Image forming apparatus

Abstract

To obtain good secondary transferability of both a toner primarily transferred to an intermediate transfer belt on an upstream side and a toner primarily transferred to the intermediate transfer belt on a downstream side, in a configuration using electric potential regulation members.SOLUTION: An image forming apparatus 1 has: first and second photoreceptors 11c, 11k that can be charged to have a predetermined polarity; an intermediate transfer belt 6; first and second primary transfer members 15c, 15k; first and second electrode members 8c, 8k; application units 80c, 80k that apply a bias having a same polarity as the predetermined polarity to the first and second electrode members 8c, 8k; and a control unit 3. The first photoreceptor 11c is arranged on an upstream side of the second photoreceptor 11k in a direction of movement of the intermediate transfer belt 6. The control unit 3 controls the application units 80c, 80k so that an absolute value of a bias applied to the second electrode member 8k becomes smaller than the absolute value of the bias applied to the first electrode member 8c during image formation.SELECTED DRAWING: Figure 7

Description

[Technical field]

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

[0002] As image forming apparatuses 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, as an image forming apparatus using the intermediate transfer method, there is a tandem type having a plurality of photosensitive drums (drum-shaped photosensitive bodies) and an intermediate transfer belt (an endless belt-shaped intermediate transfer body). In such an image forming apparatus, the toner images formed on the plurality of photosensitive drums are electrostatically primarily transferred onto the intermediate transfer belt in sequence at the 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 the secondary transfer section. In addition, with regard to the arrangement of members around the primary transfer section, unless otherwise specified, the upstream and downstream refer to the upstream and downstream in the conveying direction of the intermediate transfer belt. In addition, for convenience, the magnitude (high and low) of voltage and potential refers to the magnitude (high and low) when compared in absolute value unless otherwise specified.

[0003] In such an image forming apparatus, 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. The inventors of the present invention have found that the increase in the charge amount of the toner on the intermediate transfer belt increases the mirror force between the intermediate transfer belt and the photosensitive drum, 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.

[0004] 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]

[0005] [Patent Document 1] JP 2003-57963 A Summary of the Invention [Problem to be solved by the invention]

[0006] In order to suppress the increase in the amount of charge of the toner downstream of the primary transfer portion as described above, it is effective to suppress discharge downstream of the primary transfer portion.

[0007] 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.

[0008] In order to effectively suppress discharge downstream of the primary transfer section and improve secondary transfer performance, it is considered desirable to move the potential regulating member closer to the primary transfer section and to increase the bias of the same polarity as the charging polarity of the photosensitive drum.

[0009] However, the inventors of the present invention have further studied and found the following. In other words, by using a potential regulating member, it is possible to ensure the secondary transfer efficiency of a chromatic toner image such as a two-color solid (e.g., magenta + cyan) toner image with a relatively low secondary transfer electric field strength. However, it has been found that the secondary transfer efficiency of a black toner image may decrease when a potential regulating member is used. According to the inventors' studies, this is believed to be due to the arrangement of the image forming unit that forms the black toner image or the characteristics of the black toner.

[0010] Therefore, an object of the present invention is to obtain good secondary transfer properties for both toner that is primarily transferred to an intermediate transfer belt on the upstream side and toner that is primarily transferred to an intermediate transfer belt on the downstream side in a configuration in which a bias can be applied to an electrode member arranged downstream of a primary transfer section.

[0011] Another object of the present invention is to obtain good secondary transferability of both non-black toner and black toner in a configuration capable of applying a bias to an electrode member arranged downstream of a primary transfer unit. [Means for solving the problem]

[0012] The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention includes first and second photoconductors that can be charged to a predetermined polarity and that carry a toner image, a circumferentially movable intermediate transfer belt that contacts the first and second photoconductors to form first and second primary transfer sections, respectively, and transports the toner images that have been primarily transferred from the first and second photoconductors at the first and second primary transfer sections to a recording material at a secondary transfer section, first and second primary transfer members that contact the inner circumferential surface of the intermediate transfer belt and are applied with a primary transfer bias of a polarity opposite to the predetermined polarity to primarily transfer the toner images from the first and second photoconductors to the intermediate transfer belt, respectively, and a first and second primary transfer member that contacts the inner circumferential surface of the intermediate transfer belt and is applied with a primary transfer bias of a polarity opposite to the predetermined polarity to primarily transfer the toner images from the first and second photoconductors to the intermediate transfer belt, respectively, and a first and second primary transfer section in the moving direction of the intermediate transfer belt. The image forming apparatus comprises first and second electrode members contacting the inner surface of the intermediate transfer belt at adjacent positions downstream of the first and second electrode members, an application unit applying a bias of the same polarity as the predetermined polarity to the first and second electrode members, and a control unit capable of controlling the application unit, wherein the first photoconductor is disposed upstream of the second photoconductor in the movement direction of the intermediate transfer belt, and the control unit controls the application unit so that the absolute value of the bias applied to the second electrode member is smaller than the absolute value of the bias applied to the first electrode member when performing image formation in which a toner image formed on the first and second photoconductors is transferred to a recording material to form an image on the recording material.

[0013] According to another aspect of the present invention, there is provided a recording medium comprising: first and second photoconductors capable of being charged to a predetermined polarity and carrying a toner image; an intermediate transfer belt which contacts the first and second photoconductors to form first and second primary transfer sections, respectively, and which conveys the toner images primarily transferred from the first and second photoconductors at the first and second primary transfer sections so as to be secondarily transferred to a recording material at a secondary transfer section; first and second primary transfer members which contact an inner circumferential surface of the intermediate transfer belt and are applied with a primary transfer bias having a polarity opposite to the predetermined polarity, thereby primarily transferring the toner images from the first and second photoconductors to the intermediate transfer belt, respectively; and first transfer members which contact the inner circumferential surface of the intermediate transfer belt at adjacent positions downstream of the first and second primary transfer sections in the moving direction of the intermediate transfer belt. and a second electrode member; an application unit that applies a bias of the same polarity as the predetermined polarity to the first and second electrode members; and a control unit capable of controlling the application unit, wherein the first photoconductor is disposed upstream of the second photoconductor in the moving direction of the intermediate transfer belt, a toner image is formed on the first photoconductor with black toner, and a toner image is formed on the second photoconductor with toner of a color other than black, and the control unit controls the application unit so that the absolute value of the bias applied to the first electrode member is smaller than the absolute value of the bias applied to the second electrode member when performing image formation in which the toner images formed on the first and second photoconductors are transferred to a recording material to form an image on the recording material.

[0014] According to another aspect of the present invention, there is provided a recording medium storing a toner image on a recording medium, the recording medium including: first and second photoconductors that can be charged to a predetermined polarity and that carry a toner image; an intermediate transfer belt that contacts the first and second photoconductors to form first and second primary transfer sections, respectively, and that conveys the toner images that have been primarily transferred from the first and second photoconductors at the first and second primary transfer sections, respectively, to a recording material in a secondary transfer section, and first and second primary transfer members that contact an inner circumferential surface of the intermediate transfer belt and are applied with a primary transfer bias having a polarity opposite to the predetermined polarity, thereby primarily transferring the toner images from the first and second photoconductors to the intermediate transfer belt, respectively; an electrode member that contacts the inner surface of the intermediate transfer belt at an adjacent position downstream of one of the first and second primary transfer units, and an application unit that applies a bias of the same polarity as the predetermined polarity to the electrode member, wherein the first photoconductor is disposed upstream of the second photoconductor in the movement direction of the intermediate transfer belt, the electrode member is provided at an adjacent position downstream of the first primary transfer unit in the movement direction of the intermediate transfer belt, and the electrode member is not provided at an adjacent position downstream of the second primary transfer unit in the movement direction of the intermediate transfer belt.

[0015] According to another aspect of the present invention, there is provided a recording medium storing a toner image on a recording medium, the recording medium including: first and second photoconductors that can be charged to a predetermined polarity and that carry a toner image; an intermediate transfer belt that contacts the first and second photoconductors to form first and second primary transfer sections, respectively, and that conveys the toner images that have been primarily transferred from the first and second photoconductors at the first and second primary transfer sections so that they can be secondarily transferred to a recording material at a secondary transfer section; first and second primary transfer members that contact an inner circumferential surface of the intermediate transfer belt and are applied with a primary transfer bias having a polarity opposite to the predetermined polarity, thereby primarily transferring the toner images from the first and second photoconductors to the intermediate transfer belt; and a first transfer bias bias that is adjacent to the first and second primary transfer sections in the moving direction of the intermediate transfer belt and that is disposed downstream of the first and second primary transfer sections. and an application unit that applies a bias of the same polarity as the predetermined polarity to the electrode member, wherein the first photoconductor is disposed upstream of the second photoconductor in the moving direction of the intermediate transfer belt, a toner image is formed on the first photoconductor with black toner, and a toner image is formed on the second photoconductor with toner of a color other than black, the electrode member is not provided at an adjacent position downstream of the first primary transfer unit in the moving direction of the intermediate transfer belt, and the electrode member is provided at an adjacent position downstream of the second primary transfer unit in the moving direction of the intermediate transfer belt. Effect of the Invention

[0016] According to the present invention, in a configuration using a potential regulating member, it is possible to obtain good secondary transferability of both the toner that is primarily transferred to the intermediate transfer belt on the upstream side and the toner that is primarily transferred to the intermediate transfer belt on the downstream side. Also, according to the present invention, in a configuration using a potential regulating member, it is possible to obtain good secondary transferability of both the toner of a color other than black and the black toner, for example, when the toner that is primarily transferred on the downstream side is black toner, although this is not limited thereto. [Brief description of the drawings]

[0017] [Figure 1]FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Diagram 2] FIG. 2 is a schematic block diagram of a control system of the image forming apparatus. [Diagram 3] 3A and 3B are a cross-sectional view and a perspective view of a potential restricting member; [Figure 4] FIG. 11 is a cross-sectional view of another example of a potential restricting member. [Diagram 5] FIG. 11 is a cross-sectional view of another example of a potential restricting member. [Figure 6] 4 is a cross-sectional view for explaining the arrangement of a potential restricting member. FIG. [Figure 7] FIG. 4 is a flowchart showing an outline of a job procedure. [Figure 8] FIG. 4 is a graph for explaining the effect of the first embodiment. [Figure 9] FIG. 4 is a graph for explaining the effect of the first embodiment. [Figure 10] FIG. 4 is a graph for explaining the effect of the first embodiment. [Figure 11] FIG. 11 is a graph for explaining the effect of the second embodiment. [Figure 12] FIG. 11 is a graph for explaining the effect of the second embodiment. [Figure 13] FIG. 2 is a schematic diagram showing a configuration example of a potential regulating power supply; [Figure 14] FIG. 11 is a schematic cross-sectional view of another example of an image forming apparatus. [Figure 15] FIG. 13 is a schematic diagram of yet another example of an image forming apparatus. [Figure 16] FIG. 11 is a graph for explaining the problem. [Figure 17] FIG. 11 is a graph for explaining the problem. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

[0019] [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.

