Electrophotography – Image formation – Development
Reexamination Certificate
2000-05-30
2001-06-12
Beattty, Robert (Department: 2852)
Electrophotography
Image formation
Development
Reexamination Certificate
active
06246855
ABSTRACT:
This invention relates generally to a development apparatus for ionographic or electrophotographic imaging and printing apparatuses and machines, and more particularly is directed to an apparatus and method for loading dry xerographic toner onto a traveling wave grid or donor member, or for development onto a photoreceptor.
Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image from either a scanning laser beam or an original document being reproduced. This records an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed. Two component and single component developer materials are commonly used for development. A typical two component developer comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single component developer material typically comprises toner particles. Toner particles are attracted to the latent image forming a toner powder image on the photoconductive surface, the toner powder image is subsequently transferred to a copy sheet, and finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration.
The electrophotographic marking process given above can be modified to produce color images. One color electrophotographic marking process, called image on image processing, superimposes toner powder images of different color toners onto the photoreceptor prior to the transfer of the composite toner powder image onto the substrate. In the application of the toner to the latent electrostatic images contained on the charge-retentive surface, it is necessary to transport the toner from a developer housing to the surface. A limitation of conventional xerographic development systems, including both magnetic brush and single component, is the inability to deliver toner (i.e. charged pigment) to the latent images without creating large adhesive forces between the toner and the conveyor on which the toner rests and which transports the toner to latent images. As will be appreciated, large fluctuation in the adhesive forces that cause the pigment to tenaciously adhere to the carrier severely limits the sensitivity of the developer system thereby necessitating higher contrast voltages forming the images. Accordingly, it is desirable to reduce the large adhesion particularly in connection with latent images formed by contrasting voltages.
In order to minimize adhesive forces, it has been found that a toner conveyor including means for generating traveling electrostatic waves which can constantly move the toner about the surface of the conveyor with minimal static contact therewith.
Traveling waves have been employed for transporting toner particles in a development system, for example U.S. Pat. No. 4,647,179 to Schmidlin which is hereby incorporated by reference. In that patent, the traveling wave is generated by alternating voltages of three or more phases applied to a linear array of conductors placed abut the outer periphery of the conveyor. The force F for moving the toner about the conveyor is equal QE
t
where Q is the charge on the toner and E
t
is the tangential field supplied by a multi-phase AC voltage applied to the array of conductors.
In that Patent and in other typical approaches, toner is presented to the traveling wave grid by means of a magnetic brush which is rotated in the same direction as the traveling wave propagates. This is suppose to give an initial velocity to the toner particles which enables toner having much lower velocities to catch the wave. Increase of the traveling wave's phase velocity is therefore limited by the brush rotation speed making the loading less efficient. Another limitation on the loading arises because of the amount of toner exposed to stripping- the magnetic brush tips tend to be sparse for large brush spacing and the stripping field on the traveling wave grid decreases exponentially with distance from the grid surface. The methods to increase the amount of toner loaded to the grid (with the magnetic brush in this mode) include speeding up the magnetic roll, decreasing the spacing, increasing the loading zone length, and increasing the number of rolls. All these approaches impose additional requirements on the system thereby limiting their applicability, and will mechanically wear the traveling wave device at the loading zone (grinding at a stationary loading zone on the grid).
SUMMARY OF THE INVENTION
Briefly, in the present invention, the impact of the above problems is significantly reduced and the overall loading efficiency is increased by separating the function of toner extraction from the function of toner transport toward the development zone. This is achieved by introducing an additional loading traveling wave grid(s) between the magnetic brush and the main transport traveling wave grid. The loading grid and the transport grid can be operated under different conditions (such as traveling wave's frequency and amplitude) more appropriate for their functionality. Electrostatic transfer of toners from the loading grid(s) to the transport grid is driven by the voltage applied between them. An extra advantage is gained by miniaturizing the loading grids (minigrids) so that several (many) of them can be used to efficiently strip toner from a single magnetic brush and transfer it to the transport grid. Loading grids may be coated with a more wear-resistant coating and are less expensive to replace than the main transport grid.
REFERENCES:
patent: 4558941 (1985-12-01), Nosaki et al.
patent: 4647179 (1987-03-01), Schmidlin
patent: 4777106 (1988-10-01), Fotland et al.
patent: 5532100 (1996-07-01), Christy et al.
patent: 5717986 (1998-02-01), Vo et al.
patent: 5842094 (1998-11-01), Bruyndonckx et al.
patent: 6070036 (2000-05-01), Thompson et al.
patent: 6112044 (2000-08-01), Thompson et al.
patent: 6134412 (2000-10-01), Thompson
patent: 61-073167 (1986-04-01), None
Gartstein Yuri
Ramesh Palghat S.
Bean II Lloyd F.
Beattty Robert
Xerox Corporation
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