Image forming apparatus

Abstract

The present invention provides an image forming apparatus that can effectively prevent the image density of any of the toner images formed on multiple image carriers from falling below the appropriate image density, thereby suppressing a decrease in the image quality of the toner image transferred to the sheet. [Solution] The image forming apparatus 100 is characterized in that it performs image quality adjustment processing each time the number of sheets P reaches a predetermined number, and based on the image quality adjustment processing results obtained in the image quality adjustment processing, it is possible to change the setting of a first common development bias Vb1, which is a common development bias Vb supplied from the development power supply 61 while the first image forming mode is being executed, and a second common development bias Vb2, which is a common development bias Vb supplied from the development power supply 61 while the second image forming mode is being executed.

Description

Technical Field

[0001] The present disclosure relates to image forming apparatuses such as copiers, multifunction peripherals, printers, and facsimile apparatuses.

[0002] As an image forming apparatus (color image forming apparatus), a tandem type image forming apparatus including a plurality of image carriers has been conventionally known.

[0003] As a tandem type image forming apparatus, for example, there is one including a plurality of image carriers (photosensitive drums), a plurality of developing devices, a single developing power supply, an intermediate transfer device, and a control unit (see, for example, Patent Document 1). Specifically, electrostatic latent images are respectively formed on the plurality of image carriers. The plurality of developing devices each have a developer carrier (developing roller) that carries a developer containing toner on its surface. The plurality of developing devices form toner images on the image carriers by supplying the toner carried on the developer carrier to the electrostatic latent images respectively formed on the plurality of image carriers. The intermediate transfer device has an intermediate transfer body to which the toner image is intermediately transferred, and transfers the toner image intermediately transferred to the intermediate transfer body to a sheet. The single developing power supply supplies a common developing bias, which is a predetermined voltage value common (identical) to each of the developer carriers in the plurality of developing devices. The control unit can selectively execute a first image forming mode and a second image forming mode. The first image forming mode forms a first toner image (monochrome toner image) on a first image carrier, which is an image carrier developed by a first developing device that is one of the plurality of developing devices, and transfers the formed first toner image to a sheet via the intermediate transfer body. The second image forming mode forms a second toner image (color toner image) on a second image carrier, which is an image carrier developed by a second developing device that is at least two developing devices including the first developing device among the plurality of developing devices, and transfers the formed second toner image to a sheet via the intermediate transfer body.

Background Art

[0004] Incidentally, in image forming apparatuses, the development characteristics, such as the charge amount of toner, usually change as the number of image formation cycles increases, or more specifically, as the number of sheets to be image formed increases. Therefore, it is necessary to change the development bias supplied to the developer carriers in each of the multiple developing apparatuses so that the toner images formed on each of the multiple image carriers all have the appropriate image density.

[0005] However, in an image forming apparatus equipped with a single developing power supply, the single developing power supply provides a common developing bias to each developer carrier in multiple developing devices. As the number of sheets to be image formed increases, even if the developing characteristics, such as the amount of charge of the toner, change, the single developing power supply cannot individually change the developing bias supplied to each developer carrier in multiple developing devices. Consequently, it may not be possible to achieve the correct image density for all of the toner images formed on the multiple image carriers.

[0006] In this regard, Patent Document 1 describes an image forming apparatus equipped with a single developing power supply, in which the exposure light amount of multiple exposure means is adjusted according to the density detection result of a density detection means that detects the density of toner image transferred to an intermediate transfer material. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2013-174903 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] However, in the image forming apparatus described in Patent Document 1, there is a limit to the adjustment range of the exposure light intensity of the multiple exposure means, and depending on the value of the common development bias supplied from a single development power supply, the image density of any of the toner images formed on the multiple image carriers may become lower than the appropriate image density. This leads to a decrease in the image quality of the toner image transferred to the sheet.

[0009] Therefore, the present disclosure aims to provide an image forming apparatus that can effectively prevent any toner image formed on multiple image carriers from having an image density lower than the appropriate image density, thereby suppressing a decrease in the image quality of the toner image transferred to the sheet. [Means for solving the problem]

[0010] To solve the aforementioned problems, the present disclosure provides an image forming apparatus comprising: a plurality of image carriers on which electrostatic latent images are formed based on image data; a plurality of developing apparatuses having developer carriers on which a developer containing toner is carried on the surface, for which toner is supplied to each of the electrostatic latent images for development and for which toner is supplied to each of the plurality of image carriers to form toner images; an intermediate transfer apparatus having an intermediate transfer body for intermediately transferring the toner images formed on the plurality of image carriers to the intermediate transfer body and then transferring them to a sheet; a single developing power supply that supplies a common developing bias, which is a predetermined voltage value common to each of the developer carriers in the plurality of developing apparatuses; and a first toner image formed on a first image carrier, which is an image carrier developed by a first developing apparatus, which is one of the plurality of developing apparatuses, and the formed first toner image transferred to the sheet via the intermediate transfer body. The device comprises a control unit capable of selectively executing a first image forming mode and a second image forming mode, in which a second image carrier is developed by a second developing device which is at least two developing devices, including the first developing device, among the plurality of developing devices, and the formed second toner image is transferred to the sheet via the intermediate transfer body, wherein the control unit executes an image quality adjustment process each time the number of sheets to which the toner image has been transferred reaches a predetermined number, and can change the setting of a first common development bias, which is the common development bias supplied from the developing power supply while the first image forming mode is being executed, and a second common development bias, which is the common development bias supplied from the developing power supply while the second image forming mode is being executed, based on the image quality adjustment process result obtained in the image quality adjustment process. [Effects of the Invention]

[0011] According to this disclosure, it is possible to effectively prevent the image density of any of the toner images formed on each of the multiple image carriers from becoming lower than the appropriate image density, thereby suppressing the deterioration of the image quality of the toner image transferred to the sheet. [Brief explanation of the drawing]

