Page 295 - Injection Molding Advanced Troubleshooting Guide
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288 30 Flash
Damage to parting lines can happen for many reasons including:
Use of steel tools causing damage
Excessive removal of steel from polishing or grinding of vents
Clamping the mold on a part or runner
Flakes or stringers caught on the parting line
Erosion from lack of venting
In the case of these conditions, eliminate the root cause of the problem or the flash
will return after repair. Further STOP analysis might be required to address the
true root cause of the flashing from damaged parting lines.
When a damaged spot is located on the parting line of a mold it must be fixed. The
part will have flash at this location until the repair has been completed. Trying to
process around parting line damage will lead to a severe restriction of the process
capability for a mold.
Laser welding is a very effective means of repairing parting line damage. A skilled
laser welder can weld such fine detail that very little work will need to be done to
finish polish the parting line detail.
30.3.2.2 Mold: Vent Depth
In most cases more venting is a good thing; however, vents can only be deepened
so far based on the material being used. It is vital to determine the maximum vent
depth for a given material but it is also important to understand that vent depth is
not a “one size fits all” application.
The lower the viscosity of a material the easier it will be for vents to flash. This is
important to keep in mind when dealing with materials such as nylon that will
experience viscosity variation due to moisture content. Nylons can be very touchy
on vent depth because they usually need to be well vented but will flash easily.
When needing more venting remember that depth is only one of the parameters of
a vent; width can be added to vents without risking flash.
If vents are too deep it will usually be very apparent because the flash will mimic
the vent locations and widths. If tabs of flash are located at the vents the vents will
need to be addressed.
30.3.2.3 Mold: Trapped Plastic
There are cases when plastic can stick or break and become trapped in a mold. If
someone starts up a process without reducing second-stage pressure, it is common
for the mold to flash excessively. The excessive flash can often end up stuck in
holes, screw heads, and insert lines. This stuck plastic may prevent a complete
lockup of the mold because it is acting as a spacer on the parting line.
