WO2020161014A1

WO2020161014A1 - Method for increasing the quality of an inkjet printed image

Info

Publication number: WO2020161014A1
Application number: PCT/EP2020/052428
Authority: WO
Inventors: Thomas Boosmann; Jens VOELSCHOW; Sven Michael
Original assignee: Windmöller & Hölscher Kg
Priority date: 2019-02-04
Filing date: 2020-01-31
Publication date: 2020-08-13
Also published as: EP3921172A1

Abstract

The invention relates to a method for increasing the quality of a printed image of a digital printing machine, which printed image is printed onto a material web in the inkjet printing process, wherein the digital printing machine comprises: a printing head having inkjet nozzles; an image-capturing unit for capturing at least one part of the printed image; and a control unit for controlling the printing head. In order to keep the quality of the printed image constant over the entire printing length during the print job, the solution according to the invention is based on a method in which: a test chart is printed at the beginning of and/or during a print job, the first image data of which test chart is captured by the image-capturing unit and transferred to the control unit, wherein an error correction of defective inkjet nozzles and/or an alignment of the misaligned printing head is carried out according to the first image data; after the error correction and/or alignment has ended, a target printed image is printed, the target image data of which is captured by the image-capturing unit, transferred to the control unit and stored there; and after the target printed image has been printed, additional actual printed images are printed, the actual image data of which is captured by the image-capturing unit, compared with the target image data in the control unit and stored as target-actual comparison image data, wherein an error correction of defective inkjet nozzles is carried out according to the target-actual comparison image data.

Description

Process for increasing the quality of an inkjet print image

_

The invention relates to a method for increasing the quality of a print image of a digital printing machine that is printed on a web of material using the inkjet printing process, the digital printing machine comprising a print head with inkjet nozzles, an image acquisition unit for acquiring at least part of the print image and a control unit for controlling the print head . The invention also relates to a digital printing machine for carrying out such a method. In the inkjet printing process on a moving web of material, the printing lengths usually vary between 10 meters and a few 1000 meters. Especially with the print lengths in the upper range, the problem arises that the quality of the print image no longer corresponds to the quality at the beginning of the print job, since the external interference parameters (such as temperature, viscosity of the printing ink, etc.) vary during the course of the print job. In particular, the following three influences have a negative effect on the quality of the printed image:

• Change of color locations and tone value increase (TWZ)

· Blockage of individual inkjet nozzles

• Print head misalignment

The object of the invention is therefore to keep the quality of the print image constant over the entire print length during the print job. This object is achieved by a method according to claim 1 and a digital printing press according to claim 9.

The invention makes it possible to identify and correct the above-mentioned changes in the print image in a simple manner.

The solution according to the invention is based on a method in which a test chart is printed at the beginning and / or during a print job, the first image data of which is recorded by the image acquisition unit and transmitted to the control unit, with an error correction of defective inkjet media as a function of the first image data. Nozzles and / or and / or an alignment of the misaligned print head is carried out, in which, after the error correction and / or alignment has been completed, a target print image is printed, the target image data of which is captured by the image acquisition unit, transmitted to the control unit and stored there, in which, after printing the target print image, further actual print images are printed, the actual image data of which is captured by the image acquisition unit and compared with the target image data in the control unit and stored as target / actual comparison image data, whereby depending on the target image Actual comparison image data is an error correction of defective inkjet nozzle sen is carried out.

In the simplest case, the error correction consists in switching off the defective inkjet nozzles. In addition, it is conceivable that neighboring inkjet nozzles take over the printing area of the respective switched off inkjet nozzle and print. This technique is also called "software-based inkjet nozzle overlap".

Before the preferred embodiments are listed, a few technical terms are first explained:

Dot gain

The technical term dot gain (abbreviation: TWZ) describes the effect that halftone dots of the print template (e.g. a digital image file or a film) appear larger on the printed sheet due to the process, the print image so it is darker than intended in the template. More precisely, raster points are never transferred in exactly the same way with every technical transfer, but rather deformed in some way that is typical for the process. Deformations, enlargements, but also reductions occur.

Spectral sensor and spectral value function

Every person has three types of cones in the retina, which differ in their spectral sensitivity. In order to compensate for the individual differences in human perception of color, a "normal viewer" was defined in various standards (in particular IS0 12647 and DIN 5033). The normal color values are calculated from the spectrum of the received light, the spectral reflectance of the color and the standardized spectral value functions of the normal viewer. With the normal color values calculated in this way, the color measured with a spectral sensor can then be described exactly.

