FLAWS AND DEFECTS.docx
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FLAWS AND DEFECTS.docx
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FLAWSANDDEFECTS
FLAWSANDDEFECTS
Manyflawsanddefectscanbeavoidedbyusinggoodpartandtooldesigntechniques.AsapartdesigneritisaverygoodIdeatobeawareofyouroptionsintoolingandtoconsiderthosewhiledesigningyourpart.Forexample,havepotentialgatelocationsinmind.Trytoguesswhereknitlineswilloccurandhowdifferentgatelocationswillaffectthem.Howeasywillitbetotrimthegate?
Themorepartsyoustudy,thebetteryouwillgetatpredictingflow.Besuretocommunicateyourintentionstothetooldesignerandgethisfeedbacktoinfluenceyourfuturedesigns.
Avoidingthicksectionscanadduptohugesavingsoverthelifetimeofatool.Thicksectionsincreasethecycletimeofeachshot,narrowtheprocessingwindow,requireoverpacking,andcauserejectparts.
BLUSH
DEFINITION:
Dulldiscoloredorwhitishareaonthesurfaceofthepart,usuallyatthegate.
CAUSE:
Shearstressbetweenpolymermoleculesduringinjection.Thegatemaybetoosmallorinjectionspeedtoofast.
LOCATION:
Usuallyatthegate.Mayalsooccurwherethereisasuddenchangeinpartthickness.
CURE:
Adjustinjectionspeedandifnecessaryadjustgatedimensions.Anindependentwatercircuitinthemoldthatallowspinpointtemperaturecontrolatthegatecanalsohelp.
BLUSH
BURN
DEFINITION:
Discolorationusuallyblack,brownordarkyellow/browndependinguponseverity.Feelsroughandcrunchy.Frequentlyaccompaniedbyshortshotinburnarea.
CAUSE:
Usuallyindicatesaneedformoreventingorheatbuildupintool.Whenairistrappedinthetoolandcannotescape,theextremepressurecausestheairtoignite,burningtheedgeofthepart.
LOCATION:
Mostoftenseenindeep,blindribswherealotofaircanbeforcedintoasmallspace.
CURE:
Addmorepartinglineventsnearburnorventpinsindeepribs.Ventpinsarejustejectorpinsthatfitalittleloose.Theymayalsohaveaflatgrounddownonesidetolettheairescape.
BURN
COLDFLOW
DEFINITION:
Wavyorstreakedappearanceonpartsurface.Lookslikeafingerprintorsmallwaveslikeyouwouldseeonthesurfaceofwater.
CAUSE:
Lowmelttemperature,lowinjectionspeedorlowinjectionpressure.
LOCATION:
Hardtofillorlasttofillareas.
COLDFLOW
COLDSLUG
DEFINITION:
Coldpieceofplasticthathasbeenforcedintothepartalongwiththemelt.
CAUSE:
1.Plasticfromlastshotleftinnozzlesolidifiesbetweenshots.Thetooldesignerusuallyisabletoallowfora"coldslugwell"intherunnertocatchthispiece.2.Coldslugeffectscanalsooccurattheendofalongrunner.
LOCATION:
Ifallowedtoenterthepartitcantravelanywhere.
CURE:
Addacoldslugwellateachintersectionintherunner.Additionofashortenedejectorpinontherunnerveryclosetothegatemaydivertthecoldslug.Fordirectspruegatingtrytomakeafeatureintheparttocatchtheslugoruseaheatednozzle.
COLDSLUG
CONTAMINATION
DEFINITION:
Foreignparticlesembeddedinthepart.
CAUSE:
1.Burnedmaterialinthepressbarrel.2.Contaminatedregrind.3.Greaseorparticlesthathavenotbeencleanedfromthemold.
LOCATION:
Anywhere.
CONTAMINATION
DELAMINATION
DEFINITION:
Separationofplasticsurfacelayergivingaflakingoronionskineffect.
