Correction of misalignment of nozzles of inkjet printers

Abstract

A method of correcting misalignment of nozzles of a multicolor inkjet printer (12) for printing an image on a print medium movable in a transport direction (C) relative to the inkjet printer (12) is disclosed. An inkjet printer (12) has a plurality of printheads (40) including nozzles (42) for each print color. A plurality of printheads (40) are mounted in a matrix configuration, the method comprising the steps of: determining a relative droplet position ejected by each nozzle; determining a relative nozzle deviation from the relative droplet position; and assigning the correction factor to each of the nozzles of the printhead.

Description

[0001] This invention relates to inkjet printing, and more specifically, to the correction of nozzle performance in an inkjet printer. In particular, this invention relates to methods and systems for correcting nozzle misalignment in a printhead of a multicolor inkjet printer, inkjet printing methods, and computer programs for performing nozzle misalignment correction methods.

[0002] In a scanning inkjet printer, a printhead mounted on a printing strip performs a reciprocating scanning movement along the width of a non-moving printing medium, such as one carried by a drive carriage, during which an image, or a portion thereof, is printed onto the medium. During the printhead's stroke in the width direction, the nozzles print along lines in the width direction of the printing medium. Between subsequent strokes, the printing medium is transported in the length direction to position it for the next stroke of the printhead. In the multi-pass mode of a scanning printer, portions of the image are printed onto the medium during multiple reciprocating strokes, where the medium moves intermittently between strokes. In the single-pass mode of a scanning printer, portions of the image are printed during a single stroke. The printing strip, moving in the scanning direction perpendicular to the transport direction of the printing medium, is much smaller than the maximum width of the printing medium.

[0003] In stationary inkjet printers, the printhead is mounted on a frame fixed relative to the conveyed printing media. The printhead, which prints colors, covers at least the width of the printing media to be printed. The nozzles of the printhead print lines in the longitudinal direction of the printing media, parallel to the conveying direction. Typically, printing media, such as a web of textile temporarily attached to the conveyor, is conveyed continuously.

[0004] In inkjet printing, the quality of the printed image depends on many factors, including nozzle performance. If the nozzle does not perform according to predetermined specifications, image quality deteriorates. Furthermore, nozzles undergo wear, thus altering their performance over time. Visually inspecting and evaluating test images to identify deviations and establish corrections is laborious, tedious, and subjective, as it depends on the operator's skill and experience. Software corrections for failed nozzles, and corrections for grayscale inhomogeneity and oversaturation, are known from WO2020239820A1 under the applicant's name, by evaluating scanned test patterns already printed on a dedicated test substrate.

[0005] Regarding image quality, proper alignment of the printhead and its nozzles, as well as timing control to trigger the correct nozzle at the right time, also play a crucial role in nozzle performance and the resulting image quality. Mechanical alignment is limited to aligning the printhead as a whole.

[0006] EP2853399A1 discloses a fixed printing apparatus for printing on printing media and a method for correcting step shift. The printing apparatus is configured to correct a step shift in the printing offset in the transport direction of the printing media. The printing apparatus includes a printer having at least two line heads spaced apart in the transport direction of the printing media. Each line head has multiple recording modules having a series of recording devices (nozzles) linearly arranged in the width direction of the printing media. The printing apparatus further includes: a scanner configured to scan an image to be printed by the printer to obtain a scanned image; a calibration chart printing unit configured to cause a first line printhead of the printer to print a first calibration chart in a first line printhead printing area and to cause a second line printhead to print a second calibration chart in a second line printhead printing area, the second line printhead printing area being away from the first line printhead printing area in the transport direction; a scanned image collection unit configured to operate the scanner to read the first line printhead printing area and the second line printhead printing area to collect a first scanned image and a second scanned image, respectively; a calibration data calculation device configured to calculate calibration data; and an adjustment device configured to adjust timing based on the calibration data when printing with the printer. The calibration data calculation device considers the line figure in a first scanned image formed by one of a plurality of recording modules as a reference line figure, and determines the difference in the transport direction between the reference line figure and a line figure formed by another recording module as a reference printhead difference, and determines the difference in the transport direction between the first line figure and a second line figure in a second scanned image as a separate printhead difference. The first line figure is formed by a first line printhead, and the second line figure is formed by a second line printhead. The reference printhead difference and the separate printhead difference are added to obtain a sum for each recording module. The reference printhead difference is considered as calibration data in the transport direction for each recording module in the first line printhead, and the sum is considered as calibration data for each recording module in the second line printhead. In the disclosed embodiment, the second line figure is printed by printing half of the line figure for each recording module in the corresponding line printhead, thereby allowing calibration data for each recording module to be obtained. Therefore, EP2853399A1 discloses distributing the same calibration data to each nozzle in the nozzles of the printhead.

[0007] US2012 / 0044291A1 discloses a scanning printing apparatus and method that allows for the printing of high-quality images using a time-division driving method (block driving method), even in cases of misaligned printheads and / or printing media delivery errors. The nozzles are divided into multiple blocks, wherein the driving order of the multiple nozzle blocks changes according to the block displacement to print on the same grid. Therefore, US2012 / 0044291A1 discloses assigning the same driving order (correction data) to each nozzle in the block.

[0008] Neither EP2853399A1 nor US2012 / 0044291A1 are publicly disclosed, nor do they allow for individual nozzle misalignment correction.

[0009] Further improvements in image quality can be achieved by applying corrections based on nozzle misalignment identification to the nozzle control data to obtain corrected nozzle control data, which can be used to control each nozzle when printing images by an inkjet printer.

[0010] The present invention aims to provide an automatic nozzle misalignment correction method for each individual nozzle, thereby compensating for deviations in nozzle performance.

[0011] In particular, the object of the present invention is to provide a method and system for correcting nozzle misalignment of each nozzle or group of nozzles in each printhead of a scanning multicolor inkjet printer or a stationary multicolor inkjet printer, and more particularly to provide an automated method in which operator involvement is minimal.

[0012] Another object of the present invention is to provide a method and system for correcting such nozzle misalignment for each nozzle or set of nozzles in each printhead of a scanning printer and for each printing direction.

[0013] In a first aspect, the present invention relates to a method for correcting misalignment of nozzles in a multicolor inkjet printer for printing images on a printing medium movable relative to the inkjet printer in a transport direction (C), the inkjet printer having a plurality of printheads for each printing color, the plurality of printheads being mounted in a matrix configuration of columns and rows, the printheads having an array of nozzles configured to eject ink droplets of the printing color (Clr), wherein: printheads of the same printing color are arranged in columns in a matrix configuration extending in a direction parallel to the transport direction (C) of the printing medium, and printheads for different printing colors are arranged in rows in a direction perpendicular to the transport direction (C) on a carriage that can reciprocate in a scanning direction perpendicular to the transport direction (C) of the printing medium; or printheads for different printing colors are fixedly arranged in columns in a matrix configuration extending in a direction parallel to the transport direction (C), and printheads for the same printing color are arranged in rows in a direction perpendicular to the transport direction (C).

[0014] The method includes the following steps:

[0015] a) Determine the relative position of the droplets ejected by each nozzle in the nozzle of each printhead in the printhead used for the reference print color (Clr_ref) relative to the reference element.

[0016] b) Determine the relative position of the droplets ejected by each nozzle of each printhead in the printhead for each print color other than the reference print color (Clr_ref) (Clr≠Clr_ref) relative to the relative position of the droplets ejected by each nozzle of the corresponding printhead for the reference print color (Clr_ref).

