Molding of hygroscopic thermoplastics has traditionally been a problem because of the ability of these materials to quickly re-absorb moisture. Even dry, “out of the bag” virgin resins can absorb moisture due to indoor humidities. This paper deals with a comprehensive investigation of mechanical properties of moisture-sensitive, hygroscopic thermoplastic materials. Five different materials were subjected to four different processing conditions resulting in 20 different combinations. Each of 19 out of 20 combinations were subjected to three different ASTM standard tests for tension, flexure, and impact properties. All experiments were conducted under strict adherence to the ASTM standards. The main objective of this research was to investigate if the unvented injection molding process could be replaced by the vented injection molding process, without sacrificing the mechanical properties of the materials. All the experimental results were analyzed both graphically and statistically. It was found with a 95% confidence interval that there exists no significant difference in mechanical properties between the investigated hygroscopic thermoplastics, when the unvented injection molding process is replaced by the vented injection molding process. On the contrary, some materials showed an improvement in their mechanical properties when the samples were made by the vented injection molding process. This result is of extreme value to designers of moisture-sensitive thermoplastic materials, as well as to those in the plastics industry interested in reducing manufacturing costs.