JP5114107B2 - Developing device and image forming apparatus equipped with the same - Google Patents

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, this developing apparatus has a so-called hybrid development system. Specifically, this method is a combination of the two-component development method and the one-component development method, and a non-magnetic toner is charged using a magnetic carrier, and only this toner is supplied to the drum, and the toner is applied to the surface The image is being developed.

  And the technique using the said system is disclosed (for example, refer patent document 1), It is equipped with the image development roll which faces a drum, and the magnetic brush roll which faces this roll. A magnet that forms a magnetic brush made of toner is provided. A magnet is also provided in the sleeve of the developing roll, and the density of the magnetic brush is increased. A thin layer of toner only is formed on the sleeve.

JP 2005-274924 A

However, in the developing roll described above, the width of the magnet is shorter than the width of the sleeve. This is because it is necessary to support both ends of the magnet with flanges in order to arrange the magnet.
Further, in the magnetic brush roll described above, the width of the magnet is formed longer than the width of the sleeve in order to scrape off the toner layer formed over the width of the sleeve after development.

  That is, the width of the magnet of the magnetic brush roll is larger than the width of the sleeve of the developing roll, and the width of this sleeve is larger than the width of the magnet of the developing roll. However, in this case, the toner layers formed at both end portions are difficult to be collected, and there is a problem that a difference occurs in the next development and image distortion is caused.

  As described above, in the above-described conventional technology, there is still a problem with respect to prevention of image disturbance at both end portions of the sleeve of the developing roll, and special consideration is given to making the sleeve width a large printable width. Not. In order to make the sleeve width a large printable width, it may be possible to change the sleeve to a sleeve having a longer width, but this causes another problem that the developing device is increased in size.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a developing device capable of solving the above-described problems and preventing image disturbance at both end portions of the sleeve of the developing roll, and an image forming apparatus equipped with the developing device.

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 magnet, a magnetic roll provided with a magnet for forming a magnetic brush made of a carrier and toner, and a parallel opening substantially parallel to the rotation axis of the magnetic roll at the opening; A developing roll comprising a sleeve having a width shorter than the width in the axial direction of the magnet and a magnet formed to a width shorter than the width in the rotational axis direction of the sleeve and forming a thin layer of toner transferred from the magnetic brush If, in order to recover in both the magnetic brush and the toner layer of the toner layer formed on the end portion of the sleeve after development is formed in a substantially central portion of the sleeve of the toner image, the sleeve Comprising a brush density reinforcing means for relatively high density of magnetic brush facing the end portion of the sleeve with respect to the density of the magnetic brush facing the substantially central portion.
Further, the brush density strengthening means is configured so that the value of the main pole magnetic force of the magnet that forms the magnetic brush facing the end portion of the sleeve is larger than the value of the main pole magnetic force of the magnet that forms the magnetic brush facing the substantially central portion of the sleeve. It is characterized by being set to a large value of about 10 mT or more and 20 mT or less.

  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 roll. Specifically, this brush is formed by a magnet in a magnetic roll, and forms a thin layer of toner on the developing roll. 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 roll after development.

Here, since the width of the magnetic brush in the axial direction is longer than the width of the sleeve of the developing roll, all the toner layers after development can be scraped off, and the toner layer in the developing roll is not yet removed. The collection area can be eliminated.
On the other hand, when a magnet is provided on the developing roll, for example, when the width of the magnet of the developing roll is formed to be shorter than the width of the sleeve, as in the case of providing a flange supporting both ends of the sleeve, this sleeve There will be areas without magnets at both ends. That is, the toner layer formed at the substantially central portion of the sleeve is collected, but the toner layer formed at both end portions is difficult to collect.

However, the brush density enhancing means has a magnetic brush facing the end portion of the sleeve so that the toner layer formed on the end portion of the sleeve of the developing roll can be collected in the same manner as the toner layer formed on the substantially central portion thereof. Therefore, the toner layer formed at both end portions of the sleeve can be recovered in the same manner as the toner layer formed at the substantially central portion thereof. As a result, a difference is less likely to occur in the next development, and image disturbance or the like can be avoided.
In addition, the toner layer formed at the end of the sleeve can be recovered in the same manner as the toner layer formed at the substantially central portion without increasing the width of the sleeve of the developing roll, thereby contributing to downsizing of the developing device. To do.

