JP2020034800A - Image forming apparatus - Google Patents

Abstract

To prevent the occurrence of toner scattering during an image forming operation after replacement of a toner supply unit without unnecessarily keeping a user waiting, in an image forming apparatus including a developing device applied with a two-component developing system.SOLUTION: A replacement control unit 985, when a toner supply unit 15 is replaced, executes a toner supply operation of supplying a toner to a developing device 23 with the toner supply unit 15 until a toner concentration sensor 101 detects a toner concentration larger than a first threshold TH1, and an aging operation of causing first and second screw feeders 233, 232 to perform a stirring operation in a state where a developing bias is not applied to a developing roller 231. After completion of the toner supply operation, the replacement control unit causes a mode control unit 984 to repeatedly execute a charge amount acquisition mode until the amount of charge Q/M of toner acquired in a charge amount measurement mode becomes larger than a second threshold TH2, and subsequently ends the aging and outputs a permission signal permitting an image forming 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, an image forming apparatus including a developing device that develops an electrostatic latent image formed on an image carrier into a toner image using a two-component developer including a toner and a carrier has been known. Such an image forming apparatus generally includes a replaceable toner replenishing unit that stores replenishing toner and replenishes the stored toner to the developing device, and a toner amount based on a carrier amount included in the developer. And a toner density detecting unit for detecting the toner density which is the ratio of the toner density. Each time an image is formed, the toner replenishing unit replenishes the developing device with a replenishing amount of toner according to the toner consumption based on the image data to be formed and the toner density value detected by the toner density detecting unit.

  Such an image forming apparatus once prohibits execution of the image forming operation in order to replace the toner supply unit. Then, the image forming apparatus supplies a predetermined amount of toner to the developing device by the toner replenishing unit, and agitates and mixes the toner supplied to the developing device and the developer in the developing device. Then, when the toner density detected by the toner density sensor reaches a predetermined target value, execution of the image forming operation is permitted. Therefore, in order to start the image forming operation after the replacement of the toner replenishing unit, it is necessary to stir and mix the replenished toner with the developer in the developing device until the toner concentration reaches a predetermined target value. Met.

  Therefore, for example, in Patent Document 1, the target value of the toner density for permitting the execution of the image forming operation is set to a target value lower than the target value of the toner density at the time of performing the image forming operation. Techniques for reducing the time required for stirring and mixing have been proposed.

  On the other hand, a technique for accurately estimating the charge amount of toner has conventionally been proposed. In Patent Documents 2 and 3, while the surface potential of the photosensitive drum before development and the surface potential of the toner layer on the photosensitive drum after development are respectively measured, based on the image density measurement results of the developed toner layer 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 4 and 5, 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 2005-62848 A JP 2003-345075 A JP 2004-37952 A Japanese Patent No. 5024192 Patent No. 5273542

  However, in the technique disclosed in Patent Document 1, the target value of the toner density for permitting the image forming operation is set to a value lower than the target value of the toner density at the time of performing the image forming operation. For this reason, if high-density printing is performed after resuming the image forming operation, there is a possibility that toner supply may not be able to catch up.

  In the techniques described in Patent Documents 2 and 3, 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 4 and 5, the current flowing into the developing roller includes the current flowing in the carrier in addition to the current flowing in the toner. 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. Therefore, with the techniques described in Patent Documents 4 and 5, it is difficult to accurately measure the charge amount of the toner from the current flowing into the developing roller.

  As described above, it is difficult to accurately measure the charge amount of the toner with the conventional technique of measuring the charge amount of the toner.

  SUMMARY An advantage of some aspects of the invention is to solve a problem as described above, and in an image forming apparatus including a developing device to which a two-component developing method is applied, after replacing a toner supply unit without waiting a user more than necessary. It is an object of the present invention to suppress the occurrence of toner scattering during the image forming operation.

  An image forming apparatus according to an aspect of the present invention is an image forming apparatus that performs an image forming operation of forming an image on a sheet, and includes an apparatus main body, which is rotated to form an electrostatic latent image on a surface, An image carrier that carries the toner image in which the electrostatic latent image is revealed, a charging device that charges the image carrier to a predetermined charging potential, and a surface of the image carrier that is charged to the charging potential. An exposure device that forms the electrostatic latent image by exposing according to predetermined image information, and a developing device that is disposed to face the image carrier at a predetermined development nip portion, and includes a toner and a carrier. A developer transporting unit that performs a stirring operation of transporting the developer while stirring, and rotates the developer transported by the developer transporting unit on the peripheral surface and supplies the toner to the image carrier. The toner image A developing device including a developing roller to be formed, a toner replenishing unit that is exchangeably mounted on the device main body, stores the toner therein, and replenishes the stored toner to the developing device; A transfer unit for transferring the toner image carried on the image carrier to a sheet, a developing bias applying unit capable of applying a developing bias in which an AC voltage is superimposed on a DC voltage to the developing roller, and a density of the toner image A toner concentration detector that detects a toner concentration indicating a ratio of a toner amount to a carrier amount contained in the developer, and controls the charging device, the exposure device, and the developing bias applying unit. Forming a plurality of measurement toner images having different toner development amounts from each other on the image carrier, and determining the density of the plurality of measurement toner images detected by the density detection unit. Based on the DC component of the developing current flowing between the developing roller and the developing bias applying unit when the plurality of measuring toner images are formed in addition to the densities of the plurality of measuring toner images. A charge acquisition unit that performs a charge acquisition operation for acquiring a charge amount of the toner based on the toner supply unit, and outputs a prohibition signal that prohibits the image forming operation with replacement of the toner supply unit. An exchange control unit that executes a predetermined preparation operation when exchanged, wherein the exchange control unit detects in the preparation operation that the toner concentration detection unit detects a toner concentration larger than a predetermined first threshold value Until the toner replenishing unit supplies the toner to the developing device, the toner replenishing unit supplies the toner to the developing unit. Performing an aging operation for performing the operation, and after the toner replenishment operation is completed, the charge amount obtaining unit until the charge amount of the toner obtained in the charge amount obtaining operation becomes larger than a second predetermined threshold value. After the charge amount obtaining operation is repeatedly performed, the aging operation is terminated, and a permission signal for permitting the image forming operation is output.

  According to this configuration, in the charge amount acquisition operation, the charge amount acquisition unit uses the densities of the plurality of measurement toner images, or the densities of the plurality of measurement toner images and the formation of the plurality of measurement toner images. Based on the DC component of the obtained development current, the charge amount of the toner included in the plurality of measurement toner images can be accurately acquired.

  For this reason, in the aging operation when the toner replenishment unit is replaced, the replacement control unit causes the stirring operation to be performed only for an appropriate period in accordance with the charge amount of the toner accurately acquired by the charge amount acquisition operation. After that, the image forming operation can be permitted. This can reduce the risk that the stirring operation is performed for an unnecessarily long time or the risk that the stirring operation is not performed properly and the image forming operation is performed while the charge amount of the toner is unstable. As a result, it is possible to suppress the occurrence of toner scattering during the image forming operation after the replacement of the toner supply unit without waiting for the user more than necessary.

  Further, in the preparation operation, when the charge amount of the toner acquired in the charge amount acquisition operation is equal to or less than the second threshold value, and in the preparation operation, and the time elapsed since the start of the aging operation, May be larger than a third threshold value, the charge amount acquisition unit may terminate the repeated execution of the charge amount acquisition operation, and then terminate the aging operation and output the permission signal.

