Developing device and image forming apparatus equipped with same

Abstract

To provide a developing device that can improve the charge rising properties of toner and prevent the occurrence of image fogging and carrier development in a two-component development system, and an image forming apparatus including the same.SOLUTION: A developing device comprises a developer container, a first stirring and conveying member, a second stirring and conveying member, and a developer carrier. The developer container has a plurality of conveying chambers including a first conveying chamber and a second conveying chamber that are arranged in parallel to each other and partitioned from each other by a partition wall and communication parts that communicate the first conveying chamber and the second conveying chamber with each other at both ends of the partition wall, and stores two-component developer including carrier and toner. The first stirring and conveying member stirs and conveys the developer in the first conveying chamber in a first direction. The second stirring and conveying member stirs and conveys the developer in the second conveying chamber in a second direction. The developer container includes a toner supply part that directly feeds supply toner into the developer in the first conveying chamber from the upstream side of the first conveying chamber with respect to the first direction. The carrier satisfies 0.73≤FR×AD / shape factor≤2.10.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a developing device used in an image forming apparatus such as a copying machine, a printer, a facsimile machine, or a combination machine thereof that uses an electrophotographic system, and an image forming apparatus equipped with the same, and in particular to a developing device of a two-component development system that uses a two-component developer containing toner and carrier. [Background technology]

[0002] In an image forming apparatus, an electrostatic latent image formed on an image carrier such as a photosensitive drum is developed by a developing device to be visualized as a toner image. One such developing device employs a two-component development method using a two-component developer. This type of developing device contains a two-component developer (hereinafter simply referred to as developer) consisting of a carrier and a toner in a developer container, and is provided with a developing roller (developer carrier) that supplies the developer to the image carrier, as well as a stirring and transporting member that stirs and transports the developer in the developer container and supplies it to the developing roller.

[0003] In a two-component development device, a development device has been proposed in which the toner supply unit is located on the side where the developer is not supplied to the development roller. With this configuration, the supplied toner reaches the development roller after being sufficiently mixed with the developer in the development container, thereby suppressing the occurrence of fogging and carrier development due to insufficient charge on the toner.

[0004] On the other hand, since only the amount of toner in the two-component developer contained in the developer container is consumed for development, the toner may be stored in the developer container for a long period of time. Therefore, if blocking occurs in the developer container, where the toner aggregates and solidifies, the toner cannot be sufficiently supplied to the electrostatic latent image using the developing roller, which may result in image degradation.

[0005] Therefore, developers with excellent blocking resistance have been proposed. For example, Patent Document 1 discloses a developer that is made of copolymer particles obtained by polymerizing a binder resin containing an amorphous resin and a crystalline resin and a monomer containing 99 to 80% by weight of a radically polymerizable monomer and 1 to 20% by weight of a sulfonic acid monomer, and that has flowability indices FR (flow rate) × AD (apparent density) and AD × Hc (coercive force) defined within predetermined ranges. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-164647 Summary of the Invention [Problem to be solved by the invention]

[0007] In the developer of Patent Document 1, the carrier has low saturation magnetization, making carrier development more likely to occur, and therefore it is necessary to set the potential difference between the development potential and the surface potential of the photosensitive member (development potential difference) low. As a result, image fogging worsens, and it is necessary to increase the toner charge amount. Meanwhile, in developing devices, a method of replenishing toner by dropping it from above the stirring section is common. With this replenishing method, the replenished toner is difficult to absorb and mix with the developer in the developing device. As a result, the toner may arrive at the developing roller before the toner charge amount reaches the target value, which may result in image fogging.

[0008] In particular, in the case of high-speed printing where there is a narrow margin for carrier development and image fogging, the stirring and transporting members within the developing device also rotate at high speed, increasing the probability that undercharged toner will reach the developing roller, and, as mentioned above, the narrow design margin makes it more likely that problems will occur.

[0009] In view of the above problems, the present invention aims to provide a developing device that can improve the charge build-up property of toner in a two-component development system and suppress the occurrence of image fogging and carrier development, and an image forming apparatus equipped with the same. [Means for solving the problem]

[0010] To achieve the above object, a first aspect of the present invention is a developing device including a developing container, a first agitating / transporting member, a second agitating / transporting member, and a developer carrier. The developing container has multiple transport chambers, including a first transport chamber and a second transport chamber, arranged in parallel with each other, a partition wall that separates the first transport chamber and the second transport chamber along the longitudinal direction, and communication sections that connect the first transport chamber and the second transport chamber at both ends of the partition wall, and contains a two-component developer containing carrier and toner. The first agitating / transporting member agitates and transports the developer in the first transport chamber in a first direction. The second agitating / transporting member agitates and transports the developer in the second transport chamber in a second direction that is opposite to the first direction. The developer carrier is rotatably supported in the developing container and carries the developer in the second transport chamber on its surface. The developing container is equipped with a toner supply section that directly feeds replenishment toner into the developer in the first transport chamber from the upstream side of the first transport chamber in the first direction. The carrier satisfies the following formula (1). 0.73≦FR×AD / shape factor≦2.10 (1) however, FR: Time required for 50g of carrier to be discharged [s / 50g] AD: Bulk density of carrier [g / cm 3 ] Shape factor: Carrier particle size calculated from the measured carrier volume average particle size / BET specific surface area is. [Effects of the Invention]

