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1、SANDCASTINGMostmetalcastingsaremadebypouringmoltenmetalintoapreparedcavityandallowingittosolidify.Theprocessdatesfromantiquity.ThelargestbronzestatueinexistencetodayisthegreatSunBuddhainNara,Japan.Castintheeighthcentury,itweighs551tons(500metrictons)andismorethan71ft(21m)high.Artis
2、ansoftheShangDynastyinChina(1766-1222B.C.)createdartworksofbronzewithdelicatefiligreeassophisticatedasanythingthatisdesignedandproducedtoday.Therearemanycastingprocessesavailabletoday,andselectingthebestonetoproduceaparticularpartdependsonseveralbasicfactors,suchascost,size,product
3、ionrate,finish,tolerance,sectionthickness,physical-mechanicalproperties,intricacyofdesign,machinability,andweldability.Sandcasting,theoldestandstillthemostwidelyusedcastingprocess,willbepresentedinmoredetailthantheotherprocessessincemanyoftheconceptscarryoverintothoseprocessesaswel
4、l.GreenSandGreensandgenerallyconsistsofsilicasandandadditivescoatedbyrubbingthesandgrainstogetherwithclayuniformlywettedwithwater.Morestableandrefractorysandshavebeendeveloped,suchasfusedsilica,zircon,andmullite,whichreplacelower-costsilicasandandhaveonly2%linearexpansionatferrousm
5、etaltemperatures.Also,relativelyunstablewaterandclaybondsarebeingreplacedwithsyntheticresins,whicharemuchmorestableatelevatedtemperatures.Greensandmoldingisusedtoproduceawidevarietyofcastingsinsizesoflessthanapoundtoaslargeasseveraltons.Thisversatileprocessisapplicabletobothferrous
6、andnonferrousmaterials.Greensandcanbeusedtoproduceintricatemoldssinceitprovidesforrapidcollapsibility;thatis,themoldismuchlessresistanttothecontractionofthecastingasitsolidifiesthanareothermoldingprocesses.Thisresultsinlessstressandstraininthecasting.Thesandisrammedorcompactedaroun
7、dthepatternbyavarietyofmethods,includinghandorpneumatic-toolramming,jolting(abruptmechanicalshaking),squeezing(compressingthetopandbottommoldsurfaces),anddrivingthesandintothemoldathighvelocities(sandslinging).Sandslingersareusuallyreservedforuseinmakingverylargestingswheregreatvol
8、umesofsandarehandled.Forsm
