Page 178 - Injection Molding Advanced Troubleshooting Guide

168 18 Bubbles

plastic pressure. If running with back pressure lower than 1000 psi plastic pres-
sure the process should be evaluated to determine if the setting can be changed.
Low back pressure will lead to a poor uniformity of the melt. Normally, low back
pressures are used for shear-sensitive or glass-filled materials. Verify that the back
pressure is set to the documented process.
One commonly occurring mistake is when people are purging a machine they will
drop the back pressure to help the screw recover, but forget to reset the back pres-
sure to the set point. With these low back pressures, the air that is located between
the pellets in the barrel will not be forced out, resulting in a melt that is full of
trapped gas. If lowering back pressure to help with screw recovery make sure to
reset the value to the required set point.

18.3.1.3„ Molding Process: High Decompression
Decompression is an important setting on a molding machine to help with con-
trolling drool and for helping a non-return valve seat. However, remember that
excessive decompression may pull air into the plastic melt, which in turn can lead
to bubbles in the molded part.

Verify that the decompression setting is correct. Evaluate if the decompression can
be lowered without risking other problems. Avoid the habit of using more decom-
pression than is required for producing a quality part.
If excessive decompression is being used to control drool or stringing problems
review the nozzle heater actual values to ensure that temperatures are not the true
cause of the problem. There will be cases where dropping the nozzle temperatures
will allow the decompression to be reduced, which in turn leads to an elimination
of bubbles and drooling problems.

18.3.2„ Bubbles Troubleshooting Mold Issues

Potential reasons for bubble defects relating to mold issues include:
ƒ Venting
ƒ Hot runner temperatures
ƒ Cracked water line
ƒ Venturi effect

18.3.2.1„ Mold: Venting
The most common mold-related problem that can lead to bubble problems is a lack
of venting. If gas is trapped in the melt stream and not allowed to vent properly the
gas can very easily lead to a bubble.
See Chapter 7 on venting for more details.