Image forming apparatus

Abstract

To provide a technique that can maintain good image quality in a long-term use of an image forming apparatus.SOLUTION: In an image forming apparatus, an area, of a surface of an image carrier, where a second potential difference is formed which generates an electrostatic force to move developer electrified to have a polarity opposite to a normal electrification polarity of the developer from the image carrier to a developer carrier, in a second operation to move a developer from the surface of the image carrier to the developer carrier, includes an area where a first potential difference is formed which generates an electrostatic force to move the developer electrified to have the polarity opposite to the normal electrification polarity of the developer from an electrifying member to the image carrier, in a first operation to move a developer from a surface of the electrifying member to the surface of the image carrier.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus for forming an image on a recording material. [Background technology]

[0002] One of the image defects of image forming devices that use electrophotographic recording is called fogging. Fogging is a phenomenon in which toner as developer adheres to non-image areas where the surface potential of the photosensitive drum has a higher potential toward the normal polarity of the toner than the image area where an electrostatic latent image is formed on the surface of the photosensitive drum as an image carrier. It is known that the amount of fogging is largely influenced by the charge amount of the developer, and in order to suppress fogging, it is necessary to properly control the charge amount of the normal polarity of the developer.

[0003] As a means for increasing the chargeability of a toner, a technology has been reported that uses inorganic particles that exhibit a charge polarity opposite to that of the toner as an external additive to stabilize the chargeability. For example, as in Patent Document 1, a technology is known that uses a titanate compound, such as strontium titanate, as an external additive to stabilize the chargeability. By using the configuration disclosed in Patent Document 1, the occurrence of fogging can be suppressed to a certain extent.

[0004] However, when an image forming apparatus is used for a long period of time, the fogging may worsen, causing image problems. This is because the external additives migrate to non-image forming areas of the photosensitive drum during image formation, and the amount of external additives in the developing container decreases, causing a decrease in the amount of charge on the toner. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2001-290302 A Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide a technique capable of maintaining good image quality even when an image forming apparatus is used for a long period of time. [Means for solving the problem]

[0007] In order to solve the above problems, the image forming apparatus of the present invention comprises: A rotating image carrier; a charging member that rotates while contacting the image carrier to form a charging portion, the charging member charging the surface of the image carrier in the charging portion; a charging voltage application unit that applies a charging voltage to the charging member; an exposure unit for exposing the surface of the image carrier charged by the charging member; a developer carrier that carries a developer and faces the image carrier in a developing section to supply the developer to a surface of the image carrier; a developing voltage applying section that applies a developing voltage to the developer carrying member; a transfer member that transfers the developer supplied to the surface of the image carrier to a transfer target in a transfer section; a transfer voltage application unit that applies a transfer voltage to the transfer member; a cleaning member that contacts the image carrier in a cleaning section that is formed downstream of the transfer section and upstream of the charging section in the rotation direction of the image carrier; a control unit that controls the charging voltage application unit, the developing voltage application unit, and the transfer voltage application unit; Equipped with The control unit is a first operation for moving a developer from a surface of the charging member to a surface of the image carrier by controlling the charging voltage application unit; a second operation for moving the developer from the surface of the image carrier to the developer carrier by controlling the charging voltage application unit and the developing voltage application unit; a third operation for supplying developer to the cleaning unit by controlling the charging voltage application unit, the developing voltage application unit, and the transfer voltage application unit; It is possible to execute In the first operation, A first potential difference is formed between the charging member and the image carrier, which generates an electrostatic force that causes the developer charged to a polarity opposite to the normal charging polarity of the developer to move from the charging member to the image carrier. Controlling the charging voltage application unit; In the second operation, a second potential difference is formed between the image carrier and the developer carrier, which generates an electrostatic force that causes the developer charged with a polarity opposite to the normal charging polarity of the developer to move from the image carrier to the developer carrier; Controlling the developing voltage application unit; The region of the surface of the image carrier where the second potential difference is formed in the second operation includes a region where the first potential difference is formed in the first operation. Effect of the Invention

[0008] According to the present invention, it is possible to maintain good image quality even when the image forming apparatus is used for a long period of time. [Brief description of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view of an image forming apparatus according to a first embodiment of the present invention; [Diagram 2] Schematic diagram of the developing and recovery sequence operation in the first embodiment [Diagram 3] Sequence chart of the developing and recovery sequence in the first embodiment [Figure 4] Schematic diagram of a process unit at (t1) in Example 1 [Diagram 5] Schematic diagram of the process unit at (t2) in Example 1 [Figure 6] Schematic diagram of the process unit at (t3) in Example 1 [Figure 7] Schematic diagram of the process unit at (t4) in Example 1 [Figure 8] Sequence chart of developing and collecting sequence in the second embodiment [Figure 9] Sequence chart of developing and collecting sequence in the third embodiment [Figure 10] Sequence chart of developing and collecting sequence in the fourth embodiment [Figure 11] Sequence chart of developing and collecting sequence operation in Comparative Example 1 [Figure 12] Sequence chart of developing and collecting sequence operation in Comparative Example 2 [Figure 13] System configuration diagram of an image forming apparatus [Figure 14] Schematic diagram of voltage application configuration [Figure 15] FIG. 1 is a diagram illustrating the configuration of a scanner unit according to a first embodiment of the present invention; [Figure 16] A diagram showing the relationship between image height and partial magnification [Figure 17] Image of exposure on the photosensitive drum surface before and after correction processing DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] <Embodiment 1> Hereinafter, the embodiment of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, etc. of the components described in the embodiment may vary. The relative arrangement of these components and components should be appropriately changed depending on the configuration of the device to which the invention is applied and various conditions, and therefore the scope of the invention is not limited to the following embodiments.

[0011] <Example 1> (Overall configuration of image forming apparatus) The overall configuration of an image forming apparatus will be described with reference to Fig. 1. Fig. 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus according to an embodiment of the present invention, and each component is shown in a simplified manner.

[0012] Here, the image forming apparatus to which the present invention is supposed to be applied is typically an image forming apparatus that utilizes an electrophotographic recording method, such as a laser printer, a copying machine, or a facsimile.

[0013] In this embodiment, a so-called monochrome type image forming apparatus, that is, an image forming apparatus having a single image forming unit, is exemplified, but the image forming apparatus to which the present invention can be applied is not limited to this. For example, the present invention is also suitably applied to a so-called full-color type image forming apparatus, that is, an image forming apparatus having a plurality of image forming units. The difference in device configuration between a monochrome image forming apparatus and a color image forming apparatus is, first, that the color and number of colors of the image formed are different. That is, a monochrome image forming apparatus can form an image of a single color (typically black), but a color image forming apparatus can form an image of any color by superimposing a plurality of developer images of different colors on each other. In addition, in an intermediate transfer type color image forming apparatus, a developer image is first transferred from a photosensitive body as an image carrier to an intermediate transfer body (which can also be called a second image carrier) as a first transferee, and then transferred from the intermediate transfer body to a recording material as a second transferee. In contrast, in a monochrome image forming apparatus or a direct transfer type color image forming apparatus, a developer image is transferred directly from a photosensitive body to a recording material.

[0014] The image forming apparatus 100 according to this embodiment includes a process cartridge 10. The process cartridge 10 includes a photosensitive drum 1 as an image carrier. Around the photosensitive drum 1 of the process cartridge 10, a charging roller 2 as a charging member for charging the surface of the photosensitive drum 1, and a developing roller 41 as a developer carrier for developing an electrostatic latent image formed on the surface of the photosensitive drum 1 with a developer 44 are provided. The process cartridge 10 further includes a developing blade 43 as a developer regulating member for regulating and charging the toner on the developing roller 41, and a supply roller (toner supply member) 42 for supplying and peeling off the toner to the developing roller 41. The electrostatic latent image formed on the developing roller 41 is developed into a toner image (developer image) by the toner that holds a normal charge.

[0015] The image forming apparatus 100 is equipped with a transfer roller 5 as a transfer member that contacts the photosensitive drum 1 and transfers toner to a recording material P, and a scanner unit (laser exposure unit) 3 as an exposure section for forming an electrostatic latent image corresponding to image data on the charged photosensitive drum 1.

[0016] The image forming apparatus 100 is equipped with a power supply (see FIG. 13) for applying a predetermined voltage to each of the charging roller 2, the developing roller 41, the developing blade 43, the supply roller 42, and the transfer roller 5.

[0017] The photosensitive drum 1 is a cylindrically shaped photosensitive member. The photosensitive drum 1 of this embodiment has a photosensitive layer formed of a negatively charged organic photosensitive member on a drum-shaped base body formed of aluminum. The photosensitive drum 1 has a diameter of 24 mm and is driven to rotate in a predetermined direction (clockwise direction in the figure) at a predetermined process speed by a motor. The machine 1 is rotated at a process speed of 260 mm / sec.

