JP4421553B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine, and more specifically, a developer including a magnetic carrier and toner is used, and only toner is formed in a thin layer on a developing roll. The present invention relates to an image forming apparatus having a so-called hybrid type developing device that develops toner by causing toner on a developing roll to fly to an electrostatic latent image by an AC superimposed DC voltage.

  For image forming devices such as copiers, printers, facsimiles, and multifunction machines that have these functions using electrophotographic methods, two-component development methods using toner and magnetic carrier, insulating toner and conductive toner are used. 1-component development method, using a 2-component developer that charges a non-magnetic toner using a magnetic carrier, holds only the charged toner on the developing roll, and flies it to the electrostatic latent image. There is a hybrid development system that develops the image.

  The two-component development method has excellent toner chargeability with a carrier, and can have a long lifespan and has the advantage of uniform solid images, but the developing device becomes large and complicated, and toner scattering and carrier pulling occur. However, there is a drawback that 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 has low durability due to deterioration of the developing roll and charge roller, and the cost of consumables is high because the developing device is replaced. There is a disadvantage that selective development occurs. The hybrid development method is a method that has excellent dot reproducibility and can achieve a long life and high-speed image formation. Conventionally, fine powder toner has been added to the developer due to the development ghost and the change in the toner particle size distribution. As a result, the surface of the carrier is contaminated with the fine powder toner, the charge amount is reduced, and there are problems such as scattering of the toner from the developing device and adhesion of the fine powder toner on the developing roll.

  The hybrid developing method has been conventionally studied as a non-contact one-component developing means. However, as a developing method capable of forming a high-speed image in recent years, a plurality of color images are formed on a photoconductor (image carrier). Have been studied for use in a one-drum color superposition method for sequentially forming the. In this method, it is possible to form a color image with little color misregistration by accurately superimposing toner on the photoconductor, and has been attracting attention as a technique for improving the color image quality.

  However, in this one-drum color superposition method, since the developing devices for the number of colors to be used must be arranged around the photoconductor, the photoconductor becomes large, which hinders downsizing of the image forming apparatus. Therefore, a tandem method in which a plurality of electrophotographic process members corresponding to the color of the toner to be used are arranged side by side, a color image is formed in synchronization with the transfer of the transfer member, and the color is superimposed on the transfer member has attracted attention. It was. However, this method has the advantage of being excellent in high speed, but has the disadvantage of increasing the size because the electrophotographic process members for each color must be arranged side by side. As a countermeasure, there has been proposed a small tandem type image forming apparatus in which the intervals between the photosensitive members are narrowed and a downsized image forming unit is arranged.

  In the small tandem type image forming apparatus configured as described above, it is advantageous to make the developing device a vertical type in order to minimize the size of the image forming unit in the width direction. That is, it is desirable in terms of layout that the developing device is arranged in the upper direction of the photosensitive member. However, in the conventional two-component development system, when the developing device is arranged vertically as described above, the reflux of the developer, that is, the supply from the developer agitating unit to the developing member adjacent to the photosensitive member becomes complicated. There has been a problem that there is a limit to downsizing, and carrier adhesion to the photoreceptor and toner scattering are unavoidable.

  As another method, a one-component developing method that does not use a carrier has been proposed, but the method in which the developing roll is in contact with the photosensitive member causes torque fluctuation of the photosensitive member, and promotes color misregistration that is a weak point of the tandem type. There was a drawback. Further, in the method in which the photosensitive member is not contacted, the toner is charged by the charge roll, and the layer thickness on the developing roll is regulated by the elastic regulation blade, so that the toner additive adheres to the charge roll and is charged. In some cases, the ability is reduced, or toner adheres to the regulating blade, resulting in non-uniform layer formation and image defects.

  For this reason, the above-described hybrid development method has attracted attention as one means for solving these problems. In other words, the hybrid developing system can provide a high-speed image forming apparatus that does not cause torque fluctuation because the developing roll is not in contact with the photosensitive member, has excellent dot reproducibility, and can extend the life.

  However, in this hybrid type developing device, coarse particles of highly developable toner are easily selectively developed on the photoconductor, and selective development occurs and the toner particle size distribution changes as described above. The amount of fine toner increases and the surface of the carrier is contaminated to lower the charge amount. There are problems such as scattering of toner from the developing device and reduction of image density due to adhesion of fine toner on the developing roll. Most of the toner on the developing roll is collected by the magnetic brush outside the development time, but if a relatively large amount of fine powder in the toner adheres firmly to the surface of the developing roll, the fine powder is collected between short images during continuous printing. It cannot be left on the developing roll. The adhering fine powder toner remaining on the developing roll causes a decrease in the bias of the developing roll, resulting in a decrease in density, a potential difference with the magnetic roll increases, causing carrier pulling, and image defects and toner scattering. Cause. In addition, when a toner consumption area and a non-consumption area are generated on the developing roll, a part of the previous developed image is changed to the next development due to a variation in the toner adhesion state and the toner potential difference on the developing roll. There is also a problem that a phenomenon that sometimes appears as an afterimage (ghost), that is, a so-called history phenomenon is likely to occur.

Of these, the hysteresis phenomenon (ghost) eliminates the potential difference between the magnetic roll and the developing roll outside the development time and collects most of the toner on the developing roll with a magnetic brush, leaving a development history at the next development timing. A method for obtaining no image has been proposed. A technique disclosed in Patent Document 1 is known as a countermeasure against an increase in fine powder toner due to selective development. This technique includes a first step (first means) for forming a thin toner layer containing a large amount of fine powder toner on the surface of the developing roll when required other than during printing, and a state in which the developing roll is stopped or the developing roll is stepped. The second step (second means) is carried out by applying an AC voltage to the developing roll while repeating rotation and stopping, and causing the toner thin layer containing a large amount of fine powder toner on the surface of the developing roll to fly to the photoreceptor. Thus, an increase in fine toner due to selective development is to be suppressed.
JP 2005-10290 A

  However, the conventional apparatus disclosed in Patent Document 1 has a problem in that it takes a long time to discharge fine powder toner and adjust the toner particle size, which is inefficient. In addition, for this reason, the second process must be in a state in which the developing roll is stopped, or a state in which the developing roll is repeatedly rotated and stopped in steps, and the rotation control of the developing roll becomes complicated. there were.

  SUMMARY An advantage of some aspects of the invention is that an image forming apparatus capable of efficiently discharging fine toner in an apparatus that tends to increase or increase due to selective development and preventing an increase in fine toner is provided. The purpose is to provide.

