JP2009086112A - Developing apparatus and image forming apparatus mounted with the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a developing apparatus that eliminates the difference between a charge amount of toner during developing and a charge amount of the toner in a non-developing period, and to provide an image forming apparatus mounted with the device. <P>SOLUTION: The developing apparatus includes: a housing having an opening opposing to an image carrier; a magnetic roller (50) having a magnet for forming a magnetic brush including a carrier and a toner; a developing roller (60) having a magnet (66) for forming a toner thin layer transferred from the magnetic brush, the developing roller disposed at the opening and approximately parallel to the rotation axial line of the magnetic roller; and a magnetic force adjusting means (92) adjusting the position of the main pole angle of the magnet for forming the toner thin layer. The magnetic force adjusting means moves the position of the main pole angle of the magnet for forming the toner thin layer toward an upstream side with respect to the rotation direction of the developing roller during developing, and moves the position from the upstream side to a downstream side in a non-developing period. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a developing device using a two-component developer having a carrier and a toner, and an image forming apparatus equipped with the developing device.

In this type of developing device, a toner image is developed on the surface of a photosensitive drum on which an electrostatic latent image is formed, and the toner image is transferred and fixed on a sheet.
Here, the developing device includes a method in which a two-component developing method and a one-component developing method are combined. Specifically, in this method, a non-magnetic toner is charged using a magnetic carrier, only this toner is supplied to the drum, and a toner image is developed on the surface thereof.

  A technique using the method is disclosed (see, for example, Patent Document 1), which includes a developing roller facing the drum and a magnetic brush roller facing the roller. The brush roller includes a carrier and a roller. A magnet that forms a magnetic brush made of toner is provided. A thin layer of only toner is formed on the developing roller.

JP 2003-280357 A

By the way, the density of the magnetic brush can be increased by providing a magnet on the developing roller as compared with the conventional technique.
However, when a magnet is simply provided on the developing roller, there is a problem that image distortion occurs.
Specifically, even if the magnet on the developing roller side is configured to be swingable within the developing roller, the magnet is disposed near the position where the gap between the developing roller and the magnetic brush roller forms the shortest distance. Then, of the developer conveyed by the brush roller, the developer that cannot pass through the gap stays in front of the gap, which disturbs the toner layer on the developing roller or causes the developer to leak. . On the other hand, if the magnet is disposed at a position apart from the gap, the toner layer is hardly scraped off by the brush, and a development ghost is generated at the next development.

This image disturbance is caused by a difference between the toner charge amount during development and the toner charge amount during non-development. In other words, some measure is required to eliminate the difference in charge amount between these toners, but the above-described conventional technology does not give any special consideration to this point.
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a developing device that eliminates the above-mentioned problems and eliminates the difference between the toner charge amount during development and the toner charge amount during non-development, and an image forming apparatus equipped with the same. is there.

  A first invention for achieving the above object is a developing device for developing a toner image by charging a toner using a carrier and supplying the toner to the image carrier, and having an opening facing the image carrier. A housing having a magnetic roller having a magnet for forming a magnetic brush formed of a carrier and toner, and a housing having a magnet and a magnetic roller formed in parallel with the rotation axis of the magnetic roller at an opening; A developing roller having a magnet for forming a thin layer of toner transferred from the magnetic brush; and a magnetic force adjusting means for adjusting the position of the main pole angle of the magnet for forming the thin layer of toner. When developing the toner image, the position of the main pole angle of the magnet that forms the toner thin layer is moved toward the upstream side when viewed in the rotation direction of the developing roller, while the toner image is developed. If not, the position of the main pole angles of the magnet to form a thin layer of toner is moved from the upstream side toward the downstream side.

  According to the first invention, a two-component developer composed of a carrier and a toner is used, and a magnetic brush made of this developer is formed on the magnetic roller. Specifically, this brush is formed by a magnet in a magnetic roller, and causes a thin layer of toner to be formed on the developing roller. The toner image is developed by supplying the toner to the image carrier. On the other hand, this brush scrapes off and collects the toner layer of the developing roller after development.

