Image forming apparatus

Abstract

In an image forming apparatus that recovers toner from a photosensitive drum using a developing roller, the invention aims to achieve both development and recovery, as well as a reduction in the amount of toner affected by inversion fouling. [Solution] An image forming apparatus that charges and exposes an image carrier to form an electrostatic latent image, develops the image to form a toner image, and transfers it to a transfer target, wherein development and recovery can be performed to return the toner image on the image carrier to the development means, and in the recovery area that is the target of development and recovery, along the direction of the rotation axis, there is a first region and a second region with a higher density than the first region, depending on the toner density An image forming apparatus is used that sets the parameters such that, during development and retrieval, the second back contrast of the second region becomes greater than the first back contrast of the first region.

Description

[Technical Field]

[0001] This invention relates to an image forming apparatus. [Background technology]

[0002] Conventionally, image forming devices that use an intermediate transfer body have been known as image forming devices such as photocopiers and laser beam printers. In this image forming device, as a primary transfer step, the toner image formed on the surface of the photosensitive drum, which acts as the image carrier, is transferred onto the intermediate transfer body by applying a voltage from a voltage power supply to a primary transfer member positioned opposite the photosensitive drum. In full-color printers that form color images consisting of multiple colors, this primary transfer step is performed for each color, and the toner images of each color are superimposed on each other to form a toner image consisting of multiple colors on the surface of the intermediate transfer body. Then, as a secondary transfer step, the toner images of multiple colors formed on the surface of the intermediate transfer body are transferred to the surface of a recording material such as paper by applying a voltage to a secondary transfer member. The transferred toner image is then permanently fixed to the recording material by a fixing means, thereby forming a color image.

[0003] In such image forming apparatuses, toner may remain on the photosensitive drum without moving to the paper. For example, in the primary transfer process, toner may remain on the photosensitive drum because it cannot be transferred to the intermediate transfer medium (referred to as residual toner), or an incomplete toner image may remain on the photosensitive drum if the image forming operation is interrupted due to a jam in the paper during transport. It is necessary to remove (clean) this toner that remains on the paper without moving (defined as residual toner) from the photosensitive drum in order to prepare for the next image forming.

[0004] Patent Document 1 describes a means for removing residual toner from a photosensitive drum. By controlling the voltage applied to the photosensitive drum and the developing roller, the potential difference between the surface potential Vd of the photosensitive drum and the developing roller potential Vdc (called the back contrast ΔVb) is used to recover the residual toner on the photosensitive drum into the developing roller (called development recovery). It is also stated that the larger the potential difference between the photosensitive drum potential Vd and the developing roller potential Vdc (back contrast ΔVb), the higher the recovery capacity into the developing roller. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2023-074989 [Overview of the project] [Problems that the invention aims to solve]

[0006] Even if the normal charging polarity of toner is negative, and most of the toner adhering to the surface of the developing roller has a negative charge, a certain proportion of toner will have a positive charge, which is the opposite of the normal charging polarity. In such toner, for example, if the photosensitive drum potential Vd is set to -600V, the developing roller potential Vdc to -300V, and the back contrast ΔVb is controlled to 300V, and development and recovery is performed, the toner with a positive charge will be attracted to the surface of the photosensitive drum where the negative potential is higher. This phenomenon is called inversion fouling, and the attracted toner is called inversion fouling toner.

[0007] Increasing the back contrast ΔVb to improve the efficiency of development and recovery increases this reverse fouling toner. This reverse fouling toner adheres to and accumulates on the surface of the charging means that charges the photosensitive drum. This may reduce the charging performance of the charging means.

[0008] The present invention has been made in view of the above problems, and aims to achieve both development and recovery and reduction of the amount of inverted toner in an image forming apparatus that recovers toner on a photosensitive drum using a developing roller. [Means for solving the problem]

[0009] This invention employs the following configuration: Image carrier and, A charging means for charging the image carrier, An exposure means for exposing the image carrier to form an electrostatic latent image, A developing means that supplies toner to the electrostatic latent image on the image carrier to form a toner image, A transfer means for transferring the toner image formed on the image carrier to a transfer target, A control unit that controls the charging voltage applied to the charging means, the developing voltage applied to the developing means, the transfer voltage applied to the transfer means, and the exposure amount of the exposure means. Image information acquisition means for acquiring information related to the density of the toner image formed on the image carrier, Equipped with, The control unit is capable of developing and recovering the toner image formed on the image carrier in a recovery area which is a predetermined range along the rotation axis of the image carrier, without transferring the toner image to the transfer target, and returning it to the developing means. When the absolute value of the difference between the surface potential of the image carrier and the development voltage when the toner image is returned to the developing means is defined as the back contrast, the image information acquisition means arranges the first region and the first region so as to be aligned in the direction along the rotation axis in the recovery region. Also, set a second area with a higher toner density. The control unit controls the second back contrast of the second region during development and recovery to be greater than the first back contrast of the first region. This is an image forming apparatus characterized by the following features.

[0010] The present invention also employs the following configuration: a rotatable image carrier, charging means for charging the image carrier, exposure means for exposing the image carrier to form an electrostatic latent image, developing means for supplying toner to the electrostatic latent image on the image carrier to form a toner image, transfer means for transferring the toner image formed on the image carrier to a transfer medium, a control unit for controlling a charging voltage applied to the charging means, a developing voltage applied to the developing means, a transfer voltage applied to the transfer means, and an exposure amount of the exposure means, image information acquisition means for acquiring information related to the toner image formed on the image carrier, comprising: the charging means, the exposure means, the developing means, and the transfer means are arranged in this order from the upstream side in the rotation direction of the image carrier, an image forming apparatus capable of operating in an image forming mode for transferring the toner image formed on the image carrier to the transfer medium and a discharge mode for supplying discharge toner to the image carrier by the developing means, recovering a part of the discharge toner by the developing means, and transferring another part of the discharge toner to the transfer medium, in the discharge mode, the image information acquisition means divides the toner image into a plurality of regions, calculates an average printing rate in each of the plurality of regions as information related to the toner image, and the control unit controls the density of the discharge toner for each region based on the average printing rate An image forming apparatus characterized by the above.

Advantages of the Invention

[0011] According to the present invention, in an image forming apparatus that recovers toner on a photosensitive drum by a developing roller, it is possible to achieve both development recovery and reduction of the amount of reverse fog toner.

Brief Description of the Drawings

[0012] [Figure 1]Explanatory diagram of pixel information detection result and background contrast [Figure 2] Cross-sectional view explaining an image forming apparatus having the features of Example 1 [Figure 3] Diagram explaining the control of an image forming process according to an embodiment [Figure 4] Diagram explaining the control of weak exposure in an image forming process according to an embodiment [Figure 5] Diagram explaining the outline of developing means [Figure 6] Diagram explaining toner according to an embodiment [Figure 7] Diagram explaining the operations of development and development recovery [Figure 8] Diagram explaining the toner image at the time of interruption of image forming operation according to an embodiment [Figure 9] Diagram explaining the development recovery process after interruption of image forming operation according to an embodiment [Figure 10] Diagram explaining the image used for verification of Example 1 [Figure 11] Diagram explaining the pixel information detection result of region Q in Example 1 [Figure 12] Diagram explaining the contribution degree of background contrast in Example 1 [Figure 13] Diagram explaining the pixel information detection result of region Q in Example 2 [Figure 14] Diagram explaining the contribution degree of background contrast in Example 2 [Figure 15] Diagram explaining the operation of ejection control according to Example 3 [Figure 16] Diagram explaining the voltage and potential control of ejection control according to Example 3 [Figure 17] Diagram explaining the division of the paper area according to Example 3 [Figure 18] Diagram explaining the number of sheets passed, printing rate, and deteriorated toner ratio according to Example 3 [Figure 19] Diagram explaining the average printing rate and the density of ejected toner according to Example 3

Modes for Carrying Out the Invention

[0013] The embodiments of the present invention will be described in detail below with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in the following embodiments should be appropriately modified depending on the configuration and various conditions of the apparatus to which the present invention is applied. Therefore, unless otherwise specifically stated, this is not intended to limit the scope of the present invention. Although multiple features are described in the embodiments, not all of these features are necessarily essential to the invention, and the features may be combined in any way.

[0014] [Example 1] <Description of the image forming apparatus> First, the overall configuration of the electrophotographic image forming apparatus (hereinafter referred to as the image forming apparatus) according to the present invention will be described. Figure 2 is a schematic cross-sectional view of the image forming apparatus 100 of this embodiment. The configuration, operation, and control of the image forming apparatus of this embodiment will be explained using Figure 2.

[0015] The image forming apparatus 100 in this embodiment is a full-color laser printer employing an in-line method and an intermediate transfer method. The image forming apparatus 100 can form a full-color image on a recording material P (e.g., recording paper, plastic sheet) according to image information. Image information is input to the image forming apparatus 100 from an image reading device or a host computer 199 such as a personal computer that is communicatively connected to the image forming apparatus 100.

[0016] The image forming apparatus 100 has a plurality of image forming units, each consisting of first, second, third, and fourth process cartridges Sa, Sb, Sc, and Sd for forming images of yellow (Y), magenta (M), cyan (C), and black (K), respectively. In this embodiment, the first to fourth process cartridges Sa, Sb, Sc, and Sd are arranged in a row in a direction intersecting the vertical direction. They are located in [location]. In this embodiment, the configuration and operation of the first to fourth process cartridges Sa, Sb, Sc, and Sd are substantially the same except for the different colors of the images they form. Therefore, unless otherwise specified, the subscripts a, b, c, and d given to the symbols to indicate that an element is provided for one of the colors will be omitted, and the explanation will be general.

[0017] In this embodiment, the image forming apparatus 100 has four drum-shaped electrophotographic photoreceptors, namely photosensitive drums 1 (1a, 1b, 1c, 1d), arranged in parallel in a direction intersecting the vertical direction, as multiple image carriers. The photosensitive drums 1 are rotationally driven by a drive source 550. Around the photosensitive drums 1 are charging rollers 2 (2a, 2b, 2c, 2d), a scanner unit (exposure device) 3 (3a, 3b, 3c, 3d) as an exposure means, and a developing unit (developing device) 4 (4a, 4b, 4c, 4d). The charging rollers 2 are charging means that uniformly charge the surface of the photosensitive drums 1.

[0018] The scanner unit 3 is an exposure means that irradiates a laser to form an electrostatic image (electrostatic latent image) on the photosensitive drum 1 based on the output calculated by the CPU from image information input from a host computer 199 such as a personal computer. When forming an electrostatic latent image corresponding to the image signal by the exposure means, the photosensitive drum potential is stabilized by uniformly and weakly exposing the non-image area of ​​the surface of the photosensitive drum 1. Hereinafter, the formation of an electrostatic image by the exposure means will be distinguished as exposure (first exposure), and uniformly exposing the non-image area will be distinguished as weak exposure (second exposure). The developing unit 4 is a developing means that develops the electrostatic image as a developer (hereinafter referred to as toner) image. The photosensitive drum 1, the charging roller 2 which is a process means that acts on the photosensitive drum 1, and the developing unit 4 are integrated to form a process cartridge S. The process cartridge S is detachable from the image forming apparatus 100 via mounting means such as mounting guides and positioning members provided on the image forming apparatus 100.

