Page 342 - Injection Molding Advanced Troubleshooting Guide

338 35 Part Sticking on Cover

„ „35.3„ Part Sticking on Cover Troubleshooting

Sometimes a part will completely stick to the cover side of the mold and other
times it may stick and distort but stay on the ejector half of the mold. Often, stick-
ing to the cover will cause cracking noises during mold open. If cover sticking is
suspected stop the mold before ejection and look for areas of the part that have
pulled away from the ejector because this indicates that the part was sticking in
the cover and tried to pull away from the ejector.

35.3.1„ Part Sticking on Cover Troubleshooting Molding Process Issues

Some process conditions that can lead to parts sticking to the cover are:

ƒ High second-stage pressure
ƒ High melt temperature
ƒ Low or high mold temperature
ƒ Mold open breakaway speed

35.3.1.1„ Molding Process: High Second-Stage Pressure
Sticking on the cover side of the mold often indicates high cavity pressure. If the
cavity pressure is high the part may not be shrinking enough to be effectively
pulled away from the cavity. High second-stage pressure can over-pack plastic into
details in the mold and molds with a lot of cover side detail will be more prone to
over-packing.

Verify the second-stage pressure against the documented process and make
adjustments based on it. When evaluating pressure, it should always be viewed as
plastic pressure, which means that the intensification ratio of the machine must be
accounted for.

35.3.1.2„ Molding Process: High Melt Temperature
If the actual melt temperature is running on the high side it can impact sticking for
two reasons:
1. High melt temperatures can lead to increased material degradation, which can
make a material more prone to stick.
2. High melt temperatures lead to an enhanced ability to distribute cavity pressure
in the mold. See Section 35.3.1.1 for more on high pressure concerns for stick-
ing.