Developing device and image forming apparatus including the same

Abstract

To provide a developing device that can ensure regulation stability while reducing stress on developer and improve a developer draw-up performance, and an image forming apparatus including the same.SOLUTION: A developing device comprises: a developer container that has a first conveying chamber and a second conveying chamber; a first stirring and conveying member that conveys developer in the first conveying chamber; a second stirring and conveying member that conveys developer in the second conveying chamber; a developer carrier that has a developing sleeve and a magnet; and a regulation member. The magnet has a regulation pole and a draw-up pole that are formed of one magnetic pole. In the regulation pole, the maximum value of vertical magnetic force is 65 [mT] or less, and an area with a vertical magnetic force gradient of 0.3 [mT / °] or less is present by 10° or more. A peak position of the vertical magnetic force gradient of the regulation pole is arranged at a position opposite to the second stirring and conveying member. When A [mT / °] is a vertical magnetic force gradient at the peak position of the vertical magnetic force gradient, and K [mT] is the vertical magnetic force, A≥2.8 and A×K≥62.5 are satisfied.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a developing device mounted on an image forming apparatus such as a copying machine, a printer, a facsimile machine, or a multifunction machine including an image carrier, and more particularly to a two-component developing system developing device using a two-component developer containing toner and carrier, and an image forming apparatus including the same.

Background Art

[0002] In an image forming apparatus, an electrostatic latent image formed on an image carrier made of a photoreceptor or the like is developed by a developing device and visualized as a toner image. As one such developing device, a two-component developing system using a two-component developer containing a magnetic carrier and toner is adopted.

[0003] In the two-component developing system, by using a single magnetic pole as a pickup electrode (catch electrode) that picks up the developer supplied from the stirring member onto a developing roller (developer carrier) and a regulating electrode that regulates the amount of developer transported on the developing roller, a method is known for reducing the mechanical stress on the developer generated in the opposing portion (regulation portion) between the regulating member and the developing roller and preventing toner deterioration. At this time, by reducing the magnetic force of the regulating electrode, it is possible to further reduce the stress, but at low magnetic force, the stability of the magnetic regulation force (regulation stability) in the regulation portion decreases.

[0004] Therefore, a method has been proposed to ensure regulation stability by making the regulating electrode have a low magnetic force and widening the pole width. For example, Patent Document 1 discloses a developing device in which a magnetic pole member located in a portion facing a regulating member has a flat vertical magnetic force distribution portion with little change in vertical magnetic force, and the regulating member is provided in a portion where the fluctuation width of the vertical magnetic force in this magnetic pole is 10 Gauss or less.

[0005] On the other hand, when using a regulating pole with low magnetic force, there is also a problem that the supply of the developer from the stirring member decreases. By widening the pole width of the regulating pole as in the configuration of Patent Document 1, the supply performance is slightly improved, but still, sufficient supply of the developer cannot be achieved, and unevenness (screw pitch unevenness) of the stirring member occurs in the image.

[0006] Therefore, Patent Document 2 discloses an image forming apparatus having a retention regulating member on the immediate upstream side of the developer regulating member, where the developer carrier and the retention regulating member are at the same potential, and the distance between the non-magnetic sleeve surface and the retention regulating member is 1.8 mm or less.

[0007] Patent Document 3 discloses an image forming apparatus in which a regulating member is disposed between two adjacent magnetic poles, from the point where the vertical magnetic field is 0 [mT] to the peak of the vertical magnetic field, and a horizontal magnetic field peak exists on the upstream side of the peak of the vertical magnetic field. In the configuration of Patent Document 3, since the peak of the horizontal magnetic field is on the upstream side of the regulating portion, it is easier to return the excess developer at the regulating portion to the stirring portion.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0009] The method of Patent Document 2 includes attaching a retention control member immediately upstream of the regulation member to eliminate the unevenness of the stirring member. However, this method has a problem that the mechanical stress at the regulation part increases and the deterioration of the developer is accelerated. Further, in the configuration of Patent Document 3, since the pumping electrode and the regulation electrode are formed of different magnetic poles, it cannot be applied to a configuration in which the pumping electrode and the regulation electrode are the same magnetic pole.

[0010] In view of the above problems, an object of the present invention is to provide an image forming apparatus and an image forming apparatus including the same, which can ensure regulation stability while reducing stress on the developer and improve the pumping performance of the developer when the regulation electrode and the pumping electrode are formed of the same magnetic pole in a two-component development system.

