Developmentofa3DPrintingMethodforProductionofDentalApplicationLiYanga,ShanshanZhanga,GustavoOliveirab,BrentStuckera

aDepartmentofIndustrialEngineering,J.B.SpeedSchoolofEngineeringbDepartmentofGeneralDentistryandOralMedicine,SchoolofDentistry

UniversityofLouisville,KY,40292AbstractTraditionally,themanufacturingofdentalrestorations,includingcrowns,veneersandotherstructuresmadebyceramics,isalaborintensiveandtimeconsumingprocess.Additivemanufacturinghasthepotentialtosignificantlydecreasethetimeandcostassociatedwiththisprocess.ThisworkperformedpreliminaryinvestigationforthefeasibilityofdentalrestorationpartsprintingusingtheExOneMLabsystemwithacommercializeddentalporcelainpowder.Theporcelainpowderswerecharacterized,andtwomeasurements,includingpresinteringandadditionofflowagent,weretakenintheattempttoimprovetheprocessabilityoftheoriginalpowderfeedstock.Theresultsshowedthatwhiletheadditionofflowagenthasmoresignificanteffectsinimprovingtheflowabilityofthepowderused,thepostsinteredpartsexhibitconsiderableshrinkageandresidualporositythatnecessitatesfurtherinvestigation.IntroductionOverthepastdecades,therehasbeenatrendofusingmetalfreerestorationsinthedentalfield(DenryandHolloway,2010,Guessetal.,2011).Ceramicdentalrestorationshavebeenincreasinglyusedduetotheiroutstandingaestheticfeaturesandresistivitytochemicals(Conradetal.,2007,Kelly,1999).Traditionally,thedentalceramicswereproducedbyhotpressing,sinteringorslipcastingprocesses(DenryandHolloway,2010),whichingenerallacksufficientflexibilityintheaccuratecustomizationoftheparts.Machiningwasusedaswell,butoftensuffersfromthehighhardnessandlowtoughnessoftheceramicfeedstock.Inrecentyears,aCAD/CAMbasedmethodwasdevelopedtoproducedentalrestorationswithgeometriesaccuratelymatchingthatofthepatients(Filseretal.,2003,Guessetal.,2011).ThebasicproceduresofthisprocessinvolvesthepresinteringofceramicblocksfollowedbyaCNCmachiningprocessthatproducesaccurateshapes,thenthegreenmachinedpartsaresubjecttoadensificationsinteringprocesstogeneratethefinaldensitiesaswellasmechanicalperformance.Theuseofthetwostagesinteringhelpsreducetheissuesassociatedwithmachiningofhardceramics,thereforeincreasingtheefficiencyoftheprocesswithlowercost.However,duetotheshrinkageinthesecondsinteringstage,theaccuracyofthemachiningcouldbesignificantlyreduced,whichcompromisesthebenefitintendedtobeprovidedviamachining.Inaddition,therelativelycomplicatedproceduresalsoresultinlongermanufacturingtimeandhighercost.WiththeabilityofmanufacturingpartsdirectlyfromaCADmodelwithadequateaccuracyandminimalwaste,additivemanufacturingholdsgreatpotentialforthefutureproductionofcustomdentalrestorationparts.Severallasersinteringbasedstudieshavebeenpresentedintheattempttofabricatedentalrestorationswithvariousmaterials(Lietal.,2000,Hagedornetal.,2011).Theuseofalaserforthedirect

