Image forming apparatus

Abstract

To allow a recording material to be effectively charged in a recording material charging part, by restraining current from flowing between a secondary transfer part and the recording material charging part.SOLUTION: An image forming apparatus 100 includes: an image carrier 1; an intermediate transfer belt 70; a plurality of stretching rollers containing an inner roller 71; a secondary transfer member 81; a first application part E4 for applying a secondary transfer bias of the same polarity as a normal charge polarity of a toner, to the inner roller 71; a recording material charge member 84 which is arranged on a surface side opposite to a toner image transfer surface being a surface on which a toner image of a recording material S to be conveyed to a secondary transfer part N2 will be transferred, and which forms a recording material charge part N3 for charging the recording material S; an opposing member 91 which is arranged opposite to the recording material charge member 84, and which forms the recording material charge part N3 together with the recording material charge member 84; and a second application part E5 for applying a recording material charge bias of the same polarity as the secondary transfer bias, to the recording material charge member 84.SELECTED DRAWING: Figure 4

Description

[Technical Field]

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

[0002] Conventionally, there are image forming apparatuses of an intermediate transfer type, such as electrophotographic copying machines. In an image forming apparatus of the intermediate transfer type, a toner image formed on an image carrier is primarily transferred onto an intermediate transfer body at a primary transfer unit, and the toner image on the intermediate transfer body is secondarily transferred onto a recording material such as paper at a secondary transfer unit. An intermediate transfer belt, which is an endless belt stretched over multiple tension rollers, is often used as the intermediate transfer body.

[0003] In such an image forming apparatus, depending on the specifications of the recording material, image degradation such as transfer defects due to insufficient transfer current may occur. For example, when a toner image is transferred to a recording material with high electrical resistance in a low-humidity environment, the absolute value of the voltage of the secondary transfer bias for passing the necessary transfer current may exceed the high-voltage capacity, and transfer defects may occur due to insufficient transfer current.

[0004] Patent Document 1 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 secondary transfer portion. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 3517621 Summary of the Invention [Problem to be solved by the invention]

[0006] Recently, there has been a trend toward an increasing variety of recording materials used for image formation, for example, in production machines using intermediate transfer methods. For example, to properly transfer a toner image onto 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 secondary transfer bias with a very large absolute value of voltage. To properly transfer a toner image onto such a recording material without reducing productivity (image formation speed), it is effective to charge the surface of the recording material onto which the toner image is to be transferred in advance with a polarity opposite to the normal charging polarity of the toner, thereby compensating for the insufficient transfer current.

[0007] Here, it is desirable that the pre-charging section (recording material charging section), which pre-charges the surface of the recording material onto which the toner image is transferred, be as close to the secondary transfer section as possible. The farther the pre-charging section is from the secondary transfer section, the more the charge on the surface of the recording material onto which the toner image is transferred decreases, and the less effective it is at compensating for a lack of transfer current. However, if the pre-charging section is placed too close to the secondary transfer section, current may flow between the secondary transfer section and the pre-charging section through the recording material (interference between the transfer current and the pre-charging current), which could result in problems such as insufficient transfer current due to an insufficient transfer current.

[0008] Therefore, an object of the present invention is to suppress the flow of current between the secondary transfer section and the recording material charging section, thereby making it possible to effectively charge the recording material in the recording material charging section. [Means for solving the problem]

[0009] The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides an image forming apparatus including an image carrier that carries a toner image, an intermediate transfer belt onto which the toner image is transferred from the image carrier, a plurality of tension rollers that stretch the intermediate transfer belt, the plurality of tension rollers including an inner roller that forms a secondary transfer section that transfers the toner image from the intermediate transfer belt to a recording material, a secondary transfer member that is disposed on the outer peripheral surface of the intermediate transfer belt and forms the secondary transfer section together with the inner roller, a first application section that applies a secondary transfer bias to the inner roller, the secondary transfer bias having the same polarity as the normal charging polarity of the toner, in order to transfer the toner image from the intermediate transfer belt to the recording material, and a second application section that applies a second transfer bias to the inner roller, the second application section applying a second transfer bias having the same polarity as the normal charging polarity of the toner, in order to transfer the toner image from the intermediate transfer belt to the recording material, and a second application section that applies a second transfer bias to the inner roller in a direction in which the recording material is transported. a recording material charging member that is arranged upstream of the secondary transfer portion on the side opposite to a toner image transfer surface, onto which a toner image of a recording material transported to the secondary transfer portion is transferred, and that forms a recording material charging portion that charges the recording material; an opposing member that is arranged opposite the recording material charging member and forms the recording material charging portion together with the recording material charging member; and a second application portion that applies a recording material charging bias of the same polarity as the secondary transfer bias to the recording material charging member in order to charge the toner image transfer surface of the recording material to a polarity opposite to the normal charging polarity of the toner. [Effects of the Invention]

