Injection Molding2.810Professor Tim GutowskiShort history of plastics1862 first synthetic plastic1866 Celluloid1891 Rayon1907 Bakelite1913 Cellophane1926 PVC1933 Polyethylene1938 Teflon1939 Nylon stockings1957 velcro1967 “The Graduate”Ref Kalpakjian and SchmidMcCrum, Buckley, BuckknallOutline• Basic operation• Cycle time and heat transfer• Flow and solidification• Part design• Tooling• New developments• EnvironmentReadings• Tadmore and Gogos– Molding and Casting pp 584 -610• Boothroyd Dewhurst– Design for Injection Molding pp 319 - 359• Kalpakjian Ch 7 & 19• Thiriez et al, "An Environmental Analysis of InjectionMolding“• "Injection Molding Case Study“ (Gas Assist)30 ton, 1.5 oz (45 cm3) EngelInjection Molding Machinefor wheel fabricationProcess & machineschematics** Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htm *Schematic of thermoplastic Injection molding machineProcess Operation• Temperature: barrel zones, tool, die zone• Pressures: injection max, hold• Times: injection, hold, tool opening• Shot size: screw travelFlashMeltThermaldegradationShort-shotTemp.PressureProcessing windowTypical pressure/temperature cycle( )polymersfor sec10 thicknesshalf232cmtcool!==""Time(sec)Cooling time generally dominates cycle timeTime(sec)* Source: http://islnotes.cps.msu.edu/trp/inj/inj_time.html **Calculate clamp force, & shotsizeF=P X A = 420 tons3.8 lbs = 2245 cm3 =75 oz Actual ; 2 cavity 800 tonClamp force and machine costHeat transfer Note; αTool > αpolymer!!t("# c #T )$x$y = %!qx!x$x$y)()'( kind 3rdconstant)'( kind 2ndconstant)'( kind1st !"==##"==##"==TThxxxTkxxxTkxxTBoundary Conditions:1-dimensional heat conduction equation :The boundary condition of 1st kind applies to injection molding since thetool is often maintained at a constant temperature xTkqx!!"=qxqx + Δqx!" c "#T#t= k#2T#x2 or #T#t=$#2T#x2Fourier’s lawHeat transferTWTiitx+L-LLet Lch = H/2 (half thickness) = L ; tch = L2/α ; ΔTch = Ti – TW (initial temp. – wall temp.)Non-dimensionalize:2 ;1 ;LtFLxTTTTOWiW!=+=""=#$%22 !""##=##OFDimensionless equation:Initial condition1 0 ==!OFBoundary condition0 20 0====!"!"Separation of variables ; matching B.C.; matching I.C.!= )()(),( ""#gFfFOOCenterline, θ = 0.1, Fo = αt/L2 = 1Temperature in a slabBi-1 =k/hLSee Heat Transfer TextBy Lienhard on lineReynolds Number* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htmµ!µ!VLLVLV==s viscouinertia Re22Reynolds Number:For typical injection molding!= 1g cm3= 103N m4s2; LZ= 10"3m thicknessV #Part lengthFill time= 10"11s; µ= 103N $ s m2410Re!=For Die casting300101010103Re3313=!!"#$$$Viscous Shearing of FluidsvFhhvAF!F/Av/h1µNewtonian Viscosityhvµ!=Generalization:!µ"!=)(!"!#!!=!!Injection moldingrateshear :!!Typical shear rate forPolymer processes (sec)-1Extrusion 102~103Calendering 10~102Injection molding 103~104Comp. Molding 1~10“Shear Thinning”~ 1 sec-1 for PEViscous HeatingRate of Heating = Rate of Viscous Work2!"#$%&='='=hvhvAFVolvFVolPµRate of Temperature rise22or !"#$%&'=!"#$%&='hvcdtdThvdtdTcpp(µµ(Rate of Conduction out222~hTckdxTdckdtdTpp!""=##Tkv!=2Conductionheating ViscousµBrinkman numberFor injection molding, order of magnitude ~ 0.1 to 10Non-Isothermal FlowvFlow rate: 1/t ~V/LxHeat transfer rate: 1/t ~a/(Lz/2)2xzzxzLLVLLLV!=!!""414~rateHeat xfer rate Flow2For injection molding5.2101.0/101.0/1041~rateHeat xfer rate Flow23=!"#cmcmscmcmscmFor Die casting of aluminum2210101.0/3.01.0/1041~rateHeat xfer rate Flow!"#$cmcmscmcmscm* Very small, therefore it requires thick runnersSmall value=> Short shotInjection mold die cast moldFountain Flow* Source: http://islnotes.cps.msu.edu/trp/inj/flw_froz.html ; ** Z. Tadmore and C. Gogos, “Principles of Polymer Processing” ***Shrinkage distributions* Source: G. Menges and W. Wubken, “Influence of processing conditions on Molecular Orientation in Injection Molds” V=3.5cm/sV=8cm/ssampleTransverse directionGate Location and WarpingCenter gate: radial flow – severe distortionDiagonal gate: radial flow – twisting End gates: linear flow – minimum warpingGateAir entrapmentEdge gate: warp free, air entrapmentSprue2.02.060°Before shrinkage60.32°1.961.976After shrinkageShrinkageDirection of flow – 0.020 in/inPerpendicular to flow – 0.012Effects of mold temperature andpressure on shrinkage0.0300.0000.0100.0050.0150.0200.025100 120 140 160 180 200 220 240Mold Temperature (F)LDPE PPNylon 6/6PMMAAcetalShrinkage0.0300.0000.0100.0050.0150.0200.025Shrinkage6000800010000120001400016000Pressure on injection plunger (psi)AcetalLDPENylon6/6PP withflow18000PP acrossflowPMMAWhere would you gate this part?Weld line, Sink mark* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_design_7.htm Weld lineMold FillingGateSolidified partSink markBasic rules in designing ribsto minimize sink marksInjection Molding*** Source: http://www.idsa-mp.org/proc/plastic/injection/injection_design_2.htmWhere is injection molding?Controlled by shrinkageand warping. Hence,polymer, fillers, moldgeometry and processingconditions can allinfluence the finaltolerance.Shrinkage is of order10-100/1000 for unfilledand1-10/1000 for filled acrossthe thicknessEffects of mold pressure onshrinkage0.0300.0000.0100.0050.0150.0200.025Shrinkage6000800010000120001400016000Pressure on injection plunger (psi)AcetalLDPENylon6/6PP withflow18000PP acrossflowPMMATooling BasicsCavity PlateCavityMouldingCoreCore PlateBasic mould consisting of cavity and core plateRunnerCavityGateNozzleSprueMelt DeliveryPartCavityCoreStripper plateTooling for a plastic cupRunnerKnobNozzleTooling for a plastic cupRunnerPartCavityNozzlePartCavityKnobStripperplateRunnerPartCavityNozzleTooling* Source: http://www.idsa-mp.org/proc/plastic/injection/; ** http://www.hzs.co.jp/english/products/e_trainer/mold/basic/basic.htm (E-trainer by HZS Co.,Ltd.)****** **Part design rules• Simple shapes to reduce tooling cost– No undercuts, etc.• Draft angle to remove part– In some cases, small angles (1/4°) will do– Problem for gears• Even wall thickness• Minimum wall thickness ~ 0.025 in• Avoid sharp corners• Hide weld lines– Holes may be molded 2/3 of the way through thewall only, with final drilling to eliminate weld linesNew developments- Gasassisted injection moldingNew developments ; injectionmolding with coresCores and Part Molded in Clear Plastic Cores used in Injection Molding Injection
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