EP1251011B1 - Method for varying full-tone colour density in offset printing inside a rotation printing machine - Google Patents

Description

For digital printing processes, i. H. in processes for the production of printing forms in the binary sense, in which a local color range is either accepted or not, such as. B. in planographic printing, d. H. in offset printing, the color density of uninterrupted color layers, called full tone, is controlled by the color supply of the ink supply system to the printing plate.

In conventional offset printing, it is known that the color offer and thus the thickness of the ink layer offered to the printing form are regulated by what are known as ink fountain keys. The printing form then only decreases, where it is ink accepting, according to the color splitting, the color in proportion to the quantity offered. More color range leads to a higher ink layer thickness and thus to a higher solid density.

However, the controllability of the inking unit with respect to the color range has disadvantages both with regard to the regular effort and with regard to the resulting complexity of the inking unit, as well as with regard to the desired freedom from reaction of different color decreases on subsequent printed copies.

There are now inking, z. B. in offset printing the anilox inking, so a short inking unit for printing with low-viscosity inks for example, the newspaper printing, which bring the color directly on an anilox roller and a few intermediate cylinders on the printing plate and thus have a significantly reduced complexity with all the resulting advantages. However, this form of inking only allows a very limited control of the color range.

Each substrate now requires a specific amount of ink for a defined solid density, depending on the surface roughness, absorbency, impact behavior and other. A non-ink-adjustable inking unit in connection with a binary printing form can therefore only certain Volltondichten that vary depending on Bedruckstoffart realize should not be used depending on the substrate another anilox roller or a color other pigment concentrations or viscosity.

FR 2 660 245 A1 describes a printing plate production in which the ink-accepting areas are reduced with regard to the predetermined area ratio, for which purpose suitable screening is used.

Furthermore, WO-A 96/02868 discloses a use of frequency-modulated screening or stochastic screening for attenuating the printing area in the case of dry or wet-offset printing technology.

DE 199 53 145 A1 discloses a method for compensating an optical density increase when generating a color correction flag of halftone prints, in which a halftone print image for the photomechanical production of structured or patterned surfaces is simulated.

Object of the present invention is to develop a method for varying the color density of the solid when printing within a rotary printing machine, despite a constant range of colors of the inking unit, ie an inking unit without ink zone setting, or the color-promoting elements control of Volltondichte, or an adjustment the raster tone values allowed in the print.

The object is achieved by the measures of claim 1. The printing method itself used here can preferably be lithographic offset, high-pressure, flexographic printing or electrophotographic or electrographic printing. However, the invention is not limited to these methods.

In particular, the geometric tone value increase during ink transfer from the printing plate to the substrate is taken into account. The term dot gain is based on the concept of area coverage. Area coverage is defined as the proportion of area at a given location covered in color. This is measured either by optical-geometric measurement methods, which measure the purely geometric area coverage or via the measurement of the transmission ratios of fully covered area (full tone) and teilgeseckent area (halftone), which then measure the effective or optical surface coverage.

As is known, in addition to the solid density and thus the ink layer thickness, the dot size (in a basic grid) is a determining factor for the print quality. Lighter color shades are usually displayed in print by rasterizing the three primary colors cyan, magenta and yellow together with black. The dot size is used in binary imaging of the Set printing form according to the tonal values of the respective image information. During halftoning, bright image areas in small and dark image areas are split into larger halftone dots (binary, area-variable image information).
This applies to both a periodic autotypic grid and a stochastic grid.

The area coverage in% serves for the numerical capture and definition of the various binary image information. The halftone dot value can be specified in percent area coverage, ie 0% for white and 100% for full area. However, as is well known, the halftone tone value in printing does not correspond to the geometric area coverage on the printing form, since both geometrical and optical effects lead to a so-called dot gain.

Tonwertzunahme in the sense meant here is thus the increase of the area coverage of the printing plate to the substrate. The dot gain splits into two parts, one optical and one geometric. The optical component is caused by light migration in the substrate (light trap) from the uncovered areas to the covered areas. The geometrical component which plays a role for the method according to the invention is caused by crushing effects in the ink transfer points from the printing form to the printing material or in the electrophotography by toner clouds around the actual image areas. Due to this effect, the surface not covered on the printing form is geometrically reduced from the edges of the covered surface.

