Extruder for a 3D printer with a variable material throughput

An extruder (1) for a 3D printer, the extruder comprising at least one outer nozzle (2) and a conveying screw (3) for feeding liquid and/or plasticized starting material (4) into an interior space (21) of the outer nozzle (2), characterized in that an inner nozzle (5) is arranged in the interior space (21) of the outer nozzle (2), wherein the inner nozzle (5) is mounted rotatably on the conveying screw (3) via a bearing element (6), wherein an interior space (51) of the inner nozzle (5) is connected to the interior space (21) of the outer nozzle (2) via at least one duct (52, 52a, 52b), wherein the at least one duct (52, 52a, 52b) allows the starting material (4) to pass from the interior space (21) of the outer nozzle (2) to the interior space (51) of the inner nozzle (5), wherein the inner nozzle (5) is mounted in a manner linearly movable along a longitudinal axis (2a) of the outer nozzle (2), wherein the outer nozzle has opposite front and rear ends spaced apart along the longitudinal axis, wherein the outer nozzle (2) has an outlet opening (23) at the front end and an inner surface (22) defining the interior space (21) of the outer nozzle (2), wherein a portion of the inner surface (22) of the outer nozzle (2) faces toward the rear end, wherein the inner nozzle (5) is movable along the longitudinal axis (2a) toward and away from a position in which an outer surface (53) of the inner nozzle (5) is brought into a sealing engagement (24a, 24b) with the portion of the inner surface (22) of the outer nozzle (2), and wherein the sealing engagement (24a, 24b) is configured to prevent direct passage of the starting material (4) from the interior space (21) of the outer nozzle (2) to the outlet opening (23) of the outer nozzle (2) and to allow indirect passage of the starting material (4) from the interior space (21) of the outer nozzle (2) to the outlet opening (23) of the outer nozzle (2) via the at least one duct (52, 52a, 52b), the interior space (51) of the inner nozzle (5) and an outlet opening (54) of the inner nozzle (5).