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15.3 Black Specks Troubleshooting 133
pound or something stable like polypropylene). It is vital to establish a documented
machine shutdown process for all materials.
In cases where clear polycarbonate (PC) is being molded an effective shutdown
procedure is to bank the heats to a lower set point rather than completely turning
them off. By banking the heats with clear PC the residence time that occurs during
cool down and warm up is eliminated.
Case Study: Clear Polycarbonate Shutdown
In this particular case the part being molded was a clear PC dirt bin in a
700-ton molding machine. Over the history of running this part black specks
were a major scrap problem. After trialing numerous process changes it was
decided to bank the heaters on the molding machine to 300 °F whenever
the job was shut down. After making this change in shutdown procedure
black specks were no longer an issue.
15.3.1.2 Molding Process: High Melt Temperature
While high melt temperature is not typically the root cause of black specks it can
be a contributor to the issue. If plastic is exposed to higher than recommended
process temperatures it is more likely to degrade. Black specks are material that
has been severely degraded usually through an extended exposure to increased
temperature. If the material is exposed to excessive temperature the likelihood that
it will degrade to the point of producing black specks is increased. Again when
dealing with black specks the issue usually goes back to length of time at tempera-
ture, but the higher the temperature the less time before the material will degrade.
15.3.1.3 Molding Process: Long Residence Time
Excess time at temperature is what typically creates black specks. Normally, to
reach the point where material turns to black specks, it takes more time than the
residence time that the plastic sees moving through the barrel. However, keep in
mind that black specks are typically a time and temperature problem, so it is
important to evaluate what the material is experiencing from a heat history view-
point.
15.3.2 Black Specks Troubleshooting Mold Issues
Some of the potential areas related to the mold can include:
Debris from mold wear or shutoff surfaces
Hot runner temperatures
Hot runner hang up areas