CAUSE:
1.Contaminatedresin.Maybecausedbyincompletemachinepurging,uncleanmaterialhandlingequipmentorimpureregrind.
LOCATION:
Anywhere.
DELAMINATIONorPEELING
DISCOLORATION
DEFINITION:
Deviationfromtheoriginalintendedcolorofthematerialascomparedtothemanufacturerscolorchip.
CAUSE:
1.Contaminatedresin.2.Overheatedresin.3.Incorrectregrindratio.4.Incorrectcolormixingorblending.
LOCATION:
Entirepart.
DISCOLORATION
DRAG
DEFINITION:
Fine,straightlinesscrapedinthelineofdraw.
CAUSE:
Dependsuponlocation.1.Cavitysidehappensduringmoldopeningandisusuallyfrominsufficientdraftforthetextureusedorfromoverpacking.2.Coresidedraghappensduringejectionandisusuallyfrominadequatedraft,roughcore,oroverpacking.
LOCATION:
Maybeinopeningdirectionorsideactiondirection.Cavity,core,slide,orlifter.
CURE:
Solveoverpackingproblem.Cavitysidedrag,tonedownthetexturebystoningthenbeadblast.Coresidedrag,polishcore,adddraft.
DRAG
FLASH
DEFINITION:
Excessplasticsqueezingoutperpendiculartothepartatpartingline.
CAUSE:
1.Plasticinjectionforceexceedingtheclampingpressureofthepress.(Overpacking.)2.Mayhappenatfirstshotswhilemoldisbeingdialedin.3.Poorlyconstructedorwornoutmold.
LOCATION:
Alonganypartingline.
CURE:
Runthemoldinabiggerpress.Relieveareasofthepartingsurfacesthatarenotimmediatelyadjacenttothepart.Leave0.500in.ofshutofflandaroundthepart.
FLASH
GLOSS
DEFINITION:
Smoothshinyareasonthepartsurface.
CAUSE:
1.Underpacking2.Seesectiononplateoutbelow.
LOCATION:
Hardtofillareas.
GLOSS
JETTING
DEFINITION:
Squigglylineinpartpointingtogate.Lookslikeaworminthepart.
CAUSE:
1.Incorrectgateplacementorsize.Thegateispositionedinsuchamannerastoaimtheplasticstraightintoanopenarea.Theplasticlaunchesoutintotheopenlikeapieceof"sillystring"andthenstacksupinsquiggles.
LOCATION:
Neargate.
CURE:
Aimthegateatanobstaclethatinterruptstheflow.Usedifferentgatestyleorlargergate.
JETTING
KNITLINE
DEFINITION:
Alinewherethemoltenpolymerflowfrontsmeetinthemold.Incompleteadhesionoccursalongtheknitlineandcausesaweakpointintheplasticpart.
CAUSE:
Coldfrontsmeetinginthetoolwheremoltenplasticfrontsmeet.
LOCATION:
Coldfrontsmeetinginthetoolwhereplasticflowsaroundobstaclesoroverraisedareasinthemetal.Itisguaranteedthatyouwillhaveaknitlineastheplasticflowsaroundanyopeninginthepart.RecessedtextorIconscanalsocausesmallcosmeticknits.
CURE:
Frequentlyitispossibletoplacethegateinsuchamannerastopushtheknitlinesintoobscureareas.Ifthisisnotpossibleuse"flowdirectors"onthenoncosmeticsideoftheparttopushtheknitlineintoacorner,crease,orshadowtohideit.Flowdirectorsareusuallyjustshallowraisedareasintheplasticthatarecutintothetoolwithalargeballendmill.Sometimesitispossibletoadda"sump"thatthecoldmaterialflowsoutinto.Thissumpisthenclippedoff.
KNITLINE
BADGATETRIM
DEFINITION:
Eithertoomuchortoolittleplasticwherethegatehasbeentrimmedoff.
