A planetary gear extruder for the compounding and extrusion of thermoplastic materials, in which an annular groove is formed in the metallic stop ring to receive a ceramic body which is engageable by the forward ends of the planetary spindles in order to reduce friction. A ceramic body may additionally also be arranged in a recess in the end face of each of the planetary spindles, aligned with and adapted to engage the ceramic body mounted in the stop ring. As a result, the coefficients of friction between the stop ring and the end faces of the spindles are reduced considerably, consequently reducing the wear on the planetary spindles themselves and the central spindle.