Developing device, process cartridge and image forming apparatus to inhibit the increase of the rate of uncharged toner during prolonged operation

A developing device, which can inhibit the increase of the rate of uncharged toner caused by long hours of operation and develop an excellent image, and a process cartridge and an image forming apparatus that use such a developing device. A developing sleeve is rotary driven, and has, as magnetic field generating means that is fixedly disposed within the developing sleeve, a magnet roll composed of a plurality of stationary magnets. The magnet roll has five magnetic poles, P1 (south pole), P2 (north pole), P3 (north pole), P4 (south pole) and P5 (north pole), which are arranged in the direction of rotation of the developing sleeve in this order starting from a developing position that is a region facing a photoreceptor. On the periphery of the developing sleeve, there is disposed blade-shaped developer accumulation means, which is attached to a developer container at a position between a doctor blade and a stirring section and in the vicinity of the stirring section, with a predetermined space between the developing sleeve and the developer accumulation means.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a developing device for use in a copying machine, a facsimile device, a printer or the like, and to a process cartridge and an image forming apparatus using the developing device. Particularly, the present invention relates to a developing device that performs development using a two-component developer composed of a toner and a magnetic carrier, and to a process cartridge and an image forming apparatus using the developing device.

2. Description of the Related Art

There has been conventionally known a developing device that uses a two-component developer composed of a magnetic carrier and a toner. Such a developing device using the two-component developer generally deposits the developer on a developer carrier as a so-called magnetic brush, regulates the thickness of the developer layer to a predetermined thickness, and then develops an electrostatic latent image on a latent image carrier such as a photoreceptor. As the developer carrier, for example, the one having a cylindrical developing sleeve, which is disposed rotatably and carries and conveys the two-component developer, and a magnet roll, which is fixedly disposed within the developing sleeve and has a plurality of magnetic poles arranged around the magnet roll, are used.

Incidentally, the distance between the latent image carrier and the developer carrier needs to be reduced in order to faithfully develop the electrostatic latent image held on the latent image carrier. If the distance between the latent image carrier and the developer carrier is reduced, the thickness of the developer layer formed on the developer carrier accordingly needs to be reduced.

As a conventional technique for regulating the thickness of the developer layer on the developer carrier, there has been widely used a method of regulating the thickness of the developer layer by using a developer regulating member against the developer carrier and using a fixed space formed between a leading end of the developer regulating member and the surface of the developing sleeve (referred to as “doctor gap” hereinafter). When using this method, the amount of developer passing through the developer regulating member can be adjusted by controlling the doctor gap, but the doctor gap needs to be set extremely small in order to form a thin layer of the developer. However, if the doctor gap is small, the amount of developer passing through the doctor gap fluctuates more significantly than when the doctor gap is large, and thereby the layer thickness of the developer formed on the developer carrier becomes nonuniform. Moreover, great stress is applied to the developer when the developer passes through the doctor gap, reducing the life of the developer.

In order to solve such problems, Japanese Unexamined Patent Publication No. H5-6103, for example, proposes a developing device in which magnetic poles of the same polarity are adjacently disposed on a magnet roll of a developer carrier, a developer regulating member is provided between the magnetic poles of the same polarity, and the packing density of a developer in the doctor gap is reduced by using repulsive magnetic fields of these magnetic poles so as to reduce the amount of developer passing through the doctor gap.

However, in this conventional developing device, the developer tends to accumulate in the doctor gap due to long hours of operation and, as a result, the rate of uncharged toner increases, causing a problem that an abnormal image with a scumming or the like is generated easily.

SUMMARY OF THE INVENTION

The present invention is contrived in order to solve the above problems. It is an object of the present invention to provide a developing device capable of preventing the rate of uncharged toner from being increased by long hours of operation, to develop an excellent image, and a process cartridge and an image forming apparatus that use the developing device.

