Page 265 - Injection Molding Advanced Troubleshooting Guide

27.3 Excessive Cycle Time Troubleshooting 257

melt temperature the rest of the process will need to be dialed in to accommo-
date the melt temperature.
2. Melt temperature is run high to compensate for something else.
A typical example of this is a situation where a part is designed without ade-
quate gating, which in turn leads to a pressure limited machine as the part is
filled. In such a situation a typical process adjustment is to raise the melt tem-
perature to try to process a lower pressure to fill the mold.
To help avoid excess cycle time due to high melt temperature it is important to
make sure that first of all people understand the impact of high melt temperature
on cycle time. Also during part and mold design look for conditions like long flow
lengths, thin wall sections, and lack of gating. If these part design issues are not
addressed before steel is cut the process will be left to suffer.

Case Study: Nonuniform Wall Stock

In this part the nominal wall stock was thinner than certain areas with a
much thicker cross section. As a result of the variation in nominal wall stock
the material temperature was run on the high side of the material supplier’s
recommended temperature range. As a result, the cycle time to cool the
thick areas was longer than necessary. A secondary problem with this type
of part design is that the thinner nominal wall stock freezes early preventing
full pack of the thick sections, which leads to sink, nonuniform cooling, and
shrinkage resulting in warp.
Maintaining nominal wall stock is critical for quality molding processes!


If the melt temperature can be dropped without impacting part quality there is a
good chance of reducing the cycle time for the part. If reducing the melt tempera-
ture causes other problems dig deeper and try to resolve them, which in turn will
allow the part to be molded with a cooler melt temperature and potentially a faster
cycle time.

27.3.1.2„ Molding Process: High Mold Temperature
The mold acts as a heat exchanger to remove the heat from the melted plastic. Lack
of cooling in a mold will lead to longer cycle time to allow the part to adequately
cool. It is important to understand that one hot spot in a mold can drive the overall
required cooling time for a part. Therefore, uniform cooling will lead to a better
opportunity to minimize the cycle time for any molded part.
Verify that the thermolator is running to the specified set point. Surface tempera-
ture checks or thermal imaging of the temperature of the part exiting the mold
may indicate cooling problems. Also surface temperature checks of the mold sur-
face can indicate cooling-related concerns.