[0012] [Figure 1]This is a cross-sectional view showing the schematic configuration of the image forming apparatus according to this embodiment. [Figure 2] Figure 1 is a cross-sectional view showing an example of an image forming apparatus equipped with a single developing power supply. [Figure 3] This graph shows the surface potential at different positions along the axial direction of the photoreceptor drum's surface. [Figure 4] This is a schematic block diagram showing the system configuration of the control unit in an image forming apparatus. [Figure 5] This is a first development bias table showing the first common development bias when the first image formation mode is set based on the image quality adjustment processing results. [Figure 6] This is a second development bias table showing the second common development bias when the second image formation mode is set based on the image quality adjustment processing results. [Figure 7] This is a bottom view showing the toner patterns for adjusting the image quality of each color transferred to the intermediate transfer medium. [Figure 8] This is a front view showing the positional relationship between the detection unit and the intermediate transfer body. [Figure 9] This is a side view showing the positional relationship between the detection unit and the intermediate transfer material. [Figure 10] This graph shows the image density of yellow toner patterns formed by changing the common development bias to multiple values. [Figure 11] This graph shows the image density of magenta toner patterns formed by changing the common development bias to multiple values. [Figure 12] This graph shows the image density of cyan toner patterns formed by changing the common development bias to multiple values. [Figure 13] This graph shows the image density of black toner patterns formed by changing the common development bias to multiple values. [Figure 14] This is the first development bias table, which shows the first common development bias when binarization is performed in the first image formation mode. [Figure 15]The first development bias table showing the first common development bias when performing processes other than the binarization process in the first image formation mode. [Figure 16] FIG. 1 is a cross-sectional view showing an example in which a single developing power supply and a single charging power supply are provided in the image forming apparatus shown in FIG. 1. [Figure 17] FIG. 6 is a flowchart of a main routine showing an example of an image forming process. [Figure 18] FIG. 17 is a flowchart of a subroutine showing an example of the image quality adjustment process.

Embodiments for Carrying Out the Invention

[0013] Hereinafter, embodiments according to the present disclosure will be described with reference to the drawings. In the following description, the same parts are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed descriptions thereof will not be repeated.

[0014] 〔Image Forming Apparatus〕 FIG. 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus 100 according to the present embodiment. In the figure, the width direction is X, the left side is X1, the right side is X2, the direction orthogonal to the width direction X is the depth direction Y, the front side is Y1, the back side is Y2, and the vertical direction orthogonal to the width direction X and the depth direction Y is Z. The following description will be made with reference to these directions.

[0015] FIG. 2 is a cross-sectional view showing an example in which a single developing power supply 61 is provided in the image forming apparatus 100 shown in FIG. 1. FIG. 3 is a graph showing the surface potential at positions in the axial direction on the surfaces of the photoreceptor drums 3y, 3m, 3c, and 3k.

[0016] The image forming apparatus 100 is a multifunctional device having a copying function, a scanner function, a facsimile function, and a printer function, and transmits the image of the document G read by the image reading device 102 to the outside. Further, the image forming apparatus 100 forms an image of the document G read by the image reading device 102 or an image received from the outside in color or monochrome on a sheet P such as paper.

[0017] The image reading device 102 includes a document placement table 130a (document setting table) on which the document G is placed, and an image reading unit 130 that reads the document G placed on the document placement table 130a. The image reading unit 130 creates image data by moving the scanning optical system 130c in the scanning direction, thereby reading the image of the document G placed on the document placement table 130a with the document reading unit 130b.

[0018] The image reading device 102 further includes a document feeder 160. The document feeder 160 is supported above the image reading unit 130 so as to be openable and closable relative to the image reading unit 130. The document feeder 160 transports one or more documents G one by one to the image reading position 130d. The documents G sent to the image reading position 130d are read by the document reading unit 130b via the scanning optical system 130c which is stopped at the image reading position 130d, and image data is created. As described above, the image reading unit 130 generates image data by scanning the scanning optical system 130c and reading the documents G placed on the document tray 130a with the document reading unit 130b, or by reading the documents G transported by the document feeder 160 with the document reading unit 130b.

[0019] The image forming apparatus main unit 101 includes a light scanning device 1, developing devices 2y, 2m, 2c, 2k, photoreceptor drums 3y, 3m, 3c, 3k (an example of an image carrier), drum cleaning devices 4y, 4m, 4c, 4k, charging devices 5y, 5m, 5c, 5k (chargers), intermediate transfer device 70 (primary transfer belt device), secondary transfer device 11, fixing device 12, sheet transport path S, paper feed cassette 18, and sheet discharge trays 141 (141a, 141b).

[0020] The image forming apparatus 100 first transfers a toner image formed using multiple colors of toner to an intermediate transfer body 71 (primary transfer belt), and then secondarily transfers the toner image transferred to the intermediate transfer body 71 to a sheet P.

[0021] In this embodiment, image data corresponding to color images using yellow (Y), magenta (M), and cyan (C), or monochrome images using a single color [e.g., black (K)], is handled. In the following description, yellow, magenta, cyan, and black will simply be referred to as Y, M, C, and K, respectively.

[0022] The image forming unit 50 of the image forming apparatus 100 is equipped with developing devices 2y, 2m, 2c, 2k for forming four types of toner images, photoreceptor drums 3y, 3m, 3c, 3k, drum cleaning devices 4y, 4m, 4c, 4k, and charging devices 5y, 5m, 5c, 5k, which are corresponding to Y, M, C, and K respectively, forming four image stations Py, Pm, Pc, and Pk.

[0023] Each charging device 5y, 5m, 5c, and 5k has a charging member 51y, 51m, 51c, and 51k (charging rollers in this example) (see Figure 2) that uniformly charges the surface of the photoreceptor drums 3y, 3m, 3c, and 3k to a predetermined charging potential Vo (for example, about -600V to -400V, in this example, -525V) (see Figure 3). The optical scanning device 1 exposes the surface of the photoreceptor drums 3y, 3m, 3c, and 3k, which are charged to the predetermined charging potential, to form an electrostatic latent image. At this time, as shown in Figure 3, the image potential Vi (for example, about -50V to -100V) of the image portion (the portion exposed by the optical scanning device 1) on the surface of the photoreceptor drums 3y, 3m, 3c, and 3k has an absolute value smaller than the absolute value of the charging potential Vo (for example, about -600V to -400V). The developing units 2y, 2m, 2c, and 2k develop the electrostatic latent image on the surface of the photoreceptor drums 3y, 3m, 3c, and 3k with toner of each color, forming a toner image on the surface of the photoreceptor drums 3y, 3m, 3c, and 3k. Therefore, the developing units 2y, 2m, 2c, and 2k have developing rollers 21y, 21m, 21c, and 21k (an example of a developer carrier) (see Figure 2) that supply toner of each color to the surface of the photoreceptor drums 3y, 3m, 3c, and 3k. The drum cleaning units 4y, 4m, 4c, and 4k remove and recover any residual toner remaining on the surface of the photoreceptor drums 3y, 3m, 3c, and 3k after the primary transfer described later. Through the series of operations described above, toner images of each color are formed on the surface of each photoreceptor drum 3y, 3m, 3c, and 3k.

