Image forming apparatus

Abstract

To prevent a situation where, in a configuration in which pre-electrification is performed on a belt by using a belt-type transfer member, switching the settings of the pre-electrification makes it difficult to appropriately set a transfer bias.SOLUTION: An image forming apparatus has: a transfer belt; a transfer bias application unit; a first member and a second member that are arranged on an inner peripheral surface side and an outer peripheral surface side of the transfer belt, respectively, and sandwich the transfer belt to form an electrification unit that electrifies a transfer surface of a recording material to have a polarity opposite to the normal polarity of toner; an electrification bias application unit; a detection unit that detects a current flowing in a transfer part or a voltage applied to the transfer part; and a control unit that sets a transfer bias on the basis of a result of detection performed by the detection unit. The control unit sets the transfer bias in transferring a toner image to the recording material having the transfer surface electrified by the electrification unit, on the basis of the result of detection performed by the detection unit when an area of the transfer belt passing through the electrification unit while an electrification bias is applied to the electrification unit passes through the transfer part immediately thereafter.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a copying machine, printer, plotter, facsimile machine, or multifunction machine having two or more of the functions of these machines, 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 photosensitive member or an 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 applying a transfer bias to the transfer section. Also known in high-speed machines, for example, is a belt-type transfer member that transports the recording material to the transfer section using a belt stretched around multiple rollers, including a transfer roller.

[0003] In such image forming devices, if the transfer current supplied to the transfer section by the transfer bias is insufficient, image defects such as "transfer gaps" or "low density" may occur, where transfer is not performed sufficiently and the desired image density is not achieved. Furthermore, if the transfer current supplied to the transfer section by the transfer bias is excessive, discharge may occur in the transfer section, and the polarity of the charge of the toner in the toner image may be reversed due to the effects of this discharge, resulting in image defects such as "white gaps" where the toner image is not partially transferred. Therefore, in order to form high-quality images, it is necessary to apply an appropriate transfer bias to the transfer section.

[0004] Therefore, one method is to apply a test bias to the transfer section when there is no recording material in the transfer section, obtain the voltage-current characteristics, determine the voltage value that will obtain a specified target current, and add the recording material share voltage according to the type of recording material to this voltage value to set the voltage value of the transfer bias.

[0005] However, there are cases where high-resistivity recording materials, such as synthetic paper, which has a higher volume resistivity than plain paper or coated paper, are used as recording materials. When such high-resistivity recording materials are used, insufficient high voltage (insufficient transfer current) at the transfer section becomes an issue.

[0006] Patent Document 1 proposes that the surface of the recording material onto which the toner image is transferred (the "toner image transfer surface") be pre-charged to a polarity opposite to the normal charging polarity of the toner before the recording material reaches the transfer section. In particular, Patent Document 1 proposes that in a configuration using a belt-type transfer member, pre-charging is performed on the belt upstream of the transfer section. [Prior art documents] [Patent documents]

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

[0008] In the configuration described in Patent Document 1, if pre-charging is performed when a low-resistivity recording material with a low volume resistivity, such as thin paper, is used, the transfer bias (transfer current) becomes excessive at the transfer section. This can cause the polarity of the toner to reverse at the transfer section, resulting in the toner being re-transferred to the image carrier. Therefore, it is considered to switch whether or not to perform pre-charging depending on the type of recording material.

[0009] However, in a configuration using a belt-type transfer member in which pre-charging is performed on the belt, switching the pre-charging settings, such as turning the pre-charging output ON / OFF, causes the potential of the belt to fluctuate, making it difficult to apply an appropriate transfer bias at the transfer section.

[0010] Therefore, an object of the present invention is to prevent the difficulty of appropriately setting the transfer bias when switching the pre-charging setting in a configuration using a belt-type transfer member and performing pre-charging on the belt. [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 rotatable transfer belt that is an endless belt and contacts the image carrier to form a transfer section, and carries and transports a recording material to the transfer section, a first applying section that applies a transfer bias to the transfer section to transfer the toner image from the image carrier to the recording material, a first member that is arranged on the inner peripheral surface of the transfer belt upstream of the transfer section in the transport direction of the recording material and that forms a charging section that charges a transfer surface onto which the toner image of the recording material transported to the transfer section by the transfer belt is transferred, to a polarity opposite to the normal charging polarity of the toner, and a first member that is arranged on the outer peripheral surface of the transfer belt and that sandwiches the transfer belt between the first member and the first member. a second application unit that applies a charging bias to the charging unit to charge the transfer surface of the recording material to the opposite polarity; a detection unit that detects a current flowing through the transfer unit or a voltage applied to the transfer unit; and a control unit that sets the transfer bias based on the detection result by the detection unit, wherein the control unit sets the transfer bias when a toner image is transferred to the recording material whose transfer surface has been charged to the opposite polarity by the charging unit based on the detection result by the detection unit when a region of the transfer belt that passed through the charging unit while the charging bias is being applied to the charging unit passes through the transfer unit immediately afterwards. [Effects of the Invention]

[0012] According to the present invention, in a configuration in which a belt-type transfer member is used and pre-charging is performed on the belt, it is possible to prevent the difficulty of appropriately setting the transfer bias due to switching of the pre-charging setting. [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. [Figure 5] FIG. 1 is a flowchart showing an outline of the ATVC control procedure. [Figure 6] FIG. 10 is a graph showing voltage-current characteristics acquired under ATVC control. [Figure 7] FIG. 10 is a schematic diagram showing an example of a table of recording material distribution voltages. [Figure 8] FIG. 2 is a flowchart showing an outline of the control procedure of the first embodiment. [Figure 9(a)] FIG. 10 is a flowchart showing an outline of the control procedure of the second embodiment. [Figure 9(b)] FIG. 10 is a flowchart showing an outline of the control procedure of the second embodiment. [Figure 10(a)] FIG. 10 is a flowchart showing an outline of the control procedure of the third embodiment. [Figure 10(b)] FIG. 10 is a flowchart showing an outline of the control procedure of the third embodiment. [Figure 11] FIG. 11 is a graph showing voltage-current characteristics obtained in the pre-multiple rotation process in Example 3. [Figure 12(a)] FIG. 10 is a flowchart showing an outline of the control procedure of the fourth embodiment. [Figure 12(b)] FIG. 10 is a flowchart showing an outline of the control procedure of the fourth embodiment. [Figure 13] FIG. 10 is an explanatory diagram showing control timings in the fourth embodiment. [Figure 14] 10 is a schematic cross-sectional view of the vicinity of the secondary transfer portion, illustrating another example of a voltage application configuration. 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 1 is a schematic cross-sectional view of an image forming apparatus 100 according to this embodiment. The image forming apparatus 100 according to this embodiment is a tandem full-color printer that employs an intermediate transfer method and is capable of forming full-color images using an electrophotographic method.

[0016] 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 200 (FIG. 3) or image information input via an operation unit 130 (or an image reading device) provided in the image forming apparatus 100. Examples of the external device 200 include a host device such as a personal computer, a digital camera, and a smartphone.

[0017] In the image forming apparatus 100, paper is mainly used as the recording material S, and therefore 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 be any material on which a toner image can be formed, and specific examples include plain paper, synthetic resin sheets that are substitutes for plain paper, cardboard, and overhead projector sheets. In this way, 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-deposited paper with a metal layer.

[0018] 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.

[0019] 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 source 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.

[0020] 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 source 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.

[0021] 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.

[0022] The toner image formed on the photosensitive drum 1 is transferred (primary transfer) onto the rotating intermediate transfer belt 70 at the primary transfer portion N1 by the action of the primary transfer roller 5. During the primary transfer, a primary transfer bias (primary transfer voltage), which is a DC voltage of the opposite polarity (positive in this embodiment) to the normal charging polarity of the toner, is applied to the primary transfer roller 5 by a primary transfer power supply E3 (FIG. 3) serving as a primary transfer voltage application means (primary transfer voltage application portion). By applying the positive primary transfer bias to the primary transfer roller 5, the toner image made of negative 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 transferred sequentially onto the intermediate transfer belt 70 so as to be superimposed on top of each other, forming a multiple toner image on the intermediate transfer belt 70.

[0023] 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.

[0024] 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 14 Ω·cm (23°C, 50% RH). Although the intermediate transfer belt 70 has a three-layer structure in this embodiment, 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.

[0025] A secondary transfer unit 8 is disposed on the outer peripheral surface of the intermediate transfer belt 70, facing the inner secondary transfer roller 71. The secondary transfer unit 8 includes a secondary transfer belt 80 formed as an endless belt, and an outer secondary transfer roller 81 disposed on the inner peripheral surface of the secondary transfer belt 80, facing the inner secondary transfer roller 71. The outer secondary transfer roller 81 is pressed against the inner secondary transfer roller 71 and contacts the inner secondary transfer roller 71 via the secondary transfer belt 80 and the intermediate transfer belt 70. This allows the outer secondary transfer roller 81 to form a secondary transfer portion (secondary transfer nip portion) N2, which is the contact portion between the intermediate transfer belt 70 and the secondary transfer belt 80. The inner secondary transfer roller 71 and the outer secondary transfer roller 81 are an example of a roller-type secondary transfer member serving as a 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, during secondary transfer, a secondary transfer bias (secondary transfer voltage), which is a DC voltage of the same polarity (negative in this embodiment) as the normal charging polarity of the toner, is applied by a secondary transfer power source E4 serving as a secondary transfer voltage application means (secondary transfer voltage application unit) to the inner secondary transfer roller 71. Also, in this embodiment, the outer secondary transfer roller 81 is connected to ground (ground potential) (electrically grounded).

