A dampening unit is used in the offset printing machine. It wets the non-printing parts of the printing form with a thin film of dampening solution , which prevents it from being colored with printing ink. This is done with a number of touching rollers that apply a dampening solution evenly to the printing plate . It consists mainly of water and is transferred from a water box with rollers, brushes, nozzles or in a similar way to the first roller and then as a film evenly conveyed over all rollers to the printing plate.
The dampening units are a further development of the wiper roller , which was used to manually dampen the lithographic stone . Since part of the dampening solution is consumed via the printing ink , printing plate, rubber blanket and paper and another part evaporates, new dampening solution must be continuously added via the dampening unit. The dampening solution consists mainly of water and 2 to 30% of dissolved additives. These include alcohol ( IPA ) or alcohol substitutes and various conditioners such as pH buffers, softeners and biocides (against algae, fungi, etc.). Dampening systems are differentiated according to their design:
Siphon dampening system
This is an older design in which the dampening solution is supplied discontinuously by a rotating ductor and an oscillating lifter, which removes the dampening solution from the ductor and transfers it to another roller. Here it is evenly distributed and transferred to the application rollers, which dampen the printing plate accordingly. The ductor in the water tank and the application rollers are covered with an absorbent material such as plush , molton or terrycloth . However, this system has a high degree of inertia in the event of changes in the supply of the amount of dampening solution, since the substance reference acts as a water reservoir. Further disadvantages are lint on the covers, uneven dampening solution distribution and high maintenance costs. During printing, the covers are increasingly contaminated with ink and therefore often have to be cleaned outside the printing press by hand or with a dampening roller washing machine. Because of these disadvantages, there are hardly any siphon dampening systems of this type in practice today.
So-called alcohol moistening was introduced to compensate for the disadvantages of the fabric covers. The fabric-related dampening form rollers are exchanged for unrelated, soft rubber rollers.
Another variant are the spray dampening units, in which the required amount of dampening water is applied to a roller via brushes, nozzles or spinners. These systems are also known as contactless dampening systems and are preferably used in newspaper printing. Their advantage is that emulsified ink and dissolved paper dust do not get into the dampening solution supply. In addition, the amount of dampening solution can not only be controlled across the entire width of the machine, but also partially influenced. This is done by means of a flap, the opening of which can be adjusted, so that more or less dampening solution reaches the plate. The partial dampening solution setting is particularly advantageous in newspaper printing presses, which often have very different color assignments and also use different colors in a printing unit.
These systems are very common today and work without a master roller or fabric covers, but require alcohol or other special additives in the fountain solution. The dampening solution film is scooped out of the water tank by the rubber-covered immersion roller. Next comes a roller with a hard surface, mostly made of ceramic , rarely made of chrome . The chrome and ceramic rollers are driven separately and, thanks to the adjustable speed, the amount of dampening solution required is controlled and transferred from the application rollers to the printing plates. Film dampening systems are quick to react because every change in the dampening solution flow takes effect immediately. The ink-water balance required for printing is achieved quickly and reduces the amount of waste .
- Helmut Kipphan (Hrsg.): Handbuch der Printmedien. Technologies and production processes. Springer, Berlin et al. 2000, ISBN 3-540-66941-8 .
- Wolfgang Walenski: The web offset printing. History. Modern technology. Materials. Fachschriften-Verlag, Fellbach 1995. ISBN 978-3-931436-01-8
- Helmut Kipphan (Ed.): Handbook of Print Media: Technologies and Production Processes, p. 227 f. Springer-Verlag, Berlin, November 2000.