JP2008292847A - Developing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a developing device having stable developing performance over a long period of time, and an image forming apparatus equipped with the developing device. <P>SOLUTION: In the developing device, toner supply bias voltage is applied to a magnetic roller 73 by a toner supply bias voltage applying means 94 and a developing bias voltage is applied to a developing roller 72 by a developing bias voltage applying means 91, thereby shifting toner 81 in two-component developer 83 carried on the magnetic roller 73 to the surface of the developing roller 72, and further the toner 81 shifted to the developing roller 72 is made to fly to the surface of a photoreceptor drum 37, thereby developing an image. In the developing device, the absolute value of a difference between a first coefficient of variation in the number distribution of particle size of the toner of a toner image developed on the surface of the photoreceptor drum 37 and a second coefficient of variation in the number distribution of particle size of the toner 81 in the two-component developer 83 carried on the magnetic roller 73 is within 6%. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a developing device used in an image forming apparatus using an electrophotographic system such as a copying machine, a printer, a facsimile machine, and a multifunction machine thereof, and an image forming apparatus including the developing device.

  A developing device used in an image forming apparatus using an electrophotographic system is configured to apply toner conveyed by a developing roller toward the surface of a photosensitive drum which is an image carrier on which an electrostatic latent image based on image data is formed. Fly to form a toner image. An image forming apparatus provided with such a developing device transfers a toner image formed on a photosensitive drum to a recording medium such as paper. The transferred toner image is fixed on the recording medium by heating with a fixing device. By doing so, the image forming apparatus forms an image based on the image data on a recording medium (see, for example, Patent Document 1).

  Among such image forming apparatuses, the image forming apparatus is excellent in high speed, and a plurality of image forming units corresponding to the color of the toner to be used are arranged side by side to form a color image in synchronization with the feeding of the intermediate transfer belt. A tandem image forming apparatus that performs color superposition on an intermediate transfer belt is known. Although this type of image forming apparatus has excellent high-speed performance, it has the disadvantage of increasing the size because the image forming units of the respective colors must be arranged side by side. As a countermeasure, there has been proposed a small tandem type image forming apparatus in which a space between the photosensitive drums is narrowed and a downsized image forming unit is arranged.

  As a developing device provided in such a small tandem type image forming apparatus, for example, a touch-down developing type developing device is used. The touchdown development type developing device uses a two-component developer containing toner and carrier as a developer, and applies a toner supply bias voltage to a magnetic roller that carries and conveys the two-component developer. Thus, the toner of the two-component developer is transferred to the developing roller, and the developing bias voltage is applied to the developing roller to cause the toner on the developing roller to fly to the surface of the photosensitive member for development.

  As a conventional developing device of the touchdown development system, for example, a superimposed voltage obtained by superimposing a rectangular waveform AC component on a DC component is applied to a developing roller as a developing bias voltage, and a DC voltage is applied to a magnetic roller as a toner supply bias voltage. (See Patent Documents 2 to 4).

  In addition, a superimposed voltage obtained by superimposing a rectangular waveform alternating current component on the direct current component is applied to the developing roller as the developing bias voltage, and the toner supply bias voltage has an opposite phase at the same frequency as the alternating current component of the developing bias voltage and a duty ratio. There is a developing device that applies a superimposed voltage obtained by superimposing a reversed alternating current component of a rectangular waveform on a direct current component to a magnetic roller (see Patent Document 5).

Further, the charge quantity distribution of the toner carried on the toner carrier (developing roller) is different from the charge quantity distribution of the toner in the two-component developer carried on the toner supply member (magnetic roller). An example of the developing device is described (see Patent Document 6).
U.S. Pat. No. 3,929,098 JP 2003-211961 A JP 2003-21966 A JP 2003-280357 A JP-A-2005-242281 JP 2001-272857 A

  According to the developing devices described in Patent Literatures 2 to 4, the potential difference between the DC component of the developing bias voltage and the toner supply bias voltage, the duty ratio of the AC component of the developing bias voltage, and the like are optimized, so that the previous development is performed. It is disclosed that a phenomenon in which a part of an image appears as an afterimage (ghost) at the next development, a so-called history phenomenon, occurrence of fogging, and the like can be suppressed. This is considered to be because, for example, a force in the direction of pulling back unnecessary toner from the surface of the photosensitive member can be applied by superimposing an alternating current component on the direct current component as the developing bias voltage.

  However, in such a developing device, a phenomenon called so-called selective development has occurred in which toner of a specific particle size is selectively moved and toner of a specific particle size is preferentially consumed. Therefore, when image formation is performed, the particle size distribution of the toner in the developing device changes, so that stable development performance cannot be exhibited for a long period of time. Further, in order to suppress the selective movement of the toner, when a strong developing electric field is formed between the photosensitive member and the developing roller, the toner is moved to the photosensitive member regardless of the particle diameter of the toner. The toner supply bias voltage applied to the magnetic roller has to be weakened due to the relationship with the developing bias voltage applied to the developing roller, and the undeveloped toner is weakly peeled off from the developing roller, and from the magnetic roller to the developing roller. Since the toner supply is insufficient, it is difficult to suppress the hysteresis phenomenon.

  According to the developing device described in Patent Document 5, the voltage (developing electric field) between the photosensitive member and the developing roller is superimposed by superimposing an alternating current component corresponding to the alternating current component of the developing bias voltage as the toner supply bias voltage. It is disclosed that the voltage (the voltage between the developing roller and the magnetic roller) for moving the toner from the magnetic roller to the developing roller can be increased without increasing it.

  However, as described in detail later, such a developing device must control the voltage for the toner to move from the magnetic roller to the developing roller in relation to the developing bias voltage and the toner supply bias voltage. The selective movement of the toner could not be sufficiently suppressed. Furthermore, if the phases of the developing bias voltage and the toner supply bias voltage are not strictly controlled, the developing efficiency is lowered.

  In addition, as in the developing device described in Patent Document 6, the charge amount number distribution of the toner carried on the developing roller is different from the charge amount number distribution of the toner in the two-component developer carried on the magnetic roller. This means that the toner of the two-component developer on the magnetic roller selectively moves to the developing roller. Therefore, this developing device does not focus on suppressing the selective movement of toner, and is not intended to enable stable image formation over a long period of time.

  SUMMARY An advantage of some aspects of the invention is that it provides a developing device having stable development performance over a long period of time and an image forming apparatus including the developing device. It is said.

