JP5849037B2 - Developing device and image forming apparatus having the same - Google Patents

Description

  The present invention relates to a developing device employing a developer containing toner and an image forming apparatus provided with 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 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.

JP-A-7-92804

  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 added mainly for the purpose of maintaining the chargeability of the toner and promoting the polishing action on the surface of the image carrier. The external additive is not only consumed on the image carrier side together with the toner during the developing operation, but also in the non-developing operation represented by the paper, the external additive is consumed alone on the image carrier side by the AC bias. There was a case. When the external additive is detached from the toner and consumed alone, the stress applied to the toner increases and the density tends to decrease.

  In order to suppress such consumption of external additives, it is conceivable to reduce the amplitude value of the AC bias applied to the developing roller during the non-developing operation. However, if the amplitude value of the AC bias is reduced, the toner recoverability from the developing roller to the magnetic roller tends to deteriorate. For this reason, there is a problem that, particularly in a low printing rate condition in which the toner on the developing roller is hardly consumed, the density is lowered due to the development ghost and the rotation of the developing roller with the deterioration of the recoverability. On the other hand, development ghosts as described above are unlikely to occur under the high printing rate conditions where the toner on the developing roller is likely to be consumed. However, since the frequency of toner passing through the surface of the developing roller increases, the toner adheres to the surface of the developing roller. There was a problem to do.

  The present invention has been made in order to solve the above-described problems, and a developing device in which toner is stably non-developed from a developing roller to a magnetic roller and toner is prevented from sticking to the developing roller, It is another object of the present invention to provide an image forming apparatus including the same.

A developing device according to one aspect of the present invention includes: a developing housing that stores a developer containing toner to which an external additive is added; and a developer that receives the developer in the developing housing while rotating in a predetermined direction. A developer carrying body that carries the toner layer, and a toner layer that receives the toner from the developer layer and carries the toner layer while rotating while being in contact with the developer layer; A bias in which an AC bias is superimposed on a DC bias is applied to the toner carrier that supplies the toner, the developer carrier, and the toner carrier, and a predetermined gap is provided between the developer carrier and the toner carrier. a bias applying means for forming a potential difference at the time of developing operation of the toner image is formed on the image bearing member, a first developing bias is applied to said bias applying means, Do not the developing operation is performed During non-developing operation, the bias control means for applying a second developing bias to said bias application means, the printing rate calculation means for calculating a printing rate of the toner image formed on said image bearing member for a predetermined period In accordance with the printing rate calculated by the printing rate calculation means, the amplitude of the AC bias in the second development bias is changed during the non-development operation, so that the external additive is in the second development. possess a consumption condition changing means for changing the consumption condition by said AC bias bias from the toner carrying member is consumed on the image bearing member, wherein the consumption condition changing means, the non-developing operation said at second The amplitude value of the AC bias in the developing bias is set to be smaller than the amplitude value of the AC bias in the first developing bias during the developing operation. When the printing rate is equal to or higher than a preset threshold, the amplitude value of the AC bias in the second developing bias is smaller than the first amplitude value of the AC bias in the first developing bias. The amplitude value of the AC bias in the second developing bias is smaller than the first amplitude value and the second amplitude when the printing rate is less than the threshold value. The third amplitude value is set to be larger than the value .

  According to this configuration, after the external additive is separated from the toner by the first development bias and the second development bias applied to the toner carrier and the developer carrier during the development operation and the non-development operation, In some cases, the image carrier may be moved. Even in such a case, the consumption condition changing unit changes the consumption condition in which the external additive is consumed from the toner carrier to the image carrier during the non-development operation according to the printing rate. For this reason, the consumption condition of the external additive during the non-development operation is suitably changed without affecting the development operation. As a result, the consumption of the external additive in the developing device is suitably adjusted in response to low printing rate conditions in which toner recoverability tends to be reduced and high printing rate conditions in which toner is likely to adhere to the surface of the toner carrier. It becomes possible to adjust.

Further , according to this configuration, the consumption condition in which the external additive is consumed from the toner carrier to the image carrier side is changed by changing the amplitude value of the AC bias during the non-development operation.

Further , according to this configuration, it is possible to suppress the consumption of the external additive corresponding to the high printing rate condition in which the toner is likely to adhere to the surface of the toner carrier. For this reason, more external additives are interposed between the surface of the toner carrier and the surface of the toner. As a result, the toner is prevented from sticking to the surface of the toner carrier.

