The presented paper aims to determine the influence of the length of the plasticating system on the course of an autothermal extrusion process. Three special screws were used, of the relationship of the length of the operating part to diameter of the screw L/D = 20, 25, and 30, with a tip for intensive shearing and mixing, assigned for polyethylene processing, and five screw rotation frequencies were applied. The conducted research consisted of the measurement, during the extrusion, of the factors studied directly and in the calculation, and on their basis, the values characterizing this process. It is stated that the change of the length of the plasticating system in a significant way influences the dependences determining polymer flow rate, temperature and pressure of the processed polymer, extrusion velocity, specific energy consumption, and energy efficiency of the extruder; that is, the whole characteristic of an autothermal extrusion process. (C) 2005 Wiley Periodicals, Inc.