Image forming apparatus

Abstract

To provide an image forming apparatus that can always determine an appropriate developing voltage to prevent occurrence of a white dot image regardless of a layer thickness of a photosensitive layer of an image carrier.SOLUTION: An image forming apparatus comprises: an image forming unit that has an image carrier, a charging device, and a developing device, and performs image formation by using toner; an exposure device; an image density sensor; a developing voltage power supply; a control unit; and a layer thickness detection mechanism. The control unit can execute calibration for adjusting density of a toner image by detecting density of a reference image formed by the image forming unit with the image density sensor, and adjusting a developing voltage on the basis of a result of the detection. The layer thickness detection mechanism detects a layer thickness of a photosensitive layer. The control unit gradually reduces, from a reference value, an upper limit value of a DC voltage in adjusting the developing voltage in the calibration as the layer thickness of the photosensitive layer detected by the layer thickness detection mechanism is reduced.SELECTED DRAWING: Figure 8

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, or a combination machine thereof, which is equipped with an image carrier, and more particularly to an image forming apparatus equipped with a developing device of a two-component development system using a two-component developer. [Background technology]

[0002] In an image forming apparatus, an electrostatic latent image formed on an image carrier such as a photoconductor is developed by a developing device to be visualized as a toner image. One such developing device employs a two-component developing method using a two-component developer.

[0003] In a two-component development type image forming apparatus, 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 electrostatic capacitance of the photosensitive layer, so the amount of toner that moves during development decreases, and developability deteriorates. Therefore, the thicker the photosensitive layer, the higher the development voltage must be, but if the development voltage is high, toner aggregates tend to move to the photosensitive drum during development, resulting in the generation of white spots on the image. On the other hand, from the viewpoint of extending the life of the image forming apparatus, it is necessary to use a photosensitive drum with a large layer thickness, that is, a photosensitive layer with a certain thickness or more at the start of use.

[0004] In order to solve the above-mentioned problems, for example, Patent Document 1 discloses an image forming apparatus that controls the operation of a supply device based on the detection results of a film thickness detection device so that the toner concentration in the developing unit is lower when the film thickness on the photosensitive drum is thin compared to when the film thickness is thick. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2012-18212 A Summary of the Invention [Problem to be solved by the invention]

[0006] Patent Document 1 describes correcting the development contrast potential (for example, to -10 V) when the thickness of the photoconductor drum is thin. However, since the image changes depending on whether the development contrast potential is corrected (-10 V), there is a problem in that the output image differs depending on whether the thickness of the photoconductor drum is thick or thin.

[0007] SUMMARY OF THE PRESENT DISCLOSURE In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of always determining an appropriate developing voltage and suppressing the occurrence of white spot images, regardless of the thickness of the photosensitive layer of the image carrier. [Means for solving the problem]

[0008] In order to achieve the above object, the first configuration of the present invention includes an image forming unit, an exposure device, an image density sensor, a development voltage power supply, a control unit, and a layer thickness detection mechanism. The image forming unit has an image carrier, a charging device, and a developing device, and forms an image using toner. The image carrier has a photosensitive layer formed on its surface. The charging device charges the surface of the image carrier. The developing device has a developer carrier that carries a developer containing toner, and develops an electrostatic latent image formed on the image carrier into a toner image. The exposure device exposes the surface of the image carrier charged by the charging device to form an electrostatic latent image whose charge is attenuated. The image density sensor detects the density of the toner image formed in the image forming unit. The development voltage power supply applies a development voltage, which is a DC voltage superimposed on an AC voltage, to the developer carrier. The control unit controls the development voltage power supply. The control unit is capable of performing calibration to adjust the density of the toner image by detecting the density of a reference image formed in the image forming unit using the image density sensor and adjusting the development voltage based on the detection result. The layer thickness detection mechanism detects the layer thickness of the photosensitive layer, and the control unit gradually decreases the upper limit value of the DC voltage when adjusting the development voltage by calibration from a reference value as the layer thickness of the photosensitive layer detected by the layer thickness detection mechanism decreases. Effect of the Invention

[0009] According to the first configuration of the present invention, the upper limit value set for the DC voltage of the developing voltage is changed based on the thickness of the photosensitive layer. As a result, an appropriate upper limit value according to the thickness of the photosensitive layer is set, and an optimal DC voltage is determined within a range that does not exceed the upper limit value when performing calibration. Therefore, even when an image carrier with a thick photosensitive layer is used from the viewpoint of extending the life of the image carrier, the occurrence of white spots can be suppressed while maintaining an appropriate image density as much as possible. [Brief description of the drawings]

