Abstract: An extruder apparatus for the extrusion of a strand of building material for 3D printing of a structural part includes an extruder nozzle and at least one inner element. The extruder nozzle has a discharge opening for the discharge of the strand of building material out of the extruder apparatus. The at least one inner element is designed for arrangement within the extruder nozzle for purposes of defining an inner edge of a flow cross section of building material within the extruder nozzle in order to specify an inner edge of a strand cross section of the discharged strand of building material.