Developing device and image forming apparatus therewith

A developing device includes a developing container, a first stirring member, a second stirring member, and a developer discharge port. The developing container has a first and a second transport chamber. The first stirring member has a rotary shaft and a first helical blade. The second stirring member has a rotary shaft and a second helical blade. Through the developer discharge port, surplus developer inside the developing container is discharged. On the second stirring member, a restricting portion for restricting movement of developer toward the developer discharge port is formed downstream of the second helical blade. The restricting portion has two or more turns of a restricting blade wound in the direction opposite to the second helical blade, and the restricting blade has an increasingly large outside diameter from upstream to downstream with respect to the developer transport direction inside the second transport chamber.

INCORPORATION BY REFERENCE

This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2015-176805 filed on Sep. 8, 2015, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present disclosure relates to developing devices used in image forming apparatuses adopting an electrophotographic system, such as copiers, printers, facsimile machines, and multifunction peripherals having combined functions of those, and to image forming apparatuses provided with such developing devices. More particularly, the present disclosure relates to developing devices that, while being supplied with fresh two-component developer containing toner and carrier, discharge surplus developer, and to image forming apparatuses provided with such developing devices.

In an image forming apparatus, an electrostatic latent image formed on an image carrier comprising a photosensitive body or the like is developed by a developing device into a toner image so as to be made visible. There is a type of developing device that adopts a two-component development system using two-component developer. In this type of developing device, two-component developer containing carrier and toner is stored in a developing container, a developing roller for feeding the developer to the image carrier is arranged, and a stirring member for feeding, while stirring and transporting, the developer inside the developing container to the developing roller is arranged.

In this developing device, as toner is consumed in developing operation, carrier remains unconsumed inside the developing container. Thus, the carrier, which is stirred with the toner inside the developing container, deteriorates due to mechanical stress from being stirred repeatedly. As a result, the carrier's ability to electrostatically charge the toner gradually diminishes.

As a solution, there have been proposed developing devices in which, while fresh developer containing carrier and toner is supplied into a developing container, surplus developer is discharged with a view to suppressing a drop in electrostatic charging performance.

For example, in a known developing device, two stirring members each having a rotary shaft and a helical blade formed in a helical shape on its circumference are arranged parallel to each other in transport chambers respectively. The transport chambers are divided by a partition portion, and in opposite end parts of the partition portion, communicating portions for delivering developer are provided. A developer discharge port is formed downstream of one of the transport chambers with respect to the developer transport direction, and between the stirring member there and the developer discharge port, an opposite helical blade formed in a helical shape wound in the direction opposite to the helical blade on the stirring member is provided, as a restricting portion, integrally with the rotary shaft.

With this developing device, when developer is supplied into the developing container, as the stirring member rotates, the developer is, while being stirred, transported to downstream of the transport chamber. As the opposite helical blade rotates in the same direction as the stirring member, the opposite helical blade applies to the developer a transporting force that acts in the direction opposite to the direction of developer transport by the stirring member. With this opposite transporting force, on the downstream side of the transport chamber, developer is blocked to have an increased height. Thus, surplus developer passes over the opposite helical blade (restricting portion) to move to the developer discharge port, and is discharged. In this way, developer can be replaced while the height of the developer inside the developing container is kept substantially constant.

SUMMARY

According to one aspect of the present disclosure, a developing device includes a developing container, a first stirring member, a second stirring member, a developer carrier, a developer supply port, and a developer discharge port. The developing container has a plurality of transport chambers including a first transport chamber and a second transport chamber arranged side by side, and communicating portions through which the first and second transport chambers communicate with each other at opposite longitudinal-direction end parts thereof. The developing container stores two-component developer containing carrier and toner. The first stirring member is composed of a rotary shaft and a first helical blade formed on the circumferential face of the rotary shaft, and stirs and transports developer inside the first transport chamber in the rotary-shaft direction. The second stirring member is composed of a rotary shaft and a second helical blade formed on the circumferential face of the rotary shaft, and stirs and transports developer inside the second transport chamber in the direction opposite to the first stirring member. The developer carrier is rotatably supported on the developing container, and carries on the surface thereof the developer inside the second transport chamber. Through the developer supply port, developer is supplied into the developing container. The developer discharge port is provided in a downstream-side end part of the second transport chamber with respect to the developer transport direction inside the second transport chamber, and through it, surplus developer inside the developing container is discharged. On the second stirring member, a restricting portion which restricts the movement of developer toward the developer discharge port is formed so as to be opposed to the developer discharge port downstream of the second helical blade with respect to the developer transport direction inside the second transport chamber. The restricting portion is composed of two or more turns of a restricting blade wound in the direction opposite to the second helical blade. The second transport chamber has a uniform inside diameter in the part thereof opposed to the restricting blade. The restricting blade has an increasingly large outside diameter from upstream to downstream with respect to the developer transport direction inside the second transport chamber.