In an aspect of the present invention, a developing device comprises a developer carrier that has therein a magnetic field generating device having a plurality of magnetic poles, and carries and conveys a two-component developer composed of a toner and a magnetic carrier; a developer regulating member that regulates a layer thickness of the two-component developer carried and conveyed by the developer carrier; a stirring section that stirs the toner and the two-component developer removed from the developer carrier; and a developer accumulation device for accumulating the two-component developer between the stirring section and the developer regulating member. Some of the two-component developer is fed back to the stirring section by the developer accumulation device.

In another aspect of the present invention, a process cartridge comprises a developing device. The developing device comprises a developer carrier that has therein a magnetic field generating device having a plurality of magnetic poles, and carries and conveys a two-component developer composed of a toner and a magnetic carrier; a developer regulating member that regulates a layer thickness of the two-component developer carried and conveyed by the developer carrier; a stirring section that stirs the toner and the two-component developer removed from the developer carrier; and a developer accumulation device for accumulating the two-component developer between the stirring section and the developer regulating member. Some of the two-component developer is fed back to the stirring section by the developer accumulation device.

In another aspect of the present invention, an image forming apparatus comprises a developing device. The developing device comprises a developer carrier that has therein a magnetic field generating device having a plurality of magnetic poles, and carries and conveys a two-component developer composed of a toner and a magnetic carrier; a developer regulating member that regulates a layer thickness of the two-component developer carried and conveyed by the developer carrier; a stirring section that stirs the toner and the two-component developer removed from the developer carrier; and a developer accumulation device for accumulating the two-component developer between the stirring section and the developer regulating member. Some of the two-component developer is fed back to the stirring section by the developer accumulation device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, the inventor of the present invention has investigated the occurrence of an abnormal image having a scumming or the like in a developing device, which has: a developer carrier that has magnetic field generating means having fixed therein a plurality of magnetic poles and carries and conveys a two-component developer as a magnetic brush, which is composed of a toner and a magnetic carrier; and a developer regulating member that regulates the thickness of a layer of the developer carried and conveyed by the developer carrier. As a result of the investigation, the inventor has discovered that when the developer carried by the developer carrier accumulates before a doctor gap regulating the thickness of the developer layer, the rate of uncharged toner increases in the toner of the developer accumulated during long hours of operation, and that this uncharged toner is one of the reasons for the occurrence of scumming and the like.

As a result of further investigation performed for solving this problem, the inventor has discovered that it is necessary to inhibit the accumulation of the developer before the doctor gap as much as possible, and to sufficiently stir the developer removed from the developer carrier by the magnetic field generating means and a new replenished toner by means of a stirring section so as to uniformly mix the toner and developer. As a result of investigation performed based on this result, the inventor has discovered that the increase of the uncharged toner rate can be inhibited by providing developer accumulation means between the stirring section and the doctor gap, causing the developer accumulation means to accumulate the developer, and feeding the accumulated developer to the stirring section.

It is desired that the developer accumulation means be disposed between the stirring section and doctor gap and particularly adjacent to the stirring gap in order to prevent the developer from being accumulated before the doctor gap. Moreover, it is desired that the mixture of the developer removed from the developer carrier and the toner be accumulated properly between the stirring section and the doctor gap, particularly in the vicinity of the stirring section, by the developer accumulation means. As with the developer regulating member that forms a doctor gap, the following members can be used effectively as a member that satisfies the abovementioned conditions: a blade-like developer regulating member that physically regulates partial conveyance of the developer carried and conveyed by the developer carrier, or, as described hereinafter, a member that magnetically applies a transfer force to the direction that causes the developer to flow backward in the conveyance direction of the developer carrier, to accumulate the developer.

Hereinafter, the present invention is described in detail with reference to the drawings.

First Embodiment

FIG. 1shows a schematic configuration of a process cartridge of the present embodiment. In the drawing, reference numeral1designates a drum-like photoreceptor, which is a latent image carrier for carrying a latent image, reference numeral2designates a developing device that makes the latent image visible, and reference numeral3designates a cleaning device that cleans the photoreceptor1by removing transferred residual toner remaining on the photoreceptor1after a visible toner image is transferred to a recording paper, the toner image being obtained by making the latent image visible by the developing device.

