Vacuum removal of liquid toner from a record member

Abstract: A development apparatus (10) in association with an electrostatic record member (16) is provided with a shroud (14, 50) surrounding a development electrode (12, 40, 68) and a toner return including a vacuum source to increase circulation of the toner. The vacuum source also draws air around the shroud (14, 50) to strike the record member (16) and remove excess toner.

Description: Field of The Invention This invention relates to liquid developing apparatus used to deposit electroscopic toner particles dispersed in a fluid onto one surface of a moving recording member in accordance with a latent electrostatic image formed thereon. Background of The Invention Several techniques are known for applying electroscopic toner to latent electrostatic image bearing record members so that toner will adhere to the record member in desired areas to develop the image. One technique is by cascading the particulate toner in dry form onto the surface of the record member, and removing the excess toner either by allowing it to slide off the record member by the force of gravity, or by blowing the excess toner with a fan. Another technique is to entrain the toner particles in a finely divided magnetic powder and to use a magnetic brush for distributing the toner over the surface of the record member. A third technique for distributing the toner is to entrain the toner in a dielectric liquid which is then brought in contact with the surface bearing the latent image. The developer apparatus of the present invention utilizes the latter technique. A significant problem encountered in developer apparatus that uses liquid entrained toner is commonly known as "boundary layer depletion." This may occur even after a very short period of use, since the concentration of toner in the liquid near where the liquid is brought into contact with the recording member may lower very rapidly when developing a "dark area" of the latent image. Other than the total immersion of the recording member in the liquid entrained toner for the purpose of development, which is not very effective with flat record members, there has existed the problem of supplying an adequate concentration of toner to ensure complete development. One of the earliest attempts to solve this problem was to use a roller dipped into a liquid toner bath. The roller, which was also conducting, was then rotated to provide a constant changing and replenishing action to the liquid entrained toner at the processing nip. Such an apparatus is illustrated in U.S. patent no. 3,367,791. As processing speeds increased, equipment modifications were required to keep pace with the increased through-put. It was found that as the recording member was passed over a rotating developing electrode, the developing zone was relatively small and that effective development was limited by the volume of liquid dispersed toner that the rotating electrode could deliver to the processing nip. U.S. patent no. 3,561,400 relates to an improved apparatus that attempted to solve many of the problems mentioned above. Generally, the apparatus comprises a shroud that surrounds a cylindrical roller. The shroud has an opening therein which permits a portion of the roller to project beyond the shroud. A pump provides a supply of liquid entrained toner into the space between the roller and the shroud. The roller is rotated to carry a film of the developer liquid over the projecting portion of the roller to contact the record bearing members. An attempt to increase the through-put of such an apparatus would necessitate a drastic increase in the diameter of the rotating electrode to obtain a proper development zone as well as an adequate electrical field in the development zone. While providing adequate toner to the image is one problem, the removal of excess toner from the image area after toning is another. Excess toner clings to the record member, and unless reliably removed, produces an undesirably dark background to the image. Summary of the Invention The objects of the present invention are to improve development through-put by increasing the circulation of liquid toner through the development zone while providing for the removal of excess toner to prevent or reduce background effects on the image surface. According to this invention there is provided a development apparatus for the application of liquid toner to the surface of an electrographic record bearing member, comprising: a development electrode in spaced, operative liquid toner applying proximity to said record bearing member; a shroud surrounding said development electrode; and means for providing a continuous supply of said liquid toner and forcing said liquid toner into said space between said development electrode and said record member; characterised by a blade extending into said space between said development electrode and said record bearing member; means associated with said shroud for removing said liquid toner from said space including a source of vacuum for drawing air around said blade in addition to removing said liquid toner from said space, wherein; said blade is positioned with respect to said record member so as to cause said air drawn around said blade to strike said record member and remove excess liquid toner from the surface of said record member. Brief Description of the Drawings The present invention will be more thoroughly described with reference to the drawings, wherein like numbers refer to like parts in the several views, and wherein: Figure 1 is a schematic, cross-sectional view of a development apparatus according to the present invention and a portion of a cylindrical recording member; and Figure 2 is an enlarged, partial cross-sectional view of a portion of the development apparatus and recording member of Figure 1. Detailed Description of the Invention Figures 1 and 2 illustrate a first embodiment of a development apparatus, generally indicated as 10, which includes a development roller 12 surrounded by a shroud 14 which is closed at its ends to completely enclose the roller 12. The development apparatus is located adjacent an electrostatic record bearing member 16 which is illustrated as being in cylindrical form, although the record member 16 may be in the form of a plate or web. The shroud 14 is open at the top in order to expose the development roller 12 and allow the development roller to approach the record member 16 in spaced, operative proximity, usually on the order of 0.38 mm, to form a development gap. The shroud surrounding the development roller 12 is longitudinally divided by a wiper blade 18 into a supply chamber 20 and a return chamber 22. The development roller 12 and the record member 16 rotate in the directions of arrows 24 and 26 such that their surfaces both move from the supply chamber 20 toward the return chamber 22. Liquid toner is supplied to the supply chamber 20 by means of a pump 28 and forced toward the development gap 17. The supply of liquid toner forms a meniscus 30, the width of which defines a development zone. Liquid toner in the development zone overflows the shroud 14 to fall into a catch tray 32 for return to the supply chamber 20. Toner is also withdrawn from the development zone by a vacuum pump 34 which enhances the circulation of liquid toner through the development zone. Toner withdrawn by the vacuum pump 34 is returned to the supply pump 28 for further circulation. The vacuum produced by the vacuum pump 34 also produces a flow of air into the return chamber from outside the shroud 14. The downstream side of the shroud 14 with respect to the rotation of the development roller 12 and the record member 16 is provided with a blade 36 which projects into the development zone. The blade 36 is positioned so as to cause the above mentioned flow of air from outside the shroud 14 to strike the record member 16 adjacent the development zone and strip excess liquid toner from the surface of the record member 16. The liquid toner consists of pigmented, insoluble particles having diameters up to two microns, that are stabilized in ISOPAR G (Registered Trademark of Humble Oil & Refining Company), a nonpolar, branched, liquid hydrocarbon. The particles are positively charged and are attracted to areas of the record member 16 which, as a result of charging and exposure of the record member 16, are more electrically negative than the particles. Toned density is dependent upon three factors, toner contact time, the concentration of the toner in its carrier and uniformity of the electrical field existing between the development roller 12 and the record member 16. Images developed using lower concentration toners have a cleaner background in the toned image. In addition, lower concentration toners are capable of developing small image detail with greater density and sharper edges. Thus, the developed images have high resolutions and better contrast when lower concentration toners are used. In order to use lower concentration toners, however, the supply of liquid toner to the development zone must be increased to avoid depletion of toner particles below an acceptable level in the development zone. The provision of the vacuum source 34 on the return side of the shroud 14 has been found to greatly increase the circulation of liquid toner through the development zone and thus ensure an adequate supply of toner particles to the image.

