Patent GB-755486-A (GB)
Title: Improvements in electrostatic printing
IPC: G03G15/22
Filed: 1954-09-14
Abstract: 755,486. Xerography; development. HORIZONS, Inc. Sept. 14, 1954 [Sept. 15, 1953], No. 26633/54. Class 98 (2). An electrostatic image is formed on an insulating surface backed by a conductive metal layer by contacting the insulating surface with a transparent photo-conductive surface mounted on a conductive backing while maintaining a potential difference between the conductive backing layers and while forming a light-and-shade pattern on the photo-conductive layer, separating the insulating surface and either developing the electrostatic image formed thereon or transferring it to a second insulating surface on which it is developed. The layers between the light source and the photo-conductive layer must of course be transparent. The photo-conductive element and the image-receiving insulating element may be in the form of a cylinder or a plain surface. Fig. 1 shows a cylinder 6 comprising a glass or transparent plastic cylinder 8, a transparent conducting layer 9 and a transparent photoconductive layer 10 in rolling contact with a cylinder 7 comprising a metal core 11 carrying an insulating layer 12. A battery 15 maintains a potential difference between the conductors 9 and 11. A transparent original 14 is placed inside the cylinder 8 and is illuminated by a lamp 13. As the cylinders rotate, an electrostatic image is formed on the insulating surface 12 and developed at 22 with carbon black. At 16 the powder image is transferred to paper strip 17 and the surface 12 is cleaned in zone 18 and electrically discharged in zone 19. The transparent conducting layer 9 may be a thin layer of tin oxide or a mixture of gold and bismuth. The photo-conductive layer 10 may be of selenium, arsenic selenide, or zinc-cadmium sulphide deposited by vacuum deposition so as to give a relatively transparent layer and the insulating layer 12 may be of polystyrene, methyl methacrylate, cellulose acetate or other transparent cellulose resins, rubber-like materials or a layer of zinc sulphide. Alternatively the light source and original may be within the drum 7, which then comprises a transparent conductive layer on a transparent cylinder, with the insulating layer 12 made of transparent material. In this case, layers 8 and 9 of drum 6 may be opaque (e.g. a single metal core). In one modification the drum 6 contains the light-source as before but drum 7 comprises a central metal core, a rubber layer, and then a conducting layer (which is connected to the battery) and the insulating layer 12. In a further modification the transferred latent image is not developed on layer 12 but transferred to another cylinder constructed as cylinder 7 in Fig. 1 and on which the image is developed and transferred to the paper strip 17 as described above. In another modification the drum 6 is replaced by a flat element which is rolled past the drum 7, the latter having a rubber backing layer as described above. In the last modification both elements are flat and are simply laid in contact, development being carried out in a separate zone, after separation of the plates. The developer may comprise a conventional powder such as carbon black dispersed in an electrically-insulating liquid such as carbon tetrachloride or conventional hydrocarbon insulating oils. The layer bearing the electrostatic latent image may be immersed in the dispersion or a metal electrode may be placed close to the image surface and the developer dispersion flowed between the surfaces. Alternatively, an insulating roller such as of rubber, may be coated with the dispersion, for instance by dipping and draining off the excess liquid, and then rolled in wet or dry condition over the image surface to be developed. U.S.A. Specification 2,277,013 is referred to.