Abstract: The invention relates to a multi-lobe composite casing, the inner wall of said casing being constituted by several adjacent cylindrical sectors, each enveloping a screw over the portion of the latter outside the penetration zone. According to the invention the hardest metal layer deposited on the inner wall of the body is applied in the form of a powder containing the desired constituents. This powder is welded to the metallic inner base wall by means of a plasma torch operating on the principle of a transferred arc or a semi-transferred arc. The invention is applied to machines with several overlapping screws of the extruder type.