Image forming apparatus

Abstract

An image forming apparatus includes a photosensitive member, a first charging member, a second charging member, a developing member, a transfer member, a first charging voltage applying portion, a second charging voltage applying portion, a transfer voltage applying portion, and a controller. The controller: controls the transfer voltage applying portion such that a region of at least a part of a surface of the photosensitive member in contact with the transfer member at the transfer position has a potential of a polarity opposite to a predetermined polarity; controlling the first charging voltage applying portion such that a surface of the photosensitive member is charged at a first charging position by applying a first charging voltage smaller than the discharge start voltage; and control the second charging voltage applying portion such that the surface of the photosensitive member is charged at the second charging position by applying a second charging voltage not less than the discharge start voltage.

Description

Technical Field

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

[0002] Generally, in an electrophotographic type image forming apparatus (e.g., a laser printer), the surface of a photosensitive member is charged by a charging unit, and the charged surface of the photosensitive member is exposed (image development position) by an exposure unit, thereby forming an electrostatic latent image on the photosensitive member. Then, toner is deposited on the electrostatic latent image by a developing unit, thereby forming a toner image on the photosensitive member, and the toner image is transferred onto a sheet-like recording material by a transfer unit.

[0003] In many cases, a rotatable photosensitive drum is used as the photosensitive member. In addition, in many cases, a transfer member such as a transfer roller for forming a transfer portion in contact with the photosensitive member is used as the transfer unit. An image forming apparatus including a photosensitive drum and a transfer roller will be described as an example below. When a transfer voltage having a polarity opposite to the charge polarity of the photosensitive drum is applied to the transfer roller, the toner image on the photosensitive drum is transferred onto the recording material.

[0004] Here, the charge polarity (normal charge polarity) of the photosensitive drum is the polarity of the potential formed on the surface of the photosensitive drum charged by the charging unit 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 in some cases, the surface of the photosensitive drum is non-uniformly charged. This phenomenon is called "transfer memory". When transfer memory occurs and the surface of the photosensitive drum is non-uniformly charged, it is sometimes difficult to make the surface potential of the photosensitive drum uniform when the surface of the photosensitive drum is subsequently charged by the charging unit.

[0005] For example, as conditions where transfer memory is likely to occur, in the case of a direct transfer type image forming apparatus in which a toner image is directly transferred from a photosensitive member to a recording material, a state where a transfer voltage is not applied to the transfer roller in a state where the recording material is not inserted between the photosensitive drum and the transfer roller can be cited. In a state where the recording material as a resistor is not inserted between the photosensitive drum and the transfer roller, when a transfer voltage similar to the transfer voltage when the recording material is inserted between the photosensitive drum and the transfer roller is applied to the transfer roller, discharge is likely to occur between the photosensitive drum and the transfer roller in some cases.

[0006] As a technique for reducing transfer memory, a method is known in which the surface of a photosensitive drum is discharged by irradiating the surface of the photosensitive drum with light after the photosensitive drum has passed through a transfer portion. Japanese Patent Application Laid-Open No. 2016-218155 discloses an image forming apparatus in which a pre-exposure unit (pre-charging exposure unit) is provided for immediately exposing the surface of the photosensitive drum after the photosensitive drum has passed through the transfer portion.

[0007] When transfer memory occurs in a case where the absolute value of the transfer voltage is large in a state where a recording material is not inserted between the photosensitive drum and the transfer roller, potential non-uniformity is generated on the surface of the photosensitive drum due to discharge non-uniformity depending on the surface shape of the transfer roller. For example, in the case of using foamed rubber as the surface layer of the transfer roller, the surface shape of the transfer roller is formed according to the presence or absence of foam cells. That is, the potential non-uniformity generated on the surface of the transfer roller has a distribution of the same degree as the size of the foam cells. Due to this potential non-uniformity, the surface potential of the photosensitive drum on which transfer memory is generated fluctuates.

[0008] When the surface potential of the photosensitive drum on which transfer memory is generated is, for example, the same in polarity as the charge polarity of the photosensitive drum and becomes lower in absolute value than the potential after image exposure, it is difficult to make the potential of a portion where the absolute value of the surface potential has become lower than the potential after image exposure uniform even when the surface of the photosensitive drum is exposed by the above-described pre-exposure unit. In addition, it is also considered to make the exposure intensity of the pre-exposure unit stronger than the exposure intensity of the exposure unit for performing image exposure. However, particularly in a case where the polarity of at least a part of the surface potential of the photosensitive drum on which transfer memory is generated is reversed to a polarity opposite to the charge polarity (charge potential), it is difficult to sufficiently make the transfer memory uniform. Summary of the Invention

[0009] A main object of the present invention is to provide an image forming apparatus capable of suppressing the occurrence of image defects caused by transfer memory.

[0010] This object is achieved by an image forming apparatus according to the present invention.

[0011] According to one aspect of the present invention, an image forming apparatus includes: a rotatable photosensitive member; a first charging member configured to contact a surface of the photosensitive member at a first charging position with respect to a rotation direction of the photosensitive member to charge the surface of the photosensitive member; a second charging member configured to charge the surface of the photosensitive member at a second charging position with respect to the rotation direction of the photosensitive member; a developing member configured to form a toner image on the surface of the photosensitive member by supplying toner charged to a predetermined polarity to the surface of the photosensitive member at a developing position with respect to the rotation direction of the photosensitive member; a transfer member that contacts the surface of the photosensitive member at a transfer position with respect to the rotation direction of the photosensitive member and is configured to transfer the toner image from the photosensitive member to a recording material passing between the photosensitive member and the transfer member; a first charging voltage applying portion configured to apply a first charging voltage of a predetermined polarity to the first charging member; a second charging voltage applying portion configured to apply a second charging voltage of a predetermined polarity to the second charging member; a transfer voltage applying portion configured to apply a transfer voltage having a polarity opposite to the predetermined polarity to the transfer member; and a controller configured to control the first charging voltage applying portion, the second charging voltage applying portion, and the transfer voltage applying portion, wherein, with respect to the rotation direction of the photosensitive member, 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 developing position, the developing position is located downstream of the second charging position and upstream of the transfer position, and the transfer position is located downstream of the developing position and upstream of the first charging position, and wherein the controller: controls the transfer voltage applying portion such that, on the surface of the photosensitive member in direct contact with the transfer member at the transfer position, a region corresponding to between the recording material and a subsequent recording material after the recording material has a potential of a polarity opposite to the predetermined polarity; controls the first charging voltage applying portion such that the surface of the photosensitive member is charged at the first charging position by applying a first charging voltage less than a discharge start voltage to the first charging member at the first charging position; and controls the second charging voltage applying portion such that the surface of the photosensitive member is charged at the second charging position by applying a second charging voltage not less than the discharge start voltage to the second charging member at the second charging position.

[0012] Other features of the present invention will become apparent from the following description of exemplary embodiments with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a schematic cross-sectional view of the image forming apparatus according to Embodiment 1.

[0014] Figure 2 is a schematic cross-sectional view of the image forming apparatus according to Conventional Example 1.

[0015] Figure 3 Parts (a) to (d) in [description] are schematic diagrams for explaining the progress of the surface potential of the photosensitive drum in Conventional Example 1.

[0016] Figure 4 Parts (a) and (b) in [description] are schematic diagrams for explaining the method of measuring the cell diameter of the foam material.

[0017] Figure 5 Parts (a) to (d) in [description] are schematic diagrams for explaining the progress of the surface potential of the photosensitive drum in Example 1.

[0018] Figure 6 is a graph showing an example of the surface potential of the photosensitive drum after passing through the transfer position.

[0019] Figure 7 is a schematic cross-sectional view of an image forming apparatus according to a modified example of Example 1.

[0020] Figure 8 is a schematic cross-sectional view of the image forming apparatus according to Example 2.

[0021] Figure 9 is a schematic diagram for explaining the layer structure of the photosensitive drum in Example 3. Detailed Description

[0022] Hereinafter, the image forming apparatus according to the present invention will be specifically described with reference to the accompanying drawings.

[0023] (1) Image Forming Apparatus

[0024] (1-1) Structure of the Image Forming Apparatus

[0025] Figure 1 is a schematic cross-sectional view of the image forming apparatus 100 according to Example 1. The image forming apparatus 100 of this example is an electrophotographic type laser beam printer and is capable of forming a black (monochrome) image on a recording material P depending on image information input from an external device 200 such as a personal computer. First, the structure of the image forming apparatus 100 of this example will be described.

[0026] The image forming apparatus 100 includes, within the main apparatus unit, a photosensitive drum 1 as an image bearing member, and the photosensitive drum 1 is a drum-shaped (cylindrical) photosensitive member. The photosensitive drum 1 is formed by disposing a photosensitive material such as OPC (organic photoconductor), amorphous selenium, or amorphous silicon on a cylindrical drum substrate formed of aluminum, nickel, etc. In this example, the outer diameter of the photosensitive drum 1 is The negatively chargeable OPC photosensitive member. That is, in the present embodiment, the charge polarity (normal charge polarity) of the photosensitive drum 1 is negative. The photosensitive drum 1 is formed by sequentially laminating a conductive layer, an undercoat layer, and a photosensitive layer composed of a charge generation layer and a charge transport layer on the surface of a conductive support member made of an aluminum cylinder, starting from the conductive support member side. The photosensitive drum 1 is rotationally driven along Figure 1 the arrow Rd direction (clockwise direction) in

[0027] Around the photosensitive drum 1, the following components are sequentially arranged along the rotation direction Rd of the photosensitive drum 1.

