Image forming apparatus

Abstract

To reduce the time from the start of correction image forming processing until the end of pattern image forming processing.SOLUTION: An image forming apparatus comprises: an image forming unit that forms images in a plurality of colors on a transfer medium; a detection unit that detects the images on the transfer medium; and a control unit. The image forming unit has a first photoconductor drum, a second photoconductor drum, a third photoconductor drum, and a fourth photoconductor drum corresponding to a first color, a second color, a third color, and a fourth color. The photoconductor drums are arranged side by side in order of the first color, the second color, the third color, and the fourth color from the downstream side toward the upstream side in a movement direction in which the transfer medium moves toward the detection unit. The control unit executes correction image forming processing such that, in the movement direction, a first bias correction image K1 for the first color is located before first bias correction images Y1, M1, and C1 for the second color, the third color, and the fourth color, and a second bias correction image K2 for the first color is located between the first bias correction images M1 and C1 for the two colors of the second color, the third color, and the fourth color.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to an image forming apparatus including a control unit that executes processes such as adjustment of printing density based on the result of reading an image formed on a transfer medium such as a belt by a detection unit.

Background Art

[0002] Conventionally, as an image forming apparatus, there is known one including an image forming unit that forms images of a plurality of colors on a belt, a detection unit that detects the images formed on the belt, and a control unit that executes correction image forming processing, bias setting processing, and pattern image forming processing (see Patent Document 1). The correction image forming processing is a process of forming a first bias correction image on the belt while applying a first developing bias to a developing roller, and then forming a second bias correction image on the belt while applying a second developing bias having a value different from that of the first developing bias to the developing roller.

[0003] The bias setting processing is a process of setting a developing bias based on the results of reading the first bias correction image and the second bias correction image by the detection unit. The pattern image forming processing is a process of forming a plurality of pattern images on the belt while applying the developing bias set in the bias setting processing to the developing roller.

[0004] Further, the image forming apparatus includes four photosensitive drums. The photosensitive drums are arranged in the color order of K, C, M, and Y from the side closer to the detection unit.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] By the way, conventionally, in the correction image formation process and the pattern image formation process, a plurality of images formed on the belt are regularly arranged in the order of Y, M, C, and K from the side closer to the detection unit. However, in this case, since the black (K) image formed on the photosensitive drum closest to the detection unit is read by the detection unit later than the images of other colors, there is a problem that the time from the start of the correction image formation process to the end of the pattern image formation process becomes long.

[0007] Therefore, an object of the present invention is to shorten the time from the start of the correction image formation process to the end of the pattern image formation process.

Means for Solving the Problems

[0008] In order to solve the above problems, an image forming apparatus according to the present invention includes an image forming unit that forms images of a plurality of colors on a transfer medium, a detection unit that detects an image formed on the transfer medium, and a control unit. The image forming unit includes a developing roller and a photosensitive drum corresponding to a first color, a developing roller and a photosensitive drum corresponding to a second color, a developing roller and a photosensitive drum corresponding to a third color, and a developing roller and a photosensitive drum corresponding to a fourth color. Each photosensitive drum is arranged in the order of the first color, the second color, the third color, and the fourth color from the downstream side to the upstream side in the moving direction in which the transfer medium moves toward the detection unit. The control unit is capable of executing a correction image formation process, a bias setting process, and a pattern image formation process for each of the first color, the second color, the third color, and the fourth color. The correction image formation process is a process of forming a first bias correction image on the transfer medium by the image forming unit with a first developing bias applied to the developing roller, and then forming a second bias correction image on the transfer medium by the image forming unit with a second developing bias having a value different from that of the first developing bias applied to the developing roller. The bias setting process is a process of setting a developing bias based on the results of reading the first bias correction image and the second bias correction image by the detection unit. The pattern image forming process is a process of forming a plurality of pattern images on the transfer medium by the image forming unit with the developing bias set in the bias setting process applied to the developing roller. The control unit executes the correction image forming process such that, in the moving direction, the first bias correction image of the first color is positioned in front of the first bias correction images of the second color, the third color, and the fourth color, and the second bias correction image of the first color is positioned between the first bias correction images of two of the second color, the third color, and the fourth color.

[0009] According to this configuration, the bias setting process for the first color can be completed quickly, so the start of the pattern image forming process for the first color can be advanced, and as a result, the time from the start of the correction image forming process to the end of the pattern image forming process can be shortened.

[0010] Also, the image forming apparatus according to the present invention includes an image forming unit that forms images of a plurality of colors on a transfer medium, a detection unit that detects an image formed on the transfer medium, and a control unit. The image forming unit includes a developing roller and a photosensitive drum corresponding to the first color, a developing roller and a photosensitive drum corresponding to the second color, a developing roller and a photosensitive drum corresponding to the third color, and a developing roller and a photosensitive drum corresponding to the fourth color. Each photosensitive drum is arranged in the order of the first color, the second color, the third color, and the fourth color from the downstream side to the upstream side in the moving direction in which the transfer medium moves toward the detection unit. The control unit is capable of executing a correction image forming process, a bias setting process, and a pattern image forming process for each of the first color, the second color, the third color, and the fourth color. The correction image formation process is a process in which, with a first development bias applied to a development roller, a first bias correction image is formed on the transfer medium by the image forming unit, and then, with a second development bias having a value different from that of the first development bias applied to the development roller, a second bias correction image is formed on the transfer medium by the image forming unit. The bias setting process is a process of setting a development bias based on the results of reading the first bias correction image and the second bias correction image by the detection unit. The pattern image formation process is a process in which, with the development bias set in the bias setting process applied to the development roller, a plurality of pattern images are formed on the transfer medium by the image forming unit. The control unit executes the correction image formation process and the pattern image formation process such that the color of the first bias correction image located at the forefront of a first image group composed of a plurality of colors of the first bias correction images and a plurality of colors of the second bias correction images is the first color, and the color of the pattern image located at the forefront of a second image group composed of a plurality of colors of the pattern images is the first color, and the color of the second bias correction image located at the rearmost of the first image group is different from the color of the pattern image located at the rearmost of the second image group.