[0011] According to the first configuration of the present invention, by using a carrier that satisfies formula (1) in a developing device that directly feeds toner into the developer in the first transport chamber from the upstream side of the first transport chamber, the fluidity of the developer is improved, and the replenishment toner fed into the first transport chamber is less likely to be pushed back at the junction with the developer circulating in the developing container. As a result, the replenishment toner is quickly absorbed into the developer in the developing container, and the charge amount of the toner can be quickly and stably raised to the target value. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic cross-sectional view of an image forming apparatus 100 in which developing devices 3a to 3d of the present invention are mounted. [Figure 2] 1 is a perspective view of a developing device 3a according to an embodiment of the present invention; [Figure 3] Side cross-sectional view of developing device 3a [Figure 4] 1 is a plan cross-sectional view showing an agitation section of a developing device 3a; [Figure 5] A side cross-sectional view of the stirring and transport chamber 21 of the developing device 3a [Figure 6] FIG. 10 is a plan cross-sectional view showing the agitation unit of the developing device 3a (comparative examples 2 and 3) that drops the toner upstream of the agitation and transport unit 21 in Example 2. [Figure 7] Graph showing the relationship between the combination of carrier (FR x AD / shape factor) and toner supply method and toner charge buildup DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment 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 according to one embodiment of the present invention. Within the main body of the image forming apparatus 100 (here, a color printer), four image forming units Pa, Pb, Pc, and Pd are arranged in this order from the upstream side in the transport 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 sequentially form images of yellow, cyan, magenta, and black through the processes of charging, exposure, development, and transfer, respectively.

[0014] Each of the image forming stations Pa through Pd is provided with photosensitive drums (image carriers) 1a, 1b, 1c, and 1d, which carry visible images (toner images) of each color. An intermediate transfer belt (intermediate transfer member) 8, which rotates counterclockwise in FIG. 1 by a belt drive motor (not shown), is provided adjacent to each of the image forming stations Pa through Pd. The toner images formed on the photosensitive drums 1a through 1d are sequentially transferred (primary transfer) onto the intermediate transfer belt 8, which moves while contacting the photosensitive drums 1a through 1d, and then superimposed on each other. The toner images primarily transferred onto the intermediate transfer belt 8 are then secondarily transferred onto a transfer sheet P (an example of a recording medium) by a secondary transfer roller 9. The transfer sheet P onto which the toner images have been secondarily transferred is then fixed in a fixing unit 13, and then ejected from the image forming apparatus 100. While the photosensitive drums 1a through 1d are rotating clockwise in FIG. 1, an image formation process is performed on each of the photosensitive drums 1a through 1d.

[0015] The transfer paper P onto which the toner image is secondarily transferred is stored in a paper cassette 16 located at the bottom of the main body of the image forming apparatus 100, and is transported via a paper feed roller 12a and a pair of registration rollers 12b to the nip between the secondary transfer roller 9 and the drive roller 11 of the intermediate transfer belt 8. A sheet made of dielectric resin is used for the intermediate transfer belt 8, and a seamless belt is usually used. In addition, a blade-shaped belt cleaner 19 is located downstream of the secondary transfer roller 9 to remove toner and other particles remaining on the surface of the intermediate transfer belt 8.

[0016] Next, the image forming units Pa to Pd will be described. Around and below the rotatably arranged photosensitive drums 1a to 1d, there are provided charging devices 2a, 2b, 2c, and 2d that charge the photosensitive drums 1a to 1d, an exposure device 5 that exposes image information onto each of the photosensitive drums 1a to 1d, developing devices 3a, 3b, 3c, and 3d that form toner images on the photosensitive drums 1a to 1d, and cleaning devices 7a, 7b, 7c, and 7d that remove developer (toner) and the like remaining on the photosensitive drums 1a to 1d.

[0017] When image data is input from a host device such as a personal computer, the charging devices 2a-2d first uniformly charge the surfaces of the photosensitive drums 1a-1d. Next, the exposure device 5 irradiates light according to the image data, forming electrostatic latent images on each of the photosensitive drums 1a-1d. The developing devices 3a-3d are filled with a predetermined amount of two-component developer containing yellow, cyan, magenta, and black toner, respectively. If the toner content in the two-component developer in each of the developing devices 3a-3d falls below a specified value due to the formation of a toner image (described below), toner is replenished from toner containers 4a-4d to each of the developing devices 3a-3d. The toner in the developer is supplied to the photosensitive drums 1a-1d by the developing devices 3a-3d and electrostatically adheres to them. This results in the formation of a toner image corresponding to the electrostatic latent image formed by exposure from the exposure device 5.

[0018] Then, primary transfer rollers 6a-6d apply an electric field at a predetermined transfer voltage between the primary transfer rollers 6a-6d and the photosensitive drums 1a-1d, and the yellow, magenta, cyan, and black toner images on the photosensitive drums 1a-1d are primarily transferred onto the intermediate transfer belt 8. These images are formed with a predetermined positional relationship. After that, in preparation for the subsequent formation of a new electrostatic latent image, toner and the like remaining on the surfaces of the photosensitive drums 1a-1d after the primary transfer are removed by cleaning devices 7a-7d.

[0019] The intermediate transfer belt 8 is stretched over a driven roller 10 on the upstream side and a drive roller 11 on the downstream side. When the intermediate transfer belt 8 starts to rotate counterclockwise as the drive roller 11 is rotated by a belt drive motor (not shown), the transfer paper P is transported from the registration roller pair 12b to a nip portion (secondary transfer nip portion) between the drive roller 11 and the secondary transfer roller 9 provided adjacent thereto at a predetermined timing. The toner image on the intermediate transfer belt 8 is then secondarily transferred onto the transfer paper P passing through the nip portion. The transfer paper P onto which the toner image has been secondarily transferred is transported to a fixing unit 13.

