Method of compensating for failed print nozzle of ink printhead

Abstract

To further improve compensation for failed or incorrectly printing nozzles.SOLUTION: A method is provided for compensation for a failed print nozzle of an ink printhead for the printing of a printed image onto a print material conveyed in a transport direction, the method including printing the printed image (9) with at least two different print droplet volumes, one of which is a maximum print droplet volume, and at least one compensation nozzle (5), i.e. a neighboring nozzle of the failed nozzle (4), prints compensation droplets with an elevated compensation droplet volume compared to the maximum print droplet volume. A frequency for the creation of compensation droplets is defined, and the compensation nozzle (5) is actuated and controlled such that the defined frequency is observed and the compensation droplets successive to each other printed by the compensation nozzle (5) have, in this case, a prescribed minimum separation from each other in the transport direction.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for compensating for failed print nozzles of an ink print head having the features of the preamble of claim 1.

[0002] Technical Field The technical field of the invention is the graphic industry, in particular the field of industrial, i.e. highly productive, ink printing (inkjet) on flat substrates, i.e. the image-wise application of extremely fine droplets of liquid ink - ejected from nozzles - to a sheet-, web-, film- or label-like substrate, preferably made of paper, cardboard, paperboard, plastic, metal or composite material, in which non-ejecting or erroneously ejecting nozzles are compensated for, for example, by their neighboring nozzles, thereby avoiding printing errors. [Background technology]

[0003] Inkjet printheads usually have several nozzles that can be controlled individually. The nozzles are usually arranged in a nozzle row or in several nozzle rows parallel (or slightly tilted) to the substrate transport direction. Each nozzle generates one or more ink droplets when it is activated. If the nozzles are activated permanently, they print a visible line of the ink color on the substrate. On the other hand, if a nozzle is defective, for example because the nozzle opening is clogged with hardened ink, or if a nozzle only ejects droplets obliquely or in a mist, a light line that is also visible (in English, called "missing nozzle", "missing line" or "white line") may result. As an undesirable consequence, the color of the substrate, usually a white background, shows through in the printed image as a line. Furthermore, if, for example, a defective nozzle is overcompensated, a dark visible line (in English, also called "darkline") may be present.

[0004] Since "white lines" in high-quality printed products are not accepted by customers, in the industry such nozzles that do not print or print erroneously must be compensated. In known methods, the properties of the print head nozzles can be used to form droplets of different sizes depending on the drive control of the print head nozzles. In this way, the neighboring nozzles ("to the right and to the left" of the defective nozzle) can be used to generate larger compensation droplets, i.e. with a larger ink volume, with the surplus ink to completely fill the "white line" or at least to such an extent that it is no longer perceived as a defect by the naked eye.

[0005] The abbreviation MNC used in the art stands for "missing nozzle compensation" or, more commonly, "malfunctioning nozzle compensation", i.e. compensation for printing nozzles that do not print or that print erroneously.

[0006] In order to identify the nozzles which do not print or which print erroneously, or the respective positions of the nozzles in the nozzle row, it is already known to print a test pattern designed for this purpose, to photograph this, for example by means of a camera, and to evaluate the photographed image by means of a computer, whereby for the nozzles which have been identified as requiring compensation, a compensation strength can be defined which determines the degree of compensation, for example low or high compensation.

[0007] DE 102015220716 A1 describes a method for automatically compensating for missing print nozzles in an inkjet printing system by a control computer, comprising the following steps: printing a test form for a specific material combination; evaluating the printing of the test form and creating a look-up table with a number of compensation probabilities with values ​​between 0 and 1 depending on the local areal density; detecting the missing print nozzle; reading out from the print data a target drop size to be compensated at the location of the missing print nozzle; calculating the local areal density at the location of the missing print nozzle; a compensation probability read from a lookup table using the determined local areal density; calculating pseudo-random numbers between 0 and 1 for each neighboring pixel to the right and left of the pixel to be compensated; increasing the drop sizes for the neighboring pixels to the right and left of the pixel to be compensated if the pseudo-random numbers for the neighboring pixels to the right and left of the pixel to be compensated are smaller than the compensation probability read from the lookup table; calculating neighboring drop sizes for all target drops to be compensated for at the missing print nozzle and applying the modified drop sizes to the print data; and executing the print job using the modified print data.

