Image forming apparatus

Abstract

To prevent an adverse effect on an image due to attachment of an external additive to a charging member resulting from a process subsequent to an image forming operation.SOLUTION: In an image forming apparatus 200, a control unit 110 executes a static elimination operation of eliminating static electricity from a surface of a photoreceptor 1 after image formation and before the rotation of the photoreceptor 1 stops, in the static elimination operation, causes an exposure device 3 to expose the surface of the photoreceptor 1 to form an exposure surface on the surface of the photoreceptor 1, while the exposure surface passes through a developing unit P4, applies, to a developing member 41, developing voltage on a predetermined polarity side having a normal charging polarity relative to the potential of the exposure surface, while the exposure surface passes through a transfer part P5, applies, to the transfer part P5, transfer voltage in which the absolute value of the potential difference between the potential of the exposure surface and the potential of the transfer voltage is less than discharge start voltage, and while the exposure surface passes through a charging part P2, applies, to a charging member 2, charging voltage on the predetermined polarity side relative to the potential of the exposure surface.SELECTED DRAWING: Figure 2

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 or electrostatic recording method. [Background technology]

[0002] In a conventional image forming apparatus such as a printer using an electrophotographic method, the surface of a photoconductor is uniformly charged by a charging device, and the charged surface of the photoconductor is exposed by an exposure device to form an electrostatic latent image on the surface of the photoconductor. The electrostatic latent image formed on the surface of the photoconductor is then developed by a developing device that supplies toner as a developer, thereby forming a toner image on the surface of the photoconductor.

[0003] A contact charging method is widely used, in which a rotatable charging roller is brought into contact with the photosensitive drum and a charging bias is applied to the core of the charging roller to charge the surface of the photosensitive drum. The contact charging method has the advantages of less discharge and suppressing wear of the surface layer (resin film) of the photosensitive drum.

[0004] Furthermore, it is common to add fine particles (external additive) such as silicon oxide to the surface of the toner in advance to improve the fluidity of the toner and, for example, improve the circulation of the toner within a developing device.

[0005] However, particularly in the contact charging method, if the external additive adheres excessively to the surface of the charging roller, image defects due to insufficient charging may occur. In response to this, it is known that the toner adhered to the surface of the photosensitive drum can be prevented from adhering to the surface of the charging roller by removing electricity from the surface of the photosensitive drum before charging with the charging roller (Patent Document 1).

[0006] Furthermore, in recent years, as image forming apparatuses become smaller, a development separation-less configuration has been widely adopted. That is, in an image forming apparatus that adopts a contact development method in which a development roller as a developer carrier provided in a development device is brought into contact with a photosensitive drum to perform development, a contact / separation mechanism that brings the development roller into contact with and separates it from the photosensitive drum may be provided. The development separation-less configuration refers to a configuration in which the contact / separation mechanism is not provided and the development roller and the photosensitive drum are substantially always in contact with each other. In this development separation-less configuration, it is known that fog at start-up can be reduced by applying a development bias of a polarity opposite to the normal charging polarity of the toner to the development roller when the rotation of the photosensitive drum starts (Patent Document 2). In order to suppress fog at start-up, an appropriate development bias is set assuming that a predetermined time has passed since the previous image forming operation and the surface potential of the photosensitive drum dark decays. Alternatively, the surface of the photosensitive drum may be de-electrified when the image forming operation is completed (Patent Document 3). This prevents a large change in the surface potential of the photosensitive drum at startup due to differences in the amount of time that has elapsed since the previous image forming operation, and allows an appropriate developing bias to be set at startup, thereby suppressing fogging at startup. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2009-192941 A [Patent Document 2] JP 2005-345915 A [Patent Document 3] JP 2020-160361 A Summary of the Invention [Problem to be solved by the invention]

[0008] In order to sufficiently remove static electricity from the surface of the photosensitive drum, it is desirable to set the length of the static electricity removal operation to be equal to or longer than the time it takes for the photosensitive drum to rotate once. In general, when the surface of the photosensitive drum from which static electricity has been removed passes through the charging section, no charging bias is applied to the charging roller.

[0009] In this case, however, the potential difference between the surface potential of the photosensitive drum and the charging bias becomes small, and the adhesion of external additives to the surface of the charging roller is no longer suppressed, which may result in poor image quality due to poor charging.

[0010] SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENT An object of the present invention is to suppress image deterioration caused by adhesion of external additives or the like to a charging member resulting from post-processing of an image forming operation. [Means for solving the problem]

[0011] The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides an image forming apparatus having a rotatable photoconductor, a charging member that contacts the photoconductor to form a charging section and charges the surface of the rotating photoconductor, a charging voltage application section that applies a charging voltage to the charging member, an exposure device that exposes the surface of the photoconductor charged by the charging member to light to form an electrostatic latent image on the surface of the photoconductor, a developing member that contacts the photoconductor to form a developing section and supplies a developer charged to a predetermined polarity to the electrostatic latent image on the surface of the photoconductor in the developing section to form a developer image on the surface of the photoconductor, a developing voltage application section that applies a developing voltage to the developing member, a transfer member that contacts the photoconductor to form a transfer section, a transfer voltage application section that applies a transfer voltage to the transfer section, and a control section that can control the charging voltage application section, the developing voltage application section, the transfer voltage application section, and the exposure device, and a transfer member that transfers a transferred image from the photoconductor to the electrostatic latent image on the surface of the photoconductor in the transfer section. and a developing unit that controls the developing member to be in contact with the photoconductor after image formation and a developer image is transferred to the photoconductor, and the rotation of the photoconductor stops with the developing member in contact with the photoconductor after image formation. The control unit controls the developing member to be in contact with the photoconductor after image formation and before the rotation of the photoconductor stops after image formation, and in the charge removal operation, the control unit exposes the surface of the photoconductor using the exposure device to form an exposure surface on the surface of the photoconductor, applies a development voltage of the predetermined polarity with respect to the potential of the exposure surface to the developing member while the exposure surface passes through the developing unit, applies a transfer voltage to the transfer unit while the exposure surface passes through the transfer unit, the absolute value of the potential difference between the potential of the exposure surface and the development voltage being less than a discharge start voltage, and applies a charging voltage of the predetermined polarity with respect to the potential of the exposure surface to the charging member while the exposure surface passes through the charging unit.

[0012] According to another aspect of the present invention, a photosensitive drum includes a rotatable photosensitive drum, a charging member that contacts the photosensitive drum to form a charging portion and charges a surface of the rotating photosensitive drum, a charging voltage application unit that applies a charging voltage to the charging member, an exposure device that exposes the surface of the photosensitive drum charged by the charging member to form an electrostatic latent image on the surface of the photosensitive drum, a developing member that contacts the photosensitive drum to form a developing portion and supplies a developer charged to a predetermined polarity to the electrostatic latent image on the surface of the photosensitive drum in the developing portion to form a developer image on the surface of the photosensitive drum, a developing voltage application unit that applies a developing voltage to the developing member, a transfer member that contacts the photosensitive drum to form a transfer portion, a transfer voltage application unit that applies a transfer voltage to the transfer portion, a cleaning member that is disposed in contact with the photosensitive drum and removes the developer from the photosensitive drum, and a control unit that is capable of controlling the charging voltage application unit, the developing voltage application unit, the transfer voltage application unit, and the exposure device, and the developer image is transferred from the photosensitive drum to a transfer receiving member in the transfer portion. and an exposure device for exposing the surface of the photoconductor to light to form an exposed surface on the surface of the photoconductor, the control unit controls the exposure device to expose the surface of the photoconductor to light to form an exposed surface on the surface of the photoconductor, the control unit applies a developing voltage of the predetermined polarity with respect to a potential of the exposed surface to the developing member while the exposed surface passes through the developing unit, the control unit applies a transfer voltage to the transfer unit while the exposed surface passes through the transfer unit, the control unit applies a charging voltage of the predetermined polarity with respect to a potential of the exposed surface to the charging member while the exposed surface passes through the charging unit, and the control unit controls the transfer voltage applied to the transfer unit while the exposed surface passes through the transfer unit to be an opposite polarity to the predetermined polarity with respect to the charging voltage applied to the charging member while the exposed surface passes through the charging unit.

[0013] According to another aspect of the present invention, a photosensitive drum includes a rotatable photosensitive drum, a charging member that contacts the photosensitive drum to form a charging section and charges the surface of the rotating photosensitive drum, a charging voltage application section that applies a charging voltage to the charging member, an exposure device that exposes the surface of the photosensitive drum charged by the charging member to form an electrostatic latent image on the surface of the photosensitive drum, a developing member that contacts the photosensitive drum to form a developing section and supplies a developer charged to a predetermined polarity to the electrostatic latent image on the surface of the photosensitive drum in the developing section to form a developer image on the surface of the photosensitive drum, a contact and separation mechanism that contacts and separates the developing member from the photosensitive drum, a transfer member that contacts the photosensitive drum to form a transfer section, a transfer voltage application section that applies a transfer voltage to the transfer section, a cleaning member that is disposed in contact with the photosensitive drum and removes the developer from the photosensitive drum, and a control section that is capable of controlling the charging voltage application section, the transfer voltage application section, and the exposure device, In an image forming apparatus in which the rotation of the photoconductor stops with the developing member separated from the photoconductor by the contact / separation mechanism after image formation in which a developer image is transferred from the photoconductor to a transferee, the control unit executes a discharge operation to discharge the surface of the photoconductor before the rotation of the photoconductor stops after image formation, and in the discharge operation, exposes the surface of the photoconductor using the exposure device to form an exposed surface on the surface of the photoconductor, applies a transfer voltage to the transfer unit while the exposed surface passes through the transfer unit, applies a charging voltage of the predetermined polarity with respect to the potential of the exposed surface to the charging member while the exposed surface passes through the charging unit, and controls the transfer voltage applied to the transfer unit while the exposed surface passes through the transfer unit to be the polarity opposite to the predetermined polarity with respect to the charging voltage applied to the charging member while the exposed surface passes through the charging unit. Effect of the Invention

[0014] According to the present invention, it is possible to suppress image defects caused by adhesion of external additives or the like to a charging member resulting from post-processing of an image forming operation. [Brief description of the drawings]

[0015] [Figure 1] 1 is a schematic cross-sectional view of an image forming apparatus according to a first embodiment of the present invention. [Diagram 2] FIG. 4 is a timing chart for explaining a post-process in the first embodiment. [Diagram 3] FIG. 4 is a schematic diagram for explaining an exposure width in the first embodiment. [Figure 4] FIG. 11 is a timing chart for explaining a post-process in a modified example of the first embodiment. [Diagram 5] FIG. 11 is a schematic cross-sectional view of an image forming apparatus according to a second embodiment. [Figure 6] FIG. 11 is a schematic cross-sectional view of an image forming apparatus according to a third embodiment. [Figure 7] FIG. 11 is a graph illustrating an example of a method for controlling a transfer bias. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The image forming apparatus according to the present invention will be described in more detail below with reference to the drawings. The dimensions, materials, shapes, and relative positions of the components described in the following examples may be changed as appropriate depending on the configuration of the apparatus to which the present invention is applied and various conditions, and the scope of the present invention is not limited to the following examples.

[0017] For convenience, the magnitude (high / low) of a voltage or potential (or potential difference) refers to the magnitude (high / low) when compared in absolute value, unless otherwise specified.

[0018] [Example 1] 1. Configuration of image forming device First, the configuration of an image forming apparatus 200 of this embodiment will be described with reference to Fig. 1. Fig. 1 is a schematic cross-sectional view of the image forming apparatus 200 of this embodiment.

[0019] The image forming apparatus 200 of this embodiment is configured such that the process cartridge 100 is detachably attached to the apparatus main body 210. In this embodiment, the process cartridge 100 is configured by integrating a photosensitive drum 1, a charging roller 2 as a process means acting on the photosensitive drum 1, and a developing device (developing unit) 4. Here, the apparatus main body 210 is the portion of the image forming apparatus 200 excluding the process cartridge 100. The process cartridge 100 performs a part of the image forming operation while attached to the apparatus main body 210.

