Patent Application
20090245889
The present invention claims priority pursuant to 35 U.S.C. §119 from Japanese Patent Application No. 2008-095302, filed on Apr. 1, 2008 in the Japan Patent Office, the contents and disclosures of each of which are hereby incorporated by reference herein in their entirety.
1. Field of the Invention
Exemplary embodiments of the present invention generally relate to a developing unit containing a two-component developer including magnetic carrier particles and toner particles, a process cartridge including the developing unit, and an image forming apparatus, such as a copier, printer, facsimile machine, and the like, incorporating the developing unit.
2. Discussion of the Related Art
Developing units that develop toner images for electrophotographic printing generally employ either a one-component developer or a two-component developer. While the one-component developer includes toner particles only, the two-component developer includes toner particles and magnetic carrier particles.
Such developing units include a developer bearing member for bearing the developer to convey it to a development region where the developer bearing member faces an image bearing member. The developer bearing member may include a cylindrical development sleeve, for example, constituted as a hollow cylinder the interior of which contains a magnetic field generator capable of generating a magnetic field sufficient to hold the magnetic carrier particles of the developer on the exterior perimeter surface of the development sleeve. Toner particles are then electrostatically attracted to the magnetic carrier particles. As the development sleeve rotates, the toner particles attached to the magnetic carrier particles that are held on the exterior perimeter surface of the development sleeve are conveyed to the development region and then supplied to a latent image formed on a surface of the image bearing member at the development region.
The magnetic field generator has multiple magnetic poles along a direction of rotation of the development sleeve. Examples of such magnetic field generator are a roller-shaped member having magnetic pole-forming parts magnetized by external magnetic fields, a member in which multiple magnets are held by a common holding member so that each of the magnets faces a given direction, and the like.
Developer carried on the exterior perimeter surface of the development sleeve by the magnetic force generated by the magnetic field generator is conveyed in a direction of movement of the surface of the development sleeve as the development sleeve rotates.
The first conventional developing unit 1214 includes a developer roller 1240 that serves as a developer bearing member and includes an outer development sleeve 1241 serving as a nonmagnetic hollow body and an inner magnetic roller 1247 serving as a magnetic field generator. That is, the developer roller 1240 is formed by the hollow cylindrical development sleeve 1241 made of some non-magnetic material surrounding the magnetic roller 1247, so as to hold developer on an exterior perimeter surface of the development sleeve 1241 by a magnetic force generated by the magnetic roller 1247.
The developing unit 1214 further includes a developer container 1249 for containing developer, screw-shaped agitation/conveyance members 1242 and 1243 for agitating and conveying the developer axially along a direction of a rotary shaft of the development sleeve 1241, and a developer regulating member 1246 for regulating the thickness of a layer of developer carried on the development sleeve 1241.
The developer container 1249 is separated in a first container (i.e., a developer storing chamber) 1249A and a second container (i.e., a developer agitating chamber) 1249B. The first container 1249A is positioned lower than the development sleeve 1241 and extends in an axial direction of the development sleeve 1241. The second container 1249B is disposed adjacent the first container 1249A and also extends in the axial direction of the development sleeve 1241. The first container 1249A includes the agitation/conveyance member 1242 and the second container 249B includes the agitation/conveyance member 1243 that rotates in a direction indicated by arrow “R1” in
Toner is generally supplied from a toner bottle, not shown, to the second container 1249B for replenishment, that is, replacing an amount of toner consumed for development. During conveyance of the developer, the magnetic force generated by the magnetic roller 1247 scoops up, or attracts, the developer contained in the first container 1249A, which is then supplied to the development sleeve 1241. Then, the thickness of the layer of thus-supplied developer on the development sleeve 1241 is regulated by the developer regulating member 1246, and the developer passes the development region facing an image bearing member 1012, and returns to the developer container 1249.
The magnetic roller 1247 includes five magnetic poles, which are a magnetic pole S1 for development, a magnetic pole N1 for conveyance, a magnetic pole S2 for developer release at an upstream portion, a magnetic pole S3 for developer release and attraction, and a magnetic pole N2 for regulation. Where the magnetic poles S1, S2, and S3 are implemented as south poles, for example, the magnetic poles N1 and N2 are implemented as north poles, for example.
As the development sleeve 1241 rotates in a direction indicated by arrow “R2” in
When the magnetic pole S2 and the magnetic pole S3 having an identical polarity are disposed adjacent to each other, a developer-releasing region P is formed between the magnetic poles S2 and S3 in the developing unit 1214 shown in
The first conventional developing unit 1214 shown in
However, such a continuous high-density state of developer in the vicinity of the polarity inversion point Q imposes a constant mechanical stress on the developer particles, causing them to deteriorate. Therefore, an amount of torque to drive the agitation/conveyance member 1243 of the first container 1249A has to be increased and the agitation/conveyance member 1243 has to be more rigid in strength and larger in size, which can lead to an increase both in cost and in size of the first conventional developing unit 1214.
Further, since the developer is subject to a great amount of stress, a speed of progression of implantation of external additives from the toner into the surface of each carrier particle and abrasion of a surface layer film of each carrier particle, both of which are undesirable, may be accelerated. These actions easily can degrade toner chargeability and powder flowability of developer, which in turn can make it difficult to maintain good image quality over an extended period of time. Since the powder properties of developer can degrade easily, an amount of developer conveyed to the development region may decrease especially when the ability of the development sleeve 1241 to convey developer has deteriorated, and good image quality cannot be maintained for an extended period of time.
Similar to the first conventional developing unit 1214, the second conventional developing unit 1314 includes a developer roller 1340 that serves as a developer bearing member and is disposed facing the image bearing member 1012, and includes an outer development sleeve 1341 serving as a nonmagnetic hollow body and an inner magnetic roller 1347 serving as a magnetic field generator. The development unit 1314 further includes a developer container 1349 for containing developer, screw-shaped agitation/conveyance members 1342 and 1343, and the developer regulating member 1346 for regulating the thickness of layer of developer carried on the development sleeve 1341 that rotates in a direction indicated by arrow “R2” in
According to the second conventional developing unit 1314 shown in
Although not disclosed in the first conventional developing unit 1214 and the second conventional developing unit 1314, the developer tends to accumulate in an area from at least a downstream part of the developer-releasing region P that is located upstream from the developer-regulating region to the developer-regulating region in the developing units 1214 and 1314. Whit this arrangement, in the second conventional developing unit 1314, while the developer released from the development sleeve 1341 in an upstream part of the developer-releasing region P may fall onto the agitation/conveyance screw 1343, the developer remaining on the development sleeve 1341 after passing the downstream part of the developer-releasing region P may be taken in developer accumulated in the area to be released or removed therefrom. That is, similar to the first conventional developing unit 1214 shown in
However, the above-described configuration, in which the developer in the developer container 1349 is used for removing the developer remaining on the development sleeve 1341, may impose a certain amount of mechanical stress on the developer when the developer on the development sleeve 1341 is scraped therefrom. In light of market demands to reduce stress on the developer as much as possible, it is also desired to reduce the above-described stress on the developer when scraping the developer off the development sleeve 1341.
