Regardless the type of the recycled plastic, the final product will always contain impurities and pollutants that will affect the mechanical and physical properties of the processed pieces. Given that polypropylene (PP) and polyethylene (PE) represent over 40% of the world consumption of plastic, the study of behavior of recycled matter derived from these two polymers is of a great interest (Colbeaux, 2001). PE is the majority pollutant in recycled PP, which tends to form a heterogeneous mixture with it due to the incompatible nature of those two polymers (Wignall, 1978) (Joseph, Renardy, & Renardy, 1984). For that, the determination of the composition of the plastic waste streams is therefore important, because it determines the appropriate management procedure for the end-of-life material. The aim of the current study is to develop an instrumented device that is able to detect the presence of the impurities as well as their concentration fluctuations in recycled thermoplastics during their processing. This device is equipped with pressure and temperature sensors allowing an online measurement of pressure drop variations as well as viscous dissipation variations through the temperature field within the flow. These detections might hopefully lead to a future control of the processing parameters thus controlling the final parts qualities. In the present study, PP and PE were extruded with different percentage in weight of PE. The pressure drops and the temperature within the flow in cylindrical die –using a measuring temperature cell (MTC) – were measured (LAUNAY, 2013). A mixing law was proposed to describe the rheological response on immiscible mixtures (HAN, 1981). A level set based finite element model is set up using Comsol Multiphysics software in order to predict the various rheological behaviors of different polymer mixtures flowing in the cylindrical die (VORONETSKA, 2012). The primary results of the model agree with the experimental results. The results obtained in this work show that the rheological response of a mixture is very sensitive to the viscosity of the minor polymer when the viscosity of the latter is lower than the major one.