A plurality of screws
Among extruders of the type wherein a screw is rotatably housed in a cylinder, some of those having a plurality of screws are adapted to divide a flow into rearranged striations as described above. For example, an extruder having two screws which are not fully in meshing engagement with each other generally has a construction such that the two screws are housed in two cylindrical spaces partly communicating with each other, and the clearances defined by the respective cylindrical inner surfaces and screw flights communicate with each other at the junction of the two cylinders.Accordingly, by virtue of rotation of the two screws, a portion of the material in one clearance is separated from the main flow in the form of striation by the movement of the flight and passes through the communicating junction to join the flow in the other clearance within the adjacent cylinder. In this way, part of the material in the clearance of one cylinder is separated to join the material in the clearance of the other cylinder, and vice versa. Thus, the extruder effects the rearrangement of divided striations by repeating the division and union of the material conveyed.
Further with single screw extruders of the standard type, the solid material charged therein is melted while being sent toward the direction of extrusion, with the solid material positioned behind the screw flight and the melt in front of the screw flight in a separate manner. Accordingly, even after the solid in the screw channel is reduced with the advance of the material and the material having eventually left the distal end of the screw in entirely molten state, periodic variations are generally found in the temperature and kneading degree of the resulting helically continuous molten material, in corresponding relation to the pitch of the helix. A highly viscous fluid such as molten plastics flows along the wall of a static device generally in the form of a laminar flow element. The laminar flow has a stream line which remains uncharged in spite of the lapse of time. It therefore follows that although the material is immobile in directions transverse to the stream line. Mixing by the standard-type single screw extruder resembles that achieved by such movement of material in a manner which is known as laminar flow mixing.
Homogeneity in directions twin screw extruder to a flow may be improved by dividing the flow in the transverse plane into some tubular flow elements whose surfaces are surrounded by a number of stream lines (hereinafter referred to briefly as "striations") and joining the striations in a rearranged fashion in an order different from the original arrangment of the group of striations. (The latter step is hereinafter referred to as "rearrangement of divided striations".) The striations described above can be produced by forming their surfaces with a plate, groove, tube, projection or like means placed into the flow and capable of dividing the flow.