[0024] The intermediate transfer device 70 comprises an intermediate transfer body 71, primary transfer rollers 6y, 6m, 6c, 6k, a plurality of belt tension rollers 72-72 (including a drive roller 721, a driven roller 722, and a tension roller 723), and a belt cleaning device 9. The intermediate transfer body 71 is stretched over the plurality of belt tension rollers 72-72, and when the drive roller 721, one of the belt tension rollers 72-72, is rotationally driven by a drive device (not shown), it rotates in a predetermined rotational direction R. At this time, the driven roller 722 and the tension roller 723 rotate in conjunction with the intermediate transfer body 71. The primary transfer rollers 6y, 6m, 6c, 6k are provided inside the intermediate transfer body 71 to bring it into contact with the surfaces of the corresponding photosensitive drums 3y, 3m, 3c, 3k. The toner images of each color formed on the surface of the photoreceptor drums 3y, 3m, 3c, and 3k are primary transferred to the intermediate transfer body 71 by the primary transfer rollers 6y, 6m, 6c, and 6k.

[0025] The secondary transfer device 11 has a secondary transfer roller 11a. The secondary transfer device 11 forms a transfer nip section TN (transfer nip region) between the secondary transfer roller 11a and the intermediate transfer body 71, and transports the sheet P that has been transported through the sheet transport path S by sandwiching it in the transfer nip section TN. As the sheet P passes through the transfer nip section TN, the toner image on the surface of the intermediate transfer body 71 is secondary transferred to the sheet P and the sheet P is transported to the fixing device 12. The belt cleaning device 9 removes and recovers waste toner that remains on the surface of the intermediate transfer body 71 without being transferred to the sheet P.

[0026] The fixing device 12 is equipped with a fixing roller 31 and a pressure roller 32 that rotate with the sheet P in between. The fixing device 12 places the sheet P, on which the toner image has been transferred, between the fixing roller 31 and the pressure roller 32, heats and pressurizes it, and fixes the toner image to the sheet P.

[0027] The paper feed cassette 18 is for storing sheets P used for image formation and is located below the optical scanning device 1. The sheets P are pulled out of the paper feed cassette 18 by the pickup roller 16 and transported to the sheet transport path S. The sheets P transported to the sheet transport path S pass through the secondary transfer device 11 and the fixing device 12, are transported to the discharge rollers 17 (17a, 17b), and discharged to the sheet discharge trays 141 (141a, 141b) in the discharge section 140. The sheet transport path S is equipped with transport rollers 13, registration rollers 14, and discharge rollers 17 (17a, 17b). In this example, the discharge rollers 17 consist of two discharge rollers 17a and 17b located vertically, and the sheet discharge trays 141 consist of two sheet discharge trays 141a and 141b, corresponding to the two discharge rollers 17a and 17b, respectively. The transport rollers 13-13 facilitate the transport of the sheets P. The registration roller 14 temporarily stops the sheet P and aligns the leading edge of the sheet P. The registration roller 14 then transports the temporarily stopped sheet P in time with the toner image on the intermediate transfer body 71. The toner image on the intermediate transfer body 71 is secondarily transferred to the sheet P at the transfer nip TN between the intermediate transfer body 71 and the secondary transfer roller 11a.

[0028] Although Figure 1 shows only one paper feed cassette 18, the system is not limited to this configuration. Multiple paper feed cassettes 18 may be provided, each loaded with a different type of sheet P.

[0029] Furthermore, if the image forming apparatus 100 is to perform image formation not only on the front surface but also on the back surface of the sheet P, it transports the sheet P in the reverse direction from the upper discharge roller 17b to the sheet reversal path Sr. The image forming apparatus 100 reverses the front and back surfaces of the sheet P that has been transported in the reverse direction and guides it back to the register roller 14. The image forming apparatus 100 then forms an image on the back surface of the sheet P guided to the register roller 14 in the same way as the front surface, and transports it to the sheet discharge tray 141 (141a, 141b).

[0030] When forming a color image, the image forming apparatus 100 rotates the intermediate transfer body 71, the photosensitive drums 3y, 3m, 3c, 3k, and the developing rollers 21y, 21m, 21c, 21k of the developing apparatus 2y, 2m, 2c, 2k (an example of a developer carrier). When forming a monochrome image, the intermediate transfer body 71, the photosensitive drums 3y, 3m, 3c, 3k, and the developing roller 21k of the developing apparatus 2k are rotated, but the rotation of the developing rollers 21y, 21m, 21c of the developing apparatus 2y, 2m, 2c is stopped.

[0031] [About this embodiment] Figure 4 is a schematic block diagram showing the system configuration of the control unit 110 in the image forming apparatus 100. Figure 5 is the first development bias table TB1 showing the first common development bias Vb1 when the first image forming mode is set based on the image quality adjustment processing results. Figure 6 is the second development bias table TB2 showing the second common development bias Vb2 when the second image forming mode is set based on the image quality adjustment processing results.

[0032] The image forming apparatus 100 (color image forming apparatus) according to this embodiment comprises a plurality (four in this example) of photoreceptor drums 3y, 3m, 3c, 3k, a plurality (four in this example) of developing devices 2y, 2m, 2c, 2k, a single developing power supply 61 (see Figure 2), an intermediate transfer device 70, and a control unit 110 (see Figure 4).

