06polymer_3_6 - 2.008 Design Manufacturing II Spring 2004...

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1 2.008-spring-2004 S. Kim 1 2.008 Design & Manufacturing II Spring 2004 Polymer Processing III Thermoforming 2.008-spring-2004 S. Kim 2 Design for Manufacturing ± Moldable: flow path ratio, machine size ± Draft angle ± Shrinkage ± Reinforcements (ribs and bosses) ± Cycle time ± Appearance, defects ± Balance, balance, balance!! 2.008-spring-2004 S. Kim 3 Gate -Restricts the flow and the direction of molten plastics. -Quickly cools and solidifies to avoid backflow after molten plastics has filled up in the cavity. -Simplifies cutting of a runner and moldings to simple finishing of parts. Side gate Submarine gate 2.008-spring-2004 S. Kim 4 Gate Positioning Point 1: Set a gate position where molten plastics finish filling up in each cavity simultaneously. Same as multiple points gate. Point 2: Basically set a gate position to the thickest area of a part. This can avoid sink marks due to molding (part) shrinkage . Point 3: Set a gate position to an unexposed area of part or where finishing process can be easily done. Point 4: Consider degasing, weldline, molecular orientation. Point5: Fill up molten plastics using the wall surface in order not to generate jetting. Die swell > Thickness, t t 2.008-spring-2004 S. Kim 5 2.008-spring-2004 S. Kim 6 Molecular orientation Gate 1 Gate 2 C C CC C C C C C C C C C σ σ
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2 2.008-spring-2004 S. Kim 7 Defects Molding defects are caused by related and complicated reasons as follows: * Malfunctions of molding machine * Inappropriate molding conditions * Bad product and mold design * Improper Selection of molding material 2.008-spring-2004 S. Kim 8 Sink marks -Equal cooling from the surface -Secondary flow -Collapsed surface Æ Sink Mark ts t ts < t 2.008-spring-2004 S. Kim 9 Weldline It is a boundary between flows caused by incomplete fusion of molten plastics. It often develops around the far edge of the gate. Cause Low temperature of the mold causes incomplete dissolution of the molten plastics. Solution Increase injection speed and raise the mold temperature. Increase the melt temperature and increase the injection pressure. Change the gate position to prevent development of weldline at high stress area. 2.008-spring-2004 S. Kim 10 Jetting This is the phenomenon where the part has a toothpaste flow pattern on the surface. Cause Due to inappropriate gate position, a flow of molten plastics into the cavity is cooled in a line shape and remains unfused with other plastics flow coming later. Solution Change the gate position to make the molten plastics touch the facing side before making a line shape. Die swell > Thickness, t t 2.008-spring-2004 S. Kim 11 Die swell ± Exit zone- die ± die imparts shape on the material, e.g., rod, tube, sheet, channel ± exit material is called extrudate ± extrudate swells at end of die due to normal forces from the polymer flow, called die swell Die Swell 2.008-spring-2004 S. Kim 12 Viscousity ± Shear thinning: paints Shear Rate V i s c o t y Newtonian Shear Thinning Shear Thickening
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3 2.008-spring-2004 S. Kim 13 Flow mark This is a phenomenon where the initial flow of molten plastics which solidifies mixes with a later flow and remains undissolved. It develops distinctive
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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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06polymer_3_6 - 2.008 Design Manufacturing II Spring 2004...

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