Water-assisted injection molding (WAIM) is one of the latest techniques to produce hollow or partially hollow products. A shorter cycle time in combination with a lower material and process cost are the main advantages. However, the occurrence of part defects remains unresolved. This study investigated the influence of the polymer shrinkage behavior on the occurrence of the part defects. Therefore, two polypropylene resins were used of which one resin was filled with nucleating agents, whereas the other was unfilled. The results cleared out that the presence of nucleating agents, inducing a higher polymer shrinkage, leads to a higher amount of part defects.