Image forming apparatus

Abstract

To provide an image forming apparatus that can prevent the occurrence of density unevenness while reducing manufacturing cost.SOLUTION: A control unit recognizes the cumulative number of rotations of an image carrier, and a correction image forming mode is executed until the cumulative number of rotations reaches a predetermined threshold. A standard image forming mode is executed after the cumulative number of rotations of the image carrier reaches the threshold. A correction surface potential at the start of execution of the correction image forming mode, is higher than a standard surface potential at the start of execution of the standard image forming mode. The frequency of an AC component of a developing bias in the correction image forming mode, is lower than the frequency of the AC component of the developing bias in the standard image forming mode.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to an image forming apparatus.

Background Art

[0002] Conventional image forming apparatuses include an image forming unit, a developing bias power supply, a charging bias power supply, and a control unit. The image forming unit includes an image carrier, a charging device, an exposure device, and a developing device. The image carrier has a photosensitive layer formed on its surface. The charging device is disposed opposite the image carrier and has a charging member for charging the image carrier. The exposure device forms an electrostatic latent image by exposing the image carrier charged by the charging device. The developing device is disposed opposite the image carrier and has a developer carrier for carrying a developer, and forms a toner image by attaching toner to the electrostatic latent image formed on the image carrier.

[0003] The developing bias power supply applies a developing bias to the developer carrier. The charging bias power supply applies a voltage to the charging member. The control unit controls the image forming unit, the developing bias power supply, and the charging bias power supply.

[0004] In the image forming apparatus of Patent Document 1, the casing that houses the charging member is covered with a metal shield member and is in close contact with the shield member. Further, in the image forming apparatus of Patent Document 2, the developing bias power supply is covered and hidden from the charging member by a conductive member.

[0005] Thereby, the influence of the electric field of the developing bias applied by the developing bias power supply on the charging bias applied by the charging bias power supply can be suppressed. Therefore, unevenness in the surface potential of the image carrier can be reduced, and the occurrence of density unevenness (stained image) can be suppressed.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] In the configurations of Patent Document 1 and Patent Document 2, there was a problem that the number of parts increased and the manufacturing cost rose.

[0008] In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of suppressing the occurrence of density unevenness while suppressing the manufacturing cost.

Means for Solving the Problems

[0009] To achieve the above object, a first configuration of the present invention is an image forming apparatus including an image forming unit, a developing bias power supply, a charging bias power supply, and a control unit. The image forming unit includes an image carrier, a charging device, an exposure device, and a developing device. The image carrier has a photosensitive layer formed on its surface. The charging device is disposed opposite to the image carrier and has a charging member for charging the image carrier. The exposure device forms an electrostatic latent image by exposing the image carrier charged by the charging device. The developing device is disposed opposite to the image carrier and has a developer carrier for carrying a developer, and forms a toner image by attaching toner to the electrostatic latent image formed on the image carrier. The developing bias power supply applies a developing bias obtained by superimposing an AC voltage on a DC voltage to the developer carrier. The charging bias power supply applies a charging bias including a DC voltage to the charging member. The control unit controls the image forming unit, the developing bias power supply, and the charging bias power supply. The control unit is capable of executing a standard image forming mode and a correction image forming mode. In the standard image forming mode, the developer carrier is charged with a predetermined standard surface potential as a target to perform image formation. In the correction image forming mode, the developer carrier is charged with a correction surface potential different from the standard surface potential as a target to perform image formation. The control unit recognizes the cumulative rotation number of the image carrier, and the correction image forming mode is executed until the cumulative rotation number of the image carrier reaches a predetermined threshold value. The standard image forming mode is executed after the cumulative rotation number of the image carrier reaches the threshold value. The correction surface potential at the start of execution of the correction image forming mode is higher than the standard surface potential at the start of execution of the standard image forming mode. The frequency of the AC component of the developing bias in the correction image forming mode is lower than the frequency of the AC component of the developing bias in the standard image forming mode.

Effects of the Invention

[0010] According to the first configuration of the present invention, it is possible to provide an image forming apparatus capable of suppressing the occurrence of density unevenness while suppressing the manufacturing cost.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing the internal structure of the image forming apparatus 100 according to the first embodiment of the present invention, and FIG. 2 is a partial enlarged view of the periphery of the image forming unit Pa including the control path of the image forming apparatus 100. Note that the configurations of the image forming units Pb to Pd and the control paths of the charging devices 2a to 2d are the same as those of the image forming unit Pa and the control path of the charging device 2a, and thus the description thereof is omitted.

