JP2007121940A - Electrophotographic image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To keep image density at a fixed target value over a long period, regarding a hybrid developing apparatus wherein two-component developer is electrified on a magnetic roll, and only the thin toner layer is formed on the developing roll and the toner is made to fly to an electrostatic latent image. <P>SOLUTION: The image forming apparatus includes a density detecting means for detecting the image density of a toner image formed based on a prescribed image density pattern original with target image density imparted, and calibration straightness is formed based on the detection result of the image density, and a DC voltage to be separately applied on the magnetic roll and the developing roll is controlled so that the image density may become equal to the target density. A ratio of variation of DC voltage applied on the magnetic roll to that on a developing roll is 2.5 in Fig.5, the allowable ratio is ≥1.5 and ≤6.0. The allowable voltage difference between the DC voltage applied on the magnetic roll and the DC voltage applied on the developing roll is ≥50 V and ≤350 V. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile machine, or a composite machine using an electrophotographic method, and more particularly, a toner is charged by holding and mixing a two-component developer on a magnetic roll. A hybrid developing device that uniformly forms a thin layer of toner only on the developing roll, superimposes DC and AC voltages in the space with the electrostatic latent image carrier, and causes the toner to fly to the electrostatic latent image. In an image forming apparatus for forming a toner image formed on an electrostatic latent image carrier on a recording material, the image forming apparatus includes density detecting means for detecting an image density of a toner image formed by a pattern original, and each of a magnetic roll and a developing roll is subjected to direct current. Electrophotography that applies an AC superimposed voltage and controls the DC voltage applied to the magnetic roll and the developing roll so that the image density becomes the target density based on the detection result of the image density About the image forming apparatus.

  In a conventional hybrid developing device, a toner image is formed from a pattern original, the image density is detected, and a direct current applied to a magnetic roll based on the image density in order to maintain a constant image density even after many image formations. The applied DC voltage (DC shift bias value) of the AC superimposed voltage was controlled.

  On the other hand, for example, in the developing method in the image forming apparatus disclosed in Patent Document 1, the density of the toner image on the photosensitive member is detected to control the DC applied voltage to the developing roll. Specifically, by writing a solid pattern at the timing of development by the toner layer formed in the second turn after the application of the developing roll application DC / AC superimposed voltage, the toner layer on the developing roller is consumed. Whether or not the toner layer formed in the second round has reached a predetermined layer thickness is written by writing a halftone pattern at the timing of development by the toner layer formed in the week and detecting the toner density. It is to detect.

  The electrophotographic recording apparatus disclosed in Patent Document 2 detects the image density on the photoreceptor in the one-component two-stage development method, and controls the bias of the first toner conveying means in the two-stage development method, Suppresses changes in image density over time.

In JP 2005-55841 A (FIG. 4, paragraph 0055), JP-A-7-261468 (FIG. 1, paragraph 0009)

  However, when the toner charge amount changes with time, the electrostatic latent image on the photosensitive member by the toner thin layer on the developing roll surface is changed in the same manner as the toner thin layer forming ability on the developing roll surface by the magnetic roll changes. The developing ability for images has also changed. In Patent Document 1 and Patent Document 2, only the developing roll and the bias of the first toner conveying means are controlled independently to control the change in image density over time. Is high, the toner thin layer forming ability of the magnetic roll is lowered, and the developing ability of the developing roll is also lowered. Here, if the bias value of the magnetic roll is controlled to maintain the toner image density constant, the thin layer forming ability of the magnetic roll is improved, including the reduction in the developing ability of the developing roll. The layer thickness will be excessively increased. Conversely, when the toner charge amount is low, the toner thin layer forming ability by the magnetic roll is improved, and the developing ability of the developing roll is also improved. Here, if the bias value of the magnetic roll is controlled to maintain the toner image density constant, the thin roll forming ability of the magnetic roll, including the development capacity improvement of the developing roll, is reduced. The toner layer thickness will be excessively reduced. As described above, when the toner layer thickness on the developing roll becomes too thin at 0.5 mg / cm 2 or less, density reduction and image unevenness are likely to occur in continuous output of a high density image. Further, when the toner layer is too thick at 1.5 mg / cm 2 or more, development ghost is generated and toner scattering occurs.

