Page 397 - Injection Molding Advanced Troubleshooting Guide

40.3 Short Shot Troubleshooting 395

40.3.4.4„ Material: Unmelts
Unmelts can lead to short shots due to the potential to block gates. This is a bigger
concern on a multicavity mold because the unmelts will impact cavity balance. An
unmelt is an unmelted pellet that has made it through the barrel. This issue seems
to be most common when running acetal materials, but may occur with other
semi-crystalline materials. Unmelts will often hang in a gate until the pressure is
high enough to force them through the gate; often leading to a short shot. An un-
melt can also impact the seating of the non-return valve and lead to inconsistent
fill weights.

Unmelt Test

1. After running the molding process for a minimum of 10 minutes, back
the barrel away from the mold and purge out an air shot.
2. Immediately recover another shot and purge it out.
3. Continue to recover and purge shots until unmelted pellets appear in the
purge. If this occurs after three shots or fewer the quality of the melt is
not good, and the odds of unmelt problems will be high.


40.3.4.5„ Material: Contamination
If the material becomes contaminated with foreign material a short shot can result.
If the foreign contamination moves through the melt stream as a solid it may be-
come trapped in a gate and lead to a short shot.
Check the material for any potential contamination. Review Chapter 23 on contam-
ination for additional details.

References

[1] Beaumont, J., Questioning the Science in Injection Molding: What are you Actually Getting in a
“Pressure Drop Study” Originally published in SPE Plastics Engineering Magazine; http://www.
beaumontinc.com/questioning-science-injection-molding-actually-getting-pressure-loss-study/