Page 464 - Injection Molding Advanced Troubleshooting Guide

464 46 Warp

In some cases running without gate seal will allow the plastic near the gate to
equalize with the rest of cavity. This equalization will help reduce the pressure
differential in the cavity. Another concern is on a rectangular lid-style design that
has different flow and pack distances as the plastic travels away from the center
gate. A center gate on this shape of part will guarantee that there are areas filling
at different rates as well as packing differently.
If a part is warping during process development try running with gate seal and
without gate seal to determine if there is any impact on the warp. If the warp is
improved by running without gate seal and all other factors are acceptable (sink,
voids, gloss, and dimensions) run the mold without gate seal and document as
such. The assumption that every mold has to run with gate seal is wrong.
When troubleshooting a production problem with warp verify whether the mold is
supposed to run with or without gate seal. Check the process to determine that the
second-stage time is set correctly. If in doubt that the gate is sealed or not conduct
a gate seal study (see Chapter 3 on the Decoupled Molding™ Process).

46.3.1.5„ Molding Process: Cooling Time
When a part is removed from the mold too soon warp can develop. A molded part
must be allowed to cool long enough to allow the plastic to reach a temperature
below the heat deflection temperature (HDT) of the material. If a part is ejected too
soon the plastic will still be pliable and can easily deform resulting in a warped
part.
Verify that cooling time and overall cycle time are consistent with the established
and documented process. If the time is wrong make the appropriate adjustment to
bring the process back to the baseline. Check the part ejection temperature to de-
termine if the part is not being cooled as effectively because a higher part tempera-
ture after ejection can indicate cooling problems that need to be addressed. Adding
cooling time to compensate for a mold cooling issue is a costly means of working
around the problem: always try to drive to the root cause rather than process
around a problem.

46.3.1.6„ Molding Process: Post-Mold Handling
Sometimes warp occurs during and/or after part removal from the mold or during
handling by the robot or operator. Utilize the STOP process to observe potential
causes that are impacting the part during or after ejection.
Look for situations where the operator is stacking parts on top of each other prior
to packing. Also make sure that the parts are being packed correctly because im-
proper pack can lead to warp. Another thing to keep an eye on is how quickly the
parts are being packed because in some cases the operator may be clearing the
conveyor belt and packing parts that are hotter than normal.