Here, when a magnet is simply provided on the developing roller as described above, a toner thin layer can be satisfactorily formed on the developing roller, but the toner charge amount during development and the toner charge amount during non-development There will be a difference.
However, since the magnetic force adjusting means moves the position of the main pole angle of the magnet toward the upstream side when viewed in the rotation direction of the developing roller at the time of development, the toner charge amount at the time of development is smaller than that of the conventional case. Although less, this toner is difficult to leak from the developing roller. Therefore, the toner layer is likely to be present on the developing roller, and image density defects are suppressed.

On the other hand, the magnetic force adjusting means moves the position of the main pole angle of the magnet from the upstream side to the downstream side during non-development. Become more. Therefore, the toner layer is easily scraped off by the brush, and the occurrence of development ghost is suppressed.
As a result of the elimination of the difference between the toner charge amount during development and the toner charge amount during non-development, image disturbance can be avoided.

According to a second aspect of the present invention, in the configuration of the first aspect, when the magnetic force adjusting means develops the toner image, the position of the main pole angle of the magnet for forming the toner thin layer is determined in the rotation direction of the developing roller. It is characterized by being set to a position moved about 10 ° or more upstream as viewed.
According to the second invention, in addition to the action of the first invention, the magnetic force adjusting means further comprises about 10 upstream of the position of the main pole angle of the magnet in the rotation direction of the developing roller during development. Since the position is set at a position moved by more than 0 °, the toner layer is surely present on the developing roller, and image density defects can be reliably suppressed.

According to a third aspect of the present invention, in the configuration of the first or second aspect, when the magnetic force adjusting means does not develop the toner image, the position of the main pole angle of the magnet that forms the thin toner layer is set to the position of the developing roller. It is characterized in that it is set at a position moved about 5 ° or less upstream in the rotational direction.
According to the third aspect of the invention, in addition to the effects of the first and second aspects, the magnetic force adjusting means, when not developing, can be seen upstream by looking at the position of the main pole angle of the magnet in the rotation direction of the developing roller. Therefore, the toner layer is surely scraped off by the brush, and development ghosts can be reliably suppressed.

A fourth invention is characterized in that, in the configuration of the third invention, the developing roller is not applied with a predetermined AC bias when the toner image is not developed.
According to the fourth invention, in addition to the action of the third invention, when a developing bias, that is, an AC bias superimposed with a predetermined DC bias is applied, the toner layer of the developing roller is directed toward the image carrier. However, since the AC bias is not applied during non-development, the toner layer of the developing roller is difficult to fly toward the image carrier. Therefore, the toner layer is more reliably scraped off by the brush.

According to a fifth aspect of the present invention, in the configurations of the first to fourth aspects of the invention, the position of the main pole angle of the magnet that forms the magnetic brush is about 5 ° or more on the downstream side in the rotation direction of the magnetic roller, and about It is characterized by being set at a position moved within a range of 15 ° or less.
According to the fifth invention, in addition to the actions of the first to fourth inventions, the position of the main pole angle of the magnet is about 5 ° or more downstream in the rotation direction of the magnetic roller. The position is set within a range of about 15 ° or less. This is because when the angle is less than about 5 °, a large amount of current flows through the developing roller, and the toner is likely to be scattered.
However, if the angle exceeds about 15 °, the magnetic force becomes small and the toner layer formed on the developing roller cannot be collected. Therefore, as described above, when the value is set within the range of about 5 ° to about 15 °, an appropriate magnetic brush having high density and strong scraping force is formed.

A sixth invention is characterized in that, in the configurations of the first to fifth inventions, the toner has a sphericity of about 0.95 or more and about 0.98 or less.
According to the sixth aspect, in addition to the effects of the first to fifth aspects, the toner has a sphericity of about 0.95 or more and about 0.98 or less. The contact area becomes smaller than that of the prior art, and the toner layer is more easily scraped off by the brush.

A seventh invention is an image forming apparatus equipped with the first to sixth developing devices.
According to the seventh aspect, in addition to the effects of the first to sixth aspects, image disturbance is avoided and good image formation is performed, which contributes to improving the reliability of the image forming apparatus.

  According to the present invention, it is possible to provide a developing device that eliminates the difference between the charge amount of toner during development and the charge amount of toner during non-development and prevents image disturbance, and an image forming apparatus equipped with the same.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram of a tandem color printer according to the present embodiment, in which the right direction corresponds to the front of the printer 1 and the left direction corresponds to the back.
As shown in the figure, a paper cassette 3 is arranged at the lower part of the apparatus main body 2, and paper P before image formation is accommodated in the cassette 3 in a stacked state. Are separated one by one and sent from the cassette 3 toward the left.

