Image forming apparatus

Abstract

The present invention provides an image forming apparatus that allows for simple and low-cost adjustment of the density and gradation of halftone images in 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 halftone image based on the developing current when a halftone image is formed.

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 after 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, a fixing process is generally performed.

[0003] Conventionally, a technique for adjusting an image using a developing current flowing between a photosensitive drum and a developing roller (developer carrier) has been proposed. For example, in Patent Document 1, an image carrier on which an electrostatic latent image is formed, a developing member provided to face the image carrier and transporting a developer containing toner and carrier to a facing region with the image carrier, a detection processing unit that detects a non-developing current flowing when a specific electric field that moves toner to the developing member side is formed in the facing region where the developer exists, and a determination processing unit that determines whether or not the execution timing of an adjustment process for adjusting image forming conditions has arrived based on the non-developing current detected by the detection processing unit are provided.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The technology described in Patent Document 1 allows for adjustment of the density and gradation of halftone images by adjusting the light intensity, charging voltage, and transfer voltage in addition to the development voltage, thereby improving image quality. However, in the method described in Patent Document 1, the development current only plays a role in determining the timing of image adjustment, and an image density sensor is indispensable for actual image adjustment. Therefore, monochrome cameras that do not have an image density sensor require the separate installation of one, which leads to increased costs.

[0006] In view of the above problems, the present invention aims to provide an image forming apparatus that can easily and inexpensively adjust the density and gradation of halftone images 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 developing device having a developer carrier positioned opposite the image carrier and carrying a two-component developer containing a magnetic carrier and toner, which deposits toner onto an electrostatic latent image to form a toner image. 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 a halftone image is formed as a toner image using the current detection mechanism, and adjusts the image density of the halftone image based on the detection result. [Effects of the Invention]

[0008] According to the first configuration of the present invention, by adjusting the image density of the halftone image using the development current that flows during the development of the halftone image, the image density of the halftone image 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 of the halftone image can be accurately adjusted 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 unit 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] A graph showing the correlation between the current in the white area and the amount of toner charge. [Figure 5] Graph showing the relationship between the tonal area ratio of a halftone image and the development current. [Figure 6] A flowchart showing an example of adjusting and controlling the gradation of a halftone image 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 by 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 photoreceptor 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] Towards the photosensitive drum 5 on which the toner image is formed as described above, the paper (recording medium) is conveyed from the paper feed cassette 10 or the manual paper feed device 11 through the paper conveyance path 12 and the registration roller pair 13. When the paper passes through the nip portion (transfer nip portion) between the transfer roller 14 and the photosensitive drum 5, the toner image formed on the surface of the photosensitive drum 5 is transferred onto the paper. The paper onto which the toner image is transferred is separated from the photosensitive drum 5 and conveyed to the fixing device 15 where the toner image is fixed. The paper that has passed through the fixing device 15 is conveyed to the upper part of the device by the paper conveyance path 16. When forming an image 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 images on both sides of the paper (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 diverted to the reverse conveyance path 21 that branches from the branch portion 20 and is re-conveyed to the registration 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 onto the surface of the paper where no image has been formed. The paper onto which the toner image is 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 arranged to contact the photosensitive drum 5 and charge the photosensitive drum 5.

[0016] The charging roller 41 is formed by coating a core metal 41a with a conductive layer 41b and is arranged to abut against 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 photoreceptor drum 5 is formed, for example, with an organic photoreceptor layer (OPC) 5b, which is a positively-charged photoconductor as a photosensitive layer, on the surface of a drum base tube (conductive substrate) 5a made of aluminum. The photoreceptor drum 5 is rotationally driven at a constant speed about a support shaft by a drum drive unit (not shown).

[0018] As shown in FIG. 2, when the photoreceptor drum 5 rotates in the counterclockwise direction, the charging roller 41 that contacts the surface of the photoreceptor 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 photoreceptor drum 5 is uniformly charged.

[0019] The charging roller 41 is connected to a charging voltage power source 43 that generates a charging voltage in which an AC voltage is superimposed on a DC voltage. The charging voltage power source 43 includes an AC constant voltage power source and a DC constant voltage power source (both not shown). The AC constant voltage power source outputs a sine-wave AC voltage generated from a low-voltage DC voltage modulated in a pulse shape using a step-up transformer (not shown). The DC constant voltage power source outputs a DC voltage obtained by rectifying a sine-wave AC voltage generated from a low-voltage DC voltage modulated in a pulse shape using a step-up transformer.

[0020] The developing device 8 is disposed opposite to the photoreceptor drum 5 and includes a developing roller 30 that carries a two-component developer (hereinafter also simply referred to as a developer) containing a magnetic carrier and toner. The developing roller 30 is connected to a developing voltage power source 44 that generates a developing voltage in which an AC voltage is superimposed on a DC voltage. The configuration of the developing voltage power source 44 is the same as that of the charging voltage power source 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 41 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 storage 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. Adjusting the image density of halftone images based on the development current] The following describes a feature of the present invention: a method for adjusting the image density of a halftone image based on the development current. In this embodiment, the development current when developing a halftone image is measured, and the image density of the halftone image is adjusted based on the measurement result.

