The three main types of folding machine are the knife folder, the buckle or plate folder, and the combination folder, which uses both knife and buckle folding in different stages.
With a knife folder, the fold is made with a blunt steel blade, which pushes the internal part of the paper between two rollers. A buckler folder creates a fold when the paper, taken through a set of rollers into a plate, hits a backstop that causes it to buckle and forces it downward into a set of nip rollers. The nip rollers grab the sheet at the buckle and pull it through, compressing a fold into it. In both cases, the sheet can be propelled along by the machine to further units for further folds. Knife folders can make folds that are parallel or at right angles to the previous fold. Buckle folders often incorporate knife folding units to make folds at right angles to the parallel buckle fold.
Buckles folders are higher-speed folding machines, but knife folders can handle a greater diversity of papers. Knife folders consequently are used in book work, while buckle folders are generally used for folding advertising matter.
The knife folder is more precise on the folds since the pages are kept in close registration. Composite machines, used for large quantities and more complex jobs, combine speed and close registration.
Modern folding machines can fold sheets that measure from 4 by 4 inches to 26 by 60 inches, or even 50 by 74 inches in special cases. The largest knife folders can fold a flat sheet of 128 pages to produce sections of 32 pages each.