Image forming device

Abstract

To reduce the amount of exposed light with which a photoreceptor is irradiated, while maintaining image quality.SOLUTION: An image forming apparatus comprises: rotating photoconductor drums 4; electrifying rollers 5 that electrify the photoconductor drums 4; a laser exposure unit 9 that irradiates the photoconductor drums 4 electrified by the electrifying rollers 5 with emitted light to expose the photoconductor drums to the light, and thereby forms latent images; and developing rollers 6 that develop the latent images with toner. In a non-printing area Y1 that is a non-printing area in an area where a printing area and the non-printing area are provided in a main scanning direction of the photoconductor drum 4, the electrifying roller 5 performs electrification at a predetermined electrification voltage (post-electrification voltage), and the laser exposure unit 9 emits light at a second light emission amount. In a non-printing area Y2 that is an area where only the non-printing area is provided in the main scanning direction of the photoconductor drum 4, the electrifying roller 5 performs electrification at a voltage lower than the predetermined electrification voltage (post-exposure voltage), and the laser exposure unit 9 does not perform the light exposure.SELECTED DRAWING: Figure 5

Description

[Technical Field]

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

[0002] Conventionally, electrophotographic image forming apparatuses have used contact charging devices, which have advantages such as low ozone and low power consumption, as charging means for uniformly charging a photoconductor (image carrier) to a predetermined polarity and potential. Contact charging devices charge the photoconductor by applying a voltage to a charging member in contact with the photoconductor. In particular, roller charging contact charging devices that use a charging roller (conductive roller) as the charging member are widely used due to their charging stability. Methods for applying a charging voltage to the charging roller include AC charging methods, which apply a voltage obtained by superimposing a DC voltage (direct current voltage) and an AC voltage (alternating current voltage), and DC charging methods, which apply only a DC voltage. In recent years, DC charging methods have become popular due to their low cost and space-saving features.

[0003] However, with DC charging, it is difficult to equalize the potential unevenness of the photoconductor after transfer, and the potential unevenness can manifest as image defects (also known as transfer memory). When a toner image is transferred from the photoconductor to a recording medium (recording material or secondary transfer body), the amount of transfer current flowing to the photoconductor differs between toner-bearing and non-toner-bearing areas on the photoconductor surface, resulting in potential unevenness on the photoconductor after transfer. This potential unevenness on the photoconductor then prevents the potential on the photoconductor from being sufficiently uniform in the charging process, the next step in the image formation cycle, and the potential unevenness manifests itself as image defects on the image. Therefore, conventionally, to equalize the potential on the photoconductor surface after transfer, optical neutralization (full-surface eraser exposure) has been performed using a potential unevenness elimination device. However, providing a potential unevenness elimination device increases the size and cost of the device.

[0004] Therefore, Patent Document 1, for example, proposes so-called background exposure as a method for suppressing transfer memory without providing a separate potential unevenness removal device. In background exposure, an exposure device exposes the print area where a toner image is formed on a photoreceptor charged to a predetermined potential during a charging process, while also exposing the non-print area where no toner image is formed with a weak amount of light. However, if the photoreceptor is exposed to light exceeding a certain amount, a phenomenon known as light fatigue occurs, in which the charge potential decreases. Therefore, in a method of constantly exposing the photoreceptor surface as in Patent Document 1, it is necessary to consider the decrease in sensitivity of the photoreceptor due to light fatigue. Therefore, Patent Document 2, for example, proposes exposure control that does not perform background exposure or reduces laser output in non-image-forming areas other than the image-forming area where an image to be transferred to a recording material is formed. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-8991 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-123017 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in recent years, there has been a demand for longer product life and higher image quality, and in order to achieve even longer product life, it has become necessary to minimize light fatigue of the photoreceptor and to prevent a decrease in sensitivity of the photoreceptor.

[0007] The present invention has been made under these circumstances, and an object of the present invention is to reduce the amount of exposure light irradiated onto a photosensitive member while maintaining image quality. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems, the present invention has the following configuration.

[0009] (1) An image forming apparatus comprising: a rotating photosensitive member; a charging means for charging the surface of the photosensitive member; an exposure means for irradiating the surface of the photosensitive member charged by the charging means with light to expose it and form a latent image; and a developing means for developing the latent image with toner, wherein the exposure means emits light at a first light emission amount in a print area where the latent image is developed with toner among areas of the photosensitive member corresponding to a recording material on which an image is formed, and emits light at a second light emission amount smaller than the first light emission amount in a non-print area where the latent image is not formed, thereby forming a toner image on the recording material; wherein the charging means charges the first non-print area, which is a non-print area in a main scanning direction that is the rotation axis of the photosensitive member, with a predetermined charging voltage, and the exposure means emits light at the second light emission amount; and wherein the charging means charges the second non-print area, which is a region on the photosensitive member where only the non-print area is provided in the main scanning direction, with a voltage lower than the predetermined charging voltage, and the exposure means does not perform exposure. First, the charging means charges the second non-printing area, which is the most downstream in the sub-scanning direction, which is the rotation direction of the photosensitive member, with a voltage lower than the predetermined charging voltage. An image forming apparatus characterized by: (2) An image forming apparatus comprising: a rotating photosensitive member; a charging means for charging the surface of the photosensitive member; an exposure means for irradiating the surface of the photosensitive member charged by the charging means with light to expose it and form a latent image; and a developing means for developing the latent image with toner, wherein the exposure means emits light at a first light emission amount in a print area where the latent image is developed with toner among areas of the photosensitive member corresponding to a recording material on which an image is formed, and emits light at a second light emission amount smaller than the first light emission amount in a non-print area where the latent image is not formed, thereby forming a toner image on the recording material, and wherein the exposure means emits light at a first light emission amount in a non-print area where the latent image is not formed, and forms a toner image on the recording material, and the photosensitive member has a rotation axis in a main scanning direction, i.e., a rotation axis of the photosensitive member, and a front side of the print area. the charging means charges a first non-printing area, which is a non-printing area in an area where the non-printing area and the non-printing area are provided, with a predetermined charging voltage, and the exposure means emits light at the second light emission amount; the charging means charges a second non-printing area, which is a area where only the non-printing area is provided in the main scanning direction of the photosensitive body, with the predetermined charging voltage, and the exposure means emits light at a third light emission amount that is smaller than the second light emission amount; and the exposure means emits light at the third light emission amount in the second non-printing area, which is the second non-printing area that is the most downstream in the sub-scanning direction, which is the rotation direction of the photosensitive body, within the second non-printing area. [Effects of the Invention]

