Image forming apparatus

Abstract

To appropriately set a bias to be applied to an electrode member arranged downstream of a primary transfer unit, in consideration of a current flowing from a primary transfer member to a potential regulation member.SOLUTION: An image forming apparatus 1 has: a photoreceptor 11; an intermediate transfer belt 6; a primary transfer member 15; a first application unit 75 that applies a bias to the primary transfer member 15; an electrode member 8 that is in contact with an inner peripheral surface of the intermediate transfer belt 6 on the downstream side of a primary transfer unit N1; a second application unit 80 that applies a bias to the electrode member 8; detection units 75a, 75b that detect a current flowing in the primary transfer member 15 or a voltage applied to the primary transfer member 15; and a control unit 3 that, on the basis of results of detection performed by the detection units 75a, 75b, performs control of executing setting operations to set an electrode bias that is applied to the electrode member 8 by the second application unit 80 during image formation.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a copying machine, printer, facsimile machine, or multifunction machine having a plurality of functions of these machines, which uses an electrophotographic system or an electrostatic recording system. [Background technology]

[0002] Among electrophotographic image forming devices such as color copiers, color printers, and color multifunction devices, intermediate transfer image forming devices have become mainstream due to their advantages of compact size and ease of compatibility with various recording materials. Intermediate transfer image forming devices typically include multiple photosensitive drums and an intermediate transfer belt. In such image forming devices, toner images formed on the multiple photosensitive drums are electrostatically transferred sequentially onto the intermediate transfer belt at a primary transfer unit. The toner images transferred onto the intermediate transfer belt are then electrostatically transferred onto a recording material such as paper at a secondary transfer unit. Regarding the arrangement of components around the primary transfer unit, the terms "upstream" and "downstream" refer to the upstream and downstream directions of the intermediate transfer belt, unless otherwise specified.

[0003] In the image forming apparatus described above, the toner on the intermediate transfer belt tends to increase in charge due to discharge between the intermediate transfer belt and the photosensitive drum downstream of the primary transfer section. The inventors' further investigation revealed that the increased charge on the toner on the intermediate transfer belt makes it difficult to transfer the toner to the recording material at the secondary transfer section. For example, the increased charge on the toner on the intermediate transfer belt after the primary transfer is completed increases the secondary transfer electric field required to transfer the toner to the recording material at the secondary transfer section, resulting in poor image graininess. Alternatively, it becomes difficult to uniformly transfer the toner to embossed paper or other surfaces with uneven surfaces.

[0004] Here, Patent Document 1 discloses a configuration in which a conductive contact plate is provided downstream of the primary transfer unit and on the inner peripheral side of the intermediate transfer belt, and a bias of the same polarity as the charging polarity of the photosensitive drum is applied to the contact plate. Patent Document 1 proposes that this configuration prevents degradation of image quality caused by toner scattering after primary transfer. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-57963 Summary of the Invention [Problem to be solved by the invention]

[0006] In order to suppress the increase in the amount of charge on the toner downstream of the primary transfer unit as described above, it is effective to suppress discharge downstream of the primary transfer unit (herein also referred to as "downstream discharge").To achieve this, it is effective to reduce the potential difference between the photosensitive drum and the intermediate transfer belt after primary transfer.

[0007] Patent Document 1 does not mention suppressing downstream discharge. However, the inventors of the present invention have conducted further research and found that an effective way to suppress downstream discharge is to place a conductive electrode member downstream of the primary transfer unit and on the inner peripheral surface side of the intermediate transfer belt, and to apply a bias of the same polarity as the charging polarity of the photosensitive drum to this electrode member. Here, this conductive electrode member is also referred to as a "potential regulating member."

[0008] However, it has been found that when a potential regulating member is used, a "leakage current" flows from the primary transfer member to the potential regulating member, reducing the primary transfer current flowing from the primary transfer member to the photosensitive drum, which may impair primary transfer performance. In order to maintain primary transfer performance, it is important to pass the necessary primary transfer current from the primary transfer member to the photosensitive drum.

[0009] On the other hand, according to the inventors' investigations, in order to suppress downstream discharge, it is important to release the charge of the opposite polarity to the surface potential of the photosensitive drum through the intermediate transfer belt downstream of the primary transfer portion. This is thought to reduce the potential difference between the photosensitive drum and the intermediate transfer belt after primary transfer. In other words, in order to suppress downstream discharge, it is also important to flow the necessary current equivalent to the above-mentioned leakage current from the primary transfer member to the potential regulating member.

[0010] Here, the primary transfer bias for passing the required primary transfer current varies depending on the environment, usage conditions (accumulated usage of the image forming apparatus), etc. Furthermore, the leakage current varies depending on the environment and usage conditions of the image forming apparatus, as well as the primary transfer bias.

[0011] Therefore, to improve image quality, it is necessary to maintain primary transfer performance by supplying a primary transfer current required according to the environment, usage conditions, etc., while suppressing downstream discharge and improving secondary transfer performance by supplying a leakage current required according to the primary transfer bias in addition to the environment and usage conditions. In other words, it is necessary to appropriately set the bias applied to the potential regulating member, taking into account the current flowing from the primary transfer member to the potential regulating member (current flowing in the potential regulating member).

[0012] Patent document 1 does not describe correcting the current flowing from the primary transfer member to the contact plate or correcting the voltage applied to the contact plate, taking into account the current flowing from the primary transfer member to the potential control member.

[0013] Therefore, an object of the present invention is to appropriately set the bias to be applied to the electrode member disposed downstream of the primary transfer portion, taking into consideration the current flowing from the primary transfer member to the potential regulating member. [Means for solving the problem]

[0014] The above object is achieved by an image forming apparatus according to the present invention. In summary, according to one aspect of the present invention, the image forming apparatus includes a photosensitive member that can be charged to a predetermined polarity and that carries a toner image, a rotatable intermediate transfer belt that conveys the toner image that has been primarily transferred from the photosensitive member in a primary transfer section so that the toner image can be secondarily transferred to a recording material in a secondary transfer section, a primary transfer member that contacts the inner peripheral surface of the intermediate transfer belt to form the primary transfer section where the photosensitive member and the intermediate transfer belt abut, and that is applied with a bias to transfer the toner image from the photosensitive member to the intermediate transfer belt, and a second transfer member that applies a bias of a polarity opposite to the predetermined polarity to the primary transfer member. an electrode member that contacts the inner surface of the intermediate transfer belt downstream of the primary transfer unit in the movement direction of the intermediate transfer belt; a second application unit that applies a bias of the same polarity as the predetermined polarity to the electrode member; a detection unit that detects the current flowing through the primary transfer member or the voltage applied to the primary transfer member; and a control unit that controls, based on the detection result by the detection unit, to execute a setting operation to set the electrode bias to be applied to the electrode member by the second application unit during image formation. [Effects of the Invention]

[0015] According to the present invention, the bias applied to the electrode member disposed downstream of the primary transfer portion can be appropriately set in consideration of the current flowing from the primary transfer member to the potential regulating member. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Figure 2] FIG. 2 is a schematic block diagram of a control system of the image forming apparatus according to the first embodiment. [Figure 3] 3A and 3B are a cross-sectional view and a perspective view of a potential regulation member; [Figure 4] FIG. 10 is a cross-sectional view of another example of a potential restricting member. [Figure 5] FIG. 10 is a cross-sectional view of another example of a potential restricting member. [Figure 6] 10 is a cross-sectional view of a main part for explaining the arrangement of a potential regulation member. FIG. [Figure 7] FIG. 4 is a schematic cross-sectional view for explaining a current path around a primary transfer portion. [Figure 8] 10 is a table showing an example of target values of effective current of a primary transfer bias and target values of current of a potential regulating bias. [Figure 9] FIG. 10 is a schematic graph for explaining the primary transfer ATVC. [Figure 10] FIG. 10 is a schematic graph for explaining the potential regulation ATVC. [Figure 11] FIG. 3 is a timing chart for explaining the control of the first embodiment. [Figure 12] FIG. 3 is a flowchart illustrating the control of the first embodiment. [Figure 13] FIG. 10 is a timing chart for explaining control of a modified example of the first embodiment. [Figure 14] FIG. 10 is a flowchart for explaining control of a modified example of the first embodiment. [Figure 15] FIG. 10 is a schematic block diagram of a control system of an image forming apparatus according to a second embodiment. [Figure 16] 10 is a table showing an example of a target value of an effective current of a primary transfer bias, a target value of a current of a potential regulating bias, and a target value of a voltage of the potential regulating bias. [Figure 17] FIG. 10 is a timing chart for explaining the control in the second and third embodiments. [Figure 18] 10 is a table showing an example of a reference value of the voltage of the primary transfer bias, a reference value of the voltage of the potential regulating bias, and a correction value; [Figure 19] FIG. 10 is a flowchart illustrating the control of the second embodiment. [Figure 20] FIG. 11 is a schematic block diagram of a control system of an image forming apparatus according to a third embodiment. [Figure 21] 10 is a table showing an example of an additional correction value of the potential regulating bias depending on the number of printed sheets. [Figure 22] 10 is a table showing an example of additional correction values for the potential regulating bias depending on the basis weight and type of recording material. [Figure 23]FIG. 10 is a flowchart illustrating the control of the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0017] The image forming apparatus according to the present invention will be described in more detail below with reference to the drawings.

[0018] [Example 1] 1. Overall configuration and operation of the image forming apparatus First, the overall configuration and operation of the image forming apparatus of this embodiment will be described. Figure 1 is a schematic cross-sectional view of the image forming apparatus 1 of this embodiment. The image forming apparatus 1 of this embodiment is a tandem full-color printer that employs an intermediate transfer system and is capable of forming a full-color image on a sheet-shaped recording material S using an electrophotographic system.

[0019] The image forming apparatus 1 includes an image forming unit 2, a control unit 3, a feeding unit 4 for a recording material S, and an ejection unit 5 for the recording material S. The image forming apparatus 1 is provided with a temperature sensor 71 (FIG. 2) capable of detecting the temperature inside the apparatus and a humidity sensor 72 (FIG. 2) capable of detecting the humidity (relative humidity) inside the apparatus. The temperature sensor 71 and the humidity sensor 72 are each an example of an environment detection unit (environment detection unit) for detecting environmental information related to the environment, i.e., at least one of the temperature and humidity inside or outside the image forming apparatus 1. The image forming apparatus 1 can form an image on the recording material S based on image information (image signal) acquired by a document reading device (not shown) provided in or connected to the image forming apparatus 1. The image forming apparatus 1 can also form an image on the recording material S based on image information (image signal) from an external device (not shown) connected to the image forming apparatus 1, such as a personal computer (host device), a digital camera, or a smartphone.

[0020] The recording material (transfer material, recording medium, sheet, paper) S is a material on which an image is formed using toner. Specific examples of the recording material S include plain paper, cardboard, gloss coated paper, matte coated paper, embossed paper, and synthetic resin sheets (synthetic paper) and overhead projector sheets (transparent resin films) that are substitutes for plain paper. Here, the recording material S is sometimes referred to as "paper," but even in this case, the recording material S includes materials other than paper or materials containing materials other than paper.

[0021] The image forming section 2 forms an image based on image information on the recording material S fed from the feeding section 4. The image forming section 2 includes image forming units 10y, 10m, 10c, and 10k, toner bottles 18y, 18m, 18c, and 18k, exposure devices 13y, 13m, 13c, and 13k, an intermediate transfer unit 20, a secondary transfer device 26, and a fixing device 27. The image forming units 10y, 10m, 10c, and 10k form toner images of yellow (y), magenta (m), cyan (c), and black (k), respectively. Note that elements having the same or corresponding functions or configurations and provided for each color may be generally described by omitting the suffixes y, m, c, and k, which indicate that the element is for one of the colors.

[0022] Furthermore, the image forming apparatus 1 can also form a monochrome image, such as a black monochrome image, or a multicolor image, using one image forming unit 10 or some of the four image forming units 10.

[0023] The image forming unit 10 has a photosensitive drum 11, which is a drum-type (cylindrical) photosensitive member (electrophotographic photosensitive member) serving as an image carrier. The image forming unit 10 also has a charging roller 12, which is a roller-type charging member serving as a charging means. The image forming unit 10 also has a developing device 14 serving as a developing means. The image forming unit 10 also has a pre-exposure device 16 serving as a discharging means. The image forming unit 10 also has a drum cleaning device 17 serving as a photosensitive member cleaning means. In FIG. 1, only a cleaning blade (elastic blade) of the drum cleaning device 17 is depicted. The image forming unit 10 forms a toner image on an intermediate transfer belt 6, which will be described later.

[0024] The photosensitive drum 11 is movable (rotatable) and carries an electrostatic image (electrostatic latent image) or a toner image. In this embodiment, the photosensitive drum 11 is a negatively charged organic photoconductor (OPC) with an outer diameter of 30 mm. The photosensitive drum 11 has an aluminum cylinder as a base and a surface layer formed on the surface of the aluminum cylinder. In this embodiment, the surface layer has three layers: an undercoat layer, a photocharge generation layer, and a charge transport layer, which are coated and stacked on the base in the following order. When an image formation operation starts, the photosensitive drum 11 is rotated in the direction of arrow R1 (counterclockwise) in FIG. 1 at a predetermined peripheral speed (process speed) by a drive motor (not shown) as a drive means.

[0025] The surface of the rotating photosensitive drum 11 is uniformly charged to a predetermined potential of a predetermined polarity (negative in this embodiment) by the charging roller 12. In this embodiment, the charging roller 12 is a rubber roller that contacts the surface of the photosensitive drum 11 and rotates in accordance with the rotation of the photosensitive drum 11. A charging power supply 73 (FIG. 2) serving as a charging bias application means (charging bias application unit) is connected to the charging roller 12. During charging, the charging power supply 73 applies a predetermined charging bias (charging voltage) to the charging roller 12. In this embodiment, a bias in which AC (alternating current) is superimposed on DC (direct current) is used as the charging bias. However, the charging bias may be a bias consisting of only DC (direct current).

[0026] The surface of the charged photosensitive drum 11 is scanned and exposed by the exposure device 13 based on image information, and an electrostatic image is formed on the photosensitive drum 11. In this embodiment, the exposure device 13 is a laser scanner that uses a laser as a light source. The exposure device 13 emits laser light in accordance with image information of separated colors output from the control unit 3, and scans and exposes the surface (outer peripheral surface) of the photosensitive drum 11. Note that the exposure device 13 may also be an LED print head that uses an LED as a light source.

[0027] The electrostatic image formed on the photosensitive drum 11 is developed (visualized) by the developing device 14 by supplying toner, and a toner image (toner image, developer image) is formed on the photosensitive drum 11. In this embodiment, the developing device 14 is a two-component developing device that uses a two-component developer containing toner (non-magnetic toner particles) and carrier (magnetic carrier particles) as the developer. A developer container (developer container body) 14b of the developing device 14 contains the two-component developer, and an amount of toner corresponding to the consumed toner is replenished from a toner bottle 18. The developing device 14 has a developing sleeve 14a as a developing member (developer carrier). The developing sleeve 14a is made of a non-magnetic material such as aluminum or non-magnetic stainless steel (aluminum in this embodiment). Inside the developing sleeve 14, a roller-shaped magnet (not shown) serving as a magnetic field generating means (magnetic field generating member) is fixed and arranged so as not to rotate relative to the developing container 14b. The developing sleeve 14a rotates while carrying a two-component developer, transporting toner to a development area facing the photosensitive drum 11. In the development area, toner moves from the two-component developer on the developing sleeve 14a and adheres to the image area of the electrostatic image on the photosensitive drum 11. A developing power supply 74 (FIG. 2) serving as a developing bias application unit (developing bias application unit) is connected to the developing sleeve 14a. During development, the developing power supply 74 applies a predetermined developing bias (developing voltage) to the developing sleeve 14a. In this embodiment, a bias in which AC (alternating current) is superimposed on DC (direct current) is used as the developing bias. In this embodiment, toner charged with the same polarity as the charge polarity of the photosensitive drum 11 (negative polarity in this embodiment) adheres to the exposed area (image area) of the photosensitive drum 11, which has been uniformly charged and then exposed to light, reducing the absolute value of the potential (reverse development method). In this embodiment, the normal charging polarity of the toner, which is the main charging polarity of the toner during development, is negative polarity.

[0028] An intermediate transfer unit 20 is disposed facing the four photosensitive drums 11y, 11m, 11c, and 11k. The intermediate transfer unit 20 has an intermediate transfer belt 6, which is an endless belt serving as an intermediate transfer body. The intermediate transfer belt 6 is wound around a plurality of tension rollers, including a drive roller 21, a tension roller 22, and a secondary transfer inner roller 23, and is tensioned at a predetermined tension. The intermediate transfer belt 6 is movable (rotatable) while carrying a toner image. The drive roller 21 is driven to rotate by a drive motor (not shown) serving as a driving means, transmitting a driving force to the intermediate transfer belt 6, causing the intermediate transfer belt 6 to rotate (circumferentially move) in the direction of arrow R2 (clockwise) in the figure at a predetermined peripheral speed (process speed) corresponding to the peripheral speed of the photosensitive drums 11. A tension roller 22 controls the tension of the intermediate transfer belt 6 to be constant. The tension roller 22 applies a force to the intermediate transfer belt 6 from its inner peripheral surface (back surface) to its outer peripheral surface (front surface) by the biasing force of a tension spring (not shown) composed of a compression coil spring, which serves as a biasing member. This force applies a tension of approximately 2 to 5 kgf to the intermediate transfer belt 6 in its conveyance direction (process progress direction, movement direction). Note that the symbol "to" used in numerical ranges includes the preceding and following values. The inner secondary transfer roller 23, together with the outer secondary transfer roller 25 (described later), constitutes a secondary transfer device 26. Primary transfer rollers 15y, 15m, 15c, and 15k, which are roller-type primary transfer members serving as primary transfer means, are disposed on the inner peripheral surface of the intermediate transfer belt 6, corresponding to the photosensitive drums 11y, 11m, 11c, and 11k, respectively. In this embodiment, the primary transfer roller 15 faces the photosensitive drum 11 and sandwiches the intermediate transfer belt 6 between itself and the photosensitive drum 11. The primary transfer roller 15 is pressed against the photosensitive drum 11 and contacts the photosensitive drum 11 via the intermediate transfer belt 6, forming a primary transfer portion (primary transfer nip portion) N1, which is the contact portion between the photosensitive drum 11 and the intermediate transfer belt 6.

