Design of gates 86069_probe.pdf

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1、6DesignofGates6.1TheSprueGateThespruegateisthesimplestandoldestkindofgate.Ithasacircularcross-section,isslightlytapered,andmergeswithitslargestcross-sectionintothepart.Thespruegateshouldalwaysbeplacedatthethickestsectionofthemoldedpart.Providedpropersize,theholding

2、pressurecanthusremaineffectiveduringtheentiretimethemoldedpartsolidifies,andthevolumecontractionduringcoolingiscompensatedbyadditionalmaterialforcedintothecavity.Noformationofvoidsorsinkmarkscanoccur.Thediameterofthespruegatedependsonthelocationatthemoldedpart.Itha

3、stobealittlelargerthanthesectionthicknessofthemoldedpartsothatthemeltinthespruesolidifieslast.Thefollowingholds(Figure5.9):dFSmax+1.0(mm).(6.1)Itshouldnotbethicker,though,becauseitthenthemeltsolidifiestoolateandextendsthecoolingtimeunnecessarily.Todemoldthespruewi

4、thouttroubleitshouldtaperofftowardstheorificeonthesideofthenozzle.Thetaperis1–4°.(6.2)Americanstandardspruebushingshaveauniformtaperof1/2inchperfoot,whichisequivalenttoabout2.4°.Theorificetowardsthenozzlehastobewiderthanthecorrespondingorificeofthenozzle.Therefor

5、edAdD+1.5mm(6.3)(RefertoFigure5.9forexplanationofsymbols)Iftheserequirementsarenotmet,undercutsattheupperendareformed(Figure5.8).Verylongsprues,thatisifthemoldplatensareverythick,callforacheckonthetaper.Possiblyanothernozzlehastobeusedintheinjectionmoldingmachine.

6、Toalargedegreethereleasepropertiesofthespruealsodependonthesurfacefinishofthetaperedhole.Scoresfromgrindingorfinishingperpendiculartothedirectionofdemoldinghavetobeavoidedbyallmeans.Materialwouldstickinsuchscoresandpreventthedemolding.Asaruletheinteriorofspruebushi

7、ngsishighlypolished.Aradiusr2(Figure5.9)atthebaseofthesprueisrecommendedtocreateasharpnotchbetweensprueandmoldingandtopermitthematerialtoswellintothemoldduringinjection.Toitsdisadvantage,thespruealwayshastobemachinedoff.Evenwiththemostcarefulpostoperation,thisspotr

8、emainsvisible.Thisisannoyinginsomecases,andonecouldtrytopositionthesprueatalocationthatwillbecoveredafterassemblyofthearticle.Sincethisisoftenimp