EP3216611A1 - Method for compensating defective nozzles in an inkjet printing machine - Google Patents

Abstract

A method for compensating failed nozzles in an inkjet printing machine, wherein failed nozzles are compensated by an increased ink drop volume of adjacent nozzles and which is characterized in that the real positions of the pressure points of all the nozzles are determined and for each pressure nozzle to compensate for their failure, the necessary ink droplet volumes of the respective adjacent pressure nozzles as a function of the real position of the pressure point of the pressure nozzle and of the real positions of the pressure points of the respective adjacent pressure nozzles are calculated.

Description

The invention lies in the technical field of digital printing.

Generally, inkjet printing machines include one or more printheads and each printhead includes a plurality of printing nozzles. The inkjet printing machines use the nozzles for printing by emitting ink. These printing machines have nozzle plates with specific arrangements of the individual nozzles. In the event of failure of a single pressure nozzle areas arise that can not be imaged by the nozzle provided in the single-color separation. This results in color-free areas, which can show up as white lines. If it is a multicolor print, the corresponding color is missing at this point and the color values are distorted. It should also be noted that the beam path of a single nozzle is not ideal but may deviate more or less, and the size of a jetted point must be taken into account. Thus, a malfunctioning nozzle affects the print quality of each printed document. The reasons for the failure of individual nozzles are diverse, it may be a temporary failure or a permanent failure.

In order to reduce the effects on the printed image, especially in solid areas, several approaches to compensation are known from the prior art.

One known approach to compensating failed nozzles is to replace the failed nozzle with the nozzles of the other used inks at the same location. An attempt is made, by targeted and controlled overprinting of the remaining available colors, to come as close as possible to the failed printing ink. As a result, neither a redundancy of pressure nozzles or heads is required nor is the failure of adjacent pressure nozzles a problem. The main disadvantage of this compensation method, however, is that it is only for the Multi-color printing can be used. In addition, an increased computing and taxation needs by the computer of the printing press required to determine the necessary color combinations. In addition, depending on the color difference of the color that has fallen out of the still printable color space of the residual colors, the resulting print result may deviate significantly from the desired values.

Another approach to compensating failed nozzles is to use dual nozzles in the same color to compensate for redundancy in individual nozzles. Examples of this procedure are from the patent applications US 20060256157 A1 and US 20060268034 A1 known. Although this is efficient, but also expensive, needs additional space and brings more problems, such as the complicated control of the double units with it.

Another known approach is to perform the compensation via pressure nozzles from other systems. This means that several positionable print heads are used to print an image. If print nozzles fail, the print heads are repositioned to replace the failed nozzle as well as possible. The patents or patent applications US 20120075373 A1 and US 7607752 B2 disclose methods that realize this approach. Again, a de facto redundancy printheads of the same color is required, which brings the problems already mentioned with it.

However, the most important known approach, with which the present invention is concerned, is to cover the error by adjacent printing nozzles in the same color and the same inkjet unit. That is, to compensate for individual failed inkjet printing nozzles after determining which individual nozzle, the adjacent nozzles are driven so that the dot sizes of these nozzles are increased so that the location of the failed nozzle is covered with. The neighboring nozzles thus write the picture of the failed nozzle. White lines, which result from the non-printing of individual nozzles, can thus be prevented. The patent application US 020060125850 A1 describes a method and a printing press which operate on this principle. However, the process has an impact on the printed image - in particular, it becomes problematic if several directly adjacent nozzles fail. The compensation over the double or multiple distance is only possible with difficulty.

Another implementation of this approach is from the European patent specification EP 1157840 B1 known. Here, image values associated with the defective print nozzle are redistributed to one or more image values associated with non-defective print nozzles located immediately adjacent to the defective print nozzle and the same color as the defective print nozzle. Again, however, it is a problem if several, directly adjacent nozzles fail.

However, a fundamental problem, which is particularly noticeable in this approach, is the deviating position of the pressure points of individual pressure nozzles. A pressure point is the color wheel generated by the ink droplet. The positions of the pressure points of the printing nozzles, measured at their circle center point, are in reality not always the same distance from the pressure points of adjacent printing nozzles. They are more or less distracted, due to production or by partial blockage, transversely or longitudinally to the printing direction. This leads to the approach of compensation failed nozzles by increased ink drop volume of the adjacent pressure nozzles to the fact that if the pressure point of one or both pressure points of the adjacent pressure nozzles is shifted away from the failed pressure nozzle, despite compensation, albeit less, white line is formed. On the other hand, if the pressure points are shifted towards the failed pressure nozzle, overcompensation occurs. Then the opposite of a White Line - a Black Line. To solve this problem, the real positions of the pressure points of all the pressure nozzles would have to be taken into account.

