Waterless offset printing

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Scheme of the plate process on a planographic printing plate in waterless offset printing

In contrast to conventional offset printing , which uses two separate media ( printing ink and dampening solution ), there are applications in which the flat printing process can be operated as waterless offset printing with only one medium, namely ink.

history

A good century after the invention of planographic printing by Alois Senefelder (1798 in Munich), the printing researcher Cašpar Hermann experimented in Vienna and Leipzig between 1926 and 1931 to develop a planographic printing process without dampening solution. He tried making complicated modifications to the color. Heinrich Renck took a different approach: in 1930 in Hamburg he developed the first special printing plate for printing without dampening solution. Waterless offset printing began commercially in the 1960s under the name driography . The 3M company developed, patented and sold plates, but encountered technical difficulties in the practical implementation of the process.

The Japanese company Toray Industries bought the rights and supplied printing plates. Their initiative promoted the commercial distribution and technical development of this flat printing variant. The patents protected Toray and hindered competitors, which prevented the process from spreading more widely for many years. In Japan, however, waterless offset printing achieved a large share of the offset printing market.

In Germany, the Marks-3zet company offered Toray records, which were only slowly becoming popular. With positive and negative plates , attempts were made to replace wet offset practically 1: 1, and only a few print shops adopted this technology. With the expiry of the Toray patent, other record manufacturers came onto the market, who also offered their own developments for special market segments. There were also developments in printing machine construction and the development of more suitable printing inks.

The Badische Zeitung was the first to use this printing process in newspaper printing, and in 2008 it won the award for the world's best newspaper printing for its print quality.

Printing principle and plate structure

Waterless offset printing is a flat printing process with indirect ink application. In planographic printing, the printing (ink-carrying) areas of the printing plate can be wetted with ink, whereas the non-image (non-printing) areas cannot.

The wetting processes that take place are described by surface and interfacial tensions. According to general consensus, a wetting liquid (here the color) should have a lower surface tension than the solid to be wetted (here the plate). The interfacial tensions between the ink and the respective surface parts of the plate, printed or non-image, are difficult to determine and are neglected as influencing variables for the sake of simplicity: It is assumed that they occur in the substance contacts between printing inks and printing plates - depending on the design , stay the same. In a simplified representation, this means that the printing areas on the plate can be covered with paint because of their high surface tension of around 35 mN / m, and because of their lower surface tension of around 30 mN / m. The non-printing areas are coated with silicone and are not covered with ink because of its even lower surface tension of around 20 mN / m.

The imaging of the plates can be done photomechanically with subsequent development by solidifying ( positive copy ) of the silicone layer, then it is part of the modern CtP work processes ( computer to press , direct exposure from data). On the other hand, the illustration can be done by removing the silicone layer ( negative copy ).

This can be done photomechanically, by spark erosion (obsolete, earlier DI technology) or thermal ablation (removal) with laser beams (IR laser). The negative plate with thermal ablation gives (according to the current state of the art) the best images, because they have the sharpest drawing, and is particularly suitable for modern fine screens .

properties

Typically, the application rollers deliver a 6-8 µm thick layer of ink to the plate surface, and a 3 to 4 µm thick layer of ink adheres to the image areas. The halftone dots are colored sharply and precisely, which is one of the strengths of waterless offset printing. A process-typical increase in tonal value (over-coloring of the printing areas into the non-image areas) as with "wet offset" is not known, i.e. nonexistent or very slight. This means that it cannot fluctuate, and in comparison to wet offset, the constancy of the run of the screen dot sizes increases. Since only ink is applied to the plate, the equilibrium (without dampening solution) is stabilized within a few turns and the print image is formed; this results in a very small amount of start-up waste . According to current understanding, the waterless planographic printing process works by virtue of the fact that silicone with a low surface tension repels wetting by paint. The difference between the silicone and the printing surface is only about 15 mN / m. The surface tension of the paint (here the tension of the paint surface in relation to the air) decreases more strongly than that of the silicone used as it heats up. Even at relatively low temperatures, from 32 ° C, color can be passed on through the silicone in places. Then the plate tones point-like or even flat. For this reason, printing plates and inking rollers must be cooled in waterless offset printing. The more energy is fed into the machine, depending on the speed and duration of the run, the greater the problem with sound and the more effectively the plate and rollers have to be cooled.

