Image forming apparatus

Abstract

An image forming apparatus includes an image forming unit, an exposure device, an image density sensor, a developing voltage power supply, a control unit, and a layer thickness detection mechanism, the image forming unit includes an image carrier, a charging device, and a developing device, and performs image formation using toner. The control section detects, using the image density sensor, the density of the reference image formed by the image forming section, and adjusts the developing voltage on the basis of the detection result, so that calibration for adjusting the density of the toner image can be performed. The layer thickness detection mechanism detects the layer thickness of the photosensitive layer. As the layer thickness of the photosensitive layer detected by the layer thickness detection mechanism becomes smaller, the control unit gradually reduces the upper limit value of the DC voltage when the developing voltage is adjusted by calibration from the reference value.

Description

Technical Field

[0001] The present invention relates to an image forming device, which is an image forming device such as a copier, printer, fax machine and multifunctional all-in-one machine equipped with an image carrier, and in particular to an image forming device equipped with a developing device of a two-component developing method using a two-component developer. Background Art

[0002] In an image forming apparatus, an electrostatic latent image formed on an image carrier such as a photosensitive body is developed by a developing device to be visualized as a toner image. As one of such developing devices, a two-component developing method using a two-component developer is adopted.

[0003] In an image forming apparatus using a two-component development method, the development characteristics vary greatly depending on the thickness of the photosensitive layer of the photosensitive drum. Specifically, the thicker the photosensitive layer, the greater the capacitance of the photosensitive layer, so the amount of toner that moves during development decreases, and the developability decreases. Therefore, the thicker the photosensitive layer, the higher the development voltage needs to be, but if the development voltage is increased, the toner aggregates are more likely to move to the photosensitive drum during development, resulting in white spots on the image. On the other hand, from the perspective of extending the life of the image forming apparatus, it is necessary to use a photosensitive drum with a thicker layer, and the thickness of the photosensitive layer of the photosensitive drum at the beginning of use is above a certain level. Summary of the invention

[0004] 1. Technical issues to be resolved

[0005] An object of the present invention is to provide an image forming apparatus capable of always determining an appropriate development voltage regardless of the thickness of a photosensitive layer of an image carrier, thereby suppressing the occurrence of white spot images.

[0006] (II) Technical solution

[0007] The image forming device of the first structure of the present invention is characterized in that it comprises: an image forming unit, an exposure device, an image density sensor, a developing voltage power supply, and a control unit, wherein the image forming unit has: an image bearing member having a photosensitive layer formed on the surface, a charging device for charging the surface of the image bearing member, and a developing device, wherein the developing device has a developer bearing member, the developer bearing member carries a developer containing a colorant, the developing device develops an electrostatic latent image formed on the image bearing member into a colorant image, the image forming unit uses the colorant to form an image, the exposure device exposes the surface of the image bearing member charged by the charging device to form the electrostatic latent image with attenuated charge, and the image density sensor detects: The concentration of the colorant image formed by the image forming unit, the developing voltage power supply applies to the developer carrier: a developing voltage in which an AC voltage is superimposed on a DC voltage, the control unit controls the developing voltage power supply, the control unit uses the image density sensor to detect: the concentration of the reference image formed by the image forming unit, and adjusts the developing voltage based on the detection result, thereby being able to perform calibration for adjusting the concentration of the colorant image, the image forming device has a layer thickness detection mechanism, the layer thickness detection mechanism detects the layer thickness of the photosensitive layer, as the layer thickness of the photosensitive layer detected by the layer thickness detection mechanism becomes smaller, the control unit causes the upper limit value of the DC voltage when adjusting the developing voltage by the calibration to be reduced in stages from the reference value.

