ROTARY PRINTING: OFFSET LITHOGRAPHY

There a four main areas of the Commercial Print Production, the first of these is:

ROTARY PRINTING

ROTARY PRINTING PROCESSES

In this process the image printing plates are wrapped around a cylinder. This is an automated print process and the material to be printed can be sheet fed or on a roll.

The three Rotary Printing processes are

-Offset Lithography

-Rotogravure

&

-Flexography

OFFSET LITHOGRAPHY

Offset lithography is a printing technique which is widely used around the world because it is fast, efficient, cheap, and relatively easy. It is called ‘Offset’ Printing because the ink is transferred to a separate surface before being applied to the paper.

This means the image on the plate is the normal way around, reversed when transferred to the middle or ’blanket cylinder’ and finally the right way again when on the material to be printed.

The first step in offset lithography is making a plate with the image to be printed. The plate itself is aluminium which has been etched into. If the image is in black and white, only a single plate is required, because the plate can simply be inked with black ink.

Colour images are produced using a four-color separation process, in which four different plates are made for the cyan, magenta, yellow, and key (black) inks; when the plates are printed, the colours blend together visually, creating a colour image.

There is either Sheet fed printing or Web Offset. Web Offset is a roll of paper, to continuously print whereas Sheet Fed is process where printing will happen one after the other, separately. Sheet fed would be used for medium volume runs, and in contrast Web offset will be used for high volume production. This would be more appropriate for books, newspapers, magazines etc.

This printing technique takes advantage of the fact that oil and water do not mix. The plate is brushed with rollers coated in water, and then with rollers covered in ink. The right balance of water and ink is essential to get an even coating of ink on the image plate.

The first press for offset lithography was developed in 1903, and the concept quickly caught on in the printing community. Offset presses vary in size from the massive presses used at commercial publishers, which can be bigger than a house, to smaller models around the size of a mini bus used at smaller printing companies.