[0020] The image forming apparatus 1 has an image forming section 2, a control section 3, a feeding section 4 for a recording material S, and a discharge section 5 for a recording material S. Inside the image forming apparatus 1, a temperature sensor 71 (FIG. 2) capable of detecting the temperature inside the apparatus and a humidity sensor 72 (FIG. 2) capable of detecting the humidity inside the apparatus are provided. The temperature sensor 71 and the humidity sensor 72 are each an example of an environment detection means for detecting at least one of the environment inside and outside the image forming apparatus 1. The image forming apparatus 1 can form an image on the recording material S based on image information (image signal) acquired by a document reading device (not shown) provided in the image forming apparatus 1 or connected to the image forming apparatus 1. The image forming apparatus 1 can also form an image on the recording material S based on image information (image signal) from an external device (not shown) such as a personal computer (host device), a digital camera, or a smartphone connected to the image forming apparatus 1. Incidentally, the recording material (transfer material, recording medium, sheet) S is a material on which an image is formed using toner, and specific examples include plain paper, thick paper, gloss coated paper, matte coated paper, embossed paper, or a synthetic resin sheet (synthetic paper) or overhead projector sheet (resin film) that is a substitute for plain paper, etc. Here, the recording material S may be referred to as "paper" (such as "paper," "embossed paper," or "high resistance paper"), but even in such cases, the recording material S includes materials other than paper or materials that contain materials other than paper.

[0021] 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 has image forming units 10y, 10m, 10c, 10k, toner bottles 18y, 18m, 18c, 18k, exposure devices 13y, 13m, 13c, 13k, an intermediate transfer unit 20, a secondary transfer device 26, and a fixing device 27. The image forming units 10y, 10m, 10c, 10k form toner images of the colors yellow (Y), magenta (M), cyan (C), and black (K), respectively. Note that 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 indicating that the elements are for any one of the colors. Also, elements for each color may be simply referred to as elements for each color, for example, "yellow image forming unit 10y." 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.

[0022] The image forming unit 10 has a photosensitive drum 11 which is a drum-type (cylindrical) photosensitive member (electrophotographic photosensitive member) serving as an image carrier. The image forming unit 10 also has a charging roller 12 which is a roller-type charging member serving as a charging means. The image forming unit 10 also has a developing device 14 serving as a developing means. The image forming unit 10 also has a pre-exposure device 16 serving as a discharging means. The image forming unit 10 also has a drum cleaning device 17 serving as a photosensitive member cleaning means. The image forming unit 10 forms a toner image on an intermediate transfer belt 6 which will be described later.

[0023] 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 the figure (counterclockwise direction) at a predetermined peripheral speed (process speed) by a drive motor (not shown) as a drive means.

[0024] 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 comes into contact with the surface of the photosensitive drum 11 and rotates in accordance with the rotation of the photosensitive drum 11. A charging power source 73 (FIG. 2) serving as a charging bias application means (charging bias application section) is connected to the charging roller 12. During charging, the charging power source 73 applies a predetermined charging bias (charging voltage) to the charging roller 12.

[0025] 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 circumferential surface) of the photosensitive drum 11.

[0026] 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. The developing sleeve 14a carries a two-component developer and transports it to a developing area facing the photosensitive drum 11. In the developing 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 to it. A developing power source 74 (FIG. 2) is connected to the developing sleeve 14a as a developing bias application means (developing bias application portion). During development, the developing power source 74 applies a predetermined developing bias (developing voltage) to the developing sleeve 14a. In this embodiment, toner charged to the same polarity as the charging polarity of the photosensitive drum 11 (negative polarity in this embodiment) adheres to the 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 (reversal development method). In this embodiment, the normal charging polarity of the toner, which is the main charging polarity of the toner during development, is negative polarity.

[0027] 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, which 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 intermediate transfer belt 6 is rotated (moves orbitally) in the direction of arrow R2 in the figure (clockwise direction) at a predetermined peripheral speed corresponding to the peripheral speed of the photosensitive drum 11 by a driving force transmitted by the drive roller 21 being driven to rotate by a drive motor (not shown) serving as a driving means. 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.

[0028] The toner image formed on the photosensitive drum 11 is transferred (primary transfer) onto the rotating intermediate transfer belt 6 by the action of the primary transfer roller 15 in the primary transfer portion N1. For example, when a full-color image is formed, the toner images of each color, yellow, magenta, cyan, and black, formed on each photosensitive drum 11 are superimposed on the intermediate transfer belt 6 in order to be multi-transferred. A primary transfer power supply 75 (FIG. 2) serving as a primary transfer bias application means (primary transfer bias application unit) is connected to the primary transfer roller 15. During the primary transfer, the primary transfer power supply 75 applies a predetermined primary transfer bias (primary transfer voltage), which is a DC voltage of the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, to the primary transfer roller 15. As a result, the negative polarity toner image on the photosensitive drum 11 is primarily transferred onto the intermediate transfer belt 6. The primary transfer power supply 75 is connected to a voltage detection sensor 75a (FIG. 2) as a voltage detection means (voltage detection unit) for detecting the output voltage, and a current detection sensor 75b (FIG. 2) as a current detection means (current detection unit) for detecting the output current. In this embodiment, a primary transfer bias of, for example, about 1 to 2 kV is applied to the primary transfer roller 15 ("~" indicates a range including the numerical values ​​before and after it. The same applies below). In this embodiment, the primary transfer bias is controlled to 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.

[0029] 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 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.

[0030] 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.

[0031] On the outer peripheral surface side of the intermediate transfer belt 6, a secondary transfer outer roller 25, which is a roller-type secondary transfer member serving as a secondary transfer means, is disposed. The secondary transfer outer roller 25 as a secondary transfer member constitutes a secondary transfer device 26 together with the secondary transfer inner roller 23 as an opposing member (opposing electrode). The secondary transfer outer roller 25 is pressed toward the secondary transfer inner roller 23 and abuts against the secondary transfer inner roller 23 via the intermediate transfer belt 6 to form a secondary transfer portion (secondary transfer nip portion) N2, which is an abutment portion between the intermediate transfer belt 6 and the secondary transfer outer roller 25. The toner image formed on the intermediate transfer belt 6 is transferred (secondary transfer) onto the recording material S, which is being conveyed while being sandwiched between the intermediate transfer belt 6 and the secondary transfer outer roller 25, by the action of the secondary transfer device 26 at the secondary transfer portion N2. A secondary transfer power source 76 (FIG. 2) serving as a secondary transfer bias application means (secondary transfer bias application portion) is connected to the secondary transfer outer roller 25. During the secondary transfer, the secondary transfer power source 76 applies a predetermined secondary transfer bias (secondary transfer voltage), which is a DC voltage of the polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment), to the secondary transfer outer roller 25. As a result, the negative polarity toner image on the intermediate transfer belt 6 is secondarily transferred onto the recording material S. The secondary transfer power source 76 is connected to a voltage detection sensor 76a (FIG. 2) as a voltage detection means (voltage detection unit) for detecting the output voltage, and a current detection sensor 76b (FIG. 2) as a current detection means (current detection unit) for detecting 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.

[0032] 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.

[0033] 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.

[0034] The recording material S onto which the toner image has been transferred is conveyed to a fixing device 27 as a fixing means. The fixing device 27 has a fixing roller 27a and a pressure roller 27b. The fixing roller 27a incorporates a heater as a heating means. The pressure roller 27b presses against the fixing roller 27a to form a fixing section (fixing nip section). The fixing device 27 conveys the recording material S carrying an unfixed toner image by pinching it between the fixing roller 27a and the pressure roller 27b, thereby heating and pressurizing it, and fixing (melting and fixing) the toner image onto the recording material S. The temperature (fixing temperature) of the fixing roller 27a is detected by a fixing temperature sensor 77 (FIG. 2). The recording material S onto which the toner image has been fixed is conveyed by a discharge roller 51 and the like in the discharge section 5, and is discharged (output) from a discharge port (not shown) onto a discharge tray 52 provided outside the device main body (hereinafter also simply referred to as the "device main body") 1a of the image forming apparatus 1.

[0035] 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 17. In this embodiment, the drum cleaning device 17 scrapes off the primary transfer residual toner from the surface of the rotating photosensitive drum 11 by a cleaning blade as a cleaning member and collects it in a collection container (not shown). The cleaning blade is a plate-shaped member that abuts against the photosensitive drum 11 with a predetermined pressing force. The cleaning blade abuts against the surface of the photosensitive drum 11 in a counter direction to the rotation direction of the photosensitive drum 11 so that the tip of the free end side faces the upstream side of the rotation direction of the photosensitive drum 11. 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 24 as an intermediate transfer body cleaning means.

[0036] The image forming unit 10 may be configured as a cartridge (process cartridge) that is detachable from the apparatus main body 1a as a whole. In this embodiment, the intermediate transfer belt 6, the tension roller of the intermediate transfer belt 6, the primary transfer rollers 15, the belt cleaning device 24, and the potential regulating members 8 described below constitute an intermediate transfer unit 20. The intermediate transfer unit 20 may be configured as a cartridge that is detachable from the apparatus main body 1a as a whole.

