JP5891158B2 - Developing device and image forming apparatus having the same - Google Patents

Description

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

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

  In such a touch-down development method, when a toner with poor charging property stays on the developing roller, an image defect such as a development ghost occurs. In order to solve such a problem, Patent Document 1 discloses a technique in which a collecting operation for collecting the toner from a developing roller to a magnetic roller is performed during non-printing. A voltage in which an AC component is superimposed on a DC component is applied to the developing roller and the magnetic roller. By changing the voltage between the developing operation and the collecting operation, the moving direction of the toner is changed. Patent Documents 2 and 3 disclose techniques for collecting toner from the developing roller to the magnetic roller by changing the voltage applied to the magnetic roller.

JP-A-7-92804 JP 2005-55839 A JP 2006-221107 A

  In the touchdown development system as described above, when a DC bias and an AC bias are applied to the developing roller and the magnetic roller, the external additive may be detached from the toner due to an oscillating electric field caused by the AC bias. Such a phenomenon is caused not only by the developing operation but also by the voltage applied during non-printing as described above. If the removal of the external additive is accelerated, the transferability of the toner is lowered and the density of the image is lowered. Further, when the external additive released from the toner is moved from the developing roller to the photosensitive drum as a single unit, the amount of the external additive in the developing housing decreases. In this case, the image density reduction as described above is further accelerated.

  The present invention has been made to solve the above-described problems, and includes a developing roller and a magnetic roller, and a developing device to which a bias in which an AC bias is superimposed on a DC bias is applied, and In the image forming apparatus having the above, an object is to suppress the detachment of the external additive from the toner and the reduction of the external additive in the developing device as much as possible.

A developing device according to one aspect of the present invention receives a developer in a developer housing that stores a developer containing toner and a carrier to which an external additive is added, and the developer in the developer housing while rotating in a predetermined direction. A developer carrying member carrying the developer layer; and while rotating in contact with the developer layer, receiving a toner from the developer layer to carry the toner layer, and forming an electrostatic latent image on the surface; A toner carrier that supplies toner is applied to an image carrier that carries a toner image that is manifested by toner, and a bias in which an AC bias is superimposed on a DC bias is applied to the developer carrier and the toner carrier. And a bias applying means for forming a predetermined potential difference between them and a developing operation in which the toner image is formed on the image carrier, a first amount of toner from the developer carrier to the toner carrier. As toner is supplied, together to apply the DC bias and the first developing bias including the AC bias for each of said developer carrying member and the toner carrying member to the bias applying means, said developing operation The DC bias is applied to the bias applying means so that a second amount of toner smaller than the first amount is supplied from the developer carrying member to the toner carrying member during a non-development operation in which toner is not performed. And bias control means for applying a second development bias including the AC bias to each of the developer carrier and the toner carrier, the bias control means between the peaks of the AC bias. In the voltage, the peak-to-peak voltage of the second developing bias is set smaller than the peak-to-peak voltage of the first developing bias. In addition, the difference between the first DC bias applied to the toner carrier among the first development bias and the background potential of the electrostatic latent image is greater than the second development bias. The difference between the second DC bias applied to the toner carrier and the background portion potential is large , and the second DC bias is higher than the image portion potential with respect to the maximum image density of the electrostatic latent image. Thus, the first and second developing biases are set as described above.