Consequently, the present inventors have conducted extensive research designed to eliminate stress on the developer when scraping it off a development sleeve, and as a result have developed a developing unit 1414 as shown in
As illustrated in
In the developing unit 1414, the developing roller 1440 is shifted upward in relation to the developer container 1449 as shown in
However, the inventors have found that the developing unit 1414 can cause the following problems.
As described above, the configuration of the developing unit 1414 prevents the developer in the developer-releasing region P on the development sleeve 1441 from contacting the developer stored in the developer storing chamber 1449A. With this configuration, the developer released from the development sleeve 1441 in the developer-releasing region P is subject to the action of a rotative force or torque of the development sleeve 1441 and a release force of the magnetic force generated by the magnetic poles, and consequently flies into the developer container 1449 to be taken and mixed into the developer therein.
Adjacent to and on the downstream side of the developer-releasing region P, a developer-attracting region R is provided to attract the developer with the magnetic force generated by the magnetic pole N3 for developer release, attraction, and regulation. The developer released in the developer-releasing region P, especially an end part thereof, is also subjected to the torque of the development sleeve 1441 and consequently moves toward a downstream side of the developer-releasing region P, which can cause the developer to fly toward the developer-attracting region R while moving away from the development sleeve 1441. Therefore, the developer flying toward the developer-attracting region R after being released from the development sleeve 1441 is affected by the magnetic force generated by the magnetic pole N3 and reattaches to the development region by being taken in with other developer attracted to the developer-attracting region R or by directly adhering or attaching to the developer-attracting region R. As with the above-described developer carryover, such developer reattachment hinders, stable image development, but with this difference:
Whereas the developer carryover causes unevenness in image density with streaks that may extend in a circumferential direction of the development sleeve 1441, the developer reattachment causes unevenness in image density with spots. However, both the developer carryover and the developer reattachment can degrade image quality.
Further, the developer in the downstream part of the developer-releasing region P contacts the surface of the developer stored in the developer storing chamber 1349A and can act as a wall to protect the developer released from the upstream part of the developer-releasing region P, so that the developer may not be taken by the developer in the developer attracting region R and/or may directly adhere to the developer attracting region R, for example.
Further, in the developing unit shown in
Exemplary aspects of the present invention have been made in view of the above-described circumstances.
Exemplary aspects of the present invention provide a novel developing unit that can effectively decrease mechanical stress on developer in a developer-regulating region in which a height or thickness of the developer is regulated by the developer regulating member and can prevent developer carryover and developer reattachment with respect to a development sleeve.
Another exemplary aspect of the present invention provide an image forming apparatus that incorporates the above-described novel developing unit.
Yet another exemplary aspect of the present invention provide a process cartridge that includes the above-described novel developing unit.
In one exemplary embodiment, a novel developing unit includes a developer bearing member including a magnetic field generator and a nonmagnetic hollow body containing the magnetic field generator for bearing a two-component developer including magnetic carrier particles and toner particles on an exterior perimeter surface thereof by a magnetic force generated by the magnetic field generator, a developer container including a developer storing chamber to store the two-component developer, an agitation/conveyance member to convey the two-component developer in an axial direction of the developer bearing member while agitating the two-component developer, and a developer regulating member to regulate a thickness of layer of the two-component developer held on the developer bearing member. The two-component developer conveyed in the developer container is attracted by the magnetic force exerted by the magnetic field generator to the developer bearing member, is regulated by the developer regulating member, then passes through a development region of the developer bearing member facing an image bearing member, and returns to the developer container. The magnetic field generator includes first and second magnetic poles with an identical polarity disposed adjacent to each other and downstream from the development region in a direction of rotation of the developer bearing member to generate respective magnetic forces for removing the two-component developer from the developer bearing member after the developer passes through the development region. The second magnetic pole is disposed downstream from the first magnetic pole in a direction of conveyance of developer by the developer bearing member and proximate to the developer regulating member to generate a magnetic force to attract the two-component developer from the developer storing chamber in the developer container for forming a magnetic brush of the two-component developer on the developer bearing member regulated by the developer bearing member. The developer bearing member includes a developer-releasing region to release the two-component developer from the developer bearing member using a release force corresponding to magnetic forces generated by the first and second magnetic poles. The developer is disposed higher than a top surface of the two-component developer stored in the developer storing chamber so that the developer-releasing region on the developer bearing member remains separated from the top surface of the two-component developer in the developer storing chamber as the developer bearing member rotates. A component of a magnetic flux density of the magnetic field generated by the magnetic field generator in a direction normal to the developer-releasing region on the developer bearing member is directed to a same direction as the first and second magnetic poles across the developer-releasing region without forming a local maximum point.
The magnetic field generator may be disposed such that the release force exerted on the two-component developer in the developer-releasing region on the developer bearing member has two local maximum points, and a release force at a local minimum point between the two local maximum points is at least 50% as strong as a release force at the local maximum point.
The magnetic field generator may be disposed such that the release force exerted on the two-component developer in the developer-releasing region on the developer bearing member has a single local maximum point.
The developer-releasing region on the developer bearing member may include a first point where the magnetic flux density in a normal direction of the first magnetic pole reaches a maximum on the developer bearing member in the direction of conveyance of developer thereon, a second point where the magnetic flux density in a normal direction of the second magnetic pole reaches a maximum on the developer bearing member in the direction of conveyance of developer thereon, and a third point where the magnetic flux density in a direction normal to the developer bearing member reaches a minimum on the developer bearing member. The magnetic field generator may be disposed such that the third point is located closer to the second point than to the first point from a center point between the first point and the second point.
A speed of surface movement of the nonmagnetic hollow body may be 350 mm/sec or greater.
Multiple elliptic dents may be formed randomly on the exterior perimeter surface of the nonmagnetic hollow body of the developer bearing member.
The volume average particle diameter of each of the magnetic carrier particles may be 20 μm to 50 μm.
