EP4008561A1 - Method and device for printing a surface of a paper - Google Patents

Abstract

The invention relates to a method for printing a decoration on paper (2) by means of a digital printing system which has - at least one primer application unit (4) for applying a primer, - at least one printing unit (8) for printing the decoration on the applied primer, and - at least a dryer (6) for drying at least part of the paper (2) after the primer and/or the decoration has been applied, the method having the following steps:a) providing an unprinted base paper (2),b) applying a primer on the surface of the unprinted base paper (2),c) imprinting at least one grid marking and at least one decoration on the base paper (2) provided with the primer, so that a printed base paper (2) is produced,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 d he grid marking of the printed raw paper according to the actual state with a grid spacing for the printed decoration stored digitally as a target state,f1) changing the stored target grid spacing on the basis of the comparison carried out by using a changed target grid spacing and a modified decor is created and/orf2) changing 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 predetermined criterion is met.

Description

The invention relates to a method for printing a surface of a paper 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 on the applied primer and at least one dryer for drying at least part of the paper afterwards the primer and/or the decoration has been applied. The invention also relates to a method for printing a surface of a paper with a decoration using a digital printing system that has at least one printing unit for printing the decoration on the applied primer and at least one dryer for drying at least part of the paper after the primer and/or the decoration has been applied. The invention also relates to a digital printing system for carrying out such a method.

The surface of a paper is now printed with a decor in many different applications and for many different purposes. In the present case, a decoration is understood to mean, in particular, a graphic, very particularly a pictorial representation, with which the optical impression that the paper creates is intended to be changed. The paper can be in the form of individual sheets, so-called sheets, or can be used in the form of a roll. If the paper is printed in roll form, it is preferably trimmed after printing to the correct size needed for the particular application.

Using a digital printing system to print the surface of a paper has a number of advantages. For example, you can switch from one decor to another very quickly and easily. No rollers or similar elements have to be removed or exchanged for this, so that the changeover times are very short. Here the paper is clamped into the digital printing system and held at the right tension by rollers. It is essential to prevent creasing if a clean print image is to be produced and the amount of rejects is to be kept as low as possible.

It is known that the dimensions of the paper web to be printed can change during the printing process in web printing. This can already happen due to the tension under which the paper web is held in the printing system, which leads to an elongation of the paper web in the transport direction, which is also called the running direction. Particularly in the case of intensely colored decors, in which a relatively large quantity of ink is applied in liquid form to the paper web to be printed, so-called wet stretching of the paper can occur. It is well known that moist or even wet paper expands. This usually happens transversely to the direction of travel more than in the direction of travel. Different grammages of the paper, irregularities and deviations in the manufacture of the paper and/or the colors used can also mean that the raw paper to be printed is first coated with a primer. This achieves a uniform, reproducible and previously determined basic color of the paper to be printed. Like the printing inks, the primer is mostly in liquid form, so that the quantity applied can also lead to wet stretching of the paper. The paper moisture content of the base paper, which can be determined or at least influenced, for example, by the air humidity in the room in which the paper is stored, can also influence the expansion of the paper.

In particular, the moisture content of the paper, which, as already explained, can be influenced by the ink or the primer, also depends on how well and to what extent the applied liquid can penetrate the respective paper at the given moisture content. This can be influenced by additives that are added to the base paper during production, for example, or by the production process for the base paper.

A digital printing system of the type described here therefore has at least one dryer, through which at least part of the surface to be printed, but preferably the entire surface to be printed, can be dried. This is done, for example, by subjecting the surface to an air flow, in particular a warm air flow, or exposure to 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 in the process direction behind the primer application unit and one behind the printing unit. This makes it possible to dry the paper after the primer has been applied and after the grid marking and the at least one decoration have been printed on. In both application steps, in addition to the actual active substances, water is also introduced, which has an effect on the unresinated base paper.

Preferably, any paper stretching that may occur, particularly in the running direction, can be adjusted by adjusting the tension applied by the guide rollers of the digital printing system. The disadvantage is that dryers, air discharges or guide rollers and their tension are often set manually and according to empirical values and individual assessment of an operator of the system, for example via a control panel. A reproducible control is often not available.

