Page 280 - Injection Molding Advanced Troubleshooting Guide
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272 28 High Fill Pressure
when doing machine side assembly with fasteners there is a chance that a fas-
tener could be dropped into an open material container. Magnets will not catch
everything but they should be used to catch what they can.
Unmelts. Unmelts are pellets that make their way through the screw and barrel
without actually melting. In some cases running a semi-crystalline material like
nylon or acetal will pass unmelts through the screw potentially plugging the
nozzle or tip. Another case is where a material of higher melting temperature
such as nylon contaminates a lower melting temperature material like poly-
propylene.
Dispersion tip. If running with a dispersion tip there is already going to be a
large increase in pressure. The other concern with a dispersion tip is that it will
trap items in the holes further restricting flow.
Case Study: Intentional Sabotage
In this case the machine started running short shots, and the fill pressure
became pressure limited. Troubleshooting through the problem showed that
the purge pressure on the machine was very high. The nozzle tip was removed
and a distorted penny was found jammed against the tip. The penny was
blocking much of the material flow through the nozzle. The guess was that a
disgruntled employee had decided to toss a penny in the material gaylord.
In another case a similar problem occurred and upon removing the nozzle
assembly screws were found in the nozzle and tip.
28.3.3.4 Machine: Machine Performance
It is always critical to understand if a molding machine is actually achieving the
process set points that are input into the controller. Just assuming that the ma-
chine will achieve a set point can be dangerous. The need for verification on what
the machine is actually doing is why it is so important to document as many plastic
condition outputs as is possible.
By documenting the fill time of a part, it really does not matter what the set value
for fill speed is. A much better measure for fill rate would be volumetric flow rate
because that would apply regardless of the screw size. For process consistency,
repeating the documented fill only weight at the documented fill time means that
the mold was filled with the same flow rate.
If the process is set at 500 psi hydraulic pressure, is that what the machine is
achieving? Every press should be providing output that corresponds to the inputs.
See Chapter 8 on machine performance for more information on evaluating ma-
chine performance.
