Developing device and image forming apparatus

A developing device includes: a developing member including a magnetism-generating member having plural magnetic poles polarized in a circumferential-direction thereof, and a rotating member that is rotated about the magnetism-generating member; a developer-pumping pole provided in the magnetism-generating member, and that pumps up the developer, which is supplied from a developer accommodating portion, to the rotating member surface; a developer-peeling-off pole provided in the magnetism-generating member below a rotation axis of the rotating member in a vertical direction on an upstream side of the developer-pumping pole in a rotational-direction of the rotating member, that peels off the developer from the rotating member surface; and a magnetic body provided in at least a portion between a facing position facing the developer-peeling-off pole in a circumferential-direction of the rotating member and an intermediate position between the developer-peeling-off pole and the developer-pumping pole in the circumferential direction, facing the rotating member surface.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2010-181598 filed Aug. 16, 2010.

BACKGROUND

Technical Field

The present invention relates to a developing device and an image forming apparatus.

SUMMARY

According to an aspect of the invention, a developing device includes: a developing member including a magnetism generating member having plural magnetic poles polarized in a circumferential direction thereof, and a rotating member that is rotated about the magnetism generating member while holding a developer on a surface thereof, the developing member supplying a toner included in the developer to an image carrier on which an electrostatic latent image is formed to develop the electrostatic latent image; a developer pumping pole that is provided in the magnetism generating member, and that pumps up the developer, which is supplied from a developer accommodating portion accommodating the developer, to the surface of the rotating member; a developer peeling-off pole that is provided in the magnetism generating member below a rotation axis of the rotating member in a vertical direction on an upstream side of the developer pumping pole in a rotational direction of the rotating member, and that peels off the developer from the surface of the rotating member; and a magnetic body that is provided in at least a portion between a facing position facing the developer peeling-off pole in a circumferential direction of the rotating member and an intermediate position between the developer peeling-off pole and the developer pumping pole in the circumferential direction, and that faces the surface of the rotating member.

DETAILED DESCRIPTION

An example of a developing device and an image forming apparatus according to a first exemplary embodiment of the invention will be described with reference toFIGS. 1 to 5. In the drawings, an arrow UP indicates the upper side in the vertical direction.

As shown inFIG. 5, an image forming apparatus10includes an image processing unit11that forms an image, a recording medium supply unit15that supplies a sheet member P as a recording medium to the image processing unit11, and a recording medium discharge unit13that discharges the sheet member P having an image formed thereon.

The image processing unit11includes an image carrier12on which an electrostatic latent image is formed by a laser beam after the image carrier is uniformly charged, a charging device14that uniformly charges the image carrier12, an exposure device16that emits a laser beam to the image carrier12to form an electrostatic latent image on the basis of image data, a developing device18that selectively transfers toner to the electrostatic latent image to visualize the electrostatic latent image as a toner image, a transfer roller22that transfers the toner image formed on the surface of the image carrier12to the sheet member P supplied along a transport path20, a fixing device24that heats and presses the toner image on the sheet member P and fixes the toner image to the sheet member P, and a cleaning device26that cleans the toner remaining on the image carrier12after the toner image is transferred. The developing device18will be described in detail below.

The image forming apparatus10is covered with a main body side cover108and a top board10A. A shaft10C that rotatably connects the top board10A to the main body side cover10B is provided at the upper corner of the main body side cover10B. When the top board10A is rotated about the shaft10C in the direction of an arrow A, the image forming apparatus10is opened.

The charging device14and the cleaning device26are formed as one charging unit28. When the top board10A of the image forming apparatus10is opened, the charging unit28may be attached or removed in or from a main frame (not shown in the drawings) in the image forming apparatus10.

A manual feed tray32, structuring the recording medium supply unit13, that enables the user to manually supply the sheet member P to the image forming section31including the image carrier12and the transfer roller22is provided on the side of the image forming apparatus10. A semicircular transport (feeding) roller34is provided in the manual feed tray32. A separating roller36is provided so as to face the transport roller34with the sheet member P interposed therebetween.

The separating roller36is rotatably supported by a supporting member (not shown in the drawings) provided at both ends thereof, and is urged to the transport roller34by the urging force of a coil spring provided in the supporting member. In this way, when the transport roller34is rotated, the sheet members P placed on the manual feed tray32are fed one by one to the image forming section31by the transport roller34and the separating roller36.

