Developing device and image forming apparatus

A developing device for developing an electrostatic latent image on an image bearing member including a developer bearing member opposed to the image bearing member to bear thereon a developer including a toner and a magnetic carrier; a developer containing part to contain the developer to supply the developer to the developer bearing member; an agitating or feeding member to agitate or feed the developer in the developer containing part; a magnetic permeability sensor to measure magnetic permeability of the developer to determine toner concentration of the developer; a developer feeder; a feeding member to feed the developer to the developer containing part through the developer feeder; and a driver to drive the feeding member independently of the agitating or feeding member. The magnetic permeability sensor is arranged on the developer feeder.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is based on and claims priority pursuant to 35 U.S.C. §119 to Japanese Patent Applications Nos. 2011-147207 and 2012-073296, filed on Jul. 1, 2011 and Mar. 28, 2012, respectively, in the Japan Patent Office, the entire disclosure of which is hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a developing device. In addition, the present invention also relates to an image forming apparatus using the developing device.

BACKGROUND OF THE INVENTION

Recently, a developing device using a two-component developer including a toner and a magnetic carrier is mainly used for image forming apparatuses such as copiers, printers, facsimiles and multifunction products having two or more functions of a copier function, a printer function, and a facsimile function. A developing device illustrated inFIG. 4is an example of such a developing device.

Referring toFIG. 4, a developing device100includes a developing sleeve101serving as a developer bearing member, a first feeding screw102and a second feeding screw103, which feed a two-component developer to the developing sleeve101while circulating the developer, and a first developer containing part104and a second developer containing part105, which contain the two-component developer including a toner and a magnetic carrier. In this regard, central portions of the first and second developer containing parts104and105are separated from each other by a partition106provided on a casing of the developing device100, and communication holes107and108are formed on both end portions of the first and second developer containing parts104and105to communicate the developer containing parts104and105with each other.

A drive motor109is provided on a shaft of the developing sleeve101to directly transfer a rotation driving force of the drive motor109to the shaft of the developing sleeve101, thereby rotating the developing sleeve101. An axle gear110provided on the shaft of the developing sleeve101is engaged with a first receiver gear111provided on one end portion of the first feeding screw102. Since the gears110and111are engaged with each other, the rotation driving force of the drive motor109is transmitted to the first feeding screw102, thereby rotating the first feeding screw102. In addition, an intermediate gear112is provided on the other end portion of the first feeding screw102so as to be engaged with a second receiver gear113. Since the gears112and113are engaged with each other, the rotation driving force of the drive motor109is transmitted to the second feeding screw103, thereby rotating the second feeding screw103.

The two-component developer (hereinafter referred to as a developer) contained in the second developer containing part105is fed from left to right inFIG. 4by the second feeding screw103. The developer thus fed by the second feeding screw103is then fed to the first developer containing part104through the right communication hole107. The first feeding screw102, which is arranged in the vicinity of the developing sleeve101, feeds the developer in the first developer containing part104from right to left inFIG. 4while feeding the developer to the developing sleeve101. In addition, the first feeding screw102feeds the developer in the first developer containing part104from right to left inFIG. 4while receiving the developer, which has been used for developing an electrostatic latent image on a photoreceptor drum1, from the developing sleeve101. The developer fed to the left end of the first developer containing part104by the first feeding screw102is returned to the second developer containing part105through the communication hole108. Thus, the developer in the developing device100is circulated in the first and second developer containing parts104and105.

The developing sleeve101bears thereon the developer fed by the first feeding screw102by means of a magnetic force of a magnet roller provided in the developing sleeve101. An electrostatic latent image formed on the photoreceptor1serving as a latent image bearer is developed with the toner included in the developer on the developing sleeve101. The developer used for development, in which the toner therein is consumed for development, is returned to the first developer containing part104to be mixed with the developer fed by the first feeding screw102. The mixed developer is fed to the second developer containing part105, and is then mixed with a toner supplied from a toner supply opening114to increase the toner concentration of the developer. In this case, a magnetic permeability sensor to detect the toner concentration of the developer is provided on a proper portion of the second developer containing part105to determine the mixing ratio of the magnetic carrier to the toner from the measured magnetic permeability. A toner supplying device supplies the toner to the second developer containing part105of the developing device100if desired so that the developer mixed with the toner has a toner concentration in the predetermined concentration range.

