JP2013109087A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress deterioration and scattering of toner in an image forming apparatus that includes a development device adopting a touch-down development method.SOLUTION: When the integrated value of a printing ratio difference being a difference in printing ratios between a standard printing ratio and an actual printing ratio reaches a first threshold, a discharge operation is performed for forcibly discharging toner on a development roller to a photoreceptor drum. Further, when the number of prints reaches a second threshold, a recovery operation is performed for forcibly recovering toner on the development roller to a magnetic roller. When the integration value of the printing ratio difference reaches a first threshold as indicated in a clock time T3, the count value of the number of prints is reset. While a certain number of prints is counted between the clock-time T2 and T3, the count value is rewritten to zero (0) at the clock-time T3. This configuration puts the discharge operation at higher priority than the recovery operation, thereby performing the aforementioned two operations at different timings.

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.

  An image forming apparatus having a function of executing a refresh operation (discharge operation) for forcibly discharging toner staying on a developing roller to an image carrier is also known. 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 collection operation and the refresh operation described above are the same for the purpose of removing the toner staying on the developing roller for a long time, although the destination of the toner is different. However, in the conventional image forming apparatus, the collection operation and the refresh operation are individually executed based on each index. The collection operation is executed each time printing is performed on a certain number of sheets, for example, using the number of printed sheets as an index. The refresh operation is executed when, for example, the above-mentioned average printing rate is used as an index and a certain condition is satisfied. For this reason, both may reach the execution time at the same timing. In this case, the time interval between sheets is extended, and both the collection operation and the refresh operation are executed. This leads to a decrease in print processing capability.

  The refresh operation is an operation for discharging toner that may be deteriorated to the outside of the housing of the developing device. Therefore, although there is a problem that the amount of toner consumption increases, it is excellent from the viewpoint of preventing the generation of deteriorated toner. Yes. On the other hand, since the collecting operation is an operation for returning the toner on the developing roller into the housing, the degree of toner deterioration can be suppressed, but it is not a perfect countermeasure for preventing the deterioration. When toner collection from the developing roller and toner re-loading to the developing roller are repeated, the deterioration of the toner proceeds.

  Another problem is toner scattering during the collecting operation. This is because the toner density of the magnetic brush increases when the toner is peeled off from the developing roller by the magnetic brush. The scattered toner causes image quality deterioration and internal contamination. In recent years, there has been a demand for an increase in printing speed, or in order to prevent a reduction in printing productivity even if the collection operation and the refresh operation are performed between sheets, it is required to increase the linear speed of the image forming apparatus. When the speed is increased in order to satisfy such a requirement, the above-described problem of toner scattering becomes more prominent.

  The present invention has been made in view of the above problems, and provides an image forming apparatus capable of suppressing toner deterioration and toner scattering in an image forming apparatus including a developing device employing a touch-down developing method. Objective.

  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 carrying body having a circumferential surface to carry, and supplying the toner of the toner layer to the image carrying body for developing the electrostatic latent image; and the toner carried on the toner carrying body to the image carrying body A discharging means for forcibly discharging the toner to the developer carrying body, a collecting means for forcibly collecting the toner carried on the toner carrying body to the developer carrying body, the discharging action and the collecting action Execution Control means for controlling image forming, and the control means executes the discharge operation when the first count value for the first index related to image formation reaches a first threshold value, or The collection operation is executed when a second count value for two indexes reaches a second threshold value, and the first count value is set to the first threshold value before the second count value reaches the second threshold value. When the threshold value is reached, the second count value counted so far is reset.

  According to this configuration, the control means resets the second count value as described above. For this reason, the collection operation is not executed simultaneously with the discharge operation or immediately after the execution of the discharge operation. In this way, by giving priority to the discharging operation for discharging the toner out of the developing housing, it is possible to reduce the number of times the collecting operation is performed, and to suppress toner deterioration and toner scattering. Further, since the collecting operation and the discharging operation are not performed simultaneously between one sheet, it is not necessary to increase the time interval between sheets. In other words, in order to maintain printing productivity, it is not necessary to increase the linear velocity by increasing the time interval between sheets. This contributes to suppression of toner scattering.