[0010] According to the present invention, it is possible to suppress the flow of current between the secondary transfer portion and the recording material charging portion, and to effectively charge the recording material in the recording material charging portion. [Brief explanation of the drawings]

[0011] [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] 2 is a schematic cross-sectional view of the vicinity of a secondary transfer unit in the image forming apparatus of the first embodiment. [Figure 5]FIG. 10 is a schematic cross-sectional view of the vicinity of a secondary transfer unit in an image forming apparatus of Comparative Example 1-2. [Figure 6] FIG. 10 is a schematic cross-sectional view of the vicinity of a secondary transfer unit in an image forming apparatus according to a second embodiment. [Figure 7] 10 is a schematic cross-sectional view of the vicinity of a secondary transfer unit in an image forming apparatus of Comparative Example 2. FIG. [Figure 8] FIG. 11 is a schematic cross-sectional view of the vicinity of a secondary transfer unit in an image forming apparatus according to a third embodiment. [Figure 9] FIG. 11 is a schematic cross-sectional view of the vicinity of a secondary transfer unit in an image forming apparatus of Comparative Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0012] The image forming apparatus according to the present invention will be described in more detail below with reference to the drawings.

[0013] [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.

[0014] 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.

[0015] The drum-shaped (cylindrical) photosensitive drum 1, which serves as an 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) 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 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) 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.

[0016] 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.

[0017] An intermediate transfer unit 7 is disposed facing the four photosensitive drums 1Y, 1M, 1C, and 1K. The intermediate transfer unit 7 includes an intermediate transfer belt 70, a secondary transfer inner roller 71, a drive roller 72, a tension roller 73, and primary transfer rollers 5Y, 5M, 5C, and 5K. The intermediate transfer belt 70, an endless belt serving as an intermediate transfer member, 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) serving as a drive unit. 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] A secondary transfer unit 8 is disposed on the outer peripheral surface of the intermediate transfer belt 70 so as to face the inner secondary transfer roller 71. The secondary transfer unit 8 includes a secondary transfer belt 80 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 so as to face the inner secondary transfer roller 71. The outer secondary transfer roller 81, a roller-type secondary transfer member serving as a secondary transfer means, is pressed against the inner secondary transfer roller 71 and comes into contact with the inner secondary transfer roller 71 via the secondary transfer belt 80 and the intermediate transfer belt 70. This causes the outer secondary transfer roller 81 to form a secondary transfer portion (secondary transfer nip portion) N2, which is a contact portion between the intermediate transfer belt 70 and the secondary transfer belt 80. The toner image formed on the intermediate transfer belt 70 is transferred (secondary transfer) onto the recording material S 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.

[0022] 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 serves as a conveying member. The recording material S is conveyed by the pair of registration rollers 14 toward a secondary transfer unit N2 in synchronization with the toner image on the intermediate transfer belt 70. 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] An environmental sensor 18 is also connected to the control unit 120. The environmental sensor 18 is an example of an environmental detection unit that detects at least one of the temperature and humidity of at least one of the inside and outside of 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 (near the cassettes 11a and 11b). The environmental sensor 18 inputs a signal indicating the detection results of the 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.

[0030] 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.

[0031] 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. The drum drive motor D1, the intermediate transfer belt drive motor D2, and the secondary transfer belt drive motor D3 may all or some of them be shared.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] In this embodiment, the secondary transfer bias is applied using constant voltage control. The voltage value (target voltage) of the secondary transfer bias is determined by adding a base voltage Vb for obtaining a predetermined transfer current and a recording material voltage Vp, which is determined depending on the type of recording material S. The recording material voltage Vp is preset according to the type of recording material S and the environment (e.g., absolute moisture content), and is stored in memory 122 as table data or the like. The base voltage Vb can be determined 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 no toner image or recording material S is present at the secondary transfer unit N2. For example, constant current control is performed with a predetermined transfer current as the target current, and the base voltage Vb can be determined based on the voltage generated at that time. The predetermined transfer current is preset according to the environment (e.g., absolute moisture content), and is stored in memory 122 as table data or the like. 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. Here, constant current control is control that adjusts the output of a power supply so that the current supplied to the target is kept substantially constant at a target current. Constant voltage control is control that adjusts the output of a power supply so that the voltage applied to the target is kept substantially 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, or synthetic paper) (so-called paper type category), numerical values ​​or numerical ranges for basis weight and thickness, and brand (including manufacturer, product number, etc.). Generally, the type of recording material S is often identified by the paper type category and thickness (or basis weight).

