is that it is possible to turn the screws 2 3 at the same rotational speed this is because each of the screw flights drives the material along in a translational movement with respect to a fixed wall this effect is obtained by the flight 21 with respect to the interior wall of the body 1 and in the same way by the flight 31 with respect to the interior wall of the cylindrical tube 4 it is then easy to secure the screws 2 3 to one another by positioning them concentrically and coaxially with respect to one another and interconnecting them preferably in axial abutment with one another thus only the larger diameter screw 2 needs to be connected to a driving member thereby greatly simplifying the production of the device it is also possible to produce an assembly comprising several successive stages by extending the head 5 of the body 1 and by making orifices in the downstream part of the screw 3 to allow the space swept by the flight 31 of the screw 3 to communicate with an internal space formed in the core of the screw 3 which would work in conjunction with a smaller diameter.