Further features and advantages of the present disclosure will become apparent from the description of embodiments given below.

DETAILED DESCRIPTION

An embodiment of the present disclosure will be described below with reference to the accompanying drawings.FIG. 1is a sectional view schematically showing a construction of an image forming apparatus1incorporating developing devices2ato2daccording to the present disclosure. The image forming apparatus1is a tandem-type color printer. There are arranged rotatable photosensitive drums (image carriers)11ato11dthat are, for example, organic photosensitive bodies (OPC photosensitive bodies) having an organic photosensitive layer formed on them or amorphous silicon photosensitive bodies having an amorphous silicon layer formed on them and that are arranged to correspond to different colors, namely black, yellow, cyan, and magenta. Around the photosensitive drums11ato11d, there are arranged developing devices2ato2d, an exposure unit12, electrostatic chargers13ato13d, and cleaning devices14ato14d.

The developing devices2ato2dare arranged beside (to the right of) the photosensitive drums11ato11d, respectively, so as to be opposed to them, and feed toner to the photosensitive drums11ato11d. The electrostatic chargers13ato13dare arranged upstream of the developing devices2ato2dwith respect to the rotation direction of the photosensitive drums11ato11d(inFIG. 1, the counter-clockwise direction) so as to be opposed to the surfaces of the photosensitive drums11ato11d, and electrostatically charge the surfaces of the photosensitive drums11ato11duniformly.

The exposure unit12scans the photosensitive drums11ato11dto expose them to light based on image data, conveying characters and figures, fed to an image input portion (unillustrated) from a personal computer or the like, and is arranged under the developing devices2ato2d. Inside the exposure unit12, there are arranged a laser light source and a polygon mirror, and there are also arranged reflective mirrors and lenses to correspond to the photosensitive drums11ato11d. The laser light emitted from the laser light source is shone, via the polygon mirror, reflective mirrors, and lenses, onto the surfaces of the photosensitive drums11ato11dfrom downstream of the electrostatic chargers13ato13dwith respect to the rotation direction of the photosensitive drums11ato11d. The laser light thus shone forms electrostatic latent images on the surfaces of the photosensitive drums11ato11d, and these electrostatic latent images are developed into toner images by the developing devices2ato2d.

An endless intermediary transfer belt17is wound around a tension roller6, a driving roller25, and a following roller27. The driving roller25is driven to rotate by an unillustrated motor, and as the driving roller25rotates, the intermediary transfer belt17is driven to circulate in the clockwise direction inFIG. 1.

The photosensitive drums11ato11dare arranged next to each other along the belt movement direction (the direction indicated by arrows inFIG. 1) under the intermediary transfer belt17so as to remain in contact with the intermediary transfer belt17. Primary transfer rollers26ato26dare arranged so as to be opposed to the photosensitive drums11ato11drespectively across the intermediary transfer belt17, and are kept in pressed contact with the intermediary transfer belt17to form a primary transfer portion. At this primary transfer portion, as the intermediary transfer belt17rotates, the toner images on the photosensitive drums11ato11dare successively transferred to the intermediary transfer belt17with predetermined timing. As a result, on the surface of the intermediary transfer belt17, a full-color toner image is formed that has toner images of four colors, namely cyan, magenta, yellow, and black, superimposed together.

A secondary transfer roller34is opposed to the driving roller25across the intermediary transfer belt17, and is kept in pressed contact with the intermediary transfer belt17to form a secondary transfer portion. At this secondary transfer portion, the toner image on the surface of the intermediary transfer belt17is transferred to a paper or other sheet P. After the transfer, a belt cleaning device31clears the surface of the intermediary transfer belt17of toner that is left unused.

