JP5860792B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that forms an image on a sheet, and more particularly to an image forming apparatus that uses a toner to which an external additive having an abrasive action is added.

  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 type developing device 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 is moved from the two-component developer layer on the development roller to carry the toner layer. Further, the electrostatic latent image is visualized by supplying toner from the toner layer to the image carrier.

  Patent Document 1 discloses a technique in which a developing bias in which an AC bias is superimposed on a DC bias is applied to a developing roller during a developing operation and a non-developing operation in which the developing operation is not performed. Patent Document 2 discloses a technique in which titanium oxide added to a toner and having a polishing function is selectively supplied from a developing device to an image carrier and the surface of the image carrier is polished. .

JP-A-7-92804 JP 2011-169995 A

  When a DC bias and an AC bias are applied to the developing roller as in the technique described in Patent Document 1, the external additive added to the toner is separated from the toner by the AC bias and moved to the image carrier side. There was. Examples of such external additives include titanium oxide described in Patent Document 2. The external additive is not only consumed on the image carrier side with the toner during the developing operation, but also may be consumed alone on the image carrier side by the AC bias during the non-developing operation. In particular, when the printing rate of the toner image is low, the external additive is easily consumed from the developing device alone. On the other hand, when the printing rate is high, the external additive is supplied together with new toner to the developing device, so that the amount of the external additive in the developing device is relatively increased. Further, there has been a problem that the change in the amount of the external additive in the developing device changes the chargeability of the toner, resulting in density fluctuations and toner fogging.

  When the image forming apparatus is installed in a high temperature and high humidity environment, when the power is turned off or the sleep mode is continued, the temperature of the image carrier is temporarily lowered. Then, when the image forming apparatus is used when the power is turned on next time or when returning from the sleep mode, the discharge product adhering to the surface of the image carrier obtains moisture generated by condensation. And adsorbed on the surface of the image carrier. In order to remove the discharge products adsorbed on the surface of the image carrier as described above, when an external additive is selectively supplied from the developing device to the image carrier side as in the technique described in Patent Document 2. Further, the amount of the external additive in the developing device is further changed, and the problems such as density fluctuation and toner fog as described above become more remarkable.

  The present invention has been made to solve the above-described problems, and is capable of refreshing the surface of the image carrier while stably maintaining the amount of the external additive in the developing device. An object is to provide a forming apparatus.

An image forming apparatus according to an aspect of the present invention includes an image carrier that carries a toner image in which an electrostatic latent image is formed on a surface, and the electrostatic latent image is manifested by toner; A developing housing for storing toner to which an additive has been added; a toner carrier which is driven to rotate and carries a toner layer made of the toner and supplies the toner to the image carrier during the developing operation; and during the developing operation Refreshing means for supplying the external additive from the toner carrier to the surface of the image carrier and performing a refresh operation for polishing the surface of the image carrier during a non-development operation different from A printing rate calculation means for calculating a printing rate of the toner image formed on the image carrier during a period of time, and according to the printing rate calculated by the printing rate calculation means, A supply condition changing means for changing the supply conditions of the external additive is supplied to the image bearing member from the toner carrier, the toner carrier, and applying a bias AC bias is superimposed on a DC bias, the image A bias applying means for forming a predetermined potential difference between the carrier and the toner carrier; a first developing bias is applied to the bias applying means during the developing operation; and a bias applying means is applied to the bias applying means during the refresh operation. Bias control means for applying a second developing bias, and the supply condition changing means has the second developing bias in an amplitude value of the AC bias when the printing rate is equal to or greater than a preset threshold value. Is set to a second amplitude value larger than the first amplitude value of the first developing bias .

According to this configuration, during the non-development operation, the refresh unit supplies the external additive from the toner carrier to the surface of the image carrier, and executes a refresh operation for polishing the surface of the image carrier. When a low-printing-rate toner image is printed and the amount of external additive in the developing housing is reduced, if a large amount of external additive is supplied to the image carrier for refreshing operation, The toner stress is increased, and the density of the toner image may be lowered. Even in such a case, the supply condition changing means changes the supply condition for supplying the external additive from the toner carrier to the image carrier during the refresh operation according to the printing rate calculated by the printing rate calculation means. change. For this reason, even when the printing rate fluctuates, it is possible to suitably adjust the consumption amount of the external additive in the developing housing accompanying the refresh operation. Further, the surface of the image carrier is suitably polished by the external additive supplied to the image carrier. In addition, according to this configuration, under a high printing rate condition in which a relatively large number of external additives are present in the developing housing, the amplitude value of the AC bias during the refresh operation is larger than that during the developing operation. The external additive in the housing can be positively supplied to the image carrier side. For this reason, the refresh operation of the image carrier is stably realized. Further, since a relatively large amount of external additive exists in the developing housing, the external additive is not exhausted.

  In the above configuration, the supply condition changing unit sets the amplitude value of the second developing bias to the first amplitude value of the first developing bias when the printing rate is less than the threshold value. Is desirable.

  According to this configuration, the amplitude value of the AC bias during the developing operation is maintained even during the refresh operation under the low printing rate condition in which the amount of the external additive in the developing housing tends to be small. For this reason, when a large amount of external additive is supplied to the image carrier during the refresh operation, reduction of the external additive in the developing housing is suppressed. As a result, it is possible to suppress the toner chargeability from deteriorating and the toner image density from being lowered.

  In the above-described configuration, the supply condition changing unit is configured to reduce the amplitude value of the second developing bias to a third amplitude value smaller than the second amplitude value when the printing rate is less than the preset threshold value. It is desirable to set the amplitude value.

  According to this configuration, in the low printing rate condition in which the amount of the external additive in the developing housing tends to be small, the AC bias amplitude value during the refresh operation is set smaller than in the high printing rate condition. For this reason, when a large amount of external additive is supplied to the image carrier during the refresh operation, reduction of the external additive in the developing housing is suppressed. As a result, it is possible to suppress the toner chargeability from deteriorating and the toner image density from being lowered.

