JP4452544B2 - Development device - Google Patents

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a developing device used in an image forming apparatus such as a copying machine, a printer, a facsimile, or a composite machine using an electrophotographic method, and particularly uses a two-component developer containing a magnetic carrier and a non-magnetic toner. The present invention relates to a developing device that performs development.

  In general, in an image forming apparatus, an electrostatic latent image is formed on a photosensitive drum, which is an image carrier, and the electrostatic latent image is developed by a developing device to become a toner image. The toner image on the photosensitive drum is transferred to a recording sheet, and the toner image on the recording sheet is fixed by a fixing device. As a developing method in such a developing apparatus, there are a two-component developing method using a carrier and a toner, and a one-component developing method using no carrier. The two-component development method is excellent in the charging property of the toner by the carrier and can extend the life, but has disadvantages such as a large and complicated developing device and unevenness of the image by the magnetic brush. In addition, the one-component development method has a compact development device and is excellent in dot reproducibility and image uniformity. However, in the non-magnetic one-component development method that does not use magnetic powder used in color machines, it is suitable for the developing roller. The life is shortened because the blade is brought into contact. Taking advantage of the characteristics of both development methods, a toner layer is formed on the development roller with a magnetic brush formed by a magnetic roller using a two-component developer composed of a magnetic carrier and toner, and photosensitive as in the one-component development method. A developing system that develops an electrostatic latent image formed on a body is drawing attention.

  In such a developing device, when continuous printing is performed, toner having a high charge amount accumulates on the developing roller, so-called selective development occurs, and the image density tends to decrease. Further, when a toner consumption area and a non-consumption area are generated on the developing roller, the toner adhesion state and the toner potential difference are varied on the developing roller, and as shown in FIG. A phenomenon history may occur in which the portion appears as an afterimage (ghost) during the next development.

  As shown in FIG. 9A, when a halftone image 12 wider than the solid image 11 is printed following the rectangular solid black image 11, a toner consumption area and a non-consumption area are generated on the developing roller. 9B, an afterimage portion (ghost) 13 is generated in the halftone image 12 as shown in FIG. Further, when a high density development pattern is continuously printed, image non-uniformity such as unevenness in image density is likely to occur.

  In order to prevent such development history, the magnetic pole member provided in the magnetic roller is provided with a magnetic pole for toner supply facing the developing roller, and the gap between the magnetic roller and the developing roller is at least 5 times the shortest gap. There is a configuration in which a full width at half maximum in which at least both sides of the magnetic pole for supplying toner are half the peak value of the magnetic flux density is set (see Patent Document 1).

  On the other hand, in a developing device using a one-component developer different from the developing device using the two-component developer described above, the charge removing member is used as a residual developer on the developing roller in order to transport toner having no development history to the developing region. The bias is set in a direction to apply a force to electrically remove the residual developer on the developing roller from the developing roller to the discharging member without causing mechanical peeling. (See Patent Document 2).

Japanese Patent Laid-Open No. 7-128983 (pages 4 to 7, FIGS. 1 to 9) Japanese Patent Laid-Open No. 11-219032 (pages 5 to 9, FIGS. 1 to 7)

  By the way, in the developing device disclosed in Patent Document 2 described above, a single component developer is used to press the static elimination member against the developing roller so as not to cause mechanical peeling with respect to the residual developer. Further, when applied to a developing device using a two-component developer as disclosed in Patent Document 1, stress is applied to the toner, which causes deterioration of the toner. That is, the toner (collected toner) collected from the developing roller to the magnetic roller side deteriorates.

  Further, in a developing device using a two-component developer as disclosed in Patent Document 1, the magnetic carrier inevitably deteriorates due to long-term use, and as a result, the toner charge amount changes over time. For this reason, the collected toner is deteriorated in charging characteristics due to its deterioration, and the toner newly supplied to the developing device (supplementary toner) has good charging characteristics. Therefore, the toner in the toner thin layer formed on the developing roller has a good charging characteristic. The charge amount distribution covers a wide range.

  As described above, when toners having different charge amounts exist on the developing roller, that is, when the amount of poorly charged toner increases, image defects such as toner scattering and image fogging are caused. In particular, when the potential difference between the developing roller and the magnetic roller is fixed, the layer thickness of the toner thin layer formed on the developing roller changes according to the toner charge amount, and when the toner charge amount increases, the toner layer thickness decreases. When the toner charge amount is reduced, the toner layer thickness is increased. If the toner layer thickness is not kept within an appropriate range, there is a problem that an image defect occurs.