The process cartridge of the present embodiment is obtained by integrally assembling the photoreceptor1, developing device2and cleaning device3. The process cartridge can be incorporated in an after-mentioned image forming apparatus to perform image formation processing, or can be removed from the image forming apparatus so that the photoreceptor1, developing device2and cleaning device3can be easily maintained, repaired or replaced. The members incorporated in the process cartridge are not limited to the photoreceptor1, developing device2and cleaning device3, thus, for example, a member obtained by integrally assembling only the developing device2and photoreceptor1, or a charging device for uniformly charging the surface of the photoreceptor may be added to the process cartridge described in the above embodiment.

The present embodiment is characterized by the structure of the developing device2, thus the developing device2is mainly described.

The developing device2of the present embodiment is disposed on the side of the photoreceptor1, as shown inFIG. 1. A developing sleeve5serving as the developer carrier is disposed such that a part thereof is exposed to the outside of the photoreceptor1from an opening section6of a developer container4accommodating a removed developer and a toner for replenishment, which are described hereinafter. The developing sleeve5is rotary driven in the direction of an arrow B by driving means, not shown, and has, as magnetic field generating means, a magnet roll7that is fixedly disposed in the magnetic sleeve and composed of a plurality of stationary magnets. The magnet roll7has five magnetic poles, P1(south pole), P2(north pole), P3(north pole), P4(south pole) and P5(north pole), which are arranged in the direction of rotation of the developing sleeve in this order starting from a developing position8that is a region facing the photoreceptor1.

These five magnetic poles, P1, P2, P3, P4and P5, show a distribution of normal magnetic flux density as shown inFIG. 2. The magnetic pole P1facing the photoreceptor1has the largest magnetic field in order to reliably supply the toner to the photoreceptor1rotating in the direction of an arrow A. The magnetic poles P2and P3are of the same polarity and thus generate a repulsive magnetic force to remove a residual developer11acarried on the developing sleeve5and return it into the developer container4.

It should be noted that the distribution of the magnetic flux density shown inFIG. 2and the arrangement of the magnetic poles are merely an example, thus the number of magnetic poles and the arrangement thereof are not limited to the pattern described above. Therefore, the developing sleeve5can carry, on the surface thereof, a two-component developer11(referred to as “developer” hereinafter) having a toner and a magnetic carrier, as a magnetic brush.

A doctor blade10, which is the developer regulating member for regulating the amount of developer11to be supplied to the photoreceptor1, is placed on the periphery of the developing sleeve5, whereby the thickness of a layer of the developer11carried on the developing sleeve5is regulated by a doctor gap formed between the doctor blade10and the developing sleeve5.

Inside the developer container4, there is placed a stirring member9, such as a screw, which is rotary driven by unshown driving means in the direction of an arrow C. This stirring member9forms a region for stirring and mixing the removed toner11a,which is removed from the developing sleeve5by the magnetic poles P2and P3, and a toner to be replenished (stirring section15). The toner to be replenished is fed from a toner replenishing section12to the stirring section15by an auger13via a toner replenishing opening4aformed on the developer container4. In this manner, the toner and the magnetic carrier are stirred and mixed by the stirring member9, whereby a predetermined toner charge amount is obtained. It is preferred that the toner charge amount on the developing sleeve5be in a range of −10 to −25 (μC/g). The toner can have a magnetic material and be used as a magnetic toner.

On the periphery of the developing sleeve5, there is disposed blade-shaped developer accumulation means16, which is attached to the developer container4at a position between the doctor blade10and the stirring section15and in the vicinity of the stirring section15, with a predetermined space between the developing sleeve5and the developer accumulation means16. The developer accumulation means16prevents partial conveyance of a mixture11bof the removed toner11aand toner carried on the developing sleeve5so that the mixture accumulates, and the accumulated mixture11bis sent back to the stirring section15, stirred, and mixed by the stirring section15again, whereby sufficient toner charge amount can be obtained.

