Image formation apparatus, ink discharge control method, control device and program

Abstract

To eliminate occurrence of density unevenness in an overlapping region of multiple recording heads and enable formation of high-quality images.SOLUTION: An image formation apparatus 1 includes: recording head units 25Y, 25M, 25C, 25K that have a plurality of recording heads 41, 42 in which a plurality of discharge nozzles 45 is arranged in a main scanning direction X, and are arranged in a state where portions of the recording heads 41, 42 overlap with each other in the main scanning direction X; a switching unit 60 that switches the discharge nozzles 45 that discharge ink from the plurality of recording heads 41, 42 in the overlapping region R1 of the plurality of recording heads 41, 42 in the main scanning direction X; a storage unit 62 that stores a plurality of pieces of correction data 63, 64 for equalizing density unevenness in the main scanning direction X caused by each of the plurality of recording heads 41, 42; and a density correction unit 52 that corrects the density unevenness that occurs in the overlapping region R1 on the basis of the plurality of pieces of correction data 63, 64.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus, an ink ejection control method, a control device, and a program, and more particularly to an image forming technique that uses a plurality of printheads to increase the print width in the main scanning direction. [Background technology]

[0002] Inkjet recording type image forming apparatuses are known that use multiple recording heads with multiple ejection nozzles in the main scanning direction to increase the recording width in the main scanning direction. In this type of image forming apparatus, the ends of two recording heads are arranged overlapping each other in the main scanning direction. The image forming apparatus ejects ink from the ejection nozzles of one of the recording heads in the overlapping area of ​​the two recording heads to form an image.

[0003] Conventionally, a technique has been proposed for making the seam between the ink ejected from one print head and the ink ejected from the other print head less noticeable in the overlapping region of the two print heads (for example, Patent Document 1). In this conventional technique, the position of the seam between the ink ejected from each of the two print heads is changed for each line in the main scanning direction. This makes it possible to disperse the ink seam in the main scanning direction. This suppresses image quality degradation caused by the ink seam appearing as a single stripe along the sub-scanning direction.

[0004] The above conventional technology also proposes suppressing image quality degradation caused by differences in ink ejection density from two adjacent recording heads. That is, the above conventional technology proposes reading the density of a test output image and performing density correction (shading correction) to correct the difference in ink ejection density. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2003-165263 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, the density correction in the above conventional technology converts the image data value based on correction data in which a correction value is associated with each position in the main scanning direction, regardless of the position of the joint between the inks ejected from each of the two recording heads. The correction value corresponding to each position in the main scanning direction is calculated based on a value obtained by averaging the density of the test output image in the sub-scanning direction. Therefore, the above conventional technology has a problem that the density difference between the inks ejected from each of the multiple recording heads cannot be corrected completely at the joint between the inks ejected from each of the multiple recording heads, resulting in uneven density. This problem will be specifically described below.

[0007] 11 and 12 are diagrams showing the concept of density correction in the conventional technology. As shown in FIG. 11(a), two print heads 101 and 102 are arranged in a state where their ends overlap each other along the main scanning direction X. The print head 101 has a plurality of ejection nozzles 103 arranged along the main scanning direction X. The print head 102 has a plurality of ejection nozzles 104 arranged along the main scanning direction X. The nozzle diameters of the ejection nozzles 103 and 104 are not necessarily uniform due to manufacturing errors and the like. Therefore, the amount of ink ejected from each of the ejection nozzles 103 and 104 is not constant. In FIG. 11(a), the ejection nozzles with nozzle diameters larger than the standard size are labeled "large," the ejection nozzles with standard size are labeled "medium," and the ejection nozzles with smaller than the standard size are labeled "small."

[0008] An ejection nozzle with a standard nozzle diameter ejects a predetermined amount of ink. In contrast, an ejection nozzle with a large nozzle diameter ejects more than the predetermined amount of ink. As a result, the density of dots formed by ink ejected from an ejection nozzle with a large nozzle diameter becomes higher. In addition, an ejection nozzle with a small nozzle diameter ejects less than the predetermined amount of ink. As a result, the density of dots formed by ink ejected from an ejection nozzle with a small nozzle diameter becomes lower.

[0009] In the conventional technology, in order to make the seam between the ink ejected from one print head and the ink ejected from the other print head in the overlapping region Rx of the two print heads 101, 102 less noticeable, the seam position Px of the ink ejected from each of the two print heads 101, 102 is switched to a different position in the main scanning direction X for each line based on seam position data (division line shape data) 105, as shown in Fig. 11(b). This makes it possible to distribute the ink seam positions Px in the main scanning direction X.

[0010] Here, assume that an image with uniform density is formed by two print heads 101, 102 as shown in Fig. 11(b). In this case, since there is a difference in the amount of ink ejected from each ejection nozzle, an image formed by the two print heads 101, 102 without density correction will be an image as shown in Fig. 11(c). In the image in Fig. 11(c), density unevenness occurs due to the difference in the amount of ink ejected from each ejection nozzle 103, 104 along the main scanning direction X.

[0011] In the conventional technology, in order to correct the above-mentioned uneven density, the image shown in FIG. 11(c) is output as a test image and the density of the test image is read. Then, in the conventional technology, a correction value at each position in the main scanning direction X is generated based on the density read from the test image. Here, the image scanner that reads the density of the test image reads the density value averaged along the sub-scanning direction of the test image. Therefore, in the conventional technology, a correction value at each position in the main scanning direction X is calculated based on the density value averaged in the sub-scanning direction. FIG. 12(a) shows correction data 106 that records the correction value calculated in the conventional technology. When the density value averaged in the sub-scanning direction in the image of FIG. 11(c) is the standard density, the correction value in the correction data 106 in FIG. 12(a) is "0". Also, when the density value averaged in the sub-scanning direction in the image of FIG. 11(c) is darker than the standard density, the correction value in the correction data 106 in FIG. 12(a) is "-1". Furthermore, if the density value averaged in the sub-scanning direction in the image of Fig. 11(c) is lighter than the standard density, the correction value in the correction data 106 of Fig. 12(a) becomes "+1." In the conventional technology, density correction is performed based on the correction value of one correction data 106 shown in Fig. 12(a).

[0012] However, as described above, when the joint position Px of the ink ejected from each of the multiple print heads 101, 102 is switched to a different position in the main scanning direction X for each line based on the joint position data 105, even if density correction is performed based on the correction value of one correction data 106, dots 107 that are darker than the standard density and light dots 108 appear in the overlapping region Rx of the two print heads 101, 102 as shown in Fig. 12(b). In other words, the conventional technology cannot completely correct the density unevenness in the overlapping region Rx of the two print heads 101, 102, and there is still a problem that image quality degradation due to the density unevenness occurs.

[0013] In order to solve the above-mentioned conventional problems, the present invention aims to provide an image forming apparatus, an ink ejection control method, a control device, and a program that eliminate the occurrence of uneven density in overlapping areas of multiple recording heads and enable the formation of high-quality images. [Means for solving the problem]

[0014] In order to achieve the above-mentioned object, the invention of claim 1 is an image forming device comprising: a recording head unit having a plurality of recording heads each having a plurality of ejection nozzles in a main scanning direction, and arranged in a state in which the plurality of recording heads are partially overlapped in the main scanning direction; a switching unit that switches the ejection nozzles from the plurality of recording heads that eject ink in an overlapping area of ​​the plurality of recording heads in the main scanning direction; a memory unit that stores a plurality of correction data for uniformly distributing density unevenness in the main scanning direction caused by each of the plurality of recording heads; and a density correction unit that corrects density unevenness that occurs in the overlapping area based on the plurality of correction data.

