Image forming apparatus

Abstract

To appropriately charge a surface on which a toner image of a recording material will be transferred before the recording material arrives at a transfer part, to a reverse polarity of a normal charge polarity of a toner, according to a recording material to be used in image formation.SOLUTION: An image forming apparatus 100 includes: an image carrier 70; transfer members 71, 81 for forming a transfer part N2; a first application part E4 for applying a transfer bias to the transfer part N2; a recording material charge member 84 for charging a toner image transfer surface of a recording material S to a reverse polarity of a normal charge polarity of a toner; a second application part E5 for applying a recording material charge bias to the recording material charge member 84; a detection part 25b for detecting a current flowing in the transfer part N2; and a control part 120 for executing control of changing a setting of the recording material charge bias, on the basis of a detection result by the detection part 25b when the recording material S passes through the transfer part N2.SELECTED DRAWING: Figure 6

Description

[Technical Field]

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

[0002] Conventionally, in image forming devices such as electrophotographic copiers, a toner image is electrostatically transferred from an image carrier, such as a photoreceptor 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 then applying a transfer bias to the transfer section. Insufficient transfer current supplied to the transfer section by the transfer bias can result in image defects such as "transfer voids" or "low density," where transfer is insufficient and the desired image density is not achieved. Furthermore, excessive transfer current supplied to the transfer section by the transfer bias can cause discharge at the transfer section, which can reverse the polarity of the toner charge in the toner image, resulting in image defects such as "whiteouts," where the toner image is not transferred in parts. Therefore, applying an appropriate transfer bias to the transfer section is required to produce high-quality images.

[0003] Patent document 1 discloses that a test bias is applied to the transfer section when there is no recording material in the transfer section to obtain the voltage-current characteristics, a voltage value that will give a predetermined target current is determined, and a recording material share voltage according to the type of recording material is added to this voltage value to set the voltage value of the transfer bias.

[0004] Here, the types of recording materials include types that differ in the smoothness of the surface of the recording material, such as high-quality paper and coated paper, and types that differ in the thickness of the recording material, such as thin paper and thick paper. The recording material distribution voltage can be determined in advance, for example, depending on the type of recording material. However, depending on the specifications of the recording material, if the electrical resistance of the recording material is high, the absolute value of the transfer bias voltage for passing the necessary transfer current in, for example, a low-humidity environment may exceed the high-voltage capacity, and image defects may occur due to insufficient transfer current.

[0005] Patent Document 2 proposes that the surface of the recording material onto which the toner image is transferred is charged in advance to a polarity opposite to the normal charging polarity of the toner before the recording material reaches the transfer section. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-117920 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-171282 Summary of the Invention [Problem to be solved by the invention]

[0007] Recently, there has been a trend toward an increasing variety of recording materials used in image formation in production machines and the like. For example, to properly transfer a toner image to a recording material with high electrical resistance, such as synthetic paper (a recording material primarily composed of plastic) or extra-thick paper (thick paper with a basis weight exceeding 250 gsm), it is necessary to apply a transfer bias with a larger absolute value of voltage. To properly transfer a toner image to such a recording material without reducing productivity (image formation speed), it is effective to compensate for the insufficient transfer current by pre-charging the surface of the recording material onto which the toner image is to be transferred with a polarity opposite to the normal charging polarity of the toner.

[0008] However, there are a great many types of recording materials on the market. Furthermore, the electrical resistance of a recording material can vary depending on factors such as the time it is left in the environment, even if the environment (temperature and humidity) is the same. For these reasons, it can be difficult to accurately determine in advance the charge that needs to be applied to the recording material before transfer. For example, in the case of a recording material with high electrical resistance, even a small change in electrical resistance can significantly change the charge that needs to be applied to the recording material before transfer.

[0009] In addition, it is possible that the amount of charge applied to the recording material before transfer may be insufficient or excessive due to a mismatch between the information on the type of recording material set by the user or the like and the type (characteristics) of the recording material actually used for image formation.

[0010] If the charge applied to the recording material before transfer is insufficient, resulting in an insufficient transfer current, image defects such as transfer voids and low density may occur as described above.Furthermore, if the charge applied to the recording material before transfer is excessive, resulting in an excessive transfer current, image defects such as white spots may occur as described above.

[0011] Therefore, the object of the present invention is to make it possible to appropriately charge the surface of the recording material onto which the toner image is transferred to a polarity opposite to the normal charging polarity of the toner before the recording material reaches the transfer section, depending on the recording material used in image formation. [Means for solving the problem]

[0012] 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 comprising: an image carrier that carries a toner image; a transfer member that forms a transfer section that transfers the toner image from the image carrier to a recording material; a first applying section that applies a transfer bias to the transfer section; a recording material charging member that is located upstream of the transfer section in the recording material transport direction and that charges a toner image transfer surface of the recording material transported to the transfer section, onto which the toner image is transferred, to a polarity opposite to the normal charging polarity of the toner; a second applying section that applies a recording material charging bias to the recording material charging member to charge the toner image transfer surface of the recording material; a detection section that detects a current flowing through the transfer section or a voltage applied to the transfer section; and a control section that executes control to change the setting of the recording material charging bias based on the detection result of the detection section while the recording material is passing through the transfer section. [Effects of the Invention]

[0013] According to the present invention, depending on the recording material used for image formation, it is possible to appropriately charge the surface of the recording material onto which the toner image is transferred to a polarity opposite to the normal charging polarity of the toner before the recording material reaches the transfer section. [Brief explanation of the drawings]

[0014] [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. 10 is a graph showing an example of the change in the charge amount of a recording material. [Figure 6] FIG. 2 is a flowchart of control in the first embodiment. [Figure 7] FIG. 10 is a flowchart of the control in the second embodiment. [Figure 8] FIG. 10 is a schematic cross-sectional view showing another example of the configuration in the vicinity of the secondary transfer portion. [Figure 9] FIG. 10 is a schematic cross-sectional view showing another example of the configuration in the vicinity of the secondary transfer portion. [Figure 10] FIG. 10 is a schematic cross-sectional view showing another example of the configuration in the vicinity of the secondary transfer portion. DETAILED DESCRIPTION OF THE INVENTION

[0015] The image forming apparatus according to the present invention will be described in more detail below with reference to the drawings.

[0016] [Example 1] 1. Configuration and operation of image forming apparatus FIG. 1 is a schematic cross-sectional view of an image forming apparatus 100 according to this embodiment. The image forming apparatus 100 is a tandem-type full-color printer employing an intermediate transfer method, capable of forming full-color images using an electrophotographic method. The image forming apparatus 100 is capable of forming and outputting an image on a sheet-like recording material S based on image information input from an external device such as a personal computer or image information input via an operation unit 130 provided on the image forming apparatus 100. Because the image forming apparatus 100 primarily uses paper as the recording material S, the recording material S is sometimes referred to as paper, but the recording material S is not limited to paper. The recording material S may also be made of materials other than paper or materials containing materials other than paper, such as synthetic paper or film made primarily of a synthetic resin, or special paper such as metal-coated paper.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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 intermediate transfer member formed of an endless belt as a first image carrier, is movable (rotatable) while carrying a toner image. The intermediate transfer belt 70 is stretched over and tensioned by a plurality of support rollers (support rollers)—the secondary transfer inner roller 71, the drive roller 72, and the tension roller 73—under a predetermined tension. The drive roller 72 is driven to rotate by an intermediate transfer belt drive motor D2 (FIG. 3) 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.

