JP5244357B2 - Developing device and image forming apparatus including the same - Google Patents

Description

  The present invention uses a two-component developer composed of a magnetic carrier and a toner, and develops an electrostatic latent image on an image carrier in a non-contact manner by holding only the charged toner on the developing roller and developing the image on the image bearing member in a non-contact manner. The present invention relates to an apparatus and an image forming apparatus such as a copying machine, a facsimile, and a printer provided with the apparatus.

  Conventionally, as a development method using dry toner in an image forming apparatus using an electrophotographic process, a one-component development method using no carrier and a two-component developer charging a nonmagnetic toner using a magnetic carrier are used. A two-component development system is known in which an electrostatic latent image on an electrostatic latent image carrier (photoconductor) is developed by a magnetic brush made of toner and carrier formed on a developing roller.

  The one-component development method is suitable for high image quality because the electrostatic latent image on the electrostatic latent image carrier is not disturbed by the magnetic brush. On the other hand, the toner is charged by the charge roller and developed by the elastic regulation blade. In order to regulate the layer thickness on the roller, the toner additive adheres to the charge roller and the charging ability is lowered, and it is difficult to stably maintain the charge amount of the toner. In addition, toner may adhere to the regulating blade, resulting in non-uniform layer formation and image defects.

  Further, in the case of color printing in which color superposition is performed, since the toner is required to be transparent, it needs to be a non-magnetic toner. Therefore, full-color image forming apparatuses often employ a two-component development system that charges and conveys only toner that does not contain a carrier component. However, the two-component development method can maintain a stable charge amount for a long time and is suitable for extending the life of the toner, but the magnetic brush described above may affect the image quality.

  As one of the means for solving these problems, for example, as disclosed in Patent Documents 1 and 2, the developer is brought into non-contact with the electrostatic latent image carrier (photoconductor) using a magnetic roller. Transfer to the installed developing roller, transfer the toner onto the developing roller to form a thin layer with non-magnetic toner, and fly the toner to the latent image on the electrostatic latent image carrier (photosensitive member) by an AC electric field A so-called touch-down development method has been proposed.

  According to this technology, the two-component development system as described above is adopted for the toner charging area in consideration of the extension of the life of the toner, and the subsequent development area is non-contact with the photoconductor for the purpose of improving the image quality. Since the one-component development method in which only the toner is ejected is adopted, the advantages of the one-component development method and the two-component development method can be utilized. Further, since the toner is charged with the carrier, the toner charge amount is relatively high and stable, which is advantageous for toner scattering and fogging. Therefore, it is the most suitable development method for a full-color image forming apparatus that is required to have high speed, high image quality, and long life.

  In such a touch-down development method, the toner layer on the developing roller is charged up by friction with the cutting blade for regulating the toner layer or the surface of the developing roller as the output is accumulated. In addition, high-charged toner that has not been developed gradually increases, and eventually the toner thin layer is disturbed, resulting in image density defects and image unevenness.

Therefore, a method for increasing the charge amount of the toner layer has been proposed. For example, Patent Document 3 uses a one-component toner made of a non-magnetic or weak magnetic material, and is provided with a toner layer regulating means and a charge injection means along the moving direction of the toner conveying body (developing roller). At least the surface of the body is formed of a material containing a charge control agent having the same polarity as that of the toner, and the triboelectric charge amounts TECr and TECc of the material forming the toner and the toner transport body are set to | TECr−TECc | = 5 to 40 μc An electrostatic charge image developing device formed to be / g is disclosed. In Patent Document 3, a forming phenomenon (toner fusion) on the surface of the toner transport body is prevented, and the triboelectric charge amount of the toner is increased to ensure a stable high-quality image.
JP 2003-211961 A JP 2003-21966 A JP-A-2-79062

  However, in the method of Patent Document 3, it is possible to prevent toner fusion by increasing the charge amount of the toner, but it is necessary to charge the toner thin layer only by defining the toner charge amount of the entire toner thin layer. It is difficult to prevent image density defects and fogging due to charge-up of a layer close to the surface of the developing roller (lower layer).

  In the touchdown development system as described above, the toner charge amount of the lowermost layer and the other layers becomes different when the output of the toner layer on the developing roller is repeated. This is because only the toner that is easy to fly is developed, and particularly, the toner existing in the lower layer cannot be moved from the surface of the developing roller and is repeatedly charged up due to friction with the cutting blade or the surface of the developing roller.

  For example, when the volume average particle diameter of the toner is about 6 μm, the toner layer on the developing roller is preferably formed of about 3 layers, and the layer thickness is about 6 to 18 μm. If print output is continued in this state, the toner layer is in a state where the toner with a small particle size with high charge accumulates in the lower layer (about 1 to 1.5 toner layers, that is, about 30 to 50% from the lower layer). Therefore, there is a toner with a lower charge than that.

  Since the toner with a small particle size with a high charge has a strong adhesion to the developing roller, if a high electric field is applied to develop the toner with a high charge on the photosensitive drum, a leak occurs between the developing roller and the drum. There is a risk of toner layer disturbance and image defects. Further, a method of developing only the toner in the upper layer portion by increasing the toner layer is also conceivable, but in this method, there is a possibility that a problem such as thickening of the character may occur due to environmental fluctuations. Also, for example, in a compact and high-speed image forming apparatus using a developing roller of φ25 mm or less and a photosensitive drum peripheral speed of 300 mm / sec, toner shortage or toner unevenness may occur and image density defects may occur. It was.

  Furthermore, in the touch-down method, the undeveloped toner that has once passed through the development zone is drawn into the developing machine and peeled off by a magnetic brush. However, uncharged toner with a small charge and high charge is developed. Due to strong adhesion to the roller, it tends to remain on the surface of the developing roller. As a result, highly charged toner increases on the developing roller, eventually disturbing the thin toner layer and may cause image density defects and image unevenness.

  In view of the above problems, the present invention suppresses charge-up of the lower layer in the toner layer formed on the developing roller, prevents image density defects, image unevenness and fogging, and provides a stable image over a long period of time. It is an object of the present invention to provide a touchdown developing type developing device and an image forming apparatus including the same.

In order to achieve the above object, the present invention uses a two-component developer containing at least a carrier and a toner, a toner carrier disposed opposite to the image carrier, and a toner using a magnetic brush on the toner carrier. A toner supply member that forms a thin layer, and applying a developing bias to the toner carrier and the toner supply member causes the toner to fly from the toner carrier to the surface of the image carrier. In the developing device for developing an electrostatic latent image, q / d (a) is an average charge amount per toner particle diameter in the toner layer formed on the toner carrier, and the upper toner layer to the toner layer When the average charge amount per particle diameter of the toner in the lower layer from which the toner layer corresponding to 50 to 70% by weight of the toner is removed is q / d (b), the following conditional expression (1) is satisfied. It is characterized in.
| Q / d (a) −q / d (b) | ≦ 0.5 (femto-C / μm) (1)

  Further, the present invention provides a first bias applying unit that applies a first bias to the toner carrier, and is electrically connected to a ground common to the first bias applying unit. And a second bias applying means for applying a second bias superimposed on the first bias.

