Injection Molding Troubleshooting Tips

Tip #10: Parts Sticking

Occasionally a molder is confronted with a part that continuously or intermittently wants to stick in the mold. Aside from being a nuisance, a stuck part can cause damage to the mold and the sticking issue should be investigated and corrected.

There are a number of reasons that a part can stick in the mold. It can be due to part design, mold design, mold construction, processing or material. From a part design standpoint, does the part have undercuts that could cause it to hang-up in the mold? Or perhaps there is not enough draft on the part to allow it to be released off of the mold?

From a material standpoint one should verify whether the material contains a mold release additive or not. If it does not, a mold release / lubricant masterbatch could be added to help eliminate the problem or a spray on mold release could be evaluated. One should also ensure that the melt flow index of the material is correct and that an excessive amount of regrind is not being used. Both of these could cause the melt flow of the material to be higher which could result in over-packing of the part, leading to sticking.

From a tooling standpoint one should check to make sure there are no undercuts in the mold and that the part has adequate draft, especially if the part has a long pull. If the part design dictates an undercut, it may be difficult to change this feature. Likewise, if the mold does not have adequate draft, this may be difficult or impossible to modify.

From a processing standpoint, reducing the injection pressure, hold / pack pressure, hold / pack time, melt temperature and mold temperature can all be tried to help eliminate the part sticking issues. Increasing cooling time may also help. Using these process changes, singly or in combination will generally eliminate the sticking problem. If changes to the processing conditions doesn’t eliminate the problem, try adding a mold release masterbatch or using a mold release spray to see if this works. If not, then most likely the cause of part sticking may be due to an undercut or inadequate draft.

Entec Polymers Technical Service Team