Page 357 - Injection Molding Advanced Troubleshooting Guide
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36.3 Part Sticking on Ejector Trouble shooting 353
Case Study: Sticking TPU
TPU is thermoplastic polyurethane, which becomes very sticky when allowed
to degrade. TPU is extremely sensitive to moisture content, temperature,
and residence time.
In this case the part was having off-and-on sticking problems. When sticking
would occur it would be so bad that the material would leave behind a resi-
due on the mold surface. To effectively eliminate the residue the mold would
need to be sand blasted.
Evaluation of startup procedures showed that people were having two
different issues:
1. Starting the press with wet material
This guaranteed that the part would stick every time. All technicians were
trained to verify the moisture content before starting the mold.
2. Not purging the barrel thoroughly prior to start up
Because the material was so sensitive to temperature and residence
time the technicians had to learn to appreciate how little down time they
could afford with this particular mold.
Clear training and startup procedures were critical to avoiding an ongoing
problem with this mold.
36.3.4.2 Material: Moisture Content
Most of the materials mentioned in Section 36.3.4.1 will be sensitive to moisture
content. Ensure that materials are adequately dried before molding (see Chapter 9
on material drying for more details).
36.3.4.3 Material: Release Agents
Most molding materials have release agents and lubricants added to help with
processing. If a given lot of material starts to stick it may indicate that the level of
release agent is not the same as in previous lots. If adding anything to a material
that may contain additional release agents or lubricants verify that the correct let-
down ratio is being used for the additive.
Use of a spray release may be required during process startup but is poor practice
for ongoing production. If a mold will not run without using spray mold release on
a continuous basis there is a problem that must be identified and corrected. Add-
ing an operator with a can of mold release to get parts to come off a mold is an ex-
pensive Band-Aid to get a mold to run; instead, find the root cause of the sticking
and correct it.