[0011] According to the present invention, it is possible to reduce the amount of exposure light irradiated onto the photosensitive member while maintaining image quality. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus according to first and second embodiments. [Figure 2] Schematic diagram illustrating an image forming area and a non-image forming area on the surface of a photosensitive drum in Examples 1 and 2. [Figure 3] 1 is a schematic diagram illustrating a print area and a non-print area on the surface of a photosensitive drum in Example 1, and a diagram showing the potential of the surface of the photosensitive drum when exposure control is performed; [Figure 4] Graph showing the relationship between back contrast and fogging toner density in Examples 1 and 2 [Figure 5] FIG. 10 is a diagram showing the potential of the photosensitive drum surface when the exposure control and the charge control of the first embodiment are performed. [Figure 6] Schematic diagram showing image data used in the experiment of Example 1 [Figure 7] FIG. 10 is a diagram showing the potential of the photosensitive drum surface when switching to a non-printing area in Example 1. [Figure 8] FIG. 10 is a diagram showing the potential of the photosensitive drum surface when the charge control switching timing when switching to the non-printing portion in Example 2 is changed. [Figure 9] 1 is a schematic diagram showing image data used in an experiment of Example 2, and a diagram showing the potential of the photosensitive drum surface. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. [Example]

[0014] [Configuration of image forming device] FIG. 1 is a cross-sectional view showing the configuration of an image forming apparatus P used in Example 1. The image forming apparatus P is an intermediate transfer color laser beam printer having four image forming stations Y, M, C, and K. The four image forming stations Y, M, C, and K are image forming units that form toner images using yellow, magenta, cyan, and black toner, respectively. As shown in FIG. 1, the image forming stations Y, M, C, and K are arranged in parallel at regular intervals from left to right inside the image forming apparatus P. The main part of each image forming station is configured as a process cartridge 3 (3Y, 3M, 3C, 3K) that can be removed from a predetermined mounting portion of the main body of the image forming apparatus P.

[0015] Each process cartridge 3 (3Y, 3M, 3C, 3K) has the same configuration, although the color of the developer (toner) stored therein is different. Note that the Y, M, C, and K suffixes to the reference numerals in FIG. 1 indicate that the components are components of the image forming stations Y, M, C, and K. Hereinafter, the Y, M, C, and K suffixes to the reference numerals will be omitted except when indicating that the components are components of a specific image forming station.

[0016] In this embodiment, each process cartridge 3 has a rotating drum-type photosensitive drum 4 as an image carrier. Each process cartridge 3 also has a charging roller 5 that charges the photosensitive drum 4 to a predetermined potential, a developing roller 6 that is a developing member that develops the electrostatic latent image on the photosensitive drum 4 (photoconductor) with toner, and a cleaning unit 7 that removes toner from the photosensitive drum 4. The charging roller 5 is disposed opposite the photosensitive drum 4 and contacts the photosensitive drum 4 to charge the surface of the photosensitive drum 4 to a uniform potential. The developing device visualizes the electrostatic latent image formed on the photosensitive drum 4 with developer (toner), and includes a developer container 8 that contains the developer and a developing roller 6 disposed opposite the photosensitive drum 4. The developing device in this embodiment is a one-component contact development type developing device that uses non-magnetic one-component toner (negative charging characteristic) as the developer. The developer container 8Y of the process cartridge 3Y contains yellow (Y) toner as the developer, and the developer container 8M of the process cartridge 3M contains magenta (M) toner as the developer. Similarly, the developer container 8C of the process cartridge 3C contains cyan (C) toner as the developer, and the developer container 8K of the process cartridge 3K contains black (K) toner as the developer. The cleaning unit 7 is a cleaner that cleans toner that remains on the surface of the photosensitive drum 4 without being transferred after the toner image formed on the photosensitive drum 4 is transferred to the intermediate transfer belt 10d (described later), and in this embodiment, a blade is used as the cleaning member.

[0017] The photosensitive drum 4 is driven to rotate in the direction of the arrow in the figure (counterclockwise) at a surface movement speed (process speed) of 150 mm / sec by a drive unit (not shown) provided in the main body of the image forming apparatus P. The photosensitive drum 4 has a base material, an aluminum cylinder with an outer diameter of 20 mm, coated with a thin film of a charge generation layer and a charge transport layer, and the aluminum cylinder is grounded.

[0018] The charging roller 5, which serves as a charging means, has a core and a conductive elastic layer formed concentrically around the core, with both ends of the core rotatably supported by bearings. The charging roller 5 is disposed parallel to the photosensitive drum 4, and the conductive elastic layer abuts against the photosensitive drum 4 with a predetermined pressing force, rotating in response to the rotation of the photosensitive drum 4. In this embodiment, a DC voltage of approximately -1000 V is applied as a charging voltage to the core of the charging roller 5 from a power supply (not shown) provided in the main body of the image forming apparatus P.

[0019] The developing roller 6, which serves as a developing means, has a core and a conductive elastic layer formed concentrically around the core. The developing roller 6 is disposed parallel to the photosensitive drum 4 and is driven to rotate at a predetermined surface movement speed by a drive unit (not shown) provided in the main body of the image forming apparatus P. The developing roller 6 carries and transports negatively charged toner (developer) to a developing position opposite the photosensitive drum 4. In this embodiment, a DC voltage of approximately -350 V is applied to the core of the developing roller 6 as a developing voltage from a power supply unit (not shown) provided in the main body of the image forming apparatus P.

[0020] As shown in FIG. 1, a laser exposure unit 9, which is an exposure means for exposing the photosensitive drum 4 of each process cartridge 3, is provided above the image forming stations Y, M, C, and K. A printer controller 200, which issues a print request to the image forming apparatus P, transmits a print request signal containing image information to a video controller 100 within the image forming apparatus P. The video controller 100 converts the image information received from the printer controller 200 along with the print request signal into an image-processed video signal and outputs it to the laser exposure unit 9. The laser exposure unit 9 outputs laser light L from a laser output section having a laser element, with the light emission amount (first light emission amount) modulated in accordance with the video signal input from the video controller 100 (referred to as normal light emission). The output laser light L scans the photosensitive drum 4 of each process cartridge 3 in the longitudinal direction (depth direction in FIG. 1), forming an electrostatic latent image on the surface of the photosensitive drum 4 according to the image information. The electrostatic latent image formed on the photosensitive drum 4 is developed by the development roller 6 adhering (supplying) toner to form a visible toner image.

[0021] As shown in FIG. 1, an intermediate transfer belt unit 10 is disposed below the image forming stations Y, M, C, and K. The intermediate transfer belt unit 10 includes a secondary transfer opposing roller 10a, a drive roller 10b, a suspension roller 10c, and an intermediate transfer belt 10d suspended between these rollers. The intermediate transfer belt 10d is an endless resin film. The resin film has an electrical resistance (volume resistivity) of approximately 10E+11 to 10E+16 (Ω·cm) and a thickness of 100 to 200 μm, and is made of PVdf (polyvinylidene fluoride), nylon, PET (polyethylene terephthalate), PC (polycarbonate), or the like. The intermediate transfer belt 10d is driven at a predetermined process speed (a predetermined speed corresponding to the surface movement speed of the photosensitive drum 4) by the drive roller 10b being driven to rotate in the clockwise direction indicated by the arrow in the figure by a drive unit (not shown).

[0022] 1, primary transfer rollers 11 corresponding to the photosensitive drums 4 of each process cartridge 3 are disposed inside the intermediate transfer belt 10d. Each primary transfer roller 11 is disposed opposite its corresponding photosensitive drum 4 and is in contact with the lower part of the photosensitive drum 4 with a predetermined pressure via the intermediate transfer belt 10d. In each of the image forming stations Y, M, C, and K, the contact point between the photosensitive drum 4 and the intermediate transfer belt 10d is referred to as the primary transfer position T1. A positive DC voltage is applied as a primary transfer voltage from a power supply (not shown) to the shaft of each primary transfer roller 11, whereby the toner image formed on the photosensitive drum 4 is transferred to the intermediate transfer belt 10d.

