Developing apparatus

Abstract

To provide a configuration in which developer is delivered from a first developing sleeve to a second developing sleeve against the direction of gravity, and the occurrence of an image defect can be prevented.SOLUTION: A first developing sleeve 33 carries and conveys developer to a first developing position where an electrostatic latent image formed on a photoconductor drum 28Y is developed with the developer. A second developing sleeve 34 carries and conveys developer to a second developing position where the electrostatic latent image passing through the first developing position is developed. The axis of rotation of the second developing sleeve 34 is present above the axis of rotation of the first developing sleeve 33 in a vertical direction, and the developer is delivered from the first developing sleeve 33 to the second developing sleeve 34 against the direction of gravity. A plurality of first grooves are formed in an outer peripheral surface of the first developing sleeve 33 along a circumferential direction of the first developing sleeve 33. A plurality of second grooves are formed in an outer peripheral surface of the second developing sleeve 34 along a circumferential direction of the second developing sleeve 34.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] A known developing device configuration is one in which developer is supplied from multiple developing sleeves to an electrostatic latent image formed on an image carrier (Patent Document 1). Another proposed configuration is one in which multiple grooves are provided on the surfaces of a first developing sleeve and a second developing sleeve that supply developer to the electrostatic latent image (Patent Document 2). In the configuration described in Patent Document 2, the first developing sleeve is positioned vertically above the second developing sleeve, and the developer is transferred from the first developing sleeve to the second developing sleeve along the direction of gravity. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-257468 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-217237 Summary of the Invention [Problem to be solved by the invention]

[0004] In an area where high speed and high productivity are required, such as production printing, in order to achieve stable image density, the developer transport performance of the multiple developing sleeves must have a transport capacity above a certain level. If the transport capacity is insufficient, the developer cannot be sufficiently transferred between the multiple developing sleeves, and there is a risk that the developer will be carried around on the developing sleeve. The "carry-around of the developer" refers to a phenomenon in which the developer carried on the developing sleeve is not separated in the separation area where it is to be separated from the developing sleeve, and is transported on the developing sleeve, or a phenomenon in which the developer cannot be transferred between the multiple developing sleeves, and is transported directly on the developing sleeve from which it is to be handed over.

[0005] When the drag phenomenon occurs, a decrease in the TD ratio (the weight ratio of toner to the total weight of carrier and toner) of the developer used in the development process affects the distribution of the TD ratio of the developer in the development container, resulting in image defects such as fluctuations in the color of the toner image in the subsequent image formation process.

[0006] In particular, unlike Patent Document 2, in a configuration in which the developer is transferred from the first developing sleeve to the second developing sleeve against the direction of gravity, the developer transport capacity on the developing sleeve surface has a large effect on the co-rotation.

[0007] An object of the present invention is to provide a configuration in which developer is transferred from a first developing sleeve to a second developing sleeve against the direction of gravity, and in which the occurrence of image defects can be suppressed. [Means for solving the problem]

[0008] One aspect of the present invention is a development device comprising: a development container that contains a developer containing toner and a carrier; a first development rotor to which the developer contained in the development container is supplied, the first development rotor carrying and transporting the developer to a first development position where an electrostatic latent image formed on a rotatable image carrier is developed; a second development rotor that is disposed opposite the first development rotor and to which the developer is transferred from the first development rotor, the second development rotor carrying and transporting the developer to a second development position where the electrostatic latent image that has passed the first development position is developed; and a first magnet that is disposed inside the first development rotor in a non-rotating manner. and a second magnet arranged inside the second developing rotor in a non-rotating fixed manner, wherein the rotation axis of the second developing rotor is higher than the rotation axis of the first developing rotor, the developer is transferred from the first developing rotor to the second developing rotor against the direction of gravity by a magnetic field generated between the first magnet and the second magnet, a plurality of first grooves are formed on the outer peripheral surface of the first developing rotor along the circumferential direction of the first developing rotor, and a plurality of second grooves are formed on the outer peripheral surface of the second developing rotor along the circumferential direction of the second developing rotor. [Effects of the Invention]

[0009] According to the present invention, in a configuration in which the developer is transferred from the first developing sleeve to the second developing sleeve against the direction of gravity, the occurrence of image defects can be suppressed. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus according to a first embodiment. [Figure 2] FIG. 2 is a cross-sectional view showing the schematic configuration of the developing device according to the first embodiment. [Figure 3] FIG. 4 is a diagram illustrating a groove ratio according to the first embodiment. [Figure 4] 10 is a table showing verification results of the configuration according to the first embodiment. [Figure 5]1A is a graph showing the relationship between the developer carrying amount and the groove ratio in the configuration according to the first embodiment, and FIG. 1B is a graph showing the relationship between the developer carrying amount and the peripheral speed of the developing sleeve in the configuration according to the first embodiment. [Figure 6] 10 is a graph for explaining groove pitch unevenness according to the second embodiment. [Figure 7] 10 is a graph showing the verification results of the configuration according to the second embodiment. [Figure 8] 10 is a graph showing the verification results of the configuration according to the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] First Embodiment The first embodiment will be described with reference to Figures 1 to 5(b) First, the schematic configuration of an image forming apparatus according to this embodiment will be described with reference to Figure 1.

[0012] [Image forming equipment] Image forming apparatus 100 is a full-color image forming apparatus, and in this embodiment, it is, for example, an MFP (Multi-Function Peripheral) having copy, printer, and scan functions. As shown in FIG. 1, image forming apparatus 100 has parallel image forming units PY, PM, PC, and PK, which respectively perform image forming processes for four colors of toner images: yellow, magenta, cyan, and black. In this embodiment, image forming apparatus 100 is communicably connected to a host device, such as a document reader connected to the image forming apparatus main body (main body) or a personal computer. Therefore, in accordance with image information from the host device, a four-color full-color image of yellow (Y), magenta (M), cyan (C), and black (K) can be formed on recording material S (recording paper, plastic sheet, cloth, etc.) using electrophotography.

[0013] The image forming units PY, PM, PC, and PK for each color include primary chargers 21Y, 21M, 21C, and 21K, developing units 1Y, 1M, 1C, and 1K, optical writing units (exposure units) 22Y, 22M, 22C, and 22K, photosensitive drums 28Y, 28M, 28C, and 28K, and cleaning units 26Y, 26M, 26C, and 26K. The image forming apparatus 100 also includes a transfer unit 2 and a fixing unit 3. Since the image forming units PY, PM, PC, and PK for each color have the same configuration, the following description will be given using the image forming unit PY as a representative.

[0014] The photosensitive drum 28Y, which serves as an image carrier, is a photosensitive member having a photosensitive layer made of a resin such as polycarbonate containing an organic photoconductor (OPC), and is configured to rotate at a predetermined speed. The primary charger 21Y is made of a corona discharge electrode disposed around the photosensitive drum 28Y, and charges the surface of the photosensitive drum 28Y with generated ions.

[0015] The optical writing unit 22Y incorporates a scanning optical device and exposes the charged photosensitive drum 28Y based on image data, thereby reducing the potential of the exposed area and forming a charge pattern (electrostatic latent image) corresponding to the image data. The developing device 1Y transfers the contained developer to the photosensitive drum 28Y to develop the electrostatic latent image formed on the photosensitive drum 28Y. The developer is a mixture of carrier and toner corresponding to each color, and the electrostatic latent image is visualized by the toner.

[0016] The transfer device 2 has primary transfer rollers 23Y, 23M, 23C, and 23K, an intermediate transfer belt 24, and a secondary transfer roller 25. The intermediate transfer belt 24 is wound around the primary transfer rollers 23Y, 23M, 23C, and 23K and multiple rollers and is supported so that it can run. The primary transfer rollers 23Y, 23M, 23C, and 23K, which serve as primary transfer members, correspond to the colors Y (yellow), M (magenta), C (cyan), and K (black), respectively, from top to bottom in FIG. 1. The secondary transfer roller 25 is disposed outside the intermediate transfer belt 24 and is configured so that the recording material can pass between it and the intermediate transfer belt 24.

[0017] The toner images of each color formed on the photosensitive drums 28Y, 28M, 28C, and 28K are transferred (primary transfer) onto the intermediate transfer belt 24 by the action of a primary transfer bias applied to the primary transfer rollers 23Y, 23M, 23C, and 23K at a primary transfer portion (primary transfer nip) T1 where the intermediate transfer belt 24 and the photosensitive drums 28Y, 28M, 28C, and 28K come into contact. For example, in the case of a four-color full-color image, the toner images are transferred onto the intermediate transfer belt 24 in order from the photosensitive drum 28Y, and a color toner image in which yellow, magenta, cyan, and black layers are superimposed is formed.

[0018] Meanwhile, recording material S stored in a cassette 110 serving as a recording material storage unit is transported toward transfer device 2 via pickup roller 111 and registration roller 112. Recording material S is transported in synchronization with the toner image on intermediate transfer belt 24 to secondary transfer unit (nip unit) T2, where intermediate transfer belt 24 and secondary transfer roller 25 serving as a secondary transfer member come into contact. The toner image formed on intermediate transfer belt 24 is then secondarily transferred onto recording material S at secondary transfer unit T2 by the action of a secondary transfer bias applied to secondary transfer roller 25. Pressure and heat are applied to the recording material onto which the toner image has been transferred in fixing device 3. This melts the toner on the recording material, and the color image is fixed to the recording material. Thereafter, recording material S is discharged outside the apparatus.

