Gravure plate and gravure process for full printing of large surfaces

IS 2 188 246 T3

DESCRIPTION

Etching plate and etching process for complete large-area printing.

The present invention relates to a printing plate for the complete printing of large areas by means of a gravure printing process, to a method for producing the printing plate, and to a data carrier element for large format printed images produced. through the intaglio printing process.

In line gravure, planar representations are known to be produced by closely etched adjacent lines, the individual etched lines usually being fractions of a millimeter wide and spaced apart by unengraved surfaces.

For the printing operation, the engraved lines of the printing plate are filled with ink. The excess ink is removed from the printing plate with the help of a cleaning cylinder or a knife that acts in tangent arrangement, in such a way that the engraved lines are filled with ink up to the edge. The surfaces that separate the etched lines are cleaned in this operation at the same time.

Finally, during printing, the data-bearing element to be printed, generally paper, is pressed towards the printing plate under high pressure by means of a pressure cylinder having an elastic surface. In this way, the data-bearing element is pressed towards the ink-filled engraved edges of the printing plate, thereby establishing contact with the ink. When the data-bearing element is detached, it passes the ink out of the depressions of the etched lones. The resulting printed image has printed lines that vary in the thickness of the ink layer depending on the depth of the engraving.

If translucent inks are used in gravure lone, light tones are obtained when printing a white data carrier with low ink layer thicknesses, and dark tones when printing with thick ink layers.

Compared to other common printing processes, the intaglio printing process can produce images printed with a very large ink layer thickness. The resulting printed images are even manually perceptible if the engravings are deep enough. However, according to the above, by using fine engravings, extremely fine and sharp printed lones can also be obtained.

Although the intaglio printing process can produce very high quality printed images, resolved in lonely structures, it has the disadvantage of not being able to produce large continuous print areas, i.e. lonely ones with a width of about one millimeter or more. maos. This is because when the inked printing plate is cleaned, not only is the excess ink in the large format engraving area removed, but ink is also removed from the engraving. This decreases the ink surface below the level of the printing plate surface in such etched areas. Since the paper pressed into the etched areas of the printing plate does not reach the ink surface everywhere, gaps are produced in the printed image that render the print unusable.

The problem of the present invention is therefore to provide measures that allow areas of large format printed images to be fully printed by means of the intaglio printing process to produce a uniform color effect for the viewer.

This problem is solved according to the invention by means of the features of the independent claims. The developments will be found in the subclaims.

The invention starts from the discovery that the ink can be prevented from being cleaned from the engraving area, when the cylinder or plate for printing is cleaned by arranging so-called separating surfaces or partitions in the engraving, which prevent o they minimize the action of the cleaning cylinder on the ink incorporated in the printing plate engraving. It can be assumed that the excess ink wave propelled on the surface of the printing plate by the cleaning cylinder during cleaning, draws parts of the ink out of the engraving, also for the reason of hydrodynamic effects. The separating reliefs apparently prevent the ink in the engraving from being displaced with the full volume and entrained with the ink wave from the cleaning cylinder. The separating reliefs thus divide a large-format engraving into contiguous "chambers" or channels, which allow the ink to be drawn outward in a direction perpendicular to the surface of the printing plate during printing, but not during cleaning. direction parallel to the build plate surface.

The spacer reliefs are preferably arranged transversely to the direction of rotation of the impression cylinder. In this arrangement, they apparently cause a slip of the ink wave during the cleaning process, and therefore, on the surface of the printing plate, a hydrodinaomic decoupling of ink occurs in the engraving from the cleaning process.

In cases where it is not possible to arrange the separating reliefs transversely to the cleaning direction, the separating reliefs perform at least one division of the large-format engravings, giving them a similar function with respect to the cleaning of ink as it exists. in fine structure engravings.

Taking into account the basic inventive idea of ​​an optimized shape, it is preferable that the etched areas are equipped with spacer reliefs transversely to the cleaning direction. For etched lines extending along the cleaning direction, a division of the etched lines into adjacent partial areas occurs. Engravings that extend transversely or diagonally across

The cleaning direction is divided at least in the longitudinal direction of the engraved line, preferably the separating reliefs extending parallel to the engraved edges.

In cases where the engraving does not only consist of very wide engraved lines but also contains large-format engraved elements that have similar extensions in the x and y directions, it is also possible to make the separating reliefs like a screen, i.e. to provide separating reliefs. that intersect, extending, for example longitudinally or transversely with respect to the cleaning direction. It is also possible to provide spacer reliefs in the form of concentric circles in the shape of alveoli or the like. A formation of spacer ridges of this type not only has the advantage of guaranteeing in any case the function of the spacer ridges regardless of the cleaning direction, but also ensures that the spacer ridges have increased mechanical stability.

