Page 192 - Injection Molding Advanced Troubleshooting Guide

182 20 Burns

„ „20.2„ Burns Troubleshooting Chart

Table 20.1 shows the burns troubleshooting chart.

Table 20.1 Burns Troubleshooting Chart
Molding Process Mold Machine Material
too fast fill velocity venting buildup on screw, end moisture content
cap, etc.
high melt temperature grease, oil, or other screw design type
contamination
high back pressure barrel heater control additives
problems
decompression damage to screw,
barrel, tip, etc.

„ „20.3„ Burns Troubleshooting

Burns are areas of the molded part that have a brown or black discoloration that is
often sooty (like charcoal). The two main questions when troubleshooting burns
are:

1. Where is the gas coming from? Keep in mind that the cavity is full of gas (air) at
the start of fill.
2. Why is the gas not escaping the mold?

Burn troubleshooting often comes down to venting-related issues; do not process
around tooling issues!

20.3.1„ Burns Troubleshooting Molding Process Issues

As mentioned above, the root cause of burns is frequently a lack of venting. Pro-
cessing around a burning issue should be the last choice; instead address tooling
issues before adjusting the process!
Process settings that can either create excess gas byproducts or cause the gas to be
trapped include:
ƒ Fill velocity
ƒ Melt temperature