  Accordingly, an object of the present invention is to form a magnetic brush made of a two-component developer on a magnetic roll to which a DC / AC superimposed voltage is applied, and to apply only the toner in the magnetic brush to the DC / AC superimposed voltage. In a hybrid developing type electrophotographic image forming apparatus that transfers to a developing roll and develops an electrostatic latent image with a thin toner layer on the developing roll, a high-quality toner image is obtained at a constant density over a long period of time.

  In the present invention, a toner image is formed from a pattern original, the image density is detected by an image density sensor, and the DC voltage applied to the magnetic roll and the developing roll is controlled based on the image density sensor to maintain the toner image at a predetermined image density. To do. By controlling the DC voltage of the magnetic roll and the developing roll, the toner layer thickness on the developing roll is prevented from being too thin and too thick, resulting in a decrease in image density in continuous output of high density images, image unevenness and development ghosting. Generation of toner is prevented, and toner scattering in the developing device is prevented.

  The first means for solving the above-mentioned problem is to form a magnetic brush made of a two-component developer on a magnetic roll to which a DC / AC superimposed voltage is applied, and to apply only the toner in the magnetic brush to the DC / AC superimposed. A hybrid developer that develops an electrostatic latent image with a thin toner layer on the developing roll by transferring voltage to the developing roll, and an image that detects the image density of a toner image formed from a predetermined calibration pattern document An electrophotographic image forming apparatus comprising: a density detecting unit; and controlling a DC voltage applied to each of the magnetic roll and the developing roll so that the image density becomes a target density based on a detection result. That is.

  Thereby, it is possible to prevent the toner thin layer on the developing roll from being too thin or too thick and maintain a good image.

  The ratio ΔVm / ΔVd of the magnetic roll applied DC voltage change amount ΔVm to the developing roll applied DC voltage change amount ΔVd is 1.5 or more and 6.0 or less.

  Thereby, the relationship between the applied voltage change amount and the image density is determined, and it is possible to automatically perform control to reach the target density based on the detected image density.

  A difference (Vm−Vd) between the DC voltage Vm applied to the magnetic roll and the DC voltage Vd applied to the developing roll is 50 V or more and 300 V or less.

  As a result, the relationship between the applied voltage difference and the image density is determined, and it is possible to automatically perform control to reach the target density based on the detected image density.

  Embodiments of the present invention will be described below 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.

[Configuration of this embodiment]
FIG. 1 shows a front view of an example of a color copying machine (tandem machine) having a hybrid developing device as an example of an electrophotographic image forming apparatus. Note that the present invention can also be applied to a monochrome image forming apparatus.

  In the main body 1 of the tandem machine, for each color of yellow, magenta, cyan, and black (in no particular order), an image forming unit (a charger 70, an exposure unit 20, a developer 40, a transfer unit 90, It has a cleaning device 80. It also has a transfer belt 50 and a fixing device 120 for conveying paper.

  A recording medium (such as paper) (not shown) is supplied onto the transfer belt 50 from the right in the drawing, the paper comes into contact with the photoconductor 30, and the toner image on the photoconductor is transferred onto the paper by the transfer means 9. After the toner image of each color is transferred, the toner image is fixed on the paper by the fixing device 120, and the paper is discharged out of the main body 11. In a tandem type printer, it is important to design them compactly.

Next, each part will be described with reference to FIG. FIG. 2 is a conceptual diagram of the developing device 40.
In the figure, reference numeral 3 denotes a photoreceptor electrostatic latent image carrier, 2 denotes a developing roll, 1 denotes a magnetic roll, 5 denotes toner, 4 denotes a carrier, and a bias is applied between the photoreceptor 3 and the developing roll 2. The power supplies 7a and 7b output a DC bias voltage Vdc2 and an AC voltage applied to the developing roll 2, respectively. The power supplies 8a and 8b output a DC bias voltage Vdc1 and an AC voltage applied to the magnetic roll 1, respectively. 6 is a thin toner layer on the developing roll 2, 10 is a magnetic brush, and 9 is a regulating blade for controlling the thickness of the magnetic brush.

  A semiconductor laser or LED can be used as a light source (not shown) for exposing the electrostatic latent image carrier 3. A wavelength around 770 nm is effective for a positively charged organic photosensitive material (positive OPC), and a wavelength around 685 nm is effective for an a-Si photoreceptor.

