Engineering Polyurethanes – RIM Part and Mold Design Guide

Parameters

Elastomers Rigid Rim CompositesSolid Bayflex

System (Unfilled)

Foamed Baydur System

Solid Baydur GS

System

Solid PRISM System

(Thin Walled)

Foamed Baydur STR/F

System

Solid Baydur STR/C

System

Part Design(t)Wall Thickness (in) 0.2 – 1.0 0.2 – 1.5 0.25 – 4.0 0.125 – 0.5 0.125 – 0.25 0.09 – 0.15

Rib Thickness at Root a 0.75t t t 0.75tUse box beam or corrugation

Use box beam or corrugation

Draft (degrees) This is function of Part Draw or Depth

0.5 min +0.25/inch

greater than one inch of

draw

0.5 min +0.25/inch

greater than one inch of

draw

0.5 min +0.25/inch

greater than one inch of

draw

0.5 min +0.25/inch

greater than one inch of

draw

1.0 min 1.0 min

Molded Holes/Slots Yes Yes Yes Yes No No

Undercuts Slight undercutsUse side pull or removable

insert

Use side pull or removable

insert

Use side pull or removable

insertNo No

Snap Fits Possible Possible Possible Possible No No

Fillets (Inner Radius)(in) 1/16 1/8 1/8 1/16 1/4 1/4

Finishing

Best Surface Class A Class A N/A Class ANo Class A, use

textureClass A with veil

Screw Assembly Use bolt and nutb Thread cuts skin Thread cutting Thread cutting Thread cutting Thread cutting

Mold Design and Processing Parameters

Material of ConstructionMetal preffered (depending on prod. volume)

Metal preffered (depending on prod. volume)

Metal preffered (depending on prod. volume)

Metal preffered (depending on prod. volume)

Steel Steel

Preferred Gating Fan Dam Dam Dam Center/Direct Center/Direct

Shot Time, Max. (sec) 2 (9c) 9 15 5 5 15

Mold Pressure (psi) 100 100 100 100 200 200

Mold Temperature (°F) 140 – 160 140 – 160 140 – 160 140 – 160 130 – 140 175 – 185

Physical Properties

Flexural Modulus (psi) 5,000 – 100,00053.000 – 240,000

130,000 – 190,000

270,000 – 310,000

150,000 – 750,000

Up to 2,000,000

Part Density (lb/in3) 60 – 65 15 – 55 63 – 68 61 – 67 20 – 40 90 – 110

Flexural Strength (psi) N/A 3,000 – 12,000 5,300 – 7,700 9,300 – 10.300 3,500 – 17,000 50,000

Tensile Strength (psi) 1,900 – 4,000 1,000 – 4,800 3,600 – 5,300 5,500 – 6,600 2,500 – 9,000 26,000

Elongation at Break (%) 100 – 360 6 – 10 16 – 29 11 – 12 2.5 2.5

DTUL at 66 psi (°F) N/A 160 – 212 140 – 215 190 – 205 205 400

Hardness (Shore D) 30 – 69 40 – 81 70 – 75 73 – 75 60 – 70 60 – 70

% Reinforcement 0 – 25 N/A N/A N/A 20(mat) 55(mat)

aRoot includes both radii. bCan also screw through to metal substrate. cLonger shot times are possible with a Bayflex XGT system, which has an extended gel time.

Quick Design Reference Guide for RIM

1

TABLE OF CONTENTS

IntroductionPART DESIGN5 TheRIMProcess6 MaterialDescriptions

Chapter1MATERIAL SELECTION CRITERIA9 AestheticConsiderations10 FunctionalConsiderations10 EconomicConsiderations

Chapter2GENERAL PART DESIGN13 PartStiffness13 WallThickness15 RibbingDesignandConfiguration17 RibbingDirection18 Draft19 Bosses21 Holes,GroovesandSlots23 Inserts23 MetalStiffeningInserts24 WoodStiffeningInserts24 ThreadedInserts25 Undercuts25 SnapFits,WireGuidesandHinges26 Fillers27 WarpageinPartDesign27 CreepConsiderations27 FatigueConsiderations28 BackMolding

Chapter3SOLID MATERIALS29 WallThickness29 RibDesignandConfiguration30 Radii/Fillets

Chapter4FOAMED MATERIALS31 FoamRiseandFlow31 WallThickness32 RibDesignandConfiguration33 Radii/Fillets33 Bosses34 StructuralAnalysisConsiderations

Chapter5COMPOSITE MATERIALS35 GlassMat36 Reinforcements36 Radii/Fillets37 Pads37 Preforms38 Finishes

Chapter6POSTMOLDING OPERATIONS39 Finishing39 Pigmentation40 In-MoldCoatings40 Patching40 PostmoldPainting40 Textures41 DecalsandSilk-Screening41 AssemblyOperations41 Screws42 Adhesives43 P0stfabrication43 Nailing/Stapling/Planing43 RecyclingPolyurethanes

3

TABLE OF CONTENTS

IntroductionPART DESIGN5 TheRIMProcess6 MaterialDescriptions

Chapter1MATERIAL SELECTION CRITERIA9 AestheticConsiderations10 FunctionalConsiderations10 EconomicConsiderations

Chapter2GENERAL PART DESIGN13 PartStiffness13 WallThickness15 RibbingDesignandConfiguration17 RibbingDirection18 Draft19 Bosses21 Holes,GroovesandSlots23 Inserts23 MetalStiffeningInserts24 WoodStiffeningInserts24 ThreadedInserts25 Undercuts25 SnapFits,WireGuidesandHinges26 Fillers27 WarpageinPartDesign27 CreepConsiderations27 FatigueConsiderations28 BackMolding

Chapter3SOLID MATERIALS29 WallThickness29 RibDesignandConfiguration30 Radii/Fillets

Chapter4FOAMED MATERIALS31 FoamRiseandFlow31 WallThickness32 RibDesignandConfiguration33 Radii/Fillets33 Bosses34 StructuralAnalysisConsiderations

Chapter5COMPOSITE MATERIALS35 GlassMat36 Reinforcements36 Radii/Fillets37 Pads37 Preforms38 Finishes

Chapter6POSTMOLDING OPERATIONS39 Finishing39 Pigmentation40 In-MoldCoatings40 Patching40 PostmoldPainting40 Textures41 DecalsandSilk-Screening41 AssemblyOperations41 Screws42 Adhesives43 P0stfabrication43 Nailing/Stapling/Planing43 RecyclingPolyurethanes

4

TABLE OF CONTENTS

IntroductionMOLD DESIGN

Chapter7GENERAL MOLD DESIGN CONSIDERATIONS47 PartSize/ClampingPressure48 MoldCosts49 Shrinkage49 DimensionalTolerances

Chapter8GATE DESIGN51 MixingHead52 Aftermixers53 EdgeGating54 FoamedSystems54 DamGates55 SolidSystems56 FanGates58 BallCheck58 Center-GatedDirectFill

Chapter9PARTING-LINE CONSIDERATIONS62 MoldSealing63 MoldVenting64 MoldFilling

Chapter10OTHER MOLD DESIGN CONSIDERATIONS65 MoldTemperatureControl66 DemoldingMethods68 MovableCoresandInserts69 MoldDesignforSlots69 ShearEdges

Chapter11SPECIAL MOLDS71 Multiple-CavityMolds72 Self-ContainedMolds

Chapter12MOLD FINISHING73 MoldConstructionMaterialsandFabricationTechniques73 MaterialSelection74 Steel74 Aluminum74 ZincAlloys(Kirksite)74 NickelShells74 EpoxyMolds75 MoldConstructionTechniques75 MilledBlock75 StructuralComponents75 Cast75 ExtrudedAluminumProfiles75 NickelPlating76 SurfaceTreatmentsforMolds76 TexturesandFinishes

Chapter13TECHNICAL SUPPORT77 HealthandSafetyInformation77 DesignandEngineeringExpertise78 TechnicalSupport78 DesignReviewAssistance78 ApplicationDevelopmentAssistance78 ProductSupportAssistance78 RegulatoryCompliance79 RIMPlasticsRecycling79 ForMoreInformation

APPENDICES80 ListofFiguresandTables82 Index

TABLE OF CONTENTSIntroduction

PART DESIGN

5

Today,variousReactionInjectionMolding(RIM)polyurethanesystemsarereplacingmanytraditionalmaterialsbecauseoftheirinherentadvantages,including:

•Large–partmolding;.Wall–thicknessvariations;

•Excellentencapsulationcapabilities;

•Excellentsurfacereproducibilityandin–moldpaintfinishing;

•Gooddimensionalstability;

•Goodchemicalresistance;

•Goodweatherability.

Generally,RIMprocessinguseslessexpensivemolds,lessenergyandlower–tonnagepressesthanthermoplasticprocessing.Thesecharacteristicsadduptosuperiordesign,economicandprocessingflexibility.

TheextensivenumberofRIMpolyurethanesystemswiththeirvariousphysicalandmechanicalpropertiescanmakeselectingtherightsystemdifficult.Thepurposeofthismanualistohelpyou–thedesignengineer,productdesignerandotherswhoworkwithRIMpolyurethanematerials–makepracticaldesigndecisions.

Thismanualisdividedintotwosections:partdesignandmolddesign.ThesectiononpartdesignbeginswithabriefdiscussionoftheRIMpolyurethaneprocessanddesignconsiderationscommontoallRIMpolyurethanesystems.Uniquepropertiesanddesignguidelinesforvariouspolyurethanesystemsarediscussednext,followedbydesignconsiderationsforassemblyandpostmoldingoperations.Thesecondsectionofthisbookcontainsinformationonmold,gateandrunnerdesign.

Manyrulesofthumbappearinthetext.Naturally,theremaybesomeexceptionstotheserulesofthumbortimeswhenoneconflictswithanother.Ifthishappens,talkwithyourmoldmaker/designerandBaySystemspersonnelforappropriateaction.Whilethismanualprovidesagoodoverviewofthetopicsyoushouldaddresswhendesigningforpolyurethaneparts,youshouldalsohaveagoodunderstandingofpolyurethanesystemsbeforemakingfinaldecisions.Foraquickreference,seethepart–designmatrixontheinsidebackcoverforkeyinformation.

Specificsystemdataandtypicalpropertyinformationhavenotbeenincludedinthismanualexceptasexamplesforgeneralinformation.Allvaluesthatappearinthismanualareapproximateandarenotpartoftheproductspecifications.Donotusethisdataforproductspecification.Formorespecificinformationonaparticularsystem,pleasereadtheappropriateBaySystemsProductInformationBulletin(PIB).Publisheddatashouldbeusedtoscreenpotentialmaterialcandidates.Yourunderstandingofmaterialsandprocessingandyourpart’srequirementsdeterminesthesuitabilityofamaterialforyourproductorapplication.Ultimately,materialselectionmustbebaseduponyourprototypetestingunderactual,end–useconditions.

BayerMaterialScienceoffersafullrangeofRIMpolyurethanesystems,includingfoamed,solidandstructuralcompositematerials.Asaservicetoourcustomers,wealsohavetechnicalpersonnelreadytohelpyouwithpartdesignandproduction.Alistoftheseservicesappearsinthebackofthisbooklet.Pleasefeelfreetocontactuswithspecificquestionsat412111-2000.

6

Introduction

PART DESIGN

7

THE RIM PROCESS

Incontrasttothermoplasticswhereamoltenplasticiscompactedinamoldandthensolidifiesuponcooling,RIMsystemsarecomposedoftwoliquidcomponentsthatchemicallyreactinthemold.Theliquids,isocyanateandpolyol,arecommonlyreferredtoasthe“Aside”and“Bside”components,respectively,intheUnitedStates.InEurope,thesedemarcationsarereversed.Amatchedsetof“A”and“B”componentsisreferredtoasa“RIMsystem.”

Generally,the“B”componentcontainsadditivessuchasstabilizers,flowmodifiers,catalysts,combustionmodifiers,blowingagents,filler,pigmentsandreleaseagentstomodifyphysicalcharacteristicsinthefinalpart.Whenthe“A”and“B”componentscombine,theisocyanatereactswiththehydroxylinthepolyoltoformathermosettingpolyurethanepolymer.Thisreactionisexothermic:whenthe“A”and“B”componentscombine,heatisreleased.Topreventscorchingand/orotherpartdefects,coolinglinesonthemoldhelpdissipatethisheat.

InfigureP–1,thepressurizeddaytankstypicallyholdfrom30to250gallonsofisocyanateorpolyol.Recirculationpumpsandagitatorsmaintainahomogeneousblendoftheindividualcomponents.Theheatexchangersmaintaincomponents’temperature.Eitherhigh–pressure,meteringpumpsorhydraulicallydrivenchemicalcylinders–commonlyreferredtoas“lances”–meterisocyanateandpolyolintothemixinghead.Flowratesandpressuresarepreciselycontrolledtoensurehigh–qualityparts.

Themixheadcontainsinjectornozzleswhichimpingetheisocyanateandpolyolatultra–highvelocitytoprovideexcellentmixing.Additionalmixingisaccomplishedusinganafter–mixer,typicallyconstructedinsidethemold.

8

MATERIAL DESCRIPTIONS

Thedegreeofrigidityusuallydefinesapolyurethanesystem,placingitinoneoftwocategories:rigidorelastomeric.Arigid polyurethane materialgenerallyhasahigherflexuralmodulusanddegreeofhardness.Thisclassofmaterialsnormallyoffersgoodthermalresistance,electricalproperties,chemicalresistanceandacousticalinsulation.Anelastomeric polyurethane systemisoftenfoundinapplicationsrequiringsuperiorimpactstrength.Elastomericpolyurethane

•Foamed polyurethane systemsuseablowingagenttomakepartswithahigher–densityskinandalowerdensity,microcellularcoreinasandwich–likecomposition.Baydurstructuralfoamandotherrigidsystemshavehard,solidskinsandarefoundinbusinessmachines,electronicandmedicalhousings,automobilespoilers,skisandotherload–bearingapplications.AllfoamedRIMmaterialsaremicrocellularsystems,havingcellsassmallas0.001inch.

Figure P-2Polyurethane Systems Classified by Flexural Modulus

Flexural Modulus (103 psi)

Type

s of

Poly

uret

hane

Mat

eria

ls

0 15001200900600300 1800

BAYDUR STR/CSolid

Composites

BAYDUR STR/CFoamed

Composites

PRISMRigidSolids

BAYDURRigidFoams

BAYFLEXElastomericSolids

RRIM

systemsexhibitgoodtoughnessanddimensionalstabilitythroughoutawidetemperaturerangeandhaveexcellentcorrosion,abrasion,wearandcutresistance.

Physicalpropertiesforthesetwocategoriesarenotabsoluteandtheflexuralmodulirangesofthesematerialsoverlap(seefigureP–2).Withinthesetwoclassifications,thearethreetypesofpolyurethanesystems(seefigureP–3):

Introduction

PART DESIGN

9

•Solid polyurethane systemsdonotuseblowingagents,resultinginahomogeneous,rigidorelastomericplastic.Bayflexsolidelastomericmaterialsareusedinmanyapplications,includingtheautomotive,specialtytransportation,construction,agricultureandrecreationalindustries.Commonpartsincludefenders,fascias,trimsandverticalpanels.Glassormineralfillerscanbeaddedtosolidelastomersforimprovedstiffness.ThisiscommonlyreferredtoasReinforcedRIMorRRIM.

PRISMsolidpolyurethanesystemshavemanypropertiessimilartothoseofengineeringthermoplastics.Theycanbeusedtomoldmanythickerorthin–walledpartsandmaybemoreeconomicalthanthermoplastics.

•Structural composite polyurethane systemsaresolidorfoamedmaterials,moldedincombinationwithlong–fiberreinforcements,suchasglassmat,toimprovethesystem’smechanicalcharacteristics.Sometimesreferredtoas“SRIM,”forStructuralRIM,thesesystemshaveextremelyhighstiffnessandhighimpactstrengthbecauseofthemat.Typicalapplicationsincludedoorpanels,shelves,automotivehorizontal/load–bearingpanelsandrecreationalequipmentparts

Figure P-3

Solid

Self-Skinning Foam

Composite

Types of Polyurethane Materials

10

Bayflex® polyurethane systems impart excellent durability and impact resistance to molded automotive panels and fenders.

Chapter1

MATERIAL SELECTION CRITERIA

11

When designing parts made of RIM polyurethane systems, make sure that your part or assembly meets all performance parameters and can be molded successfully and cost effectively. Before selecting a polyurethane system for part production, you must address three areas : aesthetic considerations, the part’s functional needs and your economic concerns. Involving the material supplier, mold maker and molder/ processor throughout a project will make the development process from concept to finished part much easier. Look in the back of this manual for a Quick Design Reference Guide for RIM Materials. See BaySystems’s EngineeringPolymers:PropertyGuide for material property information.

AESTHETIC CONSIDERATIONS

Whenestablishingaestheticrequirementsforyourpart,remembertheoldadage,“formfollowsfunction.”Ifyouaredesigningacarfasciaorfender,yourpartwillneeda“classA”finish,becausemostautomotiveapplicationsareaestheticallysensitive.Ontheotherhand,ifyouaredesigninganunexposedstructuralmember,aestheticsmaynotbeasimportantasload–bearingcharacteristics.Whileitisimportanttomakeyourpartlookgood,specifyingunusualfinishesorpaintscanincreaseyourpostmoldingcosts.Someaestheticguidelinestoconsiderwhendesigningpartsinclude:

•Determineifyourpartwillneedasmooth,mirror–like,“classA”finish,atexturedfinish,orothertypeoffinishbecausesomepolyurethanematerialsmaybemoresuitableforyourpart(seephoto).

•Decideifyourpartwillneedcolor.Polyurethanepartscanbepainted,pigmentedand/orin–moldcoated.

•Determineifpigmentationwithoutpaintingisacceptable.Darkercolors–blacks,graysorbrowns–maynotneedpaintingbecausecolorshiftscausedbyultravioletlightwillbelessnoticeable.

•ConsiderapplyingagoodpolyurethanepaintorclearcoattopreventchalkingorcolorshiftscausedbyUVexposure.Polyurethanecoatingsinherentlyhaveexcellentadhesion.