  According to this configuration, the bias applying unit applies a bias in which an AC bias is superimposed on the DC bias to the toner carrier and the developer carrier, and forms a predetermined potential difference therebetween. The bias control unit causes the bias applying unit to apply the first and second developing biases. As a result, during the developing operation, the first amount of toner is supplied from the toner carrier to the developer carrier. In a non-development operation, a second amount of toner smaller than the first amount is supplied from the toner carrier to the developer carrier. At this time, the bias control means sets the peak-to-peak voltage of the second developing bias smaller than the peak-to-peak voltage of the first developing bias in the peak-to-peak voltage of the AC bias. For this reason, it is possible to prevent the external additive from being detached from the toner during the non-development operation. In addition, the bias control means is configured to determine whether the toner carrying of the second developing bias is greater than the difference between the first DC bias applied to the toner carrying member of the first developing bias and the background potential of the electrostatic latent image. The first and second developing biases are set so that the difference between the second DC bias applied to the body and the background potential is increased. As a result, during the non-development operation, the external additive is suppressed from flying from the toner carrier toward the region having the background potential of the image carrier. Accordingly, excessive consumption of the external additive from within the developing device is suppressed.

  In the above-described configuration, the bias control unit is configured to determine the toner carrying amount at the DC bias of the second developing bias based on a potential difference between the toner carrying member and the developer carrying member at the DC bias of the first developing bias. It is desirable to set the first and second developing biases so that the potential difference between the body and the developer carrier is small.

  According to this configuration, the potential difference between the toner carrier and the developer carrier at the DC bias of the second development bias is larger than the potential difference between the toner carrier and the developer carrier at the DC bias of the first development bias. Get smaller. For this reason, in the non-development operation, the potential difference of the DC bias is set smaller than that in the development operation in addition to the potential difference of the peak voltage of the AC bias applied between the toner carrier and the developer carrier. . Therefore, during the non-development operation, the second amount of toner is suitably supplied from the developer carrier to the toner carrier.

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

  Therefore, according to this configuration, it is possible to suppress the external additive from being detached from the toner during the non-development operation. Further, during the non-development operation, the external additive is prevented from flying from the toner carrier toward the region having the background potential of the image carrier. Accordingly, excessive consumption of the external additive from within the developing device is suppressed. As a result, a decrease in toner image density is preferably 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 suppress the external additive from being detached from the toner before or after the execution of the image forming operation or between sheets. Further, excessive consumption of the external additive from within the developing device is suppressed.

  According to the present invention, a developing device including a developing roller and a magnetic roller, to which a bias in which an AC bias is superimposed on a DC bias is applied, and an external additive from toner in the image forming apparatus including the developing device. And the reduction of external additives in the developing device are suppressed as much as possible.

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. FIG. 6 is a schematic diagram illustrating a developing operation in the developing device according to the embodiment of the present invention. FIG. 9 is a diagram showing a change in bias in a developing operation and a developing preparation operation (non-developing operation) in the developing device according to the embodiment of the present invention. FIG. 6 is a diagram illustrating a change in density of a printed image with respect to the number of printed sheets.

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

<Description of Image Forming Apparatus>
The image forming apparatus 1 includes an apparatus main body 10 having a substantially rectangular parallelepiped housing structure, and an automatic document feeder 20 disposed on the apparatus main body 10. In the apparatus main body 10, a reading unit 25 that optically reads a document image to be copied, an image forming unit 30 that forms a toner image on a sheet, a fixing unit 60 that fixes the toner image on a sheet, and image formation A sheet feeding unit 40 for storing sheets conveyed to the unit 30, and a conveyance path 50 for conveying the sheet from the sheet feeding unit 40 or the sheet feeding tray 46 to the sheet discharge port 10E via the image forming unit 30 and the fixing unit 60. And a conveyance unit 55 having therein a sheet conveyance path constituting a part of the conveyance path 50 is accommodated.

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

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

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

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

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

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

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

  The intermediate transfer unit 33 includes an intermediate transfer belt 331, a driving roller 332, and a driven roller 333. The intermediate transfer belt 331 is an endless belt that is stretched around a driving roller 332 and a driven roller 333, and toner images from a plurality of photosensitive drums 321 are superimposed on the same portion on the outer peripheral surface of the intermediate transfer belt 331. Is transcribed. The intermediate transfer unit 33 is rotated counterclockwise in FIG.