Means for Solving the Problems

[0011] In order to achieve the above object, a first configuration of the present invention is a developing device including a developing container, a first stirring and conveying member, a second stirring and conveying member, a developer carrier, and a regulating member, for developing an electrostatic latent image formed on the surface of an image carrier into a toner image. The developing container has a plurality of conveying chambers including a first conveying chamber and a second conveying chamber arranged in parallel with each other, and accommodates a two-component developer including a magnetic carrier and toner. The first stirring and conveying member conveys the developer in the first conveying chamber in the first direction while stirring it. The second stirring and conveying member conveys the developer in the second conveying chamber in the second direction, which is opposite to the first stirring and conveying member, while stirring it. The developer carrier is rotatably supported by the developing container and carries the developer in the second conveying chamber on its outer peripheral surface. The regulating member is disposed to face the developer carrier with a predetermined interval therebetween. The developer carrier has a developing sleeve and a magnet. The developing sleeve is rotatable, and a magnetic brush is formed on the surface by carrying the developer. The magnet is fixedly arranged non-rotatably in the developing sleeve, and includes a regulating pole disposed at a position facing the regulating member and a plurality of magnetic poles including a main pole disposed on the downstream side of the regulating pole in the rotation direction of the developing sleeve, which are arranged at a predetermined interval in the circumferential direction. The magnet is constituted by one magnetic pole including the regulating pole and a pumping pole for pumping up the developer supplied from the second stirring and conveying member to the developer carrier. The regulating pole has a maximum value of the vertical magnetic force of 65 [mT] or less, and there is a region where the vertical magnetic force gradient is 0.3 [mT / °] or less and exists for 10° or more. When the peak position of the vertical magnetic force gradient of the regulating pole is arranged at a position facing the second stirring and conveying member, and the vertical magnetic force gradient at the peak position of the vertical magnetic force gradient is A [mT / °] and the vertical magnetic force is K [mT], A≧2.8 and A×K≧62.5 are satisfied.

Effect of the Invention

[0012] According to the first configuration of the present invention, when the regulating pole and the pumping pole are constituted by the same magnetic pole, by setting the vertical magnetic force gradient, the peak position, and the magnitude of the vertical magnetic force of the regulating pole to satisfy predetermined conditions, it is possible to ensure the regulating stability in the regulating portion and to ensure the pumping performance of the developer. As a result, the developability in the developing region can also be maintained.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing the internal structure of the image forming apparatus 100 including the developing devices 3a to 3d of the present invention. Inside the image forming apparatus 100 (here, a color printer), four image forming units Pa, Pb, Pc, and Pd are arranged in order from the upstream side in the conveyance direction (left side in FIG. 1). These image forming units Pa to Pd are provided corresponding to images of four different colors (yellow, cyan, magenta, and black), and yellow, cyan, magenta, and black images are sequentially formed by the respective steps of charging, exposure, development, and transfer.

[0015] In these image forming units Pa to Pd, photosensitive drums (image carriers) 1a, 1b, 1c, and 1d that carry visible images (toner images) of respective colors are disposed. Further, an intermediate transfer belt (intermediate transfer member) 8 that rotates in the counterclockwise direction in FIG. 1 by a belt drive motor (not shown) is provided adjacent to each of the image forming units Pa to Pd. The toner images formed on these photosensitive drums 1a to 1d are sequentially primary transferred and superimposed onto the intermediate transfer belt 8 that moves while contacting the respective photosensitive drums 1a to 1d. Thereafter, the toner image primary transferred onto the intermediate transfer belt 8 is secondary transferred onto transfer paper P as an example of a recording medium by a secondary transfer roller 9. Further, after the toner image is fixed in the fixing unit 13, the transfer paper P onto which the toner image has been secondary transferred is discharged from the main body of the image forming apparatus 100. While rotating the photosensitive drums 1a to 1d in the clockwise direction in FIG. 1, an image forming process for each of the photosensitive drums 1a to 1d is executed.

[0016] The transfer paper P onto which the toner image is secondary transferred is housed in a paper cassette 16 disposed at the lower part of the main body of the image forming apparatus 100, and is conveyed to the nip portion between the secondary transfer roller 9 and the drive roller 11 of the intermediate transfer belt 8 via a paper feed roller 12a and a registration roller pair 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a seamless belt having no seam is mainly used. Further, a blade-shaped belt cleaner 19 for removing toner and the like remaining on the surface of the intermediate transfer belt 8 is disposed on the downstream side of the secondary transfer roller 9.

[0017] Next, the image forming units Pa to Pd will be described. Around and below the rotatably disposed photosensitive drums 1a to 1d, charging devices 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d, an exposure device 5 for exposing image information onto the respective photosensitive drums 1a to 1d, developing devices 3a, 3b, 3c, and 3d for forming toner images on the photosensitive drums 1a to 1d, and cleaning devices 7a, 7b, 7c, and 7d for removing developer (toner) and the like remaining on the photosensitive drums 1a to 1d are provided.

[0018] When image data is input from a host device such as a personal computer, first, the charging devices 2a to 2d uniformly charge the surfaces of the photosensitive drums 1a to 1d. Next, the exposure device 5 irradiates light according to the image data, and an electrostatic latent image corresponding to the image data is formed on each of the photosensitive drums 1a to 1d. The developing devices 3a to 3d are each filled with a predetermined amount of a two-component developer containing yellow, cyan, magenta, and black toner. When the ratio of the toner in the two-component developer filled in each of the developing devices 3a to 3d falls below a specified value due to the formation of the toner image described later, toner is replenished from the toner containers 4a to 4d to each of the developing devices 3a to 3d. The toner in this developer is supplied onto the photosensitive drums 1a to 1d by the developing devices 3a to 3d and adheres electrostatically. Thereby, a toner image corresponding to the electrostatic latent image formed by the exposure from the exposure device 5 is formed.