[0037] In this embodiment, the image forming apparatus 1 can perform image formation in a full-color mode (first mode) and a black monochrome mode (monochrome mode, second mode) as image forming modes. In the full-color mode, it is possible to form a full-color image by forming yellow, magenta, cyan, and black toner images using the four image forming units 10y, 10m, 10c, and 10k. In the black monochrome mode, it is possible to form a black toner image by using only the black image forming unit 10k among the four image forming units 10y, 10m, 10c, and 10k to form a black monochrome image. The control unit 3 can switch the image forming mode between the full-color mode and the black monochrome mode based on a mode selection instruction and image information included in the job information (image forming information).

[0038] Here, in this embodiment, the image forming apparatus 1 has a contact / separation mechanism 90 (FIG. 2) that causes the intermediate transfer belt 6 to contact and separate from the yellow, magenta, and cyan photosensitive drums 11y, 11m, and 11c. The contact / separation mechanism 90 can bring the intermediate transfer belt 6 into contact with the yellow, magenta, and cyan photosensitive drums 11y, 11m, and 11c to create a contact / separation state in which the intermediate transfer belt 6 contacts all of the photosensitive drums 11y, 11m, 11c, and 11k ("full contact state"). The contact / separation mechanism 90 can also separate the intermediate transfer belt 6 from the yellow, magenta, and cyan photosensitive drums 11y, 11m, and 11c to create a contact / separation state in which the intermediate transfer belt 6 contacts only the black photosensitive drum 11k among the four photosensitive drums 11 ("black contact state"). The control unit 3 can switch the contact / separation state between the photosensitive drum 11 and the intermediate transfer belt 6 between a full contact state and a black contact state by controlling the contact / separation mechanism 90 according to the image forming mode, for example. In the full-color mode, the control unit 3 sets the contact / separation state between the photosensitive drum 11 and the intermediate transfer belt 6 to a full contact state. In the black monochrome mode, the control unit 3 can select the black contact state as the contact / separation state between the photosensitive drum 11 and the intermediate transfer belt 6. However, in this embodiment, the image forming apparatus 1 can form only a black toner image to form a black monochrome image in either the full contact state or the black contact state. When an image is formed in the black contact state, the rotation of the photosensitive drum 11 and the developing sleeve 14a of the yellow, magenta, and cyan image forming units 10y, 10m, and 10c is stopped.

[0039] 2. Control configuration FIG. 2 is a block diagram showing a schematic configuration of a control system of the image forming apparatus 1 of this embodiment. The image forming apparatus 1 is provided with a control unit (control circuit) 3 as a control means. The control unit 3 is configured to 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 unit 3 and devices external to the control unit 3, and the like. The ROM 32 stores programs and the like for controlling each part 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 part such as the feeding unit 4, the image forming unit 2, and the discharge unit 5, and exchanges signals with each of these parts and controls the operation of each of these parts. The ROM 32 stores an image formation control sequence for forming an image on the recording material S, and the like.

[0040] 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 contact / separation mechanism 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). 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 a primary transfer power supply 75, a voltage detection sensor 76a and a current detection sensor 76b of a secondary transfer power supply 76, a voltage detection sensor 80a and a current detection sensor 80b of a potential regulating power supply 80, a fixing temperature sensor 77, and the like. Signals (information) indicating the detection results of each sensor are input to the control unit 3.

[0041] 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.

[0042] 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.

[0043] 3. Effects of discharge downstream of the primary transfer section Next, the influence of discharge downstream of the primary transfer portion N1 will be further described. For convenience, the magnitude (high / low) of voltage or potential refers to the magnitude (high / low) when compared in absolute value unless otherwise specified. Furthermore, with respect to the arrangement of the primary transfer portion N1, the photosensitive drum 11, the primary transfer roller 15, and the potential regulating member 8 described below, the upstream and downstream refer to the upstream and downstream in the transport direction (process progress direction, movement direction) of the intermediate transfer belt 6 unless otherwise specified.

[0044] 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.

[0045] In order to suppress the increase in the charge amount of the toner downstream of the primary transfer portion N1 as described above, it is effective to suppress discharge downstream of the primary transfer portion N1. 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.

[0046] 4. Potential control material Next, the configuration of the potential regulating member 8 in this embodiment will be described. As shown in Fig. 1, in the image forming apparatus 1 of this embodiment, potential regulating members 8y, 8m, 8c, and 8k, 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.

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

[0048] 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.

[0049] 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].

[0050] A potential regulating power supply 80 is connected to the potential regulating member 8 as a potential regulating bias application means (potential regulating bias application section). 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) which is a DC voltage of 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 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 section N1. This makes it possible to suppress discharge between the intermediate transfer belt 6 and the photosensitive drum 11 downstream of the primary transfer section N1. In this embodiment, the potential regulating bias is a DC voltage of negative polarity. In this embodiment, the potential regulating bias is controlled to a constant voltage. In the configuration of this embodiment, the potential regulating bias (positive constant voltage) is preferably about −500 to −5000 [V], and more preferably about −1000 to −3000 [V].

[0051] 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.

[0052] 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.

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

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

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

[0056] In this embodiment, the outer diameter of the photosensitive drum 11 is 30 [mm], the outer diameter of the primary transfer roller 15 is 18 [mm], and the thickness of the intermediate transfer belt 6 is 0.350 [mm]. In this embodiment, the primary transfer roller 15 is 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.

[0057] Here, in order to explain the arrangement of the potential regulating member 8, it is assumed that the potential regulating member 8 is removed. In a cross section substantially perpendicular to the rotation axis direction of the photosensitive drum 11, a straight line passing through the tension surface on the inner peripheral side of the intermediate transfer belt 6 downstream of the primary transfer portion N1 in the absence of the potential regulating member 8 is defined as a straight line L. More specifically, this straight line L corresponds to the tension surface in a state in which only the potential regulating member 8 is substantially removed from the configuration of the image forming apparatus 1 in the image forming operation state. Also, on the straight line L, a point where the inner peripheral surface of the intermediate transfer belt 6 separates from the tension member immediately adjacent to the upstream side of the potential regulating member 8 is defined as "C (or upstream tension portion C)", and a point where the inner peripheral surface of the intermediate transfer belt 6 separates from the tension member immediately adjacent to the downstream side of the potential regulating member 8 is defined as "D (or downstream tension portion D)". In addition, although the straight line L is shown generally horizontally in FIG. 6, if the surface of the primary transfer roller 15 is lifted toward the photosensitive drum 11 due to deformation of the elastic layer of the primary transfer roller 15 or the like, the straight line L may be inclined so as to descend downward in the figure as it approaches the downstream side.

[0058] 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.

[0059] 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.

[0060] Furthermore, for any primary transfer portion N1, if there is another tension roller that regulates the posture of the intermediate transfer belt 6 during image formation 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.

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

[0062] In this embodiment, the potential regulating member 8 is pressed against the inner peripheral surface of the intermediate transfer belt 6 at both ends in the longitudinal direction by a pressing spring 87 (FIG. 3(b)) constituted by a compression coil spring, which is a pressing member serving as a pressing means. At this time, it is preferable that the contact portion of the potential regulating member 8 that contacts the inner peripheral surface of the intermediate transfer belt 6 is caused to enter the photosensitive drum 11 side from the straight line L. This allows the potential regulating member 8 to be more stably brought into contact with the intermediate transfer belt 6 even if waving or vibration occurs in the intermediate transfer belt 6 during the image forming operation (while the intermediate transfer belt 6 is running). In this embodiment, the pressing force of the pressing spring 87 is set (adjusted) so that the upstream end A and downstream end B of the contact surface 83, which is the contact portion of the potential regulating member 8 that contacts the inner peripheral surface of the intermediate transfer belt 6, enter the photosensitive drum 11 side by about 0.5 [mm] from the straight line L. By thus causing the contact surface 83 of the potential regulating member 8 to intrude into the photosensitive drum 11 side with respect to the straight line L, the potential regulating member 8 can be brought into surface contact with the intermediate transfer belt 6 more stably even if the intermediate transfer belt 6 is wavy or vibrated during the image forming operation (while the intermediate transfer belt 6 is running). Although not limited thereto, the intrusion amount of the contact surface 83 of the potential regulating member 8 into the straight line L is preferably about 0.3 to 5 mm, and typically about 0.5 to 3 mm. If this intrusion amount is too small, it may be difficult to bring the potential regulating member 8 into stable contact with the intermediate transfer belt 6, and if it is too large, it may be difficult to transport the intermediate transfer belt 6 stably.

[0063] 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.

[0064] 6, the potential regulating member 8 is disposed so that the straight line M and the straight line L are substantially parallel, but the potential regulating member 8 may be disposed so that the straight line M is inclined with respect to the straight line L as long as the straight line M does not intersect with the line segment CD of the straight line L. For example, by inclining the straight line M with respect to the straight line L so that the upstream end A side is closer to the straight line L than the downstream end B side, it is possible to reduce the curvature generated in the intermediate transfer belt 6 due to the intermediate transfer belt 6 running around in the vicinity of the upstream end A. This is therefore advantageous in reducing the possibility that scratches will occur on the inner peripheral surface of the intermediate transfer belt 6 due to friction with the potential regulating member 8.

[0065] 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.

[0066] 5. Challenges FIG. 16 is a graph showing the relationship between the secondary transfer current and the secondary transfer efficiency of a two-color solid (magenta (M) + cyan (C)) toner image and a black (K) toner image. FIG. 16(a) shows the relationship when no potential regulating member 8 is provided in any of the image forming units 10. FIG. 16(b) shows the relationship when the same potential regulating bias is applied to the potential regulating members 8 of all the image forming units 10. The method for measuring the secondary transfer efficiency will be described later.

[0067] 16(a) and (b), the secondary transfer current at which the secondary transfer efficiency of a two-color solid (M+C) toner image is 95% is smaller and the required strength of the secondary transfer electric field can be reduced when the potential regulating member 8 is used than when the potential regulating member 8 is not used. Therefore, by using the potential regulating member 8, it is possible to ensure the secondary transfer efficiency of a chromatic toner image such as a two-color solid (M+C) toner image at a relatively low strength of the secondary transfer electric field while improving the graininess of the toner image and the secondary transferability to embossed paper.

