Page 346 - Injection Molding Advanced Troubleshooting Guide
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342 35 Part Sticking on Cover
the buildup. Some material buildup can be cleaned off better with resin remover,
citrus-based mold cleaners, or a mold cleaner such as Zapox. There are also cases
with materials such as TPU where the mold may actually need to be bead blasted
to remove the buildup sticking issues.
Case Study: TPU Sticking
In this case a part was being molded from an 85-shore durometer TPU. This
TPU tended to become very sticky when degraded from either moisture or
residence time. The worst aspect of this problem was that when anyone
molded a part with degraded material a buildup was left behind that would
not clean off with any commercial cleaners. To remove the buildup and keep
parts from sticking, the mold had to be removed from the machine and
bead blasted. After blasting, the surface would be fine until someone shot
degraded TPU again.
The solution to this problem was training of all process technicians to ob-
serve the viscosity of the material when purging. If the material was runny it
was likely degraded and they needed to purge longer.
35.3.2.7 Mold: Ejector Half Detail
If the ejector side of the mold does not have adequate detail to retain the part
during mold opening the part will tend to stick to the cover. Proper selection of the
“A” versus “B” side is important during design and the ejector side should have
detail that the plastic is shrinking onto.
If the ejector side does not have adequate detail there are some tricks that can im-
prove the sticking problem including:
1. Undercuts ground into the ejector side.
2. Texture added to the ejector side.
3. Sand-blasted finish on the ejector side.
4. Preloaded push pins on the cover half to push the part off.
35.3.3 Part Sticking on Cover Troubleshooting Machine Issues
Process control is the main machine concern relating to sticking in the mold. All
process settings on the machine must be accurately controlled by the machine. If
the machine is not controlling the set points it is the same as entering the wrong
set points. The machine must provide the temperatures, pressures, and speeds
accurately.
