Image forming device

Abstract

To increase an ability to move a foreign substance to an upstream-side image carrier to reduce the amount of foreign substance moving to a downstream-side image carrier, thereby preventing an image defect caused by the foreign substance in association with the extension of life of units including the image carrier and a developing device.SOLUTION: An image forming apparatus 1 has a first image forming section PY, a second image forming section PM, a belt 12, an electrification voltage application section E1, and a transfer voltage application section E3. In the direction of movement of a recording material S, the first image forming section PY is arranged upstream of the second image forming section PM. In an image forming operation, the potential difference between a first transfer voltage and a potential at which an image formed on a first image carrier 4Y in a first transfer section NY is not formed, is larger than the potential difference between a second transfer voltage and a potential at which an image formed on a second image carrier 4M in a second transfer section NM is not formed.SELECTED DRAWING: Figure 5

Description

[Technical field]

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

[0002] Conventionally, there is an image forming apparatus using an electrophotographic method, which transfers toner images formed on a plurality of image carriers to a recording material carried and transported by a recording material carrier to form an image on the recording material. In this image forming apparatus, a charging member for charging the image carrier, a developing device for supplying toner as a developer to the image carrier to form a toner image, and a transfer member for transferring the toner image from the image carrier to the recording material are provided for each image carrier. A transfer belt made of an endless belt is often used as the recording material carrier. Then, toner images are sequentially transferred from the plurality of image carriers arranged in line along the moving direction of the recording material by the recording material carrier onto the recording material that is electrostatically attracted to the transfer belt and transported. As the transfer member, a transfer roller or the like arranged on the opposite side of the image carrier across the transfer belt is used, and the toner image is transferred by applying a transfer voltage to the transfer member.

[0003] Moreover, the image forming apparatus as described above is provided with a cleaning means for removing the toner (transfer residual toner) remaining on the image carrier without being transferred to the recording material from the image carrier. The following cleaning means are known. A cleaning roller is provided as a cleaning member that contacts the image carrier, and during image formation, the transfer residual toner is caused to adhere to the cleaning roller and temporarily collected. This prevents the transfer residual toner from affecting the next image formation process and causing image defects. In addition, the transfer residual toner temporarily collected by the cleaning roller is discharged from the cleaning roller onto the image carrier at a predetermined timing during non-image formation. Then, the transfer residual toner discharged onto the image carrier is collected into a developing device. The toner collected into the developing device is reused to develop a toner image.

[0004] Furthermore, when the recording material comes into contact with the image carrier, foreign matter adheres to the surface of the image carrier. This foreign matter is mainly the fibers and fillers that make up the paper (so-called "paper dust"), or dust that adheres to the recording material. If such foreign matter adheres to the surface of the image carrier, it may affect the subsequent image formation process and cause image defects.

[0005] Patent Document 1 discloses a configuration in which a foreign matter recovery member that attracts and recovers foreign matter from the cleaning roller is provided only for the most upstream image carrier in the direction of movement of the recording material, which is the most affected by foreign matter. In this configuration, foreign matter that has moved to the most upstream image carrier during image formation is recovered by the cleaning roller during non-image formation, and is further recovered by the foreign matter recovery member and stored in a storage section. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2010-165000 A Summary of the Invention [Problem to be solved by the invention]

[0007] As in the above-mentioned conventional configuration, by providing a foreign matter recovery member for the most upstream image carrier in the moving direction of the recording material, most of the foreign matter can be recovered. This is because most of the foreign matter generated from the recording material is moved to the most upstream image carrier with which the recording material first comes into contact and removed. However, a small amount of foreign matter may not move to the most upstream image carrier, but may adhere to an image carrier downstream of the most upstream image carrier in the moving direction of the recording material.

[0008] Up until now, even if a small amount of foreign matter that has moved to the downstream image carrier is carried around on the image carrier, carried around on a cleaning roller, or collected in a developing device together with the transfer residual toner, no problem has been evident. However, in recent years, there has been a trend toward longer life for image carriers and developing devices as consumables, or for image forming apparatuses. In other words, there is a trend toward an increase in the total amount of foreign matter that moves to the downstream image carrier. As a result, there has been a possibility that image defects will occur due to the influence of foreign matter that has not been able to move to the most upstream image carrier and has moved to the downstream image carrier, even if only in small amounts. This problem is prominent in the configuration in which a foreign matter recovery member is provided only for the most upstream image carrier as described above, but the same problem will occur even if a foreign matter recovery member is provided for the downstream image carrier as well. As a result, foreign matter that has been carried around on the image carrier for a long period of time may adhere to the image carrier, causing streak-like image defects or dot-like image defects on the image. In addition, if foreign matter is carried around on the cleaning roller for a long period of time, the foreign matter may adhere to the cleaning roller, reducing the cleaning performance of the transfer residual toner, and image defects may occur due to poor cleaning. Furthermore, for example, when the amount of foreign matter collected in the developing device together with the transfer residual toner gradually increases and exceeds a certain threshold, the charge of the toner may be disturbed, causing image defects.

[0009] To address the above-mentioned problem, it is important to improve the ability of the upstream image forming section to remove foreign matter from the recording material, and to reduce the amount of foreign matter sent to the downstream image forming section.

[0010] Therefore, the present invention aims to suppress image defects caused by foreign matter due to the extended life of units including image carriers and developing devices by increasing the ability to move foreign matter to the upstream image carrier and reducing the amount of foreign matter that moves to the downstream image carrier. [Means for solving the problem]

[0011] The above object is achieved by an image forming apparatus according to the present invention. ClearlyAccording to the invention, an image forming apparatus capable of performing an image forming operation for forming an image on a recording material includes a rotatable first image carrier, a first charging member for charging a surface of the first image carrier, a first developing device for supplying a first developer to the surface of the first image carrier, a first transfer member for transferring the first developer supplied to the surface of the first image carrier to a recording material, a first cleaning member for contacting the first image carrier to clean the surface of the first image carrier, and a cleaning device for contacting the first cleaning member to clean the first cleaning member. a second image forming unit having a rotatable second image carrier, a second charging member for charging a surface of the second image carrier, a second developing device for supplying a second developer to the surface of the second image carrier, a second transfer member for transferring the second developer supplied to the surface of the second image carrier to a recording material, and a second cleaning member for contacting the second image carrier and cleaning the surface of the second image carrier; a belt that forms a first transfer portion and contacts the second image carrier to form a second transfer portion, the belt nipping and conveying a recording material between the first image carrier and the second image carrier in the first transfer portion and the second transfer portion, respectively; a charging voltage application portion that applies a first charging voltage to the first charging member and a second charging voltage to the second charging member; and a transfer voltage application portion that applies a first transfer voltage to the first transfer member and a second transfer voltage to the second transfer member, The image forming apparatus is characterized in that the first image forming unit is disposed upstream of the second image forming unit, and in the image forming operation, a potential difference between a potential at which an image is not formed on the first image carrier in the first transfer unit and the first transfer voltage is larger than a potential difference between a potential at which an image is not formed on the second image carrier in the second transfer unit and the second transfer voltage, and a film thickness of a surface layer of the first image carrier is smaller than a film thickness of a surface layer of the second image carrier. Effect of the Invention

[0012] According to the present invention, by increasing the ability to move foreign matter to the upstream image carrier and reducing the amount of foreign matter that moves to the downstream image carrier, it is possible to suppress image defects caused by foreign matter that occur as a result of the extended life of units including image carriers and developing devices. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Diagram 2] 2 is a schematic cross-sectional view of the drum unit and the developing cartridge. [Diagram 3] FIG. 2 is a schematic block diagram for explaining a control mode of the image forming apparatus. [Figure 4] 5A and 5B are schematic diagrams for explaining collection of transfer residual toner and foreign matter. [Diagram 5] 11 is a graph showing the relationship between the potential difference between the non-image portion potential and the transfer voltage and the amount of foreign matter that moves to the downstream photosensitive drum. FIG. [Figure 6] 6 is a graph illustrating the relationship between the electrical resistance of the transfer roller and transfer efficiency. FIG. [Figure 7] FIG. 11 is a graph illustrating the relationship between the charge amount of toner and transfer efficiency. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

[0015] [Example 1] <Configuration of Image Forming Apparatus> 1 is a schematic cross-sectional view of an image forming apparatus 1 according to this embodiment. The image forming apparatus 1 according to this embodiment is a color laser printer that uses an electrophotographic image forming process, and can form a color image on a recording material S according to image information from an external device such as a personal computer.

[0016] The image forming apparatus 1 has four image forming units (stations) PY, PM, PC, and PK that form images of the colors yellow (Y), magenta (M), cyan (C), and black (K), respectively. Elements having the same or corresponding functions or configurations in the four image forming units PY, PM, PC, and PK may be generally described by omitting the Y, M, C, or K at the end of the reference numerals indicating that the element is for one of the colors.

