Method and device for preventing typographical errors during digital printing due to faulty printing nozzles
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2024-05-08
- Publication Date
- 2026-03-18
AI Technical Summary
Digital printing processes are prone to errors due to faulty printing nozzles, leading to reduced print quality and increased economic losses from downtime and rejected goods, as existing methods can only react to defects after they occur and do not prevent them effectively.
A method and device that monitor the performance of printing nozzles, set target and tolerance values, and automatically correct deviations in real-time by adjusting the voltage, separation data, and printing angle of adjacent nozzles to prevent print errors, allowing for continuous high-quality printing.
This approach minimizes rejects and maintains consistent print quality by detecting and correcting nozzle malfunctions during the printing process, reducing downtime and economic losses by preventing defects from occurring in the first place.
Smart Images
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Abstract
Description
[0001] Method and device for preventing printing errors in digital printing caused by faulty print nozzles
[0002] The invention relates to a method for preventing printing errors in digital printing caused by faulty print nozzles, comprising a digital printer, the method comprising the steps of: i. monitoring at least one print nozzle of the digital printer and recording the printing performance of the at least one print nozzle and each further monitored print nozzle; ii. specifying a target value and a tolerance value for the printing performance of each monitored print nozzle of the digital printer; iii. calculating and storing the deviation of the printing performance of at least one monitored print nozzle from the target value of this print nozzle; iv. correcting the printing performance of at least one print nozzle if the deviation of the printing performance of at least one monitored print nozzle is greater than the tolerance value.
[0003] Furthermore, the invention provides a device for preventing printing errors in digital printing due to faulty printing nozzles and a method for printing a print decoration with a digital printer onto a carrier material.
[0004] There are a variety of technical solutions for reproducing or duplicating print patterns. The term "printing technology" encompasses all processes for reproducing print patterns, such as letterpress, offset printing, gravure printing, flexographic printing, screen printing, and digital printing. These printing techniques use different methods to transfer a printing medium, such as an ink, to a substrate. Each printing technique offers different advantages and is used accordingly in different areas of application.
[0005] The areas of application for printing technology are extremely diverse. Well-known examples include the printing of print media, wallpaper, and similar materials. Decorative printing is used in the production of laminate flooring or wall and ceiling paneling, for example. 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 often used, although the variety of different patterned decorative papers is endless. As an alternative to using decorative paper on wood-based panels, the option of direct printing on wood-based panels has emerged. This eliminates the need for printing on paper and subsequently laminating or directly coating it onto the wood-based panels. The main printing techniques used here are gravure and digital printing.For these printing processes, the print design is available as a digital template that depicts the colors and color distribution of the print design. At the beginning of the printing process, a separation file is created in which the print pattern is broken down into the primary colors to be printed.
[0006] With digital printing, the print pattern is transferred directly from a computer to a digital printer, such as a laser or inkjet printer. Digital printing eliminates the use of static printing forms and thus the limitations on the length and width of the print patterns. Digital printing thus offers a high degree of flexibility and variability in terms of print motifs and imposes no restrictions on repeat.
[0007] Digital printing is also known for printing on wood-based panels. EP 2 181 852 B1, for example, relates to a digital printing process for printing on flat, wood-based panels. Here, the flat panels are printed directly with a digital printer.
[0008] In digital printing, the primary colors cyan, magenta, yellow, and black (CMYK) are typically used. The CMYK color model is a subtractive color model, where the abbreviation CMYK stands for the three color components cyan, magenta, yellow, and the black component, the key, representing the color depth. This color system can be used to represent a color space (gamut) that meets many requirements from a wide variety of fields. In digital printing, the printing medium is applied to the substrate drop by drop. The print head has a multitude of print nozzles, each of which can dispense a print medium drop by drop.
[0009] So-called piezo nozzles are known in the prior art, in which the pressure medium is shot out of the pressure nozzle by pressure (US 2005 / 0063016 A1). The pressure is generated by a piezo element. In piezoelectric materials, the application of pressure causes charges to develop on the surfaces of the material. 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 piezo actuators. Piezo actuators known from the prior art enable travel distances between a few tens of pm and a few millimeters, depending on their design. Examples known to those skilled in the art include longitudinal actuators, shear actuators, tubular actuators, contraction actuators, and bending actuators.While longitudinal actuators can achieve travel ranges in the range of a few tens of pm to a few hundred pm, bending actuators are characterized by travel ranges of up to a few millimeters.
