JP2020034828A - Image forming apparatus - Google Patents

Abstract

To set a suitable developing bias for image formation through a calibration operation even when the amount of charge of toner is easily changed, in an image forming apparatus including a developing device applied with a two-component developing system.SOLUTION: A mode control unit 984 executes a calibration operation of forming a plurality of electrostatic latent images for calibration on a photoreceptor drum and controlling a developing bias application unit 971 to make different at least DC voltages of developing biases for the electrostatic latent images for calibration to form a plurality of toner images for calibration, and determining a developing bias applied to a developing roller 231 in an image forming operation. A determination unit 985 determines as to whether correction of a developing bias determined in the calibration operation or re-execution of the calibration operation is necessary from the amounts of charge of toner before and after the execution of the calibration operation.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an image forming apparatus that forms an image on a sheet.

  2. Description of the Related Art Conventionally, as an image forming apparatus for forming an image on a sheet, an image forming apparatus including a photosensitive drum (image carrier), a developing device including a developing roller, and a transfer member has been known. When the electrostatic latent image formed on the photosensitive drum is revealed by the developing device in the developing nip portion, a toner image is formed on the photosensitive drum. The toner image is transferred to the sheet by the transfer member. As a developing device applied to such an image forming apparatus, a two-component developing technique using a developer containing a toner and a carrier is known.

  In an image forming apparatus using such a developing technique, a so-called calibration operation is performed to improve the quality of image density. In the calibration operation, electrostatic latent images of a plurality of preset image patterns are formed on the image carrier, and the developing bias applied to the developing roller is changed according to each image pattern. Then, by measuring the density of the toner image of each image pattern, a developing bias corresponding to a suitable image density is selected.

  On the other hand, a technique for accurately estimating the charge amount of toner has conventionally been proposed. In Patent Literatures 1 and 2, the surface potential of the photosensitive drum before development and the surface potential of the toner layer on the photosensitive drum after development are measured, respectively. A development amount of the toner is calculated. Then, the toner charge amount is calculated from the measured surface potentials and the toner development amount.

  Further, in Patent Documents 3 and 4, a current value flowing into a developing roller carrying a developer is measured, and the measured current value is assumed to be a charge amount of the toner moved from the developing roller to the photosensitive drum. . Further, the amount of toner development is calculated from the image density measurement result of the developed toner layer. Then, the charge amount of the toner is calculated from the charge amount of the toner and the development amount of the toner.

JP 2003-345075 A JP 2004-37952 A Japanese Patent No. 5024192 Patent No. 5273542

  A preferable developing bias for obtaining a predetermined image density varies depending on the charge amount of the toner. The calibration operation as described above is often performed to maintain image quality when the image forming apparatus is left unattended for a long time. In this case, the toner in the developing device that has been left for a long period of time undergoes a stir operation, and thus the charge amount tends to change. Therefore, during and after the calibration operation, the charge amount of the toner may change every moment. With the developing bias selected by the conventional calibration operation, a preferable image density is obtained in the image forming operation after the calibration. There was something I could not do.

  In the techniques described in Patent Documents 1 and 2, a surface potential sensor is required to measure the surface potential on the photosensitive drum. Here, in order to measure the surface potential of the toner layer formed on the photoconductor drum, it is necessary to install a surface potential sensor downstream of the development nip in the rotation direction of the photoconductor drum. However, when the surface potential sensor is installed at this position, the surface of the surface potential sensor is easily contaminated by the toner scattered from the developing roller, and it becomes difficult to accurately measure the surface potential over a long period of time.

  In the techniques described in Patent Documents 3 and 4, the current flowing into the developing roller includes the current flowing in the carrier in addition to the current flowing in the toner. Therefore, it is difficult to accurately calculate the charge amount of the toner from the current value. Further, when the resistance value of the carrier changes due to the peeling of the coat of the carrier or the contamination of the coat due to the repeated printing in the image forming apparatus, the current flowing through the carrier also changes. As described above, in the conventional method, it is difficult to correctly measure the charge amount of the toner from the current flowing into the developing roller.

  As described above, even if the conventional calibration operation is combined with the technique of measuring the charge amount of the toner, it is difficult to set an appropriate developing bias due to the change in the charge amount of the toner. There was a problem that it was not possible to obtain a suitable image quality when forming an image.

  SUMMARY An advantage of some aspects of the invention is to solve the above-described problem. In an image forming apparatus including a developing device to which a two-component developing method is applied, even when the charge amount of a toner is easily changed, an image is formed through a calibration operation. An object is to set a suitable developing bias for formation.

  An image forming apparatus according to an aspect of the present invention includes an image carrier that is rotated to form an electrostatic latent image on a surface thereof, and that carries a toner image in which the electrostatic latent image is made visible, A charging device that charges the body to a predetermined charging potential; and a charging device that is disposed downstream of the charging device in the rotation direction of the image carrier, and converts the surface of the image carrier charged to the charging potential into predetermined image information. An exposure device that forms the electrostatic latent image by performing exposure in response to the exposure device, and a development device that is disposed to face the image carrier at a predetermined development nip portion downstream of the exposure device in the rotation direction. A developing device that includes a developing roller that forms a toner image by rotating and carrying a developer including a toner and a carrier on a peripheral surface and supplying toner to the image carrier; Said toner image A transfer unit for transferring to a sheet, a development bias application unit capable of applying a development bias in which an AC voltage is superimposed on a DC voltage to the development roller, a density detection unit for detecting the density of the toner image, the charging device, Controlling the exposure device to form a plurality of electrostatic latent images for calibration on the image carrier and controlling the developing bias applying unit to control at least the developing bias for each electrostatic latent image for calibration; A plurality of calibration toner images are formed by changing the DC voltage, and an image forming operation is performed based on a relationship between the densities of the plurality of calibration toner images detected by the density detection unit and the developing bias. A calibrator that executes a calibration operation for determining the developing bias applied to the developing roller in Controlling the charging device, the exposing device, and the developing bias applying unit before and after the execution of the calibration operation, and controlling the amount of toner development on the image bearing member with respect to each other. Forming a plurality of different measurement toner images, based on the densities of the plurality of measurement toner images detected by the density detector, or in addition to the densities of the plurality of measurement toner images, Based on a DC component of a developing current flowing between the developing roller and the developing bias applying unit when a toner image is formed, the toner included in the measurement toner image formed on the image carrier is A charge amount acquisition unit that performs a charge amount acquisition operation for acquiring a charge amount, and before the calibration operation that is acquired by the charge amount acquisition unit From the charge amount of the toner and the charge amount of the toner after the execution of the calibration operation, it is determined whether the correction of the developing bias determined in the calibration operation or the re-execution of the calibration operation is necessary. A calibration operation determination unit.

  According to this configuration, the calibration operation execution unit determines an appropriate developing bias according to the image density of the calibration toner image. On the other hand, the calibration operation largely depends on the charge amount of the toner in the developing device. Therefore, if the charge amount of the toner greatly changes during the execution of the calibration operation, a deviation occurs between the actual charge amount of the toner and the determined developing bias. Even in such a case, the charge amount acquisition unit acquires the charge amount of the toner before and after the execution of the calibration operation. Then, the calibration operation determination unit determines whether the correction of the developing bias determined in the calibration operation or the re-execution of the calibration operation is necessary based on the acquired toner charge amount. For this reason, a large difference between the developing bias determined in the calibration operation and the actual charge amount of the toner is suppressed, and a stable image can be formed in the image forming operation.

  In the above configuration, the calibration operation determination unit may obtain the charge amount of the toner after the execution of the calibration operation with respect to the charge amount of the toner before the execution of the calibration operation, obtained by the charge amount acquisition unit. When the difference or the ratio is larger than a preset threshold value, it is desirable to determine that correction of the developing bias determined in the calibration operation or re-execution of the calibration operation is necessary.

  According to this configuration, the calibration operation determination unit compares the charge amounts of the toners before and after the execution of the calibration operation based on the difference or ratio thereof, thereby correcting the determined development bias or performing the calibration. The necessity of re-executing the operation can be easily determined.

  In the above configuration, the calibration operation determination unit may obtain the charge amount of the toner after the execution of the calibration operation with respect to the charge amount of the toner before the execution of the calibration operation, obtained by the charge amount acquisition unit. It is desirable to correct the developing bias determined in the calibration operation so as to change the difference in the DC voltage between the image carrier and the developing roller according to the ratio of the developing bias.

  According to this configuration, the determined developing bias is corrected in accordance with the change (ratio) of the toner charge amount during the execution of the calibration operation. Therefore, a stable image can be formed in the image forming operation.

