GB872387A - Improvements in or relating to apparatus and methods for electrostatically spray coating articles - Google Patents

Abstract

A method of electrostatically spray-coating an article which is at earth potential by employing an atomizer which is maintained at a high potential and which produces an annular spray of particles having a central void region, comprises maintaining by means of an electrode a zone of particle-attracting electric potential in the region substantially to <PICT:0872387/III/1> <PICT:0872387/III/2> reduce the void in the spray pattern produced, the potential of the region being intermediate the high potential and earth. An apparatus for carrying out the method comprises a gun 10 having an insulating atomizer 28 through the centre of which projects a metal electrode 32, the atomizer 28 and electrode 32 being mounted on an insulating shaft 22 which is rotated by a suitably driven flexible shaft 18. The atomizer 28 is connected to a high voltage source by a wire 38, a high resistance 42, e.g. of 1,000 meg ohms, and a metal wire spring 44 to a resistance coating 28c, e.g. of 20 to 100 meg ohms. The electrode 32 is connected through a high resistance 34, e.g. of 1,500 meg ohms, and a spring 35 to the shaft 18 which is earthed. In operation, manipulation of a trigger 50 delivers liquid coating material through a line 46 and a needle valve to a conduit terminating in a nozzle adjacent the p inner surface 28b of the rotating atomizer 28 and behind an insulating baffle ring 26 mounted by insulating pins 29 to leave a narrow annular gap 30 between the parts 26, 28. The liquid flows through the gap 30 and is atomized at the annular edge 28a. A 100 kilovolt source will ensure an electrostatic field of 7,000 volts per inch between the atomizer 28 and an article 12 inches away, and the electrode 32 protruding 3 inches beyond the edge 28a of the atomizer is at a point in the field of 70,000 volts, the high resistance 34 maintaining the potential of the tip of the electrode 32 at 30,000 volts, which, while attracting liquid particles, is insufficient to cause much deposition on the electrode 32 or the enclosing shaft 22. The shaft 18 is rotated at 900 r.p.m. and liquid is fed to the gun at 100 c.c. per minute. In a modification, an electrical connection is made between the shaft 18 and the end of resistance 42 adjacent the atomizer 26, the electrode 32 thus being connected to the voltage source through the resistances 34, 42, the values of which may be chosen to give a desired charge on the tip of the electrode 32. Values mentioned are 5,000 meg ohms and 500 meg ohms respectively. In another embodiment, Fig. 9, the electrode comprises a concave disc 138 mounted on a baffle ring 131 which supports an atomizer 125, the ring 131 being mounted on a rotatable shaft 130. The parts 125, 130, 131, 138 are insulating, e.g. of nylon, and a resistance coating 125a, e.g. of carbon mixed with an alkyd resin is put on the atomizer 125 and on front and back of the electrode 138. The electrical connections are as described in the first embodiment, the resistance in the circuit to the electrode 138 having a value of 10,000 meg ohms, and the resistance 122 in the high voltage circuit being 1,000 meg ohms. The high voltage source is 120 kilovolts and the atomizer is rotated at 600 r.p.m. Specification 865,765 is referred to.