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JournaloftheEuropeanCeramicSociety31(2011)2543–2550Inkjetprintingceramics:FromdropstosolidB.DerbySchoolofMaterials,UniversityofManchester,OxfordRoad,ManchesterM139PL,UKAvailableonline16February2011AbstractInkjetprintingisapowerfulmicrofabricationtoolthathasbeenappliedtothemanufactureofceramiccomponents.Tosuccessfullyfabricateceramicobjectsanumberofconditionsmustbesatisfiedconcerningfluidpropertiesanddropplacementaccuracy.Ithasbeenproposedthatfluidsareprintablewithinthebounds1Z10(whereZistheinverseoftheOhnesorgenumber)andtheselimitsareshowntobeconsistentwithceramicsuspensionsdeliveredbypiezoelectricdrop-on-demandinkjetprinters.Thephysicalprocessesthatoccurduringdropimpactandspreadingarereviewedandtheseareshowntodefinetheminimumfeaturesizeattainableforagivenprinteddropdiameter.Finallythedefectsthatcanoccurduringthedryingofprinteddropsarereviewed(coffeestaining)andmechanismsandmethodologiestoreducethisphenomenonarediscussed.Keywords:Inkjetprinting;Shaping;Drying;Suspensions1.IntroductionInkjetprintinghasmajorcommercialapplicationsingraph-icsoutputandotherconventionalprintingoperations.However,therehasbeendevelopinginterestinusinginkjetprintingtomanufacturecomponentswithapplicationsfor:displays,1plas-ticelectronics,2rapidprototyping,3tissueengineering,4andceramiccomponentmanufacture.5Asignificantandfundamen-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,thesesuspensionsneedtobedeliveredontoasubstrateorontoapreviouslyprintedlayerofsolidifiedceramicink,withdropsinsufficientproximitytoeachothertoallowthemtointeractandformdesired2-Dfeatures.Next,theprintedceramicinkmustundergophasetransitiontoasoliddeposit.Finally,toproduce3-Dstructuresthedeposi-tionanddrying/solidificationprocessesneedtoberepeatedonalayerofpre-depositedanddriedmaterial.Herewewillconsidereachoftheserequirementsandtheiroptimizationforthedirectprintingofceramics.2.CeramicinksManufacturersofDODinkjetprintingequipmentnormallystatearangeofviscosityandsurfacetensionwithinwhichinksmaybesuccessfullyprinted.However,thisinformationisnor-mallyprovidedforthebenefitofformulatinggraphicsinksandmaynotbedirectlyapplicabletothedevelopmentofceramicinks.Thisisbecauseinkscontainingasignificantvolumefrac-tionofceramicparticlesinsuspensionhavemuchhigherdensityvaluesthantypicalgraphicsinks,whichtypicallyhavedensitiesintherange800–1000kgm−3andthebehaviourofafluidduringprintingdependsstronglyonitsinertialbehaviour.Thefluidrheologicalrequirementsforaprintableinkaredeterminedbythephysicsandfluidmechanicsofthedropgen-erationprocess.6,18ThebehaviouroffluidsduringinkjetprintingcanberepresentedbytheReynolds,WeberandOhnesorgenum-