CAUSE:
1.locationofthegateonaconcaveorconvexpartsurfacecanmakeitdifficulttoaccuratelytrimthegate.2.Anothercausecanbeinadequatelytrainedoruncoordinatedpeopledoingthetrimming.
LOCATION:
Gate.
CURE:
Placethegateonastraightedgeifpossible.Agoodtrimjobtakesgoodeye/handcoordination.Useofagatetrimmingfixturecanworkwellforhighvolumegatetrimming.Useselfdegatingtechniqueswherepossiblesuchastunnelgatesorbananagates.Ofcoursegatingtoanoncosmeticareaisalwayspreferred,butnotalwaysfeasible.
BADGATETRIM
MISMATCH
DEFINITION:
Thecavitysideofthetooldoesnotfallinperfectregistrywiththecoresideresultinginastepatpartingline.Itmaylooklikeflashifitisslight.Ifitissmoothasyourfingerrunsacrossonewayandfeelssharptheotherwayitismismatch.Ifyoucanfeelitbothwaysitisflash.
CAUSE:
1.Unevenpressureinthemoldcavitycanpushthecavityonedirectionandthecoretheother.Thisusuallyhappensinveryasymmetricalpartsorpartswithapartingsurfacethatslopesonlyoneway.2.Moldmakerdidnotproperlypositionthecavityrelativetothecore.3.Inoldertoolsmismatchmayoccuraslockingfaceswear.
LOCATION:
Atpartinglines.
CURE:
Straightlocksatpartingline.ThebestarethosemadebyProgressiveComponents.
MISMATCH
PINPUSH
DEFINITION:
Circularorsemicircularwhitestressringsonthesideofthepartoppositeanejectorpin.Mayevenberaisedcircularbumps.Inseriouscasespinsmaypushrightthroughthepart!
CAUSE:
1.Overpacking.2.Stickingonthecore.3.Inadequateejection.
LOCATION:
Onthecosmeticsideofthepartoppositeanejectorpin.
CURE:
Solveoverpackingproblem.Polishcoreorincreasedraftoncore.Addmoreejectorpins.Moresmallpinsarebetterthanafewbigones.
PINPUSH
PLATEOUT
DEFINITION:
Achangeofmoldtextureovertimethatisnotduetowear.
CAUSE:
1.Buildupofchemicalresiduefromoutgassing.2.Buildupofmoldrelease.
LOCATION:
Anywhere
CURE:
Havethemoldcleaned.
PLATEOUT
PULLING
DEFINITION:
Deformed,twistedandsmearedplasticinthepartusuallyon,oradjacenttosteepverticalfaces.
CAUSE:
1.Cavityside:
Aportionofthepartstickingtothecavityontoolopening.Listentothemoldasitopenstoseeifyoucanhearitpopfree.2.Coreside:
Unevenpartejectionisnotpushingthepartoutstraight.Thepartgetsskewedasitejects,theresultingdamageiscalledpulling.
LOCATION:
Anywhereinpart.
CURE:
Cavitysidepulling,addundercutsortextureoncoresidesopartpullscleanlyfromthecavity.Coresidepulling,addejection.Moresmallpinsarebetterthanafewbigones.
PULLING
SHORTSHOT
DEFINITION:
Missingplasticorfeaturesthatarenotfullyformed.Missingcornersorfeatureshaveasmooth,roundedappearance.
CAUSE:
Underpacking,lowinjectionpressure,trappedgas.
LOCATION:
Areasofthepartfarthestfromthegate,thinareasordelicatefeatures.Generallythelastpartofthemoldtofill.
CURE:
Makesurethetoolisadequatelyventedandpushmoreplasticin.
SHORTSHOT
SINKS
DEFINITION:
Depressionsordimplesinthepartthatareusuallyadjacenttothickareas.Inclearparts,bubblescanbeseeninthickareas.Thesebubblescanbetheprecur
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- FLAWS AND DEFECTS