DEVELOPING DEVICE, PROCESS CARTRIDGE, AND IMAGE FORMING APPARATUS

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus such as a copier, printer, facsimile device or a combination machine thereof that uses an electrophotographic system, and to a developing device and process cartridge installed therein, and more particularly relates to a developing device, process cartridge and image forming apparatus of a premix developing system that suitably supplies new carrier into the developing device.

2. Description of the Related Art

Disclosed in the past (for example, Japanese Patent Application Laid-open No. 2001-183893 (Prior Art 1) and Japanese Patent Application Laid-open No. 2000-112238 (Prior Art 2)) was technology (called a premix developing system) to suitably supplement new carrier into a developing device of an image forming apparatus such as a copier or a printer housing two-component developer comprising toner and carrier (this shall include developers to which additives are added).

In developing devices that use two-component developer, toner is suitably supplemented into the developing device from a toner supplement port provided on one part of the developing device corresponding to the consumption of toner in the developing device. The supplemental toner is agitated and mixed with the developer in the imaging apparatus by a transport member (agitating member) such as a transport screw. Part of the agitated and mixed developer is supplied to a developing roller. The developer supported on the developing roller is restricted to a suitable amount by a doctor blade, and then, in a position opposite a photosensitive drum, the toner in that two-component developer adheres to a latent image on the photosensitive drum.

In this way, carrier degrades over time because the carrier in the two-component developer housed in the developing device remains in the developing device without being consumed in the normal developing process. In more detail, this generates the “film scraping phenomenon”, in which the coating layer of the carrier is abraded or peeled off due to the carrier being agitated and mixed over a long time in the developing device causing a reduction in the charge capacity of the carrier, and the “spent phenomenon”, in which the toner components and additives adhere to the surface of the carrier causing a reduction in the charge capacity.

The premix developing system is for the purpose of preventing the reduction of the output image quality by this kind of carrier deterioration over time. Specifically, by suitably supplementing new carrier (or new two-component developer) into the developing device and suitably discharging some of the two-component developer housed in the developing device to outside of the developing device, the amount of deteriorated carrier in the developing device is reduced and the volume and charge capacity of the carrier housed in the developing device is maintained.

Image forming apparatuses that use this kind of premix developing system have more stable output image quality over time than do apparatuses that require replacement of the developing device and carrier with new products every time the carrier deteriorates over time.

Meanwhile, disclosed in Prior Art 1 and Prior Art 2, and the like, is an apparatus in a premix developing system that uses an overflow system as means for discharging developer to the outside of the developing device. In more detail, a discharge port (hole) is provided in a wall of the developing device, and when the developer surface that is carried to that location exceeds a specified height, that developer (developer that has become the surplus portion after supplementing carrier) is discharged to the outside from the discharge port.

Moreover, disclosed in Prior Art 2 and the like is a technology that, for the purpose of reducing the amount of developer discharged from the discharge port (developer discharge port), eliminates (or decreases the radius of) the screw part (blade) of the transport member (agitation and transport member) facing the discharge port.

In the developing device of the premix developing system of Prior Art 1 and the like described above, developer is added to the developer to be discharged until the necessary amount of developer has been discharged from the discharge port, and the amount of developer inside the developing device can become insufficient by discharging an excessive amount of the developer. Concretely, as opposed to only the developer that exceeds the specified height of the developer surface and that should be discharged from the discharge port, developer splashed by the transport member is also discharged from the discharge port.

When the amount of developer in the developing device becomes insufficient in this way, the deterioration status of the developer becomes unstable, the amount of static electricity of the toner drops, and problems occur such as an output image with decreased image concentration.

In order to resolve this kind of problem, the screw part of the transport member facing the discharge port was removed (or made a smaller diameter) in Prior Art 2. However, as a result of assiduous research, the three inventors of the present application discovered that it is not possible to fully suppress the problem of developer splashed by the transport member being discharged from the discharge port simply by removing the screw part of the transport member facing the discharge port.

SUMMARY OF THE INVENTION

An object of the present invention is to resolve the problems described above, and to provide a developing device of a premix developing system, a process cartridge, and an image forming apparatus that do not discharge from the apparatus developer splashed by the transport member, that do not produce fluctuations in the amount of developer to be discharged to the outside, and that have stable output image quality.

In an aspect of the present invention, a developing device houses developer having carrier and toner, and develops a latent image formed on an image support unit. The developing device comprises a plurality of transport members for longitudinally transporting the developer housed in the device to form a circulation route; a supply device for supplying new carrier into the device; and a pocket part, which is installed in a depressed manner facing the outside in relation to the transport route formed by one of the plurality of transport members, and into and out of which part of the developer transported in the transport route flows. The pocket part comprises a wall part that covers the side corresponding to the downstream side of the transport route, and a discharge port for discharging developer to the outside of the device when the surface of the developer flowing into the pocket part has exceeded a specified height.

