Page 438 - Injection Molding Advanced Troubleshooting Guide

438 43 Sprue Sticking

Figure 43.5 Sprue from a mold with a broken Z puller

43.3.2.4„ Mold: Cooling of Sprue Bushing

Because the sprue base is typically the thickest area of plastic in any mold, cooling
is critical for ensuring that the sprue is solid enough for removal. Cooling lines in
the mold should be designed to provide uniform cooling around the sprue bushing.
Also ensure that the sprue bushing has good contact with the cavity block to allow
heat transfer from the sprue bushing to the mold. Any reduction in contact be-
tween the sprue bushing and the cavity block can lead to reduced cooling capabil-
ity for the sprue.
High-performance sprue bushing made from copper alloys can improve the cooling
of sprues if used correctly. Any time a high-conductive material such as copper
alloy is used in a mold a cooling path must be provided to allow the heat to transfer
out of the copper component. If cooling is not provided to high conductive inserts
or sprue bushings the heat will transfer from the steel into the copper alloy but
will have nowhere to go from there, the result of which is the alloy will heat up and
make things worse.
Another consideration is to replace the sprue bushing with a hot sprue. The two
big advantages of a hot sprue are:
1. Reduced cycle time due to not needing to wait for the sprue to cool.
2. Reduced waste in the runner system because the sprue stays molten instead of
being cooled and ejected every shot.