  In the above configuration, when the frequency of the AC voltage of the developing bias is changed in a state where the potential difference of the DC voltage between the developing roller and the image carrier is kept constant, the amount of change in the frequency is changed. The image forming apparatus further includes a storage unit that stores, in advance, reference information relating to a slope of a reference straight line indicating a relationship of a density change amount of the toner image for each charge amount of the toner, wherein the charge amount acquisition unit includes the developing roller and the image carrier. Forming the plurality of measurement toner images on the image carrier while changing the frequency of the AC voltage of the developing bias in a state where the potential difference of the DC voltage with the body is kept constant; The slope of the measurement straight line indicating the relationship between the amount of change in the density of the toner image for measurement with respect to the amount of change in the frequency and the density detection result of the toner image for measurement by the density detector. Acquires, it is desirable to obtain the charge amount of the toner contained from the slope of the measurement line is the acquisition and reference information of the storage unit to the toner image for measurement formed on said image bearing member.

  According to this configuration, the charge amount of the toner can be obtained without using a surface potential sensor that measures the potential on the image carrier or an ammeter that measures the developing current flowing into the developing roller.

  In the above configuration, the reference information stored in the storage unit is such that when the charge amount of the toner is the first charge amount, the slope of the reference straight line is negative, and the charge amount of the toner is When the second charge amount is smaller than the first charge amount, the inclination of the reference straight line is positive, and the inclination of the reference straight line is set to increase as the charge amount of the toner decreases. It is desirable to have been.

  According to this configuration, the charge amount of the toner can be accurately acquired from the relationship between the frequency of the AC voltage of the developing bias and the density of the toner image formed on the image carrier (the amount of the developed toner).

  In the above-described configuration, the charge amount acquiring unit may change the frequency of the AC voltage of the developing bias while maintaining the potential difference of the DC voltage between the developing roller and the image carrier constant. Forming the plurality of measurement toner images on the body, and forming the plurality of measurement toner images with respect to a difference in density between the plurality of measurement toner images detected by the density detection unit; A charge amount of the toner contained in the measurement toner image formed on the image carrier, based on a ratio of a difference between a DC component of the development current flowing between the image carrier and the development bias application unit. Good.

  Further, in the above configuration, the charge amount acquisition unit controls the exposure device in a state where a potential difference of a DC voltage between the developing roller and the image carrier is kept constant, and controls a printing rate per unit area. Forming the plurality of measurement toner images on the image carrier while changing the density, and forming the plurality of measurement toner images with respect to the difference between the densities of the plurality of measurement toner images detected by the density detection unit. The toner contained in the measurement toner image formed on the image carrier based on the ratio of the difference between the DC components of the developing current flowing between the developing roller and the developing bias applying unit. What acquires the charge amount may be used.

  According to the present invention, in an image forming apparatus including a developing device to which a two-component developing method is applied, a suitable developing bias for image formation is set through a calibration operation even when the charge amount of toner is apt to change. Becomes possible.

FIG. 1 is a cross-sectional view illustrating an internal structure of an image forming apparatus according to an embodiment of the present disclosure. FIG. 2 is a cross-sectional view of a developing device according to an embodiment of the present invention and a block diagram illustrating an electrical configuration of a control unit. FIG. 3 is a schematic diagram illustrating a developing operation of the image forming apparatus according to the embodiment of the present disclosure. FIG. 4 is a schematic diagram illustrating a magnitude relationship between potentials of an image carrier and a developing roller according to an embodiment of the present invention. 4 is a graph showing a relationship between a developing bias frequency and an image density in the image forming apparatus according to the embodiment of the present disclosure. 5 is a graph showing the relationship between the inclination of the graph of FIG. 4 and the toner charge amount in the image forming apparatus according to one embodiment of the present invention. 6 is a flowchart of a charge amount measurement mode executed in the image forming apparatus according to the embodiment of the present disclosure. FIG. 3 is a schematic diagram of a measurement toner image formed on an image carrier in a charge amount measurement mode executed in the image forming apparatus according to one embodiment of the present disclosure. 6 is a flowchart of a calibration operation and a calibration correction operation performed in the image forming apparatus according to the embodiment of the present disclosure. FIG. 9 is a cross-sectional view of a developing device according to a modified embodiment of the present invention and a block diagram illustrating an electrical configuration of a control unit. 9 is a flowchart of a charge amount measurement mode executed in an image forming apparatus according to a modified embodiment of the present invention.

  Hereinafter, an image forming apparatus 10 according to an embodiment of the present invention will be described in detail with reference to the drawings. In the present embodiment, a tandem type color printer will be described as an example of the image forming apparatus. The image forming apparatus may be, for example, a copying machine, a facsimile machine, or a multifunction peripheral thereof. Further, the image forming apparatus may form a single color (monochrome) image.

  FIG. 1 is a sectional view showing an internal structure of the image forming apparatus 10. The image forming apparatus 10 includes an apparatus main body 11 having a box-shaped housing structure. In the apparatus main body 11, a sheet feeding section 12 for feeding a sheet P, an image forming section 13 for forming a toner image to be transferred onto the sheet P fed from the sheet feeding section 12, and the toner image is primarily transferred. An intermediate transfer unit 14 (transfer section), a toner replenishing section 15 for replenishing toner to the image forming section 13, and a fixing section 16 for performing processing for fixing an unfixed toner image formed on the sheet P to the sheet P. It is decorated. Further, at the upper part of the apparatus main body 11, there is provided a paper discharge unit 17 for discharging the sheet P subjected to the fixing processing by the fixing unit 16.

  An operation panel (not shown) for inputting output conditions and the like for the sheet P is provided at an appropriate position on the upper surface of the apparatus main body 11. This operation panel is provided with a power key, a touch panel for inputting output conditions, and various operation keys.

  In the apparatus main body 11, a sheet conveying path 111 extending vertically is formed at a position on the right side of the image forming section 13. The sheet conveying path 111 is provided with a pair of conveying rollers 112 for conveying a sheet to an appropriate position. Further, a pair of registration rollers 113 for correcting the skew of the sheet and feeding the sheet at a predetermined timing to a nip portion for secondary transfer described later is provided upstream of the nip portion in the sheet conveying path 111. The sheet transport path 111 is a transport path that transports the sheet P from the paper supply unit 12 to the paper discharge unit 17 via the image forming unit 13 and the fixing unit 16.

  The paper supply unit 12 includes a paper supply tray 121, a pickup roller 122, and a paper supply roller pair 123. The paper feed tray 121 is removably mounted below the apparatus main body 11, and stores a sheet bundle P1 in which a plurality of sheets P are stacked. The pickup roller 122 feeds out the uppermost sheet P of the sheet bundle P1 stored in the sheet feeding tray 121 one by one. The paper feed roller pair 123 sends out the sheet P fed by the pickup roller 122 to the sheet transport path 111.

  The paper feeding unit 12 includes a manual paper feeding unit attached to the left side of the apparatus main body 11 shown in FIG. The manual sheet feeding unit includes a manual tray 124, a pickup roller 125, and a sheet feeding roller pair 126. The manual feed tray 124 is a tray on which sheets P to be manually fed are placed, and is opened from the side of the apparatus body 11 as shown in FIG. The pickup roller 125 feeds out the sheet P placed on the manual feed tray 124. The sheet feeding roller pair 126 sends out the sheet P fed by the pickup roller 125 to the sheet conveying path 111.

  The image forming unit 13 forms a toner image to be transferred to the sheet P, and includes a plurality of image forming units that form toner images of different colors. In the present embodiment, as the image forming units, a magenta (M) color is sequentially arranged from the upstream side to the downstream side (from left to right in FIG. 1) in the rotation direction of the intermediate transfer belt 141 described later. A magenta unit 13M using a developer, a cyan unit 13C using a cyan (C) developer, a yellow unit 13Y using a yellow (Y) developer, and a black (Bk) developer are used. A black unit 13Bk is provided. Each of the units 13M, 13C, 13Y, and 13Bk includes a photosensitive drum 20 (image carrier), a charging device 21, a developing device 23, a primary transfer roller 24, and a cleaning device 25 disposed around the photosensitive drum 20. Is provided. An exposure device 22 common to the units 13M, 13C, 13Y, and 13Bk is disposed below the image forming unit.

