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43.3 Sprue Sticking Troubleshooting 441
that formed after running glass-filled nylon. The impact of glass-filled plastic on
the steel can be hard to believe, but the high shear rates combined with the glass
filler can create many issues if not accommodated.
43.3.3.2 Machine: Machine Performance
All set points on the machine are important to the process.
See Chapter 8 on machine performance.
43.3.3.3 Machine: Injection Unit Forward Pressure
If there seems to be leaking between the nozzle tip and the sprue bushing verify
that the pressure for holding the carriage forward is set appropriately. This pres-
sure may be called sled forward pressure, nozzle touch force, or nozzle forward
pressure. If this pressure is set too low there may be leaking pressure between the
nozzle tip and the sprue bushing which in turn can lead to sticking sprues.
43.3.4 Sprue Sticking Troubleshooting Material Issues
Some material concerns include:
Moisture content
Material type
Additives
43.3.4.1 Material: Moisture Content
If processing with wet material the material can degrade and become very sticky.
If processing a hygroscopic material verify that the material has been adequately
dried. Materials that are very sensitive to hydrolysis such as TPU, polyesters, and
polycarbonate tend to have more problems due to lack of drying.
See Chapter 9 for more information on drying.
43.3.4.2 Material: Material Type
One immediate thing to consider with material type is the wear caused by running
glass-filled materials. The glass content can be very abrasive and will cause ero-
sion damage to the steel in the sprue bushing. If this erosion becomes severe
enough sticking sprues may be the result. If running glass-filled material investing
in a hard sprue bushing may pay long-term benefits.
Some materials are inherently stickier than others. Materials such as nylon, poly-
esters, TPU, and polycarbonate can tend to stick harder in the sprue bushing. Flex-
ible materials such as PVC or TPEs may be soft enough that they tend to tear or
stretch and jump past the sprue puller resulting in a stuck sprue.