[0023] A secondary transfer roller 12 is disposed opposite the secondary transfer opposing roller 10a with an intermediate transfer belt 10d interposed therebetween, and is held in a state where an appropriate pressure is applied. The contact point between the secondary transfer roller 12 and the intermediate transfer belt 10d is referred to as a secondary transfer position T2. ​​A positive DC voltage is applied as a secondary transfer voltage from a power supply (not shown) to the shaft of the secondary transfer roller 12, whereby the toner image formed on the intermediate transfer belt 10d is transferred onto the recording material passing through the secondary transfer position T2. ​​In addition, at the belt loop portion of the secondary transfer opposing roller 10a, downstream in the rotation direction of the intermediate transfer belt 10d from the secondary transfer position T2, a belt cleaner 13 is disposed to remove toner that has not been transferred to the recording material and remains on the intermediate transfer belt 10d.

[0024] As shown in FIG. 1, the image forming apparatus P has a cassette 14 containing recording materials S, which are recording media, below an intermediate transfer belt unit 10. The image forming apparatus P also has a pickup roller 15 that feeds the recording materials S one by one from the cassette 14, a registration roller pair 16 that controls the transport timing of the recording materials S fed from the pickup roller 15 to a secondary transfer position T2, and other components. As shown in FIG. 1, a transport path 20 is located on the left side of the main body of the image forming apparatus P, along which the recording materials S fed from the cassette 14 lead to an ejection tray 17 provided at the top of the image forming apparatus P. The transport path 20 is arranged, from upstream to downstream in the transport direction of the recording materials S, with the registration roller pair 16, a secondary transfer roller 12, a fixing unit 18, and an ejection roller pair 19. The fixing unit 18 has a fixing roller 18a heated by a fixing heater that heats the recording material S to which a toner image has been transferred, and a pressure roller 18b that abuts against the fixing roller 18a and applies a predetermined pressure to the passing recording material S.

[0025] [Image formation operation] Next, the image forming operation of the image forming apparatus P will be described. When the video controller 100 receives a print request signal from the printer controller 200, it starts the operation of a drive unit (not shown) that rotates the photosensitive drum 4 of each process cartridge 3, the intermediate transfer belt 10d, etc., to begin image formation. When the photosensitive drum 4 starts to rotate, the surface of the photosensitive drum 4 is charged to a uniform potential by the charging roller 5, to which a charging voltage is applied. When the surface of the photosensitive drum 4 charged to a uniform potential by the charging roller 5 reaches the exposure position of the laser exposure unit 9, the surface of the photosensitive drum 4 is irradiated with laser light L emitted from the laser element of the laser exposure unit 9 according to image information. This forms an electrostatic latent image corresponding to the image information on the surface of the photosensitive drum 4. The electrostatic latent image formed on the surface of the photosensitive drum 4 is developed by the developing roller 6, which rotates in contact with the photosensitive drum 4, with toner attached, forming a visible toner image. The formed toner image is transferred onto the intermediate transfer belt 10d at the primary transfer position T1 by the primary transfer voltage applied to the primary transfer roller 11.

[0026] When forming a color image, the above-mentioned process is performed sequentially at the four image forming stations Y, M, C, and K, and toner images of multiple colors are transferred onto the intermediate transfer belt 10d in a superimposed manner. In this embodiment, the yellow, magenta, cyan, and black toner images formed on the photosensitive drums 4 at the image forming stations Y, M, C, and K are transferred sequentially onto the intermediate transfer belt 10d in a superimposed manner, forming a color toner image. The toner images formed on the intermediate transfer belt 10d are transferred at a secondary transfer position T2 onto the recording material S conveyed from the cassette 14 at a predetermined timing by the secondary transfer roller 12 to which a secondary transfer voltage is applied.

[0027] The recording material S onto which the toner image has been transferred passes through a fixing nip formed between a fixing roller 18a and a pressure roller 18b heated to a predetermined temperature in a fixing unit 18, whereby the toner melts and the toner image is fixed as a fixed image onto the recording material S. Then, the recording material S onto which the toner image has been fixed is discharged onto a discharge tray 17 by a pair of discharge rollers 19 as an image-formed product.

[0028] Furthermore, in each of the image forming stations Y, M, C, and K, toner remaining on the photosensitive drum 4 without being transferred to the intermediate transfer belt 10d at the primary transfer position T1 is collected by the cleaning unit 7. Similarly, toner remaining on the intermediate transfer belt 10d without being transferred to the recording material S at the secondary transfer position T2 is collected by the belt cleaner 13.

[0029] [Issues in exposure control] In the exposure control of this embodiment, control is performed to solve the following two problems: "drum ghost" and "transfer memory."

[0030] (Drum Ghost) In the photosensitive drum 4, a potential difference occurs in the portion exposed in the previous process compared to the portion not exposed in the previous process during the next charging process due to the influence of charges remaining in the charge transport layer, etc. Therefore, when the photosensitive drum 4 is exposed again, a potential difference occurs after exposure between the portion exposed in the previous process and the portion not exposed. In other words, the potential difference between the portion printed (exposed portion) and the portion not printed (unexposed portion) in the previous image formation remains on the photosensitive drum 4 when the next image is formed. If this potential difference becomes large, a density difference (drum ghost) will occur in the finally formed image.

[0031] To suppress drum ghosts, this embodiment employs background exposure control, which uses a weak light emission (second light emission amount) sufficient to prevent toner from adhering during the exposure process. Background exposure will now be described with reference to the accompanying drawings. FIG. 2 illustrates the manner in which the laser exposure unit 9 exposes the photosensitive drum 4. FIG. 2 is a schematic plan view of the surface of the photosensitive drum 4, with the surface of the photosensitive drum 4 moving downward (in the direction of movement of the photosensitive drum surface). The surface of the photosensitive drum 4 is irradiated with laser light L from the laser exposure unit 9, and has an image area (one page of recording material S) where an electrostatic latent image is formed, and a non-image area where no electrostatic latent image (toner image) is formed. The non-image area corresponds to, for example, a pre-rotation area, a paper gap, or a post-rotation area. Here, the paper gap refers to the distance between the trailing edge of the preceding recording material S in the transport direction and the leading edge of the recording material S following the preceding recording material S when printing on consecutive recording materials S. Background exposure control is performed as follows. 2 is formed on the surface of the photosensitive drum 4 in accordance with image information, laser light L is irradiated not only to the printed area X where toner is to be deposited, but also to the non-printed area Y where toner is not to be deposited in the image forming area. As a result, both the printed area X and the non-printed area Y are exposed to the laser light L, making it difficult for a potential difference to occur between the printed area X and the non-printed area Y, thereby suppressing the occurrence of density differences (drum ghosts).

