Method and device for printing a surface of a paper

Abstract

The invention relates to a method for printing a paper with a decorative pattern by means of a digital printing system which comprises at least one primer application unit for applying a primer, at least one printing unit for printing the decorative pattern onto the applied primer, and at least one dryer for drying at least one part of the paper after the primer and / or the decorative pattern have / has been applied, wherein the method comprises the following steps: a) providing of an unprinted raw paper, b) applying of a primer to the surface of the unprinted raw paper, c) printing of at least one grid marking and at least one decorative pattern onto the raw paper which is provided with the primer, with the result that a printed raw paper is produced, d) detecting of a grid spacing between at least two points of the grid marking of the printed raw paper in order to determine an actual state of the printed raw paper, e) comparing of the detected grid spacing of the grid marking of the printed raw paper according to the actual state with a grid spacing, stored digitally as setpoint state, for the printed decorative pattern, f1) changing of the stored setpoint grid spacing on the basis of the comparison which is carried out, by a changed setpoint grid spacing and a decorative pattern which is changed by way thereof being produced, and / or f2) changing of at least one process parameter for controlling the application of the primer and / or the printing of the grid marking and / or the drying until a predefined criterion is met.

Description

[0001] The invention relates to a method for printing a paper surface with a decoration using a digital printing system, which has at least one primer application unit for applying a primer, at least one printing unit for printing the decoration onto the applied primer, and at least one dryer for drying at least a portion of the paper after the primer and / or the decoration has been applied. The invention further relates to a method for printing a paper surface with a decoration using a digital printing system, which has at least one printing unit for printing the decoration onto the applied primer and at least one dryer for drying at least a portion of the paper after the primer and / or the decoration has been applied. The invention further relates to a digital printing system for carrying out such a method.

[0002] Such a method is known from DE 10 2013 214 014 A1.

[0003] Today, the surface of paper is printed with a decoration in many different applications and for many different purposes. In this context, a decoration is understood to be a graphic, especially a pictorial, representation intended to change the visual impression created by the paper. The paper can be in the form of individual sheets or used in a roll. If the paper is printed in a roll, it is preferably cut to the correct size required for the respective application after printing.

[0004] Using a digital printing system to print on the surface of a paper offers several advantages. For example, switching from one design to another is very quick and easy. No rollers or similar elements need to be removed or replaced, so changeover times are very short. Here, the paper is clamped into the digital printing system and held at the correct tension by rollers. Wrinkles must be avoided at all costs if a clean print image is to be produced and the amount of waste is to be kept as low as possible.

[0005] It is well known that the dimensions of the paper web to be printed can change during the web-fed printing process. This can occur simply due to the tension under which the paper web is held in the printing system, leading to stretching of the paper web in the transport direction, also known as the running direction. Particularly with color-intensive designs, which require a relatively large amount of ink in liquid form to be applied to the paper web to be printed, what is known as wet stretching of the paper can occur. It is well known that moist or even wet paper expands. This usually happens more across the running direction than in the running direction. Different paper grammages, irregularities and deviations in paper production and / or the inks used can also mean that the base paper to be printed on must first be coated with a primer.This achieves a uniform, reproducible, and predetermined base color for the paper to be printed. Like the printing inks, the primer is usually in liquid form, so its application rate can also lead to wet expansion of the paper. The moisture content of the base paper, which can be determined or at least influenced by the humidity in the room where the paper is stored, can also affect paper expansion.

[0006] In particular, paper moisture content, which, as already explained, can be influenced by the ink or primer, also depends, at a given humidity level, on how well and how deeply the applied liquid can penetrate the paper in question. This can be influenced by additives added to the base paper during production, for example, or by the base paper's manufacturing process.

[0007] A digital printing system of the type described here therefore has at least one dryer, by means of which at least part of the surface to be printed, but preferably the entire surface to be printed, can be dried. This occurs, for example, by subjecting the surface to an air stream, in particular a warm air stream, or by irradiation with heat or infrared radiation. Such drying can take place at different times in the process. The digital printing system preferably has at least two dryers, one of which is arranged downstream of the primer application unit and one downstream of the printing unit. This makes it possible to dry the paper after the primer has been applied and after the screen marking and at least one decoration have been printed. In both application steps, in addition to the actual active substances, water is also introduced, which has an effect on the unreinforced base paper.