[0038] The developing current is the amount of charge transferred per unit time, so if the developing current is I, it is expressed as I = Q / t. Also, since the developing current = toner charge amount × toner deposition amount, if the toner charge amount is Q / M, then the toner deposition amount = developing current / toner charge amount = (Q / t) / (Q / M) = M / t. When adjusting image density based on the developing current, the method used is to calculate the toner deposition amount M / t from the developing current I = Q / t and the estimated toner charge amount Q / M, and use this to adjust the image density.

[0039] Various methods have been proposed to estimate the toner charge amount Q / M. For example, one method estimates the toner charge amount based on the development current that flows when forming a reference image and the image density of the reference image detected by an image density sensor. However, since the image forming apparatus 100 of this embodiment is not equipped with an image density sensor, the method using an image density sensor cannot be adopted. As an example of a method for estimating the toner charge amount without using an image density sensor, one method is to estimate the toner charge amount Q / M from the correlation between the white area current and the toner charge amount.

[0040] Figure 4 is a graph showing the correlation between the white area current and the toner charge. In Figure 4, the horizontal axis is the product of toner concentration T / C and toner charge Q / M (T / C*Q / M), and the vertical axis is the white area current I, plotting the white area current I as T / C*Q / M is varied. As shown in Figure 4, there is a positive correlation between the white area current and the toner charge.

[0041] The relationship shown in Figure 4 is determined experimentally beforehand and stored in the memory unit 70. Then, the toner charge amount Q / M is estimated using the development current I (=Q / t) that flows when a halftone image is formed and the toner concentration T / C in the development device 8 detected by the toner concentration sensor 81 (see Figure 2). Since the amount of toner deposited is expressed as (Q / t) / (Q / M)=M / t, the amount of toner deposited M / t can be calculated using the estimated toner charge amount Q / M, and the image density of the halftone image can be adjusted.

[0042] Methods for adjusting image density 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.

[0043] Next, we will explain a method for adjusting the tonality of halftone images using the development current. As a method for adjusting the tonality, one can develop halftone images with three or more tonal area ratios and adjust the development current so that it flows linearly during the development of each halftone image.

[0044] Figure 5 is a graph showing the relationship between the tonal area ratio of a halftone image and the development current. As shown in Figure 5, the development current increases as the tonal area ratio increases. More specifically, within a certain range of tonal area ratio (between A1 and A3 in Figure 5), the increase in development current with respect to the tonal area ratio is linear. Therefore, halftone images are developed so that the tonal area ratio has three or more levels within the range where the increase in development current with respect to the tonal area ratio is linear. Then, the development current flowing during the development of each halftone image is controlled to be linear. This makes it possible to adjust the image density of halftone images without relying on estimated values ​​of the charge amount.

[0045] For example, if the slope of the developing current is steep between A1 and A2, and gentler between A2 and A3 (shown by the dashed line in Figure 5), the image density is too high compared to the target value. Therefore, the developing conditions are adjusted to lower the image density. Specifically, this can be done by lowering the DC voltage Vdc of the developing voltage, reducing the amount of light in the exposure device 7, or increasing the charging voltage applied to the charging roller 41.

[0046] Furthermore, if the slope of the developing current is gentle between A1 and A2, and steep between A2 and A3 (indicated by the dashed line in Figure 5), the image density is too low compared to the target value. Therefore, the developing conditions should be adjusted to increase the image density. Specifically, this can be done by increasing the DC voltage Vdc of the developing voltage, increasing the light intensity of the exposure device 7, or decreasing the charging voltage applied to the charging roller 41.

[0047] One method for changing the tonal area ratio of a halftone image to three or more levels is to change the image pattern of the halftone image (the arrangement and number of dots) to set three or more tonal area ratios. Then, the tonal area ratio is adjusted based on the results obtained when developing the halftone images of each set level under certain development conditions.

[0048] Furthermore, as shown in Figure 5, the relationship between the tonal area ratio and the development current deviates from a straight line in the region where the tonal area ratio is less than A1 and in the region where it exceeds A3. In order to accurately adjust the tonality of a halftone image, it is preferable that the three levels of tonal area ratio exist over as wide a range as possible within the range where the relationship between the tonal area ratio and the development current is straight. That is, as described above, it is preferable to vary the tonal area ratio to three or more levels, including the minimum value (A1) and maximum value (A3), within the range where the relationship between the tonal area ratio and the development current is straight. The range in which the relationship between the tonal area ratio and the development current is straight has been confirmed in advance through experiments, etc., and is stored in the memory unit 70.

