Image forming apparatus

Abstract

To prevent the movement of toner from a photoconductor drum to an intermediate transfer belt and scattering of the toner on the intermediate transfer belt in a restoration mode.SOLUTION: An image forming apparatus has a photoreceptor, a charging device, a developing member, an intermediate transfer belt, a primary transfer member, an electrode member that is arranged on an inner peripheral surface side of the intermediate transfer belt at an adjacent position on the downstream side of the primary transfer unit in the direction of movement of the intermediate transfer belt, first cleaning means, second cleaning means, a charging bias application unit, a primary transfer bias application unit, an electrode bias application unit that applies an electrode bias having the same polarity as a predetermined polarity to the electrode member, and a control unit. The image forming apparatus can execute an image forming operation, and a restoration operation of cleaning the surface of the photoreceptor and the surface of the intermediate transfer belt when the photoreceptor and the intermediate transfer belt are stopped during the execution of the image forming operation. In the restoration operation, the control unit performs control not to apply a charging bias to the charging device and not to apply the electrode bias to the electrode member.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 method. [Background technology]

[0002] As image forming apparatuses such as color copiers, color printers, and color multifunction machines using the electrophotographic method, image forming apparatuses using the intermediate transfer method have become 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 a plurality of 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 plurality of 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. The toner remaining on the photosensitive drum after the primary transfer is removed from the photosensitive drum by a drum cleaning device and collected. In addition, the toner remaining on the intermediate transfer belt after the secondary transfer is removed from the intermediate transfer belt by a belt cleaning device and collected.

[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 described above, the toner on the intermediate transfer belt is subject to discharge between the intermediate transfer belt and the photosensitive drum downstream of the primary transfer section, and the charge amount tends to increase. It has been 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] However, it has been found that the configuration in which a potential regulating member is disposed downstream of the primary transfer portion and a potential regulating bias is applied to this potential regulating member has the following problems.

[0010] When a recording material is not properly conveyed or an error occurs during the image forming operation, the image forming apparatus may stop with a large amount of thick toner remaining on the photosensitive drum or intermediate transfer belt. This toner may cause stains on the recording material in the subsequent image forming operation, and therefore needs to be removed from the photosensitive drum or intermediate transfer belt. In this case, the image forming apparatus may be configured to execute a recovery mode in which the toner remaining on the photosensitive drum or intermediate transfer belt is cleaned after the recording material that has caused the conveyance error is removed and the image forming apparatus is released from the stop state. In this case, it may be difficult for the belt cleaning device to sufficiently collect the large amount of thick toner on the intermediate transfer belt in one go. This is particularly noticeable when the belt cleaning device is an electrostatic cleaning type that electrostatically collects toner using a brush.

[0011] Therefore, in the recovery mode, a bias of a different polarity from that during normal image formation may be applied to the primary transfer unit, or the bias applied to the primary transfer unit may be turned off. This causes the toner remaining on the intermediate transfer belt to be returned to the photosensitive drum, or causes the toner remaining on the photosensitive drum to be left on the photosensitive drum without being transferred to the intermediate transfer belt. Then, this toner is collected by the drum cleaning device.

[0012] However, it was found that if a potential regulating bias is applied to the potential regulating member in the same manner as in normal image formation, the toner on the photosensitive drum moves to the intermediate transfer belt or the toner on the intermediate transfer belt scatters. This toner then adheres to the secondary transfer member or stains the inside of the image forming apparatus, which causes toner stains on the recording material, such as back stains, in the subsequent image forming operation.

[0013] SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENTS An object of the present invention is to suppress the movement of toner from the photosensitive drum to the intermediate transfer belt and the scattering of toner on the intermediate transfer belt in the recovery mode. [Means for solving the problem]

[0014] The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides a rotatable photoconductor that can be charged to a predetermined polarity, a charging device that charges the surface of the photoconductor by application of a charging bias, a developing member that supplies a developer to the surface of the photoconductor that has been charged by the charging device to form a developer image on the surface of the photoconductor, a rotatable intermediate transfer belt that contacts the photoconductor to form a primary transfer section and transports the developer image that has been primarily transferred from the photoconductor at the primary transfer section to a recording material for secondary transfer at a secondary transfer section, and a roller that is disposed inside the intermediate transfer belt and corresponds to the photoconductor. a primary transfer member disposed on the circumferential surface side of the intermediate transfer belt, to which a primary transfer bias of a polarity opposite to the predetermined polarity is applied to perform primary transfer of a developer image from the photoconductor to the intermediate transfer belt; an electrode member disposed on the inner circumferential surface side of the intermediate transfer belt at a position adjacent to the downstream side of the primary transfer portion in the moving direction of the intermediate transfer belt; a first cleaning means for cleaning the surface of the photoconductor; a second cleaning means for cleaning the surface of the intermediate transfer belt; a charging bias application section for applying a charging bias to the charging device; an electrode bias application unit that applies an electrode bias of the same polarity as the predetermined polarity to the electrode member; and a control unit capable of controlling the charging bias application unit, the primary transfer bias application unit, and the electrode bias application unit, and capable of executing an image forming operation of forming an image consisting of a developer image on a recording material, and a return operation of cleaning the surface of the photoconductor and the surface of the intermediate transfer belt before starting a next image forming operation when the photoconductor and the intermediate transfer belt stop during the image forming operation, wherein the control unit controls the return operation so that no charging bias is applied to the charging device or that a charging bias is applied to the charging device such that the surface potential of the photoconductor is a surface potential with an absolute value smaller than that during the image forming operation, and no electrode bias is applied to the electrode member or that an electrode bias with an absolute value smaller than that when the primary transfer bias is applied to the primary transfer member during the image forming operation is applied to the electrode member. Effect of the Invention

[0015] According to the present invention, it is possible to suppress the movement of toner from the photosensitive drum to the intermediate transfer belt and the scattering of toner on the intermediate transfer belt in the recovery mode. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Diagram 2] FIG. 2 is a schematic cross-sectional view of an example of a drum cleaning device and a belt cleaning device. [Diagram 3] FIG. 2 is a schematic block diagram of a control system of the image forming apparatus. [Figure 4] 3A and 3B are a cross-sectional view and a perspective view of a potential restricting member; [Diagram 5] FIG. 11 is a cross-sectional view of another example of a potential restricting member. [Figure 6] FIG. 11 is a cross-sectional view of another example of a potential restricting member. [Figure 7] 4 is a cross-sectional view for explaining the arrangement of a potential restricting member. FIG. [Figure 8] 11 is a sequence chart for explaining operations during image formation and in a return mode in the embodiment. FIG. [Figure 9] 5A and 5B are schematic diagrams for explaining the relationship between the surface potential of the photosensitive drum, the primary transfer bias, and the potential regulating bias during image formation and in a return mode in the embodiment. [Figure 10] FIG. 11 is a flowchart showing an example of a procedure for controlling switching of a potential regulating bias. [Figure 11] 13 is a sequence chart for explaining operations during image formation and in a return mode in the modified example. FIG. [Figure 12] 11 is a sequence chart for explaining operations during image formation and in a return mode in a comparative example. FIG. [Figure 13] 5A and 5B are schematic diagrams for explaining the relationship between the surface potential of the photosensitive drum, the primary transfer bias, and the potential regulating bias during image formation and in a return mode in a comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0018] [Example 1] 1. Overall configuration and operation of the image forming apparatus First, the overall configuration and operation of the image forming apparatus of this embodiment will be described. 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.

