Developing device and image forming apparatus

Abstract

To provide a developing device that can prevent unevenness of screw pitch, and an image forming apparatus.SOLUTION: A developing device carries, with the action of a magnetic field, a developer including toner and magnetic carrier and conveyed by a supply and conveying screw 143 being a conveying screw in a developer storage part, on a surface of a developing sleeve 141 being a developer carrier, and conveys the developer carried on the surface of the developing sleeve to a developing area. When the minimum distance between an apex 143a of a blade of the supply and conveying screw 143 and the surface of the developing sleeve is defined as L, L≤6[mm], and the screw pitch P of the supply and conveying screw 143 satisfies the relationship of 1.5×L+2.74≤P≤0.64×L+9.74.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a developing device and an image forming apparatus. [Background technology]

[0002] Conventionally, a developing device is known in which a developer containing toner and magnetic carriers is transported by a transport screw in a developer storage section and carried on the surface of a developer carrier by the action of a magnetic field, and the developer carried on the surface of the developer carrier is transported to a development area.

[0003] Patent Document 1 describes a developing device in which the distance between the conveying screw and the developing roller serving as a developer carrier is greater than 0 mm and is 15 mm or less, and the screw pitch of the conveying screw is 20 mm. Summary of the Invention [Problem to be solved by the invention]

[0004] However, there is a risk of screw pitch unevenness, which is image density unevenness that depends on the screw pitch of the conveying screw, occurring. [Means for solving the problem]

[0005] In order to solve the above-mentioned problems, the present invention provides a developing device that carries developer, including toner and magnetic carriers, on the surface of a developer carrier by the action of a magnetic field, which is carried by a transport screw in a developer accommodating section, and transports the developer carried on the surface of the developer carrier to a development area, wherein the transport screw is disposed at a position where it contacts the developer carried on the surface of the developer carrier against gravity by the action of the magnetic field, and the transport screw is a multi-thread screw with multiple blades wound around it in a spiral shape, When the minimum distance between the top of the blade of the conveying screw and the surface of the developer carrier is L, L≦6 [mm], and a screw pitch P, which means the misalignment of the blades of the conveying screw, satisfies the relationship 1.5×L+2.74≦P≦0.64×L+9.74. It is something. [Effects of the Invention]

[0006] According to the present invention, it is possible to suppress the screw pitch unevenness. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram illustrating the configuration of a printer according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing the configuration of an imaging device. [Figure 3] FIG. 2 is a cross-sectional view of the developing device taken along a direction perpendicular to the rotation axis of the developing sleeve. [Figure 4] 3A and 3B are explanatory diagrams showing the schematic configuration of a developing device and the distribution of magnetic flux density (absolute value) in the normal direction on the surface of a developing sleeve. [Figure 5] 6 is a diagram showing the relationship between the developer on the developing sleeve carried by the drawing-up magnetic pole and the supply / conveyance screw; FIG. [Figure 6] FIG. 2A is a diagram showing the relationship between the developer in the conventional supply chamber and the developer on the developing sleeve, and FIG. 2B is a diagram showing the relationship between the developer in the conventional supply chamber and the developer on the developing sleeve of the present embodiment. [Figure 7] Graph showing the results of an evaluation test. [Figure 8] 5A and 5B are diagrams illustrating the behavior of developer that has fallen from a developing sleeve. [Figure 9] FIG. 10 is a diagram showing the magnetic force of the fifth magnetic pole at the position where the supply / conveyance screw and the developing sleeve are closest to each other. [Figure 10] 10 is a graph comparing the magnetic force of a conventional fifth magnetic pole in the normal direction at each position in the rotation direction of the developing sleeve with the magnetic force of the fifth magnetic pole of this embodiment in the normal direction at each position in the rotation direction of the developing sleeve. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, an embodiment in which the present invention is applied to an electrophotographic printer (hereinafter simply referred to as a "printer"), which is an image forming apparatus, will be described. 1 is a schematic diagram of a printer according to this embodiment, where Y, C, M, and K represent components for yellow, cyan, magenta, and black, respectively. In this printer, four color image forming devices 10Y, 10C, 10M, and 10K are detachably mounted in image forming stations formed on the device main body 1 as process cartridges. These devices use different color Y, M, C, and K toners as image forming materials, but are otherwise identical in configuration and are replaced when their lifespan expires. The printer also includes an optical unit 20 as an exposure means capable of emitting laser light, an intermediate transfer unit 30, a paper feed unit 40, a fixing unit 50, and the like.

[0009] The imaging devices 10Y, 10C, 10M, and 10K have the same structure, and each is integrally configured with a photosensitive drum 12Y, 12C, 12M, or 12K as a latent image carrier and a processing means acting on the photosensitive drum. The processing means include charging devices 13Y, 13C, 13M, or 13K that charge the photosensitive drums, and cleaning devices 15Y, 15C, 15M, or 15K that remove toner and other residue from the photosensitive drums. These are also connected to developing devices 14Y, 14C, 14M, or 14K that develop the latent images formed on the photosensitive drums.

[0010] The intermediate transfer unit 30 includes an intermediate transfer belt 31 serving as an intermediate transfer body. It also includes multiple (three in this example) rollers 32, 33, and 34 that rotatably support the intermediate transfer belt 31, and a primary transfer roller 35 that transfers the toner images formed on each photosensitive drum 12 onto the intermediate transfer belt 31. It also includes a secondary transfer roller 36 that further transfers the toner images transferred onto the intermediate transfer belt 31 onto recording paper P serving as a recording material. The paper feed unit 40 includes a paper feed roller 43 that transports recording paper P from a paper feed cassette 41 or a manual paper feed tray 42 to a secondary transfer area, a registration roller 44, and the like. The fixing unit 50 includes a fixing roller 51 and a pressure roller 52, and employs a well-known configuration in which the toner image on the recording paper P is fixed by applying heat and pressure.

[0011] In addition, toner bottles 60Y, 60C, 60M, and 60K, each containing toner to be supplied to the toner supply port 145 described later, are attached to the top of the device main body 1 so as to be detachable from the device main body 1 individually for each of the image forming devices 10Y, 10C, 10M, and 10K.