[0008] (III) Beneficial effects

[0009] According to the first configuration of the present invention, the upper limit value set for the DC voltage of the development voltage is changed based on the thickness of the photosensitive layer. Thus, an appropriate upper limit value corresponding to the thickness of the photosensitive layer is set, so that when performing calibration, the optimal DC voltage can be determined within a range that does not exceed the upper limit value. Therefore, even when an image carrier having a thicker photosensitive layer is used from the viewpoint of extending the life, an appropriate image density can be maintained as much as possible, and the generation of white spots can be suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 It is a schematic structural diagram of a color printer 100 according to an embodiment of the present invention.

[0011] Figure 2 yes Figure 1 An enlarged view of the vicinity of the image forming portion Pa in FIG.

[0012] Figure 3 This is a diagram showing an example of a patch image (reference image) for image density correction.

[0013] Figure 41 is a block diagram showing an example of a control path for the color printer 100 .

[0014] Figure 5 It is a graph showing the relationship between the layer thickness of the photosensitive layer 19 b of the photosensitive drums 1 a to 1 d and the DC voltage of the appropriate development voltage determined by calibration.

[0015] Figure 6 It is a graph showing the relationship between the layer thickness of the photosensitive layer 19b of the photosensitive drums 1a to 1d and the upper limit value of the DC voltage Vdc of the development voltage, and is a graph showing the case where the absolute humidity is low.

[0016] Figure 7 It is a graph showing the relationship between the layer thickness of the photosensitive layer 19b of the photosensitive drums 1a to 1d and the upper limit value of the DC voltage Vdc of the development voltage, and is a graph showing the case where the absolute humidity is high.

[0017] Figure 8 1 is a flowchart showing an example of control for setting the upper limit value of the DC voltage Vdc when calibration is executed in the color printer 100 . DETAILED DESCRIPTION

[0018] Embodiments of the present invention will be described below with reference to the accompanying drawings. Figure 1 It is a schematic cross-sectional view of a color printer 100 according to an embodiment of the present invention. Figure 2 yes Figure 1 1 is an enlarged view of the vicinity of the image forming section Pa in FIG. The image forming sections Pb to Pd also have basically the same configuration, and thus their description is omitted.

[0019] In the main body of the color printer 100, from the upstream side in the conveying direction (in Figure 1 The image forming units Pa, Pb, Pc, and Pd are arranged in sequence. These image forming units Pa to Pd are arranged corresponding to images of different four colors (yellow, magenta, cyan, and black), and form yellow, magenta, cyan, and black images in sequence through charging, exposure, development, and transfer processes.

[0020] The image forming sections Pa to Pd are provided with photosensitive drums 1a, 1b, 1c, and 1d for carrying visible images (toner images) of respective colors. Figure 1 The intermediate transfer belt 8 rotates counterclockwise as shown. The intermediate transfer belt 8 is wound around a driving roller 10 on the downstream side and a tension roller 11 on the upstream side. With respect to the rotation direction of the intermediate transfer belt 8, a belt cleaning device 30 is arranged on the upstream side of the image forming section Pa so as to face the tension roller 11 with the intermediate transfer belt 8 interposed therebetween.

[0021] like Figure 2As shown, around the photosensitive drum 1a, along the drum rotation direction ( Figure 2 In the clockwise direction of the image processing apparatus, a charging device 2a, a developing device 3a, a cleaning device 7a, and a neutralizing device 20 are provided, and a primary transfer roller 6a is provided across the intermediate transfer belt 8.

[0022] The photosensitive drums 1a to 1d are composed of a conductive substrate 19a and a photosensitive layer 19b formed on the surface of the conductive substrate 19a. In this embodiment, a single organic photosensitive layer is laminated on the surface of a cylindrical conductive substrate 19a made of aluminum as the photosensitive layer 19b.

[0023] The charging devices 2 a to 2 d include a charging roller 21 that contacts the photosensitive drums 1 a to 1 d and applies a charging voltage (DC voltage+AC voltage) to the drum surfaces, and a charging cleaning roller 24 for cleaning the charging roller 21 .

