Abstract This study investigates the relation between water penetration behavior and product quality in water-assisted injection molding (WAIM) for different process and material parameters. The principle mechanisms responsible for both building up the residual wall thickness (RWT) and formation of part defects as well as the influence of these process and material parameters on the RWT and occurrence of part defects could be revealed. This is done by relating the measurements of water pressure and water volume flow rate, visualizing in-mold variations during water injection, to both the part weight and a visual inspection of part cross-sections, for different process settings and materials. It was found that the RWT as well as the formation of part defects depend on the polymer resistance, since the variation of the latter leads to a change in both the water bubble width and velocity. On the one hand, the resistance is determined by the process parameters having an influence on the amount of melt ahead of the water bubble, depending on injector opening time, water volume flow rate and water injection delay time. On the other hand, the resistance also depends on the friction between the polymer chains, which is determined by the zero-shear viscosity and the shear thinning behavior of the polymer. In addition, the occurrence of part defects is directly related to the polymer crystallization behavior. The resulting product quality therefore depends on a combination of the process parameters and the applied material.