Image forming device

Abstract

To efficiently apply a voltage having the same polarity as a normal electrification polarity of toner to a transfer member, while reducing a size and cost of an apparatus by not providing a separate power supply for applying a voltage having the same polarity as the normal electrification polarity of toner to the transfer member.SOLUTION: An image forming apparatus 1 has a photoreceptor 2, an electrifying member 3, an exposure device 4, a developing member 21, a developing voltage application unit E2, a transfer member 8, a first transfer voltage application unit E3, a second transfer voltage application unit E4, a common power supply 50 that supplies a voltage to the developing voltage application unit E2 and the second transfer voltage application unit E4, and a control unit 100 that can control the common power supply 50. The control unit 100 controls to execute image forming operation to form a toner image on a recording material P and non-image forming operation different from the image forming operation, and controls the common power supply 50 in the non-image forming operation.SELECTED DRAWING: Figure 4

Description

[Technical Field]

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

[0002] In conventional electrophotographic image forming apparatuses, the surface of a generally drum-shaped electrophotographic photoreceptor (hereinafter simply referred to as "photoreceptor") is charged by a charging means, and the charged surface of the photoreceptor is exposed by an exposure means to form an electrostatic latent image on the photoreceptor. A developing means then applies toner to the electrostatic latent image formed on the photoreceptor to form a toner image on the photoreceptor, and the toner image formed on the photoreceptor is then transferred by a transfer means onto a sheet-like recording material such as recording paper. Here, the recording material on which an image is formed in an image forming apparatus is sometimes referred to as "paper," but the recording material is not limited to paper. A transfer roller, a roller-shaped transfer member, is widely used as the transfer means. The transfer roller is positioned opposite the photoreceptor and pressed against the photoreceptor to form a transfer nip (transfer portion) between the photoreceptor and the recording material. In this case, the recording material is fed into the transfer nip, and a transfer voltage of a polarity opposite to the normal charge polarity of the toner is applied to the transfer roller, thereby imparting an electric charge to the recording material and transferring the toner from the photoreceptor onto the recording material.

[0003] In such an image forming apparatus, when image forming operations are performed repeatedly or when a recording material jam occurs (paper jam), the toner on the photosensitive member may be transferred directly to the transfer roller and adhere to the transfer roller. If the amount of toner adhering to the transfer roller is relatively large, the toner adhering to the transfer roller may be transferred to the back side of the recording material (the surface facing the transfer roller) during the next or subsequent image forming operation, resulting in a phenomenon known as "backside paper soiling," in which the back side of the recording material is soiled.

[0004] Therefore, a configuration is known in which the following cleaning operation of the transfer roller (hereinafter simply referred to as "cleaning operation") is performed (Patent Document 1). That is, when no recording material is present in the transfer nip portion and paper is not passing through, a voltage of the same polarity as the normal charging polarity of the toner is applied to the transfer roller, causing the toner adhering to the transfer roller to be transferred (reverse transferred) onto the photosensitive member, and cleaning the toner adhering to the transfer roller. By performing such a cleaning operation, it is possible to suppress contamination of the back of the paper. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-29281 Summary of the Invention [Problem to be solved by the invention]

[0006] For example, when performing the cleaning operation described above, a power supply is required to apply a voltage of the same polarity as the normal charge polarity of the toner to the transfer member in order to transfer the toner of the normal charge polarity attached to the transfer member from the transfer member to the photosensitive member. In conventional configurations, a separate power supply is provided to apply a cleaning voltage to the transfer roller for cleaning the transfer roller. However, in recent years, due to demands for further miniaturization and cost reduction in image forming apparatuses, there has been a demand for a configuration that does not require a separate power supply to apply a voltage of the same polarity as the normal charge polarity of the toner, such as the cleaning voltage, to the transfer roller.

[0007] To address this issue, it is conceivable to share a common power supply for both the cleaning voltage and the charging voltage. However, in such a configuration, if an attempt is made to change the cleaning voltage to a value suitable for cleaning the transfer roller during cleaning, the surface potential of the photosensitive member may be changed from the appropriate value. In this case, the potential difference between the transfer roller and the photosensitive member, which is required to electrostatically transfer the toner adhering to the transfer roller to the photosensitive member, is changed, which may result in unstable cleaning performance of the transfer roller.

[0008] In this way, it is desirable to achieve both miniaturization and cost reduction of the device by configuring it so that a separate power supply for applying a cleaning voltage to the transfer roller is not provided, and stable cleaning of the transfer roller is possible. Similar problems can arise when performing a non-image forming operation that is different from the image forming operation for forming a toner image on a recording material, which requires a power supply for applying a voltage of the same polarity as the normal charging polarity of the toner to the transfer member.

[0009] Therefore, the present invention aims to effectively apply a voltage of the same polarity as the normal charging polarity of the toner to the transfer member while achieving miniaturization and cost reduction of the device by not providing a separate power source for applying a voltage of the same polarity as the normal charging polarity of the toner to the transfer member. [Means for solving the problem]

[0010] The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention comprises a rotatable photosensitive member, a charging member for charging the surface of the photosensitive member, an exposure device for exposing the surface of the photosensitive member that has been charged to light to form an electrostatic latent image on the surface of the photosensitive member, a developing member for attaching toner to the electrostatic latent image to form a toner image, a developing voltage application unit for applying a developing voltage to the developing member, a transfer member that contacts the surface of the photosensitive member to form a transfer section and transfers the toner image from the surface of the photosensitive member to a recording material passing through the transfer section, a first transfer voltage application unit for applying a transfer voltage of a polarity opposite to the normal charging polarity of the toner to the transfer member, and a second transfer voltage application unit for applying a transfer voltage of the same polarity as the normal charging polarity of the toner to the transfer member. a second transfer voltage application unit that applies a transfer voltage of the same polarity, a common power source that supplies voltage to the development voltage application unit and the second transfer voltage application unit, and a control unit that can control the common power source, wherein the control unit controls the execution of an image forming operation that forms a toner image on a recording material and a non-image forming operation that is different from the image forming operation, and as the non-image forming operation, when there is no recording material in the transfer unit, the second transfer voltage application unit applies a voltage of the same polarity to the transfer member to move the toner from the transfer member to the photosensitive member, and controls the common power source in the non-image forming operation, and the control unit i) an output of the common power source so that a value of a voltage applied to the developing member by the developing voltage application unit during the cleaning operation is made different from a value of a voltage applied to the developing member by the developing voltage application unit during the formation of the toner image; of change To do so control ii) controlling the absolute value of the voltage applied to the transfer member by the second transfer voltage application unit during the cleaning operation to be larger than that in a case where the value of the voltage applied to the development member by the development voltage application unit is not changed from that in the image forming operation. do 、 The image forming apparatus is characterized by the above.

[0011] According to another aspect of the present invention, there is provided a rotatable photosensitive body, a charging member for charging the surface of the photosensitive body, a charging voltage application unit for applying a charging voltage to the charging member, an exposure device for exposing the surface of the photosensitive body that has been charged to light to form an electrostatic latent image on the surface of the photosensitive body, a developing member for attaching toner to the electrostatic latent image to form a toner image, a developing voltage application unit for applying a developing voltage to the developing member, a transfer member that contacts the surface of the photosensitive body to form a transfer section and transfers the toner image from the surface of the photosensitive body to a recording material that passes through the transfer section, a first transfer voltage application unit for applying a transfer voltage of a polarity opposite to the normal charging polarity of the toner to the transfer member, and a second transfer voltage application unit for applying a transfer voltage of a polarity opposite to the normal charging polarity of the toner to the transfer member. a second transfer voltage application unit that applies a transfer voltage of the same polarity as a normal charging polarity; a common power source that supplies voltage to the developing voltage application unit, the charging voltage application unit, and the second transfer voltage application unit; and a control unit that can control the common power source, wherein the control unit controls the execution of an image forming operation that forms a toner image on a recording material and a non-image forming operation that is different from the image forming operation, and as the non-image forming operation, controls the execution of a cleaning operation that moves the toner from the transfer member to the photosensitive member by applying a voltage of the same polarity to the transfer member when there is no recording material in the transfer unit, and controls the common power source in the non-image forming operation, and the control unit i) the output of the common power source is controlled so as to perform at least one of: making the value of the voltage applied to the developing member by the developing voltage application unit during the cleaning operation different from the value of the voltage applied to the developing member by the developing voltage application unit during the formation of the toner image; or making the value of the voltage applied to the charging member by the charging voltage application unit during the cleaning operation different from the value of the voltage applied to the charging member by the charging voltage application unit during the charging process. of change To do so control ii) controlling the absolute value of the voltage applied to the transfer member by the second transfer voltage application unit during the cleaning operation to be larger than that in a case where the value of the voltage applied to the development member by the development voltage application unit is not changed from that in the image forming operation. do 、 An image forming apparatus characterized by the above features is provided. [Effects of the Invention]

[0012] According to the present invention, by not providing a separate power supply for applying a voltage of the same polarity as the normal charging polarity of the toner to the transfer member, it is possible to reduce the size and cost of the device while effectively applying a voltage of the same polarity as the normal charging polarity of the toner to the transfer member. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Figure 2] FIG. 2 is a schematic cross-sectional view of an image forming unit. [Figure 3] FIG. 2 is a schematic block diagram showing a control mode of the image forming apparatus. [Figure 4] FIG. 2 is a schematic circuit diagram showing an example of a high-voltage circuit configuration of an image forming apparatus. [Figure 5] FIG. 10 is a graph showing an example of the relationship between a cleaning voltage and a developing voltage. [Figure 6] FIG. 10 is a graph showing an example of the relationship between the development voltage and the amount of fog toner. [Figure 7] FIG. 10 is a timing chart for explaining an example of a cleaning operation. [Figure 8] FIG. 10 is a graph showing an example of the relationship between development voltage and cleaning performance. [Figure 9] FIG. [Figure 10] FIG. 10 is a graph showing another example of the relationship between the development voltage and the cleaning performance. [Figure 11] FIG. 10 is a timing chart for explaining another example of the cleaning operation. [Figure 12] FIG. 10 is a graph showing another example of the relationship between the development voltage and the cleaning performance. [Figure 13] FIG. 10 is a schematic circuit diagram showing another example of a high-voltage circuit configuration of an image forming apparatus. [Figure 14] FIG. 10 is a graph showing another example of the relationship between the cleaning voltage and the developing voltage. [Figure 15] FIG. 10 is a graph showing another example of the relationship between the development voltage and the cleaning performance. [Figure 16]FIG. 10 is a timing chart for explaining another example of the cleaning operation. [Figure 17] FIG. 10 is a graph showing another example of the relationship between the development voltage and the cleaning performance. [Figure 18] FIG. 10 is a graph showing an example of the relationship between cleaning voltage and charging voltage. [Figure 19] FIG. 10 is a timing chart for explaining another example of the cleaning operation. [Figure 20] FIG. 10 is a schematic flowchart of control for switching between the developer contact and separation states during a cleaning operation. DETAILED DESCRIPTION OF THE INVENTION

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

[0015] [Example 1] (1) Image forming device The overall configuration and operation of an image forming apparatus 1 of this embodiment will be described using Figure 1. Figure 1 is a schematic cross-sectional view of the image forming apparatus 1 of this embodiment. The image forming apparatus 1 of this embodiment is an electrophotographic laser printer that forms an image on a recording material P such as paper or plastic film in accordance with image information input from an external device 200 (Figure 3) such as a host computer.

[0016] The image forming apparatus 1 has a rotatable drum-shaped (cylindrical) photoconductor (photosensitive drum) 2 that serves as an image carrier. When a print command (a command to start a print job) is input from an external device 200 to the image forming apparatus 1, the photoconductor 2 is driven to rotate counterclockwise in FIG. 1 at a predetermined peripheral speed (process speed) by a driving force transmitted from a drive source (not shown). In this embodiment, the photoconductor 2 is configured by forming an OPC layer (organic photoconductor) on an aluminum cylinder. In this embodiment, the OPC layer has a 20 μm-thick CT layer (charge transfer layer) made primarily of a polycarbonate binder. In this embodiment, the photoconductor 2 has an outer diameter of 30 mm.

[0017] The surface (outer periphery) of the rotating photoreceptor 2 is uniformly charged to a predetermined potential of a predetermined polarity (negative in this embodiment) by the charging roller 3, a rotatable roller-shaped charging member serving as a charging means. In this embodiment, the charging roller 3 is a single-layer elastic roller with a conductive core metal coated with a conductive elastic layer. In this embodiment, both longitudinal ends of the conductive core metal of the charging roller 3 are pressed toward the photoreceptor 2 by a pressing means (not shown). The charging roller 3 contacts the surface of the photoreceptor 2 and rotates in accordance with the rotation of the photoreceptor 2. In this embodiment, a predetermined charging voltage (charging bias), which is a negative DC voltage, is applied to the charging roller 3 during charging. The position on the photoreceptor 2 relative to the rotation direction of the photoreceptor 2 where charging is performed by the charging roller 3 is the charging position. The charging roller 3 charges the surface of the photosensitive member 2 by discharging in at least one of the minute gaps formed between the photosensitive member 2 and the charging roller 3 on the upstream and downstream sides of the contact point between the photosensitive member 2 and the charging roller 3 in the rotation direction of the photosensitive member 2. However, the position on the photosensitive member 2 where the charging roller 3 contacts in the rotation direction of the photosensitive member 2 may be considered as the charging position.

[0018] The surface of the charged photoreceptor 2 is scanned and exposed according to image information by a laser scanner (exposure device) 4 serving as exposure means. The laser scanner 4 outputs laser light L modulated according to time-series electric digital pixel signals of image information input to the image forming apparatus 1 from an external device 200. The laser scanner 4 then scans and exposes the charged surface of the photoreceptor 2 with the laser light L. As a result, an electrostatic latent image (electrostatic image) according to the image information is formed on the photoreceptor 2.

[0019] The electrostatic latent image formed on the photoreceptor 2 is developed (visualized, visualized) by the developing device 5, which serves as a developing means, by supplying toner as developer, and a toner image (toner image, developer image) is formed on the photoreceptor 2. In this embodiment, the developing device 5 deposits toner charged with the same polarity as the charge polarity of the photoreceptor 2 (negative in this embodiment) to the exposed portion (image portion) of the photoreceptor 2, which has been uniformly charged and then exposed to light to reduce the absolute value of the potential (reverse development method). In this embodiment, during development, a predetermined developing voltage (developing bias), which is a negative DC voltage, is applied to the developing roller of the developing device 5, which will be described later. In this embodiment, the normal charging polarity (normal polarity) of the toner, which is the charge polarity of the toner during development, is negative. In addition, in this embodiment, the developing device 5 uses a non-magnetic one-component developer. However, the developing device 5 may also use a magnetic one-component developer or a two-component developer comprising toner and carrier. The position in the rotation direction of the photosensitive member 2 where the electrostatic latent image on the photosensitive member 2 is developed by the developer 5 (in this embodiment, the position on the photosensitive member 2 where the developing roller comes into contact) is the development position.