Further , according to this configuration, it is possible to prevent the toner recoverability from deteriorating by reducing the amplitude value of the AC bias excessively under the low printing rate condition in which the toner recoverability is likely to be reduced.

  In the above configuration, it is preferable that the consumption condition changing unit sets the second amplitude value to be smaller as the printing rate becomes larger than the threshold value.

  According to this configuration, even when the printing rate is increased and the toner easily adheres to the surface of the toner carrier, the amount of external additive consumed from the developing device can be reduced according to the printing rate. . As a result, a lot of external additives are interposed between the surface of the toner carrier and the toner under the high printing rate condition, and toner sticking can be suppressed.

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

  According to this configuration, the consumption conditions of titanium oxide during the non-development operation are suitably changed without affecting the development operation. As a result, the consumption of titanium oxide in the developing device is suitably adjusted in response to low printing rate conditions in which toner recoverability tends to be low and high printing rate conditions in which toner tends to adhere to the surface of the toner carrier. It becomes possible to do. In particular, in response to high printing rate conditions where the toner is likely to adhere to the surface of the toner carrier, the consumption of titanium oxide having an abrasive action is suppressed, and the toner is detached from the surface of the toner carrier with the abrasive action. Can do.

  In the above configuration, it is desirable that the external additive includes resin fine particles.

  According to this configuration, it is possible to suitably adjust the consumption of the external additive in the developing device in response to a high printing rate condition in which the toner is likely to adhere to the surface of the toner carrier by the adsorption action of the resin fine particles. It becomes.

  In the above configuration, the developing device according to any one of the above, the image carrier that carries the electrostatic latent image and the toner image formed by the toner supplied from the toner carrier, and the sheet from the image carrier. And a transfer means for transferring the toner image.

  According to this configuration, the consumption condition of the external additive during the non-development operation is suitably changed without affecting the development operation. As a result, the consumption of the external additive in the developing device is suitably adjusted in response to low printing rate conditions in which toner recoverability tends to be reduced and high printing rate conditions in which toner is likely to adhere to the surface of the toner carrier. It becomes possible to adjust. As a result, the development ghost associated with the non-development failure of the toner and the decrease in density associated with the rotation of the toner carrier are suppressed from appearing on the sheet. In addition, image quality defects caused by toner adhering to the surface of the toner carrier are suppressed.

  In the above-described configuration, the non-development operation is performed before or after execution of an image forming operation for forming an image on a sheet, or when one image is formed on a plurality of sheets and subsequent sheets. It is desirable to be executed correspondingly between paper sheets.

  According to this configuration, it is possible to suitably adjust the consumption of the external additive before, after or after the image forming operation.

  According to the present invention, a developing device in which toner is stably undeveloped from a developing roller to a magnetic roller and toner is prevented from sticking to the developing roller, and an image forming apparatus including the developing device are provided.

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. 6 is a graph showing the adhesion force of toner and the polishing force of external additives when the printing rate is changed. It is a flowchart of the consumption condition change operation | movement which concerns on embodiment of this invention. It is a schematic diagram which shows the relationship between the printing rate and amplitude value of an alternating current bias in embodiment of this invention. It is a schematic diagram which shows the relationship between the printing rate and amplitude value of an alternating current bias in other embodiment of this 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 (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 resin fine particles have a function of stabilizing the chargeability of the toner. 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, 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 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 and a magnetic roller 82 in the developing device 324 are described during a developing operation and a non-developing operation described later. Further, the 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 drives the photosensitive drum 321 to rotate during the developing operation and the non-developing 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 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 (consumption condition changing means).

  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 control unit 92 determines a bias setting value for the developing roller 83 and the magnetic roller 82 and controls the first application unit 88 and the second application unit 89 during the non-development operation when the development operation is not performed. Then, the second developing bias Bp is applied.

  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 an average printing rate per sheet from image data formed on the photosensitive drum 321 during the past one minute.

  The consumption condition changing unit 94 changes the consumption condition in which the external additive is consumed from the developing roller 83 to the photosensitive drum 321 during the non-development operation according to the printing rate calculated by the printing rate calculating unit 93. . Specifically, the consumption condition changing unit 94 changes the maximum amplitude value Vpp (peak-to-peak) of the AC bias of the later-described non-development bias Bp applied during the non-development 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. Further, in the non-development operation, different bias values are applied to the magnetic roller 82 and the development 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 control unit 92 controls the first application unit 88 and the second application unit 89 during the development operation, and an example of the development bias Bd applied to the magnetic roller 82 and the development roller 83, and others. The conditions are as follows.