[0020] The transfer paper P transported to the fixing unit 13 is heated and pressurized by the fixing roller pair 13a, and the toner image is fixed to the surface of the transfer paper P, forming a predetermined full-color image. The transfer paper P on which the full-color image has been formed is then transported in different directions by the branching unit 14, which branches into multiple directions, and is then discharged directly (or after being sent to the double-sided transport path 18 and having images formed on both sides) onto the discharge tray 17 by the discharge roller pair 15.

[0021] Furthermore, an image density sensor 40 is disposed downstream of the image forming unit 1d at a position facing the intermediate transfer belt 8. The image density sensor 40 is generally an optical sensor equipped with a light-emitting element such as an LED and a light-receiving element such as a photodiode. When measuring the amount of toner adhesion on the intermediate transfer belt 8, the light-emitting element irradiates each reference image formed on the intermediate transfer belt 8 with measurement light, and the measurement light is reflected by the toner and the belt surface and enters the light-receiving element.

[0022] The light reflected from the toner and belt surface includes specularly reflected light and diffusely reflected light. This specularly reflected light and diffusely reflected light are separated by a polarizing separation prism and then incident on separate light receiving elements. Each light receiving element photoelectrically converts the received specularly reflected light and diffusely reflected light and outputs an output signal to a control unit (not shown). The toner amount is then detected from the change in the characteristics of the output signals for the specularly reflected light and diffusely reflected light, and density correction (calibration) is performed by comparing them with a predetermined reference density and adjusting the characteristic values of the developing voltage, etc.

[0023] Next, the configuration of developing devices 3a to 3d will be described. FIG. 2 is a perspective view of developing device 3a according to one embodiment of the present invention, which is mounted on image forming apparatus 100. FIG. 3 is a side cross-sectional view of developing device 3a. FIG. 4 is a plan cross-sectional view (cross-sectional view taken along arrow XX' in FIG. 3) showing the agitation section of developing device 3a. FIG. 5 is a side cross-sectional view (cross-sectional view taken along arrow YY' in FIG. 4) of agitation transport chamber 21 of developing device 3a. Note that FIG. 2 shows a state in which cover member 20a of developer container 20 and developing roller 31 have been removed. In the following description, developing device 3a disposed in image forming section Pa in FIG. 1 will be exemplified, but the configurations of developing devices 3b to 3d disposed in image forming sections Pb to Pd are basically the same, and therefore description thereof will be omitted.

[0024] 2 and 3, the developing device 3a includes a developing container 20 that contains a two-component developer (hereinafter simply referred to as developer) containing a magnetic carrier and a toner. The developing container 20 includes a cover member 20a, a partition wall 20b, a first communication portion 20c, a second communication portion 20d, an agitation conveying chamber 21 (first conveying chamber), a supply conveying chamber 22 (second conveying chamber), and a toner replenishing portion 32.

[0025] The cover member 20a is detachable from the main body of the developing container 20 and forms the upper part of the developing container 20. The partition wall 20b divides the interior of the developing container 20 into a stirring and conveying chamber 21 and a supply and conveying chamber 22, which are arranged in parallel. The first communication portion 20c and the second communication portion 20d communicate the stirring and conveying chamber 21 and the supply and conveying chamber 22 at both longitudinal ends of the partition wall 20b.

[0026] The mixing and conveying chamber 21 and the supply and conveying chamber 22 are rotatably provided with a mixing and conveying screw 25 and a supply and conveying screw 26, respectively, for mixing the toner supplied from the toner container 4a (see FIG. 1) with the magnetic carrier, stirring and charging the toner. In this embodiment, a two-component developer consisting of positively charged toner and a ferrite-resin coated carrier is used. The detailed configurations of the toner and carrier will be described later.

[0027] The agitating and conveying screw 25 has a rotating shaft 25a and a first conveying blade 25b formed in a spiral shape at a constant pitch in the axial direction of the rotating shaft 25a. The rotating shaft 25a and the first conveying blade 25b are integrally molded from synthetic resin. The first conveying blade 25b extends to both longitudinal end sides of the agitating and conveying chamber 21 and is also provided facing the first communicating portion 20c and the second communicating portion 20d. The rotating shaft 25a is rotatably supported by the first side wall portion 20f and the second side wall portion 20g of the developing container 20. The agitating and conveying screw 25 conveys the developer in the agitating and conveying chamber 21 in a constant direction (first direction, direction of arrow A1) while agitating the developer.

[0028] The supply / conveyor screw 26 has a rotating shaft 26a and a second conveying blade 26b formed in a spiral shape at a constant pitch in the axial direction of the rotating shaft 26a. The rotating shaft 26a and the second conveying blade 26b are integrally molded from synthetic resin. The second conveying blade 26b has a length equal to or greater than the axial length of the developing roller 30 and further extends to a position facing the first communicating portion 20c. The rotating shaft 26a is disposed parallel to the rotating shaft 25a and is rotatably supported by the first side wall portion 20f and the second side wall portion 20g of the developing container 20. The supply / conveyor screw 26 conveys the developer in the supply / conveyor chamber 22 in the opposite direction (second direction, arrow A2 direction) to the agitating / conveyor screw 25 while agitating the developer.

[0029] The developer is then stirred and transported in the axial direction (the direction perpendicular to the paper surface of FIG. 3) by the stirring and transport screw 25 and the supply and transport screw 26, and circulates between the stirring and transport chamber 21 and the supply and transport chamber 22 via the first communication portion 20c and the second communication portion 20d formed at both ends of the partition wall 20b. That is, a circulation path for the developer is formed inside the developing container 20 by the stirring and transport chamber 21, the first communication portion 20c, the supply and transport chamber 22, and the second communication portion 20d.