[0013] The image forming apparatus 100 includes image forming units Pa to Pd, primary transfer rollers (transfer members) 6a to 6d, a developing bias power source 43, a charging bias power source 45, a transfer voltage power source 47, current detection units 43 and 46, a main control unit (control unit) 80, and a detection sensor 48.

[0014] In the image forming apparatus 100 (here, a color printer) body, four image forming units Pa, Pb, Pc, and Pd are arranged in order from the upstream side (the left side in FIG. 1) in the conveyance direction. The image forming units Pa to Pd are provided corresponding to images of four different colors (cyan, magenta, yellow, and black), and cyan, magenta, yellow, and black images are sequentially formed by the respective steps of charging, exposure, development, and transfer.

[0015] The image forming units Pa to Pd include primary transfer rollers (transfer members) 6a to 6d, photosensitive drums (image carriers) 1a to 1d that carry visible images (toner images) of respective colors, charging devices 2a to 2d, an exposure device 5, and developing devices 3a to 3d. Further, an intermediate transfer belt (transfer body) 8 that rotates in the counterclockwise direction in FIG. 1 by a driving means (not shown) is provided adjacent to each of the image forming units Pa to Pd.

[0016] The primary transfer rollers (transfer members) 6a to 6d are disposed to face the photosensitive drums (image carriers) 1a to 1d, and a predetermined transfer voltage is applied to transfer the visible images (toner images) of respective colors formed on the photosensitive drums (image carriers) 1a to 1d to the intermediate transfer belt (transfer body) 8. Thereby, the toner images formed on the photosensitive drums 1a to 1d are sequentially primary transferred and superimposed on the intermediate transfer belt 8 while moving in contact with the respective photosensitive drums 1a to 1d.

[0017] The toner image primary-transferred onto the intermediate transfer belt 8 is secondary-transferred onto a sheet S as an example of a recording medium by a secondary transfer roller 9. The sheet S onto which the toner image is secondary-transferred is housed in a paper cassette 16 arranged at the lower part of the main body of the image forming apparatus 100. The sheet S is conveyed to the nip portion between the secondary transfer roller 9 and the driving roller 11 of the intermediate transfer belt 8 via a paper feed roller 12a and a resist roller pair 12b.

[0018] A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a seamless belt having no seam is mainly used. Further, a blade-shaped belt cleaner 19 for removing toner and the like remaining on the surface of the intermediate transfer belt 8 is arranged on the downstream side of the secondary transfer roller 9.

[0019] The photoreceptor drums (image carriers) 1a to 1d have a photosensitive layer 111 formed on their surfaces (see FIG. 2). In the present embodiment, the photoreceptor drums (image carriers) 1a to 1d have the photosensitive layer 111 formed on the surface of an aluminum cylinder, and the photosensitive layer 111 is formed by vapor-depositing amorphous silicon, which is a positively chargeable photoconductor. That is, the photosensitive layer 111 is a single-layer type for static charge. The photosensitive layer 111 preferably has a film thickness of 37 μm or more in an unused state. By increasing the film thickness, the durability of the photoreceptor drums 1a to 1d against wear is improved.

[0020] The charging devices 2a to 2d each have a charging roller (charging member) 34. Each charging roller 34 is arranged to face the photoreceptor drum (image carrier) 1a to 1d and charges the photoreceptor drum (image carrier) 1a to 1d. The charging roller 34 is formed, for example, by coating a core metal with an elastic material having conductivity, an epichlorohydrin rubber layer. In the present embodiment, the charging roller 34 is in contact with the photoreceptor drums 1a to 1d respectively. Note that the charging roller 34 may be non-contact with the photoreceptor drums 1a to 1d respectively.

[0021] The exposure device 5 forms an electrostatic latent image by exposing the photoreceptor drums (image carriers) 1a to 1d charged by the charging devices 2a to 2d.

[0022] The developing devices 3a to 3d are arranged to face the photoreceptor drums (image carriers) 1a to 1d and have developing rollers (developer carriers) 31. The developing devices 3a to 3d apply a predetermined developing bias to the developing rollers (developer carriers) 31 to attach toner to the electrostatic latent images formed on the photoreceptor drums (image carriers) 1a to 1d, thereby forming toner images. The developing rollers (developer carriers) 31 are arranged to face the photoreceptor drums (image carriers) 1a to 1d and carry a two-component developer containing a magnetic carrier and toner.