[0018] The charging roller 2 has a diameter of φ8.5 mm and contacts the photosensitive drum 1 with a predetermined pressure to form a charging portion. A desired charging voltage is applied to the charging roller 2 by a high-voltage charging power supply, thereby uniformly charging the surface of the photosensitive drum 1 to a predetermined potential. In this embodiment, the photosensitive drum 1 is negatively charged by the charging roller 2.

[0019] The scanner unit 3 as an exposure means uses a polygon mirror to irradiate the photosensitive drum 1 with laser light corresponding to image information input from an external device or a reading device, thereby scanning and exposing the surface of the photosensitive drum 1. This exposure forms an electrostatic latent image corresponding to the image information on the surface of the photosensitive drum 1. Note that the scanner unit 3 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 1.

[0020] The developing device section of the process cartridge 10 includes a developing roller 41 as a developer carrier that carries a developer, a developing container 4 that is a frame that constitutes a developer storage section, and a supply roller 42 that can supply developer 44 to the developing roller 41. The developing roller 41 and the supply roller 42 are rotatably supported by the developing container 4. The developing roller 41 has a diameter of φ10 mm and is disposed at the opening of the developing container 4 so as to face the photosensitive drum 1. The supply roller 42 rotatably contacts the developing roller 41, and the toner contained in the developing container 4 as the developer 44 is applied to the surface of the developing roller 41 by the supply roller 42. Note that the supply roller 42 is not necessarily required as long as the developing roller 41 is configured to be sufficiently supplied with toner.

[0021] The process cartridge 10 of this embodiment uses a contact development method as a development method. That is, the toner layer carried by the development roller 41 comes into contact with the photosensitive drum 1 in a development section (development area) where the photosensitive drum 1 and the development roller 41 face each other. A development voltage is applied to the development roller 41 by a development high-voltage power supply. Under the development voltage, the toner carried by the development roller 41 is transferred from the development roller 41 to the drum surface according to the potential distribution on the surface of the photosensitive drum 1, so that the electrostatic latent image is developed into a toner image. Note that this embodiment employs a reversal development method. That is, the toner image is formed by adhering to the surface area of ​​the photosensitive drum 1, which is charged in a charging process and then exposed in an exposure process, where the charge amount is attenuated.

[0022] In this embodiment, a toner having a particle size of 7 μm and a normal negative charge polarity is used. As an example of the toner in this embodiment, a polymerized toner produced by a polymerization method is used. The toner in this embodiment does not contain a magnetic component, and is a so-called non-magnetic one-component developer in which the toner is carried on the developing roller 31 mainly by intermolecular forces and electrostatic forces (image forces). However, a one-component developer containing a magnetic component may also be used.

[0023] In addition to the toner particles, the one-component developer may contain additives as external additives for the purpose of modifying the surface properties in order to adjust the fluidity and charging performance of the toner. In order to modify the surface properties of the toner, inorganic salts can be formed on the toner surface and used. Examples of inorganic salts formed on the toner surface used in this embodiment include silica, alumina, titanium oxide, aluminum oxide, barium titanate, magnesium titanate, calcium titanate, and strontium titanate. Alternatively, zinc oxide, tin oxide, silica sand, clay, mica, wollastonite, diatomaceous earth, chromium oxide, cerium oxide, red ocher, antimony trioxide, and magnesium oxide can also be used. Alternatively, zirconium oxide, barium sulfate, barium carbonate, calcium carbonate, silicon carbide, silicon nitride, and the like can also be used. In the following description, these external additives are used as additives having the opposite polarity to the toner having the normal polarity of negative charging property. An external additive having a certain positive charging property may be described as a positive external additive. A two-component developer consisting of a non-magnetic toner and a magnetic carrier may also be used as the developer. When a magnetic developer is used, a cylindrical developing sleeve with a magnet disposed inside is used as the developer carrier. That is, the developer may be formed of only toner particles, or may be formed of toner particles, additives, and carriers, and the developer including these various forms may be described as toner 44 in the following description.

[0024] The developing container 4 is provided with an agitating member 45 as an agitating means disposed therein. The agitating member is driven to rotate by a motor 250 shown in FIG. 13 to agitate the toner in the developing container 4 and to send the toner toward the developing roller 41 and the supply roller 42. The agitating member 45 is not limited to a rotating type. For example, an agitating member in a swinging type may be used.

[0025] The developing roller 41 is disposed in contact with and facing the photosensitive drum 1 at a developing portion, and is driven to rotate at a predetermined speed. The developing roller 41 in this embodiment is always in contact with the photosensitive drum 1 even when not forming an image while the process cartridge 10 is mounted in the main body of the image forming apparatus 100. In other words, the image forming apparatus 100 is not provided with a contact / separation mechanism for separating the developing roller 41 from the photosensitive drum 1.

[0026] The supply roller 42 rotates in contact with the developing roller 31 to supply the toner 44. The developing blade 43 is an elastic member, and is arranged in contact with the developing roller 41 while being bent against its elasticity. The stirring member 45 rotates at a predetermined speed in conjunction with the rotation of the developing roller 41 to stir the developer 44 in the developing container 4 and to supply the toner 44 to the supply roller 42.

[0027] Toner 44 is carried on the developing roller 41, formed into a predetermined layer thickness by the developing blade 43, and conveyed to a developing section facing the photosensitive drum 1. In this embodiment, the developing roller 41 rotates at a speed 1.4 times faster than the surface movement speed of the photosensitive drum 1. A predetermined developing voltage is applied to the developing roller 41 by a developing voltage application section provided in the image forming apparatus 100, thereby developing the electrostatic latent image. A DC high voltage power supply was used as the output source of the developing voltage.

[0028] The fixing unit 6 is of a thermal fixing type that fixes an image by heating and melting the toner on the recording material P. The fixing unit 6 includes a fixing film, a fixing heater such as a ceramic heater that heats the fixing film, a thermistor that measures the temperature of the fixing heater, and a pressure roller that is pressed against the fixing film.

[0029] <Description of Image Forming Apparatus System Configuration> The system configuration of the image forming apparatus 100 according to this embodiment will be described with reference to FIG. 13. The controller 201 is capable of communicating with the host computer 200 and the engine control unit 202. When print data is input from the host computer 200, the controller 201 develops the print data and converts it into image data for image formation. Then, the controller 201 generates a video signal for exposure to perform exposure based on the image data. When the controller 201 completes the generation of the video signal, it instructs the video interface unit 210 of the engine control unit 202 to start image formation by a command. After that, when the CPU 220 receives an instruction to start image formation from the video interface unit 210, it starts various actuators such as the main motor 250 and starts preparation for image formation. When the engine control unit 202 completes preparation for image formation, it starts outputting a / BD signal, which is a reference timing for outputting a video signal, to the controller 201, and sequentially executes the above-mentioned image formation operations.

[0030] During image formation operation, the engine control unit 202 starts the main motor 250 as a drive source, and controls the conveyance of the recording material P by driving each roller involved in the conveyance of the recording material P. The rollers involved in the conveyance of the recording material P include a feed roller 102, a conveyance roller 103, a registration roller 104, a transfer roller 106, and a discharge roller 110. The registration sensor 105 measures the paper interval during continuous paper passing (when printing is continuously performed on a plurality of recording materials P) based on the detection timing of the leading end and trailing end of the recording material P accompanying the conveyance of the recording material P. The engine control unit 202 determines the next paper feed timing from, for example, the paper length and the paper interval, and drives the paper feed solenoid 113 at that paper feed timing to feed the next recording material P.

[0031] The voltage control section 230 is configured to be able to control the application of voltages to the charging roller 2, the developing roller 41, the transfer roller 5, the supply roller 42, and the developing blade 43. As shown in FIG. 14, the voltage control section 230 is configured to be able to apply a negative voltage to the charging roller 2 from a high-voltage power source CP for applying a charging voltage (charging voltage application section). The voltage control section 230 is also configured to be able to apply a negative voltage to the developing roller 41 from a high-voltage power source DP for applying a developing voltage (developing voltage application section). The voltage control section 230 is also configured to be able to apply a positive voltage to the transfer roller 5 from a high-voltage power source TPp for applying a transfer voltage (positive voltage application section) and to apply a negative voltage to the transfer roller 5 from a power source TPn for applying a non-transfer voltage (negative voltage application section) (transfer voltage application section). The polarities of these voltages are based on the premise that the normal charging polarity of the toner is negative. Therefore, it goes without saying that when the normal charging polarity of the toner is positive, the polarities of the above-mentioned voltages are reversed.

[0032] During charging roller cleaning, a cleaning control means 231 applies a voltage of positive polarity and a voltage of negative polarity with respect to the charging potential of the photosensitive drum 1 to the charging roller 2 at a predetermined timing. By applying a voltage of positive polarity and a voltage of negative polarity to the charging roller 2, respectively, deposits (including external additives of toner, etc.) on the charging roller 2 are transferred to the photosensitive drum 1 regardless of the charging polarity, thereby performing cleaning.

