Basic Systems of Sheetfed Offset Presses

Posted on 10-07-2013 by Admin

A sheetfed press consists of a feeder, one or more printing units, a sheet register system including transfer devices to move the paper through the press, a delivery, and various auxiliary devices (such as a control console).

The printing unit of a sheetfed offset lithographic press generally consists of three primary cylinders and systems for dampening and inking the plate (figure 3-1):

. Plate cylinder. A cylinder that carries the printing plate, a flexible image carrier with ink-receptive image areas and, when moistened with a water-based solution, ink-repellent nonimage areas.

. Blanket cylinder. A cylinder that carries the offset blanket, a fabric coated with synthetic rubber that transfers the image from the printing plate to the substrate.

. Impression cylinder. A cylinder running in contact with the blanket cylinder that transports the paper or other substrate and forces the paper against the inked blanket.

. Dampening system. A series of rollers that carry a metered film of water mixed with other important chemicals (together called dampening solution), "dampening" the printing plate. The water-based dampening solution contains additives such as acid, gum arabic, and isopropyl alcohol or other wetting agents.

. Inking system. An ink fountain that functions as ink reservoir and a series (set) of rollers that transport and mill the ink, finally depositing a metered film of ink onto the printing plate.

In addition to one or more printing units, a press also includes the following:

. Feeder, which lifts and forwards the sheets of paper or other substrate from a pile to the first printing unit.

. Sheet register system, which provides (1) alignment guides and devices that enable every sheet to be registered and printed in the exact same location and (2)transfer devices -- often auxiliary cylinders with sheet grippers -- that facilitate sheet transports through the press.

. Delivery, which receives and stacks the printed sheet.