Image forming apparatus

Abstract

To prevent electric discharge occurring between a photoconductor drum and a charging member.SOLUTION: An image forming apparatus has: a first charging member that is in contact with a surface of a photoconductor drum to form a first charging part, and upon application of first charging voltage in the first charging part, electrically charges the surface of the photoconductor drum; and a second charging member that, upon application of second charging voltage in a second charging part facing the surface of the photoconductor drum, electrically charges the surface of the photoconductor drum. In a direction of rotation of the photoconductor drum, the first charging part is formed on the downstream of a transfer unit and on the upstream of the second charging part. When the potential difference formed between the second charging voltage and a second surface potential formed on the surface of the photoconductor drum is defined as a charging potential difference in the second charging part, in an image forming operation, the image forming apparatus controls the charging potential difference to be equal to or larger than discharge start voltage, rotates the photoconductor drum, and with the first charging voltage applied, controls the charging potential difference in a non-image forming operation to be smaller than that in the image forming operation.SELECTED DRAWING: Figure 9

Description

[Technical field]

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

[0002] Conventionally, in electrophotographic or electrostatic recording image forming apparatuses, a charging method has been adopted as a charging means for an electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) in which a voltage is applied to a charging member in contact with the photosensitive drum, and the surface of the photosensitive drum is charged by discharge.

[0003] Specifically, when a voltage equal to or higher than a certain threshold value is applied to the charging member, a discharge occurs between the charging member and the photosensitive drum in accordance with Paschen's law, and as a result, the surface of the photosensitive drum becomes charged.

[0004] Here, as the number of printed sheets increases, the toner and external additives acting as the developer adhere to and contaminate the surface of the photosensitive drum and the charging member, causing the charging potential on the photosensitive drum surface to become non-uniform, which may result in a failure to obtain a stable image.

[0005] Patent Document 1 discloses a configuration for solving such problems. Patent Document 1 discloses a charging configuration that combines a charging method by discharge and an injection charging method that does not use discharge. The injection charging method is a method for charging the surface of a photosensitive drum by directly injecting charge from a charging member into the surface of the photosensitive drum. However, the injection charging method may result in non-uniform charging potential on the surface of the photosensitive drum. Therefore, Patent Document 1 discloses a configuration in which a charging member using the injection charging method is provided on the upstream side in the rotation direction of the photosensitive drum, and a charging member using a charging method by discharge is provided on the downstream side, so that the photosensitive drum surface is uniformly charged by the charging member on the downstream side. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2023-56470 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, Patent Document 1 has the following problems. The charging method by discharge may cause image defects due to the generation of discharge products by discharge or the surface deterioration of the photosensitive drum. The surface of the photosensitive drum on which discharge products have accumulated or the surface of the photosensitive drum has been deteriorated becomes less resistive, particularly in a high-temperature and high-humidity environment, and therefore the surface potential of the photosensitive drum required for image formation cannot be maintained, and it may be difficult to develop the desired toner image by the developing device.

[0008] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to suppress discharge occurring between the photosensitive drum and the charging member. [Means for solving the problem]

[0009] An image forming apparatus capable of performing an image forming operation for forming an image on a transfer material, the image forming apparatus comprising: a rotatable photosensitive drum; a first charging member that contacts a surface of the photosensitive drum to form a first charging section, the first charging section charging the surface of the photosensitive drum by applying a first charging voltage; a second charging member that faces the surface of the photosensitive drum, the second charging member that charges the surface of the photosensitive drum by applying a second charging voltage; an exposure unit that forms an electrostatic latent image on the surface of the photosensitive drum charged by the first charging member and the second charging member; a developing member that faces the surface of the photosensitive drum, the developing member that develops the electrostatic latent image with a developer to form a developer image on the surface of the photosensitive drum; a second charging voltage application unit that applies a charging voltage of the first charging voltage and a control unit that controls the first charging voltage and the second charging voltage, wherein the first charging unit is formed downstream of the transfer unit and upstream of the second charging unit in a rotation direction of the photosensitive drum, and a potential difference formed between a first surface potential formed on the surface of the photosensitive drum and the first charging voltage in the first charging unit is defined as a first charging potential difference, and a potential difference formed between a second surface potential formed on the surface of the photosensitive drum and the second charging voltage in the second charging unit is defined as a second charging potential difference, wherein the control unit i) controls the second charging potential difference to be equal to or greater than a discharge start voltage in the image forming operation, and ii) controls the second charging potential difference in a non-image forming operation different from the image forming operation while the photosensitive drum is rotated and the first charging voltage is applied, so that the second charging potential difference in the image forming operation is smaller. Effect of the Invention

[0010] As described above, according to the present invention, it is possible to suppress discharge occurring between the photosensitive drum and the charging member. [Brief description of the drawings]

[0011] [Figure 1] 1 is a schematic cross-sectional view of an image forming apparatus according to a first embodiment. [Diagram 2] 2 is a schematic cross-sectional view of an image forming unit in the first embodiment. FIG. [Diagram 3] FIG. 2 is a control block diagram according to the first embodiment. [Figure 4] 2 is a schematic diagram of a cross section near a charging portion in Example 1. FIG. [Diagram 5] FIG. 2 is a schematic diagram showing a layer structure of a photosensitive drum in the first embodiment. [Figure 6] FIG. 4 is a diagram showing the charging characteristics of the charging roller in Example 1. [Figure 7] FIG. 4 is a diagram showing the charging characteristics of the charging brush in Example 1. [Figure 8] 5 is a diagram showing potential unevenness on the surface of the photosensitive drum in the first embodiment. FIG. [Figure 9] 4 is a timing chart of charging voltage control in the first embodiment. [Figure 10] 11 is a timing chart of charging voltage control in Comparative Example 1. [Figure 11] FIG. 11 is a schematic diagram of a cross section near a charging portion in Example 2. [Figure 12] 10 is a timing chart of charging voltage control in the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, the embodiment of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative positions of the components described in the embodiment should be appropriately changed depending on the configuration of the device to which the invention is applied and various conditions. In other words, the scope of the present invention is not limited to the following embodiment. For convenience, the magnitude (high / low) of voltage or potential refers to the magnitude (high / low) when compared in absolute value, unless otherwise specified. EXAMPLES

[0013] [Image forming device] Fig. 1 shows an image forming apparatus 1 in this embodiment. Fig. 1 is a cross-sectional view of the image forming apparatus 1. As shown in Fig. 1, the image forming apparatus 1 has an image forming section 10 that forms a toner image on a recording material P, a feeding section 60 that feeds the recording material P to the image forming section 10, a fixing section 90 that fixes the toner image formed by the image forming section 10 onto the recording material, and a pair of discharge rollers 80.

[0014] The image forming section 10 has a scanner unit 11, an electrophotographic process cartridge 20, and a transfer roller 12 that transfers a toner image formed on a photosensitive drum 21 of the process cartridge 20 to a recording material P. FIG. 2 shows a detailed view of the process cartridge 20. The process cartridge 20 has the photosensitive drum 21, a drum unit 20 including a charging brush 22 and a charging roller 23 arranged around the photosensitive drum 21, a pre-exposure device 24, and a developing device 30 including a developing roller 31. The process cartridge 20 is configured to be detachable from the image forming apparatus 1. The process cartridge 20 may be configured to be attached to the image forming apparatus 1, or the drum unit 20 may be configured as a drum cartridge and the developing device 30 may be configured as a developing cartridge that can be detachable from the image forming apparatus 1 separately.

[0015] When an image formation command is input to the image forming apparatus 1, an image forming process is started by the image forming section 10 based on image information input from an external computer connected to the image forming apparatus 1.

[0016] The photosensitive drum 21 serving as an image carrier is rotationally driven by a motor 60 serving as a drive unit in a predetermined direction (clockwise direction in FIG. 2) at a predetermined process speed.

[0017] The charging brush 22 and charging roller 23 as charging members each come into contact with the photosensitive drum 21 with a predetermined pressure, and a desired charging voltage is applied by a charging power source 71 as a charging high voltage application unit in Fig. 2, thereby uniformly charging the surface of the photosensitive drum 21 to a predetermined potential. The pre-exposure device 24 neutralizes the surface of the photosensitive drum 21 before it enters the charging unit, for stable charging by the charging brush 22 and charging roller 23. The charging brush 22 and charging roller 23 will be described in detail later.

[0018] The scanner unit 11, which serves as an exposure device or exposure unit, uses a polygon mirror to irradiate laser light toward the photosensitive drum 21 based on input image information, and forms an electrostatic latent image on the photosensitive drum 21 by scanning and exposing it. Note that the scanner unit 11 is not limited to a laser scanner device, and may be, for example, an LED exposure device having an LED array in which a plurality of LEDs are arranged along the longitudinal direction of the photosensitive drum 21.

[0019] The electrostatic latent image formed on the photosensitive drum 21 is developed by the developing device 30 to form a toner image on the photosensitive drum 21 .

[0020] The developing device 30 will be described in detail with reference to FIG. 2. The developing unit 30 as the developing device includes a toner carrier that carries toner and developer, a developing roller 31 as a developer carrier, a developing container 32 that is a frame of the developing device 30, and a supply roller 33 that can supply toner to the developing roller 31. The developing roller 31 and the supply roller 33 are rotatably supported by the developing container 32. The developing roller 31 is disposed at the opening of the developing container 32 so as to face the photosensitive drum 21. In this embodiment, the surface of the developing roller 31 is in contact with the surface of the photosensitive drum 21 to form a developing section. The toner carried by the developing roller 31 is supplied to the surface of the photosensitive drum 21 in the developing section. The moving directions of the surfaces of the developing roller 31 and the photosensitive drum 21 are the same. That is, the rotating direction of the developing roller 31 and the rotating direction of the photosensitive drum 21 are opposite. Furthermore, in this embodiment, a configuration is adopted in which the surface of the photosensitive drum 21 and the surface of the developing roller 31 are in constant contact with each other. That is, there is no contact / separation mechanism for separating and contacting the surface of the photosensitive drum 21 and the surface of the developing roller 31. The supply roller 33 rotatably contacts the developing roller 31, and the toner contained in the developing container 32 is applied to the surface of the developing roller 31 by the supply roller 33. A developing voltage is applied to the developing roller 31 from a developing power source 72 serving as a developing voltage application unit in FIG. 3, and a supply voltage is applied to the supply roller 33 from a supply power source 75 serving as a supply voltage application unit in FIG. 3.

