Injection molding is one of the most important methods for the manufacture of plastic products; however, there are several unresolved problems that confound the overall success of this technique. Sinkmarks occurring on the surface of molded parts caused by inappropriate mold design and processing conditions is one problem. In this report, an L'18 orthogonal array design based on the Taguchi method was conducted to minimize the sinkmarks of injection-molded thermoplastic parts. The polymeric materials used were general-purpose polystyrene and low-density polyethylene. A plate cavity with various ribs was used for molding. Experiments were carried out on an 80-ton reciprocating injection-molding machine. After molding, the sinkmarks on the surface of molded parts were characterized by a profile meter. For the factors selected in the main experiments, the corner geometry and the width of the rib were found to be the principal factors affecting sinkmark formation in injection-molded thermoplastics. A rib of an undercut geometry and a small width produces parts with the least sinkmark. Experimental investigation of an injection-molding problem can help illuminate the formation mechanism of sinkmarks so that steps can be taken to optimize the surface quality of molded parts.