[0010] [Figure 1] 1 is a schematic diagram of a color printer 100 according to an embodiment of the present invention; [Diagram 2] An enlarged view of the image forming section Pa in FIG. [Diagram 3] FIG. 13 is a diagram showing an example of a patch image (reference image) for image density correction; [Figure 4] A block diagram showing an example of a control path used in a color printer 100. [Diagram 5] 1 is a graph showing the relationship between the thickness of the photosensitive layer 19b of the photosensitive drums 1a to 1d and the DC voltage of the appropriate developing voltage determined by calibration. [Figure 6] FIG. 1 is a graph showing the relationship between the thickness of the photosensitive layer 19b of the photosensitive drums 1a to 1d and the upper limit of the DC voltage Vdc of the developing voltage, in the case where the absolute humidity is low. [Figure 7] FIG. 1 is a graph showing the relationship between the thickness of the photosensitive layer 19b of the photosensitive drums 1a to 1d and the upper limit of the DC voltage Vdc of the developing voltage, when the absolute humidity is high. [Figure 8] 1 is a flowchart showing an example of control for setting an upper limit of the DC voltage Vdc when performing calibration in the color printer 100. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a schematic cross-sectional view of a color printer 100 according to one embodiment of the present invention. Fig. 2 is an enlarged view of the vicinity of image forming unit Pa in Fig. 1. Note that image forming units Pb to Pd have a basically similar configuration, so their description will be omitted.

[0012] Four image forming units Pa, Pb, Pc, and Pd are arranged in this order from the upstream side in the transport direction (the left side in FIG. 1) inside the main body of color printer 100. 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, magenta, cyan, and black through the processes of charging, exposing, developing, and transferring, respectively.

[0013] Each of the image forming stations Pa to Pd is provided with a photoconductor drum 1a, 1b, 1c, and 1d that carries a visible image (toner image) of each color. An intermediate transfer belt 8 that rotates counterclockwise in Fig. 1 is provided adjacent to each of the image forming stations Pa to Pd. 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, and a belt cleaning device 30 is provided upstream of the image forming station Pa with respect to the rotation direction of the intermediate transfer belt 8, facing the tension roller 11 with the intermediate transfer belt 8 sandwiched therebetween.

[0014] As shown in FIG. 2, a charging device 2a, a developing device 3a, a cleaning device 7a, and a discharging device 20 are arranged around the photosensitive drum 1a in the drum rotation direction (clockwise in FIG. 2), and a primary transfer roller 6a is disposed with an intermediate transfer belt 8 sandwiched therebetween.

[0015] The photosensitive drums 1a to 1d are each 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 as the photosensitive layer 19b on the surface of the cylindrical conductive substrate 19a made of aluminum.

[0016] The charging devices 2a to 2d each have a charging roller 21 that contacts the photoconductor drums 1a to 1d and applies a charging voltage (DC voltage+AC voltage) to the drum surface, and a charging cleaning roller 24 for cleaning the charging roller 21.

[0017] The developing devices 3a to 3d are of a two-component development type having two stirring and conveying screws 25 and a developing roller 29, and are filled with a predetermined amount of two-component developer containing toner of each color, yellow, magenta, cyan, 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 the magnetic brush is brought into contact with the surface of the photosensitive drum 1a while a developing voltage of the same polarity as the toner (positive polarity in this case) is applied to the developing roller 29, thereby adhering the toner and forming a toner image. When the ratio of toner in the two-component developer filled in each developing device 3a to 3d falls below a specified value due to the formation of the toner image, toner is replenished from the toner containers 4a to 4d to each developing device 3a to 3d.

[0018] The cleaning devices 7a to 7d each have a cleaning blade 31 and a recovery screw 33. The cleaning blade 31 removes toner and other substances remaining on the surfaces of the photoconductor drums 1a to 1d. The recovery screw 33 discharges the toner and other substances 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 eliminator 20 irradiates the surfaces of the photoconductor drums 1a to 1d with static elimination light to remove residual charges.

