Web offset printing is the most frequently used rotary printing process and, in comparison with sheet-fed offset printing, it has a much higher printing speed. Here is some more information for anyone who has been wondering how very high print runs are managed.
The basics of web offset
Printing in sheet-fed offset and web offset is an indirect flat printing process. It is indirect because the transfer of the colour from the printing plate first takes place via an additional roller (rubber blanket cylinder) and subsequently on the printing material. The term flat printing comes about because the elements which are to be printed and those which are not to be printed are found on one level on the rubber blanket.
For printing machines used for web offset there is a difference between models which use a heatset and coldset procedure in the drying process which follows printing. The heatset process is used, for example, in the production of (advertising) brochures, catalogues and magazines, while coldset printing machines are used to produce newspapers and paperback books. In the case of these two types of machine we also often talk of commercial web offset and newspaper web offset. As with sheet-fed offset, there are usually four inking systems set up consecutively in web offset, while the paper web, unlike the individual sheets used in sheet-fed offset, is always printed on both sides. This perfecting printing process means that the paper web can sweep through the rollers and impression cylinders at speeds of up to 50km/h.
High speed printing
Web offset is one of the most popular processes in the print industry. This is due in particular to the speed with which this type of machine can produce. These machines can achieve 65,000 cylinder revolutions/h and over. This makes them extremely economical, as millions of print product runs can be produced in a short space of time. The paper web is constantly processed by a roll and as soon as it reaches the end of one roll, if it is in operation, a new roll is directly attached.
Therefore the machines can run almost without a break, resulting in huge time savings. However, this high speed printing is only cost-effective for large print runs.
Heatset or Coldset?
As already mentioned, in web offset printing there are two separate variations. Broadly speaking the difference is in the drying of the paper web. Due to the fast speed of the machine the printing material must be supported in the drying process in order to keep the production process running.
The coldset process is used in newspaper printing machines. They have a vertical web lead and the drying takes place in a purely physical manner via the absorption of the colour. Drying in coldset web offset usually takes longer and results in black marks on your fingers when reading a newspaper.
In heatset web offset printing, the printing takes place via a horizontal web lead and with printing ink which has been dried through heat. After the final printing unit the paper web passes through the dryer. Temperatures of around 250 °C cause the paper web to be heated to around 120 °C. In the next cooling roller unit the paper is suddenly cooled to around 20-30 °C on chrome-plated roll surfaces, which results in a hardening of the ink. This brings about the typical heatset printing shine of the printing ink. Finally the paper is provided with additional protection in the form of a water silicone mixture. This provides the paper with moisture following drying and at the same time increases the scratch resistance of the surface.
Unlike sheet-fed offset, there is the possibility to directly process the paper web in the web offset machine following printing and drying. In what is known as the folder superstructure the slitting of the paper web takes place along with the placing of the resulting part strands on top of each other. Then the first longitudinal fold takes place in the folder unit along with a transverse section of the collected strands. After this there is further folding of the sheets. These include the cross fold, longitudinal fold and the quarter fold. It is also possible to integrate longitudinal and transverse pasting, sizing, trimming edges and numbering in the folding unit.