Page 389 - Injection Molding Advanced Troubleshooting Guide

40.3 Short Shot Troubleshooting 387

Case Study: “There’s what in the hot tip?”
In this case the part had eight direct hot drops. Shorts were starting to show
in a location that pointed to two of the gates having some kind of issue.
Short shots showed that indeed there was no flow coming from these two
gates. The temperatures of the drops were checked and everything looked
OK, so the mold was pulled from the machine and taken to the tool room.
Examination of the mold in the tool room showed that the drops seemed to
be coming to temperature without issue. An attempt to insert a thin probe
into the drop failed due to hitting something hard in the tip. The tool room
decided to drill out the drop and eventually hit the hard spot. They were able
to get the foreign material out of the drop and examination of the material
showed it was stone!
Discussions with the material supplier led to the possible root cause that the
transfer lines used to load the material from the tanker truck to the silo
was dropped in the parking lot and picked up a piece of gravel that then was
loaded into the silo and made it all the way through the system until it
reached the drops where it could not get any further.


If the suspected culprit is contamination from a higher melt temperature material,
raising the drop and tip temperature may allow the material to melt and flow
through the restriction of the tip.

40.3.2.4„ Mold: Stuck Plastic or Buildup

Sometimes a part will have some detail such as a rib missing and look like a short
shot. Before trying to resolve a short shot problem check to make sure that plastic
is not stuck in the mold detail. Sometimes the detail breaks off due to sticking and
stays trapped in the mold creating parts with the detail missing.
If this happens on a mold it may well indicate that the detail in the mold is lacking
in polish and more prone to stick as a result. Inspect the rib for electrical discharge
machining (EDM) marks or poor polish. Sometimes cleaning up the EDM marks in
a rib will correct this problem from reoccurring.
Another factor that can magnify this problem is a mold started up with degraded
plastic, which will be more brittle and sticky. Thorough purging is necessary when
molding materials that are prone to degradation. Also start a mold using a fill only
shot rather than using full pack pressure during startup.
Materials that are very gassy and not vented well can actually build up in deep
mold details and leave what looks like a short shot. It may be necessary to clean
out the buildup and add venting to prevent this problem from reoccurring.