  SUMMARY OF THE INVENTION In view of the above circumstances, an object of the present invention is to obtain a developing device that has a simple configuration and maintains a toner layer thickness on a developing roller within an appropriate range and does not cause image defects.

In order to solve the above problem, the present invention is to form a magnetic brush composed of magnetic carrier and preparative toner on the outer circumference of the feed roller containing the fixed magnetic pole member, said supply while rubbing the magnetic brush on the developing roller A toner thin layer is formed on the developing roller by transferring toner to the developing roller in accordance with a potential difference between the roller and the developing roller (hereinafter referred to as a potential difference of a supply bias) , and the developing roller, the image carrier, In accordance with the potential difference between the toners (hereinafter referred to as the potential difference of the developing bias), the toner is caused to fly from the toner thin layer to the electrostatic latent image formed on the image carrier, and the toner is transferred onto the electrostatic latent image. In a developing device for developing an image ,
Pattern forming means for forming an image pattern in which the potential difference supply bias varied than the layer thickness of the plurality of different potential toner thin layer formed on each, corresponding to a toner image developed on the ZoTanji body When,
Image density detection means for obtaining an image density by detecting the density of each image pattern with an image density sensor ;
It is characterized in that it has a voltage difference deciding unit supply the bias determining the set potential difference in accordance with the image density for each potential of the toner amount and the supply bias of the toner thin layer for each potential difference of the supply bias .

In the present invention, for example, a first DC bias is applied to the supply roller, and a second DC bias and an AC bias are applied to the developing roller,
The potential difference of the supply bias is a difference between the first and second DC biases,
The pattern forming means fixes the first DC bias and changes the second DC bias to form the image pattern for each potential difference of the supply bias .

In the present invention, the supply bias potential difference determining means selects the second DC bias corresponding to the toner amount corresponding to the maximum density as the selection bias at the maximum density of the image density, and selects the selection bias. The difference between the bias and the first DC bias is the set potential difference, and the image density is calibrated by changing the first and second DC bias with the set potential difference fixed. It is desirable to have calibration means.

  As described above, the developing device of the present invention changes the potential difference between the developing roller and the supply roller to form a density detection pattern as a toner image for each of a plurality of different potential differences, and for each density detection pattern. The density is detected to obtain the image density, and the set potential difference is determined according to the toner amount of the toner thin layer for each potential difference and the image density for each potential difference. The layer thickness of the toner thin layer can be maintained in an appropriate range, and as a result, there is an effect that no image defect occurs.

  In the present invention, at the maximum image density, the second DC bias corresponding to the toner amount corresponding to the maximum density is selected as the selection bias, and the difference between the selection bias and the first DC bias is set as the set potential difference. Since the image density calibration is performed by changing the first and second DC biases with the set potential difference fixed, there is an effect that the desired image density can be easily adjusted.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention unless otherwise specified, but are merely illustrative examples. Not too much.

  Referring to FIG. 1, FIG. 1 is a diagram showing an example of a color image forming apparatus in which a developing device according to Embodiment 1 of the present invention is used. This image forming apparatus 20 includes an endless belt (conveying belt) 21. The endless belt 21 conveys the recording paper fed from the paper feed cassette 22 toward the fixing device 23. Above the endless belt 21, a black developing device 24a, a yellow developing device 24b, a cyan developing device 24c, and a magenta developing device 24d are arranged along the conveyance direction of the recording paper. The developing devices 24a to 24d are respectively provided with supply rollers (magnetic rollers) 25a to 25d and developing rollers 26a to 26d, and a toner thin layer is formed on the developing rollers 26a to 26d as described later. .

  Photosensitive drums 27a to 27d, which are image carriers, are arranged facing the developing rollers 26a to 26d, and chargers 28a to 28d and exposure devices 29a to 29d are arranged around the photosensitive drums 27a to 27d, respectively. Has been placed. Then, after the photosensitive drums 27a to 27d are charged by the chargers 28a to 28d, the photosensitive drums 27a to 27d are exposed by the exposure devices 29a to 29d in accordance with the image data, and the photosensitive drums 27a to 27d are exposed on the photosensitive drums 27a to 27d. An electrostatic latent image is formed. Then, as will be described later, the electrostatic latent images on the photosensitive drums 27a to 27d are developed by the developing rollers 26a to 26d to form toner images of respective colors.

