Image forming apparatus

Abstract

The present invention provides an image forming apparatus capable of maintaining the toner concentration in the developer appropriately over a long period of time in a two-component developing system using a two-component developer. [Solution] The image forming apparatus comprises an image carrier, a charging device, an exposure device, a developing device, a toner container, a toner density sensor, a toner charge amount detection unit, and a control unit. The developing device includes a developer carrier that carries a two-component developer containing a magnetic carrier and toner, and develops the electrostatic latent image formed on the surface of the image carrier into a toner image. The toner density sensor detects the toner density in the developing device. The toner charge amount detection unit detects the amount of toner charge in the developing device. The control unit controls the supply of toner from the toner container to the developing device based on the detection result of the toner density sensor. The control unit performs a first target value change control to change the target value of the toner density based on the amount of toner charge detected by the toner charge amount detection unit.

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 multifunction machine having an image carrier, and particularly to an image forming apparatus of an intermediate transfer type using a two-component developer containing toner and carrier.

Background Art

[0002] In an image forming apparatus, an electrostatic latent image formed on an image carrier made of a photoreceptor or the like is developed by a developing device and visualized as a toner image. As one of such developing devices, a two-component development method using a two-component developer containing a magnetic carrier and toner is adopted.

[0003] In the two-component development method, the developer deteriorates under the influence of factors such as the number of printed sheets, environmental (temperature and humidity) fluctuations, printing mode, and printing ratio on the image (the ratio of the printed area to the image-formable area), and the charging characteristics of the toner in the developer change. As a result, the toner cannot be sufficiently charged, and problems such as a decrease in image density, image fogging, and toner scattering occur.

[0004] Therefore, methods for calculating the toner charge amount have been proposed. For example, in Patent Documents 1 and 2, there are disclosed image forming apparatuses capable of measuring the toner current included in the developing current and calculating the toner charge amount based on the measurement results.

[0005] Further, in Patent Documents 3 and 4, there are disclosed image forming apparatuses that predict the causes and occurrence levels of defects such as transfer memory and developing ghost based on the estimated results of the toner charge amount and feed back to the control of the process conditions.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

[0007] In image forming apparatuses equipped with toner charge detection means, as described in Patent Documents 1 and 2, the toner charge can be adjusted to an appropriate range by changing the toner concentration (T / C; mass ratio of toner to carrier) and development conditions based on the toner charge detection results. However, this method was not necessarily sufficient for stabilizing image density over a long period of time.

[0008] In the methods described in Patent Documents 3 and 4, control is based on the prediction of toner charge amount. Therefore, if the prediction accuracy of the toner charge amount is low, the toner charge amount will change due to environmental fluctuations or changes in toner durability. As a result, the toner charge amount is unstable, and appropriate feedback cannot be provided for controlling the process conditions.

[0009] In view of the above problems, the present invention aims to provide an image forming apparatus that can properly maintain the toner concentration in the developer over a long period of time in a two-component developing method using a two-component developer. [Means for solving the problem]

[0010] To achieve the above objective, the first configuration of the present invention is an image forming apparatus comprising an image carrier, a charging device, an exposure device, a developing device, a toner container, a toner density sensor, a toner charge amount detection unit, and a control unit. A photosensitive layer is formed on the surface of the image carrier. The charging device charges the surface of the image carrier to a predetermined surface potential. The exposure device irradiates light onto the image carrier charged by the charging device to form an electrostatic latent image as the charge decays. The developing device comprises a developer carrier that carries a two-component developer containing a magnetic carrier and toner, and develops the electrostatic latent image formed on the surface of the image carrier into a toner image. The toner container contains toner to be supplied to the developing device. The toner density sensor detects the toner density, which is the ratio of toner to magnetic carriers in the developing device. The toner charge amount detection unit detects the toner charge amount, which is the amount of charge per unit mass of toner in the developing device. The control unit controls the supply of toner from the toner container to the developing device based on the toner density detection result by the toner density sensor. The control unit performs a first target value change control to change the target value of the toner concentration based on the toner charge amount detected by the toner charge amount detection unit. [Effects of the Invention]

[0011] According to the first configuration of the present invention, by performing a first target value change control that changes the target value of toner density based on the amount of toner charge, even when the carrier deteriorates due to durable printing and the amount of toner charge decreases, the toner density in the developing device can be maintained within an appropriate range and a decrease in image density can be suppressed. [Brief explanation of the drawing]