[0017] c) Based on the determined relative positions of the droplets ejected by the nozzles of the printhead, determine the deviation for each nozzle in each nozzle of each printhead in the printhead, and

[0018] d) Based on the determined nozzle deviation of the printhead, a correction factor is assigned to each nozzle of each printhead in a plurality of printheads.

[0019] In the method of the first aspect of the invention, a correction factor for misaligned nozzles is obtained for each nozzle of all printheads. The printheads are arranged in a matrix configuration of columns in a direction parallel to the transport direction of the printing medium and rows in a vertical direction. In a scanning inkjet printer, these columns represent the printing color indicated by (Clr), or in a stationary inkjet printer, they represent printheads for different printing colors. In a scanning inkjet printer, these rows represent the number of printheads for a printing color, indicated by the number n, or in a stationary inkjet printer, they represent printheads for the same printing color. For clarity and illustration, the position of a printhead in the matrix configuration of a scanning inkjet printer is represented by (Clr, n), where Clr indicates the printing color, and n indicates the number of printheads in the column for color Clr. The position of a printhead in the matrix configuration of a stationary inkjet printer is represented by (n, Clr), where n indicates the column number of the printhead for printing color Clr in the matrix configuration, and Clr indicates the printing color in the row of the matrix configuration of the printheads. Therefore, a specific printhead can be identified by its position. In a matrix configuration of a fixed inkjet printer, printheads used for printing colors can be arranged in a staggered configuration, such that adjacent printheads partially overlap each other.

[0020] Typically, printing colors include at least black (K), cyan (C), magenta (M), and yellow (Y), with the option to include one or more additional colors, such as blue (B1) and orange (O). The number of printheads used for printing colors is unlimited, but in scanning inkjet printers, it is typically in the range of 4 to 12 (e.g., 6 or 8). Because in stationary inkjet printers, the printheads need to cover the width of the printing media, the number is larger in industrial printers with print widths up to several meters (e.g., 1 to 2 meters), for example, in the range of 30 to 60.

[0021] In the first step a), the relative position of the droplets ejected from each nozzle of each printhead in one of the printheads selected as the reference print color (Clr_ref) relative to the reference element is determined, for example, by applying an imaginary line (e.g., a spline—a mathematical function defined by piecewise polynomials) to the actual line printed by ejecting droplets from each nozzle of these printheads of the reference print color and measuring the distance to the reference element (e.g., its desired (fixed) position). Advantageously, black (K) is chosen as the reference print color because black (K) is the color with the highest contrast, i.e., the most dominant color in the image, and typically exhibits the greatest color deviation.

[0022] In step b), the relative position of the droplets ejected from each nozzle of the printhead of the printhead for a print color other than the reference print color (Clr_ref) (Clr≠Clr_ref) is determined relative to the droplets ejected from the nozzle of the corresponding printhead of the reference print color. In the case of a scanning inkjet printer, typically, the printhead corresponding to the reference print color (Clr_ref) is the printhead of the reference print color (Clr_ref) that is in the same row as the corresponding printhead of the print color other than the reference print color (Clr_ref) (Clr≠Clr_ref). In the case of a stationary inkjet printer, typically, the printhead corresponding to the reference print color (Clr_ref) is the printhead of the reference print color (Clr_ref) that is in the same column as the corresponding printhead of the print color other than the reference print color (Clr≠Clr_ref).

[0023] Based on the relative positions of the droplets, the determined deviation for each nozzle in all printheads is determined in step c), and this deviation is converted into a correction factor for each nozzle in step d). Typically, the correction factors are stored in a table of graphs. Advantageously, the correction factors are rounded to integers representing pixel translations, which can be used in the calibrated nozzle control data for the controller during the actual printing of the image. The method according to the invention determines individual correction factors for each nozzle in all printheads. Using calibrated nozzle control data that takes into account these correction factors determined individually for each nozzle by the method according to the invention allows for improved image quality when printing images compared to images printed using uncalibrated nozzle control data.

[0024] In this embodiment, steps a) to b) include: printing a test pattern on a test substrate, the test pattern including a test pattern portion for each printhead; and scanning the printed test pattern to determine the relative positions of droplets ejected by each nozzle in the nozzles of each printhead. In this embodiment, the test pattern is designed to include a test pattern portion for each printhead for each print color. Such a test pattern portion includes image elements (such as one or more lines) to be printed by the printhead whose correction factor for each nozzle is to be determined, and reference image elements (such as one or more reference lines) to be printed by the printhead of the reference print color. In this embodiment, lines printed by the printhead of the print color other than the reference print color (Clr ≠ Clr_ref) (Clr ≠ Clr_ref) for which the correction factor for the nozzles is to be determined are positioned between lines printed by the corresponding printhead of the reference print color (Clr_ref) to eliminate influences from the test substrate itself, from its processing, and from the scanner. In this way, the correction factor derived from the relative positions of the printed elements is essentially only related to the nozzles of the inkjet printer's printhead. The test pattern is printed on a test substrate (typically a test substrate with a receiving layer suitable for inkjet inks), such as (reactive) dye inks, solvent-based inks, or water-based inks. The test substrate with the test pattern printed on it is scanned, and the relative position of the scan and its deviation are analyzed as described above to obtain an individual correction factor for each nozzle in the nozzles. This individual correction factor is stored for use in calibrated nozzle control data during actual printing.

[0025] In an embodiment of the method for a scanning inkjet printer, printheads of the same printing color are arranged in columns of a matrix configuration extending in a direction parallel to the transport direction (C) on a carriage, and printheads for different printing colors are arranged in rows extending in a direction perpendicular to the transport direction (C). The carriage can reciprocate in a scanning direction (S) perpendicular to the transport direction (C) of the printing medium. The printhead position in the matrix configuration is represented by (Clr, n), where Clr indicates the printing color in the column of the matrix configuration of the printhead, and n indicates the row number of the printhead for color Clr in the matrix configuration. Step a) includes the following sub-steps:

[0026] a1) Determine the relative position of the droplets ejected by each nozzle in the nozzle of the reference printhead used for referencing the print color (Clr_ref) at the printhead position (Clr_ref) relative to the reference element.

[0027] a2) Determine the relative position of the droplet ejected by each nozzle of the remaining printheads at printhead position (Clr_ref, n ≠ r) for reference print color (Clr_ref) relative to the relative position of the droplet ejected by the nozzle of the reference printhead at printhead position (Clr_ref, r) for reference print color.

[0028] In this embodiment, a printhead of one of the printing colors is selected as the base. The correction factors for the remaining printheads of the same printing color, as well as the correction factors for printheads of printing colors other than the reference printing color, depend directly or indirectly on this base. This printhead is designated as the reference printhead, and its position is (Clr_ref,r). The selected printing color is called the reference printing color (Clr_ref). Advantageously, the reference printhead is the printhead of the reference printing color, which moves as the last (in the direction of transport of the printing medium) on the printing medium.

[0029] In substep a1), the position of the droplets relative to the reference element is determined, for example, by applying an imaginary line (e.g., a spline (a mathematical function defined by piecewise polynomials)) to the actual line printed by ejecting droplets from each nozzle of the nozzles of the reference printhead, and measuring the distance to the desired (fixed) position. In substep a2), the relative position of the droplets ejected from each nozzle of the remaining printheads of the reference print color at position (Clr ≠ Clr_ref. n ≠ r) relative to the reference printhead is determined. This allows for the correction of the nozzles of all printheads of the reference print color (Clr_ref) that can print at any position on the printing medium during the reciprocating scan movement, ensuring that the line is correctly addressed by the nozzles of all printheads of the reference print color (Clr_ref). Relative to the corresponding printhead of the reference print color in the same row n, determine the relative position of the droplets ejected from the nozzle of the printhead of the print color other than the reference print color (Clr≠Clr_ref) at position (Clr ≠ Clr_ref, n). That is, compare the printhead of the corresponding print color at position (Clr ≠ Clr_ref, n) with the printhead of the reference print color in the same row (n is equal) at position (Clr_ref, n).

