Developing device and image forming apparatus including the same

Abstract

To provide a developing device that can prevent occurrence of carrier development while maintaining stability of magnetic regulation force of a regulation member in a two-component development system, and an image forming apparatus including the same.SOLUTION: A developing device comprises: a developer container that stores two-component developer; a developer carrier; and a regulation member that is arranged opposite to the developer carrier. The developer carrier has a rotatable developing sleeve that has magnetic brushes formed on its surface, and a magnet that has a plurality of magnetic poles arranged in a circumferential direction, the magnetic poles including a regulation pole opposite to the regulation member and a main pole opposite to an image carrier. The regulation member is arranged on a downstream side of a position at which vertical magnetic force of the regulation pole becomes 0 [mT] and on an upstream side of a position at which the vertical magnetic force and the horizontal magnetic force of the regulation pole become equal to each other. When A is a horizontal magnetic force gradient at a position at which the horizontal magnetic force of the main pole becomes 0 [mT], and B is a horizontal magnetic force gradient on an upstream side surface of the regulation member with respect to a direction of rotation of the developing sleeve, |A|>2.45, and |B|<1.30 are satisfied.SELECTED DRAWING: Figure 5

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 method 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 of such developing devices, a two-component developing method using a two-component developer containing a magnetic carrier and toner is adopted.

[0003] In the two-component developing method, so-called carrier development in which a carrier having the opposite polarity to the toner develops on the blank portion of the photoreceptor is an important problem to be solved. It is known that this carrier development deteriorates when the charge amount of the carrier is large, when the electric field applied to the development region is large, when the magnetic force of the carrier is small, when the resistance of the carrier is low, and when the vertical magnetic force of the main pole is low. Since these items affect various image qualities such as image density, fogging, and developing ghost, it is necessary to perform design while considering the overall balance.

[0004] For example, Patent Document 1 discloses a configuration in which a magnetic device used in an electrophotographic developing device and having a magnet roll rotatably installed in a cylindrical sleeve has a maximum value of magnetic attraction force on the downstream side of the developing position. In the magnetic device of Patent Document 1, the force for pulling back the carrier is maximized on the downstream side of the developing position where the magnetic brush is separated from the photoreceptor, and carrier development is suppressed.

[0005] Patent Document 2 discloses a developing device in which a magnetic pole in a magnet 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 range of the vertical magnetic force in this magnetic pole is 10 Gauss or less. In the configuration of Patent Document 2, since the portion where the vertical magnetic force of the regulating magnetic pole is flat faces the regulating member, even if the angle of the magnet member changes, the change in vertical magnetic force is small, and it becomes possible to regulate a stable amount of developer.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] As described in Patent Documents 1 and 2, conventionally, attempts have been made to obtain stable conditions by regulating the vertical magnetic force and magnetic attraction force, but it has not been possible to sufficiently suppress carrier development only by regulating the vertical magnetic force and magnetic attraction force.

[0008] In view of the above problems, an object of the present invention is to provide a developing device capable of suppressing the occurrence of carrier development while maintaining the stability of the magnetic regulation force of a regulating member in a two-component developing system, and an image forming apparatus including the same.

Means for Solving the Problems

[0009] In order to achieve the above object, a first configuration of the present invention is a developing device that includes a developing container, a developer carrier, and a regulating member, and develops an electrostatic latent image formed on the surface of an image carrier into a toner image. The developing container houses a two-component developer containing a magnetic carrier and toner. The developer carrier is rotatably supported by the developing container and carries the developer 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 carries the developer to form a magnetic brush on its surface. The magnet is fixedly arranged non-rotatably within the developing sleeve, and includes a regulating pole disposed in a regulating portion facing the regulating member and a main pole disposed in a developing region facing the image carrier on the downstream side of the regulating pole in the rotational direction of the developing sleeve. A plurality of magnetic poles are arranged at predetermined intervals in the circumferential direction. The regulating member is disposed on the downstream side of the position where the vertical magnetic force of the regulating pole becomes 0 [mT] with respect to the rotational direction of the developing sleeve, and on the upstream side of the position where the vertical magnetic force and the horizontal magnetic force of the regulating pole become equal. When the horizontal magnetic force gradient at the position where the horizontal magnetic force of the main pole becomes 0 [mT] is A, and the horizontal magnetic force gradient on the upstream side surface of the regulating member with respect to the rotational direction of the developing sleeve is B, |A|>2.45 and |B|<1.30 are satisfied.