In the present invention, the duty ratio of the waveform on the same polarity side as the toner in the AC component of the first bias is Dslv, and the duty ratio of the waveform on the same polarity side as the toner in the AC component of the second bias is Dmag. In this case, the first and second biases are applied so that the duty ratio satisfies the following conditional expressions (2) and (3).
100-Duty (mag) <Duty (slv) (2)
Duty (slv) ≧ 40% (3)

  According to the present invention, the toner is a positively chargeable toner, and the surface of the toner carrier is coated with a silicon-modified urethane resin.

  The present invention also provides an image forming apparatus equipped with the developing device having the above-described configuration.

  According to the first configuration of the present invention, the average charge amount q / d (a) of the toner layer formed on the toner carrier, and the average charge amount q of the lower layer formed on the toner carrier side of the toner layer. When the relationship with / d (b) satisfies the conditional expression (1), it is possible to prevent charge-up of the lower layer portion and to reduce the charge difference between the upper layer and the lower layer of the toner thin layer. Therefore, it is possible to prevent image density defects, image unevenness and fogging, and obtain a stable image over a long period of time.

  Further, according to the second configuration of the present invention, in the developing device having the first configuration, the first bias is applied to the toner carrier by the first bias applying unit, and the second bias applying unit is provided. The first bias applying means is electrically connected to a common ground, and the second bias is superimposed on the first bias and applied to the toner supply member, whereby the alternating current component of the first bias and the first bias are applied. The AC components of the two biases can be set independently, and application conditions that improve the toner layer forming efficiency on the toner carrier and the toner recovery efficiency on the toner supply member can be set. It becomes possible.

  According to the third configuration of the present invention, in the developing device of the second configuration, the duty ratio Dslv of the waveform on the same polarity side as the toner in the AC component bias of the first bias, and the second bias When the relationship between the toner component in the AC component and the duty ratio Dmag of the waveform on the same polarity side satisfies the conditional expressions (2) and (3), the toner layer formation efficiency on the toner carrier and the toner recovery to the toner supply member Efficiency can be improved and friction between the toner carrier and the lower layer of the toner layer can be reduced. Thereby, a toner layer satisfying the relationship between q / d (a) and q / d (b) can be formed. Therefore, the charging difference between the upper layer portion and the lower layer portion of the toner layer can be reduced only by changing the application of the first and second AC biases.

  According to the fourth configuration of the present invention, in the developing device having any one of the first to third configurations, when the toner is a positively chargeable toner, the surface of the toner carrier is coated with the silicon-modified urethane resin. By doing so, the adhesion force between the toner carrier and the toner can be reduced, so that the efficiency of collecting the toner from the toner carrier can be improved.

  In addition, according to the fifth configuration of the present invention, by mounting the developing device having any one of the first to fourth configurations, it is possible to prevent image density defects, image unevenness and fogging, and for a long period of time. And an image forming apparatus capable of stable image formation.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view of an image forming apparatus on which the developing device of the present invention is mounted. Here, a tandem color image forming apparatus is shown. In the main body of the color image forming apparatus 100, four image forming portions Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (the right side in FIG. 1). These image forming portions Pa to Pd are provided corresponding to images of four different colors (cyan, magenta, yellow, and black), and cyan, magenta, and yellow are respectively performed by charging, exposure, development, and transfer processes. And a black image are sequentially formed.

  Photosensitive drums 1a, 1b, 1c, and 1d that carry visible images (toner images) of the respective colors are disposed in the image forming portions Pa to Pd, and are formed on the photosensitive drums 1a to 1d. The transferred toner image is rotated clockwise in FIG. 1 by a driving means (not shown) and sequentially transferred onto the intermediate transfer belt 8 moving adjacent to each image forming unit, and then the secondary transfer roller 9. In this case, the toner image is transferred onto the transfer paper P at once, and further fixed on the transfer paper P in the fixing unit 7 and then discharged from the apparatus main body. An image forming process for each of the photosensitive drums 1a to 1d is executed while rotating the photosensitive drums 1a to 1d counterclockwise in FIG.

  The transfer paper P onto which the toner image is transferred is accommodated in a paper cassette 16 at the lower part of the apparatus, and is conveyed to the secondary transfer roller 9 via a paper feed roller 12a and a registration roller pair 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a belt in which both ends thereof are overlapped and joined to form an endless shape, or a belt without a seam (seamless) is used. A blade-shaped belt cleaner 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed on the downstream side of the secondary transfer roller 9.

  Next, the image forming units Pa to Pd will be described. There are chargers 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d and image information on the photosensitive drums 1a to 1d around and below the photosensitive drums 1a to 1d that are rotatably arranged. The exposure unit 4 for exposing the toner, the developing devices 3a, 3b, 3c and 3d for forming toner images on the photosensitive drums 1a to 1d, and the developer (toner) remaining on the photosensitive drums 1a to 1d are removed. Cleaning units 5a, 5b, 5c and 5d are provided.

  When the image formation start is input by the user, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the chargers 2a to 2d, and then light is irradiated by the exposure unit 4 to each of the photosensitive drums 1a to 1d. An electrostatic latent image corresponding to the image signal is formed on the top. Each of the developing devices 3a to 3d is filled with a predetermined amount of cyan, magenta, yellow, and black toner by a replenishing device (not shown). The toner is supplied onto the photosensitive drums 1a to 1d by the developing devices 3a to 3d and electrostatically attached, whereby a toner image corresponding to the electrostatic latent image formed by exposure from the exposure unit 4 is formed. It is formed.

  After an electric field is applied to the intermediate transfer belt 8 at a predetermined transfer voltage, cyan, magenta, yellow, and black toner images on the photosensitive drums 1a to 1d are transferred to the intermediate transfer belt 8 by the intermediate transfer rollers 6a to 6d. Transcribed above. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, the toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the cleaning units 5a to 5d in preparation for the subsequent formation of a new electrostatic latent image. The photosensitive drums 1a to 1d will be described later.