[0010] In the print nozzles of printer heads, a voltage is applied to the piezo actuator, causing it to deform, and in turn, the wall of a conduit for the print medium in the print nozzle. This changes the volume of the conduit, causing a drop of the print medium of the desired size to be shot out of the print nozzle. A so-called "waveform" is used to control the print nozzle. The "waveform" describes the voltage curve that is applied to a piezo actuator over time, thus causing its deformation.
[0011] As the droplet exits the printhead nozzle, it typically forms a ligament, which, ideally, is drawn into the droplet. However, it can also atomize and form a fine mist. This spray contaminates the nozzles of the printhead and other elements of the printing device. Over time, this deteriorates print quality, making cleaning the printer necessary. Another problem is the condensation of water vapor on the printhead, which can impair print quality or even damage the printhead.
[0012] The print nozzles' functionality can also be limited by paper dust, air bubbles, or dried-out 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 print image on the substrate.
[0013] Although digital printing can be used very advantageously in many areas, the quality of the print image depends on the faultless functioning of the print heads and, in particular, the print nozzles.
[0014] EP 3 691 904 B1, for example, is known from the prior art and deals with a wood simulation process. The subject of the patent specification is a method intended to prevent individual print head nozzles from becoming clogged due to dust or heat. The performance of the individual print head nozzles is monitored by monitoring the printed decoration. Each printed pixel is assigned to one or more nozzles, allowing a conclusion to be drawn about which nozzles have not been printing for what period of time. If the time T1 during which a nozzle was inactive exceeds a predetermined limit, this nozzle is designated for "cleaning."
[0015] The "cleaning" of the nozzles identified as inactive, and possibly also neighboring nozzles, is performed by printing a "mask." This involves printing with smaller droplet sizes in the branch structures, pore structures, etc. of the decoration ("mask") to clean the inactive nozzles and prevent clogging. The printed "mask" is intended to be visually unnoticeable in the print image and does not serve to compensate for or correct printing errors; it only serves to clean the print nozzles.
[0016] However, EP 3 691 901 B1 only enables the detection of inactive print nozzles based on quality defects in the print image. While further quality defects can be avoided during subsequent printing by cleaning the print nozzle, the designs already printed are usually rejects. Furthermore, printing system downtime occurs when print nozzles need to be cleaned. EP 3 656 571 A1 deals with the compensation of printing errors in a print design during printing. Nozzle monitoring is described in such a way that defective areas in the print are identified based on a previous print of a design or a reference print. According to EP 3 656 571 A1, corrections are then planned for subsequent printing. The droplet sizes of individual nozzles or neighboring nozzles are adjusted.However, even with this process, it is only possible to react to an already faulty print of the design or reference. This is time-consuming and costly, as rejects cannot be avoided.
[0017] US 2022 / 0161551 A1 relates to a printing device in which piezoelectric print nozzles are monitored. After ink droplets are ejected from a print nozzle, the piezo element of the respective print nozzle continues to oscillate. This oscillation is monitored for intensity and frequency, and conclusions about a malfunction of the print nozzle are derived from the signals by comparing them with reference values. If a malfunction of a print nozzle is detected, it can be compensated for by regulating neighboring print nozzles (complementary printing) or the faulty nozzle can be cleaned (ejection operation). According to US 2022 / 0161551 A1, a response to malfunctioning print nozzles is only possible to a very limited extent.If the faulty printing performance of a printing nozzle cannot be compensated in this way, the printing device must be serviced or repaired, which leads to interruptions in the printing process and thus to economic losses.
[0018] US 2007 / 0103500 A1 also relates to a method for preventing printing errors in digital printing caused by faulty print nozzles. The print nozzles are monitored by a pressure measurement using a pressure sensor. Each pressure sensor is arranged in a pressure chamber, which contains a piezo actuator. The measured values of the pressure sensor are compared with a target value. Deviations from this target value are evaluated, and maintenance instructions are provided. Suitable maintenance processes can include vacuuming or rinsing the nozzles. However, error correction without interrupting the ongoing printing process is not provided.