Introducing the separating reliefs in gravure printing plate engraving in a novel way already highlights in a special way the advantages of the width of an engraved line greater than 0.5 millimeters. It is proven that for engraved lines with a width of a millimeter and more, it is almost imperative.

The height of the spacer ridges can vary over a relatively large amount of time, as tests have shown. If the spacer reliefs end at the surface level of the printing plate, it can be assured that the spacer relief acquires sharp cradle-shaped shapes, seen in cross section. On the one hand, this ensures that the engraving is divided into separate channels or chambers in a seventh constitution, while, on the other hand, the sharp edge separating reliefs do not cause interruption of the printing area.

If the upper edges of the spacer relief are lowered below the level of the surface of the printing plate, the cross-sectional shape of the spacer reliefs can deviate almost at will from the cradle shape, i.e. trapezoid, rounded. or a different way. Since the upper edges of the separating relief are always arranged below the level of the surface of the printing plate in this case, and thus always covered with ink, the production of a continuous printing area is ensured in any case. .

It turns out that when separating reliefs whose upper edges end exactly on the level of the printing plate surface are used, the surface of the cleaning cylinder wears relatively quickly. Decreasing the top edge of the spacer relief by at least 2 microns to 5 microns eliminates this problem. For this reason, a minimal decrease of this type is recommended in any case.

Furthermore, tests have shown that a much greater taper of the upper edges of the spacer relief is also possible. Accordingly, a decrease of up to about 50% below the level of the surface of the printing plate is possible, based on the depth of the engraving.

It also turns out that if the separating reliefs have a height (also referred to below as amplitude) based on the depth of the engraving of more than 50%, they can cause "notches" in the surface of the ink layer on the printing area produced. in this way. Although the printing area produced with such a large format engraving is continuously printed with ink, it therefore has a surface relief caused by the separating reliefs. The surface relief is pronounced in a special way if the amplitude of the separating relief is selected in the range of 75% to 100% of the engraving depth. With smaller amplitudes, for example in the range of about 60%, this surface relief always becomes weaker, eventually disappearing completely with an amplitude of about 50%. Below the 50% value, one can increasingly expect printing errors in the form of gaps or jumps that render the printing useless, particularly with deeper engravings.

Tests have finally shown that etching depths from 5 microns to around 150 microns are excellent to use according to the invention. The preferred depth of etching found for the production of typical printed images was in the range of 10 microns to 60 microns. Using common intaglio printing inks, ink layers with a fairly translucent color effect can be obtained in this way, and even slight changes in the depth of the engraving easily lead to visible changes in tone. Etchings with a depth in the range of about 60 microns to 100 microns are particularly suitable for printing layers of ink with a saturated, opaque color effect. The exact values ​​vary of course depending on whether a light or dark color is involved.

Engravings to a depth of 100 microns or more are particularly well suited for producing ink layered structures with an easily noticeable relief to the touch.

The finer the fine structure of the printed area represented by the surface relief, the less it stands out when viewed without aids (magnification lens). This applies at least to fine structures that are produced from spacer reliefs with a spacing of about 20 microns to 150 microns and shaped like a cradle. Separating reliefs with a distance of 150 microns to around 400 microns, are already recognizable with the naked eye, but do not affect in any way the overall flat impression of the printed color area. If a trapezoidal cross-sectional profile is used instead of cradle-shaped spacer reliefs, the notches in the surface relief become wider, that is, more extensive. Structures of this type allow a creative influence on the area to be printed, since, for example, the screen

ES 2 188 246 T3 made up of the separating reliefs also appears as a display element. If the separating reliefs are not worked within the engraving as a screen, but in the form of characters, graphic symbols or the like, these characters or graphic symbols are also recognizable in the printed area.

If the distance of the spacer reliefs is widened clearly above 500 microns, the above-mentioned printing errors increasingly occur in the form of ink gaps, skips, smudges or the like.

Considering that the production of printing plates for intaglio is already one of the most elaborate methods for producing printing plates, it is easy to see that further disclosure of spacer reliefs in engraving achieves considerable additional problems. This maintains everything, since not only the shape, width and arrangement of the spacer reliefs are necessary for the inventive function, but also the precision in the micron range. Impression plates of this type cannot be produced manually or by etching. The printing plates and impressions of the invention thus ensure a high measure of protection against forgery and imitation.

However, printing plates of this type can be produced by means of an engraving apparatus from the applicant, as described in WO 97/48555. This apparatus makes it possible to mill intaglio printing plates by computer control. The lines of a two-dimensional line workstation are detected by means of a computer and the area of ​​each individual line exactly defined. Using an engraving tool, for example a rotary chisel or laser beam, the outer contour of the contour of these areas is first etched to outline the area cleanly. Subsequently, the area of ​​the margin of the aerea is cleaned using the same or another engraving tool, so that the entire line is recorded exactly according to the original line. Depending on the nature and control of the engraving tool, both a certain roughness (rather than a smooth surface) can be produced on the basis of the engraving, and the spacer reliefs of the invention with any desired amplitude, different lateral angles. or shapes given in cross section accurately. The important issue, as mentioned above, is that the spacer reliefs have a minimum width of about 50% of the engraving depth for the function of the invention. If this value clearly decreases, the ink adheres to the engraving base better than with a smooth engraving base, but the printing errors mentioned above are unavoidable with large-format engraving elements.

