the method comprises a ) an extrusion stage wherein a extruded tube made of a thermoplastic material is formed by extrusion with the aid of an extrusion die having a diameter d 0 , a slit width e 0 and a section s 0 ; b ) a stage wherein the extruded tube is radially expanded in order to form a radially expanded tube having a diameter d 2 , a thickness e 2 and corresponding section of area s 1 ; c ) a truncating stage wherein the expanded tube is broken down into tube portions ; d ) a stage in which the tube portions are formed in such a way that a heat - shrinkable cap is obtained and in which an axial drawing stage is incorporated , between extrusion stage a ) and expansion stage b ), at the output of the extrusion die , in order to obtain an axially drawn tube having a diameter d 1 which is typically smaller than d 0 and d 2 , and a thickness e 1 which is typically lower than e 0 , such that s 0 / s 1 typically ranges between 2 and 10 . the advantages of the invention are as follows : it is possible to obtain caps which are economic , easily heat - shrinkable and which have a stable axial dimension , thereby avoiding any axial distortion , i . e . any axial distortion of the impression .