Page 459 - Injection Molding Advanced Troubleshooting Guide
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46.3 Warp Troubleshooting 459
stress concentrations, which in turn can lead to warp as the plastic tries to
relieve stress during cooling.
Many warp problems can be significantly improved during part and mold design.
Addressing the above concerns during part design is a foundation to provide warp-
free parts. Modern molding CAE software packages can help to predict warp and
should be used as an iterative tool to evaluate design impact to warp.
Another factor to consider is that even if a part is stiff enough to resist warping
after ejection from the mold the part may end up with high internal stress. Internal
molded-in stress may lead to failure of the part especially when subjected to en-
vironmental stress cracking conditions because the environmental stress crack
resistance (ESCR) of a part is impacted by the molded-in stress.
46.3.1 Warp Troubleshooting Molding Process Issues
Molding process concerns that can lead to warp include:
Mold temperature
Melt temperature
Pressure differential
Gate seal
Cooling time
Post-mold handling
46.3.1.1 Molding Process: Mold Temperature
An important factor relative to warp of a part is the heat deflection temperature
(HDT) of the material being molded. When a thermoplastic is above its HDT the
material will not maintain its shape when exposed to loading. If a molded part is
ejected at a temperature above the HDT the part will be easily distorted.
Plastic cooling rate is one of the key process variables in injection molding. The
mold ends up acting as a heat exchanger to remove the heat energy from the
molded part. To bring the molded part temperature down to levels where the part
can be ejected without excess warp requires the mold temperature to be correct.
Every material will have a recommended mold temperature which is a vital
reference point for any process. Questions should definitely be asked if the mold
is running at 180 °F but the supplier recommends 80–120 °F for the mold tem-
perature.
The most obvious contributor to mold temperature is the set point on the tempera-
ture control unit or thermolator as they are commonly called. The water tempera-
ture will help determine what the mold temperature will run at with the actual
steel temperature normally running warmer than the water set point. Actual steel
