EP4461541B1 - Method and apparatus for preventing printing errors in digital printing by means of defective printing nozzles - Google Patents

Description

The invention relates to a method for preventing printing errors in digital printing caused by faulty print nozzles.

Furthermore, the invention provides a device for preventing printing errors in digital printing caused by faulty print nozzles and a method for printing a print design onto a substrate using a digital printer.

There are numerous technical solutions for reproducing or duplicating printed designs. The term "printing technology" encompasses all these processes. Printing techniques for reproducing printed designs include letterpress, offset printing, gravure printing, flexographic printing, screen printing, and digital printing. These techniques employ different methods to transfer a printing medium, such as ink, onto a substrate. Each technique offers distinct advantages and is therefore used in various applications.

The applications of printing technology are very diverse. For example, it is well-known for printing on printed materials, wallpaper, and similar items. In the production of laminate flooring or wall and ceiling cladding elements, decorative printing is used. This often involves printing on wood-based panels. There are several approaches to decorating wood-based panels. In the past, coating wood-based panels with decorative paper was frequently used, and there are virtually no limits to the variety of patterned decorative papers available. As an alternative to using decorative paper on wood-based panels, direct printing on the panels has been developed, eliminating the need to print on paper and then laminate or directly coat it onto the wood-based panels. The main printing techniques used here are gravure and digital printing. For these printing processes, the design is available as a digital template that represents the colors and color distribution of the design. At the beginning of the printing process, a separation file is created in which the design is broken down into the primary colors to be printed.

In digital printing, the print design is transferred directly from a computer to a digital printer, such as a laser or inkjet printer. Digital printing therefore eliminates the need for static printing plates and thus also the limitations on the length and width of the print data. Digital printing therefore offers a high degree of flexibility and variability with regard to print motifs and imposes no restrictions on repeating patterns.

Digital printing is also known for printing on wood-based panels, based on current technology. EP 2 181 852 B1 For example, this involves a digital printing process for printing on flat, wood-based panels. In this process, the flat panels are printed directly using a digital printer.

The document US 2007/103500 A1 discloses a method for preventing printing errors in digital printing and a corresponding device.

Digital printing typically uses the primary colors cyan, magenta, yellow, and black (CMYK). The CMYK color model is a subtractive color model. The abbreviation CMYK stands for the three color components cyan, magenta, yellow, and the black component (key) as the color depth. This color system allows for the reproduction of a color space (gamut) that meets the requirements of many different fields. In digital printing, the printing medium is applied to the substrate drop by drop. For this purpose, the printhead has a multitude of print nozzles, each capable of dispensing a drop of printing medium.

Piezo nozzles are known in the prior art, in which the pressure medium is ejected from the nozzle by pressure ( US 2005/0063016 A1 The pressure is generated by a piezoelectric element. In piezoelectric materials, pressure causes charges to form on the material's surfaces. Conversely, in these materials, the inverse piezoelectric effect causes a change in length when an electrical voltage is applied. This actuator effect converts electrical energy into mechanical energy and is utilized in so-called piezoelectric actuators. Piezoelectric actuators known from the prior art enable travel distances ranging from a few tens of micrometers to several millimeters, depending on their design. Examples known to those skilled in the art include longitudinal actuators, shear actuators, tube actuators, contractors, and bending actuators. While longitudinal actuators can achieve travel distances in the range of a few tens of micrometers to several hundred micrometers, bending actuators are characterized by travel distances of up to several millimeters.

In the nozzles of printheads, a voltage is applied to the piezoelectric actuator, causing it to deform and, in turn, deforming the wall of a conduit carrying the printing medium within the nozzle. This changes the volume of the conduit, which then ejects a droplet of the printing medium of the desired size from the nozzle. A so-called "waveform" is used to control the nozzle. The waveform describes the voltage profile applied to the piezoelectric actuator over time, thus causing its deformation.