  The developing device of the present invention is disposed opposite to an image carrier on which an electrostatic latent image is formed, and carries a two-component developer including a developing roller for carrying and transporting toner on the surface, and toner and a carrier. A magnetic roller that conveys the toner, and by applying a toner supply bias voltage to the magnetic roller, the toner in the two-component developer conveyed by the magnetic roller is transferred to the surface of the developing roller, and the developing By applying a developing bias voltage to the roller, the toner conveyed by the developing roller is caused to fly to the surface of the image carrier, and an electrostatic latent image previously formed on the surface of the image carrier is converted into a toner image. And a first variation coefficient in the number distribution of the toner particle diameter of the toner image visualized on the surface of the image carrier, and the magnetic roller carried on the surface of the image carrier. A developing apparatus, wherein the absolute value of the difference between the second variation coefficient in the number particle size distribution of the toner 2 in component developer that is within 6%.

  According to this configuration, the selective movement of the toner is suppressed between the magnetic roller and the image carrier, and the specific particle diameter of the toner in the two-component developer carried on the magnetic roller is reduced. This toner is rarely used for development preferentially. Therefore, this developing device can exhibit stable development performance over a long period of time, even when image formation is performed, because there is little change in the toner particle size distribution in the developing device.

  Moreover, it is preferable that the thickness of the toner conveyed by the developing roller is 6 to 14 μm. By doing so, most of the toner conveyed by the developing roller can be transferred to the image carrier, so that the selective movement of the toner between the magnetic roller and the image carrier can be further suppressed.

  Further, it is preferable that the sum of the positive duty ratios of the toner supply bias voltage and the development bias voltage is larger than 100. According to this configuration, the time for applying the toner supply bias voltage in the direction in which the toner of the two-component developer on the magnetic roller moves to the developing roller, and the developing bias in the direction in which the toner on the developing roller moves to the image carrier. Both the voltage application time and the voltage application time can be increased. Therefore, since the toner can be effectively transferred, the selective movement of the toner can be further suppressed, and a higher quality image can be formed.

  The positive duty ratio of the toner supply bias voltage is preferably larger than the positive duty ratio of the development bias voltage. According to this configuration, it is possible to smoothly transfer the toner of the two-component developer carried on the magnetic roller, which is more difficult to transfer than the toner transfer from the developing roller to the image carrier. The selective movement of the toner between the rollers can be further suppressed.

  The toner supply bias voltage is preferably superimposed on the development bias voltage. By doing so, the movement of the toner from the magnetic roller to the developing roller is almost independent of the developing bias voltage and depends on the toner supply bias voltage. On the other hand, the movement of the toner from the developing roller to the image carrier hardly depends on the toner supply bias voltage but depends on the developing bias voltage.

  Therefore, the selective movement of the toner between the magnetic roller and the developing roller and the selective movement of the toner between the developing roller and the image carrier control the toner supply bias voltage and the developing bias voltage. Can be individually optimized. Therefore, a configuration that can suppress both of the selective movement of the two toners can be easily realized.

  The developing roller preferably has a magnetic pole formed at a position facing the magnetic roller. According to this configuration, the magnetic brush made of the two-component developer formed between the developing roller and the magnetic roller can be formed more firmly, and the toner that has not been used for development on the developing roller can be more peeled off. Therefore, the hysteresis phenomenon can be suppressed and higher development performance can be exhibited. In addition, since a strong magnetic brush is formed, the selective movement of toner between the magnetic roller and the developing roller can be further suppressed, and more stable development performance can be exhibited.

  The image forming apparatus of the present invention includes the developing device and the image carrier, and visualizes an electrostatic latent image previously formed on the surface of the image carrier by the developing device as a toner image. An image forming apparatus is characterized by being formed. According to this configuration, the image forming apparatus enjoys the effects of the developing device of the present invention, and can form a high-quality image stably over a long period of time.

  The developing device of the present invention can exhibit stable developing performance over a long period of time. Therefore, when the developing device of the present invention is provided, an image forming apparatus capable of forming a high-quality image stably over a long period of time can be obtained.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, a developing device according to an embodiment of the invention and an image forming apparatus including the developing device will be described in detail with reference to the drawings.

  First, a color printer as an example of an image forming apparatus to which a developing device according to an embodiment of the present invention is applied will be described with reference to FIG. FIG. 1 is a schematic cross-sectional view showing the overall configuration of a color printer 1 including a developing device 71 according to an embodiment of the present invention.

  As shown in FIG. 1, the color printer 1 has a box-shaped device body 1a. In the apparatus main body 1a, a toner image based on image data and the like is transferred to the paper P while feeding the paper P fed from the paper feeding unit 2 and the paper P fed from the paper feeding unit 2. An image forming unit 3 and a fixing unit 4 for performing a fixing process for fixing the unfixed toner image transferred onto the paper P by the image forming unit 3 to the paper P are provided. Further, on the upper surface of the apparatus main body 1a, a paper discharge unit 5 for discharging the paper P subjected to the fixing process by the fixing unit 4 is provided.

  The paper feed unit 2 includes a paper feed cassette 21, a pickup roller 22, paper feed rollers 23, 24, 25 and a registration roller 26. The paper feed cassette 21 is provided so as to be detachable from the apparatus main body 1a, and stores the paper P of each size. The pickup roller 22 is provided at the upper left position of the paper feed cassette 21 shown in FIG. 1 and takes out the paper P stored in the paper feed cassette 21 one by one. The paper feed rollers 23, 24 and 25 send out the paper P taken out by the pickup roller 22 to the paper transport path. The registration roller 26 temporarily waits for the paper P sent to the paper transport path by the paper feed rollers 23, 24, 25, and then supplies it to the image forming unit 3 at a predetermined timing.

  The paper feeding unit 2 further includes a manual feed tray (not shown) and a pickup roller 27 that are attached to the left side surface of the device main body 1a shown in FIG. The pickup roller 27 takes out the paper P placed on the manual feed tray. The paper P taken out by the pickup roller 27 is sent out to the paper conveyance path by the paper feed rollers 23 and 25 and is supplied to the image forming unit 3 by the registration roller 26 at a predetermined timing.

  The image forming unit 3 includes an image forming unit 7, an intermediate transfer belt 31 on which a toner image based on image data transmitted from a computer or the like to the surface (contact surface) of the image forming unit 7 is primarily transferred, A secondary transfer roller 32 is provided for secondary transfer of the toner image on the intermediate transfer belt 31 onto the paper P fed from the paper feed cassette 21.

