Burn Marks on Injection Molded Parts: Diagnosis Path

Burn marks usually appear as dark streaks, brown areas, black spots, or scorched edges. They are often blamed on temperature first, but the real cause may be trapped air, vent blockage, excessive residence condition, material contamination, or flow behavior around an insert.

Quick Answer

Treat burn marks as an air, material, heat, and flow investigation. If the burn appears at the end of flow, start with venting and trapped air. If it appears after stoppage or startup, review material residence and warm-up condition. If it follows an insert feature, review flow split and air escape around that insert.

What You May See

• Dark marks appear near the flow end, gate area, rib, boss, or insert edge.
• Burning appears more often during startup, after a pause, or after material changeover.
• The same cavity burns repeatedly while other cavities look acceptable.
• A smell, smoke, or unusual material appearance appears during molding.
• Burn marks appear together with short shot, weld lines, or bubbles.

Likely Cause Areas

• Air may be trapped because vents are blocked, undersized, dirty, or located away from the real air escape path.
• Material may be degraded by residence condition, contamination, previous material residue, or inappropriate drying state.
• Temperature control may drift at the barrel, nozzle, hot runner if used, or mold heating area.
• Injection behavior may compress trapped air quickly at the flow end.
• Insert geometry may split the flow and create a pocket where air cannot leave easily.
• Startup and pause conditions may leave material in a less stable state before production becomes repeatable.

Safe Checks Before Adjustment

Stop automatic operation if movement, alarm, guarding, or operator safety is uncertain. Do not place hands in the mold, table, injection, or guarded area while movement is possible. Use photos, controller screenshots, cycle logs, and safe observation before changing several settings.

Diagnosis Path

1. Record whether burn marks appear at startup, during stable running, after pause, or after material changeover.
2. Map the burn location by cavity, flow end, insert position, and table station.
3. Inspect vents, parting-line exhaust areas, and local shutoff features under safe stopped conditions.
4. Check material drying, batch, colorant, contamination risk, and purge history.
5. Review temperature alarms or temperature drift before the burn appeared.
6. Compare burn marks with short shot, weld line, or bubble locations to see whether trapped air is involved.
7. Record the original process state before making a single controlled adjustment.
8. Escalate if burn marks appear together with repeated alarms, smoke, abnormal smell, or unstable movement.

Evidence to Collect

Check	Why It Matters	Evidence to Collect
Burn location	End-of-flow burns often point to trapped air or venting	Marked part photos by cavity and flow path
Timing pattern	Startup burns and running burns often have different causes	Shift notes, pause time, startup scrap record
Material condition	Contamination or degradation can create dark marks	Material batch, drying record, purge notes
Temperature history	Drift or alarm records help identify heat-related causes	Controller screenshots and alarm history
Vent condition	Poor air escape can scorch the surface before fill completes	Safe mold inspection photos

What Not to Change Too Quickly

• Do not treat burn marks only as a barrel temperature issue.
• Do not continue production if there is smoke, unusual smell, or repeated temperature alarm.
• Do not clean vents or inspect the mold while the machine can move.
• Do not change temperature, speed, and pressure together without a record.

When to Contact Deboge

Contact Deboge when burn marks continue after vent, material, and temperature checks, or when they appear with alarms or machine instability. Send defect photos, material records, temperature screenshots, mold vent photos, machine model, and recent changeover notes.

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