[0008] DE 10 2018201674 A1 discloses a method for performing a printing process in an inkjet printer with computer-assisted compensation of non-normal printing nozzles using a modified halftone screen image, in which after a halftone screening process creating a halftone screen image for the print image to be printed, the non-normal printing nozzles are compensated for by increased ink application of adjacent printing nozzles, characterized in that at least one modified halftone screen image is pre-calculated by the computer for adjacent printing nozzles, which modified halftone screen image replaces the halftone screen image created in the halftone screening process in at least one column of adjacent printing nozzles, and the printing process is performed with this modified halftone screen in the inkjet printer.

[0009] When identifying the maximum printable tone value (within the bounds of the achievable color space expansion), it is important to keep in mind that this tone value can be compensated for in the event of a possible nozzle failure, and, if compensated, the compensating nozzle does not also fail due to overloading with the larger ink volume that would normally result from the compensation. Summary of the Invention [Problem to be solved by the invention]

[0010] The object of the present invention over such prior art is to provide an improvement over the prior art, in particular in which the compensation for faulty or incorrectly printing nozzles can be further improved while the achievable color space is enlarged, and thus the print quality can be further improved. [Means for solving the problem]

[0011] This problem is solved according to the invention by a method as claimed in claim 1.

[0012] Advantageous and therefore preferred developments of the invention emerge from the dependent claims as well as the description and the drawings.

[0013] A method according to the present invention for compensating for a failed printing nozzle of an ink printing head for printing a printing image on a substrate transported in a transport direction, which prints the printing image with at least two mutually different printing drop volumes, one of the different printing drop volumes being a maximum printing drop volume, and at least one compensation nozzle, i.e. an adjacent nozzle of the failed nozzle, prints compensation drops having a compensation drop volume larger than the maximum printing drop volume, is distinguished by setting a frequency for generating compensation drops and driving and controlling the compensation nozzle such that the set frequency is maintained and successive compensation drops printed by the compensation nozzle have in this case a predetermined minimum distance from each other in the transport direction. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Advantageous Configurations and Functions of the Invention The present invention advantageously allows for further improved compensation for failed or incorrectly printing nozzles, in particular while expanding the achievable color space, thereby further improving print quality.

[0015] The present invention makes it possible to expand the color space and, in this case, to effectively compensate for printing errors that may be caused by printing nozzles that do not print or print erroneously, while at the same time ensuring that the compensating nozzles themselves are not persistently overloaded and do not fail.

[0016] The feature "printing the print image with at least two different print drop volumes" means, for example, that when printing the print image, small drops (S), medium drops (M), and large drops (L) are used, in which case the printed ink volumes can be in the range of 2 picoliters, 4 picoliters, and 6 picoliters.

[0017] The feature "one of the different printed drop volumes is the maximum printed drop volume" means, for example, that the maximum volume L drop is the largest ink drop used when printing.

[0018] The feature "at least one compensation nozzle prints compensation drops having a compensation drop volume larger than the maximum printed drop volume" means, for example, that the compensation drops are larger than an L drop, i.e., have a drop volume larger than, for example, about 6 picoliters.

[0019] The feature "setting the frequency for generating compensation drops" may mean, for example, that the frequency value is stored digitally in the print head or in a controller for the compensation process.

[0020] The feature "driving and controlling the compensation nozzle so as to maintain the set frequency" means, for example, that the compensation nozzle (on the left or right side of the missing or failed nozzle) deposits compensation droplets in the transport direction at a frequency corresponding to the set frequency.

[0021] The feature "successive compensation drops printed by the compensation nozzle have in this case a predefined minimum distance from each other in the transport direction" means, for example, that with a set frequency of 50% every second possible compensation drop is placed, with a set frequency of 33% every third possible compensation drop is placed, and with a set frequency of 25% every third possible compensation drop is placed. The compensation drops are therefore printed regularly, i.e. equidistant from each other and without any particular stochastic distribution. This also improves the flow properties of the ink, so that the disruptive white lines are filled in better.

[0022] The placement of compensation pixels in the printing direction is according to the invention carried out taking into account a set frequency, which is supplemented by the requirement to maintain a minimum distance between the compensation pixels to be placed. An example of this: if a frequency of 50% is set, then exactly every other pixel is printed with a compensation drop, so that the minimum distance between compensation pixels is always one pixel.