As the droplet leaves the printhead nozzle, it typically forms a ligament, which ideally is drawn into the droplet. However, sometimes the ligament atomizes, forming a fine mist. This spray contaminates the printhead nozzles and other components of the printing device. Over time, this deteriorates print quality, necessitating printer cleaning.

Another problem is the condensation of water vapor on the printhead, which can lead to an impairment of print quality or even damage to the printhead.

The print nozzles can also be impaired by paper dust, air bubbles, or dried ink. Furthermore, due to their technical design, digital printers are often prone to printhead malfunctions. If a printhead fails, this is immediately noticeable as streaking in the printed image on the substrate.

Digital printing can therefore be very advantageous in many areas, but the quality of the printed image depends on the flawless functioning of the printheads and especially the print nozzles.

For example, the state of the art includes the EP 3 691 904 B1 The patent is known for its use of a wood simulation process. The patent describes a method designed to prevent individual printhead nozzles from becoming clogged by dust or heat. The performance of each printhead nozzle is monitored by tracking the printed design. Each printed pixel is assigned to one or more nozzles, allowing the system to determine which nozzles have been inactive for a given period. If the inactivity time (T1) of a nozzle exceeds a predefined limit, that nozzle is scheduled for cleaning.

The "cleaning" of nozzles identified as inactive, and possibly also of adjacent nozzles, is performed by printing a "mask." This involves printing with smaller droplet sizes specifically within branch structures, pore structures, or similar features of the design ("mask") to clean the inactive nozzles and prevent clogging. The printed "mask" should not be visually noticeable in the printed image and is not intended to compensate for or correct printing errors, but solely for cleaning the print nozzles.

The EP 3 691 901 B1 However, this only allows for the detection of inactive print nozzles based on quality defects in the printed image. While further quality defects can be avoided during subsequent printing by cleaning the print nozzle, the already printed designs are usually scrap. Furthermore, the printing system experiences downtime when print nozzles need to be cleaned.

The EP 3 656 571 A1 This deals with compensating for printing errors in a printed design during the printing process. Nozzle monitoring is described as follows: based on a previous print of the design or a reference print, defective areas in the print are identified. For the subsequent print, adjustments are made according to the... EP 3 656 571 A1 Corrections are then made. This involves adjusting the droplet sizes of individual nozzles or even neighboring nozzles. However, this method can only be used to react to an already incorrectly printed decoration or reference. This is time-consuming and costly, as some rejects cannot be avoided.

The object of the invention is therefore to provide a method in which the print quality can be kept consistently high and thus the economic efficiency of printing processes can be increased.

For this purpose, the invention provides a method according to claim 1 and a device according to claim 8.

Detailed description

The invention provides a method in which defective prints or printouts with quality defects caused by faulty print nozzles are avoided. The method for preventing printing errors in digital printing due to faulty print nozzles includes a digital printer.

The digital printer has at least one printhead with at least one print nozzle, the print nozzle being a piezoelectric actuator. Digital printers with piezoelectric nozzles are well known to those skilled in the art. Generally, a printhead has more than one print nozzle. Commercially available printheads can, for example, have 1200 print nozzles per inch.

According to the invention, at least one print nozzle of the digital printer is monitored and its printing output is recorded. If several print nozzles are monitored, the printing output of each monitored print nozzle is recorded.

In one embodiment of the method according to the invention, between 20% and 100%, preferably between 50% and 100% of the print nozzles of the digital printer are monitored.

• die Ausdehnung der Piezoaktoren der Druckdüse überwacht werden und/oder
• die Spannung, die am Piezoaktor der Druckdüse anliegt, überwacht wird.

In one embodiment of the present invention, at least one pressure nozzle is monitored by

• the expansion of the piezo actuators of the pressure nozzle is monitored and/or
• The voltage applied to the piezo actuator of the print nozzle is monitored.