[0023] When image data is input from a host device such as a personal computer, first, the charging devices 2a to 2d uniformly charge the surfaces of the photoreceptor drums 1a to 1d. Next, the exposure device 5 irradiates light according to the image data to form electrostatic latent images corresponding to the image data on the respective photoreceptor drums 1a to 1d.

[0024] The developing devices 3a to 3d are each filled with a predetermined amount of a two-component developer containing toner of each color: cyan, magenta, yellow, and black. The toner in the developer is supplied onto the photoreceptor drums 1a to 1d by the developing devices 3a to 3d and electrostatically adheres thereto, thereby forming toner images corresponding to the electrostatic latent images formed by the exposure from the exposure device 5.

[0025] Then, an electric field is applied at a predetermined transfer voltage between the primary transfer rollers 6a to 6d and the photoreceptor drums 1a to 1d by the primary transfer rollers 6a to 6d, and the cyan, magenta, yellow, and black toner images on the photoreceptor drums 1a to 1d are primarily transferred onto the intermediate transfer belt 8. These four-color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, in preparation for the subsequent formation of new electrostatic latent images, the toner and the like remaining on the surfaces of the photoreceptor drums 1a to 1d after primary transfer are removed by the cleaning devices 7a to 7d.

[0026] The intermediate transfer belt 8 is stretched between the upstream driven roller 10 and the downstream driving roller 11. When the intermediate transfer belt 8 starts to rotate counterclockwise as the driving roller 11 rotates by a driving motor (not shown), the sheet S is conveyed from the resist roller pair 12b to the nip portion (secondary transfer nip portion) between the driving roller 11 and the secondary transfer roller 9 provided adjacent thereto at a predetermined timing, and the full-color image on the intermediate transfer belt 8 is secondarily transferred onto the sheet S. The sheet S onto which the toner image has been secondarily transferred is conveyed to the fixing unit 13.

[0027] The sheet S conveyed to the fixing unit 13 is heated and pressurized by the fixing roller pair 13a, and the toner image is fixed on the surface of the sheet S, forming a predetermined full-color image. The sheet S on which the full-color image has been formed has its conveyance direction distributed by the branching portion 14 branched in a plurality of directions, and is then discharged to the discharge tray 17 by the discharge roller pair 15 (either as it is or after being sent to the duplex conveyance path 18 and having images formed on both sides).

[0028] Furthermore, an image density sensor 40 is disposed at a position facing the driving roller 11 with the intermediate transfer belt 8 interposed therebetween. The image density sensor 40 measures the toner adhesion amount on the intermediate transfer belt 8. The detection sensor 48 detects the temperature and humidity around the charging roller 34.

[0029] The developing bias power source 43 is connected to the developing roller 31 (see FIG. 2). The developing bias power source 43 includes an AC constant voltage power source 43a and a DC constant voltage power source 43b. The AC constant voltage power source 43a outputs a sine-wave AC voltage generated from a low-voltage DC voltage modulated in a pulse shape using a boost transformer (not shown). The DC constant voltage power source 43b outputs a DC voltage obtained by rectifying a sine-wave AC voltage generated from a low-voltage DC voltage modulated in a pulse shape using a boost transformer.

[0030] During image formation, the developing bias power supply 43 applies a developing bias, which is an AC voltage superimposed on a DC voltage, from the AC constant voltage power supply 43a and the DC constant voltage power supply 43b to the developing roller 31. The current detection unit 44 detects the value of the DC current flowing between the developing roller 31 and the photosensitive drum 1a. By applying a developing bias, which is an AC voltage superimposed on a DC voltage, to the developing roller 31, it becomes easier to control the developability of the toner during image formation and the image quality is improved.

[0031] The charging roller 34 is connected to the charging bias power supply 45 (see Fig. 2). The charging bias power supply 45 includes a DC constant voltage power supply 45b. The DC constant voltage power supply 45b outputs a DC voltage obtained by rectifying a sine wave AC voltage generated from a low-voltage DC voltage modulated in a pulse shape using a step-up transformer.

[0032] In this embodiment, during image formation, the charging bias power supply 45 applies a charging bias composed only of a DC voltage from the DC constant voltage power supply 45b to the charging roller (charging member) 34. By configuring the charging bias with only a DC voltage, the wear of the photosensitive layer 111 can be reduced, the ozone generation amount can be reduced, and the charging noise can be reduced.

[0033] The transfer voltage power supply 47 applies a primary transfer voltage and a secondary transfer voltage to the primary transfer rollers 6a to 6d and the secondary transfer roller 9 (see Fig. 1), respectively.