  According to this configuration, the replacement control unit can prevent the aging operation when the toner supply unit is replaced from being performed for a time longer than the third threshold. Accordingly, it is possible to reduce the risk that the time during which the image forming operation cannot be performed after the replacement of the toner replenishing unit becomes excessively long while allowing a certain amount of toner scattering.

  Alternatively, the exchange control unit may be configured such that, in the preparation operation, when the charge amount of the toner acquired in the charge amount acquisition operation is equal to or less than the second threshold, and the number of times the charge amount acquisition operation is repeated is a predetermined number. When the charge amount is larger than the fourth threshold value, the charge amount acquisition unit may terminate the repetitive execution of the charge amount acquisition operation, and then terminate the aging operation and output the permission signal.

  According to this configuration, during the aging operation when the toner supply unit is replaced, the replacement control unit repeats the charge amount acquisition operation for acquiring the charge amount of the toner a number of times greater than the fourth threshold. That can be avoided. Accordingly, it is possible to reduce the risk that the time during which the image forming operation cannot be performed after the replacement of the toner replenishing unit becomes excessively long while allowing a certain amount of toner scattering.

  In the above configuration, it is preferable that the second threshold value is predetermined in accordance with a driving time of the developing device and a temperature and a humidity around the developing device.

  According to this configuration, when the toner replenishment unit is replaced, the replacement control unit appropriately performs the aging operation until the charge amount of the toner reaches a value corresponding to the driving time of the developing device and the temperature and humidity of the surroundings. It can be performed.

  In the above configuration, it is preferable that the toner concentration detection unit is a toner concentration sensor that detects the toner concentration of the developer conveyed by the developer conveyance unit.

  According to this configuration, in the toner supply operation, the exchange control unit supplies the developing device with an amount of toner corresponding to the toner concentration of the developer conveyed by the developer conveyance unit, which is actually detected by the toner concentration sensor. Can be replenished. For this reason, the replacement control unit estimates the toner density from the toner consumption amount and the toner supply amount estimated from the image data and the operation time of the toner replenishing unit, and determines the amount of toner corresponding to the estimated toner concentration. An appropriate amount of toner can be supplied to the developing device as compared with the case where the toner is supplied to the developing device.

  In the above configuration, the developing device includes a toner supply port that receives toner supplied by the toner supply unit, and the toner concentration sensor faces the developing roller from the toner supply port by the developer transport unit. It is desirable to detect the toner concentration of the developer conveyed toward a position.

  According to this configuration, the toner concentration sensor detects the toner concentration of the developer that is conveyed from the toner supply port toward the position facing the developing roller when the toner is not supplied and consumed, so that a stable toner having a small variation is provided. The concentration can be detected. Accordingly, the replacement control unit can supply a more appropriate amount of toner to the developing device in the toner supply 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, toner scattering occurs during an image forming operation after replacing a toner supply unit without waiting for a user more than necessary. Can be suppressed.

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. 1 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. 1 is a plan view of a developing device according to an embodiment of the present invention. FIG. 3 is a schematic diagram illustrating a developing operation of the image forming apparatus according to the embodiment of the present disclosure. FIG. 3 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. 7 is a graph showing the relationship between the inclination of the graph of FIG. 6 and the charge amount of toner 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. 4 is a schematic diagram of a measurement toner image formed on an image carrier when a charge amount measurement mode is executed. 6 is a flowchart of a container exchange mode executed in the image forming apparatus according to the embodiment of the present disclosure. It is a figure showing an example of the 2nd threshold. FIG. 4 is a cross-sectional view of a developing device according to a first 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 the image forming apparatus according to the first modified embodiment of the present invention. 9 is a flowchart of a container exchange mode executed in the image forming apparatus according to the second embodiment of the present invention. 9 is a flowchart of a container exchange mode executed in the image forming apparatus according to the third embodiment of the present invention. It is a figure showing an example of an experiment result of container exchange mode.

  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.

<About the image forming apparatus>
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, a toner replenishing unit 15 for replenishing toner to the image forming unit 13, and a fixing unit 16 for fixing an unfixed toner image formed on the sheet P to the sheet P are provided therein. . 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 18 for allowing a user to operate the image forming apparatus 10 is provided at an appropriate position on the upper surface of the apparatus main body 11. On the operation panel 18, a liquid crystal display that displays information about the image forming apparatus 10, such as an operation state of the image forming apparatus 10 (for example, during printing), an output condition of an image on a sheet P, and the like, relate to operations of the image forming apparatus 10. A touch panel for inputting information and various operation keys are provided.

  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 arranged to face the photosensitive drum 20 at a predetermined developing nip portion NP (FIG. 4) 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 is rotated 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 a plurality of density sensors 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 plurality of density sensors 100 (density detection units) are positioned at predetermined intervals in the main scanning direction (the axial direction of the photosensitive drum 20) at a position downstream of the units 13M, 13C, 13Y, and 13Bk and facing the intermediate transfer belt 141. Are arranged side by side. Each density sensor 100 detects the density of a toner image formed on the intermediate transfer belt 141 at a position facing itself. In the control unit 980 described later, the average value of the density of the toner image detected by the plurality of density sensors 100 is used as the density of the toner image.

  In another embodiment, the plurality of density sensors 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. Good. Hereinafter, for convenience of description, the average value of the density of the toner image detected by the plurality of density sensors 100 will be referred to as the density of the toner image detected by the density sensor 100.

  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 section 15 includes a magenta toner container 15M, a cyan toner container 15C, a yellow toner container 15Y, and a black toner container 15Bk that store replenishment toners of each color of M / C / Y / Bk. These toner containers 15M, 15C, 15Y, and 15Bk supply the replenishment toner contained therein to the developing devices 23 of the image forming units 13M, 13C, 13Y, and 13Bk corresponding to the respective colors of M / C / Y / Bk.

  More specifically, each of the toner containers 15M, 15C, 15Y, and 15Bk is configured to be detachable in the apparatus main body 11, and is replaceable. The toner containers 15M, 15C, 15Y, and 15Bk each include a rotating member 15S for transporting the stored toner of each color, and a toner outlet 15H connected to the developing device 23 by a toner transport path (not shown). Is provided.

  The rotating member 15S is configured to include a rotating shaft extending in a direction perpendicular to the paper surface of FIG. 1 and spiral wings (screw wings) protruding in a spiral shape around the rotating shaft. The rotation member 15S rotates around the rotation axis by a designated angle under the control of the control unit 980 described later. Accordingly, the toner containers 15M, 15C, 15Y, and 15Bk are respectively transferred from the toner discharge port 15H to the developing devices 23 of the image forming units 13M, 13C, 13Y, and 13Bk corresponding to the respective colors of M / C / Y / Bk. The supply amount of each color toner is supplied according to the angle.

  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. FIG. 3 is a plan view of the developing device 23. The developing device 23 includes a developing housing 230, a first screw feeder 233 (developer conveying unit), a second screw feeder 232 (developer conveying unit), a developing roller 231, 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 portion 230H is covered by a top plate 235 (FIG. 2) and is divided into a first transport portion 230A and a second transport portion 230B by a partition plate 230E extending in the front-rear direction. The partition plate 230E is shorter than the width of the developing housing 230 in the front-rear direction. The first transport unit 230A and the second transport unit 230B are provided in front of the front end (hereinafter, the front end) of the partition plate 230E and behind the rear end (hereinafter, the rear end) of the partition plate 230E. A first communication portion 230C and a second communication portion 230D are provided to communicate with each other. As a result, a circulation path is formed in the developer accommodating portion 230H for repeatedly moving the developer in the order of the first transport portion 230A, the first communication portion 230C, the second transport portion 230B, and the second communication portion 230D. That is, the toner is conveyed clockwise in FIG. 3 in the circulation path. Further, a toner supply port 236 is provided in the developer accommodating section 230H. The toner supply port 236 is connected to a toner discharge port 15H provided in each of the toner containers 15M, 15C, 15Y, and 15Bk (FIG. 1) by a toner transport path (not shown).