Among image forming apparatus using such a developing device100, an image forming apparatus is known which has multiple print modes and changes the process linear velocity thereof when a user changes the print mode. In this regard, for example, change of the process linear velocity is performed as follows. Specifically, when a standard mode is selected from the multiple print modes using an operation part, image forming members such as the photoreceptor1and the developing sleeve101are rotated at a predetermined reference linear velocity. When an image quality oriented mode is selected, the image forming members are rotated at a linear velocity lower than the reference linear velocity. In contrast, when a print speed oriented mode is selected, the image forming members are rotated at a linear velocity higher than the reference linear velocity.

There is a proposal in which the developer feeding speed of the first developer feeding screw is controlled independently of that of the second developer feeding screw. Specifically, the developer feeding speed of the second developer feeding screw, which is rotated in synchronization with the developing roller, is controlled by controlling the rotation speed of a development motor via a second motor driver, which is performed by a controller. In addition, the developer feeding speed of the first developer feeding screw is controlled by controlling the rotation speed of a first feeding motor via a first motor driver, which is also performed by the controller.

Further, there is a proposal in which when the revolution of the first feeding screw is R1 (rps) and the revolution of the second feeding screw is R2 (rps), the output from a toner concentration sensor is periodically read in a cycle of not less than a least common of 1/R1 and 1/R2.

Furthermore, there is a proposal for an image forming apparatus having various processing speeds and performing inductance detection to detect the developer concentration. In the developing device, the concentration detection is carried out only when the linear velocity of a screw for agitating and feeding developer is a predetermined first rotating velocity.

In these image forming apparatuses, the rotation speeds of the first and second feeding screws102and103depend on the rotation speed of the developing sleeve101. Therefore, the screws feed the developer at a speed corresponding to the selected mode. Namely, the developer feeding speed is changed depending on the modes. Therefore, even when the developer feeding speed is changed due to change of the mode, the speed at which the developing sleeve101uses the developer is also changed depending on the mode, and a proper amount of developer is generally supplied to the developing sleeve101.

However, as a result of an experiment of the present inventors, there is a case in which when the feeding speed of a developer is changed, the output from the magnetic permeability sensor is changed even though the toner concentration of the developer is not changed. The result of the experiment is illustrated inFIG. 5.FIG. 5is a graph showing a relation between the revolution of the feeding screw102and the output from the magnetic permeability sensor.

In the experiment, the toner concentration of the developer is controlled to 7% by weight. It can be understood fromFIG. 5that when the revolution of the first feeding screw102changes, the output from the magnetic permeability sensor changes.

It is also confirmed from this experiment that since the output from the magnetic permeability sensor changes even when the toner concentration is not changed, the toner concentration determining operation has a large margin of error.

When the toner concentration determining operation has a large margin of error, the toner concentration controlling operation is performed based on the incorrect toner concentration, which is largely different from the correct toner concentration, a problem in that the developer has too high or too low toner concentration is caused.

In attempting to solve the problem, there is a proposal in which the developer feeding direction and speed at the magnetic permeability detecting part are set so as to be constant independently of the image forming modes (such as print speed oriented mode, image quality oriented mode, and the like) by maintaining the revolution of the feeding screw feeding the developer at the magnetic permeability detecting part so as to be constant independently of the revolution of the developing roller.

By using this technique, change of the output from a magnetic permeability sensor due to change of the print modes can be reduced. However, the technique has a drawback such that at a low image forming speed, the developer is not well-balanced in the developer container, thereby changing the developer drawing conditions of the developing roller.