[0021] 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.

[0022] 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.

[0023] 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). In this embodiment, the intermediate transfer belt 70 has a three-layer structure, but it may also 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.

[0024] 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 (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 (secondarily transferred) 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 to the inner secondary transfer roller 71 by a secondary transfer power source E4 serving as a secondary transfer voltage application means (secondary transfer voltage application unit). Also, in this embodiment, the outer secondary transfer roller 81 is connected (electrically grounded) to ground (ground potential). Details of the secondary transfer unit 8 will be described later.

[0025] The 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 conveyance drive motor D4 (FIG. 3) serving as a driving means. In this embodiment, a pre-charging device 9 is provided upstream of the secondary transfer unit N2 (downstream of the pair of registration rollers 14) in the conveying direction of the recording material S. The pre-charging device 9 pre-charges the surface of the recording material S onto which the toner image is transferred to a polarity opposite to the normal charging polarity of the toner before the recording material S reaches the secondary transfer unit N2. Details of the pre-charging device 9 will be described later.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] For example, various power sources such as a charging power source E1, a developing power source E2, a primary transfer power source E3, a secondary transfer power source E4, and a pre-charging power source E5 (described later) are connected to the control unit 120. Also, various drive units such as a drum drive motor D1, an intermediate transfer belt drive motor D2, and a secondary transfer belt drive motor D3 (described later) are connected to the control unit 120.

[0032] The control unit 120 is also connected to an environmental sensor 18. The environmental sensor 18 is an example of an environmental detection unit that detects at least one of the temperature and humidity inside or outside the image forming apparatus 100 (the installation environment of the image forming apparatus 100). In this embodiment, the environmental sensor 18 is configured as a temperature and humidity sensor that detects the temperature and humidity (relative humidity) inside the image forming apparatus 100 (inside the cassettes 11a and 11b or near the cassettes 11a and 11b). The environmental sensor 18 inputs signals indicating the detected temperature and humidity to the control unit 120. Based on the temperature and humidity detected by the environmental sensor 18, the control unit 120 can calculate the absolute moisture content (absolute humidity) as environmental temperature and humidity information and use it for control. Even if the ambient environment of the image forming apparatus 100 changes suddenly, the electrical resistance value of the recording material S often does not change suddenly. Therefore, by installing the environment sensor 18 in or near the cassettes 11a and 11b, it is possible to grasp the change in the electrical resistance of the recording material S more accurately.

[0033] 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 voltage and current detection results to the control unit 120. The control unit 120 can execute secondary transfer voltage determination control (described later) and other operations based on the detection results of the voltage detection sensor 25a and the current detection sensor 25b. The control unit 120 can also execute pre-charging bias adjustment control (described later) and other operations based on the detection results of the current detection sensor 25b.

[0034] The image forming apparatus 100 also has a conveyance sensor 26 as a recording material detection unit for controlling the timing of conveying the recording material S to the secondary transfer portion N2 or the timing of image formation. The conveyance sensor 26 is disposed, for example, downstream of and adjacent to the registration roller pair 14 in the conveyance direction of the recording material S. The conveyance sensor 26 can detect at least one of the arrival of the leading edge of the recording material S in the conveyance direction and the passing of the trailing edge of the recording material S in the conveyance direction. The conveyance sensor 26 inputs a signal indicating the detection result of the recording material S to the control unit 120. Based on the detection result of the conveyance sensor 26, the control unit 120 can control the drive of the registration roller pair 14 by the conveyance drive motor D4 and the current detection timing in the pre-charging bias adjustment control, which will be described later.

[0035] 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 such as an image reading device (not shown) or a personal computer that is provided in or connected to the image forming apparatus 100.

[0036] 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.

[0037] The image forming apparatus 100 executes a job (print job), which is a series of operations that starts with a single start command and forms and outputs an image on one or more recording materials S. The job generally includes an image formation process, a pre-rotation process, a sheet spacing process, and a post-rotation process. The image formation process is a period during which electrostatic image formation (exposure), toner image formation (development), and toner image transfer of the image that will actually be formed and output on the recording material S are performed. This period is referred to as the image formation time. More specifically, the timing of the image formation process differs depending on the position where each of the electrostatic image formation, toner image formation, and toner image transfer processes is performed. The pre-rotation process is a period during which preparatory operations are performed before the image formation process, from when a start command is input until the actual start of image formation. The sheet spacing process is a period corresponding to the interval between recording materials S when image formation is performed continuously on multiple recording materials S (continuous image formation). The post-rotation process is a period during which tidying up operations (preparatory operations) are performed after the image formation process. Non-image formation refers to periods other than image formation, and includes the above-mentioned pre-rotation process, paper interval process, post-rotation process, and also the pre-multi-rotation process, which is a preparatory operation when the image forming device 100 is turned on or when it returns from a sleep state.

[0038] 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.

[0039] 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.

[0040] The secondary transfer belt 80 can be configured as an endless belt member having a layer formed of a resin material or a metal material. For example, the secondary transfer belt 80 can be made of a resin such as polyimide or polycarbonate containing an appropriate amount of carbon black as an antistatic agent, so that the volume resistivity is 1×10 9 ~1×10 14 It is formed of a resin material adjusted to Ω·cm (23°C, 50% RH). The secondary transfer belt 80 may have a single-layer structure or a multi-layer structure. The thickness of the secondary transfer belt 80 is, for example, about 0.07 to 0.1 mm. The circumferential length of the secondary transfer belt 80 is, for example, about 300 mm.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] In this embodiment, the secondary transfer bias is applied by constant voltage control. The voltage value (target voltage) of the secondary transfer bias is determined as the sum of a base voltage Vb for obtaining a predetermined transfer current and a recording material distribution voltage Vp determined by the type of recording material S. The recording material distribution voltage Vp is set in advance according to the type of recording material S and further the environment (for example, absolute moisture content), and is stored in memory 122 as table data or the like.