[0028] First, a charging brush 2 is provided as the first charging unit, and the charging brush 2 is a brush-shaped charging member. In this embodiment, a fabric with a width of 5 mm obtained by making conductive nylon fibers into a flocked fabric is adhered and fixed to a conductive support portion (also used as a power supply electrode) made of a stainless steel (metal) plate to form the charging brush 2. In this embodiment, the conductive nylon fibers (brush fibers) constituting the charging brush 2 have a fineness of 2 denier, a bristle (yarn) planting density of 200 kF / inch 2 and a pile length of 4 mm. In this embodiment, the charging brush 2 contacts the surface (outer peripheral surface) of the photosensitive drum 1 such that the penetration amount of the bristle tips of the brush fibers into the photosensitive drum 1 is 0.6 mm. Further, in this embodiment, the charging brush 2 is fixedly arranged and rubs the surface of the photosensitive drum 1 as the photosensitive drum 1 rotates. Incidentally, the width of the charging brush 2 is the length of the charging brush 2 along the moving direction of the surface of the photosensitive drum 1. Further, "kF / inch" as the unit of the bristle planting density 2" indicates the number of filaments per square inch. The charging brush 2 substantially uniformly contacts the surface of the photosensitive drum 1 at the tips of its brush fibers, and is arranged such that the variation in the bristles of the tips of its brush fibers is reduced due to the rotation of the photosensitive drum 1. Here, the above-mentioned entry amount is represented by the difference between the length (fluff length) of the brush fibers and the gap between the brush fiber support portion and the photosensitive drum 1 in a state where no force for bending the brush fibers is applied from the outside. In this embodiment, the resistance on the outer peripheral surface side of the brush fibers of the charging brush 2 in the cross-sectional direction of the brush fibers is higher than that on the center side. For this reason, in this embodiment, in order to satisfactorily perform the injection charging described later, the charging brush 2 is arranged such that the tips of the brush fibers contact the surface of the photosensitive drum 1. Incidentally, for example, when the resistance on the outer peripheral surface side of the brush fibers in the cross-sectional direction is low enough, this does not apply, and preferably, the charging brush 2 is inserted into the photosensitive drum 1 such that the first charging ratio (brush charging ratio) described later becomes a predetermined value or more, and thus the brush fibers can be laid. The position on the photosensitive drum 1 where the charging brush 2 performs the charging process in the rotation direction of the photosensitive drum 1 is the first charging position Pa. The charging brush 2 mainly charges the surface of the photosensitive drum 1 by direct injection charging. The charging brush 2 charges the surface of the photosensitive drum 1 by causing a current depending on the potential difference between the charging brush 2 and the photosensitive drum 1 to flow through the portion of the brush fibers in direct contact with the surface of the photosensitive drum 1. The charging brush 2 is arranged along the rotation axis direction of the photosensitive drum 1, and the length of the region where the brush fibers are provided in this rotation axis direction is greater than the length of the image forming region (the region where a toner image can be formed) on the photosensitive drum 1 in this rotation axis direction.

[0029] Next, a charging roller 3 is provided as the second charging unit, and the charging roller 3 is a roller-shaped charging member. In this embodiment, the charging roller 3 is composed of a conductive base shaft (core metal, core portion) that also serves as a power supply electrode and an elastic layer that is cylindrically wound around the outer peripheral surface of the core metal. In this embodiment, the charging roller 3 has a roller outer diameter of The core metal diameter is and an elastic roller with an elastic layer thickness of 2.5 mm. In this embodiment, SUS (stainless steel) is used as the material of the core metal of the charging roller 3, and a mixed rubber material of NBR (nitrile rubber) and epichlorohydrin is used as the material of the elastic layer of the charging roller 3. The charging roller 3 is pressed against the photosensitive drum 1 and rotates as the photosensitive drum 1 rotates.

[0030] Regarding the rotation direction of the photosensitive drum 1, the position on the photosensitive drum 1 where the surface of the photosensitive drum is charged 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 discharging generated in at least one of the minute gaps between the photosensitive drum 1 and the charging roller 3, and these minute gaps are formed on the upstream side and the downstream side of the contact portion between the photosensitive drum 1 and the charging roller 3 with respect to the rotation direction of the photosensitive drum 1. For simplicity, the contact portion between the photosensitive drum 1 and the charging roller 3 can be considered as the second charging position Pb. The rotation axis direction of the charging roller 3 is substantially 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 greater than the length of the image forming area on the photosensitive drum 1 in the same direction.

[0031] Next, an exposure device 4 is provided as the exposure unit. In this embodiment, the exposure device 4 is composed of a laser scanner device (laser optical system). Regarding the rotation direction of the photosensitive drum 1, the position on the surface of the photosensitive drum 1 where the surface is exposed by the exposure device 4 is the exposure position Pc.

[0032] Next, a developing device 5 is provided as the developing unit. In this embodiment, in the developing device 5, a non-magnetic one-component developer (toner) is used as the developer. The developing device 5 includes a developing roller 5a as a developer carrying member (developing member), and a developing container 5b as a container for accommodating the developer. The developing roller 5a is in contact with the surface of the photosensitive drum 1, and supplies toner to the developing portion which is the portion (contact portion) opposite to the photosensitive drum 1. The toner in the developing container 5b is supplied to the developing roller 5a. Incidentally, the developing device 5 can use a magnetic one-component developer (toner) or a two-component developer containing toner and a carrier as the developer. Regarding the rotation direction of the photosensitive drum 1, the position on the photosensitive drum 1 where toner is supplied by the developing roller 5a (in this embodiment, the position where the developing roller 5a is in contact with the photosensitive drum 1) is the developing position Pd. In this embodiment, as the main charge polarity during development, the normal charge polarity of the toner is negative.

[0033] Next, a transfer roller 6 is provided as a transfer unit. The transfer roller 6 is a roller-shaped transfer member (a rotatable transfer member). The transfer roller 6 is pushed (pressed) against the photosensitive drum 1 by a transfer pressing spring (not shown) serving as a pushing member (as a pushing unit), and is in pressure contact with the photosensitive drum 1. Thus, a transfer clamping portion (transfer portion, transfer clamping portion) Nt is formed, which is the contact portion between the photosensitive drum 1 and the transfer roller 6. The transfer roller 6 rotates as the photosensitive drum 1 rotates. The transfer roller 6 not only clamps and feeds the recording material P between itself and the photosensitive drum 1, but also transfers the toner image from the photosensitive drum 1 to the recording material P when a voltage is applied. 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 excitation electrode and an elastic layer that surrounds the outer peripheral surface of the conductive base shaft in a cylindrical shape. As the material of this elastic layer, a semiconductor rubber material formed by using EPDM (ethylene propylene diene methyl rubber), NBR (nitrile rubber), SBR (styrene butadiene rubber), polyurethane rubber, chloromethyl ether rubber, silicone rubber, etc. is generally used. The material of the elastic layer may contain an appropriate amount of a conductive agent, such as an ionic conductive agent. In addition, in order to make the outer peripheral surfaces of the photosensitive drum 1 and the transfer roller 6 contact each other evenly, in some cases, the elastic layer of the transfer roller 6 is formed of a foam member (elastic foam member), and a unit structure is formed near the surface of the transfer roller 6. The transfer roller 6 used in this embodiment is a foamed elastic roller, and its roller outer diameter is The core metal diameter is The thickness of the elastic layer is 4.5 mm, and the elastic layer is composed of an elastic foam layer. In this embodiment, when the unit diameter of the surface of the transfer roller 6 is measured by the measurement method described later, the unit diameter is 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. Regarding the rotation direction of the photosensitive drum 1, the position on the photosensitive drum 1 where the toner image is transferred to the recording material P (the position corresponding to the above transfer clamping portion Nt) is the transfer position Pe.

[0034] Next, a charge removal needle 19 is provided as a charge removal member, which is not only used to remove the excessive charge on the surface of the recording material P after transfer, but also used to reduce the degree of potential non-uniformity on the photosensitive drum 1 caused by peeling discharge. As the charge removal needle 19, a charge removal needle provided with a serrated sharp end and formed of a thin metal plate material having good conductivity (such as a SUS plate or an aluminum plate) can be used. The charge removal needle 19 is provided on the downstream side of the transfer roller 6 with respect to the feeding (transporting) direction of the recording material P, so that the tip of the needle faces the surface of the photosensitive drum 1.

[0035] In addition, a recording material cassette 7 for accommodating a recording material P such as paper (transfer material, recording medium, sheet) is provided at the lower part of the image forming apparatus 100. Further, along the feeding path of the recording material P starting from the recording material cassette 7, a feeding roller 8, a conveying roller 9, a top sensor 10, a pre-transfer conveying guide 15, a transfer-to-fixing conveying guide 11, a fixing device 12, a discharging roller 13, and a discharging tray 14 are arranged in sequence. In addition, the image forming apparatus 100 is provided with a controller 40 for performing control of the operation of the image forming apparatus 100.

[0036] Incidentally, the photosensitive drum 1 and the charging brush 2, the charging roller 3, and the developing device 5 as processing units that can act on the photosensitive drum 1 can be integrally assembled into a processing cartridge that is detachably mounted to the main assembly of the image forming apparatus 100.

[0037] (1-2) Image forming operation

[0038] Next, the image forming operation in the image forming apparatus 100 of the present embodiment will be described.