[0012] In this configuration, first, in the imaging device 10Y for the first color, yellow, the photosensitive drum 12Y is uniformly charged by the charging device 13Y. Next, a latent image is formed by laser light emitted from the optical unit 20, which serves as a latent image forming means, and the latent image is developed by the developing device 14Y to form a toner image. The Y toner image formed on the photosensitive drum 12Y is transferred onto the intermediate transfer belt 31 by the action of the primary transfer roller 35Y. After the primary transfer is completed, the photosensitive drum 12Y is cleaned by the cleaning device 15Y in preparation for the next image formation. The residual toner collected by the cleaning device 15Y is stored in the waste toner collection bottle 16, which is installed in the removal direction of the imaging device 10Y (in the direction of the rotational axis of the photosensitive drum). The waste toner collection bottle 16 is detachable from the image forming apparatus main body 1 so that it can be replaced when it becomes full.

[0013] A similar image forming process is performed in each of the image forming devices 10C, 10M, and 10K for C, M, and K, whereby a toner image of each color is formed and transferred in succession onto the previously formed toner image. Meanwhile, the toner image formed on the intermediate transfer belt 31 is transferred by the action of the secondary transfer roller 36 onto recording paper P transported to the secondary transfer area from paper feed cassette 41 or manual paper feed tray 42. The recording paper P with the transferred toner image is transported to fixing unit 50, where the toner image is fixed at the nip between fixing roller 51 and pressure roller 52 of fixing unit 50, and the recording paper is then discharged by paper discharge roller 55 to paper discharge tray 56 at the top of the device.

[0014] Next, a specific configuration of the image forming device will be described. The imaging devices 10Y, 10C, 10M, and 10K have the same configuration except for the color of toner they use, so the following description will be given taking the yellow imaging device 10Y as an example. FIG. 2 is a schematic diagram showing the configuration of an image forming device 10Y for forming a Y toner image. The charging device 13Y provided in the image forming device 10Y includes a charging roller 131 and a cleaning roller 132 that cleans the surface of the charging roller 131. The cleaning device 15Y includes a cleaning brush 151 and a cleaning blade 152 that come into contact with the surface of the photosensitive drum. It also includes a toner recovery coil 153 that transports the toner scraped off by the cleaning brush 151 and the cleaning blade 152 toward the waste toner recovery bottle 16.

[0015] The developing device 14Y is equipped with a non-magnetic developing sleeve 141 that constitutes a hollow member of a developer carrier that carries a two-component developer (hereinafter simply referred to as "developer") consisting of a magnetic carrier and toner and transports the developer to a development region facing the photosensitive drum 12Y by rotating counterclockwise in FIG. 2. A magnet roller 147 is fixedly disposed inside the developing sleeve 141 and serves as a magnetic field generating means having multiple magnetic poles arranged in the circumferential direction. The developing sleeve 141 and the magnet roller 147 constitute the developer carrier.

[0016] The developing device 14Y also includes a developer regulating member 146 that is disposed opposite the developing sleeve 141 and that forms a doctor gap between the developing sleeve 141 and the surface of the developing sleeve 141 to regulate the layer thickness of the developer carried on the surface of the developing sleeve 141. The developing device 14Y also includes a supply conveying screw 143 and a recovery conveying screw 142 that convey the magnetic carrier stored in the developing device 14Y and the replenishment toner supplied from a toner replenishing port 145 back and forth in the axial direction of the photosensitive drum 12Y while stirring them. These members are housed and supported in a developing case 144.

[0017] Next, the structure and operation of the developing device will be further described. FIG. 3 is a cross-sectional view of the yellow developing device 14Y in this embodiment taken in a direction perpendicular to the rotation axis of the developing sleeve. FIG. 4 is an explanatory diagram showing the distribution of magnetic flux density (absolute value) in the normal direction on the surface of the developing sleeve 141, along with a schematic configuration of the yellow developing device 14Y in this embodiment.

[0018] The developing case 144 forms a developer storage section for storing developer inside the developing device 14Y. The developer storage section is divided into a supply chamber 149A located below the developing sleeve 141 and extending in the axial direction of the developing sleeve, and an agitation chamber 149B adjacent to the supply chamber 149A and extending in the axial direction of the developing sleeve. A supply conveying screw 143 is provided in the supply chamber 149A, and a recovery conveying screw 142 is provided in the agitation chamber 149B.

[0019] The diameter D1 of the developing sleeve 141 is φ16 to 22 mm, the rotation speed of the developing sleeve is 70 to 700 rpm, and the rotation speed ratio between the developing sleeve 141 and the supply / conveyor screw (developing sleeve / supply / conveyor screw) is set to 0.7 to 0.85. If the rotation speed is lower than 0.7, problems may occur due to temperature rise of the supply / conveyor screw (such as aggregation due to softening of toner in the supply chamber). On the other hand, if the rotation speed ratio exceeds 0.85, the rotation speed of the supply / conveyor screw 143 becomes too slow compared to the rotation speed of the developing sleeve 141, and the amount of developer pumped up to the developing sleeve 141 per unit time increases relative to the amount of developer moving axially in the supply chamber per unit time. As a result, developer is pumped up to the developing sleeve 141 on the upstream side of the developer movement direction in the supply chamber, resulting in a shortage of developer on the downstream side of the supply chamber 149A. This may result in deviations in the amount of developer in the axial direction on the surface of the developing sleeve, resulting in uneven image density.

[0020] In this embodiment, the amount of developer contained in the developer container is 170 g to 280 g. The magnetic carrier may have a weight-average particle size of 20 μm or more and 50 μm or less. If the weight-average particle size is less than 20 μm, particle uniformity decreases, making carrier adhesion more likely. On the other hand, if the weight-average particle size exceeds 50 μm, the reproducibility of image details decreases, making it difficult to obtain fine images. The weight-average particle size of the carrier can be measured using a Microtrac particle size analyzer (manufactured by Nikkiso Co., Ltd.) SRA type, with a range setting of 0.7 μm to 125 μm. Methanol is used as the solvent for the dispersion, and the refractive index is set to 1.33, and the refractive index of the carrier and core material is set to 2.42.

[0021] In this embodiment, the amount of developer accommodated in the developer accommodating portion is reduced compared to conventional devices. The supply conveying screw 143 is disposed close to the developing sleeve 141 so that the developer in the supply chamber 149A can be adequately supplied to the surface of the developing sleeve 141 even with a small amount of developer. Specifically, the supply chamber 149A is positioned above the stirring chamber 149B, and the rotation center O2 of the supply conveying screw 143 is positioned above the rotation center O3 of the recovery conveying screw 142.