  The paper P delivered from the cassette 3 is conveyed upward along the left side surface of the main body 2. Inside the main body 2, a feed roller 22, a registration roller 24, an image forming unit 4, and a transfer unit 71 are sequentially arranged on the downstream side in the sheet conveyance direction. An exposure unit 20 is provided below the image forming unit 4, and laser light is irradiated from the unit 20 toward the photosensitive drum (image carrier) 5 of the image forming unit 4.

In addition, a fixing unit 72 and a discharge branching unit 74 are sequentially arranged on the downstream side of the transfer unit 71 as viewed in the paper transport direction. In the case of single-sided printing, the paper P discharged from the fixing unit 72 is discharged from the discharge branching unit. The sheet is discharged to the sheet discharge tray 76 through 74.
On the other hand, a duplex printing unit 78 is disposed between the discharge branching unit 74 and the cassette 3. In this unit 78, the sheet P discharged from the fixing unit 72 is returned to the registration roller 24, and the image forming unit 4. Send again towards

  Here, the image forming unit 4 according to the present exemplary embodiment includes four image forming units 4a, 4b, 4c, and 4d. These units 4a to 4d are arranged in order from the front side to the back side of the printer 1, and are provided corresponding to images of four different colors (yellow, magenta, cyan and black), and are respectively charged and exposed. The yellow, magenta, cyan and black images are sequentially formed through the development and transfer processes.

Specifically, each of the units 4a to 4d is provided with drums 5a, 5b, 5c, and 5d that carry visible images (toner images) of the corresponding colors. Each of the drums 5a to 5d is installed to be rotatable with respect to the main body 2, and is driven counterclockwise in the figure by a drive motor (not shown).
Further, at appropriate positions around the respective drums 5a to 5d, corresponding chargers 6a, 6b, 6c, 6d, developing devices 7a, 7b, 7c, 7d, cleaning units 8a, 8b, 8c, 8d, intermediate Transfer rollers 9a, 9b, 9c, and 9d are provided.

  In the chargers 6a to 6d, the surfaces of the corresponding drums 5a to 5d are uniformly charged. In the developing devices 7a to 7d, yellow, magenta, cyan, and black toners are used to electrostatically adhere to the surfaces of the drums 5a to 5d, respectively. Thereby, toner images corresponding to the electrostatic latent images by the exposure unit 20 are developed on the surfaces of the drums 5a to 5d. The toner images formed on these drums 5a to 5d are sequentially transferred onto the intermediate transfer belt 10 and synthesized as one page of toner images.

Specifically, the belt 10 is an endless belt in which both end portions of a dielectric resin sheet material are overlapped and joined, or a seamless belt without a seam, and includes a driving roller 12 and a conveying roller 11. And is driven clockwise by a drive motor (not shown).
The belt 10 travels between the drums 5a to 5d and the intermediate transfer rollers 9a to 9d, whereby the toner images formed on the drums 5a to 5d are primarily transferred onto the belt 10. In the cleaning units 8a to 8d, the toner remaining on the drums 5a to 5d is removed.

The transfer unit 71 described above is provided adjacent to the driving roller 12, and when the paper P passes between the roller of the transfer unit 71 and the belt 10, the toner image transferred onto the belt 10 is secondary to the paper P. The paper P is transferred and conveyed toward the fixing unit 72.
Here, the developing device 7 and the like described above are electrically connected to a controller (main ECU) 90 in the apparatus main body 2 (FIG. 2).

This controller 90 detects whether image data to be printed is obtained. Specifically, the input port 80 of this embodiment is configured to receive image data from an external computer (not shown), and this image data is input to the controller 90.
When the controller 90 detects reception of image data, the controller 90 operates the exposure unit 20, the image forming unit 4, the transfer unit 71, and the fixing unit 72.

That is, the controller 90 drives the registration roller 24 to feed the waiting paper P toward the transfer unit 71 in time for the image transfer timing. The controller 90 also drives the rollers 21 and 22 when waiting for the paper P in the cassette 3.
Here, the developing device 7 of the present embodiment employs a method in which a two-component developing method and a one-component developing method are combined.

