Image forming apparatus

Abstract

To suppress the occurrence of transfer failure due to a current flowing from a transfer part to a recording material charging part.SOLUTION: An image forming device 100 includes an image carrier 12, a transfer member 81, a first application part N2 for applying a transfer bias to a transfer part E4, a first member 84 and a second member 91 for forming a recording material charging part N3 for charging a toner image transfer surface of a recording material S to a polarity opposite to a normal charging polarity of toner, a second application part N3 for applying a recording material charging bias to the recording material charging part E5 by applying a bias to the first member, and a control part 120. The image forming apparatus includes a control unit 120 capable of performing control so as to execute image formation in a first mode in which a toner image is transferred in a state in which a recording material charging bias is applied to the recording material charging portion N3 and a second mode in which a toner image is transferred in a state in which a recording material charging bias is not applied to the recording material charging portion N3, and a current suppression mechanism 141 and a E5 that suppress a current from flowing from the transfer portion N2 to the first member 84 and the second member 91 via the recording material S when image formation is executed in the second mode.SELECTED DRAWING: Figure 4

Description

[Technical Field]

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

[0002] Conventionally, in image forming devices such as electrophotographic copying machines, a toner image is electrostatically transferred from an image carrier, such as a photoreceptor or intermediate transfer member, to a recording material, such as paper. Transfer is often performed by contacting a transfer member, such as a transfer roller, with the image carrier to form a transfer section, and then applying a transfer bias to the transfer section. If the transfer current supplied to the transfer section by the transfer bias is insufficient, image defects such as "transfer voids" or "low density," in which transfer is insufficient and the desired image density is not achieved, may occur. Therefore, in order to form high-quality images, it is necessary to apply an appropriate transfer bias to the transfer section.

[0003] Therefore, a test bias is applied to the transfer section when there is no recording material in the transfer section to obtain the voltage-current characteristics, and the voltage value at which a predetermined target current is obtained is determined.Then, a recording material share voltage according to the type of recording material is added to this voltage value to set the voltage value of the transfer bias.

[0004] Recently, production machines and the like often use recording materials with high electrical resistance (high-resistivity recording materials), such as textured paper, synthetic paper (recording materials primarily composed of plastic), and extra-thick paper (cardboard with a basis weight exceeding 250 gsm). The recording material distribution voltage described above can be determined in advance depending on the type of recording material. However, depending on the specifications of the recording material, a very large transfer bias may be required to properly transfer a toner image. In order to properly transfer a toner image to such a recording material without reducing the productivity of image formation, it is effective to charge the surface of the recording material onto which the toner image is to be transferred in advance to a polarity opposite to the normal charging polarity of the toner in order to compensate for the insufficient transfer current.

[0005] Patent document 1 proposes improving transferability by charging a recording material that is attracted to a conveying member and transported toward a transfer section by applying a charge of the opposite polarity to the normal charging polarity of the toner at the attraction section, thereby charging the recording material.

[0006] In addition, Patent Document 2 proposes a configuration in which the adsorption bias is set to a float potential just before the leading edge of the recording material arrives at the transfer section in order to suppress interference between the transfer bias and the adsorption bias at the transfer section in a configuration in which the recording material is adsorbed to a conveying member using the adsorption bias. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-171282 [Patent Document 2] Japanese Patent Application Publication No. 9-179419 Summary of the Invention [Problem to be solved by the invention]

[0008] It is desirable that the pre-charging section (recording material charging section), which charges the surface of the recording material onto which the toner image is transferred to a polarity opposite to the normal charging polarity of the toner, be as close to the transfer section as possible. The farther the pre-charging section is from the transfer section, the more the charge on the surface of the recording material onto which the toner image is transferred decreases, and the less effective it is in compensating for the lack of transfer current.

[0009] However, if the pre-charging unit is placed close to the transfer unit, the current supplied to the transfer unit may flow to the pre-charging unit through the recording material, resulting in an insufficient transfer current and resulting in poor transfer. This may occur particularly when a recording material with low electrical resistance, such as foil paper or metallized paper, is used as the recording material, or when the recording material's electrical resistance decreases due to moisture absorption in a high-humidity environment. The configuration described in Patent Document 2, in which the attraction bias is set to a float potential, may not be sufficiently effective.

[0010] SUMMARY OF THE INVENTION An object of the present invention is to prevent transfer defects from occurring due to current flowing from the transfer section to the recording material charging section. [Means for solving the problem]

[0011] The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides an image forming apparatus including an image carrier that carries a toner image, a transfer member that forms a transfer section that transfers the toner image from the image carrier to a recording material, a first applying section that applies a transfer bias to the transfer section, a first member and a second member that are arranged upstream of the transfer section in the recording material conveyance direction, and that form a recording material charging section that sandwiches the recording material conveyed to the transfer section and charges a toner image transfer surface of the recording material, onto which the toner image is transferred, to a polarity opposite to the normal charging polarity of the toner, and a bias that is applied to the first member to apply a recording material charging bias to the recording material charging section to charge the toner image transfer surface of the recording material to the polarity opposite to the normal charging polarity of the toner. a control unit capable of controlling the second application unit to perform image formation in two modes: a first mode in which the transfer unit transfers a toner image to a recording material that has passed through the recording material charging unit with the recording material charging bias applied to the recording material charging unit, and a second mode in which the transfer unit transfers a toner image to a recording material that has passed through the recording material charging unit with the recording material charging bias not being applied to the recording material charging unit; and a current suppression mechanism that suppresses current from flowing from the transfer unit to the first member and the second member via the recording material when image formation is performed in the second mode. [Effects of the Invention]

[0012] According to the present invention, it is possible to prevent transfer defects from occurring due to current flowing from the transfer section to the recording material charging section. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Figure 2] FIG. 2 is a schematic cross-sectional view of an image forming unit. [Figure 3] FIG. 2 is a block diagram showing an outline of a control configuration of the image forming apparatus. [Figure 4] FIG. 2 is a schematic cross-sectional view showing a configuration in the vicinity of a secondary transfer unit in the first embodiment. [Figure 5] FIG. 10 is a flowchart illustrating an example of control for switching whether pre-charging is required. [Figure 6] FIG. 10 is a schematic cross-sectional view showing a configuration in the vicinity of a secondary transfer unit in a second embodiment. [Figure 7] FIG. 11 is a schematic cross-sectional view showing a configuration in the vicinity of a secondary transfer unit in a third embodiment. [Figure 8] FIG. 10 is a schematic cross-sectional view showing a configuration in the vicinity of a secondary transfer unit in a fourth embodiment. [Figure 9] FIG. 10 is a schematic cross-sectional view showing a configuration in the vicinity of a secondary transfer unit in a modified example. [Figure 10] 10 is a schematic cross-sectional view showing a configuration in the vicinity of a secondary transfer portion in Comparative Example 1. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0014] The image forming apparatus according to the present invention will be described in more detail below with reference to the drawings.

[0015] [Example 1] 1. Configuration and operation of image forming apparatus FIG. 1 is a schematic cross-sectional view of an image forming apparatus 100 according to this embodiment. The image forming apparatus 100 is a tandem-type full-color printer employing an intermediate transfer method, capable of forming full-color images using an electrophotographic method. The image forming apparatus 100 is capable of forming and outputting an image on a sheet-like recording material S based on image information input from an external device such as a personal computer or image information input via an operation unit 130 provided on the image forming apparatus 100. Because the image forming apparatus 100 primarily uses paper as the recording material S, the recording material S is sometimes referred to as paper, but the recording material S is not limited to paper. The recording material S may also be made of materials other than paper or materials containing materials other than paper, such as synthetic paper or film made primarily of a synthetic resin, or special paper such as metal-coated paper.

[0016] The image forming apparatus 100 has four image forming units 10Y, 10M, 10C, and 10K that form images in the colors yellow (Y), magenta (M), cyan (C), and black (K). The image forming units 10Y, 10M, 10C, and 10K are arranged in series along the direction of movement of the image transfer surface of an intermediate transfer belt 70, which is disposed substantially horizontally. Elements having the same or corresponding functions or configurations for each color may be generally described by omitting the Y, M, C, or K suffixes to designate the elements for a specific color. In this embodiment, the image forming unit 10 includes a photosensitive drum 1, a charging device 2, an exposure device 3, a developing device 4, and a drum cleaning device 6, which will be described later. Figure 2 is a schematic cross-sectional view of the image forming unit 10.

[0017] The drum-shaped (cylindrical) photosensitive drum 1, serving as a first image carrier, is movable (rotatable) and carries an electrostatic image (electrostatic latent image) or a toner image. The photosensitive drum 1 has an aluminum cylinder as a base and a surface layer (photosensitive layer) formed on the surface. When an image formation operation begins, the photosensitive drum 1 is rotated at a predetermined peripheral speed (process speed) in the direction of arrow R1 (counterclockwise) by a drum drive motor D1 (FIG. 3) serving as a driving means. The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential of a predetermined polarity (negative in this embodiment) by a charging device 2 serving as a charging means. In this embodiment, the charging device 2 is a scorotron charger disposed opposite the photosensitive drum 1. During charging, a predetermined charging bias (charging voltage) is applied to the charging wire of the charging device 2 by a charging power supply (high-voltage power supply circuit) E1 (FIG. 3) serving as a charging voltage application means (charging voltage application unit). This causes the charging device 2 to generate a discharge, and electrons generated by this discharge charge the surface of the photosensitive drum 1. The charged surface of the photosensitive drum 1 is scanned and exposed by the exposure device 3 as an exposure means based on image information (image signals), and an electrostatic image is formed on the photosensitive drum 1. In this embodiment, the exposure device 3 is a laser scanner. The exposure device 3 emits laser light in accordance with image information of separated colors output from the control unit 120 (FIG. 3), and scans and exposes the surface (outer peripheral surface) of the photosensitive drum 1.

[0018] The electrostatic image formed on the photosensitive drum 1 is developed (visualized) by the developing device 4 as a developing means, which supplies toner, and a toner image (toner image, developer image) is formed on the photosensitive drum 1. In this embodiment, the developing device 4 develops the electrostatic image using a two-component developer containing non-magnetic toner particles (toner) and magnetic carrier particles (carrier). The developing device 4 has a developing sleeve 41 as a developer carrier (developing member) and a developing container 42 that contains the developer. The developing sleeve 41 carries the developer in the developing container 42 and transports it to a development area facing the photosensitive drum 1. During development, a predetermined developing bias (developing voltage) is applied to the developing sleeve 41 by a developing power supply (high-voltage power supply circuit) E2 (FIG. 3) as a developing voltage applying means (developing voltage applying unit). In this embodiment, toner charged with the same polarity as the charge polarity of the photosensitive drum 1 (negative in this embodiment) adheres to the exposed area (image area) on the photosensitive drum 1, where the absolute value of the potential has been reduced by exposure after being uniformly charged (reverse development method). In this embodiment, the normal charge polarity of the toner, which is the main charge polarity of the toner during development, is negative.

[0019] An intermediate transfer unit 7 is disposed facing the four photosensitive drums 1Y, 1M, 1C, and 1K. The intermediate transfer unit 7 includes an intermediate transfer belt 70, a secondary transfer inner roller 71, a drive roller 72, a tension roller 73, and primary transfer rollers 5Y, 5M, 5C, and 5K. The intermediate transfer belt 70, an endless belt serving as a second image carrier, is movable (rotatable) while carrying a toner image. The intermediate transfer belt 70 is stretched over multiple support rollers (support rollers)—the secondary transfer inner roller 71, the drive roller 72, and the tension roller 73—and stretched with a predetermined tension. The drive roller 72 is driven to rotate by an intermediate transfer belt drive motor D2 (FIG. 3) serving as a driving means. The drive roller 72 transmits a driving force to the intermediate transfer belt 70, causing it to rotate (circumferentially move) in the direction of arrow R2 (clockwise) in the figure at a predetermined peripheral speed (process speed) corresponding to the peripheral speed of the photosensitive drums 1. A tension roller 73 applies a predetermined tension to the intermediate transfer belt 70. The inner secondary transfer roller 71 cooperates with an outer secondary transfer roller 81 (described later) to form a secondary transfer portion N2. Primary transfer rollers 5Y, 5M, 5C, and 5K, which are roller-type primary transfer members serving as primary transfer means, are disposed on the inner circumferential surface of the intermediate transfer belt 70, corresponding to the photosensitive drums 1Y, 1M, 1C, and 1K, respectively. The primary transfer rollers 5 press the intermediate transfer belt 70 toward the photosensitive drums 1, forming a primary transfer portion (primary transfer nip portion) N1, which is the contact portion between the photosensitive drums 1 and the intermediate transfer belt 70. The tension rollers of the intermediate transfer belt 70 other than the drive roller 72 and the primary transfer rollers 5 are rotated in accordance with the rotation of the intermediate transfer belt 70.