[0022] The minimum distance L between the surface of the developing sleeve and the top of the blade 143a of the supply / conveyor screw 143 is preferably 3 mm or more and 6 mm or less. By setting the minimum distance L to 6 mm or less, the blade 143a of the supply / conveyor screw 143 can be brought into contact with the developer held on the developing sleeve 141 by the magnetic force of the P5 pole (see FIG. 4), which is a drawing-up magnetic pole. This allows the developer in the supply chamber 149A to be efficiently supplied to the developing sleeve 141, even if the amount of developer contained in the developer containing portion is small and the height of the developer in the supply chamber 149A is below the top of the shaft of the supply / conveyor screw 143 (see FIG. 6(b)). On the other hand, if the minimum distance L is less than 3 mm, the contact pressure of the developer in the developing sleeve 141, which is exerted by the top of the blade 143a of the supply / conveyor screw, on the surface of the developing sleeve increases. As a result, as the supply conveying screw 143 rotates, the load fluctuation of the rotation of the developing sleeve 141 increases when the tops of the blades 143a of the supply conveying screw 143 slide over the surface of the developer on the developing sleeve 141. This can cause abnormal vibration of the developing sleeve 141, resulting in abnormal images. For this reason, in this embodiment, the minimum distance L between the surface of the developing sleeve and the tops of the blades of the supply conveying screw 143 is set to 3 mm or more. In this embodiment, the minimum distance L is set to 4.3±0.5 mm.

[0023] The diameter D2 (diameter of the blade) of the supply / conveyor screw 143 is 14 to 20 mm, the diameter of the shaft is 6 to 8 mm, and the screw is a three-threaded screw with three blades 143a. The clearance d between the top of the blade 143a of the supply / conveyor screw 143 and the inner wall surface of the supply chamber 149A is set to 0.5 to 1.0 mm. By using a three-threaded supply / conveyor screw 143, the decrease in developer transport speed can be suppressed, the screw pitch can be narrowed, the occurrence of screw pitch unevenness can be suppressed, and density deviation in the main scanning direction can be suppressed. The larger of the two angles β between a line connecting the rotation center O1 of the developing sleeve 141 and the rotation center O2 of the supply / conveyor screw 143 and a reference line γ extending horizontally from the rotation center O1 of the developing sleeve 141 toward the development zone is 230° to 250°.

[0024] The three blades 143a of the supply conveying screw 143 have the same lead. The screw pitch is 7 mm to 14 mm, and more preferably 8 to 11 mm, and the relationship between the minimum distance L and the screw pitch P is set to satisfy the following formula (1). 1.5×L+2.74≦P≦0.64×L+9.74 (1) However, L≦6mm

[0025] By configuring the minimum distance L and screw pitch P to satisfy the relationship shown in formula (1), it is possible to suppress the occurrence of screw pitch irregularities in a configuration in which the minimum distance L is 6 mm or less, as will be described later.

[0026] The recovery and conveyance screw 142 is a single-thread screw having one blade 142a, and has a shaft portion with a larger diameter than the supply and conveyance screw 143.

[0027] The developer transported to the downstream end of supply chamber 149A (the rear side in the figure) by supply transport screw 143 is transferred to stirring chamber 149B, and is transported toward the downstream end of stirring chamber 149B (the front side in the figure) by recovery transport screw 142 inside the stirring chamber. The developer transported to the downstream end of stirring chamber 149B is then transferred to supply chamber 149A, and is transported toward the downstream end of supply chamber 149A by supply transport screw 143 inside supply chamber 149A. In this way, the developer is circulated and transported within the developer accommodating section.

[0028] Replenishing toner to replenish the toner consumed by development is supplied to the developer in the stirring chamber 149B from a toner supply port. The developer in the supply chamber 149A is drawn up onto the developing sleeve 141 by the magnetic force of the magnet roller 147 (fifth magnetic pole P5 shown in FIG. 4) while being transported. Thereafter, the developer drawn up onto the developing sleeve 141 is regulated by the developer regulating member 146, passes through the development area facing the photosensitive drum 12Y, and returns to the developer accommodating section.

[0029] As shown in Fig. 4, the magnet roller 147 has five magnetic poles: a first magnetic pole P1 (north pole), a second magnetic pole P2 (south pole), a third magnetic pole P3 (north pole), a fourth magnetic pole P4 (north pole), and a fifth magnetic pole P5 (south pole). P1 to P5 in Fig. 4 indicate the distribution of magnetic flux density (absolute value) in the normal direction of the magnetic field formed by each magnetic pole on the surface of the developing sleeve 141. Hereinafter, the magnetic poles will be referred to as P1 to P5.

[0030] In this embodiment, the developer is drawn up from the supply chamber 149A by the magnetic force of the P5 pole, which is a drawing-up magnetic pole, and attracted to the developing sleeve 141. The developer is transported counterclockwise in the drawing as the developing sleeve 141 rotates. The developer, which is restricted to a predetermined amount by the developer regulating member 146, is formed into chains by the magnetic force of the P1 pole in the development region. The toner is then supplied from the chains to the electrostatic latent image on the surface of the photosensitive drum 12Y by the development electric field, thereby carrying out the development process. The developed developer is held on the developing sleeve 141 by the magnetic forces of the P2 and P3 poles and transported as the developing sleeve 141 rotates. The developer then reaches the position of the developer release pole, which is formed by the third and fourth magnetic poles P3 and P4, which are adjacent magnetic poles of the same polarity (N pole). The magnetic force of the developer release pole causes the developer to separate from the surface of the developing sleeve 141 and drop into the supply chamber 149A in the developer storage section.