  Hereinafter, an example in the case of using positive OPC will be described. The positive OPC 3 that is the electrostatic latent image carrier is charged to 400 V by a charger (not shown). Thereafter, when exposure is performed with an LED having a wavelength of 770 nm, the post-exposure potential becomes 70V. The positive OPC photosensitive member is disposed with a space of about 250 μm with respect to the developing roll 2. No wire electrode or the like is used in this space. When positive OPC is used as the photosensitive material for the photoconductor 3, the generation of ozone and the like is small and charging is stable. In particular, even when the positive OPC having a single layer structure is used over a long period of time and the film thickness changes It is ideal for long-life systems because it has little change in characteristics and stable image quality. In addition, it is also possible to use an a-Si photoconductor. When a long-life system is used, the film thickness of the positive OPC is set to about 20 μm to 40 μm. In the case of 20 μm or less, when the film decreases and reaches 10 μm, black spots are noticeably generated due to dielectric breakdown. On the other hand, if the film thickness is thicker than 40 μm, the sensitivity is lowered and the image quality is lowered.

Next, the hybrid developer will be described. First, the developer composed of the toner 5 and the carrier 4 held on the surface of the magnetic roll 1 is held, and the toner 5 is charged to an appropriate level while stirring. The developer passes through the regulating blade 9 and contacts the developing roll 2 with a constant layer thickness. The gap between the regulating blade 9 and the magnetic roll 2 is, for example, 0.3 mm to 1.5 mm, and the gap between the magnetic roll 1 and the developing roll 2 is, for example, 0.3 mm to 0.5 mm, preferably 0.2 mm to 0.4 mm. Degree. The toner thin layer 6 on the developing roll 2 is set to a thickness of, for example, 20 μm to 100 μm, preferably 30 μm to 70 μm. This thickness is a value corresponding to about 5 to 10 layers of the toner 5 when the average particle size of the toner 5 is 7 μm. The gap between the developing roll 2 and the electrostatic latent image carrier 3 is, for example, 150 μm to 400 μm, preferably 200 μm to 300 μm. If it is smaller than 150 μm, it causes fogging. If it is larger than 400 μm, it becomes difficult to cause the toner 5 to fly to the photoreceptor 3, and a sufficient image density cannot be obtained. It also becomes a factor that causes selective development. Here, the selective development means that a highly developable toner (a toner having a relatively large particle diameter and a low charge amount) is selectively developed on the electrostatic latent image carrier 3, and the charge amount is increased when continuous printing is performed. This is a phenomenon in which high toner (toner having a relatively small particle diameter) accumulates on the developing roll 2 and remains, and when toner having a large charge amount and a relatively small particle diameter accumulates on the developing roll 2, Since these toners are not developed, a sufficient image density cannot be obtained even after development, which is an undesirable phenomenon.
On the other hand, the surfaces of the magnetic roll 1 and the developing roll 2 are rotating bodies (conductive sleeves) made of conductive aluminum. The material of the conductive sleeve may be a uniform conductor, and SUS, conductive resin coating, etc. can be applied.
Then, by applying an alternating electric field to the conductive developing roll 2 and the magnetic roll 1 with the power supplies 7b and 8b, the development on the electrostatic latent image body (photosensitive body) 3 can be accurately performed, It is easy to collect the development residual toner.
The DC voltage and AC voltage from the power supplies 7 a and 7 b are transmitted to the conductive sleeve via the shaft of the developing roll 2. The output of the power supply 7a is 100V, and the output of the power supply 7b is Vpp 1.6 kV, frequency 2.7 kHz, duty 27%. The magnetic roll rotating body receives the DC voltage and the AC voltage from the power supplies 8a and 8b on the shaft of the magnetic roll 1 and transmits them to the magnetic roll rotating body. The output of the power supply 8a is 350 to 500v (variable by calibration), and the power supply 8b has a Vpp of 300V, a frequency of 2.7 kHz, and a duty of 73%. The waveform of the AC component is preferably a rectangular wave. By applying these superimposed biases to the developing roll and the magnetic roll, it is possible to develop the electrostatic latent image on the electrostatic latent image carrier 3 with good developability and to form the toner thin layer 6 on the magnetic roll 1. The recoverability is improved and the stability of continuous printing is improved.
In order to stabilize the image density in continuous printing, in order to prevent the above-described selective development, for example, based on the print data, the development roll is periodically checked by grasping the development status of the developer on the development roll. It is preferable to remove the toner from 2 and refresh. The reason for this is that toner with a large charge amount that is difficult to be developed (overcharged toner) deposited on the developing roll is forcibly applied from the developing roll 2 to the electrostatic latent image carrier 3 by adjusting the developing bias voltage. By shifting, the excessively charged toner on the developing roll is removed.
The toner is always refreshed if the toner is peeled off from the developing roll at the end of development, but it takes time to form a stable toner layer again, and a sufficient printing speed cannot be achieved. In order to supply sufficient toner to the latent image on the photosensitive member without increasing the sheet interval, the peripheral speed of the developing roll 2 is set to 1.5 times or more with respect to the electrostatic latent image carrier 3, and the time is shortened. The toner can be taken in and out. Further, when the magnetic roll 1 is set at a speed 1 to 2 times that of the developing roll 2, toner replacement is promoted. At this time, it is preferable that the rotating direction of the magnetic roll is opposite to the developing roll. In order to replace the toner thin layer 6 on the developing roll 2, the toner thin layer 6 of the developing sleeve is recovered by the magnetic brush 10 by changing the DC voltage in a state where AC is applied at the end of development.

