EP3882036A1 - Coalescence of compensation nozzles - Google Patents

Abstract

A method for compensating for defective printing nozzles in an inkjet printing machine (7) by a computer, the computer correspondingly controlling an enlarged ink drop volume from directly adjacent printing nozzles and these adjacent printing nozzles of the inkjet printing machine to compensate for the defective printing nozzles, which is characterized in that the Defective print nozzles, provided that the print nozzles are printing crookedly, are not deactivated by the computer, but are controlled in such a way that they continue to print more intensely neighboring pressure nozzles.

Description

The present invention is concerned with a method for compensating for defective printing nozzles in an inkjet printing machine by utilizing the coalescence effect.

The invention lies in the technical field of inkjet printing.

In inkjet printing, the status of the print nozzles of the inkjet heads used in the printing units is a decisive criterion for the print quality to be achieved. It is relatively common for individual print nozzles to change their printing behavior over time, be it within the scope of a print job or across several print jobs. This can, for example, relate to the pressure level or a differently printing pressure nozzle. As soon as these deviations exceed certain limit values, the relevant pressure nozzles must be deactivated. It can also happen that pressure nozzles fail completely. The cause of these deviations is mostly due to clogged pressure nozzles. This occurs when ink dries in the nozzles if they are not used for too long. Problems in the ink supply channels can also lead to the error patterns described. Print nozzles that print differently or no longer print at all, or deactivated print nozzles, cause so-called "white line" errors in the print image to be generated. They can be seen most clearly in single-colored full-tone areas, as the failed print nozzle causes linear image artifacts through which the color of the underlying print substrate shines through. Since it is mostly white printing paper, a so-called "white line". Errors of this kind are usually compensated for by activating the adjacent print nozzles again in such a way that they deliver an increased ink output, as a result of which the "white line" of the failed or deactivated print nozzle between them runs again. An exact compensation of the defective pressure nozzle, so that no visible "white line" arises, is very difficult to carry out, since it is of course not only from the Print hardware depends on the print nozzles used, but also on the print image to be generated.

With some of the printing inks used, there is also the problem of so-called coalescence. This affects the attraction between individual ink droplets on the print substrate. With particularly viscous inks, e.g. UV inks, the coalescence effect can be very strong, especially in connection with print heads that have a time difference between adjacent nozzles. Here it can happen that due to the coalescence effect, a "white line" generated by a defective pressure nozzle that has been switched off cannot be closed. In addition, the printing error appears even more pronounced, as the ink collects to the right and left of the "white line" and thus increases the difference in contrast even more. If, on the other hand, the compensation drop is made so large that the gap is closed, the printing speed is reduced due to the time required to generate a large drop. In addition, overcompensation in the form of a "dark line" can result.

The object of the present invention is therefore to disclose a method for compensating for defective printing nozzles which works reliably and efficiently despite the coalescence effect of the printing inks used.

This object is achieved by a method for compensating for defective print nozzles in an inkjet printing machine by means of a computer, the computer correspondingly controlling an increased ink drop volume from directly adjacent print nozzles and these adjacent print nozzles of the inkjet printing machine to compensate for the defective print nozzles, which is characterized accordingly that the defective print nozzles, provided that the print nozzles are printing crookedly, are not deactivated by the computer, but are controlled in such a way that they continue to print more intensely, whereby the intensified ink drops placed between the two ink drops of the directly adjacent print nozzles via a coalescence effect Attracts ink droplets from the directly adjacent print nozzles. The present proceedings therefore have two central points. On the one hand, only defective pressure nozzles can be compensated for, which have not completely failed and their inclination with regard to the deviation of their pressure point certain limit values does not exceed. Usually the hardest limit value is that the relevant pressure nozzles are not allowed to press into the area of the next adjacent pressure nozzles. It is now decisive for the compensation according to the invention that these defective print nozzles are not deactivated as in the prior art or continue to print at a reduced rate as in the next prior art, but that these defective, i.e. deviating print nozzles continue to print more intensively, thus creating the coalescence effect with the correspondingly enlarged ink droplets and attract the neighboring ink droplets in order to close the white line created by the defective print nozzle. Logically, this method is mainly used for solid areas, since a white line that occurs in these areas would be extremely disruptive and noticeable and, in addition, an overcompensation, i.e. a dark line, can easily occur in grid areas.

Advantageous and therefore preferred developments of the method emerge from the associated subclaims and from the description with the associated drawings.

A preferred development of the method according to the invention is that the print nozzles that print crookedly print only part of the pixels to be set according to the image data with intensified ink drops in the printing direction. This prevents overcompensation and a corresponding dark line from occurring. The proportion of the pixels to be set with intensified ink droplets depends on the printing conditions, the printing ink used and the printing substrate used.

