JP2005242281A - Developing device in image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a developing device in an image forming apparatus in which an alternating electric field formed between a developing roll and a photoreceptor and a DC bias or the like applied to the developing roll and a magnetic roll can be easily balanced with each other and a toner layer is satisfactorily formed on the developing roll to prevent ghost and toner sticking to the developing roll and reduction in image density resulting from the toner sticking are prevented to prevent the occurrence of leak between the photoreceptor and the developing roll and between the developing roll and the magnetic roll. <P>SOLUTION: In the developing device, the developing roll is caused to carry the toner layer thereon by rubbing with a magnetic brush formed of a two-component developer, and toner is flown from the developing roll by an alternating electric field which is composed of of square waves and is formed between the developing toll and the photoreceptor by a first power source, to develop a latent image on the photoreceptor, and a second power source for applying an alternating electric field which is composed of square waves and has the same frequency as the alternating electric field formed by the first power source, an opposite phase and an inverted duty ratio is provided between the magnetic roll and the developing roll. <P>COPYRIGHT: (C)2005,JPO&NCIPI

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 having a magnetic carrier and a nonmagnetic toner. The present invention relates to a developing device in which toner is held on a developing roller to develop an electrostatic latent image.

  As a developing method in this type of image forming apparatus, a two-component developing method using a toner and a carrier, a one-component developing method not using a carrier, and a two-component developer that charges a toner using a carrier are used. There is a so-called hybrid development system in which only the charged toner is held to develop the electrostatic latent image.

  The two-component development method is excellent in the charging property of the toner by the carrier and can extend the life, but has a drawback that the developing device becomes large and complicated and the image quality changes depending on the durability of the carrier. In addition, the one-component development system has a compact developing device and excellent dot reproducibility, but the durability of the developing roller and the replenishing roller is generally low, and the cost of consumables increases because the developing device is periodically replaced. Become.

  Taking advantage of the characteristics of both of these development methods, a two-component developer having a toner and a carrier is used to hold only the toner on the developing roller, and the electrostatic latent image formed on the photosensitive member with the toner can be contacted without contact. The so-called hybrid development system in which development is performed has attracted attention. Particularly, this hybrid development system is used as a development system capable of high-speed image formation and for a one-drum color superposition system that sequentially forms a plurality of color images on a photoreceptor. As a development device for a tandem system, a plurality of electrophotographic process members are arranged side by side, a color image is formed in synchronization with feeding of the transfer member, and color is superimposed on the transfer member. .

  Of these, in the case of a tandem image forming apparatus, a plurality of electrophotographic process members are arranged side by side. Therefore, if the developing roller and the magnetic roller are arranged side by side with respect to the photoreceptor, the width of the electrophotographic process member itself is increased. This hinders downsizing. Therefore, it is preferable to arrange the developing roller and magnetic roller constituting the electrophotographic process member above the photosensitive member so that the developing device is a vertical type so that the depth of the image forming apparatus can be reduced.

  As a conventional technique related to such a technique, Patent Document 1 discloses a developing apparatus that uses a magnetic roller to advance a developer to a donor roller and transfers toner onto the donor roller to form a toner layer. . However, in this system, toner charging control is complicated, and it is necessary to apply a high surface potential and a large developing electric field to the photoreceptor, and it is difficult to refresh undeveloped toner on the donor roller. When a toner consumption area and a non-consumption area are generated on the upper side, a part of the previous developed image becomes an afterimage (ghost image) during the next development because of a relationship in which the toner adhesion state on the developing roller and the toner potential difference vary. ), A so-called history phenomenon is likely to occur.

  Therefore, for example, in Patent Documents 2 and 3, a magnetic roller that holds a magnetic brush formed of a two-component developer having a carrier and toner by a magnetic pole member fixed inside, and a slide by the magnetic brush held by the magnetic roller. A developing roller that forms a toner layer by rubbing and a power source that forms an alternating electric field between the developing roller and the photosensitive member are provided, and the alternating electric field causes the toner layer formed on the developing roller to fly. There is shown a developing device in which a latent image on the photosensitive member is developed with toner to prevent occurrence of a residual image (ghost) during development while avoiding occurrence of fog.

  Patent Document 4 discloses an image forming apparatus that uses a one-component developer and includes a developing roller that is in contact with the photosensitive member and a supply roller that is in contact with the developing roller. The supply roller supplies toner to the developing roller. However, in a developing device in which a latent image on a photosensitive member is developed in a thin layer state by frictional charging with a regulating blade, the density of a low density image or a fine line image is difficult to develop, and the toner charge amount increases. In order to prevent unevenness, if the developing electric field applied to the developing roller is alternating current, low density images and fine line images can be developed well, and it is easy to scrape off unconsumed toner. In order to solve the problem that fogging occurs when the value is high, and the effect of scraping off unconsumed toner becomes low when the value is low, an AC voltage is applied to the supply roller. An AC voltage is developed apparatus of phase at the same frequency is shown.

U.S. Pat. No. 3,929,098 JP 2003-211961 A JP 2003-21966 A JP 2001-134050 A

  However, the developing devices disclosed in Patent Documents 2 and 3 also balance the alternating electric field formed between the developing roller and the photoconductor and the DC bias applied to the developing roller and the magnetic roller, respectively. Therefore, highly accurate control is required, and a technology with more margin is desired.

  That is, the toner layer is formed on the developing roller by a potential difference (hereinafter referred to as a first potential difference) between the minimum voltage of the alternating electric field applied to the developing roller and the DC bias applied to the magnetic roller. When the potential difference is large, the toner layer is well formed and ghost is reduced. Further, the development residual toner on the developing roller is peeled off by a potential difference (hereinafter referred to as a second potential difference) between the maximum voltage of this alternating electric field and a DC bias applied to the magnetic roller, and if this potential difference is large, the developing roller Toner adhesion to the toner can be prevented.

  On the other hand, development from the developing roller to the photosensitive member is performed by a potential difference (hereinafter referred to as a third potential difference) between the maximum voltage of the alternating electric field applied to the developing roller and the bright potential (post-exposure potential) of the photosensitive member. A potential difference (hereinafter referred to as a fourth potential difference) between the light potential of the photoconductor and the minimum voltage of the alternating electric field is a potential difference in the toner collecting direction on the photoconductor. Further, the potential difference (hereinafter referred to as the fifth potential difference) between the dark potential (unexposed portion potential) on the photosensitive member and the minimum voltage of the alternating electric field is referred to as a so-called fog removal potential. Works.