Therefore, in the developer11having the sufficiently charged toner, only a small amount of developer11that passes through the space between the developer accumulation means16and the developing sleeve5is conveyed to the doctor blade10. The layer thickness of the developer11is regulated by the doctor blade10, and the developer11can be conveyed to the developing position8without generating a large quantity of accumulated developer before the doctor blade10. Therefore, the increase of the uncharged toner can be inhibited, and excellent image formation can be maintained.

Incidentally, the developing device2of the present embodiment sends excess developer back to the stirring section15by means of the developer accumulation means16that is disposed on the upstream side of the developer conveyance direction (direction B) higher than the doctor blade10. By disposing this developer accumulation means16, the amount of the developer that is held by the magnetic pole P4facing the doctor blade10(referred to as “amount of accumulated developer” hereinafter) is adjusted to be equal to or less than twice the amount of developer passing through the developer regulating member. It should be noted that the amount of accumulated developer is the amount of the developer that is held between, on the developing sleeve5, the position of the magnetic pole P4facing the doctor blade10and the position where the normal magnetic flux density distribution value of the magnetic pole P4becomes zero (P0). This amount of developer may be measured by taking the developing sleeve5outside the developing device2, with the doctor blade10attached thereto, after the developing sleeve5is rotated once, and then collecting the developer accumulated in the developing sleeve5.

Regarding the amount of accumulated developer in front of the doctor blade10, there are methods of reducing the magnetic field of the magnetic pole P4shown in, for example,FIG. 1and controlling the amount of developer to be supplied from the developer container4to the developing sleeve5, but it is difficult to stably control the amount supplied using these methods, due to the effects of the stirring member9disposed on the stirring section15. Therefore, the developer accumulation means16that accumulates the excess developer after the developer is supplied from the stirring section15once and controls the amount of the accumulated developer is provided between the stirring section15and the doctor blade10to adjust the amount of the accumulated developer, whereby the amount of developer passing through the doctor blade10is stabilized without being influenced by the amount supplied by the stirring member9, whereby the degree of stress applied to the developer11can be reduced.

In the developing device having the above-described configuration, the developer11on the developing sleeve5is conveyed as the developing sleeve5rotates in the direction of the arrow B, and the layer thickness of the developer11is regulated by the doctor blade10to be thinned. The developer11, the layer thickness of which is reduced, is conveyed to the developing position8that faces the photoreceptor1rotating in the direction of the arrow A. A developing bias is applied to the developing sleeve5by a power source, which is not shown. In the developing position8, the toner in the developer11is supplied to the electrostatic latent image formed on the surface of the photoreceptor1, the electrostatic latent image is then made visible, and development is performed. A development gap GP, which is a gap between the photoreceptor1and the developing sleeve, can be set within a conventional range of 0.8 mm to 0.4 mm, and development efficiency can be improved by setting the value of the development gap small.

Second Embodiment

FIG. 3shows a schematic configuration of the developing device of the present embodiment. In the drawing, the same reference numerals are applied to the same components used in the first embodiment, thus the overlapping explanations are omitted.

The developing device2of the present embodiment has basically the same configuration as the one described in the first embodiment, but the number and arrangement of the magnetic poles within the magnet roll7are different. Also, regarding the developer accumulation means16, a magnetic force is used in place of the blade-shaped developer regulating member to cause accumulation of the developer11. Specifically, the magnet roll7has seven magnetic poles, P1(south pole), P2(north pole), P3(north pole), P4(south pole), P5(north pole), P6(south pole), and P7(north pole), as shown inFIG. 4, wherein the magnetic pole P1faces the photoreceptor1and P7faces the doctor blade10. The magnetic poles P4and P5that have a minute magnetic field of 20 mT (militesla) or lower are provided between the magnetic pole P7and the magnetic pole P3adjacent to the stirring member9. The endurance of the developer on the developing sleeve5is weakened by the magnetic force generated between these minute magnetic fields P4and P5, causing accumulation of the developer11. Therefore, P4and P5cause the same developer accumulation as the developer accumulation means16of the above-described first embodiment.

