Patent ID: 7691317

Claim:
A method for manufacturing a double-walled, thermoplastic tube, where a) a first tube ( 1 ) is extruded into a mould tunnel ( 4 ) having at least one row of moulds ( 5 ) guided on a path, b) the first tube ( 1 ) is given a corrugated shape in at least one first section and expanded into a connecting sleeve in at least one second section, c) a second tube ( 6 ) is extruded into the first tube and pressed against the corrugation troughs ( 8 ) of the first tube ( 1 ), d) subjecting the space (A) between the two tubes ( 1 , 6 ) to a first pressure (p 1 ) that is above atmospheric pressure while the first tube ( 1 ) is being given the corrugated shape and the second tube ( 6 ) is being extruded into the first tube, e) subjecting the space (A) between the two tubes ( 1 , 6 ) to a second pressure (p 2 ) at a specified time proximate each expansion of the first tube ( 1 ) into the connecting sleeve including switching the first pressure (p 1 ) to the second pressure (p 2 ) with a compressed gas controller, the second pressure (p 2 ) an essentially constant pressure that is less than or equal to the first pressure (p 1 ) and above atmospheric pressure, or to a variable, but not continuously declining pressure (p 2 ), f) subjecting a third pressure (p 3 ) above atmospheric pressure against an interior of the second tube ( 6 ) where the second tube ( 6 ) is pressed against the first tube ( 1 ) during extrusion of the second tube ( 6 ) into the first tube ( 1 ) to form the connecting sleeve, and g) applying the first pressure (p 1 ) to space (A) between the two tubes ( 1 , 6 ) proximate completion of the connecting sleeve by the second tube ( 6 ).