Image forming device

Abstract

To provide an image forming device with which, while reducing a cyclically occurring unevenness in image density, it is possible to reduce a decrease in the quality of deliverables due to fogging or the risk of a device fault due to carrier adhesion.SOLUTION: An image forming device 200 comprises an image forming part P, an image density sensor 70, and a CPU. The CPU generates a correction bias to correct at least one of a charging bias for an electrifier 2 to electrify a photoconductor drum 1 and a developing bias for a developer 4 to perform development, corrects the correction bias so that the amplitude of the correction bias becomes smaller than or equal to a threshold when the amplitude of the correction bias exceeds the threshold, and causes an image forming part P to form an image under an image formation condition where at least one of the charging bias and the developing bias is corrected by the corrected correction bias.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Electrophotographic image forming apparatuses are beginning to spread in the printing industry, and the demand for high-speed output and high image quality is rapidly increasing. Among the requirements for high image quality, there is a high demand for uniform image density within a page. Therefore, it is important to minimize image density variations within a page. While there are various causes of image density variations, periodic image density variations that occur during development are known to be particularly visible. Periodic image density variations are thought to be caused by periodic fluctuations in the development field strength due to rotational fluctuations of the photosensitive drum or developing sleeve.

[0003] Patent Document 1 discloses an image forming apparatus that corrects image density unevenness caused by rotational fluctuations of the photosensitive drum or developing sleeve by modulating a development bias in accordance with the rotational period of the photosensitive drum. Specifically, this image forming apparatus uses a rotational position detection sensor that detects the rotational position of the photosensitive drum and a density detection sensor that detects image density. The image forming apparatus detects image density unevenness based on the detection results of the density detection sensor. Image density unevenness is isolated based on the rotational period of the photosensitive drum and suppressed by periodically changing the development bias using a signal from the rotational position detection sensor as a trigger. The development bias suppresses image density unevenness by canceling out electric field fluctuations caused by rotational fluctuations and maintaining a constant electric field. Furthermore, a similar effect can be achieved by modulating not only the development bias but also the charging bias used when charging the photosensitive drum. Hereinafter, this technique for correcting image density unevenness caused by rotational fluctuations of the photosensitive drum or developing sleeve is referred to as "sub-scanning density unevenness correction." [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-98675 Summary of the Invention [Problem to be solved by the invention]

[0005] However, modulating the charging bias or developing bias to correct periodic image density variations that occur during development can be affected by fluctuations in the "fogging potential," which is the difference between the potential of the non-exposed portion of the photosensitive drum and the potential of the developing sleeve. Generally, a low fogging potential increases the amount of toner adhering to the non-exposed portion of the photosensitive drum, while a high fogging potential increases the amount of carrier adhering to the non-exposed portion of the photosensitive drum. Toner adhering to the non-exposed portion of the photosensitive drum reduces image quality by causing fog in the white background. Carrier adhering to the non-exposed portion of the photosensitive drum can cause image defects at the primary transfer unit and poor cleaning by the drum cleaner. For this reason, the fogging potential must be set within an appropriate range. However, if the fogging potential deviates from the appropriate range due to modulation of the charging bias or developing bias, there is a risk of degraded quality in the finished product due to fogging and device failure due to carrier adhering.

[0006] In view of the above-mentioned problems, the main object of the present invention is to provide an image forming apparatus that can reduce the periodically occurring unevenness in image density while reducing the risk of deterioration in the quality of the finished product due to fogging and device failure due to carrier adhesion. [Means for solving the problem]

[0007] The image forming apparatus of the present invention comprises a rotating photosensitive member, a charger that charges the photosensitive member based on a charging bias, and the photosensitive member charged by the charger. to Forming an electrostatic latent image exposing the photoreceptor to lightThe image forming device includes an exposure device and a developing device that develops the electrostatic latent image based on a developing bias to form an image on the photosensitive member, a reading device that reads the test image formed by the image forming device, an environmental sensor that detects environmental conditions, and a control device, and the control device The fogging potential is set so that the amount of toner and the amount of carrier attached to the non-exposed portion of the photosensitive member are within a specified range. a threshold value is determined, and the control means determines a charge correction bias to be superimposed on the charge bias and a development correction bias to be superimposed on the development bias based on the reading result of the test image by the reading means so that the sum of the amplitude of the charge correction bias and the amplitude of the development correction bias is equal to or less than the threshold value, the control means controls the charge bias based on the charge correction bias and controls the development bias based on the development correction bias, the charge correction bias is a bias that suppresses periodic fluctuations in density of an image formed in the rotation direction of the photosensitive body, and the development correction bias is a bias that suppresses periodic fluctuations in density of an image formed in the rotation direction of the photosensitive body. [Effects of the Invention]

[0008] According to the present invention, it is possible to reduce the risk of deterioration in the quality of the finished product due to fogging and of device failure due to carrier adhesion, while reducing the periodically occurring unevenness in image density. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram illustrating the configuration of an image forming apparatus. [Figure 2] 5A and 5B are diagrams illustrating the relationship between the potential of the photosensitive drum in the developing portion and the developing bias. [Figure 3] 1 is a graph showing development gamma characteristics. [Figure 4] 10A and 10B are diagrams illustrating the relationship between the fog removal potential and the fogging and carrier adhesion. [Figure 5] FIG. 2 is a diagram illustrating the configuration of an image density sensor. [Figure 6] FIG. [Figure 7] FIG. 10 is an example diagram of an output from a photointerrupter. [Figure 8] FIG. [Figure 9] 10 is a flowchart showing a sub-scanning density unevenness correction process. [Figure 10] FIG. 10 is an example of a test image. [Figure 11] FIG. 4 is a diagram showing the relationship between laser power and the potential of a photosensitive drum. [Figure 12] 10 is a table showing the effects of the first embodiment. [Figure 13] FIG. [Figure 14] 10 is a flowchart showing a sub-scanning density unevenness correction process. [Figure 15] 10A and 10B are diagrams illustrating the relationship between the fog removal potential and the fogging and carrier adhesion. [Figure 16] 1 is an explanatory diagram of the absolute moisture content and threshold value of the installation environment conditions. DETAILED DESCRIPTION OF THE INVENTION

[0010] Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. The present invention will be described in more detail using examples. These examples are examples of preferred embodiments of the present invention, but the present invention is not limited to the configurations of these examples.