  In the above-described configuration, the supply condition changing unit is configured so that, when the printing rate is equal to or greater than the threshold, the duty ratio of the AC bias having a polarity on the side where the toner moves from the toner carrier to the image carrier. It is desirable that the duty ratio of the second developing bias is set smaller than the duty ratio of the first developing bias.

  According to this configuration, under a high printing rate condition in which a relatively large amount of external additives are present in the developing housing, the duty ratio of the AC bias during the refresh operation is reduced as compared with that during the developing operation. The external additive can be positively supplied to the image carrier side. For this reason, the refresh operation of the image carrier is realized more stably.

  In the above configuration, at the time of the developing operation and the refresh operation, the image bearing member and the toner carrier are rotated in the same direction in a region facing each other, and the supply condition changing unit includes: When the printing rate is equal to or higher than the threshold value, it is desirable to set a rotation speed ratio of the toner carrier to the image carrier during the refresh operation to be larger than the rotation speed ratio during the development operation.

  According to this configuration, the rotation speed ratio of the toner carrier to the image carrier is increased under a high printing rate condition in which a relatively large amount of external additive is present in the developing housing. For this reason, the contact time during which the toner layer of the toner carrier contacts the surface of the image carrier is increased, so that the external additive in the developing housing can be positively supplied to the image carrier. For this reason, the refresh operation of the image carrier is realized more stably.

  In the above configuration, when the printing rate is less than the threshold value, the refresh unit controls at least the bias control unit, and the toner carrier between the image carrier and the toner image carrier. It is desirable to set a potential difference of the DC bias having a polarity in which the toner moves to the image carrier.

  According to this configuration, the toner can be supplied to the image carrier together with the external additive during the refresh operation under the low printing rate condition in which the amount of the external additive in the developing housing tends to be small. For this reason, it is suppressed that the balance between the toner and the external additive in the developing housing is further deteriorated by supplying the external additive alone to the image carrier.

  In the above configuration, the refresh unit controls at least the bias control unit between the image carrier and the toner image carrier when the printing rate is equal to or greater than the threshold, and from the image carrier. It is desirable to set the potential difference of the DC bias of the polarity that the toner moves to the toner carrier, or set the potential of the DC bias of the image carrier and the toner image carrier to the same potential.

  According to this configuration, the toner can be held on the developing housing side during the refresh operation under the high printing rate condition in which a relatively large amount of external additives exist in the developing housing. For this reason, it is possible to selectively supply the external additive present in the developing housing to the image carrier side while suppressing toner consumption.

  In the above configuration, the image forming apparatus includes a rotation driving unit that rotates the image carrier during the developing operation and the refreshing operation, and the refreshing unit is less than the threshold value than when the printing rate is equal to or higher than the threshold value. In this case, it is desirable to lengthen the rotation time of the image carrier accompanying the refresh operation.

  According to this configuration, even when the supply of the external additive to the image carrier is suppressed under the low printing rate condition, the rotation time of the image carrier accompanying the refresh operation is set to be long. For this reason, it is possible to suitably realize the refresh operation even with a small amount of external additive.

  In the above configuration, the external additive preferably contains titanium oxide.

  According to this configuration, even when the printing rate fluctuates, it is possible to suitably adjust the consumption amount of titanium oxide in the developing housing accompanying the refresh operation. For this reason, it is suppressed that the charging property of the toner in the developing housing varies.

  In the above configuration, the developer in the developing housing is received while rotating in a predetermined direction, and the developer carrying member carrying the developer layer and the developing member while rotating in contact with the developer layer. It is desirable to have the toner carrier that receives the toner from the agent layer and carries the toner layer.

  According to this configuration, even when a touch-down type developing technique in which toner is supplied from the developer carrier to the image carrier via the toner carrier, the external additive in the development housing is provided. The amount can be maintained stably, and the surface of the image carrier can be suitably refreshed.

  In the above configuration, the image forming apparatus includes a transfer unit that transfers the toner image from the image carrier to a sheet, and the refresh operation is performed before or after execution of an image forming operation for forming an image on the sheet, or a plurality of refresh operations. It is desirable to be executed between sheets of one sheet and the subsequent sheet when an image is formed on the sheet.

  According to this configuration, the amount of the external additive in the developing housing can be suitably adjusted before, after execution of the image forming operation, or using the space between sheets.

  According to the present invention, there is provided an image forming apparatus capable of refreshing the surface of the image carrier while stably maintaining the amount of the external additive in the developing housing.

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. FIG. 2 is a block diagram illustrating an electrical configuration of a developing device 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. 5 is a graph showing a relationship between a potential difference between a developing roller and a photosensitive drum and a flying amount of an external additive. It is a flowchart of the consumption condition change operation | movement which concerns on embodiment of this invention. FIG. 6 is a diagram schematically showing a waveform of an AC bias constituting a developing bias. It is the graph which showed the relationship between the printing rate and the amplitude value of AC bias.

  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. In addition, the surface of the photosensitive drum 321 is polished by rotating the cleaning roller in a state where toner is interposed between the cleaning roller and the photosensitive drum 321. At this time, the polishing is preferably promoted by an external additive having a polishing action such as titanium oxide contained in the toner. By polishing the surface of the photoconductive drum 321, it is possible to suppress the occurrence of image blur due to discharge products attached to the surface of the photoconductive drum 321.

  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 (transfer means) is disposed facing 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. 2 is a cross-sectional view in the vertical and horizontal directions schematically showing the internal structure of the developing device 324, and FIG. 3 is a cross-sectional view in the front and back and horizontal directions 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 formed integrally with the developing housing 80 and extends in the longitudinal direction. However, as shown in FIG. 3, the 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 agitate and convey the developer by rotating around the axis. The screw feeders 85 and 86 are rotationally driven by a drive unit 962 described later, and 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 nonmagnetic toner and the carrier are mixed, and the nonmagnetic toner is charged positively, for example.