Further, in one exemplary embodiment, an image forming apparatus includes an image bearing member to bear an image on a surface thereof, and the above-described developing unit. The developing unit is disposed facing the image bearing member to convey and adhere the two-component developer to the image to develop a toner image to be transferred from the image bearing member onto a recording medium.
Further, in one exemplary embodiment, a process cartridge, detachably attachable to an image forming apparatus, includes an image bearing member to bear an image on a surface thereof, and the above-described developing unit. The image bearing member and the developing unit are integrally supported by the process cartridge. The developing unit is disposed facing the image bearing member to convey and adhere the two-component developer to the image to develop a toner image to be transferred from the image bearing member onto a recording medium.
A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of the present invention is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner.
Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, preferred embodiments of the present invention are described.
Now, referring to
The image forming apparatus 1 can be any of a copier, a printer, a facsimile machine, a plotter, and a multifunction printer including at least one of copying, printing, scanning, plotter, and facsimile functions. In this non-limiting example embodiment, the image forming apparatus 1 functions as a printer for electrophotographically forming a toner image based on image data on a recording medium (e.g., a recording sheet).
Reference symbols “Y”, “C”, “M”, and “K” represent yellow color, cyan color, magenta color, and black color, respectively.
The image forming apparatus 1 includes a main body 10, an image forming unit 11, an optical writing unit 20, an intermediate transfer unit 30, a sheet feed unit 40, and a fixing unit 50.
The image forming unit 11 includes four image forming units 11Y, 11C, 11M, and 11K that serve as process cartridges and are detachably attachable to an image forming station provided in the main body 1. The image forming units 11Y, 11C, 11M, and 11K include respective consumable image forming components to perform image forming operations for producing respective toner images with toners of different colors of yellow (Y), cyan (C), magenta (M), and black (K). The image forming units 11Y, 11C, 11M, and 11K are separately disposed at positions having different heights in a stepped manner and are detachably provided to the image forming apparatus 1 so that each of the image forming units 11Y, 11C, 11M, and 11K can be replaced at once at an end of its useful life. The image forming units 11Y, 11C, 11M, and 11K have similar structures and functions, except that respective toners are of different colors, which are yellow, cyan, magenta and black toners, the discussion below will be applied to any of the image forming units 11Y, 11C, 11M, and 11K when the units and components are described without suffixes.
The image forming unit 11 includes a photoconductor drum 12, a charging unit 12, a developing unit 14, and a cleaning unit 15. As previously described, the image forming units 11Y, 11C, 11M, and 11K have similar configurations to each other, except for different toner colors, the photoconductor drum 12 corresponds to any of photoconductor drums 12Y, 12C, 12M, and 12K, the charging unit 13 corresponds to any of charging units 13Y, 13C, 13M, and 13K, the developing unit 14 corresponds to any of developing units 14Y, 14C, 14M, and 14K, and the cleaning unit 15 corresponds to any of cleaning units 15Y, 15C, 15M, and 15K.
The photoconductor drum 12 serves as an image bearing member to form an electrostatic latent image on a surface thereof.
The charging unit 13 uniformly charges the photoconductor drum 12.
The developing unit 14 develops an electrostatic latent image formed on the photoconductor drum 12.
The cleaning unit 15 cleans the photoconductor drum 12 by removing residual toner remaining thereon.
The photoconductor drum 12, the charging unit 13, and the cleaning unit 15 are integrally mounted on the image forming unit 11.
The optical writing unit 20 emits multiple laser light beams each of which irradiates the surface of the photoconductor drum 12 to form an electrostatic latent image.
The intermediate transfer unit 30 includes an intermediate transfer belt 31, multiple rollers 32, 33, and 34, a primary transfer roller 35, and a secondary transfer roller 36.
The intermediate transfer belt 31 serves as an intermediate transfer member and is spanned around and extended by the multiple rollers 32, 33, and 34.
The primary transfer roller 35 corresponds to any of primary transfer rollers 35Y, 35C, 35M, and 35K, and transfers the toner image held on the photoconductor drum 12 onto the intermediate transfer belt 31.
The secondary transfer roller 36 transfers the toner image on the intermediate transfer belt 31 onto a transfer sheet S as a recording medium.
The sheet feed unit 40 includes a sheet feed cassette 41, a manual sheet feed tray 42, a sheet feed roller 43, and a pair of registration rollers 44.
The sheet feed roller 43 feeds the transfer sheet S either from the sheet feed cassette 41 or from the manual sheet feed tray 42 and conveys the transfer sheet S to a secondary transfer region.
The pair of registration rollers 44 stops and feeds the transfer sheet S conveyed by the sheet feed roller 43.
The fixing unit 50 includes a fixing roller 51 and a pressure roller 52.
The fixing roller 51 and the pressure roller 52 fix the toner image to the transfer sheet S by applying heat and pressure, respectively.
Toner bottles 60Y, 60C, 60M, and 60K are disposed above and detachably attachable to the main body 10, separated from the image forming units 11Y, 11C, 11M, and 11K. Each of the toner bottles 60Y, 60C, 60M, and 60K includes toner of a corresponding single color to be conveyed to a toner supply port 145 (see
Next, image forming operations using the above-described configuration of the image forming apparatus 1 are described.
For example, the surface of the photoconductor drum 12Y is uniformly charged by the charging unit 13Y of the image forming unit 11Y for forming yellow toner image, and exposed to light by the optical writing unit 20 to form an electrostatic latent image thereon. The developing unit 14Y develops the electrostatic latent image to a yellow toner image by attracting yellow toner to the surface of the photoconductor drum 12Y. The yellow toner image formed on the photoconductor drum 12Y is transferred onto the intermediate transfer belt 31 by action of the primary transfer roller 35Y. After the primary transfer, the cleaning unit 15Y cleans the surface of the photoconductor drum 12Y for a subsequent image forming operation.
Residual toner collected by the cleaning unit 15Y is conveyed and stored in a wasted toner collection bottle 16 that is disposed at a lower left position in
The above-described operations are repeated for forming a cyan toner image, a magenta toner image, and a black toner image in the image forming units 11C, 11M, and 11K, respectively. The cyan toner image, the magenta toner image, and the black toner image are sequentially transferred onto the intermediate transfer belt 31 to be overlaid on the yellow toner image previously formed thereon, and thus a color toner image is formed.
