12.008-spring-2004 S. Kim 12.008 Design & Manufacturing IISpring 2004Polymer Processing IIInjection Molding2.008-spring-2004 S. Kim 2What is a mold?-Net shape manufacturing-Volume vs. cost-Life of a moldFrom Webster: a cavity in which a substance is shaped: as (1) : a matrix for casting metal (2) : a form in which food is given a decorative shape 2.008-spring-2004 S. Kim 3Casting1. Assemble a mold.2. Pour molten metal in.3. Cool down.4. Open the mold and remove the part.2.008-spring-2004 S. Kim 4Die casting1. Assemble a mold2. Inject molten metal into3. Open and removealuminum, magnesiumPrecision parts2.008-spring-2004 S. Kim a slide from B. Kim, 1982, MIT 5Injection Molding2.008-spring-2004 S. Kim 6Injection Molding Machine22.008-spring-2004 S. Kim 7Steps of Injection Molding-Mold closing-Filling, packing, holding-Cooling-Opening, part removal.2.008-spring-2004 S. Kim 8Injection 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. Kim 9Design 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 10Injection molding process windowGoodPmaxPminTmin TmaxshortshotflashPoorqualityDegradation burnBetter weldLow cost2.008-spring-2004 S. Kim 11FlashesFlashes develop at the mold parting line or ejector pin installation point. It is a phenomenon where molten polymer smears out and sticks to the gap.CausePoor quality of the mold. The molten polymer has too low viscosity. Injection pressure is too high, or clamping force is too weak.SolutionAvoiding excessive difference in thickness is most effective.Slow down the injection speed.Apply well-balanced pressure to the mold to get consistent clamping force, or increase the clamping force.Enhance the surface quality of the parting lines, ejector pins and holes.2.008-spring-2004 S. Kim 12Short shotThis is the phenomenon where molten plastics does not fill the mold cavity completely. and the portion of parts becomes incomplete shape.CauseThe shot volume or injection pressure is not sufficient. Injection speed is so slow that the molten plastics becomes solid before it flows to the end of the mold.SolutionApply higher injection pressure. Install air vent or degassing device. Change the shape of the mold or gate position for better flow of the plastics.32.008-spring-2004 S. Kim 13Injection 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 14rough surfacesmooth surfacePlasticating Extrusion2.008-spring-2004 S. Kim 15Suggested Melt Temp at nozzle• Acetal (coploymer) 400 F• Acrylic 425 F• ABS 400 F• Liquid Crystal Polymer 500 F• Nylon 6 500 F• Polyamide-imide 650 F• Polyarylate 700 F• Polycarbonate 550 F• Polyetheretherketone 720 F• Polyethylene LDPE 325 F• Polyethylene HDPE 350 F• Polypropylene 350 F• Polystyrene 350 F• Thermoplastic polyester (PBT)425 F• Urethane elastomer 425 F2.008-spring-2004 S. Kim 16Temperature Cycle2.008-spring-2004 S. Kim 17Flow path ratio-Flow path ratio is the ratio between L (the distance between the gate and the farthest point in the molding dimension) and T (the thickness 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 = 160-90Acrylonitrile butadienstylene (ABS) L/T = 280-120 Polyamide (PA) L/T = 320-200Rule of thumb2.008-spring-2004 S. Kim 18Mold 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.42.008-spring-2004 S. Kim 19Mold 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 20WarpageThis deformation appears when the part is removed from the mold and pressure is released.CauseUneven shrinkage due to the mold temperature difference (surfacetemperature difference at cavity and core), and the thickness difference in the part. Injection pressure was too low and insufficient packing.SolutionTake a longer cooling time and lower the ejection speed. Adjust the ejector pin position or enlarge the draft angle. Examine the part thickness or dimension. Balance cooling lines. Increase packing pressure.2.008-spring-2004 S. Kim 21Pressure 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 22Pressure ControlEnd of filling2.008-spring-2004 S. Kim 23Pressure 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 closure2.008-spring-2004 S. Kim 24Clamp 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 ?52.008-spring-2004