Image forming apparatus

Abstract

To prevent the occurrence of image density unevenness caused by changing a discharge current during execution of a print job.SOLUTION: An image forming apparatus 100 is configured to, when a control unit 50 changes a discharge current during execution of a print job of continuous image formation of continuously transferring toner images to a plurality of recording materials P, control a discharge power supply E5 to change the discharge current, while an area of an intermediate transfer belt 6 being an inter-image area between an image forming area of an image transferred to a preceding recording material P when passing through a primary transfer unit N1 and an image forming area of an image transferred to a subsequent recording material P, passes through a discharge part D immediately before passing through the primary transfer unit N1.SELECTED DRAWING: Figure 11

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, the discharge current is set to bring the current balance as close to zero as possible. For example, the discharge current can be set based on target values ​​for the primary transfer current, secondary transfer current, and cleaning current.

[0011] Furthermore, during the execution of a print job, the primary and secondary transfer currents, for example, may change depending on factors such as the presence or absence of toner, or the primary and secondary transfer voltages may change due to readjustment. Therefore, it is sometimes desirable to change the discharge current during the execution of a print job based on the detection results of the primary and secondary transfer currents.

[0012] However, changing the discharge current during the execution of a print job can alter the charge state of the intermediate transfer belt, which in turn changes the primary transfer current and can result in uneven image density.

[0013] Therefore, the objective of the present invention is to suppress the occurrence of image density unevenness caused by changing the discharge current during the execution of a print job. [Means for solving the problem]

[0014] The above objective is achieved by the image forming apparatus according to the present invention. In summary, the present invention comprises an image carrier that carries a toner image, a rotatable endless intermediate transfer belt onto which the toner image is transferred from the image carrier, a primary transfer member that supplies a primary transfer current to the intermediate transfer belt in a primary transfer section to transfer the toner image from the image carrier to the intermediate transfer belt, a secondary transfer member that supplies a secondary transfer current to the intermediate transfer belt in a secondary transfer section to transfer the toner image from the intermediate transfer belt to a recording material, and a discharge section that supplies a discharge current to the intermediate transfer belt downstream of the secondary transfer section and upstream of the primary transfer section in the rotational direction of the intermediate transfer belt, thereby supplying a current in the direction from the outer circumferential surface side to the inner circumferential surface side of the intermediate transfer belt and a current in the direction from the inner circumferential surface side to the outer circumferential surface side of the intermediate transfer belt An image forming apparatus having a discharge member that controls the relationship with a current supplied in the opposite direction, a discharge power supply that supplies the discharge current, and a control unit that can control the discharge power supply, wherein when the control unit changes the discharge current during the execution of a print job for continuous image formation in which toner images are transferred to a plurality of recording materials in succession, the control unit controls the discharge power supply so as to change the discharge current while the region on the intermediate transfer belt, which is the inter-image region between the image forming region of the image transferred to the preceding recording material and the image forming region of the image transferred to the next recording material as it passes through the primary transfer section, is passing through the discharge section immediately before passing through the primary transfer section. [Effects of the Invention]

[0015] According to the present invention, it is possible to suppress the occurrence of image density unevenness caused by changing the discharge current during the execution of a print job. [Brief explanation of the drawing]

[0016] [Figure 1] This is a schematic cross-sectional view of an image forming apparatus. [Figure 2] This is a schematic cross-sectional view of the vicinity of 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 block diagram showing the control mode of the image forming apparatus. [Figure 6] It is a flowchart showing an outline of the procedure for correcting the primary transfer current between sheets of paper. [Figure 7] It is a graph showing an example of the relationship between the voltage application time (current supply time) of the intermediate transfer belt and the electrical resistance. [Figure 8] It is a schematic diagram of a measuring device for measuring the relationship of FIG. 7. [Figure 9] It is a graph showing an example of the relationship between the discharge current and the primary transfer current. [Figure 10] It is a graph showing an example of the relationship between the primary transfer current and the primary transfer efficiency. [Figure 11] It is a timing chart for explaining the timing of changing the discharge current. [Figure 12] It is a schematic diagram showing the positional relationship between the discharge device and the primary transfer device. [Figure 13] It is a timing chart for explaining the timing of changing the discharge current. [Figure 14] It is a flowchart of the control of Example 1. [Figure 15] It is a flowchart of the control of Example 2. [Embodiments for Carrying Out the Invention] <00001​​​​​​​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 5). 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. In the configuration of this embodiment, the ion-conductive agent is considered to facilitate the movement of negative ions. 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 in the rotational direction of the intermediate transfer belt 6, downstream of the secondary transfer portion N2 and upstream of the primary transfer portion N1 (the most upstream primary transfer portion N1Y), 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 members (current supply members), namely, first and second cleaning brushes 122 and 123, are provided. Also, first and second recovery members, namely, first and second recovery rollers 124 and 125, are provided. Further, first and second scraping members, namely, first and second blades 126 and 127, 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 a yarn electrical resistance value 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 flocking 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. However, in a configuration where the discharge unit D is located adjacent to the upstream of the primary transfer unit N1, the impact on the image due to changes in the discharge current, as described later, tends to be more pronounced.

[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. Control Modes Figure 5 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 for performing 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.

[0050] 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.

