Burn Mark Troubleshooting in Injection Molding: Causes, Diagnosis & Solutions
Key Takeaway: Burn marks are always caused by trapped air compressing to combustion temperature. Step 1: Locate the gas trap (last-to-fill analysis). Step 2: Add venting at that location. For deep holes and blind cavities, gas release core pins are the definitive solution — process adjustments alone are temporary fixes.
Root Cause: The Diesel Effect
When molten resin enters the cavity, it pushes air ahead of it. If the air has no escape path (blocked by the advancing resin front), it compresses in a shrinking pocket. The adiabatic compression raises the air temperature to 300–500°C — well above the auto-ignition point of most polymer off-gases. The result is localized combustion that leaves black or brown carbonized marks on the part surface.
This phenomenon is identical to the compression ignition in a diesel engine — hence the name "diesel marks."
Burn Mark Location Diagnosis
| Burn Mark Location | Likely Gas Trap | Recommended Fix |
|---|---|---|
| Bottom of deep holes | Air trapped between core pin tip and advancing resin | Gas release core pins (SVC for severe cases) |
| End of flow path (far gate) | Air compressed at last-to-fill cavity wall | Add parting line vents at last-to-fill location |
| Rib intersections | Air trapped where ribs meet walls | Parting line vents + possible vacuum assist |
| Boss base (inside) | Air trapped between boss core pin and surrounding resin | Gas release boss core pins |
| Weld line locations | Air trapped between merging flow fronts | Vent at weld line + overflow wells |
| Random locations (cycle-to-cycle) | Degraded resin (not trapped air) | Reduce barrel temperature, check residence time |
Systematic Troubleshooting Procedure
- Step 1: Identify burn location — Is it consistent (same spot every shot) or random? Consistent = gas trap geometry. Random = material degradation.
- Step 2: Short-shot analysis — Run progressively shorter shots (reduce hold time/pressure) to map the fill pattern. The last area to fill is where air accumulates.
- Step 3: Check existing vents — Are parting line vents clogged with deposits? Clean all vents. Verify vent depth (should be 0.02–0.05 mm for most resins).
- Step 4: Process adjustment (temporary) — Reduce injection speed by 10–20%. Reduce barrel temperature by 5–10°C. These reduce burn severity but do not eliminate the root cause.
- Step 5: Add venting (permanent fix) — For deep holes/blind cavities, replace standard core pins with gas release variants. For surface locations, add new parting line vents.
Process Adjustments vs Tooling Solutions
| Approach | Effectiveness | Sustainability | Cost |
|---|---|---|---|
| Reduce injection speed | Moderate — reduces compression rate | Temporary — may cause short shots | Free (process change) |
| Lower melt temperature | Low — less gas but slower fill | Temporary — risks incomplete fill | Free (process change) |
| Clean/deepen parting line vents | Good for surface burns | Semi-permanent — vents re-clog | Low (maintenance) |
| Gas release core pins | Excellent for deep holes | Permanent — vents along pin shaft | $15–30 per pin |
| Vacuum-assisted venting | Excellent for all locations | Permanent — active air removal | $2,000–5,000 (system) |
According to ScienceDirect's burn mark analysis, the combination of proper venting at the last-to-fill location plus optimized injection speed profile eliminates burn marks in 95% of cases without requiring vacuum-assisted systems.
Prevention During Mold Design
- Run mold-fill simulation to identify last-to-fill locations before mold manufacture
- Specify gas release core pins for any feature with depth/diameter ratio > 3:1
- Design parting line vent channels at all predicted last-to-fill locations
- Include overflow wells at weld line locations for resins with high gas content (POM, PA6)
Frequently Asked Questions
What causes burn marks in injection molded parts?+
Trapped air compresses to 300–500°C during injection fill, auto-igniting residual gases. The root cause is always inadequate venting at the last-to-fill location.
How do gas release core pins prevent burn marks?+
They provide a vent path along the pin shaft from the cavity bottom to atmosphere, allowing trapped air to escape before reaching combustion temperature.
Can I fix burn marks with process adjustments alone?+
Partially — reducing injection speed and barrel temperature helps but doesn't eliminate geometric gas traps. The definitive fix is adding proper venting at the trapped air location.
Gas Release Solutions
Struggling with Burn Marks?
Send us photos of the burn marks, part drawing, and current core pin specifications — our engineers will diagnose the root cause and recommend the fix.
✓ Free burn mark diagnosis✓ Gas release pins in stock✓ Same-day shipping