[0049] 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 unit N2 when there is no toner image or recording material S at the secondary transfer unit N2. For example, when there is no toner image or recording material S at the secondary transfer unit N2, a test bias is applied to the secondary transfer unit N2 (inner secondary transfer roller 71) under constant current control so that the current detected by the current detection sensor 25b is equal to 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, when there is no toner image or recording material S at the secondary transfer unit N2, multiple test biases are applied to the secondary transfer unit N2 (inner secondary transfer roller 71) under constant current control or constant voltage control. The voltage-current characteristic (linear or curved) is then obtained based on the voltage generated or current detected by the voltage detection sensor 25a or current detection sensor 25b. Based on the voltage-current characteristics, a base voltage Vb that provides a predetermined transfer current can be calculated. The predetermined transfer current is preset, for example, according to the environment (e.g., absolute moisture content) and stored in memory 122 as table data. This type of control is called secondary transfer voltage determination control or ATVC (Active Transfer Voltage Control). This allows the secondary transfer bias to be changed in response to the electrical resistance of the secondary transfer unit N2, which changes sequentially due to changes in the environment and the usage status (accumulated usage) of the components involved in secondary transfer. The components involved in secondary transfer include the inner secondary transfer roller 71, the outer secondary transfer roller 81, the intermediate transfer belt 70, and the secondary transfer belt 80. The secondary transfer voltage determination control is controlled and executed by the control unit 120. The secondary transfer voltage determination control is typically executed during the pre-rotation process or pre-multiple rotation process for each job, but can also be executed at any timing (e.g., during the sheet-to-sheet process) during non-image formation when there are no toner images or recording material S at the secondary transfer unit N2.

[0050] 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).

[0051] 4.Pre-charging device Next, the pre-charging device (recording material charging device) 8 in this embodiment will be further described.

[0052] 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.

[0053] 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 high-voltage power supplies are used, their placement may not allow for proper surface clearance, making it impossible to apply a secondary transfer bias with a high absolute value. Furthermore, increasing the absolute value of the secondary transfer bias voltage can cause image defects due to discharge at the secondary transfer section, 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 example, with recording materials with high volume resistivity, such as synthetic paper with a resin layer or high-resistivity paper such as extra-thick paper, it is difficult to sufficiently transfer toner to recording material S using only the secondary transfer bias at secondary transfer unit N2, particularly in low-humidity environments, and transfer defects may occur. Also, if the absolute value of the voltage of the secondary transfer bias needs to be 10 kV or more, and taking into account the creepage distance near secondary transfer unit N2, increasing the output of the secondary transfer power supply will lead to an increase in the size of the image forming apparatus.

[0054] 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 (also referred to as the "toner image transfer surface") 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. 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.

[0055] 4, in this embodiment, a pre-charging device 9 is provided upstream of the secondary transfer portion N2 in the conveyance direction of the recording material S (downstream of the registration roller pair 14) to pre-charge 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. This improves the transferability of the toner image onto extra-thick paper and synthetic paper.

[0056] 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 (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).

[0057] 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.

[0058] 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.

[0059] The recording material S conveyed by the pair of registration rollers 14 is conveyed to a nip portion (pre-charging portion) N3 between a pre-charging opposing roller 91 and a secondary transfer belt 82 wound around a drive roller 84. In this embodiment, guide members 22 (upper guide member 22a and lower guide member 22b) that guide the recording material S are provided upstream of the pre-charging portion N3 and downstream of the pair of registration rollers 14 in the conveyance direction of the recording material S. The recording material S conveyed by the pair of registration rollers 14 is conveyed to the pre-charging portion N3 while being guided by the guide members 22. Then, in the pre-charging portion N3, the recording material S is charged (pre-charged) and electrostatically attracted (by electrostatic force) to the secondary transfer belt 80. The recording material S attracted to the secondary transfer belt 80 is conveyed to the secondary transfer portion N2, where a toner image is transferred (secondary transfer) onto the recording material S.

[0060] 5. Control of pre-charging bias In this embodiment, the pre-charging bias is applied under constant current control. The pre-charging power supply E5 incorporates a current detection unit (not shown) as a current detection means, and can perform constant current control of the output voltage so that the current value detected by this current detection unit remains approximately constant. The appropriate target current for 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 current for 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 current for 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 experiment, etc. as an appropriate charge amount that will provide appropriate transferability. Also, for example, the pre-charging bias may be applied only when a predetermined type of recording material S is used, or only when a predetermined type of recording material S is used and the absolute moisture content is within a predetermined range (for example, smaller than a predetermined value).

[0061] In this embodiment, to facilitate understanding of the present invention, it is assumed that when a job is started, it is determined whether or not a pre-charging bias is applied to the drive roller 84 (whether or not pre-charging is necessary) depending on the type of recording material S. As an example, in this embodiment, it is assumed that when 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, it is determined that pre-charging is necessary. In this embodiment, it is determined that pre-charging is unnecessary when a paper type category other than synthetic paper or a predetermined high resistance paper (such as extra-thick paper) is selected.

[0062] In the configuration of this embodiment, for example, when synthetic paper Yupo YPI250 (a product name of Yupo Corporation) is used as the recording material S, the secondary transfer bias alone is insufficient to provide a sufficient transfer current in an environment with a temperature of 23°C and a humidity of 5% RH. Therefore, in this case, a pre-charging bias is applied to the drive roller 84 to supply a current of -60 μA to the drive roller 84. This allows the toner image transfer surface of the recording material S to be charged to approximately 5 kV.

[0063] FIG. 5 is a graph showing the change in charge amount over time on the toner image transfer surface of a recording material S, YUPO YPI250. The graph plots the change in charge amount over time immediately after the toner image transfer surface of the recording material S is charged to 5 kV in two environments: a temperature of 23°C and a humidity of 5% and a temperature of 23°C and a humidity of 60%. In the 23°C and 5% humidity environment, the toner image transfer surface of the recording material S remains charged to 5 kV. In contrast, in the 23°C and 60% humidity environment, the charge amount (absolute value) on the toner image transfer surface of the recording material S rapidly decreases from 5 kV over time. This is thought to be due to the charge stored on the recording material S being neutralized by natural discharge and being drawn into the conveying section and roller pair that come into contact with the recording material S. Therefore, for example, if pre-charging is performed with a target current set for an environment with a temperature of 23° C. and a humidity of 5% RH, the transfer current will be insufficient in an environment with a temperature of 23° C. and a humidity of 60% RH.

[0064] As described above, the target current of the pre-charging bias can be preset based on the characteristics shown in FIG. 5 so as to obtain appropriate transfer performance depending on the environment. For example, in a low-temperature, low-humidity environment where the moisture content of the recording material S tends to decrease, the volume resistivity of the recording material S increases, making it difficult to inject charge into the recording material S and thus difficult to apply charge. Furthermore, in a high-temperature, high-humidity environment where the recording material S tends to absorb moisture, the volume resistivity of the recording material S decreases, making charge leakage more likely. Based on these changes, the target current of the pre-charging bias can be changed in stages depending on the temperature and humidity environment. For example, the absolute value of the target current of the pre-charging bias when the absolute moisture content is a second value greater than the absolute value of the target current of the pre-charging bias when the absolute moisture content is a first value can be set to be larger than the absolute value of the target current of the pre-charging bias when the absolute moisture content is a second value greater than the first value. Setting the target current of the pre-charging bias according to the environment can be performed for each range of temperature and humidity environments (absolute moisture content, etc.), such as a normal temperature and low humidity environment (or a low-temperature, low-humidity environment), a normal temperature and normal humidity environment, or a high temperature and high humidity environment. Furthermore, the target current of the pre-charging bias can be set according to the environment for each type of recording material S. By determining whether pre-charging is necessary or setting the target current of the pre-charging bias according to the environment in this way, it is possible to perform control such as increasing the absolute value of the voltage of the secondary transfer bias in a low-temperature, low-humidity environment and reducing the absolute value of the voltage of the secondary transfer bias in a high-temperature, high-humidity environment.

