Image forming apparatus

Abstract

To prevent increase in electric resistance of an intermediate transfer belt even when a variation in an output value occurs due to an individual difference of a discharge power supply.SOLUTION: In an image forming apparatus, Ia represents a total sum of positive polarity currents per a unit length in a current supply area supplied in an internal direction to an intermediate transfer belt during image formation, Ib represents a total sum of positive polarity currents per a unit length in the current supply area supplied to an external direction, and a value of Ia-Ib represents a current balance. Within a range where the electric resistance of the intermediate transfer belt is equal to or less than a predetermined value, a relationship of the electric resistance of the intermediate transfer belt with the current balance satisfies a relationship of |Sb|>|Sa|, wherein Sa represents a ratio of a change in the electric resistance of the intermediate transfer belt to a change in the current balance when the value of the current balance is within a positive range, and Sb represents the ratio when the current balance is within a negative range. A control unit controls a discharge power supply so as to adjust an average value of the current balance to be a positive value when image formation is continuously performed on a predetermined number of recording materials in a continuous image forming job.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] This invention relates to image forming apparatuses such as photocopiers, printers, and facsimile machines that use electrophotographic or electrostatic recording methods. [Background technology]

[0002] Conventionally, some image forming apparatuses using electrophotography and other methods employ an intermediate transfer method in which toner images formed on multiple image carriers are first transferred onto an intermediate transfer body, and then secondarily transferred onto a recording material such as paper. As the intermediate transfer body, an intermediate transfer belt composed of an endless belt (hereinafter simply referred to as "belt") is widely used.

[0003] Primary transfer is often performed by applying a primary transfer voltage to primary transfer members, which are provided so as to be able to contact the inner circumferential surface of the intermediate transfer belt, corresponding to each of the multiple image carriers, and supplying a primary transfer current to the primary transfer section where the image carrier and the intermediate transfer belt are in contact. Secondary transfer is often performed by applying a secondary transfer voltage to secondary transfer members, which are provided so as to be able to contact the outer circumferential surface of the intermediate transfer belt, and supplying a secondary transfer current to the secondary transfer section where the intermediate transfer belt and the secondary transfer members are in contact.

[0004] Toner and other deposits remaining on the intermediate transfer belt after the secondary transfer process (secondary transfer residue toner) are removed and recovered from the intermediate transfer belt by a belt cleaning device, which serves as an intermediate transfer body cleaning means. As a belt cleaning device, an electrostatic cleaning device is known that electrostatically recovers toner from the intermediate transfer belt. Cleaning by this device is performed, for example, by applying a cleaning voltage to a cleaning member that is provided so as to be in contact with the outer surface of the intermediate transfer belt, and supplying a cleaning current to the cleaning portion where the intermediate transfer belt and the cleaning member are in contact.

[0005] Furthermore, in markets such as commercial printing, intermediate transfer belts with an elastic layer are sometimes used. The presence of an elastic layer in the intermediate transfer belt improves transferability to recording materials with uneven surfaces, such as embossed paper.

[0006] In image forming apparatuses using the intermediate transfer method, there is a problem in that the electrical resistance of the intermediate transfer belt, which has many electrically conductive points, increases as described above. This is particularly noticeable when the intermediate transfer belt has an elastic layer and an ionic conductive agent (ionic conductive agent) is used to adjust the electrical resistance of that elastic layer.

[0007] In an ion-conductive belt containing an ion-conducting agent, the electric field generated within the belt by the flow of electric current causes a force on the positive and negative ions responsible for ion conductivity. Positively charged cations move in the direction of the electric field, while negatively charged anions move in the opposite direction. For example, consider a configuration in which a toner with a normal charge polarity of negative polarity is used. In this case, for primary transfer, a positive voltage is applied to the primary transfer member in contact with the inner surface of the intermediate transfer belt, and a positive current is supplied in the primary transfer section in the direction from the inner surface to the outer surface of the intermediate transfer belt (hereinafter also referred to as the "outward direction"). As a result, cations move towards the outer surface of the intermediate transfer belt, and anions move towards the inner surface of the intermediate transfer belt. Furthermore, for secondary transfer, a positive voltage is applied to the secondary transfer member in contact with the outer surface of the intermediate transfer belt, and a positive current is supplied in the secondary transfer section in the direction from the outer surface to the inner surface of the intermediate transfer belt (hereinafter also referred to as the "inward direction"). Thus, in the secondary transfer section, an electric field is generated in the opposite direction to that in the primary transfer section. As a result, during secondary transfer, ions in the intermediate transfer belt move in the opposite direction to that during primary transfer (cations move towards the inner surface, and anions move towards the outer surface). When the balance between the total outward charge supplied to the intermediate transfer belt and the total inward charge is significantly disrupted, an imbalance occurs in the ions within the intermediate transfer belt, increasing the electrical resistance of the intermediate transfer belt. As the electrical resistance of the intermediate transfer belt increases with use, and the voltage required to be applied for primary and secondary transfer increases (as the absolute value increases), image defects caused by discharge in the primary and secondary transfer sections become more likely to occur.

[0008] To suppress the increase in electrical resistance of the intermediate transfer belt and extend its lifespan, a configuration using a discharge device has been proposed (Patent Document 1). The discharge device, for example, contacts a discharge member such as a conductive fur brush roller with the intermediate transfer belt and supplies a discharge current to the intermediate transfer belt so as to even out the ion imbalance within the intermediate transfer belt. For example, in a tandem-type full-color image forming apparatus that uses toner with a normal charge polarity of negative polarity, positive current is supplied outward at four primary transfer sections and positive current is supplied inward at one secondary transfer section. In this case, since the supply of positive current outward is greater, it is desirable to supply a positive discharge current inward using a discharge member. In configurations that include an electrostatic cleaning device, the current supplied by the cleaning section is also taken into consideration. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2020-34699 [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] Here, the current balance is defined as the sum of positive currents supplied in the direction from the inner surface to the outer surface (outward direction) of the intermediate transfer belt, compared to the sum of positive currents supplied in the direction from the outer surface to the inner surface (inward direction) of the intermediate transfer belt. In this case, it is possible to set a discharge current that makes the current balance zero. For example, the discharge current that makes the current balance zero can be set based on the target values ​​and detection results of the primary transfer current, secondary transfer current, and cleaning current.

[0011] However, due to individual variations, the output value of discharge current power supplies exhibits a certain degree of variation (fluctuation). Therefore, even when the output of the power supply is controlled to achieve a zero current balance, the actual current balance may swing to the positive or negative side. It has been found that there is a difference in the increase in the electrical resistance of the intermediate transfer belt depending on whether the current balance is positive or negative. Although the detailed mechanism is unknown, this is thought to be due to the fact that some ionic conductive agents facilitate the movement of negative ions, while others facilitate the movement of positive ions. Therefore, even if a discharge component is provided as a countermeasure against the increase in the electrical resistance of the intermediate transfer belt, if the current balance deviates from zero to the side where the increase in the electrical resistance of the intermediate transfer belt is greater, the current balance will be accumulated on the side where the increase in electrical resistance is greater. For this reason, the desired effect may not be obtained.

[0012] Therefore, the objective of the present invention is to suppress the increase in electrical resistance of the intermediate transfer belt even when there is variation in output values ​​due to individual differences in discharge power supplies. [Means for solving the problem]

[0013] The above objective is achieved by the image forming apparatus according to the present invention. In summary, the present invention relates to an image forming apparatus comprising: an image carrier that carries a toner image; a rotatable endless intermediate transfer belt onto which a toner image is transferred from the image carrier; a primary transfer member, which is a current supply member that supplies a primary transfer current to the intermediate transfer belt in a primary transfer section to transfer a toner image from the image carrier to the intermediate transfer belt; a secondary transfer member, which is a current supply member that supplies a secondary transfer current to the intermediate transfer belt in a secondary transfer section to transfer a toner image from the intermediate transfer belt to a recording material in a secondary transfer section; a discharge member, which is a current supply member that supplies a discharge current to the intermediate transfer belt in a discharge section downstream of the secondary transfer section and upstream of the primary transfer section in the rotational direction of the intermediate transfer belt; a discharge power supply that supplies the discharge current; and a control unit that can control the discharge power supply, wherein during image forming, the current supply member supplies an electric current to the intermediate transfer belt in a direction from the outer peripheral surface to the inner peripheral surface. The image forming apparatus is characterized in that, within a range where the electrical resistance of the intermediate transfer belt is less than or equal to a predetermined value, the relationship between the electrical resistance of the intermediate transfer belt and the current balance is such that, when the current balance is in a positive range, Sa is the ratio of the change in the electrical resistance of the intermediate transfer belt to the change in the current balance within a positive range, and Sb is the ratio within a negative range, the control unit controls the discharge power supply so that the average value of the current balance when images are formed on a predetermined number of recording materials in a continuous image forming job is a positive value.

[0014] According to another aspect of the present invention, an image forming apparatus comprising: an image carrier for carrying a toner image; a rotatable endless intermediate transfer belt onto which a toner image is transferred from the image carrier; a primary transfer member, which is a current supply member that supplies a primary transfer current to the intermediate transfer belt in a primary transfer section to transfer a toner image from the image carrier to the intermediate transfer belt; a secondary transfer member, which is a current supply member that supplies a secondary transfer current to the intermediate transfer belt in a secondary transfer section to transfer a toner image from the intermediate transfer belt to a recording material; a discharge member, which is a current supply member that supplies a discharge current to the intermediate transfer belt in a discharge section downstream of the secondary transfer section and upstream of the primary transfer section in the rotational direction of the intermediate transfer belt; a discharge power supply that supplies the discharge current; and a control unit capable of controlling the discharge power supply, wherein during image forming, the current supplied by the current supply member is supplied in a direction from the outer peripheral surface side to the inner peripheral surface side of the intermediate transfer belt by the current supply member, An image forming apparatus is provided, characterized in that, within a range where the electrical resistance of the intermediate transfer belt is less than or equal to a predetermined value, the relationship between the electrical resistance of the intermediate transfer belt and the current balance is such that, when the current balance is in a positive range, Sa is the ratio of the change in the electrical resistance of the intermediate transfer belt to the change in the current balance in the positive range, and Sb is the ratio in a negative range, the control unit controls the discharge power supply so that the average value of the current balance when images are formed on a predetermined number of recording materials in a continuous image forming job becomes a negative value. [Effects of the Invention]

[0015] According to the present invention, even when there is variation in the output value due to individual differences in the discharge power supply, it is possible to suppress an increase in the electrical resistance of the intermediate transfer belt.

