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ORIGINALARTICLESurfacemicrostructurereplicationininjectionmoldingUffeArlTheilade&HansNrgaardHansenReceived:13January2006/Accepted:20July2006/Publishedonline:24October2006#Springer-VerlagLondonLimited2006AbstractInrecentyears,polymercomponentswithsur-facemicrostructureshavebeeninrisingdemandforapplicationssuchaslab-on-a-chipandopticalcomponents.Injectionmoldinghasproventobeafeasibleandefficientwaytomanufacturesuchcomponents.Ininjectionmolding,themoldsurfacetopographyistranscribedontotheplasticpartthroughcomplexmechanisms.Thisreplication,how-ever,isnotperfect,andthereplicationqualitydependsontheplasticmaterialproperties,thetopographyitself,andtheprocessconditions.Thispaperdescribesanddiscussesaninvestigationofinjectionmoldingofsurfacemicrostruc-tures.Thefundamentalproblemofsurfacemicrostructurereplicationhasbeenstudied.Theresearchisbasedonspecificmicrostructuresasfoundinlab-on-a-chipproductsandonroughsurfacesgeneratedfromEDM(electrodischargemachining)moldcavities.Emphasisisputontheabilitytoreplicatesurfacemicrostructuresundernormalinjection-moldingconditions,i.e.,withcommoditymateri-alswithintypicalprocesswindows.Itwasfoundthatwithintypicalprocesswindowsthereplicationqualitydependssignificantlyonseveralprocessparameters,andespeciallythemoldtemperature.Forthespecificmicrostructures,evidencesuggeststhatstep-heightreplicationqualitydependslinearlyonstructurewidthinacertainrange.KeywordsMicrostructures.Molding.Microinjectionmolding.Microtopography1IntroductionTheuseofmicroproductsandmicrocomponentshasbeenstronglyincreasingoverthepastdecade.Applicationspointattheuseofpolymersasfeasibleengineeringmaterials.Microtechnologyapplications,suchaslab-on-a-chipproductsofteninvolvesurfacemicrostructures,e.g.,intheformoffluidchannels.Moreover,asproductsarescaleddown,thesurfaceingeneralbecomesincreasinglyimportant.Hence,whetherintheformoffunctionalstructuressuchasfluidchannelsormoregeneralsurfacepropertiessuchasroughness,theabilitytocontrolthesurfacetopographyisimportantinthefieldofmicrotechnology.Injectionmoldinghasproventobeafeasibleandefficientwaytomanufacturecomponentslikelab-on-a-chippartsinpolymermaterials.Ininjectionmolding,themoldsurfacetopographyistranscribedontotheplasticpartthroughcomplexmechanisms.Thisreplication,however,isnotperfect,andthereplicationqualitydependsonthepolymermaterialproperties,thetopogra-phyitself,andtheprocessconditions.Thispaperdescribesanddiscussesaninvestigationofinjectionmoldingofsurfacemicrostructures.Thescopeisthegeneralreplicationproblempertainingbothreplicationofspecificstructuresandreplicationofroughsurfaces.Emphasisisputontheabilitytoreplicatesurfacemicrostructuresundernormalinjection-moldingcondi-tions,notablywithlow-costmaterialsatmoderatemoldtemperatures.Overthelast10yearsorso,impressivereplicationresultshavebeenreportedforinjectionmoldingwithhighaspectratiomicrostructures.Figure1providesanoverviewofsomeofthedimensionalscalesrecentlyreported19.ThepreferredmaterialsforinjectionmoldingwithmicrostructuredsurfacesseemtobePC(polycarbonate)16IntJAdvManufTechnol(2007)33:157166DOI10.1007/s00170-006-0732-yU.A.Theilade:H.N.Hansen(*)DepartmentofManufacturingEngineeringandManagement,TechnicalUniversityofDenmark,Produktionstorvet,Building427S,2800Kgs.Lyngby,Denmarke-mail:hnhipl.dtu.dkandPMMA(polymethylmetacrylate)3,4,6,8,butnovelmaterialssuchasCOC(cyclicolefincopolymer)3,6havealsoreceivedfocus.Asthemoldtemperaturehasbeenidentifiedasthemostcriticalprocessparameter13toensurehighfidelityreplication,theconventionalinjection-moldingprocesshastypicallybeenmodifiedtotheso-calledvario-thermconcept1orrunwithpermanentlyhighmoldtemperatures2.Replicationofsub-micronstructureswithaspectratiosofmorethantencanbeachievedbyapplyinghotmoldsurfaces2.Undervario-therm,themoldsurfaceisheatedtoatemperatureabovetheplasticmaterialstransitiontemperatureandsubsequent-lycooledinordertofacilitatedemolding.Thevario-thermprocessisrelativelycomplicatedcomparedtoconventionalinjectionmoldingandresultsincycletimesoftenlongerthana1min1.Incomparison,equivalentcycletimesinconventionalinjectionmoldingdowntoapproximately5scanbeachieved.Withconventionalinjectionmoldingandwithintypicalprocesswindows,near-perfectreplicationofrectangular0.20.2mprofileshasbeenachieved3.However,inthisprocess,specializedmaterialgradeswereemployed.2SurfacetechnologyThesurfacegeometryofanartefactcanbedescribedatdifferentgeometricallevelscommonlystratifiedasform,waviness,androughness.Attheextremes,formdescribesthemacroscopicgeometryofthesurface,whileroughnessdescribesthesurfacemicrogeometryofthepart.Theconceptoftopographyconcernsallgeometricsurfacefeatures10.Engineeredsurfacescanbeclassifiedasstructuredorunstructuredsurfaces.Structuredsurfacescontainadeterministicandsystematicstructurewithorwithoutdirection.Unstructuredengineeredsurfacesappearasarandomstructure,butareresultsofdeliberatesurfacealterationthroughamanufacturingprocess10.Thetopicofsurfacetechnologyinvolvesthreemainelements:Generation,function,andcharacterization,asdescribedinthefollowingtext11.2.1GenerationInjectionmoldingisinherentlyareplicationprocesswheretheplasticpartisproducedasanegativereplicaofthemoldcavity.Thereplicationprocessdefinesthegeometricalboundariesoftheplasticpartandoccursatdifferentgeometricallevels.Ingeneral,thereplicationisnotperfect,andtheplasticpartdiffersgeometricallyfromtheinversegeometryofthemoldcavity.Itisdesirabletobeabletocontrolthedegreeofreplicationperfectionorreplicationquality.Thisrequiressomeunderstandingofthephysicalmechanismsofreplicationorsimilarempiricallybasedknowledge.Thenatureofreplicationimperfectiondiffersbetweenthegeometricallevels.Atthemacrolevel,replicationimperfectionistypicallyobservedasshrinkageandwarp,andinmorespecialcases,assinkmarks.Suchphenomenaarerelativelywellunderstoodandcanbepredictedanalytically12,13ornumerically14,15withconsiderableaccuracy.Atthemicrolevel,replicationisaquestionofmold-to-partsurfacetopographytranscription.Thesemechanismsare,however,lesswellunderstood.2.2FunctionThesurfacetopographyofinjection-moldedplasticpartscanbeimportantforaestheticalandtechnicalreasons.TheFig.1Replicatedsurface-fea-turedimensionsforasampleofpublishedresearchinthefieldofmicroinjectionmolding19.Bubblesizeindicatesreplicatedaspectratio；shadedbubbleindicatesthatelevatedmoldtemperaturewasapplied158IntJAdvManufTechnol(2007)33:157166aestheticalimplicationsofsurfacetopographyrelatetovisualandtactileperceptionissuessuchasgloss,colorperception,andgeneral“look-and-feel”experience.Theseparametershaveahighpriorityinmanyelectronicconsumerproductslikemobilephonesandaudio-visualequipment16.Surfacemicrotopographycanalsohaveaestheticalrelevancewhenusedtoconcealsurfacedefectssuchassinkmarksandweldlines17,18.ThetechnicalrelevanceofsurfacemicrotopographyiscomprisedofabroadspectrumofperformancerelatedfunctionsandmechanismsasdemonstratedinTable119.Thesetopography-dependentpropertiesarerelevantforalargenumberoftraditionaltechnicalcomponents.Withtheemergenceofmicroengineeringandnanotechnology,additionalfunctionalaspectsofsurfacetopographyfollow.Importantapplicationsinthesefieldswheresurfacetopographyiscrucialincludecomputercomponents,microelectro-mechanicalsystems(MEMS),biomedicalsystems,opticalapplications,andchemicalsystems20.Inconnec-tionwithinjectionmolding,manyoftheseapplicationsarerelevant.AccordingtoMnkknenetal.3,prominentexamplesincludeTAS(micrototalanalysissystems)orlab-on-a-chipcomponents,CDs,DVDs,securityanddecorativeholograms,brightness-enhancementfoils,lightcollimators,andDOEs(diffractiveopticalelements).AspotentialapplicationsforHARMs(highaspectratiomicro-structures),Despaetal.7mentionheatexchangers,catalystsubstrates,andsealfaces.AdditionalexamplesofapplicationsareshowninTable28.MEMSandopticalsurfacescangenerallyberegardedasengineeredstructuredsurfacesand,assuch,fallinanothercategorythane.g.,EDMsurfaces.However,replicationofthestructuredandunstructuredsurfaceswithinjectionmoldingconceptuallyembodiesthesameproblem.Asubstantialnumberofarticlesaboutthereplicationofstructuredsurfaceshavebeenpublished,buttheliteratureonroughnessreplicationininjectionmoldingisquitescarce.2.3CharacterizationThetopographicalcharacterizationofplasticpartsrepre-sentsachallengeofitsown.Theweaklyreflectingandrelativelysoftplasticsurfacesposetoughrequirementsforthecharacterizationinstruments10,21,andcontact-lesscharacterizationispreferred.For21/2Dstructures,theISO5436step-heightdefinitionlendsitselfwellasatopographicalamplitudemeasure(Fig.2).Concerningroughnesscharacterization,three-dimensionaltopographycharacterizationisarelativelynovelareathatisstillbeingdeveloped.Standardizedcharacterizationproce-duresdonotexistandcarefulmetrologicalconsiderationsmustbegiventotheindividualcases.Inthetwo-dimensionalregime,topographyparametersarewellestablishedandstandardizedasinISO4287.Asimilarbodyofstandardshasnotyetbeenestablishedforthree-dimensionalparameters.Aprimarysetofthree-dimensionalparameterswasproposedbyStoutandBlunt19.Theseso-calledBirmingham-14Table1ExamplesoffunctionalimplicationsofsurfacemicrotopographySurface-usecategoryFunction/mechanismTranslationalsurfacesFrictionWearSealingStaticcontactsurfacesAdhesionandbondingFatigueStressFractureNon-contactsurfacesReflectivityGlossPlatingPaintingHygieneBasedon19Table2OverviewandexamplesofMEMSandMEMS-likeapplicationsTypeofapplicationExamplesElectro-opticalcomponentsSwitchesDiffractiongratingsMiniaturelensesMirrorsMechanicaldevicesWatchcomponentsPrinterheadsAutomotivesensorsMicro-heatexchangersMicropumpsMedicalandchemicalchipsFuelcellsHearingaidsGenechipsDrugdeliverysystemsBio-sensorsCompiledfrom8Fig.2FatlinesindicateISO5436step-heightreferences.Axisunits:mIntJAdvManufTechnol(2007)33:157166159parameterscanberegardedasadefactostandard.Inthecurrentpaper,thescopeislimitedtotheamplitudeparametersoftheBirmingham-14parameters(excludingSz)aslistedinTable3.3MicroinjectionmoldingandreplicationMicroinjectionmoldingcanbeusedastheheadlineforinjectionmoldingofcomponentswithoneofthefollowingcharacteristics:Verylowshotweightswithcriticaldimensionsinthemrange.Largerproductswithfunctionalfeaturesandatleastonecriticaldimensioninthemrange.Conceptually,topographicalreplicationqualitycanbedefinedasthedegreeofsimilaritybetweentheplasticandtheinvertedmoldsurface.Asthereplicationprocesstrans-formspositivetopographytonegative,perfectreplicationcorrespondstotheinvertedmoldsurface.Thereplicationofsurfacemicrostructuresininjectionmoldingisbelievedtobedeterminedbythefollowingthreemainfactors:DrivingforceMaterialdeformabilityMicrostructuregeometryThedrivingforceisestablishedbythecavitypressurethatarisesduetothecavityfillingandlatertheholdingpressure.Materialdeformabilityiscontrolledbymaterialpropertiessuchasviscosityandelasticityofthematerial,whichagainarestronglyinfluencedbythetemperature.Insomecases,thematerialdeformabilitymaybeattributedtothesizeofthefrozenlayerofplasticmaterialagainstthemoldwall.Themicrostructuregeometryaffectstherepli-cationinsuchawaythatsmallerstructureswithhigheraspectratiosareincreasinglymorechallengingtoreplicate.4Replicationofaspecificstructure4.1Experimentalset-up:specificstructureTheexperimentalworkwasbasedonasimple100241mmruler-typepartmoldedinatwo-platemoldwithaconven-tionalcoolingsystemandacoldrunnersystemincludinga0.6-mmfilm/fangate.Thisgeometricalconfigurationensuredanevenandessentiallyone-dimensionalmeltfrontadvancementinthecavity.Thecavitywasequippedwithnickelinsertscontaining21/2Drectangularstructureswithheightsof9mandaspectratiosfrom0.2to1,manufacturedbylithographyandsubsequentelectrochem-icalplating(Table4).ProductiontookplacewithanEngelES80/25HLinjection-moldingmachineandthePP(polypropylene)gradeBasellMoplenHP501H(Type:Homopolymer；meltflowrate(MFR):2.1g/10min(230C/2.16kgISO1133)；heatdeflectiontemperatureB(0.45MPa):85C(ISO75B-1,2).Themoldtemperaturewaskeptconstantatapprox-imately50C,whilebarreltemperaturesof220,250,and280Cwereemployed.Injectionflowrateandholdingpressure(switch-overatapproximately99%partfilling)weresetat35cm3/sand44MPa(melt),respectively.Atallbarreltemperaturelevels,additionalserieswererunwithinjectionflowratesof20and50cm3/s.Finally,theeffectofhighholdingpressure(89MPa)wasexploredat220Cbarreltemperature(Table5).DetailedprocessanalysiswascarriedoutwiththesimulationsoftwareMoldFlowMPI