Patent ID: 6519851
Filing Date: 2003-02-18
Classification: B21D,F01N,Y10T

Abstract:
A method for producing an air-gap-insulated exhaust pipe with a branch stub for a vehicle exhaust line having an inner tube with a branch for carrying exhaust surrounded at a distance by an outer jacket to form an insulating air gap, said method comprising:providing a first tube and a second tube, each having a corresponding shape, inserting said first tube into said second tube with limited play to form a double tube, having an inner tube and a outer tube, placing said double tube in a first internal high-pressure shaping tool having a first engraving including a branch, sealing off both ends of said double tube to be tight to a high-pressure fluid, closing the first shaping tool and introducing a pressure fluid into an interior of the inner tube of the double tube such that the double tube expands to match the contours of the first engraving to form a shaped double tube including a double-walled branch stub blown out of the double tube into the branch, relieving the pressure fluid in the first shaping tool, removing the shaped double tube from the first shaping tool, placing the shaped double tube in a second internal high-pressure shaping tool having a second engraving which holds the shaped double tube at axial end areas in a fit with play, the second engraving being spaced apart from the shaped double tube between the axial end areas including the branch stub to define an expansion chamber, closing the second shaping tool and introducing a pressure fluid between the two tubes that form the shaped double tube and simultaneously into the interior of the inner tube, such that the outer tube expands into said expansion chamber and engages said second engraving of the second shaping tool to define an insulating air gap between the outer tube and the inner tube, an end of the branch stub facing away from the rest of the double tube being externally supported without yielding via a second tool counterpunch located in a branch of said second engraving, relieving the pressure fluid in the second shaping tool, removing the finished double tube from the second shaping tool, and cutting off a cap area at the end of the branch stub to form a through opening between the interior of the inner tube and the outside of the air-gap-insulated exhaust pipe.