EP3882036B1 - Coalescence of compensation nozzles - Google Patents

Description

The present invention relates to a method for compensating defective print nozzles of 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, whether within the scope of a print job or across several print jobs. This can affect the print strength, for example, or a print nozzle that prints differently. As soon as these deviations exceed certain limits, the print nozzles in question must be deactivated. It can also happen that print nozzles fail completely. The cause of these deviations is usually clogged print 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 produced. They are most clearly visible in solid color areas, as the defective print nozzle causes line-shaped image artifacts through which the color of the underlying printing substrate shines through. Since this is usually white printing paper, a so-called "white line" is created. Such errors are usually compensated by controlling the neighboring print nozzles so that they deliver an increased ink output, whereby the "white line" between them returns to the defective or deactivated print nozzle. Compensating the defective print nozzle precisely so that no visible "white line" is created is very difficult to carry out, as it naturally depends not only on the printing hardware of the printing nozzles used, but also on the print image to be produced.

Some printing inks also have the problem of coalescence. This affects the attraction between individual ink drops on the printing substrate. With particularly viscous inks, e.g. UV ink, the coalescence effect can be very strong, especially in conjunction with print heads that have a time difference between neighboring nozzles. In this case, it can happen that a "white line" created by a switched-off, faulty print nozzle cannot be closed due to the coalescence effect. In addition, the printing error appears even more severe because the ink accumulates to the right and left of the "white line", thus increasing the difference in contrast. 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 create a large drop. Overcompensation in the form of a "dark line" can also be the result.

US 2006/244774 A1 describes a method for compensating for inkjet printing nozzles that print at an angle. When white lines occur, a neighboring inkjet printing nozzle prints with an increased ink drop volume, and when dark lines occur, a neighboring inkjet printing nozzle prints with a reduced ink drop volume.

JP 2006 051617 A describes a method for compensating defective print nozzles in which optical abnormalities on the printing substrate are reduced by controlling individual print nozzles with an increased ink drop volume and thus printing enlarged areas that partially overlap with the non-enlarged areas of directly adjacent print nozzles.

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

This problem is solved by a method for compensating defective print nozzles in an inkjet printing machine using a computer, wherein the defective print nozzles, if they are printing at an angle, are not deactivated by the computer but are controlled in such a way that they continue printing with an increased ink drop volume, wherein the defective print nozzles are not allowed to print into the area of the directly adjacent print nozzles and wherein the directly adjacent print nozzles are those print nozzles which are arranged orthogonally to the printing direction directly adjacent to the respective defective print nozzle, wherein the computer controls both print nozzles of the inkjet printing machine directly adjacent to the respective defective print nozzle in order to compensate for the defective print nozzles in such a way that they each eject an increased ink drop volume and wherein the ink drop with the increased ink drop volume thus placed between the two ink drops of the two directly adjacent print nozzles attracts the ink drops of the two directly adjacent print nozzles via a coalescence effect. The present method therefore has two central points. On the one hand, it can only compensate for defective print nozzles that have not completely failed and whose skewness with regard to the deviation of their pressure point exceeds certain limits. According to the invention, the strictest limit is that the print nozzles in question must not print into the area of the next neighboring print nozzles. The decisive factor for the compensation according to the invention is that these defective print nozzles are not deactivated as in the prior art or continue to print at a reduced level as in the next prior art, but that these defective, i.e. deviating, print nozzles continue to print at an increased level in order to exploit the coalescence effect with the correspondingly enlarged ink droplet and attract the neighboring ink drops in order to close the white line created by the defective print nozzle. This method is logically used mainly for solid areas, since a white line would be extremely disruptive and noticeable in these areas in particular and, in addition, overcompensation, i.e. a dark line, can easily occur in raster 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 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 be useful to have the next but one adjacent print nozzles print with an increased ink drop volume in order to apply enough ink to actually close the white line that is created. Here, too, the problem of overcompensation with a dark line is of course inherent. In this case, it is recommended to increase the frequency of the ink drop to be set. amplified ink droplet of the defective print nozzle to be compensated.

A further preferred development of the method according to the invention is that the obliquely printing print nozzles only print with increased ink drops in areas of the print image with high area coverage. As already mentioned, it is recommended to use the method according to the invention mainly for areas of the print image with high area coverage, i.e. in solid areas. On the one hand, these are less susceptible to overcompensation with dark lines than halftone areas and, on the other hand, a white line is far more disruptive in these areas than in corresponding halftone areas.