[0012] [Figure 1] Side cross-sectional view showing the internal configuration of an image forming apparatus 100 according to one embodiment of the present invention. [Figure 2] Side cross-sectional view of the developing device 3a mounted on the image forming apparatus 100 of this embodiment. [Figure 3] Enlarged view of the area around the image forming unit Pa, including the control path of the image forming unit Pa. [Figure 4]A graph showing the relationship between toner concentration and toner charge amount as the cumulative number of printed pages changes. [Figure 5] A graph showing the relationship between the cumulative number of printed pages and the amount of toner charge when the target value of toner concentration is changed. [Figure 6] A graph showing the relationship between the cumulative number of printed pages and image density when the target value of toner density is changed. [Figure 7] Graph showing the change in toner density relative to the dot count in the second target value change control. [Figure 8] A graph showing the change in toner density relative to the dot count when the first target value change control and the second target value change control are executed in conjunction. [Figure 9] A flowchart showing the execution procedures for the first target value change control and the second target value change control in the image forming apparatus 100 of this embodiment. [Modes for carrying out the invention]

[0013] [1. Overall configuration of the image forming apparatus] Embodiments of the present invention will be described below with reference to the drawings. Figure 1 is a cross-sectional view showing the internal structure of an image forming apparatus 100 according to one embodiment of the present invention. Inside the main body of the image forming apparatus 100 (here, a color printer), four image forming units Pa, Pb, Pc, and Pd are arranged in order from the upstream side in the transport direction (left side in Figure 1). These image forming units Pa to Pd are provided to correspond to images of four different colors (cyan, magenta, yellow, and black), and each sequentially forms images of cyan, magenta, yellow, and black through the processes of charging, exposure, development, and transfer, respectively.

[0014] In these image forming units Pa to Pd, photosensitive drums (image carriers) 1a, 1b, 1c, and 1d that carry visible images (toner images) of respective colors are disposed. Further, an intermediate transfer belt (intermediate transfer member) 8 that rotates in the counterclockwise direction in FIG. 1 by a driving motor (not shown) is provided adjacent to each of the image forming units Pa to Pd. The toner images formed on these photosensitive drums 1a to 1d are sequentially primary transferred and superimposed onto the intermediate transfer belt 8 that moves while contacting each of the photosensitive drums 1a to 1d. Thereafter, the toner image primary transferred onto the intermediate transfer belt 8 is secondary transferred onto transfer paper P as an example of a recording medium by a secondary transfer roller 9. Further, after the toner image is fixed on the transfer paper P onto which the toner image has been secondary transferred, the transfer paper P is discharged from the main body of the image forming apparatus 100. While rotating the photosensitive drums 1a to 1d in the clockwise direction in FIG. 1, an image forming process for each of the photosensitive drums 1a to 1d is executed.

[0015] The transfer paper P onto which the toner image is secondary transferred is housed in a paper cassette 16 disposed at the lower part of the main body of the image forming apparatus 100. The transfer paper P is conveyed to the nip portion between the secondary transfer roller 9 and the driving roller 11 of the intermediate transfer belt 8 via a paper feed roller 12a and a registration roller pair 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a seamless belt having no seam is mainly used. Also, a blade-shaped belt cleaner 19 for removing toner and the like remaining on the surface of the intermediate transfer belt 8 is disposed on the downstream side of the secondary transfer roller 9. ]]

[0016] Next, the image forming units Pa to Pd will be described. Around and below the photosensitive drums 1a to 1d that are rotatably disposed, charging devices 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d, an exposure device 5 for exposing image information onto each of the photosensitive drums 1a to 1d, developing devices 3a, 3b, 3c, and 3d for forming a toner image on the photosensitive drums 1a to 1d, and cleaning devices 7a, 7b, 7c, and 7d for removing developer (toner) and the like remaining on the photosensitive drums 1a to 1d are provided.

[0017] 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, and an electrostatic latent image corresponding to the image data is formed on each of the photoreceptor drums 1a to 1d. The developing devices 3a to 3d are each filled with a predetermined amount of a two-component developer containing cyan, magenta, yellow, and black toners. When the ratio of the toner in the two-component developer filled in each of the developing devices 3a to 3d falls below a specified value due to the formation of a toner image described later, toner is replenished from the toner containers 4a to 4d to each of the developing devices 3a to 3d. The toner in this developer is supplied onto the photoreceptor drums 1a to 1d by the developing devices 3a to 3d and adheres electrostatically. Thereby, a toner image corresponding to the electrostatic latent image formed by the exposure from the exposure device 5 is formed.