[0034] The cleaning device 7a includes a cleaning blade 32 for removing residual toner on the surface of the photosensitive drum 1a, a rubbing roller 33 for removing residual toner on the surface of the photosensitive drum 1a and rubbing and polishing the surface of the photosensitive drum 1a, and a transport spiral 35 for discharging the residual toner removed from the photosensitive drum 1a by the cleaning blade 32 and the rubbing roller 33 to the outside of the cleaning device 7a.

[0035] The image forming apparatus 100 is provided with a main control unit 80 composed of a CPU or the like. The main control unit 80 is connected to a storage unit 70 composed of a ROM, a RAM, or the like. The main control unit 80 controls each part of the image forming apparatus 100 (charging devices 2a to 2d, developing devices 3a to 3d, exposure device 5, primary transfer rollers 6a to 6d, cleaning devices 7a to 7d, secondary transfer roller 9, fixing unit 13, developing bias power source 43, current detection unit 46, charging bias power source 45, transfer voltage power source 47, voltage control unit 50, etc.) based on the control program and control data stored in the storage unit 70.

[0036] The voltage control unit 50 controls a developing bias power source 43 that applies a developing bias to the developing roller 31, a charging bias power source 45 that applies a charging bias to the charging roller 34, and a transfer voltage power source 47 that applies a transfer voltage to the primary transfer rollers 6a to 6d and the secondary transfer roller 9. Note that the voltage control unit 50 may be composed of a control program stored in the storage unit 70.

[0037] A liquid crystal display unit 90 and a transmission / reception unit 91 are connected to the main control unit 80. The liquid crystal display unit 90 functions as a touch panel for the user to perform various settings of the image forming apparatus 100, and also displays the state of the image forming apparatus 100, the image forming status, the number of printed sheets, etc. The transmission / reception unit 91 communicates with the outside using a telephone line or an Internet line.

[0038] As described above, the charging bias applied by the charging bias power source 45 during image formation is affected by the AC component of the developing bias. As a result, unevenness may occur in the surface potential of the photosensitive drums (image carriers) 1a to 1d, and density unevenness (fog image) may occur in the toner images of each color formed on the photosensitive drums (image carriers) 1a to 1d.

[0039] In addition, the photosensitive layers 111 of the photosensitive drums (image carriers) 1a to 1d wear (film collapse) and the film thickness becomes thinner as the cumulative rotation number increases compared to when not in use. On the other hand, the capacitance of the photosensitive drums 1a to 1d in the initial state where the film thickness of the photosensitive layer 111 is thick is lower than that in the state where the film thickness is thin. As a result, the photosensitive drums 1a to 1d in the initial state are more susceptible to the influence of the AC component of the developing bias compared to the photosensitive drums 1a to 1d in which the film thickness of the photosensitive layer 111 has become thin. Thereby, unevenness in surface potential is likely to occur in the photosensitive drums (image carriers) 1a to 1d in the initial state.

[0040] Also, the resistance value of the charging roller (charging member) 34 increases as the ambient temperature and ambient humidity decrease. For this reason, when the applied charging bias is increased, discharge from the charging roller 34 to the photosensitive drums 1a to 1d is likely to become non-uniform. Thereby, unevenness is likely to occur in the surface potential of the photosensitive drums (image carriers) 1a to 1d.

[0041] Also, as the ambient humidity around the charging roller (charging member) 34 increases, the charging roller 34 absorbs water and the resistance value decreases. At this time, discharge from the charging roller 34 to the photosensitive drums 1a to 1d is likely to become non-uniform.

[0042] In this embodiment, the main control unit (control unit) 80 can execute a correction image forming mode and a standard image forming mode. The correction image forming mode is executed until the cumulative rotation number of the photosensitive drums 1a to 1d reaches a predetermined threshold value. That is, the main control unit (control unit) 80 determines that the film thickness of the photosensitive layer 111 is equal to or greater than a predetermined thickness until the cumulative rotation number of the photosensitive drums 1a to 1d reaches the predetermined threshold value, and executes the correction image forming mode. Thereby, the influence of the AC component of the developing bias can be reduced, and the occurrence of density unevenness (foggy image) can be suppressed.

[0043] The standard image forming mode is executed after the cumulative rotation numbers of the photoreceptor drums 1a to 1d reach a predetermined threshold value. That is, the main control unit (control unit) 80 determines that the film thickness of the photosensitive layer 111 is worn to less than a predetermined thickness after the cumulative rotation numbers of the photoreceptor drums 1a to 1d reach the predetermined threshold value, and executes the normal standard image forming mode.

