Patent ID: 7130178

Claim:
A corona charging device comprising: a powder feed having an outlet; an internal charging cavity having an inlet and a charged powder outlet, the powder feed outlet being positioned at the internal charging cavity inlet, the device being adapted to guide a powder stream downstream from the powder feed outlet to the charged powder outlet; a corona charger comprising at least one needle projection, the at least one needle projection having a tip positioned and adapted to facilitate a corona ion flow from the needle projection and intersecting the powder stream, the corona charger optionally comprising a staggered array of three or more needle projections, each needle projection each having a tip positioned and adapted to facilitate a corona ion flow from the needle projection and intersecting the powder stream; a ground electrode adapted to be charged or grounded to attract the corona flow from the needle projection, which can be a rotating ground electrode adapted to rotate segments of the ground electrode between the internal charging cavity; and one or more of: (a) a ground electrode cleaner, wherein the ground electrode is a rotating ground electrode; or (b) a field electrode located downstream of the corona charger and positioned and adapted to induce free ions entrained in the powder stream to contact the ground electrode or a second ground electrode; or (c) one or more sheath conduits positioned around the powder feed to provide (i) a sheathing gas stream between the powder stream and the tip of the needle projection and (ii) a sheathing gas stream between the powder stream and the ground electrode, and (iii) a sheathing gas stream between the powder stream and the side walls of the charging chamber; or (d) one or more sheath conduits positioned around the powder feed to provide (i) a sheathing gas stream between the powder stream and the tip of the needle projection and (ii) a sheathing gas stream between the powder stream and the ground electrode; or (e) one or more sheath ducts positioned around portions of one or more of the needle projections from an interior part of the device to the portion of the needle projections that protrudes into the charging cavity; or (f) the combination of one or more sheath ducts positioned around portions of one or more of the needle projections from an interior part of the device to the portion of the needle projections that protrudes into the charging cavity; a manifold connected to the sheath conduit(s) and positioned for directing gas through the sheath conduit(s); and a controllable source of gas pressure connected to the manifold.