Brief Description of the Drawings

[0016] [Figure 1] It is a schematic cross-sectional view of an image forming apparatus. [Figure 2] It is a schematic cross-sectional view near the belt cleaning device. [Figure 3] It is a schematic cross-sectional view near the discharge device. [Figure 4] It is a schematic cross-sectional view of the intermediate transfer belt. [Figure 5] It is a schematic diagram of a measuring device for measuring the relationship between the current balance and the electrical resistance of the intermediate transfer belt. [Figure 6] It is a graph showing an example of the waveform of the current supplied by the measuring device of FIG. 5. [Figure 7] It is a graph showing an example of the relationship between the current balance and the electrical resistance of the intermediate transfer belt. [Figure 8] It is a schematic block diagram showing the control mode of the image forming apparatus. [Figure 9] It is a flowchart showing an outline of the procedure of a print job. [Figure 10] It is a graph showing the effects of the examples. [Figure 11] It is a graph showing another example of the relationship between the current balance and the electrical resistance of the intermediate transfer belt.

Modes for Carrying Out the Invention

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

[0018] [Example 1] 1. Overall Configuration and Operation of the Image Forming Apparatus Figure 1 is a schematic cross-sectional view of the image forming apparatus 100 of this embodiment. The image forming apparatus 100 of this embodiment is a tandem type printer employing an intermediate transfer method that is capable of forming full-color images using an electrophotographic method.

[0019] The image forming apparatus 100 has four image forming units (stations) 10Y, 10M, 10C, and 10K, each forming images of yellow (Y), magenta (M), cyan (C), and black (K). Elements with the same or corresponding functions or configurations in each image forming unit 10Y, 10M, 10C, and 10K may be described collectively by omitting the Y, M, C, and K at the end of the symbols indicating that they are elements for one of the colors. In this embodiment, the image forming unit 10 is composed of a photosensitive drum 1, a charging device 2, an exposure device 3, a developing device 4, a primary transfer roller 5, a drum cleaning device 11, and the like, which will be described later.

[0020] The photosensitive drum 1, a rotatable drum-shaped (cylindrical) photoreceptor (electrophotographic photoreceptor) that carries the toner image, is driven to rotate at a predetermined peripheral speed in the direction of arrow R1 (counterclockwise) in the figure. The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential with a predetermined polarity (negative polarity in this embodiment) by a charging device 2, which is a charging means. During the charging process, a predetermined charging voltage (charging bias) is applied to the charging device 2. The charged surface of the photosensitive drum 1 is scanned and exposed by an exposure device (laser beam scanner) 3, which is an exposure means, based on image information, and an electrostatic image (electrostatic latent image) corresponding to the desired image information is formed on the photosensitive drum 1. The exposure device 3 outputs on / off modulated laser light according to image information input from an external device such as an image scanner or computer, and scans and exposes the charged surface on the photosensitive drum 1. The electrostatic image formed on the photosensitive drum 1 is developed (visualized) by the developing device 4, which is a developing means, when toner is supplied as a developer, and a toner image is formed on the photosensitive drum 1. In this embodiment, toner charged with the same polarity as the charging polarity of the photosensitive drum 1 (negative polarity in this embodiment) adheres to the exposed area (image area) on the photosensitive drum 1, where the absolute value of the potential has decreased after being uniformly charged and then exposed based on image information (reverse development method). In this embodiment, the normal charging polarity of the toner, which is the main charging polarity of the toner during development, is negative polarity. During development, a predetermined developing voltage (developing bias) is applied to the developing roller, which is a developer carrier (developing member) provided in the developing device 4.

[0021] An intermediate transfer belt 6, composed of an endless belt, is positioned opposite four photosensitive drums 1 as an intermediate transfer body. The intermediate transfer belt 6 is positioned so as to be able to contact the surfaces of the four photosensitive drums 1. The intermediate transfer belt 6 is stretched over a plurality of tension rollers, the first to sixth tension rollers 21 to 26, and is taut with a predetermined tension. In this embodiment, the first tension roller 21 is a secondary transfer opposing roller (secondary transfer internal roller) that functions as an opposing member (opposing electrode) of the secondary transfer roller 9, which will be described later. The second tension roller 22 is a drive roller for the intermediate transfer belt 6. The third and fourth tension rollers 23 and 24 are first and second auxiliary rollers that form the image transfer surface of the intermediate transfer belt 6, on which the toner image is primary transferred from each photosensitive drum 1, as will be described later. The fifth tension roller 25 is a tension roller configured to control the tension of the intermediate transfer belt 6 to be approximately constant. Furthermore, the sixth tension roller 26 is a pre-secondary transfer roller that forms the surface of the intermediate transfer belt 6 that enters the secondary transfer section N2, which will be described later. The intermediate transfer belt 6 is driven by the rotational drive of the drive roller 22, and rotates (moves in a circular motion) at a peripheral speed of 150 to 470 mm / sec in the direction of arrow R2 (clockwise direction) in the figure. On the inner circumferential surface (back side) of the intermediate transfer belt 6, primary transfer rollers 5Y, 5M, 5C, and 5K, which are roller-type primary transfer members (current supply members) as primary transfer means, are arranged, corresponding to each photosensitive drum 1Y, 1M, 1C, and 1K. The primary transfer roller 5 is pressed toward the photosensitive drum 1 via the intermediate transfer belt 6, forming a primary transfer section (primary transfer nip section, primary transfer position) N1 where the photosensitive drum 1 and the intermediate transfer belt 6 come into contact.

[0022] As described above, the toner image formed on the photosensitive drum 1 is transferred (primary transfer) to the rotating intermediate transfer belt 6 in the primary transfer section N1 by the action of the primary transfer roller 5. During primary transfer, the primary transfer roller 5 is supplied with a primary transfer voltage (primary transfer bias), which is a constant voltage controlled DC voltage with the opposite polarity to the normal charging polarity of the toner (positive polarity in this embodiment), by the primary transfer power supply (high voltage power supply) E1 (Figure 8). This supplies a primary transfer current to the primary transfer section N1. For example, during primary transfer, a constant voltage controlled primary transfer voltage of approximately +1 to +3 kV is applied to each primary transfer roller 5, and a current of approximately +20 to +100 μA flows outward in each primary transfer section N1. For example, when forming a full-color image, the toner images of yellow, magenta, cyan, and black formed on each photosensitive drum 1 are sequentially transferred onto the intermediate transfer belt 6 so as to be superimposed. In this embodiment, a primary transfer voltage is applied to each primary transfer roller 5 in synchronization with the transport of the toner image of each color to the primary transfer section N1. In this embodiment, the primary transfer roller 5 is composed of a core metal (base material) and an elastic layer formed of ion-conductive foamed rubber on the outer circumference of the core metal. In this embodiment, the outer diameter of the primary transfer roller 5 is 15 to 20 mm. In this embodiment, the electrical resistance of the primary transfer roller 5 is 1 × 10⁻¹⁰ when measured with a voltage of 2 kV applied in an N / N environment (23°C, 50% RH). 5 ~1 × 10 8 It is Omega.

[0023] On the outer circumferential surface of the intermediate transfer belt 6, a secondary transfer roller (secondary transfer outer roller) 9, which is a roller-type secondary transfer member (current supply member) as a secondary transfer means, is positioned opposite the secondary transfer opposing roller 21. The secondary transfer roller 9 is pressed toward the secondary transfer opposing roller 21 via the intermediate transfer belt 6, forming a secondary transfer section (secondary transfer nip, secondary transfer position) N2 where the intermediate transfer belt 6 and the secondary transfer roller 9 come into contact (direct contact or grip the recording material P). As described above, the toner image formed on the intermediate transfer belt 6 is transferred (secondary transfer) in the secondary transfer section N2 onto the recording material P, which is being transported while being gripped between the intermediate transfer belt 6 and the secondary transfer roller 9, by the action of the secondary transfer roller 9. In this embodiment, the secondary transfer roller 9 is composed of a core metal (base material) and an elastic layer formed of ion-conductive foamed rubber on the outer circumference of the core metal. In this embodiment, the outer diameter of the secondary transfer roller 9 is 20 to 25 mm. Furthermore, in this embodiment, the electrical resistance of the secondary transfer roller 9 was measured by applying a voltage of 2kV in an N / N environment (23°C, 50%RH) and was 1 × 10⁻¹⁰. 5 ~1 × 10 8 The value is Ω. In this embodiment, the secondary transfer opposing roller 21 is composed of a core metal (base material) and an elastic layer made of electronically conductive rubber on the outer circumference of the core metal. In this embodiment, the outer diameter of the secondary transfer opposing roller 21 is 20 to 22 mm. In this embodiment, the electrical resistance of the secondary transfer opposing roller 21 is 1 × 10⁻¹⁰ when measured by applying a voltage of 50 V in an N / N environment (23°C, 50% RH). 5 ~1 × 10 8 It is Omega.

[0024] During secondary transfer, the secondary transfer roller 9 is supplied with a secondary transfer voltage (secondary transfer bias), which is a constant-voltage controlled DC voltage with the opposite polarity to the normal charging polarity of the toner (positive polarity in this embodiment), by a secondary transfer power supply (high-voltage power supply) E2. This supplies a secondary transfer current to the secondary transfer section N2. For example, during secondary transfer, a constant-voltage controlled secondary transfer voltage of approximately +1 to +7 kV is applied to the secondary transfer roller 9, and a current of approximately +40 to +120 μA flows inward in the secondary transfer section N2. In this embodiment, the secondary transfer opposing roller 21 is electrically grounded (connected to ground). The recording material (transfer material, recording medium, sheet) P is stored in a recording material storage section (not shown), such as a feed cassette. Based on a feed start signal, a feed member (not shown), such as a feed roller, is driven to feed the recording material P one sheet at a time from the recording material storage section. The recording material P is then transported to the secondary transfer section N2 by the register roller 8. The register roller 8 is controlled to transport the recording material P to the secondary transfer section N2 in synchronization with the timing at which the leading edge of the toner image on the intermediate transfer belt 6 reaches the secondary transfer section N2. The recording material P is typically paper, but may also be synthetic paper, a resin sheet (film) such as an OHP sheet, etc. In this embodiment, the inner roller corresponding to the secondary transfer opposing roller 21 may be used as a secondary transfer member (current supply member), and a voltage with the opposite polarity to the voltage applied to the secondary transfer roller 9 in this embodiment may be applied to it. In this case, the outer roller corresponding to the secondary transfer roller 9 in this embodiment may be used as the opposing member and electrically grounded.

