Image forming apparatus

Abstract

To provide an image forming apparatus that can prevent the occurrence of density unevenness while reducing manufacturing cost.SOLUTION: In a control unit, during execution of an image forming operation to form an image, the amplitude Vpp of an electrification bias affected by an AC component of a developing bias satisfies the following formula (1). In the following formula (1), Vr is a first electrical potential (V) of an outer peripheral surface of an image carrier before being electrified in an electrification step. V0 is a second electrical potential (V) of the outer peripheral surface of the image carrier at a developing position after being electrified in the electrification step. Q is the amount of electrification charges of the image carrier. S is an electrification area (m2) of the image carrier. d is a film thickness (m) of a photosensitive layer. εr is the relative dielectric constant of the photosensitive layer. ε0 is the dielectric constant of vacuum (F / m).SELECTED DRAWING: Figure 2

Description

Technical Field

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

Background Art

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

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

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

[0005] Thereby, the influence of the electric field of the developing bias applied by the developing bias power source on the charging bias applied by the charging bias power source can be suppressed. Therefore, it is possible to reduce the occurrence of unevenness in the surface potential of the image carrier and suppress the occurrence of density unevenness (stained image).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

[0007] The configurations of Patent Documents 1 and 2 have the problem of increasing the number of parts and increasing manufacturing costs.

[0008] 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 suppress the occurrence of density unevenness while suppressing manufacturing costs. [Means for solving the problem]

[0009] In order to achieve the above object, a first aspect of the present invention is an image forming apparatus including an image forming unit, a developing bias power supply, a charging bias power supply, a transfer bias power supply, and a control unit. The image forming unit includes an image carrier, a charging device, an exposure device, and a developing device. The image carrier has a photosensitive layer formed on its surface. The charging device is disposed opposite the image carrier and has a charging member that charges the image carrier. The exposure device forms an electrostatic latent image by exposing the image carrier charged by the charging device. The developing device is disposed opposite the image carrier and has a developer carrier that carries developer, and forms a toner image by attaching toner to the electrostatic latent image formed on the image carrier. The developing bias power supply applies a developing bias, which is a DC voltage superimposed on an AC voltage, to the developer carrier. The charging bias power supply applies a charging bias including a DC voltage to the charging member. The control unit controls the image forming unit, the developing bias power supply, and the charging bias power supply. The control unit, during the image forming operation for forming an image, determines that the amplitude Vpp of the charging bias, influenced by the AC component of the developing bias, satisfies the following formula (1): In the formula (1), Vr is the first potential [V] of the outer peripheral surface of the image carrier before being charged in the charging process; V0 is the second potential [V] of the outer peripheral surface of the image carrier at the development position after being charged in the charging process; Q is the amount of charge [C] of the image carrier; S is the charging area [m 2]. d is the film thickness of the photosensitive layer [m]. Εr is the relative dielectric constant of the photosensitive layer. ε0 is the dielectric constant of a vacuum [F / m].

number

[0010] According to the first aspect of the present invention, it is possible to provide an image forming apparatus that can suppress the occurrence of density unevenness while suppressing manufacturing costs. [Brief explanation of the drawings]

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

[0012] 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 a first embodiment of the present invention, and Fig. 2 is a partially enlarged view of the periphery of image forming unit Pa, including the control path of image forming apparatus 100. Note that the configuration of image forming units Pb-Pd and the control path of charging devices 2a-2d are similar to the configuration of image forming unit Pa and the control path of charging device 2a, and therefore description thereof will be omitted.

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

[0014] Within the main body of 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). Image forming units Pa to Pd are provided corresponding to images of four different colors (cyan, magenta, yellow, and black), and sequentially form cyan, magenta, yellow, and black images through the processes of charging, exposure, development, and transfer, respectively.

