The observation and origin of micro flow marks in the precision injection molding of polycarbonate

Abstract

Micro flow marks have been found to occur in injection molding of polycarbonate Their minimum depth is ∼ 5 nm on the mold smooth surface. Such marks seem to be formed when the molten polymer near the mold wall at the advancing front is cooled to the no-flow temperature of resin before it comes into contact with the mold wall, preventing the precision transcription of the mold smooth surface. We investigated the molding factors affecting their occurrence by heat transfer analysis. It was found that the size of micro flow marks is related to the melt temperature, mold temperature, no-flow temperature, thermal diffusivity of the resin, and time pitch formation of flow mark ridges and valleys. The relationships between those factors and the sizes of micro flow marks are clarified quantitatively, and the guidelines for precision transcription in injection molding are formulated.