[0026] Here, the toner image on the intermediate transfer belt 70 is secondarily transferred onto the recording material S at the secondary transfer portion N2, and the recording material S is attracted to the secondary transfer belt 80 by the supplied electrostatic force. For example, a current of 40 to 60 μA is passed through the secondary transfer portion N2. The inner secondary transfer roller 71 may be, for example, a roller made of an elastic layer of ion-conductive foam rubber (NBR rubber) and a core metal, with an outer diameter of 24 mm and a roller surface roughness Rz of 6.0 to 12.0 (μm). The inner secondary transfer roller 71 may also be, for example, a roller having a resistance measured at N / N (23°C, 50% RH) and 1×10 when 2 kV is applied. 5 ~1×10 7A roller having a hardness of Ω and an Asker-C hardness of the elastic layer of about 30 to 40 can be used. A secondary transfer power supply E4 that supplies a variable bias is connected to the inner secondary transfer roller 71. Details of the secondary transfer unit 8 will be described later.

[0027] Recording material (transfer material, recording medium, paper, sheet) S is stored in cassettes 11a and 11b, which are recording material storage units that function as feeding units. 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 conveying members. The recording material S is conveyed by the pair of registration rollers 14 toward the secondary transfer unit 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 conveying drive motor D4 (FIG. 3), which serves as a driving means. In this embodiment, the image forming apparatus 100 is also provided with a pre-charging device 9 that pre-charges (pre-charges) the surface of the recording material S, onto which the toner image is transferred, to a polarity opposite to the normal charging polarity of the toner before the recording material S reaches the secondary transfer unit N2. The pre-charging device 9 is provided upstream of the secondary transfer portion N2 (downstream of the pair of registration rollers 14) in the conveying direction of the recording material S. The pre-charging device 9 will be described in detail later.

[0028] 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.

[0029] 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 so that the toner image can be transferred 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 conveyed 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.

[0030] 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.

[0031] 2. Control Configuration 3 is a block diagram showing an outline of the control configuration of image forming apparatus 100 in this embodiment. Image forming apparatus 100 has a control unit (control circuit) 120 that controls image forming apparatus 100. Control unit 120 is configured to have a CPU 121 as an arithmetic processing means (arithmetic processing unit), a memory (storage medium) 122 such as ROM or RAM as a storage means (storage unit), and an input / output unit (not shown) for inputting and outputting information to and from devices external to control unit 120. CPU 121 and memory 122 are capable of transferring and reading data from and to each other. Control programs, pre-determined data tables, etc. are stored in ROM. Information input to control unit 120, detected information, calculation results, etc. are stored in RAM, which is a rewritable memory.

[0032] The control unit 120 is connected to each unit of the image forming apparatus 100. The control unit 120 controls the operation of each unit of the image forming apparatus 100, and causes the image forming apparatus 100 to perform various operations such as an image forming operation.

[0033] 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, a pre-charging power source E5 (described later), a first cleaning power source E6, a second cleaning power source E7, etc. are connected to the control unit 120. Also connected to the control unit 120 are various drive units such as a drum drive motor D1, an intermediate transfer belt drive motor D2, and a secondary transfer belt drive motor D3 (described later).

[0034] 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 of the environment (the installation environment of the image forming apparatus 100) inside or outside 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). That is, the environmental sensor 18 has a temperature sensor and a humidity sensor. 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.

[0035] The secondary transfer power supply E4 is connected to a voltage detection sensor 25a as a voltage detection means (voltage detection unit) that detects the output voltage of the secondary transfer power supply E4, and a current detection sensor 25b as a current detection means (current detection unit) that detects the output current of the secondary transfer power supply E4. The voltage detection sensor 25a can detect the voltage applied to the inner secondary transfer roller 71 (secondary transfer unit N2). The current detection sensor 25b can detect the current flowing through the inner secondary transfer roller 71 (secondary transfer unit N2). The voltage detection sensor 25a and the current detection sensor 25b input signals indicating the detection results of the voltage and current, respectively, to the control unit 120. The control unit 120 can perform secondary transfer voltage control (ATVC control), which will be described later, based on the detection results of the voltage detection sensor 25a and the current detection sensor 25b.

[0036] Furthermore, the control unit 120 can recognize the timing at which an area that was at a pre-charging portion N3 (described later) on the secondary transfer belt 80 reaches the secondary transfer portion N2 when a pre-charging bias (described later) on the secondary transfer belt 80 is turned on or off. This can be determined from the timing at which the pre-charging bias is turned on or off, the driving speed of the secondary transfer belt 80, the distance from the pre-charging portion N3 to the secondary transfer portion N2, etc. Information regarding this timing may be determined in advance and stored in the memory 122, or may be incorporated into the control program.

[0037] 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 have 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 with a touch panel or the like that has the functions of a display unit and an input unit. The control unit 120 may also be connected to an external device 200, such as an image reading device (not shown) provided in or connected to the image forming apparatus 100, or a personal computer connected to the image forming apparatus 100.

[0038] 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.

[0039] Here, the image forming apparatus 100 executes a job (image output operation, print job) that 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-to-sheet process when forming images on multiple recording materials S, and a post-rotation process. The image formation process is a period during which electrostatic image formation, toner image formation, primary transfer of the toner image, and secondary transfer of the toner image are performed for the image that is actually formed and output on the recording materials S. This is referred to as the image formation period. More specifically, the timing of the image formation process differs depending on the positions where the electrostatic image formation, toner image formation, primary transfer of the toner image, and secondary transfer are performed. The pre-rotation process is a period from when a start command is input until the actual start of image formation, during which preparatory operations are performed before the image formation process. The sheet-to-sheet 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 a tidying up operation (preparatory operation) is performed after the image formation process. Non-image formation time (non-image formation period) refers to a period other than image formation time, 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.

[0040] In this embodiment, the control unit 120 has, as its functional blocks, an image formation preparation processor, an ATVC control processor, an image formation processor, etc. The control unit 120 also has, as its functional blocks, a primary transfer voltage storage / calculation unit, a secondary transfer voltage storage / calculation unit, etc. These processing units and storage / calculation units may be provided as part of the CPU 121 or memory 122 (RAM). For example, the control unit 120 (more specifically, the image formation processor) can execute jobs as described above. The control unit 120 (more specifically, the ATVC control processor) can execute ATVC control of the secondary transfer unit (and the primary transfer unit), which will be described later.

[0041] 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.

[0042] The secondary transfer unit 8 has 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 has, as tension rollers arranged on the inner circumferential surface side of the secondary transfer belt 80, an outer secondary transfer roller 81, a separation roller 82, a tension roller 83, and a drive roller (secondary transfer belt drive roller) 84. In this embodiment, the secondary transfer unit 8 also has, as tension rollers arranged on the inner circumferential surface side of the secondary transfer belt 80, first and second cleaning opposing rollers 85 and 86. The rotational axes of the outer secondary transfer 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.

[0043] The secondary transfer belt 80 is made of a resin such as polyimide or polycarbonate containing an appropriate amount of carbon black as an antistatic agent, and has a volume resistivity of 1×10 9 ~1×10 14The secondary transfer belt 80 may be formed from a resin material adjusted to Ω·cm (23°C, 50% RH). The secondary transfer belt 80 may be sufficiently hard, for example, with a Young's modulus of 100 MPa or more and 10 GPa or less, as measured by a tensile test method (JIS K 6301). 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 peripheral length of the secondary transfer belt 80 is, for example, about 300 mm.

[0044] 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 outer secondary transfer roller 81 has a core metal and an elastic layer of ion-conductive foam rubber (NBR rubber and ECO rubber) formed so as to cover the outer periphery of the core metal. The outer diameter of the outer secondary transfer 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. The outer secondary transfer 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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) of the same polarity (negative in this embodiment) as the normal charging polarity of the toner is applied to the first brush roller 87 from a first cleaning power source E6. A cleaning bias (cleaning voltage) of the opposite polarity (positive in this embodiment) to the normal charging polarity of the toner 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 a polarity opposite to 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 same polarity as 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.

[0049] 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.

[0050] The internal power supply system improves the transferability of toner images to, for example, a recording material S with low electrical resistance (low-resistance recording material), 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 transfer current leaks through the recording material S to a member near the secondary transfer unit N2, the transfer current escapes to the member 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 transfer current leaks through the recording material S to a member near the secondary transfer unit N2, the transfer current escapes to the member 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, for example, a recording material S with low electrical resistance, compared to the external power supply system.

[0051] In this embodiment, the secondary transfer bias is applied using constant voltage control. The voltage value Vtr (set voltage, target voltage) of the secondary transfer bias is determined by adding a base voltage Vb for obtaining a predetermined transfer current and a recording material distribution voltage Vp, which is determined depending on the type of recording material S. The recording material distribution voltage Vp is preset according to the type of recording material S and the environment (e.g., absolute moisture content) and is stored in memory 122 as table data. The base voltage Vb is determined, for example, by control called secondary transfer voltage control or ATVC (Active Transfer Voltage Control). ATVC control is typically performed during the pre-rotation process or pre-multi-rotation process for each job, but can also be performed at any timing (such as the sheet-to-sheet process) during non-image formation when there is no toner image or recording material S at the secondary transfer unit N2. Details of ATVC control will be described later.