346

sinteringofceramicsenablesonestepprocessingforthemanufacturingoftheseparts,andprovidesthepotentialfordirectmanufacturingofceramicovermetaldentalparts.Ontheflipside,thermalcrackingisacommonissuefortheseprocesses(Wilkes,2010),andthefeatureresolutionisalsooftenlimitedduetothepartialsinteringofthesurroundingceramicpowders(Hagedornetal.,2011).Recently,adirectwritebasedprintingprocesswasinvestigatedforthefabricationofzirconiadentalprostheses(Ebertetal.,2009).Theprocessselectivelydepositsazirconiabasedsuspensionongraphitesubstrates,followedbyapostsinteringprocesstoachievethefinaldensity.Theresultingpartsexhibithighgeometricalaccuracyanddensity,aswellasgoodmechanicalstrength.Inthisworkitwasalsoreportedthepotentialissuewiththenozzleclogging(Ebertetal.,2009). Inthisstudy,theExOneMLabwasusedinanattempttofabricateceramicpartsfromofftheshelfcommercialceramicpowdersusedfordentalapplications.Whilethesystemisrelativelynew,therehavebeenstudiesthatutilizebinderjettingtomakeceramicparts(Cimaetal.,1995,Uhlandetal.,1999).Thisprocessofferssomepotentialadvantagesinceramicprinting,suchastheflexibilitywithdifferentceramicmaterials,therelativelyhighfeatureresolution,andeasyprocesscontrol;thereforebinderjettingwasadoptedforthisstudywithfuturedevelopmentsinmind.Thegoalofthisstudyistoidentifythefeasibilityofthesystemforquickandaccuratefabricationofcomplexshaped,3Ddentalrestorationparts.Basedonthepreliminarystudy,futureworkcanbedeterminedintheefforttoproducehighqualitypartswithminimaldefects.MaterialsandMethodsTheprimarymaterialusedforthisstudyisVITAVM13Base(VitaZahnfabrik,Germany),whichisaleucitereinforcedglassceramicwidelyusedfordentalpractice.ThechemicalcompositionandbasicpropertiesofthepowderprovidedbythemanufacturerisshowninTable1(VitaVM13,2009).Powder Chemicalcomposition(wt%) Particlesize

Linearcoefficientofthermalexpansion

Flexuralstrength

VM13Base

SiO2:5963%,Al2O3:1316%,

K2O:911%,Na2O:46%

~18m 13.614x106K1 ~120MPa

Table1GeneralinformationofVM13BaseTheoriginalpowdersexhibitsignificantaggregationwhichmadeitunsuitablefortheprocess.Preliminaryparticlesizeanalysis(MicrotracS3000)indicatedthatthecharacteristicsizeoftheaggregationwasaround100105m,asshowninFig.1.Twofactorswereconsideredtocontributetotheaggregation.FromFig.2,itcouldbeseenthatthepowdershaveaveryirregularmorphologyandaratherwidesizedistribution,whichcouldsignificantlyreduceitsflowability.Inaddition,thelargesurfaceactivityof

347

thesilicapowdersalsomakesthempronetoclumping(Bagweetal.,2006).Therefore,Presinteringandflowagentadditionwereimplementedinanattempttoimprovetheissue.

Fig.1ParticlesizeanalysisresultsfortheVM13Basepowder

Fig.2MorphologyoftheoriginalVM13powderThroughapreliminarytrialitwasfoundthatatabout600Cofsinteringtemperature,significantdensificationdoesnotoccurforthepowder.Therefore,600Cwaschosentobethepresinteringtemperatureinordertoachievefasteffects.Theholdingtimeforpresinteringwaskeptat30minutes,followedbyfurnacecooling.SurfacemodifiedR972SiO2powder(COSMOSPlastic&Chemicals)wasusedastheflowagent.Thepowderiscomposedof>99.8%fumedsilicatreatedwithdimethyldichlorosilane(DDS),withanaverageparticlesizeof16nm.TheflowagentwasaddedtotheoriginalVM13powderandmixedbyhanduntilsatisfactoryimprovementofflowabilitywasobserved.Theflowabilitywasevaluatedbytheangleofreposefortheoriginalandtreatedpowders.Inaddition,micrographsweretakenforthepresinteredpowdertoevaluatetheevolutionofpowders.Afterthetreatment,theprocessablepowderswereusedto

348

printsamplecrownpartsusingtheMLab.ThebinderusedfortheprocesswastheExOnePMBSR104,anethersolventbasedbinder.Afterprinting,thegreenpartsweredriedintheovenat70Covernight.TheschematicofthepostsinteringprocessisshowninFig.3.Thegreenpartswereheldat500Cfor1hourtoburnoutthebinders,followedbyatwostepsinteringsequenceat700Cand850C,respectively.Thedimensionsofthefinalpartsweremeasuredwithcaliperbeforeandaftersintering,andresultswerecomparedtoevaluatetheshrinkage.Thecrosssectionalmicrostructureofthesinteredsampleswerealsopreparedandobservedusingopticalmicroscopy.ResultsanddiscussionPresinteringdidnotseemtohavesignificanteffectontheflowabilityofthepowder.Fig.3showsthemorphologyofthepowdersafterthepresintering.FromFig.3itcouldbeclearlyobservedthatsignificantsinteringoccurredwhichresultedinthegenerationoflargepowderaggregations.Ontheotherhand,theflowagenthadamoresignificanteffectontheflowabilityofthepowderattheratioofapproximately7%involume.Fig.4showstheanglesofreposeforeachsample,andTable2showstheresultsofthemeasurement.Presinteringwasusedforceramicpowderswithfineparticlesizesinpreviousstudiesusingselectivelasersintering(Harlanetal.,1999).However,fromthisstudyitseemsapparentthatwithhighlyirregularpowders,evenifthepresinteringcouldreducethetotalsurfacearea,theincreaseofparticleirregularitywillcanceloutallthebenefitsandpotentiallyresultinthereductionofflowability.ItwasexpectedthattheadditionofflowagentcouldimprovetheoverallpowderflowabilitybyservingasalubricationinterfacebetweenthelargeVM13powders,whichwasverifiedbythestudy.

