JP2013064782A - Developing device, and image forming apparatus having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To collect tonner remaining on a developing roller to the side of a magnetic roller and to prevent the collected toner from accumulating on the magnetic roller, in a developing device comprising the developing roller and the magnetic roller.SOLUTION: A control unit 90 applies a collection bias Br including an alternate-current component and performs a collecting operation that collects toner from a developing roller 83 to a magnetic roller 82 at a predetermined timing when no developing operation is performed. The control 90 checks, in advance of performance of the collecting operation, a set value of the most recent developing bias and determines a value of the collection bias Br corresponding to the checked developing bias. In this case, a Duty ratio component, of a polarity on the side where toner moves from the developing roller to the magnetic roller, can be changed among alternate-current biases constituting the collection bias Br.

Description

  The present invention relates to a developing device used in an image forming apparatus such as a printer, and more particularly to a developing device employing a two-component developer including a carrier and a toner, and an image forming apparatus including the developing device.

  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 case, a two-component developer layer (so-called magnetic brush layer) is carried on the magnetic roller, and the toner layer is carried on the developing roller by receiving toner from the two-component developer layer. Further, the electrostatic latent image is visualized by supplying toner from the toner layer to the image carrier.

  In a developing device employing the touch-down development method, if toner that has not been supplied to the image carrier side stays in the toner layer of the developing roller, the toner is not sufficiently developed during the next development, resulting in a decrease in density. In order to prevent this, a technology is known in which an auxiliary electrode is provided on the developing roller, and this auxiliary electrode collects toner on the magnetic roller side (see, for example, Patent Document 1). Further, a technique is known in which a staying toner is collected on the magnetic roller side by applying a bias having a polarity opposite to that during development between the developing roller and the magnetic roller (for example, Patent Documents 2 to 4). .

JP-A-6-67546 JP 2000-66508 A JP 2003-345134 A JP 2005-55837 A

  However, with the technique described in Patent Document 1, it is possible to temporarily collect toner on the magnetic roller side, but if the auxiliary electrode is contaminated with toner, the recoverability may be reduced. In each of the techniques described in Patent Documents 2 to 4, since a bias having a polarity opposite to that at the time of development is applied at the time of toner collection, a large amount of toner tends to accumulate on the magnetic roller side. As a result, the apparent surface potential of the magnetic roller changes, and problems such as density reduction may occur during the next development.

  As described above, when the toner remaining on the developing roller is not sufficiently collected, the image bearing member is not sufficiently developed, and an image defect is generated. In addition, if a large amount of collected toner accumulates on the magnetic roller, it may affect the potential during the next developing operation, resulting in a decrease in density.

  The present invention has been made to solve the above-described problems, and improves the recoverability of the toner remaining on the developing roller to the magnetic roller side, and deposits the collected toner on the magnetic roller. The purpose is to prevent.

  A developing device according to an aspect of the present invention includes a developing housing that stores a developer including toner and a carrier, and a developer that receives the developer in the developing housing while rotating in a predetermined direction and carries the developer layer. A toner that is rotated while being in contact with the developer carrier and a toner layer that receives the toner from the developer layer and carries the toner layer, forms an electrostatic latent image on the surface, and is manifested by the toner A bias applying means for applying a bias to at least one of the toner carrier for supplying the toner and at least one of the developer carrier and the toner carrier and forming a predetermined potential difference between the toner carrier for supplying the toner to the image carrier that carries the image And applying a developing bias to the bias applying means during a developing operation in which toner is supplied from the developer carrier to the toner carrier. A bias control unit that applies a recovery bias to the bias applying unit during a recovery operation for forcibly returning the held toner to the developer carrier, and the developer carrier and the developer carrier during the development operation and the recovery operation. A rotation drive unit that rotates the toner carrier, and the bias control unit changes the development bias according to a change in development conditions and collects the development bias as the development bias increases. The collection bias is controlled so that the toner collection performance during operation is increased.

  According to this configuration, for example, even when the development condition changes according to durability (use of the apparatus), the bias control unit controls the bias application unit to change the development bias. Further, the bias control unit controls the collection bias so that the toner collection performance is increased when the development bias is changed to increase. As a result, the toner recovery performance is adjusted in accordance with changes in the development conditions, so that the toner recovery from the toner carrier to the developer carrier can be performed stably. In particular, even when the toner is charged up by use of the apparatus, the toner collection performance is controlled to increase, so that the toner is prevented from remaining and accumulating on the toner carrier.

  In the above configuration, the bias applying unit forms the recovery bias by superimposing an AC bias on a DC bias, and the bias control unit increases the developing bias according to a change in the developing condition. Accordingly, it is preferable to increase the duty ratio of the polarity on the side where the toner moves from the toner carrier to the developer carrier in the AC bias constituting the recovery bias.

  According to this configuration, as the development bias is changed so that the potential difference increases, the bias control unit changes the duty ratio of the AC bias constituting the recovery bias. At this time, since the duty ratio of the polarity on the side where the toner moves from the toner carrier to the developer carrier is increased, the toner collection performance is increased. For this reason, even when the toner is charged up, the toner recovery performance is increased, and it is possible to prevent the specific toner such as the small particle size toner from remaining and accumulating on the toner carrier.

  In the above-described configuration, it is preferable that the bias control unit controls the collection bias so that the toner collection performance is lowered as the developer carrying member rotates during the collection operation.

  According to this configuration, the bias applying unit is controlled so that the toner collecting performance is lowered with the rotation of the developer carrying member during the collecting operation. For this reason, the amount of collected toner per rotation of the developer carrying member is reduced, and it is possible to prevent toner from being excessively deposited on the developer carrying member.

  In the above configuration, the bias applying unit forms the recovery bias by superimposing an AC bias on a DC bias, and the bias control unit is accompanied by rotation of the developer carrier during the recovery operation. It is preferable that the duty ratio of the polarity on the side where the toner moves from the toner carrier to the developer carrier in the AC bias constituting the recovery bias is reduced.

