Image formation device

Abstract

To provide an image formation device which has an electrode member for suppressing electric discharge on a downstream side of a primary transfer part, the image formation device capable of suppressing electric discharge on the downstream side of the primary transfer part efficiently according to a use state of a photoreceptor.SOLUTION: An image formation device 1 comprises: a photoreceptor 11 which can be electrified to a prescribed polarity; an electrification unit 12; an intermediate transfer belt 6; a primary transfer member 15; an electrode member 8 contacting an inner peripheral face of the intermediate transfer belt 6 at a position where the downstream side of the primary transfer part N1 in a movement direction of the intermediate transfer belt 6 is adjacent thereto; an electrode bias application part 80 for applying the electrode bias which is equal to the prescribed polarity to the electrode member 8; a control part 3 capable of controlling the electrode bias application part 80; and an acquisition part (electrification travel distance integration part) 78 for acquiring information which correlates with a thickness of the photoreceptor 11. The control part 3 changes the electrode bias on the basis of the information.SELECTED DRAWING: Figure 8

Description

[Technical field]

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

[0002] As image forming apparatuses using the electrophotographic method, such as color copiers, color printers, and color multifunction machines, image forming apparatuses using the intermediate transfer method are mainstream because of the advantages of being compact and being relatively easy to handle various recording materials. In addition, image forming apparatuses using the intermediate transfer method generally have multiple photosensitive drums (drum-shaped photosensitive bodies) and an intermediate transfer belt (an endless belt-shaped intermediate transfer body). In such image forming apparatuses, toner images formed on the multiple photosensitive drums are electrostatically primarily transferred onto the intermediate transfer belt in sequence at a primary transfer section. In addition, the toner images primarily transferred onto the intermediate transfer belt are electrostatically secondarily transferred onto a recording material such as paper at a secondary transfer section.

[0003] In addition, with regard to the arrangement of members around the primary transfer unit, unless otherwise specified, upstream and downstream refer to upstream and downstream in the transport direction of the intermediate transfer belt. Furthermore, for convenience, unless otherwise specified, the magnitude (high and low) of voltage or potential refers to the magnitude (high and low) when compared in absolute value.

[0004] In the image forming apparatus as described above, the toner on the intermediate transfer belt tends to increase in charge amount due to discharge between the intermediate transfer belt and the photosensitive drum downstream of the primary transfer section. The inventors have further studied this and found that the increase in the charge amount of the toner on the intermediate transfer belt increases the adhesion force between the intermediate transfer belt and the toner, making it difficult to transfer the toner to the recording material in the secondary transfer section. For example, the secondary transfer electric field required to transfer the toner to the recording material in the secondary transfer section becomes large, which deteriorates the graininess of the image and makes it difficult to transfer the toner uniformly to embossed paper with an uneven surface.

[0005] Here, Patent Document 1 discloses a configuration in which a conductive contact plate is provided downstream of the primary transfer section and on the inner surface side of the intermediate transfer belt, and a bias of the same polarity as the charging polarity of the photosensitive drum is applied to this contact plate. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2003-57963 A Summary of the Invention [Problem to be solved by the invention]

[0007] In order to suppress the increase in the amount of charge on the toner downstream of the primary transfer portion as described above, it is effective to suppress discharge downstream of the primary transfer portion.

[0008] Patent Document 1 does not mention suppressing discharge downstream of the primary transfer unit. However, as a result of the inventors' further investigation, it was found that in order to suppress discharge downstream of the primary transfer unit, it is effective to dispose a potential regulating member, which is a conductive electrode member, downstream of the primary transfer unit and on the inner peripheral side of the intermediate transfer belt, and to apply a bias (herein also referred to as a "potential regulating bias") of the same polarity as the charging polarity of the photosensitive drum to this potential regulating member.

[0009] At this time, in order to effectively suppress discharge downstream of the primary transfer section, it is desirable to set the potential regulating bias so that the potential difference between the intermediate transfer belt and the photosensitive drum immediately after passing through the primary transfer section is below a predetermined potential difference (discharge start voltage).

[0010] However, it is known that the surface potential of the photosensitive drum fluctuates with use of the photosensitive drum. This is because the surface layer of the photosensitive drum is scraped off with use of the photosensitive drum, and the capacity of the charge transport layer, which contributes to the formation of the surface potential of the photosensitive drum as a capacitor, fluctuates. The surface layer of the photosensitive drum is scraped off with use of the photosensitive drum, for example, by a cleaning member such as a cleaning blade that cleans the residual toner on the photosensitive drum. When the surface potential of the photosensitive drum immediately after passing through the primary transfer section fluctuates, the potential difference between the intermediate transfer belt and the photosensitive drum immediately after passing through the primary transfer section also fluctuates. As a result, when the potential difference between the intermediate transfer belt and the photosensitive drum immediately after passing through the primary transfer section exceeds the above-mentioned predetermined potential difference, it may be difficult to sufficiently suppress discharge downstream of the primary transfer section.

[0011] Therefore, an object of the present invention is to effectively suppress discharge downstream of a primary transfer portion in accordance with the usage conditions of a photosensitive member in a configuration in which an electrode member is provided for suppressing discharge downstream of a primary transfer portion. [Means for solving the problem]

[0012] The above object can be achieved by an image forming apparatus according to the present invention. In summary, the present invention is an image forming apparatus comprising: a photoconductor that can be charged to a predetermined polarity and carries a toner image; a charging device that charges the surface of the photoconductor by applying a charging bias; a rotatable intermediate transfer belt that contacts the photoconductor to form a primary transfer section and transports the toner image that has been primarily transferred from the photoconductor at the primary transfer section to a recording material at a secondary transfer section; a primary transfer member that contacts the inner surface of the intermediate transfer belt and is applied with a primary transfer bias of an opposite polarity to the predetermined polarity to perform primary transfer of the toner image from the photoconductor to the intermediate transfer belt; an electrode member that contacts the inner surface of the intermediate transfer belt at an adjacent position downstream of the primary transfer section in the movement direction of the intermediate transfer belt; an electrode bias application unit that applies an electrode bias of the same polarity as the predetermined polarity to the electrode member; a control unit that can control the electrode bias application unit; and an acquisition unit that acquires information correlated with the film thickness of the photoconductor, wherein the control unit controls to change the electrode bias based on the information. Effect of the Invention

[0013] According to the present invention, in a configuration in which an electrode member is provided for suppressing discharge downstream of a primary transfer portion, discharge downstream of the primary transfer portion can be effectively suppressed depending on the usage conditions of the photosensitive member. [Brief description of the drawings]

[0014] [Figure 1] 1 is a schematic cross-sectional view of an image forming apparatus according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic block diagram of a control system of the image forming apparatus according to the embodiment. [Diagram 3] 3A and 3B are a cross-sectional view and a perspective view of an example of a potential restricting member. [Figure 4] FIG. 11 is a cross-sectional view of another example of a potential restricting member. [Diagram 5] FIG. 11 is a cross-sectional view of another example of a potential restricting member. [Figure 6] 4 is a cross-sectional view for explaining the arrangement of a potential restricting member. FIG. [Figure 7] FIG. 4 is a flow chart showing an outline of a procedure for film thickness prediction control in the embodiment. [Figure 8] FIG. 11 is a flow chart showing an outline of a procedure for potential regulating bias adjustment control in the embodiment. [Figure 9] 6 is a graph showing the relationship between the film thickness of the photosensitive drum and the potential of the photosensitive drum immediately after passing through a primary transfer portion. [Figure 10] 6 is a graph showing the relationship between the potential regulating bias and the potential of the intermediate transfer belt immediately after passing through the primary transfer portion. FIG. [Figure 11] FIG. 11 is a graph showing, in Comparative Example 1, (a) a method for controlling the potential regulating bias, (b) the relationship between the number of sheets output and the potential of the photosensitive drum and the potential of the intermediate transfer belt immediately after passing through the primary transfer section, and (c) the relationship between the number of sheets output and the potential difference between the intermediate transfer belt and the photosensitive drum immediately after passing through the primary transfer section. [Figure 12] FIG. 4 is a graph showing the relationship between the film thickness of the photosensitive drum and the potential regulating bias in the embodiment. [Figure 13] 11 is a graph showing the relationship between the number of output sheets and the potential difference between the intermediate transfer belt and the photosensitive drum immediately after passing through the primary transfer portion in the embodiment. FIG. [Figure 14] FIG. 11 is a graph showing, in Comparative Example 2, (a) a method for controlling the potential regulating bias, (b) the relationship between the number of sheets output and the potential of the photosensitive drum and the potential of the intermediate transfer belt immediately after passing through the primary transfer section, and (c) the relationship between the number of sheets output and the potential difference between the intermediate transfer belt and the photosensitive drum immediately after passing through the primary transfer section. [Figure 15] FIG. 11 is a schematic block diagram of a control system of an image forming apparatus according to another embodiment. [Figure 16] FIG. 11 is a flowchart showing an outline of a procedure for film thickness detection control in another embodiment. [Figure 17] FIG. 11 is a flow chart showing an outline of a procedure for potential regulating bias adjustment control in another embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

[0016] [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. Fig. 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 type full-color printer that employs an intermediate transfer method and is capable of forming a full-color image on a sheet-shaped recording material S using an electrophotographic method.

[0017] The image forming apparatus 1 has an image forming section 2, a control section 3, a feeding section 4 for a recording material S, and a discharge section 5 for a recording material S. Inside the image forming apparatus 1, 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 inside the apparatus are provided. The temperature sensor 71 and the humidity sensor 72 are each an example of an environment detection means for detecting at least one of the environment inside and 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 the image forming apparatus 1 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) such as a personal computer (host device), a digital camera, or a smartphone connected to the image forming apparatus 1. Incidentally, the recording material (transfer material, recording medium, sheet) S is a material on which an image is formed using toner, and specific examples include plain paper, thick paper, gloss coated paper, matte coated paper, embossed paper, or synthetic resin sheets (synthetic paper) and overhead projector sheets (resin films) that are substitutes for plain paper. Here, the recording material S is sometimes referred to as "paper" (such as "paper" and "embossed paper"), but even in such cases, the recording material S includes materials other than paper or materials that contain materials other than paper.

[0018] The image forming section 2 forms an image on the recording material S fed from the feeding section 4 based on image information. 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. Elements having the same or corresponding functions or configurations provided for each color may be generally described by omitting the suffixes y, m, c, and k of the reference numerals indicating that the elements are for any one of the colors. The image forming apparatus 1 can also form a monochrome image, such as a black monochrome image, or a multi-color image using image forming units 10 for a desired single color or for some of four colors.

[0019] 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 (charging device) 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. The image forming unit 10 forms a toner image on an intermediate transfer belt 6 which will be described later.

[0020] The photosensitive drum 11 is movable (rotatable) while carrying an electrostatic image (electrostatic latent image) or a toner image. In this embodiment, the photosensitive drum 11 is a negatively charged organic photoconductor (OPC) having an outer diameter of 30 mm. The photosensitive drum 11 has an aluminum cylinder as a base and a surface layer (photoconductive layer) made of an organic material formed on the surface. In this embodiment, the surface layer has three layers, an undercoat layer, a photocharge generation layer, and a charge transport layer, which are applied and laminated on the base in the following order. When an image forming operation is started, the photosensitive drum 11 is rotated in the direction of the arrow R1 in the figure (counterclockwise direction) at a predetermined peripheral speed (process speed) by a driving force transmitted from a driving motor (not shown) as a driving means. The photosensitive drum 11 may have a protective layer (which may or may not have a charge transport function) constituting the surface layer. Furthermore, the photosensitive drum 11 is not limited to an organic photosensitive member, but may be an amorphous silicon photosensitive member having a surface layer (photoconductive layer) mainly composed of amorphous silicon.

