What is offset printing and it's history

Despite the digital age, print media are still extremely popular. Advertising in the form of flyers, posters and brochures are commissioned and given out in large numbers every day in Nigerian online print shops. The printing of large amounts of paper would be inconceivable without offset printing. I would like to talk about this print process, which we use daily, in detail in today’s article.

The definition of offset printing

Offset printing is – in comparison with letterpress printing – an indirect print process, in which the ink is not applied directly from the printing plate to the material which is to be printed (e.g. paper), but rather indirectly via a roller. This method is part of a flat printing process.

Historical facts about the print process

The definition and the process of offset printing has been shaped – as is often the case – by different people.

The Czech Alois Senefelder, inventor of the lithography printing technique, was looking for a cost-effective method of copying sheets of music in the 18th century. He used high pressure forms made of limestone, and he covered the areas which were to be printed with fatty printing-ink and the areas which were to remain free were etched with a slightly acidic solution made from polysaccharide (gum arabic) in the smooth stone surface. When moistening with water no ink remained stuck to the freshly etched areas. Therefore only the fat image areas were dyed. This fact makes etching to a high-pressure form superfluous. This discovery was the breakthrough and signified an important milestone in the principle of flat printing. It is still the basis of offset printing to this day.

In 1904 the American Ira Washington Rubel and the German immigrant Caspar Hermann constructed the first prototypes of offset printing machines independently of each other. They mirrored the indirect printing of the printing plate via a rubber blanket cylinder on the sheet of paper. When Hermann returned to Germany in 1907, he developed the first offset printing machine based on his plans, which was presented to the public in Leipzig in 1912.

Offset printing put into practice

As already mentioned, offset printing is an indirect print process. This means that in the print process a cylinder covered with a rubber blanket is set up between the printing plate and the paper. This is so the printing ink is transferred indirectly on the printed sheet. This ensures an even print, so that the same quality can be produced in high print runs. Unlike the lithography printing process described above, in offset printing the processes are largely automated. Instead of stone, millimetre-thin aluminium plates are used as the print template.

Before printing, a printing plate is created for each printing ink, so there is a total of 4 plates for the 4 colours C-M-Y-K. The image surfaces, which will later be filled with ink during the printing process or remain free, are placed on one level on the printing plate. Here the physical basis is the different surface structure, which comes about due to the exposure of the plates. This means that colour only remains where this is intended. Therefore there is a printing plate for each colour, as only the motif is exposed on the plate which is to be printed in the respective colour. The plates are then inserted into the respective printing unit (matching the colour) in the offset printing machine. The printed sheet passes through all 4 units after each other which results in all colours being printed on top of each other.

The actual printing takes place with the aid of three printing cylinders, which are identified in the diagram below with the numbers 4, 5 and 7. The prepared printing plate is stretched on the first cylinder, known as the plate cylinder, and absorbs printing ink from the inking system on the areas which are to be printed. The areas which are not to be printed are moistened with water with the aid of the rollers of the dampening unit, see No. 3 in the diagram. From the printing plate the right-reading image is first of all transferred back to front on a rubber blanket, which is fitted on the rubber blanket cylinder. Finally the reversed image is once again transferred to the paper the correct way round, which passes between the printing cylinder and the rubber blanket cylinder.

After the print process the colour registers (position of the individual colours on top of each other) and the ink application are checked via line testers (strong lenses with three to twelvefold enlargement).

Distinction of offset printing

With regard to the use of the paper which is to be printed, there is a distinction between sheet-fed offset and web offset. While this process is suitable for the printing of newspapers, telephone books or catalogues in very high print runs, sheet-fed offset printing is used more for small and medium-sized runs.

When considering the development of the past few years it is clear that this type of printing has made a massive contribution to the simplification and speeding up of printing. It would be unthinkable to print newspapers, brochures or flyers produced in large print runs without offset printing Printing?