[0033] In this embodiment, the charging voltage application unit is configured to include only a power source CP that applies a negative voltage. Therefore, application of a positive voltage to the charging roller 2 with respect to the charging potential of the photosensitive drum 1 is substituted by not applying a voltage to the charging roller 2. In other words, by setting the potential of the charging roller 2 to 0 V, the potential of the charging roller 2 is controlled to the positive side with respect to the negative surface potential of the photosensitive drum 1. Note that a positive voltage application configuration may be added to the charging voltage application unit.

[0034] In the following explanation, in order to simplify the explanation, the time required for the voltage control unit 230 to rise and fall when applying and stopping the voltage application to the charging roller 2, the developing roller 41, and the transfer roller 5 will not be taken into consideration.

[0035] The exposure control unit 240 is configured to perform settings for the scanner unit 3 and expose the photosensitive drum 1 with a predetermined amount of light. During toner purging, the discharge control means 241 exposes the photosensitive drum 1 with a predetermined amount of light and timing to form a toner image, which can then be sent to the cleaning blade 7 as a cleaning member.

[0036] The positional relationship of each member around the circumferential surface of the rotating photosensitive drum 1 will be described. Around the circumferential surface of the rotating photosensitive drum 1, the position where the photosensitive drum 1 and the charging roller 2 come into contact is defined as a charging position (charging section a). With respect to this charging position, downstream in the direction of rotation of the photosensitive drum 1 around the circumferential surface of the photosensitive drum 1, an exposure position (scanning exposure section b) where laser light emitted from the scanner unit 3 is irradiated is formed. With respect to this exposure position, A development nip portion (developing portion c) where the photosensitive drum 1 and the developing roller 41 come into contact is formed downstream in the rotation direction of the roller 1 (this portion is an opposing portion in a non-contact development type device). A transfer nip portion where the photosensitive drum 1 and the transfer roller 5 come into contact is formed downstream in the rotation direction of the photosensitive drum 1 around the circumferential surface of the photosensitive drum 1 from the position where this development nip portion is formed (developing position). A contact portion (cleaning position, cleaning portion) where the cleaning blade 7 comes into contact with the photosensitive drum 1 is formed downstream in the rotation direction of the photosensitive drum 1 around the circumferential surface of the photosensitive drum 1 from the position where this transfer nip portion is formed (transfer position) and upstream of the charging position.

[0037] The image forming operation of the image forming apparatus 100 will be described. When an image forming command is input to the image forming apparatus 100, an image forming process is started by the image forming unit based on image information input from an external computer 200 or a reading device connected to the image forming apparatus 100. By driving the paper feed solenoid 113, the top sheet of a stack of recording materials P is fed from a cassette by a paper feed roller 102, and is conveyed by a conveyance roller 103 and a registration roller 104. The leading end and trailing end of the conveyed recording material P are detected by a registration sensor 105 installed on the conveyance path. The scanner unit 3 irradiates a laser beam toward the photosensitive drum 1 based on the input image information. At this time, the photosensitive drum 1 is charged in advance by the charging roller 2, and an electrostatic latent image is formed on the photosensitive drum 1 by irradiating the laser beam. Thereafter, the electrostatic latent image is developed by a developing roller 41, and a toner image is formed on the photosensitive drum 1.

[0038] A transfer voltage is applied to the transfer roller 5 as a transfer means from a transfer high voltage power supply TPp, and the toner image carried on the photosensitive drum 1 is transferred to the recording material P being transported by the registration roller pair. The recording material P with the transferred toner image is transported to the fixing section 6, where the toner image is heated and pressurized as it passes through a nip between the fixing film and pressure roller of the fixing section 6. This causes the toner particles to melt and then adhere, thereby fixing the toner image to the recording material P. The recording material P that has passed through the fixing section 6 is detected by a fixing paper discharge sensor 109, and is discharged to the outside (outside the machine) of the image forming apparatus 100 by a paper discharge roller 110 as a discharge means, and is loaded onto a discharge tray as a stacking section formed on the top of the printer main body.

[0039] Next, the potential relationship around the photosensitive drum 1 in the image forming process of this embodiment will be described.

[0040] 1. Potential relationship around the photosensitive drum In this embodiment, the surface of the photosensitive drum 1 is charged to a uniform charging potential Vd (dark area potential: -550V) by the charging roller 21 to which a charging voltage of -1140V is applied, and is exposed for image formation, and the exposure amount and exposure area are determined according to the image signal. The image forming section is exposed by the scanner unit 3, and is adjusted to a post-exposure potential Vl (light area potential: -170V), which is the image area potential. The exposure amount E0 that forms this Vl is 0.24μJ / cm 2 It was decided.

[0041] Here, the scanner unit 3 in this embodiment will be described. Fig. 15 is a configuration diagram of the scanner unit 3 in this embodiment, Fig. 15(a) shows a cross-sectional view in the main scanning direction, and Fig. 15(b) shows a cross-sectional view in the sub-scanning direction. The main scanning direction is a direction parallel to the surface of the photosensitive drum 1 and perpendicular to the moving direction of the surface of the photosensitive drum 1. The sub-scanning direction is the moving direction of the surface of the photosensitive drum 1.

[0042] The laser light 208 emitted by the light source 401 is shaped into an elliptical shape by the aperture stop 402 and enters the coupling lens 403. The laser light 208 that passes through the coupling lens 403 is converted into substantially parallel light and enters the anamorphic lens 404. Note that substantially parallel light includes weakly convergent light and weakly divergent light. The anamorphic lens 404 has a positive refractive power in the main scanning cross section, and converts the incident light beam into convergent light in the main scanning cross section. Also, the anamorphic lens 404 condenses the light beam in the vicinity of a reflecting surface 405a of a deflector (polygon mirror) 405 in the sub-scanning cross section, and forms a long line image in the main scanning direction.

[0043] Then, the light beam passing through the anamorphic lens 404 is reflected by a reflecting surface 405a of a deflector 405. The laser beam 208 reflected by the reflecting surface 405a passes through an imaging lens 406 and is imaged on the surface of the photosensitive drum 1 to form a predetermined spot-shaped image (hereinafter, referred to as a spot). By rotating the deflector 405 in the direction of the arrow Ao at a constant angular velocity by a driving unit (not shown), the spot moves in the main scanning direction on a scanned surface 407 of the photosensitive drum 1, and an electrostatic latent image is formed on the scanned surface 407.

[0044] A beam detect (hereinafter, abbreviated as BD) sensor 409 and a BD lens 408 constitute a synchronization optical system that determines the timing of writing an electrostatic latent image on a scanned surface 407. The laser light 208 that passes through the BD lens 408 is incident on and detected by the BD sensor 409 that includes a photodiode. The writing timing is controlled based on the timing at which the laser light 208 is detected by the BD sensor 409. The light source 401 in this embodiment has one light emitting unit, but the light source 401 may also have a plurality of light emitting units that can be independently controlled for light emission.

[0045] 15, the imaging lens 406 has two optical surfaces (lens surfaces), an incident surface 406a and an exit surface 406b. The imaging lens 406 is configured so that the light beam deflected by the reflecting surface 405a scans the surface 407 to be scanned with desired scanning characteristics in the main scanning cross section. The imaging lens 406 is also configured so that the spot of the laser light 208 on the surface 407 to be scanned has a desired shape.

[0046] The imaging lens 406 of this embodiment does not have so-called fθ characteristics. In other words, when the deflector 405 rotates at a constant angular velocity, it does not have a scanning characteristic that moves the spot of the light beam passing through the imaging lens 406 at a constant speed on the scanned surface 407. In this way, by using the imaging lens 406 that does not have the fθ characteristics, it is possible to arrange the imaging lens 406 close to the deflector 405. In other words, the deflector 405 can be arranged at a position where the distance D1 shown in FIG. 15 is small. In addition, the imaging lens 406 that does not have the fθ characteristics can be made smaller in the main scanning direction (width LW) and the optical axis direction (thickness LT) than the imaging lens that has the fθ characteristics as shown in FIG. 15, thereby realizing a compact housing of the optical scanning device 400. In addition, in the case of a lens that has the fθ characteristics, there may be a sharp change in the shape of the entrance surface and the exit surface of the lens when viewed in the main scanning cross section. Therefore, if there is a restriction on the shape, there is a possibility that good imaging performance cannot be obtained. In contrast, the imaging lens 406 does not have fθ characteristics, and therefore there is little abrupt change in the shapes of the entrance surface and exit surface of the lens when viewed in the main scanning section, thereby achieving good imaging performance.

[0047] The scanning characteristic of the imaging lens 406 according to this embodiment is expressed by the following formula (1).