  The toner supply port 236 is an opening formed in the top plate 235 (FIG. 2), and is disposed above the vicinity of the upstream end (front end) in the developer conveyance direction (D1 direction) in the first conveyance section 230A. (FIG. 3). The toner supply port 236 is arranged to face the above-described circulation path, and receives the supply toner supplied from the toner supply section 15 (FIG. 1) into the developer accommodating section 230H. For example, the toner supply port 236 has an opening of 14 mm × 8 mm in plan view.

  The first screw feeder 233 is provided in the first transport section 230A. The first screw feeder 233 includes a first rotating shaft 233A, and first spiral blades 233B (screw blades) protruding in a spiral shape around the first rotating shaft 233A. The first screw feeder 233 is rotated in the direction of the arrow D23, and is transported in the first transport direction D1 while stirring and mixing the new toner flowing from the toner supply port 236 and the developer in the developer accommodating portion 230H. I do.

  A first paddle 233c is disposed downstream of the first screw feeder 233 in the toner transport direction (direction D1). The first paddle 233c is a rib member extending in the axial direction for one pitch of the first spiral blade 233B. The first paddle 233c is rotated together with the first rotation shaft 233A, and transfers the toner from the first transport unit 230A to the second transport unit 230B in the direction of arrow D3 in FIG.

  The second screw feeder 232 is disposed to the left and below the developing roller 231 so as to face the developing roller 231. As the second screw feeder 232 rotates in the direction of the arrow D22, toner is supplied from the second screw feeder 232 to the developing roller 231. In the present embodiment, the toner supply path to the developing roller 231 is formed only by the path supplied from the second screw feeder 232. Therefore, when the toner is pumped upward from below by the second screw feeder 232, the toner is supplied to the developing roller 231.

  A second paddle 232C is disposed downstream of the second screw feeder 232 in the toner transport direction (D2 direction). The second paddle 232C is a plate-like member provided on the rotation axis of the second screw feeder 232. The second paddle 232C is rotated together with the rotation shaft, and transfers the toner from the second transport unit 230B to the first transport unit 230A in the direction of arrow D4.

  That is, the first screw feeder 233 and the second screw feeder 232 function as a developer transport unit that performs a stirring operation of transporting the developer including the toner and the carrier while stirring. Hereinafter, the first screw feeder 233 and the second screw feeder 232 are collectively referred to as a developer transport unit.

  Further, on the outer surface of the developing housing 230, a toner density sensor 101 (toner density detecting unit) is arranged. The toner density sensor 101 detects the toner density of the developer conveyed by the developer conveyance units (the first screw feeder 233 and the second screw feeder 232). Specifically, the toner density sensor 101 detects the toner density of the developer conveyed from the toner supply port 236 to a position facing the developing roller 231 by the developer conveying section. More specifically, the toner density sensor 101 is disposed near the downstream end (rear end) of the outer surface of the developing housing 230 in the developer conveying direction (D1 direction), and is moved from the first conveying unit 230A to the second conveying unit by the first paddle 233c. The toner concentration of the developer immediately before being delivered to 230B is detected. The toner density is the ratio of the amount of toner to the amount of carrier contained in the developer (= toner amount / carrier amount).

  The developing roller 231 is arranged to face the photosensitive drum 20 at the developing nip NP (FIG. 4). 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 this embodiment, the developing roller 231 rotates in the same direction (with direction) at a position facing the photosensitive drum 20.

  The regulating blade 234 is arranged at a predetermined interval on the developing roller 231, and regulates the layer thickness of the developer supplied from the second 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 drive unit 972 is composed of a motor and a gear mechanism for transmitting the torque thereof, and receives light during the execution of an image forming operation, a charge amount measurement mode, and a container exchange mode described below in response to a control signal from a drive control unit 981 described below. In addition to the body drum 20 and the like, the rotating member 15S in the toner supply unit 15, the developing roller 231 in the developing device 23, the first screw feeder 233, and the second screw feeder 232 are driven to rotate. The driving unit 972 further generates a driving force for driving (rotating) other members of the image forming apparatus 10. The image forming operation includes driving the photosensitive drum 20, the charging device 21, the exposing device 22, the developing device 23, the primary transfer roller 24, the intermediate transfer unit 14, the secondary transfer roller 145, and the fixing unit 16 to form a sheet P. The operation of forming an image will be described.

  The control unit 980 controls the drive control unit 981, the bias control unit 982, the storage unit 983, the mode control unit 984 (charge acquisition unit), and the exchange control unit 985 by the CPU executing the control program stored in the ROM. It works to have

  The drive control unit 981 controls the drive unit 972 to rotationally drive the rotating member 15S, the developing roller 231, the first screw feeder 233, and the second screw feeder 232. 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 exchange control unit 985. For example, the storage unit 983 stores the value of the developing bias adjusted according to the rotation speed of the developing roller 231 and the environment.

  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 relating to the inclination of the reference straight line indicating the relationship between the toner image density change amount and the toner image density change amount 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. 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).

  In the charge amount measurement mode, the mode control unit 984 controls the charging device 21, the exposure device 22, the developing bias applying unit 971, and the like, to form a plurality of measurement toner images having different toner development amounts on the photosensitive drum 20 from each other. Form. The mode control unit 984 forms the plurality of measurement toner images based on the density 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. At this time, the charge amount of the toner is obtained based on the DC component of the developing current flowing between the developing roller 231 and the developing bias applying unit 971.

  More specifically, in the charge amount measurement mode, the mode control unit 984 changes the photosensitive bias while changing 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 body drum 20. Thereafter, the mode control unit 984 calculates the inclination of the measurement straight line indicating the relationship between the amount of change in the frequency and the amount of change in the density of the toner image for measurement. The charge amount of the toner contained in the measurement toner image formed on the photoconductor drum 20 is acquired from the density detection result and from the acquired inclination of the measurement straight line and the reference information in the storage unit 983. .

  The replacement control unit 985 outputs a prohibition signal for prohibiting the image forming operation in accordance with replacement of one or more of the four toner containers 15M, 15C, 15Y, and 15Bk provided in the toner replenishing unit 15. When one or more of the toner containers 15M, 15C, 15Y, and 15Bk are replaced, a container replacement mode (preparation operation) is executed.

  More specifically, the exchange control unit 985 can be used, for example, when a tag attached to each toner container (for example, an RFID tag or an IC tag) cannot be read by a tag reader provided near the arrangement position of each toner container. Then, it is detected that the respective toner containers have been detached from the apparatus main body 11. Upon detecting that each toner container has been detached from the apparatus main body 11, the replacement control unit 985 outputs a prohibition signal for prohibiting the image forming operation.