[0018] 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 a predetermined full-color image formation. Then, in preparation for the formation of a new electrostatic latent image that is subsequently performed, toner and the like remaining on the surfaces of the photoreceptor drums 1a to 1d after the primary transfer are removed by the cleaning devices 7a to 7d.

[0019] The intermediate transfer belt 8 is stretched between the upstream driven roller 10 and the downstream driving roller 11. When the intermediate transfer belt 8 starts to rotate counterclockwise as the driving roller 11 rotates by a driving motor (not shown), the transfer paper P is conveyed from the registration roller pair 12b at a predetermined timing to the nip portion (secondary transfer nip portion) between the driving roller 11 and the secondary transfer roller 9 provided adjacent thereto. When the transfer paper P passes through the secondary transfer nip portion, the toner image on the intermediate transfer belt 8 is secondarily transferred onto the transfer paper P. The transfer paper P onto which the toner image has been secondarily transferred is conveyed to the fixing unit 13.

[0020] The transfer paper P, transported to the fixing unit 13, is heated and pressurized by the fixing roller pair 13a, fixing the toner image to the surface of the transfer paper P and forming a predetermined full-color image. The transfer paper P on which the full-color image has been formed is then transported in a direction determined by the branching unit 14 which branches in multiple directions, and is discharged into the discharge tray 17 by the discharge roller pair 15 (or after being sent to the double-sided transport path 18 for double-sided printing).

[0021] An image density sensor 40 is positioned downstream of the image forming unit Pd and opposite the intermediate transfer belt 8. Generally, an optical sensor is used as the image density sensor 40, which includes a light-emitting element such as an LED and a light-receiving element such as a photodiode. When measuring the amount of toner deposited on the intermediate transfer belt 8, measurement light is irradiated from the light-emitting element onto each reference image formed on the intermediate transfer belt 8, and the measurement light is incident on the light-receiving element as light reflected by the toner and light reflected by the belt surface.

[0022] The light reflected from the toner and the belt surface includes specular and diffuse reflection. This specular and diffuse reflection is separated by a polarization separation prism and then incident on separate photodetectors. Each photodetector converts the received specular and diffuse reflection into photoelectric signals and outputs them to the main control unit 80 (see Figure 3). The amount of toner is then detected from the characteristic changes of the specular and diffuse reflection output signals, and density correction (calibration) is performed for each color by adjusting the characteristic values ​​of the development voltage, etc., by comparing it with a predetermined reference density.

[0023] [2. Configuration of the developing device] Figure 2 is a side cross-sectional view of the developing apparatus 3a mounted on the image forming apparatus 100 of this embodiment. In the following description, the developing apparatus 3a located in the image forming section Pa of Figure 1 is used as an example, but the configuration of the developing apparatuses 3b to 3d located in the image forming sections Pb to Pd is basically the same, so their description is omitted.

[0024] As shown in Figure 2, the developing apparatus 3a includes a developing container 20 in which a two-component developing agent (hereinafter simply referred to as "developing agent") containing a magnetic carrier and toner is stored. The developing container 20 is divided into an agitation and conveying chamber 21 and a supply and conveying chamber 22 by a partition wall 20a. Rotatable agitation and conveying screws 25a and 25b are installed in the agitation and conveying chamber 21 and the supply and conveying chamber 22, respectively, to mix and agitate the toner supplied from the toner container 4a (see Figure 1) with the magnetic carrier and charge it.

[0025] The developer is then agitated and transported axially (perpendicular to the plane of the paper in Figure 2) by the agitation and transport screw 25a and the supply and transport screw 25b, and circulates between the agitation and transport chamber 21 and the supply and transport chamber 22 through developer passages (not shown) formed at both ends of the partition wall 20a. In other words, a developer circulation path is formed within the developing container 20 by the agitation and transport chamber 21, the supply and transport chamber 22, and the developer passages.