  Therefore, for toner adhesion to the ghost and the developing roller, it is preferable to increase the first potential difference and the second potential difference, that is, increase the peak-to-peak voltage of the alternating electric field, but the image density and dot reproducibility are Depending on the balance between the third potential difference and the fourth potential difference, if the third potential difference increases by increasing the peak-to-peak voltage of the alternating electric field, leakage from the developing roller to the photosensitive member is likely to occur, and black spots There may be a problem with the circuit system such as an image, a terrible and a white band or a black band, and the bias output instantaneously drops.

  Further, the developing device disclosed in Patent Document 4 is a developing device using a one-component developer and a developing device of a type in which a photosensitive member and a supply roller are in contact with a developing roller. When a developing device of a type in which a roller contacts is used in a tandem type image forming apparatus, there is a risk of causing color fluctuation and promoting color misregistration, which is a weak point of the tandem type.

  Therefore, in the present invention, it is possible to easily balance the alternating electric field formed between the developing roller and the photosensitive member and the DC bias applied to the developing roller and the magnetic roller, and the toner layer on the developing roller. Prevents the ghosting and prevents the toner from adhering to the developing roller and the resulting decrease in image density, thereby causing a leak between the photoconductor and the developing roller and a leak between the developing roller and the magnetic roller. It is an object of the present invention to provide a developing device in an image forming apparatus that does not exist.

In order to solve the above problems, the developing device in the image forming apparatus of the present invention is:
A magnetic roller for holding a magnetic brush formed of a two-component developer having carrier and toner by a magnetic pole member fixed inside, a developing roller for carrying a toner layer by rubbing with the magnetic brush, and a rectangular shape on the developing roller An image forming unit that develops a latent image on the photosensitive member with toner that has been ejected from a toner layer formed on the developing roller by the alternating electric field. In the developing device in the apparatus,
A second power supply is provided for forming an alternating electric field on the magnetic roller, which is formed of a rectangular wave having the same frequency as that of the alternating electric field formed by the first power supply but having the opposite phase and a reversed duty ratio. To do.

  A third power source for applying a DC bias to the developing roller is provided, and a bias voltage in the third power source is made larger than a minimum voltage of an alternating electric field formed on the magnetic roller.

  In this way, the toner layer is carried on the developing roller by rubbing with a magnetic brush formed of a two-component developer, and the toner is ejected from the developing roller by an alternating electric field composed of a rectangular wave formed by the first power source on the developing roller. An alternating electric field by a rectangular wave having the same frequency as that of the alternating electric field formed by the first power supply, but having the opposite phase and the duty ratio reversed, is applied to the magnetic roller in the developing device that develops the latent image on the photosensitive member. By providing the second power source for applying the voltage, the potential difference for forming the toner layer on the developing roller without changing the potential difference between the photosensitive member and the developing roller, that is, the alternating electric field formed on the developing roller. The potential difference (corresponding to the first potential difference) between the minimum voltage and the maximum voltage of the alternating electric field formed on the magnetic roller and the potential difference for removing the toner from the developing roller, that is, the developing low The potential difference (corresponding to the second potential difference) between the maximum voltage of the alternating electric field formed on the magnetic field and the minimum voltage of the alternating electric field formed on the magnetic roller increases, and is formed between the developing roller and the photosensitive member. A balance between the alternating electric field and a DC bias applied to the developing roller and the magnetic roller can be easily balanced, and a good toner layer is formed on the developing roller to reduce ghosting and prevent toner adhesion to the developing roller. And can solve problems such as a decrease in density.

  Moreover, since the potential difference between the photosensitive member and the developing roller is the same as the conventional one in spite of the fact that the potential difference corresponding to both the first and second potential differences is increased, leakage between the developing roller and the photosensitive member Leakage between the developing roller and the magnetic roller can be prevented, and it is possible to prevent a black spot image or an instantaneous bias output from dropping to cause a problem with the circuit system. In addition, by making the alternating electric field formed by the first power source and the second power source a rectangular wave, it is possible to obtain an optimum duty ratio for forming and scraping the toner layer on the developing roller, and By reversing the duty ratio of the alternating electric field generated by the power source 2 with respect to the alternating electric field generated by the first power source, the ghost and density decrease due to toner adhesion to the developing roller, leakage between the developing roller and the photosensitive member, and the developing roller Leakage between magnetic rollers can be prevented more effectively.

  Also, a third power source for applying a DC bias to the developing roller is provided, and the bias voltage at the third power source is made larger than the minimum voltage of the alternating electric field formed on the magnetic roller, whereby It is possible to provide a developing device that can further suppress toner adhesion and can output a good image with no decrease in density over a long period of time.

  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.

  FIG. 1 is a diagram showing an alternating bias and a DC bias applied to the developing roller and the magnetic roller of the developing device in the image forming apparatus according to the present invention, and FIG. 2 is an alternating bias and a DC bias applied to the developing roller and the magnetic roller. FIG. 3 is a diagram for explaining the action when a DC bias is applied, FIG. 3 is a diagram for explaining the relationship among the alternating bias applied to the developing roller, the DC bias, and the photoreceptor potential, and FIG. 4 is applied to the developing roller. FIG. 5 is a diagram for explaining the action when the voltage of the DC bias applied to the magnetic roller is increased while leaving the alternating bias and the DC bias unchanged, and FIG. 5 is the same as the DC bias voltage applied to the developing roller and the magnetic roller. FIG. 6 is a diagram for explaining the action when the peak-to-peak voltage of the alternating bias applied to the developing roller is increased. FIG. FIG. 7 is a diagram for explaining the relationship between the developing roller and the photosensitive member potential when the voltage between the alternating bias peaks applied to the developing roller is increased without changing the bias voltage. FIG. 7 shows the alternating bias applied to the developing roller and the direct current. FIG. 8 is a table in which the state of ghost, leak, and image density when changing the peak-to-peak voltage of the alternating bias applied to the magnetic roller while keeping the bias and the DC bias applied to the magnetic roller constant is shown. FIG. 9 is a schematic diagram of an embodiment of an image forming apparatus having the developing device of the present invention.

