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19.3 Buildup Troubleshooting 177
times just reducing an excessive land length will improve a buildup issue. Also be
aware that if the vent relief channel has not been squared to the mold edge you will
only have the desired land length at the center of the radius from the cutter.
Applying diamond chrome with PC materials can help with buildup. Other mold
coatings may help to minimize buildup and improve mold cleaning.
In some cases with buildup, adding vacuum to the vents can improve the overall
ability of the mold to vent. There are ways to do this with seals at the parting line
but in some cases just adding a vacuum line to the vent relief can provide good
results. Connect the vacuum line to a vacuum generator and use a mold closed
switch to actuate the vacuum to draw the volatiles out of the vents.
19.3.3 Buildup Troubleshooting Machine Issues
The main machine-related cause of buildup is machine performance.
See Chapter 8 on machine performance for details.
19.3.4 Buildup Troubleshooting Material Issues
Possible material-related problems include:
Regrind
Moisture content
Material type
Additives
19.3.4.1 Material: Regrind
Use of regrind can contribute to buildup, especially in a case where the regrind
has been degraded or is molded wet. Anything that causes an increase in the
amount of gas generated during material plastication can lead to problems with
buildup.
Note that there are also cases where using regrind may help reduce a buildup
problem because some of the components leading to buildup have been burned out
with the extra heat history. Things like low molecular weight resins, release agents,
residual monomers, or other additives may actually be reduced after the first pass
through the molding machine. Regrind can have a significant impact on the pro-
cess and a good way to determine the impact is to mold 100% virgin material and
evaluate if the problem changes.