  The image forming unit 7 includes a black unit 7K, a yellow unit 7Y, a cyan unit 7C, and a magenta unit 7M, which are sequentially arranged from the upstream side (right side in FIG. 1) to the downstream side. I have. In each of the units 7K, 7Y, 7C, and 7M, a photosensitive drum 37 as an image carrier is disposed at the center position so as to be rotatable in the arrow (clockwise) direction. Around each photosensitive drum 37, a charger 39, an exposure device 38, a developing device 71, a cleaning device (not shown), a static eliminator, and the like are sequentially arranged from the upstream side in the rotation direction.

  The charger 39 uniformly charges the peripheral surface of the photosensitive drum 37 rotated in the direction of the arrow. Examples of the charger 39 include a non-contact discharge type corotron type and scorotron type charger, a contact type charging roller, a charging brush, and the like. The exposure device 38 is a so-called laser scanning unit, which irradiates the peripheral surface of the photosensitive drum 37 uniformly charged by the charger 39 with laser light based on the image data input from the image reading device or the like, and thereby the photosensitive member. An electrostatic latent image based on the image data is formed on the drum 37. The developing device 71 supplies toner to the peripheral surface of the photosensitive drum 37 on which the electrostatic latent image is formed, thereby forming a toner image based on the image data. The toner image is primarily transferred to the intermediate transfer belt 31. The cleaning device cleans the toner remaining on the peripheral surface of the photosensitive drum 37 after the primary transfer of the toner image to the intermediate transfer belt 31 is completed. The static eliminator neutralizes the peripheral surface of the photosensitive drum 37 after the primary transfer is completed. The peripheral surface of the photosensitive drum 37 cleaned by the cleaning device and the charge eliminator goes to the charger for a new charging process, and a new primary transfer is performed.

  The intermediate transfer belt 31 is an endless belt-like rotating body, and includes a driving roller 33, a driven roller 34, a backup roller 35, and a surface (contact surface) side so as to abut on the circumferential surface of each photosensitive drum 37. It is spanned across a plurality of rollers such as the primary transfer roller 36. The intermediate transfer belt 31 is configured to rotate endlessly by the plurality of rollers in a state in which the intermediate transfer belt 31 is pressed against the photosensitive drum 37 by a primary transfer roller 36 disposed to face each photosensitive drum 37. The driving roller 33 is rotationally driven by a driving source such as a stepping motor, and gives a driving force for rotating the intermediate transfer belt 31 endlessly. The driven roller 34, the backup roller 35, and the primary transfer roller 36 are rotatably provided, and are driven to rotate with the endless rotation of the intermediate transfer belt 31 by the driving roller 33. These rollers 34, 35, 36 are driven to rotate via the intermediate transfer belt 31 in accordance with the main rotation of the drive roller 33 and support the intermediate transfer belt 31.

  The primary transfer roller 36 applies a primary transfer bias (a polarity opposite to the toner charging polarity) to the intermediate transfer belt 31. By doing so, the toner image formed on each photoconductive drum 37 is moved in the direction of the arrow (counterclockwise) by the drive roller 33 between each photoconductive drum 37 and the primary transfer roller 36. The images are sequentially transferred (primary transfer) to the rotating intermediate transfer belt 31 in an overcoated state.

  The secondary transfer roller 32 applies a secondary transfer bias having a polarity opposite to that of the toner image to the paper P. By doing so, the toner image primarily transferred onto the intermediate transfer belt 31 is transferred to the paper P between the secondary transfer roller 32 and the backup roller 35, and thereby, a color transfer image ( An unfixed toner image) is transferred.

  The fixing unit 4 performs a fixing process on the transfer image transferred to the paper P by the image forming unit 3. The fixing unit 4 is disposed opposite to the heating roller 41 heated by the energized heating element and the heating roller 41. And a pressure roller 42 whose surface is pressed against the peripheral surface of the heating roller 41.

  Then, the transfer image transferred to the paper P by the secondary transfer roller 32 in the image forming unit 3 is subjected to a fixing process by heating when the paper P passes between the heating roller 41 and the pressure roller 42. Fixed to P. Then, the paper P subjected to the fixing process is discharged to the paper discharge unit 5. Further, in the color printer 1 of the present embodiment, a transport roller 6 is disposed at an appropriate position between the fixing unit 4 and the paper discharge unit 5.

  The paper discharge unit 5 is formed by recessing the top of the apparatus main body 1a of the color printer 1, and a paper discharge tray 51 for receiving the discharged paper P is formed at the bottom of the concave portion. .

  Next, the configuration of the developing device 71 according to the embodiment of the present invention will be described. FIG. 2 is a schematic cross-sectional view showing a configuration of the developing device 71 according to the embodiment of the present invention, and shows an enlarged peripheral portion of the developing device 71 provided in the color printer 1 shown in FIG. .

  The developing device 71 includes a developing roller 72, a magnetic roller 73, a paddle mixer 74, a stirring mixer 75, a spike cutting blade 76, a partition plate 77, and a voltage applying unit 90.

  The developing roller 72 carries (conveys) toner on the surface thereof, and visualizes (develops) an electrostatic latent image formed in advance on the surface of the photosensitive drum 37 as a toner image. The developing roller 72 has a built-in magnet so that a magnetic pole is formed at a position facing the magnetic roller 73. The magnetic roller 73 attracts the two-component developer by a magnet disposed therein to generate a magnetic brush, and supplies toner to the developing roller 72. Since the developing roller 72 has a built-in magnet, a magnetic brush made of a two-component developer formed between the developing roller 72 and the magnetic roller 73 can be formed more firmly and used for development on the developing roller 72. Since the toner that has not been removed can be peeled off more, hysteresis and fog can be suppressed, and high development performance can be exhibited. For example, the developing roller 72 uses a roller having a diameter of 20 mm, and the magnetic roller 73 uses a roller having a diameter of 25 mm, for example.

  The paddle mixer 74 and the agitation mixer 75 have spiral blades and agitate the two-component developer in opposite directions to charge the toner. Further, the paddle mixer 74 supplies a two-component developer containing charged toner and a carrier to the magnetic roller 73. The ear cutting blade 76 regulates the thickness of the magnetic brush formed on the magnetic roller 73. The partition plate 77 is provided between the paddle mixer 74 and the stirring mixer 75 so that the two-component developer can freely pass outside the both end sides of the partition plate 77.

  The voltage applying unit 90 has a plurality of power supplies for applying voltages as described later to the developing roller 72 and the magnetic roller 73. The voltage applying unit 90 includes a developing bias voltage applying unit 91 that applies a developing bias voltage to the developing roller 72, and a toner supply bias voltage applying unit 94 that applies a toner supply bias voltage to the magnetic roller 73. The toner supply bias voltage application unit 94 of the voltage application unit 90 applies the toner supply bias voltage based on the development bias voltage applied by the development bias voltage application unit 91. That is, the voltage application unit 90 is configured such that the toner supply bias voltage is superimposed on the development bias voltage.