[0051] 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 the 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 (hereinafter also referred to as the "paper passing period"). The pre-rotation process is the period during which preparatory operations are performed before the image forming process, from when a start instruction is input to the image forming apparatus 100 until the image is actually formed. 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) (hereinafter also referred to as the "inter-paper period"). 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 region 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. In this embodiment, the primary transfer voltage may be controlled during the inter-paper process, as described later. In this embodiment, the primary transfer voltage is controlled during the inter-paper process so that the primary transfer current reaches a target value. The image-forming regions on the photosensitive drum 1 and the intermediate transfer belt 6 are the regions where an image can be formed after being transferred to the recording material P and output from the image-forming apparatus 100, while the non-image-forming regions are the regions other than the image-forming regions.

[0052] 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. The control for readjusting the primary transfer voltage in the inter-paper process (inter-paper primary transfer current correction control) will be described in more detail later. 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. Furthermore, 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 the output value to a constant current so that the value of the current detected by the current detection unit F5 is approximately constant to the target value of the discharge current determined as described later.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.

[0053] 6. Paper-to-paper primary transfer current correction control The primary transfer current correction control between sheets of paper (also simply referred to as "inter-sheet control") in this embodiment will be described below. Figure 6 is a flowchart showing an overview of the inter-sheet control procedure in this embodiment. In this embodiment, during the execution of a print job, the control unit 50 performs a control in the inter-sheet process to correct the primary transfer current (readjust the primary transfer voltage) each time the number of recording materials P on which images have been formed consecutively exceeds a predetermined value (for example, 10 to 15 sheets), as information related to the image formation time.

[0054] When the control unit 50 starts a print job and begins image formation (S101), it determines whether or not it is time to execute inter-paper control (S102). In this embodiment, each time an image is formed on one sheet of recording material P (for example, secondary transfer), the control unit 50 accumulates the number of images formed and stores it in the sheet counter 70. The control unit 50 then determines whether or not the number of images formed has reached a predetermined number (for example, 10 to 15 sheets), and if it has, it determines that it is time to execute inter-paper control. If the control unit 50 determines in S102 that it is not time to execute inter-paper control ("No"), it proceeds to the process in S109. If the control unit 50 determines in S102 that it is time to execute inter-paper control ("Yes"), it obtains the detection result of the primary transfer current in the inter-paper process (S103). In this embodiment, inter-paper control is executed independently in each image forming unit 10Y, 10M, 10C, and 10K. In other words, in this embodiment, as the same non-image-forming region, the inter-paper region (inter-image region), sequentially passes through the primary transfer section N1 of each image-forming section 10 from the upstreammost image-forming section 10Y to the downstreammost image-forming section 10K in the direction of movement of the surface of the intermediate transfer belt 6, inter-paper control (detection of primary transfer current, change of primary transfer voltage) is performed for each image-forming section 10Y, 10M, 10C, and 10K. Thus, in this embodiment, the inter-paper control for each image-forming section 10Y, 10M, 10C, and 10K is performed synchronously in a single inter-paper process.

[0055] Next, the control unit 50 compares the detection result of the primary transfer current acquired in S103 (hereinafter also referred to as the "detected current") with the target current (S104). The control unit 50 determines whether the target current and the detected current are equal (S105). If the control unit 50 determines in S105 that the target current and the detected current are equal ("Yes"), it proceeds to the process in S109. If the control unit 50 determines in S105 that the target current and the detected current are not equal ("No"), it then determines whether the detected current is smaller than the target current (S106). If the control unit 50 determines in S106 that the detected current is smaller than the target current ("Yes"), it increases the primary transfer voltage (absolute value) applied to the primary transfer roller 5 in the inter-paper process so that the detected current approaches the target current (S107). Furthermore, if the control unit 50 determines in S106 that the detected current is greater than the target current ("No"), it lowers the primary transfer voltage (absolute value) applied to the primary transfer roller 5 in the inter-paper process so that the detected current approaches the target current (S108). In this embodiment, the control unit 50 adjusts the primary transfer voltage using the correspondence between the difference between the target current and the detected current, which is stored in advance in the ROM 53, and the voltage change (correction voltage) made in S107 and S108. After adjusting the primary transfer voltage in S107 or S108, the control unit 50 proceeds to the process in S109.

[0056] In S109, the control unit 50 determines whether all image formation for the print job has been completed. If the control unit 50 determines in S109 that image formation is complete ("Yes"), it terminates the print job. If the control unit 50 determines in S109 that image formation is not complete ("No"), it returns to the process in S101.

[0057] In Figure 6, the processes from S105 to S108 are shown focusing on one image forming unit 10 as a representative example, but similar processes are performed for each image forming unit 10Y, 10M, 10C, and 10K. Furthermore, if the control unit 50 determines in S102 that it is time to execute the subsequent processes, it resets the count value of the number of images formed related to the execution timing of inter-paper control stored in the sheet counter 70 to its initial value (0 in this embodiment). Also, in the example in Figure 6, it is determined in S105 whether the target current and the detected current are equal, but for example, if the difference between the target current and the detected current exceeds a predetermined value, the primary transfer voltage may be adjusted so that the difference becomes less than or equal to the predetermined value.