[0032] (transcription memory) When printing a color image, the toner image formed on the intermediate transfer belt 10d at the image forming station upstream in the direction of movement of the intermediate transfer belt 10d disturbs the potential of the photosensitive drum 4 at the image forming station downstream. This can result in an image defect known as transfer memory. For example, desired colors are formed by superimposing multiple color toners: yellow and magenta for printing red, magenta and cyan for printing blue, and yellow and cyan for printing green. The red, blue, and green images are formed at the yellow, magenta, and cyan image forming stations Y, M, and C, which are located upstream of the black image forming station K shown in FIG. 1. Therefore, when the black image forming station K forms an image, the yellow, magenta, and cyan toner images have already been transferred onto the intermediate transfer belt 10d. When multiple toner images are superimposed on the intermediate transfer belt 10d, especially when a large amount of multiple color toners are superimposed (hereinafter referred to as a multi-color state), the following situation occurs at the primary transfer position T1. That is, in a multi-color state, the primary transfer current that flows from the primary transfer roller 11 to the photosensitive drum 4 via the intermediate transfer belt 10d flows very slowly. Therefore, due to the difference in the amount of primary transfer current that flows to the photosensitive drum 4 between the multi-color portion on the intermediate transfer belt 10d where a large amount of toner of multiple colors overlaps and the portion where no toner is present, a large potential difference occurs in the surface potential of the photosensitive drum 4 after passing through the primary transfer position T1. In particular, in a multi-color state where a large amount of toner of multiple colors overlaps, if the surface potential of the photosensitive drum 4 after passing through the primary transfer position T1 is close to the predetermined potential in the subsequent charging process, the surface of the photosensitive drum 4 cannot be stably charged. Therefore, there is a possibility that the surface potential of the photosensitive drum 4 will become overcharged, that is, a potential higher than the predetermined post-charging potential.

[0033] If the development process is performed under such a potential difference, the amount of toner transferred from the development roller 6 to the photosensitive drum 4 will differ depending on the potential difference on the surface of the photosensitive drum 4. Ultimately, the difference in the amount of transferred toner will manifest as a difference in density on the image, resulting in image defects (transfer memory). The background exposure described above is an effective means for suppressing transfer memory. In this embodiment, the background exposure always reduces (makes smaller) the surface potential of the photosensitive drum 4 after charging by approximately 100 V in absolute value compared to the potential after charging. This ensures a potential difference with the surface potential of the photosensitive drum 4 after charging when the next charging process is performed, and the surface potential of the photosensitive drum 4 after charging can be maintained at a predetermined charging potential.

[0034] (Background exposure issue) However, background exposure, which is performed to suppress the aforementioned drum ghost and transfer memory, requires irradiating the photosensitive drum 4 with laser light L from the laser exposure unit 9 so as to constantly lower the surface potential of the photosensitive drum 4 by approximately 100 V. As a result, the photosensitive drum 4 is constantly irradiated with a somewhat strong amount of laser light L. This poses a problem, particularly when aiming for a long product life, as it can cause light fatigue of the charge transport layer of the photosensitive drum 4 and the underlying charge generation layer. A light-fatigued photosensitive drum 4 reduces its sensitivity, which can prevent the required development voltage from being sufficiently high and the potential difference between the surface potential of the print area on the photosensitive drum 4 where the electrostatic latent image is formed (hereinafter referred to as development contrast), resulting in a low density. Another issue in achieving a long life for the image forming apparatus P is the degradation of the laser element due to the extended laser emission time caused by background exposure. Deterioration of the laser element reduces the amount of laser light, making it impossible to ensure sufficient development contrast, resulting in a decrease in the density of the formed image.

[0035] [Background Exposure Control in this Example] To solve the above-mentioned problems with background exposure control, a method for reducing the amount of laser light irradiated onto the photosensitive drum 4 in this embodiment will be described. FIG. 3(a) is a schematic diagram illustrating the printed area, non-printed area, and non-image-forming area in the image-forming area on the surface of the photosensitive drum 4, showing the surface of the photosensitive drum 4 expanded in the direction of rotation of the photosensitive drum 4 (the direction of movement of the photosensitive drum surface). In FIG. 3(a), the surface of the photosensitive drum 4 is composed of an image-forming area where a toner image to be transferred to the intermediate transfer belt 10d is formed, and a non-image-forming area corresponding to areas such as pre-rotation, post-rotation, and the gap between sheets, which is the gap between recording materials. X (the black area in the figure) indicates the printed area in the image-forming area where an electrostatic latent image is formed, and Y1 and Y2 indicate the non-printed areas in the image-forming area where an electrostatic latent image is not formed. In detail, the non-printed area Y1 (the cross-hatched area in the figure) indicates the non-printed area when the area of ​​the printed area X exists in the main scanning direction (left-right direction in the figure) in which the laser light scans the photosensitive drum 4, and background exposure is performed (BG exposure ON). On the other hand, the non-printed area Y2 (the diagonally hatched area in the figure) indicates the non-printed area when a printed area such as the printed area X does not exist in the main scanning direction, and background exposure is not performed (BG exposure OFF). The main scanning direction is also the direction of the rotational axis of the photosensitive drum 4.

[0036] Here, the only area that requires background exposure to suppress the aforementioned transfer memory is the non-printed area Y1, where the printed area X exists in the main scanning direction. As described above, transfer memory occurs due to the difference in the amount of current flowing through the photosensitive drum 4 at the primary transfer position T1 between the area on the photosensitive drum 4 where toner is present (printed area X) and the area where toner is not present (non-printed area Y1). The difference in the amount of current flowing through the photosensitive drum 4 at the primary transfer position T1 causes a large potential difference in the surface potential of the photosensitive drum 4 after passing through the primary transfer position T1, and this potential difference manifests as a density difference in the image. Therefore, when the printed area X does not exist in the main scanning direction of the photosensitive drum 4, transfer memory is not apparent, and therefore background exposure is not necessary. In other words, by limiting background exposure to only the non-printed area where the printed area X exists in the main scanning direction, it is possible to suppress photodegradation of the photosensitive drum while maintaining image quality.

[0037] As described above, in this embodiment, as shown in Fig. 3(a), of the non-printing areas present within the image forming area of ​​the photosensitive drum 4, background exposure is not performed in the non-printing area Y2 that is located furthest downstream in the surface movement direction of the photosensitive drum 4. The position at which the background exposure control is switched to be stopped is determined based on the bitmap data after image data processing, which will be described later.

[0038] [Image data processing for exposure control] In this embodiment, the video controller 100 in the image forming apparatus P receives a print request signal containing image information from the printer controller 200, which issues a print request to the image forming apparatus P. At this time, the printer controller 200 sends image data as image information, along with commands such as the paper size on which the image is to be formed and the operating mode (single-sided printing, double-sided printing). The image data is processed by the video controller 100. If the image data is a color image, the image data is in the form of color information using RGB (red, green, blue) data. The color information using each RGB (red, green, blue) data is converted by the video controller 100 from device RGB data reproducible by the image forming apparatus P to device YMCK (yellow, magenta, cyan, black) data. The image forming apparatus P in this embodiment has a pixel count of 600 dpi, and the video controller 100 creates bitmap data (image density data for each CMYK color) corresponding to the pixel count. The exposure amount for each color is then converted into an exposure pattern to be actually used for each pixel, and the laser exposure unit 9 outputs laser light L corresponding to the image information.

