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, or a facsimile machine, that uses an electrophotographic system. [Background technology]

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

[0003] The toner image formed on the photoreceptor is transferred onto the 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 contacts the photoreceptor to form a transfer portion (transfer nip portion). The transfer roller conveys the recording material by sandwiching it between the photoreceptor and the transfer roller, and transfers the toner on the photoreceptor onto the recording material. During transfer, a transfer voltage of 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. Note that the recording material is sometimes called "paper", but the recording material is not limited to paper, and may be a material mainly composed of synthetic resin, such as an overhead projector sheet or synthetic paper. For convenience, the high and low (large and small) or increase and decrease of the electric potential or voltage refers to the high and low (large and small) or increase and decrease when compared in terms of the absolute value of the electric potential or voltage.

[0004] Here, the photoconductor charging method includes a method in which a conductive charging member that contacts the photoconductor is used as a charging means, and a voltage is applied to this charging member to perform charging processing. A charging roller, which is a roller-shaped charging member, is often used as the charging member. In addition, such charging methods include an AC / DC charging method in which an oscillating voltage in which a direct current voltage (DC voltage) and an alternating current voltage (AC voltage) are superimposed on each other is applied to the charging member, and a DC charging method in which only a direct current voltage (DC voltage) is applied. The DC charging method has the advantage that it does not require an AC power source, and therefore can reduce the size and cost of the device.

[0005] Also, a pre-exposure means for exposing the surface of the photoconductor to light may be provided downstream of the transfer position by the transfer means and upstream of the charging position by the charging means in the rotation direction of the photoconductor, to remove residual charges on the surface of the photoconductor after the transfer process. LED chip arrays, fuse lamps, halogen lamps, fluorescent lamps, etc. are used as pre-exposure means (discharging means). On the other hand, there is a pre-exposure-less method that omits the pre-exposure means 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 above-mentioned DC charging method and pre-exposure-less method.

[0007] In addition, Patent Document 2 proposes a configuration in which the amount of exposure by an exposure device to non-paper passing areas on the photosensitive body is adjusted to lower the surface potential of the non-paper passing areas on the photosensitive body, thereby suppressing toner adhesion to the surface of the photosensitive body. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2003-302808 A [Patent Document 2] JP 2019-194650 A Summary of the Invention [Problem to be solved by the invention]

[0009] However, in conventional image forming apparatuses, it has been found that when the contact area of ​​the surface of the photoconductor with the transfer roller is shorter than the contact area of ​​the surface of the photoconductor with the charging roller in a direction substantially perpendicular to the moving direction of the surface of the photoconductor (the conveying direction of the recording material), the following problem occurs. Note that the direction substantially perpendicular to the moving direction of the surface of the photoconductor (the conveying direction of the recording material) (i.e., the direction substantially parallel to the rotation axis direction of the charging roller) is sometimes referred to as the "longitudinal direction." Also, the length of the contact area of ​​the surface of the photoconductor with the charging roller is sometimes simply referred to as the length of the charging roller, and the length of the contact area of ​​the surface of the photoconductor with the transfer roller is sometimes simply referred to as the length of the transfer roller.

[0010] When the transfer roller is shorter than the charging roller in the longitudinal direction, an area where the charging roller contacts the photoconductor and the transfer roller does not contact the photoconductor is generated at the longitudinal end. Here, the area of ​​the photoconductor surface that contacts the transfer roller is called the "transfer area", and the area of ​​the photoconductor surface that contacts the charging roller but does not contact the transfer roller is called the "non-transfer area". When considering the surface potential of the photoconductor after transfer, in the transfer area, a transfer voltage is applied when transferring a toner image from the photoconductor to a recording material, so the surface potential of the photoconductor is low. On the other hand, in the non-transfer area, a transfer voltage is not applied, so the surface potential of the photoconductor remains high. As a result, a potential difference occurs between the transfer area and the non-transfer area in the surface potential of the photoconductor after transfer. This potential difference becomes smaller during the subsequent charging process, but gradually increases by repeatedly passing through the transfer section. For example, in a configuration that employs a reversal development method using negatively charged toner, the non-transfer area is negatively charged by the charging roller but is not positively charged by the transfer roller. Therefore, when charging is repeated during continuous image formation, etc., the charge removal effect of the positive charging of the transfer roller cannot be obtained in the non-transfer area, and the surface potential of the photoconductor may rise to an excessively negative potential.

[0011] The phenomenon described above, in which the surface potential of the non-transfer area at the longitudinal end of the photosensitive member rises to an excessive potential, tends to be more pronounced when the image forming apparatus employs a DC charging method in which the potential leveling effect of an AC voltage cannot be obtained, and further when a pre-exposure-less method is employed.

[0012] If the surface potential of the non-transfer area at the longitudinal end of the photoconductor rises to an excessive potential as described above, the following problems may occur, for example.

[0013] For example, if the surface potential of the non-transfer area at the longitudinal end of the photoconductor rises to an excessive potential, discharge may occur between the photoconductor in that area and the core metal part of the transfer roller, causing damage such as leakage marks due to insulation breakdown on the surface of the photoconductor. If a charging voltage is applied to the charging member while this damage is on the photoconductor, current may concentrate on the damaged area, causing a drop in the applied voltage to the charging member. As a result, the photoconductor, including other areas, may not be able to reach the desired surface potential, and striped images may occur in the longitudinal direction due to poor charging.

[0014] In addition, in the longitudinal direction, there is a configuration in which the toner coat area (development area) on the development roller is longer than the contact area of ​​the surface of the photoconductor with the transfer roller. In this configuration, the development area faces both the transfer area and the non-transfer area of ​​the photoconductor. In this case, if the surface potential of the photoconductor in the non-transfer area rises to an excessive potential as described above, "reverse fogging" may occur, in which "reverse toner" charged with a polarity opposite to the normal charging polarity adheres to the photoconductor. If the toner adhered to the surface of the photoconductor in the non-transfer area increases due to this "reverse fogging," a cleaning failure may occur. And, due to this cleaning failure, there is a possibility that "edge contamination" may occur, in which the edge of the recording material in the direction approximately perpendicular to the conveying direction of the recording material is stained with toner.

[0015] In the method described in Patent Document 2, the amount of exposure to the photoconductor in the area where the transfer roller contacts is adjusted, and therefore the problem of the increase in the surface potential of the photoconductor outside the area where the transfer roller contacts cannot be addressed.

[0016] Therefore, the object of the present invention is to suppress an excessive increase in the surface potential at the longitudinal end of a photosensitive body in a configuration in which the contact area of ​​the photosensitive body surface with a transfer member is shorter in the longitudinal direction than the contact area of ​​the photosensitive body surface with a charging member. [Means for solving the problem]

[0017] The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides an image forming apparatus having a rotatable photoconductor, a rotatable charging member that contacts the photoconductor to form a charging section and charges the surface of the photoconductor in the charging section, an exposure device that exposes the surface of the photoconductor charged by the charging member to form an electrostatic image on the surface of the photoconductor, a developing member that supplies toner to the electrostatic image formed on the surface of the photoconductor to form a toner image, a transfer member that contacts the surface of the photoconductor to form a transfer section and transfers the toner image from the surface of the photoconductor to a recording material in the transfer section by applying a voltage, and a control unit that can control the exposure device, the width of the transfer section being shorter than the width of the charging section in the direction of the rotation axis of the charging member, and a non-transfer area that contacts the charging member and does not contact the transfer member at an end of the surface of the photoconductor in the direction of the rotation axis. and when, in the direction of the rotation axis, the area on the surface of the photosensitive body that comes into contact with the recording material at the transfer section is defined as a paper passing area, and the area outside the paper passing area and inside the transfer section is defined as a paper-outside transfer area, the control unit is capable of performing an exposure operation to expose at least the non-transfer area of ​​the photosensitive body using the exposure device while the photosensitive body is rotating, and controls the exposure device so that, by the exposure operation, a surface potential is formed on the surface of the photosensitive body downstream of the exposure section where the surface of the photosensitive body is exposed in the rotation direction of the photosensitive body and upstream of the transfer section, and controls the exposure device so that the absolute value of the surface potential formed in the non-transfer area is smaller than the absolute value of the surface potential formed in the paper-outside transfer area downstream of the exposure section and upstream of the transfer section in the rotation direction of the photosensitive body.

[0018] According to another aspect of the present invention, a recording medium storing image pickup device includes a rotatable photoconductor, a rotatable charging member that contacts the photoconductor to form a charging portion and charges the surface of the photoconductor in the charging portion, an exposure device that exposes the surface of the photoconductor charged by the charging member to form an electrostatic image on the surface of the photoconductor, a developing member that supplies toner to the electrostatic image formed on the surface of the photoconductor to form a toner image, a transfer member that contacts the surface of the photoconductor to form a transfer portion and transfers the toner image from the surface of the photoconductor to a recording material in the transfer portion by application of a voltage, and a control unit that can control the exposure device, wherein the width of the transfer portion is shorter than the width of the charging portion in the direction of the rotation axis of the charging member, In an image forming apparatus having a non-transfer area at the end of the surface of the photosensitive member in the direction of the rotation axis, which contacts the charging member but does not contact the transfer member, when the area of ​​the surface of the photosensitive member that contacts the recording material at the transfer section in the direction of the rotation axis is defined as a paper passing area, and the area outside the paper passing area and inside the transfer section is defined as an outer-paper passing transfer area, the control unit is capable of performing an exposure operation to expose at least the non-transfer area of ​​the photosensitive member using the exposure device while the photosensitive member is rotating, and the image forming apparatus is characterized in that, in the exposure operation, the control unit controls the exposure device so that the amount of exposure for the non-transfer area is greater than the amount of exposure for the outer-paper passing transfer area.

