Image forming apparatus

Abstract

In a configuration in which a bias of the same polarity as the charging polarity of the photoreceptor is applied to an electrode member positioned downstream of the primary transfer section, discharge through the space between the primary transfer section and the electrode member is suppressed. [Solution] The image forming apparatus 1 comprises a photoreceptor 11, an intermediate transfer belt 6, a primary transfer member 15, an electrode member 8 provided on the inner circumferential surface side of the intermediate transfer belt 6 downstream of the primary transfer section N1 in the direction of movement of the intermediate transfer belt 6, a power supply that applies a voltage with the same polarity as the charging polarity of the photoreceptor 11 to the electrode member 8, and an electrically insulating shielding member 9 provided on the inner circumferential surface side of the intermediate transfer belt 6 downstream of the primary transfer section N1 in the direction of movement, and positioned between the primary transfer member 15 and the electrode member 8 to electrically shield the primary transfer member 15 and the electrode member 8.

Description

[Technical Field]

[0001] The present invention relates to image forming apparatus such as photocopiers, printers, facsimile machines, or multifunction devices that have multiple functions among these, using electrophotographic or electrostatic recording methods. [Background technology]

[0002] Image forming devices such as color copiers, color printers, and color multifunction devices using electrophotography have become mainstream due to their advantages such as miniaturization of the device body and relatively easy compatibility with various recording materials. Image forming devices using the intermediate transfer method generally have a configuration comprising multiple photosensitive drums and an intermediate transfer belt. In this image forming device, toner images formed on the multiple photosensitive drums are sequentially electrostatically transferred onto the intermediate transfer belt in the primary transfer section. The toner images transferred onto the intermediate transfer belt are then electrostatically transferred onto a recording material such as paper in the secondary transfer section. When referring to the arrangement of components around the primary transfer section, upstream and downstream refer to the upstream and downstream directions of transport of the intermediate transfer belt unless otherwise specified.

[0003] In the image forming apparatus described above, the toner on the intermediate transfer belt tends to become charged due to discharge between the intermediate transfer belt and the photosensitive drum downstream of the primary transfer section. This increased charge on the intermediate transfer belt can make it difficult to transfer the toner to the recording material in the secondary transfer section. For example, a larger secondary transfer electric field required to transfer the toner to the recording material in the secondary transfer section can worsen the granularity of the image or make it difficult to uniformly transfer toner to embossed paper or other materials with uneven surfaces.

[0004] In Patent Document 1, a conductive contact plate is provided downstream of the primary transfer section and on the inner circumferential 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] Japanese Patent Publication No. 2003-57963 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] To suppress the increase in toner charge downstream of the primary transfer section as described above, it is effective to suppress discharge downstream of the primary transfer section. Furthermore, to suppress this discharge, it is effective to reduce the potential difference between the photosensitive drum after passing through the primary transfer section and the intermediate transfer belt. To achieve this, it is effective to place a conductive electrode member downstream of the primary transfer section and on the inner surface of the intermediate transfer belt, and to apply a bias to this electrode member with the same polarity as the charge polarity of the photosensitive drum.

[0007] However, the polarity of the bias applied to the electrode member is the same as the charging polarity of the photosensitive drum, and is the opposite polarity to the bias applied to the primary transfer member that forms the primary transfer section. Therefore, when the electrode member is placed close to the primary transfer section and the bias applied to the electrode member is increased, a large potential difference is generated between the electrode member and the primary transfer member. This potential difference can cause discharge (leakage) through the space between the primary transfer member and the electrode member. As a result, for example, the effect of the electrode member in suppressing discharge between the intermediate transfer belt and the photosensitive drum may be reduced.

[0008] Therefore, an object of the present invention is to suppress discharge through the space between the primary transfer member and the electrode member in a configuration in which a bias of the same polarity as the charging polarity of the photoreceptor is applied to an electrode member placed downstream of the primary transfer member. [Means for solving the problem]

[0009] The above objective is achieved by the image forming apparatus according to the present invention. In summary, the present invention is an image forming apparatus characterized by comprising: a photoreceptor capable of being charged to a predetermined polarity and carrying a toner image; a circumferentially movable intermediate transfer belt for transporting a toner image primarily transferred from the photoreceptor to a recording material for secondary transfer; a primary transfer member that contacts the inner circumferential surface of the intermediate transfer belt to form a primary transfer section where the photoreceptor and the intermediate transfer belt come into contact, and to which a voltage is applied to transfer a toner image from the photoreceptor to the intermediate transfer belt in the primary transfer section; an electrode member provided on the inner circumferential surface side of the intermediate transfer belt downstream of the primary transfer section in the direction of movement of the intermediate transfer belt; a power supply for applying a voltage of the same polarity as the predetermined polarity to the electrode member; and an electrically insulating shielding member provided on the inner circumferential surface side of the intermediate transfer belt downstream of the primary transfer section in the direction of movement, and positioned between the primary transfer member and the electrode member to electrically shield the primary transfer member and the electrode member. [Effects of the Invention]

[0010] According to the present invention, in a configuration in which a bias of the same polarity as the charging polarity of the photoreceptor is applied to an electrode member placed downstream of the primary transfer section, discharge through the space between the primary transfer section and the electrode member can be suppressed. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic cross-sectional view of an image forming apparatus. [Figure 2] This is a schematic block diagram of the control system of an image forming apparatus. [Figure 3] These are schematic cross-sectional and perspective views of the potential regulating member. [Figure 4] This is a schematic cross-sectional view of another example of a potential regulating member. [Figure 5] This is a schematic cross-sectional view of another example of a potential regulating member. [Figure 6] This is a schematic cross-sectional view illustrating the arrangement of the potential regulating members. [Figure 7]It is a schematic cross-sectional view for explaining the configuration and arrangement of the shielding member. [Figure 8] It is a plan view for explaining the longitudinal configuration and arrangement of the shielding member. [Figure 9] It is a schematic cross-sectional view for explaining the force relationship between the shielding member and the potential regulating member. [Figure 10] It is a schematic cross-sectional view for explaining the configuration and arrangement of a modified example of the shielding member. [Figure 11] It is a schematic cross-sectional view for explaining the force relationship between the shielding member and the potential regulating member in another embodiment.

Mode for Carrying Out the Invention

[0012] Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

[0013] [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 employing an intermediate transfer method, which can form a full-color image on a sheet-like recording material S using an electrophotographic method.

[0014] The image forming apparatus 1 comprises an image forming unit 2, a control unit 3, a recording material S feeding unit 4, and a recording material S discharge unit 5. Inside the image forming apparatus 1, there is also a temperature sensor 71 (Figure 2) capable of detecting the internal temperature and a humidity sensor 72 (Figure 2) capable of detecting the internal humidity. The image forming apparatus 1 can form an image on the recording material S based on image information (image signals) acquired by a document reader (not shown) provided in or connected to the image forming apparatus 1. Furthermore, the image forming apparatus 1 can form an image on the recording material S based on image information (image signals) from external devices (not shown) such as a personal computer (host device), digital camera, or smartphone connected to the image forming apparatus 1. The recording material (transfer material, recording medium, sheet) S is one on which an image is formed using toner. Specific examples of recording material S include plain paper, cardboard, gloss-coated paper, matte-coated paper, embossed paper, or substitutes for plain paper such as synthetic resin sheets (synthetic paper) or overhead projector sheets (resin film). Here, recording material S is sometimes referred to as "paper" (e.g., "paper," "embossed paper," "high-resistance paper"), but even in that case, recording material S includes materials other than paper or materials that include materials other than paper.

[0015] The image forming unit 2 forms an image on the recording material S supplied from the feeding unit 4 based on image information. The image forming unit 2 includes image forming units 10y, 10m, 10c, and 10k, toner bottles 18y, 18m, 18c, and 18k, exposure devices 13y, 13m, 13c, and 13k, an intermediate transfer unit 20, a secondary transfer device 26, and a fixing device 27. The image forming units 10y, 10m, 10c, and 10k each form toner images of yellow (y), magenta (m), cyan (c), and black (k), respectively. In addition, elements with the same or corresponding functions or configurations provided for each color may be described collectively by omitting the suffixes y, m, c, and k that indicate that they are elements for any of the colors. Furthermore, the image forming apparatus 1 can also form monochrome images, such as a monochrome black image, or multicolor images using image forming units 10 for several of the desired single colors or four colors.

[0016] The image forming unit 10 has a photosensitive drum 11, which is a drum-shaped (cylindrical) photoreceptor (electrophotographic photoreceptor) that serves as an image carrier. The image forming unit 10 also has a charging roller 12, which is a roller-shaped charging member that serves as a charging means. The image forming unit 10 also has a developing device 14 as a developing means. The image forming unit 10 also has a pre-exposure device 16 as a static elimination means. The image forming unit 10 also has a drum cleaning device 17 as a photoreceptor cleaning means. The image forming unit 10 forms a toner image on the intermediate transfer belt 6, which will be described later.

[0017] The photosensitive drum 11 is movable (rotatable) and carries an electrostatic image (electrostatic latent image) or toner image. In this embodiment, the photosensitive drum 11 is a drum-type photoreceptor equipped with a negatively charged organic photoreceptor (OPC), and has an outer diameter of 30 mm. This photosensitive drum 11 can be charged to a negative polarity as a predetermined polarity. In this embodiment, the photosensitive drum 11 has an aluminum cylinder as a substrate and a surface layer (photosensitive layer) formed on its surface. In this embodiment, the surface layer has three layers, which are coated and laminated on the substrate in the following order: an undercoat layer, a photocharge generation layer, and a charge transport layer. When the image forming operation is started, the photosensitive drum 11 is rotated by a drive motor (not shown) as a driving means at a predetermined peripheral speed (process speed) in the direction of arrow R1 in the figure (counterclockwise direction).

[0018] The surface of the rotating photosensitive drum 11 is uniformly charged by the charging roller 12. In this embodiment, the charging roller 12 is a rubber roller that contacts the surface of the photosensitive drum 11 and rotates in conjunction with the rotation of the photosensitive drum 11. A charging power supply 73 (Figure 2), which serves as a means for applying a charging voltage (charging voltage application unit), is connected to the charging roller 12. During the charging process, the charging power supply 73 applies a predetermined charging voltage (charging bias) to the charging roller 12.