[0019] An intermediate transfer belt 10 is positioned opposite multiple (in this case, four) photosensitive drums 1 to transfer the toner image on the photosensitive drums 1 to the recording material P. The intermediate transfer belt 10 is the transfer target onto which the toner image is transferred. In this embodiment, an intermediate transfer method is employed, and the intermediate transfer belt 10 is the intermediate transfer target. It may also be a recording material transport member that transports a recording medium instead of an intermediate transfer target, or, in the case of a monochrome printer being used as the image forming apparatus 100, a recording medium can be used as the transfer target instead of an intermediate transfer target. The intermediate transfer belt 10, formed as an endless belt, contacts all of the photosensitive drums 1 and moves (rotates) in a circular motion in the direction of arrow R3 (clockwise) shown in the figure. The intermediate transfer belt 10 is stretched across multiple support members: a drive roller 11, a tension roller 12, and a secondary transfer opposing roller 13. The intermediate transfer belt 10 rotates as the drive roller 11 rotates in direction R2.

[0020] On the inner circumferential surface of the intermediate transfer belt 10, four primary transfer rollers 14 (14a, 14b, 14c, 14d) are arranged in parallel, facing each photosensitive drum 1, as primary transfer means. The primary transfer rollers 14 press the intermediate transfer belt 10 toward the photosensitive drum 1, forming a primary transfer section where the intermediate transfer belt 10 and the photosensitive drum 1 come into contact. A voltage opposite to the normal charging polarity of the toner (Vtr = +100V in this embodiment) is applied to the primary transfer rollers 14 from a primary transfer power supply 15 (high-voltage power supply) which serves as a primary transfer voltage application means. As a result, the toner image on the photosensitive drum 1 is primary transferred onto the intermediate transfer belt 10. When forming a full-color image, the above process is performed sequentially in the first to fourth process cartridges Sa, Sb, Sc, and Sd, and the toner images of each color are superimposed and primary transferred onto the intermediate transfer belt 10.

[0021] The primary transfer roller 14 is a cylindrical metal roller with a diameter of Φ6 mm, and is made of nickel-plated SUM material.

[0022] The intermediate transfer belt 10 is an endless belt in which conductivity is imparted by adding a conductive agent to a resin material. It has a circumference of 700 mm and a thickness of 70 μm, and uses an endless polyimide resin mixed with carbon as the conductive agent as the base layer. In terms of electrical properties, it exhibits electronic conductivity and is characterized by small fluctuations in resistance value with respect to temperature and humidity in the atmosphere. In this example, the volume resistivity of the intermediate transfer belt 10 is 1 × 10⁻⁶ 9 The value is Ω·cm. Volume resistivity was measured using a HIRESTA-UP (MCP-HT450) from Mitsubishi Chemical Corporation with a ring probe of type UR (model MCP-HTP12). The measurement was performed under the following conditions: room temperature set to 23°C, room humidity 50%, applied voltage 100V, and measurement time 10 sec. Here, volume resistivity is a measure of the conductivity of the intermediate transfer belt 10 as a material.

[0023] A secondary transfer roller 20, acting as a secondary transfer means, is positioned on the outer circumferential surface of the intermediate transfer belt 10, facing the secondary transfer opposing roller 13. The secondary transfer roller 20 presses against the secondary transfer opposing roller 13 via the intermediate transfer belt 10, forming a secondary transfer section where the intermediate transfer belt 10 and the secondary transfer roller 20 are in contact. A voltage opposite to the normal charging polarity of the toner is applied to the secondary transfer roller 20 from a secondary transfer power supply 21 (high-voltage power supply), which acts as a secondary transfer voltage application means. As a result, the four-color toner image on the intermediate transfer belt 10 is transferred collectively onto the recording material P by the action of the secondary transfer roller 20, which is in contact with the intermediate transfer belt 10 via the recording material P. The recording material P stored in the cassette 51 is transported to the secondary transfer section by the feeding means 50 in synchronization with the movement of the intermediate transfer belt 10.

[0024] The secondary transfer opposing roller 13 is cleaned and removed by an intermediate transfer belt cleaning device 16 via an intermediate transfer belt 10, and the residual secondary transfer toner remaining on the intermediate transfer belt 10 is collected in a waste toner container 17.

[0025] After the secondary transfer is complete, the recording material P, which carries the four-color toner images, is transported to the fixing device 30, specifically the fixing nip section formed by the fixing roller 31 and the pressure roller 32. There, the recording material P is heated and pressurized, causing the four colors of toner to melt and mix, fixing the toners to the recording material P, and it is then discharged from the image forming apparatus 100.

[0026] Furthermore, the image forming apparatus 100 can also form monochrome or multicolor images using only one desired image forming unit, or using only some (but not all) of the image forming units.

[0027] In this embodiment, the image forming apparatus 100 is a printer with a process speed of 148 mm / sec and compatible with A4 size paper.

[0028] The configuration of the engine control unit 210, which controls the entire image forming apparatus, will be explained with reference to Figure 3. As shown in Figure 3, the engine control unit 210 incorporates a CPU circuit unit 150, a ROM 151, and a RAM 152. The CPU circuit unit 150 comprehensively controls the primary transfer control unit 201, the secondary transfer control unit 202, the development control unit 203, the exposure control unit 204, the charging control unit 205, the pre-charging exposure control unit 206, the development blade control unit 401, and the supply roller control unit 403 according to the control program stored in the ROM 151. The RAM 152 temporarily holds control data and is used as a workspace for calculation processing associated with the control. When the controller 200 receives print information and print commands from the host computer 199, it transmits them to the engine control unit 210 as a video signal that can be processed. The engine control unit 210 controls each component (primary transfer control unit 201, secondary transfer control unit 202, development control unit 203, exposure control unit 204, charging control unit 205, pre-charging exposure control unit 206, development blade control unit 401, supply roller control unit 403, drive source 550) to perform the necessary operations for printing. The image forming operation is performed. The environmental sensor 300 is a sensor that acquires environmental information in the vicinity of the device, and includes a temperature sensor 301 that detects temperature information and a humidity sensor 302 that detects humidity information.

[0029] The control of weak exposure in the non-image area implemented in this embodiment will be explained using Figure 4. In Figure 4, the image signal sent from the controller 200 is a multi-level signal (0 to 255) with 8 bits = 256 gradations in the depth direction. When this signal is 0, the laser light is off; when it is 255, it is fully on; and between 1 and 254, it has a value intermediate between the two. Here, the exposure level of the non-image area can be arbitrarily set by the level of the multi-level signal. In the following explanation, 32 will be used as the level of this multi-level signal to perform exposure of the non-image area. Non-image areas where the image signal sent from the controller 200 is 0 are converted to 32 by the image signal conversion circuit 68 in the exposure control unit 204, and for image signals with values ​​from 1 to 255, they are compressed and converted from 33 to 255. Subsequently, the frequency modulation circuit 61 converts it into a serial time-axis direction signal, which is used for pulse width modulation of each dot pulse with a resolution of 600 dots / inch in this example.

[0030] This signal drives the laser driver 62, causing the laser diode 63 to emit light and the laser beam L to be emitted. This laser beam L passes through a correction optical system 67, which includes a polygon mirror 64, a lens 65, and a folding mirror 66, and is then irradiated onto the photosensitive drum 1 as scanning light. The frequency modulation circuit 61 may be located on the controller side, separate from the laser driver 62.

[0031] When the scanner unit 3, acting as an exposure device, performs weak exposure, it exposes a predetermined area on the surface of the photosensitive drum 1 by repeatedly performing a main scan in the main scanning direction and a sub-scan in the sub-scanning direction. The main scanning direction is the direction along the rotation axis of the photosensitive drum 1. By performing a main scan, the scanner unit 3 can weakly expose one line in the main scanning direction. That is, one line is weakly exposed with each main scan. The sub-scanning direction is the circumferential direction of the surface of the photosensitive drum 1, intersecting the main scanning direction. The scanner unit 3 can move the exposure position between lines by performing sub-scans.

[0032] <Description of Process Cartridges> Next, the overall configuration of the process cartridge S installed in the image forming apparatus 100 of this embodiment will be described. The process cartridge S is composed of a photosensitive unit equipped with a photosensitive drum 1 and a rotatable charging roller 2, and a developing unit (developing apparatus) 4 equipped with a rotatable developing roller 22, etc., integrated together.

[0033] The photosensitive drum 1 is rotatably supported via bearings. The photosensitive drum 1 is configured to rotate in the direction of arrow R1 (counterclockwise) according to the image forming operation when the driving force of the drive source 550 is transmitted to the photoreceptor unit. The charging roller 2 is configured to rotate by pressure contact of the photosensitive drum 1 with a conductive rubber roller portion. In this embodiment, a charging roller 2 that makes pressure contact with the photosensitive drum 1 is used, but this is not limited to this, and a non-contact charging method such as a corona charger may also be used. In the rotation direction R1 of the photosensitive drum 1, with the charging roller 2 as the starting point, the components are arranged in the following order from the upstream side: charging roller 2, scanner unit 3, developing roller 22, and primary transfer roller 14.

[0034] The photosensitive drum 1 consists of a φ20mm aluminum tube on which a photosensitive layer and a surface layer are provided. The surface layer is a thin film made of polyarylate with a thickness of 23 μm.

[0035] The electrostatic roller 2 has a thickness of 1.5 mm and a volume resistivity of 1 × 10⁻¹⁰ on a metal shaft with a diameter of 5.5 mm. 6 A 228mm long, φ8.5mm diameter object with an elastic layer made of conductive rubber with an approximate Ωcm diameter. A roller was used. The charged roller 2 is pressed against the photosensitive drum 1 with a total pressure of 600 gf (300 gf on each side) from bearings (not shown) at both ends of the metal shaft, and rotates while forming a nip width of about 300 μm.

[0036] On the other hand, as shown in Figure 5, the developing unit 4 includes a developing roller 22 that carries toner T, a developing blade 23 (regulating member), and a developing frame 24 that fixes them. The developing frame 24 includes a developing chamber 241 (developing container) in which the developing roller 22 is arranged, and an anti-blowing sheet 242 that seals the developing opening (opening) that connects the developing chamber 241 to the outside world. One end of the developing blade 23 is fixed to a fixing member 25 fixed to the developing frame 24, and the other end of the developing blade 23 is brought into contact with the developing roller 22, so that the amount of toner coating on the developing roller 22 can be regulated and an electric charge can be applied. The developing roller 22 is positioned in the developing opening and is capable of contacting the photosensitive drum 1.

[0037] As shown in Figure 5, the developing roller 22 has a metal core 221 with a diameter of 6.0 mm and a thickness of 2.0 mm, with a volume resistivity of 1 × 10⁻⁶. 7 A φ10mm roller was used, consisting of a base layer 222 made of conductive silicone rubber with a thickness of approximately Ωcm, and a surface layer 223 made of urethane, which are sequentially laminated. It is positioned to rotate in the direction R4 indicated by the arrow in the figure. The developing roller 22 rotates with a speed difference relative to the photosensitive drum 1 in order to control the amount of toner developed onto the photosensitive drum 1. In this embodiment, the developing control unit 203 shown in Figure 3 controls the developing roller drive source 405, and by controlling the developing roller drive source 405, the rotation speed of the developing roller 22 can be changed relative to the rotation speed of the photosensitive drum 1. In this embodiment, the developing roller 22 rotates at 140% of the rotation speed of the photosensitive drum 1. The developing roller drive source 405 is a driving member such as a motor. Here, an independent motor is used as the developing roller drive source 405. However, a motor common to the drive source 550 that drives the photosensitive drum 1 and the charging roller 2 may also be used as the developing roller drive source 405.