In a lower part inside the image forming apparatus1, a sheet feed cassette32for storing sheets P is arranged, and to the right of the sheet feed cassette32, a stack tray35for manual feeding of sheets is arranged. To the left of the sheet feed cassette32, there is arranged a first sheet transfer passage33through which a sheet P fed out of the sheet feed cassette32is transferred to the secondary transfer portion on the intermediary transfer belt17. To the left of the stack tray35, there is arranged a second sheet transfer passage36through which a sheet fed out of the stack tray35is transported to the secondary transfer portion. Furthermore, in an upper left part of the image forming apparatus1, there are arranged a fixing portion18which performs fixing operation on a sheet P having an image formed on it and a third sheet transport passage39through which a sheet having undergone fixing operation is transported to a sheet discharge portion37.

The sheet feed cassette32, when drawn out of the apparatus (toward the near side inFIG. 1), can be replenished with sheets, and feeds out sheets P stored in it into the first sheet transfer passage33, one sheet after another, by the action of a pickup roller33band a separating roller33a.

The first sheet transfer passage33and the second sheet transfer passage36meet just downstream of a registration roller pair33c, and by the registration roller pair33c, a sheet P is transported to the secondary transfer portion with appropriate timing with regard to image forming operation on the intermediary transfer belt17and sheet feeding operation. The sheet P thus transported to the secondary transfer portion then has the full-color toner image on the intermediary transfer belt17secondarily transferred to it by a secondary transfer roller34to which a bias potential is applied, and is then transported to the fixing portion18.

The fixing portion18includes, among others, a fixing belt which is heated by a heating roller, a fixing roller which is kept in contact with the fixing belt from inside, and a pressing roller which is arranged in pressed contact with the fixing roller, and performs fixing operation by applying heat and pressure to the sheet P having the toner image transferred to it. After the fixing of the toner image by the fixing portion18, the sheet P is, as necessary, reversed in a fourth sheet transport passage40so that a toner image is secondarily transferred to the reverse side of the sheet as well and is fixed in the fixing portion18. The sheet having the toner image or images fixed to it passes through the third sheet transport passage39, and is discharged onto the sheet discharge portion37by discharge rollers19a.

FIG. 2is a side sectional view showing a construction of a developing device2that is used in the image forming apparatus1described above. The following description deals with the construction and operation of the developing device2acorresponding to the photosensitive drum11ashown inFIG. 1. Since the construction and operation of the developing devices2bto2dare similar to those of the developing device2a, no overlapping description will be repeated, and the suffixes “a” to “d” distinguishing the developing devices and the photosensitive bodies for different colors will be omitted.

As shown inFIG. 2, the developing device2is composed of a developing roller20, a magnetic roller (developer carrier)21, a restricting blade24, stirring members42, a developing container22, etc.

The developing container22forms a casing of the developing device2, and a lower part of the developing container22is divided into a first transport chamber22cand a second transport chamber22dby a partition portion22b. In the first and second transport chambers22cand22d, developer containing carrier and toner (here, positive-charge toner) is stored. The developing container22rotatably holds the stirring members42, the magnetic roller21, and the developing roller20. The developing device2has an opening22aformed in it through which the developing roller20is exposed toward the photosensitive drum11.

The developing roller20is opposed to the photosensitive drum11, and is arranged to the right of the photosensitive drum11across a predetermined distance. In the developing roller20, at a position close to and opposed to the photosensitive drum11, a developing region D is formed through in toner is fed to the photosensitive drum11. The magnetic roller21is opposed to the developing roller20across a predetermined distance, and is arranged obliquely to the lower right of the developing roller20. At a position close to and opposed to the developing roller20, the magnetic roller21feeds toner to the developing roller20. The stirring members42are arranged substantially under the magnetic roller21. The restricting blade24is fixedly held by the developing container22obliquely to the lower left of the magnetic roller21.

The stirring member42is composed of two members, namely a first spiral (first stirring member)43and a second spiral (second stirring member)44. The second spiral44is arranged under the magnetic roller21, inside the second transport chamber22d, and the stirring member42is arranged next to, to the right of, the second spiral44, inside the first transport chamber22c.

The first and second spirals43and44stir developer to electrostatically charge the toner in the developer to a predetermined level. This permits the toner to be held by the carrier. In opposite end parts of the partition portion22b, which divides between the first and second transport chambers22cand22d, in its longitudinal direction (inFIG. 2, the direction perpendicular to the plane of the figure), communicating portions (unillustrated) are provided; thus, as the first spiral43rotates, electrostatically charged developer is transported to the second spiral44via one of the communicating portions formed in the partition portion22bso that the developer circulates through the first and second transport chambers22cand22d. The developer is then fed from the second spiral44to the magnetic roller21.

