Developing device and image forming apparatus therewith

A developing device includes a developer container, a developer carrier, and first and second members for stirring/conveying. The developer container includes a first partition wall between first and second chambers, a communication portion through which those chambers communicate at opposite ends of the first partition wall, a developer supply port, and a developer discharge portion. The first member stirs and conveys developer in the first chamber in a first direction. The second member stirs and conveys developer in the second chamber in a second direction and includes a regulating portion and a discharging blade. The developer container includes a second partition wall between the first chamber and the regulating portion. The gap from the upper end of the second partition wall to the inner surface of the developer container is larger than the gap thereto from the upper ends of a first helical blade and the regulating portion.

INCORPORATION BY REFERENCE

This application is based on and claims the benefit of Japanese Patent Application No. 2020-088775 filed on May 21, 2020, the contents of which are hereby incorporated by reference.

BACKGROUND

The present disclosure relates to developing devices used in image forming apparatuses employing electrophotography, such as copiers, printers, facsimile machines, and multifunction peripherals incorporating their functions, and to image forming apparatuses provided with such a developing device. More particularly, the present disclosure relates to developing devices which supply two-component developer containing toner and carrier and which discharge excessive developer, and to image forming apparatuses provided with such a developing device.

In image forming apparatuses, a latent image formed on an image carrying member composed of a photosensitive member or the like is developed by a developing device to be visualized as a toner image. In one type of such developing devices, a two-component development system that uses two-component developer is adopted. This type of developing device stores two-component developer containing carrier and toner in a developer container, and includes a developing roller (developer carrying member) which supplies the developer to the image carrying member and a stirring/conveying member which supplies, while conveying and stirring, the developer in the developer container to the developing roller.

In the developing device employing the two-component development system, while toner is consumed as development operation proceeds, carrier remains in the developing device unconsumed. Thus stirred together with toner in the developer container, carrier degrades as it is stirred repeatedly. As a result, the electrostatic charging performance of carrier with respect to toner gradually degrades.

To cope with that, there has been proposed a developing device that employs a CASS (carrier auto streaming system) to supply developer containing carrier to the developer container and that discharges excessive developer, thereby preventing degradation in electrostatic charging performance.

Incidentally, the height of developer tends to decrease in a high humidity environment and to increase in a low humidity environment. This causes the weight of developer in the developer container to vary depending on the environment in which the image forming apparatus is used. There is thus concern for, when the environment changes from a high humidity one to a low humidity one, a sudden increase in the amount of developer discharged and, when the environment changes from a low humidity one to a high humidity one, development failure due to an insufficient height of developer.

For example, in a known developing device, a disk portion is provided in a path for discharging developer in the developing device toward a discharge port, and an end part of a reversing screw provided upstream of the disk portion in the discharging direction is arranged so as not to be joined to the disk portion. In this developing device, developer is restrained from being lifted up due to the disk portion being joined to a conveying portion upstream of the disk portion, and helps stabilize the amount of developer discharged.

SUMMARY

According to one aspect of the present disclosure, a developing device includes a developer container, a developer carrying member, and first and second stirring/conveying members. The developer container includes first and second conveying chambers which are arranged parallel to each other, a first partition wall which partitions between the first and second conveying chambers along the longitudinal direction, a communication portion through which the first and second conveying chambers communicate with each other in opposite end parts of the first partition wall, a developer supply port through which developer containing magnetic carrier and toner is supplied, and a developer discharge portion which is provided in a downstream-side end part of the second conveying chamber and through which excessive developer is discharged. The developer carrying member is rotatably supported on the developer container and carries, on its surface, the developer in the second conveying chamber. The first stirring/conveying member includes a rotary shaft and a first conveying blade formed on the outer circumferential face of the rotary shaft and stirs and conveys the developer in the first conveying chamber in the first direction. The second stirring/conveying member includes a rotary shaft and a second conveying blade formed on the outer circumferential face of the rotary shaft and stirs and conveys the developer in the second conveying chamber in the second direction opposite to the first direction. The second stirring/conveying member includes a regulating portion and a discharging blade. The regulating portion is formed adjacent to the second conveying blade on its downstream side in the second direction and is composed of a conveying blade for conveying the developer in the direction opposite to the second conveying blade. The discharging blade is formed adjacent to the regulating portion on its downstream side in the second direction and conveys the developer in the same direction as the second conveying blade to discharge the developer through the developer discharge portion. The communication portion is composed of a first communication portion which, at the downstream side in the first direction, passes the developer from the first conveying chamber to the second conveying chamber, and a second communication portion which, at the downstream side in the second direction, passes the developer from the second conveying chamber to the first conveying chamber. The developer container includes a second partition wall which is arranged adjacent to the regulating portion on the downstream side of the second communication portion in the second direction to partition between the first conveying chamber and the regulating portion. The gap from an upper end part of the second partition wall to the inner surface of the developer container is larger than the gap from upper end parts of the first conveying blade and the regulating portion to the inner surface of the developer container.