Further, the brush density enhancing means sets the value of the main pole magnetic force of the magnet that forms the magnetic brush to a value that is about 10 mT or more and 20 mT or less greater than the value that faces the end portion of the sleeve than the value that faces the substantially central portion of the sleeve. Since the brush density at the end portion of the sleeve is higher than the brush density at the substantially central portion, it is possible to collect the toner layer in the portion where the magnets at both ends of the sleeve of the developing roll are not present. Therefore, the width of the sleeve of the developing roll becomes a larger printable width.

According to a second aspect of the invention, in the configuration of the first aspect of the invention, the brush density enhancing means is configured such that the position of the main pole angle of the magnet that forms the magnetic brush facing the end portion of the sleeve faces the substantially central portion of the sleeve. It is characterized in that it is set at a position moved about 5 ° or more upstream from the position of the main pole angle of the magnet forming the brush in the rotation direction of the magnetic roll.
According to the second invention, in addition to the action of the first invention, the brush density enhancing means further sets the position of the sleeve opposite to the end portion of the sleeve with respect to the position of the main pole angle of the magnet that forms the magnetic brush. It is set at a position moved about 5 ° or more upstream in the rotation direction of the magnetic roll from the position facing the substantially central portion, and the brush density at the end portion of this sleeve is set to be higher than the brush density at the central portion. Since the height is raised, it is possible to collect the toner layer in the portion where the magnets at both ends of the sleeve of the developing roll do not exist, so that the width of the sleeve of the developing roll becomes a larger printable width.

The third invention is an image forming apparatus equipped with the first and second developing devices.
According to the third invention, in addition to the effects of the first and second inventions, the width of the sleeve of the developing roll becomes a larger printable width, and it is difficult to make a difference in the next development. Image formation is performed and contributes to improving the reliability of the image forming apparatus, and the printable width is increased without increasing the width of the sleeve of the developing roll. Contribute.

  According to the present invention, it is possible to provide a developing device and an image forming apparatus equipped with the developing device in which the sleeve width is made larger without using a long sleeve.

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 so as to be rotatable with respect to the main body 2, and is driven counterclockwise in FIG.
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.
By the way, amorphous silicon layers are formed on the outer peripheral surfaces of the drums 5a to 5d of this embodiment, and developing devices 7a to 7d are installed on the right side of the drums 5a to 5d in the drawing. Yes.

Each developing device 7 of this embodiment employs a so-called hybrid development method. That is, a non-magnetic toner is charged using a magnetic carrier, and only the charged toner is caused to fly toward the drum 5, and a toner image is developed on the drum 5 by a non-contact developing method.
More specifically, the housing 30 shown in FIG. 2 is provided. The housing 30 of this embodiment has an opening 32 facing the drum 5 and a peripheral wall 34 extending downward from the opening 32. 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 roll (magnetic roll) 50 via a mixer 44.

  The magnetic brush roll 50 is disposed above the mixer 44, and the shaft 52 is rotatably supported with respect to the housing 30 as shown in FIG. 3. The shaft 52 of this embodiment has a flange portion 58, and a tubular brush-side sleeve 54 is fitted to the outer periphery of the flange portion 58. Thereby, when the shaft 52 is rotated by a drive motor (not shown), the sleeve 54 rotates clockwise in FIG.

  On the other hand, a holding shaft 53 is disposed on the same line as the shaft 52 in the brush side sleeve 54. A brush-side magnet (magnet) 56 is held on the outer peripheral side of the holding shaft 53 with a predetermined main pole magnetic force and main pole angle, and a magnetic brush is formed by attracting the carrier and toner from the mixer 44. Let Specifically, in this embodiment, both end sides of the brush side magnet 56 are respectively held by the flange 58, and the width of the sleeve 54 in the rotation axis direction is longer than the width of the brush side magnet 56 in the axial direction. 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 pan blade 38 (FIG. 2), and when a predetermined direct current (DC) bias is applied, the magnetic brush conveys the toner toward the developing roll 60. On the other hand, this magnetic brush scrapes off the developed toner layer from the developing roll 60.
As described above, in the magnetic brush, the toner layer firmly attached is peeled off from the developing roll 60 by a mechanical force, and new toner necessary for development is supplied to the developing roll 60, and the developer is supplied to the developer. The role of toner collection and supply is required. In order to fulfill this role, it is desirable to use a carrier having a volume resistivity of 10 ⁶ to 10 ¹³ Ωcm and to increase the number of contacts with the toner using a carrier having an average particle size of 50 μm or less. Therefore, a coated ferrite carrier having a volume resistivity of 10 ¹⁰ Ωcm, a saturation magnetization of 65 emu / g, and an average particle size of 45 μm is used for the carrier of this example.

