I@ Following an initial operation possibly required for separation from other types of refuse, waste plastics materials are triturated or crushed and cold-mixed. The triturated and mixed wastes undergo a first heating stage at a first temperature, with a view to obtaining a mass in an initial state of plastification. This mass subsequently undergoes a second heating stage at a second temperature, generally different from said first temperature, until there is obtained thereform a homogeneously melted mass. The apparatus comprises a generally tubular housing (21) carrying at one end thereof a triturating device (2) within which the wastes are reduced to fragments and cold-mixed. The triturating device (2) comprises a toothed rotor (9) fitted onto a rotary element (16) rotatably mounted within the housing (21) and provided with thrust surfaces for the material with a helicoidal development (24, 25, 26, 28). The hub (10) of the triturator (2) is provided with apertures (15) which permit the triturating material to penetrate radially into this hub (10), so as to be brought into contact with the thrust surfaces of the rotary element (16) which functions as a mixing member. On at least one longitudinal portion (19) of the rotary element (16), the thrust surfaces for the material are defined by angular screw-threaded sectors (25, 26) arranged according to at least two oppositely directed helicoidal trajectories.