Structure Design.docx

Structure Design.docx

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StructureDesign

PlasticPartDesign

Toinsureaqualityfinalproduct,itisnecessarytostartoutwithqualitycomponents.Injectionmoldedpartscanbemoldedtoahighqualitystandardbyfocusingontheseareasofplastictechnology:

1）CorrectPartDesign

2）AccurateSelectionofMaterial

3）ProcessingPlasticProcessing

Onlybydrawingonexpertisefromthesethreeareasofplastictechnologycanaproductdesignercreatequalitymoldedpartsthatmaximizeperformanceandarecosteffective.Thepurposeofthisdesignguidedealswiththefirstofthesethreeissues-partdesign.

CorrectPartDesignGuidelines

Iftherewasonlyonerulefortheinjectionmoldingprocessitwouldhavetobe

tomaintainuniformwallthickness.Herearesomeexamplesofproblemsassociated

withpartdesignsthatfeatureanon-uniformwallthickness.

Sinkmarksdueto

unevenshrinkage

Sinkmarksresultfromawallyieldingtothestillshrinkinginteriormass.

Stressdueto

unevenshrinkage

Apartwithnon-uniformwallthicknesswillcoolunevenly,resultinginhighmolded-instress.

Voidsduetouneven

shrinkage

Thealreadycooledsectionwillnotyieldtotheshrinkingactionofthecoolinginteriormasscausing

voidsinthethickportionofthepart.

Warpingdueto

unevenshrinkage

Concentratedstressatthejunctionofhigh&lowshrinkageareamaycauseaparttowarp.

Draft

Plasticpartsarealwaysdesignedwithataperordraftinthedirectionofmoldmovementtoallowpartejectionorremovalfromthemold.Sinceplasticsshrinkwhencooled,itiscommonforpartstoshrink（orgriptightly）tocores.Agooddefinitionfordraftwouldbe:

thedegreeoftaperofasidewallorribneededtoallowthemoldedplasticparttoberemovedfromthemetalmold.Withoutproperdraft,plasticpartsmaybedifficulttoremovefromthemold.Adraftangleof1/2degreesisregardedasminimumformostapplications.Draftanglesof11/2to2degreespersideareconsiderednormalforplasticinjectionmolding.

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Ribs

Manytimesthestiffnessofapartmustincreasebecauseoftheloadappliedtothepartdesign.Oneoftheeasiestwaystocurethisproblemischangethepartgeometrybyaddingribs.Theuseofribsisapracticalwayandeconomicalmeansofincreasingthestructuralstrengthofapart.Butthereareguidelinesthatgovernaddingribswithoutcausingsinkmarksorsurfaceblemishestoyourparts.

1）Ribthicknessshouldbelessthanwallthickness.Aribthicknessof60%to80%ofnominalwallthicknessisrecommended.

2）Toincreasestiffnessincreasethenumberofribsor"gussetplates",anotherfeaturedesignedtostrengthentheplasticpart.

3）Foragivenstiffness,itisbettertoincreasethenumberofribs,nottheheight.

4）Forthickribs"coreout"theribfromtheback.Thiscreatesahollowspaceunderneaththepart

andmaintainsauniformwallthickness.

Height:

Maximumheightofthreetimenominalwallthicknessofpart.

Spacing:

Minimumoftwotimesnominalwallthicknessofpartbetweenribs.

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Holes

Holesareeasytoproduceinmoldedparts.Corepinsthatprotrudeintothemoldcavitymaketheholeswhenthepartismolded.Throughholesinthemoldedpartsareeasiertoproducethanblindholes,whichdonotgoallthewaythroughapart.Blindholesaremadebycorepinssupportedononeendonly.Thepinscanbedeflectedandpushedoffcenterbythepressuresofthemoltenplasticmaterialduringthemoldingprocess.Agoodruleofthumb:

thedepthoftheblindholeshouldbeabouttwicethediameterofcorepinsupto3/16",anduptofourtimesthediameterofcorepinsover3/16".Theguidelinesforblindandthroughholesareseenbelow.

BlindHole（shownwithdraft）

L=2DforDiametersLessthan3/16"dia.corepins

L=4DforDiametersMorethan3/16"dia.corepins

ThroughHole（shownwithdraft）

L=4DforDiameterslessthan3/16"dia.corepins

L=6DforDiametersmorethan3/16"dia.corepins

Therearedefiniterulesfortheplacementofcoredholesinamoldedpart.Iftheseminimumdistancesshownbelowarenotfollowed,theholeswillbeeggshapedorthepartwilldeformintheareasaroundtheholes.

T=wallthicknessofpart

D=diameterofhole

Asdepictedabove,thedistancefromtheedgeofaholetoaverticalsurface（i.e.rib）ortheedgeofapartshouldbetwicethepartthickness（ormore）,oratleastonediameterofthehole.Thissameruleappliesbetweenholes-atleasttwopartthicknessesoroneholediametershouldbespecified.

Holes（part2）

Aseasyasitistomakeholesinmoldedpartsitdoesnotcomewithoutsomeconcernsforthestrengthofthepart.Foreverycoredormoldedholetherewillbeaweldline.Theweldlinesarecausedbytheflowofthemeltedplasticaroundthecorepins.Theseweldlinesarenotasstrongasthesurroundingplasticmaterial,andalsomaydetractfromtheoverallappearanceofthemoldedpart.Thepartdesignershouldconsiderthesepointswhendesigningholesinamoldedpart.