  According to this configuration, the duty ratio of the AC bias constituting the recovery bias is changed as the developer carrying member rotates during the recovery operation. At this time, since the duty ratio of the polarity on the side where the toner moves from the toner carrier to the developer carrier is reduced, the toner collection performance is lowered with the rotation of the developer carrier. For this reason, the amount of collected toner per rotation of the developer carrying member is reduced, and it is possible to prevent toner from being excessively deposited on the developer carrying member.

  In the above configuration, the rotation driving unit rotates the developer carrying member a plurality of times during the collecting operation, and the bias control unit rotates the developer carrying member by one rotation unit during the collecting operation. It is preferable to control the recovery bias.

  According to this configuration, the developer carrying member is rotated a plurality of times during the collecting operation, and a sufficient collecting period can be secured. At this time, since the collection bias is controlled so that the collection performance is deteriorated in units of one rotation, the toner can be collected uniformly from the peripheral surface of the toner carrier.

  In the above configuration, it is preferable that the rotation driving unit drives the developer carrier to rotate three or more times during the collecting operation.

  According to this configuration, in order to prevent excessive accumulation of toner on the developer carrying member, the developer carrying member rotates three or more times even when the toner collection performance is controlled to decrease. Thus, a sufficient collection period can be secured.

  In the above configuration, the bias control unit may be configured to apply the bias application unit so that the recovery bias is intermittently applied in the rotation unit as the developer carrying member rotates during the recovery operation. It is preferable to control.

  According to this configuration, the developer carrying member can be rotated during the collecting operation in a state where the collecting bias is not applied. For this reason, the collected toner can be moved to the outside of the developer carrying member at any time, and the toner can be prevented from being excessively deposited on the developer carrying member.

  An image forming apparatus according to another aspect of the present invention includes an image carrier that carries an electrostatic latent image and a toner image formed by toner supplied from the toner carrier, and any one of the above. And a developing device.

  According to this configuration, it is possible to prevent the toner having a small particle diameter from accumulating on the toner carrier, and to prevent the large particle toner from being selectively developed from the toner carrier to the image carrier side. it can. Therefore, it is possible to suppress a decrease in density on the image caused by selective development. Further, it is possible to suppress a decrease in density on the image caused by excessive accumulation of the collected toner on the developer carrying member.

  According to the present invention, it is possible to improve the recoverability of the toner remaining on the toner carrier to the developer carrier and to prevent the collected toner from being deposited on the developer carrier.

1 is a cross-sectional view illustrating an internal structure of an image forming apparatus according to an embodiment of the present invention. 1 is a cross-sectional view of a developing device according to an embodiment of the present invention. 1 is a diagram illustrating a structure inside a developing device according to an embodiment of the present invention. 1 is a block diagram showing an electrical configuration according to an embodiment of the present invention. It is the schematic diagram which showed the image development operation | movement which concerns on embodiment of this invention. 6 is a flowchart of a developing bias adjustment operation according to an embodiment of the present invention. It is the schematic diagram which showed the collection | recovery operation | movement which concerns on one Embodiment of this invention. It is a flowchart of recovery mode determination concerning one embodiment of the present invention. It is a timing chart of collection mode concerning one embodiment of the present invention. It is a flowchart of the collection | recovery bias adjustment operation | movement which concerns on the 1st Embodiment of this invention. It is the schematic diagram which showed the waveform of the collection | recovery bias which concerns on the 2nd Embodiment of this invention. It is the schematic diagram which showed the waveform of the collection | recovery bias which concerns on the 3rd Embodiment of this invention. It is the schematic diagram which showed the waveform of the collection | recovery bias in the Example and comparative example of this invention. It is the graph which showed transition of the image density in the example and comparative example of the present invention.

  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, as the image forming apparatus 1, a multi-function machine having a printer function and a copying function is illustrated, but the image forming apparatus may be a printer, a copier, or a facsimile machine.

<Description of Image Forming Apparatus>
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 for storing sheets conveyed to the unit 30, and a conveyance path 50 for conveying the sheet from the sheet feeding unit 40 or the sheet feeding tray 46 to the sheet discharge port 10E via the image forming unit 30 and the fixing unit 60. And a conveyance unit 55 having therein a sheet conveyance path constituting a part of the conveyance path 50 is accommodated.

  An automatic document feeder (ADF) 20 is rotatably attached to the upper surface of the apparatus body 10. The ADF 20 automatically feeds a document sheet to be copied toward a predetermined document reading position in the apparatus main body 10. On the other hand, when the user manually places the document sheet on a predetermined document reading position, the ADF 20 is opened upward. The ADF 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 tray 23 that discharges a document sheet after reading. .

  The reading unit 25 optically reads an image of a document sheet automatically fed from the ADF 20 on the upper surface of the apparatus main body 10 or a manually placed document sheet. 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 (image carrier) and a charger 322, an exposure unit 323, a developing device 324, and a primary transfer arranged around the photosensitive drum 321. A roller 325 and a cleaning device 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 for a two-component developer 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, and a driven roller 333. The intermediate transfer belt 331 is an endless belt that is stretched around a driving roller 332 and a driven roller 333, and toner images from a plurality of photosensitive drums 321 are superimposed on the same portion on the outer peripheral surface of the intermediate transfer belt 331. Is transcribed. The intermediate transfer unit 33 is rotated counterclockwise in FIG.

  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 serves as a secondary transfer portion 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. In addition, the density sensor 35 </ b> A is disposed opposite to the circumferential surface of the intermediate transfer belt 331 at a position upstream of the drive roller 332 in the rotation direction of the intermediate transfer belt 331. The density sensor 35A outputs an electrical signal corresponding to the density of the image formed on the intermediate transfer belt 331.

