Design Guides for Plastics原版完整文件

July2005July2005DesignGuidesforPlasticsTANGRAMTECHNOLOGYCliveMaier,EconologyLtdTANGRAMTECHNOLOGYθcon^ogyThispublicationismadeupofaseriesofarticlespublishedinPlasticsandRubberWeeklyasapiecework.ThekindassistanceoftheauthorandPRWisacknowledgedinthepublicationofthework.ThepublicationwillbeupdatedinaregularbasisasnewsectionsoftheguidearepublishedbyPRW.Thedesignhintsinthisbookletaregiveningoodfaithandrepresentcurrentgoodpractice.Theshortnatureofthehintsmeansthatnotallinformationcanbeincluded.Noresponsibilitycanbetakenforanyerrorsorconsequentialdamagesresultingfromusingthesehints.Thispublicationmaybefreelyreproducedexceptforsaleoradvertisingpurposes.Itmaybehostedonwebsitesforfreedownloadingprovidingthatitisusedinit'sentiretyandthatreferenceismadetotheoriginalpublication.?CliveMaier2004TypesetbyTangramTechnologyLtd.ContentsTOC\o"1-5"\h\z\o"CurrentDocument"Preface 1\o"CurrentDocument"Introduction 2Injectionmoulding 4Basics\o"CurrentDocument"Wallthickness 5\o"CurrentDocument"Corners 6\o"CurrentDocument"Ribs 7\o"CurrentDocument"Bosses 10Designforrecycling PlannedSpecialfeatures\o"CurrentDocument"Livinghinge 13\o"CurrentDocument"Bearings 15\o"CurrentDocument"Gears 18Assembly\o"CurrentDocument"Pressfits 23\o"CurrentDocument"Snap-fits 24\o"CurrentDocument"Hotairstaking 28\o"CurrentDocument"Ultrasonicwelding 30Hotplatewelding PlannedSpinwelding PlannedFrictionwelding PlannedInductionwelding PlannedLaserwelding PlannedAdhesiveandsolventbonding PlannedSpecialtechniquesDesignforoutsertmoulding PlannedDesignforgasassistinjectionmoulding PlannedExtrusion Designforprofileextrusion PlannedBlowmoulding Designforextrusionblowmoulding PlannedThermoforming Designforthermoforming PlannedJuly2005July2005July2005#PrefaceThissetofhintsandtipsforplasticsproductdesignersisintendedasasourcebookandan'aidememoire'forgooddesignideasandpractices.Itisasourcebookforplasticsproductdesignersatalllevelsbutitisprimarilyaimedat:studentdesignerscarryingoutdesignworkforalllevelsofacademicstudies;non-plasticsspecialistsinvolvedinthedesignofplasticsproducts;plasticsspecialistswhoneedtoexplaintheirdesigndecisionsandthedesignlimitationstononplasticsspecialists.Thebookcoverseachtopicinasinglepagetoprovideabasicreferencetoeachtopic.Thisspaceconstraintmeansthateachtopicisonlycoveredtoabasiclevel.Detailedplasticproductdesignwillalwaysrequiredetailedknowledgeoftheapplication,theprocessingmethodandtheselectedplastic.Thisinformationcanonlybeprovidedbyrawmaterialssuppliers,specialistplasticsproductdesignersandplasticsprocessorsbutthereisaneedtogetthebasicsoftheproductdesignrightinthefirstinstance.Usingthehintsandtipsprovidedinthisguidewillenabledesignerstoreduceinitialerrorsandwillleadtobetterandmoreeconomicdesignwithplastics.Ihopethisshortworkwillimprovethebasicdesignofplasticsproductsandifitcandothisthenitwillhaveservedit,sobjectives.CliveMaierEconologyLtd.INTRODUCTIONGooddesignisimportantforanymanufacturedproductbutforplasticsitisabsolutelyvital.Wehavenoinstinctforplastics.Mostofthoseweusetodayhavebeenaroundforlittlemorethantwogenerations.Comparethatwiththethousandsofyearsofexperiencewehavewithmetals.Andplasticsaremorevaried,morecomplicated.Formostdesignsinmetals,thereisnoneedtoworryabouttheeffectsoftime,temperatureorenvironment.Itisadifferentstoryforplastics.Theycreepandshrinkastimepasses;theirpropertieschangeoverthetemperaturerangeofeverydaylife;theymaybeaffectedbycommonhouseholdandindustrialmaterials.ThephilosopherHeideggerdefinedtechnologyasawayofarrangingtheworldsothatonedoesnothavetoexperienceit.Wecanextendhisthoughttodefinedesignasawayofarrangingtechnologysothatwedonothavetoexperienceit.Inotherwords,gooddesigndeliversfunction,formandtechnologyinobjectsthatmeettheneedsofuserswithoutmakingdemandsonthem.Thewell-designedobjectgivespleasureoratleastsatisfactioninuse,anddoeswhatitshoulddowithoutundueconcern.IntheseDesignGuideswewillsetoutthebasicsofgooddesignforplastics.Therulesandrecommendationswegivewillnecessarilybegeneralisations.Theywillapplyoftenbutnotinvariablytothermoplastics,frequentlybutnotexclusivelytoinjectionmoulding.Thebasicadvicewillbegoodbutbecauseplasticsaresocomplexandvariedthegoldenrulemustalwaysbetoconsidercarefullywhethertheadviceneedsadjustingtosuityourparticularapplication.Gooddesigncombinesconceptwithembodiment.Unlessthetwoareconsideredtogether,theresultwillbeanarticlethatcannotbemadeeconomicallyoronethatfailsinuse.Thisisparticularlyimportantforplastics.Itisvitaltochoosetherightmaterialforthejob.Whenthatisdone,itisequallyimportanttoadaptthedetailsofthedesigntosuitthecharacteristicsofthematerialandthelimitationsoftheproductionprocess.Plasticscomeinabewilderingvariety.Thereareahundredormoredistinctgenerictypes.Ontopofthat,advancedtechniqueswithcatalystsandcompoundingarecreatingnewalloys,blendsandmolecularforms.AllofthesematerialscanhavetheirpropertiesDESIGNCONSIDERATIONSmodifiedbycontrolofmolecularweightandbyadditivessuchasreinforcements.Thenumberofdifferentgradesofplasticsmaterialsavailabletothedesignernowapproaches50,000.Theimportance-andthedifficulty-ofmakingtherightchoiceisobvious.Plasticscanbegroupedintocategoriesthathaveroughlysimilarbehaviour.Thermoplasticsundergoaphysicalchangewhenprocessed;theprocessisrepeatable.Thermosetsundergoachemicalchange;theprocessisirreversible.Akeydistinctionbetweenthermoplasticsrelatestothemoleculararrangement.Thosewithrandomtangledmoleculesarecalledamorphous.Thosewithadegreeofmoleculararrangementandorderingarecalledsemicrystalline.Thedifferenceissignificant.Forexample,mostamorphousmaterialscanbefullytransparent.Andthemorecrystallineamaterialis,thelesslikelyitistohaveawide'rubbery'processingregion,somakingitlesssuitableforstretchingprocesseslikeblowmouldingandthermoformingDesignersmustdesignforprocessaswellaspurposeandmaterial.Insingle-surfaceprocessesforexample,thereisonlyindirectcontrolovertheformofthesecondsurface.Designmusttakethislimitationintoaccount.SOMECOMMONPLASTICSALLSURFACESDEFINEDSINGLESURFACEDEFINEDBATCHPROCESSInjectionmouldingCompressionmouldingTransfermouldingBlowmouldingThermoformingRotationalmouldingCONTINUOUSPROCESSExtrusionCalenderingPultrusionCOMMONPLASTICSFORMINGPROCESSESPart1Injectionmoulding1WALLTHICKNESSPartsthatmightbemadeassolidshapesintraditionalmaterialsmustbeformedquitedifferentlyinplastics.Mouldedplasticsdonotlendthemselvestosolidforms.Therearetwoprincipalreasonsforthis.First,plasticsareprocessedwithheatbutarepoorconductorsofheat.Thismeansthatthicksectionstakeaverylongtimetocoolandsoarecostlytomake.Theproblemsposedbyshrinkageareequallysevere.Duringcooling,plasticsundergoavolumereduction.Inthicksections,thiseithercausesthesurfaceoftheparttocaveintoformanunsightlysinkmark,orproducesaninternalvoid.Furthermore,plasticsmaterialsareexpensive;itisonlyhigh-speedproductionmethodsandnet-shapeformingthatmakemouldingsviable.Thicksectionswastematerialandaresimplyuneconomic.Sosolidshapesthatwoulddothejobwellinwoodormetalmustbetransformedtoa'shell'forminplastics.Thisisdonebyhollowingoutor'coring'thickpartssoyouareleftwithacomponentwhichregardlessofcomplexityiscomposedessentiallyofrelativelythinWallsjoinedbycurves,angles,corners,ribs,stepsandoffsets.Asfaraspossible,allthesewallsshouldbethesamethickness.Itisnoteasytogeneralisewhatthewallthicknessshouldbe.Thewallplaysapartbothindesignconceptandembodiment.Thewallmustbethickenoughtodoitsjob;itmustbestrongenoughorstiffenoughorcheapenough.Butitmustalsobethinenoughtocoolquicklyandthickenoughtoallowefficientmouldfilling.Ifthematerialisinherentlystrongorstiffthewallcanbethinner.Asageneralguide,wallthicknessesforreinforcedmaterialsshouldbe0.75mmto3mm,andthoseforunfilledmaterialsshouldbe0.5mmto5mm.Ideally,theentirecomponentshouldbeauniformthickness-thenominalwallthickness.Inpracticethatisoftennotpossible:theremustbesomevariationinthicknesstoaccommodatefunctionsoraesthetics.Itisveryimportanttokeepthisvariationtoaminimum.Aplasticspartwiththicknessvariationswillexperiencedifferingratesofcoolingandshrinkage.Theresultislikelytobeapartthatiswarpedanddistorted,oneinwhichclosetolerancesbecomeimpossibletohold.Wherevariationsinthicknessareunavoidable,thetransformationbetweenthetwoshouldbegradualnotsuddensoinsteadofastep,usearamporacurvetomovefromthicktothin.ASDESIGNED-withthicksectionsanddissim?rv∕a∣lthicknessASMOULDED-withsinkmarks,voidsanddistortionThicksectionsandnon-uniformwallscause

problemsWRONG-thicksolidsectionRIGHT-coredoutto

thinuniformwallRIGHT-coredoutto

thinuniformwallSolidshapesmustberedesignedas'shells'Gradualtransitionsbetweenthickandthin

sectionsDESlGNERSNOTEBOOKKeepwallthicknessasuniformaspossible.Usegradualtransitionsbetweenthickandthinsections.Wallthicknessmustsuitbothfunctionandprocess.Wallthicknessguiderangeis:0.75mmto3mmforreinforcedmaterials0.5mmto5mmforunreinforcedmaterials2CORNERSStressconcentrationfactorsforcantileverloadingStressconcentrationfactorsforcantileverloadingDESIGNERSNOTEBOOKAvoidsharpinternalcorners.Internalradiishouldbeatleast0.5andpreferably0.6to0.75timesthewallthickness.Keepcornerwallthicknessascloseaspossibletothenominalwallthickness.Ideally,externalradiishouldbeequaltotheinternalradiiplusthewallthickness.Whentheideasofcorrectanduniformwallthicknessareputintopracticetheresultisaplasticspartcomposedofrelativelythinsurfaces.Thewayinwhichthesesurfacesarejoinedisequallyvitaltothequalityofamouldedpart.Wallsusuallymeetatrightangles,atthecornersofaboxforexample.Wheretheboxwallsmeetthebase,theanglewillgenerallybeslightlymorethan90degreesbecauseofadraftangleonthewalls.Theeasiestway,andtheworst,tojointhewallsistobringthemtogetherwithsharpcornersinsideandout.Thiscausestwoproblems.Thefirstdifficultyisthattheincreaseinthicknessatthecornerbreakstheruleofuniformwallthickness.Themaximumthicknessatasharpcornerisabout1.4timesthenominalwallthickness.Theresultisalongercoolingtimeaccompaniedbyariskofsinkmarksandwarpingduetodifferentialshrinkage.Theotherproblemisevenmoreserious.Sharpcornersconcentratestressandgreatlyincreasetheriskofthepartfailinginservice.Thisistrueforallmaterialsandespeciallysoforplastics.Plasticsaresaidtobenotch-sensitivebecauseoftheirmarkedtendencytobreakatsharpcorners.Thishappensbecausethestressconcentrationatthecornerissufficienttoinitiateamicroscopiccrackwhichspreadsrightthroughthewalltocausetotalfailureofthepart.Sharpinternalcornersandnotchesarethesinglemostcommoncauseofmechanicalfailureinmouldedparts.Theansweristoradiustheinternalcorner,butwhatsizeshouldtheradiusbe?Mostwallsapproximatetoaclassicalcantileverstructuresoitispossibletocalculatestressconcentrationfactorsforarangeofwallthicknessesandradii.Theresultinggraphshowsthatthestressconcentrationincreasesverysharplywhentheratioofradiustowallthicknessfallsbelow0.4.Sotheinternalradius(r)shouldbeatleasthalfthewallthickness(t)andpreferablybeintherange0.6to0.75timeswallthickness.Iftheinnercornerisradiussedandtheoutercornerleftsharp,thereisstillathickpointatthecorner.Foraninternalradiusof0.6t,themaximumthicknessincreasestoabout1.7timesthewallthickness.Wecanputthisrightbyaddingaradiustotheoutsidecorneraswell.Theoutsideradiusshouldbeequaltotheinsideradiusplusthewallthickness.ThisresultsinaconstantWallthicknessaroundthecorner.Properlydesignedcornerswillmakeabigdifferencetothequality,strengthanddimensionalaccuracyofamoulding.Butthereisanotherbenefittoo.Smoothcurvedcornershelpplasticflowinthemouldbyreducingpressuredropsinthecavityandminimisingflow-frontbreak-up.IntgmaI:ShIntemal:r■O.6t internal:r■0.6tmternal:r■0.61External:Sharp Eilemal.Sharp External,r≈0.6t Externalr≈0βt?*IGoodandbadcornerdesign3.1RIBSSofarinthisdesignserieswehaveseenthatplasticspartsshouldbemadewithrelativelythinanduniformwallslinkedbycornerradii,notsharpcorners.Bothideasareimportantinthedesignofribs.Whenthenormalwallthicknessisnotstiffenoughorstrongenoughtostanduptoserviceconditionsthepartshouldbestrengthenedbyaddingribsratherthanmakingthewholewallthicker.Theprincipleisthefamiliaroneusedinsteelgirderswhere'Γand,T'sectionsarealmostasrigidassolidbeamsbutareonlyafractionoftheweightandcost.Athickersectionisinevitablewheretheribjoinsthemainwall.Thisribrootthicknessisusuallydefinedbythebiggestcircle(D)thatcanbeinscribedinthecross-section,anditdependsontheribthickness(w)andthesizeofthefilletradius(r).Toavoidsinkmarks,thisthickregionmustbekepttoaminimumbutthereareconstraints.Iftheribistoothinitwillhavetobemadedeepertogiveadequaterigidityandthenitmaybuckleunderload.Thereareotherproblemstoo;themouldbecomesdifficulttomachineandfill.Andribsfilledunderhighinjectionpressuretendtostickinthemould.Thefilletradiusmustnotbemadetoosmalleither,oritwillnotsucceedinreducingstressconcentrationswheretheribjoinsthemainwall.Ideally,thefilletradiusshouldnotbelessthan40percentoftheribthickness.Theribsthemselvesshouldbebetweenahalfandthree-quartersofthewallthickness.Thehighendofthisrangeisbestconfinedtoplasticsthathavealowshrinkagefactorandarelesspronetosinkmarks.Asimplecomparisonshowsthebenefitofgoodribdesign.Aribthatis65percentofthewallthicknessandhasa40percentfilletradius,resultsinarootthicknessthatisabout1.23timesthewallthickness.Bycontrast,therootthicknesssoarsto1.75timesthewallthicknesswhentheribisasthickasthewallandhasanequalradius.Ribsofcoursemustbeextractedfromthemould,sotheymustbeplacedinthedirectionofdraworprovidedwithmovingmouldpartstofreethem.Ribsshouldbetaperedtoimproveejection;onedegreeofdraftpersideisideal.Iftheribisverydeepthedraftanglemustbereducedortheribbecomestoothin.Forthisreasonribsareoftenlimitedtoamaximumdepthoffivetimestheribthickness.Sofar,sogood.Buthowmanyribsareneededtomakeapartstrongenoughandhowshouldtheybearranged?WewillexaminethatinthenextDesignGuide.RΛdraftangleRlbfilletradiusr-SeiQS_?三d@p^RibrootthicknessDRibthicknessWRibscreatethicksectionsattheroot060α≈Q070o?sαβoQMoso%ιωRibthicknessretotivotowallthickness(Mt)(Vg)￥uxq￡-WXO*W-WOgUMaWOOJ≡αHowπbrootthicknessincreasesDESIGNER'SNOTEBOOKRibthicknessshouldbe50-75%ofthewallthickness.Filletradiusshouldbe40-60%oftheribthickness.Ribrootthicknessshouldnotbemorethan25%greaterthanthewallthickness.Ribdepthshouldnotbemorethan5timestheribthickness.Taperribsformouldrelease.R舊SDESIGNERSNOTEBOOKDESIGNERSNOTEBOOKDeepribsarestifferthanthickribs.Followthebasicrulesforribthicknessandfilletradii.Calculateribdepthandspacingwithareckoner,orbyusingmathsoftwareorfiniteelementanalysis.Ribsareusedtoimprovetherigidityofaplasticspartwithoutincreasingthewallthicknesssomuchthatitbecomesunsuitableforinjectionmoulding.Inthepreviousguidewelookedatthebasicsofribdesign.Thistimewewillseehowtoputribsintopractice.Usuallywewantaparttobeequallyrigidinalldirections,justlikeasolidplate.Wecangetalmostthiseffectbyrunningribsalongandacrossthepart,sotheycrossatrightangles.Thiscreatesathicksectionwheretheribscrossbutifwefollowthedesignrulesforribsandfilletradiitheincreaseiswithinacceptablelimits-about1.3timesthewallthicknessattheworst.Thiscanbereducedalmosttothebasicwallthicknessbyformingacored-outbossatthejunction,butabettersolutionistouseanormaljunctionwithribsthatarelessthan0.75timesthewallthickness.Buthowmanyribsdoweneedandhowdeepshouldtheybe?Rigidityisafunctionofthemomentofinertiaoftheribsection.Thistellsusthatthestiffeningeffectofaribisproportionaltoitsthicknessbutproportionaltothecubeofitsdepth.Sodeepribsarestructurallymuchmoreefficientthanthickribs.Acommontaskistodeveloparelativelythinribbedplatethathasthesamerigidityasathicksolidplate.Standardengineeringtextbooksprovidethebasicformulaetomakethecalculationbutthemathematicscanbeachoretomanagemanually.Tominimisetheworkanumberof'readyreckoners,havebeendevised,includinganelegantcross-ribsolutiondevelopedbyDuPont.Mostofthesereckonersorcalculatorsarebasedonaparticularsetofassumptionssousewithcautionifyourdesignvaries.Forexample,theDuPontribbedplatecalculatorassumestheribsarethesamethicknessasthewall.Toseehowitworks,let'simaginethatwewanttodesignacrossribbedplatewitha2.5mmwall(tB)thatwillbeasstiffasasolidplateof5mmthick(tA).Calculatetβ∕tA-thevalueis0.5-andfindthisvalueontheleft-handscale.Rulealineacrosstotheright-handscaleandreadoffthevaluewhichis1.75.ThisvalueisT/tAwhereTistheribdepthincludingthewallthickness.Soinourexample,T=1.75timestAwhichis8.75mm.Nowreadoffthevalueonthebasescaleverticallybelowthepointofintersectionbetweenthe0.5lineandthecurve.Thefigureis0.16anditrepresentstheproductoftAandthenumber(N)ofribsperunitofplatewidth(W).ThecurveassumesthatWisunity.SoNequals0.16dividedbytAwhichis0.032ribspermmofwidth,oroneribper31.25mm.Wecanmakeaprorataadjustmentforribsthatarecorrectlydesignedtobethinnerthanthewall.Iftheribsare65percentofthewallthickness,theribspacingbecomes65percentof31.25,makingit20mmforpracticalpurposes.AlternativeribjunctionsSOLIDPLATE