[0039] In this embodiment, the bitmap data sent from the video controller 100 is used as print section rear end position information within the image forming area of ​​the photosensitive drum 4. Specifically, the print section rear end position information represents the distance (number of pixels) from the front end of the image forming area in the sub-scanning direction of the photosensitive drum 4 to the pixel position of the rear end pixel of the print section. Based on the print section rear end position information, the laser exposure unit 9 stops controlling the background exposure that is currently being performed.

[0040] [Back Contrast by Background Exposure Control in this Example] In this embodiment, the laser light intensity irradiated onto the printed section X region within the image formation area is set to 0.320 (μJ / cm), and the laser light intensity irradiated onto the non-printed section region for background exposure is set to 0.055 (μJ / cm). Furthermore, among the non-printed section regions within the image formation area, only the non-printed section Y2 region located furthest downstream in the direction of movement of the surface of the photosensitive drum 4 is set to not undergo background exposure. Specifically, for the image shown in FIG. 3(a), the printed section X region is exposed with a laser light intensity of 0.320 (μJ / cm). Meanwhile, the non-printed section Y1 region extending up to the rear end of the printed section X region in the sub-scanning direction of the photosensitive drum 4 (the direction of movement of the photosensitive drum surface) is exposed with a laser light intensity of 0.055 (μJ / cm), while the non-printed section Y2 region beyond the rear end of the printed section X in the sub-scanning direction is not exposed.

[0041] Fig. 3(b) is a graph showing the surface potential of the photosensitive drum 4 at line L in Fig. 3(a) when the above-described exposure control is performed. In Fig. 3(b), the horizontal axis indicates the position in the sub-scanning direction (rotation direction) of the photosensitive drum 4, and Y1, X, and Y2 in the figure indicate the names of areas within the image forming area shown in Fig. 3(a). On the other hand, the vertical axis indicates the surface potential of the photosensitive drum 4 (unit: V).

[0042] After being charged by the charging roller 5, the surface potential of the photosensitive drum 4 is approximately −600 V (potential after charging) regardless of the position in the sub-scanning direction. Background exposure is performed in the non-printed area Y1. Therefore, the surface potential of the photosensitive drum 4 after background exposure by the laser exposure unit 9 drops to approximately −500 V (potential after exposure). In this embodiment, a constant development voltage of −350 V is applied. Therefore, the back contrast Vback, which is the potential difference between the development voltage and the surface potential of the non-printed area Y1 of the photosensitive drum 4, is approximately 150 V (=|−500 V|−|−350 V|). Meanwhile, in the printed area X, the surface potential of the photosensitive drum 4 drops to approximately −100 V due to exposure by the laser exposure unit 9. Therefore, the development contrast Vcont, which is the potential difference between the development voltage required for adhering and developing toner and the surface potential of the print area X area of ​​the photosensitive drum 4, is ensured to be approximately 250V (=|-350V|-|-100V|). In addition, the non-print area Y2 area behind the rear end of the print area X area is not subjected to background exposure, so the surface potential of the photosensitive drum 4 after charging remains at approximately -600V. Therefore, the back contrast Vback is approximately 250V (=|-600V|-|-350V|).

[0043] [Charge Voltage Control in this Example] FIG. 4 is a graph showing the relationship between the back contrast Vback and the toner fog density on the photosensitive drum 4. Here, fog refers to the phenomenon in which toner adheres to non-printed areas due to the development operation by the developing roller 6, resulting in an increase in density. In FIG. 4, the horizontal axis represents the voltage (unit: V) of the back contrast Vback, and the vertical axis represents the toner fog density. The fog density increases upward on the vertical axis and decreases downward on the vertical axis. In FIG. 4, the fog density is lowest when the back contrast Vback is 100 V. As the back contrast Vback increases from 100 V to 150 V, 200 V, and 250 V, the fog density increases due to reverse fog, which occurs when toner charged with the opposite polarity (positive polarity) is developed. On the other hand, as the back contrast Vback decreases from 100 V to 50 V, the fog density increases due to background fog, which occurs when toner charged with the normal polarity (negative polarity) is developed.

[0044] When the back contrast Vback increases, reverse fog occurs, which is caused by toner charged to the opposite polarity (positive polarity) due to insufficient toner charging. This results in more toner being consumed than expected, and more toner being collected in the cleaner container by the cleaning unit 7. As a result, the cleaner container may become larger, which may lead to an increase in the size and cost of the image forming apparatus P.

[0045] Therefore, in this embodiment, as shown in FIG. 3(a), background exposure is not performed in the non-printed area Y2 within the image forming area, and charging control is performed to lower the charging voltage used to charge the non-printed area Y2. FIG. 5 is a diagram showing the photosensitive drum surface potential along line L in FIG. 3(a) when the charging voltage is lowered. In FIG. 5, the horizontal axis indicates the position of the photosensitive drum 4 in the sub-scanning direction (rotation direction), and Y1, X, and Y2 in the diagram indicate the names of areas within the image forming area shown in FIG. 3(a). Meanwhile, the vertical axis indicates the surface potential (unit: V) of the photosensitive drum 4.

[0046] As shown in FIG. 5, the surface potential of the photosensitive drum 4 in the non-printed area Y1 is −500 V (potential after exposure) due to background exposure by the laser exposure unit 9. The surface potential of the photosensitive drum 4 in the printed area X is approximately −100 V due to exposure by the laser exposure unit 9. On the other hand, the surface potential of the photosensitive drum 4 in the non-printed area Y2 is −600 V (potential after charging) as shown in FIG. 3(b) when background exposure is not performed. However, in this embodiment, in the non-printed area where background exposure is not performed (non-printed area Y2), the charging voltage is reduced by 100 V, so that the surface potential of the photosensitive drum 4 after charging changes as follows: That is, the surface potential of the photosensitive drum 4 after charging decreases from −600 V to approximately −500 V, which is the same potential as after background exposure. This ensures that the back contrast Vback of the non-printed area Y2 is 150 V (= |-500 V|-|-350 V|), the same as the non-printed area Y1. As a result, the back contrast Vback of the non-printed area Y2 becomes 150 V from 250 V, and as shown in the graph in Figure 4, an increase in the amount of toner due to inverted fog can be suppressed.

[0047] [Effects of this Example] An experiment was conducted using the above-described control, and the effects of reducing the amount of laser exposure irradiated onto the photosensitive drum 4 and reducing the amount of abrasion on the surface of the photosensitive drum 4 were confirmed. In this experiment, an image forming apparatus P was used to print two consecutive sheets of A4-sized recording material S (approximately 297 mm long in the conveying direction) using the image data shown in FIG. 6. FIG. 6 is a diagram showing the image data used in this experiment in a schematic diagram of the surface of the photosensitive drum 4 expanded in the direction of rotation of the photosensitive drum 4 (direction of movement of the photosensitive drum surface). In FIG. 6, the image forming area indicates the print area and non-print area printed on the A4-sized recording material S, and the non-image forming area corresponds to the area corresponding to the pre-rotation, paper gap, post-rotation, etc.