  That is, non-magnetic toner is charged using a magnetic carrier, and only the charged toner is caused to fly toward each drum 5, and a toner image is developed on the drum 5 by a non-contact developing method. More specifically, the developing device 7 has a housing 30 shown in FIG. 3, and the housing 30 of this embodiment has an opening 32 facing the drum 5, and extends downward from the opening 32. The peripheral wall 34 is provided, and a bottom wall 36 formed at the lower end portion of the peripheral wall 34.

  The bottom wall 36 accommodates the above-described carrier and toner, and an agitating mixer 40 and a paddle mixer 44 are disposed respectively. The shafts 42 and 46 of the mixers 40 and 44 are rotatably supported with respect to the housing 30. When the mixers 40 and 44 are rotated by a drive motor (not shown), the carrier and the toner are agitated to charge the toner. These carriers and toner are conveyed to a magnetic brush roller (magnetic roller) 50 via a mixer 44.

  The magnetic brush roller 50 is disposed above the mixer 44, and the brush roller 50 is rotatably supported with respect to the housing 30. Specifically, the brush roller 50 has a shaft 52 at the center, and flanges are disposed at both ends of the shaft 52 via bearings, and a tubular brush-side sleeve 54 is integrally fitted on the outer periphery of the flange. Are combined. When the flange is rotated by the drive motor via the gear, the sleeve 54 rotates clockwise in FIG.

  On the other hand, in the brush-side sleeve 54, a brush-side magnet (magnet) 56 is fixed to the outer peripheral side of the shaft 52 with a predetermined main pole magnetic force and main pole angle. A magnetic brush is formed by suction. Specifically, the magnetic brush is formed in a region having the brush-side magnet 56 on the outer peripheral surface of the sleeve 54.

Further, the thickness of the magnetic brush is regulated by the ear cutting blade 38, and when a predetermined direct current (DC) bias is applied, the magnetic brush conveys the toner toward the developing roller 60. On the other hand, this magnetic brush scrapes off the developed toner layer from the developing roller 60.
As described above, in the magnetic brush, the toner layer firmly attached is peeled off from the developing roller 60 by a mechanical force, and new toner necessary for development is supplied to the developing roller 60.

  According to the magnetic brush described above, it is not necessary to provide a special device such as a scraping blade for the toner after development. This magnetic brush contacts the toner layer of the developing roller 60, and the rollers 50, 60 The toner collection and conveyance are facilitated by the brush effect due to the difference in rotational speed and the replacement of the developer of the magnetic brush by agitation in the mixers 40 and 44.

The developing roller 60 is disposed in the vicinity of the opening 32 above the brush roller 50.
The shaft 62 of the roller 60 is juxtaposed in parallel with the rotation axis of the brush roller 50, and the developing roller 60 is rotatably supported by the housing 30. Specifically, the developing roller 60 also has a shaft 62 at the center, and flanges are disposed at both ends of the shaft 62 via bearings, and a tubular developing-side sleeve 64 is integrally fitted on the outer periphery of the flange. Are combined. When the flange is rotated by the drive motor via the gear, the sleeve 64 rotates clockwise in FIG.

  Also in the sleeve 64, a developing side magnet (magnet) 66 is fixed to the outer peripheral side of the shaft 62 with a predetermined main pole magnetic force and main pole angle. Thereby, the density of the magnetic brush of the part which opposes each roller 50 and 60 is raised. A thin layer of only the toner transferred from the brush roller 50 is formed by the potential difference between the brush roller 50 and the developing roller 60.

  On the other hand, the shaft 62 of the present embodiment is configured to be swingable, and as will be described later, the developing-side magnet 66 can appropriately change the above-described main pole magnetic force and main pole angle. When a developing bias, that is, an alternating current (AC) bias superimposed with a predetermined DC bias is applied, the toner on the developing roller 60 flies toward the drum 5. As a result, a toner image is developed on the surface of each drum 5.

By the way, the toner charge amount at the time of development on the drum 5 (at the time of printing on the paper P) and at the time of non-development on the drum 5 (at the time of non-printing on the paper P), in other words, of the developing roller 60. There is a concern that a difference may occur between the charge amount of the toner when the toner layer is scraped off.
Therefore, in this embodiment, the controller 90 includes a magnetic force adjusting unit (magnetic force adjusting unit) 92 (FIG. 2), and the magnetic force adjusting unit 92 adjusts the position of the main pole angle of the developing side magnet 66.