  Then, the color toner images are sequentially transferred onto the recording paper conveyed by the endless belt 21 by the transfer devices 30a to 30d, and a color toner image is formed on the recording paper. Thereafter, the recording paper is sent to the fixing device 23, where the color toner image is fixed, and the recording paper is discharged via a paper discharge path.

  With reference to FIGS. 2 and 3, now, when attention is paid to the developing device 24a (the developing devices 24b to 24d have the same configuration as the developing device 24a, the description thereof is omitted here). As described above, it has the magnetic roller (supply roller) 25a and the developing roller 26a, and the magnetic roller 25a has a cylindrical rotating sleeve 31a made of a nonmagnetic metal material and a fixed magnet body 31b disposed therein. A plurality of magnetic poles are formed on the fixed magnet body 31b.

  The magnetic roller 25a and the developing roller 26a are disposed in the developing container 32, and a DC bias Vdc1 is applied to the developing roller 26a from a direct current (DC) bias power source 33a and an alternating current (AC) bias as described later. An AC bias Vac is applied from the power supply 33b. A DC bias Vdc2 is applied to the magnetic roller 25a from a direct current (DC) bias power source 34. The bias power supplies 33a and 34 are controlled by a control device (not shown) as described later.

  A paddle mixer 35 and a stirring mixer 36 are disposed in the developing container 32, and a partition plate 37 is disposed between the paddle mixer 35 and the stirring mixer 36. The two-component developer (hereinafter simply referred to as developer) in the developing container 32 is charged while being agitated and conveyed by the agitating mixer 36, and is supplied to the magnetic roller 25a while being agitated and charged by the paddle mixer 35. . A spike cutting blade (layer thickness regulating blade) 38 is provided facing the magnetic roller 25a, and the height of the magnetic brush formed on the magnetic roller 25a is regulated by the layer thickness regulating blade 38. As shown in FIG. 3, the length of the partition plate 37 (the axial direction of the developing roller 26 a) is shorter than the width of the developing container 32, and the developer can freely pass through both ends of the partition plate 37. It is like that.

  As shown in FIG. 3, the width H1 of the magnetic brush forming region of the magnetic roller 25a is a width for collecting the toner on the developing roller 26a, and the width H2 of the developing roller 26a is set to the magnetic brush forming region of the magnetic roller 25a. If the width H1 is shorter than the width H1, the uncollected area can be surely eliminated. Further, a toner concentration sensor 39 is attached to the developing container 32 so as to face the stirring mixer 36 (see FIG. 2: not shown in FIG. 3).

  The toner layer thickness on the developing roller 26a is regulated according to the potential difference | (Vdc2) − (Vdc1) | (hereinafter referred to as Δ) between the DC bias Vdc2 and the DC bias Vdc1, and for example, if Δ is increased, the toner layer thickness on the developing roller 26a When the toner thin layer becomes thicker and Δ is decreased, the toner thin layer becomes thinner. Then, in accordance with the potential difference between the photosensitive drum 27a and the developing roller 26a, the toner jumps from the toner thin layer on the developing roller 26a to the electrostatic latent image formed on the photosensitive drum 27a, and development is performed.

As the magnetic carrier, a carrier having a resistance value of 10 ⁶ Ωcm ³ to 10 ⁹ Ωcm ³ is used. In order to increase the contact point with the toner, it is necessary to increase the surface area of the carrier by using a carrier having a small diameter of 40 μm or less. preferable. As the magnetic carrier, magnetite carrier, Mn ferrite, Mn-Mg ferrite or the like is used. In the illustrated example, a ferrite carrier having a volume resistivity of 10 ⁷ Ωcm and a silicone resin coating, a saturation magnetization of 70 emu / g, and a weight average particle size of 35 μm was used.

  The mixing ratio of the toner and the carrier is 5 to 20% by weight, preferably 5 to 15% by weight, based on the total amount of the carrier and the toner. When the mixing ratio of the toner is less than 5% by weight, the charge amount of the toner becomes high and a sufficient image density cannot be obtained, and when it exceeds 20% by weight, a sufficient charge amount cannot be obtained.