[0019] 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. 3) capable of detecting the temperature inside the apparatus and a humidity sensor 72 (FIG. 3) capable of detecting the humidity inside the apparatus are provided. The temperature sensor 71 and the humidity sensor 72 are examples of an environment detection means for detecting at least one of the temperature and humidity inside or outside the image forming apparatus 1. The image forming apparatus 1 can form an image on the recording material S based on image information (image signal) acquired by a document reading device (not shown) provided in the image forming apparatus 1 or connected to 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) 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.

[0020] 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 the four colors.

[0021] 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 50 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.

[0022] 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 formed on the surface of the cylinder. 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 FIG. 1 (counterclockwise direction) at a predetermined peripheral speed (process speed) by a driving force transmitted from a driving motor provided in a driving unit 90 (FIG. 3) as a driving means.

[0023] 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. 3) is connected to the charging roller 12 as a charging bias application means (charging bias application unit). During image formation (charging process), 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 is configured to have a DC power source and an AC power source. In this embodiment, during image formation (charging process), the charging power source 73 applies a superimposed bias (oscillating voltage) to the charging roller 12, in which a DC bias ("charging DC bias"), which is a DC voltage component having the same polarity as the charging polarity of the photosensitive drum 11, and an AC bias ("charging AC bias"), which is an AC voltage component, are superimposed. For example, in this embodiment, the voltage of the charging DC bias ("charging DC voltage") is set to -900V, and the voltage of the charging AC bias ("charging AC voltage": peak-to-peak voltage in this embodiment) is set to more than twice the discharge start voltage (the voltage at which discharge starts when a DC voltage is applied), and the surface of the rotating photosensitive drum 11 is uniformly charged to about -900V. As the waveform of the charging AC bias, a sine wave, a square wave, a triangular wave, etc. can be appropriately applied. Alternatively, a square wave voltage formed by periodically turning on and off a DC power source may be applied. The frequency of the charging AC bias is about 1 kHz to 3 kHz. The charging DC bias and charging AC bias applied to the charging roller 12 during image formation are not limited to the above values, and are appropriately set to potentials suitable for good image formation depending on the environment or the usage status (usage amount) of the photosensitive drum 11 and the charging roller 12.

[0024] The surface of the charged photosensitive drum 11 is scanned and exposed by the exposure device 13 based on image information, and an electrostatic image is formed on the photosensitive drum 11. In this embodiment, the exposure device 13 is a laser scanner. 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 peripheral surface) of the photosensitive drum 11.

[0025] 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 image formation (development), the developing sleeve 14a is rotationally driven by a driving force transmitted from a drive motor provided in a drive unit 90 (FIG. 3) as a drive 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. 3) as a developing bias application means (developing bias application unit) is connected to the developing sleeve 14a. During image formation (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 have a DC power source and an AC power source. In this embodiment, during image formation (development), a superimposed bias (oscillating voltage) in which a DC bias ("developing DC bias"), which is a DC voltage component having the same polarity as the charging polarity of the photosensitive drum 11, and an AC bias ("developing AC bias"), which is an AC voltage component, are superimposed is applied to the developing sleeve 14a by the developing power source 74. The developing DC bias is appropriately set so as to become an appropriate fog removal potential with respect to the potential of the non-image portion on the photosensitive drum 11 in the development area.In this embodiment, toner charged with the same polarity (negative in this embodiment) as the charge polarity of the photosensitive drum 11 adheres to an exposed portion (image portion) on the photosensitive drum 11, the absolute value of the potential of which has been reduced by exposure after being uniformly charged (reverse development method). In this embodiment, the normal charge polarity of the toner, which is the main charge polarity of the toner during development, is negative.

[0026] 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 drive force transmitted from a drive motor provided in a drive unit 90 (FIG. 3) serving as a drive means. The intermediate transfer belt 6 rotates (moves around) in the direction of arrow R2 in FIG. 1 (clockwise direction) at a predetermined circumferential speed corresponding to the circumferential speed of the photosensitive drum 11 by the drive force transmitted 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 circumferential surface (back surface) side to the outer circumferential 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, 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 circumferential 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 to 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.

[0027] 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. 3) serving as a primary transfer bias application means (primary transfer bias application unit) is connected to the primary transfer roller 15. During image formation (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. 3) as a voltage detection means (voltage detection unit) for detecting the output voltage, and a current detection sensor 75b (FIG. 3) 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 at a constant voltage. 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.

[0028] 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 1×10 5 ~1×10 8A roller with a resistance of [Ω] (measured at N / N (23° C., 50% RH), applied voltage of 2 kV) can be suitably used.

[0029] 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.

[0030] 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. 3) serving as a secondary transfer bias application means (secondary transfer bias application portion) is connected to the secondary transfer outer roller 25. During image formation (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, a toner image made of negative 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. 3) as a voltage detection means (voltage detection unit) that detects the output voltage, and a current detection sensor 76b (FIG. 3) 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.

[0031] 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.

[0032] 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.

[0033] 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 applying heat and pressure to fix (melt, fix) 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. 3). 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.

[0034] The surface of the photosensitive drum 11 after the primary transfer is neutralized by a pre-exposure device 16. In addition, toner (primary transfer residual toner) remaining 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 and collected by a drum cleaning device 50. In addition, deposits such as toner (secondary transfer residual toner) remaining 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 and collected by a belt cleaning device 60 serving as an intermediate transfer body cleaning means. The drum cleaning device 50 and the belt cleaning device 60 will be described in further detail later.

[0035] 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 60, 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.

[0036] 2.Cleaning device 2-1.Drum cleaning device FIG. 2(a) is a schematic cross-sectional view of a drum cleaning device 50 in this embodiment.

[0037] The drum cleaning device 50 has a housing 51 that is long in the direction of the rotation axis of the photosensitive drum 11. The drum cleaning device 50 also has a cleaning blade 52, a cleaning metal plate 53, a collected toner transport screw 54, and a scattering prevention sheet 55.