Advantages of the Invention

[0010] According to the first configuration of the present invention, at the position of the main pole (developing region), the horizontal magnetic force gradient is increased to increase the scraping force of the developer by the magnetic brush, and at the position of the regulating pole (regulating portion), the horizontal magnetic force gradient is decreased to improve the robustness, so that carrier development can be stably suppressed. Therefore, while effectively suppressing carrier development, the magnetic regulation force in the regulating portion can be ensured to stably convey the developer.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0012] 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 an image forming apparatus 100 including 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 respective processes of charging, exposure, development, and transfer.

[0013] Photoconductor drums (image carriers) 1a, 1b, 1c, and 1d for carrying visible images (toner images) of respective colors are arranged in these image forming units Pa to Pd. Further, an intermediate transfer belt (intermediate transfer member) 8 that rotates counterclockwise 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 photoconductor drums 1a to 1d are sequentially primary transferred and superimposed on the intermediate transfer belt 8 that moves while contacting each of the photoconductor 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, the transfer paper P onto which the toner image is secondary transferred is discharged from the main body of the image forming apparatus 100 after the toner image is fixed in the fixing unit 13. While rotating the photoconductor drums 1a to 1d clockwise in FIG. 1, an image forming process for each of the photoconductor drums 1a to 1d is executed.

[0014] The transfer paper P onto which the toner image is secondarily 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 driving 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 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.

[0015] Next, the image forming units Pa to Pd will be described. Around and below the rotatably arranged 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 the photosensitive drums 1a to 1d with image information, developing devices 3a, 3b, 3c, and 3d for forming a toner image on the photosensitive drums 1a to 1d, and cleaning devices 7a, 7b, 7c, and 7d for removing the developer (toner) and the like remaining on the photosensitive drums 1a to 1d are provided.

[0016] 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 to form an electrostatic latent image corresponding to the image data 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 supplied 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.

[0017] Then, a predetermined transfer voltage is applied between the primary transfer rollers 6a to 6d and the photosensitive drums 1a to 1d by the primary transfer rollers 6a to 6d, and an electric field is applied, 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. 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 primary transfer are removed by the cleaning devices 7a to 7d.

[0018] The intermediate transfer belt 8 is stretched between the upstream driven roller 10 and the downstream driving roller 11. When the intermediate transfer belt 8 starts to rotate counterclockwise as the driving roller 11 rotates by a belt driving motor (not shown), the transfer paper P is conveyed from the registration roller pair 12b to the nip portion (secondary transfer nip portion) between the driving roller 11 and the secondary transfer roller 9 provided adjacent thereto at a predetermined timing, 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.

[0019] The transfer paper P conveyed to the fixing unit 13 is heated and pressurized by the fixing roller pair 13a, and the toner image is fixed on the surface of the transfer paper P, and a predetermined full-color image is formed. The transfer paper P on which the full-color image has been formed has its conveyance direction distributed by the branching portion 14 that branches in a plurality of directions, and is then discharged to the discharge tray 17 by the discharge roller pair 15 (either as it is or after being sent to the duplex conveyance path 18 and having images formed on both sides).

[0020] 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. However, since the configurations of the developing devices 3b to 3d disposed in the image forming units Pb to Pd are basically the same, the description thereof is omitted.

[0021] 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 and conveying chamber 21 and a supply and conveying chamber 22 by a partition wall 20a. In the stirring and conveying chamber 21 and the supply and conveying chamber 22, a stirring and conveying screw 25a and a supply and conveying screw 25b 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 this 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.

[0022] Then, the developer is stirred by the stirring and conveying screw 25a and the supply and conveying screw 25b and conveyed in the axial direction (the direction perpendicular to the plane of FIG. 2), and circulates between the stirring and conveying chamber 21 and the supply and conveying 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 and conveying chamber 21, the supply and conveying chamber 22, and the developer passage.

[0023] 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 conveying screw 25b in 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, 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).