  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 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, and an induction heating unit 65. 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.

<Configuration of developing device>
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 for storing a developer containing a non-magnetic toner and a magnetic carrier. 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 disposed.

  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 charged positively, 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 unit 962 (rotational driving means). 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 dimension is also formed between the peripheral surface 83A and the peripheral surface of the photosensitive drum 321. Yes.

<Electrical configuration, block diagram>
Next, the main 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. 4 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. In addition to the image forming unit 30 described above, the control unit 90 includes a first application unit 88 (bias application unit), a second application unit 89 (bias application unit), a drive unit 962 (rotation drive unit), and a density sensor. 35A, an image memory 963, an I / F 964, and the like are electrically connected.

  The first application unit 88 includes a DC power source and an AC power source, and applies a bias to the magnetic roller 82 in the developing device 324 based on a control signal from the bias control unit 94 (bias control unit). Similarly, the second application unit 89 includes a DC power supply and an AC power supply, and applies a bias to the developing roller 83 in the developing device 324 based on a control signal from the bias control unit 94.

  The drive unit 962 includes a motor and a gear mechanism that transmits torque thereof, and in accordance with a control signal from the control unit 90, a developing roller 83, a magnetic roller 82, and a Screw feeders 85 and 86 are driven to rotate.

  The density sensor 35A detects the image density of the toner image formed on the intermediate transfer belt 331 and converts it into an electrical signal. The density sensor 35A includes a light emitting unit that emits light to the belt surface of the rotationally driven intermediate transfer belt 125, and a light receiving unit that receives reflected light from the belt surface (not shown). Information relating to the image density output from the density sensor 35A is referred to by the bias controller 94 and is reflected in the developing bias adjustment operation described later.

  The image memory 963 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. The image memory 963 temporarily stores image data optically read by the ADF 20 when the image forming apparatus 1 functions as a copying machine.

  The I / F 964 is an interface circuit for realizing data communication with an external device. For example, the I / F 964 creates a communication signal in accordance with a network communication protocol for 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 964, and image data is stored in the image memory 963 via the I / F 964.

  The control unit 90 executes a control program stored in the ROM by the CPU, whereby a mode determination unit 91, a time integration unit 92, a density information storage unit 93, a bias control unit 94 (bias control unit), and a potential difference storage unit. 95 to function.

  The mode determination part 91 performs the determination operation | movement of whether to perform collection | recovery mode. In the present embodiment, the collection mode 1 or the collection mode 2 is selected and executed according to the development conditions. In the collection mode 1, the toner on the developing roller 83 is collected on the magnetic roller 82 side for one rotation in the rotation time of the developing roller 83 and the magnetic roller 82. In the collection mode 2, the toner on the developing roller 83 is collected on the magnetic roller 82 side with respect to two or more rotations of the developing roller 83 and the magnetic roller 82. In order to estimate the development conditions, the mode determination unit 91 refers to the driving accumulated time T of the low printing rate of the developing roller 83 accumulated by the time accumulating unit 92, and the collection mode needs to be executed according to the magnitude of the value. Determine no.

  In order to estimate the development conditions, the time integration unit 92 integrates the drive accumulation time T with a low printing rate of the development roller 83 and temporarily stores it. Here, the driving accumulation time T with a low printing rate means a rotation time when an image with a low printing rate is formed in the rotation of the developing roller 83. The time integration unit 92 receives image information from the image memory 963 for grasping an image with a low printing rate, and integrates the rotation time of the developing roller 83 only when the printing rate is equal to or lower than a predetermined printing rate. The drive accumulation time T with a low printing rate is reset to zero when the collection mode 2 described later is executed.

  The density information storage unit 93 stores density threshold values D1 and D2 to be compared with image density information on the intermediate transfer belt 331 detected by the density sensor 35A in the developing bias adjustment operation. The density threshold values D1 and D2 are allowable upper and lower threshold values to be compared with the density of the reference image created on the intermediate transfer belt 331. The density of the image density is determined according to the comparison result, and the developing bias is adjusted by the control unit 90 in order to keep the image density within an allowable range.

  The bias controller 94 (bias controller) determines a bias setting value for the developing roller 83 and the magnetic roller 82 when executing the developing operation, and controls the first application unit 88 and the second application unit 89 to perform development. Apply a bias. In addition, the bias controller 94 executes a developing bias adjustment operation and changes the developing bias according to the density detection result by the density sensor 35A. Further, the bias control unit 94 determines the bias setting values for the developing roller 83 and the magnetic roller 82 when executing the recovery operation, and controls the first application unit 88 and the second application unit 89 to apply the recovery bias. Let

  The potential difference storage unit 95 stores a developing bias during a developing operation and a collecting bias during a collecting operation set by the bias control unit 94 under various conditions, and a set value of a potential difference between the magnetic roller 82 and the developing roller 83.

<Application of development bias and development operation>
Next, with reference to FIG. 5, a configuration for bias application of the developing device 324 and a developing operation will be described. The developing device 324 controls the first application unit 88, the second application unit 89 (both bias application means), 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. 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 values of the direct current bias and the alternating current bias applied to the magnetic roller 82 and the developing roller 83 are determined when the developing device 324 supplies toner onto the peripheral surface of the photosensitive drum 321 (develops the electrostatic latent image). It is changed according to the charging property of the supplied toner. Further, in the collecting operation for collecting the toner adhering to the developing roller 83 to the magnetic roller 82 after completion of the developing operation, different bias values are applied to the magnetic roller 82 and the developing roller 83.

