Image forming apparatus and image forming system

Abstract

To reduce density unevenness of a toner image compared to before.SOLUTION: An image forming apparatus has: a photoreceptor; charging means that electrostatically charges the surface of the photoreceptor; exposure means that exposes the surface of the photoreceptor; developing means that develops an electrostatic latent image with toner to form a toner image; and transfer means that transfers the toner image to a sheet. The image forming apparatus stores a cumulative image formation amount for each of N exposure areas on the surface of the photoreceptor, determines an exposure condition on the basis of the cumulative image formation amount for each of the N exposure areas, and exposes the N exposure areas.SELECTED DRAWING: Figure 13

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus and an image forming system. [Background technology]

[0002] An electrophotographic image forming apparatus uniformly charges a photosensitive drum, exposes the photosensitive drum to light to form an electrostatic latent image, and applies toner to the electrostatic latent image to form a toner image. The photosensitive layer provided on the core metal of the photosensitive drum gradually becomes thinner each time a toner image is formed. As a result, the density of the toner image gradually decreases. Patent Document 1 proposes that the film thickness of the photosensitive layer is calculated according to the number of sheets fed to the image forming apparatus, and exposure conditions are changed according to the film thickness. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 9-120245 Summary of the Invention [Problem to be solved by the invention]

[0004] According to the conventional technology, it is assumed that the thickness of the photosensitive layer is uniformly thinned at any position on the surface of the photosensitive body according to the number of sheets on which images are printed. However, the amount of reduction in the thickness of the photosensitive layer depends on the type of image to be formed. Therefore, unevenness in the thickness of the photosensitive layer occurs on the entire surface of the photosensitive body. For example, an adhesive toner image for bonding multiple sheets may be frequently formed at a specific position in the main scanning direction of the photosensitive body (a direction parallel to the rotation axis of the photosensitive body). In this case, the thickness of the photosensitive layer at a specific position in the main scanning direction becomes thinner than the thickness at other positions. Therefore, the present invention aims to reduce the density unevenness of the toner image more than ever before. [Means for solving the problem]

[0005] The present invention relates to, for example, A photoconductor that is rotated; A charging means for charging the surface of the photoreceptor; an exposure unit for exposing the surface of the photoreceptor to light to form an electrostatic latent image; a developing means for developing the electrostatic latent image with a toner to form a toner image; a transfer means for transferring the toner image to a sheet; a storage means for storing an accumulated image formation amount for each of the N exposure areas on the surface of the photoconductor; a determining means for determining an exposure condition for each of the N exposure regions based on the accumulated image formation amount; a control means for controlling the exposure means so as to expose each of the N exposure areas according to the exposure conditions; The present invention provides an image forming apparatus having the above structure. Effect of the Invention

[0006] According to the present invention, density unevenness in a toner image is reduced more than ever before. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram illustrating an image forming system. [Diagram 2] Diagram to explain the image [Diagram 3] Diagram explaining the thermocompression unit [Figure 4] FIG. 1 is a diagram illustrating a process cartridge. [Diagram 5] Diagram explaining the photosensitive layer [Figure 6] A diagram explaining the surface potential [Figure 7] A graph showing the exposure amount versus the VD fluctuation amount and a graph showing the film thickness reduction amount versus the number of sheets passed [Figure 8] Diagram explaining test images [Figure 9] FIG. 1 is a diagram for explaining memory addresses assigned to an exposure area; [Figure 10]Graph showing film thickness reduction and exposure amount versus cumulative pixel count [Figure 11] Table explaining the experimental results [Figure 12] Diagram explaining test images [Figure 13] Diagram explaining the functions of a CPU [Figure 14] Flowchart showing a method for counting the cumulative number of pixels [Figure 15] Flowchart showing a method for determining exposure conditions [Figure 16] Diagram explaining test images [Figure 17] FIG. 1 is a diagram illustrating the relationship between the rotation period of a photoconductor drum and a test image. [Figure 18] Flowchart showing a method for determining exposure conditions [Figure 19] Graph showing the film thickness reduction amount and exposure amount according to the cumulative number of pixels [Figure 20] Flowchart showing a method for determining exposure conditions [Figure 21] Flowchart showing a method for determining image processing conditions DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, the embodiments will be described in detail with reference to the attached drawings. Note that the following embodiments do not limit the invention according to the claims. Although the embodiments describe a number of features, not all of these features are essential to the invention, and the features may be combined in any manner. Furthermore, in the attached drawings, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0009] <Example 1> (1) Image forming system 1, the image forming system 1 includes an image forming apparatus 100 and a post-processing apparatus 130. The post-processing apparatus 130 is a sheet processing apparatus connected to the image forming apparatus 100. The image forming apparatus 100 forms an image on a sheet S, which is a recording material. The intermediate conveying unit 120 conveys the sheet S on which the image has been formed to the post-processing apparatus 130. The post-processing apparatus 130 performs post-processing on the sheet S as necessary and outputs it.

[0010] The image forming apparatus 100 includes a sheet cassette 8, an image forming unit 10, a fixing unit 6, and a housing 19 that houses these components. The image forming unit 10 forms a toner image on a sheet S fed from the sheet cassette 8. The fixing unit 6 performs a fixing process to fix the toner image on the sheet S.

[0011] The sheet cassette 8 is provided at the bottom of the image forming apparatus 100. The sheet cassette 8 is inserted into the housing 19 so as to be removable, and can store a large number of sheets S. In this embodiment, it is assumed that the maximum size of the sheet S on which an image can be formed is A4 size (297 mm length x 210 mm width). The long side of the A4 size sheet S is parallel to the conveying direction of the sheet S. The feed roller 81 feeds the sheet S from the sheet cassette 8, and passes the sheet S to a pair of conveying rollers 82. The multi-tray 20 can also feed the sheets S one by one.

[0012] The image forming section 10 is a tandem-type electrophotographic unit including four process cartridges 7n, 7y, 7m, and 7c, an exposure device 2, and a transfer unit 3. ymc means yellow, magenta, and cyan, respectively. n means adhesive toner. The letters ymnc, which indicate the color of the toner, may be omitted from the reference symbol. The color of the adhesive toner may be transparent or black. When the color of the adhesive toner is transparent, black is realized by appropriately mixing yellow, magenta, and cyan (process black). The toner type (material) used as the yellow, magenta, and cyan toner and the adhesive toner is, for example, a thermoplastic resin. Examples of thermoplastic resins include polyester resin, vinyl resin, acrylic resin, and styrene acrylic resin. The process cartridges 7n, 7y, 7m, and 7c allow multiple parts that are responsible for the image forming process to be replaced as a whole. In other words, the process cartridges 7n, 7y, 7m, and 7c are formed by integrating multiple parts.

[0013] The process cartridges 7n, 7y, 7m, and 7c have corresponding developing devices Kn, Ky, Km, and Kc, photoconductor drums Dn, Dy, Dm, and Dc, and charging rollers Cn, Cy, Cm, and Cc. The structure of the process cartridges 7n, 7y, 7m, and 7c is substantially the same, except for the type of toner.

[0014] The developing devices Ky, Km, Kc contain yellow, magenta, and cyan toners for forming a visible image on the sheet S. The developing device Kn contains adhesive toner Tn. The adhesive toner Tn is used to form a user image (original image) and to thermally press a plurality of sheets S together in the post-processing device 130. An image made of the adhesive toner Tn is formed on the photoconductor drum Dn by development using the adhesive toner Tn.

[0015] The image forming unit 10 may have a fifth process cartridge that uses a toner exclusively for adhesion or a black toner. The type and number of printing toners can be changed depending on the application of the image forming apparatus 100.

[0016] The charging rollers Cn, Cy, Cm, and Cc are chargers that uniformly charge the surfaces of the corresponding charging rollers Cn, Cy, Cm, and Cc. The exposure device 2 is disposed below the process cartridges 7n, 7y, 7m, and 7c and above the sheet cassette 8. The exposure device 2 irradiates the photosensitive drums Dn, Dy, Dm, and Dc with corresponding laser beams Jn, Jy, Jm, and Jc to form electrostatic latent images. The exposure device 2 may also be called an optical scanning device.

