Development device

Abstract

The maximum value of the absolute value of the magnetic flux density of a first development upstream pole (104) is greater than the maximum value of the absolute value of the magnetic flux density of a first development downstream pole (106). The maximum value of the absolute value of the magnetic flux density of a second development downstream pole (204) is greater than the maximum value of the absolute value of the magnetic flux density of a second development upstream pole (202). When the angle in the rotational direction of a second sleeve (34) from the position on an outer circumferential surface of the second sleeve (34) at which the absolute value of the magnetic flux density of the second development upstream pole (202) is at a maximum to the position on the outer circumferential surface of the second sleeve (34) at which the absolute value of the magnetic flux density of a second development pole (203) is at a maximum is θ1 (°) and the angle in the rotational direction of the second sleeve (34) from the position on the outer circumferential surface of the second sleeve (34) at which the absolute value of the magnetic flux density of the second development pole (203) is at a maximum to the position on the outer circumferential surface of the second sleeve (34) at which the absolute value of the magnetic flux density of the second development downstream pole (204) is at a maximum is θ2 (°), θ1>θ2.

Description

Developing device

[0001] The present invention relates to a developing device that develops an electrostatic latent image formed on an image carrier with a developer.

[0002] A typical developing device using a two-component developer consisting of toner and magnetic carriers has a magnet with multiple magnetic poles inside a developing roller, which carries the developer on the developing roller and develops an electrostatic latent image formed on an image carrier. To achieve higher image quality, a configuration using a magnet with seven or more magnetic poles has also been proposed (see JP 2008-241997 A). In such a configuration, the distance between the developing pole facing the image carrier and the magnetic poles upstream and downstream of it is short, resulting in a stronger magnetic force and denser magnetic chains that contact the image carrier near the developing pole, enabling higher image quality.

[0003] Furthermore, a developing device having two developing rollers arranged side by side in the rotation direction of an image carrier has been proposed (Japanese Patent Laid-Open No. 2013-254107). In the developing device described in Japanese Patent Laid-Open No. 2013-254107, developer is supplied from a supply unit to a first developing roller that is vertically lower than the first developing roller, and developer is transferred from the first developing roller that is vertically upper to a second developing roller that is vertically upper.

[0004] In a configuration having two developing rollers as in JP 2013-254107 A, if a magnet with seven or more magnetic poles as in Patent Document 1 is to be used as the magnet for each developing roller, it is possible to configure the magnets as follows, for example. First, another magnetic pole (transport pole) is provided between the developing pole of the first developing roller and the delivery pole that delivers the developer from the first developing roller to the second developing roller. Also, another magnetic pole (transport pole) is provided between the developing pole of the second developing roller and the receiving pole that receives the developer from the first developing roller to the second developing roller. By adding a transport pole to each developing roller in this way, the effect of improving image quality can be further achieved.

[0005] However, when a new magnetic pole (transport pole) is placed between the development pole and the transfer pole or receiving pole of each developing roller as described above, the transport pole of the first developing roller and the transport pole of the second developing roller are configured with opposite polarities. In other words, because the transfer pole and the receiving pole are opposite polarities, the transport poles of each adjacent developing roller are also opposite polarities. This results in a magnetic field being generated between the transport poles of each developing roller. If this magnetic field between the transport poles causes developer to move between the transport poles, the developer moving between the transport poles will come into contact with the image carrier, resulting in streaky fogging on the output image. To prevent this streaky fogging, it is effective to reduce the magnetic flux density of the transport pole of each developing roller, making it more difficult for a magnetic field to be generated between the transport poles.

[0006] On the other hand, when developing an electrostatic latent image on an image carrier, magnetic chains of developer are formed on the surface of a developing roller, and toner is deposited from the magnetic chains onto a photosensitive drum to develop the electrostatic latent image with the toner. During this process, carrier may adhere from the magnetic chains to the photosensitive drum (hereinafter also referred to as "carrier adhesion"). This carrier adhesion can result in image defects such as small white areas on the output image. Regarding carrier adhesion, if the magnetic chains of developer are in contact with the image carrier downstream of the portion facing the developing roller in the rotational direction, magnetic carrier is likely to remain on the image carrier, making carrier adhesion more likely.

[0007] This makes it easier for the magnetic wires near the development pole to extend upstream, while on the downstream side they quickly fold, making them less likely to come into contact, thereby making it less likely for carrier adhesion to occur. To make it easier for the magnetic wires near the development pole to extend upstream, it is advisable to make the magnetic flux density of the magnetic pole downstream of the development pole greater than the magnetic flux density of the magnetic pole upstream of the development pole. In this way, the magnetic field lines from the development pole extend downstream while wrapping around upstream, making it easier for the magnetic wires near the development pole to extend upstream.

[0008] When such a countermeasure against carrier adhesion is applied to a configuration in which a transport pole is provided between the development pole and the delivery pole or receiving pole of each developing roller, it is preferable to make the magnetic flux density of the transport pole downstream of the development pole of the first developing roller and the second developing roller greater than the magnetic flux density of the transport pole upstream of the development pole. However, in order to suppress the occurrence of the above-mentioned streak-like fogging, it is desirable to make the magnetic flux density of the transport pole downstream of the development pole of the first developing roller as small as possible.

[0009] An object of the present invention is to provide a configuration that can achieve both suppression of streak-like fogging and suppression of carrier adhesion.

[0010] One aspect of the present invention is a development container that contains a developer containing toner and a carrier; a first rotor to which the developer contained in the development container is supplied, the first rotor rotating in the same direction as the image carrier at a position on the outer circumferential surface of the first rotor that is closest to the rotating image carrier, and carrying and transporting the developer to a first development position where an electrostatic latent image formed on the image carrier is developed; a first magnet that is fixedly disposed inside the first rotor so as not to rotate, the first magnet being disposed opposite the image carrier at the first development position; a first magnet having a first upstream pole disposed adjacent to the first developing pole and having a polarity different from that of the first developing pole, a first downstream pole disposed adjacent to the first developing pole and having a polarity different from that of the first developing pole, and a delivery pole disposed downstream of the first downstream pole and upstream of the first upstream pole in the rotation direction of the first rotating body; and a developer transfer pole disposed opposite the first rotating body, and a developer transfer pole through a magnetic field generated by the first magnet. a second rotating body that rotates in the same direction as the image carrier at a position on the outer circumferential surface of the second rotating body that is closest to the image carrier and that carries and transports the developer to a second developing position where the electrostatic latent image is developed; a second magnet that is fixedly disposed inside the second rotating body so as not to rotate, the second rotating body having a second developing pole that is disposed facing the image carrier at the second developing position; a second upstream pole that is disposed adjacent to the second developing pole upstream of the second developing pole in the rotation direction of the second rotating body and has a polarity different from that of the second developing pole; a second magnet having: a second downstream pole arranged adjacent to the second developing pole downstream of the second developing pole in the rotation direction and having a polarity opposite to that of the second developing pole; and a receiving pole arranged downstream of the second downstream pole and upstream of the second upstream pole in the rotation direction of the second rotor and adjacent to the delivering pole and having a polarity opposite to that of the delivering pole, wherein the maximum absolute value of the magnetic flux density of the first upstream pole in the normal direction to the outer circumferential surface of the first rotor is greater than the maximum absolute value of the magnetic flux density of the first downstream pole in the normal direction to the outer circumferential surface of the first rotor,The maximum absolute value of the magnetic flux density of the second downstream pole in the normal direction to the outer peripheral surface of the second rotor is greater than the maximum absolute value of the magnetic flux density of the second upstream pole in the normal direction to the outer peripheral surface of the second rotor, and when the angle in the rotation direction of the second rotor from the position on the outer peripheral surface of the second rotor where the absolute value of the magnetic flux density of the second upstream pole in the normal direction to the outer peripheral surface of the second rotor is maximum to the position on the outer peripheral surface of the second rotor where the absolute value of the magnetic flux density of the second developing pole in the normal direction to the outer peripheral surface of the second rotor is maximum is θ1 [°], and the angle in the rotation direction of the second rotor from the position on the outer peripheral surface of the second rotor where the absolute value of the magnetic flux density of the second developing pole in the normal direction to the outer peripheral surface of the second rotor is maximum to the position on the outer peripheral surface of the second rotor where the absolute value of the magnetic flux density of the second downstream pole in the normal direction to the outer peripheral surface of the second rotor is θ2 [°], the relationship θ1 > θ2 is satisfied.

[0011] One aspect of the present invention is a development device comprising: a development container that contains a developer containing toner and a carrier; a first rotor to which the developer contained in the development container is supplied, the first rotor rotating in the same direction as the image carrier at a position on the outer circumferential surface of the first rotor that is closest to the rotating image carrier, and carrying and transporting the developer to a first development position where an electrostatic latent image formed on the image carrier is developed; a first magnet that is fixedly disposed inside the first rotor so as not to rotate, the first magnet being disposed opposite the image carrier at the first development position; a first magnet having a first upstream pole disposed adjacent to the first developing pole upstream of the first developing pole in the rotation direction of the rotating body and having a polarity different from that of the first developing pole, a first downstream pole disposed adjacent to the first developing pole downstream of the first developing pole in the rotation direction of the first rotating body and having a polarity different from that of the first developing pole, and a delivery pole disposed downstream of the first downstream pole and upstream of the first upstream pole in the rotation direction of the first rotating body; a second rotating body to which the developer is transferred from the first rotating body, the second rotating body rotating in the same direction as the image carrier at a position on the outer circumferential surface of the second rotating body closest to the image carrier, and carrying and transporting the developer to a second developing position where the electrostatic latent image is developed; a second magnet disposed inside the second rotating body in a non-rotating fixed manner, the second rotating body having a second developing pole disposed opposite the image carrier at the second developing position; and a second magnet disposed adjacent to the second developing pole upstream of the second developing pole in the rotation direction of the second rotating body, the second developing pole being disposed adjacent to the second developing pole. a second magnet having a second upstream pole having a polarity opposite to that of the second developing pole, a second downstream pole arranged adjacent to the second developing pole downstream of the second developing pole in the rotation direction of the second rotor and having a polarity opposite to that of the second developing pole, and a receiving pole arranged downstream of the second downstream pole and upstream of the second upstream pole in the rotation direction of the second rotor and adjacent to the delivering pole and having a polarity opposite to that of the delivering pole, from a position on the outer circumferential surface of the first rotor where the absolute value of the magnetic flux density of the first upstream pole in the normal direction to the outer circumferential surface of the first rotor is maximum,is a maximum value of the absolute value of the magnetic flux density in a tangential direction to the outer circumferential surface of the first rotor in a section in the rotation direction of the first rotor from a position on the outer circumferential surface of the first rotor where the absolute value of the magnetic flux density of the first development pole in the normal direction to the outer circumferential surface of the first rotor is maximum, and is a maximum value of the absolute value of the magnetic flux density in a tangential direction to the outer circumferential surface of the first rotor in a section in the rotation direction of the first rotor from a position on the outer circumferential surface of the first rotor where the absolute value of the magnetic flux density of the first development pole in the normal direction to the outer circumferential surface of the first rotor is maximum to a position on the outer circumferential surface of the first rotor where the absolute value of the magnetic flux density of the first downstream pole in the normal direction to the outer circumferential surface of the first rotor is maximum, |Bθ1-1| where |Bθ2-1| is the maximum value of the absolute value of the magnetic flux density in the tangential direction to the outer peripheral surface of the second rotor in a section in the rotation direction of the second rotor from the position on the outer peripheral surface of the second rotor where the absolute value of the magnetic flux density is maximum to the position on the outer peripheral surface of the second rotor where the absolute value of the magnetic flux density of the second developing pole in the normal direction to the outer peripheral surface of the second rotor is maximum, and |Bθ2-2| is the maximum value of the absolute value of the magnetic flux density in the tangential direction to the outer peripheral surface of the second rotor in a section in the rotation direction of the second rotor from the position on the outer peripheral surface of the second rotor where the absolute value of the magnetic flux density of the second developing pole in the normal direction to the outer peripheral surface of the second rotor is maximum to the position on the outer peripheral surface of the second rotor where the absolute value of the magnetic flux density of the second downstream pole in the normal direction to the outer peripheral surface of the second rotor is maximum, and

[0012] According to the present invention, it is possible to simultaneously suppress streak-like fogging and carrier adhesion.

[0013] FIG. 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus according to an embodiment.

[0014] FIG. 2 is a cross-sectional view showing the schematic configuration of the developing device according to the embodiment.

[0015] FIG. 3 is a diagram showing the arrangement of magnetic poles of the first developing roller according to the embodiment.

[0016] FIG. 4 is a diagram showing the arrangement of magnetic poles of the second developing roller according to the embodiment.

[0017] FIG. 5 is a diagram showing the arrangement of magnetic poles of the peeling roller according to the embodiment.

[0018] FIG. 6 is a diagram showing the relationship between the magnetic pole arrangements of the first and second developing rollers according to the embodiment.

[0019] FIG. 7 is a schematic diagram showing the state of magnetic lines of force when the absolute value of the normal component of the magnetic flux density is greater in the magnetic pole located adjacent to the downstream side of the development pole than in the magnetic pole located adjacent to the upstream side of the development pole.

[0020] FIG. 8 is a schematic diagram showing the state of magnetic lines of force when the absolute value of the normal component of the magnetic flux density is greater in the magnetic pole located adjacent to the upstream side of the development pole than in the magnetic pole located adjacent to the downstream side of the development pole.

[0021] FIG. 9 is a diagram showing the relationship between the magnetic pole arrangements of the first and second developing rollers according to the embodiment, and is a diagram showing the relationship between the transport poles of the first and second developing rollers.

[0022] FIG. 10 is a graph showing (a) the magnetic characteristics of the first developing roller and (b) the magnetic characteristics of the second developing roller according to Comparative Example 1.

[0023] FIG. 11 is a graph showing (a) the magnetic characteristics of the first developing roller and (b) the magnetic characteristics of the second developing roller according to Example 1.

[0024] FIG. 12 is a graph showing (a) the magnetic characteristics of the first developing roller and (b) the magnetic characteristics of the second developing roller according to Example 2.

[0025] FIG. 13 is a graph showing the distribution of the normal component of the magnetic flux density of the second developing roller according to Comparative Example 1 and Examples 1 and 2. In FIG.

[0026] FIG. 14 is a graph showing the distribution of the tangential component of the magnetic flux density of the second developing roller according to Comparative Example 1 and Examples 1 and 2. In FIG.

[0027] FIG. 15 is a graph showing (a) the magnetic characteristics of the first developing roller and (b) the magnetic characteristics of the second developing roller according to Example 3.

[0028] FIG. 16 is a graph showing (a) the magnetic characteristics of the first developing roller and (b) the magnetic characteristics of the second developing roller according to Comparative Example 2.

[0029] FIG. 17 is a graph showing the distribution of the normal component of the magnetic flux density of the first developing roller according to Examples 1 and 3 and Comparative Example 2.

[0030] FIG. 18 is a graph showing the distribution of the tangential component of the magnetic flux density of the first developing roller according to Examples 1 and 3 and Comparative Example 2.

[0031] FIG. 19 is a graph showing (a) the magnetic characteristics of the first developing roller and (b) the magnetic characteristics of the second developing roller according to Example 4.

[0032] FIG. 20 is a graph showing the distribution of the normal component of the magnetic flux density of the first developing roller according to Examples 1 and 4.

[0033] FIG. 21 is a graph showing the distribution of the tangential component of the magnetic flux density of the first developing roller according to Examples 1 and 4. In FIG.

[0034] The embodiment will be described with reference to FIGS. 1 to 21. First, the schematic configuration of the image forming apparatus of the embodiment will be described with reference to FIG. 1. [Image forming apparatus]

[0035] The image forming apparatus 100 is a full-color image forming apparatus, and in this embodiment, is, for example, an MFP (Multi-Function Peripheral) having a copy function, a printer function, and a scan function. As shown in Figure 1, the image forming apparatus 100 has image forming units PY, PM, PC, and PK arranged in parallel, each of which performs an image forming process for forming a toner image of four colors: yellow, magenta, cyan, and black.

[0036] The image forming units PY, PM, PC, and PK for each color include primary chargers 21Y, 21M, 21C, and 21K, developing units 1Y, 1M, 1C, and 1K, optical writing units (exposure units) 22Y, 22M, 22C, and 22K, photosensitive drums 28Y, 28M, 28C, and 28K, and cleaning units 26Y, 26M, 26C, and 26K. The image forming apparatus 100 also includes a transfer unit 2 and a fixing unit 3. Since the image forming units PY, PM, PC, and PK for each color have the same configuration, the following description will be given using the image forming unit PY as a representative.

[0037] The photosensitive drum 28Y, which serves as an image carrier, is a photosensitive member having a photosensitive layer made of a resin such as polycarbonate containing an organic photoconductor (OPC), and is configured to rotate at a predetermined speed. In this embodiment, the linear velocity of the surface of the photosensitive drum 28Y is set to 650 mm / s. The primary charger 21Y is made of a corona discharge electrode disposed around the photosensitive drum 28Y, and generates ions to charge the surface of the photosensitive drum 28Y.

[0038] The optical writing unit 22Y incorporates a scanning optical device and exposes the charged photosensitive drum 28Y based on image data, thereby reducing the potential of the exposed area and forming a charge pattern (electrostatic latent image) corresponding to the image data. The developing device 1Y transfers the contained developer to the photosensitive drum 28Y to develop the electrostatic latent image formed on the photosensitive drum 28Y. The developer is a mixture of carrier and toner corresponding to each color, and the electrostatic latent image is visualized by the toner.