[0019] Then, an electric field is applied at a predetermined transfer voltage between the primary transfer rollers 6a to 6d and the photosensitive drums 1a to 1d by the primary transfer rollers 6a to 6d, and the yellow, magenta, cyan, and black toner images on the photosensitive drums 1a to 1d are primarily transferred onto the intermediate transfer belt 8. These images are formed with a predetermined positional relationship that has been determined in advance. Thereafter, in preparation for the formation of a new electrostatic latent image that is subsequently performed, toner and the like remaining on the surfaces of the photosensitive drums 1a to 1d after the primary transfer are removed by the cleaning devices 7a to 7d.

[0020] The intermediate transfer belt 8 is stretched between the upstream driven roller 10 and the downstream drive roller 11. When the intermediate transfer belt 8 starts to rotate counterclockwise as the drive roller 11 rotates by a belt drive motor (not shown), the transfer paper P is conveyed from the registration roller pair 12b at a predetermined timing to the nip portion (secondary transfer nip portion) between the drive roller 11 and the secondary transfer roller 9 provided adjacent thereto, and the toner image on the intermediate transfer belt 8 is secondarily transferred onto the transfer paper P. The transfer paper P onto which the toner image has been secondarily transferred is conveyed to the fixing unit 13.

[0021] The transfer paper P conveyed to the fixing unit 13 is heated and pressed by the fixing roller pair 13a, and the toner image is fixed on the surface of the transfer paper P, forming a predetermined full-color image. The transfer paper P on which the full-color image is formed has its conveyance direction distributed by the branching unit 14 branched in a plurality of directions, and is discharged to the discharge tray 17 by the discharge roller pair 15 as it is (or after being sent to the duplex conveyance path 18 and having images formed on both sides).

[0022] FIG. 2 is a side cross-sectional view of the developing device 3a mounted on the image forming apparatus 100. In the following description, the developing device 3a disposed in the image forming unit Pa of FIG. 1 is exemplified, but the configurations of the developing devices 3b to 3d disposed in the image forming units Pb to Pd are basically the same, and thus the description thereof is omitted.

[0023] As shown in FIG. 2, the developing device 3a includes a developing container 20 in which a two-component developer (hereinafter simply referred to as a developer) containing a magnetic carrier and toner is stored. The developing container 20 is partitioned into a stirring conveyance chamber 21 and a supply conveyance chamber 22 by a partition wall 20a. In the stirring conveyance chamber 21 and the supply conveyance chamber 22, a stirring conveyance screw 25 and a supply conveyance screw 26 for mixing, stirring, and charging the toner supplied from the toner container 4a (see FIG. 1) with the magnetic carrier are rotatably disposed respectively. In the present embodiment, a positively charged toner and a two-component developer composed of a ferrite-resin coated carrier are used. The detailed configuration of the carrier will be described later.

[0024] Then, the developer is stirred and conveyed in the axial direction (the direction perpendicular to the paper surface of FIG. 2) by the stirring conveyance screw 25 and the supply conveyance screw 26, and circulates between the stirring conveyance chamber 21 and the supply conveyance chamber 22 through a developer passage (not shown) formed at both ends of the partition wall 20a. That is, a circulation path of the developer is formed in the developing container 20 by the stirring conveyance chamber 21, the supply conveyance chamber 22, and the developer passage.

[0025] The developing container 20 extends obliquely upward to the right in FIG. 2, and a developing roller 30 is disposed obliquely upward to the right of the supply and conveyance screw 26 within the developing container 20. A part of the outer peripheral surface of the developing roller 30 is exposed from the opening of the developing container 20 and faces the photosensitive drum 1a with a predetermined interval (developing gap) therebetween, thereby forming a developing region 40. The developing roller 30 rotates in the counterclockwise direction in FIG. 2 (trailing rotation at the position facing the photosensitive drum 1a).

[0026] The agitation and conveyance screw 25 includes a rotation shaft 25a and conveyance blades 25b formed in a spiral shape at a constant pitch in the axial direction of the rotation shaft 25a. The agitation and conveyance screw 25 conveys the developer within the agitation and conveyance chamber 21 in one direction (first direction) while agitating the developer by the rotation of the rotation shaft 25a. The supply and conveyance screw 26 includes a rotation shaft 26a and conveyance blades 26b formed in a spiral shape at a constant pitch in the axial direction of the rotation shaft 26a. The supply and conveyance screw 26 conveys the developer within the supply and conveyance chamber 22 in the direction opposite to that of the agitation and conveyance screw 25 (second direction) while agitating the developer by the rotation of the rotation shaft 26a.

[0027] The developing roller 30 is composed of a cylindrical developing sleeve 31 that rotates counterclockwise in Fig. 2, and a magnet 32 having a plurality of magnetic poles fixedly and non-rotatably installed inside the developing sleeve 31. In this embodiment, a developing sleeve 31 with a knurled surface is used. However, those with a large number of concave shapes (dimples) formed on the surface, those with a blasted surface, those with both knurling and concave shape formation in addition to blasting, those with plating treatment for the purpose of improving durability, those with anodizing treatment, and those treated with the so-called secondary electrolytic coloring method in which metal salts such as Ni, Sn, and Mo are treated on the porous part of the anodized layer can also be used. In particular, those treated with anodizing or the secondary electrolytic coloring method after anodizing not only improve durability but also have the effect of suppressing the occurrence of developing leakage. This is because when the surface of the developing sleeve 31 is anodized, the leakage current generated by the magnetic brush is less likely to spread circumferentially on the surface of the developing roller 30 and will not develop into a large leakage that entraps adjacent magnetic brushes.