In order to solve the above-described problems and achieve the above-described object, according to a first aspect of the present invention, there is provided an image forming apparatus, wherein a charged surface is subjected to light irradiation modulated by an image signal and electrostatically received. Adsorbs a developer comprising an image carrier comprising a photoconductor on which a latent image is formed, a developing roll disposed opposite to the image carrier and having a toner thin layer formed on an outer peripheral surface, and a toner and a magnetic carrier. A developing device including a magnetic roll that transports and supplies the toner to the developing roll to form the toner thin layer, and a DC voltage that applies a DC voltage to the magnetic roll and superimposes an AC voltage on the developing roll. And a controller that controls the developing device and the image carrier.

Then, during the printing, the control unit receives the supply of the toner from the magnetic roll, and an AC voltage having a preset duty ratio is superimposed on the developing roll having a thin toner layer formed on the surface . A direct current voltage is applied to cause the toner thin layer on the developing roll to fly to the image carrier, thereby developing the electrostatic latent image, and the toner when required other than during printing. A first toner discharge control unit that forms a thin layer on the developing roll; and after the toner thin layer is formed on the developing roll by the first toner discharge control unit , for the serial developing roll, by applying a DC voltage to an AC voltage superimposed a high set duty ratio than the duty ratio at the time of printing, the developing roll The formed the thin toner layer which is intended to comprise a second toner discharge control unit of flying to the image bearing member.

  According to this configuration, the developer is adsorbed on the magnetic roll, and toner is supplied from the adsorbed developer to the developing roll, whereby a thin toner layer is formed on the developing roll. At the time of printing, development is realized by the development printing control unit forming a thin toner layer on the developing roll and then flying to the image carrier. Through this process, as a result of the toner having a large particle size selectively flying to the image carrier, the ratio of the toner having a small particle size to the toner remaining in the developing device increases. However, when required other than during printing, the remaining toner is collected on the developing roll by the first toner discharge control unit, and a toner thin layer is formed. Further, since the formed toner thin layer is caused to fly to the image carrier by the second toner discharge controller, the toner remaining in the developing device is discharged to the image carrier.

At this time, since the duty ratio of the AC voltage applied to the developing roll is set higher than the duty ratio at the time of printing by the second toner discharge control unit, the toner having a small particle diameter is at a higher ratio than at the time of printing. It is discharged to the image carrier. For this reason, it is possible to efficiently discharge the fine toner in the developing device which tends to increase or increase due to the selective development, and to effectively prevent the increase of the fine powder toner. As a result, when the toner thin layer formed on the developing roll is caused to fly to the image carrier by the second toner discharge controller, the developing roll is not stopped or intermittently rotated in the same manner as during printing. It is also possible to continuously rotate, thereby simplifying the rotation control of the developing roll.
Further, the second toner discharge control unit causes the toner thin layer formed on the developing roll to fly to the image carrier without charging the image carrier, so that the toner thin layer is discharged from the developing roll to the image carrier. Is done efficiently.

According to a second aspect of the present invention, there is provided an image forming apparatus comprising an image bearing member comprising a photosensitive member that forms an electrostatic latent image on a charged surface by receiving light modulated by an image signal. A developer roll disposed opposite to the image carrier and having a toner thin layer formed on the outer peripheral surface thereof, and a developer containing toner and a magnetic carrier, and adsorbed and conveyed to supply the toner to the developer roll. A developing device including a magnetic roll for forming the toner thin layer, and the developing device and the image carrier while applying a DC voltage to the magnetic roll and applying a DC voltage superimposed with an AC voltage to the developing roll. And a control unit for controlling.

Then, the control unit receives a supply of the toner from the magnetic roll during printing, and a DC voltage in which an AC voltage having a preset frequency is superimposed on the developing roll having a thin toner layer formed on the surface thereof. And a developing print control unit that causes the toner thin layer on the developing roll to fly to the image carrier, thereby developing the electrostatic latent image, and the toner thin layer when required other than during printing. A first toner discharge control unit for forming the image bearing member on the developing roll, and after the toner thin layer is formed on the developing roll by the first toner discharge control unit, the developing roller is not charged. relative to a frequency lower than the frequency at the time of printing, applying a DC voltage to an AC voltage of a high duty ratio than the duty ratio is superimposed during printing And by, and is intended to comprise a second toner discharge control unit of flying the toner thin layer formed on the developing roll to the image carrier.

  According to this configuration, as in the image forming apparatus according to the first aspect, the toner having a large particle diameter selectively jumps to the image carrier through the development printing process, and as a result, the small toner occupies the toner remaining in the developing apparatus. The ratio of the particle size toner is increased. However, when required other than during printing, the remaining toner is collected on the developing roll by the first toner discharge control unit, and a toner thin layer is formed. Further, since the formed toner thin layer is caused to fly to the image carrier by the second toner discharge controller, the toner remaining in the developing device is discharged to the image carrier.

At this time, since the frequency of the AC voltage applied to the developing roll is set lower than the frequency at the time of printing by the second toner discharge control unit, the toner having a small particle diameter is imaged at a higher ratio than at the time of printing. It is discharged to the carrier. For this reason, it is possible to efficiently discharge the fine toner in the developing device which tends to increase or increase due to the selective development, and to effectively prevent the increase of the fine powder toner. As a result, when the toner thin layer formed on the developing roll is caused to fly to the image carrier by the second toner discharge control unit, the developing roll is stopped or intermittently rotated in the same manner as at the time of printing. It is also possible to continuously rotate, thereby simplifying the rotation control of the developing roll.
Further, the second toner discharge control unit causes the toner thin layer formed on the developing roll to fly to the image carrier without charging the image carrier, so that the toner thin layer is discharged from the developing roll to the image carrier. Is done efficiently.

Further, according to the second aspect, the frequency of the AC voltage in the DC voltage applied to the developing roller by the second toner discharge control unit is greater than the frequency of the AC voltage in the DC voltage applied to the developing roller during printing. And the duty ratio of the AC voltage in the DC voltage applied to the developing roller by the second toner discharge control unit is set higher than the duty ratio of the AC voltage in the DC voltage applied to the developing roller during printing. ing.

As a result, the toner having a small particle diameter is discharged to the image carrier at a higher rate than during printing. For this reason, the fine powder toner in the developing device that tends to increase or increase due to the selective development can be discharged more efficiently, and the increase of the fine powder toner can be prevented more efficiently.

According to a third aspect of the present invention, there is provided the image forming apparatus according to the first and second aspects, wherein the second toner discharge control unit continuously rotates the developing roller, while The toner thin layer formed on the developing roll is caused to fly to the image carrier.