[0021] The surface of the rotating photosensitive drum 11 is uniformly charged 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 source 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 source 73 applies a predetermined charging bias (charging voltage) to the charging roller 12. In this embodiment, the charging power source 73 includes a charging DC power source 73a (FIG. 2) and a charging AC power source 73b (FIG. 2). During charging, the charging power source 73 applies, as a charging bias, an oscillating bias (oscillating voltage) in which a DC voltage component (charging DC bias) output from the charging DC power source 73a and an AC voltage component (charging AC bias) output from the charging AC power source 73b are superimposed to the charging roller 12. The charging portion is a position in the rotation direction of the photosensitive drum 11 where the charging process of the surface of the photosensitive drum 11 is performed by the charging roller 12. The charging roller 12 charges the surface of the photosensitive drum 11 by discharging in at least one of minute gaps formed on the upstream side and downstream side of the contact portion between the photosensitive drum 11 and the charging roller 12 in the rotation direction of the photosensitive drum 11. However, for simplicity, the contact portion between the photosensitive drum 11 and the charging roller 12 can be considered to be the charging portion.

[0022] The charged surface of the 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. The exposure device 13 emits laser light according to image information of separated colors output from the control unit 3, and scans and exposes the surface (outer circumferential surface) of the photosensitive drum 11. The position in the rotation direction of the photosensitive drum 11 where the exposure device 13 irradiates the surface of the photosensitive drum 11 with light (image light) (image exposure) is the exposure unit.

[0023] The electrostatic image formed on the photosensitive drum 11 is developed (visualized) by supplying toner by the developing device 14, 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 including toner (non-magnetic toner particles) and carrier (magnetic carrier particles) as a developer. A two-component developer is contained in a developing container (developing container body) 14b of the developing device 14, 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 14a, 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. During development, the developing sleeve 14a is rotationally driven by a driving force transmitted from a driving motor (not shown) as a driving means. The developing sleeve 14a carries a two-component developer and transports it 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 to the image portion of the electrostatic image on the photosensitive drum 11 and adheres thereto. A developing power source 74 (FIG. 2) as a developing bias application means (developing bias application section) is connected to the developing sleeve 14a. During development, the developing power source 74 applies a predetermined developing bias (developing voltage) to the developing sleeve 14a. In this embodiment, the developing power source 74 is configured to include a developing DC power source (not shown) and a developing AC power source (not shown). During development, the development power supply 74 applies, as a development bias, to the development sleeve 14a, an oscillating bias (oscillating voltage) in which a DC voltage component (development DC bias) output from a development DC power supply and an AC voltage component (development AC bias) output from a development AC power supply are superimposed. In this embodiment, toner charged with the same polarity as the charging polarity of the photosensitive drum 11 (negative polarity in this embodiment) adheres to an exposed portion (image portion) on the photosensitive drum 11 in which the absolute value of the potential has been reduced by exposure after being uniformly charged (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. In the rotation direction of the photosensitive drum 11, the position where the developing device 14 supplies the toner to the surface of the photosensitive drum 11 is the developing portion (the developing area where the photosensitive drum 11 faces the developing sleeve 14a).

[0024] An intermediate transfer unit 20 is disposed so as to face the four photosensitive drums 11y, 11m, 11c, and 11k. The intermediate transfer unit 20 has an intermediate transfer belt 6 that is an endless belt serving as an intermediate transfer body. The intermediate transfer belt 6 is wound around and tensioned by a drive roller 21, a tension roller 22, and a secondary transfer inner roller 23 serving as a plurality of tension rollers. The intermediate transfer belt 6 is movable (rotatable) while carrying a toner image. The drive roller 21 is driven to rotate by a driving force transmitted from a drive motor (not shown) serving as a driving means. The intermediate transfer belt 6 rotates (moves around) in the direction of the arrow R2 in the figure (clockwise direction) at a predetermined peripheral speed corresponding to the peripheral speed of the photosensitive drum 11 by transmitting the driving force from the drive roller 21. The tension roller 22 controls the tension of the intermediate transfer belt 6 to be constant. The tension roller 22 applies a force to push the intermediate transfer belt 6 from the inner peripheral surface (back surface) side to the outer peripheral surface (front surface) side by the biasing force of a tension spring (not shown) composed of a compression coil spring, which is a biasing member as a biasing means. This force applies a tension of about 2 to 5 kgf to the intermediate transfer belt 6 in its conveying direction (process progress direction, front surface movement direction). The secondary transfer inner roller 23, together with the secondary transfer outer roller 25 described later, constitutes a secondary transfer device 26. On the inner peripheral surface side of the intermediate transfer belt 6, primary transfer rollers 15y, 15m, 15c, and 15k, which are roller-type primary transfer members as primary transfer means, are arranged corresponding to the photosensitive drums 11y, 11m, 11c, and 11k, respectively. In this embodiment, the primary transfer roller 15 is arranged opposite the photosensitive drum 11 and holds the intermediate transfer belt 6 between the photosensitive drum 11 and the primary transfer roller 15. The primary transfer roller 15 is pressed against the photosensitive drum 11 and comes into contact with the photosensitive drum 11 via the intermediate transfer belt 6 to form 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. The tension rollers other than the drive roller 21 and each primary transfer roller 15 are rotated in accordance with the rotation of the intermediate transfer belt 6.

[0025] The toner image formed on the photosensitive drum 11 is transferred (primary transfer) onto the rotating intermediate transfer belt 6 by the action of the primary transfer roller 15 in the primary transfer portion N1. For example, when a full-color image is formed, the toner images of each color, yellow, magenta, cyan, and black, formed on each photosensitive drum 11 are superimposed on the intermediate transfer belt 6 in order to be multi-transferred. 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 roller 15. During the primary transfer, the primary transfer power supply 75 applies a predetermined 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 roller 15. As a result, the toner image made of negative polarity toner on the photosensitive drum 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) for detecting the output voltage, and a current detection sensor 75b (FIG. 2) as a current detection means (current detection unit) for detecting 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 ("~" indicates a range including the numerical values ​​before and after it. The same applies below). In this embodiment, the primary transfer bias is controlled by a constant current. In this embodiment, the primary transfer power supplies 75 (75y, 75m, 75c, 75k) are provided independently for each of the primary transfer rollers 15 (15y, 15m, 15c, 15k). In this embodiment, the primary transfer bias applied to each of the primary transfer rollers 15 (15y, 15m, 15c, 15k) by each of the primary transfer power supplies 75 (75y, 75m, 75c, 75k) can be individually controlled. The position in the rotation direction of the photosensitive drum 11 where the toner image is transferred from the surface of the photosensitive drum 11 to the intermediate transfer belt 6 by the primary transfer roller 15 is a primary transfer portion N1.

[0026] In this embodiment, the primary transfer roller 15 has a core metal and an elastic layer of ion conductive foamed rubber (NBR rubber) formed around the core metal. The outer diameter of the primary transfer roller 15 is, for example, 15 to 20 mm. The primary transfer roller 15 has an electrical resistance of 1×105 ~1×10 8 A roller with a resistance of [Ω] (measured at N / N (23° C., 50% RH), applied voltage of 2 kV) can be suitably used.

[0027] 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. The material constituting the base layer is a resin such as polyimide or polycarbonate, and a material containing an appropriate amount of carbon black as an antistatic agent can be suitably used. The thickness of the base layer is, for example, 0.05 to 0.15 [mm]. The material constituting the surface layer is suitably CR rubber provided with electrical 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). Although the intermediate transfer belt 6 has a two-layer structure in this embodiment, it may have a single layer structure of a material equivalent to the above-mentioned base layer. The surface layer may be a resin coat layer containing a resin such as a fluororesin and having a thickness of about 0.002 to 0.01 [mm]. The intermediate transfer belt 6 may have a multi-layer structure of three or more layers.

[0028] On the outer peripheral surface side of the intermediate transfer belt 6, a secondary transfer outer roller 25, which is a roller-type secondary transfer member serving as a secondary transfer means, is disposed. The secondary transfer outer roller 25 as a secondary transfer member constitutes a secondary transfer device 26 together with the secondary transfer inner roller 23 as an opposing member (opposing electrode). The secondary transfer outer roller 25 is pressed toward the secondary transfer inner roller 23 and abuts against the secondary transfer inner roller 23 via the intermediate transfer belt 6 to form a secondary transfer portion (secondary transfer nip portion) N2, which is an abutment portion between the intermediate transfer belt 6 and the secondary transfer outer roller 25. The toner image formed on the intermediate transfer belt 6 is transferred (secondary transfer) onto the recording material S, which is being conveyed while being sandwiched between the intermediate transfer belt 6 and the secondary transfer outer roller 25, by the action of the secondary transfer device 26 at the secondary transfer portion N2. A secondary transfer power source 76 (FIG. 2) serving as a secondary transfer bias application means (secondary transfer bias application portion) is connected to the secondary transfer outer roller 25. During the secondary transfer, the secondary transfer power source 76 applies a predetermined secondary transfer bias (secondary transfer voltage), which is a DC voltage of the polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment), to the secondary transfer outer roller 25. As a result, the toner image made of the negative polarity toner on the intermediate transfer belt 6 is secondarily transferred onto the recording material S. The secondary transfer power source 76 is connected to a voltage detection sensor 76a (FIG. 2) as a voltage detection means (voltage detection unit) that detects the output voltage, and a current detection sensor 76b (FIG. 2) as a current detection means (current detection unit) that detects the output current. In addition, the core metal of the secondary transfer inner roller 23 is connected to a ground potential. In this embodiment, for example, a secondary transfer bias of about 1 to 6.5 [kV] is applied to the secondary transfer outer roller 25, and a secondary transfer current of about 15 to 100 [μA] is passed through the secondary transfer section N2, so that the toner image on the intermediate transfer belt 6 is secondarily transferred onto the recording material S. In this embodiment, the secondary transfer bias is controlled to a constant voltage. 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 as the secondary transfer member, and the outer secondary transfer roller 25 as the opposing member may be connected to the ground potential. The outer secondary transfer roller 25 may be configured to rotate in accordance with the rotation of the intermediate transfer belt 6, or may be configured to be rotated by a driving means.

[0029] The recording material S is conveyed from the feeding section 4 to the secondary transfer section N2 in parallel with the formation of the toner image on the intermediate transfer belt 6. The recording material S is accommodated in a cassette 41 as a recording material accommodation section of the feeding section 4. The recording material S accommodated in the cassette 41 is separated one by one by a feeding roller 42 as a feeding member of the feeding section 4 and sent out from the cassette 41. The recording material S is conveyed to a registration roller (a pair of registration rollers) 19 as a conveying member provided on a conveying path (conveying path) 44 of the recording material S by a conveying roller 43 as a conveying member of the feeding section 4. Then, the recording material S is conveyed to the secondary transfer section N2 by the registration roller 19 in synchronization with the toner image on the intermediate transfer belt 6. Although only one cassette 41 is illustrated in FIG. 1, the image forming apparatus 1 may have a plurality of cassettes 41. The feeding section 4 may also be capable of feeding the recording material S from a recording material storage section (recording material placement section) other than the cassette 41, such as a manual feed tray.

[0030] In this embodiment, the secondary transfer outer roller 25 has a core metal and an elastic layer of ion conductive foamed rubber (NBR rubber) formed around the core metal. The outer diameter of the secondary transfer outer roller 25 is, for example, 20 to 25 mm. The secondary transfer outer roller 25 has an electrical resistance of 1×10 5 ~1×10 8 A roller with a resistance of [Ω] (measured at N / N (23° C., 50% RH), applied voltage of 2 kV) can be suitably used.

[0031] The recording material S onto which the toner image has been transferred is conveyed to a fixing device 27 as a fixing means. The fixing device 27 has a fixing roller 27a and a pressure roller 27b. The fixing roller 27a incorporates a heater as a heating means. The pressure roller 27b presses against the fixing roller 27a to form a fixing section (fixing nip section). The fixing device 27 conveys the recording material S carrying an unfixed toner image by pinching it between the fixing roller 27a and the pressure roller 27b, thereby heating and pressurizing it, and fixing (melting and fixing) 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 conveyed by a discharge roller 51 and the like in the discharge section 5, and is discharged (output) from a discharge port (not shown) onto a discharge tray 52 provided outside the device main body (hereinafter also simply referred to as the "device main body") 1a of the image forming apparatus 1.