[0015] The image forming stations Pa to Pd each include primary transfer rollers (transfer members) 6a to 6d, photosensitive drums (image carriers) 1a to 1d that carry visible images (toner images) of each color, charging devices 2a to 2d, exposure device 5, developing devices 3a to 3d, and a discharging lamp 20. Also provided adjacent to each of the image forming stations Pa to Pd is an intermediate transfer belt (transfer receiving member) 8 that rotates counterclockwise in FIG. 1 by a driving means (not shown).

[0016] Primary transfer rollers (transfer members) 6a to 6d are disposed opposite photosensitive drums (image carriers) 1a to 1d, and a predetermined transfer voltage is applied to transfer the visible images (toner images) of each color formed on the photosensitive drums (image carriers) 1a to 1d onto intermediate transfer belt (transfer recipient) 8. As a result, the toner images formed on the photosensitive drums 1a to 1d are sequentially primarily transferred and superimposed onto the intermediate transfer belt 8, which moves while contacting each of the photosensitive drums 1a to 1d.

[0017] The toner image that has been primarily transferred onto the intermediate transfer belt 8 is secondarily transferred onto a sheet S, which is an example of a recording medium, by a secondary transfer roller 9. The sheet S onto which the toner image is secondarily transferred is stored in a paper cassette 16 that is located at the bottom of the main body of the image forming apparatus 100. The sheet S is transported to a nip portion between the secondary transfer roller 9 and a drive roller 11 of the intermediate transfer belt 8 via a paper feed roller 12a and a pair of registration rollers 12b.

[0018] A sheet made of dielectric resin is used for the intermediate transfer belt 8, and a seamless belt is usually used. In addition, a blade-shaped belt cleaner 19 is disposed downstream of the secondary transfer roller 9 to remove toner and other particles remaining on the surface of the intermediate transfer belt 8.

[0019] The photosensitive drums (image carriers) 1a to 1d have a photosensitive layer 111 formed on their surfaces (see FIG. 2). In this embodiment, the photosensitive drums (image carriers) 1a to 1d have the photosensitive layer 111 formed on the surface of an aluminum cylinder, and the photosensitive layer 111 is formed by vapor deposition of amorphous silicon, a positively chargeable photoconductor. In other words, the photosensitive layer 111 is an electrostatically charged single layer type. It is preferable that the photosensitive layer 111 has a film thickness of 37 μm or more in an unused state. Increasing the film thickness improves the durability of the photosensitive drums 1a to 1d against wear.

[0020] Each of the charging devices 2a to 2d has a charging roller (charging member) 34. The charging roller 34 is disposed opposite the corresponding photosensitive drum (image carrier) 1a to 1d and charges the corresponding photosensitive drum (image carrier) 1a to 1d. The charging roller 34 is formed, for example, by coating a core metal with an epichlorohydrin rubber layer, which is a conductive elastic material. In this embodiment, the charging roller 34 is in contact with each of the photosensitive drums 1a to 1d. However, the charging roller 34 may not be in contact with each of the photosensitive drums 1a to 1d.

[0021] The exposure device 5 exposes the photosensitive drums (image carriers) 1a to 1d charged by the charging devices 2a to 2d, thereby forming electrostatic latent images.

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

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

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

[0025] Then, an electric field is applied at a predetermined transfer voltage between the primary transfer rollers 6a to 6d and the photoreceptor drums 1a to 1d by the primary transfer rollers 6a to 6d, and the cyan, magenta, yellow, and black toner images on the photoreceptor drums 1a to 1d are primarily transferred onto the intermediate transfer belt 8. These four-color images are formed with a predetermined positional relationship determined in advance for predetermined full-color image formation. Thereafter, in preparation for the formation of new electrostatic latent images that are subsequently performed, toner and the like remaining on the surfaces of the photoreceptor drums 1a to 1d after primary transfer are removed by the cleaning devices 7a to 7d. The residual charges remaining on the surfaces of the photoreceptor drums 1a to 1d after primary transfer are removed by the discharge lamp 20.

