Sheet control systems control positioning of each paper sheet. This movement of each paper sheet through press from the feeder to delivery is controlled by sheet control systems. There are a number of sheet control mechanisms. The sheet is fed into registering system. The printing unit grippers get control of the inserted sheet. The latter is then carried through printing unit in the register. The registration is kept perfect. The sheet is then delivered.
Register and feeding cycle
Register refers to sheet positioning in relation to image on blanket. The sheet's front guiding and side guiding creates fixed distance from side guide side and head of sheet to image. Any paper's dimensional instability requires that the presses could only register each sheet's two sides: the side-guide side and he head.
The feeding cycle consists of paper being piled into feeder. These sheets are then separated. They are then fed into register system. The sheet is both side and front guided. This sheet is inserted into or taken by the impression cylinder grippers. These grippers hold sheet in register while being printed. The sheet gets delivered through an endless chain. The latter has gripper bars mounted on it. Both the delivery and impression grippers grasp the sheet at the same time. Then sheet gets jogged into the correct position in delivery.
Sheet feeder and stream feeder
The feeder of a sheet fed press has multiple functions. It keeps sheets in alignment with registering mechanism. The top sheet gets separate from a pile and is lifted off from the pile. The sheet is then forwarded into forwarding device. The pile height is controlled to ensure that the feed goes on uninterrupted.
The top sheet in successive sheet feeder gets separated and then lifted art the gripper or front side by vacuum suckers and air blast. The sheet is then forwarded into the register-board which travels at identical speed rate as the press. The succeeding sheets gets forwarded and lifted once the preceding sheet's tail end has cleared feeder.
Majority of large presses have stream feeders. Vacuum suckers and air blast separates and lifts the top sheet. Front edge of the sheet, now floating on an air layer, gets forwarded into register-board. Every succeeding sheet gets lifted at its back edge and then it gets advanced as soon as preceding sheet had traveled the short distance down register-board. An under-lapped sheets stream thus gets formed. The feeder speed is less than press speed.