JP2011095516A - Developing device and image forming apparatus including the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a developing device can prevent occurrence of a developing history phenomenon by improving developability of toner for a developing roller and collecting property of development residual toner on the developing roller, and an image forming apparatus including the same. <P>SOLUTION: In the developing devices 3a to 3d, a first N pole 41a is provided at an opposed position to the developing roller 23 inside the magnetic roller 22, and a third S pole 43a arranged at the above opposed position and a fourth S pole 43b arranged adjacently to an upstream side of the third S pole 43a in a rotating direction of the developing roller 23 are provided inside the developing roller 23. The fourth S pole 43b is formed so that its magnetic flux density in a normal direction may be larger than that of the third S pole 43a. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a developing device mounted on an image forming apparatus such as a copying machine, a facsimile machine, and a printer, and an image forming apparatus including the developing device, and more particularly to a developing device that uses a two-component developer having a magnetic carrier and toner. Is.

  Conventionally, as a development method using dry toner in an image forming apparatus using an electrophotographic process, a one-component development method using no carrier and a two-component developer charging a nonmagnetic toner using a magnetic carrier are used. A two-component development system is known in which an electrostatic latent image on an image carrier (photoconductor) is developed by a developer chain (magnetic brush) composed of toner and a carrier formed on a developing roller.

  The one-component development method is suitable for high image quality because the electrostatic latent image on the image carrier is not disturbed by the magnetic brush. On the other hand, the toner is charged by the charge roller, and the elastic regulating blade is used on the development roller. In order to regulate the layer thickness, the toner additive adheres to the charge roller, the charging ability is lowered, and it is difficult to stably maintain the charge amount of the toner. In addition, toner may adhere to the regulating blade, resulting in non-uniform layer formation and image defects.

  In the case of color printing in which color superposition is performed, since the color toner is required to be transparent, it needs to be a non-magnetic toner. Therefore, in a full-color image forming apparatus, a two-component development system in which toner is charged and conveyed using a carrier is often employed. However, the two-component development method can maintain a stable charge amount for a long time and is suitable for extending the life of the toner, but is disadvantageous in terms of image quality due to the influence of the magnetic brush described above.

  As one of the means for solving these problems, the magnetic carrier is left on the magnetic roller when the developer is transferred onto the developing roller installed in a non-contact manner with respect to the image carrier using the magnetic roller. A developing system has been proposed in which only a non-magnetic toner is transferred onto a developing roller to form a toner thin layer (toner layer), and the toner is attached to an electrostatic latent image on an image carrier by an alternating electric field. In such a developing system, the development residual toner that is not used for developing the electrostatic latent image and remains on the developing roller is collected by the magnetic roller by the magnetic brush.

  Here, when the development residual toner remaining on the developing roller is not sufficiently collected by the magnetic roller, the region where the toner layer remains (exists) and the toner layer are consumed (does not exist) on the developing roller. The toner is supplied again from the magnetic roller in a state where the area coexists. As a result, the toner layer is not uniformly formed on the developing roller, and a development history phenomenon may occur. Also, when the toner is not sufficiently transferred from the magnetic roller to the developing roller, that is, when the toner developability between the magnetic roller and the developing roller is low, the toner layer is stable on the developing roller. Therefore, the development history phenomenon may occur.

  Therefore, in Patent Document 1, magnetic roller magnetic poles are arranged on the upstream side and the downstream side in the rotation direction of the magnetic roller with respect to the position facing the developing roller inside the magnetic roller, and the downstream magnetic field is located inside the developing roller. There has been proposed a method for preventing the development history phenomenon from occurring by disposing the developing roller magnetic pole at a position facing the roller magnetic pole.

JP 2005-221800 A

  With the colorization of image forming apparatuses, further improvement in image quality is demanded. For this purpose, for example, the size of toner is reduced. Further, when such a small size toner is applied to the image forming apparatus having the magnetic roller and the developing roller as described above, the adhesion force of the toner to the developing roller becomes strong. For this reason, there is a demand for further improvement in the ability to collect the development residual toner on the developing roller.

  However, in the method of Patent Document 1, the development residual toner on the developing roller can be sufficiently collected by the magnetic brush at the position facing the magnetic roller of the developing roller, but the position of the developing roller facing the magnetic roller with respect to the rotation direction of the developing roller. Since the magnetic brush is not formed on the upstream side, there is a possibility that the development residual toner conveyed from the upstream side with the rotation of the developing roller cannot be sufficiently collected.

  SUMMARY OF THE INVENTION In view of the above problems, the present invention improves the developing performance of toner with respect to the developing roller and the recovery performance of the development residual toner on the developing roller, and prevents the development history phenomenon from occurring, and image formation provided with the developing device An object is to provide an apparatus.

