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19.3 Buildup Troubleshooting 175
19.3.1.2 Molding Process: Excess Shear
Shear contributes a great deal of the energy input for melting the plastic. It is im-
portant to understand that high levels of shear can be generated during process-
ing, which in turn can lead to excessively high melt temperatures, which can lead
to material degradation. Key items to check for high shear include:
Back pressure
Screw recovery speed
Injection fill velocity
To verify the impact of these factors, check the melt temperature of the material.
Often when shear levels are high during plastication it will be possible to detect
overriding barrel zones. A good check is to examine the barrel temperatures to
determine if they are running to the set point; overriding zones may indicate that
there is excessive shear as the screw recovers.
When filling a mold with high velocity the gases generated during processing will
have less chance to escape through the vents of the mold, which results in faster
fill velocities requiring more venting. If fill time seems to contribute to the buildup
issues, first try to add additional venting to the mold rather than immediately slow
down the fill velocity. Slower fill velocities result in longer cycle times and make
the part more expensive to produce. Way too often there are cases where someone
slows down fill velocities to eliminate a defect caused by lack of venting (see Sec-
tion 19.3.2 for more on venting).
One key in processing is to avoid conditions that will damage the material. Shear
rates, temperatures, and residence time are the typical process conditions that are
most likely to damage a material.
19.3.2 Buildup Troubleshooting Mold Issues
The main issue that contributes to buildup relates to venting. If the mold does not
have adequate venting the volatiles from the plastic will be trapped in the mold,
which will lead to potential problems with buildup on the mold surfaces. See Fig-
ure 19.2 for an example of buildup on the mold surface; notice how the black
buildup has been scraped off the mold surface.