DC bias Vmag_dc of magnetic roller 82; 350V
DC bias Vslv_dc of developing roller 83; 50V
AC roller bias (Vpp) Vmag_ac; 1200 V (4.5 kHz)
AC bias of developing roller 83 (Vpp) Vslv_ac; 1200 V (4.5 kHz)
Duty ratio (Duty1) of bias between the photosensitive drum 321 and the developing roller 83; 37%
Duty ratio of bias between developing roller 83 and magnetic roller 82 (Duty 2); 63%
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 non-development operation>
Next, the non-developing operation in the developing device 324 in this embodiment will be described. This non-development operation is a timing at which the previous development operation is not performed, for example, before and after the image forming operation in which an image is formed on a sheet, or after one sheet when an image is formed on a plurality of sheets and thereafter The process is executed corresponding to the interval between the sheets between successive sheets.

  In the non-development operation, as in the development operation, each member of the developing device 324 including the developing roller 83 is rotated in addition to the photosensitive drum 321. Then, by applying the second developing bias Bp to the developing roller 83 and the magnetic roller 82, a toner layer is formed on the developing roller 83 in preparation for the developing operation. In the non-development operation, the toner is preliminarily formed on the development roller 83 to cope with the rapid consumption of the toner from the development roller 83 toward the photosensitive drum 321 when the development operation is started. Can do. In addition, in a non-development operation performed between sheets or after image formation, a part of the toner carried on the development roller 83 during the development operation is moved to the magnetic roller 82 side, so that The toner is preferably replaced, and the charging property of the toner on the developing roller 83 is stably maintained.

  Here, an example of the second developing bias Bp set by the bias controller 92 for the non-developing operation is as follows.

DC bias Vmag_dc of magnetic roller 82; 150V
DC bias Vslv_dc of developing roller 83; 50V
AC roller bias (Vpp) Vmag_ac of magnetic roller 82; 1000 V (4.5 kHz)
AC bias of developing roller 83 (Vpp) Vslv_ac; 1200 V (4.5 kHz)
Duty ratio (Duty1) of bias between the photosensitive drum 321 and the developing roller 83; 37%
Duty ratio of bias between developing roller 83 and magnetic roller 82 (Duty 2); 63%

  Thus, in this embodiment, the DC bias of the magnetic roller 82 is set to Vmag_dc; 350V and the DC bias of the developing roller 83 is set to Vslv_dc; 50V in the developing operation, and the DC bias of the magnetic roller 82 is set in the non-developing operation. Vmag_dc; 150V, and the DC bias of the developing roller 83 is set to Vslv_dc; 50V. That is, with respect to the direct current bias, in the developing operation, the positively charged toner is moved from the magnetic roller 82 toward the developing roller 83 by a first amount due to a potential difference of 300V. On the other hand, in the non-development operation, the toner is moved from the magnetic roller 82 toward the developing roller 83 by a potential difference of 100 V with a second amount smaller than the first amount.

<Consumption of external additives>
In developing and non-developing operations, toner 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. In some cases, the external additive is detached from the photosensitive drum 321 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 during the non-developing operation. When the external additive is detached from the toner and consumed alone, the stress applied to the toner increases and the density tends to decrease. In order to suppress such consumption of external additives, it is conceivable to reduce the amplitude value of the AC bias applied to the magnetic roller 82 and the developing roller 83 during the non-developing operation. However, when the amplitude value of the AC bias is reduced, the toner is not smoothly moved from the developing roller 83 to the magnetic roller 82. In particular, when images are continuously formed on a plurality of sheets, when the amplitude value of the AC bias is reduced between sheets, a development history on the developing roller 83 is generated as an image quality defect on the next sheet. (Development ghost). Such a development ghost becomes conspicuous under a low printing rate condition in which toner consumption on the development roller 83 is low. That is, since there is a large amount of toner that continues to circulate on the developing roller 83, it is necessary to collect a large amount of toner on the magnetic roller 82 side. Thus, under low printing rate conditions, reducing the amplitude value of the AC bias in order to suppress consumption of the external additive leads to deterioration of the development ghost.