In another aspect of the present invention, a process cartridge is detachably installed to an apparatus main body of an image forming apparatus. A developing device and an image support unit are formed into a single unit. The developing device houses developer having carrier and toner, and develops a latent image formed on an image support unit. The developing device comprises a plurality of transport members for longitudinally transporting the developer housed in the device to form a circulation route; a supply device for supplying new carrier into the device; and a pocket part, which is installed in a depressed manner facing the outside in relation to the transport route formed by one of the plurality of transport members, and into and out of which part of the developer transported in the transport route flows. The pocket part comprises a wall part that covers the side corresponding to the downstream side of the transport route, and a discharge port for discharging developer to the outside of the device when the surface of the developer flowing into the pocket part has exceeded a specified height.

In another aspect of the present invention, an image forming apparatus comprises a developing device and an image support unit. The developing device houses developer having carrier and toner, and develops a latent image formed on an image support unit. The developing device comprises a plurality of transport members for longitudinally transporting the developer housed in the device to form a circulation route; a supply device for supplying new carrier into the device; and a pocket part, which is installed in a depressed manner facing the outside in relation to the transport route formed by one of the plurality of transport members, and into and out of which part of the developer transported in the transport route flows. The pocket part comprises a wall part that covers the side corresponding to the downstream side of the transport route, and a discharge port for discharging developer to the outside of the device when the surface of the developer flowing into the pocket part has exceeded a specified height.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which:

FIG. 1 is a diagram indicating the overall configuration of an image forming apparatus in one embodiment of the present invention;

FIG. 2 is a diagram indicating a process cartridge provided in the same image forming apparatus;

FIG. 3 is a diagram indicating the configuration of a developing device.

FIG. 4 is a cross-sectional diagram viewing the circulation route of the same developing device longitudinally;

FIG. 5 is a cross-sectional diagram indicating the Y1-Y1 cross-section of the circulation route of FIG. 4;

FIG. 6 is a cross-sectional diagram indicating the Y2-Y2 cross-section of the circulation route of FIG. 4;

FIG. 7 is a diagram indicating the state when wave-shaped distortions are produced in the developer in the circulation route of FIG. 4;

FIG. 8 is a top-view diagram indicating the transport route formed by the first transport member;

FIG. 9 is a cross-sectional diagram indicating the configuration near the wall part;

FIG. 10 is another cross-sectional diagram indicating the configuration near the wall part; and

FIG. 11 is a diagram indicating the state when the developer splashed by the same first transport member penetrates into the discharge port.

DESCRIPTION OF THE PREFERRED EMBODIMENT(s)

Optimum forms for embodying the present invention will be explained below by referring to the diagrams. In addition, the same codes will be applied to the same or equivalent parts in the diagrams, and redundant explanations will be suitably abbreviated or omitted.

First, referring to FIG. 1 of the diagrams, the configuration and operation of the image forming apparatus of the embodiment as a whole will be explained.

Writing units 2A to 2D are devices that after a charge process use image data to write electrostatic latent images onto a photosensitive drum 21 (image support unit). Writing units 2A to 2D are optical scanning devices that use polygon mirrors 3A to 3D and optical elements 4A to 4D and the like. Further, LED arrays may be used as the writing unit instead of an optical scanning device.

A paper feed unit 61 stores transfer material P such as recording paper or acetate for OHP, and the transfer material P is sent toward a transfer belt 30 during image formation.

The transfer belt 30 is an endless belt for electrostatically adsorbing the transfer material P to the surface thereof, transporting, and transferring a toner image formed on a photosensitive drum 21 onto the transfer material P; and an adsorbing roller 64 and a belt cleaner 65 are provided on the peripheral surface thereof.

A transfer roller 24 facing the photosensitive drum 21 through the transfer belt 30 has a metal core and a conductive elastic layer that covers the metal core. The conductive elastic layer of the transfer roller 24 is an elastic body that has the electric resistance value (volume resistant modulus) adjusted to middle resistance by compounding and dispersing carbon black, zinc oxide, tin oxide or the like in an elastic material such as polyurethane rubber, ethylene-propylene-diene polyethylene (EPDM) or the like.

A fixing part 66 has a heating roller 68 and a pressure roller 67, and fixes the toner image on top of the transfer material P to the transfer material P using pressure and heat.

Four process cartridges 20Y, 20C, 20M and 20BK arranged transversely along the transfer belt 30 are for the purpose of forming yellow, cyan, magenta and black toner images respectively.

Developer cartridges 28Y, 28C, 28M and 28BK are arranged on the respective process cartridges 20Y, 20C, 20M, and 20BK as supply means for supplying carrier (magnetic carrier) and toner (toner particles) of various color (yellow, cyan, magenta and black) to the developing device 23.