[0038] As shown in Figure 5, the developing blade 23 is in contact with the developing roller 22 so as to be oriented in the counter direction to the rotation direction of the developing roller 22, thereby regulating the amount of toner coated and imparting charge through triboelectric charging. In this embodiment, a support member (not shown) made of a spring-shaped SUS plate with a thickness of 50 to 120 μm is used as the developing blade 23, which is the toner regulating member, and the surface of the blade portion is brought into contact with the developing roller 22 by utilizing the spring elasticity of the support member. The blade portion is configured such that the blade portion is formed at one end in the short direction, and the other end is fixed and supported by the developing frame 24. On the other hand, the blade portion is formed by coating the surface of the support member with a thin film made of conductive urethane resin. Furthermore, by applying a predetermined DC voltage to the developing blade 23 (developing blade voltage Vbld, Vbld = -500V during the image forming operation in this embodiment) and controlling the potential difference (developing blade contrast ΔVbld = Vbld - Vdc, ΔVbld = -200V during the image forming operation in this embodiment) with the voltage applied to the developing roller 22 (developing roller potential Vdc, Vdc = -300V during the image forming operation in this embodiment), the amount of toner coating and the amount of toner charge are controlled.

[0039] The supply roller 26 has a core metal electrode 261 with an outer diameter of φ5.5 (mm), which is a conductive support, and a foamed urethane layer 262 is provided around it. The outer diameter of the entire supply roller 26, including the foamed urethane layer 262, is φ11 (mm). The penetration depth between the supply roller 26 and the developer roller 22 is 1.2 mm. Here, if we consider the state in which the supply roller 26 and the developer roller 22 are in contact at one point on the outer circumference as the reference state, as the supply roller 26 and the developer roller 22 are brought closer together, the surface of either the supply roller 26 or the developer roller 22 deforms, and one becomes embedded in the other. The "penetration depth" above refers to the amount of embedding from the reference state. At the point of contact with the developer roller 22, the supply roller 26 rotates in a direction (direction of arrow R5 in the figure) such that the supply roller 26 and the developer roller 22 have opposite velocities. The powder pressure of the toner T present around the foamed urethane layer 262 acts on it, and as the supply roller 26 rotates, the toner T is compressed in the foamed urethane layer. It is incorporated into layer 262.

[0040] The supply roller 26 containing toner T supplies toner T to the developing roller 22 at the point of contact with the developing roller 22, and further friction imparts a preliminary triboelectric charge to the toner T. On the other hand, the supply roller 26 that supplies toner to the developing roller 22 also has the role of peeling off toner that remains on the developing roller 22 without being developed in the developing section.

[0041] A predetermined DC voltage (supply roller voltage Vrs, Vrs = -500V during the image forming operation in this embodiment) is applied to the supply roller 26, and the potential difference (supply roller contrast ΔVrs = Vrs - Vdc, ΔVrs = -200V during the image forming operation in this embodiment) between this voltage and the voltage applied to the develop roller 22 (develop develop roller potential Vdc, Vdc = -300V during the image forming operation in this embodiment) is controlled to control the toner supply amount and the amount of preliminary triboelectric charge.

[0042] Toner T is a non-magnetic toner with a normal charge polarity of negative polarity, manufactured by suspension polymerization, with a volume-average particle size of 7.0 μm, and becomes negatively charged when supported on the developing roller 22. In addition, to modify the surface properties of the toner matrix (referring to the carbon-based composition of binder resin and release agent contained in the toner particles), silicon dioxide particles at a concentration of approximately 1.5% of the toner's weight are attached to the surface of the toner as an external additive, with a volume-average particle size of approximately 20-120 nm.

[0043] Due to prolonged friction with the developing blade 23, the toner T may deform in shape, or the external additives on the toner surface may peel off or become embedded. The deformation of the toner shape increases the contact area between the toner T and the photosensitive drum 1. The peeling off of the external additives on the toner surface allows the resin components of the toner to come into contact with the photosensitive drum 1, and the embedding of the external additives into the toner further increases the contact area between the toner and the photosensitive drum 1. As a result, the non-electrostatic adhesion force with the photosensitive drum 1 becomes higher than in the new state, which in this embodiment is referred to as toner degradation. The change in non-electrostatic adhesion force due to the deformation of the toner shape can be represented by a change in the average circularity (aspect ratio) of the toner. In this embodiment, the average circularity in the new state is about 0.95, and a state where it falls below about 0.90 is considered a degraded state. The average circularity of the toner can be measured using the flow-type particle image analyzer "FPIA-3000" (manufactured by Sysmex Corporation) under the measurement and analysis conditions during calibration.

[0044] Furthermore, changes in the surface condition due to peeling or embedding of external additives contained in toner T can be quantitatively understood using the BET value. In this embodiment, the BET value of the toner was measured using QUADRA.SORB SI manufactured by CANTAChrome. The BET value of the toner, used as an indicator of the change in the adhesion state of external additives to the toner surface, indicates the amount of external additives attached to the toner surface. As the amount of external additives present on the toner surface decreases, the BET value of the toner decreases. In other words, when an external additive with a high BET value is added to the toner matrix surface, the BET value of the toner increases, but the BET value of the toner decreases due to embedding of the external additives into the toner resin or detachment from the toner surface. When the external additive is completely removed from the toner surface, the BET value of the toner becomes the same as the BET value of the toner matrix. In this embodiment, the BET value of a new product was 2.8m. 2 It is approximately / g, and 2.0m 2 A state where the value falls below approximately / g is considered a deteriorated state.

[0045] Changes in non-electrostatic adhesion can be measured using other indicators; for example, by directly measuring the amount of toner remaining after primary transfer (primary transfer residue), changes in non-electrostatic adhesion can be determined by an increase in the primary transfer residue.

[0046] In this embodiment, toner T was used in which an external additive was added to the base material, but this is not limited to this, for example, a partial structure represented by the following formula (1) may be used as toner T on the surface of the toner base material. A toner containing an organosilicon polymer having a convex portion, as shown in Figure 6, may also be used. R0-SiO 3 / 2 (1) (R0 is an alkyl group or phenyl group with 1 to 6 carbon atoms)

[0047] The convex spacing G and convex height H on the toner surface shown in Figure 6 can be measured using a scanning probe microscope (SPM). The SPM consists of a probe, a cantilever that supports the probe, and a displacement measurement system that detects the bending of the cantilever. It detects the interatomic force (attractive or repulsive force) between the probe and the sample to observe the shape of the sample surface.

[0048] Figure 7 illustrates the toner development and the collection of residual toner in the developing unit in the above configuration. In Figure 7, circles enclosed by solid lines represent the toner after movement, and circles enclosed by dashed lines represent the toner before movement.

[0049] First, the toner development process will be explained using Figures 7(a) and 7(b). Figure 7(a) is a schematic diagram of the potential relationship when the toner is developed, and Figure 7(b) is a schematic diagram of the vicinity of the development unit when the toner is developed. In this embodiment, the voltage Vpri (hereinafter referred to as the charging voltage) applied to the charging roller 2 during image formation is -1200V, and the drum potential Vp after charging is approximately -700V. In addition, the drum potential Vd after weak exposure, which is formed in the non-image area by weak exposure, is approximately -480V. The development voltage Vdc applied to the development roller 22 is -300V, and the drum potential VL in the exposure area, where the charge has decayed due to exposure, is approximately -150V. Primary transfer is performed by the potential difference ΔVtr1 between the drum potential VL in the exposure area and the primary transfer voltage Vtr.

[0050] As shown in Figures 7(a) and 7(b), the potential difference between the developing roller potential Vdc and the exposure drum potential VL (hereinafter referred to as the developing contrast ΔVc) causes the negatively charged toner to come into contact with the photosensitive drum 1, thereby revealing the electrostatic latent image and forming a toner image. Furthermore, the potential difference between the developing voltage Vdc and the post-exposure drum potential Vd formed by weak exposure (hereinafter referred to as the back contrast ΔVb) electrically holds the toner on the developing roller 22 so that it does not transfer to the non-image area.

[0051] Next, a method for processing toner that remains on the surface of the photosensitive drum 1 without being transferred to the intermediate transfer belt 10 during the primary transfer process (hereinafter referred to as residual transfer toner) will be described. In this embodiment, the residual transfer toner is recovered by the developing roller 22 and reused. The method for recovering residual transfer toner by the developing roller 22 (hereinafter referred to as developing and recovering) will be explained using Figures 7(c) and 7(d).

[0052] Figure 7(c) is a schematic diagram of the potential relationship during development and recovery, and Figure 7(d) is a schematic diagram of the vicinity of the development unit during development and recovery. Of the toner developed on the photosensitive drum 1, the toner with a low charge amount and almost neutral polarity cannot be transferred to the intermediate transfer belt 10 in the primary transfer process and remains on the surface of the photosensitive drum 1 as transfer residue toner. As shown in Figures 7(c) and 7(d), the transfer residue toner is charged to the normal charge polarity by the charging voltage Vpri when it passes through the charging roller 2. At the same time, the charging voltage Vpri forms a post-charged drum potential Vp on the surface of the photosensitive drum 1, and subsequently a post-weak exposure drum potential Vd is formed by weak exposure. The transfer residue toner (charged to the normal charge polarity) on the drum surface where the post-weak exposure drum potential Vd is formed is recovered by the developing roller 22 by the electric field created by the potential difference ΔVb between the potential Vdc of the developing roller 22 formed by applying a DC voltage to the developing roller 22 and the post-weak exposure drum potential Vd, and is reused.

[0053] In describing the features of this embodiment 1, the development cycle of residual toner formed on the photosensitive drum Let's explain the harvesting process.

[0054] <Residual toner formed on the photosensitive drum when image forming operation is interrupted> If the image formation process is interrupted during printing, such as when a paper jam is detected, a toner image may remain formed on the photosensitive drum 1. Figure 8(a) shows a cross-sectional view of the photosensitive drum 1 and Figure 8(b) shows a longitudinal view of the photosensitive drum 1 after the image formation process has been interrupted. The toner coated on the developing roller 22 is called the pre-development coated toner and is represented by a black circle plot. The point where the developing roller 22 and the photosensitive drum 1 face each other is defined as P1 (development facing point), and the point where the photosensitive drum 1 and the intermediate transfer belt 10 face each other is defined as P2 (transfer facing point), and the distance on the circumference between P1 and P2 is denoted as Y. In this embodiment 1, Y is approximately 18 mm. The toner image formed between P1 and P2 is called the post-development drum toner and is represented by a dotted circle plot. The residual toner on the photosensitive drum surface is returned to the developing roller 22, and the surface of the photosensitive drum 1 is cleaned. Furthermore, the toner transferred to the intermediate transfer belt 10 immediately before the image formation operation is interrupted is called post-transfer belt toner and is indicated by a white circle plot. This toner is collected by the waste toner container 17, which serves as a cleaning means for the intermediate transfer belt 10.