[0068] However, as shown in Figures 16(a) and (b), it has been found that the secondary transfer efficiency of the black (K) toner image may decrease when the potential regulating member 8 is used. According to the inventors' investigation, this is believed to be due to the following reasons.

[0069] The first reason is that the black image forming unit 10k is located downstream of the chromatic color (yellow, magenta, cyan in this embodiment) image forming units 10y, 10m, and 10c. That is, the toner image transferred onto the intermediate transfer belt 6 at the upstream primary transfer portion N1 is discharged downstream of the primary transfer portion N1, and is further discharged downstream of the primary transfer portion N1 downstream of the primary transfer portion N1. FIG. 17 is a graph diagram showing the distribution of the charge amount of the toner on the intermediate transfer belt 6 after passing through the most downstream primary transfer portion N1k (before reaching the secondary transfer portion N2). The solid lines in FIG. 17 show the distribution of the charge amount of the yellow, magenta, cyan, and black toner on the intermediate transfer belt 6 after passing through the most downstream primary transfer portion N1k (before reaching the secondary transfer portion N2) in the case where the potential regulating member 8 is not used. If the potential regulating member 8 is not used, the yellow, magenta, cyan, and black toners on the intermediate transfer belt 6 are subjected to the above-mentioned discharge, and the distribution of the charge amount broadens while the average value of the charge amount rises. In contrast, by using the potential regulating member 8, the distribution of the charge amount of the toner on the intermediate transfer belt 6 can be made closer to the distribution of the charge amount of the toner on the photosensitive drum 11 (the charge amount distribution can be made sharper while the average value of the charge amount can be reduced).

[0070] However, if the same potential regulating bias is applied to the upstream yellow, magenta, and cyan potential regulating members 8y, 8m, and 8c and the downstream black potential regulating member 8k, the effect of suppressing the increase in the charge amount for the black toner is too strong. The dashed lines in FIG. 17 show the distribution of the charge amount of the yellow, magenta, and cyan toners on the intermediate transfer belt 6 after passing through the most downstream primary transfer portion N1k (before reaching the secondary transfer portion N2) when the potential regulating member 8 is used. The dashed lines in FIG. 17 show the distribution of the charge amount of the black toner on the intermediate transfer belt 6 after passing through the most downstream primary transfer portion N1k (before reaching the secondary transfer portion N2) when the potential regulating member 8 is used. The black toner on the intermediate transfer belt 6 is only subjected to discharge downstream of the black primary transfer portion N1k, so the degree of increase in the charge amount due to the discharge (broadening of the charge amount distribution, increase in the average charge amount) is low. Therefore, when the same potential regulating bias as that applied to the yellow, magenta, and cyan potential regulating members 8y, 8m, and 8c is applied to the black potential regulating member 8k, the increase in the charge amount of the black toner is excessively suppressed. As a result, as shown in Fig. 16(b), when the secondary transfer current is set to achieve a secondary transfer efficiency of 95% for a chromatic toner image such as a two-color solid (M+C) toner image, the strength of the secondary transfer electric field for the black toner image is too strong, causing the toner charge polarity to be inverted, and the secondary transfer efficiency to decrease.

[0071] The second reason is the difference in components between chromatic toners (yellow, magenta, and cyan in this embodiment) and black toners. Black toners generally have a higher content (weight %) of conductive components such as carbon black than toners of colors other than black, such as chromatic toners (including transparent toners and white toners, which will be described later), so the charge amount is less likely to increase. Therefore, when the same potential regulating bias as that applied to the yellow, magenta, and cyan potential regulating members 8y, 8m, and 8c is applied to the black potential regulating member 8k, as described above, the increase in the charge amount of the black toner is excessively suppressed, and the secondary transfer efficiency is reduced. The reason why the charge amount of the black toner decreases too much compared to the color toners is that the black toner contains a large amount of carbon black, which is a pigment.

[0072] Therefore, it is desired to ensure the secondary transfer efficiency of chromatic toner images such as a two-color solid toner image with a relatively low secondary transfer electric field strength, and to suppress a decrease in the secondary transfer efficiency of a black toner image. In other words, in a configuration using the potential regulating member 8, it is desired to obtain good secondary transferability of both the toner that is primarily transferred to the intermediate transfer belt 6 on the upstream side and the toner that is primarily transferred to the intermediate transfer belt 6 on the downstream side. In yet another way, it is desired to obtain good secondary transferability of both the toner of a color other than black and the black toner in a configuration using the potential regulating member 8.

[0073] 6. Potential Regulation Bias 6-1. Determination of potential regulation bias Next, a method for determining the potential regulating bias in this embodiment will be described. Here, a case where the image forming apparatus 1 forms an image in full color mode will be described. Fig. 7 is a flow chart showing an outline of a job procedure including control for determining the potential regulating bias in this embodiment.

[0074] In this embodiment, as described above, the potential regulating power supplies 80 (80y, 80m, 80c, 80k) are provided independently for each potential regulating member 8 (8y, 8m, 8c, 8k). In addition, in this embodiment, the potential regulating bias applied to each potential regulating member 8 (8y, 8m, 8c, 8k) by each potential regulating power supply 80 (80y, 80m, 80c, 80k) can be individually controlled. In this embodiment, the potential regulating bias applied to each potential regulating member 8 (8y, 8m, 8c, 8k) during image formation is constant-voltage controlled at a preset target voltage. In this embodiment, the target voltage of the potential regulating bias applied to each potential regulating member 8 (8y, 8m, 8c, 8k) is preset according to the environment (temperature, humidity). In this embodiment, the control unit 3 controls the potential regulating bias applied to the downstream black potential regulating member 8k to be smaller than the potential regulating bias applied to the upstream yellow, magenta, and cyan potential regulating members 8y, 8m, and 8c.

[0075] First, when the control unit 3 starts a job (S101), it acquires the temperature detection result by the temperature sensor 71 and the humidity detection result by the humidity sensor 72, and judges whether the environment is low humidity or high humidity (S102). In this embodiment, the low humidity environment is an environment where the temperature is 23° C. or less and the humidity (relative humidity) is 55% or less. On the other hand, in this embodiment, the high humidity environment is an environment where the temperature is higher than 23° C. and the humidity is higher than 55%.

[0076] Next, the control unit 3 determines target voltages of the potential regulating biases to be applied to the yellow, magenta, cyan, and black potential regulating members 8y, 8m, 8c, and 8k according to the environment determined in S102 (S103). In this embodiment, the control unit 3 reads out and determines the target voltages of the potential regulating biases corresponding to the environment from table data such as that shown in Table 1, which is preset and stored in the ROM 32. Table 1 is an example of information indicating the relationship between the environment and the potential regulating bias for each potential regulating member 8 (8y, 8m, 8c, and 8k).

[0077] [Table 1]

[0078] In this embodiment, when the control unit 3 determines in S102 that the environment is low humidity, it determines to apply a potential regulating bias of −3000 V to the yellow, magenta, and cyan potential regulating members 8y, 8m, and 8c, as shown in Table 1. In this case, the control unit 3 determines to apply a potential regulating bias of −1500 V to the black potential regulating member 8k, as shown in Table 1, which is smaller than the potential regulating bias applied to the yellow, magenta, and cyan potential regulating members 8y, 8m, and 8c.

[0079] Furthermore, in this embodiment, when the control unit 3 determines in S102 that the environment is high humidity, it determines to apply a potential regulating bias of -3000V to the yellow, magenta, and cyan potential regulating members 8y, 8m, and 8c, as shown in Table 1. In this case, the control unit 3 determines to turn off the black potential regulating power supply 80k (potential regulating member 8k is electrically grounded) or to apply a potential regulating bias of approximately 0V to the black potential regulating member 8k (constant voltage control with a target voltage of approximately 0V) as shown in Table 1.

[0080] That is, in this embodiment, in each environment, the potential regulating bias applied to the downstream black potential regulating member 8k is smaller than the potential regulating bias applied to the upstream yellow, magenta and cyan potential regulating members 8y, 8m and 8c.

[0081] Then, the control unit 3 applies the potential regulating bias determined in S103 to each potential regulating member 8 (8y, 8m, 8c, 8k) (S104), performs image formation (S105), and ends the job (S106).

[0082] 6-2.Effects in low humidity environments Next, the effect of the setting of the potential regulating bias in this embodiment in a low humidity environment will be described.

[0083] Fig. 8 is a graph showing the relationship between the secondary transfer current and the secondary transfer efficiency of a two-color solid (M+C) toner image and a black (black monochromatic solid) (K) toner image in a low humidity environment. Fig. 8(a) shows the relationship when the same potential regulation bias of -3000V is applied to the yellow, magenta, cyan, and black potential regulation members 8y, 8m, 8c, and 8k as a comparative example. Fig. 8(b) shows the relationship in the present embodiment, that is, when a potential regulation bias of -3000V is applied to the yellow, magenta, and cyan potential regulation members 8y, 8m, and 8c, and a potential regulation bias of -1500V is applied to the black potential regulation member 8k.

[0084] The weight of toner per unit area of ​​the two-color solid (M+C) toner image shown in FIG. 8 is approximately 0.9 to 1.2 mg / cm. 2 The weight of toner per unit area of ​​the black (K) toner image, the results of which are shown in FIG. 8, is approximately 0.45 to 0.65 mg / cm. 2 It is.

[0085] The secondary transfer efficiency was calculated by measuring the following A, B, and C, and then calculating the following formula: Secondary transfer efficiency = {(AC) / (A+B-2×C)}×100(%) The calculation was made by the following formula. A is a value obtained by attaching a transparent tape (PET25(A)MF11 / 3L manufactured by Lintec) on the toner image that has been secondarily transferred to paper (recording material S), and measuring the density with a densitometer of 500 Series manufactured by X-Rite. B is a value obtained by peeling off the toner remaining on the intermediate transfer belt 6 that has not been secondarily transferred to the paper with the adhesive layer of the transparent tape, attaching it to an unused paper of the same brand as the paper used for the secondary transfer, and measuring the density with the densitometer. C is a value obtained by attaching the transparent tape as it is on the unused paper similar to the above, and measuring the density with the densitometer. The paper used for measuring the secondary transfer efficiency is GF-C081 manufactured by Oji Paper Co., Ltd.