The disadvantage is that this leads to a relatively large amount of rejects, especially in applications where the size of the surface to be printed and the size of the printed decoration are very important, if, for example, length and/or width values are not met can become. Such an application is, for example, the production of laminate panels, which usually have a core made of a wood-based panel. A printed paper is applied to this core, the surface of which is provided with a decoration. A structure is generally embossed or pressed into the surface of a laminate panel produced in this way, preferably synchronized with the decor printed on the surface of the paper. If the printed decor shows a wood grain, for example, this will be achieved with the corresponding structure Wood grain picked up and the haptic impression of the panel is matched to that of a real wood panel. If there are discrepancies between the dimensions of the decor and the dimensions of the structure, this is immediately apparent to the observer and such a panel must be sorted out as rejects.

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

• a) Bereitstellen eines unbedruckten Rohpapiers,
• b) Auftragen eines Primers auf die Oberfläche des unbedruckten Rohpapiers,
• c) Aufdrucken mindestens einer Rastermarkierung und vorzugsweise mindestens eines Dekors auf das mit dem Primer versehene Rohpapier, sodass ein bedrucktes Rohpapier entsteht,
• d) Erfassen eines Raster-Abstandes zwischen mindestens zwei Punkten der Rastermarkierung des bedruckten Rohpapiers zum Ermitteln eines Ist-Zustandes des bedruckten Rohpapiers,
• e) Vergleichen des erfassten Raster-Abstandes der Rastermarkierung des bedruckten Rohpapiers gemäß dem Ist-Zustand mit einem digital als Soll-Zustand hinterlegten Raster-Abstand für das gedruckte Dekor,
• f1) Verändern des hinterlegten Soll-Rasterabstandes auf der Basis des durchgeführten Vergleiches indem ein geänderter Soll-Rasterabstand und ein damit geändertes Dekor erstellt wird und/oder
• f2) Verändern wenigstens eines Prozessparameters zur Steuerung des Aufbringens des Primers und/oder des Aufdruckens der Rastermarkung und/oder der Trocknung

solange, bis ein vorbestimmtes Kriterium erfüllt ist.The invention solves the problem set by a method according to the preamble of claim 1, which has 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 preferably at least one decoration onto the base paper provided with the primer, so that a printed base paper is produced,
• d) detecting a grid distance 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 recorded grid spacing of the grid marking of the printed raw paper according to the actual state with a grid spacing for the printed decor stored digitally as the target state,
• f1) changing the stored target grid spacing on the basis of the comparison carried out by creating a changed target grid spacing and thus a modified decor and/or
• f2) changing at least one process parameter to control the application of the primer and/or the printing of the grid marking and/or the drying

until a predetermined criterion is met.

In the first step of the process, the unprinted raw paper is made available. This is preferably resin-free. On the unprinted base paper is in the second Process step applied to the primer. This can also be referred to as a primer layer. The base paper is then referred to as primed base paper. It is still unrestrained. A grid marking is applied to the primer in process step c). This is printed, for example, on 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 grid marking are printed. The grid marking is preferably designed in such a way that it can be recognized and read by machines, but is not visible to the human eye. According to the invention, the grid marking is printed with part of the decoration or with the entire decoration. The paper is then an unresinated, primed and printed base paper.

In a further method step, a raster spacing of the raster marking of the printed raw paper is then recorded, which extends between two points of the raster marking. The points do not have to be punctiform or even just circular pressure points, but can be any points of the grid marking that are identifiable. The grid spacing preferably extends along the running direction, that is to say the transport direction of the paper through the digital printing system, or transversely to this running direction. However, the grid spacing may extend along any other direction.