12

FUNCTIONAL CONSIDERATIONS

Whendefiningfunctionalrequirements,consideralltheenvironmentstowhichyourpartwillbeexposed,aswellasitsend–useconditions.Ambienttemperature,humidityandUVradiationareofparticularimportance.Considerthefollowingguidelineswhenaddressingfunctionalrequirements:

•Determineifyourpartneedshighimpactresistanceand/orhighstiffness.

•Definethepart’sloadingconditions,fasteningorattachingparametersandotherphysicalrequirements.ThephysicalpropertiesoftheRIMmaterialmustwithstandthestructuralconditionstowhichyourpartwillbesubjected.

•Determinethechemicalstowhichyourpartmaybeexposedduringprocessingandassembling,aswellasinenduse.Theseinclude,butarenotlimitedtosolvents,de–greasers,cleaningagentsandhouseholdproducts.Ensurethatthesechemicalsarecompatiblewithyourmaterialselection.

•Determineifstiffeninginsertsshouldbeencapsulatedinthepart.

•ConsiderelastomericRIMpolyurethanesystems,ifyourpartneedsgoodimpactcharacteristics.

•Considerrigidpolyurethanesystemsforpartsthatneedhighstiffness.

•Useastructuralcompositepolyurethanesystemifyourpartneedshighstiffnessandhighimpactstrength.

•Useself–skinning,rigid,foamedsystemswhenyouneedtoreducedensityandpartweight.

•UseBaySystems’Engineering Polymers: Material SelectionandconsultwithyourBaySystemsrepresentativeformoreinformationonmaterialselection.

ECONOMIC CONSIDERATIONS

Thefinalcostofapartinvolvesmorethantheper–poundcostofthematerial.Differentmaterials–steelandplastic,forinstance–havedifferentcostsassociatedwithprocessing,finishing,productivityandqualitycontrol–allofwhichcanalterthecostdramatically.RIMpolyurethanesystems,forinstance,offerquickcycletimesforlargepartsmadeofelastomersand,inmanycases,canuseless–expensiveequipmentthanthermoplastics.Additionally,partsmadeofpolyurethanemayweighlessthancomparablepartsmadeofothermaterials.

Becausethepart’sshape,notitsweight,isfixedinthedesign,youshouldalsocomparethecostpervolume($/in3)insteadofcostperpound.Atonoflow–densitymaterialwillproducemorepartsthanatonofhigh–densitymaterial.

Chapter1

MATERIAL SELECTION CRITERIA

13

Partgeometryalsoplaysanimportantrole.Whencomparingpolyurethanesystemsforaload–bearingapplication,optimizepartgeometryforeachmaterial’scharacteristics.Forexample,youmaybeabletodesignapartwiththinnerwallsandfewerribstoachievetherequiredstiffnessbyusingahigher–densitysystem.Considertheseguidelineswhendeterminingcosts:

•Identifyyourcosttarget.

•Knowthecurrentcostofmanufacturingandassembly,particularlywhenreplacingapartmadeusingadifferentprocess.

•Optimizewallthicknesstoreducepartweightandmold–cycletime.

•Corethicksectionswherepossible.Useotherreinforcingtechniques

suchasribbing,corrugatingandencapsulatingstructuralinsertstoimprovethepart’sstiffness.

•Simplifypartdesign.Complexpatswithmultiplesidepullswillincreasemoldcostssignificantly(seefigure1–1).

•Considertheaddedcostsforfinishing,painting,orcoating.

CorePin

Simple ToolComplex Tool

No SideAction

SideActionRequired

Draw

Example 1: Snap-Fit Undercut

14

This boat dashboard is molded in one piece from a BaySystems Baydur® 667 polyurethane system. The styrene-free fabrication process produces a lightweight dash with a high-strength-to-weight ratio and an excellent surface finish, all with

significantly less labor than the hand spray-up process.

Chapter2

GENERAL PART DESIGN

15

Other design parameters beyond material–specific guidelines must also be considered when designing parts, including:

• Wall thickness

• Warping

• Ribs and ribbing direction • Slots

• Vents

• Grooves

• Inserts

These design parameters are discussed in this section.

PART STIFFNESS

Whendesigningforpartstiffness,youmayhavetobalanceapart’swallthicknessandthematerial’sflexuralmodulusor,infoamedsystems,materialdensity.Toimprovestiffnesswhileminimizingwallthickness,considerusingahigher–modulussystem,improvingpartgeometry,oraddingreinforcementsandencapsulations.

Theflexuraltest,asimplysupported,three–pointloadingtest,determinesthematerial’sflexural–modulusvalue,thebasicphysicalpropertyusedtodeterminestiffnessintypicalbendingapplications(seefigure2–1).Theflexuralmodulusisdefinedastheinitialslopeofthestress–strain

curvegeneratedinthistest.Greaterflexuralmodulusvaluesindicatehigherinherentstiffness.Apartsubjectedtobendinghasastiffnessindicatordefinedastheflexuralmodulusmultipliedbythemomentofinertia,ageometricproperty.Thisindicatorgivesacomparativevaluetousewhenoptimizingdifferentmaterialsandpartgeometries.

WALL THICKNESS

Partsmadefrompolyurethanesystemscanbedesignedwithvaryingwallthicknessesmoreeffectivelythantraditionalthermoplastics.Thickerwallshavehigherstiffness.

LL/2

Cross Section

Load (P)

b

h

�

DEFLECTION(�):

� = PL3

48EI

E = Flexural Modulus

For RectangularCross Sections

I= bh3

12

16

•Ifyoudoublewallthicknessinaflatpart,thepart’sstiffnesswillincreasebyafactorofeight.

•Wallthicknessforsolidmaterialsistypically1/8inch,althoughpartswithwallsasthickas1/2inchhavebeenmoldedsuccessfully.

•WallthicknessforBaydurSTR/CorSTR/Fcompositematerialstypicallyrangesfrom1/16to1/4inch,althoughpartswiththickerwallshavebeenmolded.

•Anothermethodtostiffenasidewallinthedirectionofdrawistocurveitatthebase,orredesigntheflatsectionsothatithassteps,angles,orcorrugations(seefigure2–2).

•WallthicknessforpartsmadeofBaydurstructuralfoamcanrangefrom1/4to1–1/2inches

Becauseapart’sthickestcrosssectiondeterminesmoldingtime,excessivelythickcrosssectionsmaycauseuneconomicalandlongmoldingcycles.Thin–walledpartshavetheshortermold–cycletimes,becausetheheatofreactiondissipatesmorerapidly.

Figure 2-2

Stepped

Ribs

Corrugated

Curved

Chamfered

Part-Stiffening Techniques

Chapter2

GENERAL PART DESIGN

17

Unusuallythickcrosssectionscanalsocausedimensionaldifficulties.Becausethematerialinthickcrosssectionstakeslongertocool,partsmayshrinkmoreandcanpossiblywarp.Inextremecases,scorchingorsplittingmayoccur.Wheneverpossible,corethicksectionstoavoidthiseffect(seefigure2–3).Considerusingribsorotherlocalreinforcementstoincreasepartstiffnessasanalternative.

WhileRIMpolyurethanesystemscanbeusedtomakepartswithvaryingwallthicknesses,designingpartswithexcessivewall–thicknessvariationsmaycauseunevenfillingandracetracking.Figure2–4,showingafive–sidedbox,isagoodexampleofracetracking.Theliquidcomponentsfillthethickerwalls,leavingairentrapmentsinthethinnerbase.Tocorrectforthisracetrackingeffect,designthinnersidewallsorathickerbase.

RIB DESIGN AND CONFIGURATION

Taller,thinnerribsaremoreeffectivethanshorter,widerones(seefigure2–5).Inthisfigure,bothribshavethesamecross–sectionalarea,butthestiffeningeffectofribBisfargreaterthanthatofribA.Ribsshouldruncontinuouslyfromside–to–sideorcorner–to–cornerwheneverpossible.Thelowestribheightdeterminestheeffectivestiffnessofnotchedribs(seefigure2–6).

Figure 2-3

ThickPart

Cored/RibbedPart

Cored Part

Figure 2-4

ConsistentWall

Thickness

Correct

ThickThin

Air Trap

Incorrect

Racetracking

18

Tallerribswithdraftmayleadtoawidebase,resultinginproblemsinprocessing,withcycletimeandwithproductappearance.

Ribsandotherprotrusionsthatarethickerthanthenominalwallcancause“readthrough”–sinkmarksorvisualblemishesontheoppositeshowsurface(seefigure2–7).Ingeneral,sinkmarksaremuchlessofaproblemwithRIMpolyurethanesthanwiththermoplastics.Thesesinksappearwheretheribandmatingwallmeetbecausetheincreasedwallthicknessleadstoincreasedlocal–areashrinkingasthepartcools.Designingastepinthepartwheretheribmeetsthematingwallhelpstoavoidsinkmarks(seefigure2–8).

Ifyouneedthesupportofathickrib,designitasaseriesofthinnerribswithequivalentheightandcross–sectionalarea.Thespacebetweenthesethinnerribsshouldbenolessthanthenominalwallthickness(seefigure2–9).

t tA B

Stiffness of Rib (B) isArea (A) = Area (B) Then Much Greater than Stiffness of Rib (A)

Figure 2-5 Thick Versus Thin Ribs

Figure 2-7 Sinks Caused by Thick Ribs

Sink

Figure 2-6 Notched Rib

h

Chapter2

GENERAL PART DESIGN

19

Sidewallsmayneedtobestiffenedinthedirectionofdrawand/orperpendiculartothedirectionofdraw.Forstiffeninginthedirectionofdraw,usesimpleribbing.Whenperpendicularribbingisnecessary–suchasinwalls–youmayhavetouseslidingcores,whichmayaddsignificantlytomoldandfinishingcosts.Additionaldesignconsiderationsforribsinclude:

•Ribbingincreasesstiffnessonlyintheribbingdirectionofthatrib.

•Ifaribisnotched,thelowersectionoftheribwilldeterminestrength,unlessthenotchisbridgedwithametalstiffener(seefigure2–10).

Ribsarequitedifficulttomoldinpartsmadeofcompositematerials.Typicallytheseribsmayhaveresin–rich,potentiallybrittleareasattheirtops,becauseitisdifficulttogetmatintothistightarea.

Ribbing Direction

Figure2–11illustratesthreecommonribbingconfigurations.Ifapartneedstobereinforcedinonlyonedirection,useparallel ribbing.Usebidirectional ribbingifyourpartneedstobereinforcedinbothdirectionsoftheplanesurface.Applybidirectionalribssparinglybecauseexcessiveribbingcanmakeyourpartheavyanduneconomicaltoproduce.Additionally,avoidplacingribsperpendiculartotheanticipatedflowdirection,becausetheymaytrapairandcausefillingdifficulties(seefigure4–3).

Figure 2-8Offset Rib

Figure 2-9Rib Configuration

Sink

For Thick Ribs

Poor

No Sink,Quicker Cycle

Better

A

t

t

t

s

Make SureArea (B + B) = Area A

s ≤ 0.75t for Solidss ≤ t for Foams

Convert thick ribs into evenly spaced, thin ribs

B B

Figure 2-10 Notched Rib with Bridge

Steel

h

20

Ifyourpartneedstorsionalstiffnessaswellaslongitudinalstiffnessinbothdirectionsoftheplanesurface,usediagonalcrossribbing.Possiblythemost–economicalribbingpatternformaterialusage,thisribbingconfigurationiseasiertofillandlesslikelytotrapair.

tophalfofthemold)thanitisonthecavityside,becausepartsgenerallyshrinkontothecoreduringcooling.Otherrulesofthumbfordraftanglesinclude:

•Aminimumof1/2°isusuallyadequateforpartswithlowsidewallsorribs,typicallythoseuptoIinchdeep,

•Addatleast1/4°ofdraftforeveryadditionalinchofdraw,suchthata5–inchdrawwouldrequireaminimumof1–1/2°draft(seefigure2–12).

DRAFT

Everysurfaceparalleltothedirectionofdrawneedsadraftangletofacilitatedemolding.Therecommendeddraftangleincreaseswithpartheight.

Donotdesigndeepwoodgraintexturesonsidewalls.Evenlighttexturesinthisorientationrequireadditionaldraft.Elastomericsystemsmayrequirelessdraftintexturedparts.Generally.draftismoreimportantonthecoreside(usuallythe

Figure 2-11Different Types of Ribbing

Unidirectional

Bidirectional

Diagonal

Figure 2-12 Recommended Draft

0.5° Min.

1.5° Min.

5”

1”

Chapter2

GENERAL PART DESIGN

21

BOSSES

Usebossesforsupport,asspacersorasattachmentpoints.Attachbossesandotherprojectionsontheinsideofpartstothesidewallswithconnectorsthatallowairtoescapeduringmolding(seefigure2–13).Avoidisolatedbosses,alsoknownas“blindbosses.”Ifyoucannotattachabosstoasidewallbecauseofinterferenceordistancefromthewall,designgussetsorventthebosswithacore.Openbosses,thosecoredfromonesideandattachedtoanexteriorsidewall,arefrequentlyusedforassemblytoeliminatetheneedforconnectorsorgussets(seefigure2–14).Allbossesshouldhaveradiiattheirbases.Followstandardradiirecommendationslistedunderthesectionsonsolid,foamedandcompositematerialsinthisbrochure.

Ifyouareusingabosstoaccommodateaninsert,suchasascreworpressfit,maketheholeasdeepaspossible,preferablyleavingonlyonenominalwallthicknesstopreventsinkmarks.Otherdesignguidelinesinclude:

•Ifyoucannotavoidanisolatedboss,addgussetsthatextendfromthebasetothetoponthesideinthedirectionofflowtofacilitateairremovalandmoldfilling.

•Attachbossestosidewallswithaconnectorofnominalwallthicknessforfoamedmaterialsand3/4nominalwallthicknessforsolidmaterials.

Figure 2-13 Bosses and Venting

s

t

Boss UsingGussets orConnected to Side Walls

UnventedIsolated Boss

Avoid Correct

s ≤ 0.75t for Solidss ≤ t for Foams

Figure 2-14 Open Boss

A

A

Section A-A

22

•Designbossesawayfromcornersunlessthebosscanbeconnectedtothewalldirectly(seefigure2–15)orindirectly(seefigure2–13).Thiswillhelppreventlocalizedheatbuildupandpossiblewarpage.

•Considermoldingahollowbosstomaintainnominalwallthickness(seefigure2–16).

•Corebossesinsteadofdrillingwhenusingthread–cuttingscrewsandthread–cuttinginsertsinpartsmadeofstructuralfoamtoincreasepulloutstrength.

•Considerdesigninganelongatedbossandhavingtheexcessgroundoffasapostmoldingoperation,onlyasalastresort(seefigure2–17)

Figure 2-15 Corner Bosses

IncorrectExcessive Wall Thickness

CorrectMaintain Nominal Wall

Figure 2-16Hollow Bosses

Cored Boss

Partial Core

Figure 2-17 Elongated Boss

EntrappedAir

RemoveExcessMaterial

Chapter2

GENERAL PART DESIGN

23

HOLES, GROOVES AND SLOTS

Holescanbepostdrilled,moldedinthedirectionofdraworformedbyaretractablepinactuatedbyahydrauliccylinder.Aholeinasidewallwithenoughdraftcanalsobeformedbyhavingthemoldcoreandcavitymeetatthehole(seefigure2–18).Inthisdesign,holescanbepositionedanywhereonthewall.

Orientgroovesandslotsintheflowdirectiontominimizeairentrapmentsorknitlines.Makesurethatgroovesareroundedorchamberedratherthansharptohelpflow,ventairandreducestressconcentrations(seefigure2–19).

Groovesshouldnotreducethewallthicknesstotheextentthatfoamflow

isimpeded.Asaruleofthumb,donotrecessgroovesmorethan3/16inchforfoamedmaterialsand3/32inchforsolidmaterials.Groovewidthshouldfollowtherulesestablishedforslotsinfigure2–22.Widergroovesruntheriskofracetrackingandairentrapments.

Considerpositioningslotsinasidewall,curledaroundthebaseplane,toallowformoldingwithoutslides(seefigure2–20).Anotheroptionistodesignslotswithsteppedcutouts,positionedinaslopingwallsection(seefigure2–21).Thickerwallswillrequiremoreslope.Ifusingthislastoption,donotmakethematingsectionstoosharp,asthiscoulddamagethemold.Rulesofthumb

Figure 2-18Hole in Side Wall Figure 2-20 Slots Curled Around Corner

Figure 2-19 Grooves

1.5° Min.

Foamed = 3/16” Min.Solid = 3/32” Min.

24

forslotsorlouversareshowninfigure2–22.Designslotsandgrooveswithaminimum1–1/2°drafttohelpwithdemolding.Infoamedmaterials,grooves,slotsandholesshouldbelocatedundertheliquidlevelandlieinthedirectionoffoamrisetohelppreventairentrapment(seefigure2–23)

Figure 2-22 Basic Dimentions for Slots

t

L

b w

t = Thicknessw ≥ tb = 1.5wL ≤ 20w

FLOW

FLOW

or

Figure 2-23

Vent

FoamRise

FlowDirection

Gate

Approx. Liquid Levelfor 0.5 S.G. Density

As a good design practice, locate slots, grooves and holes under liquid level. Place in foam-rise direction.

Figure 2-21Slots or Louvers on Sloping Wall

MoldCore

MoldCavity

Direction of Draw

Direction of Draw

Mold

PartMold

Chapter2

GENERAL PART DESIGN

25

INSERTS

Polyurethaneshavelowmoldingtemperaturesandpressures,makingthemidealforencapsulatingreinforcinginserts.Theinsertshouldnotimpedematerialflow.Ifusingahollowinsert,theendsmustbesealed.Thermoplasticendcapshavebeensuccessfullyusedtosealinserts.Topromotegoodadhesionwiththepolyurethane,cleanandroughentheinsertsand,ifnecessary,treatthemwithanadhesionpromoter.

ThetypeofRIMsystemuseddeterminestherecommendedminimumdistancebetweenaninsertandthemoldwall.Forsolidmaterials,thisminimumdistanceis1/8inch;forfoamedsystems,1/4inch.Forexample,asolidmaterialwitha1/8–inchnominalwallthicknessshouldhaveaminimumdistanceof1/8inchbetweenthewallandinsert(seefigure2–24).

Encapsulatedinsertsareusedforanynumberofreasons.Forexample,they:

•Increasestiffness

•Reducewallthickness

•Absorbhighstresses

•Controlthermalexpansion

Themost–commontypesofinsertsarediscussedinthefollowingsections.

