Image forming apparatus

Abstract

The present invention provides an image forming apparatus that can achieve stable image density in a simple and low-cost manner using a two-component development method. [Solution] The image forming apparatus comprises an image forming unit, a developing voltage power supply, a current detection mechanism, and a control unit. The image forming unit includes an image carrier having a photosensitive layer formed on its surface, and a developer carrier positioned opposite the image carrier and carrying a two-component developer containing a magnetic carrier and toner, and a developing device that forms a toner image by attaching toner to an electrostatic latent image formed on the image carrier. The developing voltage power supply applies a developing voltage including at least a DC voltage to the developer carrier. The current detection mechanism detects the developing current flowing between the developer carrier and the image carrier when a developing voltage is applied to the developer carrier. The control unit adjusts the image density of the toner image based on the developing current when the developing conditions are changed to multiple levels.

Description

Technical Field

[0001] The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction machine thereof, and particularly to an image forming apparatus using a two-component development method that uses a two-component developer containing toner and carrier.

Background Art

[0002] In an image forming apparatus using an electrophotographic process, after the photosensitive layer on the surface of a photosensitive drum (image carrier) is charged to a predetermined surface potential (the same polarity as the charging polarity of the toner) by a charging device, an electrostatic latent image is formed on the photosensitive drum by an exposure device. Then, the formed electrostatic latent image is visualized with toner in a developing device, and the toner image is transferred onto a recording medium that passes through a nip portion (transfer nip portion) between the photosensitive drum and a transfer member that contacts the photosensitive drum, and then a fixing process is generally performed.

[0003] Conventionally, it has been proposed to detect changes in the developability of toner and the like and feed back and control them to image forming conditions. For example, Patent Document 1 discloses a method of obtaining the adhesion amount and charge amount of toner on a developing roller using non-developing current and developing current in a one-component development method and stabilizing image density based on this.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in two-component development systems that use a two-component developer containing both carrier and toner, the amount of toner on the developing roller is large, and the toner consumption efficiency is low. This makes it difficult to estimate the amount of toner deposited and the amount of charge based solely on the non-developing current and developing current. Consequently, it is difficult to stabilize the image density using only the developing current, which has been a major challenge, especially in situations where high-quality printing is required.

[0006] In view of the above problems, the present invention aims to provide an image forming apparatus that can achieve stable image density in a simple and low-cost manner using a two-component development method. [Means for solving the problem]

[0007] To achieve the above objective, the first configuration of the present invention is an image forming apparatus comprising an image forming unit, a developing voltage power supply, a current detection mechanism, and a control unit. The image forming unit includes an image carrier having a photosensitive layer formed on its surface, and a developer carrier positioned opposite the image carrier and carrying a two-component developer containing a magnetic carrier and toner, and a developing apparatus that forms a toner image by attaching toner to an electrostatic latent image formed on the image carrier. The developing voltage power supply applies a developing voltage including at least a DC voltage to the developer carrier. The current detection mechanism detects the developing current flowing between the developer carrier and the image carrier when a developing voltage is applied to the developer carrier. The control unit controls the image forming unit and the developing voltage power supply. The control unit detects the developing current when the developing conditions are changed to multiple levels using the current detection mechanism and adjusts the image density of the toner image based on the detection result. [Effects of the Invention]

[0008] According to the first configuration of the present invention, by correcting the image density using the development current when the development conditions are changed to multiple levels, the image density can be properly maintained over a long period of time. Furthermore, since it is not necessary to measure the image density using an image density sensor, the image density can be adjusted accurately even in an image forming apparatus that is not equipped with an image density sensor. [Brief explanation of the drawing]

[0009] [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] Enlarged view of the area around the image forming section P, including the control path of the image forming apparatus 100. [Figure 3] A schematic diagram showing the current detection mechanism 50 connected to the photoreceptor drum 5 and the developing roller 30. [Figure 4] Graph showing the relationship between image current and development potential difference. [Figure 5] Graph showing the relationship between white area current and development potential difference. [Figure 6] A flowchart showing an example of image density adjustment control based on development current in the image forming apparatus 100 of this embodiment. [Figure 7] This figure shows a method for measuring the resistance value of carriers using a bridge-type electrical resistance meter 110 and an ultra-insulation meter 120. [Modes for carrying out the invention]

