CN111196088B

CN111196088B - Method for compensating defective printing nozzles in an ink jet printer by means of a computer

Info

Publication number: CN111196088B
Application number: CN201911125450.2A
Authority: CN
Inventors: R·蒂塔; H·克勒; G·容汉斯
Original assignee: Heidelberger Druckmaschinen AG
Current assignee: Heidelberger Druckmaschinen AG
Priority date: 2018-11-19
Filing date: 2019-11-18
Publication date: 2022-12-27
Anticipated expiration: 2039-11-18
Also published as: DE102019127870A1; CN111196088A

Abstract

The invention relates to a method for compensating defective printing nozzles in an inkjet printer (7) by means of a computer (6), wherein, in order to compensate for the defective printing nozzles, the computer (6) calculates an increased ink drop volume (12) of the directly adjacent printing nozzle and a decreased ink drop volume (11) of the next adjacent printing nozzle and controls the adjacent printing nozzles of the inkjet printer (7) accordingly, characterized in that, as long as the defective printing nozzles are printing nozzles which print weakly and/or askew, they are not deactivated by the computer (6) but rather continue printing in a controlled manner.

Description

Method for compensating defective printing nozzles in an ink jet printer by means of a computer

Technical Field

The present invention relates to a method for compensating for malfunctioning printing nozzles, taking into account the coalescence effect of individual ink drops.

The invention belongs to the technical field of ink jet printing.

Background

In inkjet printing, the state of the printing nozzle of an inkjet head used in a printing mechanism is an index that is of decisive importance for the quality of the print to be achieved. For example, it is relatively frequent that individual printing nozzles change their behavior over time within the context of a print job or over the course of several print jobs. This may relate, for example, to the printing intensity or to the printing nozzles printing with deviations. As soon as these deviations exceed certain limit values, the printing nozzles concerned must be deactivated. In addition, complete failure of the printing nozzles may also occur. The reason for this type of deviation is mostly due to blocked printing nozzles. This can occur if the ink in the nozzles dries out when they are not used for too long. Problems in the ink supply path may also result in the false images described above. Such printing nozzles that print with deviations or no longer print at all (or printing nozzles that are deactivated) cause so-called "white line" errors in the printed image to be produced. Such "white line" errors are most clearly visible in monochrome full-tone surfaces, since a defective printing nozzle can cause a line-shaped image formation (Bildartefakte) through which the color of the underlying printing substrate is exposed. Since white printing paper is mostly concerned, such so-called "white lines" are formed". Such errors are usually compensated for by controlling the adjacent printing nozzles in such a way that they provide an increased ink output, so that the "white lines" of the defective or deactivated printing nozzle which are situated between the adjacent printing nozzles close again

The correct compensation of the defective printing nozzles so that no visible "white lines" occur is very difficult to perform, since this of course depends not only on the printing hardware of the printing nozzles used, but additionally also on the printed image to be produced.

There is also the problem of so-called coalescence (koaleszezz) in some of the printing inks used. This involves attraction between individual ink drops on the substrate (Anziehung). In the case of particularly viscous inks (for example UV inks), this coalescence effect can be very strong, in particular in the case of a print head incorporating: these print heads have a temporal difference between adjacent nozzles. Here, the situation may arise that: the "white line" created by the failed printing nozzle that is turned off cannot close due to the coalescence effect. Furthermore, printing errors appear even more intense, since the ink collects on the right and left sides of the "white line" and thereby also further enhances the contrast difference. Conversely, if the compensating drop is large enough to close the void, the printing speed may be reduced due to the time required to produce such a large drop. Furthermore, overcompensation in the form of "black lines" may result.

Disclosure of Invention

The object of the present invention is therefore to provide a method for compensating defective printing nozzles in an inkjet printer, which method achieves an improved printing quality compared to the methods known from the prior art.