[0029] The toner images formed on the photosensitive drums 11 are transferred (primary transfer) onto the rotating intermediate transfer belt 6 at the primary transfer portion N1 by the action of a primary transfer bias applied to the primary transfer rollers 15. For example, when forming a full-color image, the yellow, magenta, cyan, and black toner images formed on the photosensitive drums 11 are superimposed one on top of the other and transferred onto the intermediate transfer belt 6. A primary transfer power supply 75 (FIG. 2) serving as a primary transfer bias application means (primary transfer bias application unit) is connected to the primary transfer rollers 15. During primary transfer, the primary transfer power supply 75 applies a primary transfer bias (primary transfer voltage), which is a DC voltage of the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, to the primary transfer rollers 15. As a result, the toner image made of negatively charged toner on the photosensitive drums 11 is primarily transferred onto the intermediate transfer belt 6. The primary transfer power supply 75 is connected to a voltage detection sensor 75a (FIG. 2) as a voltage detection means (voltage detection unit) that detects the output voltage, and a current detection sensor 75b (FIG. 2) as a current detection means (current detection unit) that detects the output current. In this embodiment, a primary transfer bias of, for example, about 1 to 2 kV is applied to the primary transfer roller 15. In this embodiment, the primary transfer bias is controlled to a constant voltage during image formation (primary transfer). In this embodiment, primary transfer power supplies 75y, 75m, 75c, and 75k are provided independently for each of the primary transfer rollers 15y, 15m, 15c, and 15k. In this embodiment, the primary transfer bias applied to each of the primary transfer rollers 15y, 15m, 15c, and 15k can be controlled individually.

[0030] Here, the primary transfer roller 15 has, for example, a core metal and an elastic layer of ion-conductive foam rubber (NBR rubber) formed around the core metal. The outer diameter of the primary transfer roller 15 is, for example, φ15 to 20 [mm]. The primary transfer roller 15 has, for example, an electrical resistance of 1×10 5 ~1×10 8 [Ω] (measured at 23° C., 50% RH environment, against a φ30 [mm] metal drum, 2 kV applied, 4.9 N×2) rollers can be suitably used.

[0031] In this embodiment, the intermediate transfer belt 6 is an endless belt having a two-layer structure in which a base layer and a surface layer are laminated in this order from the inner peripheral surface side to the outer peripheral surface side. A suitable material for the base layer is a resin such as polyimide or polycarbonate containing an appropriate amount of carbon black as an antistatic agent. The thickness of the base layer is, for example, 0.05 to 0.15 mm. A suitable material for the surface layer is CR rubber provided with conductivity by carbon black. The thickness of the surface layer is, for example, 0.200 to 0.300 mm. In this embodiment, the volume resistivity of the intermediate transfer belt 6 is 5×10 8 ~1×10 14 [Ω·cm] (23°C, 50% RH). In this embodiment, the intermediate transfer belt 6 has a two-layer structure, but it may also have a single-layer structure made of a material equivalent to the above-mentioned base layer. The surface layer may also be a resin coating layer containing a resin such as a fluororesin and having a thickness of about 0.002 to 0.01 [mm] as a release layer. The intermediate transfer belt 6 may also have a multi-layer structure of three or more layers.

[0032] An outer secondary transfer roller 25, a roller-type secondary transfer member serving as a secondary transfer means, is disposed on the outer peripheral surface of the intermediate transfer belt 6. The outer secondary transfer roller 25 as a secondary transfer member, together with the inner secondary transfer roller 23 as an opposing member (opposing electrode), constitutes a secondary transfer device 26. The outer secondary transfer roller 25 is pressed toward the inner secondary transfer roller 23 and contacts the inner secondary transfer roller 23 via the intermediate transfer belt 6, forming a secondary transfer portion (secondary transfer nip portion) N2, which is the contact portion between the intermediate transfer belt 6 and the outer secondary transfer roller 25. At the secondary transfer portion N2, a toner image formed on the intermediate transfer belt 6 is transferred (secondarily transferred) onto the recording material S, which is being conveyed while being sandwiched between the intermediate transfer belt 6 and the outer secondary transfer roller 25, by the action of a secondary transfer bias applied to the outer secondary transfer roller 25. A secondary transfer power supply 76 (FIG. 2) serving as a secondary transfer bias application means (secondary transfer bias application portion) is connected to the outer secondary transfer roller 25. During secondary transfer, the secondary transfer power supply 76 applies a secondary transfer bias (secondary transfer voltage), which is a DC voltage of the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, to the outer secondary transfer roller 25. As a result, the toner image composed of negatively charged toner on the intermediate transfer belt 6 is secondarily transferred onto the recording material S. The secondary transfer power supply 76 is connected to a voltage detection sensor 76a (FIG. 2) as a voltage detection means (voltage detection unit) for detecting the output voltage of the secondary transfer power supply 76 and a current detection sensor 76b (FIG. 2) as a current detection means (current detection unit) for detecting the output current of the secondary transfer power supply 76. In addition, the core metal of the inner secondary transfer roller 23 is connected to a ground potential (electrically grounded). In this embodiment, for example, a secondary transfer bias of approximately 1.0 to 6.5 kV is applied to the outer secondary transfer roller 25, and a secondary transfer current of approximately 15 to 100 μA flows through the secondary transfer section N2. As a result, the toner image on the intermediate transfer belt 6 is secondarily transferred onto the recording material S (outer secondary transfer roller power supply method). In this embodiment, the secondary transfer bias is controlled to a constant voltage during image formation. Alternatively, a secondary transfer bias, which is a DC voltage having the same polarity as the normal charging polarity of the toner, may be applied from a secondary transfer power source 76 to the inner secondary transfer roller 23 serving as the secondary transfer member (inner secondary transfer roller power supply system). In this case, the outer secondary transfer roller 25 serving as the opposing member is connected to the ground potential.

[0033] The recording material S is transported from the feeding unit 4 toward the secondary transfer unit N2 in parallel with the formation of a toner image on the intermediate transfer belt 6. The recording material S is stored in a cassette 41 serving as a recording material storage unit of the feeding unit 4. The recording materials S stored in the cassette 41 are separated one by one and sent out of the cassette 41 by a feeding roller 42 or the like serving as a feeding member of the feeding unit 4. The recording material S is transported to a registration roller (pair of registration rollers) 19 serving as a transport member provided on a transport path 44 of the recording material S by a transport roller 43 or the like serving as a transport member of the feeding unit 4. The recording material S is then transported by the registration roller 19 to the secondary transfer unit N2 in synchronization with the toner image on the intermediate transfer belt 6. Note that although only one cassette 41 is shown in FIG. 1, the image forming apparatus 1 may have a plurality of cassettes 41. The feeding section 4 may also be configured to be able to feed the recording material S from a recording material storage section (recording material loading section) other than the cassette 41, such as a manual feed tray.

[0034] In this embodiment, the outer secondary transfer roller 25 has a core metal and an elastic layer of ion-conductive foam rubber (NBR rubber) formed around the core metal. The outer diameter of the outer secondary transfer roller 25 is, for example, φ25 to 30 [mm]. The outer secondary transfer roller 25 has, for example, an electrical resistance of 1×10 5 ~1×10 8 [Ω] (measured at 23° C., 50% RH environment, against a φ30 [mm] metal drum, 2 kV applied, 4.9 N×2) rollers can be suitably used.

[0035] The recording material S onto which the toner image has been transferred is transported to a fixing device 27 serving as a fixing unit. The fixing device 27 includes a fixing roller 27a and a pressure roller 27b. The fixing roller 27a incorporates a heater serving as a heating unit. The pressure roller 27b presses against the fixing roller 27a to form a fixing section (fixing nip). The fixing device 27 heats and presses the recording material S bearing the unfixed toner image by sandwiching it between the fixing roller 27a and the pressure roller 27b and transporting it, thereby fixing (melting and adhering) the toner image onto the recording material S. The temperature (fixing temperature) of the fixing roller 27a is detected by a fixing temperature sensor 77 (FIG. 2). The recording material S onto which the toner image has been fixed is transported by a discharge roller 51 in the discharge unit 5 and discharged (output) onto a discharge tray 52 provided outside the apparatus main body 1a of the image forming apparatus 1.

[0036] After the primary transfer, the surface of the photosensitive drum 11 is neutralized by a pre-exposure device 16. Toner remaining on the photosensitive drum 11 (primary transfer residual toner) that was not transferred to the intermediate transfer belt 6 during the primary transfer is removed from the photosensitive drum 11 and collected by a drum cleaning device 17. In this embodiment, the drum cleaning device 17 uses a cleaning blade as a cleaning member to scrape the primary transfer residual toner from the surface of the rotating photosensitive drum 11 and collect it in a collection container (not shown). The cleaning blade is a plate-shaped member that contacts the photosensitive drum 11 with a predetermined pressure. The cleaning blade contacts the surface of the photosensitive drum 11 in a counter direction to the rotation direction of the photosensitive drum 11, with the tip of its free end facing upstream in the rotation direction of the photosensitive drum 11. Furthermore, deposits such as toner remaining on the intermediate transfer belt 6 (secondary transfer residual toner) that was not transferred to the recording material S during the secondary transfer are removed from the intermediate transfer belt 6 and collected by a belt cleaning device 24 as an intermediate transfer body cleaning means.

[0037] The image forming unit 10 may be configured as a cartridge (process cartridge) that is detachable from the main body 1a of the image forming apparatus 1. An intermediate transfer unit 20 is configured by the intermediate transfer belt 6, tension rollers 21, 22, and 23 for the intermediate transfer belt 6, primary transfer rollers 15, a belt cleaning device 24, and potential regulating members 8 (described later). The intermediate transfer unit 20 may be configured as a cartridge that is detachable from the main body 1a of the image forming apparatus 1.

[0038] 2. Control Configuration FIG. 2 is a block diagram showing a schematic configuration of a control system of the image forming apparatus 1 of this embodiment. The image forming apparatus 1 is provided with a control unit (control circuit) 3 as a control means. The control unit 3 is configured with a CPU 31 as a processing unit, a ROM 32 and a RAM 33 as storage units, and an input / output circuit (I / F) (not shown) for inputting and outputting signals between the control unit 3 and external devices. The ROM 32 stores programs and data tables for controlling each unit of the image forming apparatus 1, and the RAM 33 temporarily stores control-related data. The CPU 31 is a microprocessor that controls the overall control of the image forming apparatus 1 and is the main system controller. The CPU 31 is connected to each unit, such as the feed unit 4, the image forming unit 2, and the discharge unit 5, and exchanges signals with these units and controls the operation of these units. The ROM 32 stores an image formation control sequence for forming an image on the recording material S.

[0039] The control unit 3 is connected to, for example, a charging power supply 73, a developing power supply 74, a primary transfer power supply 75, a secondary transfer power supply 76, and a potential regulating power supply 80 (described later), each of which is controlled by a signal from the control unit 3. Although not shown in the figure, in this embodiment, the charging power supply 73, the developing power supply 74, the primary transfer power supply 75, and the potential regulating power supply 80 are independently provided for each image forming unit 10. The control unit 3 is also connected to a temperature sensor 71, a humidity sensor 72, a voltage detection sensor 75a and a current detection sensor 75b of the primary transfer power supply 75, a voltage detection sensor 76a and a current detection sensor 76b of the secondary transfer power supply 76, a voltage detection sensor 80a and a current detection sensor 80b of the potential regulating power supply 80 (described later), and a fixing temperature sensor 77. Signals (information) indicating the detection results of each sensor are input to the control unit 3.

[0040] An operation unit 70 is also connected to the control unit 3. The operation unit 70 has an input unit (not shown) including operation buttons (keys) as input means, and a display unit 70a including a liquid crystal panel (display) as display means. In this embodiment, the display unit 70a is configured as a touch panel and also functions as input means. An operator such as a user or a service representative can operate the operation unit 70 to cause the image forming apparatus 1 to execute a job (described below). The control unit 3 receives signals from the operation unit 70 and operates various devices of the image forming apparatus 1. The image forming apparatus 1 can also execute jobs in response to signals not from the operation unit 70 but from an external device such as a personal computer.

[0041] Here, the image forming apparatus 1 executes a job (print job), which is a series of operations that starts with a single start instruction and forms and outputs an image on one or more recording materials S. The job generally includes a pre-rotation process, an image forming process, an inter-sheet process when forming images on multiple recording materials S, and a post-rotation process. The image forming process is a period during which the following processes are performed: electrostatic image formation, toner image formation, primary transfer of the toner image, secondary transfer, and fixing of the toner image, for the image that is actually formed and output on the recording material S. This period is referred to as the image formation time (image formation period). Furthermore, the timing of the electrostatic image formation, toner image formation, primary transfer of the toner image, secondary transfer, and fixing varies depending on the location where each of these processes is performed. The pre-rotation process is a period during which preparatory operations are performed before the image forming process, from when a job start instruction is input until the actual start of image formation. The inter-sheet process (inter-recording material process, inter-image process) is a period corresponding to the interval between recording materials S when image formation is performed continuously on multiple recording materials S (continuous printing, continuous image formation). The post-rotation process is a period in which a tidying up operation (preparatory operation for the next job) is performed after the image forming process. The non-image formation time (non-image formation period) is a period other than the image forming process, and includes a pre-rotation process, an inter-sheet process, a post-rotation process, and a pre-multiple rotation process which is a preparatory operation when the image forming apparatus 1 is turned on or when it returns from a sleep state.

[0042] 3. Secondary transferability issues Next, the issue of secondary transferability will be described. For convenience, unless otherwise specified, the magnitude (high / low) of voltage or potential refers to the magnitude (high / low) when compared in absolute values. Furthermore, with regard to the arrangement of the primary transfer section N1, photosensitive drum 11, primary transfer roller 15, and potential regulating member 8 (described later), unless otherwise specified, upstream and downstream refer to the upstream and downstream in the transport direction (process progress direction, movement direction) of the intermediate transfer belt 6.

[0043] As described above, downstream of the primary transfer portion N1, the toner on the intermediate transfer belt 6 tends to be subjected to discharge (downstream discharge) between the intermediate transfer belt 6 and the photosensitive drum 11, resulting in an increase in the amount of charge (charge amount). As a result of further investigations, the inventors of the present invention have found that an increase in the amount of charge of the toner on the intermediate transfer belt 6 increases the adhesive force (reflection force) with the intermediate transfer belt 6, making it difficult to transfer the toner to the recording material S at the secondary transfer portion N2.

[0044] For example, if the charge amount of the toner on the intermediate transfer belt 6 increases, the secondary transfer electric field required to transfer the toner to the recording material S at the secondary transfer portion N2 increases, which can result in a deterioration in the graininess of the image. Furthermore, uniformly transferring toner to, for example, embossed paper with an uneven surface is difficult because a gap occurs between the intermediate transfer belt 6 and the recording material S at the secondary transfer portion N2, requiring a relatively large secondary transfer electric field. Therefore, if the charge amount of the toner on the intermediate transfer belt 6 increases, it becomes even more difficult to transfer toner to embossed paper with an uneven surface. Embossed paper is paper (fancy paper) with an uneven pattern created by embossing or embossing. Furthermore, even for recording materials with relatively high electrical resistance (high-resistivity paper), such as synthetic paper primarily made of synthetic resin or resin film, if the charge amount of the toner on the intermediate transfer belt 6 increases, a higher secondary transfer voltage is required, making toner transfer difficult.

[0045] In order to suppress the increase in the charge amount of the toner downstream of the primary transfer portion N1 as described above, it is effective to suppress discharge downstream of the primary transfer portion N1 (downstream discharge). To achieve this, it is effective to reduce the potential difference between the photosensitive drum 11 and the intermediate transfer belt 6 after the primary transfer. As a result of further investigations, the present inventors have found that in order to suppress downstream discharge, it is effective to dispose a potential regulating member 8, which is a conductive electrode member, downstream of the primary transfer portion N1 and on the inner circumferential surface side of the intermediate transfer belt 6, and to apply a bias (electrode bias) of the same polarity as the charge polarity of the photosensitive drum 11 to this potential regulating member 8.

[0046] 4. Potential control material Next, the configuration of the potential regulating member 8 in this embodiment will be described. As shown in Fig. 1, in the image forming apparatus 1 of this embodiment, potential regulating members 8y, 8m, 8c, and 8k are respectively arranged downstream of the primary transfer portions N1y, N1m, N1c, and N1k in contact with the inner circumferential surface of the intermediate transfer belt 6. In this embodiment, the potential regulating members 8y, 8m, 8c, and 8k provided for the primary transfer portions N1y, N1m, N1c, and N1k have substantially the same configuration.

[0047] The shape of the potential regulating member 8 in this embodiment will be described. Fig. 3(a) is a cross-sectional view of the potential regulating member 8 in this embodiment (showing a cross section substantially perpendicular to the rotation axis direction of the photosensitive drum 11). Fig. 3(b) is a perspective view of the potential regulating member 8 in this embodiment.

