DE102009034078A1 - Printing plate producing method for printing press i.e. sheetfed offset press, involves providing data files, and determining number of elements, where values of elements are changed by evaluating regions within data fields - Google Patents

Abstract

The method involves providing data files with data needed for screening image data, prior to screening the image data for a subsequent exposure of printing forms. The data is changed such that values of individual elements in two-dimensional data fields are changed. A number of elements are determined, where values of the elements are changed by evaluating regions within the data fields. Image datasets subdivided into micro-areas are produced. The micro-areas with pixels having the same size as an integer multiple or an integer part of a size of pixels of a screened image are defined.

Description

In Offset printing machines are currently two fundamentally different Types of inking used. Especially in sheetfed offset are so-called Zone inking units spread across the sheet width Have a variety of individually adjustable zone screws. With which can be according to the subject to be printed the promoted by the rollers of the inking and the printing plate offered amount of color in the associated zonal Set areas differently. Inking units of the second type are so-called "anilox" color works, often also called short inking unit. These have an anilox roller with a fixed cup occupancy, which is in contact with a Paint reservoir turns and squeegee across the entire Width is doctored off. This inking unit transfers the printing plate the entire width a uniform amount of color available, which also only to a very small extent, for example, by tempering over influence the viscosity of the paint. Bigger changes in color needs require a relatively complex exchange the complete anilox roller against one with another cup occupancy.

Anilox inking units are more widespread in web offset, especially in Newspaper presses. In addition, however, are also recently sheetfed offset printing presses become known with anilox inking.

Depending on the paper used, the color used, etc., the sheet to be printed requires different amounts of ink in order to achieve a defined areal density or solid density. Since the color range of anilox inking units can be changed over the viscosity only in a very narrow range and the costly replacement of anilox rolls is avoided, it has also been proposed to transfer the quantity of ink to be transferred to the print sheet via a modification of the screened printed image to change. The known proposals aim to specifically reduce the area coverage of the screened printed image. This suggests the EP 0 770 228 B1 to superimpose a stochastic grid of white dots on the printed image as a whole or to set a corresponding grid of black dots on a film to be copied onto the printing form. The EP 1 251 011 B1 describes a method in which a fine hole pattern is imprinted in the already screened image or in the surface elements of the binary image information that controls the platesetter, which also as in EP 0 770 228 B1 stochastic can be applied. A very similar procedure is also in the DE 199 53 145 A1 for a laser proofer.

At the Printing run the holes or white points of the finer grid with color from the neighboring pixels too, so that they are no longer visible. This reduces the ink layer thickness of the printed image in total in the ratio of white points to the pixels occupied with color, ie in the desired Wise.

These known methods engage in or after the screening of the printed image into the already rastered print image. superficial this has the advantage that the already rasterized records for the exposure of the printing plates not in again the prepress must be rasterized and a second time need to be, if the same print image with a respect the area coverage reduced printing plate are reprinted should. However, this case does not occur frequently in practice on. Because at constant production conditions on z. B. always The same press with anilox inking gets the process the area reduction always be the same, so that one additional process for making a second plate is disadvantageous with reduced area coverage. The simple overlay the entire raster image with a grid of white dots but has other disadvantages. They are u. a. to see that for larger reductions in areal density the Pressure points become dirty and the tonal characteristics through an increased dot gain is disturbed. The uniform overlay of the printed image with a white dot grid also generates in the Print image a visually perceptible restlessness over the entire Tonal range.

Of the Invention has for its object to provide a method with the area coverage of the image to be printed in one compared to the prior art and related to as Resulting printing plate change high quality way leaves.

These Task is with the measures specified in claim 1 solved.

According to the Invention is used to reduce the surface density in the Data intervened before the actual process of rasterizing of the printed image or for the process of screening first needed.

Namely, only the solid areas, that is, for example, the image parts having a tonal range of 99% to 100%, are substantially perforated with a stochastic structure of white dots and thus reduced in tonal value. The underlying tonal values (0% to 99%) are not perforated by the stochastic white point structure, but remain unaffected. Only a tone adjustment is made on them, in they are recalibrated or re-linearized to avoid a density jump with known software tools, so that the area of the area coverage with the chosen for the print job grid for the tonal values from 0 to 99% corresponds to a range of area coverage between 0% and the value the solid density should be reduced or followed by this reduced value.

The Reduction of solid areas by overlaying with the white points can be conveniently simple be done by frequency modulation of the solid surfaces be rasterized while for the tonal values below of the solid, for example, uses an amplitude modulated raster becomes.