[0011] According to this configuration, the bias setting process for the first color can be completed quickly, so the start of the pattern image formation process for the first color can be advanced, and as a result, the time from the start of the correction image formation process to the end of the pattern image formation process can be shortened.

[0012] Further, the control unit may execute the correction image formation process such that, in the moving direction, the second bias correction image of the first color is located in front of the first bias correction image of one of the second color, the third color, and the fourth color.

[0013] According to this configuration, since the bias setting process for the first color can be completed earlier, the time from the start of the correction image formation process to the end of the pattern image formation process can be further shortened.

[0014] Further, the control unit may execute the correction image formation process such that the second bias correction image of the fourth color is positioned in front of at least one of the second color and the third color in the moving direction.

[0015] According to this configuration, since the bias setting process for the fourth color can be completed earlier, the start of the pattern image formation process for the fourth color can be advanced, and as a result, the time from the start of the correction image formation process to the end of the pattern image formation process can be shortened.

[0016] Further, the control unit may execute the correction image formation process such that the first bias correction image of the fourth color is positioned in front of at least one of the second color and the third color in the moving direction.

[0017] Further, the control unit may execute the correction image formation process such that the first bias correction image of the first color, the first bias correction image of the fourth color, the first bias correction image of the third color, the second bias correction image of the first color, and the first bias correction image of the second color are arranged in this order from the head in the moving direction.

[0018] Further, the control unit may execute the correction image formation process such that the second bias correction image is arranged in the order of the fourth color, the third color, and the second color after the first bias correction image of the second color in the moving direction.

[0019] According to this configuration, for example, compared with a form in which the second bias correction images are arranged in the order of the third color, the fourth color, and the second color, the bias setting process for the fourth color can be completed earlier.

[0020] Further, the transfer medium is an endless belt, and the distance from the front end of the foremost first bias correction image to the rear end of the rearmost pattern image in the moving direction may be smaller than the circumference of the belt.

[0021] According to this configuration, since it is not necessary to perform a belt cleaning process that rotates the belt one or more times between the bias setting process and the pattern image forming process, the time from the start of the correction image forming process to the end of the pattern image forming process can be shortened.

[0022] Further, the pattern image may be a tone setting image for setting the correspondence relationship between the target density and the exposure density.

[0023] Further, the pattern image may be a positional deviation correction image for correcting the positional deviation of the image.

Advantages of the Invention

[0024] According to the present invention, the time from the start of the correction image forming process to the end of the pattern image forming process can be shortened.

Brief Description of the Drawings

[0025]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Mode for Carrying Out the Invention

[0026] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings as appropriate. As shown in FIG. 1, a color printer 1 as an example of an image forming apparatus includes a main body housing 2, a supply unit 3, an image forming unit 4, a detection unit PS, a fixing device 8, a conveyance unit 9, and a control unit 100. The main body housing 2 has a discharge tray 21 on its upper surface.

[0027] The supply unit 3 is provided in the lower part of the main body housing 2 and includes a supply tray 31 for accommodating the sheet S and a supply mechanism 32 for supplying the sheet S in the supply tray 31 to the image forming unit 4. The supply mechanism 32 includes a pickup roller 33, a separation roller 34, a separation pad 35, and a registration roller 36.

[0028] The supply unit 3 supplies the sheet S in the supply tray 31 toward the separation roller 34 by the pickup roller 33 and separates the sheet S into one sheet between the separation roller 34 and the separation pad 35. The registration roller 36 aligns the positions of the leading ends of the sheet S and then conveys the sheet S toward the image forming unit 4.

[0029] The image forming unit 4 includes an exposure device SC, a process unit PU, and a transfer unit 7.

[0030] The exposure device SC is disposed at the upper part inside the main body housing 2 and includes a light source, a polygon mirror, etc. (not shown). The exposure device SC exposes the surface of the photosensitive drum 51 by irradiating light onto the surface of the photosensitive drum 51.

[0031] The process unit PU includes a drawer 5 and four developing cartridges 6. The drawer 5 is pullable out from the main body housing 2. The drawer 5 includes four photosensitive drums 51, chargers 52, drum cleaners 53, and discharge lamps 54 respectively.

[0032] The charger 52 is a member that charges the surface of the photosensitive drum 51. When the charged surface of the photosensitive drum 51 is exposed by the exposure device SC, an electrostatic latent image based on the image data is formed on the surface of the photosensitive drum 51. The drum cleaner 53 is a member that cleans the surface of the photosensitive drum 51. The discharge lamp 54 is a member that irradiates light onto the surface of the photosensitive drum 51 for discharging.