[0031] The magnetic force f(Mag) acting on the magnetic carrier of the magnetic poles of the magnet roller 147 can be calculated by the following formula (2) in the following case: That is, when the magnetic carrier is a soft magnetic fine powder that is approximately spherical or irregular in shape and has low magnetization, and the influence of the magnetic force of the magnetic carrier on the magnetic field formed by the magnetic poles of the magnet roller 147 is small and negligible. In the following formula (2), H0 is the magnetic field formed by the magnetic poles of the magnet roller 147, M is the magnetic moment of the magnetic carrier, and V is the volume of the magnetic carrier. f(Mag)=(M / V)H0 (2) The magnetic moment M of the magnetic carrier is expressed as M = V × Jm, where Jm is the magnetization, i.e., the magnetic force, of the magnetic carrier. As shown in equation (2), the magnetic force f (Mag) acting on the magnetic carrier from the magnetic poles of the magnet roller 147 can be approximated as a force proportional to the size of the magnetic carrier, the magnetic susceptibility of the magnetic carrier, the strength of the magnetic field formed by the magnetic poles of the magnet roller 147, and the magnetic gradient of the magnetic poles.

[0032] In this embodiment, the magnetic force of the P5 pole in the normal direction to the line segment connecting the rotation center O1 of the developing sleeve 141 and the rotation center O2 of the supply / conveyor screw 143 is set to -6E-9 [N] or less. This makes it possible to prevent the developer that has separated from the developing sleeve 141 from being collected in the supply chamber 149A and adhering to the developing sleeve 141 due to the magnetic force of the P5 pole, as will be described later.

[0033] As described above, in this embodiment, the amount of developer contained in the developer storage unit is reduced compared to conventional methods. When the amount of developer in the developer storage unit is reduced, the developer level in the supply chamber 149A is lowered. This lower developer level in the supply chamber 149A increases the likelihood of screw pitch unevenness, which is image density unevenness that corresponds to the screw pitch of the supply / conveyor screw 143. Furthermore, to facilitate pumping up the developer, a so-called lower doctor system is employed, in which a developer regulating member is positioned lower in the direction of gravity than the development area. While the lower doctor system contributes to reducing the amount of developer, it also increases the likelihood of screw pitch unevenness compared to the upper doctor system, in which a developer regulating member is positioned higher in the direction of gravity than the development area.

[0034] Therefore, in this embodiment, the supply conveying screw 143 is disposed close to the developing sleeve 141, and the blades of the supply conveying screw 143 come into contact with the developer g1 held on the developing sleeve by the magnetic force of the P5 pole, as shown in Fig. 5. Specifically, when the developer in the supply chamber is removed, the leading edge of the developer held by the magnetic force of the P5 pole does not fall due to gravity but comes into contact with the outer diameter of the supply conveying screw.

[0035] Fig. 6(a) is a schematic diagram showing a case where the blade of the supply conveying screw 143 is not in contact with the developer g1 held on the developing sleeve 141 by the magnetic force of the P5 pole. Fig. 6(b) is a schematic diagram showing a case where the blade 143a of the supply conveying screw 143 is in contact with the developer g1 held on the developing sleeve by the magnetic force of the P5 pole. As shown in FIG. 6(a), when the blade 143a of the supply conveying screw 143 does not come into contact with the developer g1 held on the developing sleeve 141, there are locations where the developer in the supply chamber 149A and the developer held on the developing sleeve 141 are separated.

[0036] The height of the developer in supply chamber 149A differs between the upstream and downstream sides of the blades in the developer transport direction when viewed from a direction perpendicular to the axial direction. The developer is higher on the upstream side in the developer transport direction, where it is lifted by the blades, and the developer in supply chamber 149A enters the magnetic field of pole P5. As a result, on the upstream side of the blades in the developer transport direction, the developer held on the developing sleeve and the developer in supply chamber 149A are connected, and the developer in supply chamber 149A is effectively drawn up by the magnetic force of pole P5.

[0037] On the other hand, the developer level in supply chamber 149A is lower on the downstream side of the blades in the developer transport direction than on the upstream side, and the developer is outside the magnetic field of pole P5. As a result, the downstream side of the blades in the developer transport direction is separated from the developer held on the developing sleeve, as shown in FIG. 6(a), and the developer in supply chamber 149A is hardly drawn up by the magnetic force of pole P5. As a result, the developer on the developing sleeve at the location corresponding to the downstream side of the blades in the developer transport direction is depleted. As a result, the amount of developer on the developing sleeve supported by the magnetic force of pole P5 varies in the axial direction, causing screw pitch unevenness (hereinafter, this screw pitch unevenness will be referred to as depletion screw pitch unevenness).

[0038] On the other hand, in this embodiment, the supply conveying screw 143 is positioned close to the developing sleeve 141 so that the blades of the supply conveying screw 143 come into contact with the developer held on the developing sleeve by the magnetic force of the P5 pole. In this embodiment, the clearance d between the top of the blade 143a of the supply conveying screw 143 and the inner wall surface of the supply chamber 149A is set to 0.5 mm to 1.0 mm. Therefore, by positioning the supply conveying screw 143 as described above, the bottom surface of the supply chamber 149A is raised compared to the configuration of FIG. 6(a), as shown in FIG. 6(b). As a result, the developer in the supply chamber 149A downstream in the developer conveyance direction between the blades also enters the magnetic field of the P5 pole, and the developer held on the developing sleeve and the developer in the supply chamber 149A are connected to each other. Therefore, the developer on the downstream side of the developer transport direction between the blades is also drawn up by the magnetic force of the P5 pole, preventing the developer on the developing sleeve at the downstream side of the developer transport direction between the blades from being depleted. This makes it possible to suppress the occurrence of depletion screw pitch unevenness. In this embodiment, the above-mentioned depletion screw pitch unevenness was successfully suppressed when the minimum distance L between the surface of the developing sleeve and the top of the blade 143a of the supply / conveyor screw 143 was 6 mm or less.

[0039] However, when the supply / conveyance screw 143 is positioned so that the blades of the supply / conveyance screw 143 come into contact with the developer held on the developing sleeve 141 by the magnetic force of the P5 pole, screw pitch unevenness occurs, resulting in a lighter image density than other areas. This screw pitch unevenness is thought to be caused by the following factors.

[0040] That is, by disposing the supply conveying screw 143 close to the developing sleeve, the distance between the blades of the supply conveying screw 143 and the developing sleeve 141 becomes short. As a result, the developer lifted by the blades 143a of the supply conveying screw pushes aside the developer carried by the magnetic force of the P5 pole on the developing sleeve, and a large amount of the developer lifted by the blades ends up being carried on the developing sleeve.