  The photoreceptor drum 20 is driven to rotate around its axis to form an electrostatic latent image on the surface thereof, and carries a toner image in which the electrostatic latent image has been revealed. As the photosensitive drum 20, for example, a known amorphous silicon (α-Si) photosensitive drum or an organic (OPC) photosensitive drum is used. The charging device 21 uniformly charges the surface of the photosensitive drum 20 to a predetermined charging potential. The charging device 21 includes a charging roller and a charging cleaning brush for removing toner attached to the charging roller. The exposure device 22 is disposed downstream of the charging device 21 in the rotation direction of the photosensitive drum 20, and has various optical system devices such as a light source, a polygon mirror, a reflection mirror, and a deflection mirror. The exposure device 22 irradiates the surface of the photosensitive drum 20 uniformly charged to the charged potential with light modulated based on image data (predetermined image information) to expose the surface, thereby forming an electrostatic latent image. Form.

  The developing device 23 is opposed to the photosensitive drum 20 at a predetermined developing nip portion NP (FIG. 3A) downstream of the exposure device 22 in the rotation direction of the photosensitive drum 20. The developing device 23 includes a developing roller 231 that rotates and carries a developer including a toner and a carrier on a peripheral surface and supplies the toner to the photosensitive drum 20 to form the toner image.

  The primary transfer roller 24 forms a nip with the photosensitive drum 20 with the intermediate transfer belt 141 provided in the intermediate transfer unit 14 interposed therebetween. Further, the primary transfer roller 24 primarily transfers the toner image on the photosensitive drum 20 onto the intermediate transfer belt 141. The cleaning device 25 cleans the peripheral surface of the photosensitive drum 20 after the transfer of the toner image.

  The intermediate transfer unit 14 is disposed in a space provided between the image forming unit 13 and the toner replenishing unit 15, and includes an intermediate transfer belt 141, a drive roller 142 rotatably supported by a unit frame (not shown), and , A driven roller 143, a backup roller 146, and the density sensor 100. The intermediate transfer belt 141 is an endless belt-shaped rotating body, and is stretched over the driving roller 142 and the driven rollers 143 and 146 such that the peripheral surface thereof comes into contact with the peripheral surface of each of the photosensitive drums 20. I have. The intermediate transfer belt 141 is driven to rotate by rotation of the driving roller 142. A belt cleaning device 144 that removes toner remaining on the peripheral surface of the intermediate transfer belt 141 is disposed near the driven roller 143. The density sensor 100 (density detecting unit) is disposed on the downstream side of the units 13M, 13C, 13Y, and 13Bk so as to face the intermediate transfer belt 141, and detects the density of the toner image formed on the intermediate transfer belt 141. To detect. In another embodiment, the density sensor 100 may detect the density of the toner image on the photosensitive drum 20 or may detect the density of the toner image fixed on the sheet P.

  A secondary transfer roller 145 is disposed outside the intermediate transfer belt 141 so as to face the drive roller 142. The secondary transfer roller 145 is pressed against the peripheral surface of the intermediate transfer belt 141 to form a transfer nip with the driving roller 142. The toner image primarily transferred on the intermediate transfer belt 141 is secondarily transferred to a sheet P supplied from the paper supply unit 12 at a transfer nip. That is, the intermediate transfer unit 14 and the secondary transfer roller 145 function as a transfer unit that transfers the toner image carried on the photosensitive drum 20 to the sheet P. The drive roller 142 is provided with a roll cleaner 200 for cleaning the peripheral surface thereof.

  The toner replenishing unit 15 stores toner used for image formation. In the present embodiment, the toner replenishing unit 15 includes a magenta toner container 15M, a cyan toner container 15C, a yellow toner container 15Y, and a black toner container 15Bk. These toner containers 15M, 15C, 15Y, and 15Bk store replenishment toner of each color of M / C / Y / Bk. The toner of each color is supplied to the developing devices 23 of the image forming units 13M, 13C, 13Y, and 13Bk corresponding to each color of M / C / Y / Bk from the toner discharge port 15H formed on the bottom surface of the container.

  The fixing unit 16 includes a heating roller 161 having a heating source therein, a fixing roller 162 disposed to face the heating roller 161, a fixing belt 163 stretched between the fixing roller 162 and the heating roller 161, a fixing belt 163. And a pressure roller 164 that is arranged to face the fixing roller 162 with the fixing roller 162 interposed therebetween to form a fixing nip portion. The sheet P supplied to the fixing unit 16 is heated and pressed by passing through the fixing nip. As a result, the toner image transferred to the sheet P at the transfer nip is fixed to the sheet P.

  The paper discharge unit 17 is formed by recessing the top of the apparatus main body 11, and a paper discharge tray 171 that receives the discharged sheet P is formed at the bottom of the concave. The sheet P on which the fixing process has been performed is discharged to a discharge tray 151 via a sheet conveyance path 111 extending from an upper portion of the fixing unit 16.

<About the developing device>
FIG. 2 is a cross-sectional view of the developing device 23 according to the present embodiment and a block diagram illustrating an electrical configuration of the control unit 980. The developing device 23 includes a developing housing 230, a developing roller 231, a first screw feeder 232, a second screw feeder 233, and a regulating blade 234. The developing device 23 employs a two-component developing method.

  The developing housing 230 is provided with a developer accommodating portion 230H. The developer accommodating section 230H accommodates a two-component developer composed of a toner and a carrier. Further, the developer accommodating portion 230H transports the developer in a first transport direction (a direction perpendicular to the plane of FIG. 2, a direction from the rear to the front) from one end of the developing roller 231 toward the other end in the axial direction. And a second transport unit 230B which is communicated with the first transport unit 230A at both ends in the axial direction and in which the developer is transported in a second transport direction opposite to the first transport direction. Including. The first screw feeder 232 and the second screw feeder 233 are rotated in the directions indicated by arrows D22 and D23 in FIG. 2, and respectively transport the developer in the first transport direction and the second transport direction. In particular, the first screw feeder 232 supplies the developer to the developing roller 231 while transporting the developer in the first transport direction.

  The developing roller 231 is arranged to face the photosensitive drum 20 at the developing nip NP (FIG. 3A). The developing roller 231 includes a sleeve 231S that is rotated, and a magnet 231M that is fixed and disposed inside the sleeve 231S. The magnet 231M has S1, N1, S2, N2 and S3 poles. The N1 pole functions as a main pole, the S1 pole and the N2 pole function as carrier poles, and the S2 pole functions as a peeling pole. The S3 pole functions as a pumping pole and a regulating pole. As an example, the magnetic flux densities of the S1, N1, S2, N2, and S3 poles are set to 54 mT, 96 mT, 35 mT, 44 mT, and 45 mT. The sleeve 231S of the developing roller 231 is rotated in the direction of arrow D21 in FIG. The developing roller 231 is rotated, receives the developer in the developing housing 230, carries the developer layer, and supplies toner to the photosensitive drum 20. In the embodiment, the developing roller 231 rotates in the same direction (with direction) at a position facing the photosensitive drum 20.

  The regulating blade 234 (layer thickness regulating member) is arranged at a predetermined interval on the developing roller 231, and regulates the layer thickness of the developer supplied from the first screw feeder 232 onto the peripheral surface of the developing roller 231.

  The image forming apparatus 10 including the developing device 23 further includes a developing bias applying unit 971, a driving unit 972, and a control unit 980. The control unit 980 includes a CPU (Central Processing Unit), a ROM (Read Only Memory) for storing a control program, a RAM (Random Access Memory) used as a work area of the CPU, and the like.

  The developing bias applying unit 971 is composed of a DC power supply and an AC power supply. Based on a control signal from a bias control unit 982 described later, the developing bias in which the AC voltage is superimposed on the DC voltage is applied to the developing roller 231 of the developing device 23. Is applied.

  The driving unit 972 is composed of a motor and a gear mechanism for transmitting the torque thereof. According to a control signal from a driving control unit 981 described below, the driving unit 972 performs a developing operation (image forming operation), a charge amount measurement mode, and a calibration operation. In addition to the photoconductor drum 20 and the like, the developing roller 231 and the first screw feeder 232 and the second screw feeder 233 in the developing device 23 are driven to rotate. The driving unit 972 further generates a driving force for driving (rotating) other members of the image forming apparatus 10.

  When the CPU executes the control program stored in the ROM, the control unit 980 controls the drive control unit 981, the bias control unit 982, the storage unit 983, and the mode control unit 984 (the charge amount acquisition unit, the calibration operation execution unit). ) And a determination unit 985 (calibration operation determination unit).

  The drive control unit 981 controls the drive unit 972 to rotationally drive the developing roller 231, the first screw feeder 232, and the second screw feeder 233. Further, the drive control unit 981 controls a drive mechanism (not shown) to rotate the photosensitive drum 20.

