METHOD AND DEVICE FOR PRINTING ON A PAPER TRAY

Description

[0001] The invention relates to a method for printing a paper web with at least one print motif using a digital printing system, comprising the following steps: Providing a digital image of a master pattern of the print motif, printing a plurality of copies of a section of the at least one print motif with different color settings, creating digital images of the printed copies using an optical sensor, comparing the digital images of the printed copies with the digital image of the master pattern by means of an electrical control, whereby a degree of similarity is determined in each case, selecting color settings based on the determined degrees of similarity and printing the print motif onto the paper web with the selected color settings.

[0002] The invention also relates to a digital printing system that is equipped to carry out such a process.

[0003] In Many different products today utilize paper products with a decorative design or motif to enhance their appearance. This applies, for example, to floor, wall, or ceiling panels, where a printed decorative paper is applied to a solid core, such as one made of wood-based material, typically by gluing or laminating. The individual decorative papers are printed in the form of a paper web, preferably wound onto a paper roll, and only cut to the desired size after printing. Paper rolls used in these processes can be several kilometers long and weigh several tons. Printed paper webs are also used in many other products, such as wallpaper or art prints.

[0004] It is crucial to ensure consistent print quality. In particular, it is essential that each print run produces a visually uniform appearance on the printed paper. This guarantees that the product always looks the same to the customer and allows for the combination of different products, even those printed on the same part of the paper web. This is immediately apparent in the case of floor panels. To increase customer acceptance, it is necessary that floor panels from different production batches can be combined without it being possible to subsequently identify which panels were printed in which print run.

[0005] It is therefore common practice to produce a master sample that achieves the desired visual impression and to compare all subsequent production runs and printed paper webs with this master sample. Many different factors can influence the visual appearance of a printed paper web. This includes, on the one hand, environmental factors at the production site, such as the temperature and / or humidity in the digital printing system. On the other hand, production parameters, such as the quantity and / or composition of the primer and / or the printing inks used, also influence the overall visual result of the printing process. Properties of the printed paper web itself, such as paper moisture content and temperature, can also alter the overall visual impression of the printed paper.It is therefore known to be insufficient to simply store the production parameters used in the production of the original sample and to base all subsequent prints of the same print motif on these production parameters.

[0006] Traditionally, a proof print of the design to be printed is first produced on a roll of paper. This proof print is then compared with the original sample to determine whether the proof print is sufficiently similar to the original. Various methods are known in the art for determining the degree of similarity. If the result of such a comparison is negative, meaning the deviation between the proof print and the original sample is too high, the production parameters, especially the color settings used for printing the proof, are changed. A second proof print is produced with these changed color settings and again compared with the original sample. By changing the color settings, an attempt is made to make the proof print more similar to the original sample. This iterative process is carried out until the similarity between the last proof print and the original sample is sufficiently high.After that, production can begin, i.e., the actual printing of the paper web with the print motif.

[0007] A disadvantage of this method is its extreme time-consuming nature. The digital printing system is used to produce the proof print. However, while the similarity between the proof print and the original is being checked, the digital printing system cannot produce and print; it must be shut down until the comparison is complete. This results in significant downtime and therefore high costs, especially when many proof prints need to be produced consecutively. Consequently, small print jobs are particularly unprofitable, as the ratio of downtime to production time is especially unfavorable. Prior art methods exist that print proofs side-by-side on the same medium; see, for example, US 5,182,638 A and US 2020 / 374410 A1.

[0008] The invention is based on the objective of accelerating the process and, in particular, reducing the proportion of downtime.

[0009] The invention solves the stated problem by a generic method characterized in that the majority of copies are printed on an end section of a first paper web and the print motif on a second paper web, wherein the first paper web is removed from the digital printing system after the copies have been printed and the second paper web is clamped into the digital printing system.