[0026] 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, and when the intermediate transfer belt 8 starts to rotate counterclockwise as the drive roller 11 is rotated by a drive motor (not shown), the sheet S is transported from the registration roller pair 12b to a nip portion (secondary transfer nip portion) between the drive roller 11 and a secondary transfer roller 9 provided adjacent thereto at a predetermined timing, and the full-color image on the intermediate transfer belt 8 is secondarily transferred onto the sheet S. The sheet S onto which the toner image has been secondarily transferred is transported to a fixing unit 13.

[0027] The sheet S conveyed to the fixing unit 13 is heated and pressurized by the fixing roller pair 13a, and the toner image is fixed to the surface of the sheet S, forming a predetermined full-color image. The sheet S on which the full-color image has been formed is then directed to a branching unit 14, which branches into multiple directions, and is then discharged directly (or after being sent to the double-sided conveying path 18 and having images formed on both sides) onto a discharge tray 17 by the discharge roller pair 15.

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

[0029] The developing bias power supply 43 is connected to the developing roller 31 (see FIG. 2). The developing bias power supply 43 includes an AC constant voltage power supply 43a and a DC constant voltage power supply 43b. The AC constant voltage power supply 43a outputs a sine wave AC voltage 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 obtained by rectifying a sine wave AC voltage generated from a low voltage DC voltage modulated into a pulse shape using a step-up transformer.

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

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

[0032] In this embodiment, during image formation, the charging bias power supply 45 applies a charging bias consisting of only a DC voltage from the DC constant voltage power supply 45b to the charging roller (charging member) 34. By configuring the charging bias to consist of only a DC voltage, it is possible to reduce wear of the photosensitive layer 111, reduce the amount of ozone generated, and reduce charging noise.

[0033] The transfer bias power supply 47 applies a primary transfer voltage (transfer voltage) and a secondary transfer voltage to the primary transfer rollers (transfer members) 6a to 6d and the secondary transfer roller 9 (see FIG. 1), respectively. The charging bias power supply 47 includes a DC constant voltage power supply 47b. The DC constant voltage power supply 47b outputs a DC voltage obtained by rectifying a sinusoidal AC voltage generated from a low-voltage DC voltage modulated into a pulse using a step-up transformer.

[0034] In this embodiment, during image formation, transfer bias power supply 47 applies a transfer bias consisting of only a DC voltage from DC constant voltage power supply 47b to primary transfer rollers (transfer members) 6a to 6d and secondary transfer roller 9 (see FIG. 1).

[0035] The cleaning device 7a includes a cleaning blade 32 that removes residual toner from the surface of the photosensitive drum 1a, a rubbing roller 33 that removes residual toner from the surface of the photosensitive drum 1a and also rubs against the surface of the photosensitive drum 1a to polish it, and a conveying spiral 35 that discharges the residual toner removed from the photosensitive drum 1a by the cleaning blade 32 and the rubbing roller 33 to the outside of the cleaning device 7a.

[0036] The image forming apparatus 100 is provided with a main control unit 80 that is configured with a CPU and the like. The main control unit 80 is connected to a storage unit 70 that is configured with a ROM, RAM and the like. The main control unit 80 controls each unit of the 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 bias power supply 43, current detection unit 46, charging bias power supply 45, transfer bias power supply 47, voltage control unit 50, etc.) based on the control program and control data stored in the storage unit 70.

[0037] The voltage control unit 50 controls a development bias power supply 43 that applies a development bias to the development roller 31, a charging bias power supply 45 that applies a charging bias to the charging roller 34, and a transfer bias power supply 47 that applies a transfer voltage to the primary transfer rollers 6a to 6d and the secondary transfer roller 9. The voltage control unit 50 may be configured by a control program stored in the storage unit 70.

[0038] 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 sheets, etc. The transmission / reception unit 91 communicates with the outside world using a telephone line or an internet line.

