In this work, processing of a unique ultra-high molecular weight polyethylene (UHMWPE) polymer, in both powder and pellet form, using regular and special injection molding technologies was investigated. In the first study, supercritical nitrogen and carbon dioxide were used as processing aids in a special full-shot microcellular injection molding (MIM) process for processing UHMWPE pellets. The mechanical properties in terms of tensile strength, Young’s modulus, and elongation-at-break were examined. It was found that the processing of UHMWPE with both gases effectively reduced the thermal degradation of the material and the injection pressure compared to the regular injection molding while retaining the mechanical properties of the resin. In the second study, UHMWPE powder was processed using injection molding (IM) and injection–compression molding (ICM). The processing parameters of feeding the powders were optimized to ensure proper dosage and to avoid damaging the UHMWPE molecular structure. Dynamic mechanical analysis (DMA) and Fourier-transform infrared spectroscopy (FTIR) tests confirmed that the thermal and oxidative degradations of the material were minimized but crosslinking was induced during melt processing. Tensile tests and impact tests showed that the ICM samples were superior to the IM samples. A delamination skin layer was formed on the IM sample surfaces, while it was absent in the ICM samples, thus suggesting two different flow behaviors between IM and ICM during the packing phase. Regardless of the pellet or powder form used, short-shot molding showed that the UHMWPE melt interestingly did not exhibit the typical progressive, smooth melt front advancements and the typical fountain flow. Due to its highly entangled polymer chain structure, it entered the cavity as an irregular porous-like structure and tended to exhibit plug flow, especial at the beginning of cavity filling as shown by short shots and micro-computed tomography (µCT) scans.