  The bias control unit 982 controls the developing bias applying unit 971 during a developing operation in which toner is supplied from the developing roller 231 to the photosensitive drum 20, so that a DC voltage and an AC voltage are applied between the photosensitive drum 20 and the developing roller 231. A voltage potential difference is provided. The toner is moved from the developing roller 231 to the photosensitive drum 20 by the potential difference.

  The storage unit 983 stores various types of information referred to by the drive control unit 981, the bias control unit 982, the mode control unit 984, and the determination unit 985. As an example, the value of the developing bias adjusted according to the rotation speed of the developing roller 981 and the environment is stored. Further, the storage unit 983 stores the amount of change in the frequency of the AC voltage of the developing bias when the frequency of the AC voltage of the developing bias is changed while the potential difference of the DC voltage between the developing roller 231 and the photosensitive drum 20 is kept constant. Reference information on the inclination of the reference straight line indicating the relationship between the toner image density change amounts is stored in advance for each toner charge amount. The reference information stored in the storage unit 983 has a negative slope of the reference line when the charge amount of the toner is the first charge amount, and the charge amount of the toner is smaller than the first charge amount. When the charge amount is the second charge amount, the inclination of the reference straight line is positive, and further, the inclination of the reference straight line is set to increase as the charge amount of the toner decreases. Note that the data stored in the storage unit 983 may be in the form of a graph, a table, or the like.

  The mode control unit 984 executes a charge amount measurement mode (charge amount acquisition operation) and calibration (calibration operation) described later.

  The mode control unit 984 controls the charging device 21, the exposure device 22, the developing bias applying unit 971, and the like before the execution of the calibration and after the execution of the calibration, so that a plurality of toner development amounts different from each other The measurement toner images are formed based on the densities of the plurality of measurement toner images detected by the density sensor 100 or in addition to the densities of the plurality of measurement toner images. Based on the DC component of the developing current flowing between the developing roller 231 and the developing bias applying unit 971 during the formation, the charge amount of the toner contained in the measurement toner image formed on the photosensitive drum 20 is determined. The acquired charge amount measurement mode is executed.

  More specifically, in the charge amount measurement mode, the mode control unit 984 changes the frequency of the AC voltage of the developing bias while maintaining the potential difference of the DC voltage between the developing roller 231 and the photosensitive drum 20 constant. A plurality of measurement toner images are formed on the photosensitive drum 20. Then, the inclination of the measurement straight line indicating the relationship between the amount of change in the density of the toner image for measurement with respect to the amount of change in the frequency is obtained from the amount of change in the frequency and the density detection result of the toner image for measurement by the density sensor 100 At the same time, the charge amount of the toner included in the measurement toner image formed on the photosensitive drum 20 is acquired from the acquired inclination of the measurement straight line and the reference information in the storage unit 983.

  Further, in the calibration operation, the mode control unit 984 controls the charging device 21 and the exposure device 22 to form a plurality of electrostatic latent images for calibration on the photosensitive drum 20 and controls the developing bias application unit 971 to operate. A plurality of toner images for calibration are formed by controlling and making at least the DC voltage of the developing bias different for each electrostatic latent image for calibration. The mode control unit 984 controls the developing bias applied to the developing roller 231 in the image forming operation based on the relationship between the densities of the plurality of calibration toner images detected by the density sensor 100 and the developing bias. To determine.

  The determining unit 985 determines the charge amount of the toner before the execution of the calibration and the charge amount of the toner after the execution of the calibration, which are obtained by the mode control unit 984, and is determined in the calibration. It is determined whether correction of the developing bias or re-execution of the calibration is necessary. Specifically, the determination unit 985 sets in advance a difference or ratio of the charge amount of the toner after the execution of the calibration with respect to the charge amount of the toner before the execution of the calibration, which is acquired by the mode control unit 984. If it is larger than the threshold value, it is determined that correction of the developing bias determined in the calibration or re-execution of the calibration is necessary. In particular, in the present embodiment, the determination unit 985 determines the ratio of the charge amount of the toner after the execution of the calibration to the charge amount of the toner before the execution of the calibration, which is acquired by the mode control unit 984. The developing bias determined in the calibration is corrected so as to change the DC voltage difference between the photosensitive drum 20 and the developing roller 231.

<Developing operation>
FIG. 3A is a schematic diagram of the developing operation of the image forming apparatus 10 according to the present embodiment, and FIG. 3B is a schematic diagram illustrating the magnitude relationship between the potentials of the photosensitive drum 20 and the developing roller 231. Referring to FIG. 3A, a developing nip NP is formed between developing roller 231 and photosensitive drum 20. The toner TN and the carrier CA carried on the developing roller 231 form a magnetic brush. In the developing nip portion NP, the toner TN is supplied from the magnetic brush to the photosensitive drum 20 side, and a toner image TI is formed. Referring to FIG. 3B, the surface potential of photoconductor drum 20 is charged by charging device 21 to background portion potential V0 (V). Thereafter, when exposure light is irradiated by the exposure device 22, the surface potential of the photosensitive drum 20 is changed from the background portion potential V0 to the image portion potential VL (V) at the maximum according to the image to be printed. On the other hand, a DC voltage Vdc of a developing bias is applied to the developing roller 231, and an AC voltage (not shown) is superimposed on the DC voltage Vdc.

  In the case of such a reversal developing method, a potential difference between the surface potential V0 and the DC component Vdc of the developing bias is a potential difference that suppresses toner fogging on the background portion of the photosensitive drum 20. On the other hand, the potential difference between the surface potential VL after the exposure and the DC component Vdc of the developing bias is a developing potential difference for moving the positive polarity toner to the image portion of the photosensitive drum 20. Further, the movement of the toner from the developing roller 231 to the photosensitive drum 20 is promoted by the AC voltage applied to the developing roller 231.

  On the other hand, each toner is frictionally charged with the carrier while being circulated and transported in the developing housing 230. The amount of charge of each toner affects the amount of toner (development amount) that moves toward the photosensitive drum 20 due to the above-described development bias. Therefore, if it becomes possible to accurately predict the charge amount of the toner in the image forming apparatus 10, by adjusting the developing bias and the toner density according to the number of prints, environmental fluctuation, print mode, print ratio, etc. Image quality can be maintained. For this reason, it has conventionally been desired to accurately predict the charge amount of the toner.

<Estimation of toner charge amount>
The inventor of the present invention has conducted intensive studies in view of the above situation, and has newly found that when the frequency of the AC voltage of the developing bias is changed, the change in the amount of developed toner varies depending on the amount of charge of the toner. did. Specifically, when the charge amount of the toner is low, increasing the frequency of the AC voltage increases the development amount of the toner. On the other hand, it was newly found that when the charge amount of the toner is high, increasing the frequency of the AC voltage decreases the development amount of the toner. By utilizing this characteristic, it is possible to accurately predict the charge amount of the toner by measuring the change in the image density when the frequency of the AC voltage is changed.

  FIG. 4 is a graph showing the relationship between the frequency of the developing bias and the image density in the image forming apparatus 10 according to the present embodiment. FIG. 5 is a graph showing the relationship between the inclination of the graph of FIG. 4 and the toner charge amount in the image forming apparatus 10 according to the present embodiment.

  The potential difference between the DC voltage of the developing bias applied to the developing roller 231 and the DC voltage between the electrostatic latent image on the photosensitive drum 20 is kept constant, and the peak-to-peak voltage Vpp of the AC voltage of the developing bias and the duty ratio are reduced. In the fixed state, the frequency of the AC voltage is changed. As a result, the image density of the toner image detected by the density sensor 100 tends to be different depending on the charge amount of the toner on the developing roller 231 (FIG. 4). That is, as shown in FIG. 4, when the charge amount of the toner is 27.5 μc / g, the image density decreases as the frequency f decreases. On the other hand, when the charge amount of the toner is 34.0 μc / g or 37.7 μc / g, the image density increases as the frequency f decreases. Then, as the charge amount of the toner decreases, the slope of the graph illustrated in FIG. 4 increases. Referring to FIG. 5, the relationship between the inclinations of the three graphs in FIG. 4 and the toner charge amounts is distributed on a straight line (approximate straight line). Therefore, if the information shown in FIG. 5 is stored in the storage unit 983 in advance, and the inclination of the straight line shown in FIG. 4 is derived in the charge amount measurement mode described later, the charge amount of the toner at that time is measured (predicted). It is possible to do.

