ATypicalInjectionMoldDesignGuideThischecklistcanbeusedasageneralreferenceguideforinjectionmolddesignengineers.Itisdividedinto3partsofamolddesignprocess.Part1-Requirementstostartyourmolddesign:Checktheinjectionmachinewherethemoldistobemounted.Thiswillhelpyoudecidethesizeandstructureofthemold.foreaseofinstallationandotherfactors.Importantnotes:Locatingringsize(orotherpositioningmethod)NozzlesizeMethodofclamping(Autoormanual)TemperaturecontrolsystemDeterminethenumberofcavitiesandvolumerequirements.Thiswillhelpyoudecidethematerialthatyouaregoingtouseandothermoldcomponentsthatyouwillchooseforcosteffectivedesign.Determinethegatelocationandsize.Determinethelocationwhereejectorpinmarksareprohibited.Part2-Moldbaselayout:Placecavitiesclosetothecenterofthemoldtominimizebasesizeandrunnerlength.Ensurethatthemoldedpartremainsonthemovablehalf(ejectorhalf)uponopeningofPLtofacilitateproperejection.Waterlinesshouldbeplacedasevenlyaspossibletothecontoursofthecavity.Usesupportpillarsunderneaththecavitypockets.Useejectorguidesformoldswithsmallejectorpinsandrectangularejectorpins.Provideeye-boltholeforeaseofmountinganddismounting.Installmoldopeningpreventionlocksontheoperatorside.Establishprybargrooveonthecornersofthemoldpartinglinetofacilitateeaseofmoldopeningduringassemblyandmaintenance.Bythistimeyoumayaskforthemoldlayoutapprovalfromthecustomer.Part3-Cavity/coredetails:Checkmaterialshrinkage.Locateportions(corners)forpossiblesignificantdeflectionanddeformation.Maintainuniformwallthickness.Draftangleshouldbewithindimensiontolerance.Dividecoreblockstosimplifymachiningandprovidegasventpath.Gate,smallcores,andcoreswithshut-offfittingsarebetterdesignedasinsertablecomponentsforeasymodificationandrepair.Watchoutforpossibledeformationofcorepins.Positiontheejectorpinsontheribsandotherhighstrengthlocations.Ensureejectorbalance.Detailing/partdrawing:Includeallparametersneededforprocessing-material,quantity,surfacefinish/texture,dimensions,tolerancesandmanymore.Donotassumethemachinistunderstandseverything.Anydesignchangeandamendmentstothemoldmustbere-approvedbythecustomerormoldowner.Standardhorizontalclamppressesdelivermoltenresintothemoldthroughaholeinthecenterofthestationarypressplaten.Amaterial-deliverysystemusuallyconsistingofasprue,runners,andgatesthenleadstheresinthroughthemoldandintothecavity.Thesecomponentsofthematerialdeliverysystemarediscussedinthissection.SpruesThesprue,orientedparalleltothepressinjectionunit,deliversresintothedesireddepthintothemold,usuallythepartingline.Thoughtheycanbecutdirectlyintothemold,spruebushingsareusuallypurchasedasoff-the-shelfitemsandinsertedintothemold(seefigure7-18).Theheadendofthespruebushingcomespremachinedwithasphericalrecesstypically0.5-or0.75-inchradiustoreceiveandsealoffagainsttheroundedtipofthepressinjectionnozzle.Thespruebushingflow-channeldiametertypicallytaperslargertowardthepartinglineatarateof0.5inchperfoot.Thiseasesremovalofthemoldedsprue.Thesprueorificesize,thediameteratthesmallend,comesstandardinodd1/32sfrom5/32to11/32inch.Spruedesigncanaffectmoldingefficiencyandeaseofprocessing.Inmanymolds,thegreatestrestrictiontomaterialflowoccursatthepressnozzletipandsprueorifice.Theseareasseethehighestvolumetricflowrateoftheentiresystem.Anexcessivelysmallsprueorificecangeneratelargeamountsofmaterialshearandleadtomaterialdegradation,cosmeticproblems,andelevatedfillingpressure.Theproblemcanbeworseinthepressnozzletipbecausethetiporificemustbeslightlysmallerthanthesprueorificetoavoidforminganundercut.Thevolumetricflowrateusedduringfillinglargelydeterminesthecorrectsprueorificesize.Shotsizeandfillingspeed,aswellastheflowpropertiesofthespecificresin,governtherequiredflowrate.*Largepartsand/orpartsneedingfastfillingspeedsrequirelargesprueorificediameterstoavoidproblemsassociatedwithexcessiveflowshear.*Asageneralrule,amorphousresinsandblendssuchasMakrolonpolycarbonate,LustranABS,andBayblendPC/ABSresinsrequirelargerspruesandrunnersthansemicrystallineresinssuchasDurethanPA6andPocanPBT.Thediameteratthebaseofthesprueincreaseswithincreasingspruelength.Standardspruetaper,typicallyone-halfinchperfoot,leadstolargebasediametersinlongsprues.Thislargebasediameterlengthenscoolingandcycletimesandalsoleadstoregrindproblems.Figure7-19showstypicalspruesizesforBayeramorphousresinsasafunctionofshotsizeandfillingtime.Becausethemaximumshearrateinasprueoccursattheorificeandthemajorityofshearheatingandpressurelosstakesplaceinthefirsttwoinches,theseguidelinesshouldapplytospruesofvariouslengths.Partgeometryinfluencesfillingtimetosomeextent.Forexample,partswithamixofthickandthinfeaturesmayneedafastfillingspeedtopreventprematurecoolingofthethinfeatures.Othergeometriesmayrequireslowerfillingspeedstopreventproblemssuchascosmeticdefectsorexcessiveclamptonnagerequirements.Hotspruebushingsprovideonesolutiontothisproblem.Hotspruebushingshaveaheatedflowchannelthattransportsmaterialalongitslengthinmoltenform,eliminatingorshorteningthemoldedcoldsprue.Additionally,somemoldsrelyonextensionpressnozzlesthatreachdeepintothemoldtoreducespruelength.RunnersUnlikesprues,whichdelivermaterialdepthwisethroughthecenterofthemoldplates,runnerstypicallytransportmaterialthroughchannelsmachinedintothepartingline.Runnerdesigninfluencespartqualityandmoldingefficiency.Overlythickrunnerscanlengthencycletimeneedlesslyandincreasecostsassociatedwithregrind.Conversely,thinrunnerscancauseexcessivefillingpressuresandrelatedprocessingproblems.Theoptimumrunnerdesignrequiresabalancebetweeneaseoffilling,molddesignfeasibility,andrunnervolume.Materialpassingthroughtherunnerduringmoldfillingformsafrozenwalllayerasthemoldsteeldrawsheatfromthemelt.Thislayerrestrictstheflowchannelandincreasesthepressuredropthroughtherunner.Roundcross-sectionrunnersminimizecontactwiththemoldsurfaceandgeneratethesmallestper-centageoffrozenlayercross-sectionalarea.