[0021] An agitating member 34 is provided inside the developing container 32 as an agitating means. The agitating member 34 is driven to rotate, thereby agitating the toner in the developing container 32 and sending the toner toward the developing roller 31 and the supply roller 33. The agitating member 34 also circulates the toner that has not been used in development and has been scraped off from the developing roller 31 within the developing container 32, thereby making the toner in the developing container 32 uniform. In this embodiment, the agitating member 34 rotates clockwise as shown in FIG. 2. The rotating direction of the agitating member 34 may be counterclockwise.

[0022] Further, at the opening of the developer container 32 in which the developing roller 31 is disposed, a developing blade 35 made of a SUS plate is disposed to regulate the amount of toner carried by the developing roller 31. It is possible to apply to the developing blade 35 a regulating voltage different from the developing voltage applied to the developing roller 31 from a regulating power source 76 serving as a regulating voltage application unit in FIG.

[0023] The toner supplied to the surface of the developing roller 31 passes through the opposing portion with the developing blade 35 as the developing roller 31 rotates, and is thereby uniformly formed into a thin layer.

[0024] The developing device 30 of this embodiment uses a contact development method. That is, the toner layer carried by the developing roller 31 comes into contact with the photosensitive drum 21 in a developing section (developing area) where the photosensitive drum 21 and the developing roller 31 face each other. In this embodiment, the photosensitive drum 21 is driven by the driving section 60 of FIG. 3 to rotate at a surface speed of 300 mm / sec, and the difference between the surface speed of the developing roller 31 and the surface speed of the photosensitive drum 21 (hereinafter referred to as the developing peripheral speed difference) is 40%. Therefore, the developing roller 31 rotates at a speed 40% faster than the photosensitive drum 21. That is, the moving speed of the surface of the developing roller 31 is set to 1.4 times the moving speed of the surface of the photosensitive drum 21. If the rotation speed of the photosensitive drum 21 is 100%, the rotation speed of the developing roller 31 is 140%. The developing peripheral speed ratio, which is the ratio of the rotation speed of the developing roller 31 to the rotation speed of the photosensitive drum 21, is 140%. As a result, the photosensitive drum and the developing roller 31 come into contact with each other with a speed difference of 120 mm / sec. The developing peripheral speed ratio is not limited to 140%, and the rotation speed of the developing roller 31 may be set appropriately. A developing voltage is applied to the developing roller 31 by a developing power source 72. Under the application of the developing voltage, the toner carried by the developing roller 31 is transferred from the developing roller 31 to the surface of the photosensitive drum 21 according to the potential distribution on the surface of the photosensitive drum 21. Thereby, the electrostatic latent image is developed into a toner image. Note that in this embodiment, a reversal development method is adopted. That is, the toner image is formed by adhering to the surface area of ​​the photosensitive drum 21, which is charged in the charging process and then exposed in the exposure process, where the charge amount is attenuated.

[0025] In this embodiment, toner with an average particle size of 6 μm and a normal negative charge polarity is used. As an example of the toner, a polymerized toner produced by a polymerization method is used. The toner does not contain a magnetic component, and is a so-called non-magnetic one-component developer in which the toner is supported on the developing roller 31 mainly by intermolecular forces and electrostatic forces (image forces). The toner and additives such as external additives are combined to form the developer. For convenience, the toner is sometimes called the developer.

[0026] The toner particles contain multiple waxes to adjust the toner's melting characteristics during the fixing process and its adhesion to the print medium and fixing roller.

[0027] Fine particles made of silica particles having a particle size on the order of submicrons are added to the surface of the toner particles in order to adjust the fluidity and charging performance of the toner.

[0028] In this embodiment, a non-magnetic one-component developer is used as an example, but a one-component developer containing a magnetic component may be used. Also, a two-component developer composed of a non-magnetic toner and a magnetic carrier may be used as the developer. When a magnetic developer is used, a cylindrical developing sleeve with a magnet disposed inside may be used as the developer carrier.

[0029] In parallel with the image forming process described above, the recording material P stored in the feeding section 60 is sent out in accordance with the transfer timing of the toner image. The feeding section 60 has a front door 61 supported by the image forming apparatus 1 so as to be openable and closable, a stacking tray 62, a middle plate 63, a tray spring 64, and a pickup roller 65. The stacking tray 62 constitutes the bottom surface of a recording material storage space that appears when the front door 61 is opened, and the middle plate 63 is supported by the stacking tray 62 so as to be movable up and down. The tray spring 64 biases the middle plate 63 upward, and presses the recording material P loaded on the middle plate 63 against the pickup roller 65. The front door 61 closes the recording material storage space when closed relative to the image forming apparatus 1, and supports the recording material P together with the stacking tray 62 and the middle plate 63 when opened relative to the image forming apparatus 1. The process of conveying the recording material P will be described below. First, the pickup roller 65 of the feeding section 60 feeds out the recording material P supported by the front door 61, the stacking tray 62, and the middle plate 63. Next, the recording material P is fed by the pickup roller 65 to the pair of registration rollers 15, and the skew of the recording material P is corrected by the recording material hitting the nip of the pair of registration rollers 15. The pair of registration rollers 15 is then driven in accordance with the transfer timing of the toner image, and conveys the recording material P toward the transfer nip formed by the transfer roller 12 and the photosensitive drum 21.

[0030] A transfer voltage is applied from a primary transfer voltage power source 73 to a transfer roller 12 as a transfer means, and the toner image carried on the photosensitive drum 21 is transferred onto a recording material P conveyed by a pair of registration rollers 15 .

[0031] The recording material P onto which the toner image has been transferred is conveyed to a fixing section 90, where the toner image is heated and pressurized as it passes through a nip between a fixing film 91 and a pressure roller 92 of the fixing section 90. This causes the toner particles to melt and then adhere to the recording material P, thereby fixing the toner image to the recording material P.

[0032] The fixing unit 90 here is of a thermal fixing type that fixes an image by heating and melting the toner on the recording material P. The fixing unit 90 includes a fixing film 91, a fixing heater (not shown) such as a ceramic heater that heats the fixing film 91, a thermistor (not shown) that measures the temperature of the fixing heater, and a pressure roller 92 that is in pressure contact with the fixing film 91.

[0033] The recording material P that has passed through the fixing unit 90 is discharged to the outside of the image forming apparatus 1 by a pair of discharge rollers 80, and is stacked on a discharge tray 81. The discharge tray 81 is inclined upward toward the downstream in the discharge direction of the recording material P, and the recording material P discharged to the discharge tray 81 slides down the discharge tray 81, so that the rear end of the recording material P is aligned by a regulating surface 82.

[0034] In this embodiment, a process cartridge that is detachably attached to the main body of the image forming apparatus 1 is used, but the present invention is not limited to this. Any process cartridge may be used as long as it can perform a predetermined image forming process. For example, although the above description concerns a developing cartridge in which the developing device is detachably attached and a drum cartridge in which the drum unit is detachably attached, a toner cartridge that supplies toner to the developing device from the outside may also be used.

[0035] In this embodiment, a cleaning member for collecting toner on the photosensitive drum 21 that has not been transferred to the recording material P in the transfer process is not provided, but the present invention is not limited to this and may be configured to include a cleaning member.

[0036] [Charge configuration] This embodiment is characterized in that the injection charging method and the contact charging method are combined to charge the surface of the photosensitive drum 21. That is, in the rotation direction of the photosensitive drum 21, a first charging member arranged on the upstream side and a second charging member arranged on the downstream side are combined to charge the surface of the photosensitive drum 21.

[0037] 4 shows the charging configuration in this embodiment. In this embodiment, a charging brush 22 as a first charging member is contacted with a predetermined pressure at a first charging portion Pa, which is a peripheral position of the photosensitive drum 21, and a charging roller 23 as a second charging member is contacted with a predetermined pressure at a second charging portion Pb, which is a peripheral position of the photosensitive drum 21. Here, the first charging portion Pa is located downstream of the transfer portion, which is a transfer nip, and upstream of the second charging portion Pb in the rotation direction of the photosensitive drum 21.

[0038] A predetermined charging voltage is applied to the charging brush 22, and the surface of the photosensitive drum 21 is charged by an injection charging method according to the magnitude of a first charging potential difference formed between the surface potential at position Pa of the photosensitive drum 21 and the brush voltage, which is the first charging voltage. A predetermined charging voltage is applied to the charging roller 23, and the surface of the photosensitive drum 21 is charged by discharge according to the magnitude of a second charging potential difference formed between the surface potential at position Pb of the photosensitive drum 21 and the charging voltage, which is the second charging voltage.

[0039] [Photosensitive drum] The photosensitive drum 21 is constructed by providing a photosensitive material such as an organic photosensitive body, amorphous selenium, or amorphous silicon on a cylindrical drum base made of aluminum, nickel, or the like.

[0040] Fig. 5 is a cross-sectional view of the photosensitive drum 21 used in this embodiment, showing its structure. The photosensitive drum 21 is a negatively charged organic photosensitive body having an outer diameter of φ24 mm. As shown in Fig. 5, the photosensitive drum 21 has a conductive support 21a made of an aluminum cylinder, a conductive layer 21b, an undercoat layer 21c, and a photosensitive layer made of two layers, a charge generating layer 21d and a charge transport layer 21e.

[0041] For the purpose of suppressing scraping of the surface of the photosensitive drum 21 and adjusting the coefficient of friction, the photosensitive drum 21 may have an additional layer coated on the charge transport layer 21e.

[0042] [Charging roller] The charging roller 23 as the second charging member has a multi-layer structure in which a stainless steel core having a diameter of 6 mm is used as a support and is covered with multiple flexible resin layers. In this embodiment, the charging roller 23 has a two-layer structure consisting of a base layer, which is a first resin layer covering the core, and a surface layer, which is a second resin layer covering the base layer. The resin material of the base layer is conductive hydrin rubber with conductive carbon dispersed therein, and is formed on the core by extrusion molding, with a film thickness of about 2 mm. In this embodiment, conductive hydrin rubber is used, but it is not limited to this as long as it is a flexible and conductive resin material.