[0044] 4.Pre-charging device Next, the pre-charging device (recording material charging device) 8 in this embodiment will be further described.

[0045] As mentioned above, image forming apparatuses may experience image degradation, such as transfer defects due to insufficient transfer current, depending on the specifications of the recording material. Recently, the number of recording material types used for image formation, particularly in production machines using intermediate transfer systems, has been increasing. For example, in production machines with high image formation speeds, it can be difficult to properly transfer a toner image onto recording materials with high electrical resistance, such as ultra-thick paper (high-resistivity paper) or synthetic paper (high-resistivity recording material) due to its resin layer, without reducing productivity. For example, in low-humidity environments, the electrical resistance of the outer secondary transfer roller increases, making it necessary to increase the absolute value of the secondary transfer bias voltage to supply the required 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 defects due to insufficient transfer current may occur. This transfer defect may occur, for example, in secondary-color toner images. Furthermore, high-voltage power supplies capable of applying such secondary transfer bias are expensive, which may increase the cost of image forming apparatuses. Furthermore, even if such a high-voltage power supply is used, it may not be possible to ensure creepage due to the layout, and it may not be possible to apply a secondary transfer bias with a large absolute value of the voltage described above. Furthermore, if the absolute value of the secondary transfer bias voltage is increased, image defects due to discharge phenomena may occur at the secondary transfer section, making it difficult to obtain a proper image. Image defects due to this discharge phenomenon may include streak-like image defects, white flowers, or punch-through images caused by part of the toner image not being transferred or part of the toner image being disturbed (scattered).

[0046] 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 (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 even if the absolute value of the voltage of the secondary transfer bias is relatively small.

[0047] 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.

[0048] 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).

[0049] 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.

[0050] 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. Thus, in this embodiment, a pre-charging bias having the same polarity as the secondary transfer bias applied to the inner secondary transfer roller 71 is applied to the drive roller 84. In other words, the inner secondary transfer roller 71 and the drive roller 84 apply biases of the same polarity to the recording material S from different sides of the recording material S. By applying a pre-charging bias of the same polarity as the normal charging polarity of the toner to the surface of the recording material S opposite to the 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 of the recording material S onto which the toner image is transferred is charged to a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment) by the charge induced from the ground. At this time, an apparent current flows through the pre-charging section N3.

[0051] 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).

[0052] 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.

[0053] 5. Evaluation As an example, the transferability of a toner image onto synthetic paper, Yupo YPI200 (a product name of Yupo Corporation), used as recording material S, was evaluated in an environment of a temperature of 23°C and a humidity of 60%RH. The evaluation was carried out for this example, Comparative Example 1-1, and Comparative Example 1-2. The evaluation results are shown in Table 1. In Table 1, "◯" indicates good, and "×" indicates bad.

[0054] <Comparative Example 1-1> As Comparative Example 1-1, in the configuration of this example, the transferability of the toner image was evaluated when pre-charging was not performed (when the voltage value of the pre-charging bias was 0 V).

[0055] In Comparative Example 1-1, even when a secondary transfer bias voltage (approximately -8.0 kV) was applied, which was the sum of the base voltage Vb (base voltage for normal processing speed) and the maximum recording material distribution voltage Vp, it was not possible to pass a transfer current of -70 μA, which is the appropriate transfer current. As a result, transfer defects (transfer omissions) sometimes occurred in the secondary color solid images of magenta and cyan. Furthermore, in Comparative Example 1-1, because the absolute value of the secondary transfer bias voltage was large, image defects (streaks) sometimes occurred due to discharge at the secondary transfer portion N2.

[0056] <Comparative Example 1-2> In Comparative Example 1-2, the transferability of a toner image when pre-charging was performed in the following configuration was evaluated. The configuration of Comparative Example 1-2 is substantially the same as the configuration of this example, except for the differences described below. In addition, in the configuration of Comparative Example 1-2, elements having the same or corresponding functions or configurations as those in this example will be described using the same reference numerals.