  According to the above-described magnetic brush, 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 on the sleeve 64, and 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.

  In order to promote the replacement of the developer, the rotational speed of the brush roll 50 may be set to about 1 to 2 times the rotational speed of the developing roll 60. As a result, the toner on the sleeve 64 is collected, and the developer set to an appropriate toner concentration is supplied to the developing roll 60, so that a uniform toner layer can be formed. In order to maintain a uniform image density, the potential difference between the rolls 50 and 60 may be set to the same potential at times other than the development timing. Thereby, the toner on the sleeve 64 can be collected in the brush roll 50 without imposing a burden on the toner.

The developing roll 60 is disposed above the brush roll 50 at the opening 32.
As shown in FIG. 3, the shaft 62 of the roll 60 is juxtaposed in parallel with the rotation axis of the brush roll 50, and is rotatably supported by the housing 30. Further, the shaft 62 of this embodiment also has a flange portion 68, and a tubular developing side sleeve 64 is fitted to the outer periphery thereof. Thus, when the shaft 62 is rotated by a drive motor (not shown), the sleeve 64 rotates clockwise in FIG.

  Also in the sleeve 64, a holding shaft 63 is disposed on the same line as the shaft 62, and a developing side magnet (magnet) 66 is held with a predetermined main pole magnetic force or main pole angle. Thereby, the density of the magnetic brush of the part which opposes each roll 50 and 60 is raised. Then, a thin layer of only the toner transferred from the brush roll 50 is formed by the potential difference between the brush roll 50 and the developing roll 60.

Although the thickness of the thin layer varies depending on the resistance of the developer and the difference in rotational speed between the rolls 50 and 60, in this embodiment, the potential difference is set from about 100V to about 350V. The layer thickness is adjusted. This is because as the potential difference is set larger, the thickness of the thin layer increases.
In this embodiment, both end sides of the developing side magnet 66 are also sandwiched between the flanges 68, and the width of the sleeve 64 in the rotational axis direction is shorter than the axial width of the brush side magnet 56. Therefore, the thin layer is formed over the entire area of the outer peripheral surface of the sleeve 64.

  The shafts 52 and 62 described above are supported by a bearing 70, and the bearing 70 is configured to be rotatable about the shaft 52 as a rotation center. When a developing bias, that is, an alternating current (AC) bias superimposed with a predetermined direct current (DC) bias is applied, the toner on the developing roll 60 flies toward the drum 5. As a result, a toner image is developed on the surface of each drum 5. In this embodiment, an alternating current (AC) bias is applied immediately before development in order to prevent toner scattering.

Here, when an amorphous silicon-based material is used as the photosensitive material of each drum 5 as in this embodiment, the surface potential after exposure has a very low characteristic of 20 V or less. When the thickness is reduced, the saturation charging potential is lowered, and the withstand voltage leading to dielectric breakdown is lowered. On the other hand, the charge density on the surface of the drum 5 when a latent image is formed tends to be improved, and the development performance tends to be improved. This characteristic is particularly remarkable when the film thickness of the drum 5 having a dielectric constant of about 10 is 25 μm or less, more preferably 20 μm or less. Therefore, development can be performed by setting the DC component of the developing bias to 150 V or less, the AC component to V _P−P = 500 to 2000 V, and the frequency 1 to 4 kHz.

  If a positively charged organic photoreceptor (OPC) is used for this drum, the film thickness is set to 25 μm or more in order to make the residual potential 100 V or less, and the amount of charge generation material added is Increasing is particularly important. In particular, OPC having a single layer structure is advantageous in that a charge generating agent is added to the photosensitive layer, so that the sensitivity change is small even when the photosensitive layer is reduced. Also in this case, the DC component of the developing bias 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.

