Folding unit for forming an empty cardboard box; the folding unit has a mandrel, around which a blank is folded to form the empty cardboard box. The mandrel has at least two parts, which are mobile in respect to one another between an expanded configuration, in which the two parts are further from one another so as to confer a maximum size to the mandrel, and a contracted configuration, in which the two parts are closer to one another so as to confer a minimum size to the mandrel.