An embodiment of the method described here that is not inventive is that it is not the defective, crooked printing nozzle that prints more intensively, but one of the two directly adjacent printing nozzles, whereby the compensation of the defective, crooked printing nozzle and its neighboring printing nozzles is shifted by one printing nozzle to the left or right. This approach is particularly useful if the pixel to nozzle mapping was not successful. This means that the defective printing nozzle identified by means of a detection method was incorrectly assigned. Most detection methods work with optical observations of a generated print image or test pattern and must then assign image pixels to real printing nozzles accordingly, 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 the wrong printing nozzle. In this case, it is advisable to compensate one of the two directly adjacent printing nozzles accordingly in an adapted method, since in most cases the incorrect pixel to nozzle mapping can be compensated for in this way.

Another embodiment of the method described here is that if several adjacent printing nozzles are printed at an angle, the computer calculates these according to a formation rule and classifies them as either increased printing or normal printing nozzles and controls them accordingly. Defective print nozzles, i.e. those that print crookedly, appear as clusters, i.e. next to each other. In this case, depending on the printing conditions, formation rules can often be created or specified with which it can be precisely determined which compensation behavior can best compensate for these groups of defective print nozzles. For example, there may be a group of three defective print nozzles in which the first print nozzle deviates to the left, the second to the right and the third to the left again. In this case, there is a specific compensation pattern for how the three print nozzles are to be controlled, which can be recorded in a fixed formation rule. In the example given, the middle print nozzle deviating to the right would print particularly strongly.

A further preferred development of the method according to the invention is that the size of the amplified ink droplet depends 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 droplet of the defective printing nozzle can be amplified. For example, yellow printing inks that print on a white substrate can be amplified much more than black printing inks, since the yellow color is much less likely to cause a dark line to form on the white printing substrate than the corresponding black printing ink.

A further 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 ink. 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 colors. In this case, overcompensation by increasing the use of white printing ink is practically impossible. At most, too high an 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 disruptive. In this case, it is therefore only a matter of closing the disruptive white/dark line and then being able to dry properly on the applied opaque white.

The invention as such and 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 embodiment. In the drawings, corresponding elements are each 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 (nicht erfindungsgemäß)

Figur 7:

ein Beispiel für ein pixelverschobenes Kompensationsverfahren (nicht erfindungsgemäß)

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 compensation works depending on functioning and defective print nozzles

Figure 4:

a pixel-based example of a state-of-the-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 (not according to the invention)

Figure 7:

an example of a pixel-shifted compensation method (not according to the invention)

The application area of the preferred embodiment is an inkjet printing machine 7. An example of the basic structure of such a machine 7, consisting of the feeder 1 for feeding the printing substrate 2 into the printing unit 4, where it is printed by the print heads 5, up to the delivery unit 3, is shown in Figure 1 This is a sheet-fed inkjet printing machine 7, which is controlled by a control computer 6. During operation of this printing machine 7, as already described, failures 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 a multi-coloured print, distorted color values. An example of such a "white line" 9 in a print image 8 is shown in Figure 2 shown.

In a current compensation method known from the state of the art, a modification of the original pixels is carried out in a group of five print nozzles arranged next to each other. Figure 4 shows this for the resulting pixels. The nozzle N in question is the nozzle to be compensated and is switched off completely. The first neighboring nozzles n±1 are amplified, or a larger amount 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 pixels two gray levels smaller.

Another approach known from the state of the art provides for nozzle N to be partially switched off as the nozzle to be compensated in a group of five print nozzles next to each other and thus printing continues only at a reduced level. The first neighboring nozzles n±1 are amplified, or a larger amount of ink / pixel 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 attenuated as in the first compensation approach, or a smaller amount of ink / pixel with smaller gray values are used.

The method according to the invention proceeds differently. In contrast to the prior art, the nozzle to be compensated is not switched off or weakened, but rather amplified in special situations. Figure 5 shows this using the already known example of the five neighboring print nozzles. This means that if a nozzle to be compensated sprays at an angle, but the target position deviates by less than 21um at 1200dpi print resolution and the nozzle belongs to the group of nozzles where 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 if the nozzle to be compensated is itself strengthened. Nozzles that have a skew of less than 21um are usually functional, i.e. they jet stable drops, but not directly to the target position. The example of Figure 5 shows the closing of a white line. Here, the nozzle N amplified by +200%. Nozzles N±1 are amplified by >0 to +50%. Nozzles N±2 are amplified by the same factor as N±1.

