EP1568490B1 - Method for the direct engraving of cups for receiving the printing ink in intaglio printing - Google Patents

Abstract

The method involves direct engraving of cells (10) for the admission of printing ink in the surface of pneumatic cylinders under low pressure. The cells are formed from a pre-determined number of engraved pixels (11). The engraving depth of the pixels can be modified during the expansion (14) of the two dimensional element (15) of the pixel (13). The engraving depth can be specified in gradated depth steps in a design from 1 to 8 bits.

Description

The invention relates to a method for direct engraving of cups for receiving printing inks in the surface of gravure printing cylinders according to the preamble of claim 1.

In contrast to offset printing, in which the conversion of all information of an image or text to be printed by means of a printing machine is necessarily done by area elements, one speaks here of area variability or autotype, since the offset plate or offset roller on its surface only pasteurized colors uniform color layer thickness can be used in gravure printing because of the very liquid colors used there, the formation of wells in the printing cylinder is required, each well transfers a precisely defined color volume during printing. In offset printing, the entire tonal variability is achieved by varying the areas, using autotypic pressure grid points. In intaglio printing, it is not necessary for the generation of halftones, which are formed in the interaction of 1 color point with the paper white surrounding him within the grid mesh paper, a brightness impression, a digitization of halftone images in the form of perform "ripped" data, because the gravure used in gravure, for example, a diamond or a laser engraving agent, which has very advantageous property to be able to be controlled directly with gray scale corresponding data.

For example, the US-A-4 013 831 a method of producing printing forms by mechanical engraving, in which individual cups are engraved with a diamond stylus in the printing plate surface, wherein the diamond stylus is driven directly with the halftone data of the respective einzuziehravierenden well to dig this deeper or less deep from the printing plate. In the known method, however, the size of the wells is directly related to the depth of the wells, since it is not possible with a diamond stylus to simultaneously engrave flat and large and small and deep wells.

From the DE-A-21 11 628 It is further known in a method for producing printing forms by laser engraving to control the volume of the wells engraved in the printing form by changing the intensity distribution of a laser beam used for engraving. Although the brightness values of the engraved cells can thus be controlled to a certain extent by the depth or shape of the cells, the intensity distribution of a single laser beam can not be changed in such a way that arbitrary depth and area variable cells can be engraved into the printing plate. For example, the known method does not allow the formation of part cups which are engraved only adjacent to one of their edges in the printing form.

Therefore, a major problem of intaglio printing is still that a sharp reproduction of contours is not yet possible to the extent that it would actually be desirable and how offset printing, which has other drawbacks, in itself makes possible.

A variant of the implementation of the information of the image to be printed on the printing cylinder in gravure printing is the so-called. Laser mask exposure of a previously applied to the printing cylinder thin thermal or photosensitive mask layer followed by etching. The mask is removed by means of a fine laser beam or it is exposed at the locations where one or no wells are to be etched. Subsequently, the cleaning in the case of a thermal mask or the development and washing out in the case of a photosensitive mask is carried out on the mask surface and then the etching of the exposed elements of the copper surface then takes place.

Although by means of the above-described laser mask exposure incl. The etching, the formation of sharper contours in edges or fonts is possible, also in this technique, the color must be enclosed in wells. However, in contrast to the electromechanical engraving Teilnäpfchen can be formed, whereby sharper contours can be achieved that are not with a so-called. "Sawtooth effect", for example, diamond-engraved fonts are afflicted.

However, since the laser beam can only be used so that it either removes or leaves the mask, i. Only a yes-no-process control by means of the laser is possible, the wells in the mask layer can also be applied only surface variable, i. they can vary only in the number and arrangement of the surface elements composing them. This means that a different depth variation of the individual wells is not possible because all wells are etched the same depth over the etching process.

From the WO-A - 89/12256 a method for the production of deep and variable area gravure forms is known in which an applied to the printing plate etch resist or Kopierlack protective layer with a high energy beam, for example a laser beam or an electron beam, is wholly or partially removed over the wells to be engraved, and in which the shape of the etched after the removal of the Ätzresist- or Kopierlack protective layer from the printing plate wells is controlled by over leaving at least a punctiform etch resist or copy resist protective layer member on a portion of the wells, thereby reducing the depth of etch beneath the protective layer member. Although limited depth variation of the wells is possible with this method, it is very severely limited by the fact that relatively wide circumferential gaps must be created around the protective layer elements so that sufficient etching liquid reaches the printing form surface during the subsequent etching process, rather than the material of the printing form wegzuätzen only in the gaps but also below the protective layer elements.

