Text 3 Printing Unit Design on Web Presses

Web presses can be designed for all of the three main printing technologies: lithographic printing (offset printing), gravure printing, and letterpress/flexographic printing, as well as for non-impact printing technologies. It is mainly the printing unit configurations of web presses for letterpress and offset printing that are explained in this section.

The image carrier (printing plate) in web-fed letterpress printing presses was, and to a certain extent still is, a round form (stereotype plate, electrotype plate, or wrap-around plate),which is mounted on a plate cylinder. This is brought into contact with an impression cylinder, a cylinder covered with a blanket. The paper web to be printed is routed through the two cylinders and the impression cylinder presses it against the plate cylinder to transfer the ink onto the web.

When lithographic printing was introduced into web printing, the only change to the above printing unit was that the letter press plate was replaced by a lithographic plate. At the beginning zinc plates were used. The dampening unit supplemented the inking unit, to enable the process of separating ink-accepting and non-ink-accepting, that is, ink-repellent, image areas on the flat form.

It was only with the invention of offset printing that a third cylinder was used as an intermediate image carrier. Accordingly the main elements of an offset printing unit therefore consisted of a plate cylinder, a blanket cylinder (a cylinder with a blanket covering), and an impression cylinder. The latter no longer needed to be coated, that is, covered with a blanket. Only with the invention of blanket-to-blanket printing could one of these additional cylinders be dispensed with again. Here two blanket cylinders are in contact with the web, which runs between the cylinders and is printed on the front and reverse side simultaneously (perfecting). This was a considerable improvement in rationalization.