This and other objects of the present disclosure, and the specific benefits obtained according to the present disclosure, will become apparent from the description of embodiments which follows.

DETAILED DESCRIPTION

Hereinafter, with reference to the accompanying drawings, embodiments of the present disclosure will be described.FIG. 1is a sectional view showing an internal structure of an image forming apparatus100mounted with developing devices3ato3daccording to the present disclosure. In the image forming apparatus100(here, a color printer), four image forming portions Pa, Pb, Pc and Pd are arranged in this order from upstream in the conveying direction (from the left side inFIG. 1). These image forming portions Pa to Pd are provided so as to correspond to images of four different colors (cyan, magenta, yellow, and black) and sequentially form images of cyan, magenta, yellow, and black through the processes of electrostatic charging, exposure, developing, and transfer.

In these image forming portions Pa to Pd, photosensitive drums (image carrying members)1a,1b,1c, and1dare respectively arranged which carry visible images (toner images) of the different colors. Further, an intermediate transfer belt8which rotates, by the action of a driving mean (unillustrated), counter-clockwise inFIG. 1is provided adjacent to the image forming portions Pa to Pd. The toner images formed on the photosensitive drums1ato1dare primarily transferred sequentially, while being superimposed on each other, to the intermediate transfer belt8that moves while keeping contact with the photosensitive drums1ato1d. Then, the toner images primarily transferred to the intermediate transfer belt8are secondarily transferred by a secondary transfer roller9to transfer paper P, which is one example of a recording medium. The transfer paper P on which the toner images have been secondarily transferred is, after having the toner images fixed on it in a fixing portion13, discharged out of the main body of the image forming apparatus100. An image forming process is performed with respect to the photosensitive drums1ato1dwhile they are rotated clockwise inFIG. 1.

The transfer paper P to which the toner image is secondarily transferred is stored in a sheet feeding cassette16arranged in a lower part of the main body of the image forming apparatus100, and is conveyed via a sheet feeding roller12aand a registration roller pair12bto a nip portion between the secondary transfer roller9and a driving roller11of the intermediate transfer belt8. Used as the intermediate transfer belt8is a sheet of dielectric resin, and typically is a belt without seams (seamless belt). On the downstream side of the secondary transfer roller9, a blade-form belt cleaner19is arranged for removing toner and the like left on the surface of the intermediate transfer belt8.

Next, the image forming portions Pa to Pd will be described. Around and below the photosensitive drums1ato1dthat are rotatably arranged, there are provided charging devices2a,2b,2c, and2dwhich electrostatically charge the photosensitive drums1ato1drespectively, an exposure device5which exposes the photosensitive drums1ato1dto light carrying image information, developing devices3a,3b,3c, and3dwhich form toner images on the photosensitive drums1ato1drespectively, and cleaning devices7a,7b,7c, and7dwhich remove developer (toner) and the like left on the photosensitive drums1ato1drespectively.

When image data is input from a host device such as a personal computer, first, the surfaces of the photosensitive drums1ato1dare electrostatically charged uniformly by the charging devices2ato2d. Next, the exposure device5irradiates the photosensitive drums1ato1dwith light based on the image data to form on them electrostatic latent images reflecting the image data. The developing devices3ato3dare loaded with predetermined amounts of two-component developer containing cyan, magenta, yellow, and black toner respectively. When, through formation of toner images, which will be described later, the proportion of toner in the two-component developer stored in the developing devices3ato3dfalls below a determined value, developer containing toner and carrier is supplied from containers4ato4dto the developing devices3ato3drespectively. The toner in the developer is fed from the developing devices3ato3dto the photosensitive drums1ato1d, and electrostatically attaches to them to form toner images based on the electrostatic latent images formed through exposure to light from the exposure device5.

Then, by primary transfer rollers6ato6d, electric fields with a predetermined transfer voltage are applied between the primary transfer rollers6ato6dand the photosensitive drums1ato1d, and the cyan, magenta, yellow, and black toner images on the photosensitive drums1ato1dare primarily transferred to the intermediate transfer belt8. These images in four colors are formed with a predetermined positional relationship with each other that is prescribed for formation of a predetermined full-color image. Then, in preparation for the subsequent formation of new electrostatic latent images, toner and the like left on the surface of the photosensitive drums1ato1dafter the primary transfer are removed by the cleaning devices7ato7d.