  According to this configuration, since the second toner discharge control unit causes the toner thin layer formed on the developing roll to fly to the image carrier while continuously rotating the developing roller, the rotation control of the developing roll is simplified. Is done.

  As described above, according to the present invention, it is possible to efficiently discharge fine powder toner in the apparatus that tends to increase or increase due to selective development, and to prevent an increase in fine powder toner.

  FIG. 1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus 1 is configured as a tandem type full-color printer that enables adjustment of toner particle size, and includes a developing device 2, a photoconductor 3 as an image carrier, an exposure unit 4, a conveyance belt 5 for a recording medium, Developer container 6, paper feed cassette 7 for storing a recording medium such as recording paper, charger 8 for charging the photoreceptor 3, and transfer for transferring the toner image on the photoreceptor 3 to the recording medium by a transfer bias. Drives and controls each part of the apparatus 9, a fixing device 10 that fixes the toner image transferred to the recording medium, a cleaning device 11 that removes unnecessary toner remaining on the photoreceptor 3 and collects it in a collection container, and the image forming apparatus 1. A control circuit 40 is provided.

  In the image forming apparatus 1 adopting the tandem type, it is important to design the charger 8, the exposure unit 4, the developing device 2, the transfer device 9, the cleaning device 11, and the like installed around the photoreceptor 3 in a compact manner. In the example of FIG. 1, the developing device 2 is disposed adjacent to the photoreceptor 3 and extending substantially upward.

  FIG. 2 is a schematic configuration diagram of the developing device 2, the photoreceptor 3, and the control circuit 40 included in the image forming apparatus 1. The developing device 2 carries a developing roller 20 that carries a toner thin layer and develops an electrostatic latent image on the photosensitive member 3, and generates a magnetic brush of a two-component developer by a magnet disposed inside the developing roller 20. A magnetic roll 21 as a developer conveying member that supplies toner to the paddle mixer 22 that has a spiral blade and agitates while conveying the developer in the opposite direction, charges the toner, and conveys the toner to the magnetic roll 21; A stirring mixer 23, a spike cutting blade 24 for regulating the thickness of the magnetic brush formed on the magnetic roll 21, a toner sensor 25 for detecting the toner concentration in the developing device 2, and a main body of the developing device. A housing 26 and a partition wall 29 are provided.

  The housing 26 defines a first developer stirring chamber 27 and a second developer stirring chamber 28 inside. The developing device 2 includes a DC bias power source 30 a that applies a direct current (DC) bias to the developing roll 20, an AC bias power source 30 b that similarly applies an alternating current (AC) bias to the developing roll 20, and a direct current ( DC) DC bias power supply 31a for applying a bias is connected.

  The control circuit 40 includes a computer circuit 41 and a drive circuit 42. The computer circuit 41 has a CPU (Central Processing Unit) (not shown) storing a RAM (Random Access Memory) and a ROM (Read Only Memory) that store a program that defines the operation of the CPU, and temporarily. Peripheral devices such as a storage unit such as a RAM for storing data are included. The drive circuit 42 is a power circuit that drives a load that requires power, such as a motor that drives the developing device 2, the photoreceptor 3, the conveyance belt 5, and the like. The drive circuit 42 adjusts power in response to a control signal from the computer circuit 41 and supplies the power to a circuit such as a required motor.

  The control circuit 40 controls each part of the image forming apparatus 1 as described above. As exemplified by the dotted lines in FIG. 2, the control circuit 40 includes, for example, the operation of the photosensitive member 3 including the exposure unit 4 and the charger 8, the operations of the power supplies 30 a, 30 b, and 31 a, and the magnetic roll 21 and the developing roll. The operation of the developing device 2 including 20 is controlled. The control circuit 40 configured as described above functions as the development printing control unit 43, the first toner discharge control unit 44, and the second toner discharge control unit 45. These control units 43 to 45 are realized by both the computer circuit 41 and the drive circuit 42.

  The development printing control unit 43 controls the developing device 2 and other parts during normal printing. The first toner discharge control unit 44 and the second toner discharge control unit 45 discharge the toner having a high ratio of fine powder accumulated in the developing device 2 to the photosensitive member 3 at a time other than printing. The other parts are controlled. Accordingly, the image forming apparatus 1 suppresses an increase in the ratio of fine powder contained in the toner in the developing device 2. The functions of the first toner discharge controller 44 and the second toner discharge controller 45 to be noted will be described in detail later.

  The computer circuit 41 reads and cooperates with the drive circuit 42 to store a program for realizing the above functions in a non-volatile and large-capacity external storage device such as an HDD (not shown). By appropriately transferring to the main storage device such as the RAM, it is possible to use it for execution by the CPU. The program can be supplied through a data recording medium such as a ROM or a CD-ROM, or can be supplied through a transmission medium such as a network connected to the interface unit 9. The transmission medium is not limited to a wired transmission medium, and may be a wireless transmission medium. The transmission medium includes not only a communication line but also a relay device that relays the communication line, such as a router.

  Conventionally, an OPC photoconductor is known as a photoconductor used in an image forming apparatus. However, the OPC photosensitive member has a problem that the surface of the photosensitive layer is soft and the photosensitive layer is easily scraped by rubbing with a cleaning blade. Therefore, since the surface is harder than the OPC photoreceptor, and the durability and function retention (maintenance-free) are excellent, the a-Si (amorphous silicon) photoreceptor having a photosensitive layer thickness of 25 μm or more. Has been used in recent years. However, since the a-Si photosensitive member is formed using a glow discharge decomposition method or the like, if the photosensitive layer is thick as described above, there is a problem that manufacturing time and manufacturing cost are increased, which is economically disadvantageous. .

  When an a-Si photosensitive member is used as the photosensitive material of the photosensitive member 3, the post-exposure potential on the surface thereof is a very low value of 10 V or less. When the film thickness is reduced, the saturation charging potential is lowered, and the withstand voltage leading to dielectric breakdown is lowered. On the other hand, the surface charge density when the latent image is formed tends to be improved and the development performance tends to be improved. This characteristic is particularly remarkable when the relative dielectric constant is as high as about 10 for an a-Si photosensitive member of 25 μm or less, more preferably 20 μm or less. The output voltage of the direct current (DC) bias power source 30a at the time of development is set to 150 V or less, more preferably 100 V or less, Vp-p (potential difference between positive and negative peaks) is set to 500 to 2000 V, and the frequency is set to 1 to 3 kHz. It is possible to develop.

