Developing device and image forming apparatus provided therewith

1. A developing device, comprising:

a developing roller incorporating a fixed magnet body having a plurality of magnetic poles in a circumferential direction, for supplying developer to a developing region facing an image carrier;

a regulating member for regulating an amount of developer on the developing roller so as to form a developer layer region on the developing roller by means of a magnetic field formed by the regulating member and the plurality of magnetic poles of the fixed magnet body, which face the regulating member; and

a magnetic-field generating member for scraping off, on an upstream of the regulating member in a rotational direction of the developing roller, developer which is not used for development on the developing roller,

wherein the following relation Bm/Br>1 is satisfied, where Bm represents a magnetic flux density of a distal end portion of the magnetic-field generating member facing a surface of the developing roller and Br represents a magnetic flux density of a distal end portion of the regulating member facing the surface of the developing roller.

2. A developing device according to claim 1 , wherein the following relation 1.2>Bm/Br>1.8 is further satisfied.

3. A developing device according to claim 1 , wherein the following relation Km/Kr<1 is satisfied, where Km represents an interval between the distal end portion of the magnetic-field generating member and the surface of the developing roller and Kr represents an interval between the distal end portion of the regulating member and the surface of the developing roller.

4. A developing device according to claim 3 , wherein the following relation 0.3>Km/Kr>0.7 is further satisfied.

5. A developing device according to claim 1 , wherein:

the magnetic-field generating member comprises:

a magnetic body facing the surface of the developing roller at a predetermined interval; and

a magnet which comprises a facing magnetic pole facing one of the plurality of magnetic poles of the fixed magnet body, the magnet being attached to the magnetic body while facing the magnetic body in the rotational direction of the developing roller; and

the facing magnetic pole of the magnet has the same polarity as that of the one of the plurality of magnetic poles of the fixed magnet body.

6. A developing device according to claim 5 , wherein the magnetic body comprises a distal end portion facing the developing roller and an opposite facing portion on an opposite side to the distal end portion, and has a width in the circumferential direction of the developing roller smaller than a width of the opposite facing portion.

7. A developing device according to claim 1 , wherein the magnetic-field generating member comprises a magnet facing one of the plurality of magnetic poles of the fixed magnet body at a predetermined interval with respect to the surface of the developing roller, the magnet facing the one of the plurality of magnetic poles of the fixed magnet body while having opposite polarity.

8. An image forming apparatus, comprising the developing device according to claim 1 .

9. A developing device, comprising:

a developing roller incorporating a fixed magnet body having a plurality of magnetic poles in a circumferential direction, for supplying developer to a developing region facing an image carrier;

a regulating member for regulating an amount of developer on the developing roller so as to form a developer layer region on the developing roller by means of a magnetic field formed by the regulating member and the plurality of magnetic poles of the fixed magnet body, which face the regulating member; and

a magnetic-field generating member for scraping off, on an upstream of the regulating member in a rotational direction of the developing roller, developer which is not used for development on the developing roller, wherein:

the magnetic-field generating member comprises:

magnetic-field generating end portions facing both-end-portions in a longitudinal direction of the developer layer region; and

a magnetic-field generating central portion sandwiched in the longitudinal direction between the magnetic-field generating end portions; and

an interval between each of the magnetic-field generating end portions and a surface of the developing roller is smaller than an interval between the magnetic-field generating central portion and the surface of the developing roller.

10. A developing device according to claim 9 , wherein:

the magnetic-field generating member comprises:

a magnetic body facing the surface of the developing roller at a predetermined interval; and

a magnet which comprises a facing magnetic pole facing the surface of the developing roller, the magnet being attached to the magnetic body while facing the magnetic body in the rotational direction of the developing roller;

the facing magnetic pole of the magnet has the same polarity as that of one of the plurality of magnetic poles of the fixed magnet body; and

the interval between each of the magnetic-field generating end portions of the magnetic body and the surface of the developing roller is smaller than the interval between the magnetic-field generating central portion of the magnetic body and the surface of the developing roller.

11. A developing device according to claim 9 , wherein:

the magnetic-field generating member comprises:

a magnetic body facing the surface of the developing roller at a predetermined interval; and

a magnet which comprises a facing magnetic pole facing the surface of the developing roller, the magnet being attached to the magnetic body while facing the magnetic body in the rotational direction of the developing roller;

the magnetic body extends to both-end-portions in the longitudinal direction of the developer layer region;

the magnet comprises a pair of magnets arranged so as to face the both-end-portions in the longitudinal direction of the developer layer region; and

the facing magnetic pole of each of the pair of magnets has the same polarity as that of one of the plurality of magnetic poles of the fixed magnet body; and

the interval between each of the magnetic-field generating end portions of the magnetic body and the surface of the developing roller is smaller than the interval between the magnetic-field generating central portion of the magnetic body and the surface of the developing roller.

12. A developing device according to claim 9 , wherein:

the magnetic-field generating member comprises a magnetic body facing the surface of the developing roller at a predetermined interval;

the magnetic body faces one of the plurality of magnetic poles of the fixed magnet body; and

the interval between each of the magnetic-field generating end portions of the magnetic body and the surface of the developing roller is smaller than the interval between the magnetic-field generating central portion of the magnetic body and the surface of the developing roller.

13. A developing device according to claim 9 , wherein:

the regulating member comprises:

end-portion regulating portions facing the both-end-portions in the longitudinal direction of the developer layer region; and

a central regulating portion sandwiched in the longitudinal direction between the end-portion regulating portions;

each of the end-portion regulating portions has a magnetic force higher than a magnetic force of the central regulating portion; and

the magnetic-field generating end portions of the magnetic-field generating member are formed so as to face the developer layer region while including a part of the developer layer region, the part corresponding to a boundary between each of the end-portion regulating portions and the central regulating portion.