[0017] In this embodiment, each image forming station P has a photosensitive drum 4, which is a drum-type (cylindrical) photosensitive member (electrophotographic photosensitive member) as a rotatable image carrier, a charging device (charging member) 5 as a charging means, and a developing device 8 as a developing means. Also, each image forming station P has a transfer roller 16, which is a roller-type transfer member as a transfer means, a discharging light source 31 (FIG. 2) as a discharging means, and a cleaning mechanism 36 (FIG. 2) as a cleaning means. The four image forming stations PY, PM, PC, and PK are arranged in a line along the moving direction of the recording material S by the transfer belt 12 described later (here, simply referred to as the "moving direction of the recording material S"). Also, in this embodiment, the exposure device 10 used as an exposure means in each image forming station P is configured as one unit. Also, in this embodiment, in each image forming station P, the drum unit 30 having the photosensitive drum 4 and the developing device (developing cartridge, developing unit) 8 are each a replaceable unit as a consumable item.

[0018] FIG. 2 is a schematic cross-sectional view showing the drum unit 30 and the developing cartridge 8. FIG. 2(a) shows the drum unit 30Y and the developing cartridge 8Y of the image forming section PY located at the most upstream position in the moving direction of the recording material S. FIG. 2(b) shows the drum units 30M, 30C, 30K and the developing cartridges 8M, 8C, 8K of the image forming sections PM, PC, PK located downstream (also simply referred to as the "downstream side" here) of the image forming section PY located at the most upstream position in the moving direction of the recording material S. Note that, with respect to the multiple image forming sections P or their elements, "upstream" and "downstream" refer to the upstream and downstream positions in the moving direction of the recording material S. As shown in FIG. 2(a) and (b), in this embodiment, the configuration of the cleaning mechanism 36 is different between the most upstream drum unit 30Y and the downstream drum units 30M, 30C, 30K. The configuration and operation of the drum unit 30 and the developing cartridge 8 will be described in detail later.

[0019] A cartridge tray 3 is detachably provided in an apparatus main body 2 of the image forming apparatus 1. The drum unit 30 and the developing cartridge 8 are each configured to be detachably attached to the cartridge tray 3. The apparatus main body 2 also includes an exposure device 10, an electrostatic transfer device 11, a paper feed unit 18, a fixing device 21, a discharge unit 22, and a front door 40.

[0020] The exposure device 10 is provided above the drum unit 30 and the developing cartridge 8 mounted on the cartridge tray 3, and outputs laser light L corresponding to image information. The exposure device 10 scans and exposes the surfaces of the four photosensitive drums 4Y, 4M, 4C, and 4K with the laser light L.

[0021] The electrostatic transfer device 11 is provided below the drum unit 30 and the developing cartridge 8 mounted on the cartridge tray 3. The electrostatic transfer device 11 has a transfer belt 12, which is an endless belt serving as a recording material carrier, so as to come into contact with the four photosensitive drums 4Y, 4M, 4C, and 4K. The transfer belt 12 is made of a film-like material such as a resin film or a multi-layer film having a resin layer provided on a rubber base layer. The transfer belt 12 is stretched over a driving roller 13 and a driven roller 14 serving as a plurality of tension rollers (support rollers) and stretched with a predetermined tension. The transfer belt 12 electrostatically attracts a sheet-like recording material (recording medium, transfer material, sheet) S such as paper to the outer peripheral surface on the upper side in FIG. 1, and circulates so as to bring the recording material S into contact with the four photosensitive drums 4Y, 4M, 4C, and 4K in sequence. Four transfer rollers 16Y, 16M, 16C, and 16K are arranged on the inner peripheral surface side of the transfer belt 12, corresponding to the four photosensitive drums 4Y, 4M, 4C, and 4K, respectively. In this embodiment, the transfer rollers 16 are arranged facing the corresponding photosensitive drums 4 via the transfer belt 12, and abut against the photosensitive drums 4 via the transfer belt 12. As a result, transfer portions NY, NM, NC, and NK are formed where the photosensitive drums 4Y, 4M, 4C, and 4K come into contact with the transfer belt 12. In this way, the transfer belt 12 comes into contact with each photosensitive drum 4 to form a transfer portion N, and the recording material S is sandwiched and conveyed between the transfer belt 12 and the photosensitive drum 4 at each transfer portion N. During transfer, a predetermined transfer voltage (transfer bias) is applied to each transfer roller 16, and an electric charge is applied to the recording material S via the transfer belt 12. The toner image on the photosensitive drum 4 is transferred to the recording material S in contact with the photosensitive drum 4 by the electric field generated at this time. In this embodiment, the transfer roller 16 is configured by coating a roller shaft (shaft core) made of metal or the like with a sponge layer made of a sponge material (such as a foamed rubber material such as foamed polyurethane) as an elastic layer.

[0022] The paper feed unit 18 is provided below the electrostatic transfer device 11. The paper feed unit 18 has a paper feed tray 19 as a recording material storage section in which the recording material S is stacked and stored, a paper feed roller 20 as a transport member, and the like.

[0023] The fixing device 21 and the discharge unit 22 are provided above the electrostatic transfer device 11. The fixing device 21 fixes the toner image transferred onto the recording material S onto the recording material S. The discharge unit 22 discharges the recording material S that has passed through the fixing device 21 onto a discharge tray 23.

[0024] As shown in Figs. 2(a) and (b), the drum unit 30 includes a charging device 5, a static electricity removing light source 31, and a cleaning mechanism 36. As shown in Fig. 2(a), the most upstream drum unit 30Y includes a first cleaning mechanism 36a as the cleaning mechanism 36. The first cleaning mechanism 36a includes a cleaning roller 32 as a cleaning member for cleaning the surface of the photosensitive drum 4, and a foreign matter recovery roller 33 as a foreign matter recovery member for recovering foreign matter adhering to the cleaning member 32. The first cleaning mechanism 36a also includes a scraping member 34 as a foreign matter removal member, and a foreign matter recovery container 35 as a storage section. As shown in Fig. 2(b), the downstream drum units 30M, 30C, and 30K include a second cleaning mechanism 36b as the cleaning mechanism 36. The second cleaning mechanism 36b includes a cleaning roller 32 as a cleaning member. In this embodiment, the second cleaning mechanism 36b does not have the foreign matter collection roller 33, scraping member 34, and foreign matter collection container 35 that the first cleaning mechanism 36a has. In this embodiment, in the first cleaning mechanism 36a, the cleaning roller 32 is connected to a cleaning power supply E4 (FIG. 3), and the foreign matter collection roller 33 is connected to a collection power supply E5 (FIG. 3). In the second cleaning mechanism 36b, the cleaning roller 32 is connected to the cleaning power supply E4 (FIG. 3).

[0025] As shown in FIGS. 2(a) and 2(b), the developing cartridge 8 has a developing frame (developing container) 28 that contains toner T as a developer, and a developing roller 6 that is a rotatable developer carrier carrying toner T on its surface. The developing cartridge 8 also has a supply roller 26 as a developer supply member that supplies toner T to the developing roller 6, and a regulating blade 60 as a developer regulating member that contacts the surface of the developing roller 6 to regulate the layer thickness of the toner T carried on the surface of the developing roller 6. The developing cartridge 8 also has a rotatable stirring shaft 50 and a stirring sheet 51 fixed to the stirring shaft 50 as stirring and transporting members for stirring and transporting the toner T in the developing frame 28. In this embodiment, the toner T is a non-magnetic one-component developer.

[0026] <Image formation process> Next, an image forming process in the image forming apparatus 1 of this embodiment will be described with reference to FIGS.

[0027] During the image forming process, the photosensitive drum 4 is rotated in the direction of arrow D (clockwise direction) in FIGS. 1 and 2 at a predetermined peripheral speed by a driving motor (not shown) as a driving means. The transfer belt 12 of the electrostatic transfer device 11 rotates (circulates) in the direction of arrow F (counterclockwise direction) in FIG. 1 at a peripheral speed corresponding to the peripheral speed of the photosensitive drum 4 by the driving roller 13 being rotated by a driving motor (not shown) as a driving means. The surface of the rotating photosensitive drum 4 is uniformly charged to a predetermined potential of a predetermined polarity (positive polarity in this embodiment) by the charging device 5. During the charging process, a predetermined charging voltage (charging bias) is applied to the charging device 5 by a charging power source E1 (FIG. 3) as a charging voltage application unit. In this embodiment, the charging power source E1 applies a positive DC voltage as the charging voltage to the charging device 5. The charged surface of the photosensitive drum 4 is scanned and exposed by the exposure device 10 with laser light L output according to the image signals of each color, and an electrostatic latent image (electrostatic image) according to the image signals of each color is formed on the photosensitive drum 4. In this embodiment, the laser light L is irradiated onto the image area (printed area, image area) on the photosensitive drum 4, which has been uniformly charged and on which a potential (non-image area potential, dark area potential) has been formed in the non-image area (non-printed area, non-image area). As a result, the absolute value of the potential at the location irradiated with the laser light L decreases, and a potential (image area potential, light area potential) is formed in the image area.