  As described above, the toner layer thickness on the developing roller 26a is defined by Δ, but the toner charge amount changes according to the printing environment (environment atmosphere where the image forming apparatus is disposed). For this reason, if Δ is fixed, the toner layer thickness also changes according to the toner charge amount. For example, when the toner charge amount increases, the toner layer thickness decreases, and when the toner charge amount decreases, the toner layer thickness increases. When the toner layer is too thick, toner scattering from the developing device, image fogging, color points, image density defects, developing roller toner adhesion, and the like occur. On the other hand, if the toner layer thickness is too thin, image solid density reduction and development ghost occur.

  For example, toner scattering occurs when the toner reciprocates due to the AC bias (particularly when the toner returns from the developing roller to the magnetic brush) at a portion where the toner thin layer on the developing roller contacts the magnetic brush. When the toner layer on the developing roller is thick, a large amount of toner reciprocates between the developing roller and the magnetic brush. Further, when the toner layer thickness is thick, a large amount of toner remains on the developing roller after development, and the magnetic brush holds a large amount of toner even when the residual toner is collected by the magnetic brush. As a result, toner scattering from the developing device increases.

  The developing effective potential is the sum of the voltage applied to the developing roller and the potential of the toner thin layer. When the toner layer thickness on the developing roller is increased, the developing effective potential is increased, such as an a-Si photoreceptor. As described above, when the development is performed with the surface potential being low, image fogging occurs.

  Further, when the toner layer thickness on the developing roller increases, electric charges are accumulated in the toner thin layer, the electric charges leak to the photosensitive drum, and the toner moves to the photosensitive drum side. As a result, it appears in the image as a color point.

  When continuous printing is performed with a thick toner layer, high-charged toner fine powder accumulates on the bottom surface of the toner thin layer on the developing roller, and the toner fine powder does not contribute to development, resulting in a decrease in image density. To do. Further, if the toner thin layer on the developing roller is thin, the solid image density is lowered because the absolute amount of toner contributing to development is small.

  Therefore, in the image forming apparatus shown in FIG. 1, it is necessary to appropriately adjust the thickness of the toner thin layer formed on each of the developing rollers 26a to 26d in order to improve the image density and the like. That is, in order to prevent image defects, it is necessary to properly adjust the toner thin layer formed on the developing rollers 26a to 26d. For this reason, the image forming apparatus shown in FIG. 1 is provided with a toner density sensor (not shown) for measuring the density of toner images formed on the photosensitive drums 27a to 27d. Controls the bias power sources 33a and 34 of the developing device 24a according to the density detection signal (voltage signal) from the toner density sensor (the bias power sources are similarly controlled in the developing devices 24b to 24d).

  4 and 5, when attention is paid to the developing device 24a shown in FIG. 2 (assuming that the photosensitive drum 27a is grounded), the DC bias Vdc2 (the first one) from the DC power source 34 is now considered. DC bias voltage of 1 (hereinafter referred to as DC bias Vmag) is fixed at 500V, the bias power supply 33a is controlled, and the DC bias voltage applied to the developing roller 26a (second DC bias: hereinafter referred to as DC bias Vslv) Is gradually decreased from about 400 V to about 100 V (for example, DC bias Vslv is set to 400 V, 360 V, 320 V, 280 V, 240 V, 220 V, 180 V, 160 V, and 120 V. Note that the AC bias is Vpp = 1.5KV, frequency 3kHz, duty ratio 30% rectangular wave). Then, at each DC bias Vslv, a solid image pattern (density detection pattern) is formed on the photosensitive drum 27a, and the image density is detected by the toner density sensor (Step S1: image density obtained by the toner density sensor hereinafter). Is referred to as the development amount).

  At this time, the amount of toner on the developing roller 26a at each DC bias Vslv was measured. Then, the relationship between the development amount and the toner amount was examined. The result is shown in FIG. In FIG. 4, a curve L1 indicates the toner amount, and a curve L2 indicates the development amount. As shown in FIG. 4, although the toner amount increases as the DC bias Vslv is decreased (that is, the layer thickness of the toner thin layer on the developing roller 26a is increased, the developing amount is about 320V when the DC bias Vslv is increased). It can be seen that it becomes almost constant at the following, and slightly decreases when it is below about 240 V. In other words, even if the toner layer thickness on the developing roller 26a becomes thick, all of it is supplied to the photosensitive drum 27a side. Instead, it remains on the developing roller 26a.

