Three main types of folding machines are in use today: the knife folder, buckle (or plate) folder, and combination folders, which are machines that employ both knife and buckle folding in different stages in the same machine. All three types are designed to take standard sizes of paper and are suitable for different types and classes of work.
In the knife folder, a blunt-edged knife is set parallel with and above the slot formed by the two rollers. 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. The knife having descended, the sheet, now folded once, is carried by rollers and tapes to a second unit of knife and rollers where a second fold is made, then a third fold, etc., the folded sheets (now sections) being delivered to a stacker. If any knife is at right angles to the previous unit a fold at right angles to the previous fold is made; however, if the machine is constructed with the folding units parallel to preceding folds then parallel folds are made. 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° 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°, the sheet buckles at the point of entry to the plates. 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. The sheet continues to be propelled by rollers and the folded edge may be deflected into a second and then third plate, producing additional folds parallel to the first. Buckle folders frequently incorporate knife folding units, the knives being used to make folds at right angles to the parallel buckle fold. As with knife folding machines, perforators and slitters are incorporated for use at various stages of folding.
Buckle folders are generally run at higher speeds than knife folders, but knife folders are used in book work because they are better adapted for handling a greater diversity of papers. The largest knife folders will fold a flat sheet of 128 pages, producing four 32-page sections, each with four folds. Buckle folders, in their most complex form, can produce almost any series of folds, but, in general, they are used more for the folding of advertising matter than for book work.
The first folding machine is believed to have been introduced in about 1850 by a man named Blake. In 1856, Cyrus Chambers, Jr., patented and sold a folding machine to the Lippincott Publishing Company. Although the accuracy of early folders was poor, with hand folding still predominating for better grade work, development of the folding machine after 1862 was rapid, and in 1873 a machine was patented that would fold a 16-page section and one of 8 pages, inset the latter, and paste it in place. That same year devices to cut and slit paper as it went through the machine were introduced. Automatic feeders were also developed, one being patented in 1855.
Modern folding machines are available in many sizes, capable of folding sheets measuring 4 by 4 inches up to 26 by 60 inches and, in special cases, even up to 50 by 74 inches. (52 , 101 , 179 , 320 , 339 )