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in printing, a machine that folds printed paper sheets into signatures.
Knife, buckle-section, and combination folders differ in the manner in which the folds are produced. In tape-and-knife folders, a thin steel blade drives the sheet through a slit and then between rollers, which form the fold. In buckle-section folders, the entering sheet reaches a stop and begins to bend, forming a loop; the loop is grabbed by rollers, which press it into a fold. Combination machines have devices of both types; the final fold is formed by a tape-and-knife mechanism.
Knife folders occupy considerable space; they have more complicated designs and are less productive than buckle-section folders, and they can perform only a limited number of folding variations. However, they are capable of folding large-format sheets of any weight paper with great accuracy. Multisignature folders cut sheets during folding into two, four, or eight parts, each of which exits from the machine as a separate signature. Printed sheets are fed into folding machines by automatic feeders. Knife folders operate at a rate of 80–100 cycles per minute; buckle-section folders, at 120–140 cycles per minute; and combination folders, at 160–180 cycles per minute.