流道设计外文翻译参考文献.docx
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流道设计外文翻译参考文献
流道设计外文翻译参考文献
(文档含中英文对照即英文原文和中文翻译)
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,