[0039] The transfer device 2 has primary transfer rollers 23Y, 23M, 23C, and 23K, an intermediate transfer belt 24, and a secondary transfer roller 25. The intermediate transfer belt 24 is wound around the primary transfer rollers 23Y, 23M, 23C, and 23K and multiple rollers and supported so that it can run. The primary transfer rollers 23Y, 23M, 23C, and 23K, from top to bottom in FIG. 1 , correspond to the colors Y (yellow), M (magenta), C (cyan), and K (black). The secondary transfer roller 25 is disposed outside the intermediate transfer belt 24 and is configured so that a recording material can pass between it and the intermediate transfer belt 24. The recording material may be, for example, a sheet of paper, a plastic sheet, or the like.

[0040] The toner images of each color formed on the photosensitive drums 28Y, 28M, 28C, and 28K are transferred successively onto the intermediate transfer belt 24 by the primary transfer rollers 23Y, 23M, 23C, and 23K, forming a color toner image in which yellow, magenta, cyan, and black layers are superimposed. The formed toner image is transferred by the secondary transfer roller 25 onto a recording material conveyed from a cassette or the like containing the recording material. The recording material onto which the toner image has been transferred is subjected to pressure and heat in the fixing device 3. This melts the toner on the recording material, and the color image is fixed to the recording material.

[0041] Developer storage units 27Y, 27M, 27C, and 27K are provided corresponding to developing devices 1Y, 1M, 1C, and 1K, respectively, and are filled with replaceable bottles containing developers corresponding to the respective colors of yellow, magenta, cyan, and black, from top to bottom. Developer storage units 27Y, 27M, 27C, and 27K are configured to be able to transport (supply) developers to developing devices 1Y, 1M, 1C, and 1K corresponding to the colors of the developers stored therein.

[0042] For example, the toner weight ratio of the developer stored in the bottle is 80 to 95%, and the toner weight ratio of the developer in the developing devices 1Y, 1M, 1C, and 1K is 5 to 10%. Therefore, when toner is consumed by development in the developing devices 1Y, 1M, 1C, and 1K, developer containing toner corresponding to the consumed amount is replenished, and the toner weight ratio of the developer in the developing devices 1Y, 1M, 1C, and 1K is maintained constant. [Developing Device]

[0043] Next, the developing devices 1Y, 1M, 1C, and 1K will be described in detail using Figures 2 to 5. Since the developing devices 1Y, 1M, 1C, and 1K have the same configuration, the following description will focus on the developing device 1Y as a representative. Figure 2 is a conceptual diagram illustrating the developing device 1Y shown in Figure 1, and Figures 3, 4, and 5 are conceptual diagrams illustrating the magnetic pole configurations of the first magnet 36, second magnet 37, and third magnet 38 arranged in the developing device 1Y.

[0044] As shown in FIG. 2, the developing device 1Y has a first developing roller 30, a second developing roller 31, a peeling roller 32, a developer supply screw 42, a developer stirring screw 43, and a developer recovery screw 44, and these components are housed in a developing container 60.

[0045] The first developing roller 30 is a developer carrier that is driven to rotate, and is disposed adjacent to the photosensitive drum 28Y so that its rotation axis is substantially parallel to the rotation axis of the photosensitive drum 28Y. The first developing roller 30 has a rotating first sleeve (first rotating body) 33 and a first magnet (fixed magnet, first magnet) 36 that is non-rotatingly disposed inside the first sleeve 33 and that magnetically attracts the developer to the surface of the first sleeve 33. The first developing roller 30 magnetically attracts (carries) the developer pumped up by a developer supply screw 42, and uses the developer to develop an electrostatic latent image formed on the rotating photosensitive drum 28Y (image carrier).

[0046] To the first sleeve 33 (and the second sleeve 34 described later) of the developing device 1Y, for example, a DC developing bias having the same polarity as the charging polarity of the primary charger 21Y, or a developing bias in which a DC voltage having the same polarity as the charging polarity of the primary charger 21Y is superimposed on an AC voltage, is applied. As a result, reversal development is performed in which toner charged with the same polarity as the charging polarity of the primary charger 21Y adheres to the electrostatic latent image formed by the optical writing unit 22Y. In this embodiment, the charging polarity of the primary charger 21Y and the DC voltage of the developing bias are negative, and reversal development is performed in which negatively charged toner adheres to the electrostatic latent image.

[0047] The first sleeve 33 is a non-magnetic cylindrical member with an outer diameter of 25 mm (radius r1 = 12.5 mm) and is driven to rotate around a rotation shaft 39. The rotation direction of the first sleeve 33 is clockwise, as indicated by the arrow in FIG. 2 , which is opposite to the rotation direction of the photosensitive drum 28Y in this embodiment. Therefore, the first sleeve 33 and the photosensitive drum 28Y rotate in the same direction while facing each other. In this embodiment, the linear velocity of the surface of the first sleeve 33 of the first developing roller 30 is set to 1.0 times (= 650 mm / s) the linear velocity of the surface of the photosensitive drum 28Y. It is advantageous from the viewpoint of toner degradation to keep the ratio of the linear velocity of the surface of the first sleeve 33 to the linear velocity of the surface of the photosensitive drum 28Y between 1.0 and 1.2 times. On the other hand, there is a concern that the amount of toner supplied to the photosensitive drum 28Y will decrease, which may affect the developability. However, in this embodiment, two developing rollers 30 and 31 are provided, and the amount of toner supplied to the photosensitive drum 28Y can be maintained even if the linear velocity ratio is reduced.

[0048] The first magnet 36 is disposed inside the first sleeve 33, and has a plurality of magnetic poles 101 to 107, as shown in Fig. 3. The solid lines of the magnetic poles 101 to 107 shown in Fig. 3 indicate the positions of the maximum values ​​(peak positions, pole positions) of the distribution of the normal component of the magnetic flux density of the first magnet 36. A space is disposed between the inner periphery of the first sleeve 33 and the outer periphery of the first magnet 36 to allow rotation of the first sleeve 33.

[0049] The developer attracted to the first sleeve 33 (on the first sleeve) is carried and transported toward the photosensitive drum 28Y by the rotation of the first sleeve 33, and develops the latent image formed on the photosensitive drum 28Y at the first development position. After developing the latent image formed on the photosensitive drum 28Y, the developer on the first sleeve 33 is transported to the vicinity of the second developing roller 31 by the rotation of the first sleeve 33. Then, near the closest position between the first developing roller 30 and the second developing roller 31, the developer is peeled off from the first sleeve 33 and transferred onto the second sleeve 34 (on the second sleeve) by the magnetic fields generated by the first magnet 36 contained in the first developing roller 30 and the second magnet 37 contained in the second developing roller 31.

[0050] As will be described below, the second developing roller 31 of the developing device 1Y of this embodiment is disposed vertically above the first developing roller 30. Therefore, the developer must be transferred from the first sleeve 33 to the second sleeve 34 vertically upward against gravity. The first sleeve 33 and the second sleeve 34 are disposed with a gap of 3 mm between them at their closest points.

[0051] The second developing roller 31 is a rotationally driven developer carrier, and is disposed downstream of the first developing roller 30 in the rotation direction of the photosensitive drum 28Y and such that the rotation center O2 of the second developing roller 31 is located vertically above the rotation center O1 of the first developing roller 30, and developer is transferred from the first developing roller 30 by magnetic force (FIG. 2). In this embodiment, the entire second developing roller 31 is located above the rotation center O1 of the first developing roller 30. Like the first developing roller 30, the second developing roller 31 is disposed adjacent to the photosensitive drum 28Y such that its rotation axis is substantially parallel to the rotation axis of the photosensitive drum 28Y. Therefore, the rotation axes of the second developing roller 31 and the first developing roller 30 are substantially parallel to each other.

[0052] The second developing roller 31 has a rotating second sleeve (second rotating body) 34 and a second magnet (fixed magnet, second magnet) 37 that is non-rotatingly disposed inside the second sleeve 34 and that magnetically attracts developer to the surface of the second sleeve 34. The second developing roller 31 receives and attracts (carries) the developer from the first developing roller 30 (first sleeve 33) based on the magnetic force, and develops the electrostatic latent image formed on the rotating photosensitive drum 28Y with the developer. A peeling roller 32, which will be described later, is located to the side of the second developing roller 31.

[0053] The second sleeve 34 is a non-magnetic cylindrical member with an outer diameter of 25 mm (radius r2 = 12.5 mm) and is driven to rotate around the rotation shaft 40. The rotation direction of the second sleeve 34 is the same clockwise direction as the first sleeve 33, as shown by the arrow in FIG. 2 , and in this embodiment, it is the opposite direction to the rotation direction of the photosensitive drum 28Y. Therefore, the second sleeve 34 and the photosensitive drum 28Y rotate in the same direction when they face each other. Furthermore, the second sleeve 34 and the first sleeve 33 rotate in opposite directions when they face each other. In this embodiment, the linear speed of the surface of the second sleeve 34 of the second developing roller 31 is set to be 1.2 times (= 780 mm / s) the linear speed of the surface of the photosensitive drum 28Y.

[0054] The second magnet 37 is disposed inside the second sleeve 34, and has a plurality of magnetic poles 201 to 207, as shown in Fig. 4. The solid lines of the magnetic poles 201 to 207 shown in Fig. 4 indicate the positions of the maximum values ​​(peak positions, pole positions) of the distribution of the normal component of the magnetic flux density of the second magnet 37. A space is disposed between the inner periphery of the second sleeve 34 and the outer periphery of the second magnet 37 to allow rotation of the second sleeve 34.

[0055] The developer attracted to the second sleeve 34 is carried and transported toward the photosensitive drum 28Y by the rotation of the second sleeve 34, and develops the latent image formed on the photosensitive drum 28Y at the second development position. After the latent image formed on the photosensitive drum 28Y is developed, the developer remaining on the second sleeve 34 is transported to the vicinity of the peeling roller 32 by the rotation of the second sleeve 34. Then, near the closest position between the second developing roller 31 and the peeling roller 32, the developer is transferred from the second sleeve 34 to the third sleeve 35 of the peeling roller 32 by the magnetic fields generated by the second magnet 37 contained in the second developing roller 31 and the third magnet 38 contained in the peeling roller 32.

[0056] The peeling roller 32 as a peeling unit is disposed on the opposite side of the photosensitive drum 28Y with respect to the rotation center of the second sleeve 34, and peels off the developer from the second developing roller 31 after the electrostatic latent image on the photosensitive drum 28Y is developed by the second developing roller 31. Specifically, the peeling roller 32 is a developer carrier that is driven to rotate, and is disposed between the second developing roller 31 and the developer recovery screw 44 so that its rotation center is above the rotation center R of the second developing roller 31.

[0057] The peeling roller 32 is disposed so that its rotation axis is substantially parallel to the rotation axis of the second developing roller 31. The peeling roller 32 has a rotating third sleeve 35 and a third magnet (fixed magnet) 38 that is disposed non-rotatingly inside the third sleeve 35 and that attracts the developer to the surface of the third sleeve 35 by magnetic force, and is configured to receive the developer from the second developing roller 31 based on the magnetic force.

[0058] The third sleeve 35 is a non-magnetic cylindrical member with an outer diameter of 18 mm (radius of 9 mm) and is driven to rotate around the rotation shaft 41. The rotation direction of the third sleeve 35 is counterclockwise as indicated by the arrow in Figure 2, which in this embodiment is the opposite direction to the rotation direction of the second sleeve 34. Therefore, the third sleeve 35 and the second sleeve 34 rotate in the same direction while facing each other.

[0059] The third magnet 38 is disposed inside the third sleeve 35 and has a plurality of magnetic poles 301 to 305 as shown in Fig. 5. The solid lines of the magnetic poles 301 to 305 shown in Fig. 5 indicate the positions of the maximum values ​​(peak positions, pole positions) of the distribution of the normal component of the magnetic flux density of the third magnet 38. A space is disposed between the inner periphery of the third sleeve 35 and the outer periphery of the third magnet 38 to allow rotation of the third sleeve 35.

[0060] The developer attracted onto the third sleeve 35 is carried and transported downstream in the rotation direction by the rotation of the third sleeve 35, and is peeled off from the third sleeve 35 by the third magnet 38 contained in the peeling roller 32 at a position close to the developer recovery screw 44, and falls by its own weight toward the guide member 45 located vertically below. The developer that has fallen onto the guide member 45 is then guided by its own weight toward the developer recovery screw 44.

[0061] The guide member 45 and the developer recovery screw 44 constitute a developer recovery section 47 that serves as a recovery section that recovers the developer peeled off from the third sleeve 35 on the peeling roller 32. In the developer recovery section 47, the developer recovery screw 44 is disposed so that its center of rotation is positioned lower than the center of rotation of the peeling roller 32 in the vertical direction, and conveys the developer handed over (recovered) from the peeling roller 32 while stirring it.

[0062] The guide member 45 serving as a guide section is disposed vertically below the peeling roller 32 and guides the developer peeled off by the peeling roller 32 toward the developer recovery screw 44. Such a guide member 45 has an inclined surface 45a along which the developer slides down under its own weight, in order to more reliably guide the peeled off developer toward the developer recovery screw 44. The inclined surface 45a is inclined with respect to the horizontal direction so that the developer recovery screw 44 side is lower than the position below the peeling roller 32.

[0063] The developer recovery screw 44, which serves as a recovery member and a transport section, transports the recovered developer to a developer circulation section 46, which will be described next. That is, the developer recovery screw 44 is a screw transport member used to transport the recovered developer in one direction while stirring it as it slides down the inclined surface of the guide member 45.

[0064] The developer circulating unit 46 is a supply unit for supplying the developer to the first developing roller 30, and includes a regulating member 50, a developer supply screw 42, and a developer stirring screw 43. In the developer circulating unit 46, the developer is stirred in the developer supply screw 42 and the developer stirring screw 43 while being transported in a substantially horizontal direction, and is then supplied to the first developing roller 30. As described above, the developer collected by the developer collecting unit 47 falls by its own weight and is introduced into the developer circulating unit 46.

[0065] The developer supply screw 42, developer agitation screw 43, and developer recovery screw 44 are screw transport members that transport the developer in one direction while agitating it, and the developer supply screw 42 and developer agitation screw 43 are located vertically below the developer recovery screw 44. The developer supply screw 42, developer agitation screw 43, and developer recovery screw 44 are also arranged so that their rotation axes are substantially parallel to each other. The rotation axis of each screw is also substantially parallel to the rotation axis of the first developing roller 30.

[0066] The developer supply screw 42 is located between the first developing roller 30 and the developer agitating screw 43, and a partition wall 48 of the developing container 60 is disposed between the developer supply screw 42 and the developer agitating screw 43. The partition wall 48 of the developing container 60 extends along the rotational axis direction of the developer supply screw 42 and the developer agitating screw 43. The partition wall 48 is provided with a communication opening (not shown) that communicates a first transport path 61, through which the developer is transported by the developer supply screw 42, with a second transport path 62, through which the developer is transported by the developer agitating screw 43.

[0067] The developer stirred by the developer recovery screw 44 passes through a communication port (not shown) formed in a partition wall 63 of the developing container 60 located between the developer recovery screw 44 and the developer supply screw 42, and falls by its own weight toward the developer supply screw 42. The guide member 45 is formed integrally with the partition wall 63, and the developer recovery screw 44 is disposed above the partition wall 63.

[0068] The position of the communication port through which the developer stirred by the developer recovery screw 44 falls under its own weight and is introduced into the developer circulation unit 46 is preferably arranged to avoid the area where the developer is supplied toward the first developing roller 30 (the middle part with respect to the rotational axis direction of the developer supply screw 42). In this embodiment, the communication port is positioned within the range of the downstream end (terminal end) in the developer transport direction of the first transport path 61 in which the developer supply screw 42 is arranged.

[0069] The developer transport directions of the developer supply screw 42 and the developer agitation screw 43 are opposite to each other. The start side (upstream end in the developer transport direction) and end side (downstream end in the developer transport direction) of the first transport path 61 in which the developer supply screw 42 is disposed communicate with the end side and start side of the second transport path 62 in which the developer agitation screw 43 is disposed via communication ports provided in the partition wall 48. Therefore, the developer circulates in the rotation direction of the developer supply screw 42 and the developer agitation screw 43, indicated by the arrows in FIG. 2 , and in a substantially horizontal direction within the developing container 60, and a portion of the developer is supplied toward the first developing roller 30.

[0070] The developer supply port 51 (see FIG. 2) is disposed above the developer stirring screw 43 in the developing container 60, and is connected to the developer storage unit 27Y (see FIG. 1). The developer supply port 51 is configured to be able to supply the developer stored in a bottle loaded in the developer storage unit 27Y to the second conveying path 62 in which the developer stirring screw 43 is disposed.

[0071] As described above, the toner weight ratio of the developer stored in the bottle of developer storage section 27Y is greater than the toner weight ratio of the developer in developing device 1Y, so by adjusting the developer supplied to developer stirring screw 43, it is possible to maintain a constant toner weight ratio of the developer in developing device 1Y.