[0011] (First embodiment) Fig. 1 is a diagram showing the configuration of an image forming apparatus according to this embodiment. The image forming apparatus 200 according to this embodiment is a four-color full-color printer using an electrophotographic system. The image forming apparatus 200 shown in Fig. 1 may be combined with other devices as appropriate to form a copier, a multifunction peripheral, or a facsimile machine.

[0012] The image forming apparatus 200 forms an image on a sheet of recording material based on a print signal received from an external device. The recording material is a recording medium on which an image can be formed, such as plain paper, coated paper, OHT, or label. Hereinafter, the recording material will be referred to as "paper S." The image forming apparatus 200 converts the received print signal into image signals separated into four colors: yellow (Y), magenta (M), cyan (C), and black (K). The image forming apparatus 200 charges multiple photoconductors corresponding to each color to a predetermined potential and exposes the charged photoconductors based on the image signals for each color to form electrostatic latent images of the corresponding color on each photoconductor. The image forming apparatus 200 develops the electrostatic latent images using toner of the corresponding color to form toner images on each photoconductor, and then transfers the toner images from each photoconductor to an intermediate transfer body in a superimposed manner. The image forming apparatus 200 transfers the toner images from the intermediate transfer body to the paper S in a batch. The image forming apparatus 200 performs a fixing process by thermocompression on the paper S onto which the toner image has been transferred, and discharges the paper S as a finished product outside the apparatus.

[0013] The image forming apparatus 200 includes an operation unit 140. The operation unit 140 is a user interface and includes, for example, a display, operation buttons, a touch panel, etc. A user can input various processing instructions to the image forming apparatus 200 using the operation unit 140. For example, the user can input, using the operation unit 140, instructions for image formation and instructions for performing sub-scanning density unevenness correction, which will be described later.

[0014] In order to perform the image formation process described above, the image forming apparatus 200 includes image forming units Pa to Pd, an intermediate transfer belt 7 serving as an intermediate transfer body, and a fixing unit 13. The image forming apparatus 200 employs a tandem intermediate transfer system in which the image forming units Pa to Pd are arranged along the intermediate transfer belt 7. The intermediate transfer belt 7 is an endless belt stretched by multiple rollers including a drive roller 18, a tension roller 17, and a secondary transfer inner roller 8, and is transported (rotated) in the direction R7. The image forming units Pa to Pd each form a toner image of a different color. In this embodiment, the image forming unit Pa forms a yellow (Y) toner image. The image forming unit Pb forms a magenta (M) toner image. The image forming unit Pc forms a cyan (C) toner image. The image forming unit Pd forms a black (K) toner image.

[0015] The image forming units Pa to Pd have the same configuration and operate in the same way, except for the color of toner they use. The following describes image forming unit Pa, which forms a yellow toner image, and a description of image forming units Pb to Pd will be omitted. In the following description, when it is not necessary to distinguish between colors, the suffixes a to d will be omitted.

[0016] The image forming section Pa has a configuration in which a charger 2a, an exposure unit 3a, a developing unit 4a, a primary transfer unit T1a, and a drum cleaner 6a are arranged around a photosensitive drum 1a, which is a photosensitive member.

[0017] The photosensitive drum 1a has a photosensitive layer formed on a grounded cylindrical conductive tube, and is driven to rotate clockwise around the drum axis in the drawing. The charger 2a is a roller with an elastic layer formed around a conductive central axis. The charger 2a is biased toward the photosensitive drum 1a, and rotates while forming a nip between itself and the photosensitive drum 1a. At this time, a charging bias is applied to the central axis of the charger 2a from a charging high-voltage power supply, and the charger 2a uniformly charges the surface (photosensitive layer) of the photosensitive drum 1a to a predetermined potential.

[0018] The exposure device 3a is a laser scanner that scans and exposes a laser beam emitted from a laser light-emitting element in the axial direction of the photosensitive drum 1a via a polygon mirror and an fθ optical system. The laser beam is modulated by a drive signal generated based on an image signal and irradiates the photosensitive drum 1a. As a result, a potential drop occurs in the area of ​​the surface of the photosensitive drum 1a exposed to the laser beam, and an electrostatic latent image corresponding to the image signal is formed on the surface of the photosensitive drum 1a.

[0019] The developing unit 4a includes a stirring and transporting section filled with a two-component developer consisting of a magnetic carrier and a non-magnetic toner, a developing sleeve, and a regulating member disposed at a predetermined distance from the developing sleeve. The developing sleeve is configured with a conductive member provided around a fixedly disposed magnet roller. The developer is stirred and transported in the stirring and transporting section, and the toner is charged to a predetermined charge. The charged developer is carried and transported on the developing sleeve by the magnetic force of the magnet roller and the rotation of the developing sleeve, and is adjusted to a predetermined thickness by the regulating member. The developer adjusted to a predetermined thickness on the developing sleeve is supplied to the photosensitive drum 1a.

[0020] The supply of developer to the photosensitive drum 1a is performed by applying a development bias to the development sleeve from a development high-voltage power supply. When the development bias is applied to the development sleeve, a driving force is generated by the potential difference between the electrostatic latent image formed on the photosensitive drum 1a and the development bias, causing toner to move from the development sleeve to the photosensitive drum 1a. The toner that has moved to the photosensitive drum 1a adheres to the electrostatic latent image, developing it into a toner image. In this embodiment, negative polarity toner is used.

[0021] The primary transfer unit T1a is provided with a primary transfer roller positioned opposite the photosensitive drum 1a with the intermediate transfer belt 7 sandwiched therebetween. A primary transfer nip is formed by biasing the primary transfer roller toward the photosensitive drum 1a. A primary transfer bias of opposite polarity to the toner is applied to the primary transfer roller, causing the toner image on the photosensitive drum 1a to be transferred to the intermediate transfer belt 7. Any toner remaining on the photosensitive drum 1a without being transferred at this time is collected by the drum cleaner 6a. The photosensitive drum 1a from which the remaining toner has been collected by the drum cleaner 6a is used again for image formation.

[0022] Image forming stations Pb to Pd form toner images of the corresponding colors on photosensitive drums 1b to 1d using the same process as image forming station Pa. A magenta toner image is formed on photosensitive drum 1b. A cyan toner image is formed on photosensitive drum 1c. A black toner image is formed on photosensitive drum 1d. The intermediate transfer belt 7 is driven to rotate at approximately the same surface speed as the photosensitive drums 1a to 1d. The toner images of each color formed by image forming stations Pa to Pd are transferred and superimposed on the intermediate transfer belt 7 so that they are aligned in position according to the rotation speed of the intermediate transfer belt 7.