An alternative procedure (not according to the invention) shows 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 amplified even further - typically with the same strength as the direct neighboring nozzles. In return, nozzle N as the nozzle to be compensated does not print with a boost, as in the first embodiment, but continues to print normally. The background is that if a nozzle is to be compensated, but the nozzle belongs to the group of nozzles where the print 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 if the second neighbors are also amplified, 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 towards the nozzle to be compensated. The example of Figure 6 shows the closing of a white line ("approach white"). In this case, nozzle N is neither switched off nor weakened (0%). Nozzles N±1 are amplified by >0 to +200%. Nozzles N±2 are amplified by the same factor as N±1.

Both approaches of the described method can be applied especially for high area coverage, where the area closure is more important than the exact achievement of the target position of the pixel.

Another variant not according to the invention consists in using the compensation algorithm described, but instead of the actual pressure nozzle N to be compensated, the first directly adjacent nozzle is selected for compensation. In this case, a nozzle is generally selected that is easy to compensate from the coalescence perspective, and the nozzle that is actually to be compensated is reinforced as a neighboring 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 reduced by one and this nozzle is now compensated, thus reinforcing the actual nozzle. Figure 7 shows this process for the example already used. The example of Figure 7 shows the closing of a white line ("approach white"). Here, instead of nozzle N, nozzle N-1 is "compensated" (switched off). Nozzle N is the actual problem nozzle. The nozzles N±1 are amplified with >0 to +200%, for example with +100%, always increasing one grey level. Now the actual problem nozzle N is amplified. The nozzles N±2 are weakened with <0 to -200%, for example -100%, always decreasing one grey level, only if N±1 is increased.

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

The described methods are part of a general approach to compensate for defective print nozzles, whereby the nozzles N, n±1 and n±2 can be amplified as desired or weakened. In this case, the following fluctuation ranges of the individual nozzles result:

• Nozzle N: Nozzle to be compensated => Change strengths from -200% to +200%
• First neighboring nozzles n±1 => Change strengths from -200% to +200%l
• Second neighboring nozzles n±2 => Change strengths 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 should be noted: The print head hardware determines the local print density. This is done according to the state of the art in order to select the setting of the compensation strengths to be used in this area coverage according to the following points:

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

Another alternative embodiment of the method described (not according to the invention) consists in not only selecting nozzles that are not switched off in groups of malfunctioning print nozzles, but also grouping nozzles that are specifically amplified. This is typically used if nozzles deviate from the target position by less than the print resolution.

b)

c)

d)

e)

f)

g)

h)

i)

j)

k)

l)

An example of a group of three neighboring, defective print nozzles is shown in the following table. Reinforced nozzles are shown in bold. M stands for a completely failed print nozzle, X for a deactivated print nozzle that is compensated for by the neighboring nozzles, otherwise an arrow shows the direction of the deviation. case Entrance Exit a)

b)

c)

d)

e)

f)

G)

H)

i)

j)

k)

l)

List of reference symbols

1

Investors

2

current printing substrate / current printing sheet

3

boom

4

Inkjet printing unit

5

Inkjet printhead

6

calculator

7

Inkjet printing machine

8th

Print image on current printing sheet

9, 9a

White Line

10

generated ink drops

11

next but one neighboring ink drops

12, 12a

to compensate for enlarged ink drops

13

Coalescence effect

14, 14a, 14b

Overcompensation / Dark Line

Claims (4)

1. Method of compensating for defective printing nozzles in an inkjet printing machine by means of a computer, wherein the defective printing nozzles are not deactivated by the computer if they are nozzles that print at an angle but are instead actuated in such a way that they continue to print at a larger ink drop volume, wherein the defective printing nozzles are not allowed to print into the region of the immediately adjacent printing nozzles and wherein the immediately adjacent printing nozzles are those printing nozzles that are disposed immediately adjacent to the respective defective printing nozzle in a direction orthogonal to the printing direction, characterized
   in that to compensate for the defective printing nozzles, the computer actuates the two printing nozzles disposed immediately adjacent to the respective defective printing nozzle in the inkjet printing machine in such a way that each one of them emits an increased ink drop volume and wherein the ink drop with the increased ink drop volume thus positioned between the two ink drops of the two immediately adjacent printing nozzles attracts the ink drops of the two immediately adjacent printing nozzles due to a coalescence effect.
2. Method according to claim 1, wherein in addition to the immediately adjacent printing nozzles, the next-but-one printing nozzles likewise print at an increased ink drop volume for compensation purposes.
3. Method according to any one of the preceding claims, wherein the printing nozzles that print at an angle only print at an increased ink drop volume in print image areas of high area coverage.
4. Method according to any one of the preceding claims, wherein white printing ink is used as the ink to be compensated for.

Images