[0043] A high-resistance resin layer with a thickness of approximately 30 μm and suitable surface roughness is spray-coated on the base layer of the charging roller 23 as a surface layer. By making the outermost surface highly resistive, it is possible to suppress the transfer of charge from the charging roller 23 to the photosensitive drum 21. In this configuration, the coating liquid for the surface layer is made by mixing a urethane resin material with roughening particles with a particle size of approximately 20 μm made of a urethane material at a weight ratio of approximately 50% to give the surface an appropriate surface roughness. The coating liquid is spray-coated on the base layer to form the surface layer. The volume resistivity of the surface layer is approximately 1×10 5 The surface roughness is approximately 2.0 μm.

[0044] The volume resistivity of the surface layer of the charging roller 23 is in the range of 1.0×10 4 Ω cm or more, 1.0×10 7It is preferable that the volume resistivity of the charging roller 23 is Ω·cm or less, and the arithmetic mean roughness Ra of the surface is preferably 0.5 to 3.0 μm. If the volume resistivity of the charging roller 23 is extremely low, a leakage current due to an overcurrent may flow to the photosensitive drum 21. If the volume resistivity of the charging roller 23 is extremely high, the charging voltage may be difficult to apply to the surface of the charging roller 23, and the discharge for uniform charging may become unstable. In view of the above, the volume resistivity of the charging roller 23 is preferably in the above range. Furthermore, the surface roughness is also preferably in the above range, since the uniform charging property is easily stabilized by setting the surface roughness in the above range.

[0045] FIG. 6 shows the charging characteristics when the surface of the photosensitive drum 21 is charged by the charging roller 23 in an environment of a temperature of 32.5° C. and a humidity of 80%. That is, FIG. 6 shows the relationship between the direct current (DC) charging voltage applied to the charging roller 23 and the surface potential of the photosensitive drum 21 when the charging roller 23 as a charging member is pressed against the photosensitive drum 21. The surface potential of the photosensitive drum 21 was measured using a surface potential meter (TREK Model 344-F) and a side view probe for surface potential meter (TREK Model 6000B-8). In the charging roller 23 of this embodiment, when a charging voltage is applied to the charging roller 23 so that the second charging voltage is −550 V or more, a negative potential can be uniformly formed on the surface of the photosensitive drum 21 by the contact charging method. Here, the discharge start voltage Vth between the charging roller 23 and the photosensitive drum 21 is −550 V. As shown in Fig. 6, when a charging voltage of about -550V or more is applied to the charging roller 23, the surface potential of the photosensitive drum 21 rises linearly with a slope of 1. This threshold value is the voltage at which charging accompanied by discharge starts (hereinafter, the discharge start voltage). In order to form a dark potential Vd, which is the surface potential of the photosensitive drum 21, if the discharge start voltage is Vth, it is necessary to apply a DC voltage of Vd+Vth or more to the charging roller 23. The above-mentioned method of charging the photosensitive drum by applying only a DC voltage to a contact charging member is called a contact DC charging method.

[0046] As mentioned above, the magnitude (high / low) of voltage or potential refers to the magnitude (high / low) when compared in absolute value, unless otherwise specified. In other words, regarding potential or applied voltage, a high potential refers to an absolute value that is large on the negative polarity side (for example, -1000V compared to -500V), and a low potential refers to an absolute value that is small on the negative polarity side (for example, -300V compared to -500V). This is because in this embodiment, negatively charged toner is considered as the standard. Also, voltage is expressed as a potential difference with respect to the earth potential (0V). Therefore, a development voltage of -300V means that there is a potential difference of -300V due to the development voltage applied to the core metal of the development roller 31 with respect to the earth potential. This is also true for other voltages such as charging voltage.

[0047] [Conductive brush] The charging brush 22 as the first charging member is constructed by attaching and fixing a 5 mm wide pile fabric of conductive nylon fibers 22b to a stainless steel sheet metal 22a that also serves as a power supply electrode. The conductive nylon fibers 22b have a fineness of 2 denier, a pile density of 200 kF / inch^2, and a pile length of 4 mm, and are in contact with the photosensitive drum 21 so that the penetration depth from the tip of the fibers is 0.6 mm. The unit of pile density, kF / inch^2, indicates the number of filaments per square inch. The brush portion composed of the conductive nylon fibers 22b is sometimes simply called the brush portion.

[0048] The resistance value of the charging brush 22 is measured by pressing a stainless steel sheet metal vertically from above the charging brush 22 with a penetration depth of 1 mm, and applying a voltage between the base sheet metal. The applied voltage is +250V, and the resistance value 5 seconds after the voltage application is taken as the brush resistance. A HIOKI ST5520 was used to measure the resistance. In this example, the brush resistance is 1.0×10 4 Ω~1.0×10 6 The brush resistance is in the range of Ω. The brush resistance can be controlled to a desired value by changing the material and amount of conductive particles dispersed in the conductive pile yarn.

[0049] The brush resistance value of the charging brush 22 is in the range of 1.0×10 2 Ω~1.0×10 8 Ω is preferable. If the brush resistance is extremely large, it becomes difficult for current to flow, and the surface of the photosensitive drum 21 cannot be appropriately charged. Also, if the brush resistance is extremely small, a large current flows locally from the charging brush 22 to the photosensitive drum 21, causing insulation breakdown of the photosensitive layers 21d and 21e of the photosensitive drum 21, which is known as pinhole leakage. In view of the above, the brush resistance value is preferably in the above range.

[0050] The contact pressure of the charging brush 22 is preferably in the range of 40 gf to 200 gf. The charging brush 22 is disposed so that the tip of the brush portion 22b is in substantially uniform contact with the photosensitive drum 21 and there is little change in the bristles due to the rotation of the photosensitive drum 21. If the contact pressure of the brush portion 22b is significantly high, the charging brush 22 may scratch the photosensitive drum 21.

[0051] Also, this embodiment employs a cleanerless configuration that does not include a cleaning member for removing developer remaining on the surface of the photosensitive drum 21. In the cleanerless configuration, if the contact pressure between the charging brush 22 and the photosensitive drum 21 is high, the untransferred toner remaining on the photosensitive drum 21 without being transferred is blocked by the charging brush 22, which may cause contamination of the brush portion 22b and a decrease in charging performance.

[0052] Furthermore, if the contact pressure of the brush portion 22b is too low, the brush portion 22b may not contact uniformly, or the brush portion 22b may tilt or move due to the rotation of the photosensitive drum 21.

[0053] In view of the above, the contact pressure of the brush portion 22b is preferably within the above range.

[0054] In addition, in this embodiment, the charging brush 22 is attached to a flexible sheet and is brought into contact with the photosensitive drum 21 at a predetermined pressure, but it may be arranged so that both ends of the support of the charging brush 22 are pressed by a spring or the like to bring the charging brush 22 into contact with the photosensitive drum 21. It may also be arranged so that the charging brush 22 is fixed to the cartridge frame and brought into contact with the photosensitive drum 21 with a predetermined intrusion amount.

[0055] 7 shows the charging characteristics when the surface of the photosensitive drum 21 is charged by the charging brush 22 in an environment of a temperature of 32.5° C. and a humidity of 80%. In the charging brush 22 of this embodiment, even when the first charging voltage is equal to or less than the discharge start voltage Vth−550 V, a potential is gradually formed on the photosensitive drum 21 by injection charging. When the first charging voltage exceeds Vth, the charging brush 22 behaves in the same manner as the charging roller 23, and in addition to injection charging, a surface potential of the photosensitive drum 21 is formed by discharging. Therefore, the slope between the first charging voltage and the surface potential of the photosensitive drum 21 changes at the discharge start voltage Vth.

[0056] <Control configuration of image forming apparatus> Next, a description will be given of the control configuration of the image forming apparatus 1 of this embodiment. Fig. 3 is a block diagram showing the control configuration of the main parts of the image forming apparatus 1 of this embodiment.

[0057] The image forming apparatus 1 is provided with a control unit 202 that controls the operation of the image forming apparatus 1. Signals indicating various information are input to and output from the control unit 202 via electrical connections. The control unit 202 processes signals input from various process devices and sensors, and processes signals output to instruct the various process devices to operate. A controller 200 provided in the image forming apparatus 1 inputs and outputs various signals to and from an external device (host device), and inputs and outputs various signals to and from the control unit 202 via an interface 201 provided in the image forming apparatus 1. The control unit 202 comprehensively controls the operation of the image forming apparatus 1 according to a predetermined control program and reference table in response to an instruction from the controller 200.

[0058] The control unit 202 has a CPU 221 as a calculation processing means which is a central element for performing various calculation processes, and a main memory 222 such as a RAM, a ROM, and a non-volatile memory which are storage elements for storing information. The RAM temporarily stores the detection results of the sensor, the count results of the counter, the calculation results, and the like. The ROM stores a control program, a data table obtained in advance by experiments, and the like. The non-volatile memory stores the count results of the counter, various setting information, the results of the sensor, and the like. The control unit 202 is connected to each control target, sensor, counter, and the like in the image forming apparatus 1. The control unit 202 controls the input and output of various signals, the timing of driving each unit, and the like, to control a predetermined image forming sequence, and the like.

[0059] The control unit 202 controls, for example, the charging power supply 71, the developing power supply 72, the supply power supply 75, the regulating power supply 76, the exposure device 11, the primary transfer power supply 73, the drive unit 60, the brush power supply 74, and the like.

[0060] The driving unit 60 is configured to have a driving motor as a driving source, a drive transmission member, etc. The driving sources for driving the rotating members of the photosensitive drum 21 and the developing device 30 may be provided independently of each other, or at least a part of them may be shared. The driving sources for driving the elements for each color may be provided independently of each other, or at least a part of them may be shared.

[0061] Here, the image forming apparatus 1 executes an image forming operation (print job) which is a series of operations for forming and outputting an image on one or more recording materials P, which is started by one start instruction. The image forming operation generally includes an image forming process, a pre-process (pre-rotation process, pre-print operation), a paper-interval process when forming images on multiple recording materials P, and a post-process (post-rotation process, post-print operation). The image forming process is a period during which an electrostatic latent image of an image to be actually formed on the recording material P and output is formed, a toner image is formed, the toner image is primarily transferred, and fixed, and the image forming time refers to this period. More specifically, the timing of the image forming time differs depending on the position where each process of charging, exposure, development, primary transfer, and fixing is performed. The pre-process is a period during which a preparatory operation is performed before the image forming process from when a start instruction is input until the image actually starts to be formed. The paper-interval process is a period corresponding to the period between recording materials P when image formation is performed continuously on multiple recording materials P (continuous image formation). The post-rotation process is a period in which a tidying operation (preparatory operation) is performed after the image forming process. The non-image forming time is a period other than the image forming time, and includes the above-mentioned pre-process, inter-sheet process, post-process, and further the pre-multiple rotation process which is a preparatory operation when the image forming apparatus 1 is turned on or when it returns from a sleep state.