[0072] The toner concentration detection sensor 49 (see FIG. 2) is disposed to detect the toner concentration in the developer contained in the developer circulation unit 46. The toner concentration detection sensor 49 is a sensor that detects the magnetic permeability of the developer. The toner concentration corresponds to the amount of toner consumed in the developing device 1Y, and is therefore used to control the supply of developer from the developer storage unit 27Y. For example, when it is detected that the toner concentration has dropped below a predetermined value, developer is replenished from the developer storage unit 27Y. Note that the magnetic permeability of the developer changes depending on the toner concentration, and therefore the toner concentration can be detected using the magnetic permeability.

[0073] The regulating member 50 is disposed adjacent to the first developing roller 30 and is used to regulate the amount of developer supplied from the developer circulating unit 46 to the first developing roller 30. The regulating member 50 can be configured to regulate the amount of developer attracted to the first developing roller 30 based on, for example, the gap between the surface of the first sleeve 33 of the first developing roller 30 and the end of the regulating member 50.

[0074] The developer circulation path in the developing container 60 is such that the developer is transported in a substantially horizontal direction while being stirred in the developer circulation section 46, and then supplied to the first developing roller 30, and is transferred from the first developing roller 30 to the second developing roller 31 above by magnetic force. Next, the developer is transferred from the second developing roller 31 to the peeling roller 32 on the side of the second developing roller 31 again by magnetic force, and is then peeled off from the peeling roller 32 by the third magnet 38 contained in the peeling roller 32, and is then collected in the developer collection section 47 and introduced into the developer circulation section 46 again.

[0075] As described above, this embodiment uses a two-component development system, and the developer is a mixture of negatively charged non-magnetic toner and magnetic carrier. The non-magnetic toner is negatively charged due to friction with the magnetic carrier, while the magnetic carrier is positively charged. The non-magnetic toner is made by incorporating colorants, wax components, etc. into resins such as polyester or styrene acrylic, which are then pulverized or polymerized to form a powder, with fine powders such as titanium oxide and silica added to the surface. The magnetic carrier is made by coating the surface of a core made of resin particles kneaded with ferrite particles or magnetic powder. In this embodiment, the toner concentration in the developer (weight ratio of toner contained in the developer) in the initial state is 8%.

[0076] The magnetic carrier has a magnetic field of 40 Am when the applied magnetic field is 1000 Oersted (79577 A / m). ２ / kg or more, 80Am ２ It is preferable that the magnetic carrier has a magnetization amount per unit weight of 63 Am / kg or less. Reducing the magnetization amount of the magnetic carrier has the effect of suppressing scavenging by the magnetic brush, but it becomes difficult for the magnetic carrier to adhere to the non-magnetic sleeve due to the magnet inside the developing roller, and image defects such as magnetic carrier adhesion to the photosensitive drum may occur. Scavenging is a phenomenon in which the developed toner is scraped off by the magnetic carrier once development has been completed. Furthermore, if the magnetization amount of the magnetic carrier is greater than the above range, image defects may occur due to the pressure of the magnetic brush as described above. In this embodiment, the magnetization amount per unit weight is 63 Am ２ / kg of magnetic carrier was used.

[0077] The amount of magnetization of the magnetic carrier was measured using a vibration magnetic field type magnetic property automatic recording device BHV-30 manufactured by Riken Denshi Co., Ltd. The magnetic property value of the magnetic carrier was measured by creating an external magnetic field of 1000 oersted and determining the magnetization strength at that time. The magnetic carrier was packed in a cylindrical plastic container so that it was sufficiently dense. In this state, the magnetization moment was measured, and the actual weight when the sample was placed inside was measured, and the magnetization strength (Am ２ / kg).

[0078] The true specific gravity of the magnetic carrier is determined using a dry automatic density analyzer, Accupyc 1330, manufactured by Shimadzu Corporation. In this embodiment, the true specific gravity (density) is 4.6 (g / cm ３ The magnetic carrier used had a weight average particle diameter of 35 μm (radius b=17.5 μm).

[0079] Generally, two-component development systems using toner and carrier charge both to a predetermined polarity through frictional contact between the toner and carrier, which means that the toner is subjected to less stress than single-component development systems using single-component developers. However, over time, the amount of contaminants (spent toner) adhering to the carrier surface increases, gradually reducing the toner's ability to charge. This results in problems such as fogging and toner scattering. While increasing the amount of carrier contained in a two-component development device could be considered to extend the life of the device, this is undesirable because it would increase the size of the development device.

[0080] In order to solve the above problems associated with two-component developers, this embodiment employs an ACR (Auto Carrier Refresh) method. The ACR method is a method for suppressing an increase in degraded carrier by gradually replenishing new developer from the developer storage section 27Y into the developing device 1Y and gradually discharging developer with degraded charging performance from a discharge port (not shown) of the developing device 1Y. This allows the degraded carrier in the developing device 1Y to be gradually replaced with new carrier, making it possible to maintain the charging performance of the carrier in the developing device 1Y approximately constant. [Regarding the magnetic poles of each magnet]

[0081] Next, the magnetic pole configurations of the first magnet 36, second magnet 37, and third magnet 38 contained in the first developing roller 30, second developing roller 31, and peeling roller 32 shown in Figures 3, 4, and 5 will be described.

[0082] As shown in FIG. 3 , the first magnet 36 contained within the first developing roller 30 has a total of seven poles, namely, multiple magnetic poles 101, 102, 103, 104, 105, 106, and 107. Of these, magnetic pole 107 is a transfer pole for transferring developer from the first developing roller 30 to the second developing roller 31. The magnetic poles 101 to 107 are arranged in numerical order in the rotational direction of the first sleeve 33. As described above, the solid lines of the magnetic poles 101 to 107 shown in FIG. 3 represent the positions (pole positions) of the peak values ​​(maximum values) of the magnitude of the normal component Br of the magnetic flux density of the first magnet 36 relative to the surface of the first sleeve 33 (magnetic flux density Br in the normal direction relative to the outer peripheral surface of the first sleeve 33; hereinafter, this may be simply referred to as "magnetic flux density Br" or "normal component Br"). This also applies to the magnetic poles 201 to 207 of the second magnet 37 shown in FIG. 4 and the magnetic poles 301 to 305 of the third magnet 38 shown in FIG.

[0083] The magnetic pole 107 as a transfer pole is a magnetic pole for transferring developer from the first sleeve 33 to the second sleeve 34 by a magnetic field generated in cooperation with the second magnet 37 of the second developing roller 31, and hereinafter, the magnetic pole 107 may be referred to as the transfer pole 107. The magnetic pole 101 is a north pole and is used to attract the developer supplied from the developer supply screw 42 onto the first sleeve 33. The magnetic poles 102, 103, 104, 105, and 106 are a south pole, north pole, south pole, north pole, and south pole and are used to transport the developer attracted by the magnetic pole 101 upward as the first sleeve 33 rotates. The magnetic pole 107 is a north pole and transfers the developer from the first sleeve 33 to the second sleeve 34 facing the first sleeve 33 by a magnetic field generated in cooperation with the magnetic pole 201 in the second magnet 37 contained in the second developing roller 31 as described above.

[0084] In this embodiment, a low magnetic force portion 110 having a magnetic force lower than that of the delivery pole 107 is formed by a repulsive magnetic field generated in cooperation between the delivery pole 107 and the magnetic pole 101, which is disposed downstream of the delivery pole 107 in the rotational direction of the first sleeve 33 and has the same polarity as the delivery pole 107. This low magnetic force portion 110 promotes the transfer of developer from the first sleeve 33 to the second sleeve 34. Note that, although the low magnetic force portion 110 has almost no magnetic force in this embodiment, it may have a low magnetic force. For example, the magnetic force (normal component Br of the magnetic flux density) may be 5 mT or less. This also applies to the low magnetic force portion 210 of the second magnet 37 shown in FIG. 4 and the low magnetic force portion 310 of the third magnet 38 shown in FIG. 5.

[0085] 4, the second magnet 37 contained in the second developing roller 31 has a total of seven poles, namely, multiple magnetic poles 201, 202, 203, 204, 205, 206, and 207. Of these, the magnetic pole 201 is a receiving pole for the second developing roller 31 to receive the developer from the first developing roller 30. The magnetic poles 201 to 207 are arranged in numerical order in the rotation direction of the second sleeve 34.

[0086] The magnetic pole 201 as a receiving pole is a magnetic pole for receiving and attracting the developer from the first sleeve 33 to the second sleeve 34 by a magnetic field generated in cooperation with the magnetic pole 107 of the first magnet 36 of the first developing roller 30, and hereinafter the magnetic pole 201 may be referred to as the receiving pole 201. The magnetic pole 207 is a magnetic pole for transferring the developer from the second sleeve 34 to the third sleeve 35 by a magnetic field generated in cooperation with the third magnet 38 of the peeling roller 32.

[0087] The receiving pole 201 is an S pole, different in polarity from the handover pole 107, and is used to attract the developer from the first developing roller 30 (first sleeve 33) onto the second sleeve 34, as described above. The magnetic poles 202, 203, 204, 205, and 206 are an N pole, an S pole, an N pole, an S pole, and an N pole, and are used to transport the developer attracted by the magnetic pole 201 upward as the second sleeve 34 rotates. The magnetic pole 207 is an S pole, and transfers the developer after passing through the development region with the photosensitive drum 28Y corresponding to the magnetic pole 203 from the second sleeve 34 to the third sleeve 35 facing the second sleeve 34 by a magnetic field generated in cooperation with the magnetic pole 303 in the third magnet 38 contained in the peeling roller 32.

[0088] In this embodiment, a magnetic pole 207, which is disposed upstream of the receiving pole 201 in the rotation direction of the second sleeve 34 and has the same polarity as the receiving pole 201, and a low magnetic force portion 210 having a magnetic force lower than that of the magnetic pole 207 are formed by a repulsive magnetic field generated in cooperation between the receiving pole 201 and the magnetic pole 207. This low magnetic force portion 210 promotes the transfer of developer from the first sleeve 33 to the second sleeve 34. Furthermore, the low magnetic force portion 210 can prevent the developer from being attracted to the closest portion of the first sleeve 33 and the second sleeve 34, thereby suppressing the pressure applied to the developer.

[0089] 5, the third magnet 38 contained in the peeling roller 32 has a plurality of magnetic poles 301, 302, 303, 304, and 305. The magnetic poles 301 to 305 are arranged in numerical order in the rotation direction of the third sleeve 35.

[0090] The magnetic pole 303 is an N pole opposite to the magnetic pole 207, and is used to attract the developer peeled off from the second sleeve 34 onto the third sleeve 35 as described above. The magnetic poles 301, 302, and 304 are N, S, and S poles, and are used to transport the developer on the third sleeve 35 as the third sleeve 35 rotates. In particular, the magnetic pole 304 is used to transport the developer attracted by the magnetic pole 303 downward as the third sleeve 35 rotates. The magnetic pole 305 is an N pole, and is a peeling pole used to peel off the developer attracted to the third sleeve 35 from the third sleeve 35 by a repulsive magnetic field generated in cooperation with the magnetic pole 301 of the same polarity. [Arrangement of the magnetic poles]

[0091] Next, using FIG. 6 , the positional relationship between the magnetic pole of the first magnet 36 disposed inside the first developing roller 30 and the magnetic pole of the second magnet 37 disposed inside the second developing roller 31 will be described. FIG. 6 is a conceptual diagram illustrating the arrangement of the first developing roller 30, the second developing roller 31, and the photosensitive drum 28Y in this embodiment. The magnetic pole 105 of the first magnet 36 of the first developing roller 30 is an N pole and is disposed in a position facing the photosensitive drum 28Y across the first sleeve 33. This magnetic pole is used to develop the electrostatic latent image formed on the photosensitive drum 28Y. Hereinafter, the magnetic pole 105 may be referred to as the first developing pole 105. The transfer pole 107 is located downstream of the first developing pole 105 with respect to the rotation direction of the first sleeve 33. This magnetic pole has the same polarity as the first developing pole 105 and transfers the developer from the first developing roller 30 to the second developing roller 31, as described above.

[0092] As described above, the first magnet 36 of the first developing roller 30 has seven magnetic poles. This is intended to improve the quality of the output image. To achieve high-quality output images, it is preferable that the magnetic chains of the developer formed by the magnetic force of the first developing pole 105, which is the magnetic pole facing the photosensitive drum 28Y via the first sleeve 33, are dense. To make the magnetic chains of the developer dense, the magnetic force of the first developing pole 105 is strengthened. By strengthening the magnetic force of the first developing pole 105, the magnetic carrier in the developer is more easily attracted to the surface of the first sleeve 33 of the first developing roller 30, making the magnetic chains denser.

[0093] To strengthen the magnetic force of the first developing pole 105, either the absolute value of the magnetic flux density of the first developing pole 105 is increased or the change in magnetic flux density (differential with respect to distance) is increased. In this embodiment, the absolute value of the magnetic flux density Br of the first developing pole 105 is increased to 150 mT or more, and the first magnet 36 has seven poles. The magnetic poles (transport poles) 104 and 106 are positioned in close proximity within 40° upstream and downstream of the first developing pole 105 with respect to the direction of rotation of the first sleeve 33, thereby increasing the change in magnetic flux density. As a result, the magnetic force of the first developing pole 105 can be increased, the magnetic chains become denser, and higher image quality can be achieved in the output image.

[0094] Therefore, in this embodiment, a magnetic pole (first developing downstream pole, first downstream pole) 106 is disposed adjacent to the first developing pole 105 between the first developing pole 105 and the delivery pole 107, downstream of the first developing pole 105 in the developer transport direction. Furthermore, a magnetic pole (first developing upstream pole, first upstream pole) 104 is disposed adjacent to the first developing pole 105 upstream of the first developing pole 105 in the developer transport direction. That is, the magnetic pole 106 is located adjacent to the downstream side of the first developing pole 105 and adjacent to the upstream side of the delivery pole 107 in the rotation direction of the first sleeve 33, and is a magnetic pole of a different polarity from the first developing pole 105. Hereinafter, the magnetic pole 106 may be referred to as the first developing downstream pole 106. Furthermore, the magnetic pole 104 is located adjacent to the upstream side of the first developing pole 105 in the rotation direction of the first sleeve 33, and is a magnetic pole of a different polarity from the first developing pole 105. Hereinafter, the magnetic pole 104 may be referred to as the first upstream developing pole 104 .

[0095] Downstream in the rotation direction of the photosensitive drum 28Y from the portion where the first developing pole 105 of the photosensitive drum 28Y faces via the first sleeve 33, the magnetic pole 203 of the second magnet 37 of the second developing roller 31 is disposed substantially opposite the photosensitive drum 28Y via the second sleeve 34. That is, the magnetic pole 203 of the second magnet 37 of the second developing roller 31 is an S pole, is disposed at a position where it faces the photosensitive drum 28Y via the second sleeve 34, and is a magnetic pole for developing the electrostatic latent image formed on the photosensitive drum 28Y. Hereinafter, the magnetic pole 203 may be referred to as the second developing pole 203. The receiving pole 201 is disposed upstream of the second developing pole 203 with respect to the rotation direction of the second sleeve 34, has the same polarity as the second developing pole 203 but is a magnetic pole opposite to the delivering pole 107, and is a magnetic pole for the second developing roller 31 to receive the developer from the first developing roller 30, as described above.

[0096] For the same reasons as for the first magnet 36 of the first developing roller 30, the second magnet 37 of the second developing roller 31 also has a large magnetic flux density Br of 150 mT or more, and the second magnet 37 has seven poles. This allows magnetic poles (transport poles) 202 and 204 to be positioned in close proximity within 40° upstream and downstream of the second developing pole 203 with respect to the rotational direction of the second sleeve 34, thereby increasing the change in magnetic flux density. In this embodiment, a magnetic pole (second upstream developing pole, second upstream pole) 202 is positioned adjacent to the second developing pole 203 and the receiving pole 201, upstream of the second developing pole 203 in the developer transport direction. Furthermore, a magnetic pole (second downstream developing pole, second downstream pole) 204 is positioned adjacent to the second developing pole 203 downstream of the second developing pole 203 in the developer transport direction.

[0097] That is, the magnetic pole 202 is located adjacent to the upstream side of the second developing pole 203 and adjacent to the downstream side of the receiving pole 201 in the rotation direction of the second sleeve 34, and is a magnetic pole of a different polarity from the second developing pole 203. Hereinafter, the magnetic pole 202 may be referred to as the second developing upstream pole 202. Furthermore, the magnetic pole 204 is located adjacent to the downstream side of the second developing pole 203 in the rotation direction of the second sleeve 34, and is a magnetic pole of a different polarity from the second developing pole 203. Hereinafter, the magnetic pole 204 may be referred to as the second developing downstream pole 204.

[0098] As described above, if the first downstream developing pole 106 is provided between the first developing pole 105 and the delivery pole 107 of the first magnet 36 of the first developing roller 30, and the second upstream developing pole 202 is provided between the second developing pole 203 and the receiving pole 201 of the second magnet 37 of the second developing roller 31, there is a concern that image defects due to streak-like fogging (abnormal image) may be more likely to occur. The streak-like fogging (abnormal image) will now be explained. Typically, developer is transferred from the first developing roller 30 to the second developing roller 31 between the nearby delivery pole 107 of the first developing roller 30 and the receiving pole 201 of the second developing roller 31. That is, because the delivery pole 107 and the receiving pole 201 have opposite polarities, a magnetic field is formed between the magnetic poles 107 and 201, causing the developer to move and be transferred from the first developing roller 30 to the second developing roller 31.