[0023] In order to feed paper S on which an image is formed, the image forming apparatus 200 is provided with a paper feed cassette 60, a pair of paper feed rollers 61, a pair of registration rollers 62, and a secondary transfer outer roller 9 in a conveyance path along which paper S is conveyed. The secondary transfer outer roller 9 constitutes a secondary transfer unit T2 between itself and the secondary transfer inner roller 8. Paper feed cassette 60 stores a stack of paper S therein. Paper S is frictionally separated by the paper feed roller pair 61 in accordance with the timing of image formation by image forming units Pa to Pd, and is fed and conveyed one sheet at a time to the conveyance path. Paper S is conveyed to the registration roller pair 62 via the conveyance path. After correcting skew of paper S, the registration roller pair 62 adjusts the timing to convey paper S to the secondary transfer unit T2.

[0024] At the secondary transfer portion T2, the outer secondary transfer roller 9 is biased toward the inner secondary transfer roller 8 with the intermediate transfer belt 7 sandwiched therebetween, thereby rotating drivenly while forming a secondary transfer nip. The paper S supplied to the secondary transfer portion T2 is sandwiched and transported through the secondary transfer nip. At this time, a secondary transfer bias of opposite polarity to the toner is applied to the outer secondary transfer roller 9, so that the toner image on the intermediate transfer belt 7 is transferred onto the paper S. Any toner remaining on the intermediate transfer belt 7 without being transferred is collected by a belt cleaner 11, which is disposed opposite the tension roller 17 across the intermediate transfer belt 7. The intermediate transfer belt 7 from which the remaining toner has been collected by the belt cleaner 11 is used again for image formation.

[0025] The fixing device 13 includes a pair of rollers with a built-in heater, and melts and fixes the toner image on the paper S by thermocompression. The toner image is fixed on the paper S to complete the finished product. The finished product is discharged onto a paper discharge tray 63 provided outside the image forming apparatus 200.

[0026] (development gamma characteristics, Vback latitude) FIG. 2 is an explanatory diagram of the relationship between the potential of the photosensitive drum 1 in the developed area and the development bias when sub-scanning density unevenness correction is not performed. As described above, the photosensitive drum 1 is scanned in the drum axial direction by the laser light. Therefore, the drum axial direction is the main scanning direction. The sub-scanning direction, which is perpendicular to the main scanning direction, is the direction of rotation of the photosensitive drum 1. Because the laser light does not scan the entire photosensitive drum 1 in the drum axial direction, there are exposed areas in the main scanning direction that are exposed to the laser light and unexposed areas that are not exposed to the laser light.

[0027] The potential of the non-exposed portion of the photosensitive drum 1 is Vd, and the potential of the exposed portion is Vl. The DC component of the development bias applied to the development sleeve is Vdc. The potential Vd of the non-exposed portion is the potential charged by the charger 2. The potential Vl of the exposed portion is the potential changed by exposure to laser light from the potential charged by the charger 2. In this embodiment, the development bias has an AC component superimposed on the DC component to improve developability. As the AC component, for example, an AC voltage with a frequency of 1.4 kHz and a peak-to-peak voltage of 1 kVpp is used.

[0028] The magnetic carrier of the two-component developer of this embodiment is composed of a ferrite-based core coated with silicone resin. The magnetic carrier has a volume resistivity of approximately 1013 [Ω·cm] and a particle size (volume average particle size) of approximately 40 [μm]. The non-magnetic toner is composed of a polyester-based resin dispersed with colorants, charge control agents, etc., and is a powder with a volume average particle size of approximately 6 [μm]. The non-magnetic toner is frictionally charged to a negative polarity by friction with the magnetic carrier. The magnetic carrier is frictionally charged to a positive polarity.

[0029] Here, the development contrast is defined as Vcont = Vl - Vdc. The development contrast Vcont is an index of the toner driving force in the development area. Figure 3 is a graph showing the relationship between the development contrast Vcont and the reflection density (image density) of the toner image on the paper S (hereinafter referred to as "development gamma characteristics"). The larger the development contrast Vcont, the more toner adheres to the photosensitive drum 1, and the higher the image density.

[0030] Also, the fog removal potential is defined as Vback = Vdc - Vd. Generally, if the fog removal potential Vback is small, the amount of toner adhesion to the non-exposed portion of the photosensitive drum 1 increases, and if the fog removal potential Vback is large, the amount of carrier adhesion to the non-exposed portion of the photosensitive drum 1 increases. Toner adhesion to the non-exposed portion of the photosensitive drum 1 causes "fog" and reduces image quality. Carrier adhesion to the non-exposed portion of the photosensitive drum 1 causes poor toner image transfer at the primary transfer portion T1 and poor cleaning by the drum cleaner 6.

[0031] For this reason, the fog removal potential Vback needs to be set within an appropriate range (hereinafter referred to as "Vback latitude"). Figure 4 is an explanatory diagram of the relationship between the fog removal potential Vback and the fog and carrier adhesion. In this embodiment, the Vback latitude is the range of the fog removal potential Vback that satisfies the fog reflection density on the photosensitive drum 1 of 1.5% or less and the number of carrier adhesion particles on the photosensitive drum 1 of 10 [particles / cm^2] or less.

[0032] (Reflective sensor) 1, an image density sensor 70 is disposed downstream of the image forming units Pa to Pd in ​​the rotation direction of the intermediate transfer belt 7. The image density sensor 70 detects the reflectance of the intermediate transfer belt 7, thereby detecting the image density of the toner image transferred onto the intermediate transfer belt 7. The image forming apparatus 200 is provided with image density sensors 70 corresponding to the colors yellow, magenta, cyan, and black, but because these have a common configuration, differences between the colors will not be described.

[0033] 5 is a diagram showing the configuration of the image density sensor 70. The image density sensor 70 is disposed opposite the surface of the intermediate transfer belt 7 onto which the toner image is transferred. The image density sensor 70 is composed of a light-emitting unit 71 that emits infrared rays, light-receiving units 72 and 73 that receive the infrared rays, and an electric board 74 on which the light-emitting unit 71 and the light-receiving units 72 and 73 are mounted. The light-emitting unit 71 is, for example, an LED (Light Emitting Diode). The light-receiving units 72 and 73 are, for example, photodiodes.

