Electrophotography – Image formation – Transfer
Reexamination Certificate
2002-07-08
2004-05-25
Ngo, Hoang (Department: 2852)
Electrophotography
Image formation
Transfer
Reexamination Certificate
active
06741827
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to electrophotographic printing and, more particularly, to an improved transfer-roll system and method for enhanced printing, digital imaging, lithographic reproduction or the like.
During electrophotographic printing, for instance, a printed image is typically produced by transferring toner from a rotating, cylinder-like photoconductor to a sheet of paper or similar medium. This is usually accomplished by placing the paper against the photoconductor at a selected transfer point. A corona then applies a charge, such as by spraying, onto selected portions of the back side of the paper, the charge causing the paper to attract toner to the corresponding charged portions of the paper. As the photoconductor is rotated, i.e., during the toner transfer step, the paper is moved in tandem with the photoconductor. Upon contact between the photoconductor and the paper, the toner image corresponding to the charged portions is transferred from the photoconductor to the paper.
Other conventional electrophotographic printers use a semiconductive transfer-roller which is placed against the back of the paper for attracting toner to the front thereof. The roller is also soft, allowing it to press gently and uniformly against the rotating photoconductor, thereby squeezing the paper evenly against the photoconductor. Simultaneously, a voltage is applied to the roller for attracting and retaining toner on the paper. Because the soft roller, by its nature, is constructed of materials that are porous, it often captures and retains toner from the photoconductor. This usually occurs at locations where there is no paper between the roller and photoconductor.
To prevent toner and corresponding portions of the desired image from coming into contact with the transfer-roller, applications software has typically been provided to insure that the image is kept away from edge portions of the paper. In addition, transfer-rollers have been utilized that are of the same or greater width than the paper being printed upon. Since the photoconductor touches the roller only where no printing occurs, it was, therefore, believed that little or no toner could be developed on the transfer-roller.
To the contrary, however, some toner often ends up on the transfer-roller, such as on lengthwise or gap regions between consecutive sheets of paper. Toner transfer also occurs when printing on envelopes and other media considerably narrower than the transfer-roller. This toner transfer is usually due to incorrectly charged toner and/or unintended development such as during paper misfeeds. As a result, relatively small amounts of toner, e.g., between about 1% and 3% of the so-called background, end up in the gap regions and, in turn, on the transfer-roller. Eventually, a significant amount of toner is captured on the roller, typically in stripe form, causing unwanted soiling of the paper being fed and shortened roller life.
While undesirable, this occurrence was not generally considered problematic for several reasons. First, toner coverage in the unprinted or background areas is frequently quite low. Second, the transfer voltage could be turned off between sheets of paper, then on again, preventing, in theory, photoconductor to transfer-roller toner contact. Third, in applications such as printing on photographic paper where toner coverage is relatively high, such paper usually comes in a roll so that there are no gaps between sheets of paper. It is noted, in any case, that conventional printers are often readily modifiable for use of rolls rather than sheets of paper.
Although not an issue with every printing job, generally speaking, it is highly desirable to allow images to be printed to the edges of the paper. This is particularly true in photo paper printing applications which use clear toner to protect ink-jet prints. In these applications, a thick, clear toner layer about 20 microns thick, for example, is applied uniformly to the photo paper all the way to the edge thereof. The toner is then laid upon an ink-jet printed image and, subsequently, electrostatically fused to the photo paper to provide protection against moisture.
To insure coverage of the entire surface of the paper, it has been found that the photoconductor itself and, hence, the photoconductor toner layer must be generally wider than the photo paper, e.g., by several millimeters. Accordingly, where only one width of paper is used, a transfer-roller several millimeters narrower than the paper can be used without toner capture on the roller. Beyond each end of the transfer-roller, i.e., where the photoconductor (or equivalent) is wider than the paper, the toner has nowhere to go and is subsequently scraped from the photoconductor by a cleaning blade. Alternatively, where no cleaning blade is provided, toner on the photoconductor simply returns to the development region and the development roller reabsorbs the toner. Because photo paper is usually sufficiently stiff and adequately conductive to receive the desired charges from the charge roller, for optimum results, toner should be transferred uniformly to the outward most edges of the photo paper, even where there is no direct support from the transfer-roller.
Problems frequently arise, however, where multiple paper widths are utilized. Specifically, wherever the paper is narrower than the transfer-roller (and the photoconductor), some toner usually develops on the transfer-roller. This problem is magnified in that the development density present is at least twice that of solid area development densities, such as background toner levels, typically transferrable in the space between paper sheets. In some cases, up to about 2 mg/cm
2
may be continuously placed on the transfer-roller, resulting in relatively heavy stripes of toner on those portions of the roller adjacent to the paper edges.
Hence, when paper of greater width is subsequently printed upon, toner buildup from previous, smaller width printing jobs reduces the resiliency of the transfer-roller at the point of contact between the roller and paper. This, in turn, results in variable contact pressure, particularly in areas away from the stripes, leading to uneven transfer of toner in regions outside the striped areas. It also causes transfer of toner onto the back side of the paper, despoiling the paper and hindering fusion of the toner thereto.
A system and a method are, therefore, desired for improving the quality of printed images on printing media as the geometry of the media changes, so as to provide not only even, uniform coverage of toner and the corresponding image to be printed, but also to the physical edges of the media, simply, practically, reliably and at a relatively low cost.
SUMMARY OF THE INVENTION
Accordingly, a specific, illustrative apparatus is provided for use in a system including a photoconductor for collecting a photostatic charge in a selected form corresponding to an image to be printed, and for retaining toner in the form of the image on a first surface of printing media. The apparatus includes a transfer roller in contact with the photoconductor at a selected transfer point for transferring the toner image from the photoconductor to the first surface of the printing media. The transfer roller has at least one groove in proximity to each roller end for accommodating overhanging edges of printing media as a setback from the paper edges and for clearance from toner on the photoconductor.
Another specific, illustrative apparatus is provided for use in a system including a photoconductor for collecting a photostatic charge in a selected form corresponding to an image to be printed, and for retaining toner in the form of the image on a first surface of printing media. The apparatus includes a transfer roller in contact with the photoconductor at a selected transfer point for transferring the toner image from the photoconductor to the first surface of the printing media. The transfer roller has extendable roller ends such that, upon their extension a selected dista
Hewlett--Packard Development Company, L.P.
Ngo Hoang
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