  FIG. 3 is a schematic diagram for explaining the development of the developing device 71. FIG. 3 is a schematic diagram for explaining the development of the developing device 71, and the positional relationship among the photosensitive drum 37, the developing roller 72, the magnetic roller 73, and the ear cutting blade 76 is different from that in FIG.

  The two-component developer 83 including the toner 81 and the carrier 82 charged by the paddle mixer 74 and the stirring mixer 75 is supplied to the magnetic roller 73. The two-component developer 83 supplied to the magnetic roller 73 is conveyed as a magnetic brush by a magnet inside the magnetic roller 73. Thereafter, the magnetic brush is moved by the rotation of the sleeve on the surface of the magnetic roller 73, and the thickness thereof is regulated when passing between the ear cutting blade 76 and the magnetic roller 73. A potential difference is generated between the developing roller 72 and the magnetic roller 73 due to the voltage applied by the voltage applying unit 90. Therefore, when the magnetic brush whose thickness is regulated moves to the vicinity of the developing roller 72, only the charged toner 81 moves to the developing roller 72 due to this potential difference. The toner 81 moved to the developing roller 72 becomes a uniform toner layer. Note that the potential difference between the developing roller 72 and the magnetic roller 73 is that the toner supply bias voltage applied by the toner supply bias voltage applying means 94 is superimposed on the development bias voltage as described above. Depends on voltage.

  A potential difference is also generated between the photosensitive drum 37 and the developing roller 72 by the voltage applying unit 90. Therefore, development based on the electrostatic latent image formed on the photosensitive drum 37 is performed by this potential difference. The potential difference between the photosensitive drum 37 and the developing roller 72 depends on the developing bias voltage applied by the developing bias voltage applying unit 91.

  As described above, the developing device 71 can perform development by applying the developing bias voltage and the toner supply bias voltage by the voltage applying unit 90 as described above.

  Further, the developing device 71 has a coefficient of variation [CV (drum)] (corresponding to the first coefficient of variation) in the number distribution of the toner particle diameter of the toner image visualized on the surface of the photosensitive drum 37, and a magnetic roller. The absolute value of the difference from the coefficient of variation [CV (mag)] (corresponding to the second coefficient of variation) in the number distribution of toner particle diameters of the two-component developer carried on the toner 73 is within 6%. If it exceeds 6%, the selective movement of the toner is too large, and a defect such as an image density defect occurs at a print output of about 1000 sheets. Therefore, the selective movement of the toner is suppressed between the magnetic roller 73 and the photosensitive drum 37, and specific particles out of the toner 81 in the two-component developer 83 carried on the magnetic roller 73. The toner of the diameter is rarely used for development preferentially.

  The coefficient of variation (CV) is an index indicating the uniformity of the particle size (diameter) of the particle product (sharpness of particle size distribution), and is the ratio of the standard deviation to the average particle size. The larger the CV, the broader the particle size distribution, and the smaller the CV, the sharper the particle size distribution. Here, the coefficient of variation in the number distribution of the toner particle diameter is a value obtained by dividing the standard deviation of the toner particle diameter by the average particle diameter of the toner, and is calculated from the following calculation formula (1).

CV (%) = standard deviation ÷ average value of particle diameter × 100 (1)
The absolute value of the difference between the coefficient of variation [CV (slv)] and the CV (mag) in the number distribution of the particle diameter of the toner carried on the developing roller 72 is preferably within 5%. More preferably, the absolute value of the difference between drum) and CV (slv) is within 3%. If these differences are too large, the selective movement of the toner is too large, and if printing is performed for a long time, there is a tendency that defects such as image density defects occur. Therefore, the selective movement of toner between the developing roller 72 and the photosensitive drum 37 and the selective movement of toner between the developing roller 72 and the magnetic roller 37 are both suppressed.

  Further, in order to suppress the selective movement of toner between the magnetic roller 73 and the developing roller 72, it is necessary to control the potential difference between the developing roller 72 and the magnetic roller 73. The toner supply bias voltage applied by the toner supply bias voltage applying means 94 may be controlled to a suitable voltage as described later. Further, in order to suppress the selective movement of toner between the developing roller 72 and the photosensitive drum 37, it is necessary to control the potential difference between the photosensitive drum 37 and the developing roller 72, and the developing device 71. In this case, the developing bias voltage applied by the developing bias voltage applying means 91 may be controlled to a suitable voltage as described later.

  The toner supply bias voltage applied by the toner supply bias voltage application means 94 by the voltage application means 90 and the development bias voltage applied by the development bias voltage application means 91 will be described. FIG. 4 is a waveform diagram showing an example of a voltage waveform applied to the developing device 71. 4A shows an example of the waveform of the toner supply bias voltage, and FIG. 4B shows an example of the waveform of the developing bias voltage.

  The toner supply bias voltage applying means 94 includes an AC power supply 95 that applies an AC voltage and a DC power supply 96 that applies a DC voltage. The toner supply bias voltage applied by the toner supply bias voltage application means 94 is preferably the following voltage. The DC voltage [Vdc (mag)] applied by the DC power source 96 is 100 to 450 V, although it varies depending on the resistance of the developer, the rotational speed difference (peripheral speed ratio) between the developing roller 72 and the magnetic roller 73, and the like. It is preferable. If this DC voltage is too low, the toner thin layer formed on the developing roller 72 tends to be thin, and if it is too high, the toner layer tends to be too thick. The peak-to-peak value [Vpp (mag)] of the AC voltage applied by the AC power supply 95 is preferably 0.5 to 5.0 kV, and is set to 1.6 kV or 2.8 kV, for example. . Vpp (mag) corresponds to the voltage difference between the maximum value (ON state) and the minimum value (OFF state) of the toner supply bias voltage shown in FIG. Further, the frequency [f (mag)] of the AC voltage applied by the AC power source 95 is preferably 1 to 6 kHz, more preferably 2.5 kHz or more, and is set to 2.7 kHz, for example. . The positive DUTY ratio [Duty (mag)] of the AC voltage applied by the AC power source 95 is preferably 40 to 70%, and more preferably 60 or more. Note that Duty (mag) is an ON state time T1 which is a voltage in the direction in which the toner moves from the magnetic roller 73 to the developing roller 72 and an OFF state which is a voltage in the direction in which the toner is pulled back from the developing roller 72 to the magnetic roller 73. It is the ratio of T1 to the total time with time T2.