[0034] The light-emitting unit 71 is positioned to irradiate the intermediate transfer belt 7 with infrared light at an incident angle of 20°. The light-receiving unit 72 is positioned to receive specularly reflected light of light irradiated onto the intermediate transfer belt 7 and the toner image transferred onto the intermediate transfer belt 7 at a reflection angle of -20°. The light-receiving unit 73 is positioned to receive diffusely reflected light of light irradiated by the light-emitting unit 71 onto the intermediate transfer belt 7 and the toner image transferred onto the intermediate transfer belt 7 at a reflection angle of 50°. The electric board 74 includes a drive circuit that supplies current to the light-emitting unit 71 and a light-receiving circuit having an IV conversion function that converts the current flowing in accordance with the amount of light received by the light-receiving units 72 and 73 into a voltage.

[0035] (Phase detection) The photosensitive drum 1, the charger 2, and the developing sleeve of the developing unit 4 each include a phase detection unit that detects the phase of rotation. Fig. 6 is an explanatory diagram of the phase detection unit that detects the rotation phase of the photosensitive drum 1. The phase detection unit 50 of this embodiment includes a photointerrupter 51.

[0036] A drum shaft 53, which is the rotation center of the photosensitive drum 1, is connected to an output shaft 55 of a drive motor 54 via a coupling (not shown). In this configuration, the photosensitive drum 1 is rotationally driven by the drive motor 54. In addition to a photointerrupter 51, the phase detection unit 50 has a light-shielding member 52 that is integral with the drum shaft 53 and rotates as the drum shaft 53 rotates. The light-shielding member 52 is detected by the photointerrupter 51 when the photosensitive drum 1 reaches a predetermined rotation position. This allows the photointerrupter 51 to detect the rotation phase of the photosensitive drum 1. The charger 2 and the developing sleeve also have approximately the same configuration, and their rotation phases are also detected.

[0037] 6, the photosensitive drum 1 is driven by a direct drive system directly connected to the drive motor 54, but a speed reduction mechanism may be provided in the power transmission from the drive motor 54. The same applies to the driving of the developing sleeve. As described above, the charger 2 rotates following the rotation of the photosensitive drum 1, and therefore does not require a drive motor.

[0038] FIG. 7 is an example diagram of the output of the photointerrupter 51. When the light blocking member 52, which rotates in synchronization with the photosensitive drum 1, passes the photointerrupter 51, the output signal of the photointerrupter 51 drops to approximately 0 [V]. The rotational phase of the photosensitive drum 1 is calculated by detecting the falling edge of the output signal at this time. The rotational phase at the timing of the falling edge of the output signal is set to zero, and the phase advances by 2π for one cycle of the photosensitive drum 1. Based on this, the rotational phase of the photosensitive drum 1 at a predetermined timing during rotational driving can be calculated. In this embodiment, the timing at which the output signal of the photointerrupter 51 drops to 0 [V] as the light blocking member 52 passes is set as the "home position."

[0039] (Control unit) 8 is a configuration diagram of a control unit that controls the operation of image forming apparatus 200. The control unit is built into image forming apparatus 200. The control unit includes a CPU (Central Processing Unit) 301. Connected to CPU 301 are controller 87, image processing unit 84, I / F unit 85, timer 90, high voltage control unit 92, image data generation unit 89, sensor drive unit 305, image density detection unit 306, and motor control unit 91.

[0040] The CPU 301 has the function of generating various command signals and executing arithmetic processing to operate various sensors, motors, etc. provided in the image forming apparatus 200. The CPU 301 has a built-in memory for storing data. The image data generation unit 89 has the function of converting various image data into control signals for laser control under the control of the CPU 301 and transmitting them to the laser driving unit 303. The image data generation unit 89 also has the function of generating a test image for detecting image density.

[0041] The number of laser driving units 303 provided corresponds to the number of exposure units 3. In this embodiment, four laser driving units 303 are provided because four exposure units 3 are provided. The laser driving units 303 have the function of driving the laser light emitting elements of the exposure units 3 based on control signals obtained from the image data generating unit 89, and controlling the lighting and light intensity of the laser.

[0042] The sensor driving unit 305 and the image density detection unit 306 are connected to the image density sensor 70. The sensor driving unit 305 has a function of controlling the light emission and drive current of the light emitting unit 71 inside the image density sensor 70 in response to a command signal obtained from the CPU 301. The image density detection unit 306 amplifies the received light electrical signal output from the image density sensor 70 and transmits it to the CPU 301 as the detection result of the image density sensor 70.

[0043] The motor control unit 91 is electrically connected to a plurality of motors 56 arranged in the image forming apparatus 200, such as the drive motor 54, and has the function of controlling the drive timing and drive speed. The motor control unit 91 controls each motor 56 in response to a command signal obtained from the CPU 301.

[0044] High voltage control unit 92 is electrically connected to high voltage output unit 93, and has the function of controlling the output of bias voltages required for the image formation process, such as charging bias, developing bias, and transfer bias, in response to command signals received from CPU 301. High voltage output unit 93 is the above-mentioned charging high voltage power supply and developing high voltage power supply.

[0045] The CPU 301 is connected to the operation unit 140 via the I / F unit 85. The operation unit 140 includes an input unit 94 and a display unit 95. The input unit 94 is, for example, an operation button, a touch panel, or the like. The operation unit 140 may be provided in the image forming apparatus 200 as described above, or may be an external terminal such as a personal computer connected to the image forming apparatus 200. The CPU 301 is electrically connected to the controller 87 and the image processing unit 84. A print signal 88 is sent to the CPU 301 via the controller 87. The CPU 301 can form an image signal by processing the acquired print signal 88 in the image processing unit 84.

[0046] (Sub-scanning density unevenness correction) FIG. 9 is a flowchart showing the sub-scanning density unevenness correction process performed by the image forming apparatus 200 configured as described above.