[0008] Preferably, any paper stretching that may occur, especially in the direction of travel, can be adjusted by adjusting the tension applied by the guide rollers of the digital printing system. A disadvantage is that dryers, air exhausts, or guide rollers and their tension are often adjusted manually, based on experience and individual assessment by the system operator, for example, via a control panel. Reproducible control is often not available.

[0009] The disadvantage is that this leads to a relatively large amount of waste, particularly in applications where it is important that the size of the surface to be printed matches the size of the printed decoration. This can result in a relatively large amount of waste if, for example, length and / or width values cannot be adhered to. One such application is the production of laminate panels, which usually have a core made of a wood-based panel. A printed paper with a decoration on its surface is applied to this core. A structure is usually embossed or pressed into the surface of a laminate panel produced in this way, preferably synchronized with the printed decoration on the surface of the paper. If the printed decoration shows a wood grain, for example, the corresponding structure picks up on this wood grain and the tactile impression of the panel is adapted to that of a real wood panel.If there are any discrepancies between the dimensions of the decor and the dimensions of the structure, this will be immediately noticeable to the observer and such a panel must be rejected as scrap.

[0010] DE 10 2018 217 362 A1 describes a process in which a design to be printed onto a paper web is pre-distorted before printing to counteract and compensate for subsequent shrinkage of the paper that may occur during drying. However, it is necessary to know the expected shrinkage in order to achieve the correct distortion of the printed image.

[0011] US 2007 / 0144375 A1 proposes using markings detected by an optical detector to adjust individual machine elements of the printing device. The individual elements are, for example, different printing units that apply different inks, and the markings are located diagonally opposite each other. Changes in the distance between the markings provide information about how the individual machine elements need to be adjusted.

[0012] EP 0 681 525 B1 addresses a different problem. When a printing roller rolls repeatedly across the substrate, a "parting line" inevitably occurs between two repeating patterns. This line should be placed in an area with the lowest possible dot density so that it is less noticeable to the human eye.

[0013] EP 3 028 132 B1 discloses a device with at least two printing units. Sensor data is used to determine how and whether the web to be printed has changed, in particular, elongated, during the first printing process. The second printing unit is adjusted based on this sensor data.

[0014] The present invention is therefore based on the object of further developing the method for printing on a surface of a paper in such a way that less waste is produced.

[0015] The invention solves the problem by a method according to claim 1.

[0016] In the first process step, the unprinted base paper is first provided. This is preferably unresinated. In the second process step, the primer is applied to the unprinted base paper. This can also be referred to as a primer layer. The base paper is then referred to as primed base paper. It is still unresinated. In process step c), a screen marking is applied to the primer. This is printed, for example, onto the paper to be printed, for which purpose the digital printing system has at least one printing unit. This preferably has a plurality of print heads with which the decoration and the at least one screen marking are printed. The screen marking is preferably designed such that it is machine-detectable and readable, but not visible to the human eye. According to the invention, the screen marking is printed with part of the decoration or with the entire decoration.The paper is then an unreinforced, primed and printed base paper.

[0017] In a further process step, a grid spacing of the grid marking on the printed base paper is recorded, extending between two points of the grid marking. The points do not have to be dot-shaped or even circular print dots, but can be any identifiable location on the grid marking. The grid spacing preferably extends along the direction of travel, i.e., the direction the paper is transported through the digital printing system, or perpendicular to this direction. However, the grid spacing can also extend along any other direction.