[0058] 7. Discharge current Figure 7 is a graph showing an example of the relationship between the voltage application time (current supply time) on the intermediate transfer belt 6 and the electrical resistance (volume resistivity) of the intermediate transfer belt 6. Figure 8 is a schematic diagram of a measuring device 200 for measuring the above relationship. As shown in Figure 8, the intermediate transfer belt 6 is wrapped around the first roller 201 and the second roller 202 is brought into contact with it. The intermediate transfer belt 6 is wrapped around the first roller 201 such that the base layer 6a is in contact with the first roller 201 and the second roller 202 is in contact with the outer circumferential surface of the surface layer 6c. Then, while rotating both the first roller 201 and the second roller 202 at a substantially constant speed, the first roller 201 is electrically grounded and current is supplied from the high-voltage power supply 203 to the second roller 202. At this time, first, current is supplied from the high-voltage power supply 203 so that a current of substantially constant value with positive polarity flows from the second roller 202 toward the first roller 201 via the intermediate transfer belt 6. As shown in Figure 7, the electrical resistance of the intermediate transfer belt 6 having an ion-conductive elastic layer 6b increases in proportion to the current supply time. After a predetermined time has elapsed, the current was changed to a negative current with approximately the same absolute value as above, but with the opposite polarity, flowing from the second roller 202 through the intermediate transfer belt 6 toward the first roller 201. Then, after approximately the same predetermined time has elapsed, the electrical resistance of the intermediate transfer belt 6 returns to almost its original value.

[0059] Here, in general terms, the current balance of the current supplied to the intermediate transfer belt 6 can be determined by subtracting the sum of positive currents supplied in the direction from the inner surface to the outer surface (outward direction) of the intermediate transfer belt 6 from the sum of positive currents supplied in the direction from the outer surface to the inner surface (inward direction) of the intermediate transfer belt 6. However, in this embodiment, the current balance is determined by subtracting the current per unit length in the width direction of the intermediate transfer belt 6 of the current supply area provided by the current supply member. The length of the current supply area provided 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. For simplicity, the length of the current supply area provided 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 area, the current balance can be evaluated more accurately regardless of the length of the current supply area. 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 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 inner 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 inner surface to the inner surface (inward direction) of the intermediate transfer belt 6. In other words, if we represent the current per unit length in the longitudinal direction of the current supply member supplied in the direction from the inner surface to the outer surface of the intermediate transfer belt 6 (outward direction) as a positive value, and the current per unit length in the longitudinal direction of the current supply member supplied in the direction from the outer surface to the inner surface of the intermediate transfer belt 6 (inward direction) as a negative value, then the sum of the above currents per unit length from each current supply member can be called the current balance. The discharge current is then set to bring the current balance as close to zero as possible.

[0060] The discharge current should be within ±50% of the target current value for which the current balance is 0 μA / mm (for example, if the target current value is 200 μA, the range should be ±100 μA, i.e., 100 μA to 300 μA), preferably within ±30%, and more preferably within ±5%. If the discharge current exceeds the ±50% range, the effect of suppressing the increase in the electrical resistance of the intermediate transfer belt 7 may become insufficient.

[0061] Next, we will explain how to calculate the discharge current supplied to the discharge brush 271, which serves as a discharge component. In this embodiment, in light of the characteristics of the intermediate transfer belt 6 as described above, the discharge current is calculated as follows.

[0062] 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 this embodiment, during image formation (during image formation), the 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 ···(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

[0063] 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.

[0064] 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 on the intermediate transfer belt 6 passes through 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 through 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 through 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 through 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 (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 such 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.

[0065] In this embodiment, once a print job is started, the values ​​of the primary transfer current and secondary transfer current are detected substantially at all times. Based on the detected current values, the discharge current is calculated at predetermined timings, for example, every predetermined number of image formations (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 in the print job become available (for example, until the first predetermined number of image formations are performed), 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 described above. 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] 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. Applying the values ​​shown in Table 1 to equation (1), the discharge current becomes 207.9 μA.

[0068] [Table 1]

[0069] 8. The effect of discharge current on image quality Figure 9 is a graph illustrating an example of the relationship between discharge current and primary transfer current, illustrating the effect of changing the discharge current on the primary transfer current. Figure 10 is a graph illustrating an example of the relationship between primary transfer current and primary transfer efficiency.

[0070] As shown in Figure 9, increasing the discharge current increases the primary transfer current. Conversely, decreasing the discharge current decreases the primary transfer current. This is thought to be due to the effect of the intermediate transfer belt 6 being charged by the current supplied from the discharge brush 271. As shown in Figure 10, the primary transfer efficiency is highest when the primary transfer current is at a certain value (around 75 μA in this embodiment). If the primary transfer current is higher than this value (around 75 μA in this embodiment), the primary transfer efficiency decreases due to re-transfer, and if the primary transfer current is lower than this value (around 75 μA in this embodiment), the primary transfer efficiency decreases due to insufficient transfer field. As the primary transfer efficiency decreases, the amount of toner transferred decreases, resulting in a phenomenon where the image density becomes lighter.

[0071] As mentioned above, the discharge current is set to bring the current balance as close to zero as possible, but it is sometimes desirable to change the discharge current during the execution of a print job. However, changing the discharge current during the execution of a print job can cause the phenomena described above. In other words, changing the discharge current during the execution of a print job changes the charge state of the intermediate transfer belt 6 before and after the change, which changes the primary transfer current, reducing the primary transfer efficiency and potentially causing differences in image density (uneven image density).

