the invention concerns a method which consists in : a ) forming by extrusion an extruded thermoplastic tube having , when coming out of the extruder , a diameter d 1 ; b ) then subjecting said extruded tube of diameter d 1 to a radiation treatment , so as to produced a cross - linked tube ; c ) after bringing said cross - linked tube to a temperature not less than a temperature substantially equal to the melting point of its crystalline phase , subjecting it , in displacement , to a radial monoaxial expansion for form an expanded tube of diameter d 2 , then cooling it down to a temperature , typically room temperature , at which it can be cut into tube portions with length substantially equal to the height of said caps ; and e ) assembling or forming a head on said tube portion , so as to obtain a cap or top , with heat - shrinkable skirt , economical and having a homogeneous skirt free of axial connection line .