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ORIGINALARTICLESurfacemicrostructurereplicationininjectionmoldingUffeArløTheiladeHansNørgaardHansenReceived13January2006/Accepted20July2006/Publishedonline24October2006SpringerVerlagLondonLimited2006AbstractInrecentyears,polymercomponentswithsurfacemicrostructureshavebeeninrisingdemandforapplicationssuchaslabonachipandopticalcomponents.Injectionmoldinghasproventobeafeasibleandefficientwaytomanufacturesuchcomponents.Ininjectionmolding,themoldsurfacetopographyistranscribedontotheplasticpartthroughcomplexmechanisms.Thisreplication,however,isnotperfect,andthereplicationqualitydependsontheplasticmaterialproperties,thetopographyitself,andtheprocessconditions.Thispaperdescribesanddiscussesaninvestigationofinjectionmoldingofsurfacemicrostructures.Thefundamentalproblemofsurfacemicrostructurereplicationhasbeenstudied.TheresearchisbasedonspecificmicrostructuresasfoundinlabonachipproductsandonroughsurfacesgeneratedfromEDMelectrodischargemachiningmoldcavities.Emphasisisputontheabilitytoreplicatesurfacemicrostructuresundernormalinjectionmoldingconditions,i.e.,withcommoditymaterialswithintypicalprocesswindows.Itwasfoundthatwithintypicalprocesswindowsthereplicationqualitydependssignificantlyonseveralprocessparameters,andespeciallythemoldtemperature.Forthespecificmicrostructures,evidencesuggeststhatstepheightreplicationqualitydependslinearlyonstructurewidthinacertainrange.KeywordsMicrostructures.Molding.Microinjectionmolding.Microtopography1IntroductionTheuseofmicroproductsandmicrocomponentshasbeenstronglyincreasingoverthepastdecade.Applicationspointattheuseofpolymersasfeasibleengineeringmaterials.Microtechnologyapplications,suchaslabonachipproductsofteninvolvesurfacemicrostructures,e.g.,intheformoffluidchannels.Moreover,asproductsarescaleddown,thesurfaceingeneralbecomesincreasinglyimportant.Hence,whetherintheformoffunctionalstructuressuchasfluidchannelsormoregeneralsurfacepropertiessuchasroughness,theabilitytocontrolthesurfacetopographyisimportantinthefieldofmicrotechnology.Injectionmoldinghasproventobeafeasibleandefficientwaytomanufacturecomponentslikelabonachippartsinpolymermaterials.Ininjectionmolding,themoldsurfacetopographyistranscribedontotheplasticpartthroughcomplexmechanisms.Thisreplication,however,isnotperfect,andthereplicationqualitydependsonthepolymermaterialproperties,thetopographyitself,andtheprocessconditions.Thispaperdescribesanddiscussesaninvestigationofinjectionmoldingofsurfacemicrostructures.Thescopeisthegeneralreplicationproblempertainingbothreplicationofspecificstructuresandreplicationofroughsurfaces.Emphasisisputontheabilitytoreplicatesurfacemicrostructuresundernormalinjectionmoldingconditions,notablywithlowcostmaterialsatmoderatemoldtemperatures.Overthelast10yearsorso,impressivereplicationresultshavebeenreportedforinjectionmoldingwithhighaspectratiomicrostructures.Figure1providesanoverviewofsomeofthedimensionalscalesrecentlyreported1–9.ThepreferredmaterialsforinjectionmoldingwithmicrostructuredsurfacesseemtobePCpolycarbonate1–6IntJAdvManufTechnol200733157–166DOI10.1007/s001700060732yU.A.TheiladeH.N.HansenDepartmentofManufacturingEngineeringandManagement,TechnicalUniversityofDenmark,Produktionstorvet,Building427S,2800Kgs.Lyngby,Denmarkemailhnhipl.dtu.dkandPMMApolymethylmetacrylate3,4,6,8,butnovelmaterialssuchasCOCcyclicolefincopolymer3,6havealsoreceivedfocus.Asthemoldtemperaturehasbeenidentifiedasthemostcriticalprocessparameter1–3toensurehighfidelityreplication,theconventionalinjectionmoldingprocesshastypicallybeenmodifiedtothesocalledvariothermconcept1orrunwithpermanentlyhighmoldtemperatures2.Replicationofsubmicronstructureswithaspectratiosofmorethantencanbeachievedbyapplyinghotmoldsurfaces2.Undervariotherm,themoldsurfaceisheatedtoatemperatureabovetheplasticmaterialstransitiontemperatureandsubsequentlycooledinordertofacilitatedemolding.Thevariothermprocessisrelativelycomplicatedcomparedtoconventionalinjectionmoldingandresultsincycletimesoftenlongerthana1min1.Incomparison,equivalentcycletimesinconventionalinjectionmoldingdowntoapproximately5scanbeachieved.Withconventionalinjectionmoldingandwithintypicalprocesswindows,nearperfectreplicationofrectangular0.20.2μmprofileshasbeenachieved3.However,inthisprocess,specializedmaterialgradeswereemployed.2SurfacetechnologyThesurfacegeometryofanartefactcanbedescribedatdifferentgeometricallevelscommonlystratifiedasform,waviness,androughness.Attheextremes,formdescribesthemacroscopicgeometryofthesurface,whileroughnessdescribesthesurfacemicrogeometryofthepart.Theconceptoftopographyconcernsallgeometricsurfacefeatures10.Engineeredsurfacescanbeclassifiedasstructuredorunstructuredsurfaces.Structuredsurfacescontainadeterministicandsystematicstructurewithorwithoutdirection.Unstructuredengineeredsurfacesappearasarandomstructure,butareresultsofdeliberatesurfacealterationthroughamanufacturingprocess10.ThetopicofsurfacetechnologyinvolvesthreemainelementsGeneration,function,andcharacterization,asdescribedinthefollowingtext11.2.1GenerationInjectionmoldingisinherentlyareplicationprocesswheretheplasticpartisproducedasanegativereplicaofthemoldcavity.Thereplicationprocessdefinesthegeometricalboundariesoftheplasticpartandoccursatdifferentgeometricallevels.Ingeneral,thereplicationisnotperfect,andtheplasticpartdiffersgeometricallyfromtheinversegeometryofthemoldcavity.Itisdesirabletobeabletocontrolthedegreeofreplicationperfectionorreplicationquality.Thisrequiressomeunderstandingofthephysicalmechanismsofreplicationorsimilarempiricallybasedknowledge.Thenatureofreplicationimperfectiondiffersbetweenthegeometricallevels.Atthemacrolevel,replicationimperfectionistypicallyobservedasshrinkageandwarp,andinmorespecialcases,assinkmarks.Suchphenomenaarerelativelywellunderstoodandcanbepredictedanalytically12,13ornumerically14,15withconsiderableaccuracy.Atthemicrolevel,replicationisaquestionofmoldtopartsurfacetopographytranscription.Thesemechanismsare,however,lesswellunderstood.2.2FunctionThesurfacetopographyofinjectionmoldedplasticpartscanbeimportantforaestheticalandtechnicalreasons.TheFig.1Replicatedsurfacefeaturedimensionsforasampleofpublishedresearchinthefieldofmicroinjectionmolding1–9.Bubblesizeindicatesreplicatedaspectratioshadedbubbleindicatesthatelevatedmoldtemperaturewasapplied158IntJAdvManufTechnol200733157–166aestheticalimplicationsofsurfacetopographyrelatetovisualandtactileperceptionissuessuchasgloss,colorperception,andgenerallookandfeelexperience.Theseparametershaveahighpriorityinmanyelectronicconsumerproductslikemobilephonesandaudiovisualequipment16.Surfacemicrotopographycanalsohaveaestheticalrelevancewhenusedtoconcealsurfacedefectssuchassinkmarksandweldlines17,18.ThetechnicalrelevanceofsurfacemicrotopographyiscomprisedofabroadspectrumofperformancerelatedfunctionsandmechanismsasdemonstratedinTable119.Thesetopographydependentpropertiesarerelevantforalargenumberoftraditionaltechnicalcomponents.Withtheemergenceofmicroengineeringandnanotechnology,additionalfunctionalaspectsofsurfacetopographyfollow.Importantapplicationsinthesefieldswheresurfacetopographyiscrucialincludecomputercomponents,microelectromechanicalsystemsMEMS,biomedicalsystems,opticalapplications,andchemicalsystems20.Inconnectionwithinjectionmolding,manyoftheseapplicationsarerelevant.AccordingtoMönkkönenetal.3,prominentexamplesincludeμTASmicrototalanalysissystemsorlabonachipcomponents,CDs,DVDs,securityanddecorativeholograms,brightnessenhancementfoils,lightcollimators,andDOEsdiffractiveopticalelements.AspotentialapplicationsforHARMshighaspectratiomicrostructures,Despaetal.7mentionheatexchangers,catalystsubstrates,andsealfaces.AdditionalexamplesofapplicationsareshowninTable28.MEMSandopticalsurfacescangenerallyberegardedasengineeredstructuredsurfacesand,assuch,fallinanothercategorythane.g.,EDMsurfaces.However,replicationofthestructuredandunstructuredsurfaceswithinjectionmoldingconceptuallyembodiesthesameproblem.Asubstantialnumberofarticlesaboutthereplicationofstructuredsurfaceshavebeenpublished,buttheliteratureonroughnessreplicationininjectionmoldingisquitescarce.2.3CharacterizationThetopographicalcharacterizationofplasticpartsrepresentsachallengeofitsown.Theweaklyreflectingandrelativelysoftplasticsurfacesposetoughrequirementsforthecharacterizationinstruments10,21,andcontactlesscharacterizationispreferred.For21/2Dstructures,theISO5436stepheightdefinitionlendsitselfwellasatopographicalamplitudemeasureFig.2.Concerningroughnesscharacterization,threedimensionaltopographycharacterizationisarelativelynovelareathatisstillbeingdeveloped.Standardizedcharacterizationproceduresdonotexistandcarefulmetrologicalconsiderationsmustbegiventotheindividualcases.Inthetwodimensionalregime,topographyparametersarewellestablishedandstandardizedasinISO4287.Asimilarbodyofstandardshasnotyetbeenestablishedforthreedimensionalparameters.AprimarysetofthreedimensionalparameterswasproposedbyStoutandBlunt19.ThesesocalledBirmingham14Table1ExamplesoffunctionalimplicationsofsurfacemicrotopographySurfaceusecategoryFunction/mechanismTranslationalsurfacesFrictionWearSealingStaticcontactsurfacesAdhesionandbondingFatigueStressFractureNoncontactsurfacesReflectivityGlossPlatingPaintingHygieneBasedon19Table2OverviewandexamplesofMEMSandMEMSlikeapplicationsTypeofapplicationExamplesElectroopticalcomponentsSwitchesDiffractiongratingsMiniaturelensesMirrorsMechanicaldevicesWatchcomponentsPrinterheadsAutomotivesensorsMicroheatexchangersMicropumpsMedicalandchemicalchipsFuelcellsHearingaidsGenechipsDrugdeliverysystemsBiosensorsCompiledfrom8Fig.2FatlinesindicateISO5436stepheightreferences.AxisunitsμmIntJAdvManufTechnol200733157–166159parameterscanberegardedasadefactostandard.Inthecurrentpaper,thescopeislimitedtotheamplitudeparametersoftheBirmingham14parametersexcludingSzaslistedinTable3.3MicroinjectionmoldingandreplicationMicroinjectionmoldingcanbeusedastheheadlineforinjectionmoldingofcomponentswithoneofthefollowingcharacteristics–Verylowshotweightswithcriticaldimensionsintheμmrange.