Image forming device

Abstract

To provide an image forming apparatus that includes a secondary transfer voltage adjustment mode of not determining an unnecessarily large secondary transfer voltage, while preventing insufficient transfer.SOLUTION: An image forming apparatus sets, from a plurality of test patterns, a plurality of sets each composed of at least two or more test patterns, selects one set from the plurality of sets based on results of detection of the test patterns belonging to the sets, and sets a transfer voltage applied during image formation based on the test patterns belonging to the selected one set.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus that transfers a toner image onto a recording material to form an image. [Background technology]

[0002] Conventionally, in image formation using electrophotography or the like, a toner image is electrostatically transferred from an image carrier such as a photoconductor or intermediate transfer body to a recording material such as paper. This transfer is often performed by applying a transfer voltage to a transfer member such as a transfer roller that contacts the image carrier to form a transfer section.

[0003] Patent Document 1 proposes an image forming apparatus equipped with a mode for adjusting a secondary transfer voltage.

[0004] In this secondary transfer voltage adjustment mode, multiple patch images are output on one sheet of recording material, and the voltage applied is switched for each patch.The density of each patch is then detected, and the optimal secondary transfer voltage conditions are selected according to the detection results. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2013-37185 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, when the secondary transfer voltage conditions are selected based on the detection results of each patch density, the secondary transfer voltage may be too high, resulting in an image defect known as "whiteout," in which part of the image is left blank.On the other hand, insufficient secondary transfer voltage may result in low density.

[0007] Conventionally, in the secondary transfer voltage adjustment mode, for example, a patch image is output by switching from a low secondary transfer voltage to a high voltage, and the density of the patch image is gradually increased. The voltage applied when the density change becomes small is determined as the optimal secondary transfer voltage. This is to avoid determining the secondary transfer voltage to be unnecessarily high in the secondary transfer voltage adjustment mode. However, since the density varies due to various reasons, there are cases where the secondary transfer voltage is calculated to be small even when the secondary transfer voltage is actually insufficient, and the voltage is determined to be the voltage. On the other hand, in order to prevent the secondary transfer voltage from being insufficient, if the secondary transfer voltage is determined to be the voltage when a patch with a high density is output among the multiple patch images output in the secondary transfer voltage adjustment mode, the secondary transfer voltage may become higher than necessary. As a result, there are cases where "whiteout" occurs.

[0008] The present invention has been made to solve the above-mentioned problems, and has an object to provide an image forming apparatus capable of preventing insufficient transfer while preventing an unnecessarily large transfer voltage from being set. [Means for solving the problem]

[0009] In order to achieve the above object, an image forming apparatus of the present invention includes an image carrier that carries a toner image, an image forming unit that forms a toner image on the image carrier, an intermediate transfer belt onto which the toner image formed on the image carrier is transferred, a transfer member that transfers the toner image transferred to the intermediate transfer belt onto a recording material, a power source that applies a transfer voltage to the transfer member, a sensor that detects the toner image formed on the recording material, and a control unit that executes an adjustment mode in which, during non-image formation, the transfer voltage to be applied when the toner image is transferred from the intermediate transfer belt to the recording material is adjusted, and in the adjustment mode, the control unit controls the power source to apply multiple levels of test voltage to the transfer member to transfer multiple test toner images, and adjusts the transfer voltage to be applied to the transfer member based on a detection result of the multiple test toner images detected by the sensor, and in the adjustment mode, the control unit (i) sets multiple groups constituted by test toner images extracted from the multiple test toner images, and (ii) acquires an index value for each of the multiple groups based on a detection result when the sensor detects the multiple test toner images belonging to each of the multiple groups, the index value is information regarding a variation in luminance information of the test toner images belonging to each of the plurality of sets, (iii) selecting from the plurality of sets based on the index value The set with the smallest variation in the luminance information is extracted as a specific set. and (iv) a test toner image belonging to the specific set. Based on the information on the variation in the brightness information of the test toner image, the test toner image having the smallest variation in the brightness information is selected as the specific test toner image. and (v) adjusting the transfer voltage to be applied to the transfer member based on a test voltage of the particular test toner image. Effect of the Invention

[0010] It is possible to provide an image forming apparatus capable of preventing insufficient transfer while preventing an unnecessarily large transfer voltage from being set. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram illustrating an image forming apparatus according to an embodiment of the present invention. [Diagram 2] Block diagram of this embodiment [Diagram 3] FIG. 1 is a diagram showing a secondary transfer control flow in an embodiment. [Figure 4] FIG. 1 is a diagram illustrating an image for adjusting a secondary transfer voltage according to an embodiment; [Diagram 5] FIG. 1 is a diagram illustrating an image for adjusting a secondary transfer voltage according to an embodiment; [Figure 6] FIG. 4 is a diagram showing a flow of a secondary transfer voltage adjustment mode in the embodiment. [Figure 7] FIG. 4 is a diagram illustrating an example of an operation screen in a secondary transfer voltage adjustment mode in the embodiment. [Figure 8] FIG. 1 is a diagram illustrating an example of a secondary transfer voltage adjustment mode in an embodiment. [Figure 9] FIG. 1 is a diagram for explaining an example of a secondary transfer voltage adjustment mode in the first embodiment. [Figure 10] FIG. 1 is a diagram for explaining an example of a secondary transfer voltage adjustment mode in the first embodiment. [Figure 11] FIG. 13 is a diagram for explaining an example of a secondary transfer voltage adjustment mode in the second embodiment. [Figure 12] FIG. 13 is a diagram for explaining an example of a secondary transfer voltage adjustment mode in the second embodiment. [Figure 13] FIG. 13 is a diagram for explaining an example of a secondary transfer voltage adjustment mode in the third embodiment. [Figure 14] FIG. 13 is a diagram showing an example of table data of recording material distribution voltages; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] A first embodiment will be described in detail below with reference to Figs. 1 to 3. In this embodiment, a tandem-type full-color printer is described as an example of an image forming apparatus 1. However, the present invention is not limited to the tandem-type image forming apparatus 1, but may be an image forming apparatus of another type, and is not limited to being full-color, but may be monochrome or mono-color. Alternatively, the present invention can be embodied in various applications, such as printers, various printing machines, copiers, FAX machines, and multifunction machines.