The expansion of piezoelectric actuators can be monitored, for example, by measuring the pressure in the line carrying the pressure medium adjacent to the actuator. However, so-called "piezo self-sensing signals" are also known from the prior art and can be used to monitor the expansion of piezoelectric actuators.

Additionally or alternatively, the print nozzle can also be monitored via the voltage applied to the piezo actuator.

The data obtained from monitoring at least one print nozzle is transmitted to a computer unit, hereinafter also referred to as a computer, running suitable software. This software allows the printing output of the monitored print nozzle to be determined.

Monitoring a print nozzle allows you to determine how it is functioning, or even if it is functioning at all (zero print output). The print output of the monitored nozzle can be determined on a predefined scale integrated into the software. Such a scale can be established, for example, through reference measurements and then stored in the software.

Monitoring the print nozzles via the piezo actuators offers the significant advantage that malfunctions of the piezo actuators, and thus of the print nozzles, can be detected during printing. This allows for immediate intervention in the printing process and correction to maintain print quality.

A particular advantage over the prior art is that it detects defects in the printed image not only those caused by faulty print nozzles. Rather, such defects are largely avoided through the corrections made. Consequently, rejects caused by printing with faulty nozzles can be minimized or even completely eliminated. This ensures consistently high print quality throughout the entire printing process.

In one embodiment, in addition to monitoring the at least one print nozzle, the printed image produced by the digital printer can also be optically monitored. Such monitoring can be carried out, for example, by a digital camera, a spectral camera, or a hyperspectral camera.

A spectral camera, for example, is a multispectral camera with 12 image channels per captured pixel, generating one pixel per image channel. This results in a color spectrum of 12 image channels for each captured pixel. A common sensor technology uses a CMOS sensor to apply different color filters to individual pixels, allowing a single image capture to record multiple spectral information.

In a hyperspectral camera, the light is spectrally split at each pixel using an optical device, such as a prism, and the individual spectral ranges are measured separately. This increases the spectral resolution compared to a multispectral camera to approximately 20 to 250 or more image channels.

In contrast to a conventional RGB camera, not just one color is captured per pixel, but a spectral distribution with significantly more information depth.

Optical monitoring also allows for quality control of the printed image. The images captured by the optical monitoring system are transmitted to a processing unit and evaluated there by suitable software.

In one embodiment, AI software can additionally be used to detect trends in print quality and/or predict developments in print quality. The results of the trend detection and/or prediction by the AI software can be used to determine the printing performance of the Pressure nozzles are taken into account, for example by considering possible results of the AI software when creating the predefined scale already described.

In a further embodiment of the present invention, the results of the AI software can be used to verify the determined pressure outputs of the print nozzles. If the results of the AI software deviate from at least one determined pressure output, a notification can be issued to a user. This allows for further quality control.

In a further embodiment, a comparison can be made with a reference image stored in the software in order to detect defects. If defects are detected, a warning message can be issued to a user and/or further corrections can be made to the printing performance of at least one print nozzle in accordance with the present invention.

According to the invention, a target value and a tolerance range are specified for each monitored print nozzle. The target value defines the full functionality of the print nozzle, and the tolerance value specifies by what percentage the target value may be exceeded or fallen short.

In one embodiment, the tolerance value is between 1% and 70%, preferably between 1% and 65%, and particularly preferably between 1% and 60%. This allows for performance variations of a print nozzle between 1% and 70%. By adjusting the tolerance value, the requirements for print quality can be regulated. The smaller the tolerance value, the smaller the performance variations of a print nozzle are tolerated, and the higher the print quality.

According to the invention, the deviation of the measured pressure output of at least one print nozzle from the target value is calculated and then stored. The deviation is compared with the tolerance value, and if the deviation is greater than the specified tolerance range, a correction is made to the pressure output of at least one print nozzle.

In one embodiment, the deviation of the printing output from the respective target value is determined for each monitored printing nozzle.