[0025] The recording material P onto which the toner image has been transferred is separated from the intermediate transfer belt 6 and transported by the pre-fixing transport device 20 to the fixing device 30, which serves as the fixing means. The pre-fixing transport device 20 has a rotatable endless belt body made of rubber material such as EPDM, with a width of 100-110 mm and a thickness of 1-3 mm, located in the center of a direction approximately perpendicular to the transport direction of the recording material P, and the recording material P is placed on top of this belt for transport. The belt body has holes with a diameter of 3-7 mm, and air is drawn in from the inside of the belt body. This increases the load-bearing force of the recording material P on the belt body, stabilizing the transport of the recording material P. The fixing device 30 uses a fixing rotating body pair to heat and pressurize the recording material P carrying the unfixed toner image, thereby fixing (melting and solidifying) the toner image onto the recording material P. The recording material P with the fixed toner image is discharged (output) to the outside of the main body of the image forming apparatus 100.

[0026] Furthermore, toner that remains on the photosensitive drum 1 without being transferred to the intermediate transfer belt 6 during the primary transfer (primary transfer residue toner) is removed from the photosensitive drum 1 and recovered by the drum cleaning device 11, which serves as a photoreceptor cleaning means. Also, any adhering substances such as toner that remains on the intermediate transfer belt 6 without being transferred to the recording material P during the secondary transfer (secondary transfer residue toner) are removed from the intermediate transfer belt 6 and recovered by the belt cleaning device 12, which serves as an intermediate transfer body cleaning means. The belt cleaning device 12 will be described in more detail later. In this embodiment, the belt cleaning device 12 electrostatically recovers the secondary transfer residue toner on the intermediate transfer belt 6.

[0027] 2. Intermediate Transfer Form Figure 4 is a schematic cross-sectional view of the intermediate transfer belt 6 in this embodiment. In this embodiment, the intermediate transfer belt 6 is composed of a base layer (layer forming the inner circumferential surface) 6a, an elastic layer (intermediate layer) 6b, and a surface layer (layer forming the outer circumferential surface) 6c. The base layer 6a is made of a material containing an appropriate amount of carbon black as an antistatic agent in a resin such as polyimide or polycarbonate, or various types of rubber, and has a thickness of 0.05 to 0.15 [mm]. The elastic layer 6b is made of a material containing an appropriate amount of an ion conductive agent in a various types of rubber such as chloroprene rubber (CR rubber), urethane rubber, or silicone rubber, and has a thickness of 0.1 to 0.500 [mm]. As the material for the elastic layer 6b, for example, a system in which an ion conductive polymer is mixed with a halogen-containing non-conductive polymer such as chloroprene rubber as the main component to adjust the electrical resistance can be used. As the ion-conductive polymer, a copolymer having a main chain and / or side chains with regularly arranged ether bonds, containing at least one of epichlorohydrin, ethylene oxide, propylene oxide, and allyl glycidyl ether, can be used. The surface layer 6c is formed of a resin such as urethane resin or fluororesin, and has a thickness of 0.0002 to 0.020 [mm].

[0028] In this example, the volume resistivity of the intermediate transfer belt 6 is 5 × 10 8 ~1 × 10 14 The hardness is [Ω·cm] (23℃, 50%RH). The hardness of the intermediate transfer belt 6 is 60~85° on the MD1 hardness scale (23℃, 50%RH). The static friction coefficient of the intermediate transfer belt 6 is 0.15~0.6 (23℃, 50%RH, HEIDON type94i).

[0029] 3. Belt cleaning device Figure 2 is a schematic enlarged cross-sectional view of the vicinity of the belt cleaning device 12 in the present embodiment. The belt cleaning device 12 is disposed at a position downstream of the secondary transfer portion N2 and upstream of the primary transfer portion N1 (the most upstream primary transfer portion N1Y) in the rotational direction of the intermediate transfer belt 6, particularly at a position facing the driving roller 22 via the intermediate transfer belt 6. In the present embodiment, the belt cleaning device 12 is constituted by an electrostatic cleaning device that electrostatically collects toner on the intermediate transfer belt 6, particularly an electrostatic brush cleaning device using a conductive fur brush roller.

[0030] In the present embodiment, the belt cleaning device 12 has a housing 121 disposed in the vicinity of the intermediate transfer belt 6. The following members are provided inside the housing 121. First, first and second cleaning brushes 122 and 123 as first and second cleaning members (current supply members) are provided. Also, first and second recovery rollers 124 and 125 as first and second recovery members are provided. Further, first and second blades 126 and 127 as first and second scraping members are provided.

[0031] The first and second cleaning brushes 122 and 123 are constituted by rotatable conductive fur brush rollers. The brush fibers of the first and second cleaning brushes 122 and 123 are carbon-dispersed nylon fibers, acrylic fibers or polyester fibers having an electrical resistance value of the yarn of 3×10 5 ~1×10 13 (Ω / cm) and a fiber thickness of 2 to 15 denier. And the first and second cleaning brushes 122 and 123 have these brush fibers with a tufting density of 50,000 to 500,000 per inch 2The cleaning brushes are constructed by implanting bristles onto a metal roller, which serves as the base material, in a specific ratio. The first and second cleaning brushes 122 and 123 are positioned to penetrate the intermediate transfer belt 6 by approximately 1.0 to 2.0 mm. The first and second cleaning brushes 122 and 123 are rotated by a drive motor (not shown) as a driving means at a peripheral speed of 20 to 80% of the peripheral speed of the intermediate transfer belt 6 in the direction of arrow R3 (clockwise) in the figure. In other words, the first and second cleaning brushes 122 and 123 rotate so as to move in the opposite direction to the movement of the intermediate transfer belt 6 at the contact point with the intermediate transfer belt 6, thereby rubbing against the surface of the intermediate transfer belt 6. In this embodiment, the first and second cleaning brushes 122 and 123 are in contact with a drive roller 22, which functions as an opposing member, via the intermediate transfer belt 6. The drive roller 22 is electrically grounded. The first and second cleaning brushes 122 and 123 are arranged so that their rotational axis directions are approximately parallel to the direction (also referred to here as the "width direction") which is approximately perpendicular to the direction of movement of the surface of the intermediate transfer belt 6. The lengths of the first and second cleaning brushes 122 and 123 in the rotational axis direction are longer than the maximum image forming width on the intermediate transfer belt 6 in the width direction of the intermediate transfer belt 6. The contact point between the first cleaning brush 122 and the intermediate transfer belt 6 is the first cleaning section (first cleaning position) CL1 where toner is recovered from the intermediate transfer belt 6 by the first cleaning brush 122. The contact point between the second cleaning brush 123 and the intermediate transfer belt 6 is the second cleaning section (second cleaning position) CL2 where toner is recovered from the intermediate transfer belt 6 by the second cleaning brush 123. The first and second cleaning sections CL1 and CL2 are located downstream of the secondary transfer section N2 and upstream of the primary transfer section N1 (the uppermost primary transfer section N1Y) in the rotational direction of the intermediate transfer belt 6. In this embodiment, the first cleaning section CL1 is located upstream of the second cleaning section CL2 in the rotational direction of the intermediate transfer belt 6.

[0032] The first and second recovery rollers 124 and 125 are composed of rotatable metal rollers (made of aluminum in this embodiment). The first and second recovery rollers 124 and 125 are positioned to maintain an insertion depth of approximately 1.5 to 2.5 mm relative to the first and second cleaning brushes 122 and 123. The first and second recovery rollers 124 and 125 are rotated by a drive motor (not shown) as a driving means in the direction of arrow R4 (counterclockwise) in the figure at a peripheral speed equivalent to that of the first and second cleaning brushes 122 and 123. In other words, the first and second recovery rollers 124 and 125 rotate so as to move in the same direction as the movement of the first and second cleaning brushes 122 and 123 at the contact point with the first and second cleaning brushes 122 and 123. The first and second recovery rollers 124 and 125 are positioned so that their rotational axis directions are approximately parallel to the width direction of the intermediate transfer belt 6. The lengths of the first and second recovery rollers 124 and 125 in the rotational axis direction are equal to the lengths of the first and second cleaning brushes 122 and 123 in the rotational axis direction.

[0033] The first and second blades 126 and 127 are positioned in contact with the first and second recovery rollers 124 and 125. The first and second blades 126 and 127 are made of a rubber material such as urethane rubber as an elastic member. The first and second blades 126 and 127 are plate-shaped members having a predetermined length in the longitudinal direction which is approximately parallel to the rotation axis direction of the first and second recovery rollers 124 and 125, and a predetermined thickness in the short direction which is approximately perpendicular to the longitudinal direction. The thickness of the first and second blades 126 and 127 is 1.6 to 2.2 mm, and the hardness is 70 to 78° on the IRHD hardness scale (23°C, 50% RH). Furthermore, the first and second blades 126 and 127 are positioned to penetrate the first and second recovery rollers 124 and 125 by 0.5 to 2.0 mm. The first and second blades 126 and 127 are in contact with the first and second recovery rollers 124 and 125 in a counter-direction (the direction in which the free ends face upstream in the direction of rotation) with respect to the rotational direction of the first and second recovery rollers 124 and 125. The longitudinal length of the first and second blades 126 and 127 is equal to the length of the first and second recovery rollers 124 and 125 in the direction of the rotational axis.

[0034] In this embodiment, a first cleaning voltage (first cleaning bias) of negative polarity, which is the same polarity as the normal charge polarity of the toner, is applied to the first cleaning brush 122 located upstream in the rotational direction of the intermediate transfer belt 6. In this embodiment, a constant-current controlled negative DC voltage is applied to the first recovery roller 124 by a first cleaning power supply (high-voltage power supply) E3, which is a DC power supply. As a result, a constant-current controlled negative DC voltage is applied to the first cleaning brush 122 via the first recovery roller 124. In this embodiment, the first cleaning voltage is applied so that a first cleaning current of -73 μA flows from the first cleaning power supply E3 to the first cleaning brush 122 (i.e., the first cleaning section CL1) via the first recovery roller 124. In this embodiment, the first cleaning current is set to -73 μA, but this is not limited to this value. In other words, when cleaning the intermediate transfer belt 6, a positive current flows outward in the first cleaning section CL1.

[0035] On the other hand, in this embodiment, a second cleaning voltage (second cleaning bias) of positive polarity, which is the opposite polarity to the normal charging polarity of the toner, is applied to the second cleaning brush 123 located downstream in the rotational direction of the intermediate transfer belt 6. In this embodiment, a constant-current controlled positive DC voltage is applied to the second recovery roller 125 by a second cleaning power supply (high-voltage power supply) E4, which is a DC power supply. As a result, a constant-current controlled positive DC voltage is applied to the second cleaning brush 123 via the second recovery roller 125. In this embodiment, the second cleaning voltage is applied so that a second cleaning current of +73 μA flows from the second cleaning power supply E4 to the second cleaning brush 123 (i.e., the second cleaning section CL2) via the second recovery roller 125. In this embodiment, the second cleaning current is set to +73 μA, but this is not limited to this. In other words, in this embodiment, when the intermediate transfer belt 6 is cleaned, a positive current is passed inward in the second cleaning section CL2.

