Page 442 - Injection Molding Advanced Troubleshooting Guide

442 43 Sprue Sticking

It may seem counterintuitive but when molding a material like TPU the sprue
bushing should be bead blasted rather than polished to ensure release. If the sprue
is sticking and the TPU material is known to be dry the sprue bushing may need to
be blasted.

43.3.4.3„ Material: Additives

A lack of release agent in a material will lead to sticking concerns. If sticking starts
when a new lot of material is molded a call to the material supplier may be war-
ranted. Do not get in the habit of “crying wolf” to the material supplier with every
problem that pops up. Make sure that all internal root causes have been investi-
gated and resolved. It is a major waste of everyone’s time to call in a material
technology representative for a splay issue with the material only to have them
point out that the dryer is running with a +30 °F dew point.
If a release agent is added as a masterbatch at the machine check that it has been
added and at the correct letdown ratio. Also evaluate if some colors stick worse
than others, because this could point to different additives in the colorant package
or a different nucleating effect depending on the color. Different nucleation in
semi-crystalline materials can lead to different cooling and shrink rates and may
impact how a sprue releases.
Glass-filled materials can have a tendency to cause erosion of the sprue bushing,
which can create undercuts and sticking concerns. When running glass-filled ma-
terials use of a sprue bushing of higher hardness may be necessary to eliminate
erosion concerns.