Abstract: Experimental study on the depth of the sink marks caused by different ribs of parts during the external gas-assisted injection molding (EGAIM) process was performed on the basis of a laboratory-developed EGAIM system. The single factor method was adopted to investigate the effects of mold temperature, melt temperature, gas delay time, gas holding pressure and gas holding time on these sink marks. The results indicated that there were different effects on the sink marks of part with different ribs. The results from an orthogonal experiment demonstrated that the gas delay time and gas holding pressure were two main factors that influenced the depth of sink mark. The EGAIM process along with the optimal process parameters could well eliminate the sink marks by designing the aspect ratio of rib. The sink mark revealed a depth of 1.2 μm at an aspect ratio of rib width/part thickness up to 1.5, which was less than the value obtained at the conventional aspect ratio of 0.6. These results indicated that the optimized processing conditions of EGAIM could effectively eliminate the sink mark caused by the thicker ribs and improved the design freedom of the parts accordingly.