a sealing device for assuring a sealed junction between two ducts for conveying plastic material at a high temperature and under high pressure in a machine for transforming plastic material , includes a first part defining the first duct and a second part defining the second duct , the second part being inserted in the first duct . the second part comprises at one end an annular lip protruding from its external cylindrical surface and connected to the one end by a thin annular zone . the lip has a cylindrical external surface which abuts the internal surface of the first part under a mechanical bias due to the dimensional tolerances of the external surface of the lip and the internal surface of the first cylindrical part .