This study starts with the definition of different defects that occur in water-assisted injection-molded products to
which subsequently responsible formation mechanisms are attributed. It is seen that the four different defect types in the current experimental setup—irregular residual wall, void, double wall, and no residual wall—are either formed by different mechanisms or by the same mechanism of which the extent decides on the actual defect type. The current insights into the occurring part defects are used in the second part of this study to explain the influence of process and material parameters on the defect occurrence in a reference experiment. The presence as well as the extent of a formation mechanism is here further linked to the water and/or polymer properties/conditions, which exist during water penetration. The water and melt temperature, water holding pressure, and the presence of nucleating agents in the polymer melt were therefore varied within the predefined reference setting. The influence on the nature and location of the part defects was investigated with a qualitative defect analysis. It is found that the proposed definitions and mechanisms are able to explain experimentally observed changes in defect occurrence physically, with which the existing indistinctness in the literature can be elucidated as well.