[0019] According to another aspect of the present invention, a recording medium storing image 100 includes a rotatable photoconductor, a rotatable charging member that contacts the photoconductor to form a charging portion and charges the surface of the photoconductor in the charging portion, an exposure device that exposes the surface of the photoconductor that has been charged by the charging member to light to form an electrostatic image on the surface of the photoconductor, a developing member that supplies toner to the electrostatic image formed on the surface of the photoconductor to form a toner image, a transfer member that contacts the surface of the photoconductor to form a transfer portion and transfers the toner image from the surface of the photoconductor to a recording material in the transfer portion by application of a voltage, and a control unit that can control the exposure device, wherein the width of the transfer portion is shorter than the width of the charging portion in the direction of the rotation axis of the charging member, and the photoconductor The image forming apparatus has a non-transfer area at an end of the surface in the direction of the rotation axis that contacts the charging member but does not contact the transfer member, and at least a portion of the toner coated area of ​​the developing member overlaps with the non-transfer area in the direction of the rotation axis, wherein when an area of ​​the surface of the photoconductor that contacts the recording material at the transfer section in the direction of the rotation axis is defined as a paper passing area, and an area outside the paper passing area and inside the transfer section is defined as an outside-paper passing transfer area, the control unit is capable of performing an exposure operation to expose at least the non-transfer area of ​​the photoconductor or at least the non-transfer area of ​​the photoconductor and the outside-paper passing transfer area by the exposure device when the photoconductor is rotating. Effect of the Invention

[0020] According to the present invention, in a configuration in which the contact area of ​​the surface of the photosensitive body with the transfer member is shorter in the longitudinal direction than the contact area of ​​the surface of the photosensitive body with the charging member, it is possible to suppress an excessive increase in the surface potential at the longitudinal end of the photosensitive body. [Brief description of the drawings]

[0021] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Diagram 2] FIG. 2 is a schematic diagram showing the positional relationship in the longitudinal direction of each part around the photosensitive drum. [Figure 3(a)]5 is an explanatory diagram of an increase in the surface potential of a photosensitive drum. [Figure 3(b)] 5 is an explanatory diagram of an increase in the surface potential of a photosensitive drum. [Figure 4(a)] 5 is an explanatory diagram of the transition of the surface potential of the photosensitive drum in the first embodiment. FIG. [Figure 4(b)] 5 is an explanatory diagram of the transition of the surface potential of the photosensitive drum in the first embodiment. FIG. [Diagram 5] 10 is a graph showing changes in surface potential at an end portion of a photosensitive drum in an example and a comparative example. FIG. [Figure 6] 10 is a schematic diagram showing the positional relationship in the longitudinal direction of each part around a photosensitive drum in Example 2. FIG. [Figure 7] FIG. 11 is a graph showing the relationship between Vback and the degree of occurrence of “fogging.” [Figure 8(a)] FIG. 11 is a graph showing the transition of the surface potential at the end portion of the photosensitive drum in the third embodiment. [Figure 8(b)] FIG. 11 is a graph showing the transition of the surface potential at the end portion of the photosensitive drum in the third embodiment. [Figure 9] 5 is an explanatory diagram of a paper-to-paper position in the circumferential direction of a photosensitive drum. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 device First, the configuration of an image forming apparatus 100 of this embodiment will be described. Fig. 1 is a schematic cross-sectional view of the image forming apparatus 100 of this embodiment. The image forming apparatus 100 of this embodiment is an electrophotographic laser printer, 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-type (cylindrical) photosensitive body (electrophotographic photosensitive body) serving as an image carrier, inside an apparatus main body M. The photosensitive drum 1 is configured by providing a photosensitive material such as an OPC (organic photoconductor), amorphous selenium, or amorphous silicon on a cylindrical drum base made of aluminum, nickel, or the like. The photosensitive drum 1 used in this embodiment is a negatively charged OPC photosensitive body with an outer diameter of φ24 mm. This photosensitive drum 1 is configured by having a photosensitive layer in which a charge generation layer and a charge transport layer are laminated in this order from the conductive base side on the surface of a conductive base made of an aluminum cylinder.

[0025] Around the photosensitive drum 1, the following means are arranged in order along the rotation direction Rd. First, a charging roller 2, which is a roller-shaped charging member serving as a charging means, is arranged. 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) serving as a transfer means, is arranged. Next, a charge eliminating needle 20 is arranged as a charge eliminating 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 the outer circumferential surface of the base shaft. The charging roller 2 used in this embodiment is an elastic roller with a roller outer diameter of φ10 mm, a core metal diameter of φ5 mm, and a thickness of the elastic layer of 2.5 mm. In this embodiment, the core metal is made of SUS, and the elastic layer is made of a mixed rubber material of NBR and epichlorohydrin. The charging roller 2 is pressed against the photosensitive drum 1 and rotates in response to the rotation of the photosensitive drum 1. The charging roller 2 is disposed so that the direction of its rotation axis is approximately parallel to the direction (width direction) that is approximately perpendicular to the moving direction of the surface of the photosensitive drum 1. With respect to the rotation direction of the photosensitive drum 1, the position on the photosensitive drum 1 where the charging process is performed by the charging roller 2 is the charging position Pa. The charging roller 2 charges the surface of the photosensitive drum 1 by discharging generated in at least one of the minute gaps formed on the upstream side and downstream side 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 is referred to as "discharge charging." The charging roller 2 also charges the surface of the photosensitive drum 1 by injecting charge at the contact portion between the charging roller 2 and the photosensitive drum 1. This is referred to as "injection charging." For simplicity, the contact portion between the charging roller 2 and the photosensitive drum 1 may be considered as a charging position (charging portion) Pa.

[0027] In this embodiment, the exposure device 3 is configured with 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 by the exposure device 3 is performed is the exposure position (exposure portion) Pb.

[0028] In this embodiment, the developing device 4 uses a non-magnetic one-component developer (toner) as a developer. The developing device 4 has a developing roller 4a as a developer carrier (developing member) and a developing container 4b. The developing roller 4a abuts against the surface of the photosensitive drum 1 during development, and supplies the toner to a developing section that faces (abuts) the photosensitive drum 1. The developing container 4b is a container that contains the developer, and the developer contained in the developing container 4b is supplied to the developing roller 4a. The developing device 4 may use a magnetic one-component developer (toner) or a two-component developer including a 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 the toner is supplied by the developing roller 4a (the position where the developing roller 4a abuts in this embodiment) 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 as a biasing means, and is pressed against the photosensitive drum 1. As a result, a transfer portion (transfer nip portion, transfer nip portion) Nt, which is a contact portion between the photosensitive drum 1 and the transfer roller 5, is formed. The transfer roller 5 rotates following the rotation of the photosensitive drum 1. The transfer roller 5 conveys the recording material P by nipping it between the photosensitive drum 1 and the transfer roller 5, and transfers the toner image from the photosensitive drum 1 to the recording material P by applying a voltage. The transfer roller 5 is composed of, for example, a conductive base shaft (core metal) which also serves as a power supply electrode, and an elastic layer which cylindrically surrounds the outer circumferential surface of the conductive base shaft. As the elastic layer, a semiconductive rubber material composed of EPDM, NBR, SBR, urethane rubber, epichlorohydrin, silicone rubber, or the like is generally used. The material of the elastic layer may contain an appropriate amount of a conductive agent, for example, an ionic conductive agent. The transfer roller 5 used in this embodiment is an elastic roller with an outer roller diameter of φ14 mm, a core diameter of φ5 mm, and an elastic layer thickness of 4.5 mm. In this embodiment, the core is made of SUS, and the elastic layer is made of a mixed rubber material of SBR and epichlorohydrin. 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 when the transfer roller 5 is pressed against an aluminum cylinder with a force of 9.8 N, rotated at 50 mm / sec, and +1000 V is applied. 8 Ω. The resistance value of the transfer roller 5 is the resistance value when the transfer roller 5 is left in a normal temperature and normal humidity environment at the beginning of 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 portion Nt) is the transfer position Pd.

[0030] The charge eliminating 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. A charge eliminating needle having a sawtooth-shaped sharp tip and made of a thin metal plate having good conductivity, such as a stainless steel plate or an aluminum plate, can be used as the charge eliminating needle 20. The charge eliminating needle 20 is disposed downstream of the transfer roller 5 in the conveying 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 deposits such as toner (transfer residual toner) remaining on the photosensitive drum 1 after transfer. In this embodiment, the cleaning device 6 has a cleaning blade 6a and a cleaning container 6b as cleaning members arranged to abut against the surface of the photosensitive drum 1. With respect to the rotation direction of the photosensitive drum 1, the position on the photosensitive drum 1 where the toner is removed by the cleaning blade 6a (in this embodiment, the position where the cleaning blade 6a abuts) is the cleaning position (cleaning portion) Pe.

[0032] Further, a recording material cassette (paper feed tray) 7 in which recording materials (transfer materials, recording media, sheets) P ​​such as paper are stored is disposed at the bottom of the apparatus main body M in the figure. Further, from the recording material cassette 7, a feed roller 8, a conveying roller 9, a top sensor 10, a pre-transfer conveying guide 15, a transfer / fixing conveying guide 11, a fixing device 12, a discharge roller 13, and a discharge tray 14 are disposed in this order along the conveying path of the recording material P. Further, the apparatus main body M is provided with a control unit 40 which controls the image forming apparatus 100, and a video controller 110 which performs image processing and the like.

