Development device, process cartridge including same, and image forming apparatus including same

A development device includes a developer carrier, a supply compartment having a downstream end portion positioned outside the development area in the axial direction of the developer carrier, a collection compartment disposed lower than the supply compartment, first and second communication ports respectively formed in the downstream end portion and an upstream end portion of the supply compartment, a developer circulation unit including first and second developer conveyance members respectively provided in the supply compartment and the collection compartment, a toner supply port formed in an upper portion of the downstream end portion of the supply compartment, and an airflow path limiter provided in the downstream end portion of the supply compartment, closer to the development area than the toner supply port in the axial direction of the developer carrier, to restrict an airflow path above the developer in the downstream end portion of the supply compartment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent specification is based on and claims priority from Japanese Patent Application No. 2009-290420, filed on Dec. 22, 2009 in the Japan Patent Office, which is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a development device using two-component developer consisting essentially of toner and carrier, a process cartridge including the same, and an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction machine having at least two of these capabilities, that includes the same.

2. Description of the Background Art

Two-component developer consisting essentially of toner and carrier is widely used in electrophotographic image forming apparatuses.

Development devices using two-component developer typically include a developer container in which developer is contained, a rotary developer carrier such as a development roller, and a developer conveyance member such as a conveyance screw provided in the developer container. The developer conveyance member supplies developer to the developer carrier while transporting the developer through a developer supply compartment (i.e., a developer supply path) in the developer container in an axial direction of the developer carrier. Then, the developer carrier rotates and supplies the developer carried thereon to a development area facing an image carrier such as a photoconductor. After having passed through the development area and toner therein has been consumed, the developer (hereinafter “used developer”) is collected either in the supply compartment (hereinafter “one-conveyance path method”) or a collection compartment separate from the supply compartment (hereinafter “supply-collection separation method).

The one-conveyance path method has a drawback in that the concentration of toner in the developer in the supply compartment decreases downstream in a direction in which the developer is transported (hereinafter “developer conveyance direction”), and accordingly the concentration of toner in the developer supplied to the development area is uneven in the axial direction of the developer carrier. Such unevenness in toner concentration causes unevenness in image density of images formed on sheets of recording media and is undesirable. It is to be noted that hereinafter “downstream” and “upstream” as used in this specification means downstream and upstream in the developer conveyance direction unless otherwise specified.

To solve the problem described above, for example, JP-2002-006599-A employs a supply-collection separation method in which used developer is collected in a collection compartment separate from the supply compartment (hereinafter “a supply-collection separation-type development device”). In the supply-collection separation-type development device, the concentration of toner in the developer in the supply compartment can be kept substantially constant in the developer conveyance direction. Although the concentration of toner in the developer supplied to the development area can be kept uniform in the axial direction of the developer carrier in the supply-collection separation-type development device, doing so makes it impossible to make such development devices compact. At present, it is preferred to make development devices more compact to make the image forming apparatus incorporating the development device compact.

Several approaches, such as those described below, have been tried to make supply-collection separation-type development devices compact. For example, a vertical agitation arrangement shown inFIG. 17is effective to make supply-collection separation-type development devices compact. More specifically, in a known supply-collection separation-type development device3Z shown inFIG. 17, a supply compartment301SZ is disposed adjacent to and to one side of a developer carrier302Z (inFIG. 17, in a lateral direction), and a collection compartment301CZ is disposed beneath the supply compartment301SZ. The collection compartment301CZ receives developer that has been transported to a downstream end portion of the supply compartment301SZ. In the development device3Z, developer is circulated in the direction indicated by outlined arrows A1and A2shown inFIG. 17.

However, the vertical agitation arrangement has a limitation regarding the location of a toner supply mechanism for supplying toner to the development device. In the development device3Z shown inFIG. 17, developer is transported from the downstream end portion of the supply compartment301SZ to an upstream end portion of the collection compartment301CZ and transported from a downstream end portion of the collection compartment301CZ to an upstream end portion of the supply compartment301SZ, thus forming a developer circulation path.

In the development device3Z, as the location of a toner supply port310Z through which toner is supplied to the developer circulating in the developer circulation path, a portion of the supply compartment301SZ facing the development area of the developer carrier302Z must be avoided. If toner is supplied to that portion, the toner just after supplied is likely to be carried by the developer carrier302Z to the development area. Since electrical charge of the toner just after supplied is insufficient, such toner can scatter on the backgrounds of output images or around the interior of the image forming apparatus if being used in image development.

In view of the foregoing, it is preferred that the toner supply position be positioned further from the upstream end portion of the supply compartment301SZ on the developer circulation path, outside the portion facing the development area of the developer carrier302Z. Such an arrangement can increase contact between the supplied toner and carrier particles in the developer, thus charging the supplied toner better, before the supplied toner reaches the upstream end portion of the supply compartment301SZ where the developer is carried onto the developer carrier302Z.

Therefore, in supply-collection separation-type development devices, toner is typically supplied to the collection compartment301CZ. However, to supply toner to the collection compartment301CZ in the vertical agitation arrangement in which the collection compartment301CZ is positioned beneath the supply compartment301SZ, the arrangement is limited to the two arrangements described below.

In a first arrangement, as shown inFIG. 17, a toner supply route350is positioned on the side of the supply compartment301SZ, opposite the developer carrier302Z, and the toner supply port310Z, which is at a downstream end of the toner supply route350, is formed in a side wall of the collection compartment301CZ positioned beneath the supply compartment301SZ. This arrangement increases the size of the development device3Z in the lateral direction inFIG. 17by a length corresponding to the width of the toner supply route350.

FIG. 18illustrates a second arrangement regarding the location of the toner supply route. As shown inFIG. 18, in a development device3Z1, a collection compartment301CZ′ is made longer than a supply compartment301SZ′ in the axial direction of a developer carrier302Z, and a toner supply port310Z is provided in the expanded portion of the collection compartment301CZ. This arrangement increases the size of the development device3Z1in the axial direction of the developer carrier302Z (inFIG. 18, the lateral direction) by a length corresponding to the width of the toner supply port310Z.