  Specifically, the shaft 62 that holds the development side magnet 66 is configured to be driven by a drive motor (not shown), and the magnetic force adjusting unit 92 is positioned at the position of the main pole angle of the development side magnet 66 when printing on the paper P. (Magnetic force peak position) is moved toward the upstream side when viewed in the rotation direction of the developing roller 60. On the other hand, when the toner layer is scraped off, the magnetic force adjusting unit 92 moves the position of the main pole angle (magnetic peak position) of the developing side magnet 66 again from the upstream position to the downstream side.

  More specifically, as shown in FIG. 4A, the magnetic force adjusting unit 92 first sets the position (magnetic peak position) of the main pole angle of the developing side magnet 66 when printing on the paper P. With respect to a reference line (indicated by a one-dot chain line in the figure) that connects the centers of the rollers 50 and 60, it is about 10 ° or more upstream and about 18 ° or less when viewed in the rotation direction of the developing roller 60. It is set to the position moved to.

The position of the main pole angle (magnetic force peak position) of the brush-side magnet 56 is a range of about 5 ° or more and about 15 ° or less on the downstream side in the rotation direction of the brush roller 50 with respect to the reference line. It is set to the position moved in.
On the other hand, as shown in FIG. 4B, the magnetic force adjusting unit 92 next sets the position of the main pole angle (magnetic force peak position) of the developing side magnet 66 when the toner layer of the developing roller 60 is scraped off. With respect to the reference line, it is set at a position that is moved in the range of about 0 ° or more and about 5 ° or less on the upstream side in the rotation direction of the developing roller 60.

In addition, when the toner layer is scraped off, the AC bias is not applied to the developing roller 60 among the developing biases described above. Also in this case, the position of the main pole angle (magnetic force peak position) of the brush-side magnet 56 is maintained in the same state as the state of FIG.
In the printer 1 equipped with the developing device 7, the paper P is separated and sent from the cassette 3 one by one, and the paper P reaches the registration roller 24. The roller 24 sends the paper P to the transfer unit 71 while correcting the oblique feeding of the paper P and measuring the timing with the toner image formed by the image forming unit 4.

  Further, based on the image data from the controller 90, the laser beam irradiation by the exposure unit 20 is controlled in the printer 1. As a result, an electrostatic latent image of the original image is created on each of the drums 5 a to 5 d in the image forming unit 4, and then a toner image is formed on each of the drums 5 a to 5 d from the latent image. Transcribed and synthesized. Subsequently, the toner image transferred and synthesized on the belt 10 is transferred to the paper P by the transfer unit 71.

Thereafter, the paper P is fed toward the fixing unit 72 while carrying an unfixed toner image. Next, the paper P discharged from the fixing unit 72 is discharged to the tray 76 through the discharge branching unit 74.
When double-sided printing is performed in contrast to this single-sided printing, the paper P discharged from the fixing unit 72 is pulled back to the unit 78 side by the discharge branching unit 74, and this paper P passes through the registration roller 24 and is again transferred to the transfer unit. Sent to 71. In this case, the toner image is transferred to the surface of the paper P that has not been printed.

  As described above, according to this embodiment, the two-component developer composed of the carrier and the toner is used, and the brush roller 50 forms a magnetic brush using this developer. Specifically, this brush is formed by brush side 56 magnets, and causes the developing roller 60 to form a thin layer of toner. The toner image is developed by supplying the toner to the drum 5. On the other hand, this brush scrapes off and collects the toner layer of the developing roller 60 after development.

Here, when the developing roller 60 is simply provided with the magnet 66, a thin layer of toner can be satisfactorily formed on the developing roller 60, but the toner charge amount at the time of printing on the paper P and the toner at the time of scraping. There is a difference in the charge amount.
However, since the magnetic force adjusting unit 92 moves the position of the main pole angle of the developing side magnet 66 toward the upstream side when viewed in the rotation direction of the developing roller 60 at the time of printing, the toner charge amount at the time of printing is Although less than in the prior art, this toner is difficult to leak from the roller 60 even when the developing roller 60 rotates. Therefore, the toner layer is likely to be present on the developing roller 60, and image density defects are suppressed.