  In the above configuration, the first index is a printing rate indicating a ratio of a toner image transfer area to a printable area of the sheet, and the first count value is a predetermined standard printing rate for one sheet. It is desirable that this is an integrated value of the printing rate difference, which is the difference between the standard printing rate and the actual printing rate, which is counted when the transfer of the toner image at a lower actual printing rate is executed.

  According to this configuration, when a so-called low printing rate toner image is transferred, a printing rate difference that is a difference from the standard printing rate is obtained, and every time such a printing rate difference occurs, The value is accumulated. Then, when the integrated value of the printing rate differences reaches a predetermined first threshold value, the discharging operation is executed.

  In this case, a relationship between the printing rate difference and an image length formed with toner forcibly discharged from the toner carrier to the image carrier in the discharging operation is determined in advance, and the first threshold is A value corresponding to the image length of one round of the peripheral surface of the toner carrier when the integrated value of the printing rate difference is converted into the image length is desirable.

  This configuration is premised on forcibly discharging the toner corresponding to the difference in printing rate to the image carrier when a toner image transfer with a low printing rate that is insufficient for the standard printing rate is performed. Then, when the toner of the printing rate difference reaches an amount corresponding to the image length of one circumference of the peripheral surface of the toner carrier, the discharging operation is executed. Therefore, the toner staying on the toner carrier can be efficiently discharged, and the toner consumption can be suppressed.

  In the above configuration, when the second count value reaches the second threshold before the first count value reaches the first threshold, the control unit resets the first count value counted so far. It is desirable to hold without.

  According to this configuration, even if the collecting operation is executed, the first count value is not reset. Accordingly, it is possible to cause the control means to execute the discharging operation with higher priority.

  In the above configuration, it is desirable that the second index is a number of printed sheets indicating the number of sheets on which the transfer of the toner image has been performed on the sheet.

  According to this configuration, since the execution timing of the collection operation is determined using a simple index such as the number of printed sheets, the control can be simplified.

  In the above configuration, it is preferable that the control unit performs the discharging operation or the collecting operation between sheets when the transfer of the toner image is continuously performed on a plurality of sheets.

  In the case where the discharging operation or the collecting operation is performed between the sheets, if both the operations are controlled separately, there may be a case where the discharging operation and the collecting operation must be performed between one sheet. However, according to the present invention, since the discharging operation and the collecting operation are not executed between one sheet, it is possible to contribute to shortening between sheets.

  According to the present invention, in the image forming apparatus including the developing device adopting the touch-down developing method, the discharging operation is prioritized over the collecting operation, so that it is possible to suppress toner deterioration and toner scattering. Therefore, it is possible to provide an image forming apparatus that can maintain 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. It is a figure of a table format which shows an example of integration | accumulation of a printing rate difference. 6 is a time chart for explaining a relationship between a toner discharging operation and a collecting operation. 6 is a flowchart for explaining a toner discharging operation and a collecting operation.

  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 has 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 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 are accommodated.

  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) and the control unit 90 (control unit) that controls the first application unit 88 and the second application unit 89 in order to control the development operation. ). 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. A discharge operation (refresh operation) forcibly discharging the toner on the developing roller 83 to the photosensitive drum 321 executed at a timing other than the time of image formation is the same operation as the above-described developing operation.

  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 discharging operation is prioritized over the collecting operation. 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.

  The control unit 90 executes an image formation control unit 91, a discharge control unit 92 (a part of the discharge unit), and a collection control unit 93 (a part of the collection unit) when the CPU executes a control program stored in the ROM. , A printing rate calculation unit 94, a first counter unit 95, a second counter unit 96, an execution timing control unit 97 (control means), and a storage unit 98.