[0019] When image data is input from a host device such as a personal computer, first, the main motor 40 (see FIG. 4) starts rotating the photoconductor drums 1a-1d. In addition, the belt drive motor 41 (see FIG. 4) starts rotating the intermediate transfer belt 8. Next, the charging devices 2a-2d uniformly charge the surfaces of the photoconductor drums 1a-1d with the same polarity as the toner (positive polarity in this case). Next, the exposure device 5 irradiates light according to the image data, and an electrostatic latent image with the charge attenuated according to the image data is formed on each of the photoconductor drums 1a-1d.

[0020] The developing devices 3a to 3d are filled with a predetermined amount of two-component developer (hereinafter, simply referred to as developer) containing toner of each color of yellow, magenta, cyan, and black by toner containers 4a to 4d, and the toner in the developer is supplied onto the photoconductor drums 1a to 1d by the developing devices 3a to 3d and electrostatically adheres to the photoconductor drums 1a to 1d. As a result, a toner image corresponding to the electrostatic latent image formed by exposure from the exposure device 5 is formed.

[0021] Then, an electric field is applied between the primary transfer rollers 6a-6d and the photoconductor drums 1a-1d by the primary transfer rollers 6a-6d at a predetermined transfer voltage, and the yellow, magenta, cyan and black toner images on the photoconductor drums 1a-1d are primarily transferred onto the intermediate transfer belt 8. Toner and the like remaining on the surfaces of the photoconductor drums 1a-1d after the primary transfer are removed by the cleaning devices 7a-7d. Residual charges remaining on the surfaces of the photoconductor drums 1a-1d after the primary transfer are removed by the charge removing device 20.

[0022] The transfer paper P onto which the toner image is transferred is stored in a paper cassette 16 located at the bottom inside the color printer 100, and is transported at a predetermined timing via a paper feed roller 12a and a pair of registration rollers 12b to a nip portion (secondary transfer nip portion) between the intermediate transfer belt 8 and a secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8. The transfer paper P onto which the toner image has been secondarily transferred is transported to a fixing unit 13.

[0023] The transfer paper P transported to the fixing section 13 is heated and pressurized by the fixing roller pair 13a, so that the toner image is fixed onto the surface of the transfer paper P, and a predetermined full-color image is formed. The transfer paper P on which the full-color image has been formed is discharged onto the discharge tray 17 by the discharge roller pair 15 as is (or after being diverted to the reversing conveyance path 18 by the branching section 14 and images are formed on both sides).

[0024] An image density sensor 25 is disposed at a position facing the drive roller 10 across the intermediate transfer belt 8. An optical sensor equipped with a light-emitting element such as an LED and a light-receiving element such as a photodiode is generally used as the image density sensor 25. When measuring the amount of toner adhesion on the intermediate transfer belt 8, when the light-emitting element irradiates each patch image (reference image) formed on the intermediate transfer belt 8 with measurement light, the measurement light is reflected by the toner and the belt surface and enters the light-receiving element.

[0025] The light reflected from the toner and the belt surface includes specular and diffuse reflections. The specular and diffuse reflections are separated by a polarizing separation prism and then enter separate light receiving elements. Each light receiving element photoelectrically converts the received specular and diffuse reflections and outputs an output signal to the control unit 90 (see FIG. 4).

[0026] Then, the image density (toner amount) and image position of the patch image are detected from the characteristic changes in the output signals of the specularly reflected light and the diffusely reflected light, and compared with a predetermined reference density and reference position, and image density correction and color shift correction (calibration) are performed for each color by adjusting the characteristic value of the development voltage, the exposure start position and timing of the exposure device 5, etc.

[0027] 3 is a diagram showing an example of a patch image (reference image) for image density correction. In a reference image forming region Rs on one side (right side) in the width direction of the intermediate transfer belt 8, reference images y consisting of patch images y1 to y10 with 10 levels of density, from the lightest image y1 to the darkest image y10, are formed in a line in order from the downstream side along the belt traveling direction (arrow X1 direction). Adjacent patch images are each formed in a single color so that the density changes at the boundary. Note that, although the yellow reference image y has been described as an example here, the cyan, magenta, and black reference images c, m, and k have exactly the same configuration.

[0028] The toner adhesion amounts (toner densities) of the reference images y to k are 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. Depending on the average value of the density differences obtained, parameter values ​​to be used for density correction are determined as described below, and density correction is performed for each color.