The intermediate transfer belt8is stretched around a driven roller10on the upstream side and the driving roller11on the downstream side. As the driving roller11is driven to rotate by a belt driving motor (unillustrated), the intermediate transfer belt8starts to rotate counter-clockwise, and the transfer paper P is conveyed from the registration roller pair12bto a nip portion (secondary transfer nip portion) between the driving roller11and the secondary transfer roller9arranged adjacent to it with predetermined timing. The full-color image on the intermediate transfer belt8is thus secondarily transferred to the transfer paper P. The transfer paper P on which the toner images have been secondarily transferred is conveyed to the fixing portion13.

The transfer paper P conveyed to the fixing portion13is heated and pressed by a fixing roller pair13a, and thereby the toner images are fixed on the surface of the transfer paper P to form a predetermined full-color image. The transfer paper P on which the full-color image has been formed has its conveying direction switched by a branch portion14which is branched into a plurality of directions, and is then directly (or after being directed to a duplex printing conveying passage18to have images formed on both its faces) discharged to a discharge tray17by a discharge roller pair15.

FIG. 2is a side sectional view of a developing device3aaccording to a first embodiment of the present disclosure incorporated in the image forming apparatus100. The following description deals with, as an example, the developing device3aarranged in the image forming portion Pa inFIG. 1. The developing devices3bto3darranged in the image forming portions Pb to Pd have a structure basically similar to that of the developing device3a, and thus no overlapping description will be repeated.

As shown inFIG. 2, the developing device3aincludes a developer container20that stores two-component developer (hereinafter also referred to simply as developer) containing magnetic carrier and toner. The developer container20is partitioned into a stirring/conveying chamber21and a feeding/conveying chamber22by a first partition wall20a. In the stirring/conveying chamber21and the feeding/conveying chamber22, there are respectively rotatably arranged a stirring/conveying screw25and a feeding/conveying screw26which mix the toner and the carrier fed from the container4a(seeFIG. 1) with the developer in the developer container20and which stir the mixture and thereby electrostatically charge it.

The stirring/conveying screw25arranged in the stirring/conveying chamber21includes a rotary shaft25aand a first conveying blade25bwhich is provided integrally with the rotary shaft25aand which is formed in a helical shape with a predetermined pitch in the axial direction of the rotary shaft25a. The rotary shaft25ais rotatably pivoted on the developer container20. The stirring/conveying screw25rotates to convey, while stirring, the developer inside the stirring/conveying chamber21in a predetermined direction (to one side of the developing roller31in its axial direction).

The feeding/conveying screw26arranged in the feeding/conveying chamber22includes a rotary shaft26aand a second conveying blade26bwhich is provided integrally with the rotary shaft26aand which is formed in a helical shape with a blade winding in the same direction (having the same phase) as the first conveying blade25b. The rotary shaft26ais arranged parallel to the rotary shaft25aof the stirring/conveying screw25, and is rotatably supported on the developer container20. The feeding/conveying screw26rotates to convey, while stirring, the developer inside the feeding/conveying chamber22in the direction opposite to the stirring/conveying screw25to supply it to the developing roller31.

The developer is, while being stirred by the stirring/conveying screw25and the feeding/conveying screw26, conveyed in the axial direction (the direction perpendicular to the plane ofFIG. 2) and, via an upstream-side communication portion20eand a downstream-side communication portion20f(for either, seeFIG. 3) formed in opposite end parts of the first partition wall20a, circulates between the stirring/conveying chamber21and the feeding/conveying chamber22. Thus, the stirring/conveying chamber21, the feeding/conveying chamber22, the upstream-side communication portion20e, and the downstream-side communication portion20fform a circulation passage for the developer inside the developer container20.

The developer container20extends obliquely to the upper right inFIG. 2. To the upper right of the feeding/conveying screw26in the developer container20, a developing roller31is arranged. Part of an outer circumferential face of the developing roller31is exposed through an opening20bin the developer container20so as to face the photosensitive drum1a. The developing roller31rotates counter-clockwise inFIG. 2. To the developing roller31, a developing voltage is applied which is produced by superimposing an alternating-current voltage on a direct-current voltage.