[0019] When forming images on both sides of the recording material, the recording material S that has passed through the fixing device 3 is conveyed to a reverse conveying path 113, and the recording material S that has been turned over is conveyed to registration rollers 112 by conveying rollers 114, and in the secondary transfer portion T2, a toner image is transferred to the back side of the recording material S in the same manner as described above. Then, again in the fixing device 3, the toner image is fixed to the back side of the recording material S.

[0020] After the primary transfer step, toner and other deposits remaining on the photosensitive drums 28Y, 28M, 28C, and 28K are collected by cleaning devices 26Y, 26M, 26C, and 26K. This prepares the photosensitive drums 28Y, 28M, 28C, and 28K for the next image forming step. Furthermore, toner and other deposits remaining on the intermediate transfer belt 24 after the secondary transfer step are removed by an intermediate transfer belt cleaner 29.

[0021] The image forming apparatus 100 of this embodiment can also form a monochrome or multicolor image using image forming units for a desired color, such as a black monochrome image, or for several of the four colors. While the image forming units PY, PM, PC, and PK for each color are vertically arranged in FIG. 1 , they may also be arranged horizontally or diagonally. Furthermore, in this embodiment, the outer diameter of the photosensitive drums 28Y, 28M, 28C, and 28K is, for example, 80 mm, and the image forming operation is performed while rotating at a peripheral speed of 513 mm / sec.

[0022] Developer storage units 27Y, 27M, 27C, and 27K are provided corresponding to developing devices 1Y, 1M, 1C, and 1K, respectively, and are filled with replaceable bottles containing developers corresponding to the respective colors of yellow, magenta, cyan, and black, from top to bottom. Developer storage units 27Y, 27M, 27C, and 27K are configured to be able to transport (supply) developers to developing devices 1Y, 1M, 1C, and 1K corresponding to the colors of the developers stored therein.

[0023] For example, the toner weight ratio of the developer stored in the bottle is 90 to 98%, and the toner weight ratio of the developer in the developing devices 1Y, 1M, 1C, and 1K is 5 to 11%. Therefore, when toner is consumed by development in the developing devices 1Y, 1M, 1C, and 1K, developer containing toner corresponding to the consumed amount is replenished, and the toner weight ratio of the developer in the developing devices 1Y, 1M, 1C, and 1K is maintained constant.

[0024] [Developing device] Next, developing devices 1Y, 1M, 1C, and 1K will be described in detail using Figure 2. Since developing devices 1Y, 1M, 1C, and 1K have the same configuration, developing device 1Y will be described below as a representative. As shown in Figure 2, developing device 1Y has a first developing roller 30, a second developing roller 31, a peeling roller 32, a developer supply screw 42, a developer stirring screw 43, and a developer recovery screw 44, and these components are contained in a developing container 60. The developing container 60 contains a two-component developer containing non-magnetic toner and magnetic carrier.

[0025] The first developing roller 30 is a developer carrier that is driven to rotate and is disposed adjacent to the photosensitive drum 28Y so that its rotation axis is substantially parallel to the rotation axis of the photosensitive drum 28Y. The first developing roller 30 includes a rotating first developing sleeve (first developing rotor) 33 and a first developing magnet (fixed magnet) 36 that is disposed non-rotatingly inside the first developing sleeve 33 and serves as a first magnet that magnetically attracts developer to the surface of the first developing sleeve 33. The first developing roller 30 (first developing sleeve 33) magnetically attracts (carries) the developer pumped up by a developer supply screw 42 that serves as a first transport screw, and develops the electrostatic latent image formed on the rotatable photosensitive drum 28Y (image carrier) with the developer at the first development position. That is, the first developing sleeve 33 carries and transports the developer to develop the electrostatic latent image formed on the photosensitive drum 28Y.

[0026] The first developing sleeve 33 is a non-magnetic cylindrical member and is driven to rotate around a rotation shaft 39. The rotation direction of the first developing sleeve 33 is clockwise as indicated by the arrow in FIG. 2, which is opposite to the rotation direction of the photosensitive drum 28Y in this embodiment. Therefore, the first developing sleeve 33 and the photosensitive drum 28Y rotate in the same direction at positions where they face each other (facing portions). That is, the first developing sleeve 33 rotates such that the surface facing the photosensitive drum 28Y moves vertically from below toward above.

[0027] The first developing magnet 36 is disposed inside the first developing sleeve 33, and has a plurality of magnetic poles as peaks of magnetic flux density in the normal direction, and non-magnetic pole portions, as shown in Fig. 2. A space is disposed between the inner periphery of the first developing sleeve 33 and the outer periphery of the first developing magnet 36 to allow the first developing sleeve 33 to rotate.

[0028] The developer attracted to the first developing sleeve 33 is coated in a thin layer on the surface of the first developing sleeve 33 by the action of the regulating member 50 and the magnetic field generated by the magnetic pole S1 of the first developing magnet 36 contained in the first developing roller 30 as the first developing sleeve 33 rotates. The developer coated in a thin layer on the first developing sleeve 33 is then transported toward the photosensitive drum 28Y, where it develops the latent image formed on the photosensitive drum 28Y by the magnetic field generated by the first developing magnetic pole N3. After the latent image formed on the photosensitive drum 28Y is developed, the developer on the first developing sleeve 33 is transported to the vicinity of the second developing roller 31 as the first developing sleeve 33 rotates. Then, near the closest position between the first developing roller 30 and the second developing roller 31, a magnetic field is generated between the delivery magnetic pole N4 of the first developing magnet 36 contained in the first developing roller 30 and the receiving magnetic pole S4 of the second developing magnet 37 contained in the second developing roller 31, causing the developer on the first developing sleeve 33 to be peeled off from the first developing sleeve 33 and transferred onto the second developing sleeve 34. The receiving magnetic pole S4 of the second developing magnet 37 and the delivery magnetic pole N4 of the first developing magnet 36 are in a polarity opposite to each other.

[0029] The second developing roller 31, which serves as a developing roller, is a developer carrier that is driven to rotate, and is disposed facing the first developing roller 30 downstream of the first developing roller 30 in the rotation direction of the photosensitive drum 28Y. The second developing roller 31 is disposed vertically above the center of rotation of the first developing roller 30, and receives developer from the first developing roller 30 by magnetic force. Like the first developing roller 30, the second developing roller 31 is disposed adjacent to the photosensitive drum 28Y so that its rotation axis is substantially parallel to the rotation axis of the photosensitive drum 28Y. Therefore, the rotation axes of the second developing roller 31 and the first developing roller 30 are substantially parallel to each other.

[0030] The second developing roller 31 includes a rotating second developing sleeve (second developing rotor) 34 and a second developing magnet (stationary magnet) 37, which is disposed non-rotatingly inside the second developing sleeve 34 and serves as a second magnet for attracting developer to the surface of the second developing sleeve 34 by magnetic force. The rotation center (rotation axis) R2 of the second developing sleeve 34 is located vertically above the rotation center (rotation axis) R1 of the first developing sleeve 33. The second developing roller 31 (second developing sleeve 34) receives the developer from the first developing roller 30 (first developing sleeve 33) by magnetic force. In this embodiment, the developer is transferred from the first developing sleeve 33 to the second developing sleeve 34 against the direction of gravity. The second developing sleeve 34 attracts (carries) the developer transferred from the first developing sleeve 33 and develops the electrostatic latent image formed on the rotating photosensitive drum 28Y with the developer at the second developing position. That is, the second developing sleeve 34 carries and transports the developer to develop the electrostatic latent image formed on the photosensitive drum 28Y. A peeling roller 32, which will be described later, is located on the side of the second developing roller 31.

[0031] The second developing sleeve 34 is a non-magnetic cylindrical member and is driven to rotate around a rotation shaft 40. The rotation direction of the second developing sleeve 34 is clockwise as indicated by the arrow in FIG. 2, which is opposite to the rotation direction of the photosensitive drum 28Y in this embodiment. Therefore, the second developing sleeve 34 and the photosensitive drum 28Y rotate in the same direction when they face each other. That is, the second developing sleeve 34 rotates such that the surface facing the photosensitive drum 28Y moves vertically from below to above. Furthermore, the second developing sleeve 34 and the first developing sleeve 33 rotate in opposite directions when they face each other.

[0032] The second developing magnet 37 is disposed inside the second developing sleeve 34, and has a plurality of magnetic poles as peaks of magnetic flux density in the normal direction, and non-magnetic pole portions, as shown in Fig. 2. A space is disposed between the inner periphery of the second developing sleeve 34 and the outer periphery of the second developing magnet 37 to allow rotation of the second developing sleeve 34.