The object of the present invention is therefore to disclose a method for compensating failed nozzles in an inkjet printing machine, which overcomes the problems known from the prior art and has a better performance.

The solution to this problem is solved by a method for compensating failed nozzles in an inkjet printing machine, wherein failed nozzles are compensated by an increased ink droplet volume of adjacent nozzles and which is characterized in that the real positions of the pressure points of all the nozzles are determined and for each pressure nozzle , To compensate for their failure, the necessary ink droplet volumes of the respective adjacent pressure nozzles depending on the real position of the pressure point of the pressure nozzle and the real positions of the pressure points of the respective adjacent pressure nozzles are calculated.

The core of the invention is the determination of the real pressure points of all pressure nozzles before it comes to compensate for possible failed pressure nozzles. Since the actual positions of the pressure points of the printing nozzles always have to be reduced by a certain amount from the ideal, ie. H. deviate from the theoretical position of the pressure point, with the determined real positions of the pressure points of the pressure nozzles, a more accurate compensation of failed pressure nozzles can be made. Decisive for this are the real positions of the pressure points of the adjacent pressure nozzles of a failed pressure nozzle. If these are shifted, for example, by a particular amount away from the failed pressure nozzle, the compensation by generally increased ink droplet volumes will bring a different result than when the pressure points of the adjacent pressure nozzles are shifted to the failed pressure nozzle.

Advantageous and therefore preferred developments of this invention will become apparent from the appended subclaims and from the description and the accompanying drawings.

A preferred development of the method according to the invention is that a greater distance of the pressure points leads to a larger ink droplet volume of respectively adjacent pressure nozzles and a smaller distance of the pressure points to a smaller ink droplet volume of the respectively adjacent pressure nozzles, the compensation of failed pressure nozzles respectively with the minimum required ink droplet volume ,

Thus, if the real positions of the printing dots of the adjacent printing nozzles are farther apart than the ideal positions of the printing dots, a larger ink drop volume is used for compensation than the standard ink drop volume; on the other hand, if the distance between the real positions of the pressure points of the two adjacent pressure nozzles is reduced, then the ink drop volume used, which is necessary for the compensation, is correspondingly reduced. In principle, only the minimum required ink drop volume is used in all cases, which is necessary to close the white line.

A further preferred development of the method according to the invention is that a nozzle pattern is printed for all printing units of the inkjet printing machine and this printed nozzle pattern is evaluated to determine the real positions of the printing dots.

To determine the real positions of the pressure points, the pressure of a nozzle pattern is most likely to be used, which is then evaluated. With the real positions of all pressure points of all pressure nozzles then all necessary parameters, which are required for a distance-dependent compensation, can be determined.

A further preferred development of the method according to the invention is that the deviation of the real positions of the pressure points from their desired position is calculated with the determined, real positions of the pressure points, including the statistical distribution of the deviation, and from this the necessary ink droplet volumes of the respectively adjacent pressure nozzles be determined.

From the actual positions of the printing nozzles and the statistical distribution of the deviation derived therefrom, the optimum ink droplet volumes of the adjacent printing nozzles necessary for the compensation of the respective printing nozzle can be determined for each printing nozzle.

A further preferred development of the method according to the invention is that the nozzle pattern for determining the real positions of the printing dots is printed several times, with a corresponding multiple evaluation of the real positions of the printing dots to determine the stability and reproducibility of the determined real positions of the pressure points and this stability and reproducibility as a parameter in the calculation of the necessary ink droplet volumes of the respective adjacent pressure nozzles to flow.

In order to exclude possible measurement inaccuracies, and to ensure the most accurate possible determination of the real positions of the pressure points, it makes sense to print and evaluate the nozzle pattern several times, so as to preclude any measurement inaccuracies and measurement errors from the outset.

A further preferred development of the method according to the invention consists in the fact that the real positions of the pressure points are re-determined at regular intervals after a certain number of print jobs have been performed.

Since the real positions of the pressure points can change in the course of the continuous operation, for example by partially clogged pressure nozzles, which then jet correspondingly obliquely, it is appropriate to repeat the determination of the positions of the pressure points at regular intervals. The frequency of repetition depends on the particular press and its print quality. In this case, the experience of the user is crucial. However, it is conceivable to perform repetitions after a fixed number of printing operations or to use an automated control which in certain situations, e.g. falls below certain print quality, performs a repetition.