[0019] The object of the invention is therefore to provide a method in which the print quality can be maintained consistently high and thus the cost-effectiveness of printing processes can be increased. To this end, the invention provides a method according to claim 1 and claim 12 and a device according to claim 9.
[0020] Detailed description
[0021] The invention provides a method in which rejects or prints with quality defects due to faulty printing nozzles are avoided by avoiding prints with quality defects due to faulty printing nozzles.
[0022] The method for preventing printing errors in digital printing caused by faulty print nozzles comprises a digital printer.
[0023] The digital printer has at least one print head with at least one print nozzle, wherein the print nozzle has a piezo actuator. Digital printers with piezo nozzles are well known to those skilled in the art. Generally, a print head has more than one print nozzle. Commercially available print heads can, for example, have 1200 print nozzles per inch.
[0024] According to the invention, at least one print nozzle of the digital printer is monitored and its print performance is recorded. If multiple print nozzles are monitored, the print performance of each of the monitored print nozzles is recorded.
[0025] 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.
[0026] In one embodiment of the present invention, at least one printing nozzle is monitored by
[0027] • the expansion of the piezo actuators of the pressure nozzle is monitored and / or
[0028] • the voltage applied to the piezo actuator of the pressure nozzle is monitored.
[0029] The expansion of the piezo actuators can be controlled, for example, by monitoring the pressure in the pressure medium line adjacent to the piezo actuator. However, so-called "piezo self-sensing signals" are also known from the state of the art, which can be used to monitor the expansion of the piezo actuators. Additionally or alternatively, the pressure nozzle can also be monitored via the voltage applied to the piezo actuator.
[0030] The data obtained from monitoring at least one print nozzle is sent to a computer unit (hereinafter referred to as a computer) with suitable software. This software can determine the printing performance of the monitored print nozzle.
[0031] By monitoring a print nozzle, it is possible to determine how the nozzle is operating, or even whether it is operating at all (pressure output equals zero). The pressure output of the monitored print nozzle can be determined using a predefined scale integrated into the software. Such a scale can be determined, for example, through reference measurements and is then stored in the software.
[0032] Monitoring the print nozzles by monitoring the piezo actuators offers the significant advantage of detecting malfunctions in the piezo actuators and thus the print nozzles during printing. This allows immediate intervention in the printing process and corrections to be made to maintain print quality.
[0033] A particular advantage over the state of the art is that defects in the print image caused by faulty print nozzles are not detected first, but rather such defects are largely eliminated through the corrections made. As a result, rejects caused by printing with faulty print nozzles can be minimized or even completely eliminated. Print quality can thus be maintained consistently high throughout the entire printing process.
[0034] In one embodiment, in addition to monitoring the at least one print nozzle, the print image printed by the digital printer can also be optically monitored. Such monitoring can be performed, for example, using a digital camera, a spectral camera, or a hyperspectral camera.
[0035] A spectral camera, for example, is a multispectral camera with 12 image channels per captured pixel, generating one pixel per image channel. This creates a color spectrum consisting of 12 image channels for each captured pixel. A common sensor technology applies different color filters to individual pixels on a CMOS sensor, allowing a multitude of spectral information to be captured from a single image.
[0036] 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.
[0037] In contrast to a conventional RGB camera, not only one color is obtained per pixel, but also a spectral distribution with considerably more information depth.
[0038] Optical monitoring also allows for print quality control. The images captured by the optical monitoring system are sent to a processing unit, where they are evaluated using suitable software.
[0039] In one embodiment, print quality trend detection and / or print quality development can additionally be performed with the aid of AI software. The result of the trend detection and / or prediction by the AI software can be taken into account when determining the print nozzle performance, for example, by considering possible results of the AI software when creating the predefined scale described above.
[0040] In a further embodiment of the present invention, the results of the AI software can be used to check the determined print performance of the print nozzles. If the results of the AI software deviate from at least one determined print performance, a warning message can be issued to a user. This allows for further quality control.
[0041] 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 according to 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 how many percent the target value may be exceeded or undershot.