[0033] Multiple photoreceptor drums 3y, 3m, 3c, and 3k each have a photosensitive layer on their surface on which an electrostatic latent image is formed based on image data. The photosensitive layer exhibits insulating properties in the dark and becomes conductive in areas irradiated with light. The electrostatic latent image is formed by exposing the surface (photosensitive layer) area of ​​the photoreceptor drums 3y, 3m, 3c, and 3k, which have been charged by the charging members 51y, 51m, 51c, and 51k (charging rollers) as described above, to the light scanning device 1. Multiple developing devices 2y, 2m, 2c, and 2k each have developing rollers 21y, 21m, 21c, and 21k on which a developer (two-component developer) containing toner and carrier is carried on the surface. Multiple developing devices 2y, 2m, 2c, and 2k develop the images by supplying a portion of the toner carried on the developing rollers 21y, 21m, 21c, and 21k to the electrostatic latent images formed on each of the multiple photoreceptor drums 3y, 3m, 3c, and 3k. As a result, toner images are formed on each of the multiple photoreceptor drums 3y, 3m, 3c, and 3k. The intermediate transfer device 70 has an intermediate transfer body 71 on which the toner image is intermediately transferred. The intermediate transfer device 70 transfers the toner image intermediately transferred on the intermediate transfer body 71 to the sheet P. A single developing power supply 61 supplies a common developing bias Vb (see Figure 2), which is a predetermined voltage value common to (the same as) each of the developing rollers 21y, 21m, 21c, and 21k in the multiple developing devices 2y, 2m, 2c, and 2k.

[0034] The control unit 110 controls the entire image forming apparatus 100. As shown in Figure 4, the control unit 110 realizes various functions by reading and executing various programs stored in the storage unit 112 (for example, storage or ROM). The control unit 110 may be realized by one or more control devices / arithmetic units [CPU (Central Processing Unit), SoC (System on a Chip)]. Alternatively, the control unit 110 may be composed of one or more control circuits. The control unit 110 has a processing unit 111 consisting of a microcomputer such as a CPU, and a storage unit 112 including non-volatile memory such as ROM and volatile memory such as RAM. The control unit 110 controls the operation of various components by having the processing unit 111 load control programs pre-stored in the ROM of the storage unit 112 onto the RAM of the storage unit 112 and execute them.

[0035] The control unit 110 includes an image formation mode execution unit Q1, an image quality adjustment processing unit Q2, and a development bias setting change unit Q3.

[0036] The image formation mode execution unit Q1 can selectively execute a first image formation mode (monochrome image formation mode) and a second image formation mode (color image formation mode).

[0037] In the first image formation mode, a first toner image (black toner image) is formed on a first photoreceptor drum (in this example, a black photoreceptor drum 3k) which is developed by a first developing device (in this example, a black developing device 2k), which is one of the multiple developing devices 2y, 2m, 2c, 2k. The formed first toner image is then transferred to the sheet P via an intermediate transfer body 71.

[0038] In the second image formation mode, a second toner image (color toner image) is formed on a second photoreceptor drum (in this example, a photoreceptor drum for yellow, magenta, cyan, and black 3y, 3m, 3c, 3k) which is developed by a second developing device (in this example, developing devices 2y, 2m, 2c, 2k for yellow, magenta, cyan, and black 3y, 3m, 3c, 3k) and the formed second toner image is transferred to the sheet P via an intermediate transfer body 71.

[0039] The image quality adjustment processing unit Q2 executes image quality adjustment processing every time the number of sheets P for image quality adjustment (the cumulative number of sheets P onto which toner images have been transferred since the last image quality adjustment processing) reaches, for example, 300 sheets. In other words, the image quality adjustment processing unit Q2 executes image quality adjustment processing every time the number of sheets P onto which toner images have been transferred reaches a predetermined number. Note that this predetermined number of sheets P for image quality adjustment may be defined separately for the first image formation mode and the second image formation mode. Here, image quality adjustment processing refers to processing (process adjustment processing) performed to effectively prevent changes in image quality regardless of changes over time, by adjusting various image formation conditions (process conditions) such as exposure amount, development bias, and transfer potential in order to respond to changes in image quality due to changes over time (for example, the environment in which the image forming apparatus 100 is installed, changes in materials such as developer over time, etc.).

[0040] The development bias setting change unit Q3 can change the settings of the first common development bias Vb1 and the second common development bias Vb2 based on the image quality adjustment processing results obtained in the image quality adjustment processing. The first common development bias Vb1 is a common development bias supplied from the development power supply 61 when the first image formation mode is being executed. The second common development bias Vb2 is a common development bias supplied from the development power supply 61 when the second image formation mode is being executed.

[0041] In this example, in the first development bias table TB1 (see Figure 5), a calculated common development bias Vc (Vck = -310 [V]) that results in an appropriate image density for the first toner image, the black toner image, is associated as the first common development bias Vb1. In the second development bias table TB2 (see Figure 6), among the calculated common development biases Vc (Vcy, Vcm, Vcc, Vck) that result in an appropriate image density for each of the toner images of the second toner colors, yellow, magenta, cyan, and black, the calculated common development bias Vc (Vcm in this example) that results in an image density of or greater for each color is associated as the second common development bias Vb2. The first common development bias Vb1 and the second common development bias Vb2 are stored in the storage unit 112 in the first development bias table TB1 and the second development bias table TB2, respectively, and are updated with each image quality adjustment process.

[0042] According to this embodiment, based on the image quality adjustment processing results obtained in the image quality adjustment processing performed each time a predetermined number of images are reached, the first common development bias Vb1 supplied from the development power supply 61 during the execution of the first image formation mode and the second common development bias Vb2 supplied from the development power supply 61 during the execution of the second image formation mode can be changed. Therefore, in the first image formation mode, the common development bias Vb can be set to the first common development bias Vb1 (in this example, the calculated common development bias Vc corresponding to the black toner image, which is -310[V]) such that the first toner image formed on the first photoreceptor drum (3k), which is developed by the first development device (2k), one of the multiple development devices 2y, 2m, 2c, 2k, has an appropriate image density. Furthermore, in the second image formation mode, the common development bias Vb can be set to a second common development bias Vb2 (in this example, the calculated common development bias Vc corresponding to the magenta toner image, which is -400[V]) such that the second toner images formed on the second photoreceptor drum [multiple photoreceptor drums 3y, 3m, 3c, 3k including the first photoreceptor drum (3k)], which are developed by a second development device that includes at least two development devices, including the first development device among the multiple development devices 2y, 2m, 2c, 2k, all have an appropriate image density or higher.