A feeding device40, structuring the recording medium supply unit15similar to the manual feed tray32, that feeds the sheet members P one by one, is provided at a lower part of the image forming apparatus10. The feeding device40includes a feed member41on which plural sheet members P are placed. The sheet members P placed on the feed member41are sequentially taken out by a takeout roller42and then transported one by one by the rotate-driven feed roller44and the separating roller46provided in the feed member41.

Plural transport rollers48are provided along the transport path20of the sheet member P such that the sheet member P is transported to the downstream side (hereinafter, simply referred to as a downstream side in a transport direction) in the transport direction of the sheet member P along the transport path20.

The above-mentioned fixing device24is provided on the downstream side of the image forming section31. The fixing device24includes a heating roller24H and a pressure roller24N. When the sheet member P passes through a nip portion (contact portion) between the heating roller24H and the pressure roller24N, the toner image on the sheet member P is fixed to the sheet member P.

A discharge rollers60that discharge the sheet member P having the toner image fixed thereon to the upper surface of the top board10A is provided on the downstream side of the fixing device24.

In the image forming apparatus10having the above-mentioned structure, an image is formed as follows.

First, a voltage is applied to the charging device14, and the charging device14uniformly charges the surface of the image carrier12to a predetermined negative potential. Then, the exposure device16emits light to the surface of the charged image carrier12to form an electrostatic latent image on the surface of the image carrier12on the basis of image data read by a scanner (not shown in the drawings) or external data.

That is, a laser of the exposure device16is turned on or off on the basis of video data supplied from a control device (not shown in the drawings), thereby forming an electrostatic latent image corresponding to image data on the image carrier12. The electrostatic latent image is visualized as a toner image by toner supplied from the developing device18.

Then, the sheet member P taken out from the feed member41by the takeout roller42is sent one by one to the transport rollers48by the feed roller44and the separating roller46and then transported to the transport path20. The sheet member P fed out to the transport path20passes through the image forming section31formed between the image carrier12and the transfer roller22and the toner image is transferred to the sheet member P. The transferred toner image passes between the heating roller24H and the pressure roller24N provided at the fixing device24and is then fixed to the sheet member P. The sheet member P is discharged to the upper surface of the top board10A by the discharge roller60.

In the image forming apparatus10according to this exemplary embodiment, one developing device18is provided. However, in a case where a color image is formed, four-color (Y (yellow), M (magenta), C (cyan), and K (black)) developing devices18are provided at positions facing the image carrier12.

Next, the developing device18will be described.

As shown inFIG. 1, the developing device18includes a developing roller50, which is an example of a developing member provided so as to face the image carrier12, a first stirring and transporting auger52that is disposed on the side of the developing roller50and supplies a binary-system developer G (hereinafter, simply referred to as a developer G) to the developing roller50, a second stirring and transporting auger54that is disposed below the first stirring and transporting auger52, and a housing56that accommodates the developing roller50, the first stirring and transporting auger52, and the second stirring and transporting auger54. The developer G includes toner and magnetic carrier particle as a main component.

The first stirring and transporting auger52and the second stirring and transporting auger54include rotating shafts52A and54A, respectively, and they are rotatably supported by a peripheral wall of the housing56(not shown in the drawings). In addition, spiral blades52B and54B are formed helically at predetermined pitches in the rotating shafts52A and54A of the first stirring and transporting auger52and the second stirring and transporting auger54, respectively.

Gears (not shown in the drawings) are fixed to the ends of the rotating shafts52A and54A respectively. Rotational force is transmitted from a motor (not shown in the drawings) to the gear, and the rotational force is transmitted to first stirring and transporting auger52and the second stirring and transporting auger54through the gears. When the first stirring and transporting auger52and the second stirring and transporting auger54are rotated, the developer G stored in the housing56is transported in the direction of the rotating shaft by the spiral blades52B and54B while being stirred.

Specifically, a partition wall58that extends from the side wall of the housing56toward the developing roller50is formed between the first stirring and transporting auger52and the second stirring and transporting auger54. A first stirring path62, which is an example of a developer accommodating portion and in which the first stirring and transporting auger52is arranged, and a second stirring path64in which the second stirring and transporting auger54is arranged, are formed by the partition wall58. In addition, both ends of the partition wall58in the longitudinal direction are opened (not shown in the drawings), and the first stirring path62and the second stirring path64are connected to each other.