[0018] This grid spacing is detected by detecting and measuring at least that part of the paper surface on which the grid marking is located using a corresponding sensor. Depending on the grid marking used, different sensors can be employed. In a preferred embodiment, the grid marking is printed onto the surface of the paper to be printed. This can preferably be done using an ink having a color that is also contained in the decoration to be printed later, so that the grid marking is no longer recognizable to the viewer after the decoration has been printed, since it has either become part of the decoration or has preferably been covered by the decoration. In the latter case in particular, the grid marking can also be printed using a color that is not contained in the decoration.Alternatively or additionally, the screen marking is printed with an ink invisible to the human eye, which can be detected, for example, by an infrared camera, which in this case is preferably used as a sensor. The infrared image then shows the applied screen marking and the screen spacing can be determined. The advantage of this type of screen marking is that the screen marking does not have any disruptive or noticeable effect on the subsequent decoration. The use of an ink that can be excited to glow, for example by exposure to electromagnetic radiation, such as an ink that fluorescent under black light, i.e. UV light, can also be advantageous. For this type of screen marking, it is usually necessary to use an additional printing unit, since a conventional printing unit in a digital printing system contains four different colors: cyan, magenta, yellow and black.The equipment required for the digital printing system is therefore lower if a raster marking is applied at the same time as the decoration, which is not visible to the human eye but can be read by machines and the information it contains can be recorded and further processed.

[0019] The resulting grid spacing determines the actual condition of the printed base paper. It is not absolutely necessary, but advantageous, for parameters of the printed base paper, such as paper moisture, to be determined, evaluated, or even stored in the process described here.

[0020] The actual state determined in this way, i.e. the recorded grid spacing, is compared with a target state. This is a grid spacing for the decoration after it has been printed. It is preferably stored digitally, for example in an electronic data storage device, which can be accessed by an electrical control system, in particular an electronic data processing device. The target state, i.e. the grid spacing of the digital decoration, extends between two points on the decoration. The decoration has been printed onto the surface of the paper. The target state is determined in the digital template of the decoration by the grid marking being underlying the decoration. The grid marking is stored with the decoration in the electrical control system of the system, for example an electronic data processing device. They are linked to one another. The stored data forms the target value, with which the measured actual value is compared.In the event of a deviation, either the target value, i.e., the digital print template, can be changed and adjusted, or the actual value can be influenced by adjusting a printing parameter. The process is carried out until the predetermined criterion is met. Preferably, the actual value is determined using the printed raster marking, and the target value is determined using the digitally stored raster marking. The printed raster marking is preferably embedded in the printed decoration, and the stored raster marking is connected to the stored decoration.

[0021] To ensure that the design to be printed fits the paper as well as possible and that further processing is possible, simply, reliably, and with high quality, a predetermined criterion must be met. Various types and configurations of criteria are conceivable here, which are described below. If the criterion is not met, the grid spacing of the digital template, i.e., the target state, which is subsequently referred to as the target grid spacing, is adjusted in process step f1) and / or the recorded grid spacing, i.e., the actual state, is adjusted in process step f2).

[0022] If the target grid spacing, i.e., the target state, is changed in step f1), this means that the distance between at least two points of the decoration is adjusted and changed. This makes it possible, for example, to change the dimensions of the decoration and adapt them to the dimensions of the printed base paper, which are known from the recorded grid spacing or can at least be calculated or estimated. It is not necessary, but advantageous, if these dimensions are actually determined. The decoration modified in this way is printed on the printed base paper.

[0023] If, based on the result of the comparison, for example in method step f2), the screen spacing, i.e., the actual state, is to be adjusted, the paper is dried using at least one dryer to change the screen spacing. This reduces the moisture content and the dimensional change caused in particular by the paper moisture, i.e., in particular, wet expansion of the paper. The digital printing system preferably has at least one electrical control system, in particular an electronic data processing device, which is configured to determine the screen spacing from the sensor data and the target screen spacing from the decoration, and to carry out the comparison. It is also capable of checking the result of the comparison to determine whether the predetermined criterion is met. Alternatively or additionally, at least one process parameter of the primer application unit can also be adjusted.However, this change will only take effect when printing the next decoration.

[0024] In an alternative embodiment, the invention solves the stated problem by a method according to claim 2.

[0025] In this case, for example, a base paper already coated with a primer is purchased and added to the process. Process steps B) to E2) correspond to steps c) to f2). The above statements also apply to steps B) to E2).

[0026] The base paper is preferably unresinated and preferably also unprinted before the primer is applied.