[0019] 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 the separated color image information output from the control unit 3 and scans and exposes the surface (outer surface) of the photosensitive drum 11.

[0020] The electrostatic image formed on the photosensitive drum 11 is developed (visualized) by the toner supplied 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 comprising toner (non-magnetic toner particles) and carrier (magnetic carrier particles) as the developer. The developing container (developing container body) 14b of the developing device 14 contains the two-component developer, and an amount of toner corresponding to the consumed toner is replenished from the 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 magnet roller (not shown), which is a roller-shaped magnet serving as a magnetic field generating means (magnetic field generating member), is fixed to the developing container 14b so as not to rotate. The developing sleeve 14a carries a two-component developer and transports it to the 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 supply 74 (Figure 2), which serves as a developing voltage application means (developing voltage application unit), is connected to the developing sleeve 14a. During development, the developing power supply 74 applies a predetermined developing voltage (developing bias) to the developing sleeve 14a. In this embodiment, toner charged with the same polarity as the charging polarity of the photosensitive drum 11 (negative polarity in this embodiment) adheres to the exposed area (image portion) on the photosensitive drum 11, where the absolute value of the potential has decreased after uniform charging treatment and exposure (reverse developing 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.

[0021] An intermediate transfer unit 20 is positioned opposite four photosensitive drums 11y, 11m, 11c, and 11k. The intermediate transfer unit 20 has an intermediate transfer belt 6, which is an endless belt that serves as an intermediate transfer body. The intermediate transfer belt 6 is wrapped around and stretched over a plurality of tension rollers: drive rollers 21, tension rollers 22, and secondary transfer inner rollers 23. The intermediate transfer belt 6 is movable (rotatable, circumferential) carrying the toner image. The intermediate transfer belt 6 is driven by a drive motor (not shown) which rotates the drive rollers 21, and rotates (circumferentially moves) in the direction of arrow R2 in the figure (clockwise) at a predetermined peripheral speed corresponding to the peripheral speed of the photosensitive drum 11. The tension rollers 22 control the tension of the intermediate transfer belt 6 to be constant. The tension roller 22 is subjected to a biasing force from a tension spring (not shown) composed of a compression coil spring, which is a biasing member acting as a biasing means. This force pushes the intermediate transfer belt 6 from the inner circumferential surface (back side) to the outer circumferential surface (front side). This force applies a tension of approximately 2 to 5 kgf to the intermediate transfer belt 6 in its conveying direction (process progression direction, movement direction). The secondary transfer inner roller 23, together with the secondary transfer outer roller 25 (described later), constitutes the secondary transfer apparatus 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 acting as primary transfer means, are arranged corresponding to each photosensitive drum 11y, 11m, 11c, and 11k. In this embodiment, the primary transfer roller 15 is positioned opposite the photosensitive drum 11 and sandwiches the intermediate transfer belt 6 between itself and the photosensitive drum 11. The primary transfer roller 15 is pressed toward the photosensitive drum 11 and comes into contact with the photosensitive drum 11 via the intermediate transfer belt 6, forming a primary transfer portion (primary transfer nip portion) N1, which is the contact area between the photosensitive drum 11 and the intermediate transfer belt 6.

[0022] The toner image formed on the photosensitive drum 11 is transferred (primary transfer) to the rotating intermediate transfer belt 6 in the primary transfer unit N1 by the action of the primary transfer roller 15. For example, when forming a full-color image, the toner images of yellow, magenta, cyan, and black formed on each photosensitive drum 11 are sequentially superimposed on the intermediate transfer belt 6 in a multiple transfer process. A primary transfer power supply 75 (Figure 2), which serves as a primary transfer voltage application means (primary transfer voltage application unit), is connected to the primary transfer roller 15. During primary transfer, the primary transfer power supply 75 applies a primary transfer voltage (primary transfer bias), which is a DC voltage with the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, to the primary transfer roller 15. As a result, the toner image formed with negatively polarized toner on the photosensitive drum 11 is primary transferred to the intermediate transfer belt 6. The primary transfer power supply 75 is connected to a voltage detection sensor 75a (Figure 2), which serves as a voltage detection means (voltage detection unit) for detecting its output voltage, and a current detection sensor 75b (Figure 2), which serves as a current detection means (current detection unit) for detecting its output current. In this embodiment, for example, a primary transfer voltage of about 1 to 2 [kV] is applied to the primary transfer roller 15 (regarding the numerical range, "~" indicates a range that includes the numbers before and after it. The same applies hereinafter.). In this embodiment, the primary transfer voltage is controlled to a constant voltage. In this embodiment, the primary transfer power supplies 75y, 75m, 75c, and 75k are provided independently for each of the primary transfer rollers 15y, 15m, 15c, and 15k. In this embodiment, the primary transfer voltage applied to each of the primary transfer rollers 15y, 15m, 15c, and 15k can be controlled individually.

[0023] 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 value of 1 × 10⁻¹⁰. 5 ~1 × 10 8 A roller with a value of [Ω] (N / N measurement at 23°C, 50%RH, 2kV applied) can be suitably used.

[0024] 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 that order from the inner circumferential surface to the outer circumferential surface. 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 can be, for example, CR rubber imparted with carbon black for conductivity, or the like. 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 × 10⁻⁶. 8 ~1 × 10 14 The resistance is [Ω·cm] (23℃, 50%RH). In this embodiment, the intermediate transfer belt 6 has a two-layer structure, but it may also have a single-layer structure of the same material as the base layer described above. The surface layer may also be a resin coating layer with a thickness of about 0.002 to 0.01 [mm] containing a resin such as fluororesin. Furthermore, the intermediate transfer belt 6 may have a multilayer structure of three or more layers.

[0025] A secondary transfer outer roller 25, which is a roller-type secondary transfer member serving as a secondary transfer means, is positioned on the outer circumferential surface of the intermediate transfer belt 6. The secondary transfer outer roller 25, as a secondary transfer member, together with the secondary transfer inner roller 23, which serves as a counter member (counter electrode), constitutes the secondary transfer apparatus 26. The secondary transfer outer roller 25 is pressed toward the secondary transfer inner roller 23 and contacts the secondary transfer inner roller 23 via the intermediate transfer belt 6, forming a secondary transfer section (secondary transfer nip section) N2, which is the contact point 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) in the secondary transfer section N2 to the recording material S, which is being transported sandwiched between the intermediate transfer belt 6 and the secondary transfer outer roller 25, by the action of the secondary transfer apparatus 26. A secondary transfer power supply 76 (Figure 2), which serves as a secondary transfer voltage application means (secondary transfer voltage application section), is connected to the secondary transfer outer roller 25. During secondary transfer, the secondary transfer power supply 76 applies a secondary transfer voltage (secondary transfer bias), which is a DC voltage with the opposite polarity (positive polarity in this embodiment) to the secondary transfer outer roller 25 of the normal charging polarity of the toner. As a result, the toner image formed by the negatively polarized toner on the intermediate transfer belt 6 is secondary transferred onto the recording material S. The secondary transfer power supply 76 is connected to a voltage detection sensor 76a (Figure 2) as a voltage detection means (voltage detection unit) for detecting its output voltage, and a current detection sensor 76b (Figure 2) as a current detection means (current detection unit) for detecting its output current. The core metal of the secondary transfer inner roller 23 is connected to ground potential. In this embodiment, for example, a secondary transfer voltage 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] flows through the secondary transfer unit N2, thereby secondary transferring the toner image on the intermediate transfer belt 6 onto the recording material S. In this embodiment, the secondary transfer voltage is controlled to a constant voltage. Alternatively, a secondary transfer voltage, which is a DC voltage with the same polarity as the normal charging polarity of the toner, may be applied from the secondary transfer power supply 76 to the secondary transfer inner roller 23, which serves as a secondary transfer member, thereby connecting the secondary transfer outer roller 25, which serves as a counter member, to ground potential.

[0026] The recording material S is transported from the feeding unit 4 towards the secondary transfer unit N2 in parallel with the toner image formation operation on the intermediate transfer belt 6. The recording material S is housed in a cassette 41, which serves as the recording material housing of the feeding unit 4. The recording material S housed in the cassette 41 is separated one sheet at a time by feeding rollers 42 and other feeding members of the feeding unit 4 and fed out of the cassette 41. This recording material S is then transported by transport rollers 43 and other transport members of the feeding unit 4 to a register roller (pair of register rollers) 19, which serves as a transport member in the transport path 44 of the recording material S. Then, the recording material S is transported to the secondary transfer unit N2 by the register roller 19, in timing with the toner image on the intermediate transfer belt 6. Although only one cassette 41 is shown in Figure 1, the image forming apparatus 1 may have multiple cassettes 41. Furthermore, the feeding unit 4 may also be configured to feed recording material S from a recording material storage unit (recording material placement unit) separate from the cassette 41, such as a manual feed tray.

[0027] 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. Furthermore, the electrical resistance value of the secondary transfer outer roller 25 is 1 × 10⁻¹⁰. 5 ~1 × 10 8 A roller with a value of [Ω] (N / N measurement at 23°C, 50%RH, 2kV applied) can be suitably used.

[0028] The recording material S onto which the toner image has been transferred is transported to a fixing device 27, which serves as a fixing means. The fixing device 27 includes a fixing roller 27a and a pressure roller 27b. The fixing roller 27a has a built-in 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 heats and pressurizes the recording material S, which carries the unfixed toner image, by sandwiching it between the fixing roller 27a and the pressure roller 27b and transporting it, thereby fixing (melting and solidifying) the toner image onto the recording material S. The temperature of the fixing roller 27a (fixing temperature) is detected by a fixing temperature sensor 77 (Figure 2). The recording material S with the fixed toner image is transported by a discharge roller 51, etc., in the discharge section 5 and discharged (output) from a discharge port (not shown) onto a discharge tray 52 located outside the main body 1a of the image forming apparatus 1.

