JP5629666B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus including a touch-down developing type developing device using a two-component developer containing toner and a carrier.

  An image forming apparatus such as a copying machine, a printer, or a facsimile using an electrophotographic system supplies a developer to an electrostatic latent image formed on an image carrier (for example, a photosensitive drum or a transfer belt) and supplies the developer. By developing the electrostatic latent image, a toner image is formed on the image carrier. As one of the developing methods, a touch-down developing method using a two-component developer containing a non-magnetic toner and a magnetic carrier is known. In this method, a two-component developer layer (so-called magnetic brush layer) is carried on a magnetic roller, toner is received from the magnetic brush layer on the development roller, and the toner layer is carried from the toner layer to the image carrier. The electrostatic latent image is visualized by supplying toner.

  In a developing device employing a touch-down development method, the toner once carried on the developing roller is transferred onto the magnetic roller by changing a bias applied to the developing roller between sheets where a printing process is performed or between printing jobs. It is known to perform a collecting operation forcibly collecting by a magnetic brush layer (see, for example, Patent Documents 1 and 2). For example, when the printing process of a document having a low printing rate is continued, the amount of toner supplied from the developing roller to the image carrier is reduced. In this case, since the toner stays on the developing roller for a long time, the toner becomes excessively charged and deteriorates. If the printing process is continued as it is, a decrease in image density and fogging will occur. However, by executing the above-described collecting operation, it is possible to suppress toner deterioration due to toner staying on the developing roller for a long time.

  There is also known an image forming apparatus having a function of executing a refresh operation for forcibly discharging toner staying on a developing roller to an image carrier. The refresh operation is performed, for example, between sheets or between print jobs when the average document printing rate falls below a predetermined threshold, and forms a solid electrostatic latent image on the image carrier. Toner is supplied from the developing roller to the electrostatic latent image.

JP 2005-55837 A JP 2003-280392 A

  The above collection operation and refresh operation are not necessarily performed frequently. First, since execution of these operations requires a certain amount of time, the print productivity of the image forming apparatus may be reduced. Further, if the collecting operation is performed many times, the characteristics of the developer may be adversely affected. Further, if the refresh operation is frequently performed, the amount of toner consumption increases.

  The present invention has been made in view of the above problems, and provides an image forming apparatus having a developing device adopting a touch-down developing method, wherein the collection operation is executed at an optimal timing. The purpose is to do.

An image forming apparatus according to an aspect of the present invention includes an image carrier that carries an electrostatic latent image and a toner image to be transferred to a sheet, a development housing that stores a developer including toner and a carrier, and a rotation in a predetermined direction. While receiving the developer in the developing housing, the developer carrying member for carrying the developer layer, and rotating while being in contact with the developer layer, the toner is received from the developer layer to remove the toner layer. A toner carrier for carrying the toner layer and supplying the toner of the toner layer to the image carrier for developing the electrostatic latent image; and forcing the toner carried on the toner carrier to the developer carrier The recovery means for performing the recovery operation for recovery, and the characteristics of the developer carried on the developer carrier are in a predetermined normal state , or a deteriorated state in which the characteristics of the developer are deteriorated more than the normal state Whether it is A judging means for, and control means for executing the recovery operation at a predetermined timing, and patch forming means for forming a patch toner image on said image bearing member, a density detecting means for detecting the concentration of the patch toner image, the The control means sets the execution interval of the collecting operation to a predetermined first interval when the determination means determines that the characteristic of the developer is in the normal state, and the determination means When it is determined that the characteristics of the developer are in the deteriorated state, an execution interval of the collecting operation is set to a second interval that is shorter than the first interval , and the patch forming unit includes the toner carrier. A first patch toner image having a predetermined area ratio is formed on the image carrier by toner supplied from a predetermined carrying position, and then the toner is supplied from the carrying position by the toner. A solid patch toner image having an area ratio of 100% or in the vicinity thereof is formed on the carrier, and then the area ratio is the same as that of the first patch toner image on the image carrier by the toner supplied from the carrying position. A second patch toner image is formed, and the determination unit distinguishes the normal state or the deteriorated state based on a density difference between the first patch toner image and the second patch toner image .

According to this configuration, the characteristic of the developer layer is determined by the determination unit, and the execution timing of the collecting operation is determined according to the determination result. That is, when it is determined that the developer characteristics are in a predetermined normal state , the collection operation execution interval is set to the first interval, and the developer characteristics are deteriorated more than the normal state. If it is determined, the collection operation execution interval is set to a second interval shorter than the first interval. Therefore, the collecting operation can be executed according to the current state of the developer, and the image forming apparatus can be prevented from executing the collecting operation and the image quality can be ensured. That is, when the characteristics of the developer deteriorate, the collection operation execution interval is set shorter. Accordingly, the deteriorated toner can be quickly recovered from the toner carrier, and the image quality can be maintained.

Also includes a patch forming means for forming a patch toner image on said image bearing member, a density detecting means for detecting the concentration of the patch toner image, wherein the determining means, based on the density of the patch toner image, The structure which distinguishes the said normal state or the said deterioration state is included .

  According to this configuration, the current state of the developer can be easily determined based on the density of the patch toner image. The patch forming means and the density detecting means as described above are generally provided in an image forming apparatus for a calibration operation, and have an advantage that the determination means can make a determination using existing elements.

The patch forming means causes the image carrier to form a first patch toner image having a predetermined area ratio with toner supplied from a predetermined carrying position of the toner carrier, and then from the carrying position. A solid patch toner image having an area ratio of 100% or in the vicinity thereof is formed on the image carrier by the supplied toner, and then the area ratio is applied to the image carrier by the toner supplied from the carrying position. A second patch toner image that is the same as the one patch toner image, and the determination unit determines whether the normal state or the second patch toner image is based on a density difference between the first patch toner image and the second patch toner image. It distinguishes between the deterioration state.

  When the toner is deteriorated, after the solid toner image is formed, the supply of the toner becomes insufficient, and the density of the toner image tends to decrease. Therefore, as described above, the first patch toner image and the second patch toner image having the same area ratio are formed before and after the formation of the solid patch toner image, and the density difference between the two is compared. The state can be accurately determined.

In the above configuration, the image forming apparatus further includes data acquisition means for acquiring at least one of data relating to the driving time of the image forming apparatus, data relating to the average printing rate, and data relating to the number of printed sheets, and the determination means includes: It is desirable to treat the data acquired by the data acquisition means as a demarcation element in the normal state .

According to this configuration, the normal state can be adjusted based on the data acquired by the data acquisition unit. That is, since the normal state can be set in consideration of the usage status of the image forming apparatus, the determination unit can more accurately determine the state.

  In the above configuration, a patch forming unit that forms a patch toner image on the image carrier, a density detection unit that detects the density of the patch toner image, and a bias applied to at least one of the developer carrier and the toner carrier. Bias applying means for forming a predetermined potential difference therebetween, and the bias applying means adjusts the bias condition based on the density of the patch toner image detected by the density detecting means. And the said determination means can be set as the structure which distinguishes the said 1st state or the said 2nd state based on the conditions of the said adjusted bias.

  According to this configuration, the current state of the developer can be determined using the result of the calibration operation executed by the patch forming unit, the density detecting unit, and the bias applying unit.

  According to the present invention, in the image forming apparatus provided with the developing device adopting the touch-down developing method, the collecting operation can be executed at an optimal timing. Therefore, it is possible to provide an image forming apparatus capable of suppressing the execution of an excessive collection operation while maintaining good image quality.

