Page 434 - Injection Molding Advanced Troubleshooting Guide

434 43 Sprue Sticking

43.3.1.3„ Molding Process: Short Cooling Time
If the cooling time of the process is too short the sprue will not have enough time
to adequately shrink and therefore may not release from the sprue bushing. Also if
the cooling time is too short the sprue may be too hot for consistent extraction from
the sprue bushing. Many times sprues that are too hot will actually separate from
the sprue puller and stick in the sprue bushing.
Check that there is adequate cooling time to allow for proper removal of the sprue.
Adding time to cooling can help validate if this is the cause of the sprue sticking. If
adding cooling time helps the problem, examine the mold for opportunities to
enhance the cooling in the region of the sprue.

43.3.1.4„ Molding Process: High Mold Temperature
A hot mold will result in the same concerns as too-short cooling times. The sprue
may not shrink enough for effective release, or the sprue may be too hot and actu-
ally tear apart as the mold opens.
Verify the mold surface temperature near the sprue bushing. If the mold is run-
ning hot determine if this is a new occurrence or if the mold has always run hot.
Check for cooling concerns including:
ƒ Water temperature
ƒ Water flow rate
ƒ Water line setup
ƒ Scale buildup in cooling channels
Any of the above cooling concerns can influence the cooling rate of the plastic and
must be controlled to maintain consistent cooling for the life of the mold. If any of
them has changed from the original process data the problem must be corrected.
Thermal imaging will help to detect hot spots on a molded part, runner, and sprue.
A baseline thermal image should be taken during process development to help
with troubleshooting in the future. Comparison of baseline images and current
images may point out cooling problems with the mold.

43.3.1.5„ Molding Process: Low Nozzle Temperature
Occasionally a low nozzle temperature will cause a large slug of frozen plastic in
the tip that does not want to separate from the sprue when the mold opens. Raising
the nozzle temperature can improve the break of the sprue to the nozzle tip.

43.3.1.6„ Molding Process: Sprue Break

Machines have a setting called sprue break that allows the injection unit to move
back prior to mold open. In some cases adding sprue break will allow the sprue to
break cleanly away from the nozzle tip and may resolve a sprue sticking issue. If