[0033] Next, an image forming operation in the image forming apparatus 100 of this embodiment will be described. The photosensitive drum 1 is rotated by a drive source (not shown) in the direction of the arrow Rd in the figure (clockwise direction) at a peripheral speed (process speed) of 300 mm / sec. The surface of the rotating photosensitive drum 1 is charged approximately uniformly by the charging roller 2 to a predetermined potential (dark potential, charging potential) of the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment). During charging, a charging voltage (charging bias) which is a negative DC voltage is applied to the charging roller 2 from the charging power source (high voltage power source) 21 via the charging current detection circuit 22. In this embodiment, as an example, a charging voltage of -1100 V is applied to the charging roller 2, and a dark potential of -500 V 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 in accordance with image information. The video controller 110 of the image forming apparatus 100 processes image information input from the external device 200 to the image forming apparatus 100 to generate a time-series electric digital pixel signal and inputs it to the control unit 40. The exposure device 3 is controlled by the control unit 40, outputs a laser beam L modulated in accordance with the time-series electric digital pixel signal, and scans and exposes the charged surface of the photosensitive drum 1 with the laser beam 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 portion exposed by the exposure device 3 is removed, 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 with a 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 supplying toner by the developing device 4, and a toner image (toner image, developer image) is formed on the photosensitive drum 1. During development, a development voltage (developing bias), which is a DC voltage having the same polarity (negative in this embodiment) as the normal charging polarity of the toner, is applied to the developing roller 4a from the development power source (high voltage power source) 16. In this embodiment, as an example, a development voltage of -380V is applied to the developing roller 4a. In this embodiment, toner charged to the same polarity (negative in this embodiment) as the charging polarity of the photosensitive drum 1 adheres to the exposed portion (image portion) on the photosensitive drum 1, which has been subjected to a substantially uniform charging process and then exposed to light, thereby decreasing the absolute value of the potential (reverse development method). In this embodiment, the normal charging polarity of the toner, which is the main charging polarity of the toner during development, is negative.

[0036] The toner image formed on the photosensitive drum 1 is transferred onto the recording material P by the action of the transfer roller 5 at the transfer portion Nt. During the transfer, a transfer voltage (transfer bias) which is a DC voltage of the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner is applied to the transfer roller 5 from a transfer power source (high voltage power source) 18 via a transfer current detection circuit 19 as a transfer current detection means. In this embodiment, as an example, a transfer voltage of about +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 as a recording material storage portion, and is sent out one sheet at a time from the recording material cassette 7 by a feed roller 8 as a feed member. The recording material P is transported by a transport roller 9 as a transport member, and is supplied to the transfer portion Nt along a pre-transfer transport guide 15 as a guide member. The conveying roller 9 is controlled based on the detection results of the leading edge of the recording material P in the conveying direction by a top sensor 10 as a recording material detection means, and supplies the recording material P to the transfer section Nt in synchronization with the toner image on the photosensitive drum 1.

[0037] The recording material P to which the toner image has been transferred at the transfer portion Nt has excess charge on its surface removed by a charge removing needle 20. The recording material P that has passed through the charge removing needle 20 is transported along a transfer / fixing transport guide 11 serving as a guide member to a fixing device 12 serving as a fixing means. The fixing device 12 has a fixing roller 12a incorporating a heater, and a pressure roller 12b that is in pressure contact with the fixing roller 12a. The fixing device 12 applies heat and pressure to the recording material P carrying an unfixed toner image that passes through a nip portion between these rollers, thereby fixing (melting and fixing) 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 is fixed on one side by the fixing device 12 is discharged (output) by discharge rollers 13 onto a discharge tray 14 formed on the upper surface of the apparatus main body M in the drawing. Note that the image forming apparatus 100 may be configured to be capable of double-sided image formation, in which the recording material P with the toner image fixed on one side is turned over, the conveying direction is reversed, and the recording material P is conveyed again to the transfer section Nt, where a toner image is transferred to and fixed on the second side of the recording material P.

[0039] Meanwhile, deposits such as toner (residual toner) that are not transferred to the recording material P during transfer and remain on the surface of the photosensitive drum 1 are removed and collected by the cleaning device 6. The cleaning device 6 scrapes off deposits such as residual toner from the surface of the rotating photosensitive drum 1 with a cleaning blade 6a and stores them in a cleaning container 6b.

[0040] By repeating the above operations, image formation can be performed one after another. In this embodiment, the image forming apparatus 100 can perform a printing operation at a print speed of 50 sheets per minute.

[0041] In this embodiment, the image forming apparatus 100 does not have a 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 in terms of the rotation direction of the photosensitive drum 1.

[0042] In addition, the photosensitive drum 1 and at least one of the charging roller 2, the developing device 4, and the cleaning device 6 acting as process means acting on the photosensitive drum 1 may be integrally configured as a cartridge (process cartridge) that is detachably attached to the main body M of the apparatus.

[0043] The control unit 40 is configured to include a CPU 41 as an arithmetic control means which is a central element for performing arithmetic processing, memories such as ROM 41a and RAM 41b as storage means, and an input / output unit (not shown) for controlling the exchange of signals between the control unit 40 and each unit outside the control unit 40. The RAM 41b, which is a rewritable memory, stores information input to the control unit 40, detected information, arithmetic results, etc., and the ROM 41a stores control programs, data tables obtained in advance, etc. The CPU 41 and memories such as the ROM 41a and RAM 41b can transfer and read data to each other. The CPU 41 can control various operations related to image formation by executing various programs stored in the ROM 41a, while using the RAM 41b as a working area. In particular, in this embodiment, the control unit 40 can execute an exposure operation for exposing at least a non-transfer area E (to be described later) of the photosensitive drum 1 by the exposure device 3.

[0044] Here, the image forming apparatus 100 executes a print job (printing operation) which is a series of operations for forming and outputting an image on a single or multiple recording materials P, which is started by one start instruction. A print job generally includes an image forming process, a pre-rotation process, a paper-to-paper process in the case of forming images on multiple recording materials P, and a post-rotation process. The image forming process is a period during which an electrostatic latent image of an image to be actually formed on a recording material P, a toner image, and a toner image are formed and transferred, and the image forming time refers to this period. More specifically, the timing during image formation differs depending on the positions at which the electrostatic latent image forming process, the toner image forming process, and the toner image transfer process are 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 a period during which a preparatory operation is performed before the image forming process, from when a start instruction is input until the image actually starts to be formed. The paper-to-paper process (image-to-paper process, recording material-to-paper process) is a period corresponding to the period between recording materials P when image formation is performed continuously on multiple recording materials P (continuous image formation, continuous printing). The post-rotation process is a period in which a sorting operation (preparatory operation) is performed after the image forming process. The non-image forming time is a period other than the image forming time, and includes the above-mentioned pre-rotation process, the sheet interval process, the post-rotation process, and the pre-multi-rotation process, which is a preparatory operation when the image forming apparatus 100 is turned on or when the image forming apparatus 100 returns from a sleep state. More specifically, the timing of the non-image forming time corresponds to a period in which the non-image forming area on the photosensitive drum 1 passes through each position where each process of forming the electrostatic latent image, forming the toner image, and transferring the toner image is performed. The image forming area on the photosensitive drum 1 or on the recording material P is an area in which a toner image that is transferred to the recording material P and output from the image forming apparatus 100 and that is preset according to the size of the recording material P and the like can be formed, and the non-image forming area is an area other than the image forming area. In this embodiment, a margin portion that is a non-image forming area is provided in a predetermined area at the leading end and trailing end of the recording material P in the conveying direction of the recording material P. In this embodiment, margins, which are non-image forming areas, are also provided in predetermined areas on both ends of the recording material P in a direction substantially perpendicular to the conveying direction of the recording material P.

[0045] (2) Longitudinal positional relationship 2 is a schematic diagram for explaining the positional relationship of each part around the photosensitive drum 1 in a direction substantially perpendicular to the moving direction of the surface of the photosensitive drum 1 (the transport direction of the recording material P). Note that the direction substantially perpendicular to the moving direction of the surface of the photosensitive drum 1 (the transport direction of the recording material P) (i.e., the direction substantially 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 in image formation (particularly the width in the direction substantially perpendicular to the transport direction), but FIG. 2 shows the positional relationship when the recording material P is LTR size.

[0046] In FIG. 2, "photosensitive body region A" indicates the region in the longitudinal direction where the photosensitive layer of the photosensitive drum 1 is formed or the width of that region. Also, "charged region (charging section) B" indicates the region in the longitudinal direction where the charging roller 2 can contact the surface of the photosensitive drum 1 or the width of that region. Also, "transfer region (transfer section) C" indicates the region in the longitudinal direction where the transfer roller 5 can contact the surface of the photosensitive drum 1 or the width of that region. Also, "paper passing region D" indicates the region in the longitudinal direction through which the recording material P passes in the transfer section Nt or the width of that region. Also, "non-transfer region E" indicates the region in 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 region of the difference between the charging region B and the transfer region C or the width of that region). Further, the "non-paper-passing transfer region F" indicates the region 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 portion Nt, or the width of that region (i.e., the region that is the difference between the transfer region C and the paper-passing region D, or the width of that region). In other words, in the longitudinal direction, the region on the surface of the photosensitive drum 1 that contacts the recording material P at the transfer portion Nt is the paper-passing region D, and the region outside the paper-passing region D and inside the transfer region C is the non-paper-passing transfer region F. For convenience, the regions on the photosensitive drum 1 that correspond to the above-mentioned "charged region B", "transfer region C", "paper-passing region D", "non-transfer region E", and "non-paper-passing transfer region F" are also referred to as the "charged region B", "transfer region C", "paper-passing region D", "non-transfer region E", and "non-paper-passing transfer region F", respectively.

[0047] In this embodiment, the photoconductor area A, charging area B, transfer area C, and paper passing area D are arranged so that their longitudinal centers are approximately aligned with the longitudinal center of the image forming area (area where a toner image can be formed) (center reference). Therefore, among the above areas, those with relatively short longitudinal widths are included inside those with relatively long widths. Note that FIG. 2 illustrates the range from the longitudinal center to one end side.

