Image formation apparatus

Abstract

To provide an image formation apparatus capable of accurately estimating the surface potential of a photosensitive body without using a surface potential sensor, and also capable of suppressing generation of a fog toner.SOLUTION: The image formation apparatus includes: an image carrier; a charging device; a developing device; a developing voltage power supply; a charging voltage power supply; a current detection unit; and a control unit, the control unit being configured to execute a surface potential estimation process for estimating the surface potential of the image carrier, the control unit setting a developing current, which is obtained by applying a direct-current voltage Vdc to a developer carrier of the developing device so that a developing potential difference ΔV, which is the difference between the surface potential of the image carrier and the direct-current voltage Vdc, becomes a predetermined value while no charging voltage is applied to a charging member, as a reference current and executing the surface potential estimation process for estimating the surface potential of the image carrier as the sum of the direct-current voltage and the developing potential difference ΔV when the developing current detected by the current detection unit becomes equal to the reference current in a state where the charging voltage is applied to the charging member to charge the image carrier to a predetermined surface potential.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. [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] For example, Patent Document 1 discloses an image forming apparatus that calculates the development voltage as the surface potential when the non-charged development current flowing through the development device in an uncharged state where the charging device has not charged the photosensitive drum is equal to the charged development current flowing through the development device in a charged state where the charging device has charged the photosensitive drum. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2021-157163 Summary of the Invention [Problem to be solved by the invention]

[0006] Patent Document 1 describes measuring a reference current that flows when 0 V is applied as the charging voltage and developing voltage (ΔV=0), and applying a developing voltage that is greater than the surface potential V0 (ΔV<0) so that a developing current greater than the reference current flows. However, when ΔV≦0, toner moves from the developer carrier to the photosensitive member (fogging), causing the intermediate transfer belt and secondary transfer roller to become contaminated with toner (fogging toner), which can cause paper to become contaminated when printing immediately afterwards.

[0007] SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide an image forming apparatus that can accurately detect the surface potential of a photosensitive member without using a surface potential sensor and can also suppress the generation of fogging toner. [Means for solving the problem]

[0008] In order 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, a developing voltage power supply, a charging voltage power supply, a current detection unit, and a control unit. The image carrier has a photosensitive layer formed on its surface. The charging device charges the surface of the image carrier to a predetermined surface potential. The developing device has a developer carrier that carries 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 thereon. 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 device. The current detection unit detects the developing current flowing between the image carrier and the developer carrier. The control unit controls the developing voltage power supply and the charging voltage power supply. The control unit performs a surface potential estimation process in which the development current when a DC voltage Vdc is applied to the developer carrier so that a development potential difference ΔV, which is the difference between the surface potential of the image carrier and the DC voltage Vdc, becomes a predetermined value without applying a charging voltage to the charging member, is used as a reference current, and the control unit performs a surface potential estimation process in which the control unit estimates the surface potential of the image carrier as the sum of the DC voltage when the development current detected by the current detection unit becomes equal to the reference current in a state in which a predetermined charging voltage is applied to the charging member and the image carrier is charged to a predetermined surface potential, and the development potential difference ΔV. [Effects of the Invention]

[0009] According to the first aspect of the present invention, the development current flowing between the image carrier and the developer carrier 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 measuring the surface potential, thereby contributing to cost reduction of the image forming apparatus. Furthermore, by measuring the development current under conditions in which fogging toner is unlikely to occur (ΔV>0), the generation of fogging toner can be reduced compared to conventional methods, and contamination of the transfer member and recording medium by fogging toner can be effectively suppressed. [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] 1 is a graph showing the relationship between the DC voltage Vdc of the development voltage and the development current in the surface potential estimation process executed in the image forming apparatus 100 of this embodiment. [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] 10 is a flowchart showing another example of the surface potential estimation process executed in the image forming apparatus 100 of the present embodiment. 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. Estimation of surface potential using development current] A method for estimating the surface potentials of the photosensitive drums 1a to 1d, which is a characteristic feature of the present invention, will be described below.