  On the other hand, the development ghost as described above is less likely to be a problem under the high printing rate condition in which the toner on the development roller 83 is easily exchanged. However, since the amount of toner passing through the peripheral surface 83A (see FIG. 5) of the developing roller 83 is large, there is a problem that the toner easily adheres to the peripheral surface 83A. It has been found by the present inventor that adhesion of toner to the peripheral surface 83A becomes remarkable when resin fine particles (also referred to as resin beads) are added to the toner. This is because when the resin fine particles are interposed between the toner and the peripheral surface 83A of the developing roller 83, the toner is easily fixed to the peripheral surface 83A due to the adsorption action of the resin fine particles. The graph shown by the dotted line in FIG. 7 is a graph in which the adhesive force of the toner to the peripheral surface 83A of the developing roller 83 is a relative value when the horizontal axis is the printing rate. As shown in the figure, when the printing rate is increased, the amount of resin fine particles, which is one of the external additives, increases as the toner is replenished, so that the adhesion force of the toner to the peripheral surface 83A of the developing roller 83 is increased. Will be increased. As a result, toner adhesion to the peripheral surface 83A becomes a problem.

  In view of this, the present inventor has focused on a phenomenon in which the problem of toner fixation is less prominent under low printing rate conditions. As a result, it has been newly discovered that among external additives added to the toner, an external additive having a polishing action represented by titanium oxide contributes to the suppression of toner fixation. The graph shown by the solid line in FIG. 7 is a graph in which the polishing force of titanium oxide on the peripheral surface 83A of the developing roller 83 is a relative value when the horizontal axis is the printing rate. When the printing rate is increased, new titanium oxide flows into the developing housing 80 together with the toner replenished from the toner replenishing unit 34 to the developing device 324. As a result, as shown in FIG. 7, the polishing force of titanium oxide on the peripheral surface 83A of the developing roller 83 is also increased.

  However, as shown in FIG. 7, the adhesion force of the toner to the peripheral surface 83A of the developing roller 83 increases proportionally with the increase of the printing rate, but the polishing force of titanium oxide tends to converge secondarily. Is provided. For this reason, it was found that under the high printing rate conditions, the polishing force of titanium oxide did not catch up with the adhesion force of the toner, resulting in the toner fixing as described above. In other words, under high printing rate conditions, titanium oxide having a polishing action is relatively insufficient in the developing housing 80.

  In the present embodiment, in order to solve the above-described problem, the consumption condition changing unit 94 develops the external additive during the non-development operation according to the printing rate calculated by the printing rate calculation unit 93. The consumption condition consumed from the roller 83 to the photosensitive drum 321 is changed. More specifically, the consumption condition changing unit changes the amplitude value of the AC bias in the second developing bias according to the printing rate. Hereinafter, the first embodiment of the consumption condition changing operation of the present invention will be described with reference to FIG.

  FIG. 8 is a first flowchart when the consumption condition changing unit 94 sets the consumption condition in the present embodiment. In the present embodiment, the amplitude value Vpp of the AC bias of the first developing bias Bd during the developing operation is referred to as Vpp (1). When printing is started in the image forming apparatus 1 (step S001), the average printing rate for the past 100 sheets is calculated by the printing rate calculation unit 93 (step S002). Thereafter, the printing condition is compared with a preset first threshold value by the consumption condition changing unit 94 (step S003). In this embodiment, a printing rate of 10% is set in advance as the first threshold value. When the average printing rate calculated by the printing rate calculation unit 93 is less than 10% (YES in step S003), the consumption condition changing unit 94 uses the second developing bias Bp during the non-development operation represented by the sheet. The amplitude value of the AC bias is set to Vpp (2). In other words, the consumption condition changing unit 94 sets the amplitude value Vpp (2) smaller than the amplitude value Vpp (1) during the developing operation. On the other hand, when the average printing rate calculated by the printing rate calculation unit 93 exceeds 10% (NO in step S003), the consumption condition changing unit 94 uses the second developing bias during the non-development operation represented by the sheet. In Bp, the amplitude value of the AC bias is set to Vpp (3). In other words, the consumption condition changing unit 94 sets the amplitude value Vpp (3) smaller than the amplitude values Vpp (1) and Vpp (2) during the developing operation. When images are continuously formed on a plurality of sheets, each time image data formed on each sheet is stored in the image memory 963, the control unit 90 performs steps S002 to S004 or S005 in FIG. The flow up to is repeated (flow indicated by a dotted line in FIG. 8). For this reason, the amplitude value Vpp of the AC bias of the second developing bias Bp is individually set corresponding to each sheet.

