Method for improving coherence of color densities on printing

Abstract

The invention relates to a method for improving the consistency of color densities over the print width of an inkjet printer (10), comprising the following steps: printing a two-dimensional pattern (32) wherein the two-dimensional pattern (32) has the same nominal print density over the print width and has a varying nominal print density in the print direction; dividing the two-dimensional pattern (32) into a number of columns (38) over the print width; recording, with a camera (24), at least one color value for each nominal print density in each of the columns (38); deriving a correction curve for each column (38) from the recorded color values; and correcting the printing signal and / or the printing pattern with the correction curve. The invention further relates to an inkjet printer (10).

Description

Technical Field

[0001] The present invention relates to a method for improving the uniformity of color density across the print width of an inkjet printer. Furthermore, the present invention relates to an inkjet printer. Background Art

[0002] Inkjet printers are commonly used to digitally print a variety of products, such as labels, textiles, tiles, etc., by dispensing micro-droplets of ink through the nozzles of the print head.

[0003] To achieve consistent and high-quality printing, the ejection of drops from each nozzle must be precisely controlled. Even small changes in ink flow rate, ink temperature or ink pressure can affect the drop formation process and may cause a nozzle to behave differently from its neighboring nozzles. Variations in the volume, shape, direction or velocity of droplets produced on a printhead or multiple printheads can affect the print pattern, especially the consistency of ink density printed onto the substrate and, therefore, the color density in multi-color inkjet printing. Summary of the invention

[0004] The object of the invention is to improve the print quality by compensating for deviations in the droplet formation process over the print width.

[0005] The object of the present invention is to achieve a method for improving the consistency of color density across the print width of an inkjet printer, the method comprising the following steps:

[0006] - printing a two-dimensional pattern, wherein the two-dimensional pattern has the same nominal print density across the print width and has a varying nominal print density in the print direction;

[0007] - Divide the two-dimensional pattern into several columns across the print width;

[0008] - recording with a camera at least one color value for each nominal print density in each of said columns;

[0009] - deriving a correction curve for each column from the recorded color values; and

[0010] -Correcting the print signal and / or the print pattern using a correction curve.

[0011] The method can be applied to piezoelectric inkjet printers and other drop-on-demand printing machines with a single or a few print heads. For example, it is suitable for improving the print quality of a single-pass inkjet printer with a plurality of printing stations through which a web and / or substrate subsequently passes and which are configured to print a plurality of ink materials, in particular a plurality of colors, onto the web and / or substrate.

[0012] For piezoelectric inkjet printers, the step of modifying the print signal may involve adjusting the amplitude, duration and / or shape of the voltage pulses applied to the print head piezoelectric elements of a single print nozzle or a set of print nozzles. The adjustment allows the droplet volume, shape and velocity to be increased or decreased locally. In practice, the voltage pulse amplitude and shape are often set at the print head level, and the timing and duration are set for each nozzle.

[0013] It is thus possible to influence the droplet formation process of individual print nozzles or sets of print nozzles and thus to homogenize the droplet formation process over the print width.

[0014] Furthermore, all steps of the method can be performed in tandem during printing without any user intervention, thus saving time, costs and being less error-prone compared to manual adjustments.

[0015] In an embodiment, the method may comprise the additional step of printing a position calibration pattern adapted to associate a print nozzle or group of print nozzles with a column printed from the print nozzle or group of print nozzles.

[0016] The calibration pattern may be recorded with a camera to obtain the position data. It is conceivable that the same camera that records the color values ​​may be used for this purpose. Alternatively, an additional calibration camera configured to record the calibration pattern may be used.

[0017] Because the distance from the substrate to the camera can vary over the print width, it is preferred that calibration pattern marks are printed and recorded at least every 5mm to 10mm, more preferably every 0.3mm to 2mm. In particular, the calibration pattern may consist of small parallel lines forming a periodic pattern over the print width.