  For example, the replacement control unit 985 stops the driving of each unit in the image forming apparatus 10 and stores a flag indicating that the image forming operation is prohibited in the storage unit 983, thereby prohibiting the image forming operation from being prohibited. Flag) is output (stored). The replacement control unit 985 is not limited to this. For example, when a toner container to be replaced is specified using the operation panel 18 and an instruction to replace the specified toner container is input, the image forming operation is performed. May be output.

  Then, after outputting the prohibition signal for prohibiting the image forming operation as described above, the replacement control unit 985 causes the respective toner containers to be removed when the tag attached to each toner container is read by the tag reader. Judge that the container has been replaced, and execute the container replacement mode. However, the present invention is not limited to this, and the exchange control unit 985 determines that each of the toner containers has been exchanged when an instruction indicating that the exchange of each toner container has been completed is input using, for example, the operation panel 18 or the like. Then, the container exchange mode may be executed, or the container exchange mode may be executed when there is a possibility that the container has been exchanged such as opening and closing the cover.

  The replacement control unit 985 performs the replacement in the container replacement mode until the toner density sensor 101 disposed in the developing device 23 corresponding to the replaced toner container detects a toner density higher than a predetermined first threshold. The toner supply operation is performed to supply the toner to the developing device 23 corresponding to the toner container by the used toner container. Further, the exchange control unit 985 executes an aging operation for causing the first screw feeder 233 and the second screw feeder 232 to perform the stirring operation in a state where the developing bias is not applied to the developing roller 231.

  After the completion of the toner supply operation, the exchange control unit 985 repeats the charge amount measurement mode by the mode control unit 984 until the charge amount of the toner acquired in the charge amount measurement mode becomes larger than the second predetermined threshold value. After the execution, the aging operation is terminated, and a permission signal for permitting the image forming operation is output. For example, the exchange control unit 985 outputs a permission signal for permitting the image forming operation by deleting the flag indicating that the image forming operation is prohibited, stored in the storage unit 983. The replacement control unit 985 is not limited to this, and may output a permission signal for permitting the image forming operation by a method according to the method of outputting the prohibition signal for prohibiting the image forming operation.

<Developing operation>
FIG. 4 is a schematic diagram of a developing operation of the image forming apparatus 10 according to the present embodiment. FIG. 5 is a schematic diagram illustrating the magnitude relationship between the potentials of the photosensitive drum 20 and the developing roller 231. As shown in FIG. 4, a developing nip NP is formed between the developing roller 231 and the 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. As shown in FIG. 5, the surface potential of the photosensitive drum 20 is charged by the charging device 21 to the 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. The background portion of the photoconductor drum 20 is an area on the surface of the photoconductor drum 20 where no electrostatic latent image is formed. 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. The image portion of the photosensitive drum 20 is an area on the surface of the photosensitive drum 20 where an electrostatic latent image is formed. 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. 6 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. 7 is a graph showing the relationship between the inclination of the graph of FIG. 6 and the charge amount of the toner 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. 6). That is, as shown in FIG. 6, 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. 6 increases.

  As shown in FIG. 7, the relationship between the slopes of the three graphs in FIG. 6 and the charge amount of each toner is distributed on a straight line (approximate straight line). Therefore, if the information shown in FIG. 7 is stored in the storage unit 983 in advance, and the inclination of the straight line shown in FIG. 6 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, the following effect is further provided in addition to the effect at the time of replacing the toner container described later. That is, it is not necessary to provide a surface potential sensor for measuring the surface potential on the photosensitive drum 20 in order to estimate 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, it is possible to stably predict the charge amount of the toner without being affected by the change in the current flowing into the developing roller 231 due to the contamination of the surface potential sensor and the change in the resistance of the carrier. For this reason, when the image density printed in the image forming apparatus 10 decreases, 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 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 “charge of toner”. Increase in the amount ”. In this case, if the image density is decreased due to factors other than the increase in the charge amount of the toner, and if the toner concentration is increased in order to reduce the charge amount of the toner, a new problem such as toner scattering occurs. there's a possibility that. It is desirable to reduce the toner charge amount by increasing the toner density to reduce the image density caused by the increase in the toner charge amount, and to develop the image density by reducing the image density due to other factors. It is preferable to increase the electric field (development bias). Further, by grasping the charge amount of the toner, 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 charge amount of the toner 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 not affected. 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 decreases as described above, the number of reciprocating movements of the toner in the developing nip portion NP decreases. Is hardly detached from the carrier, so that 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. 8 is a flowchart of the charge amount measurement mode executed in the image forming apparatus 10 according to the present embodiment. FIG. 9 is a schematic diagram of a measurement toner image formed on the photosensitive drum 20 when the charge amount measurement mode is executed.

  As shown in FIG. 8, when the charge amount measurement mode is started (step S01), the mode control unit 984 sets a 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 developing 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 previous image forming history. Normally, a bias used for printing (image formation) is applied to the reference developing bias condition. If 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 toner image for measurement 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. Note that, in order to increase the accuracy of the charge amount measurement, it is preferable that the prescribed number of times N = 2 or more, and it is more preferable that 3 ≦ N be set.

  On the other hand, if n = N (YES in step S08), mode control section 984 calculates the slope of the approximate straight line shown in FIG. 6 based on the information stored in storage section 983 (step S10). . Then, the mode control unit 984 estimates the charge amount of the toner from the inclination based on the graph (reference information) shown in FIG. 7 stored in the storage unit 983 (step S11), and sets the charge amount measurement mode. Is completed (step S12).

  FIG. 9 shows an example in which, when the prescribed number of times N = 3, the image density of the measurement toner image is increased by increasing the frequency f. 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 charge amount of the toner, and is “positive (+)” when the charge amount of the toner is low, and “negative (−)” when the charge amount of the toner 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. The reference information shown in FIG. 7 is expressed by Expression 2.
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 amount of charge of the toner per unit mass. By substituting the slope a of the approximate straight line calculated from Equation 1 in Step S10 into Equation 2, the toner charge amount Q / M is calculated.

  The charge amount measurement mode shown in FIG. 8 may be executed for each of the developing devices 23 of 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 a result of the previous toner charge amount measurement mode. In this case, the accuracy of the measured charge amount of the toner 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. For example, the charge amount measurement mode automatically started is performed after the end of the toner supply operation in the container replacement mode described later.

  In addition, it is desirable that the charge amount measurement mode be executed when the image forming apparatus 10 is shipped from the factory after manufacturing and when the image forming apparatus 10 is set up at the 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, when the idle period is long, the amount of charge of the toner tends to decrease, and the level of the tendency often differs depending on the period during which the developer is left and the environment. Therefore, by measuring the charge amount of the toner at the time of shipment from the factory and at the time of set-up of the main body, the deterioration state due to leaving the developer is predicted, and when the leaving time is extremely long or when the developer is left in a poor environment, The difference between the charge amounts of the two toners (the charge amounts of the toner 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 charge amount of the toner at the time of factory shipment and the setup of the main body is low, if the difference between the charge amounts of the two toners is small, the possibility that the developer has deteriorated is low. Therefore, it is not necessary to change the developer at the place of use, and the image quality can be improved by adjusting the toner concentration and the developing conditions (developing bias and the like). As described above, the change in the state of the developer can be grasped by executing the charge amount measurement mode according to the present embodiment after the image forming apparatus 10 is left unused for a predetermined period of time without being used. Become.

  As described above, in the charge amount measurement mode according to the present embodiment, the developing device does not use the surface potential sensor that measures the potential on the photosensitive drum 20 or the ammeter that measures the developing current flowing into the developing roller 231. The charge amount of the toner stored in the storage unit 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.