1 is a cross-sectional view illustrating an embodiment of an image forming apparatus according to the present invention. It is sectional drawing of the perpendicular direction of a developing device. It is sectional drawing of the horizontal direction of a developing device. It is a schematic diagram for explaining a developing operation of the developing device. FIG. 6 is a schematic diagram for explaining a toner collecting operation from a developing roller. It is a functional block diagram of a control part. FIG. 4 is a schematic diagram illustrating an example of forming a patch toner image on an intermediate transfer belt. 6 is a flowchart for explaining a developing operation of the image forming apparatus. 6 is a flowchart for explaining a toner collecting operation. FIG. 6 is a schematic perspective view showing a formation state of a patch toner image. It is a schematic diagram showing the order of formation of patch toner images. It is a flow chart for explaining an example of patch density detection operation. 10 is a flowchart for explaining another example of the patch density detection operation. It is a graph which shows the measurement result of image density. It is a graph which shows the measurement result of image density.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a sectional view showing an internal structure of an image forming apparatus 1 according to an embodiment of the present invention. Here, a copying machine is illustrated as the image forming apparatus 1, but the image forming apparatus may be a printer, a facsimile machine, or a multifunction machine having these functions.

  The image forming apparatus 1 includes an apparatus main body 10 having a substantially rectangular parallelepiped housing structure, and an automatic document feeder 20 disposed on the apparatus main body 10. In the apparatus main body 10, a reading unit 25 that optically reads a document image to be copied, an image forming unit 30 that forms a toner image on a sheet, a fixing unit 60 that fixes the toner image on a sheet, and image formation A sheet feeding unit 40 that stores sheets conveyed to the unit 30, a conveyance path 50 that conveys the sheet from the sheet feeding unit 40 to the sheet discharge port 10 </ b> E via the image forming unit 30 and the fixing unit 60, and the conveyance path A conveyance unit 55 having therein a sheet conveyance path constituting a part of 50 is accommodated.

  The right side surface 10 </ b> R of the apparatus main body 10 is provided with a main body cover 101 that is opened and closed when a unit such as the image forming unit 30 and the fixing unit 60 is removed from the apparatus main body 10 during jam processing or maintenance. The main body cover 101 is rotatably attached to the apparatus main body 10 at a base end portion (lower end portion) thereof.

  The automatic document feeder 20 is rotatably attached to the upper surface of the apparatus body 10. The automatic document feeder 20 automatically feeds a document sheet to be copied toward a predetermined document reading position (position where the first contact glass 241 is assembled) in the apparatus body 10. On the other hand, when the user manually places the document sheet on a predetermined document reading position (position of the second contact glass 242), the automatic document feeder 20 is opened upward. The automatic document feeder 20 includes a document tray 21 on which a document sheet is placed, a document transport unit 22 that transports a document sheet via an automatic document reading position, and a document discharge in which a document sheet after reading is discharged. A tray 23.

  The reading unit 25 is a first contact glass 241 for reading a document sheet automatically fed from the automatic document feeder 20 on the upper surface of the apparatus body 10 or a second contact glass for reading a manually placed document sheet. Through 242, the image of the original sheet is optically read. In the reading unit 25, a scanning mechanism including a light source, a movable carriage, a reflection mirror, and the like, and an image sensor are accommodated (not shown). The scanning mechanism irradiates the original sheet with light and guides the reflected light to the image sensor. The image sensor photoelectrically converts the reflected light into an analog electric signal. The analog electric signal is converted into a digital electric signal by an A / D conversion circuit and then input to the image forming unit 30.

  The image forming unit 30 performs a process of generating a full-color toner image and transferring it onto a sheet. Yellow (Y), magenta (M), cyan (C), and black (Bk) arranged in tandem. An image forming unit 32 including four units 32Y, 32M, 32C, and 32Bk for forming the respective toner images, an intermediate transfer unit 33 disposed adjacent to the image forming unit 32, and an intermediate transfer unit 33. And a toner replenishing section 34 disposed in the.

  Each of the image forming units 32Y, 32M, 32C, and 32Bk includes a photosensitive drum 321 and a charger 322, an exposure device 323, a developing device 324, a primary transfer roller 325, and a cleaning device disposed around the photosensitive drum 321. 326.

  The photosensitive drum 321 rotates about its axis, and an electrostatic latent image and a toner image are formed on its peripheral surface. As the photosensitive drum 321, a photosensitive drum using an amorphous silicon (a-Si) material can be used. The charger 322 uniformly charges the surface of the photosensitive drum 321. The exposure device 323 includes a laser light source and optical system devices such as a mirror and a lens, and forms an electrostatic latent image by irradiating the peripheral surface of the photosensitive drum 321 with light based on image data of a document image. .

  The developing device 324 supplies toner to the peripheral surface of the photosensitive drum 321 in order to develop the electrostatic latent image formed on the photosensitive drum 321. The developing device 324 is a touch-down developing type developing device, and includes a screw feeder, a magnetic roller, and a developing roller. The details of the developing device 324 will be described in detail later.

  The primary transfer roller 325 forms a nip portion with the photosensitive drum 321 across the intermediate transfer belt 331 provided in the intermediate transfer unit 33, and the toner image on the photosensitive drum 321 is primary transferred onto the intermediate transfer belt 331. To do. The cleaning device 326 includes a cleaning roller and the like, and cleans the peripheral surface of the photosensitive drum 321 after the toner image is transferred.

  The intermediate transfer unit 33 includes an intermediate transfer belt 331, a driving roller 332, a driven roller 333, and a tension roller 334. The intermediate transfer belt 331 is an endless belt that is stretched over the rollers 332, 333, and 334 and the above-described primary transfer roller 325. The intermediate transfer belt 331 is provided with toner from a plurality of photosensitive drums 321 on the outer peripheral surface thereof. The image is transferred to the same place and transferred (primary transfer).

  A secondary transfer roller 35 is disposed to face the peripheral surface of the drive roller 332. A nip portion between the driving roller 332 and the secondary transfer roller 35 becomes a secondary transfer portion 35A that transfers a full-color toner image overcoated on the intermediate transfer belt 331 to a sheet. A secondary transfer bias potential having a polarity opposite to that of the toner image is applied to one of the driving roller 332 and the secondary transfer roller 35, and the other roller is grounded. The driven roller 333 is a roller that is driven according to the rotation of the intermediate transfer belt 331, and the tension roller 334 is a roller that applies a predetermined tension to the intermediate transfer belt 331.

  A patch density sensor 336 (density detection means) that detects the density of the patch toner image formed on the peripheral surface of the intermediate transfer belt 331 is disposed in the vicinity of the secondary transfer portion 35A. The patch density sensor 336 includes a light emitting unit that emits inspection light toward the patch toner image, and a light receiving unit that receives reflected light of the inspection light. The development bias is adjusted based on the patch toner image density detection result by the patch density sensor 336 (calibration operation).

  The toner supply unit 34 includes a yellow toner container 34Y, a magenta toner container 34M, a cyan toner container 34C, and a black toner container 34Bk. These toner containers 34Y, 34C, 34M, and 34Bk respectively store toner of each color, and are supplied to the developing devices 324 of the image forming units 32Y, 32M, 32C, and 32Bk corresponding to each color of YMCBk through a supply path (not shown). Supply toner of each color.

  The paper feed unit 40 includes two-stage paper feed cassettes 40A and 40B that accommodate sheets subjected to image forming processing. These paper feed cassettes 40A and 40B can be pulled out from the front of the apparatus body 10 toward the front. The paper feed cassettes 40A and 40B are cassettes provided for automatic paper feed. On the right side surface 10R of the apparatus main body 10, a paper feed tray 46 for manual paper feed is also provided. The paper feed tray 46 is attached to the main body cover 101 at its lower end so as to be freely opened and closed. When performing manual sheet feeding, the user opens the sheet feeding tray 46 as shown in the drawing and places a sheet thereon.

  The sheet feeding cassette 40A (40B) includes a sheet storage unit 41 that stores a sheet bundle formed by stacking a plurality of sheets, and a lift plate 42 that lifts up the sheet bundle for feeding. A pickup roller 43 and a roller pair of a paper feed roller 44 and a retard roller 45 are arranged on the upper right side of the paper feed cassette 40A (40B). By driving the pickup roller 43 and the paper feed roller 44, the uppermost sheet of the sheet bundle in the paper feed cassette 40A is fed out one by one and carried into the upstream end of the transport path 50. On the other hand, the sheet placed on the paper feed tray 46 is similarly carried into the conveyance path 50 by driving the pickup roller 461 and the paper feed roller 462.