[0010] The inventive method significantly reduces downtime. It is no longer necessary to produce a proof print of the entire print design. Instead, it is sufficient to use a section of at least one print design to determine the appropriate color settings. According to the invention, this section is printed in multiple copies, each using different color settings. Preferably, no two copies are printed with the same color settings; rather, the color settings used differ for each printed copy. Thus, it is no longer necessary to perform the individual proof prints iteratively. Digital images are then generated from the printed copies and compared with a digital image of the original design.This is preferably done in an electrical control system, in particular an electronic data processing device, which is especially preferably part of the digital printing system.

[0011] The digital images of the specimens are preferably captured using an optical sensor, for example, a digital camera, and provided as electronic data to the electronic control system. The optical sensor preferably includes an optical and / or spectral colorimeter, which is particularly preferably configured for hyperspectral area measurement. The electronic control system compares the generated digital images of the specimens with the provided digital image of the original sample and generates a value that is a measure of similarity. Based on these values ​​generated for each specimen, a color setting can be selected by means of an algorithm running in the electronic control system, which then forms the basis for the subsequent printing of the design onto the paper web.Preferably, the digital image of the master pattern is not a digital image of the entire master pattern of the print design, but only a digital image of the section of the master pattern of the print design that corresponds to the section of the print design printed in a multiple of copies. This reduces the amount of data in the image of the master pattern and speeds up the process.

[0012] The selected color settings are then used to apply the print design to the paper web in the desired number of copies. This makes it possible to select a color setting that ensures sufficient similarity between the printed design and the original sample with a single proof print containing the majority of copies of the section in different color settings. Multiple proof prints are no longer necessary, significantly reducing downtime for the digital printing system.

[0013] The print design, for example, measures 205 × 140 cm and preferably contains the design of several different elements, such as panels. The size of the section printed in multiple copies is preferably 45 × 33 cm, 42 × 30 cm, or 40 × 30 cm; however, significantly smaller sections, for example, 10 × 15 cm, 11 × 15 cm, or 10 × 30 cm, can also be used. The smaller the section, the more copies of the section can be printed on the same area of ​​the paper web, and the more different color settings can be provided for selection.

[0014] In a preferred configuration, the color settings of the copy whose digital image has the highest degree of similarity to the digital image of the original artwork are selected. This ensures that the optimal color setting is chosen based on the color settings used to print the majority of copies, thus guaranteeing the best possible match between the printed image and the original artwork.

[0015] Preferably, the digital image of the original sample and / or the digital images of the specimens are hyperspectral images. A hyperspectral image contains information not only about the visible part of the color spectrum, but also beyond. The broader the color spectrum used for the digital image, the more accurate and higher quality the comparison of the different digital images will be. It is therefore advantageous if both the digital image of the original sample and the digital images of the respective specimens in the section are hyperspectral images. In a particularly preferred embodiment, the digital image is created under a standardized illuminant, for example, D65 and / or RA or D50. The use of multiple images captured and recorded under different illuminants is also possible, with these then together forming the digital image.

[0016] Advantageously, at least 5, preferably at least 10, and most preferably at least 25 copies of the section are printed. As already explained, these preferably all differ in at least one setting, and preferably in at least one color setting. The more copies of the section are printed with different color settings, the more parts of the copies can be compared with the digital image of the original design, and the better the degree of similarity and thus the print quality of the print motif subsequently printed on the paper web is expected to be.

[0017] Preferably, the color settings are specified in the so-called "Lab" color space. In this space, colors are defined by the three parameters "L" (the L value), "a" (the a value), and "b" (the b value). The L value describes the brightness of the color used. The a value describes the position of the color on a green-red axis, which can also be called the green-magenta axis. The b value describes the position of the color used on the yellow-blue axis. The three parameters L, a, and b allow the colors used to be uniquely described. In a preferred embodiment, the different color settings used for the majority of copies of the printed design differ from each other in at least one of these parameters. Particularly preferably, several copies differ from each other in exactly one parameter.For example, it makes sense to use different color settings for parameter L when choosing a combination of parameters a and b, and to apply these corresponding color settings when printing copies. Additionally or alternatively, it is advisable to vary parameter b when choosing a combination of parameters L and a, and / or to vary parameter a when choosing a combination of parameters L and b. The more copies of the section are printed with different color settings, the more precisely the variation of the various parameters can be selected.