The magnetic roller21includes a magnetic pole member M and a non-magnetic sleeve21bformed of a non-magnetic material, carries developer fed from the stirring member42, and feeds, out of the developer it carries, only the toner to the developing roller20. In the magnetic pole member M, a plurality of magnets having a fan-shaped cross-section and having different polarities in circumferential parts are arranged alternately, and are fixed to a roller shaft21awith adhesive or otherwise. The roller shaft21ais, inside the non-magnetic sleeve21b, non-rotatably supported on the developing container22, with a predetermined distance secured between the magnetic pole member M and the non-magnetic sleeve21b. The non-magnetic sleeve21brotates in the same direction (inFIG. 2, the clockwise direction) as the developing roller20by being driven by an unillustrated driving mechanism composed of a motor and gears, and a bias56that has an alternating-current voltage56bsuperposed on a direct-current voltage56ais applied to the non-magnetic sleeve21b. On the surface of the non-magnetic sleeve21b, electrostatically charged developer is carried while forming a magnetic brush under the magnetic force of the magnetic pole member M, and the height of the magnetic brush is adjusted to a predetermined height by the restricting blade24.

As the non-magnetic sleeve21brotates, the magnetic brush is transported while being carried on the surface of the non-magnetic sleeve21bby the magnetic pole member M; when it makes contact with the developing roller20, out of the magnetic brush, only the toner is fed to the developing roller20in accordance with the bias56applied to the non-magnetic sleeve21b.

The developing roller20is composed of, among others, a fixed shaft20a, a magnetic pole member20b, and a developing sleeve20cformed of a non-magnetic metal material in a cylindrical shape.

The fixed shaft20ais non-rotatably supported on the developing container22. On the fixed shaft20a, the developing sleeve20cis rotatably held, and the magnetic pole member20b, which is formed of magnets, is fixed with adhesive or otherwise, at a position opposed to the magnetic roller21, with a predetermined distance secured from the developing sleeve20c. The developing sleeve20cis rotated in the direction indicated by an arrow inFIG. 2(the clockwise direction) by an unillustrated mechanism composed of a motor and gears. A developing bias55that has an alternating-current voltage55bsuperposed on a direct-current voltage55ais applied to the developing sleeve20c.

As the developing sleeve20chaving the developing bias55applied to it rotates in the clockwise direction inFIG. 2, in the developing region D, a potential difference between the developing bias potential and the potential in the exposed part of the photosensitive drum11causes the toner carried on the surface of the developing sleeve20cto fly to the photosensitive drum11. The toner thus flown attaches to the exposed part of the photosensitive drum11rotating in the direction indicated by arrow A (the counter-clockwise direction), and thereby an electrostatic latent image on the photosensitive drum11is developed.

Next, with reference toFIG. 3, a stirring portion in the developing device2will be described in detail.FIG. 3is a sectional plan view (sectional view across line X-X′ inFIG. 2as seen from the direction indicated by arrows) showing the stirring portion in the developing device2.

As mentioned above, inside the developing container22, there are formed a first transport chamber22c, a second transport chamber22d, a partition portion22b, an upstream-side communicating portion22e, and a downstream-side communicating portion22f, and there are further formed a developer supply port22g, a developer discharge port22h, an upstream-side wall portion22i, and a downstream-side wall portion22j. Of the first transport chamber22c, the left side inFIG. 3are referred to as the upstream side, and the right side inFIG. 3as the downstream side. Of the second transport chamber22d, the right side inFIG. 3are referred to as the upstream side, and the left side inFIG. 3as the downstream side. Accordingly, with respect to the second transport chamber22d, the communicating portions are each referred to as either an upstream-side or downstream-side communicating portion, and the side wall portions are each referred to as either an upstream-side or downstream-side wall portion.

The partition portion22bextends in the longitudinal direction of the developing container22to divide between the first and second transport chambers22cand22dsuch that these lie side by side. A right end part of the partition portion22bin its longitudinal direction forms, together with an inner wall part of the upstream-side wall portion22i, the upstream-side communicating portion22e; on the other hand, a left end part of the partition portion22bin its longitudinal direction forms, together with an inner wall part of the downstream-side wall portion22j, the downstream-side communicating portion22f. Thus, the developer can circulate through the first transport chamber22c, the upstream-side communicating portion22e, the second transport chamber22d, and the downstream-side communicating portion22f.

