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, an end portion of a surface of a photoreceptor 1 in a rotational axis direction of a charging member 2 includes a non-transfer area E in contact with the charging member 2 and in no contact with a transfer member 5. A control portion 40 can perform exposure operation such that an exposure device 3 exposes at least the non-transfer area E of the photoreceptor 1 during rotation of the photoreceptor 1, and by the exposure operation, controls the exposure device 3 such that an absolute value of a surface potential of the photoreceptor 1, which is formed in the non-transfer area E, is smaller than an absolute value of a surface potential of the photoreceptor 1, which is formed in a transfer outside paper passing area F downstream of an exposure portion Pb and upstream of the transfer portion Nt in a rotational direction of the photoreceptor 1.SELECTED DRAWING: Figure 5

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. Then, 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 by 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 including 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 is sometimes referred to as "paper", the recording material is not limited to paper, and may be a synthetic resin such as an OHP sheet or synthetic paper as the main component. Also, for the sake of convenience, regarding the high and low (large and small) of potential and voltage and the raising and lowering, it shall mean the high and low (large and small) and the raising and lowering when comparing the absolute values of 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.

[0007] Furthermore, Patent Document 2 proposes a configuration in which the surface potential of the non-paper-passing area on the photoreceptor is lowered by adjusting the exposure amount by the exposure device to the non-paper-passing area on the photoreceptor, thereby suppressing the adhesion of toner to the surface of the photoreceptor. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2003-302808 [Patent Document 2] Japanese Patent Publication No. 2019-194650 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] 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.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] For example, 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 photoreceptor surface with 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] Furthermore, in the longitudinal direction, there are configurations where 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. And as a result of this cleaning failure, "edge fogging" may occur, where the edges of the recording material in a direction approximately perpendicular to the transport direction of the recording material become soiled with toner.

[0015] Furthermore, the method described in Patent Document 2 adjusts the exposure amount to the photoreceptor within the area in contact with the transfer roller. Therefore, it cannot address the problem of increased surface potential of the photoreceptor outside the area in contact with the transfer roller, as described above.

[0016] 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]

[0017] The above objective is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides an image forming apparatus comprising: a rotatable photoreceptor; a rotatable charging member that contacts the photoreceptor to form a charged portion and charges the surface of the photoreceptor in the charged portion; an exposure apparatus that exposes the surface of the photoreceptor charged by the charging member to form an electrostatic image on the surface of the photoreceptor; a developing member that supplies toner 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 portion and transfers a toner image from the surface of the photoreceptor to a recording material in the transfer portion when a voltage is applied; and a control unit that can control the exposure apparatus, wherein in the direction of the rotation axis of the charging member, 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 direction of the rotation axis has a non-transfer region that contacts the charging member but does not contact the transfer member. The image forming apparatus is characterized in that, in the direction of the rotation axis, the area on the surface of the photoreceptor that contacts the recording material at the transfer portion is defined as the paper-feeding region, and the area outside the paper-feeding region and inside the transfer portion is defined as the paper-outside transfer region, the control unit is capable of performing an exposure operation with the exposure device to expose at least the non-transfer region of the photoreceptor when the photoreceptor is rotating, the exposure device is controlled by the exposure operation to form a surface potential on the surface of the photoreceptor downstream of the exposure portion and upstream of the transfer portion in the direction of the rotation of the photoreceptor, and the exposure device is controlled so that the absolute value of the surface potential formed in the non-transfer region is smaller than the absolute value of the surface potential formed in the paper-outside transfer region downstream of the exposure portion and upstream of the transfer portion in the direction of the rotation of the photoreceptor.

[0018] According to another aspect of the present invention, the present invention comprises a rotatable photoreceptor, a rotatable charging member that contacts the photoreceptor to form a charging portion and charges the surface of the photoreceptor at the charging portion, an exposure apparatus that exposes the surface of the photoreceptor charged by the charging member to form an electrostatic image on the surface of the photoreceptor, a developing member that supplies toner 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 portion and, when a voltage is applied, transfers a toner image from the surface of the photoreceptor to a recording material at the transfer portion, and a control unit capable of controlling the exposure apparatus, wherein, in the direction of the rotation axis of the charging member, the width of the transfer portion is shorter than the width of the charging portion. An image forming apparatus is provided having a non-transfer region at the end of the surface of the photoreceptor in the direction of the rotation axis that is in contact with the charging member but not in contact with the transfer member, wherein, in the direction of the rotation axis, the region of the surface of the photoreceptor that is in contact with the recording material at the transfer portion is defined as the paper-feed region, and the region outside the paper-feed region and inside the transfer portion is defined as the paper-outside transfer region, the control unit is capable of performing an exposure operation with the exposure device to expose at least the non-transfer region of the photoreceptor when the photoreceptor is rotating, and the exposure device is controlled in such a way that the exposure amount for the non-transfer region is greater than the exposure amount for the paper-outside transfer region.

[0019] According to another aspect of the present invention, there are provided a rotatable photoreceptor, a rotatable charging member that contacts the photoreceptor to form a charging portion and charges the surface of the photoreceptor in the charging portion, an exposure device that exposes the surface of the photoreceptor charged by the charging member to form an electrostatic image on the surface of the photoreceptor, a developing member that supplies toner 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 portion and transfers the toner image from the surface of the photoreceptor to a recording material in the transfer portion when a voltage is applied, and a control unit that can control the exposure device. In the direction of the rotation axis of the charging member, the width of the transfer portion is shorter than the width of the charging portion. At an end portion of the surface of the photoreceptor in the direction of the rotation axis, there is a non-transfer region that contacts the charging member and does not contact the transfer member. In an image forming apparatus in which at least a part of the toner coating region of the developing member overlaps with the non-transfer region in the direction of the rotation axis, when a region that contacts the recording material in the transfer portion on the surface of the photoreceptor is defined as a paper passing region and a region outside the paper passing region and inside the transfer portion is defined as a non-paper-passing transfer region in the direction of the rotation axis, the control unit can perform an exposure operation of exposing at least the non-transfer region of the photoreceptor or at least the non-transfer region of the photoreceptor and the non-paper-passing transfer region by the exposure device when the photoreceptor is rotating. An image forming apparatus is provided with this feature.

Advantages of the Invention

[0020] According to the present invention, in a configuration where the contact region of the surface of the photoreceptor with the transfer member is shorter than the contact region of the surface of the photoreceptor with the charging member in the longitudinal direction, it is possible to suppress an excessive increase in the surface potential at the end portion of the surface of the photoreceptor in the longitudinal direction.

Brief Description of the Drawings

[0021] [Figure 1] It is a schematic cross-sectional view of an image forming apparatus. [Figure 2] It is a schematic diagram showing the positional relationship in the longitudinal direction of each part around the photosensitive drum. [Figure 3(a)]It is an explanatory diagram of the rise in the surface potential of the photosensitive drum. [Figure 3(b)] It is an explanatory diagram of the rise in the surface potential of the photosensitive drum. [Figure 4(a)] It is an explanatory diagram of the change in the surface potential of the photosensitive drum in Example 1. [Figure 4(b)] It is an explanatory diagram of the change in the surface potential of the photosensitive drum in Example 1. [Figure 5] It is a graph showing the change in the surface potential at the end of the photosensitive drum in the examples and comparative examples. [Figure 6] It is a schematic diagram showing the longitudinal positional relationship of each part around the photosensitive drum in Example 2. [Figure 7] It is a graph showing the relationship between Vback and the degree of occurrence of "fogging". [Figure 8(a)] It is a graph showing the change in the surface potential at the end of the photosensitive drum in Example 3. [Figure 8(b)] It is a graph showing the change in the surface potential at the end of the photosensitive drum in Example 3. [Figure 9] It is an explanatory diagram of the intersheet position regarding the circumferential direction of the photosensitive drum. [Embodiments for Carrying Out the Invention]

[0022] Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

[0023] [Example 1] (1) Image forming apparatus First, the configuration of the image forming apparatus 100 of this example will be described. FIG. 1 is a schematic cross-sectional view of the image forming apparatus 100 of this example. The image forming apparatus 100 of this example 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.