  In order to achieve the above object, the present invention uses a two-component developer containing a carrier and toner, a developing roller disposed opposite to the image carrier, and a toner using a magnetic brush on the developing roller. A magnetic roller that forms a thin layer, and a developing device that develops an electrostatic latent image on the surface of the image carrier from the developing roller, wherein the first roller is positioned at a position facing the developing roller inside the magnetic roller. A magnetic pole part, and a second magnetic pole part formed from the facing position to the upstream side in the rotation direction of the developing roller inside the developing roller and having a polarity opposite to the first magnetic pole part, The normal magnetic flux density on the surface of the developing roller in the second magnetic pole portion is characterized in that the upstream side in the rotational direction is larger than the facing position.

  According to the present invention, in the developing device configured as described above, the second magnetic pole portion is disposed adjacent to the first magnetic pole member disposed at the facing position and upstream of the facing position in the rotation direction. And the second magnetic pole member is characterized in that the normal magnetic flux density is larger than that of the first magnetic pole member.

  According to the present invention, in the developing device configured as described above, the normal magnetic flux density of the second magnetic pole portion increases from the facing position toward the upstream side in the rotational direction.

  In the developing device configured as described above, the magnetic field generated by the first magnetic pole part and the second magnetic pole part has a pole width having the normal magnetic flux density of 80% or more with respect to a peak value. It is characterized by being set to be 30 ° or more with respect to the support shaft of the developing roller.

  Further, the present invention is an image forming apparatus provided with the developing device having the above-described configuration.

  According to the first configuration of the present invention, the first magnetic pole portion is formed at a position facing the developing roller inside the magnetic roller, and is formed from the facing position inside the developing roller to the upstream side in the rotation direction of the developing roller. And a second magnetic pole portion having a polarity opposite to that of the first magnetic pole portion, and the normal magnetic flux density on the surface of the developing roller in the second magnetic pole portion is set to be larger on the upstream side in the rotational direction than on the opposing position. As a result, the magnetic brush can be formed from the position where the magnetic roller and the developing roller are opposed to the upstream side in the rotation direction of the developing roller, thereby improving the developability of the toner with respect to the developing roller and developing residual toner from the developing roller. The recovery performance can also be improved, and the development history phenomenon can be effectively prevented.

  Further, according to the second configuration of the present invention, in the developing device having the first configuration, the second magnetic pole portion includes the first magnetic pole member disposed at the facing position and the upstream side in the rotation direction of the facing position. A second magnetic pole member disposed adjacent to the second magnetic pole member, and the second magnetic pole member uses two magnetic pole members by making the normal magnetic flux density larger than that of the first magnetic pole member. It is possible to easily expand the magnetic brush forming area.

  According to the third configuration of the present invention, in the developing device having the first configuration, the normal magnetic flux density of the second magnetic pole portion is increased from the facing position toward the upstream side in the rotation direction. Since the number of members can be reduced and the magnetic field formed by the first magnetic pole portion can be set in more detail, the state of formation of the magnetic brush can be improved.

  According to the fourth configuration of the present invention, in the developing device having any one of the first to third configurations, the magnetic field generated by the first magnetic pole portion and the second magnetic pole portion is set to 80% of the peak value. A magnetic brush can be formed more satisfactorily by setting the pole width having a normal magnetic flux density of at least% to 30 ° or more with respect to the support shaft of the developing roller.

  Further, according to the fifth configuration of the present invention, the image forming apparatus including the developing device having any one of the first to fourth configurations described above prevents image defects caused by the development history phenomenon. Image formation can be performed.

1 is a schematic cross-sectional view illustrating an overall configuration of an image forming apparatus including a developing device according to an embodiment of the present invention. Side surface sectional drawing which shows the structure of the image development apparatus concerning 1st Embodiment of this invention. The figure which shows typically the magnetic brush formed between a magnetic roller and a developing roller in the image development apparatus concerning this embodiment. The figure which shows typically the magnetic field formed between a magnetic roller and a developing roller, when the magnetic pole inside a developing roller is formed from a substantially uniform permanent magnet. The figure which shows typically the magnetic brush formed of the magnetic field of FIG. The figure which shows typically the magnetic brush formed between a magnetic roller and a developing roller in the developing device which concerns on 2nd Embodiment of this invention. A graph showing the relationship between the developing roller driving time and the toner layer thickness per unit area on the developing roller

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view of an image forming apparatus including a developing device according to an embodiment of the present invention. Here, a tandem color image forming apparatus is shown. In the main body of the color image forming apparatus 100, four image forming portions Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (the right side in FIG. 1). These image forming units Pa to Pd are provided corresponding to images of four different colors (cyan, magenta, yellow, and black), and cyan, magenta, yellow, and black are respectively subjected to charging, exposure, and development processes. Are sequentially formed.

