The most common kind of offset printing is derived from the photo offset process, which involves using light-sensitive chemicals and photographic techniques to transfer images 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 litho printing on to a web (reel) of paper is commonly used for printing of newspapers and magazines for high speed production.

 

How ink is transferred from the ink duct to the paper has several steps.

 

1Offset printing ink duct roller delivers ink from the ink duct to the ink pyramid.

 

2 The drop roller transfers ink from duct roller to distribution roller. It is never in contact with both rollers.

 

3 The distribution rollers evenly distributed the offset printing inks. The first distribution roller picks up the ink from driving rollers, and the last distribution rollers transfer the ink to the form rollers.

 

4 The transfer rollers transfer ink between the ink-absorbing and ink-delivering driving rollers.

 

5 Driving rollers roll against the distribution rollers and either absorb or deliver ink, depending on their placement.

 

6 Ink form rollers transfer ink from the last distribution rollers on the printing plate.

 

7 The printing plate transfers the ink to the paper.