[0024] The developing devices 3a to 3d are two-component developing devices having two stirring and conveying screws 25 and a developing roller 29, and are respectively filled with a predetermined amount of two-component developer containing toners of cyan, magenta, yellow, and black and a magnetic carrier. A magnetic brush is formed on the surface of the developing roller 29 using the two-component developer, and a developing voltage having the same polarity as the toner (positive polarity in this case) is applied to the developing roller 29. In this state, the magnetic brush is brought into contact with the surface of the photosensitive drum 1a, and the toner is attached to form a toner image. In addition, when the proportion of the toner in the two-component developer filled in each developing device 3a to 3d is lower than a predetermined value due to the formation of the toner image, the toner is replenished from the toner container 4a to 4d to each developing device 3a to 3d.

[0025] The cleaning devices 7a to 7d include a cleaning blade 31 and a recovery screw 33. The cleaning blade 31 removes the toner and the like remaining on the surface of the photosensitive drums 1a to 1d. The recovery screw 33 discharges the toner and the like removed by the cleaning blade 31 to the outside of the cleaning devices 7a to 7d and recovers them in a waste toner recovery container (not shown). The static elimination device 20 irradiates the surface of the photosensitive drums 1a to 1d with static elimination light to eliminate the residual charge.

[0026] When image data is input from a host device such as a personal computer, first, the main motor 40 (see Figure 4 ) starts the rotation of the photosensitive drums 1a to 1d. In addition, the belt drive motor 41 (see Figure 4 ) The intermediate transfer belt 8 is started to be rotated. Then, the surfaces of the photosensitive drums 1a to 1d are uniformly charged to the same polarity as the toner (positive polarity in this case) by the charging devices 2a to 2d. Then, the exposure device 5 irradiates light according to the image data, and forms an electrostatic latent image on each photosensitive drum 1a to 1d, the charging of which is attenuated according to the image data.

[0027] The developing devices 3a to 3d are filled with a predetermined amount of two-component developer (hereinafter referred to as developer) containing yellow, magenta, cyan, and black toners by toner containers 4a to 4d, and the toners in the developer are supplied to the photosensitive drums 1a to 1d by the developing devices 3a to 3d for electrostatic adhesion, thereby forming a toner image corresponding to the electrostatic latent image formed by exposure from the exposure device 5.

[0028] Then, an electric field is applied between the primary transfer rollers 6a to 6d and the photosensitive drums 1a to 1d at a predetermined transfer voltage by the primary transfer rollers 6a to 6d, and the yellow, magenta, cyan, and black toner images on the photosensitive drums 1a to 1d are primarily transferred to the intermediate transfer belt 8. After the primary transfer, the toner and the like remaining on the surfaces of the photosensitive drums 1a to 1d are removed by the cleaning devices 7a to 7d. After the primary transfer, the residual charge remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the charge removing device 20.

[0029] The transfer paper P to which the toner image is transferred is stored in a paper cassette 16 disposed at the lower part of the color printer 100, and the transfer paper P is conveyed to a nip portion (secondary transfer nip portion) between a secondary transfer roller 9 disposed adjacent to the intermediate transfer belt 8 and the intermediate transfer belt 8 via a paper feed roller 12a and a registration roller pair 12b at a predetermined timing. The transfer paper P to which the toner image is secondarily transferred is conveyed to the fixing unit 13.

[0030] The transfer paper P conveyed to the fixing section 13 is heated and pressed by the fixing roller pair 13a, so that the toner image is fixed on the surface of the transfer paper P, thereby forming a predetermined full-color image. The transfer paper P with the full-color image formed thereon is discharged to the discharge tray 17 by the discharge roller pair 15 directly (or after being distributed to the reverse conveying path 18 by the branch section 14 and having images formed on both sides).