[0020] When an image forming operation is started, the photosensitive drum 1, which is a rotatable drum-type (cylindrical) photosensitive member (electrophotographic photosensitive member) serving as an image carrier, is rotated in the direction of arrow R1 (clockwise direction) in the figure at a predetermined peripheral speed (process speed). The photosensitive drum 1 is rotated by a driving force transmitted from a driving motor (main motor) 10 serving as a driving source constituting a driving means.

[0021] The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential of a predetermined polarity (negative in this embodiment) by the charging roller 2, which is a roller-type charging member serving as a charging means. The charging roller 2 has a conductive charging roller core metal 21 and a conductive rubber layer (elastic layer) 22 formed around the charging roller core metal 21. The charging roller 2 is disposed in contact with the photosensitive drum 1 and rotates around the charging roller core metal 21 in accordance with the rotation of the photosensitive drum 1. A negative charging power source 81 is connected to the charging roller core metal 21 as a charging bias (charging voltage) to the charging roller 2 as a charging negative bias, which is a DC voltage of negative polarity (same polarity as the charging polarity of the photosensitive drum 1). During image formation (charging process), a negative charging bias (negative charging bias) is applied to the charging roller 2 by the negative charging power source 81. The position on the photosensitive drum 1 where charging is performed by the charging roller 2 in the rotation direction of the photosensitive drum 1 is a charging portion (charging position) P2. The charging roller 2 charges the surface of the photosensitive drum 1 by discharge occurring in at least one of minute gaps between the photosensitive drum 1 and the charging roller 2 formed on the upstream side and downstream side of the contact portion between the photosensitive drum 1 and the charging roller 2 in the rotation direction of the photosensitive drum 1. However, for simplicity, it may be assumed that the contact portion between the photosensitive drum 1 and the charging roller 2 is the charging portion P2.

[0022] The surface of the charged photosensitive drum 1 is scanned and exposed by an exposure device (laser scanner) 3 as an exposure means, and an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 1. As described in detail below, the exposure device 3 has a laser output unit and a rotating polygon mirror, and forms an electrostatic latent image of a predetermined resolution on the surface of the photosensitive drum 1 by irradiating the surface of the photosensitive drum 1 with laser light in accordance with image information (image signal). The position on the photosensitive drum 1 in the rotation direction of the photosensitive drum 1 where light is irradiated by the exposure device 3 is an exposure unit (exposure position) P3.

[0023] The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by supplying toner as a developer by the developing device 4 as a developing means, and a toner image (toner image, developer image) is formed on the photosensitive drum 1. In this embodiment, toner charged with the same polarity as the charge polarity of the photosensitive drum 1 (negative polarity in this embodiment) adheres to the image portion on the photosensitive drum 1, which has been uniformly charged and then exposed to light to reduce the potential (reverse development method). In this embodiment, the normal charge polarity of the toner, which is the main charge polarity of the toner during development, is negative polarity. The developing device 4 has a developing container 45 that contains toner, which is a non-magnetic one-component developer as the developer, a developing roller 41 as a developer carrier (developing member), and a regulating blade 42 as a developer regulating member. Fine inorganic particles (external additives) are added to the surface of the toner, and the toner is given the desired chargeability and fluidity. The developing roller 41 has a conductive developing roller core metal 411 and a conductive rubber layer (elastic layer) 412 formed around the developing roller core metal 411. The developing device 4 is pressurized by a pressure spring, which is a biasing member serving as a biasing means, so that the developing roller 41 abuts against the photosensitive drum 1 with a predetermined pressure. In this embodiment, when the process cartridge 100 is mounted in the apparatus main body 210, the developing roller 41 is always in contact with the photosensitive drum 1.

[0024] The developing roller 41 is driven to rotate at a predetermined peripheral speed in the direction of the arrow R2 (counterclockwise direction) in the figure around the developing roller core metal 411. In this embodiment, the developing roller 41 is driven to rotate so that the peripheral speed ratio, which is the ratio between the peripheral speed of the photosensitive drum 1 and the peripheral speed of the developing roller 41, becomes a predetermined peripheral speed ratio. In this embodiment, the developing roller 41 is rotated by a driving force transmitted from a driving motor 10 common to the photosensitive drum 1. The rotation of the developing roller 41 is started and stopped in synchronization with the start and stop of the rotation of the photosensitive drum 1. A developing negative power source 83 is connected to the developing roller core metal 411 as a developing bias (developing voltage) to the developing roller 41, which is a developing negative bias that is a DC voltage of negative polarity (same polarity as the charging polarity of the photosensitive drum 1). During image formation (development), a developing bias (developing negative bias) of negative polarity is applied to the developing roller 41 by the developing negative power source 83. This developing bias causes negatively charged toner to move from the developing roller 41 to the surface of the photosensitive drum 1, thereby visualizing the electrostatic latent image and forming a toner image. In addition, a developing positive power source 82 is also connected to the developing roller core metal 411 as a developing bias application means (developing bias application unit) that applies a developing positive bias, which is a DC voltage of positive polarity (opposite polarity to the charging polarity of the photosensitive drum 1), to the developing roller 41 as a developing bias. By appropriately selecting the developing positive bias and the developing negative bias, a predetermined developing bias can be applied to the developing roller 41. In addition, the regulating blade 42 is disposed in contact with the developing roller 41. The regulating blade 42 regulates the amount of toner carried on the developing roller 41 and imparts a predetermined charge to the toner mainly by friction charging. The position on the photosensitive drum 1 where the developing roller 41 supplies toner in the rotation direction of the photosensitive drum 1 (the contact portion between the developing roller 41 and the photosensitive drum 1) is the developing unit (developing position) P4.

[0025] The toner image formed on the photosensitive drum 1 is transferred onto a recording material S, which is a transfer target material sandwiched and conveyed between the photosensitive drum 1 and the transfer roller 5, by the transfer roller 5, which is a roller-type transfer member serving as a transfer means. The transfer roller 5 has a conductive transfer roller core metal 51 and a urethane foam layer (elastic layer) 52 formed around the transfer roller core metal 51. The transfer roller 5 is in contact with the photosensitive drum 1 when the recording material S is not interposed. In this embodiment, the image forming apparatus 200 is not provided with a mechanism for separating the transfer roller 5 from the photosensitive drum 1. The transfer roller core metal 51 is connected to a transfer positive power source 84, which serves as a transfer bias application means (transfer bias application unit) that applies a transfer positive bias, which is a DC voltage of positive polarity (opposite polarity to the normal charging polarity of the toner), to the transfer roller 5 as a transfer bias (transfer voltage). During image formation (transfer), a positive transfer bias (positive transfer bias) is applied to the transfer roller 5 (transfer portion P5) by the positive transfer power source 84. This transfer bias transfers a toner image made of negative toner on the photosensitive drum 1 onto the recording material S. The transfer roller core metal 51 is also connected to the above-mentioned negative charging power source 81. By adjusting the output of the negative charging power source 81 and the output of the positive transfer power source 84, respectively, a predetermined transfer bias obtained by superimposing these outputs can be applied to the transfer roller 5. The position where the toner image on the photosensitive drum 1 is transferred to the recording material S by the transfer roller 5 in the rotation direction of the photosensitive drum 1 (the contact portion between the transfer roller 5 and the photosensitive drum 1) is the transfer portion (transfer position) P5.

[0026] A recording material (transfer material, recording medium, sheet) S such as paper or a plastic sheet is separated and sent out one by one from a cassette 11 serving as a recording material storage section by a feed roller 12 serving as a feed member, and is conveyed to a registration roller 13 serving as a conveying member. This recording material S is conveyed by the registration roller 13 to a transfer section P5 so as to be synchronized with the toner image on the photosensitive drum 1.

[0027] The recording material S onto which the toner image has been transferred is conveyed to a fixing device (fixing unit) 6 as a fixing means. The fixing device 6 applies pressure and heat to the recording material S carrying the unfixed toner image, thereby fixing (melting and bonding) the toner image onto the recording material S. The recording material S onto which the toner image has been fixed is discharged (output) to the outside of the apparatus main body 210.

[0028] Among the toner of the toner image formed on the photosensitive drum 1, a part of the toner (transfer residual toner) remaining on the photosensitive drum 1 without being transferred to the recording material S is transported to the charging section P2 with the rotation of the photosensitive drum 1. This transfer residual toner is negatively charged by a discharge (also referred to as "upstream discharge" here) that occurs between the surface of the photosensitive drum 1 on the upstream side of the contact portion between the photosensitive drum 1 and the charging roller 2 in the rotation direction of the photosensitive drum 1 and the surface of the charging roller 2. Therefore, an electrostatic force that moves the transfer residual toner from the charging roller 2 side to the photosensitive drum 1 side is applied to the transfer residual toner by an electric field formed by the surface potential of the photosensitive drum 1 and the charging bias. As a result, the transfer residual toner passes through the charging section P2 while adhering to the surface of the photosensitive drum 1 and is transported to the developing section P4. At that time, the transfer residual toner adhering to the non-image portion of the surface of the photosensitive drum 1 that is not exposed at the exposure section P3 moves to the surface of the developing roller 41 by an electric field formed by the dark area potential of the surface of the photosensitive drum 1 and the developing bias. Then, the toner is contained in the developer container 45 as the developing roller 41 rotates. In this manner, in this embodiment, the transfer residual toner is collected by the developing device 4. On the other hand, the transfer residual toner adhering to the image portion exposed at the exposure portion P3 on the surface of the photosensitive drum 1 is transported again to the transfer portion P5 as a toner image and can be transferred onto the recording material S.

[0029] Then, after a post-process (post-rotation process) described later, the driving of the driving motor 10 is stopped, and the image forming operation is completed.

[0030] Various sensors are provided in the image forming apparatus 200. A registration sensor 91 as a recording material detection unit for detecting the recording material S is located on the conveying path 14 of the recording material S upstream of the transfer portion P5 (and downstream of the registration rollers 13) in the conveying direction of the recording material S, and detects whether the recording material S is on the conveying path 14 or not. A signal (information) indicating the detection result by the registration sensor 91 is sent to a CPU 110, which will be described later. This enables the CPU 110 to determine the timing when the leading and trailing ends in the conveying direction of the recording material S reach the transfer portion P5, and to reflect this in the control of various biases, etc.

[0031] The image forming apparatus 200 is also provided with a CPU 110 as a control unit. A memory (storage medium) 120 as a storage means provided in the image forming apparatus 200 is connected to the CPU 110. The CPU 110 exchanges various electrical information with a host device (external device), and generally controls the operation of the image forming apparatus 200, such as an image forming operation (print job), in accordance with a predetermined control program and reference table stored in the memory 120. For example, the CPU 110 controls the output strength and change timing of various power sources, the exposure device 3, and the drive motor 10. The image forming apparatus 200 forms an image on a sheet-shaped recording material S based on print information including an electrical image signal input from the host device to the CPU 110. Examples of the host device include an image reader (original image reading device), a personal computer, a facsimile, and a smartphone.

[0032] Here, the image forming operation (print job) is a series of operations that are started by one start instruction and form and output an image on one or more recording materials S. The image forming operation generally includes an image forming step, a pre-step (pre-rotation step), a paper-interval step when forming images on multiple recording materials S, and a post-step (post-rotation step). The image forming step (image forming process) is a period in which an electrostatic latent image of an image that is actually formed on the recording material S and output is formed, a toner image is formed, the toner image is transferred, and fixed, and the image forming time refers to this period. More specifically, the timing of the image forming time differs depending on the position where each of the charging, exposing, developing, transferring, and fixing steps is performed. The pre-step is a period in which a preparatory operation is performed before the image forming step from when a start instruction is input until the image actually starts to be formed. The paper-interval step is a period corresponding to the interval between recording materials S when image formation is performed continuously on multiple recording materials S (continuous image formation). The post-rotation step is a period in which an arrangement operation (preparatory operation) is performed after the image forming step. Non-image formation refers to periods other than image formation, and includes the above-mentioned pre-process, inter-paper process, post-process, and also the pre-multiple rotation process, which is a preparatory operation when the image forming apparatus 200 is turned on or when it returns from a sleep state.