Another preferred development of the method according to the invention is that, in addition to the directly adjacent print nozzles, the next but one, adjacent print nozzles also print with an increased ink drop volume for compensation. In some cases it can make sense to also print the next but one adjacent print nozzles with an enlarged ink drop volume in order to apply enough ink to actually close the resulting white line. Here too, of course, the problem of overcompensation with a dark line is inherent. In this case it is recommended to set the frequency of the to adjust accordingly increased ink drop of the defective print nozzle to be compensated.

A further preferred development of the method according to the invention is that the print nozzles that print crookedly print with intensified ink droplets only in areas in the print image with high surface coverages. As already mentioned, it is advisable to use the method according to the invention mainly for areas in the print image with high surface coverages, ie. H. in solid areas. On the one hand, these are less prone to overcompensation with dark lines than halftone areas and, on the other hand, a white line is far more disruptive in these than in corresponding halftone areas.

A further preferred development of the method according to the invention is that it is not the defective, crooked printing nozzle, but one of the two directly adjacent printing nozzles that prints more intensely, whereby the compensation of the defective, crooked printing nozzle and its neighboring pressure nozzles is increased by one pressure nozzle to the left or the right shifts right. This approach is particularly useful if the pixel to nozzle mapping was unsuccessful. This means that the defective pressure nozzle determined by means of a detection method was assigned incorrectly. Most detection methods work with optical observations of a generated print image or test pattern and then have to assign corresponding image pixels to real print nozzles, which is done with the aforementioned pixel-to-nozzle mapping. It can happen that a white line present in a recorded digital image is assigned to a wrong pressure nozzle. In this case, it is advisable to compensate one of the two directly adjacent pressure nozzles in accordance with the invention in an adapted method according to the invention, since in most cases the faulty pixel-to-nozzle mapping can thus be compensated for.

A further preferred development of the method according to the invention is that when several adjacent printing nozzles printing askew, the computer groups them according to a formation rule and classifies them as either intensely printing or normal printing nozzles and controls them accordingly. Kick frequently defective, so crooked printing, print nozzles as a cluster, ie next to each other. In this case, depending on the prevailing printing conditions, formation rules can often be created or specified with which it is possible to determine precisely with which compensation behavior these groups of defective pressure nozzles can best be compensated. So z. B. a group of three defective pressure nozzles exist in which the first pressure nozzle deviates to the left, the second to the right and the third again to the left. For this case there is a specific compensation pattern, how the three pressure nozzles are to be controlled according to the invention, which can be recorded in a fixed formation rule. In the example mentioned, the middle pressure nozzle deviating to the right would, according to the invention, print particularly intensely.

Another preferred development of the method according to the invention is that the size of the reinforced ink drop is dependent on the color of the printing ink used and the printing substrate used. The less the color of the printing ink used is susceptible to overcompensation, the more the corresponding ink drop of the defective printing nozzle can be amplified. So z. B. yellow printing inks, which print on a white substrate, are reinforced much more than black printing inks, since a dark line can form much less on the white printing substrate due to the yellow color than with corresponding black printing inks.

Another preferred development of the method according to the invention is that a white printing ink is used as the printing ink to be compensated. White printing inks often have a particularly high coalescence behavior, which makes the method according to the invention particularly suitable for this type of printing inks. In addition, white printing ink is often used in the form of opaque white or primer in order to then continue printing on this white color with the actual printing inks. In this case, overcompensation by increasing the introduction of white printing ink is practically impossible. At most, too much area coverage would be disruptive. On the other hand, a white line, which would de facto be more of a dark line here, would be extremely annoying. Therefore, in this case it is only a matter of closing the disturbing white / dark line and then being able to dry it properly on the applied opaque white.

The invention as such as well as structurally and / or functionally advantageous developments of the invention are described in more detail below with reference to the associated drawings using at least one preferred exemplary embodiment. In the drawings, elements that correspond to one another are provided with the same reference numerals.

Figur 1:

ein Beispiel des Aufbaus einer Bogen-Inkjet-Druckmaschine

Figur 2:

ein schematisches Beispiel einer "white line", verursacht durch eine "missing nozzle"

Figur 3:

Beispiele für die Funktionsweise der Kompensation abhängig von funktionierenden und defekten Druckdüsen

Figur 4:

ein pixelbasiertes Beispiel für ein Kompensationsverfahren aus dem Stand der Technik

Figur 5:

ein pixelbasiertes Beispiel für das erfindungsgemäße Kompensationsverfahren

Figur 6:

ein pixelbasiertes Beispiel für ein alternatives Kompensationsverfahren

Figur 7:

ein Beispiel für ein pixelverschobenes Kompensationsverfahren

The drawings show:

Figure 1:

an example of the structure of a sheet-fed inkjet printing machine

Figure 2:

a schematic example of a "white line" caused by a "missing nozzle"