[0033] The developing cartridge 6 stores toner. The developing cartridge 6 includes a developing roller 61 that supplies toner to the electrostatic latent image of the photosensitive drum 51 to form an image, which is a toner image, on the photosensitive drum 51.

[0034] The developing cartridges 6 are arranged in the order of 6K, 6C, 6M, and 6Y, where 6K, 6C, 6M, and 6Y indicate the cartridges filled with toner of each color of black, cyan, magenta, and yellow, from the downstream side to the upstream side in the conveyance direction of the sheet S. In other words, the developing cartridges 6K, 6C, 6M, and 6Y are arranged in the order of 6K, 6C, 6M, and 6Y from the downstream side to the upstream side in the moving direction in which the conveyance belt 73 described later moves toward the detection unit PS. Also, each photosensitive drum 51 corresponding to each developing cartridge 6 is arranged in the order of K, C, M, and Y from the downstream side to the upstream side in the moving direction in which the conveyance belt 73 moves toward the detection unit PS. In the present specification and drawings, when identifying members such as the photosensitive drum 51 and the developing roller 61 corresponding to the color of the toner, symbols K, C, M, and Y are attached corresponding to black, cyan, magenta, and yellow respectively, and the characters "first", "second", "third", and "fourth" are attached to the beginning of the member names.

[0035] For example, the photosensitive drum 51 and the developing roller 61 corresponding to black are also referred to as the first photosensitive drum 51K and the first developing roller 61K. The photosensitive drum 51 and the developing roller 61 corresponding to cyan are also referred to as the second photosensitive drum 51C and the second developing roller 61C. The photosensitive drum 51 and the developing roller 61 corresponding to magenta are also referred to as the third photosensitive drum 51M and the third developing roller 61M. The photosensitive drum 51 and the developing roller 61 corresponding to yellow are also referred to as the fourth photosensitive drum 51Y and the fourth developing roller 61Y. In the present embodiment, black corresponds to the first color, cyan corresponds to the second color, magenta corresponds to the third color, and yellow corresponds to the fourth color.

[0036] The transfer unit 7 includes a driving roller 71, a driven roller 72, a conveyance belt 73 as an example of a transfer medium, four transfer rollers 74, and a belt cleaner BC. The conveyance belt 73 is an endless belt. The driving roller 71 and the driven roller 72 are members for rotating the conveyance belt 73. The driving roller 71 is arranged downstream of the driven roller 72 in the conveyance direction of the sheet S.

[0037] The transfer roller 74 is a member that transfers the image on the photosensitive drum 51 to the sheet S or the conveyor belt 73. The transfer roller 74 is disposed inside the conveyor belt 73 and sandwiches the conveyor belt 73 between itself and the corresponding photosensitive drum 51.

[0038] The belt cleaner BC has a function of cleaning the toner on the conveyor belt 73. The belt cleaner BC is located on the side opposite to the photosensitive drum 51 with respect to the conveyor belt 73.

[0039] The detection unit PS is a sensor that detects the printing density of the inspection image formed on the conveyor belt 73. The detection unit PS is disposed opposite to the end of the conveyor belt 73 on the driving roller 71 side. The detection unit PS is an optical reflection type sensor including a light emitting element such as an LED and a light receiving element such as a phototransistor, and irradiates light on the image on the conveyor belt 73 to detect the reflected light.

[0040] The fixing device 8 is a device that thermally fixes the toner image on the sheet S. The fixing device 8 includes a heating roller 81 that heats the sheet S and a pressure roller 82 that sandwiches the sheet S between itself and the heating roller 81.

[0041] The conveying unit 9 is mainly configured to convey the sheet S discharged from the fixing device 8 outward of the main body housing 2. The conveying unit 9 mainly includes a conveying roller 91 and a discharge roller 92. The sheet S discharged from the fixing device 8 is conveyed by the conveying roller 91 to the discharge roller 92 and discharged to the discharge tray 21 by the discharge roller 92.

[0042] The control unit 100 has a CPU, a ROM, a RAM, etc., and is configured to execute various processes according to a program prepared in advance and in response to reception of a printing instruction, etc.

[0043] The control unit 100 has a function of executing a printing process for forming an image on the sheet S based on a print job output from a computer (not shown). In the printing process, the control unit 100 executes an exposure process of exposing the photosensitive drum 51 based on a printing pattern image that is a binary image. Specifically, when executing the printing process and printing an image on the sheet S, the control unit 100 selects one printing pattern image from a plurality of printing pattern images with different exposure densities (tones) so that the printing density of this image becomes a predetermined target density, and executes the exposure process. The printing pattern image consists of a binary image as shown in FIG. 2. The exposure density refers to the area ratio of the pixels to be exposed in the printing pattern image.

[0044] Here, the printing density is the density of the toner image developed by the developing roller 61, which is the density detected by the detection unit PS. The target density refers to the target density indicated by the print command. For example, when the target density indicated by the print command is 20%, the control unit 100 selects a printing pattern image with an exposure density of 20% and executes the exposure process with this printing pattern image. However, the relationship between the target density and the exposure density is changed by the tone setting process described later. For example, when it is changed from a state where a printing pattern image with an exposure density of 20% is associated with a target density of 20% to a state where a printing pattern image with an exposure density of 10% is associated with a target density of 20%, when forming an image with a target density of 20%, the exposure process is executed with a printing pattern image with an exposure density of 10%.

[0045] As described above, after the control unit 100 selects one printing pattern image corresponding to the target density from a plurality of printing pattern images, it performs exposure and development using the printing pattern image to form a toner image with the target density on the photosensitive drum 51.