[0033] 2.Post-process Next, the post-process after the image formation process in the image forming apparatus 200 of this embodiment will be described with reference to Fig. 2. Fig. 2 is a timing chart for explaining the control of the operation of the post-process in this embodiment. Fig. 2 shows the ON / OFF of the registration sensor 91, the ON / OFF of the drive motor 10, the output of various biases (charging bias, developing bias, transfer bias), and the transition of the exposure intensity of the exposure device 3. Also, in Fig. 2, the control of the post-process is shown consecutively from the control of the image formation process (during image formation). The operation according to the timing chart shown in Fig. 2 is controlled by the CPU 110.

[0034] In particular, the "image forming process (at the time of image formation)" in the control described with reference to FIG. 2 refers to a process in which a toner image is formed on the surface of the photosensitive drum 1 and transferred to the recording material S. Therefore, even after the control of the charging bias and the like is shifted to the post-process after the image forming process in the control described with reference to FIG. 2, the fixing process by the fixing device 6 is performed in parallel. The dashed arrow in FIG. 2 indicates that a portion of the surface of the photosensitive drum 1 that was located at a predetermined phase at a certain timing has moved to the phase at the tip of the arrow with the passage of time. In addition, the numerical values ​​of various biases and the like in this embodiment are merely examples, and are not limited to these values, and are appropriately set as necessary. In addition, as described above, for convenience, the magnitude (high / low) of the voltage or potential (or potential difference) refers to the magnitude (high / low) when compared in absolute value, unless otherwise specified.

[0035] In this embodiment, the image forming apparatus 200 has a development separation-less configuration, and the developing roller 41 and the photosensitive drum 1 are substantially always in contact with each other in the image forming apparatus 200 as described above. In this embodiment, when the image forming apparatus 200 starts rotating the photosensitive drum 1, a developing bias having a polarity opposite to that during image formation (opposite to the normal charging polarity of the toner) is applied to the developing roller 41. After or approximately simultaneously with the start of application of the developing bias to the developing roller 41, the rotation of the photosensitive drum 1 and application of the charging bias to the charging roller 2 are started. Thereafter, in accordance with the timing when the surface of the photosensitive drum 1 that has been charged reaches the developing portion P4, the developing bias applied to the developing roller 41 is switched to a developing bias having a polarity during image formation (the same polarity as the normal charging polarity of the toner). This can reduce fogging at the time of startup. That is, in this embodiment, in the preparation operation for starting the image forming apparatus 200 from a stopped state, the photosensitive drum 1 starts rotating after or approximately at the same time as the application of a developing bias of a polarity opposite to that during image formation (opposite to the normal charging polarity of the toner) to the developing roller 41 is started. Therefore, in this embodiment, as described with reference to FIG. 2, the image forming apparatus 200 executes a charge removing operation for removing charge from the surface of the photosensitive drum 1 when the image forming operation is ended (when the image forming apparatus 200 is brought into a stopped state). As a result, a large change does not occur in the surface potential of the photosensitive drum 1 at the time of starting due to the difference in the elapsed time from the previous image forming operation, and an appropriate developing bias can be set at the time of starting, and fogging at the time of starting can be suppressed. Here, the charge removing of the surface of the photosensitive drum 1 is not limited to setting the surface potential of the photosensitive drum 1 to 0 V, but includes removing at least a part of the charge on the surface of the photosensitive drum 1.

[0036] First, the control during image formation will be described.

[0037] During image formation, a negative charging bias Vc1 is applied to the charging roller 2. Specifically, in this embodiment, the charging bias Vc1 is -1100V. As a result, in this embodiment, the surface potential (dark area potential) VD of the non-image area of ​​the photosensitive drum 1 becomes -550V. That is, in this embodiment, the discharge start voltage (absolute value) between the photosensitive drum 1 and the charging roller 2 is about 550V.

[0038] During image formation, the exposure device 3 can irradiate the photosensitive drum 1 with a laser beam having an exposure intensity E1. Specifically, in this embodiment, the exposure intensity E1 is 0.25 μJ / cm 2 As a result, the surface potential (light area potential) VL1 of the image area of ​​the photosensitive drum 1 becomes -100V. Note that in Fig. 2, the exposure intensity during image formation is shown to be in a state in which the photosensitive drum 1 can be irradiated with laser light, including marginal areas, at a set exposure intensity based on an image signal, etc. Meanwhile, in Fig. 2, the exposure intensity in the later process, which will be described later, is shown to be exposed over the entire area of ​​an exposure width Le, which will be described later, at a set exposure intensity.

[0039] Also, during image formation, a negative developing bias Vd1 is applied to the developing roller 41. Specifically, in this embodiment, the developing bias Vd1 is -400V. As a result, a potential difference of -300V (=Vd1-VL1) (also referred to as "development contrast" here) is formed between the light area potential VL1 on the surface of the photosensitive drum 1 and the developing bias Vd1, and the negative toner moves from the developing roller 41 to the image area on the surface of the photosensitive drum 1 and adheres thereto. On the other hand, a potential difference of 150V (=Vd1-VD) (also referred to as "back contrast" here) is formed between the dark area potential VD on the surface of the photosensitive drum 1 and the developing bias Vd1, and the negative toner is prevented from moving to the non-image area on the surface of the photosensitive drum 1 (fog) by electrical repulsion. Also, due to this back contrast, the transfer residual toner is collected by the developing roller 41 as described above. It is desirable to form an appropriate back contrast mainly in consideration of the balance between the suppression of this fogging and the collection of toner.

[0040] At this time, a part of the external additive adhering to the surface of the toner also moves from the developing roller 41 to the surface of the photosensitive drum 1. In addition to cases where the external additive moves along with the toner that has moved to the surface of the photosensitive drum 1 as a toner image and fog, there are also cases where the external additive that has been detached from the toner moves independently to the surface of the photosensitive drum 1. As a result, a certain amount of external additive is left adhering to the surface of the photosensitive drum 1.

[0041] Also, during image formation, a positive transfer bias Vt1 is applied to the transfer roller 5. Specifically, in this embodiment, the transfer bias Vt1 is feedback-controlled (constant current controlled) as needed so that a predetermined current (a constant current of 13 μA in this embodiment) flows when the recording material S is in the transfer portion P5. As a result, a toner image made of a negative toner formed on the surface of the photosensitive drum 1 is transferred to the recording material S. Also, by applying a positive transfer bias to the transfer roller 5, the surface potential (also referred to as the "post-transfer potential" here) Vpre of the photosensitive drum 1 after passing through the transfer portion P5 changes to a more positive polarity side than before passing through the transfer portion P5. Specifically, in this embodiment, the surface potential of the portion that was the dark potential VD of -550V changes to a post-transfer potential Vpre of -450V. Note that the photosensitive drum 1 rotates at a predetermined rotation speed, and the post-transfer potential Vpre of -450V formed when passing through the transfer portion P5 is maintained without being significantly attenuated even immediately before reaching the charging portion P2. Here, the surface potential of the photosensitive drum 1 immediately before the charging portion P2 (also referred to as the "pre-charge potential" here) is also expressed as "Vpre", which is the same as the post-transfer potential. Therefore, during image formation, the potential difference (=Vc1-Vpre) (also referred to as the "charge contrast" here) between the pre-charge potential Vpre, which is the surface potential of the photosensitive drum 1 immediately before the charging portion P2, and the charging bias Vc1 is -650V.

[0042] The larger this charge contrast is, the larger the upstream discharge from the charging roller 2 to the surface of the photosensitive drum 1 at the charging section P2 is. As a result, the toner and external additives remaining on the surface of the photosensitive drum 1 become more negatively charged, and the repulsive force with the negative charging bias applied to the charging roller 2 suppresses the toner and external additives on the photosensitive drum 1 from adhering to the surface of the charging roller 2.

[0043] Next, the control of the post-process will be described.

[0044] When the trailing edge of the recording material S in the transport direction passes through the transfer portion P5, the process after the position on the photosensitive drum 1 that passes through the transfer portion P5 passes through each portion (charging portion P2, exposure portion P3, development portion P4) will be described as the post-process. The transition from the image formation process to the post-process is performed in consideration of the phase difference of each portion (charging portion P2, exposure portion P3, development portion P4, transfer portion P5) in the rotation direction of the photosensitive drum 1, and as shown by the dashed arrow in Figure 2, first the charging bias, then the exposure, then the development bias, and then the transfer bias are applied in this order.

[0045] During image formation, when the recording material S is at the position of the conveying path 14 where the registration sensor 91 is provided, the registration sensor 91 sends an ON signal to the CPU 110. Thereafter, as the image formation process progresses, at a timing T0 when the trailing end of the recording material S in the conveying direction passes the position of the conveying path 14 where the registration sensor 91 is provided, the signal sent by the registration sensor 91 to the CPU 110 changes from ON to OFF. If subsequent print information has not been input to the CPU 110 and the subsequent recording material S is not conveyed, this change in the signal of the registration sensor 91 triggers the CPU 110 to proceed with the transition from the image formation process to the subsequent process.

[0046] It takes time t95 (also referred to as "inter-registration transfer time") for the recording material S to move from the detection position of the registration sensor 91 to the transfer portion P5 in the conveying path 14. That is, it takes the inter-registration transfer time t95 from the timing T0 when the registration sensor 91 detects the trailing end of the recording material S in the conveying direction to the timing T3 when the trailing end of the recording material S in the conveying direction reaches the transfer portion P5. Specifically, in this embodiment, the inter-registration transfer time t95 is 300 msec. On the other hand, it takes time t25 (also referred to as "inter-charge transfer time") for the surface of the photosensitive drum 1 to move from the charging portion P2 to the transfer portion P5 as the photosensitive drum 1 rotates. Specifically, in this embodiment, the inter-charge transfer time t25 is 150 msec. Based on these, the charging bias is first transferred to the post-process at the timing T1 that is earlier than the timing T3 when the trailing end of the recording material S in the conveying direction reaches the transfer portion P5 by the inter-charge transfer time t25. However, in this embodiment, the charging bias is kept constant even in the post-processing step from the charging bias Vc1 during image formation.

[0047] Next, exposure by the exposure device 3 moves to the post-process. It takes time t23 (herein also referred to as "inter-charge-exposure time") for the surface of the photosensitive drum 1 to move from the charging section P2 to the exposure section P3. Specifically, in this embodiment, the inter-charge-exposure time t23 is 20 msec. Therefore, exposure by the exposure device 3 moves to the post-process at timing T2, which is the inter-charge-exposure time t23 after timing T1 when the charging bias moves to the post-process. At this time, irradiation of laser light by the exposure device 3 onto the photosensitive drum 1 is turned off.

[0048] Next, the developing bias is transferred to the post-processing. However, in this embodiment, the developing bias is not changed from the developing bias Vd1 during image formation and is kept constant in the post-processing, similarly to the charging bias.

[0049] Thereafter, at timing T3 when the trailing edge of the recording material S in the transport direction passes through the transfer portion P5, the transfer bias is shifted to the post-process. At this time, the transfer bias is changed from transfer bias Vt1 to transfer bias Vt2. Specifically, in this embodiment, the voltage value at which a predetermined current (a constant current of 13 μA in this embodiment) flows when the recording material S is not at the transfer portion P5 is obtained in the pre-process before the image formation process. From timing T3, application of transfer bias Vr2 (constant voltage control) is started so that the constant voltage becomes the value obtained in the pre-process. At this time, the post-transfer potential Vpre, i.e., the pre-charging potential Vpre, is −450 V, the same as during image formation.

[0050] Next, the post-process moves to a charge removal operation (charge removal process). As shown in FIG. 2, at timing T4, the exposure device 3 starts irradiating the entire area of ​​the exposure width Le described later with a laser beam of exposure intensity E3. This starts the formation of a surface of the photosensitive drum 1 that is charge removed by exposure (also referred to as an "exposed surface" here). This causes the surface potential of the photosensitive drum 1 to become a light area potential VL3 over the entire area of ​​the exposure width Le. At this time, it is desirable to set the exposure intensity to a value as large as possible in order to make the surface potential of the photosensitive drum 1 as small as possible. During image formation, the width and height of the toner image formed on the photosensitive drum 1 by the development unit P4 vary greatly depending on the magnitude of the development contrast. Therefore, the exposure intensity E1 during image formation is controlled within the light amount settable range of the exposure device 3 based on a preset control table or the like according to, for example, the usage history of the process cartridge 100 and the detection results of temperature and humidity. It is desirable to set the exposure intensity E3 in the charge removal operation to a value larger than the exposure intensity E1 during image formation. Specifically, in this embodiment, the exposure intensity E1 during image formation is 0.25 μJ / cm 2 The exposure intensity E3 during static elimination is 0.45 μJ / cm 2As a result, the bright area potential VL1 during image formation is set to -100 V, whereas the bright area potential (surface potential of the exposed surface) VL3 during the static elimination operation is set to -50 V. In this manner, in this embodiment, the absolute value of the bright area potential VL1 during image formation is smaller than the absolute value of the bright area potential VL3 during the static elimination operation.

