the method includes the use of a closed mold consisting of two shells and an annular joint , the joint having a hole for filling the mold cavity with a polymerizable material , the introduction of the material occurring after a stage in which an external mechanical effort is exerted in order to bring the shells together and before a stage in which the effort is relaxed . in the joint , the filling hole includes two sections . a first section extends from the molding cavity to the second section which is wider . the device includes elements for exerting an external effort in order to bring the shells together and elements for introducing the polymerizable material into the molding cavity .