Electrophotography – Image formation – Development
Reexamination Certificate
2002-12-17
2004-08-03
Royer, William J. (Department: 2852)
Electrophotography
Image formation
Development
Reexamination Certificate
active
06771923
ABSTRACT:
This invention relates generally to an electrophotographic printing machine and, more particularly, to a development system for development of electrostatic images.
An electrophotographic printing machine includes a photoconductive member which is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to an optical light pattern representing a document being produced. This records an electrostatic image on the photoconductive member corresponding to the informational areas contained within the document. After the electrostatic image is formed on the photoconductive member, the image is developed by bringing a developer material into effective contact therewith. Typically, the developer material comprises toner particles bearing electrostatic charges chosen to cause them to move toward and adhere to the desired portions of the electrostatic image. The resulting physical image is subsequently transferred to a copy sheet. Finally, the copy sheet is heated or otherwise processed to permanently affix the powder image thereto in the desired image-wise configuration.
Development may be interactive or non-interactive depending on whether toner already on the image may or may not be disturbed or removed by subsequent development procedures. Sometimes the terms scavenging and non-scavenging are used interchangeably with the terms interactive and non-interactive. Non-interactive development is most useful in color systems when a given color toner must be deposited on an electrostatic image without disturbing previously applied toner deposits of a different color, or cross-contaminating the color toner supplies.
In the prior art, both interactive and non-interactive development have been accomplished with magnetic brushes. In typical interactive embodiments, the magnetic brush is in the form of a rigid cylindrical sleeve which rotates around a fixed assembly of permanent magnets. In this type of development system, the cylindrical sleeve is usually made of an electrically conductive, non-ferrous material such as aluminum or stainless steel, with its outer surface textured to improve developer adhesion. The rotation of the sleeve transports magnetically adhered developer through a development zone where there is direct contact between the magnetic brush and the imaged surface, and toner is stripped from the passing magnetic brush filaments by the electrostatic fields of the image.
U.S. Pat. No. 5,409,791 to Kaukeinen et al., the disclosure of which is hereby incorporated by reference, describes a non-interactive magnetic brush development method employing permanently magnetized carrier beads operating with a rotating multipole magnet within a conductive and nonmagnetic sleeve. Magnetic field lines form arches in the space above the sleeve surface and form chains of carrier beads. The developer chains are held in contact with the sleeve and out of direct contact with a photoreceptor by gradients provided by the multipole magnet. As the magnet rotates in one direction relative to the sleeve, the magnetic field lines beyond the sleeve surface rotate in the opposite sense, moving carrier bead chains in a tumbling action which transports developer material along the sleeve surface. The strong mechanical agitation effectively dislodges toner particles generating a rich powder cloud which can be developed to the adjacent photoreceptor surface under the influence of development fields between the sleeve and the electrostatic image. However, such radial flow of developer material occurs over the full surface of the sleeve; hence, a stripping device such as a skiving blade must be included for removing spent developer before it returns to the development zone.
U.S. Pat. No. 5,227,848 to Robinson et al. describes a toning roll assembly in which a rotatable core member with magnetic poles disposed about the periphery thereof is positioned within a hollow shell so that the shell-to-core clearance, and therefore, the magnetic field strength on the outside surface of the shell varies from point-to-point on the shell. Magnetically attractable developer particles are fed onto the shell's surface at a point in a loading zone of higher field strength and moved through a point of lower field strength on the shell. The field strength is provided solely by the rotatable core member, which, although positioned eccentrically with respect to the shell, must be positioned so as .to attract and support a requisite amount of developer material on the shell.
As illustrated in
FIG. 1
, it may be observed that the provision of agitated bead chains according to the foregoing two patents will result in linear ridges, or “piles”, of developer material distributed about the developer sleeve
100
due to the high density of bead chains approximately at the midpoint of each magnet
120
. (Corresponding valleys, or “troughs”, of developer material thus locate at the intersections of adjacent magnets
120
.) As a consequence of this effect, the resulting developer chains accumulate over the developer roll in linear piles having varying peaks and troughs that are arranged in parallel to the central longitudinal axis of the magnetic developer roll.
In typical practice, the magnetic developer roll is operated with associated devices, such as a skiving blade, that engage the developer sleeve
100
in order to remove unused developer and toner material from the developer sleeve
100
. Similar devices may be located adjacent the developer sleeve
100
to meter fresh developer material onto the developer sleeve
100
so as to effect replenishment of fresh developer material. In typical practice, such blades are oriented in parallel with the same central longitudinal axis of the magnetic developer roll.
In conceiving the present invention, I have found that the above-described development systems suffer from the following undesirable phenomena.
The magnetic brush height formed by the developer mass in the magnetic fields on the sleeve surface in the aforementioned types of development systems is periodic in thickness and is statistically noisy as a result of complex carrier bead agglomeration and filament exchange mechanisms that occur during operation. Accordingly, substantial clearance must be provided in the development gap to avoid photoreceptor interactions through direct physical contact. The use of a closely spaced developer layer, which is critical to high fidelity image development, is precluded.
The magnetic pole spacing cannot be reduced to an arbitrarily small size because allowance for the thickness of the sleeve and a reasonable mechanical clearance between the sleeve and the rotating magnetic core sets a minimum working range for the magnetic multipole forces required to both hold and tumble the layer of developer material on the sleeve. Since the internal pole geometry defining the spatial wavelength of the tumbling component also governs the magnitude of the holding forces for the developer material at any given range, there is limited design freedom available to satisfy the opposing system requirements of short spatial wavelength and strong holding force.
Relative rotation of the magnetic developer roll and the developer sleeve will rotate successive ones of the magnets within the developer sleeve and thus under the engaging edge of the skiving blade. A corresponding movement of successive linear piles of developer material move along the exterior of the developer sleeve. As a result, the skiving blade will periodically be impacted by the entire length of a linear pile of developer material, whereupon the development system undergoes substantial increase in mechanical stress, which is periodic due to the rapid succession of developer pile masses encountered by the skiving blade. During each stress peak, the skiving blade, magnetic developer roll, developer material, along with the motor drive and any respective mechanisms including motor drive bearings, will experience a significant increase in mechanical force. The motor drive
DeYoung Victoria F.
Mashtare Dale R.
Xiao Fei
Royer William J.
Xerox Corporation
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