[0020] A transfer roller 8, a rotatable roller-shaped transfer member (transfer rotor) serving as a transfer means, is disposed opposite the photoreceptor 2. In this embodiment, the transfer roller 8 is an elastic roller with an outer diameter of 14 mm, which is made of a 5 mm outer diameter stainless steel (SUS) core metal and a 4.5 mm thick sponge-like elastic layer made of NBR (acrylonitrile butadiene) hydrin. In this embodiment, the transfer roller 8 is pressed against the photoreceptor 2, forming a transfer nip (transfer portion) N, which is the contact portion between the surface (outer peripheral surface) of the photoreceptor 2 and the surface (outer peripheral surface) of the transfer roller 8. The transfer roller 8 rotates in accordance with the rotation of the photoreceptor 2. The toner image on the photoreceptor 2 is sent to the transfer nip N by the rotation of the photoreceptor 2. The transfer position is the position where the toner image is transferred to the recording material P on the photosensitive member 2 in relation to the rotation direction of the photosensitive member 2 (in this embodiment, the position where the toner image comes into contact with the transfer roller 8 on the photosensitive member 2), and corresponds to the position on the photosensitive member 2 where the transfer nip portion N is formed.

[0021] Sheet-like recording materials P, such as recording paper, loaded on a sheet stacking tray 9a of a paper feed cassette 9 are picked up one by one by a paper feed roller 10 driven at a predetermined control timing and fed to a registration section by a pair of conveying rollers 11. At the registration section, the leading edge of the recording material P is temporarily received in a nip portion between a registration roller 12 and a roller 12a, where skew of the recording material P is corrected. A registration sensor 13 serving as a recording material detection means is disposed in the registration section downstream of the registration roller 12 and the roller 12a in the conveying direction of the recording material P. The registration sensor 13 detects the arrival timing of the leading and trailing edges of the recording material P. The recording material P is then fed from the registration section to a transfer nip portion N. The recording material P fed to the transfer nip portion N is sandwiched between the photosensitive element 2 and a transfer roller 8 and conveyed. As the recording material P is transported through the transfer nip portion N, a predetermined transfer voltage (transfer bias), which is a DC voltage of the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the transfer roller 8 by the transfer voltage application unit E3 (Figure 3), which will be described later, and the toner image on the photosensitive member 2 is transferred to the recording material P.

[0022] The recording material P separated from the surface of the photoreceptor 2 is conveyed along a conveyance guide 14 to a fixing device 15 serving as a fixing means. The fixing device 15 has a fixing rotor 15a such as a fixing film, and a pressure member 15b such as a pressure roller that is in pressure contact with the fixing rotor 15a. The fixing device 15 applies heat and pressure to the recording material P carrying an unfixed toner image in a fixing nip portion between the fixing rotor 15a and the pressure rotor 15b, thereby fixing the toner image to the recording material P. After the toner image has been fixed, the recording material P is discharged from the fixing nip portion of the fixing device 15 and conveyed by discharge rollers 16. The discharge rollers 16 discharge (output) the recording material P onto a discharge tray 17 provided outside the main body of the image forming apparatus 1.

[0023] On the other hand, deposits such as toner (transfer residual toner) remaining on the surface of the photoreceptor 2 after the recording material P is separated are removed and collected from the surface of the photoreceptor 2 by a cleaner 6 serving as a photoreceptor cleaning means. As a result, images are repeatedly formed on the photoreceptor 2.

[0024] During a series of image forming operations, there is a time when no recording material P is present in the transfer nip N, a so-called "non-paper passing time." This "non-paper passing time" corresponds to the following timings: First, at the start of the image forming operation, during preparatory state (pre-rotation) until each component is ready for image formation. Second, during a situation where multiple recording materials P are continuously transported during the image forming operation, the timing between recording materials P (sheet interval) corresponds to this timing. Third, during operation stop processing (post-rotation) after the series of image forming operations is completed, the timing corresponds to this timing. At these timings, a small amount of toner, known as "fog toner," generated on the surface of the photoreceptor 2 may be transferred to the surface of the transfer roller 8. Therefore, the image forming apparatus 1 of this embodiment performs a cleaning operation (cleaning sequence) to clean toner, such as the fog toner, adhering to the transfer roller 8 during operation stop processing (post-rotation) after the series of image forming operations is completed, which is a "non-paper passing time." In the cleaning operation, a predetermined cleaning voltage (cleaning bias), which is a DC voltage of the same polarity (negative in this embodiment) as the normal charging polarity of the toner, is applied to the transfer roller 8. This causes toner such as the fog toner adhering to the transfer roller 8 to be transferred (reverse transferred) onto the photoreceptor 2. The toner transferred onto the photoreceptor 2 is removed from the photoreceptor 2 and collected by the cleaner 6. The "fog toner" will be explained further below.

[0025] The image forming apparatus 1 of this embodiment has a print speed of 55 sheets per minute (for letter-size paper) and a process speed (corresponding to the peripheral speed of the photosensitive member 2) of approximately 300 mm / s.

[0026] Next, the configuration of the image forming unit (the photoreceptor 2 and the process means acting on the photoreceptor 2) in the image forming apparatus 1 of this embodiment will be further described with reference to Fig. 2. Fig. 2 is a schematic cross-sectional view showing the configuration of the image forming unit of the image forming apparatus 1 of this embodiment.

[0027] A predetermined charging voltage (charging bias), which is a DC voltage of the same polarity (negative in this embodiment) as the normal charging polarity of the toner, is applied to the charging roller 3 by a charging voltage application unit E1 (FIG. 3) described below, and the surface of the photoreceptor 2 is uniformly charged. In this embodiment, during charging, a charging voltage of approximately −1000 V is applied to the charging roller 3 so that the surface potential of the photoreceptor 2 becomes −500 V. The surface potential (charging potential) of the photoreceptor 2 formed by charging by the charging roller 3 is called the “dark potential Vd.”

[0028] The laser scanner 4 scans and exposes the charged surface of the photoreceptor 2 with laser light L, removing the charge on the surface of the photoreceptor 2 and forming an electrostatic latent image on the surface of the photoreceptor 2. The surface potential of the photoreceptor 2 at the point exposed by the laser scanner 4 is called the "light area potential VL." In this embodiment, the light emission amount of the laser scanner 4 is adjusted so that the light area potential VL is -100V.

[0029] The developing device 5 has a developing roller 21 as a developer carrier, a developing blade 22 as a regulating member, a supply roller 23 as a supply member, a storage chamber 24 for storing toner, and toner stored in the storage chamber 24 as a developer. In this embodiment, non-magnetic spherical toner with a normal negative charge polarity and an average particle size of 7 μm is used as the toner. In this embodiment, silica particles (external particles) with an average particle size of 20 nm are added (externally added) to the surface of the toner as an external additive.

[0030] The developing blade 22 is composed of a plate-like member that is generally rectangular in plan view and has a predetermined length in both a longitudinal direction, which is disposed substantially parallel to the rotational axis of the developing roller 21, and a predetermined thickness in a transverse direction, which is substantially perpendicular to the longitudinal direction. The developing blade 22 abuts against the surface (outer peripheral surface) of the developing roller 21 in a counter-direction relative to the rotational direction of the developing roller 21. In other words, the developing blade 22 abuts against the developing roller 21 such that its free end, which is the other end of the transverse direction, is located upstream of its fixed end, which is the other end of the transverse direction, in the direction of rotation of the developing roller 21. The developing blade 22 regulates the coating amount of toner supplied onto the developing roller 21 by the supply roller 23 and imparts a charge to the toner. In this embodiment, the developing blade 22 is composed of a relatively thin plate-like member (thin plate), and the spring elasticity of this thin plate is utilized to generate a contact pressure against the developing roller 21. The surface of the developing blade 22 facing the developing roller 21 contacts the toner and the developing roller 21. In this embodiment, a 0.1 mm thick, leaf-spring-shaped thin plate made of stainless steel and coated with semiconductive resin was used as the developing blade 22. However, the developing blade 22 is not limited to that of this embodiment, and a thin metal plate made of phosphor bronze, aluminum, or the like may be used instead of the stainless steel. Furthermore, a semiconductive rubber or a thin metal plate with no surface coating may be used instead of the semiconductive resin.

[0031] In this embodiment, during development, a regulating member voltage application unit (not shown) applies to the developing blade 22 a predetermined regulating member voltage (regulating member bias), which is a DC voltage of the same polarity (negative in this embodiment) as the normal charging polarity of the toner. As a result, the toner is given a negative charge due to discharge between the developing blade 22 and the developing roller 21 and frictional charging caused by rubbing between the developing blade 22 and the developing roller 21. At the same time, the layer thickness of the toner on the developing roller 21 is regulated by the developing blade 22. In this embodiment, during development, the regulating member voltage application unit applies a regulating member voltage to the developing blade 22 so that the potential difference obtained by subtracting the potential of the developing blade 22 from the potential of the developing roller 21 is −100 V. In other words, during development, the regulating member voltage application unit applies to the developing blade 22 a regulating member voltage of the same polarity as the developing voltage but with an absolute value greater than that of the developing voltage.

[0032] The supply roller 23 is disposed in contact with the developing roller 21, forming a predetermined nip between the surface (outer periphery) of the developing roller 21 and the surface (outer periphery) of the supply roller 23. The supply roller 23 rotates counterclockwise in FIG. 2. In this embodiment, the supply roller 23 is an elastic sponge roller having an elastic layer made of elastic foam formed on the outer periphery of a conductive core. The supply roller 23 and the developing roller 21 are in contact with each other with a predetermined penetration amount. Furthermore, the supply roller 23 and the developing roller 21 rotate so as to move in the same direction at the contact portion. In this embodiment, the supply roller 23 is driven to rotate by a driving force branched and transmitted from a driving source that drives the photoreceptor 2. The supply roller 23 supplies toner to the developing roller 21 and scrapes off toner remaining on the developing roller 21 after development. The amount of toner supplied to the developing roller 21 can be adjusted by adjusting the potential difference between the supply roller 23 and the developing roller 21. In this embodiment, during development, a supply member voltage application unit (not shown) applies to the supply roller 23 a predetermined supply member voltage (supply member bias), which is a DC voltage of the same polarity (negative in this embodiment) as the normal charging polarity of the toner. In this embodiment, during development, the supply member voltage application unit applies a supply member voltage to the supply roller 23 so that the potential difference obtained by subtracting the potential of the supply roller 23 from the potential of the development roller 21 is -100 V. In other words, during development, the supply member voltage application unit applies to the supply roller 23 a supply member voltage of the same polarity as the development voltage but with an absolute value greater than the absolute value of the development voltage.

[0033] In this embodiment, the developing roller 21 is a roller in which an elastic layer made of a conductive rubber material is formed around a conductive core metal. The toner contained in the toner storage chamber 24 is taken up by the sponge portion of the supply roller 23 and transported to the developing roller 21. In this embodiment, the developing roller 21 and the supply roller 23 both have an outer diameter of 20 mm, and the intrusion amount of the supply roller 23 into the developing roller 21 is set to 1.5 mm. The developing roller 21 and the photosensitive element 2 rotate so that they move in the same direction at their opposing portions (contact portions). In this embodiment, the developing roller 21 is driven to rotate by a driving force branched from and transmitted from a driving source that drives the photosensitive element 2. During development, a predetermined developing voltage (developing bias), which is a DC voltage of the same polarity (negative in this embodiment) as the normal charging polarity of the toner, is applied to the developing roller 21 by a developing voltage application unit E2 (FIG. 3), which will be described later. In the development nip portion (development portion) where the development roller 21 contacts the photosensitive member 2, the potential difference between the development roller 21 and the photosensitive member 2 causes negatively charged toner to transfer to the image portion of the electrostatic latent image on the photosensitive member 2, thereby developing the electrostatic latent image. In this embodiment, during development, a development voltage of −350 V is applied to the development roller 21 by the development voltage application portion E2.

[0034] The developing roller 21, the developing blade 22, and the supply roller 23 each constitute a developing member for attaching toner to the electrostatic latent image on the photosensitive member 2 to form a toner image.

[0035] A predetermined transfer voltage (transfer bias), which is a DC voltage of the opposite polarity (positive in this embodiment) to the normal charging polarity of the toner, is applied to the transfer roller 8 by a transfer voltage application unit E3 (described later), thereby transferring the toner image on the photoreceptor 2 onto the recording material P. In the image forming apparatus 1 of this embodiment, a constant current circuit (not shown) is used to control (adjust) the transfer voltage so that the current supplied to the transfer roller 8 from the transfer voltage application unit E3 (described later) is approximately 16 μA. In this embodiment, a transfer roller 8 with an electrical resistance of 7.8 Log Ω was used. The electrical resistance of the transfer roller 8 was measured as follows. Specifically, the transfer roller 8 was rotated at a peripheral speed of approximately 120 mm / sec under a normal temperature and humidity environment (23°C / 50% RH) while being pressed against an electrically grounded aluminum drum with a load of 400 gf. The electrical resistance was then calculated from the current value measured when a voltage of 2.0 kV was applied to the core of the transfer roller 8.

[0036] The configuration of each member and the control voltage value are not limited to those described above, and can be changed (selected) as appropriate as long as the same function is obtained.

[0037] In addition, in this embodiment, the photosensitive element 2, the charging roller 3 as a process means acting on the photosensitive element 2, the developing device 5, and the cleaner 6 are integrally formed into a process cartridge 20 that can be attached to and detached from the main body of the image forming device 1.

[0038] FIG. 3 is a schematic block diagram showing the control mode of the main parts of the image forming apparatus 1 of this embodiment. The image forming apparatus 1 is provided with a control unit 100 for controlling the operation of the image forming apparatus 1. The control unit 100 is configured with a CPU 101 as an arithmetic control means which is a central element for performing arithmetic processing, a memory (storage medium) 102 such as ROM or RAM as a storage means, and an input / output unit (not shown) that controls the exchange of signals between the control unit 100 and each part outside the control unit 100. The RAM, which is a rewritable memory, stores information input to the control unit 100, detected information, arithmetic results, etc., while the ROM stores control programs, pre-calculated data tables, etc. The CPU 101 and the memory 102 such as ROM or RAM can transfer and read data to and from each other. The control unit 100 comprehensively controls each part of the image forming apparatus 1 to perform image formation. In addition, as will be described later, the control unit 100 can control the transfer roller 8 to perform a cleaning operation when there is no recording material P in the transfer nip N, by applying a voltage of the same polarity as the normal charging polarity of the toner to the transfer roller 8, thereby moving the toner from the transfer roller 8 to the photosensitive element 2.