The invention offers completely new design possibilities for intaglio printing plates. By using engraving printing on large areas, it is now possible to produce engraving lines with a thickness of 1 to 10 millimeters and more, with ink layer thicknesses of 40 microns and more. It is also possible to make continuous geoetric areas with a measurement of a few square centimeters per gravure line without problems.

The fine structure of the printing area can be present both in the form of a screen and in the form of graphic characters or symbols. Even if the fine structure is selected or coarser (with a spacer relief distance of the order of magnitude of 500 microns), it cannot be imitated with any known printing process, which considerably increases the resistance to counterfeiting of the elements. printed data carriers, in accordance with the above. In this way, the fine structure evidences not only the use of the intaglio printing process, which is already of high quality by itself, but also the use of the engraving apparatus described in patent WO 97/48555, which is not available. available for any counterfeiting due to its high cost.

Additional advantages may appear from the description of the following embodiments.

Figures 1 to 7 each show details of a printing plate with an engraving in cross section.

Figure 1 shows a detail of a printing plate (1), whose surface (2) was provided with an engraving (3) with a given depth (t) that is used to receive ink. The engravings shown in the cross section extend in a linear manner, perpendicular to the plane of the paper, and are constituted in such a way that there are separating reliefs (4) between parallel depressions, the upper edges (5) of the separating relief being at the level of the surface (2) of the printing plate. On the one hand, the separating reliefs (4) prevent the ink from coming out of the depressions formed by the engraving (3), and on the other hand, they cause a structuring of the layer of ink transferred to a substrate. The substrate is printed with ink over the entire area in the region of the engraving.

The displacement with which the parallel engravings (3) are produced corresponds to the distance (d) of the edges (5) of the upper part of the separating relief. In the case shown in Figures 1 to 3, in which the displacement of the engraving tool during engraving of the depressions (3) corresponds to the distance (d) of the edges of the upper part of the separating relief, the distance (d) is preferably in the range of 20 to 150 microns, with a distance of about 50 microns being especially preferred for the production of fine structures that cannot be recognized without assistance.

The modulation of the thickness of the ink layer generated by the separating reliefs produces a fine structure in the printed ink layer that is not visible to the naked eye under normal observation, and therefore can serve as a hidden security feature that does not It can be reproduced either by electrophotographic processes or by other printing processes.

Despite the fine structure of the printed ink layer, a homogeneous color effect is produced to the human eye. The intensity of the perceived color effect or tone depends on the

ES 2 188 246 T3 average thickness of the ink layer and can be adjusted by means of the engraving depth (t) according to a given lateral angle α.

Figure 2 shows a cross section of a printing plate for printing a generally thinner ink layer producing a lighter shade. The etched areas of the printing plates shown in Figures 1 and 2 are equally large, and the etchings (3) have the same lateral angle α. Due to the lower engraving depth (t) in figure 2, a smaller distance (d) is selected for the separation of the engraving lines. For the printing of contiguous areas of color it is essential that the engraving depth (t) and the distance (d) of the upper edges (5) of the separating relief are selected in consideration with the lateral angle α in such a way that they do not originate. Plateau-shaped flat plates at surface level (2) of the printing plate with the etching area.

In figure 3 the engraving area has the same extension as in the examples of figures 1 and 2. The engraving depth (t) is the same as in figure 1. Although the separating reliefs (4) have a lateral angle Different β, a layer of ink transferred with a printing plate according to figure 3 has the same thickness of the main layer as one printed with a printing plate according to figure

1. Despite the different distance (d) of the separating reliefs (5), and therefore different fine structures, the areas with the same shade are printed with the printing plates of Figures 1 and 3.

However, the printing plates according to figures 2 and 3 have the same distance (d) from the separating relief, and in this way produce a fine structure with the same periodicity, but directed to ink layers with medium thicknesses and tones. different, due to the different lateral angles (α, β).

The engravings (3) are preferably produced with a rotating chisel whose point angle, measured from the center line of the chisel, corresponds to the lateral angle of the engraving. The lateral angles are preferably in the range of 15 ° to 60 °, being particularly in the preferred range of 30 ° to 50 °. Mechanic engraving tools have increased their life especially with preferred point angles. The printing plates with the preferred side angles can be duplicated more easily by means of molding techniques and also have especially favorable printing properties. The preferred constitutions of the separating relief (in cross section) are cradle-shaped geometries. However, any other geometry can be used, especially wave or sine-shaped. The constitution of the cross section of the separating reliefs (4) is restricted only by the design possibilities of the contour of an engraving tool.