[0048] In this embodiment, the potential regulating member 8 has a planar first portion 81 that is arranged along the width direction of the intermediate transfer belt 6 (a direction substantially perpendicular to the conveyance direction, and substantially parallel to the direction of the rotation axis of the photosensitive drum 11). In addition, in this embodiment, the potential regulating member 8 has a planar second portion 82 that is arranged along the width direction of the intermediate transfer belt 6 and extends in a direction substantially perpendicular to the plane of the first portion 81. In this embodiment, the contact surface 83, which is the contact portion of the first portion 81 of the potential regulating member 8 that comes into contact with the inner circumferential surface of the intermediate transfer belt 6, is planar. In other words, in this embodiment, the first portion 81 that constitutes the contact surface 83 of the potential regulating member 8 is a flat plate.

[0049] In this example, in a cross section substantially perpendicular to the rotational axis of the photosensitive drum 11, the upstream end of the contact surface 83 is defined as "A (or upstream end A)," and the downstream end of the contact surface 83 is defined as "B (or downstream end B)." In this embodiment, the upstream end A of the contact surface 83 corresponds to the upstream end of the potential regulating member 8, and the downstream end B of the contact surface 83 corresponds to the downstream end of the potential regulating member 8. To more effectively suppress discharge (downstream discharge) between the intermediate transfer belt 6 and the photosensitive drum 11, it is preferable to bring the potential regulating member 8 into surface contact with the intermediate transfer belt 6. From this perspective, the length between the line segment AB, i.e., the "contact width," which is the length of the contact surface 83 in the transport direction of the intermediate transfer belt 6, is preferably 5 mm or more. The longer the length between the line segment AB, the greater the effect of suppressing downstream discharge. However, if the length is too long, it may be difficult to stably bring the potential regulating member 8 into contact with the intermediate transfer belt 6 due to factors such as component precision. Furthermore, if the potential regulation member 8 has only the first portion 81, it may be weak and bend. Therefore, from the viewpoint of strength, a substantially L-shaped cross section is preferable, and it is preferable to provide the second portion 82. Furthermore, the potential regulation member 8 is more effective in suppressing downstream discharge when the upstream end A is closer to the primary transfer portion N1. Therefore, if the length between the line segment AB is short, the second portion 82 may come into contact with the primary transfer roller 15. Therefore, it is preferable to set the length between the line segment AB to an appropriate length that matches the outer diameter of the primary transfer roller 15. Furthermore, a length of 50 mm or less is often sufficient for the length between the line segment AB, and typically 30 mm or less. In other words, the length between the line segment AB is preferably approximately 5 to 50 mm, and typically approximately 5 to 30 mm. From another perspective, the length between the line segment AB should be equal to or less than half the distance between the axes of adjacent photosensitive drums 11 in a cross section substantially perpendicular to the rotational axis direction of the photosensitive drums 11. In this embodiment, the potential regulation member 8 has a length of 25 mm between the line segments A and B. In this embodiment, the axial distance between the photosensitive drums 11 in a cross section substantially perpendicular to the direction of the rotation axis of the photosensitive drums 11 is approximately 100 mm.

[0050] A potential regulating power supply 80 serving as a potential regulating bias application means (potential regulating bias application section) is connected to the potential regulating member 8. A voltage detection sensor 80a (FIG. 2) serving as a voltage detection means (voltage detection section) for detecting the output voltage of the potential regulating power supply 80 and a current detection sensor 80b (FIG. 2) serving as a current detection means (current detection section) for detecting the output current of the potential regulating power supply 80 are connected to the potential regulating member 8. In this embodiment, the potential regulating power supply 80 is connected to a second portion 82 of the potential regulating member 8. During at least the primary transfer of an image forming operation, the potential regulating power supply 80 applies to the potential regulating member 8 a potential regulating bias (potential regulating voltage), which is a DC voltage having the same polarity (positive in this embodiment) as the charging polarity of the photosensitive drum 11. The primary transfer refers to the period during which the primary transfer bias is applied, and more specifically, the period during which an image area (an area onto which a toner image can be transferred) on the intermediate transfer belt 6 passes through the primary transfer section N1. This reduces the potential difference between the intermediate transfer belt 6 and the photosensitive drum 11 downstream of the primary transfer portion N1, and suppresses discharge (downstream discharge) between the intermediate transfer belt 6 and the photosensitive drum 11 downstream of the primary transfer portion N1. In addition, in this embodiment, a negative constant voltage is applied to the potential regulating member 8 as the potential regulating bias, but a certain degree of effect can be expected even if the potential regulating member 8 is grounded.

[0051] The potential regulating member 8 is a member that is elongated in the width direction of the intermediate transfer belt 6. The length of the contact surface 83 of the potential regulating member 8 in the longitudinal direction (the direction along the width direction of the intermediate transfer belt 6) is preferably longer than the maximum image width in the width direction of the intermediate transfer belt 6. The maximum image width is the length of the image area of the largest image that can be formed by the image forming apparatus 1 in the width direction of the intermediate transfer belt 6. In this embodiment, the length of the contact surface 83 of the potential regulating member 8 in the longitudinal direction is longer than the maximum image width and also longer than the width of the area where the primary transfer roller 15 contacts the intermediate transfer belt 6 in the width direction of the intermediate transfer belt 6. That is, in this embodiment, the range of the maximum image width and the range of the area where the primary transfer roller 15 contacts the intermediate transfer belt 6 in the width direction of the intermediate transfer belt 6 are both within the range of the longitudinal length of the contact surface 83 of the potential regulating member 8. As a result, regardless of the length of the toner image transferred to the intermediate transfer belt 6 in the width direction of the intermediate transfer belt 6, the effect of suppressing an increase in the charge amount of the toner on the intermediate transfer belt 6 due to suppression of downstream discharge can be obtained. On the other hand, in this embodiment, the length of the potential regulating member 8 in the longitudinal direction is shorter than the width of the intermediate transfer belt 6. In other words, in this embodiment, the range of the length of the potential regulating member 8 in the longitudinal direction is within the range of the width of the intermediate transfer belt 6. This reduces the possibility that, when the longitudinal ends of the potential regulating member 8 protrude beyond the width ends of the intermediate transfer belt 6, discharge will occur between the potential regulating member 8 and the surrounding members of the intermediate transfer belt 6, reducing the effect of suppressing downstream discharge.

[0052] The potential regulation member 8 can be made of, for example, only one conductive material. In this embodiment, the potential regulation member 8 is made essentially of only a conductive metal, such as SUS (stainless steel). More specifically, in this embodiment, the potential regulation member 8 is made by bending a metal plate (sheet metal) made of SUS or other metal to form a first portion 81 and a second portion 82. In this embodiment, neither the first portion 81 nor the second portion 82 of the potential regulation member 8 is substantially deformed when the image forming apparatus 1 is in use. By performing the bending process in this manner, the strength of the potential regulation member 8 can be increased. However, the present invention is not limited to this embodiment, and the potential regulation member 8 may be made of two or more materials.

[0053] FIG. 4 is a cross-sectional view of another example of the potential regulating member 8 (showing a cross section substantially perpendicular to the rotation axis direction of the photosensitive drum 11). For example, as shown in FIG. 4, the potential regulating member 8 may have a base 84 having a shape similar to that of the potential regulating member 8 shown in FIG. 3, and a surface layer 85 provided on the surface of the base 84. The contact surface 83 that contacts the intermediate transfer belt 6 and the surface layer 85 that forms the connection portion with the potential regulating power source 80 are made of a conductive material, such as metal or conductive resin. The base 84 may be made of a conductive material, but may also be made of a non-conductive material, such as non-conductive resin. The base 84 and the surface layer 85 can be fixed by any fixing means, such as adhesive or welding.

[0054] 5 is a cross-sectional view of yet another example of the potential regulation member 8 (showing a cross section substantially perpendicular to the rotational axis direction of the photosensitive drum 11). For example, as shown in FIG. 5, the contact surface 83 of the potential regulation member 8 that comes into contact with the intermediate transfer belt 6 may be made of a conductive nonwoven fabric 86. Note that in FIG. 5, the conductive nonwoven fabric 86 is provided on the contact surface 83 of the potential regulation member 8 configured as shown in FIG. 4, but the conductive nonwoven fabric 86 may also be provided on the contact surface 83 of the potential regulation member 8 configured as shown in FIG. 3. The conductive nonwoven fabric 86 can be fixed by any fixing means such as a conductive adhesive. Furthermore, instead of the nonwoven fabric 86, felt or pile fabric (cut pile fabric (velvet, brush) or loop pile fabric (terry cloth)) made of conductive fibers, or a sponge (elastic foam) made of a conductive rubber material, etc. may be used. In this way, by constructing the contact surface 83 of the potential regulating member 8 that comes into contact with the intermediate transfer belt 6 from a flexible or elastic material, the possibility of scratches occurring on the inner surface of the intermediate transfer belt 6 due to friction between the inner surface of the intermediate transfer belt 6 and the potential regulating member 8 is reduced, and it is easier to maintain a stable contact state between the potential regulating member 8 and the intermediate transfer belt 6.

[0055] Next, the arrangement of the potential regulating member 8 in this embodiment will be described. Fig. 6 is a cross-sectional view (showing a cross section substantially perpendicular to the rotation axis direction of the photosensitive drum 11) for explaining the arrangement of the potential regulating member 8 provided between two adjacent primary transfer portions N1 in the transport direction of the intermediate transfer belt 6. Fig. 6 shows, as an example, a potential regulating member 8c provided between the cyan and black primary transfer portions N1c and N1k.

[0056] In this embodiment, the outer diameter of the photosensitive drum 11 is 30 mm, the outer diameter of the primary transfer roller 15 is 18 mm, and the thickness of the intermediate transfer belt 6 is 0.350 mm. In this embodiment, the primary transfer roller 15 is disposed offset downstream from the photosensitive drum 11. In this embodiment, the offset amount X1 is, for example, 3.0 mm. The offset amount X1 is the distance between the rotation center of the photosensitive drum 11 and the rotation center of the primary transfer roller 15 in a direction along a common tangent line of the multiple photosensitive drums 11 on the side that contacts the intermediate transfer belt 6, in a cross section that is approximately perpendicular to the rotation axis direction of the photosensitive drum 11.

[0057] To explain the arrangement of the potential regulation member 8, let us assume that the potential regulation member 8 is removed. In a cross section substantially perpendicular to the rotational axis of the photosensitive drum 11, the line passing through the tensioned surface on the inner circumferential side of the intermediate transfer belt 6 downstream of the primary transfer unit N1 in the absence of the potential regulation member 8 is defined as line L. More specifically, line L corresponds to the tensioned surface in a state where only the potential regulation member 8 is substantially removed from the configuration of the image forming apparatus 1 during image formation (however, the photosensitive drum 11 and the intermediate transfer belt 6 are stationary). Also, on line L, the point where the inner circumferential surface of the intermediate transfer belt 6 separates from the tensioning member immediately upstream of the potential regulation member 8 is defined as "C (or upstream tensioning portion C)," and the point where the inner circumferential surface of the intermediate transfer belt 6 separates from the tensioning member immediately downstream of the potential regulation member 8 is defined as "D (or downstream tensioning portion D)." Although the straight line L is shown schematically as being approximately horizontal in FIG. 6, if the surface of the primary transfer roller 15 is lifted toward the photosensitive drum 11 due to deformation of the elastic layer of the primary transfer roller 15 or the like, the straight line L may be inclined downward in the figure as it goes downstream.

[0058] In this embodiment, the tension member closest to the upstream side of the potential regulating member 8 is the primary transfer roller 15, and the position on the inner circumferential surface of the intermediate transfer belt 6 where the intermediate transfer belt 6 separates from the primary transfer roller 15 is the upstream tension portion C. However, the tension member closest to the upstream side of the potential regulating member 8 is not limited to the primary transfer roller 15. For example, if the primary transfer roller 15 is disposed offset upstream from the photosensitive drum 11, the position on the inner circumferential surface of the intermediate transfer belt 6 where the intermediate transfer belt 6 separates from the photosensitive drum 11 is the upstream tension portion C.

[0059] In this embodiment, for the yellow, magenta, and cyan primary transfer portions N1y, N1m, and N1c, the tension members closest to the downstream side of the potential regulating member 8 are the photosensitive drums 11m, 11c, and 11k disposed adjacently on the downstream side of the respective primary transfer portions N1y, N1m, and N1c. The downstream tension member D is a position on the inner circumferential surface of the intermediate transfer belt 6 corresponding to the position where the intermediate transfer belt 6 separates from the photosensitive drums 11m, 11c, and 11k. However, the tension member closest to the downstream side of the potential regulating member 8 is not limited to the photosensitive drum 11. For example, if the primary transfer roller 15 is disposed offset upstream from the photosensitive drum 11, the downstream tension member D is a position on the inner circumferential surface of the intermediate transfer belt 6 where the intermediate transfer belt 6 separates from the primary transfer roller 15. In this embodiment, for the most downstream black primary transfer portion N1k, the tension member closest to the downstream side is a tension roller 22 (tension roller in this embodiment). The position on the inner circumferential surface of the intermediate transfer belt 6 where the intermediate transfer belt 6 separates from the tension roller 22 is the downstream tension portion D.

[0060] Furthermore, for any primary transfer portion N1, if there is another tension roller that regulates the posture of the intermediate transfer belt 6 during image formation as the nearest tension member downstream of the potential regulation member 8, the line L and the downstream tension portion D are defined based on that tension roller. Also, even if a scraper or brush, rather than a tension roller, is in contact with the inner circumferential surface of the intermediate transfer belt 6 for purposes such as cleaning the inner circumferential surface of the intermediate transfer belt 6, it can be considered to be the nearest tension member downstream of the potential regulation member 8 as long as it regulates the posture of the intermediate transfer belt 6 during image formation. A scraper is generally made of a sheet-like or film-like member.

[0061] As shown in FIG. 6, the potential regulating member 8 is disposed downstream of the primary transfer portion N1 so as not to contact the primary transfer roller 15 and not to contact the photosensitive drum 11 via the intermediate transfer belt 6. In this case, the closer the upstream end A is to the primary transfer portion N1, the greater the effect of suppressing downstream discharge. In this embodiment (FIG. 6), for example, the potential regulating member 8 is disposed downstream of the primary transfer portion N1 so that the distance X2 from the primary transfer roller 15 to the upstream end A is approximately 8.0 mm. Here, the distance X2 is the distance between the rotation center of the primary transfer roller 15 and the upstream end A in a direction along a common tangent line on the side of the multiple photosensitive drums 11 that contact the intermediate transfer belt 6, in a cross section substantially perpendicular to the rotation axis direction of the photosensitive drum 11. In other words, in this embodiment, the distance from the rotation center of the primary transfer roller 15 to the upstream end A in the direction along the common tangent line is shorter than the distance (radius) from the rotation center of the primary transfer roller 15 to the outer periphery of the primary transfer roller 15. Although not limited to this, the distance X2 is preferably about 1 to 20 mm, and typically about 5 to 10 mm.

[0062] In this embodiment, the potential regulating member 8 is pressed against the inner circumferential surface of the intermediate transfer belt 6 at both longitudinal ends thereof by pressure springs 87 (FIG. 3B) composed of compression coil springs, which serve as biasing members serving as biasing means. At this time, the contact portion of the potential regulating member 8 that contacts the inner circumferential surface of the intermediate transfer belt 6 is set to be closer to the photosensitive drum 11 than the line L. This allows the potential regulating member 8 to be in more stable contact with the intermediate transfer belt 6 even if the intermediate transfer belt 6 is wavy or vibrated during image formation (while the intermediate transfer belt 6 is running). In this embodiment, the pressure of the pressure springs 87 is set (adjusted) so that the upstream end A and downstream end B of the contact surface 83, which is the contact portion of the potential regulating member 8 that contacts the inner circumferential surface of the intermediate transfer belt 6, are pushed toward the photosensitive drum 11 by approximately 0.5 mm from the line L. By thus causing the contact surface 83 of the potential regulating member 8 to intrude toward the photosensitive drum 11 with respect to the line L, the potential regulating member 8 can be brought into more stable surface contact with the intermediate transfer belt 6 even if undulations or vibrations occur in the intermediate transfer belt 6 during image formation (while the intermediate transfer belt 6 is running). Although not limited thereto, the intrusion amount of the contact surface 83 of the potential regulating member 8 with respect to the line L is preferably about 0.3 to 3 mm, and is typically about 0.5 to 1.5 mm. If this intrusion amount is too small, it may be difficult to bring the potential regulating member 8 into stable contact with the intermediate transfer belt 6, and if it is too large, it may be difficult to transport the intermediate transfer belt 6 stably.

[0063] Here, in a cross section ( FIG. 6 ) substantially perpendicular to the rotation axis direction of the photosensitive drum 11, a line passing through the upstream end A and the downstream end B of the contact surface 83 is defined as line M. In this case, it is preferable that line M does not intersect with line segment CD on line L. This ensures more reliable surface contact between the intermediate transfer belt 6 and the potential regulating member 8 when the contact surface 83 of the potential regulating member 8 is flat. If line M intersects with line segment CD on line L, there is a possibility that only the end of the potential regulating member 8 on the upstream end A side or the end of the potential regulating member 8 on the downstream end B side may be able to contact the inner circumferential surface of the intermediate transfer belt 6. In this case, it may be difficult to enhance the effect of suppressing downstream discharge due to surface contact.

[0064] 6, the potential regulation member 8 is disposed so that the lines M and L are substantially parallel to each other. However, the line M may be disposed so that it is inclined with respect to the line L, as long as the line M does not intersect with the line segment CD of the line L. For example, by inclining the line M with respect to the line L so that the upstream end A is closer to the line L than the downstream end B, the curvature of the intermediate transfer belt 6 caused by the routing of the intermediate transfer belt 6 near the upstream end A can be reduced. This is therefore advantageous in reducing the possibility of scratches occurring on the inner circumferential surface of the intermediate transfer belt 6 due to friction with the potential regulation member 8.