[0029] 4 is a block diagram showing an example of a control path used in color printer 100. Note that, because various controls are performed on each part of color printer 100 when color printer 100 is used, the control paths for color printer 100 as a whole are complex. Therefore, the following description will focus on the parts of the control paths that are necessary for implementing the present invention.

[0030] The charging voltage power supply 52 applies a charging voltage to the charging roller 21 in the charging devices 2a to 2d. The developing voltage power supply 53 applies a developing voltage, which is a DC voltage Vdc superimposed with an AC voltage Vac, to the developing roller 29 in the developing devices 3a to 3d. The transfer voltage power supply 54 applies a predetermined primary transfer voltage and 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 each of these power supplies according to an output signal from the control unit 90.

[0031] The image input unit 60 is a receiving unit that receives image data sent from a personal computer or the like to the color printer 100. The image signal input by the image input unit 60 is converted into a digital signal and then sent to the temporary storage unit 94.

[0032] 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 status of the color printer 100, the image formation status, the number of copies to be printed, etc. The LED 72 displays various statuses and errors of the color printer 100. Various settings of the color printer 100 are performed from the printer driver of the personal computer.

[0033] In addition, the operation unit 70 is provided with a start button that the user uses to instruct the start of image formation, a stop / clear button that is used when canceling image formation, and a reset button that is used to reset various settings of the color printer 100 to their default states.

[0034] The internal temperature and humidity sensor 80 detects the temperature and humidity inside the color printer 100, particularly the temperature and humidity around the image forming units Pa to Pd, and is disposed near the image forming units Pa to Pd.

[0035] The control unit 90 at least comprises a CPU (Central Processing Unit) 91 as a central processing unit, a ROM (Read Only Memory) 92 which is a read-only memory unit, a RAM (Random Access Memory) 93 which is a readable and writable memory unit, a temporary memory unit 94 which temporarily stores image data and the like, a counter 95, and multiple (two in this case) I / Fs (interfaces) 96 which send control signals to each device within the color printer 100 and receive input signals from the operation unit 50. The control unit 90 can be placed anywhere inside the color printer 100 main body.

[0036] The ROM 92 stores control programs for the color printer 100, numerical values ​​necessary for control, and other data that will not change while the color printer 100 is in use. The RAM 93 stores necessary data that arises during the control of the color printer 100, data that is temporarily necessary for controlling the color printer 100, and the like.

[0037] The RAM 93 (or ROM 92) also stores the relationship between the thickness of the photosensitive layer 19b and the DC voltage Vdc when changing the upper limit of the DC voltage Vdc of the developing voltage applied to the developing roller 29 of the developing device 3a-3d based on the absolute humidity and the thickness of the photosensitive layer 19b of the photosensitive drums 1a-1d, as described below, and the relationship (arithmetic expression) between the accumulated driving distance and the thickness of the photosensitive layer 19b when calculating the thickness of the photosensitive layer 19b based on the accumulated driving distance of the photosensitive drums 1a-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 cumulatively counts the number of printed sheets.

[0038] Furthermore, the control unit 90 transmits control signals from the CPU 91 through the I / F 96 to each part and device in the color printer 100. Furthermore, signals indicating the state of each part and device and input signals are transmitted to the CPU 91 through the I / F 96 from each part and device. Examples of parts and devices controlled by the control unit 90 include image forming units Pa to Pd, exposure unit 5, intermediate transfer belt 8, secondary transfer roller 9, fixing unit 13, voltage control circuit 55, image input unit 60, operation unit 70, and in-machine temperature and humidity sensor 80.

[0039] From the viewpoint of extending the life of the color printer 100, it is necessary to use photoconductor drums 1a-1d in which the thickness of the photoconductor layer 19b at the start of use is at least a certain level (for example, at least 32 μm). On the other hand, in developing devices 3a-3d that use a two-component development method, the development characteristics vary greatly depending on the thickness of the photoconductor layer 19b of the photoconductor drums 1a-1d.

[0040] Fig. 5 is a graph showing the relationship between the layer thickness of the photosensitive layer 19b of the photosensitive drums 1a to 1d and the appropriate development voltage (DC voltage Vdc) determined by calibration. The layer thickness of the photosensitive layer 19b is the thickest when the photosensitive drums 1a to 1d start to be used, and becomes thinner as durable printing progresses. Therefore, in Fig. 5 and Figs. 6 and 7 described later, the origin of the horizontal axis is set to the start of use, and the thickness becomes thinner as it moves away from the origin.