  The intermediate transfer belt 8 is stretched between an upstream conveyance roller 10 and a downstream drive roller 11, and the intermediate transfer belt 8 rotates clockwise as the drive roller 11 is rotated by a drive motor (not shown). When the rotation starts, the transfer paper P is conveyed from the registration roller 12b to the secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and a full color image is transferred. The transfer paper P onto which the toner image is transferred is conveyed to the fixing unit 7.

  The transfer paper P conveyed to the fixing unit 7 is heated and pressurized by the fixing roller pair 13 so that the toner image is fixed on the surface of the transfer paper P, and a predetermined full color image is formed. The transfer paper P on which the full-color image is formed is distributed in the transport direction by the branching portion 14 that branches in a plurality of directions. When an image is formed only on one side of the transfer paper P, it is discharged as it is onto the discharge tray 17 by the discharge roller 15.

  On the other hand, when images are formed on both sides of the transfer paper P, the transfer paper P that has passed through the fixing unit 7 is distributed to the paper transport path 18 by the branching unit 14, and the secondary transfer roller 9 with the image surface reversed. Re-conveyed. Then, after the next image formed on the intermediate transfer belt 5 is transferred by the secondary transfer roller 9 to the surface of the transfer paper P where the image is not formed, and conveyed to the fixing unit 7 and the toner image is fixed. , And discharged to the discharge tray 17.

  As the photosensitive drums 1a to 1d, an amorphous silicon (a-Si) photosensitive member, an organic photosensitive member (OPC), or the like can be used. When an a-Si photosensitive member is used as the photosensitive material of the photosensitive drums 1a to 1d, it has a feature that the post-exposure potential on its surface is very low of 20V or less. The potential decreases, and the withstand voltage that leads to dielectric breakdown decreases. On the other hand, when latent images are formed, the charge density on the surfaces of the photosensitive drums 1a to 1d is improved, and the development performance tends to be improved.

  This characteristic is particularly noticeable when the film thickness is 25 μm or less, more preferably 20 μm or less, for an a-Si photoreceptor having a high dielectric constant of about 10. When a positively charged organic photoconductor (OPC) is used as the photoconductor drum 1a, the positively charged organic photoconductor (positive OPC) has a stable charge because it generates less ozone. In particular, a positively charged organic photoconductor having a single layer structure is suitable for a system having a long life because the photosensitivity changes little and the image quality is stable even when the film thickness changes due to long-term use.

In order to extend the life of the positively charged organic photoconductor, the residual potential needs to be 100 V or less. Therefore, the thickness of the photosensitive layer is set to 25 μm or more, and the amount of charge generation material added can be increased. Of particular importance. In particular, OPC having a single-layer structure is advantageous because a charge generating material is added to the photosensitive layer, and therefore the sensitivity change is small even when the photosensitive layer is reduced. Further, the peripheral speed of the photosensitive drum 1 a to 1 d when more than 180 mm / sec, the charging of the photosensitive drum 1 a to 1 d, the exposure process time such development and neutralization is shortened, the printing of the image forming apparatus at a high speed can do. However, since the developing nip time is short, it is necessary to further improve the developability, and it is important to reduce the adhesion force of the toner 26 to the developing roller 22. This correspondence will be described later.

  FIG. 2 is a side sectional view showing the structure of the developing device according to the present invention. Here, the developing device 3a disposed in the image forming unit Pa of FIG. 1 will be described, but the configuration of the developing devices 3b to 3d disposed in the image forming units Pb to Pd is basically the same, and thus described. Is omitted.

  As shown in FIG. 2, the developing device 3a includes a developing container 20 in which a two-component developer (hereinafter simply referred to as a developer) is accommodated, and the developing container 20 is divided into a first wall and a second wall by a partition wall 20a. The first and second agitating chambers 20b and 20c are divided into agitating chambers 20b and 20c, and a toner (positively charged toner) supplied from a toner container (not shown) is mixed with a carrier and agitated and charged. The stirring screw 21a and the second stirring screw 21b are rotatably arranged.

Then, the developer is conveyed in the axial direction while being stirred by the first stirring screw 21a and the second stirring screw 21b, and is supplied to the first and second via a developer passage (not shown) formed in the partition wall 20a. It circulates between the stirring chambers 20b and 20c. In FIG. 2 , the developing container 20 extends obliquely upward to the left, and a magnetic roller 23 (toner supply member) is disposed in the developing container 20 above the second stirring screw 21a. A developing roller 22 (toner carrier) is disposed oppositely to the upper left of the magnetic roller 23, and the developing roller 22 faces the photosensitive drum 1a on the opening side of the developing container 20 (left side in FIG. 2). . The developing roller 22 and the magnetic roller 23 rotate clockwise in the figure.

  Note that a toner sensor (not shown) is disposed in the developing container 20 so as to face the second stirring screw 21b, and from the toner container through the toner supply port 20d according to the toner concentration detected by the toner sensor. Thus, the toner is supplied into the developing container 20.

  The first and second stirring screws 21a and 21b in the developing container 20 mix and stir the toner supplied from the toner container with the carrier, thereby charging the toner and the carrier. A magnetic brush is formed on the magnetic roller 23 by the developer composed of the charged toner and the carrier, and the magnetic brush is in contact with or close to the developing roller 22 with a certain layer thickness, and between the magnetic roller 23 and the developing roller 22. A toner layer (toner thin layer) is formed on the developing roller 22 by the formed potential difference.

  Due to the potential difference formed between the developing roller 22 and the photosensitive drum 1a, the toner jumps from the toner layer on the developing roller 22 to the photosensitive member 1a, and the transferred toner forms an electrostatic latent image formed on the surface of the photosensitive drum 1a. A thin toner layer and a toner image are formed by adhering to the image. The bias applied to the developing roller 22 is a first bias, and the bias applied to the magnetic roller 23 is a second bias.

FIG. 3 is a schematic diagram showing a connection state of the first and second power sources in the developing device of the present embodiment. FIG. 4A shows the waveforms of the first and second biases applied to the developing roller 22 and the magnetic roller 23 used in the present embodiment, and FIG. 4B shows the combined waveform thereof. FIG. The developing device will be described in detail below with reference to FIGS.

It is important to define the particle size distribution of the toner 26 in order to avoid selective developability. In general, the spread of the particle size distribution of the toner 26 is measured with Multisizer III (manufactured by Beckman Coulter, Inc.) and an aperture diameter of 100 μm (measurement range: 2.0 to 60 μm). And the number distribution average particle size. In order to prevent selective development, it is important to reduce the ratio. When the distribution is wide, toner 26 having a relatively small particle size is accumulated on the developing roller 22 during continuous printing, and developability is deteriorated.