[0099] However, when the first downstream developing pole 106 is disposed between the first developing pole 105 and the delivery pole 107 of the first developing roller 30, and the second upstream developing pole 202 is disposed between the second developing pole 203 and the receiving pole 201 of the second developing roller 31, as in this embodiment, the first downstream developing pole 106 of the first developing roller 30 and the second upstream developing pole 202 of the second developing roller 31 are configured with opposite poles, and therefore a magnetic field is generated between the magnetic poles 106 and 202. If the developer is moved by this magnetic field between the magnetic poles 106 and 202, the developer moving between the magnetic poles 106 and 202 may come into contact with the photosensitive drum 28Y, which may cause vertical streak-like fogging on the output image.

[0100] In order to suppress the occurrence of such streaky fogging (abnormal images), it is preferable to reduce the magnetic flux density of the first downstream developing pole 106 of the first developing roller 30 and the second upstream developing pole 202 of the second developing roller 31, making it difficult for a magnetic field to be formed between the two magnetic poles 106, 202.

[0101] Next, a magnetic pole configuration advantageous for carrier adhesion will be described with reference to FIGS. 7 and 8. FIGS. 7 and 8 are diagrams schematically showing the state of magnetic field lines in the opposing region between the developing roller and the photosensitive drum. In FIGS. 7 and 8, the photosensitive drum and the developing roller rotate in the directions indicated by the arrows, respectively, and magnetic pole S1 is located upstream of developing pole N with respect to the direction of rotation of the developing roller, and magnetic pole S2 is located downstream of developing pole N. The developing rollers shown in FIGS. 7 and 8 correspond to the first developing roller 30 or the second developing roller 31.

[0102] Carrier adhesion is a phenomenon in which magnetic carrier adheres to the photosensitive drum 28Y and becomes apparent on an image. Carrier adhesion occurs when, in the opposing region between the first developing roller 30 and the second developing roller 31 and the photosensitive drum 28Y, a negative charge is injected into the magnetic carrier in the developer by the action of the developing bias, causing the magnetic carrier, which was positively charged, to become negatively charged, the same as the toner, and the magnetic carrier flies to the photosensitive drum 28Y together with the toner.

[0103] Therefore, if the magnetic carriers remain in the opposing regions of the first and second developing rollers 30 and 31 and the photosensitive drum 28Y for a long time, negative charge injection into the magnetic carriers is promoted, and carrier adhesion is likely to occur. As mentioned above, in this embodiment, in order to suppress toner deterioration, the ratio of the surface linear speed of the first and second sleeves 33 and 34 of the first and second developing rollers 30 and 31 to the surface linear speed of the photosensitive drum 28Y is kept low, at 1.0 to 1.2. As a result, the magnetic carriers tend to remain in the opposing regions of the first and second developing rollers 30 and 31 and the photosensitive drum 28Y for a long time, and negative charge injection into the magnetic carriers is likely to be promoted. For this reason, the configuration of this embodiment requires greater attention to addressing carrier adhesion.

[0104] In particular, when the ratio of the surface linear velocities is 1.4 or less, it is more important to take measures against carrier adhesion.

[0105] According to the inventors' investigations, carrier adhesion tends to occur more easily when the magnetic chains of the developer are in contact downstream in the rotation direction of the photosensitive drum 28Y with respect to the opposing region of the photosensitive drum 28Y with the first developing roller 30 and the second developing roller 31. For this reason, by making the magnetic chains near the development poles of the first developing roller 30 and the second developing roller 31 more likely to extend upstream and by making the magnetic chains more likely to fold downstream, it is possible to make it difficult for the magnetic chains of the developer to come into contact downstream of the aforementioned opposing region, thereby making it difficult for carrier adhesion to occur.

[0106] The magnetic chains of the developer are formed by magnetic carrier particles in the developer arranged along magnetic field lines. Therefore, to facilitate the extension of the magnetic chains near the development pole N in the upstream direction, it is preferable to make the magnetic field lines extend more easily upstream of the development pole N, as shown in FIG. 7 . FIG. 7 illustrates the magnetic field lines when the absolute value of the normal component Br of the magnetic flux density is greater for the magnetic pole S2 located downstream of the development pole N than for the magnetic pole S1 located upstream of the development pole N. In this case, the proportion of the magnetic field lines extending from the development pole N that extend to the downstream magnetic pole S2 increases. As a result, the magnetic field lines from the development pole N bend upstream and extend to the downstream magnetic pole S2, making it easier for the magnetic chains near the development pole N to extend upstream. When the magnetic chains extend upstream from the development pole N, they can easily contact the photosensitive drum on the upstream side (the area surrounded by the dotted line in FIG. 7 ) and can be less likely to contact the photosensitive drum on the downstream side, improving carrier adhesion.

[0107] 8 is a diagram showing the state of the magnetic field lines when the absolute value of the normal component Br of the magnetic flux density is greater for the magnetic pole S1 located adjacent to the upstream side of the development pole N than for the magnetic pole S2 located adjacent to the downstream side of the development pole N. In this case, the proportion of the magnetic field lines extending from the development pole N that extend to the upstream magnetic pole S1 increases. As a result, the magnetic field lines from the development pole N extend to the upstream magnetic pole S1 while winding around downstream, making it easier for the magnetic brush near the development pole N to extend downstream.

[0108] If the magnetic brush extends downstream from the development pole N, it is likely to come into contact with the photosensitive drum downstream (the area surrounded by the dotted line in Figure 8), raising concerns that carrier adhesion may occur. One reason why carrier adhesion is more likely to occur when the magnetic brush comes into contact with the photosensitive drum downstream is that the distance between the photosensitive drum and the developing roller gradually increases downstream of the photosensitive drum, weakening the magnetic force that pulls back the magnetic carrier that has adhered to the photosensitive drum. Another reason is that the magnetic brush spends more time in contact with the photosensitive drum. This will be explained in more detail below.

[0109] As shown in Figure 8, even when the magnetic brush easily extends downstream of the photosensitive drum, the distance between the photosensitive drum and the developing roller gradually narrows upstream of the photosensitive drum, causing developer to accumulate and making it easier for the magnetic brush to contact the photosensitive drum even on the upstream side. Therefore, the magnetic brush comes into contact with the photosensitive drum both upstream and downstream. As a result, the magnetic brush contacts the photosensitive drum for a longer period of time, which promotes the injection of negative charge into the magnetic carrier, making carrier adhesion more likely to occur. However, while a longer contact time between the magnetic brush and the photosensitive drum raises concerns about carrier adhesion as described above, it also increases the time for toner development on the photosensitive drum, which is beneficial for development performance.

[0110] 7 is applied to a configuration having two developing rollers that rotate in the same direction facing the photosensitive drum as described in FIG. 6 and a transport pole provided between the developing pole of each developing roller and the magnetic pole in the transfer area, it is effective to increase the magnetic flux density of the first downstream developing pole 106 of the first developing roller 30. Here, the magnetic poles in the transfer area refer to the transfer pole 107 of the first developing roller 30 and the receiving pole 201 of the second developing roller 31. In addition, the transport poles between the developing poles of each developing roller and the magnetic poles in the transfer area refer to the first downstream developing pole 106 located between the first developing pole 105 and the transferring pole 107 of the first developing roller 30, and the second upstream developing pole 202 located between the second developing pole 203 and the receiving pole 201 of the second developing roller 31. That is, when trying to take measures against carrier adhesion with the configuration described in Fig. 6, it is preferable to increase the magnetic flux density of the first downstream developing pole 106 downstream of the first downstream developing pole 105 of the first developing roller 30, as described in Fig. 7. However, from the viewpoint of suppressing the above-mentioned streak-like fogging (abnormal image), it is preferable that the magnetic flux density of the first downstream developing pole 106 of the first developing roller 30 be as small as possible. For this reason, it is difficult to simultaneously suppress streak-like fogging (abnormal image) and carrier adhesion with the configuration described in Fig. 6.

[0111] Therefore, in this embodiment, in a configuration in which two developing rollers are provided that rotate in the same direction in a portion facing the photosensitive drum and a transport pole is provided between the developing pole of each developing roller and the magnetic pole in the transfer area, the following configuration is adopted to simultaneously suppress streak-like fogging (abnormal images) and suppress carrier adhesion. [Regarding the first developing roller and the second developing roller]

[0112] 9 shows a schematic configuration of the periphery of the area where the first developing pole 105 of the first magnet 36 of the first developing roller 30 and the second developing pole 203 of the second magnet 37 of the second developing roller 31 face the photosensitive drum 28Y. Note that some of the magnetic poles are omitted from the illustration to avoid complication.

[0113] As described above, in order to deal with carrier adhesion, it is preferable to make the absolute value of the magnetic flux density Br of the first and second downstream developing poles 106, 204 larger than the absolute value of the magnetic flux density Br of the first and second upstream developing poles 104, 202 with respect to the first developing pole 105 of the first developing roller 30 and the second developing pole 203 of the second developing roller 31. However, because the absolute value of the magnetic flux density Br of the first downstream developing pole 106 of the first developing roller 30 is large, a magnetic field is likely to be formed between the first downstream developing pole 106 and the second upstream developing pole 202 of the second developing roller 31, which raises the concern that streaky fogging may be more likely to occur due to the movement of developer between the magnetic poles 106, 202.

[0114] For this reason, this embodiment is configured as follows. First, the maximum absolute value of the normal component Br of the magnetic flux density of the first upstream developing pole 104 is set to be greater than the maximum absolute value of the normal component Br of the magnetic flux density of the first downstream developing pole 106. Furthermore, the maximum absolute value of the normal component Br of the magnetic flux density of the second downstream developing pole 204 is set to be greater than the maximum absolute value of the normal component Br of the magnetic flux density of the second upstream developing pole 202. Furthermore, in the second developing roller 31, a line L2 is defined as a line connecting the position (peak position) on the second sleeve 34 where the absolute value of the normal component Br of the magnetic flux density of the second upstream developing pole 202 is maximum and the rotation center O2 of the second sleeve 34. A line L21 is defined as a line connecting the position (peak position) on the second sleeve 34 where the absolute value of the normal component Br of the magnetic flux density of the second upstream developing pole 202 is maximum and the rotation center O2 of the second sleeve 34. The angle (acute angle) formed by line L2 and line L21 is defined as θ1 [°]. In other words, θ1 is the angle in the rotational direction of the second sleeve 34 from the position on the outer peripheral surface of the second sleeve 34 where the absolute value of the magnetic flux density of the second developing upstream pole 202 in the normal direction to the outer peripheral surface of the second sleeve 34 is maximum to the position on the outer peripheral surface of the second sleeve 34 where the absolute value of the magnetic flux density of the second developing pole 203 in the normal direction to the outer peripheral surface of the second sleeve 34 is maximum. Furthermore, line L22 is the line connecting the position (peak position) on the second sleeve 34 where the absolute value of the normal component Br of the magnetic flux density of the second developing downstream pole 204 is maximum and the rotation center O2 of the second sleeve 34. The angle (acute angle) formed by line L2 and line L22 is defined as θ2 [°]. In other words, θ2 is the angle in the rotation direction of the second sleeve 34 from the position on the outer circumferential surface of the second sleeve 34 where the absolute value of the magnetic flux density of the second developing pole 203 in the normal direction to the outer circumferential surface of the second sleeve 34 is maximum to the position on the outer circumferential surface of the second sleeve 34 where the absolute value of the magnetic flux density of the second developing downstream pole 204 in the normal direction to the outer circumferential surface of the second sleeve 34 is maximum. In this case, θ1 > θ2 is satisfied.

[0115] First, with respect to the second developing roller 31, the absolute value of the magnetic flux density Br of the second developing downstream pole 204 is set to be greater than the absolute value of the magnetic flux density Br of the second developing upstream pole 202 of the second magnet 37. As a result, with respect to the second developing roller 31, the magnetic chains near the second developing pole 203 tend to extend toward the upstream side of the photosensitive drum 28Y, and as a result, carrier adhesion is less likely to occur.

[0116] On the other hand, in this embodiment, with respect to the first developing roller 30, the absolute value of the magnetic flux density Br of the first developing downstream pole 106 is set smaller than the absolute value of the magnetic flux density Br of the first developing upstream pole 104 of the first magnet 36. In this way, it is possible to keep the absolute values ​​of the magnetic flux density Br of both the first developing downstream pole 106 and the second developing upstream pole 202 low. As a result, a magnetic field is less likely to be formed between the first developing downstream pole 106 of the first developing roller 30 and the second developing upstream pole 202 of the second developing roller 31, and streaky fogging due to the movement of developer between the magnetic poles 106, 202 is less likely to occur.

[0117] However, with such a configuration, there is a concern that the magnetic chains near the first developing pole 105 of the first developing roller 30 will tend to extend toward the downstream side of the photosensitive drum 28Y, making carrier adhesion more likely to occur. Regarding this point, according to the inventors' investigations, it was found that, because the first developing roller 30 is disposed upstream of the second developing roller 31 in the rotation direction of the photosensitive drum 28Y, even if some carrier adhesion occurs on the first developing roller 30, it is possible to reset the carrier adhesion on the first developing roller 30 by taking proper measures to prevent carrier adhesion on the second developing roller 31.

[0118] Therefore, in this embodiment, with respect to the second developing roller 31, the absolute value of the magnetic flux density Br of the second developing downstream pole 204 is made larger than the absolute value of the magnetic flux density Br of the second developing upstream pole 202. In addition, the angle θ2 between the peak positions of the magnetic flux density Br of the second developing pole 203 and the second developing downstream pole 204 is made smaller than the angle θ1 between the peak positions of the magnetic flux density Br of the second developing pole 203 and the second developing upstream pole 202. In this way, when the angle θ2 is made smaller than the angle θ1, the magnetic field lines extending from the second developing pole 203 extend more toward the second developing downstream pole 204, which is closer than the second developing upstream pole 202. As a result, the magnetic field lines extending from the second developing pole 203 extend downstream while bending further upstream, so that the magnetic wire formed by the second developing pole 203 is more likely to extend upstream, making it less likely for this magnetic wire to come into contact downstream in the rotational direction of the photosensitive drum 28Y.

[0119] Furthermore, if the angle θ2 between the peak positions of the second developing pole 203 and the second developing downstream pole 204 is small, the magnetic field lines extending from the second developing pole 203 will quickly move toward the second developing downstream pole 204 downstream of the second developing pole 203. As a result, the magnetic chain formed by the second developing pole 203 is quickly folded on the downstream side, making it more difficult for the magnetic chains to come into contact downstream in the rotation direction of the photosensitive drum 28Y.

[0120] As described above, in this embodiment, with respect to the second developing roller 31, the absolute value of the magnetic flux density Br of the second downstream developing pole 204 is made greater than the absolute value of the magnetic flux density Br of the second upstream developing pole 202. In addition, the angle θ2 between the peak positions of the second downstream developing pole 204 and the second upstream developing pole 202 is made smaller than the angle θ1 between the peak positions of the second upstream developing pole 203 and the second downstream developing pole 204. While making the angle θ2 smaller than the angle θ1 is effective, it is preferable to make it smaller by 2° or more to achieve a greater effect, and even more preferably by 3° or more. That is, it is preferable to satisfy the relationship 2°≦θ1−θ2, and more preferably 3°≦θ1−θ2. However, it is preferable that the angle θ2 between the peak positions of the second downstream developing pole 204 be at least half the angle θ1 between the peak positions of the second upstream developing pole 202 and the second upstream developing pole 203 (θ2≧θ1 / 2). If the angle θ2 is too small, the peak position of the second downstream developing pole 204 becomes too close to the peak position of the second developing pole 203, and the volume of the magnet (piece of the second magnet 37) that contributes to the magnetic field formation of the second developing pole 203 becomes small, making it difficult to generate a magnetic field, which raises the concern that carrier adhesion by the second developing roller 31 may be more likely to occur. With the above-described configuration, this embodiment can simultaneously suppress the occurrence of streak-like fogging and suppress carrier adhesion. [Relationship between half-value widths]

[0121] A more preferable configuration is described below. In this embodiment, the half-width of the normal component Br of the magnetic flux density of the second downstream developing pole 204 is preferably larger than the half-width of the normal component Br of the magnetic flux density of the second upstream developing pole 202. Here, the half-width is the angle of the portion where the normal component Br of the magnetic flux density of each magnetic pole is half its peak value. To distinguish it from the half-width, it is sometimes called the full-width at half maximum, but in this specification, the half-width refers to the full-width at half maximum. This configuration can more reliably increase the proportion of the magnetic field lines extending from the second downstream developing pole 203 that extend to the second downstream developing pole 204, thereby improving carrier adhesion. Considering manufacturing variations, it is more preferable to make the half-width larger by 3° or more, and even more preferably by 5° or more. That is, the half-value width of the normal component Br of the magnetic flux density of the second downstream developing pole 204 is preferably 3° or more, and more preferably 5° or more, larger than the half-value width of the normal component Br of the magnetic flux density of the second upstream developing pole 202. [Relationship with the closest position of the second developing roller]

[0122] In this embodiment, the peak position of the normal component Br of the magnetic flux density of the second developing pole 203 of the second developing roller 31 is set at a position substantially opposite the photosensitive drum 28Y. More specifically, the peak position of the second developing pole 203 is disposed upstream in the rotation direction of the photosensitive drum 28Y from the opposing position (broken line L23 connecting the center of the photosensitive drum 28Y and the center of the second developing roller 31 in FIG. 9 ), which is the closest position between the photosensitive drum 28Y and the second developing roller 31. In other words, the position (peak position) on the second sleeve 34 where the absolute value of the normal component of the magnetic flux density of the second developing pole 203 is maximum is located upstream in the rotation direction of the second sleeve 34 from the closest position of the second sleeve 34 to the photosensitive drum 28Y.