  Photosensitive drums (image carriers) 1a, 1b, 1c, and 1d that carry visible images (toner images) of the respective colors are disposed in the image forming portions Pa to Pd, and these photosensitive drums 1a. After the toner images formed on ˜1d are rotated counterclockwise in FIG. 1 by driving means (not shown) and sequentially transferred onto the intermediate transfer belt 8 moving adjacent to each image forming portion. Then, the image is transferred onto the transfer paper P at a time by the action of the secondary transfer roller 9 and further fixed on the transfer paper P in the fixing unit 7 and then discharged from the apparatus main body.

  The transfer paper P onto which the toner image is transferred is accommodated in a paper cassette 16 at the lower part of the apparatus, and is interposed between the secondary transfer roller 9 and a driving roller 11 described later via a paper feed roller 12a and a registration roller pair 12b. It is conveyed to the formed secondary transfer nip. A sheet made of dielectric resin is used for the intermediate transfer belt 8, and a (seamless) belt having no seam is used. Further, a blade-like belt cleaner 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed on the downstream side in the rotation direction of the intermediate transfer belt 8 with respect to the position where the secondary transfer roller 9 contacts the intermediate transfer belt 8. Has been.

  Next, the image forming units Pa to Pd will be described. There are chargers 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d and image information on the photosensitive drums 1a to 1d around and below the photosensitive drums 1a to 1d that are rotatably arranged. The exposure unit 4 for exposing the toner, the developing devices 3a, 3b, 3c and 3d for forming toner images on the photosensitive drums 1a to 1d, and the developer (toner) remaining on the photosensitive drums 1a to 1d are removed. Cleaning units 5a, 5b, 5c and 5d are provided.

  When the image formation start is input by the user, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the chargers 2a to 2d, and then light is irradiated by the exposure unit 4 to each of the photosensitive drums 1a to 1d. An electrostatic latent image corresponding to the image signal is formed on the top. Each of the developing devices 3a to 3d is filled with a predetermined amount of cyan, magenta, yellow, and black toner together with a carrier. The toner is supplied onto the photosensitive drums 1a to 1d by the developing devices 3a to 3d and electrostatically attached, whereby a toner image corresponding to the electrostatic latent image formed by exposure from the exposure unit 4 is formed. It is formed. An amount of toner corresponding to the amount of toner consumed in the developing devices 3a to 3d is supplied to the developing devices 3a to 3d from toner containers (not shown) for the respective colors.

  Then, after a predetermined electric field is applied to the intermediate transfer belt 8, the cyan, magenta, yellow, and black toner images on the photosensitive drums 1a to 1d are intermediate transferred by the intermediate transfer rollers (primary transfer rollers) 6a to 6d. Transferred onto the belt 8. These four-color images are formed with a predetermined positional relationship predetermined for full-color image formation. Thereafter, the toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the cleaning units 5a to 5d in preparation for the subsequent formation of a new electrostatic latent image.

  The intermediate transfer belt 8 is stretched over an upstream conveying roller 10, a downstream driving roller 11, and a tension roller 51 upstream of the conveying roller 10 and downstream of the driving roller 11. When the intermediate transfer belt 8 starts to rotate clockwise as the drive roller 11 rotates (not shown), the transfer paper P is conveyed from the registration roller 12b to the secondary transfer nip at a predetermined timing, and a full color image is transferred. Transferred to paper P. The transfer paper P onto which the toner image is transferred is conveyed to the fixing unit 7.

  The transfer paper P conveyed to the fixing unit 7 is heated and pressurized by the fixing roller pair 13 so that the toner image is fixed on the surface of the transfer paper P, and a predetermined full color image is formed. The transfer paper P on which the full-color image is formed is distributed in the transport direction by the branching portion 14 that branches in a plurality of directions. When an image is formed only on one side of the transfer paper P, it is discharged as it is onto the discharge tray 17 by the discharge roller 15.

  On the other hand, when images are formed on both sides of the transfer paper P, the transfer paper P that has passed through the fixing unit 7 is distributed to the paper transport path 18 by the branching unit 14 and is transferred to the secondary transfer nip with the image surface reversed. Re-conveyed. Then, the next image formed on the intermediate transfer belt 8 is transferred to the surface of the transfer paper P where the image is not formed by the action of the secondary transfer roller 9, and is conveyed to the fixing unit 7 to fix the toner image. After that, it is discharged to the discharge tray 17.

  FIG. 2 is a side cross-sectional view showing the configuration of the developing device according to the first embodiment of the present invention, and FIG. 3 shows the magnetic force formed between the magnetic roller and the developing roller in the developing device according to the present embodiment. It is a figure which shows a brush typically. Portions common to those in FIG. 1 are denoted by common reference numerals and description thereof is omitted. Here, the developing device 3a disposed in the image forming unit Pa of FIG. 1 will be described, but the configuration of the developing devices 3b to 3d disposed in the image forming units Pb to Pd is basically the same, and thus described. Is omitted.