[0051] Here, the timing T4 at which the formation of the exposure surface of the photosensitive drum 1 starts in the post-process can be any timing as long as it is after the last image forming area (area where a toner image can be formed according to image information) in the image forming operation has passed the exposure section P3. However, it is desirable to set this timing T4 in consideration of various tolerances, such as the variation in the position of each section (charging section P2, exposure section P3, developing section P4, transfer section P5) in the rotation direction of the photosensitive drum 1, high voltage delay, detection error of the registration sensor 91, signal transmission speed and processing speed, etc. Specifically, in this embodiment, the timing T4 at which the formation of the exposure surface of the photosensitive drum 1 starts is 320 msec after the timing T0 at which the registration sensor 91 turns OFF.

[0052] It is necessary to change the developing bias to an appropriate value so as to suppress the adhesion of toner to the photosensitive drum 1 due to the formation of the exposed surface of the photosensitive drum 1. In other words, it is necessary to set the developing bias to an appropriate value so as to prevent the formation of a development contrast that causes unintended development with toner, or the increase in back contrast that makes it easier for fogging (fogging of toner with a polarity opposite to the normal charging polarity) to occur. It takes time t34 (here, also referred to as the "exposure-development time") for the surface of the photosensitive drum 1 to move from the exposure section P3 to the development section P4. Specifically, in this embodiment, the exposure-development time t34 is 50 msec. Therefore, at timing T5, which is the exposure-development time t34 after the timing T4 when the formation of the exposed surface of the photosensitive drum 1 is started, the developing bias is switched from the developing bias Vd1 to the developing bias Vd3. It is desirable to set the developing bias Vd3 so as to form an appropriate back contrast with respect to the light area potential VL3 of the exposed surface of the photosensitive drum 1. That is, the developing bias Vd3 is set to a value on the positive polarity side with respect to the light area potential VL3 of the exposed surface of the photosensitive drum 1 so that the negative polarity toner does not move to the photosensitive drum 1. Specifically, in this embodiment, the power source that outputs the developing bias is switched from the developing negative power source 83 to the developing positive power source 82, and the developing bias is switched to a developing bias Vd3 of +100 V. As a result, a back contrast of 150 V (= Vd3 - VL3) is formed, the same as during image formation, with respect to the light area potential VL3 of -50 V of the exposed surface of the photosensitive drum 1.

[0053] Next, the transfer bias is changed to an appropriate value for the exposure surface of the photosensitive drum 1. It takes time t35 (herein also referred to as "exposure-transfer time") for the surface of the photosensitive drum 1 to move from the exposure section P3 to the transfer section P5. Specifically, in this embodiment, the exposure-transfer time t35 is 130 msec. Therefore, at timing T6, which is the exposure-transfer time t35 after timing T4 when formation of the exposure surface of the photosensitive drum 1 is started, the transfer bias is switched from the transfer bias Vt2 to the transfer bias Vt3. Specifically, in this embodiment, the transfer bias is switched to the transfer bias Vt3 of -500V. The set value of the transfer bias Vt3 will be explained in more detail later.

[0054] It takes time t52 (herein also referred to as "transfer-charging time") for the surface of the photosensitive drum 1 to move from the transfer portion P5 to the charging portion P2. Specifically, in this embodiment, the transfer-charging time t52 is 140 msec. The surface of the photosensitive drum 1, which was at the transfer portion P5 when the transfer bias was switched to the transfer bias Vt3 at timing T6, reaches the charging portion P2 at timing T7 when the transfer-charging time t52 has elapsed. At this time, the exposed surface of the photosensitive drum 1 is recharged by the charging bias Vc1 applied to the charging roller 2. Accordingly, as described later, the external additives attached to the surface of the photosensitive drum 1 are prevented from adhering to the surface of the charging roller 2, and the contamination of the charging roller 2 is prevented.

[0055] It is desirable that the charge removal of the photosensitive drum 1 by exposure (formation of the exposed surface of the photosensitive drum 1) is performed for the entire circumference of the photosensitive drum 1. In other words, it is desirable that the period of the charge removal operation in which the photosensitive drum 1 is exposed to the exposure intensity E3 to remove the charge from the photosensitive drum 1 is longer than the time for the photosensitive drum 1 to rotate once. Although not limited to this, the period of the charge removal operation in which the photosensitive drum 1 is exposed to the exposure intensity E3 to remove the charge from the photosensitive drum 1 is often sufficient if it is shorter than the time for the photosensitive drum 1 to rotate five times, and is typically shorter than the time for the photosensitive drum 1 to rotate three times. In this embodiment, exposure of the photosensitive drum 1 at the exposure intensity E3 is performed for the time for the photosensitive drum 1 to rotate two times, specifically for 580 msec, and exposure of the photosensitive drum 1 by the exposure device 3 is turned OFF at timing T9. At timing T9, the drive motor 10 is also turned OFF. In other words, the drive of the photosensitive drum 1 by the drive motor 10 is also turned OFF at the same time as the exposure of the photosensitive drum 1 by the exposure device 3.

[0056] Also, at the charging / exposure time t23, specifically, at the timing T8, which is earlier than the timing T9 by 20 msec, the charging bias is turned OFF. Until the surface of the photosensitive drum 1 exposed at the exposure section P3 reaches the charging section P2, the surface potential of the photosensitive drum 1 is the light area potential VL3. On the other hand, after being recharged at the charging section P2, the surface potential of the photosensitive drum 1 is the dark area potential VD until it reaches the exposure section P3. If the charging bias is turned OFF at the same time as the exposure of the photosensitive drum 1 and the drive of the photosensitive drum 1 are turned OFF at the timing T9, the image forming operation will be completed in a state where the surface potential of the photosensitive drum 1 between the charging section P2 and the exposure section P3 is not discharged and is the dark area potential VD. If the photosensitive drum 1 is restarted in a state where the surface potential of the photosensitive drum 1 does not darkly decay, the surface potential of the photosensitive drum 1 will be large in a part of the area in the rotation direction of the photosensitive drum 1, and fogging at the time of starting up may easily occur. Therefore, it is preferable to turn OFF the charging bias before the timing T9 at which the exposure of the photosensitive drum 1 and the drive of the photosensitive drum 1 are turned OFF.

[0057] In addition, at timing T8 when the charging bias is turned OFF, the transfer bias is also turned OFF. This is because, as described above, in this embodiment, the output value of the transfer bias is controlled by superimposing the output of the charging negative power source 81 and the output of the transfer positive power source 84. If the image forming apparatus 200 is configured to have an independent power source that applies the transfer bias to the transfer roller 5, for example, the transfer bias may be turned OFF at the same time as timing T9 when the exposure of the photosensitive drum 1 and the drive of the photosensitive drum 1 are turned OFF.

[0058] The developing bias is turned off when a predetermined time has elapsed, specifically, when 50 msec has elapsed, from the timing T9 when the driving of the photosensitive drum 1 is turned off. This is because even if the CPU 110 issues a signal to stop the driving motor 10, the photosensitive drum 1 does not stop completely immediately thereafter, but rotates idly for a predetermined distance, and thus fogging during that time is suppressed.

[0059] In parallel with the above-described post-process operations, the fixing process and the process of discharging the recording material S from the apparatus main body 210 are completed, whereby the image forming operation is completed.

[0060] 3. Post-process transfer bias Next, the transfer bias Vt3 applied to the transfer roller 5 when the exposed surface of the photosensitive drum 1 passes through the transfer portion P5 in the charge removing operation in the post-process (hereinafter, simply referred to as "transfer bias Vt3 in the charge removing operation") will be further described.

[0061] As described in detail below, the transfer bias Vt3 in the charge removal operation is set so that the absolute value of the potential difference (=Vt3-VL3) between the surface potential VL3 of the exposed surface of the photosensitive drum 1 and the transfer bias Vt3 is less than the discharge start voltage between the photosensitive drum 1 and the transfer roller 5. This prevents the charging roller 2 from being soiled due to the adhesion of external additives to the charging roller 2. Furthermore, it is desirable to set this transfer bias Vt3 so that an electric field acts to move toner charged to the negative polarity, which is the normal charging polarity of the toner, from the transfer roller 5 side to the photosensitive drum 1 side. This prevents the transfer roller 5 from being soiled due to the adhesion of toner to the transfer roller 5.

[0062] [Table 1]

[0063] Table 1 shows the results of evaluating the degree of dirt on the charging roller 2 and the transfer roller 5 in four stages when the transfer bias Vt3 in the static electricity removal operation is changed. The dirt on the charging roller 2 and the transfer roller 5 were evaluated by forming 1000 images on one sheet at a time. The one sheet at a time means that after forming an image on one sheet of recording material S, the image forming apparatus 200 is stopped once, and an image is formed on one sheet of recording material S again, and the operation is repeated. Note that here, the dirt on the charging roller 2 and the transfer roller 5 were evaluated without performing a cleaning operation that may be performed appropriately in an electrophotographic image forming apparatus. The cleaning operation is an operation different from the normal image forming process and the post-process, and is an operation for cleaning various members by providing an additional time when various parameters such as the travel distance (surface movement distance) of the photosensitive drum 1 exceed a threshold value.

[0064] As described above, in the charge removal operation in this embodiment, the surface potential of the exposed surface of the photosensitive drum 1 is -50V. Also, in the charge removal operation in this embodiment, the charging bias Vc1 when recharging the exposed surface of the photosensitive drum 1 is -1100V. Table 1 shows the potential difference (=Vt3-VL3) between the surface potential VL3 of the exposed surface of the photosensitive drum 1 and the transfer bias Vt3 when the transfer bias Vt3 is changed. Also, Table 1 shows the post-transfer potential Vpre when each transfer bias Vt3 is applied. As described above, the post-transfer potential Vpre and the pre-charge potential Vpre do not change significantly. Therefore, the charge contrast (=Vc1-Vpre), which is the potential difference between the pre-charge potential Vpre and the charging bias Vc1, has the value shown in Table 1.

[0065] The dirt on the charging roller 2 was dirt caused by external additives attached to the surface of the charging roller 2, and was evaluated according to the following criteria. "◎" indicates a state with no problems at all. "X" indicates a state in which a large amount of external additives is attached to the surface of the charging roller 2, making it difficult to charge the surface of the photosensitive drum 1, making fogging more likely to occur, and uneven adhesion of the external additives causing uneven surface potential of the photosensitive drum 1, resulting in uneven density. "△" indicates a state that is less severe than "X", but does not reach a practical level. "◯" indicates a state in which slight unevenness in density occurs, but is sufficiently sufficient for practical use. In other words, it is desirable for the dirt on the charging roller 2 to be "◎" or "◯".

[0066] The stains on the transfer roller 5 were caused by toner accumulated on the surface of the transfer roller 5, and were evaluated according to the following criteria: "◎" indicates a state where there is no problem at all. "X" indicates a state where the toner accumulated on the surface of the transfer roller 5 adheres to the back surface of the recording material S, causing stains (back stains). "△" indicates a state where the condition is less severe than "X", but does not reach a practical level. "◯" indicates a state where a small amount of toner accumulates on the surface of the transfer roller 5, but does not adhere to the back surface of the recording material S, and there is no problem in practical use. In other words, it is desirable for the stains on the transfer roller 5 to be "◎" or "◯".

