How to Prevent Moisture Streaks and Surface Blisters in Injection Molded Parts
Moisture-related defects are a common problem in injection molded plastic parts. These defects usually appear as oblong silvery streaks, surface blisters, or torn marks that open in a U-shaped pattern opposite to the direction of material flow. In many cases, the plastic discharged from the barrel may also appear foamed, indicating excessive moisture in the raw material before molding.
Appearance of the Defect
This defect is typically visible as elongated silvery marks on the molded surface. The streaks often open in a U shape against the flow direction, and the part surface may show blisters or tearing. These visual defects can significantly reduce cosmetic quality, especially on smooth or visible plastic components.
Main Cause of Moisture Streaks and Blisters
The main cause is excessive moisture content in the plastic material. When wet resin is heated in the barrel, the moisture turns into water vapor during melting. As the molten plastic enters the mold cavity, the vapor disrupts the material flow and causes the surface of the molded part to tear open or form blister-like defects.
This issue is especially common with hygroscopic engineering plastics that absorb moisture from the air during storage or handling.
How to Correct the Problem
The most important correction is to dry the material properly before molding. The resin should be predried to the recommended residual moisture level, typically below 0.1%, depending on the material specification. Proper drying helps prevent vapor formation during melting and improves overall molding consistency.
Increasing back pressure can also help improve melt consistency and reduce the effect of trapped moisture during plasticizing. However, material drying remains the primary solution and should be controlled carefully before adjusting other process settings.
Why Material Handling Matters
Moisture defects are often not caused by mold design alone, but by material storage, drying conditions, and shop-floor handling. Even if resin is dried correctly, it may absorb moisture again if exposed to humid air for too long before processing. Stable drying procedures and controlled material handling are essential for maintaining part quality.
Conclusion
Silvery moisture streaks, U-shaped marks, and surface blisters in injection molded parts are usually caused by excessive moisture in the plastic resin. By predrying materials to the correct residual moisture level and optimizing back pressure when necessary, manufacturers can reduce these defects and improve molded part appearance and consistency.