  As shown in FIG. 2, the developing device 3a includes a developing container 20 in which a two-component developer (hereinafter simply referred to as a developer) is accommodated, and the developing container 20 is divided into a first wall and a second wall by a partition wall 20a. The first and second agitating chambers 20b and 20c are divided into agitating chambers 20b and 20c, and a toner (positively charged toner) supplied from a toner container (not shown) is mixed with a carrier and agitated and charged. The stirring screw 21a and the second stirring screw 21b are rotatably arranged.

  Then, the developer is conveyed in the axial direction while being stirred by the first stirring screw 21a and the second stirring screw 21b, and between the partition wall 20a and the developing container 20, that is, both ends of the partition wall 20a and the housing of the developing container 20. Circulates between the first and second stirring chambers 20b and 20c through a developer passage (not shown) formed between the first stirring chamber 20b and the second stirring chamber 20c. In the illustrated example, the developing container 20 extends obliquely upward to the left, and a magnetic roller 22 is disposed above the first stirring screw 21 a in the developing container 20, and the developing is performed obliquely upward to the left of the magnetic roller 22. Rollers 23 are arranged opposite to each other. The developing roller 23 faces the photosensitive drum 1a on the opening side (the left side in FIG. 2) of the developing container 20, and the magnetic roller 22 and the developing roller 23 respectively have a first support shaft 22b and a second support shaft 23b. It rotates clockwise in the figure as the center.

  Note that a toner concentration sensor (not shown) is disposed in the developing container 20 so as to face the second stirring screw 21b, and the toner supply port 20d from the toner container according to the toner concentration detected by the toner concentration sensor. The toner is supplied into the developing container 20 via

  The magnetic roller 22 includes a non-magnetic first rotating sleeve 22a and a first fixed magnet roller body 41 having a plurality of magnetic poles (here, five poles) contained in the first rotating sleeve 22a. The rotating sleeve 22a is rotated clockwise around the first support shaft 22b by a driving device (not shown).

  The developing roller 23 includes a non-magnetic second rotating sleeve 23a and a second fixed magnet roller body 43 having two magnetic poles contained in the second rotating sleeve 23a. Are opposed to each other with a predetermined gap at the closest position. Further, the second rotating sleeve 23a is rotated clockwise in the drawing around the second support shaft (support shaft) 23b by a driving device (not shown). Details of the magnetic roller 22 and the developing roller 23 will be described later.

  Further, a spike cutting blade 25 as a regulating member is attached to the developing container 20 along the longitudinal direction of the magnetic roller 22 (the front and back direction in FIG. 2). The spike cutting blade 25 is rotated in the rotational direction of the magnetic roller 22. (Clockwise in the figure), the roller is positioned so as to face the upstream side of the position where the developing roller 23 and the magnetic roller 22 face each other. A slight gap (gap) is formed between the front end of the spike cutting blade 25 and the surface of the magnetic roller 22.

  A developing bias composed of a DC voltage and an AC voltage is applied to the magnetic roller 22 and the developing roller 23 from a bias applying device (not shown). As described above, the toner is circulated through the developing container 20 while the developer is stirred by the first stirring screw 21a and the second stirring screw 21b to positively charge the toner, and the developer is fed to the magnetic roller 22 by the first stirring screw 21a. To be transported.

  The first fixed magnet roller body 41 of the magnetic roller 22 includes a first N pole 41a (first magnetic pole portion), a second N pole 41c, a third N pole 41d, a first S pole 41b, and a second S pole formed from permanent magnets. It has five magnetic poles consisting of 41e. The first N pole 41a has a circumferential surface along the first rotating sleeve 22a and has a substantially fan-shaped cross section centered on the first support shaft 22b. The circumferential surface is a position facing the developing roller 23. It is formed so that it may be arranged.

  When the first rotating sleeve 22a rotates clockwise in FIG. 2, the developer in the first stirring chamber 20b is applied with a magnetic field by the third N pole 41d and is carried on the first rotating sleeve 22a. Next, the developer carried on the first rotating sleeve 22 a is applied with a magnetic field attracted by the second S pole 41 e and reaches the ear cutting blade 25. Since the second S pole 41e faces the spike cutting blade 25, an N pole is induced at the tip of the spike cutting blade 25, and a magnetic field in the attracting direction is generated in the regulation gap.

  By this magnetic field, a developer chain (magnetic brush) in which a carrier and toner are connected is formed between the spike cutting blade 25 and the magnetic roller 22. Then, the layer thickness is regulated by the magnetic brush passing through the regulation gap, and a magnetic brush having a predetermined layer thickness (developer amount) is formed on the magnetic roller 22. On the other hand, the developer that has not been used to form the magnetic brush falls along the side surface of the developing container 20 on the upstream side of the ear cutting blade 25.

