5.3.2 Electromechanical recording
This is one of the first, but still used methods for automated production of print forms. To create the relief of print and blank elements on the plate material, an electromagnetic recorder is used there with a stylus as the core of its solenoid (see Figure 5.9). The image signal current flowing through the coil controls the stylus reciprocating motion and, thus, the degree of its immersion in the material. Moving the latter relative to the cutting head provides a flatbed or drum-type scanning system.
Figure 5.9 There are not only the depth but also the size of the printing elements is varied in electronic engraving (a); the image signal current chopped by the pulses of screen frequency to obtain a plate with a dot structure
In the simplest version, the image consists of grooves of variable width, i.e. has a linear screen structure (see Fig. 2.1, b). If the signal current is mixed with pulses, the amplitude of which provides a periodic complete extraction of the stylus, the halftone dot image is obtained. In the 60s and 70s, engraving was widely used not only for relief and gravure printing, but also for lithography, where halftone transparencies were obtained by removing a layer of paint with a stylus from the surface of transparent plastic. For this purpose, special materials were later developed that had an etching effect for the subsequent manual tone and color correction [5.10]. The most popular electronic engraving machine was Vario Klischograph K181 of Hell. The color-separated clichés were obtained in it in a wide range of screen rulings with smooth scaling of both transparent and opaque originals. For line work plates, the scan density reached 18 lines/mm. The machine was a unique fusion of the technology achievements of that time in the field of electronics, precision mechanics and optics, pneumatics and hydraulics, automation and industrial design. Its analogues were produced in Japan and United States, and in the late 60-ies similar machine was made in USSR [5.11]. However, by that time the progress in the prepress was not in favor of engraving illustrative forms and now it is preserved only for gravure printing. There it’s justified by relative complexity and high cost of the traditional indirect (using photo reproduction stage) method of preparing printing cylinders. However, in relief and offset printing the engraving was replaced by this seemingly less progressive, indirect way. Direct production of forms, however, had always been of great interest and remained relevant. Now it is implemented in the computer-to -plate concept [5.12]. In this regard, it is interesting to understand the reasons for its departure from wide practice in the early 70's. One of them was the low productivity of engraving. The cutting frequency did not exceed 2 kHz due to the inertia of the recorder and was more than an order of magnitude inferior to the speed of recording halftone dots in the alternative film output of that time. In the relief printing, the indirect films using method was to some extent justified by the availability of a stable and well-normalized so-called single-process of etching clichés [5.8]. The second and probably more important reason was that the engraved illustrative and text forms had to be further assembled both in the page and in the entire print sheet format. In letterpress printing, for example, the manual page stripping was greatly facilitated with illustrations and text presenting on film. The same was one of the reasons for the transition to an indirect method of obtaining text forms, i.e. replacing the "hot" set with a phototypesetting.
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