For these reasons, the inventors recognized that there is a need for a developing device in which the toner concentration of the developer can be determined by a magnetic permeability sensor without affected by change of the process linear velocity of the developing device and in which the developer is fed while well balanced.

BRIEF SUMMARY OF THE INVENTION

As an aspect of the present invention, a developing device for developing an electrostatic latent image on an image bearing member is provided which includes a developer bearing member located so as to be opposed to the image bearing member to bear thereon a developer including a toner and a magnetic carrier, a developer containing part to contain the developer to supply the developer to the developer bearing member, an agitating or feeding member to agitate or feed the developer in the developer containing part, and a magnetic permeability sensor to measure magnetic permeability of the developer to determine toner concentration of the developer. The developing device further includes a developer feeder, a feeding member located in the developer feeder to feed the developer to the developer containing part through the developer feeder, and a driver to drive the feeding member. The magnetic permeability sensor is provided on the developer feeder, and the feeding member is driven independently of the agitating or feeding member.

As another aspect of the present invention, an image forming apparatus is provided which includes an image bearing member to bear an electrostatic latent image thereon, and the above-mentioned developing device to develop the electrostatic latent image with the developer to form a toner image on the image bearing member.

The aforementioned and other aspects, features and advantages will become apparent upon consideration of the following description of the preferred embodiments taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described by reference to drawings.

FIG. 1is a schematic view illustrating an image forming section of an example of the image forming apparatus of the present invention. The image forming apparatus illustrated inFIG. 1has a photoreceptor drum1, which serves as an image bearing member and which is driven to rotate in a direction indicated by an arrow A, and a charger including a charging roller2, to which a voltage is applied to charge the photoreceptor drum1so that the photoreceptor drum has a predetermined potential with a predetermined polarity. In addition, the image forming apparatus has an optical image writing device to irradiate the charged photoreceptor drum1with an optically modulated laser beam L to form an electrostatic latent image on the photoreceptor drum1. A developing device20develops the electrostatic latent image on the photoreceptor drum1with a developer including a toner and a magnetic carrier to form a toner image on the photoreceptor drum1. The developing device20will be described later in detail.

The image forming apparatus further includes an intermediate transfer belt3, which is located above the photoreceptor drum1and which is rotated in a direction indicated by an arrow B, and a primary transfer roller4, which is arranged so as to be opposed to the photoreceptor drum1with the intermediate transfer belt3therebetween. A transfer bias voltage is applied to the primary transfer roller4to transfer the toner image on the photoreceptor drum1to the intermediate transfer belt3. The image forming apparatus further includes a cleaner5to remove residual toner particles remaining on the surface of the photoreceptor drum1even after the primary transfer operation, and a discharger to remove residual charges remaining on the surface of the photoreceptor drum1even after the primary transfer operation is performed, so that the photoreceptor drum1is ready for the next image forming operation. The image forming apparatus is a color image forming apparatus in which four image forming sections, which have the same configuration as that of the image forming section illustrated inFIG. 1and which use different color toners such as yellow, magenta, cyan and black toners, are arranged side by side.

The developing device20includes a case21in which a developer including a toner and a particulate magnetic carrier at a weight ratio (toner/carrier) of 7/93 is contained, a developing roller22, which serves as a developer bearing member and which is arranged so as to be opposed to the photoreceptor drum1at an opening of the case21while rotatably supported by the case21. The developing roller22includes a rotatable developing sleeve, and a fixed magnetic roller, which is arranged in the developing sleeve. The toner is preferably a toner prepared by a polymerization method.