Of the The advantage of this method is firstly that the usual pre-press equipment such as calibration / linearization software, frequency modulated and amplitude modulated grid, etc. used can be. The grid for the white point structure can be chosen freely and does not affect negatively the rasterization of the tonal values from 0 to 99%. The entire workflow is easy to understand for the entrusted staff and as a result, visible and measurable.

moreover the raster remains in the tonal ranges below the full tone undisturbed, so no riots on the expression of the rasterized Image arise.

Furthermore can according to the invention, the screening of the print job and the reduction of area coverage advantageous in one Process be performed.

Further advantages of the invention will become apparent from the following description of exemplary embodiments with reference to FIG 1 to 3 of the accompanying drawings.

1 FIG. 3 is a flow chart depicting the method of making modified masters of the invention. FIG.

2 is a diagram showing the relationship between the tonal values of an image to be scanned and the areal coverage of printing, and

3 is a simplified representation of individual halftone cells at different tonal values in the case of a "normal" screening (top row) and a screen with reduced area coverage according to the invention.

In 1 is the timing of individual steps in the prepress, as far as they are necessary for the understanding of the invention, shown in the form of a flowchart. The presentation begins here with the optional step, in which the pages, for example in PDF format, are available after setting 1 of the product to be printed are imposed. In this step, designated "Impositioning" in the figure, several pages are combined to form a printed sheet. Subsequently, as part of a data processing process customary in prepress, a transformation of the input data into a uniform printing color space takes place. Here are the z. B. four inks separated descriptions 3 the printed sheet on which the pages 1 are arranged as they are to be subsequently printed.

The separated descriptions in the form of halftones 3 should then be converted into raster data. For this purpose, the raster method suitable for the present print job (eg amplitude modulation or frequency modulation, rational or irrational raster, etc.), the number of lines, etc. must first be selected. This selection becomes a threshold matrix 4 which serves to adjust the tonal values in the halftone images of the descriptions 3 by comparing with the entries for the pixels in the threshold matrix to translate into pixels to be exposed or unexposed in the rasterized data set. The values in the cells of the threshold matrix 4 form for amplitude modulated raster z. B. the shape of a bell shell. For frequency modulated grids, the "mountains" that represent the values in the matrix, of course, look different.

The raster processor (RIP) 5 generated by comparing the entries in the threshold matrix 4 with the tonal values of the files 3 that describe the sheet, a rasterized record 6 with a resolution of typically 2,400 dpi or 2540 dpi. These also for each color to be printed datasets 6 serve to the platesetter 7 to drive, with which the printing plates 8b . 8 y . 8m . 8c be directly imaged for printing with the four colors black, yellow, magenta and cyan.

With the printing plates produced in this way 8th be in a printing press 9 , For example, a sheetfed offset printing press, then the sheet 10 printed. Alternatively, films can be imaged, which are then copied to the plates or it is first on a proofing a sample sheet in another printing process, eg. As by inkjet printing or laser printing.

The printing press 9 has in the present case so-called anilox inking units, as they for example in the EP 1 834 775 A2 are described. As already stated, these inking units allow the amount of ink with which the plates 8th be colored, only within narrow limits. If, for printing with one or more colors, a too high solid density is produced, the affected colors are processed as follows during a work preparation process in the print shop:
The printer rates the sheet 10 as to whether and to what extent the disturbing effect can be reduced by lowering the areal density of the image information on the printing plate. This step of the evaluation we have with 11 designated. Of course, this evaluation does not have to be done for every sheet, but can only be done on the basis of test pages. Instead of a visual assessment based on empirical values, it is of course also possible to evaluate the printed sheet with regard to the printed ink layer amount, ie to measure it densitometrically or spectrally. It is mainly about the determination of Volltondichte.

According to the prior art mentioned at the outset, it would then be possible to superimpose a micro-grid onto the rastered data record and then expose a second set of printing plates with which the print job can then be executed in the desired quality, as indicated by the corresponding arrow 20 in 1 is symbolized. According to the invention, in the production elsewhere, the processes of the pre-press intervene:
First of all, for the separated descriptions of the print sheet, it is determined how to treat the solid portions of the print image, ie, the image pixels whose tone values are at 100% or above a tone that is quite close to the tone, such as a tone. B. 98% or 99% are. For the image components with these solid values, a frequency-modulated raster method is selected, which is set up with regard to the number of pixels remaining white in the raster cells of the rasterized data set such that the tone values of z. B.> 99% instead of the full tone the desired reduced area coverage is achieved. If, for example, the coloring in full tone is to be reduced by 25%, the corresponding image components are rasterized with a frequency-modulated raster, which effectively only results in a coverage of 75% in the print. That is in the 3 in the right column shown as an example and simplified. With v, a grid cell with 10 × 10 pixels is designated there, whose individual pixels for the solid tone case all should have been set (100% area coverage). Instead, the rasterization with the frequency-modulated raster causes the raster cell designated r to be patterned with white dots and the solid areas to be "pitted" accordingly, resulting in the desired reduction of the tone value to the z. B. 75% leads. Conveniently, in order to avoid visible structures, a random distribution of the non-printing pixels is desired, and therefore a stochastic screening method for the solid tones is selected.

