Web Offset Press

Overview

A web offset press prints onto a continuous ribbon of paper, called a web, fed from large rolls rather than as cut sheets, and it does so at far higher speed than a sheet-fed press. The web runs unbroken from the roll, through the printing units, through a dryer, over chill rolls, and into a folder that cuts and folds it into finished signatures, all in one continuous pass at up to fifteen metres per second. This is how magazines, catalogs, newspapers, books, and advertising inserts are produced in long runs of tens or hundreds of thousands. Like a sheet-fed press it uses offset lithography, ink against water transferred through a rubber blanket, but the units are built blanket-to-blanket so each one prints both sides of the web at the same time.

The press is a long line of sections. The Reel Stand supplies paper, the Web Infeed sets web tension, the Print Unit units lay down the colors, the Hot-Air Dryer cures the ink, the Chill Rolls cools the web, and the Folder and Cutter turns it into copies. Running these are the Drive and Registration, the Press Control, and the Press Frame.

Feeding paper without stopping

A press running this fast cannot stop to change rolls, so the Reel Stand is built to splice on the fly. Two Reel Arm arms hold the running roll and the next roll, the Reel Drive Motor spins the new roll up to web speed, and the Flying Splicer swings the new roll into the web. A Press Brush presses the web onto a taped strip on the new roll and a Cutoff Knife cuts the tail of the old roll, joining them at speed. A Reel Brake holds back-tension on the unwinding roll, and a Dancer Roll absorbs the tension swing during the splice so the print does not shift.

Holding tension and tracking

Print register on a web depends on holding the paper at constant tension and keeping it tracking straight, because the web stretches under load and wanders if let. The Web Infeed pulls the web in through driven Infeed Roller nips set by the Infeed Motor, a Tension Sensor reads the tension so the drive can hold it, and a Web Guide steers the web straight before it reaches the first unit. From there the tension is managed unit to unit so the printed image lands in the same place on every copy.

Printing both sides at once

Each Print Unit is a blanket-to-blanket unit. The web passes between two opposed Blanket Cylinder cylinders, and each blanket acts as the impression surface for the other, so both sides print in one nip with no separate impression cylinder. Each side has its own Plate Cylinder carrying a Printing Plate, fed by an Ink Train and kept clean by a Dampening Train, with ink metered across the width by the Ink Fountain and its Ink Key zones. A four-unit press lays cyan, magenta, yellow, and black on both sides, and Register Motor adjusters on each plate cylinder bring the colors into alignment.

Drying and chilling

Heatset web offset uses ink that dries by driving off its solvent in heat, so after the units the web runs through the Hot-Air Dryer. A Dryer Burner heats recirculated air that a Recirculation Fan blows through Air Nozzle jets onto both faces of the web, a Web Temp Sensor holds the web in the band where the ink sets without scorching the paper, and an Afterburner burns the solvent fumes before exhaust. The web leaves the dryer hot and would smear if folded, so the Chill Rolls runs it over water-cooled Chill Roll rolls held cold by a Chill Circulator, and a Silicone Applicator wipes on a trace of silicone to ease the fold and cut.

Folding into copies

The Folder and Cutter turns the printed web into finished copies at full speed. The web first runs over a Former Board board that folds it in half along its length, sharpened by Nipping Roller rollers. The Cutting Cylinder then chops the web into single copies with its Cutting Knife against an Cutting Anvil once per revolution, the Folding Cylinder and Jaw Cylinder make the cross fold and tuck it, and a Fan Wheel wheel slows each copy and lays it onto the delivery.

Drive, control, and use

Holding register across a long press at high speed is a drive problem. Modern presses are shaftless: each unit and the folder has its own Servo Motor phased electronically by the Drive and Registration and its Drive Controller, with Register Camera cameras reading marks on the web and correcting register on the fly. The Press Control runs the press from a console, presetting ink keys from the job data and trimming color with an inline Color Scanner. Web offset is chosen for long runs where its speed and low cost per copy outweigh the long makeready and the wasted paper of threading and bringing color in, which is why it prints magazines, catalogs, newspapers, and inserts, while a Sheet-Fed Offset Press handles shorter, thicker, and packaging work. Coldset variants skip the dryer for newsprint that absorbs the ink, while heatset variants with the dryer and chill section give the gloss needed for coated magazine stock.