[0043] Therefore, it is possible to effectively prevent the toner images formed on the multiple photoreceptor drums 3y, 3m, 3c, and 3k from having an image density lower than the appropriate image density, thereby suppressing a decrease in the image quality of the toner image transferred to the sheet P.

[0044] <First Embodiment> Figure 7 is a bottom view showing the toner patterns PT(PTy, PTm, PTc, PTk) for image quality adjustment of each color transferred to the intermediate transfer body 71. Figures 8 and 9 are a front view and a side view, respectively, showing the positional relationship between the detection unit 120 and the intermediate transfer body 71. Figures 10 to 13 are graphs showing the image density IDs of the toner patterns PT(PTy, PTm, PTc, PTk) of each color formed by changing the common development bias Vb to several different values. In this example, the image density IDs of the toner patterns PT(PTy, PTm, PTc, PTk) shown in Figures 10 to 13 were converted as follows: The common development bias Vb is changed in increments of 40[V] between -340[V] and -460[V] to transfer the yellow toner pattern PTy, cyan toner pattern PTc, magenta toner pattern PTm, and black toner pattern PTk to the intermediate transfer body 71. The yellow toner pattern PTy, cyan toner pattern PTc, magenta toner pattern PTm, and black toner pattern PTk transferred to the intermediate transfer body 71 are detected by the detection unit 120. The detected values ​​obtained by the detection unit 120 are converted into image density IDs (values ​​corresponding to a reflectance densitometer) using a predetermined conversion formula CV1. The conversion formula CV1, which shows the correspondence between the detected values ​​of the detection unit 120 and the image density IDs, can be set in advance through experiments or other means. Specifically, the toner patterns PT (PTy, PTm, PTc, PTk) are transferred to both ends of the intermediate transfer body 71 in the depth direction Y, and the detection unit 120 consists of a pair of detection units 120, 120 that detect the toner patterns PT, PT transferred to both ends of the intermediate transfer body 71 in the depth direction Y. The detected value of the detection unit 120 is the average of the pair of detected values ​​output from the pair of detection units 120, 120.

[0045] Incidentally, in the second image formation mode, if the common development bias Vb becomes smaller in absolute value than the appropriate development bias (in this example, Vcy = -370[V] for the yellow toner image, Vcm = -400[V] for the magenta toner image, Vcc = -350[V] for the cyan toner image, and Vck = -310[V] for the black toner image) for any one of the toner images formed on the second photoreceptor drum (3y, 3m, 3c, 3k), the image density will become lower than the appropriate image density, which in turn will lead to a decrease in the image quality of the toner image transferred to sheet P.

[0046] In this embodiment, a detection unit 120 is further provided for detecting the image density of the toner pattern PT (PTy, PTm, PTc, PTk) (so-called toner patch) for image quality adjustment transferred to the intermediate transfer body 71. The detection unit 120 is electrically connected to the input system of the control unit 110 and transmits a detection signal corresponding to the image density to the control unit 110. As a result, the control unit 110 can detect the image density of the toner image transferred to the intermediate transfer body 71 based on the detection signal from the detection unit 120.

[0047] When the control unit 110 performs image quality adjustment processing, it develops the electrostatic latent images for image quality adjustment formed on the second photoreceptor drum [a plurality of photoreceptor drums 3y, 3m, 3c, 3k including the first photoreceptor drum (3k)] for each color (C, M, Y, K) by changing the value of the common development bias. The control unit 110 then transfers the obtained toner patterns PT (PTy, PTm, PTc, PTk) for image quality adjustment of each color (C, M, Y, K) to the intermediate transfer body 71 (see Figures 7 and 8).

[0048] The detection unit 120 is a process control sensor (so-called pro-control sensor) that detects the image density of the toner pattern PT (PTy, PTm, PTc, PTk) that has been primary transferred onto the intermediate transfer body 71 by the primary transfer rollers 6y, 6m, 6c, 6k in the image forming unit 50 (see Figures 10 to 13).

[0049] More specifically, as shown in Figure 9, the detection unit 120 comprises a light-emitting unit 121 including a light-emitting element (specifically, a light-emitting diode) and a light-receiving unit 122 including a light-receiving element (specifically, a photodiode). The light-emitting unit 121 irradiates the toner pattern PT (PTy, PTm, PTc, PTk) on the intermediate transfer body 71 with emitted light L1. The light-receiving unit 122 receives reflected light L2 reflected from the toner pattern PT (PTy, PTm, PTc, PTk) on the intermediate transfer body 71. The detection unit 120 is electrically connected to the input system of the control unit 110. As a result, the control unit 110 can detect an output signal corresponding to the image density of the toner pattern PT (PTy, PTm, PTc, PTk). The control unit 110 receives the detected value from the detection unit 120, which has detected the image density of the toner pattern PT (PTy, PTm, PTc, PTk) on the intermediate transfer body 71.

[0050] The control unit 110 uses the detection unit 120 to detect the image density of the toner patterns PT (PTy, PTm, PTc, PTk) for each color (C, M, Y, K) transferred to the intermediate transfer body 71 for image quality adjustment. Based on the detected values, the control unit 110 calculates the common development bias Vc (Vcy, Vcm, Vcc, Vck) for each color (C, M, Y, K), which is the common development bias that makes the image density of the toner patterns PT (PTy, PTm, PTc, PTk) for each color (C, M, Y, K) equal to the reference image density IDs (1.3 in this example). Here, the control unit 110 calculates the common development bias Vc for each color that makes the image density of the toner patterns PT for each color equal to the reference image density IDs using the conversion formula CV1 from the detected values ​​of the detection unit 120. The conversion formula CV1 is stored in advance in the storage unit 112.

[0051] In this example, the control unit 110 calculates a common development bias Vc (Vcy=-370[V], Vcm=-400[V], Vcc=-350[V], Vck=-310[V]) from the image density curves αy, αm, αc, αk (see Figures 10 to 13) obtained by second-order polynomial approximation from the detection value of the detection unit 120. In the calculated development bias table TBc (see Figures 5 and 6), the toner patterns PT (PTy, PTm, PTc, PTk) for each color (yellow, magenta, cyan, and black) are associated with the corresponding calculated common development bias Vc (Vcy, Vcm, Vcc, Vck). The calculated common development bias Vc (Vcy, Vcm, Vcc, Vck) is stored in the storage unit 112 in the calculated development bias table TBc and is updated with each image quality adjustment process.