[0028] The magnet 32 has a five-pole configuration including a main pole N1, a regulating pole (lifting pole) N2, conveying poles S1 and S2, and a peeling pole S3. When a driving force is input to the developing device 3a, the developing sleeve 31 rotates, but the magnet 32 does not rotate. A developing voltage composed of a DC voltage Vdc and an AC voltage Vac is applied to the developing roller 30 by a developing voltage power source (not shown).

[0029] Also, a regulating blade 27 is attached to the developing container 20 along the longitudinal direction of the developing roller 30 (the direction perpendicular to the plane of Fig. 2). A slight gap is provided between the tip of the regulating blade 27 and the surface of the developing roller 30 to form a regulating portion 41. In this embodiment, a magnetic blade made of stainless steel (SUS430) is used as the regulating blade 27.

[0030] A magnetic field (vertical magnetic force) in the attracting direction is generated between the regulating pole N2 of the magnet 32 and the regulating blade 27, forming a magnetic brush in which the developer is continuous between the regulating blade 27 and the developing roller 30. As the magnetic brush passes through the regulating blade 27 (regulating portion 41), layer regulation is performed to a desired height. Then, when the developing sleeve 31 rotates counterclockwise, a magnetic field (horizontal magnetic force) in the direction along the outer peripheral surface of the developing sleeve 31 is applied by the conveying pole S1, and the magnetic brush moves together with the developing sleeve 31. When the magnetic brush moves to the developing region 40, a magnetic field (vertical magnetic force) in the attracting direction is applied between the magnetic brush and the photosensitive drum 1a by the main pole N1, so that the magnetic brush contacts the surface of the photosensitive drum 1a to develop the electrostatic latent image.

[0031] Furthermore, when the developing sleeve 31 rotates counterclockwise, a magnetic field in the direction along the outer peripheral surface of the developing sleeve 31 is applied by the conveying pole S2, and the developer that has not been used for forming the toner image is recovered onto the developing sleeve 31 together with the magnetic brush. Further, at the peeling pole S3 having the same polarity as the conveying pole S2, the magnetic brush detaches from the developing roller 30 and drops into the supply and conveyance chamber 22. Then, after being agitated and conveyed by the supply and conveyance screw 26, a magnetic brush is formed again on the developing sleeve 31 by the magnetic field of the regulating pole N2.

[0032] Next, the magnetic force distribution of the magnet 32 in the circumferential direction of the developing roller 30, which is a characteristic part of the present invention, will be described. FIG. 3 is a diagram schematically showing the vertical magnetic force and the vertical magnetic force gradient of the magnet 32 in the circumferential direction of the developing roller 30. FIG. 4 is a graph showing the changes in the vertical magnetic force distribution and the vertical magnetic force gradient in the circumferential direction of the developing roller 30. In FIGS. 3 and 4, the vertical magnetic force is indicated by a solid line and the vertical magnetic force gradient is indicated by a broken line.

[0033] In a configuration where the regulating electrode N2 also serves as a pumping electrode (catch electrode) that pumps the developer onto the developing roller 30 as in this embodiment, mechanical stress on the developer generated in the regulating unit 41 can be reduced, and toner degradation can be suppressed. At this time, by reducing the vertical magnetic force of the regulating electrode N2 and having the vertical magnetic force have a flat portion, it is possible to further reduce the stress on the developer. However, if the regulating electrode N2 has a low magnetic force, the regulating stability in the regulating unit 41 (stability of the magnetic regulating force at the regulating blade 27) will decrease.

[0034] The magnetic attraction force, which is the force that attracts the developer to the developing roller 30, increases as the vertical magnetic force gradient [mT / °] of the regulating electrode N2 increases. Also, the closer the peak of the vertical magnetic force gradient is to the supply and conveyance screw 26, the easier it is to pump the developer conveyed by the supply and conveyance screw 26 onto the developing roller 30 by the magnetic attraction force.

[0035] Therefore, in this embodiment, in a configuration where the regulating electrode and the pumping electrode are one magnetic pole, a regulating electrode N2 is used in which the vertical magnetic force is 65 [mT] or less and there is a region where the vertical magnetic force gradient is 0.3 [mT / °] or less and exists for 10° or more. Then, the peak position (point Q in FIG. 3) of the vertical magnetic force gradient of the regulating electrode N2 is arranged at a position facing the supply and conveyance screw 26. Specifically, the direction of the peak of the vertical magnetic force gradient from the central axis O of the magnet 32 (the dashed arrow in FIG. 3) is made to overlap the supply and conveyance screw 26 when viewed from the axial direction.