[0065] The contact portion of the potential regulating member 8 that comes into contact with the inner circumferential surface of the intermediate transfer belt 6 is not limited to being flat. For example, the potential regulating member 8 may be formed of a bent plate or the like, the cross section of which is approximately perpendicular to the rotation axis direction of the photosensitive drum 11 being curved in a convex shape toward the photosensitive drum 11, and the contact portion of the potential regulating member 8 that comes into contact with the inner circumferential surface of the intermediate transfer belt 6 may be a curved surface that is convex toward the photosensitive drum 11. By making the contact portion (contact surface) of the potential regulating member 8 that comes into contact with the inner circumferential surface of the intermediate transfer belt 6 curved in this way, stress when the potential regulating member 8 rubs against the intermediate transfer belt 6 can be reduced. By using a roller-shaped potential regulating member 8, the contact portion of the potential regulating member 8 that comes into contact with the inner circumferential surface of the intermediate transfer belt 6 may be a curved surface.

[0066] FIG. 7 is a schematic diagram showing the current and voltage around the primary transfer portion N1. When a target current Ia is supplied from the primary transfer power supply 75 to the primary transfer roller 15, the target current Ia and the effective current I1 flowing toward the photosensitive drum 11 are approximately equal (Ia = I1) if the potential regulating member 8 is not present. However, as shown in FIG. 7, when the potential regulating member 8 is disposed downstream of the primary transfer portion N1, the current path from the primary transfer roller 15 branches into the effective current I1 and the advection current I2 flowing toward the potential regulating member 8 (Ia = I1 + I2). FIG. 7 also shows the primary transfer bias voltage Vtr, the potential regulating bias voltage Vb, and the potential regulating bias current Ib for supplying the target current Ia to the primary transfer roller 15. The primary transfer bias voltage Vtr is a voltage output by the primary transfer power supply 75. The potential regulating bias voltage Vb is a voltage output by the potential regulating power supply 80. Furthermore, the potential regulating bias current Ib is the current (detected current of the current detection sensor 80b) that flows through the potential regulating power supply 80 (potential regulating member 8). Here, the advection current I2(+) and the potential regulating bias current Ib(-) have the same absolute value but different polarities. Therefore, the following relationship approximately holds: Advection current I2 = |potential regulation bias current Ib|

[0067] 5. Overview of Primary Transfer Bias Setting Operation Next, an overview of the primary transfer bias setting operation in this embodiment will be described. In this embodiment, the image forming apparatus 1 has individual primary transfer power supplies 75y, 75m, 75c, and 75k for the primary transfer rollers 15y, 15m, 15c, and 15k, respectively, and is able to supply individual voltages to the primary transfer rollers 15y, 15m, 15c, and 15k. In this embodiment, the primary transfer bias setting operation for each of the primary transfer rollers 15y, 15m, 15c, and 15k is the same and is performed synchronously and individually for each of the primary transfer rollers 15y, 15m, 15c, and 15k. Here, the description will focus on one primary transfer roller 15.

[0068] In this embodiment, the image forming apparatus 1 performs ATVC of the primary transfer bias in order to supply a primary transfer current required for primary transfer of the toner image on the photosensitive drum 11 to the intermediate transfer belt 6 during image formation (primary transfer). ATVC (or ATVC control) is an abbreviation for "Active-Transfer-Voltage-Control." Here, the ATVC of the primary transfer bias is also referred to as "primary transfer ATVC."

[0069] In the primary transfer ATVC, a test bias (test voltage or test current) is used to obtain voltage-current characteristics in order to determine the primary transfer bias during image formation according to the total resistance value of the primary transfer unit N1, which is composed of the photosensitive drum 11, the intermediate transfer belt 6, and the primary transfer roller 15. The primary transfer ATVC is controlled and executed by the control unit 3.

[0070] Specifically, during non-image formation when no toner image is present at the primary transfer portion N1, the intermediate transfer belt 6 is rotated, and a predetermined voltage or current is supplied to the primary transfer roller 15 as a test bias from the primary transfer power supply 75. The predetermined voltage or current setting for the test bias may be one or more levels. In this embodiment, three levels of test bias are supplied to the primary transfer roller 15 while varying the setting. When a test bias of a predetermined voltage is supplied to the primary transfer roller 15 (constant voltage control), the current detection sensor 75b detects the current flowing through the primary transfer roller 15 (primary transfer power supply 75). Alternatively, when a test bias of a predetermined current is supplied to the primary transfer roller 15 (constant current control), the voltage detection sensor 75a detects the voltage applied to the primary transfer roller 15 (the voltage output by the primary transfer power supply 75). This allows the voltage-current characteristics corresponding to the impedance (total resistance value) of the primary transfer portion N1 to be determined. Based on this voltage-current characteristic, the voltage required to apply a primary transfer current suitable for the primary transfer of toner according to the impedance (total resistance value) of the primary transfer unit N1 is calculated. Then, during image formation, a primary transfer bias is applied to the primary transfer roller 15 under constant voltage control with the calculated voltage as the target voltage.

[0071] In this embodiment, the effective current I1 required for the primary transfer of toner and the advection current I2 (=|current Ib of the potential regulating bias|) required to suppress downstream discharge are determined in advance through experiments or the like and stored as a table in the ROM 32. The target values of the effective current I1 and the advection current I2 (=|current Ib of the potential regulating bias|) are set according to, for example, the environment (temperature, humidity) and the process speed (image formation speed). Note that the process speed is also referred to as "PS" here. In this embodiment, the process speed corresponds to the rotational speed (circumferential velocity) of the intermediate transfer belt 6. For example, FIG. 8 shows a table showing the target values of the effective current I1(+) and the target values of the potential regulating bias current Ib(-) according to the environment (moisture content) and the process speed (500 mm / sec and 250 mm / sec). Here, the moisture content is a value calculated from the temperature and humidity. The relationship between the moisture content and the temperature and humidity is roughly as follows: The numerical values between the moisture amounts in the table shown in FIG. 8 are determined by linear interpolation. Temperature 23℃・Humidity 5% = Moisture content 0.89[g / Kg] Temperature 23℃・Humidity 50% = Moisture content 8.90[g / Kg] Temperature 30℃・Humidity 80% = Moisture content 21.70[g / Kg]

[0072] FIG. 9 is a graph diagram showing a voltage-current characteristic, which is the relationship between the voltage (Vtr) and current (Ia) of the primary transfer bias obtained by the primary transfer ATVC. Here, to provide an overview of the primary transfer bias setting operation, the primary transfer bias setting operation without the potential regulating member 8 will be described. Under the condition without the potential regulating member 8 (I2 = Ib = 0 μA), for example, when the moisture content is 8.90 g / kg (= 23°C, 50%) and the process speed is 500 mm / sec, the primary transfer ATVC operates as follows: First, as the first test bias, a target current Ia (e.g., Ia = I1 + I2 = 50 + 0 = 50 μA) corresponding to the environment and process speed at that time is supplied to the primary transfer roller 15 under constant current control ((1) in FIG. 9 ). Then, when the first test bias is supplied to the primary transfer roller 15, the voltage applied to the primary transfer roller 15 (e.g., Vtr = 1200 [V]) is detected. Furthermore, two levels of test bias (Vtr = 1000 [V] and 1400 [V]), which are obtained by increasing or decreasing the voltage detected when the first test bias is supplied, for example, by ±200 V, are supplied to the primary transfer roller 15 under constant voltage control ((2) and (3) in FIG. 9 ). Then, the currents (e.g., 40 [μA] and 60 [μA]) that flow through the primary transfer roller 15 when the second and third test biases are supplied are detected. From these three points, the voltage-current characteristics are calculated. Furthermore, based on the calculated voltage-current characteristics, the voltage required to flow a target current (e.g., Ia = I1 + I2 = 50 + 0 = 50 [μA]) is calculated, for example, by linear approximation. The voltage required to pass the target current may be calculated by curve approximation depending on the configuration of the image forming apparatus 1. The calculated voltage is then determined as the primary transfer bias voltage Vtr to be applied during image formation. During image formation, the primary transfer bias is applied to the primary transfer roller 15 under constant voltage control with this voltage Vtr as the target voltage.

[0073] Here, the voltage (e.g., Vtr=1200V) when the first test bias (constant current control, e.g., 50μA) is supplied is an average value for one or more revolutions of the primary transfer roller 15. Similarly, the currents (e.g., 40μA and 60μA) when the second and third test biases (constant voltage control, e.g., Vtr=1000V and 1400V) are supplied are average values for one or more revolutions of the primary transfer roller 15.

[0074] 6. Overview of the setting operation of the potential regulation bias (when the current detection sensor 80b is present) Next, an overview of the setting operation of the potential regulating bias in this embodiment will be described. Note that in this embodiment, the image forming apparatus 1 has individual potential regulating power supplies 80y, 80m, 80c, and 80k for the potential regulating members 8y, 8m, 8c, and 8k, and is able to supply individual voltages to the potential regulating members 8y, 8m, 8c, and 8k. In this embodiment, the setting operation of the potential regulating bias for each potential regulating member 8y, 8m, 8c, and 8k is the same and is performed synchronously for each potential regulating member 8y, 8m, 8c, and 8k individually. Here, the description will focus on one potential regulating member 8.

[0075] In this embodiment, the image forming apparatus 1 performs ATVC of the potential-regulated bias in order to pass the advection current I2 necessary to suppress downstream discharge during image formation (primary transfer). Here, the ATVC of the potential-regulated bias is also referred to as "potential-regulated ATVC." The operation of the potential-regulated ATVC is generally similar to the operation of the primary transfer ATVC described above.

[0076] The potential regulation ATVC acquires voltage-current characteristics using a test bias (test voltage, test current) in order to determine the potential regulation bias during image formation according to the total resistance value configured by the primary transfer roller 15, the intermediate transfer belt 6, and the potential regulation member 8. The potential regulation ATVC is controlled and executed by the control unit 3.

[0077] Specifically, during non-image formation when no toner image is present at the primary transfer portion N1, the intermediate transfer belt 6 is rotated, and the primary transfer bias (voltage Vtr) during image formation determined by the primary transfer ATVC is applied to the primary transfer roller 15 under constant voltage control. The application of the primary transfer bias determined by the primary transfer ATVC to the primary transfer roller 15 in the potential regulation ATVC will be described in detail below. Then, a predetermined voltage or a predetermined current is supplied to the potential regulation member 8 as a test bias from the potential regulation power supply 80. The predetermined voltage or current setting value of the test bias may be one level or multiple levels. In this embodiment, three levels of test bias are supplied to the potential regulation member 8 while changing the setting value. Then, when a test bias of a predetermined voltage is supplied to the potential regulation member 8 (constant voltage control), the current flowing through the potential regulation member 8 (potential regulation power supply 80) is detected by the current detection sensor 80b. Alternatively, when a test bias of a predetermined current is supplied to the potential regulating member 8 (constant current control), the voltage applied to the potential regulating member 8 (the voltage output by the potential regulating power supply 80) is detected by the voltage detection sensor 80a. This makes it possible to determine the voltage-current characteristics according to the impedance (total resistance value) from the primary transfer roller 15 through the intermediate transfer belt 6 to the potential regulating member 8. Furthermore, based on this voltage-current characteristic, a voltage necessary to flow an advection current I2 suitable for suppressing downstream discharge according to the impedance (total resistance value) is calculated. Then, during image formation, a potential regulating bias is applied to the potential regulating member 8 using constant voltage control with the calculated voltage as the target voltage.

[0078] As described above, in this embodiment, the appropriate values (target values) of the effective current I1 and the advection current I2 (=|current Ib of the potential regulation bias|) are determined in advance based on experiments, etc., and are stored in ROM 32 as a table such as that shown in Figure 8.

[0079] FIG. 10 is a graph showing the voltage-current characteristics, which are the relationship between the voltage (|Vb|) and current (|Ib|) of the potential-regulating bias, acquired by the potential-regulating ATVC. Under conditions where the potential-regulating member 8 is present, for example, when the moisture content is 8.90 g / kg (= 23°C, 50%) and the process speed is 500 mm / sec, the potential-regulating ATVC operates as follows. First, as a first test bias, a potential-regulating bias current Ib (e.g., Ib = −10 μA) corresponding to the current environment and process speed is supplied to the potential-regulating member 8 under constant current control ((1) in FIG. 10). Then, when the first test bias is supplied to the potential-regulating member 8, the voltage applied to the potential-regulating member 8 (e.g., Vb = −3400 V) is detected. Furthermore, the second and third test biases (Vb = −2900 [V] and −3900 [V]) are applied to the potential regulating member 8 under constant voltage control. The second and third test biases are obtained by increasing or decreasing the voltage detected when the first test bias is applied by, for example, ±500 V (see (2) and (3) in FIG. 10 ). Then, the currents (e.g., −8 [μA] and −12 [μA]) flowing through the potential regulating member 8 when the second and third test biases are applied are detected. From these three points, the voltage-current characteristics are calculated. Based on the calculated voltage-current characteristics, the voltage required to pass a target current (e.g., Ib = −10 [μA]) is calculated by, for example, linear approximation. Note that the voltage required to pass the target current may also be calculated by curve approximation depending on the configuration of the image forming apparatus 1. Then, during image formation, the potential regulating bias is applied to the potential regulating member 8 under constant voltage control with the voltage Vb as the target voltage.

[0080] Constant current control is a control that adjusts the output (voltage) of a power supply so that the current supplied to the target is kept approximately constant at a target current, while constant voltage control is a control that adjusts the output (voltage) of a power supply so that the voltage applied to the target is kept approximately constant at a target voltage.

[0081] In this embodiment, the primary transfer ATVC and the potential regulation ATVC are performed in the pre-rotation process (or pre-multiple rotation process) of the job as a non-image formation time. However, the present invention is not limited to this, and the primary transfer ATVC and the potential regulation ATVC can be performed as long as the image formation time is not, such as by performing them in the inter-sheet process at a predetermined frequency (for every predetermined number of images formed) during continuous image formation.

[0082] 7. Setting operation of potential regulation bias in this embodiment <Issues in setting potential regulation bias> The appropriate potential regulating bias varies depending on factors such as the likelihood of discharge (downstream discharge) occurring downstream of the primary transfer portion N1. Therefore, the potential regulating bias is set so as to sufficiently suppress the increase in the charge amount of toner on the intermediate transfer belt 6. As mentioned above, in order to suppress downstream discharge, it is important to release charges of the opposite polarity to the surface potential of the photosensitive drum 11 through the intermediate transfer belt 6 downstream of the primary transfer portion N1. This is thought to reduce the potential difference between the photosensitive drum 11 and the intermediate transfer belt 6 after primary transfer.

[0083] The current of the potential regulating bias corresponds to the current that flows from the primary transfer roller 15 through the intermediate transfer belt 6 to the potential regulating member 8, or the current that flows due to the potential difference between the primary transfer bias and the potential regulating bias. In other words, the "leakage current to the potential regulating member" described above corresponds to the "advection current I2" or the "potential regulating bias current (detected current of current detection sensor 80b) Ib." As described above, the advection current I2(+) and the potential regulating bias current (detected current of current detection sensor 80b) Ib(-) have approximately the same absolute current value but different polarities.

[0084] Therefore, in order to suppress downstream discharge, it is important to pass a necessary current equivalent to the above-mentioned advection current I2 (leakage current) from the primary transfer roller 15 to the potential regulating member 8.

[0085] The electrical resistance values of the intermediate transfer belt 6 and the primary transfer roller 15 fluctuate due to factors such as fluctuations in the environment (temperature, humidity) in which the image forming apparatus 1 is installed, the temperature rise inside the apparatus due to continuous operation of the image forming apparatus 1, and the increase in the electrical resistance of components due to usage conditions (accumulated usage). This can cause the current and voltage (current in this embodiment) of the primary transfer bias to deviate from the target current and voltage (current in this embodiment), impairing primary transfer performance. Similarly, the current and voltage (current in this embodiment) of the potential regulating bias can deviate from the target current and voltage (current in this embodiment), failing to sufficiently suppress the increase in the charge amount of toner on the intermediate transfer belt 6 and impairing secondary transfer performance. Furthermore, due to factors such as individual differences in the electrical resistance values of the intermediate transfer belt 6, primary transfer roller 15, and potential regulating member 8, the degree of deviation in the current and voltage as described above can vary for each individual image forming apparatus 1, transfer roller 15, intermediate transfer belt 6, potential regulating member 8, etc., and depending on usage conditions.

[0086] Therefore, as described above, the primary transfer ATVC determines the target voltage Vtr for flowing the target current Ia corresponding to the total resistance value of the primary transfer section N1. The primary transfer bias is thus set to maintain sufficient primary transfer performance so that the primary transfer efficiency does not fall below the target value. The advection current I2 to the potential regulating member 8 fluctuates due to factors such as fluctuations in the electrical resistance of the intermediate transfer belt 6. Therefore, if the primary transfer ATVC is performed without considering the advection current I2 to the potential regulating member 8 (the current Ib of the potential regulating bias), the actual effective current I1 may decrease, resulting in poor primary transfer performance. Meanwhile, the advection current I2 to the potential regulating member 8 is affected by the primary transfer bias as well as the environment and usage conditions described above. Therefore, if the potential regulating bias is set without considering the primary transfer bias, the appropriate advection current I2 will not flow, and the increase in the charge amount of the toner on the intermediate transfer belt 6 may not be sufficiently suppressed, resulting in poor secondary transfer performance.

[0087] As mentioned above, to effectively suppress downstream discharge and improve secondary transfer performance, it is desirable to move the potential regulation member 8 closer to the primary transfer portion N1 and to set a high bias of the same polarity as the photosensitive drum 11 applied to the potential regulation member 8. However, the closer the potential regulation member 8 is to the primary transfer portion N1 and the larger the absolute value of the potential regulation bias applied to the potential regulation member 8, the greater the potential difference between the primary transfer roller 15 and the potential regulation member 8. This increases the advection current I2 (=|Ib|) from the primary transfer roller 15 to the potential regulation member 8. As a result, the effective current I1 flowing toward the photosensitive drum 11 at the primary transfer portion N1 decreases, potentially impairing primary transfer performance. Conversely, to maintain primary transfer performance, the further the potential regulation member 8 is moved away from the primary transfer portion N1 and the smaller the absolute value of the potential regulation bias applied to the potential regulation member 8, the greater the advection current I2 (the current Ib of the potential regulation bias) decreases. Therefore, the effect of suppressing downstream discharge is reduced, making it difficult to suppress an increase in the charge amount of the toner after primary transfer, and there is a possibility that secondary transfer properties may be impaired.