Figure 3:

Examples of how the compensation works depending on functioning and defective pressure nozzles

Figure 4:

a pixel-based example of a prior art compensation method

Figure 5:

a pixel-based example of the compensation method according to the invention

Figure 6:

a pixel-based example of an alternative compensation method

Figure 7:

an example of a pixel-shifted compensation method

The area of application of the preferred embodiment variant is an inkjet printing machine 7. An example of the basic structure of such a machine 7, consisting of feeder 1 for feeding the printing substrate 2 into the printing unit 4, where it is printed by the printing heads 5 up to Boom 3, is in Figure 1 shown. This is a sheet-fed inkjet printing machine 7 which is controlled by a control computer 6. During the operation of this printing press 7, as already described, failure of individual printing nozzles in the print heads 5 in the printing unit 4 can occur. The result is then "white lines" 9 or, in the case of multicolored printing, distorted color values. An example of such a "white line" 9 in a print image 8 is shown in FIG Figure 2 shown.

In a current compensation method known from the prior art, a modification of the original pixels is carried out in a group of five printing nozzles arranged next to one another. Figure 4 shows this for the resulting pixels. The relevant nozzle N is the nozzle to be compensated and is completely switched off. The first neighboring nozzles n ± 1 are reinforced, or a larger quantity of ink / pixel with larger gray values is used. The compensation strengths are 0 to + 200%, ie pixels up to two gray levels larger. The second neighboring nozzles n ± 2 are weakened, or a smaller amount of ink / pixel with smaller gray values is used. The compensation strengths here are 0 to -200%, ie from no change to two gray levels smaller pixels.

Another approach known from the prior art provides, with a group of five pressure nozzles next to one another, to only partially switch off the nozzle N as the nozzle to be compensated and thus to continue printing only in a weakened manner. The first neighboring nozzles n ± 1 are reinforced, or a larger amount of ink / pixels with larger gray values are used. This corresponds to the first compensation approach. The same applies to the second neighboring nozzles n ± 2. These are also weakened, as in the first compensation approach, or a smaller amount of ink / pixels with smaller gray values are used.

The method according to the invention now proceeds differently. In contrast to the prior art, the nozzle to be compensated is not switched off or weakened, but rather reinforced in special situations. Figure 5 shows this with the already known example with the five adjacent pressure nozzles. Ie if a nozzle to be compensated injects crooked, but the target position deviates by less than 21um at 1200dpi print resolution and the nozzle belongs to the set of nozzles in which the pressure points of the neighboring nozzles are pulled away from the position of the nozzle to be compensated according to the coalescence effect, Better compensation can thus be achieved if the nozzle to be compensated is itself reinforced. Nozzles with a skew of less than 21 µm are usually fully functional, i.e. they jet stable droplets, but not directly onto the target position.

An alternative procedure of the method according to the invention is shown Figure 6 . Here, the second neighbors of the nozzle to be compensated are not weakened or provided with the original image pixels, but in special situations are reinforced - typically with the same strength as the direct neighboring nozzles. For this, the nozzle N, as the nozzle to be compensated, does not print more intensely, as in the first embodiment, but continues printing normally. The background is that if a nozzle is to be compensated, but the nozzle belongs to the set of nozzles in which the pressure points of the neighboring nozzles are pulled away from the position of the nozzle to be compensated according to the coalescence effect, better compensation can be achieved, even if the second neighbors are reinforced, so that such an amount of ink is created that due to the high amount, in addition to the coalescence effect, a level compensation takes place, which pushes the ink back in the direction of the nozzle to be compensated.

Both approaches of the method according to the invention can be used especially in the case of high area coverages, in which the area closure is more important than the exact reaching of the target position of the pixel.

A further variant of the method according to the invention consists in using the compensation algorithm according to the invention, but instead of the actual pressure nozzle N to be compensated, the first direct neighboring nozzle to be selected for compensation. As a rule, a nozzle is selected that is easy to compensate from the point of view of coalescence and the nozzle actually to be compensated is reinforced as an adjacent nozzle. If the nozzle to be compensated belongs to the group of nozzles that are difficult to compensate, the number of the nozzle to be compensated is decreased by one and this nozzle is now compensated and thus the actual nozzle is reinforced. Figure 7 shows this process for the example already used.