[0039] As described above, the charging bias applied by the charging bias power source 45 during image formation is affected by the AC component of the developing bias. As a result, unevenness occurs in the surface potential of the photosensitive drums (image carriers) 1a to 1d, and density unevenness (stain image) may occur in the toner images of each color formed on the photosensitive drums (image carriers) 1a to 1d.

[0040] In addition, the charging ability of the photosensitive drums (image carriers) 1a to 1d is more easily affected by the AC component of the developing bias as it increases. As a result, density unevenness is more likely to occur.

[0041] In addition, the photosensitive drums (image carriers) 1a to 1d with a thick photosensitive layer 111 have a lower capacitance than the photosensitive drums 1a to 1d with a thin photosensitive layer 111. As a result, the photosensitive drums 1a to 1d with a thick photosensitive layer 111 have a higher charging ability ratio A than the photosensitive drums 1a to 1d with a thin photosensitive layer 111 and are more easily affected by the AC component of the developing bias. The charging ability ratio A indicates the ratio of the actual charging ability (measured value) of the photosensitive drums 1a to 1d to the theoretical charging ability (theoretical value) of the photosensitive drum 1a when the photosensitive layer 111 of the photosensitive drums 1a to 1d is charged by the charging roller (charging member) 34. The charging ability ratio A also changes according to the film thickness of the photosensitive layer 111. Specifically, the photosensitive drums (image carriers) 1a to 1d in the initial state where the photosensitive layer 111 is not worn have a large charging ability ratio A and are likely to have unevenness in the surface potential. Hereinafter, the photosensitive drum (image carrier) 1a will be described, and the description of the photosensitive drums 1b to 1d will be omitted.

[0042] The charging bias applied by the developing bias power source 47b to the charging roller 34 consists only of a DC voltage, but the charging bias discharged from the charging roller 34 to the photosensitive drum 1a is affected by the AC component of the developing bias. The amplitude Vpp (V) of the charging bias corresponds to the amplitude of the charging bias affected by the AC component of the developing bias. Image defects occur when the amplitude Vpp (V) of the charging bias exceeds a predetermined value (see Fig. 5).

[0043] In this actual topographical form, before executing the image forming operation, the charging ability ratio A of the photosensitive drum 1a is measured, and based on the relationship between the measured charging ability ratio A and the amplitude Vpp (V) of the charging bias, the magnitude of the charging bias applied by the developing bias power supply 47b to the charging roller 34 is determined. Specifically, the magnitude of the charging bias consisting only of the DC voltage applied to the charging roller 34 is determined so as not to exceed the amplitude Vpp (V) of the charging bias with the graph shown in FIG. 5 as the upper limit value. Thereby, the amplitude Vpp (V) of the charging bias affected by the AC component of the developing bias can be suppressed to a predetermined value or less, preventing the occurrence of image defects.

[0044] In this embodiment, the main control unit (control unit) 80 can execute an image forming operation for forming an image. During the execution of the image forming operation, the amplitude Vpp (V) of the charging bias affected by the AC component of the developing bias satisfies the following formula (1).

[0045] In formula (1), Vr represents the first potential [V] of the outer peripheral surface of the photosensitive drum 1a before being charged in the charging process. V0 represents the second potential [V] of the outer peripheral surfaces of the photosensitive drums 1a to 1d at the developing position after being charged in the charging process. Q represents the amount of charge [C] of the photosensitive drum 1. S represents the charging area [m 2 of the photosensitive drum 1. d represents the film thickness [m] of the photosensitive layer 111. εr represents the relative permittivity of the photosensitive layer 111. ε0 represents the permittivity of vacuum [F / m].

[0046] A in formula (1) indicates each charging ability ratio A of the photosensitive drum 1a. The charging ability ratio A indicates the ratio of the actual charging ability (measured value) of the photosensitive drum 1a to the theoretical charging ability (theoretical value) of the photosensitive drum 1a when the photosensitive layer 111 of the photosensitive drum 1a is charged by the charging roller (charging member) 34.