[0046] Note that the plurality of printing pattern images correspond to the exposure density and are stored in the storage unit in advance. Also, the plurality of printing pattern images are stored in the storage unit in advance corresponding to each of the colors yellow, magenta, cyan, and black.

[0047] The control unit 100 is capable of performing image quality adjustment to adjust the image quality of the printed toner image. The control unit 100 performs image quality adjustment when a predetermined condition is satisfied. Specifically, for example, the control unit 100 may be configured to perform image quality adjustment when any one of a plurality of conditions shown below is satisfied. First condition: The developing cartridge 6 or the photosensitive drum 51 is new. Second condition: An instruction to perform image quality adjustment has been received from the user. Third condition: The environment such as temperature and humidity has changed by a predetermined value or more since the previous image quality adjustment. Fourth condition: The number of printed sheets since the previous execution of image quality adjustment has reached a predetermined number or more. Fifth condition: A developing cartridge 6 with a different serial number has been installed.

[0048] In image quality adjustment, the control unit 100 has a function of performing correction image formation processing, bias setting processing, pattern image formation processing, and gradation setting processing for each of the colors yellow, magenta, cyan, and black.

[0049] The correction image formation processing is a process in which, with a first developing bias applied to the developing roller 61, the image forming unit 4 forms a first bias correction image on the conveyance belt 73, and then, with a second developing bias having a value different from that of the first developing bias applied to the developing roller 61, the image forming unit 4 forms a second bias correction image on the conveyance belt 73. Here, the developing bias is the bias applied to the developing roller 61 by a bias application unit (not shown). The image forming unit 4 has a bias application unit.

[0050] The first developing bias and the second developing bias are set to optimal values for each color. The first developing bias may have different values for each color, or may have the same value for some or all of the plurality of colors. For example, the first developing bias corresponding to black may have a different value from the first developing bias corresponding to yellow, or may have the same value. Similarly, for the second developing bias, it may have different values for each color, or may have the same value for some or all of the plurality of colors.

[0051] Both the first bias correction image and the second bias correction image are printing pattern images corresponding to a predetermined exposure density. Specifically, the first bias correction image is formed by using the first developing bias to form, for example, a printing pattern image with an exposure density of 40% shown in FIG. 2. In this case, the second bias correction image is formed by using the second developing bias to form a printing pattern image with an exposure density of 40%.

[0052] The bias setting process is a process of setting the developing bias based on the results of reading the first bias correction image and the second bias correction image by the detection unit PS. In the bias setting process, the control unit 100 converts the intensity of the reflected light from each bias correction image detected by the detection unit PS into a printing density, and calculates the developing bias at which the printing density becomes the target density based on the two types of printing densities and the two types of developing biases. The developing bias set in the bias setting process is set to an optimal value for each color.

[0053] The pattern image forming process is a process of forming a plurality of pattern images on the conveyance belt 73 by the image forming unit 4 with the developing bias set in the bias setting process applied to the developing roller 61. Here, the plurality of pattern images are tone setting images for performing tone setting processing. Specifically, the plurality of pattern images are a plurality of printing pattern images corresponding to a plurality of exposure densities. For example, five types of printing pattern images corresponding to exposure densities of 5%, 20%, 40%, 60%, and 80% are used.

[0054] The gradation setting process is a process of setting the correspondence relationship between the target density and the exposure density based on the results of reading a plurality of pattern images by the detection unit PS. Specifically, in the gradation setting process, the control unit 100 sets the correspondence relationship between the target density and the printing pattern image, so that the correspondence relationship between the target density and the density of the toner image actually printed, that is, the exposure density, is set.

[0055] Specifically, the control unit 100 detects a plurality of pattern images by the detection unit PS, and converts the intensity of the reflected light from each pattern image into the printing density respectively. Then, the control unit 100 compares each calculated printing density with the corresponding exposure density, and when the compared printing density and exposure density are different, corrects the correspondence relationship between the printing density and the exposure density to change the correspondence relationship between the target density and the printing pattern image.

[0056] Specifically, when the gradation setting process has never been performed in the past, the correspondence relationship between the target density and the printing pattern image (exposure density) is, for example, a default gamma table as shown by the two-dot chain line in FIG. 3. In the following description, the correspondence relationship between the target density and the printing pattern image is also referred to as the "gamma table".

[0057] The gamma table is a table for selecting a printing pattern of a predetermined exposure density corresponding to a predetermined target density when printing at a predetermined target density. The gamma table shown by the two-dot chain line, that is, the default gamma table, has the same value for the target density and the exposure density.

[0058] In the gradation setting process, for example, when the detection unit PS detects a printing pattern image corresponding to an exposure density of 20% as indicated by a black circle in the figure, if the printing density of the image becomes 10%, the control unit 100 stores the printing pattern image again as a pattern image corresponding to a target density of 10%. That is, in the next printing control, when forming an image at a target density of 10%, if image formation is performed using the first printing pattern image corresponding to an exposure density of 20%, the printing density will be 10%. Therefore, the control unit 100 associates the printing pattern image with an exposure density of 20% for a target density of 10%.

[0059] Also, in the gradation setting process at this time, when the printing density is 40% when the printing pattern image with an exposure density of 40% is detected by the detection unit PS, that is, when the target density, exposure density, and printing density match, as shown by the dashed line in the figure, the relationship between the target density and the exposure density is linearly interpolated. As a result, for example, in the next printing control, when forming an image at a target density of 20%, the printing pattern image corresponding to the exposure density X corresponding to the intersection of the line of the target density of 20% and the dashed line in the figure is selected. Note that the gamma table at this time is a table in which the lines with an exposure density of less than 40% in the default gamma table (two-dot chain line) are replaced with the dashed line in the figure. That is, as shown by the dashed line between the black circle shown in the figure and the white circle corresponding to the exposure density of 40%, it is supplemented.

[0060] As shown in FIG. 4, in the correction image formation process, the control unit 100 forms a first image group G1 composed of a plurality of color first bias correction images K1, C1, M1, Y1 and a plurality of color second bias correction images K2, C2, M2, Y2. In the pattern image formation process, the control unit 100 forms a second image group G2 composed of a plurality of color pattern images Kγ1 to Kγ5, Cγ1 to Cγ5, Mγ1 to Mγ5, Yγ1 to Yγ5. The order of the first bias correction images K1 to Y1, the second bias correction images K2 to Y2, and the pattern images Kγ1 to Yγ5 on the conveyance belt 73 is set in the order shown in the figure so that the time from the start of the correction image formation process to the end of the pattern image formation process is as short as possible. Specifically, the exposure order is determined so that the order of the inspection images on the conveyance belt 73 is the order shown in the figure. The exposure order is stored in a storage unit (not shown). Note that in FIG. 4, each image is shown in a simplified manner.

[0061] Each inspection image is arranged in the moving direction with a space between each other. Specifically, the first bias correction images K1 to Y1 and the second bias correction images K2 to Y2 are arranged at a first pitch Lp1, respectively. Among the plurality of pattern images Kγ1 to Yγ5, two adjacent pattern images (for example, Kγ1, Kγ2) are arranged at a second pitch Lp2.

[0062] Here, the length of each image in the moving direction is set according to the number of reading points (the number of positions to be read) of the detection unit PS. For example, when the number of reading points in the bias setting process is a first predetermined number, the length of each image in the moving direction constituting the first image group G1 is set to a first length corresponding to the first predetermined number. When the number of reading points in the gradation setting process is a second predetermined number smaller than the first predetermined number, the length of each image in the moving direction constituting the second image group G2 is a second length corresponding to the second predetermined number and smaller than the first length. The interval between each image is set in consideration of variations in color misregistration and reduction of toner consumption.

[0063] Each bias-corrected image is set to be arranged in the order of K1, Y1, M1, K2, C1, Y2, M2, C2 in sequence from the head in the moving direction. The control unit 100 executes correction image formation processing so that the bias-corrected images are arranged in the order described above.

[0064] Each pattern image is set to be arranged in the order of Kγ1, Kγ2, Kγ3, Kγ4, Kγ5, Cγ1, Cγ2, Mγ1, Mγ2, Cγ3, Yγ1, Yγ2, Mγ3, Cγ4, Yγ3, Mγ4, Mγ5, Cγ5, Yγ4, Yγ5 in sequence from the head in the moving direction. The pattern image Kγ3 is arranged after the pattern image Kγ2 with an interval of about two pattern images. Specifically, the pattern image Kγ3 is arranged with a pitch of 3×Lp2 with respect to the pattern image Kγ2. Note that the expression of converting the interval between two images into pitch is the same for other images, so it will be omitted in the following description.

[0065] The pattern image Kγ5 is arranged after the pattern image Kγ4 with an interval of about two pattern images. The pattern image Cγ1 is arranged after the pattern image Kγ5 with an interval of about one pattern image. The pattern image Mγ1 is arranged after the pattern image Cγ2 with an interval of about one pattern image. The foremost pattern image Kγ1 in the moving direction is arranged after the second bias-corrected image C2 with an interval of about three pattern images. The distance from the front end of the foremost first bias-corrected image K1 to the rear end of the rearmost pattern image Yγ5 in the moving direction is smaller than the circumference of the conveyance belt 73.

[0066] The order of the inspection images is determined to satisfy the following conditions. In the following description, the inspection images are also referred to as patches Gp. <Regarding the first image group G1> 1. Place patches Gp of the same color with an interval of 2×Lp1 or more. 2. Do not set the interval between patches Gp of the same color to be 5×Lp1.

[0067] The condition of 1 is a condition for ensuring the time required to switch the development bias from the first development bias to the second development bias and for preventing continuous development at the same position on the development roller 61. In the present embodiment, the time required to switch the development bias from the first development bias to the second development bias is approximately equal to the time when the conveyance belt 73 moves by the first pitch Lp1. Therefore, considering the condition for switching the development bias, the interval between patches Gp of the same color needs to be an interval of the first pitch Lp1 or more.

[0068] When development is continuously performed at the same position on the development roller 61, the formed patch Gp may become unclear. Hereinafter, this phenomenon will be described with reference to FIG. 5. Note that the length in the moving direction of one patch Gp will be described as being approximately 1 / 2 of the circumference of the development roller 61 and approximately 1 / 6 of the circumference of the photosensitive drum 51.

