Epoxy acrylate-based sterolithography resins have been used successfully as tools for injection molding. Molds made out of these resins fail at distinct times: during the first injection of plastic; during the first part first ejection; during either injection or ejection, but after a certain number of parts have been produced, which can be compared to a fatigue process. This paper presents correlations between measured properties of stereolithography molds and injection molding processing conditions so as to understand and predict mold failure. The study focuses on two stereolithography resins (SL 7510 and SL 7510) and one epoxy-based composite material used for the high speed machining of prototype molds (Renboard). Rapid tooling materials are studied in fatigue, tensile, and fracture at injection molding operating temperatures and at room temperature. Finally, a method to address failure of molds is proposed using the theory of fracture.