Image formation apparatus

Abstract

To provide an image formation apparatus capable of accurately estimating the surface potential of a photosensitive body without causing contamination of a transfer member by toner, and capable of adjusting the surface potential on the basis of the estimation result.SOLUTION: The image formation apparatus includes: an image carrier having a photosensitive layer formed on a surface thereof; a charging device; a developing device; a transfer member; a developing voltage power supply; a charging voltage power supply; a transfer voltage power supply; a current detection unit; a control unit; and a storage unit, the storage unit being configured to store, as a correction value, the difference between a charging voltage for obtaining a predetermined surface potential in a separated state in which the transfer member is separated from the image carrier and a charging voltage for obtaining the predetermined surface potential in a contact state in which the transfer member is brought into contact with the image carrier and a transfer voltage is applied; the control unit being configured to estimate a surface potential in the separated state on the basis of a developing current detected by the current detection unit and to obtain a charging voltage in the contact state by adding the correction value stored in the storage unit to the charging voltage for achieving the surface potential in the separated state.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an electrophotographic image forming apparatus, and more particularly to a method for estimating the surface potential of a photosensitive member, which is an image carrier, and adjusting the surface potential based on the estimation result. [Background technology]

[0002] In conventional electrophotographic image forming apparatuses, an image carrier, such as a photosensitive drum with a photosensitive layer formed on its surface, is charged to a predetermined surface potential, and an exposure device scans the image with light to form an electrostatic latent image with the charge attenuated. This electrostatic latent image is then developed by a developing device and visualized as a toner image. In such image forming apparatuses, to obtain high-quality images, development must be performed using a DC voltage Vdc that provides an appropriate potential difference (development potential difference ΔV = V0 - Vdc) with respect to the surface potential V0 of the image carrier. Therefore, the surface potential of the image carrier must be detected, and conventionally, this has been detected using a surface potential sensor.

[0003] However, surface potential sensors are expensive, and there is a problem that if scattered toner or the like adheres to the detection surface, the surface potential cannot be measured accurately. Therefore, various techniques for detecting the surface potential of an image carrier without using a surface potential sensor have been proposed.

[0004] Patent Document 1 discloses an electrophotographic device that includes a circuit that forms a pulsed electrostatic potential pattern on a photosensitive member and detects the current that flows when this electrostatic potential pattern is developed, a circuit that obtains the surface potential on the photosensitive member by converting the detected current into a surface potential based on a predetermined relational expression or using a pre-created correlation table, and a circuit that feeds back the obtained surface potential to control the charging of the photosensitive member by a charger. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-295540 Summary of the Invention [Problem to be solved by the invention]

[0006] Because the surface potential changes under the influence of the transfer voltage, accurate detection of the surface potential requires pressure on the transfer roller and application of the transfer voltage, which leads to problems such as time required to clean the toner adhering to the transfer roller and the possibility of the fogging toner adhering to the transfer roller scattering and contaminating the inside of the device.

[0007] In view of the above problems, the present invention aims to provide an image forming apparatus that can accurately estimate the surface potential of a photosensitive member without causing contamination of the transfer member with toner, and can adjust the surface potential based on the estimation results. [Means for solving the problem]

[0008] To achieve the above object, a first aspect of the present invention is an image forming apparatus including an image carrier, a charging device, a developing device, an image carrier having a photosensitive layer formed on its surface, a transfer member, a developing voltage power supply, a charging voltage power supply, a transfer voltage power supply, a current detection unit, a control unit, and a memory unit. The image carrier has a photosensitive layer formed on its surface. The charging device has a charging member that charges the surface of the image carrier to a predetermined surface potential. The developing device has a developer carrier that carries a developer containing toner, and forms a toner image by exposing the image carrier charged by the charging device to light, thereby causing the toner to adhere to an electrostatic latent image formed on the image carrier. The transfer member contacts the image carrier to form a transfer nip, and transfers the toner image formed on the surface of the image carrier to a recording medium or an intermediate transfer member. The developing voltage power supply applies a developing voltage including a DC voltage Vdc to the developer carrier. The charging voltage power supply applies a charging voltage to the charging member. The transfer voltage power supply applies a transfer voltage of opposite polarity to that of the toner to the transfer member. The current detection unit detects the development current flowing between the image carrier and the developer carrier. The control unit controls the development voltage power supply, the charging voltage power supply, and the transfer voltage power supply. The memory unit stores, as a correction value, the difference between the charging voltage required to obtain a predetermined surface potential when the transfer member is separated from the image carrier and the charging voltage required to obtain a predetermined surface potential when the transfer member is in contact with the image carrier and a transfer voltage is applied. The control unit measures the development voltage and the development current when the charging voltage is not applied to the developer carrier and the charging member, respectively, and sets these as reference currents. The control unit applies a predetermined charging voltage to the charging member in the separated state to charge the image carrier to a predetermined surface potential. The control unit estimates the DC voltage Vdc detected by the current detection unit when the development current becomes equal to the reference current as the surface potential in the separated state. The control unit then executes a surface potential adjustment process to obtain the charge voltage required to obtain the surface potential in the contact state by adding the correction value for the charging voltage stored in the memory unit to the charge voltage required to obtain the surface potential in the separated state. [Effects of the Invention]