A doctor blade23, which serves as a developer regulating member and which is supported by the case21, is arranged below the developing roller22so as to be close to the developing roller22to control the amount of the developer present on the surface of the developing roller22. The developing device20also includes a developer feeding chamber24, which serves as a developer containing part and which is located obliquely below the developing roller22so as to be opposed to the developing roller, and a developer agitating chamber25, which is located in the horizontal direction from the developing roller22and which is located over the developer feeding chamber24. A feeding screw26(hereinafter referred to as a second feeding screw) serving as an agitating or feeding member (a second feeding member) to agitate or feed the developer is provided in the developer feeding chamber24, which is located on an upstream side from the developing roller22relative to the developer feeding direction, and another feeding screw27(hereinafter referred to as a third feeding screw) also serving as an agitating or feeding member (a third feeding member) to agitate or feed the developer is provided in the developer agitating chamber25, which is located on a downstream side from the developing roller22relative to the developer feeding direction. Reference numeral21adenotes a partition separating the developer feeding chamber24from the developer agitating chamber25.

FIG. 2is a development view illustrating the developing device20. As illustrated inFIGS. 1 and 2, the second and third feeding screws26and27are arranged such that the shaft lines of the second and third feeding screws26and27are parallel to the shaft line of the developing roller22. The developing roller22and the second and third feeding screws26and27are connected with a driving motor28serving as a first driver via a drive transmitter29including plural gears so as to be driven such that the developing roller22and the second feeding screw26are rotated counterclockwise inFIG. 1, and the third feeding screw27is rotated clockwise inFIG. 1. When the second feeding screw26is rotated, the developer in the developer feeding chamber24is fed in a direction indicated by an arrow C inFIG. 2, and when the third feeding screw27is rotated, the developer in the developer agitating chamber25is fed in a direction indicated by an arrow D inFIG. 2.

As illustrated inFIG. 2, a developer circulation entrance30is provided on a downstream side of the developer agitating chamber25relative to the developer feeding direction D. The developer circulation entrance30is connected with a junction feeder40serving as a developer feeder mentioned later via a first circulation pipe31. In this example of the developing device, an air pump32is provided on the first circulation pipe31to feed the developer from the developer agitating chamber25to the junction feeder40. In this regard, not only an air pump but also a screw or a powder pump can be used for feeding the developer. In addition, it is possible to feed the developer by gravitation if condition permits.

A feeding screw42(hereinafter referred to as a first feeding screw), which serves as a first feeding member and which is rotated by a drive motor41serving as an independent second driver, is provided in the junction feeder40to feed the developer in the junction feeder40in a direction indicated by an arrow E inFIG. 2. The circulation pipe31is connected with a feeder entrance43provided on an upstream side of the junction feeder40relative to the developer feeding direction E, and a supplementary developer supplying part44, to which a supplementary developer is supplied from a developer container (not shown), is provided in the vicinity of the feeder entrance43so as to be connected with the junction feeder40.

In addition, a feeder exit45is provided in the junction feeder40so as to be located downstream from the feeder entrance43and the supplementary developer supplying part44, and is connected with a developer feed entrance34via a second circulation pipe33. A magnetic permeability sensor50is provided on a bottom portion of the junction feeder40between the supplementary developer supplying part44and the circulation exit45.

Next, flow of the developer in the developing device20will be described. Referring toFIG. 2, the developer supplied from the developer feed entrance34located on an upstream side of the developer feeding chamber24is fed by the second feeding screw26in the direction C while drawn to the entire surface of the developing roller22by a magnetic pole of the magnet roller fixed inside the developing roller22. The developer thus drawn to the developing roller22is regulated by the doctor blade23, thereby forming a developer layer on the developing roller22. The developer, which is not used for forming the developer layer because of being scraped off the developing roller22by the doctor blade23, is returned to the developer agitating chamber25from an opening of the developer feeding chamber24located on the downstream side of the developer feeding chamber24. The developer layer, which is formed on the developing roller22and which passes through a development region in which part of the toner included in the developer layer is used for developing en electrostatic latent image on the photoreceptor drum1, is released from the developing sleeve of the developing roller22by a developer releasing magnetic pole of the magnet roller in the developing roller22, and the developer is then returned to the developer agitating chamber25located over the developer feeding chamber24.