The developing roller31is composed of a cylindrical developing sleeve which rotates counter-clockwise inFIG. 2and a magnet (unillustrated) which is fixed inside the developing sleeve and which has a plurality of magnetic poles. Although a developing sleeve with a knurled surface is used here, any other developing sleeves can instead be used such as one with a number of recesses (dimples) formed on the surface or one with a blasted surface.

To the developer container20, a regulating blade27is fixed along the longitudinal direction of the developing roller31(in the direction perpendicular to the plane ofFIG. 2). A small gap is formed between a tip end portion of the regulating blade27and the surface of the developing roller31.

Next, the structure of a stirring portion in the developing device3awill be described in detail.FIG. 3is a sectional plan view (as seen from the direction of arrows AA′ inFIG. 2) showing a stirring portion in the developing device3aaccording to the first embodiment.FIG. 4is an enlarged view of and around a developer discharge portion20hinFIG. 3.

Formed in the developer container20are, the stirring/conveying chamber21, the feeding/conveying chamber22, the first partition wall20a, a second partition wall20c, the upstream-side communication portion20e, and the downstream-side communication portion20fAdditionally, there are also formed a developer supply port20g, the developer discharge portion20h, an upstream-side wall portion20i, and a downstream-side wall portion20j. It is assumed that, with respect to the stirring/conveying chamber21, the left side inFIG. 3is the upstream side and the right side inFIG. 3is the downstream side, and that, with respect to the feeding/conveying chamber22, the right side inFIG. 3is the upstream side, and the left side inFIG. 3is the downstream side. Accordingly, with respect to the communication portion and the wall portion, upstream side and downstream side denote those sides with respect to the feeding/conveying chamber22.

The first partition wall20aextends in the longitudinal direction of the developer container20and partitions it into the stirring/conveying chamber21and the feeding/conveying chamber22such that these are located side by side. The second partition wall20cprotrudes from an inner wall surface of the downstream-side wall portion20jand is formed on an extension line from the first partition wall20aso as to face the outer circumferential face of a helical blade constituting a regulating portion52.

A right-side end part of the first partition wall20ain its longitudinal direction and the inner wall portion of the upstream-side wall portion20iform the upstream-side communication portion20e. On the other hand, a left-side end part of the first partition wall20ain its longitudinal direction and the second partition wall20cform the downstream-side communication portion20f.

The developer supply port20gis an opening for supplying new toner and carrier to the developer container20from the container4a(seeFIG. 1) provided in an upper part of the developer container20, and is arranged on the upstream side of the stirring/conveying chamber21(on the left side inFIG. 3).

The developer discharge portion20hdischarges the developer which has become excessive in the stirring/conveying chamber21and the feeding/conveying chamber22due to the supply of the developer. The developer discharge portion20his provided on the downstream side of the feeding/conveying chamber22continuously in the longitudinal direction of the feeding/conveying chamber22.

The stirring/conveying screw25extends up to the opposite ends of the stirring/conveying chamber21in its longitudinal direction and the first conveying blade25bis provided so as to face the upstream-side and downstream-side communication portions20eand20fThe rotary shaft25ais rotatably pivoted on the upstream-side and downstream-side wall portions20iand20jof the developer container20.

The feeding/conveying screw26is longer than the developing roller31in its axial direction, and is provided so as to extend up to a position where it faces the upstream-side communication portion20e. The rotary shaft26ais arranged parallel to the rotary shaft25aof the stirring/conveying screw25, and is rotatably supported on the upstream-side wall portion20iand the developer discharge portion20hof the developer container20. To the rotary shaft26aof the feeding/conveying screw26, in addition to the second conveying blade26b, the regulating portion52and a discharging blade53are integrally arranged.

The regulating portion52holds back the developer conveyed to the downstream side in the feeding/conveying chamber22and conveys the developer exceeding a predetermined amount to the developer discharge portion20h. The regulating portion52is composed of a helical blade provided on the rotary shaft26a. This helical blade is formed in a helical shape with a blade winding in the direction opposite to (having the phase opposite to) the second conveying blade26b, has a substantially same outer diameter as the second conveying blade26b, and has a pitch smaller than that of the second conveying blade26b. The regulating portion52forms a predetermined clearance between the inner wall portion of the developer container20. Excessive developer moves to the developer discharge portion20hthrough this clearance.

On the rotary shaft26ain the developer discharge portion20h, the discharging blade53is provided. The discharging blade53is composed of a helical blade winding in the same direction as the second conveying blade26bwith a smaller pitch and a smaller blade outer circumference compared to the second conveying blade26b. As the rotary shaft26arotates, the discharging blade53rotates together. The excessive developer which has moved over the regulating portion52and has been conveyed into the developer discharge portion20his conveyed to the left side inFIG. 4to be discharged to outside the developer container20through an unillustrated developer discharge port.