[0009] According to the first aspect of the present invention, by measuring the development current transition while the transfer member is separated from the image carrier, contamination of the transfer member and recording medium by fogging toner can be effectively suppressed. Furthermore, by acquiring a correction value for the charging voltage due to the influence of the transfer voltage in advance and storing it in a memory unit, and adding it to the charging voltage determined without considering the influence of the transfer voltage, the charging voltage can be determined with the same accuracy as when the influence of the transfer voltage is considered, and the surface potential can be maintained stably over a long period of time. Furthermore, since cleaning of the transfer member is not required, the time required for adjusting the surface potential can be shortened. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a side cross-sectional view showing the internal configuration of an image forming apparatus 100 according to an embodiment of the present invention. [Figure 2] 1 is a partially enlarged view of the image forming unit Pa and its surroundings, including a control path of the image forming apparatus 100; [Figure 3] Graph showing the relationship between the DC voltage Vdc of the developing voltage and the developing current in a conventional method for estimating the surface potential. [Figure 4] Graph showing the change in development current with respect to charging voltage when a constant development voltage (DC voltage Vdc) is applied, with and without pressing primary transfer rollers 6a to 6d and applying primary transfer voltage. [Figure 5] 1 is a flowchart showing an example of control of a surface potential estimation process executed in the image forming apparatus 100 of this embodiment. [Figure 6] Graph showing the change in surface potential with respect to the number of sheets used in the example. DETAILED DESCRIPTION OF THE INVENTION

[0011] [1. Configuration of image forming device] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a cross-sectional view showing the internal structure of an image forming apparatus 100 according to one embodiment of the present invention. Within the main body of the image forming apparatus 100 (here, a color printer), four image forming units Pa, Pb, Pc, and Pd are arranged in this order from the upstream side in the transport direction (left side in Fig. 1). These image forming units Pa to Pd are provided corresponding to images of four different colors (yellow, magenta, cyan, and black), and sequentially form images of yellow, cyan, magenta, and black through the processes of charging, exposure, development, and transfer, respectively.

[0012] Each of the image forming stations Pa through Pd is provided with photosensitive drums (image carriers) 1a, 1b, 1c, and 1d, which carry visible images (toner images) of each color. An intermediate transfer belt 8, which rotates counterclockwise in FIG. 1 by a belt drive motor (not shown), is provided adjacent to each of the image forming stations Pa through Pd. The toner images formed on the photosensitive drums 1a through 1d are sequentially transferred (primary transfer) onto the intermediate transfer belt 8, which moves while contacting the photosensitive drums 1a through 1d, and superimposed on each other. The toner images primarily transferred onto the intermediate transfer belt 8 are then secondarily transferred onto a transfer sheet P (an example of a recording medium) by a secondary transfer roller 9. The transfer sheet P, onto which the toner images have been secondarily transferred, is then fixed in a fixing unit 13 and ejected from the image forming apparatus 100. While the photosensitive drums 1a through 1d rotate clockwise in FIG. 1, an image formation process is performed on each of the photosensitive drums 1a through 1d.

[0013] The transfer paper P onto which the toner image is secondarily transferred is stored in a paper cassette 16 located at the bottom of the main body of the image forming apparatus 100, and is transported via a paper feed roller 12a and a pair of registration rollers 12b to the nip between the secondary transfer roller 9 and the drive roller 11 of the intermediate transfer belt 8. A sheet made of dielectric resin is used for the intermediate transfer belt 8, and a seamless belt is usually used. In addition, a belt cleaning device 19 is located downstream of the secondary transfer roller 9 to remove toner and other particles remaining on the surface of the intermediate transfer belt 8.