[0029] The surface of the photosensitive drum 11 after primary transfer is destaticized by the pre-exposure device 16. Furthermore, toner remaining on the photosensitive drum 11 during primary transfer (primary transfer residue toner) that was not transferred to the intermediate transfer belt 6 is removed and collected by the drum cleaning device 17. In this embodiment, the drum cleaning device 17 uses a cleaning blade as a cleaning member to scrape the primary transfer residue toner from the surface of the rotating photosensitive drum 11 and collect it in a collection container (not shown). The cleaning blade is a plate-shaped member that contacts the photosensitive drum 11 with a predetermined pressing force. The cleaning blade contacts the surface of the photosensitive drum 11 in a counter-direction relative to the rotation direction of the photosensitive drum 11, with its free end facing upstream in the rotation direction of the photosensitive drum 11. Additionally, any deposits such as toner remaining on the intermediate transfer belt 6 during secondary transfer (secondary transfer residue toner) that was not transferred to the recording material S are removed and collected by the belt cleaning device 24, which serves as an intermediate transfer body cleaning means.

[0030] The image forming unit 10 may be integrally formed as a detachable cartridge (process cartridge) attached to the main body 1a of the image forming apparatus 1. In this embodiment, the intermediate transfer unit 20 is composed of an intermediate transfer belt 6, tension rollers for the intermediate transfer belt 6, each primary transfer roller 15, a belt cleaning device 24, and each potential regulating member 8, which will be described later. The intermediate transfer unit 20 may be integrally formed as detachable from the main body 1a of the apparatus.

[0031] 2. Control Configuration Next, the configuration of the control system of the image forming apparatus 1 in this embodiment will be described. Figure 2 is a block diagram showing the schematic configuration of the control system of the image forming apparatus 1 in this embodiment.

[0032] The image forming apparatus 1 is equipped with a control unit 3 (control circuit) as a control means. The control unit 3 is composed of a CPU 31 as an arithmetic processing means, a ROM 32 and RAM 33 as storage means, and an input / output circuit (I / F) (not shown) that performs signal input and output between the control unit 3 and external devices. The ROM 32 stores programs that control each part of the image forming apparatus 1. The RAM 33 temporarily stores control-related data. The CPU 31 is a microprocessor that oversees the overall control of the image forming apparatus 1 and is the main component 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 ejection unit 5, and exchanges signals with these parts and controls the operation of these parts. The ROM 32 stores image forming control sequences for forming images on the recording material S.

[0033] The control unit 3 is connected to, for example, a charging power supply 73, a developing power supply 74, a primary transfer power supply 75, a secondary transfer power supply 76, and a potential regulating power supply 80 (described later), and these are each controlled by signals from the control unit 3. Although not shown in the figures, in this embodiment, the charging power supply 73, the developing power supply 74, the primary transfer power supply 75, and the potential regulating power supply 80 are each provided independently for each image forming unit 10. The control unit 3 is also connected to a temperature sensor 71, a humidity sensor 72, a voltage detection sensor 75a and a current detection sensor 75b of the primary transfer power supply 75, a voltage detection sensor 76a and a current detection sensor 76b of the secondary transfer power supply 76, and a fixing temperature sensor 77. Signals (information) indicating the detection results of each sensor are input to the control unit 3. The control unit 3 is also connected to an operation unit 70. The operation unit 70 has an input unit consisting of operation buttons (keys) as an input means, and a display unit 70a consisting of a liquid crystal panel (display) as a display means. In this embodiment, the display unit 70a is configured as a touch panel and also functions as an input means. Users and service personnel can operate the operation unit 70 to cause the image forming apparatus 1 to execute a job (a series of operations that form and output an image on one or more recording materials S with a single start command). The control unit 3 receives signals from the operation unit 70 and operates various devices of the image forming apparatus 1. The image forming apparatus 1 can also execute jobs in response to signals from external devices such as a personal computer, rather than from the operation unit 70.

[0034] 3. Suppression of discharge between the intermediate transfer belt and the photosensitive drum. Next, we will explain the suppression of discharge between the intermediate transfer belt 6 and the photosensitive drum 11. For convenience, unless otherwise specified, the magnitude (high or low) of voltage and potential will refer to the magnitude (high or low) when compared in absolute value. Furthermore, regarding the arrangement of the primary transfer section N1, the photosensitive drum 11, the primary transfer roller 15, and the potential regulating member 8 described later, unless otherwise specified, upstream and downstream will refer to the upstream and downstream in the transport direction (process progression direction, movement direction) of the intermediate transfer belt 6.

[0035] As mentioned above, in the intermediate transfer type image forming apparatus 1, it is difficult to uniformly transfer toner images to, for example, embossed paper with an uneven surface. Embossed paper is paper (fancy paper) that has patterns created by raising or pressing on the surface of the paper. In particular, transferring toner images to the recesses of embossed paper is difficult because a gap is created between the intermediate transfer belt 6 and the embossed paper in the secondary transfer section N2, requiring a relatively large transfer electric field. Furthermore, if the secondary transfer electric field is increased to improve the transferability of toner images to the recesses of embossed paper, if the transfer electric field becomes too large in areas other than the recesses, transfer defects may occur where toner is not partially transferred to halftone images, etc.

[0036] The toner on the intermediate transfer belt 6 experiences an increase in charge due to a discharge between the intermediate transfer belt 6 and the photosensitive drum 11 downstream of the primary transfer section N1. More specifically, after receiving the above discharge, the distribution of the toner's charge tends to broaden compared to the distribution during development, while the average value of the charge tends to increase. Furthermore, the increase in charge also increases the mirroring force between the intermediate transfer belt 6 and the toner, making the transfer electric field required to transfer the toner to the recording material S in the secondary transfer section N2 larger, and making it more difficult to transfer the toner image into the recesses of the embossed paper.

[0037] In other words, if the above-mentioned discharge can be suppressed, the increase in the charge amount of toner on the intermediate transfer belt 6 can be suppressed, and the transferability of the toner image to recording materials S, such as embossed paper, which are relatively difficult to transfer toner images to, can be improved. To suppress the above-mentioned discharge, it is effective to reduce the potential difference between the photosensitive drum 11 after passing through the primary transfer section N1 and the intermediate transfer belt 6. To achieve this, it is effective to place a potential regulating member 8, which is an electrode member, on the inner circumferential surface (back surface) side of the intermediate transfer belt 6 downstream of the primary transfer section N1, and to apply a voltage with the same polarity as the charge polarity of the photosensitive drum 11 to this potential regulating member 8. In particular, by placing the potential regulating member 8 in contact with the inner circumferential surface of the intermediate transfer belt 6 and applying a voltage with the same polarity as the charge polarity of the photosensitive drum 11 to this potential regulating member 8, the above-mentioned discharge can be suppressed more effectively.

[0038] Furthermore, the above-mentioned discharge often occurs in a range of approximately 0.3 to 1.5 mm downstream from the primary transfer section N1. In contrast, by applying a voltage with the same polarity as the charge polarity of the photosensitive drum 11 to the potential regulating member 8, it is believed that the above-mentioned discharge can be suppressed by the action of the electric field formed in the space between the photosensitive drum 11 and the potential regulating member 8. Moreover, in the transport direction of the intermediate transfer belt 6, the effect of suppressing the above-mentioned discharge is greater when the potential regulating member 8 is in contact with the intermediate transfer belt 6 over a wide surface area than when it is in contact with the intermediate transfer belt 6 at a point (line) area. In addition, by making the potential regulating member 8 in contact with the intermediate transfer belt 6 over a surface area, the contact state between the intermediate transfer belt 6 and the potential regulating member 8 can be stabilized. This is thought to be because an electrostatic attraction force acts between the intermediate transfer belt 6 and the potential regulating member 8. Therefore, it is more preferable to make the potential regulating member 8 in surface contact with the intermediate transfer belt 6. Here, "surface contact" means that contact is not included in cases where contact occurs only linearly in a direction intersecting the conveying direction of the intermediate transfer belt 6 within a range narrower than the contact width (approximately 5-50 mm), which will be described in more detail later. Therefore, for example, it includes not only cases where substantially the entire area is in continuous contact within the contact width, which will be described in more detail later, but also cases where a large number of contact points are substantially uniformly distributed within the above range, such as with nonwoven fabrics. Further details will be explained below.

[0039] 4. Potential regulating member Next, the configuration of the potential regulating member 8 will be described. As shown in Figure 1, in this embodiment, the image forming apparatus 1 has potential regulating members 8y, 8m, 8c, and 8k, which are electrode members, positioned downstream of each primary transfer section N1y, N1m, N1c, and N1k, in contact with the inner circumferential surface of the intermediate transfer belt 6. In this embodiment, the potential regulating members 8y, 8m, 8c, and 8k provided for each primary transfer section N1y, N1m, N1c, and N1k have substantially the same configuration.

[0040] The shape of the potential regulating member 8 in this embodiment will now be described. Figure 3(a) is a schematic cross-sectional view of an example of the potential regulating member 8 in this embodiment (a cross-section approximately perpendicular to the rotation axis direction of the photosensitive drum 11), and Figure 3(b) is a schematic perspective view of the potential regulating member 8.

[0041] In this embodiment, the potential regulating member 8 has a planar first portion 81 that is arranged along the width direction of the intermediate transfer belt 6 (a direction substantially perpendicular to the transport direction and substantially parallel to the rotation axis direction of the photosensitive drum 11). In this embodiment, the potential regulating member 8 also has a planar second portion 82 that is arranged along the width direction of the intermediate transfer belt 6 and extends in a direction that intersects (substantially perpendicular in this embodiment) with the plane of the first portion 81. In this embodiment, the contact surface 83 of the first portion 81 of the potential regulating member 8, which is the contact portion that contacts the inner circumferential surface of the intermediate transfer belt 6, is planar. In other words, in this embodiment, the first portion 81 that constitutes the contact surface 83 of the potential regulating member 8 is a flat plate. Thus, in this embodiment, the potential regulating member 8 is composed of a member whose cross-section, substantially perpendicular to the rotation axis direction of the photosensitive drum 11, is substantially L-shaped.

