Abstract: A belling mandrel assembly for forming in the end of a plastic pipe a belled socket having therein an annular seal pocket. A lip mold for forming the lip of the socket and a base mold for forming the base of the socket are operable to press a plurality of form block segments trapped between them. Upon the application of such pressure, the form block segments expand to form an annular ring for molding an annular seal pocket in the socket. Upon widening the distance between the lip mold and the base mold the form block segments retract to allow the mandrel to be withdrawn from the belled socket.