[0024] The image forming apparatus 100 has a photosensitive drum 1, which is a drum-shaped (cylindrical) photoreceptor (electrophotographic photoreceptor), as an image carrier inside the apparatus body M. The photosensitive drum 1 is constructed by providing a photosensitive material such as OPC (Organic Photoconductor), amorphous selenium, or amorphous silicon on a cylindrical drum base made of aluminum or nickel. The photosensitive drum 1 used in this embodiment is a negatively charged OPC photoreceptor with an outer diameter of φ24 mm. This photosensitive drum 1 has a photosensitive layer on the surface of a conductive base made of an aluminum cylinder, in which a charge generation layer and a charge transport layer are laminated in that order from the conductive base side.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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 the developing roller 4a contacts) is the developing position (developing section) Pc.

[0029] 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 SBR 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.

[0030] 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.

[0031] 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).

[0032] 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.

[0033] 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 300 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] The control unit 40 is configured to include a CPU 41 as a calculation control means, which is the central element for performing calculations; memory such as ROM 41a and RAM 41b as 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. By executing various programs stored in ROM 41a, the CPU 41 can control various operations related to image formation while using RAM 41b as a working area. In particular, in this embodiment, the control unit 40 can perform an exposure operation in which at least the non-transfer area E of the photosensitive drum 1, described later, is exposed by the exposure device 3.

[0044] 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.

[0045] (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". This positional relationship changes depending on the size of the recording material P used for image formation (especially the width in the direction approximately perpendicular to the transport direction), but Figure 2 shows the positional relationship when the recording material P is LTR size.

[0046] In Figure 2, "Photoreceptor Region A" indicates the region of the photosensitive drum 1 where the photosensitive layer is formed, or the width of that region, in the longitudinal direction. "Charging Region (Charging Section) B" indicates the region of 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 Section) C" indicates the region of the transfer roller 5 that can contact the surface of the photosensitive drum 1, or the width of that region, in the longitudinal direction. "Paper Passing Region D" indicates the region of the transfer section Nt through which the recording material P passes, or the width of that region, in the longitudinal direction. "Non-Transfer Region E" indicates the region of the longitudinal direction where the charging roller 2 contacts the photosensitive drum 1, and the transfer roller 5 does not contact the photosensitive drum 1, or the width of that region (i.e., the difference between the charging region B and the transfer region C, or the width of that region). Furthermore, the "outside-paper-feeding transfer area F" refers to the area in the longitudinal direction where the transfer roller 5 contacts the photosensitive drum 1, and where the recording material P does not pass through the transfer area Nt, or the width of that area (i.e., the difference between the transfer area C and the paper-feeding area D, or the width of that area). In other words, in the longitudinal direction, the area on the surface of the photosensitive drum 1 that contacts the recording material P at the transfer area Nt is the paper-feeding area D, and the area outside the paper-feeding area D and inside the transfer area C is the outside-paper-feeding transfer area F. For convenience, the areas on the photosensitive drum 1 corresponding to the above-mentioned "charged area B," "transfer area C," "paper-feeding area D," "non-transferred area E," and "outside-paper-feeding transfer area F" are also referred to as "charged area B," "transfer area C," "paper-feeding area D," "non-transferred area E," and "outside-paper-feeding transfer area F," respectively.

[0047] In this embodiment, the photoreceptor area A, the charging area B, the transfer area C, and the paper feeding area D are arranged such that the center of each area in the longitudinal direction approximately coincides with the center of the image forming area (the area in which a toner image can be formed) in the longitudinal direction (center reference). Therefore, among the above areas, those with a relatively shorter width in the longitudinal direction are contained within those with a relatively longer width. Figure 2 illustrates the range from the center to one end in the longitudinal direction.

[0048] In this embodiment, the transfer region C is shorter than the charged region B in the longitudinal direction, and the surface of the photosensitive drum 1 has a non-transfer region E at its longitudinal end that is in contact with the charged roller 2 but not with the transfer roller 5.

[0049] (3) Increase in surface potential of the photosensitive drum in the non-transfer area Next, using Figure 3 (Figures 3(a) and 3(b)), we will explain how the surface potential of the photosensitive drum 1 in the non-transfer region E increases during printing when the exposure operation of this embodiment, described later, is not performed. In Figure 3, the horizontal axis shows the position on the photosensitive drum 1 in the longitudinal direction, illustrating the charged region B, transfer region C, paper-feeding region D, non-transfer region E, and non-paper-feeding transfer region F described above. Also in Figure 3, the vertical axis shows the surface potential of the photosensitive drum 1, indicating that the higher 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). Note that Figure 3 illustrates the range on one end side in the longitudinal direction. Furthermore, the surface potential of the photosensitive drum 1 shown in Figure 3, which will be described 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 exposure" means before reaching the exposure position Pb, "after exposure" means after passing the exposure position Pb, "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.

[0050] First, State 1-1 shows the surface potential of the photosensitive drum 1 immediately after the start of the printing operation, after charging (and before exposure). In State 1-1, the surface of the photosensitive drum 1 is charged substantially uniformly to a predetermined dark area potential Vd by the 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.

[0051] Next, state 1-2 shows the surface potential of the photosensitive drum 1 after exposure (and before transfer). An electrostatic latent image (electrostatic image) is formed in the image area (image area, printing area, printing area) within the paper feeding area D by exposure by irradiating it with laser light L from the exposure device 3. In the example in Figure 3, as an example, the image area within the paper feeding area D is exposed by the exposure device 3 to 0.3 μJ / cm². 2 When exposed to this amount of light, a bright area potential of -100V is formed on the surface of the photosensitive drum 1.

[0052] Next, states 1-3 show the surface potential of the photosensitive drum 1 after transfer (and before recharging). When the recording material P passes through the transfer section Nt, a positive transfer voltage is applied to the transfer roller 5 in the transfer section Nt. Therefore, the surface potential of the photosensitive drum 1 in the transfer region F outside the paper feed, where the photosensitive drum 1 and the transfer roller 5 are in direct contact during paper feeding, decreases. On the other hand, in the non-transfer region E, the transfer roller 5 is not in contact with the photosensitive drum 1, so no positive transfer voltage is applied. Furthermore, the image forming apparatus 100 in this embodiment does not have 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 pre-charging exposure means. Therefore, the surface potential of the photosensitive drum 1 in the non-transfer region E does not decrease significantly. As a result, a potential difference is created between the surface potential of the photosensitive drum 1 in the transfer region F outside the paper feed and the surface potential of the photosensitive drum 1 in the non-transfer region E. Furthermore, the surface potential (bright area potential) of the photosensitive drum 1 in the image area D within the paper feeding area D fluctuates within a range between the bright area potential and the development potential as a result of the influence received at the development position Pc and the transfer area Nt, as described later. In the example in Figure 3, as an example, after transfer, the surface potential of the photosensitive drum 1 in the transfer area F outside the paper feeding area is -400V, while the surface potential of the photosensitive drum 1 in the non-transfer area E remains at -500V. Also in the example in Figure 3, as an example, after transfer, the surface potential of the photosensitive drum 1 in the image area D within the paper feeding area D fluctuates from -100V to -250V. This is because it is affected by the supply of toner to the image area (the area with the bright area potential) by the development roller 4a at the development position Pc, and by the application of a positive voltage by the transfer roller 5 via the recording material P at the transfer area Nt. Furthermore, the potential difference between the surface potential of the photosensitive drum 1 in the non-transfer area F (or transfer area C) and the surface potential of the photosensitive drum 1 in the non-transfer area E is sometimes simply referred to as the potential difference between the non-transfer area F (or transfer area C) and the non-transfer area E.

