METHOD FOR PRINTING ON A PAPER TRAY

Description

[0001] The invention relates to a method for printing a paper web with a print motif using a digital printing system.

[0002] Paper webs are printed with a design for a wide variety of purposes. They are used, for example, to produce panels or boards that can be used as flooring, wall coverings, ceiling coverings, or furniture panels. Particularly in the production of floor panels, it is desirable and highly advantageous if the printed paper webs do not exhibit any color variations. This is especially important when panels with the same design are produced at different times, meaning that different paper webs are printed with the same design at different times. To achieve reproducibility and color fidelity in this situation, it is known in the art to use a digital print template.

[0003] This digital print template contains digital information about the print design. Using this digital print template, it is possible to control the digital printing system and thus use it to print the design onto the paper web. Various methods are known in the prior art in which the digital print template is used to print a design onto the paper web. This printed copy is then compared with the target design. Based on this comparison, it can be determined whether the control commands and parameters contained in the digital print template, such as color values ​​used during printing, ensure sufficient quality of the printed product, i.e., the paper web printed with the design.

[0004] For example, WO 2019 / 234147 A1 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 information about a higher frequency range. These target images are then compared with actual images generated by the printed copy of the paper web.

[0005] US 2005 / 0259276 A1 describes a procedure for calibrating a printer. The print result of a test print is compared with a target result to determine how accurately the printer being calibrated reproduces the specifications. If deviations are found, the printer is calibrated so that it responds differently to the unaltered digital information it is fed with, namely in a way that more closely matches the original.

[0006] From EP 3 961 166 A1 a method is known in which hyperspectral measurement data of a printing result are compared with predetermined reference measurement data in order to determine the quality of the printing result.

[0007] A disadvantage is that this "re-sampling," i.e., checking, adjusting, and setting the various production parameters of the digital printing system, is time-consuming. It is necessary because external influences, such as changes in temperature and humidity, the ink used, any primer applied, and / or the paper web, can impair the quality of the printed design. Correction, or at least a verification of the print quality, is therefore essential. However, the methods known from the prior art are time-consuming and / or material-intensive, and thus expensive. Furthermore, while deviations of the printed image on the paper web from a basic sample or a digital print template can be determined, the necessary correction is usually not precisely identifiable and therefore not practically feasible.Rather, "re-sampling" often requires several iteration steps and, if necessary, experience and expertise from an employee operating the digital printing system in order to achieve the desired result as quickly as possible.

[0008] The invention is therefore based on the objective of proposing a method that eliminates or at least mitigates these disadvantages.

[0009] The invention solves the stated problem by a method for printing a paper web with a print motif using a digital printing system, wherein the method comprises the following steps: Providing a digital printing template containing digital information about the print design and including the target tonal values ​​of a color of the print design and a target tonal value of a master sample of the print design; producing a printed copy by printing on a surface of the paper web using the digital printing system based on the printing template; determining at least one actual tonal value of the color at at least one location on the printed surface; comparing the determined actual tonal value with the target tonal value and correcting the target tonal value in the digital information of the printing template based on the result of the comparison; producing another printed copy by printing on a surface of the paper web using the digital printing system based on the corrected information of the printing template (4,4-6).

[0010] The tonal value of a color indicates the different gradations of the color between light and dark as a percentage. A tonal value of 0% means there is no color. The corresponding area with this tonal value is therefore unprinted and light. Such areas are also called highlights. They are called highlights regardless of the color of the surface onto which the print is applied. In contrast, a tonal value of 100% represents the maximum color, also called a solid tone, which is accordingly considered dark. Dark areas are also called shadows. The digital information about the print design, contained in the digital print file, also includes the target tonal values ​​of each color in the print design. Preferably, the digital information contains all the target tonal values ​​of all colors used in printing the print design.For each color used, a target tone value of the corresponding color or colors is preferably stored for each printed area of ​​the paper web, for example, each pixel of the printed motif.

[0011] In the inventive method, the digital print template is used to print the design onto a paper web, thereby producing a printed copy or sample copies. The actual tonal value of the color is then determined at at least one point on the printed surface and compared with the provided target tonal value for the corresponding color at that point. Based on this comparison, the tonal value in the digital information of the print template is corrected. Subsequently, another sample copy can be printed using the corrected information in the print template, or production can begin.