  The conveyance path 50 includes a main conveyance path 50 </ b> A that conveys a sheet from the sheet feeding unit 40 to the exit of the fixing unit 60 via the image forming unit 30, and a sheet that is printed on one side when performing duplex printing on the sheet. A reverse conveyance path 50B for returning to the image forming unit 30, a switchback conveyance path 50C for directing the sheet from the downstream end of the main conveyance path 50A to the upstream end of the reverse conveyance path 50B, and the downstream end of the main conveyance path 50A To a sheet discharge port 10E provided on the left side surface 10L of the apparatus main body 10 and a horizontal conveyance path 50D for conveying the sheet in the horizontal direction. Most of the horizontal conveyance path 50 </ b> D is configured by a sheet conveyance path provided inside the conveyance unit 55.

  A registration roller pair 51 is disposed on the upstream side of the main transfer path 50A from the secondary transfer portion 35A. The sheet is temporarily stopped by the resist roller pair 51 in a stopped state, and skew correction is performed. Thereafter, the registration roller pair 51 is rotationally driven by a drive motor (not shown) at a predetermined timing for image transfer, whereby the sheet is sent out to the secondary transfer portion 35A. In addition, a plurality of conveyance rollers 52 for conveying the sheet are arranged in the main conveyance path 50A. The same applies to the other transport paths 50B, 50C, and 50D.

  A paper discharge unit 530 having a paper discharge roller 53 is disposed adjacent to the conveyance unit 55 at the most downstream end of the conveyance path 50. The paper discharge roller 53 feeds the sheet to a post-processing device (not shown) disposed on the left side surface 10L of the apparatus body 10 through the sheet discharge port 10E. In the image forming apparatus to which the post-processing apparatus is not attached, a sheet discharge tray is provided below the sheet discharge port 10E.

  The transport unit 55 is a unit that transports the sheet unloaded from the fixing unit 60 to the sheet discharge port 10E. In the image forming apparatus 1 of the present embodiment, the fixing unit 60 is disposed on the right side 10R side of the apparatus main body 10, and the sheet discharge port 10E is disposed on the left side 10L side of the apparatus main body 10 facing the right side 10R. Yes. Accordingly, the transport unit 55 transports the sheet in the horizontal direction from the right side surface 10R of the apparatus main body 10 toward the left side surface 10L.

  The fixing unit 60 is an induction heating type fixing device that performs a fixing process for fixing a toner image on a sheet, and includes a heating roller 61, a fixing roller 62, a pressure roller 63, a fixing belt 64, an induction heating unit 65, and a conveyance roller. Includes pair 66. A pressure roller 63 is pressed against the fixing roller 62 to form a fixing nip portion. The heating roller 61 and the fixing belt 64 are induction-heated by an induction heating unit 65, and give the heat to the fixing nip portion. As the sheet passes through the fixing nip portion, the toner image transferred to the sheet is fixed to the sheet.

  Next, the developing device 324 will be described in detail. FIG. 2 is a vertical sectional view schematically showing the internal structure of the developing device 324, and FIG. 3 is a horizontal sectional view of the developing device 324. The developing device 324 includes a developing housing 80 that defines an internal space of the developing device 324. The developing housing 80 is provided with a developer storing portion 81 that is a cavity for storing a developer including a non-magnetic toner and a magnetic carrier and capable of transporting the developer while stirring. It has been. Further, inside the developing housing 80, a magnetic roller 82 (developer carrying member) disposed above the developer reservoir 81, and a developing roller disposed opposite to the magnetic roller 82 at a position obliquely above the magnetic roller 82. 83 (toner carrier) and a developer regulating blade 84 disposed opposite to the magnetic roller 82.

  The developer storage unit 81 includes two adjacent developer storage chambers 81 a and 81 b extending in the longitudinal direction of the developing device 324. The developer storage chambers 81a and 81b are separated from each other by a partition plate 801 that is integrally formed with the developing housing 80 and extends in the longitudinal direction. However, as shown in FIG. 3, communication paths 803 and 804 are formed at both ends in the longitudinal direction. Are in communication with each other. The developer storage chambers 81a and 81b accommodate screw feeders 85 and 86 that rotate and rotate the shaft to agitate and convey the developer. The screw feeders 85 and 86 are rotationally driven by a drive mechanism (not shown), but their rotational directions are set in opposite directions. As a result, the developer is circulated and conveyed between the developer storage chamber 81a and the developer storage chamber 81b as shown by the arrows in FIG. By this stirring, the toner and the carrier are mixed, and the toner is negatively charged, for example.

  The magnetic roller 82 is disposed along the longitudinal direction of the developing device 324 and can be rotated clockwise in FIG. A fixed so-called magnet roll (not shown) is arranged inside the magnetic roller 82. The magnet roll has a plurality of magnetic poles. In this embodiment, the magnet roll has a drawing pole 821, a regulation pole 822, and a main pole 823. The scooping pole 821 faces the developer storage section 81, the regulation pole 822 faces the developer regulation blade 84, and the main pole 823 faces the developing roller 83.

  The magnetic roller 82 magnetically pumps (receives) the developer from the developer reservoir 81 onto its peripheral surface 82A by the magnetic force of the pumping pole 821. The developer drawn up is magnetically held as a developer layer (magnetic brush layer) on the peripheral surface 82A of the magnetic roller 82, and is conveyed toward the developer regulating blade 84 as the magnetic roller 82 rotates. .

  The developer regulating blade 84 is disposed on the upstream side of the developing roller 83 when viewed from the rotation direction of the magnetic roller 82, and regulates the layer thickness of the developer layer magnetically attached to the peripheral surface 82 </ b> A of the magnetic roller 82. The developer regulating blade 84 is a plate member made of a magnetic material extending along the longitudinal direction of the magnetic roller 82, and is supported by a predetermined support member 841 fixed at a proper position of the developing housing 80. Further, the developer regulating blade 84 has a regulating surface 842 (that is, the leading end surface of the developer regulating blade 84) that forms a regulating gap G having a predetermined size with the peripheral surface 82A of the magnetic roller 82.

  The developer regulating blade 84 formed from a magnetic material is magnetized by the regulating pole 822 of the magnetic roller 82. As a result, a magnetic path is formed between the regulating surface 842 of the developer regulating blade 84 and the regulating pole 822, that is, in the regulating gap G. When the developer layer adhering to the peripheral surface 82A of the magnetic roller 82 by the drawing pole 821 is conveyed into the regulation gap G as the magnetic roller 82 rotates, the layer thickness of the developer layer is regulated in the regulation gap G. Is done. Thereby, a uniform developer layer having a predetermined thickness is formed on the peripheral surface 82A.

  The developing roller 83 is disposed so as to extend along the longitudinal direction of the developing device 324 and in parallel with the magnetic roller 82, and can rotate clockwise in FIG. The developing roller 83 has a peripheral surface 83A that receives toner from the developer layer and carries the toner layer while rotating in contact with the developer layer held on the peripheral surface 82A of the magnetic roller 82. During the development in which the development operation is performed, the toner in the toner layer is supplied to the peripheral surface of the photosensitive drum 321.

  The developing roller 83 and the magnetic roller 82 are rotationally driven by a driving source M. A gap S having a predetermined dimension is formed between the peripheral surface 83A of the developing roller 83 and the peripheral surface 82A of the magnetic roller 82. The gap S is set to about 130 μm, for example. The developing roller 83 is disposed so as to face the photosensitive drum 321 through an opening formed in the developing housing 80, and a gap having a predetermined size (for example, about 110 μm) is also provided between the peripheral surface 83A and the peripheral surface of the photosensitive drum 321. ) Is formed.