In this manner, by setting the magnitude of the magnetic poles P4and P5to be equal to or lower than 20 mT, the developer11is held only on the surface of the developing sleeve5, and the amount of accumulated developer can be easily adjusted to be equal to or less than twice the amount of developer passing through the developer regulating member. According to a method of obtaining such magnetic fields, a concave groove18with minute width is formed on a magnet, and the magnetic poles P3through P6are formed, as shown inFIG. 5. In this method, one magnet is processed even with a small multipole magnetic field, thus there is no problem in obtaining accuracy generated when handling a minute magnet. Therefore, use of this method is advantageous in terms of costs. It should be noted that an electric magnet may be used in place of the magnet. In this case, an advantage is that the size of the magnetic fields can be changed by changing the amount of current, thus, even if the property of the developer11fluctuates primarily, the layer thickness of the developer11can be changed appropriately by monitoring the fluctuation.

In this manner, by using the magnetic fields as the means for accumulating the excess developer11after the developer11is supplied from the stirring section15once, the amount of the developer11can be adjusted without contacting with the developer, thus the layer thickness of the developer can be thinned without applying new stress to the developer. Moreover, it is only necessary to change the design of the magnet to be disposed inside the developing sleeve5, thus it is not necessary to dispose any new members, which is advantageous in realizing a small device.

Third Embodiment

FIG. 6shows a schematic configuration of the image forming apparatus that uses the developing device2of the first or second embodiment described above. In the drawing, reference numerals1Y,1C,1M and1K designate, respectively, photoreceptors that form toner images in colors of yellow (Y), cyan (C), magenta (M) and black (K). Reference numerals2Y,2C,2M and2K designate, respectively, developing devices that have the structure described in the first and second embodiments and store the toners of these colors. Reference numeral23designates a charging deice, and reference numeral3designates a cleaning device that cleans the surface of each photoreceptor. Reference numeral25designates an exposure device that emits a laser beam L corresponding to each toner color to each of the photoreceptors1Y,1C,1M and1K to form an electrostatic latent image. Reference numeral31designates an endless intermediate transfer belt that is wrapped around rollers35so that the toner images of the respective colors are transferred from the photoreceptor drums1Y,1C,1M and1K by transfer rollers32. Reference numeral33designates cleaning means for cleaning the intermediate transfer belt31. Reference numeral34designates a secondary transfer roller that transfers the toner images from the intermediate transfer belt31to a transfer sheet22that is reeled out from a paper-feeding unit20by a reeling roller21. Reference numeral40designates a fixing device that thermally presses and fixes the toner images formed on the transfer sheet22transferred from the intermediate belt31. Reference numeral41designates a paper-discharging roller that discharges the transfer sheet on which the toner images are fixed. Reference numeral50designates a toner bottle that supplies the toner to each developing device.

In this image forming apparatus, an electrostatic latent image is formed on each of the photoreceptor drums1Y,1C,1M and1K by the laser beam L that corresponds to each toner color and is emitted from the exposure device25, and each of the toner color is adhered to the electrostatic latent image from each of the developing devices2Y,2C,2M and2K, whereby toner images corresponding to the respective toner colors are formed. The toner images formed in this manner are transferred to the intermediate transfer belt31. The toner images transferred to the intermediate transfer belt31are then transferred to the transfer sheet22and fixed by the fixing device, and printing is performed.

