JP6416432B1 - Ink density error correction method in ink jet printing apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the speed for high productivity and the accompanying enlargement in an inkjet printing apparatus for industrial use, but drawing is caused by a plurality of factors caused by the enlargement and enlargement. It has been a problem that the image quality is significantly reduced. An ink density error for each individual inkjet head is corrected by adjusting the use conditions of the inkjet head, and then a print image is converted based on correction data calculated based on a test pattern printed and read. It solves multiple causes of image quality deterioration at once, and can highly correct the ink density error of the printed matter that the color density of the original image data to be printed differs from the color density of the actual printed matter. In an inkjet printing apparatus, high-quality printing with a high degree of perfection could be realized. [Selection] Figure 8

Description

Inkjet printing apparatus having a function of improving the quality of a printed image when printing on a predetermined substrate by a predetermined printing method using a predetermined apparatus equipped with a predetermined ink by a printing apparatus using an inkjet head The present invention also relates to a method for improving print image quality in an inkjet printing apparatus.

As is well known, inkjet technology is an on-demand non-contact printing technology that is used not only for printing on paper media but also for a wide range of applications such as decoration on building materials and automotive parts, and application of functional materials to electrical components. It has begun to be used in so-called industrial applications where inkjet printing devices are used for product production.

There are two major printing methods for inkjet. One is a so-called single-pass method that requires high speed. When printing, errors in the ink discharge amount and direction due to individual differences in the characteristics of each nozzle in the inkjet head, and individual differences in the characteristics of each inkjet head itself. The color density of the original image data to be printed differs from the actual color density of the printed matter, which is caused by the error in the ink ejection amount and ejection direction due to the There has been a problem that it is difficult to ensure print quality due to an ink density error. The other is a so-called multi-pass method that requires high print quality. Details will be described below. However, there is a problem that the printing speed is slow and not suitable for industrial use. There is a problem that a certain degree of ink density error occurs.

Individual inkjet heads used in industrial inkjet printers generally have a large number of nozzles inside each, and usually have more than 1000 to produce a larger printable area at a time. I am trying to improve my sex.

For example, in a certain ink jet head model, one ink jet head has 1024 ink ejection nozzles arranged in a line. An arbitrary image can be drawn on a two-dimensional plane by ejecting ink from a nozzle at a predetermined position while the head or the substrate is moved.

As described above, in an ink jet printing apparatus, an ink density error due to a plurality of factors occurs. Regarding the error caused by the ink discharge amount, discharge directionality, discharge speed, etc. of each nozzle in each ink jet head. As a prior art related to this countermeasure, Patent Document 1 proposes this countermeasure. As an outline, in order to correct errors such as the ink discharge amount, discharge directionality, and discharge speed of each nozzle in one inkjet head, a test pattern is printed and the test pattern is read by the imaging device. Ink density correction data is calculated based on the read image data, and image data to be printed is converted based on the correction data. This prior art is mainly used in a small-sized inkjet printing apparatus for home use.

JP 5-220977

However, in industrial inkjet printing apparatuses that have been developed in recent years, it is necessary to increase the speed for high productivity and the accompanying increase in size, and it is common to use multiple heads per color. . As described above, with the increase in size and scale of the inkjet printing apparatus, in addition to the error in the ink discharge amount and discharge direction for each nozzle in each inkjet head, the mutual inkjet heads when a plurality of inkjet heads are connected. Ink discharge amount and discharge direction error in between, increase in the number of connected inkjet heads, so that non-uniformity of ink density such as so-called streaks in the connecting part between multiple inkjet heads becomes more prominent, etc. There is a problem that the image quality of a drawn image is more significantly reduced due to an ink density error caused by a plurality of factors.

In addition, in inkjet printers for industrial use, so-called thermal heads are used to eject not only water-based inks printed on paper but also solvent-based inks, UV inks, and other functional inks such as insulators, conductors, and adhesives. In general, a piezoelectric inkjet head equipped with a piezoelectric element such as a piezoelectric element is generally used. Piezoelectric inkjet heads are more conspicuous than the thermal head due to structural reasons and manufacturing methods, and the ink density error due to errors in the ink discharge amount and discharge direction between individual inkjet heads is greater than that of thermal heads. As a countermeasure, there is an example in which an error in ink density is corrected by adjusting a voltage applied to a piezoelectric element. However, when high image quality is required, the ink density may not be sufficiently corrected only by adjusting the applied voltage. The problem is that the ink density error is caused by the ink density occurring in the portion showing the high color density, the portion showing the medium color density, and the portion showing the low color density in the color density in the printed image. Where the degree of error is different The adjustment of the voltage applied to the inkjet head can only be performed by a fixed value, so although it can be corrected by the applied voltage for some color density parts, the other color density parts are different at the same time This occurs because the applied voltage cannot be applied.

In addition, when an inkjet printing apparatus is used as a production facility in industrial applications, it is assumed that an inkjet printing apparatus will be added in accordance with an increase in the production amount of a production object. Therefore, even in the same type of inkjet printing apparatus of the same design, there is a problem that an error occurs in the color and ink density of the printed matter for each individual inkjet printing apparatus actually produced. This is particularly noticeable in industrial-use inkjet printing apparatuses that are becoming larger. Such a problem is based on the premise that the ink density error of a printed matter is corrected in a small-sized ink jet printing apparatus, and the method disclosed in Patent Document 1 on the premise of correcting the ink density error of one ink jet head, and others. According to the conventional invention, it is impossible to collectively solve the ink density error generated in the large-sized industrial use inkjet printing apparatus. The problem is that in multi-pass printing, as described below, when printing a line of pixels constituting an image, one line or an adjacent line is printed using different nozzles. The ink density error that occurs mainly due to individual differences in the ink can be reduced to a certain extent, but in single-pass inkjet printing devices that pursue high-speed printing, processing such as multi-pass printing is not possible. Will occur.

Further, as a printing method of the ink jet printing apparatus, although productivity is lowered, there is a multi-pass method in which the same portion is divided and printed in order to pursue high image quality. In this regard, even in the multi-pass method, in order to improve the productivity of printed matter with the aim of speeding up printing, the so-called number of passes, which is the number of times the same part is printed, It is conceivable to reduce as much as possible. However, even with this method, there is a problem that it is difficult to achieve both the image quality and the productivity because the image quality is eventually lowered by reducing the number of passes. Such a problem cannot be solved even if the invention disclosed in Patent Document 1 is directly applied because the relationship between the nozzles and the image data becomes complicated in the multi-pass method.