[0032] The surface of the photosensitive drum 11 after the primary transfer is discharged by being irradiated with light by the pre-exposure device 16. The position in the rotation direction of the photosensitive drum 11 where the pre-exposure device 16 irradiates (pre-exposes) the surface of the photosensitive drum 11 with light (discharging light) is the pre-exposure section (discharging section). In this embodiment, the pre-exposure device 16 irradiates the entire image area (area where a toner image can be formed) on the photosensitive drum 11 in the rotation axis direction of the photosensitive drum 11 with light to discharge the surface potential of the photosensitive drum 11 in that area to approximately 0 [V]. Note that discharging is not limited to discharging the surface potential of the photosensitive drum 11 to 0 [V], but includes removing at least a part of the charge to make the surface potential of the photosensitive drum 11 more uniform. In addition, the toner (primary transfer residual toner) that remains on the photosensitive drum 11 without being transferred to the intermediate transfer belt 6 during the primary transfer is removed from the photosensitive drum 11 by the drum cleaning device 17 and collected. In this embodiment, the drum cleaning device 17 scrapes off the primary transfer residual toner from the surface of the rotating photosensitive drum 11 by using a cleaning blade as a cleaning member, and collects it in a collection container (not shown). The cleaning blade is a plate-shaped member that contacts the photosensitive drum 11 with a predetermined pressing force. The cleaning blade contacts the surface of the photosensitive drum 11 in a counter direction to the rotation direction of the photosensitive drum 11 so that the tip of the free end side faces the upstream side in the rotation direction of the photosensitive drum 11. The position in the rotation direction of the photosensitive drum 11 where the drum cleaning device 17 removes the toner from the surface of the photosensitive drum 11 is the cleaning portion (the contact portion between the photosensitive drum 11 and the cleaning blade). In addition, deposits such as toner (secondary transfer residual toner) that remain on the intermediate transfer belt 6 without being transferred to the recording material S during the secondary transfer are removed from the intermediate transfer belt 6 by a belt cleaning device 24 as an intermediate transfer body cleaning means and collected.

[0033] The image forming unit 10 may be configured as a cartridge (process cartridge) that is detachable from the apparatus main body 1a as a whole. In this embodiment, the intermediate transfer belt 6, the tension roller of the intermediate transfer belt 6, the primary transfer rollers 15, the belt cleaning device 24, and the potential regulating members 8 described below constitute an intermediate transfer unit 20. The intermediate transfer unit 20 may be configured as a cartridge that is detachable from the apparatus main body 1a as a whole.

[0034] 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 to include a CPU 31 as an arithmetic processing unit (arithmetic processing section), a ROM (including a rewritable one) 32 and a RAM 33 (which may further include a non-volatile memory) as storage means (storage section), and an input / output circuit (I / F) (not shown) for inputting and outputting signals between the control unit 3 and an external device. The ROM 32 stores programs and the like for controlling each section of the image forming apparatus 1. The RAM 33 temporarily stores data related to control and the like. The CPU 31 is a microprocessor that controls the entire control of the image forming apparatus 1, and is the main body of the system controller. The CPU 31 is connected to each section such as the feeding section 4, the image forming section 2, and the discharge section 5, and exchanges signals with each section and controls the operation of each section. The ROM 32 stores an image formation control sequence for forming an image on the recording material S and the like.

[0035] To the control unit 3, for example, a charging power supply 73, a developing power supply 74, a primary transfer power supply 75, a secondary transfer power supply 76, a potential regulating power supply 80 (described later), and the like are connected, and each of these is controlled by a signal from the control unit 3. Although not shown in the figure, in this embodiment, the charging power supplies 73 (73y, 73m, 73c, 73k), the developing power supplies 74 (74y, 74m, 74c, 74k), the primary transfer power supplies 75 (75y, 75m, 75c, 75k), and the potential regulating power supplies 80 (80y, 80m, 80c, 80k) are provided independently for each image forming unit 10 (10y, 10m, 10c, 10k). The control unit 3 is also connected to a temperature sensor 71, a humidity sensor 72, voltage detection sensors 75a (75ay, 75am, 75ac, 75ak) and current detection sensors 75b (75by, 75bm, 75bc, 75bk) 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 charging travel distance integrator 78 constituting a film thickness information acquisition means (film thickness information acquisition unit) described later, voltage detection sensors 80a (80ay, 80am, 80ac, 80ak) and current detection sensors 80b (80by, 80bm, 80bc, 80bk) of the potential regulating power supply 80, a fixing temperature sensor 77, and the like. Signals (information) indicating the detection results of each sensor are input to the control unit 3.

[0036] Further, the control unit 3 is connected to an operation unit 70. The operation unit 70 has an input unit configured with operation buttons (keys) as input means, and a display unit 70a configured with 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 an input means. An operator such as a user or a service person can operate the operation unit 70 to cause the image forming apparatus 1 to execute a job (described later). The control unit 3 receives a signal from the operation unit 70 and operates various devices of the image forming apparatus 1. Moreover, the image forming apparatus 1 can also execute a job in response to a signal not from the operation unit 70 but from an external device such as a personal computer.

[0037] Here, the image forming apparatus 1 executes a job (print job, image forming operation) which is a series of operations for forming and outputting an image on one or more recording materials S, which is started by one start instruction. The job generally includes an image forming process, a pre-rotation process, a paper-to-paper process in the case of forming images on multiple recording materials S, and a post-rotation process. The image forming process is a period during which electrostatic image formation, toner image formation, primary transfer of the toner image, and secondary transfer of the toner image are performed for the image to be actually formed on the recording material S and output, and the image formation time (image formation period) refers to this period. More specifically, the timing of the image formation time differs depending on the position where each of the processes of electrostatic image formation, toner image formation, primary transfer of the toner image, and secondary transfer is performed. The pre-rotation process is a period during which a preparatory operation is performed before the image forming process, from when a start instruction is input until the image actually starts to be formed. The paper-to-paper process (recording material-to-recording material process, image-to-image process) is a period corresponding to the period 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 operation (preparatory operation) is performed after the image forming process. The non-image forming time (non-image forming period) is a period other than the image forming time, and includes the above-mentioned pre-rotation process, the sheet interval process, the post-rotation process, and further the 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.

[0038] 3. Effects of discharge downstream of the primary transfer section Next, the influence of discharge downstream of the primary transfer portion N1 will be further described. For convenience, the magnitude (high / low) of voltage or potential refers to the magnitude (high / low) when compared in absolute value unless otherwise specified. Furthermore, with respect to the arrangement of the primary transfer portion N1, the photosensitive drum 11, the primary transfer roller 15, and the potential regulating member 8 described below, the upstream and downstream refer to the upstream and downstream in the transport direction (process progress direction, surface movement direction) of the intermediate transfer belt 6 unless otherwise specified.

[0039] As described above, the toner on the intermediate transfer belt 6 tends to increase in charge amount due to the discharge between the intermediate transfer belt 6 and the photosensitive drum 11 downstream of the primary transfer portion N1. More specifically, due to the discharge, the toner on the intermediate transfer belt 6 tends to have a broader charge amount distribution than the charge amount distribution of the toner on the photosensitive drum 11, while the average charge amount increases. As a result of the inventors' investigation, it was found that the mirror force between the intermediate transfer belt 6 and the toner increases as the charge amount of the toner on the intermediate transfer belt 6 increases, making it difficult to transfer the toner to the recording material S at the secondary transfer portion N2. For example, when 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 becomes large, which may deteriorate the graininess of the image. In addition, for example, it is difficult to uniformly transfer the toner to embossed paper having an uneven surface because a gap occurs between the intermediate transfer belt 6 and the paper at the secondary transfer portion N2, and a relatively large secondary transfer electric field is required. Therefore, when the charge amount of the toner on the intermediate transfer belt 6 increases, it becomes more difficult to uniformly transfer the toner to embossed paper or other paper with an uneven surface. Embossed paper is paper (fancy paper) that has a pattern created by embossing or embossing the surface of the paper.

[0040] 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. As a result of the inventors' investigation, it was found that in order to suppress discharge downstream of the primary transfer portion N1, it is effective to dispose a potential regulating member, 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 of the same polarity as the charge polarity of the photosensitive drum 11 to the potential regulating member.

[0041] 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, which are electrode members, are arranged in contact with the inner circumferential surface of the intermediate transfer belt 6 at adjacent positions on the downstream side of each of the primary transfer portions N1y, N1m, N1c, and N1k. In this embodiment, the potential regulating members 8y, 8m, 8c, and 8k provided for each of the primary transfer portions N1y, N1m, N1c, and N1k have substantially the same configuration.

[0042] The shape of the potential regulating member 8 in this embodiment will be described. Fig. 3(a) is a cross-sectional view (a cross-section substantially perpendicular to the rotation axis direction of the photosensitive drum 11) of the potential regulating member 8 in this embodiment. Fig. 3(b) is a perspective view of the potential regulating member 8 in this embodiment.

[0043] In this embodiment, the potential regulating member 8 has a planar first portion 81 arranged along the width direction of the intermediate transfer belt 6 (a direction substantially perpendicular to the transport direction, and a direction substantially parallel to the rotation axis direction of the photosensitive drum 11). In addition, in this embodiment, the potential regulating member 8 has a planar second portion 82 arranged along the width direction of the intermediate transfer belt 6 and extending in a direction substantially perpendicular to the plane of the first portion 81. In this embodiment, the contact surface 83, which is a contact portion of the first portion 81 of the potential regulating member 8 that contacts the inner circumferential surface of the intermediate transfer belt 6, is planar. That is, in this embodiment, the first portion 81 constituting the contact surface 83 of the potential regulating member 8 is a flat plate.

[0044] Here, in a cross section substantially perpendicular to the rotation axis direction 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. In order to more effectively suppress discharge between the intermediate transfer belt 6 and the photosensitive drum 11 by the action of the electric field formed in the space between the photosensitive drum 11 and the potential regulating member 8, it is preferable to bring the potential regulating member 8 into surface contact with the intermediate transfer belt 6. From this viewpoint, the length between the line segment AB, that is, the "contact width" which is the length of the contact surface 83 in the conveying 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 the discharge, but if it 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 the influence of the component precision and the like. The length between the line segments AB is often sufficient if it is 50 [mm] or less, typically 30 [mm] or less. That is, the length between the line segments AB is preferably about 5 to 50 [mm], typically about 5 to 30 [mm]. From another perspective, the length between the line segments AB is often sufficient if it is less than half the distance between the axes of the adjacent photosensitive drums 11 in a cross section substantially perpendicular to the rotation axis direction of the photosensitive drums 11. In this embodiment, the potential regulating member 8 with a length between the line segments AB of 25 [mm] is used. In this embodiment, the distance between the axes of the photosensitive drums 11 in a cross section substantially perpendicular to the rotation axis direction of the photosensitive drums 11 is about 100 [mm].

[0045] A potential regulating power supply 80 is connected to the potential regulating member 8 as a potential regulating bias application means (potential regulating bias application section, electrode bias application section). A voltage detection sensor 80a (FIG. 2) 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) 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. At least during the primary transfer during the image forming operation, a predetermined potential regulating bias (potential regulating voltage, electrode bias) which is a DC voltage having the same polarity as the charging polarity of the photosensitive drum 11 is applied to the potential regulating member 8 by the potential regulating power supply 80. During the primary transfer, more specifically, the period during which the primary transfer bias is applied, more specifically, the period during which the image area (area to 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 between the intermediate transfer belt 6 and the photosensitive drum 11 downstream of the primary transfer portion N1. In this embodiment, the potential regulating bias is a negative DC voltage. In this embodiment, the potential regulating bias is controlled to a constant voltage. In the configuration of this embodiment, the potential regulating bias is preferably about -500 to -5000 [V], and more preferably about -1000 to -3000 [V].