According to the invention, the pressure output correction can, but does not have to, take place at the print nozzle whose pressure output has fallen below the tolerance value. In one embodiment, this makes it possible, for example, to correct print nozzles adjacent to the malfunctioning one, thereby preventing a loss of print quality.

In one embodiment of the present invention, the printing power of a print nozzle is corrected by a correction to at least one print nozzle of the digital printer and/or by a correction of the separation data.

• durch eine Korrektur der Separationsdaten; und/oder
• eine Regelung der Spannung der Druckdüse deren Druckleistung unter den Toleranzwert ist; und/oder
• durch Regelung der Spannung von Druckdüsen, die zu der Druckdüse deren Druckleistung unter den Toleranzwert ist, benachbart sind; und/oder
• durch die Änderung des Druckwinkels mindestens einer benachbarten Druckdüse erfolgen.

Corrections to at least one print nozzle can be made according to the invention by

• by correcting the separation data; and/or
• a regulation of the voltage of the print nozzle whose printing output is below the tolerance value; and/or
• by regulating the voltage of pressure nozzles that are adjacent to the pressure nozzle whose pressure output is below the tolerance value; and/or
• by changing the pressure angle of at least one adjacent pressure nozzle.

• eine Regelung der Spannung der Druckdüse deren Druckleistung unter den Toleranzwert ist; und/oder
• durch Regelung der Spannung von Druckdüsen, die zu der Druckdüse deren Druckleistung unter den Toleranzwert ist, benachbart sind; und/oder
• durch die Änderung des Druckwinkels mindestens einer benachbarten Druckdüse erfolgen.

A correction to a print nozzle of the digital printer can be made by

• a regulation of the voltage of the print nozzle whose printing output is below the tolerance value; and/or
• by regulating the voltage of pressure nozzles that are adjacent to the pressure nozzle whose pressure output is below the tolerance value; and/or
• by changing the pressure angle of at least one adjacent pressure nozzle.

In one embodiment, the processing unit of the affected faulty print nozzle itself and/or to an adjacent print nozzle or nozzles sends a signal to compensate for the faulty nozzle's state by commanding a change in droplet size. The droplet size is controlled via the voltage, and thus the waveform, applied to the piezoelectric actuator of a print nozzle. Typical droplet sizes range from 1 to 8 picoliters.

Adjacent print nozzles are preferably arranged in the direction of the print width next to the faulty print nozzle. All conceivable combinations are possible. Thus The pressure output of the faulty print nozzle alone, only the pressure output of an adjacent print nozzle or adjacent print nozzles, or the pressure output of the faulty print nozzle and the pressure output of an adjacent print nozzle or adjacent print nozzles can be adjusted.

The print nozzle detected as defective can also be switched off and completely compensated for by the neighboring print nozzle(s).

Due to the increased ink flow from the faulty print nozzle and/or the compensation of the ink flow by one or more adjacent print nozzles, a print nozzle failure does not become visible in the printed image. Consequently, no defect is visible in the printed image.

In another embodiment, print nozzles adjacent to the faulty print nozzle print at an angle towards the faulty print nozzle. The angle of the adjacent print nozzles must be selected such that the droplet lands in the middle position and covers print areas of both print nozzles. In this way, the droplet effectively covers parts of its own print area as well as the print area of the faulty print nozzle.

In another embodiment, the printing power of a print nozzle is corrected by correcting the separation data.

The faulty print nozzle is then compensated for by adjusting the separation data of the individual color rows/printhead rows, whereby a defective print nozzle is compensated for by data adjustment in the print file and/or neighboring print nozzles of the faulty print nozzle take over this area.

Software is used to adjust the digital separation data by providing pressure information to the area(s) of the adjacent print nozzle(s) to compensate for the performance drop or even failure of the faulty print nozzle. Furthermore, the separation data can also be adjusted so that the faulty print nozzle prints at a correspondingly higher pressure. A combination of these methods is also possible.