[0030] The developing container 20 extends diagonally upward to the right in Figure 3, and the developing roller 30 is disposed diagonally upward to the right within the developing container 20 of the supply / conveyor screw 26. A portion of the outer circumferential surface of the developing roller 30 is exposed through an opening 20e of the developing container 20 and faces the photosensitive drum 1a across a predetermined gap (development gap). The developing roller 30 rotates counterclockwise in Figure 3 (trail rotation at the position facing the photosensitive drum 1a).

[0031] Developing roller 30 is composed of a cylindrical developing sleeve that rotates counterclockwise in Fig. 3 and a magnet (not shown) with multiple magnetic poles fixed inside the developing sleeve. Note that, although a developing sleeve with a knurled surface is used here, it is also possible to use a developing sleeve with a large number of recesses (dimples) formed on the surface, a developing sleeve with a blasted surface, a developing sleeve that has been blasted in addition to being knurled or recessed, a developing sleeve that has been plated to improve durability, a developing sleeve that has been anodized, or a developing sleeve that has been treated with a secondary electrolytic coloring method in which metal salts such as Ni, Sn, or Mo are applied to the porous portions of the anodized aluminum after the anodizing process.

[0032] In particular, alumite processing or alumite processed by secondary electrolytic coloring after alumite processing not only improves durability but also has the effect of suppressing the occurrence of developer leaks. This is because the surface of the developing sleeve 31 is anodized, making it difficult for leakage current generated in the magnetic brush to spread in the circumferential direction on the surface of the developing roller 30, preventing it from developing into a large leak that involves adjacent magnetic brushes. A developing voltage consisting of a DC voltage Vdc and an AC voltage Vac is applied to the developing roller 30 by a developing voltage power supply (not shown).

[0033] A regulating blade 27 is attached to the developing container 20 along the longitudinal direction of the developing roller 30 (perpendicular to the plane of the paper in FIG. 3). A small gap is formed between the tip of the regulating blade 27 and the developing roller 30. In this embodiment, a magnetic blade made of stainless steel (SUS430) is used as the regulating blade 27.

[0034] A toner concentration sensor 29 is disposed on the side of the stirring and conveying chamber 21, facing the stirring and conveying screw 25. The toner concentration sensor 29 detects the toner concentration in the developer in the developing container 20 (the mixture ratio of toner to carrier in the developer; T / C). As the toner concentration sensor 29, for example, a magnetic permeability sensor that detects the magnetic permeability of the two-component developer made up of toner and magnetic carrier in the developing container 20 is used. In accordance with the toner concentration detected by the toner concentration sensor 29, toner in the toner container 4a (see FIG. 1) is replenished into the developing container 20 via a toner replenishing unit 32.

[0035] Toner supply unit 32 is provided upstream of the developer transport direction within mixing / transporting chamber 21. Toner supply unit 32 includes toner supply port 33 and toner supply path 34. Toner supply port 33 opens at the top of toner supply unit 32 and is connected to toner container 4a (see FIG. 1). Toner supply path 34 extends horizontally below toner supply port 33 and communicates with mixing / transporting chamber 21 from the upstream side (left side in FIGS. 4 and 5) of the developer transport direction (arrow A1 direction) within mixing / transporting chamber 21.

[0036] The rotation shaft 25a of the agitating / conveying screw 25 passes through the second side wall portion 20g of the developing container 20 and extends into the toner supply path 34. A supply blade 25c is formed integrally with the portion of the rotation shaft 25a located within the toner supply path 34, and is spirally shaped at a constant pitch in the axial direction of the rotation shaft 25a. The supply blade 25c is formed by a spiral blade that winds in the same direction (in the same phase) as the first spiral blade 25b, and is formed with a smaller pitch and diameter than the first spiral blade 25b. In other words, the agitating / conveying screw 25 also serves as a conveying member that conveys the toner in the toner supply path 34 toward the agitating / conveying chamber 21.

[0037] The replenishment toner carried into the toner supply unit 32 from the toner container 4a (see FIG. 1) through the toner supply port 33 falls into the toner supply path 34. The replenishment toner that has fallen into the toner supply path 34 is transported horizontally (to the right in FIGS. 4 and 5) by the supply blade 25c of the stirring and conveying screw 25, and enters the stirring and conveying chamber 21 along the rotation shaft 25a. The replenishment toner is then stirred and mixed with the developer in the stirring and conveying chamber 21 (the developer that circulates from the supply and conveying chamber 22 after passing through the second communication portion 20d), and is thereby charged to a predetermined charge amount.

[0038] As mentioned above, in the replenishment method in which toner is dropped from directly above onto the developer circulating in the developer container 20, it takes time for the toner to sink from the top to the bottom of the developer due to the difference in specific gravity between the toner and the carrier. As a result, the replenished toner is not easily mixed with the developer in the developing device 3a, and it may arrive at the developing roller 30 before the toner charge amount reaches the target value, which may result in image fogging.

[0039] For example, if carrier development cannot be suppressed in an image forming apparatus 100 with a high printing speed (linear process speed), it is necessary to set a low development potential difference between the development potential and the surface potential of the photosensitive drums 1a-1d. On the other hand, lowering the development potential difference makes image fogging more likely to occur. Here, as the linear speed increases, the level of image fogging tends to worsen, making design even more difficult.

[0040] In this embodiment, the developer circulating in the developing container 20 is configured so that toner is directly supplied from the upstream side of the stirring and conveying chamber 21 along the rotation shaft 25a of the stirring and conveying screw 25 into the developer in the stirring and conveying chamber 21. This allows the replenished toner to be quickly taken in and mixed with the developer, thereby improving the charge build-up property of the toner.