  Next, the developing mechanism (developing operation) 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 bias controller 94 controls the first application unit 88 and the second application unit 89 to apply a predetermined DC bias and AC bias to the magnetic roller 82 and the developing roller 83, respectively. As a result, a predetermined potential difference (developing 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 and a part of the remaining toner remain 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. A superimposed voltage of a DC voltage and an AC voltage is applied to the developing roller 83. Therefore, a predetermined potential difference is generated between the peripheral surface of the photosensitive drum 321 having a potential on the surface according to the electrostatic latent image and the peripheral surface 83A of the developing roller 83. Due to this potential difference, the toner T in the toner layer TL moves to the peripheral surface of the photosensitive drum 321. Thereby, the electrostatic latent image on the peripheral surface of the photosensitive drum 321 is developed, and a toner image is formed.

  An example of the developing bias applied to the magnetic roller 82 and the developing roller 83 by the bias control unit 94 controlling the first applying unit 88 and the second applying unit 89 during the developing operation is as follows.

DC bias Vmag_dc of magnetic roller 82; 350V
DC bias Vslv_dc of developing roller 83; 50V
AC bias Vmag_ac of magnetic roller 82; 2500 V (4.7 kHz)
AC bias Vslv_ac of developing roller 83; 1500 V (4.7 kHz)
Duty ratio of bias between photosensitive drum 321 and developing roller 83; 43%
Duty ratio of bias between developing roller 83 and magnetic roller 82; 70%

<Adjustment of development bias>
Next, the developing bias adjustment operation according to the toner charge amount (developing conditions) will be described. The charge amount of the toner in the developing device 324 varies with the usage time of the image forming apparatus 1 and the surrounding temperature environment. At this time, if the development bias exemplified above is continuously adopted as a fixed value, the set development bias becomes constant despite the change in the charge amount (charge) of the toner. As a result, the development amount (toner supply amount) to the photosensitive drum 321 changes, which leads to a change in density on the finally transferred sheet. For this reason, in this embodiment, the density sensor 35A detects the density of the density detection image formed on the intermediate transfer belt 331. Depending on the detection result (development conditions), the bias controller 94 controls the development bias.

  FIG. 6 is a flowchart of the developing bias adjustment operation. With the start of the developing bias adjustment operation, the control unit 90 controls the image forming unit 30 to create a density detection image on the intermediate transfer belt 331. Further, the control unit 90 controls the density sensor 35A to detect the image density of the created density detection image (step S010).

  The density information detected by the density sensor 35A is compared with the lower limit density threshold value D1 stored in the density information storage unit 93 in advance by the control unit 90 (step S020). If the detected image density is equal to or higher than the lower limit density threshold D1 (YES in step S020), the control unit 90 proceeds to comparison with the upper limit density threshold D2 (step S030). On the other hand, when the detected image density is less than the lower limit density threshold D1 (NO in step S020), the control unit 90 determines that the image density is low (toner charge amount is high) and adjusts the developing bias. Then, the development potential difference is enlarged (step S060).

  In step S030, the control unit 90 compares the detected image density with the upper limit density threshold D2. If the detected image density is D2 or higher (NO in step S030), the control unit 90 determines that the image density is high (toner charge amount is low), adjusts the developing bias, and The development potential difference is reduced (step S050). If the detected image density is less than D2 (YES in step S030), the control unit 90 determines that the image density is within the appropriate range, does not execute the developing bias adjustment operation, and ends the control. (Step S040).

  In the present embodiment, the bias control unit 94 performs stepwise expansion and reduction of the developing potential difference in the developing bias adjustment operation. That is, when the control unit 90 determines that the reduction in step S050 or the expansion in step S060 is performed, the bias control unit 94 controls the first application unit 88 and the second application unit 89 so as to set the development potential difference. adjust. At this time, the bias controller 94 changes the DC bias component of the magnetic roller 82 by 10 V, for example, and adjusts the developing bias (developing potential difference). The control unit 90 stores the adjusted latest development bias information in the potential difference storage unit 95 after adjusting the development bias.

  As described above, in this embodiment, development is performed according to the image density of the density detection image created on the intermediate transfer belt 331 as a development condition in order to suppress a change in image density according to the toner charge amount. A bias adjustment operation is performed. The developing bias adjustment operation described above is performed after a predetermined number of sheets output from the image forming apparatus 1 are counted, a usage time of the image forming apparatus 1 is counted for a predetermined time, or the temperature environment around the apparatus is rapidly increased. It is preferable to execute the mode after detecting a change or the like.

<Description of collection operation>
Next, the collection operation in the developing device 324 in the first embodiment of the present invention will be described. FIG. 7A 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 performed at a timing when the previous developing operation is not performed, for example, a time corresponding to the interval between sheets during the image forming operation, or after the image forming operation.

  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 collecting pole (not shown) of the magnet roll. And returned to the developer reservoir 81 (FIG. 2) that houses the screw feeders 85 and 86.

  Here, an example of the recovery bias Br set by the bias control unit 94 for the recovery operation is as follows.

DC bias Vmag_dc of magnetic roller 82; 150V
DC bias Vslv_dc of developing roller 83; 50V
AC bias Vmag_ac of magnetic roller 82; 600 V (3.6 kHz)
AC bias Vslv_ac of developing roller 83; 1000 V (3.6 kHz)
Duty ratio of bias between the photosensitive drum 321 and the developing roller 83; 30%
Duty ratio of bias between developing roller 83 and magnetic roller 82; 70%

  7B is applied between the peripheral surface 83A of the developing roller 83 and the peripheral surface 82A of the magnetic roller 82 by applying the recovery bias Br to the developing roller 83 and the magnetic roller 82. In addition, a recovery potential difference having an AC waveform is set. In the figure, duty1 + duty2 corresponds to one cycle of the AC waveform. In the collecting operation, this AC waveform is applied for a plurality of cycles for one rotation of the developing roller 83 and the magnetic roller 82. Vrmv means a DC bias potential in the developing roller 83.