[0031] (Conventional method for estimating 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 V0 of the photosensitive drums 1a to 1d (ΔV=0 [V]), only the uncharged developing current (reference current Cs) flows between the developing roller 30 and the photosensitive drums 1a to 1d. Therefore, the DC voltage Vdc (460 [V] in FIG. 3) at point P1 where the curve L representing the change in the developing current intersects with the reference current Cs is estimated as the surface potential of the photosensitive drums 1a to 1d.

[0034] 3, the surface potential is estimated when the development current is large (>0 μA), which means that the surface potential is estimated when toner is moving from the development roller 30 to the photosensitive drums 1a-1d (fogging toner). As a result, the intermediate transfer belt 8 and secondary transfer roller 9 are contaminated with toner during printing immediately after the surface potential estimation, which can result in contamination of the transfer paper P.

[0035] (Surface potential estimation method of this embodiment) FIG. 4 is a graph showing the relationship between the DC voltage Vdc of the development voltage and the development current in the surface potential estimation process executed in the image forming apparatus 100 of this embodiment. FIG. 5 is a flowchart showing an example of control of the surface potential estimation process executed in the image forming apparatus 100 of this embodiment. In the method shown in FIG. 4, the development potential difference ΔV (=V0-Vdc) is set to be the same as ΔV during image formation. Here, as an example, a case will be described where positively charged toner is used and ΔV=70 [V] during image formation.

[0036] First, the main control unit 80 determines whether it is time to estimate the surface potential of the photosensitive drums 1a-1d (step S1). Examples of timing to estimate the surface potential include when the cumulative number of printed sheets since the start of use of the photosensitive drums 1a-1d or the charging roller 34 reaches a predetermined number, or when the cumulative number of printed sheets since the previous estimation of the surface potential reaches a predetermined number. If it is not time to estimate the surface potential (No in step S1), the process ends without estimating the surface potential.

[0037] If it is time to estimate the surface potential (No in step S1), the main control unit 80 does not apply a charging voltage to the charging roller 34 (applies 0 [V]), and applies Vdc (=-70 [V]) to the developing roller 30 so that the development potential difference ΔV is the same as during image formation (step S2). Then, the current detection unit 44 detects the development current that flows in this state (Vdc=-70 [V], V0=0 [V], ΔV=70 [V]), and the detected development current is set as the reference current Cs (step S3).

[0038] Next, the main control unit 80 applies a predetermined charging voltage to the charging roller 34 to charge the photosensitive drums 1a-1d (step S4), thereby placing a certain surface potential V0 on the photosensitive drums 1a-1d. 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 S5). The DC voltage Vdc+ΔV at the point where the reference current Cs intersects with the curve L that records the development current is estimated to be the surface potential of the photosensitive drums 1a-1d (step S5).

[0039] From FIG. 4, at point P2 where curve L intersects with reference current Cs, Vdc=380 [V] and ΔV=70 [V], so the surface potential V0 is estimated to be 380+70=450 [V].

[0040] According to the surface potential estimation 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 measuring the surface potential, which also contributes to reducing the cost of the image forming apparatus 100.

[0041] Furthermore, by measuring the development current under conditions (ΔV>0) in which fogging toner is unlikely to occur, it is possible to reduce the occurrence of fogging toner compared to conventional methods, and to effectively suppress toner contamination of the intermediate transfer belt 8 and secondary transfer roller 9, as well as contamination of the surface of the transfer paper P via these components.

[0042] Furthermore, if the development potential difference ΔV set when determining the reference current Cs during the surface potential estimation process is the same as that during image formation, correcting the charging voltage of the charging roller 34 so that the development current during image formation is the same as the reference current makes it possible to maintain a constant surface potential for the photosensitive drums 1a-1d. This allows for maintaining good image quality. Furthermore, when the development current becomes a value that significantly deviates from the reference current, this can function as a trigger to re-execute the surface potential estimation process. This eliminates the need to frequently execute the surface potential estimation process, which also extends the useful life of the charging roller 34.