[0052] 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).

[0053] 4. Pre-charging device Next, the pre-charging device (recording material charging device) 9 in this embodiment will be further described.

[0054] As mentioned above, in image forming apparatuses, depending on the specifications of the recording material, image degradation such as transfer defects and low density due to insufficient transfer current may occur. Recently, there has been a trend toward an increasing number of recording materials used for image formation, for example, in production machines using intermediate transfer methods. For example, in production machines with high image formation speeds, it can be difficult to properly perform secondary transfer of a toner image onto recording materials such as ultra-thick paper (high-resistance paper) with high electrical resistance or synthetic paper (high-resistance recording material) with a resin layer that has high electrical resistance without reducing productivity.

[0055] For example, in a low-humidity environment, the electrical resistance of the outer secondary transfer roller increases, necessitating a higher absolute value for the secondary transfer bias voltage to ensure the required 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 current may be insufficient, resulting in transfer defects and low density. These transfer defects and low density may occur, for example, in secondary-color toner images. Furthermore, high-voltage power supplies capable of applying such secondary transfer bias are expensive, potentially increasing the cost of image forming devices. Even if such a high-voltage power supply is used, it may not be possible to achieve sufficient creepage due to its placement, making it impossible to apply a secondary transfer bias with a high absolute value, as described above. Furthermore, increasing the absolute value of the secondary transfer bias voltage may result in image defects due to discharge at the secondary transfer section N2, making it difficult to obtain a proper image. Image defects due to discharge include streaky image defects, white flowers, or punch-through defects caused by parts of the toner image not being transferred or parts of the toner image being distorted (scattered). Thus, for recording material S (high-resistivity recording material), such as synthetic paper with a resin layer or ultra-thick paper, which has a higher volume resistivity than plain paper, it is difficult to sufficiently transfer toner to recording material S using only the secondary transfer bias at secondary transfer unit N2, especially in low-humidity environments. This can result in transfer failure. Furthermore, if the absolute value of the secondary transfer bias voltage needs to be 10 kV or more, taking into account factors such as the creepage distance near secondary transfer unit N2, increasing the output of secondary transfer power supply E4 will result in an increase in the size of the image forming apparatus.

[0056] 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 charged in advance to a polarity opposite to the normal charging polarity of the toner before the recording material S reaches the secondary transfer portion N2. The surface of the recording material S onto which the toner image is transferred is also called the "toner image transfer surface" or "transfer surface." This makes it possible to properly transfer the toner image onto the recording material S by compensating for the insufficient transfer current, even if the absolute value of the voltage of the secondary transfer bias is relatively small.

[0057] As shown in FIG. 4, in this embodiment, the image forming apparatus 100 is provided with a pre-charging device 9 that 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 before the recording material S reaches the secondary transfer portion N2. 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" or "recording material charging (or simply "charging")." This 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. This improves the transferability of the toner image to extra-thick paper and synthetic paper.

[0058] In this embodiment, the pre-charging device 9 includes a drive roller (secondary transfer belt drive roller) 84 disposed on the inner circumferential surface side of the secondary transfer belt 80 and a pre-charging opposing roller 91 disposed opposite the drive roller 84 across the secondary transfer belt 80. The drive roller (pre-charging roller, recording material charging roller) 84 in this embodiment is an example of a recording material charging member (pre-charging member). The drive roller 84 is a tension roller for the secondary transfer belt 80 and functions both as a drive roller that drives the secondary transfer belt 80 and as a recording material 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 opposing roller 91 forms a desired nip with the drive roller 84 to sandwich the recording material S. In other words, the drive roller 84 abuts against the pre-charging opposing 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 1 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).

[0059] 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 (NBR rubber and ECO 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 more 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 more preferably 30 mm or less. This prevents the attenuation of the charge on the surface of the recording material S onto which the toner image is transferred during the time the recording material S is conveyed to the secondary transfer section N2, even if the surface onto which the toner image is transferred is charged at the pre-charging section N3.

[0060] In this embodiment, a pre-charging power supply E5 serving as a pre-charging voltage application means (pre-charging voltage application unit) is connected to the core of the drive roller 84. The pre-charging power supply E5 applies a pre-charging bias (recording material charging bias, pre-charging voltage) having the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment) to the drive roller 84. In this embodiment, the core of the pre-charging opposing roller 91 is connected to ground, and the pre-charging opposing roller 91 is electrically grounded. By applying a pre-charging bias having the same polarity as the normal charging polarity of the toner to the surface of the recording material S opposite to the surface onto which the toner image is transferred, the surface of the recording material S opposite to the surface onto which the toner image is transferred is charged to the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment). As a result, the surface onto which the toner image is transferred of the recording material S is charged to the polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment) due to charges induced from the ground. At this time, an apparent current flows through the pre-charging unit N3. Details of the control of the pre-charging bias will be described later.

[0061] 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 80 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.

[0062] 5. Basic operation of ATVC control Next, the basic operation of the ATVC control of the secondary transfer unit N2 will be described. Generally, there are constant voltage control and constant current control for secondary transfer voltage control, but in this embodiment, constant voltage control is used. Figure 5 is a flowchart for explaining the basic operation of the ATVC control of the secondary transfer unit N2.

[0063] First, when the control unit 120 (pre-image formation preparation process unit) acquires job information from the operation unit 130 or the external device 200, it starts the job operation (S101). This job information includes image information specified by the operator and information about the recording material S ("recording material information"). This recording material information may include the size (width, length) of the recording material S on which the image is formed, information related to the thickness of the recording material S (thickness, basis weight, etc.), and information related to the surface properties of the recording material S, such as whether the recording material S is coated paper. In particular, in this embodiment, the recording material information includes information about the size of the recording material S and information about the category of the recording material S (so-called paper type category), such as "thin paper, plain paper, thick paper, synthetic paper, etc." The recording material information includes any information that can distinguish the recording material S, such as attributes based on general characteristics (so-called paper type categories) such as plain paper, high-quality paper, glossy paper, coated paper, embossed paper, thick paper, thin paper, and synthetic paper; numerical values ​​or numerical ranges for basis weight, thickness, size, and rigidity; or brand name (including manufacturer, product name, and product number). Each recording material S distinguished by the recording material information can be considered to constitute a type of recording material S. Furthermore, the recording material information may be included in print mode information that specifies the operational settings of the image forming apparatus 100, such as "plain paper mode" and "thick paper mode," or may be replaced by the print mode information. The control unit 120 (image formation preparation process unit) writes this job information to memory 122 (RAM) (S102).

[0064] Next, the control unit 120 (image formation preparation process unit) acquires environmental information detected by the environmental sensor 18 (S103). The memory 122 (ROM) stores information indicating the correlation between the environmental information and a target current (target current) Itarget for transferring the toner image on the intermediate transfer belt 70 onto the recording material S. The control unit 120 (secondary transfer voltage storage unit / calculation unit) calculates the target current Itarget corresponding to the environment from the information indicating the relationship between the environmental information and the target current Itarget based on the environmental information read in S103. The control unit 120 then writes this target current Itarget into the memory 122 (RAM or secondary transfer voltage storage unit / calculation unit) (S104). The reason the target current Itarget is changed in accordance with the environmental information is because the amount of charge on the toner varies depending on the environment. The information indicating the relationship between the environmental information and the target current Itarget is previously determined through experiments or the like.

[0065] Next, the control unit 120 (ATVC control processor) acquires information regarding the electrical resistance of the secondary transfer unit N2 ("electrical resistance information") through ATVC control before the toner image on the intermediate transfer belt 70 and the recording material S onto which the toner image is transferred reach the secondary transfer unit N2 (S105). That is, with the secondary transfer belt 80 and the intermediate transfer belt 70 in contact with each other, the secondary transfer power supply E4 supplies multiple levels of predetermined voltage (test bias) to the inner secondary transfer roller 71. The current value while the predetermined voltage is being supplied is detected by the current detection sensor 25b, and the relationship between voltage and current (voltage-current characteristics) as shown in FIG. 6 is acquired. Alternatively, multiple levels of predetermined current (test bias) may be supplied, and the voltage value while the predetermined current is being supplied may be detected by the voltage detection sensor 25a, to acquire the voltage-current characteristics. Both the predetermined voltage and the predetermined current may be used. The control unit 120 writes this voltage-current characteristics information to the memory 122 (RAM or secondary transfer voltage storage unit / calculation unit). This voltage-current characteristic is an example of electrical resistance information that changes depending on the electrical resistance of the secondary transfer portion N2. In the configuration of this embodiment, this voltage-current characteristic does not change linearly (proportional) with respect to the voltage, but rather changes so that the current changes in a manner that is expressed by a polynomial of the voltage that is quadratic or higher (in this embodiment, a quadratic expression). Therefore, in this embodiment, the predetermined voltage or current supplied when acquiring the electrical resistance information of the secondary transfer portion N2 is multi-staged, with three or more points, so that this voltage-current characteristic can be expressed by a polynomial.