[0053] Next, states 1-4 show the surface potential of the photosensitive drum 1 after recharging (and before exposure). As described above, the surface of the photosensitive drum 1 is charged again by the charging roller 2 with a potential difference between the transfer region C (composed of the paper-feeding region D and the transfer region F outside the paper-feeding region) and the non-transfer region E. In state 1-4, a predetermined charging voltage (-1100V) is applied to the charging roller 2, as in state 1-1. 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 in state 1-1. On the other hand, in the non-transfer region E, 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 Vd.

[0054] State 1-5 shows the surface potential of the photosensitive drum 1 after passing through the charging position Pa multiple times (after multiple recharging cycles and before exposure) while a charging voltage (-1100V) is continuously applied to the charging roller 2. In the transfer region C, the surface potential of the photosensitive drum 1 after charging returns to a predetermined dark area potential Vd (-500V), similar to state 1-1. On the other hand, in the non-transfer region E, 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 region E is -700V.

[0055] Next, State 1-6 shows the surface potential of the photosensitive drum 1 after exposure (and before transfer) when exposure with laser light L is performed by the exposure device 3, similar to State 1-2, in a situation where the surface potential of the photosensitive drum 1 in the non-transfer region E has risen after multiple recharging cycles. Similar to State 1-2, the surface potential of the photosensitive drum 1 in the image area within the paper feeding region D drops to a predetermined bright area potential. On the other hand, the surface potential of the photosensitive drum 1 in the non-transfer region E remains elevated, similar to State 1-5.

[0056] If the surface potential of the photosensitive drum 1 in the non-transfer region E rises excessively, the potential difference between the core metal portion of the transfer roller 5 and the non-transfer region E on the photosensitive drum 1 increases, potentially causing a discharge. This discharge may damage the photosensitive drum 1, such as causing leakage marks due to dielectric breakdown. If a charging voltage is applied to the charging roller 2 while this damaged area is present, the current may concentrate in the damaged area, causing the charging voltage to drop. As a result, it may not be possible to bring the photosensitive drum 1 to the desired surface potential, including other regions, potentially leading to problems such as streaky images appearing in the longitudinal direction due to insufficient charging. Therefore, it is desirable to suppress the excessive rise in the surface potential of the photosensitive drum 1 in the non-transfer region E.

[0057] (4) Changes in the surface potential of the photosensitive drum when the exposure operation of this embodiment is performed. Next, using Figure 4 (Figures 4(a) and 4(b)), the change in the surface potential of the photosensitive drum 1 during the printing operation when the exposure operation of this embodiment is performed will be explained. In this embodiment, by performing an exposure operation in which the exposure device 3 exposes the non-transfer area E on the photosensitive drum 1, an excessive rise in the surface potential of the photosensitive drum 1 in the non-transfer area E is suppressed. 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.

[0058] First, state 2-1 shows the surface potential of the photosensitive drum 1 immediately after the start of the printing operation, after charging (and before exposure). In state 2-1, similar to state 1-1 in Figure 3, the surface of the photosensitive drum 1 is charged substantially uniformly to a predetermined dark area potential Vd by the charging roller 2 to which a predetermined charging voltage is applied. In the example in Figure 4, similar to state 1-1 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.

[0059] Next, state 2-2 shows the surface potential of the photosensitive drum 1 after exposure (and before transfer). In this embodiment, at this time, an exposure operation is performed in which the exposure device 3 exposes the non-transfer area E on the photosensitive drum 1, thereby suppressing an excessive rise in the surface potential of the photosensitive drum 1 in the non-transfer area E. In other words, an electrostatic latent image (electrostatic image) is formed in the image area within the paper feeding area D by exposure, which is performed by irradiating it with laser light L by the exposure device 3. In this embodiment, at this time, the non-transfer area E is also exposed by irradiating it with laser light L by the exposure device 3, taking into account the rise in surface potential after recharging as shown in state 1-4 of Figure 3. This keeps the surface potential of the photosensitive drum 1 in the non-transfer area E lower than the surface potential of the photosensitive drum 1 in the transfer area F outside the paper feeding area. In the example of Figure 4, similar to state 1-2 of Figure 3, as an example, the image area within the paper feeding area D is exposed by the exposure device 3 to 0.3 μJ / cm 2 The surface of the photosensitive drum 1 is exposed to a certain exposure level, and a bright area potential of -100V is formed. On the other hand, the non-transfer area E is exposed by the exposure device 3 to a lower exposure level than that for the image area (this is also called "weak exposure"), and its surface potential is lowered. In the example in Figure 4, as an example, the non-transfer area E is exposed to 0.005 μJ / cm² by the exposure device 3. 2 The surface potential is lowered by exposure to a low exposure level. In this embodiment, in order to achieve this low exposure level, a weak exposure light source (not shown) is provided in addition to the light source within the exposure apparatus 3 to expose the image portion within the paper feeding area D. In the example shown in Figure 4, as an example, the surface potential of the photosensitive drum 1 in the non-transfer area E is lowered to -490V, which is smaller than the surface potential of the photosensitive drum 1 in the non-transfer area F, which is -500V. In this embodiment, the surface potential of the photosensitive drum 1 in the non-transfer area E is set to -490V, but it is not limited to any voltage with an absolute value smaller than -500V.

[0060] Thus, in this embodiment, after exposure (and before transfer), the absolute value of the surface potential of the photosensitive drum 1 is such that non-transfer region E < transfer region F outside the paper feed. Furthermore, in this embodiment, the transfer region F outside the paper feed on the photosensitive drum 1 is not exposed by the exposure device 3. That is, in this embodiment, during exposure, the exposure amount (exposure amount per unit area) by the exposure device 3 is such that non-transfer region E > transfer region F outside the paper feed. By satisfying these surface potential relationships or exposure amount relationships, it is possible to suppress the rise in the surface potential of the photosensitive drum 1 in the non-transfer region E, as shown in state 1-4 of Figure 3.

[0061] Next, state 2-3 shows the surface potential of the photosensitive drum 1 after transfer (and before recharging). The change in the surface potential of the photosensitive drum 1 in state 2-3 is the same as in state 1-3 in Figure 3. However, the surface potential of the photosensitive drum 1 in the non-transfer region E, where the transfer roller 5 does not make contact and exposure is performed by the exposure device 3, differs from state 1-3 in Figure 3, and maintains the surface potential after exposure in state 2-2. In the example in Figure 4, as an example, after transfer, the surface potential of the photosensitive drum 1 in the transfer region F outside the paper feed becomes -400V, and the surface potential of the photosensitive drum 1 in the non-transfer region E becomes -490V. Also, in the example in Figure 4, as an example, after transfer, the surface potential of the photosensitive drum 1 in the image area within the paper feed region D becomes -250V.