[0039] By the driving force transmitted from the driving source 17 that constitutes the driving unit, the photosensitive drum 1 is driven to rotate in the direction of arrow Rd (clockwise direction) in Figure 1 at a circumferential speed (processing speed) of 300 mm / sec. The surface of the rotating photosensitive drum 1 is substantially uniformly charged to a predetermined potential (dark portion potential, charge potential, non-image portion potential) by the charging brush 2 and the charging roller 3, and the polarity of the predetermined potential is the same as the normal charge polarity of the toner (negative polarity in the present embodiment). During charging, a first charging voltage (first charging bias), which is a negative-polarity DC (direct current) voltage, is applied from a first charging power source (high-voltage power source) 20, which is a first charging voltage applying unit (first charging voltage applying portion), to the charging brush 2. In addition, during charging, a second charging voltage (second charging bias), which is a negative-polarity DC voltage, is applied from a second charging power source (high-voltage power source) 21, which is a second charging voltage applying unit (second charging voltage applying portion), to the charging roller 3. 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, thereby forming a dark portion potential Vd of -500 V on the surface of the photosensitive drum 1.

[0040] The charged surface of the photosensitive drum 1 is subjected to scanning exposure depending on the image information by an exposure device (laser scanner) 4, thereby forming an electrostatic latent image (electrostatic image) on the photosensitive drum 1. The video controller 110 of the image forming apparatus 100 generates a time-series electrical digital pixel signal by processing the image information input from the external device 200 into the image forming apparatus 100. The exposure device 4 outputs a laser L1 modulated according to the time-series electrical digital pixel signal, and subjects the charged surface of the photosensitive drum 1 to the scanning exposure (image exposure) of the laser L1. In this embodiment, the charge at the portion of the photosensitive drum 1 exposed by the exposure device 4 is removed, so that a bright portion potential (post-image exposure potential, image portion potential) Vl of -100 V is formed on the surface of the photosensitive drum 1. Thus, an electrostatic latent image is formed on the photosensitive drum 1 by the contrast between the dark portion potential Vd and the bright portion potential Vl.

[0041] The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by supplying toner by a developing device 5, thereby forming a toner image (toner picture, developer image) on the photosensitive drum 1. During development, a developing voltage (developing bias) is applied to the developing roller 5a by a developing power source (high-voltage power source) 16 as a developing voltage application unit (developing voltage application portion), and the developing voltage is a DC voltage having the same polarity as the normal charge polarity of the toner (negative polarity in this embodiment). In this embodiment, as an example, a developing voltage of -350 V is applied to the developing roller 5a. In this embodiment, on the exposed portion (image portion) of the photosensitive drum 1 where the absolute value of the surface potential is reduced due to exposure after the photosensitive drum surface is substantially uniformly charged, toner charged to the same polarity as the charge polarity of the photosensitive drum 1 (negative polarity in this embodiment) is deposited (reverse development type).

[0042] The toner image formed on the photosensitive drum 1 is electrostatically transferred to a predetermined position on the recording material P in the transfer nip Nt by the action of the transfer roller 6. During transfer, a transfer voltage (transfer bias) is applied to the transfer roller 6 by a transfer power source (high-voltage power source) 18 as a transfer voltage application unit (transfer voltage application portion), and the transfer voltage is a DC voltage having the opposite polarity to the normal charge polarity of the toner (positive polarity in this embodiment).

[0043] The recording material P is accommodated in a recording material cassette 7 serving as a recording material accommodation portion, and is fed one by one from the recording material cassette 7 by a feed roller 8 serving as a feeding member. In the case where images are continuously formed on the recording material P, the interval between the trailing end of the current recording material (sheet of paper) P and the leading end of the subsequent recording material (subsequent sheet of paper) P, which is the recording material P after the current recording material P, is referred to as a sheet interval (paper interval). In this embodiment, the distance of the sheet interval is set to 20 mm. Incidentally, the time for this sheet interval is 67 msec, which is very short. Therefore, when it is desired to reduce the absolute value of the transfer voltage at the timing corresponding to the sheet interval, in some cases, the fall and rise of the transfer voltage are not timely. For this reason, in this embodiment, the same transfer voltage is set between the period during which the recording material P passes through the transfer nip portion Nt (here, this period is also referred to as "during the passage of the sheet (paper)") and the sheet interval. After the recording material P is fed by the feed roller 8, the recording material P is conveyed by a conveying roller (alignment roller) 9 serving as a conveying member, and is supplied to the transfer nip portion Nt along a pre-transfer conveying guide 15 serving as a guiding member. Based on the detection result of the leading end of the recording material P in the feed (conveying) direction by a top sensor 10 serving as a recording material detection unit, the conveying roller 9 supplies the recording material P to the transfer nip portion Nt so as to be synchronized with the toner image on the photosensitive drum 1.

[0044] Excess surface charge of the electric charge amount is removed from the recording material P on which the toner image has been transferred in the transfer nip portion Nt by a charge removal needle 19. The recording material P that has passed through the charge removal needle 19 is conveyed toward a fixing device 12 serving as a fixing portion along a transfer-to-fixing conveying guide 11 serving as a guiding member. The fixing device 12 includes a fixing roller 12a and a pressing roller 12b that is press-contacted with the fixing roller 12a. The fixing device 12 heats and presses the recording material P carrying the unfixed toner image passing through the nip portion between these rollers, so that the toner image is fixed on the recording material P.

[0045] The recording material P after the toner image is fixed by the fixing device 12 is discharged (output) onto a discharge tray 14 serving as a discharge portion by a discharge roller 13, and the discharge tray 14 is formed on the upper surface of the image forming apparatus 100.

[0046] On the other hand, the toner that remains on the surface of the photosensitive drum 1 without being transferred during transfer (transfer residual toner) is removed from the surface of the photosensitive drum 1 and collected in the following steps (cleanerless type). Among the transfer residual toner, toner charged to a positive polarity and toner charged to a negative polarity but not having sufficient charge are mixed and present. The transfer residual toner is recharged to a negative polarity by injection charging or discharging at the first charging position Pa and the second charging position Pb. The transfer residual toner recharged to a negative polarity at the first charging position Pa and the second charging position Pb reaches the developing position Pd as the photosensitive drum 1 rotates. Here, as described above, an electrostatic latent image depending on the image information is formed on the photosensitive drum 1 that reaches the developing position Pd. The behavior of the transfer residual toner will be described by dividing it into the behavior in the image portion (exposed portion) and the behavior in the non-image portion (non-exposed portion). The transfer residual toner deposited in the non-image portion on the photosensitive drum 1 is pushed and transferred to the developing roller 5a at the developing position Pd due to the potential difference between the bright portion potential Vl of the photosensitive drum 1 and the developing voltage, and then collected in the developing container 5b.

[0047] Incidentally, the toner collected in the developing container 5b is used again for image formation. On the other hand, the transfer residual toner deposited in the image portion on the photosensitive drum 1 is pushed toward the photosensitive drum 1 at the developing position Pd due to the potential difference between the dark portion potential Vd and the developing voltage, so that the transfer residual toner does not transfer from the photosensitive drum 1 to the developing roller 5a. This transfer residual toner moves to the transfer position Pe together with the toner transferred from the developing roller 5a to the photosensitive drum 1, and is transferred to the recording material P in the transfer nip portion Nt, thereby removing the transfer residual toner from the surface of the photosensitive drum 1.

[0048] By repeating the above operations, image formation can be continuously performed. In this embodiment, the image forming apparatus 100 can execute printing at a printing speed of 56 sheets per minute.

[0049] The controller 40 is configured to include a central processing unit (CPU) 41 as a calculation control unit, a read-only memory (ROM) 41a and a random access memory (RAM) 41b as storage units, an input / output portion (not shown) for controlling signal transmission between the controller 40 and each part, etc., where the CPU 41 is a central element for performing arithmetic processing. Information input to the controller 40, detected information, calculation results, etc. are stored in the RAM 41b as a rewritable memory, and a control program, a pre-acquired data table, etc. are stored in the ROM 41a.

[0050] The CPU 41 and memories such as the ROM 41a and the RAM 41b can perform data transfer and reading between each other. The controller 40 executes an image forming operation and the like by controlling the operations of respective parts 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.

[0051] The image forming apparatus 100 executes a job, which is a series of operations for forming and outputting an image on single or multiple recording materials P and starts with a single start instruction. The job generally includes an image forming step, a pre-rotation step, a sheet (paper) interval step, and a post-rotation step. The image forming step corresponds to the period during which an electrostatic latent image formation, a toner image formation, and a transfer of the toner image for actually forming and outputting the image on the recording material P are actually executed, and the image forming period refers to this period. Specifically, at respective positions where the steps of forming the electrostatic latent image, forming the toner image, and transferring the toner image are executed, the timing of the image forming period is different. The pre-rotation step corresponds to the period from input of the start instruction until actual image formation starts, during which preparation operations before the image forming step are executed. The sheet interval step corresponds to the period corresponding to the interval between two recording materials P when an image is continuously formed (continuous image formation) on multiple recording materials P. The post-rotation step corresponds to the period during which post-operations (preparation operations) after the image forming step are executed. The non-image forming period (non-image forming time period) is a period different from the image forming period, including the pre-rotation step, the sheet interval step, the post-rotation step, and in addition, includes the power-on period of the image forming apparatus 100 or a pre-multi-rotation step which is a preparation operation step during the recovery period from the sleep state.

[0052] (2) Image defects caused by transfer memory

[0053] Next, the mechanism of generating image defects due to transfer memory will be described.

