Page 200 - Injection Molding Advanced Troubleshooting Guide
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190 20 Burns
Case Study: PVC Burning
In this particular part an alternate material PVC was being sampled. When
the new material was sampled in the mold, there were several spots of
charred material showing on the surface. Numerous processing experiments
were conducted to minimize the burning of the material. After reviewing the
situation with the material supplier, it was determined that the formulation
was off and the material was not thermally stable enough to mold. Numerous
hours of molding trials could have been saved if the formulation was better
understood originally.
20.3.4.3 Material: Additives
Color concentrates can impact the likelihood of burning as well as the odds of de-
tecting the burns. Some concentrates will degrade faster and produce more of an
outgassing effect. Also, burns on lighter colors will be much more easily detected,
even for small burns. Because dark colors hide burns better it is critical when re-
viewing a part to examine any deep ribs for possible burning. There are cases
where the burning on the tip of a rib is not even noticed on the part until someone
rubs some of the black soot off onto their hands; closer inspection in turn shows
the burn location.
Other additive components in a material such as plasticizers or release agents can
lead to excess outgassing or buildup in the vents. As mentioned in Section 20.3.2.1,
venting is critical to avoid burns during processing.
It is important if color concentrates or other additives are added to the base mate-
rial prior to processing (either batch blended or at the throat) to ensure that the
correct letdown ratio is being used. If an excess amount of an additive is mixed in,
the likelihood of creating problems in the process will increase significantly. Also
verify that the additive or color concentrate is intended for use in the material
being molded.