[0018] Printing and recording the calibration pattern allows determining the position of the nozzle from which the pattern is ejected, and therefore also the position of all other nozzles relative to the substrate. Thus, it is possible to prevent erroneous assignment of the columns into which the print pattern is divided to print nozzles or groups of print nozzles.

[0019] In another embodiment, the correction curve for each column is derived from a comparison between the recorded color values ​​and the corresponding nominal print density. It is conceivable that the nominal print density describes the expected ink coverage on the substrate and that the print signal for printing the two-dimensional pattern is applied to the nozzles according to the nominal print density. In other words, the nominal print density contains information about how much ink should be at a certain position on the substrate. The comparison between this expected value and the actual value recorded with the camera allows the deviation between the two to be determined and thus to find a suitable correction method to compensate for this deviation.

[0020] In an embodiment, the camera records red, green and blue (RGB) color values ​​for each nominal print density in each column. A correction curve is then derived for each column from one of the RGB color values ​​or a linear combination of the RGB color values, such as their sum.

[0021] For example, if a two-dimensional pattern is printed using the first ink of a multicolor inkjet printer (e.g. magenta ink), the camera can record RGB color values ​​for the magenta pattern. However, only one color is needed to derive the correction curve. Therefore, only the green values ​​recorded by the camera can be used and the red and blue values ​​can be ignored. Alternatively, any linear combination of the RGB color values ​​(α*R+β*G+γ*B), such as their sum (α=β=γ=1), can be used to derive the correction curve for each column of the printed pattern. Depending on the application, certain linear combinations of RGB values ​​can provide a higher reproducibility of the method than others. Therefore, it is conceivable that the factors of the linear combination are selected according to the application, in particular the type of ink and the substrate applied. However, the method is also applicable to the use of a grayscale-single-channel-camera.

[0022] In a further embodiment, each column into which the print pattern is divided corresponds to a group of individually addressable print nozzles of the print head, in particular a group of 16 nozzles. It is conceivable that the print head comprises a plurality of such groups, in particular 128.

[0023] Dividing the nozzles into groups and correcting the print signal for each group reduces the computational effort of the method compared to single nozzle correction. Furthermore, the accuracy and pixel size of the camera and the print head drive electronics may limit the possibilities of single nozzle correction.

[0024] Furthermore, it is contemplated that the nominal print density describes the expected ink coverage on the substrate from 0% to 100%.

[0025] Preferably, the two-dimensional pattern is printed by subsequently applying ink with discrete nominal print densities on the web and / or substrate. The nominal print density can be between 0% and 100% with a step of 2.33%. This corresponds to 44 nominal print densities in each column of the printed pattern. It has been found that such a division results in a high quality of the modified printing process with a reasonable computational effort. Of course, depending on the application, a finer or coarser subdivision of the nominal print density range can also be selected.

[0026] In a further embodiment, the steps of the method are repeated periodically and / or when printing parameters, in particular printing speed or ink batch, or ambient temperature or humidity change. By repeating the method, a high print quality can be achieved in various printing processes. Furthermore, linking the execution of the method to special events such as ink batch changes reduces the overall workload.

[0027] In a further variant of the method, the two-dimensional pattern is printed on the substrate used for production. This also reduces the overall effort of the method (no additional calibration substrate is required) and allows compensation of substrate-specific effects such as ink spreading and / or soaking (e.g. on porous paperboard substrates).

[0028] The object of the present invention is also solved by an inkjet printer, which comprises: at least one print head, which is provided with multiple nozzles and is suitable for printing ink; a camera, which is located downstream of the at least one print head and is suitable for recording at least one color value of a pattern printed by ink over the entire printing width of the at least one print head; and a control unit, which is configured to execute the method according to the present invention.

[0029] The advantages of the method discussed also apply to inkjet printers.

[0030] In one embodiment, the inkjet printer camera has three rows of pixels in the printing direction. Each row is suitable for recording different colors, especially red, green and blue (RGB) color values ​​of the two-dimensional printed pattern over the entire printing width.

[0031] Using a single three-row camera is less error prone and / or reduces the computational effort and setup cost of the method compared to using several conventional cameras in parallel.