[0031] An image density sensor 25 is disposed at a position opposite to the driving roller 10 across the intermediate transfer belt 8. As the image density sensor 25, an optical sensor is generally used, and the optical sensor includes: a light emitting element composed of an LED or the like, and a light receiving element composed of a photodiode or the like. When measuring the amount of toner attached to the intermediate transfer belt 8, if the measurement light is irradiated from the light emitting element to each patch image (reference image) formed on the intermediate transfer belt 8, the measurement light is incident on the light receiving element as light reflected by the toner and light reflected by the belt surface.

[0032] The reflected light from the toner and the belt surface includes regular reflected light and diffuse reflected light. The regular reflected light and diffuse reflected light are separated by the polarization separation prism and then incident on different light receiving elements. Each light receiving element performs photoelectric conversion on the received regular reflected light and diffuse reflected light and transmits the received light to the control unit 90 (see Figure 4 ) outputs the output signal.

[0033] Furthermore, the image density (amount of colorant) and image position of the small image are detected based on the characteristic changes of the output signals of the regular reflected light and the diffuse reflected light, and compared with the predetermined reference density and reference position, and the characteristic value of the developing voltage, the exposure start position and timing of the exposure device 5, etc. are adjusted, thereby performing image density correction and color shift correction (calibration) for each color.

[0034] Figure 3 This is a diagram showing an example of a patch image (reference image) for image density correction. In the reference image forming area Rs on one side (right side) in the width direction of the intermediate transfer belt 8, a reference image y consisting of 10 levels of density, from the lightest image y1 to the darkest image y10, is formed in a row in the order from the downstream side along the belt travel direction (arrow X1 direction). Adjacent patch images are formed in monochrome so that the density changes at the boundary. In addition, the yellow reference image y is used as an example for explanation here, but the cyan, magenta, and black reference images c, m, and k are all of the same structure.

[0035] The toner adhesion amount (toner density) of the reference images y to k is detected by the image density sensor 25, and compared with a predetermined standard density, and the average value of the density difference between each toner density and the standard density is calculated. According to the obtained average value of the density difference, the parameter value for density correction is determined in the following manner, and density correction is performed on each color.

[0036] Figure 4 1 is a block diagram showing an example of a control path for the color printer 100. In addition, since various controls are performed on each part of the color printer 100 when the color printer 100 is used, the control path of the color printer 100 as a whole is complicated. Therefore, the part of the control path necessary for implementing the present invention will be described here in detail.

[0037] The charging voltage power supply 52 applies a charging voltage to the charging roller 21 in the charging device 2a to 2d. The developing voltage power supply 53 applies a developing voltage obtained by superimposing an AC voltage Vac on a DC voltage Vdc to the developing roller 29 in the developing device 3a to 3d. The transfer voltage power supply 54 applies a predetermined primary transfer voltage and a secondary transfer voltage to the primary transfer rollers 6a to 6d and the secondary transfer roller 9, respectively. The voltage control circuit 55 is connected to the charging voltage power supply 52, the developing voltage power supply 53, and the transfer voltage power supply 54, and operates the above-mentioned power supplies according to the output signal from the control unit 90.

[0038] The image input unit 60 is a receiving unit that receives image data transmitted from a personal computer or the like to the color printer 100 . The image signal input from the image input unit 60 is converted into a digital signal and then output to the temporary storage unit 94 .

[0039] The operation unit 70 is provided with a liquid crystal display unit 71 and an LED 72. The liquid crystal display unit 71 displays the operating state of the color printer 100, the image forming status, the number of copies, etc. The LED 72 displays various states and errors of the color printer 100. Various settings of the color printer 100 are performed by the printer driver of the personal computer.

[0040] The operation unit 70 also includes a start button for the user to instruct the start of image formation, a stop / clear button for suspending image formation, and a reset button for returning various settings of the color printer 100 to default.

[0041] The internal temperature and humidity sensor 80 detects the temperature and humidity inside the color printer 100 , especially the temperature and humidity around the image forming units Pa to Pd. The internal temperature and humidity sensor 80 is disposed near the image forming units Pa to Pd.