The object is achieved by a method for compensating defective printing nozzles in an inkjet printer by a computer, wherein the defective printing nozzles are compensated forThe computer calculates the increased ink drop volume and the next ink drop volume for the directly adjacent printing nozzle

Reduced ink drop volume of adjacent printing nozzles and correspondingly controlling these (direct and next) adjacent printing nozzles of the inkjet printer, and is characterized in that the computer does not deactivate these defective printing nozzles but rather allows them to continue printing in a controlled manner, as long as these defective printing nozzles are concerned with printing nozzles that print weakly and/or askew, wherein such ink drops are provided between these two ink drops of these directly adjacent printing nozzles: the ink droplets attract the ink droplets of these directly adjacent printing nozzles by a coalescence effect. This is to take advantage of the coalescence behavior of the increased ink drop volume of these adjacent printing nozzles and to build up ink bridges (tintenbrhunt) between the increased drops of these adjacent printing nozzles, so that compensation is better achieved. That is to say, the printing is continued in a controlled manner by such printing nozzles which print in a weakened or skewed manner, so that ink drops are arranged between the two adjacent ink drops of the adjacent printing nozzles: the ink droplets attract the ink droplets of these adjacent printing nozzles by a coalescence effect. In this way, the ink drops of these adjacent printing nozzles are no longer pulled to those adjacent ink drops further away, and thus no longer away from the gaps of these defective printing nozzles to be closed, but instead are directed towards the gaps that produce the "white lines". The negative coalescence behavior hindering the compensation of "white lines" can thereby be avoided or impaired, and at the same time the coalescence behavior of the increased ink droplets of these adjacent printing nozzles is exploited with regard to better closing of the "white lines" generated by defective printing nozzles. It is important to note here that: of course only such defective printing nozzles can be compensated by the present method: these defective printing nozzles do not fail completely and/or their offset printing points remain at least between these two adjacent ink drops. If a defective printing nozzle is printed askew in such a way that a printing point arrives at which the defective printing nozzle should be deactivatedThose print dots of adjacent print nozzles are then of course not usable according to the method of the invention. Furthermore, the method according to the invention can only be used for such inks used which have an associated coalescence effect.

Advantageous and therefore preferred developments of the invention result from the preferred embodiments and the description with the figures.

In this case, a preferred development of the method according to the invention provides that, in the case of skewed printing by the printing nozzles, the computer controls the drop size of the printing nozzle to be compensated in a targeted manner as a function of the measured skew. In particular in the case of defective printing nozzles which print askew (i.e. whose printing point deviates), the ink droplet size of the printing nozzle concerned should be correlated accordingly to the measured degree of skew. Because a defective printing nozzle of the kind having only a small deviation and thus printing substantially centrally between two adjacent ink drops may require a smaller size in terms of drop volume to reliably close the "white line" by coalescence than a defective printing nozzle having a very strong deviation. By manipulating the ink drop size of the defective printing nozzle, the result of the method according to the invention can thereby also be improved step by step.

In this case, a further preferred development of the method according to the invention provides that the defective printing nozzles are controlled in such a way that, for controlled further printing, they effect only a part of the printing points that they are to be set without defects. This means that: in such a line of "white lines", that defective print nozzle does not set all of the pixels it should print. After all, in order to fully exploit the coalescence effect to close the "white line", it is not necessary to arrange all ink drops between the two adjacent enlarged ink drops at all. Since for the closure of the "white lines" it is usually sufficient to replace only every xth printed dot of the defective printing nozzle. This should be based on the fact that: the increased ink drop volume through these adjacent printing nozzles has inherently led to increased ink penetration into the printed image. This improved ink introduction should not be increased too strongly by the additional introduction of more ink drops by means of an originally defective printing nozzle. After all, the compensation according to the invention should not cause an overcompensation in the sense of a "black line".

In this case, a further preferred refinement of the method according to the invention provides that the portion of the printing points which is realized in the controlled further printing comprises at least every tenth printing point to be set. In practice it follows that: it is sufficient to set at least every tenth printed dot to be originally set to close the "white line". However, it is also possible to provide more printing points than just every tenth printing point, depending on the respective situation of the printing task concerned. However, it should not be less than every tenth printed dot, since otherwise the positively utilized coalescence effect would be too small.

In this case, a further preferred refinement of the method according to the invention provides that the type of realization of the printing dots depends on the area coverage

Grid (master), ink type or substrate. That is, the frequency/frequency of setting these printed dots should depend on these mentioned parameters. Then, every tenth, every fifth, etc. print dot is set according to the scene determined by these parameters, respectively. The size of the print dot to be set (i.e. the ink drop size) also depends on these parameters. For example, the better the ink applied extends over the substrate, the smaller the ink drop size required by the defective printing nozzle to form the ink bridge.

