The free flow passage
In a known device there is provided at least one helical flute, the free flow passage of which is blocked off by a transverse wall, and this wall is provided with numerous overflow grooves. Said mass, which has been made flowable (rendered plastic), in the first section of the helix must pass through these numerous grooves, each of which has a very small transverse area, which strongly restricts the flow of the mass. The flowable mass is hereby divided into a large number of small strands, which collectively exhibit a large surface, so that an efficacious degassing takes place in the pressure-free degassing section which follows. A drawback of this known extruding device is that the transfer rate of the device (amount of mass flowing through the time unit) can only be varied through the rotation speed of the extruding helix, unless one uses another helix with a different pitch. Because the mass to be moved, e.g. rubber, is a bad heat conductor, the heat transfer necessary to make the mass plastic in said first section of the helix must be obtained through convection. This entails that the mass to be rendered plastic must be moved as much as possible along a wall which can be heated (helix and casing) so that it may carry the necessary heat along.
However, the transfer of single screw extruder through convection requires that a certain minimal rotation speed of the helix be exceeded. Therefore, in order to obtain a sufficient plastification in this first section of the helix, the rotation speed of the helix cannot be reduced at will, and so the transfer speed also cannot be arbitrarily reduced. Even if the rotation speed of the helix is chosen just low enough to obtain a sufficient plastification in said first section of the helix, the known device is not satisfactory whenever an extrusion with a small transfer rate (amount at the outlet) is needed because then a back pressure can build up as far as the degassing section. The latter then is not free of pressure anymore, so that no degassing is possible without an exiting of plastified mass. If this is to be absolutely avoided with the known devices, then the helix in the interval between the outlet and the degassing section must be very long. Both the use of several different helixes and a great helix length are undesirable.
A screw extruder having a screw including a plug formed with flutes by which the flow of material existing in the cross section of a screw channel is divided into a plurality of striations. The order of arrangement of inlets of the flutes is different from the order of arrangement of outlets communicating with the inlets with respect to the circumferential direction of the plug, whereby the flow is divided into rearranged striations.