[0032] Furthermore, the use of such an in-line camera allows periodic recalibration without any user intervention. It is also possible to record several samples with the same nominal print density in order to average and / or compensate for local substrate defects or waviness.

[0033] Note that this method is applied in a series of processes for controlling the print nozzles at the density level. Therefore, the print pattern contains values ​​that are further processed by the dithering algorithm to obtain the signal input to each individual print nozzle. However, we define a density value for each nozzle and, therefore, the dithering algorithm can be considered as a random quantization of the density value. In other words, for the sake of this disclosure, we can consider that each nozzle gets a density value as input and ignore the fact that this value is further processed by the dithering algorithm. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Further advantages and features will become apparent from the following description of the invention and from the accompanying drawings, which show non-limiting example embodiments of the invention and in which:

[0035] - Figure 1 schematically shows a top view of an inkjet printer according to the present invention; and

[0036] - Figure 2A diagram of the correction curve for several columns of a print pattern is schematically shown. DETAILED DESCRIPTION

[0037] Figure 1 There is schematically shown an embodiment of a single-pass inkjet printer 10 with four print stations 12. Each print station 12 is configured to print a different ink, such as cyan, magenta, yellow and black ink.

[0038] In the depicted embodiment, each print station 12 is equipped with six piezoelectric inkjet print heads 14 assembled in a print bar 16. The length of the print bar 16 defines the printing width of the inkjet printer 10.

[0039] Each of the print heads 14 includes a plurality of nozzles 18. In the illustrated embodiment, the print heads 14 are Dimatix Samba print heads with a plurality of print nozzles 18 arranged on a trapezoidal nozzle plate. Figure 1 In the figure, only the nozzles 18 for one of the print heads 14 are schematically shown. Of course, other print heads 14 with different numbers of nozzles 18 and different shapes can also be used.

[0040] The inkjet printer 10 also includes a roll 20 for carrying a printing substrate 22 such as a polymeric foil or paper box for packaging. Figure 1 In FIG. 1 , the direction of movement of the web 20 relative to the printing station 12 is indicated by an arrow.

[0041] The inkjet printer 10 is also equipped with a camera 24, which is located downstream of the printing station 12, in particular downstream of the print bar 16 to be analyzed. The camera 24 has three rows of pixel rows 26 in the printing direction. Each row of pixel rows 26 extends over the entire printing width and is suitable for recording different colors of the printed pattern. In particular, one of the pixel rows 26 is configured to record red values ​​R, one of the pixel rows 26 is configured to record green values ​​G, and one of the pixel rows 26 is configured to record blue values ​​B. Thus, the red, green and blue (RGB) color values ​​of the printed pattern can be recorded by the camera 24 over the entire printing width.

[0042] Preferably, there is a single camera for analyzing several print bars 16 of a print station 12. However, in systems where the ink dries between the print bars 16 (ie, the distance between the print bars is greater), we can place the camera 24 downstream of each print bar 16.

[0043] The inkjet printer 10 further comprises a control unit 28 configured to run a program causing the control unit 28 to perform a method for improving the uniformity of color density across a print width.

[0044] In a first step of the method, one of the print stations 12, for example the cyan print station 12, prints a calibration pattern 30 onto a print substrate 22 for production, for example onto paper, plastic foil or cardboard. In an embodiment, the calibration pattern 30 comprises a plurality of straight lines elongated in the print direction. These lines are parallel to each other and are spaced 1 mm apart from each other. Of course, the example given is not limiting. Other calibration patterns 30 and / or line distances can also be used. Preferably, the distance is selected so that the resulting positional accuracy is finer than the pixel resolution (i.e. the resulting positional uncertainty is less than the distance between two adjacent pixels measured at the print substrate).

[0045] In the second step of the method, the same printing station 12 prints a two-dimensional pattern 32 onto the substrate 22. The two-dimensional pattern 32 is as shown in FIG. Figure 1 It has the same nominal print density across the print width and a varying nominal print density in the print direction.

