What Causes Burn Marks in Plastic Injection Molding?

Burn marks (or “dieseling”) are dark brown or black discolorations that appear on plastic parts during injection molding. These defects not only ruin aesthetics but can also weaken structural integrity.

Primary Causes of Burn Marks

1. Trapped Air & Poor Venting

When air gets trapped in the mold cavity, it compresses during injection, reaching extremely high temperatures (400–600°F) that burn the plastic.

Signs:

Burn marks near last-to-fill areas (thin ribs, corners)
A burnt smell during molding

Solutions:

Add or enlarge venting channels (0.0005–0.002″ depth)
Use porous vent inserts for complex geometries
Reduce injection speed to let air escape

2. Overheating & Material Degradation

Excessive melt or mold temperatures can scorch the resin.

Common Culprits:

Too-high barrel temps (e.g., ABS above 500°F)
Long residence time (material stagnates in the barrel)
Hot runner malfunctions (localized overheating)

Solutions:

Lower melt temps by 20–50°F
Purge old material regularly
Use thermal sensors to monitor hot runner zones

3. Contaminated or Moist Material

Moisture or contaminants (dust, degraded resin) can carbonize under heat.

Signs:

Random black specks throughout the part
Bubbles or splay marks (from moisture turning to steam)

Solutions:

Dry hygroscopic resins properly (e.g., Nylon at 180°F for 4+ hrs)
Clean hoppers and barrels between material changes
Use metal detectors to catch contaminated regrind

4. Excessive Shear Heat

Fast injection speeds or high screw RPMs generate friction, burning the plastic.

Signs:

Burn marks near gates or flow paths
Jetting (wavy surface lines)

Solutions:

Reduce injection speed by 10–20%, Optimize gate design (avoid small, restrictive gates)
Use a melt temp sensor to verify shear heating

Less Common Causes

Oil/lubricant buildup in the mold (clean with alcohol)
Decomposed release agents (switch to non-silicone options)
Resin additives (e.g., flame retardants degrading)

How to Troubleshoot Burn Marks?

Check burn location

Flow path/gate? → Shear heat issue
End of fill? → Venting problem
Random spots? → Contamination

Inspect material

Test drying efficiency (moisture analyzer)
Examine for foreign particles

Monitor process data

Compare melt temps to resin specs
Check hot runner thermocouples

Key Takeaways

Improve venting – 90% of burn marks stem from trapped air
Control temperatures – Avoid excessive barrel/hot runner heats
Ensure material purity – Dry resins and filter regrind
Optimize injection speed – Balance fill time with shear heating risks

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