Metal Stiffening Inserts

MoldingmetalinsertsintoRIMpolyurethanematerialswillincreasestiffnesssignificantly.Insertsofalltypes–includingflatplates,extrusions,tubesandbars–havebeeneasilyandsuccessfullyencapsulated.Fullyencapsulatinginsertseliminatesmetalcorrosion,whilereducingthickcrosssections,controllingdeflectionandthermalelongationandabsorbinghighstresses.

Figure 2-24 Minimum Wall Thickness for Using Inserts

s

s = 1/4” for Foamed Systemss = 1/8” for Solid Systems

CalculatethecentersofgravityforboththeRIMmaterialandmetalinserttoreducethepotentialforwarping(seefigure2–25).Thecentersofgravityshouldcoincidetopreventthepartfrombendingbecauseofthemovementduetothedifferencesinthecoefficientsoflinearthermalexpansion.Asthetemperatureincreases,thepolyurethanematerialwillbeincompressionandthemetalinsertintension.Asthetemperaturedecreasesfromtheambient,thereverseistrue:thepolyurethanematerialisintension;theinsertincompression.Therelativecross–sectionalareasofthetwomaterialsdeterminetheultimateelongationofthepart.

26

Wood Stiffening Inserts

Woodinserts–generallylessexpensiveandlighterthanmetalinserts–canalsobeusedasstiffeninginsertsinpolyurethaneparts.Whenafinishedpartissubjectedtorepeatedloads,woodinsertsmayseparatefrommoldedpolyurethaneifthewood’smoisturecontentexceeds6%.Ifthewoodinsertcannotbedriedtomeetthislimit,itmustbesealedwithalacquerbeforemolding.

Figure 2-25Balancing the Cross-Sectional Centers of Gravity

Center of Gravity of Polyurethane

Center of Gravity of Insert

Incorrect Correct Correct

MetalInsert

C.O.GUnbalanced

C.O.GBalanced

C.O.GBalanced

Matched Insert

Figure 2-26Typical

Threaded Insert

WhenusingBaydurstructuralfoam,molded–ininsertsmayoffergreaterpulloutstrength,becauseskinformsovertheentireinsertsurface.Whenusingpress–fitinsertswithstructuralfoamparts,moldtheholesothatskinformsinside.

Generally,molderspreferpress–fitinserts,eventhoughtheseinsertsmaynotbeasstrongasmolded–inones.Placinginsertsonpinsinsidethemoldcanincreasecycletimesignificantly.Althoughrare,insertsmayalsofalloffpinsduringmolding.

Theinsertdesign,holediameter,partdensityandscrewsizedeterminethepulloutforceandstrippingtorqueofthreadedinserts.Contactyourinsertmanufacturerformoreinformationaboutobtainingwhole–diametervalues.SeeBaySystems’sPlastics: Joining Techniquesmanualformoreinformationoninserts.

Threaded Inserts

ThreadedinsertsareparticularlyusefulwhencomponentsmustbeattachedtoRlM–moldedparts(seefigure2–26).Useappropriatelysized,press–fitinsertswithrespecttoboss–holediameter.Usethreadedinsertsifyourpartisgoingtobefrequentlyassembledanddisassembled.

Chapter2

GENERAL PART DESIGN

27

UNDERCUTS

Ifpossible,avoidundercutswhendesigningpartsmadeofrigidRIMpolyurethanes(seefigure2–27).Theyaddtocostandmaycreatedemoldingproblems.Modifythepartgeometryormoldorientationordivideyourpartintotwoseparatemoldstoavoidundercuts.Forpartsmadeofelastomericmaterials–includingreinforcedRIMBayflexsystems–minorundercutscanbeadesignadvantage.Theflexiblenatureofthesematerialsaccommodateseasymoldreleaseevenwithminorundercuts.

SNAP FITS, WIRE GUIDES AND HINGES

Asimple,economicalandrapidjoiningmethod,snap–fitjointsofferawiderangeofdesignpossibilities.Allsnapfitshaveaprotrudingpartononecomponent–ahook,studorbead–whichdeflectsbrieflyduringjoiningandcatchesinarecessinthematingcomponent,thusrelievingthedeflectionforce(seefigure2–28).Formoreinformationonsnapfits,pleaseaskforacopyofBayerMaterialScience’sSnap–Fit Jointsbrochure.

Figure 2-27 Mold Configurations Showing Undercuts

Draw

DrawDraw

Draw

Undercut

UndercutRemovable

Expensive Correct

Incorrect

Figure 2-28Snap Fit Hook

Draw

Snap fit hook molded through hole to form undercut

Figure 2-29 Wire Guides

Draw

Usedextensivelyinbusiness–machineandappliancehousings,wireguidesoffersimpledesignsolutionstokeepcablesinposition(seefigure2–29).Generallymoldedintothepart,wireguidescanbedesignedasarestraintthatismoldedwithoutundercuts.Whendesigninghinges,considertheenduse:willitbeapermanentconnection,willitbeusedoftenand/orwillithavetodisengageafteracertainopeningangle.Allofthesefactorswillaffectdesign.Forpermanent,frequentlyusedjoints,considermetalhinges,whichcanbemolded–inorpostmoldassembled.Whiletheyaddtocosts,theymaybeoptimuminlong–termapplications.

28

Forpermanent,infrequentlyusedhinges,considerthelivinghinge(seefigure2–30).Typically,theyaremadeofthesamematerialasthepart,butcanbemadeofadifferentmaterial.Bayflexelastomericmaterialshaveexcellentflexuralfatiguestrength.MoldedstripsofBayflexelastomerscanbecutandplacedintoamoldtoformalivinghingeforamorerigidpart.However,ifsuchahingebreaks,itwillbevirtuallyimpossibletorepair.

FILLERS

Usingmaterialsthathaveglassorotherinertfillerswillaffectyourpart’sshrinkage,coefficientoflinearthermalexpansion(CLTE),stiffnessandimpactstrength.AfilledBayflexelastomericpolyurethanematerialcanhaveaCLTEclosertosteel.Generally,fillersincludefiberglassflakes,shortglassfibers,orothermineralfillers.Usually,fillersneedtohaveasizingtreatmenttopromoteadhesion.

Asfillercontentincreases,stiffnessincreases.Shortfibersusuallyorientinthedirectionofflow,causinggreaterstiffnessandlowerCLTEparalleltothefiberorientation.Adding15%glassfillertoaBayflexelastomercanalmostdoubleitsflexuralmodulus.Testyourparttoensurethatitperformsacceptablywiththesuggestedfillercontent.Whenspecifyingmaterialswithfillers,alwayscheckthematerialsafetydatasheetfromyourfillersupplierforsafehandlingpracticesfortheirproducts.

Figure 2-30Living Hinge

Rigid orOtherMaterial

BayflexElastomericor OtherFlexibleMaterial

Figure 2-31 Partial Hinges

Anotherhingingmethodistomoldapartthatlooksandoperateslikeametalhinge,withalternatingsectionsonoppositeparthalves.Thesepartialhingesofferadesigneramethodofforminghingeswithoutundercuts(seefigure2–31).Whiletheyhaveareducedload–carryingcapability,partialhingesofferlowertoolingcostsandusehingepins,asfullmetalhingesdo.Arodpushedthroughtheassemblycompletesthehinge.Thisdesignwilldisengagewhenthejointanglereaches180’.Ifyoudonotwantthehingetodisengage,considerdesigningfullholesattheendswitharetractablecorepin.

Chapter2

GENERAL PART DESIGN

29

Figure2–32showsthedataobtainedinaflexural–creeptestforaBaydurfoamedsystem,normalizedtoshowtheretentioninmodulusovertime.Thisdatashouldnotbetakenoutofcontextfortworeasons:1)Thedatarepresentspartssubjectedtocontinuousloading;2)Manufacturersusuallyrequireinstantaneousdisplacementstobeverysmall.Manufacturersanddesignersshoulddetermineacceptablesafetyfactorsforthepart’slife.

FATIGUE CONSIDERATIONS

Repeatedloadingcausesfatigue,aprogressive,permanentchangeinapartsubjectedtocyclingstressesandstrains.Whileatfirstnonoticeabledamagemayappear,overtimeandwithcontinuedstresses,partsmaybegintofail.Forinstance,consideradischargechuteonalawnmower.Asyouusethemower,thechuteoccasionallybumpsintoatreeorwall.Whileatfirstthereisnoapparentdamage,afterseveralyears,youwillnoticethatthechutehascracked.

Typically,fatiguetestsconsistofrepeatedlyputtingasampleundertension.Generatedresultsshowamaterial’sabilitytoenduretheserepeatedloads.

Material

Steel

Composite RIM

Nylon GF*

Polycarbonate GF*

Baydur XGT-GF

Polycarbonate

ABS

Nylon

Polyester

Baydur Structural

Foam

Polyurethane

ElastomericRIM Unfilled

* glass-filled resins

in/in/°Fx10-6

6

8

13

17

28

39

44

45

60

65

70

78

Coefficients of Linear Thermal Expansion (CLTE) for Common Materials

Table 2-1WARPAGE IN PART DESIGN

Warpagehasmanycauses,includingunevenmoldandpartcooling,incorrectpositioningofinserts,unfavorablepartgeometryandforcescausedbyincorrectstackingbeforeaparthasfullycured.Asadesigner,youshouldbeawareofthepotentialforpartwarpageearlyinthedesignprocess.

PlasticshavesignificantlyhigherCLTEsthanmetals,amajorconsiderationifyouaredesigningapartwithstructuralmetalinserts.Pleaserefertothesectiononmetalstiffeninginsertsinthisbrochureformoreinformation.

Warpageismorenoticeableinflatpartsthaninthosewithmorecomplexgeometries.Table2–1liststypicalcoefficientsofthermalexpansion.

Pleaseseethesectiononbackmoldingonthenextpageformoreinformationonhowtohelpavoidwarpagewithdissimilarmaterials.

CREEP CONSIDERATIONS

Allmaterialsshowacertainamountofirreversibledeformationunderlong–termload,knownascreep.Polymer–chainmovementunderstresscausescreepinpolyurethanematerials.Creepisusuallymeasuredintensionorflexure,withmeasurementstakenatseveraldifferenttemperaturesandatdifferentloads.

30

BACK MOLDING

OneuniquedesignfeatureofRIMsystemsisthatothermaterialtypes,suchasvinyl,metal,glass,polycarbonate,acrylicandotherscanbeplacedinthemoldpriortomolding.Polyurethanematerialswillthenmoldagainstthissecondmaterial.WhenusingRIMsystemsforbackmolding,one–sidedmoldingagainstadifferentmaterial,somewarpingcanresult,evenwhentheCLTEsaresimilar.Whilemostmethodstoaddresswarpageareapplicationspecific,considerthesegeneralsuggestions:

•Placeadditionalinsertstobalancethecentersofgravity(seefigure2–25).

•Adjustthecoolingsysteminthemoldingprocess.

•Modifydesignofthesubstrate.

•Createspecializedjigsforpost–curing.

Forinstance,becauseofitsdifferentrateofheatconduction,ametalsheet-suchasthoseusedintheproductionofsnowskis-maycausedifferentskinthicknessesontheoppositesidesofthepartmadeofstructuralfoam(seefigure2–33).Differentskinthicknessesexhibitdifferentshrinkagebehavior,possiblyleadingtowarping.Insertingsteelrodsjustundertheskinonthesideoppositeofthesheetmetalcanhelpcompensateforthiswarpage.ContactyourBaySystemsrepresentativeformoreinformationtoaddressthistopic.

Modulus Retention for Baydur Structural Foam

10x Hours

Flex

ural

Modu

lus

Rete

ntio

nUnd

er C

ont

inuo

s Lo

adin

g (%

)

1000 Hours

1 Year

10 years

20 Years

23°C

10-2 10-1 100 101 102 103 104 105 106

1009080706050

40

30

20

10

1Y 10Y20Y

Figure 2-32

Figure 2-33Waterski

Steel Rods

Thin Skin

Thick Skin

ScreenedGraphics May

Be Added

Foam, Back Moldedto an Aluminum Top

Chapter3

SOLID MATERIALS

31

Filled Bayflex and rigid PRISM systems can have high flexural moduli, making them a good choice for thin–walled walled applications. Because of their excellent impact properties, flexibility, toughness and ductility, elastomeric solid materials find many uses in automotive panels and bumper fascias. They also have excellent resistance to scratching and tearing. Rigid, solid materials are good choices for business machines, electronic and medical housings, load–bearing applications, appliances and consumer–product housings. Typically, parts made of solid polyurethane materials incorporate many of the same design principles as those made of thermoplastic resins.

WALL THICKNESS

Partsmadeofsolidpolyurethanematerialshavesimilarwallthicknessestothosemadeofthermoplasticmaterials(1/16to1/4inch).Additionally,RIMpartscanhavewallsasthickas1–1/2inches.Awallthicknessof1/8inchforsolidPRISMsystems,or1/4inchforBaydurGSsystems,istypicalforpartsthatneedaUL94VOand5Vrating.Pleasenotethatflammabilityresultsarebaseduponsmall–scalelaboratorytestsforcomparisonpurposesonlyanddonotnecessarilyrepresentthehazardpresentedbythisoranyothermaterialunderactualfireconditions.

RIB DESIGN AND CONFIGURATION

Ifyourpartrequiresribs,usethefollowingrulesofthumbwithsolidsystems:

•Forsolidmaterialsthethicknessattheribroot-includingbothsidesofradii-shouldnotexceed75%ofthenominalwallthicknessforpartsrequiringashowsurface(seefigure3-1).

•Fornon–aestheticapplications,youcandesignthickerribs,uptothenominalwallthickness.Yourpartmaydevelopsinkmarkscausingvisualblemishesonthesurfaceoppositetherib(seefigure2–7).Thesesinkmarksappearwheretheprotrusionandmatingwallmeetbecausethelocallyincreasedwallthicknessleadstoincreasedshrinkingasthepartcools.

Figure 3-1Rib/Wall Ratio

for Solids

t

s

s ≤ 0.75tt= Nominal Wall

32

RADII/FILLETS

Radiustheinnercornersofallribs,bossesandwallsatleast1/16inchtoreducestressconcentrationsandhelpavoidairentrapment(seefigure3–2).Outsidecornersarenotassusceptibletostressesandmaynotneedradii.

BOSSES

Whendesigningbossesinpartsmadeofsolidpolyurethanematerials,allowawallthicknessequalto75%ofthepart’snominalwallthicknessaroundthecoredhole(seefigure3–3).Holedepthshouldallowforamaximumofonenominalwallthicknessofmaterialatthebottom.Followrib–designsuggestionsforradiianddraftangles.

Forthediameterofthebosshole,refertothespecificationsoftheinsertorscrewmanufacturer.Forexample,insertsgreaterthanoneinchindiameterhavebeensuccessfullyusedinthefield.

Figure 3-2 Correct Radii/ Fillets for Solids

R1/16”

R1/16”

R1/16”

Figure 3-3 Boss Dimensions

0.75t (max)

t

Chapter4

FOAMED MATERIALS

33

Baydur structural foamed systems offer excellent strength–to–weight characteristics, because of their sandwich–like structure. These polyurethane materials are found in many large parts, such as electronic and business–machine housings, cab roofs, consoles, cabinets and shelves. Additionally, structural foamed materials are used extensively in aquatic sports equipment, such as skis and knee boards, because their density is lower than water, which allows them to float. They also offer designers more latitude than other materials: wall thickness can be varied; sink marks from ribs and bosses are less common.

While Bayflex flexible foams are not used in structural applications and are not subject to the same design restrictions, they offer some unique capabilities for designers. For instance, because these materials have high compressibility, you can design small undercuts without slides. For more information on design parameters, contact your BaySystems representative.

FOAM RISE AND FLOW

Theliquidsusedinfoamedsystemsfillapercentageofthecavity’svolume,dependinguponthefinalpartdensityrequired.Thelevelofliquidplacedinthemoldpriortoexpansionisreferredtoasthe“liquid level.”Theremainingvolumeisfilledastheliquidsreact,creatingfoam.Foamexpandsupwardandoutward,aprocesscalledfoam rise.Foamdensitycanrangefrom0.3to1.0g/cm3.Mostpartshaveafoamdensitybetween0.6and0.8g/cm3.

Whilefreonwasusedasablowingagentformanyyears,allBaySystems’sRIMpolyurethanesystemsareCFC–andHCFC–freetoday.Growingenvironmentalconcerns,coupledwithadvancesinwater–blownsystems,haveledtoalternatives.

WALL THICKNESS

Wallthicknessplaysanimportantroleindesigningpartsmadeoffoamedmaterials.Baydurstructuralfoamhasbeenusedinpartswithlocalizedwallthicknessesasthinas1/8inchtoasthickasover1inch,althoughthetypicalnominalwallthicknessrangesfrom1/4to1/2inch.Otherrulesofthumbinclude:

•Increasethewallthicknessandreducethepartdensitytomaintainpartweightandoptimizepartstiffness.

The flow properties of BaySystems foams allow you to mold parts of varying thicknesses at different points in the part – to meet your needs to enhance strength, increase sound attenuating properties or reduce weight at any area in the part, as you can see in this complex truck floor mat.

34

•Avoidhavingwall–thicknessincreasesattheendoffoamrise,nearthevents.Theincreasingviscosityandresultingdropinflow–abilitycanleadtoairentrapment,badknit–linesandinsufficientpacking.

•Keeptheratioofwall–thicknesschangebelowafactoroftwo,ifpossible.Whilesomeaward–winningapplicationshavedeviatedfromthisruleofthumb,thedifferentialfrictioninthecrosssectionscancauseracetracking,leadingtoventingproblemsandsurfaceimperfections(seefigure2–4).

•Keepthelargerthicknesschangesundertheliquidlevel.

•Avoidexcessivelythickcrosssectionsastheycauselongdemoldingtimes.Considerusingaspace–fillinginserttohelpfillthickcrosssections.

RIB DESIGN AND CONFIGURATION

Whendesigningribsforpartsmadeoffoamedmaterials,therib–rootthicknessincludingbothsidesofradiishouldnotbegreaterthan100%ofthenominalwallthicknesstohelpavoidsinkmarks(seefigure4–1).Whenaestheticsarenotaprimaryconcern,youcandesignribsthickerthanthenominalwallthickness.Otherconsiderationsinclude:

•Locateribsinthedirectionoffoamriseandflow.Ifthisisnotpossible,provideforventingtopreventairentrapment.

