Page 421 - Injection Molding Advanced Troubleshooting Guide
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420 42 Splay
rial past its upper limit. The screw should recover approximately 2–3 seconds prior
to clamp open. An analogy of running the screw fast is using a kitchen mixer to
whip air into a meringue.
42.3.1.7 Molding Process: Feed Throat Temperature
If the cooling water circulating through the feed throat is running too cold (below
the ambient dew point) it is possible to have condensation of water in the feed
throat. As this condensation drips into the feed throat the water gets conveyed
down the barrel with the plastic. Feed throat temperatures should be controlled
between 120–150 °F to firstly avoid condensation and secondly to avoid too much
heat causing pellets to stick together and jam the feed throat.
42.3.1.8 Molding Process: Inconsistent Material Feed
If the loading of raw material is inconsistent the rotating screw can be starved of
material as it tries to recover. This inconsistent material feed can lead to excess
recovery time, which can lead to overheating of the material and in turn to splay.
Always try to keep the hopper loading consistently. Inconsistent hopper loading
can be detected in one of two ways:
1. If the loader keeps running cycle after cycle, it is usually a sign that the loader
is having an issue keeping up with the machine. The root cause of this may be a
low supply of material, plugged loader filter, or a feed wand that is not reaching
the material.
2. Inconsistent screw recovery time.
42.3.2 Splay Troubleshooting Mold Issues
If the splay is in a consistent location the root cause will likely be a tooling issue.
The critical step in troubleshooting tooling-related splay is too look upstream in the
material flow to see what may be causing the splay. Often times the splay will
appear on the part surface several inches away from the mold detail that is the
problem. There are many factors involving the mold itself that can contribute to
splay including:
Venting
Hot runner temperatures
Cold slug wells
Sprue and nozzle tip orifice
Sharp corners
Gate flaking