[0038] The cleaning blade 52 as a cleaning member is attached to the housing 51. The cleaning blade 52 is a horizontally elongated plate-like member that is long in the direction of the rotation axis of the photosensitive drum 11, and is made of a rubber material having appropriate elasticity and hardness. In this embodiment, the cleaning blade 52 is a plate-like member that is approximately rectangular in plan view, having a predetermined length in the longitudinal direction arranged along the longitudinal direction of the photosensitive drum 11 and a predetermined thickness in the lateral direction approximately perpendicular to the longitudinal direction. The cleaning blade 52 is supported by a cleaning sheet metal 53 as a support member, and the cleaning sheet metal 53 is fixed to a mount part of the housing 51 by a screw. The cleaning blade 52 is abutted against the surface of the photosensitive drum 11 in the counter direction to the rotation direction of the photosensitive drum 11 so that the tip of the free end side, which is one end of the lateral direction, faces the upstream side in the rotation direction of the photosensitive drum 11. The cleaning blade 52 is arranged so that the edge part of the tip of the free end side abuts against the surface of the photosensitive drum 11 with a predetermined pressing force. The drum cleaning device 50 scrapes off residual toner after primary transfer from the surface of the rotating photosensitive drum 11 using a cleaning blade 52 , and collects the residual toner in a housing 51 .

[0039] A polyurethane rubber material is preferably used as the material for the cleaning blade 52 because it does not damage the member to be cleaned due to friction and has high abrasion resistance. A two-component thermosetting polyurethane may also be used as the material for the cleaning blade 52 because it has small plastic strain. In addition to polyurethane, styrene-butadiene copolymer, chloroprene, butadiene rubber, ethylene-propylene-diene rubber, chlorosulfonated polyethylene rubber, fluororubber, silicone rubber, etc. may also be used.

[0040] In this embodiment, the cleaning blade 52 is formed by molding using a polyurethane rubber material. In this embodiment, the cleaning blade 52 has a length in the longitudinal direction of 340 mm, a thickness of 2 mm, and a length in the lateral direction of 15 mm. In this embodiment, the length (free length) of the part of the cleaning blade 52 on the free end side in the lateral direction that is not supported by the cleaning metal plate 53 is 8 mm. A configuration in which the free end of the cleaning blade 52 in the lateral direction is arranged downward and rubs against the surface to be cleaned that moves upward is generally considered to have high cleaning ability.

[0041] The lower part of the housing 51 serves as a storage section for the toner collected from the surface of the photosensitive drum 11 by the cleaning blade 52, and a collected toner transport screw 54 is disposed in the storage section as a collected toner transport member. The collected toner transport screw 54 transports the toner collected from the surface of the photosensitive drum 11 to one end side of the longitudinal direction of the housing 51 and discharges it to the outside of the drum cleaning device 50. The toner discharged from the drum cleaning device 50 is transported through a transport path provided in the device main body 1a, and is collected in a collection container (not shown) disposed in the device main body 1a.

[0042] Further, the shatterproof sheet 55 as a shatterproof member prevents the toner in the housing 51 from scattering outside the drum cleaning device 50. The shatterproof sheet 55 is made of a polyethylene terephthalate resin sheet material having a thickness of 20 μm to 50 μm. The shatterproof sheet 55 is disposed so that its leading edge contacts the photosensitive drum 11 at a position upstream of the edge of the cleaning blade 52 in the rotation direction of the photosensitive drum.

[0043] 2-2.Belt cleaning device 2B is a schematic cross-sectional view of the belt cleaning device 60 in this embodiment. In this embodiment, the belt cleaning device 60 is an electrostatic cleaning type cleaning device that electrostatically removes adhesions such as secondary transfer residual toner on the intermediate transfer belt 6.

[0044] The belt cleaning device 60 has a housing 61 disposed near the intermediate transfer belt 6, downstream of the secondary transfer portion N2 and upstream of the primary transfer portion N1 (the most upstream primary transfer portion N1Y) in the transport direction of the intermediate transfer belt 6. The housing 61 is long in a width direction of the intermediate transfer belt 6 that is substantially perpendicular to the transport direction.

[0045] The belt cleaning device 60 has an upstream fur brush 62a as a first collection member (cleaning member) supported by the housing 61 and arranged on the upstream side in the transport direction of the intermediate transfer belt 6, and a downstream fur brush 62b as a second collection member (cleaning member) arranged on the downstream side in the transport direction of the intermediate transfer belt 6. The upstream fur brush 62a and the downstream fur brush 62b are in contact with the outer circumferential surface of the intermediate transfer belt 6 at positions facing the upstream facing roller 24a and the downstream facing roller 24b, respectively, via the intermediate transfer belt 6. The upstream facing roller 24a and the downstream facing roller 24b are in contact with the inner circumferential surface of the intermediate transfer belt 6.

[0046] In addition, the belt cleaning device 60 has, inside the housing 61, an upstream bias roller (recovery roller) 63a as a first voltage application member that contacts the upstream fur brush 62a, and a downstream bias roller (recovery roller) 63b as a second voltage application member that contacts the downstream fur brush 62b.

[0047] Furthermore, inside the housing 61, the belt cleaning device 60 has an upstream blade 64a as a first removing member that abuts against the upstream bias roller 63a, and a downstream blade 64b as a second removing member that abuts against the downstream bias roller 63b.

[0048] The upstream fur brush 62a and the downstream fur brush 62b are made of carbon-dispersed nylon fibers with an electric resistance of 0.3M (Ω / cm) and a fiber thickness of 6 denier, and the fur density is 500,000 fibers / inch. 2 The upstream bias roller 63a and downstream bias roller 63b are made of aluminum metal rollers. The upstream blade 64a and downstream blade 64b are made of plate-like members made of urethane rubber.

[0049] The upstream fur brush 62a and the downstream fur brush 62b are arranged to rub against the intermediate transfer belt 6 while maintaining a penetration amount of about 1.0 mm against the intermediate transfer belt 6. The upstream fur brush 62a and the downstream fur brush 62b are rotated at a peripheral speed of 50 mm / sec by a driving force transmitted from a driving motor provided in a driving unit 90 (FIG. 3) as a driving means. The upstream fur brush 62a and the downstream fur brush 62b rotate so as to move in a direction opposite to the moving direction of the intermediate transfer belt 6 at the contact portion with the intermediate transfer belt 6. The upstream bias roller 63a and the downstream bias roller 63b are arranged to maintain a penetration amount of about 1.0 mm against the upstream fur brush 62a and the downstream fur brush 62b. The upstream bias roller 63a and the downstream bias roller 63b are rotated at a peripheral speed equivalent to that of the upstream fur brush 62a and the downstream fur brush 62b by a driving force transmitted from a driving motor provided in a driving unit 90 (FIG. 3) as a driving means. The upstream bias roller 63a and the downstream bias roller 63b rotate so as to move in the same direction as the upstream fur brush 62a and the downstream fur brush 62b at the contact portion with the upstream fur brush 62a and the downstream fur brush 62b. The upstream blade 64a and the downstream blade 64b are arranged with an intrusion amount of about 1.0 mm with respect to the upstream bias roller 63a and the downstream bias roller 63b. The upstream blade 64a and the downstream blade 64b are abutted against the surfaces of the upstream bias roller 63a and the downstream bias roller 63b in the counter direction to the rotation direction of the upstream bias roller 63a and the downstream bias roller 63b so that the free end is located more upstream in the rotation direction of the upstream bias roller 63a and the downstream bias roller 63b than the fixed end.