[0062] Next, state 2-4 shows the surface potential of the photosensitive drum 1 after recharging (and before exposure). The surface of the photosensitive drum 1 is charged again by the charging roller 2, similar to state 1-4 in Figure 3. The surface potential of the photosensitive drum 1 after recharging returns to a predetermined dark area potential Vd (-500V) in the transfer region C, similar to state 1-4 in Figure 3. Furthermore, the surface potential of the photosensitive drum 1 after recharging also returns to a predetermined dark area potential Vd (-500V) in the non-transfer region E, similar to the transfer region C, because the surface potential has been lowered in advance in state 2-3 in anticipation of the rise in surface potential due to injection charging. In other words, the rise in the surface potential of the photosensitive drum 1 in the non-transfer region E, as in state 1-4 in Figure 3, is suppressed, and the surface potential of the photosensitive drum 1 in state 2-4 returns to the surface potential of the photosensitive drum 1 in state 2-1.

[0063] State 2-5 shows the surface potential of the photosensitive drum 1 after passing through the charging position Pa multiple times (after multiple recharging cycles and before exposure) while a charging voltage (-1100V) is continuously applied. As explained with respect to State 2-4, performing the exposure operation of this embodiment suppresses a further increase in the surface potential of the photosensitive drum 1 in the non-transfer region E, as shown in State 1-5 of Figure 3. That is, after charging, the surface potential of the photosensitive drum 1 maintains a flat surface potential (-500V) in the longitudinal direction, similar to States 2-1 and State 2-4.

[0064] Next, state 2-6 shows the surface potential of the photosensitive drum 1 after exposure (and before transfer) when exposure with laser light L is performed by the exposure device 3 in the same way as in state 2-2, under the conditions of state 2-5. As explained with respect to state 2-5, the rise in the surface potential of the photosensitive drum 1 in the non-transfer region E is suppressed, so the surface potential of the photosensitive drum 1 in state 2-6 is the same as the surface potential of the photosensitive drum 1 in state 2-2.

[0065] As described above, by performing the exposure operation of this embodiment, it is possible to suppress the rise in the surface potential of the photosensitive drum 1 in the non-transfer region E shown in Figure 3.

[0066] (5) Evaluation Test Next, we will describe the results of an evaluation test conducted to confirm the degree of increase in the surface potential of the photosensitive drum 1 in the non-transfer region E for this embodiment and Comparative Example 1. In this embodiment, the exposure operation described using Figure 4 was performed, and in Comparative Example 1, the exposure operation described using Figure 3 was performed. The configuration and operation of the image forming apparatus 100 in Comparative Example 1 are substantially the same as those of the image forming apparatus 100 in this embodiment, except for the points mentioned above. Figure 5 shows the change in the surface potential of the photosensitive drum 1 in the non-transfer region E after exposure and before transfer when continuous image formation was performed on 20 sheets of LTR-sized paper as recording material P. In Figure 5, the solid line shows the change in surface potential when the exposure operation of this embodiment is performed, and the dashed line shows the change in surface potential when the exposure operation of Comparative Example 1 is performed.

[0067] In Comparative Example 1, there is no means to counteract the rise in the surface potential of the photosensitive drum 1 in the non-transfer region E. As a result, the surface potential of the photosensitive drum 1 in the non-transfer region E gradually rises due to the charging injection from the charging roller 2, eventually reaching an excessively high potential.

[0068] On the other hand, in this embodiment, by exposing the non-transfer region E, the increase in surface potential due to injection charging is canceled out, and the increase in the surface potential of the photosensitive drum 1 in the non-transfer region E is suppressed.

[0069] 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 charged by the charging member 2 to form an electrostatic image on the surface of the photoreceptor 1, a developing member 4a that supplies toner to the electrostatic image formed on the surface of the photoreceptor 1 to form a toner image, a transfer member 5 that contacts the surface of the photoreceptor 1 to form a transfer section Nt and transfers a toner image from the surface of the photoreceptor 1 to the recording material P in the transfer section Nt when a voltage is applied, and a control unit 40 that can control the exposure apparatus 3. In the direction of the rotation axis of the charging member 2, 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 direction of the rotation axis has a non-transfer region E that is in contact with the charging member 2 but not in contact with the transfer member 5. In this embodiment, when the region of the photoreceptor 1 that contacts the recording material P with the transfer portion Nt on its surface is defined as the paper-feeding region D, and the region outside the paper-feeding region D and inside the transfer portion Nt is defined as the non-paper-feeding transfer region F, the control unit 40 can perform an exposure operation using the exposure device 3 to expose at least the non-transfer region E of the photoreceptor 1 when the photoreceptor 1 is rotating. The exposure operation controls the exposure device 3 to form a surface potential on the surface of the photoreceptor 1 downstream of the exposure portion Pb and upstream of the transfer portion Nt in the rotational direction of the photoreceptor 1, and controls the exposure device 3 so that the absolute value of the surface potential formed in the non-transfer region E is smaller than the absolute value of the surface potential formed in the non-transfer region E, which is downstream of the exposure portion Pb and upstream of the transfer portion Nt in the rotational direction of the photoreceptor 1. In other words, in this embodiment, the control unit 40 is capable of performing an exposure operation in which the exposure device 3 exposes at least the non-transfer region E of the photoreceptor 1 when the photoreceptor 1 is rotating, and in the exposure operation, the control unit 40 controls the exposure device 3 so that the amount of exposure to the non-transfer region E is greater than the amount of exposure to the transfer region F outside the paper feed. In this embodiment, the control unit 40 controls the exposure device 3 in the exposure operation so that at least the non-transfer region E and the transfer region F outside the paper feed of the photoreceptor 1 are exposed.In this embodiment, the control unit 40 controls the exposure device 3 to perform the exposure operation when the image-forming region on the surface of the photoreceptor 1 in the rotational direction of the photoreceptor 1 is passing through the exposure section Pb where the surface of the photoreceptor 1 is exposed. In this embodiment, the control unit 40 also controls the exposure device 3 to perform the exposure operation when the image-forming region is passing through the exposure section Pb, by exposing the surface of the photoreceptor 1 inside the paper-feeding region D in the rotational axis direction with a first exposure amount to form an electrostatic image on the surface of the photoreceptor 1, and when the image-forming region is passing through the exposure section Pb, by exposing the exposure device 3 to perform the exposure operation with a second exposure amount smaller than the first exposure amount.

[0070] As explained above, this embodiment makes it possible to suppress the rise in the surface potential of the photosensitive drum 1 in the non-transfer region E. Thus, in 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 in the longitudinal direction than the contact region B between the surface of the photosensitive drum 1 and the charging roller 2, it is possible to suppress an excessive rise in the surface potential of the longitudinal end of the photosensitive drum 1 (non-transfer region E). Therefore, as mentioned above, it is possible to suppress damage to the surface of the photosensitive drum 1 caused by discharge resulting from the rise in the surface potential of the photosensitive drum 1 in the non-transfer region E.

[0071] [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.

[0072] In this embodiment, for the purpose of miniaturizing the image forming apparatus 100, we will describe a case where the transfer region C is shorter than the development region G, which will be described later, in the longitudinal direction.

[0073] Figure 6 is a schematic diagram illustrating the positional relationship of the various parts around the photosensitive drum 1 in the longitudinal direction in this embodiment. This positional relationship changes depending on the size of the recording material P used for image formation (especially the width in the direction approximately perpendicular to the transport direction), but Figure 6 shows the positional relationship when the recording material P is LTR size.