[0041] The toner in the developer is subjected to significant stress and becomes frictionally charged by staying in front of the developer regulating member 146 for a predetermined period of time after being carried by the developing sleeve 141 due to the magnetic force of the P5 pole until it passes through the developer regulating member 146. However, in the supply chamber, the developer is not subjected to much stress, and the toner is often insufficiently tribo-charged. As described above, if a large amount of developer is carried on the developing sleeve, pushing aside the developer carried by the magnetic force of the P5 pole on the developing sleeve and lifted by the blades, this developer lifted by the blades does not stay in front of the developer regulating member 146 but passes through the developer regulating member 146. As a result, insufficiently charged toner is transported to the development area. On the other hand, on the side other than the blades (downstream side in the developer transport direction between the blades), the developer lifted by the blades as described above is not disturbed by the developer lifted by the blades, and the toner stays in front of the developer regulating member 146 for a predetermined period of time, and then the sufficiently charged toner passes through the developer regulating member 146. As a result, it is thought that screw pitch unevenness occurs, in which the image density is thinner than other areas (hereinafter, screw pitch unevenness caused by this factor will be referred to as insufficient charging screw pitch unevenness). In particular, the above-mentioned insufficient charging screw pitch unevenness occurs significantly under conditions where toner is replenished frequently, such as when images with a high image area ratio are continuously printed, although this depends on the composition of the developer.

[0042] In this embodiment, by setting the screw pitch P of the supply conveying screw 143 to 15.5 mm or less, it is possible to effectively suppress the insufficient charging and the screw pitch unevenness. In this way, by narrowing the screw pitch P of the blade 143a of the supply conveying screw 143, the inclination of the blade 143a becomes steeper. As a result, it is possible to reduce the amount of developer lifted by the blade 143a. This makes it possible to prevent a large amount of developer lifted by the blade 143a from being carried (pumped up) by the developing sleeve 141. As a result, it is possible to reduce the amount of developer that passes through the developer regulating member 146 without accumulating in front of the developer regulating member 146, and it is possible to suppress the insufficient charging and the screw pitch unevenness.

[0043] However, narrowing the screw pitch reduces the developer transport speed. As a result, there is a risk of density deviations occurring in the main scanning direction (width direction) of the developed image. For this reason, in this embodiment, a multi-thread screw is used as the supply / conveyance screw 143. By using a multi-thread screw, it is possible to narrow the screw pitch while suppressing a decrease in the developer transport speed. This makes it possible to suppress density deviations in the main scanning direction while suppressing screw pitch unevenness. In particular, in this embodiment, by using a three-thread screw as the supply / conveyance screw, it is possible to effectively suppress a decrease in the developer transport speed while narrowing the screw pitch, and to suppress density deviations in the main scanning direction.

[0044] Furthermore, even when the minimum distance L is 6 mm or less and the screw pitch P is 15.5 mm or less, depending on the relationship between the minimum distance L and the screw pitch P, the screw pitch irregularity may not be sufficiently improved.

[0045] The developer that has been released from the surface of developing sleeve 141 by the magnetic force of the developer release magnetic pole and fallen into supply chamber 149A is stirred for a period of time until it is lifted up again by blade 143a of the supply / conveyor screw, which varies depending on the position of blade 143a in the rotational direction when it fell into supply chamber 149A. Because the position of blade 143a in the rotational direction varies along the axial direction, the stirring time until the fallen developer is lifted up again by blade 143a varies along the axial direction. If the stirring time is short, there is a risk that the reduced toner concentration will not recover and the developer will be pumped up into developing sleeve 141.

[0046] The developer in supply chamber 149A is lifted to a certain height by blade 143a of supply / conveyor screw 143, and is then drawn up by the magnetic force of pole P5. While being lifted by blade 143a into the range of the magnetic field of pole P5, some of the developer falls down the inclined surface of blade 143a due to its own weight, thereby promoting agitation of the developer. Therefore, even if there are axial locations where the agitation time from when the developer falls into supply chamber 149A until it is lifted by blade 143a is short, the toner concentration can be restored by agitation while being lifted by blade 143a into the range of the magnetic field of pole P5.

[0047] However, if the minimum distance L is short and the blades of the supply / conveyor screw 143 contact the developer held on the developing sleeve, the height to which the blades 143a lift the developer up to the range of the magnetic field of the P5 pole is reduced. This reduces the agitation caused by the developer falling down the inclined surface of the blades 143a due to its own weight. As a result, the developer in the axial direction where the agitation time from when it falls into the supply chamber 149A to when it is lifted by the blades 143a is short is pumped up to the developing sleeve 141 without the toner concentration being fully restored.

[0048] Furthermore, by reducing the amount of developer contained in the developer containing section, the amount of developer held back by developer regulating member 146 is also reduced, and less developer accumulates in front of developer regulating member 146. This also reduces the agitation of the developer pumped up by developing sleeve 141 until it passes through developer regulating member 146. As a result, developer in an axial location where the agitation time from when it falls into supply chamber 149A until it is lifted by blade 143a is short passes through developer regulating member 146 without the toner concentration being sufficiently restored. This causes the image density in that location to be lighter than in other locations, which is thought to result in screw pitch unevenness (hereinafter, this screw pitch unevenness will be referred to as re-pumping screw pitch).

[0049] Narrowing the screw pitch P of the supply / conveyor screw 143 and reducing the amount of developer lifted by the blades 143a can prevent developer with an unrecovered toner concentration from being carried on the developing sleeve 141. This reduces the re-pumping screw pitch. However, if the screw pitch is too narrow, the axial movement of the developer per rotation of the supply / conveyor screw 143 becomes shorter, stirring does not progress, and the toner concentration does not sufficiently recover before the developer is lifted again by the blades 143a. This could result in a deterioration in the re-pumping screw pitch. In other words, if the axial movement of the fallen developer until it is lifted again by the blades 143a is a predetermined amount, stirring progresses through the axial movement, allowing the toner concentration of the developer to be recovered, and the re-pumping screw pitch can be reduced. Therefore, when the minimum distance L is short, there is an optimal range of screw pitch. Therefore, in this embodiment, the following evaluation test was carried out to find the optimum range of the screw pitch of the supply conveying screw 143 when the minimum distance L is 2 to 8 mm.

