Page 363 - Injection Molding Advanced Troubleshooting Guide

37.3 Pin Push Troubleshooting 359

There are cases where the machining marks must be polished out of the mold to
get adequate release. Many times, a draw polish will help the part release easier
and eliminate pin push.

Case Study: Polypropylene Pin Push

In this case a polypropylene part was being molded but the part had pin
push. Various process settings were tried to improve the pin push problem
without success. A tool maker reviewed the part and suggested that a draw
polish may help the problem. He did a paper draw polish on the core of the
mold, which solved the problem. Another example of “do not process
around a mold problem”.
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37.3.2.2„ Mold: Lack of Ejection
Every molded part will require a certain amount of force to eject from the mold.
This force is translated to the part through the ejector pins and lifters. If there is a
lack of ejection surface area the force on the pins will be higher and more likely to
create pin push.
When designing a mold it is critical to get adequate ejection surface area in the
mold. It is best to push a part detail off the core rather trying to pull a part detail
from the core. Whenever possible get pins, lifters, or sleeves under a part detail to
push with. Use large diameter pins where possible to avoid deflection and spread
the ejection force over a greater area. If there is a rib detail that a pin cannot be
located under, make sure that there is adequate ejection around the detail. On sub
gated or cashew gated parts locate ejector pins inside the part near the gate to
provide a good push as the gate is being separated.
Keep in mind that straight lifters can be used for ejection and give much more sur-
face area to push with. Look for areas in a mold where a straight lifter may make
sense.

37.3.2.3„ Mold: Lack of Draft
Draft is really as much a part design issue as anything but it also must be ad-
dressed during mold design. Trying to remove a part from the mold with no draft
means that the part will drag against wall stock the whole length of ejection. The
force required to eject a part lacking in draft will increase and either result in a
stuck part or pin push (or pins pushed right through the part).

Review the recommended draft requirements for the material being molded and
stick to the recommendations. There are cases where the customer pushes for less
than the appropriate amount of draft and they usually end up causing problems. In
some cases multiple attempts at polish end up opening up the draft to something
that allows the part to finally release.