Method for printing on a paper web

Abstract

The invention relates to a method for printing a print motif on a paper web by means of a digital printing system, the method comprising the following steps: providing a digital printing original, in which digital information about the print motif is contained, and a desired tonal value of a color of the print motif, printing of a surface of the paper web by means of the digital printing system on the basis of the printing original, determining at least one actual tonal value of the color at at least one location of the printed surface, comparing the determined actual tonal value with the desired tonal value and correcting the tonal value in the digital information of the printing original on the basis of the result of the comparison.

Description

[0001] Method for printing a paper web

[0002] The invention relates to a method for printing a paper web with a print motif using a digital printing system.

[0003] Paper webs are printed with a print motif for a wide variety of purposes. They are used, for example, to produce panels or boards that can be used as floor coverings, wall cladding, ceiling paneling, or furniture panels. Especially in the production of floor panels, it is desirable and highly advantageous if the paper webs printed with the print motif do not exhibit any color differences from one another. This is particularly the case when panels with the same decor are produced at different times, and thus different paper webs with the same decor are printed at these different times. In order to achieve reproducibility and color fidelity, it is known from the state of the art to store a digital print template.

[0004] This digital print template contains digital information about the print motif. Using this digital print template, it is possible to control the digital printing system and thus use it to print the paper web with a print motif. Various methods are known from the prior art in which the digital print template is used to print a print motif onto the paper web. The resulting copy is then compared with the target motif. Based on this comparison, a decision can be made as to whether the control commands and parameters contained in the digital print template, such as color values ​​used during printing, enable sufficient quality of the printed product, i.e. the paper web printed with the print motif.

[0005] WO 2019 / 234147 A1, for example, discloses a method in which a digital target image is generated from hyperspectral digital images. Hyperspectral images are images that contain information not only about visible colors, i.e., frequencies in the visible light range, but also about a frequency range beyond that. The target images created in this way are then compared with actual images generated from the printed copy of the paper web. The result of this comparison is crucial for the further procedure. If the comparison is satisfactory, i.e., the actual image sufficiently corresponds to the specifications of the target image, production can begin or at least the actual image can be further processed. However, if the comparison is unsatisfactory, it is not clear what needs to be changed to correct this.

[0006] The disadvantage is that this "re-sample" - i.e. checking, adjusting and setting the various production parameters of the digital printing system - is time-consuming and complex. It is necessary because external influences, such as changes in temperature and humidity, the ink used, any primer used and / or the paper web used, can impair the quality of the printed motif. A correction, or at least a check of the print quality, is therefore necessary. However, the methods known from the state of the art are time-consuming and / or material-intensive and therefore expensive. In addition, although deviations between the printed copy of the print motif on the paper web and a base sample or a digital print template can be identified, the necessary correction cannot usually be detected precisely and therefore cannot be implemented effectively.Rather, "re-sample" often requires several iterative steps and, if necessary, the experience and expertise of an employee operating the digital printing system in order to achieve the desired result as quickly as possible. The invention therefore aims to propose a method that eliminates or at least mitigates these disadvantages.

[0007] The invention solves the problem by a method for printing a paper web with a print motif using a digital printing system, the method comprising the following steps:

[0008] - Providing a digital print template containing digital information about the print motif and a target tone value of a colour of the print motif,

[0009] - Printing a surface of the paper web using the digital printing system based on the printing template,

[0010] - Determining at least one actual tone value of the color at at least one point on the printed surface,

[0011] - Comparing the determined actual tone value with the target tone value and

[0012] - Correcting the tonal value in the digital information of the print template based on the result of the comparison.