  In addition, it is generally well known that the toner volume average particle diameter is reduced in order to improve the image quality. On the other hand, if the toner volume average particle diameter is reduced, the influence of van der Waals force becomes stronger. It is known that it becomes difficult to separate from the surface of the developing roller 22 or to peel off from the surface of the developing roller 22. Therefore, it is preferable to define the volume average particle diameter Dt of the toner 26 in the range of 4.0 μm ≦ Dt ≦ 7.0 μm.

If Dt is less than 4.0 μm, the adhesive force is too strong, and the developability and the recoverability of the toner 26 from the developing roller 22 may be reduced. On the other hand, if it exceeds 7 μm, the reproducibility of one dot becomes difficult and it is difficult to achieve high image quality. The CV value in the number particle size distribution of the toner 26 is preferably 25.0% or less. When the CV value is more than 25.0%, the distribution of the particle size distribution is large and the selective developability becomes remarkable.

  As the carrier 27, a magnetite carrier, a Mn ferrite, a Mn—Mg ferrite, a Cu—Zn, a resin carrier in which a magnetic material is dispersed in a resin, and the like can be used. It is also possible to process and use. The carrier 27 has a role of collecting undeveloped toner on the developing roller 22 and supplying toner thereafter.

In order to peel off the toner 26 firmly and electrostatically attached at the nip between the developing roller 22 and the magnetic roller 23 with the magnetic brush 28 and supply the toner 26 necessary for development, the volume specific resistivity is 10 ⁶ Ωcm. It is preferable to use the carrier 27 in the range of -10 ¹³ Ωcm. Further, by using the carrier 27 having a weight average particle diameter of 50 μm or less, the surface area of the carrier 27 can be increased and the number of contacts with the toner 26 can be increased.

  The developing roller 22 carries a toner layer 29 of the toner 26 supplied from the magnetic brush 28, and causes the toner 26 to fly from the toner layer 29 to develop the electrostatic latent image on the photosensitive drum 1a. The surface of the developing roller 22 is composed of a sleeve made of uniform conductive aluminum, SUS, conductive resin coating, or the like.

By coating the surface of the developing roller 22 with resin, a leak margin can be secured. By applying a fluororesin or urethane resin with good releasability of the toner 26 as such resin, leakage can be suppressed even when a thin a- Si photosensitive drum 1a having a film thickness of 20 μm or less is used. It is possible to suppress problems such as black spots on the photosensitive drum 1a.

  In particular, when the toner 26 is positively charged, toner adhesion can be reduced by using a urethane resin having the same polarity, for example, a silicon-modified urethane resin. As a result, the toner can easily fly and the developing property can be improved, and the toner peeling property (recoverability) from the developing roller 22 to the magnetic roller 23 can be improved.

  The shaft portion of the developing roller 22 is connected to a first power source 30 including a DC power source 30a and an AC power source 30b, and a first bias is applied. As shown in FIG. 4A, the first bias is applied to the DC voltage Vdc1 of the first DC power supply 30a, the peak-to-peak voltage (AC voltage) Vpp1, the duty ratio Dslv, and the frequency f of the first AC power supply 30b. And a synthesized waveform Vslv (solid line) on which a rectangular wave composed of is superimposed.

  Here, the duty ratio Dslv indicates the duty ratio on the side (the same polarity side as the toner) on which the toner 26 is ejected from the developing roller 22 to the photosensitive drum 1a. A duty ratio Dmag, which will be described later, indicates a duty ratio on the side (the same polarity side as the toner) on which the toner 26 is caused to fly from the magnetic roller 23 to the developing roller 22. Next, the duty ratio will be described.

  FIG. 5 shows the developing roller 22 or the second bias when the first bias is applied when, for example, positively charged toner is used and the upper direction in the figure is a positive potential and the lower direction is a negative potential. The composite waveform of the magnetic roller 23 when applied is shown. At this time, assuming that the time during which the electric field for the toner flying from the developing roller 22 or the magnetic roller 23 is applied is a and the time during which the electric field for pulling the toner back to the developing roller 22 or the magnetic roller 23 is b, the duty ratio Dp Is represented by Dp = {a / (a + b)} × 100. That is, it is expressed as a percentage of the time during which a positive potential is applied with respect to the entire application time. When negatively charged toner is used, the duty ratio is Dp = {b / (a + b)} × 100.

  The magnetic roller 23 is formed in a cylindrical shape that can rotate with a nonmagnetic metal material, and a plurality of fixed magnets are disposed inside the magnetic roller 23 to generate a magnetic brush 28 by a carrier 27 contained in the developer. The layer thickness of the brush 28 is regulated by the ear cutting blade 25. In addition to the first power source 30, the shaft portion is electrically connected to a common ground with the first power source 30, and a second power source 31 comprising a DC power source 31a and an AC power source 31b is connected. Yes. Thereby, the second bias is further superimposed on the first bias.

  As shown in FIG. 4A, the second bias is applied to the DC voltage Vdc2 of the second DC power supply 31a, the peak-to-peak voltage (AC voltage) Vpp2, the duty ratio Dmag, and the frequency f2 of the second AC power supply 31b. And a composite waveform Vmag (broken line) on which a rectangular wave composed of is superimposed.

  As described above, the first bias and the second bias are applied to the developing roller 22 and the magnetic roller 23. Further, as described above, the first stirring screw 21a and the second stirring screw 21b circulate the developer container 20 while the developer is being stirred to charge the toner, and the second stirring screw 21b causes the developer to be a magnetic roller. It is conveyed to 23. Then, a magnetic brush 29 is formed on the magnetic roller 23, the layer thickness of the magnetic brush on the magnetic roller 23 is regulated by the ear cutting blade 25, and the developing roller 22 is applied by the potential difference between the magnetic roller 23 and the developing roller 22. A thin toner layer is formed.

  Then, due to the potential difference between the developing roller 22 and the photosensitive drum 1a, the toner on the developing roller 22 flies, and the electrostatic latent image is developed on the photosensitive drum 1a. Further, the toner on the developing roller 22 is caused by the potential difference between the developing roller 22 and the magnetic roller 23 so that the first bias and the second bias are superimposed between the developing roller 22 and the magnetic roller 23. Formation of the layer 29 and recovery of undeveloped toner on the developing roller 22 to the magnetic roller 23 are performed.

  Even when the toner layer 29 is formed on the developing roller 22 using such a developing device 3a, if the difference in charge amount between the upper layer and the lower layer is large, image density defects, image unevenness, and fog are likely to occur.