[0048] As shown in Figure 6, the first half (148.5 mm) of the length of the A4-sized recording material S in the transport direction is occupied by the area of ​​the printed section X of the image data, and the second half (148.5 mm) of the length of the recording material S in the transport direction is occupied by the area of ​​the non-printed section Y2. In addition, non-printed section Y1 areas are provided on both ends of the area of ​​the printed section X in the main scanning direction. In this experiment, background exposure was performed on the area of ​​the non-printed section Y1 (BG exposure ON), and exposure was not performed on the area of ​​the non-printed section Y2 or the non-image forming area (BG exposure OFF).

[0049] The results of this experiment confirmed that the laser emission amount during printing on two sheets of recording material S can be reduced to approximately 28% of the amount of laser emission when both the image-forming area and the non-image-forming area are constantly exposed to light during printing. The exposure control of this embodiment described above can extend the life of the laser element. Furthermore, by reducing the amount of laser emission, the amount of laser light received by the photosensitive drum 4 can be similarly reduced, thereby similarly suppressing a decrease in the sensitivity of the photosensitive drum 4. Furthermore, it was confirmed that the amount of surface wear on the photosensitive drum 4 when a total of 5,000 sheets of recording material S were printed under the above-described printing conditions could be reduced to approximately 18% of the amount of surface wear on the photosensitive drum 4 when both the image-forming area and the non-image-forming area are constantly exposed to light during printing.

[0050] In this experiment, the charging voltage was reduced by 100 V for the non-printed area Y2, and background exposure like that for the non-printed area Y1 was not performed. However, the following exposure control may be performed, for example. That is, the charging voltage for the non-printed area Y2 may remain at a predetermined voltage, and background exposure may be performed with a reduced exposure amount (using a third light emission amount) compared to the background exposure for the non-printed area Y1. In this case, reducing the exposure amount increases the back contrast Vback described above compared to the non-printed area Y1, but the exposure amount should be such that the potential difference ensures that an increase in the amount of toner due to inversion fog is suppressed.

[0051] As described above, by performing the exposure control and charge control of this embodiment, it is possible to suppress the occurrence of drum ghosts, transfer memory, toner fogging, etc., and reduce the exposure amount of the laser light irradiated onto the photosensitive drum 4 while maintaining image quality. This makes it possible to suppress the reduction in sensitivity and charge potential attenuation due to light fatigue of the photosensitive drum 4, deterioration of the laser element, and even abrasion of the surface of the photosensitive drum 4, which are issues in terms of the long life of the product.

[0052] Furthermore, in this embodiment, by not performing background exposure in the non-printed area Y2 where there is no print area in the main scanning direction of the photosensitive drum 4, light fatigue of the photosensitive drum can be suppressed. A similar effect can also be achieved by performing background exposure with a reduced amount of exposure compared to the non-printed area Y1 while maintaining the above-mentioned predetermined charging voltage. The exposure amount can be reduced by adjusting the laser output, for example, or by adjusting the exposure time.

[0053] As described above, according to this embodiment, it is possible to reduce the amount of exposure light irradiated onto the photosensitive member while maintaining image quality. [Example]

[0054] In the first embodiment, an example was described in which background exposure was not performed in the non-printed area that is furthest downstream in the sub-scanning direction within the image forming area, and only charging voltage switching control was performed. In the first embodiment, there was a maximum of one non-printed area in which charging voltage switching control was performed. In the second embodiment, an example is described in which background exposure is not performed in all of the non-printed areas within the image forming area, except for non-printed areas that meet predetermined conditions, and charging voltage switching control is performed. Note that the image forming apparatus to which the second embodiment is applied is similar to the image forming apparatus P of the first embodiment, and the same devices and components are designated by the same reference numerals as in the first embodiment, and detailed description thereof will be omitted.

[0055] [Exposure Control and Charging Voltage Control in this Example] In the second embodiment, when the exposure position and charging position on the photosensitive drum 4 move from a non-printed area where a print area exists in the main scanning direction of the photosensitive drum 4 to a non-printed area where no print area exists in the main scanning direction, the following control is performed. That is, control to switch background exposure from the ON state to the OFF state and control to switch charging voltage to lower the charging voltage are performed. When control to switch background exposure from the ON state to the OFF state is performed, the laser element that emits laser light is switched from the ON state to the OFF state, so the emission of laser light is immediately stopped. On the other hand, when switching charging voltage, the output charging voltage is not switched immediately, but rather changes relatively gradually.

[0056] 7 is a diagram showing changes in the surface potential of the photosensitive drum 4 when exposure control and charging voltage control are performed on a non-printed area Y1 where a print area exists in the main scanning direction of the photosensitive drum 4 and a non-printed area Y2 where no print area exists in the main scanning direction. In FIG. 7, the horizontal axis represents the position in the sub-scanning direction (the direction of rotation of the photosensitive drum 4) of the photosensitive drum 4, and the vertical axis represents the surface potential (unit: V) of the photosensitive drum 4. In FIG. 7, −600 V represents the voltage after the charging process (post-charging potential) (shown by a dotted line in the diagram), −500 V represents the voltage when background exposure is performed (post-exposure potential), and −350 V represents the development voltage applied to the development roller 6.

[0057] Here, charging voltage switching control is performed to lower the charging voltage when the non-printed area Y2 on the photosensitive drum 4 moves to a charging position where the charging voltage is applied. Furthermore, background exposure is switched from ON to OFF when the non-printed area Y2 on the photosensitive drum 4 moves to an exposure position where the laser light from the laser exposure unit 9 is irradiated. When charging voltage switching control and background exposure switching control are performed at such timing, the surface potential of the photosensitive drum 4 changes as shown in FIG. 7 . As described above, the charging voltage does not switch instantly like the laser element of the laser exposure unit 9, but rather falls gradually. Therefore, when background exposure is switched from ON to OFF, the surface potential of the photosensitive drum 4 temporarily rises to −600 V. Then, as the charging voltage falls from −600 V to −500 V, the surface potential of the photosensitive drum 4 converges to −500 V. That is, if the background exposure is switched from ON to OFF and the charging voltage control to lower the charging voltage is performed at the same time that the exposure position and charging position move from the non-printing area Y1 to the non-printing area Y2, the back contrast Vback temporarily increases. As a result, the back contrast Vback increases during the period until the surface potential of the photosensitive drum 4 decreases from -600 V to -500 V, which increases the amount of toner in the reverse fog, as shown in Figure 4. However, because this phenomenon occurs only during the temporary period when the charging voltage is switched, the amount of toner collected by the cleaning unit 7 is small.

[0058] In this embodiment, when the exposure position or charging position on the photosensitive drum 4 moves from a non-printed area where no printable area exists in the main scanning direction of the photosensitive drum 4 to a non-printed area where a printable area exists in the main scanning direction, the following control is performed. That is, background exposure is switched from OFF to ON, and charging voltage is switched to increase the charging voltage. FIG. 8A shows the change in the surface potential of the photosensitive drum 4 when exposure control and charging voltage control are performed on the non-printed area Y2 where no printable area exists in the main scanning direction of the photosensitive drum 4 and the non-printed area Y1 where a printable area exists in the main scanning direction. In FIG. 8A, the horizontal axis represents the position of the photosensitive drum 4 in the sub-scanning direction (the direction of rotation of the photosensitive drum 4), and the vertical axis represents the surface potential of the photosensitive drum 4 (unit: V).