In this case, a further preferred development of the method according to the invention provides that the compensation of defective printing nozzles is carried out by a computer online (online) during the main printing (fortdrck). The method according to the invention is preferably carried out online during the official printing (i.e. the completion of the printing task). In this case, the printing nozzles can react most flexibly to the occurrence of defects. After all, the behavior of the individual printing nozzles changes very frequently during the original printing process, so that the printing nozzles can only be classified as defective during the actual printing.

In this case, a further preferred development of the method according to the invention provides that the compensation of defective printing nozzles is carried out by the computer in the prepress phase (druckvorsfufe) before or during the rasterization process. An alternative to online compensation during formal printing is: compensation is performed in the pre-press stage before or during the rasterization process. This has the advantage over online compensation that: by specifically adapting the grid to the defective printing nozzle, possible formations which occur as a result of the manipulation of the grid during the online compensation can be avoided. However, the disadvantage here is that only defective printing nozzles which are already known during the course of the screen process in the prepress stage can of course be compensated.

In this case, a further preferred development of the method according to the invention provides that the compensation takes place while the defective printing nozzle continues printing in a controlled manner, compared to the case in which the defective printing nozzle does not continue printing in a controlled manner, which leads to a smaller increase in the ink drop volume of the adjacent printing nozzle. This is necessary in order to avoid possible overcompensation and thus the generation of "black lines". Finally, the volumes of ink drops of these adjacent print nozzles that compensate for the defective print nozzle are calculated to compensate for the defective print nozzle that was not printed anymore. However, since the continuous ink introduction is also carried out in this case according to the invention by means of such printing nozzles, this must be taken into account accordingly when calculating the increase in the adjacent ink drops. In the method according to the invention, the ink drop volume of adjacent printing nozzles is increased by a certain amount, depending on the current print job and, in particular, on the frequency of the printing dots provided by defective printing nozzles.

In this case, a further preferred development of the method according to the invention provides that, in the compensation of the controlled continuation of the printing by the defective printing nozzle, the reduction in the ink drop volume of the next adjacent printing nozzle is the greatest. Another possibility for avoiding overcompensation is to strongly reduce the ink drop volume of the next adjacent printing nozzle in such a way that these respective next adjacent printing nozzles do not print at all. This is not only to avoid overcompensation, but also to enhance the positive coalescing effect. After all, in this way, the two enlarged ink drops of those adjacent printing nozzles of the defective printing nozzle should be compensated for being pulled to the ink drops of the defective printing nozzle applied according to the invention, and no longer to the ink drops of the next adjacent printing nozzle in the wrong direction. The positive coalescing effect towards malfunctioning printing nozzles can thereby be even further enhanced.

Drawings

The invention and structurally and/or functionally advantageous refinements of the invention are further described below on the basis of at least one preferred embodiment with reference to the drawing. In the drawings, mutually corresponding elements are denoted by the same reference numerals, respectively.

The figures show:

FIG. 1: structural examples of a sheet inkjet printer;

FIG. 2: a schematic example of "white line" caused by "missing nozzles";

FIG. 3: examples of the operation manner of performing compensation according to the normally operating and defective printing nozzles;

FIG. 4 is a schematic view of: an example of a jump from a white line to a black line;

FIG. 5: illustrative examples of negative coalescence effects;

FIG. 6: the positive coalescence effect achieved by the ink bridge according to the invention;

FIG. 7: examples of white lines in a raster image;

FIG. 8: an example of white lines compensated in a raster image by sporadically arranged pixels of defective printing nozzles according to the invention.

Detailed Description

The field of application of the preferred embodiment variant is an ink jet printer 7. An example of the basic structure of such a machine 7 is shown in fig. 1, said machine 7 comprising a feeder 1 as far as a receiver 3 for supplying a substrate 2 into a printing mechanism 4 where the substrate 2 is printed by a print head 5. The present invention relates to a sheet-fed ink-jet printer 7 controlled by a control computer 6. During operation of this printing press 7, as already described, it is possible for individual printing nozzles in the printing heads 5 in the printing unit 4 to fail. The result is then a "white line" 9 (or distorted color values in the case of multicolor printing). An example of such a "white line" 9 in a printed image 8 is shown in figure 2.

