Keywords:
Inkjetprinting;Shaping;Drying;Suspensions
1.Introduction
Inkjetprintinghasmajorcommercialapplicationsingraph-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.Thephysicaloperationofthesedifferentprintingtechnologies
andthereasonsforthechoiceofpiezoelectricDODprintinghavebeendiscussedindetailelsewhere6,18;henceherewewill
confineourconsiderationstothistechnology.Wewillalsoonlyconsidertheprintingofceramicinksthatsolidifythroughsol-ventevaporation.Despiteearlierworkdemonstratingthatitispossibletosuccessfullyprintcmscaleobjectsusingawaxbasedphasechangeceramicink,11–13ceramicinkscontainrelativelylowvolumefractionsofsolidandthusthereisconsiderableshrinkageandpotentialfordistortionduringdewaxingandsintering.14
Inordertofabricateceramicobjectsusinginkjetprinting,itisnecessarytosatisfyanumberofrequirements.Firstthereis
aneedtoproducestableceramicsuspensionswithdefinedfluidpropertiessuchthattheycanbepassedthroughadropletgener-atorandformregulardrops.Second,thesesuspensionsneedtobedeliveredontoasubstrateorontoapreviouslyprintedlayerofsolidifiedceramicink,withdropsinsufficientproximitytoeachothertoallowthemtointeractandformdesired2-Dfeatures.Next,theprintedceramicinkmustundergophasetransitiontoasoliddeposit.Finally,toproduce3-Dstructuresthedeposi-tionanddrying/solidificationprocessesneedtoberepeatedonalayerofpre-depositedanddriedmaterial.Herewewillconsidereachoftheserequirementsandtheiroptimizationforthedirectprintingofceramics.
2.Ceramicinks
ManufacturersofDODinkjetprintingequipmentnormallystatearangeofviscosityandsurfacetensionwithinwhichinksmaybesuccessfullyprinted.However,thisinformationisnor-mallyprovidedforthebenefitofformulatinggraphicsinksandmaynotbedirectlyapplicabletothedevelopmentofceramicinks.Thisisbecauseinkscontainingasignificantvolumefrac-tionofceramicparticlesinsuspensionhavemuchhigherdensityvaluesthantypicalgraphicsinks,whichtypicallyhavedensitiesintherange800–1000kgm−3andthebehaviourofafluidduringprintingdependsstronglyonitsinertialbehaviour.
Thefluidrheologicalrequirementsforaprintableinkaredeterminedbythephysicsandfluidmechanicsofthedropgen-erationprocess.6,18ThebehaviouroffluidsduringinkjetprintingcanberepresentedbytheReynolds,WeberandOhnesorgenum-bers(Re,We,Oh):
Re=vρa,(1a)
η
We=v2ρa(1b)
γ
√η
Oh=We=(1c)
Re(γρa)1/2
whereρ,ηandγarethedensity,dynamicviscosityandsurfacetensionofthefluidrespectively,visthevelocityandaisacharacteristiclength.FrommidentifiedtheOhnesorgenumber,Oh,astheappro-priategroupingofphysicalconstantstocharacterisedropgenerationinaninkjetprinter.19Ohisindependentoffluidvelocityandiscommonlyusedinanalysesdescribingthebehaviourofliquiddrops.However,inFromm’spublication,hedefinedtheparameterZ=1/Ohandfromasimplemodeloffluidflowinadropgeneratorofsimplifiedgeometry,hepro-posedthatZ>2forstabledropgeneration.19Reisextendedthisthroughnumericalsimulationandproposedthefollowingrange,10>Z>1,forstabledropformation.20IfZ<1,viscousdissipa-tionpreventsdropejectionfromtheprinterandifZ>10,dropletsareaccompaniedbyunwantedsatellitedrops.Jangetal.studiedtheDODprintabilityofanumberoffluidmixturesofethanol,waterandethyleneglycol.ThroughthistheyexploredarangeofvaluesofOhanddeterminedthattherangeofprintabilitywas
Fig.1.Fromm’sparameterZ(Z=1/Oh)influencestheprintabilityoffluids.DashedlinesidentifythelimitsforprintabilityproposedbyReisetal.20Experi-mentalpointsareplottedforanumberofceramicsuspensions/inks:
greysymbolsindicatesuccessfulinkjetprinting,blacksymbolsindicatethatnodropswereformed,andwhitesymbolsindicatethepresenceofsatellitedropsalongwiththemainprinteddrop.
4Thereisnowasubstantialbodyofliteraturedescribingtheinkjetprintingofanumberofceramicsuspensionsandotherfluidsfornon-graphicsapplications;unfortunatelynotallpublicationsreportsufficientinformationontherheologi-calpropertiesoftheceramicsuspensionstotestthisproposedcriterionforprintabilityinallcases.Fig.1presentssuchdatathateitherreportedthevalueofOh(orZ)orreportedsufficientdatathatitiseasilycalculated.TheverticaldashedlinesonthefigureatOh=1andOh=10representthelimitsforstableinkjetprintingcalculatedbyReis.20Theexperimentaldataispresentedfromeightfluidsystemswithagreysymbolindicat-ingthesuccessfulprintingofindividualdrops,ablacksymbolindicatesthatfluidswiththesepropertiescouldnotbeprinted,andfinallyawhitesymbolshowsthecaseswhereafluiddropwassuccessfullyejectedbutaccompaniedbyoneormoresatel-litedrops.Itisusefultoseparatethesedataintotwosets:
fluidsystems1–6weredeliveredusingpiezoelectricDODprinters,whilefluidsystems7and8weredeliveredusingathermalDODprinter.ThedataobtainedfromexperimentsusingpiezoelectricDODprintingshowsreasonablygoodagreementwithReis’smodel,howeverthatobtainedintheonestudyusingathermalDODprintershowsverypooragreement,17atleastwiththeupperboundforthepredictionoftheonsetofsatellitedropfor-mation.ÖzkolconsideredthatonereasonforthediscrepancybetweenReis’spredictionandtheirresultscouldbethediffer-enceinactuationbetweenpiezoelectricandthermalDODinkjetdropletgenerators.17
Thehypothesisthatchangesinactuationexplainthedif-ferentbehaviourobservedbetweenthermalandpiezoelectricDODinkjetprintingissupportedbyanexperimentalstudyofdropandsatelliteformationinapiezoelectricDODprinterbyDongetal.22Theyfoundthatthedropformationmechanismandtheconditionsunderwhichagivenfluidformedsatellitesisalsocontrolledbytheshapeandamplitudeofthedriving
pulseappliedtothepiezoelectricactuator.Thedrivingpulse
inDODprintingisalsoknowntocontrolboththesizeoftheejecteddropanditsvelocity.12,22,23Reisdemonstratedthatfor
theformationofdropsusinghighlyloadedceramicsuspensions,acousticphenomenaareimportantandthattherearem