[0042] The control unit 90 includes at least a CPU (Central Processing Unit) 91 as a central processing unit, a ROM (Read Only Memory) 92 which is a storage unit dedicated to reading, a RAM (Random Access Memory) 93 which is a storage unit that can read and write, a temporary storage unit 94 which temporarily stores image data, etc., a counter 95, and a plurality of (here, two) I / Fs (interfaces) 96 which send control signals to various devices in the color printer 100 or receive input signals from the operation unit 70. The control unit 90 can be arranged at any position inside the main body of the color printer 100.

[0043] The ROM 92 stores a control program for the color printer 100 and data such as numerical values ​​required for control that do not change during use of the color printer 100. The RAM 93 stores necessary data generated during control of the color printer 100 and data temporarily required during control of the color printer 100.

[0044] In addition, the RAM 93 (or the ROM 92) also stores: the relationship between the thickness of the photosensitive layer 19b and the DC voltage Vdc when the upper limit value of the DC voltage Vdc of the developing voltage applied to the developing roller 29 of the developing device 3a to 3d is changed based on the absolute humidity and the thickness of the photosensitive layer 19b of the photosensitive drum 1a to 1d as described later; and the relationship (calculation formula) between the cumulative driving distance and the thickness of the photosensitive layer 19b when the thickness of the photosensitive layer 19b is calculated based on the cumulative driving distance of the photosensitive drum 1a to 1d. The temporary storage unit 94 temporarily stores: the image signal input from the image input unit 60 and converted into a digital signal. The counter 95 accumulates the number of printed sheets for counting.

[0045] In addition, the control unit 90 sends control signals from the CPU 91 to each part and device in the color printer 100 through the I / F 96. In addition, signals indicating the status and input signals are sent from each part and device to the CPU 91 through the I / F 96. As the parts and devices controlled by the control unit 90, for example, there can be mentioned: image forming units Pa to Pd, exposure device 5, intermediate transfer belt 8, secondary transfer roller 9, fixing unit 13, voltage control circuit 55, image input unit 60, operation unit 70, in-machine temperature and humidity sensor 80, etc.

[0046] In the color printer 100, from the viewpoint of extending the life, it is necessary to use the photosensitive drums 1a to 1d such that the thickness of the photosensitive layer 19b is at least a certain level (for example, at least 32 μm) at the beginning of use. On the other hand, in the developing devices 3a to 3d of the two-component developing method, the developing characteristics vary greatly depending on the thickness of the photosensitive layer 19b of the photosensitive drums 1a to 1d.

[0047] Figure 5 This is a graph showing the relationship between the thickness of the photosensitive layer 19b of the photosensitive drums 1a to 1d and the appropriate developing voltage (DC voltage Vdc) determined by calibration. In addition, the thickness of the photosensitive layer 19b is the thickest at the beginning of the use of the photosensitive drums 1a to 1d, and becomes thinner as printing continues. Figure 5 and the following Figure 6 , Figure 7 In FIG. 1 , the origin of the horizontal axis is set to the start of use, and the thickness is depicted as becoming thinner as the distance from the origin increases.

[0048] As described above, as the thickness of the photosensitive layer 19b increases, the capacitance of the photosensitive layer 19b increases, and thus the amount of toner moving from the developing roller 29 to the photosensitive drums 1a to 1d decreases, and the developing performance decreases. Figure 5 As shown in the figure, the DC voltage Vdc required to maintain the appropriate image density ( Figure 5 The solid line) increases as the thickness of the photosensitive layer 19b increases.

[0049] However, if the DC voltage Vdc is increased, white spots may appear on the image. This is because toner aggregates are easily transferred from the developing roller 29 to the photosensitive drums 1a to 1d during development. Figure 5 and the following Figure 6 , Figure 7 The dotted line in represents the DC voltage that generates white spots (white spot generating voltage). That is, when the DC voltage Vdc is above the dotted line, white spots will be generated on the image.