In the figure, reference numeral 1 is a non-magnetic metal material formed in a cylindrical shape, and a plurality of fixed magnets are disposed therein to be rotatable with a sleeve-like magnetic roller, and 2 is a magnetic brush 10 formed on the magnetic roller 1. Developing roller on which a thin layer 6 of toner is formed, 3 is a photosensitive drum, 4 is a carrier constituting a developer, 5 is toner, 6 is a toner thin layer formed on the developing roller 2, and 7a is a developing roller. 2 is a power source for applying a direct current (DC) bias V _dc1 , 7b is also a power source for applying an alternating current (AC) bias V _ac1 to the developing roller 2, and 8a is a power source for applying a direct current (DC) bias V _dc2 to the magnetic roller 1. , 8b is a power source for applying an alternating current (DC) bias V _ac2 to the magnetic roller 1, and 9 is a spike cutting blade (layer thickness regulation) for keeping the height of the magnetic brush 10 formed on the magnetic roller 1 constant. Blade) 20 is an image forming apparatus, 50A in FIG. 9 is a black developing apparatus of the image forming apparatus 20, 50B is also a yellow developing apparatus, 50C is also a cyan developing apparatus, 50D is also a magenta developing apparatus, and 53 is A paper feed cassette containing recording paper, 54 is an endless belt for conveying the recording paper, 56 is a charger for charging the photosensitive drum 3, and 57 is a latent image by exposing the charged photosensitive drum 3. An exposure device for forming an image, 58 is a transfer device for transferring the developed toner image to the recording paper, and 59 is a fixing device for fixing the toner image on the recording paper to which the toner image is transferred. In the tandem type image forming apparatus, as described above, it is important to design the charger 56, the exposure device 57, the developing device 50, the transfer device 58, the cleaning device, and the like installed around the photosensitive drum 3 in a compact manner. In the present invention, the developing device 50 is adjacent to the photosensitive drum 3 and is disposed in a substantially vertical direction.

  In the following description, the present invention is applied to a developing device of a tandem type color image forming apparatus as an example, but the present invention uses a two-component developer that charges a toner using a carrier. In addition, a monochrome image forming apparatus can be used as long as it is a developing apparatus in an image forming apparatus in which only the charged toner is held on the developing roller and is made to fly to an electrostatic latent image to develop the latent image. It is clear that it can be applied.

  Among these, as the material of the photoreceptor 3, an amorphous silicon (a-Si) photoreceptor, an organic photoreceptor (OPC), or the like can be used. Positively charged organic photoconductors (positive OPCs) generate less ozone and have a stable charge. In particular, positively charged organic photoconductors with a single layer structure are sensitive even when their film thickness changes over time. Since there is little change in characteristics and the image quality is stable, it is suitable for a long-life system. When the positively charged organic photoconductor is used in a long-life system, the film thickness is preferably set to about 20 μm to 40 μm. In the case of 20 μm or less, black spots are noticeably generated due to dielectric breakdown when the film thickness is reduced to about 10 μm. On the other hand, when the thickness is 40 μm or more, the sensitivity is lowered, which causes a reduction in image density.

  The exposure device 57 can use a semiconductor laser or an LED. When a positively charged organic photoconductor is used, a wavelength around 770 nm is effective, and when an amorphous silicon photoconductor is used, a wavelength around 685 nm is effective. Hereinafter, in the present invention, a case where a positively charged organic photoreceptor is used as the photoreceptor 3 and an LED is used as a light source of the exposure device 57 will be described as an example.

  The outermost surface of the developing roller 2 is composed of a sleeve made of uniform conductive aluminum, SUS, conductive resin coating, or the like. A direct current (DC) bias power source 7 a and an alternating current (AC) bias power source 7 b are connected to the shaft portion, and the direct current and the alternating current are superimposed between the rotating developing roller 2, the photosensitive member 3, and the magnetic roller 1. As a result of the applied bias, good developability of the latent image on the photoreceptor 3 and recoverability of the toner thin layer 6 with respect to the magnetic roller 1 are enhanced. The AC bias waveform is preferably a rectangular wave.

  In addition, a direct current (DC) bias power source 8a and an alternating current (AC) bias power source 8b are also connected to the magnetic roller 1 to effectively improve the stability of continuous printing.

  It is important to define the particle size distribution of the toner 5 in order to avoid selective developability. In general, the spread of the particle size distribution of the toner is measured by a coal counter, and the spread of the particle size distribution is expressed by a ratio between the volume distribution average particle size and the number distribution average particle size. In order to prevent selective development, it is important to reduce the ratio. When the distribution is wide, toner having a relatively small particle size accumulates on the developing roller 2 during continuous printing, and developability is deteriorated.

As the carrier 4, a magnetite carrier, a Mn-based ferrite, a Mn-Mg-based ferrite, or the like can be used, and the carrier 4 can be used after being surface-treated within a range that does not increase an appropriate resistance value. In the present invention, as an example, a ferrite carrier having a volume resistivity of 10 ⁸ Ωcm ³ and a saturation magnetization of 40 emu / g and an average particle size of 35 μm was used. If the average particle size exceeds 50 μm, the carrier stress increases and the toner density cannot be increased, and the amount of toner supplied to the developing roller 2 decreases. These carriers may be used as they are, but they can also be used after being surface-treated within a range in which an appropriate resistance value is not increased.

  The mixing ratio of the toner 5 and the carrier 4 is 5 to 20% by weight, preferably 5 to 15% by weight, based on the total amount of the carrier 4 and the toner 5. If the mixing ratio of the toner is less than 5% by weight, the charge amount of the toner becomes so high that a sufficient image density cannot be obtained, and if it exceeds 20% by weight, a sufficient charge amount cannot be obtained. Scattering from the developing device contaminates the inside of the image forming apparatus or causes toner fog on the image. The carrier and toner as a two-component developer can contain additives known per se for the purpose of charge control, fluidity improvement, and polishing the surface of the photoreceptor.

The saturated toner amount of the toner thin layer 6 on the developing roller is determined by the difference between the direct current (DC) bias 8a and the direct current (DC) bias 7a. If the toner thin layer 6 is too thin at 0.5 mg / cm ² or less, the followability of the density in the case of continuous high density images decreases, and image unevenness is likely to occur. ^If the thickness exceeds ² and is too thick, the development ghost tends to be noticeable and the toner scattering tends to be noticeable. The thickness of the toner thin layer 6 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. 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.

