Page 436 - Injection Molding Advanced Troubleshooting Guide

436 43 Sprue Sticking

Inspect the seat of the sprue bushing for damage. Plastic can leak between the
sprue bushing and the nozzle tip if there is damage. The sprue bushing seat can be
resurfaced if it has been damaged. Damage at the sprue bushing to nozzle tip will
often show as flash at the end of the sprue.
Glass-filled materials may lead to erosion of the steel in the sprue bushing. This
erosion creates an undercut that will cause sprues to stick. When dealing with
erosion it is usually best to replace the sprue bushing rather than trying to polish
away the erosion.

43.3.2.2„ Mold: Incorrect Finish in Sprue Bushing

As with general sticking some materials will need a draw polish and some will
need a blasted finish for the sprue bushing. The perfect example is a TPU which
will release much better with a sprue bushing that has been sand blasted. If the
sprue is sticking without an apparent reason investigate alternate finishes in the
sprue bushing.
Circular polishing of a sprue bushing will create small undercuts that may cause a
sprue to stick. Proper finish will normally eliminate this sticking problem.

Many materials including polycarbonates, ABS, and nylon will usually work best
with a sprue bushing that has been draw polished. In the case of polypropylene
often times a shiny highly polished finish can lead to sticking. Do not assume that
finer polish will resolve a sticking issue. Draw polishing a tapered sprue can be
difficult but is often the solution to a sticking sprue. A 320-grit paper draw polish
will often resolve sprue sticking issues.

43.3.2.3„ Mold: Sprue Puller
There are different styles of sprue pullers to aid in removal of the sprue from the
sprue bushing. The most common types are “Z” pullers and back drafted cold slug
wells. In general, the “Z” puller will provide a more robust pull of the sprue. Back
drafted cold slug wells can only be back drafted so far or they will not release from
the ejector side of the mold. “Z” pullers are cut into the cold slug well ejector pin
and should have ample radius to eliminate breaking at sharp corners.
Figure 43.3 shows an example of a Z-style sprue puller. The ejector pin in the cold
slug well is ground with an undercut to capture the sprue and allow it to pull from
the sprue bushing when the mold opens.