The present invention has been proposed in order to solve the above-described problems. In addition to the error in the ink discharge amount and discharge direction for each nozzle in each ink jet head, the present invention has a mutual relationship when a plurality of ink jet heads are connected. Multiple ink density and non-uniformity in ink density such as streaks at the joints between multiple inkjet heads due to an increase in the number of inkjet heads connected and the amount of ink ejected between inkjet heads The present invention proposes an ink jet printing apparatus having a function of all the ink density errors caused by the above factors and improving the print image quality more than the conventional method, and a method for improving the print image quality in the ink jet printing apparatus.

In order to solve the above problems, the following method is proposed.

That is, the present invention relates to an ink density error, which is the density of ink to be ejected by an inkjet head, in an inkjet printing apparatus that performs printing by relatively moving an inkjet head having a plurality of nozzles and a printing material. In this method, the ink density is adjusted individually by adjusting the use conditions of the inkjet head, the step of printing a test pattern, the step of imaging the test pattern, and the data of the captured test pattern And correcting the ink density error in the entire image printed on the substrate by converting the entire image to be printed based on the correction data.

In addition, the present invention provides an inkjet head group configured to have a width corresponding to the printing width by mounting two or more inkjet heads, and mounts at least one type of ink, and the substrate to be printed once. In an inkjet printing apparatus that is a single-pass method in which printing is completed only by passing through the nozzle surface side of the head group, the ink density, which is the density of the ink to be ejected by each inkjet head, is used as the use condition of the inkjet head Adjusting the data individually, adjusting the data individually based on the test pattern data, a means for printing a test pattern having the print width of the inkjet head group, a means for imaging the test pattern, By calculating means and converting the entire image to be printed based on the correction data And having a means for correcting the ink density errors in the entire image to be printed on.

Further, the present invention moves the substrate to be printed and the inkjet head relative to the basic unit which is the width to be printed by one movement scanning of the inkjet head in a direction parallel to the direction in which the nozzles of the inkjet head are arranged, Multipass equipped with one or more inkjet heads that completes printing by ejecting ink while moving the inkjet head and the substrate to be printed multiple times in a direction perpendicular to the direction in which the nozzles are arranged. In the inkjet printing apparatus according to the present invention, means for individually adjusting the ink density, which is the density of the ink to be ejected by each inkjet head, by adjusting the use conditions of the inkjet head, and two basic units Means for printing a test pattern having the above width, and hand for imaging the test pattern And correcting the ink density error of the entire image printed on the substrate by converting the entire image to be printed based on the correction data. It has the means.

According to the present invention, in a large-scale industrial inkjet printing apparatus, the ink density error of the nozzles in the inkjet head that cannot be corrected sufficiently by the conventional method, the ink density error between the individual inkjet heads constituting the inkjet head group, and the adjacent Ink density error at the connecting part of inkjet heads, ink density error due to individual differences between manufactured individual inkjet printers, and ink density error at adjacent parts of basic units described later in multi-pass printing , All can be corrected efficiently and highly at once, and high-quality printing can be realized in the single-pass method. Even in multi-pass printing, printing with a larger number of passes with a smaller number of passes as described later High quality printing equivalent to or better than can be achieved.

It is a figure explaining the general printing method in the printing of a single pass system. It is a figure explaining the ink density error correction produced between the some inkjet heads implemented by a prior art. It is a figure which shows the example of a measurement of the density data which read and measured the test pattern. It is a figure explaining the relationship between the target density value which should be made into the target in the case of ink density correction and correction. It is a figure which shows the example of a test pattern in the printing of a single pass system. It is a figure which shows the example of a measurement of the ink density data in the printing of a single pass system. It is a figure explaining the overlap process of an inkjet head. It is a flowchart of the work procedure of this invention in a single path | pass system. This is an example of ink density error measurement when the input color density levels are 9 levels in total. 6 is a graph obtained by cutting out and plotting ink density error data at n nozzles and peripheral nozzles. It is explanatory drawing of the logic which calculates correction data. It is a flowchart of the operation | work which converts image data based on the correction data produced in the single pass system. It is a figure which shows the example which converted the ink density error of n nozzles based on correction data. It is explanatory drawing of an example of a reduction gradation process. It is a figure explaining the general printing method of multipass printing. It is a figure which shows the example of a test pattern in the printing of a multipass system. It is a figure which shows the correspondence of the nozzle used for the example of the inkjet head which has 1000 nozzles, and printing, and the printed pixel. It is a flowchart of the work procedure of this invention in a multipath system. It is a flowchart of the operation | work which converts image data based on the correction data produced in the multipass system. It is a figure which shows the structural example of the inkjet printing apparatus using this invention.

Hereinafter, Example 1 will be described in detail with reference to the drawings.

FIG. 1 shows a typical printing configuration example in the single pass method. The ink jet head 1a, the ink jet head 1b, the ink jet head 1c, and the ink jet head 1d are arranged in a horizontal row in the nozzle row direction of the ink jet head to form an ink jet head group, thereby printing wider than the width of one ink jet head. Cover the area. The printing area on the substrate 2 is divided into the area 3 to the area 6, and the individual inkjet heads constituting the inkjet head group should be responsible for printing. The area 3 is the head 1a, the area 4 is the head 1b, and the area. 5 is configured to be in charge of the head 1c, and the region 6 is in charge of the head 1d. When the printing material 2 passes from the inkjet head 1a directly below the nozzle surface of the inkjet head 1d, the inkjet head 1a to the inkjet head 1d By discharging the ink, an image is printed on the printing material when the printing material passes. In FIG. 1, the inkjet heads 1a to 1d are arranged in a staggered manner, but any arrangement method other than the staggered arrangement may be used as long as they are arranged in a form that can cover the printing area, and constitute an inkjet head group. The number of inkjet heads can be added or reduced according to the width of the substrate. In FIG. 1, the individual adjacent inkjet heads are arranged so as to overlap each other so as to overlap each other. This will be described in detail below, but depending on the specifications of the inkjet heads used, An arrangement method that does not wrap can also be assumed.

