Page 253 - Injection Molding Advanced Troubleshooting Guide

244 26 Dimensions

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Figure 26.3 RJG eDART cycle graph: the slight change in recovery time led to a part that
was dimensionally out of specification on the following shot

26.3.1.6„ Molding Process: Mold Temperature
Normally an increase in mold temperature will lead to increased shrink in a part.
This increase in shrink will provide a smaller part. Much of this increased shrink
will come as post-mold shrink. If parts are too large an increase in mold tempera-
ture may help, but if they are too small decreasing mold temperature may help lock
in shrink.
Care should be taken with mold temperatures when molding semi-crystalline ma-
terials because in this case a cold mold may lock in shrink to a point, but exposure
to higher post mold temperatures may cause the part to shrink and warp to a lower
stress state.
Many factors need to be evaluated for optimal mold cooling (see Chapter 14 on
optimized mold cooling for details).

26.3.1.7„ Molding Process: Cooling Time
Increasing cooling time and thereby cycle time usually will help make a part larger.
This is however an expensive way to increase dimensions because your machine
and mold have now become an expensive cooling fixture.