When the transfer sheet S is conveyed from one of the sheet feed cassette 41 and the manual sheet feed tray 42 to the secondary transfer region, the secondary transfer roller 36 causes the color toner image formed on the intermediate transfer belt 31 to be transferred onto the transfer sheet S. The transfer sheet S having the color toner image thereon is conveyed to the fixing unit 50 so as to fix the toner image to the transfer sheet S by applying heat and pressure to the transfer sheet S at a fixing nip portion formed between the fixing roller 51 and the pressure roller 52. The transfer sheet S is then discharged by a discharging roller 55 to a sheet discharging tray 56 arranged at an upper position of the image forming apparatus 1.
Next, referring to
Since the image forming units 11Y, 11C, 11M, and 11K have similar structures and functions, except that respective toners are of different colors, which are yellow, cyan, magenta and black toners, the discussion below will be applied to any of the image forming units 11Y, 11C, 11M, and 11K and the image forming components incorporated therein.
The charging roller 131 has a surface, which is cleaned by the cleaning roller 132.
The cleaning brush 151 and the cleaning blade 152 contact the photoconductor drum 12 to clean a surface thereof.
The toner collection coil 153 conveys toner removed from the photoconductor drum 12 by the cleaning brush 151 and the cleaning blade 152 toward the wasted toner collection bottle 16.
The developing unit 14 includes a developing roller 140, a nonmagnetic outer development sleeve 141, conveyance screws 142 and 143, a casing 144, the toner supply port 145, a doctor blade 146, an inner magnetic roller 147, and a seal member 148. These members and components are housed and supported by the casing 144.
The developing roller 140 serves as a developer bearing member and includes the nonmagnetic development sleeve 141 and the magnetic roller 147.
The nonmagnetic development sleeve 141 serves as a nonmagnetic hollow body constituted as a hollow cylinder of the developing roller 140 and is disposed to face the photoconductor drum 12 in the development region while rotating in a counterclockwise direction as shown in
The magnetic roller 147 is fixedly disposed in the interior of the hollow development sleeve 141. The magnetic roller 147 serves as a magnetic field generator and contains multiple magnets or magnetic poles in a circumferential direction of the development sleeve 141.
The conveyance screws 142 and 143 are disposed to face the development sleeve 141 of the developing roller 140. The conveyance screws 142 and 143 serve as agitation conveyance member to mix and agitate magnetic carrier contained in the developing unit 14 and toner supplied through the toner supply port 145 and convey the carrier and toner in an axial direction of the photoconductor drum 12 according to respective directions of conveyance of the developer by the conveyance screws 142 and 143.
The doctor blade 146 serves as a developer regulating member to form a doctor gap G with the development sleeve 141 for regulating the thickness of a layer of developer held on the surface of the development sleeve 141. The doctor blade 146 is supported at a slot of the casing 144.
Specifically, the doctor blade 146 according to the exemplary embodiment includes a doctor base body 146a (see
The doctor base body 146a is constituted as a nonmagnetic member for mainly regulating an amount of developer to be conveyed to the development region to a constant amount, and therefore receives a pressure of developer when regulating the developer. To withstand the pressure of developer, the doctor base body 146a generally maintains a certain amount of strength or hardness. For example, the doctor base body 146a is required to have a thickness from approximately 1.5 mm to approximately 2.0 mm, which corresponds to a distance of movement of the surface of the development sleeve 141 in a direction of conveyance of developer by the development sleeve 141 and the leading edge thereof, which is an end portion facing the surface of the development sleeve 141, is required to have straightness of approximately 0.05 mm to the surface of the development sleeve 141.
The doctor supporting member 146b is constituted as a magnetic member to mainly increase an amount of toner charge to be conveyed to the development region. The doctor supporting member 146b is normally much thinner than the doctor base body 146a, for example, includes a tubular or flat metal of approximately 0.2 mm. To obtain constant toner chargeability in an axial direction of the development sleeve 141, the doctor supporting member 146b may need to maintain a positional relation with the surface of the development sleeve 141 across the development sleeve 141 in its axial direction with accuracy. Thus, the doctor supporting member 146b is attached to the doctor base body 146a by spot welding or swaging.
Referring to
It is preferable high roundness toner having an average roundness equal to or above 0.93 is adopted for use in the developing unit of the image forming apparatus 1. That is, it is known that the diameter of a toner particle is reduced to enhance image quality. However, when decreasing the diameter of a toner particle, a distribution of a conventional pulverized toner may become broad. Therefore, it is generally known to use a method for obtaining high image quality by increasing a circularity of toner by performing a polymerization reaction and making a sharp particle diameter distribution. The toner of this exemplary embodiment is typically prepared by dispersing a mixture of toner constituents including at least a polyester prepolymer having an isocyanate group, a polyester, a colorant, and a release agent in an aqueous medium in the presence of a particulate resin to perform a polymerization reaction (such as elongation and/or crosslinking). The toner constituents as described above are dissolved in an organic solvent to prepare a toner constituent solution. The dispersion is reacted with an elongation agent and/or a crosslinking agent in the aqueous medium. By using such a particulate resin, various effects can be achieved, for example, the pulverization process may not be required, the resource saving is promoted, the resultant toner has good charging ability and a sharp particle diameter distribution, and a toner shape control for changing the circularity of toner can be easily performed.
A shape factor “SF-1” of the toner used in the image forming apparatus may be in a range from approximately 100 to approximately 180, and the shape factor “SF-2” of the toner is in a range from approximately 100 to approximately 180.
Referring to
SF1={(MXLNG)2/AREA}×(100π/4) Equation 1,
where “MXLNG” represents the maximum major axis of an elliptical-shaped figure obtained by projecting a toner particle on a two dimensional plane, and “AREA” represents the projected area of elliptical-shaped figure.
When the value of the shape factor “SF-1” is 100, the particle has a perfect spherical shape. As the value of the “SF-1” increases, the shape of the particle becomes more elliptical.
Referring to
SF2={(PERI)2/AREA}×(100π/4) Equation 2,
where “PERI” represents the perimeter of a figure obtained by projecting a toner particle on a two dimensional plane.
When the value of the shape factor “SF-2” is 100, the surface of the toner is even (i.e., no convex and concave portions). As the value of the “SF-2” increases, the surface of the toner becomes uneven (i.e., the number of convex and concave portions increase).
In this exemplary embodiment of the present invention, toner images are sampled by using a field emission type scanning electron microscope (FE-SEM) S-800 manufactured by HITACHI, LTD. The toner image information is analyzed by using an image analyzer (LUSEX3) manufactured by NIREKO, LTD.