[0044] Note that the cumulative rotation numbers of the photoreceptor drums 1a to 1d can be calculated based on, for example, the cumulative number of image formation sheets on which the image forming units Pa to Pd have formed images.

[0045] In the correction image forming mode, the correction surface potential Va at the start of execution is higher than the standard surface potential Vb at the start of execution of the standard image forming mode. Also, the frequency of the AC component of the developing bias in the correction image forming mode is lower than the frequency of the AC component of the developing bias in the standard image forming mode.

[0046] When the correction image forming mode is executed, by setting the surface potential V0 of the photoreceptor drums 1a to 1d higher than when the standard image forming mode is executed and setting the frequency of the AC component of the developing bias lower, it is possible to suppress the occurrence of charging unevenness in the photoreceptor drums 1a to 1d in the initial state where the film thickness of the photosensitive layer 111 is equal to or more than a predetermined thickness, and reduce the occurrence of image defects while suppressing the manufacturing cost. Note that these operational effects will be described in detail later using examples.

[0047] Also, when the correction image forming mode is executed, the correction surface potential Va set as the cumulative rotation numbers of the photoreceptor drums 1a to 1d increase is gradually decreased.

[0048] For a set corrected surface potential Va, the actual surface potential may vary due to variations in the output of the high-voltage substrate, variations in the film thickness of the photosensitive layer 111, or variations in the resistance of the charging roller 34. Also, when the fluctuating surface potential exceeds a predetermined upper limit value, image defects such as fogging or white spots may occur. Further, the upper limit value of the surface potential at which image defects occur decreases as the film thickness of the photosensitive layer 111 becomes thinner. For this reason, as the cumulative rotation number of the photoreceptor drums 1a to 1d increases, the actual surface potential may exceed the upper limit value. Therefore, by gradually decreasing the corrected surface potential Va, it is possible to prevent the corrected surface potential Va from exceeding the upper limit value. Thus, the occurrence of image defects can be further reduced.

[0049] Also, when the standard image forming mode is executed, as the cumulative rotation number of the photoreceptor drums 1a to 1d increases, the frequency of the AC component of the developing bias is gradually increased.

[0050] When the surface potential exceeds a predetermined lower limit value, image defects such as fogging may occur. Also, the lower limit value of the surface potential at which image defects occur increases as the film thickness of the photosensitive layer 111 becomes thinner. The rate of increase of the lower limit value becomes lower as the frequency of the AC component of the developing bias is increased. For this reason, when the standard image forming mode is executed, as the cumulative rotation number of the photoreceptor drums 1a to 1d increases, the actual surface potential may exceed the lower limit value (see Fig. 8). Therefore, after the cumulative rotation number of the photoreceptor drums 1a to 1d reaches a predetermined threshold value, the frequency of the AC component of the developing bias is gradually increased to suppress the rate of increase of the lower limit value. Thereby, it is possible to prevent the surface potential from exceeding the lower limit value (see Fig. 9). Thus, the occurrence of image defects can be further reduced.

[0051] Image defects are particularly likely to occur when the charging roller 34 is in a low-temperature and low-humidity environment or a high-humidity environment and the photosensitive layer 111 is in an initial state with a thick film thickness as described above. However, image defects are less likely to occur when the charging roller 34 is not in a low-temperature and low-humidity environment or a high-humidity environment.

[0052] Therefore, when the main control unit (control unit) 80 determines based on the detection result of the detection sensor 48 that the charging roller 34 is not in a low-temperature and low-humidity environment or a high-humidity environment, the standard image forming mode may be executed before the cumulative rotation number of the photosensitive drums 1a to 1d reaches a predetermined threshold value.

[0053] When the charging roller 34 is not in a low-temperature and low-humidity environment or a high-humidity environment, switching of the image forming mode can be omitted. The low-temperature and low-humidity environment is, for example, an environment where the temperature is 8°C or higher and 23°C or lower and the relative humidity is 5% or higher and 15%. The high-humidity environment is, for example, an environment where the relative humidity is 70% or higher.

[0054] FIG. 3 is a flowchart showing an execution example of the correction image forming mode in the image forming apparatus 100. In step S1, the main control unit (control unit) 80 determines based on the detection result of the detection sensor 48 whether the periphery of the charging roller 34 is in a low-temperature and low-humidity environment or a high-humidity environment. When it is determined that it is not in a low-temperature and low-humidity environment or a high-humidity environment, the process proceeds to step S3 and the standard image forming mode is executed. On the other hand, when it is determined that it is in a low-temperature and low-humidity environment or a high-humidity environment, the process proceeds to step S2.