[0010] [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 side cross-sectional view showing the internal structure of an image forming apparatus 100 according to one embodiment of the present invention. Inside the image forming apparatus (here, a monochrome printer) 100, there is an image forming unit P that forms a monochrome image through the processes of charging, exposure, development, and transfer. In the image forming unit P, a charging device 4, an exposure device (laser scanning unit, etc.) 7, a development device 8, a transfer roller 14, a cleaning device 19, and a static elimination device 6 are arranged along the rotation direction of the photosensitive drum 5 (counterclockwise direction in Figure 1).

[0011] During the image formation process, the photoreceptor drum 5, which rotates counterclockwise as shown in Figure 1, is uniformly charged by the charging device 4. Next, an electrostatic latent image is formed on the photoreceptor drum 5 by a laser beam from the exposure device 7 based on the original image data. Then, toner is deposited onto the electrostatic latent image by the developing device 8 to form a toner image. The image data is transmitted from a personal computer (not shown), etc.

[0012] Toner is supplied to the developing unit 8 from the toner container 9. The toner concentration (T / C, the mass ratio of toner to magnetic carriers) in the developing unit 8 is detected by a toner concentration sensor 81 (see Figure 1). In addition, a static elimination device 6, which removes residual charge by irradiating the surface of the photoreceptor drum 5 with static elimination light, is provided downstream of the cleaning device 19 in the rotational direction of the photoreceptor drum 5.

[0013] As described above, paper (recording medium) is transported from the paper feed cassette 10 or manual feed device 11 via the paper transport path 12 and registration roller pair 13 toward the photoreceptor drum 5 on which the toner image has been formed. As the paper passes through the transfer roller 14 and the nip portion (transfer nip portion) of the photoreceptor drum 5, the toner image formed on the surface of the photoreceptor drum 5 is transferred to the paper. The paper on which the toner image has been transferred is separated from the photoreceptor drum 5 and transported to the fuser unit 15 where the toner image is fixed. The paper that has passed through the fuser unit 15 is transported to the top of the device via the paper transport path 16, and if an image is to be formed on only one side of the paper (single-sided printing), it is discharged to the discharge tray 18 by the discharge roller pair 17.

[0014] On the other hand, when forming an image on both sides of the paper (during double-sided printing), after the rear end of the paper passes through the branch portion 20 of the paper conveyance path 16, the conveyance direction is reversed. As a result, the paper is sorted to the reverse conveyance path 21 that branches from the branch portion 20, and is re-conveyed to the resist roller pair 13 with the image surface reversed. Then, the next toner image formed on the photosensitive drum 5 is transferred by the transfer roller 14 to the surface of the paper where the image has not been formed. The paper onto which the toner image has been transferred is conveyed to the fixing device 15 where the toner image is fixed, and then discharged to the discharge tray 18 by the discharge roller pair 17.

[0015] FIG. 2 is a partially enlarged view around the image forming unit P including the control path of the image forming apparatus 100. The charging device 4 includes a charging roller 41 disposed so as to contact the photosensitive drum 5 and charging the photosensitive drum 5.

[0016] The charging roller 41 is formed by coating a core metal 41a with a conductive layer 41b, and is disposed so as to contact the photosensitive drum 5. The conductive layer 41b has ionic conductivity by blending an ionic conductive agent in a crosslinked rubber. As the crosslinked rubber, epichlorohydrin rubber or the like is used. As the ionic conductive agent, a quaternary ammonium salt, a borate, or the like is used.