As a toner particle has a higher roundness, the toner particle is more likely to make a point-contact with the surface of the photoconductor drum 12 or another toner particle on the photoconductor drum 12. In this case, the adhesion force between these toner particles is weak, thereby making the toner particles highly flowable. Also, while weak adhesion force between the round toner particle and the photoconductive drum 12 enhances the transfer rate. Therefore, when the shape factor “SF-1” of the shape factor “SF-2” of the toner used in the image forming apparatus 1 exceeds 180, the transfer rate may decrease, which is not preferable.
Preferably, the toners according to an exemplary embodiment of the present invention have an volume average particle diameter of 3 μm to 8 μm, the ratio of (Dv/Dn) is 1.00 to 1.40, wherein Dv means a volume average particle diameter and Dn means a number average particle diameter. Further, narrower particle diameter distribution may lead to uniform distribution of toner charge and thus high quality images with less fog of background, and also higher transfer rate.
Further, the developing unit 14 according to the exemplary embodiment of the present invention can employ the magnetic carrier having a volume-based average particle diameter in a range of from 20 μm to 50 μm. By using the above-described magnetic carrier, the graininess in image can be enhanced, and therefore a good image quality can be obtained.
Generally, a gap between the development sleeve 141 and the photoconductor drum 12 (hereinafter, referred to as a “development gap”) and a diameter of magnetic carrier particle significantly affect the image quality. In the developing unit 14 according to this exemplary embodiment having the development gap, for example, in a range of from 0.1 mm to 0.4 mm, when the diameter of magnetic carrier particle is in a range of from 20 μm to 50 μm, a most preferable image quality can be obtained and the side effect is reduced.
If the development gap between the development sleeve 141 and the photoconductor drum 12 is too small, the electrical field between the development sleeve 141 and the photoconductor drum 12 becomes too strong, resulting in a problem referred to as carrier adhesion that the magnetic carrier particles are moved onto the surface of the photoconductor drum 12.
On the other hand, if the development gap is too large, the electrical field becomes small. For this reason, the developing effect is decreased, and the edge effect of the electrical field is increased in the edge of image portion, and thus it may become difficult to obtain an even image.
Further, if the diameter of magnetic carrier particle is too small, the size of magnetization of one carrier particle is reduced. Therefore, the magnetic binding force received from the magnetic roller 147 of the developing roller 140 is reduced, and the carrier adhesion is easily caused.
If the diameter of magnetic carrier particle is too large, the magnetic field between the magnetic carrier particles and an electrostatic latent image formed on the photoconductor drum 12 becomes sparse, and thus it may also become difficult to obtain an even image.
The volume-based average particle diameter distribution of the magnetic carrier can be determined by using measurement instruments for measuring particle diameter distribution of a toner particle, for example, a Coulter Counter (trademark) Model TA-II or a Coulter Multisizer II (trademark) (both available from Beckman Coulter, Inc.). More specifically, the volume-based average particle diameter distribution can be determined by the following process. Initially, a dispersant, i.e., 0.1 ml to 5 ml of surfactant (preferably alkylbenzene sulfonate) is added to 100 ml to 150 ml of electrolytic solution. The electrolytic solution is approximately 1% aqueous solution of NaCl of extra pure sodium chloride, such as ISOTON-II (trade name, available from Beckman Coulter, Inc.). Next, 2 mg to 20 mg of a test sample is added to the electrolytic solution. The electrolytic solution suspending the test sample is dispersed by an ultrasonic disperser for about 1 minute to 3 minutes. Thereafter, toner particles, or volume and number of toner are measured by the above-mentioned apparatus with an aperture of 100 μm, and the volume distribution and number distribution are calculated. The volume-average particle diameter (Dv) and the number-average particle diameter (Dn) are then determined from the determined distributions.
Further, the magnetic carrier according to an exemplary embodiment of the present invention includes a resin coating film surrounding a core of a magnetic member. The resin coating film contains charge control agent to add to a carrier-coating material of cross-linked substance of a melamine resin and a thermoplastic resin such as an acrylic resin, and the like. By using the magnetic carrier, an effect for absorbing impact or shock to reduce abrasion and retaining large carrier particles by an enhanced adhesion force and an effect for preventing impact to the resin coating film and cleaning of toner spent, in a balanced manner. Thus, the usable life of magnetic carrier can be longer, and film abrasion and toner spent can be avoided.
Next, referring to
The magnetic roller 147 in the developing unit 14 is a cylindrical member of resin with magnetic powder surrounded by an exterior perimeter surface magnetized by multiple magnetic poles (i.e., multiple magnets). A diameter of the magnetic roller 147 is approximately 18 mm. The magnetic poles formed on the magnetic roller 147 face the photoconductor drum 12 at the nip portion and are arranged in a counterclockwise direction in
The magnetic roller 147 is an integrally formed member. However, the magnetic roller 147 can be formed with multiple magnet members per magnetic pole around the axis thereof. For the integrally formed magnetic roller 147 used in this exemplary embodiment, it is preferable to use a roller in which magnetic powder is dispersed to resin such as ethylene ethyl acrylate and nylon (registered trade name). Preferable examples of the magnetic powder used in this exemplary embodiment include ferrites such as strontium ferrite and the like or rare earth magnetic particles such as NdFeB, SmFeN, and the like.
By contrast, the development sleeve 141 is development sleeve 141 is a hollow member of some nonmagnetic material. Examples of preferable material of the development sleeve 141 are aluminum, stainless steel, and the like, for workability, cost, and durability. More preferably, multiple elliptic dents are formed randomly on the outer perimeter surface of the development sleeve 141 so that the development sleeve 141 has multiple elliptic concave parts randomly on the outer perimeter surface thereof. Thus, the development sleeve 141 may have an uneven surface with multiple concave parts at random pitches, thereby presenting slippage of developer without adhering to the surface of the development sleeve 141 while the development sleeve 141 is rotating. Consequently a chain of developer beads rises on each concave part so that multiple chains of risen developer beads can form a thick magnetic brush. Further, the concave parts may not likely to abrade easily. Therefore, a good image with stable quality can be obtained without generating uneven image over an extended period of time. Such concave parts are preferably formed by using a conventional blasting, for example, colliding or bumping media of relatively large-shaped cut wires of short metallic wires to the surface of a pipe-shaped development sleeve.
It is a known method to form grooves or uneven convex and concave portions on the surface of the development sleeve by sand blasting, bead blasting, etc. so as to convey the developer easily. Specially, color image forming apparatuses typically use a development sleeve having convex and concave portions on the surface thereof by blasting for high image quality. Non-smooth processing such as groove forming, blasting, and the like prevents a decrease in image density generated due to slippage and accumulation of developer on the surface of the development sleeve 141 while the development sleeve 141 is rotating at high speed.
