Extruder having a set back gas-permeable wall portion

An extruder for processing powdery bulk materials, comprising

• a housing (2);
• at least one bore (3, 4) that
• has a bore wall (42, 43);
• a feed opening (9) for the powdery material which is disposed on one end of the housing (2);
• one screw (7, 8) each that is disposed in the at least one bore (3, 4) wherein said screw (7, 8) has
• one screw conveyor portion (11) each that is disposed downstream of the feed opening (9) so as to form an inlet zone (14), and
• at least one portion (12) that is disposed downstream of the screw conveyor portion (11) for processing the bulk material;
• a metering device (22) that is disposed upstream of the feed opening (9) for feeding in the powdery bulk material and that is designed in a way as to ensure that at least the inlet zone (14) of the extruder (1) is only partially filled during the operation;
• a vacuum housing portion (23) in the inlet zone (14) wherein said vacuum housing portion (23) has a gas-permeable porous wall portion (24) of sinter material which has a multitude of ducts;
• and
• a drive (19) for the at least one screw (7, 8),
• wherein
• the gas-permeable wall portion (24) has an inner wall (41) that defines the at least one bore (3, 4) and that is set back with respect to the bore wall (42, 43), thereby forming a gap (44, 44′, 44″) with a partially annular cross-section;
• wherein the gap (44, 44′, 44″) has a constant thickness D to which applies:
• 2.0 mm≦D≦30 mm;
• and
• wherein the gas-permeable wall portion (24) is connected to a pressure-gas source (33) by means of a pressure purge line (32) wherein a layer of powdery bulk material is formed in said gap to increase friction between the material and said inner wall to increase transport of said material and to prevent clogging of said gas-permeable porous wall portion.