  A secondary transfer roller 35 is disposed to face the peripheral surface of the drive roller 332. A nip portion between the driving roller 332 and the secondary transfer roller 35 serves as a secondary transfer portion that transfers a full-color toner image overcoated on the intermediate transfer belt 331 to a sheet. A secondary transfer bias potential having a polarity opposite to that of the toner image is applied to one of the driving roller 332 and the secondary transfer roller 35, and the other roller is grounded. In addition, the density sensor 35 </ b> A is disposed opposite to the circumferential surface of the intermediate transfer belt 331 at a position upstream of the drive roller 332 in the rotation direction of the intermediate transfer belt 331. The density sensor 35A outputs an electrical signal corresponding to the density of the image formed on the intermediate transfer belt 331.

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

  The paper feed unit 40 includes two-stage paper feed cassettes 40A and 40B that accommodate sheets subjected to image forming processing. These paper feed cassettes 40A and 40B can be pulled out from the front of the apparatus body 10 toward the front.

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

<Configuration of developing device>
Next, the developing device 324 will be described in detail. 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 mechanism (not shown), but their rotational directions are set in opposite directions. As a result, the developer is circulated and conveyed between the developer storage chamber 81a and the developer storage chamber 81b as shown by the arrows in FIG. By this stirring, the toner and the carrier are mixed, and the toner is charged positively, for example.

  The 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.

  In this embodiment, an external additive composed of titanium oxide is added to the toner surface. This external additive reduces the contact area between the surface of the toner and the surface of the photosensitive drum 321 when the toner is developed on the photosensitive drum 321 side. As a result, the toner is easily transferred from the photosensitive drum 321 to the intermediate transfer belt 331. That is, the transferability of the toner is improved by the external additive, and the image density of the image on the sheet is suitably maintained. Furthermore, titanium oxide is interposed between the cleaning roller of the cleaning device 326 and the surface of the photosensitive drum 321 so that the surface of the photosensitive drum 321 is suitably polished. As a result, it is possible to suppress discharge products and the like from adhering to the surface of the photosensitive drum 321. When the external additive is separated from the surface of the toner, the external additive is charged with a polarity opposite to the charging polarity of the toner.

  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 the driving unit 962. A gap S having a predetermined dimension is formed between the peripheral surface 83A of the developing roller 83 and the peripheral surface 82A of the magnetic roller 82. The gap S is set to about 130 microns, 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 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 and a magnetic roller 82 in the developing device 324 are described during a developing operation and a developing preparation 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 image memory 963 temporarily stores image data for printing given from an external device such as a personal computer when the image forming apparatus 1 functions as a printer. The image memory 963 temporarily stores image data optically read by the ADF 20 when the image forming apparatus 1 functions as a copying machine.

  The I / F 964 is an interface circuit for realizing data communication with an external device. For example, the I / F 964 creates a communication signal in accordance with a network communication protocol for connecting the image forming apparatus 1 and the external device, and from the network side. Are converted into data in a format that can be processed by the image forming apparatus 1. A print instruction signal transmitted from a personal computer or the like is given to the control unit 90 via the I / F 964, and image data is stored in the image memory 963 via the I / F 964.

  The control unit 90 functions to include a drive control unit 91 and a bias control unit 92 (bias control unit) by the CPU executing a control program stored in the ROM.

  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 a driving unit (not shown) to rotate the photosensitive drum 321. In the present embodiment, the drive control unit 91 rotationally drives the above members in the development operation and the development preparation operation.

  The bias controller 92 (bias controller) determines a bias setting value for the developing roller 83 and the magnetic roller 82 when executing the developing operation, and controls the first applying unit 88 and the second applying unit 89 to perform development. A bias Bd is applied. In addition, the bias controller 92 determines bias setting values for the developing roller 83 and the magnetic roller 82 when performing the development preparation operation described later, and controls the first application unit 88 and the second application unit 89 to perform development. A preparation bias Bp is applied.

