Ghost marks - gloss-related defects in injection-molded plastics

The production of injection-molded parts free from surface appearance defects is of great importance in the manufacturing of high-quality products. A particular surface defect which occurs on components manufactured from an acrylonitrile-butadiene-styrene copolymer (ABS) is here described. The defect has been called ghost marks and is characterized by a local change in gloss or lightness which is only visually detectable in certain viewing angles and conditions of illumination. By means of scanning electron microscopy, small-scale deformations of the surface texture were observed in the area of the defect which in turn alters the light scattering properties of the surface. The light scattering properties were evaluated by means of a multiangle spectrophotometer. The holding pressure during the injection molding process was shown to play a significant role in the formation of the ghost marks possibly imposing forces causing the deformation of the surface texture. The deformations may also occur from nonuniform thermal surface shrinkage during cooling. The type of texture and wall thickness also influences the occurrence of ghost marks.POLYM. ENG. SCI., 2012. \ua9 2011 Society of Plastics Engineers