[0036] When a cleaning voltage is applied to the first and second cleaning brushes 122 and 123, a cleaning electric field suitable for recovering toner from the intermediate transfer belt 6 is formed between the first and second cleaning brushes 122 and 123 and the intermediate transfer belt 6. As a result, the secondary transfer residue toner on the intermediate transfer belt 6 is electrostatically attracted to the first and second cleaning brushes 122 and 123 and removed from the intermediate transfer belt 6. The first cleaning brush 122 is coated with positively charged toner from the secondary transfer residue toner on the intermediate transfer belt 6, which is the opposite polarity to the normal charging polarity. The second cleaning brush 123 is coated with negatively charged toner from the secondary transfer residue toner on the intermediate transfer belt 6, which is the normal charging polarity. Furthermore, this toner is transferred from the first and second cleaning brushes 122 and 123 to the first and second recovery rollers 124 and 125 by an electric field formed between the first and second recovery rollers 124 and 125 and the first and second cleaning brushes 122 and 123. The toner transferred to the first and second recovery rollers 124 and 125 is then scraped off from the first and second recovery rollers 124 and 125 by the first and second blades 126 and 127. The toner scraped off from the first and second recovery rollers 124 and 125 is contained within the housing 121. The toner contained within the housing 121 is transported, for example, by a transport member (such as a screw) 128 provided within the housing 121 and discharged from the housing 121. Furthermore, this toner is transported toward a recovery container (not shown) provided within the main body of the image forming apparatus 100 or the like.

[0037] In this embodiment, the drive roller 22 is used as a common opposing roller for the first and second cleaning brushes 122 and 123. However, opposing rollers may be provided independently for each of the first and second cleaning brushes 122 and 123.

[0038] Furthermore, in this embodiment, a voltage is applied to the first and second recovery rollers 124 and 125, but the method of supplying cleaning current is not limited to this. For example, rollers facing the first and second cleaning brushes 122 and 123 can be independently provided via the intermediate transfer belt 6. These rollers can then be used as current supply members and a voltage can be applied to them. In this case, the first and second cleaning brushes 122 and 123 can be used as opposing members and electrically grounded via the first and second recovery rollers 124 and 125. In this case, a voltage with the opposite polarity to the voltage applied to the first and second recovery rollers 124 and 125 in this embodiment can be applied to each roller facing the first and second cleaning brushes 122 and 123. This also allows cleaning of the intermediate transfer belt 6 in the same manner as in this embodiment. Alternatively, the first and second cleaning brushes 122 and 123 can be configured to have a voltage applied directly (or be directly electrically grounded).

[0039] Furthermore, although an electrostatic cleaning device was used as the belt cleaning device 12 in this embodiment, the present invention is not limited to such a configuration. For example, a belt cleaning device that scrapes off toner from the intermediate transfer belt 6 using a cleaning member such as a cleaning blade may be used.

[0040] 4. Discharge device Figure 3 is a schematic enlarged cross-sectional view of the vicinity of the discharge device 27 in this embodiment. In this embodiment, the discharge device 27, as a discharge means, is positioned downstream of the secondary transfer section N2 and upstream of the primary transfer section N1 (the uppermost primary transfer section N1Y) in the rotational direction of the intermediate transfer belt 6. In this embodiment, the discharge device 27 is positioned in a location facing the first auxiliary roller 23 via the intermediate transfer belt 6. In other words, in this embodiment, the discharge device 27 is positioned downstream of the belt cleaning device 12 (first and second cleaning sections CL1 and CL2) and upstream of the primary transfer section N1 (the uppermost primary transfer section N1Y) in the rotational direction of the intermediate transfer belt 6. In this embodiment, the discharge device 27 has a configuration similar to that of an electrostatic cleaning device, in particular an electrostatic brush cleaning device using a conductive fur brush roller.

[0041] The discharge device (resistance rise suppression device) 27 has a housing 275 located near the intermediate transfer belt 6. The following components are provided inside the housing 275. First, a discharge brush 271 is provided as a discharge member (current supply member). Also, a recovery roller 272 is provided as a recovery member. Also, a blade 273 is provided as a scraping member.

[0042] The discharge fur brush 271 consists of a rotatable conductive fur brush roller. The brush fibers of the discharge brush 271 have an electrical resistance value of 3 × 10⁻¹⁰. 5 ~1 × 10 13 (Ω / cm), composed of carbon-dispersed nylon, acrylic, or polyester fibers with a fiber thickness of 2 to 15 denier. The discharge brush 271 uses these brush fibers with a bristle density of 50,000 to 500,000 bristles / inch. 2The discharge brush 271 is constructed by implanting bristles onto a metal roller, which serves as a base material, in a specific ratio. The discharge brush 271 is positioned to maintain an insertion depth of approximately 1.0 to 2.0 mm relative to the intermediate transfer belt 6. The discharge brush 271 is also driven to rotate in the direction of arrow R3 (clockwise) in the figure at a peripheral speed of 20 to 80% of the peripheral speed of the intermediate transfer belt 6 by a drive motor (not shown) as a driving means. In other words, the discharge brush 271 rotates so as to move in the opposite direction to the movement of the intermediate transfer belt 6 at the contact point with the intermediate transfer belt 6, and rubs against the surface of the intermediate transfer belt 6. In this embodiment, the discharge brush 271 is in contact with a first auxiliary roller 23, which functions as an opposing member, via the intermediate transfer belt 6. The first auxiliary roller 23 is electrically grounded. The rotation axis direction of the discharge brush 271 is positioned substantially parallel to the width direction of the intermediate transfer belt 6. The length of the discharge brush 271 in the direction of its rotation axis is longer than the maximum image formation width on the intermediate transfer belt 6 in the width direction of the intermediate transfer belt 6. The contact portion between the discharge brush 271 and the intermediate transfer belt 6 is the discharge section (discharge position) D, where the discharge brush 271 supplies current to the intermediate transfer belt 6 to equalize the ion imbalance within the intermediate transfer belt 6. In the discharge section D, the intermediate transfer belt 6 is discharged by supplying a discharge current to the intermediate transfer belt 6, thereby controlling the relationship between the current supplied inward and the current supplied outward to the intermediate transfer belt 6. In this embodiment, the discharge section D is located downstream of the belt cleaning device 12 (first and second cleaning sections CL1 and CL2) and upstream of the primary transfer section N1 (uppermost primary transfer section N1Y) in the rotation direction of the intermediate transfer belt 6. In a configuration where the discharge unit D is located downstream of the first and second cleaning units CL1 and CL2, a discharge current can be supplied to the cleaned intermediate transfer belt 6, enabling efficient discharge of the intermediate transfer belt 6.

[0043] The recovery roller 272 is composed of a rotatable metal roller (aluminum in this embodiment). The recovery roller 272 is positioned to maintain an insertion depth of approximately 1.5 to 2.5 mm relative to the discharge brush 271. The recovery roller 272 is driven to rotate in the direction of arrow R4 (counterclockwise) in the figure at a peripheral speed equivalent to that of the discharge brush 271 by a drive motor (not shown) as a driving means. In other words, the recovery roller 272 rotates so as to move in the same direction as the movement of the discharge brush 271 at the contact point with the discharge brush 271. The rotation axis direction of the recovery roller 272 is positioned approximately parallel to the width direction of the intermediate transfer belt 6. The length of the rotation axis direction of the recovery roller 272 is equivalent to the length of the rotation axis direction of the discharge brush 271.

[0044] The blade 273 is positioned in contact with the recovery roller 272. The blade 273 is made of a rubber material such as urethane rubber as an elastic member. The blade 273 is a plate-shaped member having a predetermined length in its longitudinal direction, which is positioned approximately parallel to the rotation axis direction of the recovery roller 272, and a predetermined thickness in its short direction, which is approximately perpendicular to the longitudinal direction. The thickness of the blade 273 is 1.6 to 2.2 mm, and its hardness is 70 to 78° on the IRHD hardness scale (23°C, 50% RH). The blade 273 is positioned so as to penetrate the recovery roller 272 by 0.5 to 2.0 mm. The blade 273 is in contact with the recovery roller 272 in the counter-direction with respect to the rotation direction of the recovery roller 272 (the direction in which the free end faces upstream in the rotation direction). The length of the blade 273 in the longitudinal direction is equal to the length of the recovery roller 272 in the rotation axis direction.

[0045] In this embodiment, a positive discharge voltage (discharge bias), which is the opposite polarity to the normal charging polarity of the toner, is applied to the discharge brush 271. In this embodiment, a constant-current controlled positive DC voltage is applied to the recovery roller 272 by a DC power supply, which is a discharge power supply (high-voltage power supply) E5. As a result, a constant-current controlled positive DC voltage is applied to the discharge brush 271 via the recovery roller 272. Consequently, a discharge current is supplied to the discharge brush 271 (i.e., the discharge section D) to equalize the ion imbalance in the intermediate transfer belt 6. In this embodiment, when discharge is performed to equalize the ion imbalance in the intermediate transfer belt 6, a positive current flows inward in the discharge section D. The discharge current will be explained in more detail later.

[0046] In this embodiment, at least a portion of the negatively charged toner that has passed through the belt cleaning device 12 may be recovered from the intermediate transfer belt 6 by the discharge device 27.

[0047] In this embodiment, the discharge device 27 has the same configuration as the electrostatic cleaning device and has the function of recovering at least a portion of the toner that has passed through the belt cleaning device 12. However, the discharge device 27 does not necessarily have the function of cleaning the intermediate transfer belt 6.

[0048] Furthermore, although a voltage is applied to the recovery roller 272 in this embodiment, the method of supplying the discharge current is not limited to this. For example, a roller facing the discharge brush 271 via the intermediate transfer belt 6 (corresponding to the first auxiliary roller 23 in this embodiment) may be used as a current supply member, and a voltage may be applied to it. In this case, the discharge brush 271 may be used as the opposing member, and it may be electrically grounded via the recovery roller 272. In this case, a voltage with the opposite polarity to the voltage applied to the recovery roller 272 in this embodiment may be applied to the roller facing the discharge brush 271 (corresponding to the first auxiliary roller 23 in this embodiment). This also allows for discharge that evens out the ion imbalance in the intermediate transfer belt 6, similar to this embodiment. Alternatively, a configuration in which a voltage is applied directly to the discharge brush 271 (or it is directly electrically grounded) may also be used.

[0049] 5. Discharge current Next, we will explain how to set the discharge current supplied to the intermediate transfer belt 6 by the discharge device 27.

