Page 400 - Injection Molding Advanced Troubleshooting Guide
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41.3 Sink Troubleshooting 399
Short second-stage time
Low fill only weight
Lacking cushion
Slow fill velocity
High mold temperature
High or low melt temperature
41.3.1.1 Molding Process: Low Second-Stage Pressure
The pressure applied during the second stage of injection packs out the cavity and
compensates for plastic shrink. Enough pressure must be applied to the plastic in
the cavity to eliminate sink in the part. Many people will use rules of thumb for
setting second-stage pressure but the best plan is to let the part dictate what is
required based on cosmetics and dimensions.
Verify that second-stage pressure is set to the documented process. Double check
that someone did not add a pressure profile to the machine controller. If the pres-
sure is set incorrectly the process should be adjusted to the specified set point.
On machines that have pack or hold volume, speed, or velocity settings make sure
that the setting is large enough to allow the machine to reach the set pressure. If
volume, speed, or velocity is set too low on second stage the machine will struggle
to reach the set pressure.
Watch for cases where second-stage pressure has been adjusted to process around
a tooling issue. An example is someone adjusting second-stage pressure down
because the mold has parting line damage, but the lowered pressure in turn pro-
duces sink. Fix the problem with the mold rather than trying to process around
the damage.
41.3.1.2 Molding Process: Short Second-Stage Time
Cavity pressure in a mold is built with second-stage pressure and maintained with
second-stage time. In the majority of cases second-stage time will be set to be just
longer than the required time for gate seal (gate freeze), which is the point the
plastic in the gate has solidified enough to prevent plastic in the cavity from flow-
ing back out of the cavity.
Determine if the mold should run with gate seal. If a gate seal study has not been
completed then take the time to verify gate seal. If a gate seal study has been com-
pleted compare the set second-stage time to what is required for gate seal. If neces-
sary, increase second-stage time to ensure gate seal (if gate seal is required).
Lack of gate seal will often lead to sink near the gate and many times directly at the
gate. However, running with second-stage time set too short can lead to sink
throughout the part. If the mold has a cavity pressure transducer it is possible to
