GB2217862A

GB2217862A - Offset printing plate latent image production

Info

Publication number: GB2217862A
Application number: GB8908837A
Authority: GB
Inventors: Rudolf Karl Uhrig
Original assignee: Heidelberger Druckmaschinen AG
Current assignee: Heidelberger Druckmaschinen AG
Priority date: 1988-04-21
Filing date: 1989-04-19
Publication date: 1989-11-01
Anticipated expiration: 2009-04-19
Also published as: GB2217862B; GB8908837D0; FR2630555A1; DE3813398C2; DE3813398A1; JPH0212253A

Description

DESCRIPTION

OFFSET PRINTING PLATE LATENT IMAGE PRODUCTION The invention relates to a process and apparatus for producing a latent image on a light-sensitive coating of an offset printing plate by means of a light beam, said light beam being emitted by a light source and being modulated by means of a light mask.

Conventional offset printing plates are exposed with a large quantity of light through a film that serves as a light mask and that is laid directly onto the lightsensitive layer of the printing plate.

Customary, on the one hand, are printing plates that are made ready for use by a "negative process", with such printing plates being made from sheet aluminium and having a light-sensitive coating onto which a film original, usually a silver-halide film negative, is laid, whereupon both are exposed in vacuum contact, for example with a metal-halide-doped mercury-vapour lamp. At the exposed places, there is a hardening of the coating as a result of a chemical reaction, and, after the non-exposed portions have been removed by means of a developer, the printing plate is ready for use for offset printing.

On the other hand, in the case of printing plates that are made ready for use by a "positive process", conversely, coatings of hydrophobic compounds are converted photolytically into more strongly hydrophilic compounds and the exposed parts of the coating are dissolved out by means of a developer.

11 n 22117801- According to another method, the-original image is projected or mirrored onto the printing plate on a selectable scale, with, once again, the point of departure being a film, possibly a microfilm, which may have been recorded by a computer. (Magazine: Druckindustrie (St. Gallen) 1987, 6 October, p. 116) In all cases, therefore, it is necessary to have a film, which has to be made in the form of a light mask prior to the production of the printing plate from the original.

The very manufacture of the films necessitates a timeconsuming process, which, moreover, is very exacting with regard to dust-free working environments. In addition, there is the likewise time-consuming mounting of individual films on a mounting foil.

The object of the invention, therefore, is to propose a process and an apparatus by which and with which, respectively, the procedure for the production of a light mask is simpl.ified and shortened.

The object of the invention is achieved by a process for producing a latent image on a light-sensitive coating of an offset printing plate by means of a light beam, said light beam being emitted by a light source and being modulated by means of a light mask, said process being characterized according to the invention in that a light-valve matrix is used as the light mask and the light-valve matrix is energized with signals that correlate with the image, as well as by apparatuses for the implementation of this process, said apparatuses being characterized, on the one hand, by a first liquidcrystal-layer unit, a computer-controlled writing beam, said writing beam being directed at a first side of said liquid-crystal- layer unit and transmitting impulses that Correlate with the image, by a mirror, said mirror reflecting light emitted by the light source onto-a second side, opposite the first side, of the liquidcrYstal-layer unit and by a lens system, said lens System imaging the second side of the liquidcrystallayer unit on the offset printing plate, and said apparatuses being characterized, on the other hand, by a second liquid-crystal-layer unit, said liquid-crystallayer unit being provided with an electrode matrix, with it being possible for electrical signals correlating with the image to be applied to the electrode matrix, by such a control of the light beam that said light beam strikes a first side of the second liquidcrystal-layer unit and by a lens system for imaging a portion of the light beam, said portion penetrating the second liquidcrystal-layer unit.

