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1、TheInjectionMold—ThedesignofRunnerl.ThebasicTherunnerisachannelmachinedintothemodplatetoconnectthespruewiththeentrance(gate)totheimpression.Inthebasictwoplatemodtherunnerispositionedonthesurfacewhileonthemorecomplexdesignstherunnermaybepositionedbelowthepartin
2、gsurfaceThewalloftherunnerchannelmustbesmoothtopreventanyrestrictiontoflow.Also,astherunnerhastoberemovedwiththemolding,theremustbenomachinemarksleft,whichwouldtendtoretaintherunnerinthemodplate.Toensurethatthesepointsaremet,it'sdesirableforthemoddesignertospe
3、cifythattherunner(channel)ispolished'inlineofdraw1.Therearesomeotherconsiderationsforthedesignertobearinmind:(i)theshapeofthecrosssectionoftherunner,(ii)thesizeoftherunner.Runnercross-sectionshapeThecross-sectionalshapeoftherunnerusedinamodisusuallyoneoffourfo
4、rms(Figure4.2):fullyround(a),trapezoidal(b),modifiedtrapezoidal(c)andhexagonal(d).Thereasonwhytheseparticularformsareusedinpreferencetoothersareoutlinedbelow.Thecriterionofefficientrunnerdesignisthattherunnershouldprovideamaximumcross-sectionalareafromthestand
5、pointofpressuretransferandminimumcross-sectionalareatoperipherywill,therefore,giveadirectindicationoftheefficiencyoftherunnerdesign;therunnersectionaregiverinFigure43Ascanbeseen,thevarioustypesofstandpoint;whereastheratiosexhibitedbythesemicircularandrectangul
6、artypesmaketheirusegenerallyundesirable.Unfortunately,thesquarerunnerisnotverysatisfactoryeither,butforanotherreason:itisdifficulttoeject.Inpractice,becauseofthis,anangleof10°isincorporatedontherunnerwell,thusmodifyingthesquaretothetrapezoidalsection.Thevolume
7、ofthetrapezoidalrunnerisapproximately25%greaterthanthatofaroundrunnerwithcorrespondingdimensions(W=D,Figure4.2).Toreducethisdifferenceandstillmaintaincorrespondingdimensions,amodifiedtrapezoidalformhasbeendeveloped(Figure4.2c)inwhichthevolumeisonly14%greater(a
8、pproximately)thanitsroundcounterpart.Thehexagonalrunnerisbasicallyadoubletrapezoidalrunner,wherethecross-sectionalareaofthisrunnertypeisabout82%ofthatofthecoiTespondingroundrunner.