[0046] The potential regulating member 8 is a member that is long 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 is longer than the width of the primary transfer roller 15 in the width direction of the intermediate transfer belt 6 that contacts the intermediate transfer belt 6. That is, in this embodiment, the range of the maximum image width and the range of the width of the primary transfer roller 15 in the width direction of the intermediate transfer belt 6 that contacts 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 the increase in the charge amount of the toner on the intermediate transfer belt 6 by suppressing the above-mentioned 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. That is, in this embodiment, the range of the length of the potential regulating member 8 in the longitudinal direction falls within the range of the width of the intermediate transfer belt 6. This makes it possible to suppress the occurrence of discharge between the potential regulating member 8 and the members surrounding the intermediate transfer belt 6 when the longitudinal end of the potential regulating member 8 protrudes beyond the width end of the intermediate transfer belt 6. This makes it possible to reduce the possibility that the effect of suppressing an increase in the charge amount of the toner on the intermediate transfer belt 6, which is caused by suppressing the above-mentioned discharge, will be reduced.

[0047] The potential regulating member 8 may be made of, for example, only one material having electrical conductivity. In this embodiment, the potential regulating member 8 is substantially made of only a metal having electrical conductivity, such as SUS (stainless steel). More specifically, in this embodiment, the potential regulating member 8 is made by bending a metal plate (sheet metal) such as SUS to form a first portion 81 and a second portion 82. By performing bending in this manner, the strength of the potential regulating member 8 can be increased. In this embodiment, neither the first portion 81 nor the second portion 82 of the potential regulating member 8 is substantially deformed when the image forming apparatus 1 is in use. However, the present invention is not limited to such an embodiment, and the potential regulating member 8 may be made of two or more materials.

[0048] FIG. 4 is a cross-sectional view (a cross-section substantially perpendicular to the rotation axis direction of the photosensitive drum 11) of another example of the potential regulating member 8. 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 constitutes the connection portion with the potential regulating power source 80 are made of a conductive material, for example, metal or conductive resin. The base 84 may be made of a conductive material, but may also be made of a non-conductive material, for example, non-conductive resin. The base 84 and the surface layer 85 can be fixed by any fixing means, such as an adhesive or welding.

[0049] FIG. 5 is a cross-sectional view (a cross-section substantially perpendicular to the rotation axis direction of the photosensitive drum 11) of still another example of the potential regulating member 8. For example, as shown in FIG. 5, the contact surface 83 of the potential regulating member 8 that contacts the intermediate transfer belt 6 may be made of a conductive nonwoven fabric 86. In FIG. 5, the conductive nonwoven fabric 86 is provided on the contact surface 83 of the potential regulating member 8 configured as shown in FIG. 4, but the conductive nonwoven fabric 86 may be provided on the contact surface 83 of the potential regulating 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. Instead of the nonwoven fabric 86, a felt or pile fabric (cut pile fabric (velvet, brush) or loop pile fabric (terry cloth)) made of conductive fibers, or a sponge (foamed elastic body) made of a conductive rubber material or the like 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 can be reduced.

[0050] Next, the arrangement of the potential regulating member 8 in this embodiment will be described. Fig. 6 is a cross-sectional view (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 cyan potential regulating member 8c provided adjacent to the cyan primary transfer portion N1c downstream between the cyan and black primary transfer portions N1c and N1k.

[0051] 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 offset downstream from the photosensitive drum 11. In this embodiment, the offset amount X1 is 3 [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 the direction along the common tangent of the sides of the multiple photosensitive drums 11 that contact the intermediate transfer belt 6 in a cross section that is substantially perpendicular to the rotation axis direction of the photosensitive drum 11.

[0052] Here, in order to explain the arrangement of the potential regulating member 8, it is assumed that the potential regulating member 8 is removed. In a cross section substantially perpendicular to the rotation axis direction of the photosensitive drum 11, a straight line passing through the tension surface on the inner peripheral side of the intermediate transfer belt 6 downstream of the primary transfer portion N1 in the absence of the potential regulating member 8 is defined as a straight line L. More specifically, this straight line L corresponds to the tension surface in a state in which only the potential regulating member 8 is substantially removed from the configuration of the image forming apparatus 1 in the image forming operation state. Also, on the straight line L, a point where the inner peripheral surface of the intermediate transfer belt 6 separates from the tension member immediately adjacent to the upstream side of the potential regulating member 8 is defined as "C (or upstream tension portion C)", and a point where the inner peripheral surface of the intermediate transfer belt 6 separates from the tension member immediately adjacent to the downstream side of the potential regulating member 8 is defined as "D (or downstream tension portion D)". In addition, although the straight line L is shown generally horizontally 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 so as to descend downward in the figure as it approaches the downstream side.

[0053] In this embodiment, the tension member immediately upstream 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 immediately upstream of the potential regulating member 8 is not limited to the primary transfer roller 15. For example, in the case where the primary transfer roller 15 is disposed offset upstream with respect to 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.

[0054] In this embodiment, the tension members immediately downstream of the potential regulating member 8 are the photosensitive drums 11m, 11c, and 11k arranged adjacent to the yellow, magenta, and cyan primary transfer portions N1y, N1m, and N1c, respectively. The downstream tension portion D is a position on the inner circumferential surface of the intermediate transfer belt 6 corresponding to a position where the intermediate transfer belt 6 separates from the photosensitive drums 11m, 11c, and 11k. However, the tension member immediately downstream of the potential regulating member 8 is not limited to the photosensitive drum 11. For example, when the primary transfer roller 15 is arranged offset upstream with respect to the photosensitive drum 11, the downstream tension portion 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, the downstream tension member immediately downstream of the black primary transfer portion N1k is a tension roller (tension roller in this embodiment) 22. 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.

[0055] 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 operation as the nearest tension member downstream of the potential regulating member 8, the straight line L and the downstream tension portion D are defined based on the tension roller. Also, even if a scraper or brush, rather than a tension roller, is in contact with the inner peripheral surface of the intermediate transfer belt 6 for the purpose of cleaning the inner peripheral surface of the intermediate transfer belt 6, it can be considered as the nearest downstream tension member 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.

[0056] 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. At this time, the closer the upstream end A is to the primary transfer portion N1, the greater the effect of suppressing the above-mentioned discharge. In this embodiment, 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 about 8 [mm]. Here, the distance X2 is the distance between the rotation center of the primary transfer roller 15 and the upstream end A in the direction along the common tangent line of the side of the multiple photosensitive drums 11 that contact the intermediate transfer belt 6 in a cross section that is substantially perpendicular to the rotation axis direction of the photosensitive drum 11. That is, in this embodiment, in the direction along the above-mentioned common tangent line, the distance from the rotation center of the primary transfer roller 15 to the upstream end A is smaller 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 thereto, the distance X2 is preferably about 1 to 20 mm, and typically about 1 to 10 mm.

[0057] In this embodiment, the potential regulating member 8 is pressed against the inner peripheral surface of the intermediate transfer belt 6 at both ends in the longitudinal direction by a pressing spring 87 (FIG. 3(b)) constituted by a compression coil spring, which is a pressing member serving as a pressing means. At this time, it is preferable that the contact portion of the potential regulating member 8 that contacts the inner peripheral surface of the intermediate transfer belt 6 is caused to enter the photosensitive drum 11 side from the straight line L. This allows the potential regulating member 8 to be more stably brought into contact with the intermediate transfer belt 6 even if waving or vibration occurs in the intermediate transfer belt 6 during the image forming operation (while the intermediate transfer belt 6 is running). In this embodiment, the pressing force of the pressing spring 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 peripheral surface of the intermediate transfer belt 6, enter the photosensitive drum 11 side by about 0.5 [mm] from the straight line L. By thus causing the contact surface 83 of the potential regulating member 8 to intrude into the photosensitive drum 11 side with respect to the straight line L, the potential regulating member 8 can be brought into surface contact with the intermediate transfer belt 6 more stably even if the intermediate transfer belt 6 is wavy or vibrated during the image forming operation (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 into the straight line L is preferably about 0.3 to 5 mm, and typically about 0.5 to 3 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.

[0058] Here, in a cross section substantially perpendicular to the rotation axis direction of the photosensitive drum 11, a straight line passing through the upstream end A and the downstream end B of the contact surface 83 is defined as a straight line M. In this case, it is preferable that the straight line M does not intersect with the line segment CD of the straight line L. This allows the intermediate transfer belt 6 and the potential regulating member 8 to be in surface contact with each other more reliably when the contact surface 83 of the potential regulating member 8 is flat. When the straight line M intersects with the line segment CD of the straight 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 can be in contact with the inner circumferential surface of the intermediate transfer belt 6. In this case, it may be difficult to enhance the discharge suppression effect by the surface contact.

[0059] 6, the potential regulating member 8 is disposed so that the straight line M and the straight line L are substantially parallel, but the potential regulating member 8 may be disposed so that the straight line M is inclined with respect to the straight line L as long as the straight line M does not intersect with the line segment CD of the straight line L. For example, by inclining the straight line M with respect to the straight line L so that the upstream end A side is closer to the straight line L than the downstream end B side, it is possible to reduce the curvature generated in the intermediate transfer belt 6 due to the intermediate transfer belt 6 running around in the vicinity of the upstream end A. This is therefore advantageous in reducing the possibility that scratches will occur on the inner peripheral surface of the intermediate transfer belt 6 due to friction with the potential regulating member 8.

[0060] The contact portion of the potential regulating member 8 that contacts the inner peripheral 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 in which the cross section substantially perpendicular to the rotation axis direction of the photosensitive drum 11 is curved in a convex shape toward the photosensitive drum 11, and the contact portion of the potential regulating member 8 that contacts the inner peripheral 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 contacts the inner peripheral surface of the intermediate transfer belt 6 a curved surface in this manner, it is possible to reduce stress when the potential regulating member 8 rubs against the intermediate transfer belt 6. By using a roller-shaped potential regulating member 8, the contact portion of the potential regulating member 8 that contacts the inner peripheral surface of the intermediate transfer belt 6 may be a curved surface.

[0061] 5. Surface potential of the photosensitive drum Next, the surface potential of the photosensitive drum 11 in each process of this embodiment will be described using specific set values ​​for each process.

[0062] In this embodiment, when the photosensitive drum 11 is first used (when it is brand new), the thickness of the charge transport layer constituting the surface layer of the photosensitive drum 11, which forms the surface potential of the photosensitive drum 11 as a capacitor (also simply referred to as the "film thickness of the photosensitive drum 11" here), is 30 [μm]. Also, in this embodiment, the peripheral speed of the photosensitive drum 11 when performing the image forming operation is 300 [mm / s].

[0063] In this embodiment, the DC voltage (charging DC bias) applied by the charging DC power source 73a to the charging roller 12 is -800 [V], and the AC voltage (charging AC bias) applied by the charging AC power source 73b to the charging roller 12 is 1800 [V] peak-to-peak voltage. When the AC voltage (charging AC bias) is a value that allows a sufficient stable discharge, the surface potential of the photosensitive drum 11 immediately after passing through the charging section converges to a value equivalent to the DC voltage (charging DC bias). As a result, the surface potential of the photosensitive drum 11 immediately after passing through the charging section is -800 [V]. After the surface potential of the photosensitive drum 11 is formed at the charging section, if no exposure is performed at the exposure section, the surface potential (non-image area potential, dark area potential) of the photosensitive drum 11 immediately before the primary transfer section N1 becomes -700 [V] due to attenuation of the charge over time.

[0064] In this embodiment, the width of the primary transfer roller 15 in the direction of its rotation axis (the direction from the front to the rear of the device along the width direction of the intermediate transfer belt 6) is 330 mm. In this embodiment, the primary transfer bias applied to the primary transfer roller 15 is constant current controlled so that the current flowing through the primary transfer roller 15 is substantially constant, and the set current value is 35 μA. In order to properly transfer the toner on the photosensitive drum 11 to the intermediate transfer belt 6 at the primary transfer portion N1, the amount of charge required is determined according to the amount of charge on the toner. For this reason, the primary transfer bias is generally managed by a current that is highly correlated with the amount of charge.

