Patent attributes
A duct structure (510), includes lengths of double wall square/rectangular duct (532) manufactured by pre-forming the duct in round cross-section (51) and then transforming the round cross-section into square and rectangular cross-sections utilizing expandable die structures (58). The duct (532) may be connected together end to end by flanged connectors (510, 520) formed from strip stock (71), that is notched at locations that correspond to the corners of the formed flange connector. The cross-sectional profile of the flanged connector is then formed by roll forming and/or bending and/or other techniques. Thereafter, the strip stock is bent at its notched locations to define the corners (80) of the flange connector outer and inner portions (511 and 512) and then the free ends of the strip stock are fixed together. Thereafter, the outer and inner flanged connector portions (511 and 512) are fastened together.