  The nonmagnetic toner in this embodiment is composed of a binder resin, a colorant, and the like. Examples of the binder resin include polystyrene resins, acrylic resins, styrene-acrylic copolymers, polyethylene resins, polypropylene resins, polyvinyl chloride resins, polyester resins, polyamide resins, polyurethane resins, and polyvinyl resins. It is preferable to use a thermoplastic resin such as an alcohol resin, vinyl ether resin, N-vinyl resin, or styrene-butadiene resin.

  The colorant is not particularly limited, and examples thereof include black, magenta, cyan and yellow pigments. These colorants are usually blended in an amount of 2 to 20 parts by mass, preferably 5 to 15 parts by mass with respect to 100 parts by mass of the binder resin.

  Other additives may be added to the toner as long as the effects of the exemplary embodiment are not impaired. Examples of such additives include charge control agents and waxes. A known charge control agent can be used as the charge control agent. As the positively chargeable charge control agent, for example, a nigrosine dye, a fatty acid-modified nigrosine dye, a carboxyl group-containing fatty acid-modified nigrosine dye, a quaternary ammonium salt, an amine compound, an organometallic compound, or the like can be used.

  The wax is not particularly limited, and examples thereof include carnauba wax and Fischer-Tropsch (hereinafter sometimes referred to as “FT”) wax having ester in the side chain, synthetic hydrocarbon wax such as polyethylene wax, polypropylene wax, and the like. It is done. Among these, from the viewpoint of dispersibility, use of FT wax or polyethylene wax having ester in the side chain is recommended.

  In addition, inorganic oxide fine particles are externally added to the nonmagnetic toner as necessary. As such an external additive, fine particles such as silica fine particles, alumina, titanium oxide, zinc oxide, magnesium oxide, and strontium titanate can be used. Further, if necessary, an organic external additive such as resin fine particles can also be used. The volume average diameter of the external additive is 0.001 to 1.0 μm, preferably 0.005 to 0.3 μm. The addition amount of the external additive is preferably in the range of 0.1 to 2.0 parts by mass with respect to 100 parts by mass of the toner.

  Here, the silica fine particles are mainly used as a fluidizing agent. Titanium oxide, alumina, resin fine particles, and the like are used as toner charge control agents. Titanium oxide or the like is suitably used as an abrasive for the surface of the photoreceptor.

  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 250 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 an image memory. 963, I / F 964, fixing unit temperature sensor 965, environment sensor 966, 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 92 (bias control unit). Similarly, the second application unit 89 includes a DC power source and an AC power source, and applies a bias to the developing roller 83 in the developing device 324 based on a control signal from the bias control unit 92.

  The driving 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 magnetic roller 82 in the developing device 324 are described during a developing operation and a refresh operation described later. Screw feeders 85 and 86 are driven to rotate. In the present embodiment, the developing roller 83, the magnetic roller 82, and the screw feeders 85 and 86 are rotationally driven in synchronization by the drive unit 962. The driving unit 962 rotates the photosensitive drum 321 during the developing operation and the refreshing operation. In the present embodiment, the photosensitive drum 321 and the developing roller 83 are rotationally driven in the same direction at portions facing each other. The drive unit 962 receives a control signal output from the drive control unit 91 described later, and changes the rotation speed ratio (S / D) of the developing roller 83 with respect to the photosensitive drum 321.

  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 fixing unit temperature sensor 965 is disposed in the fixing unit 60. The fixing unit temperature sensor 965 detects the temperature of the fixing unit 60. Further, the temperature of the fixing unit 60 detected by the fixing unit temperature sensor 965 is referred to by the refresh control unit 95, and it is determined whether or not the refresh operation is necessary.

  The environment sensor 966 is attached to the apparatus main body 10 of the image forming apparatus 1. The environment sensor 966H detects the temperature and humidity of the environment where the apparatus main body 10 is installed. In addition, the ambient temperature and humidity detected by the environment sensor 966 are referred to by the refresh control unit 95, and it is determined whether or not the refresh operation is necessary.

  The controller 90 executes a control program stored in the ROM by the CPU, so that a drive controller 91, a bias controller 92 (bias controller), a print rate calculator 93 (print rate calculator), a consumption condition It functions to include a changing unit 94 (supply condition changing unit) and a refresh control unit 95 (refresh unit).

  The drive control unit 91 controls the drive unit 962 to rotationally drive the developing roller 83, the magnetic roller 82, and the screw feeders 85 and 86. In addition, the drive control unit 91 controls the drive unit 962 to drive the photosensitive drum 321 to rotate. In the present embodiment, the drive control unit 91 rotationally drives the above members in the developing operation and the non-developing operation. Further, the drive control unit 91 receives a control signal from the consumption condition changing unit 94 and changes the rotation speed ratio of the developing roller 83 with respect to the photosensitive drum 321.

  The bias control unit 92 (bias control means) determines the bias setting values for the developing roller 83 and the magnetic roller 82 when executing the developing operation, and controls the first applying unit 88 and the second applying unit 89 to perform the first operation. 1 development bias Bd is applied. In addition, the bias controller 92 determines a bias setting value for the developing roller 83 and the magnetic roller 82 during the refresh operation, and controls the first applying unit 88 and the second applying unit 89 to set the second developing bias Bp. Apply.

  The printing rate calculation unit 93 calculates the printing rate based on the image data stored in the image memory 963. In the present embodiment, the printing rate calculation unit 93 calculates an average printing rate per sheet from the image data formed on the past 100 photosensitive drums 321. In another embodiment, the printing rate calculation unit 93 may calculate the average printing rate per sheet from the image data formed on the photosensitive drum 321 during the previous minute.

  The consumption condition changing unit 94 changes the supply condition for supplying the external additive from the developing roller 83 to the photosensitive drum 321 during the refresh operation according to the printing rate calculated by the printing rate calculating unit 93. Specifically, the consumption condition changing unit 94 controls the bias control unit 92 during the refresh operation so that the AC bias in the second developing bias Bp is relatively relative to the amplitude value of the AC bias in the first developing bias Bd. Change the amplitude value of. Further, the consumption condition changing unit 94 controls the bias control unit 92 so that the duty ratio of the first developing bias Bd is the duty ratio of the AC bias having the polarity where the toner moves from the developing roller 83 to the photosensitive drum 321. In contrast, the duty ratio of the second developing bias Bp is relatively changed. The consumption condition changing unit 94 changes the rotation speed ratio of the developing roller 83 to the photosensitive drum 321 during the refresh operation.