[0054] Figure 2It is a schematic cross-sectional view of the image forming apparatus 101 of Conventional Example 1. In the image forming apparatus 101 of Conventional Example 1, the charging brush 2 in the image forming apparatus 100 of this embodiment (Embodiment 1) is not provided, but a pre-exposure device 30 is provided. The pre-exposure device 30 is provided to equalize the potential non-uniformity on the surface of the photosensitive drum 1 after passing through the transfer position Pe (and 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 the laser L2 at the pre-exposure position Pf. In Conventional Example 1, the exposure intensity of the laser L2 of the pre-exposure device 30 is equal to the exposure intensity of the laser L1 of the exposure device 4, so that the surface potential of the photosensitive drum 1 can reach -100V, which is the same as the bright part potential Vl, through charge removal. Except for the above points, the structure and operation of the image forming apparatus 101 of Conventional Example 1 are basically 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 structures as the elements of the image forming apparatus 100 of this embodiment are added with the same reference numerals or symbols as those in this embodiment.

[0055] Figure 3 Parts (a) to (d) are schematic views for explaining the progress of the surface potential of the photosensitive drum 1 at positions corresponding to the sheet intervals when continuously forming an image on the recording material P by the image forming apparatus 101 of Conventional Example 1.

[0056] Figure 3 Part (a) shows the surface potential of the photosensitive drum 1 before passing through the transfer position Pe (after passing through the charging position Pb and before reaching the transfer position Pe). At the position corresponding to the sheet interval, the exposure device 4 does not perform exposure, so the dark part potential Vd is maintained.

[0057] Figure 3 Part (b) shows the surface potential of the photosensitive drum 1 after passing through the transfer position Pe (after passing through the transfer position Pe and before reaching the pre-exposure position Pf). In Conventional Example 1, the same transfer voltage control as that in this embodiment is adopted. Therefore, a positive-polarity transfer voltage with a large absolute value, which is the same as the transfer voltage during sheet passing, is applied in the sheet interval. For this reason, due to the potential difference between the transfer roller 6 and the photosensitive drum 1, discharge occurs in the transfer nip Nt, and thus, as shown in Figure 3The potential non-uniformity (transfer memory) shown in part (b). The non-uniformity of this transfer memory depends on the unit diameter of the surface of the transfer roller 6 and is caused by the change in the discharge state 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 more active. In addition, as shown in Figure 3 part (b), such areas are generated on the photosensitive drum 1 that have a surface potential with the same polarity as the charge polarity of the photosensitive drum 1 and an absolute value lower than the potential Vl of the bright part (hereinafter, this surface potential is simply referred to as "surface potential with absolute value lower than potential Vl of the bright part", etc.), or a surface potential with a polarity opposite to the normal charge polarity of the photosensitive drum 1 (hereinafter, this surface potential is simply referred to as "polarity-reversed surface potential", etc.).

[0058] Figure 3 Part (c) shows the surface potential of the photosensitive drum 1 after passing through the pre-exposure position Pf (after passing through the pre-exposure position Pf and before reaching the charging position Pb). At the pre-exposure position Pf, in the part with a surface potential having the same polarity as the charge polarity of the photosensitive drum 1 and an absolute value higher than the potential Vl of the bright part, the surface potential of the photosensitive drum 1 can be discharged. However, in the part with an absolute value lower than the potential Vl of the bright part or in the part with a polarity-reversed surface potential, the surface potential of the photosensitive drum 1 cannot be homogenized.

[0059] Figure 3 Part (d) shows the surface potential of the photosensitive drum 1 after passing through the charging position Pb (after passing through the charging position Pb and before reaching the transfer position Pe). Even when the surface of the photosensitive drum passes through the charging position Pb in the state shown in Figure 3 part (c), as shown in Figure 3 part (d), in some cases, the surface potential in the part where the absolute value of the transfer memory surface potential is lower than the absolute value of the potential Vl of the bright part and the surface potential in the part where the transfer memory surface potential is polarity-reversed cannot fully return to the potential Vd of the dark part.

[0060] In a portion where the surface potential cannot sufficiently return to the dark portion potential Vd, an adequate potential difference cannot be ensured between the surface potential of the photosensitive drum 1 and the developing voltage in the non-image portion of the image forming region for the subsequent recording material P. As a result, in some cases, such a potential difference may appear as an image defect, such as a black dot image or a fogged image. Further, in a portion where the surface potential cannot sufficiently return to the dark portion potential Vd, the potential difference between the surface potential of the photosensitive drum 1 and the developing voltage becomes large in the image portion of the image forming region for the subsequent recording material P, such that such a potential difference may appear as an image defect, such as an increase in density of a halftone image or the like. Therefore, in the image forming apparatus 101 of the conventional example 1, it is difficult to equalize the transfer memory in the portion having a surface potential with an absolute value lower than the bright portion potential Vl and in the portion having a surface potential with a reversed polarity during the sheet interval, and thus it is difficult to suppress image defects caused by transfer memory.

[0061] Here, by increasing the amount of light of the pre-exposure device 30 as compared with the amount of light in the conventional example 1, the surface potential of the photosensitive drum 1 can be reduced to 0V. However, even in this case, it is difficult to restore the surface potential with the reversed polarity to the dark portion potential Vd after passing through the charging position Pb. Further, when the amount of light of the pre-exposure device 30 is increased, the potential difference between the charging roller 3 and the photosensitive drum 1 becomes large, such that the discharge of the charging roller 3 becomes more active. When the discharge of the charging roller 3 becomes more active, damage to the photosensitive drum 1 is promoted. Therefore, this is not preferable. That is, even when the amount of light of the pre-exposure device 30 is increased, it is difficult to suppress image defects caused by transfer memory, and damage to the photosensitive drum 1 may be promoted.

[0062] (3) Method for measuring the cell diameter of the surface of the transfer roller

[0063] As described above, the non-uniformity of the surface potential of the photosensitive drum 1 caused by transfer memory depends on the cell diameter of the surface of the transfer roller 6. Figure 4 Parts (a) and (b) are schematic views for explaining a method for measuring the cell diameter of the elastic foam layer (foam member, foam material).

[0064] To measure the cell diameter, the surface of the transfer roller 6 is observed by using a digital microscope (for example, "VHX-1000" manufactured by KEYENCE Corporation) and a lens for the digital microscope (for example, "VH-Z100R" manufactured by KEYENCE Corporation). Figure 4 Part (a) schematically shows an image obtained by observing the surface of the transfer roller 6 when the magnification of the lens for the digital microscope is 100 times. As Figure 4As shown in part (a), multiple cells are observed in the image obtained from the digital microscope. Incidentally, in this embodiment, the diameters of 30 cells starting from the largest cell among all the cells in the image are measured, and the average value of these diameters is determined as the cell diameter of the surface of the transfer roller 6. Further, at this time, the viewable angle (x × y) of the digital microscope is 4 mm × 3 mm (the range with a length of 4 mm and a width of 3 mm on the surface of the transfer roller 6). Here, the shape of the cells constituting the elastic layer of the transfer roller 6 is not limited to a shape close to a perfect circle. For example, as Figure 4 shown in part (b), in some cases, the shape is distorted. In this case, the diameter of the perfect circle having the same area as the cell with an irregular shape is determined as the outer diameter of the cell.

[0065] (4) Suppression effect on image defects caused by transfer memory

[0066] Next, the suppression effect on image defects caused by transfer memory will be described.

[0067] An image is continuously formed on the recording material P by the image forming apparatus 100 of this embodiment, and it is checked whether image defects caused by transfer memory occur. A first charging voltage of -500 V is applied to the charging brush 2. The image pattern for evaluating whether image defects caused by transfer memory occur is a halftone image (an image with a toner application amount of 50% when the toner application amount of a solid image is set to 100%). Further, it is evaluated whether there is a change in density in the halftone image on the recording material P after the sheet interval with respect to the halftone image on the recording material P before the sheet interval. When a change in density of the halftone image is visually observed, the image pattern is determined to have image defects caused by transfer memory ("×"). Further, when no change in density of the halftone image is visually observed, the image pattern is determined not to have image defects caused by transfer memory ("○"). Further, for comparison with this embodiment, similar experiments were conducted on the image forming apparatus 101 of the above-described 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 below.

[0068] As shown in Table 1, in the image forming apparatus 100 of this embodiment in which the charging brush 2 is provided, even when the absolute value of the transfer voltage increases, image defects due to transfer memory do not occur. In this embodiment, the appropriate set value of the transfer voltage is +3000V (+3kV). Further, in this embodiment, at transfer voltages less than +3000V, sufficient transfer from the photosensitive drum 1 onto the recording material P is not performed, resulting in inappropriate transfer in some cases. Further, in this embodiment, even when a transfer voltage of +3000V is applied during the sheet interval, image defects due to transfer memory do not occur. For this reason, it is not necessary to change the set value of the transfer voltage between during sheet passage and the sheet interval. That is, the image forming apparatus 100 of this embodiment achieves a faster processing speed and a shorter sheet interval while suppressing the occurrence of image defects due to transfer memory.

[0069] Further, as shown in Table 1, in the image forming apparatus 101 of Conventional Example 1 in which the pre-exposure device 30 is provided, image defects due to transfer memory occurred at transfer voltages of +2000V (+2kV) or higher. When the surface potential of the photosensitive drum 1 was measured at a position corresponding to the sheet interval after passing through the transfer position Pe at a transfer voltage of +2000V, the surface potential was -80V, the absolute value of which was lower than the bright portion potential Vl (-100V). That is, this indicates that due to the mechanism Figure 3 described, image defects due to transfer memory occurred in the image forming apparatus 101 of Conventional Example 1.