[0020] The toner image formed on the photosensitive drum 1 is transferred (primary transfer) onto the rotating intermediate transfer belt 70 by the action of the primary transfer roller 5 at the primary transfer portion N1. A primary transfer power supply (high-voltage power supply circuit) E3 (FIG. 3) serving as a primary transfer voltage application means (primary transfer voltage application portion) is connected to the primary transfer roller 5. During primary transfer, a primary transfer bias (primary transfer voltage), which is a DC voltage of opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the primary transfer roller 5 by the primary transfer power supply E3 (FIG. 3). By applying the positive primary transfer bias to the primary transfer roller 5, the toner image made of negative polarity toner on the photosensitive drum 1 is transferred onto the intermediate transfer belt 70. For example, when a full-color image is formed, the toner images of yellow, magenta, cyan, and black formed on each photosensitive drum 1 are sequentially transferred onto the intermediate transfer belt 70 so as to be superimposed on top of each other, forming a multi-toner image on the intermediate transfer belt 70.

[0021] In this embodiment, the primary transfer roller 5 has a core metal and an elastic layer of ion-conductive foam rubber (NBR rubber (nitrile rubber) and ECO rubber (epichlorohydrin rubber)) formed so as to cover the outer periphery of the core metal. The outer diameter of the primary transfer roller 5 is, for example, 15 to 20 mm. Note that the symbol "to" in the numerical range means that the numerical values ​​before and after it are included. Furthermore, the primary transfer roller 5 has an electrical resistance of 1×10 5 ~1×10 8 A roller of Ω (measured at N / N (23° C., 50% RH), applied voltage of 2 kV) can be suitably used.

[0022] In this embodiment, the intermediate transfer belt 70 is an endless belt having a three-layer structure consisting of, from the inner circumferential surface side, a base layer, an elastic layer, and a surface layer. Suitable materials for the base layer include resins such as polyimide and polycarbonate, or various rubbers containing an appropriate amount of carbon black as an antistatic agent. The thickness of the base layer is, for example, 0.05 to 0.15 mm. Suitable materials for the elastic layer include natural rubber, styrene-butadiene rubber, butadiene rubber, isoprene rubber, nitrile rubber, chloroprene rubber, butyl rubber, ethylene-propylene rubber, chlorosulfonated rubber, acrylate rubber, epichlorohydrin rubber, urethane rubber, silicone rubber, and fluororubber. In this embodiment, urethane rubber was used. The thickness of the elastic layer is preferably 100 to 2000 μm, more preferably 200 to 800 μm, to fully utilize its flexibility and improve the transferability of toner images to, for example, uneven recording materials S. Suitable materials for the surface layer include resins such as fluororesins. The surface layer reduces the adhesion of toner to the surface of the intermediate transfer belt 70, facilitating the transfer of toner to the recording material S at the secondary transfer section N2. The thickness of the surface layer is, for example, 0.0002 to 0.020 mm. The base material of the surface layer can be one type of resin material, such as polyurethane, polyester, or epoxy resin, or two or more types of elastic materials, such as elastic rubber or elastomers, such as polyurethane resin. Powders or particles, such as fluororesins or silicone resins, such as PTFE, PVDF, or PFA, can be dispersed in this base material in one or more types or with different particle sizes to reduce surface energy and increase lubricity. This allows the surface layer to be formed. In this embodiment, the intermediate transfer belt 70 has a volume resistivity of 1×10 8 ~1×10 14Ω·cm (23°C, 50% RH). In this embodiment, the intermediate transfer belt 70 has a three-layer structure, but it may have a single-layer structure made of a material equivalent to the base layer, or a two-layer structure consisting of the base layer and the surface layer. The intermediate transfer belt 70 may also have a structure having multiple layers with different electrical resistances formed using a material equivalent to the base layer.

[0023] A secondary transfer unit 8 is disposed on the outer peripheral surface of the intermediate transfer belt 70 so as to face the inner secondary transfer roller 71. The secondary transfer unit 8 includes a secondary transfer belt 80 configured as an endless belt and an outer secondary transfer roller (secondary transfer roller) 81 disposed on the inner peripheral surface of the secondary transfer belt 80 in a position facing the inner secondary transfer roller 71. The outer secondary transfer roller 81 is pressed toward the inner secondary transfer roller 71 and abuts against the inner secondary transfer roller 71 via the secondary transfer belt 80 and the intermediate transfer belt 70. As a result, the outer secondary transfer roller 81 forms a secondary transfer portion (secondary transfer nip portion) N2, which is a contact portion between the intermediate transfer belt 70 and the secondary transfer belt 80. In this way, the secondary transfer unit 8 forms the secondary transfer portion N2 together with the inner secondary transfer roller 71. The inner secondary transfer roller 71 and the outer secondary transfer roller 81 are examples of roller-type secondary transfer members serving as secondary transfer means. The toner image formed on the intermediate transfer belt 70 is transferred (secondary transfer) onto the recording material S, which is being conveyed while being sandwiched between the intermediate transfer belt 70 and the secondary transfer belt 80, at the secondary transfer portion N2. In this embodiment, a secondary transfer power supply (high-voltage power supply circuit) E4 serving as a secondary transfer voltage application means (secondary transfer voltage application unit) is connected to the inner secondary transfer roller 71. During secondary transfer, a secondary transfer bias (secondary transfer voltage), which is a DC voltage having the same polarity (negative in this embodiment) as the normal charging polarity of the toner, is applied to the inner secondary transfer roller 71 by the secondary transfer power supply E4 serving as a secondary transfer voltage application means (secondary transfer voltage application unit). In this embodiment, the outer secondary transfer roller 81 is connected (electrically grounded) to ground (ground potential). Details of the secondary transfer unit 8 will be described later.

[0024] The recording material (transfer material, recording medium, paper, sheet) S is stored in cassettes 11a and 11b, which serve as a feeding section (recording material storage section). The recording material S is sent from either cassette 11a or 11b by feeding members 12a and 12b to a feeding / conveying path 13, which serves as a recording material conveying path, and then conveyed to a pair of registration rollers 14, which serve as a conveying member. The recording material S is conveyed by the pair of registration rollers 14 toward the secondary transfer section N2 in synchronization with the toner image on the intermediate transfer belt 70. The pair of registration rollers 14 is driven to rotate by a driving force transmitted from a conveyance drive motor D4 (FIG. 3) serving as a driving means. In this embodiment, a pre-charging device 9 is provided upstream of the secondary transfer section N2 (downstream of the pair of registration rollers 14) in the conveying direction of the recording material S. The pre-charging device 9 pre-charges the surface of the recording material S, onto which the toner image is to be transferred, to a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment) before the recording material S reaches the secondary transfer section N2. Details of the pre-charging device 9 will be described later.

[0025] The recording material S onto which the toner image has been transferred is transported by a transport belt 19 as a transport member to a fixing device 15 as a fixing means. The fixing device 15 has a fixing roller 15a and a pressure belt unit 15b. The fixing roller 15a has a built-in heater as a heating means. The recording material S carrying the unfixed toner image is heated and pressurized as it is sandwiched and transported between the fixing roller 15a and the pressure belt unit 15b. This causes the toner image to be fixed (melted and fixed) onto the recording material S.

[0026] In the single-sided printing mode, the recording material S, on which the toner image has been fixed on one side as described above, passes through the discharge conveyance path 16 as a recording material conveyance path, passes through the post-processing section 20, and is discharged (output) to the discharge tray 21 as a discharge section. In the double-sided printing mode, the recording material S, on which the toner image has been fixed on the first side as described above, is conveyed again to the secondary transfer section N2 to transfer the toner image to the second side. That is, in the double-sided printing mode, the recording material S, on which the toner image has been fixed on the first side, is sent to the reverse conveyance path 17 as a recording material conveyance path, and a switchback operation is performed in the reverse conveyance path 17 to switch the leading and trailing ends, and the recording material S is conveyed again to the feeding conveyance path 13. The recording material S conveyed to the feeding conveyance path 13 is conveyed to the registration roller pair 14 and supplied again to the secondary transfer section N2. Then, the toner image is transferred to the second side of this recording material S in the same manner as described above, and after the toner image is fixed, the recording material S is discharged to the discharge tray 21.

[0027] Furthermore, toner remaining on the photosensitive drum 1 after the primary transfer (primary transfer residual toner) is removed from the photosensitive drum 1 and collected by a drum cleaning device 6 serving as a photosensitive body cleaning means. Furthermore, deposits such as toner remaining on the intermediate transfer belt 70 after the secondary transfer (secondary transfer residual toner) are removed from the intermediate transfer belt 70 and collected by a belt cleaning device 74 serving as an intermediate transfer body cleaning means.

[0028] 2. Control Configuration FIG. 3 is a block diagram showing an outline of the control configuration of the image forming apparatus 100 in this embodiment. The image forming apparatus 100 has a control unit (control circuit) 120 that controls the image forming apparatus 100. The control unit 120 is configured to include a CPU 121 as an arithmetic processing unit (arithmetic processing section), a memory (storage medium) 122 such as a ROM or RAM as a storage unit (storage section), and an input / output unit (not shown) for inputting and outputting information to and from devices external to the control unit 120. The CPU 121 and the memory 122 are capable of transferring and reading data from and to each other. The ROM stores a control program, a pre-determined data table, and the like. The RAM, which is a rewritable memory, stores information input to the control unit 120, detected information, calculation results, and the like. The control unit 120 is connected to each unit of the image forming apparatus 100. The control unit 120 can control the operation of each unit of the image forming apparatus 100 so that the image forming apparatus 100 performs various operations, such as image formation.

[0029] For example, various power sources such as a charging power source E1, a developing power source E2, a primary transfer power source E3, a secondary transfer power source E4, and a pre-charging power source E5 (described later) are connected to the control unit 120. Also, various drive units such as a drum drive motor D1, an intermediate transfer belt drive motor D2, a secondary transfer belt drive motor D3 (described later), and a conveyance drive motor D4 are connected to the control unit 120.

[0030] The control unit 120 is also connected to an environmental sensor 18. The environmental sensor 18 is an example of an environmental detection unit that detects at least one of the temperature and humidity inside or outside the image forming apparatus 100 (the installation environment of the image forming apparatus 100). In this embodiment, the environmental sensor 18 is configured as a temperature and humidity sensor that detects the temperature and humidity (relative humidity) inside the image forming apparatus 100 (inside the cassettes 11a and 11b or near the cassettes 11a and 11b). The environmental sensor 18 inputs signals indicating the detected temperature and humidity to the control unit 120. Based on the temperature and humidity detected by the environmental sensor 18, the control unit 120 can calculate the absolute moisture content (absolute humidity) as environmental temperature and humidity information and use it for control. Even if the ambient environment of the image forming apparatus 100 changes suddenly, the electrical resistance value of the recording material S often does not change suddenly. Therefore, by installing the environment sensor 18 in or near the cassettes 11a and 11b, it is possible to grasp the change in the electrical resistance of the recording material S more accurately.

[0031] An operation unit (operation panel) 130 provided in the image forming apparatus 100 is also connected to the control unit 120. The operation unit 130 is configured to include a display unit that displays various information to an operator such as a user or a service representative under the control of the control unit 120, and an input unit through which the operator inputs various settings related to image formation into the control unit 120. The operation unit 130 may be configured to include a touch panel that has the functions of a display unit and an input unit. The control unit 120 may also be connected to an external device such as an image reading device (not shown) provided in or connected to the image forming apparatus 100, or a personal computer.

[0032] Although not shown in the drawings, in this embodiment, the charging power supply E1, the developing power supply E2, and the primary transfer power supply E3 are provided independently for each image forming unit 10. The drum drive motor D1 may be provided independently for each photosensitive drum 1, or may be shared by all or some of the photosensitive drums 1. All or some of the drum drive motor D1, the intermediate transfer belt drive motor D2, the secondary transfer belt drive motor D3, and the conveyance drive motor D4 may be shared.

