The plant consists of a feed post for drop formers and an extrusion post with a head (14), a calibrator and welder. The extrusion head produces a tubular sleeve (22) into which the drop formers, linked in a continuous strip, are inserted and welded ahead of the calibrator, either inside or in the immediate vicinity of the extrusion head. The extrusion head has a channel (48) that forms the sleeve, thrust and counter-thrust surfaces (50, 56) for pressing the drop formers into place while they are welded, and an inner mandrel (18) forming a cylindrical portion with walls that reduce in thickness towards the extrusion channel outlet.