[0039] Here, the image forming apparatus 1 executes a print job (printing operation) that is a series of operations that starts with a single start command and forms and outputs an image on one or multiple recording materials P. A print operation generally includes an image formation process, a pre-rotation process, a sheet-to-sheet process when forming images on multiple recording materials P, and a post-rotation process. The image formation process is a period during which an electrostatic latent image of the image to be actually formed and output on the recording material P is formed, a toner image is formed, and the toner image is transferred. This period is referred to as the image formation time. More specifically, the timing of the image formation time differs depending on the positions where the electrostatic latent image formation, toner image formation, and toner image transfer processes are performed, and corresponds to the period during which the image formation area on the photoreceptor 2 passes through each of the above positions. The pre-rotation process is a period from when a start command is input until the actual start of image formation, during which preparatory operations are performed before the image formation process. The sheet-to-sheet process (inter-image process, inter-recording material process) is a period corresponding to the period between recording materials P when image formation is performed continuously on multiple recording materials P (continuous printing, continuous image formation). The post-rotation process is a period during which a tidying operation (preparatory operation) is performed after the image formation process. Non-image formation time refers to a period other than image formation time, and includes the pre-rotation process, the sheet interval process, the post-rotation process, and the pre-multiple rotation process, which is a preparatory operation when the image forming apparatus 1 is turned on or when the image forming apparatus 1 returns from a sleep state. More specifically, the timing during non-image formation time corresponds to the period during which the non-image formation area on the photoreceptor 2 passes through each position where the electrostatic latent image formation, toner image formation, and toner image transfer processes are performed. The image formation area on the photoreceptor 2 or the recording material P is an area where a toner image, which is predetermined according to the size of the recording material P and output from the image forming apparatus 1, can be formed, and the non-image formation area is an area other than the image formation area.

[0040] (2) Circuit configuration Next, the high-voltage circuit configuration in this embodiment, which outputs the developing voltage and the cleaning voltage from a common power source, will be described with reference to Fig. 4. Fig. 4 is an explanatory diagram of the high-voltage circuit configuration in this embodiment.

[0041] First, a first boost circuit (power supply) 50, which is composed of a transformer or the like, generates a negative transfer voltage (cleaning voltage) Vtrn as a voltage of a first polarity. A second boost circuit (another power supply) 51, which is also composed of a transformer or the like, generates a positive transfer voltage Vtrp as a voltage of a second polarity opposite to the first polarity. During image formation (transfer), a transfer voltage Vtr, which is the sum (superimposition) of the negative transfer voltage (cleaning voltage) Vtrn and the positive transfer voltage Vtrp, is applied to the transfer roller 8. A voltage application unit (voltage application means) that applies a cleaning voltage (negative transfer voltage) to the transfer roller 8 using the first boost circuit 50 as a power source, is referred to as a "cleaning voltage application unit (or second transfer voltage application unit)" E4 (FIG. 3). Furthermore, a voltage application unit (voltage application means) that applies a transfer voltage (positive transfer voltage) to the transfer roller 8 using the second boost circuit 51 (and further the first boost circuit 50) as a power source is referred to as a "transfer voltage application unit (or first transfer voltage application unit)" E3. Here, in this embodiment, the first boost circuit 50 is a relatively inexpensive open-loop control. Therefore, the first boost circuit 50 has a characteristic that the absolute value of the negative transfer voltage (cleaning voltage) Vtrn decreases as the load becomes heavier.

[0042] The developing voltage Vdev is generated by dividing the 24V negative transfer voltage (cleaning voltage) Vtrn between resistor 52 and transistor 53. In this embodiment, to accurately control the developing voltage Vdev, the developing voltage Vdev is fed back to control the conduction of transistor 53. The high-voltage circuit configuration in this embodiment is configured so that when transistor 53 is on, the load on the first boost circuit 50 is heavier than when it is off. In other words, in this embodiment, increasing the absolute value of the developing voltage Vdev also increases the absolute value of the negative transfer voltage (cleaning voltage) Vtrn, and decreasing the absolute value of the developing voltage Vdev also decreases the absolute value of the negative transfer voltage (cleaning voltage) Vtrn. Therefore, in this embodiment, the negative transfer voltage (cleaning voltage) Vtrn can be changed by adjusting the developing voltage Vdev. A voltage application unit (voltage application means) that applies the developing voltage to the developing roller 21 using the first boost circuit 50 as a power source is referred to as a "developing voltage application unit" E2.

[0043] In this embodiment, the charging voltage Vpri is generated by an independent third boost circuit (another power source) 54. A voltage application unit (voltage application means) that applies a charging voltage to the charging roller 3 using the third boost circuit 54 as a power source is called a "charging voltage application unit" E1.

[0044] Next, we will explain why the developing voltage application unit E2 was selected as the voltage application unit sharing a common power source with the cleaning voltage application unit E4 in this embodiment. In other words, we will explain why the developing voltage was selected as the voltage sharing a common power source with the cleaning voltage application unit E4. As described above, when changing the cleaning voltage in this embodiment, control is performed to change the output voltage value of the voltage application unit sharing a common power source with the cleaning voltage application unit E4, i.e., the developing voltage. In other words, in this embodiment, the cleaning voltage (negative transfer voltage) during the cleaning operation is controlled (adjusted) by changing the output voltage value of the voltage application unit sharing a common power source with the cleaning voltage application unit E4. Meanwhile, the principle of the cleaning operation is to electrostatically transfer toner adhering to the transfer roller 8 to the photoconductor 2 by using the potential difference between the potential of the transfer roller 8 (the cleaning voltage applied to the transfer roller 8) and the surface potential of the photoconductor 2. Here, we will assume that the charging voltage application unit E1 is selected as the voltage application unit sharing a common power source with the cleaning voltage application unit E4. In this case, when the cleaning voltage is changed during the cleaning operation, the charging voltage is also changed. In other words, in this case, not only is the intended cleaning voltage changed, but the charging voltage is also changed. When the charging voltage is changed, the surface potential of the photosensitive member 2 is changed. As a result, the potential difference between the potential of the transfer roller 8 and the surface potential of the photosensitive member 2 is also changed. In other words, during the cleaning operation, both the cleaning voltage and the surface potential of the photosensitive member 2 are changed. As a result, in some cases, the potential difference between the potential of the transfer roller 8 and the surface potential of the photosensitive member 2 may not be the desired potential difference, which may result in ineffective cleaning of the transfer roller 8 or require a relatively long time to clean the transfer roller 8. Therefore, in this embodiment, from the perspective of enabling stable cleaning of the transfer roller 8, the development voltage application unit E2 is selected as the voltage application unit that shares a power supply with the cleaning voltage application unit E4.

[0045] The relationship between the developing voltage and the cleaning voltage in this embodiment will be explained using FIG. 5. FIG. 5 is a graph showing the relationship between the developing voltage and the cleaning voltage in this embodiment. As described above, in this embodiment, it is possible to change the cleaning voltage by adjusting the developing voltage. As can be seen from FIG. 5, in this embodiment, if the developing voltage is set to, for example, −350 V, which is the developing voltage during image formation (development), a cleaning voltage of approximately −600 V is applied to the transfer roller 8. Furthermore, if the developing voltage is changed to −380 V during the cleaning operation, for example, a cleaning voltage of approximately −780 V, which is more advantageous for cleaning the transfer roller 8, is applied to the transfer roller 8.

[0046] The high-voltage circuit configuration that can be used in this embodiment is not limited to the high-voltage circuit configuration in Fig. 4, and can be changed as appropriate as long as it has a circuit with similar functions. Furthermore, the relationship between the developing voltage and the cleaning voltage is not limited to the relationship in Fig. 5, and can be changed depending on the electrical resistance values ​​of each component on the circuit, the performance of the boost circuit, etc.

[0047] (3) Fog toner and developing voltage settings Next, the relationship between the fog toner and the set value of the developing voltage in this embodiment will be described.

[0048] First, we will explain fog toner. "Fog toner" refers to toner that transfers from the developing device 5 to the dark potential Vd portion of the photoconductor 2. Fog toner can occur for the following reasons: For example, frictional charging caused by the toner on the developing roller 21 rubbing against the photoconductor 2 can cause a decrease in the charge amount of some toner particles or a shift in the charge polarity opposite to the normal charge polarity (positive polarity in this embodiment). Another example is when the toner in the storage chamber 24 deteriorates due to wear of the developing device 5, causing the toner to lose its chargeability, resulting in the toner no longer being able to maintain the normal charge amount on the developing roller 21 or becoming charged to the polarity opposite to the normal charge polarity (positive polarity in this embodiment). Thus, the presence of (1) toner with a decreased charge amount and (2) toner charged to the polarity opposite to the normal polarity can easily lead to fog toner.

[0049] Next, we will explain the mechanism by which (1) toner with a reduced charge amount and (2) toner charged with a polarity opposite to the normal polarity transfer to the dark potential Vd area of ​​the photosensitive element 2 as fog toner, in relation to the set value of the development voltage.

[0050] In this embodiment, during image formation, the development voltage is set to -350 V and the dark potential Vd is set to -500 V. In addition, in this embodiment, the normal charge polarity of the toner present on the development roller 21 is negative. Therefore, toner with a normal charge polarity and charge amount is affected by the electric field between the development roller 21 and the photoconductor 2 in the development nip portion and is electrostatically attracted toward the development roller 21. Due to this influence, toner with a normal charge polarity and charge amount does not transfer to the dark potential Vd portion of the photoconductor 2, or even if it does, it is only a very small amount.

[0051] On the other hand, (1) the force with which toner with a reduced charge amount is electrostatically attracted to the developing roller 21 is relatively weaker than that of toner with a normal charge amount. Under these conditions, if the absolute value of the developing voltage is increased, for example to -400 V, the force of electrostatic attraction to the developing roller 21 is further reduced. In this case, some of the toner on the developing roller 21 may be peeled off toward the photoreceptor 2 due to physical friction with the photoreceptor 2, and may end up being transferred onto the photoreceptor 2. Furthermore, the amount of this transfer (the amount of fog toner generated on the photoreceptor 2) tends to increase as the absolute value of the developing voltage increases. The fog toner that occurs when the absolute value of the developing voltage is increased in this way is called "background fog toner."

[0052] Furthermore, (2) toner charged with a polarity opposite to the normal polarity is subjected to an electrostatic force that attracts it toward the photoconductor 2 due to the influence of the electric field between the developing roller 21 and the photoconductor 2. Furthermore, if the absolute value of the developing voltage is reduced, for example to -300 V, the force attracting it toward the photoconductor 2 due to the electrostatic force increases. If this electrostatic force increases to the extent that it overcomes the non-electrostatic adhesive force generated between the toner and the developing roller 21, the toner will be transferred to the photoconductor 2 as fog toner. Furthermore, the amount of this transfer (the amount of fog toner generated on the photoconductor 2) tends to increase as the absolute value of the developing voltage decreases. The fog toner that occurs when the absolute value of the developing voltage is reduced in this way is called "reverse fog toner."

[0053] FIG. 6 is a graph showing the relationship between the set value of the development voltage and the amount of fog toner transferred onto the photosensitive member 2 (hereinafter simply referred to as "fog toner amount") when the dark potential Vd is fixed at -500 V in the image forming apparatus 1 configured in this embodiment.

[0054] The amount of fog toner was measured using the following procedure. First, a solid white image, without forming an electrostatic latent image, was selected as the image to be printed, and image formation was initiated. Then, before the recording material P reached the transfer nip N, the rotation of the photoreceptor 2 was stopped, leaving fog toner on the photoreceptor 2. Next, the fog toner remaining on the photoreceptor 2 was adhered to an adhesive tape (Scotch Mending Tape, manufactured by Sumitomo 3M). This adhesive tape with the fog toner collected was attached to a white paper (product name GF-C081, manufactured by Canon Inc.). For comparison, an adhesive tape without fog toner was also attached to the same paper. Using a "REFLECTMETER MODEL TC-6DS" (manufactured by Tokyo Denshoku Co., Ltd.), the whiteness (reflectance D1 (%)) of the adhesive tape portion with the fog toner collected and the whiteness (reflectance D2 (%)) of the adhesive tape portion without the fog toner collected were measured. Then, the fog density (%) (=D2(%)-D1(%)) was calculated from the difference. The fog density (%) can represent the amount of fog toner.

[0055] From Figure 6, it can be seen that the amount of fog toner increases when the absolute value of the development voltage is increased from -350V, which is the set value during image formation. Note that the fog toner under this condition corresponds to the "background fog toner" described above. From Figure 6, it can also be seen that the amount of fog toner increases when the absolute value of the development voltage is decreased from -350V, which is the set value during image formation. Note that the fog toner under this condition corresponds to the "reverse fog toner" described above.

[0056] In this embodiment, the conditions under which fog toner is relatively unlikely to occur, i.e., the amount of fog toner at the beginning of a durability test when toner deterioration is not advanced, have been described using FIG. 6. A configuration that assumes the amount of fog toner at the end of a durability test when toner deterioration has progressed will be described in another embodiment, which will be described later. Here, "early durability test" and "undurable test" refer to the beginning of the life of the developing device 5 (toner in the storage chamber 24) or a new product state, and specifically correspond to the beginning of or before the start of a durability test, as will be described later. Furthermore, "post-durability test" refers to the end of the life of the developing device 5 (toner in the storage chamber 24) or a state where the device has reached the end of its life, and specifically correspond to the end of or after the completion of a durability test, as will be described later.

[0057] (4) Cleaning operation Next, the cleaning operation in this embodiment will be further described with reference to Figure 7. In this embodiment, the image forming apparatus 1 performs a cleaning operation at the timing after the last recording material P of one print job has passed through the transfer nip portion N, that is, during post-rotation after the transfer (image formation) of the toner image from the photosensitive member 2 to the recording material P has been completed. Note that in this embodiment, the image forming apparatus 1 is configured so that the photosensitive member 2 and the developing roller 21 are in constant contact with each other to form the developing nip portion.

[0058] 7 is a timing chart showing the operating states of each part at the timing of image formation (printing) on ​​the last recording material P of one print job and post-rotation after image formation is completed. In this embodiment, the control unit 100 controls the operation of the print job in accordance with the timing chart shown in FIG. 7. FIG. 7 shows the states of the charging voltage, light emission of the laser scanner 4, surface potential of the photosensitive member 2, development voltage, positive transfer voltage, and negative transfer voltage (cleaning voltage). Note that for the development voltage and negative transfer voltage (cleaning voltage), the set values ​​during image formation are denoted as "for image formation," and the set values ​​during cleaning operation are denoted as "for cleaning."