A magnet 155 is provided in the vicinity of the developing roller 140. Details of the magnet 155 will be described later.
The casing 144 provides separate space corresponding to a developer container 149 in the developing unit 14. The developer container 149 includes a developer storing chamber 149A, an agitation chamber 149B, and conveyance screws 142 and 143.
The developer storing chamber 149A is disposed below the development sleeve 141, extending in an axial direction of the development sleeve 141. The developer storing chamber 149A includes the conveyance screw 143 that rotates in a direction indicated by arrow “R1” in
The agitation chamber 149B is disposed adjacent and separate from the developer storing chamber 149A, extending in the axial direction of the development sleeve 141. The agitation chamber 149B includes the conveyance screw 142.
The conveyance screw 143 conveys the developer to a downstream end (far or distal side in
New or fresh toner for supplementing toner consumed for development is supplied through the toner supply port 145 to the developer in the agitation chamber 149B. While traveling in the developer storing chamber 149A, the developer is attracted to the development sleeve 141 by the action of magnetic force exerted by the magnetic pole N3 of the magnetic roller 147. Then, the developer on the development sleeve 141 is regulated by the doctor blade 146, passes the development region while facing the photoconductor drum 12, and returns to the developer container 149.
In an exemplary embodiment, the developer attracted from the developer storing chamber 149A to the development sleeve 141 by the action of the magnetic force generated by the magnetic pole N3 is conveyed in a counterclockwise direction in
The magnetic forces are calculated based on the following equations:
Fr=G×(Hr×(∂Hr/∂r)+Hr×(∂Hθ/∂r)); and
Fθ=G×(1/r×Hr×(∂Hr/∂θ)+1/r×(Hr×∂Hθ/∂θ)
where “Fr” represents a normal component of a magnetic force to the surface of a development sleeve (hereinafter, referred to as “normal component of the magnetic force Fr”), “Fθ” represents a tangential component of a magnetic force to the surface of a development sleeve (hereinafter, referred to as “tangential component of the magnetic force Fθ”), “Hr” represents a normal component of a magnetic flux density to the surface of a development sleeve, “Hθ” represents a tangential component of a magnetic flux density to the surface of a development sleeve, “r” represents a radius for calculation, and “G” represents a constant (7.8×10−15).
In the following description, when the normal component of the magnetic force Fr indicates a positive number, the magnetic force is exerted to move the magnetic carrier away from the development sleeve 141. By contrast, when the normal component of the magnetic force Fr indicates a negative number, the magnetic force is exerted to move the magnetic carrier toward the development sleeve 141.
Further, in the following description, an “upstream side” indicates an upstream side in a direction of conveyance of development on the development sleeve 141, a “downstream side” indicates a downstream side in a direction of conveyance of development on the development sleeve 141, and a “developer conveyance direction” indicates a direction of conveyance of development held on the surface of the development sleeve 141, unless otherwise specifically indicated.
In the exemplary embodiment, the magnetic pole N3 that is disposed adjacent the magnetic pole N2 is disposed in the vicinity of the doctor blade 146, as shown in
Further, the development sleeve 141 has a developer-releasing region P on a given area thereon, where the magnetic poles N2 and N3 generate a magnetic force that acts as a release force to cause the developer held on the development sleeve 141 to move away from the development sleeve 141 or toward a direction opposite to the surface of the development sleeve 141. In the exemplary embodiment, the developer-releasing region P is located so as not to be held in contact with (a top surface of) developer stored in the developer storing chamber 149A.
The development sleeve 141 of the developing roller 140 is disposed at a position higher than the development sleeve 1341 in the conventional developing unit 1314 of
By contrast, in the developing unit 14 according to the exemplary embodiment, the developer of a hard magnetic brush formed by the magnetic force generated by the magnetic pole N3 may not be subject to the above-described shearing forces, and thus the stress on the developer can be further reduced.
In the conventional developing unit 1314 of
In addition to the above-described function, the developer in the developer storing chamber 1349A in the conventional developing unit 1314 of
However, since the developer does not act as or not form such a wall in the exemplary embodiment of the present invention, if the developer released from the developer-releasing region P is not moved away from the developer-attracting region sufficiently or remains in the vicinity of the developer-attracting region, the developer can adhere to the development sleeve 141 again.
With the above-described reasons, the developing unit 14 according to the exemplary embodiment of the present invention is designed such that the normal component of the magnetic flux density Hr in the developer-releasing region P on the development sleeve 141 is directed to the north pole or N-pole direction, which is a positive direction same as the direction of the magnetic pole N2 and the magnetic pole N3, across the developer-releasing region P and does not form the local maximum point. By so doing, the release force can be effectively directed to the developer adhering to the development sleeve 141 in the developer-releasing region P. Details of this action will be described later. According to the above-described release force, the developing unit 14 according to the exemplary embodiment of the present invention can effectively reduce the developer carryover and developer reattachment on the development sleeve 141 even if the developer in the developer storing chamber 149A does not scrape off the developer in the developer-releasing region P or act as the wall to prevent developer reattachment to the development sleeve 141.
Next, descriptions are given of a relation between normal components of the magnetic flux density Hr and normal components of the magnetic force Fr with respect to respective surfaces of two different development sleeves, referring to graphs shown in
Similarly to the graph of
In these graphs of
The horizontal axis of the graphs indicates angles of the normal component of the magnetic force Fr to the development sleeve 141, when assuming that the direction of rotation of the development sleeve 141 or the counterclockwise direction is a positive direction and that a local maximum point of the normal component of the magnetic flux density Hr of the magnetic pole S1 to the development sleeve 141 has an angle of 0 degree.
The developing unit 1414 of
The comparative developing unit (the developing unit 1414 of
To eliminate the large loss, the present inventors conducted further researches and studies, and found the reason why the local minimum point of the normal component of the magnetic force Fr sharply dropped as shown in the graph of
However, the weak north pole could form a small local maximum point corresponding thereto, as shown in the graph of
Thus, as shown in
Next, descriptions are given of examples of a manufacturing method of a magnetic roller 147 having a distribution of the normal component of the magnetic flux density as described above, referring to
The magnetic roller 147 is constituted as a cylindrical member of a resin mixed with magnetic powder and has a perimeter surface surrounded by or facing magnetizing yokes 181 to 186 so as to magnetize the exterior perimeter surface to form magnetic poles S1, N1, S2, N2, and N3 in this order. The magnetizing yokes 181 to 185 corresponding to the magnetic poles S1, N1, S2, N2, and N3 are different in size, shape, and intensity of magnetic force depending on each width of the corresponding magnetic pole and intensity of the corresponding magnetic field.