[0062] [Charging voltage control during image formation] The voltage control during the image forming operation in this embodiment will be described. During the image forming operation, the surface potential of the photosensitive drum 21 is neutralized by the pre-exposure device 24, and the surface potential at the first charging portion Pa is approximately 0V. A charging voltage of -500V is applied to the charging brush 22, and the magnitude of the first charging voltage is -500V, so that the surface of the photosensitive drum 21 is charged to -200V by the injection charging method according to the characteristics of FIG. 7. The surface potential at the second charging portion Pb of the photosensitive drum 21 is charged to -200V, but a charging voltage of -1000V is applied to the charging roller 23, and the magnitude of the second charging potential difference is -800V. At this time, according to the characteristics of FIG. 6, the surface of the photosensitive drum 21 is charged to -250V by the contact DC charging method, and finally the surface of the photosensitive drum 21 becomes -450V. In other words, at the first charging portion Pa, a voltage of −1000V is applied from a state in which −200V is injected, resulting in a surface potential of −450V, which is the difference from the discharge start voltage Vth−550V. Here, a developing voltage of −300V is applied to the developing roller 31, and the back contrast, which is the potential difference between the photosensitive drum 21 and the developing voltage (surface potential of the developing roller 31), is set to −150V.

[0063] Here, in the injection charging method, the surface of the photosensitive drum 21 is charged only at the points where the photosensitive drum 21 and the charging brush 22 are in direct contact with each other, so if the photosensitive drum 21 and the charging brush 22 are not in uniform contact with each other, charging unevenness may occur. The charging unevenness will be described later with reference to FIG. 8. On the other hand, in the contact DC charging method, the photosensitive drum 21 is charged by discharge generated at non-contact points. Therefore, charging is possible even in the places where the photosensitive drum 21 and the charging roller 23 are not in contact with each other, that is, in the gap near the second charging unit Pb, and uniform charging is possible.

[0064] Therefore, the surface potential of the photosensitive drum 21 after charging by the injection charging method may be non-uniform, but the surface of the photosensitive drum 21 is uniformly charged by the contact DC charging method using the charging roller 23.

[0065] As described above, during image forming operation, it is preferable that the second charging member charges the surface of the photosensitive drum 21 by a contact DC charging method, and the second charging potential difference is preferably equal to or greater than the discharge start voltage.

[0066] [Effects of discharge products on photosensitive drum] When an image forming operation is performed using the image forming apparatus 1, a small amount of discharge products such as ozone and NOx are generated by the charging roller 23. Depending on the usage conditions, such as repeated use of the image forming apparatus 1, the discharge products may accumulate on the surface of the photosensitive drum 21.

[0067] In particular, in a high-temperature, high-humidity environment, when the discharge products accumulated on the surface of the photosensitive drum 21 absorb moisture, the electrical resistance of the surface of the photosensitive drum 21 decreases, making it impossible to maintain the formed electrostatic latent image, which may result in "image deletion," an image defect in which the toner image becomes blurred.

[0068] It is known that the amount of discharge products generated depends on the amount of discharge. Meanwhile, the discharge products attached to the photosensitive drum 21 are scraped off by a member that contacts the photosensitive drum 21. In other words, the amount of discharge products accumulated on the photosensitive drum 21 is related to the amount of discharge, the amount scraped off by a contact member such as the developing roller 31, the usage history of the image forming operation, and the like.

[0069] This embodiment is characterized in that the amount of discharge during non-image forming operations is suppressed in order to suppress the amount of accumulation of discharge products on the photosensitive drum 21. That is, the second charging potential difference during non-image forming operations is made smaller than the second charging potential difference during image forming operations.

[0070] Specifically, in non-image forming operations, the brush voltage is controlled to be greater toward the toner charging polarity (normal polarity) so that the absolute value of the surface potential between the first charging portion Pa and the second charging portion Pb in the rotation direction of the photosensitive drum 21 is greater than that in image forming operations.

[0071] Moreover, it is more preferable to lower the second charging voltage applied to the charging roller 23 during non-image forming operations, and to further reduce the second charging potential difference.

[0072] Moreover, it is more preferable to minimize the amount of discharge products generated by controlling the second charging potential difference so that it is smaller than the discharge start voltage Vth.

[0073] As described above, in the injection charging method using the charging brush 22, minute charging unevenness may occur in the surface potential between the first charging portion Pa and the second charging portion Pb of the photosensitive drum 21. At this time, even if the second charging potential difference is controlled to be equal to or lower than the discharge start voltage Vth, minute discharge may occur, making it difficult to completely suppress the generation of discharge products.

[0074] In this case, the first charging voltage, which is the brush voltage applied to the charging brush 22, causes unevenness in the injected surface potential of the photosensitive drum 21, resulting in the situation shown in FIG. 8. The state of the potential formed on the surface of the photosensitive drum 21 will be described in detail with reference to FIG. 8. FIG. 8 shows the axial (longitudinal) transition of the surface potential of the photosensitive drum 21 when the surface of the photosensitive drum 21 injected and charged by the charging brush 22 enters the charging roller 23 due to rotation. From FIG. 8, it can be seen that the potential formed on the surface of the photosensitive drum 21 injected and charged by the charging brush 22 is in an unstable state. This is a phenomenon that appears due to a difference in local impedance that occurs between the charging brush 22 and the photosensitive drum 21. In other words, the impedance is not necessarily uniform in the region where the charging brush 22 and the photosensitive drum 21 are in contact with each other, and there are parts where the current that flows toward the surface of the photosensitive drum 21 flows relatively easily and parts where it does not flow easily. The contact surface between the charging brush 22 and the photosensitive drum 21 has a difference in the surface resistance of the photosensitive drum 21 and a difference in the contact state of the charging brush 22. Therefore, although it is uniform to some extent, depending on the environment and the usage state, the situation as shown in FIG. 8 may occur. There is also a possibility that the surface potential of the photosensitive drum 21 is formed smaller than the injection potential assumed from the brush voltage applied to the charging brush 22, as shown at point A in FIG. 8, due to the injection charge from the charging brush 22. For example, in a low temperature and low humidity environment, if there is a point where the impedance is locally increased on the contact surface between the charging brush 22 and the photosensitive drum 21, the injection may be suppressed. In this case, the absolute value of the surface potential Vd of the photosensitive drum 21 is smaller than the assumed value. Therefore, depending on the charging voltage applied to the charging roller 23, a discharge may occur due to the potential difference generated between the area of ​​the surface of the photosensitive drum 21 where the injection amount is small and the charging voltage applied to the charging roller 23. Therefore, it is preferable that the charging voltage applied to the charging roller 23, which is the second charging voltage, is larger in absolute value than the brush voltage applied to the charging brush 22, which is the first charging voltage, and the potential difference is such that no discharge occurs. Specifically, it is preferable to control the second charging voltage applied during a non-image forming operation so that the absolute value of the second charging voltage applied during an image forming operation is smaller than that of the second charging voltage applied during an image forming operation.

[0075] In such a situation, the amount of discharge products generated can be further reduced by measuring the current value when a charging voltage is applied to the charging roller 23 and setting the charging voltage to be lower than the charging voltage at which a current due to a minute discharge begins to flow. Specifically, a charging current detection unit (charging current detection circuit) that detects the charging current flowing in the second charging unit Pb (charging roller 23, charging power source 71) may be provided.

[0076] Here, in this embodiment, the amount of discharge is reduced by controlling the charging voltage applied to the charging brush 22 to be large, but this is not limited to the above. It is sufficient that the second charging potential difference is smaller during non-image forming operation compared to during image forming operation, and for example, the amount of discharge may be reduced by reducing the charging voltage applied to the charging roller 23 without changing the brush voltage applied to the charging brush 22.

[0077] [Appropriate potential relationship between each component and the photosensitive drum surface] As described above, the surface of each member may be contaminated by repeated use of the image forming apparatus 1. The term "surface is contaminated" refers to the transfer of substances such as toner, external additives, and discharge products attached to the surface of the photosensitive drum 21 to each member due to the potential difference between the photosensitive drum 21 and each member during image forming operation. When the above substances are charged with the same polarity as the toner, they are mainly transferred to the transfer roller 12 to which a positive voltage, which is the opposite polarity to the toner, is applied during image forming operation. On the other hand, when the above substances are charged with the opposite polarity to the toner, they are transferred to the charging brush 22 and the charging roller 23 to which a negative voltage, which is the same polarity as the toner, is applied.

[0078] Here, during non-image forming operation, for example, if the relationship of the potential difference between the applied voltage of the charging roller 23 and the surface potential of the photosensitive drum 21 becomes reversed with respect to the image forming operation, there is a possibility that a substance charged with a polarity opposite to that of the toner adhering to the charging roller 23 may be transferred to the photosensitive drum 21. In this embodiment, the charging voltage during the image forming operation is -1000V, and the surface potential of the photosensitive drum 21 at the second charging portion Pb is -450V. Therefore, during the image forming operation, a potential difference is formed such that a substance charged with a positive polarity opposite to that of the toner adheres to the charging roller 23. From there, when the charging voltage is changed to 0V during non-image forming operation, if the surface potential of the photosensitive drum 21 at the second charging portion Pb is greater than 0V toward the normal polarity side of the toner, the substance adhering to the charging roller 23 will be transferred to the surface of the photosensitive drum 21. This may cause image defects. Therefore, even during non-image forming operation, it is preferable to form a potential on the surface of the photosensitive drum 21 and control the potential relationship between each member and the surface of the photosensitive drum 21 so that the electric field direction is the same as that during image forming operation. At this time, there is no need to form a toner image during non-image forming operation. Therefore, even if slight charging unevenness occurs in the surface potential formed on the photosensitive drum 21, it does not cause any problem in practical use.

