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A novel method of producing injection molded parts with a foamed structure has been developed. It has been named supercritical fluid-laden pellet injection molding foaming technology (SIFT). Compared with conventional microcellular foaming technologies, it lowers equipment costs without sacrificing the production rate, making it a good candidate for mass producing foamed injection molded parts. Both N2 and CO2 can be suitably used in this process as the physical blowing agent. However, due to their distinct physical properties, it is necessary to understand the influence of their differences over the process and the outcomes. Comparisons were made in this study between using CO2 and N2 as the blowing agents in terms of the part morphologies, as well as the shelf life and gas desorption process of the gas-laden pellets. After gaining a good understanding of the SIFT process and the gas-laden pellets, a novel foam injection molding approach combining the SIFT process with microcellular injection molding was proposed in this study. Both N2 and CO2 can be introduced into the same foaming process as the coblowing agents in a two-step manner. Using an optimal content ratio for the blowing agents, as well as the proper sequence of introducing the gases, foamed parts with a much better morphology can be produced by taking advantage of the benefits of both blowing agents. In this study, the theoretical background is discussed and experimental results show that this combined approach leads to significant improvements in foam cell morphology for low density polyethylene, polypropylene, and high impact polystyrene using two different mold geometries. POLYM. ENG. SCI., 54:899–913, 2014. © 2013 Society of Plastics Engineers