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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