[0069] As shown in FIGS. 5(a) and 5(b), when the first range R1 of the photosensitive surface of the photosensitive drum 51 is exposed, the toner located in the second range R2 of the development roller 61 moves to the first range R1, and a patch Gp is formed in the first range R1 of the photosensitive drum 51. As shown in FIG. 5(c), when the development roller 61 rotates half a turn from the position in FIG. 5(b) and the second range R2 reaches the development nip position Pn, which is the contact point between the photosensitive drum 51 and the development roller 61, again, the amount of toner on the second range R2 may be reduced. At this time, if the toner on the second range R2 is transferred to the photosensitive drum 51 to form a patch Gp, the patch Gp may become unclear. Therefore, in order to prevent two patches Gp of the same color from being arranged with an interval of the first pitch Lp1, the patches Gp of the same color need to be arranged with an interval of 2×Lp1 or more.

[0070] The condition of 2 is a condition for preventing the formation defect of the electrostatic latent image due to the influence of the previous exposure remaining at a predetermined position when the predetermined position of the photosensitive drum 51 is exposed again when the photosensitive drum 51 makes exactly one rotation after the exposure of the predetermined position of the photosensitive drum 51. Hereinafter, this phenomenon will be described with reference to FIG. 5. As shown in FIG. 5(d), the first range R1 which is the exposed range of the photosensitive drum 51 is discharged when passing through the charge-removing lamp 54, but the influence of the previous exposure may remain. If the first range R1 in a state where such an influence of exposure remains is exposed again, there is a possibility that a formation defect of the electrostatic latent image may occur. Therefore, it is necessary not to set the interval between the patches Gp of the same color to an interval of 5×Lp1.

[0071] <Regarding the second image group G2> The order of each pattern image of the second image group G2 is set to satisfy the following conditions in addition to the above-described conditions 1 and 2. 3. The distance from the rear end of the second bias correction image of a certain color (for example, K2) to the topmost pattern image of that color (for example, Kγ1) shall be equal to or greater than a predetermined distance La. La = Lb + Lc Lb: The distance that the patch Gp moves from the developing nip position Pn to the reading position Pr (see FIG. 6) at the detection unit PS Lc = The distance that the patch Gp moves during the time from reading the second bias correction image until the developing bias is switched to the developing bias set by the bias setting process

[0072] The distance Lb is different for each color. Specifically, the distance LbK corresponding to black (see FIG. 6(a)) is the smallest, the distance LbC corresponding to cyan is the second smallest, the distance LbM corresponding to magenta is the third smallest, and the distance LbY corresponding to yellow is the largest.

[0073] The distance Lc is the same for each color. Note that the distance Lc may be slightly different for each color.

[0074] As described above, the predetermined distance LaK corresponding to black is the smallest, and the predetermined distance LaC corresponding to cyan is the second smallest. The predetermined distance LaM corresponding to magenta is the third smallest, and the predetermined distance LaY corresponding to yellow is the largest.

[0075] As shown in FIG. 4, in order to start forming the black pattern images Kγ1 to Kγ5 corresponding to the smallest predetermined distance LaK earlier than the images of other colors, the first bias correction image K1 is arranged at the head in the moving direction, and the second bias correction image K2 is arranged such that the distance between the first bias correction image K1 and the second bias correction image K2 is the shortest 2×Lp1. In the following description, the names of the respective images may be omitted and only the reference signs may be used.

[0076] In order to speed up the completion timing of forming the yellow pattern images Yγ1 to Yγ5 corresponding to the largest predetermined distance LaY, Y1 is arranged immediately after K1, and Y2 is arranged in front of M2 and C2. Although it is also conceivable to reverse the order of C1 and Y2 and set the distance between Y1 and Y2 to the shortest 2×Lp1, in this case, since the distance between C1 and C2 must be set to 2×Lp1, a blank (an area where no image is formed) is generated between M2 and C2, and the length of the first image group G1 increases. Therefore, the arrangement as shown in the figure is preferable.

[0077] Y2, M2, and C2 are arranged in order from the front in descending order of the predetermined distance La. This makes it possible to minimize the number of blanks.

[0078] The distance from the rear end of K2 to the front end of Kγ1 is equal to or greater than the predetermined distance LaK. The distance from the rear end of Y2 to the front end of Yγ1 is equal to or greater than the predetermined distance LaY. The distance from the rear end of M2 to the front end of Mγ1 is equal to or greater than the predetermined distance LaM. The distance from the rear end of C2 to the front end of Cγ1 is equal to or greater than the predetermined distance LaC. In this embodiment, the difference between the distance from the rear end of the second bias correction image (for example, K2) to the front end of the pattern image (for example, Kγ1) at the head of its color and the predetermined distance (for example, LaK) in that color is smaller for K and Y than for M and C. As a result, it is possible to shorten the overall length from the front end of the first image group G1 formed on the conveyance belt 73 to the rear end of the second image group G2.

[0079] As described above, since the order of the images is determined, the control unit 100 starts the correction image formation process in the order of K, Y, M, C and ends it in the order of K, Y, M, C. Also, the control unit 100 starts the bias setting process in the order of K, Y, M, C and ends it in the order of K, Y, M, C. Further, the control unit 100 starts the pattern image formation process in the order of K, C, M, Y and ends it in the order of K, M, C, Y.

[0080] Next, a method for forming the first image group G1 by the control unit 100 will be described. Since the method for forming the second image group G2 is the same as the method for forming the first image group G1, the description thereof will be omitted.

[0081] When executing the correction image formation process, the control unit 100 exposes each photosensitive drum 51 according to the exposure order stored in the storage unit. As a result, as shown in FIG. 6(a), Y1 is first formed on the conveyance belt 73.