[0014] Next, the image forming units Pa to Pd will be described. Around and below the rotatably arranged photosensitive drums 1a to 1d, there are provided charging devices 2a, 2b, 2c, and 2d that charge the photosensitive drums 1a to 1d, an exposure device 5 that exposes image information onto each of the photosensitive drums 1a to 1d, developing devices 3a, 3b, 3c, and 3d that form toner images on the photosensitive drums 1a to 1d, and cleaning devices 7a, 7b, 7c, and 7d that remove toner (residual toner) remaining on the photosensitive drums 1a to 1d.

[0015] When image data is input from a host device such as a personal computer, the surfaces of the photosensitive drums 1a-1d are first uniformly charged by the charging devices 2a-2d. Next, the exposure device 5 irradiates light according to the image data, forming electrostatic latent images on the photosensitive drums 1a-1d in accordance with the image data. The developing devices 3a-3d are filled with a predetermined amount of two-component developer containing yellow, magenta, cyan, and black toner, respectively. If the toner content in the two-component developer filled in each developing device 3a-3d falls below a specified value due to the formation of a toner image (described below), toner is replenished from toner containers 4a-4d to each developing device 3a-3d. The toner in the developer is supplied to the photosensitive drums 1a-1d by the developing devices 3a-3d and electrostatically adheres to the photosensitive drums 1a-1d, forming a toner image corresponding to the electrostatic latent image formed by exposure from the exposure device 5.

[0016] Then, primary transfer rollers 6a-6d apply an electric field at a predetermined transfer voltage between the primary transfer rollers 6a-6d and the photosensitive drums 1a-1d, and the yellow, magenta, cyan, and black toner images on the photosensitive drums 1a-1d are primarily transferred onto the intermediate transfer belt 8. These four color images are formed in a predetermined positional relationship for forming a predetermined full-color image. After that, toner and other substances remaining on the surfaces of the photosensitive drums 1a-1d after the primary transfer are removed by cleaning devices 7a-7d in preparation for the subsequent formation of a new electrostatic latent image.

[0017] The intermediate transfer belt 8 is stretched over a driven roller 10 on the upstream side and a drive roller 11 on the downstream side. When the intermediate transfer belt 8 starts to rotate counterclockwise as the drive roller 11 is rotated by a belt drive motor (not shown), the transfer paper P is transported from the registration roller pair 12b to a nip portion (secondary transfer nip portion) between the drive roller 11 and the adjacent secondary transfer roller 9 at a predetermined timing, where the toner image on the intermediate transfer belt 8 is secondarily transferred onto the transfer paper P. After the secondary transfer, any toner remaining on the surface of the intermediate transfer belt 8 is removed by a belt cleaner 19. The transfer paper P onto which the toner image has been secondarily transferred is transported to a fixing unit 13.

[0018] The transfer paper P transported to the fixing unit 13 is heated and pressurized by the fixing roller pair 13a, and the toner image is fixed to the surface of the transfer paper P, forming a predetermined full-color image. The transfer paper P on which the full-color image has been formed is then transported in different directions by the branching unit 14, which branches into multiple directions, and is then discharged directly (or after being sent to the double-sided transport path 18 and having images formed on both sides) onto the discharge tray 17 by the discharge roller pair 15.

[0019] [2. Configuration of the image forming unit and its peripherals, including the control path of the image forming device] 2 is a partially enlarged view of the periphery of image forming unit Pa, including the control paths of image forming apparatus 100. In the following explanation, the configuration and control paths of image forming unit Pa will be described, but the configurations and control paths of image forming units Pb to Pd are similar, so their explanation will be omitted.

[0020] The developing device 3a includes a developing roller 30 that carries a two-component developer therein. The developing roller 30 is connected to a developing voltage power supply 43 that generates an oscillating voltage in which a DC voltage Vdc and an AC voltage Vac are superimposed.

[0021] The developing voltage power supply 43 includes an AC constant voltage power supply 43a and a DC constant voltage power supply 43b. During image formation, the developing voltage power supply 43 outputs a developing voltage in which an AC voltage Vac is superimposed on a DC voltage Vdc from the AC constant voltage power supply 43a and the DC constant voltage power supply 43b. The AC constant voltage power supply 43a outputs a sine wave AC voltage Vac generated from a low voltage DC voltage modulated into a pulse shape using a step-up transformer (not shown). The DC constant voltage power supply 43b outputs a DC voltage Vdc obtained by rectifying the sine wave AC voltage Vac generated from a low voltage DC voltage modulated into a pulse shape using a step-up transformer.

[0022] The current detection unit 44 detects the development current flowing between the photosensitive drum 1a and the development roller 30. The development current flowing to the current detection unit 44 may be caused by the movement of toner between the photosensitive drums 1a to 1d and the development roller 30, or may be caused by the movement of electric charge.