[0055] <Development and recovery of residual toner formed on the photosensitive drum> The upper part of Figure 9 shows four steps in the development and recovery process, from the residual toner formed on the photosensitive drum 1 to the toner on the drum after development being returned to the developing roller 22 (Figures 9(a) to (d)). The lower part of Figure 9 shows the surface potential and development voltage of the photosensitive drum 1 at each step (Figure 9(e)). The surface potential referred to here is the potential of the surface of the photosensitive drum 1 between P3 and P4 shown in Figure 9.

[0056] Each step will be explained. Figure 9(a) shows the state immediately after the interruption of the image forming operation. Figure 9(b) shows the state after the drive source 550, such as the photosensitive drum 1, has been restarted from the interruption of the image forming operation, and the residual toner on the photosensitive drum 1 has passed through the charging means 2. Figure 9(c) shows the state in which the exposure means 3 exposes (emits light) the surface of the photosensitive drum 1 that has been charged by the charging means. Figure 9(d) shows the residual toner being collected by the developing roller 22.

[0057] In Figures 9(a) to 9(d), "drum toner" refers to "toner on the drum." Specifically, in Figure 9(a), it refers to "toner on the drum after development," in Figure 9(b), it refers to "toner on the drum after development (after charging)," in Figure 9(c), it refers to "toner on the drum after development (after weak exposure control)," and in Figure 9(d), it refers to "toner on the drum after development (after development and recovery)."

[0058] At Figure 9(a), the exposure means 3 forms an electrostatic latent image on the photosensitive drum 1, and the toner on the developing roller 22 moves onto the photosensitive drum 1. In the electrostatic latent image formation section between P3 and P4, the photosensitive drum potential is at the exposure section potential VL (-150V). At Figure 9(b), since the charging means 2, to which a charging voltage of -1200V is applied, passes through, the photosensitive drum potential between P3 and P4 is at the post-charging drum potential Vp (-700V). At Figure 9(c), the exposure means 3 performs weak exposure control by irradiating the photosensitive drum 1 with laser light. The photosensitive drum potential between P3 and P4 is at the post-weak exposure drum potential Vd (-600V). At Figure 9(d), the back contrast ΔVb, which shows the potential difference between the developing voltage (-300V) and the post-weak exposure drum potential Vd (-600V), is 300V. Most of the residual toner has a negative charge. As these toners pass between the developing roller 22 and the photosensitive drum 1, the unwanted toners with a negative charge are attracted to the developing roller 22, which has a relatively positive potential. This allows the residual toner to be collected in the developing container 241.

[0059] <Pixel Information Detection> Figure 10(a) shows that the image forming operation was interrupted (temporarily suspended) due to a paper jam in the transport unit. Figure 10(b) shows a longitudinal view of the photosensitive drum 1 in its current state. The area between P1 and P2 where residual toner exists after development is shown in color (dot pattern). Figure 10(b) shows an example of the original image when image formation was interrupted. Under the conditions of A4 paper, 600 dpi, and mono mode, a toner image is formed on the photosensitive drum 1d. The CPU 150 can calculate the timing of the interruption of the image formation operation, and therefore queries the interruption timing against the original image. This allows the CPU to calculate which part of the original image is drawn on the photosensitive drum 1d between P1 and P2 immediately after the image formation interruption. Thus, the corresponding positions in Figure 10(b) for P1 and P2 shown in Figure 10(a) can be obtained. In Figure 10(b), the area enclosed by a dotted line between P1 and P2 is defined as area Q. The toner image shown in area Q remains on the photosensitive drum 1d.

[0060] Figure 11(a) shows the original image in region Q. Region Q is an area extracted from the A4 paper area, with X being 210 mm (A4 paper width) and Y being 18 mm (distance between P1 and P2). At 600 dpi, region Q has 4961 pixels in the X direction and 425 pixels in the Y direction. Figure 11(b) shows an enlarged view of the image in region Q. The image is partially omitted, not showing the entire vertical and horizontal direction. Figure 11(c) shows the pixel information detection result calculated from the image information of region Q. Each pixel has density information, and each pixel has density information managed with 8 bits from 0 to 255. If this density information is greater than 0, it is determined that the pixel has color (density) and is set to "1". If this density information is 0, it is determined that the pixel does not have color (density) and is set to "0". The result of converting the density information to "1" or "0" in this way is called the pixel information detection result. When acquiring information about the toner image in this way, the engine control unit 210 functions as an image information acquisition means.

[0061] <Pixel information detection results and low exposure control> The photosensitive drum potential in the process shown in Figure 9 will be explained. In Figure 9(a), the photosensitive drum potential is VL (-150V). In Figure 9(b), the charging means 2 reduces the photosensitive drum potential to Vp (-700V). In Figure 9(c), the exposure means 3 irradiates the photosensitive drum 1 with laser light, changing the photosensitive drum potential to Vd (-600V). This laser light irradiation process by the exposure means 3 is called weak exposure control.

[0062] The relationship between this weak exposure control and the pixel information detection result will be explained. Figure 1(a) (top of Figure 1) shows the pixel information detection result in region Q. Figure 1(b) (bottom left of Figure 1) shows the relationship between the light intensity of weak exposure, the photosensitive drum potential, and the development voltage when the pixel information detection result is 1. Figure 1(c) (bottom right of Figure 1) shows the relationship between the light intensity of weak exposure, the photosensitive drum potential, and the development voltage when the pixel information detection result is 0. Note that the maximum output of the exposure means is expressed as 255 / 255, and zero exposure output of the exposure means is expressed as 0 / 255. Weak exposure control is the control of the output of this exposure means within the range of 0 / 255 to 60 / 255.

[0063] In Figure 1(b), when the pixel information detection result is "1", the low exposure amount is set to 25 / 255, and the drum potential Vd1 after low exposure is controlled to -600V. The development voltage is then controlled to -300V. This allows the back contrast ΔVb1 to be controlled to a potential difference of 300V. Since the residual toner after development has a negative charge, it can be returned to the development roller 22, which has a relatively positive potential.

[0064] In Figure 1(c), when the pixel information detection result is "0", the weak exposure amount is set to 50 / 255, and the drum potential Vd0 after weak exposure is controlled to -450V. This allows the back contrast ΔVb0 to be controlled to a potential difference of 150V. Because the absolute value of the negative polarity side of the photosensitive drum potential is small, the movement of positively charged toner coated on the developing roller to the photosensitive drum 1 can be suppressed. In other words, the movement of inverted fouling toner onto the photosensitive drum 1 can be prevented.

[0065] In this embodiment, the area on the photosensitive drum 1 in which development and recovery can be performed is defined as the area to be recovered. The area to be recovered has a predetermined range in the direction along the rotation axis of the photosensitive drum 1. For example, the area to be recovered is the area facing the development roller 22 in the direction along the rotation axis of the photosensitive drum 1. Then, within the area to be recovered, a first area and a second area are set so as to be aligned in the direction along the rotation axis of the photosensitive drum 1. The area where the pixel information detection result is "0", that is, the area where no toner exists, is defined as the first area. The area where the pixel information detection result is "1", that is, the area where toner exists and the toner density is higher than that of the first area, is defined as the second area. At this time, the second back contrast (ΔVb1) of the second area ) is 300V, which is greater than the first back contrast (ΔVb0) of the first region, which is 150V.

[0066] Thus, a key feature of this embodiment 1 is that the amount of light for weak exposure is adjusted based on the pixel information detection results. The amount of light for weak exposure may be controlled for each detected pixel, or it may be controlled for each light intensity control unit that includes multiple pixels. The light intensity control unit may be of a fixed size; for example, a group of square pixels or a predetermined number of pixels in the scanning direction during exposure may be exposed with a common light intensity. When adjusting the light intensity for each single pixel, one pixel can be considered as one light intensity control unit.

[0067] <Verification of the effectiveness of low-exposure control based on pixel information detection results> Figure 12(a) shows the back contrast ΔVb and the recovery efficiency (recovery rate) of residual toner to the developing roller 22 after development. Figure 12(b) shows the density of fouled toner on the photosensitive drum 1 when the back contrast ΔVb is controlled. Figure 12(c) shows a summary of the results from Figures 12(a) and 12(b).

[0068] The experimental conditions shown in Figure 12(a) are described below. Here, a Bk toner cartridge is used. First, a toner image with a maximum density of 255 is formed within the image formation area, and the image formation process is temporarily paused. When restarting after the pause, the amount of light from a weak exposure applied to the photosensitive drum 1 that has passed through the charging means is varied to check the level of the back contrast ΔVb. Specifically, the back contrast ΔVb is varied to allow the residual toner after development to be collected by the developing roller 22. Then, the amount of toner remaining on the photosensitive drum is measured, and the collection efficiency to the developing roller 22 is calculated. According to Figure 12(a), it can be seen that the larger the back contrast ΔVb, the higher the development and collection efficiency. The back contrast ΔVb1 in the area where residual toner exists on the photosensitive drum is controlled to 300V. Therefore, it can be seen that the development and collection efficiency is close to 100%. The back contrast ΔVb0 in the area where no residual toner exists on the photosensitive drum is controlled to 150V. In this case, since there is no toner image to be developed and collected on the photosensitive drum, it is not a problem to set a back contrast with low development and collection efficiency. In this embodiment 1, the back contrast is controlled to be high for pixels that have density information, and high development and recovery efficiency can be obtained in that pixel region.

[0069] The experimental conditions shown in Figure 12(b) are described below. Here, a Bk toner cartridge is used. First, image formation with a density information of 0 is started, and the image formation process is temporarily paused during this operation. When restarting after the pause, the amount of light from a weak exposure applied to the photosensitive drum 1 that has passed through the charging means is varied to check the level fluctuation of the back contrast ΔVb. In this case, the density of fouling toner on the photosensitive drum is measured after the surface of the photosensitive drum, which has received a weak exposure, has passed through the point where the developing roller 22 and the photosensitive drum 1 face each other (developing facing section P1). The fouling toner density was measured with a Macbeth densitometer (manufacturer: Gretag Macbeth). From the graph in Figure 12(b), it can be seen that when the back contrast ΔVb is low at 50V, the fouling density is high. When the back contrast ΔVb is high at 300V, the fouling density is also high. On the other hand, it can be seen that when the back contrast ΔVb is controlled to about 120-200V, the fouling density is extremely low. In this configuration, the back contrast ΔVb1 is controlled to 300V in areas where toner is present on the drum after development, and the back contrast ΔVb0 is controlled to 150V in areas where toner is absent. In areas where no toner is present on the photosensitive drum, the back contrast ΔVb0 is controlled to 150V, so almost no fouling toner is generated on the photosensitive drum. In this embodiment 1, the back contrast is controlled to be small for pixels that do not have density information, thereby reducing the amount of fouling toner generated on the photosensitive drum. In this example, a fouling density of less than 0.1 is preferable.