[0074] In Figure 6, "Photoreceptor area A," "Charged area B," "Transfer area C," "Paper feeding area D," "Non-transfer area E," and "Transfer area outside paper feeding F" each represent the areas or their widths as described in Example 1. "Developing area (developing unit) G" represents the area on the developing roller 4a coated with toner in the longitudinal direction (toner coated area) or the width of that area (more specifically, the area on the developing roller 4a where the toner coating can contact the surface of the photosensitive drum 1 or the width of that area). In this embodiment, this developing area G can also be said to be the area or the width of that area where an opening is provided in the developing container 4b 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 area where this opening is provided. Furthermore, "Fogging area H" represents the area or the width of that area in the longitudinal direction, within the non-transfer area E and within the developing area G. For convenience, the areas on the photosensitive drum 1 corresponding to the above-mentioned "charged area B," "transfer area C," "paper feed area D," "non-transfer area E," "transfer area F outside paper feed," "developing area G," and "fogging area H" will also be referred to as "charged area B," "transfer area C," "paper feed area D," "non-transfer area E," "transfer area F outside paper feed," "developing area G," and "fogging area H," respectively. In this embodiment, the photosensitive area A, charged area B, transfer area C, paper feed area D, and developing area G are all arranged with respect to the center, as described in Embodiment 1. Figure 6 shows the range from the center to one end in the longitudinal direction.

[0075] In this embodiment, the developing region G overlaps with the non-transfer region E in at least a portion of its longitudinal direction. That is, in this embodiment, the developing region G is shorter than the charging region B and longer than the transfer region C in the longitudinal direction. The region of the developing region G that overlaps with the non-transfer region E is the fogging region H. That is, the fogging region H corresponds to a part of the non-transfer region E.

[0076] In this embodiment, the developing roller 4a is in contact with the photosensitive drum 1. Therefore, there is a possibility that "fogging" may occur in the developing area G, where toner adheres to the photosensitive drum 1. In particular, if the surface potential of the photosensitive drum 1 in the non-transfer area E increases, "fogging" ("reverse fogging") caused by "reverse toner" charged with the opposite polarity to the normal charging polarity may worsen. That is, "reverse fogging" caused by "reverse toner" may occur in the fogging area H. If the amount of "fogging" is large and continues for a long time, the cleaning blade 6a of the cleaning device 6 may not be able to completely remove the toner, resulting in cleaning failure. As a result of this cleaning failure, there is a possibility that "edge contamination" may occur, where the edges of the recording material P in a direction approximately perpendicular to the transport direction of the recording material P are contaminated with toner.

[0077] Here, we will explain "fogging" further. Figure 7 is a graph showing the relationship between Vback, which is the potential difference between the dark area potential of the photosensitive drum 1 (surface potential of the unexposed area) and the potential of the developing roller 4a (potential of the developing voltage), and the degree of 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. 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 predetermined piece of paper, and the density of the adhesive tape with toner attached (fogging density (%)) was measured to quantify the fogging. If no fogging occurs, the fogging density is 0%, and the larger the fogging density value, the greater the degree of fogging, indicating that a lot of toner is attached to the surface of the photosensitive drum 1. The types of fogging are as follows. First, when the potential difference between the dark area potential of the photosensitive drum 1 and the developing roller 4a becomes small, there is a phenomenon called "ground fogging" where toner charged with the correct polarity adheres to the surface of the photosensitive drum 1. Second, when the potential difference between the dark area potential of the photosensitive drum 1 and the developing roller 4a becomes large, there is a phenomenon called "reverse fogging" where "reverse toner" charged with the opposite polarity to the correct polarity adheres to the surface of the photosensitive drum 1.

[0078] As mentioned above, states 1-6 in Figure 3 show a state where the potential difference between the non-transfer area F and the non-transfer area E has increased. VbackE1, the potential difference between the surface potential of the photosensitive drum 1 in the non-transfer area E and the potential of the developing roller 4a, is greater than VbackF1, the potential difference between the surface potential of the photosensitive drum 1 in the non-transfer area F and the potential of the developing roller 4a. When Vback increases in this way, "reverse fogging" can occur, where reverse toner adheres to the surface of the photosensitive drum 1. As shown in Figure 7, in the configuration of this embodiment, the degree of fogging is smallest when Vback is around 120V, and the fogging density is 2%. This level of fogging is difficult to see on the recording material P and does not pose a problem. On the other hand, when Vback exceeds 220V, the degree of fogging (reverse fogging) increases, and if the fogging density continues to exceed 10%, cleaning failure may occur.

[0079] Figures 3, 4, and 5 will be used to explain the surface potential of the photosensitive drum 1 in the fogging region H and the degree of fogging (reverse fogging) in this embodiment and Comparative Example 2. In this embodiment, the same exposure operation as in Embodiment 1, explained using Figure 4, was performed, while in Comparative Example 2, the exposure operation explained using Figure 3 was performed. The configuration and operation of the image forming apparatus 100 in Comparative Example 2 are substantially the same as those of the image forming apparatus 100 in this embodiment, except for the differences mentioned above. Figure 5 shows the change in surface potential of the photosensitive drum 1 in the non-transfer region E after exposure and before transfer when continuous image formation was performed on 20 sheets of LTR-sized paper as recording material P. In Figure 5, the solid line shows the change in surface potential when the exposure operation of this embodiment is performed, and the dashed line shows the change in surface potential when the exposure operation of Comparative Example 2 is performed. The change in surface potential of the photosensitive drum 1 in the non-transfer region E for Embodiment 2 and Comparative Example 2 is the same as the change in surface potential of the photosensitive drum 1 in the non-transfer region E for Embodiment 1 and Comparative Example 1, respectively. As described above, the fogging region H corresponds to a part of the non-transfer region E.

[0080] As shown in Figures 3 (States 1-6) and 5, in Comparative Example 2, the surface potential of the photosensitive drum 1 in the fouled region H is -700V. Therefore, as shown in Figure 3 (States 1-6), the Vback E1 of the non-transfer region E, which includes the fouled region H, is 320V. From Figure 7, a fouling density of 320V exceeds 20%, which may result in cleaning failures due to fouling (reverse fouling). On the other hand, as shown in Figures 4 (States 2-6) and 5, in this embodiment, the surface potential of the photosensitive drum 1 in the fouled region H is -490V. Also, as shown in Figure 4 (States 2-6), the Vback E2 of the non-transfer region E, which includes the fouled region H, is 110V. From Figure 7, a fouling density of 110V is about 3%, which is not a problem in terms of cleaning failures.

[0081] Thus, in a configuration where the transfer region C is shorter than the development region G in the longitudinal direction, the occurrence of cleaning defects can be suppressed by performing an exposure operation that exposes the non-transfer region E, similar to that in Example 1.

[0082] Furthermore, in order to further suppress the occurrence of cleaning defects, it is preferable to make the density of the fogging that occurs in the non-transfer area F and the fogging area H as similar as possible. If there is a difference in density of fogging, a small torque difference in the longitudinal direction of the cleaning blade 6a is created at that point, making cleaning defects more likely to occur. In state 2-6 of Figure 4, the Vback E2 of the non-transfer area E, which includes the fogging area H, is 110V, but the Vback F2 of the non-transfer area F is 120V. From Figure 7, the fogging density at Vback 110V is about 3%, but the fogging density at Vback 120V is about 2%. Thus, in state 2-6 of Figure 4, there is a slight difference in the density of the fogging that occurs in the non-transfer area E and the non-transfer area F. From the viewpoint of suppressing the occurrence of cleaning defects, it is more preferable to expose the non-transfer area F in addition to the non-transfer area E, and to make the surface potential of the photosensitive drum 1 after exposure the same in the non-transfer area E and the non-transfer area F. Here, matching (making approximately identical) the surface potential of the photosensitive drum 1 in the non-transfer region E and the surface potential of the photosensitive drum 1 in the transfer region F outside the paper feed means making them sufficiently similar so as to adequately suppress density differences in fogging density, from the viewpoint of suppressing the occurrence of cleaning defects. Although not limited to this, typically, the difference in surface potential should be 5V or less, preferably 3V or less, and more preferably 1V or less (it may be 0V).

