Page 156 - Injection Molding Advanced Troubleshooting Guide

144 16 Blush

16.3.1.2„ Molding Process: Nozzle Temperature
An incorrect setting on a nozzle temperature can result in gate blush. The first
plastic in the mold is normally where blush occurs so this material is the material
that would be located in the nozzle between shots. In general, the nozzle tempera-
ture should be set at the melt temperature of the material.
Either raising or lowering the nozzle temperature can reduce the severity of blush.
Try lowering the nozzle temperature and inspecting the parts, and if the blush is
better try running this way to see if the problem is resolved, but if it is worse try
raising the nozzle temperature above the original set point.
If the nozzle temperature seems to help but there is variation over time confirm
that all controls for the nozzle temperature are being maintained. Watch the ma-
chine controller for variations in the nozzle temperature zone, and also look for
cases where the nozzle heater zone is calling for heat constantly or is never calling
for heat. Examine the nozzle heater band to ensure that it is the correct length for
the nozzle, is tightly attached, is the correct wattage, and is working.

16.3.1.3„ Molding Process: Mold Temperature
The mold temperature will impact the initial plastic entering the cavity. Both too
low and too high mold temperatures can lead to gate blush problems, so it can be
important to cover both extremes. Gate blush is a strange phenomenon that can be
magnified by the steel temperature and both too low and too high steel tempera-
tures may lead to a poor formation of the initial plastic through the gate.
Verify that the process is at the documented set point. During process develop-
ment try lowering and raising the mold temperature to determine if it is impacting
the blush.
In many cases with materials like ABS, PC/ABS, or TPO warmer mold tempera-
tures will result in a better-looking part surface inside the gate location. In some
cases, it appears that colder mold temperatures have more impact on the morphol-
ogy of these types of materials with rubber impact modifiers.

16.3.1.4„ Molding Process: Melt Temperature
Similar to mold temperature the melt temperature can impact the quality of the
initial formation of plastic after it leaves the gate.
The melt temperature should be set to the documented process. If the melt tem-
perature is at the correct setting try adjusting the material either up or down. The
change in melt temperature may impact the blush of the material entering the
cavity. If the blush improves try to run the process for an extended time to ensure
that the process adjustment works long term.
Excessive melt temperature may release gas that causes a buildup just inside the
gate that leads to blush. Also, high melt temperature may allow erosion of the first
material formed inside the gate causing a disruption to the surface.