  The developing bias voltage applying unit 91 includes an AC power source 92 that applies an AC voltage and a DC power source 93 that applies a DC voltage. The development bias voltage applied by the development bias voltage application unit 91 is preferably the following voltage. The DC voltage Vdc (slv) applied by the DC power supply 93 varies depending on the rotational speed difference (peripheral speed ratio) between the photosensitive drum 37 and the developing roller 72, but is preferably 400 V or less, and 300 V or less. More preferably, for example, it is set to 300V. When such a voltage is set, it is easy to remove the toner that is not transferred to the intermediate transfer belt 31 and remains on the photosensitive drum 37, the hysteresis phenomenon is less likely to occur, and further, a strong electric field is prevented from being applied to the toner. This is also preferable. The peak-to-peak value [Vpp (slv)] of the AC voltage applied by the AC power supply 92 is preferably 0.2 to 2 kV, and is set to 1.6 kV, for example. Vpp (slv) corresponds to the voltage difference between the maximum value (ON state) and the minimum value (OFF state) of the developing bias voltage shown in FIG. Further, the frequency [f (slv)] of the AC voltage applied by the AC power source 92 is preferably 1 to 4 kHz, and is set to 2.7 kHz, for example. The positive DUTY ratio [Duty (slv)] of the AC voltage applied by the AC power source 92 is preferably 35 to 65%, and more preferably 40 or more. Note that Duty (slv) is an ON state time T3 which is a voltage in the direction in which the toner moves from the developing roller 72 to the photosensitive drum 37, and OFF which is a voltage in the direction in which the toner is pulled back from the photosensitive drum 37 to the developing roller 72. It is the ratio of T3 to the total time with state time T4.

  Further, it is preferable that the toner supply bias voltage and the development bias voltage have the following relationship. Vpp (mag) is preferably larger than Vpp (slv). Since the potential difference between the developing roller 72 and the magnetic roller 73 depends only on the toner supply bias voltage, the toner 81 carried on the developing roller 72 is set when Vpp (mag) is larger than Vpp (slv). The toner 81 of the two-component developer 83 carried on the magnetic roller 73 that is less likely to shift can be transferred more smoothly. Therefore, the selective movement of the toner between the magnetic roller 73 and the developing roller 72 can be further suppressed.

  Moreover, it is preferable that the sum of Duty (mag) and Duty (slv) is larger than 100. By doing so, the time for applying the toner supply bias voltage in the direction in which the toner 81 of the two-component developer 83 on the magnetic roller 73 moves to the developing roller 72 and the toner 81 on the developing roller 72 to the photosensitive drum 37 are applied. Both the time for applying the developing bias voltage in the moving direction can be lengthened. Therefore, since the toner can be effectively transferred, the selective movement of the toner can be further suppressed, and a higher quality image can be formed.

  Moreover, it is preferable that Duty (mag) is larger than Duty (slv). By doing so, the transfer of the toner 81 of the two-component developer 83 carried on the magnetic roller 73 can be made smooth, so that the toner can be selectively moved between the magnetic roller 73 and the developing roller 72. It can be suppressed more.

  In addition, the thickness of the toner 81 conveyed by the developing roller 72 is preferably 6 to 14 μm, and more preferably 7 to 14 μm. By doing so, most of the toner 81 conveyed by the developing roller 72 can be transferred to the photosensitive drum 37, so that the selective movement of the toner between the developing roller 72 and the photosensitive drum 37 can be further suppressed.

  Further, f (mag) is preferably larger than f (slv) and not less than 2.5 kHz.

  The voltage applying unit 90 is preferably controlled to apply an alternating voltage immediately before development. As a result, toner scattering can be minimized and stable development performance can be maintained for a longer period of time. Further, in order to maintain a uniform image density, the voltage application unit 90 controls the potential difference between the developing roller 72 and the magnetic roller 73 to be the same potential at a time other than the development timing. It is effective to collect the toner on the developing roller 72 on the magnetic roller 73 without imposing a burden on the magnetic roller 73.

  Next, as a comparative embodiment for comparison with the present invention, a conventional technique, for example, a toner supply bias voltage described in Patent Document 5 is not superimposed on a development bias voltage, and the toner The developing device 100 that individually applies the supply bias voltage and the developing bias voltage will be described.

  FIG. 5 is a schematic diagram showing a configuration of a conventional developing device 100. Similar to the developing device 71 according to the embodiment of the present invention, the developing device 100 includes a developing roller 102 that faces the photosensitive drum 101, a magnetic roller 103 that faces the developing roller 102, and the like. Except for the voltage applied to the roller 103, the developing device 71 is the same. Specifically, instead of the voltage applying unit 90, a developing bias voltage applying unit 108 that applies a developing bias voltage to the developing roller 102 and a toner supply bias voltage applying unit 111 that applies a toner supply bias voltage to the magnetic roller 103. And independently. The developing bias voltage applying unit 108 includes an AC power source 109 that applies a rectangular waveform AC voltage and a DC power source 110 that applies a DC voltage. The toner supply bias voltage applying unit 111 is applied by the AC power source 109. An AC power source 112 that applies an AC voltage that has the same frequency as that of the AC voltage that is reversed in phase and a reverse duty cycle, and a DC power source 113 that applies a DC voltage are included.

  FIG. 6 is a waveform diagram showing an example of a waveform of a voltage applied to the developing device 100 shown in FIG. 6A shows an example of the waveform of the toner supply bias voltage applied to the magnetic roller 103, and FIG. 6B shows an example of the waveform of the development bias voltage applied to the developing roller 102. c) shows a waveform of a voltage (potential difference) between the developing roller 102 and the magnetic roller 103.

  The developing device 100 applies the voltage as described above to develop from the magnetic roller 103 without increasing the voltage (developing electric field) between the photosensitive drum 101 and the developing roller 102 as shown in FIG. The voltage for moving the toner 105 to the roller 102 (voltage between the developing roller 102 and the magnetic roller 103) can be increased. This is considered to be due to the following.

  As shown in FIGS. 6A and 6B, the developing device 100 uses an AC voltage corresponding to the AC voltage applied by the AC power supply 109 of the developing bias voltage applying unit 108 as the toner supply bias voltage. Superimpose. In the developing device 100, the voltage for the toner 105 to move from the magnetic roller 103 to the developing roller 102 includes the waveform of the toner supply bias voltage shown in FIG. 6A and the developing bias voltage shown in FIG. It is a synthetic | combination waveform shown in FIG.6 (c) which is a difference with this waveform. Therefore, the developing device 100 can increase the voltage for the toner 105 to move from the magnetic roller 103 to the developing roller 102 as shown in FIG. 6C without increasing the developing bias voltage.