[0086] 8(a) and (b), the secondary transfer efficiency of the black (K) toner image at the secondary transfer current at which the secondary transfer efficiency of the two-color solid (M+C) toner image is 95% is higher in this embodiment than in the comparative example. In the comparative example, the secondary transfer efficiency of the black (K) toner image at that secondary transfer current is approximately less than 90%, whereas in this embodiment, it exceeds 90%. Thus, in this embodiment, the secondary transfer efficiency of the black (K) toner image is improved compared to the comparative example.

[0087] As shown in Figures 8(a) and (b), when the secondary transfer efficiency of the black toner image tends to decrease with an increase in the secondary transfer current, the smaller the charge amount of the toner, the more pronounced this tendency becomes. This is because, as the charge amount of the toner is smaller, the proportion of the electric field used for secondary transfer in the secondary transfer electric field decreases, and the excess electric field contributes to reversing the charge polarity of the toner, resulting in a more pronounced decrease in the secondary transfer efficiency.

[0088] Here, Fig. 9 is a graph showing the charge amount of the toner on the photosensitive drum 11 and the charge amount of the toner on the intermediate transfer belt 6 in a low humidity environment. Fig. 9(a) shows the charge amount of the black toner on the black photosensitive drum 11k and the charge amount of the black toner on the intermediate transfer belt 6 after passing through the black primary transfer portion N1k (before reaching the secondary transfer portion N2). Fig. 9(b) shows the charge amount of the cyan toner on the cyan photosensitive drum 11c, the charge amount of the cyan toner on the intermediate transfer belt 6 after passing through the cyan primary transfer portion N1c (before reaching the black primary transfer portion N1k), and the charge amount of the cyan toner on the intermediate transfer belt 6 after passing through the black primary transfer portion N1k (before reaching the secondary transfer portion N2). Figures 9(a) and (b) respectively show the results when no potential regulating member 8 is provided in any of the image forming units 10, when the same potential regulating bias of -3000 V is applied to all potential regulating members 8y, 8m, 8c, and 8k, and in the present embodiment.

[0089] The charge amount of the toner was calculated by measuring the charge per unit weight with a coulomb meter using a suction method commonly used in the field. This method measures the weight [g] and charge amount [μC] of the sucked toner to calculate the charge amount [μC / g], and allows the average charge amount of the toner to be grasped.

[0090] As shown in FIG. 9(a), when the same potential regulating bias (-3000V) as that applied to the yellow, magenta, and cyan potential regulating members 8y, 8m, and 8c is also applied to the black potential regulating member 8k, the charge amount of the black toner on the intermediate transfer belt 6 does not increase, and the charge amount remains low, equivalent to the charge amount on the photosensitive drum 11k. On the other hand, when a potential regulating bias (-1500V) smaller than the potential regulating bias (-3000V) applied to the yellow, magenta, and cyan potential regulating members 8y, 8m, and 8c is applied to the black potential regulating member 8k as in this embodiment, the charge amount of the black toner on the intermediate transfer belt 6 increases, although not as much as when the potential regulating member 8 is not used. As a result, as described with reference to FIGS. 8(a) and 8(b), in this embodiment, the secondary transfer efficiency of the black toner image is improved compared to the comparative example.

[0091] Also, as shown in FIG. 9(b), in this embodiment, the charge amount of the cyan toner (chromatic toner) on the intermediate transfer belt 6 after passing through the black primary transfer portion N1k increases because the potential regulating bias (-1500V) applied to the black potential regulating member 8k is lower than when the same potential regulating bias (-3000V) is applied to the yellow, magenta, cyan, and black potential regulating members 8y, 8m, 8c, and 8k. However, compared to when the potential regulating member 8 is not used, the charge amount of the cyan toner on the intermediate transfer belt 6 after passing through the black primary transfer portion N1k can be reduced. The same can be said about the yellow toner and the magenta toner. As a result, as described with reference to FIGS. 8(a) and (b), the secondary transfer efficiency of the chromatic toner image, such as a two-color (M+C) solid toner image, can be ensured with a relatively low secondary transfer electric field strength.

[0092] 6-3.Effects in high humidity environments Next, the effect of the setting of the potential regulating bias in this embodiment in a high humidity environment will be described.

[0093] Fig. 10 is a graph showing the charge amount of black toner on the black photosensitive drum 11k and the charge amount of black toner on the intermediate transfer belt 6 after passing through the black primary transfer portion N1k (before reaching the secondary transfer portion N2) in a high humidity environment. Fig. 10 shows the results when no potential regulating member 8 is provided in any of the image forming units 10, when the same potential regulating bias of -3000 V is applied to all the potential regulating members 8y, 8m, 8c, and 8k, and when this embodiment is used.

[0094] As shown in FIG. 10, the charge amount of the black toner on the black photosensitive drum 11k is lower in a high humidity environment than in a low humidity environment (FIG. 9(a)). Therefore, in this embodiment, the potential regulating bias applied to the black potential regulating member 8k in a high humidity environment is lower than the potential regulating bias applied to the black potential regulating member 8k in a low humidity environment. In this embodiment, in a high humidity environment, the black potential regulating power supply 80k is turned off or a potential regulating bias of approximately 0 V is applied to the black potential regulating member 8k. This prevents the charge amount of the black toner on the intermediate transfer belt 6 after passing through the black primary transfer portion N1k from decreasing too much in a high humidity environment. As a result, it is possible to suppress a decrease in the secondary transfer efficiency of the black toner image in a high humidity environment.

[0095] In this embodiment, the difference in the charge amount on the photosensitive drum 11 of the yellow, magenta, and cyan toners is sufficiently small between the low humidity environment and the high humidity environment. Therefore, in this embodiment, as shown in Table 1, the potential regulating bias applied to the potential regulating members 8y, 8m, and 8c of yellow, magenta, and cyan is the same in the low humidity environment and the high humidity environment. However, this is not limited to this, and the potential regulating bias applied to the potential regulating members 8y, 8m, and 8c of yellow, magenta, and cyan may be different between the low humidity environment and the high humidity environment for the same reason as described above for the black toner. Table 2 shows an example of the setting of the potential regulating bias when the charge amount on the photosensitive drum 11 of the yellow, magenta, cyan, and black toners is lower in the high humidity environment than in the low humidity environment. Similarly, at least one of the potential regulating biases applied to the yellow, magenta, cyan and black potential regulating members 8y, 8m, 8c and 8k can be made different between the low humidity environment and the high humidity environment depending on the difference in the charge amount of the toner on the photosensitive drum 11 under the low humidity environment and the high humidity environment, provided that in each environment, the potential regulating bias applied to the black potential regulating member 8k on the downstream side is made smaller than the potential regulating bias applied to the yellow, magenta and cyan potential regulating members 8y, 8m and 8c on the upstream side.

[0096] [Table 2]

[0097] 7.Effects In this embodiment, the system includes first and second photoconductors (for example, the first photoconductor 11c and the second photoconductor 11k) that can be charged to a predetermined polarity and that support a toner image; a circumferentially movable intermediate transfer belt 6 that contacts the first and second photoconductors 11c and 11k to form first and second primary transfer portions N1c and N1k, respectively, and transports the toner images that have been primarily transferred from the first and second photoconductors 11c and 11k at the first and second primary transfer portions N1c and N1k, respectively, to a recording material S in a secondary transfer portion N2; first and second primary transfer members 15c and 15k that contact the inner circumferential surface of the intermediate transfer belt 6 and are applied with a primary transfer bias having a polarity opposite to the predetermined polarity, thereby primarily transferring the toner images from the first and second photoconductors 11c and 11k to the intermediate transfer belt 6, respectively; The intermediate transfer belt 6 includes first and second electrode members (potential regulating members) 8c and 8k that contact the inner surface of the intermediate transfer belt 6 at adjacent positions downstream of the secondary transfer portions N1c and N1k, application units (potential regulating power sources) 80c and 80k that apply a bias of the same polarity as the above-mentioned predetermined polarity to the first and second electrode members 8c and 8k, and a control unit 3 that can control the application units 80c and 80k, where the first photoconductor 11c is disposed upstream of the second photoconductor 11k in the movement direction of the intermediate transfer belt 6, and the control unit 3 controls the application units 80c and 80k so that the absolute value of the bias applied to the second electrode member 8k is smaller than the absolute value of the bias applied to the first electrode member 8c when performing image formation in which the toner images formed on the first and second photoconductors 11c and 11k are transferred to the recording material S to form an image on the recording material S. In this embodiment, the second photoconductor 11k is disposed at the most downstream position in the moving direction of the intermediate transfer belt 6 among the multiple photoconductors included in the image forming apparatus 1. In this embodiment, a toner image is formed on the first photoconductor with toner of a color other than black, and a toner image is formed on the second photoconductor with black toner.In this embodiment, the image forming apparatus 1 has environment detection means (temperature sensor 71, humidity sensor 72) for detecting the environment, and the control unit 3 controls the application units 80c, 80k so that the absolute value of the bias applied to the second electrode member 8k during the image formation is smaller when the humidity indicated by the detection results of the environment detection means 71, 72 is a second humidity higher than the first humidity than when the humidity is the first humidity. Also, the control unit 3 can control the application units 80c, 80k so that the absolute value of the bias applied to the first electrode member 8c during the image formation is smaller when the humidity indicated by the detection results of the environment detection means 71, 72 is a second humidity higher than the first humidity than when the humidity is the first humidity.

[0098] As described above, according to this embodiment, it is possible to ensure the secondary transfer efficiency of chromatic toner images such as two-color solid images (M+C) at a relatively low secondary transfer electric field strength while suppressing a decrease in the secondary transfer efficiency of black toner images.