[0057] FIG. 5 is a schematic cross-sectional view showing the vicinity of the secondary transfer portion N2 in Comparative Example 1-2 (showing a cross section substantially perpendicular to the rotational axis direction of the photosensitive drum 1 or the rotational axis direction of the tension roller of the secondary transfer belt 80). In Comparative Example 1-2, as in this embodiment, a secondary transfer bias of the same polarity (negative polarity) as the normal charging polarity of the toner is applied to the inner secondary transfer roller 71 by secondary transfer power source E4. In Comparative Example 1-2, a pre-charging bias of the opposite polarity (positive polarity) to the normal charging polarity of the toner is applied by pre-charging power source E5 to the pre-charging roller 92 corresponding to the pre-charging opposing roller 91 in this embodiment. In Comparative Example 1-2, the drive roller (secondary transfer belt drive roller) 84 is electrically grounded. The configuration of the pre-charging roller 92 in Comparative Example 1-2 is substantially the same as the configuration of the pre-charging opposing roller 91 in this embodiment.

[0058] In Comparative Example 1-2, the voltage value of the secondary transfer bias required to pass a transfer current of -70 μA, which is the appropriate transfer current, was approximately -6.0 kV. In Comparative Example 1-2, an attempt was made to supply a current of +30 μA to the pre-charging roller 92, but because the absolute values ​​of the negative secondary transfer bias of -6.0 kV and the positive pre-charging bias of +5.5 kV were large, the following occurred: In other words, the negative current due to the secondary transfer bias flowing through or on the surface of the recording material S and the positive current due to the pre-charging bias canceled each other out, preventing the required transfer current from being obtained, resulting in transfer defects (missing transfers) in the magenta and cyan secondary color solid images. Note that in Comparative Example 1-2, no image defects (streaks) due to discharge at the secondary transfer portion N2 occurred.

[0059] <This Example> In this example, a pre-charging bias was applied to the drive roller 84 in the pre-charging device 9, supplying a current of -30 μA to the drive roller 84. In this case, the voltage value of the pre-charging bias applied to the drive roller 84 was approximately -5.5 kV, and the surface onto which the YUPO YPI200 toner image was transferred immediately after passing through the pre-charging section N3 was charged to approximately +3.0 kV. Furthermore, the voltage value of the secondary transfer bias required to pass a transfer current of -70 μA, which is the appropriate transfer current, was approximately -6.0 kV. Furthermore, in this example, no transfer defects (transfer voids) occurred in the magenta and cyan secondary color solid images. Furthermore, in this example, no image defects (streaks) due to discharge at the secondary transfer section N2 occurred.

[0060] 6.Effects As described above, in this embodiment, the image forming apparatus 100 includes an image carrier (photosensitive drum) 1 that carries a toner image, an intermediate transfer belt 70 onto which the toner image is transferred from the image carrier 1, a plurality of tension rollers that tension the intermediate transfer belt 70, including an inner roller (secondary transfer inner roller) 71 that forms a secondary transfer portion N2 that transfers the toner image from the intermediate transfer belt 70 to a recording material S, a secondary transfer member (secondary transfer outer roller) 81 that is disposed on the outer peripheral surface side of the intermediate transfer belt 70 and forms the secondary transfer portion N2 together with the inner roller 71, and a first application unit (secondary transfer bias applying unit) that applies a secondary transfer bias of the same polarity as the normal charging polarity of the toner to the inner roller 71 in order to transfer the toner image from the intermediate transfer belt 70 to the recording material S. a recording material charging member (secondary transfer belt drive roller) 84 which is arranged upstream of the secondary transfer portion N2 in the conveying direction of the recording material S on the side opposite to the toner image transfer surface, which is the surface onto which the toner image of the recording material S conveyed to the secondary transfer portion N2 is transferred, and which forms a recording material charging portion (pre-charging portion) N3 which charges the recording material S; an opposing member (pre-charging opposing roller) 91 which is arranged opposite the recording material charging member 84 and forms the recording material charging portion N3 together with the recording material charging member 84; and a second application portion (pre-charging power supply) E5 which applies a recording material charging bias of the same polarity as the secondary transfer bias to the recording material charging member 84 in order to charge the toner image transfer surface of the recording material S to a polarity opposite to the normal charging polarity of the toner. In this embodiment, the image forming apparatus 100 has a secondary transfer belt 80 stretched over multiple rollers, including a secondary transfer roller (outer secondary transfer roller) 81, which is a secondary transfer member, and a recording material charging roller (drive roller) 84, which is a recording material charging member. The secondary transfer roller 81 contacts the inner roller 71 via the secondary transfer belt 80 and the intermediate transfer belt 70 to form a secondary transfer portion N2, and the recording material charging roller 84 contacts an opposing member 91 via the secondary transfer belt 80 to form a recording material charging portion N3. In this embodiment, the opposing member 91 is an opposing roller (pre-charging opposing roller) that contacts the recording material charging roller 84 via the secondary transfer belt 80. In this embodiment, the recording material charging bias is controlled by constant current. In this embodiment, the secondary transfer bias is controlled by constant voltage.

