Plastic Materials and Structural Design.docx
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Plastic Materials and Structural Design.docx
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PlasticMaterialsandStructuralDesign
PlasticMaterialsandStructuralDesign–ByJoseph,Tam,Dennis,Ricky
1MaterialSelection
Theselectionofmaterialisalwaysrelatedtotheparticularapplication,soitisessentialtoknowthepropertiesofeachmaterial.MaterialswearecommonlyusedareABS,PC,PC+ABS,PC+GlassFiberandNylon+GlassFiber(Polyamide).Thefollowingbrieflysummarizepointsneededtoconsiderwhenselectingapropermaterialforthedrillhousing.
1.1DrillHousingMaterial
Almostallofthemajorcomponentsofthedrill(Gearbox,chuck,motor,powerpacketc)areputinsideorconnectedtothedrillhousing,soitshouldhavethestrengthandrigiditynecessarytoholdandprotectthosecomponents.Thefollowingaresomeguidelinesforselectingapropermaterialforthedrillhousinginordertomeetparticularrequirement.
1.1.1DropTestRequirement:
DropTestHeight
SuitableMaterial(s)
Lessthan6ft
ABS
Morethan6ft
ABS-GSM,PC,PC+ABS,Nylon+GF,PC+GF
Anothersuggestionis,iftheweightofthedrill(includepowerpack)islessthan2.1kg,ABSispreferable.Iftheweightisheavierthanthat,ABS-GSM,PC+ABS,PC,PC+GForNylon+GFaresuitable.
1.1.2TemperatureRequirement:
HeatingPartTemperature
(E.g.GearBox,Motor)
SuitableMaterial(s)
Lessthan100oC
ABS
Morethan100oCbutlessthan120oC
PC
Morethan120oC
Nylon+GF,PC+GF
Moreover,UL94HBframegradematerialscanbeusedforthedrillhousingunlessspecifiedrequirement.Forexample,iftherehasnomotormountfortheACdrillsinwhichthemotorwilldirectcontactwiththehousing,theframeandtemperaturegraderequirementinthiscasearehigher.
1.2ChargerHousingMaterial
Thecomponentsinsidethechargerhousing(Transformer,PCBetc)aremostproperlyproduceheat,soinadditiontothedroptestrequirementdescribedinabove,thematerialitselfshouldhavetheabilitytoresisthightemperature.
-V0ABScanbeusewhenthetemperatureislow(Below100oC).
-Ifthetemperatureoftheheatgeneratingpartsislargerthan100oC,PC,High-temperature-PCissuitable.
-TheminimumframegraderequirementofallhousingmaterialisUL94V0.
1.3PowerPackHousingMaterial
Powerpackcanalsoproduceheatwhenitdischargecontinuously(Continueuse)orimproperusesuchaslockthemotoratalongtimeorevenshorttheterminals.Thehigherthevoltageofthedrill,thehigherthetemperaturethepowerpackmayproduce.
-Ifthevoltageofthedrillis4Vto12V,V0ABScanbeused,butifthevoltageislargerthan12V,GSMorevenNylon+GFcanbeused.
1.4GearBoxHousingandGearMaterial
Gearboxisthecomponentdesignedfortheconditionthatundercontinuousdynamicalloadandsuddenlargershock,soitmustbestrongenoughtostandinthiskindofenvironment.
-Nylonplus30%to50%GFisasuitablematerialforthegearboxhousing.
-PCshouldnotbeusedasthegearboxhousingbecauselubricantmaydamagetheinternalstructureofitandproducecrack.
-POMcanbeusedforthegears,itwillproducelittlenoisewhenthegearrunsbutitisbrittle.Delrin100/500isasubsetofPOMbutitismineralreinforced.Itprovideshigherimpactforceresistant.
1.5BlowMouldCaseMaterial
-PPorABSwillnotbeusedinblowmouldcasebecausetheyareeasilydamageundershockorimpact.High-densityPEisusedinstead.
1.6TransparentMaterial
ThematerialsweusedthataretransparentincludePMMA,SAN,PCandABS.
-PCcanresisthightemperatureandhashightransparenceissuitabletouseforlens.
-Sanissuitableformakingleveler.PMMAwillappearasyellowcolor.
-SanandPMMAaregoodforultra-sonicwelding.
1.8AdhesionofMaterial
-Insomecase,twoofmoretypesplasticmaterialsareneededtoadheretogether(e.g.Overmoulding),table1showsthedifferentadhesivepropertiesofdifferentplasticmaterials.
Key:
‘+’=GoodAdhesion‘O‘=PoorAdhesion‘-’=NoAdhesion
Table1.Thistableisonlyintendedasaguide.Ifindoubttheadhesionofthematerialsmustbetestedintrials.
1.9RemarksforDifferentPlasticMaterials
-PC+GFcanincreasetheextentofresistingimpactforce.IthaslessshrinkageeffectbuthaslowerresistanttohightemperaturethanNylon+GF.
-PC+ABScanincreasetheextentofresistingimpactforceandcanprovideshinysurface.
-ABScannotprovideshinysurface.
-Nylon+GFandPCisgoodforfireresistance.
2StructuralDesign
2.1GeneralRequirements
-Avoidnon-uniformwallthicknessofallplasticcomponents.
-Avoidsharpcorners;applyaminimumfilletofaboutR0.5tothem(Figure1).
Figure1.
-Whendesigningventilationhole,ifitistoolong,asmallsupportwithrecessof0.5mmwiththeoutsidesurfaceofthehousingshouldbeplacedinthemiddleoftheholetoincreasethestrength(Fig2).
