Image forming apparatus

Abstract

To suppress the excessive rise of a surface potential of an end portion in a longitudinal direction of a photoreceptor in a configuration where a contact area of a surface of the photoreceptor contacting with a transfer member is shorter than a contact area of the surface of the photoreceptor contacting with a charging member with respect to the longitudinal direction.SOLUTION: In an image forming apparatus 100, the width of a transfer portion Nt is shorter than the width of a developing portion E in a rotational axis direction of a charging member 2 and the width of the transfer portion Nt is shorter than the width of a charging portion B, and an end portion of a surface of a photoreceptor 1 in the rotational axis direction includes a non-transfer area D in contact with the charging member 2 and in no contact with the transfer member 5. A control portion 40 can control to apply voltage having normal polarity of toner to the transfer member 5 when there is no recording material P in the transfer portion Nt, and to perform operation in which an exposure device 3 exposes an area, including the non-transfer area D of the surface of the photoreceptor 1 which passed through the transfer portion Nt during application of the voltage, by a second exposure amount smaller than a first exposure amount for exposing an image portion.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to an image forming apparatus such as a laser beam printer, a copying machine, and a facsimile apparatus using an electrophotographic method.

Background Art

[0002] Conventionally, in an image forming apparatus using an electrophotographic method, the surface of a photoreceptor is uniformly charged by a charging means, and a dark potential is formed on the surface of the photoreceptor. Thereafter, the surface of the charged photoreceptor is exposed by an exposure means to form a bright potential on the surface of the photoreceptor, and an electrostatic latent image is formed on the photoreceptor with the contrast between the dark potential and the bright potential. Then, toner is supplied to the electrostatic latent image formed on the photoreceptor by a developing means, and a toner image is formed on the photoreceptor. As the developing means, a developing device provided with a developing roller which is a roller-shaped developing member is often used.

[0003] The toner image formed on the photoreceptor is transferred onto a recording material by a transfer means. As the transfer means, a transfer roller which is a roller-shaped transfer member is often used. The transfer roller abuts on the photoreceptor to form a transfer portion (transfer nip portion). The transfer roller sandwiches and conveys the recording material between the transfer roller and the photoreceptor, and transfers the toner on the photoreceptor onto the recording material. At the time of transfer, a transfer voltage having a polarity opposite to the normal charging polarity (normal polarity) of the toner is applied to the transfer roller, and the toner image on the photoreceptor is electrostatically transferred onto the recording material. Although the recording material may be referred to as "paper" in some cases, the recording material is not limited to paper, and may be a synthetic resin such as an OHP sheet or synthetic paper as a main component. Also, for the sake of convenience, the terms "higher" and "lower" (or "greater" and "less") for potential and voltage, and "raising" and "lowering" refer to the "higher" and "lower" (or "greater" and "less") and "raising" and "lowering" when comparing the absolute values of the potential and voltage.

[0004] Here, one method of charging a photoreceptor is to use a conductive charging member that comes into contact with the photoreceptor and apply a voltage to this charging member to perform the charging process. A charging roller, which is a roller-shaped charging member, is often used as the charging member. Furthermore, there are two types of such charging methods: the AC / DC charging method, which applies an oscillating voltage that is a superposition of a direct current voltage (DC voltage) and an alternating current voltage (AC voltage) to the charging member, and the DC charging method, which applies only a direct current voltage (DC voltage). The DC charging method has the advantage of not requiring an AC power supply, thus enabling miniaturization and cost reduction of the device.

[0005] Furthermore, a pre-exposure means is sometimes provided to expose the surface of the photoreceptor downstream of the transfer position by the transfer means and upstream of the charging position by the charging means, in relation to the rotation direction of the photoreceptor, in order to remove residual charge from the surface of the photoreceptor after the transfer process. Examples of pre-exposure means (static removal means) include LED chip arrays, fuse lamps, halogen lamps, and fluorescent lamps. In contrast, there is a pre-exposure-less method that omits this pre-exposure means in order to reduce the size and cost of the device.

[0006] Patent Document 1 proposes an image forming apparatus with a simple configuration that employs the DC charging method and pre-exposure-free method described above. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2003-302808 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] However, in conventional image forming apparatuses, the following problems have been found when the contact area between the transfer roller and the surface of the photoreceptor is shorter than the contact area between the charging roller and the surface of the photoreceptor in a direction approximately perpendicular to the direction of movement of the surface of the photoreceptor (the direction of transport of the recording material). Note that the direction approximately perpendicular to the direction of movement of the surface of the photoreceptor (the direction of transport of the recording material) (i.e., the direction approximately parallel to the rotation axis direction of the charging roller) is sometimes referred to as the "longitudinal direction". In addition, the length of the contact area between the surface of the photoreceptor and the charging roller is sometimes simply described as the length of the charging roller, and the length of the contact area between the surface of the photoreceptor and the transfer roller is sometimes simply described as the length of the transfer roller.

[0009] If the transfer roller is shorter than the charging roller in the longitudinal direction, a region will be created at the longitudinal end where the charging roller contacts the photoreceptor, but the transfer roller does not. Here, the region of the photoreceptor's surface that contacts the transfer roller is called the "transfer region," and the region of the photoreceptor's surface that contacts the charging roller but not the transfer roller is called the "non-transfer region." When considering the surface potential of the photoreceptor after transfer, in the transfer region, a transfer voltage is applied when transferring the toner image from the photoreceptor to the recording material, so the surface potential of the photoreceptor becomes low. On the other hand, in the non-transfer region, no transfer voltage is applied, so the surface potential of the photoreceptor remains high. As a result, a potential difference will occur in the surface potential of the photoreceptor after transfer between the transfer region and the non-transfer region. This potential difference decreases during the subsequent charging process, but gradually increases with repeated passage through the transfer area. For example, in a configuration employing a reverse development method using negatively charged toner, the non-transfer region is negatively charged by the charging roller but not positively charged by the transfer roller. Therefore, if charging is repeated during continuous image formation, the antistatic effect due to positive charging by the transfer roller cannot be obtained in the non-transfer region, which can cause the surface potential of the photoreceptor to rise to an excessively negative potential.

[0010] The phenomenon described above, in which the surface potential of the non-transfer region at the longitudinal edge of the photoreceptor rises to an excessive potential, tends to be more pronounced when the image forming apparatus employs a DC charging method that does not provide a potential equalization effect using AC voltage, and even more so when a pre-exposure-less method is employed.

[0011] Furthermore, as mentioned above, if the surface potential of the non-transfer region at the longitudinal end of the photoreceptor rises to an excessive potential, problems such as the following may occur.

[0012] For example, in the longitudinal direction, there is a configuration in which the toner coating area (development area) on the development roller is longer than the contact area between the photoreceptor surface and the transfer roller. In this configuration, the development area faces both the transfer area and the non-transfer area of ​​the photoreceptor. In this case, as mentioned above, if the surface potential of the photoreceptor in the non-transfer area rises to an excessive potential, "reverse fogging" may occur, where "reverse toner," which is charged with the opposite polarity to the normal charging polarity, adheres to the surface. If a large amount of toner adheres to the surface of the photoreceptor in the non-transfer area due to this "reverse fogging," cleaning failure may occur. This cleaning failure may then lead to "edge fogging," where the edges of the recording material in a direction approximately perpendicular to the direction of transport of the recording material become contaminated with toner.

[0013] Furthermore, if the surface potential of the non-transfer region at the longitudinal end of the photoreceptor rises to an excessively high potential, it can cause a discharge between the photoreceptor and the core metal of the transfer roller in that region, potentially damaging the surface of the photoreceptor, such as leakage marks due to dielectric breakdown. If a charging voltage is applied to a charged component while this damage is present on the photoreceptor, the current may concentrate in the damaged area, causing the applied voltage to the charged component to drop. As a result, the photoreceptor, including other regions, may not be able to reach the desired surface potential, potentially leading to the appearance of streaks in the longitudinal direction due to insufficient charging.

[0014] Therefore, an object of the present invention is to suppress an excessive rise in the surface potential of the longitudinal end of the photoreceptor in a configuration in which the contact area between the transfer member and the surface of the photoreceptor is shorter than the contact area between the charging member and the surface of the photoreceptor in the longitudinal direction. [Means for solving the problem]

[0015] The above objective is achieved by the image forming apparatus according to the present invention. In summary, the present invention comprises a rotatable photoreceptor, a rotatable charging member that contacts the photoreceptor to form a charging section and charges the surface of the photoreceptor in the charging section, an exposure apparatus that exposes the surface of the photoreceptor charged by the charging member with a first exposure amount to form an electrostatic image on the surface of the photoreceptor, a developing member that contacts the photoreceptor to form a developing section and supplies toner charged with normal polarity to the electrostatic image formed on the surface of the photoreceptor in the developing section to form a toner image, a transfer member that contacts the surface of the photoreceptor to form a transfer section and transfers a toner image from the surface of the photoreceptor to a recording material in the transfer section by applying a transfer voltage of the opposite polarity to the normal polarity, a transfer power supply that applies a voltage to the transfer member, and the Image forming apparatus comprising an optical device and a control unit capable of controlling the transfer power supply, wherein in the rotation axis direction of the charging member, the width of the transfer unit is shorter than the width of the developing unit, and the width of the transfer unit is shorter than the width of the charging unit, and the end of the surface of the photoreceptor in the rotation axis direction has a non-transfer region that is in contact with the charging member but not in contact with the transfer member, characterized in that the control unit can control the operation to apply the voltage of normal polarity to the transfer member when there is no recording material in the transfer unit, and to expose the region of the surface of the photoreceptor that has passed through the transfer unit, including the non-transfer region, with the exposure device at a second exposure amount smaller than the first exposure amount when the voltage is applied. [Effects of the Invention]

[0016] According to the present invention, in a configuration where the contact area between the transfer member and the surface of the photoreceptor is shorter than the contact area between the charging member and the surface of the photoreceptor in the longitudinal direction, it is possible to suppress an excessive rise in the surface potential of the longitudinal end of the photoreceptor. [Brief explanation of the drawing]

[0017] [Figure 1] This is a schematic cross-sectional view of an image forming apparatus. [Figure 2]It is a schematic diagram showing the longitudinal positional relationship of each part around the photosensitive drum. [Figure 3] It is an explanatory diagram of the transition of the surface potential of the photosensitive drum during image formation. [Figure 4] It is an explanatory diagram of the transition of the surface potential of the photosensitive drum in the potential difference reduction operation. [Figure 5] It is a graph showing the relationship between Vback and overlap. [Figure 6] It is a flowchart of the control of Example 1. [Figure 7] It is a flowchart of the control of Example 2. [Embodiments for Carrying Out the Invention]

[0018] Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

[0019] [Example 1] (1) Image Forming Apparatus First, the configuration of the image forming apparatus 100 of this embodiment will be described. FIG. 1 is a schematic cross-sectional view of the image forming apparatus 100 of this embodiment. The image forming apparatus 100 of this embodiment is a laser printer using the electrophotographic method, and can form an image on a recording material P according to image information input from an external device 200 such as a personal computer.