If the thickness of the ink layer in the transition area from a fine structure line to the adjacent line is to be reduced only to a non-zero value, it is suitable to use structures such as those shown in Figures 4 and 5.

A printing plate according to figure 4 is produced by eliminating the pointed ends of the separating reliefs that are outwardly after engraving the depressions that constitute the fine structure. Alternatively, the entire area can be first cleaned to a depth (a) to be etched and then the depressions constituting the fine structure are etched. This lowers the point ends of the outwardly located spacer reliefs below the level of the surface (2) of the printing plate by means of a value (a). The remaining height of the separating reliefs will be referred to below as amplitude (b), and results from the difference between the depth of the engraving (t) and the descent (a) of the separating relief. A substrate printed with such a printing plate is arranged over the entire area in the engraving region with an ink layer having a thickness (a), and further modulated with a fine structure having a maximum amplitude. (b). The upper ends of the spacer relief, formed as a platform in this example, produce light thin lines in the printed image. With the corresponding guidance of the engraved lines that produce the spacer reliefs (4), the thin lines produced in the printed image by means of trapezoidal spacer reliefs (4) can restore graphic patterns, characters or symbols.

According to the embodiment shown in figure 5, the descent (a) of the separating relief can also be obtained by selecting the separation between the individual engraving lines to be this small, with a given lateral angle α and a depth of engraving (t) given, so that the upper edge (5) of the separating relief is below the level of the surface (2) of the printing plate.

The reduction of the separating relief is advantageous because it prevents the plastic surface of the cleaning cylinder from being in direct contact with the separating reliefs (4) in the shape of a cradle, thus reducing wear both on the surface of the cleaning cylinder and on the surface of the cleaning cylinder. fine etched structures on the build plate. Preferably, the descent (a) of the separating relief is 2 to 5 microns below the level of the surface (2) of the printing plate. To ensure clean production of the engraving as a fine structure of the transferred ink layer, the amplitude (b) must be greater than 50% of the engraving depth (t).

Figure 6 shows a variant of the novel engraving supplemented with separating reliefs. In this embodiment, the separating reliefs (4) are arranged at a greater distance (d). In contrast to the embodiments of Figures 1 to 5, the distance (d) of the separating relief in this case does not correspond to the separation of the engraving tool during engraving of the depressions. The distance (d) is preferably less than 500 microns. The lower horizontal areas (6) of the engraving are arranged between the separating reliefs (4), which have a

ES 2 188 246 T3 selectively adjusted surface roughness to improve ink adhesion. The roughness of the surface is adjusted by means of the selected geometry of the angle and point radius of the engraving tool and by setting suitable values ​​for the separation between two engraved lines transversely to the direction of the engraving.

According to a preferred embodiment of the invention, the engraving (3) is incorporated into the surface (2) of the printing plate so that the depth of the engraving is not constant within the engraving area, but increases or decreases accordingly. Continuous in one direction (Figures 7a, 7b). A variation of the depth of the engraving is preferably carried out so that the deepest points of each engraving line lie on an inclined plane relative to the surface of the printing plate. It is also possible to change the depth of the engraving so that the deepest points that are located in a transverse plane of the printing plate are on a curve whose trajectory can be described, for example, by means of a parabola or a hypoerbola. By varying the depth of the etching, the perceived hue within a contiguous printed color area can be varied, particularly if the depth variation is between 5 and 60 microns.

In the embodiment according to figure 7a, the separation distance (d) and the height of the separating reliefs are constant throughout the engraving, while in the variant according to figure 7b, the distance and height of the separating reliefs increase with the depth of the engraving (d1> d2).

It is possible to combine engravings of different types and designs and with different constitutions of the separating relief on a printing plate. They can also be made in areas with different types of engraving or separating relief constitutions contiguous with each other, and carry out corresponding variations within an engraving area contained in itself. Additionally, a second engraving can be overprinted on top of a first engraving. If the first engraving consisted of parallel, preferably straight engraving lines, and similarly, the second engraving of parallel, preferably straight, engraving lines, a so-called transverse line screen can be obtained. If the lines of the first and second engravings form an angle between 20 ° and 90 °, in particular 40 to 70 °, the resulting engraving has especially good adhesion to ink, which has a favorable effect on the properties of Printing of a printing plate engraved in accordance with the above. The ink layers printed with it additionally have a particularly uniform shade.

The first engraving and the second overprinted engraving can be produced by engraving tools of different geometries and with different engraving depths and / or different engraving line spacings. In the case of the preferred cross-line screen, this leads to periodically interrupted spacer reliefs.