Injection Moulding Headaches Part 3: Burn Marks (and How to Fix Them) // Alliance Tooling Blog
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What’s your biggest annoyance when it comes to injection moulded parts? Here’s another issue seen far too often. This week, Alliance Tooling is looking at burn marks.
This is the third instalment in Alliance Tooling’s weekly series, where the company breaks down common moulding problems and examines how to resolve them through better design and tooling decisions.
The Problem: Burn Marks
Burn marks appear as dark, discoloured streaks or patches on the surface of a moulded part, often black or brown in appearance.
They are not just a cosmetic issue. They can point to deeper problems in the moulding process:
- Make parts unusable for cosmetic applications
- Indicate trapped gas or overheating
- Lead to scrap, rework, and customer complaints
Why do burn marks happen?
Burn marks are usually caused by trapped air or gases that become compressed and superheated during injection.
In simple terms, the air has nowhere to escape, so it overheats and burns.
Common causes include:
- Poor or insufficient venting
- High injection speeds trapping air in the cavity
- Sharp flow fronts or awkward geometry
- Material degradation from excessive heat
How to design them out (before tooling even starts)
This is where good tooling design has a direct impact. Addressing airflow and material behaviour early reduces the risk of defects later.
At Alliance Tooling, we focus on:
- Designing effective venting paths in critical areas
- Avoiding air traps through improved flow design
- Positioning gates to support smooth, progressive filling
- Considering material behaviour to reduce shear heating
Already seeing burn marks? Here’s how to fix it
If burn marks are appearing in production, there are several adjustments that can help bring the process back under control:
- Reduce injection speed to limit air compression
- Improve or add venting in problem areas
- Lower melt and barrel temperatures if degradation is suspected
- Adjust gate position or size to improve flow
- In some cases, tool modification is needed to eliminate air traps
Where Alliance Tooling comes in
Burn marks are often a symptom of underlying airflow or design issues. Fixing the visible defect without addressing the cause usually leads to repeat problems.
At Alliance Tooling, we don’t just treat the mark…we solve the cause. Whether it’s:
- A new tool design
- Troubleshooting an existing mould
- Or improving part quality
We bring practical, shop-floor experience to get it right.
Got burn marks you cannot eliminate, or want to prevent them in future projects? Book a quick call with Chris Rossell, Technical Director, and we can review it with you.
Next week: Short shots, why parts do not fully form and how to fix it.
Read more news from Alliance Tooling here.
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