Formoreinformationonribbingdirectionandconfigurations,pleaserefertothegeneraldesignconsiderationsinthismanual.

Figure 4-1Rib/Wall Ratio for Foamed Systems

t

s ≤ 0.75tt= Nominal Wall

s

Figure 4-2 Effect of Radius on Skin Formation

Correct Incorrect

R 1/8”

Poor Skin Formation on Inner Corners

Chapter4

FOAMED MATERIALS

35

RADII/FILLETS

Radiustheinnercornersofallfilletsandadjacentwallsaminimumof1/8inchtoreducestressconcentrations,promotegoodskinformationandhelpavoidairentrapment(seefigure4–2).Insidecornersonpartsaremoredifficulttocoolthanlarge,flatareas.Thistemperaturedifferencecanleadtopoorornoskinformationinsharpinnercorners,resultingininferiormechanicalproperties.Outsidecornersarenotassusceptibletopoorskinformationandwillformthickskins.Otherrulesofthumbinclude:

•Donotmakesharptransitions.Radiuscorners1/8inchtoallowforproperskinformation.

•RadiiareextremelyimportantforpartsmadeoffoamedrigidsystemsbecausetheyaregenerallymorenotchsensitivethanpartsmadeofBayflexsolidsystems.

BOSSES

Bossesfacilitatemechanicalassemblyoractassupportsorspacers.Connectthemtothepart’souterwallsordesignthemwithgussets(seefigure4–3).Whenbossesareusedtoaccommodatescrewsorinserts,considerthefollowingsuggestions:

•Maketheminimumwallthickness3/16incharoundtheholeifaninsertisused.

•Designbosses,includingradii,withawallthicknessnogreaterthanthenominalwallthicknessaroundthecoredhole(seefigure4–4).

Figure 4-3Boss Versus Flow Direction

FLOW

Figure 4-4 Boss Dimensions

t (max)

t

•Corebosseswhenusingself–tappingscrewsandinsertstoformskininthehole(seefigure4–5).

•Radiusallbosses1/8inchattheirbases.

Forthediameterofthebosshole,refertothespecificationsoftheinsertorscrewmanufacturer.Forexample,insertsgreaterthanoneinchindiameterhavebeensuccessfullyusedinthefield.

36

STRUCTURAL ANALYSIS CONSIDERATIONS

Becausethemechanicalpropertiesarenotuniformthroughoutatypicalcrosssectioninanon–isotropicmaterial,theavailablephysical–propertydatamaynotallowyoutocalculatereasonabledeflections.Combiningpropertiesobtainedfromdifferenttestingmethodsisrequiredforpartsmadeofstructuralfoam.Forexample,flexuralmodulushelpstopredictdeflectiononhorizontalsurfaces(perpendiculartotheloaddirection)andtensilemodulusforverticalsurfaces(paralleltoloaddirection).Partswithcomplexgeometriesmayrequireusingbothflexuralandtensilemodulitohelppredictreal–worldbehavior.

Figure 4-5Cored Versus

Drilled Bosses

Preferred Cored Hole(With Skin)

Drilled Hole(No Skin)Finite–elementanalysiscanuse

thesevaluestoestimatethepart’sdisplacementfield.Thistypeofanalysiswillcomplementprototypetesting,butshouldnotbesubstitutedforprototypetestingunderactual,end–useconditions.FormoreinformationonanalyzingBaydurstructuralfoam,requestacopyofThe Performance of RIM Structural Foam in Load– Bearing ApplicationsfromBaySystems.

Chapter5

COMPOSITE MATERIALS

37

Figure 5-1Flexural Properties Versus Percent/Type of Glass for Baydur STR/C

Glass (%)

Flex

ural

Modu

lus

(psi

)

Bidirec

tional F

iber

Random Mat

Flexural Strength20 30 40 50

3M

2M

1M

100

75

50

E103 psi

Flexural Strength

Flexura

l Modulus

Bidirectional Fiber

Random Mat

Composite materials offer a unique benefit to part designers: you can place localized additions of glass mat within the mold to strengthen higher–stressed areas. This flexibility allows for enhanced material properties. Load–bearing parts made of Baydur structural RIM materials offer excellent impact strength and high flexural modulus.

GLASS MAT

Compositematerials,madeofapolyurethanesystemreinforcedwithglass–matfiber,haveanextremelyhighflexuralmodulusthatdependsuponthefiberglasscontent(measuredbyweight),itslocationandthedirectionofthematfibers.Asthepercentageofglassincreases,theflexuralmodulusofthepartincreases.Thehighestmodulusattainedthusfarisapproximately3.0millionpsiinapartmadeofBaydurSTR/Ccompositewith60%glassinseverallayersofbidirectionalmat.At30%glass,aflexuralmodulusof1.2millionpsiwithrandom–fibermatand1.5millionpsiwithbidirectionalmatare

notuncommon.Theflexuralmodulusandpartstiffnessaregreaterinthedirectionoftheglassfiber.

Glassmatsareavailableinrandomordirectionalconfigurations,withvariousdiameterfibers.Glassmatsarefittedinthemoldpriortoinjection.Resistancetoflowincreasesasthepercentageofglassmatincreases.Thisresistancetoflow,causingbackpressure,canmakemoldfillingdifficult.Alwayscheckthematerialsafetydatasheets(MSDS)andconsultwithyourglass–matsupplierforsafe–handlingrecommendationsfortheirproducts.

(Opposite) These exterior spa panels molded from a BaySystems Baydur® STR polyurethane system offer a strong, lightweight alternative to traditional wood panels. They are dimensionally stable and offer excellent heat, chemical and water resistance sa well as enduring beauty.

38

Ifyourpartneedstobestiffandlight-weight,considerusingafoamedcompositewithmoreofasandwich–likeconstruction.Thistypeofcompositefeaturesglassfibersclosetothesurface,creatingaless–densecrosssection.

Whentheprocessorisusingcompositematerials,heshouldfollowtheserulesofthumb:

•Avoidspecifyingmorethan50%glassinanyarea.Atthishighpercentage,bulkyglasscanbedifficulttocompressinthemoldandmaybedifficulttofill.

•Extendthemattothemoldedge.Under–sizingmatsmayleadtolowglassareas-calledresin–richareas-aroundtheperipheryandcreateapreferentialpathformaterialflow.Theseresin–richareasareweakerunderloadandexhibitmore–brittlebehaviorthansectionsreinforcedwithglassmat.

•Useathickerwallorahigherpercentageofglasstoincreasestiffness.Figure5–1showsmodulusandflexuralstrengthasafunctionofthepercentageandtypeofglass.

•Makesurethatlocalizedglassadditionscanbe“wettedout”-thoroughlysaturatedwithliquidpolyurethanematerial-tominimizedryandunfilledareas.

Figure 5-2Corrugations and Box Beams

Low DensityFoam Spacer

Figure 5-3Continuous Integral Beam Assembly

Glass Preform

Glass Mat

Glass Preform

Low DensityFoam Spacer

Chapter5

COMPOSITE MATERIALS

39

•Positiongatesdirectlyintothehighestpercentageofglasswheneverpossible.Themixtureshouldflowfromthehigher–densityglasstothelower–densityglasstominimizedryareas.

REINFORCEMENTS

DonotdesignribsinpartsmadeofBaydurcompositesystems,astheyaredifficulttomold.Alternatively,considerdesigningincorrugationsand/orboxbeams(seefigure5–2).Tomakeaboxbeam,glassmatisplacedaroundalow–density,preformedcoreorspace–fillinginsert(seefigure5–3).ThestructuralRIMsystemismoldedaroundittoproduceacontinuousintegralbeam.Thesizeofthecross–sectionalareaofthebeam,alongwiththewallthicknessandpercentageofglasswilldeterminetheoverallstiffnessofthebox.

RADII/FILLETS

Whendesigningcorners,allowforaninnerradiusorfilletof1/8inchminimumtoallowforcontinuousglasstransition(seefigure5–4).

Figure 5-4Radii/Fillet

Configuration

R

R

R

R ≥ 1/8”

Figure 5-5 Hollow Bosses and Pads for Mounting

HolePostdrilled

HolePostdrilled

HollowBoss

Pad �

Chamfer � ≥ 45°

PADS

Whenworkingwithcompositematerials,usepadsorhollowbossesforassembly.Chamfertheendsofthepadsforbettermoldfilling(seefigure5–5).

PREFORMS

Preformsarerequiredforcomplexpartswithgeometriesthatcannotbemoldedwithastandardglassmat.TheyofferincreaseddesignflexibilityforstructuralRIMparts,particularlyinlargeproductionruns.Typicallypreformsaremadeofglassfibersthatareheldtogetherbyathermoplasticorthermosettingbinderinoneoftwoways:

•Compression–molded preformsuseaglassmattreatedwithabinder.Thematiscompressedintothedesiredshapeinaheatedmold.Asthebindercoolsorcures,itgluesthefibersintoshape.

•Spray–up preforms involvesprayingchoppedglassandthebinderontoaperforated,positiveform.Avacuumontheoppositesidedrawsthechoppedglassontotheform,creatingarandom,spray–uppattern.Thistypeofpreformcanaccommodatemorecomplexshapes,butmaybemorecostly.

40

FINISHES

StructuralRIMmaterialscanaccommodatemanysurfacefinishes,fromtexturesto“classA.”Forvisiblepartsrequiringa“classA”finish,suchasinautomotiveapplications,specifyapolishedmoldandsurface–veilmaterial.Aveilisaverythinmatmadeofthinfibersthatwillhelpkeeptheglassfibersfromprotrudingthroughthemoldedpartsurface.

In–moldcoatingispossiblewithBaydurSTRsystems.Tocreateagoodbondbetweenthecoatingandglassmatrix,allowthematerialtomoldagainstapartiallycuredcoating.Formoreinformationonin–moldcoating,pleasecontactyourBaySystemsrepresentative.Thenextchapteronpostmoldingoperationsprovidesmoreinformationonfinishing.

A roof module molded from a Baydur® LFT polyurethane system reduces weight and adds structural strength to the body of this 2007 Opel Zafira.

Chapter6

POSTMOLDING OPERATIONS

41

Most parts require postmolding operations, such as painting and assembling. These various operations are discussed in this section.

FINISHING

Partsmadeofpolyurethanematerialsofferdesignersmanyoptionsforcolor,textureandotherfinishingconsiderations.Formoreinformationonanyoftheseoptions,contactyourBaySystemsrepresentative.

Pigmentation

Pigments-organicdyesorcolorpastesaddedtothepolyol-changethenaturalcolorofpolyurethanematerials.Polyurethanesarelighttomediumopaquebrownwhenmoldedwithoutcolor.Notinherently

UV–stable,theywilleventuallyyelloworgainagreenishtintwhenexposedtosunlight.Whilethisdiscolorationdoesnotaffectphysicalproperties,itisusuallyaestheticallyobjectionable.Theamountofpigmentaddedtothesystemisexpressedasaweightpercentageofthepolyol,usuallyrangingfrom3to10%,ifyourpartwillbepainted.

Considerspecifyingpigmentstogiveyourfinalpartabasecolorsimilartothefinalsurfacecoating.Ahigherpigmentconcentrationisneededifyourfinalpartisgoingtobeadarkcolorandyoudon’twanttopaintit.

A lighter grade of steel encapsulated with a BaySystems Bayflex® polyurethane system produces a ladder rack that is lighter yet stronger than the former all-steel model. It’s also more chemical and corrosion resistant and adds a certain style and flair to the truck’s appearance.

42

MostrigidRIMsystemscannotbeusedtoproduceUV–stable,whiteorlight–coloredpartswithoutpainting.Pigmentsalsowillnothidesurfaceimperfectionsandcancausecolorstriations.

ln–Mold Coatings

Considerin–moldcoatings-specialpaintssprayedontomoldsurfaces-asanalternativetopostmoldpainting.Afterspraying,thesepaintsdryforabriefperiod,sothattheinjectedmixtureflowsoverthesemi–drycoatingduringmoldfilling.Typicallyin–moldpaintingisusedforlarge,relativelysimplemolds,withoutcomplexdetails,suchasagricultural–combinecabroofsandfenders.Otherpointstoconsiderwhenselectinganin–moldcoatinginclude:

•In–moldcoatingscansavethecostofpostmoldpainting.

•Moldsurfacecannothaveimperfections,aseverydetailisreproduced.

•In–moldcoatingcanreducemostsecondary,finishingoperations.

Patching

Occasionally,moldedpartshaveairpockets,lower–densityareasandothersmallimperfectionsthatmayneedtoberepaired.Ifyourpartwillultimatelybepainted,apatchingcompoundcan

filltheseareas.

Smallerareascanuseasingle–component,commerciallyavailablepatchingcompound;largerareaswillneedatwo–componentpolyestercompound,suchasthoseusedinautobodyrepair.Forgoodpatchadhesion,makesuretheareaisclean,freeofmold–releaseagentsordustandroughened.Afterthepatchhascured,sandtheareauntilsmoothpriortopainting.

Postmold Painting

Whilemorecostlythanpigmentation,postmoldpaintingofferstheaddedbenefitofexactcolormatchingtootherpartsandpartsmadeofothermaterials.Postmoldpaintingcoversminorsurfaceblemishesandallowssimilarpartstobepaintedindifferentcolors.Forexample,abumpermadeofpolyurethanemustmatchthesteelsidepanelsonthecartowhichitwillbeattached.However,thesamebumperdesignisusedonmanydifferent–coloredcars.Postmoldpaintingaccommodatesbothofthesedesignparameters.

Textures

Polyurethanemoldingtechniquesaccommodateanumberofdifferenttextures,includingwoodandleathergrain,pebbleandgraphics.Forwoodandleathergrainorotherfinetextures,anickel–shellmoldcanbeused.Thismaterialishard,hasgoodreleasecharacteristics,reproducestextureswellanddoesnotscratch

easily.Whilemoldsaccommodatecustomdesignedfinisheswithease,patternpreparationcanbeexpensive.

Ifconsideringawood-grainfinish,themoldmustnothaveanynicksorscratches.Blemishesonthemoldsurfacewillappearinthefinishedpart’sgrainandwood-surfacefinishescannotberetouchedaftermolding.Tocompleteawood-graineffect,thegrainisstainedinadarkercolorandthewholepartiscoveredwithaprotectivecoat.

Pebblesurfacescanbeformedinthemoldoraddedlaterwithacoatoftexturepaint.

Graphicscanbemoldedin.Simplemaskingwillallowraisedgraphicsandletteringtobepaintedinacontrastingcolor.Fordenseareasoftextorsmallletters,considerusingdecals.

Decals and Silk–Screening

Decalsworkwellwithpolyurethaneparts,aslongastheadhesionareahasnotextureandiscleanandfreeofanyreleaseagents.Whiledecalsadherewelltopaintedparts,theywill

Chapter6

POSTMOLDING OPERATIONS

43

besomewhateasiertoremovethanthoseapplieddirectlytounpaintedparts.Polyurethanepartscanalsobesilk-screened.Contactyourprintertodiscussyourneeds.

ASSEMBLY OPERATIONS

Polyurethanepartsthatrequireassemblycanbejoinedwithscrews,adhesivebondingandnailing.Thissectiongivesanoverviewofsomeofthesecommonjoiningmethods.

Screws

Oneofthemostcost-effective,reliableandcommonlyusedjoiningmethods,self-tappingorwoodscrewscanbeusedtoassemblepartsmadeofRIMpolyurethanesystems.Toinstallscrews,youcaneitherdrillormoldaholeinthepart.Ifyouchosetopostdrillahole,makeitslightlysmallerthanthescrewdiameter,asyouwouldwithwood.BothmethodsyieldrelativelyhighpulloutstrengthinpartsmadeofBaydurstructuralfoam.

•Generally,thepulloutstrengthis

proportionaltothescrewdepth.

•Molded-inpilotholeswillyieldhigherpulloutstrengththanpostdrilledholesforfoamedmaterials.

•Partsmadeofstructuralfoamedmaterialsrespondsimilarlytowoodinmostjoiningtechniques.

•Partsmadeofelastomericmaterialscanbejoinedwithscrews,butthematerialcantearorstretcharoundthehole.Testyourassembledpart.

A Mack Truck fender cover molded from a colorfast BaySystems polyurethane system offers many years of

tough, corrosion and dent resistant service.

44

InfoamedBaydursystems,partswithlowerdensitymakeinstallingscrewseasier.Themost-commonfoamdensityforscrewinstallationrangesfrom25to40lb/ft3(0.4to0.65g/cm3),roughlycomparablet0thatofwood.Whileseveralscrewsdesignedspecificallyforplasticareavailable,normalwoodorsheet-metalscrewscanbeusedwithfoamedparts(seefigure6-1).

•ForpartsmadeofBaydurstructuralfoam,usethread-cuttingorthread–formingscrews.Thread-formingscrewsmayleavehigherinternalstressesclosetothethread.

•ForpartsmadeofPRISMpolyurethane,usethread-cuttingscrews.

Screwscanbeinsertedwithoutapilotholeintopartsmadeoflow-densityfoam.Usecautionwheninstallingscrews,asthedangerofstrippingorfoambreakoutincreasesasthedensitydecreases.Specifyscrewsasapermanentattachmentmethodonpartsmadeoflow-densityfoam.Ifyourlow-densitypartwillbedisassembledwithanyfrequency,consideranotherjoiningmethod,suchasthreadedinserts.Formoreinformationoninserts,seethegeneralpartdesignsectioninthismanualorrequestacopyofBaySystems’sPlastics: Joining Techniques designguide.