[0013] As shown in FIGS. 1 and 2, the image forming apparatus 1 includes an apparatus main body 10, a sheet feeding section (not shown), an image forming section 40, a sheet discharging section (not shown), a control section 30 as a control means, and an operation section 70 (see FIG. 2). Inside the apparatus main body 10, a temperature sensor 71 (see FIG. 2) capable of detecting the temperature inside the apparatus and a humidity sensor 72 (see FIG. 2) capable of detecting the humidity inside the apparatus are provided. The image forming apparatus 1 can form a four-color full-color image on a recording material in response to an image signal from an image reading section 80 as a reading means, a host device such as a personal computer, or an external device such as a digital camera or a smartphone. The sheet S as the recording material is a sheet on which a toner image is formed, and specific examples thereof include plain paper, a synthetic resin sheet that is a substitute for plain paper, cardboard, and an overhead projector sheet.

[0014] The image forming section 40 can form an image on the sheet S fed from the sheet feeding section based on image information. The image forming section 40 includes image forming units 50y, 50m, 50c, and 50k, toner bottles 41y, 41m, 41c, and 41k, and exposure devices 42y, 42m, 42c, and 42k. The image forming section 40 also includes an intermediate transfer unit 44 as an image carrier, a secondary transfer device 45, and a fixing section 46. The image forming apparatus 1 of this embodiment is a full-color device, and the image forming units 50y, 50m, 50c, and 50k are provided separately with the same configuration for each of the four colors, yellow (y), magenta (m), cyan (c), and black (k). For this reason, in FIG. 1, the configurations of the four colors are shown with the same reference numerals followed by color identifiers, but in the specification, there are cases where the description is given only by the reference numerals without the color identifiers. Incidentally, this image forming apparatus 1 can also form a monochrome or multi-color image, such as a monochrome black image, by using image forming units 50 for a desired single color or for some of the four colors.

[0015] The image forming unit 50 has a photosensitive drum 51 that moves while carrying a toner image, a charging roller 52, a developing device 20, a pre-exposure device 54, and a cleaning blade 55. The image forming unit 50 is integrated into a process cartridge as a unit, and is configured to be detachable from the apparatus main body 10, and forms a toner image on an intermediate transfer belt 44b described later.

[0016] The photosensitive drum 51 as an image carrier is rotatable and carries an electrostatic image used for image formation. In this embodiment, the photosensitive drum 51 is a negatively charged organic photoconductor (OPC) having an outer diameter of 30 mm, and is rotated by a motor (not shown) in the direction of the arrow at a predetermined process speed (circumferential speed). The photosensitive drum 51 has an aluminum cylinder as a base, and has three layers on its surface as surface layers, namely, an undercoat layer, a photocharge generation layer, and a charge transport layer, which are coated and laminated in this order.

[0017] The charging roller 52 as a charging device is a rubber roller that comes into contact with the surface of the photosensitive drum 51 and rotates in response to the rotation, uniformly charging the surface of the photosensitive drum 51. A charging bias power supply 73 (see FIG. 2) is connected to the charging roller 52. The charging bias power supply 73 applies a charging bias to the charging roller 52, and charges the photosensitive drum 51 via the charging roller 52. The exposure device 42 is a laser scanner, and emits laser light in accordance with image information of the separated colors output from the control unit 30.

[0018] The developing device 20 develops the electrostatic image formed on the photosensitive drum 51 by applying a developing bias with toner. The developing device 20 has a developing sleeve 24. The developing device 20 contains the developer supplied from the toner bottle 41 and develops the electrostatic image formed on the photosensitive drum 51. The developing sleeve 24 is made of a non-magnetic material such as aluminum or non-magnetic stainless steel, and is made of aluminum in this embodiment. A roller-shaped magnet roller is fixedly installed inside the developing sleeve 24 in a non-rotating state relative to the developing container. The developing sleeve 24 carries the developer having non-magnetic toner and magnetic carrier, and transports it to a developing area facing the photosensitive drum 51. A developing bias power supply 74 (see FIG. 2) is connected to the developing sleeve 24. The developing bias power supply 74 applies a developing bias to the developing sleeve 24 to develop the electrostatic image formed on the photosensitive drum 51.

[0019] The toner image developed on the photosensitive drum 51 is primarily transferred to the intermediate transfer unit 44. After the primary transfer, the surface of the photosensitive drum 51 is neutralized by a pre-exposure device 54. The cleaning blade 55 is of a counter blade type and is abutted against the photosensitive drum 51 with a predetermined pressing force. After the primary transfer, toner remaining on the photosensitive drum 51 without being transferred to the intermediate transfer unit 44 is removed by the cleaning blade 55 provided in contact with the photosensitive drum 51, in preparation for the next image forming process.

[0020] The intermediate transfer unit 44 includes a plurality of rollers, such as a drive roller 44a, a driven roller 44d, and primary transfer rollers 47y, 47m, 47c, and 47k, and an intermediate transfer belt 44b that is wound around these rollers and moves while carrying a toner image. The driven roller 44d is a tension roller that controls the tension of the intermediate transfer belt 44b to a constant level. A force is applied to the driven roller 44d by a biasing spring (not shown) so as to push the intermediate transfer belt 44b toward the front side, and this force applies a tension of about 2 to 5 kg to the intermediate transfer belt 44b in the conveying direction.

[0021] The primary transfer rollers 47y, 47m, 47c, and 47k are disposed facing the photosensitive drums 51y, 51m, 51c, and 51k, respectively. The primary transfer roller 47 is disposed with the intermediate transfer belt 44b sandwiched between the primary transfer roller 47 and the photosensitive drum 51, and the toner image formed on the surface of the photosensitive drum 51 is primarily transferred to the intermediate transfer belt 44b at the primary transfer section 48 by applying a primary transfer voltage. A primary transfer power supply 75 is connected to the primary transfer power supply 75. A voltage detection sensor 75a for detecting an output voltage and a current detection sensor 75b for detecting an output current are connected to the primary transfer power supply 75 (see FIG. 2). The primary transfer power supplies 75y, 75m, 75c, and 75k are provided for the primary transfer rollers 47y, 47m, 47c, and 47k, respectively, and the primary transfer voltages applied to the primary transfer rollers 47y, 47m, 47c, and 47k can be individually controlled. The primary transfer roller 47 has an outer diameter of, for example, 15 to 20 mm, and includes an elastic layer of ion-conductive foamed rubber (NBR rubber) and a core metal. The primary transfer roller 47 has a resistance value of 1×10^5 to 1×10^8 Ω (measured at N / N (23°C, 50% RH), 2 kV applied).

