Patent Application
20050053397
The present invention relates generally to charging devices and in particular to charging devices that include grid elements such as scorotron charging devices used in imaging systems.
In electrostatographic-type copiers and printers in common use, a charged imaging member such as a photoconductive insulating layer of a photoreceptor may be electrically charged and thereafter exposed to a light image of an original document or a laser exposure of a digitally stored document. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the original document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with toner. During development, the toner particles are attracted from the carrier particles by the charge pattern of the image areas on the photoconductive insulating area to form a powder image on the photoconductive area. This image may be subsequently transferred to a support surface such as copy substrate to which it may be permanently affixed by heating or by the application of pressure. Following transfer of the toner image to the support surface, the photoconductive insulating surface may be discharged and cleaned of residual toner to prepare for the next imaging cycle. The imaging processes described above are well known in the art.
Various types of charging devices have been used to charge or precharge charge retentive surfaces such as the photoconductive insulating layers of photoreceptors or such as copy substrates prior to transfer of toner images. These charging devices include corotrons, dicorotrons, pin corotron, scorotron, discorotron, and pin scorotron. See, generally, R. M. Schaffert, “Electrophotography,” The Focal Press, New York, 1965.
A scorotron device, included within the list above, is typically comprised of one or more corona wires or pin arrays with a conductive control grid or screen of parallel wires or apertures in a plate positioned between the corona producing element and the photoconductor. A potential is applied to the control grid of the same polarity as the corona potential but with a much lower voltage, usually several hundred volts, which suppresses the electric field between the charge plate and the corona wires and markedly reduces the ion current flow to the photoreceptor.
The pin array variety of scorotron has proved to be a particularly inexpensive, durable, and effective device. Pins are often formed by forming “saw teeth” in a conductive metal sheet mounting these saw teeth edgewise facing the scorotron grid. In this arrangement, however, certain difficulties have been observed. One such difficulty is a sinusoidal wave pattern of charging thought to result from the increased charge potential located at the peaks of each pin when compared to each “valley” between pins. The scorotron grid is known to ameliorate the problem by diffusing the charge pattern through the grid pattern. Another method of ameliorating this problem is using at least two pin arrays arranged in parallel fashion such that the peaks of pins in the first array align with the valleys of the second array along the imaging path. Use of conventional scorotron grids with such dual pin arrays is known to produce charge uniformity across a process width of about plus or minus 25 volts for mid-range process speeds. In high quality printing, however, even relatively minor fluctuations in charge potential across the charged imaging surface, such as plus or minus 25 volts, cause undesirable printing irregularities.
A typical prior art scorotron device with dual pin arrays and a scorotron grid is shown in
As shown in
One approach to improving charge uniformity using scorotron charging devices is set forth in U.S. Pat. No. 6,459,873, issued to Song et al., where a pair of scorotrons cooperatively charge the charged imaging surface. The first scorotron device initially charges the imaging surface to an intermediate overshoot voltage and the second device thereafter uniformly charges the imaging surface to the final voltage. Improved uniformity is created because the first scorotron device provides a generally high percent open control grid area (a range above 70% is claimed in Song) while the second scorotron device provides a generally lower percent open grid area (a range below 70% is claimed in Song). The higher percent of opening in the first scorotron grid correlates to a greater rate of charging, or slope, while the smaller percent of scorotron grid opening correlates to a lesser slope, or lesser rate of charging. The lesser slope of the second scorotron device enables more precise control of the charging process and, as a result, greater uniformity. Song is hereby incorporated herein by reference in its entirety.
The dual scorotron device taught in Song improves charge uniformity due to the differential in percentage of openings between the first and second grids. It would be desirable, however, to further improve charging uniformity.
One embodiment of the invention is a charging system for charging a charge retentive surface having a, comprising: at least one corona producing element, spaced from the charge retentive surface and arranged generally along the width dimension; and grid elements, interposed between said corona producing element and the charge retentive surface, wherein the grid elements are arranged generally parallel to each other along the width dimension and comprise differentiated grid feature patterns.
Another embodiment of the invention is an electrostatographic imaging system, comprising: a charge retentive surface having a width dimension; at least one corona producing element, spaced from the charge retentive surface and arranged generally along the width dimension; and grid elements, interposed between the corona producing element and the charge retentive surface, wherein the grid elements are arranged generally parallel to each other along the width dimension and comprise differentiated grid feature patterns.
Yet another embodiment of the invention is a method for charging a charge retentive surface having a width dimension, comprising: electrically charging at least one corona producing element, spaced from the charge retentive surface and arranged generally along the width dimension, sufficiently to emit a corona field; affecting the corona field by interposing, between the corona producing element and the charge retentive surface, grid elements that are arranged generally parallel to each other along the width dimension and that comprise differentiated grid feature patterns.
The invention may take physical form in certain parts and arrangements of parts, an embodiment of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof, and wherein;
For a general understanding of the present invention, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate identical elements.
An exemplary electrostatographic system comprising an embodiment of the present invention is a multifunctional printer with print, copy, scan, and fax services. Such multifunctional printers are well known in the art and may comprise print engines based upon electrophotography and other imaging electrostatographic technologies. The general principles of electrophotographic imaging are well known to many skilled in the art. Generally, the process of electrophotographic reproduction is initiated by substantially uniformly charging a photoreceptive member, followed by exposing a light image of an original document thereon. Exposing the charged photoreceptive member to a light image discharges a photoconductive surface layer in areas corresponding to non-image areas in the original document, while maintaining the charge on image areas for creating an electrostatic latent image of the original document on the photoreceptive member. This latent image is subsequently developed into a visible image by a process in which a charged developing material is deposited onto the photoconductive surface layer, such that the developing material is attracted to the charged image areas on the photoreceptive member. Thereafter, the developing material is transferred from the photoreceptive member to a copy sheet or some other image support substrate to which the image may be permanently affixed for producing a reproduction of the original document. In a final step in the process, the photoconductive surface layer of the photoreceptive member is cleaned to remove any residual developing material therefrom, in preparation for successive imaging cycles.
The above described electrophotographic reproduction process is well known and is useful for both digital copying and printing as well as for light lens copying from an original. Since electrophotographic imaging technology is so well known, further description is not necessary. See, for reference, e.g., U.S. Pat. No. 6,069,624 issued to Dash, et al. and U.S. Pat. No. 5,687,297 issued to Coonan et al., both of which are hereby incorporated herein by reference.
Referring now to
Referring to
As seen in
The impact upon charging uniformity of using scorotron grid elements having differentiated patterns is shown in the bar chart of
The results confirm the advantages of using different grid patterns. Whereas the bar in
In sum, use of scorotron grid elements having differentiated grid patterns across the width dimension of an imaging substrate result in more uniform charging of the charge retentive surface. Embodiments of the invention apply to charging systems utilizing grids positioned between the charge retentive surface and the corona generating elements. Such charging systems include, without limitation, wire-based scorotrons, pin-array scorotrons, and discorotrons. Pin array scorotrons become particularly attractive with embodiments of the invention by combining the high charge uniformity achievable with the present invention with the relative inexpensiveness and robustness of pin array corona devices. Differentiated patterns can be achieved in any manner, including varying the grid pattern by geometric shape or by feature size.
While particular embodiments have been described, alternatives, modifications, variations, improvements, and substantial equivalents that are or may be presently unforeseen may arise to applicants or others skilled in the art. Accordingly, the appended claims as filed and as they may be amended are intended to embrace all such alternatives, modifications variations, improvements, and substantial equivalents.