[0015] The invention of claim 2 is characterized in that, in the image forming apparatus of claim 1, the switching unit selects an ejection nozzle from among the multiple recording heads that ejects ink based on connection position data that defines a connection position in the main scanning direction, and the density correction unit selects correction data to be used to correct density unevenness from among the multiple correction data based on the connection position data.

[0016] The invention of claim 3 is characterized in that, in the image forming apparatus of claim 2, the switching unit selects an ejection nozzle that ejects ink for each predetermined line in the main scanning direction based on the connection position data, and the density correction unit changes the selection state of correction data to be used to correct density unevenness from among the multiple correction data for each predetermined line based on the connection position data.

[0017] The invention of claim 4 is characterized in that, in the image forming apparatus of claim 1, it further comprises a test image printing unit that drives the multiple recording heads to print a test image of a predetermined density, an image reading unit that reads the density of the test image, and a correction data generation unit that generates the multiple correction data based on the density read by the image reading unit.

[0018] The invention of claim 5 is characterized in that, in the image forming apparatus of claim 4, the test image printing unit drives the multiple recording heads individually to cause each of the multiple recording heads to print the test image, the image reading unit reads the multiple test images formed by each of the multiple recording heads, and the correction data generation unit generates the multiple correction data based on the density read from each of the multiple test images.

[0019] The invention of claim 6 is characterized in that, in the image forming device of claim 4, the test image printing unit prints the test image multiple times under mutually different conditions, the image reading unit reads the density of the test image each time the test image is printed by the test image printing unit, the correction data generation unit generates multiple combinations of the multiple correction data based on the results of the multiple density readings by the image reading unit, and the density correction unit selects from the multiple sets of the multiple correction data combinations that were generated under the same conditions as the conditions when the print job is executed.

[0020] The invention of claim 7 is characterized in that, in the image forming device of claim 4, the test image printing unit prints the test image multiple times at different times, the image reading unit reads the density of the test image each time the test image is printed by the test image printing unit, the correction data generation unit generates multiple combinations of the multiple correction data based on the results of the multiple density readings by the image reading unit, and the density correction unit selects, from the multiple sets of the multiple correction data, the combination of the multiple correction data generated at a time closest to the time when the print job is executed.

[0021] The invention of claim 8 is an image forming device of any of claims 1 to 3, wherein the multiple recording heads include a first recording head and a second recording head, the multiple correction data include first correction data for uniforming density unevenness of the first recording head and second correction data for uniforming density unevenness of the second recording head, and the density correction unit corrects density unevenness based on the first correction data when ink is ejected from the ejection nozzles of the first recording head by the switching unit, and corrects density unevenness based on the second correction data when ink is ejected from the ejection nozzles of the second recording head by the switching unit.

[0022] The invention of claim 9 is the image forming apparatus of claim 8, further comprising a test image printing unit that drives the first recording head to print a first test image of a predetermined density and drives the second recording head to print a second test image of a predetermined density, an image reading unit that reads the densities of the first test image and the second test image, and a correction data generation unit that generates the first correction data based on the density of the first test image read by the image reading unit, and generates the second correction data based on the density of the second test image read by the image reading unit.

[0023] The invention of claim 10 is a control method for an image forming device having a plurality of recording heads each having a plurality of ejection nozzles in a main scanning direction, and arranged so that some of the recording heads are overlapped in the main scanning direction, characterized in that it includes a switching step of switching the ejection nozzles of the plurality of recording heads that eject ink in an overlapping area of ​​the plurality of recording heads in the main scanning direction, a storage step of storing a plurality of correction data for uniformly distributing density unevenness in the main scanning direction caused by each of the plurality of recording heads, and a density correction step of correcting density unevenness that occurs in the overlapping area based on the plurality of correction data.

[0024] The invention of claim 11 is characterized in that, in the control method of claim 10, the switching step selects an ejection nozzle from among the multiple recording heads that ejects ink based on connection position data that defines a connection position in the main scanning direction, and the density correction step selects correction data to be used to correct density unevenness from among the multiple correction data based on the connection position data.

[0025] The invention of claim 12 is the control method of claim 11, characterized in that the switching step selects an ejection nozzle that ejects ink for each predetermined line in the main scanning direction based on the connection position data, and the density correction step changes a selection state of correction data to be used to correct density unevenness from among the multiple correction data for each predetermined line based on the connection position data.

[0026] The invention of claim 13 is a control method of claim 10, further comprising a test image printing step of driving the multiple recording heads to print a test image of a predetermined density, a density reading step of reading the density of the test image, and a correction data generation step of generating the multiple correction data based on the density read by the density reading step.

[0027] The invention of claim 14 is characterized in that, in the control method of claim 13, the test image printing step drives the multiple recording heads individually to cause each of the multiple recording heads to print the test image, the density reading step reads the multiple test images formed by each of the multiple recording heads, and the correction data generating step generates the multiple correction data based on the density read from each of the multiple test images.

[0028] The invention of claim 15 is characterized in that, in the control method of claim 13, the test image printing step prints the test image multiple times under mutually different conditions, the density reading step reads the density of the test image each time the test image is printed by the test image printing step, the correction data generating step generates multiple combinations of the multiple correction data based on the results of the multiple density readings by the density reading step, and the density correction step selects, from the multiple sets of the multiple correction data, a combination of the multiple correction data generated under the same conditions as the conditions when the print job is executed.

[0029] The invention of claim 16 is characterized in that, in the control method of claim 13, the test image printing step prints the test image multiple times at different times, the density reading step reads the density of the test image each time the test image is printed by the test image printing step, the correction data generating step generates multiple combinations of the multiple correction data based on the multiple density reading results by the density reading step, and the density correction step selects, from the multiple sets of the multiple correction data, the combination of the multiple correction data generated at a time closest to the time of execution of the print job.

[0030] The invention of claim 17 is a control method of any of claims 10 to 12, wherein the multiple recording heads include a first recording head and a second recording head, the multiple correction data include first correction data for uniforming density unevenness of the first recording head and second correction data for uniforming density unevenness of the second recording head, and the density correction step corrects density unevenness based on the first correction data when ink is ejected from the ejection nozzles of the first recording head by the switching step, and corrects density unevenness based on the second correction data when ink is ejected from the ejection nozzles of the second recording head by the switching step.

[0031] The invention of claim 18 is a control method of claim 17, further comprising a test image printing step of driving the first recording head to print a first test image of a predetermined density and driving the second recording head to print a second test image of a predetermined density, a density reading step of reading the densities of the first test image and the second test image, and a correction data generating step of generating the first correction data based on the density of the first test image read by the density reading step, and generating the second correction data based on the density of the second test image read by the density reading step.

[0032] The invention of claim 19 is a control device for controlling an image forming device having a plurality of recording heads each having a plurality of ejection nozzles in a main scanning direction, and arranged with some of the recording heads overlapping in the main scanning direction, characterized in that it is configured to include a switching unit that switches the ejection nozzles from among the plurality of recording heads that eject ink in an overlapping area of ​​the plurality of recording heads in the main scanning direction, a memory unit that stores a plurality of correction data for uniformly distributing density unevenness in the main scanning direction caused by each of the plurality of recording heads, and a density correction unit that corrects density unevenness that occurs in the overlapping area based on the plurality of correction data.

