Injection molding is the most effective process for the mass production of plastic parts. However, there are various plastic part surface defects which occur during the injection molding process. Sink marks are among the most difficult surface defects to improve. The sink marks usually appear as depressions on the surface of a molded part. These depressions are typically very small; however, they are often quite visible because they reflect light in different directions to the part. In this paper, we conducted experimental studies in order to minimize sink marks that occurred on the in-mold type PAB door seam line by using the Mucell injection process. The results of the study indicate this method can be used to eliminate sink marks using the Mucell injection molding process.