[0123] This configuration allows the magnetic chains extending from the second developing pole 203 to extend further upstream of the photosensitive drum 28Y, more reliably improving carrier adhesion. However, because the magnetic force is strongest and the magnetic chains are dense near the peak position of the second developing pole 203, it is preferable for high image quality that the second developing pole 203 be positioned approximately facing the photosensitive drum 28Y. Therefore, it is preferable to position the peak position of the second developing pole 203 within 10°, preferably within 5°, and more preferably within 3° upstream of the opposing position of the photosensitive drum 28Y and the second developing roller 31. In other words, the angle (acute angle) formed by the line L2 connecting the peak position of the second developing pole 203 and the center of rotation O2 of the second sleeve and the line L23 connecting the closest position of the second sleeve 34 to the photosensitive drum 28Y and the center of rotation O2 of the second sleeve 34 is preferably 10° or less. The angle (acute angle) between the line L2 and the line L23 is more preferably 5° or less, and even more preferably 3° or less. [Surface Linear Speed ​​of Sleeve]

[0124] Furthermore, as described above, if the developer remains in the opposing region of the photosensitive drum 28Y for a long time, the injection of negative charge into the magnetic carrier in the developer due to the action of the developing bias is promoted, making carrier adhesion more likely to occur. Therefore, in this embodiment, the ratio of the surface linear speed of the second sleeve 34 of the second developing roller 31 to the surface linear speed of the photosensitive drum 28Y is set to 1.2, which is larger than the ratio of the surface linear speed of the first sleeve 33 of the first developing roller 30 to the surface linear speed of the photosensitive drum 28Y, which is 1.0. That is, in this embodiment, the surface linear speed of the second sleeve 34 is faster than the surface linear speed of the first sleeve 33.

[0125] The reason for suppressing the surface linear speed of the first sleeve 33 to suppress toner degradation is as follows: As shown in Figure 2, a regulating member 50 faces the first sleeve 33, and when this regulating member 50 restricts the amount of developer coated on the first sleeve 33, shear is applied to the developer, leading to toner degradation. If the surface linear speed of the first sleeve 33 can be suppressed, the frequency with which the developer is subjected to shear can be reduced, making it possible to suppress toner degradation.

[0126] On the other hand, the second sleeve 34 of the second developing roller 31 receives the developer from the first sleeve 33, and therefore does not face the restricting member 50 as does the first sleeve 33. Therefore, even if the surface linear velocity of the second sleeve 34 is increased, toner deterioration is unlikely to occur as occurs when the linear velocity of the first sleeve 33 is increased. Therefore, in this embodiment, the ratio of the surface linear velocity of the second sleeve 34 to the surface linear velocity of the photosensitive drum 28Y is set to 1.2, which is larger than the ratio of the surface linear velocity of the first sleeve 33 to the surface linear velocity of the photosensitive drum 28Y, which is 1.0. Increasing the surface linear velocity of the second sleeve 34 suppresses the injection of negative charge into the magnetic carrier, making it possible to further improve carrier adhesion.

[0127] In this embodiment, the linear velocity ratio of the second sleeve 34 to the first sleeve 33 is set to 1.2 (=1.2 / 1.0), but to obtain the desired effect, it is preferable to set it to 1.1 or more. On the other hand, if the linear velocity ratio of the second sleeve 34 to the first sleeve 33 is set to more than 1.3, the amount of developer coated per unit area of ​​the second sleeve will decrease, and the magnetic chains will become too coarse, which may affect image quality, etc. For this reason, it is preferable to set the linear velocity ratio of the second sleeve 34 to the first sleeve 33 to 1.3 or less. [Regarding the First Developing Roller]

[0128] As described above, if sufficient measures are taken to prevent carrier adhesion on the second developing roller 31 disposed downstream in the rotation direction of the photosensitive drum 28Y, some carrier adhesion is acceptable on the first developing roller 30. Therefore, in this embodiment, as described above, the absolute value of the normal component Br of the magnetic flux density of the first developing downstream pole 106 is made smaller than the absolute value of the normal component Br of the magnetic flux density of the first developing upstream pole 104 of the first developing roller 30. This makes it possible to easily cause carrier adhesion due to the first developing roller 30, but to make it difficult for streaky fogging to occur.

[0129] A more preferred configuration is described below. In this embodiment, the half-value width of the normal component Br of the magnetic flux density of the first downstream developing pole 106 is preferably smaller than the half-value width of the normal component Br of the magnetic flux density of the first upstream developing pole 104. This configuration further suppresses the magnetic field between the first downstream developing pole 106 and the second upstream developing pole 202, thereby further suppressing the occurrence of streaky fogging. Considering manufacturing variations, it is more preferable to set the half-value width smaller by 3° or more, and even more preferably by 5° or more. That is, the half-value width of the normal component Br of the magnetic flux density of the first downstream developing pole 106 is preferably smaller by 3° or more, and more preferably by 5° or more, than the half-value width of the normal component Br of the magnetic flux density of the first upstream developing pole 104.

[0130] If the absolute value of the magnetic flux density Br of the first downstream developing pole 106 is smaller than the absolute value of the magnetic flux density Br of the first upstream developing pole 104, or if the half-width of the first downstream developing pole 106 is smaller than the half-width of the first upstream developing pole 104, the proportion of the magnetic field lines extending from the first developing pole 105 that extend to the first upstream developing pole 104 increases. Therefore, as shown in FIG. 8 , the magnetic brush is more likely to face downstream. As described above, if the magnetic brush faces downstream, the time the magnetic brush is in contact with the photosensitive drum 28Y increases, increasing the time for toner development on the photosensitive drum 28Y, which has the advantage of improving developability. The above-described configuration is effective when there is a concern that reducing the surface linear velocity of the first sleeve 33 to prevent toner degradation may result in a decrease in developability.

[0131] In this embodiment, in the first developing roller 30, a line L1 is defined as a line connecting the position (peak position) on the first sleeve 33 where the absolute value of the normal component Br of the magnetic flux density of the first developing pole 105 is maximum and the rotation center O1 of the first sleeve 33. A line L11 is defined as a line connecting the position (peak position) on the first sleeve 33 where the absolute value of the normal component Br of the magnetic flux density of the first developing upstream pole 104 is maximum and the rotation center O1 of the first sleeve 33. The angle (acute angle) formed by lines L1 and L21 is defined as φ1 [°]. In other words, φ1 is the angle in the rotation direction of the first sleeve 33 from the position on the outer peripheral surface of the first sleeve 33 where the absolute value of the magnetic flux density of the first developing upstream pole 104 in the normal direction to the outer peripheral surface of the first sleeve 33 is maximum to the position on the outer peripheral surface of the first sleeve 33 where the absolute value of the magnetic flux density of the first developing pole 105 in the normal direction to the outer peripheral surface of the first sleeve 33 is maximum. Also, line L12 is the line connecting the position on the first sleeve 33 where the absolute value of the normal component Br of the magnetic flux density of the first developing downstream pole 106 is maximum (peak position) and the rotation center O1 of the first sleeve 33. The angle (acute angle) between line L1 and line L12 is φ2 [°]. In other words, φ2 is the angle in the rotation direction of the first sleeve 33 from the position on the outer circumferential surface of the first sleeve 33 where the absolute value of the magnetic flux density of the first developing pole 105 in the normal direction to the outer circumferential surface of the first sleeve 33 is maximum to the position on the outer circumferential surface of the first sleeve 33 where the absolute value of the magnetic flux density of the first developing downstream pole 106 in the normal direction to the outer circumferential surface of the first sleeve 33 is maximum. In this case, φ1 > φ2 is satisfied.

[0132] If a configuration satisfying φ1<φ2 were adopted, the effect of increasing the proportion of magnetic field lines extending from the first developing pole 105 that extend to the first upstream developing pole 104 would be obtained, but if φ1 is small, it would be difficult for the magnetic brush to contact the photosensitive drum 28Y on the upstream side, which could suppress the effect of improving developability. Therefore, in order to increase the proportion of magnetic field lines extending from the first developing pole 105 that extend to the first upstream developing pole 104, it is preferable to achieve this by making the absolute value and half-width of the normal component Br of the magnetic flux density of the first downstream developing pole 106 smaller than the normal component Br of the magnetic flux density of the first upstream developing pole 104 of the first developing roller 30. For the above reasons, in this embodiment, a configuration satisfying φ1>φ2 is adopted as a preferred example.

[0133] More specifically, for the first developing roller 30, the angle φ2 between the peak positions of the magnetic flux densities Br of the first developing pole 105 and the first downstream developing pole 106 is smaller than the angle φ1 between the peak positions of the magnetic flux densities Br of the first developing pole 105 and the first upstream developing pole 104 of the first magnet 36. This configuration increases the distance between the first downstream developing pole 106 and the second upstream developing pole 202. This reduces the generation of a magnetic field between the magnetic poles 105 and 202, suppresses the movement of developer between the magnetic poles 105 and 202, and makes it more difficult for streaky fogging to occur. Therefore, the angle φ2 between the peak positions of the magnetic flux densities Br of the first developing pole 105 and the first downstream developing pole 106 is preferably set to 40° or less (φ2≦40°), more preferably 35° or less (φ2≦35°). However, from the viewpoint of ensuring a sufficient volume of the magnets (pieces of second magnet 37) that contribute to the formation of the magnetic field at each magnetic pole, angle φ2 is preferably set to 15° or more (15°≦φ2), and more preferably 20° or more (20°≦φ2). In summary, angle φ2 preferably satisfies 15°≦φ2≦40°, and more preferably satisfies 20°≦φ2≦35°.

[0134] On the other hand, from the viewpoint of ensuring the aforementioned developability, it is preferable that the angle φ1 between the peak positions of the magnetic flux densities Br of the first development pole 105 and the first development upstream pole 104 of the first magnet 36 not be too small. The angle φ1 between the peak positions of the magnetic flux densities Br of the first development pole 105 and the first development upstream pole 104 of the first magnet 36 is preferably 25° or greater (25°≦φ1), more preferably 30° or greater (30°≦φ1). On the other hand, if the angle φ1 is too large, the proportion of the magnetic field lines extending from the first development pole 105 that extend to the first development upstream pole 104 may decrease. For this reason, the angle φ1 is preferably 50° or less (φ1≦50°), more preferably 45° or less (φ1≦45°). In summary, the angle φ1 preferably satisfies 25°≦φ1≦50°, and more preferably satisfies 30°≦φ1≦45°.

[0135] As in the present embodiment, when the angle φ2 between the peak positions of the first downstream developing pole 106 and the first upstream developing pole 105 is small, it becomes difficult to generate a magnetic field between the first downstream developing pole 106 and the second upstream developing pole 202, while the magnetic field lines are more likely to extend between the first downstream developing pole 106 and the first upstream developing pole 105. To obtain the above-described effect of improving developability, it is preferable to make it easier for the magnetic field lines to extend between the first upstream developing pole 104 and the first downstream developing pole 105. Therefore, in this embodiment, the maximum absolute value of the normal component Br of the magnetic flux density of the first upstream developing pole 104 is set to be 10 mT or more larger than the maximum absolute value of the normal component Br of the magnetic flux density of the first downstream developing pole 106. It is more preferable to set it to be 15 mT or more larger, and even more preferable to set it to be 20 mT or more larger. That is, the maximum absolute value of the normal component Br of the magnetic flux density of the first upstream developing pole 104 is preferably 10 mT or more, more preferably 15 mT or more, and even more preferably 20 mT or more, larger than the maximum absolute value of the normal component Br of the magnetic flux density of the first downstream developing pole 106. Note that, in order to prevent the magnetic poles 104 and 106 from affecting the developer transport performance, it is preferable that the maximum absolute value of the normal component Br of the magnetic flux density of each of the magnetic poles 104 and 106 be 20 mT or more. Therefore, it is preferable that the maximum absolute value of the normal component Br of the magnetic flux density of the first upstream developing pole 104 be 30 mT or more so as to satisfy the magnitude relationship between the magnetic flux densities of the first upstream developing pole 104 and the first downstream developing pole 106 described above.

[0136] However, in terms of the balance of magnetic flux densities, it is undesirable to increase the maximum absolute values ​​of the normal components of the magnetic flux density of the first upstream developing pole 104 and the first downstream developing pole 106 to be greater than the maximum absolute value of the normal component of the magnetic flux density, |Br|, of the first upstream developing pole 104. For this reason, the absolute value of the difference between the maximum absolute value of the normal component Br of the magnetic flux density of the first upstream developing pole 104 and the maximum absolute value of the normal component Br of the magnetic flux density of the first downstream developing pole 106 is preferably kept within 100 mT, more preferably within 50 mT. Furthermore, in consideration of manufacturing stability and cost, it is preferable to keep the maximum absolute value of the normal component Br of the magnetic flux density of each of the magnetic poles 104, 105, and 106 to 200 mT or less, so as to satisfy the magnitude relationship between the magnetic flux densities of the first upstream developing pole 104 and the first downstream developing pole 106. In the following description, the maximum value of the absolute value of the normal component of the magnetic flux density |Br| may be simply referred to as the absolute value of the normal component of the magnetic flux density |Br|.

[0137] Next, the configuration of this embodiment will be described more specifically using the magnetic flux density distributions of Comparative Example 1 and Examples 1 and 2. The Comparative Example has the same configuration as the Examples except for the following points.

[0138] Fig. 10(a) is a graph showing the magnetic flux density distribution of the first developing roller 30 according to Comparative Example 1, and Fig. 10(b) is a graph showing the magnetic flux density distribution of the second developing roller 31 according to Comparative Example 1. Fig. 11(a) is a graph showing the magnetic flux density distribution of the first developing roller 30 according to Example 1, and Fig. 11(b) is a graph showing the magnetic flux density distribution of the second developing roller 31 according to Example 1. Fig. 12(a) is a graph showing the magnetic flux density distribution of the first developing roller 30 according to Example 2, and Fig. 12(b) is a graph showing the magnetic flux density distribution of the second developing roller 31 according to Example 2. 10(a), 11(a), and 12(a) schematically show, with a solid line, the distribution of the normal component Br of the magnetic flux density (magnetic flux density Br in the normal direction to the outer peripheral surface of first sleeve 33) on first sleeve 33 due to first magnet 36, and with a dashed line, the distribution of the tangential component Bθ of the magnetic flux density (magnetic flux density Bθ in the tangential direction to the outer peripheral surface of first sleeve 33). Also, in Figures 10(b), 11(b), and 12(b), roughly show, with a solid line, the distribution of the normal component Br of the magnetic flux density on second sleeve 34 due to second magnet 37, and with a dashed line, the distribution of the tangential component Bθ of the magnetic flux density.

[0139] The normal component Br of the magnetic flux density, more precisely, refers to the component of the magnetic flux density B in the normal direction relative to the first sleeve 33 and the second sleeve 34 (magnetic flux density in the normal direction relative to the outer circumferential surfaces of the first and second rotating bodies). The normal component Br of the magnetic flux density of each magnet was measured using a magnetic field measuring device ("MS-9902" manufactured by F.W. BELL) with the distance between the probe, which is a component of the magnetic field measuring device, and the surface of the first sleeve 33 and the second sleeve 34 set to approximately 100 μm.

[0140] Furthermore, the tangential component Bθ of the magnetic flux density refers to the tangential component of the magnetic flux density B with respect to the first sleeve 33 and the second sleeve 34 (the magnetic flux density in the tangential direction with respect to the outer circumferential surfaces of the first rotating body and the second rotating body). The tangential component Bθ of the magnetic flux density can be calculated from the following equation 1 using the value of the normal component Br of the magnetic flux density.

[0141] Table 1 below shows the absolute value |Br| and half-width of the normal component of the magnetic flux density of the first upstream developing pole 104, the first downstream developing pole 105, and the first magnet 36 of the first developing roller 30 in Comparative Example 1, Examples 1, and 2, the angle φ1 between the peak positions of the first upstream developing pole 104, and the angle φ2 between the peak positions of the first downstream developing pole 106 and the first magnet 36 of the first developing roller 36 in Comparative Example 1, Examples 1, and 2. The first magnet 36 used in each of the first developing rollers 30 in Comparative Example 1, Examples 1, and 2 is the same, and all values ​​are the same.

[0142] Table 2 below shows the absolute value |Br| and half-width of the normal component of the magnetic flux density of the second development upstream pole 202, the second development pole 203, and the second development downstream pole 204 of the second magnet 37 of the second developing roller 31 in Comparative Example 1 and Examples 1 and 2, the angle θ1 between the peak positions of the second development pole 203 and the second development upstream pole 202, and the angle θ2 between the peak positions of the second development pole 203 and the second development downstream pole 204.

[0143] 13 shows the distribution of the normal component Br of the magnetic flux density in Comparative Example 1, Example 1, and Example 2 in order to clarify the difference in the magnetic flux density of the second developing roller 31. Also, FIG. 14 shows the distribution of the tangential component Bθ of the magnetic flux density in Comparative Example 1, Example 1, and Example 2.

