Page 38 - Plastics News February 2020

FeAtures

How Can Molders Achieve Effective Process Monitoring

Robert Gattshall

Process monitoring can be accomplished using the machine’s monitoring software,and the ability to monitor
the plastic and ensure that it reacts the same in the cavity from shot to shot is the ultimate form of effective
process monitoring

ffective process monitoring is achieved through proper Determining Normal Variation
Eprocess development. To monitor effectively, you The process of identifying what is normal variation may
must ensure that the process is optimal and repeatable. differ depending on the product line, but I consider three
Without repeatability, effective process monitoring will 8-hr runs with complete shutdowns between each run to
never be achieved. Effective process monitoring does not be a great start. It is extremely important to ensure no
simply mean setting upper and lower limits just before process changes are made during these runs, and you may
defects are created. This is what I call “defect limits.” want to undertake at least one with an alternate resin
Although it is important to know the “defect limits” of 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
of product at risk will be minimal, and focus can then be
placed on troubleshooting the issue at hand.

any process, these limits are not typically tight enough Process monitoring can be accomplished using the
to be considered effective for process control. machine’s monitoring software; but in my opinion,

Effective process-monitoring limits should be set so that third-party hardware is required for effective process
if any abnormality is discovered outside of the normal monitoring. This will allow for independent verification
process variation of the machine, it is segregated from of the machine outputs, as well as in-mold monitoring in
good product. The normal variation of your process should the form of pressure transducers and thermocouples. The
be well within the threshold of the defect limits. The ability to monitor the plastic and ensure that it reacts
goal of establishing process limits should be to monitor the same in the cavity from shot to shot is the ultimate
the process that has been proven to produce acceptable form of effective process monitoring. We use the same
product and not to create process windows. Wide process methods for establishing the effective upper and lower
limits that try to create process windows within which limits for peak cavity pressure, pack rate, and cooling rate,
process adjustments can be made can often cause more as stated above. This establishes machine-independent,
harm than good. The thousands of interactions taking effective process monitoring, and regardless of the
place in a molding process make it very difficult to prove machine outputs, it can ensure the plastic was within
out every scenario. For this reason, it makes more sense established limits.
to establish a robust process and use process monitoring To monitor a process effectively, you must also have
to ensure that this process repeats itself each and every an effective means of part discrimination when out-of-
time and that abnormalities are caught. process conditions arise. Automation options such as

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