<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 addition, before and after the development operation, a different bias value is applied to the magnetic roller 82 and the development roller 83 in the development preparation operation in which a smaller amount of toner is supplied from the magnetic roller 82 to the development roller 83 than before the development operation. The

  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 first amount of 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.

  An example of the developing bias Bd applied to the magnetic roller 82 and the developing roller 83 by the bias controller 92 controlling the first applying unit 88 and the second applying unit 89 during the developing operation is as follows.

DC bias Vmag_dc of magnetic roller 82; 370V
DC bias Vslv_dc of the developing roller 83; 70V
AC bias (Vpp) Vmag_ac of magnetic roller 82; 2550 V (3.6 kHz)
AC bias of developing roller 83 (Vpp) Vslv_ac; 1600 V (3.6 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%

<Description of development preparation operation (non-development operation)>
Next, a development preparation operation (non-development operation) in the developing device 324 in the present embodiment will be described. This development preparation operation is performed at a timing when the previous development operation is not performed, and is performed prior to the development operation. For example, it is executed before an image forming operation in which an image is formed on a sheet or in correspondence between a sheet between one sheet and a succeeding sheet when images are formed on a plurality of sheets. Further, at the end of a series of print jobs, a development preparation operation may be executed as a preparation operation for the next print job.

  As described above, during the developing operation, the first amount of toner is supplied from the magnetic roller 82 to the developing roller 83 by the developing bias Br applied to the magnetic roller 82 and the developing roller 83. On the other hand, in the development preparation operation, a second amount of toner smaller than the first amount is supplied from the magnetic roller 82 to the development roller 83 by the development preparation bias Bp applied to the magnetic roller 82 and the development roller 83. In the present embodiment, the second amount is set to approximately half of the first amount. Prior to execution of the developing operation, a predetermined amount of toner is already carried on the developing roller 83 in the developing preparation operation, so that it is possible to cope with rapid toner consumption at the start of the developing operation. In other words, even when an image with a high image density is formed at the leading edge of the first sheet or after the sheet, image loss in the image is suppressed. In other words, when the toner is suddenly carried on the peripheral surface 83A of the developing roller 83 corresponding to the leading end portion of the first sheet without the development preparation operation, the toner is supplied to the photosensitive drum 321. The above-mentioned image defect tends to occur without catching up. Further, after the toner on the peripheral surface 83A of the developing roller 83 is forcibly removed between the sheets, the toner is suddenly carried on the peripheral surface 83A of the developing roller 83 corresponding to the leading edge of the next sheet. Even in this case, the supply of toner to the photosensitive drum 321 cannot catch up and the above-mentioned image defect is likely to occur. As described above, in the present exemplary embodiment, the developing preparation operation is performed prior to the developing operation, so that the toner can be stably supplied from the developing roller 83 to the photosensitive drum 321. Further, after the development operation is performed, a part of the toner on the development roller 83 is moved onto the magnetic roller 82 by performing a development preparation operation. As a result, it is possible to prevent the toner having poor charging property from staying on the developing roller 83.

  Here, an example of the development preparation bias Bp set by the bias control unit 92 for the development preparation operation is as follows.

DC bias Vmag_dc of magnetic roller 82; 170V
DC bias Vslv_dc of developing roller 83; 40V
AC roller bias (Vpp) Vmag_ac of magnetic roller 82; 2000 V (3.6 kHz)
AC bias (Vpp) Vslv_ac of developing roller 83; 1350 V (3.6 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%

  As described above, in the present embodiment, the DC bias of the magnetic roller 82 is set to Vmag_dc; 370 V and the DC bias of the developing roller 83 is set to Vslv_dc; 70 V in the developing operation, and the DC bias of the magnetic roller 82 is set to Vslv_dc; Vmag_dc; 170V, and the DC bias of the developing roller 83 is set to Vslv_dc; 40V. That is, with respect to the DC bias, in the developing operation, the toner charged to the positive polarity is moved from the magnetic roller 82 toward the developing roller 83 by a potential difference of 300V. On the other hand, in the developing preparation operation, the toner is moved from the magnetic roller 82 toward the developing roller 83 by a potential difference of 130V. As a result, the amount of toner (second amount) supplied from the magnetic roller 82 to the developing roller 83 in the development preparatory operation is larger than the amount of toner (first amount) supplied from the magnetic roller 82 to the developing roller 83 in the developing operation. ) Is set low.