[0099] [Example 2] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

[0100] 1. Overview of this embodiment In the first embodiment, the potential regulating biases applied to the yellow, magenta, and cyan potential regulating members 8y, 8m, and 8c are the same, and the potential regulating bias applied to the black potential regulating member 8k is made smaller than the potential regulating bias. This prevents the difference in charge amount on the intermediate transfer belt 6 between the toner (chromatic toner) that receives discharge downstream of the primary transfer portion N1 on the more downstream side in addition to discharge downstream of the primary transfer portion N1 where the toner is primarily transferred, and the toner (black toner) that receives only discharge downstream of the primary transfer portion N1 where the toner is primarily transferred, from increasing. This ensures the secondary transfer efficiency of chromatic toner images such as a two-color solid toner image with a relatively low secondary transfer electric field strength, and prevents a decrease in the secondary transfer efficiency of the black toner image.

[0101] 17, the toner on the intermediate transfer belt 6 tends to broaden its charge distribution and increase its average charge value each time it is discharged downstream of the primary transfer portion N1. Therefore, looking at it in more detail, when the same potential regulating bias is applied to the yellow, magenta and cyan potential regulating members 8y, 8m and 8c, there is still a difference in the charge amounts on the intermediate transfer belt 6 of the yellow, magenta and cyan toners that pass through the primary transfer portion N1 different times.

[0102] Therefore, in this embodiment, the charge amount of the toner that has been primarily transferred to the intermediate transfer belt 6 on the upstream side and the charge amount of the toner that has been primarily transferred to the intermediate transfer belt 6 on the downstream side are made more uniform, thereby achieving more stable and better secondary transfer efficiency.

[0103] 2. Potential regulation bias 2-1. Determination of potential regulation bias A method for determining the potential regulating bias in this embodiment will be described below, taking the case where the image forming apparatus 1 forms images in a full-color mode.

[0104] The job procedure including the control for determining the potential regulating bias in this embodiment is similar to that in the embodiment 1 described using the flowchart of Fig. 7. In this embodiment, the control unit 3 controls the potential regulating bias to be applied to the potential regulating members 8y, 8m, 8c, and 8k of the image forming units 10 located further downstream so as to be smaller.

[0105] The control unit 3 determines the target voltage of the potential regulating bias to be applied to the yellow, magenta, cyan, and black potential regulating members 8y, 8m, 8c, and 8k in accordance with the environment determined in S102 (S103), as in the first embodiment. In this embodiment, the control unit 3 reads out and determines the target voltage of the potential regulating bias corresponding to the environment from table data such as that shown in Table 3, which is preset and stored in the ROM 32.

[0106] [Table 3]

[0107] In this embodiment, when the control unit 3 determines in S102 that the environment is low humidity, it decides to apply potential regulating biases of −3000 V, −2500 V, −2000 V, and −1500 V to the yellow, magenta, cyan, and black potential regulating members 8y, 8m, 8c, and 8k, respectively, as shown in Table 3.

[0108] Furthermore, in this embodiment, when the control unit 3 determines in S102 that the environment is high humidity, it determines to apply potential regulating biases of -3000V, -2500V, and -2000V to the yellow, magenta, and cyan potential regulating members 8y, 8m, and 8c, respectively, as shown in Table 3. In this case, the control unit 3 determines to turn off the black potential regulating power supply 80k (potential regulating member 8k is electrically grounded) or to apply a potential regulating bias of approximately 0V to the black potential regulating member 8k (constant voltage control with a target voltage of approximately 0V) as shown in Table 3.

[0109] That is, in this embodiment, in each environment, the potential regulating bias applied to the potential regulating members 8y, 8m, 8c, and 8k of the image forming units 10 located further downstream is smaller.

[0110] 2-2.Effects in low humidity environments Next, the effect of the setting of the potential regulating bias in this embodiment in a low humidity environment will be described.

[0111] FIG. 11 is a graph showing the relationship between the secondary transfer current and the secondary transfer efficiency of a two-color solid (M+C) toner image, a cyan (chromatic monochromatic solid) (C) toner image, and a black (black monochromatic solid) (K) toner image in a low humidity environment. FIG. 11(a) shows the relationship when the same potential regulation bias of -3000V is applied to the yellow, magenta, cyan, and black potential regulation members 8y, 8m, 8c, and 8k as a comparative example. FIG. 11(b) shows the relationship in this embodiment, that is, when potential regulation biases of -3000V, -2500V, -2000V, and -1500V are applied to the yellow, magenta, cyan, and black potential regulation members 8y, 8m, 8c, and 8k, respectively.

[0112] The weight of toner per unit area of ​​the two-color solid (M+C) toner image shown in FIG. 11 is approximately 0.9 to 1.2 mg / cm. 2 The weight of toner per unit area of ​​the cyan (C) toner image and black (K) toner image, the results of which are shown in FIG. 11, is approximately 0.45 to 0.65 mg / cm. 2 The method for measuring the secondary transfer efficiency is as described in Example 1.

[0113] As shown in Figures 11(a) and (b), the present embodiment has a higher secondary transfer efficiency of cyan (C) and black (K) toner images at a secondary transfer current at which the secondary transfer efficiency of a two-color solid (M+C) toner image is 95% than the comparative example. In the comparative example, the secondary transfer efficiency of a cyan (C) toner image at that secondary transfer current is less than 95%, whereas in the present embodiment, it is approximately 95%. Also, in the comparative example, the secondary transfer efficiency of a black (K) toner image at that secondary transfer current is less than 90%, whereas in the present embodiment, it exceeds 90%. Thus, in the present embodiment, the secondary transfer efficiency of cyan (C) and black (K) toner images is improved compared to the comparative example.

[0114] As shown in Figures 11(a) and (b), if the secondary transfer efficiency of the cyan toner image and the black toner image tends to decrease with an increase in the secondary transfer current, as described in Example 1, the smaller the toner charge amount, the more pronounced this tendency becomes.

[0115] Here, FIG. 12 shows the charge amount of the cyan toner on the cyan photosensitive drum 11c, the charge amount of the cyan toner on the intermediate transfer belt 6 after passing through the cyan primary transfer portion N1c (before reaching the black primary transfer portion N1k), and the charge amount of the cyan toner on the intermediate transfer belt 6 after passing through the black primary transfer portion N1k (before reaching the secondary transfer portion N2) in a low humidity environment. FIG. 12 shows the results when the potential regulating member 8 is not provided in all the image forming units 10, when the same potential regulating bias of −3000 V is applied to all the potential regulating members 8y, 8m, 8c, and 8k, and when this embodiment is used. Note that in this embodiment, the charge amount of the black toner on the black photosensitive drum 11k in a low humidity environment and the charge amount of the black toner on the intermediate transfer belt 6 after passing through the black primary transfer portion N1k (before reaching the secondary transfer portion N2) are as shown in FIG. 9(a).

[0116] As shown in FIG. 12, when the same potential regulating bias (-3000V) is applied to the potential regulating members 8y, 8m, 8c, and 8k of yellow, magenta, cyan, and black, the charge amount of the cyan toner on the intermediate transfer belt 6 does not increase, and the charge amount remains low, equivalent to the charge amount on the photosensitive drum 11c. On the other hand, when potential regulating biases of -3000V, -2500V, -2000V, and -1500V are applied to the potential regulating members 8y, 8m, 8c, and 8k of yellow, magenta, cyan, and black, respectively, as in this embodiment, the charge amount of the cyan toner on the intermediate transfer belt 6 increases, although not as much as when the potential regulating member 8 is not used. As a result, as described with reference to FIGS. 11(a) and 11(b), in this embodiment, the secondary transfer efficiency of the cyan toner image is improved compared to the comparative example. For the black toner, the potential regulating bias applied to the black potential regulating member 8k is lower than the potential regulating biases applied to the yellow, magenta, and cyan potential regulating members 8y, 8m, and 8c, so the charge amount does not decrease too much, similar to Example 1. As a result, as described with reference to Figures 11(a) and (b), in this embodiment, the secondary transfer efficiency of the black toner is improved compared to the comparative example.

[0117] Also, as shown in FIG. 12, in this embodiment, the charge amount of the cyan toner (chromatic toner) on the intermediate transfer belt 6 after passing through the black primary transfer portion N1k increases because the potential regulating bias (-1500V) applied to the black potential regulating member 8k is lower than when the same potential regulating bias (-3000V) is applied to the yellow, magenta, cyan, and black potential regulating members 8y, 8m, 8c, and 8k. However, compared to when the potential regulating member 8 is not used, the charge amount of the cyan toner on the intermediate transfer belt 6 after passing through the black primary transfer portion N1k can be reduced. The same can be said about the yellow toner and the magenta toner. As a result, as described with reference to FIGS. 11(a) and (b), the secondary transfer efficiency of the chromatic toner image, such as a two-color (M+C) solid toner image, can be ensured with a relatively low secondary transfer electric field strength.

[0118] Furthermore, as can be seen by comparing the present embodiment shown in FIG. 12 with the embodiment 1 shown in FIG. 9(b), in this embodiment, the charge amount of the cyan toner on the intermediate transfer belt 6 after passing through the black primary transfer portion N1k increases and the difference with the charge amount of the black toner on the intermediate transfer belt 6 becomes smaller than when the same potential regulating bias (-3000V) is applied to the yellow, magenta, and cyan potential regulating members 8y, 8m, and 8c. In this way, in this embodiment, the difference in the charge amount of the yellow, magenta, cyan, and black on the intermediate transfer belt 6 becomes smaller. As a result, in this embodiment, the charge amount of the toner primarily transferred to the intermediate transfer belt 6 on the upstream side and the charge amount of the toner primarily transferred to the intermediate transfer belt 6 on the downstream side are more uniformly aligned, thereby obtaining a more stable and good secondary transfer efficiency.

[0119] 2-3.Effects in high humidity environments Next, the effect of the setting of the potential regulating bias in this embodiment in a high humidity environment will be described.

[0120] In this embodiment, in a high humidity environment, the charge amount of black toner on the black photosensitive drum 11k and the charge amount of black toner on the intermediate transfer belt 6 after passing through the black primary transfer portion N1k (before reaching the secondary transfer portion N2) are as shown in Figure 10.