[0042] 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. As described above, 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 make the potential regulating member 8 in surface contact with the intermediate transfer belt 6. As a result of the investigation, in the potential regulating voltage of the configuration of this embodiment, it is preferable that the "contact width," which is the length between line segments A and B, that is, the length of the contact surface 83 in the transport direction of the intermediate transfer belt 6, be 5 [mm] or more. As will be described later, in the configuration of this embodiment, the potential regulating voltage is preferably around -500 to -8000 [V], and more preferably around -1000 to -5000 [V]. The longer the length between line segments A and B, the greater the effect of suppressing the discharge, but if it is made too long, it may become difficult to keep the potential regulating member 8 in stable contact with the intermediate transfer belt 6 due to the influence of component precision, etc. The length between line segments A and B is often sufficient at 50 [mm] or less, and is typically 30 [mm] or less. In other words, the length between line segments A and B is preferably around 5 to 50 [mm], and is typically 5 to 30 [mm]. From another point of view, the length between line segments A and B is often sufficient at half or less of the interaxial distance between adjacent photosensitive drums 11 in a cross section approximately perpendicular to the rotation axis direction of the photosensitive drum 11. In this embodiment, a potential regulating member 8 with a length of 25 [mm] between line segments A and B is used. In this embodiment, the distance between the photosensitive drums 11 in a cross-section approximately perpendicular to the rotation axis direction of the photosensitive drum 11 is about 100 mm.

[0043] A potential regulating power supply 80, which serves as a potential regulating voltage application means (potential regulating voltage application unit), is connected to the potential regulating member 8. In this embodiment, the potential regulating power supply 80 is connected to the second portion 82 of the potential regulating member 8. During at least the primary transfer in the image forming operation, a potential regulating voltage (potential regulating bias), which is a DC voltage with the same polarity as the charge polarity of the photosensitive drum 11, is applied to the potential regulating member 8 by the potential regulating power supply 80. Primary transfer refers to the period during which the primary transfer voltage is applied, specifically the period during which the image region (the region on which the toner image can be transferred) on the intermediate transfer belt 6 passes through the primary transfer unit N1. This makes it possible to suppress discharge between the intermediate transfer belt 6 and the photosensitive drum 11 downstream of the primary transfer unit N1. In this embodiment, the potential regulating voltage is a DC voltage with negative polarity. Furthermore, in the configuration of this embodiment, the potential limiting voltage is preferably around -500 to -8000 [V], more preferably around -1000 to -5000 [V], and typically around -1000 to -3000 [V].

[0044] The potential regulating member 8 is a member that is long in the width direction of the intermediate transfer belt 6. Preferably, the length of the contact surface 83 of the potential regulating member 8 in the longitudinal direction (along the width direction of the intermediate transfer belt 6) is 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 above-mentioned maximum image width and is also longer than the width of the portion of the primary transfer roller 15 that contacts the intermediate transfer belt 6 in the width direction of the intermediate transfer belt 6. In other words, in this embodiment, the range of the above-mentioned maximum image width and the range of the width of the portion of the primary transfer roller 15 that contacts the intermediate transfer belt 6 in the width direction of the intermediate transfer belt 6 are both within the range of the length of the contact surface 83 of the potential regulating member 8 in the longitudinal direction. 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 toner on the intermediate transfer belt 6 by suppressing the discharge described above can be obtained. On the other hand, in this embodiment, the longitudinal length of the potential regulating member 8 is shorter than the width of the intermediate transfer belt 6. In other words, in this embodiment, the range of the longitudinal length of the potential regulating member 8 is within the range of the width of the intermediate transfer belt 6. This makes it possible to suppress discharge between the potential regulating member 8 and surrounding members of the intermediate transfer belt 6, which may occur if the longitudinal end of the potential regulating member 8 protrudes beyond the widthwise end of the intermediate transfer belt 6. As a result, the possibility that the effect of the potential regulating member 8 in suppressing discharge between the intermediate transfer belt 6 and the photosensitive drum 11 will be reduced can be reduced.

[0045] The potential regulating member 8 can be composed of, for example, only one conductive material (material quality). In this embodiment, the potential regulating member 8 is substantially composed of only a conductive metal such as SUS (stainless steel). More specifically, in this embodiment, the potential regulating member 8 is formed by bending a metal plate (sheet metal) such as SUS to form a first portion 81 and a second portion 82. The thickness of the sheet metal used for the potential regulating member 8 may be, for example, about 0.5 to 5 mm, typically about 1 to 3 mm. By performing such bending, 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 substantially deform in the usage state of the image forming apparatus 1. However, the present invention is not limited to such a mode, and the potential regulating member 8 may be composed of two or more materials.

[0046] Note that a conductive material (conductor) generally refers to a material (substance) having a resistivity of 10 -6 (Ω·m) or less, typically about 10 -8 (Ω·m). Also, a non-conductive material (insulator) generally refers to a material (substance) having a resistance value of 10 8 (Ω·m) or more, preferably 10 10 (Ω·m) or more. However, typically, an insulator has a resistivity of 10 16 (Ω·m) or less.

[0047] Figure 4 is a schematic cross-sectional view (a cross-section approximately perpendicular to the rotation axis direction of the photosensitive drum 11) of another example of the potential regulating member 8 in this embodiment. For example, as shown in Figure 4, it can have a configuration having a base 84 having the same shape as the potential regulating member 8 shown in Figure 3, and a surface layer 85 provided on the surface of the base 84. The surface layer 85 that constitutes the contact surface 83 that contacts the intermediate transfer belt 6 and the connection part with the potential regulating power supply 80 is made of a conductive material, for example, metal or a conductive resin. The surface layer 85 may have the same configuration as the potential regulating member 8 shown in Figure 3. The base 84 may be made of a conductive material, but it may also be made of a non-conductive (electrically insulating) material, for example, a non-conductive resin. The base 84 and the surface layer 85 can be fixed together by any fixing means such as adhesive bonding or welding.

[0048] Figure 5 is a schematic cross-sectional view of yet another example of the potential regulating member 8 in this embodiment (a cross-section approximately perpendicular to the rotation axis direction of the photosensitive drum 11). For example, as shown in Figure 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 the example shown in Figure 5, the conductive nonwoven fabric 86 is provided on the contact surface 83 of the potential regulating member 8 with the configuration shown in Figure 4, but the conductive nonwoven fabric 86 may also be provided on the contact surface 83 of the potential regulating member 8 with the configuration shown in Figure 3. In other words, the potential regulating member 8 may be formed using different materials for the base portion to which the voltage is applied and the surface layer that contacts the inner circumferential surface of the intermediate transfer belt 6. The conductive nonwoven fabric 86 can be fixed by any fixing means such as a conductive adhesive. Alternatively, instead of the nonwoven fabric 86, felt or pile fabric (cut pile fabric (velvet, brush) or loop pile fabric (terry cloth)) made of conductive fibers, or a sponge (foamed elastic body) made of conductive rubber material may be used. By making the contact surface 83 of the potential regulating member 8 that contacts the intermediate transfer belt 6 a flexible or elastic material in this way, the possibility of damage occurring to 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.

[0049] Next, the arrangement of the potential regulating members 8 will be described. In this embodiment, the arrangement of the potential regulating members 8 provided for each primary transfer section N1y, N1m, N1c, and N1k is substantially the same. Figure 6 is a schematic cross-sectional view (a cross-section substantially perpendicular to the rotation axis direction of the photosensitive drum 11) illustrating the arrangement of the potential regulating members 8 provided between two adjacent primary transfer sections N1 in the transport direction of the intermediate transfer belt 6. Figure 5 shows, as an example, the potential regulating members 8c provided between the cyan and black primary transfer sections N1c and N1k.

[0050] 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 positioned offset downstream of 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 a direction along (approximately parallel to) the common tangent line on the side of the multiple photosensitive drums 11 that contact the intermediate transfer belt 6, in a cross-section perpendicular to the longitudinal direction of the photosensitive drum 11. The primary transfer roller 15 may also be positioned without offset relative to the photosensitive drum 11, or it may be positioned offset upstream of the photosensitive drum 11.

[0051] Here, in order to explain the arrangement of the potential regulating member 8, we will assume the case where the potential regulating member 8 is removed. In a cross section substantially perpendicular to the rotation axis direction of the photosensitive drum 11, we define the straight line L as the straight line through which the tensioned surface on the inner circumferential side of the intermediate transfer belt 6 downstream of the primary transfer section N1 passes when the potential regulating member 8 is absent. More specifically, this straight line L corresponds to the tensioned surface in a state where only the potential regulating member 8 is substantially removed from the configuration of the image forming apparatus 1 in the state of image forming operation (however, the photosensitive drum 11 and the intermediate transfer belt 6 are stopped). In other words, the straight line L corresponds to the tensioned surface in a state where the tensioned state of the intermediate transfer belt 6 is in a state where image forming is possible (primary transfer of the toner image from the photosensitive drum 11 to the intermediate transfer belt 6 is possible), and the intermediate transfer belt 6 is stopped (stationary). Furthermore, on the straight line L, the point where the inner surface of the intermediate transfer belt 6 separates from the tensioning member immediately upstream of the potential regulating member 8 is defined as "C (or upstream tensioning part C)," and the point where it separates from the tensioning member immediately downstream of the potential regulating member 8 is defined as "D (or downstream tensioning part D)." In Figure 6, the straight line L is schematically shown as approximately horizontal, but if the elastic layer of the primary transfer roller 15 deforms and the surface of the primary transfer roller 15 is lifted towards the photosensitive drum 11, the straight line L may be inclined downwards in the figure as it goes downstream.

[0052] In this embodiment, the tensioning member immediately upstream of the potential regulating member 8 is the primary transfer roller 15, and the upstream tensioning portion C is the position on the inner surface of the intermediate transfer belt 6 where the intermediate transfer belt 6 separates from the primary transfer roller 15. However, the tensioning member immediately upstream of the potential regulating member 8 is not limited to the primary transfer roller 15. For example, if the primary transfer roller 15 is offset upstream of the photosensitive drum 11, the upstream tensioning portion C is the position on the inner surface of the intermediate transfer belt 6 where the intermediate transfer belt 6 separates from the photosensitive drum 11.

