Page 177 - Injection Molding Advanced Troubleshooting Guide

18.3 Bubbles Troubleshooting 167

18.3.1„ Bubbles Troubleshooting Molding Process Issues

The main causes of bubble defects in molded parts come from excess gas in the
part. This excess gas can come from several root causes including:
ƒ High melt temperature
ƒ Low back pressure
ƒ High decompression

18.3.1.1„ Molding Process: High Melt Temperature
When plastic is overheated often a gas will be given off from degradation of the
components of the plastic. Much like moisture these gases can form bubbles that
can be carried in the melt stream and injected into the cavity of the mold. When
troubleshooting an issue such as bubbles it will be beneficial to conduct a melt
temperature check. If the melt temperature is above the recommended value for
the material being molded it will be time to check the following:
ƒ Barrel temperature set points correct? Verify settings to those recommended for
the material.
ƒ Barrel temperatures running to set point? Sometimes a zone will be overheating
due to a thermocouple problem such as poor contact between the thermocouple
and the steel, a bad thermocouple, a miswired zone (zone 1 thermocouple actu-
ally is located on zone 2), or a thermocouple that is located too far away from the
heat location.
ƒ Back pressure set or running excessively high? The majority of the heat required
to melt the plastic comes from shear through back pressure and recovery rate.
Verify that the back pressure is set correctly and giving the correct pressure.
Also remember that the intensification ratio will contribute to the actual pres-
sure that the plastic is seeing.
ƒ Screw recovery speed running excessively high? Ensure that the screw RPMs are
both set correctly and not running excessively fast. Under normal circumstances
the screw should recover 2–3 seconds prior to mold opening.
Often times when overheating occurs it will be caused by either an incorrect set-
ting on the machine or something running out of control. Overheating can lead to
many other issues including splay, brown streaks, or black specks. You may see a
combination of these defects if overheating is the root cause.

18.3.1.2„ Molding Process: Low Back Pressure
If the back pressure is set too low the shot will not be adequately compressed.
When the shot is not compressed adequately air voids will be present in the melt
stream which will be injected into the cavity potentially resulting in bubbles. Typi-
cal back pressure settings for most materials will be between 1000 and 2500 psi