Although methods for the filmless production of printing plates suitable for offset printing have already been described in the relevant literature (magazine: "Der Druckepiegel", No. 8/81, p. 662 ff, and No. 9/81, p. 744 to 748, as well as magazine: "Philips technische Rundschau" No. 8, 1977/78, p. 214-224), these methods necessitate printing plates that have been specially adapted for such methods. With such a process, the full-page makeup of an original for printing can be projected directly onto a semiconductor layer of a printing plate. For this purpose, the semiconductor layer is charged in the dark, is exposed according to the imago by having the original projected onto it, and is thus discharged and is then toned. The exposure of such printing plates with a computer-controlled, pulsed light boam or laser beam is described in the magazine:

"Der Polygraph", No. 9/82, p. 675-682. In this process, use is made of the high sensitivity of the photoconductivity of the semiconductor layer, with the result that weak light sources in the long-wave range can be used, to which the coatings on printing plates produced by the initially mentioned "positive" or.. negative method" are insensitive. Variants of the method of exposure with pulsed light or laser beams are described in the magazine: "Angewandte Chemie", No. 2/80. P. 95-106.

Finally, DE-OS 32 48 178 describes a process for the image-coating of a printing plate for offset printing, in which, under the influence of heat and pressure, inkaccepting surface elements are transferred according to the image onto a hydrophilic surface of a printing plate.

The writing of an image onto a hydrophilic surface of a printing plate with oleophilic surface elements of a thermal ink ribbon can be effected by means of a thermal writing head or laser beam (LogEscan process) (magazine: "Angewandte Chemie", 92 (1980) p. 95-106, Vollmann: New Technologies for the Filmless Production of Printing Formes).

The process according to the invention not only simplifies and shortens the procedure for producing a light mask, but, in comparison with the known methods for the "filmless" exposure of an offset printing plate, it also has the advantage that it can also be used for the "filmless" exposure of those printing plates whose coatings have been designed for the initially mentioned.. positive" and "negative processes".

Q:

1 The signals for energizing the light-valve matrix may be obtained by scanning an original for printing. Furthermore, it is also possible to use as the "original for printing" an image that has been generated in a computer itself. Thus, with the process according-to the invention, for example, the image of a physically existing original - it being desired to produce said image on the printing plate - can be supplemented by means of additional "originals for printing" that have been generated in a computer.

The use, according to the invention, of a light-valve matrix as the light mask obviates, in particular, the need for a chemical process in the form of the developing of a film. Such a process always requires a certain reaction time, which can thus be saved. Likewise, there is no need to dry a developed film, or, as is generally necessary, to mount individual films on a mounting foil.

Moreover, the light-valve matrix represents a light mask that is extremely flexible in use and that can be used again and again.

The energization of the light-valve matrix with signals that correlate with the image can be effected with the aid of a computer.

In the following, the invention is described in greater detail with reference to the drawings, in which:

1 Fig. 1 shows a schematic representation of an apparatus according to the invention using a light mask in the form of a reflection mask; Fig. 2 shows a schematic representation of an apparatus according to the invention using a light mask in the form of a transmission mask.

By its nature, the process according to the invent-ion is such that "light-spots" of a display corresponding to the image, said "light spots" having been selected by means of the signals, are imaged onto the lightsensitive coating of an offset printing plate.

In the hereinafter described variants of the process according to the invention, use is made of the fact that liquid-crystal layers change their optical behaviour in a controllable manner.

Thus, in the utilization of this fact according to the invention, in a first variant shown in Fig. 1, it is proposed to form the light-valve matrix from a first type of liquid-crystal layers, to direct impulses from a computer-controlled writing beam 2, said impulses correlating with the image, onto one side of a thus formed first liquid-crystal-layer unit 1 and to use the opposite side of the liquid-crystal-layer unit 1 as a reflection mask for the light beam 3, said light beam 3 being emitted by a light source 4. In this connection, it is possible to use as the light source 4, for example, a metal-halide-doped mercury-vapour lamp. A cathode beam or a laser beam can be used as the writing beam 2. Fig. 1 shows a writing beam 2 in a random, instantaneous position.