[0047] When a user or serviceman issues an instruction to execute sub-scanning density unevenness correction processing via the operation unit 140, the CPU 301 activates an adjustment mode for performing various adjustments (S11). Note that, when the CPU 301 detects replacement of a component of the image forming apparatus 200, it may activate the adjustment mode and start the sub-scanning density unevenness correction processing, just as when an instruction to execute the sub-scanning density unevenness correction processing is issued. Next, the CPU 301 detects the home positions of the photosensitive drum 1, the charger 2, and the developing sleeve using the phase detection unit 50 including the photointerrupter 51 described above (S12). The CPU 301 stores the home position detection timing and the patch image position in its built-in memory, and forms a strip-shaped test image on the intermediate transfer belt 7 (S13).

[0048] FIG. 10 is an example of a test image. Test image F is a strip-shaped image extending in the sub-scanning direction in a single tone for each of the colors yellow (Y), magenta (M), cyan (C), and black (K). Test image F is formed with tones in which the curve of the development gamma characteristic has a steep slope. By using such a test image F, it becomes possible to sensitively detect image density unevenness caused by rotational vibration of the photosensitive drum 1 or the developing sleeve, or potential fluctuation due to the charger 2. In this embodiment, the image density of test image F is set to 50% of the maximum density for each color.

[0049] The test images F for each color are arranged in parallel in a direction perpendicular to the rotation direction of the intermediate transfer belt 7 (main scanning direction) so that all four colors can be detected simultaneously. The test images F for each color are formed in positions that pass the detection positions of the image density sensors 70a to 70d as the intermediate transfer belt 7 rotates. The length of the test images F is set to twice the maximum circumferential length (circumferential length of the photosensitive drum 1) that is the cause of periodic image density unevenness. This is to reduce the effects of not only periodic image density unevenness but also sudden image density streaks and unevenness in the intermediate transfer belt 7.

[0050] The CPU 301 measures the image density of the test image F based on the detection results of the image density sensors 70a-70d and detects image density unevenness (S14). The image density of the cyan, magenta, and yellow test images is measured based on the detection results of the light-receiving unit 73, which receives diffusely reflected light. The image density of the black test image is measured based on the detection results of the light-receiving unit 72, which receives specularly reflected light. The light-receiving unit 72 detects both the specularly reflected light component and the diffusely reflected light component. Therefore, the CPU 301 obtains the specularly reflected light component by performing a correction calculation to remove the diffusely reflected light component detected by the light-receiving unit 73 from the reflected light component detected by the light-receiving unit 72. The surface of the intermediate transfer belt 7 reflects a large amount of light, but there is almost no light reflected from the toner. Therefore, as the image density of the toner image increases, the specularly reflected light component detected by the light-receiving unit 72 decreases. The CPU 301 stores in advance the relationship between the image density of the toner image and the diffusely reflected light and specularly reflected light of each color, and obtains the image density of the toner image (test image) from the detected diffusely reflected light and specularly reflected light. By sequentially detecting the image density of the toner image at a predetermined sampling rate, the CPU 301 creates an image density profile of the test image F for each color.

[0051] CPU 301 generates a correction bias to be superimposed on each of the charging bias and the developing bias based on the image density profile of each color (S15). When distinguishing between the correction biases to be superimposed on each of the charging bias and the developing bias, the correction bias superimposed on the charging bias is called a "charging correction bias" and the correction bias superimposed on the developing bias is called a "developing correction bias." The correction bias is generated, for example, as follows.

[0052] The CPU 301 first extracts the periodic component of the developing sleeve from the amplitude and phase spectrum of each frequency component obtained by Fourier transforming the image density detection result (image density profile) of the test image F. The image forming apparatus 200 of this embodiment has a process speed of 240 mm / s and a developing sleeve with a diameter of φ20 mm. The developing sleeve is rotated at a peripheral speed that is 180% of the photosensitive drum 1. Therefore, the period Tdev of the developing sleeve is 145 milliseconds. The photosensitive drum 1 has a diameter of φ30 mm and is rotated at a linear speed of 240 mm / s. Therefore, the period Tdr of the photosensitive drum 1 is 392 milliseconds. Sub-scanning density unevenness is image density unevenness that occurs in the sub-scanning direction due to the period Tdr of the photosensitive drum 1 and the period Tdev of the developing sleeve.

[0053] Next, the CPU 301 generates a correction bias ΔVdc=Va×cos(ω1×t+θ1) for the DC component of the developing bias and a correction bias ΔVd=Vb×cos(ω2×t+θ2) for the DC component of the charging bias. The correction bias is a bias voltage for offsetting the sub-scanning density unevenness that occurs due to the period Tde of the photosensitive drum 1 and the period Tdev of the developing sleeve. The potential Vd of the non-exposed portion and the DC component Vdc of the developing bias are modulated in opposite phases, taking into account the phase difference due to the time difference between development and image density detection, so as to generate a development contrast that corresponds to the amplitude of the image density unevenness using the correction bias.

[0054] A specific method for generating the development correction bias will be described. The CPU 301 calculates a development contrast difference Va corresponding to the amplitude D from the amplitude D of the periodic component of the developing sleeve extracted in the process of detecting density unevenness in the sub-scanning direction and the slope of the development gamma characteristic. The phase θ is expressed as θ=Φ-ω×Δt+π. Δt is the time difference between the timing of detecting the image density and the timing of applying the development bias, and is expressed as Δt=ds / S using the process speed S and the distance ds from the development position to the detection position of the image density sensor 70. The values ​​of the development contrast difference Va, ω, and phase θ calculated in this series of processes are stored in the memory of the CPU 301.

[0055] After generating the correction bias, the CPU 301 then determines whether the total voltage of the amplitude Vb of the charge correction bias and the amplitude Va of the development correction bias constructed as described above is equal to or less than the threshold Vth of the fog removal potential Vback (S16). The amplitude Vb of the charge correction bias is the potential difference before and after correction of the charge bias. The amplitude Va of the development correction bias is the potential difference before and after correction of the development bias. In the image forming apparatus 200 of this embodiment, the relationship between the fog characteristics and carrier adhesion characteristics of the white background with respect to the fog removal potential Vback is as shown in FIG. 4. Therefore, the fog removal potential Vback that does not cause image quality degradation is in the range of 100 to 180 [V], and the Vback latitude is 80 [V]. Therefore, the threshold Vth of the fog removal potential Vback in this embodiment is set to 70 [V], allowing for a margin of an additional 10 [V].