As described above, in the vertical agitation arrangement in which the collection compartment is disposed beneath the supply compartment, the development device becomes bulkier when toner is supplied to the collection compartment, which is not desirable.

In view of the foregoing, the inventors of the present embodiment recognize that there is a need for the development device to reduce scattering of toner in the background of output images and around the interior of the image forming apparatus without increasing the size of the device.

SUMMARY OF THE INVENTION

In view of the foregoing, one illustrative embodiment of the present invention provides a development device to develop a latent image formed on a latent image carrier with developer. The development device includes a developer carrier disposed facing the latent image carrier, to carry the developer by rotation to a development area facing the latent image carrier, a partition dividing an interior of the development device into at least a supply compartment and a collection compartment both facing the developer carrier and extending in an axial direction of the developer carrier, a developer circulation unit to circulate the developer in the development device, a toner supply port formed in an upper portion of a downstream end portion of the supply compartment in a developer conveyance direction in which the developer is circulated, positioned outside the development area in an axial direction of the developer carrier, and an airflow path limiter provided in the downstream end portion of the supply compartment and positioned closer to the development area than the toner supply port in the axial direction of the developer carrier, to restrict an airflow path above the developer in the downstream end portion of the supply compartment.

The developer is supplied from the supply compartment to the developer carrier and is collected from the developer carrier to the collection compartment disposed lower than the supply compartment. To circulate the developer inside the development device, a first communication port is formed in a bottom portion of the downstream end portion of the supply compartment, and a second communication port is formed in an upstream end portion of the supply compartment. The developer circulation unit includes a first developer conveyance member provided in the supply compartment, to supply the developer to the developer carrier while transporting the developer through the supply compartment in the axial direction of the developer carrier and a second developer conveyance member disposed in the collection compartment that faces the developer carrier, to transport in the axial direction of the developer carrier the developer separated from the developer carrier.

Another illustrative embodiment provides a process cartridge that is removably installable in an image forming apparatus and including a latent image carrier on which a latent image is formed and the development device described above.

Yet another illustrative embodiment provides an image forming apparatus including a latent image carrier on which a latent image is formed and the development device described above.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views thereof, and particularly toFIG. 1, a multicolor image forming apparatus according to an illustrative embodiment of the present invention is described.

It is to be noted that the subscripts M, C, Y, and K attached to the end of each reference numeral indicate only that components indicated thereby are used for forming black, magenta, yellow, and cyan images, respectively, and hereinafter may be omitted when color discrimination is not necessary.

FIG. 1is a schematic diagram of an image forming apparatus according to the present embodiment.

Referring toFIG. 1, an image forming apparatus100according to the present embodiment is a tandem-type printer (hereinafter also “printer100”) and includes multiple image forming units17K,17M,17Y, and17C arranged along a transport belt15that transports transfer sheets8(i.e., recording media). It is to be noted that arrangement order of the image forming units17is not limited to that shown inFIG. 1, and, for example, the image forming units17may be arranged in the order of magenta, cyan, yellow, and black with the image forming unit17K disposed extreme downstream in a direction in which the transfer sheet8is transported.

Each image forming unit17includes a photoconductor1serving as a latent image carrier and forms black, magenta, yellow, or cyan images on the photoconductor1. In the present embodiment, each image forming unit17includes multiple components housed in a common unit casing and is configured as a process cartridge removably installable in a main body of the printer100although it is not necessary that those components are united as a single unit. The image forming units17K,17M,17Y, and17C have a similar configuration except for the color of toner used therein.

The transport belt15is stretched around support rollers18and19. One of the rollers18and19is a driving roller and the other is a driven roller. As the driving roller rotates, the transport belt15rotates in the direction indicated by an arrow shown inFIG. 1.

Additionally, sheet cassettes20,21, and22are provided beneath the transport belt15, and a discharge tray25is provided on an upper surface of the main body of the printer100.

The image forming unit17is described in further detail below.

The photoconductor1rotates clockwise inFIG. 2. A charging roller of a charging device2is provided above the photoconductor1. The charging device2in the present embodiment employs a contact-charging method, and the charging roller to which a charging bias is applied is disposed in contact with a circumferential surface of the photoconductor1, thereby charging the photoconductor1uniformly to a predetermined electrical potential. Alternatively, a noncontact method may be adopted. Subsequently, the circumferential surface of the photoconductor1thus charged uniformly by the charging device2receives an optical beam L (i.e., a writing light) emitted from an exposure unit16shown inFIG. 1, and thus an electrostatic latent image is formed thereon.

In the present embodiment, the charging device2and the exposure unit16together form a latent image forming unit. The electrostatic latent image formed on the photoconductor1is transported to a development area A (shown inFIG. 2) facing a development device3as the photoconductor1rotates.

The development device3includes a casing301(shown inFIG. 2), serving as a developer container for containing two-component developer consisting essentially of toner and carrier and transports the developer to the development area A, in which toner in the developer adheres to the electrostatic latent image formed on the photoconductor1. Thus, the electrostatic latent image is developed into a toner image.

The toner image is then transported to a portion facing the transport belt15, that is, a transfer area B shown inFIG. 2, as the photoconductor1further rotates. The image forming unit17further includes a transfer device5positioned across the transport belt15from the photoconductor1, on an inner side of the transport belt15. The transfer device5in the present embodiment employs a transfer roller to which a transfer bias is applied. Alternatively, a corona discharge-type transfer device may be used. The image forming unit17further includes a cleaning unit6positioned downstream from the transfer device5in the rotational direction of the photoconductor1. The cleaning unit6includes a cleaning blade601, shown inFIG. 2, for removing any toner remaining on the circumferential surface of the photoconductor1after the toner image is transferred therefrom onto the transfer sheet8.

Multicolor image formation in the printer100is described below.

In multicolor image formation, in each image forming unit17, the circumferential surface of the photoconductor1is uniformly charged by the charging device2in the dark. Then, the exposure unit16directs the writing light L onto the charged circumferential surface of the photoconductor1, thus forming an electrostatic latent image thereon. Subsequently, the development device3develops the electrostatic latent image with toner, and thus single-color toner images are formed on the respective photoconductors1.

