Inking unit
The inking unit is a device on printing machines that is used to automatically track used ink to the printing form . The inking unit is usually topped up from the ink fountain . The difference between the used and replenished printing ink should be as small as possible and ideally balanced. It is also the task of the inking unit to cover every point of the printing form with the same amount of ink or ink film thickness.
A distinction is made between the following types of construction for inking units:
Plate inking unit
A simple plate, often rotatable and placed above the mold, onto which paint is applied by hand. The rollers go over the plate with each printing cycle and pick up new color. The simple, maintenance-free structure is advantageous here. A major disadvantage, however, is the uneven flow and trituration of paint and the low storage capacity. The plate inking unit is used almost exclusively for small hand crucibles and self-made extraction presses . The plate inking unit is unknown in offset machines.
Table inking unit
The table inking unit is a combination of plate and cylinder inking unit. The table inking unit is used almost exclusively for high-speed presses with a flat design, for example on book and stone printing machines.
Cylinder inking unit
It is the most modern and common inking unit technology in all designs. Cylinder inking units are used in letterpress , sheet-fed offset printing and web offset printing . In gravure printing , there is no ink unit, since the pressure cylinder passes through a paint tray, and the excess ink is scraped off by a doctor blade.
Sheetfed offset
The inking unit in sheet-fed offset presses is equipped with numerous rollers for evenly inking printing plates . The amount of ink required by the printing forme is fed from the ink fountain to the inking unit via the ductor and lifter. The ink feed can be regulated zone by zone parallel to the impression cylinder using zone screws . Automated color presetting results in a considerable reduction in set-up times . The data required for this are determined from the digital prepress stage or from a printing plate reader by measuring the printing plate.
All rollers in the inking unit, which is also known as the roller frame , have the same peripheral speed and work with practically no slip . Some rollers also move axially, i.e. laterally, and are called distribution rollers. This lateral movement ensures that the color profile caused by the ink zone opening is somewhat balanced out at the transitions. Through the channel of the plate cylinder, there is no continuous removal of the ink from the inking unit and this leads to fluctuations in the ink layer thickness on the printing plate in the circumferential direction. These fluctuations are referred to as color drop by the printer . Therefore, with high-quality inking units, there is the option of adjusting the phase position of the inking rollers so that the point of application of the lateral trituration is specifically changed. New inking unit designs even enable remote adjustment. A pre-setting of the optimal use of the distribution rollers can be determined from the color assignment data of the prepress stage.
Web offset
Compared to sheetfed offset, the main differences in the inking unit construction result from the requirements of the higher printing speed and the lower paper quality. Inking units in web offset do not use clocked master rollers, but differ in that they are fed continuously. In addition, the consistency of the printing ink is much thinner than in sheet-fed offset. The storage volume of the inking rollers is lower due to the lower number of inking rollers. While four inking rollers per inking unit are generally used in sheetfed offset, web offset makes do with two to three rollers because the channel of the plate cylinder is significantly smaller and thus more even inking of the plate is achieved.
Bridge roller
A further development of the inking unit, the bridge roller . The bridge roller is a driving and traversing capable roller with an adjustable contact pressure. The inking unit can optionally be connected to the dampening unit via the bridge roller and the dampening solution can be directed into the inking unit. In addition, the bridge roller has two settings. In the first, it acts as a pure rider roller , which only smooths the ink film in the inking unit. The second shot has the task of redistributing the color film. Practice has shown that the results are better if the front inking rollers transport the majority of the ink to the printing form. Nevertheless, it happens that there are printing forms that require a different color distribution. The bridge roller is then switched so that it functions as an additional intermediate roller. With the inking unit shown, the ink distribution is 25% without the bridge roller, whereas with the bridge roller switched on, from bottom to top 31% - 31% - 19% - 19%.
Individual evidence
- ↑ a b c Helmut Kipphan (Ed.): Handbook of Print Media: Technologies and Production Processes, p. 247 f. Springer-Verlag, Berlin, November 2000.
literature
- 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 978-3931436018