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Castinganditsprocesses1.1IntroductionManufacturingprocesscanbeclassifiedintotwogeneralgroupsknownasprimaryandsecondaryprocesses.Theprimaryprocesses,namely,casting,forging,coldheading,coldextruding,brakeforming,andsoon,arethosethatconvertrawmaterialintoshapes.Theseformingmethodsincludebothhotandcoldworkingprocessesand,ingeneral,willstillrequirefurtherfinishingoperationsinordertoobtainanendproduct.Thesecondaryprocessesarethosethatbringtheparttothedimensionsandsurfacefinishspecified.Onemayalsoincludeinthesecondaryprocessessuchoperationsasheattreatingandsurfacetreatmentsforcorrosionresistance,hardness,andappearance.Secondaryprocessesareplanning,turning,milling,drilling,boring,reaming,broaching,grinding,honing,lapping,polishing,andspecialmethodsofmetalremovalsuchaselectrochemicalmachining.Operationslikescrewcutting,tapping,threadmilling,gearcutting,andsoon,aresecondaryprocessesthataremerelyadaptationsofoneoftheaforementionedprocessesthataremerelyadaptionofoneoftheaforementionedprocesses.Acompletediscussionoftheseproceduresisbeyondthescopeofthistext.Therefore,wewillconfineourselvestosuccinctdescriptionsofthemostprominenttechniques.1.2CastingsCastingsareidentifiedbythetypeofmoldortheforcerequiredtoyfillthemold.Moldsareeitherpermanentorexpendable.Thepatternofsand,shell,andplastermolds,however,canbeusedrepeatedlyformakingnewmolds.Asthemetalcools,ittakestheshapeofthecavity.Thebasiccastingmethodsaredescribedbelow.Sandcastings.1.Thegreensandprocessisoneinwhichmost,bondedsandspackedaroundawoodormetalpattern.Thepatternisthenremovedandmoltenmetalispouredintothecavity.Whenthemetalsolidifies,themoldisbrokenandthecastingisremoved.Almostanymetalcanbeused,withvirtuallynolimitastothesizeorshapeofthepart.Themethodpermitscastingcomplexcomponentsatalowtoolingcostandisthemostdirectroutefrompatterntocasting.Somemachiningisalwaysnecessarywiththegreensandprocess,andlargecastingshaveroughsurfacefinish.Closetolerancesaredifficulttoachieve,andlong,thinprojectionsshouldnotbecast.Itispossible,however,todesignforbosses,undercutsandinserts.Theminimumcoreholediameteradvisablearealuminum,3|161|4in.andtheminimumsectionthicknessesadvisablearealuminum,3|16in.cooper,3|32in.iron,3|32in.magnesium,5|332inandsteel,1|41|2in.2.Thedrysandprocessissimilartothegreensandprocessexceptthatcoreboxesareusedinplaceofpatterns.Thismethodisusuallylimitedtosmallerpartsthanmethod1.Thesandisbondedwithasettingbinder,andthecoreisthenovenbakedat300400toremovethemoisture.3.Thecarbendioxidesandprocessisoneinwhichthesandmoldsarebondedwhitsodiumsilicatesolutionsandsetbyforcingcarbondioxidegasthroughthesand.Thistypeofmoldisstrongandpermitstheproductionofbetterdimensionallycontrolledcastingsthaneithermethod1or2.4.Thecoresandmoldprocessisonewherethemoldsareputtogethercompletelyfromovenbakedcoressetwithorganicbinderssuchasoilordextrines.ShellMoldCasting.Thisisaprocesswherethemoldsaremadebycoatingthesandwithathermosettingplastic.Themoldisthensupportedontheoutsaidbysandorshot,andmoltenmetalispoured.Whenthemetalhassolidified,themoldisbrokenawayfromthefinishedcasting.Shellmoldingproducescastingwithsmoothsurfaces,uniformgrainstructure,highdimensionalaccuracy,rapidproductionrateandminimalamountoffinishingoperations.Theminimumsectionthicknisscastableis1|161|4in.butsectiondifferenceswherethemaximum–minimumratioisgreaterthan5to1shouldbeavoided.Bosses,undercutsandinsertsareeasilycastandtheminimumcoredholediameteris1|81|4in.Themethodisrelativelycostlyislimitedtofewmetals.FullMoldCasting.Aprocessinwhichagreensandorcoldsettingresinbondedispackedaroundafoamedplasticpatternforexample,polystyrene.Theplasticpatternisvaporizedwiththepouringofthemoltenmetalintothemold.Animprovedcastingsurfacecanbeobtainedbyputtingarefractorytypeofcoatingonthepatternsurfacebeforesandpacking.Thepatterncanbeonepieceorseveralpieces,dependinguponthecomplexityoftheparttobecast.Theplasticpatternsarehandledeasily.Theydonotrequireanydraft,andproducenoflashonthecasting.Forsmallquantityproductionthismethodcanbeexpensive,dependinguponthesizeandcomplexityofthecasting.Theminimumrecommendedsectionthicknessis0.1in.,andthemaximumsectionthicknessisunlimited.Bosses,undercuts,andinsertspresentnoprobleminthistypeofcasting.Theminimumcoredholediameteris1|4in.PermanentMoldCastings.Theseareformedbyamoldthatcanbeusedrepeatedly.Someapplicationshavereportedasmanyas25000castingsbeingmadefromthesamemold.Usually,themoldrequiressomeredressingafterabout3000uses.Themoldsaremachinedofmetalforexample,grayironforcastingnonferrousmetalsandcastitons.Machinedgraphitemoldsareusedtocaststeel.Thismethodproducescastingsthathaveagoodsurfacefinishaswellasagoodgrainstructure,lowporosity,andhighdimensionalaccuracy.Theinitialmoldcostisrelativelyexpensive,butcastingsweighingaslittleasseveralouncestocastingsarelimitedtorelativelysimpleshapesandforms.Themaximumrecommendedsectionthicknessisabout2.0in.Theminimumthickness,however,dependsuoonthematerialbeingcastasfollows3|16in.foriton,3|321|8in.foraluminum,5|32in.formagnesium,and3|32–5|16in.forcopper.Theminimumcastableholesare3|161|4in.indiameter.DieCasting.Aprocessusedextensivelyinthequantityproductionofintricatelyshapedzinc,alumimum,leadandmagnesiumalloys.Themethodislimitedinusewithtinandcopperalloys.Castingsareformedbyforcingmoltenmetalunderpressuresof150025000psiintoanaccuratelymachinedsteeldie.Thesteeldie,whichiswatercooled,isheldtogetherbyahydraulicpressuntilthemetalcastingssolidifies.,Toeasetheejectionofthecastparts,alubricantissprayedonthedieformingsurfaceandtheejectionpins.Forsmalldiecastcomponents.,multiplecavitydiesareused.Thesurfacefinishoftheresultingcastingsisquitesmoothandhasexcellentdimensionalaccuracy.Althoughthecostofproductionisrelativelycheap,theinitialdiecostishigh.Diecastingislimitedtononferrousmetalsandbythesizeofthepartthatcanbecast.Themaximumwallsectionthicknessisusuallyrestrictedto3|16in.butcertainlytonogreaterthan0.50in.PlasterMoldCasting.Thesearemadebypouringanonferrousalloyforexample,aluminum,copper,orzincalloyintoaplastermold,whichisthenbrokentoremovethesolidifiedcasting.Thecastingsproducedbythisprocessaresmooth,havehighdimensionalaccuracy,lowporosity,andcanbemadeinmanyintricateshapes.Themethod,however,hasdisadvantagesinthatislimitedtononferrousmetals,smallcastingsandalsorequiresarelativelylongtimetomakethemolds.Theminimumwallsectionthicknessis0.0400.060in.foracastareathatislessthan2sq.in.Forlargercastareas,theminimumwallthicknessincreasedaccordingly.Theshowprocess,developedwithinthelastdecade,isacostpromisingmethod.Itemploystheuseofplastermolds.Thisprocedureproducescastingswithfinedetailandexcellentdimensionalaccuracyandsurfacefinish.CeramicMoldCasting.Thisprocessusesamoldmadeofaceramicpowder,binder,andgellingagent.Themoldcanalsobemadeofaceramicfacingreinforcedwithasandbacking.Themethodisrestrictedtocastingintricatepartsrequiringfinedetail,closetolerances,andsmoothfinishes.Theminimumwallthicknessrecommendedis0.0250.050in.Thereis,however,nolimittothemaximumwallthickness.InvestmentCasting.Thesearemadewhenpartsaredesiredthatareintricateinshape,haveexcellentsurfacefinish,andrequireahighdegreeofdimensionalaccuracy.Inaddition,thismethodofcastingpermitstheuseofavarietyofmetalalloysanddoesnothaveInaddition,thismethodofcastingpermitstheuseofavarietyofmetalalloysanddoesnothaveanymetalflashingtoberemovedfromthefinishescasting.Thetechniqueofinvestmentcastingrequirescarefulworkmanshipandexpensivepatternsandmoths.Theminimumcastablewallthicknessisabout0.0250.050in.andthemaximumthicknessshouldnotexceed3in.CentrifugalCasting.Thisisamethodofcastingashapebypoutingthemoltenmetalintoarotatingfladkcontainingthemold.Themoldsaremadeofsand,metal,orgraphitedependingonthemetalcastandarerotatedabouttheiraxialcenterseitherinahorizontalorverticalposition.Relativelylargediameterandbulkycomponentsaremadearepipe,gunbarrels,hollowshafts,machinerydriverolls,longsleeves,tubing,andsoon.Wherethediametertolengthratioisratherlarge,therotationalaxiscanbeverticallymountedmolds,themethodiscalledsemmicentrifugalcasting,anditisusedtomakepartssuchaswheels,turbosuperchargerdiaphragms,disks,flywheels,andsoon.Figure152showsaschematicsectionofasemicentrifugalcastingofacoredflywheel.Generally,themethodofcentrifugalcastingisexpensiveandisalsolimitedintheshapeswhichcanbecast.However,thecastingsmadebythismehodareverysoundandhavegooddimensionalaccuracy.Itshouldalsobenotedthatthismethodistheonlywaytoobtainalargecylindricalpart.ContinuousCasting.Thisisamethodwherebyamoltenmetaliscontinuouslygravityfedfromaladleintoaningotmoldofadesiredshapewhichisopenatbothends.Asthemetalfallsthroughthemold,ittakesitsshape,andisrapidlycooledbyawaterspray.Itisthencuttospecificlengths.Thecontinuouscastingprocesshastheadvantageofbeinglowincostandhavingahighrateofproduction.Inaddition,thismethodpermitscastingmaterialsthatcannotbeextruded.However,continuouscastingisrestrictedtoshapesofuniformcrosssectionthatis,inthedirectionofcasting,Theminimumwallthicknessdependsontheshapebeingcast.Theminimumsizeofcastingis1|2in.andthemaximumsizeisabout9in.
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