[0023] The charging voltage power supply 45 applies a charging voltage consisting of a DC voltage to the charging roller 34 of the charging device 2a. The configuration of the charging voltage power supply 45 is similar to that of the DC constant voltage power supply 43b of the developing voltage power supply 43.

[0024] Transfer voltage power supply 47 applies a primary transfer voltage and a secondary transfer voltage of opposite polarity (negative polarity) to the toner to primary transfer rollers 6a to 6d and secondary transfer roller 9 (see FIG. 1), respectively. In this embodiment, constant current control is used to apply to primary transfer rollers 6a to 6d and secondary transfer roller 9 a transfer voltage that causes a constant current (transfer current) of opposite polarity (negative polarity) to the toner to flow through primary transfer rollers 6a to 6d and secondary transfer roller 9.

[0025] The cleaning device 7a includes a collection roller 31 that temporarily holds the residual toner remaining on the surface of the photosensitive drum 1a. The collection roller 31 is connected to a cleaning voltage power supply 48 that generates a DC voltage. The cleaning voltage power supply 48 applies a voltage of the same polarity (positive polarity) as that of the toner and a voltage of the opposite polarity (negative polarity) to the collection roller 31. The configuration of the cleaning voltage power supply 48 is similar to that of the DC constant voltage power supply 43b.

[0026] Next, the control system of image forming apparatus 100 will be described with reference to Fig. 2. Image forming apparatus 100 is provided with a main control unit 80 constituted by a CPU and the like. Main control unit 80 is connected to a storage unit 70 constituted by a ROM, RAM and the like. Main control unit 80 controls each unit of image forming apparatus 100 (charging devices 2a-2d, developing devices 3a-3d, exposure device 5, primary transfer rollers 6a-6d, cleaning devices 7a-7d, secondary transfer roller 9, fixing unit 13, developing voltage power supply 43, charging voltage power supply 45, transfer voltage power supply 47, cleaning voltage power supply 48, voltage control unit 50, etc.) based on the control program and control data stored in storage unit 70.

[0027] Based on control signals sent from the main control unit 80, the voltage control unit 50 controls a development voltage power supply 43 that applies a development voltage to the development roller 30, a charging voltage power supply 45 that applies a charging voltage to the charging roller 34, a transfer voltage power supply 47 that applies a transfer voltage to the primary transfer rollers 6a to 6d and the secondary transfer roller 9, and a cleaning voltage power supply 48 that applies a voltage to the collection roller 31. The voltage control unit 50 may be configured by a control program stored in the storage unit 70.

[0028] The internal temperature and humidity sensor 60 constantly detects the temperature and relative humidity inside the image forming apparatus 100, specifically in the vicinity of the image forming units Pa to Pd. The detected temperature and humidity are sent to the main control unit 80.

[0029] The main control unit 80 is connected to a liquid crystal display unit 90 and a transmission / reception unit 91. The liquid crystal display unit 90 functions as a touch panel for the user to make various settings for the image forming apparatus 100, and also displays the status of the image forming apparatus 100, the image formation status, the number of printed pages, etc. The transmission / reception unit 91 communicates with the outside world using a telephone line or an internet line. The transmission / reception unit 91 functions as an input unit that receives print commands and image data from a higher-level device such as a personal computer.

[0030] [3. Photoconductor drum surface potential adjustment process] The surface potential adjustment process of the photosensitive drums 1a to 1d, which is a characteristic feature of the present invention, will be described below.

[0031] (Conventional methods for estimating and adjusting surface potential) 3 is a graph showing the relationship between the DC voltage Vdc of the developing voltage and the developing current in a conventional surface potential estimation method. When no DC voltage (0 V) is applied to the developing roller 30 and the charging roller 34, the current detection unit 44 detects the uncharged developing current, which is the sum of the current that flows when toner moves from the developing roller 30 to the photosensitive drums 1a-1d and the current that flows to the photosensitive drums 1a-1d via the magnetic brush formed on the developing roller 30. This uncharged developing current is defined as the reference current Cs (Vdc = 0 V, surface potential V0 = 0 V, development potential difference ΔV = V0 - Vdc = 0 V).

[0032] Then, with a predetermined charging voltage applied to the charging roller 34 to charge the photosensitive drums 1a to 1d to a surface potential V0, the DC voltage Vdc is gradually changed (increased) from an initial value (e.g., 300 V) that is smaller than the surface potential V0, and the progress of the development current detected by the current detection unit 44 is measured.