<About the effect of predicting the charge amount of the toner>
In the present embodiment, it is not necessary to provide a surface potential sensor for measuring the surface potential on the photosensitive drum 20 in order to predict the charge amount of the toner. Further, in order to estimate the charge amount of the toner, it is not necessary to measure the current flowing into the developing roller 231 according to the developing bias. Therefore, the amount of charge of the toner can be stably predicted without being affected by the change in the current flowing into the developing roller 231 due to the contamination of the surface potential sensor or the change in the resistance of the carrier. For this reason, when the image density printed in the image forming apparatus 10 is reduced, it is desirable to increase the image density by increasing the toner density of the developing device 23 to reduce the charge amount of the toner. It is easy to select whether it is desirable to increase the image density by increasing the development potential difference (Vdc-VL) in the portion NP.

  In general, the causes of the decrease in the image density in the image forming apparatus 10 include “decrease in development potential difference”, “decrease in transport amount of developer passing through the regulating blade 234”, “increase in carrier resistance”, and “toner charge amount”. Rise ". In this case, if the toner density is increased in order to reduce the toner charge amount with respect to the image density decrease due to a factor other than the increase in the toner charge amount, problems such as toner scattering will newly occur. there is a possibility. It is desirable to reduce the toner charge amount by increasing the toner density to reduce the image density due to the increase in the toner charge amount, and to reduce the development electric field ( It is preferable to increase the developing bias. Further, by grasping the toner charge amount, the transfer current applied to the secondary transfer roller 145 can be optimized, so that the entire system of the image forming apparatus 10 can be further stabilized.

<Relationship between frequency and toner charge amount>
The inventor of the present invention estimates that the toner charge amount contributes to the change in image density when the frequency of the AC voltage of the developing bias is changed as follows.
(1) When the charge amount of the toner is low When the charge amount of the toner is low, the toner is easily separated from the carrier because the electrostatic adhesion acting between the toner and the carrier is small. However, when the frequency of the AC voltage of the developing bias decreases, the number of reciprocating movements of the toner in the developing nip NP decreases. For this reason, the image density decreases. Note that when the frequency decreases, the reciprocating movement distance of the toner per one cycle of the AC voltage increases. However, when the charge amount of the toner is low, the original movement distance of the toner is small, and the influence on the reduction of the image density is small. Few. As described above, when the charge amount of the toner is low, the image density decreases when the frequency of the AC voltage of the developing bias decreases.
(2) When the charge amount of the toner is high When the frequency of the AC voltage of the developing bias is reduced as described above, the number of reciprocating movements of the toner in the developing nip portion NP is reduced. Since it is difficult to separate from the carrier, the influence of the decrease in the number of reciprocating movements is small. On the other hand, when the frequency decreases, the reciprocating distance of the toner per one cycle of the AC voltage increases, so that the image density increases according to the high toner charge amount. As described above, when the charge amount of the toner is high, the image density increases as the frequency of the AC voltage of the developing bias decreases.

<About the toner charge amount measurement mode>
FIG. 6 is a flowchart of the charge amount measurement mode executed in the image forming apparatus 10 according to the present embodiment. FIG. 7 is a schematic diagram of a measurement toner image formed on the photosensitive drum 20 in the charge amount measurement mode.

  Referring to FIG. 6, when the charge amount measurement mode is started (step S01), mode control section 984 sets variable n for changing the frequency of the AC voltage of the developing bias to n = 1 (step S02). ). Then, the mode control unit 984 controls the drive control unit 981 and the bias control unit 982 to rotate the phenomenon roller 231 by one or more rotations in a state where a preset reference phenomenon bias is applied. The frequency of the AC voltage is set to the first frequency (n = 1) (step S03). The reference phenomenon bias is set so that the charge amount measurement mode is not affected by the history of the immediately preceding image formation. Normally, a bias used for printing (image formation) is applied to the reference developing bias condition. When only a DC voltage is applied as the reference developing bias, the effect of eliminating the above history is weak. Therefore, it is preferable that the DC voltage and the AC voltage are applied in a superimposed manner.

  Next, a predetermined measurement toner image is developed with the developing bias having the frequency of the AC voltage set to the first frequency (step S04), and the toner image is transferred from the photosensitive drum 20 to the intermediate transfer belt 141. (Step S05). Then, the image density of the measurement toner image is measured by the density sensor 100 (step S06), and the acquired image density is stored in the storage unit 983 together with the value of the first frequency (step S07).

  Next, the mode control unit 984 determines whether or not the variable n relating to the frequency has reached a preset specified number N (step S08). Here, if n ≠ N (NO in step S08), the value of n is counted up by one (n = n + 1, step S09), and steps S03 to S07 are repeated. In order to increase the accuracy of the charge amount measurement, it is preferable that the specified number of times N is equal to or greater than 2, and it is more preferable that 3 ≦ N. On the other hand, when n = N (YES in step S08), mode control section 984 calculates the slope of the approximate straight line shown in FIG. 4 based on the information stored in storage section 983 (step S10). . Then, based on the graph (reference information) shown in FIG. 5 stored in the storage unit 983, the mode control unit 984 estimates the charge amount of the toner from the above inclination (step S11), and sets the charge amount measurement mode. Is completed (step S12).

  FIG. 7 illustrates an example in which the image density of the measurement toner image is increased by increasing the frequency f when the specified number of times N = 3. In this case, the charge amount of the toner is relatively low, such as 27.5 μc / g in FIG.

When N = 2, the image densities measured in step S06 are defined as ID1 and ID2, respectively. Further, the first frequency is defined as f1 (kHz), and the second frequency is defined as f2 (kHz) (f2 <f1). In this case, the slope a of the straight line shown in FIG.
Slope a = (ID1-ID2) / (f1-f2)) (Equation 1)
The slope a varies depending on the toner charge amount, and is “positive (+)” when the toner charge amount is low, and “negative (−)” when the toner charge amount is low. When the measurement is performed under the condition of 3 ≦ N, the slope of the approximate straight line of the linear expression obtained by the least square method may be used. In addition, the reference information shown in FIG.
Q / M = A × Slope of straight line + B (Equation 2)
Here, A and B are values specific to the developer, and are determined in advance by experiments. Q / M means the toner charge amount per unit mass. In step S10, the toner charge amount Q / M is calculated by substituting the slope a of the approximate straight line calculated from equation 1 into equation 2. The charge amount measurement mode shown in FIG. 6 may be executed for each developing device 23 of each color shown in FIG. 1, and the frequency set during execution of the mode may be a value unique to each developing device 23. May be set. In particular, when a desired frequency is known according to the temperature and humidity around the image forming apparatus 10 and the number of endurance sheets, the frequency set during the execution of the mode may be set near the known frequency. Further, a frequency used in a new measurement mode may be selected with reference to the result of the previous toner charge amount measurement mode. In this case, the accuracy of the measured toner charge amount can be improved.

<Execution timing of charge amount measurement mode>
The execution timing of the charge amount measurement mode according to the present embodiment includes one that is automatically started and one that is manually started. The automatic charge amount measurement mode is performed before and after execution of calibration (also referred to as setup or image quality adjustment operation) of the image forming apparatus 10 as described later.

  It is more preferable that a plurality of density sensors 100 are arranged in the main scanning direction (the axial direction of the photosensitive drum 20), and that a measurement toner image is formed in accordance with the position of the density sensor 100. That is, when the measurement toner images are formed corresponding to both ends of the photosensitive drum 20 in the axial direction, the toner charge amounts at both ends of the developing device 23 (developing roller 231) can be predicted. If the difference between the toner charge amounts at both ends is larger than a preset threshold value, the charging performance in the developing device 23 may be degraded. Therefore, the mode control unit 984 can prompt the exchange of the developing device 23 and the exchange of the developer through a display unit (not shown) of the image forming apparatus 10 or the like.

  Further, it is desirable that the above-described charge amount measurement modes are respectively executed when the image forming apparatus 10 is shipped from a factory after manufacturing and when the image forming apparatus 10 is set up at a place where the image forming apparatus 10 is used. As a result, it is possible to predict the effect of the image forming apparatus 10 during the suspension period. That is, the amount of charge of the developer tends to decrease when the idle period is long, and the level of the tendency often differs depending on the period in which the developer is left and the environment. Therefore, by measuring the toner charge amount at the time of shipment from the factory and at the time of set-up of the main body, a deterioration state due to leaving the developer is predicted.If the leaving time is very long or if the developer is left in a poor environment, A large difference between the two toner charge amounts (toner charge amount at the time of shipment from the factory and at the time of setup of the main body) is detected to be large. In such a case, replacement of the developer at the place of use can be promoted in the same manner as described above.