[0017] The developing devices Kn, Ky, Km, and Kc form toner images by attaching toner to the electrostatic latent images on the photoconductor drums Dn, Dy, Dm, and Dc. The developing devices Kn, Ky, Km, and Kc may also be called developers.

[0018] The transfer unit 3 includes a transfer belt 30 as an intermediate transfer body (secondary image carrier). The transfer belt 30 is an endless belt wound around an inner roller 31 and a tension roller 32. The outer peripheral surface (image forming surface) of the transfer belt 30 faces the photosensitive drums Dn, Dy, Dm, and Dc. Primary transfer rollers Fn, Fy, Fm, and Fc are disposed on the inner peripheral side of the transfer belt 30 so as to face the photosensitive drums Dn, Dy, Dm, and Dc.

[0019] The primary transfer rollers Fn, Fy, Fm, and Fc transfer the toner images from the corresponding photoconductor drums Dn, Dy, Dm, and Dc to the transfer belt 30. The primary transfer rollers Fn, Fy, Fm, and Fc may be called primary transfer devices. The toner images are transported to the secondary transfer section by the counterclockwise rotation of the transfer belt 30.

[0020] The secondary transfer roller 5 is disposed to face the inner roller 31, and forms a transfer nip 52 between the secondary transfer roller 5 and the transfer belt 30. The transfer nip 52 transfers the toner image from the transfer belt 30 to the sheet S. The transfer nip 52 may be referred to as a secondary transfer portion.

[0021] A fixing unit 6 is disposed above the secondary transfer roller 5 (downstream in the conveying direction of the sheet S). The fixing unit 6 applies heat and pressure to the sheet S passing through a fixing nip 61. This causes the toner image to be fixed onto the sheet S. The fixing unit 6 has a fixing heater 62 for heating the toner image and the sheet S. The fixing heater 62 is, for example, a halogen heater or a ceramic heater.

[0022] FIG. 2(A) shows a printing area 211 of adhesive toner Tn. The printing area 211 extends parallel to the long side of the sheet S. The printing area 211 is provided at the end close to the long side. As a result, the post-processing device 130 stacks multiple sheets S and heats and presses the printing areas 211 of the multiple sheets S, thereby adhering the multiple sheets S to form a booklet. The booklet in this case is a long-side bound booklet. Here, the width (length in the short side direction) of the adhesive toner image (printing area 211) is, for example, 4.0 mm.

[0023] 2B, a small print area 212 for adhesive toner Tn may be formed near the corner of the sheet S. This produces a booklet with corner fastenings. An image using adhesive toner Tn is not formed on the sheet S that will be the cover of the booklet.

[0024] The printing area 211 of the adhesive toner Tn may be formed on both sides of the sheet S, or may be formed only on one side of the sheet S. Whether the printing area 211 of the adhesive toner Tn is formed only on one side or on both sides may be selected in consideration of, for example, the adhesive capacity of the post-processing device 130, the adhesive capacity of the adhesive toner Tn, the type of the sheet S, the functions required of the booklet, and the like. Reliable adhesiveness is required for booklets that are to be kept as archives. Reliable adhesiveness is also required when thick paper or a special sheet S is used as the cover of the booklet. Therefore, in these cases, the printing area 211 of the adhesive toner Tn is provided on both sides of the sheet S. When a booklet for simple, one-time use is produced, the printing area 211 of the adhesive toner Tn is formed only on one side of the sheet S.

[0025] Returning to the description of FIG. 1, as shown in FIG. 1, the switching guide 33 is a flap-shaped guide member provided downstream of the fixing unit 6 in the conveying direction of the sheet S. When a single-sided printing mode in which an image is formed on one side of the sheet S is selected, the switching guide 33 guides the sheet S to a discharge roller 34. When a double-sided printing mode in which an image is formed on both sides of the sheet S is selected, the switching guide 33 guides the sheet S with an image formed on the first side to a switchback roller pair 35. The switchback roller pair 35 conveys the sheet S in a first direction. When the rear end of the sheet S is in a state in which it can enter the double-sided conveying path 36, the switchback roller pair 35 starts to reverse. As a result, the sheet S is conveyed to the double-sided conveying path 36. The double-sided conveying path 36 conveys the sheet S again to the secondary transfer unit. As a result, an image is formed on the second side of the sheet S.

[0026] The discharge roller 34 conveys the sheet S to an intermediate conveying unit 120. The intermediate conveying unit 120 has a pair of conveying rollers 121 and 122. The pair of conveying rollers 121 and 122 convey the sheet S to a post-processing device 130.

[0027] (2) After-treatment device The post-processing device 130 is a floor-standing type sheet processing device and has a function of buffering a plurality of sheets, a function of aligning a plurality of sheets, and a function of gluing a sheet stack.

[0028] In the following, the end of the sheet S on the front side in the transport direction is referred to as the leading end. The end of the sheet S on the rear side in the transport direction is referred to as the trailing end. Of the two ends of the sheet S, the end that enters the post-processing device 130 first is referred to as the first end. Of the two ends of the sheet S, the end that enters the post-processing device 130 later is referred to as the second end. Note that the leading end may be changed from the first end to the second end and the trailing end may be changed from the second end to the first end due to the switchback transport performed by the post-processing device 130.

[0029] The sheet S conveyed from the intermediate conveying unit 120 is delivered to the entrance rollers 21 of the post-processing device 130. A sheet sensor 27, also called an entrance sensor, is disposed downstream of the entrance rollers 21. When the sheet sensor 27 detects the rear end of the sheet S, the conveying roller pair 22 accelerates the sheet S. When the rear end of the sheet S, whose discharge destination is set to the upper tray 25, arrives between the conveying roller pair 22 and the conveying roller pair 24, the conveying roller pair 22 decelerates. This causes the conveying speed of the sheet S to become a predetermined discharge speed. The conveying roller pair 22 discharges the sheet S to the upper tray 25.

[0030] When the trailing end of the sheet S, the discharge destination of which is set to the lower tray 37, passes through the check valve 23, the conveying roller pair 22 stops conveying the sheet S. After that, the conveying roller pair 22 starts to rotate in the reverse direction. As a result, the sheet S switches back and is conveyed to the conveying roller pair 26. When the sheet sensor 60 provided downstream of the conveying roller pair 26 detects the leading end of the sheet S, the two rollers constituting the conveying roller pair 24 separate. As a result, the conveying roller pair 24 becomes capable of receiving the following sheet S. Furthermore, the conveying roller pair 26 stops with the preceding sheet S sandwiched between them. The conveying roller pair 26 starts to rotate in the reverse direction in response to the arrival of the following sheet S. As a result, the following sheet S is stacked on top of the preceding sheet S. By repeating the switchback of the sheet S by the conveying roller pair 26, a plurality of sheets S are stacked to form a sheet bundle. Such a sheet bundle forming operation may be called a buffer operation. A unit that realizes the buffer operation is called a buffer unit 80.

[0031] When the sheet stack is completed in the buffer unit 80, the conveying roller pair 26 conveys the sheet stack toward the intermediate stacking unit 42. The sheet stack passes through the conveying roller pair 28 and the sheet sensor 50. The sheet stack is further conveyed to the intermediate stacking unit 42 by the kick-out roller 29. A movable vertical alignment plate 39 is disposed in a standby position at the most downstream portion of the intermediate stacking unit 42. The sheet stack is aligned by hitting the vertical alignment plate 39.

[0032] A plurality of sheet bundles are stacked in sequence on the intermediate stacking section 42. As a result, a predetermined number of sheets S that form a booklet are stacked on the intermediate stacking section 42. When the alignment of the predetermined number of sheets S is completed, the thermocompression bonding unit 51 performs a binding operation (thermocompression bonding process) to form a booklet. The vertical alignment plate 39 moves from the standby position to the discharge position, pushing the booklet toward the discharge rollers 38. When the leading edge of the booklet is clamped by the discharge rollers 38, the vertical alignment plate 39 stops and returns to the standby position again. The discharge rollers 38 discharge the booklet received from the vertical alignment plate 39 from the discharge port 46 to the lower tray 37.