[0059] First, the operation of each component during image formation will be described. During image formation, the charging voltage is turned ON, and the surface of the photoconductor 2 is charged to a dark potential Vd. Furthermore, the laser scanner 4 emits light ON / OFF in response to image information, forming an electrostatic latent image on the photoconductor 2. As a result, a light potential VL is formed partially on the surface of the photoconductor 2. A developing voltage Vdev for image formation is applied to the developing roller 21, and a toner image is formed on the photoconductor 2. A transfer voltage Ttr, which is a combination of a positive transfer voltage Vtrp and a negative transfer voltage Vtrn for image formation, is applied to the transfer roller 8, and the toner image on the photoconductor 2 is transferred onto the recording material P. The transfer voltage Vtr has a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment). In other words, in this embodiment, the developing voltage Vdev and the negative transfer voltage Vtrn are output from a common power source, the first boost circuit 50. Therefore, during image formation, a transfer voltage Vtr, which is a superposition of a positive transfer voltage Vtr and a negative transfer voltage Vtrn for image formation, is applied to the transfer roller 8. In this embodiment, the transfer voltage Vtr is constant current controlled, with its target current value being 16 μA. During image formation, a positive voltage whose absolute value is greater than that of the negative transfer voltage Vtrn is applied as the positive transfer voltage Vtrp. In this embodiment, the control means 100 controls the transfer voltage Vtr to be constant current controlled by adjusting the positive transfer voltage output by the second boost circuit 51 so that the current flowing through the transfer roller 8, detected by a current detection circuit serving as current detection means, approaches the target current value.

[0060] Next, the operation of each component during the cleaning operation performed during post-rotation will be described. As described above, in this embodiment, changing the development voltage allows the cleaning voltage to be changed accordingly. During post-rotation, the development voltage Vdev is changed from the set value for image formation to the set value for cleaning. The positive transfer voltage Vtrp is also turned off. This operation aims to change the negative transfer voltage (cleaning voltage) Vtrn to a cleaning set value that more effectively cleans the transfer roller 8 by changing the development voltage Vdev. In other words, most toner adhering to the transfer roller 8 is negatively charged, which is the normal charging polarity. Therefore, applying a negative cleaning voltage with a large absolute value to the transfer roller 8 exerts a strong electrostatic force on the toner adhering to the transfer roller 8, thereby facilitating the transfer of the toner adhering to the transfer roller 8 to the photosensitive element 2. After the cleaning operation (application of a cleaning voltage to the transfer roller 8) is performed for a certain period of time during post-rotation, the operation of the image forming apparatus 1 (rotation of the rotating members, application of voltage) is terminated.

[0061] Here, the reason why the charging voltage is kept ON even during post-rotation in this embodiment will be explained. This is because, when a developing voltage is applied to the developing roller 21 without a charging voltage being applied to the charging roller 3, the potential of the developing roller 21 becomes higher than the surface potential of the photosensitive member 2 toward the normal charging polarity of the toner (negative polarity in this embodiment). In this state, the toner on the developing roller 21 is electrostatically transferred to the photosensitive member 2 due to the influence of the electric field between the developing roller 21 and the photosensitive member 2. In this case, unnecessary toner is used. Furthermore, in this case, some of the toner on the photosensitive member 2 may transfer to the transfer roller 8, contaminating the transfer roller 8. To prevent this from happening, the charging voltage remains ON even during post-rotation in this embodiment.

[0062] In this embodiment, the cleaning operation (application of cleaning voltage to transfer roller 8) during post-rotation is performed for approximately 0.6 seconds, which corresponds to four revolutions of transfer roller 8, and then the operation of image forming apparatus 1 (rotation of rotating members, application of voltage) is terminated. The setting value of the cleaning voltage in this embodiment will be further explained in the next section (5).

[0063] In this embodiment, the cleaning operation is performed during post-rotation, but the present invention is not limited to this. The cleaning operation can be performed at any timing as long as it is not during image formation. That is, the cleaning operation may be performed, for example, during pre-rotation before image formation begins, or may be performed between sheets of paper when no recording material P is present in the transfer nip N during continuous printing. Furthermore, the cleaning operation may be performed by predicting or detecting that toner has adhered to the transfer roller 8, for example, after a recording material P has jammed.

[0064] (5) Image output experiment results In this embodiment, when paper is not passing through (more specifically, when neither the development position nor the transfer position is in image formation mode), the set value of the development voltage is changed from the set value during image formation (development), and the cleaning voltage is controlled (adjusted) to a set value suitable for cleaning the transfer roller 8. At this time, the cleaning performance of the transfer roller 8 is affected by the set value of the cleaning voltage. Also, as described above, the amount of fog toner changes depending on the set value of the development voltage. Therefore, it is desirable to adjust the development voltage taking into consideration both the cleaning performance of the transfer roller 8 and the amount of fog toner during the cleaning operation.

[0065] First, the relationship between the development voltage and the cleaning performance of the transfer roller 8 will be described with reference to Fig. 8. Fig. 8 is a graph showing the experimental results of the cleaning performance when the development voltage (and cleaning voltage) during the cleaning operation is changed in the image forming apparatus 1 having the configuration of this embodiment.

[0066] The experiment was divided into two parts: "preliminary paper feed" in which toner stains were attached to the transfer roller 8, and "paper feed for evaluating backside stains" in which the backside stains of the paper were evaluated after the cleaning operation was performed.

[0067] The preliminary paper feed was performed under the following conditions: No cleaning operations that could be performed between sheets were performed, and 1,000 sheets of solid white images were continuously printed on one side to deposit toner stains on the transfer roller 8. A cleaning operation was performed only once during post-rotation after the continuous printing was completed, and then the operation of the image forming apparatus 1 was terminated. In addition, the development voltage during the cleaning operation was changed from -350 V, at which the amount of background fogging toner was the smallest, to the level shown in Figure 8, in the direction of increasing the absolute value of the development voltage.

[0068] The paper back stain evaluation was performed under the following conditions. After the above preliminary paper feed, one sheet of a solid white image was printed on one side without performing any cleaning operations that could be performed during pre-rotation, and the degree of paper back staining was measured. The degree of paper back staining was measured as follows. A "REFLECTMETER MODEL TC-6DS" (manufactured by Tokyo Denshoku Co., Ltd.) was used for the measurement. The whiteness (reflectance D1 (%)) of the area where paper back staining occurred and the whiteness (reflectance D2 (%)) of the area where paper back staining did not occur were measured. The paper back stain concentration (%) (= D2 (%) - D1 (%)) was then calculated from the difference. The degree of paper back staining can be expressed by this paper back stain concentration (%). The degree of paper back staining was also determined by visual judgment.

[0069] In addition, as a common condition for the preliminary paper feed and the paper feed for evaluating backside soiling, the experiment was conducted under normal temperature and humidity conditions (for example, under a room temperature and humidity (23°C / 50%RH) environment), and GF-C081 (A4 size paper, manufactured by Canon Inc., product name) was used as the recording material P.

[0070] The results in FIG. 8 show that with the configuration of this embodiment, paper back staining is most improved when the development voltage is about -380V. The results in FIG. 8 also show that with the configuration of this embodiment, paper back staining tends to worsen slightly under conditions where the development voltage is greater in absolute value than about -400V, and under conditions where the development voltage is smaller in absolute value than about -360V. Corresponding to these three conditions, the region where the development voltage is near -380V is designated Region B. Furthermore, the region where the development voltage is greater in absolute value than -400V is designated Region A. Furthermore, the region where the development voltage is smaller in absolute value than -360V is designated Region C.

[0071] In region C, a developing voltage with a relatively small absolute value is applied to the developing roller 21. As explained using Figure 5, in the configuration of this embodiment, when the absolute value of the developing voltage is small, the absolute value of the cleaning voltage tends to be small. As a result, a cleaning voltage sufficient to clean the toner that had adhered to the transfer roller 8 during the preliminary paper feed was not applied to the transfer roller 8 during the cleaning operation, and the toner remaining on the transfer roller 8 manifested as paper back stains when the paper was fed for paper back stain evaluation.

[0072] On the other hand, in region A, a developing voltage with a relatively large absolute value is applied to the developing roller 21. Therefore, during the cleaning operation, a cleaning voltage with a large absolute value that is advantageous for cleaning the transfer roller 8 is applied to the transfer roller 8. However, as explained using FIG. 6, the condition under which a developing voltage with a relatively large absolute value is applied to the developing roller 21 is also a condition under which background fogging toner is likely to be transferred onto the photosensitive member 2. Therefore, the background fogging toner generated on the photosensitive member 2 during the cleaning operation is transferred to the transfer roller 8 mainly due to physical adhesion, and this becomes apparent as paper back staining when the paper is subsequently passed through to evaluate paper back staining.

[0073] In contrast, in region B, as in region C, the amount of fog toner on the photosensitive member 2 is relatively small, and as in region A, a cleaning voltage with a relatively large absolute value is applied to the transfer roller 8. Therefore, region B can be said to be an effective condition for preventing paper back staining from the viewpoints of both transferring fog toner to the transfer roller 8 and cleaning toner adhering to the transfer roller 8.

[0074] Based on the above evaluation results, the performance evaluation results for paper back staining for the configuration of this embodiment, the configuration of the comparative example, and the configuration of the conventional example are shown in Table 1. As shown in Table 1, except for the differences in configuration and control voltage values, the configuration and operation of the image forming apparatus 1 of this embodiment, comparative examples 1 and 2, and the conventional example are substantially the same.

[0075] [Table 1]

[0076] First, the results of this example will be described. In this example, the cleaning voltage and the developing voltage are output from a common power source. The developing voltage during image formation is set to -350V. The developing voltage during cleaning is set to -380V, resulting in a cleaning voltage of -800V. Under these conditions, a preliminary paper feed and a paper feed for evaluating paper back staining were performed, and the paper back staining concentration was 0.7%, and the degree of paper back staining judged visually was "good."

[0077] Next, the results of Comparative Example 1 will be described. Comparative Example 1 is similar to this example in that the cleaning voltage and development voltage are output from a common power source and the development voltage during image formation is set to -350 V. However, Comparative Example 1 differs from this example in that the development voltage during cleaning operation is set to -350 V and is not changed from the development voltage during image formation. Under these conditions, the cleaning voltage is set to -600 V, and only a cleaning voltage with a relatively small absolute value is output, resulting in inferior cleaning performance of the transfer roller 8 compared to this example. In this case, the paper back stain concentration was 1.6%, and the degree of paper back staining determined by visual inspection was "slightly noticeable."

[0078] Next, the results of Comparative Example 2 will be described. Comparative Example 2 is similar to this example in that the cleaning voltage and development voltage are output from a common power source, and the development voltage during image formation is set to -350 V. However, Comparative Example 2 differs from this example in that the development voltage during cleaning is set to -450 V. Under these conditions, the cleaning voltage was set to -1200 V, making it possible to output a cleaning voltage with a relatively large absolute value, but the amount of background fogging toner generated during cleaning was large. As a result, the paper back stain density was 1.2%, and the degree of paper back staining as visually judged was "slightly noticeable."

[0079] Next, the results of the conventional example will be described. The conventional example is configured so that the cleaning voltage and the developing voltage are not shared by a common power source. This configuration allows the cleaning voltage and the developing voltage during the cleaning operation to be set to any voltage. Therefore, the developing voltage during the cleaning operation is set to -350 V, which is the most advantageous voltage for reducing the amount of fog toner. The cleaning voltage is also set to -1200 V, which provides sufficient cleaning performance for the transfer roller 8. Under these conditions, the paper back stain concentration was 0.6%. Furthermore, the degree of paper back staining, as determined by visual observation, was "good." Comparing the results of this example with those of the conventional example, although there was a slight difference in the paper back stain concentration, there was no difference in the visual observation, and both were "good." This demonstrates that this example can achieve sufficient cleaning performance for the transfer roller 8.

[0080] As described above, the image forming apparatus 1 of this embodiment includes the rotatable photoreceptor 2, the charging member 3 that charges the surface of the photoreceptor 2, the exposure device 4 that exposes the surface of the charged photoreceptor 2 to light to form an electrostatic latent image on the surface of the photoreceptor 2, the developing member 21 that causes toner to adhere to the electrostatic latent image to form a toner image, the developing voltage application unit E2 that applies a developing voltage to the developing member 21, the transfer member 8 that contacts the surface of the photoreceptor 2 to form a transfer unit N and transfers the toner image from the surface of the photoreceptor 2 to a recording material P that passes through the transfer unit N, and the toner applied to the transfer member 8. The image forming device includes a first transfer voltage application unit E3 that applies a transfer voltage of the opposite polarity to the normal charging polarity of the toner to the transfer member 8, a second transfer voltage application unit E4 that applies a transfer voltage of the same polarity as the normal charging polarity of the toner to the transfer member 8, a common power source 50 that supplies voltage to the development voltage application unit E2 and the second transfer voltage application unit E4, and a control unit 100 that can control the common power source 50. The control unit 100 controls the device to perform an image forming operation to form a toner image on the recording material P and a non-image forming operation that is different from the image forming operation, and controls the common power source 50 in the non-image forming operation. In this embodiment, the control unit 100 controls the device to perform a cleaning operation, as the non-image forming operation, to move toner from the transfer member N to the photosensitive element 2 by applying a voltage of the same polarity to the transfer member 8 when there is no recording material P at the transfer unit N. In this embodiment, the control unit 100 controls the change in output of the common power supply 50 so that the value of the voltage applied by the development voltage application unit E2 to the development member 21 during the cleaning operation differs from the value of the voltage applied by the development voltage application unit E2 to the development member 21 during toner image formation. In this embodiment, the control unit 100 controls the change so that the absolute value of the voltage applied by the second transfer voltage application unit E4 to the transfer member 8 during the cleaning operation is larger than when the change is not made. In this embodiment, when the first transfer voltage application unit E3 applies the opposite polarity voltage to the transfer member 8, a voltage obtained by superimposing the same polarity voltage output from the common power supply 50 and the opposite polarity voltage output from another power supply 51 is supplied.

[0081] As described above, in this embodiment, a common power supply is used for the cleaning voltage and the developing voltage, and the cleaning voltage is controlled (adjusted) by changing the set value of the developing voltage during cleaning from the set value used during image formation. This embodiment achieves the same level of cleaning performance for the transfer roller 8 as a conventional configuration in which the cleaning voltage and the developing voltage do not share a common power supply. Furthermore, since the cleaning voltage and the developing voltage share a common power supply, this embodiment requires fewer high-voltage power supplies than the conventional configuration, thereby enabling the image forming apparatus 1 to be made smaller and less expensive. Thus, this embodiment does not require a separate power supply for cleaning the transfer member 8, thereby achieving a smaller and less expensive device, while enabling stable cleaning of the transfer member 8. In other words, this embodiment does not require a separate power supply for applying a voltage to the transfer member 8 that is the same polarity as the normal charging polarity of the toner, thereby achieving a smaller and less expensive device, while effectively applying a voltage to the transfer member 8 that is the same polarity as the normal charging polarity of the toner.