[0033] When the DC voltage Vdc becomes equal to the surface potential of the photosensitive drums 1a-1d (ΔV=0 [V]), only the non-charging development current (reference current Cs) flows between the development roller 30 and the photosensitive drums 1a-1d. Therefore, the DC voltage Vdc (460 [V] in FIG. 3) at point P1, where the curve L representing the change in the development current intersects with the reference current Cs, is estimated as the surface potential of the photosensitive drums 1a-1d. The surface potential is then adjusted by correcting the charging voltage applied to the charging roller 34 based on the estimated surface potential.

[0034] As mentioned above, the surface potential of the photosensitive drums 1a to 1d is affected by the transfer voltage (primary transfer voltage). Therefore, in the conventional estimation method shown in FIG. 3, the surface potential is estimated with the primary transfer rollers 6a to 6d in contact with the photosensitive drums 1a to 1d via the intermediate transfer belt 8. Here, the surface potential is estimated with a large development current (>0 μA), which causes toner to migrate from the development roller 30 to the photosensitive drums 1a to 1d (fog toner). Therefore, during printing immediately after the surface potential estimation, the intermediate transfer belt 8 and secondary transfer roller 9 become contaminated with toner, necessitating cleaning of the intermediate transfer belt 8 and secondary transfer roller 9.

[0035] (Surface Potential Estimation and Adjustment Method of the Present Embodiment) 4 is a graph showing the change in development current versus charging voltage when a constant development voltage (DC voltage Vdc) is applied, with and without pressing the primary transfer rollers 6a to 6d and applying the primary transfer voltage. In FIG. 4, the solid line indicates the case where the primary transfer rollers 6a to 6d are pressed and the primary transfer voltage is applied, and the dashed line indicates the case where the primary transfer rollers 6a to 6d are not pressed and the primary transfer voltage is not applied.

[0036] As shown in Figure 4, the surface potential is offset by the influence of the primary transfer voltage, causing the entire graph to be offset. More specifically, when the primary transfer voltage is applied, the intersection P between the curve representing the change in development current and the charging voltage shifts to the right (intersection P') compared to when the primary transfer voltage is not applied. In other words, the intersection point between the curve representing the change in development current and the reference current Cs in Figure 3 also shifts, causing the surface potential to change due to the influence of the primary transfer voltage.

[0037] Therefore, if the influence of the primary transfer voltage (the difference d between the intersections P and P' in FIG. 4) is known in advance, it becomes possible to accurately estimate the surface potential even when the primary transfer rollers 6a to 6d and the intermediate transfer belt 8 are separated. Then, as in the conventional method, the surface potential is adjusted by correcting the charging voltage applied to the charging roller 34 based on the estimated surface potential.

[0038] The transition of the development current with respect to the change in the charging voltage when the primary transfer rollers 6a to 6d are in the pressed state and the separated state (FIG. 4) is preferably measured in advance by experiment or simulation (correction value acquisition process) during the manufacture of the image forming apparatus 100. Then, the difference d in the charging voltage due to the influence of the primary transfer voltage obtained from the transition of the development current is preferably stored in the storage unit 70 as a correction value.

[0039] 5 is a flowchart showing an example of control of the surface potential adjustment process executed in the image forming apparatus 100 of this embodiment. The execution procedure of the surface potential adjustment process will be described along the steps of FIG. 5, with reference to FIGS. 1 to 4 as needed.

[0040] First, the main control unit 80 determines whether it is time to adjust the surface potential of the photosensitive drums 1a to 1d (step S1). Examples of timing for adjusting the surface potential include when the cumulative number of printed sheets since the start of use of the photosensitive drums 1a to 1d or the charging roller 34 reaches a predetermined number, or when the cumulative number of printed sheets since the previous surface potential adjustment reaches a predetermined number. If it is not time to adjust the surface potential (No in step S1), the process ends without performing the surface potential adjustment process.

[0041] If it is time to adjust the surface potential (No in step S1), the main control unit 80 separates the primary transfer rollers 6a to 6d from the photosensitive drums 1a to 1d (step S2), which also separates the intermediate transfer belt 8 from the photosensitive drums 1a to 1d.

[0042] Next, the main control unit 80 applies 0 [V] as the DC voltage Vdc and the charging voltage to the developing roller 30 and the charging roller 34, respectively (step S3). Then, the current detection unit 44 detects the developing current flowing in this state (Vdc=0 [V], V0=0 [V], ΔV=V0-Vdc=0 [V]), and the detected developing current is set as the reference current Cs (step S4).

