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35.3 Part Sticking on Cover Troubleshooting 343
Check that actual values on the machine correspond with the set points. Things to
watch for include temperature zones running above set point and pressures that
are not quickly reaching set point after velocity to pressure (V to P) transfer.
There are cases where the set pressure does not match the gauge pressure or the
pressure reading on the machine controller. Which one to trust in this case? An
auxiliary gauge or a pressure transducer connected to a process monitoring sys-
tem will provide an accurate measure of actual pressure. It may be necessary to
replace the gauge and the pressure transducer of the machine to provide accurate
results.
35.3.4 Part Sticking on Cover Troubleshooting Material Issues
Some of the potential material factors that influence sticking include:
Moisture content
Material type
Lack of release agent
35.3.4.1 Material: Moisture Content
Some materials when processed with moisture content will degrade and become
sticky. Whenever processing a hygroscopic material, it is critical to ensure that the
material has been dried to the required moisture percentage.
Verify moisture content with a moisture analyzer. If moisture content is high allow
additional drying time to completely dry the material.
There are cases where sticking is occurring and the machine is shut down to polish
or work on the mold. During this downtime the material continues to dry and upon
restarting the machine the sticking problem is resolved. Unfortunately, the mold is
often blamed as the root cause but in many cases the extra drying time is what
corrected the problem. When dealing with sticking problems verify the moisture
content of the material before working on the mold.
For details on drying refer to Chapter 9.
35.3.4.2 Material: Material Type
Some materials are prone to sticking under varying conditions. Materials such as
TPU, polyesters, and polycarbonates are examples of materials that have an in-
creased tendency to stick.
As mentioned in Section 35.3.4.1 moisture can be a big issue for sticking. A reac-
tion called hydrolysis can occur when processing with excess moisture content;
hydrolysis is a breakdown in the molecular weight of the material. Materials that
