Page 222 - Injection Molding Advanced Troubleshooting Guide
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23.3 Contamination Troubleshooting 213
Figure 23.3 Contamination at hot tip
A manifold can be damaged and cracked which will provide a gap at the crack that
will allow material to become trapped. Cracking a manifold is usually a result of a
cold start where the manifold was not given enough time to heat soak. Cracked
manifolds often need expensive and time-consuming repairs; therefore, educate
process technicians to give a hot runner plenty of soak time before injecting plastic
through the manifold.
23.3.2.2 Mold: High Hot Runner Temperatures
When hot runners are running at higher temperatures than is required for a mate-
rial the odds of degrading material increase. As the material degrades in the hot
runner system the result can be brown streaks or black specks contaminating the
part.
Verify that the hot runner temperatures are set correctly and that actual tempera-
tures are reading accurately. If a hot runner zone is constantly calling for heat it
may indicate that a zone is either wired incorrectly or has a thermocouple mis-
placed.
23.3.2.3 Mold: Wear Surfaces
Any surfaces on a mold that rub against another surface have the opportunity to
wear. Over time the metal dust or flakes that are worn from the surface can con-
taminate the cavity of the mold. This metallic dust will contaminate the molded
part and may produce a visual defect.