Thus, for the method according to the above-described laser mask exposure, after halftone data generated by scanning or digital photography of an image to be printed has been generated, it is necessary to recode that halftone data, that is, the gray value (brightness impression) that a screen mesh prints should be generated and the example. In 255 stages ( Δ 1 byte) to be resolved, must be represented as a variable-area grid point whose elements - the pixels - are only bit-coded. This means that in fact there are no more halftones, but only pixels (surface elements) that are written or not. If a yes-no information can only be written in the printing cylinder in the manner described above, a "halftone image" (half tone image) must be generated in which the desired gray value is converted into a bit structure for the respective screen or printing mesh ( Bitmap) is implemented.

This process, which is carried out by a so-called raster image processor (RIP), is referred to in printing technology in the strict sense as "screening".

For offset and flexo printing, the so-called "rip," i. the conversion of the gray value in area-variable halftone dots, always required, which applies equally to the laser mask coating, which in itself belongs to the group of intaglio printing technology. By means of an electro-mechanical engraving process, for example by means of a diamond engraving tool, as is done in the formation of wells in gravure cylinders, the "rippling", as I said, is not necessary because this produces the well in one go, and directly with gray values (continuous tone data, English "continuous tone data") can be controlled.

However, the gravure based on the laser mask coating also has certain disadvantages, because in the mask, as mentioned, can be written only surface variable and the third dimension, i. the cup depth, is the same for all cup sizes and is formed by the same process, namely the etching. It has been found that this method also leads to quality limitations in the printed reproduction of the image. In addition, the repeatability of the etching is subject to restrictions and it is shown in the reproduction of halftone gradients that in particular in the range of high brightness (light) no smooth Tonwertverlauf is achieved.

It is therefore an object of the present invention to provide a gravure printing method, with the disadvantages of the gravure printing method by the method of laser mask exposure incl. The subsequent etching are eliminated, with a much better reproduction of halftone gradients, especially in the field of lights, is further improved, which does not limit the repeatability of the process and eliminates the many process steps required in the laser mask coating and the subsequent etching of the printing form, and otherwise preserves all the advantages of the conventional engraving process for forming the wells on the printing cylinders and that the method by means of per se in the prior art known engraving devices can be performed for these purposes.

The object is achieved according to the invention in that the engraving process is carried out such that at least a portion of the wells comprises a plurality of engraved pixels.

By means of the method according to the invention, as in laser mask exposure, a fine engraving tool is guided over the entire surface of the printing form to be engraved in a regular pattern such that the cells can be formed from the predetermined number of pixels by engraving the pixels. This writing grid is finer than the selected gravure grid containing the volume-variable gravure wells.

By means of the method according to the invention, unlike the method according to the laser mask coating, as in engraving with the aid of conventional engraving technology for intaglio printing, the gravure printing heads are engraved directly into the engraving-carrying surface of the printing cylinder. The complex process steps according to the method after the laser mask exposure and the subsequent etching and cleaning are completely eliminated in the inventive method, wherein according to the invention excellent sharpness of the image to be printed with respect to the actual image and all contours is achieved. According to the invention In addition, the method can in principle be carried out by means of existing engraving systems for engraving printing cylinders for intaglio printing, so that the method can also be used with existing engraving devices already in use.

According to an advantageous embodiment of the invention, the engraving depth of the pixels is differently formable, that is, the pixel volumes and thus the cell volumes can be varied by different selected engraving depth of the individual pixels.

The engraving depth of the pixels may preferably be formed differently for the same extent of the surface element of the pixel, that is, according to the invention, the engraving depth of the pixel is no longer correlated with the two-dimensional extent of the surface element of the pixel. It follows that the data that drive the engraving tool used in the method according to the invention, not as in the laser mask exposure, in a so-called. "Bitmap" are encoded. Rather, it is possible according to the invention that each individual pixel should be engraved deeply in a predetermined manner.