[0017] The photosensitive drum 5 is formed, for example, with an organic photosensitive layer (OPC) 5b, which is a positively chargeable photoconductor as a photosensitive layer, on the surface of an aluminum drum base tube (conductive substrate) 5a. The photosensitive drum 5 is rotationally driven at a constant speed around a support shaft by a drum drive unit (not shown).

[0018] As shown in FIG. 2, when the photosensitive drum 5 rotates in the counterclockwise direction, the charging roller 41 that contacts the surface of the photosensitive drum 5 rotates in a driven manner in the clockwise direction. At this time, by applying a predetermined charging voltage to the charging roller 41, the surface of the photosensitive drum 5 is uniformly charged.

[0019] The charging roller 41 is connected to a charging voltage power supply 43 that generates a charging voltage in which an AC voltage is superimposed on a DC voltage. The charging voltage power supply 43 includes an AC constant voltage power supply and a DC constant voltage power supply (neither of which are shown). The AC constant voltage power supply 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 outputs a DC voltage obtained by rectifying a sinusoidal AC voltage generated from a low-voltage DC voltage that has been pulsed-modulated using a step-up transformer.

[0020] The developing device 8 is positioned opposite the photoreceptor drum 5 and includes a developing roller 30 that carries a two-component developer (hereinafter also simply referred to as the developer) containing a magnetic carrier and toner. The developing roller 30 is connected to a developing voltage power supply 44 that generates a developing voltage in which an AC voltage is superimposed on a DC voltage. The configuration of the developing voltage power supply 44 is the same as that of the charging voltage power supply 43. By applying a predetermined developing voltage to the developing roller 30, the toner in the developer carried on the developing roller 30 flies onto the surface of the photoreceptor drum 5, and the electrostatic latent image is developed into a toner image. The current detection mechanism 50 detects the developing current flowing between the developing roller 30 and the photoreceptor drum 5.

[0021] Next, the control system of the image forming apparatus 100 will be described with reference to Figure 2. 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 device 4, static elimination device 6, exposure device 7, developing device 8, transfer roller 14, cleaning device 19, fixing device 15, charging voltage power supply 43, developing voltage power supply 44, voltage control unit 45, current detection mechanism 50, etc.) based on control programs and control data stored in the storage unit 70.

[0022] The voltage control unit 45 controls the charging voltage power supply 43 that applies a vibration voltage to the charging roller 41, and the developing voltage power supply 44 that applies a developing voltage to the developing roller 30. The voltage control unit 45 may also consist of a control program stored in the memory unit 70.

[0023] 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.

[0024] The internal temperature and humidity sensor 92 detects the temperature and humidity inside the image forming apparatus 100, particularly around the developing apparatus 8. The detection results are transmitted to the main control unit 80.

[0025] [2. Configuration of the current detection mechanism] Figure 3 is a schematic diagram showing a current detection mechanism 50 connected to the photoreceptor drum 5 and the developing roller 30. The current detection mechanism 50 has a first connection part A, a second connection part B, and a current detection part C.

[0026] The first connection A and the second connection B are each connected in series to the developing voltage power supply 44. The first connection A and the second connection B are each connected to the ground point G. When the developing voltage power supply 44 applies a developing voltage to the developing roller 30, current flows through each of the first connection A and the second connection B.

[0027] The current detection unit C comprises a first current detection unit 50d and a second current detection unit 50e. The first current detection unit 50d detects the current flowing through the first connection A. The second current detection unit 50e detects the current flowing through the second connection B. The current detection unit C detects the value obtained by subtracting the value detected by the first current detection unit 50d from the value detected by the second current detection unit 50e as the development current flowing through the image area (hereinafter referred to as the image area current). The current detection unit C also detects the value detected by the first current detection unit 50d (or the value detected by the second current detection unit 50e) as the development current flowing through the non-image area (hereinafter referred to as the white area current). The image area current and white area current will be described later.

[0028] The first connection section A includes a first resistor 50g, a second resistor 50h, a capacitor 50i, and a third resistor 50j. The second resistor 50h, the capacitor 50i, the first current detection unit 50d, and the third resistor 50j are connected in series with respect to the developing voltage power supply 44 in the order of second resistor 50h, capacitor 50i, first current detection unit 50d, and third resistor 50j. The third resistor 50j is connected to the ground point G.