[0043] Next, the main control unit 80 applies a predetermined charging voltage to the charging roller 34 to charge the photosensitive drums 1a-1d to a certain surface potential V0 (step S5). Then, the DC voltage Vdc is varied from an initial value (e.g., 300 V) that is smaller than the surface potential V0, and the development current is measured (step S6). The main control unit 80 estimates the DC voltage Vdc (see FIG. 3) at the point where the curve recording the development current transition intersects with the reference current Cs as the surface potential of the photosensitive drums 1a-1d (step S7).

[0044] Next, the main control unit 80 calculates the charging voltage (pre-correction charging voltage) without considering the influence of the primary transfer voltage from the estimated surface potential (step S8). Specifically, using the surface potential estimated in step S7 and the relationship between the charging voltage and the development current shown in Figure 4, the main control unit 80 calculates the charging voltage (=1200 [V]) at point P where the curve showing the transition of the development current intersects with the reference current Cs.

[0045] Finally, the main control unit 80 reads out the correction value (difference d, 10 [V] in FIG. 4) of the charging voltage due to the influence of the primary transfer voltage stored in the memory unit 70, adds it to the pre-correction charging voltage, and determines the final charging voltage (corrected charging voltage) to be applied to the charging roller 34 (step S9).

[0046] According to the surface potential adjustment process shown in Figures 4 and 5, the development current flowing between the photosensitive drums 1a to 1d and the development roller 30 is detected, and the surface potential is estimated based on the detected development current, thereby eliminating the need for a dedicated potential measuring device for estimating the surface potential, which also contributes to reducing the cost of the image forming apparatus 100.

[0047] Furthermore, by measuring the development current transition while the primary transfer rollers 6a to 6d and the intermediate transfer belt 8 are spaced apart from the photosensitive drums 1a to 1d, it is possible to effectively suppress contamination of the intermediate transfer belt 8 and the secondary transfer roller 9 by fogging toner, as well as contamination of the surface of the transfer paper P via these components.

[0048] Furthermore, by acquiring in advance a correction value (difference d) for the charging voltage due to the influence of the primary transfer voltage and storing it in the storage unit 70, and adding it to the pre-correction charging voltage determined without considering the influence of the primary transfer voltage, the charging voltage can be determined with the same accuracy as when the influence of the primary transfer voltage is considered, and the surface potential can be maintained stably for a long period of time. Furthermore, since cleaning operations for the intermediate transfer belt 8 and the secondary transfer roller 9 are not required, the time required for adjusting the surface potential can be shortened.

[0049] In addition, the present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit of the present invention. For example, in the above-described embodiment, the photosensitive drums 1a to 1d are charged to a predetermined surface potential V0, and the DC voltage Vdc is varied from an initial value smaller than the surface potential V0 to measure the change in the development current, and the sum of the DC voltage Vdc and the development potential difference ΔV at the point where the curve L recording the change in the development current intersects with the reference current Cs is estimated as the surface potential of the photosensitive drums 1a to 1d. However, for example, the development current may be measured at only two points, and the sum of the DC voltage Vdc and the development potential difference ΔV at the point where the line passing through the two points intersects with the reference current may be estimated as the surface potential of the photosensitive drums 1a to 1d.

[0050] 1 has been described as an example of image forming apparatus 100, but the present invention is not limited to color printers and may be applied to other image forming apparatuses that charge photosensitive drums 1a-1d using a charging member such as charging roller 34, such as color copiers, digital multifunction peripherals, and facsimiles.The effects of the present invention will be described more specifically below with reference to examples. [Example]

[0051] [Method for estimating the surface potential of a photoconductor drum during endurance printing and its relationship to changes in surface potential] The surface potentials of the photosensitive drums 1a to 1d were estimated using the development current, and the relationship between the estimation method and the transition of the surface potential was evaluated when the surface potentials were adjusted based on the estimation results.

[0052] The image formation conditions were a printing speed (process speed) of 55 sheets per minute, and the developing roller 30 used a developing sleeve 31 with an outer diameter of 20 mm, the outer surface of which was knurled (80 rows circumferentially) and blasted. The regulating blade 27 used a magnetic blade made of stainless steel (SUS430) with a thickness of 1.5 mm. A developing voltage was applied to the developing roller 30, which was a DC voltage Vdc superimposed with a square-wave AC voltage of 1200 V peak-to-peak (Vpp), 50% duty, and 8 kHz frequency.