The engraving depth can preferably be determined in stepped depth steps, but it is also possible in principle for the engraving depth to be defined or controlled by analogous control of the engraving tool.

Preferably, the depth steps are resolved with either 1 to 8 bits, preferably 8 bits, so that a maximum of 255 different depths can be engraved. But it is also possible to encode the depth resolution with only 2 bits, so that, for example, four depth positions possible are like full depth, medium depth, small depth and no engraving. But there are other depth patterns than 2 bit possible.

The engraving width per grid mesh can also be built up step by step. Thus, it may be advantageous for a well to consist of at least one pixel and to consist of a maximum of 255 pixels, that is, the pattern of pixels forming the well may increase from 1 to 255 pixels.

Also, the two-dimensional surface element of the pixel is preferably variably adjustable with respect to its areal extent, the areal extent advantageously being in the range of 10 to 20 μm.

The size of the areal extent of the area element of the pixel, i.e., the size of the area of the pixel on the surface of the printing form to be engraved, can be chosen depending on the engraving medium used for the engraving process.

The invention advantageously makes it possible for the engraving process to be carried out in such a way that partial cells can be formed from a predetermined number of pixels, so that in particular contours of text and images can be reproduced extremely sharply in the subsequent printing. The formation of Teilnäpfchen was previously possible only in the method according to the method of laser mask exposure in limited form or in offset printing with all the usual in these known printing method prevailing disadvantages.

Basically, it is possible as a means of performing the engraving of the pixels, in their entirety again form a cup to use any suitable engraving or engraving tools. These are preferably the classical electro-mechanical engraving, for example. A very fine diamond tool, but advantageously as an engraver laser light, in particular pulsed laser light used. In this case, the laser light can have any suitable cross-sectional shape per se by means of suitable lens systems and suitable diaphragm systems, so that the pixels can be suitably formed in accordance with the cross-sectional shape of the laser beam.

Fig. 1

vier jeweils in einer Rastermasche beispielhaft angeordnete Näpfchen unterschiedlichen Volumens, wie sie bisher nach der Methode des klassischen Gravierverfahrens von Druckzylindern für den Tiefdruck mittels einer elektromagnetischen Gravur unter Benutzung eines Diamantgravurwerkzeugs ausgebildet werden,

Fig. 2a

eine Mehrzahl von Rastermaschen im Bereich des Überganges von Kontur zu Bild, erzeugt durch ein Verfahren nach der Methode der Laser-Maskenbelichtung und anschließender Ätzung,

Fig. 2b

ein graphisches Element, aufgebaut aus einer Vielzahl von Rastermaschen,

Fig. 3

einen flächenvariablen Rasterpunkt (autotypischer Rasterpunkt) in einer Tiefdruckrastermasche, realisiert nach der Methode der Laser-Maskenbelichtung, und schematisch zugeordneter Bildlinie eines Laserstrahls zur Erzeugung der Bildpunkte,

Fig. 4

ein Beispiel einer Tiefdruck-Rastermasche gem. dem erfindungsgemäßen Verfahren erzeugt, mit einem Näpfchen, das beispielhaft aus 56 Bildpunkten zusammengesetzt ist, in unterschiedlicher Tiefe graviert,

Fig. 5a

eine Rastermasche am Rande einer Kontur, gem. dem erfindungsgemäßen Verfahren ausgebildet, und

Fig. 5b

eine Darstellung graphischen Elementes, wobei dessen Kontur aus einer Mehrzahl von entsprechend ausgebildeten Rastermaschen gem. Fig. 5a im Randbereich zusammengesetzt ist.

The invention will now be described in detail by way of example with reference to the following schematic drawings. Show:

Fig. 1

four each arranged in a grid mesh wells of different volume, as they are previously formed by the method of classical engraving of pressure cylinders for gravure by means of an electromagnetic engraving using a diamond engraving tool,

Fig. 2a

a plurality of raster meshes in the region of the transition from contour to image, produced by a method according to the method of laser mask exposure and subsequent etching,

Fig. 2b

a graphic element composed of a plurality of raster meshes,

Fig. 3

a surface-variable halftone dot (autotypic halftone dot) in a gravure grid, realized by the method of laser mask exposure, and schematically assigned Image line of a laser beam for generating the pixels,

Fig. 4

an example of a gravure raster mesh gem. produced by the method according to the invention, with a cup, which is composed of 56 pixels by way of example, engraved in different depths,

Fig. 5a

a grid on the edge of a contour, acc. formed the method according to the invention, and

Fig. 5b

a representation of graphic element, wherein the contour of a plurality of correspondingly formed raster mesh gem. Fig. 5a is assembled in the edge region.