The process cartridges 20Y, 20C, 20M and 20BK and the developer cartridges 28Y, 28C, 28M and 28BK can be attached and detached from an apparatus main body 1 by opening the transfer belt 30 pivoting on a rotational spindle.

Further, in the present application “process cartridge” is defined as a unit which is detachably installed to the image forming apparatus main body, and in which the image support unit is integrated with at least one of a charge part that charges the image support unit, a developing part (developing device) that develops the latent image formed on the support unit, and a cleaning unit to clean the top of the image support unit.

The image forming apparatus of the present embodiment is a combination image forming apparatus that has copier and printer functions. When functioning as a copier, image data read from a scanner is converted into read data by conducting a variety of imaging processes such as A/D conversion, MTF compensation, and gradation processing. When functioning as a printer, page description language and bitmap format image data sent from a computer and the like are converted into read data by conducting image processing.

When forming images, exposure light corresponding to black, magenta, cyan and yellow image data are respectively irradiated from writing units 2A to 2D onto process cartridges 20BK, 20M, 20C and 20Y. Specifically, exposure light (laser light) generated from light sources is irradiated onto the photosensitive drums 21 through polygonal mirrors 3A to 3D and optical elements 4A to 4D. A toner image corresponding to the exposure light is thereby formed on the photosensitive drums 21 (image support unit) of the process cartridges 20BK, 20M, 20C and 20Y. Then, those toner images are transferred to the transfer material P.

Once the timing matches at the resist roller 63 position, the transfer material P sent from the paper feed unit 61 is transported to the position of the transfer belt 30. The adsorption roller 64, which is arranged in the transfer belt 30 feed-in position, causes the transfer material P that has been fed in by the application of voltage, to be adsorbed to the transfer belt 30. The transfer material P moves in conjunction with the transfer belt 30 that runs in the direction of the arrow to successively pass through the positions of the process cartridges 20Y, 20C, 20M and 20BK, and the various colored toner images are laminated and transferred thereon.

The transfer material P onto which the color toner image has been transferred is released from the transfer belt 30 and arrives at the fixing part 66. The toner image on the transfer material P is fixed onto the transfer material P by heating while being squeezed between the heating roller 68 and the pressure roller 67. Meanwhile, the surface of the transfer belt 30 after the transfer material P has been released arrives at the position of the belt cleaner 65, and the contamination such as the toner adhering to the surface thereof is cleaned off.

Next, the process cartridge and the developer cartridge of the image forming apparatus will be described in detail.

Further, because the process cartridges 20Y, 20C, 20M and 20BK have nearly identical structures and the developer cartridges 28Y, 28C, 28M and 28BK have nearly identical structures, the process cartridge and the developer cartridge will be indicated without the alphabetical codes (Y, C, M and BK) in FIG. 2. In addition, the writing units will also be indicated without the alphabetical codes (A to D).

As indicated in FIG. 2, the photosensitive drum 21, which is the image support unit, a charge part 22, the developing device 23 (developing unit) and a cleaning part 25 are unified in the process cartridge 20, and the premix developing system (developing system in which carrier is suitably supplemented and discharged) has been adopted.

The photosensitive drum 21, which is the image support unit, is a negative charge organic photosensitive unit, and is rotationally driven counterclockwise by a rotational drive mechanism not indicated in the diagram.

The charge part 22 is a charge roller that has elasticity, in which a foam urethane layer with medium resistance was made by processing urethane resin, carbon black as the conductive particles, a sulfurizing agent, a foaming agent and the like, was formed into a roller shape on a metal core. A rubber material, in which conductive substances such as carbon black or metal oxides and the like, or a substance with these made into a foam are dispersed in urethane, ethylene-propylene-diene polyethylene (EPDM), acrylonitrile butadiene rubber (NBR), silicone rubber, isoprene rubber and the like in order to adjust the resistance, can be used as the substance of the medium resistance layer of the charge part 22.

The cleaning part 25 mechanically removes and recovers untransferred toner from the photosensitive drum 21 by setting up a cleaning brush (or cleaning blade) that makes sliding contact with the photosensitive drum 21.

The developing device 23 is arranged so that two developing rollers 23a1 and 23a2, which are developer support units, are arranged in close proximity to the photosensitive drum 21, and in both opposing parts, a developing region is formed where the photosensitive drum 21 and the magnetic brush make contact. Developer G (two-component developer) comprising a toner T and a carrier C are housed inside the developing device 23. Then, the developing device 23 develops the electrostatic latent image formed on the photosensitive drum 21 (forms a toner image). Further, the configuration and action of the developing device 23 will be explained in detail later.

Here, the developing device 23 of the present embodiment is the premix developing system type, and new carrier C (developer G) is suitably supplied from the developer cartridge 28 into the developing device 23, and the deteriorated developer G is discharged toward a developer storage container 70 arranged outside of the developing device 23.