[0072] 9. Timing of changes in discharge current Next, the timing of the change in the discharge current during the execution of a continuous image forming print job in this embodiment will be described.

[0073] Figure 11 is a timing chart illustrating the timing of changes in the discharge current during the execution of a print job in this embodiment. Figure 11 shows the ON / OFF status of the image forming signal (Itop signal), the level of the discharge current, and the paper feed period / paper inter-paper period of the primary transfer unit N1 (the uppermost primary transfer unit N1Y). Figure 12 is a schematic diagram showing the positional relationship between the discharge unit D and the primary transfer unit N1 (the uppermost primary transfer unit N1Y), which is located downstream of the discharge unit D in the direction of movement of the surface of the intermediate transfer belt 6.

[0074] During the execution of a print job, if the discharge current is changed while the area on the intermediate transfer belt 6 that becomes the image forming area (paper feeding area) as it passes through the primary transfer section N1 is passing through the discharge section D immediately before, the following may occur: In other words, image density unevenness may occur where the image density becomes lighter partway along the transport direction of the recording material P within the plane of the recording material P.

[0075] Therefore, in this embodiment, the control unit 50 controls the discharge current during the execution of a print job so that the area on the intermediate transfer belt 6 that becomes the inter-paper region (inter-image region) when it passes through the primary transfer unit N1 is changing while it is passing through the discharge unit D immediately before. In other words, in this embodiment, the control unit 50 controls the discharge current during the execution of a print job so that it changes at the timing corresponding to the immediately following inter-paper period.

[0076] As shown in Figure 11, when a print job starts, the discharge current is supplied (t1), and when image formation starts, the primary transfer current is supplied (t2). Subsequently, in the primary transfer unit N1Y, the paper feeding period begins (t3), and after a predetermined time has elapsed, the paper feeding period ends and the inter-paper period begins (t5). Subsequently, in the primary transfer unit N1Y, after a predetermined time has elapsed, the inter-paper period ends and the next paper feeding period begins (t6). At this time, for example, while the region on the intermediate transfer belt 6 that passes through the primary transfer unit N1Y during the inter-paper period from t5 to t6 is passing through the discharge unit D, the discharge current is changed. In other words, the discharge current is changed at a timing (t4) that is a predetermined time earlier than the timing (t5) when the region on the intermediate transfer belt 6 that passes through the primary transfer unit N1Y during that inter-paper period reaches the primary transfer unit N1Y. t4 can be set to a timing earlier than t5 by the time it takes for the intermediate transfer belt 6 to move the distance L (Figure 12) from the discharge section D to the primary transfer section N1Y. However, it is not limited to this; it is sufficient that the region on the intermediate transfer belt 6 that was in the discharge section D at t4 is in the primary transfer section N1Y during the period from t5 to t6. Note that Figures 11 and 12 focus on the upstream primary transfer section N1Y, but the same inter-paper region including the point where the discharge current is changed moves sequentially from the upstream primary transfer section N1Y to the downstream primary transfer section N1K in the direction of movement of the surface of the intermediate transfer belt 6.

[0077] In this way, by changing the discharge current during the execution of a print job at a timing corresponding to the immediately following inter-paper period, it is possible to suppress the occurrence of image density unevenness, where the image density becomes lighter partway along the transport direction of the recording material P within the surface of the recording material P.

[0078] However, even if the discharge current is changed during the execution of a print job at a timing corresponding to the immediately following inter-paper period, image density unevenness may occur between the recording material P before and after the inter-paper region (before and after the change in discharge current), resulting in a decrease in image density. This image density unevenness between the recording material P across the inter-paper region is less noticeable than the image density unevenness within the surface of the recording material P described above, but it may become more visible depending on the image being printed, and it is desirable to suppress it for further improvement of image quality.

[0079] Therefore, it is preferable to synchronize the inter-paper period during which inter-paper control is performed with the timing of changing the discharge current. In this case, the control unit 50 controls the change of the discharge current while the region on the intermediate transfer belt 6, which becomes the inter-paper region where inter-paper control is performed when passing through the primary transfer unit N1, is passing through the discharge unit D immediately before that. In other words, in this case, the control unit 50 controls the change of the discharge current at the timing corresponding to the inter-paper period during which the next inter-paper control is performed while the print job is being executed. Figure 13 is a timing chart diagram illustrating the timing of the change of the discharge current during the execution of a print job in this case. The timing chart in Figure 13 is the same as the timing chart diagram in Figure 11, except that the timing of the execution of inter-paper control is added.