[0050] [Evaluation test] First, a primary evaluation test was conducted under the following primary evaluation unit conditions, while gradually adjusting the minimum distance L between the developing sleeve 141 and the supply conveying screw 143 from 2 to 8 mm and changing the pitch of the supply conveying screw 143 from 5.5 to 15.5 mm. Next, for the conditions under which no screw pitch unevenness occurred in the results of the primary evaluation test, a secondary evaluation test was conducted under the following secondary evaluation unit conditions, while changing the positional relationship (angle) between the developing sleeve 141 and the supply conveying screw 143, the rotation speed ratio, the diameter of the developing sleeve 141, and the combination of the blade diameter and shaft diameter of the supply conveying screw 143.

[0051] <Primary evaluation unit conditions> Positional relationship between the developing sleeve and the supply / conveyor screw (Angle β): 240° based on the horizontal direction (Minimum distance L): 2 to 8 mm (evaluate while adjusting) Ratio of rotation speed between developing sleeve and supply transport screw: 0.8 Developing sleeve diameter: φ18mm -Developing sleeve pumping pole magnetic force (P5 magnetic pole) distribution specifications: Maximum magnetic flux density in the normal direction of the developing sleeve surface is 65±10mT, and the half-value central angle is 275±10° Supply and conveyance screw: Blade diameter φ16mm x shaft diameter φ6mm · Supply and conveying screw pitch: 5.5 to 15.5 mm (spiral level is a 3-thread screw created in 0.5 mm increments)

[0052] The half-value median angle is the larger of the two angles between the horizontal reference line γ (see Figure 3) and the line connecting the rotation center O1 of the developing sleeve 141 and the position of the half-value median of the pumping pole magnetic force (P5 magnetic pole) in the axial cross section.

[0053] <Secondary evaluation unit conditions> Positional relationship between the developing sleeve and the supply / conveyor screw (Angle β): 230 to 250° based on the horizontal direction - Ratio of rotation speed between developing sleeve and supply transport screw: 0.7, 0.8, 0.85 Developing sleeve diameter (3 types): φ16, 18, 22 mm -Developing sleeve pumping pole magnetic force (P5 magnetic pole) distribution specifications: Maximum magnetic flux density in the normal direction of the developing roller surface is 65±10mT, and the half-value central angle is 275±10° Supply and conveying screws (9 types in total): Combinations of blade diameters of φ14, 16, and 20 mm and shaft diameters of φ6, 7, and 8 mm

[0054] The minimum distance L between the developing sleeve and the supply / conveyor screw and the pitch of the supply / conveyor screw were combinations that did not cause unevenness in the screw pitch in the primary evaluation test. Note that in the primary evaluation test, abnormal images due to vibration of the developing sleeve were confirmed for evaluation units with the minimum distance of less than 3 mm.

[0055] [Evaluation criteria] Using the developing device under the above evaluation unit conditions, 10 A3 full solid images were continuously printed and image evaluation was carried out. The 8th to 10th sheets of the printed images were visually checked, and if screw pitch unevenness was confirmed, it was judged as "X", and if screw pitch unevenness did not occur, it was judged as "O".

[0056] Table 1 below shows the maximum and minimum characteristic values ​​of each developing device used in the evaluation test, as well as the optimum range determined based on the evaluation test. A three-lobe screw was used for the supply / conveyor screw 143. The lead of each blade 143a was the same, and the lead of each blade was three times the screw pitch. In the second evaluation test, no screw pitch irregularities were observed, and it was confirmed that the screw pitch irregularities were primarily related to the relationship between the minimum distance L between the developing sleeve 141 and the supply / conveyor screw 143 and the screw pitch of the supply / conveyor screw 143.

[0057] [Table 1]

[0058] FIG. 7 is a graph showing the results of the evaluation test. In Figure 5, "x" indicates an evaluated developing device in which screw pitch unevenness was confirmed in the evaluation test and was rated "x," while "●" indicates an evaluated developing device in which screw pitch unevenness was not confirmed in the evaluation test and was rated "o." Of the evaluated developing devices rated "x," Figure 5 plots evaluated developing devices with screw pitches near the border with the "o" rating.

[0059] 7 plots the optimum values ​​of the minimum distance L between the developing sleeve and the supply / conveyor screw, which are 3 to 6 mm, for evaluation developing devices. For evaluation units with a minimum distance L of less than 3 mm, abnormal images were observed in the primary evaluation due to abnormal vibration of the developing sleeve 141, so the lower limit of the optimum value for the minimum distance L was set to 3 mm. The upper limit of the optimum value for the minimum distance L, 6 mm, is the distance at which the developer in the supply chamber can be satisfactorily supplied to the developing sleeve 141 even when the amount of developer contained in the developer container is small.

[0060] From the graph plotting the evaluation test results shown in Figure 7, two lines could be drawn to separate the evaluation developing devices that received an "X" rating from the evaluation developing devices that received an "O" rating. Line B, which indicates the upper boundary of the screw pitch P that resulted in an "O" rating, was P = 0.64 × L + 9.74, and line A, which indicates the lower boundary of the screw pitch P that resulted in an "O" rating, was P = 1.5 × L + 2.74. Therefore, it was found that screw pitch unevenness can be suppressed by ensuring that the screw pitch P of the supply / conveyor screw satisfies the relationship 1.5 × L + 2.74 ≦ P ≦ 0.64 × L + 9.74. Furthermore, based on the trend in the results shown in Figure 7, even if the minimum distance L is less than 3 mm, screw pitch unevenness can be suppressed by ensuring that the relationship 1.5 × L + 2.74 ≦ P ≦ 0.64 × L + 9.74 is satisfied.

[0061] By setting the screw pitch P to P = 0.64 × L + 9.74 or less and 1.5 × L + 2.74 or more, it is possible to suppress the insufficient charging screw pitch unevenness and re-pumping screw pitch unevenness that occur in a configuration in which the blades of the supply transport screw 143, whose minimum distance L between the surface of the developing sleeve and the supply transport screw is 6 mm or less, come into contact with the developer held on the developing sleeve by the magnetic force of the P5 pole.