  When the DC bias Vslv is high, the toner layer is thin, so that even if all the toner on the developing roller 26a is supplied to the photosensitive drum 27a side, sufficient image density cannot be obtained. On the other hand, when the DC bias Vslv is lowered, the toner layer thickness is increased and sufficient toner is supplied to the photosensitive drum 27a side to obtain a sufficient image density, but the toner remaining on the developing roller 26a increases. Contamination such as toner scattering occurs.

Therefore, here, the DC bias Vslv in the vicinity where the maximum value and the toner amount coincide with each other in the maximum value of the development amount is selected (that is, according to the toner amount corresponding to the maximum density in the image density of the maximum density). The DC bias VslvA was selected as the selection bias). In the illustrated example, the DC bias Vslv corresponding to the toner amount of 0.6 mg / cm ² is set to VslvA (step S2).

  Subsequently, Vmag−VslvA = ΔVa (set potential difference) is used as a set value after image density (development amount) adjustment, ΔVa is fixed, DC bias Vslv and Vmag are changed, and image density calibration (image density is performed). Calibration is performed (step S3). Here, ΔVa = 150V, and the DC bias voltages Vslv and Vmag are changed to form a solid image and an image pattern of 25% half (halftone with an image area ratio of 25%) on the photosensitive drum 27a. The image density (development amount) was obtained by detecting the pattern density with an image density sensor.

Referring to FIG. 6, FIG. 6 is a diagram showing the relationship between the DC bias Vslv and the development amount, where the curve L3 indicates a solid image density (development amount) and the curve L4 indicates a half 25% image density. As shown in the figure, it can be seen that the solid image density (development amount) is 0.6 mg / cm ² at Vslv = 200 V, and the solid image density does not change even if the DC bias Vslv is increased thereafter. On the other hand, the half 25% image density gradually increases when the DC bias Vslv is increased, but the increase decreases when Vslv = 200V is exceeded.

  This means that when ΔVa = 150V, if the DC bias Vslv = 200V to 250V, almost all of the thin toner layer formed on the developing roller 26a is formed on the photosensitive drum 27a when a solid image is formed. This means that the image density is improved and toner contamination such as toner scattering can be prevented.

  FIG. 7 shows the result when the above-described image density calibration control (toner layer thickness control) is performed and image formation is performed up to 150,000 (150 k sheets) (in this case, ΔVa = 150 V, AC bias). Vac (Vslvac) was set to Vpp = 1.5 kV, f = 3 kHz, duty ratio = 30%, and DC bias Vmag = 400 V). For comparison, FIG. 8 shows the results of image formation up to 150k sheets without performing the above-described image density calibration control (in this case, DC bias Vslv = 50 V, AC bias Vac (Vslvac)). Vpp = 1.5 kV, f = 3 kHz, duty ratio = 30%, and DC bias Vmag = 400V).

In FIG. 7, the toner layer thickness is constant at 1.0 mg / cm ² up to 150 k printed sheets, and the toner charge amount is 7 μC / g after printing 150 k sheets. Further, toner scattering, image fogging, and color point were not generated, and both the image density and the image solid density were good.

On the other hand, in FIG. 8, the change in the toner charge amount is the same as that in FIG. 7, but the toner layer thickness changed, and the initial toner layer thickness of 1.5 mg / cm ² was printed on 150k sheets. Later, it was 3.0 mg / cm ² . Further, when the number of printed sheets reached 100k, toner scattering, image fogging, and color points were generated, and although the image density was kept good, a defect occurred in the image solid density. As a result, it is understood that when the toner layer thickness control is performed as described above, image defects and toner scattering do not occur.

  In the above-described example, a so-called tandem type image forming apparatus has been described as an example of the image forming apparatus. However, this type of developing apparatus has a degree of freedom in the arrangement of the developing roller and the magnetic roller, and between the image carriers. The distance can be shortened, that is, the apparatus width can be made compact, and it is particularly effective when used in a tandem type image forming apparatus. However, other types of color image forming apparatuses and monochrome image forming apparatuses can be used. In a color image forming apparatus having an intermediate transfer member such as an intermediate transfer belt or an intermediate transfer drum, the toner layer thickness control can be performed in the same manner by measuring the image density on the intermediate transfer member. Should be done. In this case, the intermediate transfer member is an image carrier.

