EP1251011A2 - Method for varying full-tone colour density in offset printing inside a rotation printing machine - Google Patents

Abstract

The method involves using a color application system that can provide a constant color quantity, and a binary illustration of a printing form. A base raster of grid points is produced for area-variable image information on the printing form to define the area coverage. A fine micro-raster is placed under the base raster so that the area coverage of the base raster is reduced by a variable percentage between 0% and 100%.

Description

In digital printing processes, i.e. H. in processes for the production of printing forms in the binary sense, where a range of colors is either accepted locally or not, such as B. in planographic printing, d. H. in offset printing, the color density of uninterrupted layers of color, called full tone, about the color range of the Ink supply system controlled to the printing form.

In conventional offset printing, it is known that the range of colors and thus the Thickness of the ink layer offered to the printing form via so-called Color zone screws regulated. The printing form then takes only where it is is color accepting, according to the color splitting the color is proportional to the offered quantity. More colors lead to a higher one Color layer thickness and thus to a higher solid color density.

However, the controllability of the inking unit with regard to the range of colors has disadvantages both in terms of the standard effort, as well as in terms of the resulting resulting complexity of the inking unit, as well as in terms of the desired Different color samples have no effect on subsequent ones Printed copies.

There are now inking systems, e.g. B. in offset printing the anilox inking unit, that is Short inking unit for printing with low-viscosity printing inks, for example for the Newspaper printing, the ink more directly over an anilox roller and a few Bring the intermediate cylinder onto the printing form and thus considerably reduce it Show complexity with all the resulting benefits. This form of However, inking units only allow a very limited regulation of the Color range.

Each substrate now needs a specific one for a defined solid color density Color quantity, depending on surface roughness, absorbency, kick-off behavior and others. An inking unit that cannot be regulated in terms of ink quantity in connection with A binary printing form can therefore only have certain solid densities, depending on The type of substrate fluctuates, it should not be different depending on the substrate Anilox roller or a color of other pigment concentrations or viscosity be used.

The object of the present invention is therefore to provide a method for varying the Color density of the full tone when printing within a rotary printing press develop that despite the constant color supply of the inking unit or the color-applying elements a control of the solid color density, or a Adjustment of the halftone values in the print allowed.

The object is achieved by the measures of claim 1. That included the printing process used itself can preferably be lithographic offset, High pressure, flexographic or electrophotographic or electrographic Be pressure. However, the invention is not limited to these methods.

In particular, the geometric tonal value increase in color transfer from the Note the printing form on the substrate. The term dot gain is based on the concept of area coverage. Area coverage is defined as the share of Area in a specific location that is covered with paint. This is measured either via optical-geometric measurement methods, which the pure measure geometric area coverage or by measuring the Transmission ratios of fully covered area (full tone) and the partially covered area (halftone), which is then the effective or optical Measure area coverage.

As is known, in addition to the solid color density and thus the ink layer thickness, the screen dot size (in a basic screen) is a decisive factor for the print quality. Lighter color nuances are usually represented in printing by rasterizing the three basic colors cyan, magenta and yellow together with black. The dot size is determined in the binary imaging of the printing form in accordance with the tonal values of the respective image information. During screening, bright image areas are broken down into small and dark image areas into larger screen dots (binary, area-variable image information).
This applies to both a periodic autotype grid and a stochatic grid.

For numerical recording and definition of the different binary The area coverage in% serves as image information. The halftone value can be in Percent area coverage are specified, i.e. 0% for white and 100% for Full surface. As is well known, the halftone tone value does not correspond to the print geometrical area coverage on the printing form, since both geometrical and optical effects also lead to a so-called dot gain.

Tonal value increase in the sense meant here is the increase in Coverage from the printing form to the substrate. The dot gain splits into two parts, an optical and a geometric. The optical component is caused by light migration in the substrate (light trap) uncovered areas to the covered areas. The geometrical The part that just plays a role in the method according to the invention is represented by Squeezing effects in the ink transfer points from the printing form to the substrate or in electrophotography using toner clouds around the actual image areas evoked around. This effect reduces that on the printing form not covered area geometrically from the edges of the covered area.