14. A developing device, comprising:

a developing roller incorporating a fixed magnet body having a plurality of magnetic poles in a circumferential direction, for supplying developer to a developing region facing an image carrier;

a regulating member for regulating an amount of developer on the developing roller so as to form a developer layer region on the developing roller by means of a magnetic field formed by the regulating member and the plurality of magnetic poles of the fixed magnet body, which face the regulating member; and

a magnetic-field generating member for scraping off, on an upstream of the regulating member in a rotational direction of the developing roller, developer which is not used for development on the developing roller, wherein:

the magnetic-field generating member comprises:

magnetic-field generating end portions facing both-end-portions in a longitudinal direction of the developer layer region; and

a magnetic-field generating central portion sandwiched in the longitudinal direction between the magnetic-field generating end portions; and

each of the magnetic-field generating end portions has a magnetic force higher than a magnetic force of the magnetic-field generating central portion.

15. A developing device according to claim 14 , wherein:

the magnetic-field generating member comprises:

a magnetic body facing a surface of the developing roller; and

a magnet which comprises a facing magnetic pole facing the surface of the developing roller, the magnet being attached to the magnetic body while facing the magnetic body in the rotational direction of the developing roller;

the facing magnetic pole of the magnet has the same polarity as that of one of the plurality of magnetic poles of the fixed magnet body; and

the magnet has a magnetic force higher on magnetic-field-generating-end-portion sides than a magnetic force on a magnetic-field-generating-central-portion side of the magnet.

16. A developing device according to claim 14 , wherein:

the magnetic-field generating member comprises:

a magnetic body facing the surface of the developing roller; and

a magnet which comprises a facing magnetic pole facing the surface of the developing roller, the magnet being attached to the magnetic body while facing the magnetic body in the rotational direction of the developing roller;

the magnetic body extends to both-end-portions in the longitudinal direction of the developer layer region;

the magnet comprises a pair of magnets arranged so as to face the both-end-portions in the longitudinal direction of the developer layer region; and

the facing magnetic pole of each of the pair of magnets has the same polarity as that of one of the plurality of magnetic poles of the fixed magnet body.

17. A developing device according to claim 14 , wherein:

the regulating member comprises:

end-portion regulating portions facing the both-end-portions in the longitudinal direction of the developer layer region; and

a central regulating portion sandwiched in the longitudinal direction between the end-portion regulating portions;

each of the end-portion regulating portions has a magnetic force higher than a magnetic force of the central regulating portion; and

the magnetic-field generating end portions of the magnetic-field generating member are formed so as to face the developer layer region while including a part of the developer layer region, the part corresponding to a boundary between each of the end-portion regulating portions and the central regulating portion.

18. A developing device according to claim 14 , further comprising:

a first stirring member facing the developing roller, for supplying the developer;

a second stirring member for stirring and conveying the developer together with the first stirring member;

a first conveyance path in which the developer is conveyed by the first stirring member;

a second conveyance path which is arranged in parallel with the first conveyance path and in which the developer is conveyed by the second stirring member;

a first communication portion for allowing the developer to flow from the first conveyance path to the second conveyance path; and

a second communication portion for allowing the developer to flow from the second conveyance path to the first conveyance path, wherein:

the magnetic-field generating member is arranged between the developing roller and the first stirring member; and

the first communication portion is arranged at a position not facing the magnetic-field generating member.

19. A developing device according to claim 18 , wherein:

the first stirring member comprises a rotary shaft and a helical blade formed about the rotary shaft; and

a part of the helical blade, which faces each of magnetic-field generating end portions of the magnetic-field generating member, is formed to exert a high developer-conveying force.

20. A developing device, comprising:

a developing roller incorporating a fixed magnet body having a plurality of magnetic poles in a circumferential direction, for supplying developer to a developing region facing an image carrier;

a regulating member for regulating an amount of developer on the developing roller so as to form a developer layer region on the developing roller by means of a magnetic field formed also by the plurality of magnetic poles of the fixed magnet body, which face the regulating member; and

a magnetic-field generating member for scraping off, on an upstream of the regulating member in a rotational direction of the developing roller, developer which is not used for development on the developing roller,

wherein the magnetic-field generating member comprises a pair of magnetic-field generating members arranged so as to face both-end-portions in a longitudinal direction of the developer layer region, the pair of magnetic-field generating members respectively having

inner surface portions arranged so as to face each other in the developer layer region, and

outer surface portions arranged out of the developer layer region or arranged so as to face boundaries of the developer layer region.

21. A developing device according to claim 20 , wherein the inner surface portion of each of the pair of magnetic-field generating members is inclined to a central side in the longitudinal direction from the upstream to a downstream in the rotational direction of the developing roller.

22. A developing device according to claim 20 , wherein the inner surface portion of each of the pair of magnetic-field generating members is provided with an inclined surface forming a predetermined angle with respect to a surface of the developing roller, the predetermined angle of the inclined surface being smaller from the upstream to a downstream in the rotational direction of the developing roller.

23. A developing device according to claim 20 , wherein each of the pair of magnetic-field generating members comprises a magnet having a facing magnetic pole facing a surface of the developing roller, the facing magnetic pole of the magnet having the same polarity as that of one of the plurality of magnetic poles of the fixed magnet body facing the facing magnetic pole.

24. A developing device according to claim 20 , wherein each of the pair of magnetic-field generating members comprises a magnet having a facing magnetic pole facing a surface of the developing roller, and a magnetic body attached to the magnet while facing the magnet in the rotational direction of the developing roller, the facing magnetic pole of the magnet having the same polarity as that of one of the plurality of magnetic poles of the fixed magnet body.