[0066] Next, the control unit 120 (secondary transfer voltage storage unit / calculation unit) calculates the voltage value of the secondary transfer bias to be applied from the secondary transfer power supply E4 to the inner secondary transfer roller 71 during secondary transfer (S106). That is, the control unit 120 calculates a voltage value (base voltage) Vb required to apply the target current Itarget when no recording material S is present at the secondary transfer portion N2, based on the target current Itarget written to the memory 122 (RAM) in S104 and the voltage-current characteristics calculated in S105. This base voltage Vb corresponds to the secondary transfer partial voltage (transfer voltage corresponding to the electrical resistance of the secondary transfer portion N2). The memory 122 (ROM) also stores information for calculating the recording material partial voltage Vp (transfer voltage corresponding to the electrical resistance of the recording material S), as shown in FIG. 7 . In this embodiment, this information is set as table data indicating, for example, the relationship between the moisture content (absolute moisture content) of the atmosphere and the recording material partial voltage Vp for each basis weight of the recording material S according to the paper type category. The control unit 120 (image formation preparation process unit) can determine the moisture content (absolute moisture content) of the atmosphere based on environmental information (temperature and humidity) detected by the environmental sensor 18. The control unit 120 (secondary transfer voltage storage unit / calculation unit) determines the recording material distribution voltage Vp corresponding to the job information acquired in S101 and the environmental information acquired in S103 from the table data. The control unit 120 (secondary transfer voltage storage unit / calculation unit) then determines the secondary transfer bias voltage value (set voltage, target voltage) Vtr to be the sum of the base voltage Vb and the recording material distribution voltage Vp. The control unit 120 (secondary transfer voltage storage unit / calculation unit) stores the determined secondary transfer bias voltage value Vtr in the memory 122 (RAM or secondary transfer voltage storage unit / calculation unit). In addition, when an adjustment value is set using an adjustment mode that adjusts the set voltage of the secondary transfer bias, the control unit 120 (secondary transfer voltage memory unit / calculation unit) can adjust the set voltage of the secondary transfer voltage using an adjustment amount ΔV corresponding to that adjustment value.

[0067] The table data for calculating the recording material voltage Vp as shown in FIG. 7 has been obtained in advance through experiments or the like. Here, the recording material voltage Vp may change depending on the surface properties of the recording material S in addition to information related to the thickness of the recording material S (thickness, basis weight, etc.). Therefore, the table data may be set so that the recording material voltage Vp also changes depending on information related to the surface properties of the recording material S. In this embodiment, information related to the thickness of the recording material S (and further information related to the surface properties of the recording material S) is included in the job information acquired in S101. However, it is also possible to provide a measuring device in the image forming apparatus 100 that detects the thickness and surface properties of the recording material S, and to calculate the recording material voltage Vp based on information obtained by this measuring device.

[0068] Next, the control unit 120 (image forming process unit) executes image formation, sends the recording material S to the secondary transfer unit N2, and controls so that secondary transfer is performed using the voltage value Vtr of the secondary transfer bias determined as described above (S107). Thereafter, the control unit 120 (image forming process unit) repeats the process of S107 until all images of the job have been transferred onto the recording material S and output (S108).

[0069] Note that with regard to the primary transfer unit N1, ATVC control similar to the ATVC control of the secondary transfer unit N2 is also performed from the time the job is started until the toner image is transported to the primary transfer unit N1, but a detailed description thereof will be omitted here. Hereinafter, the ATVC control will be assumed to be the ATVC control of the secondary transfer unit N2.

[0070] 6. Control of pre-charging bias In this embodiment, the pre-charging bias is applied under constant voltage control. The pre-charging power supply E5 incorporates a voltage detection unit (not shown) as a voltage detection means, and can perform constant voltage control of the output voltage so that the voltage value detected by this voltage detection unit remains approximately constant. The appropriate target voltage of the pre-charging bias may vary depending on the type and environment of the recording material S, and even the print 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 in single-sided printing or double-sided printing, or the second side in double-sided printing). Therefore, whether or not to apply the pre-charging bias or the target voltage of the pre-charging bias can be changed based on at least one of the type, environment, and print surface of the recording material S. For example, the target voltage of the pre-charging bias may be preset and stored in memory 122 as table data, etc., so that the surface of the recording material S to which the toner image is transferred 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 of the surface of the recording material S onto which the toner image is transferred can be determined in advance by experimentation or the like as an appropriate charge amount that will provide appropriate transferability. Alternatively, for example, the pre-charging bias may be applied only when a specific type of recording material S is used, or only when a specific type of recording material S is used and the absolute moisture content is within a specific range (for example, less than a specific value). Table 1 shows an example of setting the target voltage of the pre-charging bias.

[0071] [Table 1]

[0072] For simplicity, in this embodiment, when a job is started, it is assumed that whether or not a pre-charging bias is applied to the drive roller 84 (whether or not pre-charging is required) is determined according to paper type category information as recording material information. As an example, in this embodiment, it is assumed that pre-charging is required if a paper type category of synthetic paper or a predetermined high resistance paper (such as extra-thick paper) is selected when a job is started. In this embodiment, it is assumed that pre-charging is not required if a paper type category other than synthetic paper or a predetermined high resistance paper (such as extra-thick paper) is selected.

[0073] In the configuration of this embodiment, if synthetic paper such as Yupo YPI200 (a product name of Yupo Corporation) is used as the recording material S, the secondary transfer bias alone will not provide sufficient transfer current in an environment with an absolute moisture content of, for example, 0.9 g / kg. Yupo YPI200 is a PP (polypropylene) paper type. Therefore, in this case, a pre-charging bias of -5500 V is applied to the drive roller 84. This allows the toner image transfer surface of the recording material S to be sufficiently charged. Also, in the configuration of this embodiment, if synthetic paper such as Laser Peach WETY-210 (a product name of Daio Paper Corporation) is used as the recording material S, the secondary transfer bias alone will not provide sufficient transfer current in an environment with an absolute moisture content of, for example, 0.9 g / kg. Laser Peach WETY-210 is a PE (polyethylene) paper type. Therefore, in this case, a pre-charging bias of -1000 V is applied to the drive roller 84.

[0074] However, as described above, in a configuration in which pre-charging is performed on the secondary transfer belt 80, when the pre-charging setting, such as turning the pre-charging output ON / OFF, is switched, the potential (surface potential, etc.) of the secondary transfer belt 80 fluctuates. This makes it difficult to apply an appropriate secondary transfer bias at the secondary transfer portion N2.

[0075] Therefore, in this embodiment, the image forming apparatus 100 is configured to perform ATVC control (acquire electrical resistance information of the secondary transfer portion N2) when a region of the secondary transfer belt 80 to which a pre-charging bias is applied passes through the secondary transfer portion N2 for a job that requires pre-charging. The time when the region of the secondary transfer belt 80 to which a pre-charging bias is applied passes through the secondary transfer portion N2 refers to the time when a region of the secondary transfer belt 80 in the rotation direction that has passed through the pre-charging portion N3 to which a pre-charging bias is applied passes through the secondary transfer portion N2 immediately after (first). In this case, it is preferable to perform ATVC control when a region of the secondary transfer belt 80 to which a pre-charging bias substantially the same as that applied during image formation (secondary transfer) for that job is applied passes through the secondary transfer portion N2. On the other hand, in this embodiment, the image forming apparatus 100 is configured to perform ATVC control when a region of the secondary transfer belt 80 to which a pre-charging bias is not applied passes through the secondary transfer portion N2 for a job that does not require pre-charging. The time when an area of ​​the secondary transfer belt 80 to which no pre-charging bias is applied passes through the secondary transfer portion N2 refers to the time when an area in the rotation direction of the secondary transfer belt 80 that has passed through the pre-charging portion N3 to which no pre-charging bias is applied passes through the secondary transfer portion N2 immediately after (first). The ATVC control when no pre-charging is performed is as described above as the basic operation of the ATVC control. Table 2 summarizes the possibility of image defects occurring at the secondary transfer portion N2 depending on whether or not pre-charging is performed.

[0076] [Table 2]

[0077] In the case of (1) in Table 2, pre-charging is not performed, so ATVC control is not necessary with the pre-charging bias applied. By performing ATVC control without applying the pre-charging bias, the secondary transfer bias can be appropriately set. On the other hand, in the case of (2) in Table 2, ATVC control is performed without applying the pre-charging bias when pre-charging is performed, resulting in an inappropriate secondary transfer bias setting, which may result in image defects at the secondary transfer portion N2. When the pre-charging bias is applied, the potential of the secondary transfer belt 80 fluctuates compared to when it is not applied, which may prevent the application of an appropriate secondary transfer bias, resulting in image defects at the secondary transfer portion N2. In the case of (3) in Table 2, according to this embodiment, ATVC control is performed with the pre-charging bias applied when pre-charging is performed, so the secondary transfer bias can be appropriately set. This is because the ATVC control detects a current (or voltage) corresponding to the potential of the secondary transfer belt 80 depending on whether or not pre-charging is performed. Therefore, by performing ATVC control with pre-charging turned ON or OFF depending on whether pre-charging is present during image formation (secondary transfer) of the job, the secondary transfer bias during image formation (secondary transfer) of the job can be appropriately set.