ASmallSupportingRecess
Figure2.
2.1.1RibThicknessandHeight
-Ribthicknessshouldequaltothehalfofthewallthick,e.g.ifthehousingwallthickness=3mm,ribthicknessshouldbe1.5mm.
-Ribheightshouldbeatlast2.5-3.0timesthewallthicknessforeffectivestrength.
-Ifmultipleribsareneeded,theyshouldbespacedatleast2timesthewallthicknessaparttoreducemoldedinstressandproblemsincoolingofthemold.
-FilletsattheribbaseshouldbeR0.5topermitbetterstressdistributionattheribbase.
-Avoidtoohighandtoothickribsasthismaycauseproblemssuchassinkmarks,voidsandwarpage.Furthermore,ifribheightistoogreat,thereisariskthattheribstructurewillbulgeunderload.
2.1.2RibsPlacement
Ingeneral,ribhastwofunctions:
Constructedtoholdparticularcomponentandincreasetherigidityofthehousing.Forthefirstpoint,wehaveribstoholdtheswitch,motor,andgearboxindrillhousing,PCBandtransformerinchargerandbatteriesinpowerpack.Howtoplacetheribsisonlydeterminebythelocationwherethosecomponentsshouldbeplaced.Forthesecondpoint,thebriefguidelineistoaddribstopotentialweakpoints.Thiskindofpointsisratherdifficulttofindandmaybedeterminedbyexperienceandtestintrial.
2.2DrillHousingRequirements
-FETofswitchshouldnotmakeadirectcontactwiththedrillhousingwall.
-Thespacebetweenthehousingwallandtheshaftatthebackofthemotorshouldkeepatleast0.5mm(Themaximumendplayofmotoris0.5mm).
-Ribsusedforlocatingthegearbox,motorandheatsinkshouldstrongenoughtokeepallpartsreliablysecuredinplace.
2.2.1WallThickness
-Ingeneral,iftheweightofdrill(Includepowerpack)issmallerthan2kg,thewallthicknessofitshousingcanbe2.5mm,iftheweightisheavierthanthat,3mmissuitable.Whenitisextremelyheavy,thickerwallcanbeused.
2.2.2VentilationHoles
-Ventilationholesshouldbelocatedontheleftandrightside(Housingsupportandcover)ofthehousingandrightbesideattheairintakeandexhaustpositionofthemotor.
-Tominimizetheinterferencebetweentheventilationholesandtheinternalribs,theopeningsshouldbeorientedverticallyinthehousing.
-Therehasarequirementforanopeningintheenclosureofatoolthatishand-supportedduringintendeduseinUL45section6.6.
-ForDoubleinsulatedproducts,UL45section58.3statedtherequirement.
2.3ChargerHousingRequirements
-RibsinchargershouldbedesignedtohelptodirectthePCBintoitsposition.
-Theheatgeneratingpartsinsidethechargershouldnotmakeadirectcontactwiththehousingwall.Aminimumspaceof2mmisrecommended.
-Thechargershouldonlyadmittheappropriatepowerpack.Powerpackwithdifferentvoltageshouldnotbeallowedtoplugintothecharger(Formonocharger).
-UL1310(5.3):
Anenclosure,anopening,aframe,aguard,aknob,ahandle,orthelikeshallnotbesufficientlysharptocauseariskofinjuryinnormalmaintenanceoruse(Class2PowerUnits).
-UL1310(5.4):
Aunitshallbeconstructedsothatitwillnotbenecessarytoopenorremovetheenclosurewhentheunitisusedasintended(Class2PowerUnits).
-Forpowerunitsotherthanclass2,UL1012(7.1.3):
Considerationshallbegiventoaconstructioninwhichlivepartsarerecessedatleast1/8inch(3.2mm)fromtheplaneofthefrontofthefixedportionoftheenclosure,andtoanequivalentconstructionincorporatingprojectionsorguards.
2.3.1WallThickness
-Sincechargerisaheat-generatingcomponent,inordertostandmoreheat,itswallthicknessshouldbekeptas3mm.
2.3.2VentilationHoles
-Theventilationholesshouldbelocateddirectlyabovetheheatgeneratingpartswithnootherobjectblockingtheairflow.
-Avoiddirectopeningofventilationholeincharger(Figure3).
-
SizeofopeningsshallmeetULstandard(UL1310(14))forclass2powerunits.
Figure3(a)Figure3(b)
Figure3(a).Ventilationholesarenotopendirectly.Figure3(b).Theopeningscanonlybeseeninsideways.
2.4PowerPackHousingRequirements
2.4.1WallThickness
-Generalwallthicknessshouldbe3mm.
2.5GearBoxHousingandGearRequirements
-Formotorpinion,ifitsaccuracyorresistimpactforceabilityisconcerned,machinecutofpinionispreferredratherthanusingpowdermetal.
2.5.1WallThickness
-Generalwallthicknessofthegearboxhousingshouldbe2mm.
3HousingInjectionDailyOutCapability
Housing
AverageOutputin22hrs
Drill
1400pcs
PowerPack
2000pcs
Charger
1600pcs
BlowMouldCase
660pcs
*Discusswithtoolinggroupforbetterdesignofthetooling.
4ToolingConsideration–DesignoftheToolingandToolingMaterialSelection
4.1ToolingDesign
-Uniformwallthickness.
-Avoidsharpcorner(MinimumR0.5fillet).
-
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