003.002.001.000.005.009.003.001.008.005.002.05Therearetwosignificantimplicationsofthisresult.First,theclosedloopcontrolofcavitypressureshassignificantlyreducedthedependenceofpartdimensionsonmachinesettings,asevidencedbythereductioninthemagnitudeofcoefficientsfortheprimarymachinesettings.Thiseffecthasalsobeenevidencedbyreductionsinthestandarddeviationsofmultiplepartdimensionsbyanaveragefactoroffive,resultinginanincreaseintheprocesscapabilityindex,Cp,fromlessthan1tofarbeyond2.Second,thesecondmatrixineq.5isevidenceoftheimproveddimensionalcontrollabilityprovidedbythedynamicregulationofthecavitypressuredistribution.Ingeneral,changingthecavitypressureatthegateclosesttoadimensionprovidesthemajoreffectonpartdimensions.Additionally,independentcontrolofthevalvestemsprovidesthecapabilitytovarydimensionsatonelocationwithoutinterferingwithdimensionsatanotherlocation.Thisflexibilitydoesnotexistin4Copyright©1999byASMEconventionalmoldingbecauseholdpressurechangesintendedtoinfluenceoneareaofthepartcanbetransmittedtootherareasofthepartthroughthestaticfeedsystem.Itshouldbenoted,however,thatthetotalmagnitudeofdimensionalchangeavailablewithdynamicpressureregulationisapproximatelythesameasforconventionalmolding.Theseresultsmayhaveasignificantimpactontheproductandtoolingdevelopmentprocess.Currently,numericalmoldfillingsimulationsandexpertjudgmentsarecombinedtoestimatetheprocessbehaviorandmakecriticaldesigndecisions.Ifthesedecisionsareincorrect,thentoolingmodificationsmayberequired.Improvedcontrollabilityoftheinjectionmoldingprocesspermitscorrectionformanydesigninaccuraciesduringthemoldcommissioningstagewithoutretooling.Suchachangeinthedevelopmentprocesscouldsubstantiallyreducethetooldevelopmentcosts,shortenthedevelopmentcycle,andhastentimetomarket.Thedescribedprocessisalsosignificantinthatitmovespolymercontrolfromthemoldingmachinetothemolditself.Thisreducesthemoldingmachinetoapolymericpump.Variationsininjectionpressure,flowrates,packpressures,orpacktimesareallcompensatedthroughdynamicpressureandtemperaturecontrol.Themarketrepercussionscouldbesignificant,as1anoldmachinewithoutclosedloopcontrolcanprovideconsistencyequaltomodernmachines,and2amoldcommissionedonamoldingmachineintheUnitedStatesisensuredtoproduceconsistentpartsonamoldingmachineoverseas.Themoldbecomesitsownselfcontainedqualitycontrolmechanism.Assuch,thepotentialproductivityandqualitygainsaresubstantial.TemperatureControlThetypicalheatpathinthecoolingstageofinjectionmoldingisthatheatisconductedfromthehotpolymertothecomparativelycoldmold,thenconductedthroughthemoldtothecoolingline,whereitisconvectedawaybythecoolant.Recentresearchhasattemptedtodynamicallycontrolthethermalandfluidpropertiesofthemeltwithinthemoldingcycle.Whiledynamicpressurecontrolhasbeenprovenfeasible15andisbeingcommercialized,therelativelyslowthermaltransientshavepreventedsimilargainsinthermalmanagement.Thecoolingstageofinjectionmoldingcycleisnotidealforavarietyofreasonsimpactingboththeproductqualityandproductioneconomics.Theprocessphysicsdictatethatthemoldtemperaturemustbelessthanthepolymerheatdeflectiontemperaturesuchthatarigidpartisejected.However,thecoldmoldtemperatureconductsheatfromthehotpolymermelttothecoldmoldduringinjectioncausingthedevelopmentofaskinontheexteriorofthepartandpropagationoffrozenlayerstowardsthecoreofthepart.Thesefrozenlayersincreasetheflowresistance,makingthemoldcavitydifficulttofill.Sincefrozenlayersaredevelopedcontinuouslyduringinjectionandcooling,theylockinvaryinglevelsofstressandorientation.Thisvariationinpolymermorphologyasafunctionofthicknessreducesoptical,structural,andotherpartproperties1619.Tocompensateforthenegativeeffectsofcoldmoldwalls,manufacturersmayrunthemoldathighermoldtemperatures,highermelttemperatures,higherinjectionpressures,andhigherinjectionvelocities20,21.Alternatively,alowerviscositypolymerorhigherpartwallthicknessmayberequiredwithcostand/orperformancedisadvantages.Alloftheseoptionsnegativelyimpacttheeconomicsofproduction.Infact,theeconomicdriversdictatehighermoldtemperaturesduringinjectiontoallowthinpartwallthicknessesandlowinjectionpressuresbutlowermoldtemperaturesduringcoolingtoallowrapidsolidification.Thisoptimalmoldtemperaturecontrolstrategyisinfeasiblegivencurrentcontrolstrategiesandmaterialtechnologies.Thesizeofthemold,togetherwithitshighheatcapacityandthermalinertia,preventsdynamicclosedloopcontrolofthemoldsurface.Thisstatementisbasedonobjectiveanalysisaswellasobservationofprioracademicandindustrial2234.Forinstance,Jansen35,Chen36,andotherresearchershaveutilizedathermoelectricdevicewithinthemoldwalltodynamicallyheatandcoolaportionofthemold.However,thetimeresponseoftheseactivecontrolelementsisrelativelyslow,ontheorderofseconds.Also,thereislimitedabilitytoinducealargethermaldifferentialduetothemassandpropertiesofthemold.Alternativeresearchers25,26,31,32utilizedthininsulativecoatingsonthesurfaceofthemoldtodelaytheonsetoffreezinguntilafterpolymerinjection.Suchcoatingsdidnotprovideadequatedurability,butasimilartechniqueisbeingsuccessfullyutilizedbehindmetallicstampersinproductionofopticalmediatoreducethecycletimeby0.2seconds.Onabroaderscope,moldinsertswithhighthermalconductivity2729arebeingmorefrequentlyutilizedtoincreasetherateofheattransferinthickand/orhotsectionsofthepart.Aspreviouslystated,nothermoelectricorotherthermalactuatorexistswhichwillprovidethedesiredtransientmoldwalltemperaturecontrol.Moreover,otherpassiveelementssuchasinsulatorsorconductorscanonlydelayoraugmenttheflowofheatfromthepolymermelttothecoolingline.Itisevidencedfromthesepreviousattemptsthatdynamicclosedloopcontrolstrategieshavebeenunabletoeitherincreasetheperformanceofthemoldedpartorreducethemanufacturingcost.Coatingsandinserts–approacheswhichdonotuseactivecontrolelements–haveprovensomewhateffectiveandaregainingacceptanceandpenetrationinthemoldingindustry.Fortheplasticsindustry,anysuccessfultechnologymustrequirelittleadditionalcomplexityandcostwhilebeingsufficientrobustforhighvolumeproduction.Theobjectiveofcurrentresearchistodevelopanovelandmorecapablemethodfordynamiccontrolofmoldwalltemperaturethroughouttheinjectionmoldingprocess.Theresultingtechnologyshouldenablehighmoldwalltemperaturesduringtheinjectionandpackingstagestofacilitatepolymer
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