This grid spacing is detected by using a corresponding sensor to detect and measure at least that part of the surface of the paper on which the grid marking is located. Depending on the grid marking used, different sensors can be used. In a preferred embodiment, the grid marking is printed on the surface of the paper to be printed. This can preferably be done with an ink that has a color that is also contained in the decor to be printed later, so that the grid marking is no longer recognizable to the viewer after the decor has been printed, since it has either become part of the decor or preferably from the decor was covered. In the latter case in particular, the grid marking can also be printed with a color that is not contained in the decor. Alternatively or additionally, the grid marking is provided with one that is invisible to the human eye Printed in color, which can be captured, for example, by an infrared camera, which in this case is preferably used as a sensor. The infrared image then shows the applied grid marking and the grid spacing can be determined. The advantage of this type of grid marking is that the grid marking can in no way be disruptive and conspicuous in the later decor. The use of an ink that can be excited to glow, for example by exposure to electromagnetic radiation, for example an ink that fluoresces under black light, ie UV light, can also be advantageous. It is usually necessary to use an additional printing unit for such a raster marking, since a conventional printing unit of a digital printing system contains four different colors, cyan, magenta, yellow and black. The outlay in terms of equipment for the digital printing system is therefore lower if a raster marking is applied at the same time as the decor, which is not visible to the human eye but can be read out by machine and the information contained therein can be recorded and further processed.

The grid spacing determined in this way determines the actual state of the printed raw paper. It is not absolutely necessary, but advantageous for parameters of the printed base paper, such as paper moisture content, to be determined or even evaluated or stored in the method described here.

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 decal after it is printed. It is preferably stored digitally, for example in an electronic data memory which can be accessed by an electrical controller, in particular an electronic data processing device. The target state, i.e. the grid spacing of the digital decor, extends between two points of the decor. The decor has been printed on the surface of the paper. The target state is determined in the digital template of the decor, in that the grid marking is subject to the decor. The grid marking is stored with the decor in the electrical control system of the system, for example an electronic data processing device. They are connected to each other. The stored data form the target value with which the measured actual value is compared becomes. 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 setting a print parameter. The method is performed until the predetermined criterion is met. The actual value is preferably determined using the printed grid marking and the target value is determined using the digitally stored grid marking. The printed raster marking is preferably embedded in the printed decoration and the deposited raster marking is connected to the deposited decoration.

A predetermined criterion must be met so that the decor to be printed fits the paper as well as possible and further processing is good, simple, safe and of high quality. Various types and configurations of criteria are conceivable here, which are described below. If the criterion is not met, in method step f1) the grid spacing of the digital template, i.e. the target state, which is referred to below as the target grid spacing, and/or in method step f2) the detected grid spacing, i.e. the actual -Condition, adjusted.

If the target grid distance, ie the target state, is changed in step f1), this means that the distance between the 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 to adapt them to the dimensions of the printed base paper, which are known from the detected grid spacing or can at least be calculated or estimated. It is not necessary, but an advantage if these dimensions are actually determined. The decor modified in this way is printed on the printed base paper.

If, on the basis of the result of the comparison, for example in method step f2), the grid spacing, ie the actual state, is to be adjusted, the paper is dried by means of the at least one dryer in order to change the grid spacing. As a result, the moisture content drops and the dimensional change caused in particular by the paper moisture, ie in particular the wet stretching of the paper, is reduced. The digital printing system preferably has at least an electrical controller, in particular an electronic data processing device, which is set up to determine the grid spacing from the sensor data and the target grid spacing from the decor, and to carry out the comparison. It is also able to check the result of the comparison to determine whether the predetermined criterion is met. As an alternative or in addition to this, at least one process parameter of the primer application unit can also be adjusted. However, this change only takes effect when the next decor is printed.