Thecoringofholesiseasywhentheaxisisparalleltothepartingline.Butwhenholesandotherfeaturesrunperpendiculartothepartinglinethenretractablecores（orcams）arerequired.Splitpinsandcores（calledpassingsteelshutoffs）canbeusedtocreatesomeofthefeatures.Thedesignerneedstobeawareoftheproblemsofsideactioncoresandtheaddedexpensesassociatedwiththesetypesofmolds.Withalittleunderstandingofhowthemoldopensandwherethepartinglinewillexist,thesecostlyfeaturescanbemodified.Ruleofthumb:

wheneverpossiblealldesignfeaturesshouldbeincorporatedinthesamedirectionofthemoldopeningsothatcamactioncanbeavoided.

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Bosses

Bossesareusedforlocating,mounting,andassemblypurposes.Therearebossdesignguidelinesthatmustbefollowedtoinsurethehighestqualityinmoldedparts.Again,oneofthemainpointstoconsiderisnominalwallthickness.Toomanytimesbossesaredesignedwiththickwallsectionsthatcanaffecttheappearanceoftheplasticpartandthefinalproduct.

Ruleofthumb:

thewallthicknessaroundabossdesignfeature（t）shouldbe60%ofthenominalpartthickness（T）ifthatthicknessislessthan1/8".Ifthenominalpartthicknessisgreaterthan1/8"thebosswallthicknessshouldbe40%ofthenominalwall.

Bossdiameter,wallthickness,andheightdesignparameters.Whilebossheightsvarybydesign,thefollowingguidelineswillhelpavoidsurfaceimperfectionslikesinkmarksandvoids:

theheightofthebossshouldbenomorethan21/2timesthediameteroftheholeintheboss.

Pleaseobservethe"60/40"rule（seeabove）forthewallthicknessatthebottomoftheboss.

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Radii

Inthedesignofinjectionmoldedparts,sharpcornersshouldbeavoided.Sharp

cornersactasstressrisersorconcentrators,reducingpartstrengthandcausing

prematurefailures.Sharpcornersmayalsoeffectplasticflow,producingpartswith

objectionablesurfaceflowpatterns.

Bosses&Ribs

Nominalradiusshouldbeonequarterofthenominalpartthickness,withaminimumradiusof.015（i.e..100wall,025r）

T=wallthicknessofpartRadius-T

Corners

Theinsideradiusshouldbeatleasthalfthepartwallthickness.

Theoutsideradiusshouldequaltheinsideradiusplusthepartthicknessi.e..100wallandinsideradiusof.050equalsoutsideradiusof.150）

InsertMoldingTips

Anotherproblemconcerninghighstressoccurswithmolded-ininserts.Theplasticmeltheatsthemetaloftheinserts.Duringthecoolingstageofinjectionmolding,theplasticpartcools,buttheplasticbosssurroundingthemetalinsertisreheatedbytheheatfromtheinsert.Thisallowstheplastictocontinuetoshrinkaroundtheinsert,causingexcessivehoopstress*thatwilleventuallycausethebosstocrack.Thebetterdesignandprocesswouldbetouseultrasonicinsertionorahotprobe（suchasaheatstakingunit）afterthemoldedparthascooledthroughout.

Hoopstress:

stresswithinthecircumferenceoftheboss

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ReplacingMetalwithPlastics

Therearenumerousreasonswhyreplacingmetalpartswithplasticmakessense.PlasticsOne,Inc.hasworkedwithmanycompaniesonmetal-to-plasticconversions.Hereiswhyplasticmaybethebestoptionforyourparts.

1.DECREASEPIECEPARTPRICES

Apennysavedisapennyearned.Afterinitialtoolingcostsarepaid,thepiecepartpricingisusuallymuchlessthanthesamepartproducedinmetal,whetheritbeastamping,casting,oradiecastpartThiscostsavingsisrealizedbecausetheinjectionmoldingprocesshasfastercycletimes（morepartsmadepermachinehour）andthesepartsareidenticalfromonetotheotherwhicheliminatessecondarymachining.

2.ELIMINATETIME-CONSUMINGANDCOSTLYSECONDARYOPERATIONS

Eliminatingsecondaryoperationsfurtherreducescosts.Plasticmaterialcanbecoloredwithcolorconcentratesbeforemolding-eliminatingsecondarypaintingoperations.Injectionmoldscanbetexturedorgivenvariouslevelsofpolishedsurfacesbeforemolding.Thecostlyassemblyofseveralmetalstampingsorcastingsfastenedtogethercanoftenbereplacedbyasingleinjectionmoldedpartincorporatingthefeaturesofthetotalassembly.Ifmultipleassembliesarerequired,theplasticpartscanhavesnap-togetherfeaturestoeliminateanyfasteners.Eliminatingsub-assemblytoolingorfixturesbyusinginjectionmoldedpartscanquickendeliveryinnewproductdevelopmentprograms.

3.REDUCEPRODUCTWEIGHTANDIMPROVEUSEREASE

Oneprimaryadvantageofusingplasticsinsteado