  In the image output by the image forming apparatus 1, there is a condition that the toner consumption is relatively low such that the printing rate with respect to the sheet area is about several percent. Under such conditions, the amount of toner that continues to circulate without being consumed in the developing device 324 may increase, and the charge amount of the toner may increase (charge-up phenomenon). In this case, in the toner layer TL, there are many toners having a small particle diameter that are difficult to fly to the photosensitive drum 321 side. Therefore, it is necessary to promote the above collection operation and periodically collect the small particle diameter toner from the developing roller 83 to the magnetic roller 82.

  In the present embodiment, even under such conditions, in order to efficiently collect toner on the magnetic roller 82 side, there are two different collection modes.

<Description of collection mode>
In the present embodiment, a recovery mode 1 and a recovery mode 2 are provided as the recovery modes. In the recovery mode 1, the recovery operation to which the recovery bias Br is applied is executed for one rotation in the rotation time of the developing roller 83 and the magnetic roller 82. In principle, the collection mode 1 is executed every time the image forming operation is completed. In the collection mode 2, the collection operation is performed for two or more rotations of the developing roller 83 and the magnetic roller 82, and the collection mode 1 is substituted when the conditions of the flowchart described later are satisfied. Executed. The collection mode 2 can collect more toner than the collection mode 1.

  FIG. 8 is a flowchart when these collection modes are selected. The control unit 90 controls the start (step S110) and end (step S120) of the normal image forming operation, and then causes the mode determination unit 91 to execute the recovery mode determination (step S130). The mode determination unit 91 compares the cumulative driving time T of the developing roller 83 under the low printing rate condition integrated by the time integration unit 92 with a preset time threshold value T1. Here, when the drive accumulation time T is smaller than the time threshold value T1 (YES in step S130), the drive time of the developing roller 83 under the low printing rate condition is short. It is determined that the toner can be collected by the operation. As a result, the collection mode 1 is executed by the control unit 90 (step S140).

  On the other hand, when the driving cumulative time T is larger than the time threshold T1 (NO in step S130), it means that the driving time of the developing roller 83 under the low printing rate condition is long, and the developing roller 83 has a small particle size. A large amount of toner may remain. Therefore, the mode determination unit 91 determines that the collection mode 2 needs to be executed. As a result, the collection mode 2 is executed by the control unit 90 (step S150).

  FIG. 9 is an example of a timing chart in the collection mode. “Print” represents an on / off time of the image forming job. In the figure, P1 and P2 mean on-times when an image forming job is performed at a normal printing rate. On the other hand, P3 means an on-time in which an image forming job is performed at a low printing rate. “Motor” means the rotation driving time of the developing roller 83, and the developing roller 83 is driven to rotate during the periods M1, M2, and M3 corresponding to the image forming jobs P1, P2, and P3. M11, M21, and M31 represent rotational driving of the developing roller 83 when the collection mode is executed. Further, “Bias (Br)” means application of a collection bias, and periods B1, B2, and B3 correspond to rotation periods M11, M21, and M31 of the developing roller 83 during the collection mode, respectively.

  In FIG. 9, the collection mode 1 is executed after the end of the image forming jobs P1 and P2. The driving unit 962 drives the magnetic roller 82 and the developing roller 83 one rotation at a time during the periods M11 and M21. During this time, a recovery bias Br is applied between the magnetic roller 82 and the developing roller 83 (periods B1 and B2).

  On the other hand, after the end of the image forming job P3, the accumulated accumulation time T of the developing roller 83 under the low printing rate condition accumulated by the time accumulating unit 92 exceeds the preset time threshold value T1, thereby collecting mode 2 Is running. In this case, the magnetic roller 82 and the developing roller 83 are driven every three rotations during the period M31. Further, a recovery bias Br is applied between the magnetic roller 82 and the developing roller 83 corresponding to the period M31 (period B3). Since the toner is collected from the developing roller 83 during the period of three rotations of the magnetic roller 82 and the developing roller 83, the small particle size toner deposited on the developing roller 83 can be sufficiently collected under the low printing rate condition. When the collection mode 2 is executed, the control unit 90 resets the accumulation of the driving accumulation time T to zero, assuming that the small particle size toner remaining on the developing roller 83 is sufficiently collected.

<Adjustment of recovery bias according to toner charge amount (development conditions)>
Further, in the present embodiment, an adjustment operation of the recovery bias Br is executed in order to effectively execute the recovery operation according to the charge amount of the toner. As described above, the charge amount (charge) of the toner may change due to long-term toner circulation in the developing device 324, changes in the surrounding environment, and the like. Here, if the applied collection bias Br is constant, the toner collection from the developing roller 83 to the magnetic roller 82 becomes insufficient when the charge amount of the toner increases. In this case, the toner that is difficult to develop stays on the developing roller 83, thereby causing a decrease in image density.

  Therefore, in the present embodiment, the recovery bias adjustment operation is executed using the stored information at the time of the development bias adjustment operation according to the toner charge amount described above. FIG. 10 is a flowchart of the recovery bias adjustment operation. When the collection mode 1 or the collection mode 2 is selected in the previous collection mode selection flow (FIG. 8), the control unit 90 determines the latest bias determined in the developing bias adjustment operation prior to the execution of the collection operation. The development bias is obtained from the potential difference storage unit 95 (step S210).

  The controller 90 adjusts the recovery bias Br1 by referring to the table value stored in advance in the potential difference storage unit 95 using the latest development bias obtained (step S220). In the present embodiment, the control unit 90 changes the duty ratio (%) of the AC bias as a parameter for adjusting the toner recovery amount in the recovery bias Br1. Table 1 is an example of a table value that the control unit 90 refers to from the potential difference storage unit 95 in step S220. Corresponding to the latest development bias, the duty ratio (%) of the collection bias to be adjusted is stored stepwise. In Table 1, the lower limit value of the developing bias is Vmin, and the latest developing bias is represented by a difference from the Vmin. Moreover, duty1 means the ratio of the negative application time in the recovery bias Br1, and duty2 means the ratio of the positive application time. In this embodiment, since the toner is charged to the positive side (plus), the toner is strongly collected on the magnetic roller 82 side corresponding to duty 2 (collection performance is developed). Therefore, as the developing bias (developing potential difference) is expanded by the developing bias adjusting operation, the component (duty 2) in which the toner is collected in the collecting bias Br1 is adjusted to increase.