[0023] Another requirement may advantageously be that - even if the nozzle to be compensated does not place a pixel at the place in the image to be locally compensated - it is locally compensated by the neighboring nozzle. Considered the other way around, this means that the placement of the compensation pixels according to frequency is not done by using a halftone screen (this is the prior art); it also means that where there are pixels in the printed image and where they would typically be placed by a halftone screen in a microscopic structure, a larger compensation drop is placed instead. The frequency of placement of the compensation pixels is also not controlled by a statistically random algorithm (also the prior art), but rather the smallest possible spacing between the compensation pixels in the printing direction is sought. Thus, larger or smaller spacings may occur partially between the compensation pixels.

[0024] Thus, the present invention makes it possible: the darkest tone value to be compensated can be chosen darker than in the prior art, thus expanding the color space of the inkjet printer.

[0025] According to the present invention, larger droplets can be printed than permitted by the ink print head specifications, provided the droplets maintain a predetermined distance from one another, thereby not exceeding the maximum amount of ink per nozzle per unit time defined by the specifications.

[0026] Developments of the invention In the following, preferred developments of the invention (short: developments) are described.

[0027] One development may be advantageous in that for the print head a sustained drop volume is set, and the compensation drop is printed with a compensation drop volume larger than the sustained drop volume. The sustained drop volume is usually set within the print head specifications. Such a manufacturer's limit of the print head is advantageously exceeded by the compensation drop volume for a short time and for a small number of drops, but this is done so that damage to the print head does not occur. Thus, a volume that exceeds the originally permitted ink volume specification (i.e. if this amount is permanently placed) can be selected for the compensation drop, for example, every other pixel (or every third or third, etc.) can have such a compensation drop. This allows a relatively large amount of ink to be deposited in one area, and a printing image surface with an error without ink can be covered by (compensating) ink drops transversely to the printing direction, eliminating the error.

[0028] A development may be advantageous in that the method is carried out at or above a tone value at which at least 90% or at least 95% of the pixels required for the printed image are placed.

[0029] A development may be advantageous by carrying out the method above a tone value of at least 50% or at least 55% or at least 65%.

[0030] One development can be advantageous in that the frequency H is set as a percentage value, where H=P / Pmax, where P is the number of compensation droplets to be placed and Pmax is the maximum number of compensation droplets that can be placed in each case in the transport direction.

[0031] The features and combinations of features disclosed in the above technical field, invention and development paragraphs and in the following exemplary embodiment paragraph - in any combination with one another - form further advantageous developments of the invention.

[0032] Examples and drawings of the invention [Brief description of the drawings]

[0033] [Figure 1] FIG. 1 shows an apparatus for carrying out a preferred embodiment of the method according to the invention. [Diagram 2] FIG. 1 shows an apparatus for carrying out a preferred embodiment of the method according to the invention.

[0034] 1 and 2 each show an apparatus for carrying out a preferred embodiment of the method according to the invention, corresponding features being given the same reference numbers in the figures.

[0035] FIG. 1 shows three adjacent examples A, B and C for compensating for so-called white lines, where the central example B corresponds to the present invention.

[0036] A printing machine 1 for ink printing with several print heads 2 is shown, which are preferably arranged side by side in a print bar (preferably for printing one ink colour, e.g. C, M, Y or K). Each print head 2 has several print nozzles 3 (by way of example only three print nozzles 3 each are shown). The middle print nozzle of the three print nozzles 3 shown is a nozzle that is blocked for testing, thus simulating a missing or broken nozzle. Both adjacent nozzles 5 to the left and right of this nozzle or the resulting white line 11 act as compensation nozzles 5.

[0037] The printing press 1 prints a substrate 6, which is transported in a transport direction 7. The print heads 2 or print bars are arranged adjacent to one another in a transverse direction 8 and print ink drops, preferably from above, onto the substrate 6, which is moved past underneath the print heads 2. In this case, a print image 9 or at least one print control element 9 is produced on the substrate 6. For test purposes, the print image / print control element 9 may correspond to a full-tone image or a full-tone element, as indicated (by the transverse hatching).