[0050] A DC voltage of the same polarity (negative in this embodiment) as the normal charging polarity of the toner is applied as a cleaning bias to the upstream bias roller 63a by a first cleaning power source 66a as a first cleaning bias application means (first cleaning bias application section). A DC voltage of the opposite polarity (positive in this embodiment) to the normal charging polarity of the toner is applied as a cleaning bias to the downstream bias roller 63b by a second cleaning power source 66b as a second cleaning bias application means (second cleaning bias application section). The upstream opposing roller 24a and the downstream opposing roller 24b are each connected to a ground potential. The first and second cleaning power sources 66a and 66b apply cleaning biases to the upstream bias roller 63a and the downstream bias roller 63b under constant current control in order to correspond to the potentials of the intermediate transfer belt 6, the upstream fur brush 62a, and the downstream fur brush 62b under various conditions. In this embodiment, the current value of the cleaning bias is required to have a value suitable for the cleaning performance during normal image formation in which a small amount of thin toner (secondary transfer residual toner) is cleaned, and during adjustment operation (patch cleaning) in which a large amount of thick patches (test toner images for adjusting image density, etc.) are cleaned. The cleaning bias is applied under constant current control so that the respective currents flow in each of the above cases.

[0051] When cleaning the toner on the intermediate transfer belt 6, the upstream fur brush 62a, the downstream fur brush 62b, the upstream bias roller 63a, and the downstream bias roller 63b are each rotated. A negative cleaning bias is applied from the first cleaning power source 66a to the upstream fur brush 62a via the upstream bias roller 63a, and a positive cleaning bias is applied from the second cleaning power source 66b to the downstream fur brush 62b via the downstream bias roller 63b. As a result, the positively charged toner on the intermediate transfer belt 6 is physically scraped off by the upstream fur brush 62a and electrostatically attracted to be collected from the intermediate transfer belt 6. After moving onto the upstream bias roller 63a, the toner is scraped off from the upstream bias roller 63a by the upstream blade 64a and stored in the lower storage section in the housing 61. The negatively charged toner on the intermediate transfer belt 6 is physically scraped off by the downstream fur brush 62b and electrostatically attracted to be collected from the intermediate transfer belt 6. After moving onto the downstream bias roller 63b, the toner is scraped off from the downstream bias roller 63b by the downstream blade 64b and is stored in a storage section at the bottom of the housing 61. A recovered toner transport screw 65 is disposed in the storage section at the bottom of the housing 61 as a recovered toner transport member. The recovered toner transport screw 65 transports the toner recovered from the surface of the intermediate transfer belt 6 to one end side of the housing 61 in the longitudinal direction, and discharges it outside the belt cleaning device 60. The toner discharged from the belt cleaning device 60 is transported along a transport path provided in the apparatus main body 1a, and is collected in a collection container (not shown) disposed in the apparatus main body 1a.

[0052] 3. Control configuration FIG. 3 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 section (control circuit) 3 as a control means. The control section 3 is configured to have a CPU 31 as an arithmetic processing means, a ROM (including a rewritable one) 32 and a RAM 33 as storage means, an input / output circuit (I / F) (not shown) for inputting and outputting signals between the control section 3 and an external device, and the like. The ROM 32 stores programs and the like for controlling each section of the image forming apparatus 1. The RAM 33 temporarily stores data and the like related to control. 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.

[0053] 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), a driving unit 90, 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 driving unit 90 is configured to have a driving motor as a driving source and a drive transmission member. The drive unit 90 has a drive motor for driving the photosensitive drum 11, the developing sleeve 14a of the developing device 14, the intermediate transfer belt 6 (drive roller 21), the drum cleaning device 50, the belt cleaning device 60, and other components. The drive motors for driving these components may be provided independently, or a common drive motor may be used to drive at least some of these components. The drive motors for driving the components for each color may be provided independently, or a common drive motor may be used to drive at least some of the components for each color. In this embodiment, the photosensitive drum 11 and the developing sleeve 14a of the developing device 14 in each image forming unit 10 can rotate and stop independently.

[0054] Also connected to the control unit 3 are 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, voltage detection sensors 80a (80ay, 80am, 80ac, 80ak) and current detection sensors 80b (80by, 80bm, 80bc, 80bk) of the potential regulating power supply 80, and a fixing temperature sensor 77. Signals (information) indicating the detection results of each sensor are input to the control unit 3.

[0055] 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.

[0056] 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.

[0057] 4. 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, movement direction) of the intermediate transfer belt 6 unless otherwise specified.

[0058] 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.

[0059] 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.

[0060] 5. Electric 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.

[0061] The shape of the potential regulating member 8 in this embodiment will be described. Fig. 4(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. 4(b) is a perspective view of the potential regulating member 8 in this embodiment.

[0062] 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.

[0063] 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 B. 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].

[0064] A potential regulating power supply 80 serving as a potential regulating bias application means (potential regulating bias application section, electrode bias application section) is connected to the potential regulating power supply 80. A voltage detection sensor 80a (FIG. 3) serving as a voltage detection means (voltage detection section) for detecting the output voltage of the potential regulating power supply 80 and a current detection sensor 80b (FIG. 3) serving as a current detection means (current detection section) for detecting the output current of the potential regulating power supply 80 are connected to the potential regulating power supply 80. 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. The potential regulating bias will be described in further detail later.

[0065] 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.

[0066] 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.

[0067] FIG. 5 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. 5, 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. 4, 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.

[0068] 6 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. 6, 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. 6, the conductive nonwoven fabric 86 is provided on the contact surface 83 of the potential regulating member 8 configured as shown in FIG. 5, 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. 4. 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.

[0069] Next, the arrangement of the potential regulating member 8 in this embodiment will be described. Fig. 7 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. 7 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.

[0070] 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.

[0071] 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, in Figure 7, the straight line L is shown diagrammatically as being approximately horizontal, but 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, etc., the straight line L may be inclined so as to descend downward in the figure as it approaches the downstream side.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] As shown in FIG. 7, 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 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.

[0076] 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. 4(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 intrude toward the photosensitive drum 11 side from the straight line L. This allows the potential regulating member 8 to contact 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). 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, intrude toward 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.

[0077] 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.

[0078] 7, 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.

[0079] 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.

[0080] In addition, in this embodiment, a configuration in which the intermediate transfer belt 6 and the potential regulating member 8 are in contact with each other has been described, but a configuration in which the intermediate transfer belt 6 and the potential regulating member 8 are not in contact with each other and the same effect can be obtained.

[0081] 6. Potential Regulation Bias Next, the potential regulating bias applied to the potential regulating member 8 in this embodiment will be further described.

[0082] 6-1. During image formation First, the setting of the potential regulating bias during normal image formation in this embodiment will be further described.

