The most common kind of offset printing is derived from the photo offset process, which involves using light-sensitive chemicals and photographic techniques to transferimages and type from original materials to printing plates. In current use, original materials may be an actual photographic print and typeset text. However, it is more common — with the prevalence of computers and digital images — that the source material exists only as data in a digital publishing system.
Offset Lithographic printing on to a web (reel) of paper is commonly used for printing of newspapers and magazines for high speed production. In this process, ink is transferred from the ink duct to the paper in several steps:
- The ink duct roller delivers ink from the ink duct to the ink pyramid, also called the Ink Train.
- The ductor roller, sometimes called a vibrator roller due to its rapid back and forth motion, transfers ink from the duct roller to the first distribution roller. It is never in contact with both rollers at the same time.
- The distribution rollers evenly distribute the ink. The first distribution roller picks up the ink from driving rollers, and the last distribution rollers transfer the ink to the form rollers.
- The transfer rollers transfer ink between the ink-absorbing and ink-delivering driving rollers.
- Driving rollers roll against the distribution rollers and either absorb or deliver ink, depending on their placement.
- Ink form rollers transfer ink from the last distribution rollers on to the printing plate.
- The printing plate transfers the ink to the offset cylinder (typically called the blanket cylinder) usually covered with a rubber “blanket.”
- The paper is then pressed against the blanket cylinder by the impression cylinder, transferring the ink onto the paper to form the printed image..