Page 179 - Injection Molding Advanced Troubleshooting Guide

18.3 Bubbles Troubleshooting 169

18.3.2.2„ Mold: Hot Runner Temperatures
If hot runners are set or running with an excessively high temperature the mate-
rial may degrade in the hot runner manifold leading to generation of gas. This gas
will be entrapped in the plastic melt stream and will be carried into the mold
during injection.
Verify that the hot runner temperatures are set correctly as per the setup sheet. It
is also critical to determine if the hot runner system is running at the set point.
Often times a hot runner zone will overheat due to poor thermal control from
causes like a thermocouple being installed incorrectly. If a hot runner system has
two thermocouples swapped the hot runner controller will keep increasing the
current flow to try to get the zone to achieve the desired set point, but the thermo-
couple will be reading another location. More sophisticated hot runner controllers
have many built-in features such as broken thermocouple detection, swapped
thermocouple detection, and heater burnout detection.

18.3.2.3„ Mold: Cracked Water Line
Another tooling issue that can cause bubbles is a crack in the mold. If a mold
cracks to a water line the water can wick to the surface of the mold resulting in
bubbles. Cracks in the mold can be obvious with water drips showing on the mold
surface. However, there will be cases where the water only shows up after the mold
has been closed and clamped up, so when checking for suspected cracks always
close the mold to see if the clamp pressure opens up the crack. A short-term fix for
a cracked mold is to use a reverse-flow thermolator, as this can affect the cooling
capability of the mold. The ultimate fix will be to fix the crack. Be aware during tool
design that sharp radii will be more prone to crack.
Cracked water lines often are a result of a mold design mistake. Sharp corners in
steel will be stress risers that can lead to cracking. Also, if a water line is placed too
close to the surface of the mold the resulting thin steel condition will often lead to
cracking. Examination of mold designs is a critical step to providing a mold that
will meet production requirements.

18.3.2.4„ Mold: Venturi Effect
The Venturi effect is named after Giovanni Battista Venturi and describes a pres-
sure differential effect that occurs during flow. In injection molding we sometimes
see an effect like a Venturi when air is pulled into the melt as the mold is filled.

This effect can come from a variety of areas in the mold including mismatch or
poor seating between the nozzle tip and sprue bushing, mismatch or seating prob-
lems in the hot runner manifold, and areas in the mold with gaps such as ejector
pins and inserts. If the bubble seems to downstream to specific areas of the mold it
may indicate a problem with a mold component. This is a problem that will require