[0012] 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 inventive approach makes it possible to determine a direct corrective measure, regardless of what caused the deviation of the actual tone value from the target tone value.

[0013] The target tonal values ​​are preferably determined starting 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 captured from this printed template. This is preferably, but not necessarily, done hyperspectrally. The tonal values ​​thus captured are then referred to as the 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. It is no longer necessary to refer back to the printed template.

[0014] Preferably, a tonal value is provided as the target tone value, which is read from or determined from the digital information of the digital print template.

[0015] Preferably, target tonal values ​​for different colors are provided, and the actual tonal values ​​of several of these colors, preferably all of them, are determined. This ensures that the print quality of not just a single color and / or a single color value of one or more colors is determined. It also ensures that multiple tonal values ​​of one or more colors, or a single tonal value of multiple colors, are checked and compared with the corresponding target tonal value that has been provided.

[0016] Preferably, the actual tonal value of a color is determined by measuring the color's tonal values ​​at various locations on the printed surface that have the same target tonal value for that color. Preferably, these determined tonal values ​​are then averaged to ascertain the actual tonal value. If different deviations of the respective tonal values ​​from the target tonal value occur at different locations on the printed surface, these deviations would be less significant due to the averaging process.

[0017] When correcting tonal values, they are preferably corrected identically for the entire print design in the digital information of the print file. A specific tonal value of a particular color may be required at different locations within the print design. In principle, it is possible to determine the actual tonal value at each of these locations in the proof, compare it with the target tonal value, and correct the tonal value individually at each location according to the specific deviation. However, to correct this across the entire print, the deviation between the actual and target tonal values ​​would have to be determined at every point in the proof, for example, at every pixel of the print design. This is time-consuming and generally impractical.

[0018] It is therefore advantageous to correct the tonal value in the digital information of the print file for a specific color uniformly across the entire print design. This correction can be based on comparing a single, specific 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, and most preferably at least 20 individually determined tonal values ​​of the respective color.

[0019] In a preferred embodiment, when comparing the actual tonal value with the target tonal value, a difference is determined between the actual and target tonal values. 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 this difference from the tonal value. For example, if the actual tonal value is 10 percentage points higher than the target tonal value, the difference is consequently 10. In this particularly preferred embodiment, this difference is subtracted from the tonal value, thus reducing it by this amount. Consequently, in the next print run, the tonal value used to control the digital printing system is specified as smaller, so that the printed result more closely corresponds to the originally intended print design, i.e., the target tonal value.

[0020] Preferably, several actual tonal values ​​of a color are determined and displayed in a table and / or a gradation curve. The actual tonal values ​​are then compared to the corresponding target tonal values. In a perfect print, where there is no difference between the target and actual tonal values, the resulting gradation curve is a straight line with a slope of 1.

[0021] Preferably, to determine the actual tonal value, each location on the printed surface is captured using a scanner, an image sensor, a camera, and / or a light sensor, preferably using hyperspectral imaging. Preferably, the actual tonal value of the color is determined at different locations on the printed surface and compared with the respective target tonal value.

[0022] 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-based material, before the at least one actual tone value is determined.

Claims

1. A method for printing a paper web with a print motif by means of a digital printing facility, wherein the method comprises the following steps: - providing a digital print template, which contains digital information on the print motif, and a target tone value of a color of the print motif, and a target tone value of a master sample of the print motif, - producing a print copy by printing onto a surface of the paper web by means of the digital printing facility on the basis of the print template, - determining at least one actual tone value of the color at at least one point of the printed surface, - comparing the determined actual tone value with the target tone value of the master sample, - correcting the target tone value in the digital information of the print template on the basis of the results of the comparison, - producing a further print copy by printing onto a surface of the paper web by means of the digital printing facility on the basis of the corrected information of the print template.

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

3. The method according to claim 1 or 2, characterized in that the actual tone value of a color is determined by measuring tone values of the color at different points of the printed surface which have the same target tone value for this color, and then averaging the measured tone values.

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

5. The method according to claim 4, characterized in that the tone value is corrected by subtracting the difference from the tone value.

6. The method according to one of the preceding claims, characterized in that multiple actual tone values of a color are determined and output in the form of a value table and / or a gradation curve.

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

8. The method according to one of the preceding claims, characterized in that the actual tone value of the color is determined at various points of the printed surface and compared with the target tone value.

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