[0033] The image forming apparatus 100 executes a job (print job), which is a series of operations that starts with a single start command and forms and outputs an image on one or more recording materials S. The job generally includes an image formation process, a pre-rotation process, a sheet spacing process, and a post-rotation process. The image formation process is a period during which electrostatic image formation (exposure), toner image formation (development), and toner image transfer of the image that will actually be formed and output on the recording material S are performed. This period is referred to as the image formation time. More specifically, the timing of the image formation process differs depending on the position where each of the electrostatic image formation, toner image formation, and toner image transfer processes is performed. The pre-rotation process is a period during which preparatory operations are performed before the image formation process, from when a start command is input until the actual start of image formation. The sheet spacing process is a period corresponding to the interval between recording materials S when image formation is performed continuously on multiple recording materials S (continuous image formation). The post-rotation process is a period during which tidying up operations (preparatory operations) are performed after the image formation process. Non-image formation refers to a period other than image formation, and includes the above-mentioned pre-rotation process, paper interval process, post-rotation process, and also the pre-multi-rotation process, which is a preparatory operation when the image forming device 100 is turned on or when it returns from a sleep state.

[0034] 3. Secondary transfer unit Next, the secondary transfer unit (secondary transfer device) 8 in this embodiment will be further described. Fig. 4 is a schematic cross-sectional view showing the vicinity of the secondary transfer portion N2 in this embodiment (showing a cross section approximately perpendicular to the rotation axis direction of the photosensitive drum 1 or the rotation axis direction of the tension roller of the secondary transfer belt 80). Note that with regard to the secondary transfer belt 80 and the tension roller of the secondary transfer belt 80, "upstream" and "downstream" respectively refer to "upstream" and "downstream" in the rotation direction (direction of movement of the surface) of the secondary transfer belt 80.

[0035] The secondary transfer unit 8 includes a secondary transfer belt 80, which is an endless belt serving as a recording material carrier. The secondary transfer belt 80 is stretched around multiple tension rollers (support rollers) and tensioned with a predetermined tension. In this embodiment, the secondary transfer unit 8 includes a secondary transfer outer roller 81, a separation roller 82, a secondary transfer belt tension roller 83, and a secondary transfer belt drive roller 84 as tension rollers arranged on the inner circumferential surface of the secondary transfer belt 80. Hereinafter, the secondary transfer belt tension roller 83 will be simply referred to as the "tension roller 83," and the secondary transfer belt drive roller 84 will be simply referred to as the "drive roller 84." In this embodiment, the secondary transfer unit 8 also includes first and second cleaning opposing rollers 85 and 86 as tension rollers arranged on the inner circumferential surface of the secondary transfer belt 80. The rotational axes of the secondary transfer outer roller 81, separation roller 82, tension roller 83, drive roller 84, and first and second cleaning opposing rollers 85 and 86 are substantially parallel to each other. The rotational axis direction of the tension rollers of the secondary transfer belt 80 is substantially parallel to the rotational axis direction of the photosensitive drum 1 and the rotational axis direction of the tension rollers of the intermediate transfer belt 70.

[0036] The secondary transfer belt 80 can be configured as an endless belt member having a layer formed of a resin material or a metal material. For example, the secondary transfer belt 80 can be made of a resin such as polyimide or polycarbonate containing an appropriate amount of carbon black as an antistatic agent, so that the volume resistivity is 1×10 9 ~1×10 14It is formed of a resin material adjusted to Ω·cm (23°C, 50% RH). The secondary transfer belt 80 may have a single-layer structure or a multi-layer structure. The thickness of the secondary transfer belt 80 is, for example, about 0.07 to 0.1 mm. The circumferential length of the secondary transfer belt 80 is, for example, about 300 mm.

[0037] The outer secondary transfer roller 81 is disposed opposite the inner secondary transfer roller 71, with the secondary transfer belt 80 and the intermediate transfer belt 70 sandwiched between them. The outer secondary transfer roller 81 (secondary transfer unit 8) is pressed toward the inner secondary transfer roller 32 by a pressure mechanism (not shown). The outer secondary transfer roller 81 abuts against the inner secondary transfer roller 71 via the secondary transfer belt 80 and the intermediate transfer belt 70. As a result, the outer secondary transfer roller 81 and the inner secondary transfer roller 71 sandwich the secondary transfer belt 80 and the intermediate transfer belt 70, forming a secondary transfer portion N2, which is the contact portion between the intermediate transfer belt 70 and the secondary transfer belt 80. In this embodiment, the inner secondary transfer roller 71 has a core metal and an elastic layer of ion-conductive foam rubber (NBR rubber) formed to cover the periphery of the core metal. The outer diameter of the inner secondary transfer roller 71 is, for example, 24 mm. The inner secondary transfer roller 71 has an electrical resistance of 1×10 5 ~1×10 7 A roller with a resistance of Ω (N / N (measured at 23°C, 50% RH, 2 kV applied) can be suitably used. In this embodiment, the secondary transfer outer roller 81 has a core metal and an elastic layer of ion-conductive foamed rubber (NBR rubber and ECO rubber) formed so as to cover the outer periphery of the core metal. The outer diameter of the secondary transfer outer roller 81 is, for example, 15 to 35 mm. This allows a sufficient nip portion (secondary transfer portion) N2 to be formed in the secondary transfer portion N2. In addition, the secondary transfer outer roller 81 has an electrical resistance of 1×10 7 ~1×10 8 A roller of Ω (measured at N / N (23°C, 50% RH), applied 2 kV) can be suitably used. At the contact portion between the inner secondary transfer roller 71 and the outer secondary transfer roller 81 via the intermediate transfer belt 70 and the secondary transfer belt 80, the contact force causes elastic deformation of the elastic layer of the outer secondary transfer roller 81, which has a lower hardness than the inner secondary transfer roller 71.

[0038] The separation roller 82 is disposed adjacent to (immediately downstream from) the outer secondary transfer roller 81 on the downstream side. The separation roller 82 and the outer secondary transfer roller 81 form a recording material carrying surface (conveying surface) which is the outer peripheral surface of the secondary transfer belt 80 that carries and conveys the recording material S. After passing through the secondary transfer portion N2 and being electrostatically attracted to the recording material carrying surface of the secondary transfer belt 80, the recording material S is conveyed by the secondary transfer belt 80 and then peeled off from the secondary transfer belt 80 by utilizing the curvature of the separation roller 82. In this embodiment, the recording material S is handed over from the secondary transfer belt 80 to the conveying belt 19. In this embodiment, the separation roller 82 is made of a metal roller.

[0039] The tension roller (secondary transfer belt tension roller) 83 is disposed adjacent to (directly downstream of) the downstream side of the separation roller 82. The tension roller 83 is pressed from the inner peripheral surface side toward the outer peripheral surface side of the secondary transfer belt 80 by a pressure spring 89, which is a biasing member serving as a biasing means, and applies a predetermined tension to the secondary transfer belt 80. In this embodiment, the tension roller 83 is made of a metal roller.

[0040] The drive roller (secondary transfer belt drive roller) 84 is disposed adjacent to (directly upstream of) the outer secondary transfer roller 81 on the upstream side. The outer secondary transfer roller 81 and the drive roller 84 form a recording material carrying surface (transport surface), which is the outer surface of the secondary transfer belt 80 that carries and transports the recording material S. In this embodiment, the drive roller 84 has a core and an elastic layer of EPDM rubber (ethylene propylene rubber) with sufficiently low electrical resistance that covers the outer periphery of the core. This provides electrical conductivity between the drive roller 84 and the pre-charging counter roller 91 (described later). In this embodiment, the outer diameter of the core of the drive roller 84 is 20 mm. In this embodiment, the EPDM rubber constituting the elastic layer of the drive roller 84 is 0.5 mm thick, and its surface is polished to maintain a substantially uniform roughness. The drive roller 84 is driven to rotate by a secondary transfer belt drive motor D3 (FIG. 3) that serves as a drive unit. A driving force is transmitted to the secondary transfer belt 80 by the drive roller 84, and the secondary transfer belt 80 rotates (moves in a circular motion) in the direction of arrow R3 (counterclockwise) in the figure at a predetermined peripheral speed corresponding to the peripheral speed of the intermediate transfer belt 70. The tension rollers of the secondary transfer belt 80 other than the drive roller 84 are rotated in accordance with the rotation of the secondary transfer belt 80. Note that the roller to which the drive means for transporting the secondary transfer belt 80 is connected is not limited to the drive roller 84 in this embodiment, but may be any roller that contacts the inner peripheral surface of the secondary transfer belt 80. Furthermore, the secondary transfer unit 8 may be configured so that the secondary transfer belt 80 is rotated in accordance with the rotation of the intermediate transfer belt 70.

[0041] The first and second cleaning opposing rollers 85, 86 are disposed downstream of the tension roller 83 and upstream of the drive roller 84, with the first cleaning opposing roller 85 disposed upstream of the second cleaning opposing roller 86. The secondary transfer unit 8 has first and second brush rollers 87, 88 as first and second secondary transfer belt cleaning members, located on the outer circumferential surface of the secondary transfer belt 80 and facing the first and second cleaning opposing rollers 85, 86, respectively. A cleaning bias (cleaning voltage) having a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment) is applied to the first brush roller 87 from a first cleaning power source E6. A cleaning bias (cleaning voltage) having the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment) is applied to the second brush roller 88 from a second cleaning power source E7. The first and second cleaning opposing rollers 85, 86 are each electrically grounded. As a result, deposits such as toner having the same polarity as the normal charge polarity of the toner adhering to the surface of the secondary transfer belt 80 are collected by the first brush roller 87. Also, deposits such as toner having the opposite polarity to the normal charge polarity of the toner adhering to the surface of the secondary transfer belt 80 are collected by the second brush roller 88. The deposits collected by the first and second brush rollers 87, 88 are removed from the first and second brush rollers 87, 88 by a collection member (not shown) or the like, and are collected in a collection container (not shown). In this way, the surface of the secondary transfer belt 80 can be electrostatically cleaned.

[0042] In this embodiment, a secondary transfer power supply E4 is connected to the core of the inner secondary transfer roller 71. A secondary transfer bias having the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment) is applied to the inner secondary transfer roller 71 by the secondary transfer power supply E4. In this embodiment, the core of the outer secondary transfer roller 81 is connected to ground, and the outer secondary transfer roller 81 is electrically grounded. Here, a power supply method in which the secondary transfer bias is applied from the side of the recording material S onto which the toner image is transferred is referred to as an "internal power supply method." In contrast, a power supply method in which the secondary transfer bias is applied from the side of the recording material S opposite to the side onto which the toner image is transferred is referred to as an "external power supply method." In the external power supply method, for example, the inner secondary transfer roller 71 is electrically grounded, and a secondary transfer bias having a polarity opposite to the normal charging polarity of the toner is applied to the outer secondary transfer roller 81.

[0043] The internal power supply system improves the transferability of toner images to recording materials S with low electrical resistance (low-resistance recording materials, low-resistance paper), such as metallic foil paper, compared to the external power supply system. This is for the following reason: With the external power supply system, if the recording material S has low electrical resistance and leakage of secondary transfer current occurs through the recording material S to components near the secondary transfer unit N2, the secondary transfer current escapes to the components without contributing to transfer between the recording material S and the intermediate transfer belt 70. In contrast, with the internal power supply system, if the recording material S has low electrical resistance and leakage of secondary transfer current occurs through the recording material S to components near the secondary transfer unit N2, the secondary transfer current escapes to the components after contributing to transfer between the recording material S and the intermediate transfer belt 70. Therefore, the internal power supply system improves the transferability of toner images to recording materials S with low electrical resistance, for example, compared to the external power supply system.

[0044] In this embodiment, the secondary transfer bias is applied by constant voltage control. The voltage value (target voltage) of the secondary transfer bias is determined as the sum of a base voltage Vb for obtaining a predetermined secondary transfer current and a recording material distribution voltage Vp determined by the type (thickness, etc.) of the recording material S. The recording material distribution voltage Vp is set in advance according to the type of recording material S and further the environment (for example, absolute moisture content), and is stored in memory 122 as table data, etc.