[0055] In step S2, for example, it is determined whether the cumulative rotation number from the previous replacement of the photosensitive drums 1a to 1d is equal to or less than a predetermined number of times (for example, 30k times). When the cumulative rotation number from the previous replacement of the photosensitive drums 1a to 1d is equal to or less than the predetermined number of times, the main control unit (control unit) 80 determines that the film thickness of the photosensitive layer 111 of the photosensitive drums 1a to 1d is equal to or more than a predetermined thickness and proceeds to step S4 to execute the correction image forming mode.

[0056] On the other hand, when the cumulative rotation number from the previous replacement of the photosensitive drums 1a to 1d is more than a predetermined number of times (30k times), the main control unit (control unit) 80 determines that the film thickness of the photosensitive layer 111 of the photosensitive drums 1a to 1d is thinner than a predetermined thickness and proceeds to step S3 to execute the standard image forming mode.

[0057] Next, the effects of the present invention will be specifically described using a plurality of examples. In the following experiments, the relationship between the surface potential V0 of the photoreceptor drum (image carrier) 1a and the occurrence of image defects was evaluated.

[0058] Figures 4 and 5 are graphs showing the relationship between the surface potential V0 of the photoreceptor drum (image carrier) 1a and the density unevenness (ΔE). Figure 4 shows the image formation mode according to Example 1. Figure 5 shows the image formation modes according to Example 1 and Example 2. The density unevenness (ΔE) is calculated from the density difference between the half-tone image and the fog image formed in the image adjustment pattern by measuring the density of the image adjustment pattern (half-tone toner image) printed on the sheet S with a fluorescence spectroscopic densitometer (FD-5 manufactured by Konica Minolta).

[0059] In the image formation mode according to Example 1, the developing bias power source 43 applies a developing bias including an AC component with a frequency of 6.0 kHz to the developing roller 31. In the image formation mode according to Example 2, the developing bias power source 43 applies a developing bias including an AC component with a frequency of 3.5 kHz to the developing roller 31. Also, in the image formation modes according to Example 1 and Example 2, the surface potential V0 of the photoreceptor drum (image carrier) 1a was displaced stepwise while increasing the charging bias stepwise.

[0060] As shown in Figure 4, it was found that the density unevenness (ΔE) can be suppressed as the surface potential V0 of the photoreceptor drum 1a increases. Also, by setting the surface potential V0 of the photoreceptor drum 1a in the range of 400 (V) or more and 500 (V) or less, the density unevenness (ΔE) becomes 3.0 or less, which is preferable.

[0061] Also, as shown in Figure 5, it was found that when the surface potential V0 is in the range of 400 (V) or more and 500 (V) or less, the density unevenness (ΔE) can be suppressed by lowering the frequency of the AC component of the developing bias from 6.0 kHz to 3.5 kHz. From the above, it was found that the occurrence of image defects can be further reduced by increasing the surface potential V0 of the photoreceptor drum 1a and lowering the frequency of the AC component of the developing bias.

[0062] Next, an evaluation was conducted on the relationship between the number of image formations (cumulative rotation number) of the sheet S on the photoreceptor drum 1a and image defects.

[0063] Figs. 6 and 7 are graphs showing the relationship between the number of image formations of the sheet S (cumulative rotation number of the photoreceptor drum 1a), the surface potential V0, and image defects. Fig. 6 shows the image formation mode according to Example 3, and Fig. 7 shows the image formation mode according to Example 4.

[0064] In the image formation modes according to Example 3 and Example 4, the ambient temperature around the charging roller 34 was 28°C, and the relative humidity (RH) was 80%.

[0065] In the image formation mode according to Example 3, the developing bias power source 43 applies a developing bias including an AC component with a frequency of 6.0 kHz to the developing roller 31. In the image formation mode according to Example 4, the developing bias power source 43 applies a developing bias including an AC component with a frequency of 3.5 kHz to the developing roller 31. Other printing conditions are the same.

[0066] The film thickness of the photosensitive layer 111 of the photoreceptor drum 1a with a cumulative number of image formations of 0 sheets (unused) was 40 μm, and the film thickness of the photosensitive layer 111 of the photoreceptor drum 1a when the cumulative number of image formations reached 30 (k sheets) was 38 μm.