A further preferred development of the method according to the invention is that the determination of the real positions of the pressure points is realized by a two-dimensional pressure point determination in and across the direction of printing.

Although usually the deviations of the real from the theoretically ideal positions of the pressure points in a row, that is transversely to the printing direction, mainly of importance, also deviations in the printing direction, ie orthogonal to the row of line-shaped printing nozzles, should be taken into account. Thus, the most accurate adaptation of the ink droplet volumes can be achieved to compensate failed nozzles.

A further preferred development of the method according to the invention is that the method is used only for the printing of solid areas in the printed image.

Since the white lines lead mainly in full areas to a deterioration in print quality, it makes sense to use the method according to the invention only for solid areas. However, this does not preclude use even for halftone surfaces, of course, all the more so since the determination of the real positions of the pressure points is in any case carried out for all printing nozzles and thus all required parameters for the method according to the invention are present regardless of the type of printed image.

A further preferred development of the method according to the invention is that only detected, failed pressure nozzles are compensated by the method according to the invention.

There are two possible types of use for compensating failed nozzles by increasing the ink droplet volumes of adjacent nozzles. In a first type of use only detected failed pressure nozzles are compensated by the adjacent pressure nozzles. Ergo, the method according to the invention can in this case also be carried out only for detected failed pressure nozzles.

A further preferred development of the method according to the invention consists in that all pressure nozzles are compensated independently of their state according to the method according to the invention.

The second type of use of compensation failed nozzles by adjacent pressure nozzles is basically always to operate the pressure nozzles in compensation mode, since in practice anyway many pressure nozzles have failed and thus the complex control of individual pressure nozzles, which compensate each failed adjacent pressure nozzles, spared. In this operating mode, all pressure nozzles are thus operated in compensation mode, which leads to performance losses and slightly increased ink consumption, but makes both the control of individual pressure nozzles no longer necessary, as well as the complex detection of failed pressure nozzles superfluous. In this operating mode, logically, all pressure nozzles are therefore operated independently of their state with the method according to the invention.

A further preferred development of the method according to the invention is that the ink drop volume can not be set freely, but only in quantized steps over the pulse number of a waveform.

This activation variant for the ink droplet volumes of the corresponding printing nozzles means that in practice often only certain ink droplet volumes are available in correspondingly quantized steps for the method according to the invention. Since the control for the ink drop volumes does not allow arbitrarily small resolutions, but anyway has a lower limit for the minimum realizable ink drop volume, some form of quantization must be done. In this development, this happens via the pulse number of a waveform.

Another solution of the problem posed is an inkjet printing machine, set up for carrying out a method according to one of the preceding claims 1 to 11.

The invention as such as well as structurally or functionally advantageous developments of the invention are described below with reference to the accompanying drawings with reference to at least one preferred embodiment. In the drawings, corresponding elements are provided with the same reference numerals.

Figur 1:

das System einer Bogen-Inkjet-Druckmaschine, schematisch dargestellt

Figur 2:

einen Beispielbogen mit einem stilisiert dargestellten Missing-Nozzle-Fehler

Figur 3:

eine schematische Darstellung idealer und realer Positionen von Druckpunkten

Figur 4:

den Ablauf des erfindungsgemäßen Verfahrens

The drawings show:

FIG. 1:

the system of a sheet-fed inkjet printing machine, shown schematically

FIG. 2:

an example sheet with a stylized illustrated Missing Nozzle error

FIG. 3:

a schematic representation of ideal and real positions of pressure points

FIG. 4:

the course of the method according to the invention

The field of application of the preferred embodiment is an inkjet printing machine 10. An example of the structure of such a machine 10 is shown in FIG FIG. 1 shown. During operation of this printing press 10, as already described, individual print nozzles may fail in the print heads 4 in the printing unit 2. The result is then white lines 13, or in the case of multi-color printing, distorted color values. An example of such a white line 13 in a printed image 12 is shown in FIG FIG. 2 shown.

The printing process is monitored by a control computer 20. In this case, the sheet 11 is transported from the feeder 1 in the transport direction T via the transfer cylinder 5 to the printing cylinder 7 and to the printheads 4, wherein the printheads 4 consist of one or more rows of printing nozzles. The cylinders 5, 7 are accelerated by one or more drives 6. About the print nozzles in the printheads 4, the color is then applied to the sheet 11 to produce the printed image 12. The sheet 11 is further transported, dried and transported via the transfer cylinder 5 to the boom 3 on.

