Method of making lengths of flexible insulated air duct with end collars

A method of making lengths of flexible insulated air duct bendable around corners without collapsing and having end collars, the method comprising telescoping a tubular jacket of flexible plastic film over a free end portion of a cantilevered first mandrel adjacent one end of a machine and gathering the jacket adjacent a mounted end portion of the first mandrel, telescoping a first duct end collar over the free end portion of the first mandrel and securing it in position, telescoping a second duct end collar over a free end portion of a cantilevered second mandrel adjacent another end of the machine and coaxial with the first mandrel and securing the second duct end collar in position, telescoping opposite end portions of a flexible helically reinforced core respectively over and strapping them respectively to the duct end collars, wrapping a flexible insulation blanket around the core and taping opposite end portions of the blanket in place, pulling an end of the tubular jacket on the first mandrel over the insulation blanket all along the length thereof to extend the gathered jacket full length and encase the insulation blanket therein, releasing the duct end collars respectively from the mandrels, and removing the collared duct from the machine.