  Next, the bias applied in the development operation and the development preparation operation (non-development operation) will be described in further detail with reference to FIG. FIG. 6 is a schematic diagram showing the relationship between the potentials of the photosensitive drum 321 and the developing roller 83 when an image is formed on three sheets in the image forming apparatus 1. In FIG. 6, Vdrum indicates the time on the horizontal axis and the transition of the potential on the photosensitive drum 321 on the vertical axis. P1, P2, and P3 indicate development steps in which toner images are formed on the first, second, and third sheets, respectively. A1 indicates a development preparation operation immediately before the development operation for the first sheet, and A2 indicates a development preparation operation after the completion of the development operation for the third sheet. Furthermore, B1 and B2 correspond to a so-called development preparation operation between sheets, and correspond to between the sheets between the first sheet, the second sheet, the second sheet, and the third sheet, respectively.

  In this embodiment, the photosensitive drum 321 is charged to a background portion potential of +230 V by the charger 322. That is, the photosensitive drum 321 is charged to +230 V corresponding to A1, P1, B1, P2, B2, P3, and A2 in FIG. In the developing steps P1, P2, and P3 of each sheet, when the region corresponding to the maximum image density is exposed by the exposure device 323, the surface potential is reduced to Vimg; + 30V. At this time, as described above, a DC bias Vslv_dc1; 70V is applied to the developing roller 83. The developing roller 83 is applied with an AC bias (Vpp) Vslv_ac1; 1600 V (3.6 kHz). As a result, toner is supplied from the developing roller 83 to the + 30V image portion.

  On the other hand, in the development preparation operations (non-development operations) A1, B1, B2, and A2 where the above-described development operation is not performed, the photosensitive drum 321 is charged to +230 V as described above. Further, a DC bias Vslv_dc2; 40 V is applied to the developing roller 83. Further, an AC bias (Vpp) Vslv_ac2; 1350 V (3.6 kHz) is applied to the developing roller 83.

  In other words, the bias controller 92 determines the peak of the development preparation bias Bp during the development preparation operation in the peak-to-peak voltage of the AC bias, rather than the peak-to-peak voltage (Vslv_ac1; 1600V) of the development bias Bd during the development operation. The inter-voltage (Vslv_ac2; 1350V) is set small. Further, the bias controller 92 determines the difference (ΔV2) between the DC bias (Vslv_dc1; 70V) applied to the developing roller 83 of the developing bias Bd and the background potential (Vback; 230v) of the photosensitive drum 321. The development bias Bd and the development bias Bd are set so that the difference (ΔV1) between the DC bias (Vslv_dc2; 40V) applied to the development roller 83 and the background potential (Vback; 230v) of the photosensitive drum 321 becomes larger. A development preparation bias Bp is set.

  Thus, in the development preparation operation, the peak-to-peak voltage of the AC bias applied to the development roller 83 (magnetic roller 82) is set smaller than that during the development operation, so that the development roller 83 and the The external additive is prevented from being detached from the surface of the toner carried on the magnetic roller 82. For this reason, the contact area between the surface of the photosensitive drum 321 and the surface of the toner is reduced over a long period of time, and toner transferability is stably maintained. As a result, the image density is suppressed from being lowered as the developing device 324 is used.