[0041] In particular, when the toner supply method of this embodiment is adopted in an image forming apparatus 100 with a high printing speed (linear process speed), the toner charge amount rises quickly, making it possible to suppress image fogging and also to set the toner charge amount higher. In other words, the effects of the developing devices 3a to 3d of this embodiment become more pronounced when the printing speed (linear process speed) is high.

[0042] However, even when the developing devices 3a to 3d of this embodiment are used, the replenishment toner sent from the toner replenishment path 34 to the stirring and conveying chamber 21 may be pushed back at the junction (near the second communicating portion 20d) with the developer circulating in the developing container 20, and it may take some time for the required amount of replenishment toner to be sent. Specifically, if the fluidity of the developer circulating in the developing container 20 is poor, the replenishment toner is difficult to absorb. Therefore, in order to make it easier for the replenishment toner to be absorbed into the developer, it is necessary to use a developer with high fluidity.

[0043] Next, the two-component developer used in the developing devices 3a to 3d of the present invention will be described. The two-component developer contains toner and carrier. The toner concentration (weight ratio of toner to carrier, T / C) in the two-component developer is preferably 5 to 20 parts by weight of toner per 100 parts by weight of carrier.

[0044] [toner] The toner may be, for example, a positively charged toner. The positively charged toner is positively charged due to friction with the carrier. The toner particles contain toner base particles and, if necessary, external additives that adhere to the surfaces of the toner base particles. The composition of the toner base particles is not particularly limited. If not necessary, external additives may not be added. When no external additives are added, the toner base particles correspond to the toner particles.

[0045] The toner base particles contain a binder resin and a colorant. The toner base particles may contain a release agent, a charge control agent, a magnetic powder, etc., as needed. The weight-average particle diameter of the toner base particles is preferably 5 to 12 μm, more preferably 6 to 10 μm. The weight-average particle diameter of the toner base particles is measured using a particle size distribution measuring device (for example, Coulter's Multisizer II). The toner base particles are produced by known methods such as a pulverization classification method, a melt granulation method, a spray granulation method, or a polymerization method.

[0046] When adding an external additive, it is preferable to use inorganic particles with a number-average primary particle diameter of 5 nm to 30 nm in order to obtain a toner with excellent fluidity. To make the external additive function as a spacer between toner particles and obtain a toner with excellent heat-resistant storage stability, it is preferable to use resin particles with a number-average primary particle diameter of 50 nm to 200 nm inclusive as the external additive particles. Examples of external additives include inorganic oxides such as silica, titanium oxide, and alumina, and metal soaps such as calcium stearate. To fully utilize the function of the external additive while suppressing its detachment from the toner base particles, it is preferable to add the external additive in an amount of 1 part by mass to 10 parts by mass per 100 parts by mass of the toner base particles.

[0047] The toner particles may be toner particles without a shell layer (non-encapsulated toner particles), or toner particles with a shell layer (encapsulated toner particles). The encapsulated toner particles include toner base particles with a toner core and a shell layer covering the surface of the toner core. The configuration of the toner core is not particularly limited. The shell layer may be made essentially of a thermosetting resin, may be made essentially of a thermoplastic resin, or may contain both a thermoplastic resin and a thermosetting resin. To obtain a toner suitable for image formation, the volume average particle diameter (D50) of the toner base particles is preferably 4 μm or more and 9 μm or less.

[0048] Furthermore, the toner particles have hydrophobic silica particles and styrene-acrylic acid resin microparticles attached to the toner base particles. The hydrophobic silica particles are a charge control agent that adjusts the charge amount of the toner. The styrene-acrylic acid resin microparticles are spacers that prevent the silica particles from becoming embedded in the toner base particles. Styrene-acrylic acid resin microparticles usually adhere to the carrier surface during durability testing, causing a decrease in the carrier's charging performance. However, they have weak adhesion to the silicone resin coating layer containing ferroelectric particles (described below), preventing continued accumulation of these microparticles on the carrier. While the detailed mechanism is unknown, it is presumed that this is due to the low adhesion to the ferroelectrics exposed on the surface of the coating layer, making them easy to peel off.

[0049] [Career] The carrier used in the present invention has a coating layer of silicone resin or the like formed on the surface of a carrier core, which is a particle of a magnetic material. Silicone-based resins can be coated in a thin film, which increases the uniformity of the coating layer. Furthermore, the thinner the coating layer, the higher the electrostatic capacitance of the coating layer, making it easier to demonstrate the effects of the ferroelectric material added to the coating layer. When the carrier is a perfect sphere, the average weight per unit area of the coating layer (average film weight) is 0.2 to 2.7 g / m 2 ] is preferable.

[0050] The carrier can be of any shape, from irregular 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, increasing the amount of toner that the carrier can carry. This allows the toner concentration in the magnetic brush to be maintained at a high level, and sufficient toner is supplied to the developing roller 31, ensuring a sufficient thickness of the toner layer. As a result, a sufficient amount of toner can be dispersed from the toner layer to the electrostatic latent image on the photoreceptor, preventing a decrease in image density and further reducing density unevenness in the image. Furthermore, sufficient toner is supplied to the developing roller 31, preventing toner voids from forming in the toner layer on the developing roller 31 and reducing the occurrence of hysteresis.

[0051] If the average particle diameter of the carrier is smaller than 20 μm, carrier development occurs in which the carrier adheres to the photosensitive drums 1a to 1d, and the adhered carrier transfers to the intermediate transfer belt 8, causing transfer defects, or moves to the belt cleaning device 19, causing cleaning defects. Also, if the average particle diameter of the carrier is larger than 65 μm, the magnetic brush of the two-component developer becomes coarse when transferring the toner in the two-component developer from the development roller 31 to the photosensitive drums 1a to 1d, resulting in reduced image quality.