[0042] 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 changes in the performance of a print nozzle of between 1% and 70% to be taken into account. By adjusting the tolerance value, the quality requirements of a print image can be regulated. The smaller the tolerance value, the smaller the changes in the performance of a print nozzle are tolerated and the higher the print quality of the print image.
[0043] According to the invention, the deviation of the measured printing performance of at least one printing 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, the printing performance of at least one printing nozzle is corrected.
[0044] In one embodiment, the deviation of the printing performance from the respective target value is determined for each monitored printing nozzle.
[0045] According to the invention, the print performance correction can, but need not, take place at the print nozzle whose print performance has fallen below the tolerance value. In one embodiment, this allows, for example, neighboring print nozzles to be corrected, thereby preventing a loss of quality in the print image.
[0046] In one embodiment of the present invention, a correction of the printing performance of a printing nozzle is carried out by a correction of at least one printing nozzle of the digital printer and / or by a correction of the separation data.
[0047] Corrections to at least one printing nozzle can be made according to the invention by
[0048] • a correction of the separation data; and / or
[0049] • by controlling the voltage of the printing nozzle whose printing performance is below the tolerance value; and / or • by controlling the voltage of printing nozzles that are adjacent to the printing nozzle whose printing performance is below the tolerance value; and / or
[0050] • by changing the pressure angle of at least one adjacent pressure nozzle.
[0051] A correction to a print nozzle of the digital printer can be made by
[0052] • a regulation of the voltage of the printing nozzle whose printing performance is below the tolerance value; and / or
[0053] • by controlling the voltage of print nozzles adjacent to the print nozzle whose pressure output is below the tolerance value; and / or
[0054] • by changing the pressure angle of at least one adjacent pressure nozzle.
[0055] In particular, the correction of the printing performance of at least one printing nozzle, whose deviation of the printing performance is greater than the tolerance value, is carried out by
[0056] • a correction of the separation data, whereby the black / white pixels for the at least one print nozzle whose deviation of the print performance is greater than the tolerance value are adjusted; and / or
[0057] • a control of the voltage of the printing nozzle, the deviation of the printing performance is greater than the tolerance value; and / or
[0058] • by changing the printing angle of at least one printing nozzle adjacent to the printing nozzle whose deviation in printing performance is greater than the tolerance value.
[0059] In one embodiment, the processing unit sends a signal to the affected faulty print nozzle itself and / or to a print nozzle(s) adjacent to it, to compensate for the faulty nozzle's state value by commanding a changed droplet size. The droplet size is controlled via the voltage and thus the waveform applied to the piezo actuator of a print nozzle. Typical droplet sizes range from 1 to 8 picoliters.
[0060] Neighboring print nozzles are preferably arranged next to the defective print nozzle in the direction of the print width. All conceivable combinations are possible. Thus, the print output of the defective print nozzle alone, only the print output of a neighboring print nozzle(s), or even the print output of the defective print nozzle and the print output of a neighboring print nozzle(s) can be adjusted. Likewise, the print nozzle detected as defective can be switched off and completely compensated by the neighboring print nozzle(s).
[0061] Due to the increased ink application to the faulty print nozzle and / or the balancing of the ink application by one or more neighboring print nozzles, a print nozzle failure is not even visible in the print image. Consequently, no defect is visible in the print image.
[0062] In a further embodiment, print nozzles adjacent to the faulty print nozzle print at an angle toward the faulty print nozzle. In particular, the printing angle of at least one print nozzle adjacent to the faulty print nozzle, or even two print nozzles adjacent to the faulty print nozzle, is adjusted. The angle of the adjacent print nozzles must be selected such that the droplet lands in the middle position, thereby covering at least parts of the printing areas of both print nozzles. Thus, the droplet essentially covers parts of its own printing area as well as the printing area of the faulty print nozzle.
[0063] In one embodiment, the printing angle of the at least one adjacent print nozzle is adjusted as already described, and its printing performance is additionally adjusted by adjusting the separation data and / or by adjusting the voltage. Thus, if necessary, the ink application (in the form of droplet size) of the at least one adjacent print nozzle can be additionally increased to enable optimal correction.
[0064] In a further embodiment, the printing performance of a printing nozzle is corrected by correcting the separation data.