[0047] Next, a method for measuring the chargeability ratio A will be described. The first potential Vr and the second potential V0 are measured while rotating the photosensitive drum 1a around the rotation axis. For example, the charging bias applied to the charging roller 34 is set to one of +1000 V, +1100 V, +1200 V, +1300 V, +1400 V, and +1500 V. In addition, the light amount (amount of static elimination light) of the static elimination light irradiated from the static elimination lamp 20 when it reaches the outer peripheral surface of the photosensitive drum 1 is set to, for example, 5 μJ / cm. 2 ].

[0048] The outer peripheral surface of the photosensitive drum 1a is positively charged at a position facing the charging roller 34 (charging position). When the outer peripheral surface rotates from the charging position and moves to a position facing the static elimination lamp 20 (static elimination position), the charge on the outer peripheral surface 50a is eliminated. After the photosensitive drum 1 has rotated 10 times while sequentially charging and eliminating the charge, the first potential Vr and the second potential V0 are simultaneously measured. In this manner, the first potential Vr and the second potential V0 are measured under the following conditions: the charging bias applied to the charging roller 34 is +1000 V, +1100 V, +1200 V, +1300 V, +1400 V, and +1500 V. The first potential Vr and the second potential V0 are measured under the following conditions: 23°C, 50% RH, for example.

[0049] The amount of charge Q is determined by measuring the current value flowing through the charging roller 34 using a current-voltage meter at the same time that the first potential Vr and the second potential V0 are measured simultaneously. The current value is measured under each condition where the charging bias applied to the charging roller 34 is +1000 V, +1100 V, +1200 V, +1300 V, +1400 V, and +1500 V. From the measured current value, the amount of charge Q under each condition where the charging bias applied to the charging roller 34 is +1000 V, +1100 V, +1200 V, +1300 V, +1400 V, and +1500 V is calculated based on the equation: amount of charge Q = current value [A] × charging time t [seconds].

[0050] The charged area S is the area of the region (charged region) on the outer peripheral surface of the photoreceptor drum 1a that is charged by the charging roller 34. The charged area S is 2 calculated based on [m 2 ]=linear velocity of the photoreceptor drum 1a [m / sec]×charging width [m]×charging time [sec]. The charging width is the length in the axial direction of the photoreceptor drum 1a of the charged region of the photoreceptor drum 1a.

[0051] From the first potential Vr and the second potential V0 measured by the above method, V0 - Vr in Equation (1) is calculated. Also, from the amount of charged charge Q and the charged area S measured by the above method, the value of Q / S in Equation (1) is calculated. Then, a graph is created with Q / S on the horizontal axis and V0 - Vr on the vertical axis. Six points indicating the measurement results under the conditions where the charging bias applied to the charging roller 34 is +1000V, +1100V, +1200V, +1300V, +1400V, and +1500V are plotted on the graph. An approximate straight line is drawn through these six points, and the slope of the approximate straight line is obtained. The obtained slope is taken as V0 - Vr / (Q / S) in Equation (1).

[0052] The film thickness d of the photosensitive layer 111 is measured in an environment of 23°C and 50% RH. The film thickness d of the photosensitive layer 502 is measured using a film thickness measuring device (FISCHERSCOPE MMS, manufactured by HELMUT FISCHER). In this embodiment, the film thickness of the photosensitive layer 111 is 37×10 -6 [m].

[0053] ε0 represents the permittivity of vacuum. The permittivity of vacuum ε0 is constant and is 8.85×10 -12 [F / m].