[0065] Here, the surface potential of the photosensitive drum 11 that varies before and after passing through the primary transfer portion N1 will be considered.

[0066] The change (fluctuation) ΔV [V] in the surface potential of the photosensitive drum 11 before and after passing through the primary transfer portion N1 can be calculated from the fluctuation in the charge on the surface of the photosensitive drum 11. Specifically, when the amount of fluctuating charge is ΔQ [C] and the capacity of the photosensitive drum 11 is C [F], it is defined as shown in Equation 1. ΔQ=C ΔV (Equation 1)

[0067] Further, the current I [A] flowing through the primary transfer roller 15 is the charge amount ΔQ [C] per time t [s], and is therefore defined as in Equation 2. I=ΔQ / t (Equation 2)

[0068] On the other hand, the photosensitive drum 11 acts as a capacitor. In this case, the plate distance is d [m] and the area is S [m 2 ], the relative dielectric constant is εr, and the dielectric constant of a vacuum is ε0 [F / m], then the capacitance C [F] of the photosensitive drum 11 is defined as shown in Equation 3. C=εr·ε0·S / d (Eq. 3)

[0069] Here, the area S [m 2 ] can be calculated as a value per time t [s] from the process speed PS [m / s] and the width L [m] of the primary transfer roller 15 in the rotational axis direction, using Equation 4. S / t=PS·L (Formula 4)

[0070] Therefore, when formulas 1 to 4 are modified with respect to ΔV, formula 5 is obtained. ΔV=ΔQ / C =(I t) / (εr ε0 S / d) =I / (εr·ε0 / d) / (PS·L) (Equation 5)

[0071] Here, I is the current value flowing through the primary transfer roller 15, 35 [μA], εr is the relative dielectric constant of the photosensitive drum 11, ε0 is the dielectric constant of a vacuum, 8.854×10 -12Substituting [F / m] for d, the thickness of the charge transport layer of the photosensitive drum 11 at the start of use of 30 [μm] for PS, the peripheral speed of the photosensitive drum 11 of 300 [mm / s] for PS, and the width of the primary transfer roller 15 in the rotational axis direction of 330 [mm] for L, from equation 5, ΔV is as follows. ΔV=399.3[V]

[0072] That is, the change in the surface potential of the photosensitive drum 11 before and after passing through the primary transfer portion N1 in the early stage of use of the photosensitive drum 11 can be calculated to be about 400 [V]. As described above, the surface potential of the photosensitive drum 11 immediately before the primary transfer portion N1 is -700 [V]. Therefore, the surface potential of the photosensitive drum 11 immediately after passing through the primary transfer portion N1 at this time is about -300 [V].

[0073] Next, a similar consideration will be given to the case where the thickness of the charge transport layer of the photosensitive drum 11 decreases with use of the photosensitive drum 11. The main reason for the decrease in thickness of the charge transport layer is that the surface layer of the photosensitive drum 11 is scraped off with use of the photosensitive drum 11. With use of the photosensitive drum 11, the surface layer of the photosensitive drum 11 is scraped off by, for example, rubbing against a cleaning member such as a cleaning blade that cleans residual toner on the photosensitive drum 11.

[0074] In this embodiment, at the end (terminal stage) of the life of the photosensitive drum 11 (corresponding to 400,000 images formed on A4 paper in this embodiment), the thickness of the charge transport layer forming the surface potential of the photosensitive drum 11 as a capacitor was 26 [μm]. The other set values ​​are the same as those at the beginning of use of the photosensitive drum 11. At this time, among the surface potentials formed on the photosensitive drum 11, the surface potential of the photosensitive drum 11 immediately before the primary transfer portion N1 is the same because it is managed by the voltage set value of each member according to the usage status of the photosensitive drum 11 in order to form an appropriate image. In other words, the surface potential of the photosensitive drum 11 immediately before the primary transfer portion N1 is -700 [V] even at the end of the life of the photosensitive drum 11.

[0075] On the other hand, the variation ΔV in the surface potential of the photosensitive drum 11 before and after passing through the primary transfer portion N1 is expressed by Equation 5, which includes the film thickness d [m] of the charge transport layer of the photosensitive drum 11. Therefore, the amount of variation in the surface potential of the photosensitive drum 11 before and after passing through the primary transfer portion N1 differs between the early stage of use of the photosensitive drum 11 and the final stage of its life.

[0076] If the film thickness 26 [μm] of the charge transport layer of the photosensitive drum 11 at the end of the life of the photosensitive drum 11 is substituted for d, and the same values ​​as those in the early stage of use of the photosensitive drum 11 are substituted for the other items, then, from equation 5, ΔV is as follows: ΔV=346.1[V]

[0077] That is, the change in the surface potential of the photosensitive drum 11 before and after passing through the primary transfer portion N1 at the end of the life of the photosensitive drum 11 can be calculated to be about 350 [V]. As described above, the surface potential of the photosensitive drum 11 immediately before the primary transfer portion N1 is -700 [V]. Therefore, the surface potential of the photosensitive drum 11 immediately after passing through the primary transfer portion N1 at this time is about -350 [V].

[0078] In this way, the surface potential of the photosensitive drum 11 immediately after passing the primary transfer portion N1 varies according to the change in the film thickness of the photosensitive drum 11 due to the use of the photosensitive drum 11. As described above, as the cumulative usage amount of the photosensitive drum 11 increases, the film thickness of the photosensitive drum 11 decreases, and the absolute value of the surface potential of the photosensitive drum 11 immediately after passing the primary transfer portion N1 tends to increase. This phenomenon is more noticeable when the efficiency of the primary transfer is managed by the amount of current.

[0079] The relationship between the film thickness of the photosensitive drum 11 and the surface potential of the photosensitive drum 11 immediately after passing through the primary transfer portion N1 in this embodiment is shown in Figure 9. In Figure 9, the horizontal axis represents the film thickness of the photosensitive drum 11, and the vertical axis represents the surface potential of the photosensitive drum 11 immediately after passing through the primary transfer portion N1 (the value is larger on the positive side at the top).

[0080] 6. Prediction of Film Thickness on Photosensitive Drum Next, a film thickness prediction means (film thickness prediction mechanism) for predicting the film thickness of the photosensitive drum 11 in this embodiment will be described.

[0081] In this embodiment, the image forming apparatus 1 predicts the film thickness of the photosensitive drum 11 according to the usage status of the photosensitive drum 11, based on the charging travel distance of the photosensitive drum 11 which correlates with the usage status (accumulated usage amount) of the photosensitive drum 11. Fig. 7 is a flowchart of film thickness prediction control for predicting the film thickness of the photosensitive drum 11 in this embodiment.

[0082] In this embodiment, the CPU 31 of the control unit 3 executes the film thickness prediction control. In the film thickness prediction control, the CPU 31 uses the charging travel distance data accumulated by the charging travel distance accumulating unit 78 to calculate the film thickness of the photosensitive drum 11 based on the relationship between the charging travel distance stored in the ROM 32 and the film thickness of the photosensitive drum 11. In this embodiment, the CPU 31 and the charging travel distance accumulating unit 78 as a usage amount information acquiring unit (usage amount information acquiring unit) that acquires an index value that correlates with the usage amount of the photosensitive drum 11 constitute a film thickness prediction unit (film thickness prediction mechanism) as a film thickness information acquiring unit (film thickness information acquiring unit) that acquires information (film thickness information) that correlates with the film thickness of the photosensitive drum 11. The film thickness prediction control can be operated in parallel with the image forming operation. Therefore, it is not necessary to set the execution timing of the film thickness prediction control to a time when no image is formed. The film thickness prediction control can be executed at a predetermined frequency that is set in advance, such as every time a predetermined number of images is formed (for example, 1000 sheets).

[0083] The CPU 31 reads out the charging travel distance data accumulated by the charging travel distance accumulating unit 78 during the execution of the film thickness prediction control (S101). The charging travel distance accumulating unit 78 measures the charging time, which is the execution time of the charging process, every time the charging bias is applied to the charging roller 12 and the charging process of the photosensitive drum 11 is executed. Then, the charging travel distance accumulating unit 78 calculates the charging travel distance (the distance the surface of the photosensitive drum 11 moves during the charging time) by multiplying the charging time by the process speed, and accumulates and stores the charging travel distance in a storage unit (non-volatile memory) provided in the charging travel distance accumulating unit 78. The charging travel distance accumulating unit 78 accumulates the charging travel distance for each image forming unit 10. In addition, the charging travel distance accumulating unit 78 resets the charging travel distance data to an initial value (0 in this embodiment) when the photosensitive drum 11 is replaced with a new one for each image forming unit 10.

[0084] Next, the CPU 31 uses the charging travel distance data read out in S101 to calculate the film thickness d of the photosensitive drum 11 from the relationship between the charging travel distance and the film thickness of the photosensitive drum 11 that is previously obtained and stored in the ROM 32 (S102). Information indicating the relationship between the charging travel distance and the film thickness of the photosensitive drum 11 is previously obtained as table data or a calculation formula and stored in the ROM 32.

[0085] In this embodiment, a method for calculating the film thickness of the photosensitive drum 11 is used based on the relationship between the charging travel distance and the film thickness of the photosensitive drum 11, which is experimentally obtained in advance. The surface layer of the photosensitive drum 11 is scraped off by the discharge during charging processing as well as by the rubbing by the cleaning member. Therefore, it is effective to use the charging processing time as information correlating with the usage status of the photosensitive drum 11. However, the method for predicting the film thickness of the photosensitive drum 11 is not limited to this method. For example, any method for predicting the film thickness of the photosensitive drum 11 based on information correlating with the usage amount of the photosensitive drum 11, that is, the decrease in the film thickness of the photosensitive drum 11, such as the number of image output sheets (number of images formed), the number of rotations of the photosensitive drum 11, and the rotation time of the photosensitive drum 11 from the initial state (when the photosensitive drum 11 is new), can be used. In addition, if there is a factor (such as the environment) that affects the relationship between the charging travel distance and the decrease in the film thickness of the photosensitive drum 11, the charging travel distance may be corrected and integrated using a coefficient corresponding to the factor. For example, consider a case where the amount of decrease in the film thickness of the photosensitive drum 11 per unit charging travel distance in a high temperature and high humidity environment is greater than the amount of decrease in the film thickness of the photosensitive drum 11 per unit charging travel distance in a low temperature and low humidity environment. In this case, the coefficient in a high temperature and high humidity environment is set to be greater than the coefficient in a low temperature and low humidity environment, and the charging travel distance can be corrected and integrated using a coefficient according to the environment during charging processing. The same applies to the number of images formed, the number of rotations of the photosensitive drum 11, the rotation time of the photosensitive drum 11, etc.

[0086] The CPU 31 appropriately stores (saves) data on the film thickness of the photosensitive drum 11 predicted by the film thickness prediction control in the RAM 33 (or a non-volatile memory). Note that the data on the film thickness of the photosensitive drum 11 predicted by the film thickness prediction control may be stored in a storage unit (such as a non-volatile memory) provided in a unit (a process cartridge or a single photosensitive drum) including the photosensitive drum 11 that is detachable from the apparatus main body 1a.

[0087] 7. Potential regulation bias correction control 7-1. Overview of potential regulation bias correction control in this embodiment As described above, in order to suppress discharge downstream of the primary transfer portion N1, it is effective to apply a potential regulating bias of the same polarity as the charging polarity of the photosensitive drum 11 to the potential regulating member 8. At this time, in order to effectively suppress discharge downstream of the primary transfer portion N1, it is desirable to set the potential regulating bias so that the potential difference between the intermediate transfer belt 6 and the photosensitive drum 11 immediately after they have passed the primary transfer portion N1 falls below a predetermined potential difference (discharge start voltage).