[0022] The intermediate transfer belt 44b is rotatable and rotates at a predetermined speed in the direction of the arrow. The intermediate transfer belt 44b comes into contact with the photosensitive drum 51 to form a primary transfer section 48 between the intermediate transfer belt 44b and the photosensitive drum 51. A primary transfer voltage is applied to the primary transfer section 48 from a primary transfer power source 75 (see FIG. 2), so that the toner image formed on the photosensitive drum 51 is primarily transferred at the primary transfer section 48. A primary transfer voltage of positive polarity is applied to the intermediate transfer belt 44b by the primary transfer roller 47, so that the toner images having negative polarity on the photosensitive drum 51 are sequentially and superimposedly transferred to the intermediate transfer belt 44b.

[0023] The intermediate transfer belt 44b is an endless belt having a three-layer structure consisting of a base layer, an elastic layer, and a surface layer from the back side. The resin material constituting the base layer is a resin such as polyimide or polycarbonate, or a material obtained by adding an appropriate amount of carbon black as an antistatic agent to various rubbers, and the thickness of the base layer is 0.05 to 0.15 [mm]. The elastic material constituting the elastic layer is a material obtained by adding an appropriate amount of an ion conductive agent to various rubbers, such as urethane rubber or silicone rubber, and the thickness of the elastic layer is 0.1 to 0.500 [mm]. The material constituting the surface layer is a resin such as fluororesin, which reduces the adhesion of toner to the surface of the intermediate transfer belt 44b and makes it easier for the toner to be transferred to the sheet S at the secondary transfer portion N, and the thickness is 0.0002 to 0.020 [mm]. In this embodiment, the surface layer uses, as a base material, one type of resin material such as polyurethane, polyester, epoxy resin, etc., or two or more types of elastic materials such as elastic rubber, elastomer, butyl rubber, etc. Then, a surface layer is formed by dispersing one or more types of powder or particles such as fluororesin or the like, or particles of different particle sizes, as a material for reducing surface energy and increasing lubricity, on this base material. In the intermediate transfer belt 44b of this embodiment, the volume resistivity is 5×10^8 to 1×10^14 [Ω·cm] (23°C, 50% RH), and the hardness is 60 to 85° (23°C, 50% RH) in MD1 hardness. The static friction coefficient is 0.15 to 0.6 (23°C, 50% RH, HEIDON type 94i). In this embodiment, a three-layer structure is used, but a single layer structure of a material equivalent to the above base layer may also be used.

[0024] The secondary transfer device 45 includes an inner secondary transfer roller 45a and an outer secondary transfer roller 45b. The inner secondary transfer roller 45a is disposed opposite the outer secondary transfer roller 45b across the intermediate transfer belt 44b. A secondary transfer power source 76 (see FIG. 2) serving as a high voltage application means is connected to the outer secondary transfer roller 45b. A voltage detection sensor 76a for detecting an output voltage and a current detection sensor 76b for detecting an output current are connected to the secondary transfer power source 76 (see FIG. 2).

[0025] The secondary transfer power source 76 applies a DC voltage to the secondary transfer outer roller 45b as a secondary transfer voltage. The secondary transfer outer roller 45b comes into contact with the intermediate transfer belt 44b to form a secondary transfer section N between the intermediate transfer belt 44b and the secondary transfer outer roller 45b. When a secondary transfer voltage of a polarity opposite to that of the toner is applied to the secondary transfer section N, the secondary transfer outer roller 45b performs secondary transfer of the toner images, which are primarily transferred and carried on the intermediate transfer belt 44b, all at once to the sheet S supplied to the secondary transfer section N. The core metal of the secondary transfer inner roller 45a is connected to a ground potential. When the sheet S is supplied to the secondary transfer device 45, a constant voltage controlled secondary transfer voltage of a polarity opposite to that of the toner image is applied to the secondary transfer outer roller 45b. In this embodiment, for example, a secondary transfer voltage of 500V to 7kV is applied, and a current of 40 to 120μA is passed to perform secondary transfer of the toner image on the intermediate transfer belt 44b to the sheet S. In this embodiment, the secondary transfer power supply 76 applies the secondary transfer voltage to the secondary transfer portion N by applying a DC voltage to the outer secondary transfer roller 45b, but the present invention is not limited to this. For example, the secondary transfer voltage may be applied to the secondary transfer portion N by applying a DC voltage to the inner secondary transfer roller 45a.

[0026] The secondary transfer outer roller 45b has an outer diameter of, for example, 20 to 25 mm, and includes an elastic layer of ion conductive foamed rubber (NBR rubber) and a core metal. A roller with a resistance value of 1×10^5 to 1×10^8 Ω (measured at N / N (23°C, 50% RH), 2 kV applied) is used as the secondary transfer outer roller 45b.

[0027] The intermediate transfer unit 44 also has a belt cleaning device 60. The belt cleaning device 60 removes toner and other deposits remaining on the intermediate transfer belt 44b after the secondary transfer process. The belt cleaning device 60 includes electrostatic cleaning members 61 and 62 to which cleaning voltages of different polarities are applied.

[0028] The fixing section 46 includes a fixing roller 46a and a pressure roller 46b. The toner image transferred to the sheet S is heated and pressurized and fixed to the sheet S by sandwiching and conveying the sheet S between the fixing roller 46a and the pressure roller 46b. The temperature of the fixing roller 46a is detected by a fixing temperature sensor 77 (see FIG. 2). After fixing, the sheet discharge section feeds the sheet S conveyed from the discharge path, and discharges the sheet S from a discharge outlet and stacks it on an output tray. In addition, between the fixing section 46 and the output outlet, a reversing conveying path (not shown) is provided that can turn over the sheet S after fixing and pass it through the secondary transfer device 45 again. By operating the reversing conveying path, images can be formed on both sides of a single sheet.

[0029] At the top of the image forming apparatus main body 1, there are disposed an automatic document feeder 81 that automatically feeds a sheet S on which an image has been formed to the image reading unit 80, and an image reading unit 80 that reads the image on the sheet S fed by the automatic document feeder 81. This image reading unit 80 is configured to illuminate the sheet S placed on a platen glass 82 with a light source (not shown), and to read the image formed on the sheet S at a predetermined dot density with an image reading element (not shown).

