Patent GB-698994-A (GB)
Title: Method and apparatus for developing electrostatic images
Inventor: WALKUP LEWIS EDMUND
IPC: B05B5/08
Filed: 1951-12-14
Abstract: 698,994. Powder development of electrostatic images. BATTELLE DEVELOPMENT CORPORATION. Dec. 14, 1951, No. 29328/51. Class 98 (ii). [Also in Group XVI] A method of developing an electrostatic image on an insulating layer disposed on a conductive support comprises positioning an electrode at a distance less than 0.5 inch from the layer and introducing a charged finely-divided material between the electrode and the layer whereby a faithful development of the image is achieved. Preferably the electrode is a grid through which the developer particles are introduced into the space between the layer and the grid. Development may be effected in steps with the electrode at different potentials for each step to control contrast. As shown, a plate comprising a conducting support 26 and an insulating layer 35 bearing an electrostatic image is placed face downward on an insulating support over an inverted metal cone 21, at the apex of which is disposed a brush 22 and a developer mixture 55 comprising a powder such as a powdered resin and a granular carrier such as oil-freed soya bean protein, the powder and carrier acquiring charges tribo-electrically. The support 26 is earthed and a grid 27 adjacent the plate is raised to a suitable potential by means of an adjustable tap 52 on a potential divider 51, thereby rendering uniform the field adjacent the image-bearing portions of the plate, typically to the potential of the highlight areas of the plate if these are to remain clear after development. The brush 22 is then rotated by means of an electric motor 23 and the resulting air dispersion of the powder 55 passes through the grid 27 to develop the plate image. In order to obtain a linear relation between the print and original densities by compensating for the logarithmic type of relation between the density of the print and of the original, the electrostatic image may be developed in steps, each comprising raising the grid to the potential corresponding to a particular tone of the original and developing for a time such that the density of the deposited developer powder at a point where the departure from linearity of the image density becomes appreciable reaches a value in a chosen fixed ratio to the density of the corresponding tone in the original. Typically development is effected with the grid successively at the potential of the highlights, middle tones, and dark shadow areas. In a modification, the grid potential is varied in equal steps continuously during the developing period, the different potentials being maintained for different times, chosen for a particular kind of plate and developer, by means of a switch having a motor-driven wiper passing over stationary contact strips of different lengths connecting resistors arranged in series across a battery as a potential divider. The potential so varied may be superimposed on an additional steady potential applied to the grid by means of an adjustable tap on a potential divider disposed across a second battery. A charge may also be produced on developing powder particles by corona discharge by blowing the powder down a tube having a wire passing down its centre raised to a high potential. In a further embodiment the plate is developed by an air dispersion of charged drops of liquid, such as Indian ink, a solution of a water-soluble blue dye, an oil dispersion of colloidal graphite, printer's ink in carbon tetrachloride, an alcoholsoluble dye in ethanol, mimeograph ink, or lampblack in methanol, the drops being formed by an air brush which emits the drops through a ring insulated from it and raised to a high potential. Specification 679,715 is referred to.