[0056] If the total voltage of the amplitude Vb of the charge correction bias and the amplitude Va of the development correction bias is equal to or less than the threshold value Vth (70 V or less) (S16: Y), the CPU 301 stores the charge correction bias and the development correction bias generated in the process of S15 in the built-in memory (S17). This completes the adjustment mode.

[0057] If the total voltage of the amplitude Vb of the charge correction bias and the amplitude Va of the development correction bias exceeds the threshold Vth (exceeds 70 [V]) (S16:N), the CPU 301 performs bias correction to compensate for the insufficient correction of image density unevenness by the correction bias (S18). Here, the CPU 301 performs bias correction until the sum of the amplitude Vb of the charge correction bias and the amplitude Va of the development correction bias reaches 70 [V]. The CPU 301 creates a table showing the waveform of a laser power correction signal for the exposure unit 3 for the insufficient correction of image density unevenness calculated in the process of S18 (S19). The laser power correction signal is superimposed on the drive signal used by the laser driver 303 when driving the exposure unit 3, and corrects the light intensity of the laser light.

[0058] This section explains the case where the correction bias generated in the process of S15 is either the charging bias or the developing bias, and the amplitude of the correction bias alone exceeds the threshold value Vth of the fog removal potential Vback. For example, in the case of only the developing bias, the process of S15 generates a development correction bias of ΔVdc = Va × cos(ω1 × t + θ1). Because the amplitude Va of the development correction bias is greater than the threshold value Vth, the development correction bias is corrected so that Va = Vth. Although image density unevenness of (Va - Vth) remains, the period of the image density unevenness is ω1. θ1 is the phase difference between the detection timing of the home position of the developing sleeve and the image density unevenness. If correction is performed using laser light instead of the developing bias, the timing will be shifted by the distance between the exposure position and the development position of the photosensitive drum 1. Therefore, the waveform of the laser power correction signal from the exposure device 3 is ΔVl = (Va - Vth) × cos(ω1 × t + θ1 - θ2). Here, θ2 is the phase in which the developing sleeve rotates when the photosensitive drum 1 rotates from the exposure position to the development position of the photosensitive drum 1. The same applies when the correction bias generated in the process of S15 is only the charging bias.

[0059] The following describes a case where the correction bias generated in the process of S15 is both the charging bias and the developing bias, and the total amplitude of the correction bias exceeds the threshold value Vth of the fogging potential Vback. In this embodiment, bias correction for image density unevenness with a shorter period is prioritized. For example, if sub-scanning density unevenness with a period of the photosensitive drum 1 (392 milliseconds) and sub-scanning density unevenness with a period of the developing sleeve (145 milliseconds) occur, the image density unevenness with a period of the developing sleeve has a shorter period. Therefore, the development correction bias remains ΔVdc=Va×cos(ω1×t+θ1) generated in the process of S15.

[0060] The charging correction bias is calculated as ΔVd = (Vth - Va) × cos(ω2 × t + θ2) so that the amplitude of the waveform generated in S15 is equal to the threshold value Vth of the fogging potential Vback minus the amplitude Va of the development correction bias. Image density unevenness of Vb - (Vth - Va) remains, but the period of the image density unevenness is ω2. θ2 is the phase difference between the detection timing of the home position of the photosensitive drum 1 and the image density unevenness. When correction is performed using a laser rather than a charging bias, the timing is shifted by the distance between the charging position and the exposure position of the photosensitive drum 1. Therefore, the waveform of the laser power correction signal from the exposure device 3 is ΔVl = (Vb - Vth + Va) × cos(ω2 × t + θ2 + θ3). Here, θ3 is the phase difference of the photosensitive drum 1 from the charging position to the exposure position.

[0061] Fig. 11 is a diagram showing the relationship between the laser power during exposure by the exposure unit 3 and the potential of the photosensitive drum 1. Fig. 11 shows the relationship between the surface light amount and potential of the photosensitive drum 1 when the photosensitive drum 1 is charged to -700 [V] in the image forming apparatus 200 and exposed by changing the laser power of the exposure unit 3. Based on this relationship, the amplitude portion (Vb-Vth+Va) of the waveform of the laser power correction signal is converted from voltage to laser power to generate the waveform of the laser power correction signal.

[0062] By changing the laser power in accordance with the waveform of the image density unevenness calculated in the process of S18, the potential Vl of the exposed portion changes, which in turn changes the contrast potential Vcont, thereby correcting the image density unevenness that could not be corrected by bias correction.

[0063] The CPU 301 stores the charging correction bias and the development correction bias calculated in the process of S18 and the waveform of the laser power correction signal generated in the process of S19 in the built-in memory (S20). With the above, the adjustment mode is completed.

[0064] By performing the above-described process, the CPU 301 performs subsequent image formation processes under image formation conditions in which the bias correction waveform and the laser power correction signal waveform are superimposed. This suppresses the occurrence of periodic image density unevenness. The effect of correcting density unevenness in the sub-scanning direction using this process will be described below.

[0065] As shown in Figure 4, the upper and lower limits of the fog removal potential Vback are determined by the allowable limits of carrier adhesion and fog, respectively, and the Vback latitude is 80 [V]. In this embodiment, the threshold value Vth of the fog removal potential Vback is set to 70 [V], allowing for an additional margin of 10 [V]. As shown in Figure 3, when the slope of the development gamma characteristic is ΔD = 0.004 per development contrast of 1 [V], the development contrasts corresponding to amplitudes D = 0.2 and D = 0.4 of the periodic component of sub-scanning density unevenness are 50 [V] and 100 [V], respectively.

[0066] Fig. 12 is a table showing the effects of this embodiment. Fig. 12 shows the correction conditions for sub-scanning density unevenness correction, and the results of image density unevenness, fog, and carrier adhesion before and after sub-scanning density unevenness correction. For fog and carrier adhesion, "O" is indicated if it is within the allowable limit, and "X" if it is above the allowable limit. The allowable limits for fog and carrier adhesion are, respectively, a fog reflectance on the photosensitive drum 1 of 1.5% or less, and a carrier adhesion count on the photosensitive drum 1 of 10 particles / cm^2 or less.