[0082] [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 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 assigned the same reference numerals as those of embodiment 1, and detailed descriptions thereof will be omitted.

[0083] The configuration of Example 1 was designed for an image forming apparatus 1 with a relatively short product lifespan or an image forming apparatus 1 used in an environment with normal temperature and humidity conditions (for example, a room temperature and humidity (23°C / 50%RH) environment). In other words, the configuration of Example 1 was designed for a condition where the amount of fog toner is relatively small. On the other hand, this example differs from Example 1 in that it is designed for a condition where the amount of fog toner is relatively large.

[0084] In the first embodiment, the image forming apparatus 1 is configured so that the photosensitive member 2 and the developing roller 21 are always in contact with each other to form a development nip. In contrast, in the present embodiment, the image forming apparatus 1 is configured so that the photosensitive member 2 and the developing roller 21 can be mechanically separated from each other to cope with conditions where the amount of fog toner is relatively large. In the present embodiment, the image forming apparatus 1 cleans the transfer roller 8 (applies a cleaning voltage to the transfer roller 8) with the developing roller 21 separated from the photosensitive member 2.

[0085] FIG. 9 is a schematic diagram illustrating a separation mechanism 40 in this embodiment. In this embodiment, the image forming apparatus 1 has a separation mechanism 40 that can mechanically separate the photosensitive member 2 and the developing roller 21. The separation mechanism 40 can switch between a state in which the photosensitive member 2 and the developing roller 21 are in contact with each other (hereinafter also referred to as a "development contact state") and a state in which the photosensitive member 2 and the developing roller 21 are separated from each other (hereinafter also referred to as a "development separation state"). In this embodiment, the separation mechanism 40 has the following configuration. The developer container 5a that constitutes the storage chamber 24 of the developing unit 5 is fixed to another container (frame) that supports the photosensitive member 2 and the charging roller 3 so as to be rotatable (swingable) around a rotation shaft 5b that is disposed approximately parallel to the rotation axis direction of the photosensitive member 2. Furthermore, the developer container 5a is biased by a biasing member 5c such as a spring so that the developing roller 21 rotatably supported in the developer container 5a rotates in a direction in which it abuts against the photosensitive member 2. The separation mechanism 40 includes a separation motor 41 as a drive source, a moving member (such as a cam) 42 driven by the separation motor 41, and a receiving portion 43 provided in the developer container 5a that is acted upon by the moving member 42. The rotational operation of the separation motor 41 is controlled by the control unit 100, and the moving member 42 presses and releases the pressing force of the moving member 42 against the receiving portion 43. By pressing the moving member 42 against the receiving portion 43, the developer container 5a is rotated against the biasing force of the biasing member 5c, and the developer 5 can be placed in a separated position (developer separated state) in which the developing roller 21 is separated from the photosensitive member 2. Furthermore, by releasing the pressure of the moving member 42 on the receiving portion 43, the developer container 5a is allowed to rotate due to the biasing force of the biasing member 5c, and the developer 5 can be disposed in a contact position (development contact state) where the development roller 21 contacts the photosensitive member 2. In this embodiment, the separation mechanism 40 generally causes the development roller 21 to contact the photosensitive member 2 during development. In this embodiment, the separation mechanism 40 also separates the development roller 21 from the photosensitive member 2 during a cleaning operation. The separation mechanism 40 may also separate the development roller 21 from the photosensitive member 2 when the image forming apparatus 1 is stopped (in a standby state waiting for a print job, or in a power-off state), for example. In this embodiment, the development roller 21 is rotationally driven in the development contact state.In this embodiment, the rotation of the developing roller 21 is stopped in the development separation state.

[0086] In this embodiment, the purpose of switching between the developer contact state and the developer separated state using the separation mechanism 40 is to reduce the amount of fog toner transferred from the photoconductor 2 to the transfer roller 8 during the cleaning operation and improve the level of paper backside contamination during the next image generation. That is, in this embodiment, the cleaning voltage is adjusted by changing the development voltage during the cleaning operation, and the transfer roller 8 is cleaned. However, as described in the first embodiment, changing the development voltage may change the amount of fog toner. That is, even if a cleaning voltage with a large absolute value is desired, which is advantageous for cleaning the transfer roller 8, the range of selectable development voltages is limited due to concerns about an increase in the amount of fog toner and the resulting worsening of paper backside contamination. In contrast, the configuration including the separation mechanism 40 as in this embodiment makes it possible to mechanically separate the developer roller 21 from the photoconductor 2 during the cleaning operation. In this case, even if a development voltage setting that generates or increases the amount of fog toner is selected in the developer contact state, there is no physical path for the fog toner to transfer from the developer roller 21 to the photoconductor 2 in the developer separated state. Therefore, it is possible to prevent the occurrence of fogging toner on the photosensitive member 2.

[0087] Next, the conditions under which a cleaning operation is desirable in the developer separation state will be described in relation to the amount of fog toner. As mentioned above, toner that is likely to cause fog toner includes (1) toner with a reduced charge amount and (2) toner charged with a polarity opposite to the normal polarity. Conditions under which a large amount of such toner, i.e., fog toner, occurs include the following: For example, when the developer 5 (the toner in the storage chamber 24) is left in a high-humidity environment for a long period of time, the toner itself absorbs moisture and its charging performance deteriorates. Another example is when toner and the developer 5 are used after repeated image formation operations. In particular, when image formation operations are repeated, the toner in the developer 5 deteriorates due to repeated mechanical damage caused by flow within the storage chamber 24 and friction with the development blade 22, and electrical damage caused by current flow and charging on the development roller 21. Specifically, the external additives that contribute to the toner's chargeability fall off or become embedded within the toner, reducing the toner's chargeability. The degree of toner deterioration can be grasped, for example, by an index correlated with the amount of use of the developing device 5 (the toner in the storage chamber 24). Examples of such an index include the cumulative number of images formed using the developing device 5 (total number of images formed), the rotation distance (or rotation time) of the developing roller 21, and the duration of power supply to the developing blade 22. Furthermore, this is more pronounced when the information indicating the environment (at least one of the temperature and humidity inside or outside the image forming apparatus 1) indicates a high humidity environment. Furthermore, this toner deterioration is more pronounced the smaller the amount of toner in the storage chamber 24. This is because, compared to when the amount of toner in the storage chamber 24 is large, when the amount of toner in the storage chamber 24 is small, each toner particle is more likely to be affected by the friction and power supply. The degree to which the amount of toner in the storage chamber 24 influences toner deterioration can be grasped, for example, by the remaining amount of toner in the storage chamber 24. As described above, as toner deterioration progresses, the probability of the presence of toner with low chargeability increases, which ultimately increases the probability of fog toner.

[0088] The tendency of fog toner generation in un-durable toner and toner after durability testing will be explained using FIG. 10. FIG. 10 is a graph showing the relationship between the setting value of the development voltage and the amount of fog toner when the dark potential Vd is fixed at -500V in the image forming apparatus 1 configured in this embodiment. The legend "un-durable" in FIG. 10 refers to the results obtained using a new developer 5 and toner that have not been subjected to a durability test, and is the same as the results in FIG. 6 described in Example 1. Furthermore, the legend "after 10K durability testing" in FIG. 10 refers to the results obtained using a new developer 5 and toner that have been subjected to a durability test after 10K sheets of single-sided continuous printing ("K" stands for 10K). 3 The results were obtained using the developing device 5 and toner in a state after a durability test was performed in which the developer 5 and toner were in a state after the durability test was performed. The recording material P used in the durability test was GF-C081 (A4 size paper, product name, manufactured by Canon Inc.), and the image pattern formed during the durability test was a full-area halftone image with a printing rate of 5%. The method for measuring the fog toner density was the same as that described in Example 1.

[0089] 10, it can be seen that the amount of fog toner increases overall when the developing device 5 and toner after 10K durability are used compared to when the non-durable developing device 5 and toner are used. In this state where the amount of fog toner has increased, as described above, the generation of fog toner may substantially limit the range of developing voltages that can be selected during cleaning operation.

[0090] Next, the cleaning operation in this embodiment will be described with reference to FIG. 11. FIG. 11 is a timing chart showing the operating states of each part at the timing of image formation (printing) on ​​the last recording material P of one print job and post-rotation after image formation is completed. In this embodiment, the control unit 100 controls the operation of the print job in accordance with the timing chart shown in FIG. 11. FIG. 11 shows the charging voltage, light emission of the laser scanner 4, surface potential of the photosensitive member 2, development voltage, positive transfer voltage, negative transfer voltage (cleaning voltage), and development contact / separation state. Note that items other than the development contact / separation state and the associated voltage control are the same as those described in the first embodiment.

[0091] In this embodiment, when image formation is completed and the system transitions to the post-rotation operation, the positive transfer voltage Vtrp is turned off, and at approximately the same time, the separation mechanism 40 starts a separation operation to separate the developing roller 21 from the photosensitive member 2. After this separation operation is completed, the developing voltage Vdev is changed from its set value for image formation to its set value for cleaning, and the negative transfer voltage (cleaning voltage) Vtrn, which accompanies this change, is also changed from its set value for image formation to its set value for cleaning. In this way, by changing the developing voltage after separating the developing roller 21 from the photosensitive member 2, it becomes possible to set the cleaning voltage with a high degree of freedom while suppressing the generation of fog toner, as described above.

[0092] In this way, in this embodiment, the developing roller 21 is separated from the photosensitive element 2 during the cleaning operation, and the set value of the developing voltage is changed from the set value during image formation, thereby controlling (adjusting) the set value of the cleaning voltage. This makes it possible to adjust the cleaning voltage to a value more advantageous for cleaning the transfer roller 8 while suppressing the generation of fog toner during the cleaning operation. Therefore, even when the toner is in a state where fog toner is relatively likely to occur, it is possible to perform good cleaning of the transfer roller 8.

[0093] Next, the relationship between the developing voltage and the cleaning performance of the transfer roller 8 in this embodiment will be described with reference to FIG. 12. FIG. 12 is a graph showing the experimental results of cleaning performance when the developing voltage (and cleaning voltage) during the cleaning operation is changed in the image forming apparatus 1 configured in this embodiment. The experimental conditions in this embodiment are the same as those described in Example 1. Specifically, the experiment was divided into two parts: a "preliminary paper feed" in which toner stains are adhered to the transfer roller 8, and a "paper back stain evaluation paper feed" in which paper back stains are evaluated after the cleaning operation is performed. The developing voltage during the preliminary paper feed was -350 V, and the developing voltage during the paper back stain evaluation paper feed was changed to the levels shown in FIG. 12.

[0094] The legend "Example 1" in Fig. 12 refers to the results of the configuration described in Example 1, specifically, the experimental results under the conditions that the developing roller 21 was not separated from the photosensitive member 2 during the cleaning operation and un-used toner was used. In other words, "Example 1" in Fig. 12 is a re-presentation of the results of Fig. 8 for the sake of explanation.

[0095] The legend "Example 1 + Toner after Durability" in Figure 12 refers to the experimental results under the condition that the developing roller 21 was not separated from the photoconductor 2 during the cleaning operation, as in Example 1. However, the toner and developing device 5 were the same as those used in Example 1, which were subjected to the 10K durability test. As described in Figure 10, the use of the toner and developing device 5 subjected to the durability test was more likely to result in fogging than the use of brand-new toner and developing device 5. Therefore, the experimental results for "Example 1 + Toner after Durability" in Figure 12 show a tendency for overall paper back staining to worsen compared to the experimental results for "Example 1" in Figure 12. In particular, paper back staining tends to worsen under conditions where the absolute value of the developing voltage is greater than -400V. This is because, as described above, the effect of the increase in background fogging toner transferred to the photoconductor 2 outweighs the effect on the cleaning performance of the transfer roller 8 due to the increased absolute value of the cleaning voltage, resulting in an unfavorable situation for paper back staining.

[0096] The legend "Example 2 + Toner after Durability Test" in Figure 12 refers to the experimental results of this example, in which the developing roller 21 was separated from the photoreceptor 2 during the cleaning operation. These experimental results were obtained under the conditions in which the toner and developing device 5 after the 10K durability test described with reference to Figure 10 were used. Comparing the results of "Example 2 + Toner after Durability Test" in Figure 12 with those of "Example 1 + Toner after Durability Test" in Figure 12, it can be seen that "Example 2 + Toner after Durability Test" tends to improve paper back staining, particularly when the developing voltage has an absolute value greater than -400 V. This is due to the following reasons. First, the developing voltage is set to a relatively large absolute value, and therefore the cleaning voltage is set to a large absolute value that is advantageous for cleaning the transfer roller 8. In addition, the developing roller 21 is separated from the photoreceptor 2 during the cleaning operation, which suppresses the transfer of fog toner to the photoreceptor 2. In other words, both of these factors contribute to the improvement in the level of paper back staining.

[0097] Based on the above evaluation results, the performance evaluation results for paper back staining in the configuration of this example and the configuration of the comparative example are shown in Table 2. As shown in Table 2, except for the differences in configuration and control voltage values, the configuration and operation of the image forming apparatuses 1 in this example and comparative examples 3 and 4 are substantially the same.

[0098] [Table 2]

[0099] First, the results of this example will be described. In this example, the developing roller 21 is separated from the photosensitive member 2 during the cleaning operation. The developing voltage during image formation is set to -350V. Furthermore, the developing voltage during the cleaning operation is set to -450V, resulting in a cleaning voltage of -1200V. Under these conditions, a preliminary paper feed and a paper feed for evaluating paper back staining were performed, and the paper back staining concentration was 0.5%, and the degree of paper back staining judged visually was "good."

[0100] Next, the results of Comparative Example 3 will be described. In Comparative Example 3, the developing voltage during the cleaning operation was set to -450 V, and as a result, the cleaning voltage was set to -1200 V, which is the same as in this example. However, Comparative Example 3 differs from this example in that the developing roller 21 was not separated from the photosensitive element 2 during the cleaning operation. Under these conditions, a large amount of background fogging toner was generated during the cleaning operation, resulting in a paper back stain density of 2.2%, and the degree of paper back staining judged visually as "noticeable."

[0101] Next, the results of Comparative Example 4 will be described. Comparative Example 4 differs from this example in that the developing voltage during the cleaning operation was set to -380V, resulting in a cleaning voltage of -800V, and that the developing roller 21 was not separated from the photosensitive element 2 during the cleaning operation. Under these conditions, the amount of background fogging toner generated during the cleaning operation was kept relatively low, but considering the toner and developing unit 5 after endurance testing, the amount of suppression was insufficient. Furthermore, only a cleaning voltage with a relatively small absolute value could be applied to the transfer roller 8. As a result, the paper back staining concentration was 1.6%, and the degree of paper back staining as visually judged was "slightly noticeable."

