This work studies the flow behavior of a developing two-phase gas-polymer suspension during injection into the instrumented mold cavity of an injection molding machine. In the experiments, blowing agent type and concentration were varied along with processing conditions, to generate controlled cell structures in two different polymers, low density polyethylene and thermoplastic polyolefin. Experimental results indicate that the rheological properties of two phase gas-polymer suspensions were sensitive to shear rate, blowing agent concentration, melt temperature, and mold temperature. The viscosity of all gas-polymer suspensions revealed a reduction compared with neat polymer melt in the presence of gas bubbles, because of the reduced volume fraction of polymer matrix. A two-phase rheological model has been used for fitting with our experimental results for estimating the shear viscosity of two-phase flow in the mold cavity of the injection molding machine.