  Incidentally, as shown in FIG. 4, in this embodiment, the width in the rotation axis direction of the developing side sleeve 64 is formed to be longer than the width in the axial direction of the developing side magnet 66. In other words, there are areas (about 4 to 5 mm) where the developing side magnet 66 is not present at both end portions of the sleeve 64, and the thin layer of toner is the entire area of the outer peripheral surface of the sleeve 64 as described above. However, the property of the thin layer is different from the region where the development side magnet 66 is present and the region where the development side magnet 66 is not present.

More specifically, when the magnetic brush scrapes off the developed toner layer from the developing roll 60, the toner in a region with the developing side magnet 66 can be scraped, whereas the toner in a region without the developing side magnet 66 is scraped off. There is no concern.
Therefore, in this embodiment, a brush density enhancement unit (brush density enhancement means) 80 is provided in a controller (not shown) (FIG. 3), and the main pole magnetic force and main pole of the brush side magnet 56 facing a region where the development side magnet 66 is located. The angle is different from the main pole magnetic force and main pole angle of the brush side magnet 56 facing the region where the development side magnet 66 is not present.

  Specifically, first, the brush density strengthening unit 80 of the present embodiment determines the value of the main magnetic force of the brush side magnet 56 facing the region where the development side magnet 66 is not present, that is, both end portions of the development side sleeve 64. It is set to a value larger than the value of the main pole magnetic force of the brush side magnet 56 facing a certain area of the development side magnet 66, for example, the substantially central portion of the development side sleeve 64, by about 10 mT or more.

  In other words, if the value of the main pole magnetic force of the brush side magnet 56 facing the substantially central portion of the developing side sleeve 64 is set to XmT, the brush side magnet facing the both end portions of the sleeve 64. The main pole magnetic force 56 is set to a large value within the range of X + 10 mT to X + 20 mT to enhance the density of the magnetic brush so that the axial width of the developing side sleeve 64 has a larger printable width.

  Further, the brush density enhancing portion 80 of the present embodiment is configured so that the position of the main pole angle (magnetic force peak position) of the brush side magnet 56 facing the both ends of the development side sleeve 64, that is, the region without the development side magnet 66 is obtained. More upstream than the position of the main pole angle (magnetic force peak position) of the brush-side magnet 56 facing the region where the developing-side magnet 66 is located, for example, the substantially central portion of the developing-side sleeve 64 in the rotational direction of the magnetic brush roll 50. It is set to the position moved to + 5 ° or more to the side.

  That is, in FIG. 5A showing the substantially central portion of the developing side sleeve 64, the position of the main pole angle of the developing side magnet 66 is set to about + Y ° on the upstream side when viewed in the rotation direction of the developing roll 60. If the position of the main pole angle of the brush side magnet 56 is also set to about + Z ° on the upstream side when viewed in the rotation direction of the brush roll 50, the both end portions of the developing side sleeve 64 are shown in FIG. In B), the main pole angle of the brush-side magnet 56 is such that the position moved by + 5 ° or more upstream in the rotation direction of the brush roll 50, that is, the position from Z + 5 ° to Z + 15 ° is the magnetic force peak position. The magnetic brush density is strengthened by setting it to a large value so that the axial width of the developing side sleeve 64 becomes a larger printable width.

  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, the printer 1 controls the irradiation of the laser beam by the exposure unit 20. 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 including the magnetic carrier and the nonmagnetic toner is used, and the mixers 40 and 44 charge the toner using the carrier. Next, in the magnetic brush roll 50, a magnetic brush made of these carriers and toner is formed. Specifically, this brush is formed by a brush-side magnet 56, and a thin layer of toner only is formed on the developing roll 60 by a potential difference between the developing roll 60 and the brush roll 50. 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 roll 60 after development.

