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20.3 Burns Troubleshooting 189
create additional outgassing from the plastic that will in turn require additional
venting to be released from the mold. Sometimes, rather than damage, the issue
may be metal that is trapped somewhere along the feed system.
Unfortunately, to check for trapped metal or damage, the end cap must be pulled
from the machine and each of the components inspected. This is a very time-con-
suming endeavor and should be undertaken only when confident that there is
trapped metal or damage in the components of the injection unit.
20.3.4 Burns Troubleshooting Material Issues
Factors based on material issues that can contribute to burning include:
Moisture content
Type
Additives
20.3.4.1 Material: Moisture Content
Burns will normally be a result of volatiles in the melt stream not escaping while
the mold is filled. When hygroscopic materials are molded with excess moisture
content the water will vaporize during plastication and put gas into the melt
stream. This gas in the melt stream will normally appear as a splay related defect,
but can lead to burning as it tries to travel through the vents in the mold. See Chap-
ter 9 on drying for more information on moisture.
20.3.4.2 Material: Type
As with many defects, some materials will be more likely to burn than others. Very
temperature-sensitive materials such as PVC or acetal will degrade much faster
and can actually reach a point where the material in the barrel is already in a
burnt state. When processing temperature-sensitive materials it is critical to be
very aware of the residence time in the barrel and ensure that all of the tempera-
ture zones on the barrel and hot runner are accurately controlled with no areas for
material to hang up.
As always with any potential material issue make sure that the process is running
to the required conditions. Also verify that the correct material is being used and
is not contaminated.
Another factor to consider is when conducting a changeover, the barrel tempera-
tures must be allowed to cool sufficiently when switching to a lower melt tempera-
ture material. If switching to a temperature-sensitive material like a PVC or acetal
make sure that the barrel temperatures have dropped to the required temperatures
for the material.