[0033] The developer that is transferred from the delivery pole N4 of the first developing magnet 36 to the receiving pole S4 of the second developing magnet 37 and attracted onto the second developing sleeve 34 is transported toward the photosensitive drum 28Y by the rotation of the second developing sleeve 34, and the latent image formed on the photosensitive drum 28Y is developed by the second developing pole S5. After the latent image formed on the photosensitive drum 28Y is developed, the developer remaining on the second developing sleeve 34 is transported to the vicinity of the peeling roller 32 by the rotation of the second developing sleeve 34. Then, near the closest position between the second developing roller 31 and the peeling roller 32, the developer is transferred from the second developing sleeve 34 to the peeling sleeve 35 of the peeling roller 32 by a magnetic field generated between the delivery pole S7 of the second developing magnet 37 contained in the second developing roller 31 and the receiving pole N9 of the peeling magnet 38 contained in the peeling roller 32. The receiving magnetic pole N9 of the peeling magnet 38 and the handover magnetic pole S7 of the second developing magnet 37 are opposite in polarity.

[0034] Here, the transfer magnetic pole N4 of the first developing magnet 36 and the receiving magnetic pole S4 of the second developing magnet 37 are each located closer to the photosensitive drum 28Y than a straight line passing through the rotation center R1 of the first developing sleeve 33 and the rotation center R2 of the second developing sleeve 34. In other words, the transfer magnetic pole N4 of the first developing magnet 36 is preferably located upstream of the opposing portion of the first developing sleeve 33 and the second developing sleeve 34 in the rotation direction of the first developing sleeve 33. Furthermore, the receiving magnetic pole S4 of the second developing magnet 37 is preferably located downstream of the opposing portion of the first developing sleeve 33 and the second developing sleeve 34 in the rotation direction of the second developing sleeve 34. Furthermore, in order to transfer the developer from the first developing sleeve 33 to the second developing sleeve 34 against gravity, the absolute value of the peak value of the magnetic flux density in the normal direction of the developing magnet is preferably greater for the transfer magnetic pole N4 of the first developing sleeve 33 than for the receiving magnetic pole S4 of the second developing sleeve 34.

[0035] The peeling roller 32 is disposed on the opposite side of the photosensitive drum 28Y with respect to the rotation center R2 of the second developing sleeve 34, and peels off the developer from the second developing roller 31 after the electrostatic latent image on the photosensitive drum 28Y is developed by the second developing roller 31. Specifically, the peeling roller 32 is a developer carrier that is driven to rotate, and is disposed between the second developing roller 31 and the developer recovery screw 44 so that its rotation center R3 is vertically above the rotation center R2 of the second developing roller 31.

[0036] The peeling roller 32 is disposed such that its rotation axis is substantially parallel to the rotation axis of the second developing roller 31. The peeling roller 32 includes a rotating peeling sleeve 35 and a peeling magnet (fixed magnet) 38 that is non-rotatingly disposed inside the peeling sleeve 35 and magnetically attracts developer to the surface of the peeling sleeve 35, and is configured to transfer developer from the second developing roller 31 based on the magnetic force. That is, the peeling sleeve 35 is disposed facing the second developing sleeve 34 downstream of the region where the second developing sleeve 34 faces the photosensitive drum 28Y and upstream of the region where the second developing sleeve 34 faces the first developing sleeve 33, with respect to the rotation direction of the second developing sleeve 34. The magnetic field generated by the second developing magnet 37 peels off the developer from the second developing sleeve 34 after the electrostatic latent image has been developed by the second developing sleeve 34, and the peeled developer is carried and transported.

[0037] The stripping sleeve 35 is a non-magnetic cylindrical member, and is driven to rotate around a rotation shaft 41. The rotation direction of the stripping sleeve 35 is counterclockwise as indicated by the arrow in FIG. 2, which is opposite to the rotation direction of the second developing sleeve 34 in this embodiment. Therefore, the stripping sleeve 35 and the second developing sleeve 34 rotate in the same direction (forward direction) at positions where they face each other (facing portions).

[0038] The peeling magnet 38 is disposed inside the peeling sleeve 35 and has multiple magnetic poles as peaks of magnetic flux density in the normal direction and non-magnetic pole portions, as shown in Fig. 2. A space is disposed between the inner periphery of the peeling sleeve 35 and the outer periphery of the peeling magnet 38 to allow rotation of the peeling sleeve 35.

[0039] The developer attracted onto the peeling sleeve 35 by the receiving magnetic pole N9 of the peeling magnet 38 contained in the peeling roller 32 is transported downstream in the rotation direction by the rotation of the peeling sleeve 35. The developer transported downstream in the rotation direction is peeled off from the peeling sleeve 35 by a non-magnetic pole portion formed between the peeling magnetic pole N8 and the magnetic pole N10 of the peeling magnet 38 contained in the peeling roller 32 at a position close to the developer recovery screw 44, and falls under its own weight toward a guide member 45 located vertically below. The developer that has fallen onto the guide member 45 is then guided under its own weight toward the developer recovery screw 44.

[0040] The guide member 45 and the developer recovery screw 44 constitute a developer recovery section 47 as a recovery section that recovers the developer peeled off from the peeling sleeve 35 on the peeling roller 32. In the developer recovery section 47, the developer recovery screw 44 is positioned below the center of rotation of the peeling roller 32 in the vertical direction, and conveys the developer handed over (recovered) from the peeling roller 32 while stirring it.

[0041] The guide member 45 serving as a guide section is disposed vertically below the center of rotation of the peeling roller 32, and guides the developer peeled off by the peeling roller 32 toward the developer recovery screw 44. Such a guide member 45 has an inclined surface 45a along which the developer slides down under its own weight, in order to more reliably guide the peeled off developer toward the developer recovery screw 44. The inclined surface 45a is inclined with respect to the horizontal direction so that the developer recovery screw 44 side is lower than the position below the peeling roller 32.

[0042] The developer recovery screw 44, which serves as a recovery member and a transport unit, transports the recovered developer to a developer circulation unit 46, which will be described next. That is, the developer recovery screw 44 is a screw transport member used to transport the recovered developer in one direction while stirring it as it slides down the inclined surface of the guide member 45.

[0043] The developer circulating section 46 is a supply section for supplying the developer to the first developing roller 30, and includes a regulating member 50, a developer supply screw 42, and a developer stirring screw 43. In the developer circulating section 46, the developer is stirred in the developer supply screw 42 and the developer stirring screw 43 and transported in a substantially horizontal direction, and then supplied to the first developing roller 30. As described above, the developer collected by the developer collecting section 47 falls under its own weight and is introduced into the developer circulating section 46. That is, the developer circulating section 46 is positioned lower than the developer collecting section 47 in the vertical direction.

[0044] The developer supply screw 42, developer stirring screw 43, and developer recovery screw 44 are screw conveying members that convey the developer in one direction while stirring it, and the developer supply screw 42 and developer stirring screw 43 are located vertically below the rotation center of the developer recovery screw 44. The developer supply screw 42, developer stirring screw 43, and developer recovery screw 44 are also arranged so that their rotation axes are substantially parallel to each other. The rotation axes of these screws are also substantially parallel to the rotation axis of the first developing roller 30.

[0045] The developer supply screw 42 is located between the first developing roller 30 and the developer stirring screw 43, and a partition wall 48 of the developing container 60 is disposed between the developer supply screw 42 and the developer stirring screw 43. The partition wall 48 of the developing container 60 extends along the rotational axis direction of the developer supply screw 42 and the developer stirring screw 43. The partition wall 48 is provided with a communication port (not shown) that communicates a first conveying path 61, through which the developer is conveyed by the developer supply screw 42, with a second conveying path 62, through which the developer is conveyed by the developer stirring screw 43.

[0046] The developer stirred by the developer recovery screw 44 passes through a communication port (not shown) formed in a partition wall 63 of the developing container 60 located between the developer recovery screw 44 and the developer supply screw 42, and falls by its own weight toward the developer supply screw 42. The above-mentioned guide member 45 is formed integrally with the partition wall 63, and the developer recovery screw 44 is disposed above the partition wall 63.

[0047] The position of the communication port through which the developer stirred by the developer recovery screw 44 falls under its own weight and is introduced into the developer circulation section 46 is preferably arranged to avoid the area where the developer is supplied toward the first developing roller 30 (the middle part with respect to the rotational axis direction of the developer supply screw 42). In this embodiment, the communication port is positioned at a position included in the range of the downstream end (terminal end) in the developer transport direction of the first transport path 61 in which the developer supply screw 42 is arranged.

[0048] The developer transport directions of the developer supply screw 42 and the developer agitation screw 43 are opposite to each other. The start side (upstream end side in the developer transport direction) and end side (downstream end side in the developer transport direction) of the first transport path 61 in which the developer supply screw 42 is arranged communicate with the end side and start side of the second transport path 62 in which the developer agitation screw 43 is arranged via communication ports provided in the partition wall 48. Therefore, the developer circulates in the rotation direction of the developer supply screw 42 and the developer agitation screw 43 indicated by the arrows in FIG. 2 and in a substantially horizontal direction within the developing container 60, and a portion of the developer is supplied toward the first developing roller 30.

[0049] The developer supply port 51 (see FIG. 2) is disposed above the developer stirring screw 43 in the developing container 60, and is connected to the developer storage unit 27Y (see FIG. 1). The developer supply port 51 is configured so that the developer stored in a bottle loaded in the developer storage unit 27Y can be supplied to the second conveying path 62 in which the developer stirring screw 43 is disposed.