[0070] The verification results are summarized using Figure 12(c). The characteristic of this embodiment 1 is that the back contrast during development and recovery is controlled based on the pixel information detection result of residual toner after development formed on the photosensitive drum. The amount of weak exposure control differs depending on whether the pixel information detection result is "1" or "0". The amount of weak exposure when it is "0" is greater than the amount of weak exposure when it is "1". This makes it possible to control the back contrast ΔVb during development and recovery. When the pixel information detection result is "1", a back contrast with high development and recovery efficiency is required, so ΔVb1 = 300V is set considering the balance with the fogging density. The development and recovery efficiency is high at 96%, and the fogging toner density is low at 0.05.

[0071] If the pixel information detection result is "0", there is no need to develop and recover the image, so it can be controlled with a back contrast that has less fouling toner. Therefore, ΔVb0 = 150V. Development and recovery efficiency cannot be measured, so there is no data, and the fouling toner density is extremely low at 0.01.

[0072] If the back contrast ΔVb were controlled uniformly across the entire photosensitive drum without using image information, the toner density in areas where no toner image exists would be 0.05 instead of 0.01. A low toner density is the effect of this invention.

[0073] The features of this embodiment 1 are summarized below. When returning the residual toner on the photosensitive drum after development to the developing roller 22, the exposure amount to the photosensitive drum 1 by the exposure means is controlled according to the presence or absence of a toner image on the photosensitive drum. The exposure amount is reduced in areas where a residual toner image is present to improve development recovery efficiency, and the exposure amount is increased in areas where there is no residual toner image to reduce inverted toner fouling.

[0074] This allows for the image formation process to stop midway, enabling the recovery of residual toner formed on the photosensitive drum into the developing container with high efficiency, while simultaneously reducing the amount of reverse fouling toner that moves to the photosensitive drum 1. Since there is less reverse fouling toner, it is possible to suppress the adhesion of reverse fouling toner to the surface of the charging means and prevent the charging means from becoming dirty.

[0075] [Example 2] Example 1 described a method for controlling the back contrast ΔVb by distinguishing between areas where residual toner is present and areas where it is not present after development following an interruption of the image forming operation. Example 2 describes a method for controlling the back contrast ΔVb according to the concentration of residual toner after development following an interruption of the image forming operation. Example 2 can further improve the development recovery efficiency and further reduce the amount of inversion fouling toner compared to Example 1. The following description will focus on the differences from Example 1.

[0076] <Pixel Information Detection> The conditions and basic procedures for pixel information detection are the same as in Example 1, so the explanation will be omitted. Image information (density information) is managed with 8 bits from 0 to 255. If this image information (density information) is in the range of 1 to 128, the detection result is set to "2", if it is between 129 and 255, the detection result is set to "1", and if it is 0, the detection result is set to "0". In Example 2, the image has color information. Image information is divided into two groups, and control appropriate to each group is performed. In Example 2, cases where the concentration is 1 or greater are divided into two groups, resulting in a total of three detection results. However, the number of groups may be increased further.

[0077] <Pixel information detection results and low exposure control> A feature of this second embodiment is that, in the weak exposure control before developing and recovering residual toner after development, the amount of weak exposure is controlled based on the color information (density information) of the pixel information.

[0078] The upper part of Figure 13 (Figure 13(a)) shows an image of region Q where residual toner exists after development, a magnified view of that image, and the pixel information detection results corresponding to the magnified view. In the image of region Q, the shaded areas are midtones with a Bk density of 128. The black areas are the maximum Bk density of 255. The white areas have no Bk color information. According to the pixel information detection rules described above, the image information (density information) for each pixel is classified into one of 2, 1, or 0.

[0079] The lower part of Figure 13 (Figures 13(b) to (d)) shows the relationship between the low exposure amount, the photosensitive drum potential, and the development voltage when the pixel information detection result is 2, 1, or 0. Figure 13(b) shows the case when the detection result is "2", Figure 13(c) shows the case when the detection result is "1", and Figure 13(d) shows the case when the detection result is "0".

[0080] When the pixel information detection result in Figure 13(b) is "2", the weak exposure amount is set to 40 / 255, and the drum potential Vd2 after weak exposure is controlled to -500V. The development voltage is controlled to -300V. This allows the back contrast ΔVb2 to be controlled to a potential difference of 200V. In the region of detection result "2", the density of toner formed on the photosensitive drum is relatively low (the amount of toner is small) compared to the region of detection result "1", so even if the back contrast ΔVb is small, the negatively charged residual toner after development can be returned to the development roller 22, which has a relatively positive potential. Furthermore, when the pixel information detection result is "2", the potential difference of the back contrast ΔVb2 is small, and the absolute value of the negative polarity side of the photosensitive drum potential is small, so the positively charged toner coated on the development roller hardly moves to the photosensitive drum 1. In other words, the generation of reverse fouling toner on the photosensitive drum can be suppressed.

[0081] When the pixel information detection result in Figure 13(c) is "1", the low exposure amount is set to 25 / 255, and the drum potential Vd1 after low exposure is controlled to -600V. The development voltage is controlled to -300V. This allows the back contrast ΔVb1 to be controlled to a potential difference of 300V. The residual toner after development, which has a negative charge, can be returned to the development roller 22, which has a relatively positive potential.

[0082] When the pixel information detection result in Figure 13(d) is "0", the weak exposure amount is set to 50 / 255, and the drum potential Vd0 after weak exposure is controlled to -450V. This allows the back contrast ΔVb0 to be controlled to a potential difference of 150V. The absolute value of the negative polarity side of the photosensitive drum potential is small, and the positively charged toner coated on the developing roller hardly moves to the photosensitive drum 1. In other words, the generation of reverse fouling toner on the photosensitive drum can be suppressed.

[0083] Based on the above, the toner density increases in the order of 0 > 2 > 1 when the pixel information detection result is 0. Then, the amount of weak exposure is increased in this order, and the control is made so that the absolute value of the back contrast ΔVb increases. Similarly, when the number of detection result segments is increased, the control is made so that the back contrast is greater in areas with higher toner density.

[0084] <Verification of the effectiveness of low-exposure control based on pixel information detection results> Figure 14(a) shows the back contrast ΔVb and the recovery efficiency (recovery rate) of residual toner to the developing roller 22 after development. Figure 14(b) shows the effect when the back contrast ΔVb is controlled. This shows the density of fouled toner on the optical drum. Figure 14(c) shows a summary of the results from Figures 14(a) and 14(b).

[0085] The experimental conditions shown in Figure 14(a) are described below. Here, a Bk toner cartridge is used. Two cases were examined: one where the image information is a toner image with a midtone density of 128, and another where the image information is a toner image with a maximum density of 255. The examination involved starting image formation on the photosensitive drum 1 at each density, then pausing the image formation process. When restarting after the pause, the amount of weak exposure light applied to the photosensitive drum 1 that had passed through the charging means was varied to check the level of the back contrast ΔVb. Specifically, the back contrast ΔVb was varied to allow the residual toner after development to be collected by the developing roller 22. After that, the amount of toner remaining on the photosensitive drum was measured, and the collection efficiency to the developing roller 22 was calculated. In Figure 14(a), the white triangle plots represent the results for the midtone case with an image information of 128, and the white circle plots represent the development and collection efficiency for the maximum density case with an image information of 255. In both cases, the larger the back contrast ΔVb, the higher the development and collection efficiency. Furthermore, midtone images with an image information of 128 have higher development and recovery efficiency than maximum density images with an image information of 255, even with the same back contrast ΔVb. In other words, when developing and recovering low-density toner images, high development and recovery efficiency can be achieved even with a low back contrast ΔVb. This is because the more toner there is on the photosensitive drum, the more layers of toner accumulate, making it more difficult for the toner to return to the developing roller 22. Therefore, qualitatively, the less toner there is on the photosensitive drum, the higher the development and recovery efficiency.

[0086] Figure 14(b) shows the back contrast ΔVb and fogging density. The conditions and the plotted white squares are the same as those shown in Figure 12(b) in Example 1, so the explanation is omitted. As explained in Example 1, it can be seen that controlling the back contrast ΔVb within the range of 120 to 200V is appropriate for suppressing fogging toner.

[0087] In the case of low-density toner images, high development and recovery efficiency can be achieved even with a low back contrast ΔVb, allowing ΔVb to be controlled to a low value of around 200V. Therefore, the fogging density shown in Figure 14(b) can be reduced.

[0088] The verification results are summarized using Figure 14(c). When the image information is low in density, such as a midtone of 128, the pixel information detection result is "2", and the weak exposure is set to 40 / 255, controlling the back contrast ΔVb to 200V. The development and recovery efficiency is high at 96%, and the fouling toner density is extremely low at 0.01. Because the toner density of the toner to be developed and recovered is low, a high back contrast is not required, thus achieving both fouling toner suppression and development and recovery efficiency. When the image information is at the maximum density of 255, the pixel information detection result is "1", and the weak exposure is set to 25 / 255, controlling the back contrast ΔVb to 300V. The development and recovery efficiency is high at 96%, and the fouling toner density is low at 0.05. Because the toner density of the toner to be developed and recovered is high, a high back contrast of 300V is required. When there is no image information, the pixel information detection result is "0", and the weak exposure is set to 50 / 255, controlling the back contrast ΔVb to 150V. There is no toner image to develop and recover, and the fouled toner density is extremely low at 0.01. Since the back contrast ΔVb can be controlled to suppress fouled toner generation, fouled toner can be reduced to almost zero.

[0089] In this test, the development and recovery efficiency was the same at 96% for both a back contrast of 200V and 300V. We believe that the development and recovery efficiency is inherently lower at a back contrast of 200V than at 300V, and we will consider the reasons for this. A lower back contrast results in a lower development and recovery rate, but also a smaller amount of toner fouling. In this test, we measured the amount of toner on the photosensitive drum after it passed through the developing roller. The fact that the development and recovery efficiency was the same under different back contrast conditions is thought to be due to the influence of this amount of toner fouling.

[0090] In this embodiment, the area on the photosensitive drum 1 where development and recovery are performed is considered the area to be recovered, and the same examination as in Embodiment 1 is carried out. First, the area where the pixel information detection result is "0" is considered the first area, and the area where the pixel information detection result is "1" with a higher toner density is considered the second area. At this time, the first back contrast (ΔVb0) of the first area is 150V, and the second area The second back contrast (ΔVd1) is 300V, and the second back contrast is greater than the first back contrast.

[0091] Next, we examine the region where the pixel information detection result is "2" as the first region, and the region where the pixel information detection result is "1" with a higher toner density as the second region. At this time, the first back contrast (ΔVb2) of the first region is 200V, and the second back contrast (ΔVd1) of the second region is 300V, and the second back contrast is the same as the first back contrast. It's bigger than a street.

[0092] Next, we examine the region where the pixel information detection result is "0" as the first region, and the region where the pixel information detection result is "2" with a higher toner density as the second region. At this time, the first back contrast (ΔVb0) of the first region is 150V, and the second back contrast (ΔVd2) of the second region is 200V, and the second back contrast is the same as the first back contrast. It's bigger than a street.