[0080] As shown in Figure 13, if the paper-to-paper control execution timing occurs after the paper-to-paper feeding period (t3~t5) has elapsed, the supply of the primary transfer current is temporarily stopped, and the aforementioned paper-to-paper control is executed (t7~t8). At this time, the discharge current is changed while the region on the intermediate transfer belt 6 that passes through the primary transfer section N1Y during the paper-to-paper period from t5 to t6, when the paper-to-paper control is executed, is passing through the discharge section D. In other words, the discharge current is changed at a timing (t4) that is a predetermined time before the timing (t5) when the region on the intermediate transfer belt 6 that passes through the primary transfer section N1Y during the paper-to-paper period, when the paper-to-paper control is executed, reaches the primary transfer section N1Y. t4 can be a timing that is the time it takes for the intermediate transfer belt 6 to travel the distance L (Figure 12) from the discharge section D to the primary transfer section N1Y, which is earlier than t5. However, it is not limited to this, and it is sufficient that the detection of the primary transfer current in the paper-to-paper control is performed after the region on the intermediate transfer belt 6 that was in the discharge section D at t4 reaches the primary transfer section N1Y. In Figures 11 and 12, attention is paid to the uppermost primary transfer section N1Y, but as the same paper-to-paper region, including the location where the discharge current has been changed, passes sequentially from the uppermost primary transfer section N1Y to the lowermost primary transfer section N1K in the direction of movement of the surface of the intermediate transfer belt 6, paper-to-paper control (detection of primary transfer current, change of primary transfer voltage) for each image forming section 10Y, 10M, 10C, and 10K is performed.

[0081] In this way, by changing the discharge current at a timing corresponding to the inter-paper period in which the subsequent inter-paper control is performed during the execution of a print job, the primary transfer current is corrected (the primary transfer voltage is readjusted) based on the charge state of the intermediate transfer belt 6 with the changed discharge current. Therefore, it is possible to suppress the occurrence of image density unevenness, where the image density becomes lighter between the recording material P before and after the inter-paper region (before and after the change in discharge current).

[0082] 10. Control Procedure Next, an example of the operation of a print job in this embodiment will be described. Figure 14 is a flowchart illustrating an example of the print job procedure in this embodiment. In the example in Figure 14, as described above, the case in which the paper spacing period for performing paper spacing control and the timing for changing the discharge current are synchronized will be explained.

[0083] When the control unit 50 starts a print job and begins image formation (S201), 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 (S202). 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, a secondary transfer), the control unit 50 accumulates the number of images formed and stores it in the image counter 70.

[0084] Next, the control unit 50 determines whether all image formation for the print job has been completed (S203). If the control unit 50 determines in S203 that image formation has been completed ("Yes"), it proceeds to the process in S209. If the control unit 50 determines in S203 that image formation has not been completed ("No"), it determines whether it is time to execute inter-paper control (S204). In this embodiment, the control unit 50 determines whether the number of images formed has reached a predetermined number (14 in this embodiment), and if it has, it determines that it is time to execute inter-paper control. If the control unit 50 determines in S204 that it is not time to execute inter-paper control ("No"), it returns to the process in S201. If the control unit 50 determines in S204 that it is time to execute inter-paper control ("Yes"), it averages the primary transfer current and secondary transfer current for the number of images stored in the RAM 52 (S205). The control unit 50 then calculates the target value of the discharge current (target current) using the calculation method described above (S206). As described above, the control unit 50 calculates the discharge current using the detection result (average value) of the primary transfer current, the detection result (average value) of the secondary transfer current, and the target values ​​(which may also be detection results) of the first and second cleaning currents using the formula (1) described above. The control unit 50 also changes the discharge current to the value calculated this time at the timing corresponding to the inter-paper period during which the inter-paper control described above is executed (S207). In S207, the control unit 50 also erases the detection results of the primary and secondary transfer currents stored in the RAM 52 and resets the count value of the number of image formations related to the execution timing of the inter-paper control stored in the sheet counter 70 to its initial value (0 in this embodiment). After that, the control unit 50 executes the inter-paper control (S208) and returns to the process of S201.

[0085] In S209, the control unit 50 erases the detection results of the primary and secondary transfer currents stored in the RAM 52, and resets the count value of the number of image formations related to the execution timing of inter-paper control stored in the sheet counter 70 to its initial value (0 in this embodiment), and then terminates the print job.

[0086] 11. Evaluation Test As an evaluation test to confirm the effects of this embodiment, a paper-feed durability test was conducted. The test was performed for each of the following embodiments: Embodiment 1-1, Embodiment 1-2, Comparative Example 1, and Comparative Example 2. Embodiment 1-1 was defined as the case where the timing of the change in discharge current was set to correspond to the period between sheets of paper during which inter-sheet control was not performed. Embodiment 1-2 was defined as the case where the timing of the change in discharge current was set to correspond to the period between sheets of paper during which inter-sheet control was performed. Comparative Example 1 was defined as the case where the timing of the change in discharge current was set to correspond to the period of paper-feeding. Comparative Example 2 was defined as the case where the discharge current was not changed and the discharge current was kept approximately constant during the paper-feed durability test. In Comparative Example 2, a constant current of 210 μA was continuously supplied as the discharge current during the paper-feed durability test.

[0087] The paper feed durability test was conducted as follows: Under the conditions of Example 1-1, Example 1-2, Comparative Example 1, and Comparative Example 2 described above, continuous image formation was performed using A3 size plain paper as the recording material P, forming a halftone image with a reflectance density of 0.6 across the entire surface of the paper.