[0050] The present inventors will now describe the experiment they conducted. Figure 5 is a schematic diagram showing the measuring device 200 used in the experiment. As shown in Figure 5, an intermediate transfer belt 6 cut to an appropriate size is wrapped around a metal roller 201 with an outer diameter of 30 mm, and a current supply roller 202 is brought into contact with it, causing the metal roller 201 to rotate at 76 rpm. The intermediate transfer belt 6 is wrapped around the metal roller 201 such that the base layer 6a is in contact with the metal roller 201 and the current supply roller 202 is in contact with the outer circumferential surface of the surface layer 6c. The metal roller 201 is electrically grounded, and a high-voltage power supply 203 is used to alternately apply positive and negative voltages to the current supply roller 202. The current supply roller 202 has the same configuration as the primary transfer roller 5 in the image forming apparatus 100 of this embodiment.

[0051] The voltage applied from the high-voltage power supply 203 to the current supply roller 202 was a square wave. Figure 6 is a schematic diagram showing the waveform of the current flowing from the high-voltage power supply 203 to the intermediate transfer belt 6 via the current supply roller 202. As shown in Figure 6, a positive current and a negative current were supplied from the high-voltage power supply 203 to the intermediate transfer belt 6 via the current supply roller 202, with a difference in the magnitude of the two currents, and the volume resistivity of the intermediate transfer belt 6 was measured after 330 minutes. The volume resistivity was measured using a Hi-Resta UX manufactured by Nitto Seiko Analytech Co., Ltd., with a UR probe applied at a voltage of 1000V.

[0052] Figure 7 is a graph showing an example of the relationship (characteristics) between the current balance of the current supplied to the intermediate transfer belt 6 and the volume resistivity of the intermediate transfer belt 6 when current is supplied to the intermediate transfer belt 6 by the measuring device 200 as described above.

[0053] The horizontal axis of Figure 7 shows the current balance values ​​obtained as follows. Specifically, "Ia" is the absolute value of the positive current flowing from the high-voltage power supply 203 to the intermediate transfer belt 6 via the current supply roller 202. Also, "Ib" is the absolute value of the negative current flowing from the high-voltage power supply 203 to the intermediate transfer belt 6 via the current supply roller 202 (i.e., the absolute value of the positive current flowing from the metal roller 201 side to the high-voltage power supply 203 side via the intermediate transfer belt 6). In this case, the current balance can be calculated by subtracting "Ib", which is the positive current (absolute value) supplied from the inner surface side to the outer surface side of the intermediate transfer belt 6 (outward direction), from "Ia", which is the positive current (absolute value) supplied from the outer surface side to the inner surface side of the intermediate transfer belt 6 (inward direction). However, in this embodiment, the value of "Ia-Ib" is divided by the length of the current supply region in the width direction of the intermediate transfer belt 6 so that it becomes the current balance per unit length in the width direction of the current supply region by the current supply roller 202. Here, the length of the current supply region by the current supply member in the width direction of the intermediate transfer belt 6 is the shorter of the length of the current supply member and the opposing member facing the current supply member across the intermediate transfer belt 6. In the case of the measuring device 200 shown in Figure 5, the length of the current supply region by the current supply roller 202 in the width direction of the intermediate transfer belt 6 is the length of the current supply roller 202 in the longitudinal direction (direction of the rotation axis). For simplicity, the length of the current supply region by the current supply member in the width direction of the intermediate transfer belt 6 is also simply called the "longitudinal length of the current supply member" or the "longitudinal width of the current supply member". By using the current balance per unit length of the current supply region, the current balance can be evaluated more accurately regardless of the length of the current supply region. The horizontal axis of Figure 7 shows the current balance values ​​obtained in this manner. Note that the same result can be obtained by subtracting the value obtained by dividing "Ib" by the length of the current supply roller 202 from the value obtained by dividing "Ia" by the length of the current supply roller 202. Hereafter, unless otherwise specified, the current balance refers to the current balance per unit length of the current supply area as described above.In other words, the current balance is the value obtained by subtracting the sum of the positive current (absolute value) per unit length in the longitudinal direction of the current supply member supplied by the current supply member in the direction from the inner surface to the outer surface (outward direction) of the intermediate transfer belt 6 from the sum of the positive current (absolute value) per unit length in the longitudinal direction of the current supply member supplied by the current supply member in the direction from the outer surface to the inner surface (inward direction) of the intermediate transfer belt 6. The current balance on the horizontal axis of Figure 7 is plotted as a positive value when "Ia" is greater than "Ib" (when "Ia-Ib" is a positive value), and conversely, it is plotted as a negative value when "Ia" is less than "Ib" (when "Ia-Ib" is a negative value). On the other hand, the vertical axis of Figure 7 is the common logarithm (log(volume resistivity)) of the volume resistivity of the intermediate transfer belt 6.

[0054] In Figure 7, the degree of increase in volume resistivity due to an increase in the absolute value of the current balance is compared when the current balance is positive and when it is negative. In the configuration of this embodiment, the increase in volume resistivity when the current balance is positive is gradual, while the increase in volume resistivity when the current balance is negative is steep. When the current balance is negative, the volume resistivity increases sharply to 1 × 10⁻⁶ due to an increase in the absolute value of the current balance. 12 The resistance becomes Ω·cm or higher, and then nearly saturates. In the configuration of this embodiment, as will be described later, from the viewpoint of image quality (suppressing mesh-like abnormal images and maintaining sufficient transferability), the acceptable upper limit of the volume resistivity of the intermediate transfer belt 6 is 1 × 10⁻⁶. 12 It is approximately Ω·cm. On the other hand, when the current balance is positive, the volume resistivity gradually increases due to the increase in the absolute value of the current balance, reaching the upper limit of 1 × 10⁻⁶. 12In many cases, it does not reach Ω·cm. In Figure 7, let "Sa" be the slope of the approximate straight line relating the current balance and volume resistivity when the current balance is on the positive side, and "Sb" be the slope of the approximate straight line relating the current balance and volume resistivity when the current balance is on the negative side. Slope Sa is the slope in the range where the current balance is a positive value near 0 (the range where the volume resistivity is smaller than the upper limit above). That is, slope Sa is the slope in the range where the current balance is a positive value when the volume resistivity of the intermediate transfer belt 6 is below a predetermined value. Slope Sb is the slope in the range where the current balance is a negative value when the volume resistivity of the intermediate transfer belt 6 is below a predetermined value. In this example shown in Figure 7, the absolute value of the slope Sb, |Sb|, was approximately 7 times the absolute value of the slope Sa, |Sa|.

[0055] Although the mechanism of this phenomenon is not yet fully understood, the following can be predicted. Ionic conductive agents include those that easily move negative ions and those that easily move positive ions. In the configuration of this embodiment, the ionic conductive agent is considered to be one that easily moves negative ions. Looking at a cross-sectional photograph of the elastic layer 6b of the intermediate transfer belt 6, it is as follows. In other words, in the initial stages of use of the intermediate transfer belt 6, the negative ions of the ionic conductive agent are uniformly dispersed, but when the electrical resistance increases, the negative ions of the ionic conductive agent become unevenly distributed on the base layer 6a side or the surface layer 6c side of the elastic layer 6b. The base layer 6a side of the elastic layer 6b is the inner circumferential surface side of the intermediate transfer belt 6, which is the metal roller 201 (or tension roller) side, and the elongation of the elastic layer 6b is extremely small. Therefore, a negative current flows in the direction from the outer circumferential surface side to the inner circumferential surface side (inward direction) of the intermediate transfer belt 6, and it is considered that the negative ions of the ionic conductive agent that have gathered on the base layer 6a side remain almost in place. As a result, when the current balance is negative, the degree of increase in volume resistivity due to the increase in the absolute value of the current balance (absolute value of the slope Sb) is considered to be large. On the other hand, the surface layer 6c side of the elastic layer 6b is on the outer surface side of the intermediate transfer belt 6, and is therefore easily stretched. For this reason, even if a positive current flows in the direction from the outer surface side to the inner surface side of the intermediate transfer belt 6 (inward direction), and the negative ions of the ionic conductive agent are unevenly distributed on the surface layer 6c side, the elastic layer 6b is considered to stretch, and the uneven distribution is considered to be alleviated. As a result, when the current balance is positive, the degree of increase in volume resistivity due to the increase in the absolute value of the current balance (absolute value of the slope Sa) is considered to be small.

[0056] Thus, when |Sb| > |Sa|, if the current balance becomes negative, the electrical resistance of the intermediate transfer belt 6 increases sharply. In particular, when |Sb| exceeds twice |Sa|, the electrical resistance of the intermediate transfer belt 6 increases sharply. Therefore, it can be seen that the discharge current should be set in such a way that the current balance is offset to the positive side so that the current balance does not become negative due to factors such as fluctuations in the output of the discharge power supply E5.

[0057] Next, we will explain how to calculate the discharge current supplied to the discharge brush 271, which serves as a discharge component.

[0058] In this embodiment, the intermediate transfer belt 6 is supplied with current by the primary transfer roller 5, secondary transfer roller 9, and first and second cleaning brushes 122 and 123 in the primary transfer section N1, secondary transfer section N2, and first and second cleaning sections CL1 and CL2, respectively. In this embodiment, the discharge current supplied to the discharge brush 271 during image formation is determined by the following formula (1). That is, the current density is determined by dividing the absolute value of the current supplied to the intermediate transfer belt 6 by the primary transfer roller 5, secondary transfer roller 9, and first and second cleaning brushes 122 and 123 during image formation by the longitudinal width of each component. Furthermore, the sum of the current densities of the positive currents supplied in the direction from the outer circumferential surface to the inner circumferential surface (inward direction) of the intermediate transfer belt 6 is subtracted from the sum of the current densities of the positive currents supplied in the direction from the outer circumferential surface to the outer circumferential surface (outward direction) of the intermediate transfer belt 6 to obtain the value obtained by subtracting each of the above current densities. Then, this value is multiplied by the longitudinal width of the discharge brush 271. In addition, a discharge correction current is added to offset the current balance as described above. In this embodiment, during image formation (during image formation), a discharge current of the value obtained by the following formula (1) as described above is supplied to the discharge brush 271. Idis={(It1y+It1m+It1c+It1k) / Rt1 -It2 / Rt2 +Icl1 / Rcl1 -Icl2 / Rcl2}×Rdis +Idis_offset ···(1) Rt1: Longitude of the primary transfer roller Rt2: Longitudinal width of the secondary transfer roller Rcl1: Longitude of the first cleaning brush Rcl2: Longitude of the second cleaning brush Rdis: Discharge brush length It1y, It1m, It1c, It1k: Primary transfer current it2: Secondary transfer current Icl1: First cleaning current Icl2: Second cleaning current Idis: Discharge current Idis_offset: Discharge correction current

[0059] In this embodiment, the length of the current supply area by the primary transfer roller 5 in the width direction of the intermediate transfer belt 6 is the longitudinal length of the primary transfer roller 5, which is shorter in that direction than the primary transfer roller 5 and the photosensitive drum 1. The length of the current supply area by the secondary transfer roller 9 in the width direction of the intermediate transfer belt 6 is the longitudinal length of the secondary transfer roller 9, which is shorter in that direction than the secondary transfer roller 9 and the secondary transfer opposing roller 21. The length of the current supply area by the first and second cleaning brushes 122 and 123 in the width direction of the intermediate transfer belt 6 is the longitudinal length of the first and second cleaning brushes 122 and 123, which is shorter in that direction than the first and second cleaning brushes 122 and 123 and the drive roller 22. Furthermore, the length of the current supply area by the discharge brush 271 in the width direction of the intermediate transfer belt 6 is the longitudinal length of the discharge brush 271, which has a shorter length in that direction than the discharge brush 271 and the first auxiliary roller 23.

