Image forming apparatus

Abstract

To prevent the occurrence of an image defect due to a transfer memory.SOLUTION: An image forming apparatus has: a first electrifying member 2; a second electrifying member 3; a developing member 5a that supplies a toner electrified to have a predetermined polarity to a photoreceptor 1; a transfer member 6; a first electrification voltage application unit 20 that applies first electrification voltage having the predetermined polarity; a second electrification voltage application unit 21 that applies second electrification voltage having the predetermined polarity; a transfer voltage application unit 18 that applies transfer voltage having a polarity opposite to the predetermined polarity; and a control unit 40. The control unit 40 controls the transfer voltage application unit 18 so that at least part of an area on a surface of the photoreceptor 1 brought into direct contact with the transfer member 6 at a transfer position Pe has an electric potential having a polarity opposite to the predetermined polarity, controls the first electrification voltage application unit 20 to apply the first electrification voltage less than discharge start voltage to electrify the surface of the photoreceptor 1 at a first electrification position Pa, and controls the second electrification voltage application unit 21 to apply the second electrification voltage equal to or more than the discharge start voltage to electrify the surface of the photoreceptor 1 at a second electrification position Pb.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus, such as a printer, a copying machine, or a facsimile machine, that uses an electrophotographic method. [Background technology]

[0002] Conventionally, in electrophotographic image forming devices such as laser printers, the surface of a photoconductor is charged by a charging means, and the charged surface of the photoconductor is exposed (imagewise exposed) by an exposure means to form an electrostatic latent image on the photoconductor. A toner is then attached to the electrostatic latent image by a developing means to form a toner image on the photoconductor, and the toner image is then transferred to a sheet-like recording material by a transfer means.

[0003] As the photosensitive body, a rotatable photosensitive drum is often used. As the transfer means, a transfer member such as a transfer roller that contacts the photosensitive body to form a transfer portion is often used. Hereinafter, an image forming apparatus having a photosensitive drum and a transfer roller will be described as an example. The toner image on the photosensitive drum is transferred from the photosensitive drum to a recording material by applying a transfer voltage of a polarity opposite to the charge polarity of the photosensitive drum to the transfer roller. Here, the charge polarity of the photosensitive drum (normal charge polarity) is the polarity of the potential formed on the surface of the photosensitive drum by charging the surface with a charging means for image formation. At this time, particularly when the absolute value of the transfer voltage is large, discharge occurs between the photosensitive drum and the transfer roller, and the surface of the photosensitive drum may be charged unevenly. This phenomenon is called "transfer memory". When the surface of the photosensitive drum is charged unevenly due to the occurrence of transfer memory, it may be difficult to equalize the surface potential of the photosensitive drum when the charging means is used for the next charging process.

[0004] For example, in the case of a direct transfer type image forming apparatus that directly transfers a toner image from a photosensitive body to a recording material, a condition that makes transfer memory likely to occur is when a transfer voltage is applied to the transfer roller with no recording material interposed between the photosensitive drum and the transfer roller.When no recording material, which is an electrical resistor, is interposed between the photosensitive drum and the transfer roller, and a transfer voltage similar to that when a recording material is interposed is applied to the transfer roller, discharge may easily occur between the photosensitive drum and the transfer roller.

[0005] As a technique for reducing transfer memory, a method is known in which the surface of the photosensitive drum is irradiated with light after passing through a transfer section to eliminate static electricity on the surface of the photosensitive drum. Patent Document 1 discloses an image forming apparatus provided with a pre-exposure means (pre-charge exposure means) that exposes the surface of the photosensitive drum immediately after passing through a transfer section. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2016-218155 A Summary of the Invention [Problem to be solved by the invention]

[0007] In the case of transfer memory, which occurs when the absolute value of the transfer voltage is large when no recording material is interposed between the photosensitive drum and the transfer roller, potential unevenness occurs on the surface of the photosensitive drum due to discharge unevenness according to the surface shape of the transfer roller. For example, when foam rubber is used as the surface layer of the transfer roller, the surface shape of the transfer roller is determined by the presence or absence of foam cells. In other words, the potential unevenness that occurs on the surface of the transfer roller has a distribution that is approximately the same as the size of the foam cells. Due to this potential unevenness, the surface potential of the photosensitive drum on which the transfer memory occurs varies.

[0008] For example, when the surface potential of the photosensitive drum on which the transfer memory has occurred has the same polarity as the charge polarity of the photosensitive drum and is smaller in absolute value than the potential after image exposure, it is difficult to equalize the potential at the portion where the absolute value is already smaller than the potential after image exposure even if the surface of the photosensitive drum is exposed by the above-mentioned pre-exposure means. It is also possible to increase the exposure intensity of the pre-exposure means to be higher than the exposure intensity of the exposure means that performs image exposure. However, it is difficult to sufficiently equalize the transfer memory, especially when at least a part of the surface potential of the photosensitive drum on which the transfer memory has occurred is reversed to the polarity opposite to the charge polarity (charge potential) of the photosensitive drum.

[0009] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to suppress the occurrence of image defects due to transfer memory. [Means for solving the problem]

[0010] The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides a rotatable photoconductor, a first charging member that contacts the surface of the photoconductor at a first charging position in the rotation direction of the photoconductor to charge the surface of the photoconductor, a second charging member that charges the surface of the photoconductor at a second charging position in the rotation direction of the photoconductor, a developing member that supplies toner charged to a predetermined polarity to the surface of the photoconductor at a development position in the rotation direction of the photoconductor to form a toner image on the surface of the photoconductor, and a transfer member that contacts the surface of the photoconductor at a transfer position in the rotation direction of the photoconductor. a transfer member for transferring a toner image from the photoconductor to a recording material passing between the photoconductor and the transfer member, a first charging voltage application section for applying a first charging voltage of the predetermined polarity to the first charging member, a second charging voltage application section for applying a second charging voltage of the predetermined polarity to the second charging member, a transfer voltage application section for applying a transfer voltage of a polarity opposite to the predetermined polarity to the transfer member, and a control section for controlling the first charging voltage application section, the second charging voltage application section and the transfer voltage application section, the control unit controls the transfer voltage application unit so that at least a part of a surface area of ​​the photoconductor that is in direct contact with the transfer member at the transfer position has a potential of a polarity opposite to the predetermined polarity, controls the first charging voltage application unit so as to apply the first charging voltage to the first charging member with a voltage less than a discharge start voltage at the first charging position to charge the surface of the photoconductor at the first charging position, and controls the second charging voltage application unit so as to apply the second charging voltage to the second charging member with a voltage equal to or greater than a discharge start voltage at the second charging position to charge the surface of the photoconductor at the second charging position. Effect of the Invention

[0011] According to the present invention, it is possible to suppress the occurrence of image defects due to transfer memory. [Brief description of the drawings]

[0012] [Figure 1] 1 is a schematic cross-sectional view of an image forming apparatus according to a first embodiment of the present invention. [Diagram 2] FIG. 1 is a schematic cross-sectional view of an image forming apparatus according to a first conventional example. [Diagram 3] 10 is a schematic diagram for explaining a transition of a surface potential of a photosensitive drum in Conventional Example 1. FIG. [Figure 4] FIG. 2 is a schematic diagram for explaining a method for measuring the cell diameter of a foamed material. [Diagram 5] 5 is a schematic diagram for explaining the transition of the surface potential of the photosensitive drum in the first embodiment. FIG. [Figure 6] 6 is a graph showing an example of the surface potential of the photosensitive drum after passing a transfer position. FIG. [Figure 7] FIG. 4 is a schematic cross-sectional view of an image forming apparatus according to a modified example of the first embodiment. [Figure 8] FIG. 11 is a schematic cross-sectional view of an image forming apparatus according to a second embodiment. [Figure 9] FIG. 11 is a schematic diagram for explaining a layer structure of a photosensitive drum in Example 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

[0014] [Example 1] (1) Image forming device (1-1) Configuration of Image Forming Apparatus 1 is a schematic cross-sectional view of an image forming apparatus 100 of this embodiment. The image forming apparatus 100 of this embodiment is an electrophotographic laser printer, and can form a black monochrome image on a recording material P according to image information input from an external device 200 such as a personal computer. First, the configuration of the image forming apparatus 100 of this embodiment will be described.

[0015] The image forming apparatus 100 has a photosensitive drum 1, which is a drum-type (cylindrical) photosensitive body (electrophotographic photosensitive body) serving as an image carrier, inside the apparatus body. The photosensitive drum 1 is configured by providing a photosensitive material such as an OPC (organic photoconductor), amorphous selenium, or amorphous silicon on a cylindrical drum base made of aluminum, nickel, or the like. In this embodiment, the photosensitive drum 1 is a negatively charged OPC photosensitive body with an outer diameter of φ24 mm. That is, in this embodiment, the charging polarity (normal charging polarity) of the photosensitive drum 1 is negative. The photosensitive drum 1 has a configuration in which a conductive layer, an undercoat layer, and a photosensitive layer consisting of two layers, a charge generating layer, and a charge transport layer, are laminated in this order from the conductive support side on the surface of a conductive support made of an aluminum cylinder. The photosensitive drum 1 is driven to rotate in the direction of the arrow Rd (clockwise direction) in the figure.

[0016] Around the photosensitive drum 1, the following means are arranged in order along the rotation direction Rd.