  When a positively charged organic photoconductor (OPC) is used as the photosensitive material of the photoconductor 3, the thickness of the photosensitive layer is set to 25 μm or more in order to reduce the residual potential to 100 V or less, and the charge generating material is added. It is particularly important to increase the amount. In particular, OPC having a single-layer structure is advantageous because a charge generating material is added to the photosensitive layer, and therefore the sensitivity change is small even when the photosensitive layer is reduced. Even in this case, the output voltage of the direct current (DC) bias power supply 30a for development is preferably set to 400 V or less, more preferably 300 V or less, from the viewpoint of preventing a strong electric field from being applied to the toner.

  Setting the developing bias to be low in this way is effective for suppressing the dielectric breakdown of the thin film a-Si photosensitive member, preventing excessive charging of the toner, and suppressing the development history phenomenon. Further, a toner layer of 10 to 100 μm, preferably 30 to 70 μm, is formed on the developing roll 20, and the gap between the developing roll 20 and the photoreceptor 3 is 150 to 400 μm, preferably 200 to 300 μm. A clear image can be obtained by causing the toner to fly on the photoreceptor 3 by applying an alternating electric field in a superimposed manner.

The developer plays a role of supplying and collecting toner, and uses a carrier having a volume resistivity of 10 ⁶ Ωcm to 10 ⁹ Ωcm, and strongly adheres to the developing roll at the nip between the developing roll 20 and the magnetic roll 21. The toner adhering to the surface is peeled off by a magnetic brush carried on a magnetic roll, and the toner necessary for development is supplied. At this time, in order to increase the chance of contact with the toner, it is preferable to use a carrier having a small diameter of 40 μm or less and to increase the surface area per unit volume of the carrier. The low resistance carrier having a volume resistivity of 10 ⁶ Ωcm or less, where the carrier resistance places importance on toner recovery, is effective as a countermeasure against development ghosts, but gives the toner an accurate charge and suppresses the occurrence of fog. It is difficult to maintain the development, and the toner scatters from the surface of the developing roll 20 when operated for a long period of time, causing a problem of contaminating the charger 8 and the exposure unit 4. In a carrier having a volume resistivity of 10 ⁹ Ωcm or more, charging performance can be imparted, but there is a problem that charging tends to increase. By making the resistance value of the carrier appropriate, it is possible to recharge the charged toner to the developing roller 20 while collecting the toner on the developing roller 20. The toner charge amount is controlled to 5 to 20 μC / g to prevent toner scattering and fogging, and by developing at a low electric field, the development history is not left on the developing roll 20 and the toner recoverability is improved. An excellent development system can be provided.

Magnetite carrier, Mn-based ferrite, Mn-Mg-based ferrite, etc. can be used as a high-magnetic force and low-resistance carrier. These carriers may be used as they are, but the surface does not excessively increase the resistance value. It is also possible to process and use. In the present invention, as an example, a ferrite carrier having a volume resistivity of 10 ⁷ Ωcm, a saturation magnetization of 70 emu / g, and an average particle size of 35 μm was used. When the average particle size exceeds 50 μm, the carrier stress increases and the toner concentration cannot be increased, and the amount of toner supplied to the developing roll 20 decreases. By setting the average particle size of the carrier to 50 μm or less, sufficient charge can be imparted even when the toner concentration in the developer is set to 5 to 20%, and stable development can be performed.

  The mixing ratio of the toner and carrier of the developer used in the present embodiment is 2 to 20% by weight, preferably 5 to 15% by weight, based on the total amount of toner and carrier. If the mixing ratio of the toner is less than 2% by weight, the toner charge amount becomes high and a sufficient image density cannot be obtained, and if it exceeds 20% by weight, a sufficient charge amount cannot be obtained. Scattered and contaminated the inside of the image forming apparatus, or toner fog occurs on the image.

  In the image forming apparatus 1, for example, the diameter of the developing roll 20 is 16.8 mm, the diameter of the magnetic roll 21 is 16.0 mm, the diameter of the photosensitive member 3 is 30.0 mm, and the number of rotations of the developing roll 20 and the magnetic roll 21 is The ratio is set so that the rotational speed of the magnetic roll 21 is about 1.5 times the rotational speed of the developing roll 20. The gap between the ear cutting blade 24 and the magnetic roll 21 is about 0.3 to 1.5 mm, and the gap between the magnetic roll 21 and the developing roll 20 is about 0.3 to 1.5 mm. The thin layer of toner on the developing roll is set to a thickness of 10 to 100 μm, preferably 30 to 70 μm. This thickness is a value corresponding to about 5 to 12 layers of toner when the average particle diameter of the toner is 6 μm. The gap between the developing roll 20 and the photoreceptor 3 is 150 to 400 μm, preferably 200 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 to fly to the photoreceptor 3, and a sufficient image density cannot be obtained. Further, it becomes a factor that causes selective development. In the embodiment, the gap between the ear cutting blade 24 and the magnetic roll 21 is 0.55 mm, the gap between the magnetic roll 21 and the developing roll 20 is 0.40 mm, and the thin layer of toner on the developing roll is about It is designed to be maintained at 60 μm.

  First, a normal printing operation of the image forming apparatus 1 will be briefly described. In the following description, a case where positively charged toner is used as an example will be described. However, it is obvious that the present invention can be similarly implemented even when negatively charged toner is used.

  First, a two-component developer composed of toner and carrier corresponding to each color such as yellow, cyan, magenta, and black is stirred and charged by the stirring mixer 23 and the paddle mixer 22 shown in FIG. Is done. Then, a magnetic brush is formed on the magnetic roll 21, the layer thickness is regulated by the pan blade 24, and for example, +400 V by the DC bias power supply 31 a applied to the magnetic roll 21 and a DC bias power supply applied to the developing roll 20. A thin layer of toner only is formed on the developing roll 20 by the potential difference from +100 V, for example, by 30a. The thickness of the toner thin layer on the developing roll 20 varies depending on the resistance of the developer and the rotational speed difference between the developing roll 20 and the magnetic roll 21, but can also be controlled by the above-described potential difference. When the potential difference is increased, the toner layer on the developing roll 20 becomes thicker, and when the potential difference is decreased, the toner layer becomes a thinner layer.

