The rollers revolve continuously so that when a sheet is placed over them and the knife descends, the paper is caught between the rollers and carried away, a fold being made where the knife made contact. In practice the sheets are fed one at a time to stops, either by hand or by mechanical feeders, and are carried by moving tapes beneath the knife where they are precisely positioned mechanically for folding. If it is desired to slit the sheet into individual sections during folding, the sheet will travel through rotary slitters when passing from one folding unit to the next, and the individual sections of the sheet will be conveyed to individual folding units. In this manner, a sheet printed with 64 pages can be slit into four parts producing four individual 16-page sections. Machines are normally provided with perforators which perforate the bolts, thus preventing wrinkling and buckling.
Buckle folders work on a different principle.
The sheet is fed end first between a pair of continuously revolving rollers and the leading edge is guided between two closely spaced plates, the plane of the plates being at an angle of 45 deg. to the plane of feeding. The plates are fitted with internal adjustable stops, and when the leading edge of the sheet hits these stops further forward motion is prevented. The latter half of the sheet, however, is still being propelled forward by the rollers and, being already bent at an angle of 45 deg. The buckle is gripped between the lower roller and a third roller in contact with it and the buckle passes between these rollers, thus forming a fold. The portion of the sheet between the plates is immediately withdrawn by this action, leaving the mechanism clear for the next sheet.