Page 188 - Injection Molding Advanced Troubleshooting Guide
P. 188
178 19 Buildup
Regrind can be a source of contamination. If a part or runner of another material
is dropped into a grinder the result will be contaminated regrind. Contaminated
regrind could degrade due to differing melt temperatures, which then can lead to
buildup. See Figure 19.3 for an example of contaminated regrind. If contamination
is suspected take some material and spread it out across a table sorting through
the pellets looking for contamination.
Figure 19.3 Contaminated regrind: notice light gray material
19.3.4.2 Material: Moisture Content
When plastic is processed without being adequately dried the moisture will volatil-
ize in the barrel of the molding machine producing gas. This extra gas generated
during the melting process will need to be vented from the mold while the mold is
being filled. The venting on the mold may not be adequate to handle the extra gas
that is being injected during fill. As this gas tries to escape the cavity via the vents
buildup may be left behind in the vents. Hence, ensure that the material has been
adequately dried prior to processing. For more details on drying of materials see
Chapter 9.