[0030] In another embodiment, the test pattern portion of the reference printhead printing a reference print color (Clr_ref) (typically black (K)) includes: - during the movement of the carriage in a first stroke in the scanning direction, e.g., during forward movement from left to right, - printing at least one line, preferably at least two lines, e.g., three spaced lines, by ejecting ink droplets from each nozzle of the reference printhead for the reference print color at position (Clr_ref, r). The reference printhead preferably prints three lines of the reference print color, which are used to calculate individual nozzle deviations.

[0031] In this implementation, the test pattern portion of each of the remaining printheads printing the reference color (Clr_ref) includes: - during the movement of the carriage in a first stroke in the scanning direction, - printing at least one line, preferably at least two spaced lines, by ejecting ink droplets from each nozzle of each of the remaining printheads printing the reference color at printhead position (Clr_ref,n, where n ≠ r), and printing at least one reference line by ejecting ink droplets from each nozzle of the reference printhead for the reference color at printhead position (Clr_ref,r). Advantageously, the reference line printed by the reference printhead at printhead position (Clr_ref,r) is printed between the lines printed by the remaining printheads printing the reference color (Clr_ref), because the basic design of this implementation of the test pattern portion (lines 1, 3, and 5 printed by the corresponding printheads of the reference color in the first stroke) is the same for all printheads. Alternatively, in an implementation, the lines printed by the remaining printheads of the reference print color (Clr_ref) are printed between the two reference lines printed by the reference printhead at the printhead position (Clr_ref, r).

[0032] The relative position of the line printed by the corresponding remaining printheads with respect to the reference line printed by the reference printhead can be found, for example, by applying a spline function to the line printed by the corresponding remaining printheads. Then, the individual deviation of each nozzle in the remaining printheads of the reference print color relative to the reference printhead can be calculated using the equation ((actual position line 1 + actual position line 3) / 2) - actual position reference line 2, where actual position lines 1 and 3 indicate the positions of the two lines printed by the remaining printheads at position (Clr_ref, n ≠ r), and actual position reference line 2 indicates the position of the reference line printed by the reference printhead at position (Clr_ref, r). Therefore, this equation presents the distance between the actual position and the reference position of each nozzle. The deviation of the nozzles of the remaining printheads increases with the deviation calculated for the reference head nozzles of the reference print color, because the positions of the remaining printheads of the reference print color and their nozzles are relative to the position of the reference head of the reference print color.

[0033] In an implementation, the printing test pattern portion for each printing color (Clr ≠ Clr_ref) other than the reference printing color (Clr_ref) includes: - during the movement of the carriage in a first stroke in the scanning direction, - printing at least one line by ejecting ink droplets from each nozzle of each printhead of the printing color at the printhead position (Clr, where Clr ≠ Clr_ref, n), and printing at least one reference line, preferably two spaced-apart reference lines, by ejecting ink droplets from each nozzle of the corresponding printhead for the reference printing color (Clr_ref) at the printhead position (Clr_ref, n) having the same n. Advantageously, the line printed by the printhead whose relative position is to be determined is printed between the two reference lines. Then, the deviation can be calculated from the relative position already found by applying, for example, a spline function, using the equation: Actual Position Line 2 - ((Actual Position Reference Line 1 + Actual Position Reference Line 3) / 2), where Actual Position Reference Line 1 and Actual Position Reference Line 3 indicate the positions of the two lines printed by the printhead at position (Clr_ref, n), and Actual Position Line 2 indicates the lines printed by the remaining printheads at position (Clr ≠ Clr_ref, n). For this determined distance, the deviation of the printhead nozzles of the reference print color at position (Clr_ref, n) is added, since this position is determined relative to the printhead nozzles of the reference print color at position (Clr_ref, n).

[0034] Since the function of the nozzles in a bidirectional scanning inkjet printer may differ in the first stroke (e.g., forward) of the carriage along the scanning direction from its function in the second (opposite) stroke (e.g., reverse) of the carriage along the scanning direction, in its implementation, printing a test pattern further includes: - during the movement of the carriage in the second stroke along the scanning direction opposite to the first stroke, printing at least one line for each printing color by ejecting ink droplets from each nozzle of each of the nozzles of each of the plurality of printheads, the implementation further includes: determining the relative position of the droplets ejected by each nozzle of each of the nozzles of each of the printheads for each of the printing colors at position (Clr, n) during the movement of the carriage in the second stroke along the scanning direction opposite to the first stroke, the relative position being relative to the position (Clr_ref, n) of the printheads having the same n during the movement of the carriage in the scanning direction. The relative positions of the droplets ejected by each nozzle in the corresponding printhead for the reference print color (Clr_ref) at point n are determined; based on the determined relative positions of the droplets ejected by each nozzle in the printhead, a deviation for each nozzle in the printhead is determined; and based on the determined relative positions of the nozzles in the printhead, correction factors for printing in the first stroke and for printing in the second stroke are assigned to each nozzle in the printhead of the plurality of printheads. By adding the line printed in the second (reverse) stroke to the test pattern portion for each printhead in the plurality of printheads, printing it on a test substrate, scanning the test substrate with the printed test pattern, and analyzing the scan, correction factors for each nozzle for printing in the reverse stroke can be implemented in addition to the correction factors for the forward stroke. Advantageously, the line to be printed in the second stroke is positioned between the lines to be printed by the same printhead in the first stroke. For example, in the above embodiment regarding the test pattern portion to be printed in the forward stroke, a fourth line to be printed in the reverse stroke and a fifth line to be printed in the forward stroke are added, such that the fourth line to be printed in the reverse stroke will be printed between the two lines printed in the forward stroke by the print head of the reference print color at position (Clr_ref, n).

[0035] Specifically, for the remaining printheads of the reference printing color, the relative deviation of each nozzle in the reverse stroke can be calculated using the equation Actual Position Line 4 Reverse - ((Actual Position Line 3 + Actual Position Line 5) / 2) based on their relative positions. Here, Actual Position Line 4 Reverse indicates the position of the line printed in the reverse stroke, and Actual Position Lines 3 and 5 indicate the positions of two adjacent lines printed by the printhead of the reference printing color in the forward stroke. The nozzle deviations determined for the forward stroke are then added to the distance determined for the reverse stroke.

[0036] Similarly, the relative nozzle deviation of other print colors in the reverse stroke can be calculated using the equation Actual Position Line 4 Reverse - ((Actual Position Line 3 + Actual Position Line 5) / 2) based on the relative position, where Actual Position Line 4 Reverse indicates the position of the line printed by the printhead at position (Clr ≠ Clr_ref, n) in the reverse stroke, and Actual Position Line 3 and Actual Position Line 5 indicate the positions of two adjacent lines printed by the printhead of the reference print color at position (Clr_ref, n) of the same n in the forward stroke.

[0037] Therefore, based on the relative position, for example, spline functions can be used to calculate the individual deviation of each nozzle in the reverse stroke. When the position in the reverse stroke is determined relative to the position in the forward stroke, the nozzle deviation of the reference color in the forward stroke is added to the nozzle deviation in the reverse stroke.

[0038] Advantageously, all deviations of each nozzle in the printhead of the reference printing color during the forward stroke are stored, because these deviations are used as a reference to determine the individual nozzle deviations of all other printing colors in both the first direction (forward stroke) and the second direction opposite to the first direction (reverse stroke).