  When the first rotating sleeve 22a of the magnetic roller 22 rotates clockwise in the drawing and the magnetic brush reaches the vicinity of the position facing the developing roller 23, the first N pole 41a of the magnetic roller 22 and the second of the developing roller 23 It passes through the magnetic field formed by the fixed magnet roller body 43.

  On the other hand, as shown in FIG. 3, the second fixed magnet roller body 43 of the developing roller 23 includes a third S pole 43a (second magnetic pole portion, first magnetic pole member) and a fourth S pole 43b formed from permanent magnets. The third S pole 43a and the fourth S pole 43b have peripheral surfaces along the second rotating sleeve 23a, and have a second support. The section is formed in a substantially fan shape with the shaft 23b as the center. These fan-shaped radii are formed equally.

  Further, the circumferential surface of the third S pole 43a is disposed at a position facing the first N pole 41a of the magnetic roller 22, and the circumferential length is substantially the same as the circumferential length of the first N pole 41a. Is set. The fourth S pole 43b is disposed on the upstream side of the third S pole 43a with respect to the rotation direction of the developing roller 23 so as to be in contact therewith. The fourth S pole 43b has a normal direction magnetic flux density on the surface of the developing roller 23 (a radial direction magnetic flux density of the developing roller 23, hereinafter referred to as a normal direction magnetic flux density), and a normal direction magnetic flux density of the third S pole 43a. It is formed so as to be larger.

  Further, the distribution of the magnetic flux density of the magnetic field formed between the magnetic roller 22 and the developing roller 23 has a peak value (the first supporting shaft 22b of the magnetic roller 22 and the second supporting shaft 23b of the developing roller 23 in FIG. 3). Is set so that the pole width having a normal magnetic flux density of 80% or more with respect to the second support shaft 23b of the developing roller 23 is 45 ° or more. did.

  Thereby, a magnetic brush can be satisfactorily formed between the first N pole 41a as well as the fourth S pole 43b as well as the position facing the third S pole 43a. Such a magnetic brush can be formed satisfactorily by setting the above-mentioned pole width to 30 ° or more, and can be formed more satisfactorily by setting it to 45 ° or more as described above. Thus, by making the normal direction magnetic flux density of the 4th S pole 43b larger than the 3rd S pole 43a, not only between the 1st N pole 41a and the 3rd S pole 43a but the 1st N pole 41a and the 4th S pole A magnetic brush can be generated also between 43b.

  In the region where the magnetic brush is formed in this way, the magnetic brush on the first rotating sleeve 22a rises under the influence of a magnetic field having a strong magnetic binding force and contacts the second rotating sleeve 23a as shown in FIG. To do. The toner in the magnetic brush moves away from the toner due to the potential difference between the DC voltage applied to the magnetic roller 22 and the DC voltage applied to the developing roller 23, and a thin toner layer (on the developing roller 23 ( Toner layer) is formed.

  Here, FIG. 4 schematically shows a magnetic field formed between the magnetic roller 22 and the developing roller 23 when the magnetic pole in the developing roller 23 is formed of a permanent magnet, and FIG. 5 shows the magnetic field of FIG. The magnetic brush formed by is schematically shown. 2 and 3 are denoted by common reference numerals, and description thereof is omitted. In FIG. 4, the fifth S pole 43 c inside the developing roller 23 is formed from a permanent magnet, and is formed from the position facing the magnetic roller 22 to the upstream side in the rotation direction of the developing roller 23.

  When the fifth S pole 43c is formed of a permanent magnet in this way, a magnetic field is generated between the fifth S pole 43c and the Nth pole 41a at a position where the magnetic roller 22 and the developing roller 23 face each other. A magnetic field is not generated between the fifth S pole 43c and the first N pole 41a on the upstream side in the rotation direction of the developing roller 23 from the facing position.

  At this time, the magnetic flux density distribution of the magnetic field on the surface of the developing roller 23 has a peak value (normal direction in the vicinity of a straight line connecting the first support shaft 22b of the magnetic roller 22 and the second support shaft 23b of the developing roller 23 in FIG. The pole width having a normal magnetic flux density of 80% or more with respect to the second support shaft 23b of the developing roller 23 is set to 20 °.

  For this reason, as shown in FIG. 5, the magnetic brush on the magnetic roller 22 is formed only in a portion of the first N pole 41a and the fifth S pole 43c facing the first N pole 41a, and the magnetic brush forming region. Cannot be increased in the circumferential direction. Therefore, it is difficult to improve the mobility (developability) of the toner from the magnetic roller 22 to the developing roller 23 even if the fifth S pole 43c is formed upstream from the position facing the first N pole. It is also difficult to improve the toner recovery from the developing roller 23 to the magnetic roller 22.