[0050] Therefore, in the color printer 100 of this embodiment, an upper limit value is set for the DC voltage Vdc of the development voltage determined by calibration. And when the thickness of the photosensitive layer 19b of the photosensitive drums 1a to 1d is greater than a predetermined value, the upper limit value of the DC voltage Vdc is reduced as the thickness of the photosensitive layer 19b becomes thinner.

[0051] In addition, the DC voltage Vdc required to maintain an appropriate image density (the optimal value of the DC voltage Vdc) varies depending on the absolute humidity (ambient moisture content). More specifically, the optimal value of the DC voltage Vdc increases in a low humidity environment and decreases in a high humidity environment.

[0052] Figure 6 and Figure 7 The graphs show the relationship between the thickness of the photosensitive layer 19 b of the photosensitive drums 1 a to 1 d and the upper limit value of the DC voltage Vdc of the development voltage, respectively showing the case where the absolute humidity (ambient moisture content) is low and the case where it is high. Figure 6 , 7 The broken line in is the upper limit value of the DC voltage Vdc. The upper limit value of the DC voltage Vdc is set to be lower than the white dot generation voltage (dotted line).

[0053] When the absolute humidity is low (e.g. less than 18 mg / m 3 ),like Figure 6 As shown in FIG. 1 , the optimum value (solid line) of the DC voltage Vdc generated is higher than the white point generated voltage (dotted line) in the region where the thickness of the photosensitive layer 19b is thicker than the predetermined value A. Therefore, in the region where the thickness of the photosensitive layer 19b is thicker than the predetermined value A, the upper limit value (broken line) of the DC voltage Vdc is controlled to be smaller as the thickness of the photosensitive layer 19b becomes thinner.

[0054] In addition, in the region where the thickness of the photosensitive layer 19b is thinner than the predetermined value A, the optimal value of the DC voltage Vdc (solid line) is always lower than the white spot generation voltage (dotted line). Therefore, the upper limit value (broken line) of the DC voltage Vdc is made constant, and the optimal value of the DC voltage Vdc is avoided from being set lower than necessary.

[0055] In addition, in the region where the thickness of the photosensitive layer 19b is thicker than the predetermined value A, the image density may be slightly reduced because the upper limit value (broken line) of the DC voltage Vdc is set lower than the optimal value (solid line) of the DC voltage Vdc. However, since the generation of white spots on the image is suppressed, the image can be in a state without problems (no image noise).

[0056] When the absolute humidity is high (for example, 18 mg / m 3 above), such as Figure 7 As shown in FIG. 1 , the optimum value of the DC voltage Vdc is lower than the white spot generation voltage regardless of the thickness of the photosensitive layer 19b. Therefore, even if the upper limit value of the DC voltage Vdc is not changed according to the thickness of the photosensitive layer 19b, white spots will not be generated and an appropriate image density can be obtained. In other words, there is no problem in the control of making the upper limit value of the DC voltage Vdc a constant value.

[0057] Figure 8 This is a flowchart showing an example of setting control of the upper limit value of the DC voltage Vdc when the calibration is executed in the printer 100. Figure 1 to Figure 7 , and follow Figure 8 The steps of setting the upper limit value of the DC voltage Vdc are described below.

[0058] First, the control unit 90 determines whether the calibration execution time has arrived (step S1). Calibration is executed when the color printer 100 is powered on, when the cumulative number of printed sheets since the last calibration reaches a predetermined number, or when the installation environment (temperature and humidity) changes to a certain extent.

[0059] When it is time to execute calibration (Yes in step S1), the control unit 90 detects the temperature and humidity in the machine based on the temperature and humidity sensor 80 (see Figure 4 ) to determine whether the absolute humidity H is less than the specified value H1 (e.g. 18 mg / m 3 ) (step S2).