[0024] The developing roller 30 is composed of a cylindrical developing sleeve 31 that rotates in the counterclockwise direction in FIG. 2, and a magnet 32 having a plurality of magnetic poles fixedly and non-rotatably arranged within the developing sleeve 31. In the present 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, developing sleeves with a blasted surface, those with a blasted surface in addition to knurling or forming concave shapes, those with plating treatment for the purpose of improving durability, those with anodizing treatment, and those treated by 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 after anodizing can also be used. In particular, those with anodizing treatment or those treated by the secondary electrolytic coloring method after anodizing not only have improved durability but also have the effect of suppressing the occurrence of developing leakage. This is because the surface of the developing sleeve 31 is anodized, making it difficult for the leakage current generated by the magnetic brush to spread horizontally on the surface of the developing roller 30 and preventing it from developing into a large leakage that entraps adjacent magnetic brushes.

[0025] The magnet 32 has a five-pole configuration including a main pole N1, a regulating pole (lifting pole) S1, a conveying pole S2, N2, and a peeling pole N3. 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).

[0026] In addition, 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 (gap) is provided between the tip of the regulating blade 27 and the surface of the developing roller 30, forming a regulating portion 41. In the present embodiment, a magnetic blade made of stainless steel (SUS430) is used as the regulating blade 27.

[0027] A magnetic field in the attracting direction is generated between the regulating pole S1 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 and the magnetic brush moves to the developing region 40, a magnetic field is applied by the main pole N1, so that the magnetic brush contacts the surface of the photosensitive drum 1a to develop the electrostatic latent image.

[0028] 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 poles S2 and N2 this time, and the developer that was not used for forming the toner image is recovered onto the developing sleeve 31 together with the magnetic brush. Further, at the separating pole N3 having the same polarity as the conveying pole N2, the magnetic brush detaches from the developing roller 30 and falls into the supply and conveyance chamber 22. Then, after being agitated and conveyed by the supply and conveyance screw 25b, a magnetic brush is formed again on the developing sleeve 31 by the magnetic field of the regulating pole S1.

[0029] 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 graph showing the vertical magnetic force distribution and the horizontal magnetic force distribution in the circumferential direction of the developing roller 30. FIG. 4 is an enlarged view from the regulating blade 27 to the main pole N1 in FIG. 3. FIG. 5 is a graph showing the change in the horizontal magnetic force distribution and the horizontal magnetic force gradient in FIG. 4. The horizontal magnetic force gradient of the magnet 32 in the developing devices 3a to 3d of the present embodiment will be described in detail with reference to FIGS. 3 to 5. In FIGS. 3 to 5, the vertical magnetic force is indicated by a solid line, the horizontal magnetic force is indicated by a broken line, and the horizontal magnetic force gradient is indicated by a dotted line.

[0030] In the developing devices 3a to 3d of the present embodiment, by adjusting the magnetic force distribution of the magnet 32 in the circumferential direction of the developing roller 30, more specifically, by adjusting the circumferential magnetic force gradient of the horizontal magnetic force, which is the circumferential magnetic force of the developing roller 30 (hereinafter referred to as the horizontal magnetic force gradient), the scraping effect of the developer by the magnetic brush is improved and carrier development is suppressed.

[0031] As shown in FIGS. 3 and 4, near the peak of the vertical magnetic force of the main pole N1 (point S in FIGS. 4 and 5), the horizontal magnetic force becomes 0 [mT]. It is considered that the horizontal magnetic force gradient in the vicinity is related to the scraping of the developer from the surface of the photoreceptor drums 1a to 1d by the magnetic brush. Further, the scraping effect on the surfaces of the photoreceptor drums 1a to 1d by this magnetic brush is affected by the amount of developer fed into the development region 40 (developer conveyance amount). Therefore, for those with an easily fluctuating developer conveyance amount, the scraping effect also varies, and stable scraping performance cannot be obtained.

[0032] This fluctuation in the developer conveyance amount is affected by the horizontal magnetic force gradient at the position where the developing roller 30 faces the regulating blade 27 (regulating portion 41). Specifically, when the horizontal magnetic force gradient at the regulating portion 41 is small, the change in the vertical magnetic force also becomes small, and the influence of the magnetic force on the change in the fluidity of the developer due to the change in the toner concentration and charge amount in the developer can be reduced.

[0033] That is, by increasing the horizontal magnetic force gradient at the position of the main pole N1 (development region 40) to increase the scraping force and decreasing the horizontal magnetic force gradient at the position of the regulating pole S1 (regulating portion 41) to improve the robustness (stability against noise), it becomes possible to stably suppress carrier development. More specifically, when the horizontal magnetic force gradient at the position of the horizontal magnetic force 0 [mT] of the main pole N1 (point S in FIG. 5) is A [mT / °] and the horizontal magnetic force gradient on the upstream side surface of the regulating blade 27 (T1 in FIG. 5) is B [mT / °], |A|>2.45 and |B|<1.30 are satisfied.

