Residual wall thickness is an important indicator which aims at measuring the quality of water-assisted injection molding (WAIM) parts. The changes of residual wall thickness around dimensional transitions and curved sections are particularly significant. Free interface of the water/melt two-phase was tracked by volume of fluid (VOF) method. Computational fluid dynamics (CFD) method was used to simulate the residual wall thickness, and the results corresponded with that of experiments. The results showed that the penetration of water at the long straight sections was steady, and the distribution of the residual wall thickness was uniform. However, there was melt accumulation phenomenon at the dimensional transitions, and the distribution of the residual wall thickness wasn't uniform. Adding fillet at the dimensional transitions could improve the uniformity of the residual wall thickness distribution, and effectively reduce water fingering. Additionally, at the curved sections, the residual wall thickness of the outer wall was always greater than that of the inner wall, and the fluctuations of the residual wall thickness difference were small. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013