Fig.3MorphologyofVM13powderafterpresintering

349

(a)Original (b)Presintered (c)Flowagentaddition

Fig.4Anglesofreposeforeachtypeofpowder

Powder Angleofrepose(degree)Original 64

Presintering 667%vadditionofflowagent 56

Table2AnglesofreposeforeachtypeofpowderThepowderwithflowagentadditionwassuccessfullyusedfortheprintingofcrownpartsintheMLabandsinteredsubsequently.Fig.5showstheoriginalpartaswellasthepostsinteredparts.Significantshrinkagecouldbeobserved.ThedimensionsasshowninFig.6weremeasuredforthreesamples,andtheresultsareshowninTable3.Theresultsarequiteconsistentforeachdimension,andanaverageof25%32%shrinkagewasobservedineachdimensioncomparedtothegreenparts.Theshrinkagevaluesareconsiderablebutreasonableconsideringthatthepowderishighlyirregular.Aslightdifferenceofshrinkagebetweenthebuilddirection(alignedwiththedimensionH)andtheothertwodirectionswereobserved,whichcouldbecausedbytheburnoutofthebinder.Thereisalsoaslightdifferenceofshrinkagebetweenthetwoplanardirectionsperpendiculartothebuilddirection,ascouldbeseenfromTable3.Thiscouldbeassociatedwiththespecificgeometryofthesesamplessuchaswallthickness.However,withoutfurtherinvestigation,itisdifficulttodeterminethelikelycauseofthisphenomenon

350

Fig.5Greenandpostsinteredcrownparts

Fig.6Measurementofdimensions

Sample L(mm) W(mm) H(mm)

Beforesintering1 11.50 11.55 6.002 11.45 11.55 6.073 11.43 11.58 6.03

Postsintering1 8.80 8.16 4.122 8.61 8.11 4.083 8.60 8.09 4.13

Linearshrinkage1 23.48% 29.35% 31.33%2 24.80% 29.78% 32.78%3 24.76% 30.14% 31.51%

Average 24.35% 29.76% 31.88%Table3Shrinkageofthesamples

Fig.7showsthemicrostructureoftheceramicsamplesaftersintering.Ingeneralthesinteringseemedtohaveachievedarelativelyhomogeneousdensification.Largevoidsaswellassomelightparticlescouldbeclearlyobserved.Thesizeofthevoidsvariedbut

Before

After

351

wereatthemagnitudeof50100m.Thisseemedtohaveacorrelationwiththesizeoftheaggregationoftheoriginalpowder.Onepossiblecausecouldbethatduringthepowderspreadingprocess,thelargeaggregationwasdisplacedbytherollerduetothetemporaryadhesiontotherollersurface,whichresultedinsurfacevoidsonthenewlyspreadlayer.Anotherpotentialsourcethatcouldcontributetothevoidformationistheadditionoftheflowagent.Themechanismofthesurfacemodificationintheflowagentwastocreateahydrophobicsurfaceonthesilicaparticles.Sincesufficientwettingisrequiredtoformacontinuousbondingbetweenthebinderandthepowder,theexistenceoftheflowagentcouldpotentiallyaffectthecontinuityofthegreenpart,leavingvoidsthatbecomedefectsafterthesubsequentprocesses.Furtherstudiesareneededtoidentifythepotentialimpactoftheflowagentinthefinaldensitiesoftheparts.Inaddition,theuseof850Casthedensificationsinteringtemperaturewaslargelyduetothelimitationofthefurnace,whileinthereferenceinstructionfortheVM13powder(VitaVM13,2009),itwasrecommendthatthegreenpartbesinteredat920C.Thelowertemperaturecouldalsocontributedtotheporositiesobservedinthesample.

