Page 352 - Injection Molding Advanced Troubleshooting Guide

348 36 Part Sticking on Ejector

36.3.1.3„ Molding Process: Long or Short Cooling Time
On some occasions, running with too much cooling time will allow the molded part
to shrink too much to the ejector detail resulting in a stuck part. Processes some-
times are run with more cooling time than is required to produce a quality part.
Generically once the part is below the materials glass transition temperature
(many molders may also use the heat deflection temperature or HDT) it can be
ejected without warp; note that using a single data point for choosing part ejection
temperature may be challenging. Many molders end up using excess cooling time
for no good reason. Let the part data make the decision, and if the part is stable at
the faster cycle time then the process can run here.

If the cooling time is too short the plastic may be too soft to eject from the mold.
Make sure that the part has adequate cooling time to prevent sticking. In an exag-
gerated condition the ejector pins will push through the soft part leaving the part
stuck on the ejector.
Evaluate the running process against the documented process, and correct if cool-
ing time is set incorrectly. During process development look for opportunities to
run the mold faster and potentially eliminate excess shrink on the mold.

36.3.1.4„ Molding Process: Fast Ejection

Fast ejection speeds can lead to issues with a part binding on the ejector side of the
mold. There are cases when the fast ejection speed can cause the part to cock on
the mold resulting in sticking. Also fast ejection speeds may cause a part to adhere
to a lifter and not travel evenly away from the ejector half of the mold. Also remem-
ber that there are cases where a vacuum forms between the mold and the part;
slower ejection speeds may allow this vacuum to break and allow better release of
the part.
If slowing the ejection speed improves the release of the part from the ejector side
of the mold use the STOP process to determine if any other root causes are leading
to the sticking problem. Do not slow the cycle time to process around a mold issue!
Adding undercut keepers to limit the opportunity for the part to travel with a lifter
may lead to complete resolution of the problem without slowing down the ejectors.

36.3.1.5„ Molding Process: Melt Temperature
With a part sticking on the ejector side, either low or high melt temperature can
lead to problems. A low melt temperature may result in a lack of cavity pressure,
which in turn can lead to extra shrink on the ejector. High melt temperatures can
lead to either more or less shrink depending on the mold; in some cases higher
melt temperature leads to less pressure and less shrink and in other cases it leads
to increased shrink due to higher crystallinity from slower cooling.