[0034] Next, the arrangement of the regulation blade 27 will be described. Between the position where the horizontal magnetic force of the regulation pole S1 is 0 [mT] (point P in FIGS. 4 and 5) and the position where the horizontal magnetic force reaches the maximum value (point R in FIGS. 4 and 5), the horizontal magnetic force acts in the direction from R to P. Therefore, if the regulation blade 27 is on the downstream side of point P, a force that pulls the developer in the P direction acts on the developer in the regulation section, and the developer conveyance force generated by (vertical magnetic force × friction coefficient between the developer and the developing sleeve 31) can be relaxed. As a result, robust regulation can be performed against fluctuations in the fluidity of the developer and the like.

[0035] On the other hand, if the downstream side surface (T2 in FIG. 5) of the regulation blade 27 is arranged on the downstream side of the position where the horizontal magnetic force and the vertical magnetic force are equal (point Q in FIG. 4), the vertical magnetic force in the regulation section 41 becomes weak, so that the magnetic regulation force cannot be sufficiently ensured. As a result, stable conveyance of the developer becomes impossible. Therefore, the upstream side surface (T1 in FIG. 5) of the regulation blade 27 with respect to the rotation direction of the developing sleeve 31 should be arranged on the downstream side of the position where the horizontal magnetic force of the regulation pole S1 becomes 0 [mT] (point P in FIG. 5), and the downstream side surface (T2 in FIG. 5) of the regulation blade 27 should be arranged on the upstream side of the position where the vertical magnetic force and the horizontal magnetic force are equal (point Q in FIG. 5).

[0036] As described above, by defining the horizontal magnetic force gradient and the arrangement of the regulation blade 27, it is possible to effectively suppress carrier development while ensuring the magnetic regulation force in the regulation section and stably convey the developer.

[0037] Next, a method for measuring the horizontal magnetic force gradient of the magnet 32 of the developing roller 30 will be described. In the present embodiment, the developing roller 30 was mounted on a jig for angle adjustment, and measurement was 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 horizontal magnetic force gradient can be obtained by dividing the difference in the horizontal 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 horizontal magnetic force gradient cannot be accurately obtained. Therefore, in the present invention, the horizontal magnetic force was measured by changing the measurement angle by 0.02°, and (horizontal magnetic force difference at 0.08° difference / 0.08°) was defined as the gradient 1 at the midpoint within that 0.08°. Then, the average gradient per 2° of the gradient 1 was defined as the horizontal magnetic force gradient. A measurement example of the horizontal magnetic force gradient is shown in Table 1.

[0038]

Table 1

[0039] In Table 1, for example, the gradient 1 (2.50 [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 horizontal magnetic force G2 at 10.04° by 0.08°. Also, the average gradient (2.59 [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°.

[0040] 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 its 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 becomes higher, and the effect of the ferroelectric added to the coating layer is more easily exhibited.

[0041] The shape of the carrier can be used from an amorphous shape to a spherical shape. Further, 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, a decrease in image density can be suppressed, and furthermore, 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.

[0042] When 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. Further, when the average particle diameter of the carrier is larger than 65 μm, when the toner in the two-component developer is moved 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.

[0043] Examples of the carrier core include magnetic metal such as iron, nickel, and cobalt, alloys thereof, 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.

[0044] 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 in order to also uniformize the charging performance of the carrier.

[0045] The volume average particle size of barium titanate is preferably 100 nm or more and 500 nm or less. When the particle size 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 size of barium titanate becomes 500 nm or more, uniform dispersion in the coating layer becomes difficult.

[0046] When 5 parts by mass or more of barium titanate is added with respect to the coating weight, the stabilizing effect of the charge amount starts to appear, and when 25 parts by mass or more is added, the stabilizing effect of the charge amount appears more significantly. 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 blade 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 barium titanate separated through use is supplied to the developing devices 3a to 3d, which increases the load on the cleaning blade 32. Therefore, it is preferable that the addition amount of barium titanate is 5 parts by mass or more and 45 parts by mass or less.

[0047] Carbon black is added to the coating layer as a conductor. If the amount of carbon black added is too large, the carbon black released from the coating layer will adhere to the toner, causing color turbidity of the toner other than black. On the other hand, if the amount of carbon black added is too small, the transfer of charges from the carrier to the toner is likely to occur, and the increase in the toner charge amount cannot be smoothly performed. 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 amount of carbon black added by the amount corresponding to the reduction in the carrier resistance.

