Page 32 - Plastics News April 2020

The Flash-and-Shorts Chase

Robert Gattshall process window significantly. Our goal as a processor is to
find that middle ground far enough away from either side
Injection molding's most common defects can have so that normal process variation doesn't result in either
inverse correlation, where correcting one causes the defect.
other, leading to the “chase.” So don't get caught in the
Flash-and-Shorts Chase Flash's Root Causes
The technology available today for injection molding is Flash can have many different causes, but two of the
very impressive, providing molders with tools to most common are excess cavity pressure and mold
i n c r e a s e p a r t damage. Cavity pressure can cause flash when it exceeds
complexity and the available clamp force. Once the clamp force is
i m p r o v e t h e exceeded by the cavity pressure, the mold can physically
q u a l i t y a n d open, allowing plastic to escape the cavity-forming
a c c u r a c y o f steel. Excess cavity pressure can have many causes of
production. Yet its own, including a blocked gate preventing some of the
even with these cavities from being filled. Mold damage can also allow
advances there for plastic to escape the cavity-forming steel by
a r e s t i l l t w o preventing the parting line from sealing properly.
obstacles that
molders have to Coming Up 'Short'
deal with. flash and short shots. These are by far the
Shorts or short shots are under-filled parts that are
most common defects in injection molding. One reason
for this is that in most cases they are direct opposites of typically missing features due to lack of plastic. Sinks
one another. Many of the actions we take to eliminate and under-packed parts can sometimes be related to
shorts can result in flash and vice versa. These actions shorts or can be an indication that shorts are starting.
can be made via changes to the process, mold, or Shorts can be caused by a number of reasons, and it is
combination of the two.
critical to identify the root cause before trying to
If the machine was running good parts and suddenly is address them. Common causes include improper venting
producing shorts, something changed in that process, that prevents gasses from escaping so that plastic can't
and it must be identified to resolve the issue. flow into the cavity; or the nonreturn valve on the screw
For example: Shorts can be caused by inadequate not closing properly and plastic flowing backwards as the
venting in the mold, and to help eliminate these shorts, machine injects. As I have stated in a previous column,
we may need to increase the depth of existing vents or understanding the why is critical to correcting the issue.
add more. However, it is possible that by doing so, the
result could be excessive flash. Now that flash is present, Finding the Root Cause, Avoiding the Chase
we decide to resolve our new problem by compromising
the process to increase material viscosity. This can be Keep in mind that both shorts and flash are symptoms of
done by lowering the melt temperature, causing an a deeper problem. We must understand what is causing
injection-pressure increase. This increased pressure the symptom before trying to fix it. For example,
increases our Delta P—the difference between maximum increasing shot size when shorts suddenly appear can
pressure used vs. maximum pressure required—so that
result in flash, if the root cause isn't understood before
the pressure now exceeds the maximum available
injection pressure of the machine, which results in—you making the change. If the shorts are caused by debris
guessed it—sporadic short shots due to being pressure stuck in the nonreturn valve, forcing it to remain open
limited. So we're during injection, increasing your shot size may eliminate
back to where we the shorts until the debris works free. Once the debris is
started.
freed from the nonreturn valve, and it is once again
So, as you can allowed to close properly, the extra shot size we added
imagine, there
could cause the cavity pressure to exceed the clamping
can be a fine line pressure and result in flashing the mold.
between flash and
shorts, and that
fact can shrink our Just because you can make a few good-looking parts by

APRIL 2020 29 Plastics News