[0082] Thereafter, as shown in FIG. 6(b), Y2 is formed at a position 3×Lp1 behind Y1. Thereafter, as shown in FIG. 6(c), M1 is formed behind Y1 at the first pitch Lp1.

[0083] Thereafter, as shown in FIG. 7(a), M2 is formed behind Y2 at the first pitch Lp1. Thereafter, as shown in FIG. 7(b), K1 is formed in front of Y1 and C1 is formed in front of Y2 at the first pitch Lp1. Thereafter, as shown in FIG. 7(c), K2 is formed behind M1 and C2 is formed behind M2 at the first pitch Lp1.

[0084] According to the above, the following effects can be obtained in this embodiment. Since the bias setting process for black can be completed quickly, the start of the pattern image formation process for black can be advanced. As a result, the time from the start of the first correction image formation process for all colors to the end of the last pattern image formation process can be shortened.

[0085] In the moving direction, since the second bias correction image K2 for black is positioned in front of the first bias correction image C1 for cyan, the bias setting process for black can be completed even more quickly. As a result, the time from the start of the first correction image formation process for all colors to the end of the last pattern image formation process can be further shortened.

[0086] Since the second bias correction image Y2 for yellow is positioned in front of the second bias correction images M2 and C2 for magenta and cyan in the moving direction, the bias setting process for yellow can be completed quickly. As a result, the start of the pattern image formation process for yellow can be advanced, and thus the time from the start of the first correction image formation process for all colors to the end of the last pattern image formation process can be shortened.

[0087] By making the distance from the front end of the foremost first bias correction image K1 to the rear end of the rearmost pattern image Yγ5 in the moving direction smaller than the circumference of the conveyance belt 73, it is not necessary to perform a belt cleaning process that rotates the conveyance belt 73 one or more times between the bias setting process and the pattern image formation process. As a result, the time from the start of the first correction image formation process for all colors to the end of the last pattern image formation process can be shortened. In the conventional belt cleaning process, the conveyance belt 73 is rotated twice, so a significant time reduction is possible compared to this prior art.

[0088] Note that the present invention is not limited to the above-described embodiments and can be used in various forms as exemplified below. In the following description, members having substantially the same structure as those in the above-described embodiments are denoted by the same reference numerals, and the description thereof is omitted.

[0089] The pattern image is not limited to the tone setting image as in the above-described embodiment. For example, as shown in FIG. 8, the pattern image may be misregistration correction images Kp1, Yp1, Mp1, Cp1, Kp2, Yp2, Mp2, Cp2 for correcting misregistration of the image. Specifically, the first misregistration correction images Kp1, Yp1, Mp1, Cp1 extend in a predetermined direction. Here, the predetermined direction may be a direction inclined with respect to the width direction of the conveyance belt 73 or a parallel direction.

[0090] The second misregistration correction images Kp2, Yp2, Mp2, Cp2 are inclined with respect to the predetermined direction. And an image group G11 composed of the misregistration correction images Kp1, Yp1, Mp1, Cp1, Kp2, Yp2, Mp2, Cp2 is arranged in a plurality in the moving direction. Even in such a form, the time from the start of the first correction image formation process to the end of the last pattern image formation process for all colors can be shortened.

[0091] The order of each image is not limited to the above-described embodiment. The second bias correction image of the first color may be located between the first bias correction images of two colors among the second, third, and fourth colors. For example, in the above-described embodiment, K2 may be arranged between Y1 and M1.

[0092] Further, it is sufficient that the color of the first bias correction image located at the forefront of the first image group and the color of the pattern image located at the forefront of the second image group are the first color, and the color of the second bias correction image located at the rear of the first image group and the color of the pattern image located at the rear of the second image group are different. For example, in the above-described embodiment, the color of the second bias correction image located at the rear of the first image group G1 may be magenta, and the color of the pattern image located at the rear of the second image group G2 may be yellow.

[0093] Also, in the moving direction, the second bias correction image of the first color may be positioned in front of the first bias correction image of one of the second color, the third color, and the fourth color. For example, in the above embodiment, K2 may be positioned in front of M2.

[0094] Also, the second bias correction image of the fourth color may be positioned in front of the second bias correction image of at least one of the second color and the third color in the moving direction. For example, in the above embodiment, Y2 may be positioned between M2 and C2.

[0095] Also, the first bias correction image of the fourth color may be positioned in front of the first bias correction image of at least one of the second color and the third color in the moving direction. For example, in the above embodiment, Y1 may be positioned between M1 and K2.

[0096] The first developing bias for a predetermined color and the first developing bias for a color different from the predetermined color may be the same value or different values. The second developing bias for a predetermined color and the second developing bias for a color different from the predetermined color may be the same value or different values.

[0097] In addition to the first bias correction image and the second bias correction image, a third bias correction image may be formed between the first bias correction image and the second bias correction image, and the developing bias may be corrected using these three bias correction images.

[0098] The transfer medium may be a sheet such as paper. The belt may be an intermediate transfer belt.

[0099] In the above embodiment, the present invention is applied to the color printer 1, but the present invention is not limited to this, and the present invention may be applied to other image forming apparatuses such as copiers and multifunction machines. In the above embodiment, the color printer 1 exposes the photosensitive drum with a laser scanner, but as the exposure device, other exposure devices such as an LED exposure head may be used. Also, the first photosensitive drum, the second photosensitive drum, the third photosensitive drum, and the fourth photosensitive drum of the color printer 1 may be supported by one cartridge, or may be supported by individual cartridges.