[0088] The results are shown in Table 2. In the case where the discharge current was changed at a timing corresponding to the paper feeding period (Comparative Example 1), if image density unevenness occurred, where the image density became lighter from the middle of the paper feeding direction within the same sheet of paper, it was marked as "×", and if it did not occur, it was marked as "〇". In the case where the discharge current was changed at a timing corresponding to the inter-sheet period (Examples 1-1 and 1-2), if image density unevenness occurred, where the image density became lighter before and after the change, it was marked as "×", and if it did not occur, it was marked as "〇". Furthermore, after 10,000 sheets of paper were fed, a red image (secondary color of yellow and magenta) was formed, and if a mesh-like abnormal image occurred, it was marked as "×", and if it did not occur, it was marked as "〇". The mesh-like abnormal image is a phenomenon caused by discharge, where the ion conductive agent (especially negative ions in this example) of the intermediate transfer belt 6 is unevenly distributed on the base layer 6a side of the elastic layer 6b, causing an increase in the electrical resistance of the intermediate transfer belt 6.

[0089] [Table 2]

[0090] In Comparative Example 1, the discharge current was changed at a timing corresponding to the paper feeding period, resulting in uneven image density within the same page before and after the change. Also, in Comparative Example 1, no mesh-like abnormal images occurred due to an increase in the electrical resistance of the intermediate transfer belt 6. In Comparative Example 2, since the discharge current was not changed during the paper feeding durability test, no uneven image density occurred, but a mesh-like abnormal image occurred due to an increase in the electrical resistance of the intermediate transfer belt 6.

[0091] In Example 1-1, no unevenness in image density occurred within the same page, but unevenness in image density occurred between images. However, the unevenness in image density between images in Example 1-1 was minor and not noticeable depending on the image being printed. In Example 1-2, neither unevenness in image density within the same page nor unevenness in image density between images occurred. Furthermore, in Examples 1-1 and 1-2, no abnormal mesh-like images occurred due to an increase in the electrical resistance of the intermediate transfer belt 6.

[0092] Thus, 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 that supplies a primary transfer current to the intermediate transfer belt 6 in a primary transfer unit N1 to transfer the toner image from the image carrier 1 to the intermediate transfer belt 6, a secondary transfer member 9 that supplies a secondary transfer current to the intermediate transfer belt 6 in a secondary transfer unit N2 to transfer the toner image from the intermediate transfer belt 6 to the recording material P, and the rotation direction of the intermediate transfer belt 6. The device includes a discharge member 271 that controls the relationship between the current supplied in the direction from the outer circumferential surface to the inner circumferential surface (inward direction) and the current supplied in the direction from the inner circumferential surface to the outer circumferential surface (outward direction) of the intermediate transfer belt 6 by supplying 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; a discharge power supply E5 that supplies the discharge current; and a control unit 50 that can control the discharge power supply E5. When the control unit 50 changes the discharge current during the execution of a print job for continuous image formation in which toner images are transferred to multiple recording materials P in succession, it controls the discharge power supply E5 so as to change the discharge current while the region on the intermediate transfer belt that becomes the inter-image region (inter-paper region) between the image formation region of the image transferred to the preceding recording material P and the image formation region of the image transferred to the next recording material P is passing through the discharge section D just before passing through the primary transfer section N1. In this embodiment, the control unit 50 can perform adjustment control (inter-paper primary transfer current correction control, inter-paper control) to adjust the primary transfer voltage applied to the primary transfer member 5 to supply the primary transfer current while the inter-image region is passing through the primary transfer unit N1 during the execution of a continuous image forming print job. When changing the discharge current during the execution of a continuous image forming print job, the control unit 50 controls the discharge power supply E5 so that the discharge current is changed while the region on the intermediate transfer belt, which is the inter-image region where the adjustment control is performed when passing through the primary transfer unit N1, is passing through the discharge unit D just before passing through the primary transfer unit N1.

[0093] The control unit 50 can control the change of the discharge current at a timing earlier than the timing at which the region on the intermediate transfer belt, which is the inter-image region where the adjustment control is performed, reaches the primary transfer unit N1, by the time it takes for the intermediate transfer belt 6 to travel the distance from the discharge unit D to the primary transfer unit N1. However, the control unit 50 only needs to control the system so that the detection of the primary transfer current in the adjustment control is performed after the region on the intermediate transfer belt that was in the discharge unit D has reached the primary transfer unit N1 when the discharge current is changed. In this embodiment, the control unit 50 sets the discharge current so that the current balance obtained by subtracting 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 surface to the outer surface (outward direction) during image formation from 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 surface to the inner surface (inward direction) during image formation is brought closer to 0. In this embodiment, the image forming apparatus 100 has detection units F1 and F2 for monitoring the above 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 image forming apparatus 100 has cleaning members 122 and 123 that supply a cleaning current to 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 rotational direction of the intermediate transfer belt 6 to remove toner from the intermediate transfer belt 6. In this embodiment, the discharge section D is located downstream of the cleaning sections CL1 and CL2 and upstream of the primary transfer section N1 in the rotational direction of the intermediate transfer belt 6. In this embodiment, the intermediate transfer belt 6 is ionically conductive. In particular, in this embodiment, the intermediate transfer belt 6 has an elastic layer 6b containing an ionic conductive agent.

[0094] As described above, this embodiment makes it possible to suppress the occurrence of image defects due to an increase in the electrical resistance of the intermediate transfer belt 6, while also suppressing the occurrence of uneven image density caused by changing the discharge current during the execution of a print job.

[0095] [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.