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

[0049] (3) Increase in surface potential of the photosensitive drum in the non-transfer area Next, with reference to FIG. 3 (FIG. 3(a) and FIG. 3(b)), the process by which the surface potential of the photosensitive drum 1 in the non-transfer region E rises during a printing operation when the exposure operation of this embodiment described later is not performed will be described. In FIG. 3, the horizontal axis indicates the position on the photosensitive drum 1 in the longitudinal direction, and illustrates the above-mentioned charging region B, transfer region C, paper passing region D, non-transfer region E, and non-paper passing transfer region F. Also, in FIG. 3, the vertical axis indicates the surface potential of the photosensitive drum 1, and indicates that the surface potential of the photosensitive drum 1 is higher on the negative side (i.e., the absolute value of the negative surface potential is larger) toward the upper side in the figure. Note that FIG. 3 illustrates the range of one end side in the longitudinal direction. Also, the surface potential of the photosensitive drum 1 shown in FIG. 3 described below is a value that can change depending on various conditions such as the environment and the type of recording material P. In addition, in the following description, "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 section Nt), "after transfer" means after passing the transfer position Pd (transfer section 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 after charging (and before exposure) immediately after the start of a printing operation. In state 1-1, the surface of the photosensitive drum 1 is almost uniformly charged to a predetermined dark potential Vd by the charging roller 2 to which a predetermined charging voltage is applied. In the example of Fig. 3, as an example, a charging voltage of -1100V is applied to the charging roller 2 during charging, and the surface of the photosensitive drum 1 is charged to a dark potential Vd of -500V.

[0051] Next, state 1-2 shows the surface potential of the photosensitive drum 1 after exposure (and before transfer). An image portion (image area, printing location, printing area) in the paper passing area D is irradiated with laser light L by the exposure device 3, and an electrostatic latent image (electrostatic image) is formed by exposure. In the example of FIG. 3, as an example, the image portion in the paper passing area D is irradiated with 0.3 μJ / cm by the exposure device 3. 2 4, a light portion potential of −100 V is formed on the surface of the photosensitive drum 1.

[0052] Next, state 1-3 shows the surface potential of the photosensitive drum 1 after transfer (and before recharging). When the recording material P passes through the transfer portion Nt, a positive transfer voltage is applied to the transfer roller 5 at the transfer portion Nt. Therefore, the surface potential of the photosensitive drum 1 in the outside transfer region F where the photosensitive drum 1 and the transfer roller 5 are in direct contact during paper passing drops. On the other hand, the transfer roller 5 is not in contact with the photosensitive drum 1 in the non-transfer region E, so the positive transfer voltage is not applied. In addition, the image forming apparatus 100 of this embodiment does not have a means for lowering the surface potential of the photosensitive drum 1 by irradiating the surface of the photosensitive drum 1 with light after transfer and before charging, such as a pre-charge exposure means. Therefore, the surface potential of the photosensitive drum 1 in the non-transfer region E does not drop much. As a result, a potential difference occurs between the surface potential of the photosensitive drum 1 in the outside transfer region F and the surface potential of the photosensitive drum 1 in the non-transfer region E. Also, the surface potential (light area potential) of the photosensitive drum 1 in the image area in the paper passing area D varies within a range between the light area potential and the development potential as a result of the influence at the development position Pc and the transfer area Nt as described later. In the example of FIG. 3, as an example, after transfer, the surface potential of the photosensitive drum 1 in the transfer area F outside the paper passing is −400V, and the surface potential of the photosensitive drum 1 in the non-transfer area E remains at −500V. Also, in the example of FIG. 3, as an example, after transfer, the surface potential of the photosensitive drum 1 in the image area in the paper passing area D varies from −100V to −250V. This is because it is influenced by the supply of toner to the image area (the area of ​​the light area potential) by the development roller 4a at the development position Pc and the application of a positive polarity voltage by the transfer roller 5 through the recording material P at the transfer area Nt. In addition, the potential difference between the surface potential of the photosensitive drum 1 in the non-transfer area E and the surface potential of the photosensitive drum 1 in the non-transfer area F (or transfer area C) is sometimes simply referred to as the potential difference between the non-transfer area E and the non-transfer area F (or transfer area C).

[0053] Next, state 1-4 shows 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 in a state where a potential difference occurs between the transfer area C (consisting of the paper passing area D and the non-paper passing transfer area F) and the non-transfer area E. In state 1-4, a predetermined charging voltage (-1100V) is applied to the charging roller 2, as in state 1-1. The surface potential of the photosensitive drum 1 after recharging returns to the predetermined dark area potential Vd (-500V) in the transfer area C, as in state 1-1. On the other hand, the surface potential of the photosensitive drum 1 after recharging is already equivalent to the dark area potential Vd in the non-transfer area E, so no discharge charging occurs, but the potential rises due to injection charging 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 the charging position Pa multiple times (after multiple rechargings and before exposure) in a state where a charging voltage (-1100V) is constantly (continuously) applied to the charging roller 2. The surface potential of the photosensitive drum 1 after charging returns to the predetermined dark potential Vd (-500V) in the transfer region C, 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 the charging position Pa. In the example of FIG. 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 to laser light L is performed by the exposure device 3 as in state 1-2 in a situation where the surface potential of the photosensitive drum 1 in the non-transfer area E has increased after multiple recharging. As in state 1-2, the surface potential of the photosensitive drum 1 in the image area in the paper passing area D drops to a predetermined light area potential. Meanwhile, the surface potential of the photosensitive drum 1 in the non-transfer area E remains elevated as in state 1-5.

[0056] If the surface potential of the photosensitive drum 1 in the non-transfer area E rises excessively, the potential difference between the core metal part of the transfer roller 5 and the non-transfer area E on the photosensitive drum 1 becomes large, and discharge may occur. This discharge may cause damage to the photosensitive drum 1, such as leakage marks due to insulation breakdown. If a charging voltage is applied to the charging roller 2 in the state where this damaged area exists, current may concentrate on the damaged area, causing a drop in the charging voltage. As a result, the photosensitive drum 1, including other areas, may not be able to have a desired surface potential, and a problem of striped images occurring in the longitudinal direction due to poor charging may occur. For this reason, it is desirable to suppress the surface potential of the photosensitive drum 1 in the non-transfer area E from rising excessively.

[0057] (4) Change in surface potential of the photosensitive drum when the exposure operation of this embodiment is performed Next, using FIG. 4 (FIGS. 4(a) and 4(b)), we will explain the transition of the surface potential of the photosensitive drum 1 during the printing operation when the exposure operation of this embodiment is performed. In this embodiment, 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 increase in the surface potential of the photosensitive drum 1 in the non-transfer area E. The horizontal and vertical axes in FIG. 4 have the same meanings as those of the horizontal and vertical axes in FIG. 3, respectively.

[0058] First, state 2-1 shows the surface potential of the photosensitive drum 1 after charging (and before exposure) immediately after the start of a printing operation. In state 2-1, similar to state 1-1 in Fig. 3, the surface of the photosensitive drum 1 is approximately uniformly charged to a predetermined dark potential Vd by the charging roller 2 to which a predetermined charging voltage is applied. In the example of Fig. 4, as an example, similar to state 1-1 in Fig. 3, a charging voltage of -1100V is applied to the charging roller 2 during charging, and the surface of the photosensitive drum 1 is charged to a dark 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 non-transfer region E on the photosensitive drum 1 is exposed by the exposure device 3, thereby suppressing an excessive rise in the surface potential of the photosensitive drum 1 in the non-transfer region E. That is, an electrostatic latent image (electrostatic image) is formed on the image portion in the paper passing region D by irradiating the laser light L by the exposure device 3 and performing exposure. Also, in this embodiment, at this time, the non-transfer region E is also irradiated with the laser light L by the exposure device 3 in anticipation of the rise in the surface potential after recharging as in state 1-4 of FIG. 3, and exposure is performed. As a result, the surface potential of the photosensitive drum 1 in the non-transfer region E is lowered below the surface potential of the photosensitive drum 1 in the non-paper passing transfer region F. In the example of FIG. 4, as in state 1-2 of FIG. 3, as an example, the image portion in the paper passing region D is exposed to 0.3 μJ / cm by the exposure device 3. 2 4, the non-transfer area E is exposed to an exposure amount of 0.005 μJ / cm by the exposure device 3, forming a bright area potential of −100 V on the surface of the photosensitive drum 1. On the other hand, the non-transfer area E is exposed to an exposure amount lower than the exposure amount for the image area (herein also referred to as “weak exposure”), thereby lowering the surface potential. 2 The surface potential of the photosensitive drum 1 in the non-transfer region E is lowered to −490 V, which is smaller than the surface potential of the photosensitive drum 1 in the non-transfer region F of −500 V. In this embodiment, the surface potential of the photosensitive drum 1 in the non-transfer region E is −490 V, but is not limited to −500 V as long as the absolute value of the surface potential is smaller than −500 V.

