Electrophotography – Cleaning of imaging surface – Fibrous brush
Reexamination Certificate
2003-06-03
2004-11-02
Ngo, Hoang (Department: 2852)
Electrophotography
Cleaning of imaging surface
Fibrous brush
C399S350000, C399S071000
Reexamination Certificate
active
06813467
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus for forming a toner image on an image carrier with an electrophotographic process and transferring the toner image to a sheet or recording medium either directly or via an intermediate image transfer body. Also, the present invention relates to a developing device included in an image forming apparatus and using a plurality of developing rollers arranged side by side in the direction of rotation of an image carrier and operable with a two-ingredient type developer, i.e., a toner and carrier mixture, and a cleaning device also included in the image forming apparatus for removing residual toner and impurities left on the image carrier with a cleaning blade.
2. Description of the Background Art
It is a common practice with a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus to charge and then scan an image carrier imagewise for thereby forming a latent image and develop the latent image with toner. The resulting toner image is transferred to a sheet or recording medium and then fixed on the sheet.
Toner with a small grain size enhances image quality, but is defectively charged due to an increase in the carrier coating ratio of the toner, as known in the art. To solve this problem, it has been customary to increase the surface area of the individual carrier grain for a unit weight for thereby reducing the carrier coating ratio of the toner, so that the probability that the toner contacts the carrier increases. However, another problem with toner having a small grain size is that the carrier easily deposits on the image carrier.
On the other hand, considering the increasing demand for high-speed image formation, developing ability available with a single developing roller is short. In light of this, a plurality of developing rollers may be used. However, if the diameter of each developing roller is reduced to meet the demand for the size reduction of an image forming apparatus, then the rotation speed of the developing roller and therefore a centrifugal force to act on the carrier increases, aggravating carrier deposition on the image carrier. The carrier deposited on the image carrier damages the edge of a cleaning blade expected to remove residual toner from the image carrier. Further, if such carrier is transferred from the image carrier to a sheet, then it damages the surfaces of a pair of fixing rollers when being conveyed via the nip of the fixing rollers. In this manner, the carrier deposited on the image carrier degrades the reliability of the image forming apparatus. Moreover, the carrier deposited on the image carrier increases image density on a sheet and thereby smears an image, lowering image quality.
To obviate carrier deposition stated above, Japanese Patent Laid-Open Publication No. 6-51628 and Japanese Patent No. 2,930,812, for example, each define a specific linear velocity of an image carrier and that of a developing roller by using a magnet roller whose magnetic force is weak. However, developing ability available with the above documents is short because use is made of only one developing roller.
Further, to obviate carrier deposition, the flux density of a main pole for development included in a developing roller maybe increased, as proposed in the past. This scheme is directed mainly toward a developing device of the type using a single developing roller. If such a scheme is applied to a developing device of the type using a plurality of developing rollers, then it effects the flow of a developer between the developing rollers and causes an excessive amount of developer to be conveyed, resulting in overflow and other troubles.
For example, assume that two developing rollers are positioned side by side in the direction of rotation of the image carrier, and that the flux density of the main pole of the downstream developing roller is increased. Then, such an intense magnetic force scoops up the developer even via the gap between the two developing rollers with the result that an excessive amount of developer deposits on the rollers and brings about various problems including the smearing of an image.
Further, the intense magnetic force of the main pole intensifies even a magnetic force at the rear of the main pole, preventing the developer from parting from the downstream developing roller. Consequently, the developer moves in accordance with the rotation of the downstream developing roller and again reaches the upstream developing roller. This is also apt to bring about the smearing of an image and the overflow of the developer. It is therefore difficult to obviate carrier deposition on the image carrier by intensifying the magnetic force.
To increase the magnetic force of the main pole of the developing roller, use may be made of a rare earth magnet exerting an intense magnetic force, as known in the art. Such a magnet, however, intensifies the magnetic forces of the other poles as well and therefore makes it difficult to establish optimum balance between the poles while increasing cost.
To establish optimum balance between poles, we prepared a cylindrical magnet roller by combining magnets each forming a particular pole and used a rare earth magnet for one of the magnets forming the main pole. However, the rare earth magnet with an intense magnetic force caused a developer to follow the rotation of a developing roller and overflow.
Japanese Patent No. 2,545,601 and Japanese Patent Laid-Open Publication No. 2000-81789, for example, each also propose a cylindrical magnet roller in which a rare earth magnet is buried at the main pole for development. However, in Patent No. 2,545,601, the rare earth magnet is 1.15 mm long or thick in the radial direction of the roller and 5 mm long or wide in the circumferential direction of the roller. In this case, although the length in the radial direction is small, the length in the circumferential direction is great and causes the intense magnetic force to effect the other poles, again resulting in the problem stated above. Conversely, in Laid-Open Publication No. 2000-81789, although the rare earth magnet is as short or thin as 3 mm in the radial direction of the roller, it is as long or wide as 4 mm in the circumferential direction of the roller, also resulting in the above problem.
On the other hand, after the transfer of a toner image from the image carrier to a sheet, some toner is left on the image carrier as residual toner. It is therefore a common practice to remove, before the formation a new latent image, the residual toner as well as impurities including paper dust and rosin, Mg, Al, K, and other additives contained in a sheet from the image carrier. Such additives are contained not only in a sheet but also in toner for implementing various characteristics, including chargeability, fixability and fluidity, required of toner.
To remove the residual toner and impurities left on the image carrier, use is often made of a cleaning blade formed of polyurethane or similar elastic material and having its edge pressed against the surface of the image carrier by preselected pressure. However, a problem with the cleaning blade is that as cleaning is repeated, the toner and impurities tend to accumulate between the image carrier and the cleaning blade and vary the pressing condition of the blade, preventing the expected cleaning effect from being achieved. The toner and impurities so caught between the image carrier and the cleaning blade sometimes include even masses of toner. Consequently, if such toner and impurities get through the cleaning blade, cleaning efficiency is lowered and brings about defective images ascribable to the background contamination of the image carrier. In this connection, when a mass of toner is caught between the image carrier and the cleaning blade, the residual toner on the image carrier gets through the cleaning blade at both sides of the mass.
On the other hand, while the cleaning blade is pressed against the image carrier with preselecte
Arai Yuji
Ishii Hirokazu
Kawahara Shin-ichi
Saitoh Hiroshi
Sugiyama Toshihiro
Ngo Hoang
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Ricoh & Company, Ltd.
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