Injection Molding - MIT

1 Injection GutowskiD. Roylance1V-6 air intake manifoldWater control valveBrass Vs Injection history of plastics1866 Celluloid1891 Rayon1907 Bakelite1913 Cellophane1926 PVC1933 Polyethylene1938 Teflon1939 Nylon stockings1957 velcro1967 The Graduate 1970 Earth Day recycling 3 Ref Kalpakjian and SchmidMcCrum, Buckley, Bucknall4 OutlineBasic operationCycle time and heat transferFlow and solidificationPart designToolingNew developmentsEnvironment530 ton, fl oz (45 cm3) EngelInjection Molding Machinefor wheel fabrication6 Process & machine schematics** Source: *Schematic of thermoplastic Injection Molding machine7 Process OperationTemperature: barrel zones, tool, die zonePressures: Injection max, holdTimes: Injection , hold, tool openingShot size.

2 Screw window8 Typical pressure/temperature cycleTime(sec)Cooling time generally dominates cycle timeTime(sec)* Source: **9 Calculate clamp force, & shot sizeF=P X A = 420 lbs = 2245 cm3=75 ozActual ; 2 cavity 800 ton10 Clamp force and machine costBoothroyd/Busch11 Heat transfer Note; aTool>apolymerBoundary Conditions:1-dimensional heat conduction equation :The boundary condition of 1stkind applies to Injection Molding since the tool is often maintained at a constant temperatureqxqx+ DqxFourier s law12 Heat transferTWTiitx+L-LLet Lch= H/2 (half thickness) = L ; tch= L2/a ;DTch= Ti TW(initial temp. wall temp.)Non-dimensionalize:Dimensionless equation:Initial conditionBoundary conditionSeparation of variables ; matching ; matching , q = , Fo= at/L2= 1 Temperature in a slabBi-1=k/hLSee Heat Transfer TextBy Lienhard on line 14 Reynolds Number* Source: Number:For typical Injection moldingFor Die casting15 Viscous Shearing of FluidsvFhF/Av/h1mNewtonian ViscosityGeneralization: Injection moldingTypical shear rate for Polymer processes (sec)-1 Extrusion102~103 Calendering10~102 Injection molding103~104 Comp.

3 Molding1~10 Shear Thinning ~ 1 sec-1for PE16 Viscous HeatingRate of Heating = Rate of Viscous WorkRate of Temperature riseRate of Conduction outBrinkman numberFor Injection Molding , order of magnitude ~ to 1017 Non-Isothermal FlowvFlow rate: 1/t ~V/LxHeat transfer rate: 1/t ~a/(Lz/2)2 For Injection moldingFor Die casting of aluminum* Very small, therefore it requires thick runnersSmall value=> Short shotP clet FlowvFlow rate: 1/t ~V/LxHeat transfer rate: 1/t ~a/(Lz/2)2 For Injection moldingFor Die casting of aluminumVery small value for aluminum requires thicker runnersSmall value=> Short shotP clet mold die cast mold20 Fountain Flow* Source: ; ** Z. Tadmore and C. Gogos, Principles of Polymer Processing **21 Shrinkage distributions * Source: G.

4 Menges and W. Wubken, Influence of processing conditions on Molecular Orientation in Injection Molds V= direction22 Gate Location and WarpingCenter gate: radial flow severe distortionDiagonal gate: radial flow twistingEnd gates: linear flow minimum warpingGateAir entrapmentEdge gate: warp free, air Before shrinkageShrinkageDirection of flow in/inPerpendicular to flow of mold temperature and pressure on Temperature (F)LDPEPPN ylon 6 on Injection plunger (psi)AcetalLDPEN ylon 6/6PP with flow18000PP across flowPMMA24 Where would you gate this part?25 Design Features** Source: line, Sink mark* Source: lineMold FillingGateSolidified partSink markBasic rules in designing ribs to minimize sink marks2728 Where is Injection Molding ?

5 Controlled by shrinkageand warping. Hence,polymer, fillers, moldgeometry and processingconditions can allinfluence the is of order10-100/1000 for unfilledand1-10/1000 for filled acrossthe thickness29 Where is Injection Molding ?Controlled by shrinkageand warping. Hence,polymer, fillers, moldgeometry and processingconditions can allinfluence the is of order10-100/1000 for unfilledand1-10/1000 for filled acrossthe thicknessFrom Boothroyd et al 30 Tooling BasicsCavity PlateCavityMouldingCoreCore PlateBasic mould consisting of cavity and core plateRunnerCavityGateNozzleSprueMelt Delivery31 PartCavityCoreStripper plateTooling for a plastic cupRunnerKnobNozzle32 Tooling for a plastic cupRunnerPartCavityNozzlePartCavityKnobS tripper plateRunnerPartCavityNozzle33 Toy tooling examplefrom ProtomoldpartSide pullSide pull in position34 Tooling* Source: ; ** (E-trainer by HZS Co.)

6 ,Ltd.)**35 Tooling AlternativesKalpakjian & Schmid36 Undercut features to hold tape onEvidence of toolingfeatureInjection Molding Homework37 Part design rulesSimple shapes to reduce tooling cost No undercuts, angle to remove part In some cases, small angles (1/4 )will do Problem for gearsEven wall thicknessMinimum wall thickness ~ inAvoid sharp cornersHide weld lines Holes may be molded 2/3 of the way through the wall only, with final drilling to eliminate weld lines38 Novel development-Gas assisted Injection molding39 Novel development ; Injection Molding with coresCores and Part Molded in Clear PlasticCores used in Injection MoldingInjection Molded Housing40 Micro Injection molding41 Conformal Cooling built usingAdditive Manufacturing42 Environmental issuesEnergy Polymer production Compounding Machine typesRecycling4344 Polymer ProductionLargest Player in the Injection Molding LCIWhat is a polymer:How much energy does it take to make 1 kg of polymer = a lot !

7 !!Values are in MJ per kg of polymer 0645 Gold~ 250 GJ/kgAshby 200946 Compounding -extrusionAn extruder is used to mix additives with a polymer base, to bestow the polymer with the required to an Injection Molding machine, but without a mold and continuous it has a similar energy consumption Unfriendly Additives: Fluorinated blowing agents (GHG s) Phalates (some toxic to human liver, kidney and testicles) Organotin stabilizers (toxic and damage marine wildlife)47 Injection Molding ProcessSource: types: Hydraulic, electric, hydro-electric48 All-electric vs. hybridThe hydraulic plot would be even higher than the hybrid curveSource: [Thiriez]49 For Hydraulics and Hybrids as throughput increases, SEC value to melt plastics is just to MJ/kg !

8 !!Does not account for the electric grid. Source: [Thiriez]50 All-electrics have very low fixed energy costs (small idling power). SEC is constant as throughput : [Thiriez]51 DriersUsed to dry internal moisture in hygroscopic polymers and external moisture in non-hygroscopic ones. It is done before extruding and Injection : [Thiriez]Same as52 LCI Summarized Results53 Source: [Thiriez]5455Do Polymers get recycled?Ref Ashby 200956 The printer goes in the comes (first 3) & RefsTadmore 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 Injection Molding " Injection Molding Case Study (Gas Assist)59