[0033] The invention of claim 20 is a program executed in an image forming device having a plurality of recording heads each provided with a plurality of ejection nozzles in a main scanning direction, and arranged in a state in which the plurality of recording heads are partially overlapped in the main scanning direction, and is characterized in that the image forming device is configured to execute a switching step of switching the ejection nozzles among the plurality of recording heads that eject ink in an overlapping area of ​​the plurality of recording heads in the main scanning direction, a storage step of storing a plurality of correction data for uniformly distributing density unevenness in the main scanning direction caused by each of the plurality of recording heads, and a density correction step of correcting density unevenness that occurs in the overlapping area based on the plurality of correction data. Effect of the Invention

[0034] According to the present invention, it is possible to eliminate the occurrence of uneven density in the overlapping area of ​​a plurality of recording heads, and to form a high quality image. [Brief description of the drawings]

[0035] [Figure 1] FIG. 1 is a schematic diagram illustrating an example of a configuration of an image forming apparatus. [Diagram 2] FIG. 2 is a diagram showing an ink ejection surface of a recording head portion. [Diagram 3] 1A is an enlarged view of an overlapping region of two recording heads, and FIG. 1B is a diagram illustrating the concept of switching the joint position. [Figure 4] FIG. 2 is a block diagram showing a configuration example of a control device. [Diagram 5] FIG. 13 is a diagram showing an example of generating correction data from a test image printed by a first printhead. [Figure 6] 13A and 13B are diagrams illustrating an example of generating correction data from a test image printed by a second printhead. [Figure 7] 10A and 10B are diagrams illustrating the concept of density correction in an overlapping region of two print heads. [Figure 8] FIG. 13 is a diagram illustrating another configuration example of the control device. [Figure 9]5 is a flowchart showing an example of a processing procedure performed by a control device. [Figure 10] FIG. 2 is a diagram showing an example of a configuration in which four recording heads are arranged in the main scanning direction. [Figure 11] FIG. 1 is a diagram illustrating a concept of density correction in the prior art. [Figure 12] FIG. 1 is a diagram illustrating a concept of density correction in the prior art. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the following embodiments, elements common to each other are designated by the same reference numerals, and redundant description thereof will be omitted.

[0037] (First embodiment) Fig. 1 is a schematic diagram showing an example of the configuration of an image forming apparatus 1 in a first embodiment of the present invention. This image forming apparatus 1 transports a sheet-like medium 9, such as printing paper, along a predetermined transport path, and forms an image on the medium 9 by inkjet recording when the medium 9 passes a predetermined position. Note that Fig. 1 shows an example of an apparatus capable of forming a color image on the medium 9 using four colors of ink: yellow (Y), magenta (M), cyan (C), and black (K).

[0038] 1, the image forming apparatus 1 has a media supply unit 10, an image forming unit 20, a media discharge unit 30, and a control device 50. This image forming apparatus 1 is connected to an external terminal device (not shown) via a network, obtains print data to be printed from the terminal device, and forms an image on a medium 9.

[0039] The media supply unit 10 has a media tray 11 and a supply mechanism 12. The media tray 11 is a plate-shaped member on which multiple pieces of media 9 can be placed. The media tray 11 is provided so that it can move up and down depending on the number of pieces of media 9 placed on it. Of the multiple pieces of media 9 placed on the media tray 11, the media 9 at the top in the vertical direction are taken out one by one by the supply mechanism 12 and transported along a transport path.

[0040] The supply mechanism 12 has two rollers 12a and 12b and a conveyor belt 12c. The conveyor belt 12c is formed as an endless belt with both ends in the longitudinal direction connected. The conveyor belt 12c is stretched over the two rollers 12a and 12b. When one of the two rollers 12a and 12b is driven to rotate, the conveyor belt 12c moves in a circular motion between the two rollers 12a and 12b. As a result, one sheet of media 9 placed on the conveyor belt 12c is conveyed toward the image forming unit 20.

[0041] The image forming section 20 includes a medium holding section 21 , a delivery unit 22 , a printing unit 25 , a fixing section 24 , an image reading section 26 , and a discharge mechanism 27 .

[0042] The media holding unit 21 is formed as a cylindrical drum. The media holding unit 21 rotates in the direction of the arrow R by a drive motor (not shown). The outer peripheral surface of the media holding unit 21 holds the media 9 supplied from the media supply unit 10. For example, the media 9 is held in a state where it is attached to the outer peripheral surface of the media holding unit 21. Therefore, the surface (outer peripheral surface) of the media holding unit 21 functions as a media holding surface that holds the media 9. By rotating in the direction of the arrow R, the media holding unit 21 transports the media 9 along an arc-shaped transport path formed by the outer peripheral surface of the media holding unit 21. A printing unit 25, a fixing unit 24, and an image reading unit 26 are arranged facing the outer peripheral surface of the media holding unit 21.

[0043] The transfer unit 22 is provided between the supply mechanism 12 of the media supply section 10 and the media holding section 21. The transfer unit 22 has a claw portion 22a and a cylindrical transfer drum 22b. The claw portion 22a holds one end of the media 9 transported by the supply mechanism 12. The transfer drum 22b guides the media 9 held by the claw portion 22a toward the media holding section 21. As a result, the media 9 is transferred from the supply mechanism 12 to the outer circumferential surface of the media holding section 21 via the transfer unit 22.

[0044] The printing unit 25 has, from the upstream side in the transport direction of the media 9, a recording head portion 25Y that ejects yellow (Y) ink, a recording head portion 25M that ejects magenta (M) ink, a recording head portion 25C that ejects cyan (C) ink, and a recording head portion 25K that ejects black (K) ink.

[0045] The recording heads 25Y, 25M, 25C, and 25K are set to a length (width) that covers the entire medium 9 in the axial direction (width direction of the transport path) of the medium holding unit 21. That is, the image forming device 1 is a one-pass type line head inkjet recording device. The recording heads 25Y, 25M, 25C, and 25K hold the ink of each color in a molten state, and are driven by the control device 50 to eject the molten ink toward the surface of the medium 9 transported by the medium holding unit 21. The axial direction of the medium holding unit 21 coincides with the main scanning direction X when an image is formed by each of the recording heads 25Y, 25M, 25C, and 25K. The transport direction of the medium 9 coincides with the sub-scanning direction Y when an image is formed by each of the recording heads 25Y, 25M, 25C, and 25K.

[0046] A fixing unit 24 is disposed downstream of the printing unit 25 in the transport direction of the medium 9. The fixing unit 24 hardens the ink ejected onto the medium 9 to fix it to the medium 9. As such a fixing unit 24, for example, one that irradiates the medium 9 with ultraviolet light can be used.

[0047] An image reading unit 26 is provided downstream of the fixing unit 24 in the transport direction of the medium 9. For example, the image reading unit 26 is configured with an image scanner. The image reading unit 26 reads the density of the image formed on the surface of the medium 9 by the printing unit 25. At this time, the image reading unit 26 reads the density of the image at the position of each pixel arranged along the main scanning direction X.

[0048] A discharge mechanism 27 is provided downstream of the image reading unit 26 in the transport direction of the media 9. The discharge mechanism 27 transports the media 9 transported by the media holding unit 21 toward the media discharge unit 30. The discharge mechanism 27 has a cylindrical separation drum 27a and a discharge belt 27b. The separation drum 27a separates the media 9 held by the media holding unit 21 from the outer circumferential surface of the media holding unit 21. The separation drum 27a then guides the media 9 to the discharge belt 27b. The discharge belt 27b is formed as an endless belt, similar to the conveyor belt 12c of the supply mechanism 12. The discharge belt 27b is rotatably supported by a number of rollers. The discharge belt 27b sends the media 9 passed from the separation drum 27a to the media discharge unit 30.

[0049] The media discharge section 30 stores the media 9 sent out from the image forming section 20 by the discharge mechanism 27. The media discharge section 30 has a flat collection tray 31. The media discharge section 30 places the media 9 on which an image has been formed on the collection tray 31.