[0060] When H

[0061] ​Next, the control unit 90 determines whether the thickness T of the photosensitive layer 19b calculated in step S3 exceeds a predetermined value A (step S4). When T>A (yes in step S4), the upper limit value of the DC voltage Vdc of the development voltage is changed based on the thickness T of the photosensitive layer 19b calculated in step S3 (step S5). More specifically, as the thickness T of the photosensitive layer 19b becomes thinner, the upper limit value of the DC voltage Vdc is gradually reduced from the reference value.

[0062] On the other hand, when T≦A in step S4 (No in step S4), the thickness T of the photosensitive layer 19b is sufficiently thinned, and the optimal value of the DC voltage Vdc ( Figure 6 The solid line) produces a voltage higher than the white point ( Figure 6 Therefore, the upper limit value of the DC voltage Vdc set in the most recent calibration is maintained (step S6).

[0063] After that, the control unit 90 performs calibration (step S7). Specifically, in the reference image forming area Rs (refer to Figure 3 ), the developing voltage (DC voltage Vdc) is changed stepwise to form reference images y to k, and the reference images y to k are read by the image density sensor 25. The control unit 90 sets the DC voltage Vdc within a range below the upper limit value set in steps S5 and S6 based on the detection voltage of the image density sensor 25 or by linearly interpolating the detection voltage.

[0064] In addition to the setting of the development voltage described above, the calibration also includes, as necessary, determination of the exposure amount of the exposure device 5 and correction of a lookup table representing the gamma characteristics of each color.

[0065] On the other hand, when H≧H1 in step S2 (No in step S2), the thickness T of the photosensitive layer 19b and the upper limit value of the DC voltage Vdc are not calculated and changed, but calibration is performed using the upper limit value of the DC voltage Vdc as a reference value regardless of the thickness of the photosensitive layer 19b (step S7).

[0066] according to Figure 8 In the control example shown, the upper limit value set for the DC voltage Vdc of the development voltage is changed based on the thickness of the photosensitive layer 19b. Thus, an appropriate upper limit value corresponding to the thickness of the photosensitive layer 19b is set, so that when performing calibration, the optimal DC voltage Vdc can be determined within the range that does not exceed the upper limit value. Therefore, even when the photosensitive drums 1a to 1d with thicker photosensitive layers 19b are used from the viewpoint of extending the life, the appropriate image density can be maintained as much as possible, and the generation of white spots can be suppressed.

[0067] When the thickness of the photosensitive layer 19b is less than a predetermined value, the upper limit determined in the most recent calibration is maintained. Thus, the DC voltage Vdc is not set lower than necessary, and thus the image density can be maintained as much as possible.

[0068] Furthermore, when the absolute humidity is above a predetermined value, the optimum value of the DC voltage Vdc is lower than the white spot generation voltage regardless of the thickness of the photosensitive layer 19b, so the upper limit of the DC voltage Vdc is not changed from the reference value. Thus, it is avoided that the DC voltage Vdc is unnecessarily set to a low value in a high humidity environment where white spots are not generated, so that a sufficient image density can be maintained.

[0069] Furthermore, by calculating the thickness of the photosensitive layer 19b based on the cumulative drive distance of the photosensitive drums 1a to 1d, the thickness of the photosensitive layer 19b can be simply and accurately calculated, and the upper limit value of the DC voltage Vdc can be set more appropriately.

[0070] In addition, Figure 8 In the example shown, the control unit 90 calculates the thickness of the photosensitive layer 19b based on the accumulated driving distance (the control unit 90 also serves as a layer thickness detection mechanism), but a layer thickness detection mechanism may be separately provided outside the control unit 90 to calculate the thickness of the photosensitive layer 19b.

[0071] In addition, the present invention is not limited to the above-mentioned embodiment, and various changes can be made without departing from the scope of the present invention. For example, in the above-mentioned embodiment, the color printer 100 of the intermediate transfer method in which the toner image formed on the photosensitive drum 1a~1d is transferred to the intermediate transfer belt 8 and further transferred to the transfer paper P is described, but it is not limited to this, and the present invention can also be applied to the color printer of the direct transfer method in which the toner image formed on the photosensitive drum 1a~1d is directly transferred to the transfer paper P.