Figure 6-1

Foam Density (pcf)

Pull

out

Forc

e (lb)

#12 Meta

l Scre

w

#10 M

etal S

crew

#8 Wood

Screw

Polyurethane Systems Classified by Flexural Modulus

300

200

100

10 20 30 40 50

Adhesives

PolyurethaneorepoxyadhesivesworkwellwithRIMpolyurethanesystems.Theadhesionareainalapjointshouldbeatleastthreetimesthewallthickness.Bondscanhavehighstrengthinbothtensionandbending.Cleanandroughentheadhesionareastopromotegoodbonding.Formoreinformationonadhesives,contactoneofthefollowingproducers:

3M Industrial SpecialtiesSt.Paul,MN55144612133-1110

Ciba-Geigy CorporationEastLansing,MI48823800875-1363

Loctite CorporationNewington,CT06111203278-1280

Lord CorporationErie,PA16541814868-3611

Ashland ChemicalsColumbus,OH43216614889-3639

Chapter6

POSTMOLDING OPERATIONS

45

POSTFABRICATION

NaiIing/StapIing/PIaning ManystandardwoodworkingtechniquescanbeusedwithBaydurstructuralfoam,includingsawing,drilling,nailing,stapling,sandingandrouting.Ifyourpartwillbefabricatedviaoneofthesemethods,designforcommonwoodworkingtechniques.Avoidthesetechniquesformass-producedproductsastheyarecrudeandlabor-intensive.

Donotplanefoamedpolyurethaneparts.Planingwillcauseskinlossandpossibleexposureoffoamcore,withtheresultinglossinstructuralintegrity,aswellasphysicalandmechanicalproperties.

Recycling PoIyurethanes

Whendesigningapart,considerdesignfordisassembly(DFD),aconceptthatisgainingemphasisbecauseofrecycling.

Becauseofrecentadvances,severalmethodscanbeusedtorecyclepolyurethanematerials,dependinguponthetypeofmaterial.Mostpolyurethaneresinscanbegranulatedandgroundintopowderforuseasafillerinnewparts.Theamountoffillerthatcanbeusedwillbebaseduponyourfinalpartrequirements.Granulatedelastomericcontinuedmaterialcanalsobecompressionmoldedunderhighpressureandtemperaturetoproducenewparts.Partsmadethiswaymayretaintheiroriginalelongationandover50%oftheirtensilestrength.

Glycolysis,anewwaytoconvertpolyurethanematerialsbacktotheiroriginalrawmaterials,isalsoshowinggreatpromise.Polyurethanematerialscanbeconvertedintoenergy:theheatofcombustionforRIMpolyurethanematerialsisbetween12,000and15,000BTUsperpound,approximatelythesameasoilorcoal.TalktoyourBaySystemsrepresentativeforthelatestinformationonpolyurethanerecycling.

Roof shakes molded from a BaySystems polyurethane technology offer the texture, color and beauty of

real wood without wood’s inherent susceptibility to rot.

46

Introduction

MOLD DESIGN

47

Tomakegoodpolyurethaneparts,youmusthaveagoodmold.Acorrectlydesignedmoldisthesinglemostimportantfactoringainingmaximumproductivity,uniformpartqualityandtrouble-freeproduction(seefigureM-1).Improvementsingate,mixingheadandaftermixerdesignsarecontinuingtoaddtopartqualityanduniformity.Moldsaresometimesreferredtoas“tools.”Inthismanual,weuse“tools”and“molds”interchangeably.

Thissectionofthemanualprovidesguidelinestohelpyousuccessfullydesignandbuildmolds,offeringsomepracticalrulesofthumb.Itbeginswithadiscussionofgeneralmold-designparameters,followedbysuggestionsforgateandparting-linepositioning,molddetails,finishingandspecialtools.Usetheinformationpresentedhereinasgeneralguidelines.Yourmoldmakerisresponsibleforproducingafunctionalmold.ContactyourBaySystemsrepresentativeforinformationonyourspecificmold.

LifeTime Composites of Carlsbad, CA, produces LifeTime Lumber® for decking and fencing — a great “made of polyurethane” alternative to natural wood that will not rot, warp, discolor or mildew and can be sawed, drilled and worked with regular

woodworking tools. And of course, LifeTime Lumber saves trees.

48

Chapter7

GENERAL MOLD DESIGN CONSIDERATIONS

49

When designing molds for use with RIM polyurethane systems, you must address several issues, including mold size and cost, clamping pressures, part shrinkage, dimensional tolerances and part repeatability. These general considerations are discussed in this section.

PART SIZE/CLAMPING PRESSURE

WithRIMmaterials,therearenouppersizelimitationsonparts,otherthanequipmentcapabilities.Becauseofmetering-machinecapabilities,theminimumpartweightorshotsizeisapproximately0.5pounds(225grams).Forexample,thisweightroughlycorrelatestoa6–inchsquarewithathicknessof3/8inchatadensityof64lb/ft3.Whilespecialtymachinestomakesmallpartsareavailable,typicalgatingandaftermixerrequirementswouldcausetoomuchwastetomakesmallerpartspracticaloreconomical.

RIMpolyurethanesareidealforlargeparts,withpartsheavierthan100poundshavingbeenmade.Ifonemeteringmachinecannotfillthemoldfastenough,intheorytwoormorecanbeconnectedtoamold.Becauseofpracticalpress-sizeandotherequipmentlimitations,considerredesigningpartsthatexceed50poundsintosmallercomponentsforlaterassembly.Formodulardesigns,twoormorecomponentsofan

50

assemblymaybeproducedfromthesamemold.Practicallimitationsonpartsizeinclude:

•Thecapacityofthemeteringmachinesandmixingheads

•Presscapacityandclampingpressures,ifself-containmentisn’tpossible

•Gel-timeandcream-timelimitations

Table7-1liststypicalmoldingpressuresforRIMsystems.Noticemoldingpressuresaremorethananorderofmagnitudesmallerthanthoseusedinthermoplasticinjectionmolding.Makesurethatclampingpressuresforyourpart’sprojectedareaexceedthemoldingpressures.

MOLD COSTS

Becauseoflowerin-moldpressuresRIMsystemsusemoldsthatarelessexpensivethanconventionalinjectionmolds.Low-pressureRIMsystemscanbemoldedinsoftermold-constructionmaterialswhichareeasiertomachine.Table1-2showsacomparisonofthecostsofdifferentmaterialsandfabricationtechniquesforamoldtomakeasimplepart.

Tofurtherreducemoldcosts,simplifypartdesignandavoidundercutsandotherelementsthataddsignificantlytomoldandpostmoldingcosts.Themold’scomplexityandconstructionmaterialsdeterminethetotalmold–makingcostandconsequentlyalargeshareoftheeventualfinished-partcost.

Otherfactorsthatinfluencemoldcostinclude:

•Numberandtypeofhydraulicslides

•Numberandtypeofdifferentsurfacefinishes

•Partdepthandcomplexity

•Parttolerances

Whendesigningmolds,trytoweighthemoldcostagainsttheproductionvolumeandthecostofpostmoldinglaborneededtofinishthepart.Postmoldingoperations-suchastrimming,drilling,bonding,sandingandpainting-caladdsignificantcosttoapart.Designingamorecomplexmoldmayreduceoverallcycletimeandpostmoldinglabor.Whilethemoldmaycostmoreinitially,itcouldsavemoneyovertheproductionlifeofthepart,justifyingthehigherinitialexpense.Generally,moldsforrelativelyflatpartswithaminimumdrawandwithoutundercutsorspecialsurfacetreatmentshavethelowestcosts.

Finally,letyourmoldmakerknowwhichdimensionsarecriticalandwhichhaveloosertolerances.Prioritizethemfrommostcriticaltonominal.Specifieddimensionscaninfluencequotesfrommoldmakers.Usingstandardfractionalinchescanbelessexpensivebecauseamoldmakercanusestandardmachiningtools.Specifyingdecimal-numericalformatswithhighprecisioncouldsignificantlyincreasemoldcost.

System

Baydur Structural

Foam

Bayflex Elastomeric

Solids

PRISM Rigid Solids

Baydur STR

Composite

Baydur GS

* Based upon projecting part area in direction of draw. Clamping pressure must be greater than molding pressure

TypicalMolding Pressure*

(psi)

100

100

100

200

100

Typical Molding Pressures

Table 7-1

Material/Fabrication Technique

Steel, Machined

Aluminum, Machined

Nickel Shell, Electro-or Vapor-Deposited

Aluminum, Cast

Kirksite, Cast

Zinc, Spray Metal

Epoxy, Cast(Prototyping Only)

RelativeCost %

100

80

70

60

60

40

30

Relative Mold-Cost Comparison

Table 7-2

Chapter7

GENERAL MOLD DESIGN CONSIDERATIONS

51

SHRINKAGE CONSIDERATIONS

Allplastics,includingpolyurethanematerials,shrinkduringcooling.Manyfactorsinfluencetheexactamountofshrinkage.ReviewBaySystemsProductInformationBulletins(PIBs)forestimatedshrinkagepercentagesforyourselectedsystem.Addtheshrinkagedimensionperinchtoeverynominaldimensionofthepartwhendesigningmolds.Forexample,iftheshrinkageofaBaydursystem0.7%andthepartis50incheslong,thenthemold’slengthdimensionshouldbeincreased0.35inchesto50.35.

Also,letyourmoldmakerknowwhichpolyurethanesystemyouhaveselectedearlyinthemold-designingprocess.Differentsystemshavedifferentshrinkagevaluesandthereforerequiredifferentadjustmentstomolddimensionstoproduceyourfinalpart.Changingsystemsoradditivesduringoraftermoldconstructioncanleadtoanincreaseordecreaseinfinalpartdimensions.

DIMENSIONAL TOLERANCES

Anumberofenvironmental,processingandmaterialvariablesdeterminesapart’svariationfromspecifiedsize.Amongthemost-commonvariablesaffectingpart-to-paftreproducibilityare:

•Partdensity

•Moldtemperature

•Moldalignment

•Moldwear

•lnjectionrate

•Demoldingtime

•Componentratiosandtemperatures

•Ambienttemperatureandhumidity

•Inhibitedorunevenpartcuring

•Moldpressure

•Otherproductionconditions

BecauseofCAD/CAMandotheradvances,moldprecisionhasimprovedandshouldnotbeamajorconcern.Generally,expectamoldertoguaranteeadimensionaltoleranceof0.1%orless,butparttolerancescanbegreater.

Forbestresults,discussallmaterialselectionandprocessingparameterswithyourmoldmakerbeforeconstruction,sothatthefinaltoolaccommodatesyourneeds.

Oftenitismoreimportantthatyourpartsfittogetherproperlythanthattheyconformtoabsolutedimensions.Manytimes,moldersmakepartsthatfitwithmatingparts,eventhoughbothmaybeslightlyoutoftolerance.Inmostcases,thissimplematchingisfunctionallysatisfactory.Ifyourpartmustassembleexactlyandbeaccuratetothedrawing,explainthistoyourmoldmakerasearlyaspossible.Tosaveoncosts,adaptyourpartdesigntofitwithinpracticaldimensionaltolerances.

52

Polyurethane systems from BaySystems can be molded into very complex shapes ideal for hard-wearing, weathering applications.

Chapter8

GATE DESIGN

53

Gating design is a major difference between thermoplastic injection molding and RIM polyurethane molding. Gating is the way the low-viscosity liquid transfers from the mixing head to the mold cavity. Typically there is only one gate per mold in polyurethane molding systems. Throughout this manual we use the word “ gating” to refer to the combination of runners, aftermixer and the gate proper. This section discusses common design and placement parameters. For more information about gating placement, contact your BaySystems representative for a copy of our software, RIMgate@. This program helps you determine gate type, placement and size, as well as good runner and aftermixer design.

MIXING HEAD

Acriticalpartofmolding,themixingheadistheareainwhichtheisocyanateandpolyolcombinetoformaliquidpolyurethanematerialjustpriortoenteringthemold.Self-cleaningmixingheadsallowpolyurethanematerialstobeusedinlarge-scale,automatedproductionruns.Mixingheadscomeindifferentsizes,eachwitharangeofflowcapacities(seefigure8-1).Materialflowsthroughopposinginjectornozzlesinthesehigh-pressureheads,usuallyatimpingingpressuresof1,500to3,000psi(10to20MPa).Inthemixingchamber,materialreachesultra-highvelocitypriortoenteringtheaftermixerandmoldcavity.Generally,themaximumoutputofagivenmix

headequalsthreetimesitsminimum.Forexample,a12-mmmixinghead(i.e.,theinsidediameterofthemixingchamberoroutlettube)canbeusedformaterialoutputsbetween0.8and2.4lb/sec(0.4to1.1kg/sec).

Variousequipmentsuppliersmakehigh-pressureimpingementmixingheads.Becausethemixingheadsvaryinsizeandhavedifferentboltpatternstoconnectthemtothemold,theequipmentsuppliershouldhavethenecessarydimensionalinformationforthespecificmixingheadyouareconsidering.Formoreinformationonmixingheads,contactHenneckeMachineryat412777-2000.

54

Makesurethatthemixingheadistightlyfastenedtothemoldduringfilling.Aloose-fittingmixingheadmayallowthehigh-velocitystreamtobringairintothemoldcavity,leadingtodefectiveparts.

Mostcommonly,theheadmountstothesideofthemold,paralleltothepartingline.Mountingonthesideofthemoldgivesyouthegreatestaccesstothemixinghead,aswellasmoreflexibilitywhendeterminingitslocation.Alwaystrytomountmixingheadsonthestationaryhalfofthemoldtominimizehosemovement.Side-mountedmixingheadsareoftentheonlychoiceforapresswithclosedplatensandlimiteddaylight.Usually,partandmolddesignandtheavailablepressdeterminethefinalpositionofthemixinghead.

Mostmixingheadshaveacylindricalsnout,whichcanbeflushwiththemoldsideatthepartingline.Whenpossible,designthemoldsothatthesnoutintrudesintothemoldtocreateanadditionalsealingsurfacewhenthemoldcloses(seefigure8-2).

AFTERMIXERS

Anydeviationinmixqualitycancauseimperfections.Toensurecompletemixing,useanaftermixer.Whilemanyaftermixerdesignsareavailable,the“peanut”aftermixerhasbecomethepreferredchoice(seefigure8-3).BecausethemolddesignercanselectthenumberofV-shapesinthisaftermixerdesign,thepeanutaftermixercomplementsmixheadperfomance.Forinstance,increasingthenumberofV-shapeswillhelpmixdifficult-to-mixmaterials.Additionally,

itcanbeusedwithanyrunnerdiameter.Thisdesigncausesthemixturetochurnasitpassesthroughit,toensurecompletemixing.Afterthemoldingcycle,themoldedpeanutshape(slicedinhalf )visuallyindicatesthequalityofmixing,whichcanhelptodeterminemixingproblems.

Whencuttingapeanutaftermixerinbothmoldhalves,makeidenticalcuttingpatterns,notmirrorimages,topreventblindalleysthatcantrapair.Theaftermixershouldbecutintothemolditselforcutintoaplatewhichinsertsintothemold.Keepitasclosetothemixingheadaspossibletominimizegatewaste.Becauseoffactorsconcerningtemperaturecontrol

Chapter8

GATE DESIGN

55

andsealingproblems,donotusegateblocks,inwhichtheaftermixeriscutintoaseparateblockandexternallyattachedtothemold.Ifyourmoldrequiresaseparateblock,placeheatingchannelsintheblocktocontroltemperature.Toadequatelysealtheaftermixerassembly,theblockshouldbefullysupportedinthemold.

Overtheyears,awidevarietyofaftermixershavebeendeveloped.Althoughmolderstendtohavetheirownpreferences,certaintypesofaftermixershavecausedprocessingproblems.The“harp”aftermixer,forexample,mayhaveblindareaswhichtrapair.Afterfilling,thetrappedairmayexpand,causingbubblesinthestream,leadingtodefectiveparts.

EDGE GATING

AlwaysedgegatemoldsforRIMpolyurethanesystems,unlessyouareusingaBaydurSTRcomposite,whichshouldbecentergatedtopreventtheglassmatfrommoving.Themixtureshouldenterthemoldcavityasalaminarstream,flowingontothemoldwallwhenenteringthemold(seefigure8-4)toavoidairentrapment,thesinglelargestcauseofdefectiveparts.Itshouldnotbedirectedintofreespaceorperpendiculartoawallorotherobstructiontoavoidsplashingandresultingairentrapments.

Therunnerthatisadjacenttothemixing-headexitshouldhavethesamediameterasthemixinghead.Whenreworkingamold,therunnerdiameter

maynotmatchthenewmixing-headdiameter.Topreventbubblesandcavitation,makeasmoothtransitionwithnosharpedges(seefigure8-5).

Thetypeofmaterial(solidorfoamed),mixing-meteringmachineoutputandavailableroomatthepartinglinedeterminethegatedimensions.Todeterminethecorrectgatingconfiguration,youmustknowthehighestexpectedmachineoutput.Thespeedatwhichmaterialentersthemoldmustalsobekeptwithinlimits.

•Theupperlimitsforentrancespeedsare5ft/secforfoamedsystemsand25ft/secforsolidsystems.

Figure 8-4

UniformFlow Front

FlowSplitter

FLOW

Typical Dam Gating

56

•Determininglower-limitentrancevelocitiesisdifficult,becauseofthematerial’sreactivity.

•Iftheinjectionrateistoolowandthegateistoolong,highlyreactivematerialsmaybegintogelbeforereachingthemold.

Gatescanbedesignedaspartofthemoldorattachedasaseparateblock.Again,theseblockscanbedifficulttoseal,especiallywhenthehighestpressuresarelocatedintheblockandaredifficulttoheat.

Fittinggatelengthtothepartperipheryandbalancingflowlengthsgenerallylimitthechoiceofgatelocations.

Considerationsforgateplacementinclude:

•Locategatesatthelowestpossiblepointinthemold.

•Trytolocatethegateonastraightpartinglinesection.

•Trytolocatethegateinapositionthatminimizesthelongestflowlength.

•Positionthegateonnon-aestheticorless-noticeablearea.

•Positionthegatesothatflowwillbeparalleltodesigned-inribs.

•Placethegateclosesttocutoutsormost-detailedsections.Thisallowsknitlinestoformearlyintheflow.

Foamed Systems

Whenusingfoamed,self-skinningmaterials,keepthegatethicknessasthinaspossiblesothatitcanbeeasilyremovedfromthedemoldedpart.Thematerialinathingatewillcuretosolidmaterial.Whenitisremoved,nofoamcoreshouldbeexposed.

Forfoamedsystems,theruleofthumbforgatedimensionsis6inchesofgatelength,withagatethicknessof0.060to0.080inches,foreverypound-per-secondoutput.Maximumstreamvelocityshouldnotexceed5ft/sec.Table8-1showscalculationstodeterminegatelengthstoproducea10poundpartmadeofdifferentpolyurethanesystems.Figures8-6and8-7showcompletegateandrunnerdimensionsfordamgatesusedwithfoamedsystems.

