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In computer aided analyses and designs of modular co-rotating twin screw extruders, it is necessary to consider the melt flow of mixtures produced by the addition of quantities of additives/fillers to thermoplastics at various downstream positions. The question arises as to what will the rheological properties of the mixtures be under such conditions. We present an experimental study of the downstream homogenization of (i) model filled compound (high density polyethylene (//D/ > £)/calcium carbonate (CaC0 3 )) and (ii) a blend (high density polyethylene/polystyrene (PS)). HDPE was added in the hopper of the machine and either CaC0 3 or PS was added in a second downstream feed port. We measured the shear viscosity of the samples collected from various positions along the screw axis to investigate the downstream homogenization of the compound and the blend. The compound and bland were seen to be rapidly homogenized. The material distribution along the screw in the twin screw extruder was also determined in experiments with both the compound and blend. The material distribution in both cases was compared with predictions based on modeling the flow in self wiping screws and kneading disc blocks presuming the rheological properties to be those of well mixed compounds. Generally, good agreement was iound.