[0053] In this embodiment, the tension member immediately downstream of the potential regulating member 8 is the photosensitive drum 11m, 11c, and 11k, which are located adjacent to the downstream side of the primary transfer sections N1y, N1m, and N1c for yellow, magenta, and cyan, respectively. The downstream tension member D is the position on the inner surface of the intermediate transfer belt 6 at the point where the intermediate transfer belt 6 separates from the photosensitive drum 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, if the primary transfer roller 15 is offset upstream of the photosensitive drum 11, the downstream tension member D is the position on the inner surface of the intermediate transfer belt 6 at the point where the intermediate transfer belt 6 separates from the primary transfer roller 15. In this embodiment, for the furthest downstream primary transfer section N1k for black, the tension member immediately downstream is the tension roller (tension roller in this embodiment) 22. The downstream tensioning section D is the position on the inner surface of the intermediate transfer belt 6 where the intermediate transfer belt 6 separates from the tensioning roller 22.

[0054] As shown in Figure 6, the potential regulating member 8 is positioned downstream of the primary transfer section N1 and close to the primary transfer section N1 so as not to contact the primary transfer roller 15 and not to contact the photosensitive drum 11 via the intermediate transfer belt 6. In this case, the closer the upstream end A is to the primary transfer section N1, the greater the effect of the potential regulating member 8 in suppressing discharge between the intermediate transfer belt 6 and the photosensitive drum 11. In this embodiment, the potential regulating member 8 is positioned downstream of the primary transfer section N1 such that the distance X2 from the primary transfer roller 15 to the upstream end A is approximately 8 [mm]. Here, distance X2 is the distance between the rotation center of the primary transfer roller 15 and the upstream end A in a direction along (approximately parallel to) the common tangent line on the side of the multiple photosensitive drums 11 that contact the intermediate transfer belt 6, in a cross-section approximately perpendicular to the rotation axis direction of the photosensitive drum 11. In other words, in this embodiment, in the direction along the common tangent, 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 circumference of the primary transfer roller 15. Although not limited to this, the above distance X2 is preferably about 1 to 20 mm, and typically about 1 to 10 mm.

[0055] In this embodiment, the potential regulating member 8 is pressed against the inner circumferential surface of the intermediate transfer belt 6 at both ends in its longitudinal direction by compression springs 89 (Figure 7), which are biasing members composed of compression coil springs that act as biasing means. At this time, the contact portion of the potential regulating member 8 that contacts the inner circumferential surface of the intermediate transfer belt 6 is positioned to penetrate further toward the photosensitive drum 11 than the straight line L. This allows the potential regulating member 8 to contact the intermediate transfer belt 6 more stably, even if undulation 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 compression springs 89 is set (adjusted) so that the upstream end A and downstream end B of the contact surface 83 of the potential regulating member 8 penetrate approximately 0.5 [mm] toward the photosensitive drum 11 relative to the straight line L. By allowing the contact surface 83 of the potential regulating member 8 to penetrate the straight line L toward the photosensitive drum 11 in this manner, even if undulation or vibration occurs in the intermediate transfer belt 6 during the image forming operation (while the intermediate transfer belt 6 is running), the potential regulating member 8 can be more stably brought into surface contact with the intermediate transfer belt 6. Although not limited to this, the penetration depth of the contact surface 83 of the potential regulating member 8 toward the straight line L is preferably about 0.3 to 5 mm, more preferably about 0.5 to 3 mm, and typically about 0.5 to 1.0 mm. If this penetration depth is too small, it may become difficult to stably bring the potential regulating member 8 into contact with the intermediate transfer belt 6. Conversely, if this penetration depth is too large, it may become difficult to stably transport the intermediate transfer belt 6, or discharge between the intermediate transfer belt 6 and the photosensitive drum 11 may increase.

[0056] Here, in a cross-section substantially perpendicular to the rotation axis direction of the photosensitive drum 11, the straight line passing through the upstream end A and the downstream end B of the contact surface 83 is defined as straight line M. In this case, it is preferable that straight line M does not intersect with the line segment CD in straight line L. This allows for more reliable surface contact between the intermediate transfer belt 6 and the potential regulating member 8 when the contact surface 83 of the potential regulating member 8 is planar. If straight line M intersects with the line segment CD in straight line L, it is possible that only one of the ends 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 contact the inner circumferential surface of the intermediate transfer belt 6. In this case, it may become difficult to enhance the discharge suppression effect through surface contact. Also, in Figure 6, the potential regulating member 8 is arranged so that straight line M and straight line L are substantially parallel, but the potential regulating member 8 may be arranged so that straight line M is tilted relative to straight line L, as long as straight line M does not intersect with the line segment CD in straight line L.

[0057] Furthermore, the contact portion of the potential regulating member 8 that contacts the inner circumferential surface of the intermediate transfer belt 6 is not limited to being flat. For example, the potential regulating member 8 may be made of a curved plate or the like, with a cross-section that is substantially perpendicular to the rotation axis direction of the photosensitive drum 11 and curved in a convex shape toward the photosensitive drum 11, and the contact portion of the potential regulating member 8 that contacts the inner circumferential surface of the intermediate transfer belt 6 may be a curved surface that is convex toward the photosensitive drum 11. By making the contact portion (contact surface) of the potential regulating member 8 that contacts the inner circumferential surface of the intermediate transfer belt 6 a curved shape, the stress when rubbing against the intermediate transfer belt 6 can be reduced. The contact portion of the potential regulating member 8 that contacts the inner circumferential surface of the intermediate transfer belt 6 may also be curved by using a roller-shaped potential regulating member 8.

[0058] Furthermore, in this embodiment, embossed paper was given as an example of a recording material S that is relatively difficult to transfer toner images onto, but similar effects can be expected with recording materials with relatively high electrical resistance (high-resistance paper), such as synthetic paper mainly composed of synthetic resin or resin film.

[0059] 5. Suppression of discharge between the primary transfer roller and the potential regulating member. Next, we will explain the suppression of discharge between the primary transfer roller 15 and the potential regulating member 8.

[0060] As mentioned above, the potential regulating member 8 is most effective when placed as close as possible to the primary transfer section N1 and when a relatively high bias of the same polarity as the charging polarity of the photosensitive drum 11 is applied.

[0061] However, the bias applied to the potential regulating member 8, which has the same polarity as the charge polarity of the photosensitive drum 11, is of opposite polarity to the bias applied to the primary transfer roller 15. Therefore, when the potential regulating member 8 is placed close to the primary transfer section N1 and the bias applied to the potential regulating member 8 is increased, a large potential difference is generated between the potential regulating member 8 and the primary transfer roller 15. This potential difference can cause discharge (abnormal discharge, leakage) through the space between the potential regulating member 8 and the primary transfer roller 15. Generally, to sufficiently suppress discharge through the space, it is desirable to provide a space distance of 3 mm per 1 kV of potential difference. If such a space distance is not secured, discharge through the space may occur between the potential regulating member 8 and the primary transfer roller 15. This may reduce the effectiveness of the potential regulating member 8 in suppressing discharge between the intermediate transfer belt 6 and the photosensitive drum 11. It may also affect the primary transfer. Furthermore, there is a possibility that the potential regulating member 8 and the primary transfer roller 15 may be damaged.

[0062] For example, if the primary transfer voltage applied to the primary transfer roller 15 is +2kV and the potential regulating voltage applied to the potential regulating member 8 is -2kV, the potential difference between the primary transfer roller 15 and the potential regulating member 8 is 4kV. In this case, it is desirable to provide a space of 12mm in order to sufficiently suppress discharge through the space between the primary transfer roller 15 and the potential regulating member 8. If such a space is not secured, discharge may occur through the space between the primary transfer roller 15 and the potential regulating member 8.

[0063] Therefore, in this embodiment, the image forming apparatus 1 is configured to have a shielding member for electrically blocking the space between the potential regulating member 8 and the primary transfer roller 15.

[0064] 6. Shielding member 6-1. Configuration of Shielding Members Next, the shielding member in this embodiment will be described. Figure 7 is a schematic cross-sectional view of the vicinity of the primary transfer section N1 for illustrating the shielding member 9 in this embodiment.

[0065] In this embodiment, the configuration and arrangement of the potential regulating member 8 and shielding member 9 provided for each primary transfer section N1y, N1m, N1c, and N1k are substantially the same. Furthermore, in this embodiment, the potential regulating member 8 has a base portion 84 and a surface layer 85 provided on the surface of the base portion 84, with the base portion 84 being made of a non-conductive material and the surface layer 85 being made of a conductive material (such as sheet metal).

[0066] In this embodiment, the image forming apparatus 1 is provided with a shielding member 9 made of a non-conductive material for electrically blocking the space between the potential regulating member 8 and the primary transfer roller 15. The shielding member 9 is provided in a region that spans a straight line drawn to connect the primary transfer roller 15 and the potential regulating member 8 (the conductive portion (surface layer 85 in this embodiment)) by the shortest distance, when viewed substantially parallel to the width direction of the intermediate transfer belt 6 (i.e., in a cross section substantially perpendicular to the rotation axis of the photosensitive drum 11). In this embodiment, the shielding member 9 is made of an elastically deformable elastic material and is provided so as to contact the inner circumferential surface of the intermediate transfer belt 6. Further details will be described below.

[0067] When the potential regulating member 8 is positioned close to the primary transfer section N1, the space between the potential regulating member 8 and the primary transfer roller 15 becomes narrower. Therefore, in this embodiment, a sheet-like member having electrical insulating properties and elasticity (flexibility) is used as the shielding member 9. This sheet-like shielding member 9 is provided such that one side faces the primary transfer roller 15 and the other side faces the potential regulating member 8. In this embodiment, the shielding member 9 is composed of a first sheet 91 and a second sheet 92 made of different materials.

[0068] The first sheet 91 is formed from a material with lower rigidity than the second sheet 92. The tip of the first sheet 91 protrudes further towards the inner circumferential surface of the intermediate transfer belt 6 than the tip of the second sheet 92, and is positioned to contact the inner circumferential surface of the intermediate transfer belt 6. In this embodiment, the first sheet 91 is formed from a resin sheet with electrical insulation properties and a thickness of approximately 50 μm. More specifically, in this embodiment, the first sheet 91 is formed using the F series of "Superio®" (manufactured by Mitsubishi Chemical Corporation), which is a polyetherimide resin sheet. However, the material constituting the first sheet 91 is not limited to the material used in this embodiment, and any material (typically a resin material) with sufficiently low rigidity and sufficiently high electrical insulation properties can be appropriately selected and used.

