Patent GB-1025534-A (GB)
Title: Electrostatic printing system with controlled powder feed
IPC: B41M1/12
Filed: 1964-09-17
Abstract: 1,025,534. Printing apparatus. ELECTROSTATIC PRINTING CORPORATION OF AMERICA. Sept. 17, 1964 [Oct. 4, 1963; July 14, 1964], No. 37963/64. Heading B6C. In an electrostatic printing apparatus in which powder particles are brushed through a stencil screen 10, Fig. 3, on to a copy sheet 18 placed between the screen and an electrode 14 the intensity of the electrostatic field provided between the screen and the electrode by a potential source 16 is augmented by applying a potential to the bush 28 through a metal brush fibre mounting ferrule 26. In order to control the particle flow through the screen 10 the potential source 30 associated with the brush 28 may be made variable. In the apparatus of Fig. 5 a current is caused to flow through the brush 28 and the screen 10 via a resistor 36 the field across the copy sheet 18 deriving from the potential drop across the resistor. In a modification of this apparatus the screen 10 is made of a high resistance material, such as nylon, mounted in a conductive frame, the frame and the electrode 14 being connected to the negative pole of the potential source 34. The apparatus shown in Fig. 9 has a powder container 60 mounted above a rotary brush 62, the latter comprising a sleeve of brush fibres over a cylindrical metal core. If required the container and/or the brush may be moved relative to the screen 68, copy sheet and electrode 70 by suitable mechanisms. The potential applied to the container, brush and electrode 72 is derived from respective variable sources 64, 66 and 70. This apparatus may be modified by eliminating the connection to the rotary brush 62, the necessary potential being applied thereto by mounting the brush in contact with the container. In order that particle density control may be applied over selected areas of the screen the brush 102, Fig. 13, is divided into conductive portions 110 and 112 each of which is in contact with its own source of potential 116, 118. Powder is fed from a conductive reservoir 90 connected to a potential source 114 and is passed to the brush 102 by a spirally wound fibre brush 98 and a conductive transfer roller 100. The brush 102 may be divided into as many portions as required, each of which may be given a potential different from the others. Although Fig. 13 shows commutator rings 104 and 106 for connecting the brush portions to their respective potential sources, slip rings may be used these being mounted on an extension of the brush shaft and electrically connected to the various brush portions internally of the shaft and the brush core. In a modified form of the apparatus of Fig. 13 the brush is in one piece, varying degrees of potential being applied to selective parts of its surface through a transfer roller which is divided axially into a plurality of portions each of which is connected to its own potential source. In the apparatus of Fig. 15 selective particle density control is effected on the screen 94 by forming the rotary brush 142 with surface fibre segments 144 each of which is connected internally of the brush with a stud 148. Through contact of the studs 148 with electrical brushes 150 and 152, potentials may be applied to the segments 144 thereby controlling the rate of powder feed on to the brush 142 (from the transfer roller 100) independently of the feed rate from the brush to the screen 94. In yet another form of apparatus in which the transfer roller 174, Fig. 17, is divided into portions, powder feed from the container 180 on to the transfer roller is aided by compressed air introduced into the container beneath a porous partition 182.