[0079] As described above, even during non-image forming operations, it is preferable that the absolute value of the voltage applied to the charging brush 22 is greater on the normal polarity side of the toner, and the absolute value of the voltage applied to the charging roller 23 is greater on the normal polarity side of the toner, relative to the surface potential of the photosensitive drum 21. In other words, during non-image forming operations, it is preferable that an electric field is formed between the charging brush 22 and the surface of the photosensitive drum 21 in a direction such that toner charged to the normal polarity moves from the charging brush 22 to the surface of the photosensitive drum 21.

[0080] Furthermore, during non-image forming operations, it is preferable that an electric field is formed between the charging roller 23 and the surface of the photosensitive drum 21 in a direction in which toner charged to the normal polarity moves from the charging roller 23 to the surface of the photosensitive drum 21. Although the toner is described as the object to be moved, it goes without saying that it may be any of the above-mentioned substances. Furthermore, in the case of a substance charged to the polarity opposite to the normal polarity, it goes without saying that the direction of the electric field between the surface of the photosensitive drum 21 and the charging brush 22 and charging roller 23 is controlled to be opposite to the above-mentioned relationship.

[0081] In addition, in this embodiment, the surface of the developing roller 31 and the surface of the photosensitive drum 21 are in contact with each other and do not separate even during non-image forming operation. Therefore, even during non-image forming operation, when a voltage of the same polarity as the normal polarity of the toner is applied as a condition of the developing voltage, it is preferable to control the absolute value of the surface potential of the photosensitive drum 21 to be small and set the back contrast. Here, during non-image forming operation, the surface potential of the photosensitive drum 21 is formed by the charging brush 22. In this case, since the surface potential of the photosensitive drum 21 also includes a potential component formed by injection charging, the unevenness of Vd, which is the surface potential of the photosensitive drum 21, may be large depending on the environment and usage conditions. Therefore, in this embodiment, when a voltage of the normal polarity of the toner is applied to the developing roller 31 during non-image forming operation, it is preferable to control the absolute value of the surface potential of the photosensitive drum 21 to be small.

[0082] Also, it is more preferable to form the same surface potential of the photosensitive drum 21 in the image forming operation and the non-image forming operation. That is, in this embodiment, the amount of discharge caused by the discharge during the non-image forming operation is reduced compared to the image forming operation, and the non-image potential Vd of the same degree as the non-image potential Vd formed during the image forming operation is also formed during the non-image forming operation. This is one of the reasons for simplifying the control other than charging, and at the same time, the surface of the developing roller 31 and the surface of the photosensitive drum 21 are configured to remain in contact and not separate even during the non-image forming operation. That is, it is because it is necessary to set the back contrast, which is the Δ of the surface potential of the photosensitive drum 21 and the developing voltage applied to the developing roller 31, to be the same as during the image forming operation. If the surface potential Vd of the photosensitive drum 21 is changed, the developing voltage must be changed in order to maintain the back contrast at the developing section. Of course, the developing voltage may be changed in accordance with the change in Vd, but the control becomes somewhat complicated. Also, as described above, if the absolute value of the surface potential Vd of the photosensitive drum 21 is small, the potential formed becomes unstable, and there is a possibility that an appropriate back contrast is not formed. Therefore, it is more preferable to perform control so that the surface potential of the photosensitive drum 21 is formed to the same degree during the image forming operation and the non-image forming operation.

[0083] Depending on the usage environment and usage history of the image forming apparatus 1, the surface resistance of the photosensitive drum 21 may decrease, making it easier for the first charging member to inject and charge the photosensitive drum 21. As a result, the absolute value of the surface potential at the second charging portion Pb of the photosensitive drum 21 may be greater toward the normal polarity of the toner than usual. Even in such a situation, it is preferable to control as follows so that the substance attached to the second charging member and charged with a polarity opposite to the normal polarity of the toner is not transferred to the photosensitive drum 21. That is, it is preferable that a charging voltage of the same polarity as the normal polarity of the toner is applied to the first charging member and the second charging member, and that the absolute value of the charging voltage applied to the second charging member is greater than that of the charging voltage applied to the first charging member.

[0084] A more detailed explanation will be given with reference to FIG. 8. FIG. 8 shows the transition of the surface potential of the photosensitive drum 21 when the surface of the photosensitive drum 21 injected with charge by the charging brush 22 enters the charging roller 23 by rotation as described above. Although the surface potential of the photosensitive drum 21 formed by the charging brush 22 is uniform to some extent, as described above, depending on the environment and the usage state, it is possible that the situation shown in FIG. 8 may occur. The injected charge from the charging brush 22 may form the surface potential of the photosensitive drum 21 to the same extent as the brush voltage applied to the charging brush 22, as shown at point B in FIG. 8. For example, in a high-temperature and high-humidity environment, if there is a portion where the impedance is locally reduced on the contact surface between the charging brush 22 and the photosensitive drum 21, the brush voltage may be injected. In this case, the absolute value of the surface potential Vd of the photosensitive drum 21 is larger than expected. Therefore, in this embodiment, the charging voltage applied to the charging roller 23 is set to have the same polarity as the brush voltage and to have an absolute value larger than the brush voltage, so that the material charged to the opposite polarity is not transferred to the photosensitive drum 21. In this embodiment, when a brush voltage of -500 V was applied, the unevenness of Vd after passing through the first charging portion Pa was approximately 100 V. Also, when a brush voltage of -780 V was applied, the unevenness of Vd after passing through the first charging portion Pa was approximately 100 V. Therefore, the potential difference between points A and B in FIG.

[0085] [Charging voltage control during non-image formation] In view of the above, the voltage control during non-image forming operation in this embodiment will be described below. Even during non-image forming operation, the surface potential at the first charging portion Pa of the photosensitive drum 21 is neutralized to about 0V by the pre-exposure device 24. A charging voltage of -780V is applied to the charging brush 22, and the magnitude of the first charging potential difference is -780V. Therefore, according to the characteristics in Fig. 7, the surface of the photosensitive drum 21 is charged to -450V due to the effect of discharge in addition to the injection charging method.

[0086] When the potential difference formed between the charging brush 22 and the surface of the photosensitive drum 21 exceeds 550 V, a discharge occurs between the charging brush 22 and the photosensitive drum 21, but charging by direct charge injection also occurs at the same time. Therefore, even if the potential difference between the charging brush 22 and the surface of the photosensitive drum 21 exceeds 550 V, the injection charging method can reduce the total amount of discharge and reduce the amount of discharge products compared to the contact DC charging method alone.

[0087] Here, the surface potential at the second charging portion Pb of the photosensitive drum 21 is charged to -450V, but a charging voltage of -800V is applied to the charging roller 23, and the magnitude of the second charging potential difference is -350V. Therefore, the characteristics of FIG. 6 correspond to the surface potential of the photosensitive drum 21 with a charging voltage of -350V. Therefore, the surface of the photosensitive drum 21 is not charged, and the surface of the photosensitive drum 21 ultimately remains at -450V. That is, the magnitude of the second charging potential difference is smaller than the discharge start voltage Vth, and no discharge occurs, so no discharge products are generated. Furthermore, it is higher on the normal polarity side of the toner than the surface potential of the photosensitive drum 21. Therefore, fine particles charged to a polarity opposite to the normal polarity of the toner attached to the charging brush 22 or the charging roller 23 are not transferred to the photosensitive drum 21. Here, a developing voltage of -300V is applied to the developing roller 31, and the back contrast is set to -150V. Since there is no need to form a toner image during non-image forming operations, the back contrast can be set to a value that minimizes the amount of toner transferred from the developing roller 31 to the photosensitive drum 21. For example, the back contrast may be set to different values ​​during image forming operations and non-image forming operations. The control unit 202 may control the back contrast to be larger during non-image forming operations than during image forming operations, or may control the back contrast to be smaller during non-image forming operations than during image forming operations.

[0088] [Specific voltage control in embodiment 1] Fig. 9 shows a specific voltage control example of Example 1-1 described later when printing one A4-sized recording material P as an example. For convenience, the voltage application timing in Fig. 9 is described as changing simultaneously, but the timing may change slightly as long as the preparation for the image forming operation is completed. In addition, the voltage application timing and voltage application time can be set appropriately.

[0089] In this embodiment, the photosensitive drum 21 starts to be driven one second after the timing when an image formation command is input from the controller 200 to the control unit 202 of the image forming apparatus 1 via the interface 201. Then, for two seconds until the image forming operation starts, the surface potential of the photosensitive drum 21 is formed by the voltage control during non-image forming operation, and a developing voltage is applied to the developing roller 31. After that, when the image forming operation starts, the charging voltage applied to the charging brush 22 and the charging roller 23 is changed, and for about one second until the image forming operation ends, the surface potential of the photosensitive drum 21 is formed by the charging voltage control during the image forming operation, and the developing voltage is also controlled to a predetermined value. For two seconds until the image forming operation ends and the driving of the photosensitive drum 21 stops, the surface potential of the photosensitive drum 21 is formed by the charging voltage control during non-image forming operation, and the developing voltage is also controlled to a predetermined value. When the driving of the photosensitive drum 21 stops, the developing voltage control and the charging voltage control are each stopped.

[0090] The specific charging voltages in Example 1 are described below. The brush voltage, which is the first charging voltage applied during image formation operation, is set to -500 V, and the charging voltage, which is the second charging voltage, is set to -1000 V. Below, the first charging voltage and the second charging voltage during non-image formation operation in Example 1 were each changed.

[0091] (Example 1-1) In Example 1-1, the first charging voltage during non-image forming operation was −700 V, and the second charging voltage was −500 V. The surface potential of the photosensitive drum 21 when passing through the first charging portion Pa was −370, so the potential difference at the second charging portion Pb was −130 V.

[0092] (Example 1-2) In Example 1-2, the first charging voltage during non-image forming operation was −500 V, and the second charging voltage was −500 V. The surface potential of the photosensitive drum 21 when passing through the first charging portion Pa was −200, so the potential difference at the second charging portion Pb was −300 V.

[0093] (Examples 1-3) In Example 1-3, the first charging voltage during non-image forming operation was −780 V, and the second charging voltage was −1000 V. The surface potential of the photosensitive drum 21 when passing through the first charging portion Pa was −450, so the potential difference at the second charging portion Pb was −550 V.

[0094] (Examples 1 to 4) In Example 1-4, the first charging voltage during non-image forming operation was −780 V, and the second charging voltage was −800 V. The surface potential of the photosensitive drum 21 when passing through the first charging portion Pa was −450, so the potential difference at the second charging portion Pb was −350 V.

