Method for improving consistency of color density across the printing width and inkjet printer

Abstract

The present invention relates to a method for improving color density consistency across the printing width of an inkjet printer (10), the method comprising the steps of: printing a two-dimensional pattern (32), the two-dimensional pattern (32) having the same nominal print density across the printing width and different nominal print densities in the printing direction; dividing the two-dimensional pattern (32) into a plurality of columns (38) across the printing width; recording at least one color value for each nominal print density in each of the columns (38) with a camera (24); 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 present invention further relates to an inkjet printer (10).

Description

[Technical Field]

[0001] The present invention relates to a method for improving the consistency of color density across the swath of an inkjet printer. [Background technology]

[0002] Inkjet printers are typically used to digitally print on a variety of products such as labels, textiles, ceramic tiles, and many others by dispensing small droplets of ink through nozzles in a printhead.

[0003] To achieve consistent, high-quality printing, precise control of droplet ejection from each nozzle is necessary. Even slight variations in ink flow rate, ink temperature, or ink pressure can affect the droplet formation process and cause a nozzle to perform differently from adjacent nozzles. The resulting variations in droplet volume, shape, direction, or velocity across a printhead or multiple printheads can affect the print pattern, particularly the consistency of ink density printed on the substrate, and consequently, color density in multicolor inkjet printing. Summary of the Invention [Problem to be solved by the invention]

[0004] The object of the present invention is to improve print quality by correcting for variations in the drop formation process across the print width. [Means for solving the problem]

[0005] The objects of the present invention are achieved by a method for improving consistency of color density across the swath of an inkjet printer, the method comprising: printing a two-dimensional pattern, the two-dimensional pattern having the same nominal print density across the print width and a different nominal print density in the print direction; Dividing the two-dimensional pattern into a plurality of columns across the print width; recording with a camera at least one color value for each nominal print density in each of the rows; deriving a correction curve for each row from the recorded color values; correcting the printing signal and / or the printing pattern with a correction curve; Includes:

[0006] The method can be applied to piezoelectric inkjet printers and other drop-on-demand printers having single or multiple printheads, for example to improve print quality in a single-pass inkjet printer having multiple print stations configured to print multiple inks, particularly multiple colors, onto the web and / or substrate, through which the web and / or substrate passes sequentially.

[0007] For piezoelectric inkjet printers, correcting the print signal can include adjusting the amplitude, duration, and / or shape of the voltage pulses applied to the printhead piezoelectric elements of a single print nozzle or set of print nozzles. This adjustment can locally increase or decrease drop volume, shape, and velocity. In practice, the amplitude and shape of the voltage pulses are often set at the printhead level, with the timing and duration set on a nozzle-by-nozzle basis.

[0008] It is therefore possible to influence the drop formation process of individual printing nozzles or sets of printing nozzles and thus homogenise the drop formation process across the printing width.

[0009] Furthermore, all steps of the method can be performed inline during printing without user intervention, making the method time-saving, cost-saving, and less error-prone compared to manual adjustments.

[0010] In one embodiment, the method may include the further step of printing a position calibration pattern suitable for correlating a print nozzle or group of print nozzles with a column printed from the print nozzle or group of print nozzles.

[0011] The calibration pattern can be recorded by a camera to obtain position data. It is envisioned that the same camera that records the color values ​​can be used for this purpose. Alternatively, an additional calibration camera configured to record the calibration pattern can be used.

[0012] Since the distance from the substrate to the camera may vary across the print width, it is preferred to print and record a calibration pattern marking at least every 5 to 10 mm, more preferably every 0.3 to 2 mm. In particular, the calibration pattern may consist of small parallel lines that form a periodic pattern across the print width.

[0013] By printing and recording the calibration pattern, the position of the nozzle from which the pattern was ejected, and consequently the positions of all other nozzles relative to the substrate, can be determined, thus preventing incorrect assignment of printing nozzles or groups of printing nozzles to columns into which the printed pattern is divided.