Similarly, the magnetic roller 447 is constituted as a cylindrical member of a resin mixed with magnetic powder and has a perimeter surface surrounded by or facing magnetizing yokes and 481 to 486 so as to magnetize the perimeter surface to form magnetic poles S1, N1, S2, N2, and N3 in this order. The magnetizing yokes 481 to 485 corresponding to the magnetic poles S1, N1, S2, N2, and N3 are different in size, shape, and intensity of magnetic force depending on each width of the corresponding magnetic pole and intensity of the corresponding magnetic field.
As shown in
By contrast, the developing unit 14 according to an exemplary embodiment of the present invention employs the magnetizing yoke 186 as shown in
The method of manufacturing the magnetic roller 147 descri The method of manufacturing the magnetic roller 147 described here is an example and is not limited to. The present invention can be applied to any other method capable of manufacturing a magnetic roller such that the normal component of the magnetic flux density Hr to the developer-releasing region P on the development sleeve 141 can be magnetized to surely be directed to the same positive direction as the magnetic pole N2 and the magnetic pole N3 across the developer-releasing region P and the local maximum point may not be formed therein.
Further, the present invention can be applied to the magnetic roller 147 and any other roller or member disposed such that the normal component of the magnetic flux density Hr to the developer-releasing region P on the development sleeve 141 can be magnetized to surely be directed to the same positive direction as the magnetic pole N2 and the magnetic pole N3 across the developer-releasing region P and the local maximum point may not be formed therein.
As previously described, the greater the local minimum point of the normal component of the magnetic force (i.e., the release force) Fr falls or drops, the greater the loss becomes when the developer is removed from the development sleeve 141 in the developer-releasing region P. As shown in the graph of
The inventors of the present invention, which can be applied to solve the developer attachment, have found that the above-described developer reattachment is remarkably observed when a speed of the surface movement of the development sleeve 141 is 350 mm/sec or greater. The present invention can achieve a significant effect under the above-described condition.
The developing unit 14 according to the exemplary embodiment, the developing roller 140 includes the magnet 155 that serves as a repulsive magnetic field generator. The magnet 155 is disposed between the magnetic pole N2 and the magnetic pole N3 as shown in
For details, a description is given to a positional relation of the magnet 155 with respect to the magnetic poles of the development sleeve 141 with reference to
As illustrated in
Also as illustrated in
When the above-described magnet 155 is not incorporated, the previously described developer carryover and developer reattachment on the development sleeve 141 can occur in each end region in a direction along a shaft 141a of the development sleeve 141 in the opposed region of the effective development region of the magnetic roller 147 on the exterior perimeter surface of the development sleeve 141. Such a phenomenon may occur since, in the developer-releasing region P, magnetic field lines generated in the end region in the axial direction of the development sleeve 141 in the opposed region of the magnetic roller 147 may direct to the outside in the axial direction of the development sleeve 141. Therefore, the magnetic force exerting on the developer in the end regions has components directing toward the outside in the axial direction of the development sleeve 141. Therefore, the magnetic force serving as a release force cannot effectively exert the release force on the developer, and thereby causing the developer carryover and/or developer reattachment on the development sleeve 141.
Since the development sleeve 141 and the magnetic roller 147 are coaxially and integrally mounted as the developer roller 140, the shaft 141a of the development sleeve 141 corresponds to a shaft 147a of the magnetic roller 147.
As previously described, the configuration according to this exemplary embodiment of the present invention includes the magnet 155. Therefore, in the developer-releasing region P on the development sleeve 141, a direction of magnetic field lines in the each end region in the axial direction of the development sleeve 141 in a region opposite to the magnetic roller 147 can be close to a direction perpendicular to the direction of the shaft 141a of the development sleeve 141. This can increase in the release force in the end regions, which can cause the release force to be effectively exerted on the developer even in the end regions, so as to remove the developer from the outer perimeter surface of the development sleeve 141. As a result, the developer carryover and/or developer reattachment can be effectively reduced even in the end regions.
A magnetic pole face, which is the north pole face of the magnet 155, can be disposed at each end region of the magnetic roller 147 across the development sleeve 141 in the axial direction thereof. In this case, however, a part of the magnetic pole face disposed outside the end regions of the magnetic roller 147 may be arranged to generate a magnetic field greater than a different part of the magnetic pole face disposed inside the end regions of the magnetic roller 147 (i.e., a magnetic pole face facing the opposed region of the magnetic roller 147). For example, if the N-pole face of the magnet 155 has a magnetic force evenly on the N-pole face, the magnetic pole face of the magnet 155 can be arranged such that a part disposed outside the end regions of the magnetic roller 147 has area wider than other part disposed inside the end regions of the magnetic roller 147. With this configuration, even if the N-pole face of the magnet 155 is disposed across each end region of the magnetic roller 147 in the axial direction of the development sleeve 141, the direction of magnetic field lines at each end region of the magnetic roller 147 can be directed closer to a direction perpendicular to the axial direction of the development sleeve 141.
However, as described in the first exemplary embodiment, the configuration in which the N-pole face of the magnet 155 is disposed other than a position that faces the opposed region of the magnetic roller 147 is more effective to cause the direction of the magnetic field lines to make close to the direction perpendicular to the axis of the development sleeve 141, and therefore developer carryover can be reduced or prevented effectively.
Further, the developing unit 14 includes a seal member 148 to seal or eliminate space between the perimeter surface of the development sleeve 141 and the casing 144 of the developing unit 14. As shown in
Further, in the exemplary embodiment, the N-pole face of the magnet 155 is disposed so as to face the exterior perimeter surface of the development sleeve 141. However, the N-pole surface is not necessarily or limited to be disposed as above. For example, the N-pole face of the magnet 155 an be disposed outside the end region of the axis of the development sleeve 141 along the axis of the development sleeve 141. Specifically, for example, the magnet 155 can be disposed at the outer surface of the seal member 148 such that the N-pole face faces toward the center part of the axis of the development sleeve 141. Even with this configuration, the direction of magnetic field lines in each end region of the magnetic roller 147 in the axial direction of the development sleeve 141 can be close to a direction perpendicular to the axial direction of the development sleeve 141.