[0144] In the magnetic flux density distribution of the first developing roller 30 of Comparative Example 1 shown in FIG. 10A and Table 1, the absolute value |Br| of the normal component of the magnetic flux density of the first upstream developing pole 104 is larger than the absolute value |Br| of the normal component of the magnetic flux density of the first downstream developing pole 106. In addition, in the magnetic flux density distribution of the second developing roller 31 of Comparative Example 1 shown in FIG. 10B and Table 2, the absolute value |Br| of the normal component of the magnetic flux density of the second upstream developing pole 202 is smaller than the absolute value |Br| of the normal component of the magnetic flux density of the second downstream developing pole 204. Therefore, it is possible to keep the absolute values ​​|Br| of the normal components of the magnetic flux density of each of the first downstream developing pole 106 and the second upstream developing pole 202 low, thereby preventing streaky fogging.

[0145] In this regard, the magnetic flux density distributions of the first developing roller 30 in Examples 1 and 2 shown in Figures 11(a), 12(a) and Table 1, and the magnetic flux density distributions of the second developing roller 31 in Examples 1 and 2 shown in Figures 11(b), 12(b) and Table 2 are configured similarly to those in Comparative Example 1. Therefore, in Examples 1 and 2 as well, it is possible to keep low the absolute value |Br| of the normal component of the magnetic flux density of each of the first downstream developing pole 106 and the second upstream developing pole 202, and to prevent streaky fogging from occurring.

[0146] In order to more reliably prevent the occurrence of streaky fogging, it is preferable to keep the absolute value |Br| of the normal component of the magnetic flux density of the first downstream developing pole 106 and the second upstream developing pole 202 at 100 mT or less, more preferably 90 mT or less. However, if it is less than 20 mT, it may affect the transportability of the developer, so it is preferable to keep it at 20 mT or more.

[0147] Continuing with the explanation of Comparative Example 1. As described above, if the absolute value |Br| of the normal component of the magnetic flux density of the first upstream development pole 104 of the first developing roller 30 is made larger than the absolute value |Br| of the normal component of the magnetic flux density of the first downstream development pole 106, the occurrence of streak-like fogging is suppressed, but carrier adhesion to the first developing roller 30 becomes more likely to occur. For this reason, it is required that the second developing roller 31 sufficiently recovers carrier adhesion to the photosensitive drum 28Y on the first developing roller 30. However, it is considered that the configuration of the second developing roller 31 of Comparative Example 1 is not sufficient.

[0148] As already mentioned, to prevent carrier adhesion, it is preferable that the developer contact the opposing portion of the developing pole facing the sleeve on the upstream side of the photosensitive drum 28Y but not on the downstream side. To achieve this configuration, it is preferable not only to make the absolute value |Br| of the normal component of the magnetic flux density of the second upstream developing pole 202 smaller than the absolute value |Br| of the normal component of the magnetic flux density of the second downstream developing pole 204, but also to make the angle θ2 between the peak positions of the second downstream developing pole 204 smaller than the angle θ1 between the peak positions of the second upstream developing pole 202 and the second downstream developing pole 203. This increases the proportion of the magnetic field lines extending from the second downstream developing pole 204 that extend to the second downstream developing pole 204, and the magnetic field lines extend downstream while bending further upstream, making it easier for the magnetic brush to contact the opposing portion of the second downstream developing pole 203 on the upstream side of the photosensitive drum 28Y.

[0149] Furthermore, when the angle θ2 is reduced downstream of the photosensitive drum 28Y relative to the portion facing the second developing pole 203 via the second sleeve 34, the magnetic brush quickly folds, making it less likely for the magnetic brush to come into contact downstream of the photosensitive drum 28Y. However, in the magnetic flux density distribution of the second developing roller 31 of Comparative Example 1 shown in FIG. 10B and Table 2, the angle θ2 between the peak positions of the second developing pole 203 and the second developing downstream pole 204 is larger than the angle θ1 between the peak positions of the second developing pole 203 and the second developing upstream pole 202. Therefore, this is not necessarily sufficient to recover carrier adhesion on the first developing roller 30. As a result, while the configuration of Comparative Example 1 can suppress the occurrence of streaky fogging, it cannot sufficiently suppress the occurrence of carrier adhesion.

[0150] 11B, 13, and Table 2, the angle θ2 between the peak positions of the second developing pole 203 and the second downstream developing pole 204 is smaller than the angle θ1 between the peak positions of the second developing pole 203 and the second upstream developing pole 202. Therefore, a larger proportion of the magnetic field lines extending from the second developing pole 203 extend to the second downstream developing pole 204, making it easier for the magnetic wire to contact the portion of the second developing pole 203 facing the second sleeve 34 on the upstream side of the photosensitive drum 28Y. On the other hand, the magnetic wire quickly folds on the downstream side of the photosensitive drum 28Y, making it more difficult for the magnetic wire to contact the portion on the downstream side of the photosensitive drum 28Y. Therefore, in Example 1, the carrier adhesion on the first developing roller 30 can be sufficiently collected by the second developing roller 31. As a result, the configuration of Example 1 can simultaneously suppress the occurrence of streaky fogging and carrier adhesion.

[0151] To more reliably suppress carrier adhesion, it is preferable to limit the angle θ2 between the peak positions of the second developing pole 203 and the second downstream developing pole 204 to 30° or less (θ2≦30°), more preferably 28° or less (θ2≦28°). This configuration effectively prevents contact of the magnetic brush on the downstream side of the photosensitive drum 28Y with the opposing portion of the second developing pole 203 facing the second sleeve 34. However, if the angle θ2 is less than 15°, it becomes difficult to ensure sufficient volume for the magnets (pieces of the second magnet 37) that contribute to the magnetic field formation of each magnetic pole, which may result in insufficient magnetic flux density. For this reason, it is preferable to set the angle θ2 to 15° or more (15°≦θ2), more preferably 20° or more (20°≦θ2). In summary, it is preferable for the angle θ2 to satisfy 15°≦θ2≦30°, and even more preferable for it to satisfy 20°≦θ2≦28°.

[0152] On the other hand, if the angle θ1 between the peak positions of the second developing pole 203 and the second developing upstream pole 202 is too large, the second developing upstream pole 202 will approach the first developing downstream pole 106, which is unfavorable for the occurrence of streaky fogging. Therefore, it is preferable to limit the angle θ1 between the peak positions of the second developing pole 203 and the second developing upstream pole 202 to 35° or less (θ1≦35°), more preferably 30° or less (θ1≦30°). However, from the viewpoint of ensuring sufficient volume for the magnets (pieces of the second magnet 37) that contribute to the formation of the magnetic field of each magnetic pole, it is preferable to set the angle θ1 to 15° or more (15°≦θ1), more preferably 20° or more (20°≦θ1). In summary, it is preferable for the angle θ1 to satisfy 15°≦θ1≦35°, and more preferably 20°≦θ1≦30°.

[0153] Next, Example 2 will be described. The magnetic flux density distribution of the second developing roller 31 in Example 2, shown in FIGS. 12B, 13, and Table 2, has a larger absolute value |Br| of the normal component of the magnetic flux density of the second developing downstream pole 204 than in Example 1. As a result, the proportion of the magnetic field lines extending from the second developing pole 203 that extend to the second developing downstream pole 204 is greater than in Example 1. This makes it easier for the magnetic wires to contact the opposing portion of the second developing pole 203 facing the second developing pole 203 via the second sleeve 34 on the upstream side of the photosensitive drum 28Y, thereby further suppressing the occurrence of carrier adhesion. To achieve a greater effect in suppressing carrier adhesion, it is preferable to make the absolute value |Br| of the normal component of the magnetic flux density of the second developing downstream pole 204 10 mT or more larger than the absolute value |Br| of the normal component of the magnetic flux density of the second developing upstream pole 202, more preferably 15 mT or more, and even more preferably 20 mT or more. That is, the maximum absolute value of the normal component Br of the magnetic flux density of the second downstream developing pole 204 is preferably 10 mT or more, more preferably 15 mT or more, and even more preferably 20 mT or more, greater than the maximum absolute value of the normal component Br of the magnetic flux density of the second upstream developing pole 202. In order not to affect the developer transport performance of each of the magnetic poles 202, 204, it is preferable that the maximum absolute value of the normal component Br of the magnetic flux density of each of the magnetic poles 202, 204 be 20 mT or more. Therefore, it is preferable that the maximum absolute value of the normal component Br of the magnetic flux density of the second downstream developing pole 204 be 30 mT or more, thereby satisfying the magnitude relationship between the magnetic flux densities of the second upstream developing pole 202 and the second downstream developing pole 204 described above.

[0154] In Example 2, as shown in Table 2, the absolute value |Br| of the normal component of the magnetic flux density of the second developing downstream pole 204 is set to be 20 mT or more greater than the absolute value |Br| of the normal component of the magnetic flux density of the second developing upstream pole 202. However, in terms of the balance of magnetic flux densities, it is not desirable to make the maximum absolute values ​​of the normal components of the magnetic flux density of the second developing upstream pole 202 and the second developing downstream pole 204 greater than the maximum absolute value |Br| of the normal component of the magnetic flux density of the second developing pole 203. For this reason, the absolute value of the difference between the maximum absolute value of the normal component Br of the magnetic flux density of the second developing upstream pole 202 and the maximum absolute value of the normal component Br of the magnetic flux density of the second developing downstream pole 204 is preferably kept within 100 mT, more preferably within 50 mT. Furthermore, in consideration of manufacturing stability and costs, it is preferable to limit the maximum absolute value of the normal component Br of the magnetic flux density of each of the magnetic poles 202, 203, and 204 to 200 mT or less, and to satisfy the magnitude relationship between the magnetic flux densities of the second developing upstream pole 202 and the second developing downstream pole 204 described above.

[0155] As described above, in order to deal with carrier adhesion on the second developing roller 31, it is important to increase the proportion of the magnetic field lines extending from the second developing pole 203 that extend to the second developing downstream pole 204 rather than the second developing upstream pole 202, and to have the magnetic brush contact the photosensitive drum 28Y as upstream as possible. The proportion of the magnetic field lines extending from the second developing pole 203 that extend to each of the second developing upstream pole 202 and the second developing downstream pole 204 is mainly determined by the relationship between the maximum value |Br| of the absolute value of the normal component of the magnetic flux density of the second developing upstream pole 202 and the second developing downstream pole 204, the half-width, and the angles θ1 and θ2 between the peak positions with the second developing pole 203.

[0156] In the configurations of Examples 1 and 2, the maximum value |Br| of the absolute value of the normal component of the magnetic flux density of the second downstream developing pole 204 is set larger than the maximum value |Br| of the absolute value of the normal component of the magnetic flux density of the second upstream developing pole 202. Also, the angle θ2 between the peak positions of the second downstream developing pole 204 and the second upstream developing pole 202 is set smaller than the angle θ1 between the peak positions of the second upstream developing pole 202 and the second downstream developing pole 204. Furthermore, the half-value width of the normal component Br of the magnetic flux density of the second downstream developing pole 204 is set larger than the half-value width of the normal component Br of the magnetic flux density of the second upstream developing pole 202.

[0157] The proportion of the magnetic field lines extending from the second developing pole 203 to the second upstream developing pole 202 and the second downstream developing pole 204 can be predicted from the distribution of the tangential component Bθ of the magnetic flux density. Figures 10(b), 11(b), 12(b), 13, and 14 show the magnetic flux density distribution of the second developing roller 31. Between the peak positions of the normal component Br between the second developing pole 203 and the second upstream developing pole 202 and the second downstream developing pole 204, there are peaks (maximum values) Bθ2-1 and Bθ2-2 of the tangential component Bθ. It is considered that the larger the peak of the tangential component Bθ, the more magnetic field lines there are. Therefore, by comparing the magnitudes (absolute values) of the peaks Bθ2-1 and Bθ2-2 of the tangential component Bθ of the magnetic flux density upstream and downstream of the second developing pole 203, it is possible to predict the proportion of the magnetic lines of force extending from the second developing pole 203 that extend to each of the second developing upstream pole 202 and the second developing downstream pole 204.

[0158] The right side of Table 3 below shows the absolute values ​​|Bθ2-1| and |Bθ2-2| of the peaks of the tangential component Bθ of the magnetic flux density upstream and downstream of the second developing pole 203 for Comparative Example 1, Example 1, and Example 2. Table 3 also shows the ratio |Bθ2-2| / |Bθ2-1| of the absolute value of the downstream peak to the absolute value of the upstream peak. |Bθ2-1| is the maximum absolute value of the magnetic flux density in the tangential direction to the outer peripheral surface of the second sleeve 34 in the section in the rotational direction of the second sleeve 34 from the position on the outer peripheral surface of the second sleeve 34 where the absolute value of the magnetic flux density of the second developing upstream pole 202 in the normal direction to the outer peripheral surface of the second sleeve 34 is maximum to the position on the outer peripheral surface of the second sleeve 34 where the absolute value of the magnetic flux density of the second developing pole 203 in the normal direction to the outer peripheral surface of the second sleeve 34 is maximum. Furthermore, |Bθ2-2| is the maximum absolute value of the magnetic flux density in the tangential direction to the outer peripheral surface of the second sleeve 34 in the section in the rotational direction of the second sleeve 34 from the position on the outer peripheral surface of the second sleeve 34 where the absolute value of the magnetic flux density of the second developing pole 203 in the normal direction to the outer peripheral surface of the second sleeve 34 is maximum to the position on the outer peripheral surface of the second sleeve 34 where the absolute value of the magnetic flux density of the second developing pole 204 in the normal direction to the outer peripheral surface of the second sleeve 34 is maximum. It can be said that the larger this value, the greater the proportion of the magnetic field lines extending from the second developing pole 203 that extend toward the second developing pole downstream 204 rather than toward the second developing pole upstream 202. Note that information regarding the first developing roller 30 is also listed on the left side of Table 3, which will be described later.

[0159] In Comparative Example 1, |Bθ2-2| / |Bθ2-1| was 1.08. From this, it can be said that in Comparative Example 1 as well, a larger proportion of the magnetic lines of force extending from the second developing pole 203 extend to the second developing downstream pole 204 than to the second developing upstream pole 202. However, as mentioned above, there is a concern that the configuration of Comparative Example 1 may be more susceptible to carrier adhesion. This is thought to be because |Bθ2-2| / |Bθ2-1| in Comparative Example 1 was only 1.08 times, which left concerns lingering that carrier adhesion caused by the first developing roller 30 may not be sufficiently collected.

[0160] 14 and Table 3, in Example 1, |Bθ2-1| slightly decreased and |Bθ2-2| increased compared to Comparative Example 1, and |Bθ2-2| / |Bθ2-1| was 1.15. As a result, it is thought that in Example 1, the proportion of the magnetic field lines extending from the second developing pole 203 that extend toward the second developing downstream pole 204 rather than the second developing upstream pole 202 increased compared to Comparative Example 1. It is thought that this was why carrier adhesion was less likely to occur, as described above.

[0161] 14 and Table 3, in Example 2, |Bθ2-1| further decreased and |Bθ2-2| further increased compared to Example 1, and |Bθ2-2| / |Bθ2-1| was 1.34. As a result, it is thought that in Example 2, the proportion of the magnetic field lines extending from the second developing pole 203 that extend toward the second developing downstream pole 204 rather than the second developing upstream pole 202 increased more than in Example 1. This is thought to have made carrier adhesion even less likely to occur, as described above.

[0162] From the above, it is preferable to adjust the relationship between the absolute value |Br| of the normal component of the magnetic flux density of the second upstream developing pole 202 and the second downstream developing pole 204, the half-width, and the angle θ between the peak positions with respect to the second developing pole 203, so that |Bθ2-2| / |Bθ2-1| is 1.10 or more, more preferably 1.15 or more, and even more preferably 1.30 or more. That is, on the second sleeve 34, the absolute value of the magnitude at which the tangential component of the magnetic flux density between the second upstream developing pole 202 and the second upstream developing pole 202 reaches its maximum (peak) in the rotational direction of the second sleeve 34 is defined as |Bθ2-1|. Furthermore, the absolute value of the magnitude at which the tangential component of the magnetic flux density between the second downstream developing pole 204 and the second upstream developing pole 203 reaches its maximum (peak) in the rotational direction of the second sleeve 34 is defined as |Bθ2-2|. In this case, the relationship |Bθ2-2| / |Bθ2-1|≧1.1 is satisfied. Furthermore, it is preferable to satisfy |Bθ2-2| / |Bθ2-1|≧1.15, and it is more preferable to satisfy |Bθ2-2| / |Bθ2-1|≧1.30. The larger |Bθ2-2| / |Bθ2-1| is, the more effective it is in suppressing the occurrence of carrier adhesion. However, from the viewpoint of the balance of developer transport, it is not preferable for |Bθ2-2| / |Bθ2-1| to be 10.0 or more. Therefore, it is preferable to satisfy |Bθ2-2| / |Bθ2-1|<10.0.

[0163] Next, Example 3 and Comparative Example 2 will be described with reference to Figures 15(a) to 16(b). Figure 15(a) is a graph showing the magnetic flux density distribution of the first developing roller 30 according to Example 3, and Figure 15(b) is a graph showing the magnetic flux density distribution of the second developing roller 31 according to Example 3. Figure 16(a) is a graph showing the magnetic flux density distribution of the first developing roller 30 according to Comparative Example 2, and Figure 16(b) is a graph showing the magnetic flux density distribution of the second developing roller 31 according to Comparative Example 2.