[0061] According to this embodiment, it is possible to improve the transferability of toner images onto extra-thick paper or synthetic paper with high electrical resistance without reducing productivity. In this way, according to this embodiment, it is possible to effectively charge the recording material S at the pre-charging section N3 by suppressing the flow of current between the secondary transfer section N2 and the pre-charging section N3.

[0062] [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.

[0063] 1. Configuration of this embodiment FIG. 6 is a schematic cross-sectional view showing the vicinity of the secondary transfer portion N2 in this embodiment (showing a cross section substantially perpendicular to the rotation axis direction of the photosensitive drum 1 or the rotation axis direction of the tension roller of the intermediate transfer belt 70).

[0064] In this embodiment, the image forming apparatus 100 has a secondary outer transfer roller 81 that directly contacts the outer peripheral surface of the intermediate transfer belt 70. The secondary outer transfer roller 81 is pressed toward the secondary inner transfer roller 71 and abuts against the secondary inner transfer roller 71 via the intermediate transfer belt 70, forming a secondary transfer portion N2 that is a contact portion between the intermediate transfer belt 70 and the secondary outer transfer roller 81. The configuration of the secondary outer transfer roller 81 in this embodiment is substantially the same as the configuration of the secondary outer transfer roller 81 in the first embodiment.

[0065] In this embodiment, as in the first embodiment, a secondary transfer power supply D4 is connected to the core of the inner secondary transfer roller 71, and a secondary transfer bias of the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment) is applied from a secondary transfer power supply E4 to the inner secondary transfer roller 71. Also, in this embodiment, the core of the outer secondary transfer roller 81 is connected to ground, and the outer secondary transfer roller 81 is electrically grounded.

[0066] In this embodiment, the pre-charging device 9 includes 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. The pre-charging roller 93 in this embodiment is an example of a recording material charging member (pre-charging member). The pre-charging opposing roller 91 in this embodiment is an example of an opposing member (pre-charging opposing member). The pre-charging roller 93 and the pre-charging opposing roller 91 form a pre-charging portion (pre-charging nip portion) N3, which is their contact portion. The configuration of the pre-charging roller 3 in this embodiment is substantially the same as the configuration of the drive roller (secondary transfer belt drive roller) 84 in the first embodiment. The configuration of the pre-charging opposing roller 91 in this embodiment is substantially the same as the configuration of the pre-charging opposing roller 91 in the first embodiment. In this embodiment, neither the pre-charging roller 93 nor the pre-charging opposing roller 91 is driven by a driving unit, but is rotated in response to the movement of the recording material S transported by the registration roller pair 14. However, at least one of the pre-charging roller 93 and the pre-charging counter roller 91 may be rotated by a driving means.

[0067] In this embodiment, a pre-charging power supply E5 is connected to the core of the pre-charging roller 93, and a pre-charging bias of the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment) is applied to the pre-charging roller 94 by the pre-charging power supply E5. Also, 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. Thus, in this embodiment, a pre-charging bias of the same polarity as the secondary transfer bias applied to the inner secondary transfer roller 71 is applied to the pre-charging roller 93. In other words, the inner secondary transfer roller 71 and the pre-charging roller 93 apply biases of the same polarity to the recording material S from different sides of the recording material S.

[0068] The recording material S conveyed by the registration roller pair 14 is charged in the pre-charging section N3 to a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment) on the surface onto which the toner image is transferred by a negative pre-charging bias applied to the pre-charging roller 93. The recording material S that has passed through the pre-charging section N3 is conveyed to the secondary transfer section N2, where the toner image is transferred (secondary transfer) onto the recording material S.

[0069] In this embodiment, a first guide member 22 (upper guide member 22a, lower guide member 22b) that guides the recording material S is provided upstream of the pre-charging portion N3 and downstream of the pair of registration rollers 14 in the conveying direction of the recording material S. The recording material S conveyed by the pair of registration rollers 14 is conveyed to the pre-charging portion N3 while being guided by the first guide member 22. In this embodiment, a second guide member 23 (upper guide member 23a, lower guide member 23b) that guides the recording material S is provided upstream of the secondary transfer portion N2 and downstream of the pre-charging portion N3 in the conveying direction of the recording material S. The recording material S that has passed through the pre-charging portion N3 is conveyed to the secondary transfer portion N2 while being guided by the second guide member 23.

[0070] 2. Evaluation As an example, the transferability of a toner image onto Yupo YPI200 as recording material S was evaluated in an environment of a temperature of 23° C. and a humidity of 60% RH. The evaluation was carried out for this example and Comparative Example 2. The evaluation results are shown in Table 1.

