Overpacking

Overpacking occurs when extra material is compressed in one flow path while other flow paths are still filling.

Overpacking

Overpacking, which usually occurs in sections with the shortest fill time, 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.

Overpacking occurs when the easiest, that is the shortest or thickest, flow paths fill first. When these flow paths have filled, they are still under pressure as extra plastic is injected into the cavity to fill the remaining flow paths. This pressure pushes 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 freezes under pressure so stresses are frozen in. In the following diagram, the white lines represent the polymer molecules. Note that the flow paths are not balanced and overpacking will occur to the left side of the part.

What to do

The key result that identifies overpacking is the fill time result. Display the fill time at 100 percent fill, and look for any flow paths that do not finish at the same time as the first path. Solving overpacking requires a rebalancing of the flow paths, which can be achieved by the following:

• Thicken or make thinner the parts of the model that 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.