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1、Lesson11EjectionAmoldingisformedinmouldbyinjectingaplasticmelt,underpressure,intoanimpressionviaafeedsystem.Itmustthereforeberemovedmanually.Furthermore,allthermoplasticmaterialscontractastheysolidify,whichmeansthatthemoldingwillshrinkontothecorewhichformsit①.Thiss
2、hrinkagemakesthemoldingdifficulttoremove.Facilitiesareprovidedontheinjectionmachineforautomaticactuationofanejectorsystem,andthisissituatedbehindthemovingplaten.Becauseofthis,themould’sejectorsystemwillbemosteffectivelyoperatedifplacedinthemovinghalfofthemould,i.e.
3、thehalfattachedtothemovingplaten.Wehavestatedpreviouslythatweneedtoejectthemoldingfromthecoreanditthereforefollowsthatthecore,too,willmostsatisfactorilybelocatedinthemovinghalf.Theejectorsysteminamouldwillbediscussedunderthreeheadings,namely:(i)theejectorgrid;(ii)t
4、heejectorplateassembly;and(iii)themethodofejection.EJECTORGRIDTheejectorgrid(Figure11-1)isthatpartofthemouldwhichsupportsthemouldplateandprovidesaspaceintowhichtheejectorplateassemblycanbefittedandoperated.Thegridnormallyconsistsofabackplateontowhichismountedanumbe
5、rofconvenientlyshaped‘supportblocks’.Figure11-1EjectorgridEJECTORPLATEASSEMBLYTheejectorplateassemblyisthatpartofthemouldtowhichtheejectorelementisattached.Theassemblyiscontainedinapocket,formedbytheejectorgrid,directlybehindthemouldplate.Theassembly(Figure11-2)con
6、sistsofanejectorplate,aretainingplateandanejectorrod.Oneendofthislattermemberisthreadedanditisscrewedintotheejectorplate.Inthisparticulardesigntheejectorrodfunctionsnotonlyasanactuatingmemberbutalsoasamethodofguidingtheassembly.Notethattheparallelportionoftheejecto
7、rrodpassesthroughanejectorrodbushfittedinthebackplateofthemould.Figure11-2EjectorplateassemblyFigure11-2EjectorplateassemblyEJECTORTECHNIQUESWhenamoldingcools,itcontractsbyanamountdependingonthematerialbeingprocessed.Foramoldingwhichhasnointernalform,forexampleasol
8、idrectangularblock,themoldingwillshrinkawayfromthecavitywalls,therebypermittingasimpleejectiontechniquetobeadopted.However,whenthemoldinghasinter