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THEMANUFACTORING OFATWOLAYEREDI NJECTIONMOLDBYWELDINGThearticlepresentsthetechnologyofdepositcladdingdifferentmaterials,usingtheinjectionmoldsforther-moplasticasacasestudy.Theaimofthestudyistosurfaceweldtotheworkingsurfaceofthemoldadifferentmaterialwithcorrespondingphysicalproperties.Steel(1.1141)andacopperalloywereusedasthebase,ontowhichdifferent materialsweresurface-welded.Tungsteninertgas(TIG)weldingwasemployedtomakemoldsinserts.Ananalysisofcross-sectionedspecimenswasmadebyopticalmicroscopy,andchemicalandhardnessprofilesweremeasuredtoo.Thethermalconductivityofbaseand yerwasalsotested.Finally,ather-malfatiguetestwasemployedtoinvestigatethethermalfatiguepropertiesofsuchsurfaces.Keywords:injectionmold,surfacewelding,copperalloy,thermalandchemicalanalysisINTRODUCTIONTooptimize thesurfacepropertiesofthe,specialcladdingtechniqueshavetobeusedbecausethemajority of tool steelsare commonly considered asnon-weldableduetotheirhigh carbonand alloy content.Theparametersinfluencingtheselectionofcladdingtechnologyare:thepropertiesofbasemate-rial,structureandstateofheattreatmentofbasemate-rial,mouldfunctionandloadsactingupontheweld,treatmentafterthewelding,anddefectsthatmayoccuruponwelding(weldundercutsandcracks).Inthecaseofweldingofinjectionmoldsforthermoplasticbothla (finewelding)andTIG-welding process(larger-scalerepairs)arecommonlyused1-3.Inbothprocessesweldingisdoneinaprotectiveatmosphereofinertgasthatshieldstheweldfromtheinfluenceofsur-roundings(atmosphericgases).Inbothcases,thefillermaterialThe injectionmoldsforthermoplastic areusuallymadeoftwohalvesafixedpartandaslidingpart.Thelatterhousesanejectorpackagethatejectstheproductwhentheinjectioncycleiscompleted.Thefixedpartin-cludesarunnersystemusedtofeedthethermoplastic,heatedabove the flowingtemperature,intothemoldcavitysituatedbetweenthefixedandslidingpartofthemold.Afterthemoltenthermoplasticfillsupthecavity,secondarypressureisappliedtotheinjection-moldedparttoensurethepartisstableandcompletelyfilled(noairinclusionsorsinkmarks).Thisisfollowedbythelongestpartofthecyclecooling4.Theareawherebothmoldhalvescontactiscalledthepartingline.Thepartinglineisexposedtomechanic(melt abrasion, secondary pressure during injectionmolding),thermal(temperaturedifferencesduringtheinjectionprocesselongationandshrinkage)andchem-icalinfluencescausedbythethermoplastic5.Figure1 Reductionofinjectionmouldingcyclebyswit-chingfromasteelmould(a)toamouldwithcopperalloyinserts(b).METALURGIJA50(2011)4,231-234A.KONIKetal.:THEMANUFACTORINGOFATWOLAYEREDINJECTIONMOLDBYWELDINGa)b)Figure2Partwarpagevs.timeThe useofdifferent alloys resultsinshortercycletimes(Figure1)anduniformheatremovalfromthe which,inturn,createslowerunitproductioncostsoverthelifecycleofthetoolandleadstomoreuniformdimensionalparametersfortheplasticparts(Figure2).Thisiswhymaterialswithalargethermalhavetobeused.dlayersalsohavetobewearresistantinordertoenduretheabrasion6.Mainoccurringininjectionmoldingaresinkmarksontheproduct.Sinkmarksoccurintheareaswheretheprod-uctwallthicknessistoobig,causingunevencooling.Sinkmarkscanbeavoidedbycorresponding ofgeometryinthedesignphase,orthroughmorerapidcoolingofanexisting