Page 244 - Injection Molding Advanced Troubleshooting Guide

25.3 Delamination Troubleshooting 235

25.3.1.2„ Molding Process: Fill Velocity
On rare occasions excessive shear at a gate can leave a delamination in the mate-
rial near the gate. This is more likely to occur on a material blend such as a PC/ABS
material. Slowing down the fill speed will decrease the shear through the gate and
may eliminate the delamination.
Sometimes with sub gates or cashew gates there will be delamination showing
where the gate separates from the part. This can sometimes be fixed with a change
in velocity through this portion of the process.

25.3.1.3„ Molding Process: Residence Time in Barrel
Much like excess melt temperature, excessive residence time can lead to degrada-
tion of the material, which in turn can produce areas of separation or delamination
on the skin of the part. Depending on who you ask, the recommended barrel utili-
zation number may vary but it will usually be a range of approximately 25–75%
utilized per shot. If the shot turnover rate through the barrel is low (under 25%)
there will be many shots sitting at temperature. This becomes more of an issue as
the cycle time lengthens because the time at temperature will grow longer with
the increased cycle time. The more temperature sensitive a material is, the easier
it will degrade with excessive residence time.
If a mold has to run in a press with poor shot turnover, one of the best things that
can be done would be to profile the barrel temperature settings (low in the rear)
and try to keep the melt temperature at the low end of the recommended specifica-
tion. These changes will at least minimize how much heat the material is exposed
to as it travels through the barrel.
With materials that are highly temperature sensitive such as PVC or acetal the
residence time in the barrel is even more critical to ensure that the material is not
degraded. One goal of molding is to try to deliver a plastic melt that will provide as
much retention of the physical properties of the material as is possible.

25.3.2„ Delamination Troubleshooting Mold Issues

There are a couple of basic things to look for as contributors from the mold for
delamination:
ƒ Hang up spots
ƒ Lubricant contamination

25.3.2.1„ Mold: Hang Up Spots
If a hot runner system has spots where the material can hang up and become
“dead” the material may degrade over time and lead to delamination. When mate-