[0036] With this configuration, the developer supply performance (pumping performance of the regulating electrode N2) from the supply and conveyance screw 26 can be ensured. In particular, as shown in FIG. 3, by arranging the peak position of the vertical magnetic force gradient of the regulating electrode N2 at a position facing the rotation axis 26a of the supply and conveyance screw 26, it becomes easier to further ensure the developer supply performance.

[0037] In addition to the configuration in which the peak position of the vertical magnetic force gradient (point Q in FIG. 3) faces the rotation axis 26a of the supply and conveyance screw 26, as shown in FIG. 3, when the peak position of the vertical magnetic force (point P in FIG. 3) faces the supply and conveyance screw 26, the suction of the developer becomes more reliable.

[0038] Furthermore, the greater the vertical magnetic force gradient and the vertical magnetic force at the peak position of the vertical magnetic force gradient, the better the suction performance at the regulating pole N2. In the present embodiment, when the vertical magnetic force gradient at the peak position of the vertical magnetic force gradient of the regulating pole N2 (point Q in FIG. 3) is A [mT / °] and the vertical magnetic force is K [mT], it is set to satisfy A≧2.8 and A×K≧62.5. Thereby, as shown in the examples described later, uneven supply of the developer (pitch unevenness of the supply and conveyance screw 26) can be suppressed.

[0039] As described above, in the configuration in which the regulating pole N2 also serves as a suction pole, by setting the peak position and magnitude of the vertical magnetic force gradient and the vertical magnetic force of the regulating pole N2 to satisfy predetermined conditions, the regulation stability in the regulating unit 41 can be ensured, and the suction performance of the developer can be ensured. As a result, the developability in the development region 40 can also be maintained.

[0040] Also, in the developing apparatuses 3a to 3d of the present embodiment, as shown in FIG. 3, a perpendicular line L passing through the central axis O of the magnet 32 of the developing roller 30 is located on the left side of the outer edge (the right end in FIG. 3) of the conveying blade 26b of the supply and conveyance screw 26. That is, the perpendicular line L passing through the central axis O of the magnet 32 is at a position overlapping the supply and conveyance screw 26 when viewed from the axial direction.

[0041] Such a positional relationship between the developing roller 30 and the supply and conveyance screw 26 can reduce the dimensions in the width direction (the left-right direction in FIG. 2) of the developing apparatuses 3a to 3d, but it is an arrangement in which it is difficult for the regulating pole N2 to suck up the developer. Therefore, it is particularly effective to set the peak position and magnitude of the vertical magnetic force gradient and the vertical magnetic force of the regulating pole N2 as in the present embodiment.

[0042] Next, a method for measuring the vertical magnetic force gradient of the magnet 32 of the developing roller 30 will be described. In the present embodiment, the developing roller 30 is mounted on a jig for angle adjustment, and measurement is performed using a magnetic force measuring device (GAUSS METER Model GX-100, manufactured by Nihon Denshi Sokki Co., Ltd.) while rotating it by a fixed angle. The vertical magnetic force gradient can be obtained by dividing the difference in the vertical magnetic force measured at different angles by the measurement angle difference when the measurement accuracy is very high. However, when the measurement accuracy is low, the vertical magnetic force gradient cannot be accurately obtained. Therefore, in the present invention, the measurement angle is changed by 0.02° each time to measure the vertical magnetic force, and (vertical magnetic force difference at 0.08° difference / 0.08°) is defined as the gradient 1 at the midpoint within that 0.08°. Then, the average gradient per 2° of the gradient 1 is defined as the vertical magnetic force gradient. A measurement example of the vertical magnetic force gradient is shown in Table 1.

[0043]

Table 1

[0044] In Table 1, for example, the gradient 1 (6.25 [mT / °]) at an angle of 10.00° is obtained by dividing the difference G1 - G2 between the vertical magnetic force G1 at 9.96° and the vertical magnetic force G2 at 10.04° by 0.08°. Also, the average gradient (6.25 [mT / °]) at 10.00° is the average value of the gradient 1 (2° / 0.02° = 100 pieces) per 2° from 9.00° to 11.00°.

[0045] Next, the carrier used in the developing devices 3a to 3d of the present embodiment will be described. As the carrier, a carrier core made of magnetic particles with a coating layer such as a silicone resin formed on the surface is used. The silicone-based resin can be coated in a thin film, and the uniformity of the coating layer is high. Also, when the thickness of the coating layer is thinner, the capacitance of the coating layer is higher, and the effect of the ferroelectric added to the coating layer is more easily exerted.

[0046] The shape of the carrier can be used from amorphous to spherical. Furthermore, the average particle diameter of the carrier can be 20 μm or more and 65 μm or less. By setting the number average particle diameter of the carrier to 65 μm or less, the specific surface area of the carrier increases, and the amount of toner that the carrier can carry increases. As a result, the toner concentration in the magnetic brush can be maintained at a high level, and the toner supply to the developing roller 30 is sufficiently performed, so that the thickness of the toner layer can be sufficiently ensured. As a result, the amount of toner flying from the toner layer to the electrostatic latent image on the photoreceptor can be sufficiently ensured, the decrease in image density can be suppressed, and further, the density unevenness of the image can be suppressed. In addition, since the toner supply to the developing roller 30 is sufficiently performed, it becomes difficult to form a toner missing portion in the toner layer of the developing roller 30, and the occurrence of history development can be suppressed.