[0054] The relative permittivity εr of the photosensitive layer 111 corresponds to the relative permittivity of the photosensitive layer 111 when it is assumed that there are no charge traps inside the photosensitive layer 111 and all of the amount of charge supplied from the charging roller 34 changes to the surface potential of the photoreceptor drum 1. Since materials other than the first binder resin constituting the photosensitive layer 111 are also organic substances, the relative permittivity εr of the photosensitive layer 111 is almost the same as the relative permittivity of the first binder resin. Therefore, for the measurement of the relative permittivity εr of the photosensitive layer 111, a photoreceptor for relative permittivity measurement including a photosensitive layer containing only the first binder resin is used. Note that the photoreceptor for relative permittivity measurement can be manufactured in the same manner as the photoreceptor drum 1a except that a charge generating agent, a hole transporting agent, an electron transporting agent, and an additive are not added.

[0055] Based on the relationship between the charging ability ratio A of the photoreceptor drum 1a measured in advance and the amplitude Vpp of the charging bias affected by the AC component of the developing bias, the magnitude of the charging bias applied by the charging bias power supply 47b to the charging roller 34 is determined. Thereby, the amplitude Vpp (V) of the charging bias affected by the AC component of the developing bias can be suppressed to a predetermined value or less to prevent the occurrence of image defects. At this time, the capacitor of the charging bias power supply 45 can be designed to have the minimum necessary capacitance. Therefore, an image forming apparatus 100 that can suppress the occurrence of density unevenness while suppressing the manufacturing cost can be provided.

[0056] FIG. 3 is a flowchart showing an execution example of the image forming operation in the image forming apparatus 100. In step S1, the main control unit (control unit) 80 determines whether a measurement operation for measuring the charging ability ratio A has been performed on the photoreceptor drums 1a to 1d in the past. If the measurement operation has been performed in the past, the process proceeds to step S3 and a normal image forming operation is performed. If the measurement operation has not been performed in the past, the process proceeds to step S2. Note that even if any of the photoreceptor drums 1a to 1d has been replaced and the measurement operation has not been performed, the process proceeds to step S2.

[0057] In step S2, a measurement operation is performed to measure the charging ability ratio of the photoreceptor drums 1a to 1d. Based on the measured charging ability ratio and a graph (Fig. 5) stored in advance, the magnitude of the charging bias applied by the developing bias power supply 47b to the charging roller 34 during the image forming operation is determined. When the measurement operation ends, the process proceeds to step S3.

[0058] In step S3, a normal image forming operation is performed. The charging bias applied by the developing bias power supply 47b to the charging roller 34 during the execution of the image forming operation is determined in step S2. Thereby, the occurrence of density unevenness (foggy image) can be suppressed while applying an optimal charging bias.

[0059] Next, the effects of the present invention will be specifically described using a plurality of embodiments. In the following evaluation, the relationship between the surface potential V0 of the photoreceptor drum (image carrier) 1a and the occurrence of image defects was evaluated. In the following evaluation, the ambient temperature around the charging roller 34 was 28°C, and the relative humidity (RH) was 80%.

[0060] Fig. 4 is a graph showing the charging ability (V / μA) from the relationship between the charging current Ia (μA) and the surface potential V0 (V).

[0061] As shown in Fig. 4, the rate of increase in the surface potential with respect to the charging current Ia supplied to the photoreceptor drum 1a is higher for the photoreceptor drum 1a according to Example 1 than for the photoreceptor drum 1a according to Example 2. Thereby, it was found that the charging ability (V / μA) indicating the ease of charging is higher for the photoreceptor drum according to Example 1 than for the photoreceptor drum according to the example. Therefore, the charging ability ratio A can be calculated from the rate of increase in the surface potential with respect to the charging current Ia supplied to the photoreceptor drum 1a.