[0102] As described above, in this embodiment, the developing roller 21 is separated from the photosensitive element 2 during the cleaning operation. This reduces the amount of fog toner during the cleaning operation, while allowing the cleaning voltage setting value to be set to a value more favorable for cleaning the transfer roller 8. Therefore, even when using toner that is prone to generating fog toner, such as toner after a long period of use, the transfer roller 8 can be cleaned satisfactorily.

[0103] In this embodiment, the developing roller 21 is always separated from the photosensitive member 2 during the cleaning operation in order to improve the cleaning performance of the transfer roller 8. However, the present invention is not limited to this configuration. For example, performing the separation operation may result in a relatively long downtime (a period during which images cannot be formed) or may generate operating noise. Therefore, it may be desirable to avoid performing the separation operation as much as possible. Therefore, as described above, it is also possible to switch between performing and not performing the separation of the developing roller 21 from the photosensitive member 2 during the cleaning operation based on an index indicating the likelihood of fog toner generation, such as the toner durability status or installation environment information of the image forming apparatus 1.

[0104] FIG. 20 is a schematic flowchart of the control for switching whether or not to separate the developing roller 21 from the photoconductor 2 when a cleaning operation is performed during post-rotation of a print job. When image formation specified in a print job is completed (S101), the control unit 100 determines whether or not it is necessary to separate the developing roller 21 from the photoconductor 2 during a cleaning operation when transitioning to the post-rotation operation (S102). The control unit 100 successively updates and stores in the memory 102, which functions as a counter, the accumulated number of images formed using the developing unit 5 as an index correlated with the amount of toner used in the developing unit 5 (the toner in the storage chamber 24) and indicating the degree of toner deterioration. Then, the control unit 100 determines that it is necessary to separate the developing roller 21 from the photoconductor 2 during a cleaning operation when the number of images formed stored in the memory 102 exceeds a predetermined threshold, for example. If the control unit 100 determines in S102 that the toner is required ("Yes"), it performs the separation operation to separate the developing roller 21 from the photosensitive element 2 as described above (S103), and then performs the cleaning operation during post-rotation (S104). On the other hand, if the control unit 100 determines in S102 that the toner is not required ("No"), it performs the cleaning operation during post-rotation without performing the separation operation (S104). As described above, the indicator of the degree of toner deterioration is not limited to the number of images formed, but may be the rotation distance (or rotation time) of the developing roller 21, the power supply time of the developing blade 22, or the like. Furthermore, the separation operation may be performed based on the environmental detection results of an environmental sensor (such as a temperature and humidity sensor) provided in the image forming apparatus 1, for example, when the environment is high humidity. Furthermore, the separation operation may be performed based on the detection results of a remaining amount detection sensor that detects the remaining amount of toner in the toner storage chamber 24, when the remaining amount of toner in the toner storage chamber 24 falls below a predetermined threshold. The control of whether or not to perform the separation operation based on these indexes can be arbitrarily combined. Furthermore, when a cleaning operation is performed after a jam of the recording material P occurs, the separation operation may be performed so that the absolute value of the cleaning voltage can be as large as possible.

[0105] Furthermore, in this embodiment, the voltage application unit for the developing roller 21 (the aforementioned developing voltage application unit E2) was selected as the voltage application unit that shares a common power source with the cleaning voltage application unit, but the present invention is not limited to this configuration. A configuration in which the developing roller 21 can be separated from the photoconductor 2, as in this embodiment, reduces the generation of fogging toner during cleaning operations. Therefore, the voltage application unit that shares a common power source with the cleaning voltage application unit may be, for example, the aforementioned regulating member voltage application unit or supply member voltage application unit. In other words, the voltage application unit that shares a common power source with the cleaning voltage application unit may be any voltage application unit that applies voltage to any developing member involved in image formation (toner image formation) by the developing device 5, such as the developing roller 21, developing blade 22, or supply roller 23. Here, voltages that are applied to developing members such as the developing roller 21, developing blade 22, or supply roller 23 and that are involved in image formation (toner image formation) by the developing device 5 may be collectively referred to as "developing voltages." Here, the voltage application section (voltage application means) that applies voltage to the developing members such as the developing roller 21, the developing blade 22, and the supply roller 23 may be collectively referred to as the "developing voltage application section."

[0106] In addition, in this embodiment, the developing roller 21 is separated from the photosensitive member 2 for the entire period during which the cleaning voltage is applied to the transfer roller 8 in the cleaning operation, but the present invention is not limited to this embodiment. By separating the developing roller 21 from the photosensitive member 2 for at least part of the period during which the cleaning voltage is applied to the transfer roller 8 in the cleaning operation, a corresponding effect can be obtained.

[0107] In this manner, the developing member may have a developer carrier that carries and transports toner and supplies the toner to the photoconductor 2, and the developing voltage application unit E2 may apply a voltage to the developer carrier. The developing member may also have a developer carrier that carries and transports toner and supplies the toner to the photoconductor 2, and a regulating member that regulates the amount of toner carried on the developer carrier, and the developing voltage application unit E2 may apply a voltage to the regulating member. The developing member may also have a developer carrier that carries and transports toner and supplies the toner to the photoconductor 2, and a supply member that supplies the toner to the developer carrier, and the developing voltage application unit E2 may apply a voltage to the supply member. The image forming apparatus 1 may also have a separating mechanism 40 that can move the developer carrier between a contact position where the developer carrier abuts against the photoconductor 2 and a separation position where the developer carrier is separated from the photoconductor 2. The control unit 100 can control the separation mechanism 40 so that the developer carrier is positioned at the separation position during at least part of the period during which a voltage of the same polarity as the normal charging polarity of the toner is applied to the transfer member 8 during non-image forming operations.

[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 embodiments 1 and 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 embodiments 1 and 2 are assigned the same reference numerals as those of embodiments 1 and 2, and detailed descriptions thereof will be omitted.

[0109] In Examples 1 and 2, the developing voltage application unit E2 was selected as the voltage application unit that shares a power supply with the cleaning voltage application unit E4. In contrast, in this example, the power supply for the cleaning voltage application unit E4 is shared not only with the developing voltage application unit E2 but also with the charging voltage application unit E1. That is, in this example, the cleaning voltage, developing voltage, and charging voltage are supplied from a common power supply. Similar to the image forming apparatus 1 in Example 2, the image forming apparatus 1 in this example has a separation mechanism 40, which allows the developing roller 21 to be separated from the photosensitive member 2 during the cleaning operation, similar to Example 2.

[0110] The high voltage circuit configuration in this embodiment, which outputs the developing voltage, charging voltage, and cleaning voltage from a common power source, will be described with reference to Fig. 13. Fig. 13 is an explanatory diagram of the high voltage circuit configuration in this embodiment.

[0111] First, a first boost circuit (power supply) 60, which is comprised of a transformer or the like, generates a charging voltage Vpri and a negative transfer voltage (cleaning voltage) Vtrn. The charging voltage Vpri is applied to the charging roller 3. A second boost circuit (another power supply) 61, which is comprised of a transformer or the like, generates a positive transfer voltage Vtrp. Then, during image formation (transfer), a transfer voltage Vtr, which is the sum (superimposition) of the negative transfer voltage (cleaning voltage) Vtrn and the positive transfer voltage Vtrp, is applied to the transfer roller 8. In this embodiment, a voltage application unit (voltage application means) that applies a cleaning voltage to the transfer roller 8 using the first boost circuit 60 as a power source corresponds to a "cleaning voltage application unit (or second transfer voltage application unit)" E4. In this embodiment, a voltage application unit (voltage application means) that applies a charging voltage to the charging roller 3 using the first boost circuit 60 as a power source corresponds to a "charging voltage application unit" E1. In addition, in this embodiment, the voltage application unit (voltage application means) that applies a transfer voltage to the transfer roller 8 using the second boost circuit 61 (and further the first boost circuit 60) as a power source corresponds to the "transfer voltage application unit (or first transfer voltage application unit)" E3.

[0112] In this embodiment, the first boost circuit 60 performs feedback control of the charging voltage Vpri to accurately control the charging voltage Vpri. The high-voltage circuit configuration of this embodiment separates the transfer negative voltage (cleaning voltage) Vtrn and the charging voltage Vpri, but outputs voltages that are linked to each other. That is, in this embodiment, increasing the absolute value of the charging voltage Vpri also increases the absolute value of the transfer negative voltage (cleaning voltage) Vtrn, and decreasing the absolute value of the charging voltage Vpri also decreases the absolute value of the transfer negative voltage (cleaning voltage) Vtrn. Therefore, in this embodiment, adjusting the charging voltage Vpri makes it possible to change the transfer negative voltage (cleaning voltage) Vtrn.

[0113] Here, the influence of the load on the first boost circuit 60 in this embodiment will be described. In the high-voltage circuit configuration of this embodiment, when the load on the charging roller 3 is heavy, the output voltage value of the first boost circuit 60 is increased, and control is performed to maintain the charging voltage Vpri at the control value. This increases the absolute value of the negative transfer voltage (cleaning voltage) Vtrn. Conversely, when the load on the charging roller 3 is light, control is performed to decrease the output voltage value of the first boost circuit 60, and the absolute value of the negative transfer voltage (cleaning voltage) Vtrn decreases.

[0114] The developing voltage Vdev is generated by dividing the charging voltage Vpri (24 V) using a resistor 62 and a transistor 63. In this embodiment, in order to precisely control the developing voltage Vdev, the developing voltage Vdev is fed back to control the conduction of the transistor 63. Here, the high-voltage circuit configuration of this embodiment is configured so that when the transistor 63 is on, the load on the first boost circuit 60 is heavier than when the transistor 63 is off. In other words, in this embodiment, when the absolute value of the developing voltage Vdev is reduced, the absolute value of the transfer negative voltage (cleaning voltage) Vtrn increases, and when the absolute value of the developing voltage Vdev is increased, the absolute value of the transfer negative voltage (cleaning voltage) Vtrn decreases. In this embodiment, the voltage application unit (voltage application means) that applies the developing voltage to the developing roller 21 using the first boost circuit 60 as a power source corresponds to the "developing voltage application unit" E2.

[0115] The relationship between the developing voltage and the cleaning voltage in this embodiment will be described using FIG. 14. FIG. 14 is a graph showing the relationship between the developing voltage and the cleaning voltage in this embodiment. As described above, in this embodiment, the cleaning voltage can be changed by adjusting the developing voltage. As can be seen from FIG. 14, in this embodiment, if the developing voltage is set to, for example, −350 V, which is the developing voltage during image formation, a cleaning voltage of approximately −600 V is applied to the transfer roller 8. Furthermore, if the developing voltage is changed to −300 V during cleaning operation, a cleaning voltage of approximately −800 V, which is more advantageous for cleaning the transfer roller 8, is applied to the transfer roller 8. Note that FIG. 14 shows results obtained under conditions where the load on the charging roller 3 is relatively stable. Conditions where the load on the charging roller 3 fluctuates will be described in another embodiment described later.

[0116] The high-voltage circuit configuration that can be used in this embodiment is not limited to the high-voltage circuit configuration in Fig. 13, and can be changed as appropriate as long as it has a circuit with similar functions. Furthermore, the relationship between the developing voltage and the cleaning voltage is not limited to the relationship in Fig. 14, and can be changed depending on the electrical resistance values ​​of each component on the circuit, the performance of the boost circuit, etc.

[0117] Next, the relationship between the developing voltage and the cleaning performance of the transfer roller 8 in this embodiment will be described with reference to FIG. 15. FIG. 15 is a graph showing the experimental results of cleaning performance when the developing voltage (and cleaning voltage) during the cleaning operation is changed in the image forming apparatus 1 configured in this embodiment. The experimental conditions in this embodiment are the same as those described in Example 1. Specifically, the experiment was divided into two parts: a "preliminary paper feed" in which toner stains are adhered to the transfer roller 8, and a "paper back stain evaluation paper feed" in which paper back stains are evaluated after the cleaning operation is performed. The developing voltage during the preliminary paper feed was -350 V, and the developing voltage during the paper back stain evaluation paper feed was changed to the levels shown in FIG. 15.

[0118] In the configuration of this embodiment, as described above, the smaller the absolute value of the developing voltage, the larger the absolute value of the cleaning voltage, and the more effective the cleaning of the transfer roller 8. On the other hand, as described with reference to FIG. 6, when the absolute value of the developing voltage is reduced, the amount of inverted fog toner transferred to the photosensitive member 2 also increases.

[0119] First, we will explain the experimental results for "No developer separation + un-used toner" in the legend in Figure 15. Similar to Example 1, these experimental results were obtained under conditions in which the developing roller 21 was not separated from the photosensitive element 2 during cleaning and un-used toner was used. Under these conditions, it can be seen that paper back staining was most improved when the developing voltage was set to -300V. On the other hand, it can be seen that paper back staining tends to worsen slightly under conditions in which the developing voltage had an absolute value greater than -320V and a developing voltage had an absolute value less than -250V.

[0120] In the region where the absolute value of the developing voltage is greater than −320 V, a relatively large absolute value of the developing voltage is applied to the developing roller 21. As explained with reference to FIG. 14, in the configuration of this embodiment, when the absolute value of the developing voltage is large, the absolute value of the cleaning voltage tends to be small. As a result, a cleaning voltage sufficient to clean the toner adhering to the transfer roller 8 during the preliminary paper feed was not applied to the transfer roller 8 during the cleaning operation, and the toner remaining on the transfer roller 8 manifested as paper back stains when the paper was fed for paper back stain evaluation.

[0121] On the other hand, in the region where the absolute value of the developing voltage is smaller than −250 V, a developing voltage with a relatively small absolute value is applied. Therefore, during the cleaning operation, a cleaning voltage with a large absolute value, which is advantageous for cleaning the transfer roller 8, is applied to the transfer roller 8. However, as explained using FIG. 6, the condition in which a developing voltage with a relatively small absolute value is applied to the developing roller 21 is also a condition in which the inverted fog toner is likely to be transferred onto the photoreceptor 2. Therefore, the inverted fog toner generated on the photoreceptor 2 during the cleaning operation is transferred to the transfer roller 8, and then becomes apparent as paper back stains when paper is passed through to evaluate the paper back stains.

[0122] In contrast, in the region where the development voltage is around -300 V, there is relatively little fog toner on the photoconductor 2, just as in the region where the development voltage has an absolute value greater than -320 V. Also, in the region where the development voltage is around -300 V, a cleaning voltage with a relatively large absolute value is applied to the transfer roller 8, just as in the region where the development voltage has an absolute value less than -250 V. Therefore, the region where the development voltage is around -300 V can be said to be an effective condition for preventing paper back staining, from the viewpoints of both transferring fog toner to the transfer roller 8 and cleaning toner adhering to the transfer roller.

