Page 33 - Plastics News April 2020

reducing the hold pressure, that doesn't mean that the of the overall process window, and the actual root cause.
process is robust enough to accommodate normal
When I think about this type of chase, as a processor, it
process variation in production.
does feel like something is working against you. I've been
In my experience flash and shorts are among the main there. I can recall several shifts spent chasing a process
problems that can start the “chase.” That refers to early in my career … not knowing any better and being
making process changes on a machine multiple times in responsible for 12 aging machines with poorly designed
reaction to the current defect. Example: A short is molds and just trying to move on to the next firefight. In
discovered and the machine operator informs the these cases, we bring this on ourselves to some extent,
process technician of the issue. This busy technician but it can take you off your game, especially if you're
stops what he or she is working on to come over to the trying to be in multiple places at the same time.
machine and increases the hold pressure without doing
When it comes to troubleshooting flash and shorts, the
any sort of root-cause investigation. The technician
first step is identifying which one is really the issue.
looks at a shot to verify the shorts are gone and tells the
Since they can be direct opposites of each other, it is key
operator the parts are good to pack.
to know which defect is present at the optimal
Let me be clear here and say that process changes should processing conditions. For example: Even though shorts
be the last thing we do to correct an issue. If the machine are the issue we are currently seeing, flash could
was running good parts and suddenly is producing shorts, ultimately be what needs to be addressed. Flash could
something changed in that process, and it must be be preventing us from increasing our hold pressure to an
identified to resolve the issue. That said, we all get busy optimal setting to compensate for normal process
and pressure can push us towards the path of least variation, which is resulting in short shots. So the issue
resistance in order to move on to the next problem. We we should be addressing is the flash at our optimal
should always troubleshoot an issue and identify the root process setting and not the shorts that are currently
cause before making modifications, but that doesn't present. The above example is important to remember
always happen—it should, but it doesn't. during process development, as well. Just because you
can make a few good-looking parts by reducing the hold
Back to our example: A little time goes by and the
pressure, that doesn't mean that the process is robust
operator starts seeing flash on the parts—that's right,
enough to accommodate normal process variation in
flash on the machine that was just producing shorts. It
production. If we can develop a process that can prevent
turns out that whatever was causing the shorts is
normal process variation from causing defects,
sporadic and the additional hold pressure that was
troubleshooting becomes much more simplified and
added is now causing the mold to flash. The operator
helps those process technicians make better decisions.
gets a different technician, who notices that someone
increased the hold pressure and now there is flash, so of Finding the optimal process between flash and shorts
course this technician lowers the hold pressure. No more will always be an obstacle for molders. I would also
flash, problem solved. What happens next? The sporadic venture to say that they will continue to be the top scrap
issue that's causing the shorts hits again and they return, reasons for most molders. But how you approach
forcing the operator to seek help again to address the problem solving will determine whether you chase these
shorts. This chase can go on for hours or even longer, defects or will solve them.
depending on the skill level of the process team, the size

APRIL 2020 30 Plastics News