For a better understanding of the method according to the invention reference is first made to Fig. 1, in which the principle of the formation of wells in a printing cylinder using a known in the art, conventional engraving is shown. From the entire printing cylinder, for example, a small area is selected, which consists of 4 different sized wells. The actual engraving agent is normally a diamond shaped like an engraving stylus. This known engraving method for printing cylinders used in gravure printing is generally referred to as electromagnetic engraving and has long been known in the art, so that need not be further discussed here on the description of the control of the engraving to form the wells.

A specific feature of the wells produced by the electromagnetic engraving is that due to the depth of the solid geometry of the engraving stylus and the diameter, ie the extent 14, in Fig. 1 by way of example with respect to the diagonal of a cup 10, to the engraving depth 12 in a certain fixed ratio. The printing form obtained by electromagnetic engraving is called semi-autotypic intaglio printing, since the cups 10 are variable in their extent 14 and in their engraving depth 12, but always in a fixed relationship to each other, the only by differently shaped engraving tools, eg. By a Different cut a diamond stylus, can be changed, but is always fixed during the actual engraving process.

Another specific feature of the electromechanical engraving is also that due to the thin liquid ink used in gravure full-tone contours (text and line contents) are reproduced in principle the same print grid of wells as the images (image contents), in the selected for each color print grid , As a result, a certain blurring of contours (text and line content) is achieved in gravure printing.

In the case of the formation of wells on gravure printing cylinders according to the method of laser mask exposure and the subsequent etching, this method or this method is also known to the experts for a long time, it is possible, cf. Figs. 2a and 2b, in contrast to the electromechanical engraving, form Teilnäpfchen 15, which is true both for the area of the contour 18 and for the image. In the method of laser mask exposure and subsequent etching sharper contours 18 are achieved with respect to the achieved with the electromechanical engraving of wells 10 contours 18, not with the relatively strong in Appearance occurring "sawtooth effect", for example, by means of diamond styli engraved contours 18 (text and line contents) are afflicted.

FIG. 3 shows, by way of example, a surface-variable halftone dot 20 which is formed by means of the laser mask exposure method and is formed from a plurality of pixels, said dots forming in their entirety the said cup 10. Since the laser light writing beam 19 can only say "yes" or "no" in this known method, that is, can remove or leave the mask previously applied to the printing cylinder, the thus formed well can only be applied variable in area. The engraving depth 12 is generated in common for all cups 10 together via the etching process.

A in a grid 17 inventively designed cup 10 is shown schematically in Fig. 4. In this case, the method is carried out such that by means of the engraving process, the wells 10 by a suitable engraving, which may be an electromechanical engraving but also laser light, were formed from a predetermined number of pixels 11, ie, the actual well 10 is active in the engraving of a predetermined number of pixels 11, also called pixels.

The method can be carried out so that each cup 10, depending on the predetermined tonal values of the image to be printed, can be composed of any number of pixels 11, wherein a cup 10 consists of at least one pixel 11 and a maximum of the number of pixels 11, which are desired due to the program-related screening, for example, 255 pixels.

In many cases, a raster mesh is resolved in 255 steps corresponding to 1 byte in gravure printing, so that the formation of the wells 10 by 255 different patterns of pixels 11 represents a meaningful number or relationship.

4, the engraving depths 12 of the centrally located pixels 11, which appear darkest, are the deepest, the pixels 11 surrounding the central region 11 are, on the other hand, less deeply formed, and the outer, surrounding pixels are formed to an even smaller depth , Here, for example, formed in the form of a predetermined fixed depth raster, for example, represented by 2 bits. For example, full depth, average depth, shallow depth, and no engraving could follow bit code 11, 10, 01, and 00. It should be noted, however, that also preferably finer depth levels than with 2 bit can be implemented feasible, such. an 8 bit deep depth graduation.

