Patent ID: 7238317

Claim:
A method of continuously producing a twin-wall pipe ( 10 ) comprising a smooth internal pipe ( 39 ′) and an external pipe ( 37 ′) that is united with the internal pipe ( 39 ′) by welding and provided with elevations ( 38 ); a pipe socket ( 41 ); and a central longitudinal axis ( 18 ); the method comprising the following steps: extruding an external tube ( 37 ) concentrically of the central longitudinal axis ( 18 ); corrugating the external tube ( 37 ) with elevations ( 38 ) and troughs ( 40 ) by applying partial vacuum from outside; extruding an internal tube ( 39 ) into the external tube ( 37 ) concentrically of the central longitudinal axis ( 18 ); welding together the internal tube ( 39 ) and the troughs ( 40 )of the external tube ( 37 ); expanding the external tube ( 37 ) at given distances by the applying a partial vacuum from outside, to form an expanded area for a pipe socket ( 41 ) to be produced; actuating the internal tube ( 39 ) inwardly by gas of a pressure above atmospheric pressure and expanding and pressing the internal tube ( 39 ) full face against the expanded area of the external tube ( 37 ) for the pipe socket ( 41 ) to be finished; and forming a transition portion ( 61 , 64 ) between the pipe socket ( 41 ) and an adjacent trough ( 40 ), the transition portion ( 61 , 64 ) being comprised of the internal tube ( 39 ) and external tube ( 37 ) and directed outwards in relation to the central longitudinal axis ( 18 ); wherein the transition portion ( 61 , 64 ), in an area between the internal tube ( 39 ) and the external tube ( 37 ), is vented into an adjacent elevation ( 38 ).