[0029] The first resistor 50g is connected in parallel with the capacitor 50i. The first resistor 50g is connected in series with the second resistor 50h, the first current detection unit 50d, and the third resistor 50j. The first resistor 50g is positioned between the second resistor 50h and the first current detection unit 50d.

[0030] The second connection section B has a developing area 50k and a fourth resistor 50m. The developing area 50k is the area located between the developing roller 30 and the photosensitive drum 5.

[0031] The developing region 50k, the second current detection unit 50e, and the fourth resistor 50m are connected in series with respect to the developing voltage power supply 44 in the order of developing region 50k, second current detection unit 50e, and fourth resistor 50m. The fourth resistor 50m is connected to the ground point G.

[0032] The developing voltage power supply 44 applies a developing voltage of the same polarity as the toner (positive in this case) to the developing roller 30, creating a potential difference between the developing roller 30 and the photoreceptor drum 5 (hereinafter also referred to as the DS-to-DS relationship). The charged toner moves between the developing roller 30 and the photoreceptor drum 5 via the developing area 50k, and the toner carried on the developing roller 30 is supplied to the photoreceptor drum 5. As a result, the electrostatic latent image formed on the surface of the photoreceptor drum 5 is developed into a toner image. In addition, the movement of the charged toner between the developing roller 30 and the photoreceptor drum 5 causes a current to flow in the developing area 50k.

[0033] As toner moves between the developing roller 30 and the photoreceptor drum 5, the developing area 50k is filled with developer, so the toner moves through the developer. Therefore, in addition to the capacitor component, there is also a resistive component due to the developer between the developing roller 30 and the photoreceptor drum 5. The capacitor component is the capacitance between the developing roller 30 and the photoreceptor drum 5.

[0034] The image current refers to the current generated by the movement of toner between the developing roller 30 and the photoreceptor drum 5 (hereinafter referred to as the DS-to-DS space). The image current is the current generated solely by the movement of toner between the DS-to-DS space, and is the current value when the current flowing between the DS-to-DS space is not affected by capacitive and resistive components. The image current is correlated with the amount of toner moving between the DS-to-DS space (moving toner), and has a value corresponding to the amount of moving toner. Specifically, the image current increases as the amount of moving toner increases.

[0035] The white area current refers to the current that flows between the Ds when the toner does not move between them. More specifically, when the non-image area (white area) of the photoreceptor drum 5 faces the developing roller 30, the developing area 50k is filled with carriers (magnetic brushes), forming a circuit in which the first connection part A and the second connection part B are connected in a ring. In this state, the current that flows as the toner moves through the magnetic brushes toward the developing roller 30 is the white area current (developing current of the non-image area), and the detected value of the first current detection unit 50d and the detected value of the second current detection unit 50e are equal.

[0036] In the current detection mechanism 50 shown in Figure 3, a capacitor 50i is arranged in parallel in the development area 50k between the developing roller 30 and the photosensitive drum 5, and a first resistor 50g and a second resistor 50h are connected in parallel and series with the capacitor 50i, thereby enabling accurate detection of the image area current and the white area current.

[0037] [3. Image density adjustment based on development current] The following describes a feature of the present invention: a method for estimating changes in toner developability based on the developing current and adjusting image density based on the estimation result. The developing current measured using the current detection mechanism 50 may be the image current that flows when a solid image or halftone image is developed, or it may be the white background current that flows when the image is not developed.