[0041] As described above, the electrostatic capacitance of the photosensitive layer 19b increases as the thickness of the photosensitive layer 19b increases, so the amount of toner moving from the developing roller 29 to the photosensitive drums 1a-1d decreases, and the developability decreases. Therefore, as shown in Fig. 5, the DC voltage Vdc (solid line in Fig. 5) required to maintain an appropriate image density increases as the thickness of the photosensitive layer 19b increases.

[0042] However, when the DC voltage Vdc becomes high, white dots appear on the image. This is because toner aggregates are more likely to move from the developing roller 29 to the photoconductor drums 1a-1d during development. The dotted lines in Fig. 5 and Figs. 6 and 7 described below indicate the DC voltages at which white dots appear (white dot generating voltages). In other words, when the DC voltage Vdc becomes higher than the dotted lines, white dots appear on the image.

[0043] Therefore, in the color printer 100 of this embodiment, an upper limit is set for the DC voltage Vdc of the developing voltage determined by calibration. Then, when the thickness of the photosensitive layer 19b of the photosensitive drums 1a to 1d is equal to or greater than a predetermined value, the upper limit of the DC voltage Vdc is decreased as the thickness of the photosensitive layer 19b decreases.

[0044] In addition, the DC voltage Vdc required to maintain an appropriate image density (the optimum value of the DC voltage Vdc) varies depending on the absolute humidity (amount of moisture in the atmosphere). More specifically, the optimum value of the DC voltage Vdc increases in a low humidity environment and decreases in a high humidity environment.

[0045] 6 and 7 are graphs showing the relationship between the thickness of the photosensitive layer 19b of the photosensitive drums 1a to 1d and the upper limit of the DC voltage Vdc of the developing voltage, showing the cases where the absolute humidity (amount of moisture in the atmosphere) is low and high, respectively. The dashed lines in Figs. 6 and 7 indicate the upper limit of the DC voltage Vdc. The upper limit of the DC voltage Vdc is set lower than the white spot generating voltage (dotted line).

[0046] When the absolute humidity is low (for example, 18 mg / m 36, in the region where the thickness of the photosensitive layer 19b is thicker than the predetermined value A, the optimum value of the DC voltage Vdc (solid line) may be higher than the white spot generating voltage (dotted line). Therefore, in the region where the thickness of the photosensitive layer 19b is thicker than the predetermined value A, the upper limit value (dashed line) of the DC voltage Vdc is controlled to become smaller as the thickness of the photosensitive layer 19b becomes thinner.

[0047] In addition, in an area where the layer thickness of the photosensitive layer 19b is thinner than a predetermined value A, the optimum value of the DC voltage Vdc (solid line) is always lower than the white spot generating voltage (dotted line). Therefore, the upper limit value (dashed line) of the DC voltage Vdc is set to a constant value so that the optimum value of the DC voltage Vdc is not set lower than necessary.

[0048] In the region where the thickness of the photosensitive layer 19b is thicker than the predetermined value A, the upper limit of the DC voltage Vdc (dashed line) may be set lower than the optimum value of the DC voltage Vdc (solid line), which may result in a slight decrease in image density. However, the occurrence of white dots on the image is suppressed, so that the image is free of problems (no image noise).

[0049] When the absolute humidity is high (for example, 18 mg / m 3 7, the optimum value of the DC voltage Vdc is lower than the voltage at which white spots occur, regardless of the thickness of the photosensitive layer 19b. Therefore, even if the upper limit of the DC voltage Vdc is not changed according to the thickness of the photosensitive layer 19b, an appropriate image density can be obtained without causing white spots. In other words, there is no problem with controlling the upper limit of the DC voltage Vdc to a constant value.

[0050] Fig. 8 is a flowchart showing an example of control for setting the upper limit of the DC voltage Vdc when calibration is performed in the color printer 100. The procedure for setting the upper limit of the DC voltage Vdc will be described following the steps in Fig. 8, with reference to Figs. 1 to 7 as necessary.

[0051] First, the control unit 90 determines whether the calibration execution timing has been reached (step S1). Calibration is performed when the color printer 100 is powered on, when the cumulative number of printed sheets since the previous calibration reaches a predetermined number, when the installation environment (temperature and humidity) changes by a certain amount or more, and so on.