[0048]

number

[0049] In formula (1), θ is the scanning angle (scanning angle of view) by the deflector 405, Y [mm] is the light-converging position (image height) in the main scanning direction on the scanned surface 407 of the light beam, K [mm] is the imaging coefficient at the axial image height, and B is the coefficient (scanning characteristic coefficient) that determines the scanning characteristics of the imaging lens 406. Note that in this embodiment, the axial image height refers to the image height on the optical axis (Y=0=Ymin), and corresponds to the scanning angle θ=0. In other words, in this embodiment, the longitudinal axis of the photosensitive drum 1, which is the main scanning direction, is The image height is located at the center of the image on the surface to be scanned 407. The off-axis image height refers to the image height (Y≠0) outside the central optical axis (when the scanning angle θ=0), and corresponds to the scanning angle θ≠0. The most off-axis image height refers to the image height (Y=+Ymax, -Ymax) when the scanning angle θ is maximum (maximum scanning angle of view). The scanning width W, which is the width in the main scanning direction of a predetermined area (scanning area) on the scanned surface 407 where a latent image can be formed, is expressed as W=|+Ymax|+|-Ymax|. In other words, the center of the predetermined area on the photosensitive drum 1 is the on-axis image height, and the end is the most off-axis image height.

[0050] Here, the imaging coefficient K is a coefficient equivalent to f in the scanning characteristic (fθ characteristic) Y=fθ when parallel light is incident on the imaging lens 406. In other words, the imaging coefficient K is a coefficient for making the light collection position Y and the scanning angle θ proportional to each other, similar to the fθ characteristic, when a light beam other than parallel light is incident on the imaging lens 406.

[0051] To add a bit more about the scanning characteristic coefficient, when B=0, formula (1) gives Y=Kθ, which corresponds to the scanning characteristic Y=fθ of an imaging lens used in a conventional optical scanning device. When B=1, formula (1) gives Y=Ktanθ, which corresponds to the projection characteristic Y=ftanθ of a lens used in an imaging device (generally a camera). In other words, by setting the scanning characteristic coefficient B in the range of 0≦B≦1 in formula (1), it is possible to obtain a scanning characteristic between the projection characteristic Y=ftanθ and the fθ characteristic Y=fθ.

[0052] Here, by differentiating equation (1) with respect to the scanning angle θ, the scanning speed of the light beam on the scanned surface 407 with respect to the scanning angle θ can be obtained as shown in equation (2).

[0053]

number

[0054] Furthermore, by transforming equation (2), we obtain equation (3).

[0055]

number

[0056] Equation (3) expresses the deviation (partial magnification) of the scanning speed of each off-axis image height from the scanning speed of the on-axis image height. In the optical scanning device 400 according to this embodiment, the scanning speed of the light beam differs between the on-axis image height and the off-axis image height except when B=0.

[0057] FIG. 16 shows the relationship between the image height and the partial magnification when the scanning position on the scanned surface 407 according to this embodiment is fitted with the characteristic of Y=Kθ. In this embodiment, the scanning characteristic shown in formula (1) is given to the imaging lens 406, so that the scanning speed gradually increases from the on-axis image height to the off-axis image height, as shown in FIG. 16, and the partial magnification increases. For example, a partial magnification of 30% means that when light is irradiated for a unit time, the irradiation length in the main scanning direction on the scanned surface 407 is 1.3 times the on-axis image height at the off-axis image height. In the example of FIG. 16, the scanning speed at the on-axis image height is the lowest, and the scanning speed increases as the absolute value of the image height increases. Therefore, if the pixel width in the main scanning direction is determined at a constant time interval determined by the clock cycle, the pixel density will differ between the on-axis image height and the off-axis image height. Therefore, in this embodiment, partial magnification correction is performed. Specifically, the clock frequency is adjusted according to the image height so that the pixel width is approximately constant regardless of the image height.

[0058] In this embodiment, as shown in FIG. 15, the point where the laser light 208 on the deflector 405 is reflected The distance from the to-be-scanned surface is D2=130 mm, W=216 mm, and the distance to the most off-axis image height is W / 2=108 mm. Therefore, as shown in FIG. 16, at the most off-axis image height in this embodiment, the partial magnification Dmax=30%. At this time, B=0.734. The maximum value of the scanning angle θ is 40°.

[0059] Also, the time required to scan a unit length when the image height on the scanned surface 407 is near the most off-axis image height is shorter than the time required to scan a unit length when the image height is near the on-axis image height. This means that, when the emission luminance of the light source 401 is constant, the exposure amount (Ee) per unit length when the image height is near the most off-axis image height is smaller than the exposure amount (Ec) per unit length when the image height is near the on-axis image height. That is, the exposure amount of the laser light 208 reaching the on-axis image height region of the photosensitive drum 1 in the axial direction of the photosensitive drum 1 is different from the exposure amount of the laser light 208 reaching the most off-axis image height region. The ratio of Ec to Ee, Er=Ec / Ee, is approximately close to Dmax+100%, so Er=Dmax+100%=130%. This means that the light amount near the on-axis image height is 30% stronger than the light amount near the most off-axis image height.

[0060] As described above, in this embodiment, a microscanner is used without an fθ lens for the scanner laser, and the size and interval of unit dots exposed on the photosensitive drum differ in the main scanning direction of the scanner laser. The size of one dot is smaller and the dot interval is smaller in the center compared to the ends of the photosensitive drum. Therefore, if left as is, the surface potential of the photosensitive drum formed after exposure will be lower in the center compared to the ends. Therefore, in this embodiment, as shown in FIG. 12, correction processing is performed on the dot interval and shape so that the surface potential of the photosensitive drum after exposure during image formation has the same value at the center and ends. FIG. 17(a) is an exposure image of the surface of the photosensitive drum 1 before the correction processing, and FIG. 17(b) is an exposure image of the surface of the photosensitive drum 1 after the correction processing. With this processing, the exposure amount E0=0.24 μJ / cm as described above is obtained. 2The surface potential of the photosensitive drum is uniformly controlled to -170V in the main scanning direction of the scanner laser.

[0061] A developing voltage Vdc (developing potential: -400V) is applied to the developing roller 41 which develops a toner image with respect to the light area potential Vl on the photosensitive drum 1. The image forming portion and a non-image forming portion, which will be described later, are formed in an image formable area on the surface of the photosensitive drum 1. The image formable area is an area where toner 44 can be supplied from the developing roller 41 to the surface of the photosensitive drum 1 and where the toner 44 can be carried on the surface of the developing roller 41.

[0062] That is, the development contrast Vcont, which is the potential difference (absolute value) between the light area potential Vl on the photosensitive drum 1 of the image forming unit and the development voltage Vdc, is 230 V, and the back contrast Vbc, which is the potential difference (absolute value) between the dark area potential Vd on the photosensitive drum 1 and the development voltage Vdc, is 150 V. This makes it possible to appropriately output images such as solid black images, halftones, and white characters.

[0063] Here, the surface of the photosensitive drum 1 and the developing voltage that form the development contrast Vcont and the back contrast Vbc are expressed as a potential difference between the surface potential of the photosensitive drum 1 in the developing section and the developing voltage applied to the developing roller 41. If an image is formed without setting an appropriate potential, image defects will occur on the recording material P. Specifically, if the development contrast Vcont is small, the amount of toner developed on the photosensitive drum 1 will be small, resulting in low density, and if the development contrast Vcont is large, the amount of toner developed on the photosensitive drum 1 will be large, resulting in poor fixing. Therefore, the development contrast Vcont needs to be adjusted appropriately in consideration of these factors.

[0064] In this embodiment, the voltage is expressed as a potential difference with respect to the earth potential (0 V). Therefore, the developing voltage Vdc=-400V is interpreted as having a potential difference of -400V with respect to the ground potential due to the developing voltage applied to the core metal of the developing roller 41. The same applies to the charging voltage and the like.

[0065] 2. Positive external additives and fogging Next, the reason for adding a positive external additive to a toner will be described. A positive external additive is an external additive that has a positive charge, which is the opposite polarity to a toner that has a negative charge, which is the normal polarity. The positive external additive is detached from the toner surface by the image forming operation and adheres to the surface of a member such as a developing roller. Since the positive external additive that adheres to the surface of the member has a negative charge imparting property to the toner, the negative charge amount of the toner can be appropriately controlled. By adjusting the negative charge amount of the toner appropriately, excess toner is prevented from adhering to non-image forming parts (white areas) where no image is formed. This excess toner is called fogging toner, and the phenomenon in which fogging toner occurs is called fogging. When fogging occurs, toner adheres to areas other than the areas where an image is originally to be formed, causing a color to appear in the white areas, which can be disadvantageous to the user.

[0066] The amount of fog toner was measured by taping the toner on the photosensitive drum 1 with Mylar tape, copying the image, and then attaching the tape to a reference paper, and measuring the density with a Tokyo Denshoku Corporation reflection densitometer (TC-6DS / A). The amount of fog toner was calculated from the amount of toner on the photosensitive drum 1 when an image was formed using the image forming apparatus 100 and developed without using the recording material P and with the back contrast Vbc changed. If the amount of fog toner is below a certain value, it is not visible and does not cause a problem in the image, but if the amount of fog toner increases, it becomes visible and causes image problems.

