Page 404 - Injection Molding Advanced Troubleshooting Guide
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41.3 Sink Troubleshooting 403
A thermal image of a part after ejection can highlight areas of the mold that are
running hot. If these areas do not match the original thermal images taken during
process development there is likely a problem with the mold.
Another factor that must be considered is the temperature of plant cooling water. If
running with a cooling tower the cooling water low temperature will vary over the
seasons. In climates that have cold winters the cooling water may be 65 °F in the
winter but only get down to 85 °F in the summer. If a molding process is estab-
lished in the winter and the mold temperature is set at the minimum winter
temperature the process will not be repeatable in the summer when the water is
hotter. Set a minimum water temperature that is achievable all year. If the plant
has a central chiller than the lower temperature can be achieved year-round.
Stay aware when a thermolator is swapped out for replacement. The replacement
thermolator must have the same capability for flow and cooling capacity. If the
machine was running with a 7.5-hp thermolator and it is replaced with a 3-hp
thermolator the GPM capacity will be lower and there may not be enough flow to
achieve turbulent flow. If turbulent flow is not achieved the cooling capacity of the
water is greatly reduced.
41.3.1.7 Molding Process: High or Low Melt Temperature
Melt temperature is critical to a consistent process. A major part of the injection
molding process is managing heat transfer. Understanding melt temperature and
keeping it consistent and repeatable during molding runs will help minimize part
variation.
Melt temperature should be documented on the setup sheet and verified during
troubleshooting. If the melt temperature does not match the setup sheet (within
±10 °F due to measurement variation) investigate the settings that impact melt
temperature including:
Barrel temperatures
Back pressure
Screw recovery speed
If melt temperature is high the plastic will take longer to cool and will experience
additional shrink that can lead to sink. The melt temperature impact can be espe-
cially important with semi-crystalline materials because if the material is cooled
more slowly, larger crystals will be formed leading to higher shrink.
Low melt temperature can lead to an increase in pressure drop across the cavity
and create a situation where pressure is not as well distributed across the cavity.
Pressurization of the cavity is key to minimizing sink on the molded part. A se-
verely low melt temperature will lead to a short shot.