mold.In the lattercase,coolingchannelshavetobemadeinthemold(Figure3)inordertofacilitatetheflowofcoolingwater.Butinmostcaseseventhisisnotenough,asthereisalargeamountofheattoberemovedfromthecriticalareas.Insuchcases,materialswithgoodthermalcon-ductivityhavetobeused.Figure3 Removalofheatfromaninjection-mouldedproductina)mouldandsinkmarksonthefi-nalproduct(right)b)mouldwithcopper-alloyinsert,successfullyremovingsinkmarks(right).Figure4 (a)Surface-weldedspecimensaftergrindingandpolishing.Baseandfillermaterials:1cop-peralloysurfaceweldedwithAlCu,21,1141steelsurfaceweldedwithAlCu(b)Aschematicofthethermalconductivitytestset-upEXPERIMENTALPROCEDUREInordertoreachtheproposedobjectives,weldsweremadeontwobasematerials:1.1141(structurallow-car-bonsteel)andcopperalloy.Toobtainsatisfactory,thefillermaterialwaschosenaccordingtoasig-nificantdifferenceinthermal conductivitytothebasemetal.Butforaninjectionmoldthatmustbepolishedafterwelding,itisessentialthattheweldedsurfacehasasimilarhardnesswithrespecttothebasemetal.Other-wise,markscanremaininthecontouroftheweldafterthepolishingthatwouldbereproducedintheinjectionPartbronze and1.4370(tradename:InoxB18/8/6)wereusedasfillermaterials.Weldedspecimenswerepreparedwithslotsdimension15x10x1,5mmandthenfilledbyTIGweldingprocedure(Figure4a).Thespecimenswerecuttoobtainthecross-sectionofthecladpasses.Specimensforopticalmetallographywereobtainedfromthetransversedirectionoftheweld,followedby mechanical polishing bystandard andetching.EKSPERIMENTALRESULTSHeatconductionBased on the temperature measurements, it wasfoundoutthatcopperalloyandsteelbehavedifferentlyduringheating(Figure4b).Thetemperaturedifferencebetweenthetwomaterialsontheupper(cold)sideis38C,indicatingagreaterheatconductivityofcopperal-loy.Duetoalargerheatconductivity,thetemperaturegradientofcopperalloyoverthecrosssectionissmaller(thetemperaturedifferencebetweentheheatedandcoldsideisminimal,i.e.8C).Thesametemperature insteelwiththesamethickness(8,2mm)is47C.Fromtheviewpointofheattransfer(inourcase,afasterremoval ofheatfromthe mold) thismeansone canachievebettercontrolofmoldtemperature.Theuseofcoolingchannelsthereforeenablesustoachievethesired temperature acrossthe moldmore quickly andwith greater accuracy. The effectofresidual heat istherebypractically out,contributingtothestabilityoftheinjectionmoldingprocess.Figure5 Thecourseofhardnessinbasematerial,heat-affectedzoneandsurfacewelda)Basematerialiscopperal-loy,surfaceweldedwithAlCu.(b)Basematerialis1,1141steel,surfaceweldedwithAlCu.MicrostructuresandVickershardnessofcladlayerMacro specimenswere preparedfromall weldedpiecesforthe measurementofhardness.It isevidentfromFigures5aand5bthatthehardnessofAlCusur-faceweldis140to170HVinthecaseofsurfaceweld-ingontocopperalloy,and180to240HVinthecaseofsurfaceweldingontosteel.Inthefirstcasethehardnessofsurfacewelddoesnotdeviatesignificantlyfromthehardnessofbasematerialasaconsequenceofgoodremeltingandseparation.ThesituationisdifferentforsurfaceweldingofAlCuontosteel(1.1141).Thescatterislargeduetoimperfectmixingoffillerandbasemate-Figure6 Heat-affectedzonebetweenthesurfaceweldrials,whichareofaverydifferentcomposition.ThemadeofcopperalloyandAlCuandthereisnosaturation.Itcanalsobeseenthatthecon-weldishomogeneousandmore