[0027] Preferably, the at least one process parameter of the digital printing system is a process parameter of the primer application unit and / or the dryer.

[0028] The electrical control system is also preferably configured to control the at least one dryer and / or the at least one primer application unit. In doing so, it preferably accesses data stored in an electronic data storage device, from which it is determined, based on empirical values or experimental data, how much the paper must be dried by means of the at least one dryer in order to bring about a specific dimensional change and thus a change in the grid spacing. A functional relationship or a table of values from which this can be derived preferably exists for different types of base paper, different decorations, different coatings, and applications of the paper to be printed. Preferably, all of these functional relationships or tables of values are stored in the electronic data storage device.The relevant parameters are set via an input device, for example a control panel, and thus communicated to the electrical control system. The data processing device then accesses the relevant data in the electronic data storage device. Alternatively or additionally, the device has several dryer units or at least the option of using a dryer unit multiple times. In this case, the dryer is preferably used first to dry the paper to be printed. The screen spacing, i.e. the actual state, is then preferably recorded again and compared with the corresponding target screen spacing. If the predetermined criterion is still not met, the process parameter of the dryer is adjusted, for example, and the next repeat is processed with the adjusted drying process until the predetermined criterion is met. The recorded screen spacing is compared with the target screen spacing.

[0029] In a preferred embodiment, the target grid spacing is changed by stretching or compressing at least part of the decoration, preferably the entire decoration. This is particularly advantageous if, for example, the stretch of the paper to be printed is so great that the at least one existing dryer is insufficient to dry the paper sufficiently to compensate for the stretch.

[0030] The predetermined criterion is preferably met when the difference between the detected grid spacing and the target state falls below a predetermined limit value. For this purpose, it is useful if the two points between which the grid spacing is detected correspond to the points at which the two points between which the target state is determined are to be applied. In this case, the length of the grid spacing should correspond as closely as possible to the length of the target state. The predetermined limit value, which can be, for example, a maximum of 1 cm, preferably a maximum of 5 mm, particularly preferably a maximum of 1 mm, depends on the required tolerance and the application for which the printed paper is to be used. The predetermined limit value is preferably dependent on the length of the grid spacing and / or the target grid spacing.The larger the grid spacing and / or the target grid spacing, the higher the tolerance, i.e. the predetermined limit value, can be.

[0031] Preferably, the predetermined criterion is met when the ratio of the detected grid spacing to the desired state lies within a predetermined range. This means that the ratio is greater than a predetermined lower limit and less than a predetermined upper limit. As soon as the ratio of the two values lies outside this range, the actual state and / or the desired state are adjusted accordingly.

[0032] This choice of criterion is advantageous, for example, if the two points between which the grid spacing is recorded do not correspond to the points at which the points are to be applied between which the target state is determined. For example, the points can be selected such that the recorded grid spacing should be as precisely as possible twice the target grid spacing, i.e. the target state. If the ratio between the two values is not exactly 2, but deviates from this value, the predetermined criterion may not be met. For example, the upper limit can be a maximum of 2.1, preferably 2.05, particularly preferably 2.01, and the lower limit can be at least 1.9, preferably at least 1.95, particularly preferably at least 1.99. Depending on the selected grid spacing and target state, other values may also be useful and advantageous.

[0033] Advantageously, several grid spacings of the printed base paper are recorded and compared with several similarly recorded target states. The recorded grid spacings and / or the target grid spacings, i.e., the target states, preferably extend in at least two linearly independent, particularly preferably perpendicular, directions. In a preferred embodiment, one of the recorded grid spacings is compared with one of the target states. Different predetermined criteria can be used for each comparison. The use of several grid spacings and several target grid spacings is particularly advantageous when the expected elongation of the paper to be printed is inhomogeneous.This is the case, for example, when a primer or a color-receiving layer, or even an already applied decoration or part of the decoration, is applied inhomogeneously, i.e., with a locally varying application rate of primer, ink, or other liquid. This can result in certain areas exposed to a large amount of liquid, for example, exhibiting high moisture content and correspondingly high wet expansion, while other areas exposed to a small amount of liquid, for example, exhibiting low moisture content and correspondingly low wet expansion.