• A) Bereitstellen eines mit einem Primer versehenen Rohpapiers,
• B) Aufdrucken mindestens einer Rastermarkierung und mindestens eines Dekors auf das mit dem Primer versehene Rohpapier, so dass ein bedrucktes Rohpapier entsteht,
• C) Erfassen eines Raster-Abstandes zwischen mindestens zwei Punkten der Rastermarkierung des bedruckten Rohpapiers zum Ermitteln eines Ist-Zustandes des bedruckten Rohpapiers,
• D) Vergleichen des erfassten Raster-Abstandes der Rastermarkierung des bedruckten Rohpapiers gemäß dem Ist-Zustand mit einem digital als Soll-Zustand hinterlegten Raster-Abstand für das gedruckte Dekor,
• E1) Verändern des hinterlegten Soll-Rasterabstandes auf der Basis des durchgeführten Vergleiches, in dem ein geänderter Soll-Rasterabstandes und ein damit geändertes Dekor erstellt wird und/oder
• E2) Verändern wenigstens eines Prozessparameters zur Steuerung des Aufbringens des Primers und/oder des Aufdruckens der Rastermarkung und/oder der Trocknung

solange, bis ein vorbestimmtes Kriterium erfüllt ist.In an alternative embodiment, the invention solves the problem set by a method according to the preamble of claim 2, which has the following steps:

• A) providing a base paper provided with a primer,
• B) imprinting at least one grid marking and at least one decoration on the base paper provided with the primer, so that a printed base paper is produced,
• C) detecting a grid distance 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 recorded grid spacing of the grid marking of the printed base paper according to the actual state with a grid spacing for the printed decor stored digitally as the target state,
• E1) Changing the stored target grid spacing on the basis of the comparison carried out, in which a changed target grid spacing and a decor that is changed with it is created and/or
• E2) changing at least one process parameter to control the application of the primer and/or the printing of the grid marking and/or the drying

until a predetermined criterion is met.

In this case, for example, base paper that has already been primed is purchased and fed into the process. Process steps B) to E2) correspond to steps c) to f2). What was said about these also applies to steps B) to E2).

The base paper is preferably resin-free and preferably also unprinted before the primer is applied.

The at least one process parameter of the digital printing system is preferably a process parameter of the primer application unit and/or the dryer.

The electrical controller is also preferably set up to control the at least one dryer and/or the at least one primer applicator. In doing so, it preferably uses data stored in an electronic data memory, from which empirical values or experimentally determined data result how much the paper has to be dried by means of at least one dryer in order to bring about a specific change in dimensions 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 raw paper, different decorations, different coatings and applications of the paper to be printed. All of these functional relationships or tables of values are preferably stored in the electronic data memory. The corresponding parameters are set via an input device, for example a control panel, and are thus communicated to the electrical control system. The data processing device then accesses the corresponding data in the electronic data memory. Alternatively or additionally, the device has several dryer units or at least the possibility of using a dryer unit several times. In this case, the dryer is preferably used first to dry the paper to be printed. The grid spacing, ie the actual state, is then preferably recorded again and compared with the corresponding target grid spacing. If the predetermined criterion is still not met, the process parameters of the dryer are adjusted, for example, and the next repeat is processed with the drying adjusted in this way, until the predetermined criterion is met. The detected grid spacing is compared with the target grid spacing.

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 when, for example, the stretching of the paper to be printed is so great that the at least one existing dryer is not sufficient to dry the paper in such a way as to compensate for the stretching.

The predetermined criterion is preferably met when the difference between the detected grid spacing and the desired state falls below a predetermined limit value. For this purpose, it makes sense if the two points between which the grid spacing is recorded 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 precisely as possible to the length of the target state. The predetermined limit value, which can be, for example, at most 1 cm, preferably at most 5 mm, particularly preferably at most 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 greater the grid spacing and/or the target grid spacing, the higher the tolerance, ie the predetermined limit value, can be.

The predetermined criterion is preferably met when the ratio of the detected grid spacing and the target state is 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 is outside of this range, the actual state and/or the target state are adjusted accordingly.

This choice of criterion is advantageous, for example, if the two points between which the grid distance 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 in such a way that the recorded grid spacing should be as large as possible exactly 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 at most 2.1, preferably 2.05, particularly preferably 2.01, and the lower limit can be, for example, at least 1.9, preferably at least 1.95, particularly preferably at least 1.99. Depending on the grid spacing selected and the desired state, other values can also be useful and advantageous.