[0033] In the above description, the post-processing device 130 forms a sheet bundle consisting of a plurality of sheets S by using the buffer unit 80, and conveys the sheet bundle to the intermediate stacking unit 42. However, a single sheet S may be conveyed to the intermediate stacking unit 42.

[0034] (3) Booklet production operation 3A to 3D show the booklet production operation executed in the intermediate stacking unit 42. In the initial state, the intermediate stacking unit 42 is empty. As an example, a sheet bundle W consisting of five sheets S is conveyed from the buffer unit 80 to the intermediate stacking unit 42.

[0035] The Y direction is a direction parallel to the stacking surface (stack plate) of the sheets S in the intermediate stacking section 42 and parallel to the transport direction of the sheets S transported from the kick-out roller 29 to the intermediate stacking section 42. The Y direction may be called the vertical direction. The X direction is a direction parallel to the stacking surface of the sheets S in the intermediate stacking section 42 and perpendicular to the Y direction. The X direction may be called the horizontal direction. The Z direction is a direction perpendicular to the X and Y directions (the normal direction of the stacking surface, the thickness direction of the stacked sheets S). The Z direction may be called the height direction. The opposite directions of the X, Y, and Z directions may be called the -X, -Y, and -Z directions, respectively.

[0036] The vertical alignment plate 39 and the vertical alignment roller 40 function as a first alignment unit that aligns a plurality of sheets S in a first direction (Y direction). The vertical alignment plate 39 is disposed at the most downstream portion of the intermediate stacking portion 42 in the Y direction. The vertical alignment plate 39 is a reference member (first reference member) that serves as a reference for the sheet position in the Y direction. The vertical alignment roller 40 is a conveying member that conveys the sheet S in the Y direction so that the sheet S is abutted against the vertical alignment plate 39 for alignment. The vertical alignment plate 39 includes a plurality of contact portions 39a to 39c that are spaced apart in the X direction. The plurality of contact portions 39a to 39c come into contact with the end of the sheet S. The vertical alignment plate 39 and the vertical alignment roller 40 are integrally configured as a movable unit 59 that is movable in the Y direction. The movable unit 59 is movable in the Y direction by a driving source such as a motor. That is, the positions of the vertical alignment plate 39 and the vertical alignment roller 40 in the Y direction are adjustable. The lateral alignment joggers 41a to 41c function as a second alignment unit that aligns the sheet in a second direction (X direction) perpendicular to the first direction.

[0037] The lateral alignment joggers 41a to 41c are moved in the X direction by a driving source such as a motor, and press the side edges of the sheets S stacked on the intermediate stacker 42. The lateral alignment plates 72a and 72b are reference members that serve as references for the position of the sheets S in the X direction. The lateral alignment plates 72a and 72b are disposed to face the lateral alignment joggers 41a and 41b in the X direction.

[0038] (3-1) Preparation stage As shown in FIG. 3A, the sheets S1 to S5 are conveyed toward the kick-out roller 29. The sheets S1 to S5 may be conveyed to the intermediate stacking unit 42 in a state in which the lower sheet Si protrudes in the Y direction from the upper sheet Si+1. Here, i is the index of the sheets S. Before the sheets S are stacked on the intermediate stacking unit 42, the vertical alignment plate 39 moves to a predetermined standby position in advance according to the size of the sheet S to be aligned. The standby position is set so that the end position of the sheet S in the -Y direction is constant, regardless of the size of the sheet S. In other words, the standby position is a position where the distance in the Y direction from the nip position of the kick-out roller 29 to the vertical alignment plate 39 is slightly longer than the length of the sheet in the Y direction. The horizontal alignment joggers 41a to 41c wait at a position away from the sheet S being conveyed in the X direction so as not to interfere with the conveyance of the sheet S.

[0039] (3-2) Vertical alignment stage 3(B) shows that the rear end of the first sheet S1 has passed through the nip of the kick-out roller 29, and the leading end of the sheet S1 has reached the vertical alignment roller 40. The sheet S1 abuts against the vertical alignment plate 39 and is aligned based on the position of the vertical alignment plate 39. As the vertical alignment roller 40 continues to rotate, the sheets S2 to S5 which arrive at the vertical alignment roller 40 after the sheet S1 abut against the vertical alignment plate 39 in order. As a result, the five sheets S1 to S5 are aligned in the Y direction (vertical direction) based on the position of the vertical alignment plate 39.

[0040] (3-3) Lateral alignment stage 3C shows that after the alignment of the sheets S1 to S5 in the Y direction (vertical direction) is completed, the alignment in the X direction (horizontal direction) is started. The lateral alignment joggers 41a to 41c are driven in the X direction, which is the alignment direction, to come into contact with the side edges of the sheets S1 to S5 and press the sheets S1 to S5 toward the lateral alignment plates 72a and 72b. Then, the other side edges of the sheets S1 to S5 come into contact with the contact surfaces 300 of the lateral alignment plates 72a and 72b, so that the sheets S1 to S5 are aligned in the X direction (horizontal direction) based on the positions of the lateral alignment plates 72a and 72b.

[0041] (3-4) Adhesion stage (thermocompression stage) 3(D) shows a state where the alignment of the five sheets S1 to S5 in the X and Y directions has been completed. The target position (alignment position) in the alignment operation is the position of the sheet stack W when the adhesive process (thermocompression bonding) is performed by the thermocompression bonding unit 51. As described above, the image forming apparatus 100 applies adhesive toner Tn to the sheets S1 to S5 so that the side on which the adhesive toner image is formed faces the thermocompression bonding unit 51. If the sheet S1 is the cover of a booklet, adhesive toner Tn is not applied.

[0042] The thermocompression bonding unit 51 applies a thermocompression bonding operation to the sheets S1 to S5 for which alignment has been completed. During this time, the lateral alignment joggers 41a to 41c retreat in the -X direction. This allows the intermediate stacking section 42 to be in a state in which it can receive the next plurality of sheets S. After that, the sheet stack W consisting of sheets S6 to S10 generated in the buffer section 80 is stacked on top of the sheets S1 to S5.

[0043] Thereafter, the above-mentioned four steps are repeated for the sheets S1 to S10, so that the sheets S1 to S10 are bonded together in a precisely aligned state.

[0044] As an example, the sheet bundle W is composed of five sheets S. However, the number of sheets S constituting the sheet bundle W may be two or three, etc. In other words, the number of sheets S included in the sheet bundle W may be equal to or less than the maximum number of sheets S that can be stacked in the buffer unit 80.

[0045] (4) Process cartridge FIG. 4 is a cross-sectional view of the process cartridges 7n, 7y, 7m, and 7c. The cleaning blade 400 scrapes off the toner remaining on the surface of the photosensitive drum D and collects it in a collection container 401. The charging roller C charges the surface of the photosensitive drum D while a charging bias is applied from a charging power source 402. The charging roller C is a conductive elastic roller. The charging roller C has a core metal and a conductive elastic layer covering the core metal. The diameter of the core metal of the charging roller C is, for example, 6 mm. The diameter of the conductive elastic layer portion is 12 mm. The charging roller C is pressed against the photosensitive drum D with a predetermined pressing force. An electrostatic latent image is formed on the surface of the photosensitive drum D by irradiating the photosensitive drum D with a laser beam J from the exposure device 2.

[0046] The developing device K has a toner container 411 that contains toner T. An agitating member 412 charges the toner T while agitating it by rotating. A supply roller 413 supplies the toner T held in the toner container 411 to a developing roller 414. A developing bias is applied to the developing roller 414 from a developing power source 403. The developing roller 414 adheres the toner to the photosensitive drum D to form a toner image.

[0047] (5) Photoconductor drum FIG. 5 shows the structure of the photoconductor drums Dn, Dy, Dm, and Dc. The photoconductor drum D has a core metal 501 and a photosensitive layer 502 covering the core metal 501. The photosensitive layer 502 is made up of a surface layer, a charge transfer layer, a charge generation layer, and a base layer. The diameter of the photoconductor drum D is, for example, φ24 mm. The thickness of the photosensitive layer 502 is, for example, 31 um. um is an abbreviation for micrometer. The thickness of the surface layer is, for example, 17 um. When the laser light J exposes the charge generation layer, a positive charge is generated from the charge generation layer. The positive charge passes through the charge transfer layer and moves to the surface layer. This causes the negative charge on the surface of the photoconductor drum D to disappear.