[0060] Thus, in this embodiment, after exposure (and before transfer), the absolute value of the surface potential of the photosensitive drum 1 has the relationship of non-transfer area E<non-paper-passing transfer area F. Also, in this embodiment, the exposure device 3 does not expose the non-paper-passing transfer area F on the photosensitive drum 1. That is, in this embodiment, during exposure, the exposure amount (exposure amount per unit area) by the exposure device 3 has the relationship of non-transfer area E>non-paper-passing transfer area F. By satisfying such a relationship of the surface potential or the relationship of the exposure amount, it is possible to suppress an increase in the surface potential of the photosensitive drum 1 in the non-transfer area E as in state 1-4 in FIG. 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 state 1-3 in FIG. 3. However, the surface potential of the photosensitive drum 1 in the non-transfer area E where the transfer roller 5 is not in contact and exposure is performed by the exposure device 3 is different from state 1-3 in FIG. 3, and the surface potential after exposure in state 2-2 is maintained. In the example of FIG. 4, as an example, after transfer, the surface potential of the photosensitive drum 1 in the non-transfer area F outside the paper passing becomes −400V, and the surface potential of the photosensitive drum 1 in the non-transfer area E becomes −490V. Also, in the example of FIG. 4, as an example, after transfer, the surface potential of the photosensitive drum 1 in the image area in the paper passing area 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 FIG. 3. The surface potential of the photosensitive drum 1 after recharging returns to the predetermined dark potential Vd (−500 V) in the transfer area C, similar to state 1-4 in FIG. 3. Also, the surface potential of the photosensitive drum 1 after recharging is the predetermined dark potential Vd (−500 V) in the non-transfer area E as well, since the surface potential is lowered in advance in state 2-3 in anticipation of the rise in the surface potential due to injection charging. That is, the rise in the surface potential of the photosensitive drum 1 in the non-transfer area E as in state 1-4 in FIG. 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 the charging position Pa multiple times (after multiple recharging and before exposure) in a state where the charging voltage (-1100V) is constantly (continuously) applied. As described with respect to state 2-4, by performing the exposure operation of this embodiment, a further increase in the surface potential of the photosensitive drum 1 in the non-transfer region E as in state 1-5 in Fig. 3 is suppressed. In other words, 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 2-4.

[0064] Next, state 2-6 shows the surface potential of the photosensitive drum 1 after exposure (and before transfer) when exposure to laser light L is performed by the exposure device 3 in the same manner as in state 2-2 under the circumstances of state 2-5. As described with respect to state 2-5, the increase in the surface potential of the photosensitive drum 1 in the non-transfer area E is suppressed, so the surface potential of the photosensitive drum 1 in state 2-6 is similar to 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 an increase in the surface potential of the photosensitive drum 1 in the non-transfer area E as shown in FIG.

[0066] (5) Evaluation test Next, the results of an evaluation test to confirm the degree of increase in the surface potential of the photosensitive drum 1 in the non-transfer area E for this embodiment and Comparative Example 1 will be described. In this embodiment, the exposure operation described with reference to FIG. 4 was performed, and in Comparative Example 1, the exposure operation described with reference to FIG. 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 above-mentioned differences. FIG. 5 shows the transition of the surface potential of the photosensitive drum 1 in the non-transfer area E after exposure and before transfer when continuous image formation is performed on 20 sheets of LTR size paper as the recording material P. In FIG. 5, the solid line shows the transition of the surface potential when the exposure operation of this embodiment is performed, and the dashed line shows the transition of the surface potential when the exposure operation of Comparative Example 1 is performed.

[0067] In comparison example 1, since there is no means for counteracting the increase in the surface potential of the photosensitive drum 1 in the non-transfer area E, the surface potential of the photosensitive drum 1 in the non-transfer area E gradually increases due to the injection charge from the charging roller 2, and eventually becomes excessively high.

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

[0069] Thus, in this embodiment, the image forming apparatus 100 includes a rotatable photoconductor 1, a rotatable charging member 2 that contacts the photoconductor 1 to form a charging portion B and charges the surface of the photoconductor 1 at the charging portion B, an exposure device 3 that exposes the surface of the photoconductor 1 charged by the charging member 2 to form an electrostatic image on the surface of the photoconductor 1, a developing member 4a that supplies toner to the electrostatic image formed on the surface of the photoconductor 1 to form a toner image, a transfer member 5 that contacts the surface of the photoconductor 1 to form a transfer portion Nt and transfers the toner image from the surface of the photoconductor 1 to a recording material P at the transfer portion Nt when a voltage is applied, and a control unit 40 that can control the exposure device 3, and in the direction of the rotation axis of the charging member 2, the width of the transfer portion Nt is shorter than the width of the charging portion B, and a non-transfer area E that contacts the charging member 2 but does not contact the transfer member 5 is provided at the end of the surface of the photoconductor 1 in the direction of the rotation axis. In this embodiment, when the area of ​​the surface of the photoconductor 1 that comes into contact with the recording material P at the transfer section Nt in the direction of the rotation axis is defined as the paper passing area D, and the area outside the paper passing area D and inside the transfer section Nt is defined as the non-paper passing transfer area F, the control unit 40 can perform an exposure operation to expose at least the non-transfer area E of the photoconductor 1 using the exposure device 3 while the photoconductor 1 is rotating, and controls the exposure device 3 to form a surface potential on the surface of the photoconductor 1 downstream of the exposure section Pb where the surface of the photoconductor 1 is exposed in the rotation direction of the photoconductor 1 and upstream of the transfer section Nt through the exposure operation, and controls the exposure device 3 so that the absolute value of the surface potential formed in the non-transfer area E is smaller than the absolute value of the surface potential formed in the non-paper passing transfer area F downstream of the exposure section Pb and upstream of the transfer section Nt in the rotation direction of the photoconductor 1. In other words, in this embodiment, the control unit 40 can execute an exposure operation in which the exposure device 3 exposes at least the non-transfer region E of the photoconductor 1 while the photoconductor 1 is rotating, and controls the exposure device 3 so that the amount of exposure for the non-transfer region E is greater than the amount of exposure for the outside transfer region F during the exposure operation. In this embodiment, the control unit 40 controls the exposure device 3 so that the exposure device 3 exposes at least the non-transfer region E and the outside transfer region F of the photoconductor 1 during the exposure operation.In this embodiment, the control unit 40 controls the exposure device 3 to perform the exposure operation when the image forming area on the surface of the photoconductor 1 in the rotation direction of the photoconductor 1 passes through the exposure unit Pb where the surface of the photoconductor 1 is exposed. In this embodiment, the control unit 40 controls the exposure device 3 to expose the surface of the photoconductor 1 inside the paper passing area D in the rotation axis direction with a first exposure amount when the image forming area passes through the exposure unit Pb, thereby forming an electrostatic image on the surface of the photoconductor 1, and controls the exposure device 3 to perform the exposure operation with a second exposure amount smaller than the first exposure amount when the image forming area passes through the exposure unit Pb.

[0070] As described above, according to this embodiment, it is possible to suppress an increase in the surface potential of the photosensitive drum 1 in the non-transfer region E. In this manner, according to this embodiment, in a configuration in which the contact area C of the surface of the photosensitive drum 1 with the transfer roller 5 is shorter in the longitudinal direction than the contact area B of the surface of the photosensitive drum 1 with the charging roller 2, it is possible to suppress an excessive increase in the surface potential of the end portion (non-transfer region E) in the longitudinal direction of the photosensitive drum 1. Therefore, it is possible to suppress the occurrence of damage to the surface of the photosensitive drum 1 due to discharge caused by an increase in the surface potential of the photosensitive drum 1 in the non-transfer region E as described above.

[0071] [Example 2] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

[0072] In this embodiment, for the purpose of reducing the size of the image forming apparatus 100, a case will be described in which the transfer area C is shorter than a development area G (described later) in the longitudinal direction.

[0073] 6 is a schematic diagram for explaining the positional relationship of each part in the longitudinal direction around the photosensitive drum 1 in this embodiment. This positional relationship varies depending on the size (particularly the width in the direction substantially perpendicular to the conveying direction) of the recording material P used for image formation, but FIG. 6 shows the positional relationship when the recording material P is of LTR size.

[0074] In FIG. 6, the "photoconductor region A", "charging region B", "transfer region C", "paper passing region D", "non-transfer region E", and "non-paper passing transfer region F" respectively indicate the regions or the widths of the regions as described in the first embodiment. The "development region (developing section) G" indicates the region on the developing roller 4a where the toner is coated (toner coated region) in the longitudinal direction or the width of the region (more specifically, the region where the toner coat on the developing roller 4a can contact the surface of the photosensitive drum 1 or the width of the region). In this embodiment, the development region G can also be said to be the region where an opening is provided in the developing container 4b to supply the toner, which is the developer in the developing device 4, to the developing roller 4a or the width of the region. That is, in this embodiment, the toner is supplied to the developing roller 4a in the region where the opening is provided. The "fogging region H" indicates the region in the non-transfer region E and in the developing region G in the longitudinal direction or the width of the region. For convenience, the areas on the photosensitive drum 1 corresponding to the above-mentioned "charged area B", "transfer area C", "paper passing area D", "non-transfer area E", "paper outside transfer area F", "development area G", and "fog area H" are also called "charged area B", "transfer area C", "paper passing area D", "non-transfer area E", "paper outside transfer area F", "development area G", and "fog area H", respectively. In this embodiment, the photosensitive body area A, the charged area B, the transfer area C, the paper passing area D, and the development area G are arranged with the center as a reference, as in the first embodiment. Note that FIG. 6 shows the range from the center to one end side in the longitudinal direction.

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

[0076] In this embodiment, the developing roller 4a is in contact with the photosensitive drum 1. Therefore, there is a possibility that "fog" occurs in which toner adheres to the photosensitive drum 1 in the developing area G. In particular, when the surface potential of the photosensitive drum 1 in the non-transfer area E rises, there is a possibility that "fog" ("reverse fog") caused by "reverse toner" charged with a polarity opposite to the normal charging polarity may worsen. That is, there is a possibility that "reverse fog" occurs in the fog area H due to "reverse toner". When the amount of "fog" is large and "fog" continues to occur for a long time, the cleaning blade 6a of the cleaning device 6 may not be able to remove all the toner, and a cleaning failure may occur. Then, due to this cleaning failure, there is a possibility that "edge dirt" occurs in which the edge of the recording material P in the direction approximately perpendicular to the conveying direction of the recording material P is dirty with toner.