[0052] Furthermore, the control unit 110 sets the maximum calculated common development bias Vcmax (Vcm = -400 in this example) which has the largest absolute value among the calculated common development bias Vc for each color (C, M, Y, K) (in this example, Vcy = -370[V], Vcm = -400[V], Vcc = -350[V], Vck = -310[V] in the calculated development bias table TBc shown in Figure 6), as the second common development bias Vb2 in the second development bias table TB2 shown in Figure 6.

[0053] In this configuration, in the second image formation mode, the maximum calculated common development bias Vcmax (Vcm=-400[V]) among the calculated common development biases Vc (Vcy=-370[V], Vcm=-400[V], Vcc=-350[V], Vck=-310[V]) for each color (C, M, Y, K) is set as the second common development bias Vb2, and the maximum calculated common development bias Vcmax (Vcm=-400[V]) with the largest absolute value is set as the second common development bias Vb2, and the second photoreceptor drums (3y, 3m, 3c, 3k) are set accordingly. In all of the toner images formed, the common development bias Vb is never smaller in absolute value than the appropriate development bias [Vc (Vcy=-370[V], Vcm=-400[V], Vcc=-350[V], Vck=-310[V])]. This prevents the image density from becoming lower than the appropriate image density, and consequently suppresses the deterioration of the image quality of the toner image transferred to the sheet P.

[0054] <Second Embodiment> In this embodiment, when the control unit 110 executes the first image formation mode, it sets the calculated common development bias Vc of the first developing device (2k) (Vck = -310 [V]) from among the calculated common development bias Vc of each color (C, M, Y, K) (Vcy, Vcm, Vcc, Vck of the calculated development bias table TBc shown in Figure 5) as the first common development bias Vb1 in the first development bias table TB1 shown in Figure 5. When the second image formation mode is executed, it sets the maximum calculated common development bias Vcmax from among the calculated common development bias Vc of each color (C, M, Y, K) (Vcy, Vcm, Vcc, Vck) as the second common development bias Vb2.

[0055] In this configuration, in the first image formation mode, the calculated common development bias Vc(Vck) of the first developer (2k) is set as the first common development bias Vb1, thereby preventing the image density of the toner image formed on the first photoreceptor drum (3k) from being lower than the appropriate image density. Furthermore, in the second image formation mode, the maximum calculated common development bias Vcmax is set as the second common development bias Vb2, so that in any of the toner images formed on the second photoreceptor drums (3y, 3m, 3c, 3k), the common development bias Vb is never less in absolute value than the appropriate development bias [Vc(Vcy,Vcm,Vcc,Vck)], thereby preventing the image density from being lower than the appropriate image density, and consequently suppressing a decrease in the image quality of the toner image transferred to the sheet P.

[0056] <Third Embodiment> In this embodiment, the control unit 110 is capable of performing a multi-level image processing operation to form a multi-level image using image data, and a binarization operation to form a binary image by converting the image data into binary image data. Note that the multi-level image processing and binarization operation are conventionally known operations, and therefore a detailed explanation is omitted here.

[0057] Incidentally, in the second image formation mode, when the maximum calculated common development bias Vcmax (Vcm) is set as the second common development bias Vb2, there may be toner images with excessively high image density among the toner images formed on the multiple photoreceptor drums 3y, 3m, 3c, and 3k. In this case, during the multi-level processing, image processing can be performed using the multi-level image data to adjust the toner images with excessively high image density to an appropriate level.

[0058] However, in the first image formation mode, if the maximum calculated common development bias Vcmax (Vcm = -400V) is set as the first common development bias Vb1, and the common development bias Vb becomes larger in absolute value than the appropriate development bias (Vck = -310[V]) for the first toner image (black toner image), the image density of the first toner image (black toner image) may become too high. In that case, in the binarization process, it becomes difficult to perform image processing using binary image data to bring the toner image, which has become too high in image density, to an appropriate image density.

[0059] In this regard, the image forming apparatus 100 according to this embodiment has the following control configuration.

[0060] Figure 14 shows the first development bias table TB1, which represents the first common development bias Vb1 when binarization is performed in the first image formation mode.

[0061] When the control unit 110 performs binarization processing in the first image formation mode, it sets the calculated common development bias Vc(Vck) of the first development device (development device 2k for black) as the first common development bias Vb1 from among the calculated common development bias Vc(Vcy,Vcm,Vcc,Vck) of each color (C,M,Y,K).

[0062] In this configuration, in the first image formation mode, the calculated common development bias Vc(Vck), which is the appropriate development bias (Vck = -310[V]) for the first developer (2k), is set as the first common development bias Vb1, thereby preventing the image density of the toner image formed on the first photoreceptor drum (3k) from becoming too high.

[0063] In this example, the image forming apparatus 100 further includes a communication unit 150 (a facsimile communication unit in this example) that communicates image data with an external device 200, as shown in Figure 4. When the control unit 110 forms a toner image to be transferred to the sheet P based on the image data obtained via the communication unit 150, it performs a binarization process in the first image forming mode.

[0064] More specifically, the communication unit 150 is electrically connected to a network NT such as a public telephone line for its external communication system, and electrically connected to the communication control system of the control unit 110 for its internal communication system. In this example, the external device 200 is an external facsimile machine, and the communication unit 150 is a facsimile communication unit that performs facsimile communication.

[0065] In this configuration, when communicating image data with the external device 200 in the first image formation mode, the calculated common development bias Vc(Vck), which is the appropriate development bias (Vck=-310[V]) of the first developing device (2k), is set as the first common development bias Vb1, thereby preventing the image density of the toner image formed on the first photoreceptor drum (3k) from becoming too high.

[0066] <Fourth Embodiment> Figure 15 shows the first development bias table TB1, which represents the first common development bias Vb1 when performing processes other than binarization in the first image formation mode.