FIG. 2 shows an example of correction according to the conventional method for an ink density error of a printed matter that occurs between a plurality of inkjet heads and that the color density of the original image data to be printed is different from the color density of the actual printed matter. 2A is a graph showing the ink density error before correction, and FIG. 2B is a graph showing the ink density error after correction. As in FIG. 1, the printing area on the substrate 2 is divided into the area 3 to the area 6. The area 3 is the inkjet head 1a, the area 4 is the inkjet head 1b, the area 5 is the inkjet head 1c, and the area 6 is the inkjet head 1d. Shows an example in charge of printing. Although the present invention can be used even when a thermal ink jet head is used, the ink jet head in the example shown here assumes a so-called piezo ink jet head that uses a piezoelectric element and ejects ink by a piezoelectric effect. . As described above, even if the same type of inkjet head is used, there are individual differences mainly due to the use of piezo elements. As shown in FIG. 2A, the ink density error in each of the inkjet head 1a to the inkjet head 1d is large. When an image is printed without correction, the ink density varies greatly from print area to print area, which adversely affects image quality. FIG. 2B shows an example in which the ink density error between a plurality of inkjet heads is corrected by adjusting the driving voltage of the piezo element, which is the use condition, for each inkjet head. Comparing FIG. 2A and FIG. 2B, a certain degree of correction can be made, but the range of the applied voltage that can be adjusted is limited, and as described above, the adjustment of the applied voltage is a fixed value. In addition, there is a problem in that different ink density errors that occur in different color density bands cannot be corrected collectively. Also, changing the drive voltage changes the discharge speed of the ejected ink droplets and causes a change in the landing position on the substrate, which affects the quality of the printed image. Problems are also expected.

FIG. 3 is a measurement example of ink density data obtained by printing a test pattern in the configuration shown in FIG. 1 and reading and measuring it. In this example, the dark color density data example is measured data 7, and the light color density data. An example is the measurement data 8, and the measurement data 7 and the areas 3 to 6 of the measurement data 8 correspond to the printing area of the substrate in FIG. In the example of FIG. 3, the ink density error in the inkjet head 1a is the largest, there is a little in the inkjet head 1b, and there is almost no in the inkjet head 1c and the inkjet head 1d. The average density in the inkjet head is an example in which only the inkjet head 1c is darker than the others.

Further, in this example, a so-called black streak appears in a band shape on an image printed with a high ink density at the joint between the inkjet heads 1a and 1b in FIG. A so-called white streak appears in a band-like shape on the printed image where the ink density at the adjacent joints between 1b and 1c and between 1c and 1d is low and the low ink density part is printed.
The normal measurement data 7 and the measurement data 8 are usually measured using the same ink-jet head even if the measurement data has different color densities. Are often different. Therefore, test patterns for different densities, which will be described separately, are used.

FIG. 4 is a graph showing color density measurement values and target density values for explaining the relationship between ink density correction and the target density value to be targeted in the correction. The horizontal axis indicates the position in the print width direction, and the vertical axis indicates the color density. The measurement data 10 indicates the measurement value of the color density of the test pattern. It can be seen that a certain position has a high color density value and a certain position has a low color density value. Correction data is created by correcting this so as to obtain a uniform color density. First, it is necessary to determine a reference value of the color density to be targeted. In the example shown in FIG. 4, the target density value (target density) 9 indicated by the solid line is an average value of the whole density. In this case, the correction data 11 will be described in detail below with respect to the measured value, but a numerical value is calculated by a predetermined method. However, when the outline is explained, the portion where the color density value is high is lowered. Correction data is generated, correction data is generated so that a portion having a low density value becomes high, and a portion having the same density as the target density is not corrected.

Here, the target density value (target density) can be set to a designated value such as 9a and 9b. As a case where this method is to be executed, a case where a plurality of printing apparatuses of the same model for printing using the same substrate and the same ink are installed in a production factory is assumed. In this case, it is necessary to print an image having the same color density value and color even when printing is performed using any of the installed printing apparatuses. For example, in printed materials such as building materials that are used side by side, all color density values need to be uniform. It becomes a situation where quality cannot be maintained. In order to avoid such a situation, it is necessary to correct the ink density so that all installed printing apparatuses have the same color density value. For example, when there are printing apparatuses to be installed from No. 1 to No. 5, the measured value of the newly installed No. 2 machine is the measurement data 10, and the color density measured by printing a test pattern on the No. 2 machine alone The correction value calculated from the average of the values is set as the correction data 11, and the corrected density value of the already installed first machine itself is set as the target density value (target density) 9a. When the color density value of the first machine is used as the reference value, the target density value of the second machine is set as the target density value (target density) 9a. In addition, if the density value after correction of the first machine is the target density value (target density) 9b, the target density value of the second machine may be set to the target density value (target density) 9b. Then, the correction value calculated from the average value of the overall density of the Unit 2 unit is further subjected to processing for matching with a predetermined target density at the same time, so that the ink density error generated in each inkjet head itself, the inkjet head group can be reduced. Not only the ink density error between the respective inkjet heads constituting each other and the density error in the adjacent portions of the inkjet heads constituting the inkjet head group, but also the ink density error caused by the individual difference between apparatuses can be corrected at a time.

FIG. 5 shows an example of a test pattern in single-pass printing. The circular mark 12a, the circular mark 12b, the circular mark 12c, and the circular mark 12d have functions for detecting a printing position, an imaging resolution, an image inclination, and the like, respectively. Of course, it is not necessary to use a circular shape as long as it is easy to use for computer processing. The imaging conditions and specific data processing are not directly related to the present invention and will not be described in detail. However, the test pattern imaging resolution needs to be equal to or higher than the resolution and spatial frequency to be corrected. Further, the dynamic range of the image pickup apparatus needs not to be saturated in a bright direction even in a portion where there is no printing, and not saturated in a dark direction even in a portion where the maximum density is present.