[0083] During image formation (primary transfer), a predetermined potential regulating 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 source 80. The primary transfer is, more specifically, a period during which the primary transfer bias is applied, and more specifically, a period during which an image area (an area to which a toner image can be transferred) on the intermediate transfer belt 6 passes through the primary transfer portion N1. This makes it possible to suppress 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 addition, 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]. In this embodiment, during image formation (primary transfer), a potential regulating bias of -3000 [V] is applied to the potential regulating member 8 by the potential regulating power source 80.

[0084] FIG. 8(a) is a sequence chart showing the operation of each part during the image forming operation in this embodiment. In FIG. 8(a), the charging bias and the developing bias are shown as DC biases, but in this embodiment, the charging bias and the developing bias are turned on and off in synchronization with the DC bias and the AC bias. This is the same for FIG. 8(b), FIG. 11(a), (b), and FIG. 12, which will be described later. The operation of each part during the image forming operation according to the sequence chart shown in FIG. 8(a) is controlled and executed by the control unit 3. Also, FIG. 9(a) is a schematic diagram for explaining the relationship between the surface potential of the photosensitive drum 11, the primary transfer bias, and the potential regulating bias during the image forming operation in this embodiment.

[0085] As shown in FIG. 8(a), during normal image formation, the photosensitive drum 11 and the intermediate transfer belt 6 are rotated, and a charging bias is applied to the charging roller 12 by the charging power source 73 so that the surface potential (dark potential Vd) of the photosensitive drum 11 becomes −900 [V]. Then, the exposure device 13 irradiates (exposes) the image portion on the surface of the photosensitive drum 11 with laser light, and the potential of the image portion on the surface of the photosensitive drum 11 is lowered (an electrostatic image is formed). Also, each member such as the developing sleeve 14a of the developing device 14 is driven, and a developing bias is applied to the developing sleeve 14a by the developing power source 74. Then, due to the potential difference between the potential of the image portion on the surface of the photosensitive drum 11 and the developing bias (developing DC bias), toner adheres to the image portion on the surface of the photosensitive drum 11, forming a toner image. Also, a primary transfer power source 75 applies a primary transfer bias of, for example, +1000 [V], which is the polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment), to the primary transfer roller 15. Then, the toner image is transferred from the surface of the photosensitive drum 11 to the surface of the intermediate transfer belt 6 due to the potential difference between the potential of the image portion on the surface of the photosensitive drum 11 and the primary transfer bias.

[0086] In this embodiment, substantially simultaneously with the application of the primary transfer bias to the primary transfer roller 15, a potential regulating bias of −3000 [V], which is the same polarity as the charge polarity of the photosensitive drum 11 (negative polarity in this embodiment), is applied to the potential regulating member 8 by the potential regulating power source 80. As a result, due to the action of the electric field formed between the photosensitive drum 11 and the potential regulating member 8, the potential difference between the photosensitive drum 11 and the intermediate transfer belt 6 downstream of the primary transfer portion N1 becomes small, and discharge between the intermediate transfer belt 6 and the photosensitive drum 11 is suppressed. At this time, as shown in FIG. 9(a), the potential of the intermediate transfer belt 6, including the charge of the toner on the intermediate transfer belt 6, becomes approximately 0 [V]. Then, due to the relationship of the potential difference between the surface potential of the photosensitive drum 11 and the intermediate transfer belt 6 (potential regulating member 8) at this time, discharge between the intermediate transfer belt 6 and the photosensitive drum 11 is suppressed. In addition, due to the potential difference between the surface potential of the photosensitive drum 11 and the intermediate transfer belt 6 (potential regulating member 8) at this time, an electric field is formed in the direction from the intermediate transfer belt 6 to the photosensitive drum 11, which returns the scattered toner transferred to the intermediate transfer belt 6 back onto the photosensitive drum 11, thereby achieving the effect of straightening the image on the intermediate transfer belt 6.

[0087] In this embodiment, as shown in Fig. 8(a), the application of the charging bias and the application of the primary transfer bias are started at the same position in the circumferential direction of the photosensitive drum 11, so the application start timing of the charging bias and the application start timing of the primary transfer bias are shifted. However, the application start timing of each bias may be simultaneous. Also, although the start timing has been described, the same applies to the end timing, and the application of each bias may be ended simultaneously.

[0088] Furthermore, when the photosensitive drum 11 is rotating, a potential may be generated on the surface of the photosensitive drum 11 due to frictional charging, even if no bias is applied to the member in contact with the surface of the photosensitive drum 11. Such a potential may be unpredictable or may cause unevenness in the direction of the rotation axis of the photosensitive drum 11, depending on the state of the member in contact with the surface of the photosensitive drum 11. Therefore, in this embodiment, as shown in Fig. 8(a), exposure (pre-exposure) of the surface of the photosensitive drum 11 by the pre-exposure device 16 is always ON when the photosensitive drum 11 is rotating.

[0089] Here, the surface potential (Vd) of the photosensitive drum 11 is represented by the potential between the charging portion by the charging roller 12 and the primary transfer portion N1 in the rotation direction of the photosensitive drum 11, typically the potential immediately after charging by the charging roller 12. The charging roller 12 charges the surface of the photosensitive drum 11 by discharging in a minute gap formed between the charging roller 12 and the photosensitive drum 11, but for simplicity, the contact portion between the charging roller 12 and the photosensitive drum 11 may be considered as the charging portion.

[0090] 6-2.Recovery mode Next, the setting of the potential regulating bias in the recovery mode in this embodiment will be further described.

[0091] When a transport problem occurs with the recording material S during image forming operation or some other error occurs in the image forming apparatus 1, the image forming apparatus 1 may stop with a large amount of thick toner on the photosensitive drum 11 or the intermediate transfer belt 6. This toner can cause the recording material S to become dirty in the subsequent image forming operation, so it is necessary to remove it from the photosensitive drum 11 and the intermediate transfer belt 6.

[0092] Therefore, in this embodiment, after the recording material S that has caused the transport failure is removed and the error is resolved, and the image forming apparatus 1 is restarted, the image forming apparatus 1 executes a recovery mode (recovery operation) for cleaning the toner remaining on the photosensitive drum 11 and the intermediate transfer belt 6. At this time, it may be difficult for the belt cleaning device 60 to sufficiently collect a large amount of thick toner on the intermediate transfer belt 6 in one go. This is particularly noticeable when the belt cleaning device 60 is of an electrostatic cleaning type as in this embodiment.

[0093] Therefore, in the recovery mode, a bias of a different polarity from that during normal image formation is applied to the primary transfer roller 15 (primary transfer portion N1), or the bias applied to the primary transfer roller 15 (primary transfer portion N1) is turned off. As a result, the toner remaining on the intermediate transfer belt 6 is returned onto the photosensitive drum 11, or the toner remaining on the photosensitive drum 11 is left on the photosensitive drum 11 without being transferred onto the intermediate transfer belt 6. Then, this toner is collected by the drum cleaning device 50.