[0070] Further, as shown in Table 1, in the image forming apparatus of Comparative Example 1, image defects due to transfer memory occurred at transfer voltages of not less than +1500V (+1.5kV) (which is lower than the transfer voltage (+2000V) in the case of Conventional Example 1). This is because there is no potential equalization effect of the pre-exposure device 30 in the image forming apparatus of Comparative Example 1.

[0071] From the above results, it can be understood that the image forming apparatus 100 of this embodiment is more advantageous than the image forming apparatuses of Conventional Example 1 and Comparative Example 1 in suppressing the occurrence of image defects due to transfer memory.

[0072] Table 1

[0073]

[0074]

[0075] Next, the mechanism for suppressing image defects due to transfer memory in the image forming apparatus 100 of this embodiment will be described.

[0076] Figure 5 Parts (a) to (d) are schematic views for explaining the progress of the surface potential of the photosensitive drum 1 at positions corresponding to the sheet intervals when continuously forming an image on the recording material P by the image forming apparatus 100 of the present embodiment.

[0077] Figure 5 Part (a) shows the surface potential of the photosensitive drum 1 before passing through the transfer position Pe, and similar to the case of Figure 3 part (a), the exposure device 4 does not perform exposure at positions corresponding to the sheet intervals, so the dark part potential Vd is maintained.

[0078] Figure 5 Part (b) shows the surface potential of the photosensitive drum 1 after passing through the transfer position Pe, and similar to the case of Figure 3 part (b), due to transfer memory, a region having a surface potential with an absolute value lower than the bright part potential Vl or a surface potential with a polarity inversion is generated on the photosensitive drum 1.

[0079] Figure 5 Part (c) shows the surface potential of the photosensitive drum 1 after passing through the first charging position Pa (after passing through the first charging position Pa and before reaching the second charging position Pb). At the first charging position Pa, charges are injected from each brush fiber of the charging brush 2 onto the surface of the photosensitive drum 1 by injection charging. In injection charging, within the range where no discharge occurs, when there is a large potential difference between the charging brush 2 and the photosensitive drum 1, a large amount of charge is injected from the charging brush 2 onto the surface of the photosensitive drum 1. Considering this principle for a single bristle (yarn) of the brush fiber of the charging brush 2 and a minute region of the surface of the photosensitive drum 1 contacted by the single bristle. That is, the potential of each brush fiber of the charging brush 2 is the same, but the potential fluctuates due to transfer memory between minute regions of the surface of the photosensitive drum 1 contacted by each brush fiber. Further, in minute regions of the photosensitive drum 1 having a surface potential with an absolute value lower than the bright part potential Vl due to transfer memory or in minute regions of the photosensitive drum 1 having a surface potential with a polarity inversion, a large amount of charge flows from the charging brush 2 into the photosensitive drum 1. As a result, as Figure 5 shown in part (c), in regions having a surface potential with an absolute value lower than the bright part potential Vl or a surface potential with a polarity inversion, a large amount of charge flows from the charging brush 2 into the photosensitive drum 1, and the potential in this region preferentially rises, so that the surface potential of the photosensitive drum 1 is homogenized.

[0080] Figure 5Part (d) shows the surface potential of the photosensitive drum 1 after passing through the second charging position Pb (after passing through the second charging position Pb and before reaching the transfer position Pe). When the non-uniformity degree of the surface potential of the photosensitive drum 1 is reduced to Figure 5 the state shown in part (c), and further the surface potential of the photosensitive drum 1 can be restored to Figure 5 the state shown in part (c), as Figure 5 shown in part (d), by passing through the second charging position Pb, a substantially uniform dark part potential Vd can be formed on the photosensitive drum 1.

[0081] Therefore, according to this embodiment, the degree of transfer memory that would cause a surface potential with an absolute value lower than the bright part potential Vl or a surface potential with a polarity inversion is reduced, so that the occurrence of image defects caused by transfer memory can be suppressed.

[0082] (5) First charging ratio (brush charging ratio)

[0083] In order to obtain a sufficient suppression effect on image defects caused by transfer memory, preferably, the ratio of the potential amount changed at the first charging position Pa to the potential amount changed at the first charging position Pa and the second charging position Pb is greater than or equal to a predetermined value. The ratio of the potential amount changed at the first charging position Pa to the potential amount changed at the first charging position Pa and the second charging position Pb is referred to as the "first charging ratio R1". At this time, the first charging ratio R1 can be expressed by the following formula (1).

[0084] R1 [%] = {(V1 - Vt) / (Vd - Vt)} × 100 (1)

[0085] Here, V1 is the surface potential of the photosensitive drum 1 after passing through the first charging position Pa and before reaching the second charging position Pb. Vt is the surface potential of the photosensitive drum 1 after passing through the transfer position pe and before reaching the first charging position Pa. Vd is the dark part potential (the surface potential of the photosensitive drum 1 formed at the second charging position Pb).

[0086] For example, in the image forming apparatus 100 of this embodiment, when the transfer voltage is +3000V, the average value of the measurement results of each potential at the position corresponding to the sheet interval is V1 = -70V, Vt = +40V, and Vd = -500V. In this case, by substituting the average value of each potential into the above formula (1), the first charging ratio R1 can be calculated to be approximately equal to 20% (R1 ≈ 20%). Incidentally, the average value of the measurement results of each potential is represented by the average value of the measurement results of a sufficient number of points (for example, 10 points to 30 points) in the moving direction of the surface of the photosensitive drum 1.

[0087] Examine the relationship between the first charging ratio R1 and the suppression effect on image defects caused by transfer memory. By using the image forming apparatus 100 of the present embodiment, with the first charging voltage applied to the charging brush 2 being changed, evaluate whether image defects caused by transfer memory occur. Fix the transfer voltage at +3000V. Other conditions are the same as those in the experiment for obtaining the results in Table 1. Further, in the structure of the present embodiment, when the first charging voltage exceeds -500V (when the absolute value of the negative-polarity first charging voltage exceeds 500V), discharge occurs between the charging brush 2 and the photosensitive drum 1, and thus, in some cases, another defect such as uneven charging occurs. For this reason, vary the first charging voltage between 0V and -500V so that 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 below.

[0088] It can be understood from Table 2 that, in order to suppress the occurrence of image defects caused by transfer memory, the first charging ratio R1 can preferably be greater than or equal to a predetermined value, and in the structure of the present embodiment, preferably, R1 is greater than or equal to 10%.

[0089] Incidentally, in a structure in which injection charging is performed by applying a first charging voltage less than the discharge start voltage to the first charging member, the amount of potential of the photosensitive drum 1 that can be changed by the first charging member has a certain limit, and generally, the first charging ratio R1 is 50% or less. Further, a second charging voltage that causes the potential difference between the charging roller 3 and the photosensitive drum 1 at the second charging position Pb to be greater than or equal to the discharge start voltage is applied to the charging roller 3, so that the surface of the photosensitive drum 1 is charged to a predetermined dark portion potential Vd.

[0090] Table 2

[0091]

[0092] Figure 6 The curve diagram shows an example of the measurement result of the progress of the surface potential Vt of the photosensitive drum 1 after passing through the transfer position Pe and before reaching the first charging position Pa under the same conditions as in the experiment for obtaining the results in Table 2. In the case of obtaining the first charging ratio R1 by the above formula (1), Vt obtained by calculating the average value of the waveform of the surface potential of the photosensitive drum 1 at the position corresponding to the sheet interval ("paper interval") can be used as Vt at the position corresponding to the sheet interval. On the other hand, as Figure 6 shown, in the case where transfer memory occurs, the value of Vt at the position corresponding to the sheet interval fluctuates. This situation is using Figure 5the case described in part (b), and in order to suppress image defects caused by transfer memory, it is necessary to make the difference between the maximum value and the minimum value of Vt at the position corresponding to the sheet interval small at the first charging position Pa. The index R1' of the first charging ratio necessary to suppress image defects caused by transfer memory can be obtained from the following formula (2).

[0093] R1' [%] = {(Vtmax - Vtmin) / (Vd - Vtave)} × 100 (2)

[0094] Here, Vtmax is the maximum value of Vt at the position corresponding to the sheet interval, Vtmin is the minimum value of Vt at the position corresponding to the sheet interval, and Vtave is the average value of Vt at the position corresponding to the sheet interval.

[0095] In Figure 6 the case shown, Vtmax = +71V, Vtmin = +18V, Vd = -500V, Vtave = +40V, so that 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 caused by transfer memory obtained in the experiment for obtaining the results in Table 2. Therefore, the first charging ratio can be set to be not less than R1' calculated by the above formula (2). Incidentally, Vtmax and Vtmin in the above formula (2) are not limited to the maximum value and the minimum value. For example, when the difference between the maximum value and the value 1 less than the maximum value and the difference between the minimum value and the value 1 greater than the minimum value are small enough with respect to the fluctuation range of the Vt value at the position corresponding to the sheet interval, R1' can be calculated by using the value 1 less (not limited to 1) than the maximum value and the value 1 greater (not limited to 1) than the minimum value.

[0096] In addition, in order to obtain a better transfer memory reduction effect, it is desirable that the average inter-yarn distance of the charging brush 2 is smaller than the average cell diameter of the surface of the elastic foam layer of the transfer roller 6. As described above, the surface potential non-uniformity of the photosensitive drum 1 caused by transfer memory depends on the cell diameter of the surface of the transfer roller 6. When the average inter-yarn distance of the charging brush 2 is larger than such surface potential non-uniformity depending on the cell diameter, there are minute areas on the photosensitive drum 1 where the brush fibers of the charging brush 2 do not contact the photosensitive drum surface, and thus it is difficult to sufficiently reduce transfer memory in some cases. From such a viewpoint, the average inter-yarn distance of the charging brush 2 can preferably be 70% or less of the average cell diameter of the surface of the elastic foam layer of the transfer roller 6, and more preferably 50% or less. In this embodiment, when the cell diameter is measured by the above measurement method, the cell diameter of the surface of the transfer roller 6 is 300 μm. In addition, in this embodiment, the planting density of the charging brush 2 is 200 kF / inch2 , the average inter-yarn distance of the charging brush 2 can thus be obtained as 57 μm through the following calculation. Incidentally, the average inter-yarn distance can also be obtained by observations similar to those in the above case of the cell diameter of the foam member.