[0065] However, the change in charge amount as shown in Figure 5 has various characteristics depending on the material and layer structure of the recording material S, the storage conditions of the recording material S, and so it is difficult to predict with precision. Therefore, even when determining whether pre-charging is necessary or setting the target current for the pre-charging bias depending on the environment as described above, the transfer current may be insufficient with the predetermined settings. The same is true when determining whether pre-charging is necessary or setting the target current for the pre-charging bias depending on the printing surface of the recording material S as described above.

[0066] Therefore, in this embodiment, the image forming apparatus 100 is configured to adjust (correct) the pre-charging bias based on the current detected by the current detection sensor 25b when the recording material S passes through the secondary transfer portion N2 during job execution. In this embodiment, the target current (initial value) of the pre-charging bias set when the job is started is changed based on the detection result of the current detection sensor 25b. Even if pre-charging is determined to be unnecessary and set to OFF (0 V) when the job is started, pre-charging may be turned ON based on the detection result of the current detection sensor 25b. In this case, the target current of the pre-charging bias can be set based on the detection result of the current detection sensor 25b.

[0067] In this embodiment, the current detection sensor 25b detects the current each time a recording material S is conveyed to the secondary transfer portion N2 during job execution. If the target current of the pre-charging bias is changed based on the detection result of the current detection sensor 25b, the changed target current is maintained until it is determined that the next change is necessary.

[0068] In this embodiment, the timing for obtaining the detection result of the current detection sensor 25b when the recording material S is passing through the secondary transfer portion N2 is set to while the leading edge (leading edge side relative to the center in the conveyance direction) of the recording material S in the conveyance direction passes through the secondary transfer portion N2. This timing is typically while an area (margin) outside the image formation area (area onto which a toner image can be transferred) on the leading edge side in the conveyance direction of the recording material S passes through the secondary transfer portion N2, but it may also be while the image formation area in the conveyance direction of the recording material S passes through the secondary transfer portion N2.

[0069] In this embodiment, when the current detection sensor 25b detects the current, substantially the same secondary transfer bias (secondary transfer bias controlled to a constant voltage at the same target voltage) as that used when transferring the toner image to the recording material S is applied to the inner secondary transfer roller 71. As a result, the current detection sensor 25b detects a current that is the sum of the current flowing due to the secondary transfer bias and a current corresponding to the charge amount of the toner image transfer surface of the recording material S. However, in cases where the current detection sensor 25b detects the current when the leading edge margin of the recording material S in the transport direction passes through the secondary transfer portion N2, the following may be done. That is, for example, when the current detection sensor 25b detects the current, a test bias different from the secondary transfer bias used when transferring the toner image to the recording material S may be applied to the inner secondary transfer roller 71. Alternatively, when the current detection sensor 25b detects the current, no bias may be applied to the inner secondary transfer roller 71. If no bias is applied to the inner secondary transfer roller 71, the current detection sensor 25b detects a current corresponding to the charge amount of the toner image transfer surface of the recording material S. In other words, it is only necessary to detect the current that flows in accordance with the charge amount of the toner image transfer surface of the recording material S.

[0070] In this embodiment, when using YUPO YPI250 as the recording material S, for example, in an environment with a temperature of 23°C and a humidity of 5% RH, a secondary transfer current In of -100 μA or more (e.g., -100 to -120 μA) is required to properly transfer a toner image to the recording material S. Therefore, in this embodiment, a threshold value is set for comparison with the detection result of the current detection sensor 25b. If the detection result of the current detection sensor 25b exceeds (crosses) the threshold value toward the transfer current insufficiency side, it is determined that the target current of the pre-charging bias needs to be changed. Specifically, in this embodiment, the threshold value is set to -100 μA. For convenience, the magnitude relationship between the current value and the threshold value will be described in terms of absolute values. In other words, in this embodiment, if the detection result of the current detection sensor 25b is less than the threshold value of -100 μA (e.g., -80 μA), it is determined that the target current of the pre-charging bias needs to be changed. The secondary transfer current In required to properly transfer the toner image to the recording material S may vary depending on the type of recording material S, the environment, and even the printing surface (the surface to which the toner image is transferred at the secondary transfer unit N2; whether it is the first side of single-sided printing or double-sided printing, or the second side of double-sided printing). Therefore, the threshold value can be changed based on at least one of the type of recording material S, the environment, and the printing surface. For example, the absolute value of the threshold value when the absolute moisture content is a second value greater than the first value can be set to be smaller than the absolute value of the threshold value when the absolute moisture content is a first value. The threshold value can be set for each range of predetermined temperature and humidity environments (absolute moisture content, etc.), such as a normal temperature and low humidity environment (or a low temperature and low humidity environment), a normal temperature and normal humidity environment, or a high temperature and high humidity environment. The threshold value is preset and stored in memory 122 as table data or the like.

[0071] In this embodiment, the target current of the pre-charging bias is changed all at once based on information indicating the tendency of changes in the charge amount on the toner image transfer surface of the recording material S due to changes in the target current of the pre-charging bias, which has been previously determined. That is, based on this information, the amount of change in the target current of the pre-charging bias is determined so that the detection result of the current detection sensor 25b no longer exceeds the threshold value on the transfer current deficiency side. Specifically, in this embodiment, the target current of the pre-charging bias is changed all at once so that the detection result of the current detection sensor 25b becomes equal to or greater than the threshold value of −100 μA (e.g., −100 to −120 μA). However, the method of changing the target current of the pre-charging bias is not limited to this, and the target current of the pre-charging bias may also be changed gradually. When the target current of the pre-charging bias is changed gradually, it may be changed in steps by a predetermined amount (e.g., 1 to 10 μA in absolute value), or may be changed linearly or curvedly. Then, for example, when the detection result of the current detection sensor 25b no longer exceeds the threshold value on the transfer current deficiency side, the change in the target current of the pre-charging bias may be terminated.

[0072] In this embodiment, when it is determined that the target current of the pre-charging bias needs to be changed based on the current detected for a certain recording material S (typically the first recording material S in a job), the change is made while the recording material S is passing through the pre-charging section N3. However, depending on, for example, the size of the recording material S and the required amount of change in the target current of the pre-charging bias, when it is determined that the target current of the pre-charging bias needs to be changed based on the current detected for a certain recording material S, the change may be made after the recording material S has passed through the pre-charging section N3. "After the recording material S has passed through the pre-charging section N3" includes a time when there is no recording material S at the pre-charging section N3 after the recording material S has passed through the pre-charging section N3 (between sheets) and a time when the recording material S to be transported after the recording material S (for example, the next recording material S) is at the pre-charging section N3.