  As shown in FIG. 3, a toner density sensor 87 that measures the toner density in the development housing 80 is disposed in the development housing 80. The toner concentration sensor 87 includes, for example, a magnetic permeability sensor that measures the magnetic permeability, and outputs a voltage corresponding to the magnetic permeability that changes according to the toner concentration.

  Next, with reference to FIG. 4, the configuration for bias application and the developing operation of the developing device 324 will be described. The developing device 324 controls the first application unit 88 and the second application unit 89 (collection unit / bias application unit), and the control unit that controls the first application unit 88 and the second application unit 89 in order to control the development operation. 90 (control means). As shown in the figure, the first application unit 88 has a DC voltage source 881 and an AC voltage source 882 connected in series, and is connected to the magnetic roller 82. A voltage obtained by superimposing the AC bias output from the AC voltage source 882 on the DC bias output from the DC voltage source 881 is applied to the magnetic roller 82. The second application unit 89 includes a DC voltage source 891 and an AC voltage source 892 connected in series, and is connected to the developing roller 83. A voltage obtained by superimposing the AC bias output from the AC voltage source 892 on the DC bias output from the DC voltage source 891 is applied to the developing roller 83.

  The DC bias and AC bias applied to the magnetic roller 82 and the developing roller 83 at the time of development in which the developing device 324 supplies toner onto the peripheral surface of the photosensitive drum 321 (develops the electrostatic latent image) are examples of For example, it is set as follows. The DC bias applied to the magnetic roller 82 is + 300V, and the DC bias applied to the developing roller 83 is + 70V. The AC bias applied to the magnetic roller 82 has a voltage value of +2.5 kV, a frequency of 4.7 kHz, and a duty ratio of 70%. The AC bias applied to the developing roller 83 has a voltage value of +1.4 kV, a frequency of 4.7 kHz, and a duty ratio of 30%.

  A developing mechanism of the electrostatic latent image on the photosensitive drum 321 will be described. The magnetic brush layer on the peripheral surface 82 </ b> A of the magnetic roller 82 is transported toward the developing roller 83 as the magnetic roller 82 rotates after the layer thickness is uniformly regulated by the developer regulating blade 84. Thereafter, in the region of the gap S (FIG. 2), a large number of magnetic brushes DB in the magnetic brush layer come into contact with the peripheral surface 83A of the rotating developing roller 83.

  At this time, the control unit 90 controls the first application unit 88 and the second application unit 89 to apply predetermined DC bias and AC bias to the magnetic roller 82 and the developing roller 83, respectively. As a result, a predetermined potential difference is generated between the peripheral surface 82A of the magnetic roller 82 and the peripheral surface 83A of the developing roller 83. Due to this potential difference, only the toner T moves from the magnetic brush DB to the peripheral surface 83A at a position where the peripheral surface 82A and the peripheral surface 83A face each other (at a position where the main pole 823 (FIG. 2) faces the peripheral surface 83A). The carrier C of the magnetic brush DB remains on the peripheral surface 82A. Accordingly, the toner layer TL having a predetermined thickness is carried on the peripheral surface 83A of the developing roller 83.

  The toner layer TL on the circumferential surface 83A is conveyed toward the circumferential surface of the photosensitive drum 321 as the developing roller 83 rotates. Since a superimposed voltage of a DC voltage and an AC voltage is also applied to the photosensitive drum 321, a predetermined potential difference is generated between the peripheral surface of the photosensitive drum 321 and the peripheral surface 83 </ b> A of the developing roller 83. . Due to this potential difference, the toner T of the toner layer TL moves to the peripheral surface of the photosensitive drum 321 (supply of toner). Thereby, the electrostatic latent image on the peripheral surface of the photosensitive drum 321 is developed, and a toner image is formed.

  FIG. 5 is a schematic diagram for explaining the toner collecting operation from the developing roller 83 to the magnetic roller 82 side. This collection operation is executed between sheets until the transfer process of the toner image to one sheet is completed, or after one print job is completed, or the like. In the present embodiment, the execution timing of the collecting operation is controlled according to the deterioration state of the developer. In general, as the developer deteriorates, the collection operation is executed more frequently. This will be described in detail later.

  In the actual developing operation, of the toner T in the toner layer TL, residual toner RT that does not move to the photosensitive drum 321 but remains on the peripheral surface 83A is generated. When the residual toner RT is conveyed to a position where the peripheral surface 83A and the peripheral surface 82A of the magnetic roller 82 face each other as the developing roller 83 rotates, the scraping force by the magnetic brush DB and the electric power between the rollers 82 and 83 are transferred. It is recovered by the force. When the magnetic brush DB having the collected residual toner RT is conveyed to the downstream side of the main pole 823 as the magnetic roller 82 rotates, the peripheral surface 82A is generated by the magnetic force of the separation pole (not shown) of the magnet roll. And is returned to the developer reservoir 81 (FIG. 2).

  Note that the recovery operation is achieved by reversing the potential difference between the magnetic roller 82 and the developing roller 83. By temporarily reversing the potential difference between the above-described sheets and after the job is completed, the residual toner RT can be forcibly peeled off from the developing roller 83, and the peripheral surface 83A can be cleaned. As a result, at the time of developing the next sheet, the toner layer TL of the developing roller 83 is formed with the toner T newly supplied from the developer storage unit 81. That is, the replacement of the toner T that forms the toner layer TL is achieved.

  For example, the DC bias and AC bias applied to the magnetic roller 82 and the developing roller 83 during the collecting operation are set as follows. The DC bias applied to the magnetic roller 82 is −300V, and the DC bias applied to the developing roller 83 is + 70V. The AC bias applied to the magnetic roller 82 has a voltage value of +2.5 kV, a frequency of 4.7 kHz, and a duty ratio of 70%. On the other hand, no AC bias is applied to the developing roller 83 (0 V).

  Next, the electrical configuration of the image forming apparatus 1 will be described. The image forming apparatus 1 includes a control unit 90 that comprehensively controls the operation of each unit of the image forming apparatus 1. FIG. 6 is a functional block diagram of the control unit 90. The control unit 90 includes a CPU (Central Processing Unit), a ROM (Read Only Memory) that stores a control program, a RAM (Random Access Memory) that is used as a work area of the CPU, and the like. The image forming apparatus 1 includes an operation unit 961, an image memory 962, and an I / F (interface) 963 in addition to the configuration described with reference to FIGS.

  The operation unit 961 includes a liquid crystal touch panel, a numeric keypad, a start key, a setting key, and the like, and receives user operations and various settings for the image forming apparatus 1.

  The image memory 962 temporarily stores image data for printing given from an external device such as a personal computer when the image forming apparatus 1 functions as a printer. When the image forming apparatus 1 functions as a copying machine, the image data optically read by the ADF 20 is temporarily stored.

  The I / F 963 is an interface circuit for realizing data communication with an external device. For example, the I / F 963 creates a communication signal in accordance with a communication protocol of a network connecting the image forming apparatus 1 and the external device, and from the network side. Are converted into data in a format that can be processed by the image forming apparatus 1. A print instruction signal transmitted from a personal computer or the like is given to the control unit 90 via the I / F 963, and image data is stored in the image memory 962 via the I / F 963.

  When the CPU executes a control program stored in the ROM, the control unit 90 performs a refresh control unit 91, a calibration control unit 92 (patch forming unit), a determination unit 93 (determination unit), and a bias control unit 94 ( A part of the control means and the recovery means), the printing rate calculation unit 95, and the durability data management unit 96 (data acquisition unit).

  The refresh control unit 91 controls a refresh operation for forcibly discharging the toner layer TL carried on the developing roller 83 to the photosensitive drum 321. The refresh control unit 91 acquires data related to the average printing rate for the sheet from the printing rate calculation unit 95, and executes the refresh operation when the average printing rate is equal to or less than a predetermined value. For example, the refresh control unit 91 acquires average print rate data every time printing processing is performed on 50 sheets, and executes the refresh operation when the average print rate is less than 2%, for example. The refresh operation is an operation in which a solid electrostatic latent image is formed on the photosensitive drum 321 between sheets or jobs, and toner is supplied from the developing roller 83 to the electrostatic latent image.