[0014] In another embodiment, a correction curve for each row is derived from a comparison of the recorded color values ​​with the corresponding nominal print density. The nominal print density describes the intended ink coverage on the substrate, and the print signals for printing the two-dimensional pattern are assumed to be 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 present at a specific location on the substrate. By comparing this intended value with the actual value recorded by the camera, the deviation between the two can be determined, and appropriate correction measures can then be found to correct this deviation.

[0015] In one embodiment, a camera records red, green, and blue (RGB) color values ​​for each nominal print density in each of the rows. A correction curve for each of the rows is then derived from one of the RGB color values ​​or a linear combination, such as the sum, of the RGB color values.

[0016] For example, if a two-dimensional pattern is printed with a first ink (e.g., magenta ink) of a multi-color inkjet printer, the camera can record the RGB color values ​​for the magenta printed pattern. However, only one color is needed to derive the correction curve. Therefore, it is possible to use only the green color values ​​recorded by the camera and ignore the red and blue values. Alternatively, the RGB color values ​​(α * R+β * G+γ * Any linear combination of RGB values ​​(α = β = γ = 1) can be used to derive a correction curve for each column of the printed pattern. Depending on the application, a particular linear combination of RGB values ​​may provide a higher reproducibility of the method than others. Therefore, it is expected that the coefficients of the linear combination will be selected depending on the application, in particular the type of ink and substrate to be applied. Nevertheless, the method can also be applied using a grayscale (single-channel) camera.

[0017] In a further embodiment, each of the columns into which the print pattern is divided corresponds to an individually addressable group of print nozzles, specifically 16 nozzles of the print head, which is envisioned to include a plurality of groups, specifically 128 groups.

[0018] Dividing the nozzles into groups and correcting the print signal for each group reduces the computational complexity of the method compared to correcting a single nozzle. Furthermore, the accuracy and pixel size of the camera and the printhead drive electronics may limit the feasibility of single nozzle correction.

[0019] Furthermore, nominal print density is assumed to describe the intended ink coverage on the substrate from 0% to 100%.

[0020] Preferably, the two-dimensional pattern is printed by sequentially applying inks onto the web and / or substrate at individual nominal print densities. The nominal print densities can be between 0% and 100% in steps of 2.33%. This corresponds to 44 nominal print densities in all columns of the printing pattern. Such a division has been found to lead to a reasonable amount of calculation and a high quality of the corrected printing process. Of course, depending on the application, it is also possible to choose a finer or coarser subdivision of the nominal print density range.

[0021] In a further embodiment, the steps of the method are repeated periodically and / or when the printing parameters, in particular the printing speed or ink batch, or the ambient temperature or humidity, change. By repeating the method, high print quality can be achieved across various printing steps. Additionally, by linking the execution of the method to special events, such as a change in ink batch, the overall effort is reduced.

[0022] In a further variation of the method, the two-dimensional pattern is printed onto the substrate used in production, which reduces the overall effort of the method (no additional calibration substrate is required) and also allows for correction of substrate-specific effects (e.g., porous cardboard substrates), such as ink spreading and / or soaking.

[0023] The object of the present invention is also solved by an inkjet printer comprising at least one print head equipped with a plurality of nozzles and adapted to print ink, the inkjet printer comprising a camera located downstream of the at least one print head and adapted to record at least one color value of a pattern printed from the ink over the entire printing width of the at least one print head, and a control device configured to carry out the method according to the present invention.

[0024] The advantages described for this method also apply to inkjet printers.

[0025] In one embodiment, the inkjet printer camera has three rows of pixels in the print direction, each row adapted to record a different color, specifically the red, green, blue (RGB) color value, of the two-dimensional print pattern across the print width.

[0026] Compared to using multiple conventional cameras in parallel, using a single three-row camera may be less error prone and / or require less computational effort and setup costs.