[0069] On the other hand, the second sheet 92 is formed using a material with higher rigidity than the first sheet 91. The second sheet 92 is placed on top of the first sheet 91 on the side opposite to the intermediate transfer belt 6 and functions as a reinforcing member of the first sheet 91. Thus, in this embodiment, the rigidity of the first region on the tip side (9b side) of the shielding member 9 is lower than the rigidity of the second region on the base side (9a side). In this embodiment, the second sheet 92 is formed of a resin sheet with electrical insulation properties and a thickness of about 200 μm. More specifically, in this embodiment, the second sheet 92 is formed using "Lumilla®" (manufactured by Toray Industries, Inc.), which is a PET (polyethylene terephthalate) resin sheet. However, the material constituting the second sheet 92 is not limited to the material used in this embodiment, and any material (typically a resin material) that has sufficiently higher rigidity and sufficiently high electrical insulation properties than the first sheet 91 can be appropriately selected and used. Furthermore, the second sheet 92 is not limited to being made of an electrically insulating material. The second sheet 92 can be configured to function as a reinforcing member of at least the first sheet 91 and may be made of a conductive material.

[0070] The first sheet 91 is composed of a sheet-like member that is approximately rectangular in plan view when not deformed by external force. The first sheet 91 has a predetermined length in the longitudinal direction, which is arranged along the longitudinal direction of the potential regulating member 8 (the width direction of the intermediate transfer belt 6) (approximately parallel in this embodiment), and in the short direction, which is approximately perpendicular to this longitudinal direction. A portion of the first sheet 91 on the base end 91a side, which is one end in the short direction, is fixed to a base 87, which will be described later, and a portion of the other end in the short direction, from the tip 91b to the base end 91b side, is in contact with the inner circumferential surface of the intermediate transfer belt 6.

[0071] Furthermore, the second sheet 92 is composed of a sheet-like member that is approximately rectangular in plan view when not deformed by external force. The second sheet 92 has a predetermined length in the longitudinal direction, which is arranged along the longitudinal direction of the potential regulating member 8 (the width direction of the intermediate transfer belt 6) (approximately parallel in this embodiment), and in the short direction, which is approximately perpendicular to this longitudinal direction. One end of the second sheet 92 in the short direction, the base end 92a, is fixed to the base 87, which will be described later, and the other end in the short direction, the tip end 92b, is positioned close to the inner circumferential surface of the intermediate transfer belt 6.

[0072] The first sheet 91 and the second sheet 92 can be fixed (attached) to each other by any fixing means, such as adhesive bonding, welding, or fixing with double-sided tape, at least a portion of each. However, the first sheet 91 and the second sheet 92 may be fixed to the base 87 described later, or they may simply be placed on top of each other. A portion of the first sheet 91 on the tip 91b side in the short direction protrudes toward the inner circumferential surface of the intermediate transfer belt 6 than the tip 92b of the second sheet 92. In other words, a portion of the first sheet 91 from the base 91a to the tip 91b side in the short direction is overlapped with the second sheet 92. At least a portion of the portion of the first sheet 91 that protrudes from the second sheet 92 contacts the inner circumferential surface of the intermediate transfer belt 6.

[0073] In this embodiment, one end of the shielding member 9 in the short direction, which is composed of the base end 91a of the first sheet 91 and the base end 92a of the second sheet 92, is also referred to as the base end 9a of the shielding member 9. In this embodiment, the other end of the shielding member 9 in the short direction, which is composed of the tip end 91b of the first sheet 91, is also referred to as the tip end 9b of the shielding member 9.

[0074] Furthermore, in this embodiment, in order to secure a spatial distance between the longitudinal end of the potential regulating member 8 and the longitudinal end of the primary transfer roller 15, the longitudinal length of the shielding member 9 is made longer than the longitudinal length of the potential regulating member 8. Figure 8 is a schematic plan view of the potential regulating member 8 and the shielding member 9 as seen along a direction substantially perpendicular to the inner circumferential surface of the intermediate transfer belt 6. In this embodiment, the longitudinal length of the first sheet 91 and the longitudinal length of the second sheet 92 are substantially the same, and the longitudinal lengths of the first sheet 91 and the second sheet 92 are made longer than the longitudinal length of the potential regulating member 8. As a result, in the longitudinal direction of the potential regulating member 8, both ends 91c, 91c of the first sheet 91 and both ends 92c, 92c of the second sheet 92 protrude outward from both ends 8a, 8a of the potential regulating member 8 (conductive portion (surface layer 85 in this embodiment)). The longitudinal length of the shielding member 9 can be set to satisfy the condition of securing a spatial distance of 3 mm per 1 kV of potential difference, as described above, and can be calculated based on the voltage applied to the primary transfer roller 15 and the voltage applied to the potential regulating member 8. In this embodiment, the longitudinal length of the shielding member 9 is set to a length such that both ends of the shielding member 9 protrude from both ends of the potential regulating member 8 by a protrusion amount W1 of 10 mm each.

[0075] 6-2. Installation of shielding members Next, the attachment of the shielding member 9 in this embodiment will be described.

[0076] In this embodiment, the image forming apparatus 1 has a base 87 as a support member for supporting the potential regulating member 8 and the shielding member 9. The base 87 is arranged along the width direction of the intermediate transfer belt 6 (approximately parallel in this embodiment) and has a first support portion 87a for supporting the potential regulating member 8 and a second support portion 87b for supporting the shielding member 9. The potential regulating member 8 is attached to the first support portion 87a via a compression spring 89 provided between the potential regulating member 8 (base portion 84 in this embodiment) and the first support portion 87a. The shielding member 9 is attached to the second support portion 87b. As described above, in this embodiment, the shielding member 9 is attached to the second support portion 87 in a portion area on the base end 91a side in the short direction of the first sheet 91 and in a portion area on the base end 92a side in the short direction of the second sheet 92. The shielding member 9 can be fixed to the base 87 by any fixing means, such as adhesive bonding, welding, fixing with double-sided tape, or fixing with fasteners such as screws. In this embodiment, the length of the base 87 in the longitudinal direction (width direction of the intermediate transfer belt 6) is approximately the same as the length of the shielding member 9 in the longitudinal direction. In this embodiment, the base 87 is made of an electrically insulating material (a resin material in this embodiment) and is an electrically insulating material.

[0077] In this embodiment, the base 87 is provided with a positioning rib 88, which serves as a positioning part for determining the position of the shielding member 9 when attaching it. The positioning rib 88 is provided along the longitudinal direction of the base 87 (the width direction of the intermediate transfer belt 6). In this embodiment, the shielding member 9 is attached to the base 87 such that its base end 9a, which is the part to be positioned, abuts against the positioning rib 88. The relative position of the shielding member 9 with respect to the intermediate transfer belt 6 is important, just like with the potential regulating member 8. In this embodiment, the relative position between the tip 9b of the shielding member 9 and the inner circumferential surface of the intermediate transfer belt 6 is guaranteed by bringing the shielding member 9 into contact with the positioning rib 88. It should be noted that the configuration of the positioning part is not limited to that of this embodiment, as long as the relative position between the shielding member 9 and the intermediate transfer belt 6 can be guaranteed with sufficient accuracy. For example, a configuration in which an engagement hole, which is the part to be positioned, provided on the shielding member 9 is engaged with an engagement projection, which is the positioning part, provided on the base 87 may be engaged.

[0078] Furthermore, in this embodiment, the shielding member 9 is provided at an inclination such that its tip 9b is located upstream of its base end 9a in the transport direction of the intermediate transfer belt 6. In other words, when viewed substantially parallel to the width direction of the intermediate transfer belt 6, the tip 9b, which is the other end, is located closer to the intermediate transfer belt 6 than its base end 9a. The shielding member 9 is provided such that its tip 9b is located upstream of its base end 9a in the transport direction of the intermediate transfer belt 6. In this embodiment, when viewed substantially parallel to the width direction of the intermediate transfer belt 6 (the rotation axis direction of the photosensitive drum 11), the second support portion 87b of the base 87 is composed of a plane inclined toward the upstream side in the transport direction of the intermediate transfer belt 6 at an angle (root angle) θ with respect to a straight line substantially perpendicular to the transport direction of the intermediate transfer belt 6. A portion of the base end 9a side of the flat shielding member 9 is attached to this inclined second support portion 87b. Therefore, at least a portion of the area on the tip 9b side of the shielding member 9 is elastically deformed to face toward the primary transfer roller 15 (from downstream to upstream in the conveying direction of the intermediate transfer belt 6) and contacts the inner circumferential surface of the intermediate transfer belt 6. In this embodiment, the area on the tip 91b side of the first sheet 91, which protrudes further toward the inner circumferential surface of the intermediate transfer belt 6 than the tip 92b of the second sheet 92, is elastically deformed to face toward the primary transfer roller 15 and contacts the inner circumferential surface of the intermediate transfer belt 6.

[0079] 6-3. Effects of the shielding member in this embodiment By electrically blocking the space between the potential regulating member 8 and the primary transfer roller 15 with the shielding member 9, discharge through the space between the primary transfer roller 15 and the potential regulating member 8 can be suppressed.

[0080] Furthermore, by using a thin, sheet-like material as the shielding member 9, and by ensuring that at least a portion of the tip 9b side faces the primary transfer roller 15, the following effects can be obtained. First, it becomes easier to bring the shielding member 9 as close as possible to the primary transfer section N1 while avoiding interference with the primary transfer roller 15, which has a circular cross-section. This makes it easier to bring the potential regulating member 8 as close as possible to the primary transfer section N1. In addition, the tip 9b of the shielding member 9 curls up so that it faces the downstream side in the conveying direction of the intermediate transfer belt 6, preventing the shielding member 9 from interfering with the potential regulating member 8.

[0081] Furthermore, by making the shielding member 9 an elastic member, in this embodiment, composed of two elastic sheet-like members, and bringing the tip 91b of the relatively less rigid first sheet 91 into contact with the intermediate transfer belt 6, the following effects can be obtained.