[0050] FIG. 2 is a diagram showing the ink ejection surface of the recording head parts 25Y, 25M, 25C, and 25K. The ink ejection surface is a surface of the recording head parts 25Y, 25M, 25C, and 25K that faces the medium holding part 21. As shown in FIG. 2, a plurality of recording heads 40 are arranged along the main scanning direction X on the ink ejection surface of each of the recording head parts 25Y, 25M, 25C, and 25K. In FIG. 2, two recording heads 40, a first recording head 41 and a second recording head 42, are arranged along the main scanning direction X. Each of the recording heads 41 and 42 has a plurality of ejection nozzles 45 arranged along the main scanning direction X. Each ejection nozzle 45 ejects ink toward the medium 9. In addition, the two recording heads 41 and 42 are arranged in a state in which their ends overlap each other in the main scanning direction X. Therefore, the two recording heads 41 and 42 have an overlapping region R1 where they overlap each other.

[0051] When forming an image on the medium 9, the control device 50 drives one of the two recording heads 41, in an overlapping region R1 of the two recording heads 41, to form an image for one line in the main scanning direction X.

[0052] 3A is an enlarged view of the overlapping region R1 of the two recording heads 41, 42. For example, the control device 50 sets a predetermined position in the main scanning direction X in the overlapping region R1 of the two recording heads 41, 42 as a joint position Px. When forming an image on the left side of the joint position Px, the control device 50 drives the discharge nozzles 45a of the first recording head 41 to form the image. When forming an image on the right side of the joint position Px, the control device 50 drives the discharge nozzles 45b of the second recording head 42 to form the image.

[0053] As shown in FIG. 3A, the position of the discharge nozzle 45a of the recording head 41 and the position of the discharge nozzle 45b of the recording head 42 do not necessarily coincide in the main scanning direction X, and a positional deviation may occur. Such a positional deviation occurs, for example, due to an error in mounting the recording heads 41 and 42 to the recording head parts 25Y, 25M, 25C, and 25K. Therefore, it is difficult to eliminate the positional deviation. If a positional deviation occurs between the discharge nozzle 45a and the discharge nozzle 45b, the interval W at which ink is discharged at the joint position Px becomes a different interval from other parts. For example, if an image is formed in the sub-scanning direction with the joint position Px fixed, a streaky line corresponding to the interval W appears in the image formed on the medium 9.

[0054] In order to prevent such streaky lines from appearing, the control device 50 sequentially switches the joint position Px of the two recording heads 41, 42 to another position in the main scanning direction X for each line. FIG. 3B is a diagram for explaining the concept of switching the joint position Px. As shown in FIG. 3B, the control device 50 holds joint position data 59 for switching the joint position Px for each line. When forming an image in the overlapping region R1 of the two recording heads 41, 42, the control device 50 switches the joint position Px of the two recording heads 41, 42 to another position in the main scanning direction X for each line based on the joint position data 59. This causes the joint positions Px of the two recording heads 41, 42 to be distributed in the main scanning direction X. As a result, the joint between the ink ejected from the recording head 41 and the ink ejected from the recording head 42 becomes inconspicuous, and it is possible to prevent streaky lines from appearing in the image formed on the medium 9.

[0055] The control device 50 also includes a configuration for eliminating uneven density that occurs in the overlapping region R1 of the two recording heads 41 and 42. The control device 50 will be described in detail below.

[0056] Fig. 4 is a block diagram showing an example of the configuration of the control device 50. As shown in Fig. 4, the control device 50 includes a multi-value image memory 51, a density correction unit 52, a binarization processing unit 53, a binary image memory 54, an image division unit 55, a binary image memory 56, a binary image memory 57, a storage unit 58, a switching unit 60, a correction data generation unit 61, and a storage unit 62. For example, the control device 50 includes these units as hardware.

[0057] The multi-value image memory 51 stores a multi-value image to be printed. The density correction unit 52 reads out the multi-value image from the multi-value image memory 51 and corrects the density of each pixel. The binarization processing unit 53 binarizes the multi-value image output from the density correction unit 52 using a predetermined threshold value to generate a binary image. The binary image memory 54 stores the binary image output from the binarization processing unit 53. The image division unit 55 divides the binary image stored in the binary image memory 54 into an image to be recorded by the first recording head 41 and an image to be recorded by the second recording head 42. The binary image memory 56 stores the image to be recorded by the first recording head 41. The binary image memory 57 stores the image to be recorded by the second recording head 42.

[0058] 3B. The switching unit 60 switches the ejection nozzles 45 that eject ink from the two recording heads 41, 42 in the overlapping region R1 of the two recording heads 41, 42. The switching unit 60 instructs the density correction unit 52 and the image division unit 55 on the edge of the connecting position Px based on the edge position data 59 stored in the storage unit 58. For example, the switching unit 60 changes the edge position Px from the previous position to another position based on the edge position data 59 every time the scanning line in the main scanning direction X advances by one line in the sub-scanning direction.

[0059] The image dividing unit 55 divides the binary image in the binary image memory 54 into two images based on the connection position Px specified by the switching unit 60. That is, the image dividing unit 55 divides the binary image into a left image and a right image with the connection position data 59 shown in FIG. 3B as a dividing line. The image dividing unit 55 stores the left image in the binary image memory 56 as an image to be recorded by the first recording head 41. The image dividing unit 55 also stores the left image in the binary image memory 56 as an image to be recorded by the first recording head 41. The control device 50 outputs the binary images stored in the binary image memories 56 and 57 to the first recording head 41 and the second recording head 42, respectively, and drives the recording heads 41 and 42. As a result, an image drawn with ink ejected from the ejection nozzle 45a of the first recording head 41 and an image drawn with ink ejected from the ejection nozzle 45b of the second recording head 42 are joined together at the joining position Px defined by the joining position data 59, forming an image on the medium 9.

[0060] Further, in order to eliminate density unevenness in the overlap region R1, the control device 50 prints a test image when a print job is not being executed in the image forming device 1. At this time, the control device 50 generates a test image in which the entire image is filled with a predetermined density (standard density) and stores the test image in the multi-value image memory 51. When printing the test image, the density correction unit 52 outputs the test image to the binarization processing unit 53 without performing density correction on the test image. The binarization processing unit 53 binarizes the test image and stores it in the binary image memory 54. When printing the test image, the image division unit 55 does not perform image division of the binarized test image, and stores the binarized test image in each of the binary image memories 56 and 57. The control device 50 forms the test image on the medium 9 based on the test image stored in the binary image memories 56 and 57. Hereinafter, a process in which the control device 50 prints a test image and generates correction data will be described.

[0061] First, the control device 50 outputs the test image stored in the binary image memory 56 to the first recording head 41 and drives all of the ejection nozzles 45 a of the first recording head 41 to form the test image on the medium 9 .

[0062] FIG. 5 is a diagram showing an example of a test image printed by the first recording head 41. As shown in FIG. 5(a), the first recording head 41 has a plurality of discharge nozzles 45a arranged along the main scanning direction X. The nozzle diameter of the discharge nozzles 45a is not necessarily uniform due to manufacturing errors. Therefore, the amount of ink discharged from each discharge nozzle 45a is not constant. In FIG. 5(a), the discharge nozzles 45a whose nozzle diameter is larger than the standard size are marked with "large", the discharge nozzles 45a whose nozzle diameter is the standard size are marked with "medium", and the discharge nozzles 45a whose nozzle diameter is smaller than the standard size are marked with "small". When the first recording head 41 shown in FIG. 5(a) is driven to print a test image, the test image G1 becomes an image as shown in FIG. 5(b). In other words, the test image G1 becomes an image (first test image) containing shading according to the nozzle diameter of each discharge nozzle 45a arranged along the main scanning direction X.

[0063] The control device 50 drives the image reading unit 26 to read the density of the test image G1 formed on the medium 9. The image reading unit 26 reads the density of the test image G1 formed on the medium 9. For example, the image reading unit 26 reads the density of the ink ejected from each ejection nozzle 45a of the first recording head 41. At this time, the image reading unit 26 reads the density averaged in the sub-scanning direction as the density at each position in the main scanning direction X. Therefore, the image reading unit 26 can read the shading of the image in the main scanning direction X that appears in the test image G1 shown in FIG. 5(b).