<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 includes the first applying unit 88, the second applying unit 89, and the control unit 90 described above in order to control the developing 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 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. In the refresh operation, different bias values are applied to the magnetic roller 82 and the developing roller 83.

  Next, the developing mechanism of the electrostatic latent image on the photosensitive drum 321 in the developing operation 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 control unit 92 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 (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. At this time, the toner T is supplied from the peripheral surface 82A of the magnetic roller 82 to the peripheral surface 83A of the developing roller 83. 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.

  In the present embodiment, the bias controller 92 controls the first application unit 88 and the second application unit 89 during the development operation, and an example of the first development bias Bd applied to the magnetic roller 82 and the development roller 83. And other conditions are as follows.

DC bias Vmag_dc of magnetic roller 82; 200V
DC bias Vslv_dc of developing roller 83; 50V
AC roller bias (Vpp) Vmag_ac of magnetic roller 82; 300V (4.0 kHz)
AC bias of developing roller 83 (Vpp) Vslv_ac; 1700 V (4.0 kHz)
Duty ratio of the bias between the photosensitive drum 321 and the developing roller 83 (Duty 1); 43%
Duty ratio of bias between developing roller 83 and magnetic roller 82 (Duty 2); 57%
Linear speed of photosensitive drum 321; 200 mm / sec
Surface potential of photosensitive drum 321; background portion (dark potential) 230V, image portion (bright potential) 20V
Rotational speed ratio of developing roller 83 to photosensitive drum 321 (S / D); 1.5
Toner chargeability Q / m; 20 μC / g

<Description of refresh operation>
Next, the refresh operation in this embodiment will be described. The refresh operation is an operation for polishing the surface of the photosensitive drum 321. The polishing of the surface of the photosensitive drum 321 here means not only polishing of the material constituting the photosensitive drum 321 but also polishing of the discharge product adhering to the surface of the photosensitive drum 321. Including. This refresh operation is a timing at which the previous developing operation is not performed, for example, before and after the image forming operation in which an image is formed on a sheet, or one sheet when an image is formed on a plurality of sheets and its subsequent The process is executed corresponding to the gap between the sheets.

  When a discharge product typified by NOx provided by the charger 322 adheres to the surface of the photosensitive drum 321, image dripping occurs due to disturbance of the potential of the electrostatic latent image in the attached region. Such adhesion of the discharge product is particularly noticeable in a high temperature and high humidity environment. That is, when the image forming apparatus 1 is installed in a high-temperature and high-humidity environment, when the power is turned off or the sleep mode is continued, the surface temperature of the photosensitive drum 321 is temporarily lowered. Then, when the image forming apparatus 1 is used when the power is turned on next time or when the image forming apparatus 1 is returned from the sleep mode, the discharge product adhering to the surface of the photosensitive drum 321 causes moisture generated by condensation. And is adsorbed on the surface of the photosensitive drum 321.

  For this reason, in the refresh operation according to the present embodiment, an external additive typified by titanium oxide or a toner containing the external additive is supplied from the developing device 324 to the photosensitive drum 321 and then the photosensitive drum. 321 is rotationally driven. At this time, in the present embodiment, the cleaning roller of the cleaning device 326 disposed to face the photosensitive drum 321 is driven to rotate, so that the surface of the photosensitive drum 321 is suitably polished. That is, as described above, when the external roller is interposed between the cleaning roller and the photosensitive drum 321, the cleaning roller is driven to rotate so that the external additive acts as an abrasive. To do.

  In the refresh operation, similarly to the developing operation, each member of the developing device 324 including the developing roller 83 is rotated in addition to the photosensitive drum 321. A toner layer is formed on the developing roller 83 by applying the second developing bias Bp to the developing roller 83 and the magnetic roller 82. Then, the external additive or the toner containing the external additive is supplied from the developing roller 83 to the photosensitive drum 321 by a bias applied on the developing roller 83.

<Consumption of external additives>
During the developing operation and between papers, it is carried on the developing roller 83 by an AC bias electric field formed between the magnetic roller 82 and the developing roller 83 or between the developing roller 83 and the photosensitive drum 321. The external additive may be detached from the toner and moved to the photosensitive drum 321 side. 6 shows the maximum peak value Vpp-MAX of the AC bias in the polarity on the side where the toner moves from the developing roller 83 to the photosensitive drum 321 among the developing bias applied to the developing roller 83, and the dark potential of the photosensitive drum 321. 3 is a graph showing the amount of the external additive flying to the photosensitive drum 321 when the potential difference between and is changed. As described above, the external additive is consumed independently from the developing device 324 to the photosensitive drum 321 according to the AC bias component applied to the developing roller 83.

  The external additive is not only consumed with the toner during the developing operation, but is also consumed by the AC bias alone on the image carrier side due to the AC bias even between the sheets. Further, when the printing rate of the toner image is low during the developing operation, the external additive is consumed by itself on the background potential portion of the photosensitive drum 321. On the other hand, when the printing rate is high, external additive is supplied from the toner supply unit 34 to the developing device 324 together with new toner. For this reason, the amount of the external additive in the developing device 324 is relatively increased. Further, the change in the amount of the external additive in the developing device 324 causes a change in toner chargeability, resulting in a density change and a toner fog on the image. That is, when the amount of the external additive is excessive, toner fog occurs, and when the amount of the external additive is excessive, the density is lowered due to poor chargeability of the toner.

  On the other hand, regardless of the variation in the amount of the external additive in the developing device 324, when the refresh operation described above is executed and the external additive is supplied from the developing device 324 to the photosensitive drum 321 side, the development is performed. The amount of the external additive in the apparatus 324 is further changed, and the problems such as density fluctuation and toner fog as described above become more remarkable.

