Page 453 - Injection Molding Advanced Troubleshooting Guide

45.3 Voids Troubleshooting 453

45.3.1.1„ Molding Process: Low Second-Stage Pressure
When plastics are heated to a melted state the distance between the molecules
expands due to the increased energy in the plastic. During the cooling phase of the
process the distance between these molecules contracts leading to molded part
shrink. When this shrink occurs in some part details the result can be formation of
a void.
Second-stage pressure is the part of the molding process that compensates for the
plastic shrink. Enough cavity pressure must be applied to limit the amount of
shrink, which in turn can restrict formation of voids.
Confirm that the machine has been set up correctly per the documented process. If
the process is running at the correct set points look for opportunities to increase
second-stage pressure. If the mold has cavity pressure transducers verify that the
cavity pressure is matching the established process template.

Increasing second-stage pressure will lead to a more packed out part that is less
likely to have voids. If increasing pressure watch for dimensional changes in the
parts; higher second-stage pressure will create a larger part. Increased second-
stage pressure may also lead to sticking issues due to over-packing. When trouble-
shooting it is important to STOP and think through the potential negative impacts
that the process adjustment may cause.

45.3.1.2„ Molding Process: Short Second-Stage Time
If second-stage time is set too short the gate will not freeze, which can result in
material flowing back out of the cavity. If cavity pressure is not maintained the
plastic will shrink at a higher level which can result in voids in thick areas of the
part. Voids near the gate location are a very big indicator that the gate is not sealed.
Verify that a gate seal study has been completed for the mold. Evaluate the set sec-
ond-stage time against what is required to achieve gate seal. On parts that are to be
processed with gate seal ensure that the second-stage time is set approximately
10% longer than the required time for gate seal because this will ensure that the
gate is sealed even when conditions vary. As an example, if the gate seals at 10
seconds the second-stage time should be set at 11 seconds.

45.3.1.3„ Molding Process: Low or High Mold Temperature
Low mold temperature can lead to increased pressure drop across the cavity. This
increase in pressure drop can result in less effective cavity pressure being applied
throughout the cavity. Low mold temperatures will remove heat faster from the
molded part and can result in faster freezing of wall stock and gates. If the process
does not provide adequate time to effectively pack out thick sections the part will
either have sinks or voids.