[0097] 200 kF / inch 2 = 200,000 / 645.16 ≈ 310 F / mm 2

[0098] √(310 F / mm 2 ) ≈ 17.6 F / mm

[0099] 1 / (17.6 F / mm) ≈ 57 μm / F

[0100] Therefore, in this embodiment, the average inter-yarn distance of the charging brush 2 is set to a value less than the cell diameter of the surface of the elastic foam layer of the transfer roller 6.

[0101] Incidentally, regarding the rotation axis direction of the photosensitive drum 1, it is desirable that the relationship between the average inter-yarn distance of the charging brush 2 and the average cell diameter of the surface of the elastic foam layer of the transfer roller 6 becomes the above relationship. In this embodiment, regarding the rotation axis direction of the photosensitive drum 1 and the surface movement direction of the photosensitive drum 1, the average inter-yarn distance of the charging brush 2 is set to be sufficiently less than the average cell diameter of the surface of the elastic foam layer of the transfer roller 6.

[0102] (6) Modification Example

[0103] A modification example of this embodiment will be described. Figure 7 is a schematic cross-sectional view of an image forming apparatus 102 which is a modification example of this embodiment. In Figure 7 the shown image forming apparatus 102, as the first charging member, instead of Figure 1 the charging brush 2 of the image forming apparatus 100, an injection roller 25 is provided to contact the photosensitive drum 1 at the first charging position Pg. The injection roller 25 rotates as the photosensitive drum 1 rotates. The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential (dark part potential, charge potential) through the injection roller 25 and the charging roller 3. During charging, a first charging voltage (first charging bias) which is a negative-polarity DC voltage is applied from a first charging power source (high voltage power source) 26 to the injection roller 25. Except for the above points, Figure 7 the structure and operation of the shown image forming apparatus 102 are basically the same as Figure 1 the structure and operation of the shown image forming apparatus 100. In Figure 7 the shown image forming apparatus 102, elements having the same or corresponding functions or structures as the elements of Figure 1 the shown image forming apparatus 100 are added with Figure 1The same reference numerals or symbols as those in the image forming apparatus 100 shown.

[0104] Similar to the case of the transfer roller 6, the injection roller 25 is a roller prepared by forming a foamed elastic layer on a core metal. That is, the injection roller 25 is composed of a conductive base shaft (core metal, core portion) that also serves as an energy supply electrode and an elastic foam layer that cylindrically surrounds the outer peripheral surface of the conductive base shaft. A first charging voltage of -500 V is applied to the injection roller 25 so that the injection roller 25 mainly charges the photosensitive drum 1 by injection charging. Similar to the case of the charging brush 2, in order to obtain a good transfer memory reduction effect, it is desirable that the average cell diameter of the surface of the elastic foam layer of the injection roller 25 be smaller than the average cell diameter of the surface of the elastic foam layer of the transfer roller 6. Further, the average cell diameter of the surface of the elastic foam layer of the injection roller 25 may preferably be 70% or less, more preferably 50% or less, of the average cell diameter of the surface of the elastic foam layer of the transfer roller 6. In this modification, when the average cell diameter is measured by the above method, the cell diameter of the surface of the injection roller 25 is 150 μm, while the cell diameter of the surface of the transfer roller 6 is 300 μm. Therefore, in this modification, the average cell diameter of the surface of the elastic foam layer of the injection roller 25 is set to be smaller than the average cell diameter of the surface of the elastic foam layer of the transfer roller 6.

[0105] By using the image forming apparatus 102 of this modification, an experiment for evaluating whether image defects due to transfer memory occur was conducted, and this experiment was the same as the experiment that obtained the results in Table 1. As a result, similar to the image forming apparatus 100 of the present embodiment, in the image forming apparatus 102 of this modification, an inhibitory effect on the occurrence of image defects due to transfer memory was also obtained.

[0106] Therefore, also in a structure in which injection charging is performed by using a charging member having a foam structure, such as the injection roller 25 as the first charging member, the occurrence of image defects due to transfer memory can be suppressed.

[0107] (7) Effects

[0108] Therefore, in this embodiment, the image forming apparatus 100 includes: a rotatable photosensitive member (photosensitive drum) 1; a first charging member (charging brush) 2 that contacts the surface of the photosensitive member 1 at a first charging position Pa in the rotational direction of the photosensitive member 1 to charge the surface of the photosensitive member 1; a second charging member (charging roller) 3 that charges the surface of the photosensitive member 1 at a second charging position Pb in the rotational direction of the photosensitive member 1; a developing member (developing roller) 5a that forms a toner image on the surface of the photosensitive member 1 by supplying toner charged to a predetermined polarity to the surface of the photosensitive member at a developing position Pd in the rotational direction of the photosensitive member; a transfer member (transfer roller) 6 that contacts the surface of the photosensitive member 1 at a transfer position Pe in the rotational direction of the photosensitive member 1 and transfers the toner image from the photosensitive member 1 to a recording material passing between the photosensitive member 1 and the transfer member 6; a first charging voltage applying section (first charging power supply) 20 that applies a first charging voltage of a predetermined polarity to the first charging member 1; a second charging voltage applying section (second charging power supply) 21 that applies a second charging voltage of a predetermined polarity to the second charging member 3; a transfer voltage applying section (transfer power supply) 18 that applies a transfer voltage of a polarity opposite to the predetermined polarity to the transfer member 6; and a controller 40 that controls the first charging voltage applying section 20, the second charging voltage applying section 21, and the transfer voltage applying section 18.

[0109] In the rotational direction of the photosensitive member 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 developing position Pd, the developing 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 developing position Pd and upstream of the first charging position Pa. The controller 40 controls the transfer voltage applying section 18 such that at least a partial region of the surface of the photosensitive member 1 in direct contact with the transfer member 6 at the transfer position Pe has a potential of a polarity opposite to the predetermined polarity; the controller 40 controls the first charging voltage applying section 20 such that the surface of the photosensitive member 1 is charged at the first charging position Pa by applying a first charging voltage less than the discharge start voltage to the first charging member 2 at the first charging position Pa; and the controller 40 controls the second charging voltage applying section 21 such that the surface of the photosensitive member 1 is charged at the second charging position Pb by applying a second charging voltage not less than the discharge start voltage to the second charging member 3 at the second charging position Pb.

[0110] Here, regarding at least a part of the above regions, when the ratio of the surface potential of the photosensitive member 1 changed at the first charging position Pa to the surface potential of the photosensitive drum 1 changed at the first charging position Pa and the second charging position Pb is the first charging ratio, the controller 40 can preferably control the first charging voltage application section 20 so that the first charging ratio becomes 10% or more. In other words, regarding at least a part of the above regions, when the ratio of the surface potential of the photosensitive member 1 changed at the first charging position Pa to the surface potential of the photosensitive member 1 changed at the first charging position Pa and the second charging position Pb is the first charging ratio, the surface potential of the photosensitive member 1 after passing through the transfer position Pe and before reaching the first charging position Pa is Vt, the surface potential of the photosensitive member 1 formed at the second charging position Pb is Vd, the maximum value of Vt is Vtmax, the minimum value of Vt is Vtmin, and the average value of Vt is Vtave, the controller 40 can preferably control the first charging voltage application section 20 so that the first charging ratio is not less than R1' represented by the following formula: R1' [%] = {(Vtmax - Vtmin) / (Vd - Vtave)} × 100.

[0111] Specifically, in this embodiment, at least a part of the above regions is a region corresponding to between the transfer position Pe and the recording material P and the subsequent recording material P, and the subsequent recording material P is the recording material P after the recording material P.

[0112] In addition, in this embodiment, the controller 40 controls the transfer voltage application section 18 so that when the region of the surface of the photosensitive member 1 corresponding to between the recording material P and the subsequent recording material P is at the transfer position Pe, the transfer voltage applied to the transfer member 6 is the same as when the region of the surface of the photosensitive member 1 corresponding to the recording material is at the transfer position Pe. In addition, in this embodiment, the transfer member 6 is configured to include a foam member that can contact the surface of the photosensitive member 1. In addition, in this embodiment, the first charging member 2 is configured to include a brush that can contact the surface of the photosensitive member 1. In this case, the average distance between yarns of the brush of the first charging member 2 can preferably be less than the average cell diameter of the foam member of the transfer member 6.

[0113] In addition, the first charging member 2 can be configured to include a foam member that can contact the surface of the photosensitive member 1. In this case, the average cell diameter of the foam member of the first charging member 2 can preferably be less than the average cell diameter of the foam member of the transfer member 6. In addition, in this embodiment, the toner remaining on the surface of the photosensitive member 1 after the toner image is transferred from the photosensitive member 1 to the recording material P is collected by the developing member 5a.

[0114] In addition, according to this embodiment, in a structure in which transfer memory occurs such that the surface of the photosensitive member that directly contacts the transfer member in the transfer portion has a polarity opposite to the charge polarity of the photosensitive member, the occurrence of image defects caused by transfer memory can be suppressed.