In order to ensure that the amount of ink on the substrate is the same with the same range of colors Controlling the color given is the basic grid of halftone dots for the area-variable image information, which determines the area coverage, a very fine, preferably compared to the basic grid by at least a factor of two finer micro grid is subordinate to the area coverage of the basic grid the set percentage is reduced. The printing form now takes according to the geometrically covered areas from the system providing the color Ink off - in offset these are the inking rollers of the inking unit - through which However, the microgrid does not appear on the printing substrate more. The dot gain results from the difference of the known Screen tone value for the printing form illustration and the measured Screen tone value in print. The dot gain as a deviation of the Screen tone value in print from the screen tone value of the printing form can be used for Imaging and micro grid backing can be used directly in a so-called Pressure characteristic curve are shown. This generation of characteristic curves and their Use in the printing process is from densitometric measuring technology for Printing presses are well known and are not discussed further here.

In the case of the full tone, this looks like in Figure 1 A), where the effects of becoming full on the print result are shown schematically. Here has the basic grid has a micro grid backing of 50%, d. H. it will only approximately 50% of the amount of color of a full solid has been removed. On the substrate now appears due to the geometric dot gain Micraster no longer and a full tone with significantly reduced density is that Result.

This procedure can also be used in the area of halftones continue, as Fig. 1 B) shows schematically. It is of course possible in Area of the highlights according to FIG. 1 C) to dispense with a micro grid, or to set to 0% tonal value reduction. Also a smooth transition with a high one Reduction with large tonal values and little to no reduction in small tonal values are conceivable.

Another fact that supports this effect is that the transmitted Ink layer thicknesses with the diameter of the ink-transferring surface element decrease proportionally. This effect starts from about 30 µm in diameter printing element. This means that a fully covered area transmits more Color per unit area as very small halftone dots of the same geometric Area.

Of course, the entire point structure must be in its transmission characteristics be characterized and compensated. In relation to the fully covered Area lower optical density of a raster point and in particular also the Solid tone density must of course be used when determining a tone value curve be taken into account. The effective optical area coverage is then analogous to previous measurement, the ratio of remission of the grid area to Solid area, even if the printing form for both the solid and the halftone dot Can have holes.

The above procedure is also based on stochastic grids and hybrid Grid transferable. Here the points are then essentially the same size a micro grid is highlighted. This happens in an extended version of the Then the procedure after checking the environment is not, or only to a lesser extent Extent when a point stands alone or a cluster does not have a certain size exceeds. The micro grid can also be stochastic both in connection with conventional as well as stochastic Screening.

The method according to the invention for offset printing is preferably used an anilox inking unit used. The printing form, preferably a thermal one imageable plate or sleeve without chemical aftertreatment, which is a very high edge sharpness and resolution is allowed in or outside the Printing machine with a resolution of z. B. 2000 lines per cm using a Illustrated laser imagesetter (see for example DE 196 24 441 C1 or EP 0 363 842 B1). The laser imagesetter writes with continuous steel.

The basic grid is not modified for maximum color transfer, or set to 0% area coverage reduction. To reduce the transmitted Color amount by z. B. 25% in the covered areas, i.e. the Area elements of the binary image information, holes imprinted, i.e. on creates a fine hole pattern, so that about 25% of the area of the base lying point remain uncovered (see Fig. 2 A)). In this example the Laser writing beam each two pixels (halftone dots) wide, i.e. e.g. switched on for 10 µm, then one pixel (halftone dot) i.e. for 5 µm switched off. In the adjacent writing line, one pixel is then offset, written the same pattern, so that each 5 µm insulated holes arise. To reduce the amount of color by 50%, two pixels are used on and two pixels off and this in the adjacent line by two Pixels offset so that 5 µm x 10 µm holes are created (see Fig. 2 B)). This then roughly limits the applicability of what is described here Process, since the holes in the case of even larger paint quantity reductions covered areas predominate.

Another embodiment can write beams larger than 10 microns use, but is not limited to this. Is in the writing direction of the Laser beam realizes a higher addressability than the spot diameter corresponds, the addressability grid in the scanning direction is narrower than transverse to Scanning direction. It can be used to create rectangular holes that are perpendicular to the Scanning direction lie (see Fig. 3 A)) up to the square hole (see Fig. 3 B) and C)) and rectangular hole in the scanning direction.