[0050] As described above, the toner weight ratio of the developer stored in the bottle of developer storage section 27Y is greater than the toner weight ratio of the developer in developing device 1Y, so by adjusting the developer supplied to developer stirring screw 43, it is possible to maintain a constant toner weight ratio of the developer in developing device 1Y.

[0051] The toner concentration detection sensor 49 (see FIG. 2) is arranged to detect the toner concentration in the developer contained in the developer circulation unit 46. The toner concentration detection sensor 49 is a sensor that detects the magnetic permeability of the developer. The toner concentration corresponds to the amount of toner consumed in the developing device 1Y, and is therefore used to control the supply of developer from the developer storage unit 27Y. For example, when it is detected that the toner concentration has dropped below a predetermined value, developer is replenished from the developer storage unit 27Y. Note that the magnetic permeability of the developer changes depending on the toner concentration, and therefore the toner concentration can be detected using the magnetic permeability.

[0052] The regulating member 50 is disposed adjacent to the first developing roller 30 and is used to regulate the amount of developer supplied from the developer circulating unit 46 to the first developing roller 30. The regulating member 50 can be configured to regulate the amount of developer attracted to the first developing roller 30 based on, for example, the gap between the surface of the first developing sleeve 33 of the first developing roller 30 and the end of the regulating member 50.

[0053] The developer circulation path in the developing container 60 is such that the developer is transported in a substantially horizontal direction while being stirred in the developer circulation section 46, and then supplied to the first developing roller 30, and is transferred from the first developing roller 30 to the second developing roller 31 above by magnetic force. Next, the developer is transferred from the second developing roller 31 to the peeling roller 32 on the side of the second developing roller 31 by magnetic force again, and then is peeled off from the peeling roller 32 by a peeling magnet 38 contained in the peeling roller 32, and then is collected in the developer collection section 47 and introduced into the developer circulation section 46 again.

[0054] As described above, this embodiment uses a two-component development system, and the developer is a mixture of negatively charged non-magnetic toner and magnetic carrier. The non-magnetic toner is a resin such as polyester or styrene acrylic that contains colorants, wax components, etc., and is then pulverized or polymerized to form a powder, to which fine powders such as titanium oxide and silica are added. The magnetic carrier is a core made of resin particles kneaded with ferrite particles or magnetic powder, with a resin coating on the surface. In this embodiment, the toner concentration in the initial developer (the weight ratio of toner contained in the developer) is 8%.

[0055] Generally, two-component development systems using toner and carrier charge both to a predetermined polarity through frictional contact between the toner and carrier, which means that the toner is subjected to less stress than single-component development systems using single-component developers. However, over long periods of use, the amount of dirt (spent) adhering to the carrier surface increases, gradually reducing the toner's ability to charge. This results in problems such as fogging and toner scattering. While increasing the amount of carrier contained in the development device is one way to extend the life of a two-component development device, this is undesirable because it increases the size of the development device.

[0056] To solve the above problems associated with two-component developers, this embodiment employs an ACR (Auto Carrier Refresh) system. The ACR system suppresses an increase in degraded carrier by gradually replenishing new developer from the developer storage unit 27Y into the developing device 1Y and gradually discharging developer with degraded charging performance from a discharge port (not shown) of the developing device 1Y. This allows the degraded carrier in the developing device 1Y to be gradually replaced with new carrier, making it possible to maintain the charging performance of the carrier in the developing device 1Y at a substantially constant level.

[0057] In the developing device 1Y of this embodiment configured as described above, the developer in the first conveying path 61 is supplied to the first developing sleeve 33 by the developer supply screw 42, and a predetermined amount of the developer supplied to the first developing sleeve 33 is carried on the first developing sleeve 33 by the magnetic field generated by the first developing magnet 36, forming a developer reservoir. As the first developing sleeve 33 rotates, the two-component developer on the first developing sleeve 33 passes through the developer reservoir and is coated as a thin layer on the surface of the first developing sleeve 33 by the regulating member 50, and is transported to the developing region facing the photosensitive drum 28Y. In the developing region, the developer on the first developing sleeve 33 stands up to form magnetic chains.

[0058] In the first developing region where the first developing sleeve 33 and the photosensitive drum 28Y face each other, the electrostatic latent image formed on the photosensitive drum 28Y is visualized by the developing bias applied to the first developing sleeve 33. In this embodiment, the developing bias applied to the first developing sleeve 33 has a waveform in which both an AC electric field and a DC electric field are superimposed, but a developing bias of only a DC electric field may also be used.

[0059] After the two-component developer is subjected to the development process in the first development zone, it is transferred to the second development sleeve 34 at a position close to the second development sleeve 34 and transported to the second development zone where the second development sleeve 34 faces the photosensitive drum 28Y. In the second development zone, the same development bias as in the first development zone is applied, and the toner that is insufficient relative to the potential of the electrostatic latent image on the photosensitive drum 28Y is developed to compensate for the toner, and the excess toner is recovered, thereby making the toner image uniform. Here, the development bias applied to the first development sleeve 33 and the development bias applied to the second development sleeve 34 may have different waveforms.

[0060] The developer that has passed through the second developing region is peeled off in a peeling magnetic field region formed by a second developing magnet 37 contained in the second developing sleeve 34. The developer peeled off from the second developing sleeve 34 is attracted to the surface of the peeling sleeve 35 by a magnetic field formed by a peeling magnet 38 contained in the peeling sleeve 35 of the peeling roller 32, and is transported along the rotation direction of the peeling sleeve 35. The developer is then detached from the surface of the peeling sleeve 35 by the peeling magnetic field formed by the peeling magnet 38, and is collected in a developer collecting section 47.

[0061] To form high-quality images using the developing device 1Y configured as described above, it is necessary to prevent leaks when transferring developer from the first developing sleeve 33 to the second developing sleeve 34. If leaks occur during this transfer, the developer will spill vertically downward from the separation region formed by the first developing magnet 36 of the first developing sleeve 33. If this happens, the developer will fall near the regulating member 50 and be immediately supplied from the first conveying path 61 to the first developing sleeve 33 without being agitated in the developer circulation path. This results in developer with an uneven amount of toner being supplied to the developing process, causing fluctuations in density.

[0062] Furthermore, in order to form high-quality images using the developing device 1Y, it is necessary to be able to transfer the developer between the developing sleeves without the developer being carried around. If the developer transport capacity of each of the multiple developing sleeves is insufficient, the developer will not be transferred sufficiently between the developing sleeves and will be carried around, and as described above, there is a risk that the carried around developer will fall near the regulating member 50.

[0063] The above-described phenomenon is more likely to occur in a configuration in which the restricting member 50 is disposed below the developer transport path and the developer is transported upward via multiple developing sleeves against gravity, particularly in a configuration in which the image formation speed (process speed) is high. Therefore, in such a configuration, each developing sleeve is particularly required to have an appropriate developer transport capacity. In this embodiment, the first developing sleeve 33 operates at a peripheral speed of 513 mm / sec, the same as the photosensitive drum 28Y, and the second developing sleeve 34 operates at a peripheral speed of 616 mm / sec.

[0064] [Surface structure of the first and second developing sleeves] Therefore, in this embodiment, the co-rotation of the developer is suppressed by optimizing the surface shapes of the first developing sleeve 33 and the second developing sleeve 34. First, the surface shapes of the first developing sleeve 33 and the second developing sleeve 34 will be described.

[0065] In this embodiment, a plurality of grooves are formed on the surface of the first developing sleeve 33 and the surface of the second developing sleeve 34, aligned in the circumferential direction. That is, a plurality of first grooves are formed on the outer peripheral surface of the first developing sleeve 33 along the circumferential direction of the first developing sleeve 33, and a plurality of second grooves are formed on the outer peripheral surface of the second developing sleeve 34 along the circumferential direction of the second developing sleeve 34. The plurality of grooves are formed parallel to the longitudinal direction (the direction of the rotational axis of the developing sleeve) on the surface of each developing sleeve. The plurality of grooves are also formed by a cutting method. Furthermore, in this embodiment, the shape of the plurality of grooves (hereinafter also referred to as "groove shape") is V-shaped in cross section perpendicular to the longitudinal direction. Note that the groove shape may also be U-shaped, trapezoidal, or have a concave corner with an edge. The groove processing method is not limited to cutting, and other processing methods such as etching and pressing may also be used.

[0066] The "groove ratio" is defined as an index relating to the transport capacity of the groove shape on the surface of the developing sleeve. As shown in Figure 3, the groove ratio ρ is defined by the following equation 1, where N is the number of grooves 121 per revolution of a certain developing sleeve 120, d is the width per groove 121, and L is the circumferential length of the developing sleeve 120. ρ=(d×N) / L (Formula 1)

[0067] Specifically, when the width of the first grooves of the first developing sleeve 33 is d1, the number of first grooves per revolution of the first developing sleeve 33 is N1, and the circumferential length of the first developing sleeve 33 is L1, the groove ratio ρ1 of the first grooves is ρ1=(d1×N1) / L1. Similarly, when the width of the second grooves of the second developing sleeve 34 is d2, the number of second grooves per revolution of the second developing sleeve 34 is N2, and the circumferential length of the second developing sleeve 34 is L2, the groove ratio ρ2 of the second grooves is ρ2=(d2×N2) / L2.