[0088] Therefore, in order to improve secondary transfer performance while maintaining primary transfer performance, it is desirable to keep the effective current I1 flowing from the primary transfer roller 15 toward the photosensitive drum 11 and the advection current I2 flowing from the primary transfer roller 15 toward the potential regulating member 8 as constant as possible at predetermined values.

[0089] Therefore, in this embodiment, the potential-regulating ATVC is executed while applying the primary transfer bias voltage Vtr set by the primary transfer ATVC. The primary transfer ATVC is also executed by setting a target current that takes into account the advection current I2. This allows the potential-regulating bias current Ib (advection current I2) to be kept constant, and the effective current I1 flowing toward the photosensitive drum 11 to be kept constant. As a result, good and stable primary transferability and secondary transferability can be maintained.

[0090] <Details of the setting operation of the potential regulation bias in this embodiment> Next, the setting operation of the potential regulating bias in this embodiment will be further described with reference to Fig. 11. Fig. 11 is a timing chart showing the transition of the voltage and current of the primary transfer bias and the potential regulating bias during job execution in this embodiment. The operation according to the timing chart in Fig. 11 is controlled by the control unit 3.

[0091] When a job is started, the intermediate transfer belt 6 starts to be driven, and the pre-rotation process begins (T1). Thereafter, the primary transfer ATVC begins, and a test bias (also referred to as a "primary transfer test bias") is applied from the primary transfer power supply 75 to the primary transfer roller 15 under constant current control, aiming for a target current Ia (T2). In this embodiment, the primary transfer ATVC applies three levels of test bias to the primary transfer roller 15. However, for simplicity, FIG. 11 illustrates only the test bias under constant current control aiming for the target current during image formation (this also applies to other examples described later). The target current Ia at this time is determined based on a preset table such as that shown in FIG. 8. For example, when the moisture content is 8.9 g / kg (= 23°C 50%) and the process speed is 500 mm / sec, the effective current I1 is 50 μA, the potential control bias current Ib is -10 μA, and the advection current I2 is 10 μA. Therefore, the target current Ia is set to I1 + I2 = 60 μA. Based on the voltage-current characteristics (FIG. 9) acquired by constant current control and constant voltage control in the primary transfer ATVC, the primary transfer bias voltage Vtr is determined (e.g., Vtr = 1600 V). In this embodiment, three levels of values are detected in the primary transfer ATVC, but Vtr = 1600 V is determined by constant current control with Ia = 60 μA. Therefore, to shorten the time, the primary transfer ATVC may be configured to detect only one level of constant current control. At this time, the voltage Vb of the potential regulating bias applied to the potential regulating member 8 is 0 V, while the voltage Vtr of the primary transfer bias applied to the primary transfer roller 15 is 1600 V. Therefore, the current Ib of the potential regulating bias is not 0 [μA], but a current flows due to the potential difference of 1600 [V] between the primary transfer roller 15 and the potential regulating member 8 (for example, Ib1=−4.0 [μA]).

[0092] Thereafter, the primary transfer ATVC ends, and the potential-regulating ATVC starts while the primary transfer bias (e.g., Vtr=1600 [V]) is kept under constant voltage control. Then, a test bias (also referred to as a "potential-regulating test bias") is applied from the potential-regulating power supply 80 to the potential-regulating member 8 under constant current control, aiming for, for example, I2=10 [μA] (Ib2=−10 [μA]) (T3). The target value of the potential-regulating bias current at this time is determined based on a preset table such as that shown in FIG. 8. In this embodiment, the potential-regulating ATVC applies three levels of test bias to the potential-regulating member 8. However, for simplicity, FIG. 11 illustrates only the test bias under constant current control aiming for the target value of the potential-regulating bias current during image formation (this also applies to other examples described later). Based on the voltage-current characteristics (FIG. 9) obtained by constant current control and constant voltage control in the potential-regulated ATVC, the voltage Vb of the potential-regulated bias is determined from the target value Ib2=-10 [μA] (for example, Vb=-3400 [V]). Note that, as in the case of the primary transfer ATVC, the test bias of the potential-regulated ATVC may be three levels or one level.

[0093] In this way, in the pre-rotation process of the job, the voltages to be applied to the primary transfer roller 15 and the potential regulating member 8 during image formation are determined. Then, during image formation, for example, the primary transfer bias is controlled to a voltage Vtr of 1600 V and the potential regulating bias is controlled to a voltage Vb of −3400 V, under constant voltage control (T4 to T5).

[0094] Therefore, during image formation, the advection current I2 is 10 μA, and the effective current I1 is 50 μA. Therefore, the effective current I1 is sufficient to maintain primary transfer performance. Furthermore, the potential control bias current Ib2 is -10 μA (I2 = 10 μA), and the advection current I2 (=|potential control bias current Ib|) necessary to suppress downstream discharge can be maintained.

[0095] As described above, in this embodiment, the control unit 3 executes the primary transfer ATVC to determine the primary transfer bias during the pre-rotation process of the job, and then executes the potential-regulating ATVC to determine the potential-regulating bias, thereby correcting the potential-regulating bias. The primary transfer ATVC is executed by setting a target current that takes into account the advection current I2. That is, the primary transfer ATVC is executed using a target current Ia=I1+I2, which is the sum of the effective current I1 and the advection current I2 (=|potential-regulating bias current Ib|), to determine the primary transfer bias voltage Vtr. Then, while applying the voltage Vrt determined by the primary transfer ATVC, the potential-regulating ATVC is executed to correct the potential-regulating bias voltage Vb so that the advection current I2 becomes the desired value.

[0096] This allows good primary transfer performance to be maintained at a constant effective current I1 in response to fluctuations in the electrical resistance of components due to changes in the environment (temperature, humidity) and usage conditions. At the same time, a constant advection current I2 suppresses downstream discharge, suppressing an increase in the toner charge amount and maintaining good secondary transfer performance.

[0097] In this embodiment, the primary transfer bias is subjected to constant voltage control during image formation, and the voltage Vtr corresponding to the target current Ia is determined. However, this is not limiting. For example, if the primary transfer bias is subjected to constant current control during image formation, the target current Ia is determined as described above, and the primary transfer bias is subjected to constant current control using the determined target current Ia during image formation. Even when the primary transfer bias is subjected to constant current control, if the potential regulating bias is subjected to constant voltage control, the voltage Vb of the potential regulating bias is determined by the primary transfer ATVC. Even if the primary transfer bias is thus changed from constant voltage control to constant current control, the potential regulating bias can be corrected in the same manner. Furthermore, both constant voltage control and constant current control of the primary transfer bias may be performed during image formation.

[0098] Next, the procedure for setting the potential regulating bias in this embodiment will be described with reference to Fig. 12. Fig. 12 is a flowchart showing an outline of the job procedure in this embodiment. Note that the voltage and current values of the primary transfer bias and potential regulating bias are just an example (this also applies to other examples described later).

[0099] When the control unit 3 starts a job (S1), it determines the target values of the target current Ia of the primary transfer bias and the target current Ib of the potential regulating bias in advance based on the environment (temperature, humidity), process speed, etc., during the pre-rotation process (S2). The target values of the target current Ia of the primary transfer bias and the target current Ib of the potential regulating bias are determined based on a preset table, such as that shown in FIG. 8. For example, if the moisture content is 8.9 g / kg (= 23°C 50%) and the process speed is 500 mm / sec, the effective current I1 is 50 μA, the potential regulating bias current Ib is -10 μA, and the advection current I2 is 10 μA. Therefore, the target current Ia = I1 + I2 = 60 μA is determined.

[0100] Next, the control unit 3 applies a primary transfer test bias from the primary transfer power supply 75 and executes primary transfer ATVC (constant current control) at a target current Ia=60 [μA] (S3). As mentioned above, the primary transfer test bias may have one level or three levels. Then, the control unit 3 determines the primary transfer bias voltage Vtr=1600 [V] so that the current flowing through the primary transfer roller 15 becomes the target current Ia=60 [μA] (S4).

[0101] Next, the control unit 3 applies a potential regulating test bias from the potential regulating power supply 80 while applying the primary transfer bias (Vtr=1600 [V]) determined by the primary transfer ATVC to the primary transfer roller 15 under constant voltage control, thereby executing primary transfer ATVC (constant current control) at a current Ib=-10 [μA] (S5). As mentioned above, the potential regulating test bias may have one level or three levels. Then, the control unit 3 determines the voltage Vb=-3400 [V] of the potential regulating bias such that the current flowing through the potential regulating member 8 becomes Ib=-10 [μA] (S6).

[0102] Thereafter, the control unit 3 completes the pre-rotation process, and performs constant voltage control of the primary transfer bias at voltage Vtr=1600 [V] and the potential regulating bias at voltage Vb=-3400 [V] to perform image formation (S7). When image formation is completed, the control unit 3 completes the job (S8).

[0103] In this embodiment, it is important to set the target current Ia = I1 + I2 and execute the primary transfer ATVC at a current greater than the effective current I1, taking into account the advection current I2. By doing so, the potential-regulated ATVC is subsequently executed to correct the advection current I2 (the potential-regulated bias current Ib), thereby maintaining a constant advection current I2 and a constant effective current I1.

[0104] 8. Variations Next, a modified example of the first embodiment will be described. In this modified example, a primary transfer ATVC is further added after the potential regulation ATVC in the first embodiment. This further improves the accuracy of the active current I1 and the advection current I2, that is, it is possible to bring the active current I1 and the advection current I2 closer to the target values.

[0105] The setting operation of the potential regulating bias in this modified example will be described with reference to Fig. 13. Fig. 13 is a timing chart showing the transition of the voltage and current of the primary transfer bias and the potential regulating bias during job execution in this modified example. The operation according to the timing chart in Fig. 13 is controlled by the control unit 3.

[0106] The operations from T1 to T4 in FIG. 13 are the same as those from T1 to T4 in FIG. 11. In this modification, a second primary transfer ATVC is performed in the pre-rotation process before image formation (T4 to T5). That is, after the potential regulating ATVC is completed, a test bias (also referred to as "primary transfer test bias 2") is applied to the primary transfer roller 15 under constant current control, aiming for a target current Ia of 60 μA, while the voltage Vb of -3400 V determined by the potential regulating ATVC is applied to the potential regulating member 8 (T4). Note that, like the first primary transfer ATVC, the test bias for the second primary transfer ATVC may have three levels or one level. Then, a primary transfer bias voltage Vtr2 of 1550 V is determined. During image formation, constant voltage control is performed at Vtr2 of 1550 V and Vb of -3400 V (T5 to T6).

[0107] Next, the procedure for setting the potential regulating bias in this comparative example will be described with reference to Fig. 14. Fig. 14 is a flow chart showing an outline of the job procedure in this comparative example.

[0108] The operations S1 to S6 in FIG. 14 are the same as the operations S1 to S6 in FIG. 12. In this modification, the control unit 3 executes a second primary transfer ATVC in the pre-rotation process before image formation (S7 to S8). That is, the control unit 3 executes the potential regulation ATVC (S5) and determines the primary transfer bias voltage Vb=-3400 [V] (S6). Thereafter, the control unit 3 applies the primary transfer test bias 2 to the primary transfer roller 15 under constant current control, aiming for a target current Ia=60 [μA], while keeping the voltage Vb applied to the potential regulation member 8 (S7). The control unit 3 then determines the primary transfer bias voltage Vtr2=1550 [V] (S8). Thereafter, the control unit 3 executes constant voltage control of the primary transfer bias at voltage Vtr2=1550 [V] and the potential regulation bias at voltage Vb=-3400 [V], and executes image formation (S9). When the image formation is completed, the control section 3 ends the job (S10).

[0109] In this modified example, the voltage of the primary transfer bias is reduced by 50 V from Vtr1=1600 V to Vtr2=1550 V. This is because, during the first primary transfer ATVC, the voltage Vb of the potential regulating bias is 0 V, making it difficult for the advection current I2 to flow, and the voltage Vtr1 of the primary transfer bias is slightly higher. During the second primary transfer ATVC, the voltage Vb of the potential regulating bias is −3400 V, making it easy for the advection current I2 to flow, and the voltage Vtr2 of the primary transfer bias is slightly lower than the previous Vtr1. Therefore, by performing the primary transfer ATVC multiple times, the effective current I1 and the advection current I2 become closer to the preset values of effective current I1=50 μA and advection current I2=10 μA, improving accuracy relative to the target values.

[0110] Furthermore, by performing the potential-regulating ATVC multiple times, the accuracy of the potential-regulating bias current Ib relative to the target value is further improved. When the primary transfer ATVC and the potential-regulating ATVC are each performed multiple times, they are performed alternately. However, while increasing the number of times the primary transfer ATVC and the potential-regulating ATVC are performed improves the accuracy relative to the target value, the time required for the pre-rotation process increases accordingly, thereby increasing the time required for printing. In consideration of improving the accuracy relative to the target value and extending the time required for printing, it is preferable to perform the primary transfer ATVC and the potential-regulating ATVC approximately once or twice each.

[0111] As another modification, in a configuration capable of executing both the primary transfer ATVC and the potential-regulating ATVC, the primary transfer ATVC and the potential-regulating ATVC can be executed substantially simultaneously. In this case, for example, in the primary transfer ATVC, the primary transfer power supply 75 is controlled to set the primary transfer bias so that the target current Ia described in embodiment 1 flows. In this case, in the potential-regulating ATVC, the potential-regulating power supply 80 is controlled so that the target value of the potential-regulating bias current Ib described in embodiment 1 flows.

[0112] 9.Effects As described above, in this embodiment, the image forming apparatus 1 includes a photosensitive member (photosensitive drum) 11 that can be charged to a predetermined polarity and that carries a toner image, a circumferentially movable intermediate transfer belt 6 that transports the toner image that has been primarily transferred from the photosensitive member 11 at the primary transfer portion N1 to be secondarily transferred to the recording material S at the secondary transfer portion N2, a primary transfer member (primary transfer roller) 15 that contacts the inner peripheral surface of the intermediate transfer belt 6 to form the primary transfer portion N1 where the photosensitive member 11 and the intermediate transfer belt 6 come into contact, and that is applied with a bias to transfer the toner image from the photosensitive member 11 to the intermediate transfer belt 6, and a secondary transfer member that applies a bias of a polarity opposite to the predetermined polarity to the primary transfer member 15. The image forming device includes an application unit (primary transfer power supply) 75 of the first transfer member 15, an electrode member (potential regulating member) 8 that contacts the inner surface of the intermediate transfer belt 6 downstream of the primary transfer unit N1 in the movement direction of the intermediate transfer belt 6, a second application unit (potential regulating power supply) 80 that applies a bias of the same polarity as the above-mentioned predetermined polarity to the electrode member 8, detection units 75a and 75b that detect the current flowing through the primary transfer member 15 or the voltage applied to the primary transfer member 15, and a control unit 3 that controls the execution of a setting operation to set the electrode bias (potential regulating bias) to be applied to the electrode member 8 by the second application unit 80 during image formation based on the detection results by the detection units 75a and 75b. In this embodiment, the image forming apparatus 1 has second detection units 80a, 80b that detect the current flowing through the electrode member 8 or the voltage applied to the electrode member 8, and the control unit 3 controls the setting operation in such a way that, during the setting operation, the first application unit 75 applies a first test bias to the primary transfer member 15 and obtains a first detection result from the detection units 75a, 75b, and based on the first detection result, determines a first voltage at which the current flowing through the primary transfer member 15 becomes a first current, and while the first application unit 75 applies the first voltage bias to the primary transfer member 15, the second application unit 80 applies a second test bias to the electrode member 8 and obtains a second detection result from the second detection units 80a, 80b, and based on the second detection result, determines a second voltage at which the current flowing through the electrode member 8 becomes a second current, and sets the electrode bias based on the second voltage.

[0113] In this embodiment, the control unit 3 controls the setting operation so that the transfer bias to be applied to the primary transfer member 15 by the first application unit 75 during image formation is set based on the first voltage. In this embodiment, the control unit 3 controls the first current to be set based on the sum of a predetermined target value of an effective current that flows between the primary transfer member 15 and the photosensitive member 11 when the first application unit 75 applies the transfer bias to the primary transfer member 15 during image formation, and a predetermined target value of an advection current that flows between the primary transfer member 15 and the electrode member 8 when the first application unit 75 applies the transfer bias to the primary transfer member 15 during image formation. In addition, in the above-mentioned setting operation, the control unit 3 can control the second application unit 80 to apply the second voltage bias to the electrode member 8 while the first application unit 75 applies a third test bias to the primary transfer member 15 to obtain a third detection result from the detection units 75a and 75b, and based on the third detection result, determine a third voltage at which the current flowing through the primary transfer member 15 becomes the first current, and set the transfer bias to be applied to the primary transfer member 15 by the first application unit 75 during image formation based on the third voltage.

[0114] The image forming apparatus 1 may also have first detection units 75a, 75b that detect the current flowing through the primary transfer member 15 or the voltage applied to the primary transfer member 15, second detection units 80a, 80b that detect the current flowing through the electrode member 8 or the voltage applied to the electrode member 8, and a control unit that controls the execution of a setting operation to set a transfer bias to be applied to the primary transfer member 15 by the first application unit 75 during image formation based on a first detection result by the first detection units 75a, 75b obtained by having the first application unit 75 apply a first test bias to the primary transfer member 15, and to set an electrode bias to be applied to the electrode member 8 by the second application unit 80 during image formation based on a second detection result by the second detection units 80a, 80b obtained by having the second application unit 80 apply a second test bias to the electrode member 8. In this case, in the setting operation, the control unit 3 determines a first voltage at which the current flowing through the primary transfer member 15 becomes a first current based on the first detection result, and sets the transfer bias based on the first voltage, and also determines a second voltage at which the current flowing through the electrode member 8 becomes a second current based on the second detection result, and controls the setting operation so that the electrode bias is set based on the second voltage.