[0067] 3. Reduction of positive external additives in the developer container In this embodiment, strontium titanate fine powder having a number average particle diameter of primary particles of 30 to 300 nm was used as a positive external additive by uniformly attaching it to the surface of the toner. For the manufacturing method of strontium titanate fine powder, for example, the method disclosed in JP-A-2022-092546 may be appropriately used. Since strontium titanate itself has a positive chargeability with respect to the developer, when it is separated from the toner, it is likely to be supplied as an external additive alone to the non-printing portion on the photosensitive drum. When used for a long period of time with low print images, more of the positive external additive alone is consumed from the developing container 4 than the toner, and the ratio of the positive external additive to the toner in the developing container 4 decreases. This reduces the negative charge amount of the toner, which causes image problems such as fogging.

[0068] 4. Adhesion of positive external additives to the charging roller In addition, in the transfer section, the external additives attached to the non-printed section are discharged by the transfer voltage, and the positive charge amount increases. As a result, the external additives have a strong electrostatic adsorption force on the surface of the photosensitive drum 1. Normally, the external additives are scraped off by the cleaning blade 7 and stored in a waste toner container. On the other hand, when used for a long time at a low printing rate, a layer (blocking layer) of toner and external additives is not easily formed at the tip of the cleaning blade 7. By continuing to rotate the photosensitive drum 1 with a small blocking layer, the friction force between the photosensitive drum 1 and the cleaning blade 7 increases. Then, the cleaning blade 7 vibrates slightly, and the external additives contained in the blocking layer slip through the cleaning blade 7. Since the external additives that have slipped through the cleaning blade 7 are positively charged, an electrostatic force toward the charging roller 2, which applies a high negative charging voltage to the photosensitive drum 1, works, and the external additives move from the photosensitive drum 1 to the surface of the charging roller 2. As a result, the positive external additives that have moved to the photosensitive drum 1 in the development section due to long-term use move to the charging roller 2 and accumulate there.

[0069] (External additive supply) A blocking layer is formed to prevent the external additive from passing through the cleaning blade 7. To prevent this, the developer is developed on the photosensitive drum 1 during non-image formation, and the developer is actively supplied to the tip of the cleaning blade 7 (toner purging). However, if printing is performed over a long period of time, it is difficult to completely prevent the external additive from slipping through the cleaning blade 7, and the positively charged external additive adheres to and accumulates on the charging roller 2.

[0070] In this embodiment, as shown in FIG. 2, in the post-rotation operation after the image forming operation, the toner purge for forming the blocking layer is performed and a sequence operation (development recovery sequence) for developing and recovering the positive external additive attached to the charging roller 2 is performed. Here, the post-rotation operation means that even after the image forming operation for one or more predetermined sheets is completed, the main motor (driving motor) 250 continues to be driven for a predetermined time to perform a predetermined sequence operation accompanied by the rotational drive of the photosensitive drum 1. In this embodiment, the development recovery sequence is performed during the post-rotation for each surface movement distance of the developing roller 41 when printing one job, which is equivalent to 100 sheets of letter paper passing with two sheets of letter paper passing. The development recovery sequence may be performed during each post-rotation. This is because the toner on the developing roller 41 is transferred to the photosensitive drum 1 as fog toner during the process in which the developing roller 41 abuts against the photosensitive drum 1 and performs the rotation operation, and the total amount of the toner is correlated with the surface movement distance of the developing roller 41. The fog toner transferred onto the photosensitive drum 1 is mainly supplied to the tip of the cleaning blade 7, but a very small amount of it slips through the cleaning blade 7 and adheres to the charging roller 2. It has been found through research that when the amount of external additive adhering to the charging roller 2 is measured, it increases depending on the surface movement distance of the developing roller 41. For this reason, a development and recovery sequence is performed when the surface movement distance of the developing roller 41 reaches a certain distance, and the positive external additive adhering to the charging roller 2 is transferred from the charging roller 2 to the photosensitive drum 1, and development and recovery are performed.

[0071] 5. Control of the developing and recovery sequence The developer recovery sequence in this embodiment will be described in detail with reference to Fig. 3. Fig. 3 is a schematic diagram of the developer recovery sequence. Figs. 4 and 5 are schematic diagrams of the photosensitive drum 1 and its surroundings for explaining the cleaning operation of the positive external additive from the charging roller 2 in Figs. 3(t1) and (t2). Figs. 6 and 7 are schematic diagrams of the photosensitive drum and its surroundings for explaining the developer recovery operation and toner purging operation in Figs. 3(t3) and (t4).

[0072] An overview of the developer recovery sequence shown in Fig. 2 will be described. In this embodiment, the developer recovery sequence is executed during the post-rotation operation. As shown in Fig. 2, the developer recovery sequence first executes "A. Charge roller cleaning operation" as a first operation, and then executes "B. Developer recovery operation" as a second operation and "C. Toner purging operation" as a third operation in parallel.

[0073] The details of "A. Charge roller cleaning operation" will be described with reference to Figs. 3 to 5. The motor is driven continuously from the end of image formation, and the charging voltage is turned ON to control the charging voltage to -1140V in order to set the surface potential of the photosensitive drum 1 to -550V. Next, the motor drive is continued so that the photosensitive drum 1 rotates one or more revolutions so that the surface potential of the photosensitive drum 1 becomes -550V for one revolution (Figs. 3(t1) and 4). Next, the charging voltage is turned OFF to set the potential of the charging roller 2 to 0V (Fig. 3(t2)). At this time, the surface potential of the photosensitive drum 1 is maintained at -550V, so a potential difference of 550V occurs between the charging roller and the photosensitive drum in the charging section a in Fig. 5 in the opposite direction to that in Fig. 3(t1). Due to this potential difference, the positive external additive attached to the surface of the charging roller is transferred to the surface of the photosensitive drum by electrostatic force (Fig. 5). While the charging roller rotates one revolution while maintaining this potential difference, the motor is driven to clean the entire surface of the charging roller.

[0074] That is, the charging roller cleaning operation is a cleaning operation for removing the toner, external additives, and other adhering matter from the charging roller 2. In order to transfer the toner, the following potential difference is formed between the charging roller 2 and the photosensitive drum 1 with respect to the surface potential formed on the photosensitive drum 1 at the charging position (charging section). That is, it is a potential difference (first potential difference) that generates an electrostatic force that transfers the adhering matter charged with a polarity opposite to the normal charging polarity of the toner from the charging roller 2 to the photosensitive drum 1. This potential difference is a potential difference in which the potential of the charging roller 2 becomes larger in the direction of the polarity opposite to the normal charging polarity of the toner with respect to the surface potential of the photosensitive drum 1.

[0075] The period during which the first potential difference is formed is the period during which the entire surface of the charging roller 123, which is in contact with the rotating photosensitive drum 122 and rotates in a driven manner, is in contact with the surface of the photosensitive drum 122, and corresponds to one rotation of the charging roller 123.

[0076] The transfer voltage is OFF during the charging roller cleaning operation. However, the same negative bias as the charging voltage may be applied as the transfer voltage. By not changing the surface potential of the photosensitive drum 1 in the transfer section, the next steps "B. Development recovery operation" and "C. Toner purge operation" can be stably performed. If there is no effect on the potential of the photosensitive drum 1, a positive bias transfer potential may be used, but it is preferable that the potential of the photosensitive drum 1 is closer to the positive side than the transfer voltage. In an apparatus configuration in which the development roller 41 and the photosensitive drum 1 are not brought into contact with each other and separated from each other in the development section as in this embodiment, it is preferable to prevent the transfer roller 5 from being soiled because there is a possibility that toner may reach the transfer section.

[0077] Next, the details of "B. Development recovery operation" and "C. Toner purging operation" will be described with reference to Figures 6 and 7. After "A. Charge roller cleaning operation", "B. Development recovery operation" and "C. Toner purging operation" are performed in parallel.

[0078] Next, in FIG. 3(t3), the charging voltage is turned off at the charging section a, and when the surface on which the positive external additive on the charging roller 2 has been transferred onto the photosensitive drum 1 reaches the scanning exposure section b, the scanner exposure is turned on to attenuate the potential of the surface of the photosensitive drum 1 from -550V to -170V (FIG. 6). Next, in FIG. 3(t4), the surface of the photosensitive drum 1 is attenuated to -170V, and when the surface of the photosensitive drum 1 to which the positive external additive has been attached reaches the developing section c, -400V is applied to the developing voltage. As a result, the positive external additive attached to the surface of the photosensitive drum 1 is transferred to the surface of the developing roller 41 due to the potential difference between the -170V on the surface of the photosensitive drum 1 and the -400V of the developing voltage. The exposure amount at this time is E0=0.24μJ / cm, the same as during image formation. 2 The time for which a potential difference is formed between the surface of the photosensitive drum 1 and the developing voltage is controlled to be longer than the time for which the charging voltage is turned off, as shown in FIG. 3 (t3). This is to create an opportunity for all of the positive external additive that has migrated from the charging roller 2 onto the photosensitive drum 1 to be sufficiently collected by the developing roller 41. In addition, since the developing roller 41 is driven to rotate with a circumferential speed difference with respect to the photosensitive drum 1 (the speed of the circumferential surface of the developing roller 41 is made faster than the speed of the circumferential surface of the photosensitive drum 1), the positive external additive on the photosensitive drum 1 rolls due to the circumferential speed difference in the developing section, making it easier to migrate from the surface of the photosensitive drum 1 to the developing roller 41. This series of operations is the developing and collecting operation (FIG. 7).