[0067] In this embodiment, when the control unit 110 performs any image processing other than the binarization process in the first image formation mode, it sets the maximum calculated common development bias Vcmax as the first common development bias Vb1.

[0068] Thus, when forming a toner image (black toner image) in the first image formation mode, if an image processing process other than binarization (e.g., multi-level processing) is performed, even if the maximum calculated common development bias Vcmax is set as the first common development bias Vb1, the image processing other than binarization (e.g., multi-level processing) can be used to perform image processing other than binarization to adjust the image density to an appropriate level for toner images where the image density becomes too high.

[0069] <Fifth Embodiment> Incidentally, if the absolute value of the potential difference Vd between the charging potential Vo and the common charging bias Ve (see Figure 3) falls below a predetermined reference potential difference range (e.g., 150V to 180V), the phenomenon of toner migration to non-image areas (white image areas) (so-called fogging) is likely to occur. On the other hand, if the absolute value of the potential difference Vd |Vd| exceeds the reference potential difference range (e.g., 150V to 180V), the phenomenon of carriers in the developer migrating to the photoreceptor drum (3y, 3m, 3c, 3k) (so-called carrier skipping) is likely to occur.

[0070] In this regard, the image forming apparatus 100 according to this embodiment has the following control configuration.

[0071] Figure 16 is a cross-sectional view showing an example in which the image forming apparatus 100 shown in Figure 1 is equipped with a single developing power supply 61 and a single charging power supply 62.

[0072] The image forming apparatus 100 further includes a single charging power supply 62. Multiple charging devices 5y, 5m, 5c, 5k each have charging members 51y, 51m, 51c, 51k (charging rollers in this example) that charge multiple photoreceptor drums 3y, 3m, 3c, 3k, respectively. The single charging power supply 62 supplies a common charging bias Ve, which is a predetermined voltage value common to each charging member 51y, 51m, 51c, 51k in the multiple charging devices 5y, 5m, 5c, 5k. The control unit 110 can set and change the common charging bias Ve based on the first common development bias Vb1 or second common development bias Vb2 that has been modified in each of the embodiments described above.

[0073] In this configuration, the common charging bias Ve can be set and changed based on the first common development bias Vb1 or the second common development bias Vb2, so both flickering and carrier skipping phenomena can be effectively prevented when supplying charged components 51y, 51m, 51c, and 51k in multiple charging devices 5y, 5m, 5c, and 5k from the charging power supply 62.

[0074] For more details, regardless of the values ​​of the common development bias Vb (first common development bias Vb1 and second common development bias Vb2), the setting of the common charging bias Ve can be changed so that the absolute value of the potential difference Vd between the charging potential Vo and the common charging bias Ve falls within the reference potential difference range (e.g., 150V to 180V).

[0075] In this example, the common charging bias is calculated using a predetermined calculation formula CV2 so that the potential difference Vd from the common development bias Vb falls within a reference potential difference range (e.g., 150V to 180V), and the calculated common charging bias is set to the common charging bias Ve. The calculation formula CV2, which shows the correspondence between the calculated common charging bias and the common development bias Vb, can be set in advance through experiments or other means and is stored in the memory unit 112.

[0076] <Example of image forming process according to this embodiment> Figure 17 is a flowchart of the main routine, illustrating an example of image formation processing. Figure 18 is a flowchart of the subroutine, illustrating an example of image quality adjustment processing shown in Figure 17.

[0077] As shown in Figure 17, the control unit 110 first executes the image quality adjustment processing subroutine shown in Figure 18 prior to the image formation process (S10). As shown in Figure 18, in the image quality adjustment process, the control unit 110 first transfers the yellow toner pattern PTy, magenta toner pattern PTm, cyan toner pattern PTc, and black toner pattern PTk as toner patterns PT for image quality adjustment of each color to the intermediate transfer body 71 (S11) (see Figure 7). Next, the control unit 110 detects the image density of the toner patterns PT (PTy, PTm, PTc, PTk) of each color (C, M, Y, K) using the detection unit 120 (S12) (see Figures 8 and 9).

[0078] Next, the control unit 110 calculates a common development bias Vc (Vcy, Vcm, Vcc, Vck) using the image density curves αy, αm, αv, αk obtained by second-order polynomial approximation from the detected image density values ​​of each detected color (C, M, Y, K) (S13) (see Figures 10 to 13). Next, the control unit 110 stores the calculated common development bias Vc (Vcy, Vcm, Vcc, Vck) for each color in the storage unit 112 (S14) (see Figure 3).

[0079] Next, the control unit 110 performs image quality adjustment (S15), resets the number of images for image quality adjustment (S16), and returns to the main routine. Here, since image quality adjustment is a conventionally known process, a detailed explanation is omitted.

[0080] As shown in Figure 17, the control unit 110 waits until it receives an instruction to perform image formation processing (S21: No), and when it receives an instruction to perform image formation processing (S21: Yes), it determines whether the number of sheets for image quality adjustment has reached a specified number (300 sheets in this example) (S22). If the number of sheets for image quality adjustment has not reached a specified number (S22: No), the control unit 110 proceeds directly to S23. However, if the number of sheets for image quality adjustment has reached a specified number (S22: Yes), it executes the image quality adjustment processing subroutine (S10) shown in Figure 18, and then proceeds to S23.

[0081] Next, the control unit 110 determines whether it is in the first image formation mode or the second image formation mode (S23). If the control unit 110 determines that it is in the second image formation mode (S23: No), it sets the maximum calculated common development bias Vcmax, which has the largest absolute value among the calculated common development biases Vc (Vcy, Vcm, Vcc, Vck) for each color (C, M, Y, K) stored in the memory unit 112, as the second common development bias Vb2 (S24). Next, the control unit 110 calculates a calculated common charging bias from the second common development bias Vb2 such that the absolute value |Vd| of the potential difference Vd between the charging potential Vo and the common charging bias Ve is within the reference potential difference range (S25), sets the calculated calculated common charging bias to the common charging bias Ve (S26), and executes the second image formation mode (S27). The control unit 110 returns to the process in S22 until the second image formation mode is completed (S28: No), and returns to the process in S21 once the second image formation mode is completed (S28: Yes).

