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JournaloftheEuropeanCeramicSociety31(2011)2543-2550Inkjetprintingceramics:FromdropstosolidB.DerbySchoolofMaterials,UniversityofManchester,OxfordRoad,ManchesterM139P1.,UKAvailableonline16February2011AbstractInkjetprintingisapowerfulInicrofabricationtoolthathasbeenappliedtothemanufactureofceramiccomponents.Tosuccessfullyfabricateceramicobjectsanumberofconditionsmustbesatisfiedconcerningfluidpropertiesanddropplacementaccuracy.Ithasbeenproposedthatfluidsareprintablewithinthebounds1<Z<10(whereZistheinverseoftheOhnesorgenumber)andtheselimitsareshowntobeconsistentwithceramicsuspensionsdeliveredbypiezoelectricdrop-on-demandinkjetprinters.Thephysicalprocessesthatoccurduringdropimpactandspreadingarereviewedandtheseareshowntodefinetheminimumfeaturesizeattainablef()ragivenprinteddropdiameter.Finallythedefectsthatcanoccurduringthedryingofprinteddropsarereviewed(coffeestaining)andmechanismsandmethodologiestoreducethisphenomenonarediscussed.Keywords:Inkjetprinting;Shaping;Drying;Suspensions1. IntroductionInkjetprintinghasmajorcommercialapplicationsingraph-icsoutputandotherconventionalprintingoperations.However;therehasbeendevelopinginterestinusinginkjetprintingtomanufacturecomponentswithapplicationsfor:displays,1plas-ticelectronics,2rapidprototyping,3tissueengineering,4andceramiccomponentmanufacture.5Asignicantandfundamen-taldifferencebetweenthesenewapplicationsandthemorewidespreadapplicationofprintingtextorimagesisthebehaiouroftheprintedinkdropletsontheprintedsubstrate.Mostimagesareconstructedbythedepositionofdiscretedropletsand,inordertooptimiseresolutionandcontrast,thesedropletsareiso-latedanddonotcontacteachother.Incontrast,manyofthenewapplicationsforinkjetprintingenvisagethemanufactureofcontinuous1-,2-,or3-dimensionalstructures(1-,2-,or3-D).Suchstructuresrequireacontinuousdistributionofmaterialandthisnecessitatescontactandadhesionbetweenindividualdropsafterprinting.Inkjetprintingconstructsobjectsbytheprecisionplacementofpicolitrevolumesofliquidandthustheinitialinterac-tionbetweenprintedmaterialandasubstrateisaliquid/solidinteraction.Ultimately,theprinteddepositundergoesasolid-ificationprocessthatcanoccurthroughsolventevaporation,temperatureinducedsolidification/gelationorchemicalreac-tion.Considerationsoftherelativetimescalesofdropspreadingandsolidificationindicatethattherewillbeasignificantperiodoftimeafterprintingwhenaliquidispresentonasurface6andthusthemorphologicalstabilityofcoallescingliquidfilmsmustbeexamined,asmusttheeffectsofthesolidificationprocess.Therehasbeenaconsiderablenumberofpublicationsontheuseofinkjetprintinginthemanufactureofceramics.7-17Thesepriorstudieshaveusedallinkjetdropgenerationtechnologies(continuous,thermaldrop-on-demandandpiezoelectricdrop-on-demand)tosuccessfullyproduceceramicobjectsusingbothsolventevaporationandphase-changesolidification.Industrialinkjetprintingtechnologynowusespiezoelectricdrop-on-demand(DOD)generationtechnologyandthisisthechosenmethodformostapplicationsinprintingfunctionalmaterials.ThephysicaloperationofthesedifferentprintingtechnologiesandthereasonsforthechoiceofpiezoelectricDODprintinghavebeendiscussedindetailelsewhere6,18;henceherewewillconfineourconsiderationstothistechnology.Wewillalsoonlyconsidertheprintingofceramicinksthatsolidifythroughsol-ventevaporation.Despiteearlierworkdemonstratingthatitispossibletosuccessfullyprintcmscaleobjectsusingawaxbasedphasechangeceramicink,11-13ceramicinkscontainrelativelylowvolumefractionsofsolidandthusthereisconsiderableshrinkageandpotentialfordistortionduringdewaxingandsintering.14Inordertofabricateceramicobjectsusinginkjetprinting,itisnecessarytosatisfyanumberofrequirements.Firstthereisaneedtoproducestableceramicsuspensionswithdefinedfluidpropertiessuchthattheycanbepassedthroughadropletgener-atorandformregulardrops.Second,thesesuspensionsneedtobedeliveredontoasubstrateorontoapreviouslyprintedlayerofsolidiedceramicink,withdropsinsufficientproximitytoeachothertoallowthemtointeractandformdesired2-Dfeatures.Next,theprintedceramicinkmustundergophasetransitiontoasoliddeposit.Finally,toproduce3-Dstructuresthedeposi-tionanddrying/solidificationprocessesneedtoberepeatedonalayerofpre-depositedanddriedmaterial.Herewewillconsidereachoftheserequirementsandtheiroptimizationforthedirectprintingofceramics.2. CeramicinksManufacturersofDODinkjetprintingequipmentnormallystatearangeofviscosityandsurfacetensionwithinwhichinksmaybesuccessfullyprinted.However;thisinformationisnor-mallyprovidedforthebenefitofformulatinggraphicsinksandmaynotbedirectlyapplicabletothedevelopmentofceramicinks.Thisisbecauseinkscontainingasignificantvolumefrac-tionofceramicparticlesinsuspensionhavemuchhigherdensityvaluesthantypicalgraphicsinks,whichtypicallyhavedensitiesintherange800-1000kgm-3andthebehaviourofafluidduringprintingdependsstronglyonitsinertialbehaviour.Thefluidrheologicalrequirementsforaprintableinkaredeterminedbythephysicsandfluidmechanicsofthedropgen-erationprocess.6,18Thebehaviouroffluidsduringinkjetprintingcanberepresented