[0115] Next, another embodiment (Embodiment 2) of the present invention will be described. The basic structure and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus in Embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or structures as the elements in the image forming apparatus in Embodiment 1 are given the same reference numerals or symbols as those in Embodiment 1, and thus detailed descriptions of these elements are omitted.

[0116] When a durability test for further extending the life is performed on the image forming apparatus 100 of Embodiment 1, the charging brush 2 is contaminated, and thus, in some cases, the first charging ratio R1 represented by the above formula (1) decreases. On the other hand, depending on the cumulative usage amount of the charging brush 2, it is considered to increase the absolute value of the first charging voltage applied to the charging brush 2. However, when a potential difference equal to or greater than a predetermined value is generated between the charging brush 2 and the photosensitive drum 1, discharge occurs. Therefore, there is a limit to increasing the absolute value of the first charging voltage. The object of this embodiment is to more satisfactorily maintain the suppression effect on image defects caused by transfer memory by suppressing the decrease in the first charging ratio R1 as the cumulative usage amount of the charging brush 2 increases.

[0117] Figure 8FIG. 0 is a schematic cross-sectional view of the image forming apparatus 103 of the present embodiment. In this embodiment, the image forming apparatus 103 includes a cleaning device 31 as a cleaning unit for removing deposited substances (such as transfer residual toner or paper powder) remaining on the surface of the photosensitive drum 1 after passing through the transfer position Pe from the surface of the photosensitive drum 1. The cleaning device 31 includes a cleaning blade 31a as a cleaning member, which is disposed to contact the surface of the photosensitive drum 1. With respect to the rotation direction of the photosensitive drum 1, the position where the deposited substances on the photosensitive drum 1 are removed by the cleaning blade 31a (in this embodiment, the position where the surface of the photosensitive drum 1 contacts the cleaning blade 31a) is the cleaning position Ph. A charging brush 2 similar to the charging brush 2 in Embodiment 1 is disposed to contact the photosensitive drum 1 at the first charging position Pi on the photosensitive drum 1 after the photosensitive drum 1 passes through the cleaning position Ph (and before reaching the second charging position Pb). Except for the above points, the structure and operation of the image forming apparatus 103 of the present embodiment are substantially the same as those of the image forming apparatus 100 of Embodiment 1. Therefore, in this embodiment, the image forming apparatus 103 includes a cleaning member 31a for removing toner from the surface of the photosensitive drum 1 on the downstream side of the transfer position Pe and the upstream side of the first charging position Pi with respect to the rotation direction of the photosensitive member 1.

[0118] The image forming apparatus 100 of Embodiment 1 has a non-cleaner structure in which transfer residual toner is collected and reused by the developing device 5. On the other hand, in the image forming apparatus 103 of the present embodiment, a cleaning device 31 having a blade collection structure is provided so that the surface of the photosensitive drum 1 is cleaned before reaching the charging brush 2. For this reason, in the image forming apparatus 103 of the present embodiment, it is possible to suppress the charging brush 2 from being contaminated by deposited substances (such as transfer residual toner or paper powder) as the cumulative usage amount of the charging brush 2 increases. As a result, in the image forming apparatus 103 of the present embodiment, a decrease in the first charging ratio R as the cumulative usage amount of the charging brush 2 increases is suppressed, and thus the suppression effect on image defects caused by transfer memory can be maintained more satisfactorily.

[0119] Next, another embodiment (Embodiment 3) of the present invention will be described. The basic structure and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus in Embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or structures as the elements in the image forming apparatus in Embodiment 1 are added the same reference numerals or symbols as those in Embodiment 1, and thus detailed descriptions of these elements are omitted.

[0120] When evaluating the image forming apparatus 100 of Example 1 in a low temperature / low humidity environment (e.g., 15°C / 10% RH (relative humidity)), the first charging ratio R1 represented by the above formula (1) may decrease in some cases due to an increase in the resistance of the charging brush 2 and the photosensitive drum 1 itself and an increase in the contact resistance between the charging brush 2 and the photosensitive drum 1. As described in Example 2, when a potential difference equal to or greater than a predetermined value is generated between the charging brush 2 and the photosensitive drum 1, discharge occurs, and thus there is a limit to increasing the absolute value of the first charging voltage. The object of this embodiment is to obtain an inhibitory effect on image defects caused by transfer memory regardless of the environment by suppressing a decrease in the first charging ratio R1 in a low temperature / low humidity environment.

[0121] In this embodiment, the photosensitive drum 61 shown in parts (a) and (b) of Figure 9 is used instead of the photosensitive drum 1 in the image forming apparatus 100 of Example 1. Except for this point, the structure and operation of the image forming apparatus of this embodiment are basically the same as those of the image forming apparatus 100 of Example 1. Figure 9 Part (a) of Figure 9 is a schematic cross-sectional view of the photosensitive drum 61 in this embodiment, and

[0122] The photosensitive drum 61 in this embodiment will be described. The photosensitive drum 61 in this embodiment has a charge injection function on its outermost surface. As Figure 9 shown in part (b) of 9 , the photosensitive drum 61 includes a conductive support member 61a, a conductive layer 61b, an undercoat layer 61c, a photosensitive layer composed of two layers of 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 61 contains conductive particles 61g. The charge injection layer 61f is formed by dispersing the conductive particles 61g in an adhesive resin. The content of the conductive particles 61g is 5.0 vol% to 70.0 vol% of the total volume of the charge injection layer 61f. In addition, the volume resistivity of the charge injection layer 61f is 1.0×10 9 Ω·cm to 1.0×10 14 Ω·cm.

[0123] When the volume resistivity of the charge injection layer 61f is less than 1.0×10 9 Ω·cm, the resistance of the charge injection layer 61f is too low, and an electrostatic latent image cannot be properly formed, making it difficult to develop the electrostatic latent image into a predetermined image. On the other hand, when the volume resistivity of the charge injection layer 61f exceeds 1.0×10 14When the volume resistivity is Ω·cm, the resistance of the charge injection layer 61f is too high, and the charge injection characteristics from the charging brush 2 into the charge injection layer 61f deteriorate. Therefore, it is difficult to obtain a first charging ratio R1 not less than a predetermined value in a low-temperature / low-humidity environment as an object of this embodiment. In order to satisfy the above range of the volume resistivity, it is desirable that the content of the conductive particles 61g can be 5.0 vol% to 70.0 vol% of the total volume of the charge injection layer 61f. When the content of the conductive particles 61g exceeds 70.0 vol%, the charge injection layer 61f itself becomes brittle. Therefore, the surface of the photosensitive drum 61 is liable to be worn after long-term use. As a result, the charging uniformity of the photosensitive drum 61 deteriorates, and inappropriate charging is likely to occur when the image forming apparatus 100 is accelerated, and thus image defects (image failures) are likely to occur.

[0124] In addition to the content of the conductive particles 61g, the volume resistivity of the charge injection layer 61f can be controlled by, for example, the particle diameter of the conductive particles 61g. The particle diameter of the conductive particles 61g can preferably be 5 nm to 300 nm, more preferably 40 nm to 250 nm in terms of the number average particle diameter. When the number average particle diameter of the conductive particles 61g is less than 5 nm, the specific surface area of the conductive particles 61g becomes large, and the degree of water absorption near the conductive particles 61g on the surface of the charge injection layer 61f becomes large, so that the volume resistivity of the charge injection layer 61f is liable to decrease. When the number average particle diameter of the conductive particles 61g exceeds 300 nm, not only the dispersion degree of the particles in the charge injection layer 61f deteriorates, but also the area of the interface with the binder resin decreases, so that the resistance in this interface increases, and thus the charge injection characteristics are liable to deteriorate.

[0125] As the conductive particles 61g contained in the charge injection layer 61f, particles of metal oxides (such as titanium oxide, zinc oxide, tin oxide, indium oxide, etc.) can be cited. In the case where a metal oxide is used as the conductive particles 61g, the metal oxide can be doped with an element such as niobium, phosphorus or aluminum or an oxide thereof. In addition, the conductive particles 61g can have a laminated structure including core material particles and a coating covering the core material particles. As the core material particles, particles of titanium oxide, barium sulfate, zinc oxide, etc. can be cited. As the coating, layers of titanium oxide, tin oxide, etc. can be cited, and in this embodiment, a titanium oxide layer is preferred in view of the charge injection characteristics from the charging brush 2.

[0126] In addition, when titanium oxide contains niobium, the charge injection characteristics become better, and thus the charge injection characteristics can be improved in a small amount. The content of niobium can preferably be 0.5 wt% to 15.0 wt% of the total weight of the niobium-containing titanium oxide particles, more preferably 2.6 wt% to 10.0 wt%.

[0127] The niobium-containing titanium oxide particles may preferably be anatase-type or rutile-type titanium oxide particles, and may more preferably be anatase-type titanium oxide particles. By using anatase-type titanium oxide, charge transfer in the charge injection layer 61f becomes smooth, and thus, charge injection becomes better. More preferably, the titanium oxide particles are particles including anatase-type titanium oxide particles as the core material and a coating of niobium-containing titanium oxide on the surface of the core material. The anatase-type titanium oxide particles are used as the core material and coated with niobium-containing titanium oxide on their surfaces, so that charges can easily move in the charge injection layer 61f. In addition, the charge injection characteristics from the charging brush 2 to the charge injection layer 61f can be enhanced. In addition, a decrease in the volume resistivity of the charge injection layer 61f can be suppressed.