[0047] If the average particle diameter of the carrier is less than 20 μm, carrier development in which the carrier adheres to the photoreceptor drums 1a to 1d occurs. The adhered carrier migrates to the intermediate transfer belt 8, causing transfer omission or moving to the belt cleaning device 19 and causing poor cleaning. On the other hand, if the average particle diameter of the carrier is greater than 65 μm, when moving the toner in the two-component developer from the developing roller 30 to the photoreceptor drums 1a to 1d, the magnetic brush of the two-component developer becomes rough and the image quality deteriorates.

[0048] Examples of the carrier core include ferromagnetic metals such as iron, nickel, and cobalt, alloys thereof, or alloys containing rare earths, soft ferrites such as hematite, magnetite, manganese-zinc ferrite, nickel-zinc ferrite, manganese-magnesium ferrite, and lithium ferrite, iron-based oxides such as copper-zinc ferrite, and mixtures thereof. The carrier core is manufactured by a known method such as a sintering method or an atomizing method. Among the above, ferrite carriers are preferably used from the viewpoints of high image quality and long life because they have good fluidity and are chemically stable.

[0049] Barium titanate particles are added to the coating layer as a ferroelectric. Examples of the production method of barium titanate include the hydrothermal polymerization method and the oxalate method. However, barium titanate has different physical properties depending on the production method. Among them, barium titanate produced by the hydrothermal polymerization method has a small true specific gravity due to having voids inside, and the particle size distribution also becomes sharp. As a result, its dispersibility in the coating resin is better than that of those produced by other methods, and uniform dispersion is possible. Therefore, it is suitable for use in the present invention because the charging performance of the carrier is also made uniform.

[0050] The volume average particle diameter of barium titanate is preferably 100 nm or more and 500 nm or less. When the particle diameter of barium titanate becomes smaller than 100 nm, the relative permittivity of barium titanate decreases rapidly, so the effect regarding the relative permittivity becomes small. On the other hand, when the particle diameter of barium titanate becomes 500 nm or more, it becomes difficult to achieve uniform dispersion in the coating layer.

[0051] When 5 parts by mass or more of barium titanate is added based on the coating weight, the effect of stabilizing the charge amount starts to appear, and when 25 parts by mass or more is added, the effect of stabilizing the charge amount becomes more prominent. However, if the addition amount of barium titanate is too large, it cannot be completely contained in the coating layer and will separate from the coating layer. When the separated barium titanate moves to the photoreceptor drums 1a to 1d and gets caught in the edge part of the cleaning blades 32 of the cleaning devices 7a to 7d, it will cause cleaning failure. In particular, in the method of mixing the carrier with the toner in the toner containers 4a to 4d and supplying it to the developing devices 3a to 3, the load on the cleaning blade 32 increases because the barium titanate separated during use is supplied to the developing devices 3a to 3d. Therefore, the addition amount of barium titanate is preferably 5 parts by mass or more and 45 parts by mass or less.

[0052] Carbon black is added to the coating layer as a conductor. If the addition amount of carbon black is too large, the carbon black released from the coating layer adheres to the toner, causing color turbidity of the toner other than black. On the other hand, if the addition amount of carbon black is too small, the transfer of charges from the carrier to the toner is difficult to occur, and the increase in the toner charge amount cannot be smoothly carried out. In the carrier of the present invention, since the carrier resistance is reduced by adding barium titanate (a ferroelectric) to the coating layer, it is possible to reduce the addition amount of carbon black by the amount corresponding to the reduction in the carrier resistance.

[0053] By adding a ferroelectric (barium titanate) to the coating layer, the charge holding ability of the carrier is increased, and it becomes possible to impart sufficient charges to the toner. In addition, by adding a conductor (carbon black) to the coating layer, the transfer of charges from the carrier to the toner can be smoothly carried out. Due to these two synergistic effects, even when the toner concentration increases and the number of toner particles to be charged increases, it is possible to impart charges up to the saturation charge amount level of the toner particles.

[0054] In addition, by adding barium titanate, which has high hardness, as a ferroelectric to the coating layer of the carrier, the abrasion of the coating layer is reduced, and the long life of the carrier can be achieved. In addition, due to the addition of barium titanate, the carrier resistance is reduced compared to the case where only carbon black is added, so the addition amount of carbon black can be reduced. As a result, the color turbidity caused by the adhesion of carbon black to the toner can be suppressed. Furthermore, since the charge imparting performance of the carrier is improved, even when the toner concentration in the developer is increased, the change in the toner charge amount becomes small. As a result, the toner charge amount is stabilized, and the bulk fluctuation of the developer in the developing container 20 also becomes small. Therefore, the pumping performance at the regulation pole N2 is stabilized, and the developer can be stably supplied to the developing roller 30.

[0055] The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, in the above embodiment, as the magnet 32 of the developing roller 30, a configuration in which the transfer pole S1 is disposed between the regulating pole N2 and the main pole N1 is used, but a magnet in which the main pole is disposed on the downstream side of the regulating pole can also be used. In that case, the regulating pole and the main pole have different polarities.