[0048] (6) Wear of the photoconductor drum and density decrease FIG. 6 is a diagram for explaining the potential of the photoconductor drum D during image formation. The horizontal axis indicates time. The vertical axis indicates the surface potential of the photoconductor drum D. When the image formation operation starts, the charging power supply 402 applies a charging bias to the core of the charging roller C. When the potential difference between the surface potential of the photoconductor drum D and the potential of the charging roller C becomes equal to or greater than the discharge start voltage, discharging starts. This causes the surface of the photoconductor drum D to be uniformly charged. The surface potential of the photoconductor drum D becomes the dark potential VD. For example, assuming that the charging bias is -1050V, the dark potential VD becomes -500V (VD reference value).

[0049] When the laser light J is irradiated, the negative charge existing on the surface of the photoconductor drum D disappears. As a result, a bright area potential VL is formed on the surface of the photoconductor drum D. For example, when the amount of laser light (hereinafter referred to as the exposure amount) is 0.2 μJ / cm 2 In this case, the light area potential VL is 100V.

[0050] The developing bias Vdc is about -250V. The surface potential of the area of ​​the surface of the photoconductor drum D where the toner image is formed is the light area potential VL. The potential difference Vcont between the developing bias Vdc and the light area potential VL becomes the electrical force when developing the electrostatic latent image on the photoconductor drum D with toner. Note that Vback shown in FIG. 6 is the potential difference between the dark area potential VD and the developing bias Vdc. Vback is the potential difference for preventing toner from adhering to the unexposed area (toner fogging).

[0051] When the transfer of the toner image is completed, the charging roller C returns the surface potential of the photoconductor drum D from the light potential VL to the dark potential VD. At this time, a discharge current flows from the charging roller C to the photoconductor drum D. Not all of the electrical energy of the discharge current is used to charge the photoconductor drum D. Part of the electrical energy of the discharge current electrically stimulates the polymer material on the surface of the photoconductor drum D. As a result, the molecular bonds of the polymer material become more likely to break. As a result, the surface of the photoconductor drum D becomes more likely to wear down due to friction with the cleaning blade 400, etc.

[0052] When the film thickness of the photoconductor drum D decreases, the dark area potential VD of the photoconductor drum D increases in the negative direction. Accordingly, the light area potential VL of the photoconductor drum D also increases in the negative direction. Meanwhile, the development bias Vdc is constant. As a result, the potential difference Vcont decreases, and the density of the toner image formed on the sheet S decreases.

[0053] In order to maintain a constant image density, it is necessary to keep the light area potential VL constant. This is achieved by changing the amount of exposure in accordance with the amount of fluctuation in the dark area potential VD of the photoconductor drum D.

[0054] FIG. 7(A) shows the variation of the dark potential VD with respect to the amount of film thickness reduction on the photoconductor drum D. Here, the charging bias is constant. FIG. 7(B) shows the exposure amount with respect to the variation of the dark potential VD required to keep the light potential VL constant. FIG. 7(A) shows that the dark potential VD increases in the negative direction as the amount of film thickness reduction increases. FIG. 7(B) shows that the density of the toner image is maintained constant by changing the exposure amount according to the variation of the dark potential VD. Therefore, the density is maintained constant by changing the exposure amount according to the amount of film thickness reduction.

[0055] FIG. 8 shows an example of printing areas 211a-211d in which toner images are formed on an A4 size sheet S. The length of the printing areas 211a-211d in the main scanning direction is all 10 mm. The length of the printing area 211a in the sub-scanning direction is 287 mm. The length of the printing area 211b in the sub-scanning direction is 143.5 mm. The length of the printing area 211c in the sub-scanning direction is 28.7 mm. The printing area 211d is an area in which nothing is actually printed. The printing rates of the printing areas 211a-211d are 100%, 50%, 10%, and 0%, respectively.

[0056] Fig. 7(C) shows the amount of film thickness reduction versus the number of sheets S printed with the toner image shown in Fig. 8 that have been passed through. Fig. 7(C) shows that the amount of film thickness reduction increases in the surface area (exposed area) where a toner image with a higher printing rate is formed. The number of discharges that occur in the surface layer of the photosensitive drum D increases in accordance with the printing rate. Therefore, the higher the printing rate, the greater the amount of film thickness reduction.

[0057] In this way, the more sheets S are printed with an image having a high print rate, the more likely the thickness of the surface layer of the photoconductor drum D is to decrease. For example, in an image forming system 1 equipped with a booklet production device (post-processing device 130), as shown in FIG. 2(A), toner with a high print rate is repeatedly printed at a specific main scanning position (printing area 211). Therefore, the thickness of the surface layer of the photoconductor drum D at the specific main scanning position decreases significantly. As a result, when a toner image with a uniform density in the main scanning direction is printed, there are cases where noticeable density unevenness in the main scanning direction occurs.

[0058] (7) Number of printed pixels and film thickness reduction Image forming apparatus 100 may have a counter (FIG. 13) as a means for retaining the print history. The counter cumulatively counts the number of pixels on which toner is placed in the image to be printed (number of printed pixels (unit: pixel)). The accumulated number of printed pixels is called the cumulative pixel count. The counter's count value is set to zero when photoconductor drum D is new. The counter continues counting until photoconductor drum D is replaced.

[0059] For example, the number of print pixels per sheet S can be calculated as follows. As a condition for calculation, the resolution of the image forming unit 10 is assumed to be 600 dpi (dots per inch). The sheet S is an A4 size paper (297 mm length × 210 mm width). The area on the sheet S where an image can be formed is 287 mm × 200 mm. Therefore, the number of print pixels per sheet S is 4724 × 6780 (= 32,292,264). For example, when the print rate is 50%, the number of print pixels is 16,014,632. When the print rate is 10%, the number of print pixels is 3,202,926.

[0060] FIG. 9A shows an example of a counter. A memory address E is stored for every 1 mm in the main scanning direction of the photosensitive drum D. -100 ~E 100 In this example, the main scanning direction is divided into 201 exposure areas, so there are 201 memory addresses E -100 ~E 100 There are. When the main scanning direction is divided into N exposure areas, N memory addresses are provided. In this way, one memory address is assigned to one exposure area. The counter cumulatively counts the number of printed pixels for each memory address. The cumulative number of pixels correlates with the amount of film thickness reduction. Therefore, the amount of film thickness reduction may be found from the cumulative number of pixels. Note that the allocation of memory addresses is merely one example and can be changed.

[0061] FIG. 10(A) shows the amount of film thickness reduction with respect to the number of printed pixels. This is obtained by converting the print rate in the result of the amount of film thickness reduction shown in FIG. 7(C) into the number of printed pixels. For example, it is assumed that the print rate of the printed area 211a is 100% and the width of the printed area 211a in the main scanning direction is 1 mm, and the number of printed pixels in the printed area 211a is accumulated. The film thickness of the surface layer of the photoconductor drum D in the printed area 211d where no image is printed is adopted as a reference value. The amount of film thickness reduction in the printed area 211a is measured from the difference between the film thickness of the surface layer of the photoconductor drum D in the printed area 211a and the reference value.

[0062] 10A shows that as the cumulative number of pixels increases, the amount of film thickness reduction also increases. This is because discharge occurs on the surface of the photoconductor drum D every time a toner image is printed, and the amount of wear on the photoconductor drum D increases according to the number of printed pixels.

[0063] (8) Exposure correction using the number of printed pixels The image forming apparatus 100 predicts the amount of film thickness reduction from the cumulative number of pixels for each main scanning position (region), and corrects the amount of exposure based on the predicted value so that the density of the printed image is constant. FIG. 10B shows the amount of exposure relative to the cumulative number of pixels to maintain the density of the printed image constant. As described with reference to FIG. 7A, FIG. 7B, and FIG. 10A, the relationship between the cumulative number of pixels and the amount of film thickness reduction, the relationship between the amount of film thickness reduction and the amount of change in the dark potential VD, and the relationship between the amount of exposure and the amount of change in the dark potential VD are taken into consideration. As a result, the amount of exposure Pi according to the cumulative number of pixels Mi in the i-th region is calculated by the following formula.