[0020] The image forming apparatus 100 has a photosensitive drum 1, which is a drum-shaped (cylindrical) photosensitive member (electrophotographic photosensitive member) as an image carrier, inside the apparatus main body M. The photosensitive drum 1 is formed by providing a photosensitive material such as OPC (organic photoconductor), amorphous selenium, or amorphous silicon on a cylindrical drum substrate made of aluminum or nickel. The photosensitive drum 1 used in this embodiment is a negatively charged OPC photosensitive member with an outer diameter of φ24 mm. This photosensitive drum 1 is configured to have a photosensitive layer in which a charge generation layer and a charge transport layer are laminated in this order from the conductive substrate side on the surface of a conductive substrate made of an aluminum cylinder.

[0021] The following means are arranged around the photosensitive drum 1 in order along its rotational direction Rd: First, a charging roller 2, which is a roller-shaped charging member, is arranged as a charging means. Next, an exposure device 3 is arranged as an exposure means. Next, a developing device 4 is arranged as a developing means. Next, a transfer roller 5, which is a roller-shaped transfer member (transfer rotating body), is arranged as a transfer means. Next, a static elimination needle 20 is arranged as a static elimination member. Next, a cleaning device 6 is arranged as a cleaning means.

[0022] The charging roller 2 is composed of, for example, a conductive base shaft (core metal) that also serves as a power supply electrode, and an elastic layer that cylindrically surrounds its outer surface. The charging roller 2 used in this embodiment is an elastic roller with an outer diameter of φ10 mm, a core metal diameter of φ5 mm, and an elastic layer thickness of 2.5 mm. In this embodiment, SUS is used for the core metal, and a mixed rubber material of NBR and epichlorohydrin is used for the elastic layer. The charging roller 2 is pressed against the photosensitive drum 1 and rotates in conjunction with the rotation of the photosensitive drum 1. The charging roller 2 is positioned such that its rotation axis direction is approximately parallel to the direction (width direction) which is approximately perpendicular to the direction of movement of the surface of the photosensitive drum 1. With respect to the rotation direction of the photosensitive drum 1, the charging position Pa is the position on the photosensitive drum 1 where the charging process by the charging roller 2 takes place. The charging roller 2 charges the surface of the photosensitive drum 1 by a discharge that occurs in at least one of the minute gaps formed on the upstream and downstream sides of the contact portion between the charging roller 2 and the photosensitive drum 1 with respect to the rotation direction of the photosensitive drum 1. This will be called "discharge charging." In addition, the charging roller 2 also charges the surface of the photosensitive drum 1 by injecting charge at the contact point between the charging roller 2 and the photosensitive drum 1. This will be called "injection charging." For simplicity, we may consider the contact point between the charging roller 2 and the photosensitive drum 1 to be the charging position (charged area) Pa.

[0023] In this embodiment, the exposure apparatus 3 is composed of a laser scanner device (laser optical system). With respect to the rotation direction of the photosensitive drum 1, the position on the photosensitive drum 1 where exposure is performed by the exposure apparatus 3 is the exposure position (exposure area) Pb.

[0024] In this embodiment, the developing device 4 uses a non-magnetic one-component developer (toner) as the developer. The developing device 4 includes a developing roller 4a as a developer carrier (developing member) and a developing container 4b. The developing roller 4a contacts the surface of the photosensitive drum 1 during development and supplies toner to the developing section, which is the part (contact part) opposite to the photosensitive drum 1. The developing container 4b is a container that holds the developer, and the developer contained in the developing container 4b is supplied to the developing roller 4a. The developing device 4 may also use a magnetic one-component developer (toner) or a two-component developer comprising toner and a carrier as the developer. With respect to the rotation direction of the photosensitive drum 1, the position on the photosensitive drum 1 where toner is supplied by the developing roller 4a (in this embodiment, the position where it contacts the developing roller 4a) is the developing position (developing section) Pc.

[0025] The transfer roller 5 is biased (pressed) toward the photosensitive drum 1 by a transfer pressure spring (not shown), which is a biasing member acting as a biasing means, and is pressed against the photosensitive drum 1. This forms a transfer portion (transfer nip portion, transfer clamping portion) Nt, which is the contact area between the photosensitive drum 1 and the transfer roller 5. The transfer roller 5 rotates in association with the rotation of the photosensitive drum 1. The transfer roller 5 clamps and transports the recording material P between itself and the photosensitive drum 1, and when a voltage is applied, it transfers the toner image from the photosensitive drum 1 to the recording material P. The transfer roller 5 is composed of, for example, a conductive base shaft (core metal) that also serves as a power supply electrode, and an elastic layer that cylindrically surrounds its outer surface. Generally, a semiconducting rubber material such as EPDM, NBR, SBR, urethane rubber, epichlorohydrin, or silicone rubber is used as this elastic layer. The material of the elastic layer may contain an appropriate amount of a conductive agent, such as an ionic conductive agent. The transfer roller 5 used in this embodiment is an elastic roller with an outer diameter of φ14 mm, a core diameter of φ5 mm, and an elastic layer thickness of 4.5 mm. In this embodiment, SUS is used for the core, and a mixed rubber material of NBR and epichlorohydrin is used for the elastic layer. In this embodiment, the contact pressure of the transfer roller 5 against the photosensitive drum 1 is 9.8 N (1 kgf). In this embodiment, the electrical resistance value of the transfer roller 5 (hereinafter also simply referred to as "resistance value") is 2.0 × 10⁻¹⁰ when the transfer roller 5 is pressed onto an aluminum cylinder with a force of 9.8 N, rotated at 50 mm / sec, and +1000 V is applied. 8 It is Ω. Note that the resistance value of this transfer roller 5 is the resistance value when the transfer roller 5 is left in a normal temperature and humidity environment during its initial use (when new). With respect to the rotation direction of the photosensitive drum 1, the position where the toner image is transferred to the recording material P on the photosensitive drum 1 (the position corresponding to the transfer part Nt above) is the transfer position Pd.

[0026] The static elimination needle 20 eliminates excess charge on the surface of the recording material P after transfer and reduces potential unevenness on the photosensitive drum 1 caused by peeling discharge. The static elimination needle 20 can be made of a thin metal plate material such as a SUS plate or an aluminum plate that has a sawtooth-shaped sharp end and good conductivity. The static elimination needle 20 is positioned downstream of the transfer roller 5 with respect to the transport direction of the recording material P, with the needle tip facing the surface of the photosensitive drum 1.

[0027] The cleaning device 6 cleans off any deposits, such as toner (transfer residue toner), that remain on the photosensitive drum 1 after transfer. In this embodiment, the cleaning device 6 includes a cleaning blade 6a and a cleaning container 6b, which are positioned to contact the surface of the photosensitive drum 1. With respect to the rotational direction of the photosensitive drum 1, the cleaning position (cleaning section) Pe is the position on the photosensitive drum 1 where the cleaning blade 6a removes toner (in this embodiment, the position where the cleaning blade 6a contacts the drum).

[0028] Furthermore, a recording material cassette (paper feed tray) 7, which stores recording materials (transfer material, recording medium, sheet) P such as paper, is located at the bottom of the main body M in the diagram. Along the transport path from the recording material cassette 7 to the recording material P, a feed roller 8, a transport roller 9, a top sensor 10, a pre-transfer transport guide 15, a transfer-to-fixing transport guide 11, a fixing device 12, an discharge roller 13, and an discharge tray 14 are arranged in order. The main body M is also equipped with a control unit 40 that controls the image forming apparatus 100 and a video controller 110 that performs image processing and other operations.

[0029] Next, the image forming operation in the image forming apparatus 100 of this embodiment will be described. The photosensitive drum 1 is driven to rotate at a peripheral speed (process speed) of 320 mm / sec in the direction of arrow Rd (clockwise direction) in the figure by a drive source (not shown). The surface of the rotating photosensitive drum 1 is charged substantially uniformly by the charging roller 2 to a predetermined potential (dark area potential, charging potential) with the same polarity as the normal charging polarity (negative polarity in this embodiment) of the toner. During the charging process, a charging voltage (charging bias), which is a negative polarity DC voltage, is applied to the charging roller 2 from the charging power supply (high voltage power supply) 21 via the charging current detection circuit 22. In this embodiment, as an example, a charging voltage of -1100V is applied to the charging roller 2, and a dark area potential of -500V is formed on the surface of the photosensitive drum 1.

[0030] The surface of the charged photosensitive drum 1 is scanned and exposed by the exposure device 3 according to the image information. The video controller 110 of the image forming apparatus 100 processes the image information input to the image forming apparatus 100 from the external device 200 to generate a time-series electrodigital pixel signal and inputs it to the control unit 40. The exposure device 3 is controlled by the control unit 40 and outputs a modulated laser light L according to the time-series electrodigital pixel signal, and scans and exposes the charged surface of the photosensitive drum 1 with this laser light L. As a result, an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 1. In this embodiment, the charge on the photosensitive drum 1 in the exposed area is removed by the exposure device 3, and a bright area potential of -100V is formed on the surface of the photosensitive drum 1. As a result, an electrostatic latent image is formed on the photosensitive drum 1 by the contrast between the dark area potential and the bright area potential.

[0031] The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by the developing device 4 when toner is supplied, and a toner image (toner image, developer image) is formed on the photosensitive drum 1. During development, a developing voltage (developing bias), which is a DC voltage with the same polarity (negative polarity in this embodiment) as the normal charge polarity of the toner, is applied to the developing roller 4a from the developing power supply (high voltage power supply) 16. In this embodiment, as an example, a developing voltage of -380V is applied to the developing roller 4a. In this embodiment, toner charged with the same polarity (negative polarity in this embodiment) as the charge polarity of the photosensitive drum 1 adheres to the exposed area (image area) on the photosensitive drum 1, where the absolute value of the potential has decreased after being charged almost uniformly and then exposed (reverse developing method). In this embodiment, the normal charge polarity of the toner, which is the main charge polarity of the toner during development, is negative polarity.