Advantageously, several grid spacings of the printed base paper are recorded and compared with several target states that are also recorded. The detected grid spacings and/or the target grid spacings, that is to say 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 multiple grid spacings and multiple target grid spacings is particularly advantageous when the expected expansion of the paper to be printed is inhomogeneous. This is the case, for example, when a primer or an ink receiving layer or an already applied decoration or a part of the decoration is applied inhomogeneously, ie with a locally varying application quantity of primer, ink or other liquid. This can result in certain areas that are exposed to a large amount of liquid, for example, having a high level of moisture and accordingly a high wet elongation, while other areas that are exposed to a small amount of liquid, for example, have a low level of moisture and accordingly a low wet elongation.

The actual state and/or the target state are preferably changed depending on the location. In this way, the different results of the various comparisons, in which a detected grid spacing is compared with one of the target states, can be accessed will be reacted to. While the electrical control is preferably set up in this case to compress and/or stretch different areas and parts of the decoration to different extents, a location-dependent drying, for example, is necessary to change the detected grid distances. A modular dryer can be used for this purpose, which has a number of individual dryer elements. The electrical control 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 intensity, and thus to dry different areas of the surface of the paper to be printed on to different extents.

Advantageously, the grid marking has grid points that are preferably arranged equidistantly along a number of lines, with the lines preferably running in two linearly independent, preferably perpendicular, directions. In another embodiment, the grid marking is a flat marking that is, for example, a frequency-modulated grid marking. Alternatively, an amplitude modulated grid marking can also be used.

The grid marking is preferably integrated into the decoration and is not macroscopically visible.

The grid marking is preferably contained in the finished printed decor, but is not perceptible to the human eye. As a result, it cannot have a disruptive effect on the visual impression, but it is available in further processing steps. To this end, it is advantageous if the raster marking contains information which, for example, enables product tracking and makes it possible, for example, to identify the originator, a place of printing, a time of manufacture or other information. This is of particular interest for quality assurance and possible liability. Tools for a dividing saw and/or a profiling machine, but also units for sealing or painting the edges of a panel on whose carrier plate the printed paper is located, in particular in the form of decorative paper, can be aligned using the grid markings visible to sensors. In the context of the present invention, a decorative paper is one that is impregnated with a synthetic resin Understood raw paper that is suitable for pressing with a carrier to form a laminate together with the carrier, such as an HDF board

In later work 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 plate, in a pressing tool, for example a press belt or press plate. In this case, the paper has preferably already been impregnated and optionally cut to size. It is then a decorative paper.

The invention also solves the problem set 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-receptive layer, at least one printing unit for printing the decoration onto the applied ink-receptive layer and at least one dryer which is set up to dry at least part of the surface of the paper to be printed, but preferably the entire surface of the paper to be printed Paper to dry after the ink receiving layer and / or the decoration or part thereof has been applied. The at least one dryer particularly preferably has a plurality of dryer elements which are each set up to dry part of the surface of the paper to be printed. The dryer elements can preferably be controlled individually and/or in groups by the electrical control. The result of this is that the dryer output of each individual dryer element of the dryer elements can be adjusted separately and independently of other dryer elements. The digital printing system preferably has a plurality of dryers which are arranged one behind the other in the running direction of the paper to be printed. A printing unit is particularly preferably located between at least two of the dryers.

The digital printing system has at least one printing unit, preferably multiple printing units, each of which is set up to print the surface of a paper. This can be done by applying a base coat or a primer or by applying the decor.

In addition, the system preferably has at least one sensor for detecting the grid marking. Such a sensor is preferably located in front of each dryer. The measurement data recorded by the respective sensor is transmitted to the electrical control, which uses this to determine the grid spacing. If the actual state is to be adjusted as a reaction to the result of the comparison, it is advantageous if there is a dryer behind the sensor in the running direction of the paper. If, on the other hand, only the target state is to be adjusted, it is sufficient for a printing unit to be located behind the sensor.

A grid marking of the type described here can also be used to optimize further process steps in the production of laminate and the subsequent processing into laminate panels. Panels of this type are preferably suitable for floating installation and are particularly suitable for installation without glue.