<Improvement of efficiency in charge amount measurement mode>
As described above with reference to FIG. 3, the second screw feeder 232 supplies the toner to the developing roller 231 while transporting the toner in the direction of arrow D2 in the second transport unit 230B. Therefore, the toner concentration on the downstream side in the developer transport direction of the second transport unit 230B decreases in accordance with the amount of toner consumed on the upstream side (rear end side) of the developer roller 231 in the developer transport direction. Therefore, the toner concentration on the downstream side in the developer transport direction in the second transport unit 230B has a large variation. Further, since the charge amount of the toner increases when the toner concentration decreases, the measurement result of the charge amount of the toner on the downstream side in the developer transport direction in the second transport unit 230B also has large dispersion.

  Therefore, in the charge amount measurement mode, the toner charge amount is measured by detecting the density of the measurement toner image only by the density sensor 100 that is arranged at the most upstream side in the developer transport direction among the plurality of density sensors 100. It is desirable to do. In this case, the toner consumed by the execution of the charge amount measurement mode and the time required to execute the charge amount measurement mode can be reduced, and the charge amount measurement mode can be efficiently executed.

<Example of charge amount measurement mode>
Hereinafter, an example of the charge amount measurement mode will be described. The conditions of the experiment performed are 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
・ Toner: positively charged polar toner, volume average particle diameter 6.8 μm, toner concentration 8%
Carrier: volume average particle diameter 35 μm, ferrite resin-coated carrier developing bias of developing roller 231: frequency = 4.2 kHz, duty = 50%, Vpp = 1000 V AC square wave, Vdc (DC voltage) = 200 V

<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 range of “3%” to “12%” for the toner density. 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. 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, the resistance adjustment and the charge adjustment are performed by mixing and dispersing the conductive agent and the charge control agent in the range of “0 to 20 parts” with respect to “100 parts” of the coating resin, respectively. .

  Under the above conditions, an experiment was performed in which the amount of the external additive of the toner was changed to adjust the charge amount of the toner, and the printing operation was performed. The results of the experiment are shown in FIGS. 6 and 7 described above. In FIG. 6, 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 acquired in advance and the toner formed on the printing paper (paper) are obtained. Using the correlation curve with the image density of the fixed image, the image density of the toner image is calculated based on the image density of the toner-fixed image. It is represented as D.

FIG. 7 shows the relationship between the charge amount of each toner and the slope of the straight line (approximate straight line) in FIG. Equation 3 (described below) of the approximate straight line illustrated in FIG. 7 is stored in the storage unit 983 in advance. Using Equation 3, the charge amount of the toner can be predicted.
Toner charge amount Q / M (μc / g) = − 442.32 × inclination + 29.87 (Equation 3)
Note that the slope of Equation 3 = the change amount of the image density / the change amount of the frequency (slope of the graph in FIG. 6).

<Return of image forming operation after replacement of toner container>
Immediately after the replacement of the toner container, the toner supplied to the developing device 23 and the developer in the developing device 23 are not sufficiently stirred and mixed, and the image forming operation may be performed in a state where the charge amount of the toner is unstable. Was. As a result, there is a possibility that so-called toner scattering occurs in which toner is scattered in an area different from the toner image forming area. In order to suppress this, if the stirring and mixing is performed for an unnecessarily long time after the replacement of the toner container, there is a possibility that the user may wait unnecessarily.

  Therefore, the present inventor has developed a developing device 23 based on the accurately measured toner charge amount so as to prevent toner scattering during the image forming operation after replacing the toner container without waiting for the user more than necessary. The inventor of the present invention has been keenly examining that the developer is appropriately stirred and mixed in the container, and recalled that the exchange controller 985 executes the container exchange mode described later.

<Flow of container exchange mode>
Hereinafter, the flow of the container exchange mode executed by the exchange control unit 985 will be described. FIG. 10 is a flowchart of the container exchange mode executed in the image forming apparatus 10 according to the present embodiment. As shown in FIG. 10, when the replacement control unit 985 detects that each toner container has been detached from the apparatus main body 11, as described above, it outputs a prohibition signal for prohibiting the image forming operation (step S21).

  Thereafter, as described above, the exchange control unit 985 determines that the tag attached to the toner container has been read by the tag reader or that the instruction indicating that the toner container has been exchanged has been input via the operation panel 18. If it is determined that the toner container has been exchanged (step S22), the container exchange mode is executed. When the execution of the container exchange mode is started, the exchange control unit 985 starts the toner supply operation and the aging operation (Step S23).

  The replacement control unit 985 controls the toner container replaced in step S22 in the toner replenishment operation started in step S23, so that the toner density T / C detected by the toner density sensor 101 is equal to or less than the first threshold TH1. (NO in step S24), the replenishment toner stored in the toner container is supplied by a predetermined amount to the developing device 23 corresponding to the toner container. The first threshold value TH1 can be edited via the operation panel 18.

  Further, in the aging operation started in step S23, the exchange control unit 985 applies the developing bias to the developing roller 231 of the developing device 23 corresponding to the toner container exchanged in step S22 without applying a developing bias. The roller 231, the first screw feeder 233, and the second screw feeder 232 are rotationally driven. Accordingly, the exchange control unit 985 causes the first screw feeder 233 and the second screw feeder 232 to perform the stirring operation in a state where the developing bias is not applied to the developing roller 231.

  Then, when the toner density T / C detected by the toner density sensor 101 becomes larger than the predetermined first threshold value TH1 (YES in step S24), the replacement control unit 985 ends the toner supply operation started in step S23 ( Step S25).

  In this way, the exchange control unit 985 can control the toner of the developer transported by the developer transport units (the first screw feeder 233 and the second screw feeder 232) actually detected by the toner density sensor 101 in the toner supply operation. An amount of toner corresponding to the density T / C can be supplied to the developing device 23. For this reason, the replacement control unit 985 estimates the toner concentration T / C from the toner consumption amount and the toner supply amount estimated from the image data and the operation time of the toner container, and according to the estimated toner concentration T / C. An appropriate amount of toner can be supplied to the developing device 23 as compared with a case in which an excessive amount of toner is supplied to the developing device 23.

  Further, the toner density sensor 101 detects the toner density T / C of the developer conveyed from the toner supply port 236 to the position facing the developing roller 231 where the toner is not replenished and consumed. The detected toner concentration T / C can be detected. Accordingly, the replacement control unit 985 can supply a more appropriate amount of toner to the developing device 23 in the toner supply operation.

  Upon completion of the toner replenishment operation, the exchange control unit 985 causes the mode control unit 984 to operate while the charge amount Q / M of the toner acquired in the charge amount measurement mode is equal to or less than the second predetermined threshold TH2 (NO in step S27). The charge amount measurement mode is repeatedly executed (step S26).

  FIG. 11 is a diagram illustrating an example of the second threshold TH2. As shown in FIG. 11, for example, the second threshold value TH2 is a value (for example, 22 μC) corresponding to the drive time (for example, initial) of the developing device 23 and the temperature and humidity around the developing device 23 (for example, 30 ° C. 80%). ) Is predetermined. In FIG. 11, the driving time of the developing device 23 when the number of printed sheets is less than 20,000 is indicated as “initial”, and the driving time of the developing device 23 when the number of printed sheets is 20,000 or more is indicated as “durable”. I have.