In the example of the method procedure presented above, 255 pixels 11 and an engraving depth 12 (see Fig. 1) dissolved in 2 bits correspond to more than 1000 achievable values with which a well 10 can be formed according to the method.

In principle, this high number of more than 1000 adjustable values of the pixels 11 per well 10 represents an over-determination, but this is canceled out in printing practice because certain pixels 11 of the pixel pattern are taken back in their engraving depth 12 from an engraving and printing point of view so that a smooth expression of gradients is possible. This reduction of the geometric depth in certain areas is developed experimentally or empirically and stored in a so-called "folder for cup configurations".

As already indicated, by means of the method according to the invention, in particular the region of the contour 18, cf. FIGS. 5a and 5b are formed in such a way that, by means of the method according to the invention, it is also possible to form partial cells 15 from a predetermined number of pixels 11. This ensures that contours 18 can be reproduced as sharply as it was previously possible only in offset printing, but with all the advantages of gravure over offset printing.

The extension 14 of the surface element 13 of the pixel 11, cf. 1, which represents the formation of wells 10 by means of conventional electromechanical engraving, can also be variably adjustable and can be made dependent on the actual engraving means, which can advantageously, but not necessarily, be laser light 16 (FIG. 3). If the engraving medium is used in the form of laser light 16, it is in principle possible, suitably controlled, to vary the extent 14 of the surface element 13 of the pixel 11 even during the engraving process by suitable control of the laser, whereby an engraving with yet another, controllable or influenceable degree of freedom is possible.

In practice, the printing density represented by a screen mesh of the engraving screen, wherein the screen has, for example, a resolution of 70 lines / cm, interpolated from a halftone image with a higher resolution, eg 120 lines / cm, so that additional information is present, namely how the density is distributed approximately within a Gravurrastermasche. This information can be used for targeted relocation of the entire
Pattern of the pixels 11, so the composite of these cells 10, are used.

In the case of print originals where the image data are not in high resolution, for example not with 1000 lines / cm but only with 120 lines / cm, a very great improvement in the sharpness of the intaglio is achieved, in particular in the contour region, by means of the method according to the invention.

It can thus be used according to the invention all parameters contained in the image data for the engraving process.

LIST OF REFERENCE NUMBERS

10

wells

11

Pixel (pixel)

12

engraving

13

Surface element of the pixel

14

Expansion of the surface element

15

Teilnäpfchen

16

laser light

17

raster mesh

18

contour

19

Laser light / write beam

20

area-variable grid point

Claims (12)

1. Method for directly engraving cells (10) for receiving printing ink in the surface of printing cylinders intended for gravure printing wherein the engraving is executed in such a way that the cells (10) are formed from a predetermined number of engraved image dots (11), characterized in that at least a part of the cells (10) comprises a plurality of individually engraved image dots (11).
2. Method according to Claim 1, characterized in that the image dots (11) can be engraved to different depths (12).
3. Method according to Claim 2, characterized in that the image dots (11) can be engraved to different depths (12) for a given extent (14) of the surface element (15) of the image dot (13).
4. Method according to either or both of Claims 2 and 3, characterized in that the engraving depth (12) is set in graduated steps.
5. Method according to Claim 4, characterized in that the depth graduations lie in a binary pattern of optionally 1 to 8 bit.
6. Method according to Claim 5, characterized in that the binary pattern is 2 bit.
7. Method according to one or more of Claims 1 to 6, characterized in that one cell (10) consists of a maximum of 255 image dots (11).
8. Method according to one or more of Claims 1 to 7, characterized in that the surface element (13) of the image dot (11) is variable in its two-dimensional extent (14).
9. Method according to Claim 8, characterized in that the two-dimensional extent (14) lies in the range of 10 to 20 µm.
10. Method according to one or more of Claims 1 to 9, characterized in that the engraving is executed in such a way that part-cells (15) are formed from a predetermined number of image dots (11).
11. Method according to one or more of Claims 1 to 10, characterized in that the means for executing the engraving of the image dots (11) is constituted by an electro-mechanical engraving means.
12. Method according to one or more of Claims 1 to 10, characterized in that the means for executing the engraving of the image dots (11) is by constituted by laser light (16).

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