Burn Marks & Black Specks in Injection Molding — Material-Related Causes

Defect types and their material connection

• Black specks / spots: Carbonized particles or foreign polymer residue inside the granule. Cause: insufficient filtration or contaminated feedstock.
• Burn marks (diesel effect): Air trapped in the mold heats and burns the material — but moist granule increases vapor pressure and intensifies the effect.
• Silver streaks / splay: Moisture-related surface defect. Common with granule above 0.3% moisture; extending drying time is a temporary fix, dry granule is the permanent one.
• Nozzle blockage: Sand, metal and mineral particles in low-mesh granule clog nozzles and hot runner channels — causing downtime and maintenance costs.

How to control granule quality

1. Ask about filtration level: 120-mesh filtration traps particles that would clog mold pores during production.
2. Request ash content: ≤ 0.5% ash means clean material; high ash causes nozzle wear and surface defects.
3. Measure moisture: ≤ 0.1% moisture minimizes drying requirements.
4. Demand batch reports: Work with suppliers who provide MFI, density and ash reports with every shipment.

What to check on the machine side

If the material is right and problems persist: lower screw back pressure, check mold venting channels, adjust melt temperature to the material datasheet and prevent moisture transfer between hopper and mold. But our experience shows most cases start with the granule — clean raw material is often the entire solution.