[0034] Preferably, the actual state and / or the target state are changed depending on the location. This makes it possible to react to the different results of the various comparisons, in each of which a recorded grid spacing is compared with one of the target states. While the electrical control system is preferably set up to compress and / or stretch different areas and parts of the decoration to different degrees, changing the recorded grid spacing requires, for example, location-dependent drying. A modular dryer having several individual dryer elements can be used for this purpose. The electrical control system is then preferably set up to control the individual dryer elements and, for example, to operate them for different lengths of time or with different intensities, and thus to dry different areas of the paper surface to be printed to different degrees.

[0035] Advantageously, the raster marking has raster points that are preferably arranged equidistantly along several lines, with the lines preferably running in two linearly independent, preferably perpendicular, directions. In another embodiment, the raster marking is a planar marking, for example, a frequency-modulated raster marking. Alternatively, an amplitude-modulated raster marking can also be used.

[0036] According to the invention, the grid marking is integrated into the decor and is not macroscopically visible.

[0037] The grid marking is preferably included in the finished printed decoration, but is not visible to the human eye. This means that it cannot have a detrimental effect on the visual impression, but is still available in further processing steps. For this purpose, it is advantageous if the grid marking contains information that enables product tracking, for example, and makes the author, printing location, time of manufacture or other information identifiable. This is particularly important for quality assurance and potential liability. Tools on a dividing saw and / or a profiling machine, as well as units for sealing or painting the edges of a panel on whose carrier plate the printed paper, particularly in the form of decorative paper, is located, can be aligned using the grid marking that is visible to sensors.In the context of the present invention, a decorative paper is understood to mean a base paper decorated with a synthetic resin impregnated with a synthetic resin, which is suitable for pressing with a carrier in order to form a laminate together with the carrier, for example an HDF board.

[0038] In In later steps, the grid marking can be used, for example, to align the various elements to be pressed together, such as the printed paper and a carrier board, in a pressing tool, such as a press belt or press plate. In this case, the paper is preferably already impregnated and, if necessary, cut to size. It is then a decorative paper.

[0039] The invention further achieves the stated object by a digital printing system for carrying out a method according to one of the embodiments described here. The device has at least one primer application unit for applying an ink-receiving layer, at least one printing unit for printing the decoration onto the applied ink-receiving layer, and at least one dryer configured to dry at least a portion of the paper surface to be printed, but preferably the entire paper surface to be printed, after the ink-receiving layer and / or the decoration or a portion thereof has been applied. Particularly preferably, the at least one dryer has a plurality of dryer elements, each configured to dry a portion of the paper surface to be printed. The dryer elements can preferably be controlled individually and / or in groups by the electrical control system.This ensures that the drying performance of each individual dryer element can be adjusted separately and independently of other dryer elements. The digital printing system preferably has several dryers arranged one behind the other in the direction of travel of the paper to be printed. Particularly preferably, a printing unit is located between at least two of the dryers.

[0040] The digital printing system has at least one printing unit, preferably several printing units, each configured to print on the surface of a paper. This can be done by applying a primer or by applying the decoration.

[0041] The system preferably also has at least one sensor for detecting the screen marking. Preferably, such a sensor is located in front of each dryer. The measurement data acquired by the respective sensor is transmitted to the electrical control system, which then determines the screen spacing. If the actual state is to be adjusted in response to the comparison result, it is advantageous if a dryer is located behind the sensor in the direction of paper travel. However, if only the target state is to be adjusted, it is sufficient to have a printing unit behind the sensor.

[0042] A grid marking of the type described here can also be used to optimize further process steps in the production of laminate and its subsequent processing into laminate panels. Such panels are preferably suitable for floating installation, particularly for glueless installation.