[0032] The toner image formed on the photosensitive drum 1 is transferred to the recording material P in the transfer section Nt by the action of the transfer roller 5. During transfer, a transfer voltage (transfer bias), which is a DC voltage with the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the transfer roller 5 from the transfer power supply (high voltage power supply) 18 via the transfer current detection circuit 19, which serves as a transfer current detection means. In this embodiment, as an example, a transfer voltage of approximately +1000V is applied to the transfer roller 5. As a result, the toner image on the photosensitive drum 1 is electrostatically transferred to a predetermined position on the recording material P. The recording material P is stored in a recording material cassette 7, which serves as a recording material storage section, and is fed out one sheet at a time from the recording material cassette 7 by a feed roller 8, which serves as a feeding member. This recording material P is transported by a transport roller 9, which serves as a transport member, and supplied to the transfer section Nt along a pre-transfer transport guide 15, which serves as a guide member. The transport roller 9 is controlled based on the detection result of the leading edge of the transport direction of the recording material P by the top sensor 10 as a recording material detection means, and supplies the recording material P to the transfer unit Nt in a manner that matches the timing with the toner image on the photosensitive drum 1.

[0033] The recording material P onto which the toner image has been transferred in the transfer section Nt has excess charge removed from its surface by the static elimination needle 20. The recording material P that has passed through the static elimination needle 20 is transported along the transfer-fixing transport guide 11, which acts as a guide member, to the fixing device 12, which acts as a fixing means. The fixing device 12 has a fixing roller 12a with a built-in heater and a pressure roller 12b that presses against the fixing roller 12a. The fixing device 12 applies heat and pressure to the recording material P carrying the unfixed toner image that has passed through the nip between these rollers, thereby fixing (melting and solidifying) the toner image onto the recording material P.

[0034] In the case of single-sided image formation, the recording material P, after the toner image has been fixed to one side by the fixing device 12, is discharged (output) by the discharge roller 13 onto the discharge tray 14 formed on the upper surface of the main body M in the diagram. The image forming apparatus 100 may also be configured to perform double-sided image formation by reversing the front and back sides of the recording material P, which has the toner image fixed to the first side, and reversing the transport direction, transporting it again to the transfer unit Nt, and transferring and fixing the toner image to the second side of the recording material P.

[0035] On the other hand, any deposits such as toner that remain on the surface of the photosensitive drum 1 without being transferred to the recording material P during the transfer process (transfer residue toner) are removed from the surface of the photosensitive drum 1 by the cleaning device 6 and collected. The cleaning device 6 uses a cleaning blade 6a to scrape off deposits such as transfer residue toner from the surface of the rotating photosensitive drum 1 and collects them in a cleaning container 6b.

[0036] By repeating the above operations, images can be formed one after another. In this embodiment, the image forming apparatus 100 can perform printing operations at a print speed of 50 frames per minute.

[0037] In this embodiment, the image forming apparatus 100 does not have means (pre-exposure means) for lowering the surface potential of the photosensitive drum 1 by irradiating the surface of the photosensitive drum 1 with light downstream of the transfer position Pd and upstream of the charging position Pa with respect to the rotation direction of the photosensitive drum 1.

[0038] Furthermore, the photosensitive drum 1 and at least one of the charging roller 2, developing device 4, and cleaning device 6, which act as process means acting thereon, may integrally form a cartridge (process cartridge) that can be attached to and detached from the main body M of the device.

[0039] The control unit 40 is composed of a CPU 41 as a calculation control means, which is the central element for performing calculations; a memory such as ROM 41a and RAM 41b as a storage means; and an input / output unit (not shown) that controls the exchange of signals between the control unit 40 and various parts outside the control unit 40. The RAM 41b, which is a rewritable memory, stores information input to the control unit 40, detected information, and calculation results, while the ROM 41a stores control programs and pre-determined data tables. The CPU 41 and the memories such as ROM 41a and RAM 41b can transfer and read data from each other. The CPU 41 can control various operations related to image formation by executing various programs stored in ROM 41a, while using RAM 41b as a working area. The ROM 41a also stores data tables of various pre-set control values ​​(operation settings) and information on various pre-set thresholds, which are used in the potential difference reduction operation and control of image formation conditions, as described later.

[0040] Here, the image forming apparatus 100 executes a print job (print operation), which is a series of operations that form and output an image on one or more recording materials P, initiated by a single start instruction. A print job generally includes an image forming process, a pre-rotation process, a paper-to-paper process when forming an image on multiple recording materials P, and a post-rotation process. The image forming process is the period during which the electrostatic latent image, toner image, and toner image transfer of the image to be actually formed and output on the recording material P are performed, and this period is referred to as the image forming time. More specifically, the timing of the image forming time differs depending on the position in which each of the above processes of electrostatic latent image formation, toner image formation, and toner image transfer is performed, and corresponds to the period during which the image forming area on the photosensitive drum 1 passes through each of the above positions. The pre-rotation process is the period during which preparatory operations are performed before the image forming process, from when a start instruction is input until the image is actually formed. The paper-to-paper process (image-to-image process, recording material-to-recording material process) is the period corresponding to the space between recording materials P when image forming on multiple recording materials P is performed continuously (continuous image forming, continuous printing). The post-rotation process is the period during which tidying operations (preparation operations) are performed after the image formation process. Non-image formation time refers to the period other than the image formation time, and includes the pre-rotation process, inter-paper process, post-rotation process, and pre-multi-rotation process, which is the preparation operation when the image forming apparatus 100 is powered on or when it returns from sleep mode. More specifically, the timing of non-image formation time corresponds to the period during which the non-image formation area on the photosensitive drum 1 passes through the positions where the electrostatic latent image formation, toner image formation, and toner image transfer processes are performed. The image formation area on the photosensitive drum 1 or the recording material P is a region where a toner image can be formed, which is transferred to the recording material P and output from the image forming apparatus 100, and is set in advance according to the size of the recording material P, etc. The non-image formation area is the area other than the image formation area. In this embodiment, a margin area, which is a non-image formation area, is provided in predetermined areas at the leading and trailing ends of the recording material P in the transport direction of the recording material P. In addition, in this embodiment, blank areas, which are non-image forming areas, are also provided in predetermined regions at both ends of the recording material P in a direction substantially perpendicular to the transport direction of the recording material P.

[0041] (2) Positional relationship in the longitudinal direction Figure 2 is a schematic diagram illustrating the positional relationship of various parts around the photosensitive drum 1 in a direction approximately perpendicular to the direction of movement of the surface of the photosensitive drum 1 (the direction of transport of the recording material P). Note that the direction approximately perpendicular to the direction of movement of the surface of the photosensitive drum 1 (the direction of transport of the recording material P) (i.e., the direction approximately parallel to the rotation axis direction of the charging roller 2) is sometimes called the "longitudinal direction".

[0042] In Figure 2, "Photoreceptor Region A" indicates the region on the photosensitive drum 1 where the photosensitive layer is formed, or the width of that region, in the longitudinal direction. "Charging Region (Charging Area) B" indicates the region on the charging roller 2 that can contact the surface of the photosensitive drum 1, or the width of that region, in the longitudinal direction. "Transfer Region (Transfer Area) C" indicates the region on the transfer roller 5 that can contact the surface of the photosensitive drum 1, or the width of that region, in the longitudinal direction. "Non-Transfer Region D" indicates the region on the charging roller 2 that contacts the photosensitive drum 1, and the transfer roller 5 that does not contact the photosensitive drum 1, or the width of that region (i.e., the difference between charging region B and transfer region C, or the width of that region). "Developing Region (Developing Area) E" indicates the region on the developing roller 4a where toner is coated, or the width of that region, in the longitudinal direction (more specifically, the region on the developing roller 4a where the toner coating can contact the surface of the photosensitive drum 1, or the width of that region). In this embodiment, the developing region G can also be defined as the region or width of the developing container 4b where an opening is provided to supply toner, which is the developer in the developing device 4, to the developing roller 4a. In other words, in this embodiment, toner is supplied to the developing roller 4a in the region where this opening is provided. For convenience, the regions on the photosensitive drum 1 corresponding to the above-mentioned "charging region B," "transfer region C," "non-transfer region D," and "developing region E" are also referred to as "charging region B," "transfer region C," "non-transfer region D," and "developing region E," respectively.

[0043] In this embodiment, the photoreceptor region A, charging region B, transfer region C, and developing region E are arranged such that the longitudinal center of each region approximately coincides with the longitudinal center of the image forming region (the region in which a toner image can be formed) (center reference). Therefore, among the above regions, those with a relatively shorter longitudinal width are contained within those with a relatively longer width. Figure 2 illustrates the range from the center to one end in the longitudinal direction.

[0044] In this embodiment, the transfer region C is shorter than the charging region B in the longitudinal direction, and the surface of the photosensitive drum 1 has a non-transfer region D at its longitudinal end that is in contact with the charging roller 2 but not with the transfer roller 5. Also in this embodiment, at least a portion of the developing region E overlaps with the non-transfer region D in the longitudinal direction. That is, in this embodiment, the developing region E is shorter than the charging region B and longer than the transfer region C in the longitudinal direction.

[0045] (3) Surface potential difference in the longitudinal direction of the photosensitive drum Next, we will explain the change in the surface potential of the photosensitive drum 1 during printing using Figure 3. In Figure 3, the horizontal axis shows the position on the photosensitive drum 1 in the longitudinal direction, illustrating the charged region B, transferred region C, and non-transferred region D described above. In Figure 3, the vertical axis shows the surface potential of the photosensitive drum 1, indicating that the higher the position in the figure, the higher the negative surface potential of the photosensitive drum 1 (i.e., the larger the absolute value of the negative surface potential). Furthermore, the surface potential of the photosensitive drum 1 in charged region B in Figure 3 is the surface potential of the non-exposed area on the photosensitive drum 1. Note that Figure 3 illustrates the range on one end side in the longitudinal direction. Also, the surface potential of the photosensitive drum 1 shown in Figure 3, which will be explained below, is a value that can change depending on various conditions such as the environment and the type of recording material P. Furthermore, in the following explanation, "after charging" means after passing the charging position Pa, "before transfer" means before reaching the transfer position Pd (transfer area Nt), "after transfer" means after passing the transfer position Pd (transfer area Nt), and "before charging" means before reaching the charging position Pa.

[0046] First, State 1 shows the surface potential of the photosensitive drum 1 after charging (and before transfer). In State 1, the surface of the photosensitive drum 1 is charged substantially uniformly to a predetermined dark area potential Vd by a charging roller 2 to which a predetermined charging voltage is applied. In the example in Figure 3, as an example, a charging voltage of -1100V is applied to the charging roller 2 during the charging process, and the surface of the photosensitive drum 1 is charged to a dark area potential Vd of -500V.