[0060] Furthermore, the current supplied by each of the above-mentioned parts (primary transfer section N1, secondary transfer section N2, first and second cleaning sections CL1 and CL2, and discharge section D) during image formation (during image formation) can be represented by the period during which the image formation region (described later) on the intermediate transfer belt 6 passes over each of these parts. In other words, the primary transfer current during image formation can be represented by the primary transfer current when the image formation region on the intermediate transfer belt 6, which is in the process of primary image transfer, passes over the primary transfer section N1. Similarly, the secondary transfer current during image formation can be represented by the secondary transfer current when the recording material P, which is in the process of secondary image transfer, passes over the secondary transfer section N2. Finally, the first and second cleaning currents during image formation can be represented by the first and second cleaning currents when the image formation region on the intermediate transfer belt 6, immediately after the image has been transferred to the recording material P in the secondary transfer section N2, passes over the first and second cleaning sections CL1 and CL2. Furthermore, the discharge current during image formation can be represented by the discharge current when the image formation region on the intermediate transfer belt 6 passes through the discharge section D immediately after the image has been transferred to the recording material P in the secondary transfer section N2. However, a predetermined period in a print job (described later) (for example, the period from the start of feeding one sheet of recording material P to the end of ejection) may be considered as one image formation, and the average value for each period may be used as the current supplied to each section during image formation. From the viewpoint of setting a discharge current that can sufficiently suppress the increase in the electrical resistance of the intermediate transfer belt 6, it is sufficient to estimate the actual current balance of the current supplied to the intermediate transfer belt 6 with sufficient accuracy.

[0061] Table 1 shows an example of the initial values ​​of each current when the image forming apparatus 100 is powered on in this embodiment. However, the values ​​of each current are not limited to these values. In this embodiment, the discharge correction current was set to 11 μA. This is for the following reason. In other words, in the configuration of this embodiment, due to individual differences in the discharge power supply E5, there is a possibility that its output may fluctuate within a range of ±5%. For this reason, the discharge current was set to approximately 220 μA, and the discharge correction current was set to 11 μA, which is 5% of that. The above approximately 220 μA is an estimated value of the discharge current assuming that the right-hand side of equation (1) above is relatively large. Note that fluctuations in output due to individual differences in power supplies mean that, due to the accuracy of the power supply, even if the target value is set to 220 μA, it may output 209 μA or 231 μA. In other words, although there is no time variation in each device, the actual current flowing relative to the target value varies from device to device due to individual variations between devices. Thus, in this embodiment, the discharge current is set with an offset such that the current balance is on the positive side, near zero. This offset amount is set to be greater than or equal to the fluctuation of the discharge current due to fluctuations in the output of the discharge power supply E5.

[0062] [Table 1]

[0063] Applying the values ​​shown in Table 1 to equation (1), the discharge current becomes 212.8 μA. The current balance at this time is, as described above, the value obtained by subtracting the sum of the positive currents per unit length in the longitudinal direction of the current supply member supplied by the current supply member in the direction from the inner surface to the outer surface (outward direction) of the intermediate transfer belt 6 from the sum of the positive currents per unit length in the longitudinal direction of the current supply member supplied by the current supply member in the direction from the outer surface to the inner surface (inward direction) of the intermediate transfer belt 6. Calculating this, we get +0.03 μA / mm, and thus the current balance is +0.03 μA / mm. In other words, by adding a discharge correction current, the current balance is in the positive range near zero, and even if there is a fluctuation in the discharge current, it will not become negative.

[0064] In this embodiment, when the intermediate transfer belt 6 has the characteristic |Sb|>|Sa|, it is preferable that the discharge current is +2.5% or more of the target discharge current value that results in a current balance of 0 μA / mm (for example, if the target current value is 200 μA, then +5 μA or more, i.e., 205 μA or more). This makes it possible to more accurately set the current balance in the positive range near 0, so that the current balance does not become negative even if there is a fluctuation in the discharge current. On the other hand, in a system where |Sb|>|Sa| as in this embodiment, the discharge current should be +50% or less of the target discharge current value that results in a current balance of 0 μA / mm (for example, if the target current is 200 μA, then +100 μA or less, i.e., 300 μA or less), preferably +30% or less, and more preferably +10% or less. In this case, if the discharge current is too large, the effect of suppressing the increase in the electrical resistance of the intermediate transfer belt 6 may be insufficient. As explained in Example 2, depending on the characteristics of the intermediate transfer belt 6, it is possible that |Sa|>|Sb| may occur. When the characteristics of the intermediate transfer belt 6 are such, the discharge current is preferably -2.5% or less of the target current value of the discharge current that makes the current balance 0 μA / mm (for example, if the target current value is 200 μA, then -5 μA or less, i.e., 195 μA or less). This makes it possible to more accurately set the current balance in the negative range near 0, so that the current balance does not become positive even if there is a fluctuation in the discharge current. On the other hand, in a system where |Sa|>|Sb| is the case as explained in Example 2, the discharge current should be -50% or more of the target current value of the discharge current that makes the current balance 0 μA / mm (for example, if the target current is 200 μA, then -100 μA or more, i.e., 100 μA or more), preferably -30% or more, and more preferably -10% or more. In this case, if the discharge current is too small, it may not be effective enough to suppress the increase in the electrical resistance of the intermediate transfer belt 6.

[0065] In this embodiment, once a print job is started, the primary and secondary transfer currents are detected almost constantly. Based on the detected current values, the discharge current is calculated at predetermined intervals, for example, every predetermined number of images (e.g., 1 to 20 images), and the discharge current is changed to the newly calculated value. By calculating the discharge current using the detection results of the primary and secondary transfer currents, even if the primary and secondary transfer currents deviate from the values ​​at the time the image forming apparatus 100 is powered on, the discharge current can be changed to the optimal value at that time. The primary and secondary transfer currents may change unintentionally during the execution of a print job due to the presence or absence of toner or changes in the electrical resistance of the intermediate transfer belt 6 or recording material P. In addition, the primary and secondary transfer currents may be intentionally changed during the execution of a print job by readjusting the primary and secondary transfer voltages. Until the detection results of the primary and secondary transfer currents for the print job become available (for example, until the first predetermined number of images are formed), the following can be done. For example, the discharge current can be calculated using target values ​​(table values) for the primary and secondary transfer currents, or the discharge current from a previous print job (e.g., the last one) can be used.

[0066] Furthermore, in this embodiment, the discharge current is calculated using the target values ​​of the first and second cleaning currents, which are controlled by constant current, respectively. In this embodiment, the output values ​​of the first and second cleaning power supplies E3 and E4 are controlled by constant current so that the current detected by the current detection unit is approximately constant at the target value, as will be described later. Therefore, the discharge current may be calculated using the detection results of the first and second cleaning currents (for example, the average value over the number of images formed).

[0067] 6. Control Modes Figure 8 is a schematic block diagram showing the control configuration of the main parts of the image forming apparatus 100 in this embodiment. The image forming apparatus 100 has a control unit 50 as a control means. The control unit 50 is composed of a CPU 51 as an arithmetic control means, which is a central element that performs arithmetic processing, and memory (storage medium) such as RAM 52 and ROM 53 as storage means. The RAM 52, which is a rewritable memory, stores information input to the control unit 50, detected information, calculation results, etc., and the ROM 53 stores the control program, a pre-determined data table, etc. The CPU 51 and the memory such as RAM 52 and ROM 53 can transfer and read data from each other.

[0068] The control unit 50 is connected to an operation unit (not shown) provided on the image forming apparatus 100. The control unit 50 is also connected to external devices (not shown) such as an image reading unit (image scanner) or a personal computer connected to the image forming apparatus 100. Based on instructions from the operation unit of the image forming apparatus 100, image data from the image reading unit, or image forming signals (image data, control commands) from external devices, the control unit 50 comprehensively controls each part of the image forming apparatus 100 to execute a print job (described later). In this embodiment, the control unit 50 is connected to, for example, a primary transfer power supply E1, a secondary transfer power supply E2, a first cleaning power supply E3, a second cleaning power supply E4, and a discharge power supply E5. Furthermore, in this embodiment, the control unit 50 is connected to a counting means (counting means) configured with a storage means for counting the number of recording materials P on which images have been formed and output from the image forming apparatus 100. In this embodiment, the primary transfer power supply E1 is provided independently for each image forming unit 10.

[0069] Here, the image forming apparatus 100 executes a print job, which is a series of operations that form and output an image on one or more recording materials P, initiated by a single start instruction. In this embodiment, a start instruction is input to the image forming apparatus 100 from an operation unit or an external device. A print job generally includes an image forming process, a pre-rotation process, an inter-paper process when forming an image on multiple recording materials P, and a post-rotation process. The image forming process is the period during which the electrostatic image of the image to be actually formed and output on the recording material P is formed, the toner image is formed, and the toner image is transferred to the primary and secondary positions. The term "image forming time" refers to this period. More specifically, the timing of image forming time differs depending on the position where each of the above processes—electrostatic image formation, toner image formation, primary and secondary toner image transfer—is performed, and corresponds to the period during which the image forming area on the photosensitive drum 1 or the intermediate transfer belt 6 passes through each of these positions. The pre-rotation process is the period from when a start instruction is input to the image forming apparatus 100 until the image is actually formed, during which preparatory operations are performed before the image forming process. The inter-paper process (inter-recording material process, inter-image process) is the period between recording materials P when image formation is performed continuously on multiple recording materials P (continuous image formation). The post-rotation process is the period during which tidying operations (preparation operations) are performed after the image formation process. Non-image formation time is the period other than the image formation time, and includes the pre-rotation process, inter-paper process, post-rotation process, and pre-multi-rotation process, which is the preparation operation when the image forming apparatus 100 is powered on or when it returns from sleep mode. More specifically, the timing of non-image formation time corresponds to the period during which the non-image formation area on the photosensitive drum 1 or the intermediate transfer belt 6 passes through the positions where each of the electrostatic image formation, toner image formation, primary transfer of the toner image, and secondary transfer is performed. Note that the image formation area on the photosensitive drum 1 or the intermediate transfer belt 6 is the area in which an image can be formed when transferred to the recording material P and output from the image forming apparatus 100, and the non-image formation area is the area other than the image formation area.