On the outer wall of the developer container20, gears61to64are arranged. The gears61and62are fixed to the rotary shaft25a, and the gear64is fixed to the rotary shaft26a. The gear63is rotatably supported on the developer container20and meshes with the gears62and64.

As a developer driving motor (unillustrated) rotates the gear61, the stirring/conveying screw25rotates. The developer in the stirring/conveying chamber21is conveyed in the main conveying direction (first direction, arrow P direction) by the first conveying blade25b, and is then conveyed into the feeding/conveying chamber22via the upstream-side communication portion20e. As the feeding/conveying screw26rotates via the gears62to64, the developer inside the feeding/conveying chamber22is conveyed by the second conveying blade26bin the main conveying direction (second direction, arrow Q direction). During developing during which no new developer is supplied, the developer is, while greatly changing its height, conveyed into the feeding/conveying chamber22from the stirring/conveying chamber21via the upstream-side communication portion20e. Then, without moving over the regulating portion52, the developer is conveyed via the downstream-side communication portion20fto the stirring/conveying chamber21.

In this way, developer is stirred while circulating from the stirring/conveying chamber21to the upstream-side communication portion20e, then to the feeding/conveying chamber22, and then to the downstream-side communication portion20f. The stirred developer is fed to the developing roller31.

Next, a description will be given of a case where developer is supplied through the developer supply port20g. As toner is consumed in development, the developer containing toner and carrier is supplied from the container4avia the developer supply port20gto the stirring/conveying chamber21.

The supplied developer is, as during development, conveyed inside the stirring/conveying chamber21in the main conveying direction (arrow P direction) by the stirring/conveying screw25, and is then conveyed into the feeding/conveying chamber22via the upstream-side communication portion20e. Then, by the feeding/conveying screw26, the developer inside the feeding/conveying chamber22is conveyed in the main conveying direction (arrow Q direction). When the regulating portion52rotates as the rotary shaft26arotates, a conveying force in the direction opposite to the main conveying direction (reverse conveying direction) is applied to the developer by the regulating portion52. The developer is held back by the regulating portion52to bulk up, and the excessive developer (the same amount as the developer supplied from the developer supply port20g) moves over the regulating portion52and is discharged to outside the developer container20through the developer discharge portion20h.

As shown inFIG. 4, in the feeding/conveying screw26, there is arranged a disk55between the second conveying blade26band the regulating portion52. The disk55is, together with the second conveying blade26b, the regulating portion52, and the discharging blade53, molded of synthetic resin integrally with the rotary shaft26a.

The developer which is conveyed in the main conveying direction (arrow Q direction) by the second conveying blade26bis held back by the disk55, and this momentarily weakens the conveying force of the developer. Then, a conveying force in the opposite direction is applied to the developer by the regulating portion52, and the developer is pushed back in the direction opposite to the main conveying direction. That is, the disk55plays a role of reducing the conveying force (pressure) acting from the feeding/conveying chamber22to the regulating portion52. As a result, it is possible to prevent waving (fluctuation) at the surface of the developer which is moving to the regulating portion52and the downstream-side communication portion20f, and thus, regardless of the conveying speed of the developer, a nearly constant amount of developer can be retained around the regulating portion52.

Then, when the developer is supplied from the developer supply port20gto increase the height of the developer in the developer container20, the developer stagnating on the upstream side of the regulating portion52moves over the disk55and the regulating portion52to the discharging blade53(developer discharge portion20h), and excessive developer is discharged from the developer discharge portion20h. When the developer ceases to be discharged from the developer discharge portion20h, the height of the developer in the developer container20is stabilized. The volume of the developer when its height is stabilized is referred to as a stable volume.

In the image forming apparatus100according to the present disclosure, the processing speed can be switched between two speeds depending on the thickness and the kind of transfer paper P to be conveyed and the type of an output image. That is, when the transfer paper P is regular paper or when a text-based document is output, image forming processing is performed at a regular operation speed (hereinafter, referred to as a full speed mode) and, when the transfer paper P is thick paper or when a photo image is output, image forming processing is performed in a speed (hereinafter, referred to as a slowdown mode) slower than the regular speed. It is thus possible, when thick paper is used as the transfer paper P or when a photo image is output, to secure a sufficient fixing time and improve image quality.