Meanwhile, the transfer sheet8on the top, for example, in the sheet cassette20is picked up and fed along a sheet conveyance path26to a pair of registration rollers23. The registration rollers23stop the transfer sheet8and then send out the transfer sheet8, timed to coincide with image formation in the image forming unit17K. Then, the transfer sheet8is attracted to the transport belt15electrostatically and is transported to the image forming unit17K positioned extreme upstream in the sheet conveyance direction.

In the image forming unit17K, the transfer device5transfers the black toner image from the photoconductor1K onto the transfer sheet8. Then, the respective toner images on the photoconductors1M,1Y, and1C are transferred and superimposed one on another on the black image formed on the transfer sheet8carried by the transport belt15.

Thus, a multicolor toner image is formed on the transfer sheet8, after which the transfer sheet8is separated from the transport belt15and is forwarded to a fixing device24. While the transfer sheet8passes between a pair of fixing rollers provided in the fixing device24, the multicolor toner image is fixed thereon. Then, the transfer sheet8is discharged onto the discharge tray25.

Meanwhile, the cleaning unit6removes any toner remaining on the circumferential surface of the photoconductor1after image transfer in preparation for subsequent image formation. The circumferential surface of the photoconductor1that has passed through the cleaning unit6is again charged by the charging device2uniformly. Thus, image formation is repeated.

It is to be noted that features of the present embodiment can adapt to an intermediate transfer method in which toner images formed on the photoconductors1are primarily transferred and superimposed one on another on an intermediate transfer member, such as an intermediate transfer belt, and then transferred from the intermediate transfer member onto the transfer sheet. In such a configuration, the toner image formed on the photoconductor1is transferred in the transfer area B onto the intermediate transfer member.

Next, the development device3is described in further detail below.

FIG. 2is a schematic end-on axial view of the image forming unit17in the printer100according to the present embodiment. It is to be noted that, inFIG. 2, reference numeral320represents two-component developer.

Referring toFIG. 2, the development device3according to the present embodiment includes a development roller302serving as a developer carrier, and an interior of the casing301is divided by a partition306into a supply compartment301S and a collection compartment301C. The supply compartment301S extends in an axial direction of the development roller302, which is a direction perpendicular to the surface of paper on whichFIG. 2is drawn. While being transported in that direction, the two-component developer320including toner and carrier in the supply compartment301S is supplied onto a surface of the development roller302that is rotating.

The development roller302transports the developer320carried thereon to the development area A, in which toner in the developer320adheres to the electrostatic latent image formed on the photoconductor1, thus developing it into a toner image. An opening is formed in the casing301of the development device3to expose the development roller302partially. The exposed portion of the development roller302faces the photoconductor1and is positioned close to and to a side of the photoconductor1(in the lateral direction inFIG. 2). Thus, the development area A is formed in the portion where the development roller302faces the photoconductor1.

As shown inFIG. 2, the developer320that has passed through the development area A is collected from the development roller302in the collection compartment301C separate from the supply compartment301S. Thus, the development device3employs a supply-collection separation method.

The development device3includes a first conveyance screw or supply screw304provided in the supply compartment301S for transporting developer therein and a second conveyance screw or collection screw305provided in the collection compartment301C for transporting developer therein. The development device3further includes a doctor blade303for adjusting the amount of developer carried on the development roller302.

It is to be noted that, inFIG. 2, reference numeral9represents a release area in which developer is separated from the development roller302, and10represents an attraction area in which developer in the supply compartment301S is carried onto the development roller302. Additionally, reference character O-302represents center of rotation of the development roller302, O-2represents that of the photoconductor1, O-304represents that of the first conveyance screw304, and O-305represents that of the second conveyance screw305.

FIG. 3is a schematic cross-sectional view from a side of the development device3according to the present embodiment and illustrates flow of developer therein.

In the development device3, the collection compartment301C is positioned beneath the supply compartment301S, and thus employs a vertical agitation arrangement. The developer transported in the direction indicated by arrow11shown inFIG. 3(hereinafter “developer conveyance direction”) by the first conveyance screw304to a downstream end portion therein is transported through a communication port309(first communication port) formed on a bottom of that portion of the supply compartment301S, as indicated by arrow14, to an upstream end portion of the collection compartment301C in the developer conveyance direction. Then, the developer transported in the direction indicated by arrow12by the second conveyance screw305to a downstream end portion of the collection compartment301C is further transported through an opening307(second communication port) formed therein, as indicated by arrow13shown inFIG. 3, to an upstream end portion of the supply compartment301S.

It is to be noted that, inFIG. 3, reference numeral310represents a toner supply port, and308represents a bladed wheel formed in the downstream end portion, corresponding to the opening307, of the collection compartment305in the developer conveyance direction by the second conveyance screw305. The bladed wheel308is described in further detail later with reference toFIG. 8.

Next, the development roller302is described below with reference toFIGS. 5 and 6.

FIG. 5illustrates distribution of magnetic flux density superimposed on a schematic end-on axial view of the development device3.

Referring toFIG. 5, a magnet roller302dis provided inside the development roller302, and its position is fixed relative to the development device3. The magnet roller302dincludes multiple magnets MG arranged in a circumferential direction of thereof, and a cylindrical sleeve302cprovided outside the magnet roller302drotates integrally with a rotary shaft302e. It is to be noted that, inFIG. 5, only one of the multiple magnets provided in the magnet roller302dis given the reference character “MG” for simplicity. The sleeve302cis formed of nonmagnetic metal such as aluminum although other materials may be included therein. The magnet roller302dis fixed to the casing301, for example, so that each magnet MG faces in a predetermined direction. The development roller302transports the developer320carried on a surface of the sleeve302c, attracted by magnetic force exerted by the magnet MG, in the circumferential direction thereof as the sleeve302crotates.

It is to be noted that, inFIG. 5, reference characters GP1designates a gap between the photoconductor1and the development roller302(i.e., development gap), and GP2designates a gap between the development roller302and the partition306.

FIG. 6is a cross-sectional view of the development roller302along the axial direction thereof. InFIG. 6, reference character O-302adesignates a center line passing through the center of rotation O-302of the development roller302.