  In order to stabilize the image density in continuous printing, it is necessary to periodically remove the toner from the developing roller 2 and refresh. This is done by changing the direct current (DC) biases 7a and 8a while applying the alternating current (AC) bias 7b at the end of the development, and collecting the toner on the developing roller 2 to the magnetic brush 10 and refreshing. In order to supply sufficient toner to the latent image on the photosensitive member 3 without increasing the paper interval, the peripheral speed of the developing roller 2 is set to 1.5 times or more with respect to the photosensitive member 3, and the toner can be obtained in a short time. Can be taken in and out. Further, when the magnetic roller 1 is set to a speed exceeding 1 and 2 times that of the developing roller 2, toner replacement is promoted. At this time, it is preferable that the rotating direction of the magnetic roller 1 is opposite to the developing roller 2.

  First, the operation of one embodiment of the image forming apparatus 20 having the developing device according to the present invention will be described with reference to the schematic diagrams of FIGS. In the image forming apparatus 20, an endless belt 54 is disposed so as to be able to convey the recording paper from the paper feed cassette 53 toward the fixing device 59, and a black belt is disposed above the belt 54 that conveys the recording paper. A developing device 50A for yellow, a developing device 50B for yellow, a developing device 50C for cyan, and a developing device 50D for magenta are provided.

  These developing devices 50 (A, B, C, D) are developed in close proximity to the magnetic roller 1 (A, B, C, D) and the magnetic roller 1 (A, B, C, D), respectively. A roller 2 (A, B, C, D) is disposed, and a photosensitive member 3 (A, B, C, D) faces the developing roller 2, and a charging device is disposed around the photosensitive member 3. 56 (A, B, C, D) and exposure device 57 (A, B, C, D) are arranged.

  In the tandem type image forming apparatus having the developing device of the present invention configured as described above, the two-component developer having toner and carrier corresponding to each color such as yellow, cyan, magenta, and black is supplied from the toner container. The magnetic brush 10 is supplied to the developing devices 50A, 50B, 50C and 50D, and the magnetic brush 10 is formed on the magnetic roller 1 shown in FIGS. 8 and 9, and the toner is charged by stirring. The magnetic brush 10 on the magnetic roller 1 is layer-regulated by a spike cutting blade 9, and a potential difference between a direct current (DC) bias 8 a applied to the magnetic roller 1 and a direct current (DC) bias 7 a applied to the developing roller 2. And a thin layer 6 of toner only on the developing roller 2 by the AC biases 7b and 8b.

  Then, when a print start signal is received from a control circuit (not shown), first, the photosensitive member 3 composed of a positively charged organic photosensitive member (positive OPC) is charged to, for example, 400 V by the charger 56, and then, for example, 770 nm. By the exposure by the exposure device 57 using the LED having the wavelength, the post-exposure potential of the photosensitive member 3 becomes about 70 V, and a latent image is formed. The latent image is developed with the toner that has jumped from the toner thin layer 6 on the developing roller 2 to the photosensitive member 3 by a direct current (DC) bias 7a and an alternating current bias 15 applied to the developing roller 2 to form a toner image. Is done.

  When the recording paper is fed from the paper feed cassette 53 and sent by the belt 54 to reach the photosensitive member 3, a transfer bias by the transfer device 58 (A, B, C, D) is applied to the recording paper. The toner image is transferred, fixed by the fixing device 59, and discharged. Thereafter, as described above, the thin toner layer 6 on the developing roller 2 is collected on the magnetic roller 1 by periodically changing the direct current (DC) bias 8a while applying the alternating current (AC) bias 7b.

The developing device in the present invention is changed from the direct current (DC) bias V _dc1 power source 7a and the alternating current (AC) bias V _dc1 power source 7b to the developing roller 2 in the same manner as the developing devices disclosed in Patent Documents 2 and 3. A bias superimposed with a direct current is applied, and a direct current bias is applied to the magnetic roller 1 from a direct current (DC) bias 8 _dc1 power source of 8a. Further, in the present invention, the magnetic roller 1 in this conventional developing device is also supplied with an alternating current (AC) bias V _dc2 power supply of 8b with an alternating current having the same frequency, opposite phase and reverse duty as the developing roller 2. To create an alternating electric field, thereby easily balancing the alternating electric field formed between the developing roller and the photosensitive member and the DC bias applied to the developing roller and the magnetic roller, and ghosting. The problems such as the leakage between the photosensitive member 3 and the developing roller 2, the leakage between the developing roller 2 and the magnetic roller 1, the adhesion of toner to the developing roller 2 and the resulting decrease in image density can be solved. .

Here, the AC bias applied to the developing roller 2 by the alternating current (AC) bias V _dc1 power source 7b and the AC bias applied to the magnetic roller 1 by the alternating current (AC) bias V _dc2 power source 8b are respectively present. The voltage (hereinafter referred to as “area center voltage”) in which the area of the portion taking the value above the voltage is equal to the area of the portion taking the value below (hereinafter referred to as “area center voltage”) is the direct current (DC) bias V _dc1 power supply 7a and direct current ( DC) bias V _dc1 equal to the DC bias supplied by the power supply 8a, that is, {(peak voltage−area center voltage) × pulse width} becomes equal even if the duty ratio changes above and below the area center voltage. A rectangular wave is used.

That is, for example, when a rectangular wave having a Vp _-p of 1600 V and a duty ratio of 30% and a DC bias of 100 V are applied to the developing roller 2, the pulse width of the rectangular wave above the area center voltage (DC bias voltage) and lower Since the ratio of the pulse width on the side is 3: 7, the ratio from the area center voltage (DC bias) to the upper peak voltage and the lower peak voltage is 7: 3, and the upper peak voltage is (1600V × 0 .7) + 100V = 1220V
Lower peak voltage is 100V- (1600V × 0.3) =-380V
It becomes. The same applies to the AC bias applied to the magnetic roller 1.