[0045] The base voltage Vb can be calculated based on the current or voltage detection results when a single or multiple test biases (test currents or test voltages) are applied to the secondary transfer portion N2 when there is no toner image or recording material S at the secondary transfer portion N2. The secondary transfer power supply E4 includes a voltage detection unit 25a as a voltage detection unit for detecting the output voltage and a current detection unit 25b as a current detection unit for detecting the output current. The voltage detection unit 25a detects the voltage applied to the inner secondary transfer roller 71 (secondary transfer portion N2). The current detection unit 25b detects the current flowing through the inner secondary transfer roller 71 (secondary transfer portion N2). For example, when there is no toner image or recording material S at the secondary transfer portion N2, the control unit 120 applies a test bias to the secondary transfer portion N2 (inner secondary transfer roller 71) under constant current control so that the current detected by the current detection unit 25b becomes the target current. In this case, the target current is set to a value corresponding to a predetermined secondary transfer current. The control unit 120 can then determine the base voltage Vb based on the result of the voltage detection unit 25a detecting the voltage generated at that time. Alternatively, the control unit 120 controls the application of multiple test biases to the secondary transfer unit N2 (secondary transfer inner roller 71) under constant current control or constant voltage control when there is no toner image or recording material S at the secondary transfer unit N2. The control unit 120 also acquires a voltage-current characteristic (a straight line or a curve) based on the result of the voltage detection unit 25a or current detection unit 25b detecting the voltage generated or current flowing at that time. The control unit 120 can then determine the base voltage Vb that provides a predetermined secondary transfer current based on the voltage-current characteristic. The predetermined secondary transfer current is preset according to, for example, the environment (e.g., absolute moisture content) and stored in the memory 122 as table data. The recording material distribution voltage Vp is preset according to the type of recording material S and the environment so that a target current flows when the recording material S is present at the secondary transfer unit N2. This type of control is called secondary transfer voltage determination control or ATVC (Active Transfer Voltage Control).This allows the secondary transfer bias to be changed in accordance with the electrical resistance of the secondary transfer unit N2, which changes sequentially due to changes in the environment and the usage status (accumulated usage) of the members involved in the secondary transfer. The members involved in the secondary transfer include the inner secondary transfer roller 71, the outer secondary transfer roller 81, the intermediate transfer belt 70, and the secondary transfer belt 80. The secondary transfer voltage determination control is executed under the control of the control unit 120. The secondary transfer voltage determination control is typically executed in the pre-rotation process or pre-multiple rotation process for each job, but can also be executed at any timing (such as the between-sheet process) during non-image formation when there is no toner image or recording material S at the secondary transfer unit N2.

[0046] Here, constant current control is control that adjusts the output of a power source so that the current supplied to the target is approximately constant at a target current. Constant voltage control is control that adjusts the output of a power source so that the voltage applied to the target is approximately constant at a target voltage. The type of recording material S includes any information that can distinguish the recording material S, such as attributes based on general characteristics such as plain paper, coated paper, thick paper, and synthetic paper (so-called paper type category), numerical values ​​and numerical ranges for basis weight and thickness, and brand (including manufacturer, product number, etc.). In general, the type of recording material S is often specified by the paper type category and thickness (or basis weight).

[0047] 4. Pre-charging device Next, the pre-charging device (recording material charging device) 9 in this embodiment will be further described.

[0048] As mentioned above, image forming apparatuses may experience image degradation, such as transfer gaps and low density due to insufficient secondary transfer current, depending on the specifications of the recording material. For example, in production machines with high image formation speeds, it may be difficult to properly transfer a toner image onto recording materials with high electrical resistance (high-resistivity recording materials or high-resistivity paper), such as ultra-thick paper or synthetic paper with a resin layer, without reducing image formation productivity. For example, in low-humidity environments, the electrical resistance of the outer secondary transfer roller increases, making it necessary to increase the absolute value of the secondary transfer bias voltage to supply the required secondary transfer current. Depending on the type of recording material, the absolute value of the secondary transfer bias voltage may need to be 10 kV or higher. If such a secondary transfer bias exceeds the high-voltage capacity, transfer gaps and low density due to insufficient secondary transfer current may occur. These transfer gaps and low density may occur, for example, in secondary-color toner images. Furthermore, increasing the absolute value of the secondary transfer bias voltage may cause image defects due to discharge at the secondary transfer unit, making it difficult to obtain a proper image.

[0049] Therefore, in this embodiment, the image forming apparatus 100 is configured so that the surface of the recording material S onto which the toner image is transferred can be pre-charged to a polarity opposite to the normal charging polarity of the toner before the recording material S reaches the secondary transfer portion N2. Here, the surface of the recording material S onto which the toner image is transferred is also referred to as the "toner image transfer surface." Also, here, pre-charging the toner image transfer surface of the recording material S to a polarity opposite to the normal charging polarity of the toner before the recording material S reaches the secondary transfer portion N2 is also referred to as "pre-charging." This makes it possible to properly transfer the toner image onto the recording material S by compensating for the insufficient secondary transfer current, even if the absolute value of the voltage of the secondary transfer bias is relatively small.

[0050] 4, in this embodiment, a pre-charging device 9 is provided upstream of the secondary transfer portion N2 (downstream of the registration roller pair 14) in the conveyance direction of the recording material S. The pre-charging device 9 pre-charges the toner image transfer surface of the recording material S to a polarity opposite to the normal charging polarity of the toner before the recording material S reaches the secondary transfer portion N2. This improves the transferability of the toner image onto thick paper such as extra-thick paper and synthetic paper.

[0051] In this embodiment, the pre-charging device 9 includes a drive roller (recording material charging roller, pre-charging roller) 84 disposed on the inner circumferential surface side of the secondary transfer belt 80, and a pre-charging counter roller 91 disposed opposite the drive roller 84 across the secondary transfer belt 80. The drive roller 84 in this embodiment is an example of a recording material charging member (pre-charging member, pre-charging rotator, first member). The drive roller 84 is a tension roller for the secondary transfer belt 80 and functions as both a drive roller that drives the secondary transfer belt 80 and a recording material charging member. The pre-charging counter roller 91 in this embodiment is an example of a counter member (pre-charging counter member, pre-charging counter rotator, second member). The pre-charging counter roller 91, together with the drive roller 84, forms a nip portion that sandwiches the recording material S. In other words, the drive roller 84 abuts against the pre-charging counter roller 91 via the secondary transfer belt 80. As a result, the secondary transfer belt 80 is sandwiched between the drive roller 84 and the pre-charging opposing roller 91, and a pre-charging portion (pre-charging nip portion, recording material charging portion) N3 is formed, which is the contact portion between the secondary transfer belt 80 and the pre-charging opposing roller 91. Note that the length of the portions of the drive roller 84 and the pre-charging opposing roller 91 that can come into contact with the recording material S in the rotational axis direction of these rollers is longer than the length of the recording material S in the same direction that can be used in the image forming apparatus 100 (the recording material S fits within the range of the length in the rotational axis direction of each roller).

[0052] In this embodiment, the pre-charging counter roller 91 is an elastic sponge roller having a core and an elastic foam layer of ion-conductive foam rubber (such as NBR rubber, ECO rubber, or urethane rubber) with sufficiently low electrical resistance, formed to cover the outer periphery of the core. In this embodiment, the pre-charging counter roller 91 has an outer diameter of 15 mm. The outer diameter of the pre-charging counter roller 91 is, for example, approximately 5 to 30 mm, and preferably 10 to 20 mm. By configuring the pre-charging counter roller 91 as a roller with a relatively small diameter, a sufficient distance can be secured between the surface of the pre-charging counter roller 91 and the surface of the intermediate transfer belt 70. The distance from the pre-charging section N3 to the secondary transfer section N2 in the conveyance direction of the recording material S is, for example, approximately 10 to 100 mm, and preferably 30 mm or less. This prevents the attenuation of the charge on the toner image transfer surface of the recording material S during the time the recording material S is conveyed to the secondary transfer section N2, even if the toner image transfer surface of the recording material S is charged at the pre-charging section N3.

[0053] In this embodiment, a pre-charging power supply (high-voltage power supply circuit) E5, which is a recording material charging voltage application unit (pre-charging voltage application unit) that functions as a recording material charging voltage application means (pre-charging voltage application means), is connected to the core of the drive roller 84. The pre-charging power supply E5 applies a pre-charging bias (pre-charging voltage) to the drive roller 84 as a recording material charging bias (recording material charging voltage) having the same polarity (negative in this embodiment) as the normal charging polarity of the toner. In this embodiment, when the pre-charging bias is applied, the core of the pre-charging opposing roller 91 is connected to ground, and the pre-charging opposing roller 91 is electrically grounded. Thus, in this embodiment, a pre-charging bias of the same polarity as the secondary transfer bias applied to the inner secondary transfer roller 71 is applied to the drive roller 84. In other words, the inner secondary transfer roller 71 and the drive roller 84 apply biases of the same polarity to the recording material S from different sides of the recording material S. By applying a pre-charging bias of the same polarity as the normal charging polarity of the toner to the surface of the recording material S opposite to the toner image transfer surface, the surface of the recording material S opposite to the toner image transfer surface is charged to the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment). As a result, the toner image transfer surface of the recording material S is charged to the polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment) by a charge induced from the ground. At this time, an apparent current flows in the pre-charging section N3.

[0054] In this embodiment, the pre-charging bias is applied under constant current control. That is, since the charge amount of the recording material S varies depending on the type of recording material S, the environment, etc., in this embodiment, a constant current is supplied by constant current control of the pre-charging bias so that the appropriate charge amount is achieved. The electrical resistance value (volume resistance) of the pre-charging opposing roller 91 (measured at N / N (23°C, 50% RH), applied at 2 kV) is 1.0 x 10 5 ~1.0×10 9 Ω, preferably 1.0×10 6 ~1.0×10 7 The electrical resistance (volume resistance) of the drive roller 84 (measured at N / N (23°C, 50% RH), applied at 2 kV) is preferably 1.0 x 10 3 ~1.0×10 7 Ω, preferably 1.0×103 ~1.0×10 5 It is more preferable that the resistance be Ω. If the electrical resistance of the system (pre-charging section N3) is too low, in the case of a small-sized recording material S (small-size paper), a large amount of current will flow in the nip portion where the recording material S does not pass, making it difficult to apply an appropriate charge to the recording material S. Conversely, if the electrical resistance of the system (pre-charging section N3) is too high, the voltage required to apply the necessary charge to the recording material S will increase, requiring a larger power supply capacity. Also, it may be difficult to ensure sufficient creepage to suppress leakage. Details of pre-charging bias control will be described later.

[0055] The recording material S conveyed by the pair of registration rollers 14 is conveyed to a nip portion (pre-charging portion) N3 between a pre-charging opposing roller 91 and a secondary transfer belt 82 wound around a drive roller 84. In this embodiment, guide members 22 (upper guide member 22a and lower guide member 22b) that guide the recording material S are provided upstream of the pre-charging portion N3 and downstream of the pair of registration rollers 14 in the conveyance direction of the recording material S. The recording material S conveyed by the pair of registration rollers 14 is conveyed to the pre-charging portion N3 while being guided by the guide members 22. Then, in the pre-charging portion N3, the recording material S is charged (pre-charged) and electrostatically attracted (by electrostatic force) to the secondary transfer belt 80. The recording material S attracted to the secondary transfer belt 80 is conveyed to the secondary transfer portion N2, where a toner image is transferred (secondary transfer) onto the recording material S.

[0056] 5. Control of pre-charging bias In this embodiment, the pre-charging bias is applied under constant current control. The pre-charging power supply E5 is provided with a current detection unit 26 (FIG. 3) as a current detection means for detecting its output current. The control unit 120 can control the pre-charging power supply E5 to perform constant current control of the output voltage so that the value of the current detected by the current detection unit 26 becomes approximately constant. The current detection unit 26 can detect the current flowing through the drive roller 84 (pre-charging unit N3).

[0057] The appropriate target current for the pre-charging bias may vary depending on the type and environment of the recording material S, as well as the printing surface (the surface to which the toner image is transferred at the secondary transfer unit N2 immediately after passing through the pre-charging unit N3; whether it is the first side of single-sided printing or double-sided printing, or the second side of double-sided printing). Therefore, whether or not to apply the pre-charging bias, or the target current for the pre-charging bias, can be changed based on at least one of the type, environment, and printing surface of the recording material S. For example, the target current for the pre-charging bias may be preset and stored in the memory 122 as table data, etc., so that the toner image transfer surface of the recording material S has an appropriate charge amount depending on the type and environment (e.g., absolute moisture content) of the recording material S. The appropriate charge amount for the toner image transfer surface of the recording material S can be determined in advance through experiments, etc., as the charge amount that provides appropriate transferability. As an example, the target current for the pre-charging bias is approximately -40 to -100 μA. This allows the toner image transfer surface of the recording material S to be charged to approximately 5 kV, for example. Further, for example, the pre-charging bias may be applied only when a predetermined type of recording material S is used, or only when a predetermined type of recording material S is used and the absolute moisture content is within a predetermined range (for example, smaller than a predetermined value).

