Burn Marks on Molded Parts: Four Causes and Solutions

Burned parts are scrap and reduce productivity. Because burns can come from the machine, the mold or the process, the right fix depends on the cause.

1. Machine

Abnormal temperature control can overheat the barrel so that material degrades before injection. Material can also hang up in the nozzle, screw flights or check valve, degrade and then be carried into the part as dark brown burn streaks. Remove and clean the nozzle, screw and barrel.

2. Mold

Poor venting is the main cause. These burns usually appear at the same fixed location on the part, which distinguishes them from machine-related burns. Add vents or venting ejector pins.

3. Contaminated material

Dust or debris in the resin can burn. Take care when loading material, and fit a magnetic grate on the hopper to catch metal particles.

4. Process settings

Excessive back pressure can overheat the barrel and cause burns. Screw speed that is too high also generates excess heat; 40–90 rpm is generally appropriate. Where the mold has no vents or only small ones, high injection speed can cause burns from compressed hot gas.