Injection MoldingShort history of plasticsOutlineReadings30 ton, 1.5 oz (45 cm3) EngelProcess & machine schematicsProcess OperationTypical pressure/temperature cycleCalculate clamp force, & shot sizeClamp force and machine costHeat transfer Note; aTool > apolymer Heat transferReynolds NumberViscous Shearing of FluidsViscous HeatingNon-Isothermal FlowInjection mold die cast moldFountain FlowShrinkage distributions Gate Location and WarpingEffects of mold temperature and pressure on shrinkageWhere would you gate this part?Weld line, Sink markInjection MoldingEffects of mold pressure on shrinkageTooling BasicsTooling for a plastic cupTooling for a plastic cupToolingPart design rulesNew developments- Gas assisted injection moldingNew developments ; injection molding with coresEnvironmental issuesPolymer Production Largest Player in the Injection Molding LCICompounding - extrusionInjection Molding ProcessAll-electrics have very low fixed energy costs (small idling power). SEC is constant as throughput increases. For Hydraulics and Hybrids as throughput increases, SEC k.All-electric vs. hybridDriersEnergy Production IndustryScaleThe printer goes in the hopper…And comes out….ReadingsInjection Molding2.810 Fall 2008Professor Tim GutowskiShort history of plastics1862 first synthetic plastic1866 Celluloid1891 Rayon1907 Bakelite1913 Cellophane1926 PVC1933 Polyethylene1938 Teflon1939 Nylon stockings1957 velcro1967 “The Graduate”Outline• Basic operation• Cycle time and heat transfer• Flow and solidification• Part design• Tooling• New developments• EnvironmentReadings• Tadmore and Gogos– Molding and Casting pp584 -610•BoothroydDewhurst– Design for Injection Molding pp 319 - 359•Kalpakjian(5thed) see Ch 19• Injection molding case study;Washing machine augers; see on web page30 ton, 1.5 oz (45 cm3) EngelInjection Molding Machinefor wheel fabricationProcess & machine schematics** 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−==ααTime(sec)Cooling time generally dominates cycle timeTime(sec)* Source: http://islnotes.cps.msu.edu/trp/inj/inj_time.html**Calculate clamp force, & shot sizeF=P X A = 420 tons3.8 lbs = 2245 cm3=75 ozActual ; 2 cavity 800 tonClamp force and machine costHeat transfer Note; αTool> αpolymeryxxqyxTctxpΔΔ∂∂−=ΔΔ⋅⋅∂∂)(ρ)()'( kind 3rdconstant)'( kind 2ndconstant)'( kind1st ∞−==∂∂−==∂∂−==TThxxxTkxxxTkxxTBoundary Conditions:1-dimensional heat conduction equation :The boundary condition of 1stkind applies to injection molding since the tool is often maintained at a constant temperature xTkqx∂∂−=qxqx+ Δqx2222or xTtTxTktTcp∂∂=∂∂∂∂=∂∂⋅αρFourier’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⋅=+=−−=αξθ22 ξθθ∂∂=∂∂OFDimensionless equation:Initial condition1 0 ==θOFBoundary condition0 20 0====θξθξSeparation of variables ; matching B.C.; matching I.C.∑= )()(),( ξξθgFfFOOCenterline, θ = 0.1, Fo= αt/L2= 1Temperature in a slabBi-1=k/hLReynolds Number* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_process.htmμρμρVLLVLV==s viscouinertia Re22Reynolds Number:For typical injection molding2313243310 ;110 timeFilllengthPart ess thickn10 ;101msNsVmLsmNcmgZ⋅==≈===−−μρ410Re−=For Die casting300101010103Re3313=××⋅≈−−−Viscous Shearing of FluidsvFhhvAF∝F/Av/h1μNewtonian Viscosityhvμτ=Generalization:γμτ&=)(γηγτ&&=γ&Injection moldingrateshear :γ&Typical shear rate for Polymer processes (sec)-1Extrusion 102~103Calendering 10~102Injection molding 103~104Comp. Molding 1~10“Shear Thinning”~ 1 sec-1for PEViscous HeatingRate of Heating = Rate of Viscous Work2⎟⎠⎞⎜⎝⎛=⋅=⋅=hvhvAFVolvFVolPμRate of Temperature rise22or ⎟⎠⎞⎜⎝⎛⋅=⎟⎠⎞⎜⎝⎛=⋅hvcdtdThvdtdTcppρμμρRate of Conduction out222~hTckdxTdckdtdTppΔ⋅⋅=ρρTkvΔ=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⋅=⋅⋅αα414~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 and pressure 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)AcetalLDPENylon 6/6PP with flow18000PP across flowPMMAWhere would you gate this part?Weld line, Sink mark* Source: http://www.idsa-mp.org/proc/plastic/injection/injection_design_7.htmWeld lineMold FillingGateSolidified partSink markBasic rules in designing ribs to minimize sink marksInjection Molding*** Source: http://www.idsa-mp.org/proc/plastic/injection/injection_design_2.htmWhere is injection molding?Effects of mold pressure on shrinkage0.0300.0000.0100.0050.0150.0200.025Shrinkage6000800010000120001400016000Pressure on injection plunger (psi)AcetalLDPENylon 6/6PP with flow18000PP across
View Full Document
Unlocking...