  When a signal for starting printing is transmitted from the control circuit 40, the photosensitive member 3 is first charged to, for example, 400V by the charger 8, and then the post-exposure potential of the photosensitive member 3 is about 70V by exposure by the exposure unit 4. A latent image is formed. This latent image is generated by +100 V by the DC bias power source 30a applied to the developing roll 20 and Vp-p 1.6 kV by the AC bias power source 30b superimposed during development, a frequency of 3.0 kHz, and a rectangular wave with a duty ratio of 30%. Then, the toner is developed with the toner flying from the toner thin layer on the developing roll 20 to the photosensitive member 3 to form a toner image. In order to prevent toner scattering, an AC voltage from the AC bias power supply 30b is applied immediately before development. When the toner image is formed on the photosensitive member 3 in this way, the recording medium is taken out from the paper feed cassette 7 at the timing when the toner image reaches the transfer position, and is conveyed by the conveyor belt 5. Transfer is applied to the recording medium by applying a transfer bias by the transfer device 9 installed at the transfer position. When the toner images of the respective colors are sequentially transferred to the recording medium and reach the fixing device 10, the toner images are fixed and discharged by the fixing device 10.

  When the toner concentration in the developer in the developing device 2 decreases with development, the decrease in toner concentration is detected by the toner sensor 25 and the detection result is transmitted to the control circuit 40. When receiving the detection result, the control circuit 40 drives a replenishing unit (not shown) to replenish the developing device 2 with a necessary amount of toner from the developer container 6.

  The toner remaining on the developing roll 20 is not provided with a special device such as a scraping blade, but the magnetic brush on the magnetic roll 21 comes into contact with the toner layer on the developing roll 20 and the brush due to the difference in peripheral speed between the two rolls. Since the developer is replaced by the effect and stirring by the paddle mixer 22 of the magnetic brush, the toner can be easily collected and replaced. As a method for accelerating the replacement of the developer, the rotational speed of the magnetic roll 21 is set to 1.0 to 2.0 times the rotational speed of the developing roll 20, and the toner on the developing roll 20 is collected. By supplying the developer set to an appropriate toner concentration to the developing roll 20, a uniform toner layer can be formed. Further, in order to prevent an afterimage, during the image formation after development, the + 100V DC voltage of the DC bias power supply 30a is set to + 50V, the duty ratio of the AC supplied from the AC bias power supply 30b is set to 50%, and the magnetic roll 21 is supplied. And the toner on the developing roll 20 may be returned to the magnetic roll 21.

  The developing device 2 shown in FIG. 2 is provided opposite to the photoreceptor 3 capable of holding an electrostatic latent image due to a difference in charging potential, and is provided in a housing 26 that contains a two-component developer. A magnetic roll 21 that magnetically adsorbs a component developer and forms a magnetic brush on the surface, a pan blade 24 that regulates the amount of developer adsorbed on the magnetic roll 21, the magnetic roll 21 and the photoreceptor 3 and a cylindrical developing roll 20 which is disposed so as to be close to each other at a different position and rotates with toner attached thereto. A first developer agitating chamber 27 is provided in the housing 26 at a position adjacent to the magnetic roll 21, and a second developer agitating chamber partitioned by the first developer agitating chamber 27 and the partition wall 29. A chamber 28 is provided. The first developer agitating chamber 27 and the second developer agitating chamber 28 communicate with each other at both ends in the axial direction of the magnetic roll 21, and respectively agitate the developer and developer in the axial direction. A paddle mixer 22 and a stirring mixer 23 are provided. These two mixers 27 and 28 are set to rotate in such a manner as to convey the developer in opposite directions, and the developer circulates between the first developer stirring chamber 27 and the second developer stirring chamber 28. It is supposed to be.

  The magnetic roll 21 has a rotatable cylindrical sleeve arranged on the outer periphery of a magnet supported so that its position is fixed, and a two-component developer containing toner and a magnetic carrier is magnetically applied to the sleeve surface. The magnetic brush of the two-component developer can be formed by suction and the sleeve can be rotated to convey the developer. A DC voltage of +400 V can be applied to the magnetic roll 21 from a direct current (DC) bias power supply 31a for the magnetic roll. The developing roll 20 is made of an aluminum cylinder whose surface is anodized, a cylinder made of phenol resin, or a resin layer formed on the surface of an aluminum cylinder, and a direct current (DC) bias power supply 30a. To +50 to +200 V direct current and alternating current (AC) bias power supply 30 b can apply a bias in which Vp-p 1.6 kV and alternating current of frequency 3.0 kHz are superimposed.

  The developer is conveyed and agitated in the first developer agitating chamber 27 and the second developer agitating chamber 28, and the distribution of the toner in the developer is kept uniform and the toner and the carrier are mixed. A predetermined charge is imparted to the toner due to frictional charging therebetween. The agitated developer is supplied to the magnetic roll 21 by the paddle mixer 22, and the magnetic roll 21 magnetically attracts the developer to form a developer magnetic brush composed of toner and carrier, and develops by rotation. It is conveyed to a position facing the roll 20.

  As described above, a + 100V DC voltage from the direct current (DC) bias power supply 30a is applied to the developing roll 20, and a + 400V DC voltage is applied to the magnetic roll 21 from the magnetic roll direct current (DC) bias power supply 31a. Therefore, an electric field is formed between the developing roll 20 and the magnetic roll 21 by these applied voltages, and the positively charged toner is transferred to the developing roll 20, and a toner layer is formed on the surface of the developing roll 20. On the other hand, the carrier remains magnetically attracted to the magnetic roll 21 without being transferred.

  The toner layer formed on the developing roll 20 is conveyed to the developing area by the rotation of the developing roll 20, and a + 100V DC voltage applied to the developing roll 20 and Vp-p 1.6 kV applied at the time of development, frequency 3 Development is carried out by flying in the electrostatic field between the photosensitive member 3 and the developing roll 20 by a bias superimposed with alternating current by a rectangular wave of 0.0 kHz and a duty ratio of 30%, and adhering to the surface of the photosensitive member 3. At this time, the toner having a large particle diameter is more easily peeled off from the developing roll 20 than the toner having a small particle diameter, and is easily transferred to the photoreceptor 3. For this reason, the toner that does not contribute to the development and remains on the developing roll 20 and is returned to the portion facing the magnetic roll 21 again has a small particle size.

  FIG. 3 is a waveform diagram illustrating waveforms of voltages applied to the developing roll 20 and the magnetic roll 21 when printing is normally performed. In the example of FIG. 3, a DC voltage of +100 V from a direct current (DC) bias power supply 30a, a Vp-p1.6 kV from an alternating current (AC) bias power supply 30b, a frequency of 3.0 kHz, and a rectangular wave with a duty ratio of 30%. A voltage superimposed with alternating current is applied to the developing roll 20. Here, the duty ratio is a ratio of the positive voltage period T1 to the sum of the positive voltage period T1 and the negative voltage period T2, and is given by T1 / (T1 + T2). Further, a DC voltage of, for example, +400 V is applied to the magnetic roll 21 from a direct current (DC) bias power supply 31a. The development printing control unit 43 (FIG. 2) controls the developing device 2, various power supplies 30a, 30b, 31a, the photoreceptor 3, the transport belt 5, the transfer device 9 and the like when printing is performed normally.