[0017] First, the charging brush 2, which is a brush-shaped charging member serving as the first charging means, is disposed. In this embodiment, the charging brush 2 is configured by attaching and fixing a 5 mm wide pile fabric made of conductive nylon fibers to a conductive support section made of stainless steel sheet metal that also serves as a power supply electrode. In this embodiment, the conductive nylon fibers (brush fibers) that make up the charging brush 2 have a fineness of 2 denier and a hair density of 200 kF / inch 2 , and the pile length is 4 mm. In this embodiment, the charging brush 2 is in contact with the surface (outer circumferential surface) of the photosensitive drum 1 so that the penetration depth of the brush fiber tips into the photosensitive drum 1 is 0.6 mm. Also, in this embodiment, the charging brush 2 is fixedly disposed, and rubs against the surface of the photosensitive drum 1 as the photosensitive drum 1 rotates. Note that the width of the charging brush 2 is the length in the direction along the movement direction of the surface of the photosensitive drum 1. The unit of planting density, kF / inch 2indicates the number of filaments per square inch. The charging brush 2 is arranged so that the tips of the brush fibers are in almost uniform contact with the surface of the photosensitive drum 1 and the change in the tip of the brush fibers is small due to the rotation of the photosensitive drum 1. Here, the penetration amount is represented by the difference between the length of the brush fibers (pile length) when no force is applied from the outside to bend the brush fibers and the clearance between the support part of the brush fibers and the photosensitive drum 1. In this embodiment, the brush fibers of the charging brush 2 have a higher electrical resistance on the outer peripheral surface side than on the central side in the cross-sectional direction of the brush fibers. Therefore, in this embodiment, in order to perform injection charging described later well, the tips of the brush fibers are arranged so as to contact the surface of the photosensitive drum 1. Note that this is not limited to the case where, for example, a brush fiber with a sufficiently low electrical resistance on the outer peripheral surface side in the cross-sectional direction is used, and preferably, the charging brush 2 may be penetrated into the photosensitive drum 1 and the brush fibers may be laid down so that the first charging ratio (brush charging ratio) described later is equal to or higher than a predetermined value. The position on the photosensitive drum 1 in the rotation direction of the photosensitive drum 1 where charging processing is performed by the charging brush 2 is the first charging position Pa. The charging brush 2 charges the surface of the photosensitive drum 1 mainly by direct injection charging. The charging brush 2 charges the surface of the photosensitive drum 1 by causing a current corresponding to the potential difference between the charging brush 2 and the photosensitive drum 1 to flow to a point on the surface of the photosensitive drum 1 where the brush fibers are in direct contact. The charging brush 2 is disposed along the direction of the rotation axis of the photosensitive drum 1, and the length of the area where the brush fibers are provided in the direction of the rotation axis of the photosensitive drum 1 is longer than the length of an image formation area (area where a toner image can be formed) on the photosensitive drum 1 in the same direction.

[0018] Next, the charging roller 3, which is a roller-shaped charging member serving as a second charging means, is disposed. In this embodiment, the charging roller 3 is composed of a conductive base shaft (core metal, core part) that also serves as a power supply electrode, and an elastic layer that cylindrically surrounds the outer circumferential surface of the base shaft. In this embodiment, the charging roller 3 is an elastic roller with a roller outer diameter of φ10 mm, a core metal diameter of φ5 mm, and a thickness of the elastic layer of 2.5 mm. In this embodiment, the core metal of the charging roller 3 is made of SUS (stainless steel), and the elastic layer of the charging roller 3 is made of a mixed rubber material of NBR (nitrile rubber) and epichlorohydrin. The charging roller 3 is pressed against the photosensitive drum 1 and rotates in accordance with the rotation of the photosensitive drum 1. The position on the photosensitive drum 1 in the rotation direction of the photosensitive drum 1 where the charging process is performed by the charging roller 3 is the second charging position Pb. The charging roller 3 mainly charges the surface of the photosensitive drum 1 by discharge occurring in at least one of minute gaps formed upstream and downstream of the contact point between the charging roller 3 and the photosensitive drum 1 in the rotation direction of the photosensitive drum 1. For simplicity, the contact point between the charging roller 3 and the photosensitive drum 1 may be considered to be the second charging position Pb. The rotation axis direction of the charging roller 3 is approximately parallel to the rotation axis direction of the photosensitive drum 1, and the length of the elastic layer of the charging roller 3 in contact with the surface of the photosensitive drum 1 in the rotation axis direction of the photosensitive drum 1 is longer than the length of the image forming area on the photosensitive drum 1 in the same direction.

[0019] Next, an exposure device 4 is disposed as an exposure means. In this embodiment, the exposure device 4 is configured as a laser scanner device (laser optical system). The position on the photosensitive drum 1 in the rotation direction of the photosensitive drum 1 where exposure by the exposure device 4 is performed is the exposure position Pc.

[0020] Next, a developing device 5 is disposed as a developing means. In this embodiment, the developing device 5 uses a non-magnetic one-component developer (toner) as a developer. The developing device 5 has a developing roller 5a as a developer carrier (developing member) and a developing container 5b that is a container for accommodating the developer. The developing roller 5a is brought into contact with the surface of the photosensitive drum 1 during development, and supplies the toner to a developing section that is a portion facing (contacting) the photosensitive drum 1. The toner in the developing container 5b is supplied to the developing roller 5a. The developing device 5 may use a magnetic one-component developer (toner) or a two-component developer including a toner and a carrier as a developer. The position on the photosensitive drum 1 where the toner is supplied by the developing roller 5a in the rotation direction of the photosensitive drum 1 (the position where the developing roller 5a contacts in this embodiment) is the development position Pd. In this embodiment, the normal charging polarity of the toner, which is the main charging polarity of the toner during development, is negative polarity.

[0021] Next, a transfer roller 6, which is a roller-shaped transfer member (transfer rotating body) serving as a transfer means, is arranged. The transfer roller 6 is urged (pressed) toward the photosensitive drum 1 by a transfer pressure spring (not shown) serving as an urging member serving as an urging means, and is pressed against the photosensitive drum 1. This forms a transfer nip portion (transfer portion, transfer nip portion) Nt, which is a contact portion between the photosensitive drum 1 and the transfer roller 6. The transfer roller 6 rotates in accordance with the rotation of the photosensitive drum 1. The transfer roller 6 conveys the recording material P by nipping it between the photosensitive drum 1 and the transfer roller 6, and transfers the toner image from the photosensitive drum 1 to the recording material P by applying a voltage. In this embodiment, the transfer roller 6 is composed of a conductive base shaft (core metal, core portion) that also serves as a power supply electrode, and an elastic layer that cylindrically surrounds the outer circumferential surface of the base shaft. The material of the elastic layer of the transfer roller 6 is generally a semiconductive rubber material made of elastic materials such as EPDM (ethylene propylene diene rubber), NBR, SBR (styrene butadiene rubber), urethane rubber, epichlorohydrin, silicone rubber, etc. The material of the elastic layer may contain an appropriate amount of conductive agent, for example, an ionic conductive agent. In addition, in order to make the outer circumferential surface of the photosensitive drum 1 and the outer circumferential surface of the transfer roller 6 uniformly contact each other, the elastic layer of the transfer roller 6 may be formed of a foam (foamed elastic body) and a cell structure may be formed near the surface of the transfer roller 6. In this embodiment, the transfer roller 6 is a foamed elastic roller with an outer roller diameter of φ14 mm, a core metal diameter of φ5 mm, and a thickness of the elastic layer of 4.5 mm, and an elastic layer made of a foamed elastic layer. In this embodiment, when measured by a measuring method described later, the cell diameter on the surface of the transfer roller 6 was 300 μm. In this embodiment, SUS is used as the material of the core metal of the transfer roller 6, and a mixed rubber material of SBR and epichlorohydrin is used as the material of the elastic layer of the transfer roller 6. The position where the toner image is transferred to the recording material P on the photosensitive drum 1 in the rotation direction of the photosensitive drum 1 (the position corresponding to the transfer nip portion Nt) is the transfer position Pe.

[0022] Next, a charge eliminating needle 19 is arranged as a charge eliminating member for eliminating excess charge on the surface of the recording material P after transfer and reducing potential unevenness on the photosensitive drum 1 caused by peeling discharge. A charge eliminating needle having a sawtooth-shaped sharp tip and made of a thin metal plate such as a stainless steel plate or an aluminum plate having good conductivity can be used as the charge eliminating needle 19. This charge eliminating needle 19 is arranged downstream of the transfer roller 5 in the conveying direction of the recording material P so that the tip of the needle faces the surface of the photosensitive drum 1.

[0023] Further, a recording material cassette 7 in which recording material (transfer material, recording medium, sheet) P such as paper is stored is disposed at the bottom of the image forming apparatus 100. Further, from the recording material cassette 7, a feed roller 8, a conveying roller 9, a top sensor 10, a pre-transfer conveying guide 15, a transfer / fixing conveying guide 11, a fixing device 12, a discharge roller 13, and a discharge tray 14 are disposed in this order along a conveying path of the recording material P. Further, the image forming apparatus 100 is provided with a control unit 40 that controls the operation of the image forming apparatus 100.

[0024] The photosensitive drum 1 and the charging brush 2, charging roller 3, and developing device 5 acting as process means acting on the photosensitive drum 1 may be integrally formed into a process cartridge that is detachably mountable to the main body of the image forming apparatus 100.