[0026] The developing container 20 extends diagonally upward to the right in Figure 2, and within the developing container 20, the developing roller 30 is positioned diagonally upward to the right of the supply transport screw 25b. A portion of the outer surface of the developing roller 30 is exposed through the opening 20b of the developing container 20 and faces the photosensitive drum 1a. The developing roller 30 rotates counterclockwise in Figure 2.

[0027] The developing roller 30 consists of a cylindrical developing sleeve that rotates counterclockwise in Figure 2, and a magnet (not shown) with multiple magnetic poles fixed inside the developing sleeve. Here, a developing sleeve with a knurled surface is used, but developing sleeves with numerous dimples on the surface, blast-finished surfaces, or even those with knurling, dimple formation, blast finishing, or plating can also be used.

[0028] Furthermore, a regulating blade 27 is attached to the developing container 20 along the longitudinal direction of the developing roller 30 (the direction perpendicular to the plane of the paper in Figure 2). A small gap is formed between the tip of the regulating blade 27 and the surface of the developing roller 30.

[0029] The developing roller 30 is subjected to a developing voltage consisting of a direct current voltage Vslv (DC) and an alternating current voltage Vslv (AC) by a high-voltage generation circuit 43 (see Figure 3).

[0030] A toner concentration sensor 31 is positioned at the bottom of the stirring and conveying chamber 21, facing the stirring and conveying screw 25a. The toner concentration sensor 31 detects the ratio of toner to carriers in the developer (T / C), and for example, a permeability sensor that detects the permeability of the developer in the developing container 20 is used. When the permeability of the developer is detected by the toner concentration sensor 31, a voltage value corresponding to the detection result is output to the main control unit 80 (see Figure 3), which will be described later, and the toner concentration is determined by the main control unit 80 from the output value of the toner concentration sensor 31.

[0031] The sensor output value changes according to the toner concentration. As the toner concentration increases, the ratio of toner to carriers increases, and the proportion of toner that does not conduct magnetism increases, resulting in a lower output value. Conversely, as the toner concentration decreases, the ratio of toner to carriers decreases, and the proportion of carriers that conduct magnetism increases, resulting in a higher output value. The main control unit 80 transmits a control signal to the toner supply motor (not shown) according to the determined toner concentration, and a predetermined amount of toner is supplied from the toner container 4a (see Figure 1) to the agitation and transport chamber 21 via the toner supply port 20c.

[0032] [3. Control paths for the image forming unit] Figure 3 is a magnified view of the area around the image forming unit Pa, including the control path. The following explanation describes the configuration and control path of the image forming unit Pa, but the same applies to the configuration and control paths of the image forming units Pb to Pd, so the explanation is omitted.

[0033] The developing roller 30 is connected to a high-voltage generation circuit 43 that generates an oscillating voltage in which a DC voltage and an AC voltage are superimposed. The high-voltage generation circuit 43 comprises an AC constant voltage power supply 43a and a DC constant voltage power supply 43b. The AC constant voltage power supply 43a outputs a sinusoidal AC voltage generated from a low-voltage DC voltage that has been pulsed-modulated using a step-up transformer (not shown). The DC constant voltage power supply 43b outputs a DC voltage obtained by rectifying the sinusoidal AC voltage generated from a low-voltage DC voltage that has been pulsed-modulated using a step-up transformer.

[0034] During image formation, the high-voltage generation circuit 43 outputs a developing voltage obtained by superimposing an AC voltage on a DC voltage from the AC constant voltage power supply 43a and the DC constant voltage power supply 43b.

[0035] Next, the control system of the image forming apparatus 100 will be described with reference to Figure 3. The image forming apparatus 100 is equipped with a main control unit 80, which consists of a CPU and the like. The main control unit 80 is connected to a storage unit 70, which consists of a ROM, RAM, and the like. The main control unit 80 controls each part of the image forming apparatus 100 (charging devices 2a to 2d, exposure device 5, developing devices 3a to 3d, primary transfer rollers 6a to 6d, cleaning devices 7a to 7d, fixing unit 13, high-voltage generation circuit 43, voltage control unit 45, etc.) based on control programs and control data stored in the storage unit 70.

[0036] The voltage control unit 45 controls the high-voltage generation circuit 43. The voltage control unit 45 may also consist of a control program stored in the memory unit 70. The external temperature sensor 50 detects the temperature outside the image forming apparatus 100 and is installed, for example, near the intake duct (not shown) on the side of the paper cassette 16 in Figure 1, where it is less affected by heat-generating parts.