Further, in the exemplary embodiment, a minimum distance “X” (see
Next, a description is given of a first modified example of a relation between a normal component of the magnetic flux density and a normal component of the magnetic force with respect to a surface of a development sleeve, referring to graphs shown in
As shown in the graph of
Next, a description is given of a second modified example of a relation between normal components of the magnetic flux density Hr and normal components of the magnetic force Fr with respect to respective surfaces of two different development sleeves, referring to graphs shown in
Similarly to the graph of
In
As shown in the graph of
As described above, each of the developing units 14Y, 14C, 14M, and 14K according to the exemplary embodiments including the modified examples (hereinafter, referred to simply as the “exemplary embodiments”) includes the developing roller 140 serving as a developer bearing member, the developer container 149, conveyance screws 142 and 143 serving as agitation/conveyance members, and a doctor blade 146 serving as a developer regulating member. The developing roller 140 includes a magnetic roller 147 serving as a magnetic generator and a development sleeve 141 serving as a nonmagnetic hollow body containing the magnetic roller 147 to bear a two-component developer including magnetic carrier particles and toner particles on an exterior perimeter surface thereof by a magnetic force generated by the magnetic roller 147. The developer container 149 is disposed adjacent to the developing roller 140 and includes the developer storing chamber 149A to store the two-component developer therein. The conveyance screws 142 and 143 are disposed in the developer container 149 to convey the two-component developer in an axial direction of the development sleeve 141 of the developing roller 140 while agitating the two-component developer. The doctor blade 146 is disposed opposite the developing roller 140 to regulate the thickness of a layer of the two-component developer held on the development sleeve 141 of the developing roller 140. The two-component developer conveyed in the developer container 149 is attracted by the magnetic force exerted by the magnetic roller 147 to the developer bearing member, is regulated by the doctor blade 146, then passes through a development region of the development sleeve 141 of the developing roller 140 facing a corresponding one of the photoconductor drums 12Y, 12C, 12M, and 12K, and returns to the developer container 149. The magnetic roller 147 includes the magnetic pole N2 serving as a first magnetic pole and the magnetic pole N3 serving as a second magnetic pole with an identical polarity (north pole or N-pole) disposed adjacent to each other and downstream from the development region in a direction of rotation of the developing roller 140 to generate respective magnetic forces for removing the two-component developer from the development sleeve 141 of the developing roller 140 after the developer passes through the development region. The magnetic pole N3 is disposed downstream from the magnetic pole N2 in a direction of conveyance of developer by the development sleeve 141 of the developing roller 140 and proximate to the doctor blade 146 to generate a magnetic force to attract the two-component developer from the developer storing chamber 149A in the developer container 149 for forming a magnetic brush of the two-component developer on the development sleeve 141 of the developing roller 140 regulated by the doctor blade 146. In the developing unit 14, the development sleeve 141 of the developing roller 140 includes the developer-releasing region P to release the two-component developer from the development sleeve 141 of the developing roller 140 using a release force (the magnetic force in the normal direction with a positive value) corresponding to magnetic forces generated by the magnetic poles N2 and N3. The developer is disposed higher than a top surface of the two-component developer stored in the developer storing chamber 149A of the developer container 149 so that the developer-releasing region P formed on the development sleeve 141 remains separated from the top surface of the two-component developer in the developer storing chamber 149A as the development sleeve 141 rotates. The magnetic roller 147 is disposed such that a component of a magnetic flux density of the magnetic field generated by the magnetic roller 147 in a direction normal to the developer-releasing region P on the development sleeve 141 of the developing roller 140 is directed to a same positive direction (with positive values) as the magnetic poles N2 and N3 across the developer-releasing region P without forming a local maximum point. As previously described, this configuration can reduce the fall or drop of the local minimum point of the magnetic force in the normal direction Fr (i.e., the release force), which can be loss when the developer is released from the development sleeve 141 in the developer-releasing region P. Therefore, even if the developer in the developer storing chamber 149A does not scrape off the developer in the developer-releasing region P or act as a wall to prevent developer attachment to the development sleeve 141, the developing unit 14 according to the exemplary embodiments can effectively reduce developer carryover and developer reattachment on the development sleeve 141 of the developing roller 140, and therefore can effectively prevent image quality degradation caused by the above-described reasons.
Further, as shown in
Further, as shown in the first modified example, the magnetic roller 147 can be disposed such that the release force exerted on the developer in the developer-releasing region P on the development sleeve 141 of the developing roller 140 has a single local maximum point. Since the local minimum point cannot be formed with this configuration, the developing unit 14 according to the exemplary embodiments can reduce developer carryover and developer reattachment on the development sleeve 141 of the developing roller 140 more effectively, and therefore can effectively prevent image quality degradation caused by the development carryover and/or development reattachment.
Further, as shown in the second modified example, the developer-releasing region P on the development sleeve 141 of the developing roller 140 can include the first local maximum point Hr1 where the magnetic flux density in the normal direction Hr of the magnetic pole N2 reaches a maximum on the development sleeve 141 in the direction of conveyance of developer thereon, the second local maximum point Hr2 where the magnetic flux density in the normal direction Hr of the magnetic pole N3 reaches a maximum on the development sleeve 141 in the direction of conveyance of developer thereon, and the local minimum point Hr3 where the magnetic flux density in the normal direction Hr to the development sleeve 141 reaches a minimum on the development sleeve 141. The magnetic roller 147 may be disposed such that the local minimum point Hr3 is located closer to the second local maximum point Hr2 than to the first local maximum point Hr1 from a center point between the first local maximum point Hr1 and the second local maximum point Hr2. Therefore, the developing unit 14 can reduce reattachment of the removed developer to the developer sleeve 141.
Further, in the exemplary embodiments, the speed of surface movement of the development sleeve 141 is 350 mm/sec or greater, which may generally cause developer reattachment to the development sleeve 141 of the developing roller 140. However, by employing the above-described configuration, the developer reattachment can be prevented effectively, and therefore can be effectively prevent image quality degradation caused by the developer reattachment in high-speed image forming apparatuses.
Further, in the exemplary embodiments, multiple elliptic dents are formed randomly on the exterior perimeter surface of the development sleeve 141 of the developing roller 140. Therefore, as previously described, a good image with stable quality can be obtained without generating uneven image over an extended period of time.
Further, in the exemplary embodiments, the volume average particle diameter of each of the magnetic carrier particles is approximately 20 μm to approximately 50 μm, and therefore a good image with stable graininess can be obtained over an extended period of time.
The above-described exemplary embodiments are illustrative, and numerous additional modifications and variations are possible in light of the above teachings. For example, elements and/or features of different illustrative and exemplary embodiments herein may be combined with each other and/or substituted for each other within the scope of this disclosure. It is therefore to be understood that, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.
Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, the invention may be practiced otherwise than as specifically described herein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-095302 | Apr 2008 | JP | national |