Referring to FIG. 2, housed inside of the developer cartridge 28 is the developer G (toner T and carrier C) for supplying into the developing device 23. Then, the developer cartridge 28 functions as a toner cartridge to supply new toner T to the developing device 23, and functions as supply means for supplying new carrier C into the developing device 23. Concretely, developer G is suitably supplied from the developer cartridge 28, which is the supply means, into the developing device 23 by the opening and closing action of a shutter mechanism 80 based on toner concentration data (percentage of toner in the developer G) detected by a magnetic sensor 26 (refer to FIG. 4) installed on the developing device 23.

Further, in the present embodiment, the mixture percentage of toner T in relation to carrier C (toner concentration) of the developer G in the developer cartridge 28 was set comparatively high.

A supply tube 29, which is the supply means, is for the purpose of reliably leading the developer G (toner T and carrier C) supplied from the developer cartridge 28 into the developing device 23. Specifically, the developer G that is discharged from the developer cartridge 28 is supplied into the developing device 23 through the supply tube 29.

Next, the image making process that occurs on the photosensitive drum 21 will be explained.

Referring to FIG. 2, when the photosensitive drum 21 is rotationally driven, first, the surface of the photosensitive drum 21 at the position of the charge part 22 is uniformly charged. Afterwards, the surface of the charged photosensitive drum 21 arrives at the irradiation position of the exposure light L, and exposure processing is conducted by the writing unit 2. Specifically, an electrostatic latent image is formed by irradiating the photosensitive drum 21 with the exposure light L to selectively neutralize the surface thereof corresponding to the image data, thereby generating a difference (potential contrast) from the electric potential of the non-image part that is not irradiated. Further, this exposure process is one in which the load generation substance in the photosensitive layer of the photosensitive drum 21 generates a load by receiving the light, and the positive holes thereof cancel the charge load of the surface of the photosensitive drum 21.

Afterwards the surface of the photosensitive drum 21 on which the latent image is formed arrives at the position facing the developing device 23. The electrostatic latent image on the photosensitive drum 21 makes contact with the magnetic brushes on the developing rollers 23a1 and 23a2, and the image is made visible by the adherence of the negatively charged toner T from the magnetic brush.

In more detail, after the developer G, which was drawn up by magnetic force based on the magnetic pole of the upper developing roller 23a1, has been adjusted to a suitable amount by the doctor blade 23c, the developer is transported to the developing region, which is the part facing the photosensitive drum 21 (the region where the two developing rollers 23a1 and 23a2 face the photosensitive drum 21). The carrier C which is spiked up in the developing region makes sliding contact with the photosensitive drum 21. At this time, the toner T that is mixed in the carrier C is negatively charged by the friction with the carrier C. In contrast, the carrier C is positively charged. The specified developing bias is applied from a power source not indicated in the diagram to the developing rollers 23a1 and 23a2. An electric field is thereby formed between the developing rollers 23a1 and 23a2 and the photosensitive drum 21, and a toner image is formed by the negatively charged toner T selectively adhering to only the image part on the photosensitive drum 21 based on the electric field.

Afterwards, the surface of the photosensitive drum 21, on which a toner image has been formed, arrives at the position facing the transfer belt 30 and the transfer roller 24. Then, the toner image on the photosensitive drum 21 is transferred onto the transfer material P that with matching timing was transported to the opposing position. At this time, the specified voltage is applied to the transfer roller 24.

Afterwards, the transfer material P, onto which the toner image has been transferred, passes through the fixing part 66, and is discharged from the discharge roller 69 to outside of the device.

Meanwhile, the toner T remaining on the photosensitive drum 21 that has not been transferred to the transfer material P during the transfer process (untransferred toner) reaches the position facing the cleaning unit 25 with unchanged toner adhering on the photosensitive drum 21. Then, the untransferred toner on the photosensitive drum 21 is removed and collected by the cleaning part 25.

Afterwards, the surface of the photosensitive drum 21 passes through a charge removal part not indicated in the diagram, and the series of image making processes on the photosensitive drum 21 is completed.

The configuration and operation of the developing device 23 will be explained in detail below.

Referring to FIG. 3, the developing device 23 comprises developing rollers 23a1 and 23a2, which are developer support units, transport screws 23b1 to 23b3 (auger screws), which are the transport members, the doctor blade 23c, a carrier collection roller 23k, a scraper 23m, a discharge screw 23n, and the like. Moreover, three developer transport parts B1 to B3 that transport the developer G and form a circulation route are formed in the developing device 23.