  The saturated toner amount of the toner thin layer 6 is determined by the difference between Vdc1 of the power supply 8a applied to the magnetic roll 1 and Vdc2 of the power supply 7a applied to the developing roll. When Vdc2 is set to 150 V and the value of Vdc1 is set to 400 V, a toner layer of about 1.0 mg / cm 2 is obtained on the second round of the developing roll. The adjustment of the toner layer is basically obtained by a potential difference of (Vdc2−Vdc1), but factors such as the toner charge amount and the magnetic pole magnetic pole strength may also contribute.

  If Vdc2 is controlled based on an actually obtained image in order to control the toner layer thickness, a uniform and good image can be obtained at a target density. When continuously performing high density printing, it is advantageous to set the value of (Vdc2−Vdc1) slightly larger because the amount of toner transferred from the magnetic roll 1 to the developing roll 2 increases. If the toner layer is too thin at 0.5 mg / cm 2 or less, the follow-up of density when a high-density image is continuous decreases, and image unevenness is likely to occur. On the other hand, if the toner layer exceeds 1.5 mg / cm 2 and is too thick, the development ghost tends to be noticeable and toner scattering tends to be noticeable. The toner layer thickness also depends on the charge amount of the toner. When the toner charge amount is as low as 10 μC / g or less, particularly 5 μC / g or less, the toner layer thickness increases and scattering increases. Also, development ghosts become prominent. On the other hand, when the toner charge amount is 20 μC / g or more, the toner layer thickness becomes thin, the charge increases, and the developability of the toner decreases.

  The toner thin layer 6 of the developing roll 2 is collected by the magnetic brush 10 held on the magnetic roll 1, and new developer is carried to the developing roll 2 through the regulating blade 9.

  To avoid the selective developability as described above, the toner 5 is adjusted to have a particle diameter of, for example, 5.0 μm to 10.0 μm, and fine powder smaller than this range or coarse powder larger than this range should be removed. It is effective to define the particle size distribution. Generally, the particle size distribution of the toner is measured by a coal counter, and the spread of the particle size distribution is expressed by the ratio of the volume distribution average diameter to the number distribution average diameter. In order to prevent selective development, it is important to reduce the ratio. When the distribution is wide, toner having a relatively small particle size is deposited on the developing roll 2 in continuous printing, and developability is deteriorated.