[0068] In this case, a larger groove ratio ρ results in a larger developer transport capacity, while a smaller groove ratio ρ results in a smaller developer transport capacity. Furthermore, since the groove ratio ρ is the ratio of the groove width per revolution of the developing sleeve 120, the developer transport capacity can be expressed uniquely regardless of the outer diameter of the developing sleeve 120. To give an example of a basic configuration, when a developing sleeve 120 with an outer diameter of 25 mm, a groove width d of 0.144 mm, and a number of grooves N of 62 is used, the groove ratio ρ is 11.4%. Depending on the physical properties and fluidity of the developer, the groove ratio ρ often falls within a range of approximately 10% to 21% (10% or more and 21% or less).

[0069] On the other hand, the rotational speeds of the first developing sleeve 33 and the second developing sleeve 34 are also indices that greatly contribute to the developer transport capacity, and it is necessary to consider the developer transport capacity together with the above-mentioned groove ratio ρ. In this embodiment, since multiple developing sleeves facing the same photosensitive drum 28Y are considered, the contribution of the rotational speeds of the first developing sleeve 33 and the second developing sleeve 34 to the developer transport force can be expressed as a peripheral speed ratio with respect to the photosensitive drum 28Y.

[0070] Here, the developer transport force C of the developing sleeve is defined as the following formula 2. C=ρ×rate_v (Formula 2) Here, rate_v is the peripheral speed ratio of the developing sleeve to the photosensitive drum 28Y. Although the peripheral speed ratio is used because there are multiple developing sleeves in contact with the same photosensitive drum 28Y, there is no problem in calculating the conveying force using the peripheral speed.

[0071] When the groove ratio ρ is large, the developing sleeve surface has a higher ability to grip the developer, and the magnetic chains of the developer are more able to follow the rotation of the developing sleeve. Therefore, when a thin layer is coated on the developing sleeve by the regulating member 50, the larger the groove ratio ρ, the larger the amount of developer per unit area M / S after the thin layer is formed.

[0072] Here, if the developer transport force of the first developing sleeve 33 is C1, the developer transport force of the second developing sleeve 34 is C2, the groove ratio of the first developing sleeve 33 is ρ1, the groove ratio of the second developing sleeve 34 is ρ2, the circumferential speed ratio of the first developing sleeve 33 is rate_v1, and the circumferential speed ratio of the second developing sleeve 34 is rate_v2, then the transport force C1 of the first developing sleeve 33 and the transport force C2 of the second developing sleeve 34 are expressed by the following equations 3 and 4. C1=ρ1×rate_v1 (Formula 3) C2=ρ2×rate_v2 (Formula 4)

[0073] Furthermore, when an image is formed on the photosensitive drum 28Y using multiple developing sleeves, the upstream developing sleeve (first developing sleeve 33 in this embodiment) roughly completes the development process relative to the rotation of the photosensitive drum 28Y, while the downstream developing sleeve (second developing sleeve 34 in this embodiment) adjusts the unevenness of the toner layer, which can cause image defects such as density differences, edge voids, and emphasis. Therefore, the peripheral speed ratios of the first developing sleeve 33 and the second developing sleeve 34 may be changed. When there is a problem with low density, such as edge voids, the peripheral speed ratio of the second developing sleeve 34 is set to a higher value. Conversely, when there is a problem with high density, such as edge emphasis, the peripheral speed ratio of the second developing sleeve 34 is set to a lower value. In this embodiment, the default peripheral speed ratios are set as follows: the peripheral speed ratio rate_v1 of the first developing sleeve 33 is 1.0, and the peripheral speed ratio rate_v2 of the second developing sleeve 34 is 1.2.

[0074] Next, the results of verifying the phenomenon of developer entrainment on the first developing sleeve 33 will be described. In this verification, the developing device 1Y and the photosensitive drum 28Y were fixed in the same positional relationship as in the main body, rather than the image forming apparatus 100, and were installed on a jig that can drivably support them. Then, the phenomenon of developer entrainment on the first developing sleeve 33 was verified. In addition, a high-voltage power supply was connected to each of the photosensitive drum 28Y and the first developing sleeve 33 to enable the development process.

[0075] The surface of the photosensitive drum 28Y was set to 0 V, and the first developing sleeve 33 was configured to perform analog development by applying a DC bias superimposed on an AC bias formed by a blank pulse of one wavelength every two waves at a peak-to-peak voltage Vpp of 1.4 kV and a frequency of 11 kHz. The DC bias simulated printing on A3-size paper by switching from a -150 V DC bias for the 813 msec period corresponding to the image area to a +150 V DC bias for the 187 msec period corresponding to the paper gap, thereby reproducing operation similar to actual printing. The paper gap refers to the interval between multiple recording materials transported consecutively to the secondary transfer unit. In this test, the interval between A3-size sheets was simulated. A urethane blade was installed on the photosensitive drum 28Y to clean toner adhering to the surface, allowing the developed toner to be collected, enabling continuous operation.

[0076] Using the above-described verification configuration, the developer entrainment on the first developing sleeve 33 was observed by installing a miniature video camera manufactured by ENABLE, INC. at a position facing the gap between the first developing sleeve 33 and the second developing sleeve 34 inside the developing container.

[0077] Fig. 4 shows the results of examining whether or not the co-rotation phenomenon occurs in the first developing sleeve 33 with respect to C2 / C1, which is the relationship between the conveying force C1 of the first developing sleeve 33 and the conveying force C2 of the second developing sleeve 34, with the above-described configuration. In the "co-rotation" column of Fig. 4, "◯" indicates that co-rotation did not occur at all, "△" indicates that co-rotation occurred slightly, and "×" indicates that co-rotation occurred. In addition, in the "whiteout / edge emphasis" column, "◯" indicates that edge whiteout or emphasis did not occur at all, "△" indicates that edge whiteout or emphasis occurred slightly, and "×" indicates that edge whiteout or emphasis occurred.

[0078] As shown in Figure 4, the relationship between the ratio C2 / C1 of the conveying forces C1 and C2 calculated from the peripheral speed ratios rate_v1 and rate_v2 and the groove ratios ρ1 and ρ2 using Equation 3 and Equation 4 is as follows: C2 / C1≧0.70 If the above condition is satisfied, it is possible to sufficiently transfer the developer from the first developing sleeve 33 to the second developing sleeve 34, and it has been confirmed that this configuration is less likely to cause co-rotation in the first developing sleeve 33. In this embodiment, as described above, the peripheral speed ratio rate_v1 of the first developing sleeve 33 is set to 1.0, and the peripheral speed ratio rate_v2 of the second developing sleeve 34 is set to 1.2, so that the ratio C2 / C1 of the conveying forces C1 and C2 is 0.7, which is a combination of groove ratios ρ1 of 19.8% and ρ2 of 11.6%.

[0079] On the other hand, when the ratio C2 / C1 of the conveying forces becomes smaller than 0.6, which is smaller than 0.7, not only does the dragging occur but also white spots at the boundary between the light and dark areas of the latent image step tend to appear on the image, resulting in a significant impact on the image. This is because the effect of the second developing sleeve 34 in the developing process of adjusting the unevenness of the toner layer potential after development by the first developing sleeve 33 relative to the latent image potential on the photosensitive drum 28Y is weakened.

[0080] In addition, the ratio C2 / C1 of the conveying forces is 0.7, which is a condition in which the conveying force of the second developing sleeve 34 is high. C2 / C1≧0.95 It was found that by satisfying the above condition, a sufficient margin can be secured against the dragging and blank images. In this way, it is preferable that the conveying force C2 of the second developing sleeve 34 is equal to or greater than the conveying force C1 of the first developing sleeve 33, and a more preferable condition is as follows: C2 / C1>1.0 This is a configuration that satisfies the above.

[0081] This is because developer is transferred from the first developing magnet 36 to the second developing magnet 37 against gravity via opposing magnetic poles (the transfer pole N4 of the first developing magnet 36 and the receiving pole S4 of the second developing magnet 37). Therefore, if the transport force of the second developing sleeve 34 on the downstream side in the transport direction is less than the transport force of the first developing sleeve 33 on the upstream side in the transport direction, a large amount of developer tends to accumulate in the transfer area between the developing sleeves. Normally, as the amount of developer accumulation increases, the contact transfer area with the second developing sleeve 34 increases, ensuring the amount of developer transport. However, this can increase the torque at the second developing sleeve 34, accelerating developer degradation. In extreme cases, developer may flow back toward the upstream side of the first developing sleeve 33 in the transport direction. When the transport force ratio C2 / C1 is 0.7 or greater, extreme problems such as developer backflow do not occur.

[0082] Here, we will explain the levels of developer deterioration shown in the table in Figure 4. The level of developer deterioration was measured by measuring the charge amount of the developer when 1000k (1000 x 1000) print images with an image ratio of 10% were printed, and was evaluated by the amount of decrease in toner charge amount from the initial state. Regarding the levels of developer deterioration shown in the table in Figure 4, a level where almost no deterioration has occurred is indicated by "◯", a level where there is some deterioration but it is acceptable is indicated by "△", and a level where there is deterioration and it is unacceptable is indicated by "X".