[0038] Since the developing current is the amount of charge transferred per unit time, if the developing current is I, it can be expressed as I = Q / t. When the developing voltage (DC voltage) is Vdc, the developing current I and the developing voltage Vdc are proportional, so the following relationship (1) holds. I = Q / t = c * Vdc + d ... (1)

[0039] Furthermore, since the developing current = toner charge amount × toner deposition amount (toner movement amount), if the toner charge amount is Q / M, then the toner deposition amount is expressed as (Q / t) / (Q / M) = M / t. Since the toner deposition amount M / t is proportional to the developing voltage Vdc, the following relationship, equation (2), which shows the developing characteristics, holds true. M / t = a * Vdc + b ... (2)

[0040] Here, when the developability of the toner changes, the slope c in equation (1) changes. By knowing in advance the relationship between the slope c and the actual development characteristics, that is, the relationship between equations (1) and (2), it becomes possible to adjust the image density. Specifically, the current detection mechanism 50 is used to detect the development current flowing between the development roller 30 and the photosensitive drum 5 when the development conditions are changed to two or more levels. Then, the slope c in equation (1) is calculated from the detected development current.

[0041] Then, the toner deposition amount M / t is adjusted based on the slope c of the calculated equation (1) and the relationship between equation (1) and equation (2) that is pre-stored in the memory unit 70. Since the toner deposition amount M / t is proportional to the development voltage Vdc, the toner deposition amount M / t can be adjusted to the target value by adjusting the development voltage Vdc based on the slope c.

[0042] For example, if the slope c is smaller than the reference value c1, it is presumed that the developability is reduced, so the toner adhesion amount M / t is adjusted to increase it. On the other hand, if the slope c is larger than the reference value c1, it is presumed that the developability is improved, so the toner adhesion amount M / t is adjusted to decrease it.

[0043] Methods for adjusting the toner deposition amount M / t include adjusting the development voltage (DC voltage, AC voltage), adjusting the light intensity of the exposure device 7, and adjusting the charging voltage applied to the charging roller 41.

[0044] Alternatively, instead of the relationship between toner deposition amount M / t and development voltage Vdc shown in equation (2), the relationship between image density ID (image density) and development voltage Vdc shown in equation (3) below may be used. ID=a'*Vdc+b' ···(3)

[0045] Furthermore, in order to correct the relationship between the tilt and the actual development characteristics, the image density may be adjusted by taking into account durability-related parameters such as the toner density sensor, environmental sensor, cumulative print rate, and cumulative operating time of the developing device 8.

[0046] Examples of development conditions that can be varied to two or more levels include the DC voltage Vdc, the Vpp (peak-to-peak value) of the AC voltage Vac applied to the development roller 30, the frequency, and the duty cycle. However, it is preferable to vary the DC voltage Vdc. This is because the DC voltage Vdc is a common development condition that is changed when adjusting image density, and it has a strong correlation with development characteristics, making it easier to adjust image density. The following will explain in more detail the cases where the image area current and the white area current are used as the development current.

[0047] (Image density adjustment based on image current) Figure 4 schematically shows the relationship between the development potential difference and the image current. The image current is the development current that flows between the exposure area (image area) of the photoreceptor drum 5 and the development roller 30. In Figure 4, the horizontal axis represents the potential difference between the exposure area potential VL and the development voltage (DC voltage) Vdc (development potential difference VL-Vdc), and the vertical axis represents the image current Ig, showing that the image current Ig is proportional to the development potential difference VL-Vdc in the exposure area. That is, the following relationship (4) holds. Ig=Q / t=c'*(VL-Vdc)+d' ···(4)

[0048] When the developability of the toner changes, the slope c' in equation (4) changes. By knowing in advance the relationship between the slope c' and the actual development characteristics, that is, the relationship between equation (4) and the aforementioned equations (2) or (3), it becomes possible to adjust the image density.

[0049] (Image density adjustment based on white background current) Figure 5 schematically shows the relationship between the development potential difference and the white area current. The white area current is the development current that flows between the white area (background) of the photoreceptor drum 5 and the development roller 30. In Figure 5, the horizontal axis represents the potential difference between the white area potential V0 and the development voltage (DC voltage) Vdc (development potential difference V0-Vdc), and the vertical axis represents the white area current Iw, showing that the white area current Iw is proportional to the development potential difference V0-Vdc. That is, the following relationship (5) holds. Iw=c”*(V0-Vdc)+d” ···(5)

[0050] When the developability of the toner changes, the slope c'' in equation (5) changes. By understanding the relationship between the slope c'' and the actual development characteristics, that is, the relationship between equation (5) and the aforementioned equations (2) or (3), it becomes possible to adjust the image density.