  On the other hand, even if the toner charge amount is low at the time of factory shipment and when the main body is set up, if the difference between the two toner charge amounts is small, the possibility that the developer has deteriorated is low. Therefore, there is no need to replace the developer at the place of use, and the image quality can be improved by adjusting the toner concentration and the developing conditions (such as the developing bias). As described above, the toner charge amount measurement mode according to the present embodiment is executed after the image forming apparatus 10 is left unused for a predetermined period of time, so that a change in the state of the developer can be grasped. Becomes

  As described above, in the toner charge amount measurement mode according to the present embodiment, development is performed without using a surface potential sensor that measures the potential on the photosensitive drum 20 or an ammeter that measures the development current flowing into the development roller 231. The charge amount of the toner stored in the device 23 can be obtained. As a result, it is possible to accurately determine whether the developer needs to be replaced in the developing device 23 or the need for adjusting the developing bias.

  In particular, the reference information stored in the storage unit 983 has a negative slope of the reference straight line when the charge amount of the toner is the first charge amount, and the charge amount of the toner is smaller than the first charge amount. The inclination of the reference straight line is positive when the second charge amount is small, and the inclination of the reference straight line is set to increase as the charge amount of the toner decreases. According to such a configuration, the charge amount of the toner is accurately determined based on the relationship between the frequency of the AC voltage of the developing bias and the density (developed toner amount) of the toner image formed on the photosensitive drum 20 (the intermediate transfer belt 141). Can get better.

<Calibration>
Generally, calibration in an image forming apparatus includes development bias calibration for adjusting image density, laser light amount calibration for maintaining dot reproducibility, data correction calibration for gradation adjustment, and multiple calibration. This includes color registration (registration) calibration of color toner images. Note that the calibration according to the present embodiment may include at least a developing bias calibration for adjusting the image density. In other words, the calibration means an image density optimizing process for optimizing the image density on the sheet by the developing bias.

  FIG. 8 is a flowchart of a calibration operation and a calibration correction operation performed in the image forming apparatus 10 according to the present embodiment. The series of flows including the calibration is executed at predetermined intervals according to the time counted by a timer (not shown) provided in the image forming apparatus 10 and the number of printed sheets printed in the image forming apparatus 10. Further, the flow may be executed when the image forming apparatus 10 is left unused all day and night or when the surrounding environment (temperature and humidity) of the image forming apparatus 10 changes rapidly.

  When the series of flows shown in FIG. 8 is started, the mode control unit 984 executes the above-described charge amount measurement mode before executing the calibration (Step S21 in FIG. 8). Here, the charge amount of the toner before the execution of the calibration is obtained. Next, the mode control unit 984 executes calibration in order to determine a developing bias to be applied to the developing roller 231 in a subsequent image forming operation (step S22 in FIG. 8).

  When the mode control unit 984 starts the calibration, the parameters (potential, laser light amount, rotation speed, etc.) other than the background portion potential V0 and the DC component Vdc of the developing bias in FIG. After fixing, the charging device 21 and the exposure device 22 are controlled to form a plurality of electrostatic latent images for calibration on the photosensitive drum 20. Further, the mode control unit 984 controls the developing bias applying unit 971 while maintaining V0-Vdc (background portion side potential difference) in FIG. 3B at a constant value, and controls at least the developing bias for each calibration electrostatic latent image. By making the DC voltage Vdc different, a plurality of calibration toner images are formed. At this time, it is desirable that three or more DC components Vdc of the developing bias are set and three or more calibration toner images are formed. Then, based on the relationship between the densities of the plurality of calibration toner images detected by the density sensor 100 and the DC component Vdc of the developing bias, the mode control unit 984 applies the voltage to the developing roller 231 in the subsequent image forming operation. Is determined. As an example, three Vdc and image density data are linearly regressed on a graph, and a DC component Vdcm of a developing bias suitable for a preset target image density is determined from the straight line.

  After determining the DC component Vdcm of the developing bias in step S22, the mode control unit 984 executes the above-described charge amount measurement mode again (step S23 in FIG. 8). Here, the charge amount of the toner after the execution of the calibration is obtained.

Next, the determination unit 985 determines whether it is necessary to correct the developing bias (Vdcm) determined in step S22 or to re-execute the calibration in step S22 (step S24). More specifically, the determining unit 985 calculates the change amount TV of the charge amount of the toner before and after the execution of the calibration by the following Expression 3.
TV = q2 / q1 (formula 3)
Here, q1 is the toner charge amount (before performing calibration) obtained in step S21 of FIG. 8 based on the above-described charge amount measurement mode, and q2 is the step of FIG. 8 based on the above-described charge amount measurement mode. This is the charge amount of the toner obtained after the execution of the calibration in S23. Then, the determination unit 985 compares the threshold value TA previously stored in the storage unit 983 with the above-described change amount TV. When TV> TA is satisfied (YES in step S24), the determination unit 985 determines that the change in the toner charge amount during execution of the calibration in step S22 is large. In this case, the mode control unit 984 executes a calibration correction operation (step S25). On the other hand, if TV ≦ TA is satisfied in step S24 (NO in step S24), the determination unit 985 determines that the change in the toner charge amount during the execution of the calibration in step S22 is small. In this case, the development bias (Vdcm) determined in step S22 is adopted in the subsequent image forming operation without performing the above-described calibration correction operation. In step S24, when the change amount TV is very large compared to the threshold value TA, it is estimated that the deterioration of the developer is progressing, so the flow of FIG. May be notified to a predetermined service center via a display panel (not shown) or via an Internet line or a telephone line.

  In step S25, the mode control unit 984 corrects the developing bias (Vdcm) based on the change amount TV of the charge amount of the toner. Table 1 shows the state of such calibration correction (case 1, case 2).

  In Table 1, the “toner charge amount” at the time of “calibration execution” is an average value of the toner charge amounts “before calibration execution” and “after calibration execution” acquired in the charge amount measurement mode. The “image portion potential VL” is a potential of an image portion (solid image) on the photosensitive drum 20 (see FIG. 3B).

  The image density of the toner image is determined by “development bias Vdc−image portion potential VL”. When the charge amount of the toner increases during the calibration, the image density decreases. Therefore, it is necessary to correct the determined developing bias Vdcm according to the amount of increase in the charge amount (change amount TV). Therefore, the mode control unit 984 multiplies the DC component Vdcm of the developing bias determined in step S22 by the change amount TV of the toner charging amount obtained from Expression 3 to obtain the corrected DC component Vdcr of the developing bias. calculate. As a result, in the subsequent image forming operation, the DC component Vdcr of the developing bias is adopted. When the image forming operation is started after the end of the flow of FIG. 8, the change amount TV of the toner charge amount obtained from Expression 3 according to the predetermined number of printed sheets is calculated based on the developing bias corrected in step S25. By further multiplying the DC component Vdcr, a DC component Vdcr ′ of the development bias that has been further corrected may be employed. The developing bias Vdcr (Vdcr ') is used until the next calibration is performed.

  As described above, in the present embodiment, the mode control unit 984 determines an appropriate developing bias in calibration according to the image density of the calibration toner image. On the other hand, the DC component Vdcm of the developing bias determined in the calibration greatly depends on the charge amount of the toner in the developing device 23. Therefore, if the charge amount of the toner greatly changes during the execution of the calibration, a deviation occurs between the actual charge amount of the toner and the determined developing bias Vdcm. Even in such a case, the mode control unit 984 executes the charge amount measurement mode and acquires the charge amount of the toner before and after the execution of the calibration. Then, the determining unit 985 determines whether the correction of the developing bias Vdcm determined in the calibration or the re-execution of the calibration is necessary based on the acquired toner charge amount. For this reason, a large difference between the developing bias Vdcm (Vdcr) determined in the calibration and the actual toner charge amount is suppressed, and a stable image can be formed in the image forming operation.

  As a result, prior to the calibration, the first screw feeder 232 and the second screw feeder 233 of the developing device 23 are driven to stir the developer, forcibly increasing the charge amount of the toner, and stabilizing the charge amount. Thus, compared to other techniques for performing calibration (aging operation), the downtime (unusable time) of the image forming apparatus 10 does not increase, and the deterioration of the developer due to forcible stirring does not occur. There is no promotion. Further, since the minimum necessary calibration is performed, the amount of toner consumed in the calibration that is repeatedly performed can be reduced.

  Further, in the present embodiment, the determination unit 985 compares the charge amounts of the toner before and after the execution of the calibration based on the difference or ratio thereof, thereby correcting the determined development bias Vdcm or re-executing the calibration. The necessity of execution can be easily determined.