In addition, in the case of certain inks, the problem of coalescence already described occurs. How this problem arises is illustrated in fig. 3. In the uppermost first row can be seen: the ink drops 10 produced by a properly functioning print nozzle bar are aligned. In the next row can be seen: what happens when a nozzle is defective (or must be deactivated). In this case, the ink droplets that should be printed by the defective printing nozzle are missing in the arrangement consisting of the respective adjacent ink droplets 10, and thus "white lines" 9 are generated. To compensate for this white line "9", the volume of those two ink drops 12 (the drops printed by the defective print nozzle) that are adjacent is now increased, as shown in the third row. However, the coalescence 13 produced in the ink used attracts the adjacent ink droplets, which in this case results in those adjacent ink droplets having an increased volume not attracting each other to close the void of the white line "9". Instead, they are pulled to the next adjacent ink drop, as can be seen in the fourth row. Thereby making it increasingly difficult to close the "white line" 9. To counter this effect, the volume of the adjacent ink drop 12a is typically continued to increase (as seen in the lowermost row in fig. 3), thereby closing the "white line" 9 despite the coalescence effect 13. By this, the ink introduction is strongly increased, but an overcompensation in the form of a "black line" 14 is produced. This problem is again schematically illustrated in fig. 4. Here, three different images are shown: as the compensation intensity 15 increases, a "white line" 9 and an overcompensation are present. In the first image (left image) a "white line" 9 and a partially compensated "white line" 9a are seen. This partially compensated "white line" 9a is compensated for as usual by increasing the ink drop volume of the adjacent printing nozzle 12, but the dark edge (i.e. the "black line" 14) is thereby essentially formed by this increased ink introduction. The larger the ink drop volume of these adjacent printing nozzles 12 and the stronger the compensation intensity 15, the more the "white lines" 9a disappear, whereas the corresponding "black lines" 14a are also stronger. If so much ink is used to completely close the "white lines" 9a, then clearly visible "black lines" 14b will result.

In order to avoid this, the preferred embodiment variant of the method according to the invention is now used. Instead of the above-described solution in which only the two adjacent printing nozzles 12 release an increased ink drop volume, thereby causing these adjacent ink drops to be pulled further away from the adjacent ink drops by coalescence 13 and thereby increasing the difficulty of compensation (as is again correspondingly shown in fig. 5), it is employed: at certain intervals (for example, ten printing dots to be set), normally deactivated printing nozzles which are otherwise defective are reactivated. This is schematically illustrated in fig. 6, where the print head 5 with the printing nozzle 16 strips is shown as in fig. 5 when ink drops are produced. That is, in the case where the printing dot is thus activated, the printing nozzle which is originally defective continues to discharge the specific ink droplet 17. This ink drop 17 is not as large or deflected particularly severely as it should have, since the printing nozzle is defective, which is of no consequence in this case, only with regard to the formation of the bridge joint 17, in order to compensate for the coalescence 13 of the ink drops. As can be best seen in fig. 6: how the ink drop 17 provided by this originally defective printing nozzle is positioned between two adjacent ink drops 12 and here such that the two adjacent ink drops 12 with increased volume are not just pulled by coalescence 13 to those adjacent ink drops 11 that are further away, but just to the ink drop 17 of that defective printing nozzle. Adjacent ink drops 12 with increased volume are thus more easily combined with each other and also with the ink drops 17 of this defective printing nozzle, thereby closing the "white line" 9 more effectively than if the defective printing nozzle were to remain completely deactivated. The frequency with which the originally defective printing nozzle is provided with ink drops 17 depends on the actual circumstances, such as: the printing substrate 2 used, the viscosity of the ink and the coalescence 13 associated therewith, and of course also the printed image to be produced. In practice, releasing every tenth printed dot to be originally set has proven to be a way of effectively closing the "white line" 9. Releasing ink droplets too frequently should be avoided, since thereby a possible overcompensation 14 is enhanced, whereas releasing ink droplets too sparsely would of course significantly reduce the effectiveness of the method according to the invention. Fig. 7 and 8 again show exemplarily a grid structure with "white lines" 9 and with an increased ink application by placing correspondingly adjacent pixels 12 with darker grid points. Here, fig. 7 shows an example in which the method according to the invention is not employed, while fig. 8 shows an example in which the method according to the invention is employed in such a way that an otherwise defective printing nozzle is provided with a plurality of pixels 17 in order to form bridges between those adjacent pixels 12 having an increased ink drop volume. The control of the printing nozzles concerned is carried out by a computer which can be coordinated with the control computer 6 of the ink jet printer 7. However, it may also be a separate stand-alone computer. Such a computer is suitable if, for example, the ink jet print head 5 is controlled by its own inherent computer.