[0096] 1. Overview of this embodiment In this embodiment, when the amount of change in the discharge current during the execution of a print job exceeds a predetermined value, the inter-paper period during which inter-paper control is performed and the timing of changing the discharge current are synchronized. On the other hand, in this embodiment, when the amount of change in the discharge current during the execution of a print job is less than the above predetermined value, the discharge current is changed at a timing corresponding to an arbitrary inter-paper period (for example, every image formed). This is because, when the amount of change in the discharge current is small, the unevenness in image density between the recording material P across the inter-paper region may become negligible, and in such cases, priority is given to setting a more appropriate discharge current in order to suppress the increase in the electrical resistance of the intermediate transfer belt 6. In this embodiment, the predetermined value of the amount of change in the discharge current is set to 10 μA, but it is not limited to this value. It is sufficient if the unevenness in image density between the recording material P across the inter-paper region is sufficiently inconspicuous.

[0097] 2. Control Procedure Next, an example of the operation of a print job in this embodiment will be described. Figure 15 is a flowchart illustrating an example of the print job procedure in this embodiment.

[0098] When the control unit 50 starts a print job and begins image formation (S301), 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 accumulated number of images formed in the image counter 70 (S302). In this embodiment, as in Embodiment 1, the control unit 50 stores the detection result (average value) of the primary transfer current during primary transfer for each image transferred to one recording material P in the RAM 52 as the primary transfer current during image formation. In this embodiment, the control unit 50 also stores the detection result (average value) of the secondary transfer current during secondary transfer 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 (for example, secondary transfer) on one recording material P, the control unit 50 accumulates the number of images formed and stores it in the image counter 70.

[0099] Next, the control unit 50 determines whether all image formation for the print job has been completed (S303). If the control unit 50 determines in S303 that image formation has been completed ("Yes"), it proceeds to the process in S311. If the control unit 50 determines in S303 that image formation has not been completed ("No"), it averages the primary transfer current and secondary transfer current for the number of images stored in the RAM 52 (S304). The control unit 50 then calculates the target value of the discharge current (target current) using the calculation method described in Example 1 (S305). As described in Example 1, the control unit 50 calculates the discharge current using the above formula (1) with respect to the detection result (average value) of the primary transfer current, the detection result (average value) of the secondary transfer current, and the target values ​​of the first and second cleaning currents (which may also be detection results).

[0100] Next, the control unit 50 determines whether the difference between the previous target value of the discharge current and the target value of the discharge current calculated this time, that is, the change (amount of change) before and after the change in the discharge current, is greater than or equal to a predetermined value (10 μA in this embodiment) (S306). If the control unit 50 determines in S306 that the amount of change in the discharge current is less than the predetermined value ("No"), it changes the discharge current to the value calculated this time at the timing corresponding to the inter-paper period in which the inter-paper control described in Example 1 is not performed (S307), and returns to the process of S301. Also in S307, the control unit 50 erases the detection results of the primary transfer current and secondary transfer current stored in the RAM 52. If the control unit 50 determines in S306 that the amount of change in the discharge current is greater than or equal to the predetermined value ("Yes"), it determines whether or not it is the timing to perform inter-paper control (S308). In this embodiment, the control unit 50 determines whether the number of images to be formed has reached a predetermined number (14 in this embodiment), and if so, determines that it is time to execute inter-paper control. If the control unit 50 determines in S308 that it is not time to execute inter-paper control ("No"), it returns to the process in S301. If the control unit 50 determines in S308 that it is time to execute inter-paper control ("Yes"), it changes the discharge current to the value calculated in this case at the timing corresponding to the inter-paper period in which the inter-paper control described in Embodiment 1 is executed (S309). Also in S309, 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 to be formed related to the timing of execution of inter-paper control stored in the sheet counter 70 to its initial value (0 in this embodiment). After that, the control unit 50 executes inter-paper control (S310) and returns to the process in S301.

[0101] In S311, the control unit 50 erases the detection results of the primary and secondary transfer currents stored in the RAM 52, and resets the count value of the number of image formations related to the execution timing of inter-paper control stored in the sheet counter 70 to its initial value (0 in this embodiment), and then terminates the print job.

[0102] Thus, in this embodiment, the paper-to-paper control period is synchronized with the timing of changing the discharge current only when the amount of change in the discharge current exceeds a predetermined value. Therefore, when the amount of change in the discharge current is small, the discharge current can be corrected at a timing corresponding to any paper-to-paper period (for example, every image formed). As a result, the increase in the electrical resistance of the intermediate transfer belt 6 can be further suppressed.

[0103] 3. Evaluation Test As an evaluation test to confirm the effectiveness of this embodiment, a paper feed durability test was conducted. The test was performed on this embodiment and on the aforementioned Embodiment 1-2 (where the timing of the change in discharge current is set to the timing corresponding to the paper-to-paper period during which paper-to-paper control is performed).

[0104] The paper handling durability test was conducted as follows: As recording material P, 20,000 sheets of paper (A3 size plain paper) left for half a day at 30°C / 80% humidity and 80,000 sheets of paper (A3 size plain paper) left for half a day at 20°C / 5% humidity were prepared. These sheets of paper were then randomly stacked to form a continuous image with a 70% duty cycle (image ratio) across the entire surface of the paper. To bias the secondary transfer current, the ratio of the number of sheets of paper left in different environments was biased.