[0065] The faulty print nozzle is then compensated by adjusting the separation data of the individual color rows / print head rows, whereby a defective print nozzle is compensated by data adjustment in the print file and / or neighboring print nozzles of the faulty print nozzle also take over this area.
[0066] Using software, the digital separation data is adjusted by providing the area(s) of the adjacent print nozzle(s) with printing information to compensate for the performance drop or even the failure of the faulty print nozzle. Furthermore, the separation data can be adjusted so that the faulty print nozzle prints accordingly stronger. A combination of both is also possible.
[0067] The separations contain black and white pixels for each print image, which are passed on to the print nozzles. White means no printing, 100% black means the largest droplet is printed. Thus, the black color value controls the droplet size in the print. For example, if white is not printed, up to 30% black, drop 1 (small) is printed, up to 60% black, drop 2 (medium), and above that, drop 3 (large).
[0068] In the print image and the separation file, a specific area is always assigned to a print nozzle in the print direction, depending on the resolution of the print width. This way, in the event of a faulty print nozzle, the separation file for this narrow area of the nozzle can be adjusted using software. For example, the intensity for an area can be increased by a certain amount, or the line can be deleted, thus deactivating the print nozzle. At the same time, neighboring print nozzles can print more to compensate for the deactivated print nozzle.
[0069] Furthermore, it is possible that the computation unit can reduce the droplet size of print nozzles that are printing too strongly over time, or adjust the separation data by reducing the print information to print smaller droplets. This means that the droplet size is adjusted either by the voltage on the piezo actuator, i.e., via the waveform, and / or via the separation data, i.e., via the value of the black / white pixels. Both approaches have already been described.
[0070] 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.
[0071] In one embodiment, a correction is made by changing both the separation data and the voltage of at least one printing nozzle.
[0072] The extent to which a change in a print nozzle's voltage and / or separation data is required to achieve a specific change can be specified, for example, using correction data sets. Such data sets can be created in advance under error-free printing conditions (all print nozzles functioning properly) and indicate the impact a specific correction has on the print result.
[0073] The present invention thus advantageously provides numerous possibilities for preventing faulty printing caused by one or more malfunctioning print nozzles of a digital printer during the ongoing printing process. The method according to the invention can enormously increase the cost-effectiveness of such printing processes, as printer downtimes and the production of misprints can be significantly reduced.
[0074] Furthermore, the invention provides a device for preventing printing errors in digital printing caused by faulty printing nozzles, wherein the device is designed to carry out the method according to the invention. The device has
[0075] • a digital printer;
[0076] • a computing unit; and
[0077] • at least one means for monitoring the printing performance of at least one print nozzle of the digital printer;
[0078] The device has at least one computing unit on which the monitoring data of the at least one pressure nozzle is collected and on which the deviation of the pressure output from the target value is calculated. Furthermore, the computing unit compares the deviation with the tolerance value and initiates a correction according to the method according to the invention. For this purpose, the computing unit controls
[0079] • the voltage of the piezo actuators of the print nozzles of a digital printer; and / or
[0080] • the pressure angle of at least one pressure nozzle.
[0081] Furthermore, in one embodiment, the separation data can be adjusted using the computing unit.
[0082] The device has at least one means for monitoring the printing performance of at least one printing nozzle. This can be, for example, a suitable pressure sensor in the channel of the printing medium or other suitable sensors for monitoring the expansion of the piezo actuators and / or their voltage. The means for monitoring the printing performance of at least one printing nozzle is connected to the at least one computing unit in order to forward the detected signals to the computing unit. In one embodiment, the device further has 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 computing unit.
[0083] Furthermore, the invention provides a method for printing a print decoration onto a substrate using a digital printer.
[0084] • at least one print nozzle of the digital printer is monitored using a method according to the invention while the digital printer is printing a print decoration onto a carrier material.
[0085] In one embodiment of the invention, all print nozzles of the digital printer are monitored using a method according to the invention.
[0086] In a further embodiment of the invention, the carrier material is selected from the group comprising paper, glass, metal, foils, wood materials, in particular MDF or HDF boards, WPC boards, veneers, lacquer layers, plastic boards, fiber-reinforced plastic, hard paper and inorganic carrier boards.