[0028] The electrostatic latent image formed on the photosensitive drum 4 is developed (visualized) by supplying toner from the developing cartridge 8, and a toner image (developer image) corresponding to the image signal of each color is formed on the photosensitive drum 4. The toner T in the developing frame 28 is circulated in the developing frame 28 by the stirring sheet 51 fixed to the stirring shaft 50 and conveyed to the supply roller 26. The supply roller 26 supplies the toner T to the developing roller 6. The developing roller 6 is configured by coating a rubber layer formed of a rubber material as an elastic layer around a roller shaft (shaft core) formed of metal or the like. For the rubber layer, for example, a solid rubber member or a sponge member having elasticity is used. In this embodiment, the developing roller 6 abuts against the photosensitive drum 4. The supply roller 26 is configured by coating a sponge layer formed of a sponge material (foamed rubber material such as polyurethane foam) as an elastic layer around a roller shaft (shaft core) formed of metal or the like. In this embodiment, the supply roller 26 abuts against the developing roller 6. The developing roller 6 is driven to rotate in the direction of the arrow E in FIG. 2 (counterclockwise direction) at a predetermined peripheral speed. That is, the developing roller 6 rotates so that the photosensitive drum 4 and the developing roller 6 move in the forward direction at the contact portion between the photosensitive drum 4 and the developing roller 6. In this embodiment, the developing roller 6 is driven to rotate by branching and transmitting the driving force from a driving motor (not shown) that drives the photosensitive drum 4. The developing roller 6 may be driven to rotate with a peripheral speed difference with respect to the photosensitive drum 4 (for example, the developing roller 6 is faster), or may be driven to rotate at the same peripheral speed as the photosensitive drum 4. The toner T supplied to the developing roller 6 penetrates between the developing roller 6 and the regulating blade 60 and is carried on the developing roller 6 as a thin layer of a certain thickness. The toner T is charged to a predetermined polarity (positive polarity in this embodiment) by frictional charging between the regulating blade 60 and the developing roller 6 and between the supply roller 26 and the developing roller 6. That is, in this embodiment, the normal charging polarity ("normal polarity") of the toner T, which is the charging polarity of the toner T during development, is positive polarity. The regulating blade 60 is configured by bonding a resin member to a metal plate made of stainless steel, etc. Depending on the shape and material of the resin member, the pressure and the amount of frictional charge applied to the toner T that has entered between the developing roller 6 and the regulating blade 60 can be controlled.The resin member may be made of silicone rubber or urethane rubber.

[0029] The toner T carried on the developing roller 6 and positively charged is supplied to the electrostatic latent image formed on the photosensitive drum 4. As a result, the toner T adheres to the image portion of the electrostatic latent image, and a toner image is formed on the photosensitive drum 4. During development, a predetermined developing voltage (developing bias) is applied to the developing roller 6 by a developing power source E2 (FIG. 3) as a developing voltage application unit. In this embodiment, the developing power source E2 applies a positive DC voltage to the developing roller 6 as the developing voltage. The developing voltage is set to a potential between the image portion potential and the non-image portion potential on the photosensitive drum 4. That is, the potential of the electrostatic latent image formed on the photosensitive drum 4 has a larger absolute value than the voltage applied to the developing roller 6 in the non-image portion where the toner T does not adhere (where no image is formed), and a smaller absolute value than the voltage applied to the developing roller 6 in the image portion where the toner T adheres (where an image is formed). With this setting, the toner T, which is positively charged, moves from the developing roller 6 to the image portion of the electrostatic latent image formed on the photosensitive drum 4. In this manner, in this embodiment, toner T charged with the same polarity as the charging polarity of the photosensitive drum 4 (positive polarity in this embodiment) adheres to the exposed area (image area) on the photosensitive drum 4, which has been uniformly charged and then exposed to light, thereby reducing the absolute value of the potential (reverse development).

[0030] Also, the recording material S is separated and fed one by one by the paper feed unit 18 at a predetermined control timing. The recording material S is conveyed to the transfer belt 12 at a predetermined control timing so that the timing when the leading edge of the toner image on the most upstream photosensitive drum 4Y moves to the transfer section NY and the timing when the recording material S is conveyed to the transfer section NY are synchronized (the image formation start positions are the same). The toner images are sequentially transferred from each photosensitive drum 4 to the recording material S conveyed by electrostatically adsorbing it to the transfer belt 12 by an electric field formed between each photosensitive drum 4 and each transfer roller 16. During the transfer, a predetermined transfer voltage (transfer bias) is applied to each transfer roller 16 by a transfer power source E3 (FIG. 3) as a transfer voltage application unit. In this embodiment, the transfer power source E3 applies a DC voltage of negative polarity, which is the opposite polarity to the normal polarity (positive polarity in this embodiment) of the toner T, to the transfer roller 16 as the transfer voltage. This makes it possible to electrically attract the positively charged toner T to the recording material S side.

[0031] The recording material S onto which the four color toner images have been transferred is separated from the transfer belt 12 and conveyed to a fixing device 21. The fixing device 21 applies heat and pressure to the recording material S carrying the unfixed toner image, thereby fixing (melting and bonding) the toner image to the recording material S. Thereafter, the recording material S is discharged (output) by a discharge unit 22 to a discharge tray 23 provided outside the apparatus main body 2.

[0032] On the other hand, after the toner T is transferred onto the recording material S, the surface of the photosensitive drum 4 is irradiated with a discharge light by the discharge light source 31, and discharged until the surface potential is close to 0 V. This stabilizes the surface potential of the photosensitive drum 4, and improves the cleaning performance of the cleaning roller 32 of the toner (transfer residual toner) remaining on the photosensitive drum 4 without being transferred to the recording material S, and the foreign matter that has moved from the recording material S to the photosensitive drum 4. The function of the cleaning roller 32 will be described in detail later. Furthermore, by stabilizing the surface potential of the photosensitive drum 4, the surface of the photosensitive drum 4 can be more uniformly charged by the charging device 5. Discharging includes removing (attenuating) at least a part of the charge.

[0033] <Control mode> 3 is a schematic block diagram showing the control mode of the main parts of the image forming apparatus 1 of this embodiment. The image forming apparatus 1 is provided with a control unit 150. The control unit 150 has a CPU 151 as an arithmetic control means which is a central element for performing arithmetic processing, a memory (storage element) 152 such as a ROM or RAM as a storage means, an input / output unit (not shown) for controlling the transmission and reception of signals between various elements connected to the control unit 150, and the like. The RAM stores the detection results of the sensors, the arithmetic results, and the like, and the ROM stores a control program, a data table obtained in advance, and the like.

[0034] The control unit 150 is a control unit that comprehensively controls the operation of the image forming apparatus 1. The control unit 150 executes a predetermined image forming sequence by controlling the transmission and reception of various electrical information signals and the timing of driving. Each unit of the image forming apparatus 1 is connected to the control unit 150. For example, in relation to this embodiment, the control unit 150 is connected to a charging power source E1, a developing power source E2, a transfer power source E3, a cleaning power source E4, a recovery power source E5, an exposure device 10, and the like. The control unit 150 controls the ON / OFF and output values ​​of these various power sources E1, E2, E3, E4, and E5, the amount of exposure by the exposure device 10, and the like, to control the image forming operation and the operation of a cleaning mechanism 36 described later.

[0035] In this embodiment, the charging power supply E1, the developing power supply E2, the transfer power supply E3, and the cleaning power supply E4 are each provided independently for each image forming unit P. However, at least one of the charging power supply E1, the developing power supply E2, the transfer power supply E3, and the cleaning power supply E4 may be shared by a plurality of image forming units P (or may be shared by all image forming units P). Also, in this embodiment, the recovery power supply E5 is provided only for the most upstream image forming unit PY.

[0036] In this embodiment, the image forming apparatus 1 can execute a job (printing operation) which is a series of operations for forming an image on a single or multiple recording materials S in response to a single start instruction from an external device (not shown) such as a personal computer. A job generally includes an image forming process (printing process), a pre-rotation process, a paper-interval process in the case of forming an image on multiple recording materials S, and a post-rotation process. The image forming process is a period during which an electrostatic image is actually formed on the photosensitive drum 4, the electrostatic image is developed (a toner image is formed), the toner image is transferred, the toner image is fixed, and the image forming time refers to this period. More specifically, the timing during image formation differs depending on the positions at which the electrostatic image is formed, the toner image is formed, the toner image is transferred, the toner image is fixed, and the like. The pre-rotation process is a period during which a preparatory operation is performed before the image forming process. The paper-interval process is a period corresponding to the interval between recording materials S when the image forming process is performed continuously on multiple recording materials S (during continuous image formation). The post-rotation process is a period during which an arranging operation (preparatory operation) is performed after the image forming process. The non-image forming time is a period other than the image forming time, and includes the above-mentioned pre-rotation process, inter-sheet process, post-rotation process, and further the pre-multiple-rotation process which is a preparatory operation when the image forming apparatus 1 is turned on or when returning from a sleep state, etc. In this embodiment, at a predetermined timing during the non-image forming time, an operation of expelling the transfer residual toner adhering to the cleaning roller 32 from the cleaning roller 32 to the photosensitive drum 4, as described later, is executed.

