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3Theillustrationaboveshowsaphotographoftwosnap-fitmodelstakeninpolarizedlight;bothhavethesamedisplacement(y)anddeflectiveforce(P).Top:Thecantileverarmofunsatisfactorydesignhasaconstantcrosssection.Thenon-uniformdistributionoflines(fringes)indicatesaveryunevenstrainintheouterfibers.Thisdesignuses17%morematerialandexhibits46%higherstrainthantheopti-maldesign.Bottom:Thethicknessoftheoptimalsnap-fitarmdecreaseslinearlyto30%oftheorig-inalcross-sectionalarea.Thestrainintheouterfibersisuniformthroughoutthelengthofthecantilever.ASnapJoints/General•CommonfeaturesTypesofsnapjointsCommentsondimensioningBCantileverSnapJointsHintsfordesignCalculationsPermissibleundercutDeflectionforce,matingforceCalculationexamplesCTorsionSnapJointsDeflectionDeflectionforceDAnnularSnapJointsPermissibleundercutMatingforceCalculationexampleEBothMatingPartsElasticFSymbolsContentsPage2of26Snap-FitJointsforPlastics-ADesignGuideCommonfeaturesSnapjointsareaverysimple,economicalandrapidwayofjoin-ingtwodifferentcom-ponents.Alltypesofsnapjointshaveincommontheprinciplethataprotrudingpartofonecomponent,e.g.,ahook,studorbeadisdeflectedbrieflyduringthejoiningopera-tionandcatchesinadepres-sion(undercut)inthematingcomponent.Afterthejoiningoperation,thesnap-fitfea-turesshouldreturntoastress-freecondition.Thejointmaybeseparableorinseparabledependingontheshapeoftheundercut;theforcerequiredtoseparatethecompo-nentsvariesgreatlyaccordingtothedesign.Itisparticularlyim-portanttobearthefollowingfactorsinmindwhendesigningsnapjoints:•Mechanicalloadduringtheassemblyoperation.•Forcerequiredforassembly.4SnapJointsGeneralAPage3of26Snap-FitJointsforPlastics-ADesignGuide5ATypesofsnapjointsAwiderangeofdesignpossi-bilitiesexistsforsnapjoints.Inviewoftheirhighlevelofflexibility,plasticsareusuallyverysuitablematerialsforthisjoiningtechnique.Inthefollowing,themanydesignpossibili-tieshavebeenreducedtoafewbasicshapes.Calculationprincipleshavebeenderivedforthesebasicdesigns.Themostimportantare:•CantileversnapjointsTheloadhereismainlyflexural.•U-shapedsnapjointsAvariationofthecantilevertype.•TorsionsnapjointsShearstressescarrytheload.•AnnularsnapjointsThesearerotationallysym-metricalandinvolvemultiaxialstresses.Page4of26Snap-FitJointsforPlastics-ADesignGuide6CantileversnapjointsThefourcantileversonthecontrolpanelmod-uleshowninFig.1holdthemodulefirmlyinplaceinthegridwiththeirhooks,andyetitcanstillberemovedwhenrequired.Aneconomicalandreliablesnapjointcanalsobeachievedbyrigidlugsononesideincombinationwithsnap-fittinghooksontheother(Fig.2).Thisdesignisparticularlyeffectiveforjoiningtwosimilarhalvesofahousingwhichneedtobeeasilyseparated.Thepositivesnapjointillus-tratedinFig.3cantransmitconsiderableforces.Thecovercanstillberemovedeasilyfromthechassis,however,sincethesnap-fit-tingarmscanbere-leasedbypressingonthetwotonguesinthedirectionofthearrow.TheexampleshowninFig.4hascertainsimi-laritieswithanannularsnapjoint.Thepres-enceofslits,however,meansthattheloadispredominantlyflexural;thistypeofjointisthereforeclassifiedasacantileverarmfordimen-sioningpurposes.Fig4:DiscontinuousannularsnapjointSnapJoints/GeneralAPage5of26Snap-FitJointsforPlastics-ADesignGuideTorsionsnapjointsThetor-sionsnapjointofthedesignshownforaninstrumenthousinginFig.5isstilluncom-moninthermoplastics,despitethefactthatit,too,amountstoasophisticatedandeconomicaljoin-ingmethod.ThedesignofarockerarmwhosedeflectionforceisgivenlargelybytorsionofitsshaftpermitseasyopeningofthecoverunderaforceP;thetorsionbarandsnap-fittingrockerarmareintegrallymoldedwiththelowerpartofthehousinginasingleshot.AnnularsnapjointsAtypicalapplicationforannularsnapjointsisinlamphousings(Fig.6).Here,quitesmallundercutsgivejointsofconsiderablestrength.Fig5:TorsionsnapjointonahousingmadeofMakrolonpolycarbonateFig6:Acontinuousannularsnapjointoffersasemi-hermeticsealandisbetterforsingleassemblyapplicationsAPage6of26Snap-FitJointsforPlastics-ADesignGuide8Combinationofdifferentsnapjointsystems–Thetrafficlightillus-tratedinFig.7isanexampleofaneffectivedesignforafunctionalunit.Allthecompo-nentsofthehousingarejoinedtogetherbysnapjoint.Details:•Housingandfrontaccessdoorengageatthefulcrum1a.Thelugs1b(pressurepoint)holdthedooropen,whichisusefulforchangingbulbs.•Thecantileverhook2locksthedoor.Thedoorcanbeopenedagainbypressingthehookthroughtheslitinthehousingat2.•Thereflectorcatchesatthreepointsontheperiphery.Eitherasnap-fittinhook3aorapressurepoint3bmaybechosenhere,sothatthereispolygonaldeformationoftheinnerringofthehousing.•Thelensinthefrontdooriseitherproducedinthesecondoftwomoldings4aor,ifaglasslensisdesired,thiscanbeheldbyseveralcantileversnaps4b.•Thesunvisorengagesat5likeabayonetcatch.Goodservice-abilityandlow-costproductioncanbeachievedwithcarefullythought-outdesignssuchasthis.AssumptionsThecalculationproceduresapplicabletovarioustypesofjointsarebrieflydescribedonthefollowingpages,butinsuchawayastobeasgeneralaspossible.Theusercanthereforeapplythisinformationtotypesofjointsnotdealtwithdirectly.Inallthesnap-fitdesignsthatfollow,itisassumedinitiallyt