[0025] (1-2) Image formation operation Next, the image forming operation in the image forming apparatus 100 of this embodiment will be described.

[0026] The photosensitive drum 1 is rotated at a peripheral speed (process speed) of 300 mm / sec in the direction of the arrow Rd in the figure (clockwise direction) by a driving force transmitted from a driving source 17 constituting a driving means. The surface of the rotating photosensitive drum 1 is substantially uniformly charged to a predetermined potential (dark area potential, charging potential, non-image area potential) of the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment) by the charging brush 2 and the charging roller 3. During charging, a first charging voltage (first charging bias) which is a negative DC voltage is applied to the charging brush 2 by a first charging power source (high voltage power source) 20 as a first charging voltage application means (first charging voltage application section). During charging, a second charging voltage (second charging bias) which is a negative DC voltage is applied to the charging roller 3 by a second charging power source (high voltage power source) 21 as a second charging voltage application means (second charging voltage application section). In this embodiment, as an example, a first charging voltage of −500 V is applied to the charging brush 2, and a second charging voltage of −1100 V is applied to the charging roller 3, so that a dark potential Vd of −500 V is formed on the surface of the photosensitive drum 1.

[0027] The surface of the charged photosensitive drum 1 is scanned and exposed by an exposure device (laser scanner) 4 according to image information, and an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 1. The video controller 110 of the image forming apparatus 100 processes image information input from an external device 200 to the image forming apparatus 100 to generate a time-series electric digital pixel signal. The exposure device 4 outputs a laser beam L1 modulated according to the time-series electric digital pixel signal, and the charged surface of the photosensitive drum 1 is scanned and exposed (image exposed) by the laser beam L1. In this embodiment, the charge on the photosensitive drum 1 in the portion exposed by the exposure device 4 is removed, and a light area potential (potential after image exposure, image area potential) Vl of -100V is formed on the surface of the photosensitive drum 1. As a result, an electrostatic latent image is formed on the photosensitive drum 1 with a contrast between the dark area potential Vd and the light area potential Vl.

[0028] The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by supplying toner by the developing device 5, and a toner image (toner image, developer image) is formed on the photosensitive drum 1. During development, a development voltage (developing bias) 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 developing roller 5a by a development power source (high voltage power source) 16 as a development voltage application means (developing voltage application unit). In this embodiment, as an example, a development voltage of -350V is applied to the developing roller 5a. In this embodiment, toner charged to the same polarity (negative in this embodiment) as the charging polarity of the photosensitive drum 1 adheres to the exposed portion (image portion) on the photosensitive drum 1, the absolute value of the potential of which has been reduced by exposure after being charged approximately uniformly (reverse development method).

[0029] The toner image formed on the photosensitive drum 1 is electrostatically transferred to a predetermined position on the recording material P at the transfer nip portion Nt by the action of the transfer roller 6. During transfer, a transfer voltage (transfer bias), which is a DC voltage of the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the transfer roller 5 by a transfer power source (high voltage power source) 18 serving as a transfer voltage application means (transfer voltage application portion).

[0030] The recording material P is stored in a recording material cassette 7 as a recording material storage section, and is sent out one sheet at a time from the recording material cassette 7 by a feeding roller 8 as a feeding member. When images are continuously formed on the recording materials P, the distance between the trailing end of the preceding recording material P and the leading end of the succeeding recording material P that is the next recording material P after this recording material P is called the paper gap. In this embodiment, the distance between the sheets is set to 20 mm. Since the time allotted to this paper gap is very short at 67 msec, if the absolute value of the transfer voltage is reduced only at the timing corresponding to the paper gap, the fall and rise of the transfer voltage may not be in time. For this reason, in this embodiment, the same transfer voltage is set while the recording material P passes through the transfer nip portion Nt (here, also referred to as "paper passing") and during the paper gap. After being sent out by the feeding roller 8, the recording material P is transported by a transport roller (registration roller) 9 as a transport member, and is supplied to the transfer nip portion Nt along a pre-transfer transport guide 15 as a guide member. The conveying roller 9 supplies the recording material P to the transfer nip Nt in synchronization with the toner image on the photosensitive drum 1 based on the detection result of the leading edge of the recording material P in the conveying direction by a top sensor 10 as a recording material detection means.

[0031] The recording material P to which the toner image has been transferred at the transfer nip portion Nt has excess charge on its surface removed by a charge removing needle 19. The recording material P that has passed through the charge removing needle 19 is transported along a transfer / fixing transport guide 11 as a guide member to a fixing device 12 as a fixing means. The fixing device 12 has a fixing roller 12a incorporating a heater, and a pressure roller 12b that is in pressure contact with the fixing roller 12a. The fixing device 12 applies heat and pressure to the recording material P carrying an unfixed toner image that passes through the nip portion between these rollers, thereby fixing the toner image onto the recording material P.

[0032] After the toner image is fixed by the fixing device 12, the recording material P is discharged (output) by a discharge roller 13 serving as a discharge member onto a discharge tray 14 serving as a discharge section formed on the upper surface of the image forming apparatus 100.

[0033] On the other hand, the toner (transfer residual toner) remaining on the surface of the photosensitive drum 1 without being transferred to the recording material P during transfer is removed from the surface of the photosensitive drum 1 in the following process (cleanerless method). The transfer residual toner includes a mixture of positively charged toner and negatively charged toner that does not have a sufficient charge. The transfer residual toner is charged negatively again at the first charging position Pa and the second charging position Pb by injection charging or discharge. The transfer residual toner that has been charged negatively again at the first charging position Pa and the second charging position Pb reaches the development position Pd as the photosensitive drum 1 rotates. Here, as described above, an electrostatic latent image corresponding to the image information is formed on the photosensitive drum 1 that has reached the development position Pd. The behavior of the transfer residual toner that has reached the development position Pd will be described separately for the image portion (exposed portion) and the non-image portion (non-exposed portion) of the photosensitive drum 1. At the development position Pd, the transfer residual toner adhering to the non-image portion of the photosensitive drum 1 is urged toward the development roller 5a by the potential difference between the light portion potential Vl of the photosensitive drum 1 and the development voltage, and is transferred to the development roller 5a, and is then collected in the development container 5b. The toner collected in the development container 5b is used again for image formation. On the other hand, at the development position Pd, the transfer residual toner adhering to the image portion of the photosensitive drum 1 is urged toward the photosensitive drum 1 by the potential difference between the dark portion potential Vd of the photosensitive drum 1 and the development voltage, and is not transferred from the photosensitive drum 1 to the development roller 5a. This transfer residual toner moves to the transfer position Pe together with the toner transferred from the development roller 5a to the photosensitive drum 1, is transferred to the recording material P at the transfer nip portion Nt, and is removed from the surface of the photosensitive drum 1.

[0034] By repeating the above operations, image formation can be performed one after another. In this embodiment, the image forming apparatus 100 can execute printing at a print speed of 56 sheets per minute.

[0035] The control unit 40 is configured with a CPU 41 as an arithmetic control means which is a central element for performing arithmetic processing, a ROM 41a and a RAM 41b as storage means, and an input / output unit (not shown) for controlling the transmission and reception of signals between the control unit 40 and each unit. The RAM 41b, which is a rewritable memory, stores information input to the control unit 40, detected information, arithmetic results, etc., and the ROM 41a stores a control program, a data table obtained in advance, etc. The CPU 41 and memories such as the ROM 41a and the RAM 41b can transfer and read data to and from each other. The control unit 40 controls the operation of each unit of the image forming apparatus 100 including the first charging power source 20, the second charging power source 21, the transfer power source 18, and the drive source 17 to perform image forming operations, etc.

[0036] The image forming apparatus 100 executes a job, which is a series of operations for forming and outputting an image on one or multiple recording materials P, which is started by one start instruction. A job generally includes an image forming process, a pre-rotation process, a paper-interval process, and a post-rotation process. The image forming process is a period during which an electrostatic image of an image to be actually formed on a recording material P is formed, a toner image is formed, and the toner image is transferred, and this period is referred to as the image forming time (image forming period). More specifically, the timing of the image forming time differs depending on the position where each of the electrostatic image forming, toner image forming, and toner image transferring processes is performed. The pre-rotation process is a period during which a preparatory operation is performed before the image forming process, from when a start instruction is input until an image is actually formed. The paper-interval process is a period corresponding to the period between recording materials P when image formation is performed continuously on multiple recording materials P (continuous image formation). The post-rotation process is a period during which an arranging operation (preparatory operation) is performed after the image forming process. Non-image formation (non-image formation period) refers to a period other than image formation, and includes the above-mentioned pre-rotation process, paper interval process, post-rotation process, and also the pre-multiple-rotation process, which is a preparatory operation when the image forming apparatus 100 is turned on or when it returns from a sleep state.

[0037] (2) Image defects due to transfer memory Next, the mechanism by which image defects occur due to transfer memory will be described.

