Burn Marks / Dieseling
Causes, Solutions & Prevention in Injection Molding
Two types: Diesel Effect trapped air compresses and self-ignites at 300-400 deg C. Gate shear material degrades passing through undersized gate.
Below you will find general information about this defect. To get a pinpoint solution tailored to your specific machine, material, and process parameters using the world’s most comprehensive injection molding troubleshooting database — sign up and access AI-powered troubleshooting, 7 engineering calculators, and detailed project tracking.
🔍Symptoms & Visual Identification
⚠️Root Causes by Category
⚙️ Process
- Injection speed too high Diesel effect
- Melt temperature too high
- Barrel residence time too long
- Gate velocity too high shear degradation
🔲 Mold
- Vent absent or crushed at end-of-fill location
- Gate cross-section too small
- Dead-end blind pockets with no exit for air
- Excess lubricant on ejector group burns on surface
🧪 Material
- Low thermal stability grade
- High regrind ratio lower MW degrades easier
- Moisture accelerating degradation
🔧 Mold Condition
- Grease or foreign lubricants on mold surface
📋 General
- Part geometry traps air deep ribs thin meeting thick
- Multi-gate air trapped at flow front meeting points
✅Solutions & Corrective Actions
⚙️ Process
- Type 1 Diesel Reduce injection speed slightly reduce clamp force
- Type 2 Gate shear Reduce injection speed at gate reduce melt temperature
- Reduce barrel residence time
🔲 Mold
- OPEN VENT at burn location most critical action
- Enlarge gate cross-section for Type 2
- Remove excess grease from ejector group
- Add draft to rib ends to allow air escape
🔧 Mold Condition
- Excess lubricant applied to ejector group after mold maintenance causes burn marks. Keep mold surface clean.
📋 General
- Use higher thermal stability resin grade
- Reduce regrind ratio
- Dry material correctly per TDS
🛠Step-by-Step Troubleshooting
Check Process Parameters
Type 1 Diesel Reduce injection speed slightly reduce clamp force
Inspect the Mold
OPEN VENT at burn location most critical action
Verify Material Condition
Low thermal stability grade
❓Frequently Asked Questions
What causes burn marks / dieseling in injection molding?
Burn Marks / Dieseling is caused by several factors including: Injection speed too high Diesel effect; Melt temperature too high; Vent absent or crushed at end-of-fill location; Gate cross-section too small; Low thermal stability grade; High regrind ratio lower MW degrades easier.
How do you fix burn marks / dieseling in injection molding?
Start by checking key parameters: Injection speed, Back pressure, Vent depth, Screw RPM. Type 1 Diesel Reduce injection speed slightly reduce clamp force
Which plastics are most affected by burn marks / dieseling?
Common materials include: ABS, PC, PA6, POM, PVC. Pay special attention to drying, shrinkage and mold temperature for each grade.
How do I prevent burn marks / dieseling in future production runs?
Prevention relies on three pillars: (1) process optimisation — verify Injection speed, Back pressure; (2) mold maintenance — check gates, vents and cooling channels; (3) material control — ensure correct drying and virgin/regrind ratio.
What is the difference between burn marks / dieseling and similar defects?
Brown black or charred marks on part at end of fill deep ribs or near gate. May have odor. This distinguishes it from related surface defects during visual inspection.
📐Related Calculators
Run these engineering calculators to verify your process parameters and prevent this defect.
🔗Related Defects
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