[0067] Also, when the surface potential V0 was greater than 500 V regardless of the cumulative number of image formations, image defects such as fogging or white spots were observed (indicated as "NG" in the graphs of Figs. 6 and 7). Also, when the surface potential V0 was less than 300 V regardless of the cumulative number of image formations, image defects such as fogging were observed (indicated as "NG" in the graphs of Figs. 6 and 7).

[0068] In addition, in the initial state where the cumulative number of printed images is less than 30 (k sheets), for the photosensitive drum 1a, as the number of printed images increases, the lower limit of the surface potential V0 at which image defects such as fogging occur decreases. As shown in FIG. 6, in the printing mode according to Example 3, when the film thickness of the photosensitive layer 111 is 40 μm, the lower limit of the surface potential V0 at which image defects occur is 480 V. Further, when the cumulative number of printed images reaches 30k sheets, the film thickness of the photosensitive layer 111 is 38 μm, and the lower limit of the surface potential V0 at which image defects occur is 400 V. When the cumulative number of printed images is less than 30 (k sheets), when the surface potential V0 of the photosensitive drum 1a is greater than the straight line L1 and less than 500 V, no image defects were observed (represented as "OK" in the graph of FIG. 6).

[0069] Also, as shown in FIG. 7, in the printing mode according to Example 4, when the film thickness of the photosensitive layer 111 is 40 μm, the lower limit of the surface potential V0 at which image defects occur is 440 V. Further, when the cumulative number of printed images reaches 30k sheets, the film thickness of the photosensitive layer 111 is 38 μm, and the lower limit of the surface potential V0 at which image defects occur is 400 V. When the cumulative number of printed images is less than 30 (k sheets), when the surface potential V0 of the photosensitive drum 1a is greater than the straight line L2 and less than 500 V, no image defects were observed (represented as "OK" in the graph of FIG. 7).

[0070] After the cumulative number of printed images reaches 30 (k sheets), the lower limit of the surface potential at which fogging occurs remains constant at 400 V even as the cumulative number of printed images increases.

[0071] As a result, it was found that until the cumulative number of printed images reaches 30 (k sheets), by lowering the frequency of the AC component of the development bias from 6.0 kHz to 3.5 kHz, the lower limit of the surface potential V0 at which image defects occur can be lowered.

[0072] In addition, there is a variation of -25 V to +25 V between the target surface potential V0 and the actual surface potential V0. Therefore, it is preferable to set a margin for the upper limit value and the lower limit value of the surface potential V0 at which image defects occur when setting the target surface potential V0.

[0073] As shown in FIG. 7, in the image forming mode according to Example 4, when a charging bias was applied with 470 (V) as the target value of the surface potential V0, no simulated image was observed even considering the variation in the surface potential V0 of -25 V to +25 V (represented as "OK" in the graph).

[0074] From the above, it was found that by increasing the target value of the surface potential V0 of the photosensitive drum 1a and decreasing the frequency of the AC component of the developing bias, the occurrence of image defects can be further reduced in the photosensitive drum 1a in the initial state where the cumulative number of image formations is less than 30 (k sheets).

[0075] Next, the relationship between the number of image formations (cumulative rotation number) of the sheet S on the photosensitive drum 1a and image defects was evaluated. In this evaluation, the case where the upper and lower limits of the surface potential V0 at which image defects occur vary as the cumulative number of image formations increases was examined.

[0076] FIGS. 8 and 9 are graphs showing the relationship between the number of image formations of the sheet S (cumulative rotation number of the photosensitive drum 1a), the surface potential V0, and image defects. FIG. 8 shows the image forming mode according to Example 5, and FIG. 9 shows the image forming mode according to Example 6.

[0077] As shown in FIGS. 8 and 9, the upper limit of the surface potential V0 at which image defects occur decreases as the cumulative number of image formations increases, and the lower limit of the surface potential V0 at which image defects occur increases as the cumulative number of image formations increases. For this reason, there is a possibility that the actual surface potential V0 with variations exceeds the upper limit value or the lower limit value at which image defects occur.

[0078] On the other hand, in the image forming modes according to Example 5 and Example 6, the target value of the surface potential V0 is decreased as the cumulative number of image formations increases. As a result, it was found that the actual surface potential V0 with a variation of -25 V to +25 V is suppressed from exceeding the upper limit value at which image defects occur.