In FIG. 3 In the upper figure, an example of ideally positioned pressure points 15, 16, 17 of the pressure nozzles is shown. In the middle is the failed pressure nozzle, which for compensation, the two adjacent pressure nozzles 16, 17 use a correspondingly enlarged ink droplet volume 19 to compensate for the failed pressure nozzle and to fill the thereby occurred white line 13. In the lower part of FIG. 3 is the same representation again for real positions of pressure points 16 ', 17', 18 indicated for the pressure nozzles. As can be seen, some of the pressure points 18 are shifted transversely to the respective printing direction, resulting in different overlaps between the pressure areas respectively adjacent pressure nozzles leads. The illustration is schematic and for better visibility the deviations of the pressure points from their target positions are exaggerated. As can also be seen, both adjacent pressure nozzles are shifted to the failed pressure nozzle with respect to their pressure points 16 ', 17' away from the failed pressure nozzle. The pressure point of the right adjacent pressure nozzle 16 'is further away from the failed pressure nozzle shifted than that of the left adjacent pressure nozzle 17'. According to the inventive method and as in FIG. 3 Thus, to compensate, the right adjacent pressure nozzle needs to use a larger ink drop volume 19 to compensate for the gap caused by the failed pressure nozzle than the left adjacent pressure nozzle. This is the core of the process according to the invention.

In FIG. 4 is again the flow of the method according to the invention shown schematically. The first method step is to determine the real dot positions of the pressure points 18 for the printing nozzles via a nozzle test pattern which has to be printed for all print heads 4. The printed nozzle patterns are examined and from this the deviation from the theoretically present, ideal positions of the pressure points 15 is determined. In order to rule out possible errors, the nozzle test patterns are printed several times and measured several times. From this, in addition to the actual real positions of the pressure points 18, the statistical distribution of the deviation of the real pressure points 18 from the theoretical pressure points 15 can be determined. With these parameters can then be calculated by the computer 20 for each pressure nozzle, how large the ink drop volume 19 of this pressure nozzle must be in order to compensate for each of the left adjacent or the right adjacent pressure nozzle in case of failure.

für die Vollfläche berechnet. So ist zum Beispiel bei einer Auflösung von 1200 dpi mit einem Düsenabstand 21,17 µm und einem geometrischen Standardkompensationstropfendurchmesser von 47,33 µm, mit einer Standardabweichung von S = 1,2 µm und einem Kompensationstropfendurchmesser von 52,13 µm mit einem Spreizfaktor von 1,85 gemäß dieser Formel Kompensationstropfenvolumen 19 bei maximal erlaubtem Abstand der Druckpunkte benachbarter Druckdüsen gleich 11,72 picoliter. Beim Standardabstand der Druckpunkte benachbarter Druckdüsen 16', 17' ist dagegen ein Kompensationstropfenvolumen 19 von 8,77 picoliter notwendig; bei einem minimalen Abstand der Druckpunkte zur benachbarten Druckdüse jedoch nur noch ein Tropfenvolumen 19 von 6,36 picoliter erforderlich, um die benachbarte Druckdüse bei Ausfall kompensieren zu können.The diameter of the required compensation ink droplet 19 is calculated according to the formula: $${\mathrm{D}}_{\mathrm{Comp}}={\mathrm{D}}_{\mathrm{0}}+\mathrm{4}*\mathrm{standard\; deviation}\left(\mathrm{the\; pressure\; points}\right)$$

calculated for the full area. Thus, for example, at a resolution of 1200 dpi with a nozzle pitch of 21.17 μm and a geometric standard compensation drop diameter of 47.33 μm, with a standard deviation of S = 1.2 μm and a compensation drop diameter of 52.13 μm with a spreading factor of 1 , 85 according to this formula compensation drop volume 19 at the maximum allowed distance of the pressure points of adjacent pressure nozzles equal to 11.72 picoliter. At the standard distance of the pressure points of adjacent pressure nozzles 16 ', 17', however, a compensation drop volume 19 of 8.77 picoliter is necessary; However, at a minimum distance of the pressure points to the adjacent pressure nozzle only a drop volume 19 of 6.36 picoliter required to compensate for the adjacent pressure nozzle in case of failure can.

