Abstract: A casing of stretched plastic material, whether or not filled with a fluid such as gas or air, is wound with some tension about a smooth heated mandrel. During its curved path of travel said casing is on the one hand subjected to a contact or a radiation heat and on the other hand to a convection heat, said contact radiation heat prevailing during a period of time which is greater than that of the convection heat, thus causing the inner curved portion of the said casing to set during the time that the outer portions of the casing windings have already cooled down, so that smooth curved surfaces are obtained.