There are two very different types of flexographic presses used in packaging printing. Firstly, central impression (CI) presses, in which 8-10 inking units are usually arranged around a common, large impression cylinder. And inline presses, where the individual printing units are arranged in a row one behind the other. What are these two different designs of flexo printing machines all about?
|Central cylinder press (CI), with 8 printing units arranged in series around a common, large impression cylinder. (Source: Comexi)||In inline presses, printing units are arranged in a row. (Source: Gallus Ferd. Rüesch AG)|
In both flexo printing machines press types, the printing units consist of an anilox roller plus chambered doctor blade, a printing plate cylinder and an impression cylinder. And that’s where their similarities end. Which design is used for what depends very much on the end product that is to be manufactured.
Flexible packaging: In this segment of packaging printing, mainly thin plastic films are printed. Due to the stretchability of the films, there is always a risk of register fluctuations during printing, caused by the tensile stress during web transport in the press. The central cylinder design prevents this by ensuring that the web tension around the central cylinder is the same. At the same time, the web rests on the cooled, large central cylinder during the printing process, which also prevents the web from stretching due to the warm dry air between the individual printing units. Another advantage of central impression presses is that the small distances between the printing units are beneficial for register accuracy in printing.
Label printing: Due to their siliconized paper base, label materials have good dimensional stability, even under web tension. This is why label printing, with its short runs and frequent job changes, places different demands on the design of printing presses than flexible packaging. The focus here is on solutions that allow presses to be adapted quickly and easily to frequently changing, complex printing requirements. These requirements can best be met by arranging the printing units in a modular design. With modular presses, replacement printing units can be preloaded while the job is running, or the sequence of printing units for different printing processes can be changed for a job during setup. Some label printing presses are then equipped with up to 12 printing units.
Corrugated board printing: Here, the rigid substrate corrugated board determines the straight sheet transport through the printing units and thus the row design. In addition, a large amount of space is required between the printing units when changing jobs in order to insert the large printing forms, which is given by the distances between the printing units. Today, print motifs with high quality requirements are produced on presses with up to six printing units plus a coating unit arranged one behind the other.
In general, the inline design offers much more flexible options for configuring a press according to individual requirements. To this end, completely different printing processes can also be flexibly integrated.
Individual additions are also possible with central-cylinder presses or are already standard. In the main, coating units or printing units for cold seal application are integrated into the press design.