[0059] Here, charging voltage switching control is performed to increase the charging voltage when the non-printed area Y1 on the photosensitive drum 4 moves to the charging position where the charging voltage is applied. Furthermore, background exposure is switched from OFF to ON when the non-printed area Y1 on the photosensitive drum 4 moves to the exposure position where the laser light from the laser exposure unit 9 is irradiated. When charging voltage switching control and background exposure switching control are performed at such timing, the surface potential of the photosensitive drum 4 changes as shown in FIG. 8(a). Unlike the laser element of the laser exposure unit 9, the charging voltage does not switch instantly, but rather rises gradually. Therefore, when background exposure is switched from OFF to ON, the surface potential of the photosensitive drum 4 temporarily drops to −400 V. Then, as the charging voltage rises from −500 V to −600 V, the surface potential of the photosensitive drum 4 converges to −500 V. Therefore, during the period in which the surface potential of the photosensitive drum 4 rises from -400V to -500V, the back contrast Vback decreases, and the amount of background fogging toner developed from normally (negatively) charged toner increases, as shown in Fig. 4. Because the background fogging toner developed on the photosensitive drum 4 is mainly negatively charged toner, it may be transferred to the intermediate transfer belt 10d and then to the recording material P, which may manifest as an image defect.

[0060] Therefore, in this embodiment, charging voltage switching control is performed to increase the charging voltage at a timing earlier than the timing at which the non-printing area Y1 on the photosensitive drum 4 moves to the charging position where the charging voltage is applied. This suppresses the increase in the amount of toner in the background fogging described above. A specific method is described using FIG. 8(b). FIG. 8(b) is a diagram illustrating the charging voltage switching timing of this embodiment. The configuration of the diagram is the same as FIG. 8(a) except that the graph of the surface potential of the photosensitive drum 4 is different from the graph shown in FIG. 8(a). Therefore, a description of how to read the diagram will be omitted. Compared to the conventional charging voltage switching timing shown in FIG. 8(a), the charging voltage switching control in FIG. 8(b) starts 100 msec (milliseconds) earlier than the switching timing of the background exposure switching control. Initiating the charging voltage switching control earlier than the switching timing of the background exposure actually increases the back contrast Vback. However, the period during which the back contrast Vback increases is temporary, and the amount of toner collected by the cleaning unit 7 is small, thereby suppressing the occurrence of background fogging.

[0061] Furthermore, as described above, when the charging voltage is switched, the change in voltage until the charging voltage converges to the switched voltage is relatively gradual. Therefore, if the printed area X and the non-printed areas Y1 and Y2 are repeatedly present at short intervals in the sub-scanning direction of the photosensitive drum 4, the area to which the charging voltage is applied will change by the time the charging voltage converges to the switched voltage, and the charging voltage will not be switched in time. Therefore, in this embodiment, taking into account the time it takes for the charging voltage to converge to the switched voltage, when the non-printed area Y2 exists between the printed area X and the printed area X in the sub-scanning direction of the photosensitive drum 4, the control of the non-printed area Y2 is performed as follows: That is, if the length of the non-printed area Y2 in the sub-scanning direction is shorter than the length that the surface of the photosensitive drum 4 can move in 200 msec (milliseconds) (=0.2 seconds), the switching of the charging voltage is stopped and background exposure control is performed, as with the non-printed area Y1. In this embodiment, the surface movement speed of the photosensitive drum 4 (which is also the rotation speed of the photosensitive drum 4) is 150 mm / sec. Therefore, the area of ​​the non-printing portion Y2 whose length in the sub-scanning direction is shorter than 30 mm (= 150 mm / sec × 0.2 sec) corresponds to the area where background exposure control is performed.

[0062] [Effects of this Example] An experiment was conducted using the above-described control to confirm the effects of reducing the amount of laser exposure irradiated onto the photosensitive drum 4 and reducing the amount of abrasion on the surface of the photosensitive drum 4. In this experiment, an image forming apparatus P was used to print two consecutive sheets of A4-sized recording material S (approximately 297 mm long in the conveying direction) using the image data shown in FIG. 9(a). FIG. 9(a) is a diagram showing the image data used in this experiment in a schematic diagram of the surface of the photosensitive drum 4 expanded in the direction of rotation of the photosensitive drum 4 (direction of movement of the photosensitive drum surface). In FIG. 9(a), the image forming area indicates the print area printed on the A4-sized recording material S and the non-print area (the numerical values ​​in the figure are in mm), and the non-image forming area corresponds to the pre-rotation, paper gap, post-rotation, etc.

[0063] As shown in FIG. 9(a), there are three printed area X regions, and the lengths of the printed area X in the sub-scanning direction are 26 mm, 10 mm, and 10 mm, starting from the top printed area X in the figure. Between the printed area X regions in the sub-scanning direction, there are non-printed area Y2 regions that do not have printed area X in the main scanning direction. The lengths of the non-printed area Y2 regions in the sub-scanning direction are 10 mm and 238 mm, starting from the top non-printed area Y2 in the figure. The length of the top non-printed area Y2 in the sub-scanning direction is 10 mm, which is shorter than the 30 mm length in the sub-scanning direction at which the charging voltage can be switched as described above, so background exposure (BG exposure ON) is performed, just like the non-printed area Y1. On the other hand, the length of the non-printing area Y2 at the bottom of the figure in the sub-scanning direction is 238 mm, which is longer than the 30 mm length in the sub-scanning direction at which the charging voltage can be switched, so background exposure is not performed (BG exposure OFF) and the charging voltage is switched (charging voltage control that lowers the charging voltage). Note that background exposure (BG exposure ON) is performed in the area of ​​the non-printing area Y1 where the printing area X is provided in the main scanning direction. Also, control is set so that no exposure is performed on the non-image forming area.

[0064] The results of this experiment confirmed that the laser emission amount during printing on two sheets of recording material S can be reduced to approximately 7% of the amount of laser emission when both the image-forming area and the non-image-forming area are constantly exposed to light during printing. The exposure control of this embodiment described above can extend the life of the laser element. Furthermore, by reducing the amount of laser emission, the amount of laser light received by the photosensitive drum 4 can also be reduced, thereby similarly suppressing a decrease in the sensitivity of the photosensitive drum 4. Furthermore, it was confirmed that the amount of surface wear on the photosensitive drum 4 when a total of 5,000 sheets of recording material S were printed under the above-described printing conditions could be reduced to approximately 21% of the amount of surface wear on the photosensitive drum 4 when both the image-forming area and the non-image-forming area are constantly exposed to light during printing.