[0048] 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 performed. 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.

[0049] 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. Further, due to the addition of barium titanate, the carrier resistance is reduced compared to the case where only carbon black is added, so the amount of carbon black added can be reduced. As a result, color turbidity caused by the adhesion of carbon black to the toner can be suppressed. Furthermore, since the electrification 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 stability of the magnetic regulation force at the regulating blade 27 is higher than that of the carrier without the addition of barium titanate. Therefore, the developer conveyance amount is likely to be stabilized, and stable carrier development can be suppressed.

[0050] The present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, in the above embodiment, as the magnet 32 of the developing roller 30, a configuration in which the regulating pole S1 and the main pole N1 are arranged is used, but the polarities of the regulating pole and the main pole may be reversed.

[0051] 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 equipped with two-component developing type developing devices, such as monochrome and color copiers, monochrome printers, digital multifunction machines, etc. Hereinafter, the effects of the present invention will be described more specifically with reference to examples.

Example

[0052] [Manufacture of ferroelectric particle-containing carrier] 200 parts by mass of 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 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 liquid. The obtained coating liquid 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 liquid. Then, 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.

Example

[0053] [Evaluation of regulation stability and carrier development when changing the horizontal magnetic field gradient] The regulation stability (stability of the magnetic regulation force at the regulation blade 27) and the evaluation of carrier development were performed when the horizontal magnetic force gradient of the developing roller 30 was changed. The test method was to mount developing apparatuses 3a to 3d (Inventions 1 to 3 of the present invention, Comparative Examples 1 to 6) as shown in FIG. 2, in which the vertical magnetic forces of the main pole N1 and the regulation pole S1, the horizontal magnetic force gradient, and the position of the regulation blade 27 were changed, on a testing machine as shown in FIG. 1. Using this testing machine, the regulation stability and carrier development were evaluated when the developing apparatus was driven in a high-temperature and high-humidity environment (32.5°C, 80%) and a low-temperature and low-humidity environment (10°C, 15%).

[0054] The image forming conditions were as follows: the printing speed (process speed) was 55 sheets per minute, and a developing sleeve 31 with an outer diameter of 20 mm having 80 rows of concave portions formed (knurled) on the outer peripheral surface was used for the developing roller 30. The regulation blade 27 was a magnetic blade made of stainless steel (SUS430) with a thickness of 1.5 mm, and the distance (regulation gap) between the regulation blade 27 and the developing roller 30 was set to 0.5 ± 0.03 mm. A developing voltage obtained by superimposing an AC voltage with a peak-to-peak value (Vpp) of 1125 V, a frequency of 10 kHz, and a duty = 50% on a DC voltage of 250 V was applied to the developing roller 30.

[0055] For the photoreceptor drums 1a to 1d, an amorphous silicon (a-Si) photoreceptor with a relative dielectric constant of 11 was used. The peripheral speed ratio of the developing roller 30 to the photoreceptor drums 1a to 1d was set to 1.8 (trailing rotation at the facing position), and the distance (DS distance) between the photoreceptor drums 1a to 1d and the developing roller 30 was set to 0.375 ± 0.025 mm. Also, an elastic belt was used for the intermediate transfer belt 8.

[0056] As the toner, a positively charged toner with an average particle diameter of 6.8 μm was used, and as the carrier, the resin-coated carrier manufactured in Example 1 was used. The initial toner concentration (weight ratio of toner to carrier) in the developer was set to 5% and 7%. Also, two types of developers, unused products and durable products (equivalent to printing 100,000 sheets), were used.

[0057] For the evaluation method, regarding the regulation stability, the variation rate of the developer conveyance amount ((maximum value - average value) / average value × 100) was calculated. When the variation rate was 15% or less, it was marked as 〇, and when it exceeded 15%, it was marked as ×. For the carrier development, the number of carriers moving on the photoreceptor drums 1a to 1d was observed using a loupe. When the maximum number per unit area was 5 pieces / cm 2 or less, it was marked as ◎, and when it was 10 pieces / cm 2 or less, it was marked as 〇, and when it exceeded 10 pieces / cm 2 it was marked as ×. The evaluation results are shown in Table 2 together with the vertical magnetic force, horizontal magnetic force gradient, and regulation blade position of the main pole N1 and the regulation pole S1.