<Refresh operation>
In the present embodiment, in order to solve the above-described problems, the consumption condition changing unit 94 causes the external additive to be applied to the developing roller during a refresh operation according to the printing rate calculated by the printing rate calculating unit 93. The consumption condition consumed by the photosensitive drum 321 from 83 is changed. Hereinafter, the control of the refresh operation according to the present embodiment will be described with reference to FIG.

  FIG. 7 is a flowchart when the refresh control unit 95 executes the refresh operation in the present embodiment. In the present embodiment, the first developing bias Bd described above is applied to the developing roller 83 and the magnetic roller 82 in the developing operation that is performed before and after the refresh operation. When the power is turned on or the sleep mode is canceled in the image forming apparatus 1 (step S001), the refresh control unit 95 confirms the surrounding environment (step S002). That is, the refresh control unit 95 refers to the detected humidity of the environment sensor 966 and determines whether or not the detected humidity exceeds a preset threshold value. In the present embodiment, the threshold is set to a relative humidity of 80%. When the detected humidity exceeds 80%, the refresh control unit 95 determines that the surrounding is a high humidity environment (YES in step S002). In addition, when the detected humidity is lower than 80%, the refresh control unit 95 determines that the periphery does not reach a high humidity environment (NO in step S002). In this case, the refresh control unit 95 does not execute the refresh operation of the photosensitive drum 321 and the series of control ends.

  When the refresh controller 95 determines that the surrounding is a high humidity environment (YES in step S002), the refresh controller 95 further refers to the temperature detected by the fixing unit temperature sensor 965. If the temperature detected by the fixing unit temperature sensor 965 is below 50 ° C. (YES in step S003), the refresh control unit 95 determines that the refresh operation of the photosensitive drum 321 is necessary (step). S004). On the other hand, if the temperature detected by the fixing unit temperature sensor 965 exceeds 50 ° C. (NO in step S003), the refresh control unit 95 determines that the refresh operation of the photosensitive drum 321 is unnecessary, and a series of Control ends.

  In other words, in the case where it is determined in step S004 that the refresh operation of the photoconductive drum 321 is necessary, the relative humidity around the photoconductive drum 321 is a high humidity environment of 80% or more, and the fixing unit 60. Is temporarily reduced to 50 ° C. or lower. That is, since the image forming apparatus 1 has not been used for a long time, the temperature inside the apparatus main body 10 is lowered. When the image forming apparatus 1 is used in this state, a discharge product such as NOx is adsorbed by moisture accompanying condensation and adheres to the surface of the photosensitive drum 321. Therefore, a refresh operation of the photosensitive drum 321 is required.

  When the refresh control unit 95 determines that the refresh operation is necessary, the consumption condition changing unit 94 calculates the average print rate for the past 100 sheets calculated and stored by the print rate calculating unit 93 and a preset threshold value. Compare (step S005). In the present embodiment, the threshold is set to 5%. If the average printing rate is less than 5% (YES in step S005), the consumption condition changing unit 94 sets the second developing bias Bp11 as the developing bias in the refresh operation. Specifically, the consumption condition changing unit 94 sets the AC bias of the second development bias Bp11 to the same condition as the first development bias Bd during the development operation. Further, the photosensitive drum 321 and the developing roller 83 are rotationally driven at the same rotational speed ratio (S / D) as that during the developing operation.

  When the AC bias for the refresh operation is set by the consumption condition changing unit 94, the refresh operation is executed by the refresh control unit 95 (step S007). Specifically, the refresh control unit 95 controls the drive unit 962 via the drive control unit 91 to rotationally drive each member of the photosensitive drum 321 and the developing device 324. In the present embodiment, the surface of the photosensitive drum 321 is not charged during the refresh operation. In other words, a potential difference is formed between the developing roller 83 and the photosensitive drum 321 by the second developing bias Bp11 applied to the developing roller 83, and the external additive and toner are transferred from the developing roller 83 to the photosensitive drum 321 side. To be supplied. At this time, since the past average printing rate is less than 5%, the external additive (titanium oxide) is small in the developing housing 80 of the developing device 324. For this reason, with the refresh operation of the photosensitive drum 321, the external additive and the toner are simultaneously supplied to the photosensitive drum 321, thereby deteriorating the balance between the toner and the external additive in the developing device 324. Is deterred. For this reason, the potential difference of the polarity at which the toner moves from the developing roller 83 to the photosensitive drum 321 between the photosensitive drum 321 and the developing roller 83 by the DC bias of the second developing bias Bp11 set by the consumption condition changing unit 94. Is set.

  Further, in the second developing bias Bp11, the condition of the AC bias is not changed compared to the first developing bias Bd, so that the developing roller 83 to the photosensitive drum 321 are compared with those in the developing operation. Excessive supply of external additives to the side is suppressed.

  On the other hand, if the average printing rate for the past 100 sheets exceeds 5% in step S005 (NO in step S005), the consumption condition changing unit 94 sets the second development bias Bp12 as the development bias in the refresh operation. To do. Specifically, the consumption condition changing unit 94 increases the amplitude value Vpp of the AC bias of the second developing bias Bp12 as compared with the first developing bias Bd during the developing operation. In addition, the consumption condition changing unit 94 sets the second developing bias Bp12 to be greater than the duty ratio of the first developing bias Bd in the duty ratio of the AC bias having the polarity on the side where the toner moves from the developing roller 83 to the photosensitive drum 321. Set the duty ratio small. Further, in the present embodiment, when the second developing bias Bp12 is selected, the photosensitive drum 321 and the developing roller 83 are rotationally driven at a rotational speed ratio different from that during the developing operation. In this case, the rotation speed of the developing roller 83 is increased and the rotation speed ratio (S / D) of the developing roller 83 with respect to the photosensitive drum 321 is increased as compared with the developing operation.

