In the present study, a plastic injection molding technology was investigated for a mass-replication of the biomedical product with complicated features. The current plastic biomedical product, which is one part of the novel biomedical device, was designed to have a cylindrical sidewall and rectangular hole. Because the complicated features like sidewall and hole can lead to the critical defects, such as weldline formation, air trap, short shot, and breakage, during the injection molding process, both the precise design of the injection mold and optimization of the processing parameters are required. In this regard, the injection mold to produce the biomedical product with sidewall and hole structures was designed and manufactured. The experimental and numerical investigations of the injection molding filling stage were carried out. From the investigations, the detailed filling sequence and flow behavior were identified, thereby verifying the feasibility of the current mold design.