[0082] On the other hand, when the control unit 110 determines that it is in the first image formation mode (S23: Yes), it sets the calculated common development bias Vc (Vck) of the first developing device (2k) from among the calculated common development biases Vc (Vcy, Vcm, Vcc, Vck) of each color (C, M, Y, K) stored in the memory unit 112 as the first common development bias Vb1 (S29). Next, the control unit 110 calculates the calculated common charging bias from the first common development bias Vb1 so that the absolute value |Vd| of the potential difference Vd between the charging potential Vo and the common charging bias Ve is within the reference potential difference range (S30), sets the calculated common charging bias to the common charging bias Ve (S31), and executes the first image formation mode (S32). The control unit 110 returns to the process in S22 until the first image formation mode is completed (S33: No), and returns to the process in S21 when the first image formation mode is completed (S33: Yes).

[0083] This disclosure is not limited to the embodiments described above, and can be implemented in a variety of other ways. Therefore, these embodiments are merely illustrative in all respects and should not be constrained. The scope of this disclosure is defined by the claims and is not restricted by the text of the specification. Furthermore, any variations or modifications falling within the equivalent scope of the claims are all within the scope of this disclosure. [Explanation of symbols]

[0084] 100 Image forming apparatus 110 Control Unit 111 Processing Unit 112 Storage section 120 Detection unit 121 Light-emitting part 122 Light receiving part 150 Communications Department 200 External device 2y~2k developing device 21y~21k Developing roller (an example of a developer carrier) 3y~3k Photosensitive drum (an example of an image carrier) 5y~5k electrostatic device 51y~51k charged material 61 Developing power supply 62 Electrostatic Power Supply 70 Intermediate Transfer Apparatus 71 Intermediate Transfer Form ID Image Density IDs (Reference Image Density) P Sheet PT Toner Pattern Q1 Image Forming Mode Execution Unit Q2 Image Quality Adjustment Processing Unit Q3 Development bias setting change section TB1 First Development Bias Table TB2 Second Development Bias Table TBc Calculation Development Bias Table Vb Common Development Bias Vb1 First common development bias Vb2 Second Common Development Bias Vc Calculation Common Development Bias Vcmax Maximum Calculation Common Development Bias Vd potential difference Ve Common Charging Bias Vi Image Potential Vo charge potential αy~αk Image density curve X Width direction Y (depth direction) Z vertical direction

Claims

1. Multiple image carriers, each on which an electrostatic latent image is formed based on image data, A plurality of developing apparatuses having a developer carrier on which a developer containing toner is carried on the surface, wherein toner is supplied to each of the electrostatic latent images to develop them and toner images are formed on the plurality of image carriers, An intermediate transfer device having an intermediate transfer body, which transfers the toner image formed on the plurality of image carriers to the intermediate transfer body and then transfers it to a sheet, A single developing power supply that supplies a common developing bias, which is a predetermined voltage value common to each of the developer carriers in the plurality of developing devices, A control unit capable of selectively executing a first image forming mode in which a first image carrier is developed by a first developing device, which is one of the plurality of developing devices, forms a first toner image on the first image carrier, which is the image carrier, and transfers the formed first toner image to the sheet via the intermediate transfer medium; and a second image forming mode in which a second image carrier is developed by a second developing device, which is at least two developing devices, including the first developing device, among the plurality of developing devices, forms a second toner image on the second image carrier, which is the image carrier, and transfers the formed second toner image to the sheet via the intermediate transfer medium. Equipped with, The image forming apparatus is characterized in that the control unit performs image quality adjustment processing each time the number of sheets on which the toner image has been transferred reaches a predetermined number, and can change the setting of a first common development bias, which is the common development bias supplied from the development power supply while the first image forming mode is being executed, and a second common development bias, which is the common development bias supplied from the development power supply while the second image forming mode is being executed, based on the image quality adjustment processing results obtained in the image quality adjustment processing.

2. An image forming apparatus according to claim 1, The intermediate transfer medium further comprises a detection unit for detecting the image density of the toner pattern for image quality adjustment that has been intermediate transferred to it. The image forming apparatus is characterized in that, when the control unit performs the image quality adjustment process, it develops the electrostatic latent image for image quality adjustment formed on the second image carrier for each color by changing the value of the common development bias, intermediate transfers the obtained toner patterns for image quality adjustment of each color onto the intermediate transfer body, detects the image density of the toner patterns for image quality adjustment of each color intermediately transferred onto the intermediate transfer body using the detection unit, calculates the calculated common development bias for each color which is the common development bias that makes the image density of the toner patterns for image quality adjustment of each color the reference image density based on the detected values, and sets the maximum calculated common development bias with the largest absolute value among the calculated calculated common development biases for each color as the second common development bias.

3. An image forming apparatus according to claim 2, The image forming apparatus is characterized in that, when the control unit executes the first image forming mode, it sets the calculation common development bias of the first developing apparatus from among the calculation common development biases of each color as the first common development bias, and when the second image forming mode executes, it sets the maximum calculation common development bias from among the calculation common development biases of each color as the second common development bias.

4. An image forming apparatus according to claim 2, The control unit is capable of performing a multi-level image processing operation to form a multi-level image using the image data, and a bi-level image processing operation to form a binary image by converting the image data into binary image data. An image forming apparatus characterized in that, when the binarization process is performed in the first image forming mode, the calculation common development bias of the first developing apparatus is set as the first common development bias among the calculation common development biases of each color.

5. An image forming apparatus according to claim 4, It further includes a communication unit for communicating image data with external devices, The image forming apparatus is characterized in that, when the control unit forms the toner image to be transferred to the sheet based on the image data obtained via the communication unit, it performs the binarization process in the first image forming mode.

6. An image forming apparatus according to claim 4, The image forming apparatus is characterized in that, when the control unit performs an image processing other than the binarization process in the first image forming mode, it sets the maximum calculated common development bias as the first common development bias.

7. An image forming apparatus according to claim 1, Multiple charging devices having charging members for each of the multiple image carriers, A single charging power supply that provides a common charging bias, which is a predetermined voltage value common to each of the charging members in the plurality of charging devices, Furthermore, The image forming apparatus is characterized in that the control unit can change the common charging bias based on the modified first common development bias or second common development bias.

Images