–Largerproductswithfunctionalfeaturesandatleastonecriticaldimensionintheμmrange.Conceptually,topographicalreplicationqualitycanbedefinedasthedegreeofsimilaritybetweentheplasticandtheinvertedmoldsurface.Asthereplicationprocesstransformspositivetopographytonegative,perfectreplicationcorrespondstotheinvertedmoldsurface.Thereplicationofsurfacemicrostructuresininjectionmoldingisbelievedtobedeterminedbythefollowingthreemainfactors–Drivingforce–Materialdeformability–MicrostructuregeometryThedrivingforceisestablishedbythecavitypressurethatarisesduetothecavityfillingandlatertheholdingpressure.Materialdeformabilityiscontrolledbymaterialpropertiessuchasviscosityandelasticityofthematerial,whichagainarestronglyinfluencedbythetemperature.Insomecases,thematerialdeformabilitymaybeattributedtothesizeofthefrozenlayerofplasticmaterialagainstthemoldwall.Themicrostructuregeometryaffectsthereplicationinsuchawaythatsmallerstructureswithhigheraspectratiosareincreasinglymorechallengingtoreplicate.4Replicationofaspecificstructure4.1ExperimentalsetupspecificstructureTheexperimentalworkwasbasedonasimple100241mmrulertypepartmoldedinatwoplatemoldwithaconventionalcoolingsystemandacoldrunnersystemincludinga0.6mmfilm/fangate.Thisgeometricalconfigurationensuredanevenandessentiallyonedimensionalmeltfrontadvancementinthecavity.Thecavitywasequippedwithnickelinsertscontaining21/2Drectangularstructureswithheightsof9μmandaspectratiosfrom0.2to1,manufacturedbylithographyandsubsequentelectrochemicalplatingTable4.ProductiontookplacewithanEngelES80/25HLinjectionmoldingmachineandthePPpolypropylenegradeBasellMoplenHP501HTypeHomopolymermeltflowrateMFR2.1g/10min230°C/2.16kgISO1133heatdeflectiontemperatureB0.45MPa85°CISO75B1,2.Themoldtemperaturewaskeptconstantatapproximately50°C,whilebarreltemperaturesof220,250,and280°Cwereemployed.Injectionflowrateandholdingpressureswitchoveratapproximately99partfillingweresetat35cm3/sand44MPamelt,respectively.Atallbarreltemperaturelevels,additionalserieswererunwithinjectionflowratesof20and50cm3/s.Finally,theeffectofhighholdingpressure89MPawasexploredat220°CbarreltemperatureTable5.DetailedprocessanalysiswascarriedoutwiththesimulationsoftwareMoldFlowMPI4.0midplane,CoolandFlow.Byusingsimulation,measuressuchasplasticsurfacetemperatureandfrozenlayerthicknesscouldbetrackedduringthemoldingcycle.Basedonpressurecurvestudiesthesoftwarehasbeenfoundtogivereasonablyaccurateresults21.TopographicalcharacterizationofmicrostructuresonplasticsurfacescanbeperformedwithsuchinstrumentsasAFM,SEM,interferencemicroscopes,confocalmicroscopes,focusdetectioninstrumentsandstylusbasedinstruments.Acomparisonandevaluationofsomeofthesetechniqueshasbeenreportedin22,23.Forthepurposeathand,aconfocalmicroscopewasselectedbasedonthegroundsofitsnoncontactnature,fastoperation,andreliableresults.TheinstrumentusedwasaZeissLSM5PascalconfocallaserscanningmicroscopeequippedwithEpiplanobjectives2.5–50andaHeNelaser.TheacquiredsurfacetopographieswereanalyzedinthesoftwarepackageSPIP.Thetotaluncertaintyforthemeasuredplasticmicrostructuresincludinginjectionmoldingprocessvariance,locationofreferencearea,andinstrumenterrorwasestimatedTable3TheselectionofBirmingham14threedimensionaltopographyparametersusedinthisinvestigation19CategorySymbolNameAmplitudeSqRootmeansquaredeviationSskSkewnessofsurfaceheightdistributionSkuKurtosisofsurfaceheightdistributionTable4DimensionsofrectangularmoldmicrostructureprofilesStructureWidthinmoldμmA150.0B122.1B223.7C19.6C211.3Allmoldstructuresare9μmdeep160IntJAdvManufTechnol200733157–166
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