[0058] For example, the target current of the pre-charging bias can be set according to the environment for each range of predetermined temperature and humidity environments (absolute moisture content, etc.), such as a normal temperature and low humidity environment (or a low temperature and low humidity environment), a normal temperature and normal humidity environment, or a high temperature and high humidity environment. Furthermore, the target current of the pre-charging bias can be set according to the environment for each type of recording material S. By determining whether pre-charging is necessary and setting the target current of the pre-charging bias according to the environment in this way, it is possible to perform control such as increasing the absolute value of the voltage of the secondary transfer bias in a low temperature and low humidity environment and reducing the absolute value of the voltage of the secondary transfer bias in a high temperature and high humidity environment.

[0059] 6. Suppression of secondary transfer current inflow As mentioned above, in a configuration in which pre-charging is performed, the secondary transfer current supplied to the secondary transfer section N2 flows to the pre-charging section N3 through the recording material S, and the necessary secondary transfer current cannot be obtained, which may result in poor transfer. This phenomenon is particularly noticeable when the recording material S is special paper with a metallic luster that has a layer of low electrical resistance, such as foil paper or metallized paper, or Multi Paper Select White Smooth 70 g / m (described later). 2 This phenomenon may occur when paper with low electrical resistance, such as paper with low electrical resistance, is used. This phenomenon is not limited to when special paper is used, but may also occur when the electrical resistance of the recording material S decreases due to moisture absorption in a high-humidity environment.

[0060] Therefore, in this embodiment, the image forming apparatus 100 is provided with a current suppression mechanism that can suppress the secondary transfer current from flowing from the secondary transfer portion N2 to the pre-charging portion N3 through the recording material S.

[0061] In the case of recording material S with a relatively low electrical resistance that does not require pre-charging, or in an environment that does not require pre-charging, there is no need to apply a charge to recording material S, and therefore no need to apply a pre-charging bias. On the other hand, as described above, when the electrical resistance of recording material P is very low, or when the electrical resistance of recording material S is reduced in a high-humidity environment, the secondary transfer current from secondary transfer portion N2 flows through recording material S into pre-charging portion N3, which may result in poor transfer.

[0062] Therefore, in this embodiment, the image forming apparatus 100 includes a switching unit 141 that switches the state of the pre-charging counter roller 91 between a first state (grounded state) in which the pre-charging counter roller 91 is connected to ground and a second state (floating state) in which the pre-charging counter roller 91 is disconnected from ground. The switching unit 141 is configured as a relay (power relay) and is controlled by the control unit 120. When pre-charging is required (when pre-charging is performed), the control unit 120 controls the pre-charging counter roller 91 to be connected to ground via the switching unit 141. When pre-charging is not required (when pre-charging is not performed), the control unit 120 controls the pre-charging counter roller 91 to be disconnected from ground (set to the floating state) via the switching unit 141. This prevents secondary transfer current from escaping from the secondary transfer unit N2 to the pre-charging counter roller 91 through the recording material S.

[0063] In this embodiment, as described above, the pre-charging power supply E5 is provided with a current detection unit 26, allowing constant current control so that the output current of the pre-charging power supply E5 is constant. In other words, a control device for constant current control is provided in the high-voltage power supply circuit that applies the pre-charging bias to the drive roller 84. In this embodiment, the high-voltage power supply circuit that applies the pre-charging bias to the drive roller 84 includes a constant-current power supply circuit configured as a current control unit including the current detection unit 26. When pre-charging is required (when pre-charging is performed), the control unit 120 controls the pre-charging bias to be constant current so that the required current flows to the pre-charging unit N3, as described above. When pre-charging is not required (when pre-charging is not performed), the control unit 120 controls the pre-charging power supply E5 to cancel the current flowing into the drive roller 84 via the recording material S, thereby preventing the secondary transfer current from flowing into the pre-charging power supply E5. In other words, the control unit 120 controls the pre-charging power supply E5 to sufficiently suppress the flow of negative secondary transfer current into the drive roller 84 via the recording material S. In other words, the control unit 120 controls the pre-charging power supply E5 so as to sufficiently suppress the flow of a current having a polarity opposite to that of the secondary transfer current (positive) from the drive roller 84 to the recording material S.

[0064] When the pre-charging bias is not required (when pre-charging is not performed), the control unit 120 typically controls the current flowing through the pre-charging power supply E5 to be approximately 0 μA. While the secondary transfer current flowing from the inner secondary transfer roller 71 to the recording material S is approximately −20 to −120 μA (typically −40 to −60 μA), the current flowing through the pre-charging power supply E5 should be ±10 μA or less (typically ±5 μA or less). That is, the absolute value of the current flowing through the pre-charging power supply E5 should be 10 μA or less (typically 5 μA or less). In particular, it is desirable that the absolute value of the positive current flowing from the drive roller 84 to the recording material S be 10 μA or less (typically 5 μA or less). The negative current flowing from the drive roller 84 to the recording material S may be approximately the same as when the pre-charging bias is applied, but because pre-charging is not required in this case, it is desirable that the absolute value be 10 μA or less (typically 5 μA or less). In other words, in this case, the control unit 120 controls the current flowing through the pre-charging power supply E5 to be approximately 0 μA or a negative current to flow from the drive roller 84 to the recording material S. Note that a switching means may be provided to separate the pre-charging power supply E5 from the ground and set it in a floating state.

[0065] In this embodiment, the current suppression mechanism that suppresses current flow from the secondary transfer portion N2 to the drive roller 84 and the pre-charging opposing roller 91 via the recording material S includes a switching unit 141 and a pre-charging power source E5.

[0066] 7. Control Procedure Next, a job control procedure including control of switching between pre-charging and non-pre-charging in this embodiment will be described. Fig. 5 is a flowchart showing an outline of the job control procedure including control of switching between pre-charging and non-pre-charging in this embodiment. Note that this description will be given assuming that the type of recording material S used in one job is the same.

[0067] First, when job information including information specifying the type of recording material S and image information is input from the operation unit 130 or an external device, the control unit 120 acquires information regarding the type of recording material S to be used for image formation (recording material information) (S101). Note that the information specifying the type of recording material S may be information specifying one of the cassettes 11a, 11b that contains the recording material S. In this case, the control unit 120 can acquire information regarding the type of recording material S to be used for image formation from information indicating the relationship between the preset cassettes 11a, 11b and the types of recording material S contained therein. The control unit 120 also acquires the temperature and humidity detection results (environmental information) from the environmental sensor 18 (S102). The control unit 120 can calculate the absolute moisture content based on the temperature and humidity detected by the environmental sensor 18.

[0068] The control unit 120 determines whether pre-charging is necessary based on the acquired information about the type of recording material S (S103). For example, if a paper type category such as synthetic paper or a predetermined high-resistivity paper (such as extra-thick paper) is selected, pre-charging is determined to be necessary. If any other paper type category is selected, pre-charging is determined to be unnecessary. If the control unit 120 determines that pre-charging is necessary in S103, it then determines whether pre-charging is necessary based on the absolute moisture content calculated from the acquired temperature and humidity (S104). For example, the control unit 120 determines whether the absolute moisture content is less than a predetermined threshold value indicating a predetermined high-humidity environment. For example, if the absolute moisture content is less than 21.744 g / kg, which corresponds to 30°C and 80% RH, pre-charging is determined to be necessary. If the absolute moisture content is 21.744 g / kg or greater, pre-charging is determined to be unnecessary. The range of environments in which pre-charging is unnecessary may be the same regardless of the type of recording material S, or may differ depending on the type of recording material S. Any index value that indicates the amount of moisture contained in the air can be used as the humidity for control, and for example, relative humidity may be used for control.

[0069] If the control unit 120 determines in S103 and S104 that pre-charging is necessary, it determines to apply a pre-charging bias to the drive roller 84 and to ground the pre-charging opposing roller 91 (S105). In addition, in S105, the control unit 120 sets a target current for the pre-charging bias depending on the type and environment of the recording material S. On the other hand, if the control unit 120 determines in S103 and S104 that pre-charging is unnecessary, it determines not to apply a pre-charging bias to the drive roller 84 (to control the pre-charging bias at a constant current of approximately 0 μA) and to set the pre-charging opposing roller 91 in a floating state (S106). Then, the control unit 120 starts image formation in the state set in S105 or S106 (S107). In this embodiment, if pre-charging is necessary (if pre-charging is performed), pre-charging is performed over the entire area of ​​the recording material S in the conveyance direction. On the other hand, in this embodiment, if pre-charging is unnecessary (if pre-charging is not performed), pre-charging is not performed over the entire area of ​​the recording material S in the conveyance direction. Therefore, in this embodiment, the control unit 120 controls the change to the state determined in S105 or S106 so that it is completed before the leading edge of the recording material S in the conveyance direction reaches the pre-charging unit N3. For convenience of explanation, the determination process in S106 is performed. However, if the state determined in S106 is a default setting, for example, the process in S107 can be immediately performed after S103 and S104. The same applies to the determination process in S105. For convenience of explanation, the determination processes in S105 and S106 and the start of image formation in S107 are described in series. However, the formation of a toner image on the photosensitive drum 1 or the intermediate transfer belt 70 may be started before the change to the state determined in S105 and S106 (such as the start of application of the pre-charging bias or the change of the grounded / floating state of the pre-charging opposing roller 91). Furthermore, the necessity of pre-charging may be determined based on either the type of recording material S or the environment.

[0070] Thereafter, the control unit 120 determines whether or not the output of all images specified in the job has been completed (S108), and if so, ends the operation of the job, and if not, returns to the processing of S107.

[0071] As described above, when determining whether pre-charging is necessary or setting the target current for the pre-charging bias depending on the printing surface, the control unit 120 can acquire information about the printing surface and perform the above determination and setting depending on the printing surface. Also, when multiple types of recording material S are used in a job, the necessity of pre-charging (the states of the pre-charging power source E5 and the pre-charging opposing roller 91) may be switched depending on the type of recording material S during the job.

[0072] 8. Evaluation An experiment was conducted to evaluate the occurrence of transfer defects due to insufficient secondary transfer current and transfer defects due to the inflow of secondary transfer current from the secondary transfer section N2 to the pre-charging section N3. The recording material S was Yupo YPI 200g / m, which is a representative high-resistivity recording material. 2 (Product name of Yupo Corporation), Multi Paper Select White Smooth 70g / m as a representative low-resistance recording material. 2 (product name of Askul Corporation) was used. For the evaluation experiment, Yupo YPI 200g / m 2 Simply put, "High Resistivity Recording Material", Multi Paper Select White Smooth 70g / m 2 is also referred to as the unit "low resistance recording material." As evaluation images, solid images of secondary colors of magenta and cyan were used. The target current of the pre-charging bias was set to about -60 μA. The same experiment was conducted for this example and Comparative Example 1 described below.

[0073] Here, when any of the recording materials S was used, pre-charging was performed in an environment with a temperature of 23°C and a humidity of 5% RH (low humidity environment), and pre-charging was not performed in an environment with a temperature of 30°C and a humidity of 80% RH (high humidity environment). 2 is an example of a recording material S that is prone to transfer defects due to insufficient secondary transfer current if pre-charging is not performed, particularly in a low-humidity environment. Therefore, in the above-mentioned control procedure, it can be determined that pre-charging is necessary when this recording material S is used. Also, Multi-Paper Select White Smooth 70 g / m 2is an example of a recording material S that is unlikely to suffer from transfer defects due to insufficient secondary transfer current even without pre-charging. Therefore, in the control procedure described above, it can be determined that pre-charging is not necessary when this recording material S is used. However, even if pre-charging is performed on low-resistivity recording materials, the charge tends to decay easily before the material is transported to the secondary transfer portion N2, so pre-charging had almost no effect on transferability in this experiment.

[0074] The evaluation results are shown in Table 1. When neither a secondary transfer current shortage nor transfer failure due to secondary transfer current inflow occurred, the evaluation was made as "Good (◯)", and when either occurred, the evaluation was made as "Poor (×)".

[0075] <Comparative Example 1> FIG. 10 is a schematic cross-sectional view showing the vicinity of the secondary transfer portion N2 in Comparative Example 1 (showing a cross section that is approximately perpendicular to the rotation axis direction of the photosensitive drum 1 or the rotation axis direction of the tension roller of the intermediate transfer belt 70). The configuration of Comparative Example 1 is substantially the same as the configuration of this embodiment, except for the differences described below. In addition, in the configuration of Comparative Example 1, elements that have the same or corresponding functions or configurations as those in this embodiment will be described using the same reference numerals.

[0076] In Comparative Example 1, the image forming apparatus 100 has a secondary outer transfer roller 81 that directly contacts the outer peripheral surface of the intermediate transfer belt 70. The secondary outer transfer roller 81 is pressed toward the secondary inner transfer roller 71 and abuts against the secondary inner transfer roller 71 via the intermediate transfer belt 70, forming a secondary transfer portion N2 that is a contact portion between the intermediate transfer belt 70 and the secondary outer transfer roller 81. The configurations of the secondary inner transfer roller 71 and the secondary outer transfer roller 81 in Comparative Example 1 are substantially the same as the configurations of the secondary inner transfer roller 71 and the secondary outer transfer roller 81 in this embodiment, respectively.