That is, as in the conventional shown in FIG. 2, for example, developing roller 2 to _V p-p = _1.6kV from an AC power source _{V ac1} and 7b in FIG. 8, the frequency 3.5 kHz, a duty of 30% of the square wave (V Sleeve) Then, +100 V (V sleeve offset) is applied as an offset bias from the DC power supply V _{dc1 of} 7a, and ₊₄₀₀ V (V Magroll offset) is applied as an offset bias from the DC power supply V _{dc2 of} 8a to the magnetic roller 1. A thin layer 6 of toner is formed on the developing roller 2 by the potential difference of +400 V of the V mag roll offset and −380 V of the V sleeve (min), that is, 780 V of the first potential difference 11, and the V sleeve (max). Difference between + 220V and + 400V of V Magroll offset, that is, the second voltage It had peeled development residual toner on the developing roller 2 by a 820V differential 12.

  Further, as shown in FIG. 3, for example, when the main current and exposure are set so that the dark potential 16 when the photosensitive member 3 is charged is + 430V and the bright potential 17 after exposure is + 70V, the V sleeve (max) The difference between +1220 V and +70 V of the photosensitive member (light potential) 17, that is, 1150 V of the third potential difference 13 described above becomes a developing direction potential difference that causes toner to fly from the developing roller 2 to the photosensitive member 3. The difference in potential between the +70 V and the V sleeve (min) of −380 V, that is, the fourth potential difference of 450 V is the potential difference in the toner collection direction from the photosensitive member 3 to the developing roller 2.

  The balance between the development direction potential difference 13 and the collection direction potential difference 14 maintains good image density and dot reproducibility. When the development direction potential difference 13 increases, leakage from the developing roller 2 to the photoreceptor 3 is likely to occur. In some cases, there are problems with the circuit system such as a black spot image, a terrible, a white band, or a black band. Further, the potential difference between +430 V of the photosensitive member (dark potential) 16 and −380 V of the V sleeve (min), that is, the fifth potential difference 15 is a so-called fog removal potential, and acts to prevent the white portion from being stained.

That is, when the first potential difference 11 between the V mag roll offset and the V sleeve (min) is increased, the thin layer 6 is well formed on the developing roller 2 to suppress the generation of ghost, and the V sleeve (max) When the second potential difference 12 with respect to the V mag roll offset is increased, toner adhesion on the developing roller 2 can be prevented. Further, when the third potential difference 13 in the developing direction is increased, leakage from the developing roller 2 to the photosensitive member 3 is likely to occur. Therefore, when the peak _- to _- peak voltage V _p-p in the AC power source V _ac1 of 7b is increased, the first potential difference 11 is increased and the toner thin layer 6 is formed favorably, but conversely, leakage occurs during development. In reality, it is very difficult to balance and there is not enough room.

For example, as shown in FIG. 4, when the voltage of the DC power source V _dc2 of 8a for applying an offset bias (V mag roll offset) to the magnetic roller 1 is + 550V, the V mag roll offset 8a and the V sleeve (min) The first potential difference 11 increases in the hatched portion in the figure and is good against ghost, but the V sleeve (max) and the V mag roll offset 8a, that is, the second in the toner peeling direction on the developing roller 2 are removed. Potential difference 12 becomes smaller, resulting in poor replacement of the toner thin layer 6, and when a large number of sheets are continuously printed, problems such as toner adhesion on the developing roller 2 occur.

As shown in FIGS. 5 and 6, when V _p-p in the AC power source V _ac1 of 7b is 1.9 kV, unlike the case of FIG. 4, the first of the V sleeve (min) and the V mag roll offset is the first. , And the second potential difference 12 between the V mag roll offset and the V sleeve (max) is increased only in the hatched portion in the drawing, which is good against ghosting, and the development residual toner on the developing roller 2 Is sufficiently peeled off and toner does not adhere to the developing roller 2.

  However, the third potential difference 13 in the developing direction shown in FIG. 6 increases only in the hatched portion in the drawing, thereby causing a leak, and this time, the color point and black spot images frequently appear in the white portion of the image under continuous printing evaluation. Will occur. That is, when the AC voltage applied to the developing roller 2 is increased, the action with the photosensitive member 3 is also increased, causing a problem.

Therefore, in the present invention, as described above and as shown in FIG. 1, the alternating current having the same frequency and opposite phase as the alternating current bias applied to the developing roller 2 and the reverse of the duty is applied to the magnetic roller 1. _Applied by an alternating current (AC) bias V _dc2 power supply to form an alternating electric field, thereby causing ghost, leakage between the photoreceptor 3 and the developing roller 2, leakage between the developing roller 2 and the magnetic roller 1, toner to the developing roller 2. Problems such as adhesion and reduction of image density due to the adhesion can be solved.

In other words, by applying an alternating electric field to the magnetic roller 1 by applying an alternating current having the same frequency and opposite phase as that of the alternating current bias applied to the developing roller 2 and having the duty reversed, the photosensitive member 3 and the development are developed. A sixth potential difference 18 corresponding to the first potential difference 11 for forming a toner layer on the developing roller 2 without changing the potential difference between the rollers 2, that is, an alternating current (AC) bias V _dc2 power supply of 8b. The maximum potential value of V due to the voltage difference between the V-magroll (max) and the V-sleeve (min) and the seventh potential difference 19 corresponding to the second potential difference for removing the toner from the developing roller 2, that is, the V-sleeve (max) and 8b exchanges and the potential difference between the (AC) bias _{V dc2} supply by the voltage minimum value V mag roll of (min) becomes both large, the developing roller 2 and the photosensitive member Can be easily balanced with the alternating electric field formed between them and the DC bias applied to the developing roller 2 and the magnetic roller 1, and the toner thin layer 6 is well formed on the developing roller 2 and ghosts are generated. In addition to being reduced, problems such as toner adhesion to the developing roller 2 and density reduction can be solved.

In addition, since the sixth and seventh potential differences corresponding to both the first and second potential differences are increased as described above, the potential difference between the photosensitive member 3 and the developing roller 2 is the same as the conventional one. Leakage between the roller 2 and the photosensitive member 3 and leakage between the developing roller 2 and the magnetic roller 1 can be prevented, and it is possible to prevent a black spot image or an instantaneous bias output from dropping to cause a problem in the circuit system. Further, an alternating electric field formed by these alternating current (AC) bias _{V ac1} source 7b and an AC (AC) bias _{V ac2} power 8b by a rectangular wave, the scraping with the formation of the thin toner layer 6 to the developing roller 2 It can be the optimum duty ratio, and, by reversing relative to the alternating electric field generated by the AC alternating the duty ratio of the alternating electric field by (AC) bias _{V ac2} power 8b (AC) bias _{V ac1} power 7b, these ghosts Further, it is possible to more effectively prevent density reduction due to toner adhesion to the developing roller 2, leakage between the developing roller 2 and the photosensitive member 3, leakage between the developing roller 2 and the magnetic roller 1, and the like.