  When the recovery bias Br1 is adjusted, the control unit 90 executes each recovery mode (step S230). The control unit 90 controls the drive unit 962 to rotate the developing roller 83 and the magnetic roller 82 and also controls the first application unit 88 and the second application unit 89 to adjust the collection for each roller. A bias Br1 is applied.

  Thus, in the present embodiment, the recovery bias Br1 is adjusted according to the charge amount (development condition) of the toner. Therefore, even when the charge amount of the toner changes, the toner recovery from the developing roller 83 to the magnetic roller 82 is effectively realized. In particular, even when the toner in the developing device 324 is charged up due to a low printing rate condition or the like, the toner collection performance is controlled to increase. For this reason, it is possible to prevent the specific toner such as the small particle size toner from remaining on the developing roller 83 and accumulating. At this time, the latest development bias determined by the development bias adjustment operation is employed as information corresponding to the charge amount of the toner. For this reason, since both biases for toner consumption (development operation) and toner collection are adjusted at the same time, control according to the charge amount of the toner being used is always possible.

<Change in collection bias during collection operation>
Next, a second embodiment of the present invention will be described. The present embodiment is characterized in that when the recovery mode 2 is selected, the recovery bias Br1 during the recovery operation is variably controlled as the recovery bias Br2.

  In the recovery mode 2 in which the developing roller 83 and the magnetic roller 82 are driven to rotate a plurality of times in order to cope with toner charge-up and the toner is recovered, a large amount of recovered toner adheres to the magnetic roller 82 side. In this case, the toner falling and collecting on the screw feeders 85 and 86 (FIG. 2) cannot catch up, and the collected toner may accumulate on the magnetic roller 82. As a result, the apparent surface potential on the magnetic roller 82 changes, causing problems such as a decrease in density during the next development operation.

  For this reason, in the present embodiment, the collection bias Br2 is controlled so that the toner collection performance gradually decreases at each rotation in which the developing roller 83 and the magnetic roller 82 are driven to rotate. In this embodiment, when the toner collection performance is lowered, the duty ratio (%) of the AC bias in the collection bias Br2 is controlled as a parameter for adjusting the toner collection amount.

  FIG. 11 schematically shows the waveform of the recovery bias Br2 applied between the developing roller 83 and the magnetic roller 82 during the execution of the recovery mode 2 in the present embodiment. In the figure, the developing roller 83 and the magnetic roller 82 are each rotated three times, and the recovery bias Br2 is applied at different duty ratios (%) for each turn. Here, Duty 11 and Duty 21 mean the duty ratio of the negative side and the positive side applied to the first rotation of each roller. Similarly, Duty12 and Duty22 mean the duty ratio of the negative side and the positive side of the second rotation, and Duty13 and Duty23 mean the duty ratio of the negative side and the positive side of the third rotation.

  Hereinafter, the duty ratio (%) of the recovery bias Br2 will be exemplified. As in the previous embodiment, this potential set value is also stored in the potential difference storage unit 95 in advance.

Duty 11: 34 (%), Duty 21: 66 (%)
Duty 12:40 (%), Duty 22:60 (%)
Duty 13: 46 (%), Duty 23: 54 (%)

  Thus, during execution of the collection mode 2, the duty ratio on the same polarity side as the charging polarity of the toner decreases from Duty 21 (66%) to Duty 23 (54%) as the developing roller 83 and the magnetic roller 82 rotate. To do. Therefore, the amount of toner movement from the developing roller 83 to the magnetic roller 82 is reduced (recovery performance is reduced). Accordingly, a large amount of the collected toner does not adhere to the magnetic roller 82, and even during the collecting operation, the toner can fall from the magnetic roller 82 to the screw feeder 86 with a margin and be collected. .

  The control of the recovery bias Br2 (Duty ratio) has been described above as the transition of the duty ratio in the recovery mode 2 executed at a predetermined timing. Here, when the recovery bias Br1 is controlled according to the charge amount of the toner described in the first embodiment, the table value stored in the potential difference storage unit 95 is shown in Table 2. As illustrated, it has a matrix structure.

  That is, the recovery bias Br1 (= the initial value of Br2 (first rotation)) is selected according to the developing bias, in other words, according to the charge amount of the toner. Further, when the recovery mode 2 is executed, the bias setting value (Duty ratio) for the second and third rotations is selected according to the selected first rotation value. (Note that when the recovery mode 1 for executing the recovery operation for one rotation is selected, the value of the first rotation in Table 2 is selected.)

  As described above, in this embodiment, it is possible to realize the adjustment of the recovery bias Br1 according to the charge amount of the toner and the change of the recovery bias Br2 that is performed every rotation during the execution of the recovery mode. Therefore, even when the charge amount of the toner changes, toner that is difficult to develop such as small-diameter toner does not accumulate on the developing roller 83, and the collected toner accumulates on the magnetic roller 82. Can be prevented.