  Further, in the development preparation operation, the DC bias applied to the developing roller 83 is set smaller than that during development, so that the potential difference between the developing roller 83 and the photosensitive drum 321 is expanded. Therefore, the movement of the toner external additive from the developing roller 83 toward the photosensitive drum 321 is suppressed. In particular, in the present embodiment, as described above, the external additive released from the toner is charged with a polarity opposite to the polarity of the toner. Therefore, when the potential difference between the developing roller 83 and the photosensitive drum 321 is increased in the development preparation operation, the external additive is less likely to cover the photosensitive drum 321 side. As a result, wasteful consumption of the external additive from the developing housing 80 of the developing device 324 is suppressed during the developing preparation operation such as 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.

  In the embodiment described above, the toner has a positive charging polarity. However, the present invention is not limited to this. Even when the charging polarity of the toner is negative, the bias control unit 92 adjusts the developing bias Bd and the developing preparation bias Bp, so that the external additive is preferably prevented from being detached from the toner. In addition, excessive disappearance of the external additive from the developing device 324 is suppressed.

  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: Single-layer OPC
・ Line speed: 130mm / sec
<Developing roller 83>
・ Diameter: 20mm
・ Gap with the photosensitive drum 321 (distance between the facing portions): 0.10 mm
Rotation speed: 252 rpm (peripheral speed ratio 1.5, with rotation with respect to the photosensitive drum 321)
<Magnetic roller 82>
・ Diameter: 20mm
・ Gap S between magnetic roller 82 and developing roller 83: 0.25 mm
・ Rotation speed: 285 rpm (peripheral speed ratio 1.1, counter rotation with respect to developing roller 83)
・ Regulation gap G: 0.48mm
<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
-Initial toner charge amount: 15 μC / g
-Toner / carrier weight ratio: 11%
<Test print conditions>
・ Printing rate: 0.2%
・ Total number of prints: 1000
<Potential condition of photoconductor 321>
・ Image part potential: + 30V
・ Non-image area potential: + 230V
<Development bias condition of Comparative Example 1>
DC bias Vmag_dc of magnetic roller 82; 370V
DC bias Vslv_dc of the developing roller 83; 70V
AC bias (Vpp) Vmag_ac of magnetic roller 82; 2550 V (3.6 kHz)
AC bias of developing roller 83 (Vpp) Vslv_ac; 1600 V (3.6 kHz)
<Development preparation bias condition of Comparative Example 1>
DC bias Vmag_dc of magnetic roller 82; 170V
DC bias Vslv_dc of the developing roller 83; 70V
AC bias (Vpp) Vmag_ac of magnetic roller 82; 2550 V (3.6 kHz)
AC bias of developing roller 83 (Vpp) Vslv_ac; 1600 V (3.6 kHz)
<Development bias condition of Comparative Example 2>
DC bias Vmag_dc of magnetic roller 82; 370V
DC bias Vslv_dc of the developing roller 83; 70V
AC bias (Vpp) Vmag_ac of magnetic roller 82; 2550 V (3.6 kHz)
AC bias of developing roller 83 (Vpp) Vslv_ac; 1600 V (3.6 kHz)
<Development preparation bias condition of Comparative Example 2>
DC bias Vmag_dc of magnetic roller 82; 170V
DC bias Vslv_dc of the developing roller 83; 70V
AC roller bias (Vpp) Vmag_ac of magnetic roller 82; 2000 V (3.6 kHz)
AC bias of developing roller 83 (Vpp) Vslv_ac; 1350 V (3.6 KHz)
<Development Bias Conditions in Examples>
DC bias Vmag_dc of magnetic roller 82; 370V
DC bias Vslv_dc of the developing roller 83; 70V
AC bias (Vpp) Vmag_ac of magnetic roller 82; 2550 V (3.6 kHz)
AC bias of developing roller 83 (Vpp) Vslv_ac; 1600 V (3.6 kHz)
<Development preparation bias conditions in the embodiment>
DC bias Vmag_dc of magnetic roller 82; 170V
DC bias Vslv_dc of developing roller 83; 40V
AC roller bias (Vpp) Vmag_ac of magnetic roller 82; 2000 V (3.6 kHz)
AC bias (Vpp) Vslv_ac of developing roller 83; 1350 V (3.6 kHz)