[0037] <Cleaning mechanism> Next, the cleaning mechanism 36 in this embodiment will be described in more detail. FIG. 4 is a schematic diagram for explaining the operation of the cleaning mechanism 36 in this embodiment. FIG. 4(a) shows the surroundings of the photosensitive drum 4Y of the most upstream image forming unit PY for Y and the surroundings of the photosensitive drum 4M of the image forming unit PM for M arranged adjacently on the downstream side when the toner T is transferred to the recording material S (during image formation). FIG. 4(b) shows the surroundings of the photosensitive drum 4Y of the most upstream image forming unit PY for Y and the surroundings of the photosensitive drum 4M of the image forming unit PM for M arranged adjacently on the downstream side when not transferring (during non-image formation). Hereinafter, the downstream image forming units PM, PC, and PK will be mainly described with respect to the image forming unit PM for M, but in this embodiment, the configurations and operations of the downstream image forming units PM, PC, and PK are substantially the same except that the colors of the toners used for development are different. In FIG. 4, for simplicity, the suffixes Y and M of the reference characters indicating that the elements are for any color are omitted as appropriate. As described above, the most upstream image forming station PY is provided with the first cleaning mechanism 36a as the cleaning mechanism 36. Moreover, the downstream image forming station PM is provided with the second cleaning mechanism 36b as the cleaning mechanism 36.

[0038] - Temporary collection of residual toner from transfer With reference to FIG. 4(a), the operation of the cleaning mechanism 36 when a toner image is being transferred onto the recording material S (during image formation) will be described.

[0039] First, the behavior of the transfer residual toner T1 will be described. In the image forming units PY and PM on the most upstream and downstream sides, the cleaning roller 32 contacts the photosensitive drum 4 and rotates in the direction of the arrow G in the figure (counterclockwise direction). That is, the cleaning roller 32 rotates so that the photosensitive drum 4 and the cleaning roller 32 move in the forward direction at the contact portion between the photosensitive drum 4 and the cleaning roller 32. In this embodiment, the cleaning roller 32 is rotated by branching and transmitting the driving force from a driving motor (not shown) that drives the photosensitive drum 4. The cleaning roller 32 may be rotated with a circumferential speed difference with respect to the photosensitive drum 4 (for example, the cleaning roller 32 is faster), or may be rotated at the same circumferential speed as the photosensitive drum 4. In this embodiment, the cleaning roller 32 is configured by coating a rubber layer made of a rubber material as an elastic layer around a roller shaft (shaft core) made of metal or the like. For the rubber layer, for example, a solid rubber member or a sponge member having elasticity is used. The surface potential of the photosensitive drum 4 in contact with the cleaning roller 32 is lowered to nearly 0V by the charge removal light source 31. A predetermined cleaning voltage (cleaning bias) is applied to the cleaning roller 32 by the cleaning power source E4. In this embodiment, the cleaning power source E4 applies a negative DC voltage, which is the opposite polarity to the normal polarity of the toner T, to the cleaning roller 32 as the cleaning voltage. Therefore, the transfer residual toner T1, which is positively charged, moves along the electric field from the photosensitive drum 4 to the cleaning roller 32 and is collected by the cleaning roller 32. Note that this cleaning voltage may be a voltage that is higher on the opposite polarity side (negative polarity side in this embodiment) to the normal polarity of the toner with respect to the surface potential of the photosensitive drum 4 at the contact portion between the cleaning roller 32 and the photosensitive drum 4.

[0040] In the most upstream image forming portion PY, the foreign matter collecting roller 33 comes into contact with the cleaning roller 32 and rotates in the direction of the arrow H in the figure (clockwise direction). That is, the foreign matter collecting roller 33 rotates so that the cleaning roller 32 and the foreign matter collecting roller 33 move in the forward direction at the contact portion between the cleaning roller 32 and the foreign matter collecting roller 33. In this embodiment, the foreign matter collecting roller 33 is rotated by branching and transmitting the driving force from the driving motor (not shown) that drives the photosensitive drum 4. The foreign matter collecting roller 33 may be rotated with a circumferential speed difference with respect to the cleaning roller 32 (for example, the foreign matter collecting roller 33 is faster), or may be rotated at the same circumferential speed as the cleaning roller 32. The foreign matter collecting roller 33 may be rotated by being driven by the cleaning roller 32. In this embodiment, the foreign matter collecting roller 33 is made of a metal roller. The foreign matter collecting roller 33 may be made of a roller having an elastic layer similar to that of the cleaning roller 32. A predetermined toner recovery voltage (toner recovery bias) is applied to the foreign matter recovery roller 33 by the recovery power source E5. In this embodiment, the recovery power source E5 applies a positive DC voltage, which is the same polarity as the normal polarity of the toner T, to the foreign matter recovery roller 33 as the toner recovery voltage. Therefore, the positive polarity transfer residual toner T1 recovered by the cleaning roller 32 does not move to the foreign matter recovery roller 33, but is held by the cleaning roller 32. Note that this toner recovery voltage may be a voltage higher on the normal polarity side of the toner (positive polarity side in this embodiment) than the potential of the cleaning roller 32 at the contact portion between the foreign matter recovery roller 33 and the cleaning roller 32.

[0041] Next, the behavior of foreign matter A, such as paper fibers, fillers (so-called "paper powder"), or dust, generated from the recording material S will be described. A large amount of foreign matter A moves to the photosensitive drum 4Y of the most upstream image forming unit PY, and a small amount moves to the photosensitive drum 4M of the downstream image forming unit PM. This is because most of the foreign matter A generated from the recording material S is removed at the transfer unit NY of the most upstream image forming unit PY. For this reason, in this embodiment, the foreign matter collection roller 33, scraping member 34, and foreign matter collection container 35 used for separating and collecting the foreign matter A are provided only in the first cleaning mechanism 36a of the most upstream image forming unit PY. And, these members are not provided in the second cleaning mechanism 36b of the downstream image forming unit PM. This prevents the entire device from becoming large.

[0042] In the most upstream image forming portion PY, a negative transfer voltage, for example, a transfer voltage of about -1000V, is applied to the transfer roller 16Y during transfer. This transfer voltage may be a voltage higher than the surface potential of the image portion on the photosensitive drum 4Y at the transfer portion NY on the opposite polarity side (negative polarity side in this embodiment) of the normal polarity of the toner. As a result, the positively charged toner T on the photosensitive drum 4Y is attracted to the recording material S and transferred onto the recording material S. At this time, the foreign matter A present on or inside the recording material S is charged to the negative polarity by receiving a negative discharge or injection charge due to the transfer voltage. Then, this foreign matter A moves onto the photosensitive drum 4Y along the electric field due to the transfer voltage. During transfer, a cleaning voltage on the negative polarity side is applied to the cleaning roller 32 in contact with the photosensitive drum 4Y with respect to the surface potential of the photosensitive drum 4Y at the contact portion between the cleaning roller 32 and the photosensitive drum 4Y. Therefore, the negatively charged foreign matter A is not collected by the cleaning roller 32.

[0043] In the most upstream image forming portion PY, after transfer, the photosensitive drum 4Y is uniformly charged to a positive polarity by the charging device 5Y. In addition, a developing voltage of a negative polarity side with respect to the surface potential (non-image portion potential) of the photosensitive drum 4Y at the contact portion between the developing roller 6Y and the photosensitive drum 4Y is applied to the developing roller 6Y. Therefore, the negatively charged foreign matter A is not collected by the developing roller 6Y. In addition, a negative transfer voltage is applied to the transfer roller 16Y. Therefore, the negatively charged foreign matter A does not move to the recording material S or the transfer belt 12.

[0044] Therefore, in the above-described setting of the voltage applied to each roller during transfer, the transfer residual toner T1 is collected and held by the cleaning roller 32 in each image forming station P, and the foreign matter A is carried around on the photosensitive drum 4Y in the most upstream image forming station PY. The behavior of the foreign matter A in the downstream image forming station PM will be described later.

[0045] In this embodiment, the transfer voltage, the developing voltage and the recovery voltage during image formation are set to the same values ​​in all image forming stations P.

[0046] - Collection of residual toner and foreign matter The operation of the cleaning mechanism 36 when not transferring (when not forming an image) will be described with reference to FIG. 4(b).