  However, even in such a developing device 100, the voltage for moving the toner 105 from the magnetic roller 103 to the developing roller 102 must be controlled by the relationship between the developing bias voltage and the toner supply bias voltage. The selective movement of the toner 105 could not be sufficiently suppressed.

  Further, as shown in FIG. 7, the developing device 100 has a low development efficiency when the waveform of the developing bias voltage and the waveform of the toner supply bias voltage are shifted from the preferred phase shown in FIG. FIG. 7 is a waveform diagram for explaining a case where the phase of the developing bias voltage and the toner supply bias voltage is deviated from a suitable phase in the developing device 100 shown in FIG. 7A shows an example of the waveform of the toner supply bias voltage applied to the magnetic roller 103, and FIG. 7B shows an example of the waveform of the development bias voltage applied to the developing roller 102. c) shows a waveform of a voltage (potential difference) between the developing roller 102 and the magnetic roller 103.

  When the developing bias voltage and the toner supply bias voltage are in a phase as shown in FIG. 6, the developing device 100 applies a voltage Vmax in the direction in which the toner moves from the magnetic roller 103 to the developing roller 102, and The development efficiency is high, including the time T6 during which the voltage Vmin is applied in a direction in which unnecessary toner is pulled back from the surface of the photoreceptor. On the other hand, when the developing bias voltage and the toner supply bias voltage slightly deviate from the relationship shown in FIG. 6, the developing device 100 moves the toner from the magnetic roller 103 to the developing roller 102 as shown in FIG. There are times T9 and T10 for applying a voltage between Vmax and Vmin in addition to the time T7 for applying the voltage Vmax in the direction to be applied and the time T8 for applying the voltage Vmin in the direction in which unnecessary toner is pulled back from the photoreceptor surface. Therefore, both the toner supply efficiency from the magnetic roller 103 to the developing roller 102 and the recovery efficiency of undeveloped toner from the developing roller 102 to the magnetic roller 103 are low. Therefore, in order to maintain both high supply efficiency of toner from the magnetic roller 103 to the development roller 102 and high recovery efficiency of undeveloped toner from the development roller 102 to the magnetic roller 103, the developing device 100 can maintain the development bias voltage. And the toner supply bias voltage must be strictly controlled.

  Therefore, in the case of the developing device 100, as described above, since the potential difference between the developing roller 102 and the magnetic roller 103 depends on the combined waveform of the toner supply bias voltage and the developing bias voltage, Duty (mag) and The sum of Duty (slv) is preferably 100, and if it deviates from 100, the toner supply efficiency from the magnetic roller 103 to the developing roller 102 and the undeveloped toner collection efficiency from the developing roller 102 to the magnetic roller 103 Both decrease. In contrast, in the case of the present invention, the potential difference between the developing roller 72 and the magnetic roller 73 depends only on the toner supply bias voltage, so the sum of Duty (mag) and Duty (slv) is not 100. In both cases, both the toner supply efficiency from the magnetic roller 73 to the developing roller 72 and the recovery efficiency of undeveloped toner from the developing roller 72 to the magnetic roller 73 are not lowered, and the duty (mag) and duty (slv) as described above. Can be set to 100 or more. Therefore, since the bias setting is based on the toner supply efficiency from the magnetic roller 73 to the developing roller 72 and the recovery efficiency of the undeveloped toner from the developing roller 72 to the magnetic roller 73, the developing efficiency of the developing roller 72 is increased. A development bias can be applied, selective consumption of toner can be suppressed, and the sum of Duty (mag) and Duty (slv) as described above can be set to 100 or more. In the present invention, since the potential difference between the developing roller 72 and the magnetic roller 73 depends only on the toner supply bias voltage, the magnetic roller is increased by setting Vpp (mag) to be greater than Vpp (slv). The transfer from the toner 73 to the developing roller 72 can be made smoothly.

  In the case of the developing device 100, as shown in FIG. 6, it is necessary to match the phase of the toner supply bias voltage and the development bias voltage. In the present invention, as shown in FIG. There is no need to match the phase with the development bias voltage.

  Hereinafter, configurations other than the developing device 71 of the present invention will be described.

  For the toner remaining on the developing roller 72 after development, a special device such as a scraping blade may be provided on the peripheral surface, but it may not be provided. For example, the magnetic brush on the magnetic roller 73 comes into contact with the toner layer on the developing roller 72, and the toner can be easily collected and replaced by the brush effect due to the peripheral speed difference of each roller. Further, since the two-component developer that is a magnetic brush is replaced by the agitation of the paddle mixer 74 and the agitation mixer 75, the toner can be easily collected and replaced.

  In this case, since the width of the magnetic brush becomes the width for collecting the toner on the developing roller 72, the uncollected area can be surely eliminated by making the width of the developing roller 72 shorter than the width of the magnetic brush. By doing so, there is no toner adhering to the sleeve of the developing roll outside the magnetic brush area, and toner scattering at both ends can be eliminated.

  The photoreceptor drum 37 is preferably an amorphous silicon (a-Si) photoreceptor. In such a photoreceptor, when the film thickness is reduced, the saturation charging potential is lowered and the withstand voltage leading to dielectric breakdown is lowered. However, the potential of the latent image portion (exposure portion, image forming portion) is very low, 20 V or less. The potential of the non-latent image portion (non-exposed portion, non-image forming portion) is about 350V. Further, the charge density on the surface of the photosensitive drum 37 when the latent image is formed tends to be improved, and the development performance tends to be improved. This characteristic is particularly remarkable when the dielectric constant is about 10 or less for an a-Si photoreceptor having a dielectric constant of about 10 or less, more preferably 20 μm or less. For example, a photosensitive drum having a diameter of 30 mm is used as the photosensitive drum 37.

  When a positively charged organic photoconductor (OPC) is used as the photoconductor drum 37, the thickness of the photosensitive layer is set to 25 μm or more in order to make the residual potential 100 V or less, and the amount of charge generating material added is Increasing is particularly important. In particular, OPC having a single-layer structure is advantageous because a charge generating material is added to the photosensitive layer, so that the change in sensitivity is small even when the photosensitive layer is reduced. Even in this case, setting the DC voltage value of the developing bias voltage to 400 V or lower, more preferably 300 V or lower, is preferable in terms of preventing a strong electric field from being applied to the toner.

