The wavelike flow mark phenomenon is one of the surface defects that can occur during the injection stage of the injection molding process. The generating mechanism of this phenomenon is not yet well understood. In this paper, flow visualization data shaving the way in which flow marks generate are reported for the first time. On the basis of these observations a new hypothesis for the mechanism causing now marks is proposed. A mathematical expression is developed that expresses nondimensional flow mark curvature in terms of the processing conditions, the thermal properties of the polymer and the mold, and the interval of flow mark generation. This mathematical relationship is then tested over a wide range of operating conditions and for three different materials. Finally, the practical implications and significance of this new hypothesis are discussed.