[0062] As shown in FIG. 7, in the range between line B, which indicates the upper boundary of the optimum value of the minimum distance L, 3 to 6 mm, and line A, which indicates the lower boundary, the maximum value of the screw pitch P was 14 mm and the minimum value was 7 mm. Therefore, by setting the screw pitch in the range of 7 to 14 mm according to the minimum distance L, it is possible to suppress screw pitch unevenness. In addition, in this embodiment, the minimum distance L is set to 4.3±0.5 mm in consideration of the safety factor against abnormal images due to vibration of the developing sleeve. When the minimum distance L is 4.3±0.5 mm, as can be seen from FIG. 5, it is possible to suppress screw pitch unevenness by appropriately setting the screw pitch P between 8 and 11 mm.

[0063] By making the screw pitch P satisfy the relationship 1.5×L+2.74≦P≦0.64×L+9.74, the occurrence of screw pitch unevenness could be suppressed, but it was confirmed that the image density was lower than the desired image density. This is thought to be due to the following reasons. That is, as shown in FIG. 7, the shorter the minimum distance L, the narrower the screw pitch must be. Depending on the position of the blade 143a of the supply / conveyor screw 143 in the rotational direction, some of the developer separated from the developing sleeve 141 falls directly into the supply chamber, as indicated by arrow X1 in FIG. 8, while others bounce up after hitting the blade 143a. When the minimum distance L is short and the supply / conveyor screw 143 is close to the developing sleeve 141, the developer that bounces up after hitting the blade 143a is captured by the magnetic field of the P5 pole, as indicated by arrow X2 in FIG. 8, and re-adheres to the developing sleeve 141. As a result, some of the developer bounces up because of the blade 143a and re-adheres to the developing sleeve without recovering its toner concentration, as indicated by arrow X2 in FIG. 8, and some of the developer falls into the supply chamber, as indicated by arrow X1 in FIG. 8, alternate in the axial direction. It is believed that such factors may cause uneven screw pitch. By reducing the screw pitch, almost no developer falls directly into the supply chamber as indicated by the arrow X1 in the figure, improving the screw pitch unevenness. However, it is thought that the developer does not recover its toner concentration and instead re-adheres to the developing sleeve 141, resulting in a decrease in image density.

[0064] Therefore, the present applicant conducted a re-evaluation test on developing devices with configurations that were evaluated as "good" in the screw pitch evaluation test and in which a decrease in image density was confirmed, by appropriately changing the magnetic force of the P5 pole, which is the pumping magnetic pole. As a result, as shown in FIG. 9, it was found that the decrease in image density can be suppressed by setting the magnetic force f(Mag) of the P5 pole in the normal direction on the line segment B connecting the rotation center O1 of the developing sleeve 141 and the rotation center O2 of the supply / conveyor screw to -6E-9 [N] or less. That is, as shown by arrow X2 in FIG. 9, the developer that falls from the developing sleeve 141 and is splashed up by the blade 143a of the supply / conveyor screw 143 is captured by the magnetic field of the P5 pole, preventing it from re-adhering to the developing sleeve 141.

[0065] FIG. 10 is a graph comparing the magnetic force of the conventional P5 pole in the normal direction at each position in the rotation direction of the developing sleeve with the magnetic force of the P5 pole in the normal direction at each position in the rotation direction of the developing sleeve of this embodiment. Position A in the figure is a position on line segment B connecting the rotation center O1 of the developing sleeve 141 and the rotation center O2 of the supply / conveyor screw 143. By appropriately adjusting the magnetic force of the magnet inside the developing sleeve using existing magnet member magnetization methods described in JP-A-2010-061064 and JP-A-11-162731, etc., it is possible to achieve a magnetic force distribution such as that shown by the solid line in Fig. 10. This makes it possible to reduce the magnetic force in the normal direction at position A to -6E-9 [N] or less.

[0066] The above description is merely an example, and each of the following aspects provides unique effects. (Aspect 1) In the developing device 14, developer containing toner and magnetic carrier transported by a transport screw such as a supply transport screw 143 in a developer storage section is carried on the surface of a developer carrier such as a developing sleeve 141 by the action of a magnetic field, and the developer carried on the surface of the developer carrier is transported to a development area.The transport screw is positioned so that it comes into contact with the developer carried on the surface of the developer carrier against gravity by the action of the magnetic field, and the transport screw is a multi-thread screw with multiple blades 143a wound around it in a spiral shape, and the screw pitch P of the transport screw is 15.5 mm or less. According to this, as explained in the embodiment, it is possible to suppress the screw pitch unevenness and also the image density deviation in the main scanning direction.

[0067] (Aspect 2) In aspect 1, when the minimum distance between the top of the blade 143a of a conveying screw such as the supply conveying screw 143 and the surface of a developer carrier such as the developing sleeve 141 is L, L≦6 [mm], and the screw pitch P of the conveying screw satisfies the relationship 1.5×L+2.74≦P≦0.64×L+9.74. This makes it possible to effectively suppress screw pitch irregularities, as described in the embodiment.

[0068] (Aspect 3) In developing device 14, developer containing toner and magnetic carriers transported by a transport screw such as supply transport screw 143 in a developer storage section is carried on the surface of a developer carrier such as developing sleeve 141 by the action of a magnetic field, and the developer carried on the surface of the developer carrier is transported to a development area. When the minimum distance between the top of blade 143a of the transport screw and the surface of the developer carrier is L, L≦6 mm, and the screw pitch P of the transport screw satisfies the relationship 1.5×L+2.74≦P≦0.64×L+9.74. In order to ensure that the developer being transported by the transport screw is well carried on the developer carrier even when the amount of developer in the developer storage section is small, the minimum distance L between the top of the blade of the transport screw and the surface of the developer carrier was shortened and image evaluation was performed.When the minimum distance L was 6 mm or less, screw pitch unevenness occurred. The inventors had found that screw pitch unevenness could be suppressed by narrowing the pitch of the conveying screw, so they created a testing machine with a narrowed pitch of the conveying screw. However, in a configuration where the minimum distance L is 6 mm or less, it was found that narrowing the screw pitch too much worsens screw pitch unevenness. Therefore, the inventors conducted the above-mentioned evaluation test to investigate the relationship between the screw pitch P and the above-mentioned minimum distance L, and found that by setting the screw pitch P of the conveying screw to satisfy the relationship 1.5×L+2.74≦P≦0.64×L+9.74, it is possible to suppress screw pitch unevenness with a configuration in which the above-mentioned minimum distance L is 6 mm or less. In aspect 1, in a configuration in which the minimum distance L between the top of the blade of the conveying screw and the surface of the developer carrier is 6 mm or less, the screw pitch P of the conveying screw satisfies the relationship 1.5×L+2.74≦P≦0.64×L+9.74, thereby effectively suppressing screw pitch unevenness.

