05polymer_2_6f1 - 2.008 Design & Manufacturing II Spring...

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1 2.008-spring-2004 S. Kim 1 2.008 Design & Manufacturing II Spring 2004 Polymer Processing II Injection Molding 2.008-spring-2004 S. Kim 2 What is a mold? -Net shape manufacturing -Volume vs. cost -Life of a mold From 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 3 Casting 1. 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 4 Die casting 1. Assemble a mold 2. Inject molten metal into 3. Open and remove aluminum, magnesium Precision parts 2.008-spring-2004 S. Kim a slide from B. Kim, 1982, MIT 5 Injection Molding 2.008-spring-2004 S. Kim 6 Injection Molding Machine
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2 2.008-spring-2004 S. Kim 7 Steps of Injection Molding -Mold closing -Filling, packing, holding -Cooling -Opening, part removal. 2.008-spring-2004 S. Kim 8 Injection Cycle Time –$$$ – Typical Cycle of Injection Molding •M o l d C l o s e 1 - 2 s e c • Injection 2-5 sec • Pack and Hold 8-10 sec • Part Cool 10-20 sec – Screw return 2-5 sec • Mold open 1 sec •E j e c t i on 1 s e c 2.008-spring-2004 S. Kim 9 Design for Manufacturing • Part design – Moldable –D r a f t a n g l e –S h r i n k a g e – Reinforcements (ribs and bosses) –C y c l e t im e – Appearance (defects) • Mold Design –G a t e – balancing • Process Control 2.008-spring-2004 S. Kim 10 Injection molding process window Good Pmax Pmin Tmin Tmax shortshot flash Poor quality Degradation burn Better weld Low cost 2.008-spring-2004 S. Kim 11 Flashes Flashes 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. Cause Poor quality of the mold. The molten polymer has too low viscosity. Injection pressure is too high, or clamping force is too weak. Solution Avoiding 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 12 Short shot This is the phenomenon where molten plastics does not fill the mold cavity completely. and the portion of parts becomes incomplete shape. Cause The 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. Solution Apply higher injection pressure. Install air vent or degassing device. Change the shape of the mold or gate position for better flow of the plastics.
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3 2.008-spring-2004 S. Kim 13 Injection 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) • Nozzle 2.008-spring-2004 S. Kim 14 rough surface smooth surface Plasticating Extrusion 2.008-spring-2004 S. Kim
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This note was uploaded on 02/23/2012 for the course MECHANICAL 2.008 taught by Professor Jung-hoonchun during the Spring '04 term at MIT.

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05polymer_2_6f1 - 2.008 Design & Manufacturing II Spring...

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