Further, the example of FIG. 5 has a 16-step color density pattern including a portion not printed. Although it is not necessarily limited to 16 patterns, the purpose of having a multi-stage color density pattern is that the occurrence of ink density errors that occur during printing differs depending on the color density. This is because it is necessary to perform ink density correction so that the density changes, and more accurate correction is possible by measuring each ink density error occurring at multiple color densities and generating correction data. At the same time, by setting a target density value for each of a plurality of color densities, correction with higher accuracy becomes possible.

FIG. 6 shows the measured values 13 and 14 as individual measured values in two color density regions in the measurement example of the test pattern of FIG. Here, the measured values at the printing width end portion 15a and the printing width end portion 15b both tend to decrease the color density value uniformly, but this is caused by an optical blur phenomenon of the imaging device. As a result of the blur phenomenon, the color density value at the end of the print width cannot be measured accurately. This is unavoidable no matter how expensive the imaging device is used. Note that the same phenomenon occurs even when a test pattern used in a multi-pass inkjet printing apparatus shown in FIG. In this case, the optical problem of the imaging apparatus can be avoided by applying an average process in the vicinity of the end to the end of the test pattern in FIG. In addition, when the test pattern in the case of the multi-pass method shown in FIG. 16 is imaged to calculate the ink density error, different processing is performed. Details of this point will be described later.

FIG. 7 is an explanatory diagram regarding the overlap processing of the inkjet head. In FIG. 7, two adjacent ink jet heads overlap each other by four nozzles and overlap each other. In FIG. 7, “x” indicates a pixel in charge of printing with the left inkjet head, and “□” in FIG. 7 indicates a pixel in charge of printing with the right inkjet head. In the printing for the width of the four overlapping nozzles, the right ink jet head and the left ink jet head are alternately responsible for printing, thereby reducing the ink density error in the overlap portion. In this process, after reducing the overall density difference, the density correction means described so far is performed, so that image quality can be improved with higher accuracy.

Based on the above, an embodiment of the present invention in single-pass printing will be described as Embodiment 1 which is one of the best embodiments of the present invention.

Hereinafter, Example 1 is demonstrated in detail using FIGS. 8-13. First, FIG. 8 shows a procedure for creating ink density error correction data in the single pass method, and FIG. 12 shows a work procedure flow for converting image data based on the correction data created for the image data to be printed.

First, creation of ink density error correction data will be described with reference to FIG. First, in the use condition adjustment step 31, a process for correcting an ink density error is performed on each of a plurality of inkjet heads constituting the inkjet head group by adjusting the applied voltage, which is a conventional method. The use condition adjustment step 31 corresponds to the processing described with reference to FIG. Next, in an overlap processing step 32, an overlap process is performed on the connecting portions of the inkjet heads that constitute the inkjet head group and overlap each other. The specific contents of the overlap processing correspond to the processing described in FIG. By the processing up to step 2, the ink density error between the inkjet heads constituting the inkjet group and the ink density error at the connecting portion are improved to some extent, but the image quality required as a product is insufficient. It becomes.

Next, in the test pattern preparation step 33, the test pattern image to be printed described with reference to FIG. 5 is prepared, and the print gamma correction step 34 performs print gamma correction on the test pattern image. The printer gamma correction, as will be described in detail later, refers to a means for correcting the change in the actual color density of the printed material so that it is linear for each density for the image data to be printed.

Thereafter, in a test pattern printing process 35, a test pattern for the entire printable width formed by the inkjet head group is printed. In this embodiment, the test pattern to be printed is shown in FIG. Also, the width of the test pattern to be printed is as described above. The ink density error of the nozzles in one ink jet head, the ink density error between individual ink jet heads constituting the ink jet head group, and the connecting part of adjacent ink jet heads From the viewpoint of correcting all of the ink density errors caused by the individual ink density errors and the individual differences between the manufactured individual inkjet printing apparatuses, it is desirable that the printable width is the full width.

Then, in the test pattern imaging step 36, the printed test pattern is imaged. The image pickup means may be a scanner or other image pickup means, but it is necessary to pick up the entire test pattern with a required accuracy such as a predetermined resolution. In addition, by applying the average processing for setting the color density in the vicinity of the print width end portion of the imaged test pattern image shown in FIG. 6 at this time, the optical problem of the image pickup device is avoided, This can contribute to the creation of correction data with higher accuracy.

Finally, in the correction data calculation step 37, the color density of the image of the captured test pattern is measured to calculate the ink density error, and ink density error correction data is created using the calculated numerical value.

Here, a specific method for calculating the ink density error correction data will be described with reference to FIGS.

FIG. 9 is a measurement example of ink density data different from that in FIG. 3, and is an example of ink density measurement in the case where there are a total of nine color density levels input for printing. The color density range of the input data is one of 256 levels from 0 to 255, and the higher the number, the higher the color density. In FIG. 9, the input data 255 is the highest color density, and the color density becomes lower as it decreases to 224, 192, 160, 128, 96, 64, 32, 0 below. Since 0 is not printed, it is the density of the base material, and the ink density is 0. The horizontal axis represents the position of the image in the print width direction as described above, in other words, the position of the nozzle. The position of n nozzles is shown on the horizontal axis of FIG. The vertical axis represents the density, and in the figure, the density at the time of the input data 255 is shown as the highest density, and is described according to the input data below. In this example, it is assumed that the image is configured with 8 bits, but it is also possible that the image is configured with a different number of bits such as 16 bits. FIG. 9 shows the density of each nozzle, here, the density of all nozzles and all areas in nine stages. This is the overall image of the measurement data. In FIG. 9, the measurement data is the ink density in the color density band of nine input data. However, in the case of 8-bit data, 256 points of data are necessary, and these data are estimated and created by a method such as linear interpolation. Things are also possible.

FIG. 10 shows a graph obtained by cutting out and plotting ink density error data in the n nozzle of FIG. 9 and the surrounding nozzles. In FIG. 10, the ink density error data of three nozzles, that is, the n + 1 nozzle and the adjacent n + 1 nozzle and n + 2 nozzle are plotted. The position may be grasped by the nozzle number or the position number of the image. FIG. 10 shows the ink density state at the position printed by the n nozzles in the print area. In the example of FIG. 10, the density of the n + 2 nozzle is light and the n nozzle is the darkest. For example, when printing a 500 mm width at a resolution of 600 dpi, a head width of 500 / 25.5 × 600 = 111811 nozzles is required, and 11811 ink density error data for one nozzle shown in the graph of FIG. 10 are measured. Will be. However, from the viewpoint of increasing the processing speed, two graphs or more can be used as one graph, and from the viewpoint of improving resolution, one nozzle can be used as two graphs or more. In the case of 2 nozzles 1 graph, there are 5905 graphs, and in the case of 1 nozzle 2 graphs, there are 23622 graphs.

