Abstract: A semi-rigid collapsible air duct having helically biasing reinforcement element 24, enveloped between an inner skin and an outer skin with the reinforcement element in a uniform helical spacing enveloped by ultrasonic welds. The process for manufacturing the air duct includes feeding the inner duct wall skin onto a mandrel, feeding the helically biased reinforcement element onto the mandrel having a groove with a tighter helical shape over the inner duct wall skin, feeding the outer duct wall skin onto the mandrel over the reinforcement element, then passing that composite under an ultrasonic welder which welds the inner duct wall skin to the outer duct wall skin, enveloping one or more reinforcement elements in such a way as to leave the one or more reinforcement elements free to move within their respective envelopments to develope the maximum thrust to place the reinforcing elements in tension to prevent droop of the walls of the duct.

Abstract: A machine for manufacturing metal duct or pipe having improved efficiency, reliability, and speed of operation. A continuous, thin metal strip is corrugated, helically wound and seamed to form a durable yet bendable duct or pipe. The corrugation of the strip along its longitudinal axis is achieved by passing the thin metal strip through a plurality of synchronously driven (same RPM) corrugating strip roller die stands, each stand having progressively larger diameter rollers such that as a point on the metal strip passes from one corrugating strip roller stand to the next, the next stand rollers have progressively higher circumferential surface speed than the last, creating a pull on the strip material emerging from the preceeding roller stand, the pulling force at each stand preventing wrinkling or buckling of the strip as it passes through the corrugating operation.