Page 330 - Injection Molding Advanced Troubleshooting Guide
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33.3 Gloss Variation Troubleshooting 325
Figure 33.3 Mold surface buildup (right) versus clean surface (left)
Case Study: Polish Buildup
In this case the material being molded was a wood-filled polypropylene. The
mold had a polish level of approximately SPI A3, which produced nice glossy
black parts when molded with straight polypropylene. An extended run was
conducted for a special order of wood-filled polypropylene parts that mold-
ed well; however, when the material was changed over to the black poly-
propylene the gloss level of the parts was dull and inconsistent. Examination
of the mold surface showed there was a large amount of buildup on the
mold surface and the steel had even tarnished from the gases from the
wood-filled material. After many hours of scrubbing with polishing paste, the
mold was finally ready for running at high gloss.
Often when a mold texture is used there will either be a secondary micro texture or
a sand-blasted finish that creates additional small details that the plastic must be
packed into. Under magnification these peaks and valleys will be easy to see and
examine for detail. In Figure 33.4 texture details can be viewed under 200× magni-
fication: notice the peaks and valleys that the material must replicate.