  Next, with reference to FIG. 9, the relationship between the above Vpp (1), Vpp (2), and Vpp (3) will be described in detail. FIG. 9 is a graph showing the relationship between the printing rate (horizontal axis) and the AC bias amplitude value Vpp (vertical axis). As described above, during the developing operation, the amplitude value Vpp of the AC bias of the first developing bias Bd is set to Vpp (1). On the other hand, when the printing rate falls below 10% during the non-development operation represented by the gap between sheets (YES in step S003 in FIG. 8), the consumption condition changing unit 94 causes the amplitude of the AC bias of the second development bias Bp. The value Vpp is set to Vpp (2). On the other hand, when the printing rate exceeds 10% during the non-development operation (NO in step S003 in FIG. 8), the consumption condition changing unit 94 sets the AC bias amplitude value Vpp of the second development bias Bp to Vpp ( 3). In other words, the consumption condition changing unit 94 sets the AC bias amplitude value Vpp at the second developing bias Bp to the AC bias amplitude value at the first developing bias Bd when the printing rate is equal to or greater than a preset threshold value. It is set to Vpp (3) (second amplitude value) smaller than Vpp (1) (first amplitude value). Further, the consumption condition changing unit 94 sets the amplitude value Vpp of the AC bias at the second developing bias Bp to be smaller than Vpp (1) and larger than Vpp (3) when the printing rate is less than the threshold. Set to Vpp (2) (third amplitude value). Further, in the present embodiment, the consumption condition changing unit 94 sets Vpp (3) to be smaller as the printing rate becomes larger than the threshold value (10%). For this reason, in FIG. 9, Vpp (3) changes to the right as the printing rate increases.

  According to such control, since the amplitude value Vpp of the AC bias during the non-development operation is reduced as compared with during the development operation, the external additive from the toner carried on the magnetic roller 82 and the development roller 83 is reduced. Is deterred from leaving. Further, the consumption of the external additive from the developing roller 83 to the photosensitive drum 321 side is suppressed. In particular, when the printing rate is less than 10%, the reduction width of the amplitude value Vpp is set smaller than when the printing rate is 10% or more. For this reason, it is suppressed that the recoverability of the toner from the developing roller 83 to the magnetic roller 82 is deteriorated by excessively reducing the amplitude value Vpp under the low printing rate condition. On the other hand, when the printing rate is 10% or more, the amplitude value Vpp is greatly reduced as the printing rate increases. For this reason, consumption of the external additive from the developing roller 83 to the photosensitive drum 321 side is further suppressed. As a result, many external additives are held in the developing housing 80. Therefore, even when the printing rate is high and a large amount of toner comes into contact with the peripheral surface 83A of the developing roller 83, an external additive is interposed between the peripheral surface 83A and the toner to prevent the two from sticking. . In particular, in the case of an external additive having a polishing action as typified by titanium oxide, the toner that is temporarily fixed to the peripheral surface 83A is polished by the external additive, thereby further suppressing the toner adhesion. Is done.

  According to the embodiment described above, the consumption condition changing unit 94 changes the consumption condition in which the external additive is consumed from the developing roller 83 to the photosensitive drum 321 during the non-development operation according to the printing rate. For this reason, the consumption condition of the external additive during the non-development operation is suitably changed without affecting the development operation. As a result, the consumption of the external additive in the developing device 324 is suitable for low printing rate conditions in which toner recoverability tends to decrease and high printing rate conditions in which toner tends to adhere to the surface of the developing roller 83. It becomes possible to adjust to.

  Further, according to the above embodiment, the consumption condition in which the external additive is consumed from the developing roller 83 to the photosensitive drum 321 side is changed by changing the amplitude value of the AC bias during the non-development operation. . In particular, the consumption of external additives can be suppressed in response to the high printing rate condition in which the toner tends to adhere to the surface of the developing roller 83. For this reason, more external additives are interposed between the surface of the developing roller 83 and the surface of the toner. As a result, the toner is prevented from sticking to the surface of the developing roller 83.

  In addition, according to the above-described embodiment, it is possible to prevent the toner recoverability from being deteriorated by excessively reducing the amplitude value of the AC bias under the low printing rate condition in which the toner recoverability is likely to be reduced. .