[0071] <This Example> In this example, a pre-charging bias was applied to the pre-charging roller 93 in the pre-charging device 9, supplying a current of -30 μA to the pre-charging roller 93. In this example, the voltage value of the pre-charging bias applied to the pre-charging roller 93 was approximately -5.0 kV, and the surface onto which the YUPO YPI200 toner image was transferred immediately after passing through the pre-charging section N3 was charged to approximately +3.0 kV. Furthermore, the voltage value of the secondary transfer bias required to pass a transfer current of -70 μA, which is the appropriate transfer current, was approximately -5.0 kV. Furthermore, in this example, no transfer defects (transfer voids) occurred in the magenta and cyan secondary color solid images. Furthermore, in this example, no image defects (streaks) due to discharge at the secondary transfer section N2 occurred.

[0072] <Comparative Example 2> In Comparative Example 2, the transferability of a toner image when pre-charging was performed in the following configuration was evaluated. The configuration of Comparative Example 2 is substantially the same as the configuration of this example, except for the differences described below. In addition, in the configuration of Comparative Example 2, elements having the same or corresponding functions or configurations as those in this example will be described using the same reference numerals.

[0073] FIG. 7 is a schematic cross-sectional view showing the vicinity of the secondary transfer portion N2 in Comparative Example 2 (showing a cross section substantially perpendicular to the rotational axis direction of the photosensitive drum 1 or the rotational axis direction of the tension roller of the intermediate transfer belt 70). In Comparative Example 2, as in the present embodiment, a secondary transfer bias of the same polarity (negative in this embodiment) as the normal charging polarity of the toner is applied to the inner secondary transfer roller 71 by the secondary transfer power source E4. In Comparative Example 2, a pre-charging bias of the opposite polarity (positive in this embodiment) to the normal charging polarity of the toner is applied to the pre-charging roller 92 corresponding to the pre-charging counter roller 91 in this embodiment. In Comparative Example 2, the pre-charging counter roller 94 corresponding to the pre-charging roller 93 in this embodiment is electrically grounded. The configuration of the pre-charging roller 92 in Comparative Example 2 is substantially the same as the configuration of the pre-charging counter roller 91 in this embodiment. In Comparative Example 2, the configuration of the pre-charging counter roller 94 in Comparative Example 2 is substantially the same as the configuration of the pre-charging counter roller 91 in this embodiment.

[0074] In Comparative Example 2, the voltage value of the secondary transfer bias required to pass a transfer current of -70 μA, which is the appropriate transfer current, was approximately -5.0 kV. In Comparative Example 2, an attempt was made to supply a current of +30 μA to the pre-charging roller 92, but because the absolute values ​​of the negative secondary transfer bias of -5.0 kV and the positive pre-charging bias of +5.0 kV were large, the following occurred: In other words, the negative current due to the secondary transfer bias flowing through or on the surface of the recording material S and the positive current due to the pre-charging bias canceled each other out, preventing the required transfer current from being obtained, resulting in transfer defects (missing transfers) in the magenta and cyan secondary color solid images. In Comparative Example 2, no image defects (streaks) due to discharge at the secondary transfer portion N2 occurred.

[0075] 3.Effects As described above, in this embodiment, the recording material charging member is the recording material charging roller (pre-charging roller) 93, and the opposing member is the opposing roller (pre-charging opposing roller) 91 that contacts the recording material charging roller 93. Also, in this embodiment, the secondary transfer member contacts the inner roller 71 via the intermediate transfer belt 70 to form the secondary transfer portion N2.

[0076] Furthermore, according to this embodiment, as in embodiment 1, it is possible to suppress the flow of current between the secondary transfer section N2 and the pre-charging section N3, thereby making it possible to effectively charge the recording material S at the pre-charging section N3.

[0077] [Example 3] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those of embodiment 1, and detailed descriptions thereof will be omitted.

[0078] 1. Configuration of this embodiment FIG. 8 is a schematic cross-sectional view showing the vicinity of the secondary transfer portion N2 in this embodiment (showing a cross section substantially perpendicular to the rotation axis direction of the photosensitive drum 1 or the rotation axis direction of the tension roller of the intermediate transfer belt 70).

[0079] In this embodiment, the image forming apparatus 100 has a secondary transfer unit 8 equipped with a secondary transfer belt 80. The secondary transfer unit 8 is configured to have the secondary transfer belt 80 and a plurality of tension rollers, namely, an outer secondary transfer roller 81, a separation roller 82, a tension roller 83, and a drive roller 84. However, in this embodiment, the secondary transfer belt 80 does not have a surface that supports the recording material S upstream of the secondary transfer portion N2 in the conveyance direction of the recording material S. The recording material S enters the secondary transfer portion N2 directly without being supported by the secondary transfer belt 80.