  It should be noted that the control circuit 40 includes the first toner discharge control unit 44 and the second toner discharge control unit 45 as described above. The first toner discharge control unit 44 performs normal printing for a certain period of time, such as after continuous printing or after a predetermined number of printings, by the function of the development printing control unit 43. Every time, the developing device 2 and the like are controlled so that toner containing a large amount of fine powder toner is collected on the developing roll 20. This process is temporarily referred to as the first process.

  For example, the first toner discharge control unit 44 controls the developing device 2, the power sources 30a, 30b, and 31a, the photoconductor 3 and the like under the same conditions as in printing. Therefore, in the first step, the waveform of the voltage applied to the developing roll 20 is given by, for example, the waveform diagram of FIG. 3, and is a +100 V DC voltage, Vp-p 1.6 kV, frequency 3.0 kHz, duty ratio 30%. The AC voltage generated by the rectangular wave is superimposed. For example, a DC voltage of +400 V is applied to the magnetic roll 21.

  The first toner discharge control unit 44 applies the voltage having such a waveform to the developing roll 20 and the magnetic roll 21 while the developing roll 20 normally rotated about 182 times per minute rotates about 45 to 75 times (about about 15 to 25 seconds), a toner layer is formed on the peripheral surface of the developing roll 20 (first step). Since the fine toner has a large charge amount per unit volume, the toner having a large particle diameter is detected by the voltage Vf (FIG. 3) applied in the direction of moving the toner from the magnetic roll 21 to the developing roll 20 in the first step. In comparison, it is more strongly attracted to the developing roll 20. The toner having a large particle diameter is collected by the magnetic brush of the magnetic roll 21 by the voltage Vb returned from the developing roll 20 to the magnetic roll 21, but the fine powder toner having a small particle diameter is difficult to collect, and the fine powder on the developing roll 20 is not collected. Toner is deposited. Note that the first toner discharge control unit 44 may collect finer toner on the developing roller by setting the duty ratio lower than that during printing.

  When the first step is completed, the second toner discharge control unit 45 controls the developing device 2 and the like so that the toner layer containing a large amount of fine toner collected on the developing roll 20 flies to the photoreceptor 3. This process is tentatively referred to as the second process. In the second step, the waveform of the voltage applied to the developing roll 20 is set so that the duty ratio of the AC voltage is higher or the frequency is lower than the waveform during printing. As will be shown later, by setting the duty ratio and frequency of the AC voltage applied to the developing roll 20 in this way, among the thin toner layers on the developing roll 20, those having a small particle size are printed. It becomes possible to discharge to the photoreceptor 3 at a higher ratio. It is more desirable to set both to be different from those during printing rather than only one of the duty ratio and frequency. Thereby, the small particle size is more effectively discharged to the photoreceptor 3.

  As an example, the waveform of the voltage applied to the developing roll 20 in the second step is assumed to be obtained by superimposing a +100 V DC voltage and an AC voltage generated by a rectangular wave of Vp-p 1.6 kV as shown in FIG. Although given, the frequency of the AC voltage is set to 2.0 kHz lower than 3.0 kHz, and the duty ratio is set to 40% higher than 30%. In order to promote the flying of the toner thin layer from the developing roll 20 to the photoconductor 3, the DC voltage may be set to a value higher than + 100V, for example, + 200V. For example, a DC voltage of +400 V is applied to the magnetic roll 21.

  The second toner discharge control unit 45 continuously rotates the photoreceptor 3, the developing roll 20, and the magnetic roll 21 in the same manner as during printing while applying a voltage having such a waveform to the developing roll 20 and the magnetic roll 21. In order to promote the flying of the toner thin layer from the developing roll 20 to the photoreceptor 3, the second toner discharge controller 45 desirably turns off the charger 8 that charges the photoreceptor 3. Alternatively, the second toner discharge control unit 45 keeps the charger 8 turned on, and causes the exposure unit 4 to draw a latent image of a solid image on the photoconductor 3, for example, on the developing roll 20 toward the latent image. The toner thin layer may be allowed to fly. However, it is more desirable to turn off the charger 8 because the surface potential of the photoreceptor 3 can be lowered, unnecessary power can be reduced, and control is easy.

  The second toner discharge control unit 45 continues such a second process for about several seconds, for example. Thereby, among the toner thin layers on the developing roll 20, those having a small particle diameter are discharged to the photoreceptor 3 at a high ratio. The second toner discharge control unit 45 turns off the transfer device 9 while executing the second step. For this reason, the toner thin layer that has jumped to the photoreceptor 3 in the second step is removed by the cleaning device 11 without being transferred to the conveyor belt 5 and thus without contaminating the conveyor belt 5.

  4-6 is a graph which shows the result of the verification experiment about a 1st process and a 2nd process. In these drawings, when the duty ratio, frequency, and voltage (Vp-p) in the AC voltage applied to the developing roll 20 in the second step are changed, the developing device 2, the developing roll 20 and the photosensitive roll are exposed. The actual measurement result of the particle size distribution of the toner having an average particle size of 6 μm on the body 3 is shown. In the experiment, a DC voltage of +400 V was applied to the magnetic roll 21, and the charger 8 was turned off. In each of these drawings, the characteristic curve DEV indicates the toner particle size distribution in the developing device 2, the characteristic curve ROL indicates the toner particle size distribution on the developing roll 20, and the characteristic curve DRM indicates the toner particle size distribution on the photoreceptor 3.

  FIG. 4 shows a toner particle size distribution when the duty ratio of the AC voltage is changed in a state where the DC voltage of 100 V applied to the developing roll 20 together with the AC voltage and the frequency of the AC voltage of 3.0 kHz are kept constant. It is a graph. 4A shows the toner particle size distribution at a duty ratio of 18%, FIG. 4B shows the duty ratio of 27%, and FIG. 4C shows the toner particle size distribution at a duty ratio of 38%. That is, FIG. 4 (b) shows the result under conditions substantially corresponding to the preferable conditions at the time of printing already exemplified, and FIG. 4 (a) shows the result of setting the duty ratio lower than that. On the other hand, the result of setting it high is shown in FIG.