[0047] In the most upstream and downstream image forming units PY and PM, a predetermined discharge voltage (discharge bias) is applied to the cleaning roller 32 by the cleaning power source E4 at a predetermined timing other than during transfer. In this embodiment, the cleaning power source E4 applies a DC voltage of positive polarity, which is the same polarity as the normal polarity of the toner T, to the cleaning roller 32 as the discharge voltage. Note that this discharge voltage may be a voltage higher on the normal polarity side of the toner (positive polarity side in this embodiment) than the surface potential of the photosensitive drum 4 at the contact portion between the cleaning roller 32 and the photosensitive drum 4. At this time, in the most upstream image forming unit PY, a predetermined foreign matter collection time voltage (foreign matter collection time bias) is applied to the foreign matter collection roller 33 by the collection power source E5. In this embodiment, the collection power source E5 applies a positive DC voltage having an absolute value larger than the discharge voltage applied to the cleaning roller 32 to the foreign matter collection roller 33 as the foreign matter collection time voltage. The voltage during the foreign matter collection may be higher on the normal polarity side of the toner (positive polarity side in this embodiment) than the potential of the cleaning roller 32 at the contact portion between the foreign matter collection roller 33 and the cleaning roller 32 .

[0048] By the above potential setting, the transfer residual toner T1, which is positively charged in the most upstream and downstream image forming units PY and PM, is discharged from the cleaning roller 32 onto the photosensitive drum 4. At this time, a development voltage of negative polarity is applied to the developing roller 6 with respect to the surface potential (non-image portion potential) of the photosensitive drum 4 at the contact portion between the developing roller 6 and the photosensitive drum 4. Therefore, the transfer residual toner T1 discharged onto the photosensitive drum 4 is collected by the developing roller 6 and reused as the toner T during the subsequent image formation (development). Also, by the above potential setting, the foreign matter A on the photosensitive drum 4Y, which is negatively charged in the most upstream image forming unit PY, is collected by the cleaning roller 32 and further collected by the foreign matter collecting roller 33. A scraping member 34 is in contact with the foreign matter collecting roller 33, and the surface of the foreign matter collecting roller 33 is rubbed by the scraping member 34 as the foreign matter collecting roller 33 rotates. An elastic member such as a sponge or felt is used as the scraping member 34. The scraping member 34 may be an elastic rubber blade, a plastic film, etc. The foreign matter A collected by the foreign matter collection roller 33 is scraped off from the foreign matter collection roller 33 by the scraping member 34, and falls into and is collected in a foreign matter collection container 35. The behavior of the foreign matter A in the downstream image forming station PM will be described later.

[0049] In this embodiment, the transfer voltage and development voltage are set to the same values ​​for all image forming stations P during image formation and when the transfer residual toner T1 is collected into the development cartridge 8 during non-image formation. Also, in this embodiment, the discharge voltage is set to the same value for all image forming stations P.

[0050] Furthermore, the discharge of the transfer residual toner T1 from the cleaning roller 32 and the collection by the developing cartridge 8 can be performed at any timing, such as an inter-sheet process, not limited to the post-rotation process, so long as the image is not being formed.

[0051] Behavior of foreign matter in the downstream image forming section Most of the foreign matter A generated from the recording material S is removed at the transfer portion NY of the most upstream image forming portion PY. However, when a small amount of foreign matter A moves to the photosensitive drum 4M of the downstream image forming portion PM, it behaves as follows. In this embodiment, the second cleaning mechanism 36b of the downstream image forming portion PM is not provided with a foreign matter collection roller 33 for collecting the foreign matter A. Therefore, in the downstream image forming portion PM, the foreign matter A moves around on the photosensitive drum 4 as shown in FIG. 4(a) or on the cleaning roller 32 as shown in FIG. 4(b). The principle by which the foreign matter A moves onto the photosensitive drum 4 and onto the cleaning roller 32 in the downstream image forming portion PM as shown in FIGS. 4(a) and 4(b) is the same as that described for the most upstream image forming portion PY.

[0052] As described above, the amount of the foreign matter A generated from the recording material S moving to the photosensitive drum 4 is significantly larger in the most upstream image forming unit PY and smaller in the downstream image forming unit PM. However, in recent years, there has been a trend toward longer life for the drum unit 30, the developing cartridge 8, and the image forming apparatus 1 as consumables. As a result, the foreign matter A may move around on the photosensitive drum 4 or the cleaning roller 32 for a long period of time, causing adhesion toward the latter half of the life, and may cause a deterioration in the function of the photosensitive drum 4 or the cleaning roller 32. For example, if the foreign matter A adheres to the photosensitive drum 4, a streak-like image defect or a dot-like image defect may occur on the image. Also, if the foreign matter A adheres to the cleaning roller 32, the cleaning performance of the transfer residual toner T1 may decrease, and an image defect due to a cleaning defect may occur. Furthermore, if the foreign matter A moving around on the photosensitive drum 4 is collected into the developing cartridge 8 together with, for example, the residual toner T1 after transfer, and the amount of the collected foreign matter exceeds a certain threshold, the chargeability of the toner T and the regulating force of the regulating blade 60 will change, causing the charge of the toner to become disturbed and resulting in a defective image.

[0053] In particular, in a configuration in which the recording material S and the photosensitive drum 4 are in direct contact with each other as in this embodiment, the amount of foreign matter A generated from the recording material S that moves to the photosensitive drum 4 increases, making the above problem likely to occur. Also, in a configuration in which the transfer residual toner T1 is collected in the developing cartridge 8 and reused as toner T as in this embodiment, the foreign matter A is likely to get mixed into the developing cartridge 8, making the above problem likely to occur.

[0054] To address the issue of extending the lifespan, it is important to improve the ability to move foreign matter A to the photosensitive drum 4 of the upstream image forming unit P, thereby reducing the amount of foreign matter A that moves to the photosensitive drum 4 of the downstream image forming unit P. In other words, it is important to improve the ability of the upstream image forming unit P to remove foreign matter A from the recording material S, thereby reducing the amount of foreign matter A sent to the downstream image forming unit P. In other words, it is important to improve the ability of the most upstream image forming unit PY, which is provided with the foreign matter collection roller 33, to collect foreign matter A, thereby reducing the amount of foreign matter A that remains in the downstream image forming unit PM, which is not provided with the foreign matter collection roller 33.

[0055] <Prevention of foreign matter moving downstream> Foreign matter A generated from the recording material S is negatively charged by a negative voltage applied to the transfer roller 16Y in the most upstream image forming section PY, and moves onto the photosensitive drum 4Y. The amount of foreign matter A that moves to the photosensitive drum 4Y at this time, that is, the amount of foreign matter A removed from the recording material S, increases as the potential difference between the transfer roller 16Y and the photosensitive drum 4Y increases. This is because the larger the potential difference between the transfer roller 16Y and the photosensitive drum 4Y, the more strongly foreign matter A on or inside the recording material S is charged to the negative polarity, and the more easily it moves to the photosensitive drum 4Y.

[0056] Therefore, in this embodiment, the potential difference between the photosensitive drum 4Y of the most upstream image forming unit PY and the transfer roller 16Y is made larger than the potential difference between the photosensitive drum 4M of the downstream image forming unit PM and the transfer roller 16M. This makes it possible to increase the amount of foreign matter A moved to the photosensitive drum 4Y of the most upstream image forming unit PY and reduce the amount of foreign matter A moved to the photosensitive drum 4M of the downstream image forming unit PM.

[0057] Normally, the area of ​​the non-image portion of the image formed on the recording material S by the image forming apparatus 1 is larger than the area of ​​the image portion. Furthermore, the amount of foreign matter A generated from the leading or trailing end of the recording material S in the conveying direction or both ends in the width direction (the direction substantially perpendicular to the conveying direction), which correspond to the non-image portions of the recording material S, is greater than the amount generated from other portions. This is because the leading or trailing end or both ends of the recording material S become the cut surfaces when the recording material S is produced, and therefore a large amount of foreign matter generated during cutting is present there.

[0058] Therefore, in the most upstream image forming section PY, it is important to increase the potential difference between the non-image area on the photosensitive drum 4Y and the transfer roller 16Y in order to improve the ability to move foreign matter A to the photosensitive drum 4Y of the most upstream image forming section PY.

[0059] FIG. 5 is a graph showing the change in the amount of foreign matter A moving to the photosensitive drum 4M of the downstream image forming section PM when the conditions are changed as follows. Here, the value obtained by subtracting the potential difference between the non-image area on the photosensitive drum 4M of the downstream image forming section PM and the transfer roller 16M from the potential difference between the non-image area on the photosensitive drum 4Y of the most upstream image forming section PY and the transfer roller 16Y was changed from -500 V to +1000 V. The recording material (paper) S used for printing was Xerox multipurpose paper (basis weight 75 g / m) manufactured by Xerox. 2 , LTR size). The graph also shows the mass of foreign matter A that migrated to the photosensitive drum 4M of the downstream image forming unit PM per 1000 prints. The printed image is a horizontal line image extending in the width direction of the recording material S with a printing rate of 4% for each color.

[0060] It can be seen from FIG. 5 that the amount of foreign matter A moving to the downstream photosensitive drum 4M decreases as the difference in potential difference, obtained by subtracting the latter potential difference from the former potential difference, increases.