Therefore, the weight of the developing roller 2 and the average charge amount per particle diameter of the toner layer 29 formed on the 2 q / d (a) ( femto-C / μm), of the toner layer 29, the toner layer 29 from the upper layer The relationship with the average charge amount q / d (b) (femto-C / μm) per particle diameter of the lower layer from which the toner layer corresponding to 50 to 70% by weight of the toner is removed is expressed by the following conditional expression (1).
q / d (a) −q / d (b) ≦ | 0.5 | (femto−C / μm) (1)
By adjusting the first and second biases so as to satisfy the above, it is possible to prevent charge-up of the lower layer and to reduce the charge difference between the upper layer and the lower layer of the toner layer. As a result, it is possible to prevent image density defects, image unevenness, and fogging, and obtain a stable image over a long period of time.

  The toner layer refers to the entire layer of the toner layer 29. The lower layer is a layer on the surface side of the developing roller 22 in the toner layer, and is obtained by removing the upper layer of 50 wt% to 70 wt% from the upper surface of the toner layer 29. As described above, the volume average particle diameter Dt of the toner is preferably 4.0 μm ≦ Dt ≦ 7.0 μm. When a toner having a Dt of 6.0 μm is used, the toner layer on the developing roller is formed of about three layers. Therefore, the layer thickness of the toner layer is about 6 to 18 μm.

  When print output is continued in this state, the toner layer is in a state of a small particle size toner having a high charge in the lower layer (30 to 50% by weight of the toner layer from the surface of the developing roller 23, that is, about 1.5 layers from the toner layer). Accumulates and toner with a lower charge is likely to exist other than the lower layer. Therefore, about 1.5 layers from one toner layer on the developing roller 22 are used as the lower layer, and the upper toner layer (50 to 70% by weight from the upper surface) is removed to form the lower layer.

  Further, q / d is an average charge amount q per toner particle diameter d, and q / d can be a 50% integrated value of the number charge amount distribution. The number charge distribution is, for example, using a particle charge amount distribution measuring device (E-SPART ANALYZER EST-3, manufactured by Hosokawa Micron Corporation), air pressure: 0.055-0.08 (MPa), PM VOLTAGE: -0.5 kV, FILDE VOLTAGE: It can be measured under the condition of 0.050 kV.

Further, the formation of the lower layer and the measurement of the toner number charge amount distribution can be performed as follows. That is, for example, a predetermined area portion of the toner layer 29 is sucked by a QM meter (manufactured by TREK, MODEL 210S), and the weight of the sucked toner is measured, and then the toner amount A (mg / cm ² ) per unit area. Is calculated. The average charge amount q / d (a) can be obtained by measuring the number charge amount distribution using the sucked sample.

Next, the distance between the suction nozzle and the toner layer is changed, and the toner amount B (mg / cm ² ) per unit area of the toner to be sucked in a predetermined area portion different from the above in the toner layer 29 is 50 of A. Suction is performed so that the amount is from 70% to 70% by weight. Thereby, 50 to 70% by weight of the toner layer can be removed from the upper surface, and a lower layer can be formed. Further, the average charge amount q / d (b) can be obtained by measuring the number charge amount distribution using the sucked sample.

  Next, the first and second bias adjustment methods for forming the toner layer 29 having such a relationship between the average charge amounts q / d (a) and q / d (b) will be described.

  First, the bias applied to the developing roller 22 and the magnetic roller 23 will be described with reference to FIGS.

As described above, the first bias is a combined waveform Vslv having a DC voltage of Vdc1, an AC voltage Vpp1, a duty ratio of Dslv, and a frequency f. The second bias is a composite waveform Vmag having a DC voltage of Vdc2, an AC voltage of Vpp2, a duty ratio of Dmag, and a frequency f. Further, the AC component of the second bias has the same frequency and opposite phase as the AC component of the first bias, and has a larger duty ratio than that of the first bias (D mag > D slv ).

  As shown in FIG. 3, a first bias is applied to the developing roller 22. Further, since the second bias is applied to the magnetic roller 23 so as to overlap the first bias, when viewed from the magnetic roller as a reference, as shown in FIG. 4B, the combined waveform Vmag−Vslv. Has V (max) and V (min), but the electric field between the developing roller 22 and the magnetic roller 23 is only the second bias because the first bias is canceled out. At this time, a first bias is applied between the developing roller 22 and the photosensitive drum 1a.

  As a result, even if the first power supply 30 and the second power supply 31 have different bias periods and duty ratios, the composite waveform of the bias formed between the developing roller 22 and the magnetic roller 23 is the second. The power supply 31 is not affected by the second bias of the power supply 31. Therefore, the first power supply 30 of the developing roller 22 and the second power supply 31 of the magnetic roller 23 are electrically connected to a common ground, and the first power supply 30 and the second power supply 31 are overlapped. A configuration is preferable.

  On the other hand, when the first power supply 30 is superimposed on the second power supply 31 power supply, the first bias is canceled and only the second bias is applied to the electric field between the developing roller 22 and the magnetic roller 23. However, since the first bias and the second bias are applied between the developing roller 22 and the photosensitive drum 1a, it becomes difficult to independently change the first and second biases. It becomes difficult to make the properties uniform.

  Here, as shown in FIG. 6, a case where the first and second biases applied to the developing roller 22 and the magnetic roller 23 are electrically connected to separate grounds will be considered. FIG. 6 is a schematic diagram showing a state in which the developing roller and the magnetic roller are electrically connected to separate grounds. FIG. 7A shows waveforms of the first and second biases applied to the developing roller and the magnetic roller when electrically connected to separate grounds, and FIG. 7B shows a combination of these. It is a model diagram which shows a waveform. In FIG. 6A, the duty ratio is different between Vslv (solid line) of the first power supply 30 and Vmag (broken line) of the second power supply 31, and the AC bias has the same period, the same frequency and the opposite phase as Vslv. Apply.

  In such a configuration, assuming that Dmag and Dslv are different, the combined waveform between the developing roller 22 and the magnetic roller 23 is as shown in FIG. 7B, and the voltage Vi appears between Vmax and Vmin. Therefore, the application time of Vmax and Vmin is shortened by the application time of Vi, the time for forming the toner thin layer on the developing roller is shortened, and the recovery time of the undeveloped toner from the developing roller is also shortened. Becomes worse.

  When Vpp1 of the first bias or Vpp2 of the second bias is changed, Vpp2 and Vpp2 are inevitably applied to the developing roller 22 and the magnetic roller 23, respectively. Therefore, Vpp1 and Vpp2 cannot be changed independently.

  On the other hand, as described above, in the present embodiment, the bias of the first power supply 30 is applied to the developing roller 22, and the bias of the second power supply 31 is applied to the magnetic roller 23 to the bias of the first power supply 30. Is applied in a superimposed manner, so that the combined waveform of the bias formed between the developing roller 22 and the magnetic roller 23 becomes equal to the bias of the second power supply 31, and the first power supply 31 applied to the developing roller 22 Not affected by bias.