[0039] In an embodiment of the nozzle misalignment correction method for a fixed inkjet printer, printheads for different printing colors are fixedly arranged in columns of a matrix configuration extending in a direction parallel to the transport direction (C), and printheads for the same printing color are arranged in rows extending in a direction perpendicular to the transport direction (C); wherein the printhead position in the matrix configuration is represented by (n, Clr), where n indicates the column number of the printhead for printing color Clr in the matrix configuration, and Clr indicates the printing color in a row of the matrix configuration of the printhead;

[0040] Step a) includes:

[0041] The relative position of the droplet ejected by each nozzle of each printhead in the printhead used as the reference print color (Clr_ref) at position (n, Clr_ref) is determined relative to a reference element. In a stationary inkjet printer, printheads of a specific print color are arranged adjacent to each other in the width direction of the printing medium and print along a line parallel to the transport direction of the printing medium. Therefore, unlike a scanning inkjet printer, a given position in the image to be printed can be addressed by one nozzle or a group of nozzles from one printhead for each print color. Therefore, in step a), the relative position of the droplets ejected by each nozzle of each printhead in the printhead for the reference print color (Clr_ref) at position (n, Clr_ref) is determined relative to the reference element, for example, by applying an imaginary line (e.g., a spline (a mathematical function defined by a piecewise polynomial) to the actual line printed by the nozzles of these printheads for the reference print color, and measuring the distance to the reference element, such as its desired (fixed) position. Each of these printheads for the reference print color acts as the corresponding printhead for the other print colors (Clr ≠ Clr_ref) in step b). Steps b) through d) are performed as described above.

[0042] In one implementation, printing a test pattern portion of the printhead for each print color at a printhead position (n, Clr), where n is the same for each print color, includes: printing at least one spaced line for each print color (Clr≠Clr_ref) other than the reference print color (Clr_ref) by ejecting droplets from each nozzle in the nozzle of the respective printhead, wherein the relative position of the droplets is determined according to the at least one line; and printing at least one line of the reference print color (Clr_ref) between at least one line of each print color (Clr≠Clr_ref) other than the reference print color (Clr_ref), and advantageously also printing at least one line of the reference print color (Clr_ref) before the first line of the first print color (Clr≠Clr_ref) other than the reference print color (Clr_ref), and printing at least one line of the reference print color (Clr_ref) after the last line of the last print color (Clr≠Clr_ref) other than the reference print color (Clr_ref).

[0043] In its implementation, the test pattern may include multiple lines to be printed by ejecting droplets from each nozzle of the printhead of the reference print color at position (n, Clr_ref), the number of which is at least equal to the number of print colors plus 1, and for each other color between two adjacent lines to be printed by the nozzles of the printhead of the reference print color at position (n, Clr_ref), at least one line to be printed by ejecting droplets from each nozzle of each print color other than the reference print color at position n (n, Clr ≠ Clr_ref). In this case, the lines of the reference print color can be used to determine the relative positions of the two other print colors. Such a test pattern is relatively simple in design.

[0044] In step d), the total deviation thus determined is converted into an individual nozzle correction factor, for example by multiplying by a factor *-1 (the correction factor to be applied is the opposite of the determined deviation) and rounding to an integer, since only pixels in the image printed using these correction factors from the corrected nozzle control data can be addressed. Therefore, an integer represents the translation of a pixel.

[0045] The correction factor determined in this way applies to a given speed of the inkjet printer in the scanning direction. If the inkjet printer is also configured to print at different speeds, the correction factor thus assigned must apply to the different speeds. For example, if the original correction factor is determined at a normal speed such as 1200 dpi, then for twice the speed (e.g., 600 dpi), the correction factor is adjusted by dividing by 2.

[0046] In a second aspect, the present invention relates to a system for correcting misalignment of nozzles in a multicolor inkjet printer for printing images on a printable medium movable relative to the inkjet printer (12) in a transport direction (C), the system comprising:

[0047] An inkjet printer has multiple printheads for each printing color, the printheads being mounted in a matrix configuration of columns and rows. Each printhead has an array of nozzles configured to eject ink droplets of that printing color. The printheads are arranged in columns extending in a direction parallel to the transport direction, and printheads for different printing colors are arranged in rows in the scanning direction. The position of each printhead in the matrix configuration is represented by (Clr, n), where Clr indicates the printing color and n indicates the row number of the printhead for color Clr in the matrix configuration. Alternatively, printheads for different printing colors are fixedly arranged in columns extending in a direction parallel to the transport direction (C), and printheads for the same printing color are arranged in rows extending in a direction perpendicular to the transport direction (C). The position of each printhead in the matrix configuration is represented by (n, n). Clr) represents, where n indicates the column number of the printhead used for the print color Clr in the matrix configuration, and Clr indicates the print color in the row of the matrix configuration of the printhead. The inkjet printer is equipped with a controller that is configured to control the nozzles of multiple inkjet printheads.

[0048] A scanner used to scan test patterns printed by an inkjet printer;

[0049] A computer configured to correct nozzle misalignment of a plurality of printheads of an inkjet printer according to the method of the invention according to the first aspect above, thereby generating corrected nozzle control data for a controller based on an assigned correction factor.

[0050] The system includes an inkjet printer, a scanner, a computer, and a controller, providing the same advantages as the nozzle misalignment correction method of the first aspect of the invention. The implementation of this method is also applicable to the system of the second aspect of the invention.

[0051] In one implementation, a controller is configured to print a test pattern on a test substrate, the test pattern including a test pattern portion for each printhead; a scanner is configured to scan the test pattern printed on the test substrate; and a computer is configured to evaluate the relative position of droplets ejected by each nozzle in the nozzles of each printhead during the scanning of the test pattern printed on the test substrate, and to determine the relative deviation from the relative position determined therefrom for each nozzle of all printheads, and to assign a correction factor to each nozzle in the nozzles of each printhead, and to store the correction factor.

[0052] In another embodiment of the system, the controller is configured to print lines of a color other than the reference print color, based on which the relative positions of the droplets are determined, and the lines are spaced apart from the lines of the reference print color.

[0053] In one embodiment, the system also includes a substrate having a receiving layer with properties suitable for inkjet ink.

[0054] In one implementation, the system also includes a memory that can be read by the controller and configured to store correction factors for each nozzle of the multiple printheads.

[0055] In one implementation, the controller is configured to print a test pattern on a test substrate by ejecting inkjet ink from the inkjet printhead of an inkjet printer, and is configured to print an image on a printing medium by ejecting inkjet ink from the inkjet printhead of an inkjet printer using calibrated nozzle control data derived from a calibration factor.

[0056] In one embodiment, the system includes a scanning inkjet printer, wherein the printhead is mounted on a carriage that can reciprocate in a scanning direction perpendicular to the transport direction of the printing media, and a computer is configured to:

[0057] a1) Determine the relative position of the droplets ejected by each nozzle in the nozzle of the reference printhead used for referencing the print color (Clr_ref) at the printhead position (Clr_ref) relative to the reference element.

[0058] a2) Determine the relative position of the droplet ejected by each nozzle of the remaining printheads (Clr_ref, n ≠ r) for the reference print color (Clr_ref) relative to the relative position of the droplet ejected by the nozzle of the reference printhead (Clr_ref, r) for the reference print color.

[0059] b) Determine the relative position of the droplet ejected by each nozzle of each printhead in the printhead position (Clr ≠ Clr_ref, n) of each of the print colors (Clr ≠ Clr_ref) excluding the reference print color (Clr_ref), relative to the relative position of the droplet ejected by the nozzle of the corresponding printhead with the same n printhead position (Clr_ref, n) of the reference print color (Clr_ref).