[0053] The photosensitive drums 1a to 1d were positively charged organic photosensitive drums (OPC: Organic Photo Conductors) with a 32 μm-thick organic photosensitive layer formed as the photosensitive layer, the peripheral speed ratio of the developing roller 30 to the photosensitive drums 1a to 1d was 1.8 (trail rotation at the opposing position), and the distance between the photosensitive drums 1a to 1d and the developing roller 30 (DS distance) was 0.30 mm. The charging roller 34 was a rubber roller with a conductive rubber layer 3 mm thick and a volume resistivity of 6.0 [LogΩ] laminated on the outer surface of a core metal with an outer diameter of 6 mm, and a charging voltage consisting only of a DC voltage was applied.

[0054] The toner used was a positively charged toner with an average particle diameter of 6.8 μm, and the carrier used was a ferrite-resin coated carrier with an average particle diameter of 38 μm. The initial toner concentration in the developer (weight ratio of toner to carrier) was 6%.

[0055] The test method involved printing 10,000 sheets per day for 30 days (300,000 sheets), and estimating the surface potential every 20,000 sheets. As shown in FIG. 5 , the primary transfer rollers 6a-6d were spaced from the photosensitive drums 1a-1d, detecting the development current, and adding the shift due to the primary transfer voltage to estimate the surface potential. The surface potential was then adjusted based on the estimated results (invention), the primary transfer rollers 6a-6d were in contact with the photosensitive drums 1a-1d, and the transfer voltage was applied to estimate the surface potential. The changes in the surface potential were recorded for each of the following cases: (1) when the primary transfer rollers 6a-6d were in contact with the photosensitive drums 1a-1d, applying the transfer voltage, and adjusting the charge voltage based on the estimated results (comparison example 1); (2) when the charge voltage was adjusted based on predictive control of the surface potential; and (3) when the charge voltage was fixed at a constant value. The surface potential was estimated every 1,000 sheets throughout the durability printing.

[0056] In the present invention, the influence of the transfer voltage was checked every day when the image forming apparatus 100 was started, and the shift amount (correction value) d due to the transfer voltage was determined. For example, when the results shown in Fig. 4 were obtained, the shift amount d (e.g., 10 V) due to the transfer voltage was added to the charging voltage determined by the surface potential adjustment method that does not consider the influence of the transfer voltage. The surface potential adjustment process was performed with the primary transfer rollers 6a-6d separated from the photosensitive drums 1a-1d.

[0057] In Comparative Example 1, a surface potential adjustment process was performed by applying a primary transfer voltage to the primary transfer rollers 6a to 6d while the primary transfer rollers 6a to 6d were in contact with the photosensitive drums 1a to 1d. The transition of the surface potential over the entire period of durability printing is shown in FIG.

[0058] In addition, in Comparative Example 1, because fogging toner adhered to the intermediate transfer belt 8 and secondary transfer roller 9, cleaning operations were performed on the intermediate transfer belt 8 and secondary transfer roller 9 after the surface potential estimation process was performed. In the present invention and Comparative Examples 2 and 3, because there was no effect of fogging toner, cleaning operations were not performed. Table 1 shows the processing time when surface potential estimation and adjustment processes were performed using the methods of the present invention and Comparative Examples 1 to 3, and the variation in surface potential at the end of endurance printing (300,000 sheets).

[0059] [Table 1]

[0060] As shown in Figure 6, in the present invention (data series indicated by ○ in Figure 6) and Comparative Example 1 (data series indicated by ● in Figure 6), in which the charging voltage was adjusted based on the estimated surface potential obtained by detecting the development current, the surface potential was maintained almost constant (450 [V]) throughout the entire durability printing period.

[0061] In contrast, in Comparative Example 2 (data series indicated by a triangle in Figure 6), in which the charging voltage was adjusted based on predictive control of the surface potential, the surface potential began to decrease once the number of sheets exceeded 150,000, and by the end of the endurance printing run (300,000 sheets), the surface potential had dropped to 410 [V]. Furthermore, in Comparative Example 3 (data series indicated by a triangle in Figure 6), in which the charging voltage was fixed at a constant value, the surface potential had dropped to 400 [V] by the time the number of sheets reached 200,000. Furthermore, as shown in Table 1, the variations in the surface potential were also large, at 38 [V] and 200 [V], respectively.

[0062] Furthermore, as shown in Table 1, in Comparative Example 1, in which the surface potential was estimated by applying a transfer voltage while the primary transfer rollers 6a to 6d were in contact with the photosensitive drums 1a to 1d, a cleaning operation had to be performed, resulting in a longer processing time than in the present invention and Comparative Examples 2 and 3, in which no cleaning operation was performed.