[0056] Also, in the above embodiment, the color printer as shown in FIG. 1 has been described as an example of the image forming apparatus 100. However, the present invention is not limited to color printers, and can be applied to various image forming apparatuses including two-component development type developing apparatuses such as monochrome and color copiers, monochrome printers, digital multifunction machines, and the like. Hereinafter, the effects of the present invention will be described more specifically with reference to examples.

Examples

[0057] [Production of ferroelectric particle-containing carriers] 200 parts by mass of a silicone resin (manufactured by Shin-Etsu Chemical Co., Ltd., KR-255, non-volatile content = 50%), 20 parts by mass of barium titanate (manufactured by Sakai Chemical Industry Co., Ltd., volume average particle diameter 304 nm), 7 parts by mass of carbon black (manufactured by Lion Corporation, Ketjenblack EC), and 800 parts by mass of toluene were dispersed using a homomixer to obtain a coating solution. The obtained coating solution was sprayed onto 5 kg of carrier cores (Mn ferrite carriers, volume average particle diameter 34.7 μm, saturation magnetization 80 emu / g, coercive force 8 Oe, manufactured by DOWA IP Creation Co., Ltd.) under heating at 70 to 80°C using a fluidized bed coating apparatus to coat the carrier cores with the coating solution. Thereafter, firing was performed at 200 to 250°C for 1 hour using an electric furnace, and after cooling, crushing and classification were performed using a sieve to obtain carriers containing ferroelectric particles in the coating layer.

Examples

[0058] [Evaluation of supply stability of developer when changing peak position of vertical magnetic force gradient] In the image forming apparatus 100 as shown in FIG. 1, the supply stability of the developer was evaluated when the peak position of the vertical magnetic force gradient of the regulating pole N2 of the developing roller 30 was changed. The test was conducted in the black image forming unit Pd including the photosensitive drum 1d and the developing device 3d.

[0059] The test method was as follows: The vertical magnetic force gradient A, the vertical magnetic force K, the maximum value of the vertical magnetic force of the regulating pole N2, the angle at which the vertical magnetic force gradient becomes -0.3 to 0.3 [mT / °], and the opposing state between the peak position of the vertical magnetic force gradient and the vertical magnetic force and the supply conveyance screw 26 were changed. The developing device 3d (Inventions 1 to 3, Comparative Examples 1 and 2) as shown in FIG. 2 was mounted on the testing machine as shown in FIG. 1.

[0060] Using this testing machine, when 10 consecutive black solid (solid) images were printed, the image density unevenness (pitch unevenness) generated at the pitch interval of the conveying blades 26b of the supply conveyance screw 26 was visually observed.

[0061] The image forming conditions were as follows: The printing speed (process speed) was 55 sheets / min. A developing sleeve 31 with an outer diameter of 16 mm having 120 rows of recesses formed on its outer peripheral surface (knurling) was used for the developing roller 30. The regulating blade 27 was a magnetic blade made of stainless steel (SUS430) with a thickness of 1.5 mm, and the distance (regulating gap) between the regulating blade 27 and the developing roller 30 was 0.45 ± 0.03 mm. A developing voltage obtained by superimposing an AC voltage with a peak-to-peak value (Vpp) of 1200 V, a frequency of 6 kHz, and a duty = 50% on a DC voltage of 250 V was applied to the developing roller 30.

[0062] The photosensitive drum 1d used an amorphous silicon (a-Si) photosensitive body with a relative permittivity of 11. The peripheral speed ratio of the developing roller 30 to the photosensitive drum 1d was 1.8 (trailing rotation at the opposing position), and the distance (DS distance) between the photosensitive drum 1d and the developing roller 30 was 0.350 ± 0.025 mm. Also, an elastic belt was used for the intermediate transfer belt 8.

[0063] The toner used was a positively charged toner with an average particle diameter of 6.8 μm, and the carrier used was the resin-coated carrier manufactured in Example 1. The initial toner concentration in the developer (weight ratio of toner to carrier) was set to 6%.

[0064] For the evaluation method, those without pitch unevenness were rated as ◎, those with pitch unevenness but hardly noticeable were rated as 〇, those with slight pitch unevenness were rated as △, and those with clearly visible pitch unevenness were rated as ×. The evaluation results are shown in Table 2 together with the vertical magnetic force gradient A, the vertical magnetic force K, the maximum value of the vertical magnetic force of the regulating pole N2, the angle at which the vertical magnetic force gradient is -0.3 to 0.3 [mT / °], the peak positions of the vertical magnetic force gradient and the vertical magnetic force, and the facing state with the supply and conveyance screw 26.

[0065]

Table 2

[0066] As is clear from Table 2, the peak position (point Q) of the vertical magnetic force gradient of the regulating pole N2 faces the supply and conveyance screw 26, and in the developing device 3d of the first to third aspects of the present invention where the vertical magnetic force gradient A and the vertical magnetic force K [mT] at the peak position of the vertical magnetic force gradient satisfy A ≧ 2.8 and A × K ≧ 62.5, it was confirmed that the occurrence of pitch unevenness was suppressed.