[0060] c) Determine the deviation of each nozzle in the printhead based on the determined relative positions of the droplets ejected by each nozzle in the printhead, and

[0061] d) Based on the determined deviation of each nozzle in the nozzles of the printhead, a correction factor is assigned to each nozzle in the nozzles of the printhead in a plurality of printheads.

[0062] In another embodiment, the controller is configured to:

[0063] - Printing a test pattern portion of a reference printhead for a reference printhead (Clr_ref), comprising: during a first stroke of the carriage in the scanning direction, printing at least one line, preferably at least two lines, such as three spaced lines, by ejecting ink droplets from each nozzle of the reference printhead for the reference printhead at position (Clr_ref, r); and / or

[0064] - A test pattern portion for each of the remaining printheads of the reference printhead (Clr_ref), comprising: printing at least one line, preferably at least two spaced lines, by ejecting ink droplets from each nozzle of each printhead of the remaining printheads of the reference printhead at printhead position (Clr_ref, n ≠ r), and printing at least one reference line by ejecting ink droplets from each nozzle of the reference printhead for the reference printhead at printhead position (Clr_ref, r); and / or

[0065] - Printing a test pattern portion for each of the print colors (Clr ≠ Clr_ref) other than the reference print color (Clr_ref), comprising: during the movement of the carriage in a first stroke in the scanning direction, printing at least one line by ejecting ink droplets from each nozzle of the nozzles of each print head of the print color at the print head position (Clr ≠ Clr_ref, n), and printing at least one reference line, preferably at least two spaced reference lines, by ejecting ink droplets from each nozzle of the nozzles of the corresponding print head for the reference print color (Clr_ref) at the print head position (Clr_ref, n) having the same n.

[0066] Furthermore, for bidirectional scanning inkjet printers, the controller is configured to print test patterns by: during the carriage's movement in a second stroke in the scanning direction opposite to the first stroke, printing at least one line for each print color by ejecting droplets from each nozzle of each of the nozzles of a plurality of print heads, further comprising: determining the relative position of droplets ejected by each nozzle of each of the nozzles of each of the print heads for each print color at position (Clr, n) during the carriage's movement in the second stroke in the scanning direction opposite to the first stroke, the relative position being relative to the position (Clr_ref, n) of print heads having the same n during the carriage's movement in the first stroke in the scanning direction. The relative position of the droplets ejected by the nozzles of the corresponding printhead at (n) for reference printing color (Clr_ref) is determined; the deviation of each nozzle in each printhead in the printhead is determined based on the determined relative position of the droplets ejected by the nozzles of the printhead; and based on the determined deviation of the nozzles of the printhead, a correction factor is assigned to each nozzle in the nozzles of the printhead in the plurality of printheads for printing in the first stroke and for printing in the second reverse stroke.

[0067] In an embodiment of a system including a stationary inkjet printer, printheads for different printing colors are fixedly arranged in columns of a matrix configuration extending in a direction parallel to the transport direction (C), and printheads (40) for the same printing color are arranged in rows extending in a direction perpendicular to the transport direction (C); wherein the printhead position in the matrix configuration is represented by (n, Clr), where n indicates the column number of the printhead for printing color Clr in the matrix configuration, and Clr indicates the printing color in a row of the matrix configuration of the printhead;

[0068] The controller is configured to:

[0069] - A test pattern portion for each printhead printing a reference print color (Clr_ref), comprising: printing at least one line, preferably at least two lines, such as three spaced lines, by ejecting ink droplets from each nozzle in the nozzles of the printhead for the reference print color at position (n, Clr_ref); and / or

[0070] - Printing a test pattern portion for each of the print colors (Clr ≠ Clr_ref) other than the reference print color (Clr_ref), including: printing at least one line by ejecting ink droplets from each nozzle of each print head of the print color at print head position (n, Clr ≠ Clr_ref), and printing at least one reference line, preferably at least two spaced reference lines, by ejecting ink droplets from each nozzle of the corresponding print head for the reference print color (Clr_ref) at print head position (n, Clr_ref) with the same n.

[0071] In a third aspect, the present invention relates to an inkjet printing method for inkjet printing an image on a printing medium, comprising: ejecting inkjet ink from an inkjet printhead of an inkjet printer having a controller, the ejection being controlled by the controller using corrected control nozzle data derived from a correction factor obtained according to the method of the first aspect. The type of printing medium is not limited. In an embodiment, the printing medium is a textile material.

[0072] In a fourth aspect, the present invention relates to a computer program comprising instructions which, when executed by a computing device, cause the computing device to perform the method according to the first aspect.

[0073] The invention is illustrated with reference to the accompanying drawings, in which:

[0074] Figure 1 A flowchart illustrating an embodiment of the misalignment correction method according to the present invention is shown;

[0075] Figure 2 An embodiment of the system of the present invention, including a scanning inkjet printer, is illustrated schematically;

[0076] Figure 3 An embodiment of a matrix configuration for the printhead arrangement of a scanning inkjet printer is shown.

[0077] Figure 4 An embodiment of the test pattern used in an embodiment of the misalignment correction method for a scanning inkjet printer according to the present invention is illustrated schematically.

[0078] Figures 5 to 8 It shows that according to Figure 4 Details of the test pattern;

[0079] Figure 9 A summary is shown. Figures 5 to 8 The table in the section for reference printing colors and test patterns for another printing color; and

[0080] Figure 10An embodiment of the system of the present invention, including a stationary inkjet printer, is illustrated schematically.

[0081] exist Figure 1 The diagram shows a flowchart of an embodiment of a nozzle misalignment correction method according to the present invention, applicable to both scanning inkjet printers and stationary inkjet printers. This embodiment includes step 1, in which a test pattern is printed on a test substrate by the nozzles of each printhead of a scanning inkjet printer. The test pattern includes a test pattern portion for each printhead to print a color different from a reference print color, the test pattern portion including print elements to be printed by the respective printhead and reference print elements to be printed by the reference printhead. Typically, the print elements are lines formed by ejecting ink from each nozzle in the nozzles of the respective printhead. As mentioned above, the design of the test pattern portion for the reference printhead depends on the type of printer. The test substrate may have a receiving layer compatible with the inkjet ink used. In step 2, the test substrate on which the test pattern is printed is scanned by a scanner to obtain a digital scan of the printed test pattern. In step 3, the obtained scan is analyzed, and the relative position of the printed print elements relative to the printed reference print element is determined. In step 4, the relative deviation of each nozzle in the nozzles of the respective printhead is calculated based on the relative position of the printed print elements. In step 5, for the thus determined relative nozzle positions, correction factors are assigned to each nozzle, typically stored in memory, for example, in the form of a chart or table. The correction factors are usually rounded to integers. If needed, a similar line test pattern can be printed on an additional test substrate with a receiving layer compatible with the inkjet ink used, based on the assigned correction factors, using the corrected nozzle control data. This similar line test pattern is then scanned and analyzed for inspection. In step 6, the inkjet printer executes a print job that prints an image on a printing medium using the corrected nozzle control data, based on the assigned correction factors read from the stored chart or table.