Claims: A development apparatus (10) for the application of liquid toner to the surface of an electrographic record bearing member (16), comprising: a development electrode (12) in spaced, operative liquid toner applying proximity to said record bearing member (16); a shroud (14) surrounding said development electrode (12); and means (28) for providing a continuous supply of said liquid toner and forcing said liquid toner into said space (17) between said development electrode (12) and said record member (16); characterised by a blade (36) extending into said space (17) between said development electrode (12) and said record bearing member (16); means associated with said shroud (14) for removing said liquid toner from said space (17) including a source of vacuum for drawing air around said blade (36) in addition to removing said liquid toner from said space (17); wherein; said blade (36) is positioned with respect to said record member (16) so as to cause said air drawn around said blade (36) to strike said record member (16) and remove excess liquid toner from the surface of said record member (16). A development apparatus (10) according to claim 1 wherein said development electrode is a cylindrical roller (12). A development apparatus (10) according to claim 2 wherein said record member (16) is a cylinder having an axis parallel to that of said development electrode (12). A development apparatus (10) according to claim 3 further including deposit removal means (18) for said development electrode (12). A development apparatus (10) according to claim 4 wherein said deposit removal means is a wiper blade (18) in contact with said development electrode (12).

Inventors: LUNDE GEORGE G, ZWADLO GREGORY L, LUNDE, GEORGE G., ZWADLO, GREGORY L.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1989
Main classification: G03G15
IPC codes: G03G15