Switching between the full speed mode and the slowdown mode as described above results in changing the rotation speed of the stirring/conveying screw25and the feeding/conveying screw26; thus the conveying speed of the developer inside the developer container20changes sharply. This results in an uneven distribution of developer inside the developer container20and thus a change in the height (surface level) of the developer. This changes also the amount of developer discharged from the developer discharge portion20hand hence the amount of developer inside the developer container20.

Specifically, as the conveying speed of the developer (the rotation speed of the stirring/conveying screw25and the feeding/conveying screw26) increases, even when the weight of the developer inside the developer container20is constant, the height of the developer increases. For example, when the conveying speed of the developer is increased, the developer may, before reaching the downstream side of the regulating portion52, be passed from the feeding/conveying chamber22via the downstream-side communication portion20fto the stirring/conveying chamber21. As a result, less developer reaches the regulating portion52, and this makes it difficult to discharge developer through the developer discharge portion20h. In this embodiment, the amount of developer discharged is adjusted by adjusting the height of the second partition wall20cthat is arranged adjacent to the regulating portion52.

FIG. 5is a diagram of and around the downstream-side communication portion20finFIG. 4, as seen from the stirring/conveying chamber21side.FIG. 6is a longitudinal sectional view (as seen from the direction of arrows BB′ inFIG. 4) of the stirring/conveying chamber21and the feeding/conveying chamber22including the first partition wall20ain the developing device3aof the first embodiment. As shown inFIGS. 5 and 6, the first partition wall20aextends up to the top faces of the stirring/conveying chamber21and the feeding/conveying chamber22so as to completely partition between the stirring/conveying chamber21and the feeding/conveying chamber22along the longitudinal direction (left-right direction inFIG. 5, the direction perpendicular to the plane ofFIG. 6).

FIG. 7is a longitudinal sectional view (as seen from the direction of arrows CC′ inFIG. 4) of the stirring/conveying chamber21and the feeding/conveying chamber22including the second partition wall20cin the developing device3aof the first embodiment. As shown inFIGS. 5 and 7, a gap (clearance) d1is formed between a top end part of the second partition wall20cand the inner surface of the developer container20(the stirring/conveying chamber21and the feeding/conveying chamber22).

The developer passed from the feeding/conveying chamber22via the downstream-side communication portion20fto the stirring/conveying chamber21mainly flows (as indicated by a hollow arrow inFIG. 4) in the main conveying direction (the direction of arrow P) by the action of the first conveying blade25bof the stirring/conveying screw25. However, since the conveying force of the stirring/conveying screw25hardly acts around the downstream-side communication portion20f, part of the developer stagnates in an upstream-side end part (a left end part inFIG. 4) of the stirring/conveying chamber21with respect to the main conveying direction.

When the conveying speed of the developer is high, more developer is passed from the feeding/conveying chamber22to the stirring/conveying chamber21, and less developer passes through the gap between the feeding/conveying chamber22and the disk55to reach the regulating portion52. Thus, the height of the second partition wall20cis adjusted so that the developer stagnating around the upstream-side end part of the stirring/conveying chamber21moves over the second partition wall20cto return to the feeding/conveying chamber22(regulating portion52) only when the conveying speed is high. That is, when the conveying speed is high, the amount of developer that reaches the regulating portion52is increased. This helps make the amount of developer that reaches the regulating portion52constant regardless of the conveying speed of the developer, and thus helps suppress the variation in the discharge amount of the developer and thereby stabilize the height of the developer in the developer container20.

In adjusting the amount of developer that moves over the second partition wall20cto return to the regulating portion52, how the distance d1is adjusted matters. When the gap d1is too small, the developer that is lifted by the stirring/conveying screw25hits the top face of the stirring/conveying chamber21and falls without moving over the second partition wall20c. Although it is preferable that the gap d1be minimized to make the developing device3acompact, the gap d1needs to be set such that, even when the conveying speed is high, the developer does not hit the top face of the stirring/conveying chamber21. In this embodiment, the gap d1is larger than the gap d2from upper end parts of the first conveying blade25bof the stirring/conveying screw25and the regulating portion52(blade having the opposite phase) of the feeding/conveying screw26to the inner surface of the developer container20(including the stirring/conveying chamber21and the feeding/conveying chamber22).

When a sufficiently large gap d1cannot be secured between the upper end part of the second partition wall20cand the inner surface of the developer container20, the second partition wall20ccan be made lower, but if the second partition wall20cis too low, the stirring/conveying chamber21and the feeding/conveying chamber22are no longer partitioned from each other, possibly resulting in more developer stagnating around the downstream-side communication portion20f. The lower limit height of the second partition wall20cis determined according to the diameters of the stirring/conveying screw25and the feeding/conveying screw26as well as the specification of the developer.