[0094] Particularly in this embodiment, in the recovery mode, a bias of a different polarity from that during normal image formation is applied to the primary transfer roller 15. As a result, the toner on the photosensitive drum 11 is not transferred from the photosensitive drum 11 to the intermediate transfer belt 6, and is collected by the drum cleaning device 50. In addition, the toner that cannot be collected in one go by the belt cleaning device 60 and remains on the intermediate transfer belt is returned to the photosensitive drum 11 and collected by the drum cleaning device 50. As a result, the time required for the recovery mode can be shortened, and the next image forming operation can be performed promptly.

[0095] Fig. 8(b) is a sequence chart showing the operation of each part in the return mode in this embodiment. The operation of each part in the return mode according to the sequence chart shown in Fig. 8(b) is controlled and executed by the control unit 3. Fig. 9(b) is a schematic diagram for explaining the relationship between the surface potential of the photosensitive drum 11, the primary transfer bias, and the potential regulating bias in the return mode in this embodiment.

[0096] As shown in FIG. 8(b), in the return mode, the photosensitive drum 11 and the intermediate transfer belt 6 are rotated. However, since there is no need to form an image, the drive of each member of the developing device 14, such as the developing sleeve 14a, is stopped, and the charging power source 73 does not apply a charging bias to the charging roller 12, and the developing power source 74 does not apply a developing bias to the developing sleeve 14a. That is, in this embodiment, in the return mode, the charging power source 73 and the developing power source 74 are turned off so that no potential is applied to the surface of the photosensitive drum 11. In addition, a primary transfer bias (reverse bias) in the return mode is applied to the primary transfer roller 15 by the primary transfer power source 75, which has a polarity opposite to that during normal image formation, that is, the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment). The primary transfer bias (reverse bias) in the return mode may be a polarity opposite to that during normal image formation and have the same absolute value, or may be a bias with an absolute value larger or smaller than that during normal image formation as long as it is sufficient to perform the action in the return mode as described above. In this embodiment, the primary transfer bias in the return mode is set so that a current of approximately -10 [μA] flows through the primary transfer portion N1. Also, in this embodiment, in the return mode, as in normal image formation, exposure (pre-exposure) of the surface of the photosensitive drum 11 by the pre-exposure device 16 is turned ON. Note that in the return mode, the exposure device 13 does not irradiate the surface of the photosensitive drum 11 with laser light. Due to this operation, in the return mode, the surface potential (Vd) of the photosensitive drum 11 becomes approximately 0 [V].

[0097] Here, FIG. 12 is a sequence chart showing the operation of each part in the recovery mode in the comparative example. Also, FIG. 13 is a schematic diagram for explaining the relationship between the surface potential of the photosensitive drum 11, the primary transfer bias, and the potential regulating bias in the recovery mode in the comparative example. As shown in FIG. 12, a case where a potential regulating bias is applied to the potential regulating member 8 in the recovery mode in the same manner as in normal image formation is considered. In this case, as shown in FIG. 13, the surface potential (Vd) of the photosensitive drum 11 is approximately 0 [V], whereas the potential of the intermediate transfer belt 6 becomes a relatively high negative potential (strong negative). Therefore, the direction of the electric field between the photosensitive drum 11 and the intermediate transfer belt 6 becomes opposite to that during image formation, and toner may move from the photosensitive drum 11 toward the intermediate transfer belt 6. Also, the potential difference between the photosensitive drum 11 and the intermediate transfer belt 6 becomes large, and discharge occurs from the photosensitive drum 11 toward the intermediate transfer belt 6, and toner on the intermediate transfer belt 6 may scatter. Then, these toners adhere to the secondary transfer outer roller 25 or soil the interior of the image forming apparatus 1, which can cause toner stains on the recording material S, such as back stains in which the back surface of the recording material S is stained with toner, during subsequent image forming operations.

[0098] Therefore, in this embodiment, as shown in FIG. 8(b), in the return mode, the potential regulating bias is not applied to the potential regulating member 8 by the potential regulating power supply 80. That is, in this embodiment, in the return mode, the potential regulating power supply 80 is turned off. As a result, as shown in FIG. 9(b), while the surface potential (Vd) of the photosensitive drum 11 is approximately 0 [V], the potential of the intermediate transfer belt 6 becomes approximately 0 [V] or a relatively low negative potential (weak negative), and the potential difference between the photosensitive drum 11 and the intermediate transfer belt 6 becomes small. Thus, in this embodiment, when the surface potential of the photosensitive drum 11 is set to approximately 0 [V] in the return mode, the potential difference between the photosensitive drum 11 and the intermediate transfer belt 6 becomes approximately 0 [V]. Therefore, it is possible to suppress the movement of toner from the photosensitive drum 11 toward the intermediate transfer belt 6. In addition, it is possible to suppress the occurrence of discharge from the photosensitive drum 11 toward the intermediate transfer belt 6. Therefore, it is possible to suppress the contamination of the secondary transfer outer roller 25 and the interior of the image forming apparatus 1 with toner. As a result, it is possible to suppress contamination of the recording material S with toner, such as back contamination, during the subsequent image forming operation.

[0099] In the return mode, instead of turning off the potential regulating power supply 80 (the potential regulating member 8 is electrically grounded), the potential regulating power supply 80 may apply a potential regulating bias of approximately 0 [V] to the potential regulating member 8 (constant voltage control with the target voltage being approximately 0 [V]). This also applies to the charging bias (charging DC bias) and the developing bias (developing DC bias) in the return mode. That is, in the return mode, instead of turning off the charging power supply 73 (the charging roller 12 is electrically grounded), the charging power supply 73 may apply a charging bias (charging DC bias) of approximately 0 [V] to the charging roller 12 (constant voltage control with the target voltage being approximately 0 [V]) (in this embodiment, the charging AC bias is also applied at this time). Also, in the recovery mode, instead of turning off the development power supply 74 (the development sleeve 14a is electrically grounded), the development power supply 74 may apply a development bias (development DC bias) of approximately 0 [V] to the development sleeve 14a (constant voltage control with the target voltage being approximately 0 [V]) (in this embodiment, the development AC bias is also applied at this time). Furthermore, this also applies to the primary transfer bias when the primary transfer bias is turned off in the recovery mode. That is, in the recovery mode, instead of turning off the primary transfer power supply 75 (the primary transfer roller 15 is electrically grounded), the primary transfer power supply 75 may apply a primary transfer bias of approximately 0 [V] to the primary transfer roller 15 (constant voltage control with the target voltage being approximately 0 [V]).

[0100] In addition, in this embodiment, in the return mode, the charging DC bias and the charging AC bias are not applied to the charging roller 12, but only the charging AC bias of the charging DC bias and the charging AC bias may be applied. Similarly, in this embodiment, in the return mode, the developing DC bias and the developing AC bias are not applied to the developing sleeve 14a, but only the developing AC bias of the developing DC bias and the developing AC bias may be applied. In this case, instead of turning off the DC bias, a bias of approximately 0 [V] may be applied.