Printing machines

KBA Cortina, the first waterless newspaper printing press

In the early years of waterless offset printing up to the 1990s, old, but increasingly new offset printing machines were used. The dampening system was not always completely removed or left out, but mainly plate cooling was installed, for example with blown air. It was only with the construction of the GTO DI of Heidelberger Druckmaschinen AG that there was a conceptually waterless sheet-fed offset printing machine .

Other machine manufacturers such as Koenig & Bauer AG with the Genius and Karat models also produced machines that worked purely without water. In addition to waterless offset, the Koenig & Bauer concept includes short inking units. The ink is applied to the printing plate using a chamber doctor blade and an anilox roller . This further simplifies mechanical engineering because the large roller frame and the lateral zone control are no longer required. A similar concept was implemented for a newspaper printing press in high-speed web offset with the Cortina from Koenig & Bauer.

In sheetfed printing machines for conventional offset printing, there are more and more temperature control systems for the distribution rollers , because medium and large print runs require a uniform quality that cannot be achieved with climatic fluctuations. This development also favors the waterless process described here.

Printing inks

Up until the 1980s, ink manufacturers developed printing inks for waterless offset using the raw materials they were familiar with, and there were products that could be printed in both conventional and waterless offset. The paint manufacturers could not measure the surface tension of the pasty printing inks and it went unnoticed that it increased more than that of the silicone when the inks were heated. It was found that the tendency to toning when heated decreased the viscosity of the paints: they became more fluid. This was equated with the risk of toning and only highly viscous, almost putty-like colors with a high tack were offered for the waterless process . Disadvantages of these pasty colors were that they were difficult to apply (spatula) and fibers tore out of the paper, that is, they pulled heavily.

First of all, new raw materials were used to try to solve the clay problem with a few percent of certain silicone oils , which is still practiced today. This offset ink cannot be printed without problems without water. According to the current state of the art, silicone-containing paint residues cannot be reused together with conventional paint residues.

There is another difference between the inks for the two planographic printing variants: in wet offset, a very low-speed emulsification (due to the dropwise distribution of aqueous dampening solution in oily printing ink) is printed, in waterless a pure color. This eliminates the leap in speed from emulsified to ink, which enables wet-on-wet ink acceptance in multi-color machines in wet offset . In four-color printing , the colors of the waterless offset must again have graduated peaks - like those of letterpress printing .

Typical applications

CDs and DVDs are typically printed using waterless offset

High-quality picture prints, especially with non-periodic fine grids , allow the advantages of this grid method to come out best with clean fine drawing (high resolution (photography) ). These products are printed on virtually identical machines like conventional offset products - only without dampening units and those systems for water treatment. Applications are high-quality advertising prints, catalogs and illustrated books.

The many different direct imaging (DI) machines are preferably used to print small runs of high quality commercial jobs . So there are mainly four-color prints, preferably on paper. In newspaper printing with increased image quality, the combination of waterless offset with short inking units in the Cortina from König & Bauer is now widely used.

Flat plastic objects have preferably been printed without water since the 2000s. CDs and DVDs , foils, ID-1 cards and plastic labels (for example in cosmetics) are usually printed with UV-curing inks . The colors adhere well and the objects can be processed immediately after printing. In waterless offset, there are no dampening solution residues on the smooth surface. In this area, specially designed printing machines have established themselves (KBA-Meprint Genius, CD Print ).

literature

Individual evidence

  1. ^ Helmut Teschner: Print and media technology . Dr.-Ing Paul Christiani GmbH & Co. KG, Konstanz 2010, ISBN 978-3-86522-629-7 , p. 431 .
  2. ^ RH Leach, RJ Pierce: The Printing Ink Manual . 5th edition. Blueprint, London 1993, ISBN 978-0-948905-81-0 , pp. 384 .
  3. ↑ Helmut Kipphan: Handbuch der Printmedien . 1st edition. Springer-Verlag, Berlin Heidelberg 2000, ISBN 3-540-66941-8 , pp. 369 .