[0038] FIG. 2 is a schematic cross-sectional view of an image forming apparatus 101 of conventional example 1. The image forming apparatus 101 of conventional example 1 is not provided with the charging brush 2 in the image forming apparatus 100 of this embodiment, and instead is provided with a pre-exposure device 30. The pre-exposure device 30 is provided to smooth out the potential unevenness on the surface of the photosensitive drum 1 after passing through the transfer position Pe (before reaching the charging position Pb), and exposes the surface of the photosensitive drum 1 by irradiating the surface of the photosensitive drum 1 with a laser beam L2 at the pre-exposure position Pf. In conventional example 1, the laser beam L2 of the pre-exposure device 30 has an exposure intensity equivalent to that of the laser beam L1 of the exposure device 4, and can discharge the surface potential of the photosensitive drum 1 to −100 V, which is the same as the bright area potential Vl. Except for the above points, the configuration and operation of the image forming apparatus 101 of conventional example 1 are substantially the same as those of the image forming apparatus 100 of this embodiment. In the image forming apparatus 101 of conventional example 1, elements having the same or corresponding functions or configurations as those of the image forming apparatus 100 of this embodiment are given the same reference numerals as those of this embodiment.

[0039] FIG. 3 is a schematic diagram for explaining the transition of the surface potential of the photosensitive drum 1 at a position corresponding to the paper gap when images are continuously formed on the recording material P by the image forming apparatus 101 of the conventional example 1. FIG. 3(a) is the surface potential of the photosensitive drum 1 before passing the transfer position Pe (after passing the charging position Pb and before reaching the transfer position Pe), and the dark potential Vd is maintained at the position corresponding to the paper gap because the exposure device 4 does not perform exposure at the position. FIG. 3(b) is the surface potential of the photosensitive drum 1 after passing the transfer position Pe (after passing the transfer position Pe and before reaching the pre-exposure position Pf). In the conventional example 1, since the same transfer voltage control as in this embodiment is adopted, a transfer voltage with a large absolute value of positive polarity, the same as during paper passing, is applied between the sheets. Therefore, discharge occurs due to the potential difference between the transfer roller 6 and the photosensitive drum 1 at the transfer nip portion Nt, and potential unevenness (transfer memory) as shown in FIG. 3(b) occurs on the surface of the photosensitive drum 1. The unevenness of the potential unevenness depends on the cell diameter of the surface of the transfer roller 6, and occurs because the discharge state changes between the rubber part and the foam part (void part). In addition, when the potential difference between the transfer roller 6 and the photosensitive drum 1 is large, the discharge becomes active. Then, as shown in FIG. 3(b), a region having a surface potential with the same polarity as the charging polarity of the photosensitive drum 1 and a smaller absolute value than the bright area potential Vl (hereinafter, simply referred to as "surface potential with a smaller absolute value than the bright area potential Vl"), or a surface potential with a polarity opposite to the normal charging polarity of the photosensitive drum (hereinafter, simply referred to as "surface potential with reversed polarity") occurs on the photosensitive drum 1. FIG. 3(c) shows the surface potential of the photosensitive drum 1 after passing the pre-exposure position Pf (after passing the pre-exposure position Pf and before reaching the charging position Pb). At the pre-exposure position Pf, the surface potential of the photosensitive drum 1 at the portion having the same polarity as the charging polarity of the photosensitive drum 1 and a larger absolute value than the bright area potential Vl can be neutralized. However, it is not possible to equalize the surface potential of the photosensitive drum 1 at a location having a surface potential with an absolute value smaller than the bright area potential Vl or at a location having a surface potential with an inverted polarity. Figure 3(d) shows the surface potential of the photosensitive drum 1 after passing the charging position Pb (after passing the charging position Pb and before reaching the transfer position Pe).Even if the charging position Pb is passed in the state of FIG. 3(c), as shown in FIG. 3(d), the surface potential of the portion where the absolute value becomes smaller than the light potential Vl due to the transfer memory or the surface potential of the portion where the polarity is reversed may not be sufficiently returned to the dark potential Vd. In the portion where the dark potential Vd cannot be sufficiently returned, the potential difference between the surface potential of the photosensitive drum 1 and the development voltage at the development position Pd cannot be sufficiently secured in the non-image portion of the image forming area for the succeeding recording material P. As a result, this may become apparent as an image defect such as a black dot image or a fog image. In addition, in the portion where the dark potential Vd cannot be sufficiently returned, the potential difference between the surface potential of the photosensitive drum 1 and the development voltage at the development position Pd becomes large in the image portion of the image forming area for the succeeding recording material P, and this becomes apparent as an image defect in which the density of a halftone image or the like becomes high. In this way, in the image forming apparatus 101 of conventional example 1, it is difficult to level out the transfer memory at locations where the surface potential between sheets has an absolute value smaller than the bright area potential Vl or a surface potential with reversed polarity, and it is difficult to suppress image defects caused by the transfer memory.

[0040] Here, by increasing the light amount of the pre-exposure device 30 more than that of the conventional example 1, it is possible to lower the surface potential of the photosensitive drum 1 to 0V. However, even in this case, it is difficult to restore the surface potential with the polarity reversed to the dark potential Vd after passing the charging position Pb. Furthermore, when the light amount of the pre-exposure device 30 is increased, the potential difference between the charging roller 3 and the photosensitive drum 1 increases, and the discharge by the charging roller 3 becomes active. When the discharge by the charging roller 3 becomes active, damage to the photosensitive drum 1 is promoted, which is not preferable. In other words, even when the light amount of the pre-exposure device 30 is increased, it is difficult to suppress image defects due to transfer memory, and there is a possibility that damage to the photosensitive drum 1 is promoted.

[0041] (3) Method for measuring cell diameter on the surface of the transfer roller As described above, the unevenness of the potential on the surface of the photosensitive drum 1 due to the transfer memory depends on the cell diameter on the surface of the transfer roller 6. Fig. 4 is a schematic diagram for explaining a method for measuring the cell diameter of the foamed elastic layer (foam, foam material).

[0042] In order to measure the cell diameter, the surface of the transfer roller 6 was observed using a digital microscope VHX-1000 (manufactured by Keyence Corporation) and a digital microscope lens VH-Z100R (manufactured by Keyence Corporation). FIG. 4(a) is a schematic diagram showing an image obtained when the surface of the transfer roller 6 was observed with a lens magnification of the digital microscope set to 100 times. As shown in FIG. 4(a), a plurality of cells are seen in the image obtained from the digital microscope. In this embodiment, the diameters of 30 cells in the image were measured in order from the largest to the smallest, and the average value of the diameters was taken as the cell diameter on the surface of the transfer roller 6. In addition, at this time, the viewing angle x×y of the digital microscope was 4 mm×3 mm (a range of 4 mm horizontal and 3 mm vertical on the surface of the transfer roller 6). Here, the cells constituting the elastic layer of the transfer roller 6 are not necessarily close to a perfect circle. For example, they may have a distorted shape as shown in FIG. 4(b). In this case, the diameter of a perfect circle having the same area as the cell is taken as the outer diameter of the cell.

[0043] (4) Suppression of image defects caused by transfer memory Next, the effect of suppressing image defects caused by transfer memory by the configuration of this embodiment will be described.

[0044] The image forming apparatus 100 of this embodiment continuously formed images on the recording material P, and the occurrence of image defects due to transfer memory was examined. A first charging voltage of -500V was applied to the charging brush 2. The image pattern for evaluating the occurrence of image defects due to transfer memory was a halftone image (an image with a toner amount of 50% when the toner amount of a solid image is 100%). Then, the presence or absence of a change in density of the halftone image on the subsequent recording material P corresponding to the preceding paper interval was evaluated. When there was a change in density of the halftone image by visual observation, it was judged that there was an image defect due to transfer memory ("x"). Also, when there was no change in density of the halftone image by visual observation, it was judged that there was no image defect due to transfer memory ("◯"). In addition, for comparison with this embodiment, the same experiment was also performed on the image forming apparatus 101 of the above-mentioned conventional example 1 and the image forming apparatus of Comparative Example 1 in which the charging brush 2 was removed from the image forming apparatus 100 of this embodiment. The results are shown in Table 1.

[0045] As shown in Table 1, in the image forming apparatus 100 of this embodiment in which the charging brush 2 is provided, no image defects due to transfer memory occurred even if the absolute value of the transfer voltage was increased. In this embodiment, the appropriate setting value of the transfer voltage is +3000V. In this embodiment, if the transfer voltage is smaller than +3000V, sufficient transfer from the photosensitive drum 1 to the recording material P may not be performed, resulting in transfer defects. In addition, in this embodiment, even if a transfer voltage of +3000V is applied between sheets, no image defects due to transfer memory occur. Therefore, it is not necessary to change the setting value of the transfer voltage between the paper feed and the paper interval. That is, the image forming apparatus 100 of this embodiment achieves a faster process speed and a shorter paper interval while suppressing the occurrence of image defects due to transfer memory.

[0046] Furthermore, as shown in Table 1, in the image forming apparatus 101 of the conventional example 1 provided with the pre-exposure device 30, image defects due to transfer memory occurred at a transfer voltage of +2000 V or more. When the transfer voltage was +2000 V, the surface potential of the photosensitive drum 1 after passing the transfer position Pe at the position corresponding to the paper gap was measured to be -80 V, which was smaller in absolute value than the light area potential Vl (-100 V). In other words, this shows that the image defects due to transfer memory in the image forming apparatus 101 of the conventional example 1 occur due to the mechanism explained using FIG. 3.

[0047] Also, as shown in Table 1, in the image forming apparatus of Comparative Example 1, image defects due to transfer memory occurred at a transfer voltage of +1500V or more, which is lower than that in Conventional Example 1. This is because the image forming apparatus of Comparative Example 1 does not have the potential equalization effect of the pre-exposure device 30.