[0082] When the shielding member 9 is brought into contact with the intermediate transfer belt 6, electrical shielding of the space between the potential regulating member 8 and the primary transfer roller 15 is performed by the intermediate transfer belt 6 and the shielding member 9. However, the position (height) of the surface of the intermediate transfer belt 6 may fluctuate due to displacement caused by contact with the potential regulating member 8 and displacement caused by the electrostatic attraction force between the potential regulating member 8 and the intermediate transfer belt 6 when a voltage is applied to the potential regulating member 8. In this case, by making the shielding member 9 an elastic material and bringing it into contact with the inner surface of the intermediate transfer belt 6 in a pre-elastically deformed state, it becomes possible to follow the displacement of the intermediate transfer belt 6 even when it is displaced. This allows the intermediate transfer belt 6 and the shielding member 9 to maintain a state of close contact. Therefore, the intermediate transfer belt 6 and the shielding member 9 can maintain an electrically shielded state of the space between the potential regulating member 8 and the primary transfer roller 15. Furthermore, contacting the intermediate transfer belt 6 in a pre-elastically deformed state means that the tension of the intermediate transfer belt 6 is such that image formation is possible (primary transfer of the toner image from the photosensitive drum 11 to the intermediate transfer belt 6 is possible), and the intermediate transfer belt 6 is stopped (stationary), and contacting it in an elastically deformed state is performed.

[0083] On the other hand, as described above, if the elasticity of the shielding member 9 is used to make the shielding member 9 follow the displacement of the intermediate transfer belt 6, the shielding member 9 will apply a force to the intermediate transfer belt 6 that pushes its surface upward in the contact direction. The direction in which the surface of the intermediate transfer belt 6 is pushed upward is along a direction that intersects (approximately perpendicular in this embodiment) with the transport direction of the intermediate transfer belt 6, so that the outer surface of the intermediate transfer belt 6 moves closer to the photosensitive drum 11, and the inner surface of the intermediate transfer belt 6 moves away from the potential regulating member 8. In order to make the shielding member 9 adhere to the intermediate transfer belt 6 more reliably, it is conceivable to increase the rigidity of the shielding member 9 so that it contacts the intermediate transfer belt 6 with a stronger force. However, in that case, the shielding member 9 pushing up the intermediate transfer belt 6 may cause the potential regulating member 8 to separate from the inner surface of the intermediate transfer belt 6. Furthermore, the outer surface of the intermediate transfer belt 6 rises in the direction that brings it closer to the photosensitive drum 11, which may increase the amount of discharge between the photosensitive drum 11 and the intermediate transfer belt 6, which is the target that should be suppressed by the potential regulating member 8.

[0084] In contrast, in this embodiment, the shielding member 9 is composed of two sheet-like members, and the tip 91b of the first sheet 91, which has relatively lower rigidity, is brought into contact with the intermediate transfer belt 6. This ensures that the shielding member 9 can follow the displacement of the intermediate transfer belt 6 while preventing it from excessively pushing up the surface of the intermediate transfer belt 6. Note that the shielding member 9 is not limited to being composed of two sheet-like members, but may be composed of even more (for example, 3 to 5) sheet-like members.

[0085] 6-4. Force relationship between shielding member and potential regulating member The forces exerted by the potential regulating member 8 and the shielding member 9 on the intermediate transfer belt 6 in this embodiment will be described in more detail below.

[0086] Figure 9 is a schematic cross-sectional view of the vicinity of the primary transfer section N1 to explain the relationship between the forces exerted by the shielding member 9 and the potential regulating member 8 on the intermediate transfer belt 6 and the resistance force due to the tension of the intermediate transfer belt 6.

[0087] In this embodiment, it is desirable that the potential regulating member 8 and the shielding member 9 remain in contact with the intermediate transfer belt 6 during the image formation operation (while the intermediate transfer belt 6 is moving). Therefore, it is desirable that the relationship between the force exerted by the shielding member 9 and the potential regulating member 8 on the intermediate transfer belt 6 and the resistance force due to the tension of the intermediate transfer belt 6 be such that this contact state is maintained.

[0088] Here, let DT be the force with which the potential regulating member 8 pushes up the surface of the intermediate transfer belt 6, ST be the force with which the shielding member 9 pushes up the surface of the intermediate transfer belt 6, and BT be the resistance force due to the tension in the intermediate transfer belt 6 generated by DT and ST. At this time, from the equilibrium of forces, the following equation 1 holds. DT + ST = BT ... (1)

[0089] In order for the potential regulating member 8 to maintain contact with the intermediate transfer belt 6, it is sufficient to satisfy DT > 0. Similarly, in order for the shielding member 9 to maintain contact with the intermediate transfer belt 6, it is sufficient to satisfy ST > 0. Therefore, in order for the potential regulating member 8 and the shielding member 9 to maintain contact with the intermediate transfer belt 6, in addition to equation 1 above, the configuration and shape of the shielding member 9 should be set to satisfy DT > 0 and ST > 0. In other words, it is desirable that the following equation 2 be satisfied. DT + ST = BT (where DT > 0, ST > 0) ... (2)

[0090] Furthermore, in order to more reliably bring the potential regulating member 8 into contact with the intermediate transfer belt 6, it is more preferable that the following equation 3 is satisfied. DT>ST ···(3)

[0091] By using a shielding member 9 that satisfies the above-mentioned conditions (Equation 2, or Equations 2 and 3), it becomes possible to more reliably electrically shield the space between the potential regulating member 8 and the primary transfer roller 15. In this embodiment, the shielding member 9 is configured to satisfy the above-mentioned conditions (Equations 2 and 3) by setting the rigidity (material, thickness, shape, etc.) of the first sheet 91 and the second sheet 92.

[0092] Furthermore, within the range that satisfies the above conditions (Equation 2, or Equation 2 and Equation 3), it is desirable that DT and ST be as small as possible. This is to suppress the upward movement of the outer surface of the intermediate transfer belt 6 toward the photosensitive drum 11. By suppressing the upward movement of the intermediate transfer belt 6, it is possible to suppress the increase in discharge between the photosensitive drum 11 and the intermediate transfer belt 6, and the wear of the potential regulating member 8, the shielding member 9, or the intermediate transfer belt 6 due to friction between the potential regulating member 8 and the shielding member 9 and the intermediate transfer belt 6.

[0093] 6-5. Variations As described above, in this embodiment, the shielding member 9 is configured to contact the inner circumferential surface of the intermediate transfer belt 6. This allows the shielding member 9 and the intermediate transfer belt 6 to electrically shield the space between the primary transfer roller 15 and the potential regulating member 8, thereby more reliably suppressing discharge through the space between the primary transfer roller 15 and the potential regulating member 8.

[0094] However, as mentioned above, a reasonable effect can be obtained by providing the shielding member 9 in a region that straddles a straight line drawn to connect the primary transfer roller 15 and the potential regulating member 8 (conductive portion (surface layer 85 in this example)) in the shortest distance, when viewed substantially parallel to the width direction of the intermediate transfer belt 6. For example, the shielding member 9 may have the configuration shown in Figure 10. In the example shown in Figure 10, the shielding member 9 has the same configuration as the shielding member 9 of this embodiment (Figure 7). However, in the example shown in Figure 10, the tip 91b of the first sheet 91 does not protrude from the tip 92b of the second sheet 92 and does not contact the inner circumferential surface of the intermediate transfer belt 6. The shielding member 9 is positioned in a region that straddles a straight line E drawn to connect the primary transfer roller 15 and the potential regulating member 8 in the shortest distance, when viewed substantially parallel to the width direction of the intermediate transfer belt 6. When the primary transfer member is the primary transfer roller 15, the straight line E is a straight line drawn to connect the rotation center of the primary transfer roller 15 and the potential regulating member 8 by the shortest distance, when viewed substantially parallel to the width direction of the intermediate transfer belt 6. Even in this configuration, the shielding member 9 can be positioned as close as possible to the primary transfer roller 15 by being made of a sheet-like material. Furthermore, even in this configuration, it is preferable that both ends of the shielding member 9 in the longitudinal direction protrude from both ends of the potential regulating member 8 in the longitudinal direction. In addition, in this configuration, the shielding member 9 may be made of, for example, a single sheet-like material with sufficient electrical insulation properties.

[0095] Furthermore, as described above, in this embodiment, it is preferable that the shielding member 9 is substantially always in contact with the inner circumferential surface of the intermediate transfer belt 6 during the image forming operation (while the intermediate transfer belt 6 is running). However, even in this embodiment, the shielding member 9 may temporarily separate from the inner circumferential surface of the intermediate transfer belt 6 during the image forming operation (while the intermediate transfer belt 6 is running). Even in that case, a reasonable effect can be obtained in suppressing discharge through the space between the primary transfer roller 15 and the potential regulating member 8.

[0096] 7. Effects As described above, this embodiment makes it possible to suppress discharge (leakage) through the space between the electrical regulating member 8 and the primary transfer roller 15.

[0097] [Example 2] Next, other embodiments of the present invention will be described. The basic configuration and operation of the image forming apparatus in this embodiment are the same as those of the image forming apparatus in 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 in Embodiment 1 are denoted by the same reference numerals as in Embodiment 1, and detailed descriptions are omitted.

[0098] In Example 1, the potential regulating member 8 was in contact with the inner circumferential surface of the intermediate transfer belt 6. In contrast, it is also possible to separate the potential regulating member 8 from the inner circumferential surface of the intermediate transfer belt 6. In this case as well, depending on the distance between the inner circumferential surface of the intermediate transfer belt 6 and the potential regulating member 8, the effect of suppressing discharge between the photosensitive drum 11 and the intermediate transfer belt 6 can be obtained by the action of the electric field formed in the space between the photosensitive drum 11 and the potential regulating member 8. The closer the distance between the inner circumferential surface of the intermediate transfer belt 6 and the potential regulating member 8 (the distance in a direction substantially perpendicular to the transport direction of the intermediate transfer belt 6), the better. However, for example, if the distance is about 0.5 to 3.0 [mm], a sufficient effect may be obtained.

[0099] In this embodiment, the potential regulating member 8 is positioned away from the inner circumferential surface of the intermediate transfer belt 6. The configuration of the potential regulating member 8 in this embodiment is substantially the same as that of Embodiment 1. However, in this embodiment, the potential regulating member 8 is positioned such that the surface corresponding to the contact surface 83 of the potential regulating member 8 in Embodiment 1 is substantially parallel to the inner circumferential surface of the intermediate transfer belt 6, and the distance of that surface from the inner circumferential surface of the intermediate transfer belt 6 is approximately 1.0 [mm].