The method according to the invention thus has the advantage that: the "white line" 9 can be closed more reliably and better despite the coalescence effect 13. Furthermore, the ink quantity of the compensation nozzles can be reduced, since it is no longer necessary to counteract the outwardly acting coalescence 13 so strongly; the "black lines" 14 produced are also reduced in this way. The drop size of the compensating drops 12 can also be reduced, thereby achieving higher printing speeds.

List of reference numerals

1. Feeding device

2. Current printing substrate/current printing sheet

3. Material collector

4. Ink jet printing mechanism

5. Ink jet print head

6. Computer with a memory card

7. Ink-jet printer

8. Printing image on current printing sheet

9,9a white line

10. Produced ink droplets

11. Next adjacent ink drop

12,12a ink drop enlarged for compensation

13. Coalescence effect

14,14a,14b overcompensation/black line

15. Intensity of compensation

16. Printing nozzle strip of a printing head

17. Ink drops of defective printing nozzles used to form bridge joints

Claims

1. A method for compensating defective printing nozzles in an ink jet printer (7) by means of a computer (6),

wherein, in order to compensate for defective printing nozzles, the computer (6) calculates an increased ink drop volume (12) of the directly adjacent printing nozzle and a decreased ink drop volume (11) of the next adjacent printing nozzle and controls the adjacent printing nozzles of the inkjet printer (7) accordingly,

it is characterized in that the preparation method is characterized in that,

as long as a defective printing nozzle is a printing nozzle that prints with reduced and/or skewed quality, the computer (6) does not deactivate this printing nozzle, but rather allows it to continue printing in a controlled manner, so that the following ink drops are arranged between two ink drops of directly adjacent printing nozzles: the ink drops attract the ink drops of the directly adjacent printing nozzles by a coalescence effect, and

for the compensation performed by the controlled continuation of printing by the defective printing nozzle, the reduction of the ink droplet volume (14, 14a, 14b) of the next adjacent printing nozzle is the most.

2. The method as set forth in claim 1, wherein,

it is characterized in that the preparation method is characterized in that,

in the case of printing nozzles that are printed askew, the computer (6) controls the drop size of the printing nozzle to be compensated in a targeted manner as a function of the measured degree of skew.

3. The method according to claim 1 or 2,

it is characterized in that the preparation method is characterized in that,

the defective printing nozzle is controlled in such a way that, for controlled continuation of the printing, it only carries out a part of the printing points which the printing nozzle is to set when it is defect-free.

4. The method of claim 3, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

the portion of the printed dots realized in the controlled continuation of printing comprises at least every tenth printed dot to be set.

5. The method of claim 3, wherein the first and second light sources are selected from the group consisting of,

it is characterized in that the preparation method is characterized in that,

the type of printing point is related to the surface coverage, the grid, the ink type or the printing material (2).

6. The method according to claim 1 or 2,

it is characterized in that the preparation method is characterized in that,

the compensation of defective printing nozzles is performed online by the computer (6) during the main printing.

7. The method according to claim 1 or 2,

it is characterized in that the preparation method is characterized in that,

the compensation of defective printing nozzles is performed by the computer (6) in a pre-press phase before or during the rasterization process.

8. The method according to claim 1 or 2,

it is characterized in that the preparation method is characterized in that,

for compensation performed by controlled continuation of printing by a defective printing nozzle, the increase in ink drop volume (12) of the adjacent printing nozzle is smaller than when compensation is performed by an uncontrolled continuation of printing by a defective printing nozzle.