[0067] When the image density unevenness ΔD=0.2 under condition (1) is achieved, the development contrast is 50 [V], which is below the threshold value of the fog removal potential Vback. Therefore, even if the charging bias Vd and the development bias Vd are modulated, the contrast remains within the Vback latitude range. When the image density unevenness ΔD=0.4 under condition (2) is achieved without setting a threshold value for the fog removal potential Vback, the development contrast must be corrected by 100 [V]. Therefore, correction beyond the Vback latitude can cause problems such as fog carrier deposition in non-exposed areas. When a threshold value for the fog removal potential Vback under condition (3) is set and no further correction is performed, the correction is insufficient, resulting in an image density unevenness of ΔD=1.2. Although condition (4) is achieved using the correction control of this embodiment, the image density unevenness can be suppressed to ΔD=0.06. Furthermore, problems such as fog carrier deposition in non-exposed areas do not occur.

[0068] As explained above, by correcting sub-scanning density unevenness, it is possible to reduce periodic image density fluctuations that occur in the sub-scanning direction. Furthermore, by adjusting the fogging removal potential Vback within an appropriate range, it is possible to prevent deterioration in the quality of the finished product due to fogging and device failure due to carrier adhesion.

[0069] In the above description, the test image is read when correcting image density unevenness using the image density sensor 70 on the intermediate transfer belt 7. In order to detect image density unevenness with higher accuracy, the test image may be printed on paper S, and after printing, the test image on paper S may be read using a sensor or scanner.

[0070] (Second embodiment) The configuration of the image forming apparatus 200 of the second embodiment is similar to that of the first embodiment shown in Fig. 1. In the first embodiment, the sub-scanning density unevenness correction process is performed with the threshold value of the fogging potential Vback set to 70 [V]. In contrast, in the second embodiment, the threshold value of the fogging potential Vback is changed depending on the environmental conditions of the surrounding environment of the image forming apparatus 200, and the sub-scanning density unevenness correction process is performed.

[0071] In the second embodiment, an environmental sensor 80 (see FIG. 1) is used to detect the ambient environment of the image forming apparatus 200. The environmental sensor 80 is a temperature and humidity sensor that measures both the temperature and relative humidity around the image forming apparatus 200. The environmental sensor 80 is disposed near the periphery of the image forming apparatus 200.

[0072] (Control unit) 13 is a configuration diagram of a control unit of the second embodiment. The control unit of the second embodiment has a configuration in which an environment detection unit 307 and an environment sensor 80 are added to the control unit of the first embodiment shown in FIG. 8. Description of the same configuration as in the first embodiment will be omitted. The environment detection unit 307 amplifies an electrical signal related to temperature and relative humidity output from the environment sensor 80 and transmits it to the CPU 301 as the detection result of the environment sensor 80. The CPU 301 detects the installation environment conditions (temperature, relative humidity) of the image forming apparatus 200 based on the detection result of the environment sensor 80 obtained from the environment detection unit 307.

[0073] (Sub-scanning density unevenness correction) FIG. 14 is a flowchart showing the sub-scanning density unevenness correction process performed by the image forming apparatus 200 of the second embodiment.

[0074] When a user or serviceman issues an instruction to execute sub-scanning density unevenness correction processing via the operation unit 140, the CPU 301 activates an adjustment mode for performing various adjustments (S31). The CPU 301 detects the temperature and relative humidity around the image forming apparatus 200 using the environment sensor 80 (S32). Based on the detected temperature T (°C) and relative humidity Rh (%), the CPU 301 calculates the absolute moisture content using the following (Equation 1) and (Equation 2). Absolute water content = (Rws × Rh) / (273 + T) ... (Equation 1) Rsw=6.1164×10^[{7.591×(273+T)} / {240.7+(273+T)}] …(Formula 2)

[0075] The CPU 301 sets a threshold value Vth of the fog removal potential Vback based on the calculated absolute moisture amount (S33). Fig. 15 is an explanatory diagram of the relationship between the fog removal potential Vback and the fog and carrier adhesion. In this embodiment, the Vback latitude is the range of the fog removal potential Vback that satisfies the fog reflection density on the photosensitive drum 1 of 1.5% or less and the number of carrier adhesion particles on the photosensitive drum 1 of 10 particles / cm^2 or less.

[0076] The open triangles indicate the fog reflection density for unexposed areas in a low-moisture environment (temperature 23°C / relative humidity 5%RH: absolute moisture content 1g / m^3). The closed triangles indicate the number of carrier particles adhering to unexposed areas in a low-moisture environment. The open squares indicate the fog reflection density for unexposed areas in a high-moisture environment (temperature 30°C / relative humidity 80%RH: absolute moisture content 22g / m^3). The closed squares indicate the number of carrier particles adhering to unexposed areas in a high-moisture environment.

[0077] In a low-moisture environment, there is less moisture adhering to the polyester resin and charge control agent of non-magnetic toner, and to the resin coating of magnetic carrier, resulting in higher electrical resistivity. Frictional charging of the toner and carrier results in a highly negative charge on the toner, and a highly positive charge on the carrier. As shown in Figure 2, the potential Vd of the non-exposed area is more negative than the developing sleeve potential Vdc, making it difficult for negatively charged toner to move to the non-exposed area. Conversely, positively charged carriers tend to move more easily to the non-exposed area.

[0078] In the image forming apparatus 200 of this embodiment, the fog reflection density is 1.5% or less when the fog removal potential Vback is 70 V or more, and the number of carrier particles attached is 10 or less when the fog removal potential Vback is 170 V or less. Therefore, the range in which the fog removal potential Vback can be used without any problems is 70 to 170 V, with a Vback latitude of 100 V.

[0079] In a high-moisture environment, moisture adheres to the polyester resin and charge control agent of the non-magnetic toner and the resin coating of the magnetic carrier, resulting in a low electrical resistivity. By frictionally charging the toner and carrier, the toner becomes negatively charged and the carrier becomes positively charged. As a result, the negatively charged toner easily migrates to unexposed areas, while the positively charged carrier does not easily migrate to unexposed areas. In the image forming apparatus 200 of this embodiment, the fog reflection density is 1.5% or less when the fog removal potential Vback is 130 V or higher, and the number of carrier particles attached is 10 or less when the fog removal potential Vback is 185 V or lower. Therefore, the range in which the fog removal potential Vback can be used without problems is 130 to 185 V, with a Vback latitude of 55 V.

[0080] Therefore, if the charging bias or development correction bias with large amplitudes is superimposed in an image forming apparatus 200 installed in a high moisture environment, problems such as fogging in non-exposed areas and carrier adhesion are likely to occur. Figure 16 is an explanatory diagram of the absolute moisture content of the installation environment conditions detected by the environmental sensor 80 and the Vback threshold value for the sub-scanning density unevenness correction process. As shown in Figure 16, the higher the absolute moisture content of the environment, the smaller the threshold Vth of the fogging potential Vback.

