Designguidesforplastics

CliveMaier,EconologyLtd

TANGRAM

TECHNOLOGY

Consulting

Engineers

θconology

December2021

#

ThispublicationismadeupofaseriesofarticlespublishedinPlasticsandRubberWeeklyasapiecework.ThekindassistanceoftheauthorandPRWisacknowledgedinthepublicationofthework.

Thedesignhintsinthisbookletaregiveningoodfaithandrepresentcurrentgoodpractice.Theshortnatureofthehintsmeansthatnotallinformationcanbeincluded.Noresponsibilitycanbetakenforanyerrorsorconsequentialdamagesresultingfromusingthesehints.

Thispublicationmaybefreelyreproducedexceptforsaleoradvertisingpurposes.Itmaybehostedonwebsitesforfreedownloadingprovidingthatitisusedinit’sentiretyandthatreferenceismadetotheoriginalpublication.

?CliveMaier2004

TypesetandeditedbyTangramTechnologyLtd.

Contents

TOC\o"1-5"\h\z

Preface 1

Introduction 2

Injectionmoulding 4

Basics

\o"CurrentDocument"

Wallthickness 9

\o"CurrentDocument"

Corners 10

Ribs 11

Bosses 14

Designfor recycling 17

Specialfeatures

Livinghinge 22

Bearings 24

Gears 27

Assembly

Pressfits 32

Snap-fits 33

Hotstaking 37

Ultrasonicwelding 39

Hotplatewelding 44

Spinwelding 45

Frictionwelding 46

Inductionwelding 47

Laserwelding 48

Adhesiveandsolventbonding 49

Outsertmoulding 54

4

#

Preface

Thissetofhintsandtipsforplasticsproductdesignersisintendedasasourcebookandan'aidemémoire'forgooddesignideasandpractices.Itisasourcebookforplasticsproductdesignersatalllevelsbutitisprimarilyaimedat:

studentdesignerscarryingoutdesignworkforalllevelsofacademicstudies;

non-plasticsspecialistsinvolvedinthedesignofplasticsproducts;

plasticsspecialistswhoneedtoexplaintheirdesigndecisionsandthedesignlimitationstonon

plasticsspecialists.

Thebookcoverseachtopicinasinglepagetoprovideabasicreferencetoeachtopic.Thisspaceconstraintmeansthateachtopicisonlycoveredtoabasiclevel.Detailedplasticproductdesignwillalwaysrequiredetailedknowledgeoftheapplication,theprocessingmethodandtheselectedplastic.Thisinformationcanonlybeprovidedbyrawmaterialssuppliers,specialistplasticsproductdesignersandplasticsprocessorsbutthereisaneedtogetthebasicsoftheproductdesignrightinthefirstinstance.

Usingthehintsandtipsprovidedinthisguidewillenabledesignerstoreduceinitialerrorsandwillleadtobetterandmoreeconomicdesignwithplastics.

Ihopethisshortworkwillimprovethebasicdesignofplasticsproductsandifitcandothisthenitwillhaveservedit’sobjectives.

CliveMaier

ECONOLOGYLtd.

INTRODUCTION

#

Gooddesignisimportantforanymanufacturedproductbutforplasticsitisabsolutelyvital.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.Allofthesematerialscanhavetheirpropertiesmodifiedbycontrolofmolecularweightandbyadditivessuchasreinforcements.Thenumberofdifferentgradesofplasticsmaterialsavailabletothedesignernowapproaches50,000.Theimportance-andthedifficulty-ofmakingtherightchoiceisobvious.

Plasticscanbegroupedintocategoriesthathaveroughlysimilarbehaviour.

Thermoplasticsundergoaphysicalchangewhenprocessed;theprocessisrepeatable.Thermosetsundergoachemicalchange;theprocessisirreversible.Akeydistinctionbetweenthermoplasticsrelatestothemoleculararrangement.Thosewithrandomtangledmoleculesarecalledamorphous.Thosewithadegreeofmoleculararrangementandorderingarecalledsemi-crystalline.Thedifferenceissignificant.Forexample,mostamorphousmaterialscanbefullytransparent.Andthemorecrystallineamaterialis,thelesslikelyitistohaveawide'rubbery'processingregion,somakingitlesssuitableforstretchingprocesseslikeblowmouldingandthermoforming

Designersmustdesignforprocessaswellaspurposeandmaterial.Insingle-surfaceprocessesforexample,thereisonlyindirectcontrolovertheformofthesecondsurface.Designmusttakethislimitationintoaccount.

