Page 241 - Injection Molding Advanced Troubleshooting Guide

232 24 Cracking

modifiers and release agents. Impact modifiers missing from the formula could
lead to a material that cracks when ejected from the mold and missing release
agents will lead to sticking in the mold. These are rare cases because outgoing in-
spections on material properties should catch any problems if they slip through
the production monitoring side of things.

24.3.4.5„ Material: Molecular Weight
Material is produced in a manufacturing process and as such will have variation.
One of the key items molders must deal with is variation in the average molecular
weight. If the average molecular weight of a material is low the material will tend
to get weaker from a physical property standpoint, which could lead to cracking of
the part. Some mold and material combinations will be prone to this but it will be
rare. Spending a lot of effort investigating average molecular weight will be very
costly.

Unfortunately, average molecular weight is not easy to measure and keep track of
in normal molding shops. The test that gets used to try to simply quantify a mate-
rial is the melt flow index (MFI). If the average molecular weight is varying, MFI
may detect the change.

24.3.4.6„ Material: Wrong Material
Always verify that the correct material is being used. It is important to follow all
documented standards when handling materials to ensure that materials are not
used accidently.

Also bear in mind when looking at alternate materials that just because the speci-
fications look good on paper does not mean that the materials will yield the same
molding results. If an alternate material is being reviewed anticipate that process
adjustments may be required. This is another point of STOP troubleshooting where
Observation can help resolve issues quickly.