05机械制造类外文资料.doc

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1、2.4CompressionandTransferMolds2.4.1CompressionMoldingCompressionmoldingisthebasicformingprocesswhereanappropriateamountofmaterialisintroducedintoaheatedmold,whichissubsequentlyclosedunderpressure・Themoldingmaterial,softenedbyheat,isformedintoacontinuousmasshavingthegeometricalconfigurationoft

2、hemoldcavity.Furtherheating(thermosettingplastics)resultsinhardeningofthemoldingmaterial.Ifthermoplasticsarethemoldingmaterial,hardeningisaccomplishedbycoolingthemold.Fig.2-6illustratestypesofcompressionmolding.Herethemoldingcompoundisplacedintheheatedmold.Aftertheplasticcompoundsoftensandbeco

3、mesplastic,thepunchmovesdownandcompressesthematerialtotherequireddensitybyapressure.Someexcessmaterialwillflow(verticalflash)fromthemoldasthemoldclosestoitsfinalposition.Continuedheatandpressureproducethechemicalreactionwhichhardensthecompound.Thetimerequiredforpolymerizationorcuringdependspri

4、ncipallyuponthelargestcrosssectionoftheproductandthetypeofmoldingcompound.Thetimemaybelessthanaminute,oritmaytakeseveralminutesbeforethepartisejectedfromthecavity.Sincetheplasticmaterialisplaceddirectlyintothemoldcavity,themolditselfcanbesimplerthanthoseusedforothermoldingprocesses.Gatesandspr

5、uesareunnecessary.Thisalsoresultsinasavinginmaterial,beciiusetrimmed-offgatesandsprueswouldbeacompletelossofthethermosettingplastic・Thepressusedforcompressionmoldingisusuallyaverticalhydraulicpress.Largepressesmayrequirethefullattentionofoneoperator.However,severalsmallerpressescanbeoperatedby

6、oneoperator.Thepressesareconvenientlylocatedsotheoperatorcaneasilymovefromonetothenext.Bythetimehegetsaroundtoaparticularpressagain,thatmoldwillbereadytoopen.Heatingelements(d)Fig.2・6TypesofCompressionMolding(a)positive(b)semipositive(c)flash(d)diedesignformakingacompression-moldedpartwithunde

7、rcutsThethermosettingplasticswhichhardenunderheatandpressurearesuitableforcompressionmoldingandtransfermolding.Itisnotpracticaltomoldthermoplasticmaterialsbythesemethods,sincethemoldswouldhavetobealternatelyheatedandcooled.Inordertohar