The developing rollers 23a1 and 23a2 are configured such that a cylindrical-shaped sleeve comprising a non-magnetic material such as aluminum, brass, stainless steel, conductive resin or the like rotates clockwise based on a rotational drive mechanism not indicated in the diagram. Magnets that form a magnetic field are fixed inside the developing rollers 23a1 and 23a2 in order to produce spikes of the developer G on the peripheral surface of the sleeve. The carrier C in the developer G spikes up in a chain shape on the sleeve following the normal direction of magnetic force lines generated from the magnets. A magnetic brush is formed by the charged toner T adhering to the carrier C spiked up in this chain shape. The magnetic brush travels in the same direction (clockwise) as the sleeve based on the rotation of the sleeve.

The doctor blade 23c is set up on the upstream side of the developing region, and regulates the developer on the first developing roller 23a1 to a suitable amount.

The three transport screws 23b1 to 23b3 have a spiral screw part formed on a spindle, and agitate, mix, and circulate longitudinally (the direction perpendicular to the paper surface in FIG. 2) the developer G housed in the developing device 23.

The first transport screw 23b1, which is the first transport member (one transport member), is installed in the first developer transport part B1 facing the developing roller 23a1, transports the developer G horizontally (transports to the left indicated by the white arrow in FIG. 4), and supplies developer G onto the developing roller 23a1. In other words, the first developer transport part B1 faces the developing roller 23a1, and while transporting developer longitudinally (direction of the rotating spindle of the developing roller 23a1), supplies developer G to the developing roller 23a1.

The second transport screw 23b2, which is the second transport member, is installed in the second developer transport part B2. The second transport screw 23b2 is below the first transport screw 23b1, and is arranged in a position facing the developing roller 23a2. Then, second transport screw 23b2 transports horizontally (transports to the left as indicated by the white arrow in FIG. 4) the developer G that was released from the developing roller 23a2 (developer G that was forcibly released from above the developing roller 23a2 by the developer release pole after developing processing). In other words, the second developer transport part B2 is below the first developer transport part B1, is arranged in a position facing the developing roller 23a2, and transports developer G released from the developing roller 23a2 longitudinally.

The first transport screw 23b1 and second transport screw 23b2 are arranged such that the rotating spindle is nearly horizontal in the same way as the developing rollers 23a1 and 23a2, and the photosensitive drum 21.

The third transport screw 23b3, which is the third transport member, is installed in the third developer transport part B3. The third transport screw 23b3 is arranged at a diagonal in relation to horizontal such that the downstream side of the transport route formed by the second transport screw 23b2 and the upstream side of the transport route formed by the first transport member 23b1 are directly linked (refer to FIG. 4). Then, the third transport screw 23b3 transports the developer G transported by the second transport screw 23b2 to the upstream side of the transport route formed by the first transport member 23b1, and transports the developer G that circulates from the downstream side of the transport route formed by the first transport screw 23b1 through a fall route 23f to the upstream side of the transport route formed by the first transport member 23b1 (transports to the upper right diagonal indicated by the white arrow in FIG. 4). In other words, the third developer transport part B3 transports the developer G transported by the second developer transport part B2 to the upstream side of the first developer transport part B1, and transports the developer G that has arrived at the downstream side of the first developer transport part B1 to the upstream side of the first developer transport part B1.

Further, the transport route formed by the first transport screw 23b1 (first developer transport part B1), the transport route formed by the second transport screw 23b2 (second developer transport part B2), and the transport route formed by the third transport screw 23b3 (third developer transport part B3) are separated by walls.

Referring to FIG. 4, the downstream side of the second developer transport part B2 and the upstream side of the third developer transport part B3 are connected through a first relay part 23g. Moreover, the downstream side of the third developer transport part B3 and the upstream side of the first developer transport part B1 are connected through a second relay part 23h. In addition, the downstream side of the first developer transport part B1 and the upstream side of the third developer transport part B3 are connected through a fall route 23f.

According to this kind of configuration, circulation routes for circulating the developer G longitudinally in the developing device 23 are formed by the three developing transport parts B1 to B3 (transport screws 23b1 to 23b3). Here, when the developing device 23 operates, the developer housed in the device flows in the state indicated by the slanted lines in FIG. 4. Referring to FIG. 4, the reason that the surface of the developer in the downstream side of the first developer transport part B1 is lower than the surface of the upstream side is because some of the developer being transported is supplied to the developing roller 23a1. Specifically, the developer that is not supplied to the developing roller 23a1 moves to the upstream side of the third developer transport part B3 through the fall route 23f.

Further, a magnetic sensor 26, which is a toner concentration sensor), is installed on the third developer transport part B3. Then, based on the toner concentration data detected by the magnetic sensor 26, developer G of a specified toner concentration is supplied into the developing device 23 from the developer cartridge 28, which is the supply means. In the present embodiment, the toner concentration of the developer G inside the developing device 23 is controlled to be 4 to 7 weight %.