Dam Gates

Adamgateequalizesmaterialflowoveritslength(seefigure8-7).Thegatehasasplittingnosethatdividestherunnerintotwobranchesbehindthedam.Thistriangularconfigurationensuresuniformdistributionacrossthegate.DamgatesarestronglysuggestedforrigidRIMmaterials.Table8-1andfigures8-6and8-7showatypicalcalculationforgatelength.

Usuallygatelengthandthicknessaremutuallyadjustedtokeepentrancevelocityfromexceedingspecifiedlimits.

Solid Foamed Systems Systems

Known Variables

Weight Output, Ow

3 lb/sec 3 lb/sec

Density, D 70 lb/ft3 70 lb/ft3

Maximum Injection Velocity, v 15 ft/sec 5 ft/sec

Gate Thickness, gt 0.1 in 0.08 in

Calculated Variables

Volumetric Output, Ov = O

w /D 0.043ft3/sec 0.043ft3/sec

Minimn Cross-Sectional Area, A = (Ov•144) /v 0.41 in2 1.23 in2

Minimum Gate Length, L = A / gt 4.1 in 15.4 in

Sample Dam Gate Length Calculations for Solid and Foamed Systems

Table 8-1

Chapter8

GATE DESIGN

57

Solid Systems

Whenusingsolidsystems,thegatethicknessmayapproachthepartthickness.Typically,themaximumstreamvelocityshouldnotexceed25ft/sec.Iftherearenosharpcurvesintheflowpathbetweenthegateandmaincavityandifthepartandgate

Figure 8-6Dam Gate

SSSSSSS S

0.75” typ.

Provide Land Area

w

dd = Mixing Head Diameter

R 0.50”(min.) + dR 0.50”(min.)

Recess forMix Head Shout

Provide at Least 5dBetween Aftermixer and

Dam for Quiet ZoneIdentical Pattern forPeanut on Both Halves

Option 2Option 1

Option 10.75” (min.)

A

A

d

L/2

0.125” + (0.5)d

0.125”

gt0.25”

0.50”

Dam Gate

Shutoff

s =

3d 4 w = d

P L P L

Section A-A

1.00”

even spacing

thicknessareequal,streamvelocitiesexceeding100ft/sechaveyieldedacceptableautomotivepartsusingfangates.PRISMsolidrigidpolyurethanesystemscanbeedgegatedordirectfilled.

58

Fan Gates

Thetwomost-commontypesoffangates(seefigure8-8)arethestraightsidedortriangulargateandthepreferredquadraticgate,whichhasaparabolicprofile.Infangates,therunnergraduallyflattensandbendsinthewalldirection.Thetriangulargate’sapexangleshouldnotexceed40°couldgeneratebubbles.Withthisanglelimitation,thingatescanbecomeverylong,particularlyforlargeparts.Longgatingcanleadtoexcessivewaste.

•Asaruleofthumb:practicalmaximumgatelengthis6inches.

•Fangatesarerecommendedforsolidelastomericsystemsthathaveafastcuringtime.

Inpractice,fangatelengthaveragesfourinches.Usuallygatelengthandthicknessaremutuallyadjustedtokeepentrancevelocityfromexceedingspecifiedlimits.Table8-2andfigure8-9showrelationshipsfordeterminingfangatedimensions.

Figure 8-7 Dam Gate Dimensions (Option 1)

A/2 1/2 L

gt

0.125

d/2

A

A

d

1/4”

d = Diameter of Mixing Headgt = Gate ThicknessA = Gate AreaL = Gate Length

Section A-A

Chapter8

GATE DESIGN

59

Figure 8-8Fan Gates

TransitionArea

GateThickness

Gate Length

Depth

RunnerDiameter

GenerousRadius

Cavity

Gate thickness decreases quadratically from thetransition area to the entrance of the cavity

Quadratic Fan Gate

TransitionArea

GateThickness

Gate Length

Depth

RunnerDiameter

GenerousRadius

Cavity

Gate thickness decreases linearly from thetransition area to the entrance of the cavity

Triangular Fan Gate

40°ApexAngle

Ow = Weight Output, lb/sec

Ow = Part Weight/Shot Time

D = Liquid Density, lb/ft3

Ov = Volume Output, ft3/sec

Ov = O

w / D

v = Velocity, 5 ft/sec Foam

25 ft/sec Solid

Af = Final Fan Area = [O

v /v]•144, in2

gl = Gate Length (Specify) Typically, gate length depends on the available space along the parting line at thegate location

gt = Af /gl, in

Ao = Initial Fan Area, in2

Ao = πd2/4

to = d/2

Lo = A

o /t

o

Am = Mid-Fan Area, in2

Am = (A

f + A

o)/2

tm = (t

o + gt)/2

Lm = A

m / t

m

DA = gl - (2πr/4) + r

Calcuation of Typical Fan Gate Dimensions

Table 8-2

60

BALL CHECK

Incaseswherethemixingheadisnotattachedtothemold,useaballchecktopreventthemixturefromrunningoutofthemoldbeforegelling(seefigure8-10).Typically,ballchecksaresuggestedwhenusinghand-heldmixingheads,alsoknownas“handguns,”forfillingmultiplemoldsmountedonacarousel,orwhenmoldsarepositionedinahalf-circle.

Figure 8-9Fan Gate Dimensions

SSSSSSS S

Lo

Lm

DA

tm

to

r

gt

B

gl

P L

0.75” typ.

Provide Land Area

Identical Pattern for Peanuton Both Mold Halves

w

dd = Mixing Head Diameter

Provide at Least 5dBetween Aftermixer

and Dam for Quiet Zone

B

trans R 0.50” + dR 0.50”

Recess forMix Head Shout

P L

C L

Staggered

Section B-B

s =

3d 4 w = d trans = 2.5d

Theball-checkdesignallowsflowinonedirectiononly.Inthisdesign,arubberballcanfreelymoveinachannel,withoneendofthischannelhavingbypassestoallowtheliquidtoflowaroundtheball.Whenfillingends,internalpressurepushestheballtotheotherendofthewidenedsectionwheretherearenobypasses.Theballthensealsthechannel,effectivelypreventingbackflow.

CENTER-GATED DIRECT FILL

WhendesigningmoldsforusewithstructuralRIMmaterials,alwaysplanondirectfill.Centeringthegatepreventstheliquidfrompushingtheglassmatoutofpositionandallowsforuniformflowinalldirections.Inthisfillingmethod,themixingheadattachesdirectlytothemoldwall,creatinganairtightseal.

Chapter8

GATE DESIGN

61

AcentergateisparticularlyimportantinpartsmadeofstructuralRIMsystems,suchasBaydurSTRcomposite,wherebackpressureismorelikely.Forthistypeofsystem,center-gateddirectfilloffersthesimplest,cheapestandmoststraightforwardtypeofgating.Itminimizesflowlengthsandgivesmoreuniformflowinsidethecavity(seefigure8-11).Additionally,withaself-cleaningmixingheadmountedflushinthecavitywall,gatingwasteisminimized,ifnoteliminated.Becausethemixheadislocatedinthecenterofthemold,leakingisalsominimized.

Figure 8-10Ball Check

SealingEdge

Flow

SealingEdge

Bypass

0.5” 1.25”d 0.75”

Bypass

WidenedSection

PL

NoFlow

RunnerDelivery Tube

Tube ExtensionHandgun Mold

Proper Connection of Mixing Head to Mold

Figure 8-11 Direct Fill into the Center of Mold

62

Directfillalsohasdisadvantages:itmaycauseablemishoppositetheentrylocationinthepartbecausethemixturemakesa90degreeturnoverasharpedge(seefigure8-12).Thisextremelyunfavorableflowcouldcausebubblesandscarring.

•Researchshowsthatthemixturewillbebubble-freeonlyifthewallthicknessattheentrypointislessthanone-eighthoftheentrance-areadiameter(seefigure8-13).

Forexample,a16mmentrancediametershouldnothaveawallthicknesslargerthan2mm(0.08in).Becausemostpartsarethickerthanthis,redesignthewallthicknessneartheentrance,narrowingittothisvalue(seefigure8-13).

Ifthemixingheadcannotbeflushmounted,useashortsprue.Keepitasshortaspossible,sothatmoldreleasecanbesprayedintothespruecavity.

Figure 8-12 Gate Marks

Caused by Poor Gate Dimensioning

Figure 8-13 Ratio of Wall Thickness to Mixing-Head Diameter for Direct Fill

2 mm t

d16 mm

Cosmetic Surface

d = Mixing-Head Diametert = Wall Thicknesst ≤ d/8

Figure 8-14Typical Circular Aftermixer

Note: To Be Cut IntoOne Mold Half,

Usually With a Ballmill

Fornoncompositesystems,thecenter-gatepositionmakesusingaftermixersdifficult.Whentheparthascutoutslargeenoughforanaftermixerandedgegate–suchasapictureframe–anaftermixercanbeused.Useapeanutaftermixeroronemadeofconcentricchannelswithastaggered“spoke”designtoprovidealabyrinth.Thisconfigurationsplitsandreimpingesincomingmaterialbeforeitentersthemoldcavity(seefigure8-14).

Chapter9

PARTING-LINE CONSIDERATIONS

63

Many times, part configuration limits parting-line location. However, the position of the parting line directly influences many other mold features, including gate position, mold tilting and venting (see figure 9-1). Preferably, molds should fill from the lowest point to the highest, with the parting line as high as possible to accommodate vents and prevent air entrapment (see figure 9-2). Try to design parts so that a simple two-part mold can be used. If the parting line cannot be located in the highest position in the mold cavity, use a drag plate to create a secondary parting line (see figure 9-3).

Mold makers or BaySystems personnel can help determine venting areas from part drawings or models.

Figure 9-1 Mold in Tiltable Press

Air Entrapments

Gate

Gate

Vent

Vent

Horizon

Titltable Press

Add Gussetsin the Direction of Flow

Parting Line

Incorrect

Correct

FLOW

Figure 9-2 Gate Low, Vent High

Gate

Vent

SteppedPartingLine

64

MOLD SEALING

WhenmoldingpartsmadeofRIMpolyurethanematerials,themoldsmustbeadequatelysealedtoensurepartdensityandminimizeflash(seefigure9-4),theexcessmaterialthatoccasionallyformsalongthepartingline.Sometimesflashisintentionallycreatedinselectareastohelpfillthemoldcavity.Inthesecases,themoldwillhaveadumpwelltocollectexcessmaterial(seefigure9-5).

•Makesurethatflashdoesnotgetintotheknockoutmechanism.Flashcanbindtheknockoutplate,leadingtotorn,deformedparts.

Moldsealsmustbe“liquidtight.”Toachievethisseal,internalmoldpressuresmustnotexceedclampingpressures.Thesealingedgearoundthecavityandtherunnershouldbeassmallaspossibletoreducecontactareaandprovideagoodseal(seetable9-1).

Figure 9-3Secondary Parting Line Via Drag Plate

Vent

DragPlate

PrimaryPartingLine

SecondaryPartingLine

Core

Core

Cavity

Mold Open

Mold Closed

Mold Material

Steel

Aluminum

Kirksite

Nickel Shell

Epoxy

ApproximateSealing Edge

Width(Inches)

1/2

3-4

3-4

1-2

1

Approximate Sealing edge Widths

Table 9-1

Figure 9-4Mold Sealing- Land Area

0.02”-0.04”

0.5”Land Area

Cavity

Chapter9

PARTING-LINE CONSIDERATIONS

65

MOLD VENTING

AllmoldsusedwithRIMpolyurethanesystemsmusthaveventstoensuretheairinthecavitycanescapeduringthefillingprocesses.Newmoldsshouldbeprovidedwithoutvents.Ventscanbecutintothemoldwhereneeded.Initialshotswillindicatewheretheventsarerequired.

•Makeventswideandshallow;notnarrowanddeep.

•Asaruleofthumb,designmoldventsthatare1inchwide,0.010inchdeepand3to4inchesapart,edge-to-edge(seefigure9-6).

Designribsandbossestoallowforairdisplacement.Considerconnectingthemtoapartwallorplacingatiny,taperedholeinorthroughthemoldwalltohelpventing(seefigure9-7).Ifaholeisused,itmustbeaccessiblefromthemoldexteriorsothatitcanbecleanedasneeded.Ifnecessary,useventpinsthatarepartofaknockoutmechanismtoventbosses.Allprotrusionsabovethepartinglinemustbeventedseparately.

Figure 9-5 Mold Dump Well

DumpWell

Figure 9-6 Typical Mold Venting

0.01”1”

Vent

4”

VentLeaderPin Hole

Core

Leader Pin

Typical mold venting using 4-inch spacing. Vents on high side of mold.

66

MOLD FILLING

Fillinglevelsvarywithsystemsandspecifieddensity.Solidsystemsfillthemoldcompletely.Incontrast,foamedsystemsusuallyfill40to80%ofthemoldcavity,dependinguponfinalpartdensityrequired.

Whenplanningforpropermaterialflow,consideranyobstructionsinthemoldcavity,suchascores.Toreducetheeffectofweldorknitlines,liquidpolyurethanesystemsmustflowaroundsuchobstructionsandrejoin.Ifflowfrontsjoinearlyinthefillingprocess,weldlineswillbeunnoticeableandshowverylittle,ifany,lossinproperties.

Weld-lineformationisparticularlyimportantinshort-fiber,reinforcedmaterials,becausefiberstendtoalignwithflowdirection.Wheretheflowfrontsjoin,fibercrossoverandhomogeneitywillnotoccur.Withfasterreactionandgellingtimes,thisproblemintensifies.Tominimizethiscondition,consideradifferentgatingpositionclosesttothelargestobstruction.Computerizedmold-flowanalysiscanhelpdeterminewhereproblemsmayoccur.ContactBaySystemsformoreinformationonfillinganalysis.

Figure 9-7Vent Solutions for Field Problems

Using a Tapered Hole or Knockout Pin

Chapter10

OTHER MOLD DESIGN CONSIDERATIONS

67

When designing molds, you must account for mold temperature control and cooling line placement. Additionally, this section addresses demolding methods and any special inserts or movable cores in the mold.

MOLD TEMPERATURE CONTROL

Moldcoolingdirectlyaffectsthequalityofyourfinishedpart.Whenthe“A”and“B”componentsinaRIMpolyurethanesystemreact,theygenerateheat,asmuchas150BTUsperpoundofmeteredmaterial,dependinguponthesystemused.Moldtemperaturemustbekeptataconstant,specifiedlevel,usuallybetween120°and180°F,againdependinguponthesystemused.Tomaintainacontrolledtemperatureinthemold,thisheatmustbeconductedthroughthemoldwalls,awayfromthecuringpart.Coolingchannelswithcirculatingwateristhetypicalmethodformaintainingandcontrollingmoldtemperature.

Generallymanifold-typecoolinglinesarepreferred.Theyoffermore-evencooling,minimizehotspots,aremoreefficientandusemorewateratalowerpumping-headpressurethansingle-passsystems(seefigure10-1).

Theselectedpolyurethanesystem,mold-makingmaterial,moldsizeandmoldcomplexitydeterminetheplacementandnumberofcoolingchannels.Forbestresults,coolingchannelsshouldbelocated1-1/2to2channeldiametersfromthemoldsurfaceandamaximumof2inchesapart(seefigure10-2).Channelstypicallyhaveadiameterof3/8inch.Pipeortubefittingsoncoolinglinesshouldhavediametersequaltothediametersofthelinestopreventblockages.Remembertoincorporatecoolinglinesadjacenttoinsidecorners,gateblocksandotherslow-to-coolareastoproducegoodparts(seefigure10-3).PropercoolingisespeciallyimportantwhenusingBaydurstructuralfoamswhichneedadequatecoolingtoformruggedskins.

Figure 10-1 Mold Cooling Channels

Manifold Type

PreferredParallelFlow

SinglePass

Series Type

68

DEMOLDING METHODS

Tohelpremoveparts,usedemoldingtechniquesinstrategiclocations.Thethreemost-commondemoldingmethodsare:

•Mechanical or Hydraulic Knockouts–havepinsinstrategiclocationstopushoutparts(seefigure10-4);

•Air Assists–haveprovensufficientwithsimple,flatparts(seefigure10-5);

•Vacuum Cups –areappliedtopartsformanualremoval,butarerarelyused.

Knockoutsshouldnotbeactuateduntiltheparthasachievedsufficient“greenstrength”orwhentheparthassolidifiedenoughtomaintainitsshapeandberemovedfromthemoldwithoutknockoutmarksortears.Thepart’slimitedcompressivestrengthatthedemoldingtimerequiresalargecontactareaforthesedevices.Wheneverpossible,havepinsactuateagainstarib,corner,orbosstodistributetheirforcesoverawiderareaofthepart.

Small-diameterpinscandamagepartsmadeoffoamedsystemsduringdemolding.Uselargepins,withaminimumdiameterof3/8inch.Forflatareas,1/2inchpinsarerecommended.Incontrast,solidrigidparts,suchasthosemadeofPRISMpolyurethane,canwithstandactuationby1/8inchpins,particularlyinareaswhereribsorwallsintersect.

Pinsinaknockoutplateareusuallypositionedinthemovinghalfofthemold.Mechanicalstopsinthepress

Figure 10-3 Effect ofTemperature Control on Skin Formation

Irregular SkinThin Skin Uniform Skin

Incorrect Correct

Coolant Channels Coolant Channels

Figure 10-2Typical Cooling Channel Placement

Aluminum Mold Steel Mold

Diameter Of Channel (d) 3/8 – 1/2 inch 3/8 – 1/2 inch

Maximum Depth (B) 3/4 inches 3/8 – 1/2 inch

Maximum Distance 2 inches 1 – 1/2 inches

d

A

B

Cavity Side

Themost-commondemoldingdevice,mechanical knockout pins,arerodsthatareflushwiththewallduringmoldingandprotrudewhenthemoldopens

Chapter10

OTHER MOLD DESIGN CONSIDERATIONS

69

actuatetheplatewhenthemoldopens(seefigure10-4).Thepressusuallyhasfourstops:largescrewsthatmustbeadjustedsothatallfourtouchtheknockoutplateatthesametimetoensureuniformknockoutactiontohelppreventpartbinding.