The developing device 324 according to the embodiment of the present invention has been described above. However, the present invention is not limited to this, and for example, the following modified embodiment can be adopted.
(1) In the above embodiment, the AC bias duty ratio is changed as a parameter for adjusting the toner recovery amount in the recovery bias Br. However, the present invention is not limited to this. In other words, any device capable of controlling the amount of toner collected during the collecting operation may be used. For example, the AC bias Vpp (peak-to-peak) may be changed.
(2) In the above embodiment, the recovery mode is selected based on the cumulative driving time T of the developing roller 83 under the low printing rate condition accumulated by the time accumulating unit 92. However, the selection conditions for the collection mode are not limited to this, and, for example, a mode in which the driving time of the developing roller 83 is accumulated regardless of the printing rate, and the collection mode is selected according to the accumulation time.
(3) In the above embodiment, as a change in the development condition, the development bias is controlled according to the image density information detected by the density sensor 35A, and the recovery bias is controlled from the latest value of the development bias. The embodiment has been described. The control of the collection bias is not limited to this, and as a change in the development condition, a characteristic value corresponding to the charge amount of the toner is detected in addition to the above image density information, and the collection bias (for example, duty) is detected according to the result. The ratio) may be directly controlled.
(4) Further, in the above-described embodiment, the rotation of the developing roller 83 and the magnetic roller 82 in the collection mode 2 has been described as being rotated three times each. However, the present invention is not limited to this. The embodiment may be rotated as described above, and the number of circulations is not limited to an integral multiple. Further, when setting the rotation condition in the collection mode 2, the collection bias Br may be intermittently applied in units of one rotation of the developing roller 83 and the magnetic roller 82. FIG. 12 shows, as an example, bias waveforms of the recovery biases Br31 and Br32 when the recovery mode 2 is executed. In any case, the control unit 90 rotates the developing roller 83 and the magnetic roller 82 three times, and executes a collecting operation. Here, in the recovery bias Br31, the first application unit 88 and the second application unit 89 are controlled so that the recovery bias is not applied in the second rotation of the three rotations. For this reason, the collection operation of the second rotation is executed only by the scraping force (FIG. 7) of the magnetic roller 82 by the magnetic brush DB, and the collection operation is performed on the magnetic roller 82 side as compared with the case where the collection bias Br is applied. The amount of toner used is small. Therefore, the toner collected on the magnetic roller 82 in the first rotation is collected on the screw feeder 86 side with a margin, and the magnetic roller 82 can accept the toner collected in the third rotation. As a result, excessive accumulation of toner on the peripheral surface 82A of the magnetic roller 82 is suppressed, and a stable recovery operation can be performed.

  Further, the collection bias Br32 is different from the collection bias Br31 in that the collection bias at the third rotation is changed. At the third rotation of the collection bias Br32, as in the second embodiment, the duty ratio is changed so that the toner collection performance is lowered. For this reason, the toner deposited on the magnetic roller 82 during the third collection operation is reduced, and the collected toner can be collected more stably on the screw feeder 86 side. In either case of the recovery biases Br31 and Br32, the rotation is not limited to three rotations, and these intermittent rotations may be repeatedly executed.

EXAMPLES Hereinafter, although an Example and a comparative example are given and it demonstrates still in detail about embodiment of this invention, this invention is not limited only to the following Examples. In addition, each experimental condition in the implemented comparative experiment is as follows.
[Common conditions of Examples 1 to 6 and Comparative Example]
<Photosensitive drum 321>
・ Material: Single-layer OPC
・ Line speed: 130mm / sec
<Developing roller 83>
・ Diameter: 16mm
・ Gap with the photosensitive drum 321 (distance between the opposing portions): 100 μm
・ Rotation speed: 252rpm
<Magnetic roller 82>
・ Diameter: 16mm
・ Gap S between magnetic roller 82 and developing roller 83: 280 μm
・ Rotation speed: 285rpm
<Two-component developer>
-Toner number average particle diameter: 6 μm
-Carrier number average particle diameter: 35 μm
-Initial toner charge amount: 15 μC / g
<Test print conditions>
・ Printing rate: 1%
-Number of print job units: 100-Total number of prints: 5000
<Conditions for collection mode>
Collection mode 1: Executed for one rotation of the developing roller 83 and the magnetic roller 82 every 100 sheets. The collection bias is set under specific conditions (described later).
Collection mode 2: Executed every 1000 sheets. The recovery bias and the number of laps of each roller are unique conditions (described later).
<Common conditions for collection bias>
・ DC bias Vmag_dc of magnetic roller 82; 150V
-DC bias Vslv_dc of developing roller 83; 50V
・ AC bias Vmag_ac of magnetic roller 82; 600 V (3.6 kHz)
・ AC bias Vslv_ac of developing roller 83; 1000 V (3.6 kHz)
[Examples 1 to 6 and Comparative Mode Specific Conditions of Collection Mode]
<Comparative example>
-Duty ratio control of recovery bias Br1 based on development bias: None-Duty ratio control of recovery bias Br2 during recovery mode 2 execution: None-Duty 11:30 (%), Duty 12: 70 (%)
・ Number of roller rotations in collection mode 2: 3 rotations
<Example 1>
-Duty ratio control of recovery bias Br1 based on development bias: Yes-Duty ratio control of recovery bias Br2 during recovery mode 2 execution: None-Number of roller rotations in recovery mode 2: 3 rotations
<Example 2>
-Duty ratio control of recovery bias Br1 based on development bias: Yes-Duty ratio control of recovery bias Br2 during recovery mode 2 execution: Yes-Number of roller rotations in recovery mode 2: 3 rotations
<Example 3>
-Duty ratio control of recovery bias Br1 based on development bias: Yes-Duty ratio control of recovery bias Br2 during recovery mode 2 execution: Yes-Number of roller rotations in recovery mode 2: 2 rotations
<Example 4>
-Duty ratio control of recovery bias Br1 based on development bias: Yes-Duty ratio control of recovery bias Br2 during recovery mode 2 execution: None-Number of roller rotations in recovery mode 2: 3 rotations (the recovery bias must be applied for the second rotation) )
<Example 5>
-Duty ratio control of recovery bias Br1 based on development bias: Yes-Duty ratio control of recovery bias Br2 during recovery mode 2 execution: Yes-Number of roller rotations in recovery mode 2: 3 rotations (the recovery bias must be applied in the second rotation) )
<Example 6>
-Duty ratio control of the recovery bias Br1 based on the development bias: Yes-Duty ratio control of the recovery bias Br2 during execution of the recovery mode 2: Yes-Number of roller rotations in the recovery mode 2: 5 rotations (the second and fourth rotations are the recovery bias) Not applied)