[0052] Examples of carrier cores include magnetic metals such as iron, nickel, and cobalt, alloys thereof, or alloys containing rare earth elements; 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. Carrier cores are manufactured by known methods such as sintering and atomization. Among these, ferrite carriers are preferred for their excellent fluidity and chemical stability, resulting in high image quality and long life. The carrier cores preferably have a saturation magnetization of 65 emu / g to 45 emu / g and a coercive force Hc of 120 e to 600 e when a magnetic field of 3000 Oe is applied.

[0053] Barium titanate particles are added to the coating layer as a ferroelectric. Methods for producing barium titanate include hydrothermal polymerization and oxalate processes, but barium titanate has different physical properties depending on the production method. Among these, barium titanate produced by hydrothermal polymerization has internal voids, resulting in a low true specific gravity and a sharp particle size distribution. As a result, it has better dispersibility in the coating resin than those produced by other methods, enabling uniform dispersion. Therefore, the carrier charging performance is also uniform, making it suitable for use in the present invention.

[0054] The volume average particle size of barium titanate is preferably 100 nm or more and 500 nm or less. If the particle size of barium titanate is smaller than 100 nm, the relative dielectric constant of barium titanate drops sharply, and the effect on the relative dielectric constant becomes small. On the other hand, if the particle size of barium titanate is 500 nm or more, it becomes difficult to uniformly disperse the barium titanate in the coating layer.

[0055] Adding 5 parts by weight or more of barium titanate relative to the coating weight begins to stabilize the charge amount, and adding 25 parts by weight or more makes the effect even more pronounced. However, if too much barium titanate is added, it cannot be fully incorporated into the coating layer and becomes liberated from the coating layer. If the liberated barium titanate moves to the photoreceptor drums 1a-1d and gets caught in the edge of the cleaning blade 32 of the cleaning devices 7a-7d, it can cause cleaning problems. Therefore, the amount of barium titanate added is preferably 5 parts by weight to 45 parts by weight, and more preferably 25 parts by weight to 45 parts by weight, relative to 100 parts by weight of the coating resin.

[0056] Carbon black is added to the coating layer as a conductor. If too much carbon black is added, the carbon black released from the coating layer will adhere to the toner, causing the color of non-black toner to become cloudy. On the other hand, if too little carbon black is added, charge transfer from the carrier to the toner is difficult, and the toner charge amount cannot be smoothly increased. In the carrier of the present invention, the carrier resistance is reduced by adding barium titanate (a ferroelectric) to the coating layer, so it is possible to reduce the amount of carbon black added by the amount of the reduced carrier resistance.

[0057] Adding a ferroelectric (barium titanate) to the coating layer increases the carrier's charge retention capacity, allowing for sufficient charge to be imparted to the toner. Adding a conductor (carbon black) to the coating layer also facilitates the transfer of charge from the carrier to the toner. The combined effect of these two factors makes it possible to impart charge up to the saturated charge level of the toner particles, even when the toner concentration increases and the number of toner particles to be charged increases.

[0058] In this embodiment, the amount of the ferroelectric material and the conductive agent added to the carrier coating layer is adjusted, and the particle size and coating film thickness are adjusted to satisfy the following formula (1). 0.73≦FR×AD / shape factor≦2.10 (1)

[0059] The shape coefficient in formula (1) is a coefficient representing the particle shape, and is defined by the following formula (2). Shape factor = measured carrier volume average particle size / carrier particle size calculated from BET specific surface area (2) however, Carrier particle size calculated from BET specific surface area = 6 / (BET specific surface area x true specific gravity) is.

[0060] If the shape factor is too large, the shape factor is likely to change due to wear of the coating layer during durable printing, resulting in poor durability. On the other hand, if the shape factor is too small, the toner chargeability will decrease. Therefore, there is an optimum range for the shape factor.

[0061] The BET specific surface area is a specific surface area measured by the BET method (nitrogen adsorption specific surface area method), and is specifically determined from the amount of liquid nitrogen adsorbed on the surface of the carrier. More specifically, for example, an automatic specific surface area measuring device (Macsorb model 1208, manufactured by Mountec Co., Ltd.) is used to adsorb nitrogen onto the surface of a sample, and the BET specific surface area [m 2 / g] can be measured.

[0062] In formula (1), FR×AD is an index representing the fluidity of the carrier. If the fluidity of the carrier is too high, the mixing property with the toner decreases, and the toner charging property decreases. On the other hand, if the fluidity of the carrier is too low, the transport speed of the developer in the developing container 20 decreases, and when images with a high printing rate are printed continuously, the image density decreases. Therefore, there is an optimum range for the fluidity of the carrier.

[0063] FR is the carrier fluidity, which is a value [s / 50g] that represents the time it takes for 50g of carrier to be discharged. Since the discharge amount of carrier is more closely matched to the actual behavior when considered in terms of volume rather than weight, in this embodiment, FR is used as an index of carrier fluidity by converting it to the bulk density AD [g / cm 3 ] is used as the FR×AD corrected by

[0064] FR can be measured in accordance with JIS (Japanese Industrial Standards) Z2502. Specifically, a metal funnel (cone angle: 60°, orifice diameter: 2.5 mm, orifice length: 3.2 mm) is prepared, and 50 g of sample (carrier) is placed in the funnel with the orifice blocked. The orifice of the funnel is then opened, and timing is started using a stopwatch. The timing ends when the last carrier leaves the orifice. The measured time (passage time) corresponds to FR. AD can be measured in accordance with JIS-Z2504, a test method for apparent density of metal powders.

