机械模具设计的外文翻译.docx

机械模具设计的外文翻译.docx

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机械模具设计的外文翻译

TheInjectionMold

--ThedesignofRunner

1.Thebasic

Therunnerisachannelmachinedintothemodplatetoconnectthespruewiththeentrance（gate）totheimpression.Inthebasictwo-platemodtherunnerispositionedonthesurfacewhileonthemorecomplexdesignstherunnermaybepositionedbelowthepartingsurface

Thewalloftherunnerchannelmustbesmoothtopreventanyrestrictiontoflow.Also,astherunnerhastoberemovedwiththemolding,theremustbenomachinemarksleft,whichwouldtendtoretaintherunnerinthemodplate.Toensurethatthesepointsaremet,it'sdesirableforthemoddesignertospecifythattherunner（channel）ispolished'inlineofdraw'.

Therearesomeotherconsiderationsforthedesignertobearinmind:

（i）theshapeofthecrosssectionoftherunner,（ii）thesizeoftherunner.

Runnercross-sectionshapeThecross-sectionalshapeoftherunnerusedinamodisusuallyoneoffourforms（Figure4.2）:

fullyround（a）,trapezoidal（b）,modifiedtrapezoidal（c）andhexagonal（d）.Thereasonwhytheseparticularformsareusedinpreferencetoothersareoutlinedbelow.

Thecriterionofefficientrunnerdesignisthattherunnershouldprovideamaximumcross-sectionalareafromthestandpointofpressuretransferandminimumcross-sectionalareatoperipherywill,therefore,giveadirectindicationoftheefficiencyoftherunnerdesign;therunnersectionaregiverinFigure4.3.Ascanbeseen,thevarioustypesofstandpoint;whereastheratiosexhibitedbythesemicircularandrectangulartypesmaketheirusegenerallyundesirable.

Unfortunately,thesquarerunnerisnotverysatisfactoryeither,butforanotherreason:

itisdifficulttoeject.Inpractice,becauseofthis,anangleof10°isincorporatedontherunnerwell,thusmodifyingthesquaretothetrapezoidalsection.Thevolumeofthetrapezoidalrunnerisapproximately25%greaterthanthatofaroundrunnerwithcorrespondingdimensions（W=D,Figure4.2）.Toreducethisdifferenceandstillmaintaincorrespondingdimensions,amodifiedtrapezoidalformhasbeendeveloped（Figure4.2c）inwhichthevolumeisonly14%greater（approximately）thanitsroundcounterpart.

Thehexagonalrunnerisbasicallyadoubletrapezoidalrunner,wherethecross-sectionalareaofthisrunnertypeisabout82%ofthatofthecorrespondingroundrunner.Naturallyifsimilarcross-sectionalareasarerequired,thenthevalueforD（Figure4.2c）mustbeincreasedaccordthehexagonalrunnercomparedwithmatchingthetwohalvesofaroundrunner.Thispointappliesparticularlytorunnerswhichareless3mm（1/8in）inwidth.

Astheplasticmeltprogressesthroughtherunnerandmodsystemthemeltadjacenttothecoldmodsurfacewillrapidlydecreaseintemperatureandsolidify.Thematerialwhichfollowswillpassthroughthecenterofthissolidifiedmaterialand,becauseofthelowthermalconductivitythatmostthermoplasticsposses,thesolidifiedmaterialactsasaninsulationandmaintainsthetemperatureofthecentralmeltflowregion.Ideally,thegateshouldthereforebepositionedinlinewiththecenteroftherunnertoreceivethematerialfromthecentralflowstream.Thisconditionmaybeachievedwiththefullyroundrunner（Figure4.4a）,andalsowiththehexagonalrunner

Thebasictrapezoidaldesigns（Figure4.2bandc）arenotassatisfactoryinthisrespectsincethegatecannotnormallybepositionedinlinewiththecentralflowstream.

Themainobjectiontothefullyroundrunneristhatthisrunnerisformedfromtwosemicircularchannelsmachinedoneineachofthemodplates.Itisessentialthatthesechannelsareaccuratelymatchedtopreventanundesirableandinefficientrunnersystembeingdeveloped.Asimilarargumentappliestothehexagonalrunnersystem.Thefactthatthesechannelsmustbeaccuratelymatchedmeansthatthemodcostforamodcontainingroundorhexagonalrunnerwillbegreaterthanforonecontainingtrapezoidalrunners.

Thechoiceofrunnersectionisalsoinfluencedbythequestionwhetherpositiveejectionoftherunnersystemispossible.Consider,forinstance,thecaseofatwo-platemodinwhichacircularrunnerhasbeenmachinedfrombothpartingsurface.Inthiscase,asthemodopens,therunnerispulledfromitschannelinonemodhalfanditisthenejectedfromtheothermodhalfeitherdirectly,byejectorpins,orbyrelyingonitsattachmenttothemoldingsbythegates（Figure4.5）.

Formulti-platemolds,however,positiveejectionoftherunnersystemisnotpracticable.Herethebasictrapezoidal-typerunnerisalwaysspecified,therunnerchannelbeingmachinedintotheinjectionhalffromwhichitispulledasthemodopens.Inthiswaytherunnerisfreetofallundergravitybetweenmodplates.Ifacircularrunnerhadbeenpecified,however,therunnersystemcouldwelladheretoitschannelandmakeitsremovaldifficult（Figure4.6）.