It is possible to forego a detailed description of the liquid-crystallayer units used in the invention and their manner of energization, since appropriate information can be taken from the relevant literature (see for example: "SPSE - The Society for Imaging Science and Technology" 1987, p. 108-115). Details on the use of writing'beams can be taken from "Journal of Applied Physics", American Institute of Physics, Vol 57(4) dated 15 February 1985, pages 1356 ff.

Of importance in this connection with regard to the aforementioned first variant of the process according to the invention is the use of a first type of liquidcrystal layers in which the local application of a writing beam to a first side results locally in reflecting and non-reflecting areas for light that strikes the opposite side.

In a second variant, it is proposed to form the lightvalve matrix from a second type of liquid-crystal layers, to provide a thus formed second liquid-crystallayer unit 1' with an electrode matrix, to apply electrical signals 5 to the electrode matrix, said signals 5 correlating with the image, and to use the second liquid-crystal-layer unit 1' as a transmission mask for the light beam 3 (Fig. 2).

1 Of importance in this connection with regard to this second variant and deviating from the previously mentioned first variant - is the use of a second type of liquid-crystal layers in which the local application of electrical signals results locally in light-transmitting and non-lighttransmitting areas of the liquid-crystal layer. The local application of electrical signals 5 is shown schematically in Fig. 2 by the arrow that represents the signals.

With both the aforementioned processes, a display of the desired image in the form of light spots is produced on the respective liquid-crystallayer unit 1, 1' and this display is imaged onto the light-sensitive coating 6 of the offset printing plate 7.

In a further development of the invention, a lens system is provided for such imaging. According to a further embodiment of the invention, this lens system comprises a projector lens 9, said projector lens 9 being displaceable in and opposite to a direction of projection S. Thus, an image of the display of selectable size can be projected onto the coating 6.

Fig. I shows schematically an apparatus for implementing the process according to the invention using the aforementioned first type of liquidcrystal layers. With this apparatus, the light beam 3, emitted by the light source 4 and collected with the aid of a lens 11, is projected by means of a mirror 10 onto the side, facing away from the writing beam 2, of the first liquid-crystal lay6r unit 1.

In order to achieve as high a light efficiency as possible from the light beam 3, the mirror 10 is in the form of a beam divider and is arranged in such a manner that light from the light beam 3 reflected by said mirror 10 onto the liquid-crystal-layer unit 1 strikes the liquid-crystal-layer unit 1 basically perpendicularly.

Fig. 2 shows schematically an apparatus for implementing the process according to the invention using the aforementioned second type of liquidcrystal layers. In this connection, the light beam 3 is guided in such a Z Z Z.

9 - manner that, on its way to the offset printing plate 7, it penetrates the second liquid-crystal-layer unit 1' as a function of the local application of the electrical signals 5 to said liquid-crystal-layer unit V.

The process according to the invention can, as has been mentioned, be used for the exposure of conventional offset printing plates, for example those provided with a diazo or silver-halide coating. For this purpose, it is merely necessary to produce the light beam 3 with such a high- energy light source 4 that the necessary intensity of illumination is thus achieved on the respective offset printing plate.

However, the process according to the invention can also be used for the exposure of electrically charged semiconductor coatings on offset printing plates. For this purpose, it is merely necessary to use a light beam 3 with which the necessary, correspondingly lower intensity of illumination is achieved on the respective offset printing plate. This correspondingly lower intensity of illumination can also be obtai ned with an apparatus according to the invention desig ned for the exposure of conventional offset printing plates in that a filter - not shown in the drawing - is inserted into the light beam.

It will be understood that the invention has been described above purely by way of example, and that various modifications of detail can be made within the ambit of the invention.

- 10 LIST OF REFERENCE NUMBERS 1 First liquid-crystal-layer unit 1, Second liquid-crystal-layer uni 2 Writing beam 3 Light beam 4 Light source Electrical signals 6 Light-sensitive coating 7 offset printing plate 8 Direction of projection 9 Projector lens Mirror Lens z n 1 1

Claims

1. A process for producing a latent image on a light-sensitive coating of an offset printing plate, by means of a light beam, said light beam being emitted by a light source and being modulated by means of a light mask, wherein a light-valve matrix is used as the light mask, and the lightvalve matrix is energized with signals, said signals correlating with the image.