Thus, the developer fed in the direction C in the developer feeding chamber24and the developer returned from substantially the entire surface of the developing roller22by the developer releasing magnetic pole are fed to the upstream side of the developer agitating chamber25relative to the direction D. The developer mixture is fed in the direction D by the third feeding screw27and reaches the developer circulation entrance30. The developer reaching the developer circulation entrance30is fed by the air pump32to the junction feeder40through the first circulation pipe31. The junction feeder40receives the developer fed from the developer agitating chamber25and the supplementary developer (toner) supplied from the supplementary developer supplying part44to compensate for loss of the toner in the developing operation. After the developer mixture is fed rightward (inFIG. 2) while agitated by the first feeding screw42, the developer mixture is fed from the feeder exit45to the upstream side of the developer feeding chamber24through the second circulation pipe33.

The magnetic permeability sensor50provided at a bottom portion of the junction feeder40outputs a magnetic permeability signal corresponding to the magnetic permeability of the developer to a controller. It is known that the magnetic permeability of a developer including a magnetic carrier and a nonmagnetic toner correlates with the toner concentration of the developer. The controller determines the toner concentration of the developer in the developing device20based on the magnetic permeability signal sent from the magnetic permeability sensor50, and controls rotation of the developing sleeve of the developing roller22based on the signal (toner concentration). Specifically, when the toner concentration is lower than a reference concentration, the controller drives a toner supplying device to supply the toner to the junction feeder40through the developer supplying part44. Thus, the toner concentration is recovered. In this regard, the developer supplied to the junction feeder40is the toner or a mixture of the carrier and the toner (supplementary developer).

In a printer equipped with the developing device20having the above-mentioned configuration, one of three print modes, e.g., a standard mode, an image quality oriented mode and a print speed oriented mode, is selected by a user using an operating portion. When the standard mode is selected, the process linear velocity of the printer is set to a reference linear velocity. In this regard, the process linear velocity means the linear velocity of image forming members such as the photoreceptor drum1, the developing device20, the transfer roller4, and a heating roller and a pressure roller of a fixing device of the printer. When the image quality oriented mode is selected, the process linear velocity of the printer is set to a linear velocity lower than the reference linear velocity. In this mode, the printer produces prints having higher image qualities than in the standard mode. In contrast, when the print speed oriented mode is selected, the process linear velocity of the printer is set to a linear velocity higher than the reference linear velocity. In this mode, the printer produces prints at a higher print speed than in the standard mode.

In the developing device20of such a printer, which can change the process linear velocity, the linear velocity of the developing roller22(developing sleeve), and the second and third feeding screws26and27is also changed. In conventional developing devices, a magnetic permeability sensor is typically arranged at a bottom portion of a chamber having a feeding screw (such as the developer agitating chamber25having the third feeding screw27therein), and therefore outputs from the magnetic permeability sensor are changed when the velocity (revolution) of the feeding screw is changed, resulting in deterioration of the detection precision. Therefore, it is proposed that the feeding screw is driven independently of the developing roller (sleeve) and the other feeding screw (such as the second feeding screw26) to reduce the change of the developer feeding speed of the feeding screw, thereby preventing deterioration of the detection precision. However, as mentioned above, the technique has a drawback in that when the developer feeding speed of the feeding screw is different from those of the developing roller and the other feeding screw, the developer is not well-balanced in the developer container, thereby changing the amount of the developer drawing to the developing roller. Particularly, in a vertical agitation type developing device such as the developing device20mentioned above, in which the developer on the developing roller22is returned to the developer agitating chamber25having the third feeding screw27therein, the developer cannot be smoothly flown in the developing device due to difference in velocity between the third feeding screw27, and the developing roller22and the second feeding screw26, thereby causing various problems including the above-mentioned problem.