[0073] Furthermore, in this embodiment, one threshold value is set, but multiple threshold values ​​may be set and the target current of the pre-charging bias may be changed in stages each time the detection result of the current detection sensor 25b exceeds each threshold value.

[0074] 6. Control Procedure Next, a job control procedure including pre-charging bias adjustment control in this embodiment will be described. Fig. 6 is a flowchart showing an outline of the job control procedure including pre-charging bias adjustment control in this embodiment. As described above, in this embodiment, when a job is started, if a paper type category of synthetic paper or a predetermined high resistance paper (such as extra thick paper) is selected, it is determined that pre-charging is necessary, and if any other paper type category is selected, it is determined that pre-charging is not necessary.

[0075] First, when the user inputs job information including information specifying the type of recording material S and image information from an external device such as the operation unit 130 or a personal computer (S101), the control unit 120 determines the type of recording material S to be used for image formation (S102). The information specifying the type of recording material S may be information specifying one of the cassettes 11a, 11b that contains the recording material S. In this case, the control unit 120 can 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. Then, the control unit 120 determines whether pre-charging is necessary based on the acquired information about the type of recording material S (S103).

[0076] If the control unit 120 determines in S103 that pre-charging is necessary, it starts applying a pre-charging bias to the drive roller 84 (S104) and starts image formation (S105). If the control unit 120 determines in S103 that pre-charging is necessary, it sets a target current (initial value) of the pre-charging bias in accordance with the type of recording material S in S104. As described above, the following procedure can be used to determine whether pre-charging is necessary and to set the target current of the pre-charging bias in accordance with the environment. That is, the control unit 120 acquires the detection result of the environmental sensor 18 in S102, calculates the absolute moisture content as temperature and humidity information, and then makes the above-mentioned determination and setting in accordance with the temperature and humidity information in S103 and S104. As described above, the following procedure can be used to determine whether pre-charging is necessary and to set the target current of the pre-charging bias in accordance with the print surface. That is, the control unit 120 acquires information about the printing surface in S102, and performs the above-mentioned determination and setting according to the printing surface in S103 and S104. Furthermore, if the control unit 120 determines in S103 that pre-charging is not necessary, it starts image formation (S105) without applying a pre-charging bias (0 V) to the drive roller 84. Here, for convenience of explanation, the start of application of the pre-charging bias and the start of image formation are described in series, but the formation of a toner image on the photosensitive drum 1 or the intermediate transfer belt 70 may start before the application of the pre-charging bias is started.

[0077] When the leading edge of the recording material S in the conveyance direction enters the secondary transfer portion N2, the control portion 120 acquires the detection result of the secondary transfer current In by the current detection sensor 25b while the recording material S passes through the secondary transfer portion N2 (S106). Then, the control portion 120 determines whether the detected current value is less than a threshold value (for example, −100 μA) (S107). If the control portion 120 determines in S107 that the detected current value is less than the threshold value, it changes the target current of the pre-charging bias to correct the pre-charging bias (S108). On the other hand, if the control portion 120 determines in S107 that the detected current value is equal to or greater than the threshold value, it proceeds to the process of S109.

[0078] In this embodiment, when the synthetic paper category is selected as the type of recording material S, a pre-charging bias is applied with a target current of -60 μA. This charges the toner image transfer surface of the recording material S to approximately 5 kV. Furthermore, in the configuration of this embodiment, when YUPO YPI250 is used as the recording material S, for example, in an environment with a temperature of 23°C and a humidity of 5% RH, a secondary transfer current In of -100 μA or more (e.g., -100 to -120 μA) is required to properly transfer the toner image to the recording material S. Therefore, in this embodiment, the threshold value is set to -100 μA. When YUPO YPI250 is used as the recording material S, in an environment with a temperature of 23°C and a humidity of 5% RH, the toner image transfer surface of the recording material S remains charged to approximately 5 kV when the recording material S reaches the secondary transfer portion N2 (see FIG. 5). Therefore, in this case, the secondary transfer current In while the recording material S is passing through the secondary transfer portion N2 is -100 μA or more (for example, -120 μA) (Table 1), and it is determined that a change in the target current of the pre-charging bias is not necessary (No in S107). On the other hand, if YUPO YPI250 is used as the recording material S, in an environment with a temperature of 23°C and a humidity of 60% RH, the charge amount on the toner image transfer surface of the recording material S when the recording material S reaches the secondary transfer portion N2 is approximately 3 kV (see FIG. 5). Therefore, in this case, the secondary transfer current In while the recording material S is passing through the secondary transfer portion N2 is less than -100 μA (for example, -80 μA) (Table 1), and it is determined that a change in the target current of the pre-charging bias is necessary (Yes in S107). For example, if the detected secondary transfer current In is -80 μA in an environment with a temperature of 23°C and humidity of 60% RH, the target current of the pre-charging bias is changed to -100 μA by adding -40 μA to the initial value of -60 μA (Table 1). As a result, the charge on the toner image transfer surface of the recording material S when it reaches the secondary transfer portion N2 is approximately 5 kV. Therefore, the secondary transfer current In while the recording material S is passing through the secondary transfer portion N2 is -100 μA or higher (for example, -120 μA), allowing the toner image to be properly transferred to the recording material S. In other words, by correcting the pre-charging bias, the charge required to properly transfer the toner image onto the recording material S can be applied to the toner image transfer surface of the recording material S at the pre-charging portion N3.Furthermore, in this embodiment, at the timing when the leading edge of the recording material S in the conveyance direction enters the secondary transfer portion N2, while the recording material S is passing through the secondary transfer portion N2, the pre-charging bias is corrected immediately, thereby making it possible to reduce the deterioration of image quality after the timing when the pre-charging bias is corrected.

[0079] Thereafter, the control unit 120 determines whether or not the output of all images specified in the job has been completed (S109), and if so, ends the operation of the job, and if not, returns to the processing of S106.

[0080] [Table 1]

[0081] In this embodiment, the pre-charging bias (recording material charging bias) is controlled by constant current, but the pre-charging bias may also be controlled by constant voltage. It is sufficient if the toner image transfer surface of the recording material S can be charged to a polarity opposite to the normal charging polarity of the toner. When the pre-charging bias is controlled by constant voltage, the target voltage can be adjusted instead of the target current in the above embodiment.

[0082] Furthermore, in this embodiment, the secondary transfer bias is controlled to a constant voltage, but the secondary transfer bias may also be controlled to a constant current. When the secondary transfer bias is controlled to a constant current, the voltage detection sensor 25a may detect the voltage instead of the current detection sensor 25b that detects the current when the recording material S passes through the secondary transfer portion N2 during job execution in this embodiment. The larger the absolute value of the detected voltage, the smaller the amount of charge (absolute value) of the toner image transfer surface of the recording material S due to pre-charging.