  When the AC bias for the refresh operation is set by the consumption condition changing unit 94, the refresh operation is similarly performed by the refresh control unit 95 (step S009). The refresh control unit 95 controls the drive unit 962 via the drive control unit 91 to rotationally drive each member of the photosensitive drum 321 and the developing device 324. Similarly, the surface of the photosensitive drum 321 is not charged during the refresh operation. In other words, a potential difference is formed between the developing roller 83 and the photosensitive drum 321 by the second developing bias Bp12 applied to the developing roller 83, and the external additive and toner are transferred from the developing roller 83 to the photosensitive drum 321 side. To be supplied. In step S009, since the past average printing rate exceeds 5%, the external additive (titanium oxide) is sufficient in the developing housing 80 of the developing device 324. Therefore, only the external additive can be selectively supplied to the photosensitive drum 321 side in accordance with the refresh operation of the photosensitive drum 321. For this reason, the potential difference in polarity that the toner moves from the photosensitive drum 321 to the developing roller 83 between the photosensitive drum 321 and the developing roller 83 by the DC bias of the second developing bias Bp12 set by the consumption condition changing unit 94. Is set. That is, the peripheral surface of the developing roller 83 is set to a positive potential with respect to the dark potential of the photosensitive drum 321. As a result, toner is prevented from being consumed from the developing roller 83 to the photosensitive drum 321 side. In another embodiment, the potential of the photosensitive drum 321 and the developing roller 83 may be set to the same potential by the DC bias of the second developing bias Bp12. In this case, a slight amount of toner may move (cover) from the developing roller 83 to the photosensitive drum 321 side. However, compared with the case of the low printing rate (step S007), toner consumption is suppressed. The

  Further, in the second developing bias Bp12, the supply of external additives from the developing roller 83 to the photosensitive drum 321 side is promoted by changing the AC bias condition as compared with the first developing bias Bd. Is done.

  FIG. 8 is a schematic diagram showing a waveform of one cycle of the AC bias when a developing bias having a different duty ratio is applied to the developing roller 83. In addition to the effect of reducing the duty ratio in the change from the first developing bias Bd to the second developing bias Bp12, the duty ratio is 37% in the order of FIGS. 8A, 8B, and 8C, respectively. 40% and 43% are set. In this case, Vpp-MAX (A)> Vpp-MAX (B)> Vpp regarding the maximum peak value (Vpp-MAX) of the AC bias in the polarity on the side where the toner is moved from the developing roller 83 to the photosensitive drum 321 side. -Changed to MAX (C). Therefore, the potential difference between the dark potential on the surface of the photosensitive drum 321 and the Vpp-MAX during the refresh operation is increased as the duty ratio is reduced. As a result, the supply of the external additive from the developing roller 83 to the photosensitive drum 321 is promoted by reducing the duty ratio.

  FIG. 9 is a graph of the amplitude value Vpp of the AC bias of the second development bias Bp11 and Bp12 set in steps S006 and S008 of FIG. As shown in FIG. 9, in the present embodiment, the amplitude value of the AC bias of the first developing bias Bd is set to Vpp (1) (first amplitude value) during the developing operation. The consumption condition changing unit 94 sets the amplitude value of the second developing bias Bp12 to be larger than the amplitude value Vpp (1) of the first developing bias Bd when the printing rate is equal to or greater than a preset threshold value (5%). A large Vpp (3) (second amplitude value) is set. On the other hand, when the printing rate is less than the threshold value (5%), the consumption condition changing unit 94 sets the amplitude value of the second development bias Bp11 to the same Vpp (2) as the Vpp (1) of the first development bias Bd. Set to.

  Thus, even when the amplitude value Vpp is changed from the first developing bias Bd to the second developing bias Bp12, the potential difference between the dark potential on the surface of the photosensitive drum 321 and the Vpp-MAX is enlarged. The supply of the external additive is further promoted. When the rotation speed ratio of the developing roller 83 to the photosensitive drum 321 is increased with the selection of the second developing bias Bp12, the toner layer on the developing roller 83 is applied to the surface of the photosensitive drum 321. The contact time for contact is increased. For this reason, the supply of the external additive is further promoted.

  Here, an example of the second developing bias Bp and the driving condition that the bias control unit 92 and the drive control unit 91 set for the refresh operation by the control signal of the consumption condition changing unit 94 is as follows.

<Low Development Ratio Second Development Bias Bp11 and Other Conditions>
DC bias Vmag_dc of magnetic roller 82; 30V
DC bias Vslv_dc of developing roller 83; -180V
AC roller bias (Vpp) Vmag_ac of magnetic roller 82; 300V (4.0 kHz)
AC bias of developing roller 83 (Vpp) Vslv_ac; 1700 V (4.0 kHz)
Duty ratio of the bias between the photosensitive drum 321 and the developing roller 83 (Duty 1); 43%
Duty ratio of the bias between the developing roller 83 and the magnetic roller 82 (Duty 2); 70%
Linear speed of photosensitive drum 321; 200 mm / sec
Surface potential of photosensitive drum 321; background portion (dark potential) 0V
Rotational speed ratio of developing roller 83 to photosensitive drum 321 (S / D); 1.5
Refresh operation execution time (rotation time of the photosensitive drum 321): 400 msec

<Medium, High Printing Rate Second Development Bias Bp12 and Other Conditions>
DC bias Vmag_dc of magnetic roller 82; 200V
DC bias Vslv_dc of developing roller 83; 50V
AC roller bias (Vpp) Vmag_ac of magnetic roller 82; 300V (4.0 kHz)
AC bias of developing roller 83 (Vpp) Vslv_ac; 1800 V (4.0 kHz)
Duty ratio (Duty 1) of bias between the photosensitive drum 321 and the developing roller 83; 40%
Duty ratio of the bias between the developing roller 83 and the magnetic roller 82 (Duty 2); 70%
Linear speed of photosensitive drum 321; 200 mm / sec
Surface potential of photosensitive drum 321; background portion (dark potential) 230V
Rotational speed ratio of developing roller 83 to photosensitive drum 321 (S / D); 1.8
Refresh operation execution time (rotation time of the photosensitive drum 321): 300 msec

  In the above setting example, the rotation speed ratio (S / D) of the developing roller 83 to the photosensitive drum 321 is increased at the second developing bias Bp12 as compared with the second developing bias Bp11 as described above, and the developing roller 83, the AC bias (Vpp) Vslv_ac is increased, and the duty ratio of the bias between the photosensitive drum 321 and the developing roller 83 is reduced. Further, in the refresh operation (step S007 in FIG. 7) when the printing rate is low, the photosensitive drum 321 is driven to rotate for 100 msec longer than the refresh operation (step S009 in FIG. 7) when the printing rate is medium or high. The As a result, even when a relatively small amount of external additive is supplied to the photosensitive drum 321, the surface of the photosensitive drum 321 can be polished.

