The objective of this study was to experimentally investigate the effects of various processing parameters on the water-assisted injection molding of thermoplastic materials. Experiments were carried out on a lab-developed water-assisted injection molding system, which included a water pump, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit. Two types of water injection pins designs were proposed to mold the parts. After molding, the lengths of water penetration in molded parts were measured. The effects of different processing parameters on the lengths of water penetration were determined. It was found that the shrinkage rate and the viscosity of the polymeric materials, and the void shapes of the hollowed cores mainly determined the water-penetration lengths in molded products. In addition, a comparison has been made between the parts molded by water-assisted injection molding and gas-assisted injection molding. It was found that water-assisted injection molded parts exhibit less uniform void sizes along the water channel. The cycle time for water-assisted injection molded parts was shorter than that of conventional injection molded parts and gas-assisted injection molded parts.