In contrast, all halftones of the image in the range between the tone 0% and the tone z. For example, 99% as originally scheduled for the print job rasterized with an amplitude modulated raster. It should be noted, however, that no dot jumps occur or that the dot coverage values continuously increase from 0 up to the value of here 75% for the reduced area coverage of the solid tones. Accordingly, the respective threshold value matrices should be recalibrated to the reduced value and, if necessary, be re-linearized. This is exemplary in the left and middle column of the 3 shown. There, the area coverage values of 12% and 96% are reduced to 9% and 72%, respectively, according to the recalibration, which is reflected by correspondingly smaller grid points in the cells of the amplitude grid.

This fact is clarified in the presentation 2 , In the diagram shown there, the area coverage is plotted against the tonal value, the ideal progression being represented by the straight line designated a1 and the curve denoted by a2 describing the relationship between the tonal value and area coverage taking into account the dot gain in the printing.

The Scanning the areas with 100% tone value leads to a discontinuity in the solid tones. Around to meet this is now the area of the area coverage recalibrated to 75% by subtracting the threshold matrices for the Z. B. amplitude modulated raster for the semitones be modified in the range 0 to 99%. this leads to a tilting of the straight line a1 in the position shown with b1. Of Further, there is the relationship between tonal value and area coverage to linearise according to the dot gain, resulting in then the curve labeled b2 gives.

The extent of the linearization depends on various parameters, such as the color to be printed, its temperature, the type of printing plate used, its dampening and possibly also the type of blanket used. As in the past, empirically determined progressions can be used here, which were recorded and stored in accordance with the abovementioned parameters. The same applies to the work equipment used: The tools currently available on the market for the processing of precursor data allow the calibration and linearization of tonal values to the vaults standard or are integrated as corresponding functions in the software packages. The software package "Prinect" from Heidelberger Druckmaschinen AG includes z. For example, the tools "Calibration Manager" and the linearization function in the software module "MetaDimension". There are also different different screening methods available for selection. To carry out the method, however, it must be stipulated that the area coverage assigned to the full-tone values is "down-scaled" to the desired value with the stochastic grid, and the remaining image components are "normal" with a z. B. amplitude modulated grid are rasterized after the associated threshold value matrices have been recalibrated.

1

page

2

imposition (Impositioning)

3

separated Description of the print sheet

4

threshold value

5

raster process

6

rasterized record

7

platesetter

8b

Printing plate "black"

8 y

Printing plate "yellow"

8m

Printing plate "magenta"

8c

Printing plate "Cyan"

9

Sheetfed Offset Press

10

sheet

11

rating of the bow

20

arrow

a1

Just

a2

Curve

b1

position (A1)

b2

Curve

v

grid cell Concentrated

r

grid cell

B ₂

arrow

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - EP 0770228 B1 [0003, 0003]
• - EP 1251011 B1 [0003]
• - DE 19953145 A1 [0003]
• EP 1834775 A2 [0022]

Claims (6)

A method of making reduced area printing plates, wherein prior to the step of rasterizing the image data for subsequent exposure of the printing forms in one or more files required for the rasterizing step, the data is altered such that the solid image areas are rasterized with a frequency modulated raster where the amount of pixels remaining white in the raster cells (r) leads to the desired reduction of the solid density and that the threshold matrix ( 4 ) is calibrated to the halftone halftoning to the reduced value of the solid density. The method of claim 1, wherein the frequency modulated grid is a stochastic grid. Method according to claims 1 and 2, wherein for screening the solid tones and the halftoning halftone different Raster can be used. The method of claim 1 to 3, wherein the screening Halftones with an amplitude modulated grid is done. The method of claim 1 to 4, wherein the calibration the threshold matrix for halftoning halftoning is linear. The method of claim 1 to 5, wherein the calibration halftones according to given functions (b2), whose course depends on the color to be printed and / or the printing plate used and / or the blanket used and / or is the amount of fountain solution supplied during printing.