[0037] The main control unit 80 is connected to a liquid crystal display unit 90 and a transceiver unit 91. The liquid crystal display unit 90 functions as a touch panel for the user to make various settings of the image forming apparatus 100, and also displays the status of the image forming apparatus 100, the image forming status, the number of printed sheets, etc. The transceiver unit 91 communicates with the outside world using a telephone line or an internet line.

[0038] The toner charge detection unit 81 individually detects the charge per unit mass (μC / g, hereinafter referred to as toner charge) of the toner in the developing devices 3a to 3d that constitute the image forming units Pa to Pd. The method for detecting the toner charge will be described later.

[0039] [4. Determining the target value of toner concentration based on toner charge amount] Next, we will explain the determination of the target value of toner concentration based on the amount of toner charge, which is a characteristic feature of the present invention. As mentioned above, when the charging characteristics of toner in a two-component developer change, the toner cannot be sufficiently charged, resulting in problems such as a decrease in image density, image fringing, and toner scattering.

[0040] Therefore, in this invention, the amount of toner charge is estimated, and the target value of the toner concentration (T / C) in the two-component developer is changed based on the estimated amount of toner charge (first target value change control). This makes it possible to maintain the toner concentration within an appropriate range over a long period of time and suppress the decrease in image density.

[0041] (4-1. Estimation of toner charge amount) First, the method for detecting the toner charge amount by the toner charge amount detection unit 81 will be explained. A reference image (patch image) is developed on the photoreceptor drums 1a to 1d, and the development current flowing between the photoreceptor drums 1a to 1d and the development roller 30 during the development of the reference image is measured. The developed reference image is then transferred to the intermediate transfer belt 8, and the reflectance density of the reference image on the intermediate transfer belt 8 is measured by the image density sensor 40. Since there is a correlation between the development current and the reflectance density (toner transport amount) of the reference image, the toner charge amount detection unit 81 can estimate the toner charge amount from the development current and the reflectance density of the reference image.

[0042] The method for estimating the amount of toner charge is not limited to the method using the development current and the reflectance density of the reference image; other methods can also be used. For example, a reference image is developed on the photoreceptor drums 1a to 1d, and the surface potential of the photoreceptor drums 1a to 1d is measured before and after development. The reflectance density of the developed reference image is also measured by the image density sensor 40. Since there is a correlation between the potential difference of the surface potential before and after development and the reflectance density of the reference image, the toner charge detection unit 81 can estimate the amount of toner charge from the potential difference of the surface potential and the reflectance density.

[0043] Alternatively, the frequency of the AC voltage Vslv(AC) applied to the developing roller 30 is changed (for example, from 3kHz to 10kHz) to develop a reference image, and the reflectance density of the developed reference image is measured by the image density sensor 40. Then, the amount of toner charge can be predicted from the reflectance density of the toner image. The toner charge detection unit 81 estimates that the amount of toner charge is high if the reflectance density decreases (the amount of development decreases) as the frequency increases.

[0044] (4-2. Relationship between toner concentration and toner charge) Figure 4 is a graph showing the relationship between toner density (T / C) and toner charge as the cumulative number of printed pages changes. As shown in Figure 4, the toner charge decreases as the cumulative number of printed pages increases from 0 pages (○ data series), 50k pages (△ data series), 100k pages (□ data series), and 150k pages (× data series). In other words, as the print durability increases, the developer (carrier) deteriorates, and the toner charge tends to decrease.

[0045] Furthermore, when the cumulative number of printed pages is the same, there is a negative correlation between toner concentration and toner charge, with toner charge tending to decrease as toner concentration increases. Therefore, to use a toner charge of around 30 [μC / g], it is necessary to lower the toner concentration as the print durability progresses.

[0046] Figures 5 and 6 are graphs showing the relationship between the cumulative number of printed pages and the amount of toner charge, and the relationship between the cumulative number of printed pages and image density, respectively, when the target value of toner density is changed. By changing (lowering) the target value of toner density based on the estimated amount of toner charge, the amount of toner charge can be kept within the range of 20 to 40 [μC / g] over the entire range of 500k pages of durable printing, as shown in Figure 5. Furthermore, as shown in Figure 6, it can be seen that the image density (ID) remains stable within the range of 1.3 to 1.6 over the entire range of 500k pages of durable printing.

