Method for compensating for failed print nozzles of ink print head

Abstract

The invention relates to a method for compensating for failed printing nozzles of an ink printhead for printing a print image (9) onto a printing material conveyed in a conveying direction, the print image (9) being printed with at least two different print drop volumes, one being a maximum print drop volume, and the other being a second print drop volume. At least one compensation nozzle (5) prints compensation drops with a compensation drop volume that is increased relative to the maximum printing drop volume, the compensation nozzle being an adjacent nozzle of the failed nozzle (4), the compensation nozzle (5) being actuated to generate the compensation drops in a predetermined frequency, the compensation drop volume being greater than the maximum printing drop volume; in this way, successive compensation drops following the predetermined frequency and printed by the compensation nozzles (5) have a predetermined minimum distance from each other in the conveying direction. The invention further improves the compensation of the nozzles for failed or incorrect printing, and thus further improves the printing quality, in particular with the expansion of the achievable color space. The invention is used for industrial ink-jet printing.

Description

Technical Field

[0001] The invention relates to a method for compensating for failed printing nozzles of an ink printing head, the method having the features set forth in the preamble of claim 1 .

[0002] The invention belongs to the technical field of the graphics industry and in particular to the field of industrial (i.e. high-productivity) inkjet printing on flat substrates, i.e. the image-based application of fine droplets of liquid ink ejected from nozzles to a printing material in the form of sheets, webs, films or labels (preferably made of paper, cardboard, corrugated cardboard, plastic, metal or a composite material), wherein nozzles that do not fire or fire incorrectly are compensated, for example, by their adjacent nozzles and printing errors are avoided in this way. Background Art

[0003] Inkjet print heads usually have a plurality of individually controllable nozzles. These nozzles are usually arranged in a nozzle array or in a plurality of nozzle arrays, parallel (or slightly inclined) to the direction of transport of the printing material. Each nozzle produces one or more ink droplets when it is controlled. If a nozzle is continuously controlled, it prints a visible line in the ink color on the printing material. On the other hand, if a nozzle fails (for example because the nozzle opening of the nozzle is blocked by solidified ink), or if a nozzle ejects droplets in an inclined manner or only as a spray, a bright line (also called "missing nozzle", "missing line" or "white line" in English) that is also visible may also be produced. As an undesirable result, the color of the printing material (usually white) appears as a line in the printed image. In addition, there may be visible dark lines (also called "dark lines" in English), for example in the case of overcompensation of a defective nozzle.

[0004] In an industrial environment, such non-printing or incorrectly printing nozzles must be compensated, since customers cannot tolerate "white lines" in high-quality printed products. A known method utilizes the properties of the print head nozzles, which can generate droplets of different sizes depending on the control of the nozzles. In this way, the neighboring nozzles ("to the left and to the right of the defective nozzle") can be used to generate larger compensation drops, i.e. compensation drops with a larger ink volume, and to close the "white line" completely or at least so much with the excess ink of these compensation drops that it is no longer perceived as a disturbance by the naked eye.

[0005] The abbreviation MNC often used in this context stands for "missing nozzle compensation" or more generally "malfunctioning nozzle compensation", ie compensation for non-printing or malfunctioning printing nozzles.

[0006] In order to identify these nozzles that do not print or print incorrectly (or the corresponding positions of these nozzles in the nozzle arrangement), a known method is to print a test pattern specially designed for this purpose, and to capture the test pattern, for example, with the aid of a camera, and to analyze and process the captured image with the aid of a computer. Here, for the nozzles identified as requiring compensation, a compensation intensity can be determined, which determines the degree of compensation, for example, low compensation or high compensation.

[0007] Document DE102015220716A1 discloses a method for automatically compensating for a failed printing nozzle in an inkjet printing system by means of a control computer, comprising the following steps: printing a test form for a certain material combination; analyzing the printing of the test form and creating a lookup table, which consists of compensation probabilities between 0 and 1, which depend on the local surface density. Detect a failed printing nozzle; read the size of a target droplet to be compensated at the position of the failed printing nozzle from the printing data; calculate the local surface density at the position of the failed printing nozzle; read the compensation probability from a lookup table with the help of the calculated local surface density; calculate a pseudo-random number between 0 and 1 for each of the adjacent pixels on the right and left sides of the pixel to be compensated; if the corresponding pseudo-random number assigned to the pixel is smaller than the compensation probability read from the lookup table, increase the droplet size of the adjacent pixels on the right and left sides of the pixel to be compensated; calculate the adjacent droplet size of all target drops to be compensated at the position of the failed printing nozzle, and apply the changed droplet size in the printing data; and perform a printing job with the help of the changed printing data.