In order to control the amount of ink dispensed onto the substrate while maintaining the same color range, the basic grid of halftone dots for the area-variable image information, which determines the area coverage, is superimposed on a very fine microgrid, which is at least a factor of two finer than the basic grid of the basic grid by the set percentage. The printing form now decreases in accordance with the geometrically covered areas from the color providing system color from - in offset these are the application rollers of the inking -, by the Tonwertzunahme the micro-grid on the substrate does not appear more. The tone value increase results from the difference between the known halftone tone value for the printing form illustration and the measured halftone tone value in the print. The tone value increase as a deviation of the halftone tone value in the printing from the halftone tone value of the printing form can be directly used for the imaging and micro-grid backing in a so-called pressure characteristic curve. This characteristic curve generation and its use in the printing process is well known from the densitometric measuring technology for printing machines and not discussed further here.

In the case of the full tone, this looks like in Figure 1 A), where the effects of fuller are shown schematically on the print result. Here, the basic grid has a micro grid underlay of 50%, i. H. Also, only about 50% of the amount of color of a full-coverage solid is removed. Due to the geometric tone value increase, this micro-grid no longer appears on the substrate and a full tone with significantly reduced density is the result.

This procedure can also be continued in the area of halftones (semitones), as shown schematically in FIG. 1 B). It is of course possible in the field of high lights in accordance with FIG. 1 C) to dispense with a micro-grid, or to provide 0% Tonwertreduzierung. Even a smooth transition with high reduction for large tonal values and low to no reduction for small tonal values is conceivable.

A fact supporting this effect is further that the transferred ink layer thicknesses decrease proportionally with the diameter of the ink-transferring surface element. This effect begins to occur from about 30 μm diameter of the printing element. Thus, a fully covered area transmits more color per unit area than very small halftone dots of the same geometric area.

Of course, the entire point structure must be characterized and compensated in its transfer characteristics. The lower the optical density of a grid point in relation to the fully-covered area, and in particular the Solid density must of course be taken into account when determining a tone curve. The effective optical area coverage is then, analogous to the previous measurement, the ratio of remission of the screen area to the solid surface, even if the printing form can have holes both at the solid tone and at the screen dot.

The above procedure can also be applied to stochastic screens and hybrid screens. Here the points of essentially the same size are underlaid by a micro-grid. This does not happen in an extended version of the method after examination of the environment then, or only to a lesser extent, if a point stands alone or a cluster does not exceed a certain size. Also, the micro-grid can be stochastically applied, both in connection with conventional and with stochastic screening.

The method according to the invention is preferably used for offset printing with an anilox inking unit. The printing plate, preferably a thermally imageable plate or sleeve without chemical aftertreatment, which allows a very high edge sharpness and resolution, is in or outside the printing press with a resolution of z. B. 2000 lines per cm using a laser imager illustrated (see, for example, DE 196 24 441 C1 or EP 0363 842 B1). The laser imager writes with continuous steel.

For maximum ink transfer, the basic screen is not modified or set to 0% area reduction. To reduce the amount of ink transferred by z. B. 25% are imprinted in the covered areas, ie the area elements of the binary image information, holes, d, h. creates a fine hole pattern so that about 25% of the area of the underlying point remains uncovered (see Fig. 2A)). In this example, the writing beam of the laser in each case two pixels (halftone dots) far, ie, for example, turned on for 10 microns, then a pixel (halftone dot) far, ie switched off for 5 microns. In the adjacent writing line, the same pattern is then written, offset by 1 pixel, so that in each case 5 μm large isolated holes are formed. For a reduction of the color amount by 50% two pixels each switched off and two pixels and this offset in the adjacent line by two pixels, so that 5 .mu.m x 10 .mu.m holes arise (see Fig. 2 B)). This is then approximately the limit of applicability of the method described here, since with even larger reductions in ink quantity, the holes outweigh the covered areas.