FIG. 7 shows an alternating bias (AC) bias V _ac1 applied to the developing roller 2 and a direct current bias (DC) bias V _{dc1 supplied from the} power source 7 b and a direct current (DC) bias V _dc2 applied to the magnetic roller 1 _. 7 is a table in which the state of ghost, leak, and image density when the DC bias by the power supply 8a is constant and the peak-to-peak voltage of the alternating current (AC) bias _Vac2 power supply 8b applied to the magnetic roller 1 is changed is examined. In this table, V _p-p is the peak-to-peak voltage of the alternating current (AC) bias V _ac2 power source 8b, and V _magmin : V _slv is the minimum voltage (V _magroll ) of the alternating current (AC) bias V _ac2 power source 8b applied to the magnetic roller 1. (Min)) and the direct current (DC) bias V _dc1 power supply 7 a applied to the developing roller 2, and A is the direct current (DC) bias V _dc2 power supply applied to the magnetic roller 1. The potential difference between the DC bias (V mag roll (offset)) by 8a and the alternating current (AC) bias V _ac1 applied to the developing roller 2 and the minimum value of the alternating bias by the power source 7b (V sleeve (min)), B is to the magnetic roller 1 The maximum AC bias voltage (V _magroll (max)) by the AC (AC) bias V _ac2 power supply 8b to be applied and the current What is the relationship between A and B in terms of the potential difference from the alternating bias minimum voltage (V sleeve (min)) by the alternating current (AC) bias V _ac1 power supply 7b applied to the image roller 2 and A and B? The ◯, shown in the respective columns of the equation indicating ghost, leak, and image density indicate that the ghost, leak, and image density are satisfactory after continuous printing of 10,000 sheets, and × indicates that a problem has occurred. Is shown.

First, Example 1 is a case where the peak _- to _- peak voltage V _p-p of the alternating current (AC) bias V _ac2 power supply 8b applied to the magnetic roller 1 is 600 V, and the image evaluation is also ghost, leak, image after continuous printing of 10,000 sheets. The concentration was also good. The conditions in this case are _Vmagmin : V _slv is −20V <+ 100V and B = 1.23 × A. Similarly, in the second embodiment, the peak _- to _- peak voltage V _p-p of the alternating current (AC) bias V _ac2 power supply 8b applied to the magnetic roller 1 is 700 V, _Vmagmin : V _slv is −90 V <+100 V, and B = 1.27. _XA , in Example 3, the peak _- to _- peak voltage V _p-p of the alternating current (AC) bias V _ac2 power supply 8b applied to the magnetic roller 1 is 750 V, _Vmagmin : V _slv is −125 V <+100 V, and B = 1. 29 × A, in Example 4, the peak _- to _- peak voltage V _p-p of the alternating current (AC) bias V _ac2 power supply 8b applied to the magnetic roller 1 is 500 V, _Vmagmin : V _slv is +50 V <+100 V, and B = 1. 19 × A, in Example 5, the peak _- to _- peak voltage V _p-p of the alternating current (AC) bias V _ac2 power supply 8b applied to the magnetic roller 1 is set to 450 V, and _Vmagmin : V _{The slv} was ₊₈₅ V <+100 V and B = 1.17 × A. In each of Examples 2 to 5, the image evaluation and ghost, leak, and image density after continuous printing of 10,000 sheets were good.

On the other hand, the comparative example 1 is a case where the peak _- to _- peak voltage V _p-p of the alternating current (AC) bias V _ac2 power supply 8b applied to the magnetic roller 1 is 850 V, and _Vmagmin : V _slv is −195 V <+100 V, A And B is B = 1.33 × A. In this case, an image that was not noticeable by ghost was obtained, but in the evaluation after continuous printing of 10,000 sheets, the image density did not decrease, but a leak occurred between the developing roller 2 and the magnetic roller 1. Traces were seen.

The second comparative example 2 is a case where the peak _- to _- peak voltage V _p-p of the alternating current (AC) bias V _ac2 power supply 8b applied to the magnetic roller 1 is 350V, and _Vmagmin : _Vslv is + _155V > + 100V, The minimum voltage of the alternating current (AC) bias V _ac2 power supply 8b applied to the magnetic roller 1 is larger than the direct current bias voltage applied by the direct current (DC) bias V _dc1 power supply 7a applied to the developing roller 2, and the relationship between A and B is B = 1.13 × A. In this case, an image that was not noticeable by ghost was obtained, and no trace of leakage was found even after evaluation of 10,000 sheets of continuous printing, but the image density decreased from about 5000 sheets.

The following comparative example 3 is a case where the peak _- to _- peak voltage V _p-p of the alternating current (AC) bias V _ac2 power supply 8b applied to the magnetic roller 1 is 200V, and _Vmagmin : _Vslv is + 260V> + 100V for comparison. As in Example 2, the minimum voltage of the alternating current (AC) bias V _ac2 power supply 8b applied to the magnetic roller 1 becomes larger than the direct current bias voltage of the direct current (DC) bias V _dc1 power supply 7a applied to the developing roller 2, and A And B became B = 1.08 × A. In this case, the ghost was clearly visually confirmed as a result of the image evaluation, and no leak occurred even in the evaluation after continuous printing of 10,000 sheets, but the image density decreased from about 2000 sheets.

As can be seen from the above results, the minimum voltage of the alternating current (AC) bias V _ac2 power source 8b applied to the magnetic roller 1 and the direct current (DC) bias V _dc1 power source 7a applied to the developing roller 2 are the magnetic roller It can be seen that a better result is obtained when the direct current (DC) bias V _dc1 applied to the developing roller 2 is higher than the minimum voltage of the alternating current (AC) bias V _ac2 applied to 1 and the developing roller 2 is higher than the minimum voltage of the power supply 8b. Further, A is a potential difference between a direct current (DC) bias V _dc2 power source 8a applied to the magnetic roller 1 and an alternating bias (AC) bias V _ac1 power source 7b applied to the developing roller 2 and a minimum voltage of an alternating bias A. And the maximum AC bias voltage applied to the magnetic roller 1 by the alternating current (AC) bias _Vac2 power supply 8b. The potential difference B between the pressure and the minimum voltage of the alternating bias by the alternating current (AC) bias V _ac1 power supply 7b applied to the developing roller 2 is
1.1A ≦ B ≦ 1.3A
It can be seen that better results can be obtained with the above relationship.