  Further, in the present embodiment, the determined developing bias Vdcm is corrected in accordance with the amount of change (change amount TV) of the charge amount of the toner during the execution of the calibration. Therefore, a stable image can be formed in the image forming operation.

  Hereinafter, embodiments of the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples. In addition, each experimental condition in the implemented comparative experiment is as follows.

<Common experimental conditions>
Print speed: 55 sheets / min. Photoconductor drum 20: amorphous silicon photoconductor (α-Si)
Developing roller 231: Outer diameter 20 mm, surface knurl processing, 80 rows of recesses (grooves) formed along the circumferential direction.
・ Regulatory blade 234: SUS430, magnetic, 1.5mm thick
Amount of developer transported after the regulating blade 234: 250 g / m ²
A peripheral speed of the developing roller 231 with respect to the photosensitive drum 20 is 1.8 (in the trailing direction at the opposing position).
-Distance between the photosensitive drum 20 and the developing roller 231: 0.30 mm
-White background (background) potential V0 of the photosensitive drum 20: +270 V
-Image portion potential VL of photosensitive drum 20: + 20V
A developing bias of the developing roller 231: frequency = 6.0 kHz, Duty = 50%, Vpp = 1000 V AC rectangular wave, Vdc (DC voltage) = 200 V
・ Toner: positively charged polar toner, volume average particle diameter 6.8 μm, toner concentration 8%
・ Carrier: Volume average particle diameter 35μm, ferrite / resin coated carrier

<About developer>
Similar effects have been confirmed regardless of whether the toner is a pulverized toner or a toner having a core-shell structure. It was also confirmed that the same effect was obtained in the toner concentration range of 3% to 12%. Since the movement of the toner due to the AC electric field is more likely to occur as the magnetic brush becomes finer, the volume average particle diameter of the carrier is preferably 45 μm or less, more preferably 30 μm or more and 40 μm or less. Further, a resin carrier having a smaller true specific gravity than a ferrite carrier is more preferable. The measurement (actual measurement) of the charge amount of the toner during the experiment was performed using a suction type small charge amount measuring device Model 212HS manufactured by Trek.

<About Career>
The carrier is a ferrite core having a volume average particle diameter of 35 μm coated with silicon, fluorine, or the like. Specifically, the carrier was prepared by the following procedure. A coating solution is prepared by dissolving 20 parts by mass of a silicone resin KR-271 (manufactured by Shin-Etsu Chemical Co., Ltd.) in 200 parts by mass of toluene in 1000 parts by mass of a carrier core EF-35 (manufactured by Powder Tech). Then, the coating liquid was spray-coated by a fluidized-bed coating apparatus, and then heat-treated at 200 ° C. for 60 minutes to obtain a carrier. In this coating liquid, a conductive agent and a charge control agent are mixed and dispersed in a range of 0 to 20 parts with respect to 100 parts of the coat resin, respectively, to thereby perform resistance adjustment and charge adjustment.

  Under the above conditions, the charge amount of the toner was adjusted by changing the amount of the external additive of the toner, and the printing operation was performed. The results of this experiment are shown in FIGS. 4 and 5 described above. In FIG. 4, the image density of the toner image on the intermediate transfer belt 141 is measured by the density sensor 100, and the image density (sensor output) of the toner image obtained in advance and the toner formed on the printing paper (paper) are measured. Using the correlation curve with the image density of the fixed image, the toner image density is calculated based on the I.I. It is represented as D.

FIG. 5 shows the relationship between each toner charge amount and the slope of the straight line (approximate straight line) in FIG. Equation 4 (described below) of the approximate straight line shown in FIG. 5 is stored in the storage unit 983 in advance. Using this formula 4, the toner charge amount can be predicted.
Toner charge amount Q / M (μc / g) = − 442.32 × inclination + 29.87 (Equation 4)
Note that the slope of equation 4 = Δimage density / Δfrequency (see the slope of the graph in FIG. 4)

  As described above, the embodiment of the present invention has been described, but the present invention is not limited to this. For example, the following modified embodiment can be adopted.

  (1) In the above embodiment, the mode in which the surface of the developing roller 231 is knurled is described. However, the surface of the developing roller 231 having a concave shape (dimple) or the surface of the developing roller 231 is blasted. It may be something.

  (2) It is particularly desirable that the flow shown in FIG. 8 is executed after the image forming apparatus 10 has been left in a high-temperature and high-humidity state for a long time (8 hours or more). In such a case, the difference (change amount TV) of the toner charge amount before and after the execution of the calibration tends to increase, and the toner charge amount may further increase after the execution of the calibration. For this reason, it is desirable that the second calibration (FIG. 8) is executed after a predetermined number of prints or a predetermined time has elapsed. As an example, when the change amount (here, difference, q2−q1) of the charge amount before and after the execution of the calibration is 8 μc / g or more, the second calibration is executed, and the developing bias acquired at this time is used. The DC component Vdcm is employed. At this time, the charge amount measurement mode before the execution of the second calibration may be omitted, and the toner charge amount data in the charge amount measurement mode after the execution of the first calibration may be used. In this case, the amount of toner consumed in the charge amount measurement mode is reduced. On the other hand, when the difference between the toner charge amounts before and after the execution of the calibration is less than 8 μc / g, the second calibration is unnecessary, and the DC component Vdcm of the developing bias determined in the first calibration is employed. Just do it. As described above, it may be determined whether or not the re-execution of the calibration in step S22 is necessary instead of step S25.

  (3) When the image forming apparatus 10 has a plurality of developing devices 23 as shown in FIG. 1, the charge amount measurement mode according to the above-described embodiment is performed by one or two developing devices 23, and the result is determined by another developing device 23. 23 may be used.

  (4) Further, in the above embodiment, in the charge amount measurement mode, the mode control unit 984 converts the inclination of the measurement straight line and the reference information in the storage unit 983 into the measurement toner image formed on the photosensitive drum 20. Although the embodiment has been described in which the charge amount of the contained toner is obtained, the present invention is not limited to this. FIG. 9 is a cross-sectional view of the developing device 23 according to a modified embodiment of the present invention and a block diagram showing an electrical configuration of the control unit 980. FIG. 10 is a flowchart of a charge amount measurement mode executed in the image forming apparatus 10 according to the present modified embodiment.

  In the present modified embodiment, the image forming apparatus 10 further includes an ammeter 973.

  The ammeter 973 detects a DC current flowing between the developing roller 231 and the developing bias applying unit 971.

  In the charge amount measurement mode, the mode control unit 984 changes the frequency of the AC voltage of the developing bias while maintaining the potential difference of the DC voltage between the developing roller 231 and the photosensitive drum 20 constant. A plurality of measurement toner images are formed thereon. Then, the mode control unit 984 controls the distance between the developing roller 231 and the developing bias applying unit 971 when a plurality of measurement toner images corresponding to the difference in the density of the plurality of measurement toner images detected by the density sensor 100 are formed. Of the toner contained in the measurement toner image formed on the photosensitive drum 20 is acquired based on the ratio of the difference between the DC components of the developing current flowing through the photosensitive drum 20.

  Referring to FIG. 10, when mode control unit 984 starts the charge amount measurement mode (step S31), mode control unit 984 sets variable n for forming a plurality of measurement toner images to n = 1 (n = 1). Step S32). Then, the mode control unit 984 selects the image 1 corresponding to n = 1 stored in the storage unit 983 in advance (step S33). The storage unit 983 stores image information of an electrostatic latent image for forming the image n and information on the frequency of the AC voltage of the developing bias. Note that the other parameters relating to the image forming operation are set to the same values as in the immediately preceding image forming operation. Next, the mode control unit 984 controls the exposure device 22 (FIG. 1), the drive control unit 981 and the bias control unit 982 to apply a phenomenon bias for forming the image 1 to the developing roller 231. After rotating the roller 231 by one or more rotations, an electrostatic latent image of the measurement toner image corresponding to the image 1 is formed on the photosensitive drum 20. As the photoconductor drum 20 rotates, when the measurement toner image passes through the development nip NP where the photoconductor drum 20 and the development roller 231 face each other, toner is supplied to the electrostatic latent image, The toner image is developed (Step S34). During this developing operation, the developing current (DC current) is measured by the ammeter 973 (step S35).

  Thereafter, the toner image is transferred from the photosensitive drum 20 to the intermediate transfer belt 141 (Step S36). Then, the image density of the measurement toner image is measured by the density sensor 100 (step S37), and the acquired image density is stored in the storage unit 983 together with the value of the developing current measured in step S35 (step S38). ).