[0078] When a cleaning bias is applied to the first and second brush rollers 87 and 88 during image formation (secondary transfer) of a job, it is preferable to perform ATVC control when the area of ​​the secondary transfer belt 80 to which the cleaning bias is applied passes through the secondary transfer section N2. The time when the area of ​​the secondary transfer belt 80 to which the cleaning bias is applied passes through the secondary transfer section N2 refers to the time when the area in the rotation direction of the secondary transfer belt 80 that has passed through the contact area with the first and second brush rollers 87 and 88 to which the cleaning bias is applied passes through the secondary transfer section N2 immediately (first). In this case, it is also preferable to perform ATVC control when the area of ​​the secondary transfer belt 80 to which a cleaning bias with substantially the same setting as that during image formation (secondary transfer) of the job is applied passes through the secondary transfer section N2. On the other hand, when a cleaning bias is not applied to the first and second brush rollers 87 and 88 during image formation (secondary transfer) of a job, it is preferable to perform ATVC control when the area of ​​the secondary transfer belt 80 to which the cleaning bias is not applied passes through the secondary transfer section N2. When an area of ​​the secondary transfer belt 80 to which no cleaning bias is applied passes through the secondary transfer section N2, it means when an area in the rotation direction of the secondary transfer belt 80 that has passed through the contact area with the first and second brush rollers 87 and 88 to which no cleaning bias is applied passes through the secondary transfer section N2 immediately after (first).

[0079] 7. Control Procedure Next, a job control procedure including ATVC control in this embodiment will be described. FIG. 8 is a flowchart showing an outline of the control procedure. For simplicity, it is assumed that the same type of recording material S is used in one job, and that image formation is performed in single-sided printing mode. Also, as described above, it is assumed that the need for pre-charging is determined according to paper type category information as recording material information. Furthermore, the operation of the ATVC control itself may be the same as the basic operation of the ATVC control described above, and therefore a description thereof will be omitted where appropriate.

[0080] First, the control unit 120 acquires job information, including information specifying the type of recording material S and image information, input by the user from the operation unit 130 or an external device 200 such as a personal computer (S201). The control unit 120 then acquires paper type category information (S202). The information specifying the type of recording material S may be information specifying one of the cassettes 11a, 11b containing the recording material S (the same applies below). In this case, the control unit 120 can determine 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. As described above, for simplicity, a job in single-sided printing mode is used as an example. However, in addition to the paper type category information, print side information can also be acquired in S201, and control can be performed according to the print side, as described above (the same applies below). The control unit 120 also acquires environmental information (S203) and determines the target current Itarget based on the information acquired in S202 and S203 (S204).

[0081] Next, the control unit 120 determines whether pre-charging is required for the job based on the paper type category information acquired in S202 (S205). If the control unit 120 determines that pre-charging is required in S205, it starts applying a pre-charging bias to the drive roller 84 (S206). At this time, the control unit 120 sets the same pre-charging bias as that used during image formation (pre-charging) for the job according to the paper type category of the job based on table data such as that shown in Table 1. The control unit 120 then rotates the secondary transfer belt 80, and performs ATVC control using multiple levels of test bias after the area of ​​the secondary transfer belt 80 to which the pre-charging bias has been applied reaches the secondary transfer portion N2. As a result, the control unit 120 acquires electrical resistance information (voltage-current characteristics) of the secondary transfer portion N2 and calculates the base voltage Vb (S207). At this time, in this embodiment, ATVC control is performed with a bias applied to the inner secondary transfer roller 71, the first and second brush rollers 87 and 88, and also with a bias applied to the drive roller 84. The control unit 120 then determines a voltage value Vtr of the secondary transfer bias during image formation (secondary transfer) from the base voltage Vb calculated in S207 and the recording material distribution voltage Vp acquired according to the paper type category information (S208). The control unit 120 then forms images for the job using the determined secondary transfer bias (S209). When the formation of all images for the job is completed (S210), the control unit 120 ends the operation of the job.

[0082] Furthermore, if the control unit 120 determines in S205 that pre-charging is not required, it performs ATVC control without starting application of the pre-charging bias to the drive roller 84 (S211), and proceeds to the process of S208.

[0083] In this way, in this embodiment, when setting the secondary transfer bias for forming an image on the recording material S that requires pre-charging, the ATVC control is performed with the pre-charging bias applied, thereby making it possible to set the secondary transfer bias according to the potential fluctuation of the secondary transfer belt 80 due to pre-charging.

[0084] As described above, in this embodiment, the image forming apparatus 100 includes an image carrier (intermediate transfer belt) 70 that carries a toner image, a rotatable transfer belt (secondary transfer belt) 80 that is configured as an endless belt and contacts the image carrier 70 to form a transfer section (secondary transfer section) N2, and carries and transports a recording material S to the transfer section N2, a first applying section (secondary transfer power source) E4 that applies a transfer bias to the transfer section N2 to transfer the toner image from the image carrier 70 to the recording material S, a first member (drive roller, pre-charging roller) 84 that is disposed on the inner peripheral surface of the transfer belt 80 upstream of the transfer section N2 in the transport direction of the recording material S and that forms a charging section (pre-charging section) N3 that charges a transfer surface, onto which a toner image of the recording material S transported to the transfer section N2 by the transfer belt 80 is transferred, to a polarity opposite to the normal charging polarity of the toner, and the transfer belt The control unit 120 includes a second member (pre-charging opposing roller) 91 that is arranged on the outer surface side of the transfer belt 80 and sandwiches the transfer belt 80 between itself and the first member 84 to form a charging unit N3, a second application unit (pre-charging power supply) E5 that applies a charging bias to the charging unit N3 to charge the transfer surface of the recording material S to the above-mentioned opposite polarity, a detection unit (current detection sensor in this embodiment) 25b that detects the current flowing through the transfer unit N2 or the voltage applied to the transfer unit N2, and a control unit 120 that sets the transfer bias based on the detection result by the detection unit 25b, and the control unit 120 sets the transfer bias when transferring a toner image to the recording material S whose transfer surface has been charged to the above-mentioned opposite polarity by the charging unit N3 based on the detection result by the detection unit 25b when the area of ​​the transfer belt 80 that passed through the charging unit N3 while the charging bias was being applied to the charging unit N3 passes through the transfer unit N2 immediately afterwards. In this embodiment, the control unit 120 sets the transfer bias when transferring a toner image to a recording material S whose transfer surface is not charged to the opposite polarity by the charging unit N3 based on the detection result by the detection unit 25b when an area of ​​the transfer belt that passed through the charging unit N3 when no charging bias was applied to the charging unit N3 passes through the transfer unit N2 immediately afterwards. Also, in this embodiment, the second application unit E5 applies a charging bias of the same polarity as the normal charging polarity of the toner to the first member. However, as described below, the second application unit E5 can also apply the charging bias of the opposite polarity to the normal charging polarity of the toner to the second member.In this embodiment, the image carrier 70 is an intermediate transfer body onto which a toner image is transferred from another image carrier (photosensitive drum) 1. Based on information about the recording material S, the control unit 120 sets whether or not to charge the transfer surface of the recording material S to the opposite polarity at the charging unit N3. Based on information about the recording material, the control unit 120 changes the charging bias when charging the transfer surface of the recording material S to the opposite polarity at the charging unit N3.

[0085] Furthermore, according to this embodiment, in a configuration in which a belt-type transfer member is used and pre-charging is performed on the belt, it is possible to prevent the difficulty of appropriately setting the transfer bias due to switching the pre-charging setting.

[0086] [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.

[0087] 1. Overview of this Example In the first embodiment, an example of a control procedure for executing one job has been described, but in the present embodiment, an example of a control procedure for executing successive jobs will be described.

[0088] 2. Control Procedure A job control procedure including ATVC control in this embodiment will be described. FIG. 9 is a flowchart outlining the control procedure. Here, an example of two consecutive jobs, Job 1 and Job 2, will be described. Job 1 is the first job, and Job 2 is the second job. For simplicity, it is assumed that the same type of recording material S is used in one job, and image formation is performed in single-sided printing mode. As in the first embodiment, the necessity of pre-charging is determined based on paper type category information as recording material information. The operation of the ATVC control itself may be the same as the basic operation of the ATVC control described in the first embodiment, and therefore a description thereof will be omitted. For convenience, FIG. 9 is divided into FIG. 9(a) and FIG. 9(b), and the procedures in each figure are connected at "A" and "B" in each figure.

[0089] First, the control unit 120 acquires information on consecutive jobs, Job 1 and Job 2, input by the user from the operation unit 130 or external device 200, such as a personal computer (S301), and then acquires paper type category information for each of Job 1 and Job 2 (S302).The control unit 120 also acquires environmental information (S303) and determines the target current Itarget for each of Job 1 and Job 2 based on the information acquired in S302 and S303 (S304).