  In addition, the first bias formed between the developing roller 22 and the photosensitive member 1a is not affected by the bias of the second power supply 31, and is controlled only by the bias of the first power supply 30, and the first and second biases are controlled. The second bias can set the voltage and duty ratio of each bias independently of each other. At this time, the collection of undeveloped toner from the developing roller 22 to the magnetic roller 23 depends only on the second bias.

  As described above, since the first power supply 30 and the second power supply 31 are superimposed on each other, the first bias and the second bias can be set independently. Detailed settings are possible.

  Accordingly, the first bias voltage and the duty ratio are set to be large to improve the developability, and the formation of the toner layer 29 on the developing roller 22 and the toner recovery from the developing roller 22 are favorably maintained. The bias voltage and the duty ratio of the bias 2 can be set, and the bias balance between the developing roller 22 and the photosensitive member 1a and between the developing roller 22 and the magnetic roller 23 can be easily achieved.

The relationship between the first bias Dslv applied to the developing roller 22 and the second bias Dmag applied to the magnetic roller 23 is set so as to satisfy the following conditional expression. In addition, the toner layer 29 having a relationship between the average charge amount q / d (a) and q / d (b) can be formed.
100-Dmag <Dslv (2)
Dslv ≧ 40% (3)

  By setting 100−Dmag <Dslv, the time for moving the toner from the developing roller 22 to the photosensitive drum 1a per unit time (that is, the time for forming the toner layer 29 on the photosensitive drum 1a) can be increased. In addition, by setting Dslv ≧ 40%, it is possible to secure the development time of the electric field for the toner to move to the photoreceptor 1a side, so that the formation efficiency of the toner layer 29 can be improved.

  Further, by setting 100−Dmag <Dslv, the toner collection time of the magnetic roller 23 per unit time can be extended, and the collection efficiency of undeveloped toner can be improved.

  Accordingly, charging due to friction between the photosensitive drum 1a and the lower layer of the toner layer formed on the drum can be suppressed, and the q / d (a) of the toner layer and the q / d (b) of the lower layer can be reduced. The absolute difference of the difference can be 0.5 or less. When Dslv is increased, the toner 26 may adhere to the non-exposed portion (blank portion) of the electrostatic latent image on the photosensitive drum 1a, and image fog may occur. From this viewpoint, it is preferable that Dslv <75%.

  Further, when the first power supply 30 and the second power supply 31 are electrically connected to a common ground, unlike the case where they are electrically connected to separate grounds as shown in FIG. Since Vpp1 and Vpp2 are not applied to the first bias, the absolute values of Vmax and Vmin are smaller than in the case of FIG. 7B, and the electric field for moving the toner becomes weaker. For this reason, when electrically connected to a common ground, it is preferable to make Vpp1 larger than the bias voltage of the magnetic roller 23 when electrically connected to separate grounds.

  On the other hand, if the time applied to the developing roller 22 per unit time is lengthened, it may be difficult to collect the toner by the magnetic roller 23. Therefore, as described above, it is preferable to coat the surface of the developing roller 22 with, for example, a silicon-modified urethane resin.

  Next, the operation of the developing device of the present invention will be described with reference to FIGS. A magnetic brush 28 is formed on the magnetic roller 23 by the developer composed of the charged toner 26 and the carrier 27 shown in FIG. 3, and the magnetic brush 28 is layer-regulated by the earbrush blade 25 and is shown in FIG. The composite waveform Vmag of the second bias is applied, and the toner layer 29 including only the toner 26 is formed on the developing roller 22.

Next, the latent image exposed and formed on the photosensitive drum 1a is applied with the first bias composite waveform Vslv shown in FIG. 4A, and the toner 26 flies to the photosensitive drum 1a. The developed toner image is formed on the photosensitive drum 1a. Then, the toner image on the photosensitive drum 1 a is primarily transferred to the intermediate transfer belt 8, the toner image is secondarily transferred to the sheet conveyed to the intermediate transfer belt 8, fixed by the fixing unit 7, and discharged. . Thereafter, the development residual toner on the developing roller 22 is peeled off and collected by the magnetic roller 23 by the second bias composite waveform Vmag shown in FIG.

The thickness of the toner layer 29 on the developing roller 22 varies depending on the resistance of the toner, the rotational speed difference between the developing roller 22 and the magnetic roller 23, and the like, but is controlled by the potential difference ΔV between the developing roller 22 and the magnetic roller 23. Is possible. The toner layer 29 tends to be thicker by increasing ΔV and thinner by decreasing ΔV. Considering these, in order to obtain the thickness of the toner layer 29, the range of ΔV is preferably set to 100V to 350V.

The charged toner is developed with the potential difference ΔV between the magnetic roller 23 and the developing roller 22.
2 is held in a thin layer, and developed by applying a bias in which direct current and alternating current are superimposed on the photosensitive drum 1a. In order to prevent scattering of the toner, it is preferable to apply the AC voltage from the first and second AC power supplies 30b and 31b immediately before the development.

  The toner remaining on the developing roller 22 is not provided with a special device such as a scraping blade, and the magnetic brush 28 on the magnetic roller 23 contacts the toner layer 29 on the developing roller 22. It is recovered by the brush effect generated by the difference in peripheral speed from 22. The collected toner 26 is stirred by the stirring screw 21a (see FIG. 2), and the replacement of the toner 26 is promoted.

At this time, the width of the magnetic brush 2 8, to correspond to the width for collecting toner 26 on the developing roller 22, by the width of the developing roller 22 shorter than the width of the magnetic brush 2 8, of reliably toner 26 An uncollected area can be eliminated. Thus, in the sleeve of the developing roller 22, there is no toner 26 adhered to the outside of the magnetic brush 2 8 regions, it can be eliminated toner scattering at both ends of the developing roller 22.

  As a method for promoting the replacement of the toner, the toner 26 on the developing roller 22 is collected by setting the rotation speed of the magnetic roller 23 to 1.0 to 2.0 times the speed of the developing roller 22. At the same time, toner set to an appropriate density can be supplied to the developing roller 22, and a uniform toner layer 29 can be formed.

In order to maintain a uniform image density, a potential difference ΔV between the developing roller 22 and the magnetic roller 23 before and after image formation, until the next image formation, and during non-image formation such as when the apparatus is started up.
By eliminating the potential and making the potential equipotential, recovery by only the magnetic brush 28 is possible, and the toner on the developing roller 22 can be recovered by the magnetic roller 23 without imposing a burden on the toner.