The developer supply port22gis an opening through which fresh toner and carrier are supplied from a developer supply container (unillustrated) provided over the developing container22, and is arranged in an upstream-side (inFIG. 3, left-side) part of the first transport chamber22c.

The developer discharge port22his an opening through which, as fresh developer is supplied, surplus developer inside the first and second transport chambers22cand22dis discharged, and is provided in a downstream-side part of the second transport chamber22dso as to be continuous with the second transport chamber22din its longitudinal direction.

Inside the first transport chamber22c, the first spiral43is arranged, and inside the second transport chamber22d, the second spiral44is arranged.

The first spiral43has a rotary shaft43band a first helical blade43athat is formed integrally with the rotary shaft43band that has a helical shape with a predetermined pitch in the axial direction of the rotary shaft43b. The first helical blade43aextends up to opposite end parts of the first transport chamber22cin its longitudinal direction so as to be opposed to the upstream- and downstream-side communicating portions22eand22f. The rotary shaft43bis rotatably pivoted on the upstream- and downstream-side wall portions22iand22j.

The second spiral44has a rotary shaft44band a second helical blade44athat is formed integrally with the rotary shaft44band that has a helical shape with a blade wound with the same pitch as but in the opposite direction (with the opposite phase) to the first helical blade43ain the axial direction of the rotary shaft44b. The second helical blade44ahas a length that is larger than the length of the magnetic roller21in its axial direction, and extends to reach a position opposed to the upstream-side communicating portion22e. The rotary shaft44bis arranged parallel to the rotary shaft43b, and is rotatably pivoted on the upstream- and downstream-side wall portions22iand22jof the developing container22.

On the rotary shaft44b, there are integrally arranged, in addition to the second helical blade44a, a restricting portion52and a discharge blade53.

The restricting portion52blocks the developer transported downstream inside the second transport chamber22d, but allows the developer exceeding a predetermined amount to be transported to the developer discharge port22h. The restricting portion52comprises a helical blade (restricting blade) formed on the rotary shaft44b, and has a helical shape with a blade wound in the opposite direction (with the opposite phase) to, and with a smaller pitch than, the second helical blade44a. The circumferential part of the restricting portion52has a predetermined distance (clearance) from the inner wall part of the developing container22. Through this gap, surplus developer is discharged.

The rotary shaft44bextends to reach inside the developer discharge port22h. On a part of the rotary shaft44binside the developer discharge port22h, the discharge blade53is provided. The discharge blade53comprises a blade in a helical shape wound in the same direction as the second helical blade44a, but has a smaller pitch than the second helical blade44a, and the blade has a smaller circumference. Accordingly, as the rotary shaft44brotates, the discharge blade53rotates together so that the surplus developer that has been transported into the developer discharge port22hover the restricting portion52is transported leftward inFIG. 3to be discharged out of the developing container22. The discharge blade53, the restricting portion52, and the second helical blade44aare formed of synthetic resin integrally with the rotary shaft44b.

On the outer wall of the developing container22, gears61to64are arranged. The gears61and62are fixed to the rotary shaft43b, the gear64is fixed to the rotary shaft44b, and the gear63is rotatably held on the developing container22and meshes with the gears62and64.

During development in which no fresh developer is supplied, as the gear61is rotated by a driving source such as a motor, the first helical blade43arotates together with the rotary shaft43b; thus, by the first helical blade43a, developer is transported in the direction indicated by arrow P inside the first transport chamber22c, and is then transported through the upstream-side communicating portion22einto the second transport chamber22d. Then, as the second helical blade44arotates together with the rotary shaft44bwhich is interlocked with the gear64, by the second helical blade44a, developer is transported in the direction indicated by arrow Q inside the second transport chamber22d. Thus, developer is, while greatly changing its height, transported from the first transport chamber22cthrough the upstream-side communicating portion22einto the second transport chamber22d, and is transported through the downstream-side communicating portion22finto the first transport chamber22cwithout passing over the restricting portion52.

As described above, as developer circulates through the first transport chamber22c, the upstream-side communicating portion22e, the second transport chamber22d, and the downstream-side communicating portion22f, it is stirred, and the stirred developer is fed to the magnetic roller21.

Next, a description will be given of a case where developer is supplied through the developer supply port22g. As development progresses and toner is consumed, developer containing toner and carrier is supplied into the first transport chamber22cthrough the developer supply port22g.