DESIGNCONSIDERATIONS

#

PLASTICS

THERMOSETS

THERMOPLASTICS

HDPE

Highdensitypolyethylene

LDPE

Lowdensitypolyethylene

LLDPE

Linearlowdensitypolyethylene

PA

Polyamide(Nylon)

P日T

Polγt∣Lrtyleπeterephthalate

PEEK

Polyetheretherketone

POM

Po1γoxγrnethy1bγ∣e

(Acetal)

PR

Polypropylene

PPΞ

Polyphenylenesulphide

ABΞ

.^.crylonitrileb∣.rtadienestyrene

CA

Celluloseacetate

CAB

Celluloseacetate

b∣.rtyrate

CP

Celluloseproρrionate

PC

Polycarbonate

RES

Polyethersulphone

PET

Polyethylene

terephthalate

PMMA

Polymethyl

methacrylate

F,PO

Polyphenyleneoxide

PΞ

Polystyrene

PS∣J

PolysLJ∣phone

PVC

PolyvinylChloride

SAN

Styreneacrylonitrile

EPDM

Ethyleπe-propylene-dieneterpolymer

EPT

Ethyleπe-propyleneterpolymer

NBR

Nitrilebutadiene

rubber

PEB?

Pc∣lyetherblockamide

ΞBΞ

Styrene-b∣.rtadiene

styrene

TP∪

Thermoplastic

polyurethane

DAP

Diallylphthalate

MF

Melamineformaldehyde

PF

Phenolformaldehyde

UF

Ureaformaldehyde

EP

Epoxy

UP

Unsaturatedpolyester

Semi-crystalline

Amorphous

Elastomeric

SOMECOMMONPLASTICS

ALLSURFACESDEFINED

SINGLESURFACEDEFINED

BATCHPROCESS

InjectionmouldingCompressionmould!ng

Tnmnmτ?r「n口uldi「舊

Blo,∣Λ',mouldingτr∣ermoformingRotationalmoulding

CONTINUOUSPROCESS

ExtrusionCalenderingPultrusion

COMMONPLASTICSFORMINGPROCESSES

Injectionmoulding

1

WALLTHICKNESS

Partsthatmightbemadeassolidshapesintraditionalmaterialsmustbeformedquitedifferentlyinplastics.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.

Thicksectionsandnon-uniformwallscauseproblems

Solidshapesmustberedesignedas‘shells

WRONG

-sharpstep

RIGHT

-gradualtransition

byplane

RIGHT

-gradualtransition

byradius

Gradualtransitionsbetweenthickandthin

sections

DESIGNER’SNOTEBOOK

Keepwallthicknessasuniformaspossible.

Usegradualtransitionsbetweenthickandthinsections.

Wallthicknessmustsuitbothfunctionandprocess.

Wallthicknessguiderangeis:

0.75mmto3mmforreinforcedmaterials

0.5mmto5mmforunreinforcedmaterials

2

CORNERS

Whentheideasofcorrectanduniformwallthicknessareputintopracticetheresultisaplasticspartcomposedofrelativelythinsurfaces.Thewayinwhichthesesurfacesarejoinedisequallyvitaltothequalityofamouldedpart.

Wallsusuallymeetatrightangles,atthecornersofaboxforexample.Wheretheboxwallsmeetthebase,theanglewillgenerallybeslightlymorethan90degreesbecauseofadraftangleonthewalls.Theeasiestway,andtheworst,tojointhewallsistobringthemtogetherwithsharpcornersinsideandout.Thiscausestwoproblems.

Thefirstdifficultyisthattheincreaseinthicknessatthecornerbreakstheruleofuniformwallthickness.Themaximumthicknessatasharpcornerisabout1.4timesthenominalwallthickness.Theresultisalongercoolingtimeaccompaniedbyariskofsinkmarksandwarpingduetodifferentialshrinkage.

Theotherproblemisevenmoreserious.Sharpcornersconcentratestressandgreatlyincreasetheriskofthepartfailinginservice.Thisistrueforallmaterialsandespeciallysoforplastics.Plasticsaresaidtobenotchsensitivebecauseoftheirmarkedtendencytobreakatsharpcorners.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.Smoothcurvedcorners

helpplasticflowinthemouldbyreducingpressuredropsinthecavityandminimisingflow-frontbreak-up.

