Sheet Metal Fabrication of Hollow Box

Depends on the side of the box, material thickness.......

possible with a brake that has a horn........... of course your box has to be bigger than the horn.

open at Both ends, I missed that........... nope.

How much of a bend, in degree's? Normally the top and bottom would be cut as curved pieces.....

The answer is . . . it depends. On the desired box dimensions and the limitations of your machine. This looks like a good reference. You might also look at how square and rectangular structural tubing is manufactured.

Is that 4ft square or 4ft long? Should be OK if you have a bending machine of suitable size.

Tear up a sheet of paper to some where near scale then bend it around a book. If the book is larger than the sides of box you will see that the paper has to flex to accomplish the bend. 2mm sheet metal will also flex but not enough if the sides of the box are small. If, for example the sides are 1 foot each and the box is 4 foot long. Please give all dimensions of your box.

It can be done but it might take a special piece of equipment depending on the box dimensions. I sourced a bender for box about 6 X 18 inches and anywhere from 6 to 48 inches from open end to open end. The upper die was mounted to a beam that the box would wrap around as it was bent. At one end of the beam was a clamp that would release so the box could be removed. We actually did four bends so there would be an overlap at one corner for the weld. I found two companies that would make the equipment. One was in the US and one was in Denmark.

A brake with a long enough horn on it, with the exception if the 'box' is large enough to accommodate the horn.

But still don't know the dimension of the 'box', material is thin enough that would require that much tonnage.... But this type of brake is rather specialized that not many shops have it.

Here is a video of a type of sheet metal machine that would have no problem with the shape (though the model shown only bends up to 16 gauge mild steel, so a more robust model would be required).

A rectangular-cross-section box that is open both ends is called a duct. That many thousands of air handling plants use ducts of these sorts of dimensions would indicate "yes" to the possibilities query.

As for how it is done, look at ducts and duct manufacturing in the air handling industry as a first step.

Usually done with a Pittsburgh Lock Former