  As described above, according to the above-described embodiment, the refresh control unit 95 supplies the external additive to the surface of the photosensitive drum 321 from the developing roller 83 during the non-developing operation in which the developing operation is not performed, and the photosensitive drum A refresh operation for polishing the surface of 321 is executed. When a low-printing-rate toner image is printed and the amount of the external additive in the developing housing 80 decreases, a large amount of external additive is supplied to the photosensitive drum 321 side for the refresh operation. The toner stress in the housing 80 is increased, and the density of the toner image may be lowered. Even in such a case, the consumption condition changing unit 94 supplies the external additive from the developing roller 83 to the photosensitive drum 321 during the refresh operation according to the printing rate calculated by the printing rate calculating unit 93. Change the supply conditions. For this reason, even when the printing rate fluctuates, it is possible to suitably adjust the consumption amount of the external additive in the developing housing 80 accompanying the refresh operation. Further, the surface of the photosensitive drum 321 is suitably polished by the external additive supplied to the photosensitive drum 321 side.

  Further, according to the above embodiment, the AC bias amplitude value Vpp during the refresh operation is larger than that during the development operation under a high printing rate condition where a relatively large amount of external additives are present in the development housing 80. As a result, the external additive in the developing housing 80 can be positively supplied to the photosensitive drum 321 side. For this reason, the refresh operation of the photosensitive drum 321 is stably realized. Further, since a relatively large amount of external additive exists in the developing housing 80, the external additive is not exhausted.

  Further, according to the above-described embodiment, the AC bias amplitude value Vpp during the developing operation is maintained even during the refresh operation under the low printing rate condition in which the amount of the external additive in the developing housing 80 tends to decrease. . For this reason, when a large amount of external additive is supplied to the photosensitive drum 321 during the refresh operation, it is possible to prevent the external additive in the developing housing 80 from being reduced. As a result, it is possible to suppress the toner chargeability from deteriorating and the toner image density from being lowered.

  Further, according to the above-described embodiment, the duty ratio of the AC bias during the refresh operation is reduced more than during the development operation under the high printing rate condition where a relatively large amount of external additives are present in the development housing 80. As a result, the external additive in the developing housing 80 can be positively supplied to the photosensitive drum 321 side. For this reason, the refresh operation of the photosensitive drum 321 is more stably realized.

  Further, according to the above-described embodiment, the rotation speed ratio of the developing roller 83 to the photosensitive drum 321 is increased under the high printing rate condition in which a relatively large number of external additives are present in the developing housing 80. For this reason, the contact time for the toner layer of the developing roller 83 to contact the surface of the photosensitive drum 321 is increased, so that the external additive in the developing housing 80 can be positively supplied to the photosensitive drum 321 side. it can. For this reason, the refresh operation of the photosensitive drum 321 is more stably realized.

  Further, according to the above embodiment, the refresh control unit 95 causes the toner from the developing roller 83 to the photosensitive drum 321 between the photosensitive drum 321 and the developing roller 83 when the printing rate is less than the threshold. Sets the potential difference of the DC bias that moves. According to this configuration, toner can be supplied to the photosensitive drum 321 together with the external additive during the refresh operation under a low printing rate condition in which the amount of the external additive in the developing housing 80 tends to be small. As a result, the external additive alone is supplied to the photosensitive drum 321, thereby suppressing the deterioration of the balance between the toner in the developing housing 80 and the external additive.

  Further, according to the above embodiment, the refresh control unit 95 moves the toner from the photosensitive drum 321 to the developing roller 83 between the photosensitive drum 321 and the developing roller 83 when the printing rate is equal to or greater than the threshold value. Set the potential difference of the DC bias of the polarity. Alternatively, the refresh control unit 95 sets the DC bias potentials of the photosensitive drum 321 and the developing roller 83 to the same potential. According to this configuration, the toner can be held on the developing housing 80 side during the refresh operation under a high printing rate condition in which a relatively large amount of external additives exist in the developing housing 80. For this reason, it is possible to selectively supply the external additive present in the developing housing 80 to the photosensitive drum 321 side while suppressing toner consumption.

  Further, according to the above-described embodiment, the refresh control unit 95 increases the rotation time of the photosensitive drum 321 associated with the refresh operation when the printing rate is less than the threshold value than when the printing rate is equal to or greater than the threshold value. According to this configuration, even when the supply of the external additive to the photosensitive drum 321 is suppressed under the low printing rate condition, the rotation time of the photosensitive drum 321 accompanying the refresh operation is set to be long. For this reason, it is possible to suitably realize the refresh operation even with a small amount of external additive.

  Further, according to the above-described embodiment, the surface of the photosensitive drum 321 is suitably polished by supplying titanium oxide having a polishing action to the photosensitive drum 321 as an external additive. In the touch-down developing device 324 in which toner is supplied from the magnetic roller 82 to the photosensitive drum 321 via the developing roller 83, the amount of the external additive in the developing housing 80 is stably maintained. The surface of the photosensitive drum 321 can be suitably refreshed. Further, the refresh operation is performed before, after or after execution of the image forming operation, and the amount of the external additive in the developing housing 80 can be suitably adjusted.

  The developing device 324 and the image forming apparatus 1 including the developing device 324 according to the embodiment of the present invention have been described above. However, the present invention is not limited to this, and for example, the following modified embodiment is adopted. Can do.

