Annular flow electrostatic powder coater

Details Annular flow electrostatic powder coater Annular flow electrostatic powder coater

1998-11-20

2001-06-05

Crispino, Richard (Department: 1734)

Coating apparatus

With means to apply electrical and/or radiant energy to work...

Electrostatic and/or electromagnetic attraction or...

C118S640000, C118S407000, C427S459000

Reexamination Certificate

active

06240873

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to powder coating systems and, more particularly, to systems that induce a swirling flow of air and electrostatically charged powder about a workpiece.
BACKGROUND OF THE INVENTION
Powder coating technology has generally evolved over several years into various types of coating techniques. Certain advanced techniques involve the use of electrostatic technology to adhere a powder, such as a resinous polymer or paint, to a desired workpiece preferably with a uniform thickness. The initial adherence of the powder to the surface of the workpiece takes place due to the attraction created by different electric charges existing on the workpiece and the particles of powder. After the initial electrostatic powder coating is formed, the coating is cured using techniques such as heat, infrared light or ultraviolet light, to fully adhere the coating to the workpiece.
Conventional electrostatic techniques for initially adhering the powder particles to a workpiece include two general types. The first type involves electrostatically charging the powder particles emanating in a cloud from a fluidized powder bed. When an electrically conductive, grounded workpiece is placed within the emanating cloud of electrostatically charged particles, the charged particles become attracted to the outer surfaces of the workpiece and form the initial layer of coating. The workpiece may be manually placed within a powder coating hood containing the electrostatically charged powder cloud or may be on a conveyor system or otherwise moved continuously through the hood or other powder coating area. The second powder coating technique utilizes a spray gun discharging electrostatically charged powder particles. An electrode at the gun nozzle may electrostatically charge the particles or the gun may emit a stream of powder particles charged by air upstream of the gun nozzle. With electrostatic guns, the workpieces are again grounded and typically placed within a spray coating hood during a coating operation to contain and collect excess sprayed powder.
A significant, continuing problem associated with electrostatically coating workpieces concerns achieving a uniform coating on the workpiece. With a workpiece oriented or moving horizontally, for example, this problem particularly exists with respect to top to bottom uniformity. That is, the lower surfaces of the workpiece tend to develop a heavier coating build than the upper surfaces. It is believed that this effect may be attributed to stratification, or a progressive decrease in density of particle distribution upwardly over the bed. Also, the charge on the particles may reduce in strength with increased remoteness from the voltage source and/or due to dissipation of the initial charge.
Various solutions have had different amounts of success in dealing with these problems. Some apparatus deal with these problems by creating a swirling or vortex-like flow of air and powder about the workpiece to more evenly distribute the powder particles on all outer surfaces of the workpiece. For example, U.S. Pat. Nos. 4,606,928; 4,808,432; and 5,773,097, each assigned to the assignee of the present invention, are all concerned with apparatus that distribute charged powder particles more uniformly about a workpiece, such as a continuous elongate strand of wire, cable, tubing or other like material. While devices such as these have met with significant success, it would still be desirable to provide improvements relative to achieving cost efficiencies, size reduction and reduced overall complexity of the devices as well as continued improvement in coating uniformity.
SUMMARY OF THE INVENTION
The present invention generally provides powder coating apparatus including a housing having a coating area positioned generally between first and second end walls with the first end wall having an inlet for receiving a workpiece and the second end wall having an outlet for allowing the workpiece to exit the coating area. A workpiece travel path extends between the inlet and the outlet and, for example, may generally define the travel path of an elongate wire, strand or other continuous or discrete workpieces moving along the travel path. In accordance with the invention, the coating area includes a surface extending lengthwise along the direction of the workpiece travel path and sloping transversely at least partially around the workpiece travel path. This transverse, sloped surface may be on an outer wall of the housing or may be a sloped interior wall or baffle structure in a conventional box-like housing or any other interior sloped surface achieving the effects of this invention. The surface is preferably sloped in at least two directions to promote an annular swirling air pattern as will be described below and is more preferably a continuously curving wall surface, such as a cylindrically-shaped wall surface. An air moving device is positioned to move air adjacent this surface to produce the swirling air pattern about the workpiece. In the preferred embodiment, the air moving device introduces pressurized air against the surface and lengthwise along the workpiece travel path. This surface may be referred to as a vortex inducing surface because the air follows the surface and generally flows in an annular swirling pattern around at least a portion of the workpiece travel path. A powder introducer is operatively connected with the coating area of the housing and introduces powder into the annular swirling pattern of air to coat the workpiece. It is contemplated that the air moving device could be a vacuum producing device and it is preferred that the powder introducer directs electrostatically charged powder into the swirling air pattern.
The pressurized air input device most preferably comprises a tube extending along the length of the vortex inducing surface and having a plurality of apertures or, for example, one or more slots for directing air into the swirling pattern. As one illustrative alternative, the pressurized air input device may instead comprise a plurality of separate air inputs, such as nozzles, mounted adjacent the vortex inducing surface. The charged powder introducer preferably includes a powder fluidizing bed communicating with the coating area and adapted to receive a supply of ionized or charged air to form a cloud of charged powder. A powder feed hopper may be provided to supply powder to the fluidizing bed. In this preferred embodiment, the annular swirling pattern of air also swirls the charged powder cloud generally around and into the workpiece travel path. As one illustrative alternative, the charged powder introducer may include a charged powder supplier, such as a triboelectric powder spray gun, connected with the pressurized air input device for supplying charged powder directly into the swirling pattern. In another alternative, a spray gun or other charged powder introducer may be otherwise connected with the coating area.
In another aspect of the invention, a pair of vacuum chambers may be connected proximate the inlet and outlet of the housing to prevent powder from exiting the housing. More specifically, one or both chambers may be connected to a source of vacuum in a manner that draws air and powder out of the respective chambers in a swirling pattern. When used in conjunction with the vortex inducing system associated with the coating area or another vortex system, the air and powder flow pattern in the vacuum chamber or chambers preferably swirls in the same direction as the swirling pattern in the coating area.
From the foregoing description, it will be recognized that a cost efficient, relatively simplified apparatus has been provided for inducing a swirling or vortex-type of charged powder and air flow around a workpiece for effecting uniform electrostatic powder coating. The use of inlet and outlet vacuum chambers also inducing a swirling pattern of powder and air further promotes a uniform coating.
Other embodiments of the invention also generally comprise a housing having a coating a

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