The plastic mold design also has to take into account the specific material
and part geometry. That sounds so basic, but someone have found that there is
usually a gap in understanding, with the process technicians not fully
understanding how the tool can impact the process, and the mold builder not
fully understanding the process and how the tool design affects it.
Combating Flash. One issue that many people will attempt to process around is
flash. When you process around flash you can put yourself at risk for short
shots and other quality issues. Some people will settle for trimming off the
flash, which adds extra labor cost and increases the potential for quality
issues if flash is not trimmed properly. I have three rules for flash. If all
are met, flash is impossible, no matter what material or process is being
Rule 1: All mating surfaces are “True Net,” meaning they match perfectly with
no gaps or anything holding the parting lines open.
Rule 2: The tool is designed and built robustly enough to minimize deflection
in line with clamp force and perpendicular to clamp force.
Rule 3: The clamp force of the machine exceeds the maximum force generated by
plastic pressure across the projected area of the cavity and runners.
The latter two rules can often be vulnerable to misdiagnosis. If the tool is
proved to have good shutoff with the mating surfaces, you determine which of the
last two rules apply. If there is flash in line with clamp force, it results
either from tool deflection or lack of clamp pressure. It is important to
understand what plastic pressures the cavity is seeing and then multiply by the
projected area to determine if you have adequate clamp force.