[0043] In the control example shown in FIG. 5, the reference current Cs is the development current that flows when Vdc (=-70 [V]) is applied to the development roller 30 so that the development potential difference ΔV is the same as that during image formation, but it is not necessary to apply Vdc so that the development potential difference ΔV is the same as that during image formation.

[0044] For example, the development current that flows when Vdc=-60 [V] or Vdc=-80 [V] is applied may be used as the reference current Cs. However, if the development potential difference ΔV deviates by a certain amount or more from the development potential difference during image formation, it becomes difficult to maintain the surface potential of the photosensitive drums 1a to 1d constant. Therefore, it is preferable to determine the reference current Cs by applying a Vdc such that the development potential difference ΔV is the same as or close to that during image formation.

[0045] Fig. 6 is a flowchart showing another control example of the surface potential estimation process executed in the image forming apparatus 100 of this embodiment. Fig. 6 shows a method for measuring the development current when two levels of DC voltage Vdc, which have different development potential differences ΔV from each other, are applied to the development roller 30, and for determining the development potential difference ΔV (reference development potential difference ΔVs) when the reference current is 0 [μA] from the measured development current.

[0046] First, the main control unit 80 determines whether it is time to estimate the surface potential of the photosensitive drums 1a to 1d (step S1). If it is not time to estimate the surface potential (No in step S1), the main control unit 80 ends the process without executing the surface potential estimation process.

[0047] If it is time to estimate the surface potential (No in step S1), the main control unit 80 applies no charging voltage (applies 0 [V]) to the charging roller 34, and applies a first DC voltage Vdc1 (here, -100 [V]) to the developing roller 30 (step S2). Then, the current detection unit 44 detects the developing current C1 that flows in this state (Vdc1=-100 [V], V0=0 [V], ΔV=100 [V]) (step S3).

[0048] Next, the main control unit 80 applies no charging voltage (applies 0 [V]) to the charging roller 34, and applies a second DC voltage Vdc2 (here, −50 [V]) to the developing roller 30 (step S4). Then, the current detection unit 44 detects the developing current C2 that flows in this state (Vdc1=−50 [V], V0=0 [V], ΔV=50 [V]) (step S5).

[0049] Next, the main control unit 80 calculates a reference development potential difference ΔVs at which the reference current becomes 0 μA, based on the development current C1 detected in step S3 and the development current C2 detected in step S5 (step S6).

[0050] Next, the main control unit 80 applies a predetermined charging voltage to the charging roller 34 to charge the photosensitive drums 1a-1d (step S7), thereby placing a certain surface potential V0 on the photosensitive drums 1a-1d. 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 S8). The reference development potential difference ΔVs calculated in step S6 is added to the DC voltage Vdc at the point where the reference current Cs intersects with the curve L that records the development current transition, and the resulting value is estimated as the surface potential of the photosensitive drums 1a-1d (step S9).

[0051] According to the surface potential estimation process shown in Figure 6, by measuring the development current under conditions where fogging toner is unlikely to occur (reference current = 0 [μA]), it is possible to reduce the occurrence of fogging toner compared to conventional methods, and to effectively suppress toner contamination of the intermediate transfer belt 8 and secondary transfer roller 9, as well as paper surface contamination of the transfer paper P via these components.

[0052] In addition, by applying a first DC voltage Vdc1 and a second DC voltage Vdc2, which have different development potential differences ΔV, to the development roller 30 and calculating the reference development potential difference ΔVs when the reference current is 0 [μA], the reference development potential difference ΔVs can be accurately determined.

[0053] In FIG. 6, the reference development potential difference ΔVs where the reference current is 0 μA is calculated from the development current when two levels of the first DC voltage Vdc1 and the second DC voltage Vdc2 are applied to the development roller 30, but the reference development potential difference ΔVs may also be calculated from the development current when three or more levels of the DC voltage Vdc are applied to the development roller 30.

[0054] 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.