  Further, according to the above-described embodiment, the amplitude value Vpp (3) is set smaller as the printing rate becomes larger than the threshold value. For this reason, even when the printing rate is increased and the toner easily adheres to the surface of the developing roller 83, the amount of the external additive consumed from the developing device 324 can be reduced according to the printing rate. As a result, many external additives are interposed between the surface of the developing roller 83 and the toner under the high printing rate condition, and the toner can be prevented from sticking.

  Further, according to the above-described embodiment, the consumption condition of titanium oxide during the non-development operation is suitably changed without affecting the development operation. As a result, the consumption amount of titanium oxide in the developing device 324 is suitably adjusted in response to low printing rate conditions in which toner recoverability is likely to be reduced and high printing rate conditions in which toner is likely to adhere to the surface of the developing roller 83. It becomes possible to adjust. In particular, the consumption of titanium oxide having an abrasive action is suppressed in response to the high printing rate conditions where the toner is likely to adhere to the surface of the developing roller 83, and the toner is detached from the surface of the developing roller 83 with the abrasive action. Can do.

  In addition, according to the above-described embodiment, the consumption of the external additive in the developing device 324 is suitably adapted to the high printing rate condition in which the toner is likely to adhere to the surface of the developing roller 83 due to the adsorption action of the resin fine particles. It becomes possible to adjust.

  Further, in the image forming apparatus 1 including the developing device 324 according to the above-described embodiment, it is possible to prevent the development ghost accompanying the toner collection failure and the density reduction accompanying the rotation of the developing roller 83 from appearing on the sheet. Further, image quality defects caused by the toner sticking to the surface of the developing roller 83 are suppressed. In particular, during the non-development operation, it is possible to suitably adjust the consumption of the external additive by using the image forming operation before, after, or between sheets.

  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-described embodiment, the aspect in which the AC bias amplitude value Vpp of the second developing bias Bp is adjusted as a consumption condition in which the external additive is consumed from the developing roller 83 to the photosensitive drum 321 side. However, the present invention is not limited to this. As a consumption condition of the external additive from the developing roller 83 to the photosensitive drum 321, the duty ratio of the AC bias in the polarity on the side where the toner moves from the developing roller 83 to the photosensitive drum 321 may be adjusted. . That is, by increasing the duty ratio, 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 is lowered. As a result, the potential difference between the maximum peak value Vpp-MAX and the dark potential of the photosensitive drum 321 is reduced, and the external force for moving the external additive toward the photosensitive drum 321 is reduced.

  (2) In the above embodiment, as shown in FIG. 9, the consumption condition changing unit 94 is described in a mode in which Vpp (3) is set smaller as the printing rate becomes larger than the threshold (10%). did. The present invention is not limited to this. FIG. 10 is a graph showing the relationship between the printing rate (horizontal axis) and the AC bias amplitude value Vpp (vertical axis) controlled by the consumption condition changing unit 94 in a modified embodiment of the present invention. As shown in FIG. 10, the consumption condition changing unit 94 may perform control so as to set a constant Vpp (3) when the printing rate exceeds a threshold value. Even in this case, the consumption of the external additive in the development housing 80 is suitably suppressed by setting the amplitude value Vpp (3) smaller than the amplitude value Vpp (1) of the AC bias during the developing operation. Is done. Furthermore, in FIGS. 9 and 10, when the printing rate is less than 10%, the consumption condition changing unit 94 sets the amplitude value Vpp of the AC bias between the sheets (during non-development operation) as Vpp (1 during the development operation). ) May be maintained. In this case, the consumption of the external additive is equivalent to that during the developing operation, but the amplitude value Vpp is not reduced, so that the recovery of toner from the developing roller 83 to the magnetic roller 82 is prevented from deteriorating. Is done.

  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 for Examples and Comparative Examples 1 and 2]
<Photosensitive drum 321>
・ Material: Silicon drum ・ Linear speed: 200mm / sec
<Developing roller 83>
・ Diameter: 16mm
・ Gap with the photosensitive drum 321 (distance between the facing portions): 0.10 mm
<Magnetic roller 82>
・ Diameter: 16mm
・ Gap S between magnetic roller 82 and developing roller 83: 0.25 mm
<Two-component developer>
-Toner number average particle diameter: 6.8 μm
-Toner polarity: positively chargeable-Toner specific gravity: 1.2
-Carrier number average particle diameter: 35 μm
・ Carrier specific gravity: 4.5
<Potential condition of photoconductor 321>
・ Image part potential: + 20V
・ Non-image area potential: + 230V
<Common conditions for the first developing bias Bd>
DC bias Vmag_dc of magnetic roller 82; 350V
DC bias Vslv_dc of developing roller 83; 50V
AC roller bias (Vpp) Vmag_ac; 1200 V (4.5 kHz)
AC bias of developing roller 83 (Vpp) Vslv_ac; 1200 V (4.5 kHz)
<Common conditions for the second development bias Bp>
DC bias Vmag_dc of magnetic roller 82; 150V
DC bias Vslv_dc of developing roller 83; 50V
AC roller bias (Vpp) Vmag_ac of magnetic roller 82; variable (4.5 kHz)
AC bias of developing roller 83 (Vpp) Vslv_ac; 1200 V (4.5 kHz)