[0027] Furthermore, the use of such an in-line camera allows for periodic recalibration without user intervention, and also allows for recording multiple samples with the same nominal print density to average out and / or correct for local substrate imperfections or waviness.

[0028] It should be noted that the method is applied at a predetermined density level in a series of processes that control the printing nozzles. Thus, the printing pattern includes numerical values, which are further processed by a dithering algorithm to obtain the signals input to each printing nozzle. Nevertheless, a density value is defined for each nozzle, and thus the dithering algorithm can be considered as a stochastic quantization of the density values. In other words, for the purposes of this disclosure, each nozzle can be considered to receive a density value as input, and the fact that this value is further processed by the dithering algorithm can be ignored.

[0029] Further advantages and features will become apparent from the following description of the invention and the accompanying drawings which show non-limiting exemplary embodiments of the invention. [Brief explanation of the drawings]

[0030] [Figure 1] 1 shows a schematic top view of an inkjet printer according to the present invention; [Figure 2] 10A and 10B show schematic diagrams of correction curves for multiple rows of a printed pattern; DETAILED DESCRIPTION OF THE INVENTION

[0031] 1 shows a schematic representation of an embodiment of a single-pass inkjet printer 10 having four print stations 12. Each print station 12 is configured to print a different ink, e.g., cyan, magenta, yellow, and black ink.

[0032] In the described embodiment, each print station 12 includes six piezoelectric inkjet printheads 14 mounted on a printbar 16. The length of the printbar 16 defines the printing width of the inkjet printer 10.

[0033] Each print head 14 includes a plurality of nozzles 18. In the illustrated embodiment, the print heads 14 are Dimatix Samba print heads, with the plurality of print nozzles 18 arranged on a trapezoidal nozzle plate. In Figure 1, the nozzles 18 are shown schematically for only one of the print heads 14. Of course, other print heads 14 having different numbers and shapes of nozzles 18 can be used.

[0034] The inkjet printer 10 further includes a web 20 carrying a print substrate 22, such as a polymer foil or a packaging carton. The direction of movement of the web 20 relative to the print station 12 is indicated by an arrow in Figure 1 .

[0035] Inkjet printer 10 further includes a camera 24 located downstream of print station 12, specifically downstream of the print bar 16 being analyzed. Camera 24 has three rows of pixels 26 in the print direction. Each row of pixels 26 extends across the entire print width and is configured to record a different color of the printed pattern. Specifically, one of pixel rows 26 is configured to record a red color value R, one of pixel rows 26 is configured to record a green color value G, and one of pixel rows 26 is configured to record a blue color value B. Thus, camera 24 can record the red, green, and blue (RGB) color values ​​of the printed pattern across the entire print width.

[0036] Preferably, there is a single camera that analyzes multiple printbars 16 of a print station 12. Nevertheless, in systems where the ink dries between printbars 16, i.e., where the distance between printbars is large, a camera 24 can be positioned downstream of each printbar 16.

[0037] The inkjet printer 10 further includes a controller 28 configured to execute a program that causes the controller 28 to execute a method for improving color density consistency across the print swath.

[0038] In a first step of the method, one of the printing stations 12, for example the cyan printing station 12, prints a calibration pattern 30 on a printing substrate 22 to be used in production, for example paper, plastic foil, or cardboard. In an embodiment, the calibration pattern 30 comprises a plurality of straight lines extending in the printing direction. The lines are parallel and spaced apart by 1 mm from each other. Of course, this particular example is not limiting. Other calibration patterns 30 and / or line distances may also be used. Preferably, the distances are selected so that the resulting positioning accuracy is finer than the pixel resolution (i.e., the resulting positioning uncertainty is smaller than the distance between two adjacent pixels measured on the printing substrate).

[0039] In a second step of the method, the same print station 12 prints a two-dimensional pattern 32 onto the substrate 22. The two-dimensional pattern 32 is shown schematically in Figure 1. The pattern has the same nominal print density across the print width and a different nominal print density in the print direction.