The separations contain black and white pixels for each print image, which are then sent to the print nozzles. White means it is not printed, 100% black. This means the largest droplet will be printed. The droplet size in printing is controlled by the black ink value. For example, white is not printed, up to 30% droplet size is 1 (small), up to 60% droplet size is 2 (medium), and above that, droplet size is 3 (large).

In the print image and the separation file, a specific area within the print width is always assigned to a print nozzle in the printing direction, depending on the resolution. This allows the separation file for this narrow nozzle area to be adjusted using software if a print nozzle malfunctions. For example, the intensity for a specific area can be increased by a certain amount, or the line can be deleted, effectively disabling the print nozzle. Simultaneously, the adjacent print nozzles can print more to compensate for the disabled nozzle.

Furthermore, it is possible that over time, excessively high-pressure print nozzles can have their droplet size reduced by the processing unit, or the separation data can be adjusted by reducing print information to print smaller droplets.

In one embodiment, the separation data is adjusted during the ongoing printing process. This has the advantage that the printing process does not need to be interrupted, thus increasing the efficiency of the printing process.

In one embodiment, a correction is made by changing both the separation data and the voltage of at least one print nozzle.

The extent to which a change in the voltage of a print nozzle and/or in separation data is required to achieve a specific change can be defined, for example, by correction data sets. Such data sets can be created beforehand under error-free printing conditions (all print nozzles functioning perfectly) and indicate the effect of a specific correction on the print result.

• einen Digitaldrucker;
• eine Recheneinheit;
• mindestens ein Mittel zur Überwachung der Druckleistung mindesten einer Druckdüse des Digitaldruckers;

auf.Furthermore, the invention provides a device for preventing printing errors in digital printing caused by faulty print nozzles, wherein the device is configured to carry out the method according to the invention. The device has

• a digital printer;
• a computing unit;
• at least one means for monitoring the printing performance of at least one print nozzle of the digital printer;

on.

• die Spannung der Piezoaktoren der Druckdüsen eines digitalen Druckers;
  und/oder
• den Druckwinkel mindestens einer Druckdüse.

The device has at least one processing unit on which the data from monitoring the at least one print nozzle are collected and on which the deviation of the print output from the target value is calculated. Furthermore, the deviation is compared with the tolerance value on the processing unit, and a correction is triggered according to the inventive method. For this purpose, the processing unit controls

• the voltage of the piezo actuators of the print nozzles of a digital printer;
  and/or
• the pressure angle of at least one print nozzle.

Furthermore, in one embodiment, the separation data can be adjusted using the computing unit.

The device includes at least one means for monitoring the pressure output of at least one print nozzle. This can be, for example, a suitable pressure sensor in the pressure medium channel or other suitable sensors to monitor the extension of the piezoelectric actuators and/or their voltage. The means for monitoring the pressure output of at least one print nozzle is connected to the at least one processing unit to forward the detected signals to the processing unit.

In one embodiment, the device further comprises at least one means for optically monitoring the print image of the digital printer. This can be, for example, a digital camera, a spectral camera, or a hyperspectral camera. The digital camera, spectral camera, or hyperspectral camera is preferably connected to the at least one processing unit.

• mindestens eine Druckdüse des Digitaldruckers mit einem erfindungsgemäßen Verfahren überwacht während durch den Digitaldrucker ein Druckdekor auf ein Trägermaterial gedruckt wird.

Furthermore, the invention provides a method for printing a printed design onto a substrate using a digital printer. In this method,

• At least one print nozzle of the digital printer is monitored using a method according to the invention while a print decoration is printed onto a substrate by the digital printer.

In one embodiment of the invention, all print nozzles of the digital printer are monitored using a method according to the invention.

In a further embodiment of the invention, the substrate material is selected from the group comprising paper, glass, metal, foils, wood-based materials, in particular MDF or HDF boards, WPC boards, veneers, lacquer layers, plastic boards, fiber-reinforced plastic, hard paper and inorganic substrate boards.