[0079] That is, the development and recovery operation is a recovery operation for removing adhered matter such as toner and external additives from the photosensitive drum 1. In order to transfer the adhered matter from the photosensitive drum 1 to the development roller 41, the following potential difference is formed between the photosensitive drum 1 and the development roller 41 with respect to the surface potential formed on the photosensitive drum 1 at the development position (development section). That is, it is a potential difference (second potential difference) that generates an electrostatic force that transfers the adhered matter, which is charged with a polarity opposite to the normal charging polarity of the toner, from the photosensitive drum 1 to the development roller 41.

[0080] At the same time, in the developing portion c, the negative toner on the developing roller 41 is transferred to the surface of the photosensitive drum 1, thereby performing a toner purging operation. The negative toner transferred to the surface of the photosensitive drum 1 passes through the transfer portion as the photosensitive drum 1 rotates, and is supplied to the tip of the cleaning blade 7. At this time, in order to prevent the purged negative toner from adhering to the transfer roller 5, the transfer voltage is controlled to a negative bias while the toner passes through the transfer section. After that, the development voltage is turned off, the motor drive is stopped, and the development and recovery sequence is completed.

[0081] That is, the toner purge is a toner discharge operation that supplies toner to the contact portion (cleaning position) where the cleaning blade 7 contacts the photosensitive drum 1. That is, it is an operation that develops a toner image on the photosensitive drum 1 and removes the toner image with the cleaning blade 7 without transferring the toner image to a transfer object at the transfer position. During this operation, at least the following periods occur. First, there is a period in which a voltage is applied to the charging roller 2 to charge the photosensitive drum 1. There is also a period in which the scanner unit 3 exposes the photosensitive drum 1. Furthermore, there is a period in which a transfer voltage is applied to the transfer roller 5 to generate an electrostatic force that does not transfer the toner image to the transfer roller 5, that is, an electrostatic force that moves the toner charged to the normal charging polarity from the transfer roller 5 to the photosensitive drum 1. Hereinafter, the applied voltage at this time is also called a non-transfer voltage.

[0082] The area on the surface of the photosensitive drum 1 where the second potential difference is formed in the developing and recovering operation includes the area where the first potential difference is formed immediately before in the charging roller cleaning operation. Also, the area on the surface of the photosensitive drum 1 where the first potential difference is formed in the charging roller cleaning operation is included in the area to which toner is supplied in the toner purging operation when passing through the developing unit for the first time.

[0083] According to this embodiment, the potential difference (second potential difference) formed between the photosensitive drum 1 and the developing roller 41 in the developing and recovering operation after the charging roller cleaning operation is a potential difference that allows a toner image to be developed on the surface of the photosensitive drum 1 by the developer 44 carried by the developing roller 41. This allows the developing and recovering operation and the toner purging to be performed simultaneously.

[0084] Here, during the charging roller cleaning operation, the charging voltage Vp applied to the charging roller 2 and the surface potential Vd of the photosensitive drum 1 charged by the charging roller satisfy the following relational expression (1). (1) |Vp|<|Vd|

[0085] The potential difference (first potential difference) between Vp and Vd is preferably 350 V or more and equal to or less than the discharge threshold value.

[0086] The surface potential Va in the area of ​​the surface of the photosensitive drum 1 where the first potential difference is formed during the charging roller cleaning operation, which constitutes the second potential difference during the development recovery operation, and the development voltage Vdc applied to the development roller 41 at least while that area is in the development position, satisfy the following relational expression (2). (2) Va-Vdc>0

[0087] By the above control, the developing and recovering operation and the toner purge can be carried out simultaneously.

[0088] <Example 2> An outline of the developer collecting sequence in the second embodiment will be described with reference to Fig. 8. Note that in the second embodiment, the same components as those in the first embodiment are given the same reference numbers as those in the first embodiment, and the description thereof will be omitted.

[0089] The difference between the second embodiment and the first embodiment is that the developing and recovering sequence is repeated multiple times, for example, twice. After the developing and recovering operation is completed at (t2) in FIG. 8, the charging voltage is turned ON at (t3) in FIG. 8 again to perform "A. Charge roller cleaning operation", thereby applying -1140V to the charging roller, and recharging the photosensitive drum potential that was lowered at (t2) to the photosensitive drum. The motor is driven until one rotation of the optical drum reaches -550V. After that, the second developer recovery sequence is performed in the same manner as the first developer recovery sequence. After that, the developing voltage is turned off, the motor drive is stopped, and the developer recovery sequence operation is completed.

[0090] <Example 3> An outline of the developer collecting sequence in the third embodiment will be described with reference to Fig. 9. Note that in the third embodiment, the same components as those in the first and second embodiments are given the same reference numbers as those in the first and second embodiments, and the description thereof will be omitted.

[0091] The difference between Example 3 and Example 2 is that during the development recovery operation, when the photosensitive drum surface to which the positive external additive is attached is exposed by the scanner, exposure is performed without thinning out the photosensitive drum to make it uniform in the main scanning direction of the scanner. As a result, in this example, the photosensitive drum surface potential after exposure at the center is smaller than in Example 2. After the surface potential of the photosensitive drum is charged to -550 V by applying -1140 V to the charging roller, the potential of the exposed area is attenuated to -170 V at the ends and -150 V at the center by scanner exposure. The scanner exposure amount at this time is E0=0.24 μJ / cm 2 It was decided.

[0092] <Example 4> An outline of the developer recovery sequence in the fourth embodiment will be described. Note that the same components in the fourth embodiment as those in the first to third embodiments are given the same numbers as those in the first to third embodiments, and the description thereof will be omitted. The timing chart of the developer recovery sequence in the fourth embodiment is the same as the timing chart of the developer recovery sequence in the second embodiment shown in FIG.

[0093] The difference between Example 4 and Example 2 is that the scanner exposure amount to the photosensitive drum surface to which the positive external additive is attached during the development recovery operation is made larger than that of Example 2. In other words, the light amount per unit area is made larger than the light amount per unit area. The surface potential of the photosensitive drum is charged to -550 V by applying -1140 V to the charging roller, and the potential of the exposed area is attenuated to -120 V by scanner exposure. The scanner exposure amount at this time is E0 = 0.45 μJ / cm 2 It was decided.

[0094] <Example 5> An outline of the developer collecting sequence in the fifth embodiment will be described with reference to Fig. 10. Note that in the fourth embodiment, the same components as those in the first to third embodiments are given the same reference numbers as those in the first to third embodiments, and the description thereof will be omitted.

[0095] The difference between the fifth embodiment and the second embodiment is that the photosensitive drum surface to which the positive external additive is attached is not exposed to a scanner during the development recovery operation, and a potential difference of 230 V is formed with respect to the photosensitive drum surface potential (−550 V) by applying −780 V to the development roller at the timing when the surface to which the positive external additive is attached reaches the development section c. The positive external additive on the photosensitive drum is recovered by the development roller due to this potential difference.

[0096] <Evaluation Method for Each Example and Comparative Example> Image evaluation was carried out for Examples 1 to 5, and Example 6 described later. Details of the image evaluation are described below.

[0097] (Durability evaluation) Fog is an image defect that appears like background staining due to a slight amount of toner being developed in white areas (unexposed areas) that are not supposed to be printed. The amount of fog was evaluated as follows.

[0098] During printing of a solid white image, the image forming apparatus is stopped. After development and before transfer, the photosensitive drum The toner above is transferred to a transparent tape, and the tape with the toner attached is attached to a recording paper or the like. A tape without toner is also attached to the same recording paper at the same time. The optical reflectance of the tape attached to the recording paper is measured using an optical reflectance meter (TC-6DS made by Tokyo Denshoku) with a green filter, and the amount of reflectance of the fog is calculated by subtracting it from the reflectance of the tape without toner, and evaluated as the amount of fog. The amount of fog is measured at three or more points on the tape and the average value is calculated.

[0099] A: The fog amount is less than 1.0%. B: The fogging amount is from 1.0 to less than 3.0%. C: The fogging amount is from 3.0 to less than 5.0%. D: The fogging amount is from 5.0 to less than 7.0%. E: The fog amount is 7.0 or more.

[0100] The fog evaluation was performed in a test environment of 32.5°C, 80% RH, after printing 7000 sheets and 14000 sheets, and after leaving the sheet for 24 hours. The printing test was performed by continuously passing a recording image of a horizontal line with an image ratio of 1.5%. Specifically, the horizontal line with an image ratio of 1.5% was an image in which a 3-dot line was printed, followed by a 197-dot line not being printed, was used.