[0064] Next, the control device 50 activates the correction data generating unit 61. The correction data generating unit 61 generates first correction data 63 for correcting the density of the ink discharged from each discharge nozzle 45a of the first recording head 41 based on the density of the test image G1 read by the image reading unit 26. The first correction data 63 is data for uniforming the density unevenness caused by the first recording head 41. FIG. 5(c) shows an example of the first correction data 63. The first correction data 63 includes a correction value corresponding to each discharge nozzle 45a of the first recording head 41. For example, when the density detected from the test image G1 is a predetermined density (standard density), the correction value of the discharge nozzle 45a that discharged the ink of the predetermined density is "0". Also, when the density detected from the test image G1 is a density darker than the predetermined density (standard density), the correction value of the discharge nozzle 45a that discharged the ink of the dark density is "-1". The correction value "-1" is a value that lightens the density of the image. Furthermore, if the density detected from the test image G1 is lighter than a predetermined density (standard density), the correction value of the ejection nozzle 45a that ejected the ink of that light density is "+1". The correction value "+1" is a value that increases the density of the image. After generating the first correction data 63 shown in FIG. 5(c), the correction data generation unit 61 stores the first correction data 63 in the storage unit 62.

[0065] Next, the control device 50 outputs the test image stored in the binary image memory 57 to the second recording head 42 , and drives all of the ejection nozzles 45 b of the second recording head 42 to form the test image on the medium 9 .

[0066] FIG. 6 is a diagram showing an example of a test image printed by the second recording head 42. As shown in FIG. 6(a), the second recording head 42 has a plurality of discharge nozzles 45b arranged along the main scanning direction X. The nozzle diameter of the discharge nozzles 45b is not necessarily uniform due to manufacturing errors and the like. Therefore, the amount of ink discharged from each discharge nozzle 45b is not constant. In FIG. 6(a), the discharge nozzles 45b whose nozzle diameter is larger than the standard size are marked with "large", the discharge nozzles 45b whose nozzle diameter is the standard size are marked with "medium", and the discharge nozzles 45b whose nozzle diameter is smaller than the standard size are marked with "small". When the second recording head 42 shown in FIG. 6(a) is driven to print a test image, the test image G2 becomes an image as shown in FIG. 6(b). In other words, the test image G2 becomes an image (second test image) containing shading according to the nozzle diameter of each discharge nozzle 45b arranged along the main scanning direction X.

[0067] The control device 50 drives the image reading unit 26 to read the density of the test image G2 formed on the medium 9. The image reading unit 26 reads the density of the test image G2 formed on the medium 9. For example, the image reading unit 26 reads the density of the ink ejected from each ejection nozzle 45b of the second recording head 42. At this time, the image reading unit 26 reads the density averaged in the sub-scanning direction as the density at each position in the main scanning direction X. Therefore, the image reading unit 26 can read the shading of the image in the main scanning direction X that appears in the test image G2 shown in FIG. 6(b).

[0068] Next, the control device 50 activates the correction data generating unit 61. The correction data generating unit 61 generates second correction data 64 for correcting the density of the ink discharged from each discharge nozzle 45b of the second recording head 42 based on the density of the test image G2 read by the image reading unit 26. The second correction data 64 is data for equalizing the density unevenness caused by the second recording head 42. FIG. 6(c) shows an example of the second correction data 64. The second correction data 64 includes a correction value corresponding to each discharge nozzle 45b of the second recording head 42. For example, when the density detected from the test image G2 is a predetermined density (standard density), the correction value of the discharge nozzle 45b that discharged the ink of the predetermined density is "0". Also, when the density detected from the test image G2 is a density darker than the predetermined density (standard density), the correction value of the discharge nozzle 45b that discharged the ink of the dark density is "-1". The correction value "-1" is a value that lightens the density of the image. Furthermore, if the density detected from the test image G2 is lighter than a predetermined density (standard density), the correction value of the ejection nozzle 45b that ejected the ink of that light density becomes "+1". The correction value "+1" is a value that increases the density of the image. After generating the second correction data 64 shown in FIG. 6(c), the correction data generation unit 61 stores the second correction data 64 in the storage unit 62.

[0069] Next, the density correction when a print job is executed in the image forming apparatus 1 will be described. FIG. 7 is a diagram for explaining the concept of density correction in the overlapping region R1 of the two recording heads 41 and 42. When a print job is executed in the image forming apparatus 1, the control device 50 drives the ejection nozzles 45a or 45b of either one of the two recording heads 41 and 42 to eject ink in the overlapping region R1 of the two recording heads 41 and 42 shown in FIG. 7(a). At this time, the first correction data 63 and the second correction data 64 as shown in FIG. 7(b) have already been stored in the storage unit 62. When the density correction unit 52 reads out the multi-value image to be printed from the multi-value image memory 51 and performs density correction for each pixel, the density correction unit 52 selects one of the first correction data 63 and the second correction data 64 based on the connection position Px (see FIG. 7(c)) instructed by the switching unit 60, and performs density correction for the multi-value image.

[0070] The density correction unit 52 specifies whether ink for each pixel included in the multi-value image is discharged from the discharge nozzle 45a of the first recording head 41 or the discharge nozzle 45b of the second recording head 42 based on the connection position Px instructed by the switching unit 60. When performing density correction of a pixel to which ink is discharged from the first recording head 41, the density correction unit 52 performs density correction using a correction value recorded in the first correction data 63. When performing density correction of a pixel to which ink is discharged from the second recording head 42, the density correction unit 52 performs density correction using a correction value recorded in the second correction data 64. This allows the density of the ink discharged from the first recording head 41 in the overlapping region R1 of the two recording heads 41 and 42 to be corrected based on the correction value of the first correction data 63. Also, the density of the ink discharged from the second recording head 42 can be corrected based on the correction value of the second correction data 64. The density correction unit 52 performs the density correction for each line in the main scanning direction X. As a result, the image G3 formed in the overlapping region R1 of the two recording heads 41, 42 becomes an image without density unevenness as shown in Fig. 7(c). In other words, even if the connection position Px is switched for each line in the main scanning direction X, the image G3 formed on the medium 9 by the print job does not have density unevenness.

[0071] As described above, the image forming apparatus 1 of the present embodiment has a plurality of recording heads 41, 42 each having a plurality of ejection nozzles 45 in the main scanning direction X, and includes recording head units 25Y, 25M, 25C, and 25K in which the plurality of recording heads 41, 42 are arranged in a state in which the plurality of recording heads 41, 42 are partially overlapped in the main scanning direction X. The image forming apparatus 1 also includes a switching unit 60 that switches the ejection nozzles 45 that eject ink from among the plurality of recording heads 41, 42 in an overlapping region R1 of the plurality of recording heads 41, 42 in the main scanning direction X. The image forming apparatus 1 also includes a storage unit 62 that stores a plurality of correction data 63, 64 for uniforming the density unevenness in the main scanning direction X caused by each of the plurality of recording heads 41, 42. The image forming apparatus 1 further includes a density correction unit 52 that corrects the density unevenness occurring in the overlapping region R1 based on the plurality of correction data 63, 64. The image forming apparatus 1 having such a configuration can correct the density unevenness occurring in the overlapping region R1 based on the plurality of correction data 63, 64. Therefore, the image forming apparatus 1 of this embodiment can appropriately eliminate density unevenness that could not be eliminated by a single correction data 106 (see FIG. 12(a)) as in the conventional case.