[0115] As described above, according to this embodiment, it is possible to maintain the effective current I1 to maintain primary transferability, and to maintain the advection current I2 to effectively suppress downstream discharge, thereby improving secondary transferability.

[0116] When the process speed is 250 [mm / sec], for example, the following can be done: I1 and I2 (Ib) in Figures 11 to 14 are replaced from the values for PS=500 [mm / sec] in Figure 8 to values for PS=250 [mm / sec], and Vtr and Vb are determined using the same primary transfer ATVC and potential regulation ATVC.

[0117] Here, when the process speed is 250 mm / sec, Vtr and Vb are each approximately half of those when the process speed is 500 mm / sec. Therefore, for example, the primary transfer ATVC and potential regulation ATVC may be performed only when the process speed is 500 mm / sec. Then, half of the values of Vtr and Vb calculated based on the voltage-current characteristics shown in FIGS. 9 and 10 may be used as Vtr and Vb when the process speed is 250 mm / sec. This shortens the control time compared to when the primary transfer ATVC and potential regulation ATVC are performed at both the 500 mm / sec and 250 mm / sec process speeds to determine Vtr and Vb.

[0118] In this way, in an image forming apparatus 1 capable of forming images at multiple process speeds, the primary transfer ATVC and potential regulation ATVC may be performed at one process speed. The voltage or current for other process speeds can be calculated using the process speed ratio based on the results of the executed primary transfer ATVC and potential regulation ATVC. This eliminates the need to perform the primary transfer ATVC and potential regulation ATVC each time the process speed changes, thereby shortening the time required for the pre-rotation process.

[0119] [Example 2] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those of embodiment 1, and detailed descriptions thereof will be omitted.

[0120] 1. Overview of this Example FIG. 15 is a block diagram showing a schematic configuration of a control system of the image forming apparatus 1 of this embodiment. The configuration of the control system of the image forming apparatus 1 of this embodiment shown in FIG. 15 is generally similar to the configuration of the control system of the image forming apparatus 1 of the first embodiment shown in FIG. 2. However, unlike the first embodiment, the image forming apparatus 1 of this embodiment does not have a current detection sensor 80b of the potential regulating power supply 80. This allows for cost reduction and miniaturization of the image forming apparatus 1 in this embodiment. However, the image forming apparatus 1 of this embodiment cannot detect the current of the potential regulating bias.

[0121] In Example 1, the advection current I2 was added to the effective current I1 to set the target current Ia = I1 + I2, and the primary transfer ATVC was performed to determine the voltage Vtr of the primary transfer bias. Thereafter, while applying the voltage Vtr determined by the primary transfer ATVC to the primary transfer roller 15, the potential regulation ATVC was performed with the advection current I2 (= |Ib|) as the target current, and the voltage Vb of the potential regulation bias was determined. This maintained the advection current I2 (current Ib of the potential regulation bias) constant.

[0122] However, in this embodiment, the image forming apparatus 1 is not provided with the current detection sensor 80b of the potential regulating power supply 80, and therefore the potential regulating ATVC cannot be performed. That is, in this embodiment, the advection current I2 (=|Ib|) cannot be measured.

[0123] Therefore, in this embodiment, the voltage Vb of the potential regulating bias is corrected in accordance with the voltage Vtr of the primary transfer bias determined by the primary transfer ATVC, thereby maintaining the advection current I2 as constant as possible. Specifically, in this embodiment, the voltage Vb of the potential regulating bias is corrected based on the difference (difference) between the voltage Vtr of the primary transfer bias determined by the primary transfer ATVC and a preset reference value. This maintains the effective current I1 as constant as possible to maintain primary transferability, while maintaining the advection current I2 as constant as possible to improve secondary transferability.

[0124] 2. Correction control of potential regulation bias For example, FIG. 16 shows the target value of the active current I1(+), the target value of the potential regulating bias current Ib(-), and the target value of the potential regulating bias voltage Vb(-) that correspond to the environment (moisture content) and process speed (500 mm / sec and 250 mm / sec), which have been set in advance based on experiments or the like. The potential regulating bias current Ib in FIG. 16 is a target value determined in advance based on experiments or the like, and is an item that cannot be measured in this embodiment. However, this potential regulating bias current Ib is used to set the target current Ia in the primary transfer ATVC, as in the first embodiment. The information shown in FIG. 16 is set in advance and stored in the ROM 32 as a table.

[0125] The correction control of the potential regulating bias in this embodiment will be described using FIG. 17. FIG. 17 is a timing chart showing the transition of the voltage and current of the primary transfer bias and the potential regulating bias during job execution in this embodiment. However, in this embodiment, the image forming apparatus 1 is not provided with a current detection sensor 80b of the potential regulating power supply 80, so the current Ib of the potential regulating bias is a reference value. The operation according to the timing chart in FIG. 17 is controlled by the control unit 3.

[0126] When a job is started, the intermediate transfer belt 6 begins to rotate, and the pre-rotation process begins (T1). Then, the potential regulating power supply 80 begins to apply a potential regulating bias to the potential regulating member 8 under constant voltage control (T2). In this embodiment, the voltage Vb1 of the potential regulating bias at this time is determined based on a preset table such as that shown in FIG. 16. For example, if the moisture content is 8.90 g / kg (= 23°C 50%) and the process speed is 500 mm / sec, Vb1 is set to -3400 V. At this time, the voltage Vtr of the primary transfer bias applied to the primary transfer roller 15 is 0 V, but the voltage Vb1 of the potential regulating bias applied to the potential regulating member 8 is -3400 V. Therefore, due to the potential difference between the primary transfer roller 15 and the potential regulating member 8, Ib1 is approximately -6.0 μA (reference value).

[0127] Thereafter, the primary transfer ATVC is initiated, and a primary transfer test bias is applied from the primary transfer power supply 75 to the primary transfer roller 15 under constant current control, aiming for a target current Ia (T3). Here, for example, based on the table of FIG. 16, the target current Ia = active current I1 + advection current I2 (= |Ib|) = 50 + 10 = 60 μA. Then, the primary transfer bias voltage Vtr (e.g., Vtr = 1700 V) is determined. At T3, the determined voltage Vtr is applied to the primary transfer roller 15, further increasing the potential difference between the potential regulating member 8 and the transfer roller 15 to, for example, Ib2 = approximately −9.0 μA (reference value). The primary transfer bias voltage Vtr determined here affects fluctuations in the current Ib flowing through the potential regulating member 8 due to fluctuations in the electrical resistance of the intermediate transfer belt 6, etc. This may result in a discrepancy between the target current Ia and the actual active current I1. Therefore, in this embodiment, the primary transfer ATVC is executed with a target current Ia=I1+I2, which is the sum of the effective current I1 and the advection current I2 (=|current Ib of the potential regulation bias|), to determine the primary transfer bias voltage Vtr. This allows the target current Ia, effective current I1, and advection current I2 to be set to stable values with little variation, thereby maintaining stable primary transfer performance.

[0128] Next, the potential regulating bias Vb is corrected based on the voltage Vtr of the primary transfer bias determined by the primary transfer ATVC (T4). The correction of the potential regulating bias Vb is performed based on information such as that shown in FIG. 18. FIG. 18 shows the reference values of the voltage Vtr of the primary transfer bias and the voltage Vb of the potential regulating bias according to the environment (moisture content) and the process speed (500 mm / sec and 250 mm / sec). The reference value of the voltage Vtr of the primary transfer bias is preset as the voltage Vtr of the primary transfer bias required to generate the target value of the effective current I1 according to the environment and the process speed in a reference configuration (such as a new intermediate transfer belt 6). In this embodiment, the reference value of the voltage Vb of the potential regulating bias is the same as the target value of the voltage Vr of the potential regulating bias, which is preset (FIG. 16). FIG. 18 also shows the correction value (correction formula) of the voltage Vb of the potential regulating bias according to the process speed. 18 is set in advance and stored in the ROM 32 as a table or the like. Here, the reference value of the voltage Vr of the primary transfer bias is also referred to as the "Vtr reference value," and the actual value of the voltage Vtr of the primary transfer bias set by the primary transfer ATVC is also referred to as the "Vtr actual measurement value." Also, the difference between the Vtr actual measurement value and the Vtr reference value (Vtr actual measurement value - Vtr reference value) is also referred to as the "actual measurement value difference ΔVtr."

[0129] For example, consider the case where the Vtr actual measurement value after the primary transfer ATVC is 1700 [V] as described above. In this case, the Vtr reference value is 1600 [V], so the actual measurement difference ΔVtr = Vtr actual measurement value - Vtr reference value = 1700 - 1600 = 100 [V]. Furthermore, the correction value = reference value - ΔVtr = -3400 - 100 = -3500 [V], so the final voltage of the potential regulating bias Vb2 is -3500 [V]. Furthermore, at this time, Ib3 = approximately -9.5 [μA] (reference value).

[0130] In this way, even if the potential regulating power supply 80 does not have a current detection sensor 80b, the voltage Vb of the potential regulating bias can be corrected based on the voltage Vtr of the primary transfer bias, so that the current of the potential regulating bias can be brought closer to the target current Ib = -10 [μA].

[0131] Here, ΔVtr in the correction value (correction formula) shown in Fig. 18 is ΔVtr when PS = 500 [mm / sec] and ΔVtr when PS = 250 [mm / sec]. For example, if ATVC when the process speed is 250 [mm / sec] is not executed and calculation is performed from the result of ATVC when the process speed is 500 [mm / sec], the following calculation can be used as a substitute. ΔVtr when PS=250[mm / sec] = (ΔVtr when PS=500[mm / sec]) / 2

[0132] As another example, let's consider the case where the Vtr actual measurement value after the primary transfer ATVC is 1500 [V]. In this case, the Vtr reference value is 1600 [V], so the actual measurement difference ΔVtr = Vtr actual measurement value - Vtr reference value = 1500 - 1600 = -100 [V]. Furthermore, the correction value = reference value - ΔVtr = -3400 - (-100) = -3300 [V], so the final potential regulating bias voltage Vb2 is -3300 [V]. Furthermore, the reference values at this time are Ib1 = approximately -7.0 [μA], Ib2 = -11.0 [μA], and Ib3 = -10.5 [μA].

[0133] In this way, even if the potential regulating power supply 80 does not have a current detection sensor 80b, the voltage Vb of the potential regulating bias can be corrected based on the voltage Vtr of the primary transfer bias, so that the current of the potential regulating bias can be brought closer to the target current Ib = -10 [μA].

[0134] Here, FIG. 17 shows a case where the voltage and current increase with respect to the correction from Vb1 to Vb2 and from Ib2 to Ib3, but there are also cases where the voltage and current decrease, as in the other examples described above.

[0135] The actual measurement value of the voltage Vtr of the primary transfer bias after ATVC (Vtr actual measurement value) becomes larger than the reference value (Vtr reference value) when the electrical resistance of the primary transfer roller 15 or the intermediate transfer belt 6 is higher than the reference value. For example, when the intermediate transfer belt 6 has a high resistance, the advection current I2 does not flow easily, and the current flowing through the potential regulating member 8 is small (e.g., Ib2 = -9.0 [μA]). Therefore, by correcting the voltage Vb of the potential regulating bias in accordance with the actual measurement difference ΔVtr, the advection current I2 (= |Ib|) flowing through the potential regulating member 8 can be made closer to the target value (e.g., Ib = -10 [μA]) (e.g., Ib3 = -9.5 [μA]). In other words, even without the current detection sensor 80b of the potential regulating power supply 80, the advection current I2 (= |Ib|) can be made closer to the target value.

[0136] Conversely, the actual measurement value of the voltage Vtr of the primary transfer bias after ATVC (Vtr actual measurement value) becomes smaller than the reference value (Vtr reference value) when the electrical resistance of the primary transfer roller 15 or the intermediate transfer belt 6 is lower than the reference value. For example, when the intermediate transfer belt 6 has a low resistance, the advection current I2 flows easily, and the current flowing through the potential regulating member 8 becomes larger (e.g., Ib2 = -11.0 μA). Therefore, by correcting the voltage Vb of the potential regulating bias in accordance with the actual measurement difference ΔVtr, the advection current I2 (= |Ib|) flowing through the potential regulating member 8 can be made closer to the target value (e.g., Ib = -10 μA) (e.g., Ib3 = -10.5 μA). In other words, even without the current detection sensor 80b of the potential regulating power supply 80, the advection current I2 (= |Ib|) can be made closer to the target value.

[0137] 17, during image formation, constant voltage control is performed with the primary transfer bias voltage Vtr=1700[V] and the corrected potential regulating bias voltage Vb2=-3500[V] (T5 to T6). Also, in the other example described above, during image formation, constant voltage control is performed with the primary transfer bias voltage Vtr=1500[V] and the corrected potential regulating bias voltage Vb2=-3300[V] (T5 to T6).

[0138] In this way, in this embodiment, the voltage Vb of the potential regulating bias is corrected according to the voltage Vtr of the primary transfer bias, thereby making it possible to bring the advection current I2 (=|Ib|) closer to the target value even without the current detection sensor 80b of the potential regulating power supply 80.

[0139] Next, the procedure for correcting and controlling the potential regulating bias in this embodiment will be described with reference to Fig. 19. Fig. 19 is a flow chart showing an outline of the job procedure in this embodiment.

[0140] When a job is started (S1), the control unit 3 determines the target current Ia of the primary transfer bias and the target voltage Vb of the potential regulating bias in advance based on the environment (temperature, humidity), process speed, etc., during the pre-rotation process (S2). The target current Ia of the primary transfer bias and the target voltage Vb of the potential regulating bias are determined based on a predetermined table, such as that shown in FIG. 16. For example, if the moisture content is 8.90 [g / kg] (=23°C·50%) and the process speed is 500 [mm / sec], the effective current I1 is 50 [μA], the potential regulating bias current Ib is −10 [μA], and the advection current I2 is 10 [μA]. Therefore, the target current Ia = I1 + I2 = 60 [μA] is determined. Furthermore, the voltage Vb of the potential regulating bias is determined to be −3400 [V] based on the table shown in FIG. 16.

[0141] Next, the control unit 3 starts applying a constant voltage of Vb1=−3400 [V] from the potential regulating power supply 80 to the potential regulating member 8 (S3). At this time, Ib1=approximately −6.0 [μA] (reference value).

[0142] Then, the control unit 3 applies a primary transfer test bias from the primary transfer power supply 75 and executes primary transfer ATVC (constant current control) at a target current Ia=60 μA (S4). As mentioned above, the primary transfer test bias may have one level or three levels. The control unit 3 then determines the primary transfer bias voltage Vtr (for example, Vtr=1700 V) at which the current flowing through the primary transfer roller 15 becomes the target current Ia=60 μA (S5). At this time, Ib2=-9.0 μA (reference value).

[0143] Next, the control unit 3 corrects the voltage Vb of the potential regulating bias based on the voltage Vtr of the primary transfer bias determined by the primary transfer ATVC (S6). The potential regulating bias Vb is corrected based on information such as that shown in FIG. 18. For example, if the measured Vtr value after ATVC is 1700 [V], the Vtr reference value is 1600 [V], and therefore, as described above, the corrected value Vb2 of the potential regulating bias Vb is −3500 [V]. In this case, Ib3 is approximately −9.5 [μA] (reference value). By correcting the voltage Vb of the potential regulating bias, the current Ib of the potential regulating bias approaches the target current Ib = −10 [μA].

[0144] Thereafter, the control unit 3 completes the pre-rotation process, and performs constant voltage control of the primary transfer bias at voltage Vtr=1700 [V] and the potential regulating bias at voltage Vb2=-3500 [V] to perform image formation (S7). When image formation is completed, the control unit 3 completes the job (S8).

[0145] As in the other examples described above, even when the measured Vtr value after ATVC is 1500 [V], the voltage Vtr of the primary transfer bias set as described above and the voltage Vb of the potential control bias after correction are different, but the procedure is the same.

[0146] Similarly, when the process speed is 250 mm / sec, the primary transfer bias voltage Vtr, correction value (correction formula), and corrected potential regulating bias voltage Vb are set differently, but the procedure is the same. When the process speed is 250 mm / sec, based on the table in FIG. 16, the effective current I1 is 25 μA and the potential regulating bias current Ib is −5 μA. Therefore, the advection current I2 is 5 μA, and the target current Ia is set to I1 + I2 = 30 μA (S2). The potential regulating bias voltage Vb is set to −1700 V (S2). The potential regulating power supply 80 then starts applying a constant voltage Vb1 of −1700 V to the potential regulating member 8 (S3). At this time, Ib1 is approximately −3.0 μA (reference value). A primary transfer test bias is applied from the primary transfer power supply 75 to perform the primary transfer ATVC (S4), and the primary transfer bias voltage Vtr (e.g., 850 [V]) is determined (S5). At this time, Ib2=-4.5 [μA] (reference value). Based on the primary transfer bias voltage Vtr determined by the primary transfer ATVC, the potential regulating bias Vb is corrected (S6). For example, if the measured Vtr value after ATVC is 850 [V], the Vtr reference value is 800 [V], so the actual measurement difference ΔVtr = measured Vtr value - reference Vtr value = 850 - 800 = 50 [V]. Furthermore, the correction value = reference value - ΔVtr = -1700 - 50 = -1750 [V]. Finally, the potential regulating bias voltage Vb2 = -1750 [V]. Because the voltage Vb2 of the potential regulating bias increases by 50 V, the current Ib3 of the potential regulating bias also increases slightly, to approximately -4.7 μA (reference value). By correcting the voltage Vb of the potential regulating bias, the current Ib of the potential regulating bias approaches the target current Ib = -5.0 μA.