[0048] From the above results, it is understood that the image forming apparatus 100 of this embodiment is more advantageous in suppressing the occurrence of image defects due to transfer memory than the image forming apparatuses of Conventional Example 1 and Comparative Example 1.

[0049] [Table 1]

[0050] Next, a mechanism for suppressing image defects caused by transfer memory in the image forming apparatus 100 of this embodiment will be described.

[0051] FIG. 5 is a schematic diagram for explaining the transition of the surface potential of the photosensitive drum 1 at a position corresponding to the paper gap when images are continuously formed on the recording material P by the image forming apparatus 100 of this embodiment. FIG. 5(a) shows the surface potential of the photosensitive drum 1 before passing the transfer position Pe. As in the case of FIG. 3(a), the exposure device 4 does not expose the photosensitive drum 1 at the position corresponding to the paper gap, so the photosensitive drum 1 holds the dark potential Vd. FIG. 5(b) shows the surface potential of the photosensitive drum 1 after passing the transfer position Pe. As in the case of FIG. 3(b), a surface potential having an absolute value smaller than the bright potential Vl and a surface potential having an inverted polarity are generated on the photosensitive drum 1 due to the transfer memory. FIG. 5(c) shows the surface potential of the photosensitive drum 1 after passing the first charging position Pa (after passing the first charging position Pa and before reaching the second charging position Pb). At the first charging position Pa, charges are injected into the surface of the photosensitive drum 1 by injection charging from each brush fiber of the charging brush 2. In the injection charging, the larger the potential difference between the charging brush 2 and the surface of the photosensitive drum 1 is, the larger the charge is injected from the charging brush 2 to the surface of the photosensitive drum 1 within the range where no discharge occurs. This principle is applied to each brush fiber of the charging brush 2 and the minute area on the surface of the photosensitive drum 1 with which it comes into contact. In other words, the potential of each brush fiber of the charging brush 2 is the same, but the potential varies due to transfer memory between the minute areas on the surface of the photosensitive drum 1 with which each brush fiber comes into contact. Then, due to the transfer memory, a larger charge flows from the charging brush 2 to the photosensitive drum 1 in the minute area of ​​the photosensitive drum 1 having a surface potential with a smaller absolute value than the bright area potential Vl or a surface potential with an inverted polarity. As a result, as shown in FIG. 5(c), a larger charge flows from the charging brush 2 to the photosensitive drum 1 in the area with a surface potential with a smaller absolute value than the bright area potential Vl or a surface potential with an inverted polarity, and the potential of the area rises preferentially, and the surface potential of the photosensitive drum 1 is equalized. FIG. 5D shows the surface potential of the photosensitive drum 1 after passing the second charging position Pb (after passing the second charging position Pb and before reaching the transfer position Pe).By reducing the unevenness in the surface potential of the photosensitive drum 1 to the state shown in Figure 5(c) and restoring the surface potential of the photosensitive drum 1 to the state shown in Figure 5(c), it is possible to form a substantially uniform dark potential Vd on the photosensitive drum 1 by passing through the second charging position Pb, as shown in Figure 5(d).

[0052] In this manner, according to this embodiment, it is possible to reduce transfer memory, such as a surface potential having an absolute value smaller than the bright area potential Vl or a surface potential having an inverted polarity, thereby suppressing the occurrence of image defects due to the transfer memory.

[0053] (5) First charge ratio (brush charge ratio) In order to obtain a sufficient effect of suppressing image defects due to transfer memory, it is preferable that the ratio of the amount of potential that changes at the first charging position Pa to the amount of potential that changes at the first charging position Pa and the second charging position Pb is equal to or greater than a predetermined value. The ratio of the amount of potential that changes at the first charging position Pa to the amount of potential that changes at the first charging position Pa and the second charging position Pb is referred to as the "first charging ratio R1." In this case, the first charging ratio R1 can be expressed by the following formula (1). R1[%]={(V1-Vt) / (Vd-Vt)}×100 ···(1)

[0054] Here, V1 is the surface potential of the photosensitive drum 1 after it has passed the first charging position Pa and before it has reached the second charging position Pb. Vt is the surface potential of the photosensitive drum 1 after it has passed the transfer position Pe and before it has reached the first charging position Pa. Vd is the dark area potential (the surface potential of the photosensitive drum 1 formed at the second charging position Pb).

[0055] For example, in the image forming apparatus 100 of this embodiment, when the transfer voltage is +3000V, the average values ​​of the measurement results of each potential at the position corresponding to the paper gap are V1=-70V, Vt=+40V, and Vd=-500V. In this case, by applying the average values ​​of each potential to the above formula (1), it is possible to calculate that the first charge ratio R1 is approximately 20%. The average value of the measurement results of each potential is represented by the average value of the measurement results of ten points (for example, 10 points to 30 points) in the moving direction of the surface of the photosensitive drum 1.

[0056] The relationship between the first charge ratio R1 and the effect of suppressing image defects caused by transfer memory was examined. Using the image forming apparatus 100 of this embodiment, the presence or absence of image defects caused by transfer memory was evaluated when the first charging voltage applied to the charging brush 2 was changed. The transfer voltage was fixed at +3000V. The other conditions were the same as those of the experiment in which the results of Table 1 were obtained. In addition, in the configuration of this embodiment, when the first charging voltage exceeds -500V (when the absolute value of the negative first charging voltage exceeds 500V), discharge may occur between the charging brush 2 and the photosensitive drum 1, causing other problems such as uneven charging. Therefore, the first charging voltage was set to 0V to -500V, at which the potential difference between the charging brush 2 and the photosensitive drum 1 at the first charging position Pa is less than the discharge start voltage. The results are shown in Table 2.

[0057] From Table 2, it can be seen that in order to suppress the occurrence of image defects due to transfer memory, it is preferable that the first charge ratio R1 is a predetermined value or more, and in the configuration of this embodiment, it is preferable that R1 = 10% or more.

[0058] In a configuration in which injection charging is performed by applying a first charging voltage less than the discharge start voltage to the first charging member, there is a certain limit to the amount of potential of the photosensitive drum 1 that can be changed by the first charging member, and typically, the first charging ratio R1 is 50% or less. Then, a second charging voltage is applied to the charging roller 3 such that the potential difference between the charging roller 3 and the photosensitive drum 1 at the second charging position Pb is equal to or greater than the discharge start voltage, whereby the surface of the photosensitive drum 1 is charged to a predetermined dark potential Vd.

[0059] [Table 2]

[0060] FIG. 6 is a graph showing an example of the results of measuring the transition of the surface potential Vt of the photosensitive drum 1 after passing the transfer position Pe and before reaching the first charging position Pa under the same conditions as the experiment that obtained the results of Table 2. When calculating the first charging ratio R1 using the above-mentioned formula (1), the average value of the waveform of the surface potential of the photosensitive drum 1 at the position corresponding to the paper gap can be used as Vt at the position corresponding to the paper gap. On the other hand, as shown in FIG. 6, under the condition where the transfer memory occurs, the value of Vt at the position corresponding to the paper gap fluctuates. This is the condition explained using FIG. 5(b), and in order to suppress the image defect due to the transfer memory, it is necessary to reduce the difference between the maximum value and the minimum value in Vt at the position corresponding to the paper gap at the first charging position Pa. The guideline R1' of the first charging ratio required to suppress the image defect due to this transfer memory can be obtained from the following formula (2). R1' [%] ={(Vtmax-Vtmin) / (Vd-Vtave)}×100 (2)

[0061] Here, Vtmax is the maximum value of Vt at the position corresponding to the paper gap, Vtmin is the minimum value of Vt at the position corresponding to the paper gap, and Vtave is the average value of Vt at the position corresponding to the paper gap. In the case of FIG. 6, Vtmax=+71V, Vtmin=+18V, Vd=-500V, and Vtave=+40V, and R1'=9.8% can be calculated. This value is roughly consistent with the threshold value of the first charging ratio R1 for suppressing image defects due to transfer memory, which was obtained in the experiment that obtained the results of Table 2. In this way, the first charging ratio may be set to R' or more calculated by the above formula (2). Note that Vtmax and Vtmin in the above formula (2) are not limited to the maximum value and minimum value, respectively. For example, if the difference between the maximum value and the value one less than the maximum value, and the difference between the minimum value and the value one more than the minimum value are sufficiently small relative to the magnitude of the fluctuation in the Vt value at the position corresponding to the paper gap, R' may be calculated using a value one (not limited to one) less than the maximum value and a value one (not limited to one) more than the minimum value.