[0121] As shown in FIG. 10, the charge amount of the black toner on the black photosensitive drum 11k is lower in a high humidity environment than in a low humidity environment (FIG. 9(a)). Therefore, in this embodiment, the potential regulating bias applied to the black potential regulating member 8k in a high humidity environment is lower than the potential regulating bias applied to the black potential regulating member 8k in a low humidity environment. In this embodiment, in a high humidity environment, the black potential regulating power supply 80k is turned off or a potential regulating bias of approximately 0 V is applied to the black potential regulating member 8k. This prevents the charge amount of the black toner on the intermediate transfer belt 6 after passing through the black primary transfer portion N1k from decreasing too much in a high humidity environment. As a result, it is possible to suppress a decrease in the secondary transfer efficiency of the black toner image in a high humidity environment.

[0122] In this embodiment, the difference in the charge amount on the photosensitive drum 11 of the yellow, magenta, and cyan toners is sufficiently small between the low humidity environment and the high humidity environment. Therefore, in this embodiment, as shown in Table 3, the potential regulating bias applied to the potential regulating members 8y, 8m, and 8c of yellow, magenta, and cyan is the same in the low humidity environment and the high humidity environment. However, this is not limited to this, and the potential regulating bias applied to the potential regulating members 8y, 8m, and 8c of yellow, magenta, and cyan may be different between the low humidity environment and the high humidity environment for the same reason as described above for the black toner. Table 4 shows an example of the setting of the potential regulating bias when the charge amount on the photosensitive drum 11 of the yellow, magenta, cyan, and black toners is lower in the high humidity environment than in the low humidity environment. Similarly, at least one of the potential regulating biases applied to the potential regulating members 8y, 8m, 8c, and 8k of yellow, magenta, cyan, and black can be made different between the low humidity environment and the high humidity environment depending on the difference in the charge amount of the toner on the photosensitive drum 11 under the low humidity environment and the high humidity environment, provided that in each environment, the potential regulating bias applied to the potential regulating members 8y, 8m, 8c, and 8k of the image forming unit 10 located further downstream is made smaller.

[0123] [Table 4]

[0124] 3.Effects In this embodiment, the third photoconductor (e.g., the third photoconductor 11m) is disposed upstream of the first photoconductor (e.g., the first photoconductor 11c) in the moving direction of the intermediate transfer belt 6, and when performing image formation in which the toner images formed on the first, second, and third photoconductors (e.g., the third photoconductor 11m, the first photoconductor 11c, and the second photoconductor 11k) are transferred to the recording material S to form an image on the recording material S, the control unit 3 controls the application units (potential regulating power sources) 80m, 80c, and 80k so that the absolute value of the bias applied to the second electrode member (potential regulating member) 8k is smaller than the absolute value of the bias applied to the first electrode member (potential regulating member) 8c and the absolute value of the bias applied to the first electrode member (potential regulating member) 8c is smaller than the absolute value of the bias applied to the third electrode member (potential regulating member) 3m.

[0125] As described above, according to this embodiment, it is possible to ensure the secondary transfer efficiency of chromatic multi-color toner images with a large amount of toner, such as two-color solid images (M+C), with a relatively low secondary transfer electric field strength, while suppressing a decrease in the secondary transfer efficiency of toner images with a small amount of toner, such as a monochromatic cyan toner image or a monochromatic black toner image.

[0126] [Example 3] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

[0127] Fig. 13 is a schematic diagram showing a configuration example of a power supply that applies a potential regulating bias to the yellow, magenta, cyan, and black potential regulating members 8y, 8m, 8c, and 8k. In Examples 1 and 2, as shown in Fig. 13(a), potential regulating power supplies 80y, 80m, 80c, and 80k were provided independently for the yellow, magenta, cyan, and black potential regulating members 8y, 8m, 8c, and 8k, respectively.

[0128] However, the present invention is not limited to such a configuration, and a common power supply may be used for at least some of the potential regulation members 8y, 8m, 8c, and 8k of yellow, magenta, cyan, and black. By using a common power supply, it is possible to simplify and miniaturize the device configuration.

[0129] For example, as shown in Fig. 13(b), the image forming apparatus 1 may be configured to include a first potential regulating power supply 80A that applies a potential regulating bias to the yellow, magenta, and cyan potential regulating members 8y, 8m, and 8c, and a second potential regulating power supply 80B that applies a potential regulating bias to the black potential regulating member 8k. In this case, the first and second potential regulating power supplies 80A and 80B can be controlled to set the potential regulating bias in the same manner as in the first embodiment. In addition, by controlling the first and second potential regulating power supplies 80A and 80B and controlling an appropriately provided variable resistor or the like, the potential regulating bias applied to the yellow, magenta, and cyan potential regulating members 8y, 8m, and 8c can be variably controlled to set the potential regulating bias in the same manner as in the second embodiment.

[0130] 13(c), the image forming apparatus 1 may be configured to include a common potential regulating power supply 80 that applies a potential regulating bias to the yellow, magenta, cyan, and black potential regulating members 8y, 8m, 8c, and 8k. In this case, by controlling the potential regulating power supply 80 and appropriately controlling a variable resistor or the like, the potential regulating bias applied to the yellow, magenta, cyan, and black potential regulating members 8y, 8m, 8c, and 8k can be variably controlled to set the potential regulating bias in the same manner as in the first and second embodiments.

[0131] [Example 4] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

[0132] As described in the first embodiment, the reasons why the secondary transfer efficiency of the black toner image decreases when the potential regulating member 8 is used include the arrangement of the black image forming unit 10k and the difference in components between the black toner and the toner of a color other than black. In the first and second embodiments, the black image forming unit 10k is arranged at the most downstream position, and the setting of the potential regulating bias in the first and second embodiments makes it possible to suppress the decrease in the secondary transfer efficiency of the black toner image due to the above two reasons. However, the present invention is not limited to such a configuration, and can also be applied to a case where the black image forming unit 10k is not arranged at the most downstream position.

[0133] For example, in order to improve the glossiness and smoothness of an image, transparent toner and white toner may be used in addition to yellow, magenta, cyan and black toners. In addition, image forming units 10 of colors other than black (including transparent) may be arranged downstream of black image forming unit 10k. At least one of yellow, magenta and cyan image forming units 10y, 10m and 10c may be arranged downstream of black image forming unit 10k.

[0134] Fig. 14 is a schematic cross-sectional view showing a part of the configuration of the image forming apparatus 1 in this case. In the configuration shown in Fig. 14, an image forming unit 10t using a transparent toner or a white toner is disposed downstream (furthest downstream) of a black image forming unit 10k. Note that the suffix t to the reference numerals in the drawing indicates that the element is provided for transparent or white toner.

[0135] In this case, similarly to the first embodiment, the potential regulating bias applied to the black potential regulating member 8k can be made smaller than the potential regulating bias applied to the potential regulating members 8y, 8m, 8c, and 8t of colors other than black, such as yellow, magenta, cyan, and transparent. This makes it possible to prevent the charge amount of the black toner on the intermediate transfer belt 6 from becoming lower than the charge amount of the toner of colors other than black, due in particular to the difference in components between the black toner and the toner of colors other than black (mainly the difference in the carbon black content). This makes it possible to prevent the secondary transfer efficiency of the black toner image from decreasing.

[0136] Thus, in this embodiment, the first photoconductor (for example, the first photoconductor 11k) is disposed upstream of the second photoconductor (for example, the second photoconductor 11t) in the movement direction of the intermediate transfer belt 6, a toner image is formed on the first photoconductor 11k with black toner, and a toner image is formed on the second photoconductor 11t with toner of a color other than black, and when performing image formation in which the toner images formed on the first and second photoconductors 11k and 11t are transferred to the recording material S to form an image on the recording material S, the control unit 3 controls the application unit (potential regulating power supply) so that the absolute value of the bias applied to the first electrode member (potential regulating member) 8k is smaller than the absolute value of the bias applied to the second electrode member (potential regulating member) 8t.

[0137] In addition, in the configuration of this embodiment, as in the above-described embodiments, the potential regulating bias applied to the black potential regulating member 8k, and further to the potential regulating members 8 of colors other than black, can be changed depending on the environment.

[0138] [Example 5] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

[0139] In the above embodiment, the potential regulating member 8k was provided in the black image forming unit 10k as the downstream image forming unit 10. The potential regulating bias applied to this potential regulating member 8k was lowered to suppress a decrease in the secondary transfer efficiency of the black toner image as the toner image primarily transferred to the intermediate transfer belt 6 on the downstream side. However, depending on the characteristics of the toner primarily transferred to the intermediate transfer belt 6 on the downstream side or the black toner, the image forming unit 10 using that toner may not be provided with the potential regulating member 8.

[0140] Fig. 15(a) is a schematic cross-sectional view showing a partial configuration of an image forming apparatus 1 in which a potential regulating member 8 is not provided in a black image forming unit 10k that is the most downstream image forming unit 10 as a downstream image forming unit 10. Fig. 15(b) is a schematic cross-sectional view showing a partial configuration of an image forming apparatus 1 in which a potential regulating member 8 is not provided in a black image forming unit 10k that is not located at the most downstream as described in Example 4.

[0141] With this configuration, a decrease in the secondary transfer efficiency of a toner image formed in an image forming unit 10 that does not have a potential regulating member 8 can be suppressed by a mechanism similar to that of the above-described embodiment.

[0142] In the configuration of this embodiment, similarly to the above-described embodiment, the potential regulating bias applied to the potential regulating member 8 can be changed depending on the environment. Also, in the configuration of this embodiment, similarly to the second embodiment, the potential regulating bias applied to the potential regulating member 8 of the downstream image forming unit 10 can be made smaller than the potential regulating bias applied to the potential regulating member 8 of the upstream image forming unit 10.

[0143] [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.

[0144] 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.

[0145] Furthermore, in the above-described embodiment, the potential regulating bias was subjected to constant voltage control, but the potential regulating bias may be subjected to constant current control. Similarly, in the above-described embodiment, the primary transfer bias and the secondary transfer bias were each subjected to constant voltage control, but at least one of them may be subjected to constant current control. Here, constant current control refers to control that adjusts the output of the power supply so that the current supplied to the supply target is approximately constant at a target current. Furthermore, constant voltage control refers to control that adjusts the output of the power supply so that the voltage applied to the application target is approximately constant at a target voltage.