[0083] As described above, in this embodiment, the image forming apparatus 100 includes an image carrier (intermediate transfer belt) 70 that carries a toner image, transfer members (secondary transfer inner roller 71, secondary transfer outer roller 81) that form a transfer section (secondary transfer section) N2 that transfers the toner image from the image carrier 70 to the recording material S, a first application section (secondary transfer power source) E4 that applies a transfer bias to the transfer section N2, and a second application section (secondary transfer power source) E5 that applies a transfer bias to the transfer section N2. The second application section E5 is located upstream of the transfer section N2 in the conveyance direction of the recording material S and applies a charge polarity opposite to the normal charge polarity of the toner to the toner image transfer surface, which is the surface onto which the toner image of the recording material S conveyed to the transfer section N2 is transferred. The image forming apparatus includes a recording material charging member (secondary transfer belt drive roller) 84 that charges the recording material S to a polarity, a second application unit (pre-charging power supply) E5 that applies a recording material charging bias (pre-charging bias) to the recording material charging member 84 to charge the toner image transfer surface of the recording material S, a detection unit (current detection sensor 25b in this embodiment) 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 executes control to change the setting of the recording material charging bias based on the detection result of the detection unit 25b when the recording material S is passing through the transfer unit N2. In this embodiment, the control unit 120 executes control to change the setting of the recording material charging bias based on the detection result of the detection unit 25b when the recording material S is passing through the transfer unit N2 during execution of a job, which is a series of operations for forming and outputting an image on one or more recording materials S. In this embodiment, the control unit 120 executes control to change the setting of the recording material charging bias so that the setting of the recording material charging bias is not changed when the absolute value of the current detected by the detection unit 25b is a first value, and the absolute value of the recording material charging bias is increased when the absolute value of the current detected by the detection unit 25b is a second value smaller than the first value. In this case, when the current is detected by the detection unit 25b, a bias controlled to a constant voltage by the first application unit E4 is applied to the transfer unit N2. The control unit 120 can also execute control to change the setting of the recording material charging bias so that the setting of the recording material charging bias is not changed when the absolute value of the voltage detected by the detection unit (voltage detection sensor) 25a is a first value, and the absolute value of the recording material charging bias is increased when the absolute value of the voltage detected by the detection unit 25a is a second value larger than the first value.In this case, when the voltage is detected by the detection unit 25a, a bias controlled by a constant current is applied to the transfer unit N2 by the first application unit E4. In this embodiment, the image carrier 70 is an intermediate transfer body (intermediate transfer belt) onto which a toner image is transferred from another image carrier (photosensitive drum) 1.

[0084] As described above, according to this embodiment, the necessary charge can be appropriately applied to the toner image transfer surface of the recording material S before transfer so as to obtain appropriate transferability depending on the recording material S actually used for image formation, regardless of the settings of the recording material S. This reduces the degradation of image quality that occurs when the toner image transfer surface of the recording material S does not retain the necessary charge when the recording material S reaches the secondary transfer portion N2. According to this embodiment, even when the charge required to be applied to the toner image transfer surface of the recording material S before transfer significantly changes due to environmental changes, for example, in the case of a recording material S with high electrical resistance, the pre-charging bias can be corrected to obtain appropriate transferability. As such, according to this embodiment, the toner image transfer surface of the recording material S can be appropriately charged to a polarity opposite to the normal charge polarity of the toner before the recording material S reaches the secondary transfer portion N2 depending on the recording material S actually used for image formation.

[0085] The threshold value of the secondary transfer current In in this embodiment is an example and is not limited to this. This threshold value can be set arbitrarily within a range that can sufficiently prevent the occurrence of image defects such as transfer defects and low density due to insufficient transfer of the toner image from the image carrier to the recording material S.

[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, the pre-charging bias is corrected when the detection result of the secondary transfer current In while the recording material S is passing through the secondary transfer portion N2 exceeds a threshold value on the side of insufficient transfer current. In contrast, in the present embodiment, the pre-charging bias is corrected when the detection result of the secondary transfer current In while the recording material S is passing through the secondary transfer portion N2 exceeds a threshold value on the side of excessive transfer current.

[0088] For example, as in the first embodiment, if a paper type category of synthetic paper or a predetermined high-resistivity paper (such as extra-thick paper) is selected when a job is started, it is determined that pre-charging is necessary, and if any other paper type category is selected, it is determined that pre-charging is not necessary. In the configuration of this embodiment, if a paper type category of synthetic paper is selected as the type of recording material S, for example, in an environment with a temperature of 23°C and a humidity of 5% RH, the transfer current will be insufficient with just the secondary transfer bias. Therefore, in this case, a pre-charging bias is applied with a target current of -60 μA.

[0089] However, it is conceivable that due to a mismatch between the set paper type category information and the type (characteristics) of the recording material S actually used for image formation, the pre-charging may result in an excessive amount of charge being applied to the toner image transfer surface of the recording material S, more than is actually necessary. In this case, an excessive current may flow at the secondary transfer unit N2, causing a discharge at the secondary transfer unit N2, which may in turn cause the polarity of the charge of the toner in the toner image to be reversed, resulting in image defects such as "whiteouts" where the toner image is not transferred in parts.

[0090] Therefore, in this embodiment, similar to the first embodiment, the image forming apparatus 100 is configured to be able to adjust (correct) the pre-charging bias based on the current detected by the current detection sensor 25b when the recording material S passes through the secondary transfer portion N2 during a job. In this embodiment, the target current (initial value) of the pre-charging bias set when the job is started is changed based on the detection result of the current detection sensor 25b. Furthermore, even if the pre-charging is turned ON because it is determined that it is necessary when the job is started, the pre-charging may be turned OFF based on the detection result of the current detection sensor 25b.

[0091] In this embodiment, the secondary transfer current In that prevents excessive current from flowing through the recording material S and resulting in image defects is less than −130 μA. Therefore, in this embodiment, a threshold value is set for comparison with the detection result of the current detection sensor 25b. If the detection result of the current detection sensor 25b exceeds (crosses) the threshold value, it is determined that the target current of the pre-charging bias needs to be changed. Specifically, in this embodiment, the threshold value is set to −130 μA. That is, in this embodiment, if the detection result of the current detection sensor 25b is equal to or greater than the threshold value of −130 μA (for example, −140 μA), it is determined that the target current of the pre-charging bias needs to be changed. The secondary transfer current In that prevents excessive current from flowing through the recording material S and resulting in image defects may vary depending on the type of recording material S, the environment, and even the print surface (the surface onto which the toner image is transferred at the secondary transfer portion N2; whether it is the first side in single-sided printing or double-sided printing, or the second side in double-sided printing). Therefore, the threshold value can be changed based on at least one of the type of recording material S, the environment, and the print surface. For example, the absolute value of the threshold value when the absolute moisture content is a first value can be set to be larger than the absolute value of the threshold value when the absolute moisture content is a second value greater than the first value. Note that such threshold value setting can be performed for each range of predetermined temperature and humidity environments (absolute moisture content, etc.), such as a normal temperature and low humidity environment (or a low temperature and low humidity environment), a normal temperature and normal humidity environment, and a high temperature and high humidity environment. The threshold values ​​are set in advance and stored in memory 122 as table data, etc.