  Tables 3 and 4 show the set duty ratios of Examples 1 to 6. Table 3 shows the collection bias (Duty ratio) setting conditions during the rotation of the developing roller 83 and the magnetic roller 82 in each example. In any of the embodiments, since the duty ratio control of the recovery bias Br1 based on the development bias is executed, the set values A, B, and C are as shown in Table 4 according to the latest development bias at the time of executing the recovery mode. Variable control. In addition, the schematic diagram of the bias waveform in Examples 1-6 and a comparative example is shown in FIG.

  Under the conditions described above, printing is performed, and at the end of the collection mode 2 executed every 1000 sheets, a monochrome solid image is collected, and the transition of the density result (ID) measured at the position of the image tip 25 mm is shown in FIG. 14 shows. From this result, compared with the comparative example in which the duty ratio control of the recovery bias Br1 based on the developing bias is not executed, each example in which the duty ratio control is executed suppresses the decrease in density after the number of printed sheets 2000 or more. You can see that This is presumably because the comparative example employs a fixed recovery bias value and therefore cannot cope with the charge-up of the toner, and the toner having a small particle diameter is accumulated on the developing roller 83.

  In addition, compared with Examples 1 and 3, Example 2 and Examples 4 to 6 further suppress the density reduction even when the number of printed sheets is 5000. In the first embodiment, the collection bias Br2 (Duty ratio) control during the execution of the collection mode 2 is not performed. Therefore, it is considered that the toner has accumulated on the magnetic roller 82 as the rotation proceeds. Further, in Example 3, since the number of laps in the collection mode 2 is set to two, it is considered that a slight decrease in density occurred as compared with Example 2 and Examples 4 to 6.

  As described above, as compared with the comparative example, in any of the examples in which the duty ratio control of the recovery bias Br1 based on the development bias is performed, the toner recovery from the developing roller 83 and the screw feeder of the toner attached on the magnetic roller 82 are performed. Recovery to the 86 side was effectively realized, and a decrease in density in the printed image was suppressed. Further, in the embodiment (Examples 2 and 4 to 6) in which the number of laps in the collection mode 2 is set to 3 rotations or more and the duty ratio control of the collection bias Br2 in the collection mode 2 is performed (Examples 2 and 4 to 6), Improvement was confirmed.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 10 Apparatus main body 30 Image forming part 32 Image forming unit 33 Intermediate transfer unit 321 Photosensitive drum (image carrier)
324 Developing device 35A Concentration sensor 80 Developing housing 81 Developer reservoir 82 Magnetic roller (Developer carrier)
821 Flush upper electrode 822 Regulating electrode 823 Main electrode 83 Developing roller (toner carrier)
84 Developer regulating blade 85 Screw feeder 86 Screw feeder 88 First application section (bias application means)
89 Second application section (bias application means)
DESCRIPTION OF SYMBOLS 90 Control part 91 Mode determination part 92 Time integration part 93 Density information storage part 94 Bias control part (bias control means)
95 Potential difference storage unit 962 Drive unit (rotation drive means)
963 Image memory 964 I / F

Claims (8)

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;
An image carrying a toner image that receives toner from the developer layer and carries the toner layer while rotating while being in contact with the developer layer, and carries a toner image that is formed by an electrostatic latent image on the surface and is manifested by the toner A toner carrier for supplying the toner to the carrier;
Bias applying means for applying a bias to at least one of the developer carrying member and the toner carrying member to form a predetermined potential difference therebetween;
During the developing operation in which the toner is supplied from the developer carrier to the toner carrier, the bias applying unit is applied with a development bias, and the toner carried on the toner carrier is forcibly applied to the developer carrier. Bias recovery means for applying a recovery bias to the bias applying means during the recovery operation to return to
A rotation driving means for rotating the developer carrier and the toner carrier during the developing operation and the collecting operation;
The bias control means changes the development bias according to a change in development conditions, and the collection performance is increased so that the toner collection performance during the collection operation increases as the development bias is increased. A developing device that controls a bias. The bias applying means forms the recovery bias by superimposing an AC bias on a DC bias,
The bias control means increases the development bias according to a change in the development condition, and the toner in the AC bias constituting the recovery bias changes from the toner carrier to the developer carrier as the development bias increases. The developing device according to claim 1, wherein the duty ratio of the polarity on the moving side is increased.   3. The bias according to claim 1, wherein the bias control unit controls the collecting bias so that the toner collecting performance is lowered as the developer carrying member rotates during the collecting operation. Development device. The bias applying means forms the recovery bias by superimposing an AC bias on a DC bias,
The bias control means is arranged on the side where the toner moves from the toner carrying member to the developer carrying member in the AC bias constituting the collecting bias as the developer carrying member rotates during the collecting operation. 4. The developing device according to claim 3, wherein the duty ratio of the polarity is decreased. The rotation driving means rotates the developer carrier a plurality of times during the collecting operation,
5. The developing device according to claim 1, wherein the bias control unit controls the recovery bias in units of one rotation of the developer carrying member.   The developing device according to claim 5, wherein the rotation driving unit rotationally drives the developer carrying member at least three times during the collecting operation.   The bias control unit controls the bias application unit so that the recovery bias is intermittently applied in units of one rotation as the developer carrying member rotates during the recovery operation. The developing device according to claim 6, wherein: An image carrier for carrying an electrostatic latent image and a toner image formed by toner supplied from the toner carrier;
An image forming apparatus comprising: the developing device according to claim 1.

Images