Gloss Marks on Plastic Parts? Causes and Quick Fixes

Gloss marks — also known as stress shadows or burnish marks — are localized shiny spots on the surface of injection molded parts. They are a form of stress concentration and typically appear as glossy patches that differ from the surrounding matte or textured surface.

Root Cause

Gloss marks are usually caused by improper process settings that create excessive stress in the molded part. The most common contributing factors include:

• Excessive injection pressure

• Excessive holding (packing) pressure

• Overly long holding time

• Low injection speed

• Low mold temperature

These conditions cause the melt to be packed too tightly against the mold surface, creating localized densification and resulting in visible gloss differences.

Solutions

Gloss marks can often be corrected through process adjustments. The following measures are typically effective:

• Reduce injection pressure as much as possible

• Reduce holding (packing) pressure

• Shorten holding time

• Increase injection speed

• Raise mold temperature

These adjustments help reduce internal stress and promote more uniform cooling, resulting in consistent surface appearance.