CIELAB color model

The CIELAB color model describes a three-dimensional color space in which corresponding color differences, which humans perceive to be the same size, are each measurably approximately the same distance apart. The most important thing about this color space is that

Device independence and the resulting objectivity. The three axes of the CIELAB color space are defined as follows: L = axis of brightness

a = red-green axis

b = blue-yellow axis

The coordinates of the CIELAB color space are often referred to as L ^* , a ^* and b ^* to distinguish them from the Hunter coordinates Lab (introduced in 1948 by Richard Sewall Hunter). For the sake of simplicity, however, the designations L ^* , a ^* and b ^{* are} omitted in this description, ie the CIELAB color space is referred to as Lab throughout. The L value is always between 0 and 100. 0 stands for absolutely black and 100 for absolutely white. A normal viewer and the normal light type were defined to ensure a clear color perception. The difference between two color locations is expressed by the color distance AE. The color difference is calculated using the following formula:

The values AL, Aa and Ab are the differences between the components of the two color loci. Depending on the application, there are also further developments to the above distance formula, for example the color distance _formulas known from the literature with the designations AE _cmc , AE ₉₄ , AE _m or AE _m . In the context of this description, the color distance AE is therefore understood to mean any color distance that has been calculated according to a specific color distance formula, the person skilled in the art being able to select the appropriate color distance formula for the particular application.

Test chart

A test chart (equivalent term: test pattern) in the sense of the present description is a pattern printed separately from the printed image, the properties of which are optimized for carrying out certain tests.

For example, a test chart can be used for ICC color calibration. In this case, the test chart consists of a limited number of color fields, the composition of which is known from the process colors cyan, magenta, yellow and black. Another example of using a test chart is the error correction of defective inkjet nozzles. In this case, the test chart is designed in such a way that defective inkjet nozzles can be easily and clearly detected. Another example of using a test chart is the detection of misaligned printheads. Check mark

A check mark (equivalent terms are "calibration field" or

In contrast to the test chart, “test strips” in the context of the present description is a pattern that is superimposed on the printed image. There are several options:

On the one hand, it is possible through suitable screening processes that the

Check mark is printed directly within the print image. In this case, the test mark is usually not visible to the naked eye, while digital imaging systems recognize and

Enable further processing. It is also possible, for example, for the test marking to consist of yellow micro dots which can be easily detected by a camera under UV light. Such yellow microdots are also used for color printer marking as a so-called machine identification code (MIC).

On the other hand, it is also possible for the test marking to be arranged between two printing sections in the printing direction. The print sections can have a repetitive print content or also vary in the print content.

According to a preferred embodiment, it is provided that the target print image comprises a partial area of the subsequent actual print images.

According to a further preferred embodiment, it is provided that an image area of the actual print image is automatically determined as the target print image.

According to a further preferred embodiment, it is provided that the image area has the largest color gamut compared to other image areas of the actual printed image.

According to a further preferred embodiment it is provided that the image acquisition unit is a line camera. According to a further preferred embodiment it is provided that the image acquisition unit is a spectral sensor.

According to a further preferred embodiment, it is provided that the comparison of the actual image data with the desired image data is based on the histogram data of the respective image data.

According to a further preferred embodiment, it is provided that further test charts are printed, the results of which are also entered in the

Incorporate error correction.

Further details and advantages of the invention are based on the

attached drawing. This shows: FIG. 1 a simplified block diagram of a digital printing machine with an inkjet print head for carrying out the method according to the invention.

Fig. 1 shows a simplified block diagram of a digital printing machine with an inkjet print head for performing the method according to the invention. The core of the digital printing machine is the inkjet print head 101, which comprises several inkjet nozzles 106 with which individual ink droplets 102 are generated from the printing ink and transferred to the moving material web 103. The ones required for operating the inkjet print head 101

Control information is calculated by the control unit 104 and transmitted to the inkjet print head 101. The control unit 104 obtains its

Information about the image to be printed in turn from the digital

Print image that is stored in the control unit 104 in the form of digital target raster points and digital target color information. Before the start of the print job, the rastered print image was generated from a standard digital image format (e.g. RGB 24 bit, TIFF or PDF) as part of the so-called raster image process (abbr .: RIP). An image capture unit 105 is arranged above the completely printed image, which can consist, for example, of a camera.

According to the sources of error mentioned in the introduction, the following three corrective measures can now be carried out individually or in combination:

• Color space correction

• Error correction of defective inkjet nozzles

• Alignment of misaligned printheads

The three measures are explained in more detail below:

Color space correction

At the beginning of and / or during a print job, a test chart is printed, the first image data of which is captured by the image capture unit 105 and transmitted to the control unit 104. Depending on the first image data, an error correction of defective inkjet nozzles 106 and / or

Alignment of the misaligned print head 101 is carried out.