[0046] In an embodiment, the nominal print density describes the expected ink coverage on the substrate 22 from 0% to 100%.

[0047] The two-dimensional pattern 32 is created by subsequently applying ink having 44 discrete nominal print densities on the substrate 22, starting at 0% and ending at 100% with 2.33% steps in between.

[0048] In the figure, parallel lines 34 perpendicular to the printing direction represent the boundaries between regions of different nominal print densities. To simplify the drawing, only five regions 36 with discrete print densities are shown.

[0049] In an embodiment, the area 36 having the highest nominal print density corresponds to 100% cyan ink coverage on the substrate 22 and is located next to the calibration pattern 30 .

[0050] In the third step of the method, the two-dimensional pattern 32 is divided into a number of columns 38 over the print width. It is conceivable that the columns 38 are equal in size. For example, the size may be defined by a fixed value stored in the control unit 28 and / or in the program. Alternatively, the size of the columns 38 may depend on the print width and / or resolution of the print head 14. Figure 1 Columns 38 are represented by dashed lines 40 extending in the web direction.

[0051] In an embodiment, each column 38 is printed by a group 42 of sixteen individually addressable print nozzles 18 of a print head 14. Each print head 14 includes 128 such groups 42. Figure 1, a simplified schematic diagram of a group 42 for one of the print heads 14 is shown. The group 42 of nozzles 18 is configured to print ink over a width of 0.33 mm. Therefore, the width of the column 38 is also 0.33 mm.

[0052] After printing, the substrate 22 with the calibration pattern 30 and the two-dimensional pattern 32 passes by the camera 24 .

[0053] In a fourth step, the camera 24 records the calibration pattern 30 and the red, green and blue (RGB) color values ​​for each nominal print density in each column 38 of the two-dimensional pattern 32. This results in 44x3 recorded discrete color values ​​for each column 38.

[0054] By analyzing the recording of the calibration pattern 30, it is possible to discover which portion of the two-dimensional pattern 32 was printed with which group 42 of nozzles 18. In particular, recording the calibration pattern 30 allows associating the columns 38 with the group 42 of printing nozzles 18 that printed the corresponding columns 38.

[0055] In a fifth step of the method, control unit 28 derives a correction curve 44 for each column 38 from the recorded color values.

[0056] The correction curve 44 may be derived from a single recorded color, such as the green value recorded by the corresponding pixel row 26 of the camera 24 .

[0057] In the embodiment, the cyan printed pattern 32 results in a clear camera signal for the red camera channel. Therefore, using the red values ​​is sufficient to calculate the correction curve 44. In the described example, the green and blue values ​​recorded by the camera 24 are disregarded.

[0058] Of course, for other printing inks and / or applications, the correction curve 44 may be calculated using the color values ​​of the green and / or blue records or a linear combination of the RGB color values ​​(α*R+β*G+γ*B). For example, for a given ink, we may consider (α, β, γ) to be equal to the absolute difference between the component-dependent RGB0 readings of the substrate without ink coverage and the RGB100 readings of the color patch with full ink coverage (i.e., α = |R0-R100|, β = |G0-G100|, γ = |B0-B100|). We may preferably normalize the values ​​of the (α, β, γ) vector so that its Euclidean norm is equal to one. The advantage of this example is that it emphasizes the color components that carry most of the information and reduces the color components that only contribute noise to the calculation.

[0059] Generally speaking, when we refer to a recorded color or color value, we mean the channel of the camera used to record the print. It can also be the (only) channel of a monochrome camera, or a mathematical combination of the channels of a multispectral device.

[0060] In the depicted example, the correction curve 44 is derived by creating an interpolation of the 44 red values ​​recorded in each column 38 .

[0061] Figure 2 The interpolated red value (camera red channel signal [aU]) on the y-axis 46 is schematically shown over the nominal print density (ink coverage [%]) on the x-axis 48 .