[0040] In embodiments, the nominal print density describes the intended ink coverage on the substrate 22 from 0% to 100%.

[0041] The two-dimensional pattern 32 is created by sequentially applying ink having 44 distinct nominal print densities onto the substrate 22, starting from 0% and ending at 100%, with multiple steps of 2.33% in between.

[0042] In the figure, the boundaries between regions of different nominal print density are indicated by parallel lines 34 perpendicular to the printing direction. To simplify the drawing, only five regions 36 with distinct print densities are shown.

[0043] In the embodiment, the area 36 with the highest nominal print density corresponds to 100% cyan ink coverage on the substrate 22 and is located next to the calibration pattern 30 .

[0044] In a third step of the method, the two-dimensional pattern 32 is divided across the printing width into a number of columns 38. The columns 38 are assumed to be equal in size. The size may be defined, for example, by a fixed value stored in the controller 28 and / or a program. Alternatively, the size of the columns 38 may depend on the printing width and / or the resolution of the print head 14. In Figure 1, the columns 38 are indicated by dashed lines 40 extending in the web direction.

[0045] In an embodiment, each of the columns 38 is printed by an individually addressable group 42 of 16 print nozzles 18 of the print head 14. Each print head 14 includes 128 such groups 42. A simplified schematic of the groups 42 is shown for one of the print heads 14 in FIG. 1. The groups 42 of nozzles 18 are configured to print ink over a width of 0.33 mm. Thus, the width of the columns 38 is also 0.33 mm.

[0046] After printing, the substrate 22 with the calibration pattern 30 as well as the two-dimensional pattern 32 passes by the camera 24 .

[0047] In a fourth step, camera 24 records red, green, and blue (RGB) color values ​​for each nominal print density in calibration pattern 30 as well as in each of columns 38 of two-dimensional pattern 32. This results in 44 x 3 recorded individual color values ​​per column 38.

[0048] By analyzing the record of the calibration pattern 30, it is possible to determine which portions of the two-dimensional pattern 32 were printed by which groups 42 of nozzles 18. In particular, the record of the calibration pattern 30 can associate columns 38 with the groups 42 of printing nozzles 18 that printed the respective columns 38.

[0049] In a fifth step of the method, the controller 28 derives a correction curve 44 for each column 38 from the recorded color values.

[0050] The correction curve 44 can be derived from a single recorded color (eg, the green color value recorded by the corresponding pixel column 26 of the camera 24).

[0051] In an embodiment, the cyan print pattern 32 results in a clear camera signal in the red camera channel, so it is sufficient to use the red color values ​​to calculate the correction curve 44. The green and blue color values ​​recorded by the camera 24 are ignored in the described embodiment.

[0052] Of course, for other printing inks and / or applications, the green and / or blue recorded color values, or a linear combination of RGB color values ​​(α * R+β * G+γ * B) can be used to calculate the correction curve 44. For example, for a particular ink, (α,β,γ) can be considered to be equal to the absolute difference, component-wise, between the RGB0 reading of a substrate with no ink coverage and the RGB100 reading of a color patch with full ink coverage (i.e., α=|R0-R100|, β=|G0-G100|, γ=|B0-B100|). Preferably, the values ​​of the (α,β,γ) vector can be normalized so that its Euclidean norm is 1. This embodiment has the advantage of emphasizing the color components that carry the most information and reducing color components that would otherwise only introduce noise into the calculation.

[0053] Generally, recorded color or color value refers to the channel of the camera used to record the print, and this can be the (only) channel of a monochrome camera, or a mathematical combination of the channels of a multispectral device.

[0054] In the described embodiment, the correction curve 44 is derived by making an interpolation of the 44 recorded red values ​​in each column 38 .

[0055] FIG. 2 shows a schematic representation of the interpolated red color value (camera red color channel signal [aU]) on the y-axis 46 against the nominal print density (ink coverage [%]) on the x-axis 48 .

