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20.3 Burns Troubleshooting 183
Back pressure
Decompression
20.3.1.1 Molding Process: Fill Velocity
With fast fill speeds it is more likely that the gas will be trapped rather than escap-
ing through the vents. Often times slowing fill velocity will improve a burning
issue; however, this is a compromise on cycle time. Typically, if slowing the fill
velocity improves the burn defect improving the venting on the mold will also fix
the problem (fix the root cause). If burns are a problem venting should always be
reviewed prior to modifying the molding process.
Slowing the fill speed will not only increase the cycle time but may also impact the
viscosity of the material as it is injected into the mold. This viscosity shift may in
turn lead to additional part defects such as uneven gloss, sink, short shots, etc.
Slowing the fill speed on an existing process to compensate for another root cause
is not acceptable, so determine the root cause and address the issue.
Do not process around a venting issue! See Chapter 7 for more information on
venting.
Case Study: Fill Velocity
This was a case with a burn appearing at an end of fill knit line. The burn had
the classic appearance of charred material and was leaving a black deposit
on the surface of the mold in this area. Slowing the velocity eliminated the
burn by giving the gas time to escape the cavity. A mold maker was brought
over and he opened the vent to a deeper depth, which allowed the fill velocity
to be set at the original value. By opening the vents it was possible to achieve
a faster fill time, which saved over 2 seconds of cycle time.
20.3.1.2 Molding Process: Melt Temperature
When processing at high melt temperatures it is possible to degrade the material
and create gas byproducts that will lead to additional gas that may become trapped
in the cavity. Verification of actual melt temperatures should be carried out if the
above steps show no impact on results. Always be aware of the recommended melt
temperature for the material being molded. All process personnel on the shop floor
should have access to the recommended processing conditions for the materials
being used.
With melt temperature concerns it is important to verify the factors that influence
the actual melt temperature, which are barrel temperatures, back pressure, and
screw rotational speed. Do not assume that the barrel temperature settings are the
actual melt temperature.