[0018] In this process, it is particularly advantageous to print multiple designs onto the paper web. This is especially beneficial when the individual print jobs, and in particular the number of copies of the design, are relatively small. Individual print jobs, such as floor panels bearing a company logo, are often relatively small, and several of these can be processed and fulfilled on a single paper web. In this case, the advantages of this process are particularly noticeable. For such a process, digital images of all master samples are first provided. A number of copies of at least one section from several, preferably all, of these designs are then printed with different color settings. Preferably, the digital image of each master sample corresponds only to the section that is printed with different color settings.

[0019] It is advantageous, but not necessary, to print the different copies of the cutouts from different print designs separately. The only important thing is that it is known which cutout belongs to which print design.

[0020] Subsequently, digital images of the printed copies are created and compared with the respective digital image of the corresponding master sample. As previously explained, degrees of similarity are then determined between the digital image of a copy and the digital image of the corresponding master sample. Based on these degrees, the color settings to be used for printing are selected, at least for the first print motif. Preferably, the color settings for all print motifs to be printed are determined and selected before the first print motif is printed onto the paper web. However, it is also possible, in principle, to determine the color settings for all subsequent print motifs printed after the first one in the electronic control system while the first print motif is being applied to the paper web in the desired number of copies.

[0021] Advantageously, several, preferably two, three, or four, sections are used of at least one print motif, preferably of several print motifs, and most preferably of all print motifs. These are preferably selected to differ significantly in color from one another. This allows the similarity of the print motif to be printed with the original sample to be determined over a broad color spectrum.

[0022] According to the invention, the majority of copies are printed on an end section of a first paper web, preferably a paper roll. The print motif itself is preferably printed on a second paper web, preferably another paper roll. It has proven advantageous if, after the copies have been printed, the first paper web is removed from the digital printing system and the second paper web is inserted. In this way, the downtime that inevitably occurs when a paper web has to be changed can be used effectively, for example, to carry out the process and determine the color settings for printing the print motif. This applies particularly to semi-automatic or manual roll changers. With a fully automatic roll changer, two axes, and thus two rolls, are clamped simultaneously.During the roll change, the new roll is glued on at full production speed, and the printed roll is cut off to form a continuous paper web. The samples can then be removed before the printed / cut-off roll is taken off the axle. Furthermore, the printed copies of the cutout do not need to be cut out of a running paper web when the printed paper web is fed to further processing. The end section of a paper web is preferably no more than 15 m, more preferably no more than 10 m, and most preferably no more than 5 m long. Most preferably, digital images of the printed copies of the cutout are created before the first paper web is removed.

[0023] The invention further solves the stated problem by means of a digital printing system configured to carry out a method of the type described herein. The working width of such a digital printing system, which can also be referred to as a digital printing system, is preferably at least 1300 mm, more preferably at least 1600 mm, more preferably at least 2000 mm and at most 2800 mm, more preferably at most 2300 mm, more preferably at most 2200 mm, and more preferably at most 2100 mm. The speed of the system, at which the surface to be printed is moved through the system, is preferably at least 80 m / min when printing on paper, more preferably at least 100 m / min, more preferably at least 130 m / min and at most 270 m / min, more preferably at most 200 m / min, and more preferably at most 140 m / min.When printing on heavier objects, for example wood-based panels, the speed is preferably at least 20 m / min, preferably at least 60 m / min, preferably at least 70 m / min and particularly preferably at least 80 m / min and at most 110 m / min, preferably at most 100 m / min and particularly preferably at most 90 m / min.

[0024] If a primer is applied to the surface to be printed, the amount applied is preferably at least 1 g / m², preferably at least 2 g / m², and particularly preferably at least 3 g / m² and at most 10 g / m², preferably at most 6 g / m², and particularly preferably at most 4 g / m².

