Meaning
A surface defect characterized by shiny, elongated marks in the direction of the melt flow is usually caused by trapped moisture or gas. Silver streaking ruins the aesthetic quality of a part and can indicate a weakness in the material structure. This problem is frequently seen in hygroscopic resins like nylon or polycarbonate.
Moisture Presence
If the resin is not dried properly before molding, the water turns into steam when it enters the hot barrel. This steam is then stretched along the surface of the melt, creating the characteristic appearance of silver streaking. Proper desiccant drying is the primary solution for this specific issue.
Even a small percentage of residual moisture can cause visible defects during the injection cycle.
Venting Efficiency
Air that is trapped in the cavity can also be compressed and forced into the surface of the melt. When the tool lacks sufficient venting, silver streaking may appear near the end of the fill or at rib intersections. Improving the mold design to allow gas to escape often eliminates these marks.
Thermal Breakdown
Excessive melt temperatures can cause the polymer or its additives to decompose and release gaseous byproducts. This chemical breakdown results in silver streaking that is often accompanied by a foul odor or discoloration. Reducing the barrel temperature or the residence time can prevent the material from overheating.