  On the other hand, the magnetic force adjusting unit 92 moves the position of the main pole angle of the developing side magnet 66 from the upstream side to the downstream side at the time of scraping. Compared to more. Therefore, the toner layer is easily scraped off by the brush, and the occurrence of development ghost is suppressed. As a result of the elimination of the difference between the toner charge amount during development and the toner charge amount during non-development, image disturbance can be avoided.

This point will be described in detail. FIG. 5 shows an experimental result in which the position of the main pole angle of the developing side magnet 66 is changed toward the upstream side during printing.
In this experiment, the number of toner layers present on the developing roller 60 was examined. The experimental conditions are as follows: first, the linear velocity of the drum 5 is 180 mm / sec, the surface potential of the drum 5 is 330 V, and the charge amount of the toner in the developer is 20 μC / g. The toner has a sphericity of about 0.95 or more and about 0.98 or less.

Further, the particle size (volume average particle size) of this toner is 7.5 μm (1 μm = 1 × 10 ⁻⁶ m), while the particle size (weight average particle size) of the carrier is 45 μm, and it is in contact with the toner. Has been increased. The distance between the rollers 50 and 60 is 350 μm. The DC component (Vdc) of the developing bias for the developing roller 60 is 50 V, the AC component (Vpp) is 1.6 kV, the frequency f is 2.7 kHz, and the duty ratio is 27%. On the other hand, the DC component (Vdc) of the developing bias with respect to the brush roller 50 is 200 V, the AC component (Vpp) during development is 300 V, the frequency f is 2.7 kHz, and the duty ratio is 27%.

  When the magnetic force adjustment unit 92 moves the position of the main pole angle of the development side magnet 66 by 0 ° toward the upstream side in the rotation direction of the development roller 60 during printing, the toner layer is Before the experiment, 100% remains on the developing roller 60, whereas it becomes about 62% on the first round of the developing roller 60, about 44% on the second round, and about 35% on the third round. (Shown by Δ in the figure (B)), it can be seen that the scraping effect of the toner layer is large.

Next, even when the position of the main pole angle of the development side magnet 66 is moved 5 ° toward the upstream side when viewed in the rotation direction of the development roller 60, the toner layer remains on the first turn of the development roller 60. It is about 60%, about 40% in the second round, and about 30% in the third round (indicated by ▽ in the same figure (B)), which shows that the toner layer scraping effect is great.
On the other hand, when the position of the main pole angle of the developing side magnet 66 is moved 10 ° toward the upstream side when viewed in the rotation direction of the developing roller 60, the toner layer becomes the first round of the developing roller 60. However, it is about 49.1% on the second round and about 41% on the third round (indicated by a circle in the figure (B)), and the scraping effect of the toner layer is effective. I understand that there are few.

  When the position of the main pole angle of the development side magnet 66 is moved 15 ° toward the upstream side when viewed in the rotation direction of the development roller 60, the toner layer is approximately about the first turn of the development roller 60. 71%, about 53% on the second round, and about 45% on the third round (indicated by ◇ in the same figure (B)), the effect of scraping off the toner layer is reduced, and the developing roller 60 The toner layer is about 94.1% on the first turn of the developing roller 60 and about 76.5% on the second turn, and three turns. It is about 58.8% visually (indicated by a square in FIG. 5B), and it can be seen that the toner layer has little scraping effect.

Further, from FIG. 5A, at 5 ° or less, a large amount of current flows from the brush roller 50 toward the developing roller 60, and the developer tends to accumulate between the brush roller 50 and the developing roller 60. As the toner 60 rotates, toner leaks from the roller 60 to the surroundings.
However, if the angle exceeds 18 °, the magnetic force becomes small and it becomes difficult to collect the toner on the developing roller 60. Therefore, as in this embodiment, if a value within the range of 10 ° to 18 ° is set at the time of printing, the toner layer of the developing roller is not disturbed and the developer leaks without causing an appropriate toner layer. As a result, defects in image density are suppressed.

  On the other hand, the magnetic force adjusting unit 92 has a position of the main pole angle of the developing side magnet 66 that is about 0 ° or more upstream and about 5 ° or less when viewed in the rotation direction of the developing roller 60 during non-printing. Since the position is set within the range, the toner layer is surely scraped off by the brush, and the development ghost during the next development can be reliably suppressed. During non-printing, it is performed to scrape off the toner layer and is not rotated for a long time as in printing, so that the developer does not leak.

