Page 348 - Injection Molding Advanced Troubleshooting Guide

344 35 Part Sticking on Cover

are prone to suffering from hydrolysis such as those listed above need to be dried
thoroughly and kept dry to avoid sticking problems.
Semi-crystalline materials will experience more shrink than amorphous materials,
which will allow the plastic to shrink away from cover side detail (providing the
shrink on core conditions were designed into the ejector side of the mold). This
increased amount of shrink can help or hurt sticking conditions and usually inter-
acts with the cooling rate that the part is experiencing while in the mold.
Also be aware that different materials will react to different mold surfaces and may
stick worse because of the mold surface finish. Most times soft materials such as
TPEs or TPUs will tend to stick to polished surfaces due to a vacuum effect between
the soft material and the mold surface. Soft materials will often release much better
with a bead-blasted surface. If a polished surface is required an air poppet in a
hidden location may help break the surface vacuum.

35.3.4.3„ Material: Lack of Release Agent
If the material of choice does not have enough release agent in its additive package,
parts will be more likely to stick. If sticking coincides with a lot change it may in-
dicate a potential material problem.
There will be cases where a material supplier offers an enhanced release package
on the same material. The correct material choice must be identified up front. If
severe sticking is occurring the material should be reviewed with the customer for
potential replacement options.
There are cases where a secondary additive such as zinc stearate is added to a ma-
terial prior to molding. If the part being molded uses a release agent additive verify
that it is being used and at the correct loading.
Remember that manually spraying mold release on a mold to eliminate sticking
issues is not solving the problem. Giving the mold a quick spray at startup is com-
mon but if the mold needs to be sprayed on a regular basis to avoid sticking the
cause of the sticking should be identified and corrected. As soon as a person for-
gets to spray the mold after x number of shots a part is going to stick and cause
downtime and potentially damage the mold. Whenever using a spray mold release
for startup sticking make sure to verify that it is the correct type, i. e. paintable or
food grade where required.