[0069] (Aspect 4) In any of the aspects 1 to 3, a developer regulating member 146 is provided to regulate the developer carried on the surface of a developer carrier such as the developing sleeve 141, and the developer regulating member 146 is disposed below the development area in the direction of gravity. According to this, as described in the embodiment, in a configuration in which the developer regulating member 146 is arranged below the development area in the direction of gravity, screw pitch unevenness is more likely to occur than in a configuration in which the developer regulating member 146 is arranged above the development area in the direction of gravity. However, even in such a configuration in which screw pitch unevenness is likely to occur, by having the configurations of the above-mentioned aspects 1 to 3, screw pitch unevenness can be effectively suppressed.

[0070] (Aspect 5) In the second or third aspect, or the fourth aspect relating to the second or third aspect, the screw pitch P is 7 mm or more and 14 mm or less. This makes it possible to suppress the irregularity in screw pitch as explained with reference to FIG.

[0071] (Aspect 6) In the fifth aspect, the screw pitch P is 8 mm or more and 11 mm or less. This reliably prevents abnormal images caused by vibration of the developer carrier such as the developing sleeve 141, and also suppresses screw pitch unevenness in a configuration in which the minimum distance L is as short as possible (minimum distance L = 4.35 ± 0.5 mm).

[0072] (Aspect 7) In any of the aspects 1 to 6, the minimum distance L between the top of the blade 143a of the conveying screw such as the supply conveying screw 143 and the surface of the developer carrier such as the developing sleeve 141 is 3 mm or more. According to this, as described in the embodiment, it is possible to suppress the occurrence of abnormal images due to vibration of the developer carrier such as the developing sleeve 141.

[0073] (Aspect 8) In any of the first to seventh embodiments, the conveying screw, such as the supply conveying screw 143, is a triple-start screw. According to this, as explained in the embodiment, it is possible to suppress the screw pitch unevenness and also the image density deviation in the main scanning direction.

[0074] (Aspect 9) In any of aspects 1 to 8, a magnetic pole such as the fifth magnetic pole P5 is arranged inside a developer carrier such as the developing sleeve 141 to support the developer transported by a transport screw such as the supply transport screw 143 on the surface of the developer carrier, and in a cross section perpendicular to the rotational axis of the transport screw, the magnetic force in the normal direction of the magnetic pole on the line segment B connecting the rotation center O1 of the developer carrier and the rotation center O2 of the transport screw is -6E-9 [N] or less. This makes it possible to prevent the image density from becoming lower than the target density, as described in the embodiment.

[0075] (Aspect 10) In any of the first to ninth embodiments, the weight average particle size of the magnetic carrier is 20 μm or more and 50 μm or less. This makes it possible to suppress carrier adhesion and obtain a fine image, as described in the embodiment.

[0076] (Aspect 11) In an image forming apparatus that forms an image by developing a latent image formed on a latent image carrier such as a photosensitive drum with a developing device 14, any one of the developing devices according to embodiments 1 to 9 was used as the developing device. This makes it possible to obtain a good image in which screw pitch irregularities are suppressed. [Explanation of symbols]

[0077] 10: Imaging device 12: Photosensitive drum 14: Developing device 141: Developing sleeve 142: Collection and transport screw 142a: Feather 143: Supply conveying screw 143a: Feather 144: Developing case 146: Developer regulating member 147: Magnetic roller 149A: Supply room 149B: Stirring chamber L: Minimum distance between the surface of the developing sleeve and the supply / conveyor screw O1: Center of rotation of developing sleeve O2: Rotation center of the supply and transport screw O3: Center of rotation of the recovery and transport screw P: Screw pitch P1: First magnetic pole P2 :Second magnetic pole P3: Third magnetic pole P4: Fourth magnetic pole P5: Fifth magnetic pole d: Clearance between the supply conveying screw and the wall surface of the supply chamber [Prior art documents] [Patent documents]

[0078] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-071734

Claims

1. A developing device in which a developer containing toner and a magnetic carrier transported by a transport screw in a developer storage section is carried on the surface of a developer carrier by the action of a magnetic field, and the developer carried on the surface of the developer carrier is transported to a development area, the conveying screw is disposed at a position where it contacts the developer carried on the surface of the developer carrier against gravity due to the action of the magnetic field, The conveying screw is a multi-start screw having a plurality of blades wound in a spiral shape, When the minimum distance between the top of the blade of the conveying screw and the surface of the developer carrier is L, L≦6 [mm], The screw pitch P, which means the misalignment of the blades of the conveying screw, is A developing device satisfying the relationship: 1.5×L+2.74≦P≦0.64×L+9.

74.

2. In the developing device according to claim 1, a developer regulating member that regulates the developer carried on the surface of the developer carrying member; The developing device is characterized in that the developer regulating member is disposed below the developing area in the direction of gravity.

3. 3. The developing device according to claim 1, The developing device is characterized in that the screw pitch P is 7 mm or more and 14 mm or less.

4. 4. The developing device according to claim 3, The developing device is characterized in that the screw pitch P is 8 mm or more and 11 mm or less.

5. 5. The developing device according to claim 1, A developing device characterized in that the minimum distance L between the top of the blade of the conveying screw and the surface of the developer carrier is 3 mm or more.

6. 6. The developing device according to claim 1, The developing device is characterized in that the conveying screw is a three-thread screw.

7. 7. The developing device according to claim 1, a magnetic pole is disposed inside the developer carrier to cause the developer being transported by the transport screw to be carried on the surface of the developer carrier; A developing device characterized in that, in a cross section perpendicular to the rotation axis direction of the conveying screw, the magnetic force in the normal direction of the magnetic pole on the line segment connecting the rotation center of the developer carrier and the rotation center of the conveying screw is -6E-9 [N] or less.

8. 8. The developing device according to claim 1, A developing device, wherein the magnetic carrier has a weight average particle diameter of 20 μm or more and 50 μm or less.

9. In an image forming apparatus, a latent image formed on a latent image carrier is developed by a developing device to form an image, 9. An image forming apparatus, comprising the developing device according to claim 1.

Images