• die Produktverfolgung im Sinne der Nachverfolgbarkeit beispielsweise aus Gründen der Produkthaftung und/oder
• einen Urhebernachweis, von welcher Produktionslinie und von welchem Hersteller ein Dekorpapier stammt,

und/oder kann vorzugsweise verwendet werden

• als Basis zum Ausrichten von Dekorpapier und Trägerplatte zu einer Struktur auf einem Presswerkzeug, wie Pressband oder Pressblech, insbesondere für so genannte embossed-in-register Oberflächen mit Strukturen die wenigstens teilweise synchron zu dem Dekor verlaufen,
• als Basis zum Ausrichten von Werkzeugen einer Aufteilsäge, mit der aus einem großformatigen Laminat Paneele ausgesägt werden,
• als Basis zum Ausrichten von Werkzeugen einer Profiliermaschine, mit der an Seitenkanten der Paneele Verbindungsprofile und Verriegelungselemente angearbeitet werden, so genannte Nut-Feder-Verbindungen, beispielsweise als so genannte Klick-Verbindungen, und/oder
• als Basis zum Ausrichten von Aggregaten zum Kantenversiegeln und/oder Kantenlackieren der Paneele.

The grid marking preferably allows

• product tracking in terms of traceability, for example for reasons of product liability and/or
• a 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 with the decor,
• as a basis for aligning the 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 connection 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 basis for aligning aggregates for edge sealing and/or edge painting of the panels.

Figur 1 -

eine schematische Draufsicht auf eine Digitaldruckanlage und

Figur 2 -

einen vergrößerten Ausschnitt aus einer Draufsicht auf eine Anlage gemäß einem Ausführungsbeispiel der vorliegenden Erfindung.

An exemplary embodiment of the present invention is explained in more detail below with the aid of the attached figures. It shows

Figure 1 -

a schematic plan view of a digital printing system and

Figure 2 -

an enlarged detail from a plan view of a system according to an embodiment of the present invention.

figure 1 shows very schematically a 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, which is to be printed, is present in the exemplary embodiment shown as a paper roll, in particular as an endless paper web, and is figure 1 embodiment shown conveyed from left to right through the printing system. The running direction consequently also extends from left to right. The system has a first printing unit 4, which is set up to apply a first order to the paper web to be printed. This is, for example, a primer that is preferably applied homogeneously. The first printing unit 4 is a primer application unit. Consequently, liquid is applied to the paper 2 in the first printing unit 4, 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, there is also stretching transverse to the running direction, since the paper moisture has increased and the paper is therefore stretching.

The paper 2 therefore then runs into the first dryer 6, in which the paper 2 is dried. This causes the paper to shrink both in the machine direction and transversely to the machine direction. Dried in this way, the paper 2 reaches a second printing unit 8, which is used in the present exemplary embodiment to print out a marking and the decoration on the upper side of the paper 2. The decor is usually inhomogeneous, so that in the second printing unit 8 an inhomogeneous amount of liquid is applied to the upper side of the paper 2 . This may result in an inhomogeneous expansion of the printed paper 2 .

The raw paper printed in this way then runs through a detection system 10 which has at least one sensor with which a grid marking applied to the top of the paper can be detected. The data determined in this way are supplied to an electrical controller 12, which uses them to determine the recorded grid spacing. Within the electrical control, this detected grid spacing is compared to a target grid spacing, ie the target state, which corresponds to a target variable in the exemplary embodiment shown. As already explained, the application of the ink in the second printing unit 8 causes the paper to expand, so that the applied decoration also expands. In particular, if the printed paper is to be processed further, as is the case, for example, in the manufacture of wood-based panels, it is important that the printed paper that leaves the plant has the expected and required dimensions as precisely as possible. The electrical control therefore transfers control signals 14 to a second dryer 16. This second dryer 16 dries the paper 2 so that it contains the required dimensions.