[0093] The features of this second embodiment are summarized below. When returning the residual toner on the photosensitive drum after development to the developing roller, the exposure amount to the photosensitive drum 1 by the exposure means is controlled using information related to the density of the residual toner image on the photosensitive drum. Similar to the first embodiment, the exposure amount is controlled to be small in areas where there is a residual toner image to improve development recovery, and large in areas where there is no residual toner image to reduce inversion toner. Furthermore, in this embodiment, the exposure amount can be controlled in detail according to the density of the toner image in areas where there is a residual toner image, and the exposure amount may be controlled to be lower as the density of the residual toner image increases. Note that the method for changing the back contrast ΔVb is not limited to changing the exposure amount, and for example, the charging voltage by the charging roller 2 or the development voltage applied to the developing roller 22 may be changed. In addition, exposure amount control, charging voltage control and development voltage control may be controlled in combination as appropriate.

[0094] This allows for the image formation process to be stopped midway, enabling the recovery of residual toner formed on the photosensitive drum after development into the developing container with high efficiency, while simultaneously reducing the amount of reverse fouling toner that moves to the photosensitive drum 1. Since there is less reverse fouling toner, it is possible to suppress the adhesion of reverse fouling toner to the surface of the charging means and prevent the charging means from becoming dirty.

[0095] In these embodiments 1 and 2, the pixel information detection result was calculated based on image information (density information) sent from the PC (host computer 199). Based on the image information sent from the PC, the CPU circuit 150 on the engine control unit 210 controls the exposure means. Similar control can also be achieved by using the output of the exposure means or the pulse width control amount of the exposure means.

[0096] [Example 3] Examples 1 and 2 described the development and recovery of residual toner formed on the photosensitive drum after development when the image formation operation was interrupted due to an error in paper transport, etc. Example 3 describes an invention in which toner images formed on the photosensitive drum are developed and recovered for reasons different from those in Examples 1 and 2.

[0097] In the process cartridge described so far, the toner coated on the developing roller repeatedly rubs against the photosensitive drum 1. The toner that has been rubbed repeatedly becomes deformed or The toner deforms or deteriorates. To remove only this deteriorated toner from the developing roller and developing container, a toner image is formed on the photosensitive drum, and this toner image is developed and collected. This section describes toner ejection control. In other words, the image forming apparatus 100 of this embodiment can operate in an image forming mode in which an image forming unit forms a toner image and transfers it to the transfer target, and in an ejection mode in which toner is ejected to the transfer target.

[0098] <Toner ejection control> In the control of the embodiment of the present invention, deteriorated toner (hereinafter referred to as deteriorated toner) generated with the use of the process cartridge can be efficiently discharged from the developing container without discharging a large amount of toner that has not deteriorated significantly (hereinafter referred to as fresh toner).

[0099] Specifically, the control of this embodiment is characterized by the operation and control method of ejecting toner (including both degraded toner and fresh toner) from the developing container onto the photosensitive drum 1, then selectively recovering the fresh toner in the developing unit, and recovering the degraded toner into the waste toner container 17 by the intermediate transfer belt cleaning device 16.

[0100] In this embodiment, the control system allows for efficient toner discharge by selectively discharging degraded toner from the developing container while recovering a portion of the fresh toner into the developing container. As a result, unnecessary toner consumption can be suppressed. At the same time, the amount of toner recovered in the waste toner container 17 can be reduced, thus reducing the frequency of replacement of the waste toner container 17. This will be explained using Figures 15 and 16. Hereafter, the toner discharged from the developing container will be referred to as discharged toner, and the series of operations will be referred to as discharge control.

[0101] Figures 15(a) to (f) are schematic diagrams illustrating the movement of ejected toner when ejection control is performed, and Figure 16 is a schematic diagram showing the potential of the photosensitive drum 1, the developing voltage, the primary transfer voltage, and the movement of toner along the time axis during ejection control. Note that (a) to (e) in Figure 16 correspond to Figures 15(a) to (e), respectively. Also, circles enclosed by solid lines in Figure 16 represent the toner after movement, and circles enclosed by dashed lines represent the toner before movement.

[0102] This control system can be broadly divided into: <1> Toner ejection from inside the developing unit, <2> The toner is ejected and passes through the primary transfer section. <3> The toner being ejected passes through the charged part. <4> Toner sorting in the developing section, <5> Transfer of degraded toner, <6> The process can be divided into steps: processing degraded toner, and handling degraded toner. Each step is explained below.

[0103] <1> Toner ejection from the developing unit (corresponding to Figures 15(a) and 16(a)) When the ejection control operation is started, the photosensitive drum 1 is uniformly charged to a predetermined negative potential (post-charging drum potential Vp = -700V) by the charging roller 2 during its rotation, and then exposed by the scanner unit 3, which acts as the exposure means. As a result, a latent image potential of exposure area potential VL (-100V in this embodiment) is formed on the photosensitive drum 1.

[0104] Subsequently, as shown in Figure 15(a), at the position where the developing roller 22 and the photosensitive drum 1 come into contact, the potential difference between the developing voltage Vdc (-300V) and the exposure potential VL (developing contrast and Then, the toner carried on the developing roller 22 is ejected onto the photosensitive drum 1 by ΔVc) in Figure 16. Here, the ejected toner contains a mixture of degraded toner and fresh toner, with degraded toner shown as white circles and fresh toner as black circles in Figure 15.

[0105] In this ejection control, the exposure amount by the exposure means is set differently from that during the image formation operation, so that the exposure potential VL and ΔVc are greater than during the image formation operation. This is to ensure that the toner on the developing roller 22 is reliably developed onto the photosensitive drum 1.

[0106] Furthermore, the length of the photosensitive drum 1 in the rotational direction for the ejected toner at one time is preferably at least the length of one rotation of the developing roller 22, and preferably within the length of one rotation of the photosensitive drum 1. Since toner near the developing roller 22 is expected to be consumed easily, it is preferable that the length be at least the length of one rotation of the developing roller 22 in order to eject all the toner coated on the developing roller 22 immediately before ejection control. On the other hand, due to the potential relationship of the photosensitive drum, if toner ejection in the developing unit and the recovery of fresh toner in the developing unit described later are performed simultaneously, the recovery efficiency may decrease, so it is preferable that the length of the photosensitive drum 1 in the rotational direction for the ejected toner is within one rotation of the photosensitive drum.

[0107] In this embodiment, the developing roller 22 discharges a length of 44.8 mm (= 10 mm × 3.14 ÷ 1.4 × 2), which is less than the length of one rotation of the photosensitive drum 1, which is 62.8 mm (= 20 mm × 3.14).

[0108] Although there is a limit to the length of the rotational direction of the toner ejected from the photosensitive drum 1 at one time, the total amount of toner ejected can be adjusted by repeating this ejection control.

[0109] <2> Toner passes through the primary transfer section (corresponding to Figures 15(b) and 16(b)). Next, as the ejected toner passes through the primary transfer section, a voltage of -600V is applied from the primary transfer power supply 15 to the primary transfer roller 14, which is a metal roller. As shown in Figure 16(b), a potential difference ΔV1 is formed between the photosensitive drum 1 and the intermediate transfer belt 10 in the primary transfer section, such that the ejected toner (normally charged polarity) remains on the photosensitive drum 1. As a result, as shown in Figure 15(b), the ejected toner remains supported on the photosensitive drum 1 as it passes through the primary transfer section.

[0110] If the potential formed on the intermediate transfer belt 10 is negative and its absolute value is greater than the absolute value of the exposure area potential VL of the photosensitive drum 1a, the ejected toner can be left on the photosensitive drum 1a. This is because the ejected toner is charged to a normal charging polarity (negative polarity) due to friction with the developing blade 23, and is therefore electrostatically attracted to the intermediate transfer belt 10, where a negative potential greater than the absolute value of the potential of the photosensitive drum 1 (exposure area drum potential VL) is formed. As a potential difference, if it is about the same as or greater than the primary transfer contrast ΔVtr1 during image formation, the ejected toner can be left on the photosensitive drum 1. However, if the potential difference ΔV1 is too large, there is a risk that abnormal discharge will occur in the primary transfer section and the polarity of the ejected toner will be reversed, so in the configuration of this embodiment, it is preferable that the potential difference ΔV1 is less than 1500V. In this embodiment, a potential of -600V was formed on the intermediate transfer belt 10 by the primary transfer power supply 15 so that the absolute value of the potential difference ΔV1 was 500V.

[0111] <3> Toner passes through the charged section during ejection (corresponding to Figures 15(c) and 16(c)). Next, the ejected toner that has passed through the primary transfer section passes through the point where the charging roller 2 and the photosensitive drum 1 come into contact (the charging section). As shown in Figure 16(c), a negative voltage of -1200V is applied to the charging roller 2 as the ejected toner passes through the charging section. As a result, the potential difference ΔV2 (hereinafter referred to as the charging contrast ΔV2) between the potential of the photosensitive drum 1 and the voltage of the charging roller 2 prevents the ejected toner from adhering to the charging roller 2. At the same time, the post-charging drum potential Vp is formed and the ejected toner is charged.

[0112] Furthermore, from any of the above viewpoints, it is preferable for the charge contrast ΔV2 to be somewhat large, and it is sufficient if it is equal to or greater than the charge contrast during image formation operation. However, if the charge contrast ΔV2 is too large, abnormal discharge may occur in the charged area, reversing the toner polarity and potentially causing it to adhere to the charged roller 2. For this reason, in this embodiment, the charge contrast ΔV2 was set to 1100V.

[0113] <4> Toner sorting in the developing section (corresponding to Figures 15(d) and 16(d)) Next, we will explain the separation of deteriorated toner and fresh toner at the point where the developing roller 22 and the photosensitive drum 1 come into contact (hereinafter referred to as the developing section), which is a characteristic feature of this embodiment. The ejected toner that has passed through the charged section passes through the developing section. At this time, as shown in Figure 16(d), a developing voltage Vdc = -300V is applied to the developing roller 22, and a Vd = -600V is formed on the surface of the photosensitive drum due to weak exposure. The ejected toner is recovered by the potential difference ΔVb between this post-weak exposure drum potential Vd and the developing voltage Vdc.

[0114] The ejected toner contains a mixture of fresh toner and deteriorated toner. The deteriorated toner is subject to long-term rubbing against the developing blade 23, which may cause deformation of the toner shape itself, or peeling or embedding of the external additives on the toner surface. When the toner shape is deformed, the contact area between the toner and the photosensitive drum 1 increases. For example, when the external additives on the toner surface are peeled off, the resin component of the toner comes into contact with the photosensitive drum 1, and when the external additives are embedded in the toner, the contact area between the toner and the photosensitive drum 1 may increase. As a result, the non-electrostatic adhesion force of the deteriorated toner to the photosensitive drum 1 is higher than that of the fresh toner. On the other hand, since the fresh toner has less deterioration, the amount of adhesion to the photosensitive drum 1 is lower than that of the deteriorated toner, which is the opposite. Therefore, in the developing section of the ejected toner, it is easier to recover the fresh toner, and it is difficult to recover the deteriorated toner.

[0115] Using the difference in non-electrostatic adhesion force between such deteriorated toner and fresh toner, as shown in Fig. 15(d), selectively recovering the fresh toner in the developing section (not selectively recovering the deteriorated toner) is a feature of this embodiment. Since a large amount of fresh toner with less deterioration can be recovered from the ejected toner, the life of the developing device can be extended without consuming unnecessary toner. In this embodiment, as an example, the BET value of the fresh toner is about 2.8 m 2 / g, and the BET value of the deteriorated toner is 2.0 m 2 / g or less. However, this process enables the sorting of toner even when the difference in non-electrostatic adhesion force is 0.8 m 2 / g or less in terms of the BET value. That is, such selective toner ejection can be implemented even at a time when the difference in non-electrostatic adhesion force is small (for example, at the initial stage of durability when the number of sheets passed is still small). Generally, since deterioration progresses as the durability progresses and the number of sheets passed increases, the effect of implementing this sequence is greater at a time when the number of sheets passed has increased (for example, from the middle to the latter half of durability).