[0056] 2, it is desirable to achieve a high degree of fit between the plotted curves 44, since this will ensure uniform color density. Thus, the interpolated red values ​​can serve as the correction curve 44. Alternatively, the correction curve 44 can be derived from a comparison of the interpolated red values ​​of the different columns 38. In another variation, the correction curve 44 can be obtained from a comparison of the recorded color values ​​with the corresponding nominal print density.

[0057] 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 effect creating a look-up table for each correction curve.

[0058] In other words, for each of the 44 print density regions 36 in each column 38, the nominal print density (intended ink coverage) is compared to the recorded color values ​​that correlate to the actual ink coverage. In a sixth step, the print signal is corrected with a correction curve 44. In an embodiment, the correction involves modifying 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, any imbalance in ink output between the groups 42 of nozzles 18 is corrected.

[0059] After improving the color density consistency of the cyan ink, the method can be repeated for the magenta, yellow, and black inks.

[0060] Furthermore, it is envisioned that steps 1 through 6 above will be repeated frequently with the same ink to ensure high print quality over long printing applications.

[0061] Alternatively or additionally, each step can be repeated when printing parameters, such as print speed or ink batch, or ambient temperature or humidity, change. [Explanation of symbols]

[0062] 10. Inkjet printer 24 Camera 32 2D patterns 38 Multiple Columns

Claims

1. A method for improving consistency of color density across the swath of an inkjet printer (10), comprising: printing a two-dimensional pattern (32), said two-dimensional pattern (32) having the same nominal print density across the print width and a different nominal print density in the print direction; Dividing the two-dimensional pattern (32) into a plurality of rows (38) across the printing width; recording with a camera (24) at least one color value for each nominal print density in each of said rows (38); deriving a correction curve (44) for each of said columns (38) from said recorded color values; correcting the printing signal and / or the printing pattern with said correction curve (44); A method comprising:

2. 2. The method of claim 1, further comprising printing a position calibration pattern suitable for correlating a printing nozzle or group of printing nozzles with a printed column from the printing nozzle or group of printing nozzles.

3. 3. The method of claim 1, wherein the correction curve (44) for each of the columns (38) is derived from a comparison of the recorded color values ​​with the corresponding nominal print densities.

4. 4. The method of claim 1, wherein red, green, and blue (RGB) color values ​​are recorded for each nominal print density in each of the columns (38), and the correction curve (44) for each of the columns (38) is derived from one of the RGB color values ​​or a linear combination, in particular a sum, of the RGB color values.

5. 5. The method of claim 1, wherein each of the rows corresponds to a printing nozzle of a print head, specifically an individually addressable group of 16 nozzles, and wherein the print head includes a plurality of groups, specifically 128 groups.

6. The method of any one of claims 1 to 5, wherein the nominal print density describes an intended ink coverage on a substrate (22) from 0% to 100%.

7. 7. The method of claim 6, wherein the two-dimensional pattern (32) is printed by sequentially applying inks having N distinct nominal print densities onto the web (20) and / or substrate (22), the densities starting from 0% and ending at 100% with percentage steps in between, and N is comprised between 20 and 65.

8. 8. The method according to claim 1, wherein at least 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.

9. 9. The method according to any one of claims 1 to 8, wherein the two-dimensional pattern (32) is printed onto a substrate (22) used for production.

10. at least one print head (14) having a plurality of nozzles (18) and adapted to print ink; a camera (24) located downstream of said at least one print head (14) and adapted to record at least one color value of a pattern printed from said ink across the entire printing width of said at least one print head (14); A control device (28) configured to carry out the method according to any one of claims 1 to 9; An inkjet printer comprising:

11. 11. An inkjet printer according to claim 10, wherein the camera (24) has three rows of pixels (26) in the printing direction, each row (26) adapted to record a different color value, in particular RGB.

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