  (1) In the above embodiment, as supply conditions for supplying the external additive from the developing roller 83 to the photosensitive drum 321, the rotation speed ratio of the developing roller 83 to the photosensitive drum 321, Vpp at the second developing bias Bp. However, one or at least two of these conditions may be set.

  (2) In the above-described embodiment, the printing rate threshold value is described as 5%, but the present invention is not limited to this. The threshold value may be set according to the characteristics of the toner used as the developer and the external additive. Further, the expressions “low printing rate” and “high printing rate” in the above embodiment are relative and do not limit the value of the printing rate in the present invention.

  (3) Further, in the above embodiment, in the step S006 of FIG. 7, the AC bias of the second developing bias Bp11 and the rotational speed ratio S / D are set to the same conditions as those during the developing operation. However, the present invention is not limited to this. The amplitude value (low printing rate condition) of the AC bias of the second developing bias Bp11 may be set smaller than the amplitude value (high printing rate condition) of the second developing bias Bp12. Even in this case, a large amount of external additive is supplied to the photosensitive drum 321 side during the refresh operation in the case of a low printing rate, compared with the case of a high printing rate, so Reduction of external additives is suppressed. As a result, it is possible to suppress the toner chargeability from deteriorating and the toner image density from being lowered.

  (4) In the above embodiment, in the refresh operation, the surface of the photosensitive drum 321 is not charged, and a potential difference is generated between the photosensitive drum 321 and the developing roller 83 due to the developing bias applied to the developing roller 83. However, the present invention is not limited to this. As in the developing operation, the surface of the photosensitive drum 321 is charged, and further, the charged region is exposed to form a so-called toner band, thereby realizing external consumption and toner consumption in the refresh operation. It may be done.

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 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)
90 control section 91 drive control section 92 bias control section (bias control means)
93 Print rate calculation unit (print rate calculation means)
94 Consumption condition changing section (supply condition changing means)
962 Drive unit (rotation drive means)
963 Image memory 964 I / F
965 Fixing part temperature sensor 966 Environmental sensor

Claims (11)

An image carrier that carries a toner image on which an electrostatic latent image is formed, and the electrostatic latent image is manifested by toner;
A developing housing for storing toner to which an external additive having an abrasive action is added;
A toner carrier that is driven to rotate, carries a toner layer made of the toner, and supplies the toner to the image carrier during a developing operation;
Refresh means for performing a refresh operation for supplying the external additive from the toner carrier to the surface of the image carrier and polishing the surface of the image carrier in a non-development operation different from the development operation; ,
A printing rate calculating means for calculating a printing rate of the toner image formed on the image carrier during a predetermined period;
Supply condition changing means for changing a supply condition for supplying the external additive from the toner carrier to the image carrier during the refresh operation according to the print ratio calculated by the print rate calculator;
A bias applying means for applying a bias in which an AC bias is superimposed on a DC bias to the toner carrier to form a predetermined potential difference between the image carrier and the toner carrier;
Bias controlling means for applying a first developing bias to the bias applying means during the developing operation and applying a second developing bias to the bias applying means during the refresh operation;
The supply condition changing means may convert the amplitude value of the second developing bias to the first amplitude value of the first developing bias in the amplitude value of the AC bias when the printing rate is equal to or greater than a preset threshold value. An image forming apparatus, wherein the second amplitude value is set to be larger than the second amplitude value . The supply condition changing means sets the amplitude value of the second developing bias to the first amplitude value of the first developing bias when the printing rate is less than the threshold value. the image forming apparatus according to 1. The supply condition changing unit sets the amplitude value of the second developing bias to a third amplitude value smaller than the second amplitude value when the printing rate is less than the threshold value. The image forming apparatus according to claim 1 . The supply condition changing means is configured to set the first developing bias at a duty ratio of the AC bias having a polarity in which the toner moves from the toner carrier to the image carrier when the printing rate is equal to or greater than the threshold. than the Duty ratio, an image forming apparatus according to any one of claims 1 to 3, characterized in that setting a small Duty ratio of the second developing bias. Rotation driving means for rotating the image carrier and the toner carrier in the same direction in regions facing each other during the development operation and the refresh operation;
The supply condition changing means has a rotation speed ratio of the toner carrier to the image carrier during the refresh operation greater than the rotation speed ratio during the development operation when the printing rate is equal to or greater than the threshold. setting image forming apparatus according to any one of claims 1 to 4, characterized in that. The refresh means controls at least the bias control means when the printing rate is less than the threshold value, so that the toner carrier carries the image carrier between the image carrier and the toner image carrier. the image forming apparatus according to any one of claims 1 to 5 wherein the toner is characterized by setting the potential difference between the DC bias of the polarity to move. The refresh unit controls at least the bias control unit when the printing rate is equal to or greater than the threshold value, and changes the image carrier to the toner carrier between the image carrier and the toner image carrier. setting the potential difference of the DC bias having a polarity the toner is moved, or claims 1 to 6 the DC bias potential of the image bearing member and the toner image carrier and sets the same potential The image forming apparatus according to any one of the above. Rotation drive means for rotating the image carrier during the development operation and the refresh operation,
The refresh means makes the rotation time of the image carrier associated with the refresh operation longer when the printing rate is lower than the threshold value than when the printing rate is equal to or higher than the threshold value. the image forming apparatus according to claim 2 or 3. The external additive is an image forming apparatus according to any one of claims 1 to 8, characterized in that it comprises titanium oxide. A developer carrying member that receives the developer in the developing housing while rotating in a predetermined direction and carries the developer layer;
The toner carrier that receives the toner from the developer layer and carries the toner layer while rotating in contact with the developer layer;
The image forming apparatus according to any one of claims 1 to 9, characterized in that it has a. Transfer means for transferring the toner image from the image carrier to a sheet;
The refresh operation is performed before or after execution of an image forming operation in which an image is formed on a sheet, or between sheets between one sheet and a succeeding sheet when images are formed on a plurality of sheets. the image forming apparatus according to any one of claims 1 to 10, characterized in that it is executed.

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