Due to this structure, the developer G is transported in the first stirring path62and the second stirring path64by the rotation of the first stirring and transporting auger52and the second stirring and transporting auger54while being stirred, and is circulated between the first stirring path62and the second stirring path64.

The developing roller50is disposed such that a clearance (a development gap) is formed between the developing roller50and the image carrier12. The developing roller50includes a cylindrical magnet roller50B, which is an example of a magnetism generating member, and a rotating sleeve50A, which is an example of a rotating member that covers the magnet roller50B and rotates about the magnet roller50B.

The rotating sleeve50A is rotated about the magnet roller50B in the direction of an arrow D (counter clockwise direction) shown inFIG. 1. That is, the rotating sleeve50A is rotated in a direction opposite to the rotational direction of the image carrier12that is rotated in the direction of an arrow B (clockwise direction).

Five permanent magnets having the S pole or the N pole formed on the surface side along the circumferential direction the magnet sleeve50A are radially provided inside the magnet roller50B. A pole S1, which is a development pole, is arranged at a position so as to face the image carrier12. A pick-off pole N3, which is an example of a developer peel-off (separation) pole, is arranged next to the development pole S1along the rotational direction of the rotating sleeve50A, and magnetic poles are arranged next to the pick-off pole N3in the order of a pick-up pole N2, which is an example of a developer pumping pole, a trimming pole S2, and a transport pole N1. Both the development pole S1and the trimming pole S2are the S pole, and the pick-off pole N3, the pick-up pole N2, and the transport pole N1are all the N pole.

Specifically, the development pole S1generates a magnetic field having a direction for transferring the developer G supplied to the surface of the developing roller50to the surface of the image carrier12. The pick-off pole N3is arranged below the rotation axis C of the rotating sleeve50A in the vertical direction and generates a magnetic field having a direction for separating the developer G remaining on the developing roller50which is not transferred to the image carrier12from the rotating sleeve50A. That is, the developer G is separated from the rotating sleeve50A by the magnetic force of the pick-off pole N3and the magnetic force of the pick-up pole N2cooperating, and further gravity.

The pick-up pole N2generates a magnetic field having a direction for holding the developer G supplied from the first stirring and transporting auger52on the surface of the rotating sleeve50A. The trimming pole S2generates a magnetic field having a direction for making the developer G between the trimming pole S2and a cylindrical layer-thickness regulating member66, which is arranged so as to face the trimming pole S2, to direct toward the surface of the developing roller50.

A magnetic body72(in this exemplary embodiment, magnetic SUS) is provided so as to face the surface of the rotating sleeve50A in at least a portion between a position E and a position F inFIG. 1(a range J inFIG. 1). The position E is a facing position E that faces the pick-off pole N3in the circumferential direction of the rotating sleeve50A, as viewed from the direction of the rotation axis of the rotating sleeve50A. The position F is an intermediate position F between the pick-off pole N3and the pick-up pole N2in the circumferential direction of the rotating sleeve50A (an intermediate position between a position of the peak of the magnetic flux density of the pick-off pole N3and a position of the peak of the magnetic flux density of the pick-up pole N2), as viewed from the direction of the rotation axis of the rotating sleeve50A. As shown inFIG. 3, the magnetic body72is provided (fitted) in a concave portion56A of the housing56and extends in the direction of the rotation axis of the rotating sleeve50A.

The magnetic body72is arranged on the downstream side of the facing position E facing the pick-off pole N3in the rotational direction of the rotating sleeve50A. In this way, the influence of the magnetic body72on the development pole S1is less than a case in which the magnetic body72is arranged at the facing position E facing the pick-off pole N3.

Next, the operation of the developing device18will be described.

As shown inFIG. 1, the developer G is transported inside the first stirring path62and the second stirring path64by the rotation of the first stirring and transporting auger52and the second stirring and transporting auger54while being stirred, and is circulated between the first stirring path62and the second stirring path64.

When the rotating sleeve50A is rotated in the rotational direction D, first, the developer G stirred by the first stirring and transporting auger52and the second stirring and transporting auger54is attracted from the first stirring and transporting auger52to the surface of the rotating sleeve50A by the pick-up pole N2.

The magnetic field from the transport pole N1toward the development pole S1, the magnetic field from the pick-off pole N3toward the development pole S1, the magnetic field from the pick-up pole N2toward the trimming pole S2, and the magnetic field from the transport pole N1toward the trimming pole S2are formed on the surface of the rotating sleeve50A.