[0077] In Comparative Example 1, similarly to the present embodiment, a secondary transfer power supply E4 is connected to the core of the inner secondary transfer roller 71, and a secondary transfer bias of the same polarity (negative polarity) as the normal charging polarity of the toner is applied from the secondary transfer power supply E4 to the inner secondary transfer roller 71. Also, in Comparative Example 1, the core of the outer secondary transfer roller 81 is connected to ground, and the outer secondary transfer roller 81 is electrically grounded.

[0078] In Comparative Example 1, the pre-charging device 9 includes a pre-charging roller 92 that contacts the surface of the recording material S opposite to the toner image transfer surface, and a pre-charging counter roller 91 that contacts the toner image transfer surface of the recording material S. The pre-charging roller 92 and the pre-charging counter roller 91 form a pre-charging portion (pre-charging nip portion) N3, which is their contact portion. The configuration of the pre-charging roller 92 in Comparative Example 1 is substantially the same as the configuration of the drive roller (secondary transfer belt drive roller) 84 in this embodiment. The configuration of the pre-charging counter roller 91 in Comparative Example 1 is substantially the same as the configuration of the pre-charging counter roller 91 in this embodiment. In Comparative Example 1, neither the pre-charging roller 92 nor the pre-charging counter roller 91 is driven by a drive unit, but is rotated in response to the movement of the recording material S conveyed by the registration roller pair 14. However, at least one of the pre-charging roller 92 and the pre-charging counter roller 91 may be rotationally driven by a drive unit.

[0079] In Comparative Example 1, a pre-charging power supply E5 is connected to the core of the pre-charging roller 92, and a pre-charging bias of the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment) is applied from the pre-charging power supply E5 to the pre-charging roller 94. In Comparative Example 1, the core of the pre-charging opposing roller 91 is connected to ground, and the pre-charging opposing roller 91 is electrically grounded.

[0080] In Comparative Example 1, the recording material S conveyed by the registration roller pair 14 is charged at the pre-charging section N3 to a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment) by a negative pre-charging bias applied to the pre-charging roller 92. The recording material S that has passed through the pre-charging section N3 is conveyed to the secondary transfer section N2, where the toner image is transferred (secondary transfer) onto the recording material S.

[0081] In Comparative Example 1, a first guide member 22 (upper guide member 22a, lower guide member 22b) that guides the recording material S is provided upstream of the pre-charging portion N3 and downstream of the pair of registration rollers 14 in the conveying direction of the recording material S. The recording material S conveyed by the pair of registration rollers 14 is conveyed to the pre-charging portion N3 while being guided by the first guide member 22. In Comparative Example 1, a second guide member 23 (upper guide member 23a, lower guide member 23b) that guides the recording material S is provided upstream of the secondary transfer portion N2 and downstream of the pre-charging portion N3 in the conveying direction of the recording material S. The recording material S that has passed through the pre-charging portion N3 is conveyed to the secondary transfer portion N2 while being guided by the second guide member 23.

[0082] In Comparative Example 1, the switching unit 141 as in this embodiment is not provided, so the pre-charging opposing roller 91 is always grounded. Also, in Comparative Example 1, as in this embodiment, when pre-charging is not required (when pre-charging is not performed), constant current control is performed so that the current flowing through the pre-charging power supply E5 is approximately 0 μA. However, in Comparative Example 1, the evaluation results when this control is not performed are the same as when it is performed.

[0083] <Result> As shown in Table 1, in Comparative Example 1, when a low-resistivity recording material was used, transfer defects due to the inflow of secondary transfer current occurred in some cases under conditions where pre-charging was not performed in a high-humidity environment. Note that in Comparative Example 1, when pre-charging was performed in a low-humidity environment, transfer defects due to insufficient secondary transfer current and transfer defects due to the inflow of secondary transfer current did not occur, regardless of whether a high-resistivity recording material or a low-resistivity recording material was used.

[0084] In contrast, in this embodiment, even when a low-resistivity recording material was used, no transfer defects due to the inflow of secondary transfer current occurred under the condition that pre-charging was not performed in a high-humidity environment. Also, in this embodiment, under the condition that pre-charging was performed in a low-humidity environment, no transfer defects due to insufficient secondary transfer current or the inflow of secondary transfer current occurred, regardless of whether a high-resistivity recording material or a low-resistivity recording material was used.

[0085] As described above, in this embodiment, the image forming apparatus 100 includes an image carrier (intermediate transfer belt) 70 that carries a toner image, transfer members (secondary transfer inner roller 71, secondary transfer outer roller 81) that form a transfer section (secondary transfer section) N2 that transfers the toner image from the image carrier 70 to the recording material S, a first application section (secondary transfer power supply) E4 that applies a transfer bias to the transfer section N2, a first member (drive roller) 84 and a second member (pre-charging opposing roller) 91 that are arranged upstream of the transfer section N2 in the conveying direction of the recording material S, and form a recording material charging section (pre-charging section) N3 that sandwiches the recording material S conveyed to the transfer section N2 and charges the toner image transfer surface, which is the surface onto which the toner image of the recording material S is transferred, to a polarity opposite to the normal charging polarity of the toner, and a bias is applied to the first member 84 to transfer the toner image of the recording material S to the recording material charging section N3. The control unit 120 is configured to control a second application unit (pre-charging power supply) E5 that applies a recording material charging bias to charge the recording material surface to the opposite polarity, and a control unit 120 that can control the second application unit E5 to perform image formation in two modes: a first mode (operation when pre-charging is performed) in which a toner image is transferred at a transfer unit N2 to a recording material S that has passed through the recording material charging unit N3 while the recording material charging bias is applied to the recording material charging unit N3, and a second mode (operation when pre-charging is not performed) in which a toner image is transferred at a transfer unit N2 to a recording material S that has passed through the recording material charging unit N3 while the recording material charging bias is not applied to the recording material charging unit N3, and a current suppression mechanism (switching unit 141, pre-charging power supply E5) that suppresses current from flowing from the transfer unit N2 to the first member 84 and the second member 91 via the recording material S when image formation is performed in the second mode. In addition, in this embodiment, the current suppression mechanism includes a switching unit 141 that switches the state of the second member 91 between a first state in which the second member 91 is connected to ground and a second state in which the second member 91 is disconnected from ground, and that switches the second member 91 to the first state when image formation is performed in the first mode and switches the second member 91 to the second state when image formation is performed in the second mode.In this embodiment, the current suppression mechanism includes a current control unit (such as a current detection unit 26) that controls the current flowing through the second application unit E5 to 0 μA or a current with an absolute value of 10 μA or less when image formation is performed in the second mode. In other words, in this embodiment, the current suppression mechanism includes a current control unit that controls the current flowing from the second application unit E5 to the first member 84 so that a current having a polarity opposite to that of the current flowing from the transfer unit N2 to the first member 84 via the recording material S does not flow from the second application unit E5 to the first member 84 when image formation is performed in the second mode. In this embodiment, the first member 84 is disposed on the side opposite to the toner image transfer surface of the recording material S, and the second member 91 is disposed on the toner image transfer surface of the recording material S. However, as described below, the image forming apparatus 100 may also be configured such that the first member is disposed on the toner image transfer surface side of the recording material S, and the second member is disposed on the side opposite to the toner image transfer surface of the recording material S. In this embodiment, the image carrier 70 is an intermediate transfer member onto which a toner image is transferred from another image carrier (photosensitive drum) 1.

[0086] In this embodiment, the image forming apparatus 100 includes an acquisition unit (such as the operation unit 130 or an input / output unit of the control unit 120) that acquires information regarding the type of recording material S. The control unit 120 controls the image forming apparatus 100 to switch between the first mode and the second mode based on the information acquired by the acquisition unit. In this embodiment, the control unit 120 controls the image forming apparatus 100 to execute image formation in the first mode when the type of recording material S indicated by the information acquired by the acquisition unit is the first type, and to execute image formation in the second mode when the type of recording material S indicated by the information acquired by the acquisition unit is the second type, which has a lower electrical resistance (for example, electrical resistance at the secondary transfer unit N2) than the first type. In this embodiment, the image forming apparatus 100 includes a detection unit (environment sensor) 18 that detects a value related to environmental humidity. The control unit 120 controls the image forming apparatus 100 to switch between the first mode and the second mode based on the value detected by the detection unit 18. In this embodiment, the control unit 120 controls the image formation to be performed in the first mode when the humidity indicated by the value detected by the detection unit 18 is a first humidity, and to be performed in the second mode when the humidity indicated by the value detected by the detection unit 18 is a second humidity higher than the first humidity.

[0087] As described above, according to this embodiment, it is possible to prevent transfer defects from occurring due to current flowing from the secondary transfer portion N2 to the pre-charging portion N3.

[0088] [Example 2] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those of embodiment 1, and detailed descriptions thereof will be omitted.

[0089] 1. Configuration of this embodiment FIG. 6 is a schematic cross-sectional view showing the vicinity of the secondary transfer portion N2 in this embodiment (showing a cross section substantially perpendicular to the rotation axis direction of the photosensitive drum 1 or the rotation axis direction of the tension roller of the intermediate transfer belt 70).

[0090] In this embodiment, the image forming apparatus 100 has a secondary transfer unit 8 equipped with a secondary transfer belt 80. The secondary transfer unit 8 is configured to have the secondary transfer belt 80 and a plurality of tension rollers, namely, an outer secondary transfer roller 81, a separation roller 82, a tension roller 83, and a drive roller 84. However, in this embodiment, the secondary transfer belt 80 does not have a surface that supports the recording material S upstream of the secondary transfer portion N2 in the conveyance direction of the recording material S. The recording material S enters the secondary transfer portion N2 directly without being supported by the secondary transfer belt 80.

[0091] In this embodiment, as in the first embodiment, a secondary transfer power supply E4 is connected to the core of the inner secondary transfer roller 71, and a secondary transfer bias of the same polarity (negative in this embodiment) as the normal charging polarity of the toner is applied to the inner secondary transfer roller 71 by the secondary transfer power supply E4. Also, in this embodiment, the core of the outer secondary transfer roller 81 is connected to ground, and the outer secondary transfer roller 81 is electrically grounded. The configurations of the inner secondary transfer roller 71 and the outer secondary transfer roller 81 in this embodiment are substantially the same as the configurations of the inner secondary transfer roller 71 and the outer secondary transfer roller 81 in the first embodiment, respectively.

[0092] In this embodiment, the pre-charging device 9 includes a pre-charging roller 92 that contacts the surface of the recording material S opposite to the toner image transfer surface, and a pre-charging opposing roller 91 that contacts the toner image transfer surface of the recording material S. The pre-charging roller 92 in this embodiment is an example of a recording material charging member (pre-charging member). The pre-charging opposing roller 91 in this embodiment is an example of an opposing member (pre-charging opposing member). The pre-charging roller 92 and the pre-charging opposing roller 91 form a pre-charging portion (pre-charging nip portion) N3, which is their contact portion. The configuration of the pre-charging roller 92 in this embodiment is substantially the same as the configuration of the drive roller (secondary transfer belt drive roller) 84 in the first embodiment. The configuration of the pre-charging opposing roller 91 in this embodiment is substantially the same as the configuration of the pre-charging opposing roller 91 in the first embodiment. In this embodiment, neither the pre-charging roller 92 nor the pre-charging opposing roller 91 is driven by a driving unit, but is rotated in response to the movement of the recording material S transported by the registration roller pair 14. However, at least one of the pre-charging roller 92 and the pre-charging counter roller 91 may be rotated by a driving means.

[0093] In this embodiment, a pre-charging power supply E5 is connected to the core of the pre-charging roller 92, and a pre-charging bias of the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment) is applied from the pre-charging power supply E5 to the pre-charging roller 94. In this embodiment, when the pre-charging bias is applied, the core of the pre-charging opposing roller 91 is connected to ground, and the pre-charging opposing roller 91 is electrically grounded.

[0094] In this embodiment, the recording material S conveyed by the registration roller pair 14 is charged at the pre-charging section N3 to a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment) by a negative pre-charging bias applied to the pre-charging roller 92. The recording material S that has passed through the pre-charging section N3 is conveyed to the secondary transfer section N2, where the toner image is transferred (secondary transfer) onto the recording material S.