As described above, according to the present invention, the toner thin layer 6 is supported on the developing roller 2 by rubbing with the magnetic brush 10 formed of the two-component developer, and the alternating current (AC) bias V _{ac1 is applied} to the developing roller 2. An alternating current (AC) bias V _ac1 is applied to the magnetic roller 1 in the developing device that develops the latent image on the photoreceptor 3 by causing toner to fly from the developing roller 2 with an alternating electric field formed by a rectangular wave formed by the power source 7b. By providing an alternating current (AC) bias V _ac2 power supply 8b for applying an alternating electric field by a rectangular wave having the same frequency as that of the alternating electric field formed by the power supply 7b and having the opposite phase and the reverse duty _cycle , the photosensitive member 3 and the developing roller are provided. 2 does not change anything but the potential difference for forming a toner layer on the developing roller 2, that is, the alternating electric field by the alternating current (AC) bias V _ac1 power source 7b. The sixth potential difference between the small voltage and the maximum voltage of the alternating electric field by the alternating current (AC) bias V _ac2 power supply 8b and the potential difference for removing the toner from the developing roller 2, that is, the alternating current (AC) bias V _ac1 power supply 7b. The seventh potential difference between the maximum voltage of the alternating electric field due to the AC and the minimum voltage of the alternating electric field due to the alternating current (AC) bias V _ac2 power supply 8b increases, and the alternating electric field formed between the developing roller 2 and the photosensitive member 3 Further, it is possible to easily balance the DC bias applied to the developing roller 2 and the magnetic roller 1, the toner thin layer 6 is well formed on the developing roller 2, the ghost is reduced, and the toner to the developing roller 2 is reduced. Adhesion can be prevented and problems such as density reduction can be solved.

In addition, since the potential difference between the photosensitive member 3 and the developing roller 2 is the same as the conventional one in spite of the fact that both the sixth and seventh potential differences are increased as described above, leakage between the developing roller 2 and the photosensitive member 3 Leakage between the developing roller 2 and the magnetic roller 3 can be prevented, and it is possible to prevent a black spot image or an instantaneous bias output from dropping to cause a problem with the circuit system. Further, an alternating electric field formed by these alternating current (AC) bias _{V ac1} source 7b and an AC (AC) bias _{V ac2} power 8b by a rectangular wave, ideal for scraping the formation of the toner layer on the developing roller 2 it is a duty ratio, and an alternating current (AC) by reversing relative to the alternating electric field created by the bias _{V ac2} alternating the duty ratio of the alternating electric field by the power supply 8b (AC) bias _{V ac1} power 7b, these ghost and development Density reduction due to toner adhesion to the roller 2, leakage between the developing roller 2 and the photoreceptor 3, leakage between the developing roller 2 and the magnetic roller 1, and the like can be more effectively prevented.

Further, a direct current (DC) bias V _dc1 power source 7a for applying a direct current bias to the developing roller 2 is provided, and a bias voltage in the direct current (DC) bias V _dc1 power source 7a is determined by the alternating current (AC) bias V _ac2 power source 8b. By making the voltage higher than the minimum voltage of the alternating electric field, it is possible to provide a developing device that can further suppress the toner adhesion to the developing roller 2 and can output a good image without a decrease in density over a long period of time.

A direct current (DC) bias V _dc2 power supply 8a for applying a direct current bias to the magnetic roller 1 is provided, and the minimum voltage of the alternating electric field by the alternating current (AC) bias V _ac1 power supply 7b and the direct current (DC) bias V _dc2 power supply 8a are provided. the potential difference between the bias voltage by a, the AC (AC) when the potential difference between the maximum voltage of the alternating electric field created by the bias _{V ac1} the exchange and minimum voltage of the alternating electric field by the power supply 7b (AC) bias _{V ac2} power 8b is B By making A and B the following relationship, problems such as development leaks can be satisfactorily prevented, image defects such as black spot images, white bands, and black bands, and circuit problems such as momentary bias output drop. It is possible to provide a developing device that prevents the above.
1.1A ≤ B ≤ 1.3A

Here, as described above, an alternating current (AC) bias for applying an alternating electric field by a rectangular wave having the same frequency as that of the alternating electric field formed by the alternating current (AC) bias V _ac1 power source 7b and having the opposite phase and the duty ratio reversed. A _Vac2 power supply 8b is provided in the magnetic roller 1, and the gap between the magnetic roller 1 and the developing roller 2 is changed while an alternating bias is applied to the magnetic roller 1 and the developing roller 2 as shown in FIG. saw. The result is shown in FIG. Here, in the AC bias applied to the developing roller 2, Vpp = 1.6 kV, the duty ratio = 27%, and in the AC bias applied to the magnetic roller 1, Vpp = 0.5 kV (note that the magnetic roller and The gap with the cutting blade was 0.5 mm).

  For comparison, the gap between the magnetic roller 1 and the developing roller 2 was changed while no alternating bias was applied to the magnetic roller 1 (that is, the alternating bias was applied only to the developing roller 2). The result is shown in FIG.

  In FIGS. 11 and 12, a circle indicates a good state, a cross indicates a bad state, and a triangle indicates a good (probably) state. In FIG. 12, the gap between the rollers is 0.36 mm or less, and the developer A leak (agent leak) occurs and a leak occurs. On the other hand, when the gap between the rollers is 0.45 mm or more, a ghost is generated. In the range of 0.36 mm to 0.60 mm between the rollers, the adhesion of the sleeve (remaining toner on the developing roller) was almost satisfactory.

  In FIG. 11, developer leakage occurs when the gap between rollers is 0.36 mm or less, but the gap between rollers is almost satisfactory when the gap between rollers is 0.36 mm to 0.60 mm. When the gap between rollers is 0.38 mm to 0.55 mm (the gap between rollers is 0.76 to 1.10 times the blade gap), almost all of the agent leakage, leakage, ghost, and sleeve adhesion are good. State. Preferably, when the roller gap is 0.45 mm to 0.50 mm (the gap between rollers is 0.90 to 1.00 times the blade gap), agent leakage, leak, ghost, and sleeve adhesion are always good. It can be in a state.