If one speaks of "rectangular" here, this is an idealized statement because practically every scan beam is round or rounded and so one more or less large mostly inward deformation of the hole center Hole edges created.

An alternative implementation of this methodology is given by the above Fact that the transferred layers of paint with the diameter of the remove the ink-transferring element. This effect starts from around 30 µm Diameter of the printing element to occur. A color quantity control in the sense of the invention then also works with stochastic grids from very small base size, e.g. B. 5 microns x 5 microns and a double regulation of effective optical density, on the one hand via the effective area coverage, such as So far this has been done with stochastic grids and secondly via the Color quantity transfer via the decreasing layered transfer small pressure points. Specifically, this means that a 50% grid of z. B. 20 µm dots more color is transferred than a 50% grid from z. B. 10 µm dots. The proportion of 20 µm points to 10 µm points can then still be an intermediate gradation can be created. In the area of the higher Area coverage can be transferred with the same effective area coverage Color amount can be controlled via the average hole size. If the holes in the Medium is larger, more color is transferred than medium-sized ones but more numerous holes, because then the continuous solid areas are smaller.

In an alternative application of the method according to the invention, this can also be used for correcting tonal value characteristics in conventional inking units which can be regulated in zones or over a wide range.
In this case, the full tone is not interspersed with holes and its effective density is reduced, but only the halftone dots according to a predetermined characteristic. For example, it can be used to produce a printing press with a linear transfer characteristic by just compensating for the effective dot gain.

Another alternative application is a local reduction in the full tone or Screen density, depending on predictable color transfer deviations from the target, e.g. Paint waste or stenciling. This is a compensation of Weaknesses in the paint application system possible, both independent of the subject can also be subject to subject.

Claims (16)

Process for varying the solid color density when printing within a rotary printing press in connection with an ink application system, that can offer a constant amount of color, and a binary one Illustration of a printing form in which a basic grid of halftone dots for the area variable image information is generated on the printing form, the determines the area coverage, and the basic grid a fine micro grid is subordinated in such a way that the area coverage of the basic grid is reduced by a percentage that can be set between 0% and 100%. A method according to claim 1, characterized by the use in lithographic offset printing. A method according to claims 1 and 2, characterized in that an inking unit which is only adjustable in the amount of ink cylinder-wide is used as the ink application system. A method according to claim 1 and 2, characterized in that an anilox inking unit is used as the ink application system. A method according to claim 1, characterized by the use in flexographic printing. A method according to claim 1, characterized by the application in high pressure. A method according to claim 1, characterized by use in electrophotography. A method according to claim 1, characterized by its use in electrography. Method according to one of the preceding claims, characterized in that holes are imaged in the area of the binary image information on the printing form by means of laser imagesetter, ie a fine hole pattern is produced, with an area proportion which corresponds to the desired variation in the color density of the full tone in order to generate the micro raster. Method according to claim 9, characterized in that for the generation of the micro raster the resolution of the laser beam in the scanning direction is chosen to be greater than the distance of the writing beams for the generation of the basic raster, ie its addressability is higher than it corresponds to the binary image information: Method according to one of the preceding claims, characterized in that the micro grid is stochastically applied. A method according to claim 1, characterized in that the maximum coverage of the amount of ink transfer, the area coverage portion of the underlying micro-grid is set to 0%. Method according to one of the preceding claims, characterized in that a desired characteristic curve which deviates from the real pressure characteristic curve of the basic grid is generated and is used for the micro-grid underlay. A method according to claim 13, characterized in that the adjustable percentage of the reduction in the area coverage of the basic grid for the micro-grid corresponds to the geometric dot gain during the transfer of the grid points of the basic grid from the printing form via the rubber blanket to the printing material. Method according to Claim 13, characterized in that the percentage reduction in the area coverage of the basic grid set for the micro grid is used to generate a linear transmission characteristic, so that the effective dot gain results in the value zero. A method according to claim 1, characterized by the application for compensating local transmission deviations from the globally set tone value characteristics of an inking unit.

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