[0008] Document DE102018201674A1 discloses a method for performing a printing process in an inkjet printer, wherein a failed printing nozzle is compensated by using a correction grid image by a computer, wherein, after a rasterization process, the failed printing nozzle is compensated by increased ink application of adjacent printing nozzles, and the rasterization process generates a raster image for a print image to be printed, characterized in that at least one correction grid image is pre-calculated by a computer for adjacent printing nozzles, the correction grid image replaces the raster image generated in the rasterization process in at least one column of adjacent printing nozzles, and the printing process is performed in the inkjet printer with the aid of the corrected grid.

[0009] When determining the maximum printable tonal value (within the scope of expanding the achievable color space), it is important to note that this tonal value should be compensable in the event of a possible nozzle failure and that, in the event of compensation, the compensating nozzles should not also fail to compensate due to overload due to the higher ink volumes that are usually produced. Summary of the invention

[0010] In relation to this state of the art, the object of the present invention is to propose a technical solution which is an improvement over the prior art and which in particular makes it possible to further improve the compensation of failed or incorrectly printed nozzles and thereby further improve the printing quality, in particular by expanding the achievable color space.

[0011] The technical solution to solve the task according to the present invention is:

[0012] This object is achieved according to the invention by a method according to claim 1 .

[0013] Advantageous and therefore preferred developments of the invention can be derived from the dependent claims as well as from the description and the drawings.

[0014] According to the method for compensating for failed printing nozzles of an ink printing head, the ink printing head is used to print a printing image on a printing material transported in a conveying direction, wherein the printing image is printed with at least two different printing drop volumes, wherein one printing drop volume is a maximum printing drop volume, and wherein at least one compensating nozzle (i.e., an adjacent nozzle relative to the failed nozzle) prints compensating drops with a compensation drop volume that is increased relative to the maximum printing drop volume, characterized in that a frequency for generating compensating drops is predetermined The compensating nozzle is actuated in such a way that a predefined frequency is followed and that successive compensating drops printed by the compensating nozzle have a predefined minimum distance from one another in the transport direction during the printing process.

[0015] Advantageous configuration and effects of the present invention:

[0016] The invention advantageously makes it possible to further improve the compensation of failed or incorrectly printing nozzles and thus to further increase the print quality, in particular by enlarging the achievable color space.

[0017] The invention makes it possible to enlarge the color space, effectively compensating for printing defects that may be caused by non-printing or erroneous printing nozzles, and at the same time ensuring that the compensating nozzles are not permanently overloaded and fail themselves.

[0018] The feature "wherein the print image is printed with at least two mutually different print drop volumes" means, for example, that when printing the print image, small (S), medium (M) and large (L) droplets are used, wherein these printed ink volumes can be in the range of 2, 4 and 6 picoliters.

[0019] The feature "wherein one printing drop volume is a maximum printing drop volume" means that, for example, a large-volume L drop is the largest ink drop used when printing.

[0020] The feature “wherein at least one compensating nozzle prints compensating drops with a compensating drop volume that is increased relative to the maximum printed drop volume” means, for example, that the compensating drops are larger than the L drops, ie have, for example, a drop volume of more than approximately 6 picoliters.

[0021] The feature “a frequency is predefined for generating compensation drops” means, for example, that the value of this frequency is stored digitally in a control device for the print head (or the compensation process).

[0022] The feature “controlling the compensating nozzle in such a way that a predefined frequency is observed” means, for example, that the compensating nozzle (to the left or right of the failed or defective nozzle) deposits compensating drops in the conveying direction at a frequency that corresponds to the predefined frequency.

[0023] The feature "and the successive compensating drops printed by the compensating nozzles have a predetermined minimum spacing relative to one another in the conveying direction" means, for example, that for a predetermined frequency of 50%, one compensating drop is provided for every other possible compensating drop, for a predetermined frequency of 33%, one compensating drop is provided for every third possible compensating drop, and for a predetermined frequency of 25%, one compensating drop is provided for every third possible compensating drop. The compensating drops are thus printed regularly, i.e. at the same spacing relative to one another, and in particular not randomly distributed. This also improves the ink flow properties, so that interfering white lines are better closed.