[0070] In this embodiment, the primary transfer power supply E1 and the secondary transfer power supply E2 each have current detection units (current detection circuits) F1 and F2, respectively, as current detection means. The control unit 50 performs control (ATVC control) to determine a voltage value such that the current value detected by the current detection units F1 and F2 becomes a predetermined target current value during non-image formation processes such as the pre-rotation process. The primary transfer power supply E1 and the secondary transfer power supply E2 then perform constant voltage control on their output values ​​so that they remain approximately constant at the above voltage value (or a voltage value determined based on the above voltage value) during image formation (primary transfer and secondary transfer). In this embodiment, control is performed to independently determine the primary transfer voltage in each image forming unit 10Y, 10M, 10C, and 10K. In this embodiment, the first and second cleaning power supplies E3 and E4 each have current detection units (current detection circuits) F3 and F4, respectively, as current detection means. The first and second cleaning power supplies E3 and E4 control their output values ​​to a constant current so that the current values ​​detected by the current detection units F3 and F4 during image formation (cleaning) remain approximately constant at predetermined target current values. The target current values ​​for the primary transfer voltage, secondary transfer voltage, and first and second cleaning voltages may be changed according to image formation conditions, such as the detection results of the environment (at least one of the temperature or humidity inside or outside the image forming apparatus 100). For example, a corresponding value may be selected from a plurality of preset values ​​(table values) according to the image formation conditions. Furthermore, in order to readjust the primary and secondary transfer voltages during the execution of a print job, control may be performed to determine the primary and secondary transfer voltages in the inter-paper process, for example, at predetermined intervals of image formation. In this embodiment, the target values ​​for the first and second cleaning currents are fixed to approximately constant values ​​during the execution of a print job. In addition, in this embodiment, the discharge power supply E5 has a current detection unit (current detection circuit) F5 as a current detection means. When the intermediate transfer belt 6 is discharged, the discharge power supply E5 controls its output value to a constant current so that the value of the current detected by the current detection unit F5 remains approximately constant at the target value of the discharge current determined as described above.Furthermore, the primary transfer power supply E1, the secondary transfer power supply E2, the first and second cleaning power supplies E3 and E4, and the discharge power supply E5 may each have a voltage detection unit for detecting the output voltage.

[0071] 7. Control Procedure Next, the operation of the print job in this embodiment will be described. Figure 9 is a flowchart illustrating the general procedure of the print job in this embodiment.

[0072] When the control unit 50 starts a print job and begins image formation (S1), it stores the detection results of the primary transfer current and secondary transfer current during image formation in the RAM 52, and also stores the cumulative number of images formed in the image counter 70 (S2). In this embodiment, the control unit 50 stores the detection result of the primary transfer current (average value for each image in this embodiment) for each image transferred to one recording material P in the RAM 52 as the primary transfer current during image formation. In addition, in this embodiment, the control unit 50 stores the detection result of the secondary transfer current (average value for each image in this embodiment) for each image transferred to one recording material P in the RAM 52 as the secondary transfer current during image formation. Furthermore, each time an image is formed on a recording material P (for example, by secondary transfer), the control unit 50 accumulates the number of images formed and stores it in the image counter 70.

[0073] Next, the control unit 50 determines whether it is time to execute the discharge current change (S3). In this embodiment, the control unit 50 determines whether the number of images formed has reached a predetermined number of images (14 in this embodiment), and if it has, it determines that it is time to execute the discharge current change. If the control unit 50 determines in S3 that it is time to execute the discharge current change ("Yes"), it averages the primary transfer current and secondary transfer current for the number of images stored in the RAM 52 (S4). Then, the control unit 50 calculates the target value of the discharge current (target current) using the calculation method described above (S5). As described above, the control unit 50 calculates the discharge current using the detected result (average value) of the primary transfer current, the detected result (average value) of the secondary transfer current, and the target values ​​of the first and second cleaning currents (which may also be the detected results) using the formula (1) described above. Then, the control unit 50 changes the discharge current to the value calculated this time (S6). In other words, during a continuous image formation job, the control unit 50 controls the discharge power supply so that the average value of the current balance when images are formed continuously on a predetermined number of recording materials P becomes a positive value. If the discharge current is changed in S6, the control unit 50 erases the detection results of the primary transfer current and secondary transfer current stored in the RAM 52, and resets the count value of the number of images formed related to the timing of the discharge current change stored in the count counter 70 to its initial value (0 in this embodiment). If the control unit 50 determines in S3 that it is not the timing to change the discharge current ("No"), it proceeds to the process in S7.

[0074] Next, the control unit 50 determines whether all image formation for the print job has been completed (S7). If it determines that image formation has not been completed ("No"), it returns to the process in S1. If it determines that image formation has been completed ("Yes"), it terminates the print job.

[0075] A paper feed test (continuous image forming durability test) was performed on the image forming apparatus 100 of this embodiment to investigate the change in the electrical resistance of the intermediate transfer belt 6. In this embodiment, a discharge device 27 was used to set the current balance to +0.03 μA / mm. The same paper feed test was also performed on the comparative image forming apparatus 100, which is not equipped with a discharge device 27. The test conditions were as follows: Ambient temperature and humidity: 23℃ / 50%, Rotation speed of the intermediate transfer belt 6 (peripheral speed): 464 mm / s, Recording material P: Basis weight 81 g / m² 2 Continuous feeding of plain paper. The configuration and operation of the comparative image forming apparatus 100 are substantially the same as those of the image forming apparatus 100 in this embodiment, except that a discharge device 27 is not provided. For the comparative image forming apparatus 100 as well, elements having the same or corresponding functions or configurations as those of the image forming apparatus 100 in this embodiment will be denoted by the same reference numerals and described accordingly. The results are shown in Figure 10. For convenience, Figure 10 also shows the results of the paper feeding test related to Example 2, which will be described later.

[0076] In the comparative example image forming apparatus 100, which is not equipped with a discharge device 27, the increase in the volume resistivity of the intermediate transfer belt 6 after the start of the paper feeding test was significant. Furthermore, when the number of sheets fed exceeded 500k sheets, the volume resistivity of the intermediate transfer belt 6 was 1.0 × 10⁻⁶. 12 The resistance exceeded Ω·cm, resulting in a mesh-like abnormal image in the red image (secondary color of yellow and magenta). This phenomenon occurs because the conductive agent (negative ions in this embodiment) is unevenly distributed on the base layer 6a side of the elastic layer 6b, and the difference in electrical resistance between areas with and without the conductive agent on the sparsely distributed surface layer 6c is the cause. Furthermore, in the comparative example image forming apparatus 100, which does not have a discharge device 27, sufficient transfer performance could no longer be maintained after 1200k sheets of paper were fed. If, in the configuration of this embodiment, the output of the discharge power supply E5 fluctuates and the current balance becomes negative, the electrical resistance of the intermediate transfer belt 6 may increase relatively quickly, similar to the comparative example, potentially shortening the lifespan of the intermediate transfer belt 6.

[0077] On the other hand, in the image forming apparatus 100 of this embodiment, when the current balance is set to +0.03 μA / mm using the discharge device 27, the increase in the volume resistivity of the intermediate transfer belt 6 is 1.6 × 10⁻⁶. 11 The resistance was kept low, down to Ω·cm. Furthermore, in this embodiment, no mesh-like abnormal images occurred during the paper feeding test. In addition, in this embodiment, sufficient transfer performance was maintained up to 2500k sheets of paper, achieving high durability for the intermediate transfer belt 6.

[0078] As described above, in this embodiment, the image forming apparatus 100 includes an image carrier 1 that carries a toner image, a rotatable endless intermediate transfer belt 6 onto which the toner image is transferred from the image carrier 1, a primary transfer member 5 which is a current supply member that supplies a primary transfer current to the intermediate transfer belt 6 in a primary transfer section N1 to transfer the toner image from the image carrier 1 to the intermediate transfer belt 6, a secondary transfer member 9 which is a current supply member that supplies a secondary transfer current to the intermediate transfer belt 6 in a secondary transfer section N2 to transfer the toner image from the intermediate transfer belt 6 to the recording material P, a discharge member 271 which is a current supply member that supplies a discharge current to the intermediate transfer belt 6 in a discharge section D located downstream of the secondary transfer section N2 and upstream of the primary transfer section N1 in the rotational direction of the intermediate transfer belt 6, a discharge power supply E5 that supplies the discharge current, and a control unit 50 that can control the discharge power supply E5. In this embodiment, Ia is the sum of positive currents per unit length in the width direction of the intermediate transfer belt 6 supplied by the current supply member in the direction from the outer circumferential surface side to the inner circumferential surface side (inward direction) during image formation, and Ib is the sum of positive currents per unit length in the width direction of the intermediate transfer belt 6 supplied by the current supply member in the direction from the inner circumferential surface side to the outer circumferential surface side (outward direction) during image formation, and the value of Ia-Ib is the current balance of the current supplied to the intermediate transfer belt 6. Assuming that the electrical resistance of the intermediate transfer belt 6 is below a predetermined value, the relationship between the electrical resistance of the intermediate transfer belt 6 and the current balance is such that, when Sa is the rate of change in the electrical resistance of the intermediate transfer belt 6 with respect to the change in the current balance when the current balance is in the positive range (the slope described above), and Sb is the rate of change when the current balance is in the negative range (the slope described above), the relationship |Sb|>|sa| is satisfied, and the control unit 50 controls the discharge power supply E5 so that the average value of the current balance when images are formed continuously on a predetermined number of recording materials P during a continuous image formation job becomes a positive value. In other words, in this embodiment, the control unit 50 sets the discharge current so that the current balance approaches 0, and also sets the discharge current so that the current balance becomes a positive value.