[0090] Next, the control unit 120 determines whether pre-charging is required for job 1 based on the paper type category information acquired in S302 (S305). If the control unit 120 determines in S305 that pre-charging is required for job 1, it starts applying a pre-charging bias for job 1 to the drive roller 84 (S306). At this time, the control unit 120 sets the same pre-charging bias as that used during image formation (pre-charging) for job 1 according to the paper type category for job 1 based on table data such as that shown in Table 1 described in the first embodiment. Then, the control unit 120 rotates the secondary transfer belt 80, and performs ATVC control using multiple levels of test bias after the area on the secondary transfer belt 80 to which the pre-charging bias for job 1 has been applied reaches the secondary transfer portion N2. As a result, the control unit 120 acquires electrical resistance information (voltage-current characteristics) of the secondary transfer portion N2 and calculates the base voltage Vb (S307). The control unit 120 also determines a voltage value Vtr1 of the secondary transfer bias during image formation (secondary transfer) of job 1 from the base voltage Vb calculated in S307 and the recording material distribution voltage Vp acquired according to the paper type category information of job 1 (S308). The control unit 120 then performs image formation of job 1 using the determined secondary transfer bias for job 1 (S309). After completing the formation of all images of job 1 (S310), the control unit 120 determines whether the paper type categories of job 1 and job 2 are different (S311). If the control unit 120 determines in S311 that the paper type categories are different, it determines whether pre-charging is required for job 2 (S312). If the control unit 120 determines in S312 that pre-charging is required for job 2, it starts applying a pre-charging bias for job 2 to the drive roller 84 (S313). At this time, the control unit 120 sets the same pre-charging bias as that used during image formation (pre-charging) for job 2 according to the paper type category for job 2, based on table data such as that shown in Table 1 described in embodiment 1. Then, the control unit 120 performs ATVC control using multiple levels of test bias after the timing at which the area on the secondary transfer belt 80 to which the pre-charging bias for job 2 has been applied reaches the secondary transfer unit N2.As a result, the control unit 120 acquires electrical resistance information (voltage-current characteristics) of the secondary transfer unit N2 and determines the base voltage Vb (S314). The control unit 120 also determines the voltage value Vtr2 of the secondary transfer bias during image formation (secondary transfer) of job 2 from the base voltage Vb determined in S314 and the recording material distribution voltage Vp acquired according to the paper type category information of job 2 (S315). Thereafter, the control unit 120 performs image formation of job 2 using the determined secondary transfer bias for job 2 (S316). Then, when formation of all images of job 2 is completed (S317), the control unit 120 ends the operation of the job.

[0091] Furthermore, if the control unit 120 determines in S311 that the paper type categories are not different (they are the same), the pre-charging bias setting is the same, so the control unit 120 proceeds to the process of S315 without performing ATVC control again. Then, the control unit 120 determines the voltage value Vtr2 of the secondary transfer bias for job 2. At this time, Vtr2 may be determined based on the electrical resistance information of the secondary transfer unit N2 acquired in S307, or Vtr1 determined in S308 may be used as Vtr2.

[0092] Furthermore, if the control unit 120 determines in S312 that pre-charging is not required for job 2, it turns off the pre-charging bias (S318). Then, the control unit 120 performs ATVC control using multiple levels of test bias after the timing when the area on the secondary transfer belt 80 when the pre-charging bias is turned off reaches the secondary transfer unit N2. As a result, the control unit 120 obtains electrical resistance information (voltage-current characteristics) of the secondary transfer unit N2 and calculates the base voltage Vb (S319). The rest is the same as above.

[0093] Furthermore, if the control unit 120 determines in S305 that pre-charging is not required for job 1, it performs ATVC control without starting application of the pre-charging bias to the drive roller 84 (S320) and determines the voltage value Vtr2 of the secondary transfer bias for job 1. Thereafter, the control unit 120 performs image formation for job 1 using the determined secondary transfer bias for job 1 (S322). Then, after formation of all images for job 1 is completed (S323), the control unit 120 determines whether the paper type categories of jobs 1 and 2 are different (S324). If the control unit 120 determines in S324 that the paper type categories are different, it determines whether pre-charging is required for job 2 (S325). If the control unit 120 determines in S325 that pre-charging is required for job 2, it starts application of the pre-charging bias for job 2 to the drive roller 84 (S326). At this time, the control unit 120 sets the same pre-charging bias as that used during image formation (pre-charging) for job 2 according to the paper type category for job 2, based on table data such as that shown in Table 1 described in the first embodiment. Then, the control unit 120 performs ATVC control using multiple levels of test bias after the timing when the area on the secondary transfer belt 80 to which the pre-charging bias for job 2 has been applied reaches the secondary transfer unit N2. As a result, the control unit 120 acquires electrical resistance information (voltage-current characteristics) of the secondary transfer unit N2 and calculates the base voltage Vb (S327). Thereafter, the control unit 120 proceeds to the process of S315.

[0094] Furthermore, if the control unit 120 determines in S324 that the paper type categories are the same, or if it determines in S325 that pre-charging is not required, the pre-charging bias remains OFF, and the process proceeds to S315 without performing ATVC control again. Then, the control unit 120 determines the voltage value Vtr2 of the secondary transfer bias for job 2. At this time, Vtr2 may be determined based on the electrical resistance information of the secondary transfer unit N2 acquired in S320, or Vtr1 determined in S321 may be used as Vtr2.

[0095] In this way, when jobs of paper type categories with different pre-charging bias settings are performed in succession, ATVC control is performed with the respective pre-charging biases applied before image formation for each job, thereby enabling the secondary transfer bias to be set according to the potential fluctuation of the secondary transfer belt 80 due to the pre-charging for each job.

[0096] [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.

[0097] 1. Overview of this Example In Example 2, when consecutive jobs were executed, ATVC control was performed each time the pre-charging setting was switched between jobs. In this example, in order to suppress a decrease in productivity due to this, the electrical resistance of the secondary transfer portion N2 is predicted when switching between the presence and absence of pre-charging.

[0098] 2. Control Procedure A job control procedure including ATVC control in this embodiment will be described. FIG. 10 is a flowchart outlining the control procedure. Here, two consecutive jobs, Job 1 and Job 2, will be described as an example. Job 1 is the first job, and Job 2 is the second job. For simplicity, it is assumed that the same type of recording material S is used in one job, and image formation is performed in single-sided printing mode. As in the first and second embodiments, the necessity of pre-charging is determined based on paper type category information as recording material information. The operation of the ATVC control itself may be the same as the basic operation of the ATVC control described in the first embodiment, and therefore a description thereof will be omitted. For convenience, FIG. 10 is divided into FIGS. 10(a) and 10(b), and the procedures in each figure are connected at "C" and "D" in each figure. The same processing in the control of this embodiment shown in FIG. 10 as that in the control of the second embodiment shown in FIG. 9 will be omitted as appropriate.

[0099] First, before a job is input, the control unit 120 performs ATVC control using multiple levels of test biases in a pre-multiple rotation process without applying a pre-charging bias to the drive roller 84, and acquires electrical resistance information (voltage-current characteristics) of the secondary transfer unit N2 (S401). The control unit 120 stores the acquired information in the memory 122. Next, the control unit 120 starts applying the pre-charging bias to the drive roller 84, and after the region on the secondary transfer belt 80 to which the pre-charging bias is applied reaches the secondary transfer unit N2, performs ATVC control using multiple levels of test biases. This acquires electrical resistance information (voltage-current characteristics) of the secondary transfer unit N2 (S402). The control unit 120 stores the acquired information in the memory 122. Here, the value of the pre-charging bias applied in S402 may be a preset representative value. For example, it may be a value corresponding to one of the paper type categories.

[0100] The subsequent processes in steps S403 to S420 are the same as those in steps S301 to S318 in FIG. 9. Because job 1 requires pre-charging and job 2 does not, S420 switches the pre-charging bias from ON to OFF after image formation for job 1 is completed. Turning the pre-charging bias OFF changes the electrical resistance (potential of the secondary transfer belt 80) of the secondary transfer portion N2. However, performing ATVC control using multiple levels of test bias during consecutive jobs reduces productivity. Therefore, in this embodiment, the control unit 120 predicts the electrical resistance of the secondary transfer portion N2 when the pre-charging bias is not applied in job 2 (S420) based on the difference in electrical resistance information between when the pre-charging bias is applied and when it is not applied, which is acquired during the pre-multiple rotation process (S401, S402) and stored in memory 122 (RAM). That is, based on the electrical resistance information when the pre-charging bias is applied acquired in S409 for Job 1 and the difference between the electrical resistance information, electrical resistance information when the pre-charging bias is not applied for Job 2 is predicted. As shown in FIG. 11 , the voltage-current characteristics change when the pre-charging bias is applied and when it is not applied. The voltage difference Δ at the target current in the voltage-current characteristics acquired in advance during the pre-rotation process between when the pre-charging bias is applied and when it is not applied is subtracted from the electrical resistance information acquired in Job 1 (S409). That is, the difference Δ is subtracted from the voltage at the target current in the voltage-current characteristics acquired in Job 1 (S409). This makes it possible to predict the electrical resistance of the secondary transfer portion N2 when the pre-charging bias is not applied for Job 2 and to determine the base voltage Vb. Therefore, based on the base voltage Vb corresponding to the predicted electrical resistance of the secondary transfer portion N2, the voltage value Vtr2 of the secondary transfer bias for Job 2 can be determined (S417), and image formation for Job 2 can be performed.