[0100] Each element described in the above embodiment and modification examples may be implemented by arbitrarily combining them.

Explanation of Reference Numerals

[0101] 1 Color printer 4 Image forming unit 51 Photosensitive drum 51K First photosensitive drum 51C Second photosensitive drum 51M Third photosensitive drum 51Y Fourth photosensitive drum 61 Developing roller 73 Conveyor belt 100 Control unit K1 First bias correction image C1 First bias correction image M1 First bias correction image Y1 First bias correction image K2 Second bias correction image Kγ1~Yγ5 Pattern image PS Detection unit

Claims

1. An image forming unit that forms images of a plurality of colors on a transfer medium, a developing roller and a photosensitive drum corresponding to a first color, a developing roller and a photosensitive drum corresponding to a second color, a developing roller and a photosensitive drum corresponding to a third color, a developing roller and a photosensitive drum corresponding to a fourth color, and an image forming unit having the same, a detection unit that detects an image formed on the transfer medium, a control unit, and is provided with, In the moving direction in which the transfer medium moves toward the detection unit, in the order of the first color, the second color, the third color, and the fourth color from the downstream side to the upstream side, an image forming apparatus in which each photosensitive drum is arranged, The control unit, For each of the first color, the second color, the third color, and the fourth color, After forming a first bias correction image on the transfer medium by the image forming unit with a first developing bias applied to the developing roller, a correction image forming process of forming a second bias correction image on the transfer medium by the image forming unit with a second developing bias having a value different from the first developing bias applied to the developing roller, A bias setting process for setting a developing bias based on the results of reading the first bias correction image and the second bias correction image by the detection unit, A pattern image forming process for forming a plurality of pattern images on the transfer medium by the image forming unit with the developing bias set in the bias setting process applied to the developing roller, and can execute, In the moving direction, the first bias correction image of the first color is located in front of the first bias correction images of the second color, the third color, and the fourth color, and the second bias correction image of the first color is located between the first bias correction images of two of the second color, the third color, and the fourth color. An image forming apparatus characterized by executing the correction image forming process.

2. An image forming unit that forms images of a plurality of colors on a transfer medium, a developing roller and a photosensitive drum corresponding to a first color, a developing roller and a photosensitive drum corresponding to a second color, a developing roller and a photosensitive drum corresponding to a third color, a developing roller and a photosensitive drum corresponding to a fourth color, and an image forming unit having the same, a detection unit that detects an image formed on the transfer medium, a control unit, and is provided with, An image forming apparatus in which the photosensitive drums are arranged in the order of the first color, the second color, the third color, and the fourth color from the downstream side to the upstream side in the moving direction in which the transfer medium moves toward the detection unit, the control unit, for each of the first color, the second color, the third color, and the fourth color, after forming a first bias correction image on the transfer medium by the image forming unit with a first developing bias applied to the developing roller, a correction image forming process of forming a second bias correction image on the transfer medium by the image forming unit with a second developing bias having a value different from that of the first developing bias applied to the developing roller, a bias setting process of setting a developing bias based on the results of reading the first bias correction image and the second bias correction image by the detection unit, and a pattern image forming process of forming a plurality of pattern images on the transfer medium by the image forming unit with the developing bias set in the bias setting process applied to the developing roller, are executable, the color of the first bias correction image located at the forefront of the first image group composed of the first bias correction images of a plurality of colors and the second bias correction images of a plurality of colors is the first color, and the color of the second bias correction image located at the rearmost of the first image group is different from the color of the pattern image located at the rearmost of the second image group composed of the pattern images of a plurality of colors, and in the moving direction, the second bias correction image of the first color is positioned in front of the first bias correction image of one of the second color, the third color, and the fourth color, the image forming apparatus characterized by executing the correction image forming process and the pattern image forming process.

3. the control unit, the image forming apparatus according to claim 1 or claim 2, characterized in that the correction image forming process is executed such that the second bias correction image of the fourth color is positioned in front of at least one of the second color and the third color in the moving direction.

4. the control unit, The image forming apparatus according to any one of claims 1 to 3, wherein the correction image forming process is executed such that the first bias correction image of the fourth color is positioned in front of the first bias correction image of at least one of the second color and the third color in the moving direction.

5. The control unit in order from the head in the moving direction, the first bias correction image of the first color, the first bias correction image of the fourth color, the first bias correction image of the third color, the second bias correction image of the first color, The image forming apparatus according to any one of claims 1 to 4, wherein the correction image forming process is executed such that each image is arranged in the order of the first bias correction image of the second color.

6. The control unit in the moving direction, after the first bias correction image of the second color, the second bias correction image is arranged in the order of the fourth color, the third color, and the second color, and the correction image forming process is executed. The image forming apparatus according to claim 5, characterized in that

7. The transfer medium is an endless belt, The image forming apparatus according to any one of claims 1 to 6, wherein a distance from the front end of the foremost first bias correction image to the rear end of the rearmost pattern image in the moving direction is smaller than the circumference of the belt.

8. The image forming apparatus according to any one of claims 1 to 7, wherein the pattern image is a tone setting image for setting a correspondence relationship between a target density and an exposure density.

9. The image forming apparatus according to any one of claims 1 to 7, wherein the pattern image is a misregistration correction image for correcting misregistration of an image.

Images