[0062] FIG. 5 is a graph showing the relationship between the amplitude Vpp (V) of the charging bias affected by the AC component of the developing bias and image defects. The line L1 represents the above-mentioned formula (1). When the amplitude Vpp (V) of the charging bias affected by the AC component of the developing bias was smaller than the line L1, no image defects were observed (represented as "OK" in the graph in FIG. 5). When the amplitude Vpp (V) of the charging bias affected by the AC component of the developing bias was larger than the line L1, image defects were observed (represented as "NG" in the graph in FIG. 5). As a result, the magnitude of the charging bias applied to the charging roller 34 by the developing bias power supply 47b during image formation is determined based on the previously measured charging capacity ratio A of the photosensitive drums 1a-1d and the amplitude Vpp of the charging bias affected by the AC component. This indicates that the occurrence of image defects can be prevented by keeping the amplitude Vpp (V) of the charging bias affected by the AC component of the developing bias below a predetermined value.

[0063] Although the embodiments of the present invention have been described above, the scope of the present invention is not limited thereto, and various modifications can be made without departing from the spirit of the invention. For example, in the above embodiment, a color printer as shown in Fig. 1 was used as an example of image forming apparatus 100, but the image forming apparatus is not limited to a color printer, and may be other image forming apparatuses such as monochrome and color copiers, digital multifunction peripherals, facsimiles, etc. [Industrial Applicability]

[0064] The present invention can be used in an image forming apparatus equipped with a charging roller. [Explanation of symbols]

[0065] 1a to 1d: photosensitive drum (image bearing member) 2a~2d Charging device 3a~3d developing device 5 Exposure equipment 6a to 6d Primary transfer roller (transfer member) 7a~7d Cleaning device 8 Intermediate transfer belt 9 Secondary transfer roller 10 Driven roller 11 Driving roller 12a Paper feed roller 12b Registration roller pair 13 Fixing unit 13a Fixing roller pair 14 Branch part 15 Discharge roller pair 16 Paper cassette 17 Discharge tray 18 Surface conveyance path 19 Belt cleaner 20 Static elimination lamp 31 Developing roller (developer carrier) 32 Cleaning blade 33 Rubbing roller 34 Charging roller (charging member) 35 Conveying spiral 40 Image density sensor 43 Developing bias power supply 43a AC constant voltage power supply 43b, 45b DC constant voltage power supply 45 Charging bias power supply 46 Current detection part 47 Transfer bias power supply 50 Voltage control part 70 Memory part 80 Main control part 90 Liquid crystal display part 91 Transmission / reception part 100 Image forming apparatus 111 Photosensitive layer

Claims

1. An image carrier having a photosensitive layer formed on its surface, A charging device disposed opposite to the image carrier and having a charging member for charging the image carrier, An exposure device for forming an electrostatic latent image by exposing the image carrier charged by the charging device, A developing device disposed opposite to the image carrier and having a developer carrier for carrying a developer, and for forming a toner image by attaching toner to the electrostatic latent image formed on the image carrier, An image forming unit including the above, A developing bias power source for applying a developing bias obtained by superimposing an AC voltage on a DC voltage to the developer carrier, A charging bias power source for applying a charging bias including a DC voltage to the charging member, In an image forming apparatus comprising the image forming unit, the developing bias power source, and the charging bias power source, and a control unit for controlling them, The control unit, During the execution of an image forming operation for forming an image, the amplitude Vpp of the charging bias affected by the AC component of the developing bias satisfies the following formula (1). An image forming apparatus. In the following formula (1), Vr is the first potential [V] of the outer peripheral surface of the image carrier before being charged in the charging process V0 is the second potential [V] of the outer peripheral surface of the image carrier at the developing position after being charged in the charging process Q is the amount of charge [C] of the image carrier S is the charged area [m 2 of the image carrier d is the film thickness [m] of the photosensitive layer εr is the relative permittivity of the photosensitive layer ε0 is the permittivity of vacuum [F / m] 【Number 1】

2. The charging bias applied by the charging bias power source consists only of a DC voltage. The image forming apparatus according to claim 1.

3. The photosensitive layer is a positive charging single layer type and has a film thickness of 37 μm or more in an unused state. The image forming apparatus according to claim 1 or claim 2.

Images