[0047] [5. Determining the target toner concentration based on cumulative toner consumption] Furthermore, the amount of toner charge may fluctuate initially and in the short term. In response to such fluctuations in the amount of toner charge, it is difficult to maintain image performance by changing the target value of the toner concentration through feedback of the estimated charge amount by the toner charge detection unit 81.

[0048] More specifically, in the initial stages of developer use, the toner additive is easily detached, and the released additive is gradually discharged through the photoreceptor drums 1a to 1d. When the additive detaches, it adheres to the surface of the carrier, reducing the fluidity of the developer and decreasing its bulk density. As a result, the toner density sensor 31 misdetects and outputs a higher toner density than the actual density. Consequently, a problem occurs where the toner density decreases and the image density becomes lighter. This phenomenon persists from the initial use of the developer until approximately 500 images with a print density of 5% are printed.

[0049] As the toner density decreases in accordance with the cumulative toner consumption, the toner density target value is increased when the cumulative print rate from the start of developer use reaches a predetermined value, in conjunction with the first target value change control described above. After a predetermined period, a control (second target value change control) is executed to return the target value to its original value.

[0050] The cumulative toner consumption can be calculated based on the cumulative number of pixels (dot count) when the electrostatic latent image is formed by the exposure device 5, the cumulative print density of the image developed by the developing devices 3a to 3d, or the amount of toner supplied from the toner containers 4a to 4d to the developing devices 3a to 3d. Therefore, a second target value change control is performed based on the dot count, cumulative print density, or toner supply amount.

[0051] Next, we will explain the rate of change in toner density when the first target value change control and the second target value change control are executed. The following explanation will focus on the case where the second target value change control is performed based on the dot count. In Figures 7, 8, and Table 1, the dot count is shown converted to toner mass [mg]. If X is the dot count and Y is the change in toner density, then the rates of change in toner density Z1 and Z2 during the first and second target value change control are expressed as Z1 = Y1 / X1 and Z2 = Y2 / X2, respectively.

[0052] Figure 7 is a graph showing the change in toner density (T / C) relative to the dot count in the second target value change control. In the example shown in Figure 7, as the second target value change control, after printing 500 images with a print density of 5% (112,500 mg), the target value of the toner density is increased by 1%, stabilized for a predetermined period (up to 160,000 mg), and then returned to its original value (decreased by 1%).

[0053] As an example of the first target value change control, if the timing for estimating the toner charge amount is set to every 4,000 prints at a print density of 5%, and the change in the target toner concentration per control is set to 0.33%, then the rate of change in toner concentration Z1 (hereinafter referred to as the first rate of change) during the execution of the first target value change control is plotted by the dashed line L in Figure 7. In the second target value change control, if the rate of change in toner concentration Z2 (hereinafter referred to as the second rate of change) when returning the target toner concentration from a state where it has been increased by 1% is the same rate as the first rate of change Z1 (a decrease of 1% for approximately 300,000 mg), then this is shown by the solid line in Figure 7.

[0054] Furthermore, the case where the second rate of change Z2 is faster than the first rate of change Z1 (a 1% decrease for approximately 100,000 mg) is shown by the dotted line in Figure 7. The case where the second rate of change Z2 is slower than the first rate of change Z1 (a 1% decrease for approximately 600,000 mg) is shown by the dashed line in Figure 7.

[0055] Next, we will explain how to set the second change rate Z2. Figure 8 is a graph showing the change in toner density with respect to the dot count when the first target value change control and the second target value change control are executed in conjunction.

[0056] When the second rate of change Z2 is faster than the first rate of change Z1 (dotted line in Figure 8), the toner density drops sharply when the second target value change control is executed, causing the toner charge to rise sharply. In this case, the toner density target value is increased at the timing of the toner charge estimation in the first target value change control, and thereafter the control is performed solely by estimating the toner charge. As a result, the toner density remains high, making toner scattering more likely.

[0057] On the other hand, when the second rate of change Z2 is slower than the first rate of change Z1 (the dashed line in Figure 8), the toner charge remains low. In this case, the toner concentration target value is actively lowered at the timing of the toner charge estimation in the first target value change control. As a result, the toner concentration remains low, causing a decrease in image density.