  The calibration controller 92 keeps the density of toner transferred to the sheet constant. To achieve this, the calibration controller 92 keeps the toner layer TL carried on the developing roller 83 constant. A calibration operation is performed every time a printing process (for example, 100 sheets) is performed on a sheet, and an appropriate developing bias is obtained. When the calibration operation is executed, the patch toner image P is transferred to the end position of the intermediate transfer belt 331 as shown in FIG. That is, the calibration control unit 92 forms an electrostatic latent image for a patch on the end of the photosensitive drum 321, supplies toner to the electrostatic latent image with a development bias that is currently set, and the patch A toner image P is formed.

  The patch density sensor 336 detects the density of the patch toner image P described above. When the patch toner image P has a sufficient density, a sufficient toner layer TL is carried on the peripheral surface 83A of the developing roller 83 under the current developing bias conditions. In this case, the development bias is maintained as it is. On the other hand, when the density of the patch toner image P is equal to or lower than the predetermined value, the thickness of the toner layer TL is insufficient under the current development bias conditions. In this case, the developing bias is changed so that the toner T easily moves from the magnetic roller 82 to the developing roller 83 side. This change is achieved, for example, by changing the DC bias applied to the magnetic roller 82.

  The determination unit 93 determines how much the property of the developer (toner) carried on the magnetic roller 82 has deteriorated. At least the determination unit 93 determines whether the developer characteristic is in a first state that is a predetermined reference range or a second state that is changed from the reference range. In this embodiment, the current state of the developer is determined based on the execution result of the calibration operation. In general, image deterioration due to a decrease in image density is caused by excessive charging of toner in a developer. The purpose of the calibration operation is to set a developing bias that can compensate for the above-described decrease in image density. Therefore, the set developing bias and the charged state of the toner are related. Therefore, for example, the deterioration degree of the developer can be determined depending on what voltage value the DC bias applied to the magnetic roller 82 is set by the calibration operation.

  For example, assuming that the default value of the DC bias applied to the magnetic roller 82 is +300 V, the developer characteristics are normal if the DC bias set by executing the calibration operation is in the range of 250 V to 350 V. It can be determined that the state is the first state. On the other hand, when the DC bias is set to 250 V or less, it can be determined that the toner is overcharged and the developer characteristics are deteriorated (second state) more deteriorated than the normal state. Further, when the DC bias is set to 350 V or higher, it can be determined that the toner is in an insufficiently charged state. In this manner, the current state of the developer can be determined by setting a threshold value for the DC bias set by executing the calibration operation.

  The bias control unit 94 controls the bias applied to the magnetic roller 82 and the developing roller 83 by the first application unit 88 and the second application unit 89, thereby performing the electrostatic latent image developing operation and the toner collecting operation described above. Let it run. Needless to say, the developing operation is executed when an image is formed on each sheet. On the other hand, the bias control unit 94 changes the execution timing of the toner collecting operation based on the determination result of the current state of the developer by the determination unit 93.

  If the bias controller 94 determines that the developer characteristic is in the “normal state”, the collection control execution interval is set to a predetermined first interval, and the developer characteristic is When it is determined that the state is “degraded”, the collection operation execution interval is set to a second interval shorter than the first interval. When it is determined that the toner is in a state of insufficient charging, the collection operation execution interval may be set to a third interval longer than the first interval.

  As described above, the collection operation is executed between sheets (between pages) and after the job is completed. When the determination result of the determination unit 93 is the “normal state” and the print job for a plurality of pages is given, the bias control unit 94 performs, for example, a print process for four pages (four sheets). , And at the end of the print job, the collection operation is executed. That is, for example, when a single print job with 15 pages printed is given, between the 4th and 5th sheet, between the 8th and 9th sheet, between the 12th and 13th sheet, And the collection operation is executed after the printing of 15 pages is completed.

  On the other hand, when the determination result of the determination unit 93 is the “degraded state”, the bias control unit 94 increases the execution frequency of the collection operation. For example, in the “normal state”, the collection operation is executed every 4 pages as in the above example. In the “degraded state”, the bias control unit 94 performs the collection operation every 3 pages or every 2 pages. Let it run. When the degree of deterioration of the developer is large, the collecting operation is executed for each page. As described above, when the characteristics of the developer are in the “degraded state”, the collecting operation is frequently executed, that is, the toner layer TL on the developing roller 83 is frequently replaced, so that the developer is deteriorated. Can be suppressed. In addition, since the excessively charged toner does not stay much on the developing roller 83 due to the frequent execution of the collecting operation, it is possible to smoothly discharge the toner by the above-described refresh operation, and as a result. The amount of toner discharged (toner consumption) by the refresh operation can be suppressed. When “the toner is in a state of insufficient charging”, the collection operation may be performed less frequently (for example, every 6 pages).

  Various ways of setting the collection operation execution timing by the bias control unit 94 can be exemplified. For example, the collection operation may not be performed at regular timing (first interval = 0), and the collection operation may be executed only when the determination unit 93 determines “degraded state”. Alternatively, the collection operation is set to be executed at a relatively long regular timing (for example, every time 10 pages are printed + at the end of the job), and when the determination unit 93 determines that the state is “degraded” The collection operation may be executed by interrupting the regular timing.

  Furthermore, the bias control unit 94 may set the execution timing of the collecting operation in consideration of durability factors such as the driving time of the developing device 324, the average printing rate, and the number of printed sheets. These durable elements affect the deterioration of the characteristics of the developer. In general, the developer characteristics deteriorate as the driving time and the number of printed sheets increase. Further, as the average printing rate is lower, the toner tends to be overcharged, and the developer characteristics deteriorate. Therefore, it is desirable to prepare a table of the collection operation execution timing in accordance with the durability element in advance and change the collection operation execution frequency along the table. As an example of the table, after printing 200K pages, the execution timing of the collection operation between sheets in the “normal state” is changed from “every 4 pages” to “every 3 pages”. Such a table.

  The printing rate calculation unit 95 obtains the printing rate of the toner image transferred to the sheet. For example, the printing rate calculation unit 95 refers to the image data stored in the image memory 962 and calculates the printing rate in units of sheets based on the number of printing dots. Further, the printing rate calculation unit 95 also calculates an average value of the printing rates (average printing rate).

  The durability data management unit 96 manages (stores) data relating to durability factors such as the driving time of the developing device 324 (image forming apparatus 1), the average printing rate obtained by the printing rate calculation unit 95, and the number of printed sheets. When the execution timing of the collecting operation is set in consideration of the durability element, the bias control unit 94 refers to the data regarding the durability element stored in the durability data management unit 96.

  Subsequently, the control operation of the control unit 90 will be described with reference to FIGS. FIG. 9 is a flowchart illustrating an example of a control operation of the image forming unit 30 by the control unit 90, and FIG. 9 is a flowchart illustrating an example of a toner collecting operation. Here, when the image data is given from the external device through the I / F 963, or when the original image is read by the reading unit 25 and the image data is acquired, the control unit 90 performs the image forming unit. Reference numeral 30 denotes a process for executing an image forming process (development process).

  The control unit 90 executes a developing operation for one sheet (step S1). In this case, the bias control unit 94 controls the first and second application units 88 and 89 to apply a developing bias so that a predetermined potential difference is formed between the magnetic roller 82 and the developing roller 83. When the developing operation for one sheet is completed, it is confirmed whether or not it is the collection timing for collecting the toner from the developing roller 83 (step S2). Here, as described above, the collection timing is at the end of one print job or between predetermined sheets (for example, every four pages).

  If it is the collection timing (YES in step S2), toner collection control is executed (step S20). This toner collection control will be described later with reference to FIG. After the execution of the toner collection control or when it is not the collection timing (NO in step S2), is the timing for executing the refresh operation for forcibly discharging the toner on the developing roller 83 toward the photosensitive drum 321 side? It is determined whether or not (step S3). This determination criterion is whether or not a printing process for, for example, 50 sheets has been executed since the previous refresh operation necessity determination was performed.