The development roller302further includes a fixed shaft302afixed to the casing301, the magnet roller302d, which is cylindrical and united to the fixed shaft302a, the sleeve302coverlaying the magnet roller302dacross a gap, and the rotary shaft302eunited to the sleeve302c. The rotary shaft302eis rotatable relative to the fixed shaft302avia bearings302f, and the rotary shaft302eis driven by a driving unit, not shown.

As shown inFIG. 6, the multiple magnets MG are fixed on an outer circumferential surface of the magnet roller302d, arranged at predetermined intervals. The sleeve302crotates around the magnets MG. The magnets MG form magnetic fields to cause developer particles to stand on end on the circumferential surface of the sleeve302cand to separate the developer particles from the sleeve302c. In the present embodiment, five magnets MG are provided inside the sleeve302c, thus generating five magnetic poles, as shown in the distribution of magnetic flux shown inFIG. 5. It is to be noted that hereinafter the magnetic pole positioned on a vertical line connecting the center of rotation O-302of the development roller302and the center of rotation O-2of the photoconductor1is referred to as a magnetic pole P1, and other magnetic poles are respectively referred to as magnetic poles P2, P3, P4, and P5from the upstream side in the rotational direction of the development roller302.

The polarity of the magnetic poles P1, P2, P3, P4, and P5are north (N), south (S), N, N, and S, respectively. Alternatively, the magnetic poles P1through P5may have the reverse polarities to those shown inFIG. 5. On the development roller302shown inFIG. 5, centers of the magnetic poles P1, P2, P3, and P4are substantially at eight o'clock, seven o'clock, five o'clock, and one o'clock, respectively.

In the development area A, the development roller302is not in direct contact with the photoconductor1, and the development gap GP1having a predetermined distance suitable for image development is kept between the development roller302and the photoconductor1. Developer particles are caused to stand on end on the circumferential surface of the development roller302in the magnetic pole P1and brought into contact with the surface of the photoconductor1. Thus, toner particles can adhere to the electrostatic latent image formed thereon, developing the latent image.

Referring toFIG. 5, a grounded power source VP for generating development bias is connected to the fixed shaft302a. Voltage from the power source VP connected to the fixed shaft302ais applied via the electroconductive bearings302fand the electroconductive rotary shaft302eto the sleeve302c. By contrast, as shown inFIG. 5, an electroconductive support body31that forms an innermost layer of the photoconductor1is grounded. Thus, an electrical field for conveying toner particles separated from carrier particles toward the photoconductor1is formed in the development area A, and accordingly the toner particles move toward the photoconductor1due to differences in electrical potential between the sleeve302cand the electrostatic latent image formed on the surface of the photoconductor1.

It is to be noted that the present embodiment describes development devices to be used in image forming apparatuses that involve an exposure process using optical writing light L. More specifically, the charging device2shown inFIG. 2charges the photoconductor1uniformly to a negative electrical potential, and the portion on which an image is to be formed (i.e., an image portion) is exposed to the writing light L so as to reduce the amount of optical writing. Then, the image portion, that is, an electrostatic latent image, that has a reduced electrical potential is developed with toner particles whose polarity is negative, which is a method so-called “reversal development”. This is just an example, and, in development methods to which the features of the present invention are applicable are not limited to the description above regarding the polarity of charging potential of the photoconductor1.

After being used in image development in the development area, the developer320is kept on the development roller302by the magnetic force exerted by the magnetic pole P2and is conveyed inside the casing301as the development roller302rotates. The portion of the casing301corresponding to the magnetic pole P2is positioned close to the circumferential surface of the sleeve302cand curved conforming to the circumferential surface of the sleeve302c. Therefore, scattering of toner particles is alleviated with sealing effects.

The magnetic poles P3and P4positioned downstream from the magnetic pole P2in the rotational direction of the development roller302have an identical polarity. Consequently, magnetic force attracting the developer320to the surface of the development roller302becomes weaker between the magnetic poles P3and P4, and effects of releasing the developer320from the development roller302act in the release area9shown inFIGS. 2 and 4. After image development, because toner therein has moved to the photoconductor1and adhered to the latent image, the developer320has a reduced toner concentration. Therefore, desired image density might not be attained if such developer320having a reduced toner concentration is not separated from the development roller302but is carried again to the development area A (hereinafter “carryover of developer”) and used in image development.

In view of the foregoing, in the present embodiment, the developer320is separated from the development roller302in the release area9(shown inFIG. 4) and collected in the collection compartment301C (shown inFIG. 5) separate from the supply compartment301S to prevent carryover of developer.

Subsequently, the surface of the development roller302from which the developer320is separated reaches a portion facing the supply compartment301S as the development roller302rotates. The magnetic pole P4is present at that position, and the developer320flowing in the supply compartment301S is attracted to the attraction area10of the development roller302and carried thereon by the magnetic force exerted by the magnetic pole P4. Then, while passing by the development doctor303, the amount of the developer320carried by the magnetic force exerted by the magnetic pole P4on the development roller302is adjusted, after which the developer320is transported to the development area A. The magnetic pole P5positioned between the magnetic poles P4and P1serves as a developer conveyance pole for keeping developer on the surface of the development roller302in an area extending from the development doctor303to the development area A.

Next, arrangement of respective components in the development device4is described below also with reference toFIGS. 7 and 8as required.

FIG. 7is a perspective view illustrating the components of the development device4assembled, andFIG. 8is an exploded view of the components of the development device4. It is to be noted that, inFIGS. 7 and 8, reference character O-304arepresents a centerline that passes through the center of rotation O-304and parallels the centerline O-302aof the development roller302.

As shown inFIGS. 7 and 8, the first conveyance screw304provided in the supply compartment301S includes a spiral blade304B fixed to a screw shaft304C and rotates clockwise inFIG. 7on the centerline O-304aas indicated by an arrow shown inFIG. 7. With this configuration, the first conveyance screw304transports developer from the back side to the front side inFIG. 7as indicated by arrow11while agitating the developer. In other words, the first conveyance screw304transports developer in the axial direction from the back side to the front side inFIG. 7.