[0164] 15(a) and 16(a) schematically show, by a solid line, the distribution of the normal component Br of the magnetic flux density on the first sleeve 33 due to the first magnet 36, and by a dashed line, the distribution of the tangential component Bθ of the magnetic flux density. Also, in Figures 15(b) and 16(b), roughly show, by a solid line, the distribution of the normal component Br of the magnetic flux density on the second sleeve 34 due to the second magnet 37, and by a dashed line, the distribution of the tangential component Bθ of the magnetic flux density.

[0165] Furthermore, Table 4 below, like Table 1, shows the absolute value |Br| and half-width of the normal component of the magnetic flux density of the first development upstream pole 104, the first development pole 105, and the first development downstream pole 106 of the first magnet 36 of the first developing roller 30 in Example 3 and Comparative Example 2, the angle φ1 between the peak positions of the first development pole 105 and the first development upstream pole 104, and the angle φ2 between the peak positions of the first development pole 105 and the first development downstream pole 106.

[0166] 17 shows the distribution of the normal component Br of the magnetic flux density in Example 1, Example 3, and Comparative Example 2 in order to clarify the difference in the magnetic flux density of the first developing roller 30. Also, FIG. 18 shows the distribution of the tangential component Bθ of the magnetic flux density in Example 1, Example 3, and Comparative Example 2.

[0167] Similar to Table 2, Table 5 below shows the absolute value |Br| and half-width of the normal component of the magnetic flux density of the second upstream developing pole 202, second downstream developing pole 203, and second magnet 37 of the second developing roller 31 in Example 3 and Comparative Example 2, the angle θ1 between the peak positions of the second upstream developing pole 202, and the angle θ2 between the peak positions of the second downstream developing pole 204. Note that the second magnet 37 used in each second developing roller 31 in Example 3 and Comparative Example 2 is the same as that in Example 1, and all values ​​are the same.

[0168] 15(a) and 16(a) show the magnetic flux density distribution of the first developing roller 30. FIG.

[0169] Peaks (maximum values) Bθ1-1 and Bθ1-2 of the tangential component Bθ are present between the peak positions of the normal component Br between the first developing pole 105 and the first developing upstream pole 104 and the first developing downstream pole 106. The left side of Table 3 and the following Table 6 show the absolute values ​​|Bθ1-1| and |Bθ1-2| of the peaks of the tangential component Bθ of the magnetic flux density upstream and downstream of the first developing pole 105 in Comparative Examples 1 and 2 and Examples 1, 2 and 3. |Bθ1-1| is the maximum absolute value of the magnetic flux density in the tangential direction to the outer surface of the first sleeve 33 in the section in the rotational direction of the first sleeve 33 from the position on the outer surface of the first sleeve 33 where the absolute value of the magnetic flux density of the first developing upstream pole 104 in the normal direction to the outer surface of the first sleeve 33 is maximum to the position on the outer surface of the first sleeve 33 where the absolute value of the magnetic flux density of the first developing pole 105 in the normal direction to the outer surface of the first sleeve 33 is maximum. Furthermore, |Bθ1-2| is the maximum absolute value of the magnetic flux density in the tangential direction to the outer peripheral surface of the first sleeve 33 in the section in the rotational direction of the first sleeve 33 from the position on the outer peripheral surface of the first sleeve 33 where the absolute value of the magnetic flux density of the first developing pole 105 in the normal direction to the outer peripheral surface of the first sleeve 33 is maximum to the position on the outer peripheral surface of the first sleeve 33 where the absolute value of the magnetic flux density of the first developing pole downstream 106 in the normal direction to the outer peripheral surface of the first sleeve 33 is maximum. Tables 3 and 6 also show the ratio of the absolute value of the upstream peak to the absolute value of the downstream peak, |Bθ1-1| / |Bθ1-2|. Based on the same considerations as those described above for the second developing roller 31, it can be said that the larger this value, the greater the proportion of the magnetic field lines extending from the first developing pole 105 that extend toward the first developing pole upstream 104 rather than the first developing pole downstream 106. As in Table 3, the right side of Table 6 also shows the absolute values ​​|Bθ2-1|, |Bθ2-2| of each peak of the tangential component Bθ of the magnetic flux density upstream and downstream of the second development pole 203 in Example 3 and Comparative Example 2, and the ratio |Bθ2-2| / |Bθ2-1| of the absolute value of the downstream peak to the absolute value of the upstream peak.

[0170] In the first developing roller 30 of Example 3 shown in Figures 15A, 17, and Table 4, the absolute value |Br| of the normal component of the magnetic flux density of the first developing downstream pole 106 is larger than that of Example 1 (Table 1). This increases the concern about the occurrence of streaky fogging. However, in Example 3, the absolute value |Br| of the normal component of the magnetic flux density of the first developing upstream pole 104 is maintained to be larger than the absolute value |Br| of the normal component of the magnetic flux density of the first developing downstream pole 106, so it is possible to just barely achieve both the suppression of streaky fogging and the suppression of carrier adhesion.

[0171] However, according to the inventors' investigations, it was found that Example 3 had slightly inferior developability compared to Example 1, although the practical impact was small. This is thought to be due to the following reason. |Bθ1-1| / |Bθ1-2| for Example 3 shown in FIG. 18 and Table 6 is 1.00. This means that the ratio of the magnetic field lines extending from the first developing pole 105 to the first developing upstream pole 104 and the magnetic field lines extending to the first developing downstream pole 106 is approximately the same.

[0172] On the other hand, the ratio |Bθ1-1| / |Bθ1-2| of Example 1 shown in FIG. 18 and Table 3 is 1.09. This means that the proportion of magnetic field lines extending from the first developing pole 105 to the first developing upstream pole 104 is greater than the proportion of magnetic field lines extending to the first developing downstream pole 106. As a result, in the configuration of Example 1, the magnetic brush extends toward the downstream side of the photosensitive drum 28Y. As described above, when the magnetic brush faces downstream, the time the magnetic brush is in contact with the photosensitive drum 28Y is longer, increasing the time for toner development on the photosensitive drum 28Y, which is advantageous for developability. Since this advantage is no longer available in the configuration of Example 3, it is believed that the developability is lower than in Example 1.

[0173] In the first developing roller 30 of Comparative Example 2 shown in Figures 16(a), 17, and Table 4, the absolute value |Br| of the normal component of the magnetic flux density of the first developing downstream pole 106 is even larger than in Examples 1 and 3. This further increases the concern about the occurrence of streaky fog. When image formation was actually performed with the configuration of Comparative Example 2, streaky fog sometimes occurred. The configuration of Comparative Example 2 does not maintain a configuration in which the absolute value |Br| of the normal component of the magnetic flux density of the first developing upstream pole 104 is larger than the absolute value |Br| of the normal component of the magnetic flux density of the first developing downstream pole 106. Furthermore, the absolute value |Br| of the normal component of the magnetic flux density of the first developing downstream pole 106 is greater than 100 mT. This is thought to be why streaky fog occurred in Comparative Example 2.

[0174] Furthermore, the configuration of Comparative Example 2 was less susceptible to carrier adhesion compared to Examples 1 and 3, but had reduced developability. The |Bθ1-1| / |Bθ1-2| ratio for Comparative Example 2 shown in FIG. 18 and Table 6 is 0.96. This means that the proportion of magnetic field lines extending from the first developing pole 105 to the first developing upstream pole 104 is greater than the proportion of magnetic field lines extending from the first developing pole 105 to the first developing upstream pole 104. This likely resulted in the magnetic chains extending more easily upstream of the photosensitive drum 28Y, preventing the benefits of developing performance that come from the magnetic chains extending downstream, resulting in reduced developability.

[0175] From the above, the ratio |Bθ1-1| / |Bθ1-2| of the absolute value of the upstream peak of the tangential component Bθ of the magnetic flux density of the first developing roller 30 to the absolute value of the downstream peak of the first developing pole 105 is preferably 1.00 or more as in Example 3, and more preferably 1.05 or more as in Example 1. That is, it is preferable to satisfy |Bθ1-1| / |Bθ1-2|≧1.0, and it is even more preferable to satisfy |Bθ1-1| / |Bθ1-2|≧1.05. However, although the greater the |Bθ1-1| / |Bθ1-2| ratio, the greater the effect, it is not preferable to set it to 10.0 or more due to the balance of development and transport. Therefore, it is preferable to satisfy |Bθ1-1| / |Bθ1-2|<10.0.

[0176] Next, Example 4 will be described using Figures 19(a) and (b). Figure 19(a) is a graph showing the magnetic flux density distribution of the first developing roller 30 according to Example 4, and Figure 19(b) is a graph showing the magnetic flux density distribution of the second developing roller 31 according to Example 4. Figure 19(a) schematically shows the distribution of the normal component Br of the magnetic flux density on the first sleeve 33 due to the first magnet 36 with a solid line, and the distribution of the tangential component Bθ of the magnetic flux density with a dashed line. Figure 19(b) also schematically shows the distribution of the normal component Br of the magnetic flux density on the second sleeve 34 due to the second magnet 37 with a solid line, and the distribution of the tangential component Bθ of the magnetic flux density with a dashed line.

[0177] In addition, Table 7 below, similar to Tables 1 and 4, shows the absolute value |Br| and half-width of the normal component of the magnetic flux density of the first developing upstream pole 104, the first developing pole 105, and the first developing downstream pole 106 of the first magnet 36 of the first developing roller 30 in Example 4, the angle φ1 between the peak positions of the first developing pole 105 and the first developing upstream pole 104, and the angle φ2 between the peak positions of the first developing pole 105 and the first developing downstream pole 106.

[0178] 20 shows the distribution of the normal component Br of the magnetic flux density in Examples 1 and 4 to clarify the difference in the magnetic flux density of the first developing roller 30. Also, FIG. 21 shows the distribution of the tangential component Bθ of the magnetic flux density in Examples 1 and 4.

[0179] Similar to Tables 2 and 5, Table 8 below shows the absolute value |Br| and half-width of the normal component of the magnetic flux density of the second upstream developing pole 202, the second downstream developing pole 203, and the second magnet 37 of the second developing roller 31 in Example 4, the angle θ1 between the peak positions of the second upstream developing pole 202, and the angle θ2 between the peak positions of the second downstream developing pole 204. The second magnet 37 of the second developing roller 31 in Example 4 is the same as that in Examples 1 and 3, and all values ​​are the same. Therefore, similar to Examples 1 and 3, carrier adhesion is less likely to occur.

[0180] Similar to Tables 3 and 6, Table 9 below shows the absolute values ​​|Bθ1-1|, |Bθ1-2| of each peak of the tangential component Bθ of the magnetic flux density upstream and downstream of the first developing pole 105 in Example 4, and the ratio |Bθ1-1| / |Bθ1-2| of the absolute value of the upstream peak to the absolute value of the downstream peak. Table 9 also shows the absolute values ​​|Bθ2-1|, |Bθ2-2| of each peak of the tangential component Bθ of the magnetic flux density upstream and downstream of the second developing pole 203 in Example 4, and the ratio |Bθ2-2| / |Bθ2-1| of the absolute value of the downstream peak to the absolute value of the upstream peak.

[0181] 20 and Table 7, in the configuration of Example 4, the angle φ2 between the peak positions of the first developing pole 105 and the first downstream developing pole 106 of the first developing roller 30 is smaller than in Example 1 (see Tables 1 and 7). In this case, the angle φ2 between the peak positions of the first developing pole 105 and the first downstream developing pole 106 of the first developing roller 30 is smaller than the angle θ2 between the peak positions of the second developing pole 203 and the second downstream developing pole 204 of the second developing roller 31 (θ2 > φ2). As a result, in Example 4, the first downstream developing pole 106 is positioned in a direction away from the second upstream developing pole 202 compared to Example 1, which has the advantage of weakening the magnetic field between the two poles and further suppressing the occurrence of streaky fogging.

[0182] On the other hand, if the angle φ2 between the peak positions of the first developing pole 105 and the first developing downstream pole 106 of the first developing roller 30 is reduced, the magnetic field lines tend to extend from the first developing pole 105 to the first developing downstream pole 106. For this reason, if the angle φ2 between the peak positions of the first developing pole 105 and the first developing downstream pole 106 of the first developing roller 30 is reduced, this can be a disadvantage in terms of developability.

[0183] However, as shown in FIG. 20 and Table 7, in the first developing roller 30 of Example 4, the absolute value |Br| of the normal component of the magnetic flux density of the first upstream developing pole 104 is set to be 15 mT or more larger than the absolute value |Br| of the normal component of the magnetic flux density of the first downstream developing pole 106. Therefore, as shown in Table 9, |Bθ1-1| / |Bθ1-2| in Example 4 is 1.0 or more, and a state is maintained in which the proportion of magnetic field lines extending from the first developing pole 105 to the first upstream developing pole 104 is greater than the proportion of magnetic field lines extending from the first developing pole 105 to the first downstream developing pole 106. With this configuration, even in Example 4, the advantage of developability obtained by the magnetic brush extending downstream of the photosensitive drum 28Y can be maintained.

[0184] As in Example 4, even if the angle φ2 between the peak positions of the first developing pole 105 and the first downstream developing pole 106 of the first developing roller 30 is made smaller than the angle θ2 between the peak positions of the second developing pole 203 and the second downstream developing pole 204 of the second developing roller 31, as long as |Bθ1-1| / |Bθ1-2| is made 1.0 or more, it is possible to achieve both suppression of carrier adhesion and suppression of the occurrence of streaky fogging, and also obtain the benefit of improved developability.

[0185] The present invention is not limited to the configurations of the above-described embodiments. For example, the image forming apparatus 100 is not limited to an MFP, but may be a copier, printer, or facsimile machine. Furthermore, the configurations of the developer supply screw 42, developer stirring screw 43, and developer recovery screw 44 are not particularly limited as long as they can transport the developer. For example, spiral blades or paddle-shaped blades can be used.

[0186] Furthermore, in the above-described embodiment, the first developing roller 30 is positioned upstream and the second developing roller 31 is positioned downstream relative to the rotation direction of the photosensitive drum 28Y, but the same effect can be obtained even when the second developing roller 31 is positioned upstream and the first developing roller 30 is positioned downstream.

[0187] According to the present invention, a developing device is provided that can simultaneously suppress streak-like fogging and carrier adhesion.

[0188] The present invention is not limited to the above-described embodiments, and various modifications and variations can be made without departing from the spirit and scope of the present invention. Therefore, the following claims are appended to apprise the public of the scope of the present invention.

[0189] This application claims priority based on Japanese Patent Application No. 2024-107390 filed on July 3, 2024, and Japanese Patent Application No. 2025-067174 filed on April 16, 2025, the entire contents of which are incorporated herein by reference.

Claims

1. A developing container that contains a developer containing toner and a carrier; a first rotating body to which the developer contained in the developing container is supplied, the first rotating body rotating in the same direction as the rotating image carrier at a position on the outer circumferential surface of the first rotating body that is closest to the image carrier, and carrying and transporting the developer to a first developing position where an electrostatic latent image formed on the image carrier is developed; a first magnet that is fixedly disposed inside the first rotating body and non-rotatable, the first magnet having: a first developing pole disposed facing the image carrier at the first developing position; a first upstream pole disposed upstream of the first developing pole in the rotational direction of the first rotating body, adjacent to the first developing pole and having a polarity opposite to that of the first developing pole; a first downstream pole disposed downstream of the first developing pole in the rotational direction of the first rotating body, adjacent to the first developing pole and having a polarity opposite to that of the first developing pole; and a delivery pole disposed downstream of the first downstream pole and upstream of the first upstream pole in the rotational direction of the first rotating body; a second rotating body that is disposed opposite the first rotating body and to which the developer is transferred from the first rotating body by a magnetic field generated by the first magnet, the second rotating body rotating in the same direction as the image carrier at a position on the outer circumferential surface of the second rotating body that is closest to the image carrier, and that carries and transports the developer to a second developing position where the electrostatic latent image is developed; a second magnet that is fixedly disposed within the second rotating body and non-rotatably, the second magnet having: a second developing pole that is disposed facing the image carrier at the second developing position; a second upstream pole that is disposed adjacent to the second developing pole upstream of the second developing pole in the rotational direction of the second rotating body and has a polarity opposite to that of the second developing pole; a second downstream pole that is disposed adjacent to the second developing pole downstream of the second developing pole in the rotational direction of the second rotating body and has a polarity opposite to that of the second developing pole; and a receiving pole that is disposed downstream of the second downstream pole and upstream of the second upstream pole in the rotational direction of the second rotating body and close to the delivering pole and has a polarity opposite to that of the delivering pole,the maximum absolute value of the magnetic flux density of the first upstream pole in the normal direction to the outer peripheral surface of the first rotor is greater than the maximum absolute value of the magnetic flux density of the first downstream pole in the normal direction to the outer peripheral surface of the first rotor; the maximum absolute value of the magnetic flux density of the second downstream pole in the normal direction to the outer peripheral surface of the second rotor is greater than the maximum absolute value of the magnetic flux density of the second upstream pole in the normal direction to the outer peripheral surface of the second rotor; and θ1 [°] is the angle in the rotational direction of the second rotor from the position on the outer peripheral surface of the second rotor where the absolute value of the magnetic flux density of the second upstream pole in the normal direction to the outer peripheral surface of the second rotor is maximum to the position on the outer peripheral surface of the second rotor where the absolute value of the magnetic flux density of the second development pole in the normal direction to the outer peripheral surface of the second rotor; A developing device that satisfies θ1 > θ2, when the angle in the rotational direction of the second rotating body from the position on the outer peripheral surface of the second rotating body where the absolute value of the magnetic flux density of the second developing pole in the normal direction to the outer peripheral surface of the second rotating body is maximum to the position on the outer peripheral surface of the second rotating body where the absolute value of the magnetic flux density of the second downstream pole in the normal direction to the outer peripheral surface of the second rotating body is maximum is θ2 [°].

