Page 428 - Injection Molding Advanced Troubleshooting Guide
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42.3 Splay Troubleshooting 427
Figure 42.4 GP screw
42.3.3.4 Machine: Damage to Screw, Barrel, or Non-Return Valve
Damage to the screw, barrel, or non-return valve can all lead to hang up areas as
well as high shear locations that can lead to splay. Any area that traps material can
result in material becoming degraded; as the material degrades it can release
gases that will be trapped in the plastic melt stream and cause splay. Whenever a
screw is checked for preventative maintenance, a part of the procedure should be
to look for any damage to the screw, barrel, and non-return valve.
Care must be taken when a screw is cleaned. There are cases where cleaning the
screw with a grinder leaves a scratched and rough surface. Copper gauze is typi-
cally recommended for cleaning of a screw because the copper will not scratch the
screw surface. If a screw becomes scratched or otherwise damaged there will be an
increased tendency for material to hang up resulting in splay, brown streaks, or
black specks.
42.3.4 Material Issues
Some splay problems are directly caused by the material itself. Below are potential
causes based on material problems:
Contamination
Excessively wet
Wrong material
Blowing agent
42.3.4.1 Material: Contamination
Contaminated material is the most likely material-related problem encountered.
The raw material can be contaminated with other materials, dirt, cardboard flakes,
or other potential sources. Any source of foreign material may lead to splay. The
