Plastic Injection Molding Defects: Burns

Welcome to our Plastic Injection Molding Defects series, where we will explain some common defects seen by injection molders and present ways to fix or prevent them.

What are Burns?

Sometimes known as gas traps or the dieseling effect, burns appear as black or brown deposits, sooty like charcoal, both on the part and the mold surface.

What Causes Burns?

There’s a good reason burns are also called gas traps in injection molding: they’re caused by trapped gases. When gases get trapped during mold filling, high pressures can cause ignition, which leads to the plastic burning. Burns in injection molding are, quite literally, burns.

To determine the cause of your issue, first determine where the gas is coming from, and then ask why isn’t the gas escaping the mold?

You could be looking at fill velocity, melt temperatures, back pressures, or decompression among the molding process. The machine itself could have buildup, damaged parts, or even the wrong screw design. The material may struggle with its moisture content or additives.

The most common culprit deals with mold venting issues. If you determine this is the cause of your problem, do not try to process around a tooling issue. Address all tooling issues before adjusting the process. Many of the other problems are either caused by or made worse by venting. This is the first thing you should check.

Table 20.1 Burns Troubleshooting Chart, found in Injection Molding Advanced Troubleshooting Guide: The 4M Approach (p. 182)

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

How to Prevent and Remove Burn Marks in Injection Molding

It bears repeating: if you have a venting issue, do not process around it. When your mold isn’t venting well enough, it can make any of the other causes of burns worse. When you improve your mold venting, you eliminate most burning issues.

Also, reach for the Cera Lube. Because over-greasing and grease breakdown can cause burning, Nanoplas Cera Lube, which neither breaks down nor bleeds, is the best solution.

Feel free to contact us if you have questions about our products and how they might solve your injection-molding problems.

Stay tuned for future installments of our plastic injection molding defects series.

Source: Injection Molding Advanced Troubleshooting Guide: The 4M Approach by Randy Kerkstra and Steve Brammer.