[0079] Also, as shown in FIGS. 8 and 9, after the cumulative number of printed images reaches 30 (k sheets), the lower limit value of the surface potential V0 at which image defects occur increases as the cumulative number of printed images increases (as the film thickness of the photosensitive layer 111 decreases). Also, it was found that the rate of increase of the lower limit value decreases as the frequency of the AC component of the developing bias is increased. Therefore, after the cumulative number of printed images reaches 30 (k sheets), by increasing the frequency of the AC component of the developing bias to suppress the rate of increase of the lower limit value, it was found that it is possible to prevent the actual surface potential V0 from exceeding the lower limit value.

[0080] As described above, the embodiments of the present invention have been described. However, the scope of the present invention is not limited thereto, and various modifications can be made without departing from the spirit of the invention. For example, in the above embodiment, a color printer as shown in FIG. 1 has been described as an example of the image forming apparatus 100. However, the present invention is not limited to a color printer, and other image forming apparatuses such as monochrome and color copiers, digital multifunction machines, and facsimiles may be used.

Industrial Applicability

[0081] The present invention can be applied to an image forming apparatus including a charging roller.

Explanation of Signs

[0082] 1a~1d Photosensitive drums 2a~2d Charging devices 3a~3d Developing devices 5 Exposure device 6a~6d Primary transfer rollers 7a~7d Cleaning devices 8 Intermediate transfer belt 9 Secondary transfer roller 10 Driven roller 11 Driving roller 12a Paper feed roller 12b Registration roller pair 13 Fixing unit 13a Fixing roller pair 14 Branch unit 15 Discharge roller pair 16 Paper cassette 17 Output tray 18 Surface conveyance path 19 Belt cleaner 31 Developing roller 32 Cleaning blade 33 Rubbing roller 34 Charging roller 35 Conveying spiral 40 Image density sensor 43 Developing bias power supply 43a AC constant voltage power supply 43b, 45b DC constant voltage power supply 45 Charging bias power supply 46 Current detection unit 47 Transfer voltage power supply 50 Voltage control unit 70 Memory unit 80 Main control unit 90 Liquid crystal display unit 91 Transmission / reception unit 100 Image forming apparatus 111 Photosensitive layer

Claims

1. an image carrier having a photosensitive layer formed on its surface; a charging device disposed opposite the image carrier and having a charging member for charging the image carrier; an exposure device that exposes the image carrier charged by the charging device to light to form an electrostatic latent image; a developing device disposed opposite the image carrier, the developing device having a developer carrier that carries a developer, and that forms a toner image by attaching toner to the electrostatic latent image formed on the image carrier; an image forming unit including: a developing bias power supply that applies a developing bias, which is a DC voltage superimposed on an AC voltage, to the developer carrier; a charging bias power supply that applies a charging bias including a DC voltage to the charging member; a control unit that controls the image forming unit, the developing bias power supply, and the charging bias power supply; In an image forming apparatus comprising: the control unit is capable of executing a standard image forming mode in which an image is formed by charging the developer carrier with a predetermined standard surface potential as a target, and a corrected image forming mode in which an image is formed by charging the developer carrier with a corrected surface potential different from the standard surface potential as a target, the control unit recognizes the cumulative number of rotations of the image carrier, The correction image forming mode is executed until the cumulative number of rotations of the image carrier reaches a predetermined threshold value, the standard image forming mode is executed after the cumulative number of rotations of the image carrier reaches the threshold value, the corrected surface potential at the start of execution of the corrected image forming mode is higher than the standard surface potential at the start of execution of the standard image forming mode; an AC component of the developing bias in the correction image forming mode having a frequency lower than that in the standard image forming mode;

2. 2. The image forming apparatus according to claim 1, wherein, when said correction image forming mode is executed, said correction surface potential to be set is gradually decreased as the cumulative number of rotations of said image carrier increases.

3. 3. The image forming apparatus according to claim 1, wherein, when the standard image forming mode is executed, the frequency of the AC component of the developing bias is increased stepwise as the cumulative number of rotations of the image carrier increases.

4. a detection sensor for detecting the temperature and humidity around the charging member; 3. The image forming apparatus according to claim 1, wherein the control unit executes the standard image forming mode before the cumulative number of rotations of the image carrier reaches a predetermined threshold value when the control unit determines that the environment is not a low-temperature, low-humidity environment or a high-humidity environment based on the detection result of the detection sensor.

5. 3. The image forming apparatus according to claim 1, wherein the charging bias is composed of only a DC voltage.

6. 3. The image forming apparatus according to claim 1, wherein the photosensitive layer is a positively charged single layer type and has a thickness of 37 [mu]m or more in an unused state.

Images