[0116] The recovery efficiency of ejected toner when it is collected by the developing roller 22 can be controlled by the potential difference ΔVb. First, as the absolute value of the potential difference ΔVb increases within the range below the discharge threshold, the electric field that moves the ejected toner toward the developing roller 22 becomes stronger, thus improving the recovery efficiency. However, if the potential difference ΔVb is greater than the discharge threshold between the developing roller 22 and the photosensitive drum 1, the polarity of the ejected toner will reverse due to discharge, and the recovery efficiency will decrease. On the other hand, if the absolute value of the potential difference ΔVb is small, not only will the recovery efficiency decrease, but the toner coated on the developing roller 22 will not be able to be held on the developing roller 22 and will be developed on the photosensitive drum 1 (this unintended development is called fogging). As described above, the potential difference ΔVb must be set to be below the discharge threshold, within a range where fogging does not occur, and to selectively collect fresh toner toward the developing roller 22. The potential difference ΔVb should be between 100V and 500V, and should be adjusted appropriately based on the properties of the toner being used (adhesion, charge, shape, and degree of degradation).

[0117] In this embodiment, the toner T used was set to -300V, so that the absolute value of the potential difference ΔVb was 300V. In this embodiment, the back contrast ΔVb was formed by weak exposure, but this is not the only way. In the case of an image forming apparatus 100 that does not have weak exposure, an appropriate charging contrast ΔV2 and back contrast ΔVb can be secured by adjusting the charging voltage.

[0118] As explained above, a large portion of the fresh toner can be collected by the developing roller 22, while a large portion of the degraded toner remains on the photosensitive drum 1.

[0119] <5> Transfer of degraded toner (corresponding to Figures 15(e) and 16(e)) Next, the degraded toner remaining on the photosensitive drum 1 after the sorting and recovery process in the developing unit is transferred to the intermediate transfer belt 10. Since the degraded toner that has passed through the developing unit is negatively charged (normal charge polarity), as shown in Figure 15(e), a positive primary transfer voltage is applied to transfer the degraded toner to the intermediate transfer belt 10. At this time, as shown in Figure 16(e), the potential of the surface of the photosensitive drum 1 where the degraded toner remains is the drum potential Vd after weak exposure, and the degraded toner is transferred to the intermediate transfer belt 10 by the potential difference ΔVtr2 between Vd and the primary transfer voltage Vtr.

[0120] As mentioned above, when transferring degraded toner to the intermediate transfer belt 10, the degraded toner has a high adhesion force to the photosensitive drum 1, making it difficult to transfer to the intermediate transfer belt 10 with the same potential difference as during normal image formation. Therefore, the potential difference ΔVtr2 needs to be set so that the degraded toner can be reliably transferred to the intermediate transfer belt 10. At this time, the degraded toner remaining on the photosensitive drum 1 is the toner that was not collected by the developing roller 22 at the potential difference ΔVb during the previous recovery by the developing roller 22, so it is preferable that the potential difference ΔVtr2 be greater than or equal to the back contrast ΔVb during the development and recovery of fresh toner. Also, as mentioned above, the degraded toner has a high adhesion force to the photosensitive drum 1 and is difficult to transfer. Therefore, it is preferable that it be greater than the transfer contrast ΔVtr1 during normal image formation operation. In other words, it is preferable to make the potential difference ΔVtr2 greater than the larger of the back contrast ΔVb during the development and recovery of fresh toner or the transfer contrast ΔVtr1 during normal image formation operation. However, if the potential difference ΔVtr2 is too large, there is a risk of abnormal discharge occurring in the primary transfer section, so it is preferable that ΔVtr2 be less than 2000V.

[0121] In this example, when transferring degraded toner to the intermediate transfer belt 10, the transfer is performed at a potential difference of ΔVtr2 = 900V between the drum potential Vd = -600V and the primary transfer voltage Vtr = +300V after weak exposure. They are doing it.

[0122] <6> Processing of degraded toner (corresponding to Figure 15(f)) Finally, the processing of the degraded toner transferred onto the intermediate transfer belt will be explained using Figure 15(f).

[0123] The transferred degraded toner is sent to the intermediate transfer belt cleaning device 16 by the rotation of the intermediate transfer belt 10 and collected and processed in the waste toner container 17. In this way, only a portion of the discharged toner, which contains a large amount of degraded toner, can be sent to the waste toner container 17, thus delaying the container's fullness compared to sending all the discharged toner to the waste toner container 17. Therefore, the frequency of the user having to replace the waste toner container 17 can be reduced.

[0124] Furthermore, once the ejected toner has finished passing through the developing section (between Figure 15(e) and Figure 15(f)), the developing roller 22 is immediately separated from the photosensitive drum 1, and the rotation of the developing roller 22 is stopped. This is because, as the developing roller 22 rotates, the toner on the developing roller 22 rubs against the developing blade 23. The purpose of immediately separating the developing roller 22 from the photosensitive drum 1 when contact is no longer necessary is to avoid unnecessary rubbing.

[0125] Furthermore, in this embodiment, when sending the negatively charged degraded toner on the intermediate transfer belt 10 of the intermediate transfer belt cleaning device 16 to the intermediate transfer belt cleaning device 16, a negative voltage is applied to the secondary transfer roller 20 so that it passes through without adhering to the secondary transfer roller 20. It is.

[0126] In this embodiment, a voltage is applied to the secondary transfer roller 20 when the degraded toner passes over it to prevent the degraded toner from adhering to the secondary transfer roller 20. If the absolute value of the voltage applied to the secondary transfer roller 20 is too low, the toner will adhere to the secondary transfer roller 20. Conversely, if it is too high, the toner polarity will reverse due to abnormal discharge, causing it to adhere to the secondary transfer roller 20. Therefore, the voltage applied to the secondary transfer roller 20 is preferably around -300 to -1000V, and in this embodiment, a voltage of -500V is applied to the secondary transfer roller 20. In this embodiment, the secondary transfer roller 20 is in contact with the intermediate transfer belt 10, but if there is a mechanism for separating the secondary transfer roller 20, the secondary transfer roller 20 may be separated from the intermediate transfer belt 10 to prevent the degraded toner from adhering to the secondary transfer roller 20.

[0127] Furthermore, the degraded toner transferred onto the intermediate transfer belt 10 does not necessarily need to be sent to the intermediate transfer belt cleaning device 16 during the ejection control. For example, the ejection control may be terminated once the degraded toner has been transferred onto the intermediate transfer belt 10. The degraded toner remaining on the intermediate transfer belt 10 may be sent to the intermediate transfer belt cleaning device 16 for collection and processing by the rotational movement of the next normal image forming operation. In this way, it becomes possible to perform the ejection control without causing unnecessary downtime.

[0128] In this embodiment, during the output mode, the development voltage is set to Vdc = -300V, the same as during the image forming operation, and the charging voltage and exposure amount are different from those during the image forming operation. However, this is not limited to this, and the development voltage Vdc may be changed in order to change ΔVb or ΔVc.

[0129] <Average printing rate> The ratio of degraded toner to fresh toner on the developing roller and in the developing container varies depending on the history of the image being printed. In the longitudinal direction perpendicular to the paper transport direction, areas with high toner usage have a low proportion of degraded toner because fresh toner is consumed during the transition to degraded toner. On the other hand, areas with low toner usage have a higher proportion of degraded toner because toner remains on the developing container and developing roller for longer periods.

[0130] In this embodiment 3, the longitudinal direction of the paper area (the direction perpendicular to the paper transport direction) is divided into several sections, the average print density for each section is calculated, and the percentage of degraded toner for each section is calculated. The method for doing so is described below.

[0131] Figure 17(a) shows an example of division within an A4 paper area. The A4 paper area is divided into multiple regions along its length (three in this case). These regions are defined as D1, D2, and D3 from left to right in the figure. The A4 paper area is 210mm wide x 297mm wide. Each region is set to the same size of 70mm x 297mm. The sizes of the regions can be different, or the number of divisions can be increased. At 600dpi, each region consists of 1654 x 7016 = 11,604,464 pixels.

[0132] The pixel counting rule is explained in the image shown in Figure 17(b). The case of a Bk process cartridge is explained. Each pixel has density information from 0 to 255. The total number of pixels with density information greater than 0 is calculated in each of the D1, D2, and D3 regions. Since this counts the number of pixels where toner is present, the calculated total is called the total toner pixel count.

[0133] The image in Figure 17(b) shows that the entire D1 region is a solid black image, half of the D2 region is a solid black image, and the D3 region has no pixels with density information. Note that "solid" refers to a collection of pixels with density information of 255. The total number of toner pixels in each region is 11,604,464 in the D1 region, and D There are 5,802,232 toner pixels in area 2 and 0 in area D3. The total number of toner pixels is divided by the total number of pixels in each area and multiplied by 100 to determine the print density. For each sheet of paper, the print density for each area is calculated, and the average print density for each area is calculated. In this case, the engine control unit 210, which functions as an image information acquisition means, divides the image area into multiple areas with boundary lines in the direction of rotation of the photosensitive drum 1, and calculates the average print density for each of the multiple areas.

[0134] <Average print coverage and percentage of degraded toner> Figure 18 shows the number of sheets fed and the percentage of degraded toner on the developing roller. Three types of images with different print density levels (1%, 5%, and 10%) were prepared, and the paper was fed repeatedly until 1,000 sheets of paper were fed. The density information of all printed pixels was set to 255. During this 1,000-sheet paper feeding process, toner was collected from the developing roller, and the percentage of deformed toner was calculated. To calculate the percentage of degraded toner, the average circularity (aspect ratio) of the collected toner was checked using an EPIA-3000. If the average degree of deformation was 0.9 or less, it was defined as degraded.

[0135] The results confirmed that the higher the print density, the smaller the proportion of degraded toner. When the print density is high, the toner coated on the surface of the developing roller moves to the photosensitive drum 1, then to the intermediate transfer belt 10, and is transferred and fixed to the paper before it can change from fresh toner to degraded toner. Therefore, it is thought that the proportion of degraded toner is low because there are fewer opportunities for the toner to degrade. Conversely, when the print density is low, the toner coated on the surface of the developing roller has fewer opportunities to move to the paper. Therefore, it is thought that there are more times and more frictions between the developing roller 22 and the photosensitive drum, resulting in a higher proportion of degraded toner.

[0136] Furthermore, we will discuss why the print coverage rate and the percentage of degraded toner were not proportional. When the print coverage rate is low, the area outside the printable area is large. Toner spills into this area. The print coverage rate does not take this spilled toner into account. The amount of spilled toner differs depending on the print coverage rate. Therefore, it is thought that the print coverage rate and the percentage of degraded toner were not proportional.