[0013] The tonal value of a color specifies the different gradations of the color between light and dark as a percentage. A tonal value of 0% means no color is present. The corresponding area with this tonal value is therefore unprinted and light. Such specialized areas are also called highlights. They are called light or highlights regardless of the color of the surface to which the print is applied. In contrast, a tonal value of 100% represents the maximum color, also known as a solid tone, which is accordingly considered dark. Dark specialized areas are also called shadows. The digital information about the print motif contained in the digital print template also includes the target tonal values ​​of a color in the print motif. Preferably, the digital information contains all target tonal values ​​of all colors used in printing the print motif.For each color used, a target tone value of the corresponding color or colors is preferably stored for each printed location on the paper web, for example each pixel of the print motif. In the method according to the invention, the digital print template is used to print the print motif onto a paper web and thus produce a print copy or sample copies. From this, an actual tone value of the color is determined at at least one location on the printed surface and compared with the provided target tone value for the corresponding color at the respective location. Based on this comparison, the tone value in the digital information of the print template is corrected. Subsequently, another sample copy can be printed using the corrected information from the print template, or production can begin.

[0014] An advantage of the method according to the invention is that a direct production parameter is used for comparison with a corresponding target value. This is not the case in the prior art. The procedure according to the invention makes it possible to determine a direct corrective measure, regardless of what caused the deviation of the actual tonal value from the target tonal value. It is preferably directly recognizable which measure can or even must be taken to eliminate or at least reduce a possible deviation of the actual image from the target image. In particularly preferred embodiments, at least one such corrective measure is proposed and output by means of a corresponding output unit, for example a monitor.Alternatively or additionally, the electrical control, preferably the electronic data processing device that controls the digital printing system, is designed to determine and, particularly preferably, also implement at least one suitable corrective measure from the results of the comparisons of the actual tonal values ​​with the target tonal values.

[0015] The target tonal values ​​are preferably determined from a printed template. A printed template is then created that displays the desired print motif and meets all quality requirements. Tonal values ​​are then recorded from this printed template. This is preferably, but not necessarily, done hyperspectrally. The tonal values ​​recorded in this way are then referred to as a master sample and stored. If a paper web is then to be printed with the print motif of the printed template, at least one tonal value from the master sample is provided as the target tonal value and used in the method according to the invention. The printed template no longer needs to be used.

[0016] According to the invention, the tonal value in the digital information of the print template is corrected based on the result of the comparison of actual tonal values ​​and target tonal values. This does not involve a calibration of the digital printing system, but rather the print template containing the print motif - in simple terms, the file to be printed - is adjusted. Calibration of the printing system remains unaffected. The parameters that can lead to deviations between the actual tonal values ​​and the target tonal values ​​are largely independent of the digital printing system. This applies, for example, to the room temperature and humidity or the type and quality of the paper used. Therefore, most deviations are not "errors" in the digital printing system, and no new calibration is necessary to correct the parameters causing these deviations and compensate for the deviations.

[0017] Preferably, a tonal value is provided as the target tonal value which is read or determined from the digital information of the digital print template.

[0018] Preferably, target tonal values ​​of different colors are provided, and the actual tonal values ​​of several of these colors, preferably all of these colors, are determined. This not only determines the print quality of a single color and / or just a single color value of one or more colors. It ensures that several tonal values ​​of one or more colors or one tonal value of several colors are checked and compared with the corresponding target tonal value that was provided. Preferably, the actual tonal value of a color is determined by measuring tonal values ​​of the color at different points on the printed surface that have the same target tonal value for this color. Preferably, these tonal values ​​of the color determined in this way are then averaged to determine the actual tonal value of the color.If different deviations of the respective tonal values ​​from the target tonal value occur at different points on the printed surface, these would be less significant due to the formation of the average value.

[0019] When correcting tonal values, this is preferably corrected identically for the entire print motif in the digital information of the print template. A tonal value of a specific color may be required at different locations on the print motif. In principle, it is possible to determine the actual tonal value at each of these locations in the test copy, compare it with the target tonal value, and correct the tonal value individually at each location depending on the specific deviation. In order to be able to correct this across the entire print motif, however, the deviation between the actual tonal value and the target tonal value would have to be determined at every location on the test copy, for example, for every pixel of the print motif. This is time-consuming and generally impractical.