[0030] As shown in FIG. 2, the control unit 30 is configured by a computer, and includes, for example, a CPU 31, a ROM 32 that stores programs for controlling each unit, a RAM 33 that temporarily stores data, and an input / output circuit (I / F) 34 that inputs and outputs signals to and from the outside. The CPU 31 is a microprocessor that controls the overall control of the image forming apparatus 1, and is the main body of the system controller. The CPU 31 is connected to the sheet feeding unit, the image forming unit 40, the sheet discharge unit, and the operation unit 70 via the input / output circuit 34, and exchanges signals with each unit and controls their operation. The ROM 32 stores an image formation control sequence for forming an image on the sheet S, etc. A charging bias power supply 73, a developing bias power supply 74, a primary transfer power supply 75, and a secondary transfer power supply 76 are connected to the control unit 30, and each is controlled by a signal from the control unit 30. Also connected to the control unit 30 are a temperature sensor 71, a humidity sensor 72, a voltage detection sensor 75a and a current detection sensor 75b of a primary transfer power supply 75, a voltage detection sensor 76a and a current detection sensor 76b of a secondary transfer power supply 76, and a fixing temperature sensor 77. Signals detected by each sensor are input to the control unit 30.

[0031] The operation unit 70 includes an operation button and a display unit 70a including a liquid crystal panel, etc. A user can execute a print job by operating the operation unit 70, and the control unit 30 receives a signal from the operation unit 70 and operates various devices of the image forming apparatus 1.

[0032] In this embodiment, the control unit 30 has an image formation preparation processor 31a, an ATVC control processor 31b, and an image formation processor 31c. The control unit 30 also has a primary transfer voltage storage / calculation unit 31d, a cleaning voltage storage / calculation unit 31e, a secondary transfer voltage storage / calculation unit 31f, an image formation counter storage / calculation unit 31g, and a timer storage / calculation unit 31h. These processing units and storage / calculation units may be provided as part of the CPU 31 or the RAM 33. The control unit 30 can switch between a multi-color mode in which a primary transfer voltage is applied to multiple primary transfer units 48 to form an image in multiple colors, and a monochrome mode in which a primary transfer voltage is applied to only one primary transfer unit 48 out of the multiple primary transfer units 48 to form an image in a single color.

[0033] Next, an image forming operation in the image forming apparatus 1 configured as above will be described.

[0034] When the image forming operation is started, first, the photosensitive drum 51 rotates and the surface thereof is charged by the charging roller 52. Then, the exposure device 42 emits laser light to the photosensitive drum 51 based on image information, and an electrostatic latent image is formed on the surface of the photosensitive drum 51. Toner adheres to this electrostatic latent image, so that it is developed and visualized as a toner image, and then transferred to the intermediate transfer belt 44b.

[0035] Meanwhile, in parallel with this toner image forming operation, a sheet S is supplied and conveyed to a secondary transfer device 45 in synchronization with the toner image on the intermediate transfer belt 44b. Furthermore, an image is transferred from the intermediate transfer belt 44b to the sheet S, and the sheet S is conveyed to a fixing section 46, where the unfixed toner image is heated and pressurized to be fixed to the surface of the sheet S, and the sheet S is discharged from the apparatus main body 10.

[0036] <Control of secondary transfer voltage> Next, the control of the secondary transfer voltage will be described in detail with reference to Figures 2 and 3. Generally, the control of the secondary transfer voltage is classified into constant voltage control and constant current control, but in this embodiment, the constant voltage control is used.

[0037] First, when the control unit 30 acquires job information from the operation unit 70 or an external device (not shown), it starts the operation of the job (S101). This job information includes image information designated by the operator, the size (width, length) of the sheet S on which the image is to be formed, information related to the thickness of the sheet S (thickness or basis weight), and information related to the surface properties of the sheet S, such as whether the sheet S is coated paper or not. In other words, information on the recording material size and recording material type category is included. The control unit 30 writes this job information to the RAM 33 (S102).

[0038] Next, the control unit 30 acquires environmental information detected by the temperature sensor 71 and the humidity sensor 72 (S103). The ROM 32 also stores information indicating the correlation between the environmental information and the target current Itarget for transferring the toner image on the intermediate transfer belt 44b onto the sheet S. The control unit 30 obtains the target current Itarget corresponding to the environment from the information indicating the relationship between the environmental information and the target current Itarget based on the environmental information read in S103, and writes this to the RAM 33 (S104). The target current Itarget is changed according to the environmental information because the charge amount of the toner changes depending on the environment. The information indicating the relationship between the environmental information and the target current Itarget is obtained in advance by experiments or the like.

[0039] Next, the control unit 30 executes ATVC control (Active Transfer Voltage Control) before the toner image on the intermediate transfer belt 44b and the sheet S to which the toner image is transferred reach the secondary transfer unit N (S105). That is, in a state in which the secondary transfer outer roller 45b and the intermediate transfer belt 44b are in contact with each other, a predetermined voltage of multiple levels is supplied from the secondary transfer power source 76 to the secondary transfer outer roller 45b. Then, the current value when the predetermined voltage is supplied is detected by the current detection sensor 76b to obtain the relationship between the voltage and the current (voltage-current characteristics). This relationship between the voltage and the current changes according to the electrical resistance of the secondary transfer unit N. In the configuration of this embodiment, the relationship between the voltage and the current is not one in which the current changes linearly (proportional) to the voltage, but one in which the current changes so as to be expressed by a polynomial of the second degree or higher of the voltage. Therefore, in this embodiment, the predetermined voltage or current supplied when obtaining information about the electrical resistance of the secondary transfer unit N is multi-staged at three or more points so that the relationship between the voltage and the current can be expressed by a polynomial.

[0040] Next, the control unit 30 determines the voltage value to be applied from the secondary transfer power source 76 to the secondary transfer outer roller 45b (S106). That is, the control unit 30 determines the voltage value Vb required to pass the target current Itarget in a state where the sheet S is not present in the secondary transfer portion N, based on the target current Itarget written in the RAM 33 in S104 and the relationship between the voltage and the current determined in S105. Also, the ROM 32 stores information for determining the recording material shared voltage Vp, as shown in FIG. 14. In this embodiment, this information is set as table data indicating the relationship between the moisture amount of the atmosphere for each classification of the basis weight of the sheet S and the recording material shared voltage Vp. The control unit 30 can determine the moisture amount of the atmosphere based on the environmental information (temperature and humidity) detected by the temperature sensor 71 and the humidity sensor 72. The control unit 30 determines the recording material shared voltage Vp from the above table data, based on the job information obtained in S101 and the environmental information obtained in S103. Also, when an adjustment value is set by a simplified adjustment mode of the secondary transfer voltage described later, the adjustment amount ΔV is obtained. Then, the control unit 30 obtains Vb+Vp+ΔV by adding the above Vb, Vp, and ΔV as the secondary transfer voltage Vtr to be applied from the secondary transfer power source 76 to the secondary transfer outer roller 45b when the sheet S passes through the secondary transfer portion N, and writes this to the RAM 33. Note that the table data for obtaining the recording material shared voltage Vp as shown in FIG. 14 is obtained in advance by experiments or the like. Here, the recording material shared voltage (transfer voltage for the electrical resistance of the sheet S) Vp may change depending on the surface property of the sheet S in addition to information related to the thickness of the sheet S (basis weight). Therefore, the table data may be set so that the recording material shared voltage Vp also changes depending on information related to the surface property of the sheet S. Also, in this embodiment, information related to the thickness of the sheet S (and further information related to the surface property of the sheet S) is included in the job information obtained in S101. However, the image forming apparatus 1 may be provided with a measuring means for detecting the thickness and surface properties of the sheet S, and the recording material shared voltage Vp may be calculated based on information obtained by this measuring means.