[0051] By measuring the white area current Iw as the development current, toner consumption other than that used for printing can be reduced. Furthermore, there is no need to set up a special mode for measuring the development current, and the development current can be measured during the printing interval (between sheets of paper), etc.

[0052] Figure 6 is a flowchart showing an example of image density adjustment control based on the development current in the image forming apparatus 100 of this embodiment. The image density adjustment procedure will be described in detail according to the steps in Figure 6, referring to Figures 1 to 5 as needed. In Figure 6, the white area current is used as the development current.

[0053] First, the main control unit 80 determines whether or not the timing for adjusting the image density has been reached (step S1). The timing for adjusting the image density may be, for example, when the cumulative number of printed sheets since the last image density adjustment has reached a predetermined number, or when the power of the image forming apparatus 100 is turned on, or when it recovers from power-saving (sleep) mode. If the timing for adjusting the image density has been reached (Yes in step S1), the white area currents Iw1 and Iw2 are measured when the development voltage (DC voltage Vdc) is changed to two levels (step S2). The white area current is measured when the photoreceptor drum 5 is charged to a predetermined surface potential V0 and the development voltage (DC voltage Vdc) is applied. When printing, it is measured between the sheets of paper. The measured white area currents Iw1 and Iw2 are transmitted to the main control unit 80.

[0054] Next, the main control unit 80 calculates the slope c'' from the measured white area currents Iw1 and Iw2 using equation (5) (step S3). Then, it adjusts the toner adhesion amount M / t based on the calculated slope c'' and the relationship between the slope c'' previously stored in the memory unit 70 and the actual development characteristics (relationship between equations (2) and (5)). Specifically, in order to adjust the toner adhesion amount M / t, the main control unit 80 determines the development conditions such as the DC voltage Vdc and AC voltage Vac of the development voltage, the exposure amount of the exposure device, and the charging voltage (step S4).

[0055] Next, the main control unit 80 determines whether or not it is necessary to correct the toner adhesion amount M / t based on the detection results of the toner density sensor 81 and the in-machine temperature and humidity sensor 92, the cumulative printing rate, and the cumulative operating time of the developing device 8 (step S5). If it is necessary to correct the toner adhesion amount M / t (Yes in step S5), it corrects the development conditions determined in step S4 (step S6) and adjusts the toner adhesion amount with the corrected development conditions (step S7). On the other hand, if it is not necessary to correct the toner adhesion amount M / t (No in step S5), it adjusts the toner adhesion amount using the development conditions determined in step S4 (step S7).

[0056] As shown in Figure 6, by correcting the image density using the white area current, the image density can be properly maintained over a long period of time. Furthermore, since there is no need to measure the image density using an image density sensor, the image density can be adjusted accurately even in image forming apparatuses that are not equipped with an image density sensor.

[0057] Furthermore, since it is possible to measure the development current without consuming toner during printing intervals (between sheets of paper), the decrease in image formation efficiency can be suppressed, and the amount of toner consumed for purposes other than printing can be reduced.

[0058] Furthermore, by incorporating the detection results from the toner density sensor 81 and the in-machine temperature and humidity sensor 92, in addition to the measurement results of the white area current, the stability of the image density can be further enhanced.

[0059] In the control example shown in Figure 6, the image density was adjusted using the white area current as the development current. However, it is also possible to adjust the image density using the image area current as the development current. When using the image area current, it is necessary to separately set up a measurement timing for the image area current (image density adjustment mode) when not printing.