The features described for the methods according to the invention apply equally to the device according to the invention and vice versa.

The present invention advantageously enables the early detection and correction of failures or performance losses of individual print nozzles, thus preventing defects in the printed image. Intervention in the printing process can therefore be carried out earlier than with prior art methods, thereby maintaining high print quality. The invention thus offers a way to proactively monitor printing processes so that faulty print nozzles are detected early and their impact on the printed image is rectified during the printing process itself. Furthermore, the efficiency of printing processes is increased because rejects can be minimized. Manufacturers of printed products thus save costs through less waste, less downtime, and fewer complaints that slipped through quality control. In particular, the present invention minimizes downtime of printing systems that would otherwise be required for cleaning print nozzles.

The invention will be explained in more detail below using three exemplary embodiments.

Example 1 - Performance drop in print nozzle, digital paper printing

At a manufacturer of laminate flooring planks, decors for further processing into decorative panels were printed on paper rolls using a digital paper printing system.

The print width, or roll width, was 2060 mm, and the printhead resolution was 1200 dpi. There were 97,322 print nozzles available across the print width (2060 mm: 25.4 mm x 1200 dpi). The print nozzles of the printheads could be individually adjusted electrically for print intensity using a processing unit. Furthermore, the individual print nozzles were monitored, and the data was recorded by the processing unit. The deviation from the target value is calculated and this deviation is compared with a tolerance value.

A tolerance value of 20% was set.

During production, the monitoring system for print nozzle 1.112 detected an initial 17% drop in performance. Since the tolerance threshold had not yet been exceeded, the print nozzle was kept under observation, but no further action was taken. A few minutes later, the nozzle suddenly lost another 20% of its performance, for a total of 37%. The tolerance threshold was thus exceeded, and the print nozzle received a higher voltage via the processing unit by modifying the waveform. This readjustment resulted in a larger droplet pressure, which was sufficient to compensate for the defect or the lighter streak.

The printed paper roll could be used directly for further processing without errors or rewinding. The manufacturer saved costs through less waste, less downtime, and fewer complaints that slipped through quality control.

Example 2 - Print nozzle failure - Paper digital printing

At a manufacturer of laminate flooring planks, designs were printed on paper rolls using a digital paper printing system for further processing into decorative panels.

The print width, or roll width, was 2060 mm, and the printhead resolution was 1200 dpi. There were 97,322 print nozzles available across the print width (2060 mm: 25.4 mm x 1200 dpi). The print nozzles of the printheads could be individually adjusted electrically for print intensity using a processing unit. Furthermore, the individual print nozzles were monitored, and the data was recorded by the processing unit. The deviation from the target value was calculated and compared to a tolerance value.

A tolerance value of 60% was set.

During production, the monitoring of print nozzle 1.112 recorded a performance drop of 63%. The print nozzle's performance had already been continuously deteriorating beforehand, and the loss in print quality was as shown in embodiment 1. The neighboring nozzle(s) compensated for the defect. The tolerance for complete nozzle shutdown was set to 60%, and the nozzle was shut down. Simultaneously, nozzles 1.111 and 1.113 were each immediately and without interrupting production allocated 50% larger droplets for the continued printing process. The neighboring nozzles compensated for the defect caused by the shut-down nozzle.

The printed paper roll could be used directly for further processing without errors or rewinding. The manufacturer saved costs through less waste, less downtime, and fewer complaints that slipped through quality control.

Example 3 - Performance drop in print nozzle, digital paper printing (separation data)

At a manufacturer of laminate flooring planks, decors for further processing into decorative panels were printed on paper rolls using a digital paper printing system.

The print width, or roll width, was 2060 mm, and the printhead resolution was 1200 dpi. There were 97,322 print nozzles available across the print width (2060 mm: 25.4 mm x 1200 dpi). The print nozzles of the printheads could be individually adjusted electrically for print intensity using a processing unit. Furthermore, the individual print nozzles were monitored, and the data was recorded by the processing unit, the deviation from the target value was calculated, and this deviation was compared with a tolerance value.