[0058] In contrast, when the second rate of change Z2 and the first rate of change Z1 are the same (solid line in Figure 8), there is no abrupt change in the toner charge amount, and it falls within the range of the first target value change control controlled by the estimation of the toner charge amount. Therefore, no correction is made to the target value of the toner density. Table 1 shows the toner charge amount and the control based on the toner charge amount estimation results when the second rate of change Z2 is faster than the first rate of change Z1, when it is the same as the first rate of change Z1, and when it is slower than the first rate of change Z1.

[0059] [Table 1]

[0060] Table 1 assumes that when the dot count from the start of developer use reaches 112,500 mg, the second target value change control is executed to increase the toner concentration target value by 1%, and when the dot count reaches 160,000 mg, the toner charge amount estimation is performed by the first target value change control.

[0061] The results shown in Figure 8 and Table 1 indicate that when the first target value change control and the second target value change control are executed in conjunction, the second change rate Z2 must be the same as the first change rate Z1.

[0062] Figure 9 is a flowchart showing the execution procedures for the first target value change control and the second target value change control in the image forming apparatus 100 of this embodiment. The execution procedures for the first target value change control and the second target value change control will be explained according to the steps in Figure 9, with reference to Figures 1 to 8 as needed.

[0063] After the image forming apparatus 100 is put into use, the main control unit 80 performs dot counting during exposure by the exposure apparatus 5 (step S1). Next, the main control unit 80 determines whether or not it is time to execute the second target value change control (step S2). Specifically, for each of the developing apparatuses 3a to 3d, it is determined whether or not the dot count stored in step S1 has reached the value obtained by printing 500 images with a print density of 5%. If it is not time to execute the second target value change control for any of the developing apparatuses 3a to 3d (No in step S2), the dot counting continues.

[0064] If at least one of the developing devices 3a to 3d is at the timing for execution of the second target value change control (Yes in step S2), the main control unit 80 increases the target value of the toner density in the developing devices 3a to 3d that is at the timing for execution of the second target value change control by a predetermined value (e.g., 1%) (step S3).

[0065] Then, the dot count is continued and it is determined whether or not a predetermined dot count has been reached (step S4). Specifically, it is determined whether or not the dot count has reached a predetermined value (for example, 160,000 dots). If the predetermined dot count has not been reached (No in step S4), the target value of the toner density that was increased in step S3 is maintained.

[0066] If a predetermined dot count is reached (Yes in step S4), the target value of the toner density that was increased in step S3 is returned to its original value (step S5). For example, if it was increased by 1% in step S3, it is decreased by 1%. At this time, as shown in Figure 8, the rate of change of the toner density when returning the target value of the toner density to its original value (second rate of change Z2) is set to the same rate as the first rate of change Z1 in the first target value change control.

[0067] Subsequently, the main control unit 80 determines whether or not it is time to execute the first target value change control (step S6). Specifically, for each of the developing devices 3a to 3d, it determines whether or not the dot count has reached the value obtained by printing 4000 images with a print density of 5%. If it is not time to execute the first target value change control for any of the developing devices 3a to 3d (No in step S6), the dot count continues.

[0068] If at least one of the developing devices 3a to 3d is at the timing for executing the first target value change control (Yes in step S6), the main control unit 80 estimates the toner charge amount Q / M in the developing devices 3a to 3d that are at the timing for executing the first target value change control (step S7). Specifically, a reference image (patch image) is formed on the photoreceptor drums 1a to 1d and primary transferred onto the intermediate transfer belt 8. The image density of this reference image is detected by the image density sensor 25, and the toner charge amount Q / M is estimated using the relationship between the detected image density (toner transport amount) and the developing current that flows when the reference image is formed.

[0069] Next, the main control unit 80 determines whether the estimated toner charge amount Q / M falls outside the threshold A (step S8). In this control example, "threshold A" is a numerical range with a predetermined width. If the toner charge amount Q / M falls outside the threshold A (Yes in step S8), the target value of the toner concentration is changed (step S9). Specifically, the target value of the toner concentration is reduced by setting the change in toner concentration per control cycle to 0.33%. On the other hand, if the toner charge amount Q / M is within the range of threshold A (No in step S8), the target value of the toner concentration is not changed.

[0070] After that, the process returns to step S6 and continues to control the change of the toner target value using the first target value change control (steps S6-S9).

