a flange for a container opening has an internally threaded aperture to receive a plug , and a plurality of external serrations in a circularly spaced - apart relationship around the periphery of the upper portion of the flange . the plug is externally threaded and , when mated with the flange , closes the aperture . both the flange and the plug are molded out of plastic material as single integral components . on the lower portion of the flange is an annular lip which is positioned beneath and extends radially beyond the external serrations . a container suitable for the flange has a boss around the container opening in a wall of the container . the boss is formed with internal serrations equal in number to the number of external serrations on the flange . this boss can be crimped onto the flange placed in the opening , such that the two groups of serrations permanently interlock with one another thereby securely locking the flange into the container . at the same time the annular lip seals against the inner face of the wall . the plug is removable from the flange when access to the contents of the container is desired .