[0072] Further, the switching unit 60 in this embodiment selects the ejection nozzles 45 that eject ink from among the multiple recording heads 41, 42 based on the joint position data 59 that defines the joint position Px in the main scanning direction X. The density correction unit 52 selects correction data to be used to correct density unevenness from among the multiple correction data 63, 64 based on the joint position data 59. The image forming apparatus 1 having such a configuration can select the correction data 63, 64 corresponding to the recording heads 41, 42 that eject ink from among the multiple recording heads 41, 42 to perform density correction. Therefore, the image forming apparatus 1 of this embodiment can appropriately perform density correction in the overlap region R1 and suppress the occurrence of density unevenness.

[0073] Further, the switching unit 60 in this embodiment selects the ejection nozzles 45 that eject ink for each predetermined line in the main scanning direction X based on the connection position data 59. The density correction unit 52 changes the selection state of the correction data to be used to correct density unevenness from among the multiple correction data 63, 64 for each predetermined line based on the connection position data 59. The image forming apparatus 1 having such a configuration can also change the correction data to be used when correcting density unevenness as the ejection nozzles 45 are switched for each predetermined line. Therefore, the image forming apparatus 1 of this embodiment can always perform appropriate density correction in the overlap region R1.

[0074] In the image forming apparatus 1 of the present embodiment, when the plurality of recording heads 41 and 42 are driven to print test images G1 and G2 of a predetermined density, the control device 50 functions as a test image printing unit. The image forming apparatus 1 also includes an image reading unit 26 that reads the density of the test images G1 and G2, and a correction data generating unit 61 that generates a plurality of correction data 63 and 64 based on the density read by the image reading unit 26. Therefore, the plurality of correction data 63 and 64 can be generated to individually correct uneven density of the ink ejected from each recording head 41 and 42. The image forming apparatus 1 generates the plurality of correction data 63 and 64 before executing a print job, and can appropriately correct uneven density using the plurality of correction data 63 and 64 when executing the print job.

[0075] The control device 50 may also print the test images G1 and G2 multiple times under different conditions to generate multiple combinations of the correction data 63 and 64. In this case, multiple combinations of the first correction data 63 and the second correction data 64 are stored in the storage unit 62. When a print job is executed, the density correction unit 52 selects a combination of the correction data 63 and 64 generated under the same conditions as those when the print job is executed from among the multiple combinations of the first correction data 63 and the second correction data 64, thereby making it possible to perform more appropriate density correction.

[0076] For example, the state of the discharge nozzle 45 may change depending on the time of day. In order to respond to such changes in the state of the discharge nozzle 45, the control device 50 prints the test images G1 and G2 multiple times at different times. The image reading unit 26 reads the density of the test images G1 and G2 each time the control device 50 prints the test images G1 and G2. The correction data generating unit 61 generates multiple sets of combinations of multiple correction data 63 and 64 based on the results of multiple density readings by the image reading unit 26. Then, when a print job is executed in the image forming device 1, the density correcting unit 52 selects, from among the multiple sets of correction data 63 and 64, the combination of correction data 63 and 64 generated at the time closest to the time of execution of the print job, and performs density correction. As a result, density correction suitable for the state of the discharge nozzle 45 at the time of execution of the print job is performed. Therefore, density unevenness in the overlapping region R1 is appropriately eliminated.

[0077] Second embodiment Next, a second embodiment of the present invention will be described. In the first embodiment, the respective units of the control device 50 are configured by hardware. In the present embodiment, a configuration example in which the respective units of the control device 50 described in the first embodiment are realized by software will be described.

[0078] FIG. 8 is a diagram showing an example of the configuration of the control device 50 in the second embodiment. The control device 50 includes a CPU 70, a storage unit 71, and an input / output interface 76. The CPU 70 is a hardware processor that reads and executes a program 72 stored in the storage unit 71. The storage unit 71 is a storage device configured by an HDD (hard disk drive) or an SSD (solid state drive). The storage unit 71 stores the first correction data 63, the second correction data 64, and the connection position data 59 in addition to the program 72. The first recording head 41, the second recording head 42, and the image reading unit 26 are connected to the input / output interface 76. The CPU 70 drives the first recording head 41 and the second recording head 42 via the input / output interface 76, and obtains the densities of the test images G1 and G2 read by the image reading unit 26.

[0079] The CPU 70 executes the program 72 to function as a test image printing unit 73, a correction data generating unit 74, and a job control unit 75. The test image printing unit 73 and the correction data generating unit 74 function when a print job is not being executed in the image forming apparatus 1, and store the first correction data 63 and the second correction data 64 in the storage unit 71. The job control unit 75 functions when a print job is being executed in the image forming apparatus 1. The job control unit 75 has the functions of the density correction unit 52, the binarization processing unit 53, the image division unit 55, and the switching unit 60 described in the first embodiment. The storage unit 71 also has the functions of the multi-value image memory 51, the binary image memory 54, the binary image memory 56, the binary image memory 57, the storage unit 58, and the storage unit 62 described in the first embodiment.

[0080] When the image forming apparatus 1 is not executing a print job, the test image printing unit 73 drives the recording heads 41 and 42, respectively, to sequentially form the test images G1 and G2 on the medium 9.

[0081] As the test images G1 and G2 are sequentially formed on the medium 9 by the test image printing unit 73, the correction data generating unit 74 drives the image reading unit 26 and acquires the densities of the test images G1 and G2 read by the image reading unit 26. When the correction data generating unit 74 acquires the density of the test image G1, it calculates a correction value for correcting the density of the ink ejected from each ejection nozzle 45a of the first recording head 41, and generates the first correction data 63. When the correction data generating unit 74 acquires the density of the test image G2, it calculates a correction value for correcting the density of the ink ejected from each ejection nozzle 45b of the second recording head 42, and generates the second correction data 64. Then, the correction data generating unit 74 stores the first correction data 63 and the second correction data 64 in the storage unit 71.

[0082] The job control unit 75 acquires a multi-value image included in a print job designated by a user, and performs density correction on the multi-value image. At this time, the job control unit 75 functions as a density correction unit 52. The density correction unit 52 specifies whether ink for each pixel included in the multi-value image is discharged from the discharge nozzle 45a of the first recording head 41 or from the discharge nozzle 45b of the second recording head 42 based on a connection position Px defined by the connection position data 59. When performing density correction on a pixel onto which ink is discharged from the first recording head 41, the density correction unit 52 performs density correction on the multi-value image using a correction value recorded in the first correction data 63. Also, when performing density correction on a pixel onto which ink is discharged from the second recording head 42, the density correction unit 52 performs density correction on the multi-value image using a correction value recorded in the second correction data 64. This allows the density of the ink ejected from the first recording head 41 in the overlapping region R1 of the two recording heads 41, 42 to be corrected based on the correction value of the first correction data 63. Also, the density of the ink ejected from the second recording head 42 can be corrected based on the correction value of the second correction data 64. The density correction unit 52 performs the above density correction for each line in the main scanning direction X. As a result, the image G3 formed in the overlapping region R1 of the two recording heads 41, 42 is an image without density unevenness as shown in FIG. 7(c).

[0083] After performing density correction on the multi-value image, the job control unit 75 functions as the binarization processing unit 53. The binarization processing unit 53 converts the density-corrected multi-value image into a binary image. After converting the multi-value image into a binary image, the job control unit 75 functions as the image dividing unit 55. The image dividing unit 55 divides the binary image into two images based on a connection position Px instructed by the switching unit 60. That is, the image dividing unit 55 divides the binary image into an image formed with ink by the first recording head 41 and an image formed with ink by the second recording head 42. Then, the job control unit 75 drives each of the first recording head 41 and the second recording head 42 based on the two divided images to form an image on the medium 9.

