Offset lithography

Offset lithography is a more complex process than rotogravure, however by far the most common. It shares as much with lithography as with rotary printing. In this process, a zinc lithographic plate is used on the plate cylinder. As the plate cylinder rotates, water is applied to the plate and then ink is applied. The lithographic plate is treated in such a way that water will be attracted to the areas of the plate where no image should be printed. This water will repel the ink in those areas. The image portion of the plate is treated to repel water and only the ink will stick to it. The ink image is transferred onto an offset cylinder, which is typically made of rubber. The rubber picks up the ink. The substrate is then pressed between the offset cylinder and an impression cylinder where the ink is transferred to the substrate.

It is extremely cost effective for medium to very long print runs. The intial expense is in making metal printing plates, but once these have been created and loaded on to the press, they can produce one or one million prints. The cost of the paper and ink is relatively very small.

There are two ways of printing using this technique. The first is process, or CMYK printing. CMYK refers to the four ink colours used (cyan, magenta, yellow and black, known as in print language as key).

By using these four colours it is possible to print almost every colour of the spectrum. It is suitable for the vast majority of colour printing and since the press is always loaded with these four colours of inks, it is surprisingly inexpensive. The image is split down into the four colours, then four plates are made (one for each colour).

The other common use of offset litho machines is called spot colour printing. This is used where precise colour matches are needed.

Here, inks are specially mixed according to a formula guide (most commonly Pantone®, but other guides exist) and are loaded into the stations of the press. The diagram above shows a two-colour letterhead being prepared with precisely-mixed green and purple inks.

Although it may seem strange, full colour process is often cheaper than spot colour printing. This is because spot colour printing is more labour-intensive.