SIMTechTechnicalReport(PT/02/033/FT)DevelopmentofMouldDesignTechnologiesandProcessParameterStudiesinMicroMouldingProcessDrZhaoJianhongRobertMayesChenGeChanPohSingXieHong(FormingTechnologyGroup,2002)DevelopmentofMouldDesignTechnologiesandProcessParameterStudiesinMicroMouldingProcessPT/02/033/FTKeywords:Micro；Moulding；Polymer；Mould；Design11BACKGROUNDWiththeglobaltrendinplasticsinjectionmouldingindustryslantstowardsprecisionmouldingandminiaturisation,therearegrowingdemandsinthemanufacturingofmirco-precisioncomponentsinplastics.Micromouldingtechnologiescanthereforeopencompletelynewpossibilitiesformanyapplicationsindifferentdisciplines.Theabilitytoproduceacceptablemeteringaccuracyandhomogeneityofverysmallquantitiesofplasticmeltisakeychallengeinthemicromouldingprocess.Intheconventionalreciprocatingscrewinjectionmouldingprocess,polymermaterialsaremeltedandinjectedintomouldcavitiesthroughascrew-barrelsystem.Itisscarcelypossibletoproducepartsweighinglessthan0.1ginacostefficientmanneronconventionalinjectionmouldingmachines1.Thisisbecausetherearelimitationstoreducethescrewdimensionsduetoscrewstrengthandplasticpelletsfeedingconsiderations,andalsobecauseofthepossiblemeltbackflowencounteredinthescrewchannelswhenveryhighinjectionpressureisused.Moreover,inmicrocomponentmouldingusingconventionalinjectionmouldingmachines,cycletimesareusuallylongerthannecessarybecausethesprueandrunnersizesarenotalwaysproportionallyreducedduetotheconsiderationtominimisematerialresidencetimesandmeltdegradation.Anewgenerationofinjectionmouldingmachinesisthereforenecessaryforthemicromouldingtechnology.Inordertocontrolmeteringaccuracyandhomogeneityoftheverysmallquantitiesofmeltinthemicromouldingprocess,anewgenerationofmachinesthatuseaseparatescrewplasticisingunitandaplungerinjectionsystemhavebeendeveloped2,3,asshowninFigure1.Duringthemouldingprocess,plasticpelletsarefedintotheextrusionscrew,wheretheyareplasticisedbytheheatingandshearingeffects,andthenenterintothedosingbarrel.Whenthemeltinthedosingbarrelreachestheset-point,itwillbetransferredintotheinjectionbarrel,theinjectionplungerwilltheninjectthemeltintothemouldcavities.Tocapitaliseonanticipateddemandsformicroinjectionmouldingmachines,thesemachinebuildershavedevelopednovelsystemsdesignedtoconsistently,accurately,andrapidlymanufacturemouldedpartsthatweighlessthan0.1g4.Withtheexpectationofrapidlygrowingapplicationsformicro-mouldedparts,machinemanufacturershavepredictedthatthemarketformicro-mouldingmachineswillhavedoubledinthenextfewyearsduetothegrowthindemandformicro-sensorsinautomotiveapplicationsalone3.Figure1MicroinjectionmouldingsystemMicrosystemtechnologyisaninterdisciplinaryengineeringscience.Themicromouldingactivityencompassesthefullspectrumofpolymermouldingfromproductdesigntotooldesignandfabrication,andtheapplicationofinnovativeinjectiontechniques.Themouldingmachine,tooling,materialandprocess,aswellascomponenthandlingandinspection,needtobespeciallyaddressed.Forthenewgenerationofmicroinjectionmouldingmachines,newmoulddesignconceptsandmethodshavetobeadoptedtomeettherequirementsofthenewmachines.2OBJECTIVETheobjectiveofthisprojectwastodevelopmoulddesigntechniquesforthenewgenerationofmicroinjectionmouldingmachinesandtostudythemouldingbehaviourofpolymermaterialsinthemicromouldingsystem.ExtrusionscrewDosingsleevewithpressurepistonInjectionpistonHeaterSpruePlasticpartMouldMachineNozzleDevelopmentofMouldDesignTechnologiesandProcessParameterStudiesinMicroMouldingProcessPT/02/033/FT23METHODOLOGY3.1MicroPartDesignMoulddesignwasstudiedfortwomicrocomponents.Onecomponentstudiedwasalensarraywithnineteen“micro”lenssurfacesdesignedonthetopandbottomsidesofthelensarray,asshowninFigure2.Theoveralldimensionofthepartwas12x3x2mm.Thistypeoflensarrayiswidelyusedintheopticalindustry.Thepolymermaterialselectedforthiscomponentwaspolycarbonate(PC)toutilisetheadvantagesofitsopticalproperties,mechanicalandprocessproperties.Figure2MicrolensarraydesignFigure3MicrogeardesignTheothercomponentstudiedwasamicrogearwithashaftasshowninFigure3.Thegearwasan8-thoothgearwithatipdiameterof1mm.Therootdiameterofthegearwas0.55mmandthegearpressureanglewas200.Thepolymermaterialusedforthegearwaspolyoxymethylene(POM).3.2MicroMouldDesignMicromoulddesignandfabricationisaveryimportantareainthemicromouldingprocessengineering.Amouldisahighlysophisticateddevicethatcomprisesmanypartsrequiringhighqualitysteel.Whetheramicrocomponentcanbesuccessfullymouldedisdependent,toalargeextent,onthedesignandfabricationoftheinjectionmoulds.3.2.1MouldStructureTheinjectionnozzlesformicroinjectionmouldingmachinesareusuallydesignedinsuchawaythattheyareabletoprotrudeouttoreachthepartingplaneofthemould.Thisprovidesthepolymermeltwithaminimumpathdistancetothecavitiesandreducestherunnerwaste.Moulddesignersmustbearthisinmindwhendesignmicromouldsforsuchinjectionmouldingmachines,thoughdifferenttypesofmouldconfigurationsmaybeused5,e.g.two-plateorthree-platemoulds,withdifferenttypesofgatesandrunnerdesigns.Athree-platemoulddesign,asshowninFigure4,wasusedforthemicrocomponentsstudiedinthiswork.Twothree-platemicromouldsweredesignedandfabricatedforthelensarrayandthegearcomponentsrespectively.Themicrolensarraymouldwasasinglecavitytoolandthemouldforthemicrogearwasatwo-cavitymould.Becauseofthestructurefeaturesofthelensarraythegatingpointwasdesignedonthesideofthelensarray.Whileforthegearmould,thegatingpointwasdesignedatoneendoftheshaft.DevelopmentofMouldDesignTechnologiesandProcessParameterStudiesinMicroMouldingProcessPT/02/033/FT3Figure4Athree-platemouldwithinjectionnozzleextendedtothemiddleplate3.2.2NozzleContactandRunnerEjectionInaconventionalinjectionmouldingmachine,theinjectionnozzlecanmoveforwardandcomeincontactwiththemouldspruebushingunderahydraulicpressureduringthemeltinjectionprocess,directingthepolymermeltintothemould.Thenozzleoperatesasaleak-proofdevice,whichprovidesameltpassagewaywithaminimumpressureandthermalloss6.ForamicromouldingmachinewithanozzledesignasshowninFigure1,thenozzlepositionisfixedonceamouldismountedontothemachine,andthenozzleisforcedincontactwiththeinjectionmouldunderamechanicallockingforce.Forthetwomouldsusedinthepresentstudy,ataperedsurfacewasdesignedonthefixedplatetocontactwiththetaperedinjectionnozzlesurface,asschematicallyshowninFigure5.Itcanbeobservedfromthisfigurethatinthisdesignthefronttipoftheinjectionnozzleissubflushtothefixedplate.Aspacebetweenthenozzletipandthemovingplatewillbeformedwhenthemouldcloses,andathindisk-shaperunnerwillbeformedinthefrontofthenozzleduringthemeltinjectionprocess.Sincethetaperedsurfaceonthefixedplateofthemouldisextendedtotheedgeoftheplate,areversetaperedanglewillbeformedatthetipofthedisk-shaperunnerduringtheformingprocess.Becauseofthereverseangleonthedisk-shapedrunner,therunnerwillbekeptonthefixedplatesidewhenthemouldopens,thusbreakingthegatefromthepartatthepartingline.Figure5Contactbetweennozzleandmouldplate3.3RheologicalPropertiesSt