[0146] In the above embodiment, the environment is divided by temperature and humidity (relative humidity) for control, but the present invention is not limited thereto. For example, the environment may be divided by absolute moisture content (absolute humidity) based on temperature and humidity for control. In the above embodiment, the charge amount of the toner is sensitive to humidity (or moisture content), but depending on the configuration of the image forming apparatus and the characteristics of the toner, the environment may be divided by at least one of temperature and humidity for control. In the above embodiment, the environment is divided into two, but may be divided into more fine categories (three or more). For example, when changing the potential regulating bias as shown in Table 2 or Table 4, the potential regulating bias applied to each potential regulating member may be changed so that the higher the humidity of the environment, the smaller the potential regulating bias. The potential regulating bias corresponding to the environment between the environments set in the table data may be obtained by interpolation (linear interpolation, etc.). The potential regulating bias is not limited to being set based on the table data, but may be set by a calculation formula as information indicating the relationship between the environment and the potential regulating bias for each potential regulating member.

[0147] 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. [Explanation of symbols]

[0148] 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 first photoconductor and a second photoconductor capable of being charged to a predetermined polarity and carrying a toner image; an intermediate transfer belt that is movable in a circulating manner and that contacts the first and second photoconductors to form first and second primary transfer sections, respectively, and that transports the toner images that have been primarily transferred from the first and second photoconductors at the first and second primary transfer sections, respectively, to a recording material in a secondary transfer section for secondary transfer; first and second primary transfer members that are in contact with an inner circumferential surface of the intermediate transfer belt and to which a primary transfer bias having a polarity opposite to the predetermined polarity is applied, thereby primarily transferring the toner images from the first and second photoconductors to the intermediate transfer belt, respectively; first and second electrode members in contact with an inner circumferential surface of the intermediate transfer belt at adjacent positions downstream of the first and second primary transfer portions in a moving direction of the intermediate transfer belt; an application unit that applies a bias having the same polarity as the predetermined polarity to the first and second electrode members; A control unit capable of controlling the application unit; having the first photoconductor is disposed upstream of the second photoconductor in a moving direction of the intermediate transfer belt; an image forming apparatus characterized in that the control unit controls the application unit so that an absolute value of the bias applied to the second electrode member is smaller than an absolute value of the bias applied to the first electrode member when performing image formation in which a toner image formed on the first and second photosensitive members is transferred to a recording material to form an image on the recording material.

2. 2. The image forming apparatus according to claim 1, wherein the second photoconductor is disposed at the most downstream position in the moving direction of the intermediate transfer belt among a plurality of photoconductors included in the image forming apparatus.

3. 3. The image forming apparatus according to claim 1, wherein a toner image is formed on the first photoconductor using a toner of a color other than black, and a toner image is formed on the second photoconductor using a toner of black.

4. An environment detection means for detecting an environment, 2. The image forming apparatus according to claim 1, wherein the control unit controls the application unit so that the absolute value of the bias applied to the second electrode member when forming the image is smaller when the humidity indicated by the detection result by the environmental detection means is a second humidity higher than the first humidity than when the humidity is a first humidity.

5. 5. The image forming apparatus according to claim 4, wherein the control unit controls the application unit so that the absolute value of the bias applied to the first electrode member when forming the image is smaller when the humidity indicated by the detection result by the environment detection means is the second humidity higher than the first humidity than when the humidity is the first humidity.

6. a third photoconductor capable of being charged to the predetermined polarity and carrying a toner image; a third primary transfer member that contacts an inner circumferential surface of the intermediate transfer belt, and that performs primary transfer of a toner image from the third photoconductor to the intermediate transfer belt at a third primary transfer portion formed by the intermediate transfer belt contacting the third photoconductor when a primary transfer bias having a polarity opposite to the predetermined polarity is applied; a third electrode member that is in contact with an inner circumferential surface of the intermediate transfer belt at a position adjacent to the third primary transfer portion on the downstream side in the moving direction of the intermediate transfer belt, and to which a bias having the same polarity as the predetermined polarity is applied by the application portion; having the third photoconductor is disposed upstream of the first photoconductor in a moving direction of the intermediate transfer belt, 2. The image forming apparatus according to claim 1, wherein the control unit controls the application unit so that, when performing image formation in which the toner images formed on the first, second, and third photoconductors are transferred to a recording material to form an image on the recording material, the absolute value of the bias applied to the second electrode member is smaller than the absolute value of the bias applied to the first electrode member, and the absolute value of the bias applied to the first electrode member is smaller than the absolute value of the bias applied to the third electrode member.

7. a first photoconductor and a second photoconductor capable of being charged to a predetermined polarity and carrying a toner image; an intermediate transfer belt that is movable in a circulating manner and that contacts the first and second photoconductors to form first and second primary transfer sections, respectively, and that transports the toner images that have been primarily transferred from the first and second photoconductors at the first and second primary transfer sections, respectively, to a recording material in a secondary transfer section for secondary transfer; first and second primary transfer members that are in contact with an inner circumferential surface of the intermediate transfer belt and to which a primary transfer bias having a polarity opposite to the predetermined polarity is applied, thereby primarily transferring the toner images from the first and second photoconductors to the intermediate transfer belt, respectively; first and second electrode members in contact with an inner circumferential surface of the intermediate transfer belt at adjacent positions downstream of the first and second primary transfer portions in a moving direction of the intermediate transfer belt; an application unit that applies a bias having the same polarity as the predetermined polarity to the first and second electrode members; A control unit capable of controlling the application unit; having the first photoconductor is disposed upstream of the second photoconductor in a moving direction of the intermediate transfer belt; A toner image is formed on the first photoconductor using black toner, and a toner image is formed on the second photoconductor using toner of a color other than black, an image forming apparatus characterized in that the control unit controls the application unit so that an absolute value of the bias applied to the first electrode member is smaller than an absolute value of the bias applied to the second electrode member when performing image formation in which a toner image formed on the first and second photosensitive members is transferred to a recording material to form an image on the recording material.

8. a first photoconductor and a second photoconductor capable of being charged to a predetermined polarity and carrying a toner image; an intermediate transfer belt that is movable in a circulating manner and that contacts the first and second photoconductors to form first and second primary transfer sections, respectively, and that transports the toner images that have been primarily transferred from the first and second photoconductors at the first and second primary transfer sections, respectively, to a recording material in a secondary transfer section for secondary transfer; first and second primary transfer members that are in contact with an inner circumferential surface of the intermediate transfer belt and to which a primary transfer bias having a polarity opposite to the predetermined polarity is applied, thereby primarily transferring the toner images from the first and second photoconductors to the intermediate transfer belt, respectively; an electrode member that contacts an inner circumferential surface of the intermediate transfer belt at a downstream adjacent position of one of the first and second primary transfer portions in a moving direction of the intermediate transfer belt; an application unit that applies a bias having the same polarity as the predetermined polarity to the electrode member; having the first photoconductor is disposed upstream of the second photoconductor in a moving direction of the intermediate transfer belt; an electrode member is provided at an adjacent position downstream of the first primary transfer unit in the movement direction of the intermediate transfer belt, and the electrode member is not provided at an adjacent position downstream of the second primary transfer unit in the movement direction of the intermediate transfer belt.

9. 9. The image forming apparatus according to claim 8, wherein the second photoconductor is disposed at the most downstream position in the moving direction of the intermediate transfer belt among a plurality of photoconductors included in the image forming apparatus.

10. 10. The image forming apparatus according to claim 8, wherein a toner image is formed on the first photoconductor using a toner of a color other than black, and a toner image is formed on the second photoconductor using a toner of black.

11. a third photoconductor capable of being charged to the predetermined polarity and carrying a toner image; a third primary transfer member that contacts an inner circumferential surface of the intermediate transfer belt, and that performs primary transfer of a toner image from the third photoconductor to the intermediate transfer belt at a third primary transfer portion formed by the intermediate transfer belt contacting the third photoconductor when a primary transfer bias having a polarity opposite to the predetermined polarity is applied; another electrode member that contacts an inner circumferential surface of the intermediate transfer belt at a position adjacent to the third primary transfer portion on the downstream side in the moving direction of the intermediate transfer belt, and to which a bias having the same polarity as the predetermined polarity is applied by the application portion; having the third photoconductor is disposed upstream of the first photoconductor in a moving direction of the intermediate transfer belt, 9. The image forming apparatus according to claim 8, wherein the control unit controls the application unit so that an absolute value of the bias applied to the electrode member is smaller than an absolute value of the bias applied to the other electrode member when performing image formation in which the toner images formed on the first, second, and third photoconductors are transferred to a recording material to form an image on the recording material.

12. a first photoconductor and a second photoconductor capable of being charged to a predetermined polarity and carrying a toner image; an intermediate transfer belt that is movable in a circulating manner and that contacts the first and second photoconductors to form first and second primary transfer sections, respectively, and that transports the toner images that have been primarily transferred from the first and second photoconductors at the first and second primary transfer sections, respectively, to a recording material in a secondary transfer section for secondary transfer; first and second primary transfer members that are in contact with an inner circumferential surface of the intermediate transfer belt and to which a primary transfer bias having a polarity opposite to the predetermined polarity is applied, thereby primarily transferring the toner images from the first and second photoconductors to the intermediate transfer belt, respectively; an electrode member that contacts an inner circumferential surface of the intermediate transfer belt at a downstream adjacent position of one of the first and second primary transfer portions in a moving direction of the intermediate transfer belt; an application unit that applies a bias having the same polarity as the predetermined polarity to the electrode member; having the first photoconductor is disposed upstream of the second photoconductor in a moving direction of the intermediate transfer belt; A toner image is formed on the first photoconductor using black toner, and a toner image is formed on the second photoconductor using toner of a color other than black, An image forming apparatus characterized in that the electrode member is not provided at an adjacent position downstream of the first primary transfer unit in the movement direction of the intermediate transfer belt, and the electrode member is provided at an adjacent position downstream of the second primary transfer unit in the movement direction of the intermediate transfer belt.

Images