[0101] 10 is a flow chart showing an example of a procedure of switching control of the potential regulating bias in this embodiment. When applying a primary transfer bias to the primary transfer roller 15 by the primary transfer power supply 75 (S1), the control unit 3 judges whether the primary transfer bias is a bias of the same polarity as that during normal image formation (S2). If the control unit 3 judges in S2 that the primary transfer bias is the same polarity as that during normal image formation, it controls the potential regulating power supply 80 to apply a potential regulating bias of -3000 [V] to the potential regulating member 8. If the control unit 3 judges in S2 that the bias is a bias of a different polarity from that during normal image formation, it controls the potential regulating power supply 80 to be turned off (or to apply a potential regulating bias of approximately 0 [V] to the potential regulating member 8 by the potential regulating power supply 80) (S4).

[0102] In order to control the potential regulating bias in the return mode, in S2, instead of judging whether the primary transfer bias has the same polarity as that during normal image formation, it may be judged whether the charging bias (charging DC bias) is lower than that during image formation (OFF in this embodiment). In other words, it may be judged whether the surface potential of the photosensitive drum 11 is lower than that during image formation (approximately 0 [V] in this embodiment). Then, if the charging bias is set to the setting during image formation, the process proceeds to S3, and if the charging bias is lower than that during image formation, the process proceeds to S4.

[0103] 7. Variations Next, a modification of this embodiment will be described.

[0104] In the recovery mode, a bias (weak negative bias) of a negative polarity (same polarity as the charging polarity of the photosensitive drum 11) lower than that during image formation may be applied to the charging roller 12 by the charging power source 73 as a charging bias (charging DC bias) so that the surface potential (Vd) of the photosensitive drum 11 is lower than that during normal image formation (FIG. 9(b)). FIG. 11(a) is a sequence chart showing the operation of each part during the recovery mode in this case. In this case, too, as shown in FIG. 9(b), by turning off the potential regulating bias (or 0 [V]), the potential difference between the photosensitive drum 11 and the intermediate transfer belt 6 becomes small, and the direction of the electric field between the photosensitive drum 11 and the intermediate transfer belt 6 becomes the same as that during normal image formation. Therefore, the movement of toner from the photosensitive drum 11 toward the intermediate transfer belt 6 and the occurrence of discharge from the photosensitive drum 11 toward the intermediate transfer belt 6 are suppressed, and it is possible to suppress the secondary transfer outer roller 25 and the interior of the image forming apparatus 1 from being soiled with toner. As a result, it is possible to suppress contamination of the recording material S by toner, such as back contamination, during the subsequent image forming operation. The charging bias (charging DC bias) during the recovery mode can be appropriately set from the viewpoint of sufficiently suppressing the movement of toner from the photosensitive drum 11 to the intermediate transfer belt 6 during the recovery mode and sufficiently promoting the movement of toner from the intermediate transfer belt 6 to the photosensitive drum 11. The charging bias (charging DC bias) during the recovery mode may be set so that the surface potential of the photosensitive drum 11 during the recovery mode is the opposite polarity to the normal charging polarity of the toner with respect to the primary transfer bias applied to the primary transfer roller 15. For example, the charging bias (charging DC bias) during the recovery mode is preferably set in a range in which the surface potential of the photosensitive drum 11 is 0 to -300 [V], and more preferably set in a range in which the surface potential of the photosensitive drum 11 is 0 to -100 [V].

[0105] In the return mode, the potential regulating power source 80 may apply a bias (weak negative bias) of a negative polarity (same polarity as the charging polarity of the photosensitive drum 11) lower than that during image formation as a potential regulating bias to the potential regulating member 8. FIG. 11(b) is a sequence chart showing the operation of each part during the return mode in this case. Note that FIG. 11(b) shows a sequence chart in the case where a weak negative bias is applied to the charging roller 12 in the return mode as in the case of FIG. 11(a), but the application of the bias to the charging roller 12 may be turned off as in the case of FIG. 8(b). That is, in the return mode, a weak negative bias can be applied to at least one of the charging roller 12 and the potential regulating member 8. In this case, the potential difference between the photosensitive drum 11 and the intermediate transfer belt 6 is also reduced, and the direction of the electric field between the photosensitive drum 11 and the intermediate transfer belt 6 becomes the same as that during normal image formation. Therefore, the movement of the toner from the photosensitive drum 11 toward the intermediate transfer belt 6 and the occurrence of discharge from the photosensitive drum 11 toward the intermediate transfer belt 6 are suppressed, and the contamination of the secondary transfer outer roller 25 and the inside of the image forming apparatus 1 with the toner can be suppressed. As a result, the contamination of the recording material S with the toner, such as back contamination, during the subsequent image forming operation can be suppressed. The potential regulating bias in the return mode can be appropriately set from the viewpoint of sufficiently suppressing the movement of the toner from the photosensitive drum 11 to the intermediate transfer belt 6 and the scattering of the toner on the intermediate transfer belt 6 during the return mode. The potential regulating bias in the return mode may be set so that the absolute value of the potential difference between the bias and the surface potential (Vd) of the photosensitive drum 11 in the return mode is smaller than the absolute value of the potential difference between the bias and the surface potential (Vd) of the photosensitive drum 11 during image formation. It is preferable that the potential regulating bias in the return mode is approximately the same potential as the surface potential (Vd) of the photosensitive drum 11 in the return mode or is a potential on the normal charging polarity side of the toner with respect to the surface potential (Vd) of the photosensitive drum 11 in the return mode. For example, the potential regulating bias in the recovery mode is preferably about 0 to −300 [V], and more preferably about 0 to −100 [V].

[0106] 8. Effects Thus, in this embodiment, the image forming apparatus 1 includes a rotatable photoconductor 11 that can be charged to a predetermined polarity, a charging device 12 that charges the surface of the photoconductor 11 by applying a charging bias, a developing member 14a that supplies a developer to the surface of the photoconductor 11 after it has been charged by the charging device 12 to form a developer image on the surface of the photoconductor 11, a circumferentially movable intermediate transfer belt 6 that contacts the photoconductor 11 to form a primary transfer portion N1 and transports the developer image that has been primarily transferred from the photoconductor 11 at the primary transfer portion N1 to a recording material S at the secondary transfer portion N2, a primary transfer member 15 that is disposed on the inner circumferential surface side of the intermediate transfer belt 6 in correspondence with the photoconductor 11 and that is applied with a primary transfer bias of a polarity opposite to the above-mentioned predetermined polarity to perform primary transfer of the developer image from the photoconductor 11 to the intermediate transfer belt 6, and an electrode member (potential regulating member) that is disposed on the inner circumferential surface side of the intermediate transfer belt 6 at a position adjacent to the downstream side of the primary transfer portion N1 in the movement direction of the intermediate transfer belt 6. 8, a first cleaning means (drum cleaning device) 50 for cleaning the surface of the photoconductor 11, a second cleaning means (belt cleaning device) 60 for cleaning the surface of the intermediate transfer belt 6, a charging bias application unit 73 for applying a charging bias to the charging device 12, a primary transfer bias application unit 75 for applying a primary transfer bias to the primary transfer member 15, an electrode bias application unit (electrical regulation power source) 80 for applying an electrode bias (potential regulating bias) of the same polarity as the above-mentioned predetermined polarity to the electrode member 8, and a control unit 3 capable of controlling the charging bias application unit 73, the primary transfer bias application unit 75, and the electrode bias application unit 80, and is capable of performing an image forming operation for forming an image consisting of a developer image on the recording material S, and a return operation (return mode) for cleaning the surface of the photoconductor 11 and the surface of the intermediate transfer belt 6 before starting the next image forming operation when the photoconductor 11 and the intermediate transfer belt 6 stop during the image forming operation. In this embodiment, in the return operation, the control unit 3 controls the charging device 12 so that either no charging bias is applied to the charging device 12 or a charging bias is applied to the charging device 12 such that the surface potential of the photosensitive member 11 has a smaller absolute value than that during the image forming operation, and the control unit 3 controls the electrode member 8 so that either no electrode bias is applied to the electrode member 8 or an electrode bias is applied to the electrode member 8 such that the absolute value is smaller than that when the primary transfer bias is applied to the primary transfer member 15 during the image forming operation.

