5.3.3 Gravure cylinders preparation

The problem of increase of productivity was solved in HelioKlischograph K200/201 (R. Hell) by the parallel use of eight or more recording heads, each of which engraves the respective page of an edition. The relief of the obtained form, in contrast to the form of classical gravure printing, is characterized not only by the variable depth of the printing elements, but, due to the specifics of the engraving process (see Fig. 5.9 a), also by their area variation, i.e. corresponds to the form of a gravure autotype [5.13].

The method of gravure printing is initially characterized by a relatively low geometric accuracy of fine detail reproduction due to the relatively large size of the printed elements and the presence of partitions between them, serving as a support for the squeegee (“doctor blade”), removing excess ink from the cylinder before it comes into contact with paper. For this reason, the typefaces of relatively simple design are used here.

Traditionally, a special input module was used in the cylinder engraving system, the optical heads of which produced signals of negative opaque original pasted up layouts with text and color-separated halftones.

To eliminate their time-consuming manual preparation in the late 70-ies there was designed the first digital system for color text-illustrative pages layout - HDP (Helio Data Processing) [1.17]. For input and functional transformation of images was used the input section of a scanner, and the text files came from a phototypesetter. Layout and sophisticated retouching in accordance with the editorial instructions was carried out on a computer workstation having the means of interactive graphic dialogue and the ability to simulate a page on its monitor.

5.3.4 Laser engraving

Initially focused on the digital layout, the system “Lasergravure” was later developed by Crosfield Electronics. Its technology included thermal, under the influence of video-modulated radiation of a powerful laser, ablation of epoxy resin filling the pre-etched cylinder cells. To increase the number of printed copies, the cylinder surface could then be subjected to metallization.

Laser engraving means not mechanical, but thermal effect of laser radiation on the copy material. Obtaining by this method of relief metal forms for letterpress or gravure is low productive and inefficient in relation to the achieved quality. Despite a number of attempts [5.14], it has not found practical application. Therefore the use of laser for direct print forms production is, as a rule, associated with the development of special technologies, including materials and copying layers. The latter can belong to the category of silverless or, using silver salts, be close in sensitivity to phototechnical films. In this case, some output devices are universal, capable of producing both transparencies and print forms.

Indicative in this sense is the schemes variety of use the laser facsimile apparatus LogEscan and silverless LaserMask technology in New York Times in the mid 80s for continuous production cycle of this more than hundred pages daily edition.

The pasted-up original pages were transmitted by means of this equipment by cable to the relief printing department located in the ground floor of the same building. Here, in the receiver, the laser beam modulated by the transmitted signal transferred a layer of dye from the transparent substrate of the LaserMask material to the paper producing the positive copy to be returned to the layout section upstairs, where it was used to prepare the pages of the next (day, evening) editions. The remaining layer of dye on the substrate formed a transparent negative image suitable for copying to a letterpress cliché. The signal of the same transmitter was simultaneously transmitted by ultra short wave to the offset print shop located a few kilometers away in New Jersey, where a copy was recorded directly on the litho plate in a similar receiver.

In various versions of the "computer - offset plate" technology, the final result of the exposure by laser or other radiation source onto the plate material is to obtain an intermediate image in the form of a set of ink receiving (oleophilic) and repelling (oleophobic) elements. This, mainly thermal, effect is used either to remove one of the plate layers or selectively change the physical properties of its surface.

In the technology of "computer - relief form of flexography" is widely used two-stage process. At the first stage, alternative to the manufacture of halftone transparencies, the laser radiation burns the mask on the auxiliary protecting coating. Due to the subsequent selective (through the mask) irradiation of the photopolymer with un-modulated ultraviolet, the relief of the printing and blank elements is more pronounced than in the traditional exposure through the film negative. The absence of the need for such negative also makes it possible to produce seamless, sleeve photopolymer forms of web flexographic printing [5.15].