figure 2 shows an enlarged section of the system in a separate representation. The grid 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. The actual decorative printing 20 then takes place, likewise in the second printing unit 8. In the process, a partial area 22 of the upper 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 rest of the printed area of the surface of the paper. This is detected in the detection system 10 by means of the at least one sensor. The control signals 14 transmitted by the electrical controller 12 are transmitted to the second dryer 16, which in the exemplary embodiment shown has a row of individual dryer elements 24, which are transverse to the running direction of the paper, which is also in figure 2 extending from left to right, are arranged side by side. The dryer elements 24, which are located in the area of the sub-area 22 and are set up to dry it, are controlled differently than the other dryer elements 24 in such a way that they produce an increased dryer output, so that the paper 2 in the sub-area 22 is dried to a greater extent will than in the remaining part. This causes the paper to shrink and gives the desired result 26.

Reference List

2

paper

4

first print

6

first dryer

8th

second printing unit

10

detection system

12

electrical control

14

control signals

16

second dryer

18

target specification

20

decor print

22

subarea

24

dryer element

26

result

Claims (13)

Method for printing a paper with a decoration using a digital printing system, the - at least one primer applicator for applying a primer, - At least one printing unit for printing the decoration on the applied primer and - at least one dryer for drying at least part of the paper after the primer and/or the decoration has been applied,
comprises, wherein the method comprises the following steps: a) providing an unprinted base paper, b) applying a primer to the surface of the unprinted base paper, c) imprinting at least one grid marking and at least one decoration on the base paper provided with the primer, so that a printed base paper is produced, d) detecting a grid distance 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 recorded grid spacing of the grid marking of the printed raw paper according to the actual state with a grid spacing for the printed decor stored digitally as the target state, f1) Changing the stored target grid spacing on the basis of the comparison carried out by creating a modified target grid spacing and a decor that has been modified with it and/or f2) changing at least one process parameter to control the application of the primer and/or the printing of the grid marking and/or the drying until a predetermined criterion is met. Method for printing a paper with a decoration using a digital printing system, the - At least one printing unit for printing the decoration on an applied primer and - at least one dryer for drying at least part of the paper after the primer and/or the decoration has been applied,
comprises, wherein the method comprises the following steps: A) providing a base paper provided with a primer, B) imprinting at least one grid marking and at least one decoration on the base paper provided with the primer, so that a printed base paper is produced, C) detecting a grid distance 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 recorded grid spacing of the grid marking of the printed base paper according to the actual state with a grid spacing for the printed decor stored digitally as the target state, E1) Changing the stored target grid spacing on the basis of the comparison carried out, in which a changed target grid spacing and a decor that is changed with it is created and/or E2) changing at least one process parameter to control the application of the primer and/or the printing of the grid marking and/or the drying until a predetermined criterion is met. Method according to Claim 2, characterized in that the at least one process parameter of the digital printing system is a process parameter of the primer application unit and/or the dryer. Method according to Claim 1, 2 or 3, characterized in that the printed base paper is dried by means of the at least one dryer in order to change the grid spacing of the printed base paper. Method according to one of the preceding claims, characterized in that the target grid spacing is changed by stretching or compressing at least part of the decoration, preferably the entire decoration. Method according to one of the preceding claims, characterized in that the predetermined criterion is met when the difference between the detected grid spacing and the target state falls below a predetermined limit value. Method according to one of the preceding claims, characterized in that the predetermined criterion is met when the ratio of the detected grid spacing and the target state is within a predetermined range. Method according to one of the preceding claims, characterized in that several grid distances of the printed base paper are detected and compared with several target states, the detected grid spacings and/or the target states in at least two linearly independent, preferably vertical directions get lost. Method according to Claim 8 , characterized in that the grid spacing of the printed base paper and/or the desired state is changed as a function of location. Method according to one of the preceding claims, characterized in that the grid marking has grid points which are preferably arranged equidistantly along a number of lines, the lines preferably running in two linearly independent, preferably perpendicular directions. Method according to one of the preceding claims, characterized in that the grid marking is a frequency-modulated grid marking. Method according to one of the preceding claims, characterized in that the grid marking is integrated into the decoration and is not macroscopically visible. Digital printing system for carrying out a method according to one of the preceding claims.

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