In this study, we attempted to visualize the flow patterns inside molded samples of glass-fibers or talc reinforced GPPS and PP especially in the vicinity of the flow front, using a rotary runner exchange system which instantaneously switches melt flowing into the cavity. From the visualization results, an extremely characteristic flow behavior was seen at the flow front area for resins with fillers which was not observed for those without fillers. Especially in the case of resins with 40 wt.% talc or 30 wt.% GF a-symmetric zigzag fountain flows were generated alternately upward and downward along the cavity width direction. Moreover along the outer layer of the flow front, one observed a large amount of residual resin remaining for a long time during the cavity filling process, in the case of 40 wt.% GF primarily due to the increase of viscosity which restricts the occurrence of diverging flow. Based on the above visualization results, we proposed a generic model of zigzag fountain flow; an a-symmetric upward flow in a certain area starts preventing the fountain flow in the upper side of the adjacent area, causing the diverging center to sink. This phenomenon subsequently causes an a-symmetric flow in the opposite direction in the next adjacent area, which continuously propagates along the cavity width direction.