[0072] In addition, in the above embodiment, the color printer 100 is described as an example, that is, the color printer 100 includes: a photosensitive drum 1a to 1d having a single organic photosensitive layer stacked as a photosensitive layer 19b, and a developing device 3a to 3d of a two-component developing method using a two-component developer, but the present invention is not limited to this. For example, in an image forming apparatus including a developing device of a single-component developing method, the present invention can also be applied to suppress the generation of white spots caused by toner aggregates and maintain the image density as much as possible. The developing device uses: a photosensitive drum having a stacked organic photosensitive layer and an amorphous silicon photosensitive layer; a magnetic single-component developer composed only of a magnetic toner, and a non-magnetic single-component developer composed only of a non-magnetic toner.

[0073] In addition, in the above embodiment, the tandem color printer 100 is used as an example of an image forming device, but the present invention is of course also applicable to image forming devices such as color copiers, color multifunction machines, monochrome printers, monochrome multifunction machines, etc.

[0074] The present invention can be used in an image forming apparatus having a developing device of a two-component developing method. The present invention can provide an image forming apparatus that can always determine an appropriate developing voltage regardless of the thickness of the photosensitive layer of the image carrier to suppress the generation of white spot images.

Claims

1. An image forming device comprising: an image forming unit, an exposure device, an image density sensor, a developing voltage power supply, and a control unit, The image forming unit includes: an image bearing member having a photosensitive layer formed on a surface thereof, a charging device for charging the surface of the image bearing member, and a developing device. The developing device includes a developer carrier that carries a developer including a toner, and develops the electrostatic latent image formed on the image carrier into a toner image. The image forming unit forms an image using the toner. The exposure device exposes the surface of the image carrier charged by the charging device to form the electrostatic latent image whose charge is attenuated. The image density sensor detects the density of the toner image formed by the image forming unit. The developing voltage power supply applies to the developer carrier: a developing voltage obtained by superimposing an alternating voltage on a direct voltage, The control unit controls the developing voltage power supply. The control unit detects the density of the reference image formed by the image forming unit using the image density sensor, and adjusts the development voltage based on the detection result, thereby being able to perform calibration for adjusting the density of the toner image. The image forming apparatus is characterized in that A layer thickness detection mechanism is provided, wherein the layer thickness detection mechanism detects the layer thickness of the photosensitive layer. The control unit reduces the upper limit value of the DC voltage when adjusting the development voltage by the calibration in a stepwise manner from a reference value as the thickness of the photosensitive layer detected by the layer thickness detection mechanism decreases.

2. The image forming apparatus according to claim 1, wherein: When the thickness of the photosensitive layer detected by the layer thickness detection mechanism is equal to or smaller than a predetermined value, the control unit maintains the upper limit value of the DC voltage at the upper limit value of the most recent calibration.

3. The image forming apparatus according to claim 1, wherein: A humidity detection device is provided for detecting the absolute humidity around the developing device. When the absolute humidity detected by the humidity detection device is equal to or higher than a predetermined value, the control unit maintains the upper limit value of the DC voltage at the reference value regardless of the thickness of the photosensitive layer.

4. The image forming apparatus according to claim 1, wherein: The layer thickness detection mechanism detects the layer thickness of the photosensitive layer based on the accumulated driving distance of the image carrier.

5. The image forming apparatus according to claim 1, wherein: The thickness of the photosensitive layer at the start of use of the image support is 32 μm or more.

6. The image forming apparatus according to any one of claims 1 to 5, characterized in that: The photosensitive layer is a single-layer organic photosensitive layer.

7. The image forming apparatus according to any one of claims 1 to 5, characterized in that: The developing device is a two-component developing method using a two-component developer, and the two-component developer contains the toner and a carrier as the developer.

Images