[0024] According to the invention, the compensation pixels are arranged in the printing direction while adhering to a predetermined frequency, and this frequency is supplemented by the requirement that a minimum spacing is maintained between the compensation pixels to be arranged. To give an example: if the frequency is set to 50%, then exactly every other pixel is printed with a compensation drop, so that the minimum spacing of the compensation pixels is permanently a single pixel.

[0025] As a further requirement, the following can be advantageously added: If the nozzle to be compensated does not set a pixel at the position to be compensated locally in the image, local compensation is still performed with the aid of adjacent nozzles. This means, in turn: the compensation pixels are set according to the frequency and not according to the grid used (this is the prior art); where pixels are present in the printed image and are usually set microscopically in a structured manner by means of a grid, larger compensation drops are set instead. Furthermore, the frequency of setting the compensation pixels is not controlled with the aid of a statistically random algorithm (this is also the prior art), but rather the minimum possible spacing between the compensation pixels in the printing direction is sought. Therefore, there may be partially larger or smaller spacings between these compensation pixels.

[0026] Therefore, the following can be achieved by means of the embodiments of the present invention: the darkest tonal value to be compensated can be selected to be darker than in the prior art, and the color space of the inkjet printer can be expanded.

[0027] According to the invention, larger droplets than those specified according to the ink print head can be printed if the droplets are kept at a distance from one another such that the maximum ink quantity per nozzle per unit time specified according to the specification is not exceeded.

[0028] Improvement scheme of the present invention:

[0029] Preferred improvements of the present invention (abbreviated as: improvements) are described below.

[0030] A feature of an improved solution may be that a continuous drop volume (Dauertropfen-Volumen) that allows continuous printing is predefined for the print head, and the compensating drop is printed with a compensating drop volume that is increased relative to the continuous drop volume. The continuous drop volume is usually predefined within the scope of the print head specification. This manufacturer limitation of the print head is temporarily exceeded for a small amount of drops in an advantageous manner by means of the compensating drop volume, but in a manner that excludes damage to the print head. Therefore, such a volume can be selected for the compensating drop, which is higher than the actual allowed ink volume specification (i.e. if this ink volume is permanently set), and can be set with such a compensating drop, for example, every other (or every two or every three, etc.) pixel. It can be achieved that a larger amount of ink is applied in a certain area, and the defective ink-free printed image surface transverse to the printing direction is covered with (compensating) ink drops, and the defects are thus eliminated.

[0031] One development can be characterized in that the method is used starting from a tonal value at which at least 90% or at least 95% of the pixels required for printing the image are provided.

[0032] One development can be characterized in that the method is used starting from a tonal value of at least 50% or at least 55% or at least 65%.

[0033] One development can be characterized in that the frequency H is predefined as a percentage value, wherein H=P / Pmax and wherein P is the number of compensating drops to be provided and Pmax is the maximum number of compensating drops that can be provided in each conveying direction.

[0034] The features and feature combinations disclosed in the above technical fields, the summary of the invention and the developments as well as in the following exemplary embodiments—in any combination with one another—form further preferred developments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 and Figure 2 The apparatus for carrying out a preferred embodiment of the method according to the invention is shown in each case. In the figures, corresponding features are marked with the same reference numerals. DETAILED DESCRIPTION

[0036] Figure 1 Three examples A, B and C for compensating for so-called white lines are shown side by side, wherein the middle example B corresponds to an embodiment of the present invention.

[0037] Here, a printing press 1 for ink printing is shown, which has a plurality of printing heads 2, which are preferably arranged side by side as a printing beam (preferably for printing one color of ink, such as C, M, Y or K). Each printing head 2 has a plurality of printing nozzles 3 (only three printing nozzles 3 are shown by way of example). The middle one of the three printing nozzles 3 shown is a nozzle 4 that is turned off in a test mode, which thus simulates a failed or defective nozzle. Two adjacent nozzles 5 located to the left and right of this nozzle 4 (or the generated white line 11) serve as compensation nozzles 5.

[0038] The printing press 1 prints a substrate 6 transported in a conveying direction 7. The print heads 2 (or print beams) are arranged adjacent to one another in a transverse direction 8 and preferably print ink drops from above onto the substrate 6, which moves past under the print heads 2. In the process, a printed image 9 or at least one printed measurement element 9 is generated on the substrate 6. For test purposes, as shown in the figure (by shading with lines), the printed image / printed measurement element 9 can correspond to a full-tone image or a full-tone element.

[0039] For controlling the printing process and in particular for compensation, the printing press 1 comprises a control device 20 .

