Prevention of excess liquid toner contamination in the formation of electrophotographic images

Abstract: Airflow is used to substantially eliminate problems cause by excess liquid toner flow in developing a latent image on a photoconductor (226) surface (126, 326). Edge effects at each end of a squeegee roller (229) are prevented by concentrating an airstream at the nip (331) interface of the squeegee roller (229) with the photoconductor (226). Drip lines are prevented by providing a laminar airflow across the squeegee roller (229) interface with the photoconductor (226) to force excess toner away from the nip (331) interface before the excess liquid can pass through the nip (331) and cause a drip line.

Description: The present invention relates to a device for preventing the formation of drip lines in a liquid toner hard copy machine, particularly color hard copy printing and plotting and, more specifically, to the control of excess toner in a liquid electrophotography color printer. Electrophotography utilizes the formation of an electrostatic latent image to create a hard copy reproduction. In its basic aspects, a laser printing engine 124, shown schematically in FIGURE 1 (Prior Art) applies a charge with a scorotron charger 136 to a moving photoconductive insulating surface area of a photoconductor 126. The surface area is exposed to a pattern of light 138, 140. A latent image of the pattern is formed on the charged surface which is then developed by application of electroscopic toner 128, 130, 132, 134 to the photoconductive material. The developed image is transferred to a hard copy medium 152 using a transfer drum 148 with a transfer corona charge unit 150 and fused, or fixed , to the medium 152 by using another transfer corona unit 154. The photoconductive material insulating surface is then erased 146, cleaned 142, 144, and reused for the next image. This basic construct is used in a variety of state of the art products such as computer printers and plotters, copiers, facsimile machines, and the like. In the field of color hard copy reproduction, such as by laser printers using liquid electrophotography ( LEP ) techniques, the use of color liquid toners (generally yellow 128, magenta 130, cyan 132 ( the subtractive primary colors ) and a black toner 134) that are difficult to process presents challenging designs problems. Each printing cycle must charge, expose, develop, and transfer colors, several being through a toner layer that has already been deposited on the photoconductor 126. One problem inherent in the process is the managing of excess liquid toner. Pneumatic pressure has been used to control excess liquid toner. For example, U.S. Patent No. 3,741,643 uses an "air knife for removing excess toner from the surface of the photoconductive drum or belt." Col. 1, II. 35-36. Essentially, the forced air is used to evaporate the "diluent" part of the liquid toner. Referring to FIGURE 2 (Prior Art), another solution to the problem of dealing with excess liquid toner has been to add a squeegee roller 229 adjacent to the developer roller 228 of each developer assembly. While effective at drying the photoconductor at the surface, the squeegee roller 229 is known to leave the imaged photoconductor wet with toner at its outside edges (also known as edge effects ), that is, along each end of the squeegee roller proximity area with the photoconductor 126 (FIGURE 2A). This area of wet photoconductor is drawn into the next different color developer where it mixes with that toner. Over time, this color mixing, known as cross-contamination , is sufficient to seriously degrade color print quality. Various devices such as having absorbent pads, suction devices, or counter-rotating end caps at each end of the squeegee roller have provided limited success at controlling edge effects. Therefore, there is a need for an apparatus to assist squeegee roller to prevent these edge effects that lead to cross-contamination. Moreover, it is known that a squeegee roller 229 retains a volume of toner across a substantial part of its surface area after wiping an image on the photoconductor 226. As demonstrated in FIGURE 2B, a drip line of retained toner forms in the downstream nip between the squeegee roller 229 and the photoconductor 226 as the toned image pulls away from the squeegee roller 229. This volume of retained toner is known to be sufficient to contaminate the colors of the adjacent developers. Over time, the wasted toner from the drip line effect will also seriously reduce the number of pages that can be printed from a given volume of toner. Such drip lines have also been found to form on the developer roller 228. Therefore, there is also a need for an apparatus to alleviate the drip line effect problem. JP-A-60 189777 relates to a squeeze roll mechanism of a developing device which comprises developing elements inbetween which a lithographic plate is conveyed and is visualized by means of the developing elements. After development of the lithographic plate, same is passed between a couple of squeeze rolls which are used for fixing the developed image on the lithographic plate 1. Compressed air is blown into gaps between the rollers to prevent a developer from leaking to avoid any disorder of the "toner visualized part" on the lithographic plate. JP-A-06 051572 relates to a planographic printing plate drying device which comprises moisture absorbing rollers for wiping away liquid drops at the peripheral edges. Further, drying air is applied to these edges after wiping away the liquid drops. EP 0 183 455 A2 concerns an apparatus for producing an electrophotographic print which uses a pneumatic knife to cause the developer on the sensitive sheet to fall in drops downstream of the rotational direction of the drum under the pneumatic force. It is the object of the present invention to provide a device for controlling excess toner in a liquid toner hard copy machine, and for preventing the formation of drip