  The image formation control unit 91 controls each part of the image forming unit 30 and the fixing unit 60 described above based on the print instruction information given from the operation unit 961 or the print instruction information given from the outside via the I / F 963. The image forming operation is performed on the sheet.

  The discharge controller 92 controls a discharge operation (refresh operation) for forcibly discharging the toner layer TL carried on the developing roller 83 to the photosensitive drum 321. This discharge operation is an operation of forming a solid electrostatic latent image on the photosensitive drum 321 between sheets or jobs and supplying toner from the developing roller 83 to the electrostatic latent image. The discharge control unit 92 uses, as an index (first index), a printing rate that is calculated by the printing rate calculation unit 94 and indicates the ratio of the transfer area of the toner image to the printable area of the sheet. Specifically, the discharge control unit 92 counts the standard print rate that is counted when the transfer of the toner image with a real print rate lower than a predetermined standard print rate is executed for one sheet. When the integrated value (first count value) of the printing rate difference, which is the difference between the actual printing rate and the actual printing rate, reaches a predetermined threshold value (first threshold value), the discharge operation is executed. The integrated value of the printing rate difference is counted by the first counter unit 95.

  The collection controller 93 controls a collection operation for forcibly collecting the toner layer TL carried on the developing roller 83 to the magnetic roller 82. The collection control unit 93 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 as described above with reference to FIG. Is executed. The collection control unit 93 uses, as an index (second index), the number of printed sheets that is the number of sheets for which image forming processing (toner image transfer processing) has been performed on the sheet, and this number of printed sheets is set to a predetermined number (second threshold). When it reaches, the collecting operation is executed. In addition to the number of printed sheets, for example, the driving time of the image forming unit 30 may be used as an index, and the collecting operation may be executed when the integrated value of the driving time exceeds a predetermined threshold.

  The printing rate calculation unit 94 obtains the printing rate of the toner image transferred to the sheet. For example, the printing rate calculation unit 94 refers to the image data stored in the image memory 962 and calculates the printing rate for each sheet based on the number of printing dots. The calculated printing rate for each sheet is output to the first counter unit 95 described below as actual printing rate data.

  The first counter unit 95 counts the integrated value (first count value) of the printing rate difference. The printing rate difference is a difference between a predetermined standard printing rate (for example, 2%) and a printing rate (actual printing rate) of a toner image actually transferred to one sheet. When the actual printing rate is lower than the standard printing rate, the first counter unit 95 adds up the printing rate differences. That is, if the standard printing rate = 2% and the actual printing rate = 0.5%, the printing rate difference = 1.5% is integrated, but the printing rate difference when the printing rate = 4.0%. = 2.0% is not accumulated.

  That is, the first counter unit 95 accumulates the printing rate difference each time a so-called low printing rate toner image is transferred. When the integrated value of the printing rate differences reaches a predetermined threshold (hereinafter referred to as a first threshold), an execution timing control unit 97 described later sends an execution instruction signal to the discharge control unit 92 to execute the discharge operation. As described above, in the image forming apparatus 1 according to the present embodiment, in the printing process for one sheet, it is assumed that at least the toner corresponding to the standard printing rate is consumed, and the standard printing rate is low. When the toner image transfer at the printing rate is performed, the toner corresponding to the printing rate difference is forcibly discharged by the discharging operation. As a result, the toner staying on the developing roller 83 can be efficiently discharged, and the toner consumption can be suppressed.

  The first threshold value will be described. In the present embodiment, the first threshold value is a value when the integrated value of the printing rate difference reaches a value corresponding to the toner amount for the image length of one circumference of the circumferential surface 83A of the developing roller 83. Here, the relationship between the printing rate difference and the image length formed by the toner that is forcibly discharged from the developing roller 83 to the photosensitive drum 321 in the discharging operation is predetermined. For example, when the printing rate difference is 1%, the above relationship is determined such that the length of the image formed on the photosensitive drum 321 in the discharging operation in the sub-scanning direction is 2 mm. In this example, when an image is formed on an A4 size sheet at a printing rate of 100% (solid image), the image length in the sub-scanning direction is 210 mm. "Is defined as a unit of image length for forced discharge.