As can be seen, the necessary compensation drop volume 19 differs in the case of a maximum distance almost twice the necessary compensation drop volume 19 at a minimum pressure point distance. It can be easily imagined what would happen if the standard compensating drop volume 19 of 8.77 picolitres were compensated for if the adjacent failed pressure nozzle had only a minimal separation with respect to its pressure point. You would have an overcompensation, which would be realized in a misprint of a black line. In order to avoid this overcompensation, therefore, the individual compensation drop volume 19 for each pressure nozzle is calculated and stored for the left and / or right adjacent pressure nozzle. With these modified compensation drop volumes 19, the actual printing of the print job in the inkjet machine 10 is then carried out. In this case, the printing machine 10 can be operated in the continuous compensation mode, in the event that no detection of failed printing nozzles is performed. In this case, all of the print nozzles would print at the average between the left adjacent nozzle compensation drop volume 19 and the right adjacent nozzle compensation drop volume 19. Of course, in the case of the pressure nozzles at the very outside, only one value would be used to compensate for the failure of the adjacent pressure nozzle.

In the event that the operating mode of the printing machine 10 is used, where only actually failed printing nozzles are compensated, the relevant values are of course used. If both adjacent pressure nozzles have failed, then the average of the then two relevant values is used accordingly.

LIST OF REFERENCE NUMBERS

T

transport direction

1

investor

2

printing unit

3

boom

4

inkjet heads

5

transfer cylinder

6

drive

7

Printing cylinder (jetting cylinder)

8th

Sheet holding area

9

channel

10

Sheetfed

11

bow

12

print image

13

White line

14

pressure point

15

ideal position of a pressure point (center)

16

ideal pressure point of the right adjacent pressure nozzle

17

ideal pressure point of the left adjacent pressure nozzle

16 '

real pressure point of the right adjacent pressure nozzle

17 '

real pressure point of the left adjacent pressure nozzle

18

real, shifted position of a pressure point (center)

19

Compensation volumes for failed pressure nozzles

20

control computer

Claims (12)

Method for compensating failed nozzles in an inkjet printing machine, wherein failed nozzles are compensated by an increased ink drop volume of adjacent nozzles,
characterized,
in that the real positions of the pressure points of all the pressure nozzles are determined and for each pressure nozzle, to compensate for their failure, the necessary ink droplet volumes of the respectively adjacent pressure nozzles are calculated as a function of the actual position of the pressure point of the pressure nozzle to the real positions of the pressure points of the respectively adjacent pressure nozzles. Method according to claim 1,
characterized,
that a greater distance of the pressure points leads to a larger ink droplet volume respectively adjacent pressure nozzles and a smaller distance of the pressure points to a smaller ink drop volume of the respectively adjacent pressure nozzles, the compensation of failed pressure nozzles takes place respectively with the minimum required ink drop volume. Method according to one of the preceding claims,
characterized,
that a printed nozzle pattern for all printing units of the inkjet print engine and for determining the real positions of the pressure points of this printed nozzle pattern is evaluated. Method according to claim 3,
characterized,
that the determined, real positions of the pressure points, the deviation of the real positions of the pressure points is calculated from their desired position, including the statistical distribution of the deviation and from the necessary ink droplet volumes of the respective adjacent pressure nozzles are determined. Method according to claim 4,
characterized,
in that the nozzle pattern for determining the real positions of the printing dots is printed several times, with corresponding multiple evaluation of the real positions of the printing dots, to determine the stability and reproducibility of the determined real positions of the printing dots and this stability and reproducibility as parameters in the calculation of the necessary ink drop volumes to incorporate the respectively adjacent pressure nozzles. Method according to one of the preceding claims,
characterized,
that the real positions of the printing dots are re-determined at regular intervals after a certain number of print jobs have been performed. Method according to one of the preceding claims,
characterized,
that the determination of the real positions of the pressure points is realized by a two-dimensional pressure point determination in and across the direction of printing. Method according to one of the preceding claims,
characterized,
that the method is used only for the printing of solid areas in the printed image. Method according to one of the preceding claims,
characterized,
that only detected, failed pressure nozzles are compensated by the method according to the invention. Method according to one of claims 1 to 8,
characterized,
that all pressure nozzles are compensated regardless of their condition by the method according to the invention. Method according to one of the preceding claims,
characterized,
that the ink drop volume can not be set freely, but only in quantized steps over the pulse number of a Waveform. Inkjet printing machine set up to carry out a method according to one of the preceding claims.

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