[0071] As shown in Figure 9, by linking the first target value change control and the second target value change control, it is possible to set an appropriate target value for toner concentration that takes into account both initial and short-term fluctuations in toner charge and long-term fluctuations in toner charge. Therefore, it is possible to effectively suppress image density defects due to a decrease in toner concentration in the developing devices 3a to 3d, and toner scattering due to an increase in toner concentration.

[0072] Although Figure 9 illustrates the case where the second target value change control is performed based on the dot count, the same explanation can be applied to cases where the second target value change control is performed based on the cumulative printing rate or the amount of toner supplied to the developing devices 3a to 3d instead of the dot count.

[0073] Furthermore, the present invention is not limited to the embodiments described above, and various modifications are possible without departing from the spirit of the invention. For example, although the above embodiments described an image forming apparatus 100 equipped with a two-component developing apparatus 3a to 3d that includes a developing roller (developer carrier) 30 carrying a two-component developer, the present invention is not limited thereto. The present invention can be applied in exactly the same way to an image forming apparatus equipped with a developing apparatus that further includes a developer carrier such as a magnetic roller between the supply transport screw 25b and the developing roller 30, and after supplying the developer from the supply transport screw 25b to the magnetic roller, supplies only toner from the magnetic roller to the developing roller 30.

[0074] Furthermore, although the above embodiment described an image forming apparatus 100 using a color printer as an example as shown in Figure 1, the present invention is not limited to color printers and can be applied to image forming apparatuses equipped with a two-component developing apparatus, such as color copiers, color multifunction printers, monochrome printers, monochrome copiers, and monochrome multifunction printers. [Industrial applicability]

[0075] The present invention is applicable to image forming apparatuses using a two-component developing system that employs a two-component developer containing toner and a carrier. By utilizing the present invention, it is possible to provide an image forming apparatus that can properly maintain the toner concentration in the two-component developer over a long period of time. [Explanation of symbols]

[0076] Pa~Pd Image Forming Unit 1a~1d Photoreceptor drum (image carrier) 2a~2d Charging device 3a~3d developing device 5. Exposure apparatus 6a~6d Primary transfer roller 8. Intermediate transfer belt 9. Secondary transfer roller 30. Developing roller (developer carrier) 31 Toner density sensor 40 Image density sensor 43 High-voltage generation circuit 45 Voltage Control Unit 80 Main control unit 81 Toner charge amount detection unit 100 Image forming apparatus

Claims

1. An image carrier having a photosensitive layer formed on its surface, A charging device for charging the surface of the image carrier to a predetermined surface potential, An exposure apparatus that irradiates light onto the image carrier charged by the charging device to form an electrostatic latent image with reduced charge, A developing apparatus comprising a developer carrier that carries a two-component developer containing a magnetic carrier and toner, and developing the electrostatic latent image formed on the surface of the image carrier into a toner image, A toner container for storing the toner to be supplied to the developing device, A toner concentration sensor that detects the toner concentration, which is the ratio of the toner to the magnetic carrier in the developing device, A toner charge amount detection unit that detects the toner charge amount, which is the amount of charge per unit mass of the toner in the developing device, A control unit that controls the supply of toner from the toner container to the developing device based on the toner concentration detection result by the toner concentration sensor, Equipped with, The image forming apparatus is characterized in that the control unit performs a first target value change control to change the target value of the toner concentration based on the toner charge amount detected by the toner charge amount detection unit.

2. The image forming apparatus according to claim 1, characterized in that the control unit, in conjunction with the first target value change control, increases the target value when the cumulative amount of toner consumed from the initial use of the two-component developer reaches a predetermined value, and returns the target value to its original value after a predetermined period of time has elapsed.

3. The image forming apparatus according to claim 2, characterized in that when the rate of change of the toner concentration during the execution of the first target value change control is called the first rate of change, and when the rate of change of the toner concentration when the target value is returned to its original state during the execution of the second target value change control is called the second rate of change, the second rate of change is the same as the first rate of change.

4. The image forming apparatus according to claim 2, characterized in that the cumulative consumption is calculated based on the dot count when forming an electrostatic latent image by the exposure apparatus, the cumulative print rate of the image developed by the developing apparatus, or the amount of toner supplied from the toner container to the developing apparatus.

5. The image forming apparatus according to any one of claims 1 to 4, characterized in that the toner charge amount detection unit estimates the toner charge amount based on the development current flowing between the image carrier and the developer carrier and the reflectance density of the reference image when a reference image is developed on the image carrier.

Images