[0095] In this embodiment, a first guide member 22 (upper guide member 22a, lower guide member 22b) that guides the recording material S is provided upstream of the pre-charging portion N3 and downstream of the pair of registration rollers 14 in the conveying direction of the recording material S. The recording material S conveyed by the pair of registration rollers 14 is conveyed to the pre-charging portion N3 while being guided by the first guide member 22. In this embodiment, a second guide member 23 (upper guide member 23a, lower guide member 23b) that guides the recording material S is provided upstream of the secondary transfer portion N2 and downstream of the pre-charging portion N3 in the conveying direction of the recording material S. The recording material S that has passed through the pre-charging portion N3 is conveyed to the secondary transfer portion N2 while being guided by the second guide member 23.

[0096] 2. Suppression of secondary transfer current inflow In this embodiment, as in Example 1, the image forming apparatus 100 is provided with a current suppression mechanism that can suppress the flow of secondary transfer current from the secondary transfer portion N2 to the pre-charging portion N3 through the recording material S.

[0097] In this embodiment, the image forming apparatus 100 includes a switching unit 142 as a switching means for switching the state of the pre-charging counter roller 91 between a first state in which the pre-charging counter roller 91 is directly connected to ground and a second state in which the pre-charging counter roller 91 is connected to ground via a current suppressing element. The current suppressing element may be a protective resistor (resistance element) 143, a varistor 144, or the like. While FIG. 6 illustrates both the protective resistor 143 and the varistor 144 as the current suppressing element for convenience, either one of them may be provided. The switching unit 142 is configured as a relay and is controlled by the control unit 120. When pre-charging is required (when pre-charging is performed), the control unit 120 controls the pre-charging counter roller 91 to be directly connected to ground via the switching unit 142. When pre-charging is not required (when pre-charging is not performed), the control unit 120 controls the pre-charging counter roller 91 to be connected to ground via a current suppressing element (protective resistor 143 or varistor 144). This prevents the secondary transfer current from escaping from the secondary transfer portion N2 to the pre-charging opposing roller 91 through the recording material S. Current suppression elements such as a protective resistor 143 and a varistor 144 prevent a current below a certain level from flowing into the pre-charging opposing roller 91. In this case, the current suppression element typically limits the current flowing through the pre-charging opposing roller 91 to approximately 0 μA. The current flowing through the pre-charging opposing roller 91 may be ±10 μA or less (typically ±5 μA or less), similar to the current flowing through the pre-charging power supply E5 described in the first embodiment. In other words, the absolute value of the current flowing through the pre-charging opposing roller 91 may be 10 μA or less (typically 5 μA or less). While not limited thereto, the electrical resistance value of the current suppression element (e.g., a resistor element) of approximately 2 kΩ to 1 MΩ is often sufficient, typically approximately 3 kΩ to 5 kΩ.

[0098] Furthermore, in this embodiment, similarly to the first embodiment, when pre-charging is required (when pre-charging is performed), the control unit 120 controls the pre-charging bias to be constant current controlled so that the necessary current flows to the pre-charging unit N3. Furthermore, when pre-charging is not required (when pre-charging is not performed), the control unit 120 controls the pre-charging power supply E5 to cancel the current flowing into the drive roller 84 via the recording material S, and controls so that the secondary transfer current does not flow into the pre-charging power supply E5.

[0099] In this embodiment, the current suppression mechanism that suppresses current flow from the secondary transfer section N2 to the pre-charging roller 92 and the pre-charging opposing roller 91 via the recording material S is composed of a switching section 142, a current suppression element 143 or 144, and a pre-charging power supply E5.

[0100] 3. Control Procedure An example of a job control procedure including control of switching whether or not pre-charging is required in this embodiment is the same as that described in embodiment 1 using Fig. 5. However, in this embodiment, if the control unit 120 determines in S103 and S104 that pre-charging is required, it determines to apply a pre-charging bias to the drive roller 84 and to directly connect the pre-charging counter roller 91 to ground (S105). Also, in this embodiment, if the control unit 120 determines in S103 and S104 that pre-charging is not required, it determines not to apply a pre-charging bias to the drive roller 84 (to perform constant current control at approximately 0 µA) and to connect the pre-charging counter roller 91 to ground via a current suppression element (protective resistor 143 or varistor 144) (S106).

[0101] 4. Evaluation The configuration of this embodiment was also subjected to evaluation experiments similar to those described in Example 1. The evaluation results are shown in Table 1.

[0102] As shown in Table 1, in this embodiment, even when a low-resistivity recording material was used, no transfer defects due to the inflow of secondary transfer current occurred under the condition that pre-charging was not performed in a high-humidity environment. Also, in this embodiment, under the condition that pre-charging was performed in a low-humidity environment, no transfer defects due to insufficient secondary transfer current or the inflow of secondary transfer current occurred, regardless of whether a high-resistivity recording material or a low-resistivity recording material was used.

[0103] Instead of providing the switching unit 142 in this embodiment, the pre-charging opposing roller 91 may be configured to be connected to ground via a variable resistance mechanism such as a varistor 144. When pre-charging is required (when pre-charging is performed), the variable resistance mechanism can be set to a first state in which the electrical resistance value is a first value, and when pre-charging is not required (when pre-charging is not performed), the variable resistance mechanism can be set to a second state in which the electrical resistance value is a second value greater than the first value.

[0104] As described above, in this embodiment, the current suppression mechanism includes a switching unit 142 that switches the state of the second member 91 between a first state in which the second member (pre-charging opposing roller) 91 is connected to ground without the current suppression element 143 or 144 and a second state in which the second member 91 is connected to ground via the current suppression element 143 or 144. The switching unit 142 switches the second member 91 to the first state when image formation is performed in a first mode in which pre-charging is performed, and switches the second member 91 to the second state when image formation is performed in a second mode in which pre-charging is not performed. The current suppression element may be a resistor element 143. Alternatively, the current suppression element may be a varistor 144. The current suppression mechanism may also be a variable resistance mechanism (such as a varistor) provided between the second member 91 and ground, which is set to a first state when image formation is performed in the first mode, and which is set to a second state having a higher electrical resistance value than the first state when image formation is performed in the second mode.

[0105] As described above, similar to the first embodiment, the configuration of this embodiment can also suppress the occurrence of transfer defects caused by current flowing from the secondary transfer portion N2 to the pre-charging portion N3.

[0106] [Example 3] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those of embodiment 1, and detailed descriptions thereof will be omitted.

[0107] 1. Configuration of this embodiment FIG. 7 is a schematic cross-sectional view showing the vicinity of the secondary transfer portion N2 in this embodiment (showing a cross section substantially perpendicular to the rotation axis direction of the photosensitive drum 1 or the rotation axis direction of the tension roller of the intermediate transfer belt 70).

[0108] The configuration in the vicinity of the secondary transfer portion N2 in this embodiment is the same as that in the second embodiment, except for the differences described below.

[0109] In this embodiment, a pre-charging bias of a polarity opposite to the normal charging polarity of the toner (positive polarity) is applied to a pre-charging roller 93 corresponding to the pre-charging opposing roller 91 in Example 2. Also, in this embodiment, a pre-charging opposing roller 94 corresponding to the pre-charging roller 92 in Example 2 is electrically grounded as described below. The configuration of the pre-charging roller 93 in this embodiment is substantially the same as the configuration of the pre-charging opposing roller 91 in Example 2. Also, the configuration of the pre-charging opposing roller 94 in this embodiment is substantially the same as the configuration of the pre-charging roller 92 in Example 2.

[0110] 2. Suppression of secondary transfer current inflow In this embodiment, as in Example 1, the image forming apparatus 100 is provided with a current suppression mechanism that can suppress the flow of secondary transfer current from the secondary transfer portion N2 to the pre-charging portion N3 through the recording material S.

[0111] In this embodiment, the pre-charging opposing roller 94 is connected to ground via a diode 145, which is a rectifying element. This prevents a negative secondary transfer current from flowing into the pre-charging opposing roller 94 via the recording material S. In other words, the diode 145 is provided between the pre-charging opposing roller 94 and ground so as to prevent a negative secondary transfer current from flowing into the pre-charging opposing roller 94 via the recording material S.

[0112] Furthermore, in this embodiment, similarly to the second embodiment, when pre-charging is required (when pre-charging is performed), the control unit 120 controls the pre-charging bias to be constant current controlled so that the necessary current flows to the pre-charging unit N3. Furthermore, when pre-charging is not required (when pre-charging is not performed), the control unit 120 controls the pre-charging power supply E5 to cancel the current flowing into the drive roller 84 via the recording material S, and controls so that the secondary transfer current does not flow into the pre-charging power supply E5.

[0113] In this embodiment, the current suppression mechanism that suppresses current flow from the secondary transfer portion N2 to the pre-charging roller 93 and the pre-charging opposing roller 94 via the recording material S includes a diode 145 and a pre-charging power source E5.

[0114] 3. Control Procedure An example of a job control procedure including control for switching whether or not pre-charging is required in this embodiment is the same as that described in the first embodiment using Fig. 5. However, in this embodiment, if the control unit 120 determines in S103 and S104 that pre-charging is required, it determines to apply a pre-charging bias to the drive roller 84 (S105). Also, in this embodiment, if the control unit 120 determines in S103 and S104 that pre-charging is not required, it determines not to apply a pre-charging bias to the drive roller 84 (to perform constant current control at approximately 0 µA) (S106).

[0115] 4. Evaluation The configuration of this embodiment was also subjected to evaluation experiments similar to those described in Example 1. The evaluation results are shown in Table 1.

[0116] As shown in Table 1, in this embodiment, even when a low-resistivity recording material was used, no transfer defects due to the inflow of secondary transfer current occurred under the condition that pre-charging was not performed in a high-humidity environment. Also, in this embodiment, under the condition that pre-charging was performed in a low-humidity environment, no transfer defects due to insufficient secondary transfer current or the inflow of secondary transfer current occurred, regardless of whether a high-resistivity recording material or a low-resistivity recording material was used.

[0117] Thus, in this embodiment, the current suppression mechanism is a rectifying element (diode) 145 provided between the second member (pre-charging opposing roller) 94 and ground, and has a rectifying element 145 provided to suppress the current flowing from the transfer section N2 via the recording material S to the ground via the second member 94.

[0118] As described above, similar to the first embodiment, the configuration of this embodiment can also suppress the occurrence of transfer defects caused by current flowing from the secondary transfer portion N2 to the pre-charging portion N3.

[0119] [Example 4] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those of embodiment 1, and detailed descriptions thereof will be omitted.

[0120] 1. Configuration of this embodiment FIG. 8 is a schematic cross-sectional view showing the vicinity of the secondary transfer portion N2 in this embodiment (showing a cross section substantially perpendicular to the rotation axis direction of the photosensitive drum 1 or the rotation axis direction of the tension roller of the intermediate transfer belt 70).

[0121] The configuration in the vicinity of the secondary transfer portion N2 in this embodiment is the same as that in the first embodiment, except for the configuration relating to suppression of inflow of the secondary transfer current, which will be described below.

[0122] 2. Suppression of secondary transfer current inflow In this embodiment, as in Example 1, the image forming apparatus 100 is provided with a current suppression mechanism that can suppress the flow of secondary transfer current from the secondary transfer portion N2 to the pre-charging portion N3 through the recording material S.

[0123] In the first embodiment, when pre-charging is not required (when pre-charging is not performed), the pre-charging opposing roller 91 is in a floating state. If the pre-charging opposing roller 91 rotates in a floating state, it is conceivable that the pre-charging opposing roller 91 will be charged by friction with the recording material S or the secondary transfer belt 80, and that this will impart unnecessary charge to the recording material S. Therefore, in this embodiment, the image forming apparatus 100 is provided with a separation mechanism 150 as a separation unit that can bring the pre-charging opposing roller 91 into contact with and away from the secondary transfer belt 80 (drive roller 84). The separation mechanism 150 operates under the control of the control unit 120. When pre-charging is required (when pre-charging is performed), the control unit 120 controls the separation mechanism 150 to bring the pre-charging opposing roller 91 into contact with the drive roller 84 via the secondary transfer belt 80. Furthermore, when pre-charging is not required (when pre-charging is not performed), the control unit 120 controls the separation mechanism 150 to separate the pre-charging opposing roller 91 from the secondary transfer belt 80 (drive roller 84). This prevents secondary transfer current from escaping from the secondary transfer portion N2 to the pre-charging opposing roller 91 through the recording material S. Note that when the separation mechanism 150 is provided in the image forming apparatus 100 as in this embodiment, the pre-charging opposing roller 91 may remain electrically grounded.