[0020] It is therefore advantageous to uniformly correct the tonal value in the digital information of the print template for a specific color for the entire print motif. The basis for this correction can be the comparison of a single, determined actual tonal value with the respective target tonal value. However, a better basis is provided if the actual tonal value to be compared with the target tonal value is determined by averaging over a plurality of, for example, at least 5, preferably at least 10, particularly preferably at least 20, individually determined tonal values ​​of the respective color.

[0021] In a preferred embodiment, when comparing the actual tonal value with the target tonal value, a difference between the actual tonal value and the target tonal value is determined. This difference is used as the basis for correcting the tonal value in the digital information. In a particularly preferred embodiment, the tonal value is corrected by subtracting the difference from the tonal value. For example, if the actual tonal value is 10 percentage points greater than the target tonal value, the difference is therefore 10. In this particularly preferred embodiment, these are subtracted from the tonal value so that the latter is reduced by this amount. The next time you print, the tonal value used to control the digital printing system is therefore specified as smaller, so that the print result corresponds more closely to the originally intended print motif, i.e. the target tonal value.

[0022] Preferably, several actual tonal values ​​of a color are determined and output in a value table and / or a gradation curve. The actual tonal values ​​are related to the corresponding target tonal values. In the case of a perfect print, in which there is no difference between the target tonal value and the actual tonal value, the resulting gradation curve is a straight line with a gradient of 1. If the gradation curve generated from the comparison of the actual tonal values ​​with the target tonal values ​​deviates from this ideal form by more than a specified tolerance, it is easy to see from the gradation curve itself how the control parameters of the digital printing system must be changed in order to achieve a better result, i.e. a gradation curve with a smaller deviation from the aforementioned optimal curve.

[0023] To determine an actual tonal value, a respective location on the printed surface is preferably captured, preferably hyperspectrally, using a scanner, an image scanner, a camera, and / or a light sensor. The actual tonal value of the color is preferably determined at different locations on the printed surface and compared with the respective target tonal value.

[0024] In a preferred embodiment, the paper web is impregnated after the surface has been printed and pressed with a core, preferably made of a wood material, before the at least one actual tonal value is determined.

Claims

Patent claims 1. Method for printing a paper web with a print motif using a digital printing system, the method comprising the following steps: - Providing a digital print template containing digital information about the print design and a target tone value for one of the colors in the print design. - Printing on the surface of the paper web using the digital printing system based on the print template, - Determine at least one actual tone value of the color at at least one point on the printed surface, - Comparing the determined actual tone value with the target tone value - Correcting the tonal value in the digital information of the print template based on the result of the comparison.

2. Method according to claim 1, characterized in that target tonal values ​​of different colors are provided and the actual tonal values ​​of several of these colors, preferably all of these colors, are determined.

3. Method according to claim 1 or 2, characterized in that the actual tonal value of a color is determined by measuring tonal values ​​of the color at different locations on the printed surface that have the same target tonal value for that color, and then averaging over the measured tonal values.

4. Method according to one of the preceding claims, characterized in that when comparing the actual tone value with the target tone value, a difference between the actual tone value and the target tone value is determined.

5. Method according to claim 4, characterized in that the tonal value is corrected by subtracting the difference from the tonal value.

6. Method according to one of the preceding claims, characterized in that several actual tonal values ​​of a color are determined and output in the form of a table of values ​​and / or a gradation curve.

7. Method according to one of the preceding claims, characterized in that, to determine an actual tone value, the respective location of the printed surface is detected with a scanner, an image sensor, a camera and / or a light sensor, preferably hyperspectrally detected.

8. Method according to one of the preceding claims, characterized in that the actual tone value of the color is determined at different locations on the printed surface and compared with the target tone value.

9. Method according to one of the preceding claims, characterized in that the paper web is impregnated after the surface has been printed and pressed with a core, preferably made of a wood-based material, before the at least one actual tone value is determined.