[0040] For all examples A, B and C, the following assumptions are made: the tonal value of the image 9 or element 9 consists of pixels that are set at 100%; the nozzle 4 to be compensated is in each case closed in a test manner (i.e. the nozzle 4 does not print, or produces a white line 11); adjacent pixels 10 are set transversely to the conveying direction 7 by the adjacent nozzles 5 of the closed nozzle 4, so that these adjacent nozzles 5 in principle print with droplets having a larger volume than pixels in the conventional 100% tonal value.

[0041] Example A (according to prior art): All compensation pixels 10 are set to the left and right of the closed nozzle 4. The drop volume of the compensation pixel 10 is within the print head specification and it allows to set each pixel with this volume.

[0042] Example B (embodiment of the method according to the invention): Compensation pixels 10 are arranged to the left and right of the closed nozzle 4 according to a predetermined frequency, wherein the spacing of these pixels 10 is kept to a minimum. The drop volume of the compensation pixels 10 is outside the print head specification and it is permitted to arrange pixels with this volume at a defined frequency of <100% at most. In the example shown, the frequency is 33%, which means: For compensation, one pixel is arranged every three pixels 10 to the left and right of the white line 11.

[0043] Example C (according to another prior art): Depending on the frequency, compensation pixels 10 are arranged on the left and right of the closed nozzle 4, wherein the arrangement is carried out in a random manner depending on the frequency and, in this case, the spacing of these compensation pixels 10 varies randomly. The drop volume of the compensation pixels 10 is outside the print head specification, which allows pixels 10 with this volume to be arranged at most with a defined frequency of <100%. In the example shown, the frequency is also 33%.

[0044] The two (compensating) ink volumes applied in case B and case C (considered over the compensating distance in the conveying direction 7 ) are equal to one another.

[0045] Figure 2 Three examples B1 , B2 and B3 according to the invention are shown side by side for compensating the so-called white line 11 .

[0046] In principle, the three examples do not differ from one another. However, in each example, a different frequency is predefined: in example B1, a frequency of 50% (a pixel is provided every other pixel), in example B2, a frequency of 33% (a pixel is provided every other pixel), and in example B3, a frequency of 25% (a pixel is provided every other pixel). It can be seen that the successive compensating drops 10 printed by the compensating nozzle 5 have a minimum spacing A from one another in the conveying direction 7 (only shown by way of example in case B3). No droplet 10 or associated pixel 10 (relative to its preceding and succeeding "neighboring" compensating droplet in the conveying direction) has a spacing greater than the minimum spacing A.

[0047] Reference numerals list

[0048] 1 Printing press

[0049] 2 printing heads, printing beam

[0050] 3One or more nozzles

[0051] 4. One or more failed or defective nozzles

[0052] 5One or more compensation nozzles, one or more adjacent nozzles

[0053] 6 Printing materials

[0054] 7 Conveying direction

[0055] 8 Horizontal direction

[0056] 9 Printed images, especially test printed images, or printed measurement and control elements

[0057] 10 Compensation pixels or drops (on the substrate)

[0058] 11 White Line

[0059] 20 Control device

[0060] A Spacing

Claims

1. A method for compensating for a failed printing nozzle of an ink printing head for printing a print image (9) onto a printing material transported in a conveying direction, in, printing the print image (9) with at least two print drop volumes that differ from one another, wherein one printing drop volume is a maximum printing drop volume, and wherein at least one compensating nozzle (5) prints compensating drops with a compensating drop volume increased relative to a maximum printed drop volume, the compensating nozzle being the adjacent nozzle relative to the inoperative nozzle (4), It is characterized in that A frequency for generating the compensating drop is predetermined, and The compensating nozzle (5) is actuated such that a predefined frequency is followed and consecutive compensating drops printed by the compensating nozzle (5) have a predefined minimum distance from one another in the conveying direction.

2. The method according to claim 1, It is characterized in that A continuous drop volume that allows continuous printing is predetermined for the print head (2), and The compensating drop is printed with a compensating drop volume that is increased relative to the sustaining drop volume.

3. The method according to any one of the preceding claims, It is characterized in that The method is used starting from a tonal value at which at least 90% or at least 95% of the pixels required for the printed image (9) are set.

4. The method according to claim 1, It is characterized in that The method is used starting from a hue value of at least 50% or at least 55% or at least 65%.

5. The method according to any one of the preceding claims, It is characterized in that The frequency H is predefined as a percentage value, wherein H=P / Pmax, and wherein P is the number of compensating drops to be provided and Pmax is the maximum number of compensating drops that can be provided in each conveying direction (7).

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