Have you ever been to a mold trial and found the part difficult to eject?  Maybe you experienced this on a production tool.  Perhaps the draft angle is too small or the shrinkage cut into the tool is not accurate.  Both of these scenarios could be true, however it could also be an over-packing issue.  Over-packing can cause non-uniform part shrinkage and potential deformation of the part when the ejector system pushes on the part and the part does not release easily.
 
Over-packing can occur if too much demand is required of a gate on a part (amount of part volume the gate is required to fill).  If the gate must fill too much cavity volume, the pressure requirement near the gate would increase at the end of the filling cycle which can cause the part to stick in the mold.  The high pressure region near the gate is where the part would be overpacked. This can occur with a single gate or multiple gates and depends on how much demand is required from each gate.
 
A Moldflow analysis can easily determine if the region near a gate will become over-packed.  If this is the case, there are a couple of potential solutions:

 

1.  If over-packing is determined to occur during the 1st stage, then the solution is to simply add more gates to the part.  More gates must be added to the part because the over-packing situation is influenced by the long flow length the material must travel through the cavity (part geometry). If the flow length becomes too long for one gate the flow length can be decreased by adding another gate.

 

2.  If over-packing is determined to occur during the 2nd stage pack phase, then it may be necessary to decay the pack profile.

 

Over-packing can be the cause of non-uniform part shrinkage and part deformation during ejection.  It is difficult to fix an over-packing issue once a tool is cut (especially if it is found to occur during 1st stage where more gates might be required).