  The two-component developer forms a magnetic brush 10 composed of toner 5 and carrier 4 on the magnetic roll 1, and the toner 5 is charged by stirring. The magnetic brush 10 on the magnetic roll 1 is layer-regulated by a regulating blade 9, and a thin layer 6 containing only toner is formed on the developing roll 2 by a potential difference between the magnetic roll 1 and the developing roll 2. As the carrier 4, a ferrite carrier having a volume resistivity of 108 Ωcm coated with a silicone resin, a saturation magnetization of 40 emu / g, and an average particle diameter of 35 μm may be used. When the average particle size exceeds 50 μm, the carrier stress increases, and the toner concentration in the developer cannot be increased, and the amount of toner supplied to the developing roll 2 decreases. As the carrier, a magnetite carrier, Mn ferrite, Mn-Mg ferrite, or the like can be used. These carriers may be used as they are, but they may be used after being surface-treated within an appropriate resistance range. The mixing ratio of the toner 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 toner charge amount becomes high and a sufficient image density cannot be obtained. When the toner content exceeds 20% by weight, a sufficient charge amount cannot be obtained. The toner is scattered from the inside and contaminates the inside of the image forming apparatus, or toner fog occurs on the image.

  FIG. 3 is a timing chart showing the DC voltage to the developing roll 2 and the magnetic roll 1 between the sheets (between images). After developing the electrostatic latent image, the DC voltage Vdc2 of the developing roll 2 is set to zero, the toner thin layer is peeled off while the developing roll 2 rotates once, and then the development of the electrostatic latent image is started. A toner thin layer is re-formed.

[Operation of this embodiment]
In the present embodiment, the developing device 40 (FIG. 1) is the hybrid developing device described above. Here, the hybrid developing device means that a two-component developer is held on a magnetic roll and mixed to charge the toner, and only a thin layer of toner is uniformly formed on the developing roll. Is a developing device that applies a DC / AC superimposed voltage to the space and causes the toner to fly to the electrostatic latent image.

  Further, density detecting means for detecting the image density of the toner image formed by the pattern original (the density sensor 60 in FIG. 1, for example, an optical density sensor that slides the light emitting element / light receiving element as a pair) is also used. Here, in the example of FIG. 1, the toner image is a toner image on a sheet (not shown in FIG. 1).

  Based on the detection result of the density sensor 60, the DC voltage applied to the magnetic roll and the developing roll is controlled so that the toner image density becomes a predetermined density. Therefore, a lookup table showing the relationship between the applied DC voltage and the toner image density is prepared for each color. When the obtained toner image density is different from the target image density, the applied DC voltage is changed according to the lookup table.

[Example]
FIG. 4 is an example of an image density pattern original to be calibrated. A calibration image density pattern document (calibration document) is composed of four colors M / C / Y / BK from the top in the figure. Each of the four colors has a halftone patch with a printing rate of 25% and a 100% solid patch. The 25% halftone patch has three conditions for the DC voltage (bias value) Vdc1 of the magnetic roll 1 and the DC voltage (bias value) Vdc2 of the developing roll 2. The bias conditions are a magnetic roll applied DC voltage Vmag and a developing roll applied DC voltage Vslv. In combination of (Vmag, Vslv), (350V, 80V), (400V, 100V), (450V, 120V). For a 100% solid patch, there is only one of (400V, 100V).

  FIG. 5 is an example of a calibration line. In order to perform density calibration, an image is formed with a 25% halftone patch that is a calibration pattern original, and the bias values of the magnetic roll 1 and developing roll 2 and the three points of the image density sensor output are connected by a straight line or an approximate curve. The result is stored in a memory (not shown). This is a predetermined calibration curve.

  Therefore, when the actual image formation result is seen and it is found that the image quality is deteriorated, the current set application voltage is changed so that the image density reaches the target density using the calibration straight line described above. . Therefore, using a 25% halftone patch, the density of each color obtained at present is measured and input to an input means (not shown). The input output of the density sensor 60 is compared with the target image density of the look-up table in an arithmetic unit (not shown) to determine an applied voltage that gives the target image density. In this way, the development conditions of each color developing device 40 (FIG. 1) are optimized.

  As shown in FIG. 5, assuming that the developing roll bias reference value is 100 V, and the change amount of Vslv is ΔVslv, the change amount of the magnetic roll bias value Vmag at that time is 2.5 times ΔVslv (change ratio α). It has become. Specifically, it is the amount of change in Vslv (ΔVslv) when Vslv is changed from 80 V to 100 V. At this time, Vmag is changed from 350 v to 400 V, so the amount of change in Vmag (ΔVmag) is 50 v. Therefore, the change ratio α (ΔVmag / ΔVslv) is 2.5. However, this is a typical example, and it may be 1.5 ≦ α ≦ 6.0.