Internal:r=0.6tExternal:r=0.6t+t

Internal:r=0.6tExternal:Sharp

Internal:SharpExternal:Sharp

Goodandbadcornerdesign

Stressconcentrationfactorsforcantilever

loading

DESIGNER’SNOTEBOOK

Avoidsharpinternalcorners.

Internalradiishouldbeatleast0.5andpreferably0.6to0.75timesthewallthickness.

Keepcornerwallthicknessascloseaspossibletothenominalwallthickness.Ideally,externalradiishouldbeequaltotheinternalradiiplusthewallthickness.

3.1RIBS

#

Ribscreatethicksectionsattheroot

wd□p3q-

-ωs?≡=raδ

Ribthicknessrelativetowallthickness(w∕t)

Howribrootthicknessincreases

DESIGNER’SNOTEBOOK

Ribthicknessshouldbe50-75%ofthewallthickness.

Filletradiusshouldbe40-60%oftheribthickness.

Ribrootthicknessshouldnotbemorethan25%greaterthanthewallthickness.

Ribdepthshouldnotbemorethan5timestheribthickness.

Taperribsformouldrelease.

Sofarinthisdesignserieswehaveseenthatplasticspartsshouldbemadewithrelativelythinanduniformwallslinkedbycornerradii,notsharpcorners.Bothideasareimportantinthedesignofribs.

Whenthenormalwallthicknessisnotstiffenoughorstrongenoughtostanduptoserviceconditionsthepartshouldbestrengthenedbyaddingribsratherthanmakingthewholewallthicker.Theprincipleisthefamiliaroneusedinsteelgirderswhere'I'and'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.

thickness,theribspacingbecomes65percentof31.25,makingit20mmforpracticalpurposes.

Alternativeribjunctions

Ribsareusedtoimprovetherigidityofaplasticspartwithoutincreasingthewallthicknesssomuchthatitbecomesunsuitableforinjectionmoulding.Inthepreviousguidewelookedatthebasicsofribdesign.Thistimewewillseehowtoputribsintopractice.

Termsforthecalculator

Usuallywewantaparttobeequallyrigidinalldirections,justlikeasolidplate.Wecangetalmostthiseffectbyrunningribsalongandacrossthepart,sotheycrossatrightangles.Thiscreatesathicksectionwheretheribscrossbutifwefollowthedesignrulesforribsandfilletradiitheincreaseiswithinacceptablelimits-about1.3timesthewallthicknessattheworst.Thiscanbereducedalmosttothebasicwallthicknessbyformingacored-outbossatthejunction,butabettersolutionistouseanormaljunctionwithribsthatarelessthan0.75timesthewallthickness.

Source:DuPont

Buthowmanyribsdoweneedandhowdeepshouldtheybe?Rigidityisafunctionofthemomentofinertiaoftheribsection.Thistellsusthatthestiffeningeffectofaribisproportionaltoitsthicknessbutproportionaltothecubeofitsdepth.Sodeepribsarestructurallymuchmoreefficientthanthickribs.Acommontaskistodeveloparelativelythinribbedplatethathasthesamerigidityasathicksolidplate.Standardengineeringtextbooksprovidethebasicformulaetomakethecalculationbutthemathematicscanbeachoretomanagemanually.Tominimisetheworkanumberof‘readyreckoners’havebeendevised,includinganelegantcross-ribsolutiondevelopedbyDuPont.Mostofthesereckonersorcalculatorsarebasedonaparticularsetofassumptionssousewithcautionifyourdesignvaries.

DESIGNER’SNOTEBOOK

Deepribsarestifferthanthickribs.

Followthebasicrulesforribthicknessandfilletradii.

Calculateribdepthandspacingwithareckoner,orbyusingmathsoftwareorfiniteelementanalysis.

Forexample,theDuPontribbedplatecalculatorassumestheribsarethesamethicknessasthewall.Toseehowitworks,let’simaginethatwewanttodesignacrossribbedplatewitha2.5mmwall(tB)thatwillbeasstiffasasolidplateof5mmthick(tA).CalculatetB/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.Iftheribsare65percentofthewall

makingthesidewallthicker,westiffenitwithbuttressribs,oftenknownintheUSAasgussets.Thesamedesignrulesapply.Itisparticularlyimportanttofollowtheruleforthicknessotherwisesinkmarkswillshowontheoutsideofthepart.