  When it is time to determine whether or not to perform the refresh operation (YES in step S3), the refresh control unit 91 acquires data relating to the average printing rate from the printing rate calculation unit 95 (step S4). The data acquired here is, for example, average print rate data for 50 sheets printed most recently. Then, based on the average printing rate data, it is determined whether or not the refresh operation is to be executed (step S5). The criterion here is, for example, whether or not the average printing rate is 2% or less.

  When it is determined that the refresh operation is necessary (YES in step S5), the refresh control unit 91 causes the image forming unit 30 to execute the refresh operation and forcibly consume the toner (step S6). On the other hand, if it is not the timing for determining whether or not the refresh operation is necessary (NO in step S3), steps S4 to S6 are skipped. If it is determined that a refresh operation is necessary (NO in step S3), step S6 is skipped.

  Next, it is confirmed whether it is the execution timing of the calibration operation (step S7). This criterion is whether or not a printing process for, for example, 100 sheets has been executed since the previous calibration operation was executed. When it is the execution timing of the calibration operation (YES in step S7), the calibration control unit 92 forms the patch toner image P (see FIG. 7) on the intermediate transfer belt 331, and the image density is sent to the patch density sensor 336. It is detected (step S8). Further, the calibration control unit 92 adjusts the developing bias based on the density of the patch toner image P (step S9). If it is not the execution timing of the calibration operation (NO in step S7), steps S8 and S9 are skipped.

  Thereafter, the durability data relating to the durability factors such as the driving time of the developing device 324 (image forming apparatus 1), the average printing rate obtained by the printing rate calculation unit 95, and the number of print sheets, which are managed by the durability data management unit 96, are updated. (Step S10). Thereafter, it is confirmed whether or not to end the image forming process (step S11). If the image forming process is continued (NO in step S11), the process returns to step S1 and the process is repeated. On the other hand, if there is no continuous image forming process (YES in step S11), the process ends.

  Next, the toner collection control in step S20 will be described with reference to FIG. If it is determined in step S2 in FIG. 8 that the recovery timing is reached, the bias control unit 94 controls the first and second application units 88 and 89, and collects toner between the magnetic roller 82 and the developing roller 83. A bias is applied so as to form a predetermined potential difference (step S21). Thereby, the toner collecting operation is executed.

  Subsequently, the determination unit 93 checks whether or not a new calibration operation has been performed after the previous calibration operation has been performed (step S22). When the calibration operation is newly executed (YES in step S22), the determination unit 93 acquires the value of the developing bias adjusted by the calibration control unit 92 (step S9 in FIG. 8) (step S23). When the calibration operation is not newly executed (NO in step S22), step S23 is skipped. Further, the determination unit 93 acquires the latest durability data managed by the durability data management unit 96 (step S24).

  Based on the acquired development bias value and endurance data, the determination unit 93 identifies the current state of the developer and determines whether or not it is necessary to change the execution timing of the toner recovery operation (step S25). As exemplified above, when the calibration control unit 92 performs adjustment to change the DC bias applied to the magnetic roller 82 according to the density of the patch toner image P in the calibration operation, the determination unit 93 The current state of the developer is identified based on the determination element based on what value the value of the DC bias is adjusted and whether correction based on the durability data is necessary.

  If the execution timing of the current toner recovery operation does not match the current state of the developer, typically, when the characteristics of the developer deteriorates and more frequent toner recovery operation is required, the determination unit 93 Is determined as “recovery timing needs to be changed” (YES in step S25). And the determination part 93 updates collection | recovery timing according to the table etc. which were prepared beforehand (step S26). On the other hand, when the execution timing of the current toner collection operation matches the current state of the developer, the determination unit 93 determines that “collection timing change is unnecessary” (NO in step S25).

<Verification example>
The specifications of the image forming unit 30 are as follows.
[Photosensitive drum 321]
A drum using an amorphous silicon-based material, diameter = φ30 mm, peripheral speed = 300 mm / sec.
[Developing roller 83]
Diameter = φ20 mm, peripheral speed = 450 mm / sec, rotation direction is forward with the photosensitive drum at the opposed portion.
[Magnetic roller 82]
Diameter = φ20 mm, peripheral speed = 450 mm / sec, rotation direction is opposite to the developing roller and counter direction.
[Development bias]
Developing roller 83; AC bias = + 1.4 kV, duty ratio = 30%, DC bias = + 70V.
Magnetic roller 82; AC bias = + 2.5 kV, duty ratio = 70%, DC bias = + 300V.
[Bias when collecting toner]
Developing roller 83; AC bias = 0V, DC bias = + 70V.
Magnetic roller 82; AC bias = + 2.5 kV, duty ratio = 70%, DC bias = −300V.
[Developer]
Toner: particle size 6.8 μm, carrier: particle size 35 μm
Toner / carrier weight ratio: 10%

The operation of the image forming unit 30 is set as follows.
[Refresh operation]
The average printing rate is determined each time 50 sheets are printed. When the average printing rate is less than 2%, the toner on the developing roller 83 is forcibly discharged to the photosensitive drum 321.
[Calibration operation]
Each time 100 sheets are printed, a patch toner image is printed, and the toner density is measured by the patch density sensor 336. The DC bias of the magnetic roller 82 is adjusted so that the image density can be kept constant.
[Toner recovery operation]
In accordance with the adjustment value (default value = + 300 V) of the DC bias of the magnetic roller 82, the timing of the toner recovery operation to be executed between the sheets is changed as follows. When the DC bias adjustment value of the magnetic roller 82 is 0 to +200 V, the toner collecting operation is executed every time printing is performed on one sheet. When the DC bias is +200 V to +250 V, the toner is printed every two sheets. Collecting operation is executed. When DC bias = + 250V to + 350V, toner collecting operation is executed every time printing is performed on four sheets (default timing), and when DC bias = + 350V or more, printing is performed on six sheets. Execute toner collection operation every time.
[Correction based on durability data]
Every time the number of printed sheets exceeds 200k, the execution timing of the toner recovery operation under the adjustment value condition of each DC bias is shortened by one sheet. For example, when DC bias = + 250V to + 350V and the number of printed sheets exceeds 200k, the toner collecting operation is executed every time printing is performed on three sheets, and when the number exceeds 400k, two sheets are printed. The toner collecting operation is executed every time printing is performed on the sheet.

  As an experimental developer, unused initial developer, developer after printing processing on 100k sheets with an average printing rate = 5%, printing processing on 300k sheets with an average printing rate = 5% And a developer after printing processing was performed on 500 k sheets at an average printing rate of 5%. Using each test developer, the number of printed sheets = 0, 1000, 2000, 5000, and 10000 sheets, image formation based on a document with a printing rate of 0.5% and a printing rate of 20% Processing was performed, and the density of the solid image at each print number step was measured. In the embodiment, the execution timing of the toner collection operation is changed according to the settings of “toner collection operation” and “correction based on durability data” described above. On the other hand, in the comparative example, the execution timing of the toner collection operation is not changed, and the default timing is maintained. Table 1 shows the density measurement results of the solid images of the examples and comparative examples.

  As is apparent from Table 1 above, no significant difference appears in the solid image density for a document with a printing rate of 20%, regardless of the type of the test developer. However, in the case of a document with a printing rate of 0.5%, a remarkable difference was observed between the two. In the example in which the execution timing of the collecting operation is appropriately changed, a constant image density can be obtained in the range of 0 to 10000 printed sheets for any of the test developers. On the other hand, in the comparative example, the density was significantly reduced for the sheets printed after 5000 sheets. In the comparative example, when approximately 2000 sheets are printed, the maximum value of the development bias adjustment range prepared in advance to keep the image density constant is reached, and thereafter, the reduction in image density is compensated by adjusting the development bias. I reached a state where I couldn't.

