Tribo-electro-gas-dynamic powder charging apparatus

#1. Field of the Invention

#The invention relates to powder coating apparatus, more particularly to tribo-electro-gas-dynamic (TEGD) powder charging apparatus. The apparatus comprises a powder gun that is especially useful for coating deep recesses, for example inside surfaces of two piece aluminum beverage cans.

#2. Description of the Prior Art

#FIG. 1 is an elevational view in partial section showing the apparatus in combination with equipment with which it may be used.

#FIG. 2 is a longitudinal sectional view of the apparatus showing its main features.

#FIG. 3 is a longitudinal sectional view of a modification of the apparatus.

#The powder coating gun 10 as seen in FIG. 1 is intended for use in conjunction with a supply of compressed gas entering the gun through carrying means such as one of tubes 11 or 12 and with a supply of powder that may be entrained in a carrier gas entering through carrying means such as the other of tubes 11 and 12. The gun 10 may be supported by bracket 13 to aim through aperture 14 in mask plate 15. A workpiece such as can 16 may be inserted in aperture 14 to be coated internally by the gun 10.

#As shown in FIG. 2, the apparatus comprises an end cap 20 which may have ports 21 and 22 and barrel 23 having throat 24 with a semi-toroidal section 25. The end cap 20 and barrel 23 are joined together by a connection such as threads 26. A seal 27 may be used to prevent escape of pressurized gas at the connection. An annular chamber 28 is formed circumferentially surrounding section 25 within circular side wall 29 of end cap 20. In addition, annular adjustable clearance 30 exists between section 25 and end wall 31 of end cap 20. Clearance 30 is adjusted by adjusting means, for example threaded connection 26. Zone 32 exists between barrel throat 24 and end wall 31 and within annular clearance 30.

#In operation pressurized gas is forced into annular chamber 28, for example through port 22, and must pass through clearance 30 and into zone 32. As the gas passes through clearance 30, the stream becomes attached to the walls of cap 20 and section 25, and as the two walls begin to diverge and the gas expands into zone 32, part of the stream is directed into throat 24 by the coanda effect. The rest of the stream remains attached to the wall of end cap 20 until it reaches the point of powder delivery, such as the bore of port 21, at which point it detaches, creating turbulence and reduced pressure. Powder to be charged is normally delivered to port 21 entrained and dispersed in a suitable gas such as air, CO.sub.2, N.sub.2, or Ar. Since the flow in zone 32 is turbulent, many collisions occur between the particles themselves and between particles and the walls of the gun. Some gases, notably CO.sub.2, also become charged and can either enhance or degrade the charge on the particle depending on the polarity of each. In this mode of operation, the powder to be charged and its entraining gas are fairly well mixed with the propellant gas by the turbulence in zone 32 and thus have access to the walls of the throat 24 of the gun for further frictional charging. The gun 10 is operating as an ejector in this mode and the highest velocity gas is at the wall of the throat 24, thus making the gun self cleaning.