Fig.7Microstructureofthepostsinteredpart

ConclusionInthisstudy,anofftheshelfdentalveneerpowderwassuccessfullyprintedusinganMLab.Theoriginalpowderhadsignificantaggregationissues,andtwomeasurementswereusedintheattempttoimprovetheflowabilityofthepowder.Additionoftheflowagentofabout7%volumewasshowntobeaneffectivemethodtoimprovetheoriginalpowderflowability.Atrelativelylowsinteringtemperature,thefinalpartsshowedhomogeneousdensificationandcontinuousmicrostructures,althoughlargevoidswere

352

presentinthefinalparts.Theshrinkageofthepartsaftersinteringwasaround2530%ineachdirections,andanisotropicshrinkagewasobservedinallthreedirections.Furtherinvestigationsareneededtoidentifythecauseoftheanisotropicshrinkageaswellasthesourceofthelargevoidsinthefinalparts,andadditionalmechanicaltestingisalsorequiredtofurthercharacterizethequalityoftheprocess.ReferenceR.P.Bagwe,L.R.Hillard,W.Tan.SurfaceModificationofSilicaNanoparticlestoReduceAggregationandNonspecificbinding.Langmuir.22(2006):43574362.M.J.Cima,J.Yoo,S.Khanuja,M.Rynerson,D.Nammour,B.Giritlioglu,J.Grau,E.M.Sachs.StructuralCeramicComponentsby3DPrinting.Proceedingsofthe6thInternationalSolidFreeformFabricationSymposium.Austin,TX,USA.1995.H.J.Conrad,W.J.Seong,I.J.Pesun.Currentceramicmaterialsandsystemswithclinicalrecommendations:asystematicreview.JournalofProstheticDentistry.98(2007):389404.I.Denry,J.A.Holloway.CeramicsforDentalApplications:AReview.Materials.3(2010):351368.J.Ebert,E.Ozkol,A.Zeichner,K.Uibel,O.Weiss,U.Koops,R.Telle,H.Fischer.DirectInkjetPrintingofDentalProsthesesMadeofZirconia,JournalofDentalResearch.88(2009):673676.F.Filser,P.Kocher,l.J.Gauckler.Netshapingofceramiccomponentsbydirectceramicmachining.AssemblyAutomation.23(2003):382390.Y.C.Hagedorn,N.Balachandran,W.Meiners,K.Wissenbach,R.Poprawe.SLMofnetshapedhighstrengthceramics:newopportunitiesforproductiondentalrestorations.Proceedingsofthe22ndInternationalSolidFreeformFabricationSymposium.Austin,TX,USA.2011.N.Harlan,S.M.Park,D.L.Bourell,J.J.Beaman.SelectiveLaserSinteringofZirconiawithMicroScaleFeatures.Proceedingsofthe10thInternationalSolidFreeformFabricationSymposium.Austin,TX,USA.1999.J.R.Kelly.Clinicallyrelevantapproachtofailuretestingofallceramicrestorations.JournalofProstheticDentistry.81(1999):652661.X.Li,J.E.Crocker,E.Geiss,L.L.Shaw,H.L.Marcus,T.b.Cameron.EvaluationofMicrostructureandPropertiesforMultiMaterialsLaserDensificationofDentalRestorations.Proceedingsofthe11thInternationalSolidFreeformFabricationSymposium.Austin,TX,USA.2000.S.Uhland,R.Holman,B.DeBear,P.Saxton,M.Cima,E.Sachs.Threedimensionalprinting,3DP,ofelectronicceramiccomponents.Proceedingsofthe10thInternationalSolidFreeformFabricationSymposium.Austin,TX,USA.1999.VITAVM13WorkInstructions.VITAVidentwebiste.http://vident.com/products/veneeringmaterials/metalceramics/vitavm13/.J.Wilkes,Y.C.Hagedorn,S.Ocylok,W.Meiners,K.Wissenbach.Rapidmanufacturingofceramicpartsbyselectivelasermelting.AdvancedProcessingandManufacturingTechnologiesforStructuralandMultifunctionalMaterialsIV.Vol.31.2010.

353