lines. This object is achieved by a device according to claim 1. It is an advantage of the present invention that the method of operation requires no contacting parts and thus there are no adverse wear factors. It is an advantage of the present invention that it operates using an economical, low pressure forced air device, such as a diaphragm pump or fan. It is an advantage of the present invention that previously wasted toner can be recycled to the developer mechanism and reused. It is still another advantage of the present invention that employs economical, commercially available system components to provide a low cost solution. It is yet another advantage of the present invention that excess toner is returned directly to the developer, substantially eliminating opportunities for clogging. Other objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description and the accompanying drawings, in which like reference designations represent like features throughout the FIGURES. FIGURE 1 (Prior Art) is a schematic drawing of a laser electrophotography engine apparatus. FIGURE 2 (Prior Art) is a schematic drawing of components of an electrophotography engine apparatus as shown in FIGURE 1 in which: FIGURE 2A depicts the problem of edge effect formation, and FIGURE 2B depicts the problem of drip line formation. FIGURE 3 is a schematic drawing, perspective view, of the present invention mechanism for substantially eliminating drip lines. FIGURE 4 is a schematic drawing of the present invention as shown in FIGURE 3 in which: FIGURE 4A is a depiction of the operation of the present invention when the developer mechanism is engaged; FIGURE 4B is a depiction of the operation of the present invention when the developer roller has completed the transfer of toner from a reservoir to a photoconductor surface; and FIGURE 4C is a depiction of the operation of the present invention when the developer mechanism has been fully disengaged. The drawings referred to in this description should be understood as not being drawn to scale except if specifically noted. Reference is made now in detail to a specific embodiment of the present invention, which illustrates the best mode presently contemplated by the inventor(s) for practicing the invention. Alternative embodiments are also briefly described as applicable. In FIGURES 3 and 4A through 4C, the use of air pressure to prevent the drip line problem discussed above is shown. The photoconductor drum 326 is provided with a slotted surface region 701. The slotted surface region 701 has a slot 703 and a clearance feature 705, opening the drum surface 327. In an assembled condition (not shown), the drum 326 is substantially a tubular construct through which air under pressure can be introduced, such as with a fan, leaving the slot 703 as an egress airflow from the drum 326. The slot 703 creates a laminar airflow outwardly from the drum surface 327. The operation of the present invention is shown in FIGURES 4A through 4C. The arrow designated "Airflow" demonstrates the egress of air from the drum 326. When the developer mechanism is engaged (FIGURE 4A), toner is transferred from a reservoir by developer roller 228 to the drum surface 327. The squeegee roller 328 is appropriately positioned to wipe the excess toner from the drum surface 327. The excess toner accumulates in the photoconductor drum-squeegee nip. As shown in FIGURE 4B, as the developer roller-photoconductor drum nip is cleared (that is, the latent image has been fully toned with the selected toner from a reservoir), the laminar air flow, a sheet of air from the slot 703, hits the toner trapped in the upstream squeegee roller-photoconductor drum nip, pushing it down the face of the squeegee roller 328. A drip line from the toned image 801 is prevented. Referring to FIGURE 4C, the excess toner 803 remaining on the squeegee roller 328 can now be removed by any suitable method and the excess toner 803 returned to the appropriate reservoir. It should be noted that the airflow will similarly prevent any drip line formed on the developer roller, although this is a less significant problem than the squeegee roller drip line effect. It has been found that only a localized low airflow rate, in the range of 0,0142 m 3 to 0,142m 3 (0.5 to 5 cubic feet) per minute through the slot 703 will accomplish the drip line prevention task. The foregoing description of the preferred embodiment of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. Similarly, any process steps described might be interchangeable with other steps in order to achieve the same result. The embodiment was chosen and described in order to best explain the principles of the invention and its best mode practical application to thereby enable others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claim appended hereto.

Claims: A device for preventing the formation of drip lines in a liquid toner hard copy machine, including a rotating drum type photoconductor means (326, 327) having a central chamber and operatively coupled with means for applying toner (228) to develop a latent image on said photoconductor means (326, 327) and means for removing excess toner (328, 329, 330) from said photoconductor means (326, 327), said device comprising: means for providing an airflow through said central chamber of said rotating drum (326); means for converting (701, 703) said airflow into a laminar airflow across said means for removing excess toner (328, 329, 330) such that excess toner is forced away from an interface of said photoconductor means (326, 327) and said means for removing excess toner (328, 329, 330), when said latent image has been fully toned; wherein said airflow is localized and has a rate in the range of 0,0142 m 3 to 0,142 m 3 per minute (0,5 to 5 cubic feet per minute).

Inventors: ARCARO DAVID J, HETTINGER JOHN L, ARCARO, DAVID J., HETTINGER, JOHN L.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1995
Main classification: G03G15
IPC codes: G03G15