FIG. 8is a diagram of and around the downstream-side communication portion20f, as seen from the stirring/conveying chamber21side, in a modified example of the developing device3aof the first embodiment where a stepped part60is provided over the second partition wall20c.FIG. 9is a longitudinal sectional view of the stirring/conveying chamber21and the feeding/conveying chamber22including the second partition wall20cin the modified example shown inFIG. 8. As shown inFIGS. 8 and 9, by forming the stepped part60that is recessed upward in the inner surface of the developer container20facing the second partition wall20c, it is possible, while keeping the second partition wall20cat a predetermined height, to secure a sufficient gap d1to prevent the developer lifted by the stirring/conveying screw25from hitting the top face of the stirring/conveying chamber21. Where the developer is lifted varies with the conveying speed of the developer, and thus the height and the width of the stepped part60can be set in accordance with the conveying speed of the developer.

FIG. 10is an enlarged cross-sectional view of and around the developer discharge portion20hin the developing device3aaccording to a second embodiment of the present disclosure.FIG. 11is a diagram of and around the downstream-side communication portion20finFIG. 10as seen from the stirring/conveying chamber21side.FIG. 12is a longitudinal sectional view (as seen from the direction of arrows CC′ inFIG. 10) of the stirring/conveying chamber21and the feeding/conveying chamber22including the second partition wall20cin the developing device3aof the second embodiment. In this embodiment, the developer supply port20gis arranged over the second partition wall20c. In other respects, the structure of the developing device3ais similar to that in the first embodiment.

By arranging the developer supply port20gover the second partition wall20c, it is possible to form a space over the second partition wall20cas shown inFIGS. 11 and 12. This results in d1>d2as in the first embodiment, and even when the conveying speed of the developer is high, the developer lifted by the stirring/conveying screw25moves, without hitting the top face of the stirring/conveying chamber21, over the second partition wall20cto return to the regulating portion52smoothly. Thus, as in the first embodiment, it is possible to keep the amount of developer that reaches the regulating portion52constant.

Around the second partition wall20cis where the developer that has passed from the feeding/conveying chamber22through the downstream-side communication portion20fis passed to the stirring/conveying chamber21and where the movement of the developer is active owing to the developer being pushed back by the regulating portion52and, when the conveying speed of the developer is high, the developer moving, over the second partition wall20c, from the stirring/conveying chamber21to the regulating portion52. Accordingly, with the developer supply port20garranged over the second partition wall20c, the supplied developer in an agglomerated state falls down to around the second partition wall20cto be loosened before being conveyed by the stirring/conveying screw25. This helps mix the supplied developer sufficiently with the developer circulating in the developer container20, and thus helps stabilize the amount of electric charge of toner in the developer.

FIG. 13is a longitudinal sectional view of the stirring/conveying chamber21and the feeding/conveying chamber22including the second partition wall20cin the modified example of the developing device3aof the second embodiment where the stepped part60is provided around the developer supply port20g. As shown inFIG. 13, by forming the stepped part60that is recessed upward around the developer supply port20g, it is possible, while keeping the second partition wall20cat a predetermined height, to secure a sufficient space so that the developer lifted by the stirring/conveying screw25does not hit the top face of the stirring/conveying chamber21. The height and the width of the stepped part60can be set in accordance with the conveying speed of the developer.

The embodiments described above are in no way meant to limit the present disclosure, which thus allows for many modifications and variations within the spirit of the present disclosure. Although the above embodiment deals with developing devices3ato3dprovided with a developing roller31as shown inFIG. 2, this is not meant to be any limitation. The present disclosure is applicable to various developing devices which use two-component developer containing toner and carrier, such as those which, for example, have a magnetic roller for carrying developer and form a toner layer by moving toner alone from the magnetic roller to a developing roller31to develop an electrostatic latent image using the toner layer on the developing roller31.

In the above embodiments, in order to retain developer on the upstream side of the developer discharge portion20h, the regulating portion52composed of a helical blade having the phase opposite to that of the second conveying blade26band the disk55are provided on the feeding/conveying screw26, but this is not meant to limit the structure of the regulating portion52. For example, the disk55may be omitted and only the regulating portion52may be provided, or the regulating portion52and a plurality of disks55may be combined, or the regulating portion52may be composed only of a plurality of disks.

