This thesis summarizes the studies undertaken to achieve the objective of manufacturing injection-molded thermoplastic polyurethane (TPU) foams with high density reductions, desir-able microstructures, and customizable properties. TPU foams are quickly gaining market share in numerous applications in the sports and recreation as well as manufacturing industries. This is because TPU foams can be easily manufactured in a variety of densities, hardnesses, and mechanical properties, ranging from highly resilient to highly compressible foams, with different processing methods, such as injection molding, casting, and extrusion. Typically, TPUs are being foamed to a minimum density of 0.40 g/cm³ (400 kg/m³). It is the aim of this thesis to expand the understanding and processing knowledge of manufacturing TPU foams with densities as low as 0.25 g/cm³ (250 kg/m³), while maintaining desirable microstructures. It is also an aim of this thesis to develop a novel process to control foam properties locally in injection molded TPU foams. In the journey toward achieving the aforementioned objectives, five studies were conducted and are presented in this manuscript. An experimental study evaluating the CBA and PBA foaming of TPU has been undertaken. A novel foam injection molding process using CO₂ + N₂ as co-blowing agents to exploit the synergistic benefits of co-blowing agents was also evaluated in this study. It was found that PBA foaming can be vastly advantageous for foaming TPU by enabling large reductions in bulk densities without the harmful effects of chemical residue. This novel process of using co-blowing agents yielded the lowest bulk density while achieving the highest cell density and finest average cell size. To resolve the issue of low melt strength at higher temperatures, which leads to cell coalescence and undesirable microstructures, the use of a physical cross-linking agent was considered. It was proposed that a physical cross-linking agent could increase the melt strength of the TPU at higher temperatures, thus enabling aggressive foaming strategies to give enhanced density reductions. It is shown here that extremely low-density TPU foams of bulk density 0.16 g/cm³ (160 kg/m³) can be achieved consistently. In Chapter 3, nanoclay as a reinforcing filler was employed to increase the melt viscosity and to trigger heterogeneous nucleation in the TPU matrix, thereby improving the foamability of TPU while achieving a homogenous microstructure with low average cell size and high cell density. It is generally known that TPU's rebound resilience values decrease with decreasing foam density. It was observed that rebound resilience values could increase significantly if a tunable alignment of microcells can be produced within the part, even while reducing bulk densities. Through the action of mold inserts, the expanding microcells will thereby stretch and align with the intercellular TPU walls and individual molecules in the direction of mold opening. With aligned microcells, the softness of the foams improved significantly, while achieving lower hysteresis loss ratios or energy loss (dissipation). In Chapter 5, a core retraction mold capable of increasing cavity volume was designed and manufactured to enable further study of the phenomenon observed in Chapter 4, with better parameter control. Polypropylene (PP) was the material chosen to validate the capabilities of the mold and the applicability of this technology to materials other than TPU. Core retraction was used with the conventional micro¬cellular injection molding (MIM) process to foam thick PP parts with high density reductions of 30% and 55%. The cavity volume was modified by chang¬ing the retraction distance, which resulted in varying den¬sity reductions. The lowest densities were achieved with a core retraction-aided microcellular injection molding (CR-MIM) process, the results of which could not have been achieved by the conventional MIM process alone. It was shown that the CR-MIM process yielded a better microstructure and a higher tensile modulus than the conventional MIM process. Use of core retraction also yielded more consistent densities and tensile properties at different distances from the gate location. It was shown that by using delay times and holding pressures, cell nucleation and cell growth can be delayed to a more suitable time and melt temperature. Mechanical and energy absorption properties of TPU foam can be varied with the density and microstructure of the foam. In Chapter 6, a new process is developed using core retraction and out-of-mold foaming to develop injection molded parts of varying cross-sections with varying mechanical properties. It was shown that, using this process, very low density foams of less than 0.25 g/cm³ can be produced while maintaining uniform and good microstructures. It was also shown that this technology can be effectively used to vary microstructures, densities, and mechanical properties within an injection molded part locally, depending on the application. In summary, the aforementioned approaches have helped to advance the knowledge of manufacturing TPU foams and other regular thermoplastics with customizable microstructures and mechanical properties for various specific applications.