[0062] In addition, in order to obtain a better effect of reducing the transfer memory, it is desirable that the average inter-thread distance of the charging brush 2 is smaller than the average cell diameter of the surface of the foamed elastic layer of the transfer roller 6. As described above, the unevenness of the potential unevenness on the surface of the photosensitive drum 1 due to the transfer memory depends on the cell diameter of the surface of the transfer roller 6. If the average inter-thread distance of the charging brush 2 is larger than the unevenness of the potential unevenness depending on the cell diameter, a minute area where the brush fibers of the charging brush 2 do not contact is generated on the photosensitive drum 1, and it may be difficult to sufficiently reduce the transfer memory. From this viewpoint, it is more preferable that the average inter-thread distance of the charging brush 2 is 70% or less of the average cell diameter of the surface of the foamed elastic layer of the transfer roller 6, and even more preferable that it is 50% or less. In this embodiment, the cell diameter of the surface of the transfer roller 6 is 300 μm when measured by the above-mentioned measuring method. In this embodiment, the bristle density of the charging brush 2 is 200 kF / inch 2 Therefore, the average inter-fiber distance of the charging brush 2 can be calculated as 57 μm by the following calculation. Note that the average inter-fiber distance may be determined by observation in the same manner as the cell diameter of the foam described above. 200kF / inch 2= 200,000 ÷ 645.16 ≒ 310 F / mm 2 √(310F / mm 2 ) ≒ 17.6F / mm 1 / (17.6F / mm) ≒ 57μm / F

[0063] In this manner, in this embodiment, the average inter-yarn distance of the charging brush 2 is set to a value smaller than the average cell diameter of the surface of the foamed elastic layer of the transfer roller 6.

[0064] It is desirable that the relationship between the average inter-thread distance of the charging brush 2 and the average cell diameter on the surface of the foamed elastic layer of the transfer roller 6 be as described above, at least in the direction of the rotation axis of the photosensitive drum 1. In this embodiment, the average inter-thread distance of the charging brush 2 is set sufficiently smaller than the average cell diameter on the surface of the foamed elastic layer of the transfer roller 6 in the direction of the rotation axis of the photosensitive drum 1 and in the moving direction of the surface of the photosensitive drum 1.

[0065] (6) Variations A modified example of this embodiment will be described. FIG. 7 is a schematic cross-sectional view of an image forming apparatus 102 of the modified example of this embodiment. In the image forming apparatus 102 of FIG. 7, instead of the charging brush 2 of the image forming apparatus 100 of FIG. 1, an injection roller 25 is arranged in contact with the photosensitive drum 1 at a first charging position Pg as a first charging member. The injection roller 25 rotates in accordance with the rotation of the photosensitive drum 1. The surface of the rotating photosensitive drum 1 is charged substantially uniformly to a predetermined potential (dark potential, charging potential) by the injection roller 25 and the charging roller 3. During charging, a first charging voltage (first charging bias), which is a negative DC voltage, is applied to the injection roller 25 from a first charging power source (high voltage power source) 26. Except for the above points, the configuration and operation of the image forming apparatus 102 of FIG. 7 are substantially the same as those of the image forming apparatus 100 of FIG. 1. In the image forming apparatus 102 in FIG. 7, elements having the same or corresponding functions or configurations as those in the image forming apparatus 100 in FIG. 1 are denoted by the same reference numerals as those in the image forming apparatus 100 in FIG.

[0066] The injection roller 25 is a roller in which a foamed elastic layer is formed on a core metal, similar to the transfer roller 6. That is, the injection roller 25 is composed of a conductive base shaft (core metal, core part) that also serves as a power supply electrode, and a foamed elastic layer that cylindrically surrounds the outer circumferential surface of the core metal. A first charging voltage of -500V is applied to the injection roller 25, and the photosensitive drum 1 is charged mainly by injection charging. As in the case of the charging brush 2, in order to obtain a better transfer memory reduction effect, it is desirable that the average cell diameter of the surface of the foamed elastic layer of the injection roller 25 is smaller than the average cell diameter of the surface of the foamed elastic layer of the transfer roller 6. Moreover, it is more preferable that the average cell diameter of the surface of the foamed elastic layer of the injection roller 25 is 70% or less of the average cell diameter of the surface of the foamed elastic layer of the transfer roller 6, and it is even more preferable that the average cell diameter of the surface of the foamed elastic layer of the injection roller 25 is 50% or less. In this modified example, when measured by the above-mentioned measurement method, the cell diameter of the surface of the injection roller 25 is 150 μm, and the cell diameter of the surface of the transfer roller 6 is 300 μm. In this manner, in this modified example, the average cell diameter on the surface of the foamed elastic layer of the injection roller 25 is set to be smaller than the average cell diameter on the surface of the foamed elastic layer of the transfer roller 6 .

[0067] Using the image forming apparatus 102 of this modified example, an experiment was conducted to evaluate the occurrence of image defects due to transfer memory, similar to the experiment that obtained the results in Table 1. As a result, it was found that the image forming apparatus 102 of this modified example also has the effect of suppressing the occurrence of image defects due to transfer memory, similar to the image forming apparatus 100 of this embodiment.

[0068] In this manner, even in a configuration in which injection charging is performed using a charging member having a foam structure such as injection roller 25 as the first charging member, it is possible to suppress the occurrence of image defects due to transfer memory.

[0069] (7) Effects Thus, in this embodiment, the image forming apparatus 100 includes a rotatable photoconductor (photoconductor drum) 1, a first charging member (charging brush) 2 that contacts the surface of the photoconductor 1 at a first charging position Pa in the rotation direction of the photoconductor 1 to charge the surface of the photoconductor 1, a second charging member (charging roller) 3 that charges the surface of the photoconductor 1 at a second charging position Pb in the rotation direction of the photoconductor 1, a developing member (developing roller) 5a that supplies toner charged to a predetermined polarity to the surface of the photoconductor 1 at a development position Pd in ​​the rotation direction of the photoconductor 1 to form a toner image on the surface of the photoconductor 1, and the photoconductor a transfer member (transfer roller) 6 that contacts the surface of the photoconductor 1 at a transfer position Pe in the rotation direction of the photoconductor 1 and transfers a toner image from the photoconductor 1 to a recording material P passing between the photoconductor 1 and the transfer member 6; a first charging voltage application section (first charging power source) 20 that applies a first charging voltage of the above-mentioned predetermined polarity to the first charging member 2; a second charging voltage application section (second charging power source) 21 that applies a second charging voltage of the above-mentioned predetermined polarity to the second charging member 3; a transfer voltage application section (transfer power source) 18 that applies a transfer voltage of a polarity opposite to the above-mentioned predetermined polarity to the transfer member 6; and a control unit 40 that controls the charging voltage application unit 20, the second charging voltage application unit 21, and the transfer voltage application unit 18. In the rotation direction of the photoconductor 1, the first charging position Pa is located downstream of the transfer position Pe and upstream of the second charging position Pb, the second charging position Pb is located downstream of the first charging position Pa and upstream of the development position Pd, the development position Pd is located downstream of the second charging position Pb and upstream of the transfer position Pe, and the transfer position Pe is located downstream of the development position Pd and upstream of the first charging position Pa. The control unit 40 controls the transfer voltage application unit 20, the second charging voltage application unit 21, and the transfer voltage application unit 18. The transfer voltage application unit 18 is controlled so that at least a portion of the surface of the photoconductor 1 in direct contact with the transfer member 6 has a potential of the opposite polarity to the above-mentioned predetermined polarity, the first charging voltage application unit 20 is controlled so that a first charging voltage less than the discharge start voltage at the first charging position Pa is applied to the first charging member 2 to charge the surface of the photoconductor 1 at the first charging position Pa, and the second charging voltage application unit 21 is controlled so that a second charging voltage equal to or greater than the discharge start voltage at the second charging position Pb is applied to the second charging member 3 to charge the surface of the photoconductor 1 at the second charging position Pb.

[0070] Here, when the ratio of the surface potential of the photoconductor 1 that is changed at the first charging position Pa to the surface potential of the photoconductor 1 that is changed at the first charging position Pa and the second charging position Pb for at least a portion of the above-mentioned region is defined as a first charging ratio, it is preferable that the control unit 40 controls the first charging voltage application unit 20 so that the first charging ratio is 10% or more. In other words, for at least a portion of the above-mentioned region, when the ratio of the surface potential of the photoconductor 1 that is changed at the first charging position Pa among the surface potentials of the photoconductor 1 that are changed at the first charging position Pa and the second charging position Pb is defined as the first charging ratio, the surface potential of the photoconductor 1 after passing the transfer position Pe and before reaching the first charging position Pa is defined as Vt, the surface potential of the photoconductor 1 formed at the second charging position Pb is defined as Vd, the maximum value of Vt is Vtmax, the minimum value of Vt is Vtmin, and the average value of Vt is Vtave, it is preferable that the control unit 40 controls the first charging voltage application unit 20 so that the above-mentioned first charging ratio is greater than or equal to R1' expressed by the following formula, R1' [%] = {(Vtmax-Vtmin) / (Vd-Vtave)} × 100.

[0071] In particular, in this embodiment, the at least part of the region corresponds to the region between the preceding recording material P at the transfer position Pe and the succeeding recording material P which is the next recording material P to the preceding recording material P. In this embodiment, the control unit 40 controls the transfer voltage application unit 18 so as to apply the same transfer voltage to the transfer member 6 when the region of the surface of the photoconductor 1 corresponding to the region between the preceding recording material P and the succeeding recording material P is at the transfer position Pe and when the region of the surface of the photoconductor 1 corresponding to the preceding recording material P is at the transfer position Pe. In this embodiment, the transfer member 6 is configured to have a foam that can come into contact with the surface of the photoconductor 1. In this embodiment, the first charging member 2 is configured to have a brush that can come into contact with the surface of the photoconductor 1. In this case, it is preferable that the average inter-yarn distance of the brush of the first charging member 2 is smaller than the average cell diameter of the foam of the transfer member 6. In addition, the first charging member 2 may be configured to have a foam that can come into contact with the surface of the photoconductor 1. In this case, it is preferable that the average cell diameter of the foam of the first charging member 2 is smaller than the average cell diameter of the foam of the transfer member 6. In this embodiment, the toner remaining on the surface of the photoreceptor 1 after the toner image is transferred from the photoreceptor 1 to the recording material P is collected by the developing member 5a.