[0079] In this embodiment, the image forming apparatus 100 has a plurality of primary transfer members 5 along the rotation direction of the intermediate transfer belt 6. In this embodiment, the image forming apparatus 100 also has cleaning members 122 and 123, which are current supply members that supply cleaning current to the intermediate transfer belt 6 to remove toner from the intermediate transfer belt 6 in cleaning sections CL1 and CL2 located downstream of the secondary transfer section N2 and upstream of the primary transfer section N1 in the rotation direction of the intermediate transfer belt 6. In this embodiment, the control unit 50 sets the discharge current such that the offset amount with respect to the value of the discharge current that makes the current balance zero is greater than or equal to the fluctuation of the output of the discharge power supply E5. In this embodiment, the image forming apparatus 100 also has detection units F1 and F2 for monitoring the current balance, and the control unit 50 sets the discharge current based on the detection results of the detection units F1 and F2. In this embodiment, the detection units F1 and F2 detect at least the primary transfer current and the secondary transfer current. In this embodiment, the intermediate transfer belt 6 is ionic conductive. In particular, in this embodiment, the intermediate transfer belt 6 has an elastic layer 6b containing an ionic conductive agent. Also, in this embodiment, |Sb| is greater than twice |Sa|. However, it is not limited to this, but typically |Sb| is less than 10 times |Sa|.

[0080] As explained above, in this embodiment, the discharge current is set with an offset so that the current balance is on the positive side, near zero. This prevents the current balance from becoming negative and causing a sudden increase in the electrical resistance of the intermediate transfer belt 6. Furthermore, in this embodiment, the amount of this offset is set to be greater than or equal to the fluctuation in the discharge current caused by fluctuations in the output of the discharge power supply E5. This prevents the current balance from becoming negative even if there are fluctuations in the discharge current due to fluctuations in the output of the discharge power supply E5. In addition, in this embodiment, the current balance during image formation (more specifically, the primary transfer current, secondary transfer current, and first and second cleaning currents) is monitored during the execution of the print job, and the discharge current is controlled (changed) based on the results. This allows the discharge current to be set more appropriately and the intermediate transfer belt 6 to discharge more appropriately, even when the primary or secondary transfer current changes due to, for example, the presence or absence of toner.Therefore, according to this embodiment, the discharge current can be optimized and the increase in the electrical resistance of the intermediate transfer belt 6 can be suppressed.

[0081] [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.

[0082] In this embodiment, we describe a configuration in which the relationship between current balance and volume resistivity shown in Figure 11 is obtained when tested with the measuring device 200 shown in Figure 5, which was described in Example 1. In other words, in this embodiment, unlike Example 1, the slope of the above relationship is steeper when the current balance is on the positive side, and gentler when the current balance is on the negative side. For example, in the example shown in Figure 11, the absolute value of the slope Sa |Sa| is approximately 7 times the absolute value of the slope Sb |Sb|. Depending on the type of ionic conductive agent contained in the elastic layer 6b of the intermediate transfer belt 6, the relationship between the steepness of the above slope between the regions of current balance around 0 may differ. In Example 1, the ionic conductive agent was one in which negative ions move easily, but in this embodiment, it is considered to be a conductive agent in which positive ions move easily.

[0083] Thus, in the case where |Sa|>|Sb|, the discharge current should be set so that the current balance is on the negative side near zero. In the image forming apparatus 100 of this embodiment, the current balance was set to -0.03 μA / mm, and the same paper feeding test as described in Example 1 was performed. As a result, as shown in Figure 10, the increase in the volume resistivity of the intermediate transfer belt 6 was 1.6 × 10, similar to the case in Example 1. 11 The pressure was kept low, down to Ω·cm, and no mesh-like abnormal images occurred during the paper feeding test. Furthermore, sufficient transfer performance was maintained up to 2500k sheets, demonstrating the high durability of the intermediate transfer belt 6.

[0084] Thus, in this embodiment, the control unit 50 controls the discharge power supply E5 so that the average value of the current balance when images are formed continuously on a predetermined number of recording materials P during a continuous image forming job becomes a negative value. In other words, in this embodiment, the control unit 50 sets the discharge current so that the above-mentioned current balance approaches 0, and also sets the discharge current so that the above-mentioned current balance becomes a negative value. Furthermore, in this embodiment, |Sa| is greater than twice |Sb|. Although not limited to this, typically |Sa| is less than 10 times |Sb|.

[0085] As described above, in the configuration of this embodiment, as in Embodiment 1, the discharge current can be optimized to suppress the increase in the electrical resistance of the intermediate transfer belt 6.

[0086] [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.

[0087] For example, the discharge member is not limited to a brush roller, but may be in the form of a solid rubber roller, sponge rubber roller, metal roller, sheet, film, pad, or other similar material. The same applies to the primary transfer member, secondary transfer member, and cleaning member.

[0088] Furthermore, the discharge voltage is not limited to constant current control. A target value for the discharge current can be set in the same manner as in the above embodiment, and the voltage value when the current of the target value flows can be determined. The discharge voltage can then be controlled at a constant voltage so that it remains approximately constant at that voltage. The same applies to the cleaning voltage; it is not limited to constant current control, but may also be controlled at a constant voltage. Similarly, the primary and secondary transfer voltages are not limited to constant voltage control, and at least one of them may be controlled at a constant current. When the primary and secondary transfer voltages are controlled at a constant current, their target values ​​may be used to determine the discharge current. [Explanation of symbols]

[0089] 1 Photosensitive drum 5. Primary transfer roller 9. Secondary transfer roller 6. Intermediate transfer belt 10 Image forming unit 12 Belt cleaning device 27 Discharge device 50 Control Unit 100 Image forming apparatus 271 Discharge Brush E1 Primary Transfer Power Supply E2 Secondary Transfer Power Supply E3 First Cleaning Power Supply E4 Second Cleaning Power Supply E5 Discharge Power Supply P recording material

Claims

1. An image carrier that holds the toner image, A rotatable, endless intermediate transfer belt onto which a toner image is transferred from the image carrier, A primary transfer member is a current supply member that supplies a primary transfer current to the intermediate transfer belt in the primary transfer section to transfer a toner image from the image carrier to the intermediate transfer belt, A secondary transfer member is a current supply member that supplies a secondary transfer current to the intermediate transfer belt in the secondary transfer section, thereby secondary transferring a toner image from the intermediate transfer belt to the recording material. A discharge member is a current supply member that supplies a discharge current to the intermediate transfer belt in a discharge section downstream of the secondary transfer section and upstream of the primary transfer section in the rotational direction of the intermediate transfer belt, A discharge power supply that supplies the aforementioned discharge current, A control unit capable of controlling the discharge power supply, In an image forming apparatus having, During image formation, the sum of positive currents per unit length in the width direction of the intermediate transfer belt supplied by the current supply member in the direction from the outer circumferential surface to the inner circumferential surface of the intermediate transfer belt is Ia, and the sum of positive currents per unit length in the width direction of the intermediate transfer belt supplied by the current supply member in the direction from the inner circumferential surface to the outer circumferential surface of the intermediate transfer belt supplied by the current supply member during image formation is Ib. The value of Ia - Ib is defined as the current balance of the current supplied to the intermediate transfer belt, and the intermediate transfer The image forming apparatus is characterized in that, within a range where the electrical resistance of the belt is less than or equal to a predetermined value, the relationship between the electrical resistance of the intermediate transfer belt and the current balance satisfies the relationship |Sb| > |Sa|, where Sa is the ratio of the change in the electrical resistance of the intermediate transfer belt to the change in the current balance when the current balance is in a positive range, and Sb is the ratio when the current balance is in a negative range, and the control unit controls the discharge power supply such that the average value of the current balance when images are formed continuously on a predetermined number of recording materials during a continuous image forming job becomes a positive value.

2. An image carrier that holds the toner image, A rotatable, endless intermediate transfer belt onto which a toner image is transferred from the image carrier, A primary transfer member is a current supply member that supplies a primary transfer current to the intermediate transfer belt in the primary transfer section to transfer a toner image from the image carrier to the intermediate transfer belt, A secondary transfer member is a current supply member that supplies a secondary transfer current to the intermediate transfer belt in the secondary transfer section, thereby secondary transferring a toner image from the intermediate transfer belt to the recording material. A discharge member is a current supply member that supplies a discharge current to the intermediate transfer belt in a discharge section downstream of the secondary transfer section and upstream of the primary transfer section in the rotational direction of the intermediate transfer belt, A discharge power supply that supplies the aforementioned discharge current, A control unit capable of controlling the discharge power supply, In an image forming apparatus having, During image formation, the sum of positive currents per unit length in the width direction of the intermediate transfer belt supplied by the current supply member in the direction from the outer circumferential surface to the inner circumferential surface of the intermediate transfer belt is Ia, and the sum of positive currents per unit length in the width direction of the intermediate transfer belt supplied by the current supply member in the direction from the inner circumferential surface to the outer circumferential surface of the intermediate transfer belt supplied by the current supply member during image formation is Ib. The value of Ia - Ib is defined as the current balance of the current supplied to the intermediate transfer belt, and the intermediate transfer The image forming apparatus is characterized in that, within a range where the electrical resistance of the belt is less than or equal to a predetermined value, the relationship between the electrical resistance of the intermediate transfer belt and the current balance satisfies the relationship |Sa| > |Sb|, where Sa is the ratio of the change in the electrical resistance of the intermediate transfer belt to the change in the current balance when the current balance is in a positive range, and Sb is the ratio when the current balance is in a negative range, and the control unit controls the discharge power supply such that the average value of the current balance when images are formed continuously on a predetermined number of recording materials during a continuous image forming job becomes a negative value.

3. The image forming apparatus according to claim 1 or 2, characterized in that it has a plurality of primary transfer members along the rotation direction of the intermediate transfer belt.

4. The image forming apparatus according to claim 1 or 2, further comprising a cleaning member which is a current supply member that supplies a cleaning current to the intermediate transfer belt in a cleaning section downstream of the secondary transfer section and upstream of the primary transfer section in the rotational direction of the intermediate transfer belt to remove toner from the intermediate transfer belt.

5. The image forming apparatus according to claim 1 or 2, characterized in that the control unit sets the discharge current such that the offset amount with respect to the value of the discharge current that makes the current balance zero is greater than or equal to the fluctuation of the output of the discharge power supply.

6. The image forming apparatus according to claim 1 or 2, further comprising a detection unit for monitoring the current balance, wherein the control unit sets the discharge current based on the detection result of the detection unit.

7. The image forming apparatus according to claim 6, characterized in that the detection unit detects at least the primary transfer current and the secondary transfer current.

8. The image forming apparatus according to claim 1 or 2, characterized in that the intermediate transfer belt has ionic conductivity.

9. The image forming apparatus according to claim 8, characterized in that the intermediate transfer belt has an elastic layer containing an ion conductive agent.

10. The image forming apparatus according to claim 1, characterized in that |Sb| is greater than twice |Sa|.

11. The image forming apparatus according to claim 2, characterized in that |Sa| is greater than twice |Sb|.

Images