  As can be seen by comparing the characteristic curve DEV and the characteristic curve ROL in FIG. 4B, the toner particle size distribution on the developing roll 20 when the duty ratio of the AC voltage is set to 27% is the toner in the developing device 2. It approximates the particle size distribution, and no significant selectivity for particle size is observed. However, as can be seen by comparing the characteristic curve DRM in FIG. 4B with other curves, coarse toner particles having a large particle size are selectively collected in the toner thin layer on the photoreceptor 3. That is, selective development on the photosensitive member 3 is also recognized in the experimental data.

  On the other hand, as shown in FIG. 4A, even when the duty ratio is set to 18%, the toner thin layer on the developing roll 20 shows almost no selectivity, while the photosensitive member 3 has On the other hand, the occurrence of selective development is observed. Further, as can be seen from a comparison between FIG. 4A and FIG. 4B, the selective development on the photosensitive member 3 is more remarkable in FIG. 4A where the duty ratio is lower. On the other hand, as shown in FIG. 4C, when the duty ratio is set to 38%, the toner thin layer on the developing roll 20 shows almost no selectivity, but the selection with respect to the photoreceptor 3 is performed. Development has almost disappeared. That is, no change is observed in the toner particle size distribution in the developing device 2 and between the developing rolls 20 with respect to the duty ratio, while the duty ratio is increased to promote the flying of fine powder toner from the developing roll 20 to the photoreceptor 3. It became clear that it was possible.

  FIG. 5 is a graph showing the toner particle size distribution when the frequency of the AC voltage is changed in a state where the DC voltage 100V applied to the developing roll 20 together with the AC voltage and the duty ratio of 27% of the AC voltage are kept constant. It is. 5A shows a toner particle size distribution at a frequency of 2.0 kHz, FIG. 5B shows a frequency of 3.0 kHz, and FIG. 5C shows a toner particle size distribution at a frequency of 4.0 kHz. That is, FIG. 5 (b) is the same as FIG. 4 (b), and FIG. 5 (a) shows the result of setting the frequency lower than that, and FIG. 5 (c) shows the result of setting the frequency higher. It shows.

  Through FIG. 5A to FIG. 5C, the toner particle size distribution on the developing roll 20 is close to the toner particle size distribution in the developing device 2, and no significant selectivity to the particle size is recognized. On the other hand, when the frequency is set to 20 kHz as shown in FIG. 5A, the selective development on the photoreceptor 3 is suppressed as compared with the case of FIG. On the contrary, when the frequency is set to 40 kHz as shown in FIG. 5C, the selective development on the photosensitive member 3 is strengthened compared to the case of FIG. That is, no change is observed in the toner particle size distribution in the developing device 2 and between the developing rolls 20 with respect to the frequency, while the frequency is lowered to promote the flying of fine toner from the developing roll 20 to the photoreceptor 3. It became clear that it was possible.

  FIG. 6 is a graph showing the toner particle size distribution when the DC voltage superimposed on the AC voltage is changed in a state where the frequency of AC voltage is 3.0 kHz and the duty ratio is 27%. 6A shows a toner particle size distribution at a DC voltage of 50 V, FIG. 6B shows a DC voltage of 100 V, and FIG. 6C shows a DC voltage of 180 V, respectively. That is, FIG. 6 (b) is the same as FIG. 4 (b) and FIG. 5 (b), and FIG. 6 (a) shows the result of setting the DC voltage lower than that. The result is shown in FIG.

  In FIGS. 6 (a) and 6 (b) where the DC voltage is low, almost no selectivity for the particle size of the toner thin layer on the developing roll 20 is observed, whereas in FIG. 6 (c) where the DC voltage is high. The particle size distribution on the developing roll 20 is shifted to a lower particle size than the particle size distribution of the developing device 2. That is, it can be seen that when the DC voltage is increased, selectivity appears with respect to the particle size of the toner thin layer on the developing device 2. On the other hand, regarding the selective development on the photosensitive member 3, it is difficult to recognize a significant difference from each other through FIGS. 6 (a) to 6 (c).

  From the experimental results shown in FIGS. 4 to 6, in order to effectively discharge the fine powder contained in the toner to the photosensitive member 3, the AC voltage applied to the developing roll 20 is compared with the value at the time of printing. It can be concluded that it is desirable to set the duty ratio high and it is desirable to set the frequency low.

  FIG. 7 is a graph showing actual measurement results of the change in the amount of fine powder toner on the developing roll 20 when the duty ratio of the AC voltage applied to the developing roll 20 is changed in the first step. The graph shows (a) when only a direct current is applied between the developing roll 20 and the magnetic roll 21, (b) when an alternating current with a duty ratio of 50% is superimposed, and (c) when an alternating current with a duty ratio of 30% is superimposed. , The amount of fine powder toner of 5 μm or less deposited on the developing roll 20 is shown. In the graph, the horizontal axis represents the rotation time (seconds) of the developing roll 20, and the vertical axis represents the frequency of fine powder toner of 5 μm or less, that is, the number ratio (%). As is apparent from the graph, the amount of fine powder toner hardly increases when only a direct current is applied, and the amount of fine powder toner increases when an alternating current is applied, but the duty ratio is 50% as in the first step described above. It is understood that a duty ratio of 30% is more desirable because it collects more fine toner.

  FIG. 8 shows the actual measurement of the change in the amount of fine powder toner when the durable printing test (referred to as “printing durability”) is performed for the case where the toner is discharged in the first and second steps and the case where the toner is not discharged. It is a graph which shows a result. In the graph, (a) when continuous printing is performed without performing the first and second steps, and (b) when the first and second steps are performed every time continuous printing is performed for 100 recording media, The experimental result about each of is shown. In FIG. 8, the horizontal axis represents the number of printed sheets in the durable printing test (referred to as “printing durability” number), and the vertical axis represents the number ratio (%) of fine powder toner of 5 μm or less, as in the vertical axis of FIG. ing.

  In the case of (b), in the first step, a voltage obtained by superimposing a DC voltage of +100 V and an AC voltage by a rectangular wave of Vp−p 1.6 kV is applied to the developing roll 20, and the frequency of the AC voltage is It was set to 3.0 kHz and the duty ratio was set to 30%. A DC voltage of +400 V was applied to the magnetic roll 21. Further, the photosensitive member 3 was charged to +400 V by the charger 8, and a latent image was not formed. On the other hand, the second step is the same as the first step except that the duty ratio of the AC voltage applied to the developing roll 20 is changed to 38%, the frequency is changed to 2.0 kHz, and the charger 8 is turned off. Met. Unlike the prior art disclosed in Patent Document 1, the developing roll 20 was continuously rotated in the second step as in the normal printing step and the first step. Regarding the execution time for every 100 sheets of continuous printing, the first step was about 20 seconds and the second step was about 2 seconds.