Ifknockoutsarelocatedinthestationarymoldhalf,hydrauliccylindersmovetheknockoutplate.Alternatively,chain-activatedknockoutplatescanbeused.Returnpinspushtheknockoutpinsbacktotheirflushpositionwhenthemoldcloses.

Unconnectedknockoutpinscanoccasionallybeusediftheyareactuatedwithdouble-actinghydrauliccylinders.Donotusecylinderswithspringreturnsastheymaynotretractcompletelywhenthemoldcloses.

Acombinationofmechanicalknockoutpinsand air assistsisanothercommondemoldingtechnique.Avacuummaydevelopbetweenthecoreandapartastheknockoutpinsactivate.Compressedairblownintothisvacuumreleasesthepart(seefigure10-5).Theairvalvemustsealperfectlyandbeflushwiththemoldsothataircannotescapeandcreatebubblesduringmoldfilling.

Vacuum cups canbeusedtoremovepartsfromcoresections.Rulesofthumbforusingvacuumcupsinclude:

•Distributelargevacuumcupsuniformlyoverthepartsurface.

•Useaquickmotiontoremoveparts,ratherthanalongpullingmotionwithslowlyincreasingforce.

Figure 10-4 Typical Mold Arrangement

HydraullicCylinder

MovingPlaten

KnockoutPlate

ReturnPin

KnockoutPin

LandArea

LeaderPin

Cavity

Stop

70

MOVABLE CORES AND INSERTS

Usemovablecoresforpartswithundercuts-snapfits,holes,orcutouts-locatedperpendiculartothedirectionofdraw.Movablecoresmustbeliquid-tighttopreventmaterialflashingintotheactuatorsandlockingtheslides(seefigure10-6).ConsiderusingO-ringsonthepinsandactuatorshaftstopreventleakage.

Removableinsertsareanothermethodformakingundercuts(seefigure10-7).Generallymoldersdonotliketousetheseinserts,astheyarelabor-intensive,canfalloutofpositionandmaydamagethemold.Pinsalignedinthedirectionofdrawholdinsertsinposition,allowingtheinserttoberemovedwiththepart.Discussanymovablecoresandinsertswithyourmoldmakerduringthemolddesignprocess.

Figure 10-5 Air Assist Demolders

Part

Air

Part

Air

Figure 10-6 Slides

Engaged Retracted

Figure 10-7 Undercut Molded With a Removable Insert

Magnet

Pin

Removable InsertUndercut

Chapter10

OTHER MOLD DESIGN CONSIDERATIONS

71

MOLD DESIGN FOR SLOTS

Whendesigningforapartwithslots,donotdesignamoldsuchthatthecoresjusttouchtheoppositewall;rather,seatthemapproximately1/8inchintothewall(seefigure10-8).Thisdesignpracticewillfacilitateflashremoval,becausetheflashwillbeperpendiculartothepart’ssurface.Ifpossible,designamoldwithasteppedpartinglineforslots(seefigure10-9).

Figure 10-8Recommended Shut-Off for Slots

Core

Cavity

Core

Incorrect

Correct

Cavity

~0.12”

Figure 10-9 Mold for Louvers Without Side Pulls

MoldCore

MoldCavity

Direction of Draw

Direction of Draw

Mold

Mold

Part

SHEAR EDGES

Fiberglassmat,usedinpartsmadeofBaydurSTRcompositesystems,shouldfilltheentiremoldcavity,leavingnoemptyareasbetweenthematandmoldwall.Tomeetthisrequirement,thematneedstobeslightlyoversized.Themoldwillneedarenewable-steel

shearedgetocutawayanyglass-fiberoverhang,apockettoholdthisexcessandamoldsealexternaltotheshearedge(seefigure10-10).Becauseglassfiberscanerodesoftermetalmolds,westronglysuggestusingsteelmoldsforstructuralRIM.

72

Figure 10-10 Mold Shear Edge

ReplaceableKnife Edges

Pocket forCut Excess

SealingEdge

Chapter11

SPECIAL MOLDS

73

Occasionally, a multiple-cavity or self-contained mold is used for economic or processing considerations. These special molds have unique requirements, which are discussed in this section.

MULTIPLE CAVITY MOLDS

Moldscanbedesignedwithmultiplecavitiestoproduceseveralpartssimultaneously(seephoto).Multiple-cavitymoldsrarelyhavemorethanfourcavities,withatwo-cavitymoldbeingmorecommon.Aspecialtypeofmultiple-cavitymold-calleda“familymold”-producesmatingpartsofanassembly.Typically,multiple-cavitymoldsareeconomicalforlargerproductionruns.Whilethetoolingmaybemoreexpensivethansingle-cavitymolds,productiontimeandcostscanbelowerwithmultiple-cavitymolds.

Inatypicalmultiple-cavitymold,thereactionmixtureentersthemoldthroughacentrallylocatededge-gatingsystem.Thefilltimeforallcavities

shouldbeidentical.Maintainanequalpressuredropthroughallgatestoensureacceptableparts.UseY-splitsinsteadofT-splitstohelppreventbubblesinyourpart(seefigure11-1).Amultiple-cavitymoldhavingdifferent-sizedcavitiesmaybefilledthroughasinglemix-headsystemwithbalancedrunners.

Whenusingfoamedsystemsinamultiple-cavitymold,payspecialattentiontothemoldtilt.Youmayalsohavetouseadjustablerestrictorsintherunnerbranchestocontrolthedegreeoffillinginindividualcavitiestoensureequivalentfillinglevels.ContactyourBaySystemsrepresentativeforassistanceonyourspecificapplication.

74

SELF-CONTAINED MOLDS

Self-containedmoldsdonotrequireapress.Typically,thesemoldsaremoreexpensivethanthemoldinamold-and-presssetup,butgenerallycostlessthanatraditionalmoldandpress.Theyareoftenusedforsimplyshapedpartsrequiringlowmoldingpressure.ContactyourBaySystemsrepresentativeformoreinformationonself-containedmolds.

Incorrect

CorrectTo Cavity 1

To Cavity 2

FromAftermixer

Fender of the Volkswagen Touareg

Chapter12

MOLD FINISHING

75

When designing a mold, pay special attention to mold-construction materials, mold-surface treatments and any textures or finishes to be applied to the mold. These topics are discussed in this section.

MOLD CONSTRUCTION MATERIALS AND FABRICATION TECHNIQUES

BecauseRIMpolyurethanesgenerateheatwhentheyreact,youshouldchooseamoldmaterialthatisconductive,todissipateheatfromthemoldingpart.Forthisreason,metalmoldsarestronglysuggested.Considerepoxyandspray-metalmoldsonlyforprototypingorlow-volumeproductionwherecycletimeandsurfacequalityareoflessimportance.Forhigh-volumeruns,particularlythoseusingreinforcingfillers,steelisusuallythemoldmaterialofchoice.Ifyourmoldneedsmechanicalorhydraulicknockouts,metalmoldscanaccommodatetheseactuatorsmuchbetterthanepoxyones.

Generally,moldsshouldbeabletowithstand200psiofpressureasasafetymeasure,eventhoughtypicalmoldingpressuresdonotexceed100psi.Anynumberofmaterialscanbeusedtomakemolds,includingsteel,aluminum,zincalloys,copperalloysandnickel.Determiningwhichmaterialisthebestforyourparticularmolddependsuponseveralparameters,suchas:

•Numberofpartstobemade

•Surfacefinishrequirements

•Timeavailableformoldconstruction

•Parttolerances,dimensionsandshape

•Single–ormultiple-cavitymolds

•Moldcycletimeandheatconduction

•Qualityofpartstobemade

MATERIAL SELECTION

Becausesurfacetextureswillbecloselyduplicated,nonporousmoldsurfacesareessential.Asmoothmoldsurfacegreatlyimprovespartrelease.Pits,gougesandothersurfaceimperfectionsinthemoldcanleadtopoorpartreleaseandbreakage.Asrulesofthumb:

•TheRIMsystemandproductionparametersinfluencethemold-constructionmaterial.

•Partgeometryinfluencesthemold-constructiontechnique.

76

Steel

Becauseoftheirhighdegreeofproductionreliability,machinedsteelmoldsareespeciallyadvantageousformass-producedparts.Theyofferlongmoldlife,canbeoutfittedwithelaborateautomatedejectionsystemsandarelesslikelytobescratchedordamagedthansoftermaterials.Widelyusedforautomotivepartswhichrequirelongproductionruns,steelmoldslastlongerandmaybecostly.Typically,steelmoldsareusedforpartsmadeofshort-fiber-filledmaterialsorcomposites,assteelresistsabrasion.

Aluminum

Offeringlighterweight,goodheatconductivityandlowermachiningcoststhansteel,aluminumhaslongbeenthematerialofchoiceformoldmakersspecializinginpolyurethanemolds.Itissofterthansteelandmaynotbesuitableforverylongrunsorforusewithcompositesystems.

Zinc Alloys (Kirksite)

High-qualitycastzincmoldsofferexcellent,nonporoussurfaces.Theyarerelativelyheavyandrequirecloselyspacedcoolinglinesforaccuratetemperaturecontrol,becausetheyarenotasheatconductiveasaluminum.

Nickel Shells

Forhigh-qualitysurfacereproductions,considerusingnickelshells.Theseshellshaveahighsurfacehardnessandoffergoodreleasecharacteristics.Smallmoldsmaynotneedbacksupportfortheshell.Mountlargermoldsinasteeloraluminumsupportframeandbackfillwithacastingmaterialforstructuralrigidity.Coolinglinescanbeattachedorplatedontothebacksideoftheshellpriortomounting.

Epoxy Molds

Usedmostlyforshort-run,prototypeparts,epoxymoldshavepoortemperaturecontrol,arefragileandcanhavesurfaceroughness.Theyarepoorheatconductors,sometimescausingpartstostick.Epoxyshouldbeusedonlyforshortrunsofprototypeparts,whenqualityisnotimportantandlowcostisaprimaryconcern.Foamedpartsmadeinepoxymoldstendtohavethin,nonuniformskin.Applyingaspray-metalsurfacetoanepoxymoldcaneliminatesomeoftheseshortcomings.

Beforefillinganepoxymoldwithapolyurethanesystem,carefullyconditionthesurfacewithareactantsuchasisocyanate.Beforeusinganyreactant,pleasecheckappropriateliteratureonproperuse,storageandpersonalsafetyequipment.Thiscleaningshouldremoveanyaminecatalystsinthemoldcavities.Waxingallcavitysurfacesbeforethefirstmoldingmayhelpthedemoldingprocess.Ifanyreactivematerialremainsonthemoldsurface,youmayhavedifficultyremovingyourfirstprototypepart.

Chapter12

MOLD FINISHING

77

MOLD CONSTRUCTION TECHNIQUES

Aspreviouslymentioned,finalpartgeometrygenerallyinfluencesthemold-constructiontechnique.Thissectiondiscussesseveralofthemost-commonconstructionmethods.

Milled Block

Inthisconstructiontechnique,themoldcavityismachineddirectlyintoametalblock.Althoughitmaybemorecostly,amilled-blockmoldgivesthemostaccuraterepresentationofthepart,doesnotshowlineswherethemoldpartsmeetandincorporatescoolinglineseasily;however,theyaremoredifficulttomodify.

Structural Components

Milledmetalplatesarejoinedwithscrewsorpins,orweldedinthisconstructiontechnique.Joinedplateandbaristhemethodofchoiceforlarge,flatparts.

Cast

Widelyusedinmoldmaking,castmoldsarerelativelyinexpensive.Theymakeexcellentmoldsandareusedparticularlyforcurvedparts.Steelmoldsarenotnormallycast.Surfaceporosity,oftenjustundertheskin,cancausedifficultieswhenpolishingcastaluminummolds.Tomakeamoldcasting,apositivepatternwithapredeterminedpartinglineismade.Thispatternisthencast.Afterwards,thesurfaceisconditionedtoremovebubblesandimperfections.Coolinglinescanbecastintothemold.

Extruded Aluminum Profiles

Usedforbothsolidandfoamedsystems,extrudedprofilesfindspecialapplicationinbuildinginexpensivemoldsforprofilegeometriessuchaswindowframesordoorsashings.

Nickel Plating

Nickelplatingcanbeformedeitherelectrolyticallyorbyelectrolessdeposition.Inthislattertechnique,a0.04to0.08inchlayerofnickelisdepositedonapositivepattern,witha3/8to1/2inchcopperbackuplayerelectrolyticallydepositedontothenickel.Coolinglinescanalsobeplatedontothisbackuplayer.

78

SURFACE TREATMENTS FOR MOLDS

BecauseRIMpolyurethanesystemsreproducefinesurfacedetails,thetypeoffinishonamoldsurfaceiscritical.Considerusingplastic-industrystandardssuchastheSPI-SPEMoldFinishComparisonKit,availablefromtheSocietyofPlasticsIndustry,asaguideline.

Chromeandnickelplating,excellentsurfacetreatmentsformolds,improvethemold’sscratchresistanceandreducenecessarydemoldingforces.Chromeplatinggivesbetterresults.Beforeplatinganymoldsurface,closelyexamineittoensurethatitissmoothandnonporous.

OthertreatmentssuchasTefloncoatingoranickel-polymercoatingareusedtoimprovemoldreleaseproperties.Surfacehardeningmethods–suchasnitrideminimizeminordamageandtheeffectsofabuseduringproductionoperations.

Dependinguponyourpart’ssurfacespecifications,considerusingthefollowingfinishes:

•No.2Grit(#15micronrange)

•No.3320EmeryCloth•No.4280Stone

Forcosmeticsurfaces,considerusingafinishbetweenNo.2and3;fornon-cosmeticsurfaces,afinishbetween3and4shouldsuffice.

TEXTURES AND FINISHES

Lettering,texturesandgraphicscanbemoldedintoapart.Typicallythesevisualelementsaremilledintothemoldresultinginaraisedappearanceonthefinishedpart.Forlargeareasoffinetext,considerusingdecals.

Texturesenlargetheeffectivesurfacearea,requiringincreaseddemoldingforces.Surfacesparalleltothedirectionofdrawhavelowerlimitsontexturedepth.

Nickelshellsofferhighsurfacehardnessandgoodreleasecharacteristicsforhigh-qualitysurfacereproductionsoftextures,suchasleatherorwoodgrain.

Photo-etchingandmechanicaltexturingofferawidevarietyoffinishes.Inphoto-etching,chemicalsetchmoldsurfacesinagivenpattern.Somemoldmakershaveexampleswithdifferenttexturestohelpyouselectone.Inmechanicaltexturing,smallmetalballsareplacedinthemold,whichisthenshaken,toachieveapebblefinish.

Beforeapplyinganytexturetoamoldsurface,ensurethatthemoldistofinaldimensions,becausethesedimensionscannotbechangedaftertexturizing.Texturesinmoldsmustbeveryaccurate,asthepartscannotberetouchedaftermolding.Thesemoldsareverysensitivetonicksandscratches,requiringspecialcareduringdemolding.

Chapter13

TECHNICAL SUPPORT

79

HEALTH AND SAFETY INFORMATION

AppropriateliteraturehasbeenassembledwhichprovidesinformationconcerningthehealthandsafetyprecautionsthatmustbeobservedwhenhandlingBaySystemsthermosettingresinsmentionedinthispublication.Beforeworkingwithanyoftheseproducts,youmustreadandbecomefamiliarwiththeavailableinformationontheirhazards,properuseandhandling.Thiscannotbeoveremphasized.Informationisavailableinseveralforms,e.g.,materialsafetydatasheetsandproductlabels.ConsultyourlocalBaySystemsrepresentativeorcontacttheProductSafetyManagerinPittsburgh,PA.

DESIGN AND ENGINEERING EXPERTISE

Togetmaterialselectionand/ordesignassistance,justwriteorcallyourBaySystemsrepresentativeintheregionalofficeslistedonthebackcoverofthisbrochure.Tobesthelpyou,wewillneedtoknowthefollowinginformation:

•Physicaldescriptionofyourpart(s)andengineeringdrawings,ifpossible

•Currentmaterialbeingused

•Servicerequirements,suchasmechanicalstressand/orstrain,peakandcontinualservicetemperature,typesandconcentrationsofchemicalstowhichthepart(s)maybeexposed,stiffnessrequiredtosupportthepartitselforanotheritem,impactresistanceandassemblytechniques

•Applicablegovernmentorregulatoryagencyteststandards

•Tolerancesthatmustbeheldinthefunctioningenvironmentofthepart(s)

•Anyotherrestrictivefactorsorpertinentinformationofwhichweshouldbeaware

Uponrequest,BaySystemswillfurnishsuchtechnicaladviceorassistanceitdeemstobeappropriateinreferencetoyouruseofourproducts.Itisexpresslyunderstoodandagreedthatbecauseallsuchtechnicaladviceorassistanceisrenderedwithoutcompensationandisbaseduponinformationbelievedtobereliable,thecustomerassumesandherebyreleasesBaySystemsfromallliabilityandobligationforanyadviceorassistancegivenorresultsobtained.Moreover,itisyourresponsibilitytoconductend-usetestingandtootherwisedeterminetoyourownsatisfactionwhetherBaySystems’sproductsandinformationaresuitableforyourintendedusesandapplications.

80

TECHNICAL SUPPORT

Weprovideourcustomerswithdesignandengineeringinformationinseveralways:Applicationsandprocessingadvice,availablebyphone,at412777-2000;processingassistance,throughanationwidenetworkorregionalfieldtechnicalservicerepresentative(seelistonbackcover);technicalproductliterature;andperiodicpresentationsandseminars.ThetypesofexpertiseyoucanobtainfromBaySystemsincludethoselistedinthissection.

Design Review Assistance

•Conceptdevelopment

•Product/partreview

•Molddesignreview

•Partfailureanalysis

•Finiteelementstressanalysis

•Moldfillinganalysis

•Experimentalstressanalysis

•Shrinkageandwarpageanalysis

Application Development Assistance

• Productdevelopment

• Partcostestimates

• Colormatching

• Prototyping

• Materialselection

• Moldingtrials

• Physicaltesting

• Secondaryoperationadvice

Product Support Assistance

• On-siteprocessingaudits

• Start-upassistance

• On-timematerialdelivery

• Troubleshooting

• Processing/SPCSeminars

• Productivityaudits

REGULATORY COMPLIANCE

Someoftheendusesoftheproductsdescribedinthispublicationmustcomplywithapplicableregulations,suchastheFDA,USDA,NSFandCPSC.Ifyouhaveanyquestionsontheregulatorystatusoftheseproducts,contactyourlocalBaySystemsrepresentativeortheRegulatoryAffairsManagerinPittsburgh,PA.

