Reducing ghost marks in injection-molded plastic parts by using external gas-assisted holding pressure

Ghost marks are gloss-related defects in injection-molded plastics that cause uncertain visual darkness depending on the viewing angle. Conventionally, these minor defects are not substantial, but they cannot be overlooked when they diminish the appearance quality of plastic parts such as dashboards in luxury automobiles. This work investigated the causes and effects of ghost marks through experimental observations of a tensile-test sample, which was assessed to determine the improvement of its appearance quality with respect to processing parameters including mold and melt temperatures as well as injection speed and pressure. To reduce defects, external gas-assisted injection molding (eGAIM) that engages the packs on the reverse of in-molded parts with external gas pressure was further employed. For objectively evaluating the gloss qualities of the tested samples, the captured images of part surfaces were converted into grayscale images, and the degrees of defect were then quantized as the deviation values of the grayscale images. The results showed that high mold and melt temperatures as well as high injection speed and pressure facilitated increasing the quality of the parts, but exhibited limitations. Furthermore, in this study, employing external GAIM with gas packing pressure above 3 MPa successfully eliminated ghost marks.