Here, referring to FIGS. 3 to 5, a discharge port 23d (discharge means) for discharging some of the developer G housed in the developing device 23 to the outside (developer storage container 70) is provided on the separation wall 23r of the first developer transport part B1. In more detail, the discharge port 23d is for the purpose of discharging the excessive developer G toward the developer storage container 70 when the amount of developer in the device increases by supplying the developer G into the developing device 23 using the supply means 28 and 29, and the surface (top surface) of the developer being transported to that position exceeds a specified height. Specifically, the surplus developer G exceeds the height of the lower part of the discharge port 23d, is discharged from the discharge port 23d, is relayed through the discharge route 71, and falls by gravity toward the developer storage container 70. In this way, deterioration of the image quality over time can be restrained because carrier that has deteriorated and has become contaminated by the matrix resin of the toner T and other additives is automatically discharged outside of the developing unit.

Further, the diagrams in FIGS. 2 and 4 are abbreviated, but a discharge screw 23n for horizontally transporting the developer discharged from the discharge port 23d is installed in the discharge route 71 (refer to FIGS. 3 and 8).

Moreover, a bypass route for returning some of the developer G to the upstream side of the circulation route without passing through the position where the previously described discharge port 23d (discharge means) is arranged is formed in the circulation route of the developer in the developing device 23. Concretely, referring to FIGS. 4 and 6, an opening 23e is provided on the upstream side of the discharge port 23d (a position relatively adjacent to the discharge port 23d), which is the first developer transport part B1. Then, this opening 23e is an inlet of the bypass route, and the outlet of the bypass route is arranged in the transport route (adjacent to the center longitudinally) formed by the third transport screw 23b3.

In this way, by providing a bypass route in the circulation route of the developer in the developing device 23, even if wave-shaped distortions are produced in the developer in the developing device, the problems of producing fluctuations in the amount of developer discharged from the discharge port 23d and of discharging more than the necessary amount of developer from the developing device 23 can be suppressed.

FIG. 7 indicates the state when wave-shaped distortions are produced in the developer in the developer circulation route of the developing device 23. Wave-shaped distortions with a large high-low difference may be produced in the developer circulation route in this way. These wave-shaped distortions appear notably immediately after beginning operation of the developing device 23 (immediately after restarting). In the past, when these wave-shaped distortions were produced, then all of the developer which was in a position higher than the bottom of the discharge port 23 (the developer at height H2 in FIG. 7) was discharged from the discharge port 23d. Because the developer that is discharged in this way is not originally scheduled for discharge, when this phenomenon happens repeatedly, the amount of developer in the developing device 23 becomes insufficient leading to such problems as instability in the state of deterioration of the developer, a decrease in the amount of charge of the toner, and a reduction in the image concentration on the output image.

In contrast, in the present embodiment, part of the developer that is in the position higher than the lower part of the discharge port 23d is returned to the transport route of the third transport screw 23b3 through the opening 23e without being discharged from the discharge port 23d because the opening 23e that passes through to the bypass route is provided on the upstream side of the discharge port 23d. The problem of discharging excessive developer from the discharge port 23d can thereby be avoided.

Here, the height of the lower part of the discharge port 23e of the bypass route is configured to be just a height H1 higher than the height of the lower part of the discharge port 23d.

Of the developer that is in a position higher than the lower part of the discharge port 23d, the portion of developer with a height of (H2-H1) will thereby be returned to the transport route of the third transport screw 23b3 through the opening 23e without being discharged from the discharge port 23d. The problems of excessive developer being discharged from the discharge port 23d can be reliably prevented while maintaining the original function of the discharge means. Here, preferably the longitudinal distance W between the discharge port 23d and the opening 23e is as short as possible.

Referring to FIG. 3 (the indications are omitted from FIGS. 2 and 4), here in the present embodiment, carrier collection roller 23k is installed below the second developing roller 23a2 (downstream side in the direction of rotation) at a position facing the photosensitive drum 21. Further, the scraper 23m is installed in position contacting the carrier collection roller 23k.

The carrier collection roller 23k has a magnet that forms a specified magnetic field and that is fixed inside the cylinder made of stainless steel, etc., and is for the purpose of collecting the carrier that has moved (has flown) from within the developing device 23 and has adhered to the photosensitive drum 21. The carrier collection roller 23k is driven counterclockwise in FIG. 3. Nearly all of the carrier that is collected and supported by the carrier collection roller 23k moves onto the developing roller 23a2 at the position of facing the second developing roller 23a2, is released from the developing roller 23a2 at the position of the developer release pole of the developing roller 23a2, and is recovered into the second developer transport part B2. Meanwhile, the carrier that remains on and is supported by the carrier collection roller 23k without moving to the developing roller 23a2 is mechanically scraped off by the scraper 23m, and is recovered in the second developer transport part B2. In this way, the generation of abnormal images (firefly images and images with white blanks) can be avoided, and the problem of insufficient carrier in the developing device 23 can also be avoided because carrier adhering on top of the photosensitive drum 21 can be recovered into the developing device 23 by installing the carrier collection roller 23k.