[0043] The grid marking preferably allows product tracking in the sense of traceability, for example for reasons of product liability and / or proof of origin, from which production line and from which manufacturer a decorative paper originates, and / or can preferably be used as a basis for aligning decorative paper and carrier board to form a structure on a pressing tool, such as a press belt or press plate, in particular for so-called embossed-in-register surfaces with structures that are at least partially synchronous to the decor, as a basis for aligning tools of a dividing saw with which panels are sawn out of a large-format laminate, as a basis for aligning tools of a profiling machine with which connecting profiles and locking elements are processed on the side edges of the panels, so-called tongue and groove connections, for example as so-called click connections,and / or as a base for aligning units for edge sealing and / or edge painting of the panels.

[0044] An embodiment of the present invention is explained in more detail below with the aid of the accompanying figures. Figure 1 - a schematic plan view of a digital printing system and Figure 2 - an enlarged section of a plan view of a system according to an embodiment of the present invention.

[0045] Figure 1 shows a very schematic plan view of a digital printing system with which a method of the type described here can be carried out. The unprinted base paper 2 to be printed is in the embodiment shown as a paper roll, in particular as a continuous paper web, and is Figure 1In the embodiment shown, the paper is conveyed from left to right through the printing system. The running direction therefore also extends from left to right. The system has a first printing unit 4, which is set up to apply a first coat to the paper web to be printed. This is, for example, a primer, which is preferably applied homogeneously. The first printing unit 4 is a primer application unit. In the first printing unit 4, liquid is therefore applied to the paper 2, so that the moisture content of the paper 2 changes. After the paper 2 has left the first printing unit 4 to the right, there is a change in the expansion of the paper 2. On the one hand, the tension in the paper web 2, which must be maintained for printing, causes expansion in the running direction. On the other hand, expansion also occurs transversely to the running direction because the paper moisture content has increased and the paper therefore expands.

[0046] The paper 2 then runs into the first dryer 6, where it is dried. This causes the paper to shrink both in the running direction and transversely to the running direction. Dried in this way, the paper 2 reaches a second printing unit 8, which in the present embodiment is used to print a marking and the decoration onto the top side of the paper 2. The decoration is generally inhomogeneous, so that an inhomogeneous amount of liquid is applied to the top side of the paper 2 in the second printing unit 8. This results in a possibly inhomogeneous expansion of the printed paper 2. The raw paper printed in this way then passes through a detection system 10, which has at least one sensor with which a raster marking applied to the top side of the paper can be detected. The data thus determined is fed to an electrical controller 12, which uses this to determine the detected raster spacing.Within the electrical control system, this detected grid spacing is compared with a target grid spacing, i.e. the target state, which in the illustrated embodiment corresponds to a target size. As already explained, the application of ink in the second printing unit 8 causes the paper to stretch, so that the applied decoration also expands. Particularly in the event that the printed paper is to be further processed, as is the case, for example, in the production of wood-based panels, it is important that the printed paper leaving the system has the expected and required dimensions as precisely as possible. The electrical control system therefore transmits control signals 14 to a second dryer 16. This second dryer 16 dries the paper 2 so that it has the required dimensions.

[0047] Figure 2shows an enlarged section of the system in a separate illustration. The raster marking is applied in a target specification 18. For this purpose, either a first part of the second printing unit 8 or a separate printing unit is used. Subsequently, the actual decorative printing 20 also takes place in the second printing unit 8. In this case, a partial area 22 of the top side of the paper 2 to be printed is printed particularly heavily and intensively. This means that a comparatively large amount of ink is applied, so that the paper moisture in this area has increased significantly. The partial area 22 therefore expands more than the remaining printed area of the paper surface. This is detected in the detection system 10 by means of at least one sensor.The control signals 14 transmitted by the electrical control 12 are transmitted to the second dryer 16, which in the embodiment shown has a row of individual dryer elements 24 which are arranged transversely to the running direction of the paper, which is also shown in . Figure 2 extending from left to right, are arranged side by side. The dryer elements 24 located in the area of sub-area 22 and configured to dry it are controlled, unlike the other dryer elements 24, to provide increased drying performance, so that the paper 2 is dried more thoroughly in sub-area 22 than in the remaining part. This causes paper shrinkage, producing the desired result 26. List of reference symbols

[0048] 2Paper 4First printing unit 6First dryer 8Second printing unit 10Detection system 12Electrical control 14Control signals 16Second dryer 18Target value 20Decorative print 22Sub-area 24Dryer element 26Result

