Nowadays, two different processes are used in web offset printing: heatset and coldset.
The coldset process is mostly used to print newspapers and paperbacks, with the printing ink drying purely by absorption.
In the heatset process, the paper is passed through a large dryer and a chill roll unit after the last printing unit. The length of the printing press is almost doubled by these two units. To ensure that the ink dries optimally, special heat-drying inks are used here.
The paper running through the printing machine at high speed is heated up in the dryer as the mineral oils contained in the printing ink only evaporate at around 200 degrees. The dry air that is blown onto the paper therefore has a temperature of around 250 degrees Celsius and heats not only the printing ink but also the paper to 100° to 120° Celsius. This process removes a great deal of moisture from the paper, as the water contained in the paper simply evaporates. As a result, difficulties such as the paper curling, bubbles or even static charging of the paper web arise. In addition, the resins in the printing ink become liquid, i.e. the ink becomes soft and sticky. Only when the ink is cooled in a flash at the end of the dryer by specially coated chill rolls does it become solid and acquire its characteristic heatset gloss.
The paper, which is very much affected by the heat, is moistened with a water-silicone mixture after the sudden cooling to 20° to 30° Celsius. The water moistens the paper, the silicone ensures that the print is scratch-resistant.
Papers used in heatset must therefore be highly resistant to heat, liquid loss, bubbles and swelling.
Due to the high energy consumption and stricter environmental regulations, heatset plants today are mainly operated with heat recovery systems.
A good video showing a heatset system can be found on youtube: