This paper investigates the viscosity behavior of melt polymer during the plastication stage in an injection molding machine (IMM). The objective of this paper is to develop theoretical viscosity-based models for evaluating card predicting the bulk viscosity of the melt polymer near the tip of the screw during the plastication stage. The uniqueness of these models is that they include the effects of material properties and machine processing parameters, such as screw rotational speed, hydraulic back pressure, barrel pressure drop, melt pressure: and melt temperature. Several open loop tests,,ere conducted on air industrial IMM in order to quantify process parameter effects on the plastication cycle. Die derived model was used to predict bulk viscosity using the open loop tests data. The viscosity-based models in this work will be used for developing future optimal controllers for controlling the polymer viscosity during the plastication stage of the injection molding cycle.