FIG. 11 illustrates the logic for calculating the correction data. n nozzle data and correction target density target density value (target density) data are shown. When converting the measurement data of the n nozzles into the target density value, it can be seen from FIG. 11 that the ink density output from the n nozzles at the time of the input data Di is P1 and the target density value is P2. Therefore, in order to output the ink density of P2 using n nozzles, it is necessary to set the input data to Do. Therefore, by creating a table for converting Di to Dо, it is possible to output the ink density of P2 from the n nozzles.

By performing the above steps, not only the ink density error that occurs in each inkjet head itself that constitutes the inkjet head group, but also the ink density error between the individual inkjet heads that constitute the inkjet head group, the adjacent inkjet heads Ink density error correction data that can collectively correct all of the ink density errors caused by the ink density error in the connecting portion and the individual differences between the manufactured individual inkjet printing apparatuses can be obtained by executing the procedure once. Can be calculated.

Next, based on FIG. 12, the image data conversion operation will be described based on the correction data created for the image data to be printed.

First, in step 38, image data to be printed is prepared. In step 39, the RGB image is converted into a CMYK image and color profile conversion is performed on the image data. Then, in step 40, print gamma conversion is performed on the image data. Run against.

Here, an example of image data processing performed between Step 38 and Step 40 will be described in detail.

In this example, the image data to be printed is RGB multi-value data before the start of processing, and each color is usually composed of 8 bits. This is merely an example, and other gradations such as 16 bits and 12 bits are assumed, and different data formats such as palettes are also assumed. The image data is converted into multi-value data (usually 8 bits) of ink color CMYK through a color profile. If the printing device is a different model, the mounted ink and other functions are different, so there is a problem that even if the same image data is printed, there is a problem that the color will be different. Thus, color reproducibility independent of the printing apparatus is realized.

Printer gamma conversion is a means for correcting image data to be printed so that the change in color density of the actual printed matter is linear, and the actual state is often table conversion. In addition, since the ink density error correction according to the present invention is often performed using a plurality of table conversions, it is assumed that the table for printer gamma conversion and the table for ink density error correction are combined and performed in one table. it can. By using one table, the amount of information to be processed can be minimized, and the data processing speed can be increased.

After the above steps 40 are completed, in the image conversion (ink density error correction) step 41, the entire print image is converted using the ink density error correction data created in the correction data calculation step 37. Here, an example of converting an image based on the calculated ink density error correction data will be described in detail. FIG. 13 shows an example in which an image is converted in 256 steps of input data for an ink density error of n nozzles. Plotting the ink density error correction data of all density bands from 0 to 255 calculated based on the calculation method shown in FIG. 11 gives an example as shown in FIG. Therefore, if the color density of the image to be printed is converted with the corrected output data plotted in FIG. 13, the n nozzle can perform printing with the same ink density as the target density value. By performing this process for all nozzles or all position data, it is possible to correct ink density errors in all areas and all color density areas. If the density error correction in the image conversion (ink density error correction) step 41 is executed before executing the step 38 to the step 40 in FIG. 12, the density after the correction is changed by the conversion process from the step 38 to the step 40 after all. Therefore, it is desirable that the image conversion (ink density error correction) step 41 is executed after step 40 is completed.

Finally, the gradation adjustment processing is executed on the print image corrected as the gradation adjustment processing step 42 in FIG. Here, the gradation adjustment process will be described in detail. FIG. 14 is an explanatory diagram of an example of the gradation reduction process. In general, image data is 8 bits and has a density gradation of 256, but the gradation that can be realized with an inkjet head is not ejected, for example, in the case of a model that can eject droplets of three sizes, small droplets, medium droplets, and large droplets Since there are only 4 gradations including the case, it is necessary to reduce the gradation process of the image data of the density gradation of 256 gradations to 4 gradations. The horizontal axis of FIG. 14 indicates input data, and has 256 gradations from 0 to 255. The vertical axis represents the printing rate of each droplet. 100% indicates that all dots are printed in a predetermined area. In the case of 50%, dots are printed in 50% of the predetermined area. It shows the state. In FIG. 14, while the input data is 0-a, only the droplets change from 0% to 100%. While the input data is a-b, the droplet printing rate changes from 100% to 0%, and the medium droplet printing rate changes from 0% to 100%. While the input data is b-255, the medium drop printing rate changes from 100% to 0%, and the large drop printing rate changes from 0% to 100%. That is, it can be said that the printing rate of 100% for the small droplets corresponds to the input data a, the printing rate of 100% for the medium droplets corresponds to the input data b, and the printing rate of 100% for the large droplets corresponds to the input data 255. Of course, this relationship is due to the characteristics of the base material and ink. If 100% large droplets are used, the printed ink density may be too dark. In this case, the large droplets are adjusted not to use 100%. In some cases.

After performing all the above procedures, the corrected print image data is printed by the ink jet printing apparatus, so that the ink density error of the nozzles of the individual ink jet heads constituting the ink jet head group, and the individual constituting the ink jet head group Ink density errors between inkjet heads, ink density errors at the joints of adjacent inkjet heads, and ink density errors caused by individual differences between individual manufactured inkjet printers can be highly corrected collectively. As a result, high-quality printing can be realized.

Hereinafter, Example 2 will be described in detail with reference to the drawings.

FIG. 15 shows a general print configuration example in normal multi-pass printing. As described above, the multi-pass method is a method in which the same portion is divided and printed multiple times. In FIG. 15, the first scan 16 represents the first drawing scan, and ink is ejected while moving in the X direction indicated by the arrow, and this operation is repeated with the second scan 17 and the third scan 18. It is. This example is a method called three-pass printing in which the same portion is divided into three, and shows a method in which a part of the print image is completed by three times of overlapping printing from the first scan 16 to the third scan 18. In the first scan 16, the first drawing is performed using the lower third portion of the inkjet head shown in FIG. Usually, about one third of the final density, which is the ink density of a printed image that is finally finally completed, is printed in one scan.