  As is apparent from the above description, the control device functions as a pattern forming unit, a potential difference determining unit, and an image density calibration unit.

An image pattern corresponding to a toner image developed on the image carrier is formed for each of a plurality of different potential differences by changing a potential difference between the developing roller and the supply roller (magnetic roller), and the image the concentration for each pattern is detected by the image density sensor to obtain a image density, so as to determine a set potential difference supply bias in accordance with the image density of the toner amount and each potential of the toner thin layer for each potential difference supply bias As a result, the thickness of the toner thin layer on the developing roller can be maintained within an appropriate range with a simple configuration, and can be applied to developing devices of various image forming apparatuses such as copying machines.

1 is a diagram schematically showing an example of an image forming apparatus in which a first embodiment of a developing device according to the present invention is used. It is sectional drawing which shows Example 1 of the developing device by this invention. FIG. 3 is a diagram showing the developing device shown in FIG. In the developing device shown in FIG. 2, when the DC bias applied to the developing roller is changed while the DC bias applied to the magnetic roller is fixed, the toner amount and the image density (development) of the toner layer formed on the developing roller are changed. It is a figure which shows quantity. 3 is a flowchart showing potential difference determination and image density calibration between a developing roller and a magnetic roller in the developing device shown in FIG. 2. It is a figure which shows the change of the image density (development amount) at the time of fixing the potential difference of a magnetic roller and a developing roller, and changing the direct current bias applied to a developing roller. It is a figure which shows the endurance test result at the time of performing image density calibration shown in FIG. It is a figure which shows the endurance test result when not performing image density calibration shown in FIG. It is a figure for demonstrating the afterimage generation situation in the conventional developing device.

20 Image forming apparatus 21 Endless belt 22 Paper feed cassette 23 Fixing devices 24a to 24d Developing devices 25a to 25d Supply roller (magnetic roller)
26a to 26d Developing rollers 27a to 27d Photosensitive drums 28a to 28d Chargers 29a to 29d Exposure devices 30a to 30d Transfer devices 32 Developing containers 33a, 34 DC (DC) bias power supply 33b AC (AC) bias power supply 35 Paddle mixer 36 Stirring Mixer 37 Partition plate 38 Cutting blade (layer thickness regulating blade)

Claims (4)

A magnetic brush composed of a magnetic carrier and toner is formed on the outer periphery of the supply roller containing the fixed magnetic pole member, and a potential difference (hereinafter referred to as supply bias) between the supply roller and the development roller is rubbed against the development roller . The toner is transferred to the developing roller in accordance with the potential difference) to form a thin toner layer on the developing roller, and the potential difference between the developing roller and the image carrier (hereinafter referred to as the potential difference of the developing bias) Accordingly, in the developing device for developing the toner image on the electrostatic latent image by causing the toner to fly from the toner thin layer to the electrostatic latent image formed on the image carrier .
Pattern forming means for forming an image pattern in which the potential difference supply bias varied than the layer thickness of the plurality of different potential toner thin layer formed on each, corresponding to a toner image developed on the ZoTanji body When,
Image density detection means for obtaining an image density by detecting the density of each image pattern with an image density sensor ;
A developing device, characterized in that it comprises a voltage difference deciding unit supply the bias determining the set potential difference in accordance with the image density for each potential of the toner amount and the supply bias of the toner thin layer for each potential difference of the supply bias. A first DC bias is applied to the supply roller, and a second DC bias and an AC bias are applied to the developing roller,
The potential difference of the supply bias is a difference between the first and second DC biases,
2. The image forming apparatus according to claim 1, wherein the pattern forming unit fixes the first DC bias and changes the second DC bias to form the image pattern for each potential difference of the supply bias. Development device. The supply bias potential difference determining means selects the second DC bias corresponding to the toner amount corresponding to the maximum density as the selection bias at the maximum image density, and selects the selection bias and the first bias. 3. The developing device according to claim 2, wherein a difference from a direct current bias is set to the set potential difference.   4. The developing device according to claim 3, further comprising image density calibration means for performing image density calibration by changing the first and second DC biases in a state where the set potential difference is fixed.

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