Similarly, as shown inFIGS. 7 and 8, the second conveyance screw305provided in the collection compartment301C includes a spiral blade305B fixed to a screw shaft305J and rotates counterclockwise inFIG. 7as indicated by an arrow shown inFIG. 7on a centerline O-305athat passes through its center of rotation O-305and parallels the centerline O-302aof the development roller302. With this configuration, the second conveyance screw305transports developer from the front side to the back side inFIG. 7as indicated by arrow12while agitating the developer. In other words, the second conveyance screw305transports developer in the axial direction from the front side to the back side inFIG. 7, which is opposite the developer conveyance direction of the first conveyance screw304.

The partition306provided between the first conveyance screw304and the second conveyance screw305, thus separating the supply compartment301S including the first conveyance screw304from the collection compartment301C including the second conveyance screw305, is shaped like a cantilever supporter with one side thereof united to an inner face of the casing301on the side opposite the development roller302. The partition306is provided only in a center portion and is not present in both end portions in the long axis direction of the development roller302. By contrast, the first and second conveyance screws304and305extend into both end portions in the long axis direction of the development roller302.

In the collection compartment301C, developer is transported in the direction indicated by arrow12and then is blocked by a side wall of the casing301in the downstream end portion in the developer conveyance direction therein. Thus, the developer is piled against the side wall of the casing301and then moves in the direction indicated by arrow13through the opening307to the supply compartment301S. In the supply compartment301S, developer is transported in the direction indicated by arrow11and then flows down in the direction indicated by arrow14through the communication port309to the collection compartment301C.

Thus, the developer can move between the supply compartment301S and the collection compartment301C in the both end portions in the long axis direction where the partition306is not present, and accordingly a developer circulation path in which developer flows in the order of arrows11,14,12, and13is formed. Additionally, the partition306supports the developer320agitated by the first conveyance screw304from below, thus forming the supply compartment301S. Simultaneously, the partition306inhibits the developer separated from the development roller302in the release area9and collected in the collection compartment301C from moving to the supply compartment301S.

To secure these functions, the gap GP2between the circumferential surface of the development roller302and the partition306is preferably within a range of about 0.2 mm to 1 mm. If the gap GP2is less than 0.2 mm, the development roller302might contact the partition306due to eccentricities in rotation of the development roller302. If the gap GP2is greater than 1 mm, the effect of inhibiting the collected developer from moving to the supply compartment301S might be insufficient.

Further, although the partition306functions even if positioned shifted from the release area9, the partition306might regulate a relatively large amount of developer in such an arrangement, thus increasing stress to the developer. Therefore, such an arrangement is not preferred unless arrangement described below is considered. Therefore, in the present embodiment, the release area9is disposed around the development roller302on the side opposite the photoconductor1and lower than the attraction area10, and the attraction area10is disposed adjacent to and downstream from the release area9in the rotational direction of the development roller302. Additionally, the partition306is disposed in a portion between the release area9and the attraction area10, where the amount of developer adhering to the surface of the development roller302is extremely small so as to separate the space around the first conveyance screw304from the space around the second conveyance screw305. Moreover, an edge face of the partition306on the side of the development roller302faces the development roller302.

With this configuration, even if the gap GP2is out of the range from 0.2 mm to 1 mm, the partition306can function because the partition306is provided in the portion where the amount of developer adhering to the surface of the development roller302is small. Additionally, stress to the developer caused by the partition306can be reduced. That is, the limitation in the size of the gap GP2between the development roller302and the partition306can be alleviated in designing the device. It is to be noted that stress to the developer can be further reduced by satisfying the range from about 0.2 mm to 1 mm as the size of the gap GP2, in addition to the above-described arrangement.

In addition, in the present embodiment, the partition306is positioned so that the partition306is closest to the development roller302in a portion between the first conveyance screw304and the second conveyance screw305where the amount of developer adhering to the surface of the development roller302is extremely small, that is, within the release area9between the magnetic pole P3and the magnetic pole P4, where the density of magnetic flux on the surface of the development roller302is less than 10 mT.

Additionally, in the present embodiment, as shown inFIG. 8, the bladed wheel308is provided instead of the spiral blade305B in the downstream end portion, in the area corresponding to the opening307, of the collection compartment301C in the developer conveyance direction of the second conveyance screw305. The bladed wheel308includes multiple planar blades radially extending from the screw shaft305J (centerline O-305a) of the second conveyance screw305and flips up developer as the second conveyance screw305rotates.

As shown inFIG. 4, the centers of rotation O-304and O-305of the first and second conveyance screws304and305are substantially on an identical vertical line, and the bladed wheel308rotates counterclockwise inFIG. 4while flipping up developer against the inner face of the casing301. The opening307extends from the position slightly shifted toward the inner face of the casing301from the vertical line connecting the centers of axes O-304and O-305to the inner face of the casing301so that the route in which the flipped developer moves is not blocked, and simultaneously the flipped developer does not fall toward the second conveyance screw305. In other words, although the opening307connecting the supply compartment301S and the collection compartment301C is formed in the partition306in the portion where developer is brought up from the second conveyance screw305to the first conveyance screw304, the partition306is present on the side closer to the development roller302in the portion corresponding to the opening307similarly to the center portion in the longitudinal direction of the development roller302. With that portion of the partition306, the developer brought up through the opening307does not fall down to the second conveyance screw305but is attracted to the development roller302. Then, the developer is either transported by the development roller302to the second conveyance screw305or to the first conveyance screw304. Thus, the developer can be circulated efficiently.

The first conveyance screw304rotates clockwiseFIG. 4, opposite the direction in which the development roller302rotates. Generally, while transporting objects in their rotational directions, screws draw the object to be transported to the rotational direction. Therefore, the first conveyance screw304transports developer while drawing the developer to the side of the development roller302. Consequently, the amount of developer in contact with the development roller302can be increased, and developer can be supplied to the development roller302reliably.

By contrast, the second conveyance screw305rotates counterclockwise inFIG. 4, which is identical to the direction in which the development roller302rotates. With this configuration, the second conveyance screw305transports developer while drawing the developer to the opposite side of the development roller302. Therefore, the developer once separated from the development roller302in the release area9by the magnetic force or the partition306can be prevented from adhering to the development roller302again. Thus, the developer having a reduced toner concentration after image development can be prevented from moving to the supply compartment301S in which the first conveyance screw304is provided.