Further, when a- Si is used as the photosensitive material of the photosensitive drum 1a, due to the above-mentioned characteristics of the a- Si photosensitive member, when the film thickness of the photosensitive member is 25 μm or less, more preferably 20 μm or less, For example, development can be performed by setting Vdc1 of the first bias to 150 V or less, Vpp1 to 200 to 2000 V, and frequency to 1 to 4 kHz. When a positively charged organic photoreceptor (OPC) is used as the photosensitive material, the first bias Vdc1 is set to 400 V or less, more preferably 300 V or less in order to prevent a strong electric field from being applied to the toner. It is preferable to do. In order to prevent leakage, it is preferable to set Vdc1 and Vpp1 so that the potential difference from the photosensitive drum 1a does not exceed 1500V.

  In this embodiment, the surface of the developing roller 22 is coated with the silicon-modified urethane resin, but other materials may be coated as long as it has the same polarity as the toner. Moreover, q / d changes with coating conditions, such as coating layer thickness and surface roughness. Therefore, the coating material and the coating conditions may be appropriately set according to the type of toner to be used and the q / d target value. Further, the frequencies f of the first and second AC biases are made equal, but the present invention is not limited to this, and may be set as appropriate depending on the formation state of the toner layer 29 on the developing roller 22.

  In addition, this invention is not limited to the said embodiment, A various change is possible in the range which does not deviate from the meaning of this invention. For example, in the above embodiment, the developing device using positively charged toner whose charging direction is positive (plus side) has been described as an example. However, the developing device using negatively charged toner whose charging direction is negative (minus side) is described. Is equally applicable. In that case, a negatively chargeable resin may be used as the coating material.

  Further, here, a tandem color image forming apparatus using an intermediate transfer belt has been described as an example. However, the present invention is not limited to an image forming apparatus provided with a touch-down developing type developing unit. The present invention can be applied in exactly the same manner to other image forming apparatuses such as a tandem color image forming apparatus, a digital multifunction peripheral, an analog monochrome image forming apparatus, or a facsimile or printer that directly transfer to the recording medium.

  The tandem color image forming apparatus shown in FIG. 1 is equipped with the developing devices 3a to 3d (see FIGS. 1 and 2) of the above-described embodiment in which the developing roller 22 is coated with a silicon-modified urethane resin. The case where the layer 29 was formed was examined as an example of the present invention, and the charge amount q / d and the transfer performance during continuous image printing were investigated. Further, a case where a conventional developing device in which the surface of the developing roller 22 is anodized and a toner layer 29 was formed under the following conditions was mounted was similarly investigated as a comparative example.

  As test conditions common to the examples of the present invention and comparative examples, an amorphous silicon photosensitive drum having a diameter of 30 mm was used, the developing roller diameter was 20 mm, and the magnetic roller diameter was 25 mm. The development conditions were set such that the peripheral speeds of the photosensitive drum, the development roller, and the magnetic roller were 300 mm / sec, 450 mm / sec, and 675 mm / sec, respectively, and the gap between the development roller and the magnetic roller was 350 μm.

As a developer, a positively charged toner having a volume average particle diameter of 6.8 μm, a CV value of 23.5% in the number distribution, a volume resistivity of 10 ¹⁰ Ωcm, a weight average particle diameter of 45 μm, and a saturation magnetization of 65 emu / g. A two-component developer composed of a coated ferrite carrier was used. The toner volume average particle diameter and the CV value of the number distribution were measured using Multisizer III (manufactured by Beckman Coulter, Inc.) with an aperture diameter of 100 μm (measurement range: 2.0 to 60 μm). The saturation magnetization was measured using a vibrating sample magnetometer (“VSM-P7” manufactured by TOEI) at a magnetic field of 79.6 kA / m (1 kOe).

  In the example of the present invention, a developing roller having a surface of an aluminum substrate coated with a silicon-modified urethane resin layer having a thickness of 0.8 μm was used. Further, the voltage applied to the developing roller at the time of forming the toner thin layer was set to DC bias value (Vdc): 300 V, AC bias Vpp: 1.6 kV, frequency f: 2.7 kHz, and duty ratio Dslv: 40%. The applied voltage to the magnetic roller was a DC bias value (Vdc2): 400 V, an AC bias Vpp2 of 2.8 kV in the same cycle as the developing roller, a frequency f: 2.7 kHz, and a duty ratio Dmag: 70%. .

Under such conditions, a toner thin layer having a layer thickness of 13.4 μm (0.62 mg / cm ² ) was formed. Further, a predetermined area portion of the toner thin layer is sucked using a QM meter (manufactured by TREK, MODEL210S), and the toner amount (mg / cm ² ) per unit area of the sucked sample is calculated. The suction nozzle and the toner layer are separated from each other, and a predetermined area portion of the toner layer is removed by suction so that the toner amount per unit area of the toner to be sucked is about 60% by weight of the toner thin layer, The lower layer was used. As a result, the lower layer thickness was 5.4 μm (0.24 mg / cm ² ), and this was used for evaluation. The layer thickness was measured using a length measuring device (LASER SCAN DIAMETER LS-3100, manufactured by Keyence Corporation).

On the other hand, in the comparative example, a developing roller in which the surface of the aluminum base was anodized was used. Further, the applied voltage to the developing roller is set to an AC bias duty ratio Dslv: 35%, the applied voltage to the magnetic roller is set to an AC bias Vpp2 of 2.8 kV in the same cycle and opposite phase as the developing roller, and the duty ratio Dmag. : The conditions were the same as those in the examples of the present invention except that the content was 65%. Under such conditions, a toner thin layer having a layer thickness of 13.8 μm (0.62 mg / cm ² ) was formed. Further, in the same manner as in the above example of the present invention, about 60% by weight of the upper layer of the toner thin layer was removed by suction to form a lower layer. As a result, the lower layer thickness was 5.4 μm (0.24 mg / cm ² ), and this was used for evaluation.

As an evaluation method, A4 size test images (printing rate of 3%) were continuously output for 1000 sheets and 10,000 sheets for all and lower layers of the toner thin layer (toner layer) formed on the developing roller. The layer thickness (μm) and the toner charge amount q / d (femto-C / m) were measured, and the image density was measured. The test results are shown in Table 1. Table 1 also shows the toner amount (mg / cm ² ) per unit area measured when the lower layer was formed.

  As is apparent from Table 1, the toner weight and layer thickness per unit area are about 40% by weight of the toner thin layer in both the present invention and the comparative example, and the upper layer of about 60% by weight is removed. It was.

