Page 234 - Injection Molding Advanced Troubleshooting Guide

24.3 Cracking Troubleshooting 225

ƒ Mold break away
ƒ Gate seal

24.3.1.1„ Molding Process: Second-Stage Pressure
If a part is over-packed it will stick more in the mold. This increased amount of
sticking can lead to issues with parts that crack when the mold opens or during
ejection. To check for sticking in the cover half of the mold, listen (STOP: Observa-
tion) for cracking and popping noises during mold open. Stop the mold before the
part is ejected and examine the part as it sits on the ejector side of the mold be-
cause areas that stick in the cover will often be lifted away from the ejector. Note
that over-packing is one potential cause for sticking that needs to be verified.
If second-stage pressure has been set high accidently, a simple process verification
should catch the issue. If the mold is running in a different machine remember to
account for intensification ratio differences.
During process development it is important to understand how much process win-
dow is available before the part starts to stick. This can be determined by conduct-
ing a simple experiment of adjusting second-stage pressure to determine at what
pressure sticking becomes a problem. Remember to listen for noise during mold
open and inspect the part on the ejector half of the mold to ensure it has not been
pulled away from the core due to sticking in the cover.

24.3.1.2„ Molding Process: Melt Temperature
Verification of melt temperature is a reasonable step when checking for cracking
issues. If a material is being processed outside of its temperature range it will have
the potential to degrade and lead to parts that are brittle enough to crack during
ejection or subsequent part handling.
Bear in mind that higher melt temperatures could lead to over packing either
throughout the part or in localized areas. This potential of over packing could lead
to sticking that in turn could create cracks on the molded part.

24.3.1.3„ Molding Process: Residence Time
Similar to high melt temperature, excessive residence time can lead to degradation
of material, which in turn could create part cracking. Verification of shot size ver-
sus barrel capacity is important to ensure that the material is not spending exces-
sive time at temperature. The typical suggestion is that the shot size to barrel ratio
should be somewhere greater than 20%. If the shot to barrel percentage becomes
too small then there will be an increased length of residence time. Materials will
normally degrade thermally based on time at temperature, so the longer the resi-
dence time the less temperature the material can tolerate before degrading.