[0123] 15, the performance evaluation result for paper back staining for "no developer separation + unusable toner" was that the paper back staining concentration was 0.8% under the condition that the development voltage was set to -300 V, and the degree of paper back staining judged by visual observation was "good." This result is similar to that of Example 1, and it can be seen that even with a configuration in which the cleaning voltage, development voltage, and charging voltage are supplied from a common power source as in this example, it is possible to clean the transfer roller 8 as well as in Example 1.

[0124] Next, the experimental results for "No developer separation interval + toner after durability" in the legend in FIG. 15 will be described. As with Example 1, these experimental results were obtained under the condition that the developing roller 21 was not separated from the photosensitive member 2 during the cleaning operation. However, the toner and developing device 5 were the same as those used in the 10K durability test described with reference to FIG. 10. As described with reference to FIG. 10, when the durability test was used, the toner and developing device 5 were more likely to produce fog toner than when new toner and developing device 5 were used. Therefore, the experimental results for "No developer separation interval + toner after durability" in FIG. 15 show that the paper backside staining tends to worsen overall compared to the condition of "No developer separation interval + un-durable toner" in FIG.

[0125] Next, we will explain the experimental results for the "With developer separation + toner after durability test" legend in Figure 15. Similar to Example 2, these experimental results were obtained under the condition that the developing roller 21 was separated from the photoreceptor 2 during the cleaning operation. Furthermore, these experimental results were obtained under the condition that the toner and developing device 5 after the 10K durability test described with reference to Figure 10 were used. Under these conditions, it can be seen that the paper back staining tends to be improved, particularly when the developing voltage is smaller in absolute value than -300V. This is due to the following reasons. First, the developing voltage is set to a relatively small absolute value, and therefore the cleaning voltage is set to a large absolute value that is advantageous for cleaning the transfer roller 8. In addition, the developing roller 21 is separated from the photoreceptor 2 during the cleaning operation, which suppresses the transfer of fog toner to the photoreceptor 2. In other words, both of these points contribute to the improvement in the level of paper back staining.

[0126] 15, the performance evaluation result for paper back staining for "with developer separation + toner after durability test" was that the paper back staining concentration was 0.8% under the condition that the development voltage was set to -150 V, and the degree of paper back staining judged by visual observation was "good." This result is similar to that of Example 2, and it can be seen that in a configuration in which the cleaning voltage, development voltage, and charging voltage are supplied from a common power source as in this example, even when toner with further deteriorated charging performance is assumed, it is possible to clean the transfer roller 8 as well as in Example 2.

[0127] In this embodiment, as in the second embodiment, a configuration has been described in which the developing roller 21 can be separated from the photosensitive member 2 during cleaning operation. However, as in the first embodiment, in a configuration in which such separation is not performed, a common power supply may be used for the cleaning voltage, the developing voltage, and the charging voltage.

[0128] [Example 4] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of embodiments 1 to 3. 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 embodiments 1 to 3 are assigned the same reference numerals as those of embodiments 1 to 3, and detailed description thereof will be omitted.

[0129] In Examples 2 and 3, the developing roller 21 is separated from the photosensitive member 2 during the cleaning operation, thereby suppressing the transfer of fog toner to the photosensitive member 2 during the cleaning operation. In this example, similar to Examples 2 and 3, the developing roller 21 is separated from the photosensitive member 2 during the cleaning operation. Furthermore, in this example, the laser scanner 4 emits light during the cleaning operation, and the surface potential of the photosensitive member 2 is changed to the light area potential VL. The high-voltage circuit configuration of the image forming apparatus 1 in this example is the same as the high-voltage circuit configuration of the image forming apparatus 1 in Example 3.

[0130] First, the reason why the laser scanner 4 emits light during the cleaning operation will be explained. The cleaning operation is an operation to remove the toner adhering to the transfer roller 8 by applying a cleaning voltage of the same polarity (negative in this embodiment) as the normal charging polarity of the toner to the transfer roller 8 and transferring the toner adhering to the transfer roller 8 to the photosensitive member 2. This transfer of the toner to the photosensitive member 2 is performed mainly by using electrostatic force, and the larger the potential difference between the transfer roller 8 (cleaning voltage) and the photosensitive member 2, the better the cleaning ability of the transfer roller 8.

[0131] Taking the configuration of Example 2 as an example, during the cleaning operation, the laser scanner 4 does not emit light, so the surface potential of the photosensitive member 2 is −500 V, which is the dark potential Vd. For example, under these conditions, if a cleaning voltage of −1000 V is applied to the transfer roller 8, the potential difference obtained by subtracting the potential of the transfer roller 8 (cleaning voltage) from the surface potential of the photosensitive member 2 is 500 V (= −500 V − (−1000 V)). In other words, this potential difference of 500 V becomes the driving force that transfers the toner adhering to the transfer roller 8 to the photosensitive member 2 during the cleaning operation.

[0132] On the other hand, when the laser scanner 4 emits light during cleaning operation, the surface potential of the photosensitive element 2 is changed to the light area potential VL of -100 V. Under these conditions, if a cleaning voltage of -1000 V is applied to the transfer roller 8 as described above, the potential difference obtained by subtracting the potential of the transfer roller 8 (cleaning voltage) from the surface potential of the photosensitive element 2 is 900 V (= -100 V - (-1000 V)). In other words, compared to when the laser scanner 4 does not emit light, a larger potential difference can be set, and the cleaning performance of the transfer roller 8 can be improved accordingly.

[0133] Next, the reason for separating the developing roller 21 from the photoconductor 2 when the laser scanner 4 emits light during the cleaning operation will be explained. As described above, when the laser scanner 4 emits light during the cleaning operation, the surface potential of the photoconductor 2 becomes the light-area potential VL. If the developing roller 21 is not separated from the photoconductor 2 and the developing contact state is maintained in this state, the potential difference between the photoconductor 2 and the developing roller 21 becomes a potential difference in the direction in which toner is transferred from the developing roller 21 to the photoconductor 2. In other words, the potential of the developing roller 21 has the same polarity as the surface potential of the photoconductor 2 and is greater than the absolute value of the surface potential of the photoconductor 2. If cleaning of the transfer roller 8 (application of a cleaning voltage to the transfer roller 8) is performed in this state, the toner transferred to the photoconductor 2 will further transfer to the transfer roller 8, contaminating the transfer roller 8. To prevent this from happening, in this embodiment, the developing roller 21 is separated from the photoconductor 2 when the laser scanner 4 emits light during the cleaning operation.

[0134] Next, the cleaning operation in this embodiment will be described with reference to FIG. 16. FIG. 16 is a timing chart showing the operating states of each part at the timing of image formation (printing) on ​​the last recording material P of one print job and post-rotation after image formation is completed. In this embodiment, the control unit 100 controls the operation of the print job in accordance with the timing chart shown in FIG. 16. FIG. 16 shows the charging voltage, light emission of the laser scanner 4, surface potential of the photosensitive member 2, development voltage, positive transfer voltage, negative transfer voltage (cleaning voltage), and development contact / separation state. Note that items other than the development contact / separation state, the associated voltage control, and the light emission state of the laser scanner 4 are the same as those described in embodiments 1 to 3.

[0135] In this embodiment, when image formation is completed and the post-rotation operation begins, the positive transfer voltage Vtrp is turned off. At approximately the same time, the separation mechanism 40 starts a separation operation to separate the developing roller 21 from the photoconductor 2. After this separation operation is completed, the developing voltage Vdev is changed from its image formation setting to its cleaning setting, and the negative transfer voltage (cleaning voltage) Vtrn is also changed from its image formation setting to its cleaning setting. At approximately the same time, the laser scanner 4 is turned on to expose the entire surface of the photoconductor 2 (the entire image formation area in a direction approximately perpendicular to the movement direction of the surface of the photoconductor 2) to the light potential VL. By emitting light from the laser scanner 4 after separating the developing roller 21 from the photoconductor 2, unnecessary toner can be prevented from transferring from the developing roller 21 to the photoconductor 2. Furthermore, the potential difference between the photoconductor 2 and the transfer roller 8 (cleaning voltage) during the cleaning operation can be significantly changed, improving the cleaning performance of the transfer roller 8.

[0136] Next, the relationship between the developing voltage and the cleaning performance of the transfer roller 8 in this embodiment will be described with reference to FIG. 17. FIG. 17 is a graph showing the experimental results of cleaning performance when the developing voltage (and cleaning voltage) during the cleaning operation in the image forming apparatus 1 configured in this embodiment is changed. The experimental conditions in this embodiment are the same as those described in Examples 1 to 3. Specifically, the experiment was divided into two parts: a "preliminary paper feed" in which toner stains are adhered to the transfer roller 8, and a "paper back stain evaluation paper feed" in which paper back stains are evaluated after the cleaning operation is performed. The developing voltage during the preliminary paper feed was -350 V, and the developing voltage during the paper back stain evaluation paper feed was changed to the levels shown in FIG. 17.

[0137] The legend "Example 3 (Vd)" in Figure 17 refers to the results of the configuration described in Example 3, specifically, the experimental results under the conditions that the developing roller 21 was separated from the photosensitive member 2 during the cleaning operation and toner after 10K durability testing was used. In other words, "Example 3 (Vd)" in Figure 17 is a re-presentation of the results of Figure 15 for the sake of explanation.

[0138] The legend "Example 4 (VL)" in Figure 17 refers to the experimental results of this example, in which the laser scanner 4 exposes the photoconductor 2 during the cleaning operation. These experimental results were obtained under the conditions that the developing roller 21 was separated from the photoconductor 2 during the cleaning operation, and toner after 10K durability testing was used. Comparing the results of Example 3 with those of this example, it can be seen that the results of this example achieve a similar level of paper back staining at a developing voltage with a larger absolute value (i.e., a cleaning voltage with a smaller absolute value). This indicates that, because the laser scanner 4 exposes the photoconductor 2 during the cleaning operation in this example, a sufficient potential difference can be generated, enabling good cleaning of the transfer roller 8, even at a cleaning voltage with a smaller absolute value.

[0139] Based on the above evaluation results, Table 3 shows the performance evaluation results for paper back staining in the configurations of this example and Example 3.

[0140] [Table 3]

[0141] First, the results of this example will be described. In this example, the laser scanner 4 exposes the photoconductor 2 during the cleaning operation. The development voltage during image formation is set to −350 V. The development voltage during the cleaning operation is set to −250 V, resulting in a cleaning voltage of −1000 V. Under these conditions, a preliminary paper feed and a paper feed for evaluating backside contamination were performed. The backside contamination concentration was 0.8%, and the degree of backside contamination was visually evaluated as “good.” This result indicates that the level of backside contamination is similar to that of Example 3, where the development voltage during the cleaning operation is set to −150 V, resulting in a cleaning voltage of −1400 V. Note that Example 3 does not expose the photoconductor 2 using the laser scanner 4 during the cleaning operation.

[0142] As explained above, in this embodiment, good cleaning of the transfer roller 8 is possible with fewer changes in the development voltage. The configuration of this embodiment is considered effective in terms of increasing the degree of freedom of the high-voltage circuit, for example, in the following cases: In other words, when it is desired to shorten the convergence time of the development voltage by keeping the change range of the development voltage small during the cleaning operation, or when it is desirable to narrow the voltage range used from the viewpoint of the voltage output performance of the development voltage application unit E2.

[0143] In this embodiment, a case where a high-voltage circuit configuration similar to that of embodiment 3 is used has been described. However, when a high-voltage circuit configuration similar to that of embodiments 1 and 2 is used, the photosensitive element 2 may be exposed to light by the laser scanner 4 during the cleaning operation as in this embodiment.

[0144] In addition, in this embodiment, the laser scanner 4 is described as exposing the entire area in the rotational direction of the photoreceptor 2 that passes through the transfer nip N during the period when a cleaning voltage is applied to the transfer roller 8 during the cleaning operation. However, the present invention is not limited to this embodiment. By exposing the surface of the photoreceptor 2 that passes through the transfer nip N during at least a portion of the period when a cleaning voltage is applied to the transfer roller 8 during the cleaning operation, a suitable effect can be obtained. In other words, the control unit 100 can control the exposure device 4 to expose the surface of the photoreceptor 2 that passes through the transfer nip N during at least a portion of the period when a voltage of the same polarity as the normal charging polarity of the toner is applied to the transfer member 8 during a non-image forming operation.

[0145] [Example 5] 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 embodiments 1 to 4. 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 embodiments 1 to 4 are assigned the same reference numerals as those of embodiments 1 to 4, and detailed description thereof will be omitted.

[0146] In the fourth embodiment, a method was adopted in which a voltage effective as a cleaning voltage was output by changing the developing voltage during the cleaning operation. In contrast, in the present embodiment, a method is adopted in which a voltage effective as a cleaning voltage is output by changing the charging voltage. Note that the high-voltage circuit configuration of the image forming apparatus 1 in this embodiment is the same as the high-voltage circuit configuration of the image forming apparatus 1 in the third and fourth embodiments.

[0147] The method of changing the cleaning voltage by changing the charging voltage has been described in the third embodiment with reference to Fig. 13, so a detailed description will be omitted. When the cleaning voltage is changed by changing the charging voltage as in this embodiment, the set value of the cleaning voltage can be changed depending on the set value of the charging voltage and the load state of the charging roller 3.

[0148] Fig. 18 shows the relationship between the set value of the charging voltage and the set value of the cleaning voltage when the set value of the charging voltage is changed in the configuration of this embodiment. As can be seen from Fig. 18, in this embodiment, if the charging voltage is set to, for example, -1000 V, which is the charging voltage during image formation, a cleaning voltage of approximately -700 V is applied to the transfer roller 8. Also, if the charging voltage is changed to -1210 V during cleaning operation, for example, a cleaning voltage of approximately -1000 V, which is more advantageous for cleaning the transfer roller 8, is applied to the transfer roller 8.

[0149] However, there are some points to be aware of when changing the cleaning voltage by changing the charging voltage. Specifically, the cleaning operation removes toner from the transfer roller 8 by electrostatically transferring it to the photoconductor 2 using the potential difference between the transfer roller 8 and the photoconductor 2. However, simply changing the charging voltage, i.e., the surface potential of the photoconductor 2, also changes the potential difference between the transfer roller 8 and the photoconductor 2. This could result in a potential relationship that makes cleaning of the transfer roller 8 ineffective. To prevent this from happening, in this embodiment, the laser scanner 4 exposes the photoconductor 2 during the cleaning operation, stably maintaining the surface potential of the photoconductor 2 at a predetermined light-area potential VL. In other words, while the charging voltage is changed to adjust the cleaning voltage during the cleaning operation, the exposure operation stably sets the surface potential of the photoconductor 2 to a predetermined light-area potential VL to prevent this change from being affected. This enables stable cleaning of the transfer roller 8.