[0047] Next, state 2 shows the surface potential of the photosensitive drum 1 after transfer (and before recharging). When the recording material P passes through the transfer area Nt, a positive transfer voltage is applied to the transfer roller 5 in the transfer area Nt. As a result, the surface potential of the photosensitive drum 1 in the transfer area C decreases. On the other hand, in the non-transfer area D, the transfer roller 5 is not in contact with the photosensitive drum 1, so no positive transfer voltage is applied. Furthermore, in this embodiment, there is no means to lower the surface potential of the photosensitive drum 1 by irradiating the surface of the photosensitive drum 1 with light after transfer and before charging, such as a pre-charging exposure means. As a result, the surface potential of the photosensitive drum 1 in the non-transfer area D does not decrease significantly. This creates a potential difference between the surface potential of the photosensitive drum 1 in the transfer area C and the surface potential of the photosensitive drum 1 in the non-transfer area D. In the example in Figure 3, as an example, after transfer, the surface potential of the photosensitive drum 1 in the transfer area C is -400V, and the surface potential of the photosensitive drum 1 in the non-transfer area D remains at -500V. The potential difference between the surface potential of the photosensitive drum 1 in the transfer region C and the surface potential of the photosensitive drum 1 in the non-transfer region D is sometimes simply referred to as the potential difference between the transfer region C and the non-transfer region D.

[0048] Next, state 3 shows the surface potential of the photosensitive drum 1 after recharging (and before transfer). The surface of the photosensitive drum 1 is charged again by the charging roller 2 with a potential difference existing between the transfer region C and the non-transfer region D, as described above. In state 3, a predetermined charging voltage (-1100V) is applied to the charging roller 2, as described above. After recharging, the surface potential of the photosensitive drum 1 returns to the predetermined dark area potential Vd (-500V) in the transfer region C, as described above. On the other hand, in the non-transfer region D, the surface potential of the photosensitive drum 1 after recharging is already equivalent to the dark area potential Vd, so no discharge charging occurs. However, due to injection charging, the potential rises to -510V, which is higher than the predetermined dark area potential.

[0049] State 4 shows the surface potential of the photosensitive drum 1 after passing through the transfer area Nt and the charging position Pa multiple times (after multiple recharging cycles and before transfer) while a charging voltage (-1100V) is continuously applied to the charging roller 2. In the transfer area C, the surface potential of the photosensitive drum 1 after charging returns to the predetermined dark area potential Vd (-500V) as described above. On the other hand, in the non-transfer area D, the surface potential of the photosensitive drum 1 after charging gradually increases due to injection charging each time it passes through the charging position Pa. In the example in Figure 3, as an example, the surface potential of the photosensitive drum 1 in the non-transfer area D is -620V, and the potential difference between this and the surface potential of the photosensitive drum 1 in the transfer area C is 120V.

[0050] As described above, the surface potential of the photosensitive drum 1 may have a potential difference between the transfer region C and the non-transfer region D. Furthermore, as the potential difference between the transfer region C and the non-transfer region D increases, the surface potential of the photosensitive drum 1 in the non-transfer region D may rise excessively.

[0051] (4) Potential difference reduction operation Next, using Figure 4, we will explain the potential difference reduction operation (potential difference reduction sequence, potential difference improvement sequence, potential equalization control) that reduces the potential difference between the transfer region C and the non-transfer region D in this embodiment. In this embodiment, the image forming apparatus 100 can perform a potential difference reduction operation by applying a negative polarity voltage to the transfer roller 5 and exposing the photosensitive drum 1 by the exposure apparatus 3 when the toner image is not transferred to the recording material P, such as in the pre-rotation process, inter-paper process, and post-rotation process. The negative polarity voltage applied to the transfer roller 5 in the potential difference reduction operation is a DC voltage with the same polarity as the normal charging polarity of the toner, that is, a DC voltage with the opposite polarity to the transfer voltage during image formation (transfer) (hereinafter simply referred to as "negative bias"). This makes it possible to reduce the potential difference between the transfer region C and the non-transfer region D. Further details will be explained below.

[0052] We will explain, as an example, the case in which a potential difference reduction operation is performed to reduce the potential difference between the transfer region C and the non-transfer region D, starting from a state (state 4) where the surface potential of the photosensitive drum 1 in the non-transfer region C is higher than the surface potential of the photosensitive drum 1 in the transfer region C.

[0053] First, in the potential difference reduction operation, in addition to negatively charging the photosensitive drum 1 with the charging roller 2, a negative voltage (negative bias) is applied to the transfer roller 5. At this time, it is desirable to apply a negative bias to the transfer roller 5 such that the potential difference between the photosensitive drum 1 and the transfer roller 5 is less than or equal to the discharge threshold (discharge initiation voltage). In this embodiment, this discharge threshold is approximately 600V. As a result, the surface of the photosensitive drum 1 in the transfer region C is charged more strongly by the negative polarity compared to the surface of the photosensitive drum 1 in the non-transfer region D, and the potential difference between the transfer region C and the non-transfer region D becomes relatively smaller. However, the surface potential of the photosensitive drum 1 becomes higher than the predetermined dark area potential because it has been negatively charged by the transfer roller 5.

[0054] Next, in the potential difference reduction operation, the exposure device 3 exposes the photosensitive drum 1. At this time, the exposure device 3 exposes the drum with a lower light intensity than the light intensity used to expose the image area during image formation (exposure). As described above, the surface potential of the photosensitive drum 1, which is higher than a predetermined dark area potential due to the application of a negative bias to the transfer roller 5, is optimized by exposure by the exposure device 3. Here, the amount of change in the surface potential of the photosensitive drum 1 due to this exposure is greater the higher the surface potential before exposure. Therefore, even if there is a potential difference between the transfer area C and the non-transfer area D before exposure, and the surface potential of the photosensitive drum 1 in the non-transfer area D is higher than the surface potential of the photosensitive drum 1 in the transfer area C, it is possible to reduce the potential difference by exposure.

[0055] Figure 4 shows the change in surface potential of the photosensitive drum 1 during the potential difference reduction operation in this embodiment. The meaning of the horizontal and vertical axes in Figure 4 is the same as that of the horizontal and vertical axes in Figure 3, respectively.

[0056] First, state 4 shows the surface potential of the photosensitive drum 1 after passing through the transfer area Nt and the charging position Pa multiple times (after multiple recharging cycles and before transfer) while a charging voltage (-1100V) is continuously applied to the charging roller 2. This state is the same as state 4 in Figure 3, where the potential of the photosensitive drum 1 in the transfer area C is -500V, the surface potential of the photosensitive drum 1 in the non-transfer area D is -620V, and the potential difference between the transfer area C and the non-transfer area D is 120V.

[0057] Next, state 5 shows the surface potential of the photosensitive drum 1 after passing through the transfer area Nt (and before charging). In state 5, a negative voltage (negative bias) of -1000V is applied to the transfer roller 5, for example. As a result, the surface potential of the photosensitive drum 1 in the transfer area C rises. On the other hand, in the non-transfer area D, the transfer roller 5 is not in contact with the photosensitive drum 1, so no negative voltage is applied. As a result, the surface potential of the photosensitive drum 1 does not rise. Consequently, the potential difference between the transfer area C and the non-transfer area D is smaller than in state 4. In the example in Figure 4, for example, the surface potential of the photosensitive drum 1 in the transfer area C is -520V, while the surface potential of the photosensitive drum 1 in the non-transfer area D remains at -620V.

[0058] Next, state 6 shows the surface potential of the photosensitive drum 1 after recharging (and before transfer). As described above, the surface of the photosensitive drum 1 is charged again by the charging roller 2 with a potential difference created between the transfer region C and the non-transfer region D. In state 6, a predetermined charging voltage (-1100V) is applied to the charging roller 2, as described above. After recharging, the surface potential of the photosensitive drum 1 becomes -540V in the transfer region C and -630V in the non-transfer region D. In this recharging process, the lower the surface potential before recharging, the greater the change in surface potential after recharging. That is, the lower the surface potential, the greater the amount of charge injected from the charging roller 2. Therefore, the potential difference between the transfer region C and the non-transfer region D becomes smaller than in state 5.

[0059] Next, state 7 shows the surface potential of the photosensitive drum 1 after exposure (and before transfer). The surface of the photosensitive drum 1 is exposed by the exposure device 3 while a potential difference is generated between the transfer region C and the non-transfer region D, as described above. In this embodiment, in state 7, the exposure device 3 exposes substantially the entire longitudinal area of ​​the photosensitive drum 1 (substantially the entire area of ​​the photoreceptor region A or the charged region B) with a low amount of light. In this embodiment, as an example, the exposure device 3 uses 0.3 μJ / cm² for the image area during image formation. 2 An electrostatic latent image is formed by exposure with this exposure amount. In contrast, as an example, in the potential difference reduction operation, the exposure apparatus 3 reduces the potential difference of approximately the entire longitudinal area of ​​the photosensitive drum 1 to 0.03 μJ / cm². 2The surface potential is lowered by exposure with the specified exposure dose. After exposure, the surface potential of the photosensitive drum 1 becomes a predetermined dark area potential Vd (-500V) in the transfer region C and -560V in the non-transfer region D. In this exposure process, the higher the surface potential before exposure, the greater the change in surface potential after exposure. Therefore, the potential difference between the transfer region C and the non-transfer region D becomes even smaller than in state 6.

[0060] As described above, in the state after the potential difference reduction operation (state 7), the potential difference between the transfer region C and the non-transfer region D is 60V smaller compared to the state before the potential difference reduction operation (state 4). In this way, by performing the potential difference reduction operation, applying a negative voltage (negative bias) to the transfer roller 5, and performing exposure with low light intensity by the exposure device 3, the potential difference between the transfer region C and the non-transfer region D can be reduced. In other words, the surface potential of the photosensitive drum 1 can be equalized.