  However, in the present embodiment, the magnetic flux density in the normal direction of the fourth S pole 43b is larger than that of the third S pole 43a, so that a magnetic field is also generated between the fourth S pole 43b and the first N pole 41a. As a result, as shown in FIG. 3, the magnetic brush on the magnetic roller 22 rises not only from the first N pole 41a toward the third S pole 43a but also toward the fourth S pole 43b. Contact. Accordingly, since the magnetic brush forming region can be enlarged in the circumferential direction, the toner can easily move from the magnetic roller 22 to the developing roller 23, and the developability thereof can be improved.

  The thickness of the thin toner layer formed on the developing roller 23 varies depending on the resistance of the developer and the rotational speed difference between the magnetic roller 22 and the developing roller 23, but the magnetic roller 22 and the developing roller 23 It can be controlled by the direct-current potential difference (effective potential). When the effective potential is increased, the toner layer on the developing roller 23 becomes thicker, and when the effective potential is decreased, the toner layer becomes thinner.

  As shown in FIG. 2, the toner thin layer on the developing roller 23 is conveyed to a portion (developing region) opposite to the photosensitive drum 1 a and the developing roller 23 by the rotation of the developing roller 23, and is applied to the developing roller 23. Due to the bias, the toner flies due to a potential difference from the electrostatic latent image formed on the photosensitive drum 1a, and the electrostatic latent image on the photosensitive drum 1a is developed. Undeveloped residual toner that is not used for development and remains on the developing roller 23 is transported again to a position where the developing roller 23 and the magnetic roller 22 face each other, and is collected by the magnetic brush on the magnetic roller 22. The

  At this time, as shown in FIG. 3 described above, since the magnetic brush forming area extends to the fourth S pole 43b of the developing roller 23, the remaining toner on the developing roller 23 is physically removed by more magnetic brushes. Can be scraped off. Accordingly, it is possible to improve the recoverability of the residual toner.

  As shown in FIG. 2, the developer that has not been used for the formation and development of the toner thin layer is given a magnetic field attracted by the first S pole 41b when the first rotating sleeve 22a further rotates clockwise. A magnetic field attracted by the second N pole 41c is applied, and the magnetic field moves above the first stirring screw 21a. Further, when the first rotating sleeve 22a rotates clockwise in the drawing, a magnetic field repelling is applied at the boundary between the second N pole 41c and the third N pole 41d, so that the developer separates from the first rotating sleeve 22a.

  The separated developer is agitated and conveyed by the first agitation screw 21a and the second agitation screw 21b, and again becomes a two-component developer uniformly charged with an appropriate toner concentration. Thereafter, as described above, a magnetic field is applied by the third N pole 41d and is again carried on the magnetic roller 22.

  As described above, the magnetic brush can be effectively formed widely from the facing position of the magnetic roller 22 and the developing roller 23 to the upstream side in the rotation direction of the developing roller 23. In addition to improving the developability of the toner, it is possible to improve the recoverability of the development residual toner from the developing roller 23 to the magnetic roller 22. Thereby, the development history phenomenon can be more effectively prevented.

  In the above embodiment, since the developing roller 23 is provided with the third S pole 43a and the fourth S pole 43b having a normal magnetic flux density larger than that of the third S pole 43a, only two permanent magnets (magnetic pole members) are used. With this, the formation area of the magnetic brush can be easily expanded. However, the magnetic pole portion provided in the developing roller 23 is not particularly limited to this embodiment. Moreover, although the 3rd S pole 43a and the 4th S pole 43b can be fixed by sticking etc. by an adhesive agent etc., these can also be formed integrally.

  FIG. 6 is a diagram schematically showing a magnetic brush formed between the magnetic roller and the developing roller in the developing device according to the second embodiment of the present invention. As shown in FIG. 6, in this embodiment, instead of providing the third S pole 43a and the fourth S pole 43b on the developing roller 23, a sixth S pole 43d (second magnetic pole portion) is provided. The sixth S pole 43d has a circumferential surface along the second rotation sleeve 23a and has a substantially sectoral cross section around the second support shaft 23b of the developing roller 23. The circumferential surface is a rotation of the developing roller 23. It is formed from the facing position to the upstream side with respect to the direction.

  Further, the sixth S pole 43d is formed such that the normal direction magnetic flux density increases from the position facing the magnetic roller 22 toward the upstream side with respect to the rotation direction of the developing roller 23. Accordingly, a magnetic field can be generated by the first N pole 41a from the portion of the sixth S pole 43d facing the first N pole 41a to the upstream end in the rotation direction of the developing roller 23. Therefore, the magnetic brush forming region can be expanded from the position facing the magnetic roller 22 to the upstream side in the rotation direction of the developing roller 23.