[0060] [4. Setting the carrier resistance] Since the development current is the current flowing between the DS (distributed studs), fluctuations in the resistance component between the DS cause instability in the relationship between the development conditions and the target development current. Therefore, when adjusting the image density based on the development current using the procedure described above, it is preferable to stabilize the resistance of the magnetic brushes formed between the DS. This eliminates the instability of density prediction caused by fluctuations in toner concentration (T / C) in the two-component developer and fluctuations in print density, thereby achieving stable image density control.

[0061] This study investigated the resistance value of carriers in a two-component developer necessary to stabilize the resistance of a magnetic brush. The test method involved conducting durability printing tests using a two-component developer containing carriers with adjusted resistance values, and evaluating resistance fluctuations and image density stability. The durability printing consisted of two sets of repeated printing: 4k test images at 2% coverage and 1k test images at 25% coverage, for a total of 10k images.

[0062] The carrier resistance was adjusted by the amount of conductive agent (carbon black) contained in the coating material covering the surface of the carrier core. In this embodiment, the resistance determined based on the measurement results by the bridge method is used as the carrier resistance.

[0063] Specifically, a bridge-type electrical resistance meter 110 and an ultra-insulation meter 120 (SM-8220, manufactured by HIOKI Corporation) as shown in Figure 7 are used. The bridge-type electrical resistance meter 110 has a pair of electrode plates 112 fixed to the upper surface of an acrylic resin substrate 111. The pair of electrode plates 112 are positioned opposite each other with a gap W of 1 mm between them. The surfaces of the pair of electrode plates 112 face each other. A 1000 gauss magnet 113 is placed on the back side of each of the pair of electrode plates 112. The area of ​​the opposing sides of each magnet 113 is 3 cm². 2 This creates a magnetic field between the pair of electrode plates 112.

[0064] A carrier sample Cs is set between a pair of electrode plates 112. The amount of carrier sample Cs set is 0.2g. The carrier sample Cs is packed between the pair of electrode plates 112 in a chain structure according to the magnetic field lines. In this state, a DC voltage of 1000V (electric field 1 × 10⁻¹⁰) is applied. 6 The electrical resistance value 10 seconds after applying a voltage (V / m) between the terminals is read using an insulation meter 120, and this read value (in Ω) is considered the carrier resistance.

[0065] Resistance fluctuations were measured by monitoring the development current in the white areas during durable printing under constant development potential difference (V0-Vdc) conditions, and the range of resistance fluctuations (maximum / minimum value) was obtained. Image density stability was measured by measuring the image density (ID) of test images every 100 images. A "○" was given if the image density during durable printing was within ±0.3 of the target density (ID=1.4), and a "×" was given if the density difference from the target value exceeded 0.3. The results for resistance fluctuations and image density stability, along with the resistance values ​​of the coating material and carrier, are shown in Table 1.

[0066] [Table 1]

[0067] As is clear from Table 1, the carrier resistance R is log 10 In configurations 1 and 2 of the present invention, where R=13.4 and 8.7, the resistance fluctuations during durable printing were small, at 1.8 and 2.6 respectively, and the image density was within ±0.3 of the target density.

[0068] In contrast, the carrier resistance R is log 10 In the comparative example configuration with R=7.9, the resistance fluctuation during durable printing was large at 5.2, and the image density difference from the target density exceeded 0.3. 10 We did not conduct tests using carriers with an R value of 14 or higher because development ghosting occurred in the images.

[0069] From the above results, it can be seen that in order to stabilize the resistance of the magnetic brush formed between the DS, it is preferable to use a two-component developer in which the resistance value R of the carrier satisfies the following equation (6). 8.7 ≤ log 10 R<14 ···(6)

[0070] 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, the current detection mechanism 50 is not limited to the configuration of the embodiment shown in Figure 3, and other configurations can be used as long as they are capable of accurately measuring the developing current.

[0071] Furthermore, although the above embodiment describes the case using positively charged toner, the present invention can also be applied to the case using negatively charged toner. When negatively charged toner is used, the polarity of the developing side and the recovery side is reversed compared to when positively charged toner is used, with the developing side being negative and the recovery side being positive.