A tolerance value of 30% was set.

During production, the monitoring system for print nozzle 1.112 detected an initial 20% drop in performance. Since the tolerance threshold had not yet been exceeded, the print nozzle was kept under observation, but no further action was taken. A few minutes later, the nozzle suddenly lost another 20% of its performance, for a total of 40%. The tolerance threshold was thus exceeded, and the separation data was adjusted so that the faulty print nozzle printed at a correspondingly higher rate. This compensated for the previously detected performance drop.

The printed paper roll could be used directly for further processing without errors or rewinding. The manufacturer saved costs through less waste, less downtime, and fewer complaints that slipped through quality control.

Claims (13)

1. Method for preventing printing errors during digital printing due to faulty printing nozzles comprising a digital printer, the method comprising the following steps:

   i. monitoring at least one printing nozzle of the digital printer and measuring the printing performance of the at least one printing nozzle and of each further monitored printing nozzle;

   ii. specifying a target value and a tolerance value for the printing performance of each monitored printing nozzle of the digital printer;

   iii. calculating and storing the deviation of the printing performance of at least one monitored printing nozzle from the target value of this printing nozzle;

   iv. correcting the printing performance of at least one printing nozzle when the deviation of the printing performance of at least one monitored printing nozzle is greater than the tolerance value,

   characterized in that the printing performance of at least one printing nozzle is corrected by

   a. correcting the separation data; the separation data containing black/white pixels and defining the droplet size using the pixels; and/or

   b. controlling the voltage of the printing nozzle of which the printing performance is below the tolerance value; and/or

   c. changing the printing angle of at least one adjacent printing nozzle.
2. Method according to claim 1, characterized in that between 20% and 100%, preferably between 50% and 100%, of the printing nozzles of the digital printer are monitored.
3. Method according to either of the preceding claims, characterized in that in method step iii., the deviation of the printing performance from the particular target value is determined for each monitored printing nozzle.
4. Method according to any of the preceding claims, characterized in that the at least one printing nozzle is monitored by

   • monitoring the expansion of the piezo actuators of the printing nozzle, and/or

   • monitoring the voltage applied to the piezo actuator of the printing nozzle.
5. Method according to claim 4, characterized in that the printed image printed by the digital printer is also optically monitored.
6. Method according to any of the preceding claims, characterized in that the tolerance value is between 1% and 70%, preferably between 1% and 65%, particularly preferably between 1% and 60%.
7. Method according to any of the preceding claims, characterized in that a correction is also carried out on at least one printing nozzle by controlling the voltage of printing nozzles which are adjacent to the printing nozzle of which the printing performance is below the tolerance value.
8. Device for preventing printing errors during digital printing due to faulty printing nozzles, which is configured to perform a method according to any of claims 1 to 7 and comprises:

   • a digital printer;

   • a computing unit;

   • at least one means for monitoring the printing performance of at least one printing nozzle of the digital printer.
9. Device according to claim 8, characterized in that the device also comprises at least one means for optically monitoring the printed image of the digital printer.
10. Device according to claim 9, characterized in that the at least one means for optically monitoring the printed image of the digital printer is a digital camera, a spectral camera or a hyperspectral camera.
11. Method for printing a printed pattern onto a substrate using a digital printer, characterized in that

    • at least one printing nozzle of the digital printer is monitored using a method according to any of claims 1 to 7 while a printed pattern is printed onto a substrate by the digital printer.
12. Method according to claim 11, characterized in that all printing nozzles of the digital printer are monitored using a method according to any of claims 1 to 7.
13. Method according to claims 11 or 12, characterized in that the substrate is selected from the group comprising paper, glass, metal, foils, wood materials, in particular MDF or HDF panels, WPC panels, veneers, lacquer layers, plastics panels, fiber-reinforced plastics material, laminated paper and inorganic support panels.