[0101] Further, the evaluation after 7,000 sheets was defined as the middle stage of durability, and the evaluation after 14,000 sheets was defined as the final stage of durability.

[0102] Table 1 shows the evaluation results of fog in the middle and final stages of durability tests for Comparative Examples 1 and 2 and Examples 1 to 6. [Table 1]

[0103] <Superiority of the present invention over the comparative example> The advantages of the present invention over Comparative Examples 1 and 2 will be described. First, in Comparative Example 1, the developer recovery sequence of Example 1 was not performed. In Comparative Example 1, the toner purging operation is performed by transferring the negative toner on the developing roller to the photosensitive drum surface in the developing section during the post-rotation operation (FIG. 11). By supplying the developer to the tip of the cleaning blade, a blocking layer is formed, which prevents the external additive from slipping through the contact area between the cleaning blade and the photosensitive drum and adhering to the charging roller. However, some of the external additive slips through the contact area between the cleaning blade and the photosensitive drum and adheres to the charging roller. The positive external additive that has been transferred from the developing roller to the photosensitive drum does not return to the developing roller again, so the positive external additive in the developing container is reduced. For this reason, the fog evaluation results were D in the middle of the durability test and E at the end of the durability test.

[0104] The other comparative example 2 also does not perform the developer recovery sequence of the embodiment 1. In the comparative example 2, the positive external additive adhering to the charging roller is transferred onto the photosensitive drum by performing the above-mentioned "A. Charge roller cleaning operation" during the post-rotation operation (FIG. 12). The positive external additive transferred to the photosensitive drum is transferred to the photosensitive drum by the potential difference between the photosensitive drum surface potential -550V and the development voltage -400V in the development section. Due to this, electrostatic force acts in a direction that causes the positive external additive to remain on the photosensitive drum. Therefore, the positive external additive is not collected in the developing section, but passes through the developing section and is supplied to the tip of the cleaning blade. In Comparative Example 2, like Comparative Example 1, the positive external additive that has moved from the developing roller to the photosensitive drum does not return to the developing roller again, so the amount of positive external additive in the developing container decreases. For this reason, in the fogging evaluation, like Comparative Example 1, the results were D in the middle of the durability test and E at the end of the durability test.

[0105] The superiority of the present invention over these Comparative Examples 1 and 2 will be explained using Example 1.

[0106] In Example 1, while exerting the effects of the toner purge operation of Comparative Example 1 and the charging roller cleaning operation of Comparative Example 2, there is an effect of developing and recovering the positive external additive transferred onto the photosensitive drum 1 in the charging roller cleaning operation. As a result, the positive external additive transferred from the developing roller 41 to the photosensitive drum 1 can be recovered back to the developing roller 41, and the amount of the positive external additive in the developing container 4 can be maintained throughout durability. For this reason, Example 1 has better results in the fogging evaluation than the comparative examples.

[0107] <Other more effective examples> Next, other examples that are more effective than Example 1 will be described. Example 2 is different from Example 1 in that the operation of the development recovery sequence is repeated twice. In order to transfer the positive external additive attached to the charging roller to the photosensitive drum, one charging roller cleaning operation may not be sufficient. This is because, when the amount of positive external additive attached to the charging roller 2 is large, it is difficult to transfer all of the positive external additive to the photosensitive drum 1 with one charging roller cleaning operation. In Example 2, the operation of the development recovery sequence is repeated twice, so that more positive external additive can be discharged from the charging roller 2 and collected by the developing roller 41 compared to Example 1. This makes it possible to maintain the amount of positive external additive in the developing container more than in Example 1. For this reason, the result of the fogging evaluation was one rank better than that of Example 1.

[0108] Example 3 differs from Example 2 in that no thinning process is performed during scanner light emission in the development recovery sequence. As a result, the surface potential of the photosensitive drum after scanner exposure becomes lower than that during image formation in a portion of the longitudinal direction of the photosensitive drum. Then, the potential difference between the development voltage and the photosensitive drum in the development section becomes larger than in Example 2, and the development recovery efficiency of the positive external additive is further improved. This makes it possible to maintain the amount of positive external additive in the developer container more than in Example 2. For this reason, the result of the fogging evaluation was one rank better than Example 2.

[0109] In Example 4, the same thinning process as during image formation is performed during scanner light emission in the development and recovery sequence, but the scanner exposure amount is greater than during image formation, which is different from Example 2. The potential difference between the development voltage and the photosensitive drum in the development section is greater than in Example 2, which further improves the development and recovery efficiency of the positive external additive. This makes it possible to maintain the amount of positive external additive in the developer container 4 more than in Example 2. For this reason, the result of the fogging evaluation was one rank better than Example 2.

[0110] The fifth embodiment differs from the second embodiment in that the scanner does not emit light in the development recovery sequence, but instead forms a potential difference with the surface potential of the photosensitive drum 1 by changing the development voltage, and the positive external additive on the photosensitive drum 1 is recovered onto the development roller 41. As a result, the development recovery of the positive external additive is not limited to the longitudinal width where the scanner exposure is possible, and it is possible to recover a sufficient area relative to the longitudinal width of the positive external additive on the photosensitive drum 1. This is because the longitudinal width where the positive external additive moves from the development section onto the photosensitive drum 1 is the same as the width of the toner coat on the development roller 41, and the width of the positive external additive that slips through the cleaning blade 7 and adheres to the charging roller 2 is also the same, so the width of the positive external additive discharged from the charging roller 2 onto the photosensitive drum 1 by the development recovery sequence is also the same as the width of the toner coat on the development roller 41. Control of the development voltage By forming a potential difference with the surface potential of the photosensitive drum 1, the width of the toner coat on the developing roller 41 becomes a width that can be developed and recovered, and it is possible to develop and recover the entire longitudinal width of the positive external additive discharged onto the photosensitive drum 1. In addition, since the surface potential of the photosensitive drum 1 does not decrease due to scanner exposure, when the development and recovery sequence is repeated multiple times, there is no need to take the time to charge the photosensitive drum 1 again to -550 V, which reduces the sequence time and reduces downtime.

[0111] Example 6 differs from Example 3 in that the frequency of the developer recovery sequence is different between up to the middle of the durability test (7000 sheets passed) and after the middle of the durability test. As with Example 3, up to the middle of the durability test, the developer recovery sequence is executed every 100 sheets passed, 2 sheets per job, but after the middle of the durability test, the developer recovery sequence is executed every 20 sheets passed. In other words, the frequency of the developer recovery sequence is increased after the middle of the durability test. This is because in the latter half of the durability test, the amount of positive external additive on the photosensitive drum 1 increases because the amount of positive external additive released from the toner surface due to stress on the toner increases. By increasing the frequency of the developer recovery sequence, the amount of positive external additive in the developer container can be maintained, so the fogging evaluation at the end of the durability test was also good, just like in the middle of the durability test.

[0112] For the above reasons, the present invention has been described with respect to the post-rotation operation, but it can also be applied to sequence operations during non-image formation. Also, the present invention has been described with respect to a so-called monochrome image forming apparatus in which one cartridge can be mounted on the image forming apparatus, but the effects of the present invention can be obtained by using a similar configuration with a full-color image forming apparatus that has multiple cartridges and transfers toner to an intermediate transfer body.