[0080] In this embodiment, as in the first embodiment, a secondary transfer power supply D4 is connected to the core of the inner secondary transfer roller 71, and a secondary transfer bias of the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment) is applied from a secondary transfer power supply E4 to the inner secondary transfer roller 71. Also, in this embodiment, the core of the outer secondary transfer roller 81 is connected to ground, and the outer secondary transfer roller 81 is electrically grounded.

[0081] Also, in this embodiment, the pre-charging device 9, like in Embodiment 2, 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. And, in this embodiment, like in Embodiment 2, a pre-charging power supply E5 is connected to the core of the pre-charging roller 93, and a pre-charging bias of the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment) is applied to the pre-charging roller 94 by the pre-charging power supply E5. Also, 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.

[0082] The recording material S conveyed by the registration roller pair 14 is charged in a pre-charging section N3 with a negative pre-charging bias applied to a pre-charging roller 93, so that the surface onto which the toner image is transferred is charged to a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment). The recording material S that has passed through the pre-charging section N3 is conveyed to a secondary transfer section N2, where the toner image is transferred (secondary transfer) onto the recording material S. In this embodiment, as in the second embodiment, a first guide member 22 (upper guide member 22a, lower guide member 22b) and a second guide member 23 (upper guide member 23a, lower guide member 23b) are provided on the conveying path of the recording material S.

[0083] 2. Evaluation As an example, the transferability of a toner image onto Yupo YPI200 as recording material S was evaluated in an environment of a temperature of 23° C. and a humidity of 60% RH. The evaluation was carried out for this example and Comparative Example 2. The evaluation results are shown in Table 1.

[0084] <This Example> In this example, a pre-charging bias was applied to the pre-charging roller 93 in the pre-charging device 9, supplying a current of -30 μA to the pre-charging roller 93. In this example, the voltage value of the pre-charging bias applied to the pre-charging roller 93 was approximately -5.0 kV, and the surface onto which the YUPO YPI200 toner image was transferred immediately after passing through the pre-charging section N3 was charged to approximately +3.0 kV. Furthermore, the voltage value of the secondary transfer bias required to pass an appropriate transfer current of -70 μA was approximately -6.0 kV. Furthermore, in this example, no transfer defects (transfer voids) occurred in the magenta and cyan secondary color solid images. Furthermore, in this example, no image defects (streaks) due to discharge at the secondary transfer section N2 occurred.

[0085] <Comparative Example 3> In Comparative Example 3, the transferability of a toner image when pre-charging was performed in the following configuration was evaluated. The configuration of Comparative Example 3 is substantially the same as the configuration of this example, except for the differences described below. In addition, in the configuration of Comparative Example 3, elements having the same or corresponding functions or configurations as those in this example will be described using the same reference numerals.

[0086] FIG. 9 is a schematic cross-sectional view showing the vicinity of the secondary transfer portion N2 in Comparative Example 3 (showing a cross section substantially perpendicular to the rotational axis direction of the photosensitive drum 1 or the rotational axis direction of the tension roller of the intermediate transfer belt 70). In Comparative Example 3, as in the present embodiment, a secondary transfer bias of the same polarity (negative polarity) as the normal charging polarity of the toner is applied to the inner secondary transfer roller 71 by the secondary transfer power source E4. In Comparative Example 3, a pre-charging bias of the opposite polarity (positive polarity) to the normal charging polarity of the toner is applied to the pre-charging roller 92 corresponding to the pre-charging counter roller 91 in the present embodiment. In Comparative Example 3, the pre-charging counter roller 94 corresponding to the pre-charging roller 93 in the present embodiment is electrically grounded. The configuration of the pre-charging roller 92 in Comparative Example 3 is substantially the same as the configuration of the pre-charging counter roller 91 in the present embodiment. In Comparative Example 3, the configuration of the pre-charging counter roller 94 in Comparative Example 3 is substantially the same as the configuration of the pre-charging counter roller 91 in the present embodiment.

[0087] In Comparative Example 3, the voltage value of the secondary transfer bias required to pass a transfer current of -70 μA, which is the appropriate transfer current, was approximately -6.0 kV. In Comparative Example 3, an attempt was made to supply a current of +30 μA to the pre-charging roller 92, but because the absolute values ​​of the negative secondary transfer bias of -6.0 kV and the positive pre-charging bias of +5.5 kV were large, the following occurred: In other words, the negative current due to the secondary transfer bias flowing through or on the surface of the recording material S and the positive current due to the pre-charging bias canceled each other out, preventing the required transfer current from being obtained, resulting in transfer defects (missing transfers) in the magenta and cyan solid secondary color images. Note that in Comparative Example 3, no image defects (streaks) due to discharge at the secondary transfer portion N2 occurred.