EQUIVALENTIermstorthecalculatorR舊SDESIGNERSNOTEBOOKDESIGNERSNOTEBOOKUnidirectionalribsshouldbespacedapartbyatleast2andpreferably3ormoretimesthenominalwallthicknessRibdepthshouldnotbemorethan5timesribthicknessUsethecalculatorcurvetoworkoutribheightsUsebuttressribstostiffensidewallsRibsareimportantinthedesignofplasticspartsbecausetheyallowustomakeacomponentrigidwithoutmakingittoothick.Wehavealreadylookedatthefundamentalsandseenhowtodesignacross-ribbedpart.Sometimesthough,weonlyneedrigidityinonedirection.Thisusuallyhappensonalongthinfeaturelikeahandle.Inthiscase,wecanimprovestiffnessalongthelengthofthepartbyaddinganumberofparallelribs.Thesearecalledunidirectionalribs.Thefirstconsiderationisthattheseribsmustnotbetooclosetogether.Thisisbecausethegapbetweentheribsisproducedbyanupstandingcoreinthemould.Ifthiscoreistoothinitbecomesverydifficulttocoolandtheremayalsobeashrinkageeffectthatwillcauseejectionproblems.Theusualruleismakethegapatleasttwicethenominalwallthicknessandpreferablythreetimesormore.Asinthecaseofcross-ribs,designisbasedontheprinciplethatrigidityisproportionaltothemomentofinertiaofthewallsection.Thisprovidesawayofworkingoutthinribbedsectionsthathavethesamestiffnessasthickplainsections.Calculatorcurvesmakethejobeasier.Curvesareavailableforcalculatingdeflection(strain)andstressonvariousribthicknesses.Ourexampleshowsadeflectioncurveforribthicknessesequalto60percentofthenominalwallthickness.Forsimplicity,thecalculationsplitstheunidirectionalribbedpartintoanumberofT-sectionstrips,eachwithasinglerib.Thewidthofthestripisknownasthe'equivalentwidth'orBEQ.Toseehowthecalculatorworks；wewilldesignaribbedpartwiththesamestiffnessasarectangularsection45mmwide(B)by12mmthick(Wd).Wedecideonfourribsandanominalwallthicknessof3mm(W).Therearethreesimplecalculationstomake.BEQ=B/N=45/4=11.25BEQ∕W=11.25/3=3.75W√W=12/3=4Nowfindthevalue4ontheleft-handaxisanddrawahorizontallinetointersectwiththe3.75curveshownontheright-handaxis.Dropaverticalfromthispointandreadoffthevalue,5.31onthebottomaxis.Thisfigureistheratioofribheight(H)tothenominalwallthickness(W).Sotheribheightinthisexampleis:H=3(5.3)=15.9Thisismorethan5timestheribthickness,soweshouldbeconcernedaboutbuckling.Wecan'tincreasethenumberofribswithoutspacingthemtoocloselysoouroptionsaretomaketheribsand/orthewallthicker.Designoftenrequiresafewiterationstogetthebestresult.Wecanalsouseribsonsidewalls.Insteadofmakingthesidewallthicker,westiffenitwithbuttressribs,oftenknownintheUSAasgussets.Thesamedesignrulesapply.Itisparticularlyimportanttofollowtheruleforthicknessotherwisesinkmarkswillshowontheoutsideofthepart.Usebuttressribstostiffensidewalls4.1BOSSESWRONGUsesingledidyccalflatribincomerDESIGNERSNOTEBOOKBeforedesigningaboss,consideritsfunctionandtheforcesactingonitduringassemblyandserviceIftheforcesarenotgreatrWRONGUsesingledidyccalflatribincomerDESIGNERSNOTEBOOKBeforedesigningaboss,consideritsfunctionandtheforcesactingonitduringassemblyandserviceIftheforcesarenotgreatritmaybepossibletodispensewithsupportribs,otherwise:Anchorthebosstothebasewallwithbuttressribs.Ifpossible,anchorthebosstothesidewallwithaflatrib.Avoidribarrangementsthatresultinsmallmouldcoresorcomplicatedmouldmachiningset-ups.Thebossisoneofthebasicdesignelementsofaplasticsmoulding.Bossesareusuallycylindricalbecausethatistheeasiestformtomachineinthemouldanditisalsothestrongestshapetohaveinthemouldedpart.Thebossisusedwheneverweneedamountingpoint,alocationpoint,areinforcementaroundahole1oraspacer.Thebossmayreceiveaninsert,ascrew,oraplainshafteitherasaslideorapressfit.Inotherwords,thebossisnotassimpleasitlooks.Dependingonitsuse,itmayhavetostanduptoawholecombinationofforces-tension,compression,torsion,flexing,shearandbursting-soitmustbedesignedaccordingly.Wecanstartwithsomegeneraldesignrules,usingtheprincipleswehavealreadydevelopedforribsandwalls.Thebosscanbethoughtofasaspecialcaseofarib;onethatiswrappedroundintheformofatube.An'ideal'boss,designedaccordingtoribrules,wouldnotproducesinkmarksorstickinthemouldbutunfortunatelythetubularformofthebosswouldnotbestrongenoughinmostcases.Inreallife,mostbossesbreaksomeribdesignrulesbynecessity.Thismeansthatbossdesignisacompromisebetweensinkmarksandfunctionality.Rigidityisthesimplestaspectofbossdesign.Thiscanbeachievedbysupportingthebosswithbuttressribs,andoftenbylinkingthebosstoasidewall.Thesupportribscanbedesignedtonormalribrulessothatsinkmarksandstresspointsareavoided.Whenthebossislinkedtoasidewall1eitheratanedgeorthecornerofacomponent,thereisarightandawrongwaytodoit.Thewrongwayissimplytoextendthebossoutsidediametertomeetthewall.Thisinevitablyproducesathicksectionthatwillresultinsinkmarks,voids,andlongcoolingcycles.Therightwayistolinkortiethebosstothesidewallwithaflatrib,preferablyrecessedalittlebelowthebossoredgeheightsothatitcannotinterferewithanyassemblyconditions.Theotherribsthattiethebosstothebasewallremainasbuttressribs.Foreconomicalmachiningofthemould,theribsshouldbealignedontheXrYaxesofthecomponentexceptfortheflatcornerribwhichisplacedat45degrees.ThesinglediagonalribisbetterthantwoX-Yribsbecauseitavoidsasmallmouldcorebetweentheribs.Suchsmallcoresarepronetodamageandaredifficulttocool;thismayresultinslowermouldingcyclesandmoredowntime.Sowehaveestablishedhowtoconnectthebosstotherestofthecomponent.Themoredifficultpartofbossdesignconcernstheholeandthethicknessoftheboss.BossdesignisacompromiseThereisarightandawrongwaytosupport

bosses4.2BOSSESPerhapsthemostcommonfunctionofabossistoacceptascrewfastener.Therearetwotypesofscrewinwidespreaduse.Thread-cuttingscrewsworkbycuttingawaypartofthebossinnerwallastheyaredrivenin.Thisproducesafemalethread,andsomeswarf.Thread-formingscrewsproducethefemalethreadbyacoldflowprocess;theplasticislocallydeformedratherthancutandthereisnoswarf.Generally,thread-formingscrewsarepreferredforthermoplasticswhereasthread-cuttingscrewsarebetterforhardinelasticmaterialssuchasthermosets.Therangeofscrewsonthemarketmakesitdifficulttogiveageneraldesignrule,butoneapproachistousetheflexuralmodulusofthematerialasaguidetowh