  Next, the mode control unit 984 determines whether or not the variable n for forming a plurality of measurement toner images has reached a preset specified number N (step S39). Here, if n ≠ N (NO in step S39), the value of n is counted up by one (n = n + 1, step S40), and steps S33 to S39 are repeated. In order to increase the accuracy of the charge amount measurement, it is preferable that the specified number of times N is equal to or greater than 2, and it is more preferable that 3 ≦ N. On the other hand, if n = N (YES in step S39), the mode control unit 984 estimates the charge amount of the toner (step S41), and ends the charge amount measurement mode (step S42).

As an example, when N = 2, the development currents (DC currents) of n = 1 and 2 measured in step S35 are defined as I1 and I2, respectively. The image densities of n = 1 and 2 measured in step S37 are defined as ID1 and ID2, respectively. At this time, the charge amount of the toner in step S41 corresponds to the slope a obtained from the following equation (5).
Slope a = (I1-I2) / (ID1-ID2) (Equation 5)
The above gradient a corresponds to the gradient of a straight line passing through two points when data (ID, I) at n = 1 and 2 are plotted, with the horizontal axis representing the image density ID and the vertical axis representing the developing current I. When the charge amount of the toner is measured under the condition of N = 3 or more, the slope a of the approximate straight line of the linear expression obtained by the least square method is used as the charge amount of the toner.

  As another modified embodiment, the parameter that is changed when a plurality of measurement toner images are formed is not the frequency of the AC voltage of the developing bias, but the printing of the electrostatic latent image formed by the exposure device 22. It may be a rate.

  That is, in the modified embodiment, the mode control unit 984 controls the exposure device 22 in a state where the potential difference of the DC voltage between the developing roller 231 and the photosensitive drum 20 is kept constant to control the printing rate per unit area. Are formed, a plurality of measurement toner images are formed on the photosensitive drum 20. Then, the mode control unit 984 controls the distance between the developing roller 231 and the developing bias applying unit 971 when a plurality of measurement toner images corresponding to the difference in the density of the plurality of measurement toner images detected by the density sensor 100 are formed. The charge amount of the toner included in the measurement toner image formed on the photoconductor drum 20 may be obtained based on the ratio of the difference between the DC components of the developing current flowing through the photoconductor drum 20. In this case, similarly to the above-described modified embodiment, the charge amount of the toner can be obtained based on Expression 5.

10 Image forming apparatus 100 Density sensor (density detection unit)
14 Intermediate transfer unit (transfer section)
145 Secondary transfer roller (transfer section)
20 Photoconductor drum (image carrier)
23 Developing device 231 Developing roller 971 Developing bias applying unit 972 Driving unit 980 Control unit 981 Driving control unit 982 Bias control unit 983 Storage unit 984 Mode control unit (charge amount acquisition unit, calibration operation execution unit)
985 Judgment unit (calibration operation judgment unit)

Claims (7)

Rotated, an electrostatic latent image is formed on the surface, and an image carrier that carries a toner image in which the electrostatic latent image has been revealed;
A charging device for charging the image carrier to a predetermined charging potential,
The electrostatic latent image is formed by exposing the surface of the image carrier charged to the charging potential in accordance with predetermined image information, which is disposed downstream of the charging device in the rotation direction of the image carrier. An exposure apparatus,
A developing device arranged to face the image carrier at a predetermined developing nip portion on the downstream side in the rotation direction from the exposure device, the developer device being rotated and carrying a developer composed of toner and carrier on a peripheral surface thereof. A developing device that includes a developing roller that forms the toner image by supplying toner to the image carrier;
A transfer unit that transfers the toner image carried on the image carrier to a sheet,
A developing bias application unit that can apply a developing bias in which an AC voltage is superimposed on a DC voltage to the developing roller;
A density detector for detecting the density of the toner image,
Controlling the charging device and the exposure device to form a plurality of electrostatic latent images for calibration on the image carrier, and controlling the developing bias applying unit to perform the developing for each electrostatic latent image for calibration; A plurality of calibration toner images are formed by making at least the DC voltage of a bias different, based on a relationship between the density of the plurality of calibration toner images detected by the density detection unit and the developing bias. A calibration operation execution unit that executes a calibration operation to determine the developing bias applied to the developing roller in an image forming operation;
Before the execution of the calibration operation and after the execution of the calibration operation, the control unit controls the charging device, the exposure device, and the developing bias applying unit to form a plurality of measurement toners having different toner development amounts on the image carrier. An image is formed, and the plurality of measurement toner images are formed based on the density of the plurality of measurement toner images detected by the density detection unit or in addition to the density of the plurality of measurement toner images. Charging based on a DC component of a developing current flowing between the developing roller and the developing bias applying unit to obtain a charge amount of toner contained in the measurement toner image formed on the image carrier. A charge amount acquisition unit that performs an amount acquisition operation;
The development determined in the calibration operation from the charge amount of the toner before the execution of the calibration operation and the charge amount of the toner after the execution of the calibration operation, acquired by the charge amount acquisition unit. A calibration operation determination unit that determines whether correction of bias or re-execution of the calibration operation is necessary,
An image forming apparatus comprising:   The calibration operation determination unit is configured to obtain a difference or a ratio of a charge amount of the toner after the execution of the calibration operation to a charge amount of the toner before the execution of the calibration operation, which is acquired by the charge amount acquisition unit. The image forming apparatus according to claim 1, wherein when it is larger than a preset threshold, it is determined that correction of the developing bias determined in the calibration operation or re-execution of the calibration operation is necessary.   The calibration operation determination unit is configured to obtain a charge amount of the toner after execution of the calibration operation with respect to a charge amount of the toner before execution of the calibration operation, the ratio being obtained by the charge amount acquisition unit. The image forming apparatus according to claim 2, wherein the developing bias determined in the calibration operation is corrected so as to change a DC voltage difference between the image carrier and the developing roller. When the frequency of the AC voltage of the developing bias is changed in a state where the potential difference of the DC voltage between the developing roller and the image carrier is kept constant, the density change of the toner image with respect to the amount of change of the frequency. A storage unit that stores in advance reference information on the inclination of the reference straight line indicating the relationship between the amounts, for each charge amount of the toner;
The charge amount acquisition unit is configured to change the frequency of the AC voltage of the developing bias while maintaining the potential difference of the DC voltage between the developing roller and the image carrier constant, and the plurality of the plurality of charging units are arranged on the image carrier. The measurement toner image is formed, and the inclination of the measurement straight line indicating the relationship between the amount of change in the density of the measurement toner image and the amount of change in the frequency is expressed by the amount of change in the frequency and the measurement toner by the density detection unit. The toner charge amount included in the measurement toner image formed on the image carrier is obtained from the density detection result of the image and the obtained inclination of the measurement straight line and the reference information of the storage unit. The image forming apparatus according to claim 1, wherein the image forming apparatus acquires the information.   The reference information stored in the storage unit is such that when the charge amount of the toner is the first charge amount, the slope of the reference straight line is negative, and the charge amount of the toner is the first charge amount. The inclination of the reference straight line is positive when the second charge amount is smaller than the second charge amount, and the inclination of the reference straight line is set to increase as the charge amount of the toner decreases. Item 5. The image forming apparatus according to Item 4.   The charge amount acquisition unit is configured to change the frequency of the AC voltage of the developing bias while maintaining the potential difference of the DC voltage between the developing roller and the image carrier constant, and the plurality of the plurality of charging units are arranged on the image carrier. Forming the measurement toner image, and applying the developing roller and the developing bias when the plurality of measurement toner images are formed with respect to the density difference of the plurality of measurement toner images detected by the density detection unit. 4. The charge amount of toner contained in a measurement toner image formed on the image carrier is acquired based on a ratio of a difference between DC components of the developing current flowing between the image carrier and the developing unit. 5. The image forming apparatus according to claim 1. The charge amount acquisition unit controls the exposure device in a state where the potential difference of the DC voltage between the developing roller and the image carrier is kept constant, and changes the image carrying rate while changing the printing rate per unit area. Forming the plurality of measurement toner images on the body, and forming the plurality of measurement toner images with respect to a difference in density between the plurality of measurement toner images detected by the density detection unit; Acquiring a charge amount of toner contained in a measurement toner image formed on the image carrier based on a ratio of a difference between DC components of the developing current flowing between the developing bias applying unit and the developing bias applying unit. Item 4. The image forming apparatus according to any one of Items 1 to 3.