Page 361 - Injection Molding Advanced Troubleshooting Guide

37.3 Pin Push Troubleshooting 357

stick. The sticking in turn can lead to visual problems with pin push as the ejector
pins force the part from the core steel.
Under-packing can lead to excess shrink that in turn can cause the molded part to
stick to details on the core side of the mold. This excess shrink can result in pin
push because the part has shrunk and stuck on detail.
When working on pin push try both raising and lowering second-stage pressure.
Determine if there is any improvement in the pin push on changing the pressure.
Remember in troubleshooting that a change that makes something worse can tell a
great deal about the impact of the setting.

37.3.1.2„ Molding Process: Low or High Mold Temperature
As with second-stage pressure, mold temperature can impact pin push if running
too low or too high. Usually, a too-high mold temperature will lead to more pin push
because the ejector pins are impacting a softer, more pliant surface. Examine the
part out temperature with a surface probe or a thermal imaging camera. Beware of
hot spots that may cause pin push from being too pliant during ejection.
If a mold is running too cold the part may shrink too much prior to ejection. Even
though the wall stock is more solidified the extra shrink on the core may result in
pin push.

37.3.1.3„ Molding Process: High Melt Temperature

When running with a high melt temperature the molded part will be softer and
may be more likely to display pin push. Injection molding is very dependent on
thermal management and running with excess melt temperature will cause prob-
lems with managing the heat. If the molded part is too hot the odds of seeing some
surface distortion or stress from the ejector pin will be magnified.

Verify that the melt temperature is running in line with the documented process.
Also compare the melt temperature to the recommended setting of the supplier. If
the melt temperature is high investigate the potential root causes. There are cases
where melt temperature was raised to accommodate other problems such as thin
wall areas that are difficult to fill. If possible, resolve the root cause of the high melt
temperature and see if that improves the pin push.

37.3.1.4„ Molding Process: Fast Ejector Speed
If the ejectors are travelling too fast, pin push may be the result. Some parts do not
do well with trying to eject as fast as the machine allows. Look at slowing down the
ejector speed to determine if the pin push is improved. Slowing down the ejectors
may allow the part to ease off the mold rather than using just brute force. A slight
increase in cycle time will result from slowing down the ejectors but if it resolves
the defect it may be necessary.