[0025] The process can be described as "selection." In one embodiment, woodgrain decors are produced on a digital paper printing press with a working width of 2070 mm at a speed of 135 m / min. For example, white, CRYK water-based ink, and a primer of approximately 3 g / m² are applied to 65 g / m² decor paper. The woodgrain decors have repeat dimensions of 1400 mm in length and 2070 mm in width. To check the color of the decors on the specific digital paper printing press and to establish a color master sample, a fixed section with dimensions of 30 x 42 cm is defined on the right edge of the repeat. This section is saved in a new print file. The section is processed with six different color variations, which are then saved individually as print files. All six print files are combined side-by-side in a single print file with a total width of 6 x 30 cm = 180 cm.

[0026] This ensures optimal use of the print width with cutouts. After the gang print file has been processed with a color profile using software, sent to the printing press, and output, the samples are visually evaluated in a light booth under standard D65 illuminant. A decision is made as to which color scheme in the third print file is most pleasing and will serve as the master sample for future productions. This completes the first step: providing the digital image of the master sample. Of course, this process does not need to be repeated every time the procedure is carried out. The master sample, once defined for a given print design, is reused for future print jobs involving the same design. Example of implementation: Production start 1

[0027] Before production begins, only several copies of at least one section of the print design are printed. Preferably, different sections of various designs are pre-placed and printed on a assemblage sheet. In the present embodiment, several print designs are to be printed on the paper web in varying or the same number of copies. A plurality of copies of at least one section of each print design are placed on the assemblage sheet and printed. The number of copies can vary from one print design to another.

[0028] The samples of the cutouts are compared to the provided digital image of the original sample using a hyperspectral measuring device and corresponding software (e.g., ACMS: Advanced Colour Measurement System). The tolerance / similarity index is set to 92%. The degree of similarity must therefore be at least 92% so that the color settings underlying the respective cutout can also be used as the basis for printing the artwork. In the aforementioned embodiment, the tolerance exceeds the specified limit for four artworks. Thus, for each of these artworks, a sample was found that exhibits a higher degree of similarity. However, for one artwork, the print file must be corrected before production begins. Multiple samples of a cutout from this artwork are printed again with different color settings. The process is then repeated.The correction can be done manually or semi-automatically using suitable software (e.g., ColorGate and ACMS with the ColorCorrection Loop). Example of production start 2

[0029] In the second embodiment, six identical sections of a single print motif in different color combinations are placed on a stave sheet and printed for quality control before production begins. The color combinations differ in the b-color axis (yellow-blue). After the proof print, digital images of the sections are created using a hyperspectral measuring device and compared with a digital image of the original sample using suitable software (e.g., ACMS). The similarity index is set to 92%. In this embodiment, sections 1 and 2 are found to be too blue, and sections 4, 5, and 6 are found to be too yellow. The degree of similarity for these sections is below 92%. The degree of similarity for section number 3, at 94%, is within the tolerance range.The production file for this section, including its color settings, will be used for production, i.e., printing the design onto the paper web. These copies were printed the previous day at the end of a paper roll from the same batch to ensure an immediate production start with the correct print file. Example of prototype production

[0030] Wood decors are produced on a digital paper printing press with a working width of 2070 mm and a speed of 135 m / min. For example, white, CRYK water-based ink and a primer of approximately 3 g / m² are applied to 65 g / m² decor paper. The wood decors have repeat dimensions of 1400 mm in length and 2070 mm in width. The conventional method of taking master samples in digital decor printing (roll / sheet) is well-known. For example, during decor printing, master sample sections for quality control are always taken from the same location and only from a portion of the decor / repeat. These can usually be checked and evaluated for color fidelity / similarity index using ACMS. Master samples can be taken more efficiently if the same digital master sample section of the decor is arranged as many times as possible onto the printable format using software (e.g., Photoshop / ColorGate).In digital printing, it's possible to create a new print file consisting solely of master sample sections, optimally utilizing the print width. These "master sample print files" can also be produced outside of the actual decor production process, in dedicated master sample runs. Color fidelity / similarity index to other master samples or production runs can be checked using ACMS.