[0087] The features described for the methods according to the invention apply equally to the device according to the invention and vice versa.
[0088] With the present invention, failures or performance losses of individual print nozzles can advantageously be detected and corrected at an early stage, thus avoiding defects in the print image. This allows intervention in the printing process to take place earlier than with prior art methods, thereby maintaining high print quality. The invention thus offers a way of preventively monitoring printing processes so that faulty print nozzles can be detected at an early stage and their impact on the print image can be remedied during the printing process. Furthermore, the cost-effectiveness of printing processes is increased because rejects can be minimized. Manufacturers of printed products thus save costs through less rejects, less downtime, and fewer complaints that slipped through quality control.The present invention can, above all, minimize downtimes of printing systems that would otherwise be incurred for cleaning print nozzles. The invention is explained in more detail below using three exemplary embodiments.
[0089] Example 1 - Performance drop in print nozzle paper digital printing
[0090] At a manufacturer of laminate flooring, decors were printed on paper rolls on a digital paper printing system for further processing into decorative panels.
[0091] The print width, or rather, roll width, was 2060 mm, and the print head resolution was 1200 dpi. 97,322 print nozzles were available across the print width (2060 mm: 25.4 mm x 1200 dpi). The print intensity of the print nozzles used could be individually adjusted electrically using a processing unit. This meant that the voltage of each print nozzle could be individually controlled by adjusting the waveform. 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 with a tolerance value.
[0092] The tolerance value was set at 20%.
[0093] During production, the monitor for print nozzle 1.112 initially detected a 17% drop in performance. Since the tolerance value had not yet been exceeded, the print nozzle was monitored but no further action was taken. A few minutes later, the nozzle suddenly lost another 20% of its performance, resulting in a total loss of 37%. The tolerance value was thus exceeded, and the print nozzle received a higher voltage from the processing unit by changing the waveform. The readjustment resulted in the printing of a larger droplet, which was sufficient to compensate for the defect. Without correction, a light band would have appeared in the print image.
[0094] 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.
[0095] Example 2 - Print nozzle failure paper digital printing
[0096] A manufacturer of laminate flooring used a digital paper printing system to print on paper rolls for further processing into decorative panels. The print width, or roll width, was 2060 mm, and the print head resolution was 1200 dpi. 97,322 print nozzles were available across the print width (2060 mm: 25.4 mm x 1200 dpi). The print intensity of the print nozzles of the print heads used could be individually adjusted electrically using a computer unit. In addition, the individual print nozzles were monitored, and the data was recorded by the computer unit. The deviation from the target value was calculated and compared with a tolerance value.
[0097] The tolerance value was set at 60%.
[0098] During production, the monitoring system for print nozzle 1.112 detected a 63% drop in performance. The print nozzle's performance had already been steadily deteriorating beforehand, and the loss in the print image was compensated for by the neighboring print nozzle(s). The tolerance for completely shutting down the print nozzle was set to 60%, and the print nozzle was shut down. At the same time, both print nozzles 1.111 and 1.113 were immediately and without stopping production allocated 50% larger droplets for the subsequent printing process. The neighboring nozzles compensated for the defect caused by the switched-off print nozzle. Without this compensation, a light stripe would have appeared in the print image.
[0099] 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.
[0100] Example 3 - Performance drop in print nozzle paper digital printing
[0101] (separation data)
[0102] At a manufacturer of laminate flooring, decors were printed on paper rolls on a digital paper printing system for further processing into decorative panels.
[0103] The print width, or rather, roll width, was 2060 mm, and the print head resolution was 1200 dpi. 97,322 print nozzles were available across the print width (2060 mm: 25.4 mm x 1200 dpi). The print intensity of the print nozzles used could be individually adjusted electrically using a computer. Furthermore, the individual print nozzles were monitored, and the data was recorded by the computer. The deviation from the target value was calculated and compared with a tolerance value. The tolerance value was set to 30%.
[0104] During production, the monitor for print nozzle 1.112 initially detected a 20% drop in performance. Since the tolerance value had not yet been exceeded, the print nozzle was monitored but no further action was taken. A few minutes later, the nozzle suddenly lost another 20% of its performance, resulting in a total loss of 40%. The tolerance value was thus exceeded, and the separation data was adjusted so that the faulty print nozzle printed accordingly more strongly. This compensated for the previously detected drop in performance.