[0124] In the configuration of the first embodiment, a separation mechanism 150 may be provided as in this embodiment, so that it is possible to switch between separating the pre-charging opposing roller 91 from the secondary transfer belt 80 and disconnecting the pre-charging opposing roller 91 from the ground by the switching unit 141. For example, depending on the type of recording material S or the environment, it is possible to switch between separating the pre-charging opposing roller 91 from the secondary transfer belt 80 and disconnecting the pre-charging opposing roller 91 from the ground by the switching unit 141. For example, if the pre-charging opposing roller 91 is prone to frictional charging depending on the type of recording material S or the environment, it is possible to separate the pre-charging opposing roller 91 from the secondary transfer belt 80 (drive roller 84) by the separation mechanism 150.

[0125] Furthermore, in the configuration near the secondary transfer portion N2 similar to that of the second embodiment, a separation mechanism 150 may be provided that can move the pre-charging counter roller 91 in contact with and away from the pre-charging roller 92, similar to that of the present embodiment. In this case, the pre-charging counter roller 91 may remain electrically grounded. Alternatively, similar to the above, the pre-charging counter roller 91 may be configured to be able to switch between being separated from the pre-charging roller 92 and being grounded via a current suppressing element (protective resistor 143 or varistor 144) depending on, for example, the type of recording material S or the environment. For example, if the pre-charging counter roller 91 is prone to frictional charging depending on the type of recording material S or the environment, the separation mechanism 150 can be used to separate the pre-charging counter roller 91 from the pre-charging roller 92.

[0126] Furthermore, in this embodiment, similarly to the first embodiment, when pre-charging is required (when pre-charging is performed), the control unit 120 controls the pre-charging bias to be constant current controlled so that the necessary current flows to the pre-charging unit N3. Furthermore, when pre-charging is not required (when pre-charging is not performed), the control unit 120 controls the pre-charging power supply E5 to cancel the current flowing into the drive roller 84 via the recording material S, and controls so that the secondary transfer current does not flow into the pre-charging power supply E5.

[0127] In this embodiment, the current suppression mechanism that suppresses current flow from the secondary transfer portion N2 to the drive roller 84 and the pre-charging opposing roller 91 via the recording material S includes a separation mechanism 150 and a pre-charging power source E5.

[0128] 3. Control Procedure An example of a job control procedure including control of switching whether pre-charging is necessary in this embodiment is the same as that described in embodiment 1 using Fig. 5. However, in this embodiment, if the control unit 120 determines in S103 and S104 that pre-charging is necessary, it determines to apply a pre-charging bias to the drive roller 84 and to bring the pre-charging opposing roller 91 into contact with the secondary transfer belt 80 (drive roller 84) (S105). Also, in this embodiment, if the control unit 120 determines in S103 and S104 that pre-charging is unnecessary, it determines not to apply a pre-charging bias to the drive roller 84 (to perform constant current control at approximately 0 µA) and to separate the pre-charging opposing roller 91 from the secondary transfer belt 80 (drive roller 84) (S106).

[0129] 4. Evaluation The configuration of this embodiment was also subjected to evaluation experiments similar to those described in Example 1. The evaluation results are shown in Table 1.

[0130] As shown in Table 1, in this embodiment, even when a low-resistivity recording material was used, no transfer defects due to the inflow of secondary transfer current occurred under the condition that pre-charging was not performed in a high-humidity environment. Also, in this embodiment, under the condition that pre-charging was performed in a low-humidity environment, no transfer defects due to insufficient secondary transfer current or the inflow of secondary transfer current occurred, regardless of whether a high-resistivity recording material or a low-resistivity recording material was used.

[0131] As described above, similar to the first embodiment, the configuration of this embodiment can also suppress the occurrence of transfer defects caused by current flowing from the secondary transfer portion N2 to the pre-charging portion N3.

[0132] [Table 1]

[0133] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

[0134] 9 is a schematic cross-sectional view showing the configuration in the vicinity of the secondary transfer unit of a modified example of Example 1. As shown in FIG. 9, the present invention can also be applied to an image forming apparatus having a configuration in the vicinity of the secondary transfer unit similar to that described in relation to Comparative Example 1. The configuration in FIG. 9 shows a configuration having a switching unit 141 similar to that of Example 1, but it may also be a configuration having a switching unit 142 similar to that of Example 2, a diode 145 similar to that of Example 3, or a spacing mechanism 150 similar to that of Example 4.

[0135] Similarly, the configurations for suppressing the inflow of secondary transfer current described in Examples 1 to 4 can be used in any combination with the configurations near the secondary transfer unit described in Examples 1 to 4 and the above-mentioned modified examples. For example, the positional relationship between the first member (drive roller, pre-charging roller) connected to the pre-charging power supply and the second member (pre-charging opposing member) that forms the pre-charging unit together with the first member can be reversed from that in Examples 1 to 4. When the first member connected to the pre-charging power supply is provided on the toner image transfer surface side of the recording material, a pre-charging bias of opposite polarity to the normal charging polarity of the toner can be applied. Then, a current suppression mechanism for suppressing the inflow of secondary transfer current described in Examples 1 to 4 can be provided for the second member. Note that when the positional relationship between the first member and the second member is reversed in Example 3, the rectifying element can be provided between the second member and ground with its orientation reversed from that in Example 3. In either case, if pre-charging is not required (if pre-charging is not performed), the pre-charging power supply can be controlled (constant current control at approximately 0 μA) to cancel the current flowing into the first member via the recording material.

[0136] In addition, in the above-described embodiment, the image forming apparatus employs an internal power supply system for supplying the secondary transfer bias, but the image forming apparatus may employ an external power supply system for supplying the secondary transfer bias. That is, for example, the inner secondary transfer roller 71 in the above-described embodiment may be electrically grounded, and a secondary transfer bias of a polarity opposite to the normal charging polarity of the toner may be applied to the outer secondary transfer roller 81 in the above-described embodiment.

[0137] In the above-described embodiment, the recording material charging member (pre-charging member, first member) and the opposing member (pre-charging opposing member, second member) are each roller-shaped members, but the present invention is not limited to this. The recording material charging member (pre-charging member) and the opposing member (pre-charging opposing member) may each independently be a roller-shaped member, a brush-shaped member, a sheet-shaped member, a pad-shaped member, or the like.

[0138] In the above embodiment, the secondary transfer bias is controlled to a constant voltage, but the secondary transfer bias may be controlled to a constant current.

[0139] Furthermore, in the above-described embodiment, the recording material charging bias (pre-charging bias) is controlled by constant current, but the recording material charging bias (pre-charging bias) may be controlled by constant voltage.

[0140] In the above-described embodiment, the image forming apparatus is a tandem-type color image forming apparatus employing an intermediate transfer method capable of forming full-color images. However, the image forming apparatus is not limited to a tandem-type image forming apparatus. The image forming apparatus may be configured, for example, as a single-drum type in which a toner image is sequentially transferred from a first image carrier to a second image carrier (intermediate transfer member), and then transferred from the second image carrier to a recording material. The image forming apparatus may also be a direct transfer-type image forming apparatus in which a toner image is transferred from an image carrier to a recording material carried and conveyed on a conveyor belt. The image forming apparatus is not limited to an image forming apparatus capable of forming full-color images, but may also be an image forming apparatus capable of forming only monochrome (black and white or monochromatic) images. In this case, the present invention is applicable to, for example, a transfer unit, which is a contact portion between an image carrier (such as a photosensitive drum) and a transfer member (such as a transfer roller) to which a transfer bias is applied. The image forming apparatus may be an image forming apparatus for various purposes, such as a printer, various printing machines, a copying machine, a FAX machine, or a multifunction machine. [Explanation of symbols]

[0141] 1 Photosensitive drum 7 Intermediate transfer unit 8 Secondary transfer unit 9 Pre-charging device (recording material charging device) 70 Intermediate transfer belt 71 Secondary transfer inner roller 81 Secondary transfer outer roller 80 Secondary transfer belt 84 Secondary transfer belt drive roller (recording material charging member, first member) 91 Pre-charging opposing roller (opposing member, second member) 100 Image forming device S recording material

Claims

1. an image carrier that carries a toner image; a transfer member forming a transfer section that transfers a toner image from the image carrier to a recording material; a first applying unit that applies a transfer bias to the transfer unit; a first member and a second member disposed upstream of the transfer unit in the recording material conveyance direction, the first member and the second member forming a recording material charging unit that sandwiches the recording material conveyed to the transfer unit and charges a toner image transfer surface, onto which a toner image of the recording material is transferred, to a polarity opposite to the normal charging polarity of the toner; a second applying section that applies a recording material charging bias to the first member to charge the toner image transfer surface of the recording material to the recording material charging section with the opposite polarity; a control section capable of controlling the second application section, the control section being capable of controlling image formation in a first mode in which the transfer section transfers a toner image onto a recording material that has passed through the recording material charging section in a state in which the recording material charging bias is applied to the recording material charging section, and a second mode in which the transfer section transfers a toner image onto a recording material that has passed through the recording material charging section in a state in which the recording material charging bias is not applied to the recording material charging section; a current suppression mechanism that suppresses current from flowing from the transfer unit to the first member and the second member via a recording material when image formation is performed in the second mode; An image forming apparatus comprising:

2. The image forming apparatus described in claim 1, characterized in that the current suppression mechanism has a switching unit that switches the state of the second member between a first state in which the second member is connected to ground and a second state in which the second member is disconnected from ground, and that switches the second member to the first state when image formation is performed in the first mode and switches the second member to the second state when image formation is performed in the second mode.

3. The image forming apparatus according to claim 1, characterized in that the current suppression mechanism has a switching unit that switches the state of the second member between a first state in which the second member is connected to ground without a current suppression element and a second state in which the second member is connected to ground via the current suppression element, and that switches the second member to the first state when image formation is performed in the first mode and switches the second member to the second state when image formation is performed in the second mode.

4. 4. The image forming apparatus according to claim 3, wherein the current suppressing element is a resistive element.

5. 4. The image forming apparatus according to claim 3, wherein the current suppressing element is a varistor.

6. The image forming apparatus according to claim 1, characterized in that the current suppression mechanism is a variable resistance mechanism provided between the second member and ground, which is set to a first state when image formation is performed in the first mode, and is set to a second state having a higher electrical resistance value than the first state when image formation is performed in the second mode.

7. 2. The image forming apparatus according to claim 1, wherein the current suppression mechanism is a rectifying element provided between the second member and ground, and the rectifying element is provided to suppress the current flowing from the transfer unit via the recording material to ground via the second member.

8. The image forming apparatus according to any one of claims 1 to 7, characterized in that the current suppression mechanism has a current control unit that controls the current flowing to the second application unit to 0 μA or a current with an absolute value of 10 μA or less when image formation is performed in the second mode.

9. The image forming apparatus according to any one of claims 1 to 7, characterized in that the current suppression mechanism has a current control unit that controls the current so that, when image formation is performed in the second mode, a current of opposite polarity to the current flowing from the transfer unit to the first member via the recording material does not flow from the second application unit to the first member.

10. 2. The image forming apparatus according to claim 1, wherein the first member is disposed on the side of the recording material opposite to the toner image transfer surface, and the second member is disposed on the toner image transfer surface side of the recording material.

11. 2. The image forming apparatus according to claim 1, wherein the first member is disposed on the toner image transfer surface side of the recording material, and the second member is disposed on the opposite side of the recording material from the toner image transfer surface.

12. 2. The image forming apparatus according to claim 1, wherein the image carrier is an intermediate transfer member onto which a toner image is transferred from another image carrier.

13. an acquisition unit that acquires information about the type of recording material; 2. The image forming apparatus according to claim 1, wherein the control unit controls to change whether to perform image formation in the first mode or the second mode based on the information acquired by the acquisition unit.

14. The image forming apparatus according to claim 13, characterized in that the control unit controls to perform image formation in the first mode when the type of recording material indicated by the information acquired by the acquisition unit is a first type, and to perform image formation in the second mode when the type of recording material indicated by the information acquired by the acquisition unit is a second type having a lower electrical resistance than the first type.

15. A detection unit that detects a value related to the humidity of the environment, 2. The image forming apparatus according to claim 1, wherein the control unit controls to change whether to perform image formation in the first mode or the second mode based on the value detected by the detection unit.

16. The image forming apparatus according to claim 15, wherein the control unit controls the image forming apparatus to perform image formation in the first mode when the humidity indicated by the value detected by the detection unit is a first humidity, and to perform image formation in the second mode when the humidity indicated by the value detected by the detection unit is a second humidity higher than the first humidity.

Images