[0061] In this embodiment, during image formation (transfer), the same transfer voltage of -1000V is applied to the transfer rollers 16Y, 16M in the most upstream and downstream image forming stations PY and PM. On the other hand, during image formation (charging process), the surface potential (non-image portion potential) of the non-image portion on the photosensitive drum 4Y in the most upstream image forming station PY is +800V, and the surface potential (non-image portion potential) of the non-image portion on the photosensitive drum 4M in the downstream image forming station PM is +500V. In other words, to achieve such non-image portion potentials, the charging voltage applied to the charging device 5 when charging the photosensitive drum 4 is changed between the most upstream image forming station PY and the downstream image forming station PM.

[0062] By setting the above applied voltage, the amount of foreign matter A moving to the photosensitive drum 4Y of the most upstream image forming portion PY can be increased, and the amount of foreign matter A moving to the photosensitive drum 4M of the downstream image forming portion PM can be reduced.

[0063] Furthermore, in this embodiment, the light amount of the laser light L from the exposure device 10 is adjusted so that the surface potential (image area potential) of the image area on the photosensitive drums 4Y, 4M is the same +100 V in the most upstream and downstream image forming stations PY, PM during image formation (exposure). Furthermore, in this embodiment, the same development voltage of +300 V is applied to the development roller 6 in the most upstream and downstream image forming stations PY, PM during image formation (development). In this way, by aligning the potential difference (development contrast) between the image area potential and the development voltage in the most upstream and downstream image forming stations PY, PM, differences in density of the images formed in each image forming station P are prevented.

[0064] The potential settings during image formation in this embodiment are shown in Table 1 below.

[0065] [Table 1]

[0066] In this embodiment, the settings of the charging voltage, developing voltage, and transfer voltage when the transfer residual toner T1 is collected into the developing cartridge 8 during non-image formation in each image forming station P are also the same as those described above. However, this is not limited to this, and for example, the charging voltage may be set differently between when an image is formed and when the transfer residual toner T1 is collected into the developing cartridge 8 during non-image formation. For example, the charging voltage when the transfer residual toner T1 is collected into the developing cartridge 8 during non-image formation may be the same for all image forming stations P.

[0067] Here, when the potential difference (back contrast) between the non-image portion potential and the development voltage becomes equal to or less than a certain threshold, it becomes difficult to collect the transfer residual toner T1 in the development cartridge 8, and the next image forming process may be affected, resulting in a defective image. In this embodiment, in order to successfully collect the transfer residual toner T1 in the development cartridge 8 throughout the life of the development cartridge 8, this potential difference was required to be 200V or more. Conversely, when the potential difference between the non-image portion potential and the development voltage becomes equal to or more than a certain threshold, a small amount of toner T charged with a polarity opposite to the normal polarity on the development roller 6 tends to move to the non-image portion on the photosensitive drum 4, and an image defect called "fog" may occur. In this embodiment, when this potential difference exceeds 500V, the fog occurs significantly. Therefore, in this embodiment, the non-image portion potential on the photosensitive drum 4Y of the most upstream image forming unit PY is set to +800V, and the non-image portion potential on the photosensitive drum 4M of the downstream image forming unit PM is set to +500V.

[0068] As described above, according to this embodiment, it is possible to improve the ability to move the foreign matter A to the photosensitive drum PY of the most upstream image forming unit PY where the foreign matter collection roller 33 is provided. This makes it possible to reduce the amount of foreign matter A that moves to the photosensitive drum 4M of the downstream image forming unit PM where the foreign matter collection roller 33 is not provided. As a result, it is possible to suppress image defects caused by the foreign matter A due to the extended life of the drum unit 30, the developing cartridge 8, or the image forming apparatus 1, as described above.

[0069] [Example 2] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of the first embodiment. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are given the same reference numerals as those of the first embodiment, and detailed explanations are omitted. Also, in this embodiment, as for the downstream image forming units PM, PC, and PK, the image forming unit PM for M will be mainly described. However, in this embodiment, the configurations and operations of the downstream image forming units PM, PC, and PK are substantially the same, except that the colors of the toners used for development are different.

[0070] In this embodiment, a means different from that in embodiment 1 is used to make the potential difference between the transfer roller 16Y in the most upstream image forming station PY and the non-image area on the photosensitive drum 4Y larger than the potential difference between the transfer roller 16M in the downstream image forming station PM and the non-image area on the photosensitive drum 4M.

[0071] In this embodiment, different transfer voltages are applied to the transfer roller 16Y of the most upstream image forming unit PY and the transfer roller 16M of the downstream image forming unit PM. Specifically, in this embodiment, a transfer voltage of -2000V is applied to the transfer roller 16Y of the most upstream image forming unit PY, and a transfer voltage of -1000V is applied to the transfer roller 16M of the downstream image forming unit PM.

[0072] This makes it possible to set the potential difference between the non-image portion on the most upstream photosensitive drum 4Y and the transfer roller 16Y larger than the potential difference between the non-image portion on the downstream photosensitive drum 4M and the transfer roller 16M without changing the non-image portion potential. This makes it possible to increase the amount of foreign matter A moving to the photosensitive drum 4Y of the most upstream image forming unit PY where the foreign matter collection roller 33 is provided, and to reduce the amount of foreign matter A moving to the photosensitive drum 4M of the downstream image forming unit PM where the foreign matter collection roller 33 is not provided.

[0073] In this embodiment, it is not necessary to change the potential of the non-image portion on the photosensitive drum 4. Therefore, a large potential difference of 1000 V or more can be provided between the non-image portion on the photosensitive drum 4 and the transfer roller 16 between the most upstream image forming station PY and the downstream image forming station PM, while suppressing the possibility of a decrease in the collectability of the transfer residual toner T1 and the occurrence of fogging. This makes it possible to further increase the ability to move the foreign matter A to the photosensitive drum 4Y of the most upstream image forming station PY.

[0074] The potential settings during image formation in this embodiment are shown in Table 2 below.

[0075] [Table 2]

[0076] However, when the transfer voltage applied to the transfer roller 16Y of the most upstream image forming unit PY is set to be 1000V higher in absolute value than the transfer voltage applied to the transfer roller 16M of the downstream image forming unit PM in this way, the following other problem occurs. That is, the potential difference between the image portion on the photosensitive drum 4Y of the most upstream image forming unit PY and the transfer roller 16Y becomes larger than the potential difference between the image portion on the photosensitive drum 4M of the downstream image forming unit PM and the transfer roller 16M. Therefore, the transferability of the toner image on the photosensitive drum 4Y of the most upstream image forming unit PY changes with respect to the transferability of the toner image on the photosensitive drum 4M of the downstream image forming unit PM, and the transfer residual toner T1 increases in the most upstream image forming unit PY.

[0077] Therefore, in this embodiment, the problem of the change in transferability between the most upstream image forming section PY and the downstream image forming section PM is solved by the following configuration. That is, in this embodiment, the electrical resistance of the transfer roller 16Y of the most upstream image forming section PY is made higher than the electrical resistance of the transfer roller 16M of the downstream image forming section PM. Specifically, in this embodiment, the volume resistivity of the sponge material constituting the transfer roller 16Y of the most upstream image forming section PY is made higher than the volume resistivity of the sponge material constituting the transfer roller 16M of the downstream image forming section PM.

[0078] When the electric resistance of the transfer roller 16 is large, the value of the current that flows is smaller, even if the potential difference between the transfer roller 16 and the photosensitive drum 4 is increased, compared to when the electric resistance is small. Therefore, the potential difference between the transfer roller 16 and the photosensitive drum 4 for passing the transfer current required to transfer the toner image is large. As a result, even if the potential difference between the transfer roller 16Y and the photosensitive drum 4Y in the most upstream image forming station PY is larger than the potential difference between the transfer roller 16M and the photosensitive drum 4M in the downstream image forming station PM, it is possible to obtain the same high transfer efficiency in the most upstream image forming station PY and the downstream image forming station PM.

[0079] 6 is a graph showing the transfer efficiency versus the potential difference between the transfer roller 16 and the photosensitive drum 4 when using transfer rollers 16 with two types of electrical resistance (volume resistivity of the sponge material). The recording material (paper) S used for printing was Xerox multipurpose paper (basis weight 75 g / m 2 , LTR size). It also shows the transfer efficiency when transferring a solid image with the highest density as an image. The transfer efficiency is calculated as a percentage by dividing the mass of the toner T transferred onto the recording material S by the mass of the toner T on the photosensitive drum 4.

[0080] As shown in FIG. 6, the high resistance transfer roller used as the transfer roller 16Y of the most upstream image forming unit PY exhibits higher transfer efficiency at a larger potential difference than the low resistance transfer roller used as the transfer roller 16M of the downstream image forming unit PM.