[0105] 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.
[0106] Example 4 - Performance drop in paper digital printing nozzle (change in printing angle of adjacent printing nozzles)
[0107] At a manufacturer of laminate flooring, decors were printed on paper rolls on a digital paper printing system for further processing into decorative panels.
[0108] The print width, or rather, roll width, was 2060 mm, and the print head resolution was 1200 dpi. 97,322 print nozzles were available across the print width (2060 mm: 25.4 mm x 1200 dpi). The print intensity of the print nozzles used could be individually adjusted electrically using a processing unit. This meant that the voltage of each print nozzle could be individually controlled by adjusting the waveform. Furthermore, the printing angle of each print nozzle could be changed. 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 with a tolerance value.
[0109] The tolerance value was set at 25%.
[0110] During production, the monitoring system for print nozzle 1,100 detected a 30% drop in performance. Because the tolerance value was exceeded, the printing angle of the neighboring print nozzles 1,099 and 1,101 was adjusted so that the printing area of print nozzle 1,100 was at least partially covered by print nozzles 1,099 and 1,101. This correction ensured a flawless print image. Without the correction, a light band would have appeared in the print image. The print production from the paper roll could be used directly for further processing without errors or rewinding. The manufacturer saved costs through less scrap, less downtime, and recorded fewer complaints that slipped through quality control.
Claims
Claims 1. Method for preventing printing errors in digital printing caused by faulty print nozzles, comprising a digital printer, characterized in that the method comprises the steps of i. monitoring at least one print nozzle of the digital printer and recording the print performance of the at least one print nozzle and each further monitored print nozzle; ii. specifying a target value and a tolerance value for the print performance of each monitored print nozzle of the digital printer; iii. calculating and storing the deviation of the print performance of at least one monitored print nozzle from the target value of this print nozzle; iv. correcting the print performance of at least one print nozzle if the deviation of the print performance of at least one monitored print nozzle is greater than the tolerance value, wherein the correction of the print performance of at least one print nozzle is carried out by a. correcting the separation data; and / or b.a regulation of the voltage of the printing nozzle whose printing performance is below the tolerance value; and / or c. by 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 print nozzles of the digital printer are monitored.
3. Method according to one of the preceding claims, characterized in that in method step iii. the deviation of the printing performance from the respective target value is determined for each monitored printing nozzle.
4. Method according to one of the preceding claims, characterized in that the 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 pressure nozzle is monitored.
5. The method according to claim 4, characterized in that the print image printed by the digital printer is further optically monitored.
6. Method according to one 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 one of the preceding claims, characterized in that a correction of the printing performance in method step iv. is carried out by a correction of at least one printing nozzle of the digital printer and / or by a correction of the separation data.
8. Method according to one of the preceding claims, characterized in that a correction to at least one printing nozzle is further carried out by regulating the voltage of printing nozzles which are adjacent to the printing nozzle whose printing power is below the tolerance value.
9. Device for preventing printing errors in digital printing due to faulty printing nozzles, adapted to carry out a method according to one of claims 1 to 8, comprising • 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.
10. Device according to claim 9, characterized in that the device further comprises at least one means for optically monitoring the print image of the digital printer.
11. Device according to claim 10, characterized in that the at least one means for optically monitoring the print image of the digital printer is a digital camera, a spectral camera or a hyperspectral camera.
12. A method for printing a print decoration with a digital printer on a carrier material, characterized in that • at least one print nozzle of the digital printer is monitored using a method according to one of claims 1 to 8 while a print decoration is printed onto a carrier material by the digital printer.
13. Method according to claim 12, characterized in that all printing nozzles of the digital printer are monitored using a method according to one of claims 1 to 8.
14. The method according to one of claims 12 or 13, characterized in that the carrier material is selected from the group comprising paper, glass, metal, foils, wood materials, in particular MDF or HDF boards, WPC boards, veneers, lacquer layers, plastic boards, fiber-reinforced plastic, hard paper and inorganic carrier boards.