[0084] 9 is a flowchart showing an example of a processing procedure performed by the CPU 70 of the control device 50. The CPU 70 executes the program 72 to sequentially perform processing based on the flowchart shown in FIG.

[0085] When the control device 50 starts this process, it judges whether it is time to perform test printing (step S10). For example, the control device 50 judges that it is time to perform test printing at a predetermined time each day (e.g., 9:00, 13:00, 15:00, and 18:00). The control device 50 also judges that it is time to perform test printing after mass printing of a predetermined number of sheets or more has been performed in the image forming device 1. After mass printing, the state of the discharge nozzles 45 may be different from that before. Therefore, the control device 50 may judge to perform test printing after mass printing of a predetermined number of sheets or more has been performed.

[0086] When the control device 50 determines to perform test printing (YES in step S10), it drives the first recording head 41 to print a first test image G1 on the medium 9 (step S11). The control device 50 drives the image reading unit 26 to read the first test image G1 formed on the medium 9 (step S12), and generates first correction data 63 based on the density of each position in the main scanning direction X in the first test image G1 (step S13). The control device 50 stores the first correction data 63 in the storage unit 62.

[0087] Next, the control device 50 drives the second recording head 42 to print the second test image G2 on the medium 9 (step S14). The control device 50 drives the image reading unit 26 to read the second test image G2 formed on the medium 9 (step S15), and generates second correction data 64 based on the density of each position in the main scanning direction X in the second test image G2 (step S16). The control device 50 stores the second correction data 64 in the storage unit 62.

[0088] This is the end of the test printing process. If the control device 50 determines that it is not time to execute test printing (NO in step S10), it does not execute steps S11 to S16.

[0089] Next, the control device 50 judges whether or not to execute the print job (step S17). If it is judged that the print job is executed (YES in step S17), the control device 50 acquires the multi-value image included in the print job (step S18). The control device 50 reads the first correction data 63 from the storage unit 62 (step S19). The control device 50 also reads the second correction data 64 (step S20). Furthermore, the control device 50 reads the connection position data 59 from the storage unit 58 (step S21). Then, the control device 50 selects either the first correction data 63 or the second correction data 64 based on the connection position data 59, and corrects the density of the multi-value image (step S22).

[0090] Next, the control device 50 performs binarization processing on the density-corrected multi-valued image (step S23), and divides the binary image into two images based on the joint position Px of the joint position data 59 (step S24). Then, the control device 50 drives the first recording head 41 and the second recording head 42 based on the two divided images, and forms an image on the medium 9 (step S25). If the print job includes images for multiple pages, the processing of steps S18 to S25 is repeated according to the number of pages. If the control device 50 does not execute the print job (NO in step S17), the processing of steps S18 to S25 is not performed. This ends the processing by the control device 50.

[0091] By executing the above-described processing, the control device 50 of the image forming apparatus 1 can, like the first embodiment, appropriately eliminate density unevenness that could not be completely eliminated by a single correction data 106 (see FIG. 12(a)) as in the conventional case. Also, like the first embodiment, the image forming apparatus 1 of this embodiment can appropriately perform density correction in the overlap region R1, thereby suppressing the occurrence of density unevenness. In this way, the image forming apparatus 1 of this embodiment executes the operation of the control device 50 described in the first embodiment by software, and achieves the same effects as the first embodiment.

[0092] In addition, other than the above-mentioned points in this embodiment, the contents are the same as those explained in the first embodiment.

[0093] (Modification) The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the contents described in the above embodiments. Various modifications other than the above embodiments are applicable to the present invention.

[0094] For example, in the above embodiment, the image forming apparatus 1 has been described in which the two recording heads 41, 42 are arranged in the main scanning direction X in the recording head units 25Y, 25M, 25C, and 25K. However, the number of recording heads 40 arranged in the main scanning direction X in the recording head units 25Y, 25M, 25C, and 25K is not limited to two. For example, the recording head units 25Y, 25M, 25C, and 25K may have a configuration in which three or more recording heads 40 are arranged in the main scanning direction X.

[0095] FIG. 10 is a diagram showing an example of a configuration in which four recording heads 41, 42, 43, and 44 are arranged in the main scanning direction X. The recording head units 25Y, 25M, 25C, and 25K shown in FIG. 10 have an overlapping region R1 where the recording head 41 and the recording head 42 overlap, an overlapping region R2 where the recording head 42 and the recording head 43 overlap, and an overlapping region R3 where the recording head 43 and the recording head 44 overlap. Therefore, there is a possibility that uneven density occurs in each of the multiple overlapping regions R1, R2, and R3. Therefore, the control device 50 applies the above-mentioned processing to each of the multiple overlapping regions R1, R2, and R3 to prevent uneven density from occurring.

[0096] For example, when printing the test images G1 and G2, the control device 50 simultaneously drives the odd-numbered recording heads 41 and 43 in the main scanning direction X to form two test images G1 on the medium 9, and then simultaneously drives the even-numbered recording heads 42 and 44 in the main scanning direction X to form two test images G2 on the medium 9. The test image G1 formed by the recording head 41 and the test image G1 formed by the recording head 43 do not overlap each other on the surface of the medium 9. Therefore, the image reading unit 26 can simultaneously read the densities of the two test images G1. Similarly, the test image G2 formed by the recording head 42 and the test image G2 formed by the recording head 44 do not overlap each other on the surface of the medium 9. Therefore, the image reading unit 26 can simultaneously read the densities of the two test images G2. Therefore, when printing the test images, the control device 50 can efficiently generate the correction data 63 and 64 by driving the odd-numbered recording heads 41 and 43 and the even-numbered recording heads 42 and 44 at different timings.

[0097] In the above first embodiment, an example in which the control device 50 is configured by hardware has been described, and in the second embodiment, an example in which each function of the control device 50 is realized by software has been described. However, in the present invention, the functions of the control device 50 described above are not limited to being configured by only either hardware or software. In other words, the functions of the control device 50 may be configured by a combination of hardware and software.

[0098] Also, in the above embodiment, an example has been described in which the switching unit 60 switches the connection position Px for every line in the main scanning direction X. However, the switching unit 60 may switch the connection position Px for every two lines in the main scanning direction X, or may switch the connection position Px for every three or more predetermined lines.

[0099] In the above embodiment, the image forming apparatus 1 is an apparatus capable of forming a color image. However, the image forming apparatus 1 is not limited to an apparatus capable of forming a color image. In other words, the image forming apparatus 1 may be an apparatus that forms an image in a single color, for example, black.

[0100] Furthermore, the program 72 described in the second embodiment is not limited to being stored in advance in the storage unit 71 of the image forming apparatus 1. For example, the program 72 may be a transaction target by itself. In this case, the program 72 may be provided in a downloadable form via a network such as the Internet, or may be provided in a state recorded on a computer-readable recording medium such as a CD-ROM. [Explanation of symbols]

[0101] 1. Image forming device 25 Printing Unit 25Y, 25M, 25C, 25K recording head 26 Image reading unit 40, 41, 42, 43, 44 Recording head 45, 45a, 45b Discharge nozzle 52 Density correction section 59 Joint position data 60 Switching section 61 Correction data generator 62 Memory section 63 First correction data 64 Second correction data 72 Programs 73 Test image printing section 74 Correction data generator 75 Job Control Section G1,G2 test images Px Joint position R1, R2, R3 overlapping area X main scanning direction

Claims

1. a recording head unit including a plurality of recording heads each having a plurality of ejection nozzles arranged in a main scanning direction, the plurality of recording heads being arranged in a state in which parts of the recording heads are overlapped in the main scanning direction; a switching unit that switches among the plurality of recording heads which ejects ink in an overlapping region of the plurality of recording heads in the main scanning direction; a storage unit that stores a plurality of correction data for uniformly correcting uneven density in the main scanning direction caused by each of the plurality of recording heads; a density correction unit that corrects density unevenness occurring in the overlapping region based on the plurality of correction data; An image forming apparatus comprising:

2. the switching unit selects an ejection nozzle that ejects ink from among the plurality of recording heads based on joint position data that defines a joint position in the main scanning direction; 2. The image forming apparatus according to claim 1, wherein the density correction section selects correction data to be used for correcting density unevenness from among the plurality of correction data based on the connection position data.