[0077] Here, the "fog" will be further explained. FIG. 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 non-exposed area) and the potential of the developing roller 4a (potential of the developing voltage), and the occurrence degree of "fog". Note that Vback is expressed as a positive value when the dark area potential of the photosensitive drum 1 is greater on the same polarity side as the normal charging polarity of the toner than the potential of the developing roller 4a. The measurement of "fog" on the photosensitive drum 1 was performed as follows. Toner was collected by attaching the adhesive surface of a transparent adhesive tape to the photosensitive drum 1. In addition, the adhesive tape was attached to a specified paper, and the density (fog density (%)) of the adhesive tape with the toner attached thereto was measured to quantify the "fog". When "fog" does not occur, the fog density is 0%, and the higher the value of the fog density, the greater the occurrence degree of "fog" and the more toner is attached to the surface of the photosensitive drum 1. There are the following types of "fog". 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 "background fogging" in which toner charged with the normal charging polarity adheres to the surface of the photosensitive drum 1. In addition, when the potential difference between the dark area potential of the photosensitive drum 1 and the developing roller 4a becomes large, there is a "reverse fogging" in which "reverse toner" charged with the opposite polarity to the normal charging polarity adheres to the surface of the photosensitive drum 1.

[0078] As described above, state 1-6 in FIG. 3 shows a state in which the potential difference between the non-transfer area E and the transfer area F is large. 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 larger than 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, VbackE1. When Vback is large like this, "reverse fog" may occur, in which reverse toner adheres to the surface of the photosensitive drum 1. As shown in FIG. 7, in the configuration of this embodiment, when Vback is around 120V, the degree of fog occurrence is the smallest, and the fog density is 2%. This level of fog is difficult to visually recognize on the recording material P, and does not pose a problem. On the other hand, when Vback is larger than 220V, the degree of fog (reverse fog) occurrence increases, and if the fog density continues to exceed 10%, cleaning failure may occur.

[0079] 3, 4, and 5, the surface potential of the photosensitive drum 1 in the fog area H and the occurrence degree of fog (reverse fog) in this embodiment and Comparative Example 2 will be described. In this embodiment, the same exposure operation as in Example 1 described with reference to FIG. 4 was performed, and in Comparative Example 2, the exposure operation described with reference to FIG. 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 above-mentioned differences. FIG. 5 shows the transition of the surface potential of the photosensitive drum 1 in the non-transfer area E after exposure and before transfer when continuous image formation is performed on 20 sheets of LTR size paper as the recording material P. In FIG. 5, the solid line shows the transition of the surface potential when the exposure operation of this embodiment is performed, and the dashed line shows the transition of the surface potential when the exposure operation of Comparative Example 2 is performed. The transition of the surface potential of the photosensitive drum 1 in the non-transfer area E in Example 2 and Comparative Example 2 is the same as the transition of the surface potential of the photosensitive drum 1 in the non-transfer area E in Example 1 and Comparative Example 1 described above, respectively. As described above, the fog area H corresponds to a part of the non-transfer area E.

[0080] As shown in FIG. 3 (state 1-6) and FIG. 5, in Comparative Example 2, the surface potential of the photosensitive drum 1 in the fog area H is −700V. Therefore, as shown in FIG. 3 (state 1-6), VbackE1 in the non-transfer area E including the fog area H is 320V. As shown in FIG. 7, the fog density at Vback of 320V exceeds 20%, so cleaning failure due to fog (reverse fog) may occur. On the other hand, as shown in FIG. 4 (state 2-6) and FIG. 5, in this embodiment, the surface potential of the photosensitive drum 1 in the fog area H is −490V. Also, as shown in FIG. 4 (state 2-6), VbackE2 in the non-transfer area E including the fog area H is 110V. As shown in FIG. 7, the fog density at Vback of 110V is about 3%, which is a level that does not cause a problem with cleaning failure.

[0081] In this manner, in a configuration in which the transfer area C is shorter than the development area G in the longitudinal direction, by performing an exposure operation to expose the non-transfer area E similar to that in the first embodiment, the occurrence of cleaning defects can be suppressed.

[0082] In order to further suppress the occurrence of cleaning failure, it is preferable to make the fog density of the fog occurring in the non-transfer area F and the fog area H as uniform as possible. If there is a density step in the fog density, a small torque difference in the longitudinal direction of the cleaning blade 6a occurs at that location, and cleaning failure is likely to occur. In state 2-6 of FIG. 4, VbackE2 in the non-transfer area E including the fog area H is 110V, but VbackF2 in the non-transfer area F is 120V. From FIG. 7, the fog density when Vback is 110V is about 3%, but the fog density when Vback is 120V is about 2%. Thus, in state 2-6 of FIG. 4, there is a slight difference in the fog density of the fog occurring in the non-transfer area E and the non-transfer area F. From the viewpoint of suppressing the occurrence of cleaning failure, it is more preferable to perform exposure not only in the non-transfer area E but also in the non-transfer area F, and make the surface potential of the photosensitive drum 1 after exposure uniform in the non-transfer area E and the non-transfer area F. Here, making 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 non-paper passing transfer region F uniform (approximately the same) means making them sufficiently the same so as to sufficiently suppress density differences in the fog density from the viewpoint of suppressing the occurrence of cleaning defects. Although not limited thereto, typically, the difference in surface potential is set to 5 V or less, preferably 3 V or less, and more preferably 1 V or less (may be 0 V).

[0083] Thus, in this embodiment, the image forming apparatus 100 has a width of the transfer portion Nt shorter than a width of the charging portion B in the rotation axis direction of the charging member 2, has a non-transfer area E at an end of the surface of the photoconductor 1 in the rotation axis direction 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 coated area of ​​the developing member 4a overlaps with the non-transfer area E in the rotation axis direction. In this embodiment, the control unit 40 can execute an exposure operation for exposing at least the non-transfer area E of the photoconductor 1 or at least the non-transfer area E and the non-paper passing transfer area F of the photoconductor 1 by the exposure device 3 while the photoconductor 1 is rotating.

[0084] As described above, in a configuration in which the transfer area C is shorter than the development area G in the longitudinal direction, the occurrence of cleaning failures can be suppressed by performing an exposure operation to expose the non-transfer area E and even the outside transfer area F.

[0085] [Example 3] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

[0086] As described in the first embodiment, when the non-transfer region E is exposed, if the non-transfer region F is narrow and the exposure amount for the non-transfer region E is strong, toner may adhere to the longitudinal end of the photosensitive drum 1. Alternatively, as described in the second embodiment, when the non-transfer region E and the non-transfer region F are exposed, if the exposure amount for the non-transfer region F is strong, toner may adhere to the longitudinal end of the photosensitive drum 1 as described above. In such a case, if the conveying position of the recording material P in the direction substantially perpendicular to the conveying direction of the recording material P is shifted, toner may be transferred from the photosensitive drum 1 to the end of the recording material P in the direction substantially perpendicular to the conveying direction of the recording material P, and the end of the recording material P may be stained, causing "end staining." On the other hand, in order to perform weak exposure to an extent that toner does not adhere to the non-transfer region E or the non-transfer region F, it may be necessary to add a dedicated weak exposure light source or an electric board for adjusting the amount of exposure, which may lead to an increase in the size of the device. Therefore, in this embodiment, an exposure operation is described that makes it possible to expand the options for the exposure amount for exposing the non-transfer area E and even the outside transfer area F while reducing the risk of contamination occurring at the edge of the recording material P.

[0087] The image forming apparatus 100 of this embodiment is not equipped with the weak exposure light source used in the image forming apparatus 100 of the first and second embodiments, and therefore the size of the apparatus can be reduced accordingly. Note that 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, similar to the second embodiment.

[0088] In this embodiment, during non-image formation during continuous image formation in which images are formed on a plurality of sheets of recording material P with the driving of the photosensitive drum 1 not stopped, an exposure operation is performed to expose the non-transfer area E similar to that of the first or second embodiment, and further the outside transfer area F. In particular, in this embodiment, an exposure operation is performed to expose the non-transfer area E during non-image formation during continuous image formation.

[0089] In other words, by not performing an exposure operation to expose the non-transfer area E or the transfer area F outside the paper passing during image formation, the risk of toner adhering to the non-transfer area E or the transfer area F outside the paper passing and this toner being transferred to the recording material P and causing edge stains can be reduced.

[0090] In addition, the reduced risk of contamination of the edge of the recording material P expands the options for the amount of exposure to expose the non-transfer area E and the outside transfer area F. For example, if the purpose is to suppress an increase in the surface potential of the photosensitive drum 1, it becomes possible to use a strong exposure amount for the non-transfer area E and the outside transfer area F that is approximately the same as the exposure amount for the image area.

[0091] 8(a) and (b), a description will be given of the transition of the surface potential of the photosensitive drum 1 in the non-transfer area E in this embodiment. Note that the leading edge and trailing edge of the recording material P refer to the leading edge and trailing edge of the recording material P in the conveying direction, even if not otherwise specified.

[0092] 8(a) shows the transition of the surface potential of the photosensitive drum 1 in the non-transfer area E after exposure and before transfer when continuous image formation is performed on 20 sheets of LTR size paper as the recording material P with a paper-to-paper distance (the distance in the rotation direction of the photosensitive drum 1 between the position on the photosensitive drum 1 corresponding to the trailing edge of the preceding recording material P and the position on the photosensitive drum 1 corresponding to the leading edge of the succeeding recording material P) of 45 mm. In FIG. 8(a), the solid line shows the transition of the surface potential when the exposure operation of this embodiment is performed, and the dashed line shows the transition of the surface potential when the exposure operation of Comparative Example 3 is performed. In this embodiment, only during a part of the pre-rotation process when no image is formed, and during the paper-to-paper process when no image is formed, the exposure device 3 applies an exposure amount of 0.3 μJ / cm, which is the same as the exposure amount for the image area. 2 In Comparative Example 3, the exposure operation described with reference to FIG. 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 above-mentioned differences. The transition of the surface potential in Comparative Example 3 is the same as that shown in FIG. 5.

