The offset printing process is one of the oldest printing methods and one of the most commonly used in the commercial sector. Unlike inkjet printing and laser printing, which start off by taking digital signals and printing them, the offset printing process begins with the operator creating metal plates in which the image to be printed is engraved. Plates are then positioned on the printer’s cylinder and properly aligned. A rubber belt is used as a medium for the ink, which is then applied to the paper. After the paper is cut, the offset printing process is complete.
Plates are used to carry the ink in offset printing. The plates themselves are engraved with the printed image, and ink is added to the plates during the printing process. The plates traditionally were made manually, but this is difficult and can greatly hinder the offset printing process, so most plates in the early 21st century are made digitally using engraving techniques that easily mimic the image to be printed.
After the plates are complete, they are placed in the offset printer’s cylinders. There are several cylinders for different parts of the offset printing process, and the plates must be positioned in the correct order. If not, then color errors and printing errors can occur; for example, attempts to make a two-sided print may result in two different documents from two separate print jobs being printed on opposite sides of the same sheet of paper. Plate positioning will vary based on the exact offset printing method being used and the number of cylinders.
At this point, the offset printer is turned on and a rubber belt comes by to pick up the ink from the plates. To make the ink come off, water and ink are combined so the ink cleanly imprints on the rubber belt. The image in the rubber belt is backward for now. After printing, the rubber belt usually is cleaned and reused in other designs.
Ink from the rubber belt is transferred to the paper; the rubber belt is inverted before reaching the paper, so the image transfers in the correct orientation. Most offset printers have large rolls of paper, so the offset printing process is not yet complete. A cutting mechanism has to cut the offset paper roll correctly to make properly sized sheets. If the offset printer uses sheets instead of a roll — which is uncommon, but not rare — then there usually is no need for a cutter or cutting is a low-priority function.