In my endeavor to explain as many printing processes as possible I have now come to the most common method – Offset printing or Offset Lithography Printing (Lithography meaning the process of printing from a flat surface treated so as to repel the ink except where it is required for printing).
Here’s an attempt to explain it in detail: Offset printing is a technique during which an inked image is transferred, or “offset” from a plate to a rubber blanket, then to a printing surface. Offset printing is often combined with lithographic printing, which uses the repulsion of oil and water to produce a flat image carrier. This is often referred to as offset lithography.
Offset lithography is the most common type of color printing for high-volume jobs.
Each of the primary colors used in printing – cyan, magenta, yellow and black – have a separate plate.
The ink gets to the paper through a process that uses oil to repel ink. The ink is dispersed to the plates by a series of rollers. On the printing press the plates are dampened by water rollers, and then ink rollers. The rollers disperse the ink onto the plates.
The plate’s image area picks up the ink from the ink rollers and the water rollers keep the ink from transferring to the non-image areas of the plate. Each plate transfers its image to a rubber blanket that in turn transfers the image to the paper.
Offset printing presses are made up of printing bays – each bay prints 1 colour, so the one colour GTO press pictured below only prints one colour, the one beside it has 12 bays and prints 12 colours. Typically, printers will have 5 colour presses, the 4 process colours of Cyan, Magenta, Yellow and Black and the ability to print a fifth colour which can be an addional spot colour or a machine varnish. These days though, with press time being paramount for printers, they are opting for larger presses, like the 12 colour below – these are known as perfecting machines – they are able to print the 4 process colours + 2 specials on the 2 sides of the paper in one single pass.
Advantages of Offest Printing
- Consistently high quality images. Offset printing makes images and lines sharper more easily than other printing methods because the rubber blanket is flexible and therefore can conform to the texture of the printing service.
- Production plates can be made quickly and easily.
- Longer printing plate life. The plates last longer because there is no direct contact between the plate and printing surface.
- High speed and high volume printing.
- Costs less the more you print. Most of the price of printing is wrapped up in the preparation of the production plates and anything else that happens before the first page is printed. Once everything is prepared, the more you print will only cost you the price of paper and ink, which is minimal compared to setup costs.