[0065] If the carrier fluidity is too high, its mixing with the toner decreases, resulting in a decrease in its chargeability (charging ability) relative to the toner. On the other hand, if the carrier fluidity is too low, the developer transport speed in the developer container 20 decreases, resulting in a decrease in image density when continuously printing images with a high printing rate. Therefore, if the value of (FR×AD / shape factor) is less than 0.73, fluidity is likely to change during long-term printing. Furthermore, the chargeability becomes more variable, leading to image fogging. On the other hand, if the value of (FR×AD / shape factor) is greater than 2.10, image density decreases when printing images with a high printing rate, and insufficient charging (a decrease in the amount of toner charge) is likely to occur, also leading to image fogging. In other words, by satisfying the above formula (1), the carrier chargeability is stabilized, and a state with little image fogging can be maintained for a long period of time.

[0066] By using the developer containing the carrier as described above in the developing devices 3a to 3d of this embodiment, which employ a system in which toner is directly replenished into the developer in the stirring / transporting chamber 21 along the rotation shaft 25a of the stirring / transporting screw 25, the replenished toner sent from the toner replenishment path 34 to the stirring / transporting chamber 21 is less likely to be pushed back at the junction with the developer circulating in the developing container 20. As a result, the replenished toner is quickly taken into the developer in the developing container 20, and the charge amount of the toner can be quickly and stably raised to the target value.

[0067] The present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit of the present invention. For example, the present invention is not limited to a developing device equipped with the developing roller 30 as shown in FIG. 2, but is applicable to various developing devices that use a two-component developer containing toner and carrier. For example, the present invention is equally applicable to a developing device equipped with a magnetic roller (toner supply roller) that carries developer on its outer circumferential surface, and that supplies only the toner in the developer carried by the magnetic roller to the developing roller 30, thereby forming a toner layer on the outer circumferential surface of the developing roller 30 and developing an electrostatic latent image on a photosensitive drum.

[0068] In the above embodiment, the supply blade 25c is provided coaxially with the rotation shaft 25a of the stirring / conveying screw 25, and the toner is supplied along the rotation shaft 25a, but the present invention is not limited to this. As long as the supply blade 25c is a supply system in which the toner is directly sent from the upstream side of the stirring / conveying chamber 21 to the developer in the developing container 20, the supply blade 25c is not limited to being coaxial with the rotation shaft 25a, and the toner may be supplied from a position different from the rotation shaft 25a.

[0069] Furthermore, the present invention is not limited to the tandem color printer shown in Figure 1, but can also be applied to various image forming apparatuses that use a two-component development system, such as digital or analog monochrome copiers, monochrome printers, color copiers, and facsimiles. The effects of the present invention will be described more specifically below with reference to examples. [Example]

[0070] [Production of Ferroelectric Particle-Containing Carrier] [Manufacturing Example 1] 500 g of silicone resin (KR-255, manufactured by Shin-Etsu Chemical Co., Ltd.), 150 g of barium titanate (hydrothermally synthesized, manufactured by Sakai Chemical Industry Co., Ltd.), 10 g of carbon black (Ketjen Black EC, manufactured by Lion Corporation), and 1450 g of toluene were dispersed in a homomixer to obtain a coating solution. The resulting coating solution was sprayed onto 5 kg of carrier cores (Mn ferrite carrier, volume average particle diameter 34.7 μm, saturation magnetization 70 emu / g, coercive force 8 Oe, manufactured by DOWA IP Creation Co., Ltd.) using a fluidized bed coating device while heating at 200 °C, and the carrier cores were coated with the coating solution. The resulting mixture was then baked in an electric furnace at 250 °C for 1 hour. After cooling, the mixture was crushed and classified using a sieve to obtain a carrier having a volume average particle diameter (D50) of 52.3 μm and containing 30 parts by mass of ferroelectric particles (barium titanate) in the coating layer.

[0071] The volume average particle diameter (D50) of the barium titanate and the carrier core was measured using a laser diffraction / scattering particle size distribution analyzer (LA-950, manufactured by Horiba, Ltd.). [Example]

[0072] [Evaluation of toner charge build-up characteristics depending on toner supply method and carrier] The toner charge amount rise characteristics were investigated when the toner supply method to the developing devices 3a to 3d and the type of carrier in the developer were changed.

[0073] For the test method, the following were prepared: a toner supply type developing device 3a (present invention) as shown in Figures 4 and 5, filled with the two-component developer manufactured in Manufacturing Example 1 and containing a carrier with FR×AD / shape factor = 1.13; a toner supply type developing device 3a (Comparative Example 1) as shown in Figures 4 and 5, filled with a two-component developer containing a carrier with FR×AD / shape factor = 0.68; a toner supply type developing device 3a (Comparative Example 2) as shown in Figure 6, filled with a two-component developer containing a carrier with FR×AD / shape factor = 1.13, in which a toner supply port 33 is located above the stirring and conveying chamber 21 and replenishment toner falls from directly above onto the upstream side of the stirring and conveying chamber 21; and a developing device 3a (Comparative Example 3) as shown in Figure 6, filled with a two-component developer containing a carrier with FR×AD / shape factor = 0.68.

[0074] Each of the four types of developing devices 3a was filled with 200 g of developer. The agitating / conveying screw 25 and the supply / conveying screw 26 of the developing device 3a were driven at 350 rpm for a predetermined time in a normal temperature and humidity environment (R / R environment, 23°C, 50%), and the toner charge amount was measured at each position (A to E) shown in Figures 4 and 6. The amount of toner supplied from the toner supply port 33 to the toner supply path 34 was set to 0.25 g per time, and a constant amount of toner was supplied from the toner supply path 34 to the agitating / conveying chamber 21 by the rotation of the supply blade 25c.