[0031] Furthermore, various decorative patterns can be produced in a single print file or in multiple print files sequentially. This simplifies the preparation of the cutouts, as many master patterns can be extracted with just a few cuts. Since not only a small portion is taken from a repeat, but the master pattern cutouts are arranged across the entire print width, very little waste is generated. Master pattern production can be scheduled during production gaps to increase the utilization of the production line. At the same time, separate master pattern production saves time for sample extraction during the actual decorative production and minimizes downtime.

[0032] With the aid of the accompanying figures, an embodiment of the present invention will be explained in more detail below. They show Figure 1 - schematic representation of a method according to the prior art, Figures 2 and 3 - different stages in a method according to a first embodiment of the present invention.

[0033] Figure 1 Figure 1 schematically shows a paper web 2 on which multiple repeats 4 are printed. A repeat 4 originally referred to a complete cylinder impression from a printing cylinder. However, this term is still commonly used for digital printing systems and refers to the complete impression of a print design. In Figure 1 Report 4 has been applied to paper web 2 several times in succession. A section 6 is schematically represented as a black box, which, as Figure 1 As can be seen, report 4 is located in the same place.

[0034] Figure 2Figure 1 shows multiple copies of section 6, which have been combined to form a printed image 8. The individual sections 6, which are identical in terms of motif, are printed with different color settings. This is controlled by an electrical control unit 10, which is preferably designed as an electronic data processing device.

[0035] Figure 3 shows the actual printed majority of copies of section 6, each differing in color settings. Reference symbol list

[0036] 2 Paper web 4 Repeat 6 Cutout 8 Printed image 10 Electrical control

Claims

1. A method for printing a paper web (2) with at least one print motif by means of a digital printing facility, wherein the method comprises the following steps: - providing a digital image of a master sample of the print motif, - printing a plurality of copies of a section (6) of the at least one print motif with different color settings, - creating digital images of the printed copies by means of an optical sensor, - comparing the digital images of the printed copies with the digital image of the master sample by means of an electric control unit, wherein a respective degree of similarity is determined, - selecting color settings using the determined degrees of similarity and - printing the print motif onto the paper web (2) with the selected color settings characterized in that the plurality of copies is printed onto an end section of a first paper web (2) and the print motif is printed onto a second paper web, wherein the first paper web (2) is removed from the digital printing facility after the copies have been printed and the second paper web (2) is clamped into the digital printing facility.

2. The method according to claim 1, characterized in that the color settings of the copy are selected whose digital image exhibits the highest degree of similarity with the digital image of the master sample.

3. The method according to claim 1 or 2, characterized in that the digital image of the master sample and / or the digital images of the copies are hyperspectral images.

4. The method according to one of the preceding claims, characterized in that at least 5, preferably at least 10, especially preferably at least 25 copies of the section (6) are printed.

5. The method according to one of the preceding claims, characterized in that the color settings are given in the "Lab" color space and differ in the L value (lightness) and / or in the a value (green-red axis) and / or in the b value (yellow-blue axis).

6. The method according to one of the preceding claims, characterized in that paper web (2) is printed with more than one print motif, wherein - digital images of all master motifs are provided, - a plurality of copies of one section (6) from several, preferably from all, printed motifs is printed with different color settings, - digital images of the printed copies are created and compared with the digital image of the corresponding master sample before the first print motif is printed onto the paper web (2).

7. The method according to one of the preceding claims, characterized in that multiple, preferably two, three or four, sections (6) of at least one print motif, preferably of several print motifs, especially preferably of all print motifs, are used.

8. A digital printing facility configured to carry out a method according to one of the preceding claims, wherein the digital printing facility is configured to - compare the digital images of the printed copies with the digital image of the master sample and determine a degree of similarity, - select color settings using the determined degrees of similarity - wherein the color settings of the copy are selected whose digital image exhibits the highest degree of similarity with the digital image of the master sample and - print the plurality of copies onto an end section of a first paper web (2) and the print motif onto a second paper web, after the first paper web (2) is removed from the digital printing facility after the copies have been printed and the second paper web (2) is clamped into the digital printing facility.