The Master Flow 4 in. Starting Collar has a snap-together design for easy assembly. This starting collar can be used to transition from a flat surface such as a lined or duct board plenum to round pipe or insulated flexible ducts. Can also be attached to metal surfaces. Made of galvanized steel, this starting collar is made for durability. The 2 in. space between the bead and flange provides you with plenty of space for securing an insulated flexible duct.