[0061] In the potential difference reduction operation, for example, exposure with low light intensity by the exposure device 3 can be started in conjunction with the timing when the position on the photosensitive drum 1, which was in the transfer area Nt at the time the negative bias was applied, reaches the exposure position Pb. For example, exposure with low light intensity by the exposure device 3 can be started after the timing when the position on the photosensitive drum 1, which was in the transfer area Nt at the time the negative bias was applied, reaches the exposure position Pb. It is desirable that the rotation of the photosensitive drum 1 continues for a sufficient amount so that the entire circumference of the photosensitive drum 1 passes through the transfer area Nt with the negative bias applied and the exposure position Pb where exposure with low light intensity by the exposure device 3 is being performed, at least once each. Typically, after the negative bias is applied and exposure with low light intensity by the exposure device 3 is started, the application of the negative bias and exposure with low light intensity by the exposure device 3 can be continued for about 1 to 10 rotations of the photosensitive drum 1. However, it is not limited to this, and it is sufficient as long as the potential difference between the transfer area C and the non-transfer area D is sufficiently reduced. The control of the timing for executing the potential difference reduction operation will be explained further later.

[0062] Here, unlike in this embodiment, it is conceivable to eliminate the potential difference between the transfer region C and the non-transfer region D by, for example, using a pre-exposure means or by performing high-intensity exposure with the exposure device 3, similar to the exposure of the image area during image formation. In this case, the amount of discharge to the photosensitive drum 1 due to the charging process will increase. An increase in the amount of discharge can make it easier for image defects called "image flow" to occur. "Image flow" is an image defect in which discharge products generated during the discharge adhere to the surface of the photosensitive drum 1 and absorb moisture, preventing normal charging treatment of the surface of the photosensitive drum 1, resulting in a decrease in image density, etc. In recent years, the lifespan of the image forming apparatus 100 has been extended, and the tendency for the amount of discharge to the photosensitive drum 1 to increase with long-term use is also one of the reasons why image flow is more likely to occur. Thus, in methods such as using a pre-exposure means or performing high-intensity exposure with the exposure device 3, there is a trade-off relationship between "reduction of potential difference" and "image flow" from the perspective of the amount of discharge, and it is difficult to achieve both. In contrast, in the potential difference reduction operation of this embodiment, a negative voltage (negative bias) is applied to the transfer roller 5, and exposure is performed with low light intensity by the exposure device 3. This suppresses the discharge amount in the charging process while reducing the potential difference between the transfer region C and the non-transfer region D. Therefore, the potential difference between the transfer region C and the non-transfer region D can be reduced while suppressing image flow.

[0063] In this embodiment, exposure by the exposure device 3 is performed each time the surface of the photosensitive drum 1 passes between the transfer area Nt and the charged area Pa during the potential difference reduction operation. However, the present invention is not limited to this embodiment. For example, the surface of the photosensitive drum 1 may be passed between the transfer area Nt and the charged area Pa multiple times to reduce the potential difference between the transfer area C and the non-transfer area D before exposure by the exposure device 3 is performed.

[0064] Furthermore, in this embodiment, the potential difference reduction operation is performed when the toner image is not being transferred to the recording material P. However, there may not be enough time to perform this operation between sheets of paper during continuous printing. In such cases, the time between sheets of paper during continuous printing may be increased and the potential difference reduction operation may be performed only when necessary.

[0065] Furthermore, in this embodiment, a negative voltage of -1000V (negative bias) is applied to the transfer roller 5. By increasing this voltage, the time required to reduce the potential difference can be shortened. The execution time of the potential difference reduction operation (such as the rotation time, number of rotations, or rotation distance of the photosensitive drum 1 that performs the application of the negative bias and exposure by the exposure device 3) may be changed by the voltage applied to the transfer roller 5. In other words, the larger the negative bias, the shorter the execution time of the potential difference reduction operation can be. It is even more desirable that the negative bias is a voltage that does not cause discharge between the photosensitive drum 1 and the transfer roller 5.

[0066] Furthermore, the magnitude of the negative bias in the potential difference reduction operation and the execution time of the potential difference reduction operation may be changed depending on the surface potential of the photosensitive drum 1 in the non-transfer region D, or the potential difference between the transfer region C and the non-transfer region D. For example, the larger the surface potential of the photosensitive drum 1 in the non-transfer region D, or the larger the potential difference between the transfer region C and the non-transfer region D, the larger the magnitude of the negative bias can be, or the longer the execution time of the potential difference reduction operation can be. Both of these may be combined.

[0067] Furthermore, in this embodiment, the exposure amount (exposure amount per unit area) is made substantially uniform along the longitudinal direction of the photosensitive drum 1 during the potential difference reduction operation, but the present invention is not limited to this embodiment. For example, the exposure amount (exposure amount per unit area) for the non-transfer region D at the longitudinal end of the photosensitive drum 1 may be higher than the exposure amount (exposure amount per unit area) for the transfer region C. In other words, the exposure amount for the non-transfer region D can be greater than or equal to the exposure amount for the transfer region C.

[0068] (5) Poor cleaning Next, we will explain "edge contamination," which is an example of a problem that can occur when the potential difference between the transfer region C and the non-transfer region D becomes large (when the surface potential of the photosensitive drum 1 in the non-transfer region D rises excessively).

[0069] As mentioned above, when the potential difference between the transfer area C and the non-transfer area D increases, the Vback, which is the potential difference between the dark area potential (surface potential of the unexposed area) of the photosensitive drum 1 and the potential of the developing roller 4a (potential of the developing voltage), increases in the non-transfer area D. In this case, a phenomenon called "reverse fogging" may occur in the non-transfer area D, where "reverse toner," which is charged with the opposite polarity to the normal charging polarity, adheres to the surface of the photosensitive drum 1. If a large amount of toner adheres to the surface of the photosensitive drum 1 in the non-transfer area D due to this "reverse fogging," cleaning failures may occur. Furthermore, due to these cleaning failures, "edge fogging" may occur, where the edges of the recording material P in a direction approximately perpendicular to the transport direction of the recording material P become soiled with toner.

[0070] Here, we will explain the effect of the difference in surface potential of the photosensitive drum 1 on "fogging." Figure 5 is a graph showing the relationship between Vback, which is the potential difference between the dark area potential of the photosensitive drum 1 and the potential of the developing roller 4a, and "fogging." Note that Vback is represented as a positive value when the dark area potential of the photosensitive drum 1 is greater than the potential of the developing roller 4a on the side of the normal charging polarity of the toner.

[0071] The "fogging" on the photosensitive drum 1 was measured as follows: Toner was collected by attaching the adhesive side of a transparent adhesive tape to the photosensitive drum 1. Then, the adhesive tape was attached to a designated piece of paper, and the density of the adhesive tape to which the toner had adhered (fogging density (%)) was measured to quantify the "fogging." If no "fogging" occurred, the fogging density was 0%, and a higher fogging density value indicated a greater degree of "fogging" and that a large amount of toner was adhering to the surface of the photosensitive drum 1.

[0072] There are several types of "fogging." First, there is "ground fogging," which occurs when the potential difference between the dark area potential of the photosensitive drum 1 and the developing roller 4a becomes small, causing toner charged with the correct polarity to adhere to the surface of the photosensitive drum 1. Second, there is "reverse fogging," which occurs when the potential difference between the dark area potential of the photosensitive drum 1 and the developing roller 4a becomes large, causing "reverse toner," charged with the opposite polarity to the correct polarity, to adhere to the surface of the photosensitive drum 1.

[0073] As shown in Figure 5, in this embodiment, the fouling is lowest at around 2% when Vback is around 120V. This level of fouling is not problematic. On the other hand, when Vback exceeds 220V, fouling (reverse fouling) exceeding 10% occurs, which may lead to cleaning failures.

[0074] To prevent "inversion flicker" in the non-transfer region D, one method is to reduce the Vback of the non-transfer region D by lowering the dark area potential of the photosensitive drum 1 or raising the potential of the developing roller 4a. However, if the potential is changed while the potential difference between the transfer region C and the non-transfer region D is large, the Vback of the transfer region C will decrease, and then "ground flicker" will become a problem in the transfer region C. Therefore, it is desirable to reduce the potential difference between the transfer region C and the non-transfer region D.

[0075] In other words, it is even more desirable in the configuration of this embodiment to make the Vback uniform in the longitudinal direction. Alternatively, after setting the transfer bias to a negative bias, the surface potential of the photosensitive drum 1 may be changed in a direction that makes it uniform in the longitudinal direction of the photosensitive drum, and then the charging voltage and developing voltage may be controlled to adjust the Vback.

[0076] Using Figures 3 and 4, the degree of fogging (reverse fogging) in this embodiment will be explained after the initial charging (state 1), after passing through the transfer section Nt and the charging position Pa multiple times (state 4), and after the potential difference reduction operation (state 7). In Figures 3 and 4, the thick dotted line indicates the potential of the developing roller 4a. The potential difference between the surface potential of the photosensitive drum 1 and the potential of the developing roller 4a is Vback.

[0077] First, in state 1 (Figure 3), a charging voltage of -1100V is applied to the charging roller 2, and the surface potential of the photosensitive drum 1 is uniformly charged to a predetermined dark area potential of -500V. In addition, a developing voltage of -380V is applied to the developing roller 4a. Therefore, in the transfer area C and the non-transfer area D, Vback(1) is uniformly 120V, and fogging is at an acceptable level.

[0078] Next, in state 4 (Figure 3), the surface potential of the photosensitive drum 1 in the transfer area C is -500V, which is the predetermined dark area potential. Therefore, the Vback(2) of the transfer area C is approximately equal to the Vback(1) above, and as before, the fogging is at an acceptable level. On the other hand, the surface potential of the photosensitive drum 1 in the non-transfer area D is -620V, and the Vback(3) of the non-transfer area D is 240V. Since the fogging exceeds 10%, cleaning failures may occur. And, as a result of these cleaning failures, edge fogging may occur.

[0079] Next, in state 7 (Figure 4) after the potential difference reduction operation, the surface potential of the photosensitive drum 1 in the transfer area C is -500V, which is the predetermined dark area potential Vd. Therefore, the Vback(4) in the transfer area C is approximately equal to the Vback(1) above, and as before, the fogging is at an acceptable level. Also, the surface potential of the photosensitive drum 1 in the non-transfer area D is -560V, and the Vback(5) in the non-transfer area D is 180V. The fogging does not exceed 10%, so again, the fogging is at an acceptable level.

[0080] In this way, by performing a potential difference reduction operation to apply a negative voltage (negative bias) to the transfer roller 5 and performing exposure by the exposure device 3, edge fouling caused by cleaning defects due to fouling can be suppressed.

[0081] In this embodiment, during the potential difference reduction operation, a negative voltage (negative bias) is applied to the transfer roller 5 to reduce the potential difference between the transfer region C and the non-transfer region D, and then exposure is performed by the exposure device 3. This brings the surface potential of the photosensitive drum 1 closer to a predetermined dark area potential over almost the entire longitudinal region, and brings the Vback closer to 120V, thereby improving fogging that causes cleaning defects. After applying a negative voltage to the transfer roller 5, the voltage applied to the charging roller 2 may be lowered, or the voltage applied to the developing roller 4a may be increased, to bring the Vback closer to 120V and improve fogging that causes cleaning defects.