  This means that it is not necessary to strictly define the gap between the developing roller 2 and the magnetic roller 1 in the manufacture of the developing device, which facilitates manufacturing and leads to cost reduction. It was confirmed that when an alternating bias of Vpp = 300V or higher was applied to the magnetic roll 1, no ghost was generated, and when Vpp = 500V was exceeded, it was possible to prevent ghost even if the magnetic brush was shortened.

  Further, the gap between the magnetic roller 1 and the developing roller 2 was set to 0.40 mm, and as described above, an alternating bias was applied to the magnetic roller 1 and the developing roller 2 to examine the current flow into the magnetic roller and the developing roller. . For comparison, the current flow into the magnetic roller 1 and the developing roller 2 was examined without applying an alternating bias to the magnetic roller 1. The result is shown in FIG.

  Referring to FIG. 13, the alternating current Vpp applied to the magnetic roller 1 and the developing roller 2 was changed, and the inflow current value was examined. In FIG. 13, curves L1 and L2 indicate current values flowing into the magnetic roller and the developing roller when no alternating bias is applied to the magnetic roller 1, respectively, and curves L3 and L4 indicate the alternating bias for the magnetic roller 1 and the developing roller 2, respectively. The values of the current flowing into the magnetic roller and the developing roller in the state where is applied are shown.

As can be easily understood from FIG. 13, in the curves L3 and L4, it can be seen that the current value flowing into the magnetic roller and the developing roller increases without changing the gap (for example, the flow into the magnetic roller). The current value increases from 0.5 mA to about 0.7 mA). That is, the current value flowing into the magnetic roller and the developing roller can be increased. In the measurement, the magnetic force generated by the magnetic pole of the magnetic roller facing the spike blade (magnetic blade) was set to 470 G, and the developer conveyance amount in the blade gap was set to 20 to 30 mg / mm ² . The current flowing into the magnetic roller means a current flowing between the magnetic roller and the developing roller, and the current flowing into the developing roller means a current flowing from the photosensitive drum and the magnetic roller to the developing roller. .

  According to the present invention, it is possible to easily balance the alternating electric field formed between the developing roller and the photoconductor and the DC bias applied to the developing roller and the magnetic roller without complicating the developing device, It is possible to provide a developing device in which a toner layer is satisfactorily formed on the developing roller and ghosts are reduced, toner adhesion to the developing roller can be prevented, and problems such as density reduction can be solved.

FIG. 5 is a diagram showing an alternating bias and a DC bias applied to each of a developing roller and a magnetic roller of the developing device in the image forming apparatus according to the present invention. It is a figure for demonstrating an effect | action when an alternating bias and DC bias are applied to a developing roller, and DC bias is applied to a magnetic roller. It is a figure for demonstrating the relationship between the alternating bias applied to the developing roller, the direct current bias, and the photoreceptor potential. It is a figure for demonstrating an effect | action when the voltage of the direct current bias applied to a magnetic roller is made high, leaving the alternating bias and direct current bias applied to a developing roller as it is. It is a figure for demonstrating an effect | action when the peak-to-peak voltage of the alternating bias applied to a developing roller is made high, with the direct current bias voltage applied to a developing roller and a magnetic roller as it is. 7 is a diagram for explaining the relationship between the developing roller and the photosensitive member potential when the peak-to-peak voltage of the alternating bias applied to the developing roller is increased while the DC bias voltage applied to the developing roller is kept as it is. FIG. The state of ghost, leak, and image density when changing the peak-to-peak voltage of the alternating bias applied to the magnetic roller while keeping the alternating bias and DC bias applied to the developing roller and the DC bias applied to the magnetic roller constant was investigated. It is a table. 1 is a schematic diagram of a developing device in an image forming apparatus according to the present invention. 1 is a schematic diagram of an embodiment of an image forming apparatus having a developing device of the present invention. It is a figure which shows the alternating bias applied to a magnetic roller and a developing roller. It is a figure which shows the result of having investigated the state of a developer leak, a leak, a ghost, and a sleeve adhesion at the time of applying an alternating bias to a magnetic roller and a developing roller, and changing the gap of a magnetic roller and a developing roller. It is a figure which shows the result of having investigated the state of a developer leak, a leak, a ghost, and a sleeve adhesion at the time of applying an alternating bias only to a developing roller, and changing the gap of a magnetic roller and a developing roller. It is a figure which shows the inflow electric current value to a magnetic roller and a developing roller when an alternating bias is applied to a magnetic roller and a developing roller, and when an alternating bias is applied only to a developing roller.

Explanation of symbols

_{DESCRIPTION OF SYMBOLS} 1 Magnetic roller 2 Developing roller 3 Photosensitive body (electrostatic photosensitive body) drum 4 Carrier 5 Toner 6 Toner thin layer 7a Direct current (DC) bias V _dc1 power supply 7b Alternating current (AC) bias _Vac1 power supply 8a Direct current (DC) bias _Vdc2 Power supply 8b Alternating current (AC) bias _Vac2 power supply 9 _Panicle cutting blade 10 Magnetic brush 18 _{Potential difference} between V _{mag roll} (max) and V sleeve (min) 19 _Potential difference between V sleeve (max) and V _{mag roll} (min)

Claims (2)

A magnetic roller for holding a magnetic brush formed of a two-component developer having a carrier and toner by a magnetic pole member fixed inside; a developing roller for carrying a toner layer by rubbing with the magnetic brush; A first power source for forming an alternating electric field composed of rectangular waves, and developing the latent image on the photoconductor with the toner that has jumped from the toner layer formed on the developing roller by the alternating electric field. In the developing device in the forming device,
A second power supply is provided for forming an alternating electric field on the magnetic roller, which is formed of a rectangular wave having the same frequency as that of the alternating electric field formed by the first power supply but having the opposite phase and a reversed duty ratio. The developing device in the image forming apparatus. 2. A third power source for applying a DC bias to the developing roller is provided, and a bias voltage at the third power source is made larger than a minimum voltage of an alternating electric field formed on the magnetic roller. A developing device in the described image forming apparatus.

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