[0041] Next, the sheet S is sent to the secondary transfer unit N, and image formation is performed while applying the secondary transfer voltage Vtr (S107). Thereafter, the control unit 30 repeats S107 until all images of the job are transferred onto the sheet S and output (S108).

[0042] <Secondary transfer voltage simple adjustment mode> Next, the simple adjustment mode of the secondary transfer voltage will be described.

[0043] Depending on the type of sheet S used by the user, if the moisture content or resistance value of the recording material is significantly different from that of a standard recording material, optimal transfer may not be achieved with the default sheet recording material distribution voltage Vp described above.

[0044] To explain in more detail, the secondary transfer voltage must first be applied at a level necessary to transfer the toner on the intermediate transfer body. Then, the secondary transfer voltage is set at a level that will not cause abnormal discharge when it is further increased. Depending on the condition of the recording material used by the user, the resistance of the recording material may be higher than expected, resulting in a shortage of voltage required to transfer the toner. In such a case, the secondary transfer voltage must be increased. Also, if the moisture content of the recording material is reduced and discharge is more likely to occur, image defects due to abnormal discharge are more likely to occur. In this case, the secondary transfer voltage must be decreased.

[0045] Therefore, the optimum recording material shared voltage Vp+ΔV is selected by changing the recording material shared voltage Vp and outputting it. This adjustment can also be done by the user outputting the image they want to output, varying the secondary transfer voltage for each sheet, checking the image, and determining the secondary transfer voltage. However, this method can result in more recording material being discarded and take more time because the setting and output are repeated many times.

[0046] Therefore, during non-image formation, a plurality of levels of test voltage are applied to the outer secondary transfer roller 45b, a plurality of test toner images are transferred to a recording material, and a test chart for setting the transfer voltage to be applied during image formation is output. That is, a simple adjustment mode is implemented in which a test chart is output in which a test toner image is formed by varying the secondary transfer voltage for each patch using patch images of representative colors, and the secondary transfer voltage is determined.

[0047] The image chart in the simple adjustment mode will be described. Two types of charts shown in Fig. 4 and Fig. 5 are used. Fig. 4 is used for output where the length of the recording material in the transport direction is 420 to 487 mm. Fig. 5 is used for output where the length of the recording material in the transport direction is 210 to 419 mm.

[0048] The size of the patch must be large enough for the user to easily judge. Since it is difficult to judge the transferability of solid blue and solid black when the patch size is small, the patch size is preferably 10 mm square or larger, and more preferably 25 mm square or larger.

[0049] The maximum recording material size that can be used with this image forming apparatus is 13 inches x 19.2 inches. The image size of the chart in Figure 4 corresponds to this size. If the size is smaller than 13 inches x 19.2 inches and is up to A3 size, this chart is aligned to the recording material using the center of the leading edge as the reference, and a margin of 2.5 mm is added, and the chart is output, for example, A3 size is cut to a size of 292 x 415 mm. The user can output not only standard sizes but also free sizes.

[0050] The size of the blue and black patches is a 25.7 mm square, with the gray at the edge measuring 25.7 mm in the transport direction and extending to the edge of the recording material in the thrust direction. The interval between patches in the transport direction is 9.5 mm, and the secondary transfer voltage is switched between these intervals. The 11 patches in the transport direction are 387 mm apart so that they fit within the 415 mm of an A3 sheet. Patches are not formed at the leading and trailing edges of the recording material, as image defects may occur only at the leading and trailing edges, and it would be difficult to determine whether these defects were caused by the secondary transfer voltage being adjusted. If a recording material with a thrust direction smaller than 13 inches is selected, the gray image at the edge becomes smaller. In the transport direction, the trailing edge margin becomes larger.

[0051] For recording materials smaller than A3, the chart in Figure 5 corresponds to sizes smaller than A3 from A5 portrait feed, i.e. 210-419mm. The image size is 13 inches x 210mm. In the thrust direction, the gray patches get smaller according to the recording material size, and in the transport direction, the five patch output lengths are 167mm, and the trailing margin gets longer according to the recording material sizes of 210-419mm.

[0052] In the case of a recording material size of 210 to 419 mm, since only five patches can be output on one sheet, in order to increase the number of patches, output is performed on two sheets with a secondary transfer voltage of -4 to 5.

[0053] The adjustment flow will be explained using FIG.

[0054] First, the user selects the sheet feeder (not shown) containing the recording material to be adjusted, and selects the recording material type and recording material size (step S1). The user enters an adjustment screen as shown in FIG. 7 and sets the center voltage and whether to print on one side or both sides (step S2). If zero is selected, the set voltage that is set in advance in the main body for that recording material type is selected. If an adjustment value of 0 is selected, 11 patches from -5 to +5 are output by changing the secondary transfer voltage. One level of this is 150V. If the test pattern output button is selected, a polynomial of degree 2 or higher (a quadratic expression in this embodiment) of the relationship between voltage and current that represents the electrical low resistance information of the secondary transfer portion N is obtained in the same manner as the ATVC control described above (step S3). Then, 11 patches with the secondary transfer voltage changed every 150V are output by the test pattern output button (step S4). For example, if the recording material voltage Vp in this environment is 2500V and Vb obtained from the ATVC result is 1000V, image formation is performed while changing the secondary transfer voltage in increments of 150V from 2750V to 4250V.

[0055] <Adjustment flow in simple adjustment mode of secondary transfer voltage> Next, the output test pattern is read by the image reading unit 80 controlled by the CPU 31, and multiple points of RGB luminance data (8 bits) for each solid blue patch are obtained (steps S5 and S6). In this embodiment, four points of luminance data are obtained for each patch. That is, the area in which one patch is formed is divided into four, and luminance data is obtained for each divided area.