[0062] about Figure 2 , it is desirable to achieve a high degree of congruence between the depicted curves 44, as this will ensure uniform color density. Therefore, the interpolated red values ​​may be used as the correction curve 44. Alternatively, the correction curve 44 may be derived from a comparison of the interpolated red values ​​of the different columns 38. In another variation, the correction curve 44 may be obtained from a comparison between the recorded color values ​​and the corresponding nominal print density.

[0063] To use curve 44 as a correction curve, for each desired output (vertical axis), the correction consists of finding the associated input value on the X axis. In practice, we build a lookup table for each correction curve.

[0064] In other words, for each of the 44 print density areas 36 in each column 38, the nominal print density (expected ink coverage) is compared to the recorded color value associated with the actual ink coverage. In a sixth step, the print signal is corrected using the correction curve 44. In an embodiment, the correction involves a change in the voltage pulses applied to the piezoelectric elements of the print nozzle groups 42 so that the amount of cyan ink ejected from each nozzle group 42 is uniform across the print width. In other words, ink output differences between groups 42 of nozzles 18 are corrected.

[0065] After improving the color density consistency of the cyan ink, the process can be repeated for the magenta, yellow, and black inks.

[0066] Furthermore, it is also conceivable to repeat the above steps one to six frequently for the same ink material to ensure high print quality in extended printing applications.

[0067] Alternatively or additionally, this step may also be repeated when printing parameters, such as printing speed or ink batch, or ambient temperature or humidity, vary.

Claims

1. A method for improving the consistency of color density across the print width of an inkjet printer (10), comprising the following steps: - printing a two-dimensional pattern (32), wherein the two-dimensional pattern (32) has a constant nominal print density over a print width and a varying nominal print density in a print direction; - dividing the two-dimensional pattern (32) into a plurality of columns (38) over the printing width; - recording with a camera (24) at least one color value for each nominal print density in each of said columns (38); - deriving a correction curve (44) for each column (38) from the recorded color values; and - Correcting the print signal and / or the print pattern using the correction curve (44).

2. The method according to claim 1 further comprises the step of printing a position calibration pattern (30) adapted to associate a print nozzle (18) or a group (42) of print nozzles (18) with a column (38) printed from the print nozzle (18) or the group (42) of print nozzles (18).

3. A method according to claim 1 or 2, wherein the correction curve (44) is derived for each column (38) from a comparison between the recorded color values ​​and the corresponding nominal print density.

4. A method according to any of the preceding claims, wherein red, green and blue (RGB) color values ​​are recorded for each nominal print density in each of the columns (38), and wherein the correction curve (44) is derived for each of the columns (38) from one of the RGB color values, or a linear combination of the RGB color values, in particular their sum.

5. A method according to any of the preceding claims, wherein each of the columns (38) corresponds to a group (42) of individually addressable printing nozzles (18) of a print head (14), in particular a group (42) of 16 nozzles (18), wherein the print head (14) comprises a plurality of such groups (42), in particular 128.

6. A method according to any one of the preceding claims, wherein the nominal print density describes the expected ink coverage on the substrate (22) from 0% to 100%.

7. A method according to claim 6, wherein the two-dimensional pattern (32) is printed by subsequently applying ink having N discrete nominal print densities to the web (20) and / or substrate (22), the densities starting from 0% and ending at 100% with constant percentage steps in between, wherein N is between 20 and 65.

8. The method according to any of the preceding claims, wherein at least the steps of the method according to claim 1 are repeated periodically and / or when printing parameters, in particular printing speed or ink batches, or ambient temperature or humidity, are changed.

9. Method according to any of the preceding claims, wherein the two-dimensional pattern (32) is printed on a substrate (22) for production.

10. An inkjet printer comprising: at least one print head (14) provided with a plurality of nozzles (18) and adapted to print ink; a camera (24) located downstream of the at least one print head (14) and adapted to record at least one color value of a pattern printed by the ink over the entire printing width of the at least one print head (14); as well as A control unit (28) configured to perform a method according to any one of the preceding claims.

11. Inkjet printer according to claim 10, wherein the camera (24) has three pixel rows (26) in the printing direction, each row (26) being suitable for recording a different color value, in particular RGB.

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