Unidirectionalribcalculator

Source:DuPont

Usebuttressribstostiffensidewalls

Ribsareimportantinthedesignofplasticspartsbecausetheyallowustomakeacomponentrigidwithoutmakingittoothick.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=11.25

BEQ/W=11.25/3=3.75

Wd/W=12/3=4

Nowfindthevalue4ontheleft-handaxisanddrawahorizontallinetointersectwiththe3.75curveshownontheright-handaxis.Dropaverticalfromthispointandreadoffthevalue,5.3,onthebottomaxis.Thisfigureistheratioofribheight(H)tothenominalwallthickness(W).Sotheribheightinthisexampleis:H=3(5.3)=15.9

Thisismorethan5timestheribthickness,soweshouldbeconcernedaboutbuckling.Wecan’tincreasethenumberofribswithoutspacingthemtoocloselysoouroptionsaretomaketheribsand/orthewallthicker.Designoftenrequiresafewiterationstogetthebestresult.

DESIGNER’SNOTEBOOK

Unidirectionalribsshouldbespacedapartbyatleast2andpreferably3ormoretimesthenominalwallthickness

Ribdepthshouldnotbemorethan5timesribthickness

Usethecalculatorcurvetoworkoutribheights

Usebuttressribstostiffensidewalls

Wecanalsouseribsonsidewalls.Insteadof

4.1 BOSSES

#

Thereisarightandawrongwaytosupportbosses

Thebossisoneofthebasicdesignelementsofaplasticsmoulding.Bossesareusuallycylindricalbecausethatistheeasiestformtomachineinthemouldanditisalsothestrongestshapetohaveinthemouldedpart.Thebossisusedwheneverweneedamountingpoint,alocationpoint,areinforcementaroundahole,oraspacer.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.

DESIGNER’SNOTEBOOK

Beforedesigningaboss,consideritsfunctionandtheforcesactingonitduringassemblyandservice

Iftheforcesarenotgreat,itmaybepossibletodispensewithsupportribs,otherwise:

Anchorthebosstothebasewallwithbuttressribs.

Ifpossible,anchorthebosstothesidewallwithaflatrib.

Avoidribarrangementsthatresultinsmallmouldcoresorcomplicatedmouldmachiningset-ups.

Whenthebossislinkedtoasidewall,eitheratanedgeorthecornerofacomponent,thereisarightandawrongwaytodoit.Thewrongwayissimplytoextendthebossoutsidediametertomeetthewall.Thisinevitablyproducesathicksectionthatwillresultinsinkmarks,voids,andlongcoolingcycles.Therightwayistolinkortiethebosstothesidewallwithaflatrib,preferablyrecessedalittlebelowthebossoredgeheightsothatitcannotinterferewithanyassemblyconditions.Theotherribsthattiethebosstothebasewallremainasbuttressribs.Foreconomicalmachiningofthemould,theribsshouldbealignedontheX-Yaxesofthecomponentexceptfortheflatcornerribwhichisplacedat45degrees.ThesinglediagonalribisbetterthantwoX-Yribsbecauseitavoidsasmallmouldcorebetweentheribs.Suchsmallcoresarepronetodamageandaredifficulttocool;thismayresultinslowermouldingcyclesandmoredowntime.

Sowehaveestablishedhowtoconnectthebosstotherestofthecomponent.Themoredifficultpartofbossdesignconcernstheholeandthethicknessoftheboss.

FlexuralModulusofplastic(Mpa)

Preferredscrewtype

Lessthan1,400

Thread-forming

1,400to2,800

Thread-formingorThread-cutting

2,800to6,900

Thread-cutting

Greaterthan6,900

Thread-cutting,finepitch

Screwselectiondependsonmaterial

Source:DuPont

Material

HoleFactor

BossFactor

DepthFactor

ABS

0.80

2.00

2.0

ABS/PC

0.80

2.00

2.0

ASA

0.78

2.00

2.0

PA46

0.73

1.85

1.8

PA46GF30%

0.78

1.85

1.8

PA6

0.75

1.85

1.7

PA6GF30%

0.80

2.00

1.9

PA66

0.75

1.85

1.7

PA66GF30%

0.82

2.00

1.8

PBT

0.75

1.85

1.7

PBTGF30%

0.80

1.80

1.7

PC

0.85

2.50

2.2

PCGF30%

0.85

2.20

2.0

PE-HD

0.75

1.80

1.8

PE-LD

0.75

1.80

1.8

PET

0.75

1.85

1.7

PETGF30%

0.80

1.80

1.7

PM