Page 264 - Injection Molding Advanced Troubleshooting Guide
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256 27 Excessive Cycle Time
27.3 Excessive Cycle Time Troubleshooting
Many factors can impact the cycle time of a molded part. It is important to optimize
the cycle of a mold during process development and make every effort to maintain
that cycle for the life of the product.
27.3.1 Excessive Cycle Time Troubleshooting Molding Process Issues
Factors in the process that may lead to long cycle times include:
High mold temperature
High melt temperature
Timers set longer than required
Slow fill speeds
Long cooling time
27.3.1.1 Molding Process: High Melt Temperature
The dynamics of injection molding thermoplastics means that energy is added to
the plastic to melt the plastic and then after injecting the plastic into the mold we
must pull that energy back out to allow the melted plastic to resolidify. The tem-
perature of the molded part must drop to an appropriate temperature to allow the
part to be ejected from the mold in a state that will retain its molded shape; if
ejected too hot the part will warp and change shape as it continues to cool outside
of the mold. Every BTU of energy that is put into the plastic during the melting
stage must be removed during the cooling stage. It is important to remember that
the parts can be ejected at a temperature that is often well above the mold tempera-
ture, and cooling will continue outside of the mold after ejection.
If the melt temperature of the plastic is on the high side of what is required,
chances are that the cycle time will be increased due to longer cooling time and
potentially longer time required for gate seal. A high melt temperature is a double-
edged sword that costs money for the extra energy input into the machine to create
it, and requires extra process time to cool back down from the high temperature.
The two typical things to consider when detecting a high melt temperature are:
1. Melt temperature is high for no good reason.
This is often the case because a process was developed with a given set of barrel
temperatures, screw recovery speed, screw style, and back pressure. All of these
process settings will interact to create a melt temperature. Many molders will
either completely skip checking melt temperature or check the melt tempera-
ture and upon seeing that it is high do nothing. As a result of not caring about