As during development, by the first helical blade43a, the supplied developer is transported in the direction indicated by arrow P inside the first transport chamber22c, and is then transported through the upstream-side communicating portion22einto the second transport chamber22d. Then, by the second helical blade44a, the developer is transported in the direction indicated by arrow Q inside the second transport chamber22d. As the rotary shaft44brotates and thus the restricting portion52rotates together, the restricting portion52applies to the developer a transporting force that acts in the direction opposite to the direction of developer transport by the second helical blade44a. The restricting portion52thus blocks the developer and increases its height; as a result, surplus developer passes over the restricting portion52, and is discharged through the developer discharge port22hout of the developing container22.

FIG. 4is an enlarged view around the developer discharge port22hin the developing device2according to the embodiment, andFIG. 5is an enlarged view around the restricting portion52inFIG. 4. As shown inFIGS. 4 and 5, the restricting portion52is composed of one turn of an upstream-side restricting blade52awhich is located at the most upstream-side position with respect to the developer transport direction and which adjoins the second helical blade44a, a one turn of a downstream-side restricting blade52bwhich is located at the most downstream-side position with respect to the developer transport direction and which adjoins the discharge blade53, and a one turn of a middle restricting blade52cwhich is arranged between the upstream-side restricting blade52aand the downstream-side restricting blade52b.

The outside diameter of the middle restricting blade52cis larger than the outside diameter of the upstream-side restricting blade52a, and the outside diameter of the downstream-side restricting blade52bis larger than the outside diameter of the middle restricting blade52c. That is, the restricting portion52has a stepwise increasing outside diameter from upstream to downstream with respect to the developer transport direction inside the second transport chamber22d. The outside diameter of the upstream-side restricting blade52ais equal to or larger than the outside diameter of the discharge blade53. The outside diameter of the downstream-side restricting blade52bis equal to or smaller than the outside diameter of the second helical blade44a.

The inside diameter of the second transport chamber22dis uniform. Thus, the inside diameter of the part of the second transport chamber22dopposed to the second helical blade44aand that of the part of the second transport chamber22dopposed to the restricting portion52are equal.

With the construction described above, when the distance (clearance) between the second helical blade44aand the inner wall face of the second transport chamber22dis represented by La, and the distance (clearance) between the downstream-side restricting blade52band the inner wall face of the second transport chamber22dis represented by Lb, the relationship Lb≧La is fulfilled. When the distance (clearance) between the discharge blade53and the inner wall face of the developer discharge port22his represented by Lc, the relationship Lb≧Lc is fulfilled.

Owing to the outside diameter of the upstream-side restricting blade52abeing smaller than the outside diameter of the downstream-side restricting blade52b, the upstream-side restricting blade52aexerts a weaker restricting force on the developer that has been transported through the second transport chamber22dby the second helical blade44a. As a result, while the developer continues to be transported in the main developer transport direction (the direction indicated by arrow Q), the transport speed drops; thus, the developer stagnates and thereby suppresses ruffling (fluctuation) on the surface of the developer moving toward the developer discharge port22hand the downstream-side communicating portion22f. Thus, the upstream-side restricting blade52aserves to slacken the flow of developer in the restricting portion52.

Owing to the outside diameter of the downstream-side restricting blade52bbeing larger than the outside diameter of the upstream-side restricting blade52a, the developer that has been transported through the second transport chamber22dis acted on by a strong restricting force exerted by the downstream-side restricting blade52b. As a result, the developer is acted on by a transporting force that acts in the direction opposite to the main transport direction. Thus, by applying the opposite transporting force to the developer whose flow has been slackened by the upstream-side restricting blade52a, the downstream-side restricting blade52bserves to increase the height of the developer in the restricting portion52and thereby adjust the amount of developer fed to the developer discharge port22hover the downstream-side restricting blade52b(i.e., the amount of developer discharged).