Another embodiment may use larger writing beams than 10 μm, but is not limited to these. If a higher addressability is realized in the writing direction of the laser beam than corresponds to the dot diameter, the addressability grid is narrower in the scanning direction than transverse to the scanning direction. This rectangular holes can be generated, which are transverse to the scanning direction (see Fig. 3 A)) up to the square hole (see Fig. 3 B) and C)) and rectangular hole in the scanning direction.

When this is referred to as "rectangular", this is an idealized statement, since virtually every scanning beam is round or rounded and thus produces a more or less large, usually to the center of the hole inwardly directed deformation of the hole edges.

An alternative embodiment of this methodology is given by the fact that the transferred color layers decrease with the diameter of the color-transferring element. This effect begins to occur from about 30 μm diameter of the printing element. A color amount control in the context of the invention then also works with stochastic screens of very small base size, z. B. 5 microns x 5 microns and a two-fold control of the effective optical density, on the one hand on the effective area coverage, as is done in stochastic screens so far and on the other via the color quantity transfer on the decreasing color layer carry at small pressure points. Specifically, this means that a 50% grid of z. B. 20-micron dots more color transmits than a 50% grid of z. B. 10 micron points. About the proportion of 20-micron points to 10-micron points can then be created a Zwischenabstufung. In the area of higher area coverage, with the same effective area coverage, the transferred color quantity can be controlled via the average hole size. If the holes in the Medium are larger, more color is transferred as in the average smaller but more numerous holes, because then the contiguous solid areas are smaller.

In an alternative application of the method according to the invention this can also be used for the correction of Tonwertkennlinien in conventional and zweisweise or total width adjustable inking units.
In this case, the full tone is not interspersed with holes and reduced in its effective density, but only the halftone dots according to a predetermined characteristic. For example, it can be used to generate a printing machine with a linear transfer characteristic by compensating for the effective tone value increase.

Another alternative application is a local reduction of the full tone or halftone density, depending on predictable color drift deviations from the target, e.g. Color waste or stenciling. Thus, a compensation of weaknesses of the color application system is possible, which can be both subject-independent and subject-dependent.

Claims (13)

1. A method for varying the full-tone colour density when printing inside a rotary printing machine, with an ink-application system that can offer a constant quantity of ink, and with binary imaging of a printing forme, wherein the area coverage of the imaging is determined by means of a basic screen of screen dots for variable surface image information that is produced on the printing forme and by means of a micro-screen that is finer than the basic screen by at least a factor of two and is placed underneath the basic screen, in such a way that the geometric area-coverage portion of the basic screen is reduced by a percentage that can be adjusted between 0% and 100%, by using laser exposure units for the overlay exposure of holes in the geometric area coverage of the basic screen in order to produce the micro-screen on the printing forme, that is, a hole pattern is produced with an area component that corresponds to the desired variation in the full-tone colour density.
2. A method according to claim 1, characterised in that an anilox inking unit is used as the ink-application system.
3. A method according to claim 1, characterised in that in order to produce the micro-screen the resolution of the laser beam in the writing direction is selected so that it is greater than that corresponding to the pixel spacing of the basic screen so that its addressability is higher than that corresponding to the binary image information.
4. A method according to one of the preceding claims, characterised in that the micro-screen is applied stochastically.
5. A method according to one of the preceding claims, characterised in that a desired characteristic deviating from the real printing characteristic of the basic screen is produced that is used for the placement of the micro-screen underneath.
6. A method according to claim 5, characterised in that the percentage reduction in the area coverage of the basic screen that is adjustable for the micro-screen corresponds to the geometric tone-value increase in the case of the transfer of ink from the printing forme to the printing material.
7. A method according to claim 5, characterised in that the percentage reduction in the area coverage of the basic screen that is adjusted for the micro-screen is used to produce a linear transfer characteristic so that the effective tone-value increase comes to the value zero.
8. Use of the method according to claim 1 for the compensation of local transfer deviations from the globally adjusted tone-value characteristics of an inking unit.
9. Use of the method according to claim 1 in lithographic offset printing.
10. Use of the method according to claim 1 in flexo-printing.
11. Use of the method according to claim 1 in letterpress printing.
12. Use of the method according to claim 1 in electrophotography.
13. Use of the method according to claim 1 in electrography.

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