[0105] The results are shown in Table 3. After all the paper had passed through, a red image (secondary color of yellow and magenta) was formed. If a mesh-like abnormal image occurred, it was marked with "×", and if it did not occur, it was marked with "〇".

[0106] [Table 3]

[0107] In Examples 1-2, a mesh-like abnormal image was generated. However, the degree of the mesh-like abnormal image generation in Examples 1-2 was minor and not problematic in practical use. In Example 2, since the amount of change in discharge current is smaller than a predetermined value (10 μA in this example), the discharge current is corrected for each image formed. Therefore, the increase in the volume resistivity of the intermediate transfer belt 6 is 1.6 × 10⁻⁶. 11 The resistance was kept low at Ω·cm, and no mesh-like abnormal images were generated. In this embodiment, 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 The density is approximately Ω·cm. Furthermore, in both Example 1 and Example 2, no image defects due to uneven image density occurred.

[0108] Thus, in this embodiment, when the control unit 50 changes the discharge current during the execution of a continuous image forming print job, and the amount of change in the discharge current is greater than or equal to a predetermined value, the control unit 50 controls the discharge power supply E5 to change the discharge current while the region on the intermediate transfer belt, which becomes an inter-image region (inter-paper region) where adjustment control (inter-paper primary transfer current correction control, inter-paper control) is performed when passing through the primary transfer unit N1, is passing through the discharge unit D just before passing through the primary transfer unit N1.

[0109] As described above, this embodiment provides the same effects as in Example 1, while further suppressing the increase in the electrical resistance of the intermediate transfer belt 6.

[0110] [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.

[0111] 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.

[0112] 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]

[0113] 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 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 that supplies a secondary transfer current to the intermediate transfer belt in the secondary transfer section to transfer a toner image from the intermediate transfer belt to the recording material, A discharge member controls the relationship between the current supplied to the intermediate transfer belt in the direction from the outer circumferential surface to the inner circumferential surface and the current supplied to the intermediate transfer belt in the direction from the inner circumferential surface to the outer circumferential surface by supplying a discharge current to the intermediate transfer belt in the rotational direction of the intermediate transfer belt at a discharge section downstream of the secondary transfer section and upstream of the primary transfer section, 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, The image forming apparatus is characterized in that, when the control unit changes the discharge current during the execution of a print job for continuous image formation in which toner images are transferred to a plurality of recording materials in succession, the control unit controls the discharge power supply such that the discharge current is changed while the region on the intermediate transfer belt, which is the inter-image region between the image formation region of the image transferred to the preceding recording material and the image formation region of the image transferred to the next recording material as it passes through the primary transfer unit, is passing through the discharge unit immediately before passing through the primary transfer unit.

2. The control unit, During the execution of the continuous image formation print job, adjustment control can be performed to adjust the primary transfer voltage applied to the primary transfer member to supply the primary transfer current while the inter-image region is passing through the primary transfer section. The image forming apparatus according to claim 1, characterized in that when the discharge current is changed during the execution of the continuous image forming print job, the discharge power supply is controlled so as to change the discharge current while the region on the intermediate transfer belt, which is the inter-image region where the adjustment control is performed when passing through the primary transfer section, is passing through the discharge section immediately before passing through the primary transfer section.

3. The image forming apparatus according to claim 2, characterized in that the control unit controls the change of the discharge current at a timing earlier by the time it takes for the intermediate transfer belt to travel the distance from the discharge unit to the primary transfer unit than the timing at which the region on the intermediate transfer belt that is the inter-image region in which the adjustment control is performed reaches the primary transfer unit.

4. The image forming apparatus according to claim 2, characterized in that the control unit controls the primary transfer current detection in the adjustment control to occur after the region on the intermediate transfer belt that was in the discharge section has reached the primary transfer section when the discharge current is changed.

5. The image forming apparatus according to claim 2, characterized in that when the control unit changes the discharge current during the execution of the continuous image forming print job, and the amount of change in the discharge current is greater than or equal to a predetermined value, the control unit controls the discharge power supply so as to change the discharge current while the region on the intermediate transfer belt, which is the inter-image region where the adjustment control is performed when passing through the primary transfer section, is passing through the discharge section immediately before passing through the primary transfer section.

6. The control unit sets the discharge current so as close to zero as possible, obtained by subtracting the sum of positive currents per unit length in the width direction of the intermediate transfer belt in the current supply region supplied by the current supply member in the direction from the outer circumferential surface to the inner circumferential surface of the intermediate transfer belt during image formation from the sum of positive currents per unit length in the width direction of the intermediate transfer belt in the current supply region supplied by the current supply member during image formation.

7. The image forming apparatus according to claim 6, 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.

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

9. The image forming apparatus according to any one of claims 1 to 5, further comprising a cleaning 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.

10. The image forming apparatus according to claim 9, characterized in that the discharge section is located downstream of the cleaning section and upstream of the primary transfer section in the rotational direction of the intermediate transfer belt.

11. The image forming apparatus according to any one of claims 1 to 5, characterized in that the intermediate transfer belt has ionic conductivity.

12. The image forming apparatus according to claim 11, characterized in that the intermediate transfer belt has an elastic layer containing an ion conductive agent.

Images