Claims

1. A method for printing a decorative pattern onto a paper (2) by means of a digital printing facility that comprises - at least one primer application unit (4) for applying a primer, - at least one printing unit (8) for printing the decorative pattern onto the applied primer, and - at least one dryer (6, 16) for drying at least one part of the paper (2) after the primer and / or decorative pattern has been applied, the method comprising the following steps: a) providing an unprinted base paper, b) applying a primer to the surface of the unprinted base paper, c) printing at least one grid marking and at least one decorative pattern onto the primer-coated base paper, thereby obtaining a printed base paper, d) detecting a grid spacing between at least two points of the grid marking of the printed base paper to determine an actual state of the printed base paper, e) comparing the detected grid spacing of the grid marking of the printed base paper according to the actual state with a digital grid spacing saved as a target state for the printed decorative pattern, f1) amending the saved target grid spacing on the basis of the comparison by creating an amended target grid spacing and thus an amended decorative pattern and / or f2) amending at least one process parameter for controlling the application of the primer and / or the printing of the grid marking and / or the drying on the basis of the comparison carried out, until a predetermined criterion is met, characterised in that the grid marking is a frequency-modulated grid marking that is integrated into the decorative pattern and is macroscopically invisible.

2. A method for printing a decorative pattern onto a paper (2) by means of a digital printing facility that comprises - at least one printing unit (8) for printing the decorative pattern onto the applied primer, and - at least one dryer (6, 16) for drying at least one part of the paper after the primer and / or decorative pattern has been applied, the method comprising the following steps: A) providing a base paper coated in a primer, B) printing at least one grid marking and at least one decorative pattern onto the primer-coated base paper, thereby obtaining a printed base paper, C) detecting a grid spacing between at least two points of the grid marking of the printed base paper to determine an actual state of the printed base paper, D) comparing the detected grid spacing of the grid marking of the printed base paper according to the actual state with a digital grid spacing saved as a target state for the printed decorative pattern, E1) amending the saved target grid spacing on the basis of the comparison by creating an amended target grid spacing and thus an amended decorative pattern and / or E2) amending at least one process parameter for controlling the application of the primer and / or the printing of the grid marking and / or the drying on the basis of the comparison carried out, until a predetermined criterion is met, characterised in that the grid marking is a frequency-modulated grid marking that is integrated into the decorative pattern and is macroscopically invisible.

3. The method according to claim 2, characterised in that the at least one process parameter of the digital printing facility is a process parameter of the primer application unit (4) and / or the dryer (6, 16).

4. The method according to claim 1, 2 or 3, characterised in that, to amend the grid spacing of the printed base paper, the printed base paper is dried by means of the at least one dryer (6, 16).

5. The method according to one of the preceding claims, characterised in that the target grid spacing is amended by elongating or compressing at least part of the decorative pattern, preferably the whole decorative pattern.

6. The method according to one of the preceding claims, characterised in that the predetermined criterion is met when the difference between the detected grid spacing and the target state is below a predetermined limit value.

7. The method according to one of the preceding claims, characterised in that the predetermined criterion is met when the ratio of the detected grid spacing and the target state lies within a predetermined range.

8. The method according to one of the preceding claims, characterised in that multiple grid spacings of the printed base paper are detected and compared with multiple target states, the detected grid spacings and / or the target states extending in at least two linearly independent, preferably perpendicular, directions.

9. The method according to claim 8, characterised in that the grid spacing of the printed base paper and / or the target state is amended depending on location.

10. The method according to one of the preceding claims, characterised in that the grid marking features grid points arranged, preferably equidistantly, along multiple lines, the lines preferably extending in two linearly independent, preferably perpendicular, directions.

11. A digital printing facility for carrying out a method according to one of the preceding claims, wherein the digital printing facility has at least one sensor for detecting the grid marking and at least one electrical control unit that is configured to determine a grid spacing from the sensor data and to compare it with the digitally saved grid spacing for the printed decorative pattern, and to carry out the amendment on the basis of the comparison.

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