[0056] As shown in FIG. 8(a), this step S6 makes it possible to derive the average luminance transition of the patch image corresponding to each voltage level (adjustment value). As shown in FIG. 8(a), when the toner image is secondarily transferred to the sheet S and the density appears, the difference Δ between the average luminance values ​​of adjacent patches becomes small, as in the case of adjustment values ​​0 to 3. Therefore, if a location where the difference Δ between the average luminance values ​​of adjacent patches is small is selected, it can be considered that the amount of secondary transfer of toner is large. As shown in FIG. 8(a), in the case of adjustment value -5 where the secondary transfer voltage is too insufficient, it is clear that the toner is difficult to transfer secondarily, so the difference Δ with the adjacent adjustment value -4 becomes large. However, as shown in FIG. 8(b), the transition of the luminance value with respect to the adjustment value is likely to vary for adjustment values ​​of -4 to -2, which are the transitional state where the secondary transfer of toner begins. In particular, as shown in FIG. 8(a) for adjustment values ​​-4 and -3, the difference Δ between the average luminance values ​​of adjacent patches is small in the detection result, despite the unstable region where the amount of secondary transfer of toner is likely to be insufficient. In this case, if an adjustment value of -3 was automatically determined, the amount of secondary transfer would be unstable, and the density would sometimes become light during the job. Therefore, as shown in FIG. 8(a), an adjustment value of 3, which results in the lowest brightness value, was automatically determined. When an image is output with an adjustment value of 3, the average brightness is small, but there are cases where the image has locally light areas. This is because discharge begins in the secondary transfer section, and some toner that has been reversed from the normal charging polarity cannot be transferred.

[0057] In other words, we want to automatically determine the adjustment values ​​without selecting adjustment values ​​-5 to -2 in Figure 8, which result in a low secondary transfer voltage level and unstable density, and without selecting adjustment value 3 (and above), which also results in a reduction in the secondary transfer amount due to unnecessary discharge in the secondary transfer section.

[0058] <First embodiment of adjustment flow> Therefore, as a first embodiment, the standard deviation is derived as the luminance value variation for four test patterns as shown in step S7 of FIG. 6. The luminance value data of four test patterns with adjustment values ​​-5 to -2 in FIG. 8, totaling 16 data, is grouped together to derive the standard deviation. Next, the standard deviation of the 16 data for adjustment values ​​-4 to -1 is derived. FIG. 9(a) shows the transition. For example, the test patterns with adjustment values ​​-5 to -2 correspond to patch No. 1 to 4 on the horizontal axis in FIG. 9(a), the test patterns with adjustment values ​​-4 to -1 correspond to patch No. 2 to 5, and the test patterns with adjustment values ​​2 to 5 correspond to patch No. 8 to 11. That is, the CPU 31 sets multiple groups of at least two adjacent test patterns from among multiple test patterns (test toner images). The standard deviation of the luminance value for each group is plotted on the vertical axis. As shown in FIG. 9(a), the standard deviation (variation) of the luminance values ​​of patches No. 6 to No. 9, that is, the set of adjustment values ​​0 to 3, is the smallest and is determined to be the region with stable density (step S8 in FIG. 6). In this embodiment, the set with the smallest standard deviation (density variation) among a group of a plurality of patches is selected as the group with stable density. Then, the secondary transfer voltage is set based on the luminance information of the patches belonging to the selected group. In this embodiment, the standard deviation of the luminance values ​​of the set of patches No. 6 to No. 9 is small, so it is the set of test patterns for adjustment values ​​0 to 3. Of the adjustment values ​​0 to 3, the search value 0, which has the smallest secondary transfer voltage, may be determined as the optimal secondary transfer voltage, but in this embodiment, a further step is taken.

[0059] Next, the standard deviation (variation) of the four luminance values ​​of each test pattern (patch) of the set (adjustment values ​​0 to 3) is derived (step S9 in FIG. 6). FIG. 9(b) shows the transition of the standard deviation of the four luminance values ​​of each test pattern (patch). In this embodiment, adjustment values ​​0 to 3 were determined to be in the stable density region in step S8 in FIG. 6, but adjustment 2, which has the smallest standard deviation (variation) of the four luminance values ​​of each test pattern (patch) in the set, is determined to be optimal (step 10 in FIG. 6). For example, since the recording material distribution voltage Vp in this environment is 2500V, Vb obtained from the ATVC result is 1000V, and the adjustment value 1 level is 150V, the optimal secondary transfer voltage for the image formation job is determined to be 3800V (step 11 in FIG. 6).

[0060] In the first embodiment, the luminance value of the blue solid is used in steps 7 to 10 of FIG. 6, but steps 7 to 8 of FIG. 6 may be performed for the blue solid, and steps 9 to 10 may use the luminance value of the halftone shown in FIG. 4 and FIG. 5. That is, the transition from insufficient secondary transfer voltage to appropriate secondary transfer voltage to slightly excessive secondary transfer voltage (transition from adjustment value -5 to 3 in FIG. 8(a)(b)) is performed using the blue solid with a large amount of secondary transfer to determine the secondary transfer stable region and adjustment value candidates. In this embodiment, the toner amount of the blue solid is 0.8 to 1.1 mg / cm^2. In addition, in order to detect the start of being affected by discharge due to excessive secondary transfer among the adjustment value candidates in the region and the start of variation in the secondary transfer amount, the determination is performed using a halftone with a smaller amount of secondary transfer than the blue solid. The toner amount of the halftone in this embodiment is 0.18 to 0.25 mg / cm^2. This increases the sensitivity to insufficient and excessive secondary transfer voltage, improving the accuracy of determining the optimal secondary transfer voltage. Fig. 10(a) shows the change in the brightness values ​​of the four points of each test pattern (halftone patch) over time with respect to the adjustment value. Fig. 10(b) shows the change in the standard deviation of the brightness values ​​of the four points of each test pattern over time with respect to the adjustment value. As Fig. 10(b) shows, of the candidate adjustment values ​​0 to 3 selected in steps 7 and 8, the standard deviation of the four brightness data of the halftone test pattern is smallest for adjustment value 1. Compared to the case where steps 9 and 10 were performed with a solid blue color, the sensitivity of the discharge influence start point due to excessive secondary transfer voltage to the secondary transfer performance was higher, and the stability of the secondary transfer amount was improved.