[0093] Fig. 8(b) plots only the transition of the surface potential when the time domain of the horizontal axis of Fig. 8(a) is changed and the exposure operation of this embodiment is performed. The details of the exposure operation of this embodiment will be described with reference to Fig. 8(b).

[0094] First, the surface potential of the photosensitive drum 1 immediately after the start of driving (printing operation, pre-rotation process) of the photosensitive drum 1 is maintained at a dark potential Vd = -500V by the charging roller 2. The surface potential of the photosensitive drum 1 in the non-transfer area E gradually increases due to the rotation drive in the pre-rotation process before the paper is passed. Then, just before the first recording material P is passed, an exposure operation is performed in which the non-transfer area E is exposed by the exposure device 3 over a distance of 45 mm in the rotation direction of the photosensitive drum 1, which is the same as the paper distance. In this embodiment, the non-transfer area E is exposed with the same exposure amount as the exposure amount for the image area, so that the surface potential of the photosensitive drum 1 drops to a light potential Vl = -100V in the first exposure operation. After the first exposure operation is completed, the first recording material P is passed.

[0095] Here, in the configuration of this embodiment, the distance in the rotation direction of the photosensitive drum 1 exposed by one exposure operation is shorter than the circumferential length of the photosensitive drum 1 (approximately 75.4 mm), so one exposure operation cannot expose the entire circumference of the photosensitive drum 1. In other words, during paper passing without exposure of the non-transfer area E, the area previously exposed and the area not exposed in the non-transfer area E exist in the circumferential direction of the photosensitive drum 1. Therefore, in the surface potential of the photosensitive drum 1 in the non-transfer area E during the paper passing of the first recording material P, the surface potential of the area exposed during the immediately previous non-image formation is recharged by the charging roller 2 and returns to the dark potential Vd = -500 V, and further begins to rise again due to injection charging. On the other hand, the surface potential of the area not exposed during the immediately previous non-image formation continues to rise following the same transition as the transition of the surface potential of the photosensitive drum 1 at the start of the pre-rotation process (before exposure of the non-transfer area E). Therefore, the surface potential of the photosensitive drum 1 in the non-transfer area E after the first exposure operation fluctuates in a range of about −500 to −560 V in one rotation period of the photosensitive drum 1.

[0096] In the inter-paper process after the first recording material P is passed, an exposure operation similar to the exposure operation in the pre-rotation process is performed (second exposure operation). That is, in the inter-paper process after the first recording material P is passed, an exposure operation is performed in which the exposure device 3 exposes 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 recording material P is passed 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 revolution of the photosensitive drum 1. Therefore, the surface potential of the photosensitive drum 1 in the non-transfer area E rises and falls with the rotation period of the photosensitive drum 1, and the vertical range is about -500 to -580 V, which is wider than when the first recording material P was passed. In the inter-paper process after the second recording material P is passed, an exposure operation is also performed similarly (third exposure operation). When the third sheet of recording material P passes after the third exposure, there is a high possibility that there will be no area in the non-transfer area E that has not been exposed in any of the first to third exposure operations within one revolution of the photosensitive drum 1. In this embodiment, when the third sheet of recording material P passes after the third exposure, there is no area in the non-transfer area E that has not been exposed in any of the first to third exposure operations within one revolution of the photosensitive drum 1. Therefore, the vertical range of the surface potential of the photosensitive drum 1 in the non-transfer area E during the rotation period of the photosensitive drum 1 narrows, and the vertical range drops to approximately -500 to -540V.

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

[0098] On the other hand, in this embodiment, the non-transfer area E is exposed to 0.3 μJ / cm 2 , which is the same exposure amount as the image area. 2Since the photosensitive drum 1 is exposed to light at 100 V, the surface potential of the photosensitive drum 1 in the non-transfer area E after exposure is a light area potential Vl = -100 V as described above. Therefore, toner adheres to the non-transfer area E after exposure. However, unlike the situation in which the fog toner is always transported to the cleaning blade 6a as described in the second embodiment, the toner adhering to the non-transfer area E is transported to the cleaning blade 6a only during a part of the pre-rotation process and the inter-paper process. Therefore, the occurrence of cleaning defects as described in the second embodiment can be suppressed.

[0099] Next, an optimal 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 step as described above will be described. Fig. 9 is a diagram showing at what position on the circumferential length of the photosensitive drum 1 the inter-paper position (the section in the rotation direction of the photosensitive drum 1 between the position on the photosensitive drum 1 corresponding to the rear end of the preceding recording material P and the position on the photosensitive drum 1 corresponding to the front end of the succeeding recording material P) is located when the circumferential position of the photosensitive drum 1 is expressed in a linear form. Note that in this embodiment, the exposure operation is always performed throughout the entire period of the inter-paper step to suppress an increase in the surface potential of the photosensitive drum 1, but the exposure operation may be performed only during a part of the period of the inter-paper step.

[0100] FIG. 9(a) shows the paper interval position relative to the circumferential length of the photosensitive drum 1 when LTR size paper as the recording material P is passed at a paper interval distance of 45 mm in this embodiment as in the case of FIG. 8 (that is, the position where the exposure operation for suppressing the rise of the surface potential of the photosensitive drum 1 is performed). It can be seen that one revolution of the circumferential length of the photosensitive drum 1 can be exposed in the first to third paper interval processes. This coincides with the fact that the surface potential of the photosensitive drum 1 begins to stabilize when the exposure operation in the paper interval process exceeds three times (the first exposure operation is an exposure operation in a section corresponding to a part of the paper interval distance in the pre-rotation process) in FIG. 8. On the other hand, FIG. 9(b) shows the paper interval position relative to the circumferential length of the photosensitive drum 1 in Comparative Example 4. In Comparative Example 4, LTR size paper as the recording material P is passed at a paper interval 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 above points. In the case of Comparative Example 4, the deviation amount of the paper-to-paper position for each sheet (paper-to-paper position deviation amount) is small, and the paper-to-paper position is always at approximately the same position on the photosensitive drum 1. In such a case, it becomes difficult to suppress the rise in the surface potential of the photosensitive drum 1 over the entire circumference of the photosensitive drum 1 by only the exposure operation in the paper-to-paper step. This paper-to-paper position deviation amount for each sheet is expressed by the following formula (1). Amount of paper-to-paper position deviation for each sheet=(length of recording material in conveying direction+distance between sheets)-n×periphery of photosensitive drum (1)

[0101] Here, n is any integer (a positive integer equal to or greater than 1), and the value that minimizes the absolute value of formula (1) is selected. The smaller the amount of sheet-to-sheet positional deviation for each sheet, the more likely it is that it will be difficult to suppress the rise in the surface potential of the photosensitive drum 1 in the exposure operation in the sheet-to-sheet process, as in Comparative Example 4 in FIG. 9(b). In other words, it is desirable to set the length of the sheet-to-sheet distance so that the sum of the length of the recording material P in the conveying direction of the recording material P and the sheet-to-sheet distance is not approximately an integer multiple of the circumferential length of the photosensitive drum 1. For example, to avoid the rising transition of the surface potential as in Comparative Example 3 in FIG. 8, it is desirable to satisfy the relationship of the following formula (2). |Circumference of photosensitive drum ÷ misalignment amount between sheets|≦Number of pages tolerable for potential rise (2)

[0102] The absolute value of "periphery of photosensitive drum ÷ sheet-to-sheet positional deviation amount per sheet" in the above formula (2) represents the number of exposure operations required to expose the entire circumference of the photosensitive drum 1 in the exposure operation in the sheet-to-sheet process. In addition, the "number of pages for which potential rise is permitted" in the above formula (2) is the upper limit of the number of sheets (number of sheets passed, number of pages) for which continuous image formation is possible while sufficiently suppressing the defects due to the rise in surface potential described in the first and second embodiments in a situation where there is no means for canceling the rise in surface potential of the non-transfer region E on the photosensitive drum 1 as in Comparative Example 3 of FIG. 8. For example, if the surface potential reaches -700V after passing 20 sheets as in Comparative Example 3 of FIG. 8, there is a risk of poor cleaning due to the "fogging" described in the second embodiment. Therefore, in order to suppress the rise in surface potential of the photosensitive drum 1 before such a situation occurs, the number of pages for which potential rise is permitted can be set to 19 sheets (less than 20 sheets). Here, although not limited thereto, the number of exposure operations required to expose the entire circumference of the photosensitive drum 1 in the exposure operation in the paper interval process (the left side of the above formula (2)) is preferably about 1 to 10 times, and more preferably about 1 to 5 times (3 times in this embodiment).

[0103] In this way, when performing the exposure operation in the sheet interval process, it is desirable to set the sum of the length of the recording material P in the conveying direction of the recording material P and the sheet interval distance to be other than an approximate integer multiple of the circumferential length of the photosensitive drum 1. This makes it possible to suppress an increase in the surface potential of the photosensitive drum 1 over the entire circumferential length of the photosensitive drum 1.

[0104] In addition, the exposure operation in the above-mentioned paper interval process can be performed in a section corresponding to a part of the paper interval distance in the pre-rotation process as shown in Fig. 8(b) in addition to the paper interval process. Also, as described above, the exposure operation may be performed only in a part of the paper interval distance, in which case the above-mentioned formulas (1) and (2) can be applied by replacing the above-mentioned paper interval position with the section of the paper interval position where the exposure operation is performed.