Summingupthepointsconcerningcross-sectionalshape,wecansaythatforsimpletwo-platemoldswhichhaveaflatpartingsurfacethefullyroundrunnerorhexagonalrunneristobeprefaced,theincreasedmodcostbeingrelativelysmall.Formoldswhichhavecomplexpartingsurface,whereitwouldbedifficulttomatchaccuratelythesemicircularchannelsoftheroundrunneror,formulti-platemolds,thetrapezoidalormodifiedtrapezoidalsectionshouldbeused.

2.Runnersize

Whendecidingthesizeoftherunnerthedesignermustconsiderthefollowingfactors:

（i）thewallsectionandvolumeofthemolding（ii）thedistanceoftheimpressionfromthemainrunnerorsprue,（iii）runnercoolingconsiderations,（iv）therangeofmouldmaker'scuttersavailableand（v）theplasticsmaterialtobeused.

（i）Thecross-sectionalareaoftherunnermustbesufficienttopermitthefreezesandforpackingpressuretobeappliedforshrinkagecompensationifrequired,Becauseofthis,runnersbelow2mm（3/32in）diameterareseldomusedandeventhisdiameterisnormallylimitedtobranchrunnersunder25mm（1in）inlength.

（ii）Thefurthertheplasticmelthastotravelalonetherunnerthegreateristheresistancetoflow.Hencethedistancetheimpressionisfromthespruehasadirectbearingonthechoiceofcross-sectionalsizeoftherunner.Forexample,whereasa5mm（3/16in）

（iii）Thecross-sectionalareaoftherunnershouldnotbesuchthatitcontrolstheinjectioncycle,althoughthisissometimesunavoidableforverylightmoldingsThelargerhecross-sec~tionareaoftherunnerthegreateristhebulkofmaterialitcontainsandthelongertheperiodittakestocoolsufficientlytoenablethemodtobeopenedandthemoldingsandrunnerejected.Forthisreasonitisundesirabletomaketherunnerlargerthan10mm（]in）diameterformostmaterials.However,therigidPVCsandtheacrylicsareexceptionsduetotheirhighviscosityanddiametersupto13mm（1/2in）areused.

（iv）Thesizechosenfortherunnershouldbeinarangeconsistentwiththemouldmakers'snothavingtocarryinstockamultitudeofdifferent!

sizesofcutters.Inpracticethefollowingarethemorecommonsizes:

2-13mminImmstepsinthemetricrangeand~-?

inWith~instepsintheimperialunitrange.Thefollowingempiricalformulaissuggestedasaguideofthesizeoftherunnerorbranchrunnerformoldingsweighingupto200g（Ig（7oz）,andwithwallsectionslessthan3mm（0.125in）.FortherigidPVCsandtheacrylics,increasethecalculateddiameterby25%.

Theformulaisusedinconjunctionwiththenotesgivenpreviously.

（i）Therunnershouldnotbebelow2mm（3/32in）diameter,norabove10mm（3/8in）diameter（or13mm（1/2in）diameterwhereapplicable）.

（ii）Thecalculatedsizeshouldbeincreasedtothenextsuitablecuttersize

Figure4.7showsaplotofdiameterversuslengthofrunnerforvariousweightsofmolding,adoptingthemetricsystemofdimensioning.Figure4.8showsacorrespondingplotusingtheImperialdimensioningsystem.Forexample,a120g（4oz）moldinginpolyethylenebeingfedbya50mm（2in）longrunnerwillrequireadiameterof7mm（5/16in）.

Theoreticallythecross-sectionalareaofthemainrunnershouldbeequalto,orinexcessof,thecombinedcross-sectionalareasofthebranchrunnersthatitisfeeding.Thisrelationshipis,however,ignoredwhenthemaximumsuggesteddiameterisreached

Themainobjectiontothefullyroundrunneristhatthisrunnerisformedfromtwosemicircularchannelsmachinedoneineachofthemodplates.Itisessentialthatthesechannelsareaccuratelymatchedtopreventanundesirableandinefficientrunnersystembeingdeveloped.Asimilarargumentappliestothehexagonalrunnersystem.Thefactthatthesechannelsmustbeaccuratelymatchedmeansthatthemodcostforamodcontainingroundorhexagonalrunnerwillbegreaterthanforonecontainingtrapezoidalrunners.

Thechoiceofrunnersectionisalsoinfluencedbythequestionwhetherpositiveejectionoftherunnersystemispossible.Consider,forinstance,thecaseofatwo-platemodinwhichacircularrunnerhasbeenmachinedfrombothpartingsurface.Inthiscase,asthemodopens,therunnerispulledfromitschannelinonemodhalfanditisthenejectedfromtheothermodhalfeitherdirectly,byejectorpins,orbyrelyingonitsattachmenttothemoldingsbythegates（Figure4.5）.

　Formulti-platemolds,however,positiveejectionoftherunnersystemisnotpracticable.Herethebasictrapezoidal-typerunnerisalwaysspecified,therunnerchannelbeingmachinedintotheinjectionhalffromwhichitispulledasthemodopens.Inthiswaytherunnerisfreetofallundergravitybetweenmodplates.Ifacircularrunnerhadbeenspecified,however,therunnersystemcouldwelladhereto