[0067] As described above, the level of dirt on the charging roller 2 and the transfer roller 5 can be improved by performing the cleaning operation appropriately. In particular, the dirt on the transfer roller 5 to which the toner adheres is more likely to be improved by the cleaning operation than the dirt on the charging roller 2 to which the external additive adheres. This is because the external additive is smaller in size than the toner and has an irregular shape compared to the toner, which is nearly spherical, so that the external additive tends to adhere more strongly to the surface of the charging roller 2. However, as described above, the cleaning operation is generally performed in addition to the normal operation, and downtime (a period during which an image cannot be formed) occurs. In addition, depending on the frequency of the cleaning operation, there are concerns that, for example, the increase in the rotation distance of the photosensitive drum 1 will promote the scraping of the surface layer (resin film) of the photosensitive drum 1, and the increase in the rotation distance of the developing roller 41 will promote the deterioration of the toner. Therefore, it is desirable to be able to suppress the dirt on the charging roller 2 without performing the cleaning operation. It would be even better if the dirt on the transfer roller 5 could also be suppressed without performing the cleaning operation.

[0068] As shown in Table 1, if the transfer bias Vt3 is made too large on the positive side or too large on the negative side, the charge roller 2 becomes more likely to become dirty.

[0069] When the transfer bias Vt3 is increased to the negative side, the post-transfer potential Vpre increases to the negative side due to the influence of the transfer bias Vt3. This is due to the following reasons. First, charges are injected from the transfer roller 5 to the surface of the photosensitive drum 1. In addition, when the absolute value of the potential difference (=Vt3-VL3) between the surface potential VL3 of the exposed surface of the photosensitive drum 1 and the transfer bias Vt3 becomes equal to or greater than the discharge start voltage (550V or more in this embodiment), a negative discharge occurs from the transfer roller 5 to the surface of the photosensitive drum 1. Therefore, when the potential difference (=Vt3-VL3) between the transfer bias Vt3 and the surface potential VL3 of the exposed surface of the photosensitive drum 1 becomes equal to or greater than the discharge threshold start voltage on the negative side, the post-transfer potential Vpre increases to the negative side, and the charge contrast decreases. As described above, by increasing the charge contrast, it becomes easier to apply an electrostatic repulsive force to the external additives, which are mainly negatively charged and adhered to the surface of the photosensitive drum 1, so as not to adhere to the surface of the charging roller 2. Moreover, by increasing the charging contrast, the external additives attached to the surface of the photosensitive drum 1 are more likely to be charged to a negative polarity by the upstream discharge of the charging section P2. In other words, conversely, when the charging contrast is reduced, the charging roller 2 tends to become more easily soiled.

[0070] On the other hand, when the transfer bias Vt3 is increased to the positive side, a positive discharge occurs from the transfer roller 5 to the surface of the photosensitive drum 1, in contrast to when the transfer bias Vt3 is increased to the negative side. This discharge causes the external additives attached to the surface of the photosensitive drum 1 to be positively charged. Therefore, due to the potential difference between the surface potential of the photosensitive drum 1 and the charging bias, the positive external additives tend to adhere to the surface of the charging roller 2. Therefore, as shown in Table 1, when the transfer bias Vt3 is increased to the positive side, the charging roller 2 tends to become dirty.

[0071] In this way, in order to suppress contamination of the charging roller 2, the transfer bias Vt3 in the discharge operation is set so that the absolute value of the potential difference (=Vt3-VL3) between the surface potential VL3 of the exposed surface of the photosensitive drum 1 and the transfer bias Vt3 is less than the discharge start voltage.

[0072] Also, as shown in Table 1, the dirt on the transfer roller 5 is more easily suppressed as the transfer bias Vt3 is made larger on the negative side. Normally, during image formation, a large transfer bias on the positive side with respect to the surface potential of the photosensitive drum 1 is applied to the transfer roller 5 in order to transfer the toner charged to the negative polarity, which is the normal charging polarity of the toner, from the surface of the photosensitive drum 1 to the recording material S. Also, in the preparatory operation before the recording material S is transported to the transfer portion P5, an electric field of the above polarity is generally applied by the transfer bias.

[0073] Here, most of the toner (herein, also referred to as "fog toner") that moves to the surface of the photosensitive drum 1 as fog due to the electric field caused by the back contrast is positively charged, which is the opposite polarity to the normal charging polarity of the toner. However, some of the fog toner is negatively charged, and the surface of the photosensitive drum 1 is in a state where negatively charged toner and positively charged toner are mixed. The positively charged toner among the fog toner does not move to the transfer roller 5 due to the electric field formed between the transfer roller 5 and the photosensitive drum 1 by applying a positive transfer bias to the transfer roller 5 as described above, and tends to remain on the surface of the photosensitive drum 1. On the other hand, the negatively charged toner among the fog toner tends to adhere to the transfer roller 5 when there is no recording material S at the transfer portion P5. Therefore, it is desirable to set the transfer bias Vt3 in the charge removal operation so as to generate an electric field that moves the negatively charged toner, which is the normal charging polarity of the toner, from the transfer roller 5 to the photosensitive drum 1. This allows the negatively charged toner adhering to the transfer roller 5 to be moved back to the surface of the photosensitive drum 1. Therefore, as shown in Table 1, the more negative the transfer bias Vt3, the easier it is to suppress contamination of the transfer roller 5.

[0074] In this way, the transfer bias Vt3 in the charge removal operation is set so that the absolute value of the potential difference (=Vt3-VL3) between the surface potential VL3 of the exposed surface of the photosensitive drum 1 and the transfer bias Vt3 is less than the discharge start voltage in order to suppress contamination of the charging roller 2. Also, it is desirable to set the transfer bias Vt3 in the charge removal operation so that the toner charged to the normal charging polarity of the toner moves from the transfer roller 5 side to the photosensitive drum 1 side in order to suppress contamination of the transfer roller 5.

[0075] Therefore, in this embodiment, in order to simultaneously suppress the contamination of the charging roller 2 and the transfer roller 5, the transfer bias Vt3 in the charge removing operation is set to -500V.

[0076] However, the present invention is not limited to the settings of the present embodiment, and may be set to, for example, place emphasis on suppressing the contamination of the charging roller 2 rather than on the contamination of the transfer roller 5. As described above, the accumulation of toner, which is the main cause of the contamination of the transfer roller 5, is easier to suppress by performing an appropriate cleaning operation than that of external additives, which are the main cause of the contamination of the charging roller 2. Therefore, in the charge removal operation, a setting that places emphasis on suppressing the contamination of the charging roller 2, that is, the transfer bias Vt3 may be set to a value on the positive side with an absolute value less than the discharge start voltage. In other words, from the viewpoint of suppressing the contamination of the charging roller 2, the transfer bias Vt3 in the charge removal operation may be any of a positive polarity value, a negative polarity value, and 0V, as long as it is a value less than the discharge start voltage.

[0077] When 0V is selected as the transfer bias Vt3 in the charge removal operation, it is preferable to control the transfer bias application means to output 0V (constant voltage control with 0V as the target voltage) in the CPU 110, rather than turning off the output of the transfer bias application means. As shown in FIG. 1, the transfer roller 5 is in contact with the photosensitive drum 1, and even if the transfer bias is turned off, an unspecified current may flow between the charge remaining on the transfer roller 5 and the surface potential of the photosensitive drum 1. In addition, the image forming apparatus 200 is provided with various bias application means and many other circuits. Therefore, even if the transfer bias application means is turned off, an unspecified current may flow between the transfer roller 5 and the photosensitive drum 1 due to the influence of another bias application means, a residual charge of a capacitor, etc. For this reason, even if 0V is selected as the transfer bias Vt3, it is preferable to intentionally output the transfer bias as 0V. For example, in a configuration in which a transfer positive power source 84 and a charging negative power source 81 are connected to the transfer roller core 51 as shown in FIG. 1, the biases applied to the transfer roller 5 by the charging negative power source 81 and the transfer positive power source 84 can be set to have the same absolute value but different polarities, thereby applying a transfer bias of 0 V to the transfer roller 5.

[0078] 4. Exposure width Next, the exposure width of the photosensitive drum 1 when forming the exposure surface of the photosensitive drum 1 in the charge removal operation in the post-process will be further described.

[0079] When forming the exposed surface of the photosensitive drum 1 in the charge removal operation in the post-process, in order to suppress fogging at the end of the rotation axis direction of the photosensitive drum 1, it is desirable to set the exposure width of the photosensitive drum 1 as follows: In other words, it is desirable to expose an area wider than the width of the area carrying the toner on the surface of the developing roller 41 in the rotation axis direction of the photosensitive drum 1 (rotation axis direction of the developing roller 41) (here, also referred to as the "developing opening width").

[0080] Fig. 3 is a schematic diagram for explaining the positional relationship among the developing device 4, the photosensitive drum 1, and the exposure device 3 in the direction of the rotation axis of the photosensitive drum 1. The developing device 4 in Fig. 3 shows the positional relationship when the developing roller 41 is seen from the photosensitive drum 1 side along a direction substantially perpendicular to the direction of the rotation axis of the photosensitive drum 1. Also, the photosensitive drum 1 in Fig. 3 shows the positional relationship when the photosensitive drum 1 is seen from the developing roller 41 side along a direction substantially perpendicular to the direction of the rotation axis of the photosensitive drum 1.

[0081] The developing device 4 has a developing roller 41. The developing roller 41 carries the toner stored in the developing device 4 on its surface. In addition, the developing device 4 is provided with a plurality of sealing members so that the toner stored in the developing device 4 does not leak out of the developing device 4. In the rotation direction R2 of the developing roller 41, a regulating blade 42 is provided in contact with the developing roller 41 on the upstream side of the portion where the photosensitive drum 1 and the developing roller 41 contact each other. In addition, in the rotation direction R2 of the developing roller 41, a sealing sheet 44 is provided in contact with the developing roller 41 on the downstream side of the portion where the photosensitive drum 1 and the developing roller 41 contact each other. This suppresses leakage of toner from the upstream side and downstream side of the portion where the photosensitive drum 1 and the developing roller 41 contact each other in the rotation direction R2 of the developing roller 41. In addition, development end sealing members 43 are provided in contact with the developing roller 41 at both ends of the developing roller 41 in the rotation axis direction. This prevents the toner from leaking out from the end of the developing roller 41 in the rotation axis direction via the surface of the developing roller 41. Therefore, the toner is carried on the surface of the developing roller 41 in the portion between the two developing end sealing members 43, i.e., in the developing opening width Ld shown in FIG. 3. Therefore, the toner can move on the surface of the photosensitive drum 1 in the developing opening width Ld portion. The developing opening width Ld is generally set to be somewhat wider than the width of the image forming area (area where a toner image can be formed according to image information) in the rotation axis direction of the photosensitive drum 1.

[0082] On the other hand, the exposure device 3 has a laser output unit 31 and a polygon mirror 32. The laser light emitted from the laser output unit 31 is reflected by the rotating polygon mirror 32 and is corrected and polarized via various lenses and mirrors (not shown). In this way, the exposure device 3 exposes an area of ​​the surface of the photosensitive drum 1 with an exposure width Le in the direction of the rotation axis of the photosensitive drum 1. The exposure device 3 also has a BD sensor 33. The BD sensor 33 is disposed so as to receive the laser light reflected by the polygon mirror 32 to the outside of the exposure width Le when the polygon mirror 32 reaches a predetermined phase. Based on a signal input to the CPU 110 by the BD sensor 33 receiving the laser light, the CPU 110 judges whether the rotation period of the polygon mirror 32 is stable or not.

[0083] As described above, the developing bias in the charge removal operation is set to an appropriate value so as to have an appropriate potential relationship with respect to the surface potential of the exposed surface of the photosensitive drum 1. For this reason, it is desirable to configure the developing opening width Ld, through which toner can be supplied, in the rotation axis direction of the photosensitive drum 1 to be within the exposure width Le in the charge removal operation. This makes the relationship between the surface potential of the exposed surface of the photosensitive drum 1 and the developing bias appropriate even at the ends of the photosensitive drum 1 in the rotation axis direction, and makes it possible to suppress fogging at the ends of the photosensitive drum 1 in the rotation axis direction.