  Table 1 shows the amplitude value Vpp of the AC bias of the example and the comparative examples 1 and 2. In Comparative Examples 1 and 2, Vpp does not depend on the printing rate and is set to 1200 V and 1000 V, respectively. In the embodiment, when the printing rate is less than 10%, Vpp is set to 1200V, and when the printing rate exceeds 10%, it is set to gradually decrease from 1200V according to the printing rate.

  Under the above conditions, printing tests were continuously performed, and toner adhesion and development ghost were evaluated at each printing rate. As shown in Table 1, in Comparative Example 1, Vpp was set high under the high printing rate condition, so that the external additive was insufficient, and toner adhesion occurred on the peripheral surface 83A of the developing roller 83. Further, in Comparative Example 2, Vpp was set low under the low printing rate condition, so that insufficient toner collection from the developing roller 83 to the magnetic roller 82 occurred, resulting in development ghost. On the other hand, in the examples based on the present embodiment, the development ghost under the low printing rate condition and the toner adhesion under the high printing rate condition can be made compatible with the wide printing rate condition.

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 (Consumption condition changing means)
962 Drive unit 963 Image memory 964 I / F

Claims (6)

A developing housing for storing a developer containing toner to which an external additive is added;
A developer carrying member that receives the developer in the developing housing while rotating in a predetermined direction and carries the developer layer;
A toner carrier that rotates in contact with the developer layer, receives the toner from the developer layer, carries the toner layer, and supplies the toner to an image carrier that carries a toner image;
Bias applying means for applying a bias in which an AC bias is superimposed on a DC bias to the developer carrier and the toner carrier, and forming a predetermined potential difference between the developer carrier and the toner carrier; ,
A first developing bias is applied to the bias applying unit during a developing operation in which the toner image is formed on the image carrier, and a second developing is applied to the bias applying unit during a non-developing operation when the developing operation is not performed. Bias control means for applying a bias;
A printing rate calculating means for calculating a printing rate of the toner image formed on the image carrier during a predetermined period;
By changing the amplitude value of the AC bias in the second developing bias during the non-development operation in accordance with the printing rate calculated by the printing rate calculating means , the external additive can be used as the second developing bias. the AC bias by have a a consumption condition changing means for changing the consumption condition to be consumed on the image bearing member from said toner carrying member,
The consumption condition changing means is
The amplitude value of the AC bias in the second development bias during the non-development operation is set smaller than the amplitude value of the AC bias in the first development bias during the development operation;
When the printing rate is equal to or greater than a preset threshold, the amplitude value of the AC bias in the second development bias is smaller than the first amplitude value of the AC bias in the first development bias. When the printing rate is less than the threshold value, the amplitude value of the AC bias in the second developing bias is smaller than the first amplitude value and smaller than the second amplitude value. A developing device characterized in that the third amplitude value is set to be larger . The developing device according to claim 1 , wherein the consumption condition changing unit sets the second amplitude value to be smaller as the printing rate becomes larger than the threshold value. The external additive, the developing device according to claim 1 or 2, characterized in that it comprises titanium oxide. The external additive, the developing device according to any one of claims 1 to 3, characterized in that it comprises a resin particle. A developing device according to any one of claims 1 to 4 ,
The image carrier that carries an electrostatic latent image and a toner image formed by toner supplied from the toner carrier;
Transfer means for transferring the toner image from the image carrier to a sheet;
An image forming apparatus comprising: The non-development operation is performed before or after execution of an image forming operation for forming an image on a sheet, or between a sheet and a subsequent sheet when images are formed on a plurality of sheets. The image forming apparatus according to claim 5 , wherein the image forming apparatus is executed corresponding to

Images