Abstract This study investigates the influence of melt and process parameters on the quality of water-assisted injection molded tubes. The influence was determined by the aid of a design of experiments (DOE) for both a HDPE and a PP resin. This design was carried out on two different cavities, in which the residual wall thickness (RWT) and the part weight were set as response functions. This was combined with a visual inspection of the part surface to detect correlations in the occurrence of part defects such as fingering and double wall. It was found that the water injection delay time and the water volume flow rate have the largest influence on the RWT and part weight. Moreover, the shear viscosity of the applied material determines the width and shape of the water bubble and consequently the RWT and part weight. In addition, process parameters such as melt temperature, water hold time and water volume flow rate as well as melt parameters such as shear viscosity have an influence on the formation of part defects.