The present disclosure is applicable not only to tandem-type color printers like the one shown inFIG. 1but to various types of image forming apparatuses employing a two-component development system, such as digital and analogue monochrome copiers, monochrome printers, color copiers, and facsimile machines. The benefits of the present disclosure will now be described in more detail by way of practical examples.

Practical Example

In an image forming apparatus100like the one shown inFIG. 1, how the amount of developer in the developing devices3ato3dchanged as the conveying speed of developer was changed was studied. The tests were conducted on the image forming portion Pa of cyan including the photosensitive drum1aand the developing device3a.

The tests proceeded as follows. The developing device3aof the first embodiment where, as shown inFIGS. 4 to 7, the gap d1from the upper end part of the second partition wall20cto the inner surface of the developer container20was larger than the gap d2from the upper end parts of the first and second conveying blades25band26bto the inner surface of the developer container20was taken as Practical Example 1. The developing device3aof the second embodiment where, as shown inFIGS. 10 to 12, the developer supply port20gwas formed over the second partition wall20cwas taken as Practical Example 2. The developing device3awhere the gap d1equaled the gap d2was taken as Comparative Example.

The developer containers20of the developing devices3aof the Practical Examples 1 and 2 and Comparative Example were each loaded with 175 cc of developer (with a toner concentration of 6%), and the developing devices3awere each driven in a normal-temperature normal-humidity environment (25° C., 50%) with the stirring/conveying screw25and the feeding/conveying screw26rotated at three different rotation speeds of 139 rpm, 278 rpm, and 449 rpm to stir and convey the developer. When the developer ceases to be discharged from the discharge portion20h, the amount (stable weight, stable volume) of the developer in the developer container20was measured.

The first conveying blade25bof the stirring/conveying screw25and the second conveying blade26bof the feeding/conveying screw26that were used in Practical Example 1 and 2 and Comparative Example were helical blades with an outer diameter of 18 mm. The height of the first partition wall20awas 15 mm. The regulating portion52was formed of three helical blades having an outer diameter of 18 mm and the opposite phase. The height of the second partition wall20cwas 8 mm. The discharging blade53was a helical blade with an outer diameter of 8 mm. The gap d2from the upper end parts of the first conveying blade25band the regulating portion52to the inner surface of the developer container20was 1 mm. The gap from the discharging blade53to the inner surface of the developer discharge portion20hwas 1 mm.

In the developing device3aof Practical Example 1, the gap d1from the upper end part of the second partition wall20cto the inner surface of the developer container20was 7 mm (>d2). In the developing device3aof Comparative Example, the gap d1from the upper end part of the second partition wall20cto the inner surface of the developer container20was 1 mm (=d2). In the developing device3aof Practical Example 2, a space for the developer supply port20gwas formed over the second partition wall20csuch that d1>d2.

The amount of developer was measured as follows. The developing devices3aof Practical Example 1 and 2 and Comparative Example were each mounted in a testing machine and the developer was stirred as the rotation speed of the stirring/conveying screw25and the feeding/conveying screw26(i.e., the conveying speed of the developer in the stirring/conveying chamber21and the feeding/conveying chamber22) was changed, and then the developing device3awas removed and its weight was measured. The amount (stable weight) of developer was calculated by subtracting from the measured weight of the developing device3athe weight of the empty developing device3awith the developer removed. The stable volume was calculated by dividing the calculated amount of developer with the bulk density. Table 1 shows the relationship of the rotation speed of the stirring/conveying screw25and the feeding/conveying screw26with the stable volume.

As will be clear from Table 1, in Practical Example 1 where the gap d1from the upper end part of the second partition wall20cto the inner surface of the developer container20was 7 mm and in Practical Example 2 where the developer supply port20gwas formed over the second partition wall20c, even when the rotation speed of the stirring/conveying screw25and the feeding/conveying screw26was changed, the variation in the stable volume was small.

By contrast, in Comparative Example where the gap d1from the upper end part of the second partition wall20cto the inner surface of the developer container20was 1 mm, the higher the rotation speed of the stirring/conveying screw25and the feeding/conveying screw26, the larger the stable volume. These results confirm that, with the developing devices3aof the Practical Example 1 and 2, even when the conveying speed of the developer changes, it is possible to keep the stable volume of the developer constant, and thus to effectively suppress development failure and the like due to an insufficient or excessive height of developer.

The present disclosure is applicable to a developing device which supplies two-component developer containing toner and carrier and which discharges excessive developer, as well as to an image forming apparatus provided with such a developing device. Based on the present disclosure, it is possible to provide a developing device which can reduce variations in the height and weight of developer in a developer container even if the flowability and conveyance speed of developer change.