An extruder (10) for preparing a polymer-diluent mixture. The extruder includes an elongated housing (16) having an inlet end (18), an outlet end (20) and a bore (22) disposed within the housing, an elongated extruder shaft (34) having an axis of rotation, the elongated extruder shaft disposed within the bore and drivable in at least one direction of rotation, a plurality of extruder screw segments positioned along the elongated extruder shaft in a fixed angular relationship therewith, the plurality of extruder screw segments selected to form multiple extruder stages, the multiple extruder stages comprising an inlet stage (1100), a dispersion stage (1200), and at least one mixing stage (1300,1400), the plurality of extruder screw segments forming the at least one mixing stage comprising a reverse mixing screw segment (1406) having a helical flight (1306) that traverses and forms an outer periphery thereof, the helical flight having a plurality of notches (1308) positioned therealong, a material inlet adjacent the inlet end of the elongated barrel and a first fluid inlet located within the dispersion stage for introducing a diluent. A process for extruding a polymer-diluent mixture is also provided.