  Further, as shown in FIG. 5, the magnetic roll applied DC voltage Vmag is higher than the developing roll 2 applied voltage Vslv, and the voltage difference is preferably set to 50V to 350v. If it is 50V or less, the peeling from the developing roller is insufficient, and if it is 350V or more, the peeling becomes too strong, making it difficult to form the next thin layer during continuous printing, and the second image becomes thin, etc. Problem arises. In addition, by setting the duty ratio to 50% or more at the time of peeling, there is an effect of preventing the toner from adhering to the surface of the magnetic roll, and the adhesion of the toner on the surface of the developing roll can be reduced more effectively.

  FIG. 6 is a graph showing the relationship between the applied voltage difference (difference between the magnetic roll applied DC voltage Vmag and the developing roller applied DC voltage Vslv (Vmag−Vslv)) and the amount of toner on the developing roller 2. It can be seen that the toner amount on the developing roller 2 is proportionally changed with respect to the voltage difference (Vmag−Vslv).

  FIG. 7 is a graph showing the relationship between the developing roll bias value Vslv and the toner amount on the electrostatic latent image carrier 3 when the toner amount (1.0 mg / cm 2) on the developing roll 2 is kept constant. . It can be understood that the toner amount on the electrostatic latent image carrier 3 varies in proportion to the developing roller applied DC voltage (Vslv) with respect to the bias value of the developing roll 2. As a result, in consideration of the experimental result of FIG. 6, when the voltage difference (Vmag−Vslv) is controlled and supplied to the appropriate amount of toner on the developing roll 2, the applied DC voltage (Vslv) of the developing roller is It can be understood that appropriate development conditions can be set by controlling.

  From these, it can be seen that it is effective to detect the image density of the 25% halftone patch and change both the magnetic roll and developing roll bias values to the target values along the calibration line.

  The present invention can be used in electrophotographic image forming apparatuses such as copiers, printers, facsimiles, and complex machines using electrophotographic methods.

1 is a conceptual diagram of a tandem color image forming apparatus. It is a conceptual diagram of a hybrid developing device. 6 is a timing chart of toner layer peeling. It is an example of an image density pattern. It is a straight line plot (calibration straight line) showing the relationship between development conditions and toner image density for an image density pattern (25% halftone patch). 6 is a graph showing a relationship between a difference in DC applied voltage between a developing roll and a magnetic roll and a toner amount on the developing roll. 6 is a graph showing a relationship between a developing roll applied DC voltage and a toner amount on an electrostatic latent image carrier.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Magnetic roll 2 Developing roll 3 Electrostatic latent image carrier (photoreceptor)
4 Carrier 5 Toner 6 Toner thin layer 9 Regulating blade 10 Magnetic brush 11 Main body 20 of tandem type color image forming apparatus Exposure unit 30 Photoconductor 40 Developer 50 Transfer belt 60 Density sensor 70 Charger 80 Cleaning device 90 Transfer means 120 Fixing device

Claims (3)

A magnetic brush composed of a two-component developer is formed on a magnetic roll to which a DC / AC superimposed voltage is applied, and only the toner in the magnetic brush is transferred to the developing roll to which a DC / AC superimposed voltage is applied, and the development is performed. A hybrid developer for developing an electrostatic latent image with a thin toner layer on a roll;
Image density detecting means for detecting the image density of the toner image formed by the image density pattern,
An electrophotographic image forming apparatus, wherein a DC voltage applied to each of the magnetic roll and the developing roll is controlled based on the detection result so that the image density becomes a target density.   2. The electrophotographic image forming apparatus according to claim 1, wherein a ratio [Delta] Vmag / [Delta] Vslv of the magnetic roll applied DC voltage change amount [Delta] Vm to the developing roll applied DC voltage change amount [Delta] Vd is 1.5 or more and 6.0 or less. .   2. The electrophotographic image forming apparatus according to claim 1, wherein a difference (Vmag−Vslv) between the DC voltage Vm applied to the magnetic roll and the DC voltage Vd applied to the developing roll is 50 V or more and 300 V or less.

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