[0137] <Toner degradation level and toner ejection pattern> As mentioned above, we were able to confirm the relationship between the average print coverage and degraded toner. It was found that the lower the average print coverage, the higher the proportion of degraded toner. While the toner ejection pattern during toner ejection control can be the same density across the entire area, if the proportion of degraded toner can be calculated, it is desirable to implement toner ejection control using a toner ejection pattern that corresponds to this proportion.

[0138] In this embodiment 3, toner ejection control is performed based on the relationship between the average print density and the density of the toner ejection pattern shown in Figure 19(a). When the average print density is low and there is a lot of degraded toner, the density of the toner ejection pattern is controlled to be high, and when the average print density is high and there is little degraded toner, the density of the ejection pattern is controlled to be low. Figure 19(b) shows the average print density of the three divided regions D1, D2, and D3. The average print density in D1, D2, and D3 is 50%, 5%, and 50%, respectively. The toner ejection pattern at that time is shown in Figure 19(c). The ejection pattern at the positions corresponding to regions D1 and D3 is represented by the shaded area, and the ejection pattern at the position corresponding to region D2 is represented by the black area. The density information for the shaded area is 40, and the density information for the black area is 194.

[0139] This section explains the reason for adjusting the toner ejection pattern density according to the average print density. During toner ejection control, basically only degraded toner is moved to the intermediate transfer belt, so a small amount of fresh toner may also move to the transfer belt. If there is little degraded toner in the ejected toner image, it is desirable to control the density of the toner ejection pattern to a low level, considering the risk of losing fresh toner. On the other hand, if it is known that there is a lot of degraded toner in the toner ejection pattern, then with a single toner ejection control... A significant amount of degraded toner needs to be removed. Therefore, it is desirable to control the density of the toner ejection pattern to a high level.

[0140] In summary, by changing the density of the toner ejection pattern according to the percentage of degraded toner during toner ejection control, it is possible to reduce the risk of losing fresh toner while simultaneously achieving highly efficient removal of degraded toner.

[0141] In this third embodiment, we described toner ejection control that forms toner images of different densities on the photosensitive drum and returns these toner images to the developing roller 22. In this case as well, the control described in second embodiment exhibits the same effect. As described in second embodiment, by controlling the back contrast ΔVb value before development and recovery according to the density (amount) of the toner image on the photosensitive drum, it is possible to recover unwanted toner formed on the photosensitive drum into the developing container with high recovery efficiency, while simultaneously reducing the amount of reverse fouling toner that moves to the photosensitive drum 1. Since there is less reverse fouling toner, it is possible to suppress the adhesion of reverse fouling toner to the surface of the charging means and prevent the charging means from becoming dirty.

[0142] [Configuration 1] Image carrier and, A charging means for charging the image carrier, An exposure means for exposing the image carrier to form an electrostatic latent image, A developing means that supplies toner to the electrostatic latent image on the image carrier to form a toner image, A transfer means for transferring the toner image formed on the image carrier to a transfer target, A control unit that controls the charging voltage applied to the charging means, the developing voltage applied to the developing means, the transfer voltage applied to the transfer means, and the exposure amount of the exposure means. Image information acquisition means for acquiring information related to the density of the toner image formed on the image carrier, Equipped with, The control unit is capable of developing and recovering the toner image formed on the image carrier in a recovery area which is a predetermined range along the rotation axis of the image carrier, without transferring the toner image to the transfer target, and returning it to the developing means. When the absolute value of the difference between the surface potential of the image carrier and the development voltage when the toner image is returned to the developing means is defined as the back contrast, the image information acquisition means arranges the first region and the first region so as to be aligned in the direction along the rotation axis in the recovery region. Also, set a second area with a higher toner density. The control unit controls the second back contrast of the second region during development and recovery to be greater than the first back contrast of the first region. An image forming apparatus characterized by the following features. [Configuration 2] The image information acquisition means detects the toner density for each pixel in the area to be recovered, The exposure means is capable of performing a first exposure, which is the exposure when forming the toner image, and a second exposure, which is the exposure when developing and recovering the toner. The back contrast is controlled by adjusting the amount of light in the second exposure according to the density of the toner in the pixels. The image forming apparatus according to configuration 1, characterized in that it is a picture forming apparatus. [Configuration 3] When the exposure means performs the second exposure on the second region, it reduces the amount of exposure compared to when it performs the second exposure on the first region. The image forming apparatus according to configuration 2, characterized in that... [Structure 4] The image information acquisition means sets pixels where toner supplied from the developing means does not exist as the first region, and pixels where toner supplied from the developing means does exist as the second region. The image forming apparatus according to configuration 2 or 3, characterized by the above. [Composition 5] The image forming unit comprises a plurality of image forming units, each of which is supplied with a different color of toner. The transfer target is an intermediate transfer target onto which the toner image formed by the plurality of image forming units is transferred. An image forming apparatus according to any one of configurations 1 to 4, characterized by the features described above. [Composition 6] The control unit performs the development and retrieval operation if the image forming operation, which transfers the toner image formed on the image carrier to the transfer target, stops midway. An image forming apparatus according to any one of configurations 1 to 5, characterized by the above. [Composition 7] The exposure means performs exposure during development and recovery by moving the exposure position on the image carrier in a main scanning direction along the rotation axis of the image carrier and in a sub-scanning direction intersecting the main scanning direction. The control unit sets the first region and the second region for each main scan in the main scanning direction. An image forming apparatus according to any one of configurations 1 to 6, characterized by the above. [Structure 8] A rotatable image carrier, A charging means for charging the image carrier, An exposure means for exposing the image carrier to form an electrostatic latent image, A developing means that supplies toner to the electrostatic latent image on the image carrier to form a toner image, A transfer means for transferring the toner image formed on the image carrier to a transfer target, A control unit that controls the charging voltage applied to the charging means, the developing voltage applied to the developing means, the transfer voltage applied to the transfer means, and the exposure amount of the exposure means. Image information acquisition means for acquiring information related to the toner image formed on the image carrier, Equipped with, In the rotational direction of the image carrier, the charging means, exposure means, developing means, and transfer means are arranged in that order from the upstream side. An image forming apparatus capable of operating in the following modes: an image forming mode in which the toner image formed on the image carrier is transferred to the transfer target; and an ejection mode in which ejection toner is supplied to the image carrier by the developing means, a portion of the ejection toner is recovered by the developing means, and another portion of the ejection toner is transferred onto the transfer target, In the ejection mode, the image information acquisition means divides the toner image into multiple regions, calculates the average print density in each of the multiple regions as information about the toner image, and the control unit controls the density of the ejected toner for each region based on the average print density. An image forming apparatus characterized by the following features. [Composition 9] The image information acquisition means divides the toner image into a plurality of regions with boundary lines in the direction along the rotation direction. The image forming apparatus according to configuration 8, characterized by the above. [Configuration 10] The control unit supplies the ejected toner in the ejection mode such that the concentration of the ejected toner increases in the region where the average print density is higher. The image forming apparatus according to configuration 9, characterized by the features described therein. [Explanation of Symbols]

[0143] 100: Image forming apparatus 1: Photosensitive drum, 2: Charging roller, 3: Exposure device, 4: Developing means, 10: Intermediate transfer belt, 14: Primary transfer roller, 15: Primary transfer power supply, 22: Developing roller, 210: Engine control unit

Claims

1. Image carrier and, A charging means for charging the image carrier, An exposure means for exposing the image carrier to form an electrostatic latent image, A developing means that supplies toner to the electrostatic latent image on the image carrier to form a toner image, A transfer means for transferring the toner image formed on the image carrier to a transfer target, A control unit that controls the charging voltage applied to the charging means, the developing voltage applied to the developing means, the transfer voltage applied to the transfer means, and the exposure amount of the exposure means. Image information acquisition means for acquiring information related to the density of the toner image formed on the image carrier, Equipped with, The control unit is capable of developing and recovering the toner image formed on the image carrier in a recovery area which is a predetermined range along the rotation axis of the image carrier, without transferring the toner image to the transfer target, and returning it to the developing means. When the absolute value of the difference between the surface potential of the image carrier and the development voltage when the toner image is returned to the developing means is defined as the back contrast, the image information acquisition means arranges the first region and the first region so as to be aligned in the direction along the rotation axis in the recovery region. Also, set a second area with a higher toner density. The control unit controls the second back contrast of the second region during development and recovery to be greater than the first back contrast of the first region. An image forming apparatus characterized by the following:

2. The image information acquisition means detects the toner density for each pixel in the area to be recovered, The exposure means is capable of performing a first exposure, which is the exposure when forming the toner image, and a second exposure, which is the exposure when developing and recovering the toner. The back contrast is controlled by adjusting the amount of light in the second exposure according to the density of the toner in the pixels. The image forming apparatus according to feature 1.

3. When the exposure means performs the second exposure on the second region, it reduces the amount of exposure compared to when it performs the second exposure on the first region. The image forming apparatus according to feature 2.

4. The image information acquisition means sets pixels where toner supplied from the developing means does not exist as the first region, and pixels where toner supplied from the developing means exists as the second region. The image forming apparatus according to feature 2.

5. The image forming unit comprises a plurality of image forming units, each of which is supplied with a different color of toner. The transfer target is an intermediate transfer target onto which the toner image formed by the plurality of image forming units is transferred. The image forming apparatus according to any one of claims 1 to 4.

6. The control unit performs the development and retrieval operation if the image forming operation, which transfers the toner image formed on the image carrier to the transfer target, stops midway. The image forming apparatus according to any one of claims 1 to 4.

7. The exposure means performs exposure during development and recovery by moving the exposure position on the image carrier in a main scanning direction along the rotation axis of the image carrier and in a sub-scanning direction intersecting the main scanning direction. The control unit sets the first region and the second region for each main scan in the main scanning direction. The image forming apparatus according to any one of claims 1 to 4.

8. A rotatable image carrier, A charging means for charging the image carrier, An exposure means for exposing the image carrier to form an electrostatic latent image, A developing means that supplies toner to the electrostatic latent image on the image carrier to form a toner image, A transfer means for transferring the toner image formed on the image carrier to a transfer target, A control unit that controls the charging voltage applied to the charging means, the developing voltage applied to the developing means, the transfer voltage applied to the transfer means, and the exposure amount of the exposure means. Image information acquisition means for acquiring information related to the toner image formed on the image carrier, Equipped with, In the rotational direction of the image carrier, the charging means, exposure means, developing means, and transfer means are arranged in that order from the upstream side. An image forming apparatus capable of operating in the following modes: an image forming mode in which the toner image formed on the image carrier is transferred to the transfer target; and an ejection mode in which ejection toner is supplied to the image carrier by the developing means, a portion of the ejection toner is recovered by the developing means, and another portion of the ejection toner is transferred onto the transfer target, In the ejection mode, the image information acquisition means divides the toner image into multiple regions, calculates the average print density in each of the multiple regions as information about the toner image, and the control unit controls the density of the ejected toner for each region based on the average print density. An image forming apparatus characterized by the following:

9. The image information acquisition means divides the toner image into a plurality of regions with boundary lines in the direction along the rotation direction. The image forming apparatus according to feature 8.

10. The control unit supplies the ejected toner in the ejection mode such that the concentration of the ejected toner increases in the region where the average print density is higher. The image forming apparatus according to feature 9.

Images