[0084] In this embodiment, a laser scanner is used as the exposure device 3, but an LED array may be used as the exposure device 3. In this case, too, by arranging the LED array so that the development opening width Ld falls within the exposure width Le, fogging at the end of the photosensitive drum 1 in the direction of the rotation axis can be suppressed during the charge removal operation. However, when an LED array is used as the exposure device, the LEDs are arranged up to the end of the photosensitive drum 1 in the direction of the rotation axis that is not used for forming an image. Therefore, the length of the LED array becomes long, which may lead to an increase in the size and cost of the image forming apparatus. In contrast, when a laser scanner is used as the exposure device 3, the degree of freedom of the exposure width Le is greater. This is because in a laser scanner, the entire area that can be polarized by the polygon mirror 32 is not used for forming an image, and it is often configured to be able to irradiate laser light outside the exposure width Le, for example, by providing a BD sensor 33.

[0085] 5. Transfer bias control method In recent years, the image forming apparatus 200 has become smaller and more cost-effective, and the following high-voltage configuration may be used as a power source for the transfer bias, for example. In other words, the output of the high voltage when it is ON is not variable, but the high voltage is repeatedly turned ON / OFF at a predetermined interval, and the macro output value of the bias is controlled by changing the ratio of the ON / OFF time. In this configuration, it is preferable to fix the ON time and make the OFF time a variable. This allows the output value of the transfer bias to be processed as a function that does not have extreme values, making it easy to derive the OFF time for applying a specified transfer bias.

[0086] FIG. 7 is a schematic diagram showing the time change of the high voltage output value when the macro output value of the bias is changed by changing the ratio of the ON / OFF time of the high voltage. FIG. 7(a) and (b) show the time change of the high voltage output value when the ON time length (ON time) and the ON time output value are the same, and only the OFF time length (OFF time) is changed. Specifically, FIG. 7(a) shows the case where the OFF time is the same as the ON time (1:1), and FIG. 7(b) shows the case where the OFF time is five times longer than the ON time (1:5). It can be seen that the average value of the output shown by the dashed line changes with respect to the ON time output value by changing the OFF time. In that case, the longer the OFF time is made, the smaller the average value of the output bias becomes, but on the other hand, the ripple becomes worse. In addition, the OFF time is counted by the CPU 110, but due to the constraints of the CPU 110, the count number reaches a certain level and it becomes impossible to process an OFF time longer than a certain amount.

[0087] In this way, simply changing the OFF time under a constant ON time as in FIG. 7 may limit the output value to be set to a predetermined value during image formation. In such a case, multiple values ​​may be set as the ON time in advance, and the length of the ON time may be changed between image formation requiring a certain level of high output and the charge removal operation in the post-process. Here, the transfer bias output during image formation is generally set to a value larger than the discharge start voltage. Therefore, it is preferable to set the length of the ON time in the post-process to a value smaller than the ON time during image formation. When multiple values ​​are set as the ON time, it is necessary to prepare a control reference table corresponding to each ON time in the memory 120 built in the image forming apparatus 200. Therefore, it is desirable to set the number of values ​​set as the ON time to the minimum necessary.

[0088] 6. Variations Next, a modification of this embodiment will be described.

[0089] In this embodiment, as shown in FIG. 2, the switching timing of each bias is controlled based on the phase of the photosensitive drum 1. In this case, the switching timing may be shifted forward or backward in consideration of various tolerances and high voltage delays. For example, at timing T5, the developing bias is switched from a negative developing bias Vd1 to a positive developing bias Vd3, but at that time, a certain time is required for the high voltage output to stabilize at the value of the developing bias Vd3 due to a delay. On the other hand, the surface potential of the photosensitive drum 1 changes sharply by starting the formation of the exposed surface of the photosensitive drum 1 at timing T4. Therefore, there is a possibility that a section in which the potential difference between the surface potential of the exposed surface of the photosensitive drum 1 and the developing bias is not properly formed may occur during the period from when the developing bias is switched at timing T5 until the developing bias actually stabilizes at the developing bias Vd3. If the change in the developing bias is delayed due to a delay and the developing bias becomes more negative than the surface potential of the photosensitive drum 1 by a certain value or more, a development contrast may be formed and the toner carried on the developing roller 41 may move to the photosensitive drum 1. Therefore, the timing at which the developing bias is controlled to be switched to the developing bias Vd3 may be set a predetermined time earlier than the timing at which the exposure-development time t34 elapses from the timing T4 at which formation of the exposure surface of the photosensitive drum 1 begins. On the other hand, if the timing at which the developing bias is switched to the developing bias Vd3 is set too early, the back contrast may become excessive, and fogging (fogging of toner with a polarity opposite to the normal charging polarity) may be more likely to occur.

[0090] In this way, it is particularly desirable to appropriately control the timing of switching the developing bias so that the developing bias is appropriate for the exposure surface of the photosensitive drum 1. Therefore, in the post-process discharge operation, the developing bias and the charging bias can be changed in multiple stages to more appropriately control the potential difference between the surface potential of the photosensitive drum 1 and the developing bias.

[0091] Fig. 4 is a timing chart for explaining the control of the post-process operation when the developing bias and the charging bias are changed in multiple stages. In Fig. 4, the control of the post-process is shown consecutively from the control of the image forming process (during image formation) as in Fig. 2. The operation according to the timing chart shown in Fig. 4 is controlled by CPU 110.

[0092] In the case of Fig. 4, similarly to the case of Fig. 2, when it is detected at timing T0 that the signal of the registration sensor 91 has turned OFF, transition to the post-processing is started. Also, in the case of Fig. 4, at timing T5, exposure of the photosensitive drum 1 with exposure intensity E3 is started, thereby starting formation of the exposure surface of the photosensitive drum 1. Also, in the case of Fig. 4, similarly to the case of Fig. 2, at timing T6 when the exposure-development time t34 has elapsed from timing T5 when formation of the exposure surface of the photosensitive drum 1 started, the development bias is switched to the development bias Vd3. As a result, the surface potential of the exposure surface of the photosensitive drum 1 and the development bias Vd3 are controlled to have an appropriate potential relationship.

[0093] Here, in the case of FIG. 4, the developing bias is not suddenly switched from the developing bias Vd1 to the developing bias Vd3 at the timing T6, but is changed in multiple steps (stepwise) from the timing T3. In response to the multiple changes in the developing bias, the charging bias is also changed in multiple steps (stepwise) from the timing T1, which is earlier than the timing T3 at which the multiple changes in the developing bias start by the charging and developing time t24. That is, the developing bias and the charging bias are both changed by the bias difference ΔV, and then, when the time difference Δt has elapsed, the bias difference ΔV is changed again. This operation is repeated. In this example, the bias difference ΔV is set to 50V, and the time difference Δt is set to 30 msec. This bias difference ΔV and the time difference Δt can be set so that the back contrast is maintained within a predetermined range so that the fog does not occur significantly. In this case, it is desirable to set them in consideration of the transition of the actual high voltage output, such as the high voltage delay and overshoot. These can be set appropriately depending on the characteristics of the power supply, etc., but in order to maintain the back contrast within a specified range, it is preferable that the bias difference ΔV is approximately 30 V or more and 80 V or less, and the time difference Δt is approximately 10 msec or more and 50 msec or less.

[0094] By such control, the surface of the photosensitive drum 1 is gradually discharged before being abruptly discharged at timing T5, and the developing bias can be gradually changed accordingly. As a result, the potential difference between the surface potential of the photosensitive drum 1 and the developing bias can be appropriately formed, and fogging at the start of the discharge operation can be suppressed.

[0095] Also, as shown in FIG. 4, the charging bias was changed in multiple steps from charging bias Vc1 to charging bias Vc2, and then returned to charging bias Vc1 again. This is to suppress the dirt of the charging roller 2 by increasing the charging contrast, as described above. At that time, if the charging bias Vc1 is increased too much, the potential of the exposed surface of the photosensitive drum 1 also increases, and there is a possibility that the surface of the photosensitive drum 1 cannot be sufficiently discharged. Therefore, in this example, the same charging bias Vc1 as during image formation was used in consideration of the balance between the surface potential of the exposed surface of the photosensitive drum 1 and the charging contrast. Also, in returning the charging bias from charging bias Vc2 to charging bias Vc1, in this example, the charging bias was changed in multiple steps with a bias difference ΔV and a time difference Δt, as in the case of decreasing the charging bias. The bias difference ΔV and the time difference Δt when increasing the charging bias do not need to be the same as when decreasing the charging bias. Also, when increasing the charging bias, the charging bias may be increased to charging bias Vc1 at once. These may be set appropriately in consideration of the withstand voltage of the high-voltage components and the overshoot of the output value.

[0096] Furthermore, the transfer bias and charging bias set as described above can be applied to at least a portion of the area while the exposed surface passes through the transfer portion P5 and the charging portion P2, respectively, to obtain a suitable effect.

[0097] In this embodiment, the charging bias in the static elimination operation is the same as the charging bias during image formation, but is not limited to this. If the above-mentioned charge contrast can be formed and the contamination of the charging roller 2 can be sufficiently suppressed, the charging bias in the static elimination operation may be a different value from the charging bias during image formation. In this case, if the contamination of the charging roller 2 can be sufficiently suppressed, the absolute value of the charging bias in the static elimination operation may be larger or smaller than the absolute value of the charging bias during image formation.

[0098] 7.Effects In this embodiment, the image forming apparatus 200 includes a rotatable photoconductor 1, a charging member 2 that contacts the photoconductor 1 to form a charging portion P2 and charges the surface of the rotating photoconductor 1, a charging voltage application portion (negative charging power source) 81 that applies a charging voltage to the charging member 2, an exposure device 3 that exposes the surface of the photoconductor 1 charged by the charging member 2 to form an electrostatic latent image on the surface of the photoconductor 1, a developing member (developing roller) 41 that contacts the photoconductor 1 to form a developing portion P4 and supplies a developer charged to a predetermined polarity to the electrostatic latent image on the surface of the photoconductor 1 in the developing portion P4 to form a developer image on the surface of the photoconductor 1, and the developing member The developing device 1 includes a developing voltage application unit (positive development power source, negative development power source) 82, 83 that applies a developing voltage to the developing member 41, a transfer member (transfer roller) 5 that abuts against the photoconductor 1 to form a transfer portion P5, a transfer voltage application unit (positive transfer power source, negative charging power source) 81, 84 that applies a transfer voltage to the transfer portion P5, and a control unit (CPU) 110 that can control the charging voltage application unit 81, the developing voltage application units 82, 83, the transfer voltage application units 81, 84 and the exposure device 3, and after image formation in which a developer image is transferred from the photoconductor 1 to the transferred body (recording material) S at the transfer portion P5, the rotation of the photoconductor 1 stops with the developing member 41 abutting against the photoconductor 1. In this embodiment, the control unit 110 performs a discharge operation to discharge the surface of the photoconductor 1 before the rotation of the photoconductor 1 stops after image formation, and in the discharge operation, the control unit 110 controls the exposure device 3 to expose the surface of the photoconductor 1 to form an exposed surface on the surface of the photoconductor 1, applies a development voltage of the specified polarity with respect to the potential of the exposed surface to the developing member 41 while the exposed surface passes through the developing unit P4, applies a transfer voltage to the transfer unit P5, the absolute value of the potential difference between the exposed surface and the potential of the exposed surface being less than a discharge start voltage, and applies a charging voltage of the specified polarity with respect to the potential of the exposed surface to the charging member 2 while the exposed surface passes through the charging unit P2. In this embodiment, the image forming apparatus 200 is configured so that the rotation of the photoconductor 1 begins when the developing member 41 is in contact with the photoconductor 1, and the control unit 110 controls the application of a developing voltage of opposite polarity to the above-mentioned specified polarity to the developing member 41 before the photoconductor 1 starts rotating or approximately simultaneously with the photoconductor 1 starts rotating.In addition, when changing the developing voltage from the developing voltage during image formation to the developing voltage during the time when the exposed surface passes through the developing section P4 in the charge removal operation, the control section 110 can provide a period in which the absolute value of the charging voltage and the absolute value of the developing voltage are each gradually decreased so that the potential difference between the potential of the surface of the photoconductor 1 at the developing section P4 and the developing voltage is maintained within a predetermined range. In this case, the control section 110 can control the absolute value of the charging voltage to be gradually decreased during the period, and then to be increased before the exposed surface reaches the charging section P2. In addition, in this embodiment, the control section 110 provides a period in which the surface of the photoconductor 1 is exposed to light by the exposure device 3 without applying a charging voltage to the charging member 2 to form the exposed surface, at least for the time it takes for the surface of the photoconductor 1 to move from the charging section P2 to the exposure section P3, before the charge removal operation is completed and the rotation of the photoconductor 1 is stopped.

