BASF Plastics SnapFit Design Guide

BASFPlasticsSnap-FitDesignManualTopicPartIntroduction...........................................IntroductionSnap-FitDesignApplications..................................ITypesofSnap-Fits..........................................IISnap-FitBeamDesignUsingClassicalBeamTheory.............IIIImprovedCantileverSnap-FitDesign...........................IV“U“&“L“ShapedSnaps........................................VGeneralDesignGuidelines...................................VIEnglish/MetricConversionChart.......................InsideBackCoverTableofContentsAboutBASFPerformancePolymersBASFPlasticsisafullyintegrated,globalsupplierofengineeringresins“fromproductionoffeedstockstothecompounding,manufactureanddistributionofhundredsofresingrades.BASFiscommittedtocontinuousproductdevelopmenttosustainrapidgrowthinthenylonresinmarket.InourPlasticsTechnologyLaboratory,ahighlyexperiencedstaffofresearchanddevelopmentengineerscontinuestodevelopnewresinstofurtherextendthehorizonsofproductperformance.BASFoffershigh-qualityengineeringresins,including:“Ultramid®(nylon6and6/6)Nypel®(apost-industrialnylon6)“Petra“®(post-consumerrecycledPET)““Ultradur®“PBTThermoplasticPolymerUltraform®“Acetal(POM)Ultrason®“HighTempPolymersTheseresinsfromBASF,coupledwiththecompany’sconcept-through-commercializationexpertise,cancombinetohelpmakepossiblethemostefficient,cost-effectivesnap-fitforyourproduct.Ourtechnicalsupportisreadytohelpyouwithallyourneeds.Andformoreinformation,youcanalwaysvisitourwebsiteat:typesofsnap-fitdesignsandtheirapplications;howtocalculatethestrengthoftheunitandamountofforceneededforassembly;andthethreecommoncausesoffailureinsnap-fitsandhowtoovercomethem.IntroductionSnap-FitDesignApplicationsWhyusesnap-fits?Thischapterwillgiveyouathumbnailsketchofthebenefitsofsnap-fitsandthematerialsusedtomakethem.Snap-fitsarethesimplest,quickestandmostcost-effectivemethodofassemblingtwoparts.Whendesignedproperly,partswithsnap-fitscanbeassembledanddisassemblednumeroustimeswithoutanyadverseeffectontheassembly.Snap-fitsarealsothemostenvironmentallyfriendlyformofassemblybecauseoftheireaseofdisassembly,makingcomponentsofdifferentmaterialseasytorecycle.Althoughsnap-fitscanbedesignedwithmanymaterials,theidealmaterialisthermoplasticbecauseofitshighflexibilityanditsabilitytobeeasilyandinexpensivelymoldedintocomplexgeometries.Otheradvantagesincludeitsrelativelyhighelongation,lowcoefficientoffriction,andsufficientstrengthandrigiditytomeettherequirementsofmostapplications.Thedesignershouldbeawarethattheassemblymayhavesome“play“duetotolerancestack-upofthetwomatingparts.Somesnap-fitscanalsoincreasethecostofaninjectionmoldingtoolduetotheneedforslidesinthemold.Anexperienceddesignercanofteneliminatetheneedforslidesbyaddingaslotinthewalldirectlybelowtheundercutorbyplacingthesnapsontheedgeofthepart,sotheyfaceoutward(seeFigureI-1).REQUIRESSLIDEINMOLDUNDERCUTNOSLIDEREQUIREDSLOTNOSLIDEREQUIRED,MOLDLESSCOMPLEXFigureI-1I-1PartISNAP-FITDESIGNAPPLICATIONSI-2Concludingpoints:Snap-fitssolvetheproblemofcreatinganinexpensivecomponentthatcanbequicklyandeasilyjoinedwithanotherpiece.Thermoplasticsaretheidealmaterialforsnap-fitsbecausetheyhavetheflexibilityandresiliencenecessarytoallowfornumerousassemblyanddisassemblyoperations.DoorhandlebezelBacksideofbezelDetailofbacksideofbezel,cantileverdesignII-1TypesofSnap-FitsThischapterprovidesanoverviewofthedifferenttypesofcantileversnap-fitsandgivesanideaofwhentheyareused.Mostengineeringmaterialapplicationswithsnap-fitsusethecantileverdesign(seeFigureII-1)and,thus,thismanualwillfocusonthatdesign.Thecylindricaldesigncanbeemployedwhenanunfilledthermoplasticmaterialwithhigherelongationwillbeused(atypicalapplicationisanaspirinbottle/capassembly).YCANTILEVER“U”SHAPEDCANTILEVER“L”SHAPEDCANTILEVERFigureII-1Whendesigningacantileversnap,itisnotunusualforthedesignertogothroughseveraliterations(changinglength,thickness,deflectiondimensions,etc.)todesignasnap-fitwithalowerallowablestrainforagivenmaterial.Othertypesofsnap-fitswhichcanbeusedarethe“U“or“L“shapedcantileversnaps(seePartVformoredetail).Theseareusedwhenthestrainofthestraightcantileversnapcannotbedesignedbelowtheallowablestrainforthegivenmaterial.Concludingpoints:Mostapplicationscanemployacantilevertypesnap-fitinthedesign.Inapplicationswithtightpackagingrequirements,the“U“or“L“shapedsnapmayberequired.AutomotiveoilfiltersnapsCordlessscrewdriverhousing,cantileversnap-fitPartIIIII-1OVERHANGDEPTHENTRANCESIDERETRACTIONSIDEAdesignengineer’sjobistofindabalancebetweenintegrityoftheassemblyandstrengthofthecantileverbeam.Whileacantileverbeamwithadeepoverhangcanmaketheunitsecure,italsoputsmorestrainonthebeamduringassemblyanddisassembly.Thischapterexplainshowthisbalanceisachieved.Atypicalsnap-fitassemblyconsistsofacantileverbeamwithanoverhangattheendofthebeam(seeFigureIII-1).Thedepthoftheoverhangdefinestheamountofdeflectionduringassembly.FrictionCoefficientµ=tanβMatingForce=WW=Ptan(α+β)µ+tanαW=P————————1–