[0105] In this embodiment, the non-transfer area E is exposed in the exposure operation in the sheet-to-sheet step, but as described above, the non-transfer area F may also be exposed. From the viewpoint of toner consumption, it is preferable to expose only the non-transfer area E, or the non-transfer area E and the non-transfer area F, as described above, but if desired, substantially the entire area in the longitudinal direction of the photosensitive drum 1 (substantially the entire area of ​​the photosensitive area A or the charging area B) may be exposed.

[0106] Thus, in this embodiment, the control unit 40 controls the exposure device 3 to perform the above-mentioned exposure operation when the non-image forming area on the surface of the photoconductor 1 in the rotation direction of the photoconductor 1 passes through the exposure section Pb where the surface of the photoconductor 1 is exposed. Also, in this embodiment, the control unit 40 controls the exposure device 3 to expose the surface of the photoconductor 1 inside the paper passing area D in the rotation axis direction of the charging member 2 with a first exposure amount to form an electrostatic image on the surface of the photoconductor 1 when the image forming area on the surface of the photoconductor 1 in the rotation direction of the photoconductor 1 passes through the exposure section Pb, and controls the exposure device 3 to perform the above-mentioned exposure operation with an exposure amount substantially equal to the first exposure amount when the non-image forming area passes through the exposure section Pb. In this embodiment, the non-image forming area is a section between a position corresponding to the rear end of the preceding recording material P on the surface of the photoconductor 1 and a position corresponding to the front end of the succeeding recording material P during continuous image formation in which a toner image is transferred to multiple recording materials P. In this embodiment, the length of the above section is set so that the sum of the length of the recording material P in the conveying direction of the recording material P and the length of the above section in the rotation direction of the photoconductor 1 is not approximately an integer multiple of the circumferential length of the photoconductor 1. Note that it is not limited to performing the above-mentioned exposure operation in all inter-sheet processes during continuous image formation. It is sufficient to sufficiently suppress the increase in the surface potential of the photoconductor drum 1 in the non-transfer area E. For example, the above-mentioned exposure operation may be performed in the inter-sheet process for each of a plurality of recording materials P, or the exposure operation in the inter-sheet process may be started after forming images on a predetermined number of recording materials P.

[0107] As described above, according to this embodiment, it is possible to expand the options for the exposure amount for exposing the non-transfer area E and even the outside transfer area F while reducing the risk of contamination of the edge of the recording material P.

[0108] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-mentioned embodiments.

[0109] In the above embodiment, the transfer member is a transfer roller, but the transfer member is not limited to the transfer roller. The transfer member may be configured to have, for example, a rotatable endless belt that contacts the photosensitive member. On the inner peripheral surface side of the transfer belt, a voltage application member (roller, brush, sheet, etc.) that supplies a transfer voltage to the transfer section via the transfer belt may be disposed at a position facing the photosensitive member. In addition, the transfer member is not limited to a rotating body, and may be in other forms such as a pad-shaped member, a sheet-shaped (film-shaped) member, or a fixed brush-shaped member.

[0110] In the above embodiment, the photoconductor is a photoconductor drum, but the photoconductor is not limited to a photoconductor drum. The photoconductor may be a photoconductor belt having an endless belt shape.

[0111] In the above-mentioned embodiment, the image forming apparatus was not provided with a pre-exposure means. As mentioned above, the phenomenon that the surface potential of the non-transfer area at the end of the longitudinal direction of the photoconductor rises to an excessive potential tends to be prominent when the image forming apparatus adopts a pre-exposure-less method. Therefore, it can be said that the present invention is particularly effective when the image forming apparatus adopts a pre-exposure-less method. However, the present invention is not limited to such a configuration. It is also possible to apply the present invention 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-mentioned embodiment can be obtained, and the effect of reducing the amount of exposure of the pre-exposure means and reducing the amount of discharge due to the charging process can be obtained. Similarly, it can be said that the present invention is particularly effective when a DC charging method is adopted, but it can also be applied when an AC / DC charging method is adopted.

[0112] Furthermore, the present invention is not limited to application to a configuration in which the transfer area is shorter than the development area in the longitudinal direction. The present invention can be applied even to a configuration in which the length of the transfer area in the longitudinal direction is equal to or longer than the length of the development area, and the above-mentioned effects such as suppression of damage to the photoconductor can be obtained. [Explanation of symbols]

[0113] 1 Photosensitive drum 2. Charge roller 3. Exposure equipment 4. Developing device 5 Transfer roller 6 Cleaning device 40 Control section

Claims

1. 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; A control unit capable of controlling the exposure device; having; In an image forming apparatus having, in a direction of the rotation axis of the charging member, a width of the transfer portion shorter than a width of the charging portion and having a non-transfer region that contacts the charging member and does not contact the transfer member at an end portion of the surface of the photoreceptor in the direction of the rotation axis, in the direction of the rotation axis, when a region where the recording material contacts the transfer portion on the surface of the photoreceptor is defined as a paper passage region and a region outside the paper passage region and inside the transfer portion is defined as a non-paper passage transfer region, the control unit is capable of executing an exposure operation of exposing at least the non-transfer region of the photoreceptor by the exposure device when the photoreceptor is rotating, and controls the exposure device so as to form a surface potential on the surface of the photoreceptor downstream of an exposure portion where the surface of the photoreceptor is exposed in the rotation direction of the photoreceptor and upstream of the transfer portion, and controls the exposure device such that an absolute value of the surface potential formed in the non-transfer region is smaller than an absolute value of the surface potential formed in the non-paper passage transfer region formed downstream of the exposure portion and upstream of the transfer portion in the rotation direction of the photoreceptor. An image forming apparatus characterized by the above.

2. 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; A control unit capable of controlling the exposure device; having; In an image forming apparatus having a non-transfer region that contacts the charging member and does not contact the transfer member at an end portion in the rotational axis direction of the photosensitive member, in the rotational axis direction of the charging member, the width of the transfer portion is shorter than the width of the charging portion. In the rotational axis direction, when a region where the recording material contacts the transfer portion on the surface of the photosensitive member is defined as a paper passage region, and a region outside the paper passage region and inside the transfer portion is defined as a non-paper passage transfer region, the control unit can execute an exposure operation of exposing at least the non-transfer region of the photosensitive member by the exposure device when the photosensitive member is rotating. In the exposure operation, the exposure device is controlled so that the exposure amount for the non-transfer region is larger than the exposure amount for the non-paper passage transfer region. An image forming apparatus characterized by this is provided. **Claim 3** A rotatable photosensitive member; A rotatable charging member that contacts the photosensitive member to form a charging portion and charges the surface of the photosensitive member in the charging portion; An exposure device that exposes the surface of the photosensitive member charged by the charging member to form an electrostatic image on the surface of the photosensitive member; A developing member that supplies toner to the electrostatic image formed on the surface of the photosensitive member to form a toner image; A transfer member that contacts the surface of the photosensitive member to form a transfer portion, and transfers the toner image from the surface of the photosensitive member to the recording material in the transfer portion when a voltage is applied; A control unit capable of controlling the exposure device; The image forming apparatus having these components; In the rotational axis direction of the charging member, the width of the transfer portion is shorter than the width of the charging portion. The photosensitive member has a non-transfer region that contacts the charging member and does not contact the transfer member at an end portion in the rotational axis direction. In the rotational axis direction, at least a part of the toner coating region of the developing member overlaps the non-transfer region. In such an image forming apparatus, In the rotational axis direction, when a region where the recording material contacts the transfer portion on the surface of the photosensitive member is defined as a paper passage region, and a region outside the paper passage region and inside the transfer portion is defined as a non-paper passage transfer region, the control unit can execute an exposure operation of exposing at least the non-transfer region of the photosensitive member or at least the non-transfer region of the photosensitive member and the non-paper passage transfer region by the exposure device when the photosensitive member is rotating. An image forming apparatus characterized by this is provided. **Claim 4** The image forming apparatus according to any one of claims 1 to 3, wherein the control unit controls the exposure device so as to expose at least the non-transfer region and the non-paper-transfer region of the photosensitive member in the exposure operation.

5. The image forming apparatus according to any one of claims 1 to 3, wherein the control unit controls the exposure device so as to execute the exposure operation when the image forming region on the surface of the photosensitive member in the rotation direction of the photosensitive member passes through the exposure unit where the surface of the photosensitive member is exposed.

6. The image forming apparatus according to claim 5, wherein when the image forming region passes through the exposure unit, the control unit exposes the surface of the photosensitive member inside the paper passage region in the rotation axis direction by the exposure device with a first exposure amount to form the electrostatic image on the surface of the photosensitive member, and when the image forming region passes through the exposure unit, the control unit controls the exposure device so as to execute the exposure operation with a second exposure amount smaller than the first exposure amount by the exposure device.

7. The image forming apparatus according to any one of claims 1 to 3, wherein the control unit controls the exposure device so as to execute the exposure operation when the non-image forming region on the surface of the photosensitive member in the rotation direction of the photosensitive member passes through the exposure unit where the surface of the photosensitive member is exposed.

8. The image forming apparatus according to claim 7, wherein when the image forming region on the surface of the photosensitive member in the rotation direction of the photosensitive member passes through the exposure unit, the control unit exposes the surface of the photosensitive member inside the paper passage region in the rotation axis direction by the exposure device with a first exposure amount to form the electrostatic image on the surface of the photosensitive member, and when the non-image forming region passes through the exposure unit, the control unit controls the exposure device so as to execute the exposure operation with an exposure amount substantially the same as the first exposure amount by the exposure device.

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

10. The length of the recording material in the conveyance direction of the recording material and the length of the section in the rotation direction of the photoreceptor are set so that the sum thereof is not substantially an integral multiple of the circumference of the photoreceptor. The image forming apparatus according to claim 9, characterized in that the length of the section is set as such.

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