The approximate settings in the present embodiment are: the outer diameters of the developing rollers 23a1 and 23a2 are 30 mm; the linear velocities of the peripheral surfaces of the developing rollers 23a1 and 23a2 are 748 mm/second; the outer diameter of the carrier collection roller 23k is 16 mm; the linear velocity of the peripheral surface of the carrier collection roller 23k is 10.6 mm/second; and the process linear velocity (linear velocity of the peripheral surface of the photosensitive drum 21, and the transport velocity of the transfer material P) is 440 mm/second.

Moreover, the carrier C used in the present embodiment has a particle size of about 55 μm, a saturation magnetization of 96 emu/g. Further, the toner T used in the present embodiment has a particle size of about 6.8 μm.

The characteristic configuration and operation of the developing device 23 of the present embodiment will be explained in detail below using FIGS. 8 and 9.

FIG. 8 is a top-view diagram indicating the transport route formed by the first transport screw 23b1 (first developer transport part B1). FIG. 9 is a perspective diagram of the developing device indicating the vicinity of the wall part 23s.

As indicated in FIG. 8, the screw part 23b1b of the first transport screw 23b1 is formed into a spiral shape on the spindle 23b1a. Then, a cutaway region 23b10 is provided in which the screw part 23b1b is not formed in part of the longitudinal direction of the first transport screw 23b1 (the range M in the diagram, the region where only the spindle 23b1a has been formed). The cutaway region 23b10 in which this screw part 23b1b is not formed is downstream of the first developer transport part B1 (transport route formed by the first transport screw 23b1), and is provided in the non-image forming region (region that does not participate in image formation).

Moreover, the discharge port 23d faces the cutaway region 23b10 of the first transport screw 23b1 where no screw part 23b1b is formed, and is formed such that the longitudinal length N is smaller than the longitudinal length M of the cutaway region 23b10 (M>N). Further, the discharge port 23d is also downstream of the first developer transport part B1, and is provided in a non-imaging region.

The amount of developer splashed in this vicinity can be reduced by providing the cutaway region 23b10 on the first transport screw 23b1 in this way. Moreover, the longitudinal length N of the discharge port 23d is made smaller than the longitudinal length M of the cutaway region 23b10, and the discharge port is arranged opposite to and within the range of the cutaway region 23b10. Then, referring to FIGS. 8 and 9, a wall part 23s that protrudes toward the inside of the first developer transport part B1 from the partition wall 23r, on which the discharge port 23d is formed, is installed within the range of the cutaway region 23b10 on the upstream side of the first developer transport part B1 in relation to the discharge port 23d (left side of FIG. 8).

By configuring in this way, the phenomenon of the developer splashed on the upstream side of the cutaway region 23b10 penetrating the discharge port 23d (movement of the developer indicated by the arrow A in FIG. 11) can be reliably prevented. Specifically, the developer splashed on the upstream side of the cutaway region 23b10 collides with the wall part 23s and, without penetrating into the discharge port 23d, is returned into the developer being transported in the first developer transport part B1. Consequently, the problem of excessive amounts of developer inside of the developing device 23 being discharge leading to insufficient developer volume can be prevented by discharging from the discharge port 23d only the developer that should be discharged (the portion of developer in which the developer surface exceeds the lower surface of the discharge port 23d).

Here, as indicated in FIG. 9, in order to secure the above effect, preferably the wall part 23s is formed such that a projecting surface, when viewed from the upstream side in the direction of developer transport, covers the discharge port 23d. Specifically, preferably the wall part 23s is provided over the trajectory of the developer that is splashed on the upstream side of the cutaway region 23b10 and that faces the discharge port 23d. The developer that is splashed on the upstream side of the cutaway region 23b10 and that faces the discharge port 23d can thereby reliably collide with the wall part 23s.

Moreover, as indicated in FIGS. 8 and 9, in order to secure the above effect, preferably the wall part 23s is made in the vicinity of the spindle part 23b1a of the cutaway region 23b10 of the first transport screw 23b1. In other words, as indicated in FIG. 9, preferably the wall part 23s is formed such that the projecting surface, when viewed from the upstream side in the direction of developer transport, overlaps with the screw part 23b1b. The developer that is splashed on the upstream side of the cutaway region 23b10 and that faces the discharge port 23d can thereby reliably collide with the wall part 23s because the range of the wall part 23s can be made bigger without obstructing the rotation of the first transport screw 23b1.

Further, in order to secure the above effect, as indicated in FIG. 8, preferably the wall part 23s is installed near the discharge port 23d. The developer that is splashed on the upstream side of the cutaway region 23b10 and that faces the discharge port 23d can thereby reliably collide with the wall part 23s.