[0064] Pi = Pa × Mi + Pb =2.2×10 -11 ×Mi + 0.2 ...〔1〕 Here, Pa is the first correction coefficient, and Pb is the second correction coefficient. These correction coefficients may differ depending on the type of toner, the type of photoconductor drum D, the allocation of memory addresses, and the like. Therefore, these correction coefficients may be changed according to related information (e.g., toner type) input by the user. The image forming apparatus 100 can reduce density unevenness in the main scanning direction of the photoconductor drum D by correcting the exposure amount according to the accumulated pixel number for each main scanning position.

[0065] (9) Effects of Example 1 FIG. 11 is a table showing the effects of Example 1. Comparative Example I is a case where the exposure amount is not changed according to the cumulative number of pixels. Comparative Example II is a case where the exposure amount is increased uniformly in the main scanning direction according to the number of printed sheets. The printing conditions were an environmental temperature of 23° C., an environmental humidity of 50% RH, and a resolution of 600 dpi. The sheet S was A4 (length 297 mm x width 210 mm) GFC-081 (high white paper). Here, only the process cartridge 7n was used.

[0066] FIG. 12 shows a test image printed on a sheet S. Memory address E -100 ~E -96In the area corresponding to memory address E, an image with an input gradation value of 255 (maximum value of 8 bits) (hereafter referred to as solid black) was printed. -95 ~E -81 Nothing was printed in the area corresponding to memory address E (hereafter, solid white). -80 ~E 100 A halftone image with an input tone value of 100 (hereinafter referred to as HT(100) image) was printed in the area corresponding to memory address E. Under these conditions, test images were formed on 200,000 sheets S. -100 ~E -96 and the image density of the area corresponding to memory address E -80 ~E 100 The image density of the area corresponding to was measured.

[0067] To evaluate the density change caused by printing, first, reference values ​​were obtained. -95 ~E -81 The image density of each was measured at five points in the area corresponding to the HT(100) image. A Macbeth RD-918 was used as the reflection densitometer. The average value was calculated from the measurements at the five points. The image density of the solid black image was 1.44. The image density of the HT(100) image was 0.80. Using this value as the reference value, each image density was evaluated to see if it was within ±0.02.

[0068] 11, the image density of the solid black image was 1.39 in Comparative Example I. The image density of the HT(100) image was 0.74. Both of these values ​​are lower than the reference values.

[0069] In Comparative Example II 、 The image density of the solid black was 1.45, which was almost the same as the reference value. However, the image density of the HT(100) image was 0.91, which was higher than the reference value. In Comparative Example II, the exposure amount was uniformly corrected at all main scanning positions according to the number of sheets S passed through. Therefore, in the area of ​​the surface of the photoconductor drum D where no toner was placed, the exposure amount was excessive, resulting in a high image density.

[0070] In Example 1, the exposure amount was corrected according to the cumulative number of pixels in each main scanning region. As a result, the image density of the solid black image was 1.44. The image density of the HT(100) image was 0.81. Both of these were equivalent to the reference value.

[0071] In this way, according to the first embodiment, the exposure amount is corrected in accordance with the cumulative pixel count for each main scanning region, thereby making the density of each main scanning region approximately constant and reducing density unevenness of the image in the main scanning direction.

[0072] (10) Control system 13 shows functions realized by a CPU 1301 of the image forming apparatus 100. The CPU 1301 realizes various functions by executing a control program stored in a memory 1302. A communication circuit 1303 communicates with a host computer, an image reader, or the like, and receives image data. An image analysis unit 1311 analyzes the input image data, obtains the number of print pixels for each of the N main scanning regions, and adds the number of print pixels to the cumulative number of pixels held in a counter 1307. The counter 1307 is, for example, a storage area secured in the memory 1302. The cumulative number of pixels Mi for the i-th main scanning region is stored in a memory address Ei.

[0073] The image processing unit 1312 converts the input image data into bitmap data and converts the color space to generate an image signal (exposure signal) to be supplied to the exposure device 2. The dithering unit 1313 executes dithering processing on the image data.

[0074] The operation unit 1304 includes an input device and a display device. The coefficient setting unit 1314 sets the first correction coefficient Pa and the second correction coefficient Pb in the determination unit 1317 based on information input from the operation unit 1304. As described above, the coefficient setting unit 1314 may receive the first correction coefficient Pa and the second correction coefficient Pb from the operation unit 1304. The coefficient setting unit 1314 may determine the first correction coefficient Pa and the second correction coefficient Pb according to the type of toner input from the operation unit 1304. As described later, the coefficient setting unit 1314 may determine the first correction coefficient Pa and the second correction coefficient Pb according to environmental conditions (e.g., environmental temperature, environmental humidity) detected by the environmental sensor 1305.

[0075] The equation selection unit 1315 is used in other embodiments and selects an equation for calculating the exposure amount based on environmental conditions, etc. The HP sensor 1306 is used in other embodiments and detects the absolute position (home position) of the photoconductor drum D in the sub-scanning direction. The position monitoring unit 1316 is also used in other embodiments and identifies the coordinates of the surface area of ​​the photoconductor drum D based on a signal (vertical synchronization signal) output from the HP sensor 1306 and a BD signal (horizontal synchronization signal) output from the exposure device 2. In the first embodiment, the position monitoring unit 1316 informs the determination unit 1317 and the exposure control unit 1320 of the current exposure area (main scanning area in the first embodiment).

[0076] A determination unit 1317 determines an exposure amount Pi for each main scanning region based on the cumulative pixel number Mi. For example, a calculation unit 1318 calculates an exposure amount Pi in the i-th main scanning region based on a first correction coefficient Pa, a second correction coefficient Pb, and the cumulative pixel number Mi. A selection unit 1319 is used in other embodiments, and determines the content of dither processing to be applied to the i-th main scanning region according to the cumulative pixel number Mi, and sets it in the dither unit 1313.

[0077] The exposure control unit 1320 controls the exposure device 2 so as to expose the i-th main scanning region of the photoconductor drum D with the exposure amount Pi designated by the determination unit 1317. The CPU 1301 also controls the charging power supply 402 and the developing power supply 403.

[0078] (11) Flowchart (11-1) Counting the cumulative number of pixels 14 shows the counting process executed by the CPU 1301 according to the control program. In S1401, the CPU 1301 (image analysis unit 1311) obtains the number of print pixels for each exposure area (main scanning area in the first embodiment) from the image data. In S1402, the CPU 1301 (image analysis unit 1311) adds the number of print pixels for each exposure area to the cumulative number of pixels held in the counter 1307.

[0079] (11-2) Exposure control 15 shows exposure control executed by the CPU 1301 according to a control program. In S1501, the CPU 1301 (coefficient setting unit 1314) acquires a first correction coefficient Pa and a second correction coefficient Pb. In S1502, the CPU 1301 (position monitoring unit 1316) identifies an exposure area (main scanning area) in the main scanning direction.

[0080] In S1503, the CPU 1301 (determination unit 1317) acquires the cumulative pixel count of the identified exposure area from the counter 1307. In S1504, the CPU 1301 (determination unit 1317) determines the exposure condition based on the first correction coefficient Pa, the second correction coefficient Pb, and the cumulative pixel count. The exposure condition is, for example, an exposure amount Pi determined using equation (1).

[0081] In S1505, the CPU 1301 (exposure control unit 1320) exposes the specified exposure area according to the exposure conditions. Note that S1501 to S1505 are repeatedly executed while a toner image is being formed based on image data.

[0082] <Example 2> (1) Basic Concept of Example 2 In the second embodiment, matters common to the first embodiment are given the same reference numerals as in the first embodiment, and the description thereof will be omitted. The matters not described in the second embodiment are the same as those in the first embodiment.