  Subsequently, the amount of toner discharged by the refresh operation was compared. Table 1 shows the execution timing of the toner recovery operation under the adjustment value condition of each DC bias using the developer after the printing process was performed on 100 k sheets with an average printing rate = 5% as the test developer. Further, the density of the solid image was measured in the same manner as described above by shortening by one sheet from the example for “100 k developer” (for example, the default timing was printed every three sheets). Table 2 shows the density measurement result of the solid image.

  As shown in Table 2, no significant reduction in the solid image density was observed in the range of 0 to 10000 printed sheets. Further, when the amount of toner discharged by the refresh operation is compared between the example for “100 k developer” in Table 1 and the example shown in Table 2, the amount of toner discharged is reduced by 0.5%. I was able to. That is, it has been confirmed that the toner consumption by the refresh operation can be suppressed by shortening the execution timing of the toner collecting operation.

  Next, another embodiment of a method for finding out the characteristics of the developer will be exemplified. In the above-described embodiment, the mode in which the current state of the developer is determined using the steady calibration operation has been described. In another embodiment, the calibration control unit 92 performs a special patch toner image forming operation, and the determination unit 93 determines the current state of the developer based on the density detection result of the special patch toner image. Show.

  The special patch toner image is formed by sequentially using the same portion of the peripheral surface 83A of the developing roller 83 to form a first halftone patch toner image, a solid patch toner image, and a second halftone patch toner image. Is to form. Then, the developer characteristics are determined based on the density difference between the first patch toner image and the second patch toner image. Hereinafter, this embodiment will be described in detail.

  FIG. 10 is a schematic perspective view showing a patch toner image forming state. A predetermined carrying position 83P on the peripheral surface 83A of the developing roller 83 is selected as a patch toner image formation region. That is, the calibration control unit 92 forms the electrostatic latent image 321A for the patch toner image on the peripheral surface of the photosensitive drum 321 so that the toner for the patch toner image is supplied from the carrying position 83P. The electrostatic latent image 321A is developed with toner applied from the carrying position 83P, and transferred to the intermediate transfer belt as a patch toner image P. Then, the density of the patch toner image P is measured by the patch density sensor 336.

  In the present embodiment, the calibration control unit 92 forms the three patch toner images shown in FIG. 11 using the carrying position 83P continuously for each rotation of the developing roller 83. That is, first, at the first rotation of the developing roller 83, a halftone first patch toner image P11 is formed on the photosensitive drum 321 using the toner at the carrying position 83P. The first patch toner image P11 is a patch toner image having an arbitrary density selected from a range of 25% to 75% in area ratio (area ratio in which dots occupy the area of the carrying position 83P).

  Next, in the second rotation of the developing roller 83, a solid patch toner image PF having an area ratio of 100% or in the vicinity thereof is formed using the toner at the carrying position 83P. Subsequently, at the third rotation of the developing roller 83, the second patch toner image P12 having the same area ratio as the first patch toner image P11 is formed using the toner at the carrying position 83P. That is, the second patch toner image P12 is a patch toner image that is selected from the range of 25% to 75% and is formed with the same area ratio as that of the first patch toner image P11.

  The densities of the patch toner images P11, PF, and P12 are sequentially measured by the patch density sensor 336. Of these, the determination unit 93 obtains the density difference between the first patch toner image P11 and the second patch toner image P12, and determines the current state of the developer (first state or second state) based on the density difference. Another decision).

  When a solid toner image having an area ratio of 100% is formed, most of the toner carried on the peripheral surface 83A of the developing roller 83 is supplied to the peripheral surface of the photosensitive drum 321. In the subsequent toner image formation, if the developer is deteriorated, for example, if the toner is excessively charged, a sufficiently thick toner layer TL (see FIG. 4) is formed on the peripheral surface 83A of the developing roller 83. It becomes difficult to carry. As a result, the density of the toner image formed after the solid toner image is formed tends to be insufficient. In this embodiment, the characteristics of the developer are estimated using this phenomenon.

  When there is no change in developer characteristics or developer deterioration due to environmental changes, even if the solid patch toner image PF is formed in the middle, there is a difference in density between the first patch toner image P11 and the second patch toner image P12. Hardly occurs. This is because, when the toner is not overcharged, even if almost all of the toner carried on the carrying position 83P is supplied to the photosensitive drum 321 for the formation of the solid patch toner image PF, it is sufficient again at the carrying position 83P. This is because a large amount of toner is carried. On the other hand, when the toner is overcharged, the density of the second patch toner image P12 is lighter than that of the first patch toner image P11. This is because if the toner is discharged due to the formation of the solid patch toner image PF, sufficient toner is not carried on the carrying position 83P immediately. Therefore, the degree of change in the developer characteristics can be evaluated based on the degree of density difference between the first patch toner image P11 and the second patch toner image P12.

  For example, the patch density sensor 336 obtains the density by the relational expression of density = Log (1 / Q) based on the light transmittance Q of the patch toner image, and the density of the white background≈0, the highest density = 1. The case where density is evaluated in the numerical range of 5 is taken as an example (the density data shown in Table 1 and Table 2 is also based on this scale). In this case, for example, based on whether or not the density difference between the first patch toner image P11 and the second patch toner image P12 is 0.1 or more, the developer characteristic is “normal state” (first state). Or “degraded state (change state)” (second state). That is, when the density difference is less than 0.1, the determination unit 93 determines that the developer characteristic is “normal state”. On the other hand, when the density difference is 0.1 or more, the determination unit 93 determines that the developer characteristic is “degraded”, and in this case, the execution timing is changed so that the execution interval of the toner recovery operation is shortened. To do. Note that when the density difference is less than 0.1, the execution timing may be changed so that the execution interval of the toner collection operation becomes longer.

Various modes of changing the execution timing of the toner recovery operation can be exemplified. For example,
(1) It is set so that the toner recovery operation is executed at a certain interval (first interval) at a slightly longer interval, and the developer difference is detected by the above-described patch toner image density difference detection operation performed at a constant frequency. When it is determined that the characteristic is “degraded state”, a toner recovery operation is additionally interrupted and executed during the predetermined period (second interval) (Example 1).
(2) The developer characteristics are sequentially determined by the density difference detection operation performed at a constant frequency, and the execution interval (first interval) of the immediately preceding toner collection operation is set as the execution interval (second interval) according to the determination result. (Example 2) or
(3) The toner characteristic is not “deteriorated” by the density difference detection operation performed at a constant frequency of relatively short eyes without regularly performing the toner collection operation (in this case, the first interval = 0). When it is determined that the toner collection operation is performed (Example 3),
Can be exemplified.

  The aspect of the first embodiment is suitable for a high-speed printer because it can cope with a rapid change in developer characteristics due to an environmental change or the like, and the frequency of the density difference detection operation can be reduced. The aspect of the second embodiment is suitable for a high-speed printing machine because it can cope with a relatively gradual change in developer characteristics such as deterioration of the developer over time, and similarly the frequency of the density difference detection operation can be reduced. In the third embodiment, since the frequency of the density difference detection operation needs to be increased, the embodiment 3 is exclusively used for a low-speed printer. However, the number of execution times of the toner collection operation can be minimized.

  FIG. 12 is a flowchart when the control unit 90 performs the control of the first and third embodiments. When the timing for detecting the density of the patch toner image arrives, the calibration control unit 92 forms an electrostatic latent image 321A for a halftone patch toner image on the peripheral surface of the photosensitive drum 321, and this electrostatic latent image 321A is formed. Development is performed with toner supplied from the carrying position 83P of the developing roller 83. By this developing operation, a first patch toner image P11 is formed on the photosensitive drum 321 and transferred to the intermediate transfer belt 331 (step S31). Then, the density D1 of the first patch toner image P11 is measured by the patch density sensor 336 (step S32).