[0082] (6) Operating Procedure Figure 6 is a flowchart illustrating the general procedure of a print job in this embodiment. The process shown in Figure 6 is initiated when the image forming apparatus 100 receives print job information and is executed by the control unit 40 (more specifically, the CPU 41). In this embodiment, the execution of the potential difference reduction operation is controlled based on the paper feeding history information of the recording material P.

[0083] First, the control unit 40 receives information about the print job and performs image formation preparation (S101). More specifically, the control unit 40 drives the motor to drive various rotating members inside the image forming apparatus 100 (such as the photosensitive drum 1 and various rollers), and also supplies power to the heater of the fixing device 12 to preheat the fixing device 12. Next, in this embodiment, the control unit 40 sets the count value of the number of images formed in the counter (number of images counter, sequence counter) provided in the RAM 41b to an initial value (0 in this embodiment) (S102). In this embodiment, each time an image is formed on one side of the recording material P (each time the recording material P is supplied to the transfer unit Nt), the control unit 40 adds 1 to the count value of the number of images formed in the RAM 41b, and updates and stores it sequentially. Next, the control unit 40 determines whether the count value of the number of images formed in the RAM 41b is equal to or greater than a predetermined value (threshold) (S103). If the control unit 40 determines in S103 that the count value of the number of images formed is less than a predetermined value ("No"), it proceeds to the process in S107. If the control unit 40 determines in S103 that the count value of the number of images formed is equal to or greater than a predetermined value ("Yes"), it temporarily stops the transport of the recording material P, proceeds through the processes in S104 to S106, and then proceeds to the process in S107. In other words, the control unit 40 applies a negative voltage (negative bias) to the transfer roller 5 (S104), exposes the photosensitive drum 1 with the exposure device 3 (S105), resets the count value of the number of images formed in RAM 41b to an initial value (0 in this embodiment) (S106), and then proceeds to the process in S107. Here, the threshold for the number of images formed at which the potential difference reduction operation is performed can be determined by how the surface potential of the photosensitive drum 1 in the non-transfer region D rises, and may be changed according to information such as the configuration of the image forming apparatus 100, the operating environment, and the total usage history of the image forming apparatus 100.

[0084] Subsequently, the control unit 40 starts feeding the recording material P to the transfer unit Nt (S107), and then adds 1 to the count value of the number of images formed in RAM 41b (S108). Next, the control unit 40 performs the image formation operation under the specified image formation conditions (S109). Next, the control unit 40 determines whether the output of all images specified in the print job has been completed (S110). If it determines that it has not been completed ("No"), it returns to the process in S103; if it determines that it has been completed ("Yes"), it terminates the print job.

[0085] (7) Effects Next, the results of the evaluation test demonstrating the effects of this embodiment will be described. Here, using the image forming apparatus 100 configured in this embodiment, 200 consecutive images were formed under normal temperature and humidity conditions (23°C / 50%RH), and the fogging (inversion fogging) in the non-transfer region D was evaluated. The recording material P used was 75 g / m². 2 LTR-sized paper was used. The threshold for the number of images required to perform the potential difference reduction operation was set to 50.

[0086] Table 1 shows the evaluation results when image forming was performed from the 1st to the 200th image for the image forming apparatus 100 of this embodiment and the image forming apparatus 100 of Comparative Example 1. The configuration and operation of the image forming apparatus 100 of Comparative Example 1 are substantially the same as those of the image forming apparatus 100 of this embodiment, except that the control of the execution of the potential difference reduction operation is different, as will be described later. In Comparative Example 1, elements having the same or corresponding functions or configurations as those of this embodiment are denoted by the same reference numerals. In Table 1, regarding fogging, a fogging density of less than 10% in the non-transfer region D is indicated as "○ (good)", and a fogging density of 10% or more in the non-transfer region D is indicated as "× (poor)".

[0087] [Table 1]

[0088] Comparative Example 1 is an example of image formation without performing a potential difference reduction operation. In this case, when continuous image formation is performed, the potential difference between the transferred region C and the non-transferred region D gradually increases. As a result, the Vback increases in the non-transferred region D, which can worsen fogging and lead to cleaning failures.

[0089] On the other hand, in this embodiment, a potential difference reduction operation was performed every 50 images based on the paper feeding history of the recording material P. In this case, when continuous image formation is performed, the potential difference between the transfer area C and the non-transfer area D is reset every 50 images. As a result, the increase in Vback in the non-transfer area D is suppressed, and thus the occurrence of cleaning defects is suppressed.

[0090] In this embodiment, the execution of the potential difference reduction operation was controlled based on the paper feeding history of the recording material P, but the present invention is not limited to this embodiment. Any index value that can estimate the surface potential (or its change) of the photosensitive drum 1 in the non-transfer region D, or the potential difference (or its change) between the transfer region C and the non-transfer region D, can be used to control the execution of the potential difference reduction operation. This index value can be said to be an index value that correlates with the time during which the photosensitive drum 1 was charged. For example, the rotation time, number of rotations, or rotation distance of the photosensitive drum 1 may be measured by a measuring means (counter) provided in the image forming apparatus 100, and the execution of the potential difference reduction operation may be controlled based on this value. For example, the potential difference reduction operation can be executed when the value measured by the above measuring means exceeds a predetermined threshold. Alternatively, the execution of the potential difference reduction operation may be controlled based on the results of measuring the surface potential of the photosensitive drum 1 in real time, for example, by a measuring means (measuring instrument) provided in the image forming apparatus 100. For example, if the surface potential value of the non-transfer region D of the photosensitive drum 1, or the potential difference value between the transfer region C and the non-transfer region D, measured by the above-mentioned measurement means, exceeds a predetermined threshold, a potential difference reduction operation can be performed.

[0091] Furthermore, in this embodiment, the timing of the potential difference reduction operation is determined based on the potential difference between the transfer area C and the non-transfer area D, where fogging may occur beyond the acceptable range. Therefore, the timing of the potential difference reduction operation may be changed depending on the fogging situation. That is, when the image forming apparatus 100 is used in a low-temperature, low-humidity environment where fogging is likely to occur, or when a cartridge is nearing the end of its lifespan and the proportion of inverted toner in the toner in the developing container 4b tends to increase, the following may be done: The timing of the potential difference reduction operation may be set earlier, i.e., the frequency of the potential difference reduction operation may be increased. Alternatively, the execution time of the potential difference reduction operation may be increased. For example, the image forming apparatus may be provided with an environmental sensor (such as a temperature and humidity sensor) as an environmental detection means for detecting the environment (at least one of the temperature or humidity inside or outside the image forming apparatus 100). The smaller the absolute moisture content (or temperature or humidity) indicated by the detection result of the environmental sensor, the more frequently the potential difference reduction operation can be performed (for example, by reducing the threshold for the number of images formed). The frequency of the potential difference reduction operation may be changed when the absolute moisture content (or temperature or humidity) indicated by the environmental sensor's detection result falls below a predetermined threshold. Furthermore, a memory unit (for example, a non-volatile memory provided in the cartridge or the device itself) is provided to store the cartridge's usage history. The frequency of the potential difference reduction operation can be controlled to increase as the remaining lifespan of the cartridge decreases (for example, by decreasing the threshold for the number of images formed). The frequency of the potential difference reduction operation may also be changed when the remaining lifespan of the cartridge falls below a predetermined threshold. The cartridge's lifespan can be determined, for example, by detecting the usage amount of the photosensitive drum 1 (rotation time, number of rotations, or rotation distance, etc.) and the remaining amount of toner in the developing container 4b using detection means.

[0092] Thus, the control unit 40 can change the execution frequency of the potential difference reduction operation, which is correlated with the number of recording materials P on which the toner image is transferred between the execution of one potential difference reduction operation and the execution of the next potential difference reduction operation. In addition, the control unit 40 can change the execution time of the potential difference reduction operation, which is correlated with the rotation time of the photosensitive drum 1 during the potential difference reduction operation.

[0093] Thus, in this embodiment, the image forming apparatus 100 includes a rotatable photoreceptor 1, a rotatable charging member 2 that contacts the photoreceptor 1 to form a charging section B and charges the surface of the photoreceptor 1 in the charging section B, an exposure apparatus 3 that exposes the surface of the photoreceptor 1, which has been charged by the charging member 2, with a first exposure amount to form an electrostatic image on the surface of the photoreceptor 1, a developing member 4a that contacts the photoreceptor 1 to form a developing section E and supplies toner charged with normal polarity to the electrostatic image formed on the surface of the photoreceptor 1 in the developing section E to form a toner image, and a transfer section Nt that contacts the surface of the photoreceptor 1 The device comprises a transfer member 5 that transfers a toner image from the surface of the photoreceptor 1 to the recording material P in the transfer section Nt by applying a transfer voltage with the opposite polarity to the normal polarity, a transfer power supply 18 that applies a voltage to the transfer member 5, and a control unit 40 that can control the exposure device 3 and the transfer power supply 18. In the rotation axis direction of the charging member 2, the width of the transfer section Nt is shorter than the width of the developing section E, and the width of the transfer section Nt is shorter than the width of the charging section B, and the end of the surface of the photoreceptor 1 in the rotation axis direction has a non-transfer region D that is in contact with the charging member 2 but not in contact with the transfer member 5. In this embodiment, the control unit 40 can control the device to apply a voltage with the normal polarity to the transfer member 5 when there is no recording material P in the transfer section Nt, and to perform an operation in which the exposure device 3 exposes the area of ​​the surface of the photoreceptor 1 that has passed through the transfer section Nt, including the non-transfer region D, with a second exposure amount smaller than the first exposure amount when the voltage is applied. In this embodiment, the control unit 40 controls the exposure device 3 to expose the region on the surface of the photoreceptor 1 that includes the transfer region C in contact with the transfer member 5 and the non-transfer region D. The control unit 40 can also control the exposure amount of the exposure device 3 to the non-transfer region D to be greater than or equal to the exposure amount of the exposure device 3 to the transfer region C. In addition, in this embodiment, the control unit 40 controls the application of a voltage with the normal polarity, which has an absolute value greater than the surface potential of the photoreceptor 1 at the transfer portion Nt, to the transfer member 5. In addition, in this embodiment, the control unit 40 controls the application of a voltage with the normal polarity, which has an absolute value greater than the surface potential of the photoreceptor 1 at the transfer portion Nt, to the transfer member 5.Here, the control unit 40 can perform the above operation based on an index value correlated with the time the photoreceptor 1 has been charged. The index value may be the paper passage history of the recording material P, the rotation time of the photoreceptor 1, the number of rotations of the photoreceptor 1, or the rotation distance of the photoreceptor 1. The control unit 40 can also perform the above operation based on the surface potential of the photoreceptor 1 in the non-transfer region D. In this case, the control unit 40 can perform the above operation based on the potential difference between the surface potential of the photoreceptor 1 in the non-transfer region D and the surface potential of the photoreceptor 1 in the transfer region C that is in contact with the transfer member 5 on the surface of the photoreceptor 1.