[0100] By positioning the potential regulating member 8 at a distance from the intermediate transfer belt 6, the possibility of wear of the potential regulating member 8 and damage to the inner surface of the intermediate transfer belt 6 due to contact between the potential regulating member 8 and the intermediate transfer belt 6 can be reduced.

[0101] Figure 11 is a schematic cross-sectional view of the vicinity of the primary transfer section N1 to illustrate the relationship between the forces exerted by the shielding member 9 and the potential restricting member 8 on the intermediate transfer belt 6, respectively, and the resistance force due to the tension of the intermediate transfer belt 6, when the potential restricting member 8 does not come into contact with the intermediate transfer belt 6.

[0102] As shown in Figure 11, when the potential regulating member 8 does not come into contact with the intermediate transfer belt 6, no reaction force is generated between the potential regulating member 8 and the intermediate transfer belt 6. Instead, a reaction force is generated on the intermediate transfer belt 6 due to the force received from the shielding member 9, and a displacement of the intermediate transfer belt 6 occurs at that time. In this case, if the displacement of the intermediate transfer belt 6 due to the force pushed up by the shielding member 9 becomes large, the distance between the potential regulating member 8 and the intermediate transfer belt 6 will increase, leading to a reduction in the effect of suppressing discharge between the photosensitive drum 11 and the intermediate transfer belt 6. Therefore, in this embodiment, it is desirable to configure the shielding member 9 so as to satisfy the following equation 4. As in Embodiment 1, let ST be the force that the shielding member 9 pushes up the surface of the intermediate transfer belt 6, and BT be the resistance force due to the tension of the intermediate transfer belt 6 generated by ST. ST = BT (where ST > 0) ... (4)

[0103] Furthermore, in order to keep the distance between the potential regulating member 8 and the intermediate transfer belt 6 approximately constant, it is preferable to satisfy the following equation 5. Note that BS [mm] is the displacement of the surface of the intermediate transfer belt 6 when the surface of the intermediate transfer belt 6 is pushed up by ST. This displacement is the displacement in a direction that intersects (approximately perpendicular in this embodiment) with the conveying direction of the intermediate transfer belt 6. 0 [mm] <BS≦0.5[mm] ···(5)

[0104] According to the inventors' research, in the configuration of this embodiment, if BS is 0.5 [mm] or less, fluctuations in the effect of the potential regulating member 8 in suppressing discharge between the intermediate transfer belt 6 and the photosensitive drum 11 can be sufficiently suppressed. BS may be approximately 0 [mm]. This also applies to the configuration in which the potential regulating member 8 contacts the inner circumferential surface of the intermediate transfer belt 6, as in Embodiment 1. However, the upper limit of BS may be changed depending on the configuration of the potential regulating member 8.

[0105] By using a shielding member 9 that satisfies the above conditions, it becomes possible to more reliably electrically shield the space between the potential regulating member 8 and the primary transfer roller 15, even in a configuration where the potential regulating member 8 is positioned at a distance from the intermediate transfer belt 6.

[0106] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the embodiments described above.

[0107] In the above-described embodiment, the configuration and arrangement of the shielding member 9 were explained assuming that the potential regulating member 8 has the configuration shown in Figure 4, but the potential regulating member 8 may have the configurations shown in Figures 3 and 5. Also, in the above-described embodiment, the potential regulating member 8 was configured to have a first portion 81 and a second portion 82, but the potential regulating member 8 may be composed of a flat plate corresponding to the first portion 81 in the above-described embodiment, for example. Furthermore, the potential regulating member 8 may take other forms, such as a block-shaped member with a rectangular cross-section.

[0108] Furthermore, the shielding member may be made of a single sheet-like material, and the thickness of the leading end and trailing end in the short direction may be varied so that the rigidity of the leading end is lower than that of the base end.

[0109] Furthermore, if the first sheet and the second sheet constituting the shielding member are fixed to each other, the overlapping order of the first sheet and the second sheet may be reversed from that of the above-described embodiment.

[0110] Furthermore, the shielding member can be made of an elastic material such as rubber, which has electrical insulating properties and elasticity, and can be made into any shape.

[0111] Furthermore, although the primary transfer member in the above-described embodiment was a roller-shaped member, it may also be a brush-shaped member, a sheet-shaped member, a pad-shaped member, or the like.

[0112] Furthermore, in the above-described embodiment, the potential regulating power supply was provided independently for each image forming unit, but it may be shared with multiple image forming units (or all of them). The same applies to the charging power supply, developing power supply, and primary transfer power supply.

[0113] Furthermore, although the predetermined charging polarity of the photoreceptor was negative in the above-described embodiment, it is not limited to this, and the predetermined charging polarity of the photoreceptor may be positive. Similarly, although the normal charging polarity of the toner was negative in the above-described embodiment, the normal charging polarity of the toner may be positive. When the predetermined charging polarity of the photoreceptor and the normal charging polarity of the toner are positive, the various applied voltages may be appropriately changed, for example, by setting them to the opposite polarity to those in the above-described embodiment, in accordance with the above-described embodiment.

[0114] Furthermore, the image forming apparatus can be configured to have a potential regulating member and a shielding member with respect to at least one of the multiple image forming units. In other words, the image forming apparatus can be configured to have a potential regulating member and a shielding member provided on the inner circumferential surface side of the intermediate transfer belt immediately downstream of at least one of the multiple primary transfer units.

[0115] Furthermore, the image forming apparatus is not limited to an image forming apparatus capable of forming full-color images; it may also be an image forming apparatus capable of forming only monochrome (black and white or monocolor) images. [Explanation of Symbols]

[0116] 1. Image forming apparatus 6. Intermediate transfer belt 8. Potential regulating member (electrode member) 9 Shielding member 11. Photosensitive drum (photoconductor) 15. Primary transfer roller (primary transfer member) 80. Potential-regulated power supply

Claims

1. A photoreceptor that can be charged to a predetermined polarity and carries a toner image, A circulating intermediate transfer belt for transporting the toner image, which has been primarily transferred from the photoreceptor, to a recording material for secondary transfer, A primary transfer member is provided, which contacts the inner circumferential surface of the intermediate transfer belt to form a primary transfer section in which the photoreceptor and the intermediate transfer belt come into contact, and to which a voltage is applied to transfer a toner image from the photoreceptor to the intermediate transfer belt in the primary transfer section. An electrode member provided on the inner circumferential surface side of the intermediate transfer belt downstream of the primary transfer section in the direction of movement of the intermediate transfer belt, A power supply that applies a voltage with the same polarity as the predetermined polarity to the electrode member, An electrically insulating shielding member is provided on the inner circumferential surface side of the intermediate transfer belt downstream of the primary transfer portion in the aforementioned direction of movement, and is positioned between the primary transfer member and the electrode member to electrically shield the primary transfer member and the electrode member. An image forming apparatus characterized by having the following features.

2. The image forming apparatus according to claim 1, characterized in that the shielding member is provided in a region that spans a straight line drawn to connect the primary transfer member and the electrode member by the shortest distance, when viewed substantially parallel to the width direction of the intermediate transfer belt.

3. The image forming apparatus according to claim 1, characterized in that the shielding member is made of an elastic sheet-like material, and is provided such that one side faces the primary transfer member and the other side faces the electrode member.

4. The image forming apparatus according to claim 3, characterized in that the shielding member is provided such that, when viewed substantially parallel to the width direction of the intermediate transfer belt, the tip end is located closer to the intermediate transfer belt than the base end, and the tip end is located upstream in the direction of movement than the base end.

5. The image forming apparatus according to claim 4, characterized in that the shielding member is provided such that at least a portion of the tip side contacts the inner circumferential surface of the intermediate transfer belt.

6. The image forming apparatus according to claim 5, characterized in that the rigidity of the first region on the tip side of the shielding member is lower than the rigidity of the second region on the base side.

7. The image forming apparatus according to claim 5, wherein the shielding member comprises a first sheet and a second sheet, the rigidity of the first sheet is lower than that of the second sheet, a portion of the first sheet on the tip side of the shielding member protrudes toward the inner circumferential surface of the intermediate transfer belt than the tip of the second sheet on the tip side of the shielding member, and at least a portion of the protruding portion of the first sheet contacts the inner circumferential surface of the intermediate transfer belt.

8. The image forming apparatus according to claim 7, characterized in that at least a portion of the protruding region of the first sheet contacts the inner circumferential surface of the intermediate transfer belt in an elastically deformed state.

9. The image forming apparatus according to claim 1, characterized in that the shielding member is made of an elastic material and contacts the inner circumferential surface of the intermediate transfer belt in an elastically deformed state.

10. The electrode member is provided so as to contact the inner circumferential surface of the intermediate transfer belt, The shielding member is provided so as to contact the inner circumferential surface of the intermediate transfer belt, The image forming apparatus according to claim 1, characterized in that the force exerted by the shielding member, which causes the surface of the intermediate transfer belt to displace in a direction intersecting the direction of movement, when the shielding member contacts the inner circumferential surface of the intermediate transfer belt, is smaller than the force exerted by the electrode member, which causes the surface of the intermediate transfer belt to displace in a direction intersecting the direction of movement, when the electrode member contacts the inner circumferential surface of the intermediate transfer belt.

11. The shielding member is provided so as to contact the inner circumferential surface of the intermediate transfer belt, The image forming apparatus according to claim 1, characterized in that the amount by which the shielding member contacts the inner circumferential surface of the intermediate transfer belt, causing the surface of the intermediate transfer belt to be displaced in a direction intersecting the direction of movement, is 0.5 mm or less.

12. The image forming apparatus according to claim 11, characterized in that the electrode member is provided so as not to contact the inner circumferential surface of the intermediate transfer belt.

13. The image forming apparatus according to claim 1, characterized in that, in the width direction of the intermediate transfer belt, both ends of the shielding member each protrude outward more than both ends of the electrode member.

14. The image forming apparatus according to claim 1, comprising: a plurality of photoreceptors arranged along the direction of movement; a plurality of primary transfer members provided corresponding to each of the plurality of photoreceptors; and an electrode member and a shielding member provided on the inner circumferential surface side of the intermediate transfer belt immediately downstream of at least one of the primary transfer sections in the direction of movement.

Images