  Next, the calibration control unit 92 forms a solid patch toner image PF on the photosensitive drum 321 with the toner supplied from the same carrying position 83P in the second rotation of the developing roller 83 (step S33). . The solid patch toner image PF may be transferred to the intermediate transfer belt 331 or may be removed by the cleaning device 326 (FIG. 1) without applying the primary transfer bias.

  Subsequently, in the third rotation of the developing roller 83, the calibration control unit 92 uses the toner supplied from the same carrying position 83P, and the first patch toner image P11 has the same area ratio as that of the first patch toner image P11. A two-patch toner image P12 is formed (step S34). Then, the density D2 of the second patch toner image P12 is measured by the patch density sensor 336 (step S35).

  Thereafter, the determination unit 93 obtains the density difference between the density D1 of the first patch toner image P11 and the density D2 of the second patch toner image P12, and determines the characteristics of the developer (step S36). If the conditional expression D1−D2 ≦ 0.1 is not satisfied (NO in step S36), the determination unit 93 determines that the developer characteristic is “degraded”. Then, the bias controller 94 interrupts and executes a separate toner collection operation in addition to the toner collection operation executed at regular timing (Example 1) or causes the toner collection operation to be executed (Example). 3) (Step S37).

  On the other hand, when the conditional expression of density difference D1-D2 ≦ 0.1 is satisfied (YES in step S36), the determination unit determines that the developer characteristic is “normal state”. In this case, the bias control unit 94 executes only the toner recovery operation that is executed at the regular timing (Example 1) or does not execute the toner recovery operation (Example 3).

  FIG. 13 is a flowchart when the control unit 90 performs the control of the second embodiment. In FIG. 13, the processing from step S41 to step S45 is the same as the processing from step S31 to step S35 shown in FIG.

  The determination unit 93 obtains the density difference between the density D1 of the first patch toner image P11 and the density D2 of the second patch toner image P12, and determines the developer characteristics (step S46). When the conditional expression of density difference D1-D2 ≦ 0.1 is satisfied (YES in step S46), the determination unit 93 determines that the developer characteristic is “normal state”. In this case, the bias controller 94 sets the execution interval of the toner collection operation longer than immediately before. For example, if the execution frequency of the toner collection operation immediately before the detection of the density difference of the patch toner image is performed every time printing processing is performed on four sheets, the frequency is changed every five sheets.

  On the other hand, when the conditional expression of density difference D1-D2 ≦ 0.1 is not satisfied (NO in step S46), the determination unit 93 determines that the developer characteristic is “degraded state”. In this case, the bias control unit 94 sets the execution interval of the toner collection operation to be shorter than before. For example, if the execution frequency of the toner collection operation immediately before the detection of the density difference of the patch toner image is performed every time printing processing is performed on four sheets, the frequency is changed every three sheets. .

  It describes about the confirmation experiment matched with the said Examples 1-3. The image forming unit 30 having the same specifications as the above-described verification example continuously performs image forming operation on an original with a printing rate = 2%, and the toners in the embodiments 1 to 3 and the comparative example described below. The collection operation was executed.

<Example 1>
As a regular timing toner collection operation, every time printing is performed on 10 sheets, the toner collection operation is executed between sheets. The patch toner image density difference is detected every time printing is performed on 100 sheets, and when the developer characteristic state determination result is “degraded”, until the next patch toner image density difference detection timing is reached. In addition to the regular timing, an additional toner collecting operation is performed. Accordingly, for example, every time printing processing is performed on five sheets, a toner collecting operation is performed between the sheets.

<Example 2>
As an initial setting, every time printing is performed on 10 sheets, a toner collecting operation is executed between the sheets. The patch toner image density difference is detected every time printing is performed on 100 sheets. When the developer characteristic state determination result is “degraded”, the toner collection operation execution interval is shortened by one (every nine sheets), and when it is “normal state”, the toner collection operation execution interval. Is controlled to be longer by one sheet (every 11 sheets).

<Example 3>
The density difference detection of the patch toner image is performed every time printing processing is performed on 50 sheets and at the end of one printing job. When the developer characteristic state determination result is “degraded state”, the toner collecting operation is performed once immediately after that. When the developer characteristic state determination result is “normal state”, the toner recovery operation is not performed.

<Comparative example>
Each time printing processing is performed on four sheets, a toner collecting operation is executed between the sheets. The execution interval of the toner collecting operation is unchanged.

  FIG. 14 shows a predetermined number of sheets when the toner collection operation is executed in the modes of the first embodiment, the third embodiment, and the comparative example while the image forming process is continuously performed on 1 to 5000 sheets. It is a graph which shows the measurement result of image density. FIG. 15 shows the measurement of the image density for each predetermined number of sheets when the toner collecting operation is executed in the mode of Example 2 and the comparative example while image forming processing is continuously performed on 1 to 250,000 sheets. It is a graph which shows a result. As apparent from FIGS. 14 and 15, it was confirmed that when the toner collecting operation was executed in the modes of Examples 1 to 3, no significant decrease in image density occurred.

  As described above, in the image forming apparatus 1 of the present embodiment, the characteristics of the developer layer are determined by the determination unit 93, and the execution timing of the toner recovery operation is determined according to the determination result. Accordingly, the toner collecting operation can be executed according to the current state of the developer, and the image forming apparatus 1 including the developing device 324 adopting the touch-down developing method is not allowed to execute the collecting operation. Quality can be guaranteed.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 10 Apparatus main body 30 Image forming part 32Y, 32M, 32C, 32Bk Image forming unit 321 Photosensitive drum (image carrier)
324 Development Device 34 Toner Supply Unit (Toner Supply Unit)
80 Developing housing 82 Magnetic roller (Developer carrier)
83 Developing roller (toner carrier)
88 1st application part (collection means / bias application means)
89 Second application unit (recovery means / bias application means)
90 control unit 91 refresh control unit 92 calibration control unit (patch forming means)
93 Determination unit (determination means)
94 Bias control part (part of control means and recovery means)
95 Print rate calculation unit 96 Durability data management unit (data acquisition means)

Claims (2)

An image carrier for carrying an electrostatic latent image and a toner image to be transferred to the sheet;
A development housing for storing developer including toner and carrier;
A developer carrying member that receives the developer in the developing housing while rotating in a predetermined direction and carries the developer layer;
While rotating in contact with the developer layer, it receives toner from the developer layer and carries the toner layer, and supplies the toner in the toner layer to the image carrier for developing the electrostatic latent image. A toner carrier,
A collecting means for performing a collecting operation for forcibly collecting the toner carried on the toner carrying member on the developer carrying member;
Determining means for determining whether the property of the developer carried on the developer carrying member is in a predetermined normal state or a deteriorated state in which the property of the developer is deteriorated from the normal state ;
Control means for executing the recovery operation at a predetermined timing;
Patch forming means for forming a patch toner image on the image carrier;
Density detecting means for detecting the density of the patch toner image ,
The control means includes
When the determination unit determines that the developer characteristic is in the normal state, the execution interval of the collecting operation is set to a predetermined first interval;
It said determining means, when the characteristics of the developer is determined to be the deteriorated state, the interval of the recovery operation is intended to set to a shorter second interval than the first distance,
The patch forming means includes
Forming a first patch toner image having a predetermined area ratio on the image carrier with toner supplied from a predetermined carrying position of the toner carrier;
Next, a solid patch toner image having an area ratio of 100% or its vicinity is formed on the image carrier by the toner supplied from the carrying position,
After that, the second patch toner image having the same area ratio as the first patch toner image is formed on the image carrier by the toner supplied from the carrying position,
The image forming apparatus , wherein the determination unit distinguishes the normal state or the deteriorated state based on a density difference between the first patch toner image and the second patch toner image . The image forming apparatus according to claim 1 , further comprising:
Further comprising data acquisition means for acquiring at least one of data relating to the driving time of the image forming apparatus, data relating to the average printing rate and data relating to the number of printed sheets,
The determination unit is an image forming apparatus that handles the data acquired by the data acquisition unit as a demarcation element in the normal state.

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