3. the switching unit selects an ejection nozzle that ejects ink for each predetermined line in the main scanning direction based on the connection position data; 3. The image forming apparatus according to claim 2, wherein the density correction unit changes a selection state of correction data to be used for correcting density unevenness from among the plurality of correction data for each of the predetermined lines based on the connection position data.

4. a test image printing unit that drives the plurality of recording heads to print a test image of a predetermined density; an image reading unit for reading the density of the test image; a correction data generating unit that generates the plurality of correction data based on the densities read by the image reading unit; 2. The image forming apparatus according to claim 1, further comprising:

5. the test image printing unit drives the plurality of recording heads individually to cause each of the plurality of recording heads to print the test image; the image reading unit reads a plurality of test images formed by the plurality of recording heads, 5. The image forming apparatus according to claim 4, wherein the correction data generating section generates the plurality of correction data based on densities read from the plurality of test images.

6. the test image printing unit prints the test image a plurality of times under mutually different conditions; the image reading unit reads the density of the test image every time the test image printing unit prints the test image; the correction data generating unit generates a plurality of combinations of the plurality of correction data based on a result of a plurality of density readings performed by the image reading unit; 5. The image forming apparatus according to claim 4, wherein the density correction unit selects, from among the plurality of sets of correction data, a combination of the plurality of correction data generated under the same conditions as those under which the print job is executed.

7. the test image printing unit prints the test image a plurality of times at different times; the image reading unit reads the density of the test image every time the test image printing unit prints the test image; the correction data generating unit generates a plurality of combinations of the plurality of correction data based on a result of a plurality of density readings performed by the image reading unit; 5. The image forming apparatus according to claim 4, wherein the density correction unit selects, from among the plurality of sets of correction data, a combination of the plurality of correction data generated closest to a time when the print job is executed.

8. the plurality of recording heads include a first recording head and a second recording head, the plurality of correction data include first correction data for uniforming density unevenness of the first recording head and second correction data for uniforming density unevenness of the second recording head, 4. An image forming apparatus according to claim 1, wherein the density correction unit corrects density unevenness based on the first correction data when ink is ejected from the ejection nozzles of the first recording head by the switching unit, and corrects density unevenness based on the second correction data when ink is ejected from the ejection nozzles of the second recording head by the switching unit.

9. a test image printing unit that drives the first recording head to print a first test image of a predetermined density and drives the second recording head to print a second test image of a predetermined density; an image reading unit that reads the densities of the first test image and the second test image; a correction data generating unit that generates the first correction data based on a density of the first test image read by the image reading unit, and generates the second correction data based on a density of the second test image read by the image reading unit; 9. The image forming apparatus according to claim 8, further comprising:

10. 1. A method for controlling an image forming apparatus having a plurality of recording heads, each having a plurality of ejection nozzles in a main scanning direction, the plurality of recording heads being arranged in a state in which parts of the recording heads overlap in the main scanning direction, the method comprising the steps of: a switching step of switching ejection nozzles that eject ink from among the plurality of recording heads in an overlapping region of the plurality of recording heads in the main scanning direction; a storage step of storing a plurality of correction data for uniformly correcting uneven density in the main scanning direction caused by each of the plurality of recording heads; a density correction step of correcting density unevenness occurring in the overlapping region based on the plurality of correction data; A control method comprising the steps of:

11. The switching step includes selecting nozzles from among the plurality of recording heads that eject ink based on joint position data that defines joint positions in the main scanning direction, 11. The control method according to claim 10, wherein said density correction step selects correction data to be used for correcting density unevenness from among said plurality of correction data based on said connection position data.

12. the switching step selects an ejection nozzle that ejects ink for each predetermined line in the main scanning direction based on the connection position data; 12. The control method according to claim 11, wherein the density correction step changes a selection state of correction data to be used for correcting density unevenness from among the plurality of correction data for each of the predetermined lines based on the connection position data.

13. a test image printing step of driving the plurality of recording heads to print a test image of a predetermined density; a density reading step of reading the density of the test image; a correction data generating step of generating the plurality of correction data based on the densities read in the density reading step; 11. The method of claim 10, further comprising:

14. the test image printing step includes driving the plurality of recording heads individually to cause each of the plurality of recording heads to print the test image; The density reading step includes reading a plurality of test images formed by the plurality of recording heads, 14. The control method according to claim 13, wherein the correction data generating step generates the plurality of correction data based on densities read from the plurality of test images.

15. The test image printing step includes printing the test image a plurality of times under different conditions; the density reading step includes reading the density of the test image every time the test image is printed in the test image printing step; The correction data generating step generates a plurality of combinations of the plurality of correction data based on a plurality of density reading results obtained by the density reading step, 14. The control method according to claim 13, wherein the density correction step selects, from among the plurality of sets of correction data, a combination of the plurality of correction data generated under the same conditions as those under which the print job is executed.

16. The test image printing step includes printing the test image a plurality of times at different times; the density reading step includes reading the density of the test image every time the test image is printed in the test image printing step; The correction data generating step generates a plurality of combinations of the plurality of correction data based on a plurality of density reading results obtained by the density reading step, 14. The control method according to claim 13, wherein the density correction step selects, from among the plurality of sets of correction data, a combination of the plurality of correction data generated at a time closest to a time when the print job is executed.

17. the plurality of recording heads include a first recording head and a second recording head, the plurality of correction data include first correction data for uniforming density unevenness of the first recording head and second correction data for uniforming density unevenness of the second recording head, A control method according to any one of claims 10 to 12, characterized in that the density correction step corrects density unevenness based on the first correction data when ink is ejected from the ejection nozzles of the first recording head by the switching step, and corrects density unevenness based on the second correction data when ink is ejected from the ejection nozzles of the second recording head by the switching unit.

18. a test image printing step of driving the first recording head to print a first test image of a predetermined density and driving the second recording head to print a second test image of a predetermined density; a density reading step of reading the densities of the first test image and the second test image; a correction data generating step of generating the first correction data based on the density of the first test image read by the density reading step, and generating the second correction data based on the density of the second test image read by the density reading step; 20. The method of claim 17, further comprising:

19. A control device for controlling an image forming apparatus having a plurality of recording heads, each having a plurality of ejection nozzles in a main scanning direction, the plurality of recording heads being arranged in a state where parts of the recording heads are overlapped in the main scanning direction, the control device comprising: a switching unit that switches among the plurality of recording heads which ejects ink in an overlapping region of the plurality of recording heads in the main scanning direction; a storage unit that stores a plurality of correction data for uniformly correcting uneven density in the main scanning direction caused by each of the plurality of recording heads; a density correction unit that corrects density unevenness occurring in the overlapping region based on the plurality of correction data; A control device comprising:

20. A program executed in an image forming apparatus having a plurality of recording heads, each having a plurality of ejection nozzles in a main scanning direction, the plurality of recording heads being arranged in a state in which parts of the recording heads are overlapped in the main scanning direction, the program comprising: a switching step of switching ejection nozzles that eject ink from among the plurality of recording heads in an overlapping region of the plurality of recording heads in the main scanning direction; a storage step of storing a plurality of correction data for uniformly correcting uneven density in the main scanning direction caused by each of the plurality of recording heads; a density correction step of correcting density unevenness occurring in the overlapping region based on the plurality of correction data; A program characterized by executing the above.

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