[0095] (Examples 1 to 5) In Example 1-5, the first charging voltage during non-image forming operation was −500 V, and the second charging voltage was −750 V. The surface potential of the photosensitive drum 21 when passing through the first charging portion Pa was −2000, so the potential difference at the second charging portion Pb was −550 V.

[0096] Comparative Example 1 In order to explain the effect of Example 1 in more detail, Comparative Example 1 will be given.

[0097] The configuration of Comparative Example 1 is common to that of Example 1, except for the voltage control during non-image formation, which will be described below.

[0098] [Voltage control in Comparative Example 1] 10 shows a specific voltage control example of Comparative Example 1 when printing an A4 size recording material P as an example. The charging voltage control during non-image formation in Comparative Example 1 is the same as the voltage control during image formation in Example 1, and a charging voltage of -500V is applied to the charging brush 22 as the first charging voltage. A charging voltage of -1000V is applied to the charging roller 23 as the second charging voltage. A developing voltage of -300V is applied to the developing roller 31. In other words, the voltage relationship during non-image formation in Comparative Example 1 is the same as the voltage relationship during image formation operation.

[0099] [Superiority of Example 1 over Comparative Example 1] <Durability comparison evaluation> Table 1 shows the results of a durability evaluation using the above image forming apparatus.

[0100] <Durability evaluation method> Test environment: Temperature 32.5℃, humidity 80% Evaluation items: 3,000 sheets of A4 size recording material P were passed through with 2 sheets per second intermittent operation, and after leaving it for one day, halftone images were output, and evaluations of image flow and dirt on the charging roller 23 were performed. As mentioned above, image flow is an image defect in which the toner image becomes blurred, so it can be evaluated that image flow has occurred when the density of the halftone image decreases. In other words, since it depends on the amount of discharge generated on the surface of the photosensitive drum 21, the image flow becomes worse when the total amount of discharge increases. In addition, if the potential difference between the charging roller 23 and the surface of the photosensitive drum 21 cannot be secured in the second charging section Pb, positive toner, external additives, etc. will be transferred from the surface of the charging roller 23. The level of transfer from the charging roller 23 was also confirmed.

[0101] [Table 1]

[0102] In the table, ◎ is particularly excellent, 〇 is effective in suppressing the effect and is problem-free, △ is no problem in practical use, and × is problematic in practical use. The results are discussed below.

[0103] <Discussion on the results> In the first embodiment, the total discharge amount can be suppressed more than in the first comparative example. This is because the discharge caused by the second charging voltage is controlled to be suppressed in the non-image forming operation more than in the image forming operation. That is, the potential difference formed in the second charging portion Pb is made smaller in the non-image forming operation than in the image forming operation. In the first embodiment, the discharge amount by the charging brush 22 is slightly larger than in the first embodiment and the first embodiment because the charging brush 22 is used for discharging, but this is at a level that does not cause any problems in practical use. This is because the brush voltage that causes the charging brush 22 to discharge is applied to inject charge, and as a result, the injected charge amount is also larger in the non-image forming operation than in the image forming operation. Therefore, the total discharge amount can be reduced. On the other hand, in the first comparative example, the same charging voltage as that in the image forming operation is applied, so that the discharge amount in the charging roller 23, which is the second charging member, is large, and image deletion occurs. In other words, the total discharge amount exceeds the allowable range.

[0104] Regarding Example 1, items other than discharge will be examined in detail.

[0105] In Example 1-1, the discharge amount was usable without any problem. However, since the absolute value of the second charging voltage was lower than the absolute value of the first charging voltage, the potential difference at Pb, which is the second charging portion, was -130V. Therefore, compared to other Example 1, some transfer of positive toner from the charging roller 23 was observed. This is due to the fact that the potential difference could not be secured in the area where the amount of injected charge was locally increased, as shown in FIG. 8. The result this time was caused by the fact that the test environment was a high temperature and high humidity environment in which impedance is likely to decrease. If the test environment is an environment with a relatively high impedance, such as a low temperature and low humidity environment, local injection charging hardly occurs, and transfer of positive toner from the charging roller 23 hardly occurs. In addition, from the viewpoint of Vd formation, since the absolute value is smaller than Vd during image formation operation, Vd = -370V, there are some constraints on development. Specifically, the development voltage needs to be reduced by 80V in absolute value compared to image formation operation. If the above viewpoints are taken into consideration, it is possible to effectively suppress discharge and image defects in Example 1-1 as well.

[0106] In Example 1-2, the second charging voltage is the same as the first charging voltage, and the absolute values ​​of both the brush voltage and the charging voltage are kept small, so the total discharge amount can be particularly suppressed compared to other examples. On the other hand, as in Example 1-1, the potential difference at Pb, which is the second charging portion, is -300V, which is slightly small. As a result, there are areas where the potential difference is locally small. However, this is at a level that does not cause problems in practical use. In addition, from the viewpoint of Vd formation, the absolute value is smaller than Vd during image formation operation, Vd=-200V. Therefore, the constraints on development are the greatest among other Examples 1. Specifically, the development voltage needs to be lowered by 250V in absolute value compared to the image formation operation. If attention is paid to the above viewpoints, Example 1-2 can also effectively suppress discharge and image impairment.

[0107] In Example 1-3, the second charging voltage is higher than the first charging voltage, and the second charging voltage is the same as that in the image forming operation, but the first charging voltage is higher than that in the image forming operation by that amount, so that the total discharge amount can be suppressed. As described above, by applying a brush voltage that discharges the charging brush 22 to inject charging, the injected charge amount is also greater in the non-image forming operation than in the image forming operation. Therefore, the total discharge amount can be reduced as a result. Here, in Example 1-3, the potential difference at Pb, which is the second charging portion, is -550V. From the viewpoint of the potential difference formed between the second charging voltage and the surface of the photosensitive drum 21, as shown in B of FIG. 8, in some areas where the injected charge amount is small, a slight discharge occurs locally. On the other hand, since the potential difference at Pb, which is the second charging portion, is -550V, a sufficient potential difference can be secured between the charging roller 23 and the surface of the photosensitive drum 21, so that the transfer of positive polarity toner from the charging roller 23 can be suppressed. Also, from the viewpoint of Vd formation, by setting the first charging voltage to -780V, Vd=-450V is set only by the influence of the first charging voltage, which is the same potential setting as during image formation. This allows the same potential relationship as during image formation, so there are no restrictions on control of development, etc. Therefore, in addition to the discharge suppression effect, Example 1-3 has the advantage of being easy to control and being able to sufficiently secure the potential difference at Pb, which is the second charging portion.

[0108] In Example 1-4, as in Example 1-3, the second charging voltage is higher than the first charging voltage, and the first charging voltage is higher than the image forming operation, so that the total discharge amount can be suppressed. Furthermore, in Example 1-4, the potential difference at Pb, which is the second charging portion, is set to -350V. This allows an appropriate potential difference to be secured between the charging roller 23 and the surface of the photosensitive drum 21, so that the transfer of positive toner from the charging roller 23 can be suppressed while suppressing discharge at Pb, which is the second charging portion. Also, in terms of Vd formation, as in Example 1-3, the first charging voltage is set to -780V. This allows the same potential relationship as in the image forming operation, so there are no restrictions on control of development, etc. Therefore, in addition to the discharge suppression effect, Example 1-4 has the further advantage of being easy to control, being able to appropriately secure the potential difference at Pb, which is the second charging portion, and being able to suppress discharge due to the second charging voltage.

[0109] In Example 1-5, the second charging voltage is larger than the first charging voltage, and both the first charging voltage and the second charging voltage are controlled so that no discharge occurs, so that the total discharge amount can be suppressed. In addition, the potential difference at Pb, which is the second charging portion, is -550V as in Example 1-3, so that a sufficient potential difference can be secured between the charging roller 23 and the surface of the photosensitive drum 21. On the other hand, as in Example 1-3, in a part of the region where the injected charge amount is small, some discharge occurs locally. In terms of Vd formation, since Vd=-200V, which is smaller in absolute value than Vd during image formation operation, there are some restrictions on control as in Example 1-2. However, in Example 1-5, in addition to the discharge suppression effect, there is an advantage that discharge due to the first charging voltage is suppressed and a sufficient potential difference can be secured at Pb, which is the second charging portion.

[0110] As described above, the configuration of the first embodiment has the following features, and thus can provide the above-mentioned effects.

[0111] The image forming apparatus 1 is capable of performing an image forming operation for forming an image on a recording material P, and includes a rotatable photosensitive drum 21. The image forming apparatus 1 contacts the surface of the photosensitive drum 21 to form a first charging portion Pa, and includes a charging brush 22 as a first charging member that charges the surface of the photosensitive drum 21 by applying a brush voltage as a first charging voltage to the first charging portion Pa. The image forming apparatus 1 includes a charging roller 23 that charges the surface of the photosensitive drum 21 by applying a charging voltage as a second charging voltage to the second charging portion Pb that faces the surface of the photosensitive drum 21. The image forming apparatus 1 includes an exposure unit 11 that forms an electrostatic latent image on the surface of the photosensitive drum 21 that has been charged by the charging brush 22 and the charging roller 23. The image forming apparatus 1 includes a developing roller 31 as a developing member that develops a developer on the electrostatic latent image to form a developer image on the surface of the photosensitive drum 21 in a developing portion that faces the surface of the photosensitive drum 21. The image forming apparatus 1 includes a transfer roller 12 as a transfer member that transfers the developer image to a recording material P that is a transfer material in a transfer portion that faces the surface of the photosensitive drum 21. The charging roller 23 includes a brush voltage application unit 74 as a brush power supply serving as a first charging voltage application unit that applies a brush voltage to the charging brush 22, and a charging voltage application unit 71 as a charging power supply serving as a second charging voltage application unit that applies a charging voltage to the charging roller 23. The charging roller 23 includes a control unit 202 that controls the brush voltage and the charging voltage. In the rotation direction of the photosensitive drum 21, a first charging unit Pa is formed downstream of the transfer unit and upstream of the second charging unit Pb. In the first charging unit Pa, a potential difference formed between a first surface potential formed on the surface of the photosensitive drum 21 and the brush voltage is defined as a first charging potential difference. In the second charging unit Pb, a potential difference formed between a second surface potential formed on the surface of the photosensitive drum 21 and the charging voltage is defined as a second charging potential difference. Then, the control unit 202 controls the second charging potential difference to be equal to or greater than a discharge start voltage in the image forming operation. Then, the photosensitive drum 21 is rotated, and in a state in which the brush voltage is applied, the second charging potential difference in a non-image forming operation different from the image forming operation is controlled so that the second charging potential difference in the image forming operation is smaller.