• Düse N: Zu kompensierende Düse => Änderungsstärken von -200% bis +200%
• Erste Nachbardüsen n±1 => Änderungsstärken von -200% bis +200%1
• Zweite Nachbardüsen n±2 => Änderungsstärken von -200% bis +200%

The methods according to the invention are part of a general approach to compensate for defective pressure nozzles, the nozzles N, n ± 1 and n ± 2 being amplified as required or be weakened. In this case the following fluctuation ranges result for the individual nozzles:

• Nozzle N: Nozzle to be compensated => Variations from -200% to + 200%
• First neighboring nozzles n ± 1 => strengths of change from -200% to + 200% 1
• Second neighboring nozzles n ± 2 => strengths of change from -200% to + 200%

• Ist die Flächendeckung höher als eine Schwelle (dichtere Töne)
• gehört die Düse von der Fehlfunktion nicht zur Kategorie "Missing" = Totalausfall oder zu schwach
• Ist die Düse weniger als eine Schwelle schief wird vom Algorithmus gemäß dem Stand der Technik in das erfindungsgemäße Verfahren umgeschaltet => Verstärkung der zu kompensierenden Düse

In addition, the following things must be observed: The printhead hardware determines the local print density. According to the state of the art, this is also done in order to select the setting of the compensation strengths to be used in this area coverage according to the following points:

• Is the area coverage higher than a threshold (denser tones)
• the nozzle of the malfunction does not belong to the category "Missing" = total failure or too weak
• If the nozzle is skewed by less than a threshold, the algorithm according to the prior art switches to the method according to the invention => amplification of the nozzle to be compensated

A further alternative embodiment of the method according to the invention consists in not only selecting nozzles that are not switched off in groups of malfunctioning pressure nozzles, but also groups nozzles that are specially reinforced. The application for this is typically if nozzles deviate from the target position by less than the print resolution.

An example of a group of three adjacent, defective print nozzles is shown in the following table. Nozzles that are reinforced according to the invention are each shown in "bold". M stands for a completely failed pressure nozzle, X for a deactivated pressure nozzle that is compensated for by the neighboring nozzles, otherwise an arrow shows the direction of the deviation. case entry exit a) → → → → → → b) → → ← → → ← c) ← → ← → d) → ← → → ← → e) ← → → ← → → f) ← → ← ← → ← G) ← → → X ← → H) ← → → X ← → i) M → → X → → j) M → ← X → → k) M ← → X ← → l) M ← → X ← →

List of reference symbols

1

Anleger

2

aktuelles Drucksubstrat / aktueller Druckbogen

3

Ausleger

4

Inkjet-Druckwerk

5

Inkjet-Druckkopf

6

Rechner

7

Inkj et-Druckmaschine

8

Druckbild auf aktuellem Druckbogen

9, 9a

White Line

10

erzeugte Tintentropfen

11

übernächste benachbarte Tintentropfen

12, 12a

zur Kompensation vergrößerte Tintentropfen

13

Koaleszenzeffekt

14, 14a, 14b

Überkompensation / Dark Line

...

1

Investors

2

current print substrate / current print sheet

3

boom

4th

Inkjet printing unit

5

Inkjet printhead

6th

computer

7th

Inkj et printing machine

8th

Print image on the current print sheet

9, 9a

White Line

10

generated ink drops

11

Next but one adjacent ink drop

12, 12a

ink droplets enlarged to compensate

13th

Coalescence effect

14, 14a, 14b

Overcompensation / dark line

...

Claims (8)

Method for compensating defective print nozzles in an inkjet printing machine by means of a computer, the computer correspondingly activating an enlarged ink drop volume from directly adjacent print nozzles and these adjacent print nozzles of the inkjet printing machine to compensate for the defective print nozzles, characterized, that the defective print nozzles, provided that the print nozzles are printing crookedly, are not deactivated by the computer, but are controlled in such a way that they continue to print more intensely, whereby the intensified ink drops placed between the two ink drops of the directly adjacent print nozzles via a coalescence effect, the ink drops that attracts directly adjacent pressure nozzles. Method according to claim 1, characterized, that the print nozzles that print askew in the printing direction only print a part of the pixels to be set according to the image data with intensified ink droplets. Method according to one of the preceding claims, characterized, that in addition to the directly adjacent print nozzles, the next but one, adjacent print nozzles also print with increased ink drop volume for compensation. Method according to one of the preceding claims, characterized, that the print nozzles that print crooked print only in areas of the print image with high surface coverage with thick ink droplets. Method according to one of the preceding claims, characterized, that it is not the defective, crooked print nozzle, but one of the two directly adjacent print nozzles that prints more intensely, whereby the compensation of the defective, crooked print nozzle and its adjacent print nozzles is shifted by one print nozzle to the left or right. Method according to one of the preceding claims, characterized, that when several adjacent print nozzles printing askew, the computer groups them according to a formation rule and classifies them as either intensely printing or normal printing nozzles and controls them accordingly. Method according to one of the preceding claims, characterized, that the size of the enhanced ink drop is dependent on the color of the printing ink used and the printing substrate used. Method according to one of the preceding claims, characterized, that a white printing ink is used as the printing ink to be compensated.

Images