Corner piece-less duct, Method of producing the same, and duct-forming machine

摘要

This invention is aimed at providing a cornerplate-less duct having a rational construction, which can significantly reduce the number of parts by effectively utilizing scraps conventionally discarded. Plate members are joined to each other at side end portions by a seam joint structure to form a tubular duct 10 having a rectangular cross-section. A starting end portion and a terminal end portion of a plate are bent in a direction substantially perpendicular to a longitudinal direction Za of the plate to form flange forming portions 3 integral with the plate. Corner forming portions 4 are formed integrally to protrude from side ends of the flange forming portions in a width direction to form corner portions at the starting and terminal end portions of the duct. In an expanded shape of plate 1 A or 1 B, a cut portion 2 is provided closer to a center in the longitudinal direction Za than a starting end 1 s or a terminal end 1 e of the plate member such that the cut portion 2 extends in a width direction Ya of the plate member and has a depth substantially equal to an overlapping portion W 1 of the corner forming portion 4.