[0081] In this embodiment, the volume resistivity of the sponge material constituting the transfer roller 16M of the downstream image forming unit PM is 1.0×10 7 On the other hand, the volume resistivity of the sponge material constituting the transfer roller 16Y of the most upstream image forming unit PY is 1.0×10 8 [Ω cm].

[0082] As described above, according to this embodiment, the same effects as those of the first embodiment can be obtained, and the ability to move foreign matter A to the photosensitive drum 4Y of the most upstream image forming section PY can be further improved while suppressing the decrease in the recoverability of the residual toner T1 after transfer and the possibility of fogging occurring.

[0083] [Example 3] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of the first embodiment. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are given the same reference numerals as those of the first embodiment, and detailed explanations are omitted. Also, in this embodiment, as for the downstream image forming units PM, PC, and PK, the image forming unit PM for M will be mainly described. However, in this embodiment, the configurations and operations of the downstream image forming units PM, PC, and PK are substantially the same, except that the colors of the toners used for development are different.

[0084] In this embodiment, similar to the second embodiment, different transfer voltages are applied to the transfer roller 16Y of the most upstream image forming unit PY and the transfer roller 16M of the downstream image forming unit PM (most upstream: -2000V, downstream: -1000V). Therefore, in this embodiment, similar to the second embodiment, the potential difference between the image portion on the photosensitive drum 4Y in the most upstream image forming unit PY and the transfer roller 16Y is larger than the potential difference between the image portion on the photosensitive drum 4M in the downstream image forming unit PM and the transfer roller 16M. In this embodiment, the problem of the change in transferability between the most upstream image forming unit PY and the downstream image forming unit PM caused by this is solved by a means different from that in the second embodiment.

[0085] In this embodiment, the photosensitive drum 4Y of the most upstream image forming section PY and the photosensitive drum 4M of the downstream image forming section PM have different types of surface layer materials and film thicknesses. In this embodiment, the film thickness of the surface layer of the photosensitive drum 4Y of the most upstream image forming section PY is made smaller than the film thickness of the surface layer of the photosensitive drum 4M of the downstream image forming section PM. Also, in this embodiment, the average molecular weight of the binder resin material constituting the surface layer of the photosensitive drum 4Y of the most upstream image forming section PY is made larger than the average molecular weight of the binder resin material constituting the surface layer of the photosensitive drum 4M of the downstream image forming section PM. Specifically, in this embodiment, the photosensitive drum 4Y of the most upstream image forming section PY has a surface layer film thickness (initial) of 25 μm, and uses a material with an average molecular weight of about 50,000 as the binder resin material of the surface layer. On the other hand, the photosensitive drum 4M of the downstream image forming section PM has a surface layer film thickness (initial) of 30 μm, and uses a material with an average molecular weight of about 20,000 as the binder resin material of the surface layer.

[0086] The positively charged toner T on the photosensitive drum 4 adheres to the photosensitive drum 4 due to a mirror force between the photosensitive drum 4 and the photosensitive drum 4. This mirror force increases as the thickness of the surface layer of the photosensitive drum 4 decreases, and a larger potential difference (transfer contrast) between the photosensitive drum 4 and the transfer roller 16 is required to transfer the toner T. In other words, to ensure high transferability, it is necessary to increase the potential difference between the transfer roller 16 and the photosensitive drum 4.

[0087] In this embodiment, the thickness of the surface layer of the photosensitive drum 4Y of the most upstream image forming unit PY is made smaller than the thickness of the surface layer of the photosensitive drum 4M of the downstream image forming unit PM. This makes it possible to obtain high transferability equivalent to that of the most upstream image forming unit PY and the downstream image forming unit PM even if the potential difference between the transfer roller 16Y and the photosensitive drum 4Y in the most upstream image forming unit PY is larger than the potential difference between the transfer roller 16M and the photosensitive drum 4M in the downstream image forming unit PM.

[0088] Here, as printing continues using the photosensitive drum 4, the surface layer of the photosensitive drum 4 is gradually worn away by friction with the recording material S, cleaning roller 32, developing roller 6, toner T, etc. It is desirable to determine the film thickness of the surface layer of the initial (new) photosensitive drum 4 so that the photosensitive drum 4 satisfies its function even with the amount of wear when the number of sheets corresponding to the life of the photosensitive drum 4 is printed. In this embodiment, the film thickness of the surface layer is small in the photosensitive drum 4Y of the most upstream image forming unit PY, so it is desirable to reduce the amount of wear of the surface layer in order to satisfy the function throughout its life.

[0089] Therefore, in this embodiment, the material of the surface layer of the photosensitive drum 4Y of the most upstream image forming unit PY is made to be less easily scraped than the material of the surface layer of the photosensitive drum 4M of the downstream image forming unit PM. In this embodiment, in the most upstream and downstream image forming units PY and PM, a single-layer type photosensitive body that is an organic photosensitive body with a positive charging polarity is used as the photosensitive drum 4. That is, the photosensitive drum 4 is configured by forming a single-layer surface layer, which is a photosensitive layer mainly made of resin, around a cylindrical base body formed of a conductive material such as metal. In this embodiment, the photosensitive drum 4Y of the most upstream image forming unit PY uses polycarbonate resin with an average molecular weight of about 50,000 as the binder resin material of the surface layer. On the other hand, the photosensitive drum 4M of the downstream image forming unit PM uses polycarbonate resin with an average molecular weight of about 20,000 as the binder resin material of the surface layer.

[0090] In this manner, in this embodiment, the photosensitive drum 4Y of the most upstream image forming section PY uses a resin material that is larger in average molecular weight and harder as a binder resin than the photosensitive drum 4M of the downstream image forming section PM. As a result, in this embodiment, the surface layer of the photosensitive drum 4Y of the most upstream image forming section PY is less susceptible to abrasion than the photosensitive drum 4M of the downstream image forming section PM. Specifically, in this embodiment, the amount of abrasion of the surface layer of the photosensitive drum 4Y of the most upstream image forming section PY per 1000 prints is 0.2 μm on average. On the other hand, the amount of abrasion of the surface layer of the photosensitive drum 4M of the downstream image forming section PM per 1000 prints is 0.4 μm on average. The recording material (paper) S used for printing was Xerox multipurpose paper (basis weight 75 g / m2) manufactured by Xerox. 2 , LTR size). In this embodiment, the life of the photosensitive drum 4 is about 25,000 prints. The surface layer of the photosensitive drum 4Y of the most upstream image forming unit PY is worn down by about 5 μm from the initial film thickness at the end of its life. On the other hand, the surface layer of the photosensitive drum 4M of the downstream image forming unit PM is worn down by about 10 μm from the initial film thickness at the end of its life. As described above, in this embodiment, the initial film thickness of the surface layer of the photosensitive drum 4 is 25 μm for the photosensitive drum 4Y of the most upstream image forming unit PY and 30 μm for the photosensitive drum 4M of the downstream image forming unit PM. Therefore, the remaining surface layer film thickness at the end of its life is about 20 μm for both the photosensitive drums 4 of the most upstream and downstream image forming units PY and PM, which is approximately the same.

[0091] In addition, for example, in the case where a material that is sufficiently resistant to abrasion is used as the material for the surface layer of the photosensitive drum 4, the type of material and the thickness of the film of the surface layer of the photosensitive drum 4 alone may be different between the most upstream image forming unit PY and the downstream image forming unit PM.

[0092] As described above, according to this embodiment, it is possible to obtain the same effects as in embodiment 1, and also to further improve the ability to move foreign matter A to the photosensitive drum 4Y of the most upstream image forming section PY, as in embodiment 2.

[0093] [Example 4] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of the first embodiment. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are given the same reference numerals as those of the first embodiment, and detailed explanations are omitted. Also, in this embodiment, as for the downstream image forming units PM, PC, and PK, the image forming unit PM for M will be mainly described. However, in this embodiment, the configurations and operations of the downstream image forming units PM, PC, and PK are substantially the same, except that the colors of the toners used for development are different.

[0094] In this embodiment, similar to the second embodiment, different transfer voltages are applied to the transfer roller 16Y of the most upstream image forming unit PY and the transfer roller 16M of the downstream image forming unit PM (most upstream: -2000V, downstream: -1000V). Therefore, in this embodiment, similar to the second embodiment, the potential difference between the image portion on the photosensitive drum 4Y in the most upstream image forming unit PY and the transfer roller 16Y is larger than the potential difference between the image portion on the photosensitive drum 4M in the downstream image forming unit PM and the transfer roller 16M. In this embodiment, the problem of the change in transferability between the most upstream image forming unit PY and the downstream image forming unit PM caused by this is solved by a means different from that in the second and third embodiments.

[0095] In this embodiment, the charge amount of toner T supplied to the photosensitive drum 4Y in the most upstream image forming station PY is made larger than the charge amount of toner T supplied to the photosensitive drum 4M in the downstream image forming station PM. Here, the charge amount of toner T is the charge amount of toner T per unit area on the photosensitive drum 4 when a solid image is formed. In other words, the charge amount of toner T is the total charge amount of toner T that forms a solid image of a unit area on the photosensitive drum 4. In this embodiment, the charge amount of toner T per unit area of ​​a solid image is 230 μC / m on the photosensitive drum 4Y of the most upstream image forming station PY. 2 and 150 μC / m on the photosensitive drum 4M of the downstream image forming unit PM. 2 It is.