[0063] From the above results, it was confirmed that by separating the primary transfer rollers 6a to 6d from the photosensitive drums 1a to 1d, detecting the development current, and estimating the surface potential by adding the shift due to the primary transfer voltage, it is possible to maintain a constant surface potential over the entire period of durable printing of 300,000 sheets, and cleaning operations for the intermediate transfer belt 8 and secondary transfer roller 9 are no longer necessary, thereby shortening the time required for the surface potential adjustment process. [Industrial Applicability]

[0064] The present invention can be used in an electrophotographic image forming apparatus that charges a photosensitive member to a predetermined surface potential. By using the present invention, it is possible to provide an image forming apparatus that can accurately estimate the surface potential of a photosensitive member without using a surface potential sensor, adjust the surface potential based on the estimation result, and suppress contamination of a transfer member by toner. [Explanation of symbols]

[0065] 1a to 1d Photosensitive drum (image carrier) 2a~2d Charging device 3a~3d developing device 5 Exposure equipment 6a~6d Primary transfer rollers 8 Intermediate transfer belt 9 Secondary transfer roller 19 Belt cleaner 30 Developing roller (developer carrier) 34 Charging roller (charging member) 43 Development voltage power supply 44 Current detection unit 45 Charge voltage power supply 47 Transfer voltage power supply 48 Cleaning voltage power supply 50 Voltage control section 70 Memory section 80 Main control unit (control unit) 90 LCD display section 100 Image forming device

Claims

1. an image carrier having a photosensitive layer formed on its surface; a charging device having a charging member for charging the surface of the image bearing member to a predetermined surface potential; a developing device having a developer carrier that carries a developer containing toner, and that forms a toner image by adhering the toner to an electrostatic latent image formed by exposing the image carrier that has been charged by the charging device; a transfer member that contacts the image carrier to form a transfer nip and transfers the toner image formed on the surface of the image carrier to a recording medium or an intermediate transfer member; a developing voltage power source that applies a developing voltage including a DC voltage Vdc to the developer carrier; a charging voltage power source that applies a charging voltage to the charging member; a transfer voltage power supply that applies a transfer voltage having a polarity opposite to that of the toner to the transfer member; a current detection unit that detects a development current flowing between the image carrier and the developer carrier; a control unit that controls the developing voltage power supply, the charging voltage power supply, and the transfer voltage power supply; In an image forming apparatus comprising: a storage unit that stores, as a correction value, a difference between the charging voltage for obtaining the predetermined surface potential in a spaced state where the transfer member is spaced from the image carrier and the charging voltage for obtaining the predetermined surface potential in a contact state where the transfer member is in contact with the image carrier and the transfer voltage is applied; The control unit the developing current when the developing voltage and the charging voltage are not applied to the developer carrier and the charging member, respectively, is measured and used as a reference current; an image forming apparatus, characterized in that a surface potential adjustment process is performed to obtain the charging voltage for obtaining the surface potential in the contact state by applying a predetermined charging voltage to the charging member in the separated state and charging the image carrier to the predetermined surface potential, and estimating the DC voltage Vdc when the development current detected by the current detection unit becomes equal to the reference current when the development current is equal to the reference current, and adding a correction value for the charging voltage stored in the memory unit to the charging voltage for obtaining the surface potential in the separated state.

2. The control unit, when executing the surface potential estimation process, 2. The image forming apparatus according to claim 1, wherein, in the separated state, a predetermined charging voltage is applied to the charging member to charge the image carrier to the predetermined surface potential, and when the DC voltage Vdc is increased stepwise from an initial value smaller than the surface potential, the change in the development current detected by the current detection unit is measured, and the DC voltage Vdc at a point where a curve representing the change in the development current intersects with the reference current is estimated to be the surface potential in the separated state.

3. The control unit 2. The image forming apparatus according to claim 1, wherein a correction value acquisition process is executed to acquire a correction value for the charging voltage based on a change in the development current with respect to the charging voltage when a constant DC voltage Vdc is applied in the separated state and a change in the development current with respect to the charging voltage when a constant DC voltage Vdc is applied in the contact state.

4. 4. The image forming apparatus according to claim 3, wherein the control unit executes the correction value acquisition process when the image forming apparatus is turned on.

5. 4. The image forming apparatus according to claim 3, wherein the correction value acquisition process is performed when the image forming apparatus is manufactured, and the correction value for the charging voltage is stored in advance in the storage unit.

6. 6. The image forming apparatus according to claim 1, wherein the control unit re-executes the surface potential adjustment process when the development current deviates from the reference current by a certain value or more.

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