the disclosure includes a cap having a first portion made of a plastic material and forming a bottom for covering a container neck and a protective side wall for surrounding the container neck , and a second portion made of a flexible material that is compatible with the plastic material of the first portion and separate from the plastic material of the first portion , the second portion being annular and consisting of a flexible seal , which projects from the bottom toward the interior of the side wall and which is welded to the first portion . thus , the manufacture of the stopper by molding includes injecting a flexible material to produce the flexible annular seal and then the movable portions of the mold are placed such that the flexible material is in a groove that is set back from the planar surface of the mold .