  It is considered that the longer the driving time of the developing device 23, the greater the degree of carrier deterioration due to coating peeling and coating contamination. When the degree of deterioration of the carrier is large, it is considered that the charge amount of the toner can be increased only to a certain amount even if the time for stirring and mixing the toner and the carrier is lengthened. For this reason, in the example shown in FIG. 11, the second threshold value TH2 is set to be smaller as the driving time of the developing device 23 is longer, so that the charge amount measurement mode is not repeatedly executed. Further, when the environment around the developing device 23 is a high-temperature and high-humidity environment, the toner is considered to be less likely to be charged than when the environment around the developing device 23 is a low-temperature and low-humidity environment. For this reason, in the example shown in FIG. 11, the second threshold value TH2 is set to be smaller as the temperature and humidity around the developing device 23 are higher in temperature and humidity so that the charge amount measurement mode is not repeatedly executed in vain. However, the present invention is not limited to this, and the second threshold value TH2 may be editable via the operation panel 18 similarly to the first threshold value TH1.

  Thereafter, when the charge amount Q / M of the toner acquired in the charge amount measurement mode becomes larger than the second threshold value TH2 (YES in step S27), the exchange control unit 985 ends the aging operation started in step S23. (Step S28). Then, the exchange control unit 985 permits the image forming operation (step S29).

  Accordingly, the replacement control unit 985 can appropriately perform the aging operation until the toner charge amount Q / M becomes a value corresponding to the driving time of the developing device 23 and the surrounding temperature and humidity when the toner container is replaced. It can be performed.

  If the plurality of toner containers are replaced and the replacement control unit 985 determines in step S22 that the plurality of toner containers have been replaced, after steps S23 to S28 are performed for each replaced toner container, Step S29 is performed.

  As described above, in the present embodiment, in the charge amount measurement mode, the mode control unit 984 determines the densities of the plurality of measurement toner images, or the densities of the plurality of measurement toner images, and the plurality of measurement toner images. Based on the DC component of the developing current used for the formation, the charge amount Q / M of the toner included in the plurality of measurement toner images can be acquired with high accuracy.

  Therefore, in the aging operation when the toner container is exchanged, the exchange control unit 985 performs the stirring operation only for an appropriate period in accordance with the toner charge amount Q / M accurately acquired in the charge amount measurement mode. After the operation, the image forming operation can be permitted. This reduces the risk that the stirring operation is performed for an unnecessarily long time or the risk that the stirring operation is not performed properly and the image forming operation is performed while the charge amount Q / M of the toner is unstable. Can be. As a result, it is possible to suppress the occurrence of toner scattering during the image forming operation after the replacement of the toner container without waiting the user more than necessary.

  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.

<First Modified Embodiment>
In the above-described embodiment, in the charge amount measurement mode, the mode control unit 984 charges the toner included in the measurement toner image formed on the photosensitive drum 20 from the inclination of the measurement straight line and the reference information in the storage unit 983. Although described in the aspect of obtaining the amount, the present invention is not limited to this. FIG. 12 is a cross-sectional view of the developing device 23 and a block diagram showing an electrical configuration of the control unit 980 according to the first modified embodiment of the present invention. FIG. 13 is a flowchart of the charge amount measurement mode executed in the image forming apparatus 10 according to the first modified embodiment of the present invention.

  As shown in FIG. 12, 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 a 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.

  As shown in FIG. 13, when the mode control unit 984 starts the charge amount measurement mode (step S31), the mode control unit 984 sets a 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 perform development while applying the phenomenon bias for forming the image 1 to the development 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 Expression 4.
Slope a = (I1-I2) / (ID1-ID2) (Equation 4)
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 a 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 4.

<Second modified embodiment>
Even if the aging operation is continued when the toner container is replaced, the charge amount of the toner may not be larger than the second threshold value TH2 depending on the deterioration state of the toner and the carrier contained in the developer. Alternatively, it may take a long time before the charge amount of the toner becomes larger than the second threshold value TH2. In consideration of this, the image forming apparatus 10 of the above embodiment and each of the modified embodiments may be configured as follows. FIG. 14 is a flowchart of a container exchange mode executed in the image forming apparatus 10 according to the second modified embodiment of the present invention.

  In the present modified embodiment, as shown in FIG. 14, in the container exchange mode, unlike the flow shown in FIG. 10, the exchange control unit 985 sets the charge amount of the toner acquired in the charge amount measurement mode to the second threshold value TH2. Mode control (NO in step S27), and while the elapsed time (hereinafter, aging time) Ta from the start of aging in step S23 is equal to or less than a predetermined third threshold TH3 (NO in step S51), the mode control is performed. The unit 984 repeatedly executes the charge amount measurement mode (step S26).

  On the other hand, when the charge amount of the toner acquired in the charge amount measurement mode is equal to or less than the second threshold value TH2 (NO in step S27), the exchange control unit 985 determines that the aging time Ta is longer than the third threshold value TH3. (YES in step S51), after terminating the repeated execution of the charge amount measurement mode by the mode control unit 984, terminating the aging operation started in step S23 (step S28), and permitting the image forming operation (step S29). ).

  According to the configuration of the present embodiment, the replacement control unit 985 can prevent the aging operation when the toner container is replaced from being performed for a time longer than the third threshold value TH3. Accordingly, it is possible to reduce the possibility that the time during which the image forming operation cannot be performed after the toner container is replaced becomes excessively long while allowing the toner scattering to some extent.

<Third Modified Embodiment>
As described above, even if the aging operation is continuously performed when the toner container is replaced, the charge amount of the toner may not be larger than the second threshold value TH2 depending on the deterioration state of the toner and the carrier included in the developer. . Alternatively, it may take a long time before the charge amount of the toner becomes larger than the second threshold value TH2. In consideration of this, unlike the second modified embodiment, the image forming apparatus 10 may be configured as follows. FIG. 15 is a flowchart of the container exchange mode executed in the image forming apparatus 10 according to the third modified embodiment of the present invention.

  In the present modified embodiment, as shown in FIG. 15, unlike the flow shown in FIG. 14, in the container exchange mode, the exchange control unit 985 sets the charge amount of the toner acquired in the charge amount measurement mode to the second threshold value TH2. (No in step S27), and after the aging is started in step S23, the number of times the mode control unit 984 repeats the charge amount measurement mode (hereinafter, the charge amount measurement number) Cq is equal to or less than a predetermined fourth threshold value TH4. During this period (NO in step S61), the mode control unit 984 repeatedly executes the charge amount measurement mode (step S26).

  On the other hand, when the charge amount of the toner acquired in the charge amount measurement mode is equal to or less than the second threshold value TH2 (NO in step S27), the exchange control unit 985 sets the charge amount measurement number Cq to be smaller than the fourth threshold value TH4. If it becomes larger (YES in step S61), after the repetitive execution of the charge amount measurement mode by the mode control unit 984 is terminated, the aging operation started in step S23 is terminated (step S28), and the image forming operation is permitted (step S28). Step S29).

  According to the configuration of the present modified embodiment, the replacement control unit 985 performs the charge amount acquisition operation for acquiring the charge amount Q / M of the toner during the aging operation when the toner container is replaced with the fourth threshold value. It is possible to avoid being repeated a number of times greater than TH4. Accordingly, it is possible to reduce the possibility that the time during which the image forming operation cannot be performed after the toner container is replaced becomes excessively long while allowing the toner scattering to some extent.

<Other modified embodiments>
(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) 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 may be used.