[0083] In the first embodiment, the exposure amount is determined for each main scanning region using the cumulative pixel number Mi. As shown in FIG. 2A, in the case where an image for adhesion is always drawn in a specific print region 211 in the main scanning direction, the first embodiment is effective. This is because wear (thickness reduction) of the surface layer occurs almost uniformly in the sub-scanning direction of the photosensitive drum D. However, when images are continuously printed at a period of about 76 mm similar to the outer circumferential length of the photosensitive drum D, wear may be accelerated only in a part of the sub-scanning direction of the photosensitive drum D. Therefore, in the second embodiment, memory addresses are allocated to both the main scanning direction and the sub-scanning direction of the photosensitive drum D. That is, the peripheral surface (surface) of the photosensitive drum D is divided into a plurality of exposure regions arranged in a matrix, and the cumulative pixel number for each exposure region is stored in the counter 1307. As a result, even if the thickness reduction occurs in a specific exposure region in the sub-scanning direction, density unevenness is reduced over the entire surface of the sheet S.

[0084] 9B shows that memory addresses are allocated every 1 mm in both the main scanning direction and the sub-scanning direction of the photoconductor drum D. Multiple exposure areas, each having a size of 1 mm x 1 mm, are specified by coordinates (x, y) consisting of the main scanning position x and the sub-scanning position y. Ex,y is the memory address where the cumulative pixel count Mx,y of the exposure area specified by the coordinates (x, y) is stored.

[0085] The image analysis unit 1311 obtains the number of print pixels at the coordinates (x, y) by analyzing the image data, and adds this to the cumulative number of pixels Mx, y held in the counter 1307 .

[0086] The position monitoring unit 1316 identifies the coordinates (x, y) of the exposure area based on the signal from the HP sensor 1306 and the BD signal from the exposure device 2. The calculation unit 1318 of the determination unit 1317 calculates the exposure amount Px, y at the coordinates (x, y) from the accumulated pixel number Mx, y based on the equation (1). Note that the exposure amount Pi in the equation (1) is replaced with the exposure amount Px, y. The accumulated pixel number Mi is replaced with the accumulated pixel number Mx, y. The exposure control unit 1320 exposes the exposure area at the coordinates (x, y) based on the exposure amount Px, y determined by the determination unit 1317.

[0087] (2) Explanation of the Effects of Example 2 In Example 2, an experiment is performed using the same experimental conditions as in Example 1. FIG. 16 shows a test image formed on a sheet S. The image forming apparatus 100 starts printing an HT(100) image, which is always a solid image (value=255) and a halftone image (value=100), from a specific position (home position) in the sub-scanning direction. FIG. 17 shows the period of the photosensitive drum D with respect to the sheet S. Di indicates the position on the sheet S corresponding to the i-th rotation of the photosensitive drum D. i is an integer from 1 to 4.

[0088] Memory address E of counter 1307 -100,-38 From E -80,-18 The memory address E stores the cumulative number of pixels in the exposure area A. -100,0 From E -80,20 Memory address E stores the cumulative number of pixels in exposure area B. 80,-38 From E 100,-18 The memory address E stores the cumulative number of pixels in the exposure area C. 80,0 From E 100,20 stores the cumulative number of pixels in the exposure area D.

[0089] In exposure area A, two solid black images are printed per sheet S. In exposure area B, one solid black image is printed per sheet S. Because the number of solid black images printed varies depending on the position in the sub-scanning direction, the cumulative number of pixels varies for each exposure area in the sub-scanning direction. In exposure area C, two HT(100) images are printed per sheet S. In exposure area D, one HT(100) image is printed per sheet S.

[0090] Test images were formed on 400,000 sheets S, and the image density of each exposure area was measured. The densitometer, statistical method, and evaluation method were the same as in Example 1. According to Example 2, it was confirmed that the image density of each of the exposure areas ABCD was approximately the same as the reference value.

[0091] (3) Flowchart 18 shows an exposure control method executed by the CPU 1301 in accordance with a control program. In S1801, the CPU 1301 (coefficient setting unit 1314) acquires a first correction coefficient Pa and a second correction coefficient Pb. In S1802, the CPU 1301 (position monitoring unit 1316) identifies an exposure area based on the main scanning position x and the sub-scanning position y. In other words, the coordinates (x, y) of the exposure area to be processed are identified.

[0092] In S1803, the CPU 1301 (decision unit 1317) acquires the cumulative pixel count of the identified exposure area from the counter 1307. In S1804, the CPU 1301 (decision unit 1317) decides the exposure condition based on the first correction coefficient Pa, the second correction coefficient Pb, and the cumulative pixel count. The exposure condition is, for example, the exposure amount Px,y.

[0093] In S1805, the CPU 1301 (exposure control unit 1320) exposes the specified exposure area according to the exposure conditions. Note that S1801 to S1805 are repeatedly executed while a toner image is being formed based on image data.

[0094] According to the second embodiment, memory addresses are assigned to combinations of main scanning position x and sub-scanning position y on the photoconductor drum D. The peripheral surface of the photoconductor drum D is divided into a plurality of exposure areas in a matrix, and the cumulative number of pixels is stored for each exposure area. Then, exposure conditions are determined for each exposure area. This reduces density unevenness not only in the main scanning direction but also in the sub-scanning direction.

[0095] In the first embodiment, the main scanning direction is divided into a plurality of exposure areas, and in the second embodiment, the entire surface of the photoconductor drum D is divided into a plurality of exposure areas. However, this is merely one example. The ideas of the first and second embodiments are also applicable to the case where the sub-scanning direction is divided into a plurality of exposure areas (sub-scanning areas). In other words, the cumulative number of pixels may be stored for each sub-scanning area, and the exposure amount of each sub-scanning area may be determined according to the cumulative number of pixels.

[0096] <Example 3> (1) Basic Concept of Example 3 In the third embodiment, matters common to the first or second embodiment are given the same reference numerals as in the first or second embodiment, and the description thereof will be omitted. The matters not described in the third embodiment are the same as those in the first or second embodiment.

[0097] The environmental conditions in Examples 1 and 2 are assumed to be constant (temperature = 23°C, humidity = 50% RH). However, the actual environmental conditions may change. For example, the environmental conditions may be low temperature and low humidity (e.g., temperature = 15°C, humidity = 10% RH) or high temperature and high humidity (e.g., temperature = 30°C, humidity = 80% RH). The discharge voltage of the photoconductor drum D depends on the environmental conditions. Therefore, the amount of film thickness reduction relative to the cumulative number of pixels may change depending on the environmental conditions.

[0098] The amount of film thickness reduction relative to the cumulative number of pixels was measured under three different environmental conditions. The normal condition (hereafter referred to as NN) is an environmental temperature of 23°C and an environmental humidity of 50%RH. The low temperature and low humidity condition (hereafter referred to as LL) is an environmental temperature of 15°C and an environmental humidity of 10%RH. The high temperature and high humidity condition (hereafter referred to as HH) is an environmental temperature of 30°C and an environmental humidity of 80%RH. The test image shown in Figure 8 was printed, and the amount of film thickness reduction relative to the cumulative number of pixels was measured.

[0099] Figure 19(A) shows the amount of film thickness reduction relative to the cumulative number of pixels according to different environmental conditions. In the LL environment, the amount of film thickness reduction relative to the cumulative number of pixels is greater than in the NN environment. When the absolute humidity calculated from the environmental temperature and relative humidity decreases, the discharge voltage in the photoconductor drum D increases, and the surface layer of the photoconductor drum D becomes more susceptible to wear. On the other hand, in the HH environment, the amount of film thickness reduction relative to the cumulative number of pixels is smaller than in the NN environment.

[0100] FIG. 19B shows the exposure amount required to keep the density of the print image constant for the cumulative pixel count for each environmental condition. As shown in FIG. 19B, the exposure amount according to the cumulative pixel count depends on the environmental condition. For example, the memory 1302 may store a plurality of correction formulas (equations) for calculating the exposure amount from the cumulative pixel count. Each of the plurality of equations is previously associated with a different environmental condition (e.g., environmental temperature and environmental humidity). The CPU 1301 (equation selection unit 1315) selects an equation corresponding to a combination of the environmental temperature and environmental humidity acquired by the environmental sensor 1305 from the plurality of equations. The calculation unit 1318 determines the exposure condition by substituting the cumulative pixel count for each exposure area into the selected equation.

