An extruder-mixer has a plurality of rotor rings provided on the downstream end of a motor driven extruder shaft mounted for rotation in a conventional heated barrel or stator. The rings comprise a plurality of spaced driven rotor rings spaced apart from each other and a plurality of non-driven but rotatable floating rings interleaved between each pair of driven rotor rings. Both rings have parallel upstream and downstream faces between which polymer flow passageways extend so that the polymer melt moves downstream first through one type of ring followed by movement through the other type of ring and the viscosity of the melt causes the rotatable floating rings to be rotated by the driven rings at a slower speed than the driven rings so that shearing force on the melt effects mixing of the melt.