[0112] The control unit 202 may then control the absolute value of the charging voltage in the non-image forming operation so that it is smaller than the absolute value of the charging voltage in the image forming operation, and control the second charging potential difference in the non-image forming operation so that it is less than the discharge start voltage.

[0113] Moreover, it is preferable that the control unit 202 controls the charging voltage in the non-image forming operation so that the absolute value is greater than the brush voltage. The control unit 202 controls the charging voltage in the non-image forming operation so that the first charging potential difference is such that an electric field is formed so that the developer charged to the normal polarity moves from the charging brush 22 to the surface of the photosensitive drum 21. The control unit 202 controls the charging voltage in the non-image forming operation so that the second charging potential difference is such that an electric field is formed so that the developer charged to the normal polarity moves from the charging roller 23 to the surface of the photosensitive drum 21. The control unit 202 controls the absolute value of the brush voltage in the non-image forming operation so that the absolute value is greater than the brush voltage in the image forming operation. The control unit 202 controls the first charging potential difference to be greater than the second charging potential difference in the non-image forming operation.

[0114] In this embodiment, the resistance of the charging brush 22 is 1.0×10 2 Ω cm or more 1.0×10 8 The volume resistivity of the surface layer of the photosensitive drum 21 is 1.0×10 9 Ω cm or more 1.0×10 14 The electrical resistance is Ω·cm or less, and the charging brush 22 is configured to inject electric charge into the surface of the photosensitive drum 21, thereby enabling injection charging.

[0115] With the above-described configuration, it is possible to suppress discharge occurring between the photosensitive drum and the charging member. EXAMPLES

[0116] An image forming apparatus in the second embodiment will be described below. Here, a description of parts common to the first embodiment will be omitted.

[0117] [Charge Configuration] 11 shows a charging configuration in Example 2. In this example, a charging roller 25 as a first charging member is brought into contact with the photosensitive drum 21 at a peripheral position Pa, and a charging roller 23 as a second charging member is brought into contact with the photosensitive drum 21 at a peripheral position Pb, both with a predetermined pressure contact force.

[0118] A predetermined charging voltage is applied to each of the charging rollers 25 and 23, and each of them charges the surface of the photosensitive drum 21 by a contact DC charging method.

[0119] The charging roller 25 has the same structure as the charging roller 23. This embodiment is characterized in that the first discharge is performed by the charging roller 25, and the second discharge is performed by the charging roller 23. Although the charging roller 25 has the same structure as the charging roller 23, it is not limited to the same structure as long as the charging roller 25 is configured to perform discharge mainly in the first charging portion Pa and the second charging portion Pb.

[0120] [Voltage control] FIG. 12 shows a specific voltage control example of the second embodiment in the case where one A4-sized recording material P is printed as an example. The timing when an image formation command is input to the image forming apparatus 1 is set as the origin, and one second later, the driving of the photosensitive drum 21 starts. Then, for two seconds until the image forming operation starts, the surface potential of the photosensitive drum 21 is formed by the voltage control during the non-image forming operation, and the developing voltage is applied to the developing roller 31. After that, when the image forming operation starts, the charging voltage applied to the charging roller 25 and the charging roller 23 changes, and for about one second until the image forming operation ends, the surface potential of the photosensitive drum 21 is formed by the charging voltage control during the image forming operation, and the developing voltage is also controlled to a predetermined value. For two seconds until the image forming operation ends and the driving of the photosensitive drum 21 stops, the surface potential of the photosensitive drum 21 is formed by the charging voltage control during the non-image forming operation, and the developing voltage is also controlled to a predetermined value. When the driving of the photosensitive drum 21 stops, the developing voltage control and the charging voltage control are each stopped.

[0121] Next, the voltage control during the image forming operation in this embodiment will be described. During the image forming operation, the surface potential of the photosensitive drum 21 is neutralized by the pre-exposure device 24, and the surface potential immediately before the first charging portion Pa is about 0V. As shown in FIG. 12, a charging voltage of -800V is applied to the charging roller 25, and the magnitude of the first charging potential difference is -800V, so that the surface of the photosensitive drum 21 is charged to -250V by the contact DC charging method according to the characteristics of FIG. 6. The surface potential of the photosensitive drum 21 at the second charging portion Pb is charged to -250V, and a charging voltage of -1000V is applied to the charging roller 23, so that the magnitude of the second charging potential difference is -750V. At this time, according to the characteristics of FIG. 6, the surface of the photosensitive drum 21 is further charged by -200V by the contact DC charging method, and finally the surface of the photosensitive drum 21 becomes -450V. Here, a developing voltage of -300V was applied to the developing roller 31, and the back contrast was set to -150V.

[0122] Next, the voltage control during non-image formation in this embodiment will be described. Even during non-image formation, the surface potential at the first charging portion Pa of the photosensitive drum 21 is neutralized to about 0V by the pre-exposure device 24. As shown in FIG. 12, a charging voltage of -750V is applied to the charging roller 25, and the magnitude of the first charging potential difference is -750V, so that the surface of the photosensitive drum 21 is charged to -200V by the contact DC charging method according to the characteristics of FIG. 6. The surface potential at the second charging portion Pb of the photosensitive drum 21 is -200V, and a charging voltage of -500V is applied to the charging roller 23, and the magnitude of the second charging potential difference is -300V. At this time, according to the characteristics of FIG. 6, the surface of the photosensitive drum 21 is not charged by the charging roller 23, and the surface potential of the photosensitive drum 21 is finally -200V. That is, the magnitude of the second charging potential difference is smaller than the discharge start voltage Vth, and no discharge occurs, so no discharge products are generated. Furthermore, since the surface potential of the photosensitive drum 21 is higher on the toner polarity side, the fine particles adhering to the charging roller 25 and the charging roller 23 are not transferred to the photosensitive drum 21. Here, a developing voltage of -50V is applied to the developing roller 31, and the back contrast is set to -150V.

[0123] By implementing the above-mentioned control, the total amount of discharge during non-image forming operations can be reduced compared to the amount of discharge during image forming operations. Therefore, even when charging rollers are used as the first charging member and the second charging member and the surface of the photosensitive drum is formed by discharging, it is possible to suppress discharge occurring between the photosensitive drum and the charging members.

[0124] The disclosure of this embodiment includes the following configuration.

[0125] (Configuration 1) An image forming apparatus capable of performing an image forming operation for forming an image on a transfer material, A rotatable photosensitive drum; a first charging member that contacts the surface of the photosensitive drum to form a first charging section, and that charges the surface of the photosensitive drum by applying a first charging voltage to the first charging section; a second charging member that charges the surface of the photosensitive drum by applying a second charging voltage to the second charging portion that faces the surface of the photosensitive drum; an exposure unit that forms an electrostatic latent image on the surface of the photosensitive drum that is charged by the first charging member and the second charging member; a developing member that develops the electrostatic latent image with a developer in a developing section facing the surface of the photosensitive drum to form a developer image on the surface of the photosensitive drum; a transfer member that transfers the developer image to the transfer material in a transfer portion that faces the surface of the photosensitive drum; a first charging voltage application unit that applies the first charging voltage to the first charging member; a second charging voltage application unit that applies the second charging voltage to the second charging member; A control unit that controls the first charging voltage and the second charging voltage, the first charging portion is formed downstream of the transfer portion and upstream of the second charging portion in a rotation direction of the photosensitive drum, In the first charging section, a potential difference formed between a first surface potential formed on the surface of the photosensitive drum and the first charging voltage is defined as a first charging potential difference, and in the second charging section, a potential difference formed between a second surface potential formed on the surface of the photosensitive drum and the second charging voltage is defined as a second charging potential difference, The control unit is i) controlling the second charging potential difference to be equal to or higher than a discharge start voltage during the image forming operation; ii) rotating the photosensitive drum and controlling, in a state in which the first charging voltage is applied, the second charging potential difference in a non-image forming operation different from the image forming operation such that the second charging potential difference in the image forming operation is smaller; 1. An image forming apparatus comprising:

[0126] (Configuration 2) The image forming apparatus according to configuration 1, wherein the control unit controls the absolute value of the second charging voltage in the non-image forming operation so that the absolute value of the second charging voltage in the image forming operation is smaller than that of the second charging voltage in the image forming operation.

[0127] (Configuration 3) 3. The image forming apparatus according to claim 2, wherein the control unit controls the second charging potential difference to be less than a discharge start voltage during the non-image forming operation.

[0128] (Configuration 4) 2. The image forming apparatus according to claim 1, wherein the control unit controls the second charging voltage in the non-image forming operation so that the second charging voltage has an absolute value greater than that of the first charging voltage.

[0129] (Configuration 5) The image forming apparatus according to configuration 1, characterized in that the control unit controls the first charging potential difference so that an electric field is formed in which the developer charged to a normal polarity moves from the first charging member to the surface of the photosensitive drum during the non-image forming operation.

[0130] (Configuration 6) The image forming apparatus according to configuration 1, characterized in that the control unit controls the second charging potential difference so that an electric field is formed in which the developer charged to a normal polarity moves from the second charging member to the surface of the photosensitive drum during the non-image forming operation.

[0131] (Configuration 7) The image forming apparatus according to configuration 1, wherein the control unit controls the absolute value of the first charging voltage in the non-image forming operation so that the absolute value of the first charging voltage in the image forming operation is greater than the absolute value of the first charging voltage in the image forming operation.

[0132] (Configuration 8) 2. The image forming apparatus according to claim 1, wherein the control unit controls the first charging potential difference to be larger than the second charging potential difference during the non-image forming operation.

