This video shows how packing pressure influences the dimensions of the mold.
Plastic melt has a higher volume compared to solid unmolten plastic. Consider a mold cavity of dimension A. Refer to the video above. If it is filled with melt it will have less number of molecules compared to if it was solid unmolten plastic. As the melt cools down, it will occupy less space because of the shrinkage resulting in a part with a dimension smaller than the cavity dimension. Let us call value of the value of this dimension as B. During the shrinkage if more plastic is added (packing phase) and the added molecules reduce the inter molecular distance restricting the molecules to shrink to the previous levels. The part will therefore have a larger dimension and will be B which is greater than A. If the pressure is further increased during the packing even more plastic is added further reducing the inter molecular distance and therefore the shrinkage resulting in C which is greater than B.
So technically speaking, the part dimension did not increase but ‘The part shrinkage was reduced with an increase of packing pressure’.
There are 11+2 molding parameters that make a difference in the quality of the part. Why the +2? The first phase is the Injection Phase where the cavity is filled with molten plastic. The second phase is called the compensation phase where the shrinkage is being compensated. The compensation phase is further divided into the pack and the hold phase. Most processors set only one number for the compensation pressure and time and call this as the holding phase. Not accurate, but it has become acceptable. So if you consider pack and hold as different phases then you end up with 13.
Following are the 13 molding parameters. These are machine inputs.
There are a few other parameters that are secondary parameters that do not get adjusted as often as the 13 mentioned in the previous section. They also do not have a substantial effect on the part quality.
Here are the machine outputs that should be monitored. Outputs are more important than the inputs.
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