FLAWS AND DEFECTS.docx

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