Chapter13

TECHNICAL SUPPORT

81

RIM PLASTICS RECYCLING

Polyurethanesarenowbeingrecycled.Recycledpolyurethaneshavepracticaluses,likehousingsforelectronicequipmentandexteriorbodypartsfortransportationvehicles.

•Regrinding–Thistechnologyallowsfora“secondlife”formanytypesofpolyurethanepartssuchasbusinessmachinesandbumpers,whicharethengroundintoagranulateorpowderforuseasafiller.

•Adhesive Pressing–Polyurethanegranulateissurfacetreatedwithabinder,thencuredunderheatandpressure.Theresultingmaterialscanbereinforcedandmolded.

•Compression Molding–Thistechniqueallowsfor100%reuseofRIMpolyurethaneelastomersinwhichnovirginmaterialneedstobeadded.Canretainupto50%oftensileproperties.

•Energy Recovery–TechnologyexiststopyrolyzepolyurethanepolymerscleanlyandthecombustionproductscanmeetEPAstandards.OnepoundofRIMpolyurethanecontainsbetween12,000and15,000BTUs,aboutthesameenergypotentialasoilorcoal.

•Injection Molding–Thisprocessissuitableforcompositecomponents,suchasinstrumentpanelsthatcontainathermoplasticsupport,foamanddecorativeskin.Theentiremodulecanbegroundandinjectionmolded.

•Glycolysis–Thisisachemicalrecyclingprocessinwhichthepolymerisbrokendownintoamixtureofliquidpolyols.Manydifferentkindsofpolyurethanepartscanbeused.

Polyurethanes.Theydoaworldofgoodwhenyouusethem.Andwhenyoureusethem.

FOR MORE INFORMATION

Thedatapresentedinthisbrochureareforgeneralinformationonly.Theyareapproximatevaluesanddonotnecessarilyrepresenttheperformanceofanyofourmaterialsinyourspecificapplication.Formoredetailedinformation,contactMaterialScienceMarketingCommunicationsat412777-2000,oryournearestdistrictoffice.

Themannerinwhichyouuseandthepurposetowhichyouputandutilizeourproducts,technicalassistanceandinformation(whetherverbal,writtenorbywayofproductionevaluations),includinganysuggestedformulationsandrecommendationsarebeyondourcontrol.Therefore,itisimperativethatyoutestourproducts,technicalassistanceandinformationtodeterminetoyourownsatisfactionwhethertheyaresuitableforyourintendedusesandapplications.Thisapplication-specificanalysismustatleastincludetestingtodeterminesuitabilityfromatechnicalaswellashealth,safety,andenvironmentalstandpoint.Suchtestinghasnotnecessarilybeendonebyus.Unlessweotherwiseagreeinwriting,allproductsaresoldstrictlypursuanttothetermsofourstandardconditionsofsale.Allinformationandtechnicalassistanceisgivenwithoutwarrantyorguaranteeandissubjecttochangewithoutnotice.Itisexpresslyunderstoodandagreedthatyouassumeandherebyexpresslyreleaseusfromallliability,intort,contractorotherwise,incurredinconnectionwiththeuseofourproducts,technicalassistance,andinformation.Anystatementorrecommendationnotcontainedhereinisunauthorizedandshallnotbindus.Nothinghereinshallbeconstruedasarecommendationtouseanyproductinconflictwithpatentscoveringanymaterialoritsuse.Nolicenseisimpliedorinfactgrantedundertheclaimsofanypatent.

IntroductionPART DESIGNFigure P-1 RIMprocess 4Figure P-2 Polyurethanesystemsclassifiedby flexuralmodulus 6Figure P-3 Typesofpolyurethanematerials 7

Chapter1MATERIAL SELECTION CRITERIAFigure 1-1 Simple/complexpartdesignforundercuts 11

Chapter2GENERAL PART DESIGNFigure 2-1 Three-pointloadingtest 13Figure 2-2 Part-stiffeningtechniques14Figure 2-3 Coredpart 15Figure 2-4 Racetracking 15Figure 2-5 Thickversusthinribs 16Figure 2-6 Notchedrib 16Figure 2-7 Sinkscausedbythickribs 16Figure 2-8 Offsetrib 17Figure 2-9 Ribconfiguration 17Figure 2-10 Notchedribwithbridge 17Figure 2-11 Differenttypesofribbing 18Figure 2-12 Recommendeddraft 18Figure 2-13 Bossesandventing 19Figure 2-14 Openboss 19Figure 2-15 Comerbosses 20Figure 2-16 Hollowbosses 20Figure 2-17 Elongatedboss 20Figure 2-18 Holeinsidewall 21Figure 2-19 Grooves 21Figure 2-20 Slotscurledaroundcomer 21Figure 2-21 Slotsorlouversonslopingwall 22Figure 2-22 Basicdimensionsforslots 22Figure 2-23 Slot,groove,andholelocationsfor foamedmaterials 22Figure 2-24Minimumwallthicknessforusinginserts 23Figure 2-25Balancingthecross-sectional centersofgravity 24Figure 2-26 Typicalthreadedinsert 24Figure 2-27 Moldconfigurationsshowingundercuts 25

Chapter2GENERAL PART DESIGN (continued)Figure 2-28 Snap-fithookmoldedthroughhole toformundercut 25Figure 2-29 Wireguides 25Figure 2-30 Livinghinge 26Figure 2-31 Partialhinges 26Figure 2-32 ModulusretentionforBaydur structuralfoam 28Figure 2-33 Waterski 28

Table 2-1 Coefficientsoflinearthermalexpansion 27

Chapter3SOLID MATERIALSFigure 3-1 Ribwallratioforsolids 29Figure 3-2 Correctradii/filletsforsolids 30Figure 3-3 Bossdimensionsforsolidmaterials 30

Chapter4FOAMED MATERIALSFigure 4-1 Riblwallratioforfoamedsystems 32Figure 4-2 Effectofradiusonskinformation 32Figure 4-3 Bossversusflowdirection 33Figure 4-4 Bossdimensionsforfoamedmaterials 33Figure 4-5 Coredversusdrilledbosses 34

Chapter5COMPOSITE MATERIALSFigure 5-1 Flexuralpropertiesversuspercent/type ofglassforBaydurSTR 35Figure 5-2 Corrugationsandboxbeams 36Figure 5-3 Continuousintegralbeamassembly 37Figure 5-4 Radiijfilletconfiguration 37Figure 5-5 Hollowbossesandpadsformounting 37

Chapter6POSTMOLDING OPERATIONSFigure 6-1 Screwpulloutstrengthversus foamdensity 42

82

LIST OF FIGURES AND TABLES

IntroductionMOLD DESIGNFigure M-1 TypicalRIMmold 46

Chapter7GENERAL MOLD DESIGN CONSIDERATIONSTable 7-1 Typicalmoldingpressures 48Table 7-2 Relativemold-costcomparison 48

Chapter8GATE DESIGNFigure 8-1 Mixingheads:diameterversusoutput 51Figure 8-2 Cylindricalmix-headsnout,flushwithmold52Figure 8-3 Typesofaftermixers 52Figure 8-4 Typicaldamgating 53Figure 8-5 Runnertransition 53Figure 8-6 Damgate 55Figure 8-7 Damgatedimensions 56Figure 8-8 Quadraticandtriangularfangates 57Figure 8-9 Fangatedimensions 58Figure 8-10 Ballcheck 59Figure 8-11 Directfillintocenterofmold 59Figure 8-12 Gatemarkscausedbypoor gatedimensioning 60Figure 8-13 Ratioofwallthicknesstomixing-head diameterfordirectfill 60Figure 8-14 Typicalcircularaftermixer 60

Table 8-1 Sampledamgatelengthcalculationsfor solidandfoamedsystems 54Table 8-2 Calculationoftypicalfangatedimensions 57

Chapter9PARTING-LINE CONSIDERATIONSFigure 9-1 Moldintiltablepress 61Figure 9-2 Gatelow,venthigh 61Figure 9-3 Secondarypartinglineviadrag-plate 62Figure 9-4 Moldsealing-landarea 62Figure 9-5 Molddumpwell 63Figure 9-6 Typicalmoldventing 63Figure 9-7 Ventsolutionsforfieldproblemsvia taperedholeorknockoutpin 64

Table 9-1 Approximatesealing-edgewidths 62

Chapter10OTHER MOLD DESIGN CONSIDERATIONSFigure 10-1 Moldcoolingchannels 65Figure 10-2 Typicalcoolingchannelplacement 66Figure 10-3 Effectoftemperaturecontrolon skinformation 66Figure 10-4 Typicalmoldarrangement 67Figure 10-5 Air-assistdemolders 68Figure 10-6 Slides 68Figure 10-7 Undercutmoldedwitharemovableinsert 68Figure 10-8 Recommendedorientationforslots 69Figure 10-9 Moldforlouverswithoutsidepulls 69Figure 10-10Moldshearedge 70

Chapter11SPECIAL MOLDSFigure 11-1 T-splitandY-split 72

LIST OF FIGURES AND TABLES

83

A“Aside”components5,65adhesionpromoters23adhesivebonding41,42,48aestheticconsiderations9,39aftermixers5,45,47,51-53,60 circular60 harp53 peanut52,60agitators5airassists66,67airentrapments15,17,18,21,22,30, 32,33,52,53,61applicationdevelopmentassistance78assemblyoperations41

B“Bside”components5,65backmolding27,28backpressure35,53,59ballcheck58blowingagents5-7,31bosscoring19.20,30,33,41bossventing18,19,63,64bosses19,20,30,33,63,66blind19 elongated20 hollow20,36 isolated19 open19,21radiisuggestions19,20,30,33boxbeams36

Ccavity,cavityside18,21,53,59,60, 62,64,74center-gateddirectfill53,54,58-60CFCs,HCFCs31chalking9chemicalexposure10clampingpressure47,48,62“classA”finish9,38clearcoating10coating,in-mold3,9,38,40

coefficientsoflinearthermal expansion23,26-28compositematerials7,10,17-19,35- 38,48,53,58,59,69.74 descriptionof7 finishes38 glassmat7,35-37,69 ribs17,18,36coolingchannels5,65,74,75coolinglines,manifold-type65coreside18,21,67cores36,64,65,67-69movable68preformed36corrugations11,36creepconsiderations27creep,measuring27crosssections14,15,23,32cycletimes10,11

Ddamgates54-56daytanks5decals41,76degreeofrigidity6demoldingmethods65-68,76designandengineeringexpertise77designfordisassembly(DFD)43designreviewassistance78dimensionaltolerances49draft18,21,30draft,wood-graintextures18dragplate61,62drilling20,43,48dumpwells62,63

Eeconomicconsiderations10,11edgegating53,54,60,71entrancespeeds53,54 gatedimensions53,54 runnerdiameters53 solidsystems53,54elastomericmaterials6,7,10,18,22, 25,26,40,41,43,57

Ffamilymolds71fangates54-57 gatelength56 quadraticgate56 straight-sidedgate56fatigueconsiderations27,28fatiguetest28fiberorientation26,27fiberglass7,26,35-38,53,58,69,74 content35,36flakes26,37 location35,36 matfiberdirection35 matfibers,shortglass.26,64fillers5,7,26,27,43,73,74 mineral7,26fillets30,33finishes39finishing,part3,9,10,17,38,39,41flash62,68,69flexiblefoamsystems6flexuralcreeptest27flexuralmodulus13,34,35flexuraltest13flowlengths54.59foam6,16,20.22.24,31-34,41-43, 54,55 breakout42flow22,31,32 rise22,31,32foamedmaterials6,7,18,20,22,23, 27,31-34,36,40-43,53- 55,64,71 advantages31 applications6,31 description6,31 finite-elementanalysis34,78 foamriseandflow,22,31,32 gating54 ribdesignandconfiguration 16,32,33foamedpolyurethanesystems, description6,7

84

INDEX

formoreinformation79functionalconsiderations10

Ggateblocks53,54 heatingchannelsfor53gates3,36,45,47,51-60,64 design3,45,51-60 position45,51,52,54,61,64gelling54,58,64generalmolddesignconsiderations 45,47-49generalpartdesignconsiderations 13-30glassmatsee fiberglassglycolysis43graphics40,41,76greenstrength66grooves13,21,22 draft21 radius21

Hhandguns58healthandsafetyinformation77heatexchangers5hingepins26hinges25,26 living26 metal26 partial26 snap-in26holes21,22,26,30,41,63,68hydraulicslides48hydroxylcontent5

Iinjectionrates54inserts,10,11,13,19,20,23,24,27, 28,32,33,36,42,65,68 designof,23,24 encapsulated13,23 holediameter24 hollow23 metalstiffening23,27,28 molded-in24

press-fit19,24 reinforcing23 removable68 screwsize24 space-filling36 strippingtorque24 threaded24,42 woodstiffening24isocyanate5,51,74

KKirksitesee molds, zinc alloyknitlines21,32,54,64knockoutmarks66knockoutmechanisms62,63,66,67, 73

Llance5lettering,40,41,76liquidlevel,22,31,32liquidtight,62,67

Mmaterialdensity13,31,49,62materialselectioncriteria3,9-11,49, 78mechanicaletching76microcellular7,47mixingheads5,45,47,48,51-53, 58-60 flowcapacities51 impingingpressure51mounting52moldingtimes32molds aluminum73-75 cast75 cavityobstructions64 constructionmaterials48,73, 74,77,78 constructiontechniques75 cooling65 copperalloy73 costs11,17,47,48 design3,45,51,52,65,68,

69,78 forslots69 introductionto,45 dimensionaltolerances47-49, 76 epoxy73,74 extrudedaluminumprofiles, 75 fabricationtechniques48,73 filling15,17,19,35,36,40, 51,61,64,67,71,78 finishing17,39,73,76 milledblock75 multiple-cavity58,71,73 nickelplating75,76 nickelshells40,73,74,76 pressures48,49,62,73 sealing52,62 shrinkageconsiderations49 spray-metal73,74 steel69,74,75 surfacetreatments48,73,76 temperaturecontrol53,65,74 texturesandfinishes76 tilting61 venting61,63 zincalloys(Kirksite)73,74movablecoresandinserts65,68multiple-cavitymolds71,73

Nnailing,41,43

Ppads36paint,textures9,40,41painting9,39,40,48partingline45,53,61,62,69 considerations61 secondary61 stepped69parts finishing39-41 patching40 repeatability47 size47,48

INDEX

85

stiffness13-15,17,18,23,26, 27,31,35,36 weightlimitations47,48photoetching76pigments5,39,39,40pins,smalldiameter66planing43polyol5,39,51postfabrication43postmoldpainting40postmoldingoperations3,20,39,48preforms37presscapacity48ProductInformationBulletins(PIBs) 3-49productsupportassistance78prototypetesting3,34,73,74,78

Rracetracking15,32recirculationpumps5recyclingpolyurethanes43,79regulatorycompliance78releaseagents5resin-richareas17,36ribbing11,13,17,18,29,32 bidirectional17 diagonalcross18 direction13,17,29,32 parallel17ribs11,13,15-18,29,30,32,63 andflowdirection17 notched15-17 thick15,16rigidmaterials6,7,10,25,33,40,54, 66RIMmaterialdescriptions6,7RIMmaterialselectioncriteria9-11RIMpolyurethanematerials3,23,27, 41,43,47,51,53,62,73 inherentadvantages3

physicalproperties6,7,10, 39RIMprocess5RIMsystem5,6,9,15,23,28,36,40, 42,48,59,63,65,73,76RIMgatem51routing43runners3,51-55,57,62,71

Ssanding40,43,48sawing43screws19,20,24,33,41,42 pilotholefor41,42 pulloutstrength20,24,41 self-tapping33,41,42 strippingtorque24 thread-cutting20,42 thread-forming42self-containedmolds72self-skinningfoamsystems10shearedges69shrinkage16,23,28,29,47,49,78shrinkagebehavior28,49sidepulls11sidewalls14,17,18,21silkscreening41sinkmarks16,19,29,32slidingcores17,21,25,31,68slots13,21,,22,25snapfits25,68solidelastomericsystems7solidmaterials7,16,18,23,29,30, 33,53,55,57,64 advantages7,29 descriptionof7,29 inserts23 radii30 ribdesign16,29 sinkmarks16 wallthickness14,16,23,29, 30

solidpolyurethanesystems7solid,rigidsystems7,10specialmolds71sprues60stapling43structuralcompositepolyurethanesystems(SRIM)7,10,36-38,69structuralfoam6,14,23,31,34,41, 55,59 wallthickness14surfaceblemishes32,33,40,60

Ttechnicalsupport78textures18,39-41,73,74,76 leathergrain40,74,76 pebble40,41,76 woodgrain40,41,74,76thermalelongation23thermalexpansion23thermoset5,37,77thicksections,coring11,15

Uundercuts25,26,31,48,68UVstability9,10,39,40

Vvacuumcups66,68veil38ventpins19,63vents13,19,32,61,63

Wwallthickness11,13-16,19-23,25, 29-33,36,42,60 varying13,15warpageinpartdesign27warping13,23,27,28,78weldlines21,32,54,64wettingout36wireguides25

86

INDEX

The manner in which you use and the purpose to which you put and utilize our products, technical assistance and information (whether verbal, written or by way of production evaluations), including any suggested formulations and recommendations are beyond our control. Therefore, it is imperative that you test our products, technical assistance and information to determine to your own satisfaction whether they are suitable for your intended uses and applications. This application-specific analysis must at least include testing to determine suitability from a technical as well as health, safety, and environmental standpoint. Such testing has not necessarily been done by us. Unless we otherwise agree in writing, all products are sold strictly pursuant to the terms of our standard conditions of sale. All information and technical assistance is given without warranty or guarantee and is subject to change without notice. It is expressly understood and agreed that you assume and hereby expressly release us from all liability, in tort, contract or otherwise, incurred in connection with the use of our products, technical assistance, and information. Any statement or recommendation not contained herein is unauthorized and shall not bind us. Nothing herein shall be construed as a recommendation to use any product in conflict with patents covering any material or its use. No license is implied or in fact granted under the claims of any patent.

Print & Design by Bayer Print & Design Services Group Literature #20667 Edition November 2008

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