[0082] Figure 2 An embodiment of a system for correcting misalignment of printhead nozzles in a scanning inkjet printer is schematically illustrated. System 10 includes an inkjet printer 12, in this case a bidirectional scanning printer, wherein the printhead (see...) Figure 3A test substrate 18 is mounted on a carrier 14, which is reciprocally arranged on a track 16 in the width direction (scanning direction; see arrow S) of the test substrate 18, which is intermittently conveyed from an inlet 20 to an outlet 22 along a conveyor direction C, for example, temporarily attached to a conveyor 24 such as a loop belt. The reciprocating movement of the carrier 14 along the edge of the conveyor 24 between positions A and B, perpendicular to the conveyor direction, and the firing of the printhead are controlled by a controller 26, which also synchronizes these actions with the movement of the conveyor 24 to print a test pattern 28 having a test pattern portion 30 on the test substrate 18. A scanner 32 is configured to scan the test substrate 18 with the test pattern 28 already printed. The digital scanning of the test pattern is processed by a computer 34 with a processor, which is configured to identify the relative positions of the printed elements and reference printed elements from the digital scan of the test pattern 26 to determine the printhead nozzle deviation and assign correction factors to each nozzle. These correction factors are recorded, for example, in the memory 36 of the computer 34, and are subsequently used by the controller 26 to perform the actual print job.

[0083] Figure 3 An embodiment of the printhead 40 of an inkjet printer 12 is shown. In the illustrated embodiment, the printheads are arranged in a matrix configuration with columns representing printheads for a single print color Clr and rows representing the number n of printheads used for print colors. The position of the printheads in the matrix configuration is indicated by (Clr, n). Figure 3 In the illustrated embodiment, the number of printheads for each printing color is six (n=6). In this illustrated embodiment, the printing colors are black (K), cyan (C), magenta (M), yellow (Y), blue (B1), and orange (O). Specific printheads can be identified by their positions. The position of the first black printhead is indicated as (K, 1), and the position of the last black printhead is indicated as (K, 6). Similar indications are used for other printing colors. Printheads for one color are arranged adjacent to each other in the conveyor travel direction C, typically on a printing color bar releasably mounted on a carriage. Printheads for different colors are arranged in the width direction of the conveyor (i.e., in the scanning direction S perpendicular to the conveyor travel direction C). Each printhead 40 includes an array of nozzles 42. The (piezoelectric) printhead is configured to eject ink droplets (typical droplet size, e.g., 1-4 picoliters) onto the printing medium conveyed by the conveyor.

[0084] Figure 4 An embodiment is shown with a printed test pattern 28 having a test pattern portion 30 for each printhead. In this case, there are eight printheads for each printing color. Figure 5 The print test pattern section for the eight black printheads is shown in more detail.

[0085] Each test pattern portion 30 includes one or more lines as printed elements printed by ejecting droplets from each nozzle of a corresponding printhead, and one or more reference lines as reference printed elements printed by ejecting droplets from each nozzle of a reference printhead. The digital scan of the printed test pattern 28 is analyzed for nozzle misalignment of the printheads. In this embodiment, the black printhead at position (K, 8) is used as a reference printhead (Clr_ref = K, r = 8). The relative position and deviation of each nozzle in the nozzle of this black reference printhead at position (K, 8) is determined relative to a reference (e.g., an imaginary straight line). The relative position and deviation of each nozzle in the nozzles of the other black printheads at positions (K, n=1-7) are determined by one or more lines printed by these other black printheads relative to the reference lines printed by the black reference printhead at position (K, 8). The relative position and offset of each nozzle in the nozzles of the printheads for other printing colors (Clr≠K) at position (Clr, n = 1-8) is determined by one or more lines printed by ejecting droplets through the nozzles of these printheads and one or more reference lines printed by the corresponding black printheads in the same row (n equal).

[0086] In this implementation, the test pattern portion for the black reference head (K, 8) includes five parallel, spaced lines, wherein the top three lines and the bottom line are printed in a first direction (forward stroke), and the fourth line is printed in the opposite direction (reverse stroke). See also Figure 6 The position of the line is determined relative to a reference, and the relative position of each nozzle in the nozzle is measured relative to that reference, thereby calculating the corresponding measurement deviation for each individual nozzle.

[0087] Figure 7The diagram shows portions of the test pattern printed for the remaining black printheads (K, n=1-7), where, viewed from top to bottom, the first, third, and fifth lines are printed by each nozzle in the nozzles during the forward stroke of the respective black printhead, and the fourth line is similarly printed during the reverse stroke. The second line is a reference line printed by the black reference head (K, 8). For each nozzle, the relative position of the line in the first stroke with respect to the reference line is determined using the equation ((actual position line 1 + actual position line 3) / 2) - actual position reference line 2, where actual position lines 1 and 3 indicate the positions of the two lines printed by the remaining black printheads, and actual position reference line 2 indicates the position of the reference line printed by the reference black printhead (K, 8). To calculate the relative deviation, a spline function is applied to the found relative positions. The deviation of the nozzles of the remaining black printheads in the forward stroke increases with the deviation calculated for the reference black printhead. For each nozzle, the relative position of the line printed in the reverse stroke is determined by the equation Actual Position Line 4 Reverse - ((Actual Position Line 3 + Actual Position Line 5) / 2), where Actual Position Line 4 Reverse indicates the position of the line printed in the reverse stroke, and Actual Position Lines 3 and 5 indicate the positions of two adjacent lines printed by the remaining printheads of the black reference printing color in the forward stroke. To calculate the relative deviation, a spline function is applied to the found relative positions. The deviation of the nozzles of the remaining black printheads in the reverse stroke increases with the deviation calculated for that black printhead in the forward stroke.

[0088] Figure 8The diagram shows a portion of the test pattern printed at position (Clr ≠ K), for a color other than the black reference color (Clr ≠ K). Viewed from top to bottom, the first, third, and fifth lines are printed by a printhead of the black reference color at position (K, n = 1-8). The second line is printed in the first stroke direction by each nozzle in the nozzles of the corresponding printheads of the corresponding other colors at position (Clr ≠ K, n = 1-8) ejecting droplets for the same n value. The fourth line is printed in the reverse stroke direction by each nozzle in the nozzles of the corresponding printheads of the corresponding other colors at position (Clr ≠ K, n = 1-8) ejecting droplets for the same n value. For each nozzle, the relative position of the line printed by the corresponding printhead for the corresponding color (excluding black) in the forward stroke can be calculated using the equation Actual Position Line 2 - ((Actual Position Reference Line 1 + Actual Position Reference Line 3) / 2), where Actual Position Reference Line 1 and Actual Position Reference Line 3 indicate the positions of the two lines printed by the printhead at position (Clr_ref, n), and Actual Position Line 2 indicates the position of the line printed by the black printhead at position (K, n). To determine the relative deviation of the nozzles, for example, a spline function is applied to the relative positions. For these relative deviations, the deviations of the reference black printhead at position (K, n) are summed, since this position is determined relative to the black printhead at that position. Similarly, the deviation of each nozzle in the nozzle during the reverse stroke can be calculated based on the relative position calculated by the equation Actual Position Line 4 - ((Actual Position Reference Line 3 + Actual Position Reference Line 5) / 2), where Actual Position Reference Line 3 and Actual Position Reference Line 5 indicate the positions of the two lines printed by the printhead of the black reference color at position (K, n), and Actual Position Line 4 indicates the position of the line printed by the corresponding printhead of one of the other printing colors at position (Clr, n) during the reverse stroke, for example, by applying a spline function to this position. The relative deviation is supplemented by the deviation of the black printhead at position (K, n).

[0089] Figure 9 Table 1, as shown, summarizes the aforementioned test pattern portions for eight (n=1-8) printheads using one of the reference color (Clr_ref=K (black)) and other print colors (Clr≠Clr_ref). F indicates printing the corresponding line in the first (forward) stroke of the scanning direction, and B indicates printing the corresponding line in the second (reverse) stroke of the scanning direction opposite to the first stroke.