In this way, as shown inFIG. 4, the developer G attracted to the surface of the rotating sleeve50A is arranged in the direction of the line of magnetic force on the surface of the rotating sleeve50A and a magnetic brush in ear-standing state is formed.

With the rotation of the rotating sleeve50A in the rotational direction, the magnetic brush of the developer G formed on the surface of the rotating sleeve50A in the vicinity of the pick-up pole N2is transported in the counterclockwise direction inFIG. 4, that is, in the direction of the trimming pole S2→the transport pole N1→the development pole S1→the pick-off pole N3.

In the vicinity of the trimming pole S2, the developer G supplied onto the rotating sleeve50A bits the layer-thickness regulating member66and the thickness of the layer of the developer G is regulated (seeFIG. 1).

In the vicinity of the development pole S1, the toner on the magnetic brush is transferred to the image carrier12and a magnetic brush including substantially only the magnetic carriers (the developer G whose toner density is reduced) remains on the surface of the rotating sleeve50A. With the rotation of the rotating sleeve50A, the magnetic brush whose toner density is lowered (small) comes off from the rotating sleeve50A by the pick-off pole N3and is separated from the surface of the rotating sleeve50A by gravity.

As shown inFIGS. 1 and 2, in a case where the rotating sleeve50A is rotated at a high speed, it is considered that the developer G separated from the rotating sleeve50A is scattered to a region where it receives the attraction force of the pick-up pole N2(seeFIG. 1) in the tangential direction (a direction H inFIG. 2) of the rotating sleeve50A by inertial force received from the rotating sleeve50A and is then attracted to the rotating sleeve50A again.

However, the magnetic body72that is arranged so as to face the surface of the rotating sleeve50A in the above-mentioned range J is magnetized by influence of the pick-off pole N3. Accordingly, the magnetic carriers included in the developer G are attracted to the pick-off pole N3and the magnetic body72(a magnetic field is formed between the pick-off pole N3and the magnetic body72and permeability is increased).

Here, the magnetic carriers are attracted to the pick-off pole N3and the magnetic body72, so the developer G stays and remains (is in congested state) between the pick-off pole N3and the magnetic body72. Therefore, the transport speed of the developer G becomes lower than the circumferential speed of the rotating sleeve50A. In this way, the scatter-speed of the developer G separated from the rotating sleeve50A is reduced and the developer G is not scattered to the region where it receives the attraction force of the pick-up pole N2(seeFIG. 1).

Therefore, the developer G whose toner density is reduced is prevented (suppressed) from being adhered to the rotating sleeve50A again. In this way, the electrostatic latent image formed on the surface of the image carrier12is visualized as a toner image by the developer G with appropriate toner density.

Next, an example of a developing device and an image forming apparatus according to a second exemplary embodiment of the invention will be described with reference toFIG. 6. The same members as those in the first exemplary embodiment are denoted by the same reference numerals and a description thereof will be omitted.

In the second exemplary embodiment, plural magnetic bodies80are provided in a line along the direction of the rotation axis of the rotating sleeve50A, unlike the first exemplary embodiment. That is, the plural same components are arranged.

The exemplary embodiments of the invention have been described in detail above, but the invention is not limited to the exemplary embodiments. It will be understood by those skilled in the art that various kinds of exemplary embodiments may be made within the scope of the invention. For example, in the above-described exemplary embodiments, although not particularly described, the magnetic body72may be coated, and it may be enough that the magnetic body generates magnetic force to attract the magnetic carriers.

In the above-described exemplary embodiments, SUS (stainless steel) is used as the magnetic bodies72and80. However, the material forming the magnetic body is not particularly limited to SUS, but the magnetic bodies72and80may be magnets or other magnetic bodies such as iron.

In the above-described exemplary embodiments, the magnetic bodies72and80are arranged on the downstream side of the facing position E facing the pick-off pole N3in the rotational direction of the rotating sleeve50A. However, the magnetic bodies may be arranged at the facing position E.

In the above-described exemplary embodiments, no magnet is arranged between the pick-off pole N3and the pick-up pole N2, and the pick-off pole N3and the pick-up pole N2are the same pole. However, a magnet may be arranged between the pick-off pole N3and the pick-up pole N2. In this case, the pick-off pole N3and the pick-up pole N2may be different poles.