[0128] Therefore, in the image forming apparatus 100 of the present embodiment, by using the photosensitive drum 61 including the charge injection layer 61f as the surface layer, good charge injection can be performed between the charging brush 2 and the photosensitive drum 1 even in a low temperature / low humidity environment. As a result, in the image forming apparatus 100 of the present embodiment, a decrease in the first charging ratio R1 in a low temperature / low humidity environment can be suppressed, and thus, an inhibitory effect on the occurrence of image defects due to transfer memory can be obtained regardless of the environment.

[0129] In the above, the present invention has been described based on specific embodiments, but the present invention is not limited to the above embodiments.

[0130] In the above embodiment, the case where the transfer voltage is the same during the passage of the sheet and in the sheet interval has been described, but the present invention is not limited to such a structure. For example, even when the absolute value of the transfer voltage in the sheet interval is smaller than the absolute value of the transfer voltage during the passage of the sheet, when the surface potential of the photosensitive member is reversed in polarity to be opposite to the normal charge polarity at a position corresponding to the sheet interval, the present invention can also suppress the occurrence of image defects due to transfer memory.

[0131] In addition, for example, in at least a part of the pre-rotation step and the post-rotation step, when the surface potential of the photosensitive element is reversed in polarity to be opposite to the normal charge polarity, the occurrence of image defects due to transfer memory can be suppressed.

[0132] In addition, when an image is formed on a recording material whose width in the rotational axis direction of the photosensitive member (the width direction substantially perpendicular to the recording material conveyance direction) is smaller than the maximum width on which an image can be formed in the image forming apparatus, a region of the photosensitive member that directly contacts the transfer member at the transfer position may be generated. In this region, the surface potential of the photosensitive member is inverted in polarity to be opposite to the normal charge polarity, and in some cases, it is similar to the situation in the region corresponding to the sheet interval described in the above embodiments. In addition, when an image is continuously formed on a large-size paper sheet after a small-size paper sheet or in a similar situation, there is a possibility of image defects due to transfer memory in this region. According to the present invention, the transfer memory in this region can also be alleviated similar to the situation in the region corresponding to the sheet interval described in the above embodiments, so that the occurrence of image defects caused by the transfer memory in this region can be suppressed.

[0133] In addition, in a region of the photosensitive member where the recording material is inserted between the photosensitive member and the transfer member during sheet passage, although the surface potential of the photosensitive member is not inverted in polarity to be opposite to the normal charge polarity, the surface potential of the photosensitive member can also become a surface potential whose absolute value is smaller than the potential of the bright portion (see Figure 6 ). According to the present invention, the surface potential in this region can also be equalized at the first charging position. Therefore, it is advantageous that the dark portion potential is formed more uniformly at the second charging position.

[0134] In addition, the photosensitive member is not limited to a drum-shaped member, and may also be a belt-shaped member or a similar member.

[0135] In addition, the transfer member is not limited to a roller-shaped member, and may also be a brush-shaped member, a sheet-shaped member, or a similar member. When the transfer member is a brush-shaped member, it is preferable that the average distance between yarns of the charging brush is smaller than the average distance between yarns of the transfer brush (more preferably, it is not greater than 70% of the average distance between yarns of the transfer brush, and further preferably, it is not greater than 50% of the average distance between yarns of the transfer brush).

[0136] In addition, in the above embodiments, the charging brush as the first charging member is a fixed brush member. However, for example, the charging brush may also be a brush roller configured to include a core portion and a brush portion provided around the core portion.

[0137] In addition, the image forming apparatus is not limited to a monochromatic image forming apparatus. For example, the image forming apparatus may also be a color image forming apparatus including a plurality of image forming portions, each image forming portion including a photosensitive member and a processing unit that can act on the photosensitive member. In this case, the present invention can be applied to at least one of the plurality of photosensitive members.

[0138] In addition, in the above-described embodiments, a case where the image forming apparatus does not include a pre-exposure device has been described. According to the present invention, even when the pre-exposure device is not provided, transfer memory is reduced, and thus occurrence of image defects due to transfer memory can be suppressed. However, the present invention is not limited thereto, and even when a pre-exposure device is provided in the image forming apparatus, transfer memory can be effectively reduced by equalizing the surface potential of the photosensitive member whose polarity is inverted to be opposite to the normal charge polarity.

[0139] In addition, in the above-described embodiments, the normal charge polarity of the photosensitive member and the normal charge polarity of the toner are both negative polarities, but they may also be positive polarities. In this case, those skilled in the art can appropriately change the polarities in such a manner that the polarity of each of the various applied voltages is changed to be opposite to the relevant polarity in the above-described embodiments or in a similar manner.

[0140] In addition, the dimensions, materials, shapes, relative arrangements, etc. of the constituent components described in the above-described embodiments should be appropriately changed according to the structure and various conditions of the apparatus or device to which the present invention is applied.

[0141] That is, it is not intended to limit the scope of the present invention to the above-described embodiments.

[0142] According to the present invention, occurrence of image defects due to transfer memory can be suppressed.

[0143] Although the present invention has been described with reference to exemplary embodiments, it should be understood that the present invention is not limited to the disclosed exemplary embodiments. The scope of the appended claims should be given the broadest interpretation so as to include all such modifications as well as equivalent structures and functions.

Claims

1. An image forming device, comprising: a rotatable photosensitive member; a first charging member configured to contact the surface of the photosensitive member at a first charging position with respect to the rotational direction of the photosensitive member to charge the surface of the photosensitive member; a second charging member configured to charge the surface of the photosensitive member at a second charging position with respect to the rotational direction of the photosensitive member; a developing member configured to form a toner image on the surface of the photosensitive member by supplying toner charged to a predetermined polarity to the surface of the photosensitive member at a developing position with respect to the rotational direction of the photosensitive member; a transfer member contacting the surface of the photosensitive member at a transfer position with respect to the rotational direction of the photosensitive member and configured to transfer the toner image from the photosensitive member to a recording material passing between the photosensitive member and the transfer member; a first charging voltage applying section configured to apply a first charging voltage of the predetermined polarity to the first charging member; a second charging voltage applying section configured to apply a second charging voltage of the predetermined polarity to the second charging member; a transfer voltage applying portion configured to apply a transfer voltage of a polarity opposite to the predetermined polarity to the transfer member; as well as a controller configured to control the first charging voltage applying section, the second charging voltage applying section, and the transfer voltage applying section, wherein, with respect to the rotation direction of the photosensitive member, 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 developing position, The developing 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, and Wherein, the controller controls: the transfer voltage applying portion causing an area corresponding to between the recording material and a subsequent recording material after the recording material, on the surface of the photosensitive member directly contacting the transfer member at the transfer position, to have a potential of a polarity opposite to the predetermined polarity, the first charging voltage applying section charges the surface of the photosensitive member at the first charging position by applying the first charging voltage which is lower than a discharge start voltage to the first charging member at the first charging position, and The second charging voltage applying section charges the surface of the photosensitive member at the second charging position by applying the second charging voltage not less than the discharge start voltage to the second charging member at the second charging position.

2. The image forming apparatus according to claim 1, wherein: With respect to the area corresponding to between the recording material and the subsequent recording material, when a ratio of the surface potential of the photosensitive member changed at the first charging position to the surface potential of the photosensitive member changed at the first charging position and the second charging position is a first charging ratio, The controller controls the first charging voltage applying part so that the first charging ratio is 10% or more.

3. The image forming apparatus according to claim 1, wherein: With respect to the area corresponding to between the recording material and the subsequent recording material, When the ratio of the surface potential of the photosensitive member changed at the first charging position to the surface potentials of the photosensitive member changed at the first charging position and the second charging position is a first charging ratio, the surface potential of the photosensitive member after passing the transfer position and before reaching the first charging position is Vt, the surface potential of the photosensitive member formed at the second charging position is Vd, the maximum value of Vt is Vtmax, the minimum value of Vt is VTmin, and the average value of Vt is Vtave, The controller controls the first charging voltage applying part so that the first charging ratio is not less than R1' represented by the following formula: R1'[%]={(Vtmax-Vtmin) / (Vd-Vtave)}×100.

4. The image forming apparatus according to claim 1, wherein: The controller controls the transfer voltage applying part so that the same transfer voltage is applied to the transfer member when the area of ​​the surface of the photosensitive member corresponding to the recording material and the subsequent recording material is in the transfer position and when the area of ​​the surface of the photosensitive member corresponding to the recording material is in the transfer position.

5. The image forming apparatus according to claim 1, wherein: The transfer member is configured to include a foam member capable of contacting the surface of the photosensitive member.

6. The image forming apparatus according to claim 5, wherein: The first charging member is configured to include a brush capable of contacting the surface of the photosensitive member.

7. The image forming apparatus according to claim 6, wherein: An average inter-yarn distance of the brush of the first charging member is smaller than an average cell diameter of the foam member of the transfer member.

8. The image forming apparatus according to claim 5, wherein: The first charging member is configured to include a foam member capable of contacting the surface of the photosensitive member.

9. The image forming apparatus according to claim 8, wherein: An average cell diameter of the foam member of the first charging member is smaller than an average cell diameter of the foam member of the transfer member.

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

11. The image forming apparatus according to claim 1, further comprising: A cleaning member is configured to remove toner from the surface of the photosensitive member at a position downstream of the transfer position and upstream of the first charging position with respect to the rotational direction of the photosensitive member.

12. The image forming apparatus according to claim 1, wherein: The photosensitive member includes a charge injection layer forming the surface of the photosensitive member.

13. The image forming apparatus according to claim 12, wherein: The charge injection layer is constituted by dispersing conductive particles in a binder resin.

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