Next, supply of toner to the development device3is described below.

Because toner in the developer302contained in the development device3is consumed in image development, toner must be externally supplied to the developer in the development device3. It is preferred that the toner supply position be positioned further from the upstream end portion of the supply compartment301S on the developer circulation path, outside the portion facing the development area A of the development roller302, to prevent insufficiently charged toner from being supplied to the development area A.

Herein, in the vertical agitation arrangement, the communication port309is provided to transport the developer from the downstream end portion of the supply compartment301S to the upstream end portion of the collection compartment301C. Because the communication port309is preferably positioned outside the development area A so as not to cause shortage of developer supplied to the development area. Thus, the supply compartment301S is made longer than the development roller302in the axial direction of the developer carrier302so that its downstream end portion is positioned outside the development area A. In such a configuration, it is not necessary to expand the supply compartment301S further in the axial direction of the developer carrier302when toner is supplied to the extended portion.

Therefore, in the present embodiment, as shown inFIG. 8, the toner supply port310is provided above the downstream end portion of the supply compartment301S (on the front side of the development device3) where the communication port309is formed, outside the development area A. More specifically, the toner supply port310is formed in the casing301above the front-side end portion where the partition306is not present.

Referring toFIGS. 5 and 8, toner supplied through the toner supply port310falls from the downstream end portion of the supply compartment301S, outside the area facing the development roller302(i.e., the development area A), through the communication port309to the upstream end portion of the collection compartment301C together with the developer. The toner is then transported in the collection compartment301C by the second conveyance screw305while being mixed with the developer.

In the present embodiment, because supply and collection of the developer are performed in different developer conveyance compartments as described above, the developer flowing through the collection compartment301C is not supplied to the development roller302. Therefore, insufficiently charged developer in which the concentration of toner is uneven due to the toner newly supplied through the toner supply port310is not supplied to the development roller302nor is used in image development as is. In the collection compartment301C, the supplied toner is mixed with the developer collected from the development roller302, having a reduced toner concentration. Before reaching the downstream end portion of the collection compartment301C, the developer thus mixed can be charged sufficiently and the toner concentration can be equalized. Subsequently, the developer is brought up through the opening307to the supply compartment301S by the bladed wheel308or the like. While being transported to the front side of the device by the first conveyance screw304, the developer is supplied to the development roller302and used in image development.

It is to be noted that, although the description above concerns the configuration in which developer is circulated in two developer conveyance compartments, namely, the supply compartment301S and the collection compartment301C positioned beneath the supply compartment301C, the above-described features can adapt to configurations in which developer is circulated in three or greater developer conveyance compartments as long as one of them is positioned beneath the supply compartment and developer transported to the downstream end portion of the supply compartment falls to that developer conveyance compartment through the communication port.

Description will be given below of inhibiting developer from being supplied to the development area immediately after receiving supplied toner, thereby preventing or reducing scattering of toner in the backgrounds of output images.

Referring toFIG. 3, in the present embodiment, the downstream end portion of the supply compartment301S where the communication port309is provided, outside the area facing the development roller302(i.e., development area A), is relatively short in the axial direction of the development roller302to make the supply compartment301S shorter in the longitudinal direction, thereby making the device relatively compact. Toner supply is performed in such a downstream end portion of the supply compartment301S, that is, in an area adjacent to the development area A of the development roller302.

Additionally, toner is supplied downward to the downstream end portion of the supply compartment301S. Therefore, before being mixed in developer, the supplied toner particles can partly float in the air because they are fine particles having a small particle diameter. Moreover, in the present embodiment that employs the supply-collection separation method, the amount of developer is smaller in the downstream end portion of the supply compartment301S than in the upstream end portion of the supply compartment301S. Therefore, space above the developer is larger on the downstream side than the upstream side in the supply compartment301S. Therefore, a greater amount of toner particles can float in the downstream end portion of the supply compartment301S compared with a case in which toner is supplied in the upstream end portion thereof. Because the downstream end portion of the supply compartment301S is adjacent to the area facing the development area A of the development roller302as described above, the floating toner particles just after supplied to the supply compartment301S are likely to move to the area facing the development area A and further be carried by the development roller302to the development area A. If such toner particles just after supplied are transported to the development area A, it is possible that the toner particles scatter in the background of output images or around the interior of the image forming apparatus.

It is to be noted that the phenomenon described above can occur also in supply-collection separation-type development devices in which another developer conveyance compartment separate from the collection compartment301C is provided beneath the supply compartment301S and developer is transported from the downstream end portion of the supply compartment301S through a communication port to the developer conveyance compartment.

It is to be noted that providing the toner supply port310in the downstream end portion of the supply compartment301S as in the present embodiment has an advantage that supplied toner can be mixed in developer promptly, compared with a comparative development device in which the toner supply port is provided in the collection compartment.

More specifically, if the toner supply port is provided in the collection compartment, developer is transported in the collection compartment together with the supplied toner accumulated on the surface of the developer, and most of the supplied toner can remain on the surface of the developer until transported to the downstream end of the collection compartment. In an experiment using the comparative development device, it was visually confirmed that about one third of supplied toner was not mixed in the developer but remained on the surface of the developer until transported to the downstream end of the collection compartment. It is to be noted that the supplied toner accumulating on the surface of the developer is mixed with developer in the downstream end of the collection compartment where developer is piled up and then is brought up through the opening to the supply compartment.

By contrast, in the present embodiment in which the toner supply port310is provided in the downstream end portion of the supply compartment301S, developer flowing in the supply compartment301S is supplied to the toner supplied through the toner supply port310down on the surface of the developer. Subsequently, the supplied toner is further mixed with developer while falling through the communication port309. The mixed developer is then transported through the collection compartment301C by the second conveyance screw305to the downstream end portion of the collection compartment301C while agitated also with the developer collected from the development roller302, having a reduced toner concentration. In the downstream end portion of the collection compartment301C, the developer is further agitated in the portion where developer is piled up through the opening307, thus brought up to the supply compartment301S. In an experiment using the development device3according to the present embodiment, it was visually confirmed that about half the supplied toner was mixed in the developer while being transported to the upstream end portion of the collection compartment301C. Before being transported to the downstream end of the collection compartment301C, almost all the supplied toner was mixed in the developer.