[0055] 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]

[0056] [Method for estimating the surface potential of the photosensitive drum and its relationship to stains on the transfer paper] An evaluation was made on stains on transfer paper when the surface potentials of the photosensitive drums 1a to 1d were estimated using the development current.

[0057] 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.

[0058] 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.

[0059] 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%.

[0060] The test method was to perform endurance printing of 300,000 sheets, and estimate the surface potential every 20,000 sheets. The state of paper surface contamination of the transfer paper P was evaluated visually when the surface potential was estimated at ΔV = 70 [V] as shown in Figure 4 (present invention) and when the surface potential was estimated at ΔV = 0 [V] as shown in Figure 3 (comparative example). The evaluation criteria were ◯ when no paper surface contamination was observed and × when paper surface contamination was observed. The results are shown in Table 1.

[0061] [Table 1]

[0062] As shown in Table 1, when the surface potential was estimated with ΔV = 70 [V], no paper staining was observed throughout the entire durability printing period (300k sheets). In contrast, when the surface potential was estimated with ΔV = 0 [V], no paper staining was observed up to 140k sheets printed, but paper staining occurred when the number of printed sheets exceeded 160k sheets.

[0063] Although not described here, it has also been confirmed that no staining of the paper surface occurs in the surface potential estimation process of FIG. 6, in which the reference development potential difference ΔVs when the reference current is 0 [μA] is calculated, and the surface potential is calculated by adding the reference development potential difference ΔVs to the DC voltage Vdc at the point where the curve L recording the change in the development current intersects with the reference current Cs.

[0064] From the above results, it was confirmed that by making the difference between the surface potential V0 when the reference current is obtained and the DC voltage Vdc (development potential difference ΔV) the same as during image formation, or by using a reference development potential difference ΔVs where the reference current is 0 [μA], it is possible to suppress the occurrence of fogging toner, and to suppress the occurrence of paper stains over the entire period of endurance printing of 300,000 sheets. [Industrial Applicability]

[0065] The present invention can be used in an electrophotographic image forming apparatus that charges a photoconductor to a predetermined surface potential, and can provide an image forming apparatus that can accurately estimate the surface potential of the photoconductor without using a surface potential sensor and can also suppress the generation of fogging toner. [Explanation of symbols]

[0066] 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 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 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 and the charging voltage power supply; In an image forming apparatus comprising: The control unit the developing current when the charging voltage is not applied to the charging member and the DC voltage Vdc is applied to the developer carrier so that a developing potential difference ΔV, which is the difference between the surface potential of the image carrier and the DC voltage Vdc, becomes a predetermined value, is defined as a reference current; an image forming apparatus characterized in that a surface potential estimation process is performed to estimate the surface potential of the image carrier as the sum of the DC voltage when the development current detected by the current detection unit becomes equal to the reference current in a state in which a predetermined charging voltage is applied to the charging member and the image carrier is charged to the predetermined surface potential, and the development potential difference ΔV.

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

3. 2. The image forming apparatus according to claim 1, wherein the development potential difference .DELTA.V set when determining the reference current is the same as the development potential difference during image formation.

4. The control unit, when executing the surface potential estimation process, measuring the developing current when the charging voltage is not applied to the charging member and two or more levels of the DC voltage Vdc that are different from each other in the developing potential difference ΔV are applied to the developer carrier, and calculating, from the measured developing current, a reference developing potential difference ΔVs that is the developing potential difference ΔV when the reference current is 0 [μA]; a predetermined charging voltage is applied to the charging member to charge the image carrier to the predetermined surface potential, and then the DC voltage Vdc is increased stepwise from an initial value that is smaller than the surface potential, and a transition of the development current detected by the current detection unit is measured; 2. The image forming apparatus according to claim 1, wherein the surface potential of the image carrier is estimated as the sum of the DC voltage at the point where the curve representing the change in the development current intersects with the reference current and the reference development potential difference ΔVs.

5. 5. The image forming apparatus according to claim 1, wherein the control unit executes the surface potential estimation process when the development current detected by the current detection unit deviates from the reference current by a certain amount or more.

Images