2. A process according to claim 1, wherein the lightvalve matrix is afforded by a liquid-crystal-layer unit, at one side of which are directed computer-controlled writing beam impulses correlating with the image, the opposite side of said liquid-crystal-layer unit being used as a reflection mask for the light beam.

3. A process according to claim 1, wherein the lightvalve matrix is afforded by a liquid-crystal-layer. unit, said liquid-crystal-layer unit being provided with an electrode matrix, electrical signals correlating with the image being applied to the electrode matrix, and the liquidcrystal-layer unit being used as a transmission mask for the light beam.

4. A process according to claim 1, 2 or 3, wherein the light beam is passed through a lens system.

5. A process according to claim 4, wherein use is made of a lens system which permits the size of the image produced on the offset printing plate to be varied.

6. A process according to any of claims 1 to 5, wherein the light beam is produced with a high-energy light source, this being a light source such that the light beam produced by it generates on the offset printing plate an intensity of illumination - 12 sufficient to provide, with a normal offset printing plate, this term comprehending plates Provided with a diazo or silver-halide coating, the coating being overlaid by a light mask in the form of a normal silver- halide film, an exposure such that a latent image in the silver-halide film is produced in the coating.

7. A process according to any of claims 1 to 5, wherein use is made of a light beam with which there is achieved on the offset printing plate an intensity of illumination that is sufficient for --he discharge, upon exposure to this light beam, of a semiconductor coating of the offset printing plate, said semiconductor coating having been electricaIly charged in the dark.

8. A process according to any of claims 1 to 5, wherein an intensity of illumination appropriate to the discharging of an electrically charged semiconductor coating of the offset printing plate is achieved, on the offset printing plate, by inserting a filter into a light beam.

9. An apparatus suitable for use in producing a latent image on a lightsensitive coating of an offset printing plate by means of a light beam, said light -beam being emitted by a light source and being modulated by means of a light mask, comprising a liquidcrystal-layer unit, a computer-controlled writing beam, said writing beam being directed at a first side of said liquid-crystal-layer unit and transmitting z 1 11 impulses that correlate with the image, a mirror, said mirror projecting light emitted by the light source on to a second side, opposite to the first side, of the liquid-crystal-layer unit, and a lens system, said lens system imaging the second side of the liquidcrystal-layer unit on the offset printing plate.

10. An apparatus according to claim 9, wherein the mirror is in the nature of a beam divider, and light -from the light beam, Said light being reflected by the mirror on to the liquid-crystal-layer unit, strikes the liquid-crystal-layer unit substantially perpendicularly.

11. An apparatus suitable for use in producing a latent image on a li-ghtsensitiv-e coating of an offset printing plate by means of a light beam, said light beam being emitted by a light source and being modulated by means of a light mask, comprising a liquid-crystal-layer unit, said liquid-crystal-layer unit being provided with an electrode matrix, it being possible for electrical signals that correlate with the image to be applied to the electrode matrix, means for the control of the light beam in such a sense as to direct that said beam on to a first side of the liquid-crystal-layer unit, and a lens system for imaging a portion of the light beam, said portion being the portion passing through the liquidcrystallayer unit.

12. A process according to claim 1, substantially as described with reference to Figure 1 or 2 of the accompanying drawings.

14 - 13. An apparatus according to claim 9, substantiallly as described with reference to Figure 1 of the accompanying drawings.

14. An apparatus according to claim 11, substantially as described with reference to Figure 2 of the accompanying drawings.

PubUshed 1989 atThefttentOfnoe. State House, 66.71 R'9b HolbornLOn40n W01R 4TP-71'rLherooples maybe obt tainedfroM Sales Branch, St 3LLry C-.sy, Orpington, Xent BR5 3RD. Pr1nted by MultlPlex twbnlQues Itd, St MW7 Cray, Kent, -To2n'- PIA&Bt7entOM-Oe.