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THEPREPARATIONANDUSEOFPLASTICMOLDSM.B.BorisovUDC666.5666.3.032Plastermoldsarewidelyusedforcastingwaresintheceramicsindustry.Apartfromcertainadvantages,theyhavealowmechanicalstrengthandshortworkinglifeduetowearoftheworksurfaceundertheactionofwaterandelectrolytesinthebody.Besidesthis,thetemperatureinthedryercannotbehigherthan65asabovethistemperaturetheplasterisdestroyed.Thenecessityoffindingasubstituteforplasterwasincreasedwhentheporcelainindustrybeganinstallingsemiautomaticmachinesformoldinganddrying,withintensifieddryingconditions.Thesearchforasubstituteforplasterbeganinourfactoryin1967,andtothisendagroupwasorganizedcontainingthefollowingworkersA.V.Saveleva,P.S.Polyakov,T.A.Pashkova,V.A.Kholkin,A.V.Sarichev,F.I.Lisov,andothers.AtthestartoftheirworktheresearchgroupwereguidedbydatafromtheStateCeramicsResearchInstitute\1\.However,thisdidnotansweranumberofquestionsarisingfromtheworkingoftheplant,suchashowtodesignandmakepressmoldssoastoavoidcrackingintheplaneofintersectionoftwosurfaces,whatbatchtotakeperunitvolumeofmold,whatscheduletousefortheheattreatment,howtoobtainuniformdistributionofporesandincreasegeneralporosity,andhowtoavoidadheringbythebody,etc.Followingontheresultsoftheirresearch,in1969thegroupbegandevelopingthepreparationofPVCmolds,andconvertingtheproductionlineofthefactorytotheiruse.Atthepresenttimethereareinthefactoryfiveproductionlinesforplates,threeofwhichareforshallow200mmplatesandtwofordeep200mmplates,aswellassevenlinesforcups.Usingtheplasticmoldsatotalof24,000platesand3537,000cupsareproducedperday24h.Theprovisionalannualsavingaccruingfromtheintroductionofonlyoneproductionlineis2500rubles.Theworkinglifeoftheplasticmoldsincupproductionreaches4000cyclesandcanstillbeincreased.Thelifeofthemoldsinplateproductionhasstillnotbeendetermined.Asectionforpreparingthemoldswasorganizedinthefactoryin1969Fig.1.Therawmaterialwasdomesticpolyvinylchloridelatex,gradeL5,oremulsifiedpolyvinylchloride,gradeE62.Polyvinylchlorideistheproductofpolymerizationofvinylchloride.ItisawhitefinelydispersednoresidueonNo.0056sievepowderofdensity1.41g/cm3.Atatemperatureof170180andunderslightpressurespecificpressure0.05kg/cm2thepolyvinylchlorideparticlesmeltandcohere.Thefirststageinthepreparationofthemoldsisapreliminaryvibromoldingonthevibrotablewhichensuresuniformporosity.Thevibrotableperforms50oscillations/secwithanamplitudeof0.5ram.Thevibromoldingiscarriedoutfor1520sec.Thispreliminaryvibromoldingisanessentialpartoftheprocessformakingthemolds,since,duringmolding,internalstressesaregeneratedwhichareconcentratednearthepointsofcontactofmolecularcohesionofthefineparticles.Thisinternalstressisespeciallydangerousduringtheprocessofsintering.Itcanberemovedbyvibration,whichalmostcompletelybreaksallconnectionsbetweenparticlesandensurestheiruniformdistribution.ThebasicpieceofequipmentinthepreparationoftheplasticmoldsisthemetalpressmoldFig.2whichismadefromgrade45orSt.3steel.Itsinternalsurfacemustbechromiumplated.DulevoPorcelainFactory.TranslatedfromStekloiKeramika,No.3,pp.79,March,1972.91972ConsultantsBureau,adivisionofPlenumPublishingCorporation,227g/est17thStreet,NewYork,N.Y.10011.Allrightsreserved.Thisarticlecannotbereproducedforanypurposewhatsoeverwithoutpermissionofthepublisher.Acopyofthisarticleisavailablefromthepublisherfori5.00.147,.13500l23456}fiIO987Fig.IFig.2Fig.I.iHandpress2SNOLelectricallyheatedcabinetforheattreatmentofthemolds3cabinetforcoolingthemolds4tableforassemblyanddismantling5standforpreparedmolds6storagefortheplastics7electricallyheatedcabinetfordrying8standforfinishingthemolds9tableforpackingplasticsi0tableformeasuringoutplasticsiivibratingtable.Fig.2.Pressmold.aAssembledbdismantled.TheprocessoffillingthepressmoldformakingflatwareplatesplasticmoldsisshowninFig.3.L5powder,whichhaspreviouslybeendriedintheeleetrodryingcabinetatll0andthenscreenedthroughaNo.05sieve,ischargedintothematrixofthepressmoldandleveledwithaspecialtool.Theamountofplasticdependsonthetypevolumeofthemold.Forshallow200mmplatesitis600g,fordeep200mmplates660g,forsaucers400g,andforcups500515g.Thepressmoldpunchistheninsertedintothematrixinsuchawaythatitentersunderitsownweightuntilitisincontactwiththeplastic.Tocheckthecorrectnessofassemblyatthispoint,thepunchisrotatedrelativetothematrix.Theassembledpressmoldisplacedonthevibrotableandcompactedfor1520sec.Aftercompacting,thegapbetweenthematrixflangeandthepunchflangeshouldbe810ram.ThepressmoldisnextinsertedintoSNOLelectrocabinetfortheheatprocessandaweightisplacedonthetopsoastogiveapressureof0.05kg/cm2.Tomonitorthetemperature,athermometerisinsertedsothatittouchesthecenterofthepunch.Asmallamountoftheplasticisheapedonthepunchflange,anditschangeincolorisusedtoassessthereadinessofthemold.Theheatprocesscontinuesfor23handatitscompletionthetemperatureofthepressmoldis170175Afterthecompletionoftheheatprocess,thethermometerisremovedandthepressmoldtakenfromthecabinetandplacedintotheventilatedcabinettocool.Thepressmoldisthendismantledusinganextractionscrew.Thepreparedplasticmoldisremoved,trimmedsmoothattheedges,andafterinspectionisreadyforuse.Themoldsforthehollowwarescupswerepreparedaccordingtothefollowingschedule.ThecorrectamountofPVCplasticwasputintothematrix{Fig.4andleveledoff.Two2224mmthickspacerswereplacedontheflangeofthematrix1,andthematrix2loweredontoitsothattheringflangewasrestingonthespacers.Thepunch3wasthenloweredintotheringsothatitsflangewaslevelwiththeringflange.Theassembledpressmoldwasthenturnedoverandplacedflangedownonthevibrotable,withthepunchflangestandingona50x50mmblock.Thespacerswerethen.removedfromundertheringflange,andaweightof810kgplacedonthebottomofthematrix,positionedtothatitscenterlayontheaxisofthepressmold.Thevibratorwasturnedonandtheringheldbehindtheflangevibratedtheplasticfor1520sec.Aftervibratingtheclearancebetweentheflangesofthematrixandpunchshouldbeabout57ram,andbetweenthepunchflangeandtheringabout10mm.Thepressmoldwasthenturnedover,and56mmthickpackinglaidonthepunchflange,andthentheweightplacedonthiswhichgavethepressureof0.05kg/cm2fortheheatprocess.Thepressmold,withtheweight,wasplacedintheelectrodryingcabinetonafireclaybase68cmthick.TheheatingprocesswascarriedoutaccordingtothegraphFig.5for1.52huptoatemperatureofof180185Thetemperaturewasmonitoredbyamercurythermometer.Attheendoftheheattreatmenttheloadwasremovedandthepressmoldtakenfromtheelectrocabinetandpressedonthehandpressuntil148Fig.3tc160_..............iu/\020406080lOOTime,rainFig.4Fig.5Fig.3.1Stopwasher2ring3punch4die5polyvinylchloride.Fig.4.1Matrix2ring3punch4screw5mold.Fig.5.Heattreatmentgraphsforprocessingplasticmolds.200mmdiameterplates.1Forcups2forshallowtheringandpunchflangesmadecontactwiththematrixflange.Afterretainingunderthepressfor10minthepressmoldwasremovedandplacedintheventilatedcabinettocooldownto4045Thepressmoldwasdismantledandthepreparedplasticmoldextracted.Aftertrimmingitwasreadyforuse.Experienceinthepreparationofthesepolyvinylchloridemoldsandintheiruseinthefactoryhasshownthatdeviationfromtheabovepreparationschedulesleadstotheappearanceofvariousfaultsinthemoldswhichshowupduringtheprocessofformingwareswiththemunderpressing,porosityofthematerialundertheshouldersofthehollowmold,ringcracksonthestemofthehollowmold.Constantcontroloftheelectrodryingcabinetbymeansofthethermoregulatorensuresauniformtemperatureinthechamberduringtheheattreatmentofthepressings,andensuresuniformheatingoftheplasticforallsectionthicknesses.Theresultantuniformityofsinteringpreventstheproductionofamoldwithdifferentcoefficientsofthermalexpansionbetweentheedgesandcenteroftheplastic.Thisgreatlyreducestheoccurrenceofcracksintheworkingsurfaceofthemoldduringuseandincreasesitsworkinglife.Accumulatedexperienceintheuseofthemoldsinthefactoryhasshownthatitisnecessarytopayattentiontocarefuladjustmentofthemoldholderunderthepedestalofthesemiautomaticmachineselectionofthemoldingscheduleavoidingsuddenloweringoftherolleratthemomentofcontactwiththebodytheselectionofthecorrectspeedforthemoldingroller500550rpmforcupsand250275rpmforplatesandforthemoldholder750800rpmforcupsand300500forplatesinordertoavoidtheproductionofscrapintheformofhumpersandwhirlers.Failuretoobservetheserequirementscanalsoleadtodamagetothemoldchippingofftheshoulder,anddiggingoutthebottomwithhollowware.Themoldswhenproducedvaryincolorfromrosetodarkbrown,accordingtothedurationoftheheattreatmentatconstantpressureandthefinaltemperatureintheelectrocabinet.Thedarkbrownoneshavelowporosity,andinpracticecannotbeused.Therosecoloredoneshaveadequateworkingproperties,buttheirlowmechanicalstrengthsignificantlyreducestheirworkinglife.Obtainingthenormalworkingporosityof3135,withuniformlydistributedpores,andatthesametimeretainingthenecessarymechanicalstrength,wasoneofthemostdifficultproblemsintheproductionofthemolds.Iftheheattreatmentwasforced,thisrapidlyledtotheappearanceofasurfacecrustwitharawinteriorinthemold.Ifthefinaltemperaturewasraisedabove210inordertospeeduptheheattreatment,thisledtodecompositionoftheplasticwiththeevolutionofgaseswhichdestroyedthesurfaceofthemold.Afterextensiveinvestigation,theoptimumheatingschedulefortheplasticinthepressmold,whichgavemoldsofthedesiredquality,wasfoundandisshowninFig.5.Fromexperienceinthepreparationanduseofthemolds,thecorrectamountofplasticperunitvolumeofmoldwasestablishedas0.9g/cm3.Thisenables,withotherfactorsremainingconstant,tomaintainaconstantdensityforthemolds.Thetechnicalcriteriaforacceptanceofthemoldswereestablishedinthefactory.Theymusthaveasmoothdenseworkingsurfaceandahomogeneouscolormatchingtheestablishedstandard.Dimensional149variationsmustnotexceed0.05.Themoldsmusthavenodeformations.Theirporositymustfallwithinthelimits3135.Theirrateofwaterabsorptionisdeterminedbyamethoddevelopedinthefactory.Thesurfaceofwaresmadeinthesemoldsrequiresnofurthertreatment,whichisacommercialadvantage.Theconsumptionofgypsumbythefactoryisreducedby500tonsperyear.Thelaborrequirementonheavyloading/unloadingworkisreduced,theworkofthelaborer/moldersisrelieved,andtransportisreleased.Thefactoryteamcontinuetoworkontheintroductionofmoreprogressivemethodsofpreparingplasticmolds,onspeedingtheheattreatmentprocesstogiveincreasedoutputofmoldsperpressmold,andonincreasingtheirmechanicalstrengthwhilstpreservingahighporosity.1.LITERATURECITEDS.M.Tsenteretal.,StekloiKeram.,No.11969.150
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