[0096] 7 is a graph showing the transfer efficiency versus the potential difference between the transfer roller 16 and the photosensitive drum 4 when two types of toner T are used. The recording material (paper) S used for printing was Xerox multipurpose paper (basis weight 75 g / m 2 , LTR size). It also shows the transfer efficiency when transferring a solid image with the highest density as an image. The transfer efficiency is calculated as a percentage by dividing the mass of the toner T transferred onto the recording material S by the mass of the toner T on the photosensitive drum 4.

[0097] As shown in FIG. 7, in order to transfer the toner T with a larger charge amount from the photosensitive drum 4 to the recording material S with high transfer efficiency, it is necessary to increase the potential difference between the photosensitive drum 4 and the transfer roller 16. In this embodiment, in the most upstream image forming unit PY using the toner T with a larger charge amount, in order to obtain a high transferability equivalent to that of the downstream image forming unit PM, the following is done. That is, the potential difference between the photosensitive drum 4Y and the transfer roller 16Y of the most upstream image forming unit PY is set to be larger than the potential difference between the photosensitive drum 4M and the transfer roller 16Y of the downstream image forming unit PM. As a result, the ability to move the foreign matter A to the photosensitive drum 4Y of the most upstream image forming unit PY can be improved, and the amount of the foreign matter A moving to the photosensitive drum 4M of the downstream image forming unit PM can be reduced.

[0098] Here, in order to increase the charge amount of the toner T supplied to the photosensitive drum 4Y in the most upstream image forming station PY without transferring an unnecessarily large amount of toner T to the recording material S, the present embodiment is configured as follows: That is, the average particle size of the toner T supplied to the photosensitive drum 4Y in the most upstream image forming station PY is made 1 μm larger than the average particle size of the toner T supplied to the photosensitive drum 4M in the downstream image forming station PM. If the average particle size of the toner T is larger, the charge amount per toner T increases, and therefore the total charge amount of the toner T on the photosensitive drum 4 also increases.

[0099] The means for increasing the charge amount of the toner T supplied to the photosensitive drum 4 is not limited to the means employed in this embodiment. For example, the external additive of the toner T may be changed to one with high chargeability. Also, for example, the material of the surface layer of the regulating blade 60 or the developing roller 6 may be changed to a material with high chargeability to the toner T. Also, for example, the chargeability to the toner T may be improved by applying a large voltage to the regulating blade 60 on the normal polarity side (positive polarity side in this embodiment) with respect to the potential of the developing roller 6.

[0100] As described above, according to this embodiment, it is possible to obtain the same effects as in embodiment 1, and also to further improve the ability to move foreign matter A to the photosensitive drum 4Y of the most upstream image forming section PY, as in embodiment 2.

[0101] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-mentioned embodiments.

[0102] In the above embodiment, the foreign matter recovery member is provided only in the most upstream image forming section, but the foreign matter recovery member may be provided in the downstream image forming section. As described above, the problem caused by the foreign matter moving to the downstream image carrier due to the long life of the unit including the image carrier and the developing device, or the image forming apparatus, is prominent in the configuration in which the foreign matter recovery member is provided only in the most upstream image forming section. However, even if the foreign matter recovery member is provided in the downstream image forming section, problems may occur due to the foreign matter sent to the downstream image forming section being fixed to the image carrier or cleaning member, or being recovered in the developing device. Therefore, by applying the present invention to the configuration in which the foreign matter recovery member is provided in the downstream image forming section, the same effect as in the above embodiment can be obtained.

[0103] In the above embodiment, the charging voltage and the developing voltage are described as DC voltage components, but the charging voltage and the developing voltage may be oscillating voltages in which a DC voltage (DC component) and an AC voltage (AC component) are superimposed. The same applies to the collecting voltage and the discharging voltage.

[0104] In the above embodiment, the static elimination light source is used as the static elimination means, but when, for example, a charging means with sufficiently small charging unevenness is used, the static elimination means does not need to be provided.

[0105] In addition, in the above-described embodiment, a one-component non-magnetic contact development method is adopted, but the present invention is not limited to such an embodiment, and a two-component non-magnetic contact development method, a non-contact development method, a magnetic development method, etc. may also be adopted.

[0106] In the above embodiment, the cleaning member is a rotatable roller-shaped member, but it may be, for example, a rotatable brush-shaped member (brush roller), etc. In the above embodiment, the foreign matter collection member is a rotatable roller-shaped member, but it may be, for example, a rotatable brush-shaped member (brush roller), etc.

[0107] In the above embodiment, the image carrier is a rotatable drum-shaped member, but may be, for example, an endless belt. In the above embodiment, the transfer member is a rotatable roller-shaped member, but is not limited to this. For example, the transfer member may be a pad-shaped member, a sheet-shaped member, a brush-shaped member (such as a fixed brush or a rotatable brush roller), a rotatable endless belt (a pressing member that contacts the photoconductor via the belt may be provided), etc. In addition, the charging member may be a rotatable roller-shaped member, a brush-shaped member (such as a rotatable brush roller), a rotatable endless belt, etc.

[0108] In addition, in the above-described embodiment, the image forming device has four image forming units, but the number of image forming units may be two or more depending on the number of colors used, for example, it may be three, or it may be five or more. [Explanation of symbols]

[0109] 1. Image forming device 4 Photosensitive drum 5. Charging device 6 Developing roller 8 Developing device (developing cartridge) 10 Exposure equipment 12 Transfer belt 16 Transfer roller 30 Drum unit 32 Cleaning roller 33 Foreign object collection roller 34 Scraping member 35 Foreign matter collection container A Foreign body S recording material T Toner T1 Residual toner

Claims

1. An image forming apparatus capable of performing an image forming operation for forming an image on a recording material, a first image forming section including a rotatable first image carrier, a first charging member for charging a surface of the first image carrier, a first developing device for supplying a first developer to the surface of the first image carrier, a first transfer member for transferring the first developer supplied to the surface of the first image carrier to a recording material, a first cleaning member for contacting the first image carrier to clean the surface of the first image carrier, and a foreign matter recovery member for contacting the first cleaning member to recover foreign matter adhering to the first cleaning member; a second image forming section including a rotatable second image carrier, a second charging member for charging a surface of the second image carrier, a second developing device for supplying a second developer to the surface of the second image carrier, a second transfer member for transferring the second developer supplied to the surface of the second image carrier to a recording material, and a second cleaning member for contacting the second image carrier to clean the surface of the second image carrier; a belt that contacts the first image carrier to form a first transfer portion and contacts the second image carrier to form a second transfer portion, the belt nipping and conveying a recording material between the first image carrier and the second image carrier at the first transfer portion and the second transfer portion, respectively; a charging voltage application section that applies a first charging voltage to the first charging member and applies a second charging voltage to the second charging member; a transfer voltage application unit that applies a first transfer voltage to the first transfer member and applies a second transfer voltage to the second transfer member; the first image forming unit is disposed upstream of the second image forming unit in a moving direction of the recording material; in the image forming operation, a potential difference between a potential at which an image formed on the first image carrier in the first transfer portion is not formed and the first transfer voltage is larger than a potential difference between a potential at which an image formed on the second image carrier in the second transfer portion is not formed and the second transfer voltage, 13. An image forming apparatus, comprising: a surface layer of said first image carrier having a thickness smaller than that of said second image carrier.

2. 2. The image forming apparatus according to claim 1, wherein the average molecular weight of the binder resin material constituting the surface layer of the first image carrier is greater than the average molecular weight of the binder resin material constituting the surface layer of the second image carrier.

3. 3. The image forming apparatus according to claim 1, wherein the second image forming unit is not provided with the foreign matter recovery member.

4. An image forming apparatus according to any one of claims 1 to 3, characterized in that, during the image forming operation, the difference between the potential at which an image is formed on the first image carrier in the first transfer section and the potential at which an image is not formed is greater than the difference between the potential at which an image is formed on the second image carrier in the second transfer section and the potential at which an image is not formed.

5. 4. The image forming apparatus according to claim 1, wherein in the image forming operation, a potential difference between a potential for forming an image on the first image carrier in the first transfer portion and the first transfer voltage is greater than a potential difference between a potential for forming an image on the second image carrier in the second transfer portion and the second transfer voltage.

6. 6. An image forming apparatus according to claim 1, wherein a total charge amount of the first developer forming a solid image of a unit area on the first image carrier is greater than a total charge amount of the second developer forming a solid image of a unit area on the second image carrier.

7. 7. The image forming apparatus according to claim 6, wherein an average particle diameter of the first developer is larger than an average particle diameter of the second developer.

Images