In the embodiment, as described above, the restricting portion52has an increasingly large outside diameter from upstream to downstream with respect to the developer transport direction inside the second transport chamber22d. That is, the outside diameter of the upstream-side restricting blade52ais smaller than the outside diameters of the middle regulating blade52cand the downstream-side restricting blade52b. Thus, even when the transport speed of developer is low (the stirring speed is low) or the fluidity of developer is low (the toner concentration is high, or the absolute humidity is high), it is possible to alleviate the difficulty of developer reaching the downstream-side restricting blade52b, and thus to alleviate the difficulty of developer being discharged to the developer discharge port22h. Moreover, the outside diameter of the downstream-side restricting blade52bis larger than outside diameters of the upstream-side restricting blade52aand the middle restricting blade52c. Thus, even when the transport speed of developer is high (the stirring speed is high) or the fluidity of developer is high (the toner concentration is low, or the absolute humidity is low), it is possible to reduce the ease with which developer is discharged to the developer discharge port22h. As described above, it is possible to stabilize the amount of developer discharged to the developer discharge port22hagainst changes in the transport condition of the developer in the developing container22resulting from variations in the fluidity of developer due to variations in environmental conditions (humidity), variations in the toner concentration during development, variations in the developer transport speed, etc. It is thus possible to obtain a developing device2that can suppress variation of the amount of developer inside the developing container22and that thus provides stable developing performance.

Moreover, as described above, the outside diameter of the downstream-side restricting blade52bis equal to or smaller than the outside diameter of the second helical blade44a. This helps alleviate the difficulty of developer being discharged to the developer discharge port22h.

Moreover, as described above, the outside diameter of the upstream-side restricting blade52ais equal to or larger than the outside diameter of the discharge blade53. This helps prevent the upstream-side restricting blade52afrom having an excessively small outside diameter, and it is thus possible, when the fluidity of developer is high or the transport speed of developer is high, to prevent an excessive amount of developer from reaching the downstream-side restricting blade52b.

Moreover, as described above, by fulfilling Lb≧La, it is possible to alleviate the difficulty of developer being discharged to the developer discharge port22h.

Moreover, as described above, the restricting portion52is arranged between the upstream-side restricting blade52aand the downstream-side restricting blade52b, and includes a middle restricting blade52cof which the outside diameter is larger than the outside diameter of the upstream-side regulating blade52abut smaller than the outside diameter of the downstream-side restricting blade52b. It is thus possible to gradually vary the transport speed of the developer moving from the upstream-side restricting blade52ato the downstream-side restricting blade52b.

Although the embodiment deals with a case where the restricting portion52is composed of three turns of an opposite helical blade, specifically an upstream-side restricting blade52a, a downstream-side restricting blade52b, a the middle restricting blade52c, the restricting portion52may be composed of any number of turns of an opposite helical blade other than three, for example, two turns or four or more turns. Although the embodiment deals with a case where the restricting portion52has a stepwise increasing outside diameter from upstream to downstream with respect to the developer transport direction inside the second transport chamber22d, it may instead have a gradually (continuously) increasing outside diameter from upstream to downstream with respect to the developer transport direction inside the second transport chamber22d.

The present disclosure can be implemented in any manner other than specifically described by way of an embodiment above, and allows for many modifications without departing from the spirit of the present disclosure. For example, the application of the present disclosure is not limited to developing devices incorporating a magnetic roller21and a developing roller20as shown inFIG. 2; the present disclosure finds application in various developing devices that use two-component developer containing toner and carrier. For example, although the embodiment described above deals with a developing device adopting a two-axis transport system including, as a developer circulation passage inside the developing container22, a first transport chamber22cand a second transport chamber22dthat are arranged side by side, application is also possible to developing devices adopting a three-axis transport system including a collection transport chamber for collecting developer separated from the magnetic roller21for gathering it into the second transport chamber22d.

The application of the present disclosure is not limited to tandem-type color printers like the one shown inFIG. 1; the present disclosure finds application in various image forming apparatuses adopting a two-component development system, such as digital or analog monochrome copiers, monochrome printers, color copiers, and facsimile machines. Now, the effects of the present disclosure will be described in more detail by way of a practical example.

With the developing device2described above, experiments were conducted to see how the amount of developer inside the developing container22changed when the restricting portion52had an increasingly large outside diameter from upstream to downstream with respect to the developer transport direction inside the second transport chamber22d. The experiments were conducted in the black image formation portion that included the photosensitive drum11aand the developing device2a.