[0088] Meanwhile, as described above, the surface potential of the photosensitive drum 11 immediately after passing the primary transfer portion N1 varies in accordance with the change in the film thickness of the photosensitive drum 11 accompanying use of the photosensitive drum 11, and the potential difference between the intermediate transfer belt 6 and the photosensitive drum 11 immediately after passing the primary transfer portion N1 also varies. As a result, when the potential difference between the intermediate transfer belt 6 and the photosensitive drum 11 immediately after passing the primary transfer portion N1 exceeds the above-mentioned predetermined potential difference, it may become difficult to sufficiently suppress discharge downstream of the primary transfer portion N1.

[0089] It is possible to set the potential regulating bias to a large value in advance so that the potential difference between the intermediate transfer belt 6 and the photosensitive drum 11 immediately after passing the primary transfer portion N1 falls below a predetermined potential difference even if the surface potential of the photosensitive drum 11 fluctuates with use. However, in this case, more power than necessary will be consumed during the initial use of the photosensitive drum 11, etc.

[0090] Therefore, in this embodiment, the image forming apparatus 1 is configured to obtain information (film thickness information) that correlates with the film thickness of the photosensitive drum 11, and adjust the potential regulating bias applied to the potential regulating member 8 based on this information (film thickness information). In particular, in this embodiment, the image forming apparatus 1 is configured to obtain an index value that correlates with the usage amount of the photosensitive drum 11 as film thickness information, and predict the film thickness of the photosensitive drum 11 according to the usage status of the photosensitive drum 11. In this embodiment, the image forming apparatus 1 is configured to predict the surface potential of the photosensitive drum 11 immediately after passing through the primary transfer portion N1, and adjust the potential regulating bias applied to the potential regulating member 8 according to the prediction result. As a result, according to this embodiment, discharge downstream of the primary transfer portion N1 can be effectively suppressed according to the usage status of the photosensitive drum 11, and good image quality can be maintained throughout the life of the photosensitive drum 11.

[0091] 7-2.Effect of potential regulation bias First, the influence of adjusting the potential regulating bias was investigated. Fig. 10 shows the relationship between the potential regulating bias and the potential of the intermediate transfer belt 6 in the configuration of the image forming apparatus 1 of this embodiment. In Fig. 10, the horizontal axis shows the potential regulating bias, and the vertical axis shows the potential of the intermediate transfer belt 6 immediately after passing through the primary transfer section N1. The potential of the intermediate transfer belt 6 was obtained by simulation based on the design values ​​of each part in the configuration of the image forming apparatus 1 of this embodiment. Since the potential regulating bias has a negative polarity, the potential of the intermediate transfer belt 6 tends to fluctuate in the negative direction as the absolute value of the potential regulating bias increases.

[0092] Here, as Comparative Example 1, a case where the potential regulating bias is not changed according to the usage status of the photosensitive drum 11 will be described. The configuration and operation of the image forming apparatus 1 of Comparative Example 1 are substantially the same as those of this embodiment except for the above points. FIG. 11(a) shows the relationship between the output number (accumulated usage amount of the photosensitive drum 11) and the potential regulating bias in Comparative Example 1. FIG. 11(b) shows the relationship between the output number and the potential of the intermediate transfer belt 6 and the potential of the photosensitive drum 11 immediately after passing through the primary transfer portion N1 in Comparative Example 1. FIG. 11(c) shows the relationship between the output number and the potential difference between the intermediate transfer belt 6 and the photosensitive drum 11 immediately after passing through the primary transfer portion N1 in Comparative Example 1.

[0093] In Comparative Example 1, as shown in FIG. 11(a), the potential of the potential regulating member 8 is constant (−2000 [V], the same as the initial value in this embodiment described later) regardless of the usage status of the photosensitive drum 11 (the potential regulating bias is not changed depending on the usage status of the photosensitive drum 11). At this time, as shown in FIG. 11(b), the surface potential of the photosensitive drum 11 immediately after passing through the primary transfer portion N1 changes due to the change in the film thickness of the photosensitive drum 11 depending on the usage status of the photosensitive drum 11. On the other hand, as shown in FIG. 11(b), the potential of the intermediate transfer belt 6 does not change regardless of the usage status of the photosensitive drum 11. Therefore, as shown in FIG. 11(c), as the cumulative usage amount (output number) of the photosensitive drum 11 increases, the potential difference between the intermediate transfer belt 6 immediately after passing through the primary transfer portion N1 and the photosensitive drum 11 increases.

[0094] This potential difference is the potential difference related to the discharge downstream of the primary transfer portion N1 described above. In the configuration of the image forming apparatus 1 of this embodiment, the discharge start voltage (the threshold value above which discharge starts) of the discharge downstream of the primary transfer portion N1 is 630 [V]. In Comparative Example 1, it was found that when the cumulative usage amount (output number of sheets) of the photosensitive drum 11 increases, the potential difference between the intermediate transfer belt 6 and the photosensitive drum 11 immediately after passing the primary transfer portion N1 exceeds the discharge start voltage, causing discharge, which may adversely affect image quality.

[0095] On the other hand, in this embodiment, the CPU 31 of the control unit 3 controls the potential regulating bias to change as shown in FIG. 12 according to the result of prediction of the film thickness of the photosensitive drum 11 by the film thickness prediction mechanism described above. In FIG. 12, the horizontal axis indicates the film thickness of the photosensitive drum 11, and the vertical axis indicates the potential regulating bias (the value is larger on the positive side toward the upper side). Specifically, in this embodiment, the CPU 31 controls the potential regulating bias to be adjusted so that the potential difference between the intermediate transfer belt 6 and the photosensitive drum 11 immediately after passing through the primary transfer portion N1 becomes 600 [V] based on the surface potential of the photosensitive drum 11 immediately after passing through the primary transfer portion N1 predicted from the film thickness of the photosensitive drum 11. In this embodiment, the smaller the film thickness of the photosensitive drum 11 indicated by the index value (charging travel distance in this embodiment) correlated with the usage amount of the photosensitive drum 11 as film thickness information, the larger the absolute value of the potential regulating bias is made. Information indicating the relationship as shown in FIG. 12 is obtained in advance as table data or a calculation formula and stored in the ROM 32. The information may be information that directly relates the index value to the potential regulating bias.

[0096] 7-3. Procedure for potential regulation bias adjustment control FIG. 8 is a flow chart of the potential regulating bias adjustment control for adjusting the potential regulating bias in this embodiment.

[0097] First, the CPU 31 of the control unit 3 judges whether it is time to change the potential regulating bias (S201). The change of the potential regulating bias (judgment on whether the change is necessary) can be performed at a predetermined frequency, such as every time a predetermined number of images are formed (for example, 1000 sheets). The change of the potential regulating bias (judgment on whether the change is necessary) may be performed in synchronization with the above-mentioned film thickness prediction control, or the film thickness prediction control may be performed more frequently than the change of the potential regulating bias (judgment on whether the change is necessary).

[0098] When the CPU 31 determines that it is not the time to change the potential regulating bias (No in S201), it continues the operation such as image formation (S202) and repeats the operation until it determines that it is the time to change the potential regulating bias. The situation in which it is determined that it is not the time to change the potential regulating bias corresponds to, for example, a case where changing the potential regulating bias may change the image quality or density, such as during execution of a job or during execution of detection control of image density. It is desirable to change the potential regulating bias during a non-image formation time such as a pre-multiple rotation process (e.g., during an adjustment operation first thing in the morning) or a pre-rotation process. The determination of whether or not the potential regulating bias needs to be changed (S201, S202) can be made during a non-image formation time such as a pre-multiple rotation process or a pre-rotation process, as well as during execution of the image formation process of a job (during image formation).

[0099] On the other hand, when the CPU 31 judges that it is time to change the potential regulating bias (Yes in S201), it reads data on the film thickness of the photosensitive drum 11 predicted by the film thickness prediction control from the RAM 33 (or non-volatile memory) (S203). Then, the CPU 31 calculates the voltage value of the potential regulating bias based on the data on the film thickness of the photosensitive drum 11 (S204). Specifically, the voltage value of the potential regulating bias is calculated using the relationship between the film thickness of the photosensitive drum 11 and the potential regulating bias shown in FIG. 12, which is preset and stored in the ROM 32. That is, when the film thickness of the photosensitive drum 11 is predicted to be 30 [μm], the voltage value of the potential regulating bias is −2000 [V], and when the film thickness of the photosensitive drum 11 is predicted to be 26 [μm], the voltage value of the potential regulating bias is −2200 [V].

[0100] Then, the CPU 31 performs control so that the potential regulating bias of the voltage value calculated in S204 is applied to the potential regulating member 8 (S205), and ends the potential regulating bias adjustment control. Thereafter, the potential regulating bias determined in the current potential regulating bias adjustment control is used until the potential regulating bias is changed in the next potential regulating bias adjustment control.

[0101] FIG. 13 shows the relationship between the number of output sheets (the cumulative usage amount of the photosensitive drum 11) and the potential difference between the intermediate transfer belt 6 and the photosensitive drum 11 immediately after passing the primary transfer portion N1 in this embodiment. As a result of the above-mentioned control in this embodiment, as shown in FIG. 13, it is possible to make the potential difference between the intermediate transfer belt 6 and the photosensitive drum 11 immediately after passing the primary transfer portion N1 approximately constant at 600 [V] depending on the usage status of the photosensitive drum 11. That is, in this embodiment, even if the film thickness of the photosensitive drum 11 changes due to the use of the photosensitive drum 11 and the surface potential of the photosensitive drum 11 changes, the potential regulating bias is adjusted accordingly. Therefore, when the surface potential of the photosensitive drum 11 changes, it is possible to suppress the potential difference between the intermediate transfer belt 6 and the photosensitive drum 11 downstream of the primary transfer portion N1 from exceeding the discharge start voltage and causing discharge.

[0102] 8. Comparison between Examples and Comparative Examples Other possible control methods will be described below as comparative examples.

[0103] In Comparative Example 2, the potential regulating bias is not changed according to the usage status of the photosensitive drum 11 as in Comparative Example 1, but the absolute value of the potential regulating bias is set to -3000V, which is larger than that of Comparative Example 1, in anticipation of the decrease in the film thickness of the photosensitive drum 11 as the photosensitive drum 11 is used. The configuration and operation of the image forming apparatus 1 in Comparative Example 2 are substantially the same as those in this embodiment except for the above points. FIG. 14(a) shows the relationship between the number of output sheets (the cumulative usage amount of the photosensitive drum 11) and the potential regulating bias in Comparative Example 2. FIG. 14(b) shows the relationship between the number of output sheets and the potential of the intermediate transfer belt 6 and the potential of the photosensitive drum 11 immediately after passing through the primary transfer portion N1 in Comparative Example 2. FIG. 14(c) shows the relationship between the number of output sheets and the potential difference between the intermediate transfer belt 6 and the photosensitive drum 11 immediately after passing through the primary transfer portion N1 in Comparative Example 2.

[0104] In Comparative Example 2, as shown in FIG. 14(a), the potential of the potential regulating member 8 is constant (-3000 [V]) regardless of the usage status of the photosensitive drum 11 (it is not changed according to the usage status of the photosensitive drum 11). At this time, as shown in FIG. 14(b), the surface potential of the photosensitive drum 11 immediately after passing through the primary transfer portion N1 changes due to the change in the film thickness of the photosensitive drum 11 depending on the usage status of the photosensitive drum 11. On the other hand, as shown in FIG. 14(b), the potential of the intermediate transfer belt 6 does not change regardless of the usage status of the photosensitive drum 11. Therefore, as shown in FIG. 14(c), as the cumulative usage amount (output number) of the photosensitive drum 11 increases, the potential difference between the intermediate transfer belt 6 immediately after passing through the primary transfer portion N1 and the photosensitive drum 11 increases.