[0147] 3.Effects As described above, in this embodiment, the control unit 3 controls the first application unit 75 to apply a first test bias to the primary transfer member 15, obtains a first detection result from the detection units 75a and 75b, calculates a first voltage based on the first detection result so that the current flowing through the primary transfer member 15 becomes a first current, and sets the electrode bias to be applied by the second application unit 80 to the electrode member 8 during image formation based on the first voltage. In this embodiment, the control unit 3 controls the setting operation so that the absolute value of the electrode bias voltage when the absolute value of the first voltage is a second value greater than the first value is greater than the absolute value of the electrode bias voltage when the absolute value of the first voltage is a first value. In this embodiment, the control unit 3 also controls the setting operation so that the transfer bias to be applied by the first application unit 75 to the primary transfer member 15 during image formation is set based on the first voltage. In addition, in this embodiment, the control unit 3 controls the first current to be set based on the sum of a predetermined target value of the effective current that flows between the primary transfer member 15 and the photosensitive member 11 when the first application unit 75 applies the transfer bias to the primary transfer member 15 during image formation, and a predetermined target value of the advection current that flows between the primary transfer member 15 and the electrode member 8 when the first application unit 75 applies the transfer bias to the primary transfer member 15 during image formation. In addition, in the above-mentioned setting operation, the control unit 3 can control the second application unit 80 to apply a bias voltage of the electrode bias set based on the first voltage to the electrode member 8, while the first application unit 75 applies a second test bias to the primary transfer member 15 to obtain a second detection result by the detection units 75a and 75b, and based on the second detection result, determine a second voltage at which the current flowing through the primary transfer member 15 becomes the first current, and set the transfer bias to be applied to the primary transfer member 15 by the first application unit 75 during image formation based on the second voltage.

[0148] As described above, in this embodiment, similar to Embodiment 1, it is possible to maintain the effective current I1 to maintain the primary transfer performance, and to maintain the advection current I2 to effectively suppress downstream discharge, thereby improving the secondary transfer performance. Furthermore, in this embodiment, the current detection sensor 80b of the potential regulating power supply 80 is not provided, thereby reducing the cost and size of the image forming apparatus 1.

[0149] Furthermore, when the target value of the constant current control is small, for example, approximately 5 μA or less in absolute value, the ATVC (constant current control) may become unstable (the variation may become large). Therefore, correcting the voltage Vb of the potential regulating bias based on the Vtr of the primary transfer bias as in this embodiment, rather than performing the potential regulating ATVC, may stably set the effective current I1 and the advection current I2 to the target values. Therefore, depending on the configuration of the image forming apparatus 1, it is possible to arbitrarily select either the control of the first embodiment or the control (correction control) of this embodiment. For example, when the process speed is 250 mm / sec, the current Ib of the potential regulating bias may be -5 μA (FIG. 8). When the potential regulating ATVC is performed with the target current Ib = -5 μA, the voltage Vb of the potential regulating bias may become unstable (the variation may become large). Therefore, in this case, it is preferable to perform the correction control of this embodiment rather than the control of the first embodiment. Therefore, even when the current detection sensor 80b of the potential regulating power supply 80 is provided, it may be preferable to perform the correction control of this embodiment. For example, when the process speed is 500 [mm / sec], the current Ib of the potential regulating bias is -10 [μA], so the voltage Vb is determined by the ATVC of embodiment 1. On the other hand, when the process speed is 250 [mm / sec], the current Ib of the potential regulating bias is -5 [μA], so the correction control of this embodiment is performed.

[0150] Furthermore, after determining the voltage Vb of the potential regulating bias through the correction control of this embodiment, the primary transfer ATVC may be additionally executed as described in the first embodiment. This improves the accuracy of the voltage Vt of the primary transfer bias. In this case, whether or not to execute the additional primary transfer ATVC may be determined based on, for example, the difference (correction amount) between the correction value of the voltage Vb of the potential regulating bias and the reference value, or the difference (measured value difference ΔVtr) between the actual Vt value and the reference Vtr value. For example, if the difference exceeds a predetermined threshold, the additional primary transfer ATVC may be executed, but if it does not exceed the threshold, the additional primary transfer ATVC may not be executed.

[0151] [Example 3] Next, a further embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those of embodiment 1, and detailed descriptions thereof will be omitted.

[0152] 1. Overview of this Example In the second embodiment, the voltage Vb of the potential regulating bias is corrected based on the voltage Vtr of the primary transfer bias determined by the primary transfer ATVC. In particular, in the second embodiment, a method of correcting the voltage Vb of the potential regulating bias in accordance with the environment (temperature, humidity) and process speed in addition to the voltage Vtr of the primary transfer bias is described (FIG. 18).

[0153] In this embodiment, a method for correcting the potential regulating bias will be described, which refers to the voltage Vtr of the primary transfer bias as well as one or more of the environment (temperature, humidity), process speed, number of printed sheets (parts counter), basis weight of the recording material S, and type of recording material S. The type of recording material S includes any information that can distinguish the recording material S, such as attributes based on general characteristics such as plain paper, coated paper, thick paper, and synthetic paper (so-called paper type category), numerical values and numerical ranges for basis weight and thickness, and brand (including manufacturer, product number, etc.).

[0154] The electrical resistance of the primary transfer roller 15 and the intermediate transfer belt 6 tends to increase with an increase in cumulative usage. The primary transfer bias voltage Vtr is set to a value that causes the target current to flow through the primary transfer ATVC. Therefore, even if the electrical resistance of the primary transfer roller 15 and the intermediate transfer belt 6 increases with an increase in cumulative usage, the appropriate current and voltage are applied.

[0155] On the other hand, for example, as in the second embodiment, in a configuration that does not have the current detection sensor 80b of the potential regulating power supply 80, the potential regulating ATVC cannot be performed. For example, since the advection current Ia (=|Ib|) fluctuates due to fluctuations in the electrical resistance value of the intermediate transfer belt 6, it is necessary to correct the voltage of the potential regulating bias according to the usage status (accumulated usage amount). The voltage Vtr of the primary transfer bias can be used to estimate the approximate electrical resistance values of the primary transfer roller 15 and the intermediate transfer belt 6. However, this is the combined electrical resistance value of the two components, and it is not possible to estimate the electrical resistance values of each individual component.

[0156] Fig. 20 is a block diagram showing a schematic configuration of a control system of the image forming apparatus 1 of this embodiment. The configuration of the control system of the image forming apparatus 1 of this embodiment shown in Fig. 20 is generally similar to the configuration of the control system of the image forming apparatus 1 of the first embodiment shown in Fig. 2. However, in this embodiment, like the second embodiment (Fig. 15), the image forming apparatus 1 does not have a current detection sensor 80b of the potential regulating power supply 80.

[0157] In this embodiment, the image forming apparatus 1 also includes a parts counter 90 as a usage amount acquisition unit that counts (calculates) an index value that correlates with the cumulative usage amount of the image forming apparatus 1 or its components. The parts counter 90 is configured to include a storage unit (non-volatile memory) that sequentially counts (accumulates) and stores the index value. The control unit 3 is configured to read the count value of the parts counter 90 as needed and use it for control. As will be described later, in this embodiment, the parts counter 90 stores the number of printed sheets as an index value that correlates with the cumulative usage amount of the intermediate transfer belt 6 since it was new. Note that the index value that correlates with the cumulative usage amount is not limited to the number of printed sheets, and any value that correlates with the cumulative usage amount, such as the number of rotations of the intermediate transfer belt 6, the rotation time, or the image formation time, can be used.

[0158] In this embodiment, the operation unit 70 functions as an acquisition unit that acquires recording material information related to the recording material S. The recording material information may be input to the image forming apparatus 1 from an external device such as a personal computer, and in that case, an input / output unit of the control unit 3 or the like functions as an acquisition unit that acquires the recording material information.

[0159] 2. Correction control of potential regulation bias FIG. 21 shows the additional correction value of the voltage Vb of the potential regulating bias depending on the number of printed sheets (accumulated usage of the intermediate transfer belt 6).

[0160] The additional correction values in Fig. 21 are determined in advance based on experiments or the like, depending on the number of prints and the process speed. Fig. 21 particularly shows the additional correction values (additional voltages) depending on the number of prints and the process speed of the intermediate transfer belt 6. The information shown in Fig. 21 is set in advance and stored in the ROM 32 as a table.

[0161] Here, when a part (in this embodiment, the intermediate transfer belt 6) is replaced, the count value of the parts counter 90 is reset to its initial value (zero in this embodiment) and starts again from zero. In this embodiment, as described above, the parts counter 90 counts the number of pages printed since the intermediate transfer belt 6 was new. For example, one printed page corresponds to one landscape A4 page. Therefore, one double-sided A4 page and one single-sided A3 page are counted as two printed pages. The lifespan of the primary transfer roller 15 and the intermediate transfer belt 6 is, for example, approximately 2 million pages. The additional correction value in FIG. 21 is set so that +100 to +600 V is added to the reference value of the voltage Vb of the potential regulating bias (see FIGS. 16 and 18) every 500,000 pages as the increase in the electrical resistance of the intermediate transfer belt 6.

[0162] As described above, in this embodiment, even if the cumulative usage amount (number of printed sheets) of the intermediate transfer belt 6 increases and the electrical resistance value of the intermediate transfer belt 6 increases, the voltage Vb of the potential regulating bias can be corrected according to the number of printed sheets. This makes it possible to maintain the target value of the current Ib of the potential regulating bias, for example, as shown in FIG. 16. As a result, downstream discharge is suppressed, and an increase in the charge amount of the toner after primary transfer is suppressed, thereby maintaining good secondary transfer performance. In other words, good secondary transfer performance can be maintained for a long period of time, until the end of the life of the intermediate transfer belt 6.

[0163] Specifically, for example, in the control of the second embodiment, when the measured Vtr value after the primary transfer ATVC is 1700 [V], the correction value of the voltage Vb of the potential regulating bias is −3500 [V] based on the information in FIG. 18. In this embodiment, when the number of prints on the intermediate transfer belt 6 is, for example, 1,000 to 1,500,000 sheets, the voltage Vb of the potential regulating bias is set to −3500 + added voltage = −3500 − 400 = −3900 [V] based on the information in FIG. 21. This allows the current Ib of the potential regulating bias to approach the target value of −10 [μA].

[0164] 22 shows the basis weight of the recording material S as information relating to the recording material S, and additional correction values for the voltage Vb of the potential regulating bias depending on the type of the recording material S.

[0165] Basis weight of recording material S [g / m 2 As the thickness (≒ thickness) increases, the electrical resistance of the recording material S increases, and therefore the secondary transfer voltage must increase to maintain a predetermined secondary transfer current. For example, to reduce the cost and size of the high-voltage circuit for secondary transfer, it is necessary to reduce the maximum value of the secondary transfer voltage. Therefore, it is necessary to reduce the secondary transfer voltage for paper with a large basis weight (≒ thick paper) of the recording material S. Therefore, for thick paper (paper with a large basis weight) with a large electrical resistance, it is desirable to minimize the increase in the charge amount of the toner after primary transfer and reduce the secondary transfer voltage as much as possible.

[0166] The additional correction value in FIG. 22 is determined in advance based on experiments or the like in accordance with the basis weight of plain paper as the recording material S. In addition to the additional voltage, which is the additional correction value for the potential regulating bias, FIG. 22 also shows the additional current. The reason for adding the additional voltage and current to the voltage and current of the potential regulating bias is that the greater the basis weight of the recording material S, the greater the voltage Vb and current Ib of the potential regulating bias are added to increase the advection current I2. This further reduces downstream discharge and further reduces the amount of charge on the toner on the intermediate transfer belt 6 after primary transfer (before secondary transfer), thereby reducing the secondary transfer voltage. Note that in this embodiment, the current Ib of the potential regulating bias cannot be measured. Therefore, the additional current shown in FIG. 22 is a target value of the additional current that needs to be added to the reference value (see FIG. 16) of the potential regulating bias current in accordance with the basis weight (or type, described below) of the recording material S, determined in advance based on experiments or the like. However, this additional current is used to set the target current Ia in the primary transfer ATVC, as described below. The information shown in FIG. 22 is preset and stored in the ROM 32 as a table.

[0167] 22 does not show the additional correction value when the process speed is 250 mm / sec. This is because when the process speed is slow, the secondary transfer voltage is small and does not affect the maximum value of the secondary transfer voltage. For example, when the process speed is half, the secondary transfer voltage is approximately half.

[0168] Furthermore, increasing the absolute value of the voltage Vb of the potential regulating bias increases the advection current I2 and decreases the effective current I1, potentially impairing primary transfer performance. Therefore, the additional correction value in FIG. 22 is set to increase the target current Ia (= I1 + I2) when the basis weight increases by adding +3 to +6 μA to the reference value of the potential regulating bias current Ib. In other words, the target current Ia in the primary transfer ATVC is set based on the potential regulating bias current Ib (i.e., −Ib = I2) obtained by adding the additional correction value in FIG. 22 to the reference value in FIG. 16 so as to correspond to the increase in the advection current I2 (= |Ib|). This keeps the effective current I1 constant even when the advection current I2 increases, preventing primary transfer defects.

[0169] Furthermore, in this embodiment, since the target current Ia increases, the primary transfer voltage applied to the primary transfer roller 15 increases when the basis weight of the recording material S is large (in the case of thick paper). However, in general, the primary transfer voltage is smaller than the secondary transfer voltage because the recording material S is not interposed at the primary transfer portion N1. Therefore, there is a margin for the maximum value of the primary transfer voltage. If there is no margin for the maximum value of the primary transfer voltage, when the basis weight of the recording material S is large, for example, 201 to 300 [g / m 2 In the case of

[0045] , for example, the secondary transfer voltage may be reduced by lowering the process speed from 500 [mm / sec] to 250 [mm / sec].

[0170] In this way, when the basis weight of the recording material S is large (thick paper), the additional correction value in Figure 22 increases the voltage Vb of the potential regulating bias and increases the advection current I2, suppressing the increase in the toner charge amount after primary transfer and reducing the secondary transfer voltage. This allows for cost reduction and miniaturization of the high-voltage circuit for secondary transfer. Furthermore, even if the advection current I2 increases, the effective current I1 can be kept constant by increasing the target current Ia, so primary transfer performance is not impaired.

[0171] The additional correction values in Figure 22 also include the additional voltage and current when a different type (paper type category) of recording material S, such as a "high-resistivity recording material," is used as the recording material S, for example, a white resin sheet such as "Yupo (trademark, synthetic paper)." Because this type of recording material S has extremely high resistance, the secondary transfer voltage is large. Therefore, just as with a recording material S with a high basis weight, it is necessary to add and increase the potential regulation bias voltage Vb and the advection current I2 (= |Ib|). This reduces the secondary transfer voltage, enabling the secondary transfer high-voltage circuit to be reduced in cost and size.

[0172] Next, using Figs. 17 and 23, the basis weight of the recording material S is 201 to 300 [g / m 2 ] or in the case of high-resistivity recording material S, the procedure for correcting and controlling the potential regulating bias will be described. FIG. 17 is a timing chart that was also used in the description of embodiment 2 and shows the transition of the voltage and current of the primary transfer bias and the potential regulating bias during job execution in this embodiment. As mentioned above, since this embodiment does not have the current detection sensor 80b, the current Ib of the potential regulating bias in FIG. 17 is a reference value. Also, FIG. 23 is a flowchart that shows an outline of the job procedure in this embodiment.

[0173] When the control unit 3 starts a job (T1 in FIG. 17, S1 in FIG. 23), it acquires information about the recording material S (S2 in FIG. 23). The information about the recording material S includes information about the basis weight of the recording material S to be placed in the cassette 41, which has been set in advance by the user, and information about the type of recording material S. The control unit 3 acquires the information about the recording material S from the above-mentioned preset information, based on information for selecting the recording material S to be used for printing (which may be information for selecting the cassette 41), which has been input by the user from the operation unit 70 as a job setting, for example. Here, the recording material S is plain paper with a basis weight of 201 to 300 [g / m 2 ] or the case of a high-resistivity recording material S. Next, in the pre-rotation process, the control unit 3 determines the preset target current Ia of the primary transfer bias and the target voltage Vb of the potential regulating bias based on the environment (temperature, humidity), process speed, etc. (S3 in FIG. 23). The target current Ia of the primary transfer bias and the target voltage Vb of the potential regulating bias are determined based on preset tables such as those shown in FIGS. 16, 18, and 22. For example, if the moisture content is 8.90 g / kg (=23°C 50%) and the process speed is 500 mm / sec, then based on the table in FIG. 16, the effective current I1 is 50 μA, the potential regulating bias current Ib is −10 μA, and the advection current I2 is 10 μA. Furthermore, based on the table in FIG. 22, the potential regulating bias's added current Ib is −6 μA, so the added advection current I2 is 6 μA. Therefore, the target current Ia = active current I1 + advection current I2 + added advection current I2 = 50 + 10 + 6 = 66 [μA] is determined. Also, from Figures 18 and 22, the potential regulation bias voltage Vb = reference value + added voltage Vb = -3400 - 600 = -4000 [V] is determined.

[0174] Next, the control unit 3 starts applying a constant voltage control of Vb1 = -4000 [V] from the potential regulating power supply 80 to the potential regulating member 8 (T2 in FIG. 17, S4 in FIG. 23). At this time, the voltage Vtr of the primary transfer bias applied to the primary transfer roller 15 is 0 [V], but the voltage Vb1 of the potential regulating bias applied to the potential regulating member 8 is -4000 [V]. Therefore, due to the potential difference between the primary transfer roller 15 and the potential regulating member 8, Ib1 = approximately -10.0 [μA] (reference value).