[0065] In this experiment, as in the experiment of Example 1, the charging voltage for the non-printed area Y2 was reduced by 100 V, and background exposure like that for the non-printed area Y1 was not performed. For example, in Example 2, the charging voltage for the non-printed area Y2 may be kept at a predetermined voltage, and background exposure may be performed with a reduced exposure amount compared to the background exposure for the non-printed area Y1. In this case, reducing the exposure amount increases the back contrast Vback described above compared to that for the non-printed area Y1, but the exposure amount should be such that a potential difference is achieved that ensures that an increase in the amount of toner due to inversion fog is suppressed.

[0066] As described above, by performing the exposure control and charging control of the present embodiment described above, it is possible to reduce the amount of exposure of the laser light irradiated onto the photosensitive drum 4 while maintaining image quality by suppressing the occurrence of drum ghosts, transfer memory, toner fogging, etc. This has the effect of suppressing sensitivity reduction and charging potential attenuation due to light fatigue of the photosensitive drum 4, deterioration of the laser element, and even abrasion of the surface of the photosensitive drum 4, which are issues in terms of product longevity. Furthermore, a similar effect can be achieved by a method of performing background exposure in which the amount of exposure is reduced compared to the area of ​​the non-printed portion Y1 while maintaining the predetermined charging voltage described above.

[0067] [Other Examples] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments and the following modifications and variations are possible.

[0068] In the above-described embodiment, when the charging voltage is switched, the rise and fall of the charging voltage is gradual compared to the laser element that outputs laser light from the laser exposure unit 9, and it takes time for the charging voltage to converge to the voltage after switching. As a result, the back contrast Vback temporarily increases. Because the increase in the back contrast Vback is temporary, the amount of toner collected by the cleaning unit 7 is small. However, in order to reduce the amount of toner in reverse fog even if only slightly, the laser output from the laser exposure unit 9 or the exposure time of the laser light is gradually changed in accordance with the change in the voltage at the rise and fall of the charging voltage. In this way, by keeping the back contrast Vback constant, the amount of laser light irradiated onto the photosensitive drum 4 can be reduced while suppressing an increase in the amount of toner in reverse fog.

[0069] In the above-described embodiment, in the non-printed area Y2 where background exposure is not performed, the development voltage is kept constant and the charging voltage is lowered to maintain the back contrast Vback. For example, the same effect can be achieved by maintaining the charging voltage constant and increasing the development voltage to maintain the back contrast Vback. In this case, the voltage change during the rise and fall of the development voltage when switching the development voltage is relatively gradual, just like the charging voltage. Therefore, control is required to prevent the back contrast Vback from temporarily decreasing or the development contrast Vcont from changing. In particular, because changes in the development contrast Vcont are likely to affect image density and line width, it is necessary to gradually switch the laser output of the laser exposure unit 9 in accordance with the voltage change during the rise and fall of the development voltage.

[0070] As described above, according to this embodiment, it is possible to reduce the amount of exposure light irradiated onto the photosensitive member while maintaining image quality. [Explanation of symbols]

[0071] 4 Photosensitive drum 5 Charging roller 6 Developing roller 9 Laser exposure unit

Claims

1. A rotating photoreceptor; a charging means for charging the surface of the photoreceptor; an exposure unit that irradiates the surface of the photoreceptor charged by the charging unit with light to expose the surface and form a latent image; a developing means for developing the latent image with toner; the exposure unit emits light at a first light emission amount in a print area where the latent image is developed with toner among areas of the photosensitive member corresponding to the recording material on which the image is formed, and emits light at a second light emission amount smaller than the first light emission amount in a non-print area where the latent image is not formed, thereby forming a toner image on the recording material, In a first non-printing area, which is a non-printing area in an area where the printing area and the non-printing area are provided in a main scanning direction that is a rotation axis direction of the photosensitive member, the charging means charges with a predetermined charging voltage, and the exposure means emits light with the second light emission amount, In a second non-printing area, which is an area in which only the non-printing area is provided in the main scanning direction of the photosensitive member, the charging means charges the photosensitive member with a voltage lower than the predetermined charging voltage, and the exposure means does not perform exposure. The image forming apparatus is characterized in that the charging means charges the second non-printing area, which is the most downstream in the sub-scanning direction, which is the rotation direction of the photosensitive body, with a voltage lower than the specified charging voltage.

2. 2. The image forming apparatus according to claim 1, wherein when the charging means switches the charging voltage to the predetermined charging voltage in order to charge the area including the printing area from the second non-printing area, the charging means switches the charging voltage to the predetermined charging voltage earlier than the timing at which the area including the printing area reaches a charging position where the charging means charges.

3. 3. The image forming apparatus according to claim 2, wherein the timing at which the charging means switches the charging voltage to the predetermined charging voltage is the timing at which the charging voltage has converged to the predetermined charging voltage when the area including the printing area reaches the charging position.

4. 4. The image forming apparatus according to claim 2, wherein the charging means charges the second non-printing area, the length of which in the sub-scanning direction, which is the rotation direction of the photosensitive member, is shorter than a predetermined length, with the predetermined charging voltage, and the exposure means emits light with the second light emission amount.

5. 5. The image forming apparatus according to claim 4, wherein the predetermined length is determined based on the rotation speed of the photosensitive member and the time from when the charging means changes the charging voltage from a voltage lower than the predetermined charging voltage to the predetermined charging voltage until the charging voltage converges to the predetermined charging voltage.

6. 6. The image forming apparatus according to claim 1, wherein the voltage lower than the predetermined charging voltage is the same potential as the surface potential of the photosensitive member after the exposure unit emits light with the second light emission amount.

7. A rotating photoreceptor; a charging means for charging the surface of the photoreceptor; an exposure unit that irradiates the surface of the photoreceptor charged by the charging unit with light to expose the surface and form a latent image; a developing means for developing the latent image with toner; the exposure unit emits light at a first light emission amount in a print area where the latent image is developed with toner among areas of the photosensitive member corresponding to the recording material on which the image is formed, and emits light at a second light emission amount smaller than the first light emission amount in a non-print area where the latent image is not formed, thereby forming a toner image on the recording material, In a first non-printing area, which is a non-printing area in an area where the printing area and the non-printing area are provided in a main scanning direction that is a rotation axis direction of the photosensitive member, the charging means charges with a predetermined charging voltage, and the exposure means emits light with the second light emission amount, In a second non-printing area, which is an area in which only the non-printing area is provided in the main scanning direction of the photosensitive member, the charging means charges the photosensitive member with the predetermined charging voltage, and the exposure means emits light with a third light emission amount that is smaller than the second light emission amount, The image forming apparatus is characterized in that the exposure means emits light at the third light emission amount in the second non-printing area that is the most downstream second non-printing area in the sub-scanning direction, which is the rotation direction of the photosensitive body.

8. the developing means has a developing member that supplies the toner to the surface of the photosensitive member to form a toner image; 8. The image forming apparatus according to claim 7, wherein Vback, which is the absolute value of the difference between the surface potential of the second non-printing area exposed with the third light emission amount and the development voltage applied to the developing member, is a potential difference that can suppress an increase in the amount of toner due to fogging on the photosensitive member.

9. 8. The image forming apparatus according to claim 1, wherein the second non-printing area is determined based on the pixel position in the sub-scanning direction of the rear end pixel in the printing area that is the most downstream in the sub-scanning direction among the printing areas exposed by the exposure means.

Images