  Further, when a developing bias, that is, an AC bias superimposed with a DC bias is applied, the toner layer of the developing roller 60 flies toward the drum 5, but this AC bias is not applied during non-development. It becomes difficult for the 60 toner layers to fly toward the drum 5. Therefore, the toner layer is more reliably scraped off by the magnetic brush.

Further, the position of the main pole angle of the brush-side magnet 56 is set to a position that is moved to a range of about 5 ° or more and about 15 ° or less on the downstream side in the rotation direction of the brush roller 50. This is because when the angle is less than about 5 °, a large amount of current flows toward the developing roller 60, and the toner easily scatters as the developing roller 60 rotates.
However, if the angle exceeds about 15 °, the magnetic force becomes small and the toner layer formed on the developing roller 60 cannot be collected. Therefore, as in this embodiment, if the value is set within a range of about 5 ° to about 15 °, an appropriate magnetic brush having a high density and a high scraping force can be formed.

Furthermore, since the toner has a sphericity of about 0.95 or more and about 0.98 or less, the contact area with the developing roller 60 becomes smaller than the conventional one, and the toner layer is formed by a brush. It becomes easier to be scraped off.
Further, image disturbance is avoided and good image formation is performed, which contributes to improving the reliability of the printer 1.

The present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the claims.
For example, the image carrier of the present invention is not limited to four photosensitive drums. In the above embodiment, an example in which a printer is embodied as an image forming apparatus is shown. Naturally, it can also be applied to multifunction devices, copiers, facsimiles, and the like.

1 is a schematic configuration diagram of an image forming apparatus of an embodiment. FIG. 2 is a control block diagram in the image forming apparatus of FIG. 1. It is sectional drawing of the image development apparatus of FIG. FIG. 2 is an explanatory diagram of a position of a main pole angle in the developing device of FIG. 1. FIG. 2 is a diagram illustrating an experimental result by the image forming apparatus of FIG. 1.

Explanation of symbols

1 Printer (image forming device)
5 Photosensitive drum (image carrier)
7 Developing Device 30 Housing 32 Opening 50 Magnetic Brush Roller (Magnetic Roller)
56 Brush side magnet (magnet)
60 Development roller 66 Development side magnet (magnet)
92 Magnetic adjustment part (Magnetic adjustment means)

Claims (7)

A developing device for charging a toner using a carrier and supplying the toner to an image carrier to develop a toner image;
A housing having an opening facing the image carrier;
A magnetic roller provided in the housing and provided with a magnet for forming a magnetic brush made of the carrier and the toner;
A developing roller provided with a magnet that is arranged substantially parallel to the rotation axis of the magnetic roller at the opening and forms a thin layer of toner transferred from the magnetic brush;
Magnetic force adjusting means for adjusting the position of the main pole angle of the magnet for forming the toner thin layer,
When the toner image is developed, the magnetic force adjusting means moves the position of the main pole angle of the magnet for forming the toner thin layer toward the upstream side when viewed in the rotation direction of the developing roller. When the toner image is not developed, the position of the main pole angle of the magnet for forming the toner thin layer is moved from the upstream side toward the downstream side. The developing device according to claim 1,
When the toner image is developed, the magnetic force adjusting means moves the position of the main pole angle of the magnet for forming the toner thin layer about 10 ° or more upstream in the rotation direction of the developing roller. A developing device characterized in that the developing device is set at the position. The developing device according to claim 1 or 2,
When the toner image is not developed, the magnetic force adjusting means moves the position of the main pole angle of the magnet for forming the toner thin layer about 5 ° or less upstream in the rotation direction of the developing roller. A developing device characterized in that the developing device is set at the position. The developing device according to claim 3,
The developing device is characterized in that a predetermined AC bias is not applied to the developing roller when the toner image is not developed. The developing device according to any one of claims 1 to 4, wherein
The position of the main pole angle of the magnet that forms the magnetic brush is set to a position that is about 5 ° or more downstream and about 15 ° or less when viewed in the rotation direction of the magnetic roller. A developing device. The developing device according to any one of claims 1 to 5,
The developing device according to claim 1, wherein the toner has a sphericity of about 0.95 or more and about 0.98 or less.   An image forming apparatus comprising the developing device according to claim 1.

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