Next, the image forming apparatus of the present embodiment is used, and the developing device described in the first and second embodiments, and, as a comparative example, the developing device from which the developer accumulation means16is removed in the first embodiment are used to drive the developing device2K only, and the rate of uncharged toner within the developer11is measured for each driving time period. Here, the doctor gap is set such that the amount of developer passing through the doctor blade10becomes 0.5 mg/cm2. Then, only the developing devices were driven when the linear velocity of the developing sleeve5was 200 mm/sec, and the rate of the toner that was not negatively charged was measured. In the measurement, the E-Spart analyzer manufactured by Hosokawa Micron, Ltd. was used to sample 5000 toners, measure charged amount of each toner, and obtain the rate of the toners that are not negatively charged, out of the 5000 toners.

As other specific conditions, the diameter of each photoreceptor was set to 50 mm, and the linear velocity was set to 200 mm/sec. The diameter of the developing sleeve was set to 18 mm, charging potential VD of the photoreceptor1before exposure to −350 V, potential VL after exposure to −50 V, and developing bias voltage VB to −250 V, that is, a developing potential (VL−VB=200 V).

FIG. 7shows the results of measurement. A curved line1in the drawing shows the first embodiment, a curved line2shows the second embodiment, and a curved line3shows the comparative example1. According to these results, it is clear that in the first and second embodiments the rate of the uncharged toner is not much increased even when the developing devices were driven for a long time, compared to the comparative example. Furthermore, these developing devices were used to form an image, and the image was evaluated. In the comparative example, the occurrence of an abnormal image with a scumming or the like was found after 30 minutes of driving, but an excellent image with no scumming or the like was obtained in the first and second embodiments.

Fourth Embodiment

FIG. 8shows a schematic configuration of the developing device of the present embodiment. Also,FIG. 9shows an arrangement of the magnetic poles of the magnet roll7and a distribution of the normal magnetic flux density of the magnet roll7. InFIG. 8, the same reference numerals are applied to the same components used in the first embodiment, thus the overlapping explanations are omitted.

As with the developing device described in the second embodiment, in the development of the present embodiment, the magnetic fields are used to accumulate the developer11. The developer drawn up from the magnetic pole P3passes through the magnetic pole P4. The magnetic poles P4and P6are of the same polarity, thus a force applies in a direction in which the developer is separated from the developing sleeve5. At this moment, the normal magnetic flux density of the magnetic pole P5that is a repulsive magnetic field with respect to the magnetic poles P4and P6is formed to be equal to or lower than 20 mT, whereby the layer thickness of the entire developer is thinned without causing the developer to be separated at the magnetic pole P4, and the developer is conveyed to the doctor blade10disposed on the magnetic pole P7. Particularly, by disposing such a repulsive magnetic field in front of the doctor blade, the separated developer is subjected to the process in which the separated developer is disposed in the vicinity of the magnetic pole P4, stirred by a screw19rotating in the direction of an arrow D, sent back to the lower stirring member9, and captured by the developing sleeve5at the magnetic pole P3.

In this manner, by using the magnetic fields P4through P6, the developer11can be appropriately accumulated at the position adjacent to the stirring section15, thus the occurrence of the uncharged toner can be prevented. Moreover, stirring distance of the developer is increased in spite of the narrow space, thus toner dispersibility is improved, and a high-quality image with no uneven density can be produced. The image was formed using the image forming apparatus shown inFIG. 6. At this moment, two types of components that have the waveform formed by the magnetic poles as shown inFIG. 2andFIG. 9were prepared in place of the magnet roll7, to form ten solid images continuously. When the magnet roll that has the waveform formed by the magnetic poles as shown inFIG. 2was used, uneven image density was confirmed on the eighth solid image and thereafter, but almost no uneven image density was observed when the magnet roll that has the waveform formed by the magnetic poles as shown inFIG. 9was used.

As described above, by adopting the above-described configurations, the present invention can provide a developing device, which can inhibit the increase of the rate of uncharged toner caused by long hours of operation and develop an excellent image, and a process cartridge and image forming apparatus that use such a developing device.