[0061] In this embodiment, the group with the smallest standard deviation among a group of a plurality of test patterns is selected as the group with stable density, but multiple groups that satisfy a predetermined condition may be extracted as groups with stable density. For example, multiple groups with standard deviations equal to or less than a predetermined threshold may be extracted as groups with stable density, and the secondary transfer voltage may be determined based on the standard deviation of the test patterns belonging to the extracted groups.

[0062] In addition, in this embodiment, the determination of the secondary transfer stable region (selection of the candidate group) and the secondary transfer voltage are determined based on the standard deviation of the density information (luminance information) of the test pattern, but the present invention is not limited to this. For example, the average value of the luminance values ​​may be used instead of the standard deviation of the luminance values. That is, the average luminance value (index value) of each group is calculated from the average value of the luminance values ​​of the test patterns belonging to that group. Then, based on the index value calculated for each group, it may be determined whether the density has stabilized. In this way, a plurality of groups are set, and the secondary transfer stable region is determined (selection of the candidate group) based on the index value of each group, so that the secondary transfer stable region can be determined with high accuracy. In addition, the candidate groups are extracted, and the secondary transfer voltage is set from the extracted groups based on the variance information (standard deviation) of the test pattern, so that it is possible to prevent an excessive transfer voltage from being set.

[0063] <Second embodiment of adjustment flow> As a second embodiment, the difference (variation) between the maximum and minimum values ​​of four test patterns is derived as shown in step S7 of FIG. 6. The luminance value data of the four test patterns with adjustment values ​​-5 to -2 in FIG. 11(b) is treated as one set, totaling 16 data, and the difference between the maximum and minimum values ​​is derived. Next, the difference between the maximum and minimum values ​​of the 16 data in the case of adjustment values ​​-4 to -1 is derived. FIG. 11(a) shows the transition. For example, the test patterns with adjustment values ​​-5 to -2 correspond to patch No. 1 to 4 on the horizontal axis in FIG. 11(a), the test patterns with adjustment values ​​-4 to -1 correspond to patch No. 2 to 5, and the test patterns with adjustment values ​​2 to 5 correspond to patch No. 8 to 11. The difference between the maximum and minimum values ​​of the luminance value for each set is plotted on the vertical axis. As shown in FIG. 11(a), the difference between the maximum and minimum values ​​of the luminance value of the set of patches No. 6 to 9, that is, adjustment values ​​0 to 3, is the smallest, and it is determined to be the area with stable density (step S8 of FIG. 6). In this embodiment, the difference between the maximum and minimum luminance values ​​of the set of patches No. 6 to No. 9 is small, so this is a set of test patterns when the adjustment values ​​are 0 to 3. Of the adjustment values ​​0 to 3, the search value 0, which is the smallest secondary transfer voltage, may be determined as the optimal secondary transfer voltage, but in this embodiment, a further step is taken.

[0064] Next, the difference between the maximum and minimum values ​​of the four luminance values ​​of each test pattern (patch) of the set (adjustment values ​​0 to 3) is derived (step S9 in FIG. 6). FIG. 11(b) shows the transition of the difference between the maximum and minimum values ​​of the four luminance values ​​of each test pattern (patch). In this embodiment, adjustment values ​​0 to 3 are determined to be in the stable density region in step S8 in FIG. 6, but adjustment 2, which has the smallest difference between the maximum and minimum values ​​of the four luminance values ​​of each test pattern (patch) in the set, is determined to be optimal (step 10 in FIG. 6). For example, since the recording material distribution voltage Vp in this environment is 2500V, Vb obtained from the ATVC result is 1000V, and the adjustment value 1 level is 150V, the optimal secondary transfer voltage for the image formation job is determined to be 3800V (step 11 in FIG. 6).

[0065] In the second embodiment, the luminance value of the blue solid is used in steps 7 to 10 of FIG. 6, but steps 7 to 8 of FIG. 6 may be performed for the blue solid, and steps 9 to 10 may use the luminance value of the halftone shown in FIG. 4 and FIG. 5. That is, the transition from insufficient secondary transfer voltage to appropriate secondary transfer voltage to slightly excessive secondary transfer voltage (transition from adjustment value -5 to 3 in FIG. 11(b)) is determined using the blue solid with a large secondary transfer amount to determine the secondary transfer stable region and adjustment value candidates. In this embodiment, the blue solid toner amount is 0.8 to 1.1 mg / cm^2. Then, in order to detect the secondary transfer amount starting to vary due to the secondary transfer excess among the adjustment value candidates in the region starting to be affected by discharge, the secondary transfer amount is determined using a halftone less than the blue solid. (The halftone toner amount in this embodiment is 0.18 to 0.25 mg / cm^2) This increases the sensitivity to insufficient and excessive secondary transfer voltage, improving the accuracy of determining the optimal secondary transfer voltage.

[0066] Fig. 12(a) shows the change in brightness value of four points of each test pattern (halftone patch) versus adjustment value. Fig. 12(b) shows the change in difference between maximum and minimum brightness values ​​of four points of each test pattern versus adjustment value. As Fig. 12(b) shows, among the candidate adjustment values ​​0-3 selected in steps 7-8, the difference between maximum and minimum values ​​of the four brightness data of the halftone test pattern is smallest for adjustment value 1. Compared to the case where steps 9-10 were performed with a solid blue color, the sensitivity of the discharge influence start point due to excessive secondary transfer voltage to the secondary transfer performance was higher, and the stability of the secondary transfer amount was improved.

