Page 70 - Plastics News December 2019

FEAturEs

How can molders achieve effective process monitoring?

Robert Gattshall

Effective process-monitoring limits should be set so that if any abnormality is discovered outside of the normal
process variation of the machine, it is segregated from good product.

ffective process monitoring is achieved through proper 8-hr runs with complete shutdowns between each run to
Eprocess development. To monitor effectively, you must be a great start. It is extremely important to ensure no
ensure that the process is optimal and repeatable. Without process changes are made during these runs, and you may
want to undertake at least one with an alternate resin
lot number. Process outputs should be recorded every
15 minutes, and the parts should be collected for that
recorded process. Using this data, the effective process
limits can be calculated, as long as all the parts collected
are within specification.

You could set up stages of process monitoring so that any
shot that is segregated would be considered suspect, if it
is within the “defect limits,” until further review of that
part can take place. This may not be necessary if your
monitoring is set up in such a way that the machine will
alert when scrap thresholds are reached. If so, the amount
repeatability, effective process monitoring will never be of product at risk will be minimal, and focus can then be
achieved. Effective process monitoring does not simply placed on troubleshooting the issue at hand.
mean setting upper and lower limits just before defects
are created. This is what I call “defect limits.” Although Process monitoring can be accomplished using the
it is important to know the “defect limits” of any process, machine’s monitoring software; but in my opinion,
these limits are not typically tight enough to be considered third-party hardware is required for effective process
effective for process control. monitoring. This will allow for independent verification
of the machine outputs, as well as in-mold monitoring
Effective process-monitoring limits should be set so that in the form of pressure transducers and thermocouples.
if any abnormality is discovered outside of the normal
process variation of the machine, it is segregated from The ability to monitor the plastic and ensure that it reacts
good product. The normal variation of your process should the same in the cavity from shot to shot is the ultimate
be well within the threshold of the defect limits. The form of effective process monitoring. We use the same
goal of establishing process limits should be to monitor methods for establishing the effective upper and lower
the process that has been proven to produce acceptable limits for peak cavity pressure, pack rate, and cooling rate,
product and not to create process windows. Wide process as stated above. This establishes machine-independent,
limits that try to create process windows within which effective process monitoring, and regardless of the
process adjustments can be made can often cause more machine outputs, it can ensure the plastic was within
harm than good. established limits.
The thousands of interactions taking place in a molding To monitor a process effectively, you must also have
process make it very difficult to prove out every scenario. an effective means of part discrimination when out-of-
For this reason, it makes more sense to establish a robust process conditions arise. Automation options such as
process and use process monitoring to ensure that this robots, reversing conveyors, and diverters can be used
process repeats itself each and every time and that to accomplish this. If none of these is an option, the
abnormalities are caught. machine must be set up to shut down when any out-of-
process condition is identified, and ejection must be
Determining Normal Variation prohibited, leaving the suspect parts in the mold. This
The process of identifying what is normal variation may is undesirable for several reasons, one of which being
differ depending on the product line, but I consider three that a process interruption can cause instabilities in the

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