[0058]

Table 2

[0059] As is clear from Table 2, in the developing devices 3a to 3d of the first to third inventions where the horizontal magnetic force gradient A at the main pole N1 and the horizontal magnetic force gradient B at the regulation pole S1 satisfy |A| > 2.45 and |B| < 1.30, and the regulation blade 27 is arranged between the point P and the point Q, the variation rate of the developer conveyance amount is 15% or less, the number of carrier developments is 10 pieces / cm 2 or less, and it was confirmed that the regulation stability was improved and the occurrence of carrier development was suppressed.

[0060] On the other hand, in Comparative Examples 1 and 2 where the horizontal magnetic force gradient B at the regulation pole S1 is |B| ≥ 1.30, since the horizontal magnetic force gradient near the regulation blade 27 is large, the variation rate of the developer conveyance amount exceeds 15%, and the regulation stability cannot be ensured. As a result, the carrier development also deteriorated.

[0061] On the other hand, in Comparative Examples 3 to 5 where the horizontal magnetic force gradient A at the main pole N1 is |A| ≦ 2.45, the variation rate of the developer conveyance amount is 15% or less and the regulation stability is excellent. However, since the horizontal magnetic force gradient at the main pole N1 is small, the scraping force by the magnetic brush becomes insufficient, resulting in deteriorated carrier development. Further, in Comparative Example 6 where the regulation blade 27 is disposed between the point Q and the point R, since the vertical magnetic force near the regulation blade 27 becomes weak, sufficient magnetic regulation force cannot be ensured, and as a result, the carrier development deteriorates.

[0062] From the above results, it was confirmed that by disposing the regulation blade 27 between the point P and the point Q and setting the horizontal magnetic force distribution such that the horizontal magnetic force gradient A at the main pole N1 and the horizontal magnetic force gradient B at the regulation pole S1 satisfy |A| > 2.45 and |B| < 1.30, it is possible to ensure regulation stability and effectively suppress carrier development.

[0063] Here, the results in the case of using the resin-coated carrier added with barium titanate as the ferroelectric particles manufactured in Example 1 were shown, but it has been confirmed that the same effects can be obtained when using other carriers.

Industrial Applicability

[0064] 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, in a two-component developing system, it is possible to provide a developing device capable of suppressing the occurrence of carrier development while maintaining the stability of the magnetic regulation force of the regulating member, and an image forming apparatus including the same.

Explanation of Reference Numerals

[0065] Pa to Pd Image forming unit 1a to 1d Photoconductor drum (image carrier) 2a to 2d Charging device 3a to 3d Developing device 5 Exposure device 20 Developing container 27 Regulation blade (regulating member) 30 Developing roller (developer carrier) 31 Developing sleeve 32 Magnet 40 Developing area 100 Image forming apparatus S1 Regulation pole N1 Main pole S2, N2 Conveying poles N3 Peeling pole

Claims

1. A developing container that houses a two-component developer containing a magnetic carrier and toner, a developer carrier that is rotatably supported by the developing container and carries the two-component developer on its outer peripheral surface, a regulating member that is disposed 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 a rotatable developing sleeve that carries the two-component developer and forms a magnetic brush on its surface, a regulating pole that is fixedly disposed non-rotatably within the developing sleeve and is disposed in a regulating portion facing the regulating member, and a plurality of magnetic poles including a main pole disposed in a developing region facing the image carrier on the downstream side of the regulating pole with respect to the rotation direction of the developing sleeve are arranged at predetermined intervals in the circumferential direction, has, the regulating member is disposed on the downstream side of the position where the horizontal magnetic force of the regulating pole becomes 0 [mT] with respect to the rotation direction of the developing sleeve, and on the upstream side of the position where the vertical magnetic force and the horizontal magnetic force of the regulating pole become equal, When the horizontal magnetic force gradient at the position where the horizontal magnetic force of the main pole becomes 0 [mT] is A, and the horizontal magnetic force gradient on the upstream side surface of the regulating member with respect to the rotation direction of the developing sleeve is B, a developing device characterized by satisfying |A| > 2.45 and |B| < 1.

30.

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

3. The image carrier having a photosensitive layer formed on its surface, The developing device according to claim 1 or claim 2 that adheres the toner to the electrostatic latent image formed on the image carrier to form a toner image, An image forming apparatus comprising.

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