[0083] Thus, in this embodiment, the image forming apparatus 100 has a transfer portion Nt shorter than the width of the charging portion B in the rotation axis direction of the charging member 2, and the end of the surface of the photoreceptor 1 in the rotation axis direction has a non-transfer region E that is in contact with the charging member 2 but not in contact with the transfer member 5, and at least a part of the toner coating area of ​​the developing member 4a overlaps with the non-transfer region E in the rotation axis direction. Furthermore, in this embodiment, the control unit 40 is capable of performing an exposure operation by the exposure device 3 to expose at least the non-transfer region E of the photoreceptor 1 or at least the non-transfer region E and the transfer region F outside the paper feed when the photoreceptor 1 is rotating.

[0084] As explained above, in a configuration where the transfer area C is shorter than the development area G in the longitudinal direction, the occurrence of cleaning defects can be suppressed by performing an exposure operation that exposes the non-transfer area E and even the transfer area F outside the paper feed.

[0085] [Example 3] 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.

[0086] As explained in Example 1, when exposing the non-transfer area E, if the transfer area F outside the paper feed is narrow and the exposure amount to the non-transfer area E is strong, toner may adhere to the longitudinal end of the photosensitive drum 1. Alternatively, as explained in Example 2, when exposing both the non-transfer area E and the transfer area F outside the paper feed, if the exposure amount to the transfer area F outside the paper feed is strong, toner may adhere to the longitudinal end of the photosensitive drum 1, similar to the above. In such cases, if the transport position of the recording material P in a direction approximately perpendicular to the transport direction of the recording material P is misaligned, toner may be transferred from the photosensitive drum 1 to the end of the recording material P in a direction approximately perpendicular to the transport direction of the recording material P, potentially causing "end contamination" where the end of the recording material P becomes dirty. On the other hand, performing weak exposure to the extent that toner does not adhere to the non-transfer area E or the transfer area F outside the paper feed may require the addition of a dedicated weak exposure light source or an electrical circuit board for adjusting the exposure amount, which may lead to an increase in the size of the device. Therefore, in this embodiment, we will describe an exposure operation that reduces the risk of contamination at the edges of the recording material P while expanding the range of exposure options for exposing the non-transfer area E and further, the transfer area F outside the paper feed.

[0087] The image forming apparatus 100 of this embodiment does not incorporate the low-exposure light source used in the image forming apparatus 100 of Embodiments 1 and 2, thereby enabling a smaller size for the apparatus. Furthermore, similar to Embodiment 2, the image forming apparatus 100 of this embodiment is configured such that the transfer area C is shorter than the development area G in the longitudinal direction.

[0088] In this embodiment, during continuous image formation, when images are formed on multiple recording materials P while the photosensitive drum 1 is not being driven, an exposure operation is performed to expose the non-transfer area E, as in Embodiment 1 or Embodiment 2, and also the transfer area F outside the paper feed. In particular, in this embodiment, an exposure operation is performed to expose the non-transfer area E during the non-image formation phase of continuous image formation.

[0089] In other words, by not performing an exposure operation that exposes the non-transfer area E and the off-paper transfer area F during image formation, the risk of toner adhering to the non-transfer area E and the off-paper transfer area F, and this toner being transferred to the recording material P, resulting in edge staining, can be reduced.

[0090] Furthermore, by reducing the risk of edge contamination of the recording material P, the range of exposure options for exposing the non-transfer area E and the off-paper transfer area F expands. For example, if the objective is to suppress the rise in surface potential of the photosensitive drum 1, it becomes possible to use a strong exposure amount for the non-transfer area E and the off-paper transfer area F, similar to the exposure amount for the image area.

[0091] The change in the surface potential of the photosensitive drum 1 in the non-transfer region E in this embodiment will be explained using Figures 8(a) and 8(b). Note that, with respect to the recording material P, the leading edge and trailing edge refer to the leading edge and trailing edge with respect to the transport direction of the recording material P, unless otherwise specified.

[0092] Figure 8(a) shows the change in surface potential of the photosensitive drum 1 in the non-transfer region E after exposure and before transfer when continuous image formation is performed on 20 sheets of LTR-sized paper as recording material P with a paper-to-paper distance (the distance in the rotational direction of the photosensitive drum 1 between the position on the photosensitive drum 1 corresponding to the trailing end of the preceding recording material P and the position on the photosensitive drum 1 corresponding to the leading end of the subsequent recording material P) of 45 mm. In Figure 8(a), the solid line shows the change in surface potential when the exposure operation of this embodiment is performed, and the dashed line shows the change in surface potential when the exposure operation of Comparative Example 3 is performed. In this embodiment, only during a part of the pre-rotation process which is not image formation, and during the paper-to-paper process which is not image formation, the exposure device 3 applies the same exposure amount as to the image area, which is 0.3 μJ / cm². 2 An exposure operation was performed to expose the non-transfer region E with the specified exposure amount. In Comparative Example 3, the exposure operation described using Figure 3 was performed. The configuration and operation of the image forming apparatus 100 in Comparative Example 3 are substantially the same as those of the image forming apparatus 100 in this embodiment, except for the points mentioned above. The surface potential transition in Comparative Example 3 is the same as that shown in Figure 5.

[0093] Figure 8(b) plots only the surface potential change when the time domain on the horizontal axis of Figure 8(a) is changed and the exposure operation of this embodiment is performed. The details of the exposure operation of this embodiment will be explained using Figure 8(b).

[0094] First, immediately after the start of the drive of the photosensitive drum 1 (print operation, pre-rotation process), the surface potential of the photosensitive drum 1 is maintained at a dark area potential Vd = -500V by the charging roller 2. Due to the rotation drive of the pre-rotation process before paper feeding, the surface potential of the photosensitive drum 1 in the non-transfer area E gradually increases. Then, just before feeding the first recording material P, the exposure device 3 performs an exposure operation to expose the non-transfer area E over a distance of 45 mm in the rotation direction of the photosensitive drum 1, which is the same as the distance between the papers. In this embodiment, since the non-transfer area E is exposed with the same exposure amount as the image area, the surface potential of the photosensitive drum 1 drops to a bright area potential Vl = -100V after the first exposure operation. After the completion of this first exposure operation, the first recording material P is fed through.

[0095] In this embodiment, the rotational distance of the photosensitive drum 1 exposed in a single exposure operation is shorter than the circumference of the photosensitive drum 1 (approximately 75.4 mm). Therefore, a single exposure operation cannot expose the entire circumference of the photosensitive drum 1. In other words, during paper feeding when the non-transfer region E is not exposed, the previously exposed and unexposed regions of the non-transfer region E exist in the circumferential direction of the photosensitive drum 1. Consequently, in the photosensitive drum 1 surface potential of the non-transfer region E during the feeding of the first recording material P, the surface potential of the region exposed during the previous non-image formation is recharged by the charging roller 2, returning to the dark area potential Vd = -500V, and then begins to rise again due to injection charging. On the other hand, the surface potential of the region not exposed during the previous non-image formation follows the same trend as the surface potential of the photosensitive drum 1 at the start of the previous rotation process (before exposure of the non-transfer region E) and continues to rise. Therefore, the surface potential of the non-transfer region E of the photosensitive drum 1 after the first exposure operation fluctuates within a range of approximately -500 to -560V depending on the rotation period of the photosensitive drum 1.

