Polymer injection forming (PIF) is a recent advancement in manufacturing of plastic– metal hybrid products. It is a combination of injection molding and sheet metal forming in which the molten polymer additionally serves as a pressure medium. This paper presents fundamental investigations on the use of polymer melt as a pressure medium in sheet metal forming. The development of forming pressure and localized blank temperature as a result of polymer injection is analyzed. The experiments are performed using aluminum alloy. The experiments comprise the bulging of a (free form) dome and stretch forming of a cup using thermoplastic polypropylene as a pressure medium. A simple approach is presented to model a combined process. An incremental lagrangian formulation is used to describe the polymer flow and sheet metal deformation. The Newtonian behavior of the polymer melt is modeled as rigid-viscoplastic medium. Standard finite element coupling is used to model the mutual metal–polymer interaction. The simulation results in the form of forming pressure profile, localized blank temperature profile, formed shape, and strain distribution are presented and validated with experimentally obtained results.