[0038] For controlling the printing process and in particular the compensation, the printing press 1 comprises a control device 20 .

[0039] For all examples A, B and C the following assumptions are made: the tone value of the image 9 or element 9 consists of pixels that are placed at 100%; the nozzle 4 to be compensated is respectively switched off for the test (i.e. this nozzle does not print or produces a white line 11); the adjacent pixel 10 is placed such that it is printed by the adjacent nozzle 5 adjacent to the switched off nozzle 4 transversely to the transport direction 7 with a droplet that has, in principle, a larger volume than a normal 100% tone value pixel.

[0040] A (prior art): Compensation pixels 10 are placed to the left and right of all turned off nozzles 4. The drop volume of the compensation pixels 10 is within the head specifications, so it is possible to place each pixel with this volume.

[0041] B (Method according to the invention): Compensation pixels 10 are placed to the left and right of the switched off nozzles 4 with a defined frequency, where the spacing of the pixels 10 is kept to a minimum. The drop volume of the compensation pixels 10 is outside the head specifications, and pixels with this volume can be placed at a maximum defined frequency of less than 100%. In the illustrated example, the frequency is 33%, which means that every third pixel 10 is placed to the left and right of the white line 11 for compensation.

[0042] C (prior art): Compensation pixels 10 are placed to the left and right of the turned off nozzles 4 with a predefined frequency, where placement is done stochastically according to the frequency, where the spacing of the compensation pixels 10 varies stochastically. The drop volume of the compensation pixels 10 is outside the head specifications, and pixels 10 with this volume can be placed with a specified frequency up to less than 100%. In the illustrated example, the frequency is also 33%.

[0043] The two (compensating) ink volumes applied in examples B and C - viewed over the section of the compensation in the transport direction 7 - are identical to each other.

[0044] FIG. 2 shows three adjacent examples according to the invention B1, B2 and B3 for compensating for so-called white lines 11.

[0045] The three examples are not different from each other in principle. However, a different frequency is set in each example: in example B1 a frequency of 50% (every other pixel is placed), in example B2 a frequency of 33% (every other pixel is placed), and in example B3 a frequency of 25% (every other pixel is placed). It can be seen that successive compensation drops 10 printed by the compensation nozzle 5 in this case have a minimum distance A from each other in the transport direction (shown by way of example only in example B3). Neither the drops 10 nor the corresponding pixels 10 have a distance (relative to the "adjacent" compensation drops arranged one behind the other in the transport direction) greater than the minimum distance A. [Explanation of symbols]

[0046] 1 printing machine 2 Print head, print bar 3 Nozzles 4. Missing or broken nozzles 5 Compensating nozzle / adjacent nozzle 6 Printing material 7 Conveying direction 8 Horizontal 9 Printed images, especially test prints, or printed control elements 10 Compensation pixel or compensation drop (on the substrate) 11. White Line 20 Control device A Interval

Claims

1. A method for compensating for a failed print nozzle of an ink print head for printing a print image on a substrate transported in a transport direction, the method comprising the steps of: printing said print image (9) with at least two mutually different print drop volumes, one of said different print drop volumes being a maximum print drop volume, and at least one compensation nozzle (5), i.e. an adjacent nozzle of said failed nozzle (4), prints compensation drops having a compensation drop volume larger than said maximum print drop volume, A method comprising: setting a frequency for generating compensation droplets; and driving and controlling the compensation nozzle (5) such that the set frequency is maintained and that successive compensation droplets printed by the compensation nozzle (5) have a predetermined minimum distance from each other in the transport direction.

2. 2. The method of claim 1, further comprising: setting a sustained drop volume that can be printed sustainably for the print head; and printing the compensation drops with a compensation drop volume that is greater than the sustained drop volume.

3. 3. Method according to claim 1 or 2, characterized in that the method is carried out at or above a tone value at which at least 90% or at least 95% of the pixels required for the printed image (9) are placed.

4. Method according to any one of claims 1 to 3, characterized in that the method is carried out above a tone value of at least 50%, or at least 55%, or at least 65%.

5. 5. The method according to claim 1, wherein the frequency H is set as a percentage value, where H=P / Pmax, where P is the number of compensation drops to be placed and Pmax is the maximum number of compensation drops that can be placed in each case in the transport direction (7).

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