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1、TheInjectionMold—ThedesignofRunnerl.ThebasicTherunnerisachannelmachinedintothemodplatetoconnectthespruewiththeentrance(gate)totheimpression.Inthebasictwo-platemodtherunnerispositionedonthesurfacewhileonthemorecomplexdesignstherunnermaybepositionedbelowthepartingsurfaceThewallo
2、ftherunnerchannelmustbesmoothtopreventanyrestrictiontoflow.Also,astherunnerhastoberemovedwiththemolding,theremustbenomachinemarksleft,whichwouldtendtoretaintherunnerinthemodplate.Toensurethatthesepointsaremet,it'sdesirableforthemoddesignertospecifythattherunner(channel)ispolis
3、hedfinlineofdraw1.Therearesomeotherconsiderationsforthedesignertobearinmind:(i)theshapeofthecrosssectionoftherunner,(ii)thesizeoftherunner.Runnercross-sectionshapeThecross-sectionalshapeoftherunnerusedinamodisusuallyoneoffourforms(Figure4.2):fullyround(a),trapezoidal(b),modifi
4、edtrapezoidal(c)andhexagonal(d).Thereasonwhytheseparticularformsareusedinpreferencetoothersareoutlinedbelow.Thecriterionofefficientrunnerdesignisthattherunnershouldprovideamaximumcross-sectionalareafromthestandpointofpressuretransferandminimumcross-sectionalareatoperipherywill
5、,therefore,giveadirectindicationoftheefficiencyoftherunnerdesign;therunnersectionaregiverinFigure4.3.Ascanbeseen,thevarioustypesofstandpoint;whereastheratiosexhibitedbythesemicircularandrectangulartypesmaketheirusegenerallyundesirable.Unfortunately,thesquarerunnerisnotverysati
6、sfactoryeither,butforanotherreason:itisdifficulttoeject.Inpractice,becauseofthis,anangleof10°isincorporatedontherunnerwell,thusmodifyingthesquaretothetrapezoidalsection.Thevolumeofthetrapezoidalrunnerisapproximately25%greaterthanthatofaroundrunnerwithcoiTespondingdimensions(W=
7、D,Figure4.2).Toreducethisdifferenceandstillmaintaincorrespondingdimensions,amodifiedtrapezoidalformhasbeendeveloped(Figure4.2c)inwhichthevolumeisonly14%greater(approximately)thanitsroundcounterpart.Thehexagonalrunnerisbasicallyadoubletrapezoidalrunner,wherethecross-sectionalar
8、eaofthisrunnertypeisabout82%ofthatofthecoiTespondingroundrunn