[0090] Based on the deviation of each nozzle in the forward and reverse strokes thus determined, a correction factor is derived by multiplying by -1 and rounding to an integer. These correction factors for each nozzle of each printhead in the forward and reverse strokes are stored. The stored correction factors are used in the calibrated nozzle control data. Other corrections, such as non-uniformity / grayscale / oversaturation corrections known from WO2020239820A1, may also be considered in the calibrated nozzle control data.

[0091] For other printing speeds, the correction factor determined thereby should be adjusted. For example, for double speed, the correction factor should be divided by 2.

[0092] Example

[0093] Printed at a standard speed (1200 dpi; nozzle-to-test substrate height 2.8 mm) on a Javelin® inkjet printer (a scanning inkjet printer manufactured by the applicant using a Fujifilm Dimatix® printhead) as shown in the image. Figure 4 The test pattern is shown, and the scan of the printed test pattern is analyzed to obtain the correction factor for each nozzle of the printhead. Based on the correction factor thus determined, the example is repeated using the corrected nozzle control data. Table 1 below shows the mean, median, standard deviation, and the range between the minimum and maximum deviations for the initial pattern (“before”) and the patterns printed using the correction factor in the forward and reverse strokes (“after”).

[0094]

[0095] As can be seen, by printing using calibrated nozzle control data based on a correction factor, the quality of the image (in this case, the test pattern) is significantly improved. Similar results were obtained at high speed (600 dpi) and different heights (2.8 mm and 4.0 mm).

[0096] In all examples using calibrated nozzle control data, the spread of the deviation was also significantly reduced.

[0097] Figure 10 An embodiment of the system of the present invention, including a stationary inkjet printer, is schematically illustrated. In this figure, with... Figure 2The same components are labeled with the same reference numerals. System 10 includes a stationary inkjet printer 12, wherein a printhead 40 is fixedly mounted relative to a conveyor 24, which conveys printing media (in this case, a test substrate 18) temporarily adhered thereto from an inlet 20 to an outlet 22 along a conveying direction C. The firing of the printhead 40 is controlled by a controller 26, which also synchronizes these actions with the movement of the conveyor 24 to print a test pattern 28 having a test pattern portion 30 on the test substrate 18. A scanner 32 is configured to scan the test substrate 18 with the test pattern 28 already printed on it. The digital scanning of the test pattern is processed by a computer 34 with a processor, which is configured to identify the relative positions of the printed elements and reference printed elements from the digital scan of the test pattern 26 to determine the deviation of the printhead nozzles and assign individual correction factors to each nozzle. These correction factors are recorded, for example, in the memory 36 of the computer 34 and are subsequently used by the controller 26 to perform the actual printing job. In this type of inkjet printer 12, printheads 40 for a specific color Clr (the number of which is indicated by n) are typically mounted on a printing color bar that is releasably mounted in the frame of a bridging conveyor, arranged in the width direction of the conveyor and therefore perpendicular to the transport direction C. A series of printheads 40 for different printing colors are positioned in the transport direction as indicated by the arrows. Typically, black is the last color printed.

Claims

1. A method of correcting misalignment of nozzles of a multi-color inkjet printer (12) for printing an image on a print medium, the print medium being movable relative to the inkjet printer (12) along a conveyance direction (C), the inkjet printer (12) having a plurality of print heads (40) for each print color, the plurality of print heads (40) being mounted in a matrix configuration of columns and rows, a print head (40) having an array of nozzles (42) configured for ejecting droplets of print color, wherein: the print heads (40) for different print colors are arranged in rows extending in a direction perpendicular to the transport direction (C) on a carriage (14) which is reciprocally movable in a scan direction (S) perpendicular to the transport direction (C) of the print medium; or the print heads (40) for different print colors are fixedly arranged in columns of the matrix configuration extending in a direction parallel to the transport direction (C) and the print heads (40) for the same print color are arranged in rows extending in a direction perpendicular to the transport direction (C); wherein the method comprises the following steps: a) determining a relative position of a droplet ejected by each of the nozzles of each of the print heads for a reference print color (Clr_ref) relative to a reference element; b) determining a relative position of a droplet ejected by each of the nozzles of each of the print heads for each of the print colors other than the reference print color (Clr≠Clr_ref) relative to the relative position of a droplet ejected by each of the nozzles of the corresponding print head for the reference print color (Clr_ref); c) determining a deviation for each of the nozzles of each of the print heads based on the determined relative positions of the droplets ejected by the nozzles of the print heads, and d) assigning a correction factor to each of the nozzles of each of the print heads of the plurality of print heads based on the determined deviations of the nozzles of the print heads.

2. The method of claim 1, wherein, Steps a) to b) comprise printing a test pattern (28) on a test substrate (18), the test pattern (28) comprising a test pattern portion (30) for each print head (40); and scanning the printed test pattern and determining the relative positions of the droplets ejected by each of the nozzles of each print head.

3. The method of claim 1 or claim 2, wherein, the print heads (40) for different print colors are arranged in rows extending in a direction perpendicular to the transport direction (C) on a carriage (14) which is reciprocally movable in a scan direction (S) perpendicular to the transport direction (C) of the print medium; or the print heads (40) for different print colors are fixedly arranged in columns of the matrix configuration extending in a direction parallel to the transport direction (C) and the print heads (40) for the same print color are arranged in rows extending in a direction perpendicular to the transport direction (C); wherein step a) comprises the following substep: al) determining a relative position of a droplet ejected by each of the nozzles of a reference print head for a reference print color (Clr_ref) at a print head position (Clr_ref, r) relative to a reference element, b) determining a relative position of a droplet ejected by each of the nozzles of each of the print heads for each of the print colors other than the reference print color (Clr≠Clr_ref) relative to the relative position of a droplet ejected by each of the nozzles of the corresponding print head for the reference print color (Clr_ref); c) determining a deviation for each of the nozzles of each of the print heads based on the determined relative positions of the droplets ejected by the nozzles of the print heads, and d) assigning a correction factor to each of the nozzles of each of the print heads of the plurality of print heads based on the determined deviations of the nozzles of the print heads. Steps a) to b) comprise printing a test pattern (28) on a test substrate (18), the test pattern (28) comprising a test pattern portion (30) for each print head (40); and scanning the printed test pattern and determining the relative positions of the droplets ejected by each of the nozzles of each print head. the print heads (40) for different print colors are arranged in rows extending in a direction perpendicular to the transport direction (C) on a carriage (14) which is reciprocally movable in a scan direction (S) perpendicular to the transport direction (C) of the print medium; or the print heads (40) for different print colors are fixedly arranged in columns of the matrix configuration extending in a direction parallel to the transport direction (C) and the print heads (40) for the same print color are arranged in rows extending in a direction perpendicular to the transport direction (C); wherein step a) comprises the following substep: a1) determining a relative position of a droplet ejected by each of the nozzles of a reference print head for a reference print color (Clr_ref) at a print head position (Clr_ref, r) relative to a reference element, b) determining a relative position of a droplet ejected by each of the nozzles of each of the print heads for each of the print colors other than the reference print color (Clr≠Clr_ref) relative to the relative position of a droplet ejected by each of the nozzles of the corresponding print head for the reference print color (Clr_ref); c) determining a deviation for each of the nozzles of each of the print heads based on the determined relative positions of the droplets ejected by the nozzles of the print heads, and d) assigning a correction factor to each of the nozzles of each of the print heads of the plurality of print heads based on the determined deviations of the nozzles of the print heads. Steps a) to b) comprise printing a test pattern (28) on a test substrate (18), the test pattern (28) comprising a test pattern portion (30) for each print head (40); and scanning the printed test pattern and determining the relative positions of the droplets ejected by each of the nozzles of each print head. a2) determining the relative position of the droplet ejected by each of the nozzles of the remaining printheads for the reference printing color (Clr_ref) at the printhead position (Clr_ref, n≠r) with respect to the relative position of the droplet ejected by the nozzle of the reference printhead for the reference printing color at the printhead position (Clr_ref, r).