[0099] In addition, in this embodiment, the control unit 110 controls the charging member 2 to apply a charging voltage, which is on the predetermined polarity side with respect to the potential of the exposed surface and has an absolute value of a potential difference between the potential of the exposed surface and the potential of the exposed surface equal to or greater than a discharge start voltage, to the charging member 2 while the exposed surface passes through the charging unit P2 in the charge removal operation. In addition, in this embodiment, the control unit 110 controls the transfer unit P5 to apply a transfer voltage on the predetermined polarity side with respect to the potential of the exposed surface to the transfer unit P5 while the exposed surface passes through the transfer unit P5 in the charge removal operation. However, the control unit 110 may also control the transfer unit P5 to apply a transfer voltage on the opposite polarity side to the predetermined polarity with respect to the potential of the exposed surface to the transfer unit P5 while the exposed surface passes through the transfer unit P5 in the charge removal operation. In addition, preferably, the control unit 110 controls the exposure device 3 to expose the photoconductor 1 to light for at least the time it takes the photoconductor 1 to make one revolution to form the exposed surface in the charge removal operation. In addition, in this embodiment, the control unit 110 controls the charge removal operation so that the absolute value of the potential difference between the potential of the exposed surface when it reaches the charging unit P2 after passing through the transfer unit P5 and the charging voltage is greater than the absolute value of the potential difference between the potential of the non-image portion on the photoconductor when it reaches the charging unit P2 after passing through the transfer unit P5 during image formation and the charging voltage. To achieve this, in this embodiment, the control unit 110 controls the exposure intensity of the exposure device 3 in the charge removal operation so as to be greater than the exposure intensity of the exposure device 3 during image formation. In addition, in this embodiment, the transfer unit P5 is configured to be applied with a voltage obtained by superimposing the output of the charging voltage application unit (charge negative power source) 81 and the output of the transfer voltage application unit (transfer positive power source) 84. In addition, in this embodiment, the control unit 110 controls the charge removal operation so that the charging member 2 is applied with a charging voltage of the same value as that during image formation. In addition, in this embodiment, the control unit 110 controls the above-mentioned de-electrification operation so that the exposure device 3 exposes an area of ​​the surface of the photosensitive member 1 that is larger than the area in the direction of the rotation axis of the photosensitive member 1 where developer can be supplied from the developing member 41 to the photosensitive member 1, thereby forming the above-mentioned exposed surface.In addition, the control unit 110 is configured to adjust the output value of the transfer voltage application unit 84 for the transfer unit P5 by controlling the ratio between the ON time, which is the time during which the transfer voltage application unit (transfer positive power supply) 84 outputs a predetermined voltage, and the OFF time, which is the time during which the output of the transfer voltage application unit 84 is stopped, and can control the ON time while the exposed surface is passing through the transfer unit P5 in the discharge operation to be shorter than the ON time during image formation.

[0100] As described above, in this embodiment, in the charge removal operation in the post-process, an appropriate transfer bias is applied to the exposed surface formed on the photosensitive drum 1, and the exposed surface is recharged. In addition, it is preferable to set a large charge contrast when recharging the exposed surface in the charge removal operation, and it is also preferable to apply a transfer bias to the exposed surface that is less than the discharge start voltage in the charge removal operation. According to this embodiment, it is possible to prevent external additives and the like from adhering to the surface of the charging roller 2 in the post-process of the image forming operation. This makes it possible to prevent the occurrence of image defects caused by charging defects such as vertical streaks due to external additives and the like adhering to the surface of the charging roller 2 in the post-process of the image forming operation.

[0101] [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 embodiment 1. 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 embodiment 1 are given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

[0102] In the first embodiment, the image forming apparatus 200 is configured to collect the transfer residual toner by the developing roller 41. In contrast, in the present embodiment, the image forming apparatus 300 is configured to collect the transfer residual toner by a cleaning device.

[0103] FIG. 5 is a schematic cross-sectional view of an image forming apparatus 300 of this embodiment. In this embodiment, the image forming apparatus 300 has a cleaning device (cleaning unit) 7 as a cleaning means. The cleaning device 7 cleans the surface of the photosensitive drum 1 at a cleaning portion P7 downstream of the transfer portion P5 and upstream of the charging portion P2 in the rotation direction of the photosensitive drum 1. The cleaning device 7 has a cleaning container 72 and a flexible cleaning blade 71 as a cleaning member. The cleaning blade 71 is arranged in contact with the surface of the photosensitive drum 1. The position where the cleaning blade 71 removes toner from the photosensitive drum 1 in the rotation direction of the photosensitive drum 1 (the contact portion between the cleaning blade 71 and the photosensitive drum 1) is the cleaning portion (cleaning position) P7. In this embodiment, the process cartridge 100 is configured by integrally assembling the photosensitive drum 1, the charging roller 2 as a process means acting on the photosensitive drum 1, the developing device (developing unit) 4, and the cleaning device (cleaning unit) 7.

[0104] The cleaning device 7 uses the cleaning blade 71 to scrape off the toner, external additives, and even paper dust and fillers derived from the recording material S that are attached to the surface of the photosensitive drum 1 as the photosensitive drum 1 rotates. Some of the toner and other particles scraped off by the cleaning blade 71 are stored in a cleaning container 72. Some of the external additives, which are relatively small in size compared to the toner, remain at a cleaning portion P7 (the contact portion between the cleaning blade 71 and the photosensitive drum 1) and act as a blocking layer that prevents the toner and a large amount of external additives from passing through the cleaning portion P7. Meanwhile, a small amount of external additives passes through the cleaning portion P7, thereby ensuring appropriate lubrication between the cleaning blade 71 and the photosensitive drum 1.

[0105] In this way, even in the configuration in which the cleaning device 7 is provided, a certain amount of external additive is transported to the charging section P2. Therefore, even in the configuration in which the cleaning device 7 is provided, it is preferable to perform a charge removal operation in the post-process similar to that in the first embodiment. That is, in the post-process after the image formation, an exposure surface is formed by the exposure device 3, and while the exposure surface passes through the charging section P2, a charging voltage of a predetermined polarity side, which is the normal charging polarity of the toner, is applied to the charging roller 2 with respect to the potential of the exposure surface (recharging of the exposure surface). This increases the charge contrast, which is the potential difference between the charging bias Vc applied to the charging roller 2 and the potential of the exposure surface (in this case, the post-transfer potential Vpre), and thus the adhesion of the external additive to the surface of the charging roller 2 can be suppressed, thereby suppressing the contamination of the charging roller 2.

[0106] However, by providing the cleaning device 7, the amount of external additives supplied to the charging roller 2 is reduced compared to the case described in the first embodiment. Therefore, in the case of recharging the exposed surface described above, even if the setting range of the transfer bias applied to the transfer roller 5 in the charge removal operation is wider than that in the case described in the first embodiment, there is no practical problem with respect to the dirt on the charging roller 2. However, it is not preferable to set the transfer bias to a value that is more negative than the charging bias, that is, a value that is more negative than -1100V. In this case, the discharge at the transfer portion P5 causes the post-transfer potential Vpre to become more negative than the dark potential VD, specifically -550V, and the charge contrast becomes smaller than when the exposed surface is not formed, which may cause the dirt on the charging roller 2 to worsen. Therefore, in a configuration provided with the cleaning device 7, it is preferable to apply a bias that is more positive than the charging bias as the transfer bias applied in the charge removal operation in the post-process.

[0107] Thus, in this embodiment, the image forming apparatus 300 has a cleaning member 71 that is arranged in contact with the photosensitive member 1 and removes the developer from the photosensitive member, and after image formation in which the developer image is transferred from the photosensitive member 1 to the transfer material S at the transfer portion P5, the rotation of the photosensitive member 1 stops with the developing member 41 in contact with the photosensitive member 1. In this embodiment, the control unit 110 executes a discharge operation to discharge the surface of the photoconductor 1 before the rotation of the photoconductor 1 stops after image formation, and in the discharge operation, the control unit 110 exposes the surface of the photoconductor 1 using the exposure device 3 to form an exposed surface on the surface of the photoconductor 1, applies a development voltage of a predetermined polarity, which is the normal charging polarity of the developer, to the developing member 41 with respect to the potential of the exposed surface while the exposed surface passes through the developing unit P4, applies a transfer voltage to the transfer unit P5 while the exposed surface passes through the transfer unit P5, applies a charging voltage of the predetermined polarity with respect to the potential of the exposed surface to the charging member 2 while the exposed surface passes through the charging unit P2, and controls the transfer voltage applied to the transfer unit P5 while the exposed surface passes through the transfer unit P5 to be the opposite polarity to the predetermined polarity (positive with respect to the charging voltage in this embodiment) with respect to the charging voltage applied to the charging member 2 while the exposed surface passes through the charging unit P2.

[0108] [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 embodiment 2. 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 embodiment 2 are given the same reference numerals as those of embodiment 2, and detailed explanations are omitted.

[0109] In the second embodiment, the image forming apparatus 300 is configured such that the developing roller 41 is always in contact with the photosensitive drum 1. In contrast, in the present embodiment, the image forming apparatus 300 is configured such that the developing roller 41 can be brought into contact with and separated from the photosensitive drum 1.

[0110] FIG. 6 is a schematic cross-sectional view of an image forming apparatus 300 of this embodiment. In this embodiment, the image forming apparatus 300 has a contact / separation mechanism 15. The contact / separation mechanism 15 is controlled by the CPU 110 to move the developing device 4, thereby making it possible to bring the developing roller 41 into contact with and separate from the photosensitive drum 1. In this embodiment, the process cartridge 100 is configured by integrating the photosensitive drum 1, the charging roller 2 as a process means acting thereon, the developing device (developing unit) 4, and the cleaning device (cleaning unit) 7. For example, the developing device 4 is swingably coupled to a drum unit including the photosensitive drum 1, the charging roller 2, and the cleaning device 7, and is biased in a direction in which the developing roller 41 contacts the photosensitive drum 1 by a pressure spring, which is a biasing member as a biasing means. Then, the developing roller 41 can be separated from the photosensitive drum 1 by swinging the developing device 4 against the biasing force of the pressure spring by a moving member such as a cam provided in the contact / separation mechanism 15. Moreover, the developing roller 41 can be brought into contact with the photosensitive drum 1 by allowing the developing device 4 to move due to the biasing force of the pressure spring by the contact / separation mechanism 15 .

[0111] In such a configuration, by separating the developing roller 41 from the photosensitive drum 1 in a post-process after the image forming process, the supply of toner and external additives from the developing roller 41 to the photosensitive drum 1 is not performed. In addition, if the rotation of the developing roller 41 is stopped while the photosensitive drum 1 is rotated in a state in which the developing roller 41 is in contact with the photosensitive drum 1, the surface of the developing roller 41 in contact with the photosensitive drum 1 may be damaged by friction. In response to this, the occurrence of the above-mentioned damage can be suppressed by separating the developing roller 41 from the photosensitive drum 1. Furthermore, by stopping the rotation of the developing roller 41 by a clutch mechanism (not shown) when the developing roller 41 is separated from the photosensitive drum 1, it is possible to suppress the deterioration of the toner due to friction between the developing roller 41 and the regulating blade 42, for example.

[0112] However, even in a configuration in which the developing roller 41 can be separated from the photosensitive drum 1, a small amount of external additives slips through the cleaning portion P7 as described in the second embodiment. Therefore, even in a configuration in which the developing roller 41 can be separated from the photosensitive drum 1, by performing a static elimination operation in a post-process similar to that in the first embodiment, it is possible to suppress adhesion of external additives to the surface of the charging roller 2 and suppress contamination of the charging roller 2.