The experiments were conducted for each of Practical Example and Comparative Example. Practical Example incorporated a developing device2in which, as shown inFIGS. 4 and 5, the upstream-side restricting blade52ahad an outside diameter of 10 mm, the middle restricting blade52chad an outside diameter of 11 mm, and the downstream-side restricting blade52bhad an outside diameter of 12 mm. Comparative Example incorporated a developing device2in which, the upstream-side restricting blade52a, the middle restricting blade52c, and the downstream-side restricting blade52ball had an outside diameter of 12 mm. In Practical Example, the distance between the upstream-side restricting blade52aand the inner wall face of the second transport chamber22dwas 3.5 mm, the distance between the middle restricting blade52cand the inner wall face of the second transport chamber22dwas 3.0 mm, and the distance Lb between the downstream-side restricting blade52band the inner wall face of the second transport chamber22dwas 2.5 mm. In Comparative Example, the distances from the upstream-side restricting blade52a, the middle restricting blade52c, and the downstream-side restricting blade52bto the inner wall face of the second transport chamber22dall were 2.5 mm.

In the second spiral44used in both Practical Example and Comparative Example, the second helical blade44ahad an outside diameter of 14 mm and a pitch of 30 mm, and the distance La between the second helical blade44aand the inner wall face of the second transport chamber22dwas 1.5 mm. The upstream-side restricting blade52a, the middle restricting blade52c, and the downstream-side restricting blade52bhad a pitch of 5 mm. The discharge blade53had an outside diameter of 8 mm and a pitch of 5 mm, and the distance Lc between the discharge blade53and the inner wall face of the developer discharge port22hwas 1.5 mm. 150 cm3of developer was stored in the developing container22(the first and second transport chambers22cand22d).

The reference conditions were as follows: the stirring speed (rotation speed) of the first spiral43and the second spiral44was 300 rpm; the toner concentration in the developer stored in the developing container22(the mass ratio of toner to carrier, T/C) was 10%; the absolute humidity was 10 g/m3. With respect to these reference conditions, the stirring speed (rotation speed) of the first spiral43and the second spiral44was changed among three different levels of 200 rpm, 300 rpm, and 400 rpm; the toner concentration in the developer stored in the developing container22was changed among three different levels of 8%, 10% and 12%; the absolute humidity was changed among three level of 5 g/m3, 10 g/m3, and 20 g/m3. Under these different conditions, the amount of developer (stable volume) inside the developing device2at the time that the discharge of developer from the developing container22ceased was measured. The results are shown in Tables 1, 2, and 3.

Table 1 reveals the following. With the developing device2of Practical Example, a change in the stirring speed from 200 rpm to 400 rpm brought a 4 cm3variation in the volume of the developer inside developing container22. In contrast, with the developing device2of Comparative Example, the change brought a 8 cm3variation in the volume of the developer inside developing container22.

Table 2 reveals the following. With the developing device2of Practical Example, a change in the toner concentration from 8% to 12% brought a 1 cm3variation in the volume of the developer inside developing container22. In contrast, with the developing device2of Comparative Example, the change brought a 5 cm3variation in the volume of the developer inside developing container22.

Table 3 reveals the following. With the developing device2of Practical Example, a change in the absolute humidity from 5 g/m3to 20 g/m3brought a 1 cm3variation in the volume of the developer inside developing container22. In contrast, with the developing device2of Comparative Example, the change brought a 4 cm3variation in the volume of the developer inside developing container22.

Based of the results above, it is confirmed that, in the developing device according to the present disclosure where the three turns of a restricting blade constituting the restricting portion52has an increasingly large outside diameter from upstream to downstream with respect to the developer transport direction, the stable volume of developer exhibits a stable transition irrespective of the stirring speed or the developer fluidity (toner concentration, absolute humidity) as compared with that in a conventional construction where the restricting blade has a uniform outside diameter. Thus, by use of the developing device according to the present disclosure, it is possible to obtain stable developing performance, and to effectively suppress image defects and unnecessary discharge of developer.

The comparison of Practical Example with Comparative Example shows that the stable volume of developer was slightly lower in the Practical Example than in the Comparative Example. This resulted from the restricting portion52having a smaller outside diameter on the upstream side (the upstream-side restricting blade52a), and the stable volume of developer can be adjusted by modifying the outside diameter of or the number of turns in the restricting blade.

The present disclosure finds applications in developing devices used in image forming apparatuses adopting an electrophotographic system, such as copiers, printers, facsimile machines, and multifunction peripherals having combined functions of those, and in image forming apparatuses provided with such developing devices. More particularly, the present disclosure finds applications in developing devices that, while being supplied with fresh two-component developer containing toner and carrier, discharge surplus developer, and in image forming apparatuses provided with such developing devices.