[0094] As described above, according to this embodiment, by performing a potential difference reduction operation, it is possible to suppress an excessive rise in the surface potential of the photosensitive drum 1 in the non-transfer region D. Thus, according to this embodiment, in a configuration where the contact region C between the surface of the photosensitive drum 1 and the transfer roller 5 is shorter than the contact region B between the surface of the photosensitive drum 1 and the charging roller 2 in the longitudinal direction, it is possible to suppress an excessive rise in the surface potential of the longitudinal end (non-transfer region D) of the photosensitive drum 1. This makes it possible to suppress edge contamination caused by cleaning defects due to fouling.

[0095] In this embodiment, edge fouling caused by cleaning defects due to fouling was described as an example of a problem that can occur when the surface potential of the photosensitive drum 1 in the non-transfer region D rises excessively. As mentioned above, another example of a problem that can occur when the surface potential of the photosensitive drum 1 in the non-transfer region D rises excessively is the occurrence of charging defects (streaky images) due to damage to the surface of the photosensitive drum 1 caused by discharge between the photosensitive drum 1 and the core metal portion of the transfer roller 5. According to this embodiment, a similar suppression effect on other problems that can occur when the surface potential of the photosensitive drum 1 in the non-transfer region D rises excessively can also be expected.

[0096] [Example 2] Next, other embodiments of the present invention will be described. The basic configuration and operation of the image forming apparatus in 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 denoted by the same reference numerals as in Embodiment 1, and detailed descriptions are omitted.

[0097] In Example 1, the execution of the potential difference reduction operation was controlled based on the paper feeding history of the recording material P. In this example, the image forming apparatus 100 can change the positive polarity transfer voltage applied to the transfer roller 5 during transfer (image formation) based on information about the image pattern detected by the image detection means. In this example, when the positive polarity transfer voltage applied to the transfer roller 5 during transfer is reduced, the image forming apparatus 100 aims to improve productivity by reducing the frequency of the potential difference reduction operation (or shortening the execution time of the potential difference reduction operation).

[0098] Here, we will explain the effect on the potential difference between the transfer region C and the non-transfer region D of the image pattern to be formed. The amount of positive transfer voltage applied to the transfer roller 5 during transfer is determined by the image pattern to be formed. For example, if a large amount of toner is to be transferred, such as a completely black image, the required transfer voltage will be large, and if a small amount of toner is to be transferred, such as characters, the required transfer voltage will be small.

[0099] On the other hand, the potential difference between the transfer region C and the non-transfer region D is determined by the application of a positive transfer voltage to the transfer region C. This is because the magnitude of the positive transfer voltage applied to the transfer roller 5 during transfer affects how much the surface potential of the photosensitive drum 1 in the transfer region C decreases after transfer. When the positive transfer voltage applied to the transfer roller 5 during transfer is small, the above potential difference is small, and when the positive transfer voltage applied to the transfer roller 5 during transfer is large, the above potential difference is large.

[0100] In other words, when the image pattern to be formed is an image pattern with a low print density (image ratio, image duty cycle), such as a character image, the positive transfer voltage applied to the transfer roller 5 during transfer can be reduced. When the positive transfer voltage applied to the transfer roller 5 during transfer is reduced, the potential difference between the transferred area C and the non-transferred area D becomes smaller. Therefore, the timing of the potential difference reduction operation can be delayed, that is, the frequency of the potential difference reduction operation can be reduced. Alternatively, the execution time of the potential difference reduction operation can be shortened.

[0101] Next, the image detection means will be described. The image detection means is a device that analyzes image information input to the image forming apparatus 100 from an external device 200. In this embodiment, the video controller 110, acting as the image detection means, detects the print density of the image pattern to be formed. In this embodiment, the video controller 110 detects the print density for each image to be formed on one recording material P and inputs information indicating the result (print density information) to the control unit 40. The print density can be expressed as the percentage (%) of pixels in the image forming area on which a toner image is formed. Based on the print density information detected by the video controller 110, the control unit 40 determines that the print density is low if it is below a predetermined threshold and changes (reduces) the transfer voltage applied to the transfer roller 5 during transfer. In this embodiment, the threshold for the print density was set to 5%. In other words, the control unit 40 determines that the print density is low if it is 5% or less and changes (reduces) the transfer voltage applied to the transfer roller 5 during transfer.

[0102] Figure 7 is a flowchart illustrating the general procedure of a print job in this embodiment. The process shown in Figure 7 is initiated when the image forming apparatus 100 receives print job information and is executed by the control unit 40 (more specifically, the CPU 41). In this embodiment, the image forming conditions are changed and the timing of the potential difference reduction operation is changed based on the print density information of the image to be formed.

[0103] First, the control unit 40 receives information about the print job and performs image formation preparation (S201). More specifically, the control unit 40 drives the motor to drive various rotating members inside the image forming apparatus 100 (such as the photosensitive drum 1 and various rollers), and also supplies power to the heater of the fixing device 12 to preheat the fixing device 12. Next, the control unit 40 sets the count value of the number of images formed in the counter (number of images counter, sequence counter) provided in the RAM 41b to an initial value (0 in this embodiment) (S202). In this embodiment, the control unit 40 adds 1 to the count value of the number of images formed in the RAM 41b each time an image is formed on one side of the recording material P (each time the recording material P is supplied to the transfer unit Nt), and updates and stores the value sequentially. Next, the control unit 40 determines whether the count value of the number of images formed in the RAM 41b is equal to or greater than a predetermined value (threshold) (S203). If the control unit 40 determines in S203 that the count value of the number of images formed is less than a predetermined value ("No"), it proceeds to the process in S207. If the control unit 40 determines in S203 that the count value of the number of images formed is equal to or greater than a predetermined value ("Yes"), it temporarily stops the transport of the recording material P, proceeds through the processes in S204 to S206, and then proceeds to the process in S207. In other words, the control unit 40 applies a negative voltage (negative bias) to the transfer roller 5 (S204), exposes the photosensitive drum 1 with the exposure device 3 (S205), resets the count value of the number of images formed in RAM 41b to an initial value (0 in this embodiment) (S206), and then proceeds to the process in S207.

[0104] Subsequently, the control unit 40 starts feeding the recording material P to the transfer unit Nt (S207), and then adds 1 to the count value of the number of images formed in RAM 41b (S208). Next, the control unit 40 acquires the print density information of the image detected by the video controller 110 (S209). Next, the control unit 40 determines whether the print density of the image is less than a predetermined value (S210). If the control unit 40 determines in S210 that the print density is greater than or equal to the predetermined value ("No"), it proceeds to the process in S213. If the control unit 40 determines in S210 that the print density is less than a predetermined value ("Yes"), it proceeds to the process in S213 after going through the processes in S211 and S212. In other words, the control unit 40 subtracts 0.5 from the count value of the number of images formed in RAM 41b (S211). Furthermore, the control unit 40 changes the image formation conditions so that the transfer voltage applied to the transfer roller 5 during transfer is reduced to a value corresponding to a preset low print density image (S212). Then, the control unit 40 proceeds to the process in S213.

[0105] Next, the control unit 40 performs an image formation operation under the specified image formation conditions (S213). Then, the control unit 40 determines whether the output of all images specified in the print job has been completed (S214). If it determines that the output has not been completed ("No"), it returns to the process in S203. If it determines that the output has been completed ("Yes"), it terminates the print job.

[0106] Next, the results of the evaluation test demonstrating the effects of this embodiment will be described. Here, using the image forming apparatus 100 configured in this embodiment, 200 consecutive images were formed under normal temperature and humidity conditions (23°C / 50%RH), and the fogging (inversion fogging) in the non-transfer area D was evaluated. As the images to be formed, low print density images with a print density of 1% were used. The recording material P had a basis weight of 75 g / m². 2 LTR-sized paper was used. The threshold for the number of images required to perform the potential difference reduction operation was set to 50.

[0107] Table 2 shows the evaluation results for the image forming apparatus 100 of this embodiment and the image forming apparatus 100 of Comparative Example 2 when image forming was performed from the 1st to the 200th image. The configuration and operation of the image forming apparatus 100 of Comparative Example 2 are substantially the same as those of the image forming apparatus 100 of this embodiment, except that the control of the execution of the potential difference reduction operation is different, as will be described later. In Comparative Example 2, elements having the same or corresponding functions or configurations as those of this embodiment are denoted by the same reference numerals. In Table 2, regarding fogging, a fogging density of less than 10% in the non-transfer region D is indicated as "○ (good)", and a fogging density of 10% or more is indicated as "× (poor)". Table 2 also includes the count value of the number of images formed by the counter at each image forming stage, and whether or not the potential difference reduction operation was performed immediately afterward.

[0108] [Table 2]

[0109] Comparative Example 2 is an example in which a potential difference reduction operation is performed based on the paper feeding history of the recording material P, but image formation is performed without changing the control (i.e., without reducing the frequency of execution of the potential difference reduction operation) even in the case of low print density images. In this case, when continuous image formation is performed, the potential difference reduction operation is executed every 50 images formed, so although overlap is not a problem, the potential difference reduction operation will be executed 4 times during the formation of 200 images.

[0110] On the other hand, in this embodiment, if the print density is 5% or less, it is determined to be a low-print density image, and therefore the image formed in this evaluation test is determined to be a low-print density image. In this embodiment, when the image to be formed is low-print density, the transfer voltage applied to the transfer roller 5 during transfer is reduced, and the count value of the number of images formed by the counter is decreased by 0.5 each time. Therefore, in this embodiment, the potential difference reduction operation is performed only twice during the formation of 200 images: after the 100th image and after the 200th image. Thus, according to this embodiment, productivity can be improved by reducing the number of times the potential difference reduction operation is performed compared to Comparative Example 2, while maintaining the level of fogging at an acceptable level.