  The first threshold value is determined in view of the length of the peripheral surface 83A of the developing roller 83 and the rotation speed thereof. For example, when the developing roller 83 is a 20 mmφ roller, the length of one round is about 63 mm. When the rotation speed Sv of the developing roller 83 and the rotation speed Dv of the photosensitive drum 321 are the same (Sv / Dv = 1.0), the first threshold value is the integration of the printing rate difference corresponding to an image length of 63 mm. Value (≈31.5%). On the other hand, when the rotation speed Sv of the developing roller 83 is faster, the image length becomes shorter. For example, when Sv / Dv = 1.6, the first threshold value is an integrated value (≈20%) of a printing rate difference corresponding to an image length of about 40 mm.

  FIG. 7 is a tabular view showing an example of accumulation of the printing rate difference by the first counter unit 95. In FIG. 7, the integration is performed with the standard print rate = 2%, the image length converted value of the print rate difference is 1% = 2 mm, and the image length (first threshold) for one turn of the developing roller 83 is 40 mm. An example is shown. The printing rate of the first sheet is 1.0%. Therefore, the printing rate difference = 1.0%. In this case, the first counter unit 95 counts “1.0%” as the integrated value (difference integrated value) of the printing rate difference. In addition, the conversion value (image conversion value) of the difference integrated value to the image length at this time is 2 mm.

  The printing rate of the second sheet is 3.5%. Since this printing rate is larger than the standard printing rate, the first counter unit 95 does not count this. Therefore, the difference integrated value remains at 1% as before. The printing rate of the third sheet is 0.2%, and the printing rate difference = 1.8%. In this case, the first counter unit 95 performs an accumulation process of the printing rate difference, thereby increasing the difference accumulated value = 2.8%. Thereafter, the first counter unit 95 performs the same processing for each printing. In the example of FIG. 7, at the stage where the image forming process for the 34th sheet is completed, the integrated difference value exceeds 20% and the image conversion value exceeds 40 mm corresponding to the first threshold value. At this time, the discharge operation is performed by the discharge control unit 92. Then, the integrated value count of the printing rate difference by the first counter unit 95 is reset, and a new integrating process is started from the 35th sheet to be printed next.

  The second counter unit 96 counts the number of printed sheets on which the image forming unit 30 has performed image forming processing (toner image transfer processing) on the sheet. When the number of printed sheets (second count value) reaches a predetermined threshold number (hereinafter referred to as a second threshold), the collecting operation is executed. The second threshold is an appropriate number selected from a range of about 3 to 15 sheets, for example. As another embodiment, the second counter unit 96 may count the drive time of the image forming unit 30.

  The execution timing control unit 97 controls the execution timing of the discharge operation by the discharge control unit 92 and the recovery operation by the recovery control unit 93. The execution timing control unit 97 sends the discharge operation execution instruction signal to the discharge control unit 92 at a timing when the integrated value of the printing rate differences by the first counter unit 95 exceeds the first threshold value. In response to the execution instruction signal, the discharge control unit 92 executes the discharge operation. Further, the execution timing control unit 97 sends an execution instruction signal for the collection operation to the collection control unit 93 at a timing when the count value of the number of printed sheets by the second counter unit 96 exceeds the second threshold value. In response to the execution instruction signal, the collection control unit 93 executes the collection operation.

  Further, the execution timing control unit 97, if the integrated value of the printing rate difference by the first counter unit 95 reaches the first threshold before the count value of the number of printed sheets by the second counter unit 96 reaches the second threshold, Reset the count value so far of the number of printed sheets. That is, the collection operation is prevented from being executed at the same timing (between the same sheets) or at close timing. Note that the execution timing control unit 97 does not perform reverse reset. That is, even if the count value of the number of printed sheets reaches the second threshold value before the integrated value of the printing rate difference reaches the first threshold value, the execution timing control unit 97 sets the count value of the integrated value of the printing rate difference so far. Keep it without resetting.

