A device for producing tubes for packaging tubes, comprising an elongated mandrel (3), around which a substrate web can be formed in order to produce a tubular form (4) that surrounds the mandrel (3) and can be conveyed along the mandrel (3) in a conveying direction (R), as well as welding means (9) for welding the substrate web along a welding region in order to produce a welding seam (11) and at least one first welding belt (34) that can be conveyed together with the tubular form (4) and fixes the tubular form in a contact region of the substrate web with itself by clamping the substrate web between the first welding belt (34) and a clamping partner, wherein means (13) are provided for expanding the tubular form (4) fixed by the first welding belt (34) and the clamping partner by plastically elongating the substrate web in the circumferential direction, wherein said means can be adjusted into a position, in which they radially protrude over the outer surface (14) of the mandrel section (31) of the mandrel (3) that is arranged upstream of the means (13) referred to the conveying direction (R).