[0107] Here, the control unit 3 can control the charging device 12 not to apply a charging bias in the return operation, or to apply a charging bias controlled so that the voltage value is approximately 0 [V] or the current value is approximately 0 [μA] to the charging device 12 as described later. Also, the control unit 3 can control the electrode member 8 not to apply an electrode bias in the return operation, or to apply an electrode bias controlled so that the voltage value is approximately 0 [V] or the current value is approximately 0 [μA] to the electrode member 8 as described later. In this embodiment, the control unit 3 controls the primary transfer member 15 to apply a primary transfer bias of the same polarity as the above-mentioned predetermined polarity in the return operation. Also, the control unit 3 can control the primary transfer member 15 not to apply a primary transfer bias in the return operation, or to apply a primary transfer bias controlled so that the voltage value is approximately 0 [V] or the current value is approximately 0 [μA] to the primary transfer member 15 as described later. In this embodiment, the primary transfer member 15 is arranged to contact the inner circumferential surface of the intermediate transfer belt 6. In this embodiment, the electrode member 8 is disposed so as to come into contact with the inner circumferential surface of the intermediate transfer belt 6 at a position adjacent to and downstream of the primary transfer portion N1 in the movement direction of the intermediate transfer belt 6.

[0108] As described above, according to this embodiment, in the recovery mode, the movement of toner from the photosensitive drum 11 to the intermediate transfer belt 6 and the scattering of toner on the intermediate transfer belt 6 can be suppressed, thereby suppressing the occurrence of toner contamination of the recording material S.

[0109] [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.

[0110] 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.

[0111] In the above embodiment, the potential regulating bias is constant voltage controlled, but the potential regulating bias may be constant current controlled. Even if the potential regulating bias is constant current controlled, discharge between the intermediate transfer belt and the photosensitive drum downstream of the primary transfer unit can be suppressed. Similarly, in the above embodiment, the primary transfer bias and the secondary transfer bias are each constant voltage controlled, but at least one of them may be constant current controlled. Here, the constant current control is a control that adjusts the output of the power source so that the current supplied to the supply object is approximately constant at the target current. In addition, the constant voltage control is a control that adjusts the output of the power source so that the voltage applied to the application object is approximately constant at the target voltage. When the bias is constant current controlled, instead of turning off the power source, the target current can be set to approximately 0 [μA] and constant current controlled.

[0112] 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.

[0113] 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.

[0114] 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]

[0115] 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) 15 Primary transfer roller (primary transfer member) 25 Secondary transfer roller (secondary transfer member) 80 Potential Regulating Power Supply

Claims

1. a rotatable photoconductor capable of being charged to a predetermined polarity; a charging device to which a charging bias is applied to charge the surface of the photoconductor; a developing member that supplies a developer to the surface of the photoconductor that has been charged by the charging device, thereby forming a developer image on 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 developer image that has been primarily transferred from the photoconductor at the primary transfer section so that the developer image can be secondarily transferred to a recording material at a secondary transfer section; a primary transfer member that is disposed on an inner peripheral surface side of the intermediate transfer belt in correspondence with the photoconductor, and that primarily transfers a developer image from the photoconductor to the intermediate transfer belt by applying a primary transfer bias having a polarity opposite to the predetermined polarity; an electrode member disposed on an inner circumferential surface side of the intermediate transfer belt at a position adjacent to the primary transfer portion on a downstream side in a moving direction of the intermediate transfer belt; a first cleaning means for cleaning the surface of the photoreceptor; a second cleaning means for cleaning the surface of the intermediate transfer belt; a charging bias applying unit that applies a charging bias to the charging device; a primary transfer bias applying section that applies a primary transfer bias to the primary transfer member; 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 charging bias application unit, the primary transfer bias application unit, and the electrode bias application unit; having An image forming apparatus capable of performing an image forming operation for forming an image consisting of a developer image on a recording material, and a return operation for cleaning the surface of the photoconductor and the surface of the intermediate transfer belt before starting a next image forming operation when the photoconductor and the intermediate transfer belt stop during the image forming operation, The control unit controls, during the return operation, either not to apply a charging bias to the charging device or to apply a charging bias to the charging device that makes the surface potential of the photosensitive body a surface potential with an absolute value smaller than that during the image forming operation, and controls either not to apply an electrode bias to the electrode member or to apply an electrode bias to the electrode member with an absolute value smaller than that when the primary transfer bias is applied to the primary transfer member during the image forming operation.

2. The image forming apparatus according to claim 1, characterized in that, during the recovery operation, the control unit controls the charging device so that no charging bias is applied to the charging device, or so that a charging bias controlled so that a voltage value is approximately 0 [V] or a current value is approximately 0 [μA] is applied to the charging device.

3. The image forming apparatus according to claim 1, characterized in that, during the return operation, the control unit controls so that no electrode bias is applied to the electrode member, or so that an electrode bias controlled so that a voltage value is approximately 0 [V] or a current value is approximately 0 [μA] is applied to the electrode member.

4. 2. The image forming apparatus according to claim 1, wherein the control unit performs control in the return operation so as to apply a primary transfer bias having the same polarity as the predetermined polarity to the primary transfer member.

5. 2. The image forming apparatus according to claim 1, wherein the control unit controls the primary transfer member during the return operation so that no primary transfer bias is applied to the primary transfer member, or so that a primary transfer bias controlled so that a voltage value is approximately 0 [V] or a current value is approximately 0 [μA] is applied to the primary transfer member.

6. 6. The image forming apparatus according to claim 1, wherein the primary transfer member is disposed so as to be in contact with an inner circumferential surface of the intermediate transfer belt.

7. 6. An image forming apparatus according to claim 1, wherein the electrode member is arranged so as to contact an inner surface of the intermediate transfer belt at a position adjacent to the primary transfer portion on the downstream side in the direction of movement of the intermediate transfer belt.

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