[0092] Furthermore, the other points regarding the method and timing of changing the target current of the pre-charging bias, which were explained in the first embodiment when the absolute value of the target current of the pre-charging bias is increased, also apply to the present embodiment when the absolute value of the target current of the pre-charging bias is decreased.

[0093] 2. Control Procedure Next, a job control procedure including pre-charging bias adjustment control in this embodiment will be described. Fig. 7 is a flowchart showing an outline of the job control procedure including pre-charging bias adjustment control in this embodiment. As described above, in this embodiment, when a job is started, if a paper type category of synthetic paper or a predetermined high-resistivity paper (such as extra-thick paper) is selected, it is determined that pre-charging is necessary, and if any other paper type category is selected, it is determined that pre-charging is not necessary. In addition, here, the environment is assumed to be a temperature of 23°C and a humidity of 5% RH.

[0094] The processes of S201 to S209 shown in Fig. 7 in this embodiment are the same as the processes of S101 to S109 shown in Fig. 6 in embodiment 1, respectively, but the determination contents and adjustment contents in S207 and S208 are different from those in embodiment 1. In the processes in the procedure shown in Fig. 7, the description of the processes that are the same as the processes in the procedure shown in Fig. 6 will be omitted as appropriate.

[0095] In this embodiment, the control unit 120 determines whether the detected current value is equal to or greater than a threshold value (for example, −130 μA) (S207). If the control unit 120 determines in S207 that the current value is equal to or greater than the threshold value, the control unit 120 changes the target current of the pre-charging bias to correct the pre-charging bias (S208). On the other hand, if the control unit 120 determines in S207 that the current value is less than the threshold value, the control unit 120 proceeds to the process of S209.

[0096] In this embodiment, when synthetic paper is selected as the type of recording material S, a pre-charging bias is applied with a target current of -60 μA. This charges the toner image transfer surface of the recording material S to approximately 5 kV. In this embodiment, the secondary transfer current In is less than -130 μA, preventing excessive current from flowing through the recording material S and resulting in poor image quality. Therefore, in this embodiment, the threshold is set to -130 μA. Consider the case where synthetic paper is selected as the paper type category and either synthetic paper, Yupo YPI250, or non-synthetic OK Topcoat+128 gsm paper (a product name of Oji Paper Co., Ltd.), is used as the recording material S. OK Topcoat+128 gsm paper is a coated paper (glossy paper) with a coating applied to the surface of pulp-based paper. OK Topcoat+128 gsm paper has the same thickness as Yupo YPI250 but has lower electrical resistance. When YUPO YPI250 is used as the recording material S, in an environment with a temperature of 23°C and a humidity of 5% RH, the secondary transfer current In when the recording material S passes through the secondary transfer portion N2 is less than -130 μA (for example, -120 μA) (Table 2), and it is determined that a change in the target current of the pre-charging bias is not necessary (No in S207). On the other hand, when OK topcoat + 128 gsm paper is used as the recording material S, in an environment with a temperature of 23°C and a humidity of 5% RH, the secondary transfer current In when the recording material S passes through the secondary transfer portion N2 is -130 μA or more (for example, -140 μA) (Table 2), and it is determined that a change in the target current of the pre-charging bias is necessary (Yes in S207). For example, when OK topcoat + 128 gsm paper is used as the recording material S and the detected secondary transfer current In is -140 μA, the target current of the pre-charging bias is changed to -40 μA by subtracting -20 μA from the initial value of -60 μA (Table 2). As a result, the secondary transfer current In while the recording material S is passing through the secondary transfer portion N2 is -130 μA or less (for example, -120 μA), and the toner image can be properly transferred to the recording material S without causing image defects due to excessive current flow.In other words, by correcting the pre-charging bias, it is possible to apply a sufficient amount of charge to the toner image transfer surface of the recording material S at the pre-charging section N3 in order to properly transfer the toner image onto the recording material S. Note that when OK topcoat + 128 gsm paper is used as the recording material S, if coated paper is correctly set as the paper type category information, the pre-charging bias is not applied (No in S203). Therefore, in an environment with a temperature of 23°C and a humidity of 5% RH, the secondary transfer current In when the recording material S passes through the secondary transfer section N2 is less than -130 μA (for example, -120 μA), and it is determined that pre-charging is not necessary (No in S207).

[0097] [Table 2]

[0098] In this embodiment, the pre-charging bias (recording material charging bias) is controlled by constant current, but the pre-charging bias may also be controlled by constant voltage. It is sufficient if the toner image transfer surface of the recording material S can be charged to a polarity opposite to the normal charging polarity of the toner. When the pre-charging bias is controlled by constant voltage, the target voltage can be adjusted instead of the target current in the above embodiment.

[0099] Furthermore, in this embodiment, the secondary transfer bias is controlled to a constant voltage, but the secondary transfer bias may also be controlled to a constant current. When the secondary transfer bias is controlled to a constant current, the voltage detection sensor 25a may detect the voltage instead of the current detection sensor 25b that detects the current when the recording material S passes through the secondary transfer portion N2 during job execution in this embodiment. The smaller the absolute value of the detected voltage, the greater the amount of charge (absolute value) of the toner image transfer surface of the recording material S due to pre-charging.

[0100] In this embodiment, the control unit 120 controls the setting of the recording material charging bias so that it does not change when the absolute value of the current detected by the detection unit (current detection sensor) 25b is a first value, and so that it reduces the absolute value of the recording material charging bias when the absolute value of the current detected by the detection unit 25b is a second value greater than the first value. In this case, when the current is detected by the detection unit 25b, a bias controlled at a constant voltage by the first application unit (secondary transfer power supply) E4 is applied to the transfer unit N2. The control unit 120 can also control the setting of the recording material charging bias so that it does not change when the absolute value of the voltage detected by the detection unit (voltage detection sensor) 25a is a first value, and so that it reduces the absolute value of the recording material charging bias when the absolute value of the voltage detected by the detection unit 25a is a second value less than the first value. In this case, when the voltage is detected by the detection unit 25a, a bias controlled at a constant current by the first application unit E4 is applied to the transfer unit N2.

[0101] As described above, according to this embodiment, it is possible to appropriately apply a necessary and sufficient charge to the toner image transfer surface of the recording material S before transfer so that appropriate transferability can be obtained depending on the recording material S actually used for image formation, regardless of the setting state of the recording material S. This makes it possible to reduce degradation of image quality that occurs due to excessive current flow when the recording material S reaches the secondary transfer portion N2. According to this embodiment, even if excessive charge is applied by pre-charging due to, for example, a mismatch between the paper type category information and the actual type (characteristics) of the recording material S, it is possible to obtain appropriate transferability by correcting the pre-charging bias.