[0113] In this embodiment, when the exposed surface passes a position corresponding to the developing portion P4 in the charge removal operation, the developing roller 5 is separated from the photosensitive drum 1 and the developing bias is turned OFF. Also, in this embodiment, when the photosensitive drum 1 starts rotating, the developing bias is turned ON and the developing roller 5 is brought into contact with the photosensitive drum 1 after the surface of the photosensitive drum 1 that has been charged by the charging roller 2 reaches a position corresponding to the developing portion P4.

[0114] Thus, in this embodiment, the image forming apparatus 300 has a contact / separation mechanism 15 that contacts and separates the developing member 41 from the photosensitive member 1, and after image formation in which the developer image is transferred from the photosensitive member 1 to the transfer material S at the transfer section P5, the rotation of the photosensitive member 1 stops with the developing member 41 separated from the photosensitive member 1 by the contact / separation mechanism 15. In this embodiment, the control unit 110 executes a discharge operation to discharge the surface of the photoconductor 1 before the rotation of the photoconductor 1 stops after image formation, and in the discharge operation, the control unit 110 exposes the surface of the photoconductor 1 using the exposure device 3 to form an exposed surface on the surface of the photoconductor 1, applies a transfer voltage to the transfer unit P5 while the exposed surface passes through the transfer unit P5, applies a charging voltage of the specified polarity with respect to the potential of the exposed surface to the charging member 2 while the exposed surface passes through the charging unit P2, and controls the transfer voltage applied to the transfer unit P5 while the exposed surface passes through the transfer unit P5 to be the opposite polarity (positive with respect to the charging voltage in this embodiment) to the specified polarity, which is the normal charging polarity of the developer, with respect to the charging voltage applied to the charging member 2 while the exposed surface passes through the charging unit P2.

[0115] [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.

[0116] In the above-described embodiment, the image forming apparatus is configured so that the process cartridge is detachably attached, but the present invention is also applicable to an image forming apparatus in which a process unit similar to that constituting the process cartridge in the above-described embodiment is provided in the device main body.

[0117] In the above-described embodiments, the photosensitive drum, the charging roller, and the developing device (and the cleaning device in the second and third embodiments) are integrated to form a process cartridge. In contrast, for example, the drum unit in which the photosensitive drum and the charging roller (and the cleaning device) are integrated, and the developing device may each be individually detachable from the device main body. Also, a supply container (toner supply unit) containing developer (toner) to be replenished to the developing device may be detachable from the developing device. In this case, the developing device may be fixed to the image forming apparatus, and only the supply container may be detachable from the device main body.

[0118] In the above embodiment, a monochrome printer that forms an image using a single color toner has been described, but a color printer that forms an image using multiple colors of toner may also be used. For example, the present invention can also be applied to a color image forming apparatus that has multiple image forming units (photosensitive drums, etc.) and transfers multiple color toner images to a recording material using an intermediate transfer body such as an intermediate transfer belt to form a full color image. In that case, each image forming unit may control the post-process in the same manner as in the above embodiment. In an image forming apparatus with such a configuration, a transfer member that contacts the photosensitive body to form a transfer unit is composed of, for example, an intermediate transfer body and a primary transfer member that contacts the photosensitive body via the intermediate transfer body. [Explanation of symbols]

[0119] 1 Photosensitive drum 2. Charge roller 3. Exposure equipment 4. Developing device 5 Transfer roller 41 Developing roller

Claims

1. A rotatable photoreceptor; a charging member that contacts the photoconductor to form a charging portion and charges the surface of the rotating photoconductor; a charging voltage application unit that applies a charging voltage to the charging member; an exposure device that exposes the surface of the photoconductor charged by the charging member to light to form an electrostatic latent image on the surface of the photoconductor; a developing member that forms a developing section in contact with the photoconductor and supplies a developer charged to a predetermined polarity to an electrostatic latent image on the surface of the photoconductor in the developing section to form a developer image on the surface of the photoconductor; a developing voltage applying section that applies a developing voltage to the developing member; a transfer member that contacts the photoreceptor to form a transfer portion; a transfer voltage application unit that applies a transfer voltage to the transfer unit; a control unit capable of controlling the charging voltage application unit, the developing voltage application unit, the transfer voltage application unit, and the exposure device; having In an image forming apparatus, after image formation in which a developer image is transferred from the photoconductor to a transferee in the transfer section, the rotation of the photoconductor is stopped in a state in which the developing member is in contact with the photoconductor, an exposure device that exposes the surface of the photoconductor to light to form an exposed surface on the surface of the photoconductor; a developing voltage of the predetermined polarity with respect to the potential of the exposed surface is applied to the developing member while the exposed surface passes through the developing unit; a transfer voltage of the predetermined polarity with respect to the potential of the exposed surface is applied to the transfer unit while the exposed surface passes through the transfer unit; and a charging voltage of the predetermined polarity with respect to the potential of the exposed surface is applied to the charging member while the exposed surface passes through the charging unit.

2. A rotatable photoreceptor; a charging member that contacts the photoconductor to form a charging portion and charges the surface of the rotating photoconductor; a charging voltage application unit that applies a charging voltage to the charging member; an exposure device that exposes the surface of the photoconductor charged by the charging member to light to form an electrostatic latent image on the surface of the photoconductor; a developing member that forms a developing section in contact with the photoconductor and supplies a developer charged to a predetermined polarity to an electrostatic latent image on the surface of the photoconductor in the developing section to form a developer image on the surface of the photoconductor; a developing voltage applying section that applies a developing voltage to the developing member; a transfer member that contacts the photoreceptor to form a transfer portion; a transfer voltage application unit that applies a transfer voltage to the transfer unit; a cleaning member disposed in contact with the photoconductor and configured to remove developer from the photoconductor; a control unit capable of controlling the charging voltage application unit, the developing voltage application unit, the transfer voltage application unit, and the exposure device; having In an image forming apparatus, after image formation in which a developer image is transferred from the photoconductor to a transferee in the transfer section, the rotation of the photoconductor is stopped in a state in which the developing member is in contact with the photoconductor, an exposure device that exposes the surface of the photoconductor to light to form an exposed surface on the surface of the photoconductor; a developing voltage of the predetermined polarity with respect to the potential of the exposed surface is applied to the developing member while the exposed surface passes through the developing unit; a transfer voltage is applied to the transfer unit while the exposed surface passes through the transfer unit; a charging voltage of the predetermined polarity with respect to the potential of the exposed surface is applied to the charging member while the exposed surface passes through the charging unit; and a transfer voltage applied to the transfer unit while the exposed surface passes through the transfer unit is controlled to be an opposite polarity to the predetermined polarity with respect to the charging voltage applied to the charging member while the exposed surface passes through the charging unit.

3. The developing member is configured to contact the photoconductor and the photoconductor starts to rotate, 3. The image forming apparatus according to claim 1, wherein the control unit controls the application of a development voltage of a polarity opposite to the specified polarity to the developing member before the photosensitive member starts rotating or substantially simultaneously with the photosensitive member starting rotating.

4. The image forming apparatus according to claim 1 or 2, characterized in that, when changing the developing voltage from the developing voltage during image formation to the developing voltage while the exposed surface is passing through the developing unit during the discharge operation, the control unit controls to provide a period in which the absolute value of the charging voltage and the absolute value of the developing voltage are each gradually reduced so as to maintain the potential difference between the potential of the surface of the photoconductor in the developing unit and the developing voltage within a predetermined range.

5. 5. The image forming apparatus according to claim 4, wherein the control unit controls the charging voltage so as to gradually decrease the absolute value of the charging voltage during the period, and then to increase the absolute value of the charging voltage before the exposure surface reaches the charging unit.

6. The image forming apparatus according to claim 1 or 2, characterized in that the control unit provides a period during which the exposure device exposes the surface of the photoconductor to form the exposed surface without applying a charging voltage to the charging member before the de-electrification operation is completed and the rotation of the photoconductor stops, the period being at least the time it takes for the surface of the photoconductor to move from the charging unit to the exposure unit.

7. A rotatable photoreceptor; a charging member that contacts the photoconductor to form a charging portion and charges the surface of the rotating photoconductor; a charging voltage application unit that applies a charging voltage to the charging member; an exposure device that exposes the surface of the photoconductor charged by the charging member to light to form an electrostatic latent image on the surface of the photoconductor; a developing member that forms a developing section in contact with the photoconductor and supplies a developer charged to a predetermined polarity to an electrostatic latent image on the surface of the photoconductor in the developing section to form a developer image on the surface of the photoconductor; a contact / separation mechanism for contacting and separating the developing member with respect to the photoconductor; a transfer member that contacts the photoreceptor to form a transfer portion; a transfer voltage application unit that applies a transfer voltage to the transfer unit; a cleaning member disposed in contact with the photoconductor and configured to remove developer from the photoconductor; a control unit capable of controlling the charging voltage application unit, the transfer voltage application unit, and the exposure device; having an image forming apparatus in which the rotation of the photoconductor is stopped in a state in which the developing member is separated from the photoconductor by the contact / separation mechanism after image formation in which a developer image is transferred from the photoconductor to a transferee in the transfer unit; an exposure device that exposes the surface of the photoconductor to light to form an exposed surface on the surface of the photoconductor; a transfer voltage is applied to the transfer device while the exposed surface passes through the transfer device; a charging voltage of the predetermined polarity with respect to the potential of the exposed surface is applied to the charging member while the exposed surface passes through the charging unit; and a transfer voltage applied to the transfer device while the exposed surface passes through the transfer unit is controlled to be an opposite polarity to the predetermined polarity with respect to the charging voltage applied to the charging member while the exposed surface passes through the charging unit.

8. The image forming apparatus according to any one of claims 1, 2 and 7, characterized in that, during the de-electrification operation, the control unit controls the charging member to apply a charging voltage to the charging member that is on the predetermined polarity side with respect to the potential of the exposure surface and has an absolute value of a potential difference between the potential of the exposure surface and the potential of the exposure surface that is equal to or greater than a discharge start voltage while the exposure surface passes through the charging unit.

9. The image forming apparatus according to claim 1, 2 or 7, characterized in that, during the charge removal operation, the control unit controls the transfer unit to apply a transfer voltage of the predetermined polarity with respect to the potential of the exposure surface while the exposure surface is passing through the transfer unit.

10. The image forming apparatus according to any one of claims 1, 2 and 7, characterized in that, during the charge removal operation, the control unit controls the transfer unit to apply a transfer voltage of a polarity opposite to the predetermined polarity with respect to the potential of the exposure surface while the exposure surface is passing through the transfer unit.

11. The image forming apparatus according to claim 1, 2 or 7, characterized in that, during the charge removal operation, the control unit controls the exposure device to expose the photoconductor to light for at least the time it takes the photoconductor to rotate once to form the exposure surface.

12. The image forming apparatus according to any one of claims 1, 2 and 7, characterized in that the control unit controls, during the de-electrification operation, so that the absolute value of the potential difference between the potential of the exposed surface when it reaches the charging unit after passing through the transfer unit and the charging voltage is greater than the absolute value of the potential difference between the potential of the non-image area on the photoconductor when it reaches the charging unit after passing through the transfer unit during image formation and the charging voltage.

13. 8. The image forming apparatus according to claim 1, wherein the control unit controls the exposure intensity of the exposure device during the charge removal operation so as to be greater than the exposure intensity of the exposure device during image formation.

14. 8. The image forming apparatus according to claim 1, wherein the transfer section is configured to receive a voltage obtained by superimposing an output of the charging voltage application section and an output of the transfer voltage application section.

15. 8. The image forming apparatus according to claim 1, wherein the control section controls the charging member to apply, in the charge removing operation, a charging voltage having the same value as that during image formation.

16. The image forming apparatus according to any one of claims 1, 2 and 7, characterized in that, during the de-electrification operation, the control unit controls the exposure device to expose an area of ​​the surface of the photosensitive body that is larger than an area in the direction of the rotation axis of the photosensitive body where developer can be supplied from the developing member to the photosensitive body, thereby forming the exposed surface.

17. The control unit is configured to adjust the output value of the transfer voltage application unit for the transfer unit by controlling the ratio between the ON time, which is the time during which the transfer voltage application unit outputs a predetermined voltage, and the OFF time, which is the time during which the output of the transfer voltage application unit is stopped, and controls so that the ON time during which the exposure surface passes through the transfer unit during the discharge operation is shorter than the ON time during image formation.

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