[0150] Next, the cleaning operation in this embodiment will be described with reference to FIG. 19. FIG. 19 is a timing chart showing the operating states of each part at the timing of image formation (printing) on ​​the last recording material P of one print job and post-rotation after image formation is completed. In this embodiment, the control unit 100 controls the operation of the print job in accordance with the timing chart shown in FIG. 19. FIG. 19 shows the charging voltage, light emission of the laser scanner 4, surface potential of the photosensitive member 2, developing voltage, positive transfer voltage, negative transfer voltage (cleaning voltage), and the development contact / separation state. Note that items other than the development contact / separation state, the associated voltage control, and the light emission state of the laser scanner 4 are the same as those described in the first to fourth embodiments. Furthermore, with regard to the charging voltage, the set value during image formation is denoted as "for image formation," and the set value during the cleaning operation is denoted as "for cleaning."

[0151] In this embodiment, when image formation is completed and the process shifts to the post-rotation operation, the positive transfer voltage Vtrp is turned OFF, and approximately simultaneously, the separation mechanism 40 starts a separation operation to separate the developing roller 21 from the photoconductor 2. After this separation operation is completed, the charging voltage Vpri is changed from its set value for image formation to its set value for cleaning, and the negative transfer voltage (cleaning voltage) Vtrn is also changed from its set value for image formation to its set value for cleaning. Approximately simultaneously, the laser scanner 4 is turned ON to emit light, and the entire surface of the photoconductor 2 (the entire image formation area in a direction approximately perpendicular to the movement direction of the surface of the photoconductor 2) is exposed to light (light-area potential VL). By emitting light from the laser scanner 4 after separating the developing roller 21 from the photoconductor 2, unnecessary toner can be prevented from transferring from the developing roller 21 to the photoconductor 2. In addition, by significantly changing the potential difference between the photosensitive element 2 and the transfer roller 8 (cleaning voltage) during cleaning operation, it is possible to improve the cleaning performance of the transfer roller 8 while stably maintaining the surface potential of the photosensitive element 2 at a predetermined light area potential VL.

[0152] Table 4 shows the results of the performance evaluation of paper back staining in the configurations of this embodiment and Example 4. The performance evaluation conditions were the same as those described in Example 1 and others. Specifically, the evaluation was carried out in two parts: a "preliminary paper feed" in which toner stains were deposited on the transfer roller 8, and a "paper back stain evaluation paper feed" in which paper back staining was evaluated after the cleaning operation was performed. The development voltage during the preliminary paper feed and the paper back stain evaluation paper feed was set to -350V, and the charging voltage was set to -1000V.

[0153] [Table 4]

[0154] First, the results of this example will be described. In this example, during the cleaning operation, the developing voltage was set to -350 V, the same as during image formation, and the charging voltage was changed from that during image formation to approximately -1210 V. By changing the charging voltage from that during image formation in this way, the cleaning voltage during the cleaning operation was adjusted to -1000 V. Under these conditions, a preliminary paper feed and a paper feed for evaluating paper back staining were performed, and the paper back staining concentration was 0.8%, and the degree of paper back staining judged visually was "good."

[0155] Next, the results of Example 4 will be described. In the configuration of Example 4, during the cleaning operation, the charging voltage was set to -1000 V, the same as during image formation, and the developing voltage was changed from the time of image formation to -250 V. As a result, the cleaning voltage during the cleaning operation was adjusted to -1000 V. Even under these conditions, the paper back stain concentration was 0.8%, and the degree of paper back staining judged visually was "good."

[0156] That is, although the method of adjusting the cleaning voltage differs between this embodiment and embodiment 4, the cleaning voltage setting value was the same, and therefore the cleaning performance of the transfer roller 8 was equivalent.

[0157] As described above, the image forming apparatus 1 may have a common power source 60 that supplies voltages to the developing voltage application unit E2, the charging voltage application unit E1, and the second transfer voltage application unit E4. In this case, the control unit 100 may control the change in output of the common power source 60 so as to at least one of: make the value of the voltage applied by the developing voltage application unit E2 to the developing member 21 during the cleaning operation different from the value of the voltage applied by the developing voltage application unit E2 to the developing member 21 during toner image formation; or make the value of the voltage applied by the charging voltage application unit E1 to the charging member 3 during the cleaning operation different from the value of the voltage applied by the charging voltage application unit E1 to the charging member 3 during the charging process. The control unit 100 may also control the change so that the absolute value of the voltage applied by the second transfer voltage application unit E4 to the transfer member 8 during the cleaning operation is larger than when the change is not made.

[0158] As explained above, in this embodiment, the method of changing the charging voltage is used as the method of adjusting the cleaning voltage during the cleaning operation. In this case, as in the case of using the method of changing the development voltage (the voltage applied to the development members such as the developer carrier, regulating member, and supply member), good cleaning of the transfer roller 8 is possible.

[0159] In this embodiment, the cleaning voltage is adjusted by changing the charging voltage alone, but the present invention is not limited to this. To adjust the cleaning voltage, multiple voltage application units, such as a development voltage application unit (a voltage application unit that applies voltage to development members such as a developer carrier, a regulating member, and a supply member) and a charging voltage application unit, can be used. For example, the cleaning voltage can be adjusted by combining multiple voltage change methods, such as changing both the development voltage and the charging voltage.

[0160] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

[0161] In the above-described embodiment, the image forming apparatus 1 is configured to perform a cleaning operation by applying a voltage of the same polarity as the normal charge polarity of the toner to the transfer roller 8 when there is no recording material P in the transfer nip N, thereby transferring the toner from the transfer roller 8 to the photoconductor 2. However, the non-image forming operation, which is different from the image forming operation for forming a toner image on the recording material P, is not limited to the cleaning operation of the transfer roller 8. For example, the non-image forming operation may be a toner purge performed when there is no recording material P in the transfer nip N to transfer the toner carried on the developing roller 21 to the photoconductor 2 and ensure the lubrication of the cleaner 6 that contacts the photoconductor 2 to form a contact portion. Specifically, in order for the toner to reach the contact portion, the toner must pass through the transfer nip N, which is the contact portion between the photoconductor 2 and the transfer roller 8. In this case, to prevent the toner from adhering to the transfer roller 8, a transfer voltage of the same polarity as the normal charge polarity of the toner must be applied to the transfer roller 8, and the absolute value of the transfer voltage must be greater than the absolute value of the surface potential formed on the photoconductor 2 at the transfer nip N. In this configuration, as in the above-described embodiment, it is necessary to control the transfer voltage applied to the transfer roller 8 to have the same polarity as the normal charging polarity of the toner.

[0162] In the above-described embodiment, the transfer member is a transfer roller, but the transfer member is not limited to a transfer roller. The transfer member may be, for example, a rotatable endless belt that contacts the photosensitive member. A voltage application member (roller, brush, sheet, etc.) that applies a voltage to the transfer belt may be disposed on the inner peripheral surface of the transfer belt at a position facing the photosensitive member.

[0163] In the above embodiment, the photosensitive member is a photosensitive drum, but the photosensitive member is not limited to a photosensitive drum. The photosensitive member may be a photosensitive belt configured as an endless belt. [Explanation of symbols]

[0164] 1. Image forming device 2 Photoreceptor 3 Charging roller 4. Laser scanner 5. Developing unit 6. Cleaner 8 Transfer roller 21 Developing roller 22 Developing blade 23 Supply roller 24 Containment Room

Claims

1. a rotatable photoreceptor; a charging member for charging the surface of the photoreceptor; an exposure device that exposes the surface of the photoreceptor that has been charged to light to form an electrostatic latent image on the surface of the photoreceptor; a developing member for attaching toner to the electrostatic latent image to form a toner image; a developing voltage applying section that applies a developing voltage to the developing member; a transfer member that contacts the surface of the photoreceptor to form a transfer portion and transfers the toner image from the surface of the photoreceptor to a recording material that passes through the transfer portion; a first transfer voltage applying unit that applies a transfer voltage having a polarity opposite to a normal charging polarity of the toner to the transfer member; a second transfer voltage applying unit that applies a transfer voltage having the same polarity as the normal charging polarity of the toner to the transfer member; a common power source that supplies voltage to the developing voltage application unit and the second transfer voltage application unit; a control unit capable of controlling the common power supply, the control unit controls the execution of an image forming operation for forming a toner image on a recording material and a non-image forming operation different from the image forming operation, and controls the execution of, as the non-image forming operation, a cleaning operation for moving the toner from the transfer member to the photosensitive member by applying a voltage of the same polarity to the transfer member by the second transfer voltage application unit when there is no recording material in the transfer unit, and controls the common power source in the non-image forming operation; The control unit i) controlling the output of the common power source to change so that the value of the voltage applied to the developing member by the developing voltage application unit during the cleaning operation is made different from the value of the voltage applied to the developing member by the developing voltage application unit during the toner image formation; ii) controlling the absolute value of the voltage applied to the transfer member by the second transfer voltage application unit during the cleaning operation to be larger than that in a case where the value of the voltage applied to the developing member by the development voltage application unit is not changed from that in the image forming operation; An image forming apparatus characterized by:

2. the developing member has a developer carrier that carries and transports the toner and supplies the toner to the photosensitive member; 2. The image forming apparatus according to claim 1, wherein the developing voltage applying section applies a voltage to the developer carrying member.

3. the developing member includes a developer carrier that carries and transports the toner and supplies the toner to the photosensitive member, and a regulating member that regulates the amount of the toner carried on the developer carrier; 2. The image forming apparatus according to claim 1, wherein the developing voltage applying section applies a voltage to the regulating member.

4. the developing member includes a developer carrier that carries and transports the toner and supplies the toner to the photosensitive member, and a supply member that supplies the toner to the developer carrier; 2. The image forming apparatus according to claim 1, wherein the developing voltage application section applies a voltage to the supply member.

5. a rotatable photoreceptor; a charging member for charging the surface of the photoreceptor; a charging voltage applying section that applies a charging voltage to the charging member; an exposure device that exposes the surface of the photoreceptor that has been charged to light to form an electrostatic latent image on the surface of the photoreceptor; a developing member for attaching toner to the electrostatic latent image to form a toner image; a developing voltage applying section that applies a developing voltage to the developing member; a transfer member that contacts the surface of the photoreceptor to form a transfer portion and transfers the toner image from the surface of the photoreceptor to a recording material that passes through the transfer portion; a first transfer voltage applying unit that applies a transfer voltage having a polarity opposite to a normal charging polarity of the toner to the transfer member; a second transfer voltage applying unit that applies a transfer voltage having the same polarity as the normal charging polarity of the toner to the transfer member; a common power source that supplies voltages to the developing voltage application unit, the charging voltage application unit, and the second transfer voltage application unit; a control unit capable of controlling the common power supply, the control unit controls the execution of an image forming operation for forming a toner image on a recording material and a non-image forming operation different from the image forming operation, and controls the execution of, as the non-image forming operation, a cleaning operation for moving the toner from the transfer member to the photosensitive member by applying a voltage of the same polarity to the transfer member by the second transfer voltage application unit when there is no recording material in the transfer unit, and controls the common power source in the non-image forming operation; The control unit i) controlling the output of the common power source to be changed so as to perform at least one of the following: making the value of the voltage applied to the developing member by the developing voltage application unit during the cleaning operation different from the value of the voltage applied to the developing member by the developing voltage application unit during the toner image formation; or making the value of the voltage applied to the charging member by the charging voltage application unit during the cleaning operation different from the value of the voltage applied to the charging member by the charging voltage application unit during the charging process; ii) controlling the absolute value of the voltage applied to the transfer member by the second transfer voltage application unit during the cleaning operation to be larger than that in a case where the value of the voltage applied to the developing member by the development voltage application unit is not changed from that in the image forming operation; An image forming apparatus characterized by:

6. the developing member has a developer carrier that carries and transports the toner and supplies the toner to the photosensitive member; 6. The image forming apparatus according to claim 5, wherein the developing voltage applying section applies a voltage to the developer carrying member.

7. the developing member includes a developer carrier that carries and transports the toner and supplies the toner to the photosensitive member, and a regulating member that regulates the amount of the toner carried on the developer carrier; 6. The image forming apparatus according to claim 5, wherein the developing voltage application unit applies a voltage to the regulating member.

8. the developing member includes a developer carrier that carries and transports the toner and supplies the toner to the photosensitive member, and a supply member that supplies the toner to the developer carrier; 6. The image forming apparatus according to claim 5, wherein the developing voltage application section applies a voltage to the supply member.

9. 6. The image forming apparatus according to claim 5, wherein the control unit controls the first transfer voltage application unit to supply a voltage that is a superposition of the same polarity voltage output from the common power supply and the opposite polarity voltage output from another power supply when applying the opposite polarity voltage to the transfer member.

10. a separating mechanism that can move the developer carrier between a contact position where the developer carrier is in contact with the photosensitive member and a separating position where the developer carrier is separated from the photosensitive member; 9. The image forming apparatus according to claim 6, wherein the control unit is capable of controlling the separation mechanism so that the developer carrier is positioned at the separation position during at least a portion of the period during which the voltage of the same polarity is applied to the transfer member during the non-image forming operation.

11. The image forming apparatus according to claim 5, wherein the control unit controls the exposure device so as to expose the surface of the photosensitive member passing through the transfer unit during at least a portion of the period during which the voltage of the same polarity is applied to the transfer member during the non-image forming operation.

12. a rotatable photoreceptor; a charging member for charging the surface of the photoreceptor; an exposure unit that exposes the surface of the photoreceptor that has been subjected to the charging process to light to form an electrostatic latent image on the surface of the photoreceptor; a developing member for attaching toner to the electrostatic latent image to form a toner image; a developing voltage applying section that applies a developing voltage to the developing member; a transfer member that contacts the surface of the photoreceptor to form a transfer portion and transfers the toner image from the surface of the photoreceptor to a recording material that passes through the transfer portion; a first transfer voltage applying unit that applies a transfer voltage having a polarity opposite to a normal charging polarity of the toner to the transfer member; a second transfer voltage applying unit that applies a transfer voltage having the same polarity as the normal charging polarity of the toner to the transfer member; a common power source that supplies voltage to the developing voltage application unit and the second transfer voltage application unit; a control unit capable of controlling the common power supply, the control unit controls the image forming operation to form a toner image on a recording material and the non-image forming operation different from the image forming operation, and controls the common power source in the non-image forming operation; The control unit controls the first transfer voltage application unit to supply a voltage that is a superposition of the same polarity voltage output from the common power source and the opposite polarity voltage output from another power source when applying the opposite polarity voltage to the transfer member.

Images