stable withrespecttosamecontentofaluminium(5%).strength(therearenoinclusionsorprecipitatedmaterialMelt mixing, shown in Figure 7, contributes toatthetransitionthatcouldreducethestrength).jumpsinmeasuredmaterial hardness.ApartofironFigure 6 showsthe heat-affectedzone where thepenetratesthesurfaceweldandmixeswiththecopper,fractionofAlisreduced(transitionfromdarktolightwhereasthemigrationofcopperintothebasematerialisarea)ontheaccountofincreasedfractionofCu,Coandmuchweakerthanthemigrationofaluminum.Ni.ThechemicalcompositionmeasuredalongthelineMolten copper diffuses into base material in theofmeasurementisasfollows:surfaceweld(95,8%Cu,heat-affectedzone,asitisclearlyvisibleinFigure8,withMETALURGIJA50(2011)4,231-234loads.Longcrystalsarecomposedofiron(5%)andalu-includesalargerfractionofiron(upto17%)withtheheat-affectedzoneexhibitsareasofmaterialwithadif-ferentmicrostructureandpartialseparation.Theinclu-reducedinthistransition,too.Cu,2,5%Al,0,9%Ni,0,6% Co,0,4%Fe),basesionsofprecipitatedmetalhaveaconsiderableeffectonmaterial(96,6%Cu,2%Ni,1,4% Co).Hardnessisthevariationsofmeasuredhardness.ThechemicalanalysiswasconductedwithanelectronIn the case of surface welding AlCu onto steelmicroscopewhichenablestheobservationandcompari-(1.1141),themicrostructureofthesurfaceweldvariessonofcontentofindividualmetalsinthesurfaceweld,withrespecttothedepth.Beginningatthedepthof0,3heat-affectedzoneandbasematerial.Inthecaseofsur-mm, the structure changes from coarse-grained to2,5%Al,0,8%Ni,0,5%Co,0,4%Fe,HAZ(95,6%fine-grained, which is better for taking mechanicalA.KONIKetal.:THEMANUFACTORINGOFATWOLAYEREDINJECTIONMOLDBYWELDINGFigure7 Microstructure(a)anddistributionofindivi-dualmetalsinthesurfaceweld(AlCu)andbase(1.1141).thefollowingcompositions:area1(91,6%Cu,4,3%Al,3,7%Fe,0,3%Mn,0,1%Co),area2wheremoltencop-perdiffusesintobasematerial(84,4%Fe,12,4%Cu,3,2%Al),area3(92,1%Fe,6,8%Cu,1,1%Al)andarea4(99,6%Fe,0,4%Mn),asthebasematerial.Figure8alsoshowsthediffusionofcopper(lightchannelsinagreyarea)andthedendritic structure ofiron thatsolidifiesfirst.Thethicknessoftransientareais70m.Copperin-clusionsarepresentinarea3(precipitatedlightdots).ThermalFatigueTheresearchwasconductedtotestthermalfatigueresistance and thermal stability of cladded materialbasedontheconductiveheatingandsubsequentcoolingbyairyet.AschematicofthermalfatiguetestapparatusisshownonFigure9a.Itenablesacontrolledthermalfa-tiguecyclingtestofmaterials.Thetestspecimen(Fig-ure9b)wassubjectedtoupto20000thermalcycles(Figure9c).Afterthecompletionof7000,14000and20000cyclesthespecimensurfacewasinspected(Fig-ure9d).The surfaceofcladdedareashowssuperiorthermal fatigue resistance since no crackswere ob-served.CONCLUSIONSItfollowsfromtheresultsobtainedbytheoreticcon-siderationsandpracticalteststhatitispossibletoobtainverydifferentcombinationsofbaseandfillermaterialbywelding.Thisispracticalforthemanufacturingofin-jectionmoldsforthermoplastic,where bothheat re-movalandmechanicenduranceareimportant.IthastobementionedthattheeffectofabrasionisnotaslargeasFigure8 Heat-affectedzonebetweenthesurfaceweldmadeofAlCuand1.1141steelasbase