2. The developing device according to claim 1, further satisfying 2°≦θ1-θ2.

3. The developing device according to claim 1, further satisfying 3°≦θ1-θ2.

4. The developing device according to claim 1, further satisfying θ2≧θ1 / 2.

5. The developing device according to claim 1, further satisfying 15°≦θ2≦30°.

6. The developing device according to claim 1, further satisfying 15°≦θ1≦35°.

7. A developing device as described in claim 1, wherein the half-value width of the magnetic flux density of the second downstream pole in the normal direction to the outer peripheral surface of the second rotating body is greater than the half-value width of the magnetic flux density of the second upstream pole in the normal direction to the outer peripheral surface of the second rotating body.

8. A developing device as described in claim 7, wherein the half-value width of the magnetic flux density of the second downstream pole in the normal direction to the outer peripheral surface of the second rotating body is 3° or more larger than the half-value width of the magnetic flux density of the second upstream pole in the normal direction to the outer peripheral surface of the second rotating body.

9. A developing device as described in claim 7, wherein the half-value width of the magnetic flux density of the second downstream pole in the normal direction to the outer peripheral surface of the second rotating body is 5° or more larger than the half-value width of the magnetic flux density of the second upstream pole in the normal direction to the outer peripheral surface of the second rotating body.

10. A developing device as described in claim 1, wherein, with respect to the rotation direction of the second rotating body, the position on the outer peripheral surface of the second rotating body where the absolute value of the magnetic flux density of the second developing pole in the normal direction to the outer peripheral surface of the second rotating body is maximum is upstream of the position on the outer peripheral surface of the second rotating body where the second rotating body is closest to the image carrier, and downstream of the position on the outer peripheral surface of the second rotating body where the absolute value of the magnetic flux density of the second upstream pole in the normal direction to the outer peripheral surface of the second rotating body is maximum.

11. A developing device as described in claim 10, wherein the angle in the rotational direction of the second rotating body from the position on the outer peripheral surface of the second rotating body where the absolute value of the magnetic flux density of the second developing pole in the normal direction to the outer peripheral surface of the second rotating body is maximum to the position on the outer peripheral surface of the second rotating body where the second rotating body is closest to the image carrier is 10° or less.

12. A developing device as described in claim 1, wherein the maximum absolute value of the magnetic flux density of the second downstream pole in the normal direction to the outer peripheral surface of the second rotating body is 10 mT or more greater than the maximum absolute value of the magnetic flux density of the second upstream pole in the normal direction to the outer peripheral surface of the second rotating body.

13. A developing device as described in claim 12, wherein the maximum absolute value of the magnetic flux density of the second upstream pole in the normal direction to the outer peripheral surface of the second rotating body is 20 mT or more and 200 mT or less, and the maximum absolute value of the magnetic flux density of the second downstream pole in the normal direction to the outer peripheral surface of the second rotating body is 30 mT or more and 200 mT or less.

14. A developing device as described in claim 1, wherein the maximum absolute value of the magnetic flux density of the second downstream pole in the normal direction to the outer peripheral surface of the second rotating body is 15 mT or more greater than the maximum absolute value of the magnetic flux density of the second upstream pole in the normal direction to the outer peripheral surface of the second rotating body.

15. A developing device as described in claim 1, wherein the half-value width of the magnetic flux density of the first downstream pole in the normal direction to the outer peripheral surface of the first rotating body is smaller than the half-value width of the magnetic flux density of the first upstream pole in the normal direction to the outer peripheral surface of the first rotating body.

16. A developing device as described in claim 15, wherein the half-value width of the magnetic flux density of the first downstream pole in the normal direction to the outer peripheral surface of the first rotating body is smaller by 3° or more than the half-value width of the magnetic flux density of the first upstream pole in the normal direction to the outer peripheral surface of the first rotating body.

17. A developing device as described in claim 15, wherein the half-value width of the magnetic flux density of the first downstream pole in the normal direction to the outer peripheral surface of the first rotating body is smaller by 5° or more than the half-value width of the magnetic flux density of the first upstream pole in the normal direction to the outer peripheral surface of the first rotating body.

18. A developing device as described in claim 1, wherein, with respect to the rotation direction of the first rotating body, the position on the outer peripheral surface of the first rotating body at which the absolute value of the magnetic flux density of the first developing pole in the normal direction to the outer peripheral surface of the first rotating body is maximum is upstream of the position on the outer peripheral surface of the first rotating body at which the first rotating body is closest to the image carrier, and downstream of the position on the outer peripheral surface of the first rotating body at which the absolute value of the magnetic flux density of the first upstream pole in the normal direction to the outer peripheral surface of the first rotating body is maximum.

19. A developing device as described in claim 1, wherein φ1 > φ2 is satisfied when the angle in the rotational direction of the first rotor from the position on the outer peripheral surface of the first rotor where the absolute value of the magnetic flux density of the first upstream pole in the normal direction to the outer peripheral surface of the first rotor is maximum to the position on the outer peripheral surface of the first rotor where the absolute value of the magnetic flux density of the first developing pole in the normal direction to the outer peripheral surface of the first rotor is maximum is φ1 [°], and the angle in the rotational direction of the first rotor from the position on the outer peripheral surface of the first rotor where the absolute value of the magnetic flux density of the first developing pole in the normal direction to the outer peripheral surface of the first rotor is maximum to the position on the outer peripheral surface of the first rotor where the absolute value of the magnetic flux density of the first downstream pole in the normal direction to the outer peripheral surface of the first rotor is maximum is φ2 [°].

20. The developing device according to claim 19, further satisfying 15°≦φ2≦40°.

21. The developing device according to claim 19, further satisfying 20°≦φ2≦35°.

22. The developing device according to claim 19, further satisfying 25°≦φ1≦50°.

23. The developing device according to claim 19, further satisfying 30°≦φ1≦45°.

24. The developing device according to claim 19, further satisfying θ2>φ2.

25. A developing device as described in claim 1, wherein θ2 > φ2 is satisfied when the angle in the rotational direction of the first rotating body from the position on the outer peripheral surface of the first rotating body where the absolute value of the magnetic flux density of the first developing pole in the normal direction to the outer peripheral surface of the first rotating body is maximum to the position on the outer peripheral surface of the first rotating body where the absolute value of the magnetic flux density of the first downstream pole in the normal direction to the outer peripheral surface of the first rotating body is maximum is φ2 [°].

26. A developing device as described in claim 1, wherein the maximum absolute value of the magnetic flux density of the first upstream pole in the normal direction to the outer peripheral surface of the first rotating body is 10 mT or more greater than the maximum absolute value of the magnetic flux density of the first downstream pole in the normal direction to the outer peripheral surface of the first rotating body.

27. A developing device as described in claim 1, wherein the maximum absolute value of the magnetic flux density of the first upstream pole in the normal direction to the outer peripheral surface of the first rotating body is 15 mT or more greater than the maximum absolute value of the magnetic flux density of the first downstream pole in the normal direction to the outer peripheral surface of the first rotating body.

28. A developing device according to claim 1, wherein the surface linear velocity of the second rotating body is faster than the surface linear velocity of the first rotating body.

29. A developing container containing a developer containing toner and a carrier; a first rotating body to which the developer contained in the developing container is supplied, the first rotating body rotating in the same direction as the rotating image carrier at a position on the outer circumferential surface of the first rotating body closest to the image carrier, and carrying and transporting the developer to a first developing position where an electrostatic latent image formed on the image carrier is developed; a first magnet arranged non-rotatably and fixedly inside the first rotating body, the first magnet having: a first developing pole arranged facing the image carrier at the first developing position; a first upstream pole arranged adjacent to the first developing pole upstream of the first developing pole in the rotational direction of the first rotating body and having a polarity opposite to that of the first developing pole; a first downstream pole arranged adjacent to the first developing pole downstream of the first developing pole in the rotational direction of the first rotating body and having a polarity opposite to that of the first developing pole; and a delivery pole arranged downstream of the first downstream pole and upstream of the first upstream pole in the rotational direction of the first rotating body. a second rotating body that is disposed opposite the first rotating body and to which the developer is transferred from the first rotating body by a magnetic field generated by the first magnet, the second rotating body rotating in the same direction as the image carrier at a position on the outer circumferential surface of the second rotating body that is closest to the image carrier, and that carries and transports the developer to a second developing position where the electrostatic latent image is developed; a second magnet that is fixedly disposed within the second rotating body and non-rotatably, the second magnet having: a second developing pole that is disposed facing the image carrier at the second developing position; a second upstream pole that is disposed adjacent to the second developing pole upstream of the second developing pole in the rotational direction of the second rotating body and has a polarity opposite to that of the second developing pole; a second downstream pole that is disposed adjacent to the second developing pole downstream of the second developing pole in the rotational direction of the second rotating body and has a polarity opposite to that of the second developing pole; and a receiving pole that is disposed downstream of the second downstream pole and upstream of the second upstream pole in the rotational direction of the second rotating body and close to the delivering pole and has a polarity opposite to that of the delivering pole,|Bθ1-1| is the maximum value of the absolute value of the magnetic flux density in a tangential direction to the outer circumferential surface of the first rotor in a section in the rotation direction of the first rotor from a position on the outer circumferential surface of the first rotor where the absolute value of the magnetic flux density of the first upstream pole in the normal direction to the outer circumferential surface of the first rotor is maximum to a position on the outer circumferential surface of the first rotor where the absolute value of the magnetic flux density of the first development pole in the normal direction to the outer circumferential surface of the first rotor is maximum, and |Bθ1-2| is the maximum value of the absolute value of the magnetic flux density in a tangential direction to the outer circumferential surface of the first rotor in a section in the rotation direction of the first rotor from a position on the outer circumferential surface of the first rotor where the absolute value of the magnetic flux density of the first development pole in the normal direction to the outer circumferential surface of the first rotor is maximum to a position on the outer circumferential surface of the first rotor where the absolute value of the magnetic flux density of the first downstream pole in the normal direction to the outer circumferential surface of the first rotor is maximum, then |Bθ1-1| / |Bθ1-2|≧1.0 is satisfied; and a developing device that satisfies |Bθ2-2| / |Bθ2-1|≧1.1, wherein |Bθ2-2| is the maximum absolute value of the magnetic flux density in a tangential direction to the outer peripheral surface of the second rotating body in a section in the rotational direction of the second rotating body from a position on the outer peripheral surface of the second rotating body where the absolute value of the magnetic flux density of the second upstream pole in the normal direction to the outer peripheral surface of the second rotating body is maximum to a position on the outer peripheral surface of the second rotating body where the absolute value of the magnetic flux density of the second developing pole in the normal direction to the outer peripheral surface of the second rotating body is maximum; and |Bθ2-2| is the maximum absolute value of the magnetic flux density in a tangential direction to the outer peripheral surface of the second rotating body in a section in the rotational direction of the second rotating body from a position on the outer peripheral surface of the second rotating body where the absolute value of the magnetic flux density of the second developing pole in the normal direction to the outer peripheral surface of the second rotating body is maximum to a position on the outer peripheral surface of the second rotating body where the absolute value of the magnetic flux density of the second downstream pole in the normal direction to the outer peripheral surface of the second rotating body is maximum.

30. The developing device according to claim 29, further satisfying |Bθ1-1| / |Bθ1-2|≧1.

05.

31. The developing device according to claim 29, further satisfying |Bθ2-2| / |Bθ2-1|≧1.

15.

32. The developing device according to claim 29, further satisfying |Bθ2-2| / |Bθ2-1|≧1.

30.

33. A developing device according to claim 29, wherein θ1 [°] is the angle in the rotational direction of the second rotor from the position on the outer circumferential surface of the second rotor where the absolute value of the magnetic flux density of the second upstream pole in the normal direction to the outer circumferential surface of the second rotor is maximum to the position on the outer circumferential surface of the second rotor where the absolute value of the magnetic flux density of the second developing pole in the normal direction to the outer circumferential surface of the second rotor is maximum, and θ2 [°] is the angle in the rotational direction of the second rotor from the position on the outer circumferential surface of the second rotor where the absolute value of the magnetic flux density of the second developing pole in the normal direction to the outer circumferential surface of the second rotor where the absolute value of the magnetic flux density of the second downstream pole in the normal direction to the outer circumferential surface of the second rotor is maximum, and 34. A developing device according to claim 33, wherein φ1 > φ2 is satisfied when the angle in the rotational direction of the first rotor from the position on the outer peripheral surface of the first rotor where the absolute value of the magnetic flux density of the first upstream pole in the normal direction to the outer peripheral surface of the first rotor is maximum to the position on the outer peripheral surface of the first rotor where the absolute value of the magnetic flux density of the first developing pole in the normal direction to the outer peripheral surface of the first rotor is maximum is φ1 [°], and the angle in the rotational direction of the first rotor from the position on the outer peripheral surface of the first rotor where the absolute value of the magnetic flux density of the first developing pole in the normal direction to the outer peripheral surface of the first rotor is maximum to the position on the outer peripheral surface of the first rotor where the absolute value of the magnetic flux density of the first downstream pole in the normal direction to the outer peripheral surface of the first rotor is maximum is φ2 [°].

35. A developing device according to claim 29, wherein the maximum absolute value of the magnetic flux density of the first upstream pole in the normal direction to the outer peripheral surface of the first rotating body is greater than the maximum absolute value of the magnetic flux density of the first downstream pole in the normal direction to the outer peripheral surface of the first rotating body.

36. A developing device according to claim 29, wherein the maximum absolute value of the magnetic flux density of the second downstream pole in the normal direction to the outer peripheral surface of the second rotating body is greater than the maximum absolute value of the magnetic flux density of the second upstream pole in the normal direction to the outer peripheral surface of the second rotating body.

37. A developing device according to claim 29, wherein the half-value width of the magnetic flux density of the second downstream pole in the normal direction to the outer peripheral surface of the second rotating body is greater than the half-value width of the magnetic flux density of the second upstream pole in the normal direction to the outer peripheral surface of the second rotating body.

38. A developing device according to claim 29, wherein the half-value width of the magnetic flux density of the first downstream pole in the normal direction to the outer peripheral surface of the first rotating body is smaller than the half-value width of the magnetic flux density of the first upstream pole in the normal direction to the outer peripheral surface of the first rotating body.

39. A developing device as described in claim 29, wherein, with respect to the rotation direction of the second rotating body, the position on the outer peripheral surface of the second rotating body at which the absolute value of the magnetic flux density of the second developing pole in the normal direction to the outer peripheral surface of the second rotating body is maximum is upstream of the position on the outer peripheral surface of the second rotating body at which the second rotating body is closest to the image carrier, and downstream of the position on the outer peripheral surface of the second rotating body at which the absolute value of the magnetic flux density of the second upstream pole in the normal direction to the outer peripheral surface of the second rotating body is maximum.

40. A developing device as described in claim 29, wherein, with respect to the rotation direction of the first rotating body, the position on the outer peripheral surface of the first rotating body at which the absolute value of the magnetic flux density of the first developing pole in the normal direction to the outer peripheral surface of the first rotating body is maximum is upstream of the position on the outer peripheral surface of the first rotating body at which the first rotating body is closest to the image carrier, and downstream of the position on the outer peripheral surface of the first rotating body at which the absolute value of the magnetic flux density of the first upstream pole in the normal direction to the outer peripheral surface of the first rotating body is maximum.

41. A developing device as described in claim 29, wherein φ1 > φ2 is satisfied when the angle in the rotational direction of the first rotor from the position on the outer peripheral surface of the first rotor where the absolute value of the magnetic flux density of the first upstream pole in the normal direction to the outer peripheral surface of the first rotor is maximum to the position on the outer peripheral surface of the first rotor where the absolute value of the magnetic flux density of the first developing pole in the normal direction to the outer peripheral surface of the first rotor is maximum is φ1 [°], and the angle in the rotational direction of the first rotor from the position on the outer peripheral surface of the first rotor where the absolute value of the magnetic flux density of the first developing pole in the normal direction to the outer peripheral surface of the first rotor is maximum to the position on the outer peripheral surface of the first rotor where the absolute value of the magnetic flux density of the first downstream pole in the normal direction to the outer peripheral surface of the first rotor is maximum is φ2 [°].

42. A developing device as described in claim 29, wherein θ2 > φ2 is satisfied when the angle in the rotational direction of the second rotating body from the position on the outer peripheral surface of the second rotating body where the absolute value of the magnetic flux density of the second developing pole in the normal direction to the outer peripheral surface of the second rotating body is maximum to the position on the outer peripheral surface of the second rotating body where the absolute value of the magnetic flux density of the second downstream pole in the normal direction to the outer peripheral surface of the second rotating body is maximum is θ2 [°], and the angle in the rotational direction of the first rotating body from the position on the outer peripheral surface of the first rotating body where the absolute value of the magnetic flux density of the first developing pole in the normal direction to the outer peripheral surface of the first rotating body is maximum to the position on the outer peripheral surface of the first rotating body where the absolute value of the magnetic flux density of the first downstream pole in the normal direction to the outer peripheral surface of the first rotating body is maximum is φ2 [°].

43. A developing device according to claim 29, wherein the surface linear velocity of said second rotating body is faster than the surface linear velocity of said first rotating body.

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