[0083] When the conveying force ratio C2 / C1 was 1.05 or greater, the toner charge amount decreased by almost the same amount with use (the level of developer deterioration was "Good"). On the other hand, when the conveying force ratio C2 / C1 was between 0.7 and 1.0, the toner charge amount decreased by an additional 3 to 5% compared to when the conveying force ratio C2 / C1 was 1.05 or greater (the level of developer deterioration was "Good"). On the other hand, when the conveying force ratio C2 / C1 was less than 0.7, the toner charge amount decreased by an additional 10% or more compared to when the conveying force ratio C2 / C1 was 1.05 or greater (the level of developer deterioration was "Poor"). Therefore, in order to reduce developer deterioration, the amount of developer accumulation between the first developing sleeve 33 and the second developing sleeve 34 can be optimized by increasing the conveying force ratio C2 / C1 to greater than 1, as in this condition.

[0084] Here, when the ratio C2 / C1 of the conveying forces is greater than 1, the combination of the relationship between the conveying forces C1 and C2 and the peripheral speed ratio rate_v and the groove ratio ρ, which are the components of the conveying forces, will be described.

[0085] The peripheral speed ratio rate_v is the peripheral speed of the developing sleeve relative to the peripheral speed of the photosensitive drum 28Y, and is the ratio of the surface linear velocities of the first developing sleeve 33 and the second developing sleeve 34 at the portions facing the photosensitive drum 28Y. Therefore, a larger peripheral speed ratio rate_v increases the amount of developer transported per unit time. Furthermore, as the groove ratio ρ increases, the amount of developer carried per unit area increases, and even when rotating at the same peripheral speed, the amount of developer transported per unit time increases. Therefore, even when transferring developer from the first developing sleeve 33 to the second developing sleeve 34 against gravity, the developer can be transferred without stagnation between the first developing sleeve 33 and the second developing sleeve 34. Here, consider the ratio C2 / C1 of the transport forces when the peripheral speed ratio between the first developing sleeve 33 and the second developing sleeve 34 is the same and the groove ratio between the first developing sleeve 33 and the second developing sleeve 34 is different.

[0086] For example, when rate_v2=rate_v1 and ρ2>ρ1, the transport force is increased by increasing the amount of developer carried on the surface of the second developing sleeve 34. Therefore, even if the developer is transferred from the first developing sleeve 33 to the second developing sleeve 34 against gravity, the developer can be transferred without accumulating between the first developing sleeve 33 and the second developing sleeve 34.

[0087] Furthermore, for example, if the peripheral speed ratio rate_v of the first developing sleeve 33 and the second developing sleeve 34 is set to rate_v1 = rate_v2 = 1.0, and the groove ratios ρ of the first developing sleeve 33 and the second developing sleeve 34 are set to ρ1 = 11.6 [%] and ρ2 = 14.6 [%], the conveying force ratio C2 / C1 ≧ 1.0 can be satisfied.

[0088] In the configuration of this embodiment, as shown in FIG. 5A, when the groove ratio ρ changes by 1%, the amount of developer carried on the developing sleeve (the amount of developer carried when the developing sleeve is stationary) increases by 1 mg / cm 2 For example, when the groove ratios ρ of the first developing sleeve 33 and the second developing sleeve 34 are ρ1=11.6[%] and ρ2=14.6[%], the groove ratio of the second developing sleeve 34 is 3[%] higher than that of the first developing sleeve 33, so the developer carrying amount of the second developing sleeve 34 is 3[mg / cm] higher than that of the first developing sleeve 33. 2 Therefore, the developer does not accumulate between the first developing sleeve 33 and the second developing sleeve 34, and a stable transport state can be obtained.

[0089] Furthermore, as shown in FIG. 5(b), the amount of developer carried on the developing sleeve (the amount of developer carried when the developing sleeve is stationary) generally tends to decrease as the peripheral speed of the developing sleeve increases. When developer is supplied to the developing sleeve, some developer is carried and transported on the developing sleeve, leaving behind the amount supplied by shear. In contrast, the faster the peripheral speed of the developing sleeve, the greater the shear force acting on the developer carried and transported on the developing sleeve, resulting in a decrease in the amount of developer carried on the developing sleeve. On the other hand, the faster the peripheral speed of the developing sleeve, the greater the amount of developer transported per unit time passing through the opposing portion between the developing sleeve and photosensitive drum 28Y.

[0090] As can be seen from FIG. 5(b), for example, when the peripheral speed of the developing sleeve is increased by 50% from 400 [mm / sec] to 600 [mm / sec], the developer carrying amount is 33 [mg / cm 2 ] to 28 [mg / cm 2 ] is reduced by about 15%. Therefore, as the peripheral speed of the developing sleeve is increased, the amount of developer carried on the developing sleeve decreases, and the amount of developer sent per unit time to the opposing portion between the developing sleeve and photosensitive drum 28Y increases.

[0091] For example, when rate_v2>rate_v1 and ρ2=ρ1, the groove ratio of the first developing sleeve 33 and the groove ratio of the second developing sleeve 34 are the same, and the circumferential speed ratio of the second developing sleeve 34 is set to be higher than that of the first developing sleeve 33. In this way, the circumferential speed ratio of the second developing sleeve 34 is set to be higher than that of the first developing sleeve 33, thereby increasing the transport distance per unit time on the surface of the second developing sleeve 34, thereby increasing the developer transport force of the second developing sleeve 34. In this way, by setting the circumferential speed ratio of the second developing sleeve 34 higher than that of the first developing sleeve 33, the amount of developer carried on the second developing sleeve 34 is reduced, and the occurrence of developer accumulation between the first developing sleeve 33 and the second developing sleeve 34 is suppressed.

[0092] Furthermore, for example, by setting ρ1 = ρ2 = 11.6% and setting rate_v1 = 1.0 and rate_v2 = 1.2, C2 / C1 = 1.2, and the relationship C2 / C1 > 1.0 is satisfied. As mentioned above, generally, the faster the circumferential speed of the developing sleeve, the less developer is carried on the developing sleeve. If the circumferential speed of the second developing sleeve 34 is increased by 20% relative to the circumferential speed of the first developing sleeve 33, the amount of developer carried on the developing sleeve decreases by about 6.7%, as shown in FIG. 5B. However, by increasing the circumferential speed of the developing sleeve, the amount of developer transported per unit time to the opposing portion between the developing sleeve and the photosensitive drum 28Y increases by nearly 13.3%, thereby reducing the amount of developer remaining between the first developing sleeve 33 and the second developing sleeve 34.

[0093] When C2 / C1 is less than 0.7, the following is thought to be the reason why the developer tends to rotate. That is, the transfer of the developer from the first developing sleeve 33 to the second developing sleeve 34 occurs from below in the vertical direction against gravity. Therefore, if the transport force C1 of the first developing sleeve 33 is too large compared to the transport force C2 of the second developing sleeve 34, the developer is not sufficiently transferred from the first developing sleeve 33 to the second developing sleeve 34. As a result, the developer rotates on the first developing sleeve 33 and is transported downstream in the transport direction of the first developing sleeve 33.

[0094] When this dragging of developer occurs, the developer is stripped off from the first developing sleeve 33 inside the developing container 60 and falls near the regulating member 50. Then, the developer with a low TD ratio that has been subjected to the development process on the first developing sleeve 33 is immediately supplied to the first developing sleeve 33 again without being stirred, causing density fluctuations due to uneven toner concentration.

[0095] On the other hand, if the ratio C2 / C1 becomes too large—that is, if the conveying force C2 of the second developing sleeve 34 becomes too high relative to the conveying force C1 of the first developing sleeve 33—too much toner is supplied downstream of the latent image in the direction of rotation of the photosensitive drum 28Y, resulting in excessive toner development at the downstream end of the latent image. This results in image defects such as edge emphasis. This occurs particularly when the conveying speed of the second developing sleeve 34 downstream of the latent image is too fast, i.e., when rate_v2 >> rate_v1, or when the groove ratio ρ2 is too large and the amount of developer carried on the second developing sleeve 34 is too large, or both. Thus, even if the conveying force C2 of the second developing sleeve 34 is too large relative to the conveying force C1 of the first developing sleeve 33, the effect of adjusting the toner layer nonuniformity is reduced. If the ratio C2 / C1 is 1.45 or greater, dragging generally does not occur, but slight edge voids or edge emphasis may occur. Therefore, the ratio C2 / C1 of the conveying forces of the first developing sleeve 33 and the second developing sleeve 34 is 0.70≦C2 / C1<1.45 It is preferable that the following is satisfied. Furthermore, from the viewpoint of the indicator of developer deterioration described above, more preferable conditions are as follows: 1.00 <C2 / C1<1.45 The purpose is to satisfy the following.

[0096] As described above, in this embodiment, in a configuration in which the transfer of developer from the first developing sleeve 33 to the second developing sleeve 34 is performed against the direction of gravity, a plurality of grooves 121 are formed so as to be aligned in the circumferential direction on the surfaces of the first developing sleeve 33 and the second developing sleeve 34. Furthermore, in this embodiment, the developer carried on the first developing sleeve 33 is regulated by the regulating member 50, and the developer is transported from below to above by the rotation of the first developing sleeve 33, and the developer is transferred from the first developing sleeve 33 to the second developing sleeve 34.