  In other words, the comparative example 2 is different in that the peak-to-peak voltage of the AC bias applied to the developing roller 83 and the magnetic roller 82 is set smaller than that during the developing operation in the developing preparation operation. Different from Example 1. Further, the embodiment is different from the comparative example 2 in that the DC bias applied to the developing roller 83 during the developing preparation operation is set smaller than that during the developing operation.

  Under the conditions described above, in each of Comparative Example 1, Comparative Example 2, and Example, 1000 sheets were printed while the development preparation operation was performed between the sheets, and the transition of the density of the printed image was measured. Table 1 shows the transition of the concentration under each condition measured at this time, and FIG. 9 shows Table 1 as a graph. The concentration was measured with a reflection densitometer.

  As shown in Table 1 and FIG. 8, compared to Comparative Example 1 and Comparative Example 2, in the example, the density decrease after 600 sheets is gradually progressing. In the example, the density did not fall below 1.0 even after 1000 sheets were printed, and the result showed a stable density transition. In this way, in the development preparation operation, the voltage between the peaks of the AC bias applied to each roller is reduced, thereby preventing the external additive from being detached from the toner. Further, by setting the direct current bias applied to the developing roller 83 in the developing preparation operation to be low, the external additive is prevented from moving from the developing roller 83 to the photosensitive drum 321, and stable for a long time. Image density is maintained.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 10 Apparatus main body 30 Image forming part 32 Image forming unit 33 Intermediate transfer unit 321
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)
962 Drive unit (rotation drive means)
963 Image memory 964 I / F

Claims (4)

A developing housing for storing a developer including toner and carrier to which an external additive is added; and
A developer carrying member that receives the developer in the developing housing while rotating in a predetermined direction and carries the developer layer;
An image carrying a toner image that receives toner from the developer layer and carries the toner layer while rotating while being in contact with the developer layer, and carries a toner image that is formed by an electrostatic latent image on the surface and is manifested by the toner A toner carrier for supplying the toner to the carrier;
A 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 therebetween;
In the developing operation in which the toner image is formed on the image carrier, the DC bias and the bias applying unit are supplied to the bias applying unit so that a first amount of toner is supplied from the developer carrier to the toner carrier. A first developing bias including the AC bias is applied to each of the developer carrying member and the toner carrying member, and the developer carrying is performed during a non-developing operation in which the developing operation is not performed. as the toner of the small second amount than said first amount to said toner carrying member from the body is supplied, said second developing bias including the DC bias and the AC bias to the bias applying means developing Bias control means for applying to each of the agent carrier and the toner carrier ,
The bias control unit sets the peak-to-peak voltage of the second developing bias to be smaller than the peak-to-peak voltage of the first developing bias in the peak-to-peak voltage of the alternating-current bias, and the first developing The second development bias applied to the toner carrier among the second development bias is determined by the difference between the first DC bias applied to the toner carrier among the biases and the background portion potential of the electrostatic latent image. And the background portion potential is large , and the second DC bias is higher than the image portion potential with respect to the maximum image density of the electrostatic latent image . A developing device that sets a developing bias.   The bias control means is configured such that the toner carrier and the developer at the DC bias of the second development bias are greater than the potential difference between the toner carrier and the developer carrier at the DC bias of the first development bias. The developing device according to claim 1, wherein the first and second developing biases are set so that a potential difference from the carrier is small. The image carrier carrying the toner image formed by the toner supplied from the electrostatic latent image and the toner carrier;
An image forming apparatus comprising: the developing device according to claim 1.   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 3, wherein the image forming apparatus is executed in response to the process.

Images