Of all the structural shapes – angles, bars, beams, channels, tees, pipe and tube – everything else being equal, the most difficult shape to bend is channel the “hard way,” i.e. against the strong axis. As you can imagine, pressing against the flanges to curve the metal shape is likely to push them inward, especially at the “toes.” And trying to curve the web like a hard way bar (like a washer) is likely to cause the web to buckle.
Roll formed channels, i.e. channels made by bending sheet or plate, is even more difficult. Structural channels typically have flanges that get thicker closer to the web. And structural channels are reinforced where the flanges meet the web both with a square corner on the “outside” and a fillet on the “inside.” Formed channels may have a large or small radius at the corners, both of which can be challenging to roll. A precise set-up is required to insure that the corners do not move or become deformed. Formed channels have only one thickness and therefore are not reinforced as structural channels are.
A recent requirement was for channels formed from rather thin, 11-gauge material with outside corner radii of 1/4in. Four segments 7ft long with a 3-1/8 web and 1 1/2 flanges were rolled the hard way to a 61-1/2in inside radius; six segments 79in long with a 3-1/4 web and 3in flanges were rolled flanges-in to a 60in inside radius. Complicating matters, the channels had round holes and slots cut into the webs and flanges. Such cut-outs sometimes cause the channel to deform and/or are deformed themselves in the rolling process. In this particular case, everything curved correctly.
Had they not been near perfect, these channels would not function as intended: they are used in custom observatories and in enclosures for telescopes.