The second scan 17 represents the second drawing scan, and printing is performed using the lower two thirds of the head. Similar to the case of the first scan 16, the portion that is printed twice and overlapped is about two thirds of the final density, and the lower half is about one third of the final density.

The third scan 18 indicates the third drawing scan, and printing is performed using the entire inkjet head. The three times overprinted portion becomes the final image that has achieved the final density, and the printing of that portion is completed. After that, as in the fourth scan 19 and the fifth scan 20, the multi-pass printing forms the entire image by repeating the drawing scan. Here, the relationship between the printing medium and the head is relative, the head may move in the X direction indicated by the arrow, and the printing medium may move in the Y direction.

In FIG. 15, an image of one third of the length of the inkjet head is printed by one scan, but the basic unit in the multi-pass printing in the present invention is printed by one scan. Indicates the length of the image to be performed. In this example, the length of one-third of the inkjet head is the basic unit. Of course, if the number of printing divisions is increased, the basic unit becomes relatively short, and if the number of divisions is reduced, the basic unit becomes relatively long. Further, the basic unit is not necessarily limited to a fraction of the length of the inkjet head in the nozzle row direction.

FIG. 16 shows an example of a test pattern corresponding to FIG. The printing width has more than twice the basic unit of printing. FIG. 16A shows an example in which the printing width is twice the basic unit, and FIG. 16B shows an example in which the printing width is five times the basic unit. In FIG. 15, it is assumed that the head moves in the X direction and the printed material moves in the Y direction. There may be a whitish or dark streak between the Nth scan and the (n + 1) th scan. This may be due to physical phenomena due to errors in the wetting and spreading of ink on the base material due to the accuracy of the amount of movement of the substrate and the physical effects of the ink and the base material at the boundary of the basic unit. This is described by the term banding in the ink jet printer industry. In the present invention, banding is also an ink density error that should be solved. In this regard, in FIG. 16A, banding may occur by inserting two basic units of printing and printing the test pattern so that the boundary between scans is included in the test pattern. An area is secured. In FIG. 16B, five basic units of printing are inserted to further increase the amount of information. Increasing the basic unit amount enables more detailed ink density error measurement by increasing the information amount, but there is a problem that the information load increases and the processing load increases.

Note that the processing of the blur phenomenon at the print width end portion of the captured test pattern image described in the description of FIG. 6 above when the ink density error is calculated by imaging the multi-pass test pattern of FIG. explain. In the case of the multi-pass method, the presence of the ink density error in the connecting portion of the basic unit to be printed directly leads to the occurrence of the above banding, so that more accurate calculation of the ink density error is necessary. Part processing is particularly important. Therefore, in this case, the numerical value of the ink density error calculated from the both ends of the adjacent portion of the basic unit printed in the captured test pattern is applied to the print width end. That is, in the example of FIG. 16A, printing in each basic unit is performed by dividing the nozzles to be used by using the same inkjet head in all passes by the same dividing method. The state of occurrence of ink density error within a unit is theoretically the same or approximate. Therefore, in the example of FIG. 16A, the numerical value of the upper end of the lower basic unit in contact with the basic unit adjacent portion of FIG. Apply. Also, the numerical value at the lower end of the upper basic unit in contact with the basic unit adjacent portion in FIG. 16A is applied to the blur phenomenon occurrence portion at the lowermost printing width end portion in FIG. By this process, it is theoretically possible to execute a more accurate process at the end of the print width.

Here, imaging conditions in the case of imaging the test pattern of FIG. 16 will be described in detail. In multi-pass printing, the resolution of an image printed on a substrate is not uniform. In addition, the resolution of the captured test pattern depends on the banding problem peculiar to multipass printing and the necessity of matching the nozzle used and the image to be printed. It is necessary to match the resolution with the resolution of the captured test pattern image. Therefore, when imaging a test pattern, it is necessary to set conditions for matching the resolution with the resolution of the printed test pattern.

With reference to FIG. 17, it will be described that the positional relationship between the inkjet nozzle and the print image is fixed in multi-pass printing. The ink jet head 1 in FIG. 17 shows an ink jet head having 1000 nozzles, in which nozzles that eject inks assigned with numbers are shown. Assuming that 4-pass multi-pass printing is performed using this inkjet head, printing in each pass starts from the nozzle 201b, the nozzle number 1 to 250 section starting from the nozzle 201a in the drawing. It is divided into four sections, a section with nozzle numbers 251 to 500, a section with nozzle numbers 501 to 750 starting from nozzle 201c, and a section with nozzle numbers 751 to 1000 starting with nozzle 201d. .

FIGS. 17A and 17B are diagrams showing the correspondence between the nozzles used for printing and the printed pixels. The image data is shown on the upper left basis, and the numbers of the nozzles 201 and the pixels 204 are used. The correspondence relationship between the printed pixels and the nozzles used for printing is shown. FIG. 17 shows an example of two types of four-pass printing shown in FIGS. 17A and 17B although the same four-pass printing is performed. FIG. 17A shows four passes in which the resolution in the nozzle row direction is quadrupled. One pixel line is printed by one nozzle, and FIG. 17B doubles the resolution in the nozzle direction. One pixel line is printed by two nozzles. Details of these effects are not directly related to the present invention, and the description thereof is omitted.

In the example of FIG. 17A, the first line at the left writing position is the first nozzle, the second line is the 251 nozzle, the third line is the 501 nozzle, and the fourth line is the 751 nozzle. The number of pixels constituting the image corresponds to the nozzle number. In the example of FIG. 17B, the first line which is the left writing position is nozzles 1 and 501; the second line is nozzles 251 and 751; the third line is nozzle 2 and 502; As in the case of the number nozzle, the numbers of the pixels constituting the image correspond to the nozzle numbers.

In this way, the positional relationship between the image and the nozzle number can be made constant. In other words, this method makes it possible to make constant the relationship between the ink density error caused by the nozzles, the connecting portions of adjacent inkjet heads, and the positional relationship between the pixels constituting the image.