[0096] In the inter-paper process after the first sheet of recording material P has been fed, an exposure operation similar to the exposure operation in the previous rotation process is performed (second exposure operation). That is, in the inter-paper process after the first sheet of recording material P has been fed, an exposure operation is performed by the exposure device 3 to expose the non-transfer area E over a distance of 45 mm in the rotation direction of the photosensitive drum 1, which is the same as the inter-paper distance. Even when the second sheet of recording material P has been fed after the second exposure operation, there is still an area in the non-transfer area E that has not been exposed in either the first or second exposure operation within one rotation of the photosensitive drum 1. Therefore, the surface potential of the photosensitive drum 1 in the non-transfer area E fluctuates up and down with the rotation period of the photosensitive drum 1, and the range of this fluctuation is wider than when the first sheet of recording material P was fed, at approximately -500 to -580 V. The same exposure operation is performed in the inter-paper process after the second sheet of recording material P has been fed (third exposure operation). When feeding the third recording material P after the third exposure, there is a high probability that there will be no areas in the non-transfer region E within one rotation of the photosensitive drum 1 that have not been exposed in any of the first to third exposure operations. In this embodiment, when feeding the third recording material P after the third exposure, there were no areas in the non-transfer region E within one rotation of the photosensitive drum 1 that had not been exposed in any of the first to third exposure operations. Therefore, the vertical range of the surface potential of the photosensitive drum 1 in the non-transfer region E during the rotation period of the photosensitive drum 1 narrows, and this vertical range drops to about -500 to -540V.

[0097] As shown in Figure 8(a), in this embodiment, even after the number of exposure operations described above exceeds three, the surface potential of the photosensitive drum 1 in the non-transfer region E remains stable between -500 and -540V. In other words, this embodiment can suppress the rise in the surface potential of the photosensitive drum 1 in the non-transfer region E, as seen in Comparative Example 3. Furthermore, this embodiment can reduce the risk of edge contamination of the recording material P while suppressing the need to increase the size of the device by adding a weak exposure light source, etc.

[0098] On the other hand, in this embodiment, the non-transfer region E is exposed to the same amount of light as the image area, which is 0.3 μJ / cm². 2Because exposure occurs in this manner, as described above, the surface potential of the photosensitive drum 1 in the non-transfer region E after exposure becomes the bright area potential Vl = -100V. Therefore, toner adheres to the non-transfer region E after exposure. However, unlike the situation described in Example 2 where the over-filled toner is always transported to the cleaning blade 6a, in this case, the toner adhering to the non-transfer region E is transported to the cleaning blade 6a only during a part of the preceding rotation process and the inter-paper process. Therefore, the occurrence of cleaning defects as described in Example 2 can be suppressed.

[0099] Next, we will explain the optimal length of the inter-paper distance according to the length of the recording material P in the transport direction when performing the exposure operation in the inter-paper process as described above. Figure 9 is a diagram that shows where the inter-paper position (the section on the photosensitive drum 1 in the rotational direction of the photosensitive drum 1 between the position on the photosensitive drum 1 corresponding to the trailing end of the preceding recording material P and the position on the photosensitive drum 1 corresponding to the leading end of the subsequent recording material P) is located on the circumference of the photosensitive drum 1 when the circumferential position of the photosensitive drum 1 is represented linearly. In this embodiment, the exposure operation to suppress the rise in surface potential of the photosensitive drum 1 is performed at all times throughout the entire inter-paper process, but the exposure operation may be performed only during a part of the inter-paper process.

[0100] Figure 9(a) shows the position between the paper and the circumference of the photosensitive drum 1 (i.e., the position where the exposure operation to suppress the rise in the surface potential of the photosensitive drum 1 is performed) when LTR-sized paper as the recording material P is fed with a paper-to-paper distance of 45 mm, as in the case of Figure 8 in this embodiment. It can be seen that one full rotation of the circumference of the photosensitive drum 1 can be exposed in the first to third paper-to-paper steps. This is consistent with the observation in Figure 8 that the surface potential of the photosensitive drum 1 began to stabilize after the exposure operation in the paper-to-paper steps exceeded three times (the first time being an exposure operation in a section corresponding to a part of the paper-to-paper distance of the previous rotation step). On the other hand, Figure 9(b) shows the position between the paper and the circumference of the photosensitive drum 1 in Comparative Example 4. In Comparative Example 4, LTR-sized paper as the recording material P was fed with a paper-to-paper distance of 25 mm. The configuration and operation of the image forming apparatus 100 in Comparative Example 4 are substantially the same as those of the image forming apparatus 100 in this embodiment, except for the differences described above. In Comparative Example 4, the amount of inter-sheet positional displacement (inter-sheet positional displacement) is small, and the inter-sheet position on the photosensitive drum 1 arrives at almost the same position each time. In such cases, exposure operation in the inter-sheet process alone is insufficient to suppress the rise in surface potential of the photosensitive drum 1 across its entire circumference. This inter-sheet positional displacement is expressed by the following formula (1). The amount of misalignment between sheets of paper = (length in the transport direction of the recording material + distance between sheets) - n × circumference of the photosensitive drum ... (1)

[0101] Here, n is any integer (a positive integer greater than or equal to 1), and the value that minimizes the absolute value of equation (1) is selected. The smaller the amount of inter-paper positional displacement for each sheet, the more difficult it becomes to suppress the rise in surface potential of the photosensitive drum 1 during the exposure operation in the inter-paper process, as shown in Comparative Example 4 in Figure 9(b). In other words, it is desirable that the length of the inter-paper distance be set such that the sum of the length of the recording material P and the inter-paper distance in the transport direction of the recording material P is not an integer multiple of the circumference of the photosensitive drum 1. For example, to avoid the rise in surface potential shown in Comparative Example 3 in Figure 8, it is desirable that the relationship in equation (2) below be satisfied. |Circumference of the photosensitive drum ÷ Amount of misalignment between sheets| ≤ Allowable number of pages for potential rise ...(2)

[0102] In equation (2) above, the absolute value of "perimeter of the photosensitive drum ÷ amount of inter-paper positional displacement per sheet" represents the number of exposure operations required to expose the entire perimeter of the photosensitive drum 1 during the inter-paper process. Furthermore, in equation (2) above, "allowable number of pages for potential rise" is the upper limit of the number of images that can be continuously formed while sufficiently suppressing the problems caused by the rise in surface potential described in Examples 1 and 2, in a situation where there is no means to counteract the rise in surface potential of the non-transfer area E on the photosensitive drum 1, as in Comparative Example 3 in Figure 8. For example, as in Comparative Example 3 in Figure 8, if the surface potential reaches -700V after 20 sheets have been fed, there is a risk of cleaning defects caused by "fogging" as described in Example 2. Therefore, in order to suppress the rise in surface potential of the photosensitive drum 1 before such a situation occurs, the allowable number of pages for potential rise can be set to 19 sheets (less than 20 sheets). Here, although not limited to this, the number of exposure operations required to expose the entire circumference of the photosensitive drum 1 by exposure operations in the inter-paper process (left side of equation (2) above) is preferably about 1 to 10 times, and more preferably about 1 to 5 times (3 times in this embodiment).

[0103] Thus, when performing exposure operations in the inter-paper process, it is desirable to set the sum of the length of the recording material P and the inter-paper distance in the transport direction of the recording material P to be an integer multiple of the circumference of the photosensitive drum 1. This makes it possible to suppress the rise in the surface potential of the photosensitive drum 1 over the entire circumference of the photosensitive drum 1.

[0104] Furthermore, the exposure operation in the above-mentioned inter-paper process can be performed not only in the inter-paper process but also in a section corresponding to a portion of the inter-paper distance in the preceding rotation process, as shown in Figure 8(b). Also, as mentioned above, the exposure operation may be performed only in a portion of the inter-paper distance, in which case equations (1) and (2) above should be applied by substituting the inter-paper position with the section of the inter-paper position in which the exposure operation is performed.