[0175] Then, the control unit 3 applies a primary transfer test bias from the primary transfer power supply 75 and executes primary transfer ATVC (constant current control) at a target current Ia=66 [μA] (T3 in FIG. 17, S5 in FIG. 23). As mentioned above, the primary transfer test bias may have one level or three levels. Then, the control unit 3 determines the primary transfer bias voltage Vtr (for example, Vtr=1900 [V]) so that the current flowing through the primary transfer roller 15 becomes the target current Ia=66 [μA] (S6). At this time, Ib2=-14.0 [μA] (reference value).

[0176] Next, the control unit 3 corrects the voltage Vb of the potential regulating bias based on the voltage Vtr of the primary transfer bias determined by the primary transfer ATVC (T4 in FIG. 17, S7 in FIG. 23). Here, the reference value of the voltage Vtr of the primary transfer bias is 1600 [V] when the target current Ia is 60 [μA] (FIGS. 8 and 18). Therefore, when the target current Ia in this embodiment is 66 [μA] (10 [%] increase from 60 [μA]), the reference value of the voltage Vt of the primary transfer bias is 1760 [V] (reference voltage = 10 [%] increase from 1600 [V]). Therefore, the actual measurement value difference ΔVtr = Vtr actual measurement value - Vtr reference value = 1900 - 1760 = 140 [V]. Based on the information in FIG. 18, the correction value = reference value - ΔVtr = -4000 - 140 = -4140 [V], so the final correction value of the potential regulating bias is Vb2 = -4140 [V]. At this time, the current Ib3 of the potential regulating bias also increases slightly, becoming Ib3 = -15.5 [μA] (reference value). In this way, by correcting the voltage Vb of the potential regulating bias based on the voltage Vtr of the primary transfer bias, the current Ib of the potential regulating bias approaches the target current Ib = -16 [μA].

[0177] Thereafter, the control unit 3 ends the pre-rotation process, and performs constant voltage control with the primary transfer bias voltage Vtr=1900 [V] and the potential regulating bias voltage Vb2=-4140 [V] to perform image formation (T5 to T6 in FIG. 17, S8 in FIG. 23). When image formation is completed, the control unit 3 ends the job (T6 in FIG. 17, S9 in FIG. 23).

[0178] 3.Effects As described above, in this embodiment, as in the second embodiment, the control unit 3 controls the first application unit 75 to apply a first test bias to the primary transfer member 15 in the setting operation, obtains a first detection result from the detection units 75a and 75b, calculates a first voltage based on the first detection result so that the current flowing through the primary transfer member 15 becomes a first current, and sets an electrode bias to be applied to the electrode member 8 by the second application unit 80 during image formation based on the first voltage.

[0179] Here, the image forming apparatus 1 may have an environment detection unit that detects environmental information correlated with humidity, and the control unit 3 can control the setting operation to set the electrode bias based on the first voltage and the environmental information. In this case, typically, the control unit 3 controls the absolute value of the first voltage when the humidity indicated by the environmental information is a second humidity lower than the first humidity to be greater than the absolute value of the first voltage when the humidity indicated by the environmental information is the first humidity, and controls the absolute value of the electrode bias voltage when the humidity is the second humidity to be greater than the absolute value of the electrode bias voltage when the humidity is the first humidity.

[0180] Furthermore, in the setting operation, the control unit 3 can control the setting of the electrode bias based on the first voltage and information related to the rotation speed of the intermediate transfer belt 6 during image formation. In this case, typically, the control unit 3 controls the setting so that the absolute value of the first voltage when the rotation speed is a second speed faster than the first speed is larger than the absolute value of the first voltage when the rotation speed is the first speed, and also controls the setting so that the absolute value of the electrode bias voltage when the rotation speed is the second speed is larger than the absolute value of the electrode bias voltage when the rotation speed is the first speed.

[0181] Furthermore, the image forming apparatus 1 may have an acquisition unit (such as the operation unit 70) that acquires recording material information related to the recording material S, and the control unit 3 can control the setting operation to set the electrode bias based on the first voltage and the recording material information. In this case, typically, the control unit 3 controls the absolute value of the first voltage when the basis weight of the recording material S indicated by the recording material information is a second basis weight that is larger than the first basis weight, so that the absolute value of the first voltage is larger than the absolute value of the first voltage when the basis weight of the recording material S indicated by the recording material information is the first basis weight, and controls the absolute value of the electrode bias voltage when the second basis weight is the first basis weight. Furthermore, the control unit 3 controls the absolute value of the first voltage so that it is greater when the type of recording material S indicated by the recording material information is a second type having a higher electrical resistance value at the secondary transfer portion N2 than the first type, than when the type of recording material S indicated by the recording material information is a first type, and can also control the absolute value of the electrode bias voltage so that it is greater when the type is the second type than when the type is the first type.

[0182] Furthermore, the control unit 3 may include a usage amount acquisition unit (parts counter) 90 that acquires an index value correlated with the cumulative usage amount of the intermediate transfer belt 6, and the control unit 3 can control the setting operation to set the electrode bias based on the first voltage and the index value. In this case, typically, the control unit 3 controls the absolute value of the first voltage when the usage amount indicated by the index value is a second usage amount that is greater than the first usage amount to be greater than the absolute value of the first voltage when the usage amount indicated by the index value is the first usage amount, and controls the absolute value of the electrode bias voltage when the second usage amount is greater than the absolute value of the electrode bias voltage when the first usage amount is.

[0183] In this embodiment, the current Ib and voltage Vb of the potential regulating bias are corrected according to the basis weight and type of recording material S. This makes it possible to stably maintain the effective current I1 and the advection current I2 at their target values. As a result, even when the basis weight of the recording material S is large or the electrical resistance of the recording material S is high, it is possible to suppress an increase in the amount of charge on the toner after primary transfer and lower the maximum value of the secondary transfer voltage. This makes it possible to reduce the cost and size of the high-voltage circuit for secondary transfer.

[0184] As described in the second embodiment, the correction control of this embodiment can also be performed in the image forming apparatus 1 having the current detection sensor 80b of the potential regulating power supply 80 as in the first embodiment. For example, as described in the second embodiment, it may be preferable to perform the correction control of this embodiment when the target current of the constant current control is equal to or less than a predetermined value.

[0185] Furthermore, after determining the voltage Vb of the potential regulating bias by the correction control of this embodiment, the primary transfer ATVC may be additionally executed as described in embodiment 1. This makes it possible to improve the accuracy of the voltage Vt of the primary transfer bias.

[0186] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

[0187] In the above-described embodiment, the potential regulating member having a flat contact surface that comes into contact with the intermediate transfer belt is a plate-shaped member made of sheet metal or the like, but as long as it can form a similar contact surface, it may have another shape, such as a block-shaped member with a rectangular cross section. The same applies to a potential regulating member having a curved contact surface that comes into contact with the intermediate transfer belt.

[0188] Furthermore, the image forming apparatus is not limited to an image forming apparatus capable of forming a full-color image, but may be an image forming apparatus capable of forming only a monochrome (black and white or monocolor) image.

[0189] In addition, in the above-described embodiment, the predetermined charge polarity of the photoconductor is described as negative, but this is not limited thereto, and the predetermined charge polarity of the photoconductor may be positive. Similarly, in the above-described embodiment, the normal charge polarity of the toner is negative, but the normal charge polarity of the toner may be positive. When the normal charge polarity of the photoconductor or the normal charge polarity of the toner is positive, various applied voltages may be appropriately changed, such as to have the opposite polarity, in accordance with the above-described embodiment.

[0190] In the above embodiment, the potential regulating member is controlled to a constant voltage during image formation, but may be controlled to a constant current. In this case, a current detection sensor 80b for the potential regulating power supply 80 must be provided.

[0191] Furthermore, the photosensitive member is not limited to a drum-shaped member (photosensitive drum), but may be an endless belt-shaped member (photosensitive belt).

[0192] Also, in Figures 8, 16, 18, 21, and 22, the numerical values of the potential regulating bias current Ib and the potential regulating bias voltage Vb are shown as absolute values, and it should be noted that the polarity is negative. [Explanation of symbols]

[0193] 1. Image forming device 3. Control Unit 6 Intermediate transfer belt 8. Potential regulation member (electrode member) 11 Photosensitive drum (photoconductor) 15 Primary transfer roller (primary transfer member) 23 Secondary transfer inner roller 25 Secondary transfer outer roller 80 Potential Regulated Power Supply 80a voltage detection sensor 80b Current detection sensor

Claims

1. a photoreceptor that can be charged to a predetermined polarity and that carries a toner image; an intermediate transfer belt that is capable of rotating and that conveys the toner image that has been primarily transferred from the photosensitive member at the primary transfer unit so that the toner image can be secondarily transferred to a recording material at the secondary transfer unit; a primary transfer member that contacts the inner circumferential surface of the intermediate transfer belt to form the primary transfer portion where the photosensitive member and the intermediate transfer belt are in contact, and to which a bias is applied, transfers a toner image from the photosensitive member to the intermediate transfer belt; a first applying unit that applies a bias having a polarity opposite to the predetermined polarity to the primary transfer member; an electrode member that contacts the inner circumferential surface of the intermediate transfer belt downstream of the primary transfer unit in the moving direction of the intermediate transfer belt; a second applying unit that applies a bias having the same polarity as the predetermined polarity to the electrode member; a detection unit that detects a current flowing through the primary transfer member or a voltage applied to the primary transfer member; a control unit that controls execution of a setting operation to set an electrode bias to be applied to the electrode member by the second application unit during image formation based on a detection result by the detection unit; An image forming apparatus comprising:

2. a second detection unit that detects a current flowing through the electrode member or a voltage applied to the electrode member; 2. The image forming apparatus according to claim 1, wherein the control unit, in the setting operation, controls the first application unit to apply a first test bias to the primary transfer member to obtain a first detection result from the detection unit, determines a first voltage at which the current flowing through the primary transfer member becomes a first current based on the first detection result, controls the second application unit to apply a second test bias to the electrode member while applying the first voltage bias to the primary transfer member to obtain a second detection result from the second detection unit, determines a second voltage at which the current flowing through the electrode member becomes a second current based on the second detection result, and sets the electrode bias based on the second voltage.

3. 3. The image forming apparatus according to claim 2, wherein the control unit controls the setting operation to set a transfer bias to be applied to the primary transfer member by the first application unit during image formation based on the first voltage.

4. The image forming apparatus of claim 3, wherein the control unit controls the first current to be set based on the sum of a predetermined target value of an effective current that flows between the primary transfer member and the photosensitive member when the transfer bias is applied to the primary transfer member by the first application unit during image formation, and a predetermined target value of an advection current that flows between the primary transfer member and the electrode member when the transfer bias is applied to the primary transfer member by the first application unit during image formation.

5. 3. The image forming apparatus according to claim 2, wherein the control unit, in the setting operation, applies a bias of the second voltage to the electrode member using the second application unit while applying a third test bias to the primary transfer member using the first application unit to obtain a third detection result from the detection unit, calculates a third voltage at which the current flowing through the primary transfer member becomes the first current based on the third detection result, and controls the image forming apparatus to set a transfer bias to be applied to the primary transfer member by the first application unit during image formation based on the third voltage.

6. 2. The image forming apparatus according to claim 1, wherein, in the setting operation, the control unit applies a first test bias to the primary transfer member using the first application unit to obtain a first detection result from the detection unit, calculates a first voltage at which the current flowing through the primary transfer member becomes a first current based on the first detection result, and controls the electrode bias to be set based on the first voltage.

7. 7. The image forming apparatus according to claim 6, wherein the control unit controls the setting operation so that the absolute value of the electrode bias voltage when the absolute value of the first voltage is a second value greater than the first value is greater than the absolute value of the electrode bias voltage when the absolute value of the first voltage is a first value.

8. an environment detection unit that detects environmental information correlated with humidity; 7. The image forming apparatus according to claim 6, wherein the control unit controls the setting operation so as to set the electrode bias based on the first voltage and the environmental information.

9. The image forming apparatus of claim 8, wherein the control unit controls the setting operation so that the absolute value of the first voltage when the humidity indicated by the environmental information is a second humidity lower than the first humidity is greater than the absolute value of the first voltage when the humidity indicated by the environmental information is a first humidity, and also controls the absolute value of the electrode bias voltage when the humidity indicated by the environmental information is the second humidity is greater than the absolute value of the electrode bias voltage when the humidity indicated by the environmental information is the first humidity.

10. 7. The image forming apparatus according to claim 6, wherein the control unit controls the setting operation to set the electrode bias based on the first voltage and information regarding the rotation speed of the intermediate transfer belt during image formation.

11. 11. The image forming apparatus according to claim 10, wherein the control unit controls the rotational speed so that the absolute value of the first voltage when the rotational speed is a second speed faster than the first speed is greater than the absolute value of the first voltage when the rotational speed is the first speed, and controls the rotational speed so that the absolute value of the electrode bias voltage when the rotational speed is the second speed is greater than the absolute value of the electrode bias voltage when the rotational speed is the first speed.

12. an acquisition unit that acquires recording material information relating to the recording material; 7. The image forming apparatus according to claim 6, wherein the control section controls the setting operation so as to set the electrode bias based on the first voltage and the recording material information.

13. 13. The image forming apparatus according to claim 12, wherein the control unit controls, in the setting operation, the absolute value of the first voltage when the basis weight of the recording material indicated by the recording material information is a second basis weight that is greater than the absolute value of the first voltage when the basis weight of the recording material indicated by the recording material information is a first basis weight, and controls the absolute value of the electrode bias voltage when the basis weight indicated by the recording material information is the second basis weight to be greater than the absolute value of the voltage when the basis weight indicated by the recording material information is the first basis weight.

14. 13. The image forming apparatus of claim 12, wherein the control unit controls, in the setting operation, the absolute value of the first voltage when the type of recording material indicated by the recording material information is a second type having a higher electrical resistance value at the secondary transfer unit than the first type, so that the absolute value of the first voltage is greater than the absolute value of the first voltage when the type of recording material indicated by the recording material information is a first type, and also controls the absolute value of the electrode bias voltage when the type of recording material indicated by the recording material information is the second type, so that the absolute value of the electrode bias voltage is greater than the absolute value of the electrode bias voltage when the type of recording material indicated by the recording material information is the first type.

15. the control unit includes a usage amount acquisition unit that acquires an index value that correlates with a cumulative usage amount of the intermediate transfer belt, 7. The image forming apparatus according to claim 6, wherein the control unit controls the setting operation so as to set the electrode bias based on the first voltage and the index value.

16. The image forming apparatus described in claim 15, characterized in that, in the setting operation, the control unit controls so that the absolute value of the first voltage when the usage amount indicated by the index value is a second usage amount greater than the first usage amount is greater than the absolute value of the first voltage when the usage amount indicated by the index value is a first usage amount, and controls so that the absolute value of the voltage of the electrode bias when the usage amount indicated by the index value is the second usage amount is greater than the absolute value of the voltage of the electrode bias when the usage amount indicated by the index value is the first usage amount.

17. 17. The image forming apparatus according to claim 6, wherein the control unit controls, in the setting operation, to set a transfer bias to be applied to the primary transfer member by the first application unit during image formation based on the first voltage.

18. The image forming apparatus of claim 17, wherein the control unit controls the first current to be set based on the sum of a predetermined target value of an effective current that flows between the primary transfer member and the photosensitive member when the transfer bias is applied to the primary transfer member by the first application unit during image formation, and a predetermined target value of an advection current that flows between the primary transfer member and the electrode member when the transfer bias is applied to the primary transfer member by the first application unit during image formation.

19. 17. The image forming apparatus of claim 6, wherein the control unit, during the setting operation, controls the second application unit to apply a bias voltage of the electrode bias set based on the first voltage to the electrode member, while the first application unit applies a second test bias to the primary transfer member to obtain a second detection result from the detection unit, and based on the second detection result, determines a second voltage at which the current flowing through the primary transfer member becomes the first current, and based on the second detection result, controls the first application unit to set a transfer bias to be applied to the primary transfer member during image formation.

20. a photoreceptor that can be charged to a predetermined polarity and that carries a toner image; an intermediate transfer belt that is capable of rotating and that conveys the toner image that has been primarily transferred from the photosensitive member at the primary transfer unit so that the toner image can be secondarily transferred to a recording material at the secondary transfer unit; a primary transfer member that contacts the inner circumferential surface of the intermediate transfer belt to form the primary transfer portion where the photosensitive member and the intermediate transfer belt are in contact, and to which a bias is applied, transfers a toner image from the photosensitive member to the intermediate transfer belt; a first applying unit that applies a bias having a polarity opposite to the predetermined polarity to the primary transfer member; an electrode member that contacts the inner circumferential surface of the intermediate transfer belt downstream of the primary transfer unit in the moving direction of the intermediate transfer belt; a second applying unit that applies a bias having the same polarity as the predetermined polarity to the electrode member; a first detection unit that detects a current flowing through the primary transfer member or a voltage applied to the primary transfer member; a second detection unit that detects a current flowing through the electrode member or a voltage applied to the electrode member; a control unit that performs control to execute a setting operation to set a transfer bias to be applied to the primary transfer member by the first application unit during image formation based on a first detection result obtained by the first detection unit when the first application unit applies a first test bias to the primary transfer member, and to set an electrode bias to be applied to the electrode member by the second application unit during image formation based on a second detection result obtained by the second detection unit when the second application unit applies a second test bias to the electrode member; An image forming apparatus comprising:

21. 21. The image forming apparatus according to claim 20, wherein the control unit, in the setting operation, determines a first voltage at which the current flowing through the primary transfer member becomes a first current based on the first detection result, sets the transfer bias based on the first voltage, and determines a second voltage at which the current flowing through the electrode member becomes a second current based on the second detection result, and sets the electrode bias based on the second voltage.

Images