Although the supplied toner can be substantially mixed with developer promptly in the present embodiment, still, there can be toner particles not mixed in the developer but float in the air.

In particular, if the spiral blade304B of the first conveyance screw304is present in the portion where the supplied toner falls down, and is partly exposed above the surface of developer, the spiral blade304B agitates air in the space where the supplied toner falls when the first conveyance screw304rotates. Thus, the blade spiral304B stirs up the supplied toner that is not mixed in the developer. Additionally, if the portion of developer where the supplied toner falls down is agitated, it can happen that the supplied toner is stirred up again from the developer after being mixed therein.

In view of the foregoing, the present embodiment inhibits the floating supplied toner in the downstream end portion of the supply compartment301S from being supplied to the development area A as follows.

FIG. 9is a schematic top view illustrating the interior of the supply compartment301S.

In the supply compartment301S, developer is partly carried onto the surface of the development roller302while being transported downstream (inFIG. 9, to the right). Therefore, the amount of developer is greater on the upstream side and decreases downstream (to the right inFIG. 9) in the supply compartment301S.

Referring toFIG. 9, in the supply compartment301S, the developer flows downstream and, in the downstream end portion, falls down through the communication port309to the collection compartment301C. Accordingly, an airflow K0flowing in the same direction as the direction in which the developer flows is present above the developer in the downstream end portion of the supply compartment301S.

In the present embodiment, a rib311, serving as an airflow path limiter, is provided in the downstream end portion of the supply compartment301S so as to restricts an airflow path in which the airflow K0flows above the developer in the supply compartment301S, in particular, to reduce a cross-sectional area of the airflow path. The rib311is positioned closer to the development area of the development roller302than the toner supply port310, that is, upstream from the toner supply port310in the developer conveyance direction or axial direction of the development roller302.

As shown inFIG. 10, in the present embodiment, the rib311blocks partly or narrows the path through which the floating toner particles, supplied through the toner supply port310, move toward the development roller301. Typically, the velocity of airflow increases in the narrowed portion because the amount of air flowing does not change in front and the back of the narrowed portion. Consequently, an airflow K1that hinders movement of the floating toner particles to the side of the development roller302becomes stronger than the airflow K0flowing in a portion where the rib311is not provided. Therefore, the amount of floating toner particles that move to the side of the development roller302can be effectively reduced compared with a case in which the rib311is not provided.

For example, as the size of the rib311, the rib311may extend about half the width of the supply compartment301S, that is, the length of the supply compartment301S in the direction horizontal and perpendicular to the axial direction of the development roller302. It is to be noted that, in the supply compartment301S, the rib311extends not only in the space above the developer but also into the area where the developer flows as shown inFIG. 10. Although the rib311hinders the flow of developer in the supply compartment301S in this configuration to some extent, the developer can flow through the gap between the rib311and an inner face of the supply compartment301S to the downstream end portion of the supply compartment301S. Thus, circulation of developer is not inhibited. Rather, because the amount of developer is smaller on the downstream side in the supply compartment301S, restricting the flow of developer with the rib311is advantageous in that the amount of developer on the downstream side can be increased, thus attaining reliable supply of toner to the development roller302.

Additionally, because the amount of developer, hindered by the rib311, increases on the downstream side in the supply compartment301S, the surface level of the developer rises. Consequently, the cross-sectional area of the airflow path through which the floating supplied toner particles move toward the development roller302can be further restricted. Therefore, the airflow K1that inhibits the floating toner particles from moving toward the development roller302can become stronger, thus reducing the amount of the toner particles moving to the development roller302more effectively.

Additionally, as shown inFIG. 10, the rib311is positioned closer to the development roller302in the supply compartment301S on a virtual plane perpendicular to the screw shaft304C of the first conveyance screw304, that is, in the direction horizontal and perpendicular to the axial direction of the development roller302. This arrangement enables the rib311to block a linear route through which the supplied toner, fallen through the toner supply port310, moves toward the development roller302, thus attaining higher effects of inhibiting the supplied toner from moving toward the development roller302with the rib311. Moreover, even if the floating toner overstrides the rib311against the airflow K1, the toner lands on the developer in a portion away from the development roller302. Accordingly, before supplied onto the development roller302, such toner particles are transported downstream in the supply compartment301S and are not supplied to the development area A.

Additionally, as shown inFIG. 10, because the rib311extends from above the developer into the area of the developer in the supply compartment301S, it is necessary to design the device so that the spiral blade304B attached to the screw shaft304C of the first conveyance screw304does not contact the rib311. To prevent the contact between the first conveyance screw304and the rib311, in the present embodiment, the spiral blade304B is cut off in the portion corresponding to the rib311. Alternatively, other arrangement may be used. For example, the spiral blade304B of the first conveyance screw304may be reduced in size in the portion corresponding to the rib311.

Moreover, if the spiral blade304B of the first conveyance screw304is present in the downstream end portion of the supply compartment301S, the spiral blade304B flips up the developer as described above, which is not desirable. Therefore, it is more preferable that the spiral blade304B of the first conveyance screw304be removed in the area extending from the position facing the rib311to the downstream end thereof in the developer conveyance direction. It is to be noted that removing that portion of the spiral blade304B from the first conveyance screw304does not impose adverse effects in circulation of developer because developer moves down to the communication port309under its own weight.

FIG. 11is a graph illustrating results of an experiment to evaluate scattering of toner in backgrounds on the photoconductor1when the rib311is provided (first condition) and is not provided (second condition).