Here, since the width of the magnetic brush in the axial direction is longer than the width of the developing sleeve 64, all the toner layers after development including the toner layers formed at both ends of the sleeve 64 are removed. The toner can be scraped off, and the unrecovered area of the toner layer in the developing roll 60 can be eliminated to prevent the toner from scattering.
On the other hand, for example, in order to provide the developing side magnet 66, the width of the developing side magnet 66 is formed to be shorter than the width of the developing side sleeve 64, as in the case where the flanges 68 that support both end portions of the sleeve 64 are provided. When this occurs, there will be areas without magnets at both ends of the sleeve 64. That is, the toner layer formed at the substantially central portion of the developing sleeve 64 is collected, but the toner layer formed at both end portions is difficult to be collected, causing a difference in the next development, leading to image distortion and the like. It will be.

However, the brush density enhancing portion 80 enhances the density of the magnetic brush facing the end portion of the sleeve 64 so that the width of the developing sleeve 64 becomes a larger printable width. The toner layers formed at both end portions of the toner can be recovered in the same manner as the toner layer formed at the substantially central portion. As a result, a difference is less likely to occur in the next development, and image disturbance or the like can be avoided.
In addition, the sleeve 64 of the developing roll 60 is not changed to a long sleeve, and the width of the sleeve 64 becomes a larger printable width, which contributes to the downsizing of the developing device 7.

  More specifically, the brush density strengthening unit 80 sets the value of the main pole magnetic force of the brush side magnet 56 that forms the magnetic brush to a value that opposes the end portion of the development side sleeve 64. Since the brush density at the end portion of the sleeve 64 is set higher than the brush density at the substantially central portion, it is set to a value larger than the value facing the center portion. Thus, the toner layer can be collected in a portion where no magnet is present, so that the width of the sleeve 64 becomes a larger printable width.

  Further, the brush density strengthening portion 80 also has a position of the main pole angle of the brush-side magnet 56 (magnetic peak position) at a position facing the end portion of the sleeve 64 rather than a position facing the substantially central portion of the sleeve 64. The brush roll 50 is set at a position moved about 5 ° or more upstream as viewed in the rotational direction of the brush roll 50, and the brush density at the end portion of the sleeve 64 is further increased from the brush density at the central portion. Since it is possible to collect the toner layer where the magnets at both ends of the developing-side sleeve 64 do not exist, in combination with the setting of the main pole magnetic force value, the width of the sleeve 64 can be further increased. Can be.

  Specifically, in FIG. 6, the value of the main pole magnetic force of the brush-side magnet 56 facing both end portions of the developing-side sleeve 64 shows the value of the brush-side magnet 56 facing the substantially central portion of the sleeve 64. A value within the range of +5 mT to +20 mT is set on the basis of the value of the main pole magnetic force, and the position of the main pole angle of the brush side magnet 56 facing both end portions of the developing side sleeve 64 is substantially the center of the sleeve 64. The experimental results are shown in which the position within the range of −5 ° to + 15 ° is set with reference to the position of the main pole angle of the brush-side magnet 56 facing the portion.

In this experiment, an image having a width larger than the width of the developing sleeve 64 was formed, and the image was evaluated for each item of fog margin difference and half image density difference.
The fog is a phenomenon in which the toner is covered on the image due to the scattering of unrecovered toner in the developing device 7, and the fog margin is the difference between the surface potential of the drum 5 where the fog occurs and the surface potential where the fog does not occur. In this experiment, the surface potential of the drum 5 is varied to record the minimum value of the surface potential at which no fog occurs.

The fog margin difference is a difference between the minimum value of the surface potential at the end portion of the image and the minimum value of the surface potential at a substantially central portion of the image, and only when this difference is within 20V. It is evaluated that they are equivalent (indicated by ◯ in FIG. 6).
On the other hand, the half image is a printed halftone image, and the measured value of the reflection density (Macbeth RD912) is recorded. The half image density difference is a difference between the measured value of the density at the edge portion of the image and the measured value of the density at the substantially central portion of the image, and only when this difference is within 0.1. , Which are equivalent (indicated by a circle in FIG. 6).

  As shown in FIG. 6, the brush-side magnet 56 facing the both end portions of the sleeve 64 in both the fog margin difference in FIG. 6A and the half image density difference in FIG. When the value of the main pole magnetic force is larger than the value of the main pole magnetic force of the brush-side magnet 56 facing the substantially central portion of the sleeve 64 in a range less than +10 mT, the end portion of these images and the substantially central portion of the image Was clearly different. This is because if it is less than +10 mT, the toner layers formed at both end portions of the sleeve 64 cannot be recovered.