[0072] Furthermore, according to this embodiment, in a configuration in which transfer memory occurs in which the surface of a photosensitive member that comes into direct contact with a transfer member at a transfer section has a polarity opposite to the charging polarity of the photosensitive member, the occurrence of image defects due to transfer memory can be suppressed.

[0073] [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 given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

[0074] When a durability test is carried out for the purpose of further extending the life of the image forming apparatus 100 of the first embodiment, the charging brush 2 may be contaminated with adhesions such as transfer residual toner and paper powder, and the first charging ratio R1 expressed by the above-mentioned formula (1) may decrease. In response to this, it is possible to increase the absolute value of the first charging voltage applied to the charging brush 2 according to the cumulative usage amount of the charging brush 2. However, since a discharge occurs when a potential difference of a predetermined value or more occurs between the charging brush 2 and the photosensitive drum 1, there is a limit to increasing the absolute value of the first charging voltage. The purpose of this embodiment is to suppress the decrease in the first charging ratio R1 accompanying an increase in the cumulative usage amount of the charging brush 2, and to better maintain the effect of suppressing the occurrence of image defects due to transfer memory.

[0075] FIG. 8 is a schematic cross-sectional view of the image forming apparatus 103 of this embodiment. In this embodiment, the image forming apparatus 103 has a cleaning device 31 as a cleaning means for removing deposits such as residual toner and paper powder remaining on the photosensitive drum 1 after passing the transfer position Pe from the photosensitive drum 1. The cleaning device 31 has a cleaning blade 31a as a cleaning member arranged to abut against the surface of the photosensitive drum 1. The position where the cleaning blade 31a on the photosensitive drum 1 removes deposits in the rotation direction of the photosensitive drum 1 (the position where the cleaning blade 31a abuts against the photosensitive drum 1 in this embodiment) is the cleaning position Ph. The charging brush 2 similar to that of the first embodiment is arranged to abut against the photosensitive drum 1 at the first charging position Pi on the photosensitive drum 1 after passing the cleaning position Ph (before reaching the second charging position Pb). Except for the above points, the configuration and operation of the image forming apparatus 103 of this embodiment are substantially the same as those of the image forming apparatus 100 of the first embodiment. Thus, in this embodiment, the image forming apparatus 103 has the cleaning member 31a that removes toner from the surface of the photoconductor 1 downstream of the transfer position Pe and upstream of the first charging position Pi in the rotation direction of the photoconductor 1.

[0076] The image forming apparatus 100 of the first embodiment has a cleanerless configuration in which the transfer residual toner is collected and reused by the developing device 5. In contrast, the image forming apparatus 103 of this embodiment is provided with a cleaning device 31 having a blade collection configuration that cleans the surface of the photosensitive drum 1 before it reaches the charging brush 2. Therefore, the image forming apparatus 103 of this embodiment can suppress contamination of the charging brush 2 with attachments such as transfer residual toner and paper powder as the cumulative usage of the charging brush 2 increases. As a result, the image forming apparatus 103 of this embodiment can suppress a decrease in the first charging ratio R1 as the cumulative usage of the charging brush 2 increases, and can better maintain the effect of suppressing the occurrence of image defects due to transfer memory.

[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 given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

[0078] When an evaluation experiment is performed in a low-temperature, low-humidity environment (e.g., 15°C, 10% RH) in the image forming apparatus 100 of the first embodiment, the first charge ratio R1 expressed by the formula (1) may decrease due to an increase in the electrical resistance of the charging brush 2 or the photosensitive drum 1 itself, or an increase in the contact resistance between the charging brush 2 and the photosensitive drum 1. As described in the second embodiment, when a potential difference of a predetermined value or more occurs between the charging brush 2 and the photosensitive drum 1, a discharge occurs, so there is a limit to increasing the absolute value of the first charging voltage. The purpose of this embodiment is to obtain an effect of suppressing the occurrence of image defects due to transfer memory regardless of the environment by suppressing the decrease in the first charge ratio R1 in a low-temperature, low-humidity environment.

[0079] In this embodiment, a photosensitive drum 61 shown in Figures 9(a) and (b) is used instead of the photosensitive drum 1 of the image forming apparatus 100 of Example 1. Except for this point, the configuration and operation of the image forming apparatus of this embodiment are substantially the same as those of the image forming apparatus 100 of Example 1. Figure 9(a) is a schematic cross-sectional view of the photosensitive drum 61 in this embodiment, and Figure 9(b) is a schematic cross-sectional view showing the layer configuration of the photosensitive drum 61 in this embodiment.

[0080] The photosensitive drum 61 in this embodiment will be described. The photosensitive drum 61 in this embodiment has a charge injection function on the outermost surface. As shown in FIG. 9(b), the photosensitive drum 61 has a conductive support 61a, a conductive layer 61b, an undercoat layer 61c, a photosensitive layer consisting of two layers, a charge generation layer 61d and a charge transport layer 61e, and a charge injection layer 61f. The charge injection layer 61f forming the surface of the photosensitive drum 1 contains conductive particles 61g. The charge injection layer 61f is formed by dispersing the conductive particles 61g in a binder resin. The content of the conductive particles 61g is 5.0 volume % or more and 70.0 volume % or less with respect to the total volume of the charge injection layer 61f. The volume resistivity of the charge injection layer 61f is 1.0×10 9 Ω cm or more 1.0×10 14 Ω·cm or less.

[0081] The volume resistivity of the charge injection layer 61f is 1.0×10 9 If the volume resistivity of the charge injection layer 61f is less than Ω·cm, the electrical resistance of the charge injection layer 61f is too low, making it difficult to form an electrostatic latent image appropriately and to develop a desired image. 14If the resistivity exceeds Ω·cm, the electrical resistance of the charge injection layer 61f becomes too high, and the charge injection from the charging brush 2 to the charge injection layer 61f decreases, making it difficult to obtain the first charge ratio R1 of a desired value or more in the low-temperature, low-humidity environment targeted in this embodiment. In order to satisfy this range of volume resistivity, it is desirable that the content of the conductive particles 61g is 5.0 volume % or more and 70.0 volume % or less with respect to the total volume of the charge injection layer 61f. If the content of the conductive particles 61g exceeds 70.0 volume %, the charge injection layer 61f itself becomes brittle, and the surface of the photosensitive drum 61 is easily scraped off through long-term use. This reduces the charging uniformity of the photosensitive drum 61, making it easier for charging defects to occur when the image forming apparatus 100 is operated at a high speed, and image defects are more likely to occur.

[0082] The volume resistivity of the charge injection layer 61f can be controlled by, for example, the particle diameter of the conductive particles 61g in addition to the content of the conductive particles 61g. The particle diameter of the conductive particles 61g is preferably 5 nm or more and 300 nm or less, more preferably 40 nm or more and 250 nm or less, in terms of the number average particle diameter. If the number average particle diameter of the conductive particles 61g is less than 5 nm, the specific surface area of ​​the conductive particles 61g increases, and moisture adsorption increases in the vicinity of the conductive particles 61g on the surface of the charge injection layer 61f, which tends to reduce the volume resistivity of the charge injection layer 61f. If the number average particle diameter of the conductive particles 61g exceeds 300 nm, the dispersion of the particles in the charge injection layer 61f deteriorates, and the area of ​​the interface with the binder resin decreases, which increases the resistance at the interface and tends to deteriorate the charge injection property.

[0083] The conductive particles 61g contained in the charge injection layer 61f include metal oxide particles such as titanium oxide, zinc oxide, tin oxide, and indium oxide. When a metal oxide is used as the conductive particles 61g, the metal oxide may be doped with an element such as niobium, phosphorus, or aluminum or an oxide thereof. The conductive particles 61g may also have a laminated structure having a core particle and a coating layer that covers the particle. Examples of the core particle include titanium oxide, barium sulfate, and zinc oxide. Examples of the coating layer include metal oxides such as titanium oxide and tin oxide, and in this embodiment, titanium oxide is preferred from the viewpoint of charge injection from the charging brush 2.

[0084] Furthermore, when titanium oxide contains niobium, the charge injection property is improved, and the charge injection property can be improved with a small amount. The niobium content is preferably 0.5 mass% or more and 15.0 mass% or less, more preferably 2.6 mass% or more and 10.0 mass% or less, based on the total mass of the niobium-containing titanium oxide particles.

[0085] The titanium oxide particles containing niobium are preferably anatase or rutile titanium oxide particles, and more preferably anatase titanium oxide particles. By using anatase titanium oxide, charge transfer within the charge injection layer 61f becomes smooth, and charge injection becomes good. More preferably, the particles have anatase titanium oxide particles as a core material and a coating layer of titanium oxide containing niobium on the surface of the core material. By using anatase titanium oxide particles as a core material and coating the surface with titanium oxide containing niobium, charges can be easily transferred within the charge injection layer 61f and the charge injection from the charging brush 2 to the charge injection layer 61f can be improved. In addition, a decrease in the volume resistivity of the charge injection layer 61f can be suppressed.

