This paper presents an experimental and numerical study on the hybrid process of polymer injection forming (PIF) developed to manufacture sheet metal-polymer structures in one single operation. Despite the wide potential application of the PIF process, several challenges have prevented conducting an in-depth analysis of the simultaneous injection/forming condition that occurs during this hybrid process. One such challenge is the lack of a special feature in the regular injection molding machine for an independent application and control of the blank holder force (BHF) from the preset clamping force. To enable such, a new concept-to-design tool is proposed in this work. Using this specialized setup, the influence of the BHF, injection rate and their interactions are experimentally determined focusing on the transient PIF process variables and the quality of the final hybrid product. The use of the fluid-structure interaction (SFI) method to perform a multi-physics simulation on the PIF process incurs such computational costs that it is unreasonable to conduct multiple simulations to investigate the effects of several process parameters and their interactions. To achieve both efficiency and accuracy, a new combined analytical-numerical approach is presented to enable fundamental understanding of the PIF process physics considering the interaction of filling, stretching and drawing mechanisms. The feasibility of the proposed numerical and experimental methodologies is demonstrated through a comparative analysis of the deformation of the AA1100 sheet during the injection of a Polypropylene compound with different injection rates and BHF settings.