  Also in the present embodiment, the distribution of the magnetic flux density on the surface of the developing roller 23 is 45% with respect to the second support shaft 23b of the developing roller 23. It set so that it might be more than °.

  In the present embodiment, since the sixth S pole 43d is formed from one permanent magnet (magnetic pole member), the number of members can be reduced. Further, since the normal direction magnetic flux density of the sixth S pole 43d is gradually increased from the facing position toward the upstream side with respect to the rotation direction of the developing roller 23, the distribution of the magnetic field formed with the first N pole 41a is increased. More detailed settings can be made. Thereby, the formation state of a magnetic brush can be made favorable. Other configurations are the same as those in the first embodiment, and thus the description thereof is omitted.

  In addition, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, the shapes of the first N pole 41a of the magnetic roller 22, the third S pole 43a, the fourth S pole 43b, and the sixth S pole 43d of the developing roller 23, the normal direction magnetic flux density, the magnetic pole width, etc. There is no particular limitation, and the properties of the toner and the carrier, the material of the first rotating sleeve 22a of the magnetic roller 22 and the material of the second rotating sleeve 23a of the developing roller 23, the formation state of the magnetic brush, the amount of toner from the magnetic roller 22 to the developing roller 23, It can be appropriately set by a preliminary experiment or the like according to the moving state, the state of collecting the development residual toner from the developing roller 23 to the magnetic roller 22, and the like.

  Moreover, in the said embodiment, although the magnetic roller 22 was arrange | positioned diagonally upward of the 1st stirring screw 21a, this arrangement | positioning is not specifically limited to the said embodiment, For example, it is magnetic under the 1st stirring screw 21a, or diagonally downward. A roller 22 can also be arranged. Further, the arrangement of the developing roller 23 and the photosensitive drums 1a to 1d can be appropriately changed according to the arrangement of the magnetic roller 22. Moreover, in the said embodiment, although the 1st fixed magnet roller body 41 of the magnetic roller 22 was comprised from five magnetic poles 41a-41e, it is not specifically limited to this magnetic pole structure, For example, it comprises from seven poles. You can also

  In the above embodiment, the reversal development method in which positively charged toner having a positive charging direction (positive side) is used and the toner is ejected to the exposed portion of the surface of the photosensitive member is described as an example. However, the charging direction is negative ( The present invention can be applied in exactly the same manner to a developing device using negatively charged toner (minus side) and a forward developing type developing device that causes toner to fly to an unexposed portion on the surface of the photoreceptor.

  Further, the present invention is not limited to the tandem type color printer shown in FIG. 1, and includes a developing device such as a digital or analog type monochrome copying machine, a monochrome printer, a rotary developing type color printer, a color copying machine, a facsimile, or the like. The present invention can be applied to various image forming apparatuses.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention is not limited to a present Example.

  Using the image forming apparatus 100 on which the developing devices 3a to 3d according to the first embodiment of the present invention shown in FIGS. 2 and 3 are mounted, the development provided in the developing device 3a among the four developing devices 3a to 3d. The toner amount (toner layer thickness) per unit area on the roller 23 was measured. As described above, the magnetic field between the magnetic roller 22 and the developing roller 23 has a pole width with a normal direction magnetic flux density of 80% or more with respect to the peak value with respect to the second support shaft 23b of the developing roller 23. The angle was set to 45 °.

  Further, the outer diameter of the magnetic roller 22 is 20 mm, the peripheral speed of the magnetic roller 22 is 760 mm / sec, the peak value of the normal direction magnetic flux density of the third S pole 43a of the developing roller 23 is 45 mT, and the normal direction of the fourth S pole 43b. The peak value of the magnetic flux density is 60 mT, the outer diameter of the developing roller 23 is 20 mm, the peripheral speed of the developing roller 23 is 760 mm / sec, the gap between the magnetic roller 22 and the developing roller 23 is 0.3 mm, and the photosensitive drum 1a The gap with the developing roller 23 was 0.12 mm, and the peripheral speed of the photosensitive drum 1a was 460 mm / sec.

  The DC voltage Vdc applied to the magnetic roller 22 is 480 V, the AC voltage Vpp is 2.3 kV, the frequency is 4.5 kHz, the duty ratio is 70%, the DC voltage Vdc applied to the developing roller 23 is 120 V, and the AC voltage is Vpp was 1.6 kV, the frequency was 4.5 kHz, and the duty ratio was 45%.

  Under such conditions, the driving time of the developing roller 23 and the developing roller when the toner movement from the magnetic roller 22 to the developing roller 23 and the collection of the development residual toner from the developing roller 23 to the magnetic roller 22 are continuously performed, 23, the relationship between the toner layer thickness per unit area on 23 was examined. The results are shown in FIG.