[0081] After generating the threshold value Vth of the fog-removing potential Vback, the CPU 301 generates a correction bias (S35 to S38) by the same processes as S12 to S15 in Fig. 9. After generating the correction bias, the CPU 301 determines whether the total voltage of the amplitude Vb of the charging correction bias and the amplitude Va of the development correction bias is equal to or less than the threshold value Vth of the fog-removing potential Vback generated according to the absolute moisture content of the installation environment conditions (S38).

[0082] If the total voltage of the amplitude Vb of the charging correction bias and the amplitude Va of the development correction bias is equal to or less than the threshold value Vth of the fogging potential Vback (S38: Y), the CPU 301 stores the correction bias generated in the process of S37 in the built-in memory (S39). This completes the adjustment mode.

[0083] If the total voltage of the amplitude Vb of the charge correction bias and the amplitude Va of the development correction bias exceeds the threshold Vth of the fog removal potential Vback (S38:N), the CPU 301 performs bias correction to make up for the insufficient correction of image density unevenness by the correction bias (S40). Here, the CPU 301 performs bias correction until the total of the amplitude Vb of the charge correction bias and the amplitude Va of the development correction bias becomes the threshold Vth of the fog removal potential Vback generated according to the absolute moisture content of the installation environment conditions.

[0084] 9, the CPU 301 creates a table showing the waveform of the laser power correction signal of the exposure unit 3 for the insufficient correction of the image density unevenness calculated in the process of S40 (S41). By changing the laser power in accordance with the waveform of the image density unevenness calculated in the process of S40, the exposed portion potential Vl changes. As a result, the contrast potential Vcont changes, and the image density unevenness that could not be fully corrected by the bias correction is corrected.

[0085] The CPU 301 stores the charging correction bias and development correction bias calculated in the process of S37 and the waveform of the laser power correction signal generated in the process of S41 in the built-in memory (S42). With this, the adjustment mode is completed.

[0086] By performing the above-described process, the CPU 301 performs subsequent image formation processes under image formation conditions in which the bias correction waveform and the laser power correction signal waveform are superimposed. This suppresses the occurrence of periodic image density unevenness. The effect of correcting density unevenness in the sub-scanning direction using this process will be described below.

[0087] In the second embodiment, the environmental sensor 80 detects the installation environment conditions, such as temperature and relative humidity, and changes the threshold value of the fog removal potential Vback depending on the installation environment conditions. This makes it possible to reduce periodic image density fluctuations that occur in the sub-scanning direction, even in environments with different absolute moisture contents. Furthermore, by changing the fog removal potential Vback within an appropriate range, it is possible to prevent degradation of the print quality due to fog and device failure due to carrier adhesion.

[0088] As in the first embodiment, in order to detect image density unevenness with higher accuracy, the test image to be read during image density unevenness correction may be configured so that the test image is printed on paper S and then read by a sensor or scanner after printing.

[0089] As described above in the first and second embodiments, the image forming apparatus 200 acquires periodic fluctuation information indicating periodic fluctuations in image density unevenness from the detection results of the image density of a test image. Based on this periodic fluctuation information, the image forming apparatus 200 periodically varies at least one of the charging bias and the developing bias to correct the image density unevenness. In this process, the image forming apparatus 200 controls the amplitude of at least one of the charging bias and the developing bias so that the fluctuation amounts of the charging potential of the photosensitive drum 1 and the developing bias are equal to or less than a predetermined threshold. Furthermore, the image forming apparatus 200 also controls the amount of laser light output from the exposure device 3. This allows the image forming apparatus 200 to correct periodic image density unevenness that occurs during image formation within a range in which the fogging potential does not deviate from an appropriate range, thereby achieving both prevention of fogging and carrier adhesion and reduction of in-plane image density unevenness.

Claims

1. an image forming means including a rotating photosensitive member, a charger that charges the photosensitive member based on a charging bias, an exposure unit that exposes the photosensitive member to light in order to form an electrostatic latent image on the photosensitive member charged by the charger, and a developing unit that develops the electrostatic latent image based on a developing bias to form an image on the photosensitive member; reading means for reading the test image formed by the image forming means; an environmental sensor for detecting an environmental condition; a control means; the control means determines a threshold value of a fogging potential such that the amount of toner and the amount of carrier adhered to the non-exposed portion of the photosensitive member fall within a specified range, based on the environmental conditions detected by the environmental sensor; the control means determines, based on the reading result of the test image by the reading means, a charge correction bias to be superimposed on the charge bias and a development correction bias to be superimposed on the development bias so that the sum of the amplitude of the charge correction bias and the amplitude of the development correction bias is equal to or less than the threshold value; the control means controls the charging bias based on the charging correction bias, and controls the developing bias based on the developing correction bias; the charge correction bias is a bias that suppresses periodic fluctuations in density of an image formed in a rotation direction of the photosensitive member, the development correction bias is a bias that suppresses periodic fluctuations in density of an image formed in a rotation direction of the photosensitive member, Image forming device.

2. the control means controls the amount of laser light output from the exposure device based on the result of reading the test image by the reading means so as to suppress periodic fluctuations in density of the image.

2. The image forming apparatus according to claim 1.

3. when a sum of the amplitudes of the charging correction bias and the developing correction bias exceeds the threshold value, the control means corrects at least one of the charging correction bias and the developing correction bias so that the sum is equal to or less than the threshold value.

2. The image forming apparatus according to claim 1.

4. the threshold value is determined based on a difference between the potential to which the photosensitive member is charged by the charger and the developing bias.

2. The image forming apparatus according to claim 1.

5. the environmental conditions include temperature; The control means determines the threshold value based on the temperature detected by the environmental sensor.

2. The image forming apparatus according to claim 1.

6. the environmental conditions include humidity; The control means determines the threshold value based on the humidity detected by the environmental sensor.

2. The image forming apparatus according to claim 1.

7. the environmental conditions include temperature and humidity; The control means calculates an absolute moisture content based on the temperature and humidity detected by the environmental sensor, and determines the threshold value based on the calculated absolute moisture content.

2. The image forming apparatus according to claim 1.

Images