After completion of the error correction and / or the alignment, a target print image is then printed, the target image data of which is captured by the image capture unit 105, transmitted to the control unit and stored there.

Following the printing of the target print image, further actual print images are printed, the actual image data of which is captured by the image acquisition unit 105 and compared with the target image data in the control unit 104, a color space correction being carried out as a function of the comparison result. The comparison result corresponds to the delta color information (DE) in the Lab color space. The control information of the control unit 104 is corrected as a function of DE in such a way that the DE is reduced. This takes place in a control loop with correction of the control information in the control unit 104, in the raster data by image manipulation or in preseparated

Conton data through gradation curves. Two examples with numerical values are listed below:

example 1

Example 1 shows that a measured change in

Brightness L from 84 to 80 (DE from 4) in this case only affects the chromatic colors cyan, magenta and yellow. By a

Corresponding correction in the pre-separated contone data by adapting the gradation curves in the cyan, magenta and yellow channels can be a

Correction can be achieved. Then only the color separations that have changed are rasterized. In this example cyan, magenta and yellow. Example 2

In example 2, the lightness L has darkened from 37 to 30 (DE from 7), with the result that 6 colors in the 7c color space have changed. With a quality requirement of e.g. DE less than 2.5 it is sufficient to have a

Adjustment of the gradation curve for the hue black. So that no changes in the gradation curves become incorrect corrections to other color locations, a minimum number of motif colors must be monitored viewed at the same time and, accordingly, always assessed and corrected simultaneously.

Error correction of defective Inkiet nozzles

At the beginning and / or during a print job, a test chart is printed, the first image data of which is captured by the image capturing unit 105 and transmitted to the control unit 104, with an error correction of defective inkjet nozzles 106 and / or alignment of the misaligned inkjet nozzle 106 depending on the first image data Print head 101 is performed.

After the error correction has been completed, a target print image is then printed, the target image data of which is captured by the image acquisition unit 105 and sent to the

Control unit 104 are transmitted and stored there,

Following the printing of the target print image, further actual print images are printed, the actual image data of which is captured by the image acquisition unit 105 and compared with the target image data in the control unit 104, with an error correction of defective inkjet nozzles depending on the comparison result 106 is carried out. The defective inkjet nozzles are switched off and neighboring inkjet nozzles take over and print the print area of the respective switched-off inkjet nozzle (so-called

software-based inkjet nozzle overlap). Alignment of misaligned printheads

During the current print job, at least some of the

Print images printed a test mark together with the print image. The test mark is present in the control unit 104 in digital form and forms the target image data. Immediately after the test marking has been printed, its actual image data are captured by the image acquisition unit 105 and compared in the control unit 104 with the target image data and stored as target-actual comparison image data, misaligned print heads being detected as a function of the target-actual comparison image data. In one In the next step, the control unit then corrects the misaligned print heads.

Claims

1. A method for increasing the quality of a print image of a digital printing machine, which is printed in the inkjet printing process on a web of material, the digital printing machine comprising a print head with inkjet nozzles, an image acquisition unit for capturing at least part of the print image and a control unit for controlling the print head , in which a test chart is printed at the beginning and / or during a print job, the first image data of which is captured by the image acquisition unit and transmitted to the control unit, with an error correction of defective inkjet nozzles and / or and / as a function of the first image data. or an alignment of the misaligned printhead is carried out, in which, after the error correction and / or alignment has been completed, a target print image is printed, the target image data of which is captured by the image acquisition unit, transferred to the control unit and stored there Target print image further actual print printed images, the actual image data of which are captured by the image capture unit and compared in the control unit with the target image data and stored as target / actual comparison image data, an error correction of defective inkjet nozzles being carried out as a function of the target / actual comparison image data.

2. The method according to claim 1, wherein the target print image comprises a partial area of the subsequent actual print images.

3. The method according to any one of claims 1-2, wherein an image area of the actual print image is automatically determined as the target print image.

4. The method according to claim 3, wherein the image area has the largest color gamut in comparison to other image areas of the actual printed image.

5. The method according to any one of claims 1-4, wherein the image capture unit is a line camera.

6. The method according to any one of claims 1-4, wherein the image acquisition unit is a spectral sensor.

7. The method according to any one of claims 1-5, wherein when comparing the actual image data with the target image data, the flistogram data of the respective image data are used.

8. The method according to any one of claims 1-7, wherein further test charts are printed, the results of which are also included in the error correction.

9. Digital printing machine, with a print head with inkjet nozzles for printing a print image in the inkjet printing process on a web of material, with an image capture unit for capturing at least part of the print image, and with a control unit for controlling the printhead according to a procedural method of claims 1-8.