[0102] The threshold value of the secondary transfer current In in this embodiment is an example and is not limited to this. This threshold value can be set arbitrarily within a range that can sufficiently suppress the occurrence of image defects such as white spots that are caused by discharge at the transfer section due to excessive current flowing when the recording material S passes through the transfer section.

[0103] In addition, the control in Example 1 for increasing the absolute value of the target value of the pre-charging bias when the transfer current is insufficient and the control in this example for decreasing the absolute value of the target value of the pre-charging bias when the transfer current is excessive may be implemented in combination.

[0104] [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.

[0105] In the above-described embodiment, a roller (drive roller 84 in the above-described embodiment) provided on the inner circumferential surface of the secondary transfer belt 80 is used as the recording material charging member, but the present invention is not limited to this configuration. Several modified examples will be described. Note that, in the modified configuration, elements having the same or corresponding functions or configurations as those in the above-described embodiment will be described using the same reference numerals. For example, as shown in FIG. 8 , a pre-charging bias of the opposite polarity (positive polarity in the illustrated example) to the normal charging polarity of the toner may be applied from a pre-charging power source E5 to a pre-charging roller 92 serving as a recording material charging member corresponding to the pre-charging opposing roller 91 in the above-described embodiment. In this case, the driving roller (secondary transfer belt driving roller) 84 serving as the opposing member is electrically grounded. Furthermore, for example, as shown in FIG. 9 , a pair of rollers for charging the recording material S may be provided upstream of the secondary transfer portion N2 in the conveyance direction of the recording material S. 9, the pre-charging device 9 is configured to include a pre-charging roller 93 that contacts the surface of the recording material S opposite to the surface onto which the toner image is transferred, and a pre-charging opposing roller 91 that contacts the surface onto which the toner image is transferred. In this case, as in the configuration of FIG. 8, a pre-charging bias of a polarity opposite to the normal charging polarity of the toner may be applied to a roller corresponding to the pre-charging opposing roller 91 in the configuration of FIG. 9, and the roller corresponding to the pre-charging roller 93 in the configuration of FIG. 9 may be electrically grounded. Furthermore, a corona charger (having a discharging member as a recording material charging member) for charging the recording material S may be provided upstream of the secondary transfer portion N2 in the conveyance direction of the recording material S.

[0106] In the above-described embodiment, the image forming apparatus 100 includes a secondary transfer unit 8 including a secondary transfer belt 80. However, the present invention is not limited to this configuration. For example, as shown in FIG. 9, the image forming apparatus 100 may include an outer secondary transfer roller 81 that directly contacts the outer peripheral surface of the intermediate transfer belt 70. In this case, the outer secondary transfer roller 81 is pressed toward the inner secondary transfer roller 71 and abuts against the inner secondary transfer roller 71 via the intermediate transfer belt 70, forming a secondary transfer portion N2, which is a contact portion between the intermediate transfer belt 70 and the outer secondary transfer roller 81. Furthermore, as shown in FIG. 10, for example, even if the image forming apparatus 100 includes a secondary transfer unit 8 including a secondary transfer belt 80, the surface of the secondary transfer belt 80 that carries the recording material S may not be formed upstream of the secondary transfer portion N2 in the conveyance direction of the recording material S.

[0107] Furthermore, in the above-described embodiment, the image forming apparatus 100 employs an internal power supply system as the power supply system for the secondary transfer bias, but the image forming apparatus 100 may employ an external power supply system as the power supply system for the secondary transfer bias. That is, for example, the inner secondary transfer roller 71 in the above-described embodiment may be electrically grounded, and a secondary transfer bias of a polarity opposite to the normal charging polarity of the toner may be applied to the outer secondary transfer roller 81 in the above-described embodiment.

[0108] 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 members, but the present invention is not limited to this. The recording material charging member (pre-charging member) and the opposing member (pre-charging opposing member) may each independently be a roller-shaped member, a brush-shaped member, a sheet-shaped member, a pad-shaped member, or the like.

[0109] 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, for example, a single-drum type in which a toner image is transferred sequentially 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, for example, a transfer unit, which is a contact point between an image carrier (such as a photosensitive drum) and a transfer member (such as a transfer roller) to which a transfer bias is applied. The image forming apparatus may be an image forming apparatus for various purposes, such as a printer, various printing machines, copiers, fax machines, and multifunction peripherals. [Explanation of symbols]

[0110] 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 transfer member forming a transfer section that transfers a toner image from the image carrier to a recording material; a first applying unit that applies a transfer bias to the transfer unit; a recording material charging member that is located upstream of the transfer unit in the recording material conveyance direction and that charges a toner image transfer surface, onto which a toner image of the recording material conveyed to the transfer unit, to a polarity opposite to the normal charging polarity of the toner; a second applying section that applies a recording material charging bias to the recording material charging member to charge the toner image transfer surface of the recording material; a detection unit that detects a current flowing through the transfer unit or a voltage applied to the transfer unit; a control unit that executes control to change the setting of the recording material charging bias based on a detection result by the detection unit when the recording material passes through the transfer unit; An image forming apparatus comprising:

2. The image forming apparatus according to claim 1, characterized in that the control unit executes control to change the setting of the recording material charging bias based on the detection results of the detection unit when the recording material passes through the transfer unit during execution of a job, which is a series of operations to form and output an image on one or more recording materials.

3. 2. The image forming apparatus according to claim 1, wherein the control unit executes control to change the setting of the recording material charging bias so that the setting of the recording material charging bias is not changed when the absolute value of the current detected by the detection unit is a first value, and the absolute value of the recording material charging bias is increased when the absolute value of the current detected by the detection unit is a second value smaller than the first value.

4. 2. The image forming apparatus according to claim 1, wherein the control unit executes control to change the setting of the recording material charging bias so that the setting of the recording material charging bias is not changed when the absolute value of the current detected by the detection unit is a first value, and the absolute value of the recording material charging bias is reduced when the absolute value of the current detected by the detection unit is a second value greater than the first value.

5. 5. The image forming apparatus according to claim 3, wherein when the detecting section detects the current, the first applying section applies a bias voltage controlled to a constant voltage to the transfer section.

6. 2. The image forming apparatus according to claim 1, wherein the control unit executes control to change the setting of the recording material charging bias so that the setting of the recording material charging bias is not changed when the absolute value of the voltage detected by the detection unit is a first value, and the absolute value of the recording material charging bias is increased when the absolute value of the voltage detected by the detection unit is a second value greater than the first value.

7. 2. The image forming apparatus according to claim 1, wherein the control unit executes control to change the setting of the recording material charging bias so that the setting of the recording material charging bias is not changed when the absolute value of the voltage detected by the detection unit is a first value, and the absolute value of the recording material charging bias is reduced when the absolute value of the voltage detected by the detection unit is a second value smaller than the first value.

8. 8. The image forming apparatus according to claim 6, wherein when the detecting section detects the voltage, the first applying section applies a bias that is constant current controlled to the transfer section.

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

Images