Black Spots Defect in Injection Molding: Causes & Solutions

During injection molding, black spots defect are a common quality defect that directly affects product appearance and yield rate. These defects mainly include three types: black spots, localized burn marks, and black streaks.

Black Spots

Appearance: Visible impurity spots on the part surface.

Cause: Contamination in the mold or material, or degraded carbonized residue trapped in dead spots of the barrel.

Solutions:

• When black spots are present only on the surface, Clean the mold and prevent dust from entering the cavity.

• When black spots are present both inside and outside, Check for contaminated raw materials; remove carbonized deposits from the nozzle, flange, barrel, and screw; adjust barrel temperature and back pressure.

Localized Burn Marks

Appearance: Localized degradation of melted material caused by trapped gas overheating in the cavity.

Cause: Excessive melt temperature or compressed gas overheating in the cavity.

Solutions:

• Repair the temperature control system to avoid excessive barrel temperature.

• Reduce injection speed and pressure appropriately.

• Clean residual resin from the barrel.

• Choose the right barrel size to avoid prolonged melt residence time.

• Add venting grooves and improve mold venting.

Black Streaks

Appearance: Dark patches or localized discoloration on the part surface.

Cause: Resin degradation and discoloration before injection. Possible reasons include excessive barrel temperature, too high injection speed or pressure, uncleaned residual resin, overly long cycle time, or defects in the screw/barrel.

Solutions:

• Check raw materials: Remove contaminated pellets.

• Adjust process parameters: Lower barrel temperature, injection pressure, injection speed, screw speed, and back pressure.

• Control residence time: Avoid long melt residence time above 260°C; during machine stops, reduce temperature below 150°C.

• Clean equipment: Inspect and clean carbon buildup in flow channels, screw, non-return valve, flange, and other areas.

  
  

Key Takeaway

To solve black speck defects, the key is to accurately identify the defect type and systematically investigate from three angles: raw materials, equipment, and process parameters. This enables fast diagnosis and precise correction.