Sheet-Fed Offset Press

Overview

A sheet-fed offset press prints high-quality color images onto cut sheets of paper or board, one sheet at a time, at thousands of sheets per hour. It is the workhorse of commercial print for brochures, packaging, labels, books, and folding cartons. The process is offset lithography: the image is not printed directly from the plate to the paper but is offset onto a rubber blanket first and then to the sheet. This protects the plate, prints cleanly on textured stock, and gives the sharp, even ink film that offset is known for. Color images are built from separate plates, usually cyan, magenta, yellow, and black, each on its own printing unit, with the sheet carried through all units in exact register so the colors overlay precisely.

The press is a line of stations. The Sheet Feeder lifts and squares each sheet, the Printing Unit stations lay down one color each, the Sheet Transfer hands the sheet between them, and the Sheet Delivery stacks the finished sheets. Behind these run the Main Drive, the Dampening Supply, the Control Console, and the heavy Press Frame that holds it all in alignment.

Feeding one sheet at a time

Print quality starts with putting every sheet in exactly the same place. The Sheet Feeder uses a Suction Head whose Vacuum Sucker cups, fed by a Vacuum Pump, lift the top sheet while a Air Blower fans the pile so only one comes away. The sheet travels down the Feed Table to the Front Lay stops that square its lead edge, then the Side Lay pulls it to a fixed side edge so it enters the press at a known position. A Pile Lift raises the pile as sheets are taken, and a Double-Sheet Detector stops the press if two sheets feed at once, which would smash the blanket.

How an offset unit prints a color

Each Printing Unit has three cylinders working together. The Plate Cylinder carries the Printing Plate, whose imaged areas accept greasy ink and whose clear areas, kept wet, reject it. The Ink Roller Train films an even layer of ink down through many Ink Roller and Oscillating Roller stages to the Form Roller that touches the plate, while the Dampening Roller Train lays a thin water film so the non-image areas stay clean. The inked image rolls onto the Blanket Cylinder and its Rubber Blanket, which then transfers it to the sheet as it passes against the Impression Cylinder. The blanket is the offset step, and its rubber lets the press print evenly on rough or textured stock.

Ink, water, and color control

Offset balances two fluids that do not mix. Too much water washes out the ink, too little lets the plate scum, so the Dampening Supply keeps the Fountain Tank solution chilled by a Chiller, dosed by a Dosing Pump, and watched by a Conductivity Sensor so the balance holds all run. Ink supply is set across the sheet width by the Ink Fountain, whose row of motorized Ink Key screws meters more or less ink into each zone. The operator pulls a sheet, scans it on the Spectrophotometer, and the Control Console trims the ink keys to bring the color to target.

Keeping color in register

Because each color is laid by a different unit, the sheet must arrive at every unit in the same position to a fraction of a tenth of a millimetre. The Sheet Transfer hands the sheet between units on Transfer Drum drums, gripped by Gripper Finger grippers opened and closed by precise Gripper Cam timing. The Main Drive ties every cylinder to one rotation through a Main Line Shaft and matched Helical Gear Pair sets, with a Drive Encoder reporting press angle so feeding, grippers, and spray all fire at the right instant. Fine Register Adjuster controls on each plate cylinder let the operator nudge a color into perfect alignment.

Delivery, drying, and the frame

After the last unit, the Sheet Delivery takes over. The Delivery Chain pulls each sheet over a Powder Spray that dusts on fine starch so wet sheets do not stick, and an IR Drier sets the ink before the sheet drops onto the Delivery Pile Board, squared by the Jogger. Everything that turns is carried by the Press Frame, a pair of heavy cast Side Frame walls tied by Tie Bar bars, because the cylinder bearings must stay in fixed alignment for the press to hold register over millions of impressions.

Variants and use

Sheet-fed presses are specified by sheet format and number of units. A short B3 press suits small commercial work, while a B1 or B0 press with six or eight units prints packaging and folding carton in one pass with extra units for spot colors, varnish, or coating. Perfecting presses turn the sheet between units to print both sides in one run. The sheet-fed approach trades the raw speed of a Web Offset Press for the flexibility to switch jobs and stocks quickly and to print thick board, so it owns the shorter, higher-value, and packaging end of print while web presses own long runs of newspapers, magazines, and catalogs. Makeready, the time to mount plates, set ink keys, and bring color and register in, is the main cost of a short run, so modern presses automate plate loading, ink-key presetting, and inline color to cut it.