[0072] Furthermore, although the above embodiment described an image forming apparatus 100 using a monochrome printer as an example as shown in Figure 1, it is not limited to a monochrome printer and may be other image forming apparatuses such as monochrome and color copiers, digital multifunction printers, color printers, or facsimile machines.

[0073] Color copiers and color printers require an image density sensor to detect the density of a reference image in order to perform calibration, which involves correcting image density and color misalignment. Furthermore, this calibration must be performed in a special mode when not printing.

[0074] Therefore, as an aid to conventional calibration using an image density sensor, this embodiment performs image density adjustment using a development current at a different timing than the calibration. This makes it possible to shorten the time required for image density adjustment in color copiers and color printers and to operate them efficiently. [Industrial applicability]

[0075] The present invention is applicable to an image forming apparatus equipped with a developing device that uses a two-component developing system that includes a toner and a carrier. By using the present invention, it is possible to provide an image forming apparatus that can achieve stable image density in a simple and low-cost manner using a two-component developing system. [Explanation of Symbols]

[0076] 4. Charging device 5. Photosensitive drum (image carrier) 6 Static eliminator 7. Exposure apparatus 8. Developing device 14 Transfer Roller 30. Developing roller (developer carrier) 43. Charged voltage power supply 44 Developer voltage power supply 45 Voltage Control Unit 50 Current detection mechanism 70 Storage section 80 Main Control Unit (Control Unit) 81 Toner density sensor 92 In-flight temperature and humidity sensor 100 Image forming apparatus

Claims

1. An image carrier having a photosensitive layer formed on its surface, A developing apparatus having a developer carrier positioned opposite the image carrier and carrying a two-component developer including a magnetic carrier and toner, wherein the toner is attached to the electrostatic latent image formed on the image carrier to form a toner image, An image forming unit including, A developing voltage power supply that applies a developing voltage including at least a DC voltage to the developer carrier, A current detection mechanism for detecting the developing current that flows between the developer carrier and the image carrier when the developing voltage is applied to the developer carrier, The image forming unit and the control unit that controls the developing voltage power supply, In an image forming apparatus equipped with, The control unit, An image forming apparatus characterized by detecting the development current when the development conditions are changed to multiple levels using the current detection mechanism, and adjusting the image density of the toner image based on the detection result.

2. The developing current includes an image current that flows between the developer carrier and the image portion of the image carrier, and a white area current that flows between the developer carrier and the non-image portion of the image carrier. The image forming apparatus according to claim 1, characterized in that the control unit adjusts the image density of the toner image based on the detection result of the white area current when the development conditions are changed to multiple levels.

3. The image forming apparatus according to claim 1, characterized in that the control unit changes the DC voltage among the developing voltage applied to the developer carrier as the developing condition.

4. A temperature and humidity sensor for detecting the temperature and humidity inside or outside the image forming apparatus, A toner density sensor for detecting the ratio of the toner to the magnetic carrier in the developing device, Equipped with, The image forming apparatus according to claim 1, characterized in that the control unit adjusts the image density of the toner image by taking into account at least one of the detection result of the temperature and humidity sensor, the detection result of the toner density sensor, the cumulative printing rate of the toner image, and the cumulative operating time of the developing device.

5. The system includes an image density sensor for detecting the image density of the toner image formed on the image carrier, and is capable of performing calibration to adjust the image density of the toner image based on the detection result of the image density sensor. The image forming apparatus according to any one of claims 1 to 4, characterized in that the control unit detects the developing current using the current detection mechanism at a timing different from the calibration, and auxiliaryly adjusts the image density of the toner image based on the detection result.

6. The image forming apparatus according to claim 5, characterized in that it comprises a plurality of image forming units corresponding to toners of different colors.

7. The resistance value R of the magnetic carrier is 8.7 ≤ log 10 An image forming apparatus according to any one of claims 1 to 4, characterized in that R < 14.

Images