[0105] In Comparative Example 2, unlike Comparative Example 1, the absolute value of the potential regulating bias is set large in advance. Therefore, in Comparative Example 2, the potential difference between the intermediate transfer belt 6 and the photosensitive drum 11 immediately after passing through the primary transfer portion N1 is small, and it can be seen that it can be set to less than the discharge start voltage even at the end of the life period of the photosensitive drum 11. In other words, from the viewpoint of image quality, Comparative Example 2 is good even at the end of the life period of the photosensitive drum 11. However, in Comparative Example 2, the absolute value of the potential regulating bias is set large in advance, which increases power more than necessary.

[0106] Table 1 summarizes the results of the present embodiment, Comparative Example 1, and Comparative Example 2 described above. As shown in Table 1, a problem may occur in a configuration in which the potential regulating bias is not changed according to the usage status of the photosensitive drum 11, as in Comparative Example 1 and Comparative Example 2. In contrast, by setting an appropriate potential regulating bias according to the usage status of the photosensitive drum 11, as in this embodiment, it is possible to effectively suppress discharge downstream of the primary transfer portion N1 and maintain good image quality.

[0107] [Table 1]

[0108] As described above, the image forming apparatus 1 of this embodiment includes the photoconductor 11 that can be charged to a predetermined polarity and carries a toner image, the charging device 12 that charges the surface of the photoconductor 11 by applying a charging bias, the intermediate transfer belt 6 that is in contact with the photoconductor 11 to form a primary transfer portion N1 and that conveys the toner image that has been primarily transferred from the photoconductor 11 at the primary transfer portion N1 to a recording material S for secondary transfer at a secondary transfer portion N2, and the intermediate transfer belt 6 that is in contact with the inner peripheral surface of the intermediate transfer belt 6 and is applied with a primary transfer bias that is opposite in polarity to the predetermined polarity to transfer the toner image from the photoconductor 11 to the intermediate transfer belt 6. The intermediate transfer belt 6 includes a primary transfer member 15 for primarily transferring the toner image, an electrode member (potential regulating member) 8 that contacts the inner circumferential surface of the intermediate transfer belt 6 at a position adjacent to the downstream side of the primary transfer portion N1 in the moving direction of the intermediate transfer belt 6, an electrode bias application unit (potential regulating power source) 80 that applies an electrode bias of the same polarity as the above-mentioned predetermined polarity to the electrode member 8, a control unit 3 capable of controlling the electrode bias application unit 80, and an acquisition unit (charging travel distance integrating unit) 78 that acquires information that correlates with the film thickness of the photoconductor 11, and the control unit 3 controls to change the electrode bias based on the above-mentioned information. In this embodiment, the acquisition unit 78 acquires an index value that correlates with the cumulative usage amount of the photoconductor 11 as the above-mentioned information. The above-mentioned index value may be correlated with at least one of the distance traveled by the surface of the photoconductor 11 when the photoconductor 11 is charged by the charging device 12, the number of images output by the photoconductor 11, the number of rotations of the photoconductor 11, and the rotation time of the photoconductor 11. Particularly in this embodiment, the index value correlates with the distance traveled by the surface of the photoconductor 11 when the photoconductor 11 is being charged by the charging device 12. Also, in this embodiment, the control unit 3 controls so that the absolute value of the electrode bias when the film thickness indicated by the information is a second film thickness smaller than the first film thickness is greater than the absolute value of the electrode bias when the film thickness indicated by the information is a first film thickness.

[0109] As described above, according to this embodiment, in a configuration in which a potential regulating member 8 is provided for suppressing discharge downstream of the primary transfer portion N1, discharge downstream of the primary transfer portion N1 can be effectively suppressed depending on the usage status of the photosensitive drum 11.

[0110] [Example 2] Next, another embodiment of this embodiment 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 embodiment 1 are given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

[0111] 1. Overview of this embodiment In this embodiment, the image forming apparatus 1 is configured to obtain information (film thickness information) that correlates with the film thickness of the photosensitive drum 11, as in the first embodiment, and adjust the potential regulating bias applied to the potential regulating member 8 based on this information (film thickness information). In particular, in this embodiment, the image forming apparatus 1 is configured to obtain a charging current value as an index value that changes according to the film thickness of the photosensitive drum 11 as film thickness information, and detect the film thickness of the photosensitive drum 11 according to the usage status of the photosensitive drum 11. In this embodiment, the image forming apparatus 1 is configured to predict the surface potential of the photosensitive drum 11 immediately after passing through the primary transfer portion N1, and adjust the potential regulating bias applied to the potential regulating member 8 according to the prediction result. As a result, according to this embodiment, discharge downstream of the primary transfer portion N1 can be effectively suppressed according to the usage status of the photosensitive drum 11, and good image quality can be maintained throughout the life of the photosensitive drum 11.

[0112] 2. Configuration of the image forming apparatus according to this embodiment In this embodiment, similarly to the first embodiment, a charging power supply 73 (FIG. 15) including a charging DC power supply 73a (FIG. 15) and a charging AC power supply 73b (FIG. 15) is connected to the charging roller 12. During charging, the charging power supply 73 applies, as a charging bias, an oscillating bias (oscillating voltage) in which a DC voltage component (charging DC bias) output from the charging DC power supply 73a and an AC voltage component (charging AC bias) output from the charging AC power supply 73b are superimposed to the charging roller 12. A DC current detection sensor (DC current detection circuit) 73e (FIG. 15) is connected to the charging DC power supply 73a as a current detection means (current detection unit) for detecting the output current. The DC current detection sensor 73e detects the current flowing through the charging roller 12 (charging DC power supply 73a) when a charging bias is applied to the charging roller 12 and the surface of the photosensitive drum 11 is charged.

[0113] Fig. 15 is a block diagram showing a schematic configuration of a control system of the image forming apparatus 1 of this embodiment. The control configuration of the image forming apparatus 1 of this embodiment is roughly similar to the control configuration of the image forming apparatus 1 of embodiment 1 shown in Fig. 2, but in this embodiment, a DC current detection sensor 73e of a charging bias power supply 73, which constitutes a film thickness information acquisition means (film thickness information acquisition unit) described later, is connected to the control unit 3. A signal (information) indicating the detection result of the DC current detection sensor 73e is input to the control unit 3.

[0114] 3.Detection of the film thickness on the photosensitive drum Next, a film thickness detection means (film thickness detection mechanism) for detecting the film thickness of the photosensitive drum 11 in this embodiment will be described.

[0115] In this embodiment, the image forming apparatus 1 detects the film thickness of the photosensitive drum 11 according to the usage state of the photosensitive drum 11 based on the current flowing through the charging roller 12 when a charging bias is applied to the charging roller 12 as a contact member contacting the photosensitive drum 11. Fig. 16 is a flowchart of film thickness detection control for detecting the film thickness of the photosensitive drum 11 in this embodiment.

[0116] In this embodiment, the CPU 31 of the control unit 3 executes film thickness detection control. In this embodiment, the CPU 31 and the DC current detection sensor 73e as a current detection means (current detection unit) that acquires a charging current value as an index value that changes according to the film thickness of the photosensitive drum 11 constitute a film thickness detection means (film thickness detection mechanism) as a film thickness information acquisition means (film thickness information acquisition unit) that acquires information (film thickness information) correlated with the film thickness of the photosensitive drum 11.

[0117] When the timing for performing the film thickness detection control is reached, the CPU 31 controls the photosensitive drum 11 to be driven (S301). The peripheral speed (process speed) of the photosensitive drum 11 at this time is 300 [mm / s]. In this embodiment, the intermediate transfer belt 6 is also driven at this time. In this embodiment, the developing sleeve 14a is not driven and the developing bias is not applied to the developing sleeve 14a at this time, but this need not be done. In this embodiment, the primary transfer bias is not applied to the primary transfer roller 15 at this time, but this may be done. The film thickness detection control can be performed at a predetermined frequency that is set in advance, for example, every time a predetermined number of images is formed (for example, 1000 sheets). As in the case of the first embodiment, the change of the potential regulating bias (determination of whether or not the change is necessary) may be performed in accordance with the timing of the film thickness detection control, or the frequency of the film thickness detection control may be higher than the frequency of the change of the potential regulating bias (determination of whether or not the change is necessary).

[0118] Next, the CPU 31 controls the charging bias power supply 73 to apply a charging bias (oscillating bias in which a charging DC bias and a charging AC bias are superimposed) to the charging roller 12 (S302). At this time, the DC voltage (charging DC bias) applied to the charging roller 12 by the charging DC power supply 73a is -800 [V], and the AC voltage (charging AC bias) applied to the charging roller 12 by the charging AC power supply 73b is 1800 [V] peak-to-peak voltage. This AC voltage (charging AC bias) is a value that allows a sufficiently stable discharge, and the potential of the photosensitive drum 11 immediately after passing through the charging section converges to a value equivalent to the DC voltage (charging DC bias). As a result, the surface potential (non-image area potential, dark area potential) on the photosensitive drum 11 immediately after passing through the charging section becomes -800 [V].

[0119] Next, the CPU 31 operates the pre-exposure device 16 and controls it to eliminate the charge on the surface of the photosensitive drum 11 immediately before it reaches the charging portion (S303). At this time, in this embodiment, the charge elimination setting is set so that the surface potential of the photosensitive drum 11 immediately before it reaches the charging portion is approximately 0 [V].

[0120] Next, the CPU 31 acquires the current value I detected by the DC current detection sensor 73e (S304). This current value I may be the average value of a plurality of sampling results sampled during a predetermined period while the surface of the photosensitive drum 11, which has been neutralized as described above, passes through the charging unit. Then, the CPU 31 uses the current value I acquired in S304 to calculate the film thickness d of the photosensitive drum 11 from the following theoretical formula previously stored in the ROM 32 (S305).

[0121] A specific method for calculating the film thickness d of the photosensitive drum 11 will be described. From the above-mentioned formulas 1 to 3, I=ΔQ / t=(C·ΔV) / t=(εr·ε0·S / d)·ΔV / t Here, from equation 4, S / t = PS L, so it can be expressed as equation 6. I = (εr ε0 S / d) ΔV / t =εr ε0 / d ΔV (S / t) =εr·ε0 / d·ΔV·PS·L (Formula 6)

[0122] Solving this for d gives us Equation 7. d=εr·ε0·ΔV·PS·L / I (Formula 7)

[0123] Here, ΔV is the amount of change in the surface potential of the photosensitive drum 11 before and after passing through the charging portion, and in this embodiment, it is 800 [V] as described above. In addition, εr is the relative dielectric constant of the photosensitive drum 11, 3, and ε0 is the dielectric constant of a vacuum, 8.854×10 -12 [F / m], PS is the peripheral speed of the photosensitive drum 11 of 300 [mm / s], and L is the width of the charging roller 12 in the direction of the rotation axis (the front-rear direction of the device along the direction of the rotation axis of the photosensitive drum 11) of 330 [mm]. Furthermore, by using the current value I acquired by the DC current detection sensor 73e as described above, it is possible to calculate the film thickness of the photosensitive drum 11 from Equation 7. It can be seen from Equation (6) above that the smaller the film thickness of the photosensitive drum 11, the larger the current value I detected by the DC current detection sensor 73e. In other words, it can be seen from Equation (7) above that the larger the current value I detected by the DC current detection sensor 73e, the smaller the film thickness of the photosensitive drum 11.

[0124] In this embodiment, the film thickness detection control is performed using the charging roller 12, but the present invention is not limited to this configuration. For example, any mechanism may be used as long as it can detect the current flowing through a contact member that contacts the photosensitive drum 11, such as a contact-type conductive roller that changes the potential of the photosensitive drum 11. For example, the film thickness of the photosensitive drum 11 can be calculated by detecting the current flowing through the primary transfer roller 15, which serves as a contact member that contacts the photosensitive drum 11 via the intermediate transfer belt 6. In addition, the film thickness of the photosensitive drum 11 can be calculated by detecting the current flowing through a conductive roller that contacts the photosensitive drum 11 and is provided for film thickness detection control. In addition, the contact member is not limited to a roller-shaped member, and may be a pad-shaped, sheet-shaped, or brush-shaped member.

