Page 194 - Injection Molding Advanced Troubleshooting Guide

184 20 Burns

If barrel temperatures are set too high it is easily possible to overheat a material
and potentially degrade components of the material. Verify that the barrel tem-
peratures are set correctly and are running to the set point. Look for cases where a
heater zone may be calling for power 100% of the time as this may indicate that
either the heater band is not adequately heating or the thermocouple is not read-
ing correctly. Keep in mind that heater bands must be mounted tight to the barrel
and thermocouples must be located at the correct depth in the thermocouple hole.
Because shear provides much of the energy required to melt the plastic it is im-
portant to check back pressure and screw rotational speed when verifying high
melt temperatures. Just like with barrel temperatures it is possible that the back
pressure or the screw speed was set incorrectly, so always verify to the specified
value on the set-up sheet. The general rule of thumb for recovery speed is that the
screw should reach its shot size 2–3 seconds prior to mold opening. Back pressure
will normally be run in the 1000–3000 psi range depending on material and colo-
rant package.

20.3.1.3„ Molding Process: Decompression

Excessive decompression can lead to air being pulled into the melt at the nozzle
tip. When this air is injected into the mold with the plastic it must be vented. The
extra gas that the decompression pulled into the shot can be enough to overwhelm
the venting and result in a gas trap and burn.
Verify that excessive decompression is not being used. If a large decompression is
used to limit nozzle drool or stringing investigate the nozzle temperature. Excess
nozzle temperatures can lead to drool and require higher than normal decompres-
sion settings. Try lowering the decompression setting to determine impact on the
process; remember to keep enough decompression to allow the non-return valve to
function correctly.

20.3.2„ Burns Troubleshooting Mold Issues

When dealing with burning problems the mold is very often the culprit. The fol-
lowing mold-related concerns can contribute to burns:
ƒ Venting
ƒ Grease, oil, or other contaminants

20.3.2.1„ Mold: Venting

The number one root cause of burning is lack of venting in the mold! When burn-
ing is a problem always start with a review of the venting on the mold. Improved
venting will eliminate most burning issues (see Chapter 7 on venting). A classic
example of burning from lack of venting at a knit line is shown in Figure 20.2.