[0097] The relationship between the transport forces C1 and C2 of the first developing sleeve 33 and the second developing sleeve 34 is set to satisfy C2 / C1≧0.70. This makes it possible to suppress the occurrence of image defects in a configuration in which the transfer of developer from the first developing sleeve 33 to the second developing sleeve 34 is performed against the direction of gravity. In other words, by setting the relationship between the transport forces, which is formed by the groove ratio and the peripheral speed, as described above, it is possible to suppress the occurrence of the phenomenon of developer dragging around the first developing sleeve 33, and therefore the occurrence of image defects. In particular, since it is possible to suppress the occurrence of the dragging around phenomenon even when the process speed is increased, it is possible to provide an image forming apparatus that can form high-quality images.

[0098] <Second embodiment> The second embodiment will be described with reference to FIGS. 2 and 3, as well as FIGS. 6 and 7. In the first embodiment described above, the uneven shapes on the surfaces of the first developing sleeve 33 and the second developing sleeve 34 are groove-shaped, and the relationship between the conveying forces is specified. In this embodiment, the peripheral speed ratio, which is a parameter constituting the conveying force, is further specified. Since the other configurations and functions are the same as those of the first embodiment described above, the same reference numerals will be used to designate the same configurations as those of the first embodiment, and illustrations and descriptions will be omitted or simplified below, and differences from the first embodiment will be mainly described.

[0099] In this embodiment, in addition to the conditions in the first embodiment, the relationship between the peripheral speed ratio rate_v1 of the first developing sleeve 33 to the peripheral speed of the surface of the photosensitive drum 28Y and the peripheral speed ratio rate_v2 of the second developing sleeve 34 to the peripheral speed of the surface of the photosensitive drum 28Y is set as follows: rate_v2>rate_v1 We strive to satisfy the following.

[0100] Here, a problem with developing sleeves having multiple grooves on their surface (also called grooved sleeves) is that minute pitch variations in density resulting from the spacing between the grooves appear in the toner image developed on the photosensitive drum 28Y, known as groove pitch variations. This is thought to be due to the difference in the distance between the grooved and non-grooved portions of the developing sleeve surface and the surface of the photosensitive drum 28Y, which changes the electric field strength required for the development process. In the first embodiment, we described the drag phenomenon that tends to occur in solid images, which consume a lot of toner. However, groove pitch variations are more likely to become apparent in halftone images, which consume less toner, than in solid images, which carry a large amount of toner.

[0101] In a system having multiple developing sleeves, groove pitch unevenness occurs in the developing process by the first developing sleeve 33 on the upstream side in the rotation direction of the photosensitive drum 28Y, and development is performed by the downstream second developing sleeve 34 in a state where potential unevenness occurs on the toner surface on the photosensitive drum 28Y. Therefore, density unevenness that occurs in the developing process by the first developing sleeve 33 is corrected in the developing process by the second developing sleeve 34.

[0102] Measures to improve groove pitch unevenness include increasing the groove ratio on the surface of the developing sleeve and increasing the peripheral speed of the developing sleeve. Increasing the groove ratio increases the amount of developer carried per unit surface area of ​​the developing sleeve, making it more likely for the developer to clog in the area facing the photosensitive drum 28Y (development nip). This can lead to other problems, such as the toner image on the photosensitive drum 28Y developed by the first developing sleeve 33 being disturbed by the developer carried by the second developing sleeve 34. However, the groove ratio must be large enough to satisfy the C2 / C1 condition of the first embodiment. If the groove ratio is too small, groove pitch unevenness may occur due to the development process of the second developing sleeve 34.

[0103] On the other hand, if the circumferential speed of the second developing sleeve 34 is increased, in the development process carried out at the closest part between the second developing sleeve 34 and the photosensitive drum 28Y, the number of grooves passing through the closest part per unit time increases, resulting in a high frequency that makes groove pitch unevenness in the second developing sleeve 34 itself less visible. Also, if the circumferential speed of the second developing sleeve 34 is increased, the amount of toner supplied to the development process increases, thereby reducing the amount of change in the amount of developer on the second developing sleeve 34 and effectively improving groove pitch unevenness.

[0104] Next, the results of examining unevenness in groove pitch on the second developing sleeve 34 will be described. The same jig used in the examination of the first embodiment was also used in this examination. A high-speed video camera, MEMRECAM GX-4CH manufactured by NAC Image Technology, was installed downstream in the rotation direction of the photosensitive drum 28Y at the closest position between the second developing sleeve 34 and the photosensitive drum 28Y, and the developed toner image was photographed. The photographs were taken while varying the peripheral speed ratio of the second developing sleeve 34.

[0105] In addition, the image processing software ImageJ was used to time-expand the longitudinal line image at the development nip portion (the closest point between the second development sleeve 34 and the photosensitive drum 28Y) of the captured image, and then FFT processing was performed, and the brightness amplitude (brightness peak value) of the frequency of the groove pitch unevenness was used as the evaluation value of the groove pitch unevenness.

[0106] 6 shows an example of a graph obtained by FFT-processing groove pitch unevenness manifested by a single developing sleeve and calculating the brightness amplitude. Here, the peripheral speed ratio rate_v1 of the first developing sleeve 33 was 1.0, the peripheral speed ratio rate_v2 of the second developing sleeve 34 was 1.2, the outer diameter of both developing sleeves was φ25 mm, the groove ratio ρ1 of the first developing sleeve 33 was 9.1%, and the groove ratio ρ2 of the second developing sleeve 34 was 15.2%. With this configuration, the frequency of the groove pitch unevenness measured when a halftone image was formed on the photosensitive drum 28Y was 460 Hz.

[0107] The luminance amplitude at the above frequency was treated as an evaluation value for groove pitch unevenness. The luminance amplitude, which represents the magnitude of groove pitch unevenness after passing through the development processes of both the first developing sleeve 33 and the second developing sleeve 34, was 2.64. Figure 7 shows the magnitude of the luminance amplitude of groove pitch unevenness when the peripheral speed ratio rate_v2 of the second developing sleeve 34 is varied.

[0108] 7, it was confirmed that the level of groove pitch unevenness improves as the peripheral speed ratio rate_v2 of the second developing sleeve 34 increases. That is, groove pitch unevenness becomes difficult to see with the naked eye when the brightness amplitude is 3.0 or less, and becomes almost invisible when it is 2.0 or less. It can be seen from FIG. 6 that the brightness amplitude decreases as the peripheral speed ratio rate_v2 of the second developing sleeve 34 increases.

[0109] Furthermore, from Figure 7, rate_v2≧1.2 It is preferable that rate_v2≧1.4 It is more preferable that the following be satisfied.

[0110] From the above, the peripheral speed ratio rate_v2 of the second developing sleeve 34 to the peripheral speed of the surface of the photosensitive drum 28Y is set to be larger than the peripheral speed ratio rate_v1 of the first developing sleeve 33, that is, rate_v2>rate_v1 As a result, density unevenness occurring in the development process using the first developing sleeve 33 is corrected in the development process using the second developing sleeve 34, and groove pitch unevenness can be effectively improved. As a result, by satisfying the conditions of the first embodiment, the drag phenomenon occurring when toner consumption is high can be improved, and by satisfying the conditions of this embodiment, groove pitch unevenness occurring when toner consumption is low can be improved, and an image forming apparatus capable of forming higher quality images can be provided.

[0111] <Third embodiment> The third embodiment will be described using FIG. 8 with reference to FIG. 2. In the first and second embodiments described above, the relationship between the conveying forces of the groove-shaped uneven surfaces of the first developing sleeve 33 and the second developing sleeve 34 is described. In contrast, in this embodiment, the relationship between the conveying forces of the uneven surfaces of the second developing sleeve 34 and the stripping sleeve 35 is described. Since the other configurations and functions are the same as those of the first embodiment described above, the same reference numerals will be used to designate the same configurations as those of the first embodiment, and illustrations and descriptions will be omitted or simplified below, and differences from the first and second embodiments will be mainly described.

[0112] To form high-quality images using the developing device 1Y configured as described above, it is necessary to prevent developer leakage when transferring the developer from the second developing sleeve 34 to the peeling sleeve 35, as in the first embodiment. If leakage occurs during this transfer, the developer will spill vertically downward from between the second developing sleeve 34 and the developer container 60. This will cause the developer to fall into the peeling region of the first developing magnet 36 on the first developing sleeve 33 and be supplied upstream of the restricting member 50. This developer will then be supplied to the first developing sleeve 33 without being agitated in the developer circulation path. Furthermore, there is a risk that the developer will fall between the second developing sleeve 34 and the developer container 60 and into the gap between the first developing sleeve 33 and the second developing sleeve 34, be transported to the second developing sleeve 34, and be immediately supplied to the second developing region. This will result in developer with an uneven amount of toner being used in the development process, resulting in variations in density.