[0125] In addition, it is desirable to execute the above-mentioned film thickness detection control during non-image formation, such as during the pre-multiple rotation process (for example, during the first adjustment operation in the morning) or the pre-rotation process. This makes it easier to keep the charging conditions of the photosensitive drum 11 substantially constant during the film thickness detection control, thereby improving the detection accuracy of the charging current value, i.e., the film thickness of the photosensitive drum 11. However, in the case of a configuration in which the surface potential of the photosensitive drum 11 that reaches the charging section can be made uniform by the pre-exposure device 16 as described above, the film thickness detection control can also be executed during image formation (when the image area on the photosensitive drum 11 is charged).

[0126] In addition, in this embodiment, a method of calculating the film thickness of the photosensitive drum 11 from a theoretical formula is used, but the method is not limited to this. For example, a method of calculating the film thickness of the photosensitive drum 11 from the film thickness (design value) at the beginning of use of the photosensitive drum 11 and the increase in the current value acquired by film thickness detection control accompanying the decrease in the film thickness of the photosensitive drum 11 due to use may be used. That is, the film thickness of the photosensitive drum 11 can be calculated based on the film thickness (design value) at the beginning of use of the photosensitive drum 11 and the change amount from the detection result of the current at the beginning of use of the photosensitive drum 11. As the detection result of the current at the beginning of use of the photosensitive drum 11, for example, the first detection result after starting use of a new photosensitive drum 11, or the detection result until the number of image formations reaches a predetermined number of image formations (for example, 1000 sheets), can be used.

[0127] The CPU 31 appropriately stores (saves) data on the film thickness of the photosensitive drum 11 detected by the film thickness detection control in the RAM 33 (or a non-volatile memory). Note that the data on the film thickness of the photosensitive drum 11 detected by the film thickness detection control may be stored in a storage unit (such as a non-volatile memory) provided in a unit (a process cartridge or a single photosensitive drum) including the photosensitive drum 11 that is detachable from the apparatus main body 1a.

[0128] 4. Potential regulation bias correction control In this embodiment, the CPU 31 of the control unit 3 controls the potential regulating bias to change as shown in FIG. 12 in the same manner as in the first embodiment, according to the detection result of the film thickness of the photosensitive drum 11 by the film thickness detection mechanism described above. Specifically, in this embodiment, the CPU 31 of the control unit 3 controls the potential regulating bias to be adjusted so that the potential difference between the intermediate transfer belt 6 and the photosensitive drum 11 immediately after passing through the primary transfer portion N1 becomes 600 [V] based on the surface potential of the photosensitive drum 11 immediately after passing through the primary transfer portion N1 predicted from the film thickness of the photosensitive drum 11. In this embodiment, the smaller the film thickness of the photosensitive drum 11 indicated by the index value (in this embodiment, the zoning current value) that changes according to the film thickness of the photosensitive drum 11 as film thickness information, the larger the absolute value of the potential regulating bias is. Information showing the relationship as shown in FIG. 12 is obtained in advance as table data or a formula and stored in the ROM 32. The information may be information directly relating the index value and the potential regulating bias. Alternatively, the information may be information that directly correlates the amount of change in the index value from the initial stage of use of the photosensitive drum 11 with the potential regulating bias.

[0129] FIG. 17 is a flow chart of the potential regulating bias adjustment control for adjusting the potential regulating bias in this embodiment.

[0130] The processes of S401 to S405 shown in FIG. 17 are the same as the processes of S201 to S205 in the first embodiment shown in FIG. 8. However, in this embodiment, when the CPU 31 determines that it is time to change the potential regulating bias (Yes in S401), it reads out data on the film thickness of the photosensitive drum 11 detected by the film thickness detection control from the RAM 33 (or non-volatile memory) (S403). Then, the CPU 31 calculates the voltage value of the potential regulating bias based on the data on the film thickness of the photosensitive drum 11 (S404). Specifically, the voltage value of the potential regulating bias is calculated using the relationship between the film thickness of the photosensitive drum 11 and the potential regulating bias shown in FIG. 12, which is similar to that of the first embodiment and is set in advance and stored in the ROM 32. That is, when the film thickness of the photosensitive drum 11 is detected as 30 [μm], the voltage value of the potential regulating bias is −2000 [V], and when the film thickness of the photosensitive drum 111 is detected as 26 [μm], the voltage value of the potential regulating bias is −2200 [V].

[0131] In this embodiment, as in the first embodiment, as a result of the above-mentioned control, it is possible to make the potential difference between the intermediate transfer belt 6 and the photosensitive drum 11 immediately after passing the primary transfer portion N1 substantially constant at 600 [V] depending on the usage state of the photosensitive drum 11, as shown in Fig. 13. That is, in this embodiment, even if the film thickness of the photosensitive drum 11 changes due to the use of the photosensitive drum 11 and the surface potential of the photosensitive drum 11 changes, the potential regulating bias is adjusted accordingly. Therefore, when the surface potential of the photosensitive drum 11 changes, it is possible to suppress the potential difference between the intermediate transfer belt 6 and the photosensitive drum 11 downstream of the primary transfer portion N1 from exceeding the discharge start voltage and causing discharge.

[0132] Thus, in this embodiment, the acquiring unit that acquires information correlated with the film thickness of the photoconductor 11 acquires, as the information, an index value (in this embodiment, a charging current value) that changes according to the film thickness. In this embodiment, the image forming apparatus 1 has an application unit (charging power source) 73 that applies a bias to the contact member (charging roller) 12 that contacts the surface of the photoconductor 11, and a detection unit (DC current detection sensor) 73e that detects a current that flows through the contact member 12 when the application unit 73 applies a bias to the contact member 12 to change the surface potential of the photoconductor 11, and the control unit 3 controls to change the electrode bias based on the detection result by the detection unit 73e. In this embodiment, the charging device includes a charging member (charging roller) 12 as the contact member that contacts the surface of the photoconductor 11, and the detection unit 73e detects a current that flows through the charging member 12 when the application unit 73 applies a charging bias to the charging member 12 to charge the surface of the photoconductor 11. In this embodiment, the application unit 73 applies, as a charging bias, a vibration bias in which a DC bias and an AC bias are superimposed to the charging member 12. In this embodiment, the image forming apparatus 1 has a charge removing means (pre-exposure device) 16 that removes charge from the surface of the photoconductor 11 before the photoconductor 11 passes through the primary transfer portion N1 and reaches the charging portion where the charging process is performed by the charging member 12. The control unit 3 can control the electrode bias so that the absolute value of the electrode bias when the absolute value of the current indicated by the detection result by the detection unit 73 is a second value larger than the first value is larger than the absolute value of the electrode bias when the absolute value of the current indicated by the detection result by the detection unit 73 is a first value. In addition, the control unit 3 can control the electrode bias to be changed based on the amount of change in the second detection result detected by the detection unit 73 after the first detection result, relative to the first detection result by the detection unit 73. In this case, the control unit 3 can control the electrode bias so that the absolute value of the electrode bias when the amount of change is a second amount of change larger than the first amount of change is larger than the absolute value of the electrode bias when the amount of change is a first amount of change. Particularly in this embodiment, the control unit 3 obtains the film thickness of the photoconductor 11 based on the detection result by the detection unit 73, and changes the electrode bias based on the film thickness.In this embodiment, the control unit 3 controls the absolute value of the electrode bias when the film thickness is a second film thickness smaller than the first film thickness so that the absolute value of the electrode bias when the film thickness is a first film thickness is greater than the absolute value of the electrode bias when the film thickness is a second film thickness smaller than the first film thickness.

[0133] This embodiment also makes it possible to suppress the adverse effects (Table 1) that may occur in the configurations of Comparative Example 1 and Comparative Example 2 described in Example 1, and efficiently maintain good image quality.

[0134] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-mentioned embodiments.

[0135] In the above embodiment, the predetermined charge polarity of the photoconductor is negative, but this is not limited thereto, and the predetermined charge polarity of the photoconductor may be positive. Similarly, in the above embodiment, the normal charge polarity of the toner is negative, but the normal charge polarity of the toner may be positive. The predetermined charge polarity of the photoconductor and the various applied voltages when the normal charge polarity of the toner is positive may be appropriately changed, such as to have the opposite polarity to that of the above embodiment, in accordance with the above embodiment.

[0136] Furthermore, in the above-described embodiment, the potential regulating bias was subjected to constant voltage control, but the potential regulating bias may be subjected to constant current control. Furthermore, in the above-described embodiment, the primary transfer bias was subjected to constant current control, but the primary transfer bias may be subjected to constant voltage control. Furthermore, in the above-described embodiment, the secondary transfer bias was subjected to constant voltage control, but the secondary transfer bias may be subjected to constant current control. Here, constant current control refers to control that adjusts the output of a power supply so that the current supplied to the supply target is approximately constant at a target current. Furthermore, constant voltage control refers to control that adjusts the output of a power supply so that the voltage applied to the application target is approximately constant at a target voltage.

[0137] In the above embodiment, a configuration in which a superimposed bias of a DC bias and an AC bias can be applied to the charging member (charging device) and the developing member, respectively, has been described as an example, but the present invention is not limited to such a configuration. The present invention can also be applied to a configuration in which only a DC bias can be applied to the charging member (charging device) and the developing member.

[0138] Furthermore, the photoconductor is not limited to a drum-shaped one (photoconductor drum), but may be an endless belt-shaped one (photoconductor belt) or the like.

[0139] 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 monochromatic) image. [Explanation of symbols]

[0140] 1. Image forming device 3. Control Unit 6 Intermediate transfer belt 8. Electric potential regulation components (electrode components) 10 Image forming unit 11 Photosensitive drum (photoconductor) 12 Charging roller (charging device) 15 Primary transfer roller (primary transfer member) 25 Secondary transfer roller (secondary transfer member) 80 Potential Regulating Power Supply

Claims

1. a photoconductor capable of being charged to a predetermined polarity and carrying a toner image; a charging device that applies a charging bias to the surface of the photoconductor and charges the surface of the photoconductor; an intermediate transfer belt that is in contact with the photoconductor to form a primary transfer section and that conveys the toner image that has been primarily transferred from the photoconductor at the primary transfer section to a recording material at a secondary transfer section; a primary transfer member that contacts an inner peripheral surface of the intermediate transfer belt and is applied with a primary transfer bias having a polarity opposite to the predetermined polarity, and that performs primary transfer of a toner image from the photoconductor to the intermediate transfer belt; an electrode member that contacts an inner circumferential surface of the intermediate transfer belt at a position adjacent to the primary transfer portion on the downstream side in the moving direction of the intermediate transfer belt; an electrode bias application unit that applies an electrode bias having the same polarity as the predetermined polarity to the electrode member; A control unit capable of controlling the electrode bias application unit; An acquisition unit that acquires information correlated with a film thickness of the photoconductor; having The image forming apparatus according to the present invention, wherein the control unit performs control so as to change the electrode bias based on the information.

2. The image forming apparatus according to claim 1 , wherein the acquiring unit acquires, as the information, an index value that correlates with a cumulative usage amount of the photoconductor.

3. 3. The image forming apparatus according to claim 2, wherein the index value correlates with at least one of the distance traveled by the surface of the photoconductor when the photoconductor is charged by the charging device, the number of images output by the photoconductor, the number of rotations of the photoconductor, and the rotation time of the photoconductor.

4. The image forming apparatus according to claim 1 , wherein the acquiring unit acquires, as the information, an index value that changes depending on the film thickness.

5. The image forming apparatus according to any one of claims 1 to 4, characterized in that the control unit controls the absolute value of the electrode bias when the film thickness indicated by the information is a second film thickness smaller than the first film thickness so that the absolute value of the electrode bias is greater than the absolute value of the electrode bias when the film thickness indicated by the information is a first film thickness.

Images