Additive manufacturing technology has become a viable solution for making molds for plastic injection molding applications. The molds are usually made of high temperature plastic resins suitable for plastic injection molding. Molding resins have superior mechanical properties necessary to withstand the high temperatures and pressures of the injection molding process. It is known that high temperature mechanical properties of resins influence mold performance but it is not established which properties are most important and to what extent they influence the mold performance. Identifying the most important properties influencing mold performance would help resin manufacturers to develop better mold-making materials. In order to study the performance of mold materials we have built a device for measuring the mechanical properties of 3D printed resins including their strength, surface hardness, and wear resistance at molding temperatures of up to 260 oC. We then quantified the mechanical properties of three high-temperature resins along with ABS at the injection molding temperatures. This paper describes the test device and the results of characterizing the mechanical properties of the selected plastics.