[0105] Furthermore, in this embodiment, the non-transfer region E was exposed during the exposure operation in the inter-paper process, but as mentioned above, the non-transfer region F outside the paper feed may also be exposed. Also, from the viewpoint of toner consumption, it is preferable to expose only the non-transfer region E, or the non-transfer region E and the non-transfer region F outside the paper feed, as described above, but if desired, substantially the entire longitudinal area of ​​the photosensitive drum 1 (substance of the photoreceptor region A or substantially the entire charged region B) may be exposed.

[0106] Thus, in this embodiment, the control unit 40 controls the exposure device 3 to perform the above-described exposure operation when the non-image-forming region of the surface of the photoreceptor 1 in the rotational direction of the photoreceptor 1 passes through the exposure section Pb where the surface of the photoreceptor 1 is exposed. In addition, in this embodiment, when the image-forming region of the surface of the photoreceptor 1 in the rotational direction of the photoreceptor 1 passes through the exposure section Pb, the control unit 40 exposes the surface of the photoreceptor 1 inside the paper-passing region D in the rotational axis direction of the charging member 2 with a first exposure amount to form an electrostatic image on the surface of the photoreceptor 1, and when the non-image-forming region passes through the exposure section Pb, the exposure device 3 controls the exposure device 3 to perform the above-described exposure operation with an exposure amount approximately the same as the first exposure amount. In this embodiment, the non-image-forming region is the section of the surface of the photoreceptor 1 between the position corresponding to the rear end of the preceding recording material P and the position corresponding to the front end of the subsequent recording material P during continuous image formation in which toner images are transferred to a plurality of recording materials P. Furthermore, in this embodiment, the length of the section is set such that the sum of the length of the recording material P in the transport direction of the recording material P and the length of the section in the rotation direction of the photoreceptor 1 is not an integer multiple of the circumference of the photoreceptor 1. It is not limited to performing the exposure operation described above in all inter-paper steps during continuous image formation. It is sufficient to sufficiently suppress the rise in the surface potential of the photoreceptor drum 1 in the non-transfer area E. For example, the exposure operation described above may be performed in the inter-paper step for each of the multiple recording materials P, or the exposure operation in the inter-paper step may be started after an image has been formed on a predetermined number of recording materials P.

[0107] As described above, this embodiment reduces the risk of contamination at the edges of the recording material P, while expanding the range of exposure options for exposing the non-transfer area E and even the transfer area F outside the paper feed.

[0108] [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.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] Furthermore, the present invention is not limited to applications to configurations where the transfer region is shorter than the development region in the longitudinal direction. The present invention can also be applied to configurations where 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]

[0113] 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, to form an electrostatic image on the surface of the photoreceptor, A developing member that supplies toner 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 portion, and when a voltage is applied, transfers a toner image from the surface of the photoreceptor to a recording material in the transfer portion, A control unit capable of controlling the exposure apparatus, 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 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, In the direction of the rotation axis, when the region of the surface of the photoreceptor that contacts the recording material at the transfer portion is defined as the paper-feeding region, and the region outside the paper-feeding region and inside the transfer portion is defined as the paper-outside transfer region, the control unit is capable of performing an exposure operation with the exposure device to expose at least the non-transfer region of the photoreceptor when the photoreceptor is rotating, and the exposure device is controlled by the exposure operation to form a surface potential on the surface of the photoreceptor downstream of the exposure portion and upstream of the transfer portion in the rotation direction of the photoreceptor, and the exposure device is controlled so that the absolute value of the surface potential formed in the non-transfer region is smaller than the absolute value of the surface potential formed in the paper-outside transfer region downstream of the exposure portion and upstream of the transfer portion in the rotation direction of the photoreceptor.

2. 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, to form an electrostatic image on the surface of the photoreceptor, A developing member that supplies toner 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 portion, and when a voltage is applied, transfers a toner image from the surface of the photoreceptor to a recording material in the transfer portion, A control unit capable of controlling the exposure apparatus, 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 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, Image forming apparatus characterized in that, in the direction of the rotation axis, the area on the surface of the photoreceptor that contacts the recording material with the transfer portion is defined as the paper-feeding area, and the area outside the paper-feeding area and inside the transfer portion is defined as the non-paper-feeding transfer area, the control unit is capable of performing an exposure operation with the exposure device to expose at least the non-transfer area of ​​the photoreceptor when the photoreceptor is rotating, and in the exposure operation, the exposure device is controlled such that the amount of exposure to the non-transfer area is greater than the amount of exposure to the non-paper-feeding transfer area.

3. 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, to form an electrostatic image on the surface of the photoreceptor, A developing member that supplies toner 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 portion, and when a voltage is applied, transfers a toner image from the surface of the photoreceptor to a recording material in the transfer portion, A control unit capable of controlling the exposure apparatus, 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 charging portion, and the end of the surface of the photoreceptor in the rotational axis direction has a non-transfer region that contacts the charging member but does not contact the transfer member, and in the rotational axis direction, at least a portion of the toner coating region of the developing member overlaps with the non-transfer region, Image forming apparatus characterized in that, in the direction of the rotation axis, the area on the surface of the photoreceptor that contacts the recording material with the transfer portion is defined as the paper-feeding area, and the area outside the paper-feeding area and inside the transfer portion is defined as the off-paper-feeding transfer area, the control unit is capable of performing an exposure operation using the exposure device to expose at least the non-transfer area of ​​the photoreceptor, or at least the non-transfer area and the off-paper-feeding transfer area of ​​the photoreceptor, when the photoreceptor is rotating.

4. The image forming apparatus according to any one of claims 1 to 3, characterized in that the control unit controls the exposure apparatus to expose at least the non-transfer region and the paper-outside transfer region of the photoreceptor during the exposure operation.

5. The image forming apparatus according to any one of claims 1 to 3, characterized in that the control unit controls the exposure apparatus to perform the exposure operation when the image forming region on the surface of the photoreceptor in the rotational direction of the photoreceptor passes through the exposure section where the surface of the photoreceptor is exposed.

6. The image forming apparatus according to claim 5, characterized in that the control unit, when the image forming region is passing through the exposure unit, exposes the surface of the photoreceptor inside the paper-feeding region in the direction of the rotation axis with a first exposure amount to form the electrostatic image on the surface of the photoreceptor, and controls the exposure unit to perform the exposure operation with a second exposure amount smaller than the first exposure amount when the image forming region is passing through the exposure unit.

7. The image forming apparatus according to any one of claims 1 to 3, characterized in that the control unit controls the exposure apparatus to perform the exposure operation when a non-image forming region of the surface of the photoreceptor in the rotational direction of the photoreceptor passes through an exposure section in which the surface of the photoreceptor is exposed.

8. The image forming apparatus according to claim 7, characterized in that the control unit exposes the surface of the photoreceptor inside the paper-feeding area in the rotational axis direction with a first exposure amount when the image forming area on the surface of the photoreceptor in the rotational direction of the photoreceptor passes through the exposure unit, thereby forming the electrostatic image on the surface of the photoreceptor, and controls the exposure unit to perform the exposure operation with an exposure amount substantially the same as the first exposure amount when the non-image forming area passes through the exposure unit.

9. The image forming apparatus according to claim 7, characterized in that the non-image forming region is a section on the surface of the photoreceptor between a position corresponding to the rear end of a preceding recording material and a position corresponding to the front end of a subsequent recording material during continuous image formation in which a toner image is transferred to a plurality of recording materials.

10. The image forming apparatus according to claim 9, characterized in that the length of the section is set such that the sum of the length of the recording material in the transport direction of the recording material and the length of the section in the rotation direction of the photoreceptor is not an integer multiple of the circumference of the photoreceptor.

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