On the other hand, when the value of the main pole magnetic force of the brush side magnet 56 facing the both end portions of the sleeve 64 is larger than the value of the main pole magnetic force of the brush side magnet 56 facing the substantially central portion of the sleeve 64 by +10 mT or more. The edge portion of these images and the substantially central portion of the image were almost equal.
However, if it exceeds +20 mT, a large amount of current flows through both end portions of the sleeve 64, so that a large amount of toner layer is easily formed and the toner is easily scattered. Therefore, it can be seen that setting to a value within the range of about +10 mT to about +20 mT as in this embodiment results in an appropriate magnetic brush with high density and strong scraping power.

  Further, the position of the main pole angle of the brush-side magnet 56 facing the both end portions of the sleeve 64 moves within a range of less than + 5 ° than the position of the main pole angle of the brush-side magnet 56 facing the substantially central portion of the sleeve 64. Even in this case, the edge portions of these images and the substantially central portion of the images were clearly different. This is because when the angle is less than + 5 °, a large amount of current flows through both end portions of the sleeve 64, and the toner is easily scattered.

However, when the position of the main pole angle of the brush side magnet 56 facing the both end portions of the sleeve 64 is moved by + 5 ° or more than the position of the main pole angle of the brush side magnet 56 facing the substantially central portion of the sleeve 64. The edge portion of these images and the substantially central portion of the image were almost equal.
However, if it exceeds + 15 °, the magnetic force becomes small and the toner layers formed at both ends cannot be recovered. Therefore, it can be seen that setting to a value within the range of about + 5 ° to about + 15 ° as in this embodiment results in an appropriate magnetic brush with high density and strong scraping power.

Further, since the width of the sleeve 64 becomes a larger printable width, and a difference is less likely to occur in the next development, a good image formation is performed, which contributes to the improvement of the reliability of the printer 1 and this sleeve. Since the printable width becomes larger without increasing the width of 64, it contributes to the downsizing of the image forming unit 4 and the downsizing 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. It is sectional drawing of the image development apparatus of FIG. It is explanatory drawing of brush density reinforcement | strengthening in the image development apparatus of FIG. FIG. 2 is a partially enlarged view of the developing device in FIG. 1. It is arrow sectional drawing of the AA line of FIG. 4, and the BB line. 3 is a table showing experimental results 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 Roll (Magnetic Roll)
56 Brush side magnet (magnet)
60 Development Roll 64 Development Side Sleeve (Sleeve)
66 Development-side magnet (magnet)
80 Brush density enhancement part (Brush density enhancement means)

Claims (3)

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 roll provided in the housing and provided with a magnet for forming a magnetic brush with the carrier and the toner;
The opening is provided in parallel with the rotation axis of the magnetic roll, and includes a sleeve having a width shorter than the width of the magnet in the axial direction, and shorter than the width of the sleeve in the rotation axis direction. A developing roll having a magnet formed to a width and forming a thin layer of toner transferred from the magnetic brush;
Wherein in order to collect the toner layer formed on an end portion of the sleeve after the development of the toner images in both the magnetic brush and the toner layer formed at a substantially central portion of the sleeve and facing the substantially central portion of the sleeve Brush density strengthening means for relatively increasing the density of the magnetic brush opposed to the end portion of the sleeve with respect to the density of the magnetic brush ;
The brush density enhancing means is configured to determine a value of a main pole magnetic force of the magnet that forms the magnetic brush facing the end portion of the sleeve and a main pole magnetic force of the magnet that forms the magnetic brush facing the substantially central portion of the sleeve. The developing device is set to a value greater than about 10 mT and less than or equal to about 20 mT . The developing device according to claim 1,
The brush density reinforcing means, the main pole angle of the magnet to form a magnetic brush of the position of the main pole angles and faces the substantially central portion of the sleeve of the magnet to form a magnetic brush which is opposite to the end portion of the sleeve The developing device is set to a position moved about 5 ° or more upstream from the position of the magnetic roll in the rotational direction of the magnetic roll . An image forming apparatus comprising the developing device according to claim 1.

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