[0052] If it is the calibration execution timing (Yes in step S1), the control unit determines whether the absolute humidity H is less than a predetermined value H1 (for example, 18 [mg / m 3 ) based on the detection result of the in-machine temperature and humidity sensor 80 (see FIG. 4) (step S2).

[0053] If H < H1 (Yes in step S2), the control unit 90 calculates the layer thickness T of the photosensitive layer 19b of the photosensitive drums 1a to 1d (step S3). Specifically, the layer thickness T of the photosensitive layer 19b is calculated based on the relationship between the cumulative driving distance of the photosensitive drums 1a to 1d from the start of use, the cumulative driving distance stored in the RAM 93 (or ROM 92), and the layer thickness of the photosensitive layer 19b.

[0054] Next, the control unit 90 determines whether the layer thickness T of the photosensitive layer 19b calculated in step S3 exceeds a predetermined value A (step S4). If T > A (Yes in step S4), the upper limit value of the DC voltage Vdc of the developing voltage is changed based on the layer thickness T of the photosensitive layer 19b calculated in step S3 (step S5). More specifically, as the layer thickness T of the photosensitive layer 19b becomes thinner, the upper limit value of the DC voltage Vdc is gradually decreased from the reference value.

[0055] On the other hand, if T ≤ A in step S4 (No in step S4), the layer thickness T of the photosensitive layer 19b is sufficiently thin, and the optimum value of the DC voltage Vdc (solid line in FIG. 6) is lower than the white dot generation voltage (dotted line in FIG. 6). Therefore, the upper limit value of the DC voltage Vdc set in the previous calibration is maintained (step S6).

[0056] Thereafter, the control unit 90 executes calibration (step S7). Specifically, the developing voltage (DC voltage Vdc) is changed stepwise to form reference images y to k in the reference image forming area Rs (see FIG. 3) of the intermediate transfer belt 8, and the reference images are read by the image density sensor 25. The control unit 90 sets the DC voltage Vdc within the range up to the upper limit set in steps S5 and S6 based on the detected voltage of the image density sensor 25 or by linearly interpolating the detected voltage.

[0057] In addition to setting the developing voltage described above, the calibration also determines the amount of exposure of the exposure device 5 and corrects the look-up table indicating the gamma characteristics for each color as necessary.

[0058] On the other hand, if H≧H1 in step S2 (No in step S2), the layer thickness T of the photosensitive layer 19b is not calculated and the upper limit value of the DC voltage Vdc is not changed, and 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).

[0059] According to the control example shown in Fig. 8, the upper limit value set for the DC voltage Vdc of the developing voltage is changed based on the thickness of the photosensitive layer 19b. As a result, an appropriate upper limit value according to the thickness of the photosensitive layer 19b is set, and an optimal DC voltage Vdc is determined within a range that does not exceed the upper limit value when calibration is performed. Therefore, even when using photoconductor drums 1a-1d with thick photosensitive layers 19b from the viewpoint of extending the life of the photoconductor drums, it is possible to suppress the occurrence of white spots while maintaining an appropriate image density as much as possible.

[0060] Moreover, when the thickness of the photosensitive layer 19b becomes equal to or smaller than a predetermined value, the upper limit determined in the immediately preceding calibration is maintained. This prevents the DC voltage Vdc from being set lower than necessary, thereby maintaining the image density as much as possible.

[0061] Furthermore, when the absolute humidity is equal to or higher than a predetermined value, the upper limit of the DC voltage Vdc is not changed from the reference value because the optimal value of the DC voltage Vdc is lower than the voltage at which white spots occur, regardless of the thickness of the photosensitive layer 19b. This eliminates the risk of the DC voltage Vdc being set unnecessarily low in a high humidity environment where there is no risk of white spots occurring, and therefore makes it possible to maintain sufficient image density.

[0062] Furthermore, by calculating the thickness of the photosensitive layer 19b based on the accumulated driving distances of the photosensitive drums 1a to 1d, the thickness of the photosensitive layer 19b can be calculated simply and accurately, and the upper limit value of the DC voltage Vdc can be set more appropriately.

[0063] In the example shown in FIG. 8, the control unit 90 calculates the layer 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 that calculates the layer thickness of the photosensitive layer 19b may be provided separately from the control unit 90.