[0075] The toner used was a positively charged toner with an average particle diameter of 6.8 μm, and the initial toner concentration in the developer (weight ratio of toner to carrier) was 6%. The developer used was adjusted so that the saturated value of the toner charge amount under these experimental conditions was 41 μC / g, and the closer the toner charge amount at position E is to 41 μC / g, the closer the toner charge amount is to the theoretical value. The measurement results of the charge amount are shown in Figure 7.

[0076] 7, in the present invention (solid line in FIG. 7) in which a carrier with FR×AD / shape factor=1.13 was combined with the developing device 3a of this embodiment, the charge amount was high at all positions A to D, rising to 40 μC / g at position E. In contrast, in Comparative Example 1 (dashed line in FIG. 7) in which a carrier with FR×AD / shape factor=0.68 was combined with the developing device 3a of this embodiment, a delay occurred in the supply of toner from the toner supply path 34 to the stirring and conveying chamber 21, and the rise in the toner charge amount was slower at positions A to E than in the present invention.

[0077] In Comparative Example 2 (dotted line in FIG. 7), which combined a carrier with FR×AD / shape factor=1.13 with the developing device 3a in FIG. 6, it took time for the replenishment toner that dropped from directly above to be absorbed into the developer, so the rise in toner charge amount at positions A to E was slow, similar to Comparative Example 1. Furthermore, in Comparative Example 3 (dashed line in FIG. 7), which combined a carrier with FR×AD / shape factor=0.68 with the developing device 3a in FIG. 6, the rise in toner charge amount was even slower than in Comparative Examples 1 and 2 due to low fluidity of the developer.

[0078] From the above results, it was confirmed that in the toner supply type developing device 3a as shown in Figures 4 and 5, by using a developer containing a carrier that satisfies 0.73≦FR×AD / shape factor≦2.10, the toner is smoothly sent from the toner supply path 34 to the stirring and conveying chamber 21, and the charge rise property of the toner is improved, thereby effectively suppressing image fogging and carrier development. [Industrial Applicability]

[0079] The present invention can be applied to a developing device that uses a two-component developer containing toner and carrier. By utilizing the present invention, it is possible to provide a developing device that can improve the charge buildup of toner in a two-component developer and suppress the occurrence of image fogging and carrier overdevelopment, and an image forming apparatus equipped with the developing device. [Explanation of symbols]

[0080] 1a to 1d Photosensitive drum 3a~3d developing device 20 Developer container 20b Partition wall 22c 1st communication part 22d 2nd communication part 21 Mixing and transport chamber (first transport chamber) 22 Supply transport room (second transport room) 25 Agitation conveying screw (first agitation conveying member) 25a Rotating shaft 25b First conveying blade 25c supply blade 26 supply conveying screw (second stirring conveying member) 26a Rotation axis 26b Second conveying blade 27 Regulating blade (regulating member) 29 Toner density sensor 30 Developing roller (developer carrier) 32 Toner supply unit 33 Toner supply port 34 Toner supply path 100 Image forming device

Claims

1. a plurality of transfer chambers including a first transfer chamber and a second transfer chamber arranged in parallel with each other; a partition wall that divides the first transfer chamber and the second transfer chamber along a longitudinal direction; a communication portion that communicates the first transfer chamber with the second transfer chamber at both end portions of the partition wall; a developer container containing a two-component developer containing a carrier and a toner; a first stirring and transporting member that stirs and transports the two-component developer in the first transport chamber in a first direction; a second stirring and conveying member that stirs and conveys the two-component developer in the second conveying chamber in a second direction that is opposite to the first direction; a developer carrier that is rotatably supported by the developing container and that carries the two-component developer in the second transport chamber on its surface; In a developing device comprising: the developing container includes a toner supply unit that directly feeds supply toner from an upstream side of the first transport chamber in the first direction into the two-component developer in the first transport chamber, The developing device is characterized in that the carrier satisfies the following formula (1): 0.73≦FR×AD / shape factor≦2.10 (1) however, FR: Time required for 50 g of carrier to be discharged [s / 50 g] AD: Bulk density of carrier [g / cm 3 ] Shape factor: Carrier particle diameter calculated from the actually measured carrier volume average particle diameter / BET specific surface area is.

2. the toner supply unit has a toner supply port and a toner supply path that extends horizontally below the toner supply port and communicates with the first transport chamber from an upstream side in the first direction, The developing device described in claim 1, characterized in that the rotation shaft of the first agitating and transporting member extends into the toner supply path, and a supply blade is formed on the portion of the rotation shaft positioned within the toner supply path to send toner in the toner supply path along the rotation shaft into the first transport chamber.

3. The carrier has a resin coating layer formed on the surface of a carrier core, which is a particle of a magnetic material, and the coating layer contains carbon black as a conductor and barium titanate as a ferroelectric material, The coating layer has an average weight per unit area of 0.2 to 2.7 [g / m] when the carrier core is a perfect sphere. ２ 3. The developing device according to claim 1, wherein the amount of barium titanate added is 5 to 45 parts by weight per 100 parts by weight of the coating resin that forms the coating layer.

4. 4. The developing device according to claim 3, wherein the amount of barium titanate added is 25 to 45 parts by weight per 100 parts by weight of the coating resin.

5. 5. The developing device according to claim 3, wherein the barium titanate has a volume average particle diameter of 100 nm or more and 500 nm or less.

6. 6. The developing device according to claim 3, wherein the toner is toner base particles to which hydrophobic silica particles and styrene-acrylic acid resin fine particles are attached.

7. an image carrier; 7. The developing device according to claim 1, wherein the toner is attached to the electrostatic latent image formed on the image carrier to form a toner image; An image forming apparatus comprising:

Images