[0113] The disclosure of the embodiments of the present invention includes the following configurations. (Configuration 1) A rotating image carrier; a charging member that rotates while contacting the image carrier to form a charging portion, the charging member charging the surface of the image carrier in the charging portion; a charging voltage application unit that applies a charging voltage to the charging member; an exposure unit for exposing the surface of the image carrier charged by the charging member; a developer carrier that carries a developer and faces the image carrier in a developing section to supply the developer to a surface of the image carrier; a developing voltage applying section that applies a developing voltage to the developer carrying member; a transfer member that transfers the developer supplied to the surface of the image carrier to a transfer target in a transfer section; a transfer voltage application unit that applies a transfer voltage to the transfer member; a cleaning member that contacts the image carrier in a cleaning section that is formed downstream of the transfer section and upstream of the charging section in the rotation direction of the image carrier; a control unit that controls the charging voltage application unit, the developing voltage application unit, and the transfer voltage application unit; Equipped with The control unit is a first operation for moving a developer from a surface of the charging member to a surface of the image carrier by controlling the charging voltage application unit; a second operation for moving the developer from the surface of the image carrier to the developer carrier by controlling the charging voltage application unit and the developing voltage application unit; a third operation for supplying developer to the cleaning unit by controlling the charging voltage application unit, the developing voltage application unit, and the transfer voltage application unit; It is possible to execute In the first operation, A first potential difference is formed between the charging member and the image carrier, which generates an electrostatic force that causes the developer charged to a polarity opposite to the normal charging polarity of the developer to move from the charging member to the image carrier. Controlling the charging voltage application unit; In the second operation, a second potential difference is formed between the image carrier and the developer carrier, which generates an electrostatic force that causes the developer charged with a polarity opposite to the normal charging polarity of the developer to move from the image carrier to the developer carrier; Controlling the developing voltage application unit; An image forming apparatus characterized in that an area of ​​the surface of the image carrier in which the second potential difference is formed in the second operation includes an area in which the first potential difference is formed in the first operation. (Configuration 2) The image forming apparatus described in configuration 1, characterized in that the area of ​​the surface of the image carrier where the first potential difference is formed in the first operation is included in the area to which developer is supplied in the third operation when the image carrier first passes through the developing unit. (Configuration 3) The image forming apparatus according to configuration 1 or 2, characterized in that the charging voltage Vp applied to the charging member by the charging voltage application unit in the first operation and the surface potential Vd of the image carrier charged by the charging member to which the charging voltage Vp is applied, which constitute the first potential difference, satisfy the following relational expression (1). (1) |Vp|<|Vd| (Configuration 4) The image forming apparatus according to configuration 3, wherein the difference between Vp and Vd is 350 V or more and is equal to or less than a discharge threshold value. (Configuration 5) The image forming apparatus according to configuration 3 or 4, characterized in that a surface potential Va in an area of ​​the surface of the image carrier where the first potential difference is formed in the first operation, which constitutes the second potential difference, and a development voltage Vdc applied to the developer carrier by the development voltage application unit while at least the area is in the development unit, satisfy the following relational expression (2). (2) Va-Vdc>0 (Configuration 6) 6. The image forming apparatus according to claim 5, wherein the region Va is formed by exposing the region to light using the exposure unit. (Configuration 7) 7. The image forming apparatus according to configuration 5 or 6, wherein the period during which the relational expression (2) is satisfied is longer than the period during which the relational expression (1) is satisfied. (Configuration 8) The image forming apparatus according to any one of configurations 1 to 7, characterized in that the second potential difference is formed by exposing an area of ​​the surface of the image carrier where the first potential difference is formed in the first operation to the exposure unit. (Configuration 9) The image forming apparatus according to any one of configurations 5 to 7, characterized in that the amount of light per unit area in the region when the exposure unit exposes the region is greater than the amount of light per unit area in the surface of the image carrier when the exposure unit exposes the surface of the image carrier during image formation to form an image on the transfer medium. (Configuration 10) The second potential difference is formed by changing a development voltage applied to the developer carrier by the development voltage application unit while an area on the surface of the image carrier where the first potential difference is formed in the first operation is at least in the development unit. 10. An image forming apparatus according to any one of the preceding claims. (Configuration 11) The image forming apparatus according to any one of configurations 1 to 10, characterized in that a sequence operation in which the first operation is followed by the second operation and the third operation are simultaneously performed a plurality of times. (Configuration 12) The image forming apparatus according to any one of configurations 1 to 11, wherein the period during which the first potential difference is formed is a period during which the entire peripheral surface of the charging member is in contact with the peripheral surface of the image carrier, and is longer than a period corresponding to one rotation of the charging member. (Configuration 13) 13. The image forming apparatus according to any one of configurations 1 to 12, wherein the speed of the peripheral surface of the developer carrier is faster than the speed of the peripheral surface of the image carrier. (Configuration 14) The image forming apparatus according to any one of configurations 1 to 13, characterized in that the developer contains toner particles having a normal charging polarity of negative polarity and an external additive having a normal charging polarity of positive polarity that is externally added to the toner. (Configuration 15) 15. The image forming apparatus according to claim 14, wherein the external additive is a fine powder of strontium titanate. (Configuration 16) 16. The image forming apparatus according to claim 15, wherein the external additive has a number average particle size of primary particles of 30 to 300 nm. [Explanation of symbols]

[0114] 1...photosensitive drum, 2...charging roller, 3...scanner unit, 41...developing roller, 5...transfer roller, 7...cleaning blade

Claims

1. a rotating image carrier; a charging member that rotates while contacting the image carrier to form a charging portion, and that charges the surface of the image carrier in the charging portion; a charging voltage applying section that applies a charging voltage to the charging member; an exposure unit that exposes the surface of the image carrier charged by the charging member; a developer carrier that carries a developer and faces the image carrier in a developing section to supply the developer to a surface of the image carrier; a developing voltage applying section that applies a developing voltage to the developer carrier; a transfer member that transfers the developer supplied to the surface of the image carrier to a transfer target in a transfer section; a transfer voltage applying section that applies a transfer voltage to the transfer member; a cleaning blade in a cleaning section formed downstream of the transfer section and upstream of the charging section in the rotation direction of the image carrier, the cleaning blade contacting the image carrier and removing developer remaining on the surface of the image carrier without being transferred to a transferee in the transfer section from the surface of the image carrier; a control unit that controls the charging voltage application unit, the developing voltage application unit, and the transfer voltage application unit; Equipped with The control unit a first operation for moving developer from the surface of the charging member to the surface of the image carrier by controlling the charging voltage application unit; a second operation for moving the developer from the surface of the image carrier to the developer carrier by controlling the charging voltage application unit and the developing voltage application unit; a third operation for supplying developer to the cleaning unit by controlling the charging voltage application unit, the developing voltage application unit, and the transfer voltage application unit; is executable, In the first operation, a first potential difference is formed between the charging member and the image carrier, which generates an electrostatic force that causes the developer charged to a polarity opposite to the normal charging polarity of the developer to move from the charging member to the image carrier; Controlling the charging voltage application unit In the second operation, a second potential difference is formed between the image carrier and the developer carrier, which generates an electrostatic force that causes the developer charged with a polarity opposite to the normal charging polarity to move from the image carrier to the developer carrier; Controlling the developing voltage application unit; In the third operation, a third potential difference is formed on the transfer member to generate an electrostatic force that causes the developer charged to the normal charging polarity to move from the transfer member to the image carrier; Controlling the transfer voltage application unit An image forming apparatus characterized in that the area on the surface of the image carrier where the second potential difference is formed in the second operation includes the area where the first potential difference is formed in the first operation.

2. 2. The image forming apparatus according to claim 1, wherein the area on the surface of the image carrier where the first potential difference is formed in the first operation is included in the area to which developer is supplied in the third operation when the image carrier first passes through the developing unit.

3. 2. The image forming apparatus according to claim 1, wherein the first potential difference is constituted by a charging voltage Vp applied to the charging member by the charging voltage application unit in the first operation and a surface potential Vd of the image carrier charged by the charging member to which the charging voltage Vp is applied, and the charging voltage Vp satisfies the following relational expression (1): (1) |Vp|<|Vd|

4. 4. The image forming apparatus according to claim 3, wherein the difference between Vp and Vd is 350 V or more and a discharge threshold value or less.

5. 4. The image forming apparatus according to claim 3, wherein a surface potential Va in an area of ​​the surface of the image carrier where the first potential difference is formed in the first operation, which constitutes the second potential difference, and a development voltage Vdc applied to the developer carrier by the development voltage application unit while at least the area is in the development unit, satisfy the following relational expression (2): (2) Va-Vdc>0

6. 6. The image forming apparatus according to claim 5, wherein the Va is formed by exposing the area with the exposure unit.

7. 6. The image forming apparatus according to claim 5, wherein the period in which the relational expression (2) is satisfied is longer than the period in which the relational expression (1) is satisfied.

8. 2. The image forming apparatus according to claim 1, wherein the second potential difference is formed by exposing an area of ​​the surface of the image carrier where the first potential difference is formed in the first operation to the exposure unit.

9. The image forming apparatus according to claim 5, characterized in that the amount of light per unit area in the region when the exposure unit exposes the region is greater than the amount of light per unit area in the surface when the exposure unit exposes the surface of the image carrier during image formation to form an image on the transfer medium.

10. The second potential difference is generated when the developing voltage application unit applies the developing voltage to the image carrier while the area on the surface of the image carrier where the first potential difference is formed is at least in the developing unit.

2. The image forming apparatus according to claim 1, wherein the image is formed by changing a development voltage applied to the image carrier.

11. 2. The image forming apparatus according to claim 1, wherein a sequence operation in which the first operation is followed by the second operation and the third operation is simultaneously performed is repeated a plurality of times.

12. 2. The image forming apparatus according to claim 1, wherein the period during which the first potential difference is formed is a period during which the entire peripheral surface of the charging member is in contact with the peripheral surface of the image carrier, and is longer than a period corresponding to one rotation of the charging member.

13. 2. The image forming apparatus according to claim 1, wherein the speed of the peripheral surface of said developer carrier is faster than the speed of the peripheral surface of said image carrier.

14. 2. The image forming apparatus according to claim 1, wherein the developer contains toner particles having a normal negative charge polarity and an external additive having a normal positive charge polarity that is externally added to the toner particles.

15. 15. The image forming apparatus according to claim 14, wherein the external additive is a fine powder of strontium titanate.

16. 16. The image forming apparatus according to claim 15, wherein the external additive has a number average particle size of primary particles of 30 to 300 nm.

Images