Note that in the case of single-pass printing, as described above, when the printing material passes directly under the nozzle surface of the inkjet head, ink is ejected from the nozzles so that an image is printed on the printing material. In this example, the first pixel line is printed by the first nozzle, and the resolution is dependent on the nozzle of the inkjet head. This means that the pixel line is printed by the second nozzle, and as a result, the positional relationship between the inkjet nozzle and the print image is automatically fixed.

Based on the above, an embodiment of the present invention in multipass printing will be described as Embodiment 2 which is one of the best embodiments of the present invention.

FIG. 18 is a work procedure flow of the present invention in the multipath method, and will be described based on this. FIG. 18 shows a procedure for creating ink density error correction data, and FIG. 19 shows a work procedure flow for converting image data based on correction data created for image data to be printed. In addition, since it is not fundamentally different from the procedure in the case of the single path method already described in the first embodiment, common parts are appropriately omitted.

First, creation of ink density error correction data will be described with reference to FIG. First, in the use condition adjustment step 51, the applied voltage, which is a conventional method, is adjusted to perform the process of correcting the ink density error for each of the individual inkjet heads constituting the inkjet head group, and then the overlap processing step At 52, an overlapping process is performed on the connecting portion of the inkjet head. Note that the overlap processing step 52 is necessary only when the inkjet heads overlap as described with reference to FIG.

Next, in the test pattern preparation step 53, the test pattern image described with reference to FIG. 16 is prepared, and the print gamma correction step 54 performs print gamma correction on the test pattern image.
Thereafter, in a test pattern printing step 55, a test pattern for the entire printable width formed by the inkjet head group is printed. Further, the width of the test pattern to be printed is preferably at least two basic units as described in FIG.

Then, in the test pattern imaging step 56, the printed test pattern is imaged. At this time, it is important to calculate a more accurate ink density error by applying the process for the blur phenomenon to the print width end portion of the captured test pattern image described above.

Finally, in the correction data calculation step 57, the color density of the image of the captured test pattern is measured to calculate the ink density error, and ink density error correction data is created using the calculated numerical value. The specific correction data calculation method is basically the same as the ink density error correction data calculation in the case of single-pass printing, but the test pattern is also imaged collectively for the banded portion. Therefore, the correction data for the banding part can be calculated at once.

By executing the above procedure, not only the ink density error that occurs in each inkjet head itself, but also the banding that occurs at the boundary of the basic unit, or the individual inkjet heads that make up the inkjet head group when multiple inkjet heads are used Ink density error that can be corrected all at once from ink density error between each other, ink density error at the connecting part of adjacent inkjet heads and individual difference between manufactured individual inkjet printers The correction data can be calculated by executing the procedure once.

Next, based on FIG. 19, the image data conversion operation based on the correction data created for the image data to be printed will be described.

In step 58, image data to be printed is prepared. In step 59, the RGB image is converted into a CMYK image and the color profile is converted. In step 60, print gamma conversion is performed on the image data. And execute.

After the above steps 60 are completed, in the image conversion (ink density error correction) step 61, the entire print image is converted using the ink density error correction data created in the correction data calculation step 57. Note that if the density error correction in the image conversion (ink density error correction) step 61 is executed before executing the step 58 to the step 60, the density after correction is distorted by the conversion process from the step 58 to the step 60 after all. Therefore, it is desirable that the image conversion (ink density error correction) step 61 be executed after step 60 is completed, as in the first embodiment. Finally, the gradation adjustment process is executed on the print image corrected in the gradation adjustment process step 62.

After performing all of the above steps, the corrected print image data is printed by the inkjet printing apparatus, so that not only the ink density error that occurs in each inkjet head itself, but also the banding that occurs at the boundary of the basic unit or multiple In the case of using an inkjet head of this type, it results from the mutual ink density error between the individual inkjet heads constituting the inkjet head group, the ink density error at the connecting part of the adjacent inkjet heads, and the individual difference between the manufactured individual inkjet printing apparatuses. All of the ink density errors can be corrected together, and as a result, high-quality printing can be realized.

FIG. 20 shows a configuration example of the ink jet printing apparatus of the present invention. The ink jet printing apparatus in the configuration example includes an unwinding unit 101 for printing material, a printing unit 103, and a winding unit 105 for printing material. The printing unit 103 is equipped with an inkjet head, and this configuration example may be a single-pass method or a multi-pass method. A test pattern printed by the printing unit 103 is read by a scanner used as the imaging device 109 and processed by a computer. The imaging device 109 can be incorporated in an inkjet printing apparatus and automatically perform test pattern printing, test pattern imaging, correction data calculation, print data correction, and the like.

The effects described above are summarized as follows.

After correcting the ink density error, such as adjusting the applied voltage for each individual inkjet head, the print image is converted based on the correction data calculated based on the printed and read test pattern. On the other hand, the ink density can be corrected, and high-quality and high-quality printing that cannot be realized by the conventional correction method can be realized in the industrial inkjet printing apparatus.

After the correction by the overlap processing described later of the connection portion between adjacent inkjet heads, the print image correction is performed based on the correction data calculated based on the read test pattern, so that the degree of perfection in the industrial inkjet printing apparatus is improved. High image quality was achieved.

In a single-pass inkjet printing apparatus, a test pattern is printed using the entire printing width of the inkjet head group for each ink type, and image conversion is performed based on correction data calculated using the printed test pattern. Therefore, it is possible to correct all the ink density difference between the nozzles of the inkjet head, the ink density difference between the inkjet heads constituting the inkjet head group, and the ink density difference between the adjacent inkjet heads. It became possible.

By defining a reference value to be used as a reference in ink density correction and having means for switching a target value to be corrected for ink density to a reference value, it is possible to correct a density difference between apparatuses.

In a multi-pass inkjet printing apparatus, based on a test pattern having a unit width equal to or more than two basic units, which is the width to be printed by a single scanning scan of the inkjet head with the print image data and nozzle relationship fixed. By generating correction data, correction is possible even in the multi-pass method, and high-quality images can be printed with a smaller number of passes, so both high image quality and high-speed printing are possible. Became.