  As described above, the collection operation and the discharge operation are the same for the purpose of removing the toner staying in the developing roller 83 for a long time although the destination of the toner is different, and it is less meaningful to execute both at the same timing. . Further, if both operations are executed at the same timing, a long time interval between sheets is required, and the printing productivity is also deteriorated. In order to compensate for the deterioration in printing productivity, it is necessary to increase the linear velocity, but in this case, a problem of toner scattering occurs. Further, since the collecting operation is not an operation for discharging toner that may be deteriorated to the outside of the developing housing 80, if the collecting operation is repeated, the ratio of the deteriorated toner in the developing housing 80 may eventually increase. In view of these points, the execution timing control unit 97 gives priority to the discharge operation of discharging the toner out of the developing housing 80, and the collection operation is performed simultaneously with the discharge operation or immediately after the execution of the discharge operation. By preventing this, the number of collection operations is reduced. Thereby, toner deterioration and toner scattering can be suppressed. Further, since the collecting operation and the discharging operation are not performed simultaneously between one sheet, it is not necessary to increase the time interval between sheets. In other words, in order to maintain printing productivity, it is not necessary to increase the linear velocity by increasing the time interval between sheets. This contributes to suppression of toner scattering.

  The storage unit 98 stores various set values necessary for the collection operation and the discharge operation. For example, the standard printing rate, the image length conversion rate of the printing rate difference, the image length for one rotation of the developing roller 83, the first threshold value, the second threshold value, and the like are stored.

  FIG. 8 is a time chart showing an example of a reset operation by the execution timing control unit 97. It is assumed that the integration of the printing rate difference by the first counter unit 95 (the lower part of the chart) and the integration of the number of printed sheets by the second counter unit 96 (the upper part of the chart) are started from time T0. At time T1, the number of printed sheets reaches the second threshold value. The execution timing control unit 97 causes the recovery control unit 93 to execute the recovery operation at the timing of this time T1. At time T1, the count value of the number of printed sheets is reset. On the other hand, the integrated value of the printing rate difference is not reset, and the integration is continued after time T1. Also at the subsequent time T2, the number of printed sheets reaches the second threshold value. Similarly, the execution timing control unit 97 causes the recovery control unit 93 to execute the recovery operation.

  At time T3, the integrated value of the printing rate difference reaches the first threshold value. The execution timing control unit 97 causes the discharge control unit 92 to execute the discharge operation at the time T3. At time T1, the count value of the integrated value of the printing rate difference is reset. In addition to this, the execution timing control unit 97 resets the count value of the number of printed sheets at time T3. Although some number of printed sheets is counted between time T2 and time T3, the count value is rewritten to 0 at time T3.

  At time T4, the count value of the number of printed pages once reset reaches the second threshold value. Accordingly, the collecting operation is executed at the time T4. At subsequent time T5, the integrated value of the printing rate difference reaches the first threshold again. Accordingly, the discharging operation is executed at the timing of time T5. At the same time, the count value of the number of printed sheets is reset at time T5.

  Next, the toner discharging operation and the collecting operation by the control unit 90 will be described based on the flowchart shown in FIG. The image formation control unit 91 of the control unit 90 receives an image formation instruction signal from the external device through the I / F 963 or from the operation unit 961 (step S1). In response to this instruction signal, the image forming control unit 91 controls the image forming unit 30 and the fixing unit 60 to form an image (transfer a toner image) on one sheet (step S2). Then, the second counter unit 96 counts the number of printed sheets (step S3).

  Next, the execution timing control unit 97 determines whether or not the count value of the number of printed sheets is equal to or less than the second threshold value (step S4). If the second threshold value has been reached (NO in step S4), the execution timing control unit 97 sends an execution instruction signal for the recovery operation to the recovery control unit 93. In response to the execution instruction signal, the collection control unit 93 executes the collection operation (step S5). Then, the count value of the number of printed sheets of the second counter unit 96 is reset (step S6).