[0049] Furthermore, the tonal range of a halftone image can also be adjusted using other methods. For example, the DC voltage Vdc of the development voltage is varied to multiple levels to form halftone images with the same tonal area ratio, and the development current I that flows when the halftone image is formed is measured. Then, the relationship between the development current I and the development voltage (DC voltage) Vdc is given by equation (1). I = Q / t = c * Vdc + d ... (1) One method involves using a DC voltage Vdc that determines the target value for the grayscale area ratio and adjusting the image density of the halftone image.

[0050] Figure 6 is a flowchart showing an example of adjusting the gradation of a halftone image in the image forming apparatus 100 of this embodiment. The procedure for adjusting the gradation of a halftone image will be described in detail according to the steps in Figure 6, referring to Figures 1 to 5 as needed. Note that Figure 6 shows the case when adjusting the gradation of a halftone image.

[0051] First, the main control unit 80 determines whether or not the timing for adjusting the gradation of the halftone image has been reached (step S1). Examples of timing for adjusting the gradation include when the cumulative number of prints 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 gradation has been reached (Yes in step S1), the halftone image is developed so that the gradation area ratio becomes 3 levels, within the range where the increase in the development current with respect to the gradation area ratio is linear (step S2).

[0052] Next, the main control unit 80 determines whether the relationship between the development current and the tonal area ratio measured when each halftone image is developed is linear (step S3). If the relationship between the development current and the tonal area ratio is not linear (No in step S3), the development conditions are adjusted so that the relationship between the development current and the tonal area ratio becomes linear (step S4). Specifically, 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 so that the relationship between the development current and the tonal area ratio becomes linear.

[0053] Next, the main control unit 80 determines whether or not it is necessary to correct the development conditions 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 development conditions (Yes in step S5), it corrects the development conditions determined in step S4 (step S6) and adjusts the gradation of the halftone image with the corrected development conditions (step S7).

[0054] On the other hand, if no correction of the development conditions is necessary (No in step S5), the tonality of the halftone image is adjusted using the development conditions determined in step S4 (step S7). Also, if the relationship between the development current and the tonal area ratio is linear in step S3 (Yes in step S3), the process is terminated without adjusting the development conditions.

[0055] As shown in Figure 6, by adjusting the tonality of a halftone image using the development current that flows during development, the tonality of the halftone image can be properly maintained over a long period of time. Furthermore, since there is no need to measure image density using an image density sensor, the tonality of the halftone image can be accurately adjusted even in image forming apparatuses that are not equipped with an image density sensor.

[0056] Furthermore, by incorporating the detection results of the toner density sensor 81 and the in-machine temperature and humidity sensor 92, in addition to the measurement results of the development current, the stability of the gradation can be further enhanced.

[0057] [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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] [Table 1]

[0064] 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.

[0065] 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.

[0066] 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)

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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. Therefore, as an auxiliary to normal calibration using an image density sensor, this embodiment adjusts the image density and gradation of the halftone image using a development current at a different timing than calibration. This makes it possible to shorten the time required for image density adjustment and to operate color copiers and color printers more efficiently. [Industrial applicability]

[0071] The present invention is applicable to an image forming apparatus equipped with a developing device that uses 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 achieve stable image density of halftone images in a simple and low-cost manner using a two-component developing system. [Explanation of Symbols]

[0072] P Image forming section 4. Charging device 5. Photosensitive drum (image carrier) 6 Static eliminator 7. Exposure equipment 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 Memory 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 a halftone image is formed as the toner image using the current detection mechanism, and adjusting the image density of the halftone image based on the detection result.

2. The image forming apparatus according to claim 1, characterized in that the control unit adjusts the gradation of the halftone image based on the development current when a plurality of halftone images with different gradation area ratios are formed.

3. The image forming apparatus according to claim 2, characterized in that the control unit adjusts the gradation of the halftone image so that the relationship between the gradation area ratio and the development current when a plurality of halftone images are formed becomes linear.

4. A charging device for charging the surface of the image carrier, An exposure apparatus that exposes the surface of an image carrier charged by the aforementioned charging device to form an electrostatic latent image with reduced charge, A charging voltage power supply for applying a charging voltage to the aforementioned charging device, Equipped with, The image forming apparatus according to any one of claims 1 to 3, characterized in that the control unit adjusts the image density and gradation of the halftone image by changing at least one of the developing voltage, the charging voltage, and the exposure amount of the exposure apparatus.

5. The image forming apparatus according to claim 4, characterized in that the control unit calculates the amount of toner deposited based on the developing current and the estimated amount of charge of the toner.

6. The image forming apparatus according to claim 5, characterized in that the control unit detects the white area current flowing between the developer carrier and the non-image portion of the image carrier using the current detection mechanism, and estimates the amount of charge of the toner based on the detected white area current.

7. 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 halftone image 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.

8. 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 claim 1, characterized in that the control unit detects the development current by the current detection mechanism at a timing different from the calibration, and auxiliaryly adjusts the image density of the halftone image based on the detection result.

9. The image forming apparatus according to claim 8, characterized by comprising a plurality of image forming units corresponding to toners of different colors.

10. 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 3, characterized in that R < 14.

Images