[0067] Also, in the comparison of the first to third aspects of the present invention, the occurrence of pitch unevenness was suppressed as A and A × K increased. In particular, in the first aspect of the present invention where the peak position (point P) of the vertical magnetic force faces the supply and conveyance screw 26, the occurrence of pitch unevenness was not observed.

[0068] On the other hand, in the developing devices 3d of Comparative Examples 1 and 2 where the peak position (point Q) of the vertical magnetic force gradient of the regulating pole N2 faces the supply and conveyance screw 26 but does not satisfy A ≧ 2.8 and A × K ≧ 62.5, pitch unevenness occurred significantly.

[0069] From the above results, the peak position of the vertical magnetic force gradient of the regulation pole N2 faces the supply and conveyance screw 26, and by setting the vertical magnetic force distribution such that the vertical magnetic force gradient A and the vertical magnetic force K [mT] at the peak position of the vertical magnetic force gradient satisfy A ≧ 2.8 and A × K ≧ 62.5, it was confirmed that the occurrence of pitch unevenness can be effectively suppressed.

[0070] Here, the results in the case of using a resin-coated carrier to which barium titanate was added as the ferroelectric particles, manufactured in Example 1, were shown, but it has been confirmed that the same effect can be obtained even when other carriers are used.

Industrial Applicability

[0071] The present invention can be used in a developing device of a two-component developing system using a two-component developer containing toner and carrier. By using the present invention, it is possible to provide a developing device capable of suppressing the occurrence of toner scattering from a magnetic brush itself in a two-component developing system and an image forming apparatus equipped with the same.

Explanation of Signs

[0072] Pa~Pd Image forming unit 1a~1d Photoconductor drum (image carrier) 2a~2d Charging device 3a~3d Developing device 5 Exposure device 20 Developing container 21 Stirring and conveyance chamber (first conveyance chamber) 22 Supply and conveyance chamber (second conveyance chamber) 25 Stirring and conveyance screw (first stirring and conveyance member) 26 Supply and conveyance screw (second stirring and conveyance member) 26a Rotation shaft 26b Conveyance blade 27 Regulation blade (regulation member) 30 Developing roller (developer carrier) 31 Developing sleeve 32 Magnet 40 Developing region 41 Regulation unit 100 Image forming apparatus N1 Main pole N2 Regulation pole S1, S2 Conveyance poles S3 Separation pole

Claims

1. It has a plurality of transport chambers including a first transport chamber and a second transport chamber arranged in parallel with each other, a developing container that houses a two-component developer including a magnetic carrier and toner, A first stirring and transporting member that transports the developer in the first transport chamber in the first direction while stirring the developer, A second stirring and transporting member that transports the two-component developer in the second transport chamber in the second direction, which is opposite to the first stirring and transporting member, while stirring the two-component developer, A developer carrier that is rotatably supported by the developing container and supports the two-component developer on an outer peripheral surface, A regulating member that is arranged to face the developer carrier with a predetermined interval therebetween, In a developing device that develops an electrostatic latent image formed on the surface of an image carrier into a toner image, The developer carrier is A rotatable developing sleeve that supports the two-component developer and forms a magnetic brush on the surface, A magnet in which a plurality of magnetic poles including a regulating pole that is fixedly arranged non-rotatably in the developing sleeve and is arranged at a position facing the regulating member, and a main pole that is arranged on the downstream side of the regulating pole in the rotation direction of the developing sleeve are arranged at a predetermined interval in the circumferential direction, has The magnet is composed of one of the magnetic poles, the regulating pole and a pumping-up pole that pumps up the two-component developer supplied from the second stirring and transporting member to the developer carrier. The maximum value of the vertical magnetic force of the regulating pole is 65 [mT] or less, and there is a region where the vertical magnetic force gradient is 0.3 [mT / °] or less and exists for 10° or more. The peak position of the vertical magnetic force gradient of the regulating pole is arranged at a position facing the second stirring and transporting member. When the vertical magnetic force gradient at the peak position of the vertical magnetic force gradient is A [mT / °] and the vertical magnetic force is K [mT], A≧2.8 and A×K≧62.5 are satisfied. The developing device is characterized in that the peak position of the vertical magnetic force gradient of the regulating pole is arranged at a position facing the rotation axis of the second stirring and transporting member.

2. The developing device according to claim 1, wherein a peak position of the vertical magnetic force of the regulating pole is arranged at a position facing the second stirring and conveying member.

3. The developing device according to claim 1 or claim 2, wherein a perpendicular line passing through the central axis of the magnet is at a position overlapping the second stirring and conveying member when viewed from the axial direction.

4. The magnetic carrier is formed by forming a resin coat layer on the surface of a carrier core which is particles of a magnetic material, and the coat layer contains carbon black as a conductor and barium titanate as a ferroelectric. The developing device according to any one of claims 1 to 3, wherein a volume average particle diameter of the barium titanate is 100 nm or more and 500 nm or less, and an addition amount of the barium titanate is 25 to 45 parts by mass with respect to 100 parts by mass of a coat resin forming the coat layer.

5. The image carrier having a photosensitive layer formed on a surface thereof, The developing device according to any one of claims 1 to 4, which attaches the toner to the electrostatic latent image formed on the image carrier to form a toner image, and an image forming apparatus comprising the same.

Images