  As is apparent from the graph of FIG. 8, (a) when the toner is not discharged in the first and second steps, the amount of fine powder toner exceeds 15% after printing 20000 sheets, (B) When the toner is discharged in the first and second steps, the amount of fine powder toner is about 8.5% even after printing 20000 sheets, and a significant increase from 7% of the initial value. It can be seen that the increase is suppressed to the extent that is not recognized.

  As described above, even if the selective development that a large particle size toner is consumed in the process of printing is performed, the developing device 2 is processed by processing the fine toner in the developer through the first and second steps. It was proved that only small-diameter toner is not accumulated in the toner, the toner particle size is kept constant, and it is possible to obtain a good image while suppressing deterioration in image quality.

  Regarding the timing at which the first and second steps are executed, for example, as in the verification test of FIG. 8, the control circuit 40 performs the first and second steps each time 100 sheets of recording media are printed. Can be set. When the specified 100th sheet is reached during continuous printing, it is desirable to set the control circuit 40 to execute the first and second steps after the continuous printing is completed. The control circuit 40 may newly start counting the number of printed sheets after executing the first and second steps. As another example, it is possible to set the control circuit 40 so that the first and second steps are executed every time the developing roll 20 rotates for a predetermined time. Each time the toner consumption amount obtained by multiplying the number of recording media on which printing has been performed by the average printing rate (the ratio of the print image portion in the recording medium) reaches a predetermined value, the first and second It is also possible to set the control circuit 40 so as to execute the process.

  The image forming apparatus of the present invention is particularly suitable for a tandem type image forming apparatus, and the image forming apparatus 1 is configured as such a tandem type full-color printer. However, the image forming apparatus of the present invention is a tandem type. It is not limited, it is not limited to a full color type, and is not limited to a printer dedicated machine. The present invention can be widely applied to an image forming apparatus such as a printer, a facsimile machine, a copying machine, and a multifunction machine having these functions as long as it has a hybrid developing device.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a schematic configuration diagram of a developing device, a photoreceptor, and a control circuit included in the image forming apparatus of FIG. 1. It is a wave form diagram which illustrates the waveform of the voltage applied to a development roll and a magnetic roll when printing is performed normally. It is a graph which shows the result of the demonstration experiment about a 1st process and a 2nd process, and has shown especially the influence of the duty ratio of an alternating voltage. It is a graph which shows the result of the verification experiment about a 1st process and a 2nd process, and has shown especially the influence of the frequency of alternating voltage. It is a graph which shows the result of the verification experiment about a 1st process and a 2nd process, and has shown the influence of DC voltage especially. It is a graph which shows the measurement result of transition of the amount of fine powder toner on a development roll when changing the duty ratio of the alternating voltage applied to a development roll in the 1st process. It is a graph which shows the actual measurement result of transition of the amount of fine powder toner when a durable printing test is done about the case where it is the case where discharging of the toner by the 1st and 2nd processes is not so.

Explanation of symbols

2 Developing device 3 Photoconductor (image carrier)
8 Charger 20 Developing Roll 21 Magnetic Roll 30a DC Bias Power Supply 30b AC Bias 31a DC Bias Power Supply 40 Control Circuit 43 Development Printing Control Unit 44 Toner Discharge Control Unit 45 Toner Discharge Control Unit

Claims (3)

An image bearing member made of a photosensitive member on which an electrostatic latent image is formed by receiving light irradiation modulated by an image signal on a charged surface;
A developer roll that is disposed opposite to the image carrier and has a thin toner layer formed on the outer peripheral surface thereof, and a developer containing toner and a magnetic carrier is adsorbed and conveyed, and the toner is supplied to the developer roll to supply the toner. A developing device including a magnetic roll for forming a toner thin layer;
A controller for controlling the developing device and the image carrier while applying a DC voltage to the magnetic roll and applying a DC voltage superimposed with an AC voltage to the developing roll;
The control unit
By applying a DC voltage on which an alternating voltage having a preset duty ratio is superimposed to the developing roll having a thin toner layer formed on the surface by receiving the supply of toner from the magnetic roll during printing A development printing control unit for causing the toner thin layer on the developing roll to fly to the image carrier, thereby developing the electrostatic latent image;
A first toner discharge controller for forming the toner thin layer on the developing roll when required other than during printing;
After the toner thin layer is formed on the developing roll by the first toner discharge controller, the duty ratio is set higher than the duty ratio during printing for the developing roll in a state where the image carrier is not charged. And a second toner discharge control unit that causes the toner thin layer formed on the developing roll to fly to the image carrier by applying a DC voltage on which an AC voltage with a different duty ratio is superimposed. An image forming apparatus. An image bearing member made of a photosensitive member on which an electrostatic latent image is formed by receiving light irradiation modulated by an image signal on a charged surface;
A developer roll that is disposed opposite to the image carrier and has a thin toner layer formed on the outer peripheral surface thereof, and a developer containing toner and a magnetic carrier is adsorbed and conveyed, and the toner is supplied to the developer roll to supply the toner. A developing device including a magnetic roll for forming a toner thin layer;
A controller for controlling the developing device and the image carrier while applying a DC voltage to the magnetic roll and applying a DC voltage superimposed with an AC voltage to the developing roll;
The control unit
By applying a DC voltage on which an AC voltage having a preset frequency is superimposed to the developing roll having a toner thin layer formed on the surface by receiving the supply of toner from the magnetic roll during printing, A development printing control unit that causes a toner thin layer on the developing roll to fly to the image carrier, thereby developing the electrostatic latent image;
A first toner discharge controller for forming the toner thin layer on the developing roll when required other than during printing;
After the thin toner layer on the developing roll by the first toner discharging control portion is formed, in a state not to charge the image bearing member, to said developing roller, met frequency lower than the frequency at the time of printing Second toner discharge for causing the toner thin layer formed on the developing roll to fly to the image carrier by applying a DC voltage on which an AC voltage with a duty ratio higher than the duty ratio during printing is applied. An image forming apparatus comprising: a control unit; The second toner discharging control unit is configured with the developing roller rotated continuously, according to the thin layer of toner formed on the developing roll to claim 1 or claim 2 are caused to fly to the image bearing member image Forming equipment.

Images