  On the other hand, when the count value of the number of printed sheets is equal to or smaller than the second threshold (YES in step S4), the first counter unit 95 stores the actual print rate of the sheet obtained by the print rate calculating unit 94 and the storage unit 98. Comparison with the stored standard printing rate is performed (step S7). When the actual printing rate is lower than the standard printing rate (NO in step S7), the first counter unit 95 obtains a printing rate difference that is a difference between the standard printing rate and the actual printing rate, and integrates them (step S8). .

  Subsequently, the execution timing control unit 97 determines whether or not the integrated value of the printing rate difference is equal to or less than the first threshold value (step S9). When the first threshold value has been reached (YES in step S9), the execution timing control unit 97 sends a discharge operation execution instruction signal to the discharge control unit 92. In addition to this, the execution timing control unit 97 resets the count value of the number of printed sheets up to that by the second counter unit 96 (step S10). In parallel with this, the discharge control unit 92 that has received the execution instruction signal executes the discharge operation (step S11). Further, the count value of the print rate difference integrated value of the first counter unit 95 is reset (step S12). Thereafter, each count by the first counter unit 95 and the second counter unit 96 is resumed (step S13).

  Thereafter, it is confirmed whether or not one job has been completed (step S14). If the job remains (NO in step S14), the process returns to step S2 and is repeated. On the other hand, if no job remains (YES in step S14), the process ends. In step S7, when the actual printing rate is lower than the standard printing rate (YES in step S7), and in step S9, the integrated value of the printing rate difference is equal to or less than the first threshold value (NO in step S9). Steps S8 to S13 or steps S10 to S13 are skipped, and the process proceeds to step S14.

<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. The gap between the peripheral surface of the developing roller and the peripheral surface of the photosensitive drum is 110 μm.
[Magnetic roller 82]
Diameter = φ20 mm, peripheral speed = 450 mm / sec, rotation direction is opposite to the developing roller and counter direction. The gap between the peripheral surface of the magnetic roller and the peripheral surface of the developing roller is 300 μm
[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: 11%

The operation of the image forming unit 30 is set as follows.
[Example device]
With a specification for printing 75 sheets per minute, the linear velocity was 375 mm / sec, and an interval of 90 mm was provided between each sheet. The control of the discharging operation and the collecting operation relied on the flowchart of FIG.
[Comparative device]
With a specification for printing 75 sheets per minute, the linear velocity was 458 mm / sec, and an interval of 126 mm was provided between each sheet. The control of the discharging operation and the collecting operation is not linked as in the above embodiment, and both the discharging operation and the collecting operation are executed every time the printing process of four sheets is performed.

  For both the example apparatus and the comparative apparatus, after 300 k sheets were subjected to printing processing, the state of toner scattering in the machine and the state of occurrence of toner fog in the image background (white background) were visually observed. Confirmed. As a result, toner scattering and background fog were not particularly observed in the example apparatus. On the other hand, in the comparative apparatus, toner scattering and background fogging were visually observed.

  As described above, in the image forming apparatus 1 according to the present embodiment, the execution timing control unit 97 (control unit) resets the count value (second count value) of the number of printed sheets as described above. For this reason, the collection operation is not executed simultaneously with the discharge operation or immediately after the execution of the discharge operation. In this way, by giving priority to the discharging operation for discharging the toner out of the developing housing 80, it is possible to reduce the number of executions of the collecting operation and to suppress toner deterioration and toner scattering. Further, since the collecting operation and the discharging operation are not performed simultaneously between one sheet, it is not necessary to increase the time interval between sheets. In other words, in order to maintain printing productivity, it is not necessary to increase the linear velocity by increasing the time interval between sheets. This contributes to suppression of toner scattering.

