Answer to Question 1
The first step in solving the problem would be to investigate the heating. Are the temperatures too low for this resin or are there cold spots where the heaters are not operating properly? Is the polymer being mixed homogeneously or are dead spots occurring? Is there a combination of effects of heating and mixing? Lastly, is the feed homogeneous? The presence of large and small particles in the feed will complicate the melting process because the large particles take more time and energy to melt than so the small particles. This last problem may require some separation of the particles or a homogenizing step such as re-extrusion.
Answer to Question 2
Thick sections cool more slowly than thin sections because of the time required for the heat to move through the plastic material. The cooling occurs from the outside toward the middle because the heat is transferred most easily from the outside. Therefore, the inside is liquid after the outside is solid. Because of the natural shrinkage that occurs upon cooling, the liquid can still shrink whereas the outside, which is solid, cannot. This may cause a sink mark over the center
section as it cools and shrinks. This problem can be solved by making all the sections of the same thickness.
Answer to Question 3
Packing is the process of continuing to inject resin into the cavity even after the cavity is full. As the plastic shrinks from thermal processes
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