[0101] (2) Explanation of the Effects of Example 3 Printing was performed in the NN, LL, and HH environments under the same experimental conditions as in Example 1, and the effect of Example 3 was confirmed. As a result, it was confirmed that the image density of the solid black image and the image density of the HT(100) image were each within ±0.02 of the reference value in Example 3 as well.

[0102] (3) Flowchart 20 shows an exposure control method executed by the CPU 1301 according to a control program. In S2001, the CPU 1301 acquires environmental conditions using the environmental sensor 1305. In S2002, the CPU 1301 (equation selection unit 1315) acquires a first correction coefficient Pa and a second correction coefficient Pb corresponding to the environmental conditions. Note that if the equation already contains correction coefficients, S2002 is unnecessary. In S2003, the CPU 1301 (position monitoring unit 1316) identifies an exposure area. The method of identifying the exposure area may be either of the first and second embodiments.

[0103] In S2004, the CPU 1301 (determination unit 1317) acquires the cumulative pixel count of the identified exposure region from the counter 1307. In S2005, the CPU 1301 (determination unit 1317) determines the exposure condition based on the equation selected by the equation selection unit 1315 and the cumulative pixel count. The exposure condition is, for example, the exposure amount Pi or the exposure amount Px,y.

[0104] In S2006, the CPU 1301 (exposure control unit 1320) exposes the specified exposure area according to the exposure conditions. Note that S2001 to S2006 are repeatedly executed while a toner image is being formed based on image data.

[0105] According to the third embodiment, the exposure conditions are determined from the cumulative number of pixels using an equation according to the environmental conditions, which reduces uneven density of an image even if the environmental conditions change.

[0106] <Example 4> In the first to third embodiments, the exposure conditions are determined for each exposure area according to the cumulative pixel count. Alternatively, the image processing conditions (e.g., dither processing) may be changed for each exposure area according to the cumulative pixel count. The image processing conditions may be any conditions that affect the image density.

[0107] 21 shows an exposure control method executed by the CPU 1301 in accordance with a control program. In S2101, the CPU 1301 (position monitoring unit 1316) specifies an exposure area. The method for specifying the exposure area may be either the method in the first or second embodiment.

[0108] In S2102, the CPU 1301 (decision unit 1317) obtains the cumulative pixel count of the identified exposure area from the counter 1307. In S2103, the CPU 1301 (selection unit 1319) decides the image processing conditions to be applied to the image data of the identified exposure area based on the cumulative pixel count. The image processing conditions may be, for example, the contents of dither processing (e.g., error diffusion method, other dither methods).

[0109] In S2104, the CPU 1301 (dither unit 1313) applies image processing (for example, dither processing) according to the determined image processing conditions to the image data.

[0110] In S2105, the CPU 1301 (exposure control unit 1320) exposes the specified exposure area based on the image data. Note that S2101 to S2105 are repeatedly executed while the toner image is being formed based on the image data.

[0111] <Other> The CPU 1301 may determine whether to apply any of the first to fourth embodiments in response to a user instruction input from an operation unit 1304. That is, the CPU 1301 may switch between a normal mode in which correction is not performed and a correction mode in which correction is performed.

[0112] <Technical ideas derived from examples> (Item 1) A photoconductor that is rotated (e.g., photoconductor drum D), A charging means (e.g., a charging roller) for charging the surface of the photoconductor; An exposure means (e.g., exposure device 2) for exposing the surface of the photoconductor to light to form an electrostatic latent image; A developing means (e.g., a developing device K) for developing the electrostatic latent image with toner to form a toner image; A transfer means (e.g., a transfer unit 3) for transferring the toner image onto a sheet; A storage means (e.g., a memory) for storing a cumulative image formation amount (e.g., cumulative pixel numbers Mi, Mx, y) for each of the N exposure regions on the surface of the photoconductor; A determination unit (e.g., CPU 1301, determination unit 1317) that determines an exposure condition for each of the N exposure regions based on the accumulated image formation amount; A control means (e.g., CPU 1301, exposure control unit 1320) that controls the exposure means so as to expose each of the N exposure regions according to the exposure conditions; According to this embodiment, since the exposure conditions are determined based on the accumulated image formation amount for each exposure area, density unevenness of the toner image is reduced more than in the past.

[0113] (Item 2) 2. The image forming apparatus according to item 1, wherein the determining unit determines the exposure condition for each of the N exposure regions such that the greater the accumulated image formation amount, the greater the amount of exposure.

[0114] In this way, as the cumulative amount of image formation increases, the density of the toner image decreases. Therefore, by increasing the amount of exposure, the density of the toner image is maintained and density unevenness is reduced.

[0115] (Item 3) 3. The image forming apparatus according to item 1 or 2, wherein the N exposure areas are aligned along a first direction parallel to a rotation axis of the photoconductor.

[0116] This reduces density unevenness in the first direction.

[0117] (Item 4) 3. The image forming apparatus according to item 1 or 2, wherein the N exposure areas are aligned along a first direction parallel to a rotation axis of the photoconductor and a second direction perpendicular to the first direction.

[0118] This reduces density unevenness in the second direction.

[0119] (Item 5) 3. The image forming apparatus according to item 1 or 2, wherein the N exposure areas are arranged in a matrix on the surface of the photoconductor.

[0120] As a result, density unevenness over almost the entire surface of the sheet S is reduced.

[0121] (Item 6) The image forming apparatus further includes a detection unit (e.g., an environmental sensor 1305) for detecting an environmental condition of an environment in which the image forming apparatus is installed. 3. The image forming apparatus according to claim 1, wherein the determining unit determines the exposure conditions for each of the N exposure regions based on the accumulated image formation amount and the environmental conditions detected by the detecting unit.

[0122] This reduces density unevenness even when environmental conditions fluctuate.

[0123] (Item 7) Further comprising a storage means (e.g., memory 1302) storing a plurality of correction formulas (e.g., equations) each of which is associated one-to-one with a plurality of different environmental conditions; 7. The image forming apparatus according to claim 6, wherein the determination means selects a correction formula from among the plurality of correction formulas that corresponds to the environmental conditions detected by the detection means, and determines the exposure conditions for each of the N exposure areas using the selected correction formula.

[0124] This will allow density unevenness to be reduced more accurately.

[0125] (Item 8) 7. The image forming apparatus according to claim 6, wherein the environmental conditions include at least one of temperature and humidity.

[0126] Temperature and humidity affect the discharge current generated on the surface of the photoconductor drum D. In other words, temperature and humidity affect the amount of film thickness reduction on the photoconductor drum D. Therefore, density unevenness can be reduced more accurately by selecting a correction formula taking into account at least one of temperature and humidity.

[0127] (Item 9) The image forming apparatus according to item 1, wherein the determination means multiplies the accumulated image formation amount by a first correction coefficient (e.g., Pa) for each of the N exposure areas to obtain a product, and adds a second correction coefficient (e.g., Pb) to the product to determine the exposure amount under the exposure conditions.

[0128] This will allow density unevenness to be reduced more accurately.

[0129] (Item 10) 10. The image forming apparatus according to item 9, wherein the first correction coefficient and the second correction coefficient are determined according to the type of the toner.

[0130] The types of toner are mainly classified according to the material, such as polyester resin, vinyl resin, acrylic resin, and styrene-acrylic resin. In this way, by determining the exposure conditions according to the type of toner, it is possible to more accurately reduce density unevenness.

[0131] (Item 11) 10. The image forming apparatus according to item 9, wherein the first correction coefficient and the second correction coefficient are determined according to a type of the photoconductor.

[0132] The type of photoconductor includes, for example, the material and thickness of the photosensitive layer. By determining the exposure conditions taking these factors into consideration, density unevenness can be reduced more accurately.

[0133] (Item 12) 10. The image forming apparatus according to item 9, wherein the first correction coefficient and the second correction coefficient are determined according to N, which is the number of the exposure regions.

[0134] In this way, by determining the exposure conditions according to the division number (N) of the exposure area, density unevenness can be reduced more accurately.

[0135] (Item 13) 10. The image forming apparatus according to item 9, further comprising an input unit (eg, an operation unit 1304) for receiving input of the first correction coefficient and the second correction coefficient.