1 Inkjet head 2 Substrate 9 Target density value (target density)
DESCRIPTION OF SYMBOLS 10 Measurement data 11 Correction data 15 Print width edge part 31 Use condition adjustment process 32 Overlap process process 35 Test pattern printing process 36 Test pattern imaging process 37 Correction data calculation process 41 Image conversion (ink density error correction) process 42 Decrease tone process Step 201 Nozzle 204 pixels

Claims (19)

In an inkjet printing apparatus that performs printing by relatively moving an inkjet head having a plurality of nozzles and an object to be printed, a method for correcting an error in ink density, which is the density of ink to be ejected by the inkjet head, Adjusting the ink-jet head usage conditions to individually adjust the ink density, printing a test pattern, imaging the test pattern, and measuring the test pattern measurement data accurately A step of applying the color density value in the vicinity of the end portion in the print width direction to the value of the color density at the end portion in the print width direction, and a step of calculating correction data based on the measured measurement data of the test pattern And converting the entire image to be printed on the basis of the correction data, the image in the entire image printed on the substrate is printed. It characterized by having a step of correcting the click density error, ink density error correcting method. In the inkjet printing apparatus, an inkjet head group configured to have a width corresponding to a printing width by mounting two or more inkjet heads is mounted for at least one type of ink, and a substrate to be printed once 2. The ink density error correction method according to claim 1, wherein printing is completed only by passing through the nozzle surface side of the head group. The inkjet printing apparatus moves the substrate and the inkjet head relative to a basic unit that is a width to be printed by a single scanning scan of the inkjet head in a direction parallel to a direction in which the nozzles of the inkjet head are arranged, Multipass equipped with one or more inkjet heads that completes printing by ejecting ink while moving the inkjet head and the substrate to be printed multiple times in a direction perpendicular to the direction in which the nozzles are arranged. The ink density error correction method according to claim 1, wherein the ink density error correction method is used. The ink density error correction method includes: an inkjet printing apparatus having an overlap area that is formed by arranging the inkjet heads adjacent to each other so as to overlap each other by a plurality of nozzles; The ink according to any one of claims 1 to 3, further comprising a step of performing printing by sharing image data to be printed by using a part of each of the nozzles. Density error correction method. The test pattern is configured to have a plurality of color densities, and the ink density error correction method includes a step of setting a target density for the correction data for each of the plurality of color densities of the test pattern. The ink density error correction method according to any one of claims 1 to 4, wherein the ink density error correction method is performed. The ink density error correction method includes a step of specifying a positional relationship between each pixel constituting the image data to be printed and each nozzle used for printing each pixel constituting the inkjet head. The ink density error correction method according to any one of claims 1 to 5. The ink density error correction method includes a step of executing a process of setting a color density value of a portion constituting an end portion in the print width direction of the captured test pattern data as an average value in the vicinity of the end portion. The ink density error correction method according to any one of claims 1 to 6, wherein the ink density error correction method is characterized by the following. 2. The ink density error correction method according to claim 1, further comprising a step of switching the correction data when at least one of a type of a printing material, a printing mode, and an ink type is changed. The ink density error correction method according to claim 7. The ink density error correction method according to any one of claims 1 to 8, wherein the ink density error correction method is executed immediately before the gradation reduction process. The ink density error correction method according to claim 1, wherein the ink jet head is a piezoelectric type. An inkjet head group configured to have a width corresponding to the printing width by mounting two or more inkjet heads is mounted for at least one type of ink, and the substrate is printed once on the nozzle surface side of the inkjet head group In an inkjet printing apparatus that is a single-pass method that completes printing only by passing through the ink, the ink density, which is the density of ink to be ejected by each inkjet head, is adjusted individually by adjusting the use conditions of the inkjet head Means for adjusting, means for printing a test pattern having a print width of the inkjet head group, means for imaging the test pattern, and print width that could not be measured accurately among measurement data of the imaged test pattern Means for applying the color density value in the vicinity of the end portion in the printing width direction to the color density value in the direction end portion; Means for calculating correction data based on the measured data of the test pattern imaged, a means for correcting the ink density errors in the entire image to be printed on a substrate by converting the entire image to be printed based on the corrected data An ink jet printing apparatus comprising: The printing head and the inkjet head are moved relative to each other by a basic unit that is a width to be printed by one scanning of the inkjet head in a direction parallel to the direction in which the nozzles of the inkjet head are arranged. Ink jet printing apparatus having a multi-pass system in which one or more ink jet heads are mounted, and printing is completed by ejecting ink while moving and scanning a plurality of times in a direction perpendicular to the direction in which the nozzles are arranged. In this case, means for individually adjusting the ink density, which is the density of the ink to be ejected by each of the inkjet heads, by adjusting the use conditions of the inkjet head, and a test having a width of two or more basic units Means for printing a pattern; means for imaging the test pattern; and Of the test pattern measurement data, the color density calculated from the both ends of the adjacent portion of the basic unit printed on the test pattern imaged to the color density value at the edge of the print width direction that could not be measured accurately. Means for applying the value to the corresponding end portion in the print width direction, means for calculating correction data based on the measured measurement data of the test pattern, and converting the entire image to be printed based on the correction data. An ink jet printing apparatus comprising means for correcting an ink density error of an entire image printed on a substrate. The inkjet heads adjacent to each other have an overlap region generated by being arranged so as to overlap by a plurality of nozzles, and the inkjet heads adjacent to each other have nozzles to be used in the overlap region. The inkjet printing apparatus according to any one of claims 11 and 12, wherein image data to be printed is shared and printed by using a part of each of the nozzles. The test pattern is configured to have a plurality of color densities, and the correction data includes means for setting a target density for each of the plurality of color densities of the test pattern. The inkjet printing apparatus according to any one of claims 11 to 13. 15. The apparatus according to claim 11, further comprising means for specifying a positional relationship between each pixel constituting the image data to be printed and each nozzle used for printing each pixel constituting the inkjet head. The inkjet printing apparatus as described in any one. 16. The test pattern data according to claim 11, wherein the color density value of the portion constituting the end portion in the print width direction is an average value in the vicinity of the end portion. The inkjet printing apparatus according to item. The correction data includes means for switching when at least one of the type of printing material, the printing mode, and the type of ink is changed. The inkjet printing apparatus as described. 18. The inkjet printing apparatus according to claim 11, wherein the ink density error correction unit is performed immediately before the gradation reduction process. The ink jet printing apparatus according to claim 11, wherein the ink jet head is a piezoelectric type.