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Solving molding overpacking problems
Overpacking is when extra material is compressed in one flow path while other flow paths are still filling.
Overpacking occurs when the easiest (shortest/thickest) flow paths fill first. Once this flow path has filled, it will still be under pressure as extra plastic is injected into the cavity to fill the remaining flow paths. This pressure will push more material into the already full flow path, causing it to have a higher density and lower shrinkage than other regions. The overpacked fill path will have frozen under pressure, so stresses will be frozen in.
Note: The key result used to identify overpacking is the fill time result. Display the fill time at 100% fill and look for any flow paths that do not finish at the same time as the first pat
In the above diagram, the white lines represent the polymer molecules. Note that the flow paths are not balanced and overpacking will occur in the left of the model.
Overpacking generally occurs in sections with the shortest fill time. It can cause a range of problems including warpage due to non-uniform shrinkage, increased part weight due to wasted material and non-uniform density distribution throughout the part.
What to do
To solve problems caused by overpacking, balance the flow paths.
Thicken or thin parts of the model to act as flow leaders or deflectors.
Move the injection location to a position that will define similar length flow paths.
Divide the cavity into imaginary sections, and use one injection location for each section.
Remove unnecessary gates.