Page 385 - Injection Molding Advanced Troubleshooting Guide
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40.3 Short Shot Troubleshooting 383
40.3.1.6 Molding Process: No Cushion
The cushion is the plastic left in front of the screw tip after the mold has been
filled. If there is no cushion the machine cannot transfer pressure to the plastic in
the mold.
Verify that the machine is holding a cushion. A cushion reaching zero is an indi-
cator that either the shot size is too small or plastic is leaking somewhere. Plastic
may be leaking in the following areas:
Past the non-return valve
Between the nozzle tip and the sprue bushing
Inside the hot runner manifold
Through the mold parting line
If there is no cushion on the machine and there is no material leaking, the shot
size should be re-established by checking the fill only weight. Usually shot size and
transfer position will need to be adjusted.
Cushion monitoring is an excellent value for process monitoring. Alarms can be
set up if the cushion drops below an established limit.
40.3.1.7 Molding Process: No Back Pressure
This is a problem that most often occurs after purging a machine. Often the screw
will not recover due to the back pressure when purging the material. A common
way to overcome this is to reduce the back pressure. If the back pressure is dropped
to zero or a very low level the shot will be inconsistent and lacking in density,
which can result in short shots.
If back pressure is reduced to purge a machine, double check to be sure that the
pressure is returned to the required setting as this is easy to forget. Another option
is to use decompression to help pull the screw back to shot size rather than reduc-
ing the back pressure.
Case Study: No Back Pressure
In this case the process technician was fighting with inconsistent short
shots. While troubleshooting, the process technician realized that the cush-
ion was bottoming out. To combat the lack of cushion the process technician
tried adding shot and adjusting transfer while checking the fill only shot. He
continued to struggle to get a consistent shot and called for process engi-
neering support. Upon arriving at the machine, the process engineer used
STOP methodology and a 4M basic 8 check to quickly determine that the
process was running with zero back pressure. Adjusting the back pressure to
the documented pressure allowed the process to be reset at the established
settings.
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