Black Specks and Discoloration: Where They Come From

Black specks and discoloration are common defects in injection molding. The causes fall into four groups.

Machine

• Faulty heating control overheats the barrel and degrades the material.
• Defects in the screw or barrel trap material, which degrades under prolonged heating. Check the screw tip assembly for wear or metal debris.
• Some materials, such as ABS, cross-link and char under high heat while keeping their pellet shape; the screw crushes them and carries them into the part.

Mold

• Poor venting causes burning; undersized runners and gates create excessive shear and charring.
• Unsuitable oil-based lubricants or release agents in the mold.

Material

Excess volatiles or moisture, impurities, too much regrind, or contamination.

Process

Excessive pressure, injection speed, back pressure or screw speed can all degrade the material. Clean the barrel regularly and remove additives that are less heat-stable than the base resin.