[0086] In this way, in the image forming apparatus 100 of this embodiment, by using the photosensitive drum 61 having the charge injection layer 61f on the surface, good charge injection can be performed between the charging brush 2 and the photosensitive drum 1 even in a low temperature and low humidity environment. As a result, in the image forming apparatus 100 of this embodiment, it is possible to suppress the decrease in the first charging ratio R1 in a low temperature and low humidity environment, and it is possible to obtain the effect of suppressing the occurrence of image defects due to transfer memory regardless of the environment.

[0087] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-mentioned embodiments.

[0088] In the above embodiment, the transfer voltage is the same during paper passing and between papers, but the present invention is not limited to such a configuration. For example, even if the absolute value of the transfer voltage is smaller during paper passing than during paper passing, if the surface potential of the photoconductor at the position corresponding to the paper passing may be reversed to the opposite polarity from the normal charging polarity, the present invention can suppress the occurrence of image defects due to transfer memory.

[0089] Furthermore, even in cases where the surface potential of the photosensitive member is reversed to a polarity opposite to the normal charging polarity during at least a portion of the pre-rotation process or post-rotation process, the present invention can suppress the occurrence of image defects due to transfer memory.

[0090] Furthermore, when an image is formed on a recording material whose width in the direction of the rotation axis of the photoconductor (the width direction substantially perpendicular to the conveying direction of the recording material) is smaller than the maximum width on which an image can be formed in the image forming apparatus, a region of the photoconductor may be in direct contact with the transfer member at the transfer position even during paper passing. In this region, the surface potential of the photoconductor may be inverted to the opposite polarity from the normal charging polarity, as in the region corresponding to the paper gap described in the above embodiment. In addition, when an image is formed on a large size paper continuously after a small size paper, image defects due to transfer memory may occur in this region. According to the present invention, the transfer memory in this region can also be reduced in the same manner as the region corresponding to the paper gap described in the above embodiment, and the occurrence of image defects due to transfer memory in this region can be suppressed.

[0091] In addition, in the area of ​​the photoconductor where the recording material is interposed between the photoconductor and the transfer member during paper passage, the surface potential may have a smaller absolute value than the bright area potential, even if the polarity is not reversed to the opposite polarity from the normal charging polarity (see FIG. 6). According to the present invention, the potential of this area can also be made uniform at the first charging position, which is advantageous for forming a more uniform dark area potential at the second charging position.

[0092] Furthermore, the photoconductor is not limited to a drum-shaped member, but may be a belt-shaped member or the like.

[0093] Furthermore, the transfer member is not limited to a roller-shaped member, and may be a brush-shaped member, a sheet-shaped member, etc. When the transfer member is a brush-shaped member, it is preferable to make the average thread spacing of the charging brush smaller than the average thread spacing of the transfer brush (preferably 70% or less, more preferably 50% or less).

[0094] In addition, in the above-described embodiment, the charging brush as the first charging member is a fixedly disposed brush member, but it may also be, for example, a brush roller having a core portion and a brush portion provided around the core portion.

[0095] Furthermore, the image forming apparatus is not limited to a monochrome image forming apparatus, but may be, for example, a color image forming apparatus having a plurality of image forming units each having a photoconductor and a process means acting on the photoconductor. In this case, the present invention can be applied to at least one of the plurality of photoconductors.

[0096] In the above embodiment, the image forming apparatus does not have a pre-exposure device. According to the present invention, even if there is no pre-exposure device, the transfer memory can be reduced and the occurrence of image defects due to the transfer memory can be suppressed. However, the present invention is not limited to this, and even if the image forming apparatus is provided with a pre-exposure device, it is effective in reducing the transfer memory by leveling the surface potential of the photoconductor that has been inverted to the opposite polarity from the normal charging polarity.

[0097] In addition, in the above-described embodiment, the normal charging polarity of the photoconductor and the normal charging polarity of the toner are negative polarities, but these may be positive polarities, in which case a person skilled in the art can make appropriate changes, such as by making the polarities of the various applied voltages opposite to those in the above-described embodiment.

[0098] In addition, the dimensions, materials, shapes, and relative positions of the components described in the above embodiments may be changed as appropriate depending on the configuration and various conditions of the device to which the invention is applied, and the scope of the present invention is not limited to the above embodiments. [Explanation of symbols]

[0099] 1 Photosensitive drum 2 Charging Brush 3. Charging roller 4. Exposure equipment 5. Developing device 6 Transfer roller 40 Control section

Claims

1. A rotatable photoreceptor; a first charging member that contacts the surface of the photoconductor at a first charging position in the rotation direction of the photoconductor to charge the surface of the photoconductor; a second charging member for charging the surface of the photoconductor at a second charging position in the rotation direction of the photoconductor; a developing member that supplies toner charged to a predetermined polarity to the surface of the photoconductor at a development position in the rotation direction of the photoconductor to form a toner image on the surface of the photoconductor; a transfer member that contacts a surface of the photoconductor at a transfer position in a rotation direction of the photoconductor, and transfers a toner image from the photoconductor to a recording material passing between the photoconductor and the transfer member; a first charging voltage application section that applies a first charging voltage of the predetermined polarity to the first charging member; a second charging voltage applying section that applies a second charging voltage of the predetermined polarity to the second charging member; a transfer voltage application unit that applies a transfer voltage having a polarity opposite to the predetermined polarity to the transfer member; a control unit that controls the first charging voltage application unit, the second charging voltage application unit, and the transfer voltage application unit; having In a rotation direction of the photoconductor, the first charging position is located downstream of the transfer position and upstream of the second charging position, the second charging position is located downstream of the first charging position and upstream of the development position, the development position is located downstream of the second charging position and upstream of the transfer position, and the transfer position is located downstream of the development position and upstream of the first charging position, an image forming apparatus, characterized in that the control unit controls the transfer voltage application unit so that at least a portion of the surface of the photoconductor that is in direct contact with the transfer member at the transfer position has a potential of a polarity opposite to the predetermined polarity, controls the first charging voltage application unit so as to apply the first charging voltage that is less than a discharge start voltage at the first charging position to the first charging member to charge the surface of the photoconductor at the first charging position, and controls the second charging voltage application unit so as to apply the second charging voltage that is equal to or greater than a discharge start voltage at the second charging position to the second charging member to charge the surface of the photoconductor at the second charging position.

2. 2. The image forming apparatus according to claim 1, wherein, with respect to at least a portion of the region, when a ratio of the surface potential of the photoconductor that is changed at the first charging position to the surface potential of the photoconductor that is changed at the first charging position and the second charging position is defined as a first charging ratio, the control unit controls the first charging voltage application unit so that the first charging ratio is 10% or more.

3. With respect to at least a portion of the region, a ratio of the surface potential of the photoconductor changed at the first charging position among the surface potentials of the photoconductor changed at the first charging position and the second charging position is defined as a first charging ratio, a surface potential of the photoconductor after passing the transfer position and before reaching the first charging position is defined as Vt, a surface potential of the photoconductor formed at the second charging position is defined as Vd, a maximum value of Vt is defined as Vtmax, a minimum value of Vt is defined as Vtmin, and an average value of Vt is defined as Vtave, the control unit determines that the first charging ratio is expressed by the following formula: R1' [%] = {(Vtmax-Vtmin) / (Vd-Vtave)}×100 2. The image forming apparatus according to claim 1, wherein the first charging voltage application unit is controlled so that the first charging voltage application unit is equal to or larger than R1' expressed by the following formula:

4. 2. The image forming apparatus according to claim 1, wherein the at least a portion of the area corresponds to an area between a preceding recording material and a succeeding recording material which is a recording material next to the preceding recording material at the transfer position.

5. The image forming apparatus according to claim 4, characterized in that the control unit controls the transfer voltage application unit so as to apply the same transfer voltage to the transfer member when the area on the surface of the photosensitive body corresponding to between the preceding recording material and the succeeding recording material is at the transfer position and when the area on the surface of the photosensitive body corresponding to the preceding recording material is at the transfer position.

6. 2. The image forming apparatus according to claim 1, wherein the transfer member is made of a foam capable of coming into contact with the surface of the photoconductor.

7. 7. The image forming apparatus according to claim 6, wherein the first charging member has a brush capable of coming into contact with the surface of the photoconductor.

8. 8. The image forming apparatus according to claim 7, wherein an average inter-fiber distance of the brush of said first charging member is smaller than an average cell diameter of the foam of said transfer member.

9. 7. The image forming apparatus according to claim 6, wherein the first charging member is configured to have a foam capable of coming into contact with the surface of the photoconductor.

10. 10. The image forming apparatus according to claim 9, wherein the average cell diameter of the foam of the first charging member is smaller than the average cell diameter of the foam of the transfer member.

11. 11. The image forming apparatus according to claim 1, wherein the toner remaining on the surface of the photoconductor after the toner image is transferred from the photoconductor to the recording material is collected by the developing member.

12. 11. The image forming apparatus according to claim 1, further comprising a cleaning member for removing toner from the surface of the photoconductor downstream of the transfer position and upstream of the first charging position in the rotation direction of the photoconductor.

13. 11. The image forming apparatus according to claim 1, wherein the photoconductor has a charge injection layer that forms a surface thereof.

14. 14. The image forming apparatus according to claim 13, wherein the charge injection layer is formed by dispersing conductive particles in a binder resin.

Images