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MIT 2 008 - Injection Molding

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112.008 Injection Molding2.008 spring 2004 S. Kim 2Outline Polymer Equipment and process steps Considerations for process parameters Design for manufacturing, tooling and defects 2.008 spring 2004 S. Kim 3MaterialsSolid materialsmetalsceramicsPlasticsthermoplasticsthermosettselastomersPlastic: Greek,plastikos, means to form or mold2.008 spring 2004 S. Kim 4Plastics $400 billion shipments, 2002 US Music LPs, CDs No-sticking TEFLON Stre-e-e-tching SPANDEX www.plasticsdatasource.org ApplicationsName it22.008 spring 2004 S. Kim 5Automotive Plastics and Composites Use Exterior doors hoods  fenders  bumper covers (most cars have soft fascia) Interior instrument panels, door trim, seats, consoles Engine valve covers, intake manifolds, fluid containers, etc.2.008 spring 2004 S. Kim 6Plastics,Polymers, Macromolecules Poly (many) + mer (structural unit)-[C2H4]n- ,poly[ethylene] Metal: single atoms, metallic bond Ceramic: metallic oxides, ionic bond or dipole interactions, van der Waals bondsHHCCHHspaghettiHHCCHH2.008 spring 2004 S. Kim 7Thermoplastic vs. ThermosetAmorphousCrystalline(linear)Cross-linked (3D network)2.008 spring 2004 S. Kim 8CrystallinityCross-linkingStiffness32.008 spring 2004 S. Kim 9Specific Volume:Amorphous vs. CrystallineTgTg+60°CGlassy solidRubberyMeltVˆTgTmBrittleTough and flexibleMeltVˆ(a)(b)2.008 spring 2004 S. Kim 10Injection Molding Machine2.008 spring 2004 S. Kim 11Steps of Injection Molding – Mold closing2.008 spring 2004 S. Kim 12Mold filling42.008 spring 2004 S. Kim 13Packing, holding, cooling2.008 spring 2004 S. Kim 14Mold opening, part removalEjector pins2.008 spring 2004 S. Kim 15Injection Cycle Time $$$ Typical Cycle of Injection Molding Mold Close 1-2 sec Injection 2-5 sec Pack and Hold 8-10 sec Part Cool 10-20 sec Screw return 2-5 sec Mold open 1 sec Ejection 1 sec2.008-spring-2004 S. Kim2.008 spring 2004 S. KimTadmor & Gogos16Injection Molding Cycle52.008 spring 2004 S. Kim 17Injection Molding Parameters Temperature and Pressure: Function (x,y,z) Melt Temperature Control Through Cylinder(Barrel) Frictional Heating Heating bands for 3 zones Rear zone Center zone (10F-20F hotter) Front Zone (10F-20F hotter) Nozzle2.008 spring 2004 S. Kim 18Viscosity Shear thinning: paintsShear RateViscosityNewtonianShear ThinningShear Thickening2.008 spring 2004 S. Kim 19Pressure Control Pressure distribution Injection unit Initial injection pressure Applied to the molten plastic and resulting from the main hydraulic pressure pushing against the back end of the injection screw (or plunger).  Packing pressure Injection pressure inside mold Usually 1,000 psi to 5,000 psi Lower than hold and pack pressure between 10,000psi and 20,000 psi2.008 spring 2004 S. Kim 20Pressure Control Hold pressure (packing) Compensate shrinkage Rule of thumb: Hold pressure = 150% of injection pressure. Applied at the end of the initial injection stroke, and is intended to complete the final filling of the mold and hold pressure till gate closure62.008 spring 2004 S. Kim 21Pressure ControlEnd of filling2.008 spring 2004 S. Kim 22Clamp force Pressure Required Total force = projected area times injection pressure (A X P) Rule of thumb 4 to 5 tons/in2can be used for most plastics. Example, Part is 10 in by 10 in by 1 in Projected area = Surface area = 10 in x 10 in = 100 in2  Injection Pressure = 15,000 psi for PC Tonnage required to keep mold closed is  100 in2 x 15,000 psi= 1,500,000 lbs = 750 tons (note : 2000 lbs = 1 ton)10 in10 inParting line ?2.008 spring 2004 S. Kim 23Mold coolingMolding cycle can be shortened by reducing time for cooling and solidification of molten plastics. Solidification time, t thickness2/α, thermal diffusitivityWarpage or stress in a part can be generated when mold shrinkage varies due to different thickness, leading internal residual stress difference. Even cooling2.008 spring 2004 S. Kim 24Mold Temperature Control Mold Temperature Control Mold cooling with water, oil. Hot mold for less residual stresses (orientation) Low thermal inertia Uneven cooling warpage, twisting, shrinkage defects Shrinkage can progress for up to 30 days.72.008 spring 2004 S. Kim 25Flow path ratio-Flow path ratio is the ratio between L (the distance between thegate and the farthest point in the molding dimension) and T (thethickness of the part) .-When molding large or thin parts, the flow path ratio is calculated to determine if molten plastics can fill the mold cavity.Polyethylene (PE) L/T = 280-100 Polypropylene (PP) L/T = 280-150 Polyvinyl chloride (PVC) L/T = 280-70 Polystyrene (PS) L/T = 300-220 Polycarbonate (PC) L/T = 160-90 Acrylonitrile butadienstylene(ABS) L/T = 280-120 Polyamide (PA) L/T = 320-200Rule of thumb2.008 spring 2004 S. Kim 26Injection molding process windowGoodPmaxPminTminTmaxshortshotflashPoorqualityDegradation burnBetter weldLow cost2.008 spring 2004 S. Kim 27Design for Manufacturing Part design Moldable Draft angle Shrinkage Reinforcements (ribs and bosses) Cycle time Appearance (defects) Mold Design Gate balancing Process Control2.008 spring 2004 S. Kim 28Draft angle- For removing parts from the mold-1-2o, material, dimension, texture dependent- Cavity side smaller, core side larger.- Crystalline material has more shrinkage. - Amorphous material has smaller shrinkage.82.008 spring 2004 S. Kim 29Shrinkage∆L = α L∆T = α Tα: shrinkage rateResin Shrinkage (%) Polyethylene (PE) 1.5-6.0 Polypropylene (PP) 1.0-3.0 Polyvinyl chloride (PVC) 0.1-0.5 Polystyrene (PS) 0.2-0.6Polycarbonate (PC) 0.5-0.8 Acrylonitrile butadienstylene 0.3-0.8 Polyamide (PA) 0.6-2.0 2.008 spring 2004 S. Kim 30Reinforcement Thickness increase Stiffer grade material PP (unfilled), 4,400 psi tensile strength PP (20% glass filled), 7,700 psi Add secondary features, Ribs, bosses2.008 spring 2004 S. Kim 31Ribs and Bosses2.008 spring 2004 S. Kim 32Injection Molded Part Design Base feature + secondary feature (ribs, bosses, holes, etc.) Nominal wall : Keep part thickness as thin and uniform as possible. shorten the cycle time, improve dimensional stability, and eliminate surface defects.  For greater stiffness, reduce the spacing between ribs, or add more ribs.  Nominal wall thickness should be


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