4. The method of claim 3, wherein, printing a test pattern portion (30) for the reference printhead for the reference printing color (Clr_ref) comprises printing at least one line by ejecting droplets from each of the nozzles of the reference printhead for the reference printing color at the position (Clr_ref, r) during the movement of the carriage (14) in the first pass in the scanning direction.

5. The method of claim 3 or claim 4, wherein, printing a test pattern portion (30) for each of the remaining printheads for the reference color (Clr_ref) comprises printing at least one line by ejecting droplets from each of the nozzles of each of the remaining printheads for the reference printing color at the printhead position (Clr_ref, n≠r) and printing at least one reference line by ejecting droplets from the nozzle of the reference printhead for the reference printing color at the printhead position (Clr_ref, r) during the movement of the carriage (14) in the first pass in the scanning direction.

6. The method according to any of the preceding claims 3 to 5, wherein, printing a test pattern portion (30) for each of the remaining printheads for the reference color (Clr_ref) comprises printing at least one line by ejecting droplets from each of the nozzles of each of the remaining printheads for the reference printing color at the printhead position (Clr_ref, n≠r) and printing at least one reference line by ejecting droplets from the nozzle of the reference printhead for the reference printing color at the printhead position (Clr_ref, r) during the movement of the carriage (14) in the first pass in the scanning direction.

7. The method of any one of claims 3 to 6, wherein, The inkjet printer (12) is of the bidirectional scanning type, and wherein the printing of the test pattern (28) further comprises: printing at least one line for each printing color during the movement of the carriage (14) in a second stroke along the scanning direction opposite to the first stroke by ejecting drops from each of the nozzles of each of the plurality of printheads at position (Clr,n); further comprising: determining the relative position of the drops ejected by each of the nozzles of each of the printheads for each of the printing colors at position (Clr,n) during the movement of the carriage in the second stroke along the scanning direction opposite to the first stroke, the relative position being determined with respect to the relative position of the drops ejected by each of the nozzles of the corresponding printhead for the reference printing color (Clr_ref) at printing head position (Clr_ref,n) with the same n during the movement of the carriage in the first stroke along the scanning direction; determining a deviation for each of the nozzles of each of the printheads based on the determined relative position of the drops ejected by each of the nozzles of the printhead; and assigning to each of the nozzles of the printheads in the plurality of printheads a correction factor for printing in the first stroke and for printing in the second opposite stroke based on the determined deviations of the nozzles of the printhead.

8. The method of claim 1 or claim 2, wherein, The printheads (40) for different printing colors are arranged in columns of the matrix configuration extending in a direction parallel to the transport direction (C), and the printheads (40) for the same printing color are arranged in rows extending in a direction perpendicular to the transport direction (C); wherein a printing head position of a printhead in the matrix configuration is denoted by (n,Clr), wherein n indicates the column number of the printhead for printing color Clr in the matrix configuration, and Clr indicates the printing color in the row of the matrix configuration of the printhead; wherein step a) comprises: determining the relative position of the drops ejected by each of the nozzles of each of the printheads for the reference printing color (Clr_ref) at position (n,Clr_ref) with respect to the reference element.

9. The method according to any of the preceding claims, wherein, The lines of printing colors (Clr≠Clr_ref) other than the reference printing color (Clr_ref) are printed spaced apart between the lines of the reference printing color (Clr_ref), wherein the relative position of the drops is determined from the lines of the printing colors.

10. The method of claim 8, wherein, printing a test pattern portion (30) of the printheads for each printing color at a printhead position (n, Clr), wherein n is the same for each printing color, including: printing at least one line of each printing color (Clr≠ Clr_ref) except the reference printing color (Clr_ref) spaced apart, wherein the relative position of the drops is determined from said at least one line; and printing at least one line of the reference printing color (Clr_ref) between at least one line of each printing color (Clr≠ Clr_ref) except the reference printing color (Clr_ref), and at least one line of the reference printing color (Clr_ref) in front of the first line of the first printed printing color (Clr≠ Clr_ref) except the reference printing color (Clr_ref), and at least one line of the reference printing color (Clr_ref) behind the last line of the last printed printing color (Clr≠ Clr_ref) except the reference printing color (Clr_ref).

11. The method of any of the preceding claims, wherein, The reference printing color (Clr_ref) is black (K).

12. The method of any of the preceding claims, wherein, The correction factor is an integer representing a shift of a pixel.

13. The method of any of the preceding claims, wherein, The test substrate (18) is rigid, preferably a white plastic film, having a receiving layer adapted to the properties of the inkjet ink.

14. A system for correcting misalignment of nozzles of a multi-color inkjet printer for printing an image on a print medium, the print medium being movable relative to the inkjet printer (12) along a transport direction (C), the system comprising: an inkjet printer (12) having a plurality of printheads (40) for each printing color, the plurality of printheads being mounted in a matrix configuration of columns and rows, a printhead (40) having an array of nozzles (42) configured for ejecting drops of a printing color, wherein: printheads (40) of the same printing color are arranged in columns of the matrix configuration extending parallel to the transport direction (C), and printheads (40) for different printing colors are arranged in rows of the matrix configuration along a scanning direction (S), wherein a position of a printhead in the matrix configuration is denoted by (Clr, n), wherein Clr indicates the printing color and n indicates the row number of the printhead for color Clr in the matrix configuration; or printheads (40) for different printing colors are fixedly arranged in columns of the matrix configuration extending parallel to the transport direction (C), and printheads (40) for the same printing color are arranged in rows extending perpendicular to the transport direction (C), wherein a printhead position of a printhead in the matrix configuration is denoted by (n, Clr), wherein n indicates the column number of the printhead for printing color Clr in the matrix configuration and Clr indicates the printing color in the row of the matrix configuration of the printhead; the inkjet printer (12) being provided with a controller (26) configured for controlling the nozzles (42) of the plurality of inkjet printheads (40); a scanner (32) for scanning a test pattern printed by the inkjet printer; a computer (34) configured to correct misalignment of nozzles of the plurality of printheads of the inkjet printer according to the method of any of the preceding claims 1 to 13, thereby obtaining corrected nozzle control data for the controller based on assigned correction factors.

15. The system of claim 14, further comprising a test substrate (18) having a receiving layer adapted to the properties of the inkjet ink.

16. The system of claim 14 or claim 15, further comprising a memory (36) readable by the controller and configured for storing correction factors for each nozzle of the plurality of printheads.

17. The system of any one of claims 14 to 16, wherein, the controller (26) is configured to print a test pattern (28) on the test substrate (18) by ejecting inkjet ink from inkjet printheads of the inkjet printer, and to print an image on a print medium by ejecting inkjet ink from inkjet printheads of the inkjet printer using the corrected nozzle control data.

18. An inkjet printing method of printing an image on a print medium, comprising: ejecting inkjet ink from inkjet printheads (40) of an inkjet printer (12) having a controller (26), the ejection being controlled by the controller using corrected control nozzle data based on correction factors obtained by the method of any of the preceding claims 1 to 13.

19. A computer program comprising instructions which, when the program is executed by a computing device, cause the computing device (34) to perform the method of any of the preceding claims 1 to 13.

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