CN111196088A

CN111196088A - Avoid coalescence

Info

Publication number: CN111196088A
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: 2020-05-26
Anticipated expiration: 2039-11-18
Also published as: CN111196088B; DE102019127870A1

Abstract

The invention relates to a method for compensating defective printing nozzles in an inkjet printer (7) by means of a computer (6) for avoiding coalescence, 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 continue printing in a controlled manner.

Description

Avoid coalescence

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 technical field to which the invention belongs is 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 strength or to the printing strengthAnd to printing nozzles that print 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 lead to the above-mentioned false images. 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 often used, such a so-called "white line" is 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 defective printing nozzles are exactly compensated so that no visible "white lines" occur, which is very difficult to perform, since this of course not only depends 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 (such as UV inks), this coalescence effect can be very strong, in particular in combination with such print heads: these print heads have a temporal difference between adjacent nozzles. Here, the situation may arise: due to coalescence effects, leading to faulty printing by being switched offThe "white line" created by the nozzle cannot close. 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 drops are large enough to close the void, the printing speed may be reduced due to the time required to create such large drops. In addition, 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, to compensate for such defective printing nozzles, the computer calculates an increased ink drop volume and a next drop volume for the directly adjacent printing nozzle

The reduced ink drop volume of adjacent printing nozzles and the corresponding control of the (directly and next) adjacent printing nozzles of the inkjet printing machine, and the method is characterized in that the computer does not deactivate the defective printing nozzles but rather continues the printing in a controlled manner as long as the defective printing nozzles are printing nozzles that print in a reduced and/or skewed manner, wherein an ink drop is arranged between the two ink drops of the directly adjacent printing nozzles, which ink drop attracts the ink drops of the directly adjacent printing nozzles by a coalescence effect, in order to exploit the coalescence behavior of the increased ink drop volume of the adjacent printing nozzles, and in order to form an ink bridge (Tintenbr ü cken) between the increased ink drops of the adjacent printing nozzles, in order to better compensatePrinting is continued so that such ink droplets are disposed between the two adjacent ink droplets 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 the two adjacent ink drops. The method according to the invention is of course not usable if a defective printing nozzle is printed askew in such a way that the printing point of the defective printing nozzle that it should have been deactivated reaches the printing point of those neighbouring printing nozzles. 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 emerge from the dependent claims 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 development of the method according to the invention provides that the portion of the printing dots that is realized in the controlled further printing comprises at least every tenth printing dot 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 printing dots are providedShould 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 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 (druckvorstlife) 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 printing 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 printing nozzles that compensate for the defective printing nozzle are calculated to compensate for the defective printing nozzle that was not originally printed. 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 printing job and, in particular, depending on the frequency of the printing dots set by the defective printing nozzle.

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 consists in strongly reducing the ink drop volume of the respectively next adjacent printing nozzle in such a way that it is no longer printed at all. This is not only to avoid overcompensation, but also to enhance the positive coalescence 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 drawings. In the drawings, mutually corresponding elements are denoted by the same reference numerals, respectively.

The figures show:

FIG. 1: examples of the structure 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: 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 print substrate 2 into a printing unit 4 where the print 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 head 5 of 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 shown schematically in figure 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 absent in the arrangement composed 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 adjacent (to the ink drop printed by the defective print nozzle) is now increased, as shown in the third row. However, the coalescence 13 produced in the ink used attracts adjacent ink drops, which in this case results in those adjacent ink drops 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, how the "white line" 9 and overcompensation appear. 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 compensating intensity 15 here, 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), use is made of: 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 as it should have or is not deflected particularly severely, since the printing nozzle is defective, which is therefore irrelevant in this case, only with regard to the formation of the bridge 17, in order to compensate here for the coalescence 13 of the ink drop. 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 an 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 feeder

2 currently printed substrate/currently printed sheet

3 material collector

4 ink-jet printing mechanism

5 ink jet print head

6 computer

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 enlarged ink drops for compensation

13 coalescence effect

14,14a,14b overcompensation/black line

15 compensating intensity

16 print head printing nozzle strip

Ink drops for 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 the defective printing nozzle is a printing nozzle that prints weakly and/or askew, the computer (6) does not deactivate this printing nozzle but rather allows it to continue printing in a controlled manner, whereby the following ink drops are arranged between two ink drops of directly adjacent printing nozzles: the ink drops attract the ink drops directly adjacent to the printing nozzle by coalescence effects.

2. The method of claim 1, 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,

in the case of printing nozzles that are printed askew, the computer (6) controls the drop size (17) 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 any one of the preceding claims,

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 printing dots (17) realized in the controlled further printing comprises at least every tenth printing dot to be set.

5. The method according to claim 3 or 4,

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

the type of realization of the printing points (17) is related to the surface coverage, the grid, the ink type or the printing material (2).

6. The method according to any one of the preceding claims,

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 any one of claims 1 to 5,

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 any one of the preceding claims,

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 adjacent printing nozzles is smaller than for compensation performed without controlled continuation of printing by the defective printing nozzle.

9. The method according to any one of the preceding claims,

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

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