[0101] 9. The process of S422 to S428 is the same as the process of S320 to S326 in FIG. 9. Because job 1 does not require pre-charging and job 2 does, the process of S428 switches the pre-charging bias from OFF to ON after image formation for job 1 is completed. In this case, as in the above, the control unit 120 predicts the electrical resistance of the secondary transfer portion N2 when the pre-charging bias is applied in job 2 based on the difference in electrical resistance information between when the pre-charging bias is applied and when it is not applied, which is acquired during the pre-multiple rotation process (S401, S402) and stored in memory 122 (RAM) (S429). That is, the control unit 120 predicts the electrical resistance information when the pre-charging bias is applied in job 2 based on the difference between the electrical resistance information when the pre-charging bias is not applied, which is acquired in S422 for job 1, and the electrical resistance information. In this case, the voltage difference Δ at the target current in the voltage-current characteristics when the pre-charging bias is applied and when it is not applied, which was previously acquired during the pre-multiple rotation process, is added to the electrical resistance information acquired in job 1 (S422). That is, the difference Δ is added to the voltage at the target current in the voltage-current characteristics acquired in job 1 (S422). This makes it possible to predict the electrical resistance of the secondary transfer portion N2 when the pre-charging bias is applied in job 2 and to determine the base voltage Vb. Therefore, the voltage value Vtr2 of the secondary transfer bias for job 2 can be determined based on the base voltage Vb corresponding to the predicted electrical resistance of the secondary transfer portion N2 (S417), and image formation for job 2 can be performed.

[0102] In this way, when switching between pre-charging and non-pre-charging, predicting the electrical resistance of the secondary transfer unit N2 after switching can suppress a decrease in productivity due to ATVC control. Also, it is possible to set the secondary transfer bias according to the potential fluctuation of the secondary transfer belt 80 due to pre-charging.

[0103] In this embodiment, electrical resistance information of the secondary transfer section N2 when the pre-charging bias is applied and when it is not applied is obtained during the pre-rotation, but it may also be obtained during a waiting time such as temperature control of the fixing section (fixing device) 15 or during the pre-rotation of a job.

[0104] Furthermore, in this embodiment, the first time that pre-charging is switched on or off in a consecutive job has been described, but the electrical resistance of the secondary transfer portion N2 can also be predicted in the same way when pre-charging is switched on or off two or more times in a consecutive job. In this case, the prediction can typically be made based on previously acquired electrical resistance information (difference information) of the secondary transfer portion N2 when a pre-charging bias is applied and when it is not applied, and electrical resistance information of the secondary transfer portion N2 acquired during the most recent ATVC control. However, the present invention is not limited to this, and electrical resistance information of the secondary transfer portion N2 acquired during ATVC control performed before the most recent ATVC control may also be used.

[0105] In this embodiment, electrical resistance information (difference information) of the secondary transfer portion N2 when the pre-charging bias is applied and when it is not applied is obtained using a representative value of the pre-charging bias that is set in advance. Even in this case, the secondary transfer bias setting can be brought closer to the optimal value. However, this is not limiting, and the above difference information may be obtained with multiple pre-charging bias settings. Then, when switching between the presence and absence of the pre-charging bias, the above difference information obtained with the same pre-charging bias setting (or a setting closer than any other setting) as the pre-charging bias setting when the pre-charging bias is applied after or before the switching can be used.

[0106] Furthermore, the control of this embodiment switches between the presence and absence of pre-charging depending on, for example, the type of recording material S, but is also effective in further suppressing a decrease in productivity in a configuration in which the value of the pre-charging bias is constant. In this case, the processes of S413, S416, and S426 are unnecessary. Also, in this case, it is sufficient to use the constant value of the pre-charging bias to previously obtain electrical resistance information (difference information) of the secondary transfer portion N2 when the pre-charging bias is applied and when it is not applied.

[0107] [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.

[0108] 1. Overview of this Example In the third embodiment, the prediction of the electrical resistance of the secondary transfer portion N2 after switching between the presence and absence of pre-charging was described. In this embodiment, the correction control of the secondary transfer bias after this prediction will be described.

[0109] 2. Control Procedure The control procedure of a job including ATVC control in this embodiment will be described. FIG. 12 is a flowchart showing an outline of the control procedure. For convenience, FIG. 12 is divided into FIG. 12(a) and FIG. 12(b), and the procedures in each figure are connected at "E" and "F" in each figure. The control of this embodiment shown in FIG. 12 has additional processing compared to the control of embodiment 3 shown in FIG. 10, but other processing is the same, so explanations will be omitted as appropriate.

[0110] The process up to determining the voltage value Vtr2 of the secondary transfer bias for job 2 (S417) is the same as in the third embodiment. In this embodiment, if the control unit 120 determines in S419 that all image formation for job 2 has not been completed, it acquires information about the electrical resistance of the secondary transfer unit N2 (S501). For example, when there is no recording material S at the secondary transfer unit N2 (when no paper is passing), the current detection sensor 25b detects the current Itr2 when Vtr2 is being applied as information about the electrical resistance of the secondary transfer unit N2. Typically, as shown in FIG. 13 , the current detection sensor 25b detects the current Itr2 when Vtr2 is being applied between the preceding and succeeding recording materials S. If the detected Itr2 does not reach the desired current value I (target current), the correction amount ΔVtr2 is calculated using the voltage-current characteristics of the secondary transfer unit N2 acquired in advance. That is, ΔVtr2 is calculated from the difference between Itr2 and the desired current value I (target current) using the following formula: Note that the voltage-current characteristics of the secondary transfer portion N2 acquired in advance are typically the voltage-current characteristics acquired by the most recently performed ATVC control. ΔVtr2=ΔV / ΔI×(Itr2―I) Then, the control unit 120 determines the corrected voltage value Vtr2' of the secondary transfer bias from the calculated ΔVtr2 and Vtr2 (S502).Then, the control unit 120 performs image formation using the corrected voltage value Vtr2' of the secondary transfer bias (S418).

[0111] The voltage value of the secondary transfer bias applied when no paper is passing does not have to be the same as the voltage value Vtr2 of the secondary transfer bias applied immediately before. In this case, the voltage value Vtr2 of the secondary transfer bias can be corrected based on the acquired current value so as to correct for fluctuations in the electrical resistance of the secondary transfer portion N2. If necessary, the paper gap may be extended to acquire electrical resistance information of the secondary transfer portion N2. The previously acquired voltage-current characteristics of the secondary transfer portion N2 used for the correction may be the voltage-current characteristics of the secondary transfer portion N2 acquired during ATVC control performed before the most recent ATVC control.

[0112] In this way, when predicting the electrical resistance of the secondary transfer unit N2, for example, the voltage-current characteristics of the secondary transfer unit N2 between sheets of paper (in this embodiment, the detection results of the current when the target voltage of the secondary transfer bias is being applied) are acquired, and the voltage value of the secondary transfer bias is corrected. This reduces the ATVC control time after switching between the presence and absence of pre-charging, while correcting the voltage value of the secondary transfer bias to a more appropriate value, thereby suppressing the occurrence of image defects.

[0113] As described in the third embodiment, the control of this embodiment switches pre-charging on and off depending on, for example, the type of recording material S. However, this control is also effective in further suppressing a decrease in productivity in a configuration in which the pre-charging bias value is constant. In this case, the processes of S413, S416, and S426 are unnecessary. Furthermore, in this case, it is sufficient to obtain in advance electrical resistance information (difference information) of the secondary transfer portion N2 when the pre-charging bias is applied and when it is not applied using the constant pre-charging bias value. Also, in FIG. 12, the secondary transfer bias is corrected even when Vtr2 is set without prediction. However, the control unit 120 may determine whether Vtr2 is set based on prediction and correct the secondary transfer bias only when Vtr2 is set based on prediction.

[0114] In addition, in this embodiment, the correction of the voltage value of the secondary transfer bias after switching between the presence and absence of pre-charging in continuous jobs has been described, but even in the case of a single job that performs pre-charging, the voltage-current characteristics may be obtained when no paper is passing in the same manner as described above, and the secondary transfer bias may be corrected.

[0115] [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.

[0116] In the above-described embodiment, the pre-charge setting is changed on a job-by-job basis, but the present invention is not limited to this. For example, different types of recording materials may be used in a single job, and the pre-charge setting may be changed. In this case, the same effect can be achieved by performing the control performed on a job-by-job basis in the above-described embodiment on a recording material-by-recording material basis. In other words, the above description can be applied by considering job 1 (the nth job) and job 2 (the n+1th job) in the above-described embodiment as recording material 1 (the nth sheet of recording material) and recording material 2 (the n+1th sheet of recording material) in a single job.

[0117] In the above-described embodiment, a secondary transfer bias having the same polarity as the normal charging polarity of the toner is applied to the inner secondary transfer roller 71. In the above-described embodiment, a pre-charging bias having the same polarity as the normal charging polarity of the toner is applied to the drive roller 84, which is one of the rollers forming the pre-charging unit N3 and is disposed on the inner peripheral surface side of the secondary transfer belt 80. However, the present invention is not limited to this embodiment.

[0118] For example, as shown in FIG. 14, 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 281. In this case, the outer secondary transfer roller 281 may have a configuration similar to that of the inner secondary transfer roller 71 in the above-described embodiment. That is, the outer secondary transfer roller 281 in this case may be, for example, a roller made of an elastic layer of ion-conductive foamed rubber (NBR rubber) and a core metal, with an outer diameter of 24 mm and a roller surface roughness Rz of 6.0 to 12.0 (μm). In addition, the outer secondary transfer roller 281 in this case may be, for example, a roller having a resistance measured at N / N (23°C, 50% RH) and 1×10 when 2 kV is applied. 5 ~1×10 7 A roller having an Asker-C hardness of the elastic layer of about 30 to 40 can be used. In this case, the inner secondary transfer roller 271 is electrically grounded.