  As is apparent from Table 1, in the comparative example in which application was performed under the condition of 100-Dmag = Dslv, the absolute value of the difference in q / d between the toner thin layer and the lower layer was 0 by performing continuous output of 10,000 sheets. More than 0.5 (| q / d (a) −q / d (b) |> 0.5), and at this time, a decrease in image density was observed. On the other hand, in the present invention, the absolute value of the difference in q / d between the toner thin layer and the lower layer is 0.5 or less (| q / d (a) −q / d ( b) | ≦ 0.5), which satisfies the relationship of the present invention, and no change in image density was observed at this time.

  As a result, it was shown that when the relationship between the q / d (a) of the toner layer and the q / d (b) of the lower layer satisfies the conditional expression of the present invention, it is possible to prevent a decrease in image density. Further, it was shown that the toner layer having such a relationship can be obtained by setting Dslv and Dmag to be 100−Dmag <Dslv and Dslv ≧ 40%.

  The developing conditions in the above embodiment are merely examples, and the processing speed, the developing roller diameter, the magnetic roller diameter, and the like can be appropriately set according to the specifications of the image forming apparatus.

  The present invention uses a two-component developer containing at least a carrier and a toner, a toner carrier disposed opposite to the image carrier, and a toner supply for forming a toner thin layer on the toner carrier using a magnetic brush And developing a latent electrostatic image by applying a developing bias to the toner carrier and the toner supply member to cause the toner to fly from the toner carrier to the surface of the image carrier. In the apparatus, the average charge amount per particle diameter of the toner in the toner layer formed on the toner carrier is q / d (a), and the weight of the toner layer from the upper layer of the toner layer is 50 to 70. The absolute value of the difference between q / d (b) and the average charge amount per particle diameter of the toner in the lower layer from which the toner layer corresponding to% by weight has been removed is 0.5 or less.

  As a result, charge-up of the lower layer portion can be prevented, and the charge difference between the upper layer and the lower layer of the toner thin layer can be reduced. Therefore, it is possible to prevent image density defects, image unevenness and fogging, and obtain a stable image over a long period of time.

  Further, the first bias applying means applies a first bias to the toner carrier, and the second bias applying means is electrically connected to a common ground with the first bias applying means, and is connected to the toner supply member. By applying the second bias superimposed on the first bias, the alternating current component of the first bias and the alternating current component of the second bias can be set independently, and the toner carrying It is possible to set application conditions that improve the toner layer formation efficiency on the body and the toner collection efficiency on the toner supply member.

  In addition, the relationship between the duty ratio Dslv of the waveform on the same polarity side as the toner in the AC component bias of the first bias and the duty ratio Dmag of the waveform on the same polarity side as the toner in the AC component of the second bias is a conditional expression ( By satisfying 2) and (3), the toner layer formation efficiency on the toner carrier and the toner recovery efficiency on the toner supply member can be improved, and the friction between the toner carrier and the lower layer of the toner layer is reduced. can do. Thereby, a toner layer satisfying the relationship between q / d (a) and q / d (b) can be formed. Therefore, the charging difference between the upper layer portion and the lower layer portion of the toner layer can be reduced only by changing the application of the first and second AC biases.

  In addition, when the toner is a positively chargeable toner, the adhesive force between the toner carrier and the toner can be reduced by coating the surface of the toner carrier with a silicon-modified urethane resin. The recovery efficiency can be improved. Further, by mounting the developing device having the above-described configuration, it is possible to prevent image density defects, image unevenness, and fogging, and to form an image forming apparatus capable of stable image formation over a long period of time.

FIG. 1 is a schematic diagram illustrating an overall configuration of an image forming apparatus in which a developing device of the present invention is mounted. These are side surface sectional views which show the structure of the developing device of this invention. These are schematic diagrams showing the connection state of the first and second power sources in the developing device of the present embodiment. FIG. 4 is a schematic diagram showing waveforms of first and second biases applied to the developing roller and the magnetic roller used in the present embodiment, and FIG. 4A shows the waveforms of the first and second biases. FIG. 4B is a schematic diagram showing these combined waveforms. These are schematic diagrams showing a combined waveform of the developing roller when the first bias is applied or the magnetic roller when the second bias is applied. These are the schematic diagrams which show the state which connected the developing roller and the magnetic roller to the separate ground. FIG. 7A is a schematic diagram showing waveforms of first and second biases applied to the developing roller and the magnetic roller when electrically connected to separate grounds, and FIG. 7A shows the first and second biases; FIG. 7 (b) is a schematic diagram showing these combined waveforms.

Explanation of symbols

Pa to Pd Image forming section 1a to 1d Photosensitive drum (image carrier)
3a to 3d Developing device 20 Developing container 21a First stirring screw 21b Second stirring screw 22 Magnetic roller (toner supply member)
23 Developing roller (toner carrier)
25 blade cutting blade 26 toner 27 carrier 28 magnetic brush 29 toner thin layer 30 first power source 30a first DC power source 30b first AC power source 31 second power source 31a second DC power source 31b second AC power source 100 Image forming apparatus

Claims (3)

A two-component developer including at least a carrier and a toner, a toner carrier disposed opposite to the image carrier, and a toner supply member that forms a toner thin layer on the toner carrier using a magnetic brush; Have
In a developing device that develops an electrostatic latent image by applying a developing bias to the toner carrier and the toner supply member to cause the toner to fly from the toner carrier to the surface of the image carrier.
First bias applying means for applying a first bias to the toner carrier;
A second bias applying unit that is electrically connected to a common ground with the first bias applying unit and that applies a second bias superimposed on the first bias to the toner supply member; ,
The average charge amount per particle diameter of toner in the toner layer formed on the toner carrier is q / d (a), which corresponds to 50 to 70% by weight of the toner layer from the upper layer among the toner layers. When the average charge amount per particle diameter of the toner in the lower layer from which the toner layer is removed is q / d (b), the following conditional expression (1) is satisfied :
When the previous SL Dslv the duty ratio of the same polarity as the toner side of the waveform in the AC component of the first bias, Dmag the duty ratio of the second bias the same polarity as the toner side of the waveform in the AC component of the first The AC component of the bias 2 has the same frequency and antiphase as the AC component of the first bias, and the duty ratio Dmag is larger than the duty ratio Dslv of the first bias . The following conditional expression (2) and The developing device, wherein the first and second biases are applied so that the duty ratio satisfies (3).
| Q / d (a) −q / d (b) | ≦ 0.5 (femto-C / μm) (1)
100-Dmag <Dslv (2)
Dslv ≧ 40% (3)   The developing device according to claim 1, wherein the toner is a positively chargeable toner, and the surface of the toner carrier is coated with a silicon-modified urethane resin.   An image forming apparatus on which the developing device according to claim 1 is mounted.

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