Page 351 - Injection Molding Advanced Troubleshooting Guide

36.3 Part Sticking on Ejector Trouble shooting 347

36.3.1.1„ Molding Process: Second-Stage Pressure
Second-stage pressure can create sticking problems on the ejector side of the mold
if it is too low or too high.
When second-stage pressure is set too low, the plastic may shrink excessively onto
the ejector half of the mold. This excessive shrink can result in a part that is stuck
too hard to eject. Try increasing second-stage pressure to better pack out the part
and reduce shrinkage. Evaluate if the increased pressure improves the sticking. If
excessive shrink is the problem, fill only shots may also tend to shrink to the ejec-
tor side of the mold.
If second-stage pressure is too high and there is a lot of recessed part detail on the
ejector half of the mold the plastic may be over-packed into the detail. This over-
pack situation can lead to reduced shrink of the plastic and may cause the part to
stick. If recessed details seem to be sticking (look for distortion) try reducing the
second-stage pressure to determine if the sticking improves.
When troubleshooting verify that second-stage pressure is set to the documented
process. Also ensure that the machine is actually achieving set pressure. Cavity
pressure transducers will help to ensure that the actual results in the mold are
accurate to the established process.

36.3.1.2„ Molding Process: Mold Temperature
When it comes to sticking on the ejector half of the mold, the steel temperature can
be the root cause. This is another factor that can either be too low or too high de-
pending on the part and process.
When mold temperatures are too low the part may not be packed out as well, which
can lead to additional shrink resulting in sticking. Increased shrink can also occur
with high mold temperature and semi-crystalline materials. With semi-crystalline
materials larger crystals will form, which can lead to increased shrink. This impact
of either low or high can make mold sticking a difficult-to-solve problem.
If the mold temperature is too high the part may be over-packed leading to sticking
in part recesses such as ribs or bosses. Warmer molds lead to less pressure drop
across the cavity and may result in higher cavity pressures near the end of the
cavity causing sticking.
Verify the thermolator set temperature, actual mold steel temperature, and the
part ejection temperatures against the documented process. If the thermolator is
set incorrectly make the appropriate adjustment. If the thermolator is set correctly
but the steel or part ejection temperature is wrong then a deeper dive into the cool-
ing parameters is needed (see Chapter 14 on mold cooling).