[0133] (Configuration 9) An image forming apparatus capable of performing an image forming operation for forming an image on a transfer material, A rotatable photosensitive drum; a first charging member that contacts the surface of the photosensitive drum to form a first charging section, and that charges the surface of the photosensitive drum by applying a first charging voltage to the first charging section; a second charging member that charges the surface of the photosensitive drum by applying a second charging voltage to the second charging portion that faces the surface of the photosensitive drum; an exposure unit that forms an electrostatic latent image on the surface of the photosensitive drum that is charged by the first charging member and the second charging member; a developing member that develops the electrostatic latent image with a developer in a developing section facing the surface of the photosensitive drum to form a developer image on the surface of the photosensitive drum; a first charging voltage application unit that applies the first charging voltage to the first charging member; a second charging voltage application unit that applies the second charging voltage to the second charging member; A control unit that controls the first charging voltage and the second charging voltage; In the first charging section, a potential difference formed between a first surface potential formed on the surface of the photosensitive drum and the first charging voltage is defined as a first charging potential difference, and in the second charging section, a potential difference formed between a second surface potential formed on the surface of the photosensitive drum and the second charging voltage is defined as a second charging potential difference, The control unit is i) controlling the second charging potential difference to be equal to or higher than a discharge start voltage during the image forming operation; ii) controlling the photosensitive drum to rotate so that an absolute value of the first charging voltage applied in a non-image forming operation different from the image forming operation is greater than an absolute value of the first charging voltage applied in the image forming operation; 1. An image forming apparatus comprising:

[0134] (Configuration 10) a transfer member that transfers the developer image to the transfer material in a transfer portion that faces the surface of the photosensitive drum; 10. The image forming apparatus according to claim 9, wherein the first charging portion is formed downstream of the transfer portion and upstream of the second charging portion in the rotation direction of the photosensitive drum.

[0135] (Configuration 11) 11. The image forming apparatus according to configuration 10, wherein the control unit controls the second charging potential difference in the non-image forming operation to be smaller than the second charging potential difference in the image forming operation.

[0136] (Configuration 12) 12. The image forming apparatus according to claim 11, wherein the control unit controls the second charging potential difference to be less than a discharge start voltage during the non-image forming operation.

[0137] (Configuration 13) 10. The image forming apparatus according to claim 9, wherein the control unit controls the second charging voltage in the non-image forming operation so that the second charging voltage has an absolute value greater than that of the first charging voltage.

[0138] (Configuration 14) The image forming apparatus according to configuration 9, wherein the control unit controls the first charging potential difference so that an electric field is formed in the non-image forming operation such that the developer charged to a normal polarity moves from the first charging member to the surface of the photosensitive drum.

[0139] (Configuration 15) The image forming apparatus according to configuration 9, wherein the control unit controls the second charging potential difference so that an electric field is formed in the non-image forming operation such that the developer charged to a normal polarity moves from the second charging member to the surface of the photosensitive drum.

[0140] (Configuration 16) 10. The image forming apparatus according to claim 9, wherein the control unit controls the absolute value of the first charging voltage in the non-image forming operation so that the absolute value of the first charging voltage in the image forming operation is greater than the absolute value of the first charging voltage in the image forming operation.

[0141] (Configuration 17) 10. The image forming apparatus according to claim 9, wherein the control unit controls the first charging potential difference to be larger than the second charging potential difference during the non-image forming operation. (Configuration 18) The resistance value of the first charging member is 1.0×10 2 Ω cm or more 1.0×10 8 18. The image forming apparatus according to any one of configurations 1 to 17, wherein the resistivity is Ω·cm or less.

[0142] (Configuration 19) 18. The image forming apparatus according to any one of configurations 1 to 17, wherein the first charging member is in the form of a brush.

[0143] (Configuration 20) The volume resistivity of the surface layer of the photosensitive drum is 1.0×10 9 Ω cm or more 1.0×10 14 18. The image forming apparatus according to claim 1, wherein the resistivity is Ω·cm or less. [Explanation of symbols]

[0144] 1. Image forming device 10 Image forming section 11 Scanner unit 12 Transfer roller 20 Process cartridge 21 Photosensitive drum 22 Charging Brush 23 Charging roller 30 Developing device 31 Developing roller

Claims

1. An image forming apparatus capable of performing an image forming operation for forming an image on a transfer material, A rotatable photosensitive drum; a first charging member that contacts the surface of the photosensitive drum to form a first charging portion, and that charges the surface of the photosensitive drum by applying a first charging voltage to the first charging portion; a second charging member that charges the surface of the photosensitive drum by applying a second charging voltage to the second charging portion that faces the surface of the photosensitive drum; an exposure unit for forming an electrostatic latent image on the surface of the photosensitive drum charged by the first charging member and the second charging member; a developing member that develops the electrostatic latent image with a developer in a developing section facing the surface of the photosensitive drum to form a developer image on the surface of the photosensitive drum; a transfer member that transfers the developer image to the transfer material in a transfer portion that faces the surface of the photosensitive drum; a first charging voltage application unit that applies the first charging voltage to the first charging member; a second charging voltage application unit that applies the second charging voltage to the second charging member; a control unit that controls the first charging voltage and the second charging voltage, the first charging portion is formed downstream of the transfer portion and upstream of the second charging portion in a rotation direction of the photosensitive drum, In the first charging section, a potential difference formed between a first surface potential formed on the surface of the photosensitive drum and the first charging voltage is defined as a first charging potential difference, and in the second charging section, a potential difference formed between a second surface potential formed on the surface of the photosensitive drum and the second charging voltage is defined as a second charging potential difference, The control unit is i) controlling the second charging potential difference to be equal to or higher than a discharge start voltage during the image forming operation; ii) rotating the photosensitive drum, and controlling the second charging potential difference in a non-image forming operation different from the image forming operation in a state in which the first charging voltage is applied, so that the second charging potential difference in the image forming operation becomes smaller; 1. An image forming apparatus comprising:

2. 2 . The image forming apparatus according to claim 1 , wherein the control section controls the absolute value of the second charging voltage in the non-image forming operation so that the absolute value of the second charging voltage in the image forming operation is smaller than that of the second charging voltage in the image forming operation.

3. 3. The image forming apparatus according to claim 2, wherein the control unit controls the second charging potential difference to be less than a discharge start voltage during the non-image forming operation.

4. 2. The image forming apparatus according to claim 1, wherein the control section controls the second charging voltage in the non-image forming operation so that the absolute value of the second charging voltage is greater than that of the first charging voltage.

5. The image forming apparatus according to claim 1, wherein the control unit controls the first charging potential difference so that an electric field is formed in the non-image forming operation such that the developer charged to a normal polarity moves from the first charging member to the surface of the photosensitive drum.

6. The image forming apparatus according to claim 1, wherein the control unit controls the second charging potential difference so that an electric field is formed in the non-image forming operation such that the developer charged to a normal polarity moves from the second charging member to the surface of the photosensitive drum.

7. 2 . The image forming apparatus according to claim 1 , wherein the control section controls the absolute value of the first charging voltage in the non-image forming operation so that the absolute value of the first charging voltage in the image forming operation is greater than that of the first charging voltage in the image forming operation.

8. 2. The image forming apparatus according to claim 1, wherein the control unit controls the first charging potential difference to be larger than the second charging potential difference during the non-image forming operation.

9. An image forming apparatus capable of performing an image forming operation for forming an image on a transfer material, A rotatable photosensitive drum; a first charging member that contacts the surface of the photosensitive drum to form a first charging portion, and that charges the surface of the photosensitive drum by applying a first charging voltage to the first charging portion; a second charging member that charges the surface of the photosensitive drum by applying a second charging voltage to the second charging portion that faces the surface of the photosensitive drum; an exposure unit for forming an electrostatic latent image on the surface of the photosensitive drum charged by the first charging member and the second charging member; a developing member that develops the electrostatic latent image with a developer in a developing section facing the surface of the photosensitive drum to form a developer image on the surface of the photosensitive drum; a first charging voltage application unit that applies the first charging voltage to the first charging member; a second charging voltage application unit that applies the second charging voltage to the second charging member; A control unit that controls the first charging voltage and the second charging voltage; In the first charging section, a potential difference formed between a first surface potential formed on the surface of the photosensitive drum and the first charging voltage is defined as a first charging potential difference, and in the second charging section, a potential difference formed between a second surface potential formed on the surface of the photosensitive drum and the second charging voltage is defined as a second charging potential difference, The control unit is i) controlling the second charging potential difference to be equal to or higher than a discharge start voltage during the image forming operation; ii) controlling the photosensitive drum to rotate so that an absolute value of the first charging voltage applied in a non-image forming operation different from the image forming operation is greater than an absolute value of the first charging voltage applied in the image forming operation; 1. An image forming apparatus comprising:

10. a transfer member that transfers the developer image to the transfer material in a transfer portion that faces the surface of the photosensitive drum; 10. The image forming apparatus according to claim 9, wherein the first charging portion is formed downstream of the transfer portion and upstream of the second charging portion in the rotation direction of the photosensitive drum.

11. 11. The image forming apparatus according to claim 10, wherein the control section controls the second charging potential difference in the non-image forming operation to be smaller than the second charging potential difference in the image forming operation.

12. 12. The image forming apparatus according to claim 11, wherein the control unit controls the second charging potential difference to be less than a discharge start voltage during the non-image forming operation.

13. 10. The image forming apparatus according to claim 9, wherein the control section controls the second charging voltage in the non-image forming operation so that the absolute value of the second charging voltage is greater than that of the first charging voltage.

14. The image forming apparatus according to claim 9, wherein the control unit controls the first charging potential difference so that an electric field is formed in the non-image forming operation such that the developer charged to a normal polarity moves from the first charging member to the surface of the photosensitive drum.

15. The image forming apparatus according to claim 9, wherein the control unit controls the second charging potential difference so that an electric field is formed in the non-image forming operation such that the developer charged to a normal polarity moves from the second charging member to the surface of the photosensitive drum.

16. 10. The image forming apparatus according to claim 9, wherein the control section controls the absolute value of the first charging voltage in the non-image forming operation so that the absolute value of the first charging voltage in the image forming operation is greater than that of the first charging voltage in the image forming operation.

17. 10. The image forming apparatus according to claim 9, wherein the control unit controls the first charging potential difference to be larger than the second charging potential difference during the non-image forming operation.

18. The resistance value of the first charging member is 1.0×10 ２ Ω・cm or more 1.0×10 ８ 18. The image forming apparatus according to claim 1, wherein the resistivity is Ω·cm or less.

19. 18. The image forming apparatus according to claim 1, wherein the first charging member is in the form of a brush.

20. The volume resistivity of the surface layer of the photosensitive drum is 1.0×10 ９ Ω・cm or more 1.0×10 １４ 18. The image forming apparatus according to claim 1, wherein the resistivity is Ω·cm or less.

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