Experiments conducted by the four inventors of the present application confirmed that, according to the configuration described above, the phenomenon of developer splashed by the first transport screw 23b1 being discharged from discharge port 23d did not occur.

Further, it has been confirmed that, by providing the cutaway region 23b10, where the screw part 23b1b is not formed, the diagonal of the developer (the phenomenon in which the downstream developer surface becomes lower than the upstream surface as explained earlier using FIG. 4) in the first developer transport part B1 becomes smaller compared to when the cutaway region 23b10 is not provided, and it becomes more difficult for augur stripes (image irregularities caused by the pitch of the screw part 23b1b when the developer level is low downstream) to occur.

Here in the present embodiment, the discharge port 23d, the cutaway region 23b10 and wall part 23s are on the downstream side of the first developer transport part B1 and are provided in a non-imaging region, thereby eliminating an affect on the output image caused by cutting away the screw part 23b1b of the first transport screw 23b1 or an affect on the output image caused by providing the wall part 23s.

Further, the wall part 23s can be configured to be detachably installed to the developing device 23. When attaching and detaching in relation to the developing device 23 by moving the first transport screw 23b1 in the direction of axial rotation, the problem of the wall part 23s interfering with the first transport screw 23b1 can be avoided by conducting the operation to attach and detach the wall part 23s.

Moreover, the wall part 23s can be formed of a flexible material. When attaching and detaching by moving the first transport screw 23b1 in the direction of axial rotation in relation to the developing device 23, the operations can be conducted while the wall part 23s is bent.

The shape of the wall part 23s is not limited to that of the present embodiment, and the wall part 23s can be formed, for example, as indicated in FIG. 10. In this case as well, the developer that is splashed on the upstream side of the cutaway region 23b10 and that faces the discharge port 23d can reliably collide with the wall part 23s without preventing the rotation of the first transport screw 23b1.

As explained above, the developer that is splashed by the first transport screw 23b1 is blocked by the wall part 23s and is not discharged from the discharge port 23d because the discharge port 23d faces the cutaway region 23b10 of the first transport screw 23b1 (one transport member), and the wall part 23s is provided, which is on the upstream side of the discharge port 23d and protrudes toward the inside of the first developer transport part B1 (transport route) within the range of the cutaway region 23b10. The developing device 23 of a premix developing system can thereby be provided by which fluctuations in the amount of developer discharged to the outside of the developing device 23 are not produced, and the quality of the output image is stable.

Further, in the present embodiment, the present invention was applied to a developing device 23 with three developer transport parts B1 to B3 installed, but the present invention could be applied even to developing device 23 in which 2 or less or 4 or more developer transport parts are installed. In these cases as well, similar effects as those of the present embodiment can be obtained.

Moreover, in the present embodiment the third transport screw 23b3 was arranged at a diagonal to horizontal, but the third transport screw 23b3 could also be arranged horizontally.

Further, in the present embodiment the pocket part 23d was provided on the wall part of the first developer transport part B1, but the pocket part 23d could also be provided on the walls of the other developer transport parts B2 and B3.

Moreover, in the present embodiment developer G (toner T and carrier C) is provided to the developing device 23 from the developer cartridge 28, which is the supply means, but the carrier C alone could be supplied to the developing device 23 from the supply means. In this case, a toner cartridge that houses only toner is installed separately from the developer cartridge (carrier cartridge), and the toner housed in the toner cartridge suitably replenishes the developing device 23 based on the detection results of the magnetic sensor 26. Even in this case, a similar effect to that of the present embodiment can be obtained.

Moreover, in the present embodiment the present invention is applied to an image forming apparatus in which part of the image making unit is configured by a process cartridge 20. However, the use of the present invention is not limited to this, and naturally the present invention can be applied to an image forming apparatus that does not make the image making unit into a process cartridge. Concretely, the present invention can be applied even when the developing device 23 is configured by a unit that attaches and detaches to the image forming apparatus main body as a single body.

Further, in the present embodiment, the present invention was applied to a developing device 23 with two developing rollers 23a1 and 23a2 installed, but naturally the present invention could be applied to a developing device with one, or three or more developing rollers. Even in this case, a similar effect to that of the present embodiment can be obtained.

As explained above, in the present invention, a wall part is provided, which faces the discharge port in the cutaway region where the screw part of the transport member has been eliminated, which is upstream of the discharge port, and which protrudes toward the interior of the transport route within the range of the cutaway region; and therefore developer splashed by the transport member is blocked by the wall part and is not discharged from the discharge port. A developing device, process cartridge and image forming apparatus of a premix developing system, which do not produce fluctuations of the amount of developer discharged to the outside and which have stable output image quality, can thereby be offered.

Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.

DEVELOPING DEVICE, PROCESS CARTRIDGE, AND IMAGE FORMING APPARATUS

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CPC

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