  In the above embodiment, it is assumed that when a toner image transfer with a low printing rate that is insufficient for the standard printing rate is performed, toner corresponding to the printing rate difference is forcibly discharged to the image carrier. . Then, when the toner of the printing rate difference reaches an amount corresponding to the image length of one circumference of the circumferential surface 83A of the developing roller 83, the discharge operation is executed. Therefore, the toner staying on the peripheral surface 83A can be efficiently discharged, and the toner consumption can be suppressed.

  The execution timing control unit 97 also sets the difference integrated value counted so far even if the count value of the number of printed sheets reaches the second threshold value before the count value of the integrated value of the print difference reaches the first threshold value. Holds the count value without resetting it. Accordingly, the discharging operation is executed with higher priority than the collecting operation.

  In the present embodiment, the discharging operation or the collecting operation is performed between sheets when the transfer of the toner image is continuously performed on a plurality of sheets. In the case where the discharging operation or the collecting operation is performed between the sheets, if both the operations are controlled separately, there may be a case where the discharging operation and the collecting operation must be performed between one sheet. However, according to the present embodiment, the discharging operation and the collecting operation are not executed between one sheet, which can contribute to shortening between sheets.

  According to the image forming apparatus 1 of the present embodiment, as described above, in the image forming apparatus 1 including the developing device 324 adopting the touch-down developing method, the discharging operation is prioritized over the collecting operation. For this reason, toner deterioration and toner scattering can be suppressed. Therefore, it is possible to provide the image forming apparatus 1 that can maintain good image quality.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 30 Image forming part 32Y, 32M, 32C, 32Bk Image forming unit 321 Photosensitive drum (image carrier)
324 Development device 80 Development housing 82 Magnetic roller (developer carrier)
83 Developing roller (toner carrier)
88 1st application part (collection means)
89 Second application unit (recovery means)
90 control section 91 image formation control section 92 discharge control section (part of discharge means)
93 Collection control unit (part of collection means)
94 Print rate calculation unit 95 First counter unit 96 Second counter unit 97 Execution timing control unit (control means)
98 memory

Claims (6)

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;
A peripheral surface that receives toner from the developer layer and carries the toner layer while rotating in contact with the developer layer, and the toner in the toner layer is used for developing the electrostatic latent image; A toner carrier to be supplied to the image carrier;
A discharge means for performing a discharge operation for forcibly discharging the toner carried on the toner carrier to the image 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;
Control means for controlling the execution timing of the discharging operation and the collecting operation,
The control means includes
When the first count value for the first index related to image formation reaches the first threshold value, the discharge operation is executed, or when the second count value for the second index related to image formation reaches the second threshold value To perform the collecting operation,
An image forming apparatus, wherein when the first count value reaches the first threshold before the second count value reaches the second threshold, the second count value counted so far is reset. The image forming apparatus according to claim 1.
The first index is a printing rate indicating the ratio of the transfer area of the toner image to the printable area of the sheet,
The standard print rate and the actual print are counted when the transfer of the toner image with a real print rate lower than a predetermined standard print rate is executed for one sheet. An image forming apparatus that is an integrated value of a printing rate difference that is a difference from the rate. The image forming apparatus according to claim 2.
A relationship between the printing rate difference and an image length formed with toner forcibly discharged from the toner carrier to the image carrier in the discharging operation is determined in advance.
The first threshold value is a value corresponding to the image length of one round of the peripheral surface of the toner carrier when the integrated value of the printing rate difference is converted into the image length. . The image forming apparatus according to claim 1,
When the second count value reaches the second threshold before the first count value reaches the first threshold, the control means holds the first count value counted so far without resetting. An image forming apparatus. The image forming apparatus according to claim 1,
The image forming apparatus, wherein the second index is a number of printed sheets indicating a number of times that the toner image is transferred to a sheet. The image forming apparatus according to claim 1,
The image forming apparatus, wherein the control unit performs the discharging operation or the collecting operation between sheets when the transfer of the toner image is continuously performed on a plurality of sheets.

Images