In contrast, in the developing device20of the present invention, a magnetic permeability sensor50, which outputs a magnetic permeability signal corresponding to the magnetic permeability of the developer, is provided at a bottom portion of the junction feeder40, which has the first feeding screw42driven by the driving motor41, which is independent of a driver of the developing roller22, etc. Therefore, even when the print mode is changed, the developer in the junction feeder40can be fed at a constant speed or a speed in a narrow speed range. Accordingly, the magnetic permeability sensor50can detects the magnetic permeability of the developer with precision (i.e., at a small error rate) even when the image forming speed (developing speed) is changed due to change of the print mode.

When the print mode is changed in the developing device20illustrated in FIG.2and thereby the first and second feeding screws are rotated at a higher velocity than the first feeding screw42, a problem such that the developer cannot be satisfactorily supplied to the developer feeding chamber24and the developer in the developer agitating chamber25cannot be satisfactorily fed to the junction feeder40because the developer feeding speed in the junction feeder is relatively slow may be caused if the velocity difference condition is continued for a long period of time.

In order to prevent occurrence of such a problem, the developing device of the present invention can have a developer storage. Specifically, in another example of the developing device of the present invention, which is illustrated inFIG. 3, a developer storage70to store the developer fed from the developer agitating chamber25, the supplementary developer (or toner) supplied from a developer container through a supply entrance73, and the developer overflowing on an upstream side of the developer feeding chamber24, is provided. Specifically, the developer fed from the developer agitating chamber25is fed to the developer storage70through an end72of the first circulation pipe31, the supplementary developer is supplied to the developer storage70through the supply entrance73, and the developer overflowing the developer feeding chamber24is fed to the developer storage70through a return entrance74. The developer stored in the developer storage70is discharged from a developer exit71to be supplied to a feeder60serving as a developer feeder through a feeder entrance62. Similarly to the junction feeder40, the feeder60has a feeding screw driven by a driving motor61, which is an independent driver, and the magnetic permeability sensor50, which is provided on a bottom portion of the feeder60and which outputs a magnetic permeability signal corresponding to the magnetic permeability of the developer.

In this example, the developer feeding speed of the feeding screw of the feeder60is set to the highest speed so as to be able to correspond to the print speed oriented mode in which the process linear velocity is the highest. Even when the standard mode or the image quality oriented mode is selected as the print mode, in which the print speed is slower than in the print speed oriented mode, the developer feeding speed is not changed. In this case, the developer feeding speed is not changed in the feeder60, and therefore the detection precision of the magnetic permeability sensor50does not deteriorate. When the standard mode or the image quality oriented mode is selected as the print mode, an excessive amount of developer is fed to the developer feeding chamber24. In this case, excessive of the developer, which overflows the developer feeding chamber24, is returned by an air pump36serving as a developer returning member to the developer storage70via a return pipe35. Therefore, the developer is fed in the developing device20while well-balanced. In this regard, it is possible to substitute a screw or a powder pump for the air pump36similarly to the air pump32. In addition, it is possible to feed the developer by gravitation if condition permits.

Thus, this example can produce an effect such that fluctuation of outputs from the magnetic permeability sensor50can be decreased while producing an effect such that the developer in the developing device20is fed while well balanced.

Hereinbefore, several examples of the developing device of the present invention have been described. However, the present invention is not limited thereto, and additional modifications and variations of the present invention are possible in light of the above teachings. For example, although the above-mentioned developing device is a vertical agitation type developing device, the present invention can also be applied to a circulation developing device in which a developer is circulated between a first screw and a second screw while the developer fed by the first screw is supplied to a developing roller, and the developer on the developing roller, which has been used for development, is returned to the chamber in which the first screw is arranged.

In addition, although a feeding screw is exemplified as the agitating or feeding member, the agitating or feeding member is not limited thereto. For example, a feeding coil can also be used as the agitating or feeding member. Even when an elliptic plate or a paddle, which is an agitating member having no feeding function, is used instead of the agitating or feeding member, fluctuation of outputs of the magnetic permeability sensor is caused. Therefore, the present invention, in which only the developer feeder, on which a magnetic permeability sensor is provided, is independently driven so as to have a constant linear velocity, can also be applied to such an agitating member.