[0136] In this manner, the user may input or adjust the correction factor. The user input may be a numerical value itself, or may be an instruction to increase or decrease the numerical value.

[0137] (Item 14) A photoconductor that is rotated; A charging means for charging the surface of the photoreceptor; an exposure unit for exposing the surface of the photoreceptor to light to form an electrostatic latent image; a developing means for developing the electrostatic latent image with a toner to form a toner image; a transfer means for transferring the toner image to a sheet; a storage means for storing thickness information relating to the thickness of the photosensitive layer in the N exposed regions on the surface of the photoconductor or the amount of reduction in the thickness; A determination means for determining an exposure condition based on the film thickness information of each of the N exposure regions; a control means for controlling the exposure means so as to expose each of the N exposure areas according to the exposure conditions; An image forming apparatus comprising:

[0138] In the first to fourth embodiments, the cumulative number of images for each exposure region is stored in the memory 1302. The cumulative number of images is also film thickness information related to the film thickness or the film thickness reduction amount. For example, the film thickness or the film thickness reduction amount may be calculated from the cumulative number of images, and the film thickness or the film thickness reduction amount for each exposure region may be stored in the memory 1302. In this case, the formula (1) is replaced with an equation (correction formula) in which the film thickness or the film thickness reduction amount is input. In this way, density unevenness may be reduced by determining the exposure conditions for each of the N exposure regions individually.

[0139] (Item 15) A photoconductor that is rotated; A charging means for charging the surface of the photoreceptor; an exposure unit for exposing the surface of the photoreceptor to light to form an electrostatic latent image; a developing means for developing the electrostatic latent image with a toner to form a toner image; a transfer means for transferring the toner image to a sheet; a storage means for storing an accumulated image formation amount for each of the N exposure areas on the surface of the photoconductor; A determination unit (e.g., CPU 1301, selection unit 1319) that determines the content of image correction based on the accumulated image formation amount for each of the N exposure regions; an image correction unit (e.g., CPU 1301, image processing unit 1312) that corrects image data corresponding to the N exposure areas according to the content of the image correction determined by the determination unit for each of the N exposure areas; An image forming apparatus comprising:

[0140] In this way, density unevenness may be reduced by individually selecting image processing conditions for each exposure area based on the cumulative number of pixels.

[0141] (Item 16) Item 16. The image forming apparatus according to item 15, wherein the image correction includes dithering.

[0142] Dithering is an image process related to image density, so density unevenness may be reduced by selecting a dithering process for each exposure area individually based on the cumulative pixel count.

[0143] (Item 17) an image forming apparatus for forming a toner image as a user image prepared by a user and an adhesive toner image as an adhesive; and a booklet producing device (e.g., a post-processing device 130) that stacks a plurality of sheets discharged from the image forming device to form a sheet bundle, heats the adhesive toner image on the sheet bundle to remelt the adhesive toner image, and pressurizes the sheet bundle to adhere the sheet bundle to produce a booklet, 17. An image forming system, wherein the image forming apparatus is the image forming apparatus according to any one of items 1 to 16.

[0144] 2A and the like, the adhesive toner image is likely to be formed at a specific position. Therefore, density unevenness is likely to become apparent. According to the first to fourth embodiments, the density unevenness is reduced even in such an image forming system 1.

[0145] The invention is not limited to the above-described embodiments, and various modifications and variations are possible without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0146] D...photosensitive drum, C...charging roller, 2...exposure device, K...developing device, 30...intermediate transfer body, 1301...CPU, 1302...memory

Claims

1. A photoconductor that is rotated; A charging means for charging the surface of the photoreceptor; an exposure unit for exposing the surface of the photoreceptor to light to form an electrostatic latent image; a developing means for developing the electrostatic latent image with a toner to form a toner image; a transfer means for transferring the toner image to a sheet; a storage means for storing an accumulated image formation amount for each of the N exposure areas on the surface of the photoconductor; a determining means for determining an exposure condition for each of the N exposure regions based on the accumulated image formation amount; a control means for controlling the exposure means so as to expose each of the N exposure areas in accordance with the exposure conditions; An image forming apparatus comprising:

2. 2. The image forming apparatus according to claim 1, wherein the determining unit determines the exposure conditions for each of the N exposure regions such that the amount of exposure increases as the accumulated amount of image formation increases.

3. The image forming apparatus according to claim 1 , wherein the N exposure areas are aligned along a first direction parallel to a rotation axis of the photoconductor.

4. The image forming apparatus according to claim 1 , wherein the N exposure regions are arranged along a first direction parallel to a rotation axis of the photoconductor and a second direction perpendicular to the first direction.

5. 2. The image forming apparatus according to claim 1, wherein the N exposure regions are arranged in a matrix on the surface of the photoconductor.

6. The image forming apparatus further includes a detection unit for detecting an environmental condition of an environment in which the image forming apparatus is installed, 2. The image forming apparatus according to claim 1, wherein said determining means determines said exposure conditions for each of said N exposure regions based on said accumulated image formation amount and said environmental conditions detected by said detecting means.

7. Further comprising a storage means for storing a plurality of correction formulas each associated with a different plurality of environmental conditions in a one-to-one relationship; 7. The image forming apparatus according to claim 6, wherein the determination means selects a correction formula from among the plurality of correction formulas that corresponds to the environmental condition detected by the detection means, and determines the exposure conditions for each of the N exposure areas using the selected correction formula.

8. The image forming apparatus according to claim 6 , wherein the environmental conditions include at least one of temperature and humidity.

9. 2. The image forming apparatus according to claim 1, wherein the determining unit determines the exposure amount under the exposure conditions by multiplying the accumulated image formation amount by a first correction coefficient for each of the N exposure areas to obtain a product, and adding a second correction coefficient to the product.

10. The image forming apparatus according to claim 9 , wherein the first correction coefficient and the second correction coefficient are determined according to the type of the toner.

11. The image forming apparatus according to claim 9 , wherein the first correction coefficient and the second correction coefficient are determined in accordance with a type of the photoconductor.

12. The image forming apparatus according to claim 9 , wherein the first correction coefficient and the second correction coefficient are determined in accordance with N, which is the number of the exposure regions.

13. The image forming apparatus according to claim 9 , further comprising an input unit that receives input of the first correction coefficient and the second correction coefficient.

14. A photoconductor that is rotated; A charging means for charging the surface of the photoreceptor; an exposure unit for exposing the surface of the photoreceptor to light to form an electrostatic latent image; a developing means for developing the electrostatic latent image with a toner to form a toner image; a transfer means for transferring the toner image to a sheet; a storage means for storing thickness information relating to the thickness of the photosensitive layer in the N exposed regions on the surface of the photoconductor or the amount of reduction in the thickness; a determination means for determining an exposure condition based on the film thickness information of each of the N exposure regions; a control means for controlling the exposure means so as to expose each of the N exposure areas in accordance with the exposure conditions; An image forming apparatus comprising:

15. A photoconductor that is rotated; A charging means for charging the surface of the photoreceptor; an exposure unit for exposing the surface of the photoreceptor to light to form an electrostatic latent image; a developing means for developing the electrostatic latent image with a toner to form a toner image; a transfer means for transferring the toner image to a sheet; a storage means for storing an accumulated image formation amount for each of the N exposure areas on the surface of the photoconductor; a determination unit for determining the content of image correction for each of the N exposure regions based on the accumulated image formation amount; an image correction means for correcting image data corresponding to the N exposure areas in accordance with the content of the image correction determined by the determination means for each of the N exposure areas; An image forming apparatus comprising:

16. The image forming apparatus according to claim 15 , wherein the image correction includes dithering.

17. an image forming apparatus for forming a toner image as a user image prepared by a user and an adhesive toner image as an adhesive; a booklet producing device that forms a sheet bundle by stacking a plurality of sheets discharged from the image forming device, heats the adhesive toner image on the sheet bundle to remelt the adhesive toner image, and pressurizes the sheet bundle to adhere the sheet bundle to produce a booklet, An image forming system, wherein the image forming apparatus is the image forming apparatus according to claim 1 .

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