[0119] As shown in FIG. 14, among the rollers forming the pre-charging section N3, the roller positioned on the outer circumferential surface side of the secondary transfer belt 80 can be used as the pre-charging roller 291. In this case, a pre-charging bias of a polarity opposite to the normal charging polarity of the toner is applied to the pre-charging roller 291. This allows the toner image transfer surface of the recording material S to be charged to a polarity opposite to the normal charging polarity of the toner. This pre-charging roller 291 can be an elastic sponge roller made of ion-conductive foam rubber (NBR rubber, ECO rubber). The outer diameter of this pre-charging roller 291 can be, for example, 15 mm, similar to the pre-charging counter roller 91 in the above-described embodiment. This relatively small diameter allows for as much distance as possible from the surface of the intermediate transfer belt 70. In this case, among the rollers forming the pre-charging section N3, the roller positioned on the inner circumferential surface side of the secondary transfer belt 80 can be used as the pre-charging counter roller 284. This pre-charging counter roller 284 is electrically grounded. This pre-charging counter roller 284 can be, for example, a metal roller. In addition, this pre-charging opposing roller 284 can also serve as a drive roller for the secondary transfer belt 82. The combination of rollers to apply bias in the secondary transfer portion N2 and the pre-charging portion N3 can be selected arbitrarily, including those in the above-described embodiments and those shown in FIG.

[0120] In the above-described embodiment, the pre-charging bias (recording material charging bias) is controlled to a constant voltage, but the pre-charging bias may be controlled to a constant current. It is sufficient if the toner image transfer surface of the recording material can be charged to a polarity opposite to the normal charging polarity of the toner. When the pre-charging bias is controlled to a constant current, a target current can be set instead of the target voltage in the above-described embodiment.

[0121] In the above-described embodiment, the secondary transfer bias is controlled to a constant voltage, but the secondary transfer bias may be controlled to a constant current. When the secondary transfer bias is controlled to a constant current, for example, in the correction control described in the fourth embodiment, the voltage when the target current is supplied when no paper is passing may be detected by a voltage detection sensor.

[0122] Furthermore, in the above-described embodiment, the pre-charge setting is described as being changed depending on the paper type category, but as mentioned above, instead of or in addition to this, the pre-charge setting may be changed depending on the environment or the printing surface.

[0123] Although the above-described embodiment describes an example in which ATVC control is performed using multiple levels of test bias, this is not limiting. 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. For example, when there is no toner image or recording material S at the secondary transfer portion N2, a test bias is applied to the secondary transfer portion N2 using constant current control so that the current detected by the current detection sensor 25b becomes a target current corresponding to a predetermined transfer current. The base voltage Vb can then be calculated based on the voltage generated at this time, as detected by the voltage detection sensor 25a. Alternatively, as in the above-described embodiment, when there is no toner image or recording material S at the secondary transfer portion N2, multiple test biases are applied to the secondary transfer portion N2 using constant current control or constant voltage control. Furthermore, the voltage-current characteristic (linear or curved) is obtained based on the voltage generated or current detected by the voltage detection sensor 25a or current detection sensor 25b. Then, based on the voltage-current characteristics, the base voltage Vb at which a predetermined transfer current is obtained can be determined.

[0124] In the above-described embodiment, the recording material charging member (pre-charging member) and the opposing member (pre-charging opposing member) are each roller-shaped, 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. The member disposed on the inner peripheral surface of the transfer belt as the first member or the second member does not have to also serve as a tension roller for the transfer belt, and the first member and the second member may sandwich the surface of the transfer belt stretched between the tension rollers.

[0125] In the above-described embodiment, the image forming apparatus is a tandem-type color image forming apparatus employing an intermediate transfer system 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 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 a configuration in which, for example, a transfer belt is provided that contacts an image carrier (such as a photosensitive drum) to form a transfer section, and the recording material on the transfer belt is pre-charged. The image forming apparatus may also be an image forming apparatus for various purposes, such as a printer, various printing machines, copiers, fax machines, and multifunction peripherals. [Explanation of symbols]

[0126] 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) 91 Pre-charging opposing roller (opposing member) 100 Image forming device S recording material

Claims

1. an image carrier that carries a toner image; a rotatable transfer belt that is an endless belt and contacts the image carrier to form a transfer portion, carries a recording material, and transports the recording material to the transfer portion; a first applying unit that applies a transfer bias to the transfer unit to transfer a toner image from the image carrier to a recording material; a first member that is disposed on an inner peripheral surface side of the transfer belt upstream of the transfer portion in the conveying direction of the recording material, and that forms a charging portion that charges a transfer surface onto which a toner image of the recording material conveyed by the transfer belt to the transfer portion, to a polarity opposite to the normal charging polarity of the toner; a second member disposed on the outer peripheral surface side of the transfer belt, and sandwiching the transfer belt between itself and the first member to form the charging unit; a second applying section that applies a charging bias to the charging section to charge the transfer surface of the recording material to the opposite polarity; a detection unit that detects a current flowing through the transfer unit or a voltage applied to the transfer unit; a control unit that sets the transfer bias based on the detection result of the detection unit; and The control unit sets the transfer bias when transferring a toner image to a recording material whose transfer surface has been charged to the opposite polarity by the charging unit based on the detection result by the detection unit when an area of ​​the transfer belt that has passed through the charging unit while the charging bias is being applied to the charging unit passes through the transfer unit immediately afterwards.

2. a storage unit that stores a first detection result by the detection unit when a region of the transfer belt that has passed through the charging unit while the charging bias is being applied to the charging unit passes through the transfer unit immediately thereafter, and a second detection result by the detection unit when a region of the transfer belt that has passed through the charging unit while the charging bias is not being applied to the charging unit passes through the transfer unit immediately thereafter, 2. The image forming apparatus according to claim 1, wherein the control unit sets the transfer bias when transferring a toner image to a recording material whose transfer surface has been charged to the opposite polarity by the charging unit based on the first detection result, the second detection result, and a third detection result by the detection unit when an area of ​​the transfer belt that passed through the charging unit when the charging bias was not applied to the charging unit passes through the transfer unit immediately after the first detection result and the second detection result are stored in the memory unit.

3. 3. The image forming apparatus according to claim 2, wherein, when a toner image is transferred to a first recording material and a second recording material whose transfer surfaces are charged to the opposite polarities by the charging unit, the control unit sets a first transfer bias when transferring the toner image to the first recording material based on the first detection result, the second detection result, and the third detection result, and sets a second transfer bias when transferring the toner image to the second recording material based on the first transfer bias and the detection result by the detection unit after the first recording material has passed the transfer unit and before the second recording material reaches the transfer unit.

4. 2. The image forming apparatus according to claim 1, wherein the control unit sets the transfer bias when transferring a toner image to a recording material whose transfer surface is not charged to the opposite polarity by the charging unit based on the detection result by the detection unit when an area of ​​the transfer belt that passed through the charging unit when the charging bias was not applied to the charging unit passes through the transfer unit immediately afterwards.

5. a storage unit that stores a first detection result by the detection unit when a region of the transfer belt that has passed through the charging unit while the charging bias is being applied to the charging unit passes through the transfer unit immediately thereafter, and a second detection result by the detection unit when a region of the transfer belt that has passed through the charging unit while the charging bias is not being applied to the charging unit passes through the transfer unit immediately thereafter, 5. The image forming apparatus according to claim 4, wherein the control unit sets the transfer bias when transferring a toner image to a recording material whose transfer surface is not charged to the opposite polarity by the charging unit based on the first detection result, the second detection result, and a third detection result by the detection unit when a region of the transfer belt that passed through the charging unit while the charging bias was being applied to the charging unit passes through the transfer unit immediately after the first detection result and the second detection result are stored in the memory unit.

6. 6. The image forming apparatus according to claim 5, wherein, when transferring a toner image to a first recording material and a second recording material whose transfer surfaces are not charged to the opposite polarities by the charging unit, the control unit sets a first transfer bias when transferring the toner image to the first recording material based on the first detection result, the second detection result, and the third detection result, and sets a second transfer bias when transferring the toner image to the second recording material based on the first transfer bias and the detection result by the detection unit after the first recording material has passed the transfer unit and before the second recording material reaches the transfer unit.

7. 7. The image forming apparatus according to claim 1, wherein the second applying section applies the charging bias to the first member, the charging bias having the same polarity as a normal charging polarity of the toner.

8. 7. The image forming apparatus according to claim 1, wherein the second applying section applies the charging bias to the second member, the charging bias having a polarity opposite to a normal charging polarity of the toner.

9. 7. 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.

10. 7. The image forming apparatus according to claim 1, wherein the control unit sets whether or not the charging unit charges the transfer surface of the recording material to the opposite polarity based on information about the recording material.

11. 7. An image forming apparatus according to claim 1, wherein the control unit changes the charging bias when the charging unit charges the transfer surface of the recording material to the opposite polarity based on information about the recording material.

Images