Comparative example

  Instead of providing the third S pole 43a and the fourth S pole 43b on the developing roller 23, the fifth S pole 43c shown in FIGS. 4 and 5 is provided, and the magnetic field between the magnetic roller 22 and the developing roller 23 is peaked. An experiment was conducted in the same manner as in the example except that the pole width having a normal direction magnetic flux density of 80% or more with respect to the value was set to 20 ° with respect to the second support shaft 23b of the developing roller 23. . The results are shown in FIG.

  As shown in FIG. 7, in the comparative example, the toner layer thickness per unit area on the developing roller 23 is small in the first round, and there is a large difference between the first round and the subsequent rounds. On the other hand, in the example, the toner layer thickness increased in the first round, and the toner layer thickness in the first round was about 90% of the maximum toner layer thickness after 5 seconds. As a result, in the embodiment, the toner layer thickness after the first round is stable, that is, since the toner layer thickness can be stably formed on the developing roller 23 from the first round, the development history can be prevented. I understood.

  In the present invention, a first magnetic pole portion is formed at a position facing the developing roller inside the magnetic roller, and is formed from the facing position inside the developing roller to the upstream side in the rotation direction of the developing roller, and is opposite to the first magnetic pole portion. A second magnetic pole portion having a polarity, and the normal magnetic flux density on the surface of the developing roller in the second magnetic pole portion is configured such that the upstream side in the rotational direction is larger than the facing position. .

  As a result, the development history phenomenon can be effectively prevented, so that the toner diameter can be reduced and the image quality can be improved when forming a color image. In addition, the second magnetic pole portion includes a first magnetic pole member disposed at the facing position and a second magnetic pole member disposed adjacent to the upstream side of the facing position in the rotation direction, By making the magnetic pole member have a normal magnetic flux density larger than that of the first magnetic pole member, it is possible to easily widen the formation region of the magnetic brush simply by using two magnetic pole members.

  Further, by increasing the normal direction magnetic flux density of the second magnetic pole part from the opposing position toward the upstream side in the rotation direction, the number of members is reduced and the magnetic field formed by the first magnetic pole part is further increased. Can be set in detail. Further, the magnetic field generated by the first magnetic pole part and the second magnetic pole part has a pole width having a normal direction magnetic flux density of 80% or more with respect to the peak value, and is 30 ° or more with respect to the spindle of the developing roller. By setting as described above, the magnetic brush can be formed more satisfactorily. Further, by using the image forming apparatus including the developing device, it is possible to perform image formation in which image defects due to the development history phenomenon are prevented.

1a to 1d Photosensitive drum (image carrier)
3a to 3d Developing device 21a First stirring screw 21b Second stirring screw 22 Magnetic roller 22a First rotating sleeve 22b First support shaft 23 Developing roller 23a Second rotating sleeve 23b Second support shaft (support shaft)
25 Panicle cutting blade 41 First fixed magnet roller body 41a First N pole (first magnetic pole part)
43 2nd fixed magnet roller body 43a 3rd S pole (2nd magnetic pole part, 1st magnetic pole member)
43b 4th S pole (2nd magnetic pole part, 2nd magnetic pole member)
43d 6th S pole (second magnetic pole part)
100 Image forming apparatus Pa to Pd Image forming unit

Claims (5)

Two-component developer containing carrier and toner is used,
A developing roller disposed opposite the image carrier;
A developing device that develops an electrostatic latent image on the surface of the image carrier from the developing roller, and a magnetic roller that forms a toner thin layer using a magnetic brush on the developing roller.
A first magnetic pole portion at a position facing the developing roller inside the magnetic roller;
A second magnetic pole portion formed in the developing roller from the facing position to the upstream side in the rotation direction of the developing roller and having a polarity opposite to that of the first magnetic pole portion;
The developing device according to claim 1, wherein a magnetic flux density in a normal direction on the surface of the developing roller in the second magnetic pole portion is larger on the upstream side in the rotation direction than on the facing position.   The second magnetic pole portion includes a first magnetic pole member disposed at the facing position, and a second magnetic pole member disposed adjacent to the upstream side of the facing position in the rotation direction. 2. The developing device according to claim 1, wherein the magnetic pole member of 2 has a magnetic flux density in the normal direction larger than that of the first magnetic pole member.   2. The developing device according to claim 1, wherein the normal magnetic flux density of the second magnetic pole portion increases toward the upstream side in the rotation direction from the facing position.   The magnetic field generated by the first magnetic pole part and the second magnetic pole part has a pole width having the normal direction magnetic flux density of 80% or more with respect to the peak value and is 30 ° or more with respect to the support shaft of the developing roller. The developing device according to claim 1, wherein the developing device is set to be   An image forming apparatus comprising the developing device according to claim 1.

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