[0067] <Third embodiment of adjustment flow> As a third embodiment, as shown in step S7 of FIG. 6, the standard deviation is derived as the luminance value variation for four test patterns. When acquiring the luminance value data in step S6, the average value of the four luminance values ​​of each test pattern is derived. FIG. 8(a) shows the transition of the average luminance value with respect to the adjustment value. In this embodiment, when the average luminance value is 60 or more, four adjacent test patterns are not selected, but four test patterns are selected every other test pattern, and the standard deviation is derived as a set of 16 luminance value data for each of the four test patterns. For example, in this embodiment, the average luminance value at the adjustment value -5 in FIG. 8(a) is 60 or more, which is an area where the secondary transfer voltage is insufficient and the secondary transfer amount is unstable. In that case, the standard deviation of the luminance value of the 16 luminance value data for each of the four test patterns with adjustment values ​​of -5, -3, -1, and -0 is derived. It is plotted as "Patch No. 1~" in FIG. 13(a). Similarly, the average luminance value at the adjustment value -4 in FIG. 8(a) is 60 or more, which is an area where the secondary transfer voltage is insufficient and the secondary transfer amount is unstable. In this case, the standard deviation of the luminance value of 16 pieces of luminance value data for four test patterns with adjustment values ​​of -4, -2, 0, and 1 is derived. Plotted as "Patch No. 2~" in FIG. 13(a). Similarly, the average luminance value at the adjustment value -3 in FIG. 8(a) is 60 or more, which is an area where the secondary transfer voltage is insufficient and the secondary transfer amount is unstable. In this case, the standard deviation of the luminance value of 16 pieces of luminance value data for four test patterns with adjustment values ​​of -3, -1, 1, and 3 is derived. Plotted as "Patch No. 3~" in FIG. 13(a). On the other hand, the average luminance value is 60 or less for adjustment values ​​-2 to 2 in FIG. 8(a), so the standard deviation of the luminance value of 16 pieces of luminance value data for four test patterns with adjustment values ​​of -2, -1, 0, and 1 is derived. In FIG. 13(a), this is plotted as "patch No. 4~". For example, the standard deviation of 16 luminance values ​​in total is derived for the luminance value data of four test patterns with adjustment values ​​2, 3, 4, and 5. In FIG. 13(a), this is plotted as "patch No. 8~". Since it is not possible to create any more sets of four test patterns, step S7 in FIG. 6 is terminated. As FIG. 13(a) shows, patch No. 6~, that is, the set of adjustment values ​​0~3, has the smallest luminance standard deviation (variation) and is determined to be the area with the most stable density (step S8 in FIG. 6).In this embodiment, the standard deviation of the luminance values ​​of the set of patches No. 6 onwards is small, so this is a set of test patterns when the adjustment values ​​are 0 to 3. Of the adjustment values ​​0 to 3, the search value 0, which is the smallest secondary transfer voltage, may be determined as the optimal secondary transfer voltage, but in this embodiment, a further step is taken.

[0068] Next, the standard deviation (variation) of the four luminance values ​​of each test pattern (patch) of the set (adjustment values ​​0 to 3) is derived (step S9 in FIG. 6). FIG. 11(b) shows the transition of the standard deviation of the four luminance values ​​of each test pattern (patch). In this embodiment, adjustment values ​​0 to 3 were determined to be in the stable density region in step S8 in FIG. 6, but adjustment 2, which has the smallest standard deviation (variation) of the four luminance values ​​of each test pattern (patch) in the set, is determined to be optimal (step 10 in FIG. 6). For example, since the recording material distribution voltage Vp in this environment is 2500V, Vb obtained from the ATVC result is 1000V, and the adjustment value 1 level is 150V, the optimal secondary transfer voltage for the image formation job is determined to be 3800V (step 11 in FIG. 6).

[0069] In the first embodiment, the luminance value of the blue solid is used in steps 7 to 10 of FIG. 6, but steps 7 to 8 of FIG. 6 may be performed for the blue solid, and steps 9 to 10 may be performed using the luminance value of the halftone shown in FIG. 4 and FIG. 5. That is, the transition from insufficient secondary transfer voltage to appropriate secondary transfer voltage to slightly excessive secondary transfer voltage is performed in steps 7 to 8 for the blue solid with a large secondary transfer amount (0.8 to 1.1 mg / cm^2 as the blue solid toner amount in this embodiment), and the secondary transfer stable area and adjustment value candidates are determined. That is, the transition from adjustment value -5 to 3 in FIG. 8(a)(b) is determined based on the blue solid with a large secondary transfer amount. Then, in steps 9 to 10, the determination is made based on the halftone with a smaller secondary transfer amount than the blue solid (0.18 to 0.25 mg / cm^2 as the halftone toner amount in this embodiment). This makes it possible to detect the start of variation in the secondary transfer amount due to the start of being affected by discharge due to excessive secondary transfer among the adjustment value candidates in the area.

[0070] This increases the sensitivity to insufficient and excessive secondary transfer voltage, improving the accuracy of determining the optimal secondary transfer voltage. FIG. 10(a) shows the transition of the luminance values ​​of the four points of each test pattern (halftone patch) versus the adjustment value. FIG. 10(b) shows the transition of the standard deviation of the luminance values ​​of the four points of each test pattern versus the adjustment value. As FIG. 10(b) shows, of the candidate adjustment values ​​0-3 selected in steps 7-8, the standard deviation of the four luminance data of the halftone test pattern is smallest for adjustment value 1. Compared to the case where steps 9-10 were performed with a solid blue print, the sensitivity of the discharge influence start point due to excessive secondary transfer voltage to the secondary transfer performance was higher, and the stability of the secondary transfer amount was improved.

Claims

1. an image carrier that carries a toner image; an image forming unit for forming a toner image on the image carrier; an intermediate transfer belt onto which a toner image formed on the image carrier is transferred; a transfer member for transferring the toner image transferred onto the intermediate transfer belt onto a recording material; a power source that applies a transfer voltage to the transfer member; A sensor for detecting a toner image formed on a recording material; a control unit that executes an adjustment mode for adjusting the transfer voltage to be applied when a toner image is transferred from the intermediate transfer belt to a recording material during non-image formation, In the adjustment mode, the control unit controls the power source to apply a test voltage of a plurality of levels to the transfer member to transfer a plurality of test toner images, and adjusts a transfer voltage to be applied to the transfer member based on a detection result obtained by detecting the plurality of test toner images by the sensor. In the adjustment mode, the control unit (i) setting a plurality of sets composed of test toner images extracted from the plurality of test toner images; (ii) acquiring an index value for each of the plurality of groups based on a detection result when the plurality of test toner images belonging to each of the plurality of groups are detected by the sensor; the index value is information regarding a variation in luminance information of the test toner images belonging to each of the plurality of sets, (iii) extracting a pair having the smallest variation in the luminance information from the plurality of pairs as a specific pair based on the index value; (iv) extracting a specific test toner image having the smallest variation in luminance information based on information regarding the variation in luminance information of the test toner images belonging to the specific set; (v) adjusting the transfer voltage to be applied to the transfer member based on the test voltage of the specific test toner image.

2. The plurality of sets includes a first set and a second set, the first set includes a first test toner image and a second test toner image; 2. The image forming apparatus according to claim 1, wherein the second set includes the second test toner image and a third test toner image.

3. 2. The image forming apparatus according to claim 1, wherein the control unit adjusts a transfer voltage to be applied to the transfer member based on information regarding luminance dispersion of the test toner image belonging to the specific set.

4. 2. The image forming apparatus according to claim 1, wherein the index value is a standard deviation of luminance information of the test toner images belonging to each of a plurality of groups.

Images