Applying gas pressure on the reverse side of the part that called external gas-assisted injection molding (EGAIM) has the potential to solve shrinkage-related molding problems. We investigate the packing effects of EGAIM on part shrinkage and sink mark under various rib design and compare it to that of conventional injection molding (CIM). It was found that EGAIM has uniformly distributed packing pressure within the entire mold cavity. To achieve equivalent part shrinkage, CIM requires 100 MPa packing pressure from the molding machine, whereas EGAIM requires only 9 MPa. EGAIM can further reduce part shrinkage if the gas pressure and gas packing time are both increased. EGAIM can also eliminate sink marks for rib designs of an aspect ratio (rib width /part thickness) up to 1.2, whereas CIM can achieve the same sink mark level only at an aspect ratio of less than 0.5. POLYM. ENG. SCI., 50:2085-2092, 2010. (C) 2010 Society of Plastics Engineers