The carrier 82 included in the two-component developer 83 is not particularly limited as long as it is a general carrier. However, since the carrier has two roles of toner collection and supply, the volume specific resistance is 10 A small particle diameter carrier having an average particle diameter of ⁶ to 10 ¹³ Ωcm and an average particle diameter of 50 μm or less is preferable. For example, a carrier having a volume resistivity of 10 ¹⁰ Ωcm, a saturation magnetization of 65 emu / g, and a volume average particle diameter of 45 μm is used. . The saturation magnetization can be measured with a magnetic field of 79.6 kA / m (1 kOe) using VSM-P7 manufactured by Toei Industry Co., Ltd. When the volume resistivity is 10 ⁶ to 10 ¹³ Ωcm, the toner that is strongly electrostatically attached is peeled off with a magnetic brush at the nip between the developing roller 72 and the magnetic roller 73, and the toner required for development Easy to supply. Further, it is preferable that the carrier has a small particle diameter of 50 μm or less because the surface area of the carrier is increased and the number of contacts with the toner is increased.

  The peripheral speeds of the photosensitive drum 37 and the developing roller 72 are set to, for example, 300 and 450 mm / sec, respectively, and the peripheral speed ratio between the developing roller 72 and the photosensitive drum 37 (developing roller peripheral speed / photosensitive body). The drum peripheral speed is, for example, 1.5.

  The peripheral speed ratio between the magnetic roller 73 and the developing roller 72 (magnetic roller peripheral speed / developing roller peripheral speed) is preferably 1.0 to 2.0, and is set to 1.5, for example. At that time, the peripheral speed of the magnetic roller 73 is set to 675 mm / sec, for example. By doing so, the replacement of the two-component developer can be promoted, the toner on the developing roller 72 is collected, and a two-component developer set to an appropriate toner concentration is supplied to the developing roller 72 to provide a uniform toner layer. Can be formed.

  The gap between the magnetic roller 73 and the developing roller 72 is preferably 100 to 1000 μm, more preferably 150 to 500 μm, and is set to 350 μm, for example. The gap (development gap) between the developing roller 72 and the photosensitive drum 37 is preferably 100 to 1000 μm, more preferably 150 to 500 μm, and is set to 200 μm, for example. If the gap between the magnetic roller 73 and the developing roller 72 or the developing gap is too small, there is a concern that leakage or image pitch unevenness may occur, and if it is too large, there is a concern that the developing efficiency may be reduced.

  In the above embodiment, the tandem image forming apparatus is described as an example of the image forming apparatus of the present invention. However, the image forming apparatus may be any image forming apparatus using an electrophotographic method, and is limited to the tandem image forming apparatus. Not. Note that the developing device of the present invention can suppress the occurrence of image unevenness due to a change in the developing gap. Therefore, it is preferable to apply the developing device of the present invention to a small tandem type image forming apparatus in that the small tandem type image forming apparatus can easily solve the defect that the image unevenness due to the fluctuation of the developing gap easily occurs. Further, although the color printer has been described as an example of the type of the image forming apparatus, for example, a copying machine, a facsimile machine, and a multifunction machine may be used. Further, the image bearing member has been described by taking a photosensitive drum as a drum-shaped photosensitive member as an example. However, the image bearing member is not limited to this, and is not limited to this, but a belt-shaped photosensitive member, a sheet-shaped photosensitive member, or the like. Even applicable.

  Examples of the color printer 1 according to the embodiment of the present invention will be described below.

  Examples 1 to 6 and Comparative Example 1 are the following development conditions.

Photosensitive drum 37: a-Si drum, diameter 30 mm, peripheral speed 300 mm / second Developing roller 72: diameter 20 mm, peripheral speed 450 mm / second Magnetic roller 73: diameter 25 mm, peripheral speed 675 mm / second Development gap: 200 μm
Gap between magnetic roller 73 and developing roller 72: 350 μm
Toner supply bias voltage: Vdc (mag) = 400 V, f (mag) = 2.7 kHz, Vpp (mag), and Duty (mag) are the values shown in Table 1, respectively.

    Development bias voltage: Vdc (slv) = 300 V, f (mag) = 2.7 kHz, Vpp (slv), and Duty (slv) are the values shown in Table 1, respectively.

  Toner 81: volume average particle diameter 6.5 μm, initial number distribution CV value is 23.5%. CV (mag), CV (slv), and CV (drum) are values shown in Table 1, respectively.

  The volume average particle diameter and the CV value are output by Multisizer III manufactured by Beckman Coulter. Multisizer III having an aperture diameter of 100 μm is used, and the number of toners and the like are measured within a measurement range of 2 to 60 μm to calculate the volume average particle diameter and CV value.

Carrier 82: weight average particle diameter 45 μm, saturation magnetization 65 emu / g
The saturation magnetization can be measured with a magnetic field of 79.6 kA / m (1 kOe) using VSM-P7 manufactured by Toei Industry Co., Ltd.

  The image forming apparatuses according to Examples 1 to 6 and Comparative Example 1 were evaluated as shown below, and the results are shown in Table 2.

(Toner layer thickness)
Using a LASER SCAN DIAMETER LS-3100 manufactured by Keyence Corporation, the developing roller 72 carrying the toner 81 and the developing roller 72 before carrying the toner are measured, and the toner carried on the developing roller 72 The layer thickness is calculated.

(Image unevenness A)
Image unevenness A was calculated from the measurement result of the brightness P of the printed paper as follows.

  First, a 25% halftone dot area rate (600 dpi) halftone image is formed on a sheet based on image data captured at 3000 dpi using a color scanner ES8500 manufactured by Seiko Epson Corporation. The luminance Pi at the location was measured.

  The luminance Pi was measured using Dot Analyzer DA-6000 manufactured by Oji Scientific Instruments Co., Ltd., and the solid (solid portion) luminance was Pmax, and the blank paper portion luminance was Pmin.

  Next, the luminance Pi data is converted into density data Di according to the following calculation formula (2). When converting into density data, the relative density at Pi with respect to Pmax and Pmin is calculated, and the higher the density, the less the image unevenness (the less likely it is to appear in luminance).

Di = Log [(Pmax−Pi) / Pmin] (2)
Finally, the image unevenness A is calculated from the calculated Di according to the following formulas (3) to (5).

  The average value of Di is calculated according to the following formula (3).

  Next, “Fre” of Da is calculated according to the following calculation formula (4).

  Then, the image unevenness A is calculated according to the following formula (5), and this image unevenness A is used as an index for evaluating the image unevenness.

A = σ _D / Da (5)
(Image density ID)
First, the evaluation image shown in FIG. 8 is output. FIG. 8 shows an example of an evaluation image 120 for evaluating the image density ID. The evaluation image 120 is an image having five solids 121 as shown in FIG.

