Offset printing

Offset printing – fast printing in large quantities

Offset printing is an indirect printing process because the print substrate and the printing plate do not come into contact with each other during the printing process. Today, offset printing is considered the most widely used printing process in book printing, newspaper printing, advertising materials and packaging materials. In this printing process, the printing ink is transferred from the printing plate to a printing blanket. This rubber blanket then transfers the ink to the material to be printed. The process protects the printing plate, is very fast and is therefore used primarily for printing large series.

Offset printing simply explained

Unlike relief printing processes, in planographic printing processes such as offset printing, the printing and non-printing areas of the printing plate are almost on the same level. They differ in their wetting with oil or water. The printing areas are designed in such a way that water beads off them. In the areas that are not intended to print, however, the water is retained. The printing plate is mounted on a cylinder and moistened with each rotation of the cylinder. Ink rollers then apply the printing ink. Areas that have absorbed water during moistening do not accept ink. On the material to be printed, these areas remain blank, so on white paper they remain white. The remaining areas are inked by the ink roller and then transfer the ink to the printing material as lines or screen dots.

The development from sheet-fed to web offset printing

Offset printing goes back to lithography, invented by Alois Senefelder as early as 1796. Lithography is the first planographic printing process in which the printing and non-printing parts of the printing plate lie on one level. The process is based on the contrast between grease and water. Even today, the areas of an offset printing plate wetted with water do not accept ink, while the areas wetted with grease accept the oil-rich printing ink. As in lithography, individual sheets are also printed in sheet-fed offset printing. When the lithographic stone was replaced by a curved metal plate at the end of the 19th century, the way was clear for offset printing with a rotating cylinder and thus also for web offset printing.

The printing unit of an offset printing press

A sheet-fed offset printing press has a printing unit consisting of the printing plate, the printing rubber blanket and an impression cylinder. A dampening unit moistens the printing unit, and the inking unit applies the ink. In four-colour printing, see below, a separate printing unit is required for each colour used. In modern printing presses, the corresponding number of printing units is installed in an inline configuration. The sheets to be printed are taken from a stack, fed into the printing press and stacked again at the end.

Similarly, in a web offset printing press, a printing unit is also required for each colour. The paper or film to be printed is unwound from the roll and fed into the printing press through a pre-tensioning unit that ensures controlled web tension. In web offset printing, both sides are printed and the webs are dried, cut and folded after the printing process.

How are colours reproduced in offset printing?

To explain colour offset printing simply, it can be said that it is a printing process based on the primary colours cyan, magenta, yellow and black. From these basic colours, blue, red, yellow and black, almost everything that the human eye can perceive as colour can be mixed. A separate inking unit is required for each of these basic colours, as described above in the section Offset printing simply explained. The colours are mixed into the visible colour tones by means of a fine screening of the colour areas that is not visible to the naked eye. Small and large colour dots are combined, or many and few dots per area. Both methods create the desired optical impression for the human eye. The printing process using these four basic colours is also referred to as four-colour printing, which can be misleading for laypeople if they do not know that almost all colours can be produced from these four colours.

The use of spot colours and effect colours in offset printing

The colour spectrum in four-colour printing is limited. In order to reproduce solid colours, for example for art prints, or to achieve certain effects such as gold or silver, separate inking units must therefore be used in offset printing. This makes the printing process considerably more expensive, because a separate printing unit is required for each spot or effect colour. There, the ink is applied over the desired areas as a full surface without screening.

Printing films using the offset printing process

In offset printing, mainly paper and cardboard materials are printed. However, certain films are also suitable for printing using the offset printing process. For example, optimont® Syntheticpaper or the print film optimont® 501 can be printed using offset printing in addition to other printing processes. For the web offset printing process most commonly used today, the BOPET or PET films are supplied on rolls for this purpose.

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