[0113] The stripping sleeve 35 is disposed above the first developing sleeve 33 and adjacent to the second developing sleeve 34. In this embodiment, the center of rotation R3 of the stripping sleeve 35 is disposed 30 degrees above the horizontal line passing through the center of rotation R2 of the second developing sleeve 34. The center of rotation R3 of the stripping sleeve 35 is preferably positioned vertically above the center of rotation R2 of the second developing sleeve 34, but may be positioned above the lower end of the second developing sleeve 34. However, a sufficient gap must be maintained between the stripping sleeve 35 and the first developing sleeve 33 to prevent the transfer of developer from the stripping sleeve 35 to the first developing sleeve 33. The direction of rotation of the stripping sleeve 35 is set to be forward when facing the second developing sleeve 34 and counterclockwise in the cross section shown in FIG. 2.

[0114] As described above, if developer leakage occurs in the transfer from the second developing sleeve 34 to the stripping sleeve 35, the toner is repeatedly carried along and supplied to the surfaces of the first developing sleeve 33 and the second developing sleeve 34, which tends to reduce the toner concentration and cause uneven concentration. For this reason, in this embodiment, as in the first embodiment, the conveying force C3 of the stripping sleeve 35 is considered. When the peripheral speed ratio of the stripping sleeve 35 to the second developing sleeve 34 is rate_v3 and the groove ratio on the surface of the stripping sleeve 35 is ρ3, the conveying force C3 of the stripping sleeve 35 is expressed by the following equation 5. C3=ρ3×rate_v3 (Formula 5)

[0115] Here, the function required of the stripping sleeve 35 is to receive and recover all the developer from the second developing sleeve 34, and the uniformity of the developer layer carried by the stripping sleeve 35 is hardly required. For this reason, the transport force C3 of the stripping sleeve 35 can be made sufficiently larger than the transport force C2 of the second developing sleeve 34. For this reason, in this embodiment, at least C3>C2 It is desirable to satisfy the following. Also, the relationship between C3 and C2 is C3 / C2≧1.1 You can also make it so that it satisfies the following: C3 / C2≧1.3 may be set to satisfy the following.

[0116] Furthermore, if the conveying force C3 of the peeling sleeve 35 is too large compared to the conveying force C2 of the second developing sleeve 34, the friction between the developer and the surface of each sleeve becomes strong in the gap between the opposing portions of the second developing sleeve 34 and the peeling sleeve 35 where the developer is exchanged. This causes deterioration of the toner, such as external additives in the toner coming off the toner surface and migrating to the carrier surface, and deterioration such as wear on the carrier surface. As deterioration progresses, the charge amount of the toner decreases, leading to density fluctuations. For this reason, C3 / C2≦1.50 It is desirable to satisfy the following.

[0117] Figure 8 shows the change in toner charge, an indicator of developer degradation, when image formation was performed using the image forming apparatus 100, as the C3 / C2 relationship was varied. The toner charge was measured as Q / M (charge per unit mass, [-μC / g]) using a Hosokawa Micron E-Spart Analyzer. Image formation was performed using a simple configuration to minimize developer degradation: the discharge port of the ACR discharge mechanism, which refreshes the carrier, was blocked, and only toner was used as replenishment developer. To maximize developer degradation, the external environment was set to a high temperature and humidity of 30°C and 80%, and the print image was a horizontal band image in the main scanning direction with an image area ratio of 30% (assuming the consumption of an A4 full-page solid image is 100%). 20,000 sheets were printed at a constant TD ratio.

[0118] As is clear from Figure 8, the charge amount of the toner after printing tends to decrease when C3 / C2 is 1.4 or more, and when C3 / C2 is 1.8, it tends to decrease by about 20% compared to when it is 1.4. Thus, the relationship between the conveying force C3 of the stripping sleeve 35 and the conveying force C2 of the second developing sleeve 34 is as follows: 1.00 <C3 / C2≦1.50 It is desirable to satisfy the following.

[0119] Furthermore, the relationship between at least the conveying force C1 of the first developing sleeve 33 and the conveying force C2 of the peeling sleeve 35 is as follows: C3 / C1≧1.0 It is preferable that the following is satisfied.

[0120] In this way, by satisfying C3 / C1≧1.0, the amount of developer in the developing container 60 can be maintained approximately constant. 1.00 <C3 / C2≦1.50 and C3 / C1≧1.0 If the configuration satisfies the above, the developer can be used more stably in the developing device.

[0121] From the above, by configuring the conveying force C3 of the peeling sleeve 35 to be greater than the conveying force C2 of the second developing sleeve 34, it is possible to improve the developer entrainment on the second developing sleeve 34 and suppress developer deterioration, thereby providing higher quality images.

[0122] <Other embodiments> The present invention is not limited to the configurations of the above-described embodiments. For example, the image forming apparatus 100 is not limited to an MFP, but may be a copier, printer, or facsimile machine. Furthermore, the configurations of the developer supply screw 42, developer stirring screw 43, and developer recovery screw 44 are not particularly limited as long as they can transport the developer. For example, spiral blades or paddle-shaped blades can be applied. [Explanation of symbols]

[0123] 1Y, 1M, 1C, 1K...developing device 28Y, 28M, 28C, 28K...Photosensitive drum (image carrier) 33: First developing sleeve (first developing rotor) 34 Second developing sleeve (second developing rotor) 36···First developing magnet (first magnet) 37 Second developing magnet (second magnet) 60....Developing container

Claims

1. a developer container containing a developer containing toner and a carrier; a first developing rotor to which the developer contained in the developing container is supplied, the first developing rotor carrying and transporting the developer to a first developing position where an electrostatic latent image formed on a rotatable image carrier is developed; a second developing rotor disposed opposite the first developing rotor and receiving the developer from the first developing rotor, the second developing rotor carrying and transporting the developer to a second developing position where the electrostatic latent image that has passed the first developing position is developed; a first magnet fixedly disposed inside the first developing rotor so as to be non-rotatable; a second magnet fixedly disposed inside the second developing rotor so as to be non-rotatable; Equipped with a rotation axis of the second developing rotor is located above a rotation axis of the first developing rotor, the developer is transferred from the first developing rotary body to the second developing rotary body against the direction of gravity by a magnetic field generated between the first magnet and the second magnet; a plurality of first grooves are formed in an outer peripheral surface of the first developing rotor along a circumferential direction of the first developing rotor, A plurality of second grooves are formed on the outer circumferential surface of the second developing rotor along the circumferential direction of the second developing rotor. A developing device characterized by:

2. The width of the first groove is d1, the number of the first grooves per rotation of the first developing rotor is N1, The circumferential length of the first developing rotor is defined as L1, The groove ratio of the first groove is set to ρ1=(d1×N1) / L1, The width of the second groove is d2, the number of the second grooves per rotation of the second developing rotor is N2, The circumferential length of the second developing rotor is L2, The groove ratio of the second groove is ρ2=(d2×N2) / L2, a peripheral speed ratio of the first developing rotor to the image carrier is defined as rate_v1, a peripheral speed ratio of the second developing rotor to the image carrier is defined as rate_v2, The developer transport force of the first developing rotor is C1=ρ1×rate_v1, When the developer transport force of the second developing rotor is C2=ρ2×rate_v2, 0.70≦C2 / C1<1.45 fulfill 2. The developing device according to claim 1.

3. 1.0<C2 / C1 Further satisfy 3. The developing device according to claim 2.

4. ρ1 = ρ2 rate_v1<rate_v2 Further satisfy 3. The developing device according to claim 2.

5. ρ1<ρ2 rate_v1=rate_v2 Further satisfy 3. The developing device according to claim 2.

6. The width of the first groove is d1, the number of the first grooves per rotation of the first developing rotor is N1, The circumferential length of the first developing rotor is defined as L1, The groove ratio of the first groove is set to ρ1=(d1×N1) / L1, The width of the second groove is d2, the number of the second grooves per rotation of the second developing rotor is N2, The circumferential length of the second developing rotor is L2, The groove ratio of the second groove is ρ2=(d2×N2) / L2, a peripheral speed ratio of the first developing rotor to the image carrier is defined as rate_v1, When the peripheral speed ratio of the second developing rotor to the image carrier is rate_v2, ρ1 = ρ2 rate_v1<rate_v2 fulfill 2. The developing device according to claim 1.

7. The width of the first groove is d1, the number of the first grooves per rotation of the first developing rotor is N1, The circumferential length of the first developing rotor is defined as L1, The groove ratio of the first groove is set to ρ1=(d1×N1) / L1, The width of the second groove is d2, the number of the second grooves per rotation of the second developing rotor is N2, The circumferential length of the second developing rotor is L2, The groove ratio of the second groove is ρ2=(d2×N2) / L2, a peripheral speed ratio of the first developing rotor to the image carrier is defined as rate_v1, When the peripheral speed ratio of the second developing rotor to the image carrier is rate_v2, ρ1<ρ2 rate_v1=rate_v2 fulfill 2. The developing device according to claim 1.

8. The first developing rotor and the second developing rotor are opposed to each other and rotate in opposite directions.

2. The developing device according to claim 1.

9. the first developing rotor and the image carrier are opposed to each other and rotate in the same direction; The second developing rotor and the image carrier rotate in the same direction at positions facing each other.

9. The developing device according to claim 8.

Images