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23.3 Contamination Troubleshooting 221
Case Study: Incoming Contaminants
In this case a PC/ABS was the material of choice. Parts had a high-gloss
finish and were running without issues. A new lot of material was introduced
and the parts started to show surface defects. Investigation with the material
supplier showed that the TiO used to color the material was agglomerating
2
leading to visible surface defects during molding.
After a new lot of material was introduced the part defects were completely
gone.
23.3.4.4 Materials: Mixed Materials
It is crucial that materials are kept from being mixed. If a foreign material is mixed
with the desired material being molded, contamination defects are sure to occur.
Some materials can even be dangerous when mixed, such as PVC and acetal. It is
important to educate everyone in the plant that black plastic is not just black plas-
tic and all materials must be kept separate.
Figure 23.10 shows an example of a contaminated part. The black TPO part was
showing a small streak of gray on the surface. The part was cut through the streak
and a distorted pellet was clearly visible in the part cross section. This is another
example to show that one pellet of contamination is all it takes to make a defective
part.
Figure 23.10 Single-pellet contaminant in wall stock