[0088] 3.Effects As described above, in this embodiment, the recording material charging member is a recording material charging roller (pre-charging roller) 93, and the opposing member is an opposing roller (pre-charging opposing roller) 91 that contacts the recording material charging roller 93. Also, in this embodiment, the image forming apparatus 100 has a secondary transfer belt 80 that is stretched over multiple rollers including a secondary transfer roller (secondary transfer outer roller) 81 that is a secondary transfer member, and the secondary transfer roller 81 contacts an inner roller 71 via the secondary transfer belt 80 and the intermediate transfer belt 70 to form a secondary transfer portion N2.

[0089] Furthermore, according to this embodiment, as in embodiment 1, it is possible to suppress the flow of current between the secondary transfer section N2 and the pre-charging section N3, thereby making it possible to effectively charge the recording material S at the pre-charging section N3.

[0090] [Table 1]

[0091] [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.

[0092] 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.

[0093] In the above embodiment, the secondary transfer bias is controlled to a constant voltage, but the secondary transfer bias may be controlled to a constant current.

[0094] Furthermore, in the above-described embodiment, the recording material charging bias (pre-charging bias) is controlled by constant current, but the recording material charging bias (pre-charging bias) may be controlled by constant voltage.

[0095] In the above-described embodiment, the image forming apparatus is a tandem-type color image forming apparatus employing an intermediate transfer method capable of forming full-color images. However, the image forming apparatus is not limited to an image forming apparatus capable of forming full-color images, and may be an image forming apparatus capable of forming only monochrome (black and white or monochromatic) images. The image forming apparatus may also be an image forming apparatus for various purposes, such as a printer, various printing machines, copiers, fax machines, and multifunction machines. [Explanation of symbols]

[0096] 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; an intermediate transfer belt onto which a toner image is transferred from the image carrier; a plurality of tension rollers that tension the intermediate transfer belt, the plurality of tension rollers including an inner roller that forms a secondary transfer portion that transfers a toner image from the intermediate transfer belt to a recording material; a secondary transfer member disposed on an outer peripheral surface side of the intermediate transfer belt and forming the secondary transfer portion together with the inner roller; a first applying unit that applies a secondary transfer bias having the same polarity as the normal charging polarity of the toner to the inner roller in order to transfer the toner image from the intermediate transfer belt to the recording material; a recording material charging member that is disposed upstream of the secondary transfer unit in the recording material conveyance direction, on the side opposite to a toner image transfer surface, onto which a toner image is transferred, of the recording material conveyed to the secondary transfer unit, and that forms a recording material charging unit that charges the recording material; an opposing member disposed opposite the recording material charging member and forming the recording material charging portion together with the recording material charging member; a second application unit that applies a recording material charging bias having the same polarity as the secondary transfer bias to the recording material charging member in order to charge the toner image transfer surface of the recording material to a polarity opposite to the normal charging polarity of the toner; An image forming apparatus comprising:

2. a secondary transfer belt stretched by a plurality of rollers including a secondary transfer roller as the secondary transfer member and a recording material charging roller as the recording material charging member; the secondary transfer roller contacts the inner roller via the secondary transfer belt and the intermediate transfer belt to form the secondary transfer portion; 2. The image forming apparatus according to claim 1, wherein the recording material charging roller contacts the opposing member via the secondary transfer belt to form the recording material charging portion.

3. 3. The image forming apparatus according to claim 2, wherein the opposing member is an opposing roller that contacts the recording material charging roller via the secondary transfer belt.

4. 2. The image forming apparatus according to claim 1, wherein the recording material charging member is a recording material charging roller, and the opposing member is an opposing roller that contacts the recording material charging roller.

5. 5. The image forming apparatus according to claim 4, wherein the secondary transfer member contacts the inner roller via the intermediate transfer belt to form the secondary transfer portion.

6. a secondary transfer belt stretched over a plurality of rollers including a secondary transfer roller serving as the secondary transfer member; 5. The image forming apparatus according to claim 4, wherein the secondary transfer roller contacts the inner roller via the secondary transfer belt and the intermediate transfer belt to form the secondary transfer portion.

7. 7. The image forming apparatus according to claim 1, wherein the recording material charging bias is controlled by a constant current.

8. 7. The image forming apparatus according to claim 1, wherein the secondary transfer bias is controlled to a constant voltage.

Images