[0111] In this embodiment, the image formation conditions are changed based on the image print density information, and the frequency of the potential difference reduction operation is changed. However, the present invention is not limited to this embodiment. Instead of changing the frequency of the potential difference reduction operation, at least one of the conditions, such as the duration of a single potential difference reduction operation and the voltage (negative bias) applied to the transfer roller 5 during the potential difference reduction operation, may be changed. For example, instead of reducing the frequency of the potential difference reduction operation, the duration of the potential difference reduction operation can be shortened as described above. Alternatively, instead of reducing the frequency of the potential difference reduction operation, the voltage (negative bias) applied to the transfer roller 5 during the potential difference reduction operation or the exposure amount by the exposure device 3 can be reduced. Two or more of the frequency of the potential difference reduction operation, the duration of the potential difference reduction operation, and the conditions in the potential difference reduction operation may be changed in combination.

[0112] Furthermore, in this embodiment, if it is determined that the image to be formed is a low-printing-rate image, the counter's count value for the number of images formed is reduced by 0.5. However, the present invention is not limited to this embodiment. The amount by which the transfer voltage is changed during transfer in the case of low print-rate compared to the transfer voltage during transfer in the case of high print-rate differs depending on the configuration of the image forming apparatus 100. Therefore, the reduction value of the counter's count value for the number of images formed may also be appropriately changed depending on the configuration of the image forming apparatus 100.

[0113] Furthermore, the transfer voltage during transfer may be changed in multiple stages depending on the print density information of the image to be formed, and the frequency and duration of the potential difference reduction operation may be changed in multiple stages accordingly.

[0114] Furthermore, in this embodiment, a determination is made as to whether each image to be formed is an image with low print density, but the present invention is not limited to this embodiment. For example, one image may be divided into multiple regions (for example, divided into multiple regions in the transport direction of the recording material P), and the print density of the image in each region may be detected. Then, according to the print density of the image in each region, the transfer voltage during transfer may be changed, and the subtraction value of the count value of the number of images formed by the counter (i.e., the frequency and execution time of the potential difference reduction operation) may be changed accordingly. Alternatively, for example, the image formation conditions and the frequency and execution time of the potential difference reduction operation may be changed based on the average value of the print density of multiple images.

[0115] Furthermore, in this embodiment, print density information is used as information regarding the image pattern to be formed, but any index value that correlates with the amount of toner in the image to be formed can be used in the same way. For example, a pixel count value, which is the integrated value of density information for each pixel, may be used.

[0116] Thus, the control unit 40 can change the execution frequency of the above operation, which correlates with the number of recording materials P to which the toner image is transferred between the execution of one operation and the execution of the next. The control unit 40 can change the execution frequency based on the environment or the usage history of the image forming apparatus 100. The control unit 40 can also change the execution frequency based on the image information of the toner image formed between the execution of one operation and the execution of the next. At this time, the control unit 40 can change the transfer voltage during toner image transfer based on the image information. The image information may be print density information. The control unit 40 can control the execution frequency to decrease when the print density information is smaller than a predetermined threshold. The control unit 40 can also change the execution time of the above operation, which correlates with the rotation time of the photoreceptor 1 in the above operation. The control unit 40 can change the execution time based on the environment or the usage history of the image forming apparatus 100. The control unit 40 can also change the execution time based on the image information of the toner image formed between the execution of one operation and the execution of the next. At this time, the control unit 40 can change the transfer voltage during toner image transfer based on the image information. The image information may be print density information. Furthermore, if the print density information is smaller than a predetermined threshold, the control unit 40 can control the execution time to be shortened.

[0117] As described above, this embodiment provides the same effects as in Example 1, while also suppressing a decrease in productivity.

[0118] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the embodiments described above.

[0119] In the above-described embodiment, the case where the transfer member is a transfer roller was explained, but the transfer member is not limited to a transfer roller. The transfer member may be configured, for example, with a rotatable endless belt that contacts the photoreceptor. A voltage applying member (roller, brush, sheet, etc.) that supplies a transfer voltage to the transfer part via the transfer belt may be arranged on the inner circumferential surface side of this transfer belt at a position facing the photoreceptor. Furthermore, the transfer member is not limited to a rotating body, but may be in other forms such as a pad-shaped member, a sheet-shaped (film-shaped) member, or a fixed brush-shaped member.

[0120] Furthermore, although the above-described embodiment described a case where the photoreceptor is a photosensitive drum, the photoreceptor is not limited to a photosensitive drum. The photoreceptor may be a photosensitive belt configured in an endless belt shape.

[0121] Furthermore, in the above-described embodiment, the image forming apparatus was not provided with a pre-exposure means. As mentioned above, the phenomenon in which the surface potential of the non-transfer region at the longitudinal end of the photoreceptor rises to an excessive potential tends to be more pronounced when the image forming apparatus employs a pre-exposure-less method. Therefore, the present invention is particularly effective when the image forming apparatus employs a pre-exposure-less method. However, the present invention is not limited to this configuration. The present invention can also be applied to an image forming apparatus provided with a pre-exposure means. In this case, by applying the present invention, the effects described in the above-described embodiment can be obtained, as well as the effect of reducing the amount of exposure by the pre-exposure means and thereby reducing the amount of discharge due to the charging process. Similarly, the present invention is particularly effective when employing a DC charging method, but it can also be applied when employing an AC / DC charging method.

[0122] Furthermore, as in the above-described embodiment, a configuration in which the transfer region is shorter than the development region in the longitudinal direction has the effect of miniaturizing the image forming apparatus, but the present invention is not limited to such a configuration. The present invention can also be applied to configurations in which the length of the transfer region is greater than or equal to the length of the development region in the longitudinal direction, and effects such as suppression of damage to the photoreceptor as described above can be obtained. [Explanation of Symbols]

[0123] 1 Photosensitive drum 2 Charging rollers 3. Exposure apparatus 4. Developing device 5 Transfer roller 6. Cleaning device 40 Control Unit

Claims

1. A rotatable photoreceptor, A rotatable charging member that contacts the photoreceptor to form a charged portion and charges the surface of the photoreceptor at the charged portion, An exposure apparatus that exposes the surface of the photoreceptor, which has been charged by the charging member, with a first exposure amount to form an electrostatic image on the surface of the photoreceptor, A developing member that contacts the photoreceptor to form a developing section, and in the developing section supplies toner charged with normal polarity to the electrostatic image formed on the surface of the photoreceptor to form a toner image, A transfer member that contacts the surface of the photoreceptor to form a transfer area, and to which a transfer voltage with the opposite polarity to the normal polarity is applied, thereby transferring a toner image from the surface of the photoreceptor to the recording material in the transfer area, A transfer power supply for applying voltage to the transfer member, A control unit capable of controlling the exposure apparatus and the transfer power supply, It has, In an image forming apparatus in which, in the rotational axis direction of the charging member, the width of the transfer portion is shorter than the width of the developing portion, and the width of the transfer portion is shorter than the width of the charging portion, and the end of the surface of the photoreceptor in the rotational axis direction has a non-transfer region that is in contact with the charging member but not in contact with the transfer member, The image forming apparatus is characterized in that the control unit can be controlled to apply a voltage of normal polarity to the transfer member when there is no recording material in the transfer unit, and to perform an operation in which, when the voltage is applied, the exposure device exposes the area of ​​the surface of the photoreceptor that has passed through the transfer unit, including the non-transfer area, with a second exposure amount smaller than the first exposure amount.

2. The image forming apparatus according to claim 1, characterized in that the control unit controls the exposure device to expose a region on the surface of the photoreceptor that includes a transfer region in contact with the transfer member and a non-transfer region during the operation.

3. The image forming apparatus according to claim 2, characterized in that the control unit controls the exposure amount to the non-transfer area by the exposure device to be equal to or greater than the exposure amount to the transfer area by the exposure device during the operation.

4. The image forming apparatus according to claim 1, characterized in that the control unit controls the application of a voltage of normal polarity, which has an absolute value greater than the surface potential of the photoreceptor in the transfer unit, to the transfer member during the operation.

5. The image forming apparatus according to claim 1, characterized in that the control unit controls the application of a voltage of normal polarity to the transfer member such that the potential difference between the photoreceptor and the transfer member is less than or equal to a discharge threshold during the operation.

6. The image forming apparatus according to claim 1, characterized in that the control unit performs the operation based on an index value correlated with the time during which the photoreceptor was charged.

7. The image forming apparatus according to claim 6, characterized in that the index value is the paper feeding history of the recording material, the rotation time of the photoreceptor, the number of rotations of the photoreceptor, or the rotation distance of the photoreceptor.

8. The image forming apparatus according to claim 1, characterized in that the control unit performs the operation based on the surface potential of the photoreceptor in the non-transfer region.

9. The image forming apparatus according to claim 8, characterized in that the control unit performs the operation based on the potential difference between the surface potential of the photoreceptor in the non-transfer region and the surface potential of the photoreceptor in the transfer region that is in contact with the transfer member on the surface of the photoreceptor.

10. The image forming apparatus according to claim 1, characterized in that the control unit can change the frequency of execution of the operation, which correlates with the number of recording materials on which a toner image is transferred between the execution of the operation and the execution of the next operation.

11. The image forming apparatus according to claim 10, characterized in that the control unit changes the execution frequency based on the environment or the usage history of the image forming apparatus.

12. The image forming apparatus according to claim 10, characterized in that the control unit changes the execution frequency based on image information of the toner image formed between the execution of the operation and the execution of the next operation.

13. The image forming apparatus according to claim 12, characterized in that the control unit changes the transfer voltage during toner image transfer based on the image information.

14. The image forming apparatus according to claim 12, characterized in that the image information is print density information.

15. The image forming apparatus according to claim 14, characterized in that the control unit controls the execution frequency to decrease when the print rate information is smaller than a predetermined threshold.

16. The image forming apparatus according to claim 1, characterized in that the control unit can change the execution time of the operation which correlates with the rotation time of the photoreceptor in the operation.

17. The image forming apparatus according to claim 16, characterized in that the control unit changes the execution time based on the environment or the usage history of the image forming apparatus.

18. The image forming apparatus according to claim 16, characterized in that the control unit changes the execution time based on image information of the toner image formed between the execution of the operation and the execution of the next operation.

19. The image forming apparatus according to claim 18, characterized in that the control unit changes the transfer voltage during toner image transfer based on the image information.

20. The image forming apparatus according to claim 18, characterized in that the image information is print density information.

21. The image forming apparatus according to claim 20, characterized in that the control unit controls the execution time to be shortened when the print rate information is smaller than a predetermined threshold.

Images