The term “scattering of toner in backgrounds” used herein means a phenomenon that toner adheres to the non-image area of the photoconductor1where an electrostatic latent image is not formed. Scattering of toner in backgrounds occurs frequently or the degree of toner scattering increases when the ratio of insufficiently charged toner is large, that is, frictional charging between toner and carrier is insufficient. In the experiment, toner particles adhering to the non-image area of the photoconductor1was collected with transparent adhesive tape, and density of the collected toner particles was measured with an X-Rite spectrophotometric color densitometer. As can be seen fromFIG. 11, the degree of toner scattering in backgrounds was alleviated in the configuration in which the rib311was provided (first condition). The degree of toner scattering in backgrounds was thus alleviated because the rib311inhibited the supplied toner from being carried on the development roller302, thereby preventing it from being used in image development.

As described above, the rib311can not only blocks the movement of floating toner to the development area by itself but also increase the strength of the airflow inhibiting the movement of floating toner to the development area. Therefore, the amount of floating toner moving to the development area can be reduced effectively.

FIG. 12illustrates a variation of the rib311.

InFIG. 12, a face312of a rib311A on the side receiving the developer flowing in the supply compartment301S is inclined downstream in the developer conveyance direction therein. That is, a first end of the face312on the side of the gap between the rib311A and the inner wall of the supply compartment301S is positioned downstream from a second end opposite the first end of the rib311A. The rib311A may be triangular or trapeziform as shown inFIG. 12when viewed from above. With this configuration, the airflow in the supply compartment301S can be guided along the inclined surface312to the space between the rib311and the inner face of the supply compartment301S, thus increasing the strength of the airflow K1(shown inFIG. 9) flowing in that space. As a result, the effect of hindering the floating supplied toner particles from moving toward the development roller302can be increased.

The inclined surface312(shown inFIG. 12) of the rib311A can also inhibit the developer in contact with the rib311A from accumulating there in the configuration in which the rib311A extends into the area where the developer flows in the supply compartment301S as in the present embodiment.

Next, descriptions are given below of a first variation of the above-described embodiment in which the configuration of the airflow path limiter is different.

FIG. 13is a schematic top view illustrating an interior of a supply compartment301S1of a development device3A according to the first variation.FIG. 14is a schematic side view of the development device3A according to the first variation.

In the first variation, a toner guide frame313forming a toner guide path314is provided in a supply compartment301S1for guiding the supplied toner fallen through the toner supply port310to a bottom portion of the supply compartment301S1. In the first variation shown inFIGS. 13 and 14, a surface313A, which receives the developer flowing in the supply compartment301S1, of the toner guide frame313serves as the airflow path limiter instead of the above-described rib311. Referring toFIG. 14, the lower end of the toner guide frame313is positioned slightly higher than the screw shaft304C of a first conveyance screw304-1provided in the supply compartment301S1. The spiral blade304B of the first conveyance screw304does not present in the area corresponding to the toner guide frame313.

In the first variation, the toner guide frame313can better restrict diffusion of the supplied toner in the supply compartment301S1, compared with the above-described configuration in which the supplied toner falls directly to the supply compartment301S through the toner supply port310. Therefore, the floating supplied toner can be better inhibited from moving toward the development roller302and being used in image development.

It is to be noted that, as long as its lower end does not contact the screw shaft304C of the first conveyance screw304-1, the toner guide frame313may extend beneath the screw shaft304C of the first conveyance screw304-1as indicated by broken lines shown inFIG. 14.

Additionally, although the description above concern a configuration in which the toner guide frame313is disposed on the side closer to the development roller302in the supply compartment301S1on a virtual plane perpendicular to the screw shaft304C of the first conveyance screw304-1, alternatively, the toner guide frame313may be positioned on the side away from the development roller302as in a supply compartment301S2shown inFIG. 15.

Next, descriptions are given below of a second variation of the above-described embodiment in which the configuration of the developer circulation is different.

FIG. 16is a schematic end-on axial view illustrating a configuration of developer circulation in the second variation.

Although developer is circulated in two separate developer conveyance paths, namely, the supply compartment and the collection compartment disposed beneath the supply compartment, in the above-described embodiment, in the developer circulation mechanism according to the second variation, developer is circulated in three separate developer conveyance paths. It is to be noted that the configuration of the developer circulation mechanism using three developer conveyance paths is not limited to that of the second variation.

The development device3B according to the second variation includes a supply compartment301S3in which a first conveyance screw304is provided, a collection compartment301C3in which a second conveyance screw305is provided, and an agitation compartment301A that receives developer from a downstream end portion of the supply compartment301S3and that of the collection compartment301C3and returns the developer to an upstream end portion of the supply compartment301S3. In the agitation compartment301A, a third conveyance screw315is provided. In the second variation, developer is transported in an identical or similar direction in the supply compartment301S3and the collection compartment301C3, which is from the back side to the front side of the development device3B. By contrast, in the agitation compartment301A, developer is transported from the front side to the back side of the device, which is opposite the direction in which developer is transported in the supply compartment301S3and the collection compartment301C3. Although not shown inFIG. 16, a communication port309is formed in a downstream end portion of the supply compartment301S3and communicates with an upstream end portion of the agitation compartment301A, and a communication port connecting a downstream end portion of the collection compartment301C3and the upstream end portion of the agitation compartment301A is provided as well.

More specifically, developer is circulated as follows in the second variation.

A first route is from the supply compartment301S3in which the first conveyance screw304is provided to the development roller302, the collection compartment301C3in which the second conveyance screw305is provided, and the agitation compartment301A in which the third conveyance member315is provided, and then returns to the supply compartment301S3.

A second route is from the supply compartment301S3in which the first conveyance screw304is provided to the agitation compartment301A in which the third conveyance member315is provided and then returns to the supply compartment301S3.

Also in the second variation, the collection compartment301C3as well as the agitation compartment301A are positioned beneath the supply compartment301S3, and the developer transported to the downstream end portion of the supply compartment301S3is transported through the communication port309to the agitation compartment301A. Additionally, a toner supply port310is provided in the downstream end portion of the supply compartment301S3. Therefore, similarly to the above-described embodiment, to inhibit toner supplied through the toner supply port310from floating and moving to the development roller302, an airflow path limiter such as the rib311shown inFIG. 9is provided. Needless to say, the airflow path limiter is not limited to the rib311, and alternatively, for example, the toner guide frame313according to the first variation may be used.