[0064] In addition, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the spirit of the present invention. For example, in the above embodiment, an intermediate transfer type color printer 100 was described in which the toner images formed on the photosensitive drums 1a to 1d are primarily transferred to the intermediate transfer belt 8, and then secondarily transferred to the transfer paper P. However, the present invention is not limited to this, and the present invention can also be applied to a direct transfer type color printer in which the toner images formed on the photosensitive drums 1a to 1d are directly transferred to the transfer paper P.

[0065] In the above embodiment, the color printer 100 is described as having the photosensitive drums 1a-1d on which a single organic photosensitive layer is laminated as the photosensitive layer 19b, and the developing devices 3a-3d of a two-component development method using a two-component developer, but the present invention is not limited to this. For example, the present invention can be applied to an image forming apparatus equipped with a photosensitive drum on which a laminated organic photosensitive layer or an amorphous silicon photosensitive layer is formed, or a developing device of a one-component development method using a magnetic one-component developer made only of magnetic toner, or a non-magnetic one-component developer made only of non-magnetic toner, while suppressing the occurrence of white spots due to toner aggregates, and maintaining image density as much as possible.

[0066] In addition, in the above embodiment, a tandem type color printer 100 has been used as an example of an image forming apparatus, but the present invention can of course also be applied to image forming apparatuses such as color copiers, color multifunction machines, monochrome printers, and monochrome multifunction machines. [Industrial Applicability]

[0067] The present invention can be used in an image forming apparatus equipped with a two-component developing device. By using the present invention, it is possible to provide an image forming apparatus that can always determine an appropriate developing voltage and suppress the occurrence of white spot images regardless of the thickness of the photosensitive layer of the image carrier. [Explanation of symbols]

[0068] Pa~Pd Image forming section 1a to 1d Photoconductor drum (image carrier) 2a~2d Charging device 3a~3d developing device 5 Exposure equipment 6a~6d Primary transfer roller 7a~7d Cleaning device 8 Intermediate transfer belt 9 Secondary transfer roller 19a Conductive substrate 19b Photosensitive layer 29 Developing roller (developer carrier) 52 Charge voltage power supply 53 Development voltage power supply 54 Transfer voltage power supply 70 Operation section 71 LCD display section 80 In-flight temperature and humidity sensor (humidity detection device) 90 Control unit (layer thickness detection mechanism) 100 Color printers (image forming devices)

Claims

1. an image carrier having a photosensitive layer formed on its surface; a charging device for charging the surface of the image carrier; a developing device having a developer carrier that carries a developer containing a toner, and that develops an electrostatic latent image formed on the image carrier into a toner image; an image forming section which forms an image using the toner; an exposure device that exposes the surface of the image carrier charged by the charging device to light to form the electrostatic latent image whose charge has been attenuated; an image density sensor that detects the density of the toner image formed by the image forming unit; a developing voltage power source that applies a developing voltage, which is a DC voltage superimposed on an AC voltage, to the developer carrying member; A control unit that controls the developing voltage power supply; Equipped with The control unit is an image forming apparatus capable of performing calibration for adjusting the density of the toner image by detecting the density of a reference image formed by the image forming unit using the image density sensor and adjusting the developing voltage based on a detection result, a layer thickness detection mechanism for detecting a layer thickness of the photosensitive layer, The control unit is characterized in that, as the layer thickness of the photosensitive layer detected by the layer thickness detection mechanism becomes smaller, the upper limit value of the DC voltage when adjusting the developing voltage by the calibration is gradually decreased from a reference value.

2. 2. The image forming apparatus according to claim 1, wherein the control unit maintains the upper limit value of the DC voltage at the upper limit value in the immediately preceding calibration when the layer thickness of the photosensitive layer detected by the layer thickness detection mechanism becomes equal to or less than a predetermined value.

3. a humidity detection device that detects the absolute humidity around the developing device, 2. The image forming apparatus according to claim 1, wherein the control unit maintains an upper limit value of the DC voltage at the reference value regardless of a thickness of the photosensitive layer when the absolute humidity detected by the humidity detection device is equal to or higher than a predetermined value.

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

5. 2. The image forming apparatus according to claim 1, wherein the thickness of the photosensitive layer of the image carrier at the start of use is 32 [mu]m or more.

6. 6. The image forming apparatus according to claim 1, wherein the photosensitive layer is a single organic photosensitive layer.

7. 6. The image forming apparatus according to claim 1, wherein the developing device is of a two-component developing system that uses a two-component developer containing the toner and a carrier as the developer.

Images