Page 323 - Injection Molding Advanced Troubleshooting Guide

32.3 Glass Fibers on Surface Troubleshooting 317

Case Study: Glass Fibers
In this case a 33% glass-filled nylon 6 was being molded for various cos-
metic parts. The material was processing well and the parts were being pro-
duced without concerns. When a new lot of material showed up and was
introduced into the molding machine an immediate shift in surface quality
was detected. The parts now had a very glassy surface and looked terrible.
Checks were conducted on all of the 4M’s as well as moisture content but
nothing was found to be out of line.
As the material was introduced into additional molds the same problem was
experienced.
The material supplier was contacted to review the problem. The material
supplier was very transparent and communicated that on this given lot of
material they had used an alternate source for the glass fibers. Investigation
showed that the alternate glass fibers had an issue with the sizing agent
that prevented the nylon from wetting out the fibers.
A new lot of material was quickly shipped in and the problem was gone. All
material from the bad lot was returned to the supplier.
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It is also important that design engineers are given feedback on the differences in
processing as the glass percentage increases. If there is no good reason to go with
a higher glass content the processor will pay the price at the end of the line. Some-
times engineers will want to swap to a higher glass percentage or from short glass
to long glass to get a stronger part. They must understand that changes like this do
not come without a cost, and the cost is often part cosmetics.