  Next, the image density IDs of the five solids 121 were respectively measured, and the average value was set as the image density ID of the main evaluation, and the evaluation was performed according to the following criteria. The image density ID was measured using a Gretag Macbeth portable reflection densitometer RD-19 manufactured by Sakata Inx Engineering Co., Ltd.

    ○: Image density ID is 1.3 or more.

    X: Image density ID is less than 1.3.

(ghost)
First, the following evaluation image is output.

  FIG. 9 is a diagram for explaining ghost evaluation. FIG. 9A shows an example of an evaluation image 130 for ghost evaluation, and FIG. 9B shows an example of an output image 135 when a ghost occurs. This evaluation image 130 is an image including three 100% solid images 131 as shown in FIG. 9A and 10% or 25% halftone images 132 on the rear end side.

  Next, it is visually determined whether or not a ghost (afterimage) 133 is formed in the halftone image 132 as shown in FIG. 9B in the output image, and the following criteria are used. evaluated.

    A: Even if the halftone image 132 is a 10% halftone image, the ghost 133 is not confirmed.

    ◯: When the halftone image 132 is a 10% halftone image, a ghost 133 is slightly confirmed, but when the halftone image 132 is a 25% halftone image, the ghost 133 is not confirmed.

    Δ: Even if the halftone image 132 is a 25% halftone image, a slight ghost 133 is confirmed.

    X: Even if the halftone image 132 is a 25% halftone image, the ghost 133 is clearly confirmed.

(CV after printing 10,000 sheets (mag))
After 10,000 images with a printing rate of 6% were printed, CV (mag) was measured.

  As can be seen from Table 2, when the absolute value of the difference between CV (drum) and CV (mag) is within 6% (Examples 1 to 6), although the initial image density ID is high, 1 Even after printing on 10,000 sheets, the occurrence of image unevenness and ghosting can be suppressed, and stable development performance can be exhibited over a long period of time. On the other hand, when the absolute value of the difference between CV (drum) and CV (mag) is larger than 6% (Comparative Example 1), printing 10,000 sheets even though the initial image density ID is low, The occurrence of image unevenness and ghost cannot be suppressed. Further, in Examples 1 to 6, as compared with Comparative Example 1, an increase in CV (mag) is suppressed even after printing 10,000 sheets. This also indicates that even when 10,000 sheets are printed, there is little change in the toner particle size distribution in the developing device, and stable development performance can be exhibited over a long period of time. Further, the absolute value of the difference between CV (slv) and CV (mag) is within 5%, and the absolute value of the difference between CV (drum) and CV (slv) is within 3%. By comparing Examples 1 to 6 and Comparative Example 1, it can be seen that this contributes to the ability to exhibit stable development performance over a long period of time.

  Furthermore, when the sum of Duty (slv) and Duty (mag) is larger than 100 (Examples 1 to 4), the sum of Duty (slv) and Duty (mag) is 100 or less (Example 5). Compared with 6), it can be seen that it is preferable in that the occurrence of image unevenness and ghosting can be further suppressed.

1 is a schematic cross-sectional view illustrating an overall configuration of a color printer 1 including a developing device 71 according to an embodiment of the present invention. FIG. 3 is a schematic cross-sectional view illustrating a configuration of a developing device 71 according to an embodiment of the present invention. FIG. 7 is a schematic diagram for explaining development of the developing device 71. 6 is a waveform diagram showing an example of a waveform of a voltage applied to the developing device 71. FIG. FIG. 6 is a schematic diagram illustrating a configuration of a conventional developing device 100. FIG. 6 is a waveform diagram illustrating an example of a waveform of a voltage applied to the developing device 100 illustrated in FIG. 5. FIG. 6 is a waveform diagram for explaining a case where the phases of the developing bias voltage and the toner supply bias voltage 111 deviate from a suitable phase in the developing device 100 shown in FIG. An example of the evaluation image 120 for evaluation of image density ID is shown. It is drawing for demonstrating ghost evaluation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Color printer 2 Paper feed part 3 Image formation part 4 Fixing part 5 Paper discharge part 6 Conveyance roller 7 Image formation unit 21 Paper feed cassette 22, 27 Pickup roller 23, 24, 25 Paper feed roller 26 Registration roller 31 Intermediate transfer belt 32 Secondary transfer roller 33 Drive roller 34 Driven roller 35 Backup roller 36 Primary transfer roller 37 Photosensitive drum 38 Exposure device 39 Charger 41 Heating roller 42 Pressure roller 51 Paper discharge tray 71 Developing device 72 Developing roller 73 Magnetic roller 74 Paddle mixer 75 Stirring mixer 76 Ear cutting blade 77 Partition plate 81 Toner 82 Carrier 83 Two-component developer 90 Voltage application means 91 Development bias voltage application means 92, 95 AC power supply 93, 96 DC power supply 94 Toner supply bias voltage application means

Claims (7)

A developing roller disposed opposite to an image carrier on which an electrostatic latent image is formed and carrying and transporting toner on the surface; and a magnetic roller carrying and transporting a two-component developer including toner and carrier; With
By applying a toner supply bias voltage to the magnetic roller, the toner in the two-component developer conveyed by the magnetic roller is transferred to the surface of the developing roller;
By applying a developing bias voltage to the developing roller, the toner conveyed by the developing roller is caused to fly to the surface of the image carrier, and an electrostatic latent image previously formed on the surface of the image carrier is formed. A developing device that visualizes a toner image,
A first coefficient of variation in the number distribution of toner particle diameters of the toner image visualized on the surface of the image carrier, and a number distribution of toner particle diameters in the two-component developer carried on the magnetic roller. The developing device is characterized in that the absolute value of the difference from the second coefficient of variation is within 6%.   The developing device according to claim 1, wherein the thickness of the toner conveyed by the developing roller is 6 to 14 μm.   The developing device according to claim 1, wherein a sum of positive duty ratios of the toner supply bias voltage and the developing bias voltage is greater than 100. 4.   The developing device according to claim 1, wherein a positive duty ratio of the toner supply bias voltage is larger than a positive duty ratio of the developing bias voltage.   The developing device according to claim 1, wherein the toner supply bias voltage is superimposed on the development bias voltage.   The developing device according to claim 1, wherein the developing roller has a magnetic pole formed at a position facing the magnetic roller.   An image forming apparatus comprising: the developing device according to claim 1; and the image carrier. The electrostatic latent image previously formed on the surface of the image carrier by the developing device is visualized as a toner image. An image forming apparatus characterized in that an image is formed.