  (3) The image forming apparatus 10 is not provided with the toner density sensor 101, and the control unit 980 estimates the toner consumption from the image data (the number of print dots) of each electrostatic latent image formed on the photosensitive drum 20. Alternatively, the toner supply amount may be estimated from the operation time of the toner supply unit 15, and the toner concentration may be estimated from the estimated toner consumption amount and toner supply amount.

<Experimental results>
Hereinafter, the results of an experiment conducted by the present inventor to replace a toner container will be described. FIG. 16 is a diagram illustrating an example of an experiment result of replacing the toner container. The present inventor conducted the first to eighth experiments as described below under the same conditions as those used in the above-described experimental example of the charge amount measurement mode (see the section <Common Experimental Conditions>).

  In the first experiment, as shown in “Example 1” in FIG. 16, the state of the developer contained in the developing device 23 is changed to the toner image with the printing rate “5%” shown in the “Developer” column. From the “initial” state of continuous printing of less than 20,000 sheets, a toner image with a printing rate of “5%” is printed until the toner container becomes empty, and then the toner container corresponding to the developing device 23 is replaced with a new one. Was replaced with a new toner container. Here, printing a toner image means forming a toner image on the photosensitive drum 20 by an image forming operation. Then, the second threshold value TH2 was set to “22 μC” described in the “charge amount threshold value” column, and the container exchange mode was executed according to the flow shown in FIG.

  The time required for the aging operation from the start of the aging operation in step S23 (FIG. 10) to the permission of the image forming operation in step S29 (FIG. 10) is referred to as the "aging time" in the "Experiment result A" column. Column. The result of visually evaluating the degree to which the regulating blade 234 (FIG. 3) is contaminated by toner scattering is described in the “Experiment result A” column in the “Toner scattering” column. The evaluation result “◎” in the “toner scattering” column indicates that the regulating blade 234 is not stained at all, and the evaluation result “○” indicates that the regulating blade 234 is almost not stained. The evaluation result “△” indicates that the regulating blade 234 is slightly contaminated, but there is no problem in actual use, and the evaluation result “×” indicates that the regulating blade 234 is contaminated.

  In the second experiment, as shown in “Example 1” in FIG. 16, the use state of the developer contained in the developing device 23 is such that the toner image of the printing rate “20%” is continuously printed on less than 20,000 sheets. From the “initial” state, a toner image having a printing rate of “20%” is printed until the toner container becomes empty, and then replaced with a new toner container, and the second threshold value TH2 is set to “22 μC”. Set and execute container exchange mode. Thereafter, similarly to the “Experiment result A” column, the time required for the aging operation and the evaluation result visually evaluated were described in the “Experiment result B” column.

  In the third and fourth experiments, similarly to the first and second experiments, as shown in “Embodiment 2” in FIG. 16, the use state of the developer contained in the developing device 23 is changed to “initial state”. From the state, the toner images having the printing rates of "5%" and "20%" are printed until the toner container becomes empty, and then replaced with a new toner container, and the second threshold value TH2 is set. The container exchange mode was executed by setting to “25 μC”. Then, similarly to the first and second experiments, the time required for the aging operation and the evaluation result visually evaluated were described in the “Experiment results A” and “Experiment results B” columns.

  In the fifth and sixth experiments, similarly to the first and second experiments, as shown in “Example 3” in FIG. 16, the use state of the developer contained in the developing device 23 is changed to the printing state. From the “durable” state in which the toner images of the rate “5%” are continuously printed on 20,000 or more sheets, the toner images of the print rates of “5%” and “20%” respectively become the empty timing of the toner container. After printing, the container was replaced with a new toner container, the second threshold value TH2 was set to “22 μC”, and the container replacement mode was executed. Thereafter, similarly to the first and second experiments, the time required for the aging operation and the evaluation result visually evaluated were described in the “Experiment results A” and “Experiment results B” columns.

  In the seventh and eighth experiments, similarly to the first and second experiments, as shown in “Example 4” in FIG. 16, the usage state of the developer contained in the developing device 23 is “durable”. From the state, the toner images having the printing rates of "5%" and "20%" are printed until the toner container becomes empty, and then replaced with a new toner container, and the second threshold value TH2 is set. The container exchange mode was executed by setting to “25 μC”. Then, similarly to the first and second experiments, the time required for the aging operation and the evaluation result visually evaluated were described in the “Experiment results A” and “Experiment results B” columns.

  In the container exchange mode executed in the first to eighth experiments, similarly to the experimental example of the charge amount measurement mode described above, the container is formed on the photosensitive drum 20 from the inclination of the measurement straight line and the reference information in the storage unit 983. The charge amount measurement mode for acquiring the charge amount of the toner included in the obtained measurement toner image was executed.

  As a comparative example of the first to fourth experiments, as shown in “Comparative Example 1” to “Comparative Example 4” in FIG. After printing the toner images of the printing rates “5%” and “20%” until the toner container becomes empty, respectively, from the time when the used state of the developer is “initial” state, After replacing the container, the toner is supplied to the developing device 23 until the toner concentration detected by the toner concentration sensor 101 becomes larger than the first threshold value TH1, and the time (“180s”, “30s”) described in the “aging time threshold value” column , "15s", "0s").

  As shown in “Comparative Example 5” and “Comparative Example 6” in FIGS. 16A and 16B, as a comparative example of the fifth to eighth experiments, as in the first and second experiments, the developing device 23 is mounted. From the time when the used state of the developer is “durable”, the toner images of the printing rates “5%” and “20%” are printed until the toner container becomes empty, respectively. After replacing the toner container with a toner container and replenishing toner to the developing device 23 until the toner density detected by the toner density sensor 101 becomes larger than the first threshold value TH1, the time (“60s”, “ The aging operation was performed only for 30 s "). Then, similarly to the first and second experiments, the time required for the aging operation and the evaluation result visually evaluated were described in the “Experiment results A” and “Experiment results B” columns.

  When the use state of the developer contained in the developing device 23 is the “durable” state, the developer has a larger amount of the external additive attached to the carrier than in the “initial” state, The chargeability of the toner decreases. Therefore, for example, as shown in “Example 1” and “Example 3”, when the use state of the developer contained in the developing device 23 is the “durable” state, the charge amount of the toner becomes the second. The time (for example, 60 s, 80 s) required to perform the aging operation until the threshold value TH2 becomes “22 μC” or more is equal to the time (for example, 60 s, 80 s) when the use state of the developer is the “initial” state. Was also found to be long. However, in the first, second, fifth, and sixth experiments corresponding to “Example 1” and “Example 3,” the regulating blade 234 was not stained in any case. As a result, similarly to the first, second, fifth, and sixth experiments, if the container is replaced with a new toner container and the second threshold value TH2 is set to “22 μC” and the container replacement mode is executed, the regulating blade 234 is changed. It was found that the occurrence of toner scattering could be suppressed to such an extent that no toner was stained.

  Similarly, as shown in “Example 2” and “Example 4”, when the use state of the developer contained in the developing device 23 is the “durable” state, the charge amount of the toner becomes the second threshold value TH2. The time (for example, 70 s, 90 s) required to perform the aging operation until “25 μC” or more is longer than the time (for example, 90 s, 110 s) when the use state of the developer is the “initial” state. It turned out to be. However, in the third, fourth, seventh, and eighth experiments corresponding to “Example 2” and “Example 4,” the regulating blade 234 was not stained at all. As a result, similarly to the third, fourth, seventh and eighth experiments, after replacing the toner container with a new toner container, the second threshold value TH2 is set to “25 μC” which is larger than “22 μC”, and the container replacement mode is executed. Then, it was found that the occurrence of toner scattering can be more reliably suppressed to the extent that the regulating blade 234 is not stained at all.

  Further, from the time when the use state of the developer contained in the developing device 23 is the “initial” state, the toner images of the printing rates “5%” and “20%” are printed until the toner container becomes empty. As in the conventional case, it is assumed that the aging operation is performed for a predetermined time after the replacement of the toner container. In this case, in order to keep the regulating blade 234 clean, it is necessary to perform the aging operation for “180 s” after replacing the toner container as shown in “Comparative Example 1” to “Comparative Example 4”. I found it.

  However, from the time when the use state of the developer contained in the developing device 23 is the “initial” state, the toner images of the printing rates “5%” and “20%” are printed until the toner container becomes empty. As in the third and fourth experiments corresponding to “Example 2”, if the second threshold value TH2 is set to “25 μC” and the container replacement mode is executed after the replacement of the toner container, the regulating blade 234 becomes completely dirty. It turns out that it can be in a state where it is not. Further, in this case, it was found that the time required for the aging operation can be reduced to “90 s” which is half of the conventional case.

  Further, from the time when the use state of the developer contained in the developing device 23 is the “durable” state, the toner images of the printing rates “5%” and “20%” are printed until the toner container becomes empty, As in the conventional case, it is assumed that the aging operation is performed for a predetermined time after the replacement of the toner container. In this case, in order to keep the regulating blade 234 clean, as shown in “Comparative Example 5” to “Comparative Example 6,” it is necessary to perform the aging operation for at least “60 s” after replacing the toner container. I found it.

  On the other hand, from the time when the use state of the developer contained in the developing device 23 is the “durable” state, the toner images of the printing rates “5%” and “20%” are printed until the toner container becomes empty, As in the conventional case, it is assumed that the aging operation is performed for a predetermined time after the replacement of the toner container. In this case, as shown in “Comparative Example 5” to “Comparative Example 6,” it was found that even if the aging operation was performed for “60 s”, the regulating blade 234 could not be kept clean.

  However, from the time when the use state of the developer contained in the developing device 23 is the “durable” state, the toner images of the print rates “5%” and “20%” are printed until the toner container is empty. As in the fifth and sixth experiments corresponding to “Example 3”, if the second threshold value TH2 is set to “22 μC” and the container replacement mode is executed after replacing the toner container, the regulating blade 234 becomes dirty. It turned out that there could be no state. In this case, it was also found that the time required for the aging operation can be reduced from “60 s” to about “80 s”.

  To summarize the above experimental results, if the container replacement mode is executed after the replacement of the toner container, the charge amount of the toner can be accurately obtained by executing the charge amount measurement mode instead of simply performing the stirring operation for a predetermined time, It has been found that the end of the aging operation can be appropriately determined in accordance with the acquired accurate toner charge amount of the toner, so that toner scattering can be prevented in a short time.

10 Image forming apparatus 14 Intermediate transfer unit (transfer section)
15 Toner supply unit 20 Photoconductor drum (image carrier)
Reference Signs 22 Exposure device 23 Developing device 100 Density sensor (density detector)
101 Toner density sensor (toner density detector)
145 Secondary transfer roller (transfer section)
231 developing roller 232 second screw feeder (developer conveying unit)
233 First screw feeder (developer conveying section)
971 developing bias application unit 980 control unit 983 storage unit 984 mode control unit (charge amount acquisition unit)
985 Exchange control unit

Claims (10)

An image forming apparatus that performs an image forming operation of forming an image on a sheet,
The device body,
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,
An exposure device that forms the electrostatic latent image by exposing the surface of the image carrier charged to the charging potential according to predetermined image information;
A developing device disposed at a predetermined developing nip portion so as to face the image bearing member, wherein the developer conveying portion performs a stirring operation of conveying a developer including a toner and a carrier while stirring, and A developing roller configured to form the toner image by supplying the toner to the image carrier while supporting the developer conveyed by a developer conveying unit on a peripheral surface;
A toner replenishing unit that is replaceably mounted on the apparatus main body, stores the toner therein, and replenishes the stored toner to the developing device;
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,
A toner concentration detection unit that detects a toner concentration indicating a ratio of a toner amount to a carrier amount included in the developer,
The charging device, the exposure device, and the developing bias application unit are controlled to form a plurality of measurement toner images having different toner development amounts on the image carrier, and the plurality of measurement toner images are detected by the density detection unit. Based on the density of the measurement toner image, or when the plurality of measurement toner images are formed in addition to the densities of the plurality of measurement toner images, the gap between the developing roller and the developing bias applying unit is formed. A charge amount acquisition unit that performs a charge amount acquisition operation of acquiring a charge amount of the toner based on a DC component of a flowing developing current;
An exchange control unit that outputs a prohibition signal for inhibiting the image forming operation in accordance with the replacement of the toner replenishing unit, and executes a predetermined preparation operation when the toner replenishment unit is replaced;
With
The exchange control unit, in the preparation operation,
Until the toner concentration detecting section detects a toner concentration greater than a predetermined first threshold, a toner replenishing operation for replenishing the toner to the developing device with the toner replenishing section, and the developing bias is applied to the developing roller. Aging operation that causes the developer transport unit to perform the agitation operation in a state where it is not performed,
After the completion of the toner replenishment operation, the charge amount acquisition unit repeatedly performs the charge amount acquisition operation until the charge amount of the toner acquired in the charge amount acquisition operation becomes larger than a second predetermined threshold. An image forming apparatus that terminates the aging operation and outputs a permission signal for permitting the image forming operation. The exchange control unit, in the preparation operation,
When the charge amount of the toner acquired in the charge amount acquisition operation is equal to or less than the second threshold, and when the elapsed time from the start of the aging operation is larger than a predetermined third threshold, The image forming apparatus according to claim 1, wherein after terminating the charge amount acquisition unit to repeatedly execute the charge amount acquisition operation, terminating the aging operation and outputting the permission signal. The exchange control unit, in the preparation operation,
When the charge amount of the toner acquired in the charge amount acquisition operation is equal to or less than the second threshold, and when the number of times the charge amount acquisition operation is repeated is greater than a predetermined fourth threshold, the charge amount 2. The image forming apparatus according to claim 1, wherein after terminating the repetitive execution of the charge amount acquiring operation in the acquiring unit, terminating the aging operation and outputting the permission signal. 3. 4. The image forming apparatus according to claim 1, wherein the second threshold is determined in advance according to a driving time of the developing device and a temperature and a humidity around the developing device. 5. The image forming apparatus according to claim 1, wherein the toner concentration detection unit is a toner concentration sensor that detects the toner concentration of the developer conveyed by the developer conveyance unit. The developing device includes a toner supply port that receives toner supplied by the toner supply unit,
The image forming apparatus according to claim 5, wherein the toner concentration sensor detects the toner concentration of the developer conveyed from